UNITED STATES DEPARTMENT OF COMMERCE • Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS • Lawrence M. Kushner, Acting Director

Dental Materials Research

Proceedings of the 50th Anniversary Symposium

Held at the National Bureau of Standards

Gaithersburg, Md., October 6-8, 1969,

in Recognition of Fifty Years of Dental Research at NBS

George Dickson and James M. Cassel, Editors

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Sponsored by the National Bureau of Standards

and

The American Dental Association

National Bureau of Standards Special Publication 354

Nat. Bur. Stand. (U.S.), Spec. PubL 354, 238 pages (July 1972)

CODEN: XNBSAV

Issued July 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C 13.10: 354), Price S3. 75 (cloth) Stock Number 0303-0944
Abstract

A Symposium on Dental Materials Research was held at the National Bureau of

Standards, October 6-8, 1969, on the occasion of the fiftieth anniversary of the dental re- search program at NBS. The Symposium brought together outstanding researchers in the

dental materials field from throughout the world for a comprehensive examination of the

present state of research and a look at future dental needs and expectations. The program

covered the broad dental materials field—from an examination of the oral environment to a consideration of future needs from the viewpoints of dental practice, dental education,

dental industry, and basic science. Invited papers covered metals research, new developments

in nonmetallic restorative materials, dynamic methods for determining the mechanical properties of dental materials, and problems of evaluating dental materials and making such

evaluations useful to clinical dentistry through the development of specifications.

Key words : Adhesives ; composite restorative materials ; dental materials research ; dental

restorative materials ; future dental needs ; laboratory testing and clinical research

mechanical properties ; metals research ; specifications.

Library of Congress Catalog Card Number : 70-178251
Foreword

The Dental Research Section, a unit within the Polymers Division of the Institute for

Materials Research, National Bureau of Standards, renders a unique assistance to efforts to

improve dental health. Tlie research program is a cooperative effort both in personnel and funding involving several agencies of government and a major professional health association. The

specific aim is to bring physical science expertise and instrumentation to bear on those aspects of

dental research which may yield only to this approach.

The current collaborative dental research program at NBS is conducted in cooperation with

the Council on Dental Research of the American Dental Association ; the National Institute for

Dental Research ; the Dental Research Division of the U. S. Army Medical Research and Development Command ; the Dental Sciences Division of the School of Aerospace Medicine, U.S.A.F.

and the Veterans Administration.

Dental research was begun at the National Bureau of Standards in 1919 in response to a

request from the War Department for assistance in evaluating dental materials purchased by the

Government. From 1919-1922 the research staff consisted of Dr. Wilmer Souder, in whose honor

the present symposium has been dedicated. From 1922-1928, the Weitistein Research Laboratory

supported Research Associates as assistants to Dr. Souder. In 1928, the American Dental Association established its Research Associate Program and this continuing collaboration has en- couraged close rapport among this physical science laboratory, dental manufacturers, and

practicing dentists.

The achievements of the Dental Research Section during the past 50 years have transformed

the practice of dentistry in many ways. Early accomplishments included the development of

precision casting techniques for gold alloys. Later innovations included a high-speed contraangle turbine drill, a panoramic dental x-ray machine, spherical particle amalgam alloy, and

composite restorative materials. Current efforts are directed toward a better understanding of

the fundamental properties of tooth structure, the physical and chemical mechanisms relating

to initiation and development of caries, and toward the development of new and improved materials and instrumentation.

It is particularly important, in paying tribute to a half century of dental research, that

opportimity be provided in this symposium to see not only where we have been but where we

need to go. The response of dental materials experts from aroimd the world in meeting the challenge of delineating the research needs for the future is especially exhilarating.

J. D. Hoffman, Director

Institute for Materials Research

National Bureau of Standards

• • • lU
Preface

This book is the formal report of the iDroceedings of the 50th Anniversary Symposium on

Dental Materials Keseai'ch sponsored by the National Bureau of Standards and the American

Dental Association with the cooperation of Johnson and Johnson, Kerr Manufacturing Company, and Surgident, Ltd. The Symposium brought together many outstanding researchers for a comprehensive examination of the present state of research and a look at future dental needs

and expectations.

The papers included herein encompass the broad dental materials field—from an examination of the oral environment to a consideration of future needs from the viewpoint of dental

practice, dental education, dental industry, and basic science. Invited papers cover metals re- search, new developments in nonmetallic restorative materials, dynamic methods for determining the mechanical properties of dental materials, and problems of evaluating dental materials

and making such evaluations useful to clinical dentistry thi'ough the development of

specifications.

The symposium offered an opportunity to pay tribiite to the founder and for many years

guiding inspiration of the dental research program at NBS, Dr. Wilmer Souder. It also recognized the outstanding leadership of Dr. Irl C. Schoonover in the development of a unique

organization for dental research which has directly involved a government laboratory, the

National Bureau of Standards, and a private professional society, the American Dental Association, in a cooperative program supported and participated in. by the National Institute of

Dental Research, the Armed Services and the Veterans Administration Dental Corps.

Members of the Symposium Committee wish to express their appreciation to the authors

and to all participants who contributed toward making the symposium a truly memorable event.

Thanks are also given to Mrs. Marion Kumpula of the ADA Research Unit at NBS and to Mrs.

Ruth Davenport for their attention to the many details involved prior, duiing, and immediately

after the symposium. The NBS OiFice of Technical Information and Publications under the

direction of W. R. Tilley, with special help from Robert T. Cook, gave invaluable assistance in

many phases of the effort. Particularly appreciated is the assistance of Johnson and Johnson,

Kerr Manufacturing Company, and Surgident Ltd. in contributing to the funding of the

Symposium.

Committee for the 50th Anniversary

Symposium on Dental Materials Research

Gerhard M. Brauer

Walter E. Brown

James M. Cassel

Harold J. Caul

George Dickson

George C. Pafl'enbarger

William T. Sweeney, Chairman

Identification of some commercial materials and equipment has been necessary in this book. In no case does such identification impiy recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment is necessarily the best available for the purpose.

iv
Contents

Page

Foreword iii

Preface iv

I. INTRODUCTORY SESSION

Dental Research at the National Bureau of Standards—Reminiscences 3

Souder, W.

Dental Research at the National Bureau of Standards—History and Individuals 7 Sweeney, W. T.

II. FUTURE NEEDS FOR RESEARCH IN DENTAL MATERIALS

The Need lor Basic Research in Dental Materials 15

Peyton, F. A.

Research Needed by the Federal Dental Services 19

Copeland, H. I.

Research Needed by the Dental Industry 23

Glenn, J. F.

Research Needed for Dental Education and Practice 27

Phillips, R. W.

III. METALS RESEARCH

Amalgams in Dentistry 33

J0rgensen, K. D.

Basic Metallurgy of Dental Amalgams 43

Johnson, L. B., Jr., and Wilsdorf, H. G. F.

Casting Alloys in Dentistry 61

Asgar, K.

Basic Metallurgy of Dental Casting Alloys 67

Nielsen, J. P.

IV. NEW DEVELOPMENTS IN NONMETALLIC RESTORATIVE MATERIALS

Dental Porcelain 77

McLean, J. W.

Dental Silicate Cements 85

Wilson, A. D.

Composite Restorative Materials 93

Bowen, R. L., Barton, J. A., Jr., and Mullineaux, A. L.

Cements Containing o-Etlioxybenzoic Acid (EBA) 101

Brauer, G. M.

Organic Adhesives 113

Alter, H., and Fookson, A.

V
V. MECHANICAL PROPERTIES

Page

Viscoelastic Behavior 127

Oglesby, P. L.

Low Frequency Determination of Mechanical Properlies 145

Myerson, R. L.

Ultrasonic Methods for Determination of Mechanical Properties 161

Dickson, G.

Stress Analysis of Dental Structures 169

Craig, R. G.

Relations between Mechanical Properties and Clinical Behavior 177

Mahler, D. B.

VI. DEVELOPMENT OF IMPROVED METHODS FOR EVALUATING

DENTAL MATERIALS

Need for Research to Develop Performance Characteristics 183

Schoenmakers, H. P. L.

Need for Correlation between Laboratory Testing and Clinical Research 187

Hedeg§,rd, B.

Biological Evaluation of Dental Materials 191

Ryge, G.

Corrosion Testing in the Mouth 201

Nagai, K.

VII. SPECIFICATIONS

International Specification Program—Australian Experience 209

Docking, A. R.

Development of European Specifications and Testing 213

Laplaud, P.

Development of South American Specifications and Testing—Brazilian Experience 217

Siiffert, L. W.

USA Specification and Evaluation Programs 221

Stanford, J. W.

VIII. APPENDIX. NBS DENTAL RESEARCH SECTION PERSONNEL

Personnel of the Dental Research Section of the National Bureau of Standards 227

vi
I. Introductory Session

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 19T2).

Dental Research at the National Bureau of Standards—Reminiscences

Wilmer Souder*

Chief, Dental Research Section, NBS, 1919-1945

In 1918 dental amalgams had a high rate of failure and few data on their physical

properties were available. Using the interferometer for determination of the dimensional

changes of amalgam and scientific test methods for other properties, NBS began to obtain

information on the physical properties of dental materials. The specification for dental

amalgam developed in the early days with numerical limits for physical and clinical

properties and details of test methods has served as a model for specifications for dental

materials for over 40 years. Although the early results were challenged and the program

opposed by some, the dental profession soon recognized the value of the work and

requested its expansion. In time schools, dental associations, and manufacturers joined in

commending the research program.

Key words : Amalgam, dental ; American Dental Association ; dental materials ; Dental

Research Section, NBS ; interferometer, dental ; specifications, dentaL

1. Reminiscences

Thank you, Mr. Chairman, for the kind invitation to attend and the liberty to address this symposium. More than ten years ago the Bureau said

"Good Bye and Best Wishes". No attempt has been

made to have succeeding leaders change their programs. I had my opportunity and am pleased with

the results. The future opportunities and prospects

are bright. Your request for I'eminiscences is dangerous. It means looking back. One of our former

directors, Dr. Lyman J. Briggs, had a motto

"Never look back ; something might be gaining on

you". Any attempt to brag on the achievements

would flavor of conceit. To criticize would be dangerous in the presence of this audience. So what is

left? There are some personal errors, then some

confrontations and a rapprochement of all interested parties. It seems proper to document these, as they were encountered.

No honest comment on the research can be made

without an acknowledgement of the aid given by

loyal assistants. Among the first of these were,

Peters, Coleman, Hidnert, Sweeney, Swanger,

Taylor, Isaacs, and Berger ; and later Caul, Dickson, Paffenbarger, Lynch, Brauer, Jordon, Schoonover, and Eichardson. Others are named in various

publications and may be mentioned by those who

follow on this program.

2. A Dental Research Considered

A preliminary review of the dental situation in

1918 revealed a high degree of failures in dental

restorations made from silver amalgams. Claims

and endorsements were available and techniques

were numerous and elaborate but not in teclmical

Present address : Landisville, Pennsylvania 17538

terms. Tests to substantiate these claims in terms

of physical, chemical, and engineering properties

(necessary in restoring the needed functioning of a tooth) were not found. The pioneer who first

pointed out this defect was Dr. G. V. Black. He

had a clear understanding of the needs and methods

for establishing the quality of an amalgam. His

heroic efforts to create a micrometer sensitive

enough to document, with definite accuracy, the

length and volume changes in amalgams, and their

dependence on chemical compositions and operational practices, were not entirely satisfactory. His

micrometer w^as dependent on levers and gears

which introduced friction in pivots plus hysteresis

lags. This preliminary search pointed to the necessity of a more sensitive micrometer. The interferometer, having a normal sensitivity of one millionth of an inch, is such an instrument.

3. Precision Data

Mr. C. G. Peters, of the Bureau's optical laboratory, had an interferometer and gladly consented

to measure the length changes in amalgam specimens wliich I condensed. These tests established, in

definite terms, the changes and defects which Black

had suspected. It is doubted that any dental college, dentist, or manufacturer had an instrument

approaching the precision of the interferometer.

Peters and I published our findings in the Physical

Review [1].^ Our data established a difference of

three to one for the thermal expansions of amalgam to dentin; the amalgam having the higher

value. Tliis established the necessity for a positive

setting expansion of all amalgams to cancel any

separation betAveen the amalgam and the tooth

1 Figures in brackets indicate tlie literature references at ttie end of tliis paper.

3
when exposed to cold foods or drinks. Several

brands gave setting shrinkages. The dentist is called upon to create a restoration surpassing the

perfection of the natural tooth which has failed.

Furthermore he must completely exclude germs,

acids, and all other hazards; always ready to

destroy his restoration. No immediate resiDonse

came from this publication. I decided to create an

interest by dentists, manufacturers, and the public

by writing a news item for release by the Bureau

containing, among other information, a statement

"The public could better avoid the dentist who

drills out a cavity and fills it with a shrinking

amalgam". The news item was sent to the Department for approval. The item was returned with a statement (essentially) as follows: "There is no

question on the accuracy of the tests ; however, it is not considered wise to create a situation such as

this would create unless, at the same time, a remedy

is offered." So died the attempt to awaken the

public.

4. Sparrings

Unfortunately or otherwise, a metallurgist, interested in amalgam alloys, made some disparaging remarks about our budding efforts and pointed

out unimportant variations in our results when he

made a report on his own tests. These were printed

in a metallurgical journal. Peters and I set up a reply criticizing his results which revealed greater

variations than any of ours. Also we referred to

his great emphasis upon a transformation in amalgams when heated to 80 degrees Centigrade; a

condition not compatible with dental health. We

were were careful to make no reference to our

connections with the Bureau and paid for the

publication of our reply. We were elated at the

neatness of our reply—until a few days after the

appearance of the journal. We were asked to come

to the office of the Director at our convenience. We

were anticipating the approval of Director George

K. Burgess, a metallurgist of the first water. He

inquired about our work. Then he showed us a copy of the magazine containing our reply and

asked what we knew about it. We said we prepared it and paid for its printing and purposely

avoided any reference to the Bureau. Then the

sky fell in. He chewed us out as they say in the

Army. He reminded us we had stooped to put our

abilities on a level of the one we had attempted

to expose. He said the Bureau does not encour- age scraps. It looked like the last day for two

career Civil Service employees. Then his attitude

changed. He said "You have abilities, you can develop positive approaches. Go back to your laboratories and prove it". From that day we kept

within the traces, absorbing most criticisms.

5. Specifications

A request from the Army in 1918 asking for

assistance in awarding contracts for dental amalgam alloys gave the Bureau its first opportunity to

give service in this field. About twelve samples

were submitted with the request. There were nu- merous claims for superiority and endorsements

printed on the cartons, but not one numerical

datum to support the claims. After making tests

of setting expansion, compressive strength, and

ease of amalgamation, it was possible to rate most

of the samples as defective or inferior. The Army

was advised which of the remaining alloys should

give satisfaction. This experience pointed to a need for a real specification. Accordingly a study

was inaugurated and a specification [2] was set up. This specification was modeled on the pattern

of the American Medical Association's Pharmacopoeia specifications. It gives numerical limits for

physical and chemical tests and refers to instruments and techniques for making the tests. These

two are musts for any intelligible test. Otherwise

quibblings are sure to erupt. I claim credit for

this theft or appropriation of the American Medical Association's pattern. The limiting numbers

may be changed as qualities are improved by the

manufacturers. This type of specification for vari- ous dental materials has survived forty years and

is basic for the Certification Plan adopted later by

the American Dental Association.

6. Questioned Progress

The service to the Army Dental Corps was the

first signal of a serious interest in the Bureau's

work in the selection of dental materials. As might

be expected, some manufacturers (whose products

might not meet a specification) were not enthusiastic about the plan. Deans, prominent lecturers,

and dentists whose personal endorsements were

ignored in the specifications were in a clouded

area. We were disappointed when the Journal of

the American Dental Association in 1920 could

not find it possible to print our first extensive (32

page) report, "Physical Properties of Dental Materials". However, another agency was quite willing to print the report [3] with a volunteered

commending editorial. Later, consideration of the

refusal was understandable. With certain members

of the Association lukewarm and several manufacturers suspicious of the outcome of the Bureau's

work, they can be excused for the decisions to wait

for more assurance of the purposes. When con- vinced of the ability and integrity of the Bureau,

the American Dental Association became one of our loyal suppoi'ters.

7. Genuine Interest

After the publication of the first extensive re- port, interest developed rapidly. Correspondence

with dentists, schools, and associations requesting

lectures and clinics piled up. The Director approved many of these requests. (The Bureau position, relating to such requests assumed that the Bureau official would be reimbursed for travel and

4
subsistence costs and that no honorarium would be

accepted.) Requests for an extension of the program to include a study of dental gold alloys, ce- ments, and accessory materials came to the Bureau. The Bureau felt that after having set up a pattern for research in the field, it should not as- sume the responsibility and expense for extensions. However, it did ask for advice [4] on the

problem. Dr. Louis J. Weinstein, Director of the

Weinstein Research Laboratory in New York, ex- pressed a genuine interest in seeing the research

continued.

8. The Research Associate Plan

Dr. Weinstein's request was simple and direct

"I want to see data on dental gold alloys and ac- cessories which will stand up when presented before schools and private groups and not be con- fused by data in present day texts and glaring advertisements." The Bureau Research Associate plan

was explained to him whereby qualified scientists

may work at the Bureau on problems of public

interest. The Bureau directs and supervises the

work and publishes the results. The sponsor pays

the salary of the associate. Dr. Weinstein accepted

the opportunity and supported the program for six

years (1922-1928).

9. Organized Opposition

The first comprehensive report [5, 6, 7] on the

Weinstein Research Associate's achievements was

given at the Dallas meeting of the American Dental Association. It was quite evident at this meeting and in other happenmgs that a fission between

vested interests and the research associates was

developing. These interests wanted to crush the

research. They appealed to the Secretary of Commerce, insisting that the reports were creating

confusion among the dentists and in the schools

and requested that the work be stopped. The Bureau was asked for a statement on the claims. The

Director explained to the Assistant Secretary that

the research was a public operation and all findings

were announced promptly. Furthermore, he stated

that the program was a health-saving research

needed by every citizen of the United States. No

cease or desist order was issued. By some means

the dentists across the Nation become aware of

what was about to happen and were preparing for

a fight. Had this developed, with the support of

90,000 dentists across the Nation, it would have

made the Bureau's later AD-X2 confrontation

look like child's play.

10. American Dental Association

Meanwhile, the American Dental Association

had expressed an interest in and a desire to co- operate formally and actively in the program. The

details for this cooperation were completed in

1927 and the cooperative program has continued

unbroken from 1928 to date. In 1942 Dr. M. D.

Huff, Chairman of the Research Commission, ex- pressed the feelings of the American Dental Association in the following words [8] : "It was a

fortunate day when Doctors Barber, Brown, and

Vol! and of the Research Commission went to the

National Bureau of Standards in 1927 to complete

the details of the cooperative research which has

become so valuable to the dental profession. . . . Our associates have been wholeheartedly welcomed

by the Bureau. . . . The supervision of the work,

the assumption of responsibility for the data, and

the complete publication of all findings by the

Bureau, have given us an authoritative position

seldom possible in such fields of research."

11. World War II

During World II the Bui-eau found it necessary

to curtail activity on many programs as it devoted

about 90 percent of its work to problems of defense. It was declared a Restricted Area under the

supervision of the Departments of War and Navy.

The American Dental Association sent a prompt

and patriotic request to the Bureau authorizing it to feel free to transfer its dental associates to Bureau military work as needed, salaries to be con- tinued by the Association. Both, Bureau and

Association scientists, filled assigned military

positions with distinction. Dental research was

restored to its full-time activity at the close of the war. 12. Rapprochements

Within 10 years after the previously mentioned

oppositions were met, all dissenters had disappeared. The schools, societies, and manufacturers

were joining in their commendations of the work.

New and and better materials and techniques have

been discovered. Perhaps the comment of one manufacturer can be used as a summary: "The re- search and specifications are our only effective

protections against competitoi-s trying to exploit

inferior materials by glaring advertisements and

personal endorsements".

13. An Unfinished Research

One project on the possible "Remineralization

of Dentin", started by Schoonover and Souder,

was dropped by reason of the more important war

needs. The illustrations in the preliminary report

[9] show a deposit of some material in the hardness

indentations; the length of the indentation mark

is shortened after exposure to a fluoride solution.

The body's ability to repair a broken bone suggested the possibility of some such attempt to

repair injury to dentin, under favorable conditions. Dr. von Buest of the University of Louisville

Dental School was the only one to give us en- coiiragement on the idea. Quotes from recent re5
ports on the effects of fluoride are given below

Bierman, M.D. [10] "the (cancerous) bone

becomes harder and more durable". Hoffman,

NDRI [11] "topical fluoride can reverse the dental

caries process and may even heal incipient caries".

A research [12] ". . . into the molecular basis

of disease" has been announced. Even a TV

puppet declares that fluoride makes a tooth

stronger. Perhaps some budding researcher may

decide to make an independent evaluation of these

reports.

14. Fare Thee Well

With these scattered reminiscences my part in

this program must close. These are no feelings of

resentment toward anyone. Rewards and gestures

of appreciation have been excessive. My response

is, Thank You and Best Wishes for the Future.

15. References

[1] Peters, C. G., and Souder, W. H., Some physical

properties of dental materials, Phys. Rev. NS 13,

302 (April 1919).

[2] Souder, Wilmer, Measurement and application of

certain physical properties of dental amalgam.

J. Dental Res. 7:173 (June 1927).

[3] iSouder, Wilmer H., and Peters, Chauncey G., An

investigation of the physical properties of dental

materials. Dental Cosmos 62:305 (March 1920).

[4] Burgess, George K., Dental research by Department

of Commerce. J. Am. Dental Assoc. 11:249

(March 1924).

[5] Coleman, R. L., Physical properties of dental materials (wrought gold alloys). J. Am. Dental

Assoc. 12:.520 (May 1925).

[6] Coleman, R. L., Physical properties of dental materials (III) Progress Report of research on the

dental casting process. Dental Cosmos 68:743

(August 1926).

[7] Coleman, R. L., Physical properties of dental materials (gold alloys and accessory materials)

BS J. Research 1, 867 (1928) RP32.

[8] Souder, Wilmer, and Paffenbarger, George C, Physical Properties of Dental Materials, NBS Circ.

433, 222 pages (1942), Foreword.

[9] Souder, Wilmer, and Schoonover, Irl C, Experimental remineralization of dentin. J. Am. Dental

Assoc. 31:1579 (December 1944).

[10] J. Am. Med. Assoc. 208: (No 6) 953, May 12, 1969.

[11] Am. Dental Assoc. News Letter 22 : (No 7) March 31,

1969.

[12] Physics Today (We Hear That) June 1969, p. 97.

6
NATIONAL BUREAU OF STANBARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Dental Research at the National Bureau of Standards—History and Individuals

W. T. Sweeney*

Chief, Dental Research Section, NBS, 1953-1968

Dental research at NBS was initiated in 1919 by Dr. Wilmer Souder with work on

dental amalgams. Dr. Souder built the Section on the principle of cooperation between

revsearoli laboratories, manufacturers and the dental profession. In the 1920's research

associates, first from the Weinstein Research Laboratory and later from the American

Dental Association were added to the NBS dental research staff. Dr. N. O. Taylor and

Dr. George C. Paffenbarger were the first ADA Research Associates. In 1945 while Dr.

Irl C. Schoonover was Chief of the Section, the laboratory staff was further enlarged by the

addition of Guest Workers from the Armed Services. Among the many important areas of research were precision casting, dental cements, polymers for dentures bases, composite

I'estorative materials, high speed turbine handpieces, panoramic X-ray equipment, studies

of natural tooth structure and development of standards and specifications.

Key words : Amalgam, dental ; American Dental Association ; casting, dental ; dental

research ; Dental Research Section, NBS ; guest workers ; Paffenbarger, George C.

Schoonover, Irl C. ; Souder, Wilmer ; .specifications, dental.

1 . Introduction

'The objective of this paper is to' give a brief and

concise report of the most outstanding events, personnel, and accomplishments of the dental re- search program at the National Bureau of Standards in order to set the stage for the symposium

honoring the 50th Anniversary of the program.

Only a few of the many persons and events can

be discussed or even mentioned in the limited

time allowed.

The name of Dr. Wilmer Souder is the most

outstanding of all the scientific staff because he is responsible for setting the character and ideals of

the program and was chief of the Section for the

first half of its life. The merit of bringing good

scientific measurements into the health field as an

adjimct to better dental health care for the public

was thereby established. He also realized that a

cooperative effort was essential between the three

groups, dental profession, manufacturers, and

general public as represented by the government.

While this sounds trite, looking back today, it was

not a very popular idea when he first started in

1919, as there were very few who had the longrange vision to see the advantages of such cooperation.

2. Early or Formative Period, 1919-22

This period was the most important as it is the

germ from which the Section grew. The original

request from the War Department to the National

Bureau of Standards for assistance in evaluating

Present address : School of Dentistry, University of Alabama,

Birmingham, Alabama 35233.

dental products purchased for the Federal services was referred to Dr. Souder for a routine re- ply. Amalgam was the material of interest. Souder, as a young physicist fresh from his Ph.D.

studies at the University of Chicago, naturally

looked at the problem scientifically. The more he

examined the basis for selection of dental materials, the more he was convinced that the science

of measurement could make a contribution to the

dental profession and the dental treatment of the

public. He, in short, visioned the substitution of

quantitative measurements of properties for the

art of personal skill and personal recommendations. He had the zeal and forethought to advocate and demonstrate that physical science had

a great many things to offer the health professions.

A few people, notably G. V. Black, a dentist,

had carried out some very basic investigations

but the precision of a physicist was almost un- known to the science of materials used by the

dentist. Souder first turned his attention to dental

amalgams and in 1919 published his report on

the measurement of such properties as dimensional setting changes, strength, and flow. His re- sults and methods are good today even after 50

years and have led to much improvement of alloys and techniques for dental application. He

held the theory that the best way to evaluate any

material is to measure the properties that are important for its application. He meant to obtain

numerical evaluations by measurement and then

delineate his procedures so that any competent

scientist could repeat and check his results. Tliis

was a unique point of view in the field of dental

materials. He was told that it was impossible to

7
determine the elements in certain alloys, as the

analytical methods were too crude. As a matter of

fact this was correct, in some cases at least, but he

answered this by saying that if we obtained a

scientifically trained staff, we could overcome this

and he certainly proved it. Souder, realizing in this early period that a working together oi the research laboratories,

manufacturers, and profession was required if the

public were to receive maximum dental sei"vice,

tried to interest the dental profession through

their national organization to join in a cooperative program. While certain individuals gave

moral support to the idea, the orgaiiization was

not yet willing to put financial support into the

research program. When this failed he was fortunate to secure financial assistance from Weinstein

Research Laboratories, who provided sivbstantia^l

support from 1922 to 1928.

3. Expansion of the Program Via Research

Associates, 1922-28

This period was most productive as it represents

the first expansion of the dental work by the addition of research associates to the group. The re- search associates were directed by NBS and supported by the Weinstein Research Laboratory. The

first research associate, R. L. Coleman, an engineer, was selected from the NBS Staff, a member

of the Weights and Measures Division. His work

on measurements of properties of gold alloys and

precision casting techniques is dlassical in that it set

the standard for precision casting, not only in the

dental field but later in industry, such as in the

casting of blades for turbines. During this period

the first dental specification for amalgam alloys

was promulgated, based on the original work of

Souder; and the properties of accessoi'y materials

used in dental casting, such as waxes, investments,

and orthodontic wires were studied, and standard

measuring techniques were developed so that specifications could be written later.

The chemical analysis of alloys of gold and the

platinum group metals by the joint efforts of

Raleigh Gilchrist, an NBS chemist, and William

Swanger, a research associate, was an example of

how the cooperative effort produced worthwhile

scientific results. Their reports justify Souder's

prediction that well trained chemists could determine accurately the composition of dental alloys.

After these reports, it was obvious that secret formulas would be secret only until someone was willing to spend the time and money to do' precision

chemical analysis. This work created many controversial problems for NBS on the policy of publication of analyses of trade brand alloys or materials. These problems have not yet completely

disappeared because it is a moot question as to how

far a tax-supported institution should go in publishing trade secrets that individual companies

have spent much in developing. In general, the

dental profession argued that it should know ex- actly what it uses and the manufacturer thought

lie needed protection because he spent a lot of

money to advance his product. The principle has

been to use trade names in publications if they are necessary for understanding by the reader. Details

of methods of analysis were given in scientific re- ports so anyone competent could evaluate properties versus composition.

Verbal reports and many articles by the staff

of the Section during this period created such a demand by the dental profession for more scientific evaluations that by 1928 the profession was

willing to support the research program and give

official recognition to it by formal agreement between the American Dental Association and NBS.

This arrangement made for a much broader base

of operation and the liaison has proven most profitable to all concerned.

4. Cooperative Program With American

Dental Association, 1928—45

In April 1928 the coof)erative program between

the American Dental Association and the National

Bureau of Standards was initiated. The first ADA

research associate was Dr. N. O. Taylor, a chemist, who was followed in 1929 by Dr. George C.

Paffenbarger, the first dentist to work full time

in the Section. The period of the late 1920's and

1930's witnessed a great expansion of the research

program through the addition of both NBS and

ADA personnel. Basic research on the physical and

chemical reactions of many materials was undertaken. This period was productive and an improved understanding was obtained of the properties of practically all the materials used in re- storative dentistry from amalgam alloys to denture

resins. The aj^proach was basically the same for

all materials. First a study was made of the properties of the available materials and how they were

affected by composition and by techniques used in

practice. Typical examples were studies of the

effect of heat treatment on gold alloys and how the

mechanical properties were changed by cooling

rates in the dental laboratory ; how the method of

mixing cements could drastically change the useful

properties of the material; and how exposure to

light affected the color stability of pink denture

base resins.

Many special methods were developed for measuring proi^erties of these different materials. The

fact that dentistry uses small specimens in comparison to those commonly employed in industry

makes it necessary to use unique test methods. Special tests were developed to more closely simulate

dental use. The fact that the oral cavity has a varied atmosphei'e with a variety of temperatures

and humidity, makes it necessary that materials

be evaluated under conditions which are found in

clinical use. Biological requii-ements put many re- strictions on otherwise suitable materials.

8
The specification program was a major development as it made the results of research useful to the

general dentist and the public. The history of the

research program during this period followed a

consistent pattern of sufficient research on a type

of material to understand the basic reactions and

properties of dental significance followed by a sur- vey of the existing materials to ascertain what

could be reasonably supplied and to determine the

range of properties of the satisfactory materials.

A specification was then written using the information obtained to place numerical limits on the

properties thought to be important. This was re- viewed by a Specification Committee consisting

of representatives from the profession, the manufacturers, and the government (representing the

public). Following the recommendation of this

Committee the specification was adopted as an offi- cial specification of the American Dental Association. Producers were asked to voluntarily guarantee their products to comply by signing a formal

document of agreement. Specimens were purchased

on the open market and tested for compliance at

NBS by the ADA research associates. Materials

which complied were listed in the ADA journal.

Specifications developed for the Federal Government and advice provided in writing purchase

descriptions are among the many benefits of the

Section's program to the government.

The series of reports on dental casting and acces- sory materials by Taylor, Paffenbarger, and

Sweeney made it routinely possible to make clinically acceptable castings. This is in contrast to

an early investigation where only 2 of 25 leading

dentists were able to make an inlay casting that

would fit even reasonably well a standard die furnished them by NBS.

The very excellent work on analytical methods

for dental materials by Caul and Schoonover

added new and precise information to the scientific literature on such a wide variety of materials

as chrome-cobalt alloy, mercury-containing alloys,

resins, denture rubber, impression materials, etc. and made it possible for manufacturers to improve

and better control their production. These methods

are used also to define composition limits of elements in many modern specifications such as those

for amalgam alloy, and gold alloys.

5. Expansion of the Research Program to

Include Guest Workers From Federal

Dental Services and Foreign Countries,

1945-69

The period immediately following the end of

World War II saw a major expansion in the program. As a result of the large scale procurement

and utilization of dental materials by the Armed

Services, persons directly knowledgeable in dental

materials research and clinical practice had first

hand experiences which emphasized the great need

for more technical information. This made it possible for a very valuable liaison to be initiated.

Schoonover, with the help of Fischer (USAF)

and Paffenbarger (USN), cooperated to set up

working arrangements with the Federal dental

services that added great strength to the Section by

increasing both personnel and funds. This made it

possible for NBS to equip a clinical unit and correlate many laboratory findings with clinical practices. These arrangements also made it possible for

personnel from the Federal dental services to work

in the NBS Dental Research Section as Guest

Workers for periods of one to several years. The

guest workers included not only dentists with

much clinical experience but also scientifically

trained enlisted personnel (chemists, physicists,

engineers, etc.) . In addition to the research accomplished, this program resulted in providing the

services a number of dental officers trained in

materials research. Also to the benefit of NBS,

several of the enlisted guest workers remained

after their tours of duty in service to become permanent members of the NBS staff. It has been the policy to draw no sharp distinction between NBS staff members, ADA research

associates and guest workers in the operation of

the Section. A spirit of cooperation has prevailed

with the result that many are the reports co- authored by representatives of each of the three

groups. Guest workers have been encouraged to

take advanced study and frequently the research

carried out in connection with such studies involved collaboration with a Bureau senior scientist.

This period was characterized by many sharp

contrasts of opinions on the relative amount of

effort that should be put on basic in contrast to

applied research. It is my strong conviction that in

an area such as dental materials and clinical dentistry the most return will be obtained by conducting, as we have at NBS, the two side by side. Each

is an asset and a source of strength to the other.

In 1950 a very thorough review and bibliography was published by Schoonover and Souder ^ to which the reader is referred for many details

about the Section's work up to that time.

6. Summary—Resume of History and

Individuals

One cannot help but be impressed by reviewing,

such as I have, the programs since my first contact

on August 3, 1922. At that time the Section consisted of three scientists. Dr. Souder, Dr. Hidnert,

and Mr. Coleman. This is in contrast with today's

staff of thirty-five. About 175 individuals have been

associated with the Section during the past 50

years.

Space to even mention all the major projects is

not available but items that come to my mind as being specially noteworthy will be recorded with

1 NBS Cire. 497, 14 (1950).

9
the individuals connected with them. In doing this

it is realized that others may have different ideas

but it is certain that these items have been an

influence on dental practice.

1. The development of standards for dental

materials based on measurements of properties of

dental significance. This is probably the most used

and productive effort of the Section. It covered

many types of materials, in fact, most of the materials widely used in dentistry. The adoption of

specifications and the publication of a list of certified products by trade names added to the practical usefulness of the program. The combination

of professional approval and the scientific integrity of NBS combined to make a most reliable

buyers' guide for dentistry. Also, the specification

program served the manufacturers well, as it made

available a standard to compare the quality of

products without any reference to selling price.

The success of this program has spread to many

countries of the world and standards developed

here are used as models for others.

A survey a few years ago showed that only 0.1

percent of the items on the published certified

lists did not in fact meet all the requirements.

This is proof that the production control by

American manufacturers is very good. These re- sults are based on tests of materials bought in the

open market.

2. One item which is not usually emphasized

when discussing the Dental Research Section is

the effect the training of research personnel has

had on teaching of dental materials in the schools

and, in fact, on many techniques taught in restorative dentistry. The Federal dental services have

sent many of their best officers to the Section for

training and many have received advanced degrees using the research training in the Section

for credit. The cooperative program with Georgetown University has resulted in 16 master's degrees and two doctor's degrees. Eleven foreign

guest workers, for the most part representing dental schools, have been trained and returned to

teaclaing and research.

3. The publication of scientific data and the

explanation of the physical-chemical reactions of

materials, for example, the setting mechanism of

cements, the oral environmental effects on surfaces, and the effects of particle shapes of alloys

used for amalgam have lead to many material developments and to superior techniques for using

materials. The explanation of the cause of delayed

expansion of zinc-containing alloys resulted in the

elimination of the long used, undesirable, palming

technique for mixing.

4. In addition to publication, a much used

method of communication was the production of

motion pictures on the properties and techniques

for using many materials such as amalgam, ce- ments, resins, gold alloys, etc. These pictures are

in constant use in schools for explaining the basic

properties of materials and the effect handling has

on the clinical resiilts.

5. The research reports and communications

have been very productive in introducing new

materials and equipment ideas in the fields of both

clinical dentistry and dental research. The early

work of Souder produced the dental interferometer which has been adopted as standard around

the world for measuring the setting changes in

amalgam. The NBS standard MOD steel die is

probably the widest used device for evaluating

precision casting. This was the result of the precision casting work of Coleman. The fused-quartz

tube method of measuring the thermal expansion

of solids, first developed by Hidnert and Sweeney

in 1928, probably has been used more than any

other equipment for expansion measurement of

solids and especially for dental investments.

The panoramic x-ray equipment now in universal use in many clinics, was perfected by Hudson, Kumpula, and Dickson and is now commercially available with continuing expanding uses in

dentistry for recording the condition of teeth and

oral structures. This item has resulted in great

savings to the Federal dental services, more than

enough to repay the total cost of the whole Researcli Section's expenditure of public funds.

The turbine contra- angle high-speed handpiece

developed by Nelsen and Kmnpula is probably the

most important advancement in dental equipment

in this century. It has inspired manufacturers to use the principle to develop very sophisticated

clinical turbines and has revolutionized the practice of operative dentistry. It not only makes the

work of cavity preparation and tooth reduction

faster and easier for the dentist, but more important it is much more comfortable to the patient, so much so that no modern dentist today can operate

without high-speed handpieces. The number of

these instruments in use, both in this country and

foreign countries, is estimated to be in the hundreds of thousands.

The many instruments modified, or new, developed for use in specification tests are too numerous

to mention, but the method developed by Paffenbarger for testing standard consistency of cements

made it possible to compare on an equal basis the

properties of different brands of both zinc phosphate and silicate cements. The use by Sweeney of

cross index marks or pin inserts on dentures as reference points for measurement of microscopic

dimensional changes, served to evaluate the

accuracy and stability of a wide variety of denture

base materials from vulcanite to methyl methacrylate resins under clinical conditions. The technique has been used to study the effect of curing

and repairing processes on stability or warpage of

dentures in service.

The early discovery of cristobalite for use as an

investment material by Paffenbarger and Sweeney

was rewarding even though the patent was finally

10
awarded to previous investigators using an entirely

different source and method of preparation.

Schoonover developed methods for determining

the number average molecular weight of acrylic

resin which helped exj)lain many problems in cur- ing and construction of dentures. This was the first

report issued from NBS on effect of molecular

weight distribution on properties of a polymer.^

The relationship between properties and molecular

weight has been given extensive attention by re- search personnel of industry and government since

then, as it showed that chain length is as important

as elemental analysis in the area of polymeric

materials.

The development of spherical-particle alloy for

amalgam by D. F. Taylor brought a new and

promising type of alloy into general use. The

control of properties by suitable size distribution

and the low packing pressures required have made

this one of the most significant innovations in

amalgam research. Indications are that it may be

the most popular form of amalgam alloy in the

future. The explanation of the setting mechanism

of zinc oxide and eugenol cements by Copeland

and Brauer and the development of a new cement,

o-ethoxybenzoic acid (EBA), by Brauer are additional landmarks of the program. The consistent

progress made by Bowen in the field of composite

filled resins has resulted in the marketing of several products that are bringing a new era in the

quality of esthetic tooth restorations for anterior

teeth. The advancement toward adhesive materials

is also very promising and points the way to a major breakthrough in tooth restoration.

The development of methods for determining

the structure of natural teeth by Brown and

Moreno and physical properties of enamel and

dentin by Stanford have advanced the understand2 J. Amer. Dental Assoc. 25, 1487 (1938).

ing of the physical-chemical reactions that can

take place in the tooth. This will be of assistance in

obtaining a better rationale for the cause of caries.

The following special items and the names of

those most responsible for them are given—^not in

any order of relative importance

(a) Physical methods of evaluating dental

amalgam—Souder, Sweeney, Caul,

Burns, Dickson, and Oglesby

(b) Precision casting—Coleman

(c) Chemical analysis of gold aJlloys—Gilchrist and Swanger

(d) Analysis of dental cements—Isaacs,

Schoonover, Brauer, and Copeland

(e) Tests and evaluation of cements—Paffenbarger

(f ) Packaging of cements for military uses

Fischer and Bums

(g) Chemistry of resin polymers—Schoonover, Brauer and Caul

(h) Physical properties, methods of resin evaluation— Sweeney

(i) Gallium alloys as dental filling materials—^Waterstrat and D. L. Smith

(j) Turbine contra-angle high-speed handpiece—Nelsen and Kumpula

(k) Panoramic x-ray equipment—Hudson,

Kumpula and Dickson

(1) Standards and specifications for dental

materials—Souder, Taylor, Sweeney,

Paffenbarger, Beall, Caul, Bums,

Brauer, Dickson and others

(m) Promotion of national and international

standards for dental materials—Paffenbarger, Sweeney and Dickson

(n) Development of EBA cements—Brauer

(o) Composite filling material—Bowen

452-520 0—72 2 11

II. Future Needs for Research in Dental Materials

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental ^Iatekials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

The Need for Basic Research in Dental Materials

Floyd A. Peyton

School of Dentistry, University of Michigan, Ann Arbor, Mich. 48104

The field of dental materials has benefited from basic studies of the silver-tin alloy

system, of cobalt alloy systems, of polymers, and of many other areas. Future basic studies

of the surface phenomena of wetting, spreading of liquid films, adhesion, diffusion into

dental tissues, boundary interactions, and principles of viscoelasticity related to dental

materials offer productive opportunities. Basic research is essential if advances are to be

made in the improvement, modification, refinement, and development of servicable materials

for the practice of dentistry. It is anticipated that such studies will increase in importance

in the next quarter century and that basic scientists in many fields will be contributing to the improvement of dental service.

Key words : Basic research related to dental materials ; dental materials ; interdisciplinary

research ; training of dental researchers.

1. Introduction

Those who have followed the growth and development of dental materials throughout the

world during the past century readily recognize

the significant changes that have occurred in the

research activities in this field. There is no evidence to indicate that further changes will not

continue to occur in the years ahead. Certainly the

field is not dormant nor static at the present time.

The changes that have resulted since the last

part of the 19th century represent the efforts of

many persons who have conducted research

studies, made laboratory and clinical tests and

evaluations, and altered or modified materials,

techniques, and devices in order to serve the needs

of the dental profession to render more and improved service to the patient. This has required all

forms of research studies, as noted by this symposium, including basic studies ; practical, applied, or clinical testing ; and developmental projects. It

has required the effort, knowledge, and experience

of investigators throughout the world, located in

private or government supported institutes and

laboratories, universities, and dental schools, manufacturer's laboratories, as well as dentists in private or group practice. Not only have multiple

institutions contributed to the progress that has

been made, but also the training and background

of the individual investigators has been of a multidiscipline character.

At this time in history, when the multidiscipline

approach to research is being recognized and en- couraged, those in the dental materials field can

take pride in the fact that for half a century interdisciplinary studies have been common practice.

This practice has increased in recent years. The

basic scientist working with the clinician ; the biologically trained scientist working with the physical science investigator ; or the engineer, physicist, or chemist working to solve a problem of dental

practice, all have made contributions to this field.

The future holds even greater promise for interdisciplinary studies. As the subjects of bioengineering, biomaterials, and biomechanics are extended

into medical science, it is satisfying to know that

a similar relationship has existed and promises to

continue in the science of dentistry.

2. Types of Research

Perhaps no one should claim that one type of

research is more important or desirable than another. It can be recognized to advantage, however,

that different forms of research exist, and through

the combined efforts a contribution to society is made. Some forms of research appeal to certain

investigators more than others, resulting in a

greater degree of concentration on one type of

study than on another.

Basic research in the physical sciences represents

those studies that explore the fundamental nature

of a compound, a reaction, a physical state, or condition, a mathematical relationship to a physical

phenomenon, and describe the characteristics observed. Such studies explore the fundamental

physical forces and processes of nature. Basic sci- ence studies, however, are not limited to physical

phefiomena. Studies of a basic or fundamental

nature are common in the biological, social, behavioral, political, and economic fields of en- deavor. Perhaps all basic research studies attempt

to explore the fundamental nature of the subject,

in contrast to the practical, applied studies which

translate into service the fundamental concepts.

15
Generally it is recognized that basic or fundamental research studies involve investigations of a new, different, and exploratory nature. The scientific judgement of the investigator suggests that

more knowledge in a particular area will contribute to the welfare of mankind and aid in the

solution of present or future problems. The history

of scientific advances is filled with examples of

such basic and fundamental studies. One needs only

to recall the advances resulting from basic studies

in the field of relativity, x-ray and other forms of

radiation, atomic structure, and many other basic

investigations to recognize that research in the

fundamentals of science, is important as a foundation for scientific endeavor. Productive basic re- search studies provide results that can open whole

new approaches and solutions to practical problems. From the results of basic research studies,

new concepts are established which advance the

technical and scientific skills in many related

fields.

Studies in areas of basic research are confronted,

however, with numerous problems of varying magnitude. Frequently such studies are time consuming, require the service of specialized and highly

skilled personnel, or cannot be assured of success and practical application, in advance of the project

completion. The practical applications and potential significance of such basic studies frequently

are the subject of debate and speculation among

investigators having more practical interests.

Under these conditions, basic research studies frequently encounter difficulties in securing a source

of support funds, other than benevolent foundations or some government agencies. Even from

these sources the basis of justification is critical

and requires well documented and carefully

planned statements of experimental design and

procedure. This is proper, however, and only suggests that basic research studies are involved and

complex, requiring the utmost in well coordinated

efforts and skillful management for success. The ultimate gratification comes usually to the

investigator who is successful in his endeavors, and

ultimately sees his results applied to the solution

of practical problems. Perhaps there are relatively

few investigators who delight in conducting basic

research studies only for the sake of collecting such

data. Rather, it is their hope to open a whole new

solution to practical problems, when such studies are undertaken by other investigators. It is important to recognize, therefore, that normally following a successful basic research investigation

there will emerge numerous practical research

studies of varying magnitude and nature. Often

the dividing line is a thin one between basic, fundamental research studies and practical research

investigations.

3. Basic Research in Dental Materials

The field of dental materials has been fortunate

to have benefited from a variety of basic studies

on various materials in the past. The National Bureau of Standards has contributed much to these

basic studies. Frequently these have resulted in

eventual applications to the solution of practical

dental pi'oblems. Only a few need be listed to illustrate the benefits from basic research studies in

dental materials.

One that is recognized by all who are associated

with restorative materials and the practice of dentistry, is the metallurgical study of the silver-tin

alloy system and the reaction with mercury to produce dental amalgam. Many investigators have

contributed to the understanding of these alloys,

and the studies continue. The results have lead to a gradual improvement in the quality of dental

amalgam during the past 75 years, with better res- torations for the patient.

Another example of productive basic metallurgical studies involves cobalt base alloys for use in

removable cast partial restorations. From these

studies, practical prosthesis are a reality. Closely

associated with this development, and of equal

importance to the success of the small cast restoration, have been the basic studies of the precision

casting process. Dentistry has been accepted as a

leader in this specialized interest of precision casting, with numerous basic studies reported, as well

as practical applications described.

The development of the plastics industry within

the past 50 years has stimulated many basic studies

into the nature of plastics and the potential application in restoring lost tooth tissue. Another group

of materials that have special dental applications,

made possible from basic chemical research studies, are the several types of elastic impression materials presently in use in dental practice. These

hydrocolloid and synthetic rubber compounds are examples of basic chemical studies, leading to

practical applications which have resulted in altered methods and improved dental practice.

Anyone who is familiar with the investigations

that have been conducted at the National Bureau

of Standards, recognizes the contributions that

have been made to world dentistry through the development of physical test methods for various

dental materials, and the establishment of standard test procedures and specification methods. To a considerable degree, basic research studies were

involved in the development of this program. Subsequent assistance from the American Dental Association, the Federation Dentaire Internationale,

and now the United States of America Standards

Institute, has carried forward the initial basic

undertaking, and developed practical means of

evaluation. Basic studies related to this program

undoubtedly will continue.

The list might be extended to include devices

like hand pieces and x-ray machines, and the discussion expanded with documentation, but that is unnecessary to indicate that basic research studies

in dental materials have been productive in the

past, and have led to practical and applied studies

16
with benefits to dental practice. To extend this discussion would tread on the subjects of other speakers in this program.

4. Current Basic Investigations

With Promise

It might be well to list, without elaboration however, some current studies of a fundamental nature

which promise significant benefits to dentistry in

the years ahead. Some have been conducted here

at the National Bureau of Standards and others are being investigated elsewhere. Several will be

described in detail later in this program, so it is

sufficient to take note here only that unknown benefits to dentistry in the years ahead can be anticipated from these efforts.

One valuable area of study is that of composite

materials, particularly in the area of filled plastics.

This subject is being explored by many investigators. Another is the study of the basic nature of

the zinc oxide-eugenol compounds and the effects

of chemical modifiers on the characteristic properties of these materials. Practical results are beginning to emerge from each of these long and

difficult basic studies.

Studies on stress analysis in dental restorations

and supporting structures, by photoelastic and

other methods of analysis have involved numerous

basic problems. In like manner the studies that are being conducted on the friction and wear of materials used in the mouth require basic evaluations

that are new to dentistry.

In the same way studies on tissue reaction and

compatibility with restorative materials include

many basic concepts in biological and physical

sciences. The studies being conducted on clinical

evaluation and correlation with laboratory observations, presently are basic in nature but can produce long range practical benefits for the dental

profession.

5. Future Basic Investigations

As one surveys the horizon of the years ahead, one sees the opportunity for basic research investigations that can lead to productive practical problems extending into most areas of dental materials scientific endeavor. It can be anticipated that

the fundamentals of the surface phenomena of

wetting, spreading of liquid films, and adhesion to

tooth tissue will be further studied basically and

become better understood. Further studies with the

electron microscope and electron probe should provide a basic understanding of diffusion into dental

tissues, and the interaction occuring within the

material or at the boimdary surface. Future studies

will show the true potential benefit of the laser

beam and the related process of holography. Further studies in the principles of viscoelasticity

related to dental restorations, involving fundamental mathematical evaluations and interpretations to dental problems will continue.

This list might be extended into many other

areas of basic studies and investigation. Even

though other persons might choose other examples

of basic studies from the past or into the future

of dental materials research, these suggested

studies indicate the importance of basic research

to the field of dental materials.

Since basic studies related to many disciplines are involved in so much of the current dental matei'ials research and investigation, the education,

training, and interests of the investigator have

been enlarged significantly in recent years. The

training of the dentist and the dental teacher likewise has been influenced by this trend in materials

research. A broad spectrum of physical science

training presently is necessary for the application

of these principles to dentistry, whereas in the past

the biological sciences were predominant in the

training of dental investigators and practitioners.

It seems probable that this trend in training will

continue in the years ahead.

Most research investigators have a definite con- cept of what constitutes basic research, even though

it may differ to some degree from one person to

another. It is recognized by all, however, that basic

research is essential and fundamental if advances

are to be made in the improvement, modification,

refinement, and development of serviceable materials for the practice of dentistry. Basic research

studies in dental materials have come to be recognized as being essential to the further advances in

this area of dental science. It is anticipated that

such studies will increase in importance in the next

quarter century. So long as the dental profession is

concerned with improved dental service for the

patient, it seems certain that the basic scientist in

all fields will be offering his assistance to make that

service a reality.

17

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Research Needed by the Federal Dental Services*

Henry I. Copeland

Air Force Systems Command, United States Air Force, Washington, D.C. 20331

The dental research laboratories of the Federal Government function within the

Departments of Commerce ; Health, Education and Welfare ; and Defense. These laboratories

and the multidisciplined scientists therein along with those of the Veterans Administration

provide a much needed national resource. The Federal Dental Services have problems in

common with each other and with the profession at large, in addition to having problems

unique to themselves. Each Federal Dental Service needs a scientific staff capability to solve its own immediate problems or to contract for the solutions.

Key words : Capabilities and needs of Federal Dental Services ; dental research laboratories,

Federal ; dental staff ; Federal Dental Services.

1 . Introduction

Half a century ago it was an innovation at the

National Bureau of Standards to apply the scientific approach and the science of metrology to solve a specific dental problem generated by the Federal

Services—how to assure the procurement, storage,

and delivery of a dependably usable material to a Federal dental clinic.

This 50th Anniversary of the Dental Research

Section, National Bureau of Standards, is an excellent time to take a look at dental research progress, dental research resources, and how these resources may be best applied in the future—particularly to problems of the Federal Dental

Services.

Since metrology broke the ice with dentistry,

scientists of other disciplines have become intrigued with dental problems and have contributed

immensely toward the establishment of an organized body of knowledge relating specifically to

dentistry. Concurrently, there has been a proliferation of laboratories devoted to dental research and

development. As a result, there has been a tre- mendously significant improA^ement in the methods,

materials, and equipment used in dentistry.

2. Dental Materials Research in the

Federal Government

At least three cabinet departments of the Federal

government have dental research laboratories

Commerce; Health, Education, and Welfare; and

Defense. The Veterans Administration has an

extremely active research program with a number

of laboratories. These laboratories and the dedicated scientists of many disciplines who staff them

are a true national resource that has taken many

The opinions expressed herein are those of the author and

do not represent official USAF policy.

years to develop to its present capability. This

resource is needed by the nation in its struggle to

cope with the ever escalating i^roblem of assuring

oral and dental health to a growing population

and to assist the Federal Dental Services in discharging their responsibilities.

To examine dental research requirements of the

Federal Dental Services it is necessary to study

the functions of the individual service and the en- viroimient in which it operates. The Federal

Dental Services have problems in common with

each other and with the profession at large. In

addition, because of special operational conditions,

each service and agency lias problems 'and research

capabilities which are unique to itself.

The Department of Health, Education and Welfare (HEW) is the major federal instrument for

dental research support. HEW is concerned with

all aspects of the dental and oral health of the

entire population. Its National Institute of Dental

Research (NIDR) conducts broad extramual and

intramural research programs primarily of a basic

dental research nature that are designed to ad- vance fundamental techniques and to establish a

broader base of knowledge for development application. HEW's Division of Dental Health has

been assigned the task of researching ways and

means of increasing the productivity of dental

manpower; of upgrading the teaching of dental

personnel; of improving the distribution, organization, and financing of dental services; and of

developing better preventive, control, and treatment procedures. HEW also maintains a uniformed dental service as a part of the United

States Public Health Service (USPHS) which

participates in clinical investigations related to the

overall objectives of the department. The USPHS

has a special interest in indigenous ethnic groups

along with its concern with the health of the population as a whole.

19
The Veterans Administration (VA) gives special emphasis to the study of normal aging, chronic

and degenerative processes associated with aging,

the detection and localization of oral lesions, oral

tissue metabolism, and problems dealing with tis- sue restoration of war incurred wovuids or losses

caused by surgical treatment for oral cancer. The

VA research program is a continuing effort to en- hance the ability of this agency to maintain the

oral health of debilitated patients. No other

agency is so directly concerned with tliis population group. The VA program has made important

contributions to the research community and the

profession through its cooperative studies with

schools, other government agencies, and related

medical areas. In addition, it provides opportunities for performing cooperative studies as the need

arises.

The Department of Commerce (National Bureau of Standards) (NBS) with its Dental Eesearch Section, which we are saluting at this 50th

Anniversary Symposium, occupies a particular

niche in the hearts of all those interested in the

advancement of the science of dentistry. Begun in

1919 in response to the needs of the uniformed

services, its value to the services, and to the profession as a whole, is fully recognized by the

leadership of the dental profession. Its research

program is currently carried on in cooperation

with the Council on Dental Research of the American Dental Association, The National Institute

of Dental Research, the Army Institute for Dental

Research, the Dental Sciences Division of the Air

Force School of Aerospace Medicine, and the Veterans Administration.

The unique capability of NBS for physical re- search is not duplicated anywhere else in this

country. All dental health components having a

direct interest in this area contribute to this capability through the activities of the Dental Materials Group, a constituent of the International

Association for Dental Research. Representatives

of organized dentistry, the Federal Dental Services, and the dental industry unite with an interchange of ideas in this activity because it is one

of a kind. Their affiliation with different levels ex- tends also to the Federation Dentaire Internationale and the International Organization for

Standardization. All of this adds to the singular

effect of NBS on the dental research community.

Another unique feature of the cooperative program is the fact that participating agencies have

had the advantage of assigning guest workers to

the cooperative program and thus have gained

staff members with a knowledge and capability

in research disciplines.

In the Department of Defense (DOD) context,

research is thought of as research and development. The broader connotation encompasses all

research, development, test and evaluation

(RDT*&E) activities sponsored by DOD. DOD re- search in the dental area is concerned with specific

answers to particular problems of the uniformed

services and requires tangible results from funds

expended. Therefore, within the DOD, emphasis

is on development, test, and evaluation. Common

to all the dental services within the DOD—Air

Force, Army, Navy—is the problem of how to

most efficiently and effectively manage an over- whelming demand to treat oral and dental disease.

In this objective, the Federal Dental Services have

common cause with all elements of dental re- search—universities, industry, and other government agencies. However, this demand is made more

acute by the requirement that the dental services

of DOD must maintain the effectiveness of the man

in the field, the integrity of air crews, ship and

submarine crews, and the personnel of support

elements essential to all three services.

Within DOD, the Air Force, Army, Navy each

have problems peculiai- to their particular branch.

Each service requires a capability for rapid re- sponse to requests from the field for effective

answers to practical problems that may not have

application to the profession as a whole. There is a need for a capability to adapt and design equipment and facilities that meet special requirements

of the individual agency; and there is a requirement for a user test capability for the evaluation

of equipment, materials, techniques, and management concepts. For example, the Air Force has

long been concerned with dental problems asso- ciated with altitude, acceleration, and, more re- cently, man in space. The Navy has had a special

interest in problems related to extremely cold

weather and extended submarine operations. The

Army has concentrated on solutions to the need to

treat their large number of patients with traumatic

injuries and those needing prostheses as a result.

To meet its requirements, both common and

special, each Federal Dental Service needs an

informed staff representing a sufficiently broad

spectrum of professional, scientific, and technological disciplines. The staff must have a suitable

facility in which to work. The staff must maintain

an awareness of current knowledge in dental and

related sciences. It must assure a close liaison with

universities, industrial, and other government laboratories. The research staff of a Federal Dental

Service must know who is studying what and

which laboratory has the expertise and apparatus

to address particular problems. The staff must

perform investigative studies pertinent to operational problems peculiar to its own service. To

augment and complement this capability each Federal Dental Service needs a provision and funds

to negotiate contracts for special studies and for

cooperative efforts in areas of mutual interest.

3. Summary

In summary, the Dental Research Section of the

NBS triggered the impact of basic sciences on the

dental profession. This impact and the results of

20
it, have had a most beneficial effect on the profession and the services it provides the nation and

humanity everywhere. Recognition of these facts

has led to a greater interest and support of dental

research by the government. There has been a minimum of duplication due to the special objectives, interests, and requirements of the various

agencies involved. The Federal Dental Services

have a continuing need for research in areas applying to the population as a whole and to specific

problems relating to special operational conditions. Finally, each Federal Dental Service needs

a scientific staff capability to solve its own immediate problems or to contract with imiversity,

industrial, or other government agency laboratories for their solution.

21

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Resbabch, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Research Needed by the Dental Industry

John F. Glenn

Central Research Laboratories, Dentsply International, Inc., York, Pa. 17404

Dental industry requires new research information wliich must come from nonindustrial

research-oriented institutions. Two major requests of dental industry are for basic research

and better clinical coiTelation with laboratory data. Many compilations of physical test data on a variety of existing materials or ill-deflned experimental ones are being reported,

but the major need is for evaluation of the.se same materials in well controlled eUnical

applications. More rapid access to results of investigations, especially government-supported

grant. research, is needed.

Key words : Clinical and laboratory data correlation ; dental industry research needs ; dental

materials ; dental research, necessity for rapid communication of.

1. Introduction

The research needs of the dental industry could

be discussed from several viewpoints ; for example,

the need for money, for equipment, for personnel,

better management and administration, and so on. All of these needs are internal within each organization in the dental industry and are essentially our own problems.

On the other hand, I have chosen here today to

discuss the subject of the research needs of the

dental industry from the aspect oi needs that can

be satisfied by external sources. I would like to

consider how the laboratories supported by government, the schools, and others, whose only busi- ness is research and whose only products are information and research papers, can assist us in

furthering the work of the dental industry and

thereby help dentistry in the long run. As stated, this discussion is directed to the needs

of the dental industry and needs, by their very

nature, are something that we do not have or do

not have enough of. The assessment of one's needs

must always be based on a critical examination of

what is now available. This is exactly what I have

done. It is certainly not my intention to fault any

group whose services may lie within the area of

this discussion. On the other hand, it is intended

merely to review the situation with some suggestions that perhaps "something must be added."

2. Dental Industry Background

By way of background I should point out, first,

that the dental industry is in business to make and

market materials and equipment for use by the

dentist, the dental laboratory technician, and ultimately the public. It is supposed to be a profitmaking industry.

The profit motive, as a basis of operation, creates

very profound differences in research personnel

and research efforts in the dental industry compared to that of the so-called supported institutions. The goals of our research efforts must be

specific and directed towards getting answers

quickly to the multitude of problems at hand, not

just any answer, but one which will be bought by

the customers. Moreover, this answer must be obtained in the most economical way and its manner

of pursuit has to be contmually justified so that it will be acceptable and will produce the desired

profit. The attendance at meetings, the writing of

papers, the delaying completion of the project for

several months or even a year to investigate a tangential but fascinating avenue of research, all

play a very secondary role to the industrial scientist in his struggle to get materials out the front

door. I believe we, as a group, would like to be as serendipic as one can be in the academic world, but

these pursuits have to be very materially tempered

by the economic press.

The dental industry is quite small as chemical

industries go. The sale of its products is limited to

the dentist population and the quantities involved

for sale to this limited group of customers are tiny.

Therefore, the industry is just not financially able

to support extensive research programs aimed at

developing fimdamentally new information about

matter, and must of necessity look elsewhere for

this kind of data.

3. Research Needs of Dental Industry

In the last few years a great deal of money and

effort has gone into dental research. In a published

survey for 1966, PePlman said research grants were

allotted 10.2 million dollars, scholarships 1.4, and

training grants another 5.1 million. There were

reported to be 1,288 researchers in schools in that

23
year. Undoubtedly this expenditure has continued

and even increased in the last years since then, and

the likelihood is that such support will continue

into the future, even though prospects look some- what gloomy right now. This will not last—we

just cannot afford to cut off even the majority of

ithe projects sponsored by NIDR, Public Health,

the Army, the Navy, and the Air Force, and the

internally supported projects within some of the

dental schools.

Perhaps, however, this is a time when the re- search efforts that have been made in the past and

are currently going on, should be examined with

regard to goals and their real value to dentistry

and to the dental industry, which depend on and

must use much of this information to provide better dental care. With all the effort that is being

put into this type of work, I feel it is very opportune that the researcli group in the dental industry

is invited to indicate its needs.

To make this discussion more meaningful, I have

taken the liberty to carry this invitation to the industry personnel and have conducted a small sur- vey of a number of dental-materials industrial

researchers asking them to* indicate their needs

from research oriented institutions supported outside of the dental industry itself. While I must

admit that some of the comments and the frankness of those interrogated were surprising to me,

there were certain dominant clearcut ideas almost

universally expressed which could be valuable as

a guide in plaaining the nature of future research

projects.

3.1. Need for Basic Research

First, there is a crying requirement for more

fundamental or moi^e basic research in dentistry

and secondly, for more and better clinical correlation of the laiboratory test data that is being produced from many sources. In these two areas the

dental industrj^ is just not geared to function, especially, in basic work. In contrast, research institutes and organizations are usually equipped with

excellent instrumentation or at least instrumental

availability, and in my opinion they have personnel who are well oriented to and are quite capable

in the basic sciences. They should be supplying

fundamental information, for example, on teeth,

their composition, the forces involved within the

tooth, the nature of changes of oral tissue upon

application of various dental procedures, or with

time, the physical requirements which must be met

for successful restoration of the function of oral

tissues, just to mention a few. I am sure you are

well aware of the basics in dentistry.

In a number of instances research institutions

have taken upon themselves or have been directed

by someone to engage in activities which are interpreted as product development to fulfill some specific urgent need or other, which is current at the

moment. No matter how one looks at this endeavor.

it seems to be in competition with the dental industry development laboratories which generally are

better equipped for product development through

their own orientation, close contact with the dental

profession, incentive, and even the profit motive.

It seems to us from where we sit that there would

be a greater contribution to the industry and to

dentistry if the research-oriented institutions

would establish the principles on which products can be developed. In the past there has been some

history of doing this pioneering, for example,

Taylor and Demaree in the spherical alloys, Bowen

in his composite restorative materials, Brauer in

the inauguration of the EBA's, Phillips putting

fluoride in filling materials, among several others.

However, there seems to be a tendency of late

through some pressure or other to develop complete

or more or less finished products. Perhaps the industry is somewhat at fault in not taking up the

ideas coming forth fast enough and gives the appearance of inadequately fulfilling the best needs

of dentistry.

As an illustration of this need for basic work let us consider, for example, the program of the 1969

meeting of the Dental Materials Group—lADR. I

have made a tabulation of the papers that were

presented and arbitrarily divided them into four

subject categories, determined from the title, the

abstract, and listening to some of them myself.

Papers which could be considered as descriptions

of tests, new or otherwise, constituted 15 percent of

the presentations. New and more-or-less finished

materials not previously described were the subject

of 14 percent. Papers which discussed some aspect

of existing materials accounted for 59 percent, of

which 38 percent dealt with the subject of amalgam

alloys, or amalgams, or handling amalgams. Basic

research papers concerning some phenomenon or reporting facts which could be construed as con- tributing to the general fund of dental information, constituted only 12 percent of the offerings.

These percentages more or less speak for themselves and would substantiate, I think, the need

mentioned. There is an overabundance of testing,

retesting, and testing again, on current materials,

ad nauseum.

Be all this as it may, at least to industry personnel, it appears that much of the data cannot be

used and more attention could be given to acquiring basic information.

3.2 Need for Improved Clinical Correlation in

Laboratory Testing

The second area expressed as being greatly

needed is more effort directed to better correlation

of the quantities of physical data being accumulated today, with clinical function. The clinical

evaluation" of materials and processes, whether it be dental or medical is extremely difficult, probably the most difficult type of research to carry out.

It requires very careful and detailed planning, the

24
institution of extensive controls, tlie development

of careful and uniform observation techniques

which at best can be only empirical and subjective,

the selection of satisfactory material in sufficient

numbers to be really significant, and some sort of

useful and understandable tabulation of the

results. All this is tedious, time consuming, expensive, and quite frequently disappointing in the

clarity of answers that result from the experiment.

Also, more times than we would like to admit

before the experiment can be completed, the material being evaluated becomes obsolete and then

the experiment is reduced to an exercise in futility.

There have been a few brave souls who have ven- tured forth into this area and are producing some

very notable contributions such as Ryge, Paffenbarger, Myers, Phillips, and a few others. Unfortunately, the formidableness of this type of re- search has scared off, or so it seems, many who

actually are equipped to contribute greatly. I fail

to understand why the materials departments of

most dental schools, with their attendant clinical

facilities, do not take up this opportunity to contribute to dental materials development in a useful

manner.

The dental industry is not actually interested

in comparing one brand of material with another

brand of the same type of material, although this seems to be a very favorite endeavor. We are interested, however, in learning what the generic types

of dental materials do to the oral tissues and, of

course, what the oral tissues do to materials and

what are the requirements from a clinical viewpoint for satisfactory replacements.

The dental materials industrial researcher is constantly asked to produce and continually improve materials and equipment and to design procedures for replacement of tooth and other tissues,

but few guidelines exist as to what strength, hardness, fatigue, or other detailed characteristics are

actually required. It is somewhat akin to trying to

build a bridge without knowing what is going to

go over it or how long it will be. Good clinical

study will assist materially in answering the questions with facts that are needed in the design of

materials. Here then is our second major need, a

difficult one no doubt, therefore a real challenge, so

let us pick up the challenge.

3.3 Need for Improved Communication

of Current Research Results

The communication of the results of investigations by research institutions to others outside of

their cwn agency, which sponsors the research, has

not been the best. In fact, it is quite poor. In many

cases, the dental industry and other interested parties have had to wait until the information was

published in a journal or in a report iss^ied by the

National Technical Information Service a year or

so later. When something that is worthy of note is accomplished by a research institution, it should

be made available to dentistry through the industry. That is what the industry is for. There is no

need to bury research work in secrecy until the

project is completed or dropped, especially when

it is supported by public funds. It has been sugfrested that this whole matter of communication be

ooked into by the several grant-issuing agencies

and that they do some soul searching. If research

institutions wish to convey the fact that it is their

desire to aid the public and to produce better dental services ultimately through their work, then it would certainly seem that it would pay to team up

by better communication with the industrial re- search groups, who will have to produce the product eventually anyway. As it stands now the usual

channels of publication and reporting are woefully

inadequate and behind in keeping up with the

work that is being done. As suggested, the burden

of the dissemination of information lies with the

sponsoring agency and much more would be ac- complished if these agencies would recognize this

responsibility and attempt to issue informative

reports on a regular, rapid, and broad basis.

These three points constitute the major areas which were almost universally mentioned as needed by dental industrial research personnel to

pursue their efforts in the adaptation of ideas and

findings of research laboratories and make them

practical in the form of dental products.

3.4 Additional Dental Industry Research Needs

There are sevferal other areas which might be

said to be of lesser concern to industrial research

personnel, but should nevertheless, I feeil, be

mentioned.

One point is the request by many for the development of more realistic physical tests. At the

present time, the testing procedures for dental materials are more or less haphazard in their vailidity

of basis of selection. The tests that are part of the

various specifications are based on a few physical

properties of existing products and the limitations are those of the existing products. While we may

be getting by with this at the present time, we

should be striving for well organized tests that will

simulate aspects of clinical serVice and define the

service attributes of a material more specifically.

Such tests, coupled with actual clinical observations, will establish what limits are really neces- sary from a functional standpoint, and the re- quirements of the specifications will fall in line

and be more meaningful in the judgment of

materials.

It has been whispered that some of the physical

test data are being developed by individuals who

have a veiy limited background in the science of

dental materials. They sometimes arrive at implications and conclusions supported only by a bit of

statistical jugg'ling. There are many grants at present, so called training grants, issued to individuals

who carry out a few physical tests on one product

or another. Eventually the results are published.

25
Unintentionally, some misleading infonnation is put into our journals, especially the popular ones. The publication of this type of data and the

manner in which it is developed and concluded,

unfortunately for us, gives the impression right or wrong that there is some incompleteness in

dental research. Perhaps this is part of the training grant situation and, if it is, then the procedure

should be altered so that the trainee can still get

this needed support but merely write a term paper

which will be buried in a library some place or

in somebody's desk drawer. This could be quite

easily accomplished by the sponsoring agency

changing its publication requirements, so training becomes indeed a training course and not a paper mill.

Up to now I have probably sounded, although

unintentionally, somewhat critical of the present

state-of-the-art, so let us turn to a couple of farther

out thoughts for the future.

A single central research control agency, possibly in the ADA or NIDR or other governmental

institution is suggested to consider the problems of

dentistry and parcel out the projects and investigations on the basis of dental need and likelihood

of success, rather than fancy proposals. Perhaps

this is already established, but it is not evident

to us. There are several agencies at the present time

which attempt to pass out research projects, but

to industry people there appears to be a competition between these agencies even though there

probably is not. At any rate the job has the appearance of not being done and there seems to be

considerable duplication and needless effort. A central agency could control this effort more usefully.

For many years there has been much effort to

develop specifications for existing materials. Recently, with the formation of the USASI Committee from the older Dental Materials Group

Specification Committee and the addition of

others, the tempo of specification writing and test procedures development has increased notably.

A.ny good specification for a material will have behind it a battery of tests which will have been

developed through actual laboratory evaluation

and the amassing of sufficient data to prove that

the tests can be carried out with reasonable uniformity in several laboratories by a number of

different operators. Much of this work is going

on at the headquarters of the ADA, which is con- tributing very heavily and effectively to this effort.

A nurnber of schools have been cooperating and are spending time and money on test development.

Likewise, industrial research laboratories are doing the same and have contributed in a great

measure to the mass of data that backs up a number of these tests. All of this work costs money, considerable

money. Why not set up a series of grants to any

institution, including industrial laboratories, for

test development and data accumulation for specification work. Good physical facilities are available in the industrial laboratory, as is wide testing

know-how. Use should be made of it, but at the

same time it could be supported and the burden

spread.

4. Summary

I have discussed with you briefly here the re- search needs of the dental industry—the needs

that can come from outside the industry itself,

from institutions that are in the research business

and supported by grant funds from one source or another. More extensive basic dental research,

carefully controlled clinical studies to correlate

physical laboratory data and establish realistic

limits, and better communication, are the fundamental needs of the dental industry.

26
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseabch, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Research Needed for Dental Education and Practice

Ralph W. PhiUips

School of Dentistry, Indiana University, Indianapolis, Ind. 46202

The current research effort in dentistry, and that which will occur during the next

decade indicate that dental practice will be characterized by certain trends. These trends will

require an acceleration of research on adhesive film forming systems, sealants for pit and

fissures and means of improving anticariogenic characteristics of restorative materials. The

interdisciplinary aspects of dental materials will be increasingly important. Greater focus

will be on the biological properties of materials, materials and devices for oral rehabilitation,

materials for implantation and the interaction of tissue and substances used to restore lost tooth structure. Further definition of the oral cavity can be expected to become a responsibility of the materials scientist. Increased emphasis can be expected in dental education on

a more intimate basic science-clinical application orientation.

Key words : Dental education ; dental materials, research needs ; dental practice future

dental research ; materials, dental, research.

1 . Introduction

We are now living, and shall continue to exist,

in a research oriented society. While too little

attention appears to be focused on the quality of

research, the sheer magnitude of the investigative

effort in science is impressive. The health professions, and especially dentistry, are no exceptions.

It would be naive and dangerous to believe that

this ever broadening base of knowledge will not

change decidedly the nature of dental care and the

content of the dental school curriculum. It is upon

that premise that the following remarks are based.

It is indeed difficult to trace a profile of any

profession a decade hence, particularly in one such

as dentistry where such a marked transition is

occurring, not only in the specific treatment of

dental disease but also in socio-economic attitudes

that have not as yet acquired a definite character.

I am all too aware that even with much more

tangible data, predictability may sometimes go

awry.

However, it is becoming apparent that certain

trends are crystallizing in dentistry and I would

characterize them as follows : ( 1

) The present inroads that are now being made in reaching a total

comprehension of the etiology of dental caries and

designing the related therapy to arrest this disease

will soon make certain dental restorative procedures obsolete and the general nature of dental

service may be quite different from that of the

present. (2) With more emphasis upon the dental

specialties, group practice will become more popular. (3) Greater demands will be made upon re- search and teaching programs that will extend

dental treatment to the entire population. In other

words—community dentistry. (4) In order to provide a broader type of dental service, the role of

the dental auxiliaries will be expanded, as will

their training.

Let me defend these predictions and attempt to

relate them to research in dental materials science.

2. Future Trends in Dental Research

In recent years, at least one-third of the dental

research effort in the United States has been devoted to ways for preventing dental caries while

another one-third has been concerned with eliminating diseases of the soft tissues. The response of

the profession to the rapid escalation of research

in these fields has been one of increasing alarm

that coincident with any substantial reduction in

the incidence of dental caries and/or periodontal

disease will be a reduced need for the dentist, a

fear that he will eventually be phased out of the

health professions and will become somewhat of

a historical curiosity. The outcry first occurred

when fluoridation became a recognized mode for

the arrestment of dental caries on a mass scale.

To be sure, the goal of any health profession is

that research will lessen the demands that are placed upon it. Preventive measures such as fluoride therapy,

which were the fruits of dental research, have already altered the nature of dental practice. However, they have certainly not led to a lessened need

for the dentist or his auxiliaries. To the contrary,

dental research has permitted the profession to initiate programs of total oral care rather than simply playing catch-up with the overwhelming backlog of dental treatment that has existed and which

will be required by an ever expanding population.

Furthermore, in being offered superior service, the

452-525 0—72 3 27
public has become more dental conscious and motivated to seek dental care—^tlius actually increasing the demand for a lower dentist-patient ratio.

Parallel to this apprehension of the young dentist of the impact upon his profession of research

in the biological sciences, one often hears depressing predictions coming from the dental materials

teachers or researchers. Since the remaining one- third of the bodies and monies now involved in

dental research are directly concerned with improving dental materials and instrumentation, it

is predicted that this research in the physical sci- ences will shortly spill forth an avalanche of su- perior materials and that most of the former

problems associated with the clinical failure of

materials and appliances will be conquered. In

turn Utopian technics will attain such a degree of

perfection that further research will be unnecessary. It is further rationalized that as dental dis- ease becomes better controlled through preventive

measures, it must therefore follow that the need

for restorative materials and appliances will become less important, as will the personnel involved in their development.

I do not share this alarm. Conversely, the theme

of this paper is centered on an opposing premise

namely, that although the teacher or researcher in

dental materials in the year 2,000 will likely be

concerned with entirely new problems and will

be attacking them in a vastly different way, his

relevance to the profession will be even more

secure, as will the horizons for a meaningful ca- reer in research or education.

As I have suggested, any realistic evaluation

of the accomplishments that are now being made

in providing a means for irradicating dental caries

clearly indicates that the magnitude of this disease

will diminish in a somewhat regular order. If a major breakthrough should occur, restorative denistry will then be concerned principally with re- placement of the fractured tooth or the one lost

from periodontal disease. In any case, it is reason- ably safe to predict that within the forseeable

future the carious lesion will no longer be the

focus of attention, or of time, of the profession.

The arrestment of dental caries will in turn trigger other widespread changes in the nature of

dental practice. For example, the need for endodontic treatment will become less. Since fewer

teeth will be lost as a result of caries, a lower

number of prosthetic appliances will be required.

Thus it might be envisioned that the typical

dental practice of the future will emphasize : ( 1

preventive dentistry, (2) restorative measures that

will be necessitated from the loss of tooth structure due to reasons other than caries, (3) the

maintenance of the health of the supporting tis- sues, and (4) the treatment of dental disorders

that have previously been neglected by virtue

of the overwhelming preoccupation with the rav- ages of caries.

If one accepts as a basis for discussion that such

trends could be the natural outgrowth of current

programs of dental research, where does the dental materials scientist fit into such a scheme ? First, as for the dentist, the time allocated to certain

materials or technology, will be weighted entirely

different from allocations of today. Materials and

concepts that at present may receive only a casual

glance will come to the fore while some of the classical systems upon which we have lavished our

energies will be of secondary interest.

For example, although this is almost sacrilege

to say, in time dental amalgam will no longer

occupy its lofty seat as the material upon which

the attention of dental materials researchers has

for so long been concentrated. For all of its merit

as a system that has served dentistry so heroically

and one that has offered so much scientific intrigue,

the need for research on dental amalgam will

diminish since it will be replaced by other materials or therapies. The same may be said for a number of other materials, such as silicate cement

and the current resins. They will be surplanted in

time (and please don't ask me to suggest a target

date ! ) by adhesive restorative materials that will

hopefully approach or duplicate the physical and

chemical properties of the tooth itself.

It would be naive if I were not to acknowledge

that this learned audience is quite sensitive to the

importance of research in the area of dental adhesives. However, since the title of my assignment was to be centered upon the future needs of

dental materials research as they relate to practice and education, this subject affords an unusually good example of the type of investigative

effort with which we should and will be occupied.

No other affords greater opportunities for a broad

spectrum of dental applications, for the outlet of

an imaginative mind, or for the interplay between

disciplines that have historically been the trademark of the science of dental materials, and of

the 50 year leadership given to us by the dental

program at the National Bureau of Standards.

Adhesive systems afford opportunities to render

a type of dental care that is not possible with existing materials. Furthermore, they fit nicely into the

framework of the future practice of dentistry as

I have sketched it. For example, the elimination

of the microleakage phenomenon Avould alter much

of the basic biological and physical concepts that

now prevail in restorative procedures. The adhesive dental restoration would conceivably be less

complicated to place, biologically superior and

]Dhysically improved. However, the true lure of

dental adhesive molecules lies in uses other than

for the restoration of a carious tooth or the

attachment of orth,odontie brackets, important

though these may be. Adhesive films applied to

tooth structure, if dura;ble, could inhibit the formation of the dental plaque and thereby the caries

process or the deposition of calculus. Certainly

adhesive phenomena, yet to be adequately explored

28
by the dental materials scientist, are involved in

the retention of dentures, the bonding of ceramics

to metallic restorations, the retention of re- implanted teeth and of resin implants to tissue

and in the efficacy of maxillo-facial appliances.

Each of these uses is different in the sense of

the structures that are involved, the respective interfaces and phases, the conditions to which the

bond will be subjected, and the physical and biological properties of the system. The scientist in

dental materials is the most likely candidate to

lead meaningful programs in the development of

such compounds, by virtue of his experience with

the physical behavior of materials in the oral

cavity, the highly specific test methods and specifications he has developed for assessing materials

and his obvious ability to synthesize various disciplines into one that serves the multiple and complicated demands that dental treatment requires.

There is yet another example that might be

used to illustrate the need for adhesive systems

since it is one which could be part of the routine

type of therapy offered in the dental practice of

tomorrow. I am, of course, referring to a sealant

for pit and fissures in the child patient. This is a dental materials problem, even though it is essentially preventive dentistry. A number of persons

in this audience are already deeply involved in

this area of research. An adhesive and durable

restorative material that would prevent the propagation of caries in these vulnerable areas has

great import to the treatment of mass populations. As I stated earlier in this paper, I am con- vinced that the profession will be forcefully

moved, both by direct means and emotionally, to

provide dental care to all segments of the

population and in all countries. Of immediate

application in mass care would be the arrestment

of existing caries or its prevention in the young

population. Adhesive sealants, particularly if

anticariogenic, could become a major treatment

objective for such programs. Furthermore, it would not be heresy to suggest that in certain

instances this type of care might be administered

by properly trained dental auxiliaries.

I am certain that most of you are in complete

sympathy with the concept that we must lend our

assistance in every way possible to make dentistry

available to everyone and especially to those in

underdeveloped areas in this and other countries.

The mode for accomplishing this is indeed controversial and does not concern this group. What

does is the development of materials and the training of people to cope with the magnitude of the

problem. Those materials and that type of trainmg may be considerably different from what now

prevails.

If one acknowledges that the broad area of

dental adhesives is a reasonable example of the

type of research that fits into the context of "Dental Practice in the Year 2000", then it seems logical

that the interests of the dental materials scientist

will slowly and methodically, and I would emphasize the latter, shift from studies such as the

niceties of phase reactions in amalgam or the

dimensional changes that occur in gypsum products. Merely to further accumulate such information as a sort of capital investment will not be

sufficient. Instead the scientist will be explorin,g the

interaction of polymeric systems to both hard and

soft tissues, will develop criteria for relating biological characteristics of materials to their structure, will define the exact nature of tooth structure

at interfaces, and will better characterize the stress

patterns placed upon dental restoratives, appli- ances or films.

I have used the example of adhesion because it does embody so well the changing complexion of

dental materials research and its application as I

see it, as well as the responsibilities which may

thereby accrue. I have suggested, by using this

particular illustration, that the breadth of dental

materials research and education will range even

further from very complex problems to those that

are inordinately simple. For example, to develop a system that will adhere to the formidable tooth

structure in its aqueous environment will continue

to require the attention of unusual research skills

and talents. On the other hand, the application Of

that adhesive to a pit and fissure will be a very

simple treatment as compared to existing operative procedures. Thus one sees the need for so- phisticated research done in great depth, to be

paralled by training programs for the dentist and

especially for auxiliaries who may be required to

utilize the end product of that research.

As I just stated, the role of adhesive molecules

in dental application is merely one of many that

could be cited to indicate the future nature of

dental materials research and its application.

These include the evolution of implant materials

for prosthetic appliances, of materials that will be

more esthetic and superior in properties than the ones now used for maxillo-facial appliances, of

methods for providing a more precise restoration

of teeth that are lost from accident or diseases of

the soft tissue, of ways for producing mineralized

tissues or methods for protecting such tissues and

specialized materials for all of the various dental

specialties. As dental care becomes more specific,

greater emphasis will be placed upon diagnostic

aids and devices for maintaining a better control

of dental treatment. The researcher and manufacturer of dental materials long ago demonstrated

his ingenuity in that field and will no doubt be

called upon to develop technology for monitoring

physical and chemical changes in dental structure,

for minimizing the human variables in providing

dental care and for ways to reduce the cost of the

dental operation.

Whether it be fundamental research in such

divergent areas or other programs designed to

further the education of the dentist and his aids,

the point has been made that the field will remain

29
a most viable one, hopefully unfettered by previ- ous tradition as to its limits. Personally I have

every confidence that the scientists and educators

now in this field do have the wisdom to make that

adjustment and rise to meet, not obstacles, but an

everchanging science landscape. Elaborate apparatus plays an important role in science today

but we are too inclined to forget that the most

important instrument in research must always be

the mind of man. Kettering has said, "A problem

is not solved in the laboratory. It is solved in some

fellow's head. He only needs the laboratory apparatus to get his head turned around so he can

see the right thing". A multitude of opportunities

for scientific enrichment in our discipline can be

seen without much movement of the head.

3. Future Trends in Dental Education

I would like to spend just a minute to relate the

foregoing sketchy remarks to dental education. If

dental education in the future assumes the same

posture that has prevailed in the past, it by necessity will reflect changes in dental practice. Therefore, as emphasis is placed upon the dental specialties, on community dentistry and upon caring for

all types of dental disorders, the dental school

curriculum will witness a more intimate relationship between the basic sciences and the clinical subjects. No doubt this will be reflected in a more vertical alignment of courses. This will be true for

each individual science, including dental materials.

Every effort will be extended by dental school administrators to organize a proper atmosphere to

fertilize this type of orientation.

The dental materials course must eventually

change to reflect the new duties of the dentist and

the changing nature of his practice. Certain materials will become obsolete, others will receive

greater stress. In all likelihood the systems and the

techniques that will eventually be paramount are

as yet still under research. In the undergraduate

training of the dental student it seems logical that

there will be less emphasis upon those materials

and technics that will have become principally the

responsibility of the technician, assistant or hygienist. However, as dental schools assume a

greater role in the training of dental auxiliaries,

the content of the courses given to those groups

will be considerably strengthened in materials and

it will no doubt be presented at a higher level.

In addition, there will be greater emphasis upon

the biological characteristics of materials and the

interaction of materials with their environment.

The dental materials science course could very well

encompass the physical structure and physical constants of ail oral tissues.

4. Summary

In summary, I am convinced that our future lies

in a greater interdisciplinary approach, not only

in the specifics of research on materials but in re- lating that research to the other physical, biological, and clinical sciences. Furthermore, communication on an international basis at a level far

greater than that which now exists is essential in

order to make best use of the talents now available,

to prevent duplication of effort, and to concentrate

a multitude of single isolated investigations into

unified attacks on problems of mutual interest. I

guess what I am saying is that the profession will

demand more from this science than any other

discipline in turns of adjustments to newer areas

of investigation and at a high level of competency.

At the same time it will be charged with accepting

a greater responsibility in the training of individuals who will deliver dental service. The dentist

will request, and rightly so, materials and technics for handling a different type of dental practice than he is conducting today and he will ex- pect educational programs for auxiliaries that will

be heavily weighted in the subject matter pertinent

to usage of these materials. Coincident with this,

one may anticipate that the dental manufacturer

will in turn become more specialized, skillfully

developing what at present might be called bizarre

products.

I guess my major concern in this transition lies

in our capability and desire to attract future re- searchers and teachers. We are now and will con- tinue to be competing with scientists and educators

in fields that now appear to have a greater allure

and upon which the future practice of dentistry

seems to be more directly based. I contend that the

science of dental materials will comprise a pillar

for that foundation as strong as that of any other

discipline—but only if it accepts a critical, reflective attitude so that it can envision and react to

the new challenges which a changing profession

will offer. If it does, then the values and rewards

are such that this science can compete on a scientific and academic level equal or superior to that

with which we are now blessed.

30
III. Metals Research

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued Jime 1972).

Amalgams in Dentistry

Knud Dreyer j0rgenseii

Kebenhavns Tandlaegehejskole, Copenhagen, Denmark

Corrosion is a major factor in the failure of amalgam restorations through the deposition

of corrosion products which may cause chronic periodontitis or through corrosion fracture

of the margins which may cause secondary caries. Galvanic corrosion attacks only the tin- mercury 72 phase. Mercury released by this corrosion causes a mercuroscopic expansion

resulting in a deflection of the amalgam from the cavity wall at the margin. Corrosion can

be considerably reduced by minimizing the porosity of the amalgam restoration through

overfilling the cavity, burnishing the margins, eliminating excess by carving and using

zinc-containing amalgams.

Key words : Amalgam, dental ; amalgam, dental, effect of technique on corrosion ; clinical

failure of amalgam ; corrosion, dental amalgam ; dental materials ; marginal fracture, dental

a malgam ; mercuroscopic expansion, dental amalgam ; porosity of dental amalgam.

1 . Introduction

The 50th anniversary of the Dental Eesearch

Section of the National Bureau of Standards coincides with another noteworthy date: it is exactly

150 years ago this year that dental amalgams first

made their appearance in the service of dentistry.

Countless teeth have been restored and preserved

in the course of the past century and a half, thanks

to the development of these materials. There is

some uncertainty about who can claim to have invented amalgams for the restoration of human

teeth; it seems to have been the British chemist,

Charles Bell, in 1819, who produced an amalgam

of silver filings and mercury; his product was

advertised under the name "Bell's putty". A few

years later, in 1826, the French dentist, Onesiphore

Taveau, claimed to have invented an amalgam suitable for filling carious teeth; the material was

made by mixing mercury with filings cut from sil- ver coins and went under the name pate d'argent,

i.e., silver paste.

We know that amalgam as made according to

Bell and Taveau had several unfortunate pro^Derties : the consistency of the freslily made mixes was

harsh or sandy, they hardened slowly and were

subject to excessive expansion. It was soon discovered, however, that the latter two drawbacks could

be greatly reduced by adding tin filings to the sil- ver filing or by cutting filings from an alloy of sil- ver, copper and tin, i.e., the metals which even today are still the main components in dental amalgam alloys.

The dental amalgams were received with great

interest by the profession, which is not perhaps

surprising in view of the apparent simplicity of

amalgam technique; the filling materials which

amalgams could replace were primarily a variety

of metal foils, particularly gold foil. In those days

it was a very tedious task preparing a good filling

of gold foil; it frequently took more than three

hours. By comparison amalgam was advertised as

a material of Which a filling could be made in just

three minutes, painlessly and at no discomfort to

the patient. Only some of the details of the amalgam techniques of those days are known but there

is no doubt that cavity preparation techniques,

matrix techniques, and the actual amalgam technique were extremely primitive. It is scarcely sur- prising therefore that amalgam fillings frequently

caused very serious secondary injuries, and that

many dentists reacted strongly against the application of amalgam as a filling material.

The debate on the use of amalgams in operative

dentistry was confined almost exclusively to the

United States, where the material had been introduced by the Crawcour brothers in 1833 and where

the famous amalgam war raged imtil 1856. It was

in this country too that almost all the advances took

place that have since led to the amalgam alloys

and amalgam techniques of today. The people who

headed this progress were men like Josiah Flagg,

founder of the constructive critical-analytic

method in operative dentistry generally, and amalgam technique in particular ; G. V. Black, who on

the basis of extensive and detailed experiments defined the optimum composition of the amalgam

alloy, and also systematized the technique and

morphology of cavity preparation; Wilmer Souder, the first head of the Dental Section of the National Bureau of Standards, recognized as the

founder of the science of dental materials and

author of the first standard specifications for a dental material, viz., amalgam alloy ; and many other

authorities most of whom are present with us today

33
and still active in the field of research into dental

materials.

It is important for me to stress that all the re- search carried out today in the amalgam field is

based on the comprehensive and carefid studies of

these authorities—and would not have been possible without their vital contributions.

All organized research requires a specific objective in order that we may formulate and attack

the appropriate problems. The study of dental

amalgams is pure applied research, which means

that it is justified only inasmuch as it is demonstrated to be oi use to operative dentistry.

The debate in earlier decades was on whether

amalgams were of any use whatever filling

maiterial. Modem amalgam research on the other

hand is given the task of examining (1) the extent

to which amalgam fillings fail, (2) an analysis of

the physical and chemical processes leading to

failure, and (3) the influence of materials and

technique on these processes, particularly with a view to discovering whether it is possible by altering the composition and properties of the materials

and by changing the technical working methods

to halt or delay the processes.

2. Definition of Amalgam Failure

A failing amalgam filling can be defined as a

filling that has been a contributory cause of sec- ondary injuiy in the organ of the tooth, i.e., the

tooth itself and its surrounding connective tissue.

In the following discussion it is a condition that

the amalgam fillings initially Avere morphologically correct, which means among other things that

the surfaces of the fillings were smooth and that a careful inspection revealed neither marginal excess nor marginal deficiency. It is moreover a condition that the fillings have been made of alloy and

mercury, fulfilling the requirements of the relevant

ADA specifications.

The only forms of secondary damages of clinical

significance are chronic gingivitis and secondary

caries.

Chronic gingivitis occurs in this connection

when solid products of corrosion, which promote

the retention of materia alba and plaque, are deposited on the surface of amalgam fillings on the

peripheral surfaces of the tooth in close relation to

the gingiva. Figure 1 shows a typically corroded

Class II filling in which the gingival and central

one-third part are covered by a crust of dark products of corrosion, the occlusal portion remaining

clear of corrosion. Figure 2 is an enlarged view of

part of the surface of an experimental noncontaminated filling from a laboratory experiment; the

filling had originally been smooth and polished

but after four months' corrosion in artificial

materia alba its surface was roughened by small

surface blisters and solid corrosion products. The

small hemispherical nodules are drops of mercury

covered by a thin crust of deposited products of

Figure 1. Typical corroded Class II filling with the gingival and central area covered by a crust of dark products

of corrosion while the occlusal portion remains clear of

corrosion.

Figure 2. Enlarged view of the surface of an experimental

non-contaminated amalgam specimen showing small sur- face Misters and solid corrosion products after four

months exposure of the original smooth polished surface

to artificial materia alba. Magnificationr-HO X

corrosion. Although the accepted view of the

causal rdation between corrosion of fillings and

gingivitis is to some extent based upon presumption, it seems to be probable enough to provide sufficient grounds for a study of the circumstances

influencing the process of corrosion.

3. Corrosion of Amalgam

Schoonover and Souder were the first to discover

that galvanic corrosion attacks only _ the tinmercury 72 phase, and that only the tin dissolves

the released metallic mercury remaining in the
amalgam; much of the tin is deposited locally,

either within the corroded amalgam or on the

amalgam surface. Many subsequent studies have

served to confirm this view.

It is typical of amalgam corrosion thalt in oral

conditions it can occur only in the presence of a

difference of oxygen concentration on the amalgam

surface ; the area with the lowest concentration of

oxygen forms the anode. Areas of this nature regularly occur in the most gingival parts of fillings on

peripheral surfaces where the amalgam is covered

by gingiva or plaque; the surfaces of the filling

facing the cavity, and the walls in the pores of the

amalgam also have a relatively lo w oxygen tension.

Since it is only the 72 phase that is subject to

galvanic corrosion, it is importanit that we become

familiar with the morphology of this phase. If the

individual 72 grains do not anastomose, only the

surface grains in contact with the electrolyte can be

dissolved by corrosion—which would have to cease once this dissolution had terminated. Figure 3

shows a section of an amalgam with a relatively

high content of mercury. The amalgam was ex- posed for four months to corrosion in a 1 percent

NaCl solution; the corroded surface of the

amalgam—facing downward in the picture—^is partly covered by products of corrosion. The 72

phase is observed only in the upper part of the sec- tion as irregular, dendritic grains ; nearer the sur- face the phase has dissolved, and the amalgam has

become severely porous. There is no doubt that in

this case the surface 72 grains anastomosed with all

deeper lying 72 grains. Experiments in which the

72 phase was dissolved with the aid of a 10 percent

sodium citrate solution have revealed that even in

well-condensed amalgams all the 72 phase always

forms a cohesive network of crystals. Corrosion can

therefore continue into the depth of the amalgam

until the whole of this phase has disappeared. As

far as the structure of the 72 phase is concerned,

there is no essential difference between the various

types of amalgams; spherical and nonspherical,

with or without zinc, mercurized or non- mercurized ; all show the same network ^ructiire

of the 72 phase. As there is only a slight difference

in the amount of 72 phase between an amalgam

with a high content of mercury and one with a lo w

content, the corrosive properties would for all

practical purposes be independent of the amalgam's content of mercury.

The intensity of corrosion of dental amalgams

on the other hand depends very much on the porosity of the amalgams. Porosity exposes a greater area of the 72 phase and thus increases the anode

area. Experiments with a 1 percent solution of

NaCl as electrolyte have shown that intensity of

corrosion triples when porosity of the amalgam in- creases from approximately 1 to 4 percent. It was

also shown that amalgams containing zinc cor- rode less than amalgams without zinc. The primary reason for this does not appear to be the

FiGUBE 3. High mercury content amalgam after

four months exposure to 1 'percent NaCI

solution.

Corroded surface (facing downward In picture) Is partly covered by corrosion products. The 72 pliase

(irregular dendritic grains) is present only in upper

part of the section, having been dissolved in the lower portion to form porosity. Magniflcation-approx.

227 X

difference in the chemical composition of the alloys but is probably due to the fact that the zincfree alloys are not so easily wetted by mercury as those with zinc, which results in a slightly higher

porosity in the amalgams. It is a fact that zincfree amalgams characteristically have a slightly

higher porosity than those with zinc, and that this

porosity is often localized as small nests in the

amalgam.

Since otherwise the three main phases, 7, 71 and

72, present in all standard amalgams can from an

electrochemical point of view be considered identical, it is unlikely that amalgams produced from

different brands should demonstrate different degrees of corrosion.

The means at our disposal to reduce the rate

of corrosion in amalgams are therefore as follows

(1) omission of zinc-free alloys, and (2) use of a

filling technique that reduces porosity of the amalgam as much as possible. Since porosity can vary

considerably according to the filling technique,

this latter factor is the most important.
4. Effect of Technique on Porosity

of Amalgam

Detailed analysis of filling techniques have

shown that the consistency of the amalgam when

it is placed in the cavity is the factor of prime

importance as regards porosity of the condensed

amalgam. A relatively dry and more or less

crumbling amalgam will always result in greater

porosity than a soft, plastic amalgam from which

a considerable quantity of excess mercury can be

expressed.

Curve P in the diagram (fig. 4) shows the

average porosity in cylindrical, mechanically

condensed amalgam specimens, dependent on the

content of mercury in the amalgam when condensation commenced.

The vibration technique, requiring as it does a dry amalgam, will thus always produce a high

degree of porosity in the amalgam. The same applies in the case of hand-condensed fillings made

by conventional technique in which the amalgam

is triturated with excess mercury, most of the excess being expressed before the amalgam is placed

in the cavity. Fillings according to the so-called

dry or Eames technique are also fairly porous.

The lowest porosity is achieved when a significant

quantity of mercury is expressed in the cavity and

when each new increment is condensed in the free

mercury thus expressed—the mercury being re- moved only when it forms a layer of more than

approximately ^ mm on top of the condensed

amalgam. The latter method has been given the

name "wet" teclinique, wet being a reference to

the mercury content.

Porosity in fillings made by vibration or by hand

condensation with dry or with conventional techniques is concentrated at the bottom and particuP M S

.

o P X

46 50 54 58 62 7oHg

Figure 4. Relationship hetween mercury content of mix

and porosity (P), mercury content (M) and strength

(8) in cylindrical, mechanically condensed amalgam

specimens.

To convert kp/mm" to MN/m^ multiply by 9.807.

larly in the bottom corners of the individual con- densed increments. Figure 5 illustrates porosity

in the bottom corner of a well condensed, typical

Class II filling made by conventional technique;

the gingival part of the section is about % mm

long. Figure 6 shows the same part of a filling

made by Avet technique; the much lower porosity

is remarkable. The diagram (fig. 7) shows poroKity and y-phase content in gingival corners in

Class II fillings condensed by means of dry technique with two different condensers ( Ap and Af)

by conventional technique (B) or by wet technique

(C). The diagram shows that the wet technique

is an efi^ective means of reducing the porosity of

the amalgam, and that it does not lead to a higher

mercury content in the condensed amalgam than

do the other forms of amalgam technique. It may

be noted that G. V. Black many years ago stressed

the importance of using relatively plastic amalgam

during condensation.

With regard to the relation between plasticity

and porosity it is worth mentioning the rather

marked tendency of manufacturers in recent years

to mai'ket rapid-setting alloys with greatest possible one-hour strength. The clinical value of such

alloys is open to doubt. During condensation amalFiGURE 5. Porosity in the bottom comer of a well

condensed, typical Class II filling made iy a conventional technique. Magnification^approx.

115X.

36
Figure 6. Reduced porosity (compare with fig. 5)

in the 'bottom corner of a well condensed typical

Class II filling made by a wet technique. Magnification^approx. 115 Xgams of this type quickly become dry and crumbly,

and can therefore result in a significant increase in

the porosity of fillings and a similar increase in

the likelihood of corrosion. Increased porosity

means at the same time a decrease in strength of

the amalgam : a 1 percent increase in porosity has

about the same efi'ect on the strength as a rise of 10

percent in the mercury content of the hardened

amalgam. The rate at which a freshly triturated

amalgam loses its plasticity can be registered in a simple manner, for example, by measuring at vari- ous times after amalgamation how much mercucy

can be expressed from the amalgam. The difference

in the percentage mercury content between an amalgam which has been condensed as early as possible and an amalgam which is condensed later can be called the mercury absorption of the amalgam. According to a standard method used for

routine tests in Copenhagen, an amalgam which

has a mercury absorption of 2 percent five minutes

after finishing the trituration, i.e., 41/2 minutes

after the earlist possible condensation, is considered optimum for medium-size fillings. The great

majority of amalgams however have a much higher

mercury absorption during this period.

To summarize the factors that—in amalgam

Figure 7. Porosity and y-phase content in gingival comers

in Class II fillings condensed by a dry technique with

two different condensers (Ap and Af) < by a conventional

technique {B) and by wet technique (C).

fillings on peripheral dental surfaces—can lead

to minimizing secondary damage to the gingiva

we can draw up the following list

1. Avoid as far as possible use of zinc-free

alloy.

2. Do not condense the amalgam for a longer

period than that corresponding to a mercury absorption of 2 percent.

3. The amalgam should be condensed not with

a vibrator but with a hand condenser.

4. The condensation technique should follow

the main principles of the so-called "wet"

technique.

5. The surfaces of the filling should be made

as smooth as possible in order to reduce

the chances of retention of plaque and the

resulting increase in corrosion intensity.

6. Patients should be instructed to maintain the

highest level of oral hygiene because this

brings about a reduction both of the anode

area and of the time a given part of the

filling surface is covered with plaque and

can thus operate as an anode.

There is no doubt that certain details of cavity

preparation, matrix technique, and condensation

technique also affect, though indirectly, the tendency of amalgam to corrode. I do not however

intend to discuss these in detail here.

5. Marginal Inaccuracies of Amalgam

Restorations

As mentioned in the introduction, secondary

caries is the other form of damage that can result

from the failure of amalgam fillings. Secondary

caries can be defined as a type of caries with a

causal relation to marginal inaccuracies of restorations.

There are many causes of marginal inaccuracies

in amalgam fillings. A very frequent cause is marginal excess or deficiency ; but as these defects are

37
only very slightly connected with the material

properties of the amalgam, they will not be the

subject of detailed discussion at this time. Contraction of the restoration during setting, with the

resulting gap between the filling and the cavity

wall, is unlikely ever to have been the direct cause

of secondary caries ; if however the amalgam margin is not supported by the enamel wall, then this

margin if located on a loaded tooth surface will

during mastication be deflected in toward the

cavity wall ; whether marginal fracture then occurs

will depend on the relation between the width of

the gap and the maximum flexibility of the margin.

Calculations have demonstrated that weak amalgam margins can fracture under a very moderate

load and even with such a slight deflection as 2 yu,m. This shows that contracting amalgams involve an

increased risk of marginal fracture and that they

should not therefore be accepted.

It is a recognized fact that delayed expansion

can in certain circumstances produce a marginal

step of a size very likely to lead to secondary caries.

Delayed expansion can also cause crevices in cases where the side walls of the cavity diverge toward

the surface of the tooth. The cause of this is that

the delayed expansion frequently occurs at considerably varying rates in different directions in the

amalgam, and is greatest in the direction parallel

to condensation. Delayed expansion can thus be a contributory cause of marginal fractures in which

the fracture surface stretches considerably under

the margin of the cavity.

It is not too complicated a process to avoid the

causes of marginal inaccuracy already mentioned,

and it is therefore unnecessary to go into further

detail.

However, there is one further cause of marginal

defects, namely marginal corrosion, which very

frequently and seriously predisposes the margin

of the filling to fracture. Figure 8 shows an occlusal

filling with typical, marginal corrosion fractures.

An analysis of the processes leading to corrosion

fracture is rather complicated, involving both the

materials science and technology, of amalgam.

The processes are as follows: shortly after a

filling has been made, saliva penetrates between

the filling and the wall of the cavity ; because of

the relatively low concentration of oxygen on the

surface of the filling facing the cavity wall this

surface will form the anode in a galvanic corrosion

element. The mercury released by corrosion diffuses into the amalgam and causes mercuroscopic

expansion. As this expansion involves only the

surfaces facing the cavity wall, the margins of

the filling are deflected away from the walls. Figure 9 shows the state of the filling margins after

mercuroscopic deflection. Mercuroscopic expansion

is fairly considerable, even when only relatively

small quantities of mercury are absorbed; the

amalgam expands about 5 percent linearly when

it absorbs 1 percent by weight of mercury; the

expansion increases significantly with increasing

porosity of the amalgam. During mastication the

unsupported amalgam margin will be deflected in

toward the wall of the cavity, and it will fracture

if the width of the corrosion gap exceeds the maximum flexibility of the margin. Figure 10 shows a

typical corrosion fracture with the resulting Vshaped marginal defect; it has been shown that

> . • -i

FiouRE 8. Occlusal filling with typical, marginal corrosion

fractures. Figure 9. Filling margin after mercuroscopic deflection.

38
Figure 10. Typical corrosion fracture with resulting

V-shaped marginal defect.

the risk of caries is very small when the width of

the defect is less than 50 /^m; if this dimension

however exceeds 50 /xm, the risk of secondary caries

grows almost in proportion to its size.

An understanding of the processes which result

in marginal corrosion fracture permits an analysis

of the factors that influence these processes from

the point of view that if it is possible to prevent or

delay them, it will also be possible to prevent or delay corrosion fractures.

For marginal corrosion to occur requires—as already mentioned—^that saliva penetrates the gap

between filling and cavity wall. Two methods have

been used to prevent this penetration, namely an

improvement of the adaption of the amalgam to

the cavity wall, and a varnishing of the cavity wall,

primarily with copalite varnish; both of these

methods reduce saliva penetration considerably

but not enough to prevent corrosion or merely influence the rate of corrosion.

The special structure of the phase, even in

well-condensed standard amalgams containing a minimum of mercury, shows that alterations in the

quantity of this phase by modifying the condensation technique is also unable to reduce or prevent

corrosion.

In contrast, porosity of the amalgam margin

plays a decisive role in the rate of fracture. A high

degree of porosity increases to a great extent both

the rate of corrosion and, independent of this, the

mercuroscopic deflection of the margin. Increasing

porosity reduces at the same time the margin's

maximum flexibility to a marked degree. In order

to reach a state in which it can fracture during

mastication a porous amalgam margin requires less

mercuroscopic deflection and it deflects more

quickly than a margin with no or less porosity.

All such factors which influence the porosity of

the amalgam margin are thus of significance to

corrosion fractures. The diagram (fig. 11) shows

the result of experiments on the significance of

some technical factors for marginal porosity; A

represents the dry technique, B the conventional

technique, and C the wet technique. 1 means just

filling the cavity with neither excess nor deficiency

and without any further technical procedures, 2

means overfilling the cavity and eliminating the

excess by carving, while 3 means overfilling, burnishing the margins, and then removing the excess by carving. It is seen that technique no. 3 ensured

by far the lowest level of porosity ; when this technique was followed, it was unimportant whether

technique A, B, or C was employed. It may be

added that the content of mercury in the amalgam

margins examined was independent of the technique used; only in technique 1 it was somewhat

higher.

It is probable but not directly proved that the

marginal porosity will be increased by the use of

nonzinc amalgams and rapid-setting amalgams.

Figures 12 and 13 illustrate the porosity in

amalgam margins made by different techniques.

Figure 12 shows a margin with a porosity of approximately 10 percent as a result of technique

B-1, in other words, a conventional technique without overfilling or burnishing. Figure 13 is a margin

FiQTJSE 11. Effects of some technical factors on marginal porosity.

A-dry technique

B-conventional technique

C-wet technique 1-cavlty not overfilled, margins not burnished

2-cavity overfilled and excess removed by

carving

3-cavity overfilled, margins burnished and

excess removed by carving.

39
Figure 12. Amalgam margin with about 10

percent porosity produced a conventional

technique without overfilling or burnishing.

Magnification^approx. 115 X.

with about 1 percent porosity after overfilling,

burnishing, and removal of the excess. Another factor of great importance to the occur- rence of corrosion fractures is the size of the marginal angle of the tilling. The deflection that results

from mercuroscopic expansion decreases as the

marginal angle increases ; when the marginal angle

is 90°, deflection is zero. The size of the marginal

angle is decided mainly by cavity preparation. An

attempt must be made to achieve approximately

a right angle between the cavity walls and the sur- face of the tooth. This rule can often—but

not always—be obeyed without causing any

disadvantage.

6. Conclusion

The foregoing analysis of the primary causes of the failure of amalgam tillings provides the

conclusion that moderate changes in the properties

of the alloy, the amalgam technique, and cavity

preparation can produce vast differences in quality

of fillings. It is accepted that many amalgam fill- ings demonstrate excellent restorative properties;

insofar as the analysis of the causes of many

Figure 13. Amalgam margin with about 1 percent

porosity after overfilling, burnishing and re- moval of excess. Magnification^approx. 115X.

amalgam-filling failures is correct, it is also a fact

that the percentage of good quality fillings can

be increased considerably by observing the conclusions of the analysis of defects.

The main recurring theme in this analysis of defects has been the corrosion of amalgam and the

necessity of observing various details in the technical procedure ; there is no doubt that future re- search into the properties of amalgams must be

centered upon these two factors.

The deep corrosion in current standard amalgams depends j^rimarily on the network structure

of the 72 phase; if this structure can be broken,

the deep corrosion will simultaneously cease. This

appears to be possible only by a radical modification in the composition of the alloy, in which all

the tin or most of it is replaced by another metal.

Copper is not applicable in this connection because, among other things, it may form a rather

rapidly corroding cohesive copper amalgam phase

in the other amalgam ; silver seems to be unsuitable

too. On the other hand, palladium has certain

properties that are promising.

If the amalgam technique is to produce fillings

of high quality it is essential that several details

of the technique are executed with pedandic accu40
racy. For various reasons it is not always possible

to meet this condition in clinical practice—which

must be regarded as a fault in the technique. Future research into amalgams must certainly be

concentrated on this point also. The spherical

amalgams may have a promising future in this

respect; if their chemical composition were such

that, during setting, a coherent, corrosive phase

did not form, they might form the basis for a new

and simplified filling technique very close to the

ideal.

I want to close this paper with the following

general conclusion : Vast improvements have been

made in the quality of dental amalgams over the

past 50 years, particularly due to the research and

standardization work carried out by the Dental

Research Section of the National Bureau of Standards, to the benefit of the dental profession, particularly, and humanity in general. I feel con- vinced that you will continue in the future to make

significant contributions to the development of this

important material. It is a great pleasure to be

your guest here, and I thank you most warmly

for your gracious invitation.

41

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Basic Metallurgy of Dental Amalgam

L. B. Johnson, Jr. and H. G. F. Wilsdorf

Department of Materials Science, School of Engineering and Applied Science, University of Virginia,

Charlottesville, Va. 22901

An "equilibrium" mixture of components of dental amalgam would contain the phases

/3i, yi and shown by Gaylor. Because dental amalgam is formed clinically at relatively

low temperature only yi and ya are generally found. The tensile strengths of the Ag-Hg and

the Sn-Hg matrix phases of dental amalgam are considerably below the tensile strength of

the amalgam while the tensile strength of AgaSn is considerably above that of the amalgam.

Cleaning the alloy powder with 5 percent HCl promotes better bonding between matrix and

AgaSn and significantly increases the tensile strength. Tlie y: phase of amalgam is the only

phase subject to destructive corrosion. The y- phase has been successfully eliminated from

amalgam by two methods: (1) by inclusion of powder consisting of the Ag-Cu eutectic in

the alloy and (2) by substitution of 10 percent Au for Ag in the alloy.

Key words : Amalgam, dental ; amalgam, metallurgy ; amalgam, strength ; corrosion of

dental amalgam ; dental materials ; metallurgy of dental amalgam.

1 . Introduction

Probably the most successful prosthetic material

ever devised by man is dental amalgam. This ac- counts, no doubt, for the millions of man hours of

labor which have been devoted to research on this

material and the volumes of literature devoted to

rejDorting this research.

No group has contributed as much toward the

excellence of this material as the research group

at the National Bureau of Standards. It is they

who have set the high standards for the development of a fine product.

In any review of the literature covering dental

amalgam, one can only hope to scratch the surface

if he is to keep inside a reasonable time limit.

Much good work must be summarized in a sentence or perhaps not mentioned at all. This paper will

attempt to review what the authors consider to be

the most significant advances in the knowledge of

the basic science involved in the composition,

strength and chemical properties of dental

amalgam.

2. Constitution

Although the first person to use an amalgam as a dental restorative is not known, the first scientific

investigation of dental amalgam must surely be

credited to G. V. Black, beginning about 1896. It

was he who showed that the most useable dental

alloy was one containing near 75 atomic percent

Ag and 25 percent Sn, with perhaps a slight substitution of Cu for Ag (about 5%) desirable.

Since the time of Black's work only minor

changes have been made in the basic composition

of dental alloy. Eesearch efforts have been concentrated on the determination of the setting reaction or reactions and the constitution of the set amalgam. Although advances in knowledge have been

almost continuous, probably no single bit of information could be considered to be a breakthrough

for dentistry such as Black's original contribution.

Nevertheless, a number of significant advances

have been made.

Probably of foremost importance were the

accurate determinations made of the binary constitution diagrams for Ag-Sn, Ag-Hg, and Sn-Hg

and determinations of crystal structures for most

of the important phases in these systems. Outstanding among these investigations were those by

Murphy and his co-workers Almost no major

changes have been made in the constitution diagrams published by this group. Generally accepted

phase diagrams for these binary systems are shown in figures 1, 2, and 3. For the Ag-Hg system, Murphy established two

peritectic reactions resulting in two phases of re- stricted composition. The (3 phase was determined

to be hep with a = 2.98A and c/a = 1.62 and the

y phase complex bcc with a = lO.OA. Berman and

Harcoui-t [2] described two naturally occurring

phases of the Ag-Hg system (a and y) and suggested the name moschellands-bergite for the y

phase. Its composition is best represented as AgoHga and the unit cell contains 50 atoms.

Nial, Almin, and Westgren [3] determined the

structure of AgsSn to be slightly rhombically deformed hep with four atoms per unit cell and

a = 2.995A and c/a = 1.596 at 25 atomic percent

Sn.

^ Figures in brackets refer to the literature references at the end of this paper.

452-525 O—72 4 43
V. Simpson [4] reported the 7 phase of Sn-Hg

to have a simple hexagonal structure with a =

3.213A and c/a = 0.931 at 7.19 atomic percent Hg.

Although a number of researchers had studied

the ternary system of Ag-Sn-Hg between about

1910 and 1930, undoubtedly the most extensive

investigation was that by Gaylor [5] from about

1933 to 1937. Her 70° C and 100° C isotherms for

the system are shown as figures 4 and 5. Prior to Gayler's studies, McBain and Joyner

[6] had proposed the first "setting reaction" for

dental amalgam:

AgsSn + Hg Ag3Hg4 + Sn

Gayler showed that Sn did not form as a product of the reaction and explained the setting by

complex reactions which probably do not go to

completion. She proposed the following equations:

1. AgaSn + Hg ^ /3i + 72

2. /3i + 72 -> /3i + 7i + 72

Her nomenclature follows from figure 4. Troiano [7] agreed with Gayler concerning

the equilibrium phases expected to be present,

(/3i, 7i and 72), but disagreed with the steps

involved, in order to attempt to account for the

nonequilibrium manner in which dental amalgam

was prepared. He proposed the following

equations

1. Ag3Sn+Hg^7i +52+Ag3Sn (unreacted)

2. 7i +52+Ag3Sn—>7i+72->7i+72+ft

Troiano defined S2 as SuyHg which was later

shown by Fairhurst and Ryge [8] to be 7 phase

of the Sn-Hg system. He also defined 7: to be

Ag3Hg4. Extensive investigations of the setting

reactions for dental amalgam were made from

1952-60 by Moffett, Ryge and Barkow [9]. This group used a recording x-ray spectrometer

to determine the phases formed and their chronological appearance. They concluded that Hg

reacts with AgsSn to form a matrix of Ag2Hg3

(71) and the hexagonal 7 phase of the Sn-Hg

system (72). These phases could be detected within

10 minutes of the beginning of trituration and

no other new phases were identified at any time.

Schmitt [10] used selective etching techniques

to distinguish only three phases in set amalgam,

the original Ag3Sn (7), 71 and 72.

44
Sn 10 20 30 40 50 60 70 80 90 Hg

WEIGHT PER CENT MERCURY

Figure 3. Constitution diagram for the Sn-Hg system.

45
Figure 5. 100 °C isotherm for the Ag-Sv^Hg system [5].

A great deal of impetus toward further research

into the structure of set amalgam was provided

by the development of spherical-particle alloys by

Demaree and Taylor [11]. Using this type alloy,

Wing and Ryge [12] identified the same three

phases in set amalgam, (fig. 6) (7, 71 and 72) and

made the interesting observation that most of

the original alloy particles remained unreacted,

even in Targe excess of Hg (70 percent) . It appeared

that the reactions were "self limiting."

None of these latter studies of the setting of dental amalgam were able to identify at any time during setting the presence of the pi phase called for

by Gayler and Troiano. Johnson [13] showed the

presence of the fix phase in dental amalgam, but

FiGUBE 6. Microstructure of dental amalgam.

B indicates the Sn-Hg phase [12].

only after the set amalgam had been stored at elevated temperatures (38° and 60° C) for considerable periods of time. He considered that the /3i phase

formed by a diffusion controlled transformation of

the yi phase, the transformation rate being very

temperature sensitive.

More recently, Otani [14] has identified the fii phase in dental amalgam stored at room temperature for two years. Furthermore, Otani, Tsutsumi

and Yamaga [15] have demonstrated the strong

dependence of the phase relations on temperature.

In dental amalgam allowed to set at temperatures

above 80° C, in fact, no yi phase was found at any

stage, only p-^ and y^. The same amalgam, however,

when moved to room temperature for 48 hrs,

showed the formation of yi phase at the expense of

part of the ^1. Thus, the final amalgam prepared

in this manner satisfied Gayler's diagram. i

It is interesting to note that the photomicro- I

graphs of both Johnson and Otani (fig. 7 a & b)

showed the ySi phase as being continuous with the

original yparticles. An amalgam made entirely

of this "monophase" would be expected to have ex- ^

cellent mechanical and electrochemical properties. %

It appears at this point that there is no basic

disagreement among the investigations reviewed

above. It is apparent that Gayler is correct in her

assessment of the phases present in an "equilibrium" dental amalgam at, say, body temperature,

i.e., i8i+7i +72- It is equally apparent that a transition temperature exists near 80 °C, since Gayler's

100 °C isotherm shows A +72+ liquid and Otani's

dental amalgams allowed to set at any temperature

above 80 °C showed and 72 as the only solid |

46
phases. Therefore, dental amalgam prepared above

80 °C, then cooled to body temperature, will contain a near-equilibrium mixture of /3i-t-7i+72-

Dental amalgam prepared under clinical conditions, however, is not at equilibrium and the work

reviewed here has shown clearly that the /3i phase

is not present at any stage in_ the setting of the

amalgam—at least in a distinguishable quantity. In

time, however, 01 phase forms at the expense of

part of the ji phase—the higher the temperature

the more rapid the transformation. Therefore, dental amalgam prepared at room or body temperature will eventually contain a near-equilibrium

mixture of 181+71+72 as required by Gayler's

diagram. The higher the temperature the sooner

will equilibrium be established.

The exact mechanism by which the setting of

dental amalgam takes place is still not thoroughly

understood. It is greatly complicated by the fact

that one begins with a problem of sintering in a

liquid medium and ends up with a problem in

solid state diffusion. The one point that has become clear, however, is that every change undergone by the system is a striving for the attainment

of the state of equilibrium required by Gayler's

isotherm.

3. Strength

Three intermetallic phases are the main constituents of dental amalgam, designated y, yi, and

y2 as described previously. Since the alloy is produced by sintering in the presence of a liquid

phase, many metallurgical factors influence the

properties of the final material. Table 1 gives an

indication of the complexity of the manufacture of

this alloy and the effect on some properties.

Before a discussion of the strength of dental

amalgam is presented, the mechanical properties

of the three phases, yi, ya, and y, will be discussed

separately.

3.1. Silver Amalgams

The largest volume of dental amalgam is oc- cupied by the Ag-Hg phase, present as an intermetallic compound closely represented by the formula AgaHgs. At present, there is no general

theory explaining the mechanical properties of intermetallic compounds.

The fracture strength of Ag-Hg alloys was determined by Young and Wilsdorf [16, 17]. Because of the large difference between the melting

temperatures of the two components, preparation

of the cast alloys proved difficult. Suitable tensile

specimens, however, were prepared simply by

mixing Ag powder (99.99 percent purity) withHg

and compacting the mixture into a die at room

temperature. The Hg content was controlled by

varying the reaction time and the amount of Hg

available. Alloys were prepared in this manner

containing 15-70 weight percent Hg.

The tensile strength of these specimens is shown

in figure 8, ranging from more than 9,000 psi (62

MN/m 2) at low Hg contents to about 2,000 psi

(14 MN/m ^) for alloys containing 50 to 70 weight

Figure 7a. Alloy particle surrounded by /3i phase [13].

f ' til

It _' *^ •'

* *

Figure 7b. ^1 phase in dental amalgam [l^]

47
Table 1. Metallurgical factors which influence dimensional stability, flow, and strength of dental amalgam

Metallurgical factors Mechanical properties affected

Silver-tin alloy:

Composition

Particle size Particle shape

Increased tin content—^abnormal contraction

Increased silver content—>abnormal expansion

Smollfir nnrfiplp <5i9ip —>stTPri0"t,ti fi.ttflinpH in sVinrt.pr f.imp —^decrease in expansion

Spherical powders are not so sensitive to packing pressures as cut alloys and therefore

often yield more desirable over-all properties.

Alloy: mercury ratio Increasing the amount of mercury—>reduction in compressive strength —^undesirable increase in flow

Too little mercury—^expansion of amalgam decreases unduly

Trituration Increasing the time of mixing—^increased strength

Condensation Increased pressure—^decrease in expansion —>decrease in flow

Contamination Through moisture (for alloys containing zinc) —^delayed and rather excessive expansion

percent Hg. In order to assure complete bonding,

specimens were pressed at 100° C for 3 days and

tensile tested. Although the strength was greatly

improved for alloy compositions between 15 and 60

weight percent Hg (fig. 9), the strength for the

Ag-70 weight percent Hg specimen was comparable to the value shown in figure 8. Since the latter composition is near that of the Ag-Hg matrix

phase of dental amalgam, it must be concluded

that its strength is considerably lower than the

tensile strength of dental amalgam, the latter value

being 7,000-9,000 psi (48-62 MN/m

3.2. Tin Amalgams

The second constituent in the "matrix" of dental

amalgam is a Sn amalgam containing approximately 18 weight percent Hg. It has been identified as SngHg and is generally referred to as the

72 phase. Since the exact composition is not known

with certainty and since some ya grains could have

a lower Hg content, tensile specimens for Sn amalgams containing from 2 to 20 weight percent Hg

have been tested.

Alloys were prepared by casting and also by a powder metallurgical technique [16].

The cast alloys were obtained by sealing instrument grade Hg with chemically pure Sn into an

evacuated quartz tube and holding it for 30 minutes at 250° C. Frequent agitation assured homogeneity. The liquid alloy was quenched in liquid

nitrogen for solidification. After annealing for 7

days at room temperature, tensile specimens were

machined having a 1-in (25 mm) gage length and

0.250-in (6.35 mm) diam. Cylindrical compression

specimens were 0.375 in (9.52 mm) in height and

diameter.

Powder-compacted specimens were prepared by

mixing 325 mesh Sn powder with Hg in a plastic

3,000

6,000

30 40 50 WEIGHT PER CENT MERCURY

Figure 8. Tensile strength of cold-pressed Ag amalgams.

(To convert psl to MN/m^ multiply by 6.895X10-3).

48
capsule in a Spex mixer mill. The plastic mass was

immediately compacted in a die having the dimensions of a flat standard tensile specimen.

The data obtained are shown on figure 10. Rising

from the fracture stress of pure Sn at 3,600 psi (25

MN/m^) to almost 10,000 psi (70 MN/m^) at 8 or 9 weight percent Hg, the tensile strength decreased sharply as the Hg content was increased

from 12 to 20 weight percent. It should be noted

that the stress maximum coincided with the phase

change from the a+y phase field to the y phase

of the system, which at room temperature may

contain approximately 9-18 weight percent Hg.

Beyond 18 weight percent Hg, free Hg coexiste

with the solid y phase.

The information from the phase diagram offers a possible explanation for the variation of strength

and ductility with Hg content. Between 2 and 9

weight percent Hg, y is present in the a solid

solution as a second phase and thereby an effect

comparable to precipitation hardening is expected

WEIGHT PER CENT MERCURY

FiGUKE 10. Tensile strength of cast Sn amalgams.

(To convert psi to MN/m^ multiply by 6.895X10-^).

to occur. The low strength above 18 weight percent Hg is due to the presence of liquid Hg which

acts as crack nuclei in the alloy.

With the exception of the 20 weight percent Hg

alloy, powder compacted specimens showed much

lower strength. Since the reaction between Sn powder and Hg is very fast, the compacting was virtually done with y

particles. Consequently, the

bpnding between the particles was poor. Porosity

was not more than 4 percent.

Compression tests on cast specimens showed

measurable ductility, particularly for those with

low Hg content. A value of 10,000 psi (69 MN/m^)

was obtained for the 20 weight percent Hg

specimen.

As indicated above, the y phase of the Sn-Hg

system is of importance in dental amalgam, the

most likely Sn-amalgam present containing about

18 weight percent Hg. The tensile strength of this

alloy was found to be rather low, about 3,000 psi

(21 MN/m^), and the compressive strength about

10,000 psi (69 MN/m^). Both values are considerably below the corresponding strengths of dental

amalgam.

3.3. Dental Amalgam Alloy

"Dental amalgam alloy" is the name commonly

used for the powder which, when mixed with

liquid Hg, yields dental amalgam. Its chemical

composition is AggSn. It is present in appreciable

quantities as "unconsumed" alloy in the amalgam

and consequently, must play a role in its mechanical properties.

For the preparation of the alloy a vacuum technique was used [16] since alloys melted and cast

in conventional ways were found to be extremely

brittle. This latter was probably due to the introduction of oxygen since molten Ag is known to

attract a substantial amount of oxygen which, upon

solidification, precipitates at grain boundaries.

49
Dental amalgam alloy is the product of a peritectic reaction and contains 26.85 weight percent

Sn. The times required to reach phase equilibrium

are extensive and appear to be prohibitive when

manufacturing the alloy. In order to determine

the sensitivity of the strength of dental amalgam

alloy to compositional changes, two additional

alloys containing 23.85 weight percent Sn and

29.85 weight percent Sn were prepared. The former, therefore, contained more of the /3 (Ag-Sn)

phase while the latter contained more of the

Sn-rich eutectic.

None of the specimens showed any ductility

beyond 1 percent strain, so that only the fracture

strength for tensile loading and compression could

be determined.

The compressive and tensile strengths of chill

cast AgsSn were determined to be 75,000 psi (520

MN/m^) and 25,000 psi (170 MN/m-) respectively. Corresponding values for annealed dental

amalgam alloy were considerably lower. Young

[16] found evidence for precipitated films in an- nealed specimens, which could be responsible for

the reduced strength. It appears reasonable to as- sume that chill-cast or slowly cooled alloy is representative of dental amalgam alloy used in clinical

application.

The tensile strengths of chill-cast Ag-Sn alloys

with Sn contents 3 weight percent higher or lower

than the y composition were found to be slightly

increased (see table 2) though not to a marked

degree. The microstructure, of course, was different, containing /3 crystals in one alloy and 8 crystals in the other. As far as dental amalgam

is concerned, this strength difference is hardly of

concern. It should be noted, however, that the

difference in microstructure is most likely to influ- ence the amalgamation reaction.

Briefly, it has been shown that AggSn is con- siderably stronger than dental amalgam and, consequently, must have a decisive influence on the

strength of the latter.

Table 2. Fracture strength of silver-tin alloys

Alloy containing

weight-percent SnHeat treatment Tensile Compressive

23. 85

23. 85

26. 85

26. 85

26. 85

29. 85

29. 85

Chill cast.. Annealed

Slowly cooled

Chill cast. Annealed

Chill cast- . Annealed

psi

30, 600

*25, 000

23, 000

25, 000

*17, 500

26, 500

*14, 000

psi

77, 700

65, 200

75, 000

44, 800

73, 200

49, 300

Tensile specimens had a 1 in (25 mm) gage length and

either a gage diameter of 0.140 in (3.56 mm) or 0.25 in

(6.35 mm), the latter being indicated by *. Compression

specimens were 0.375 in (9.52 mm) long and 0.375 in

(9.52 mm) in diameter. (To convert psi to MN/m^ multiply

by 6.895X10-'.)

3.4. Dental Amalgam

The clinical importance of Ag-Sn amalgams in

dentistry has determined the nature of the research

that led to the excellent restorative we have today.

The restrictions in producing the amalgam under

clinical conditions are a challenge to the metallurgist. First, the amalgam must be made easily

and quickly in the dental office at room temperature. Second, it must be capable of being molded

to fit the cavity in a tooth so that mechanical forces

will keep it in place. Third, setting must occur at

37 °C, with the amalgam reaching a high strength

in a reasonably short period of time.

Two of the most important clinical requirements

should be mentioned at this point: (1) a linear

expansion of the restorative of not more than 20

iu,m/cm and (2) a high resistance to creep (in

dental literature, usually referred to as "flow").

Since dental amalgam is brittle, the compressive

strength has been used as a convenient and important criterion for assessing the effect of certain

metallurgical variables. The nature of these variables will be indicated by a selection of publications.

In 1949 Phillips [18] investigated the time dependence of the setting of amalgams. He found

that the 10-hr strength was about 80 percent of

the final, and that full strength was often attained

after setting times of 24 hr. Some alloys, however,

required about 7 days to attain full strength, so from this time on compressive strengths were usually given in terms of "7-day strength."

A detailed study showing the effect of varying

condensation methods and time, specimen size, and

deformation rate was reported by Taylor, Sweeney, Mahler and Dinger [19]. They also reported

elastic moduli ranging from 1.2 to 2.1X10^ psi

(8.3 to 14.5 X10« MN/m^) for five dental amalgams. Studies of this nature, going into considerable detail with respect to clinical factors, were

carried out through the 1950's and virtually con- cluded at the beginning of the last decade. At that

time some concern was noted about the strength of

dental amalgam in tensile loading.

Souder and Paffenbarger [20] reviewed the

early work on that subject through 1942 and emphasized that the tensile: compressive strength

ratio was extremely unfavorable, namely 1:8 or even poorer for some preparations.

The renewed interest in tensile strength in the

60's coincided with ( 1) the availability of spherical dental alloy powders, used initially at the National Bureau of Standards by Demaree and

Taylor [11], (2) the marked improvements in

metallographical techniques in handling dental

amalgam by Wing [21], and (3) the successful

application of the diametral test to this material

by Burns and Sweeney [22].

Prior to the last mentioned investigation the

measurements were made on tensile specimens the

shape of which had been conditioned by the brittle- ness and the manufacture of the material. In gen50
eral, the gage length was less than 0.5 in (13mm)

(Ward [23], Taylor [24], Kodriguez and Dickson [25], and Mahler and Mitchem [26]). Additional studies were carried out in 1964 by Hollenback and Villanyi [27] and in 1968 by Nagai and

Ohashi [28]. In general, these investigators

found a 7-day tensile strength between 6,000 and

8,400 psi (41 and 58 MN/m"). The data on each

material in each series of measurements were re- produced only over a wide range of values since the

fracture of the dumb-bell shaped specimens was

sensitive to surface flaws.

Following the work of Burns and Sweeney [22],

the diametral tensile test has been used to provide

reliable data for dental amalgam. The advantage

of this method lies in its insensitivity to surface

conditions of the specimen. The cylindrical specimen is placed between the plates of a testing machine and compressed transversely. A convenient

specimen diameter is 4mm, the length of the cylinder is usually between 5 and 10 mm. According to a treatment by Timoshenko [29], the tensile stress, o-,

is related to the compressive load, P, the diameter,

and length, Z, of a right cylinder by the equation

where <r is the tensile stress in a plane normal to the

plane of the load.

Using the above method, Eden and Waterstrat

[30] measured a tensile strength of sphericalparticle dental amalgam of almost 8,000 psi

(55 MN/m^) ; Young and Wilsdorf [17] reported

9,000 psi (62 MN/m^') (fig. 11), Modjeski and

Nuckels 10,600 psi (73 MN/m^) [31] and Nagai

et al. [32] 10,000 psi (69 MN/m^).

A comparison of tensile strengths obtained from

dumbbell shaped specimens and from diametral

tests indicates frequently a higher value for this

property from the latter test, so that the ratio of

tensile and compressive strengths is approximately

1 :6. However Burns and Sweeney [22] obtained

comparable values for the diametral and the axial

loading methods.

Recently, Lautenschlager, and Harcourt [33]

pointed out that the diametral test is only applicable to homogeneous materials and that in specimens with pores and second phase particles the

actual values may be altered. Seen in this light,

it is intei-esting to note that Nagai et al. [32] found

substantial strength differences for conventional

and spherical alloys when specimens compacted

at different pressures were subjected to the diametral tensile test. While conventional alloys

showed a strength i-eduction of about 50 percent

for a lowering of the condensation pressure from

850 to 71 psi (5.9 to 0.49 MN/m=), the strength

difference for spherical-particle alloys was in general not more than 25 percent and often less. At this time it is not possible to arrive at an understanding of the mechanical properties of

dental amalgam. After having obtained a clearer

picture of the microstructure during the past decade, it is obvious that this 3-phase alloy does not

fit into any of the alloy categories from which the

metallurgist can draw on a theory of strengthening. Most of the information on the strength of

14,000 _

4,000

2,000

ALLOY

X "AS RECEIVED" lYOUNG AND WILSDORF)

"AS RECEIVED" (MODJESKI AND NUCKLES) "CLEANED" ALLOY (YOUNG AND WILSDORFl

"CLEANED" ALLOY (MODJESKI AND NUCKLES)

42 46 50 WEIGHT PER CENT MERCURY

Jj'iGUBE 11. Tensile strength of dental amalgam [J7J.

(To convert psi to MN/m^ inuUiply by 6.895x10-3).

51
dental amalgam is based on macroscopic tests. In

order to separate different parameters, such as alloy particle size and shape, Hg content, trituration time, condensation pressure and time, specific

surface area, and others, detailed studies of the

microstructure are needed. This includes an understanding of the influence of pores on the shape

of the phases, the effect of surface contaminants,

and the atomic structure of phase and grain boundaries, to name a few topics only. Nevertheless, ad- vances in the understanding of the strength have

been made in recent years and these will be re- viewed tofifether with new measurements on relevant properties.

Strength measurements on dental amalgam nor- mally show wide scatter, exceeding by far the

experimental error. Anticipating that surface

contaminations of the AgaSn particles could cause

flaws in the amalgam and possibly would act as crack nuclei or, in any event, be detrimental to the

bonding between phases, Young and Wilsdorf

[17] prepared surface clean dental alloy. They

washed spherical alloy powder in 5 percent HCl

and dried the particles with ethanol. It was

observed that the reaction time for amalgamation

was reduced by this process and that the strength

of the resulting amalgam had increased from 9,000

to 12,500 psi (62 to 86 MN/m^). Modjeski and

Nuckels [31], using the same method, obtained

tensile strengths of 14,000 psi (96 MN/m^). An

explanation of the strength increase will be offered

in a subsequent paragraph.

The macroscopic failure of dental amalgam is

classified as brittle fracture, i.e., the separation

occurs in a catastropic fashion. Information on the

fracture mechanism is incomplete. Asgar and

Sutfin [34] employed a microbend tester and used

light microscopy in order to study the subject on a microstructural basis. They observed that cracks

went through voids and propagated through the

matrix phases (yi and yz are traditionally called

the "matrix") in an intercrystalline manner. These

results were confirmed by Caron's [35] investigation of fracture due to compressive loading. The

latter author also reported the fracturing of alloy

particles in specimens produced at high packing

pressures.

The application of electron fractography allowed additional observations at the submicroscopic level. A schematic representation of the

microstructure of dental amalgam prepared from

spherical alloy particles is shown in figure 12.

Using replica techniques, Young and Wilsdorf

[17] concluded from fracture surfaces that cracks

propagated in an intercrystalline manner for amalgams prepared from untreated alloy powder. They

noted, however, that the Sn-Hg phase showed a

certain degree of ductility during failure, as evidenced by small dimples on the surface. The fracture surface of Ag-Sn crystals showed primarily

facets, indicating intergranular and/or interphase

fracture, the latter of which was typical for cracks

Figure 12. Schematic representation of micro-structure

of dental amalgam.

going around the unconsumed AggSn particles.

Dental amalgam made from "cleaned" alloy particles also fractured in an intergranular fashion

with one remarkable difference—cracks were found

not to avoid AggSn particles but proceeded

through the particles by cleavage. This observation

gives a lead to an explanation of the increased

strength of amalgams made from "cleaned" alloy.

One could reason that the fracture of AgsSn particles is either due to a notch effect, which would

have weakened the relatively high strength AgsSn,

or the bonding between the matrix and the particles was improved by the cleaning techniques.

Since the amalgam showed a definite increase in

strength, it must be concluded that the bonding

between matrix and AggSn was improved.

At this stage of the development of the metallurgy of dental amalgam it is not possible to assess the influence of porosity on the mechanical properties in a quantitative manner. In the past, the fundamental studies have tried to minimize porosity

by using high condensation pressures (see for

example, Young and Wilsdorf [17] whose samples

had a porosity of less than three percent). However, it can be expected that porosity in clinically

prepared amalgam is higher and present techniques of investigation have to be improved in

order to make better quantitative measurements. It

is felt that the use of the scanning electron microscope could be very helpful in this respect.

Another mechanical property of considerable

importance for the clinical behavior of amalgam is creep. Extensive studies have been carried out,

but the actual mechanism in terms of advanced

solid stat« theoi'y is still elusive. The intriguing

problems associated with the behavior of dental

amalgams have attracted a larger group of solid

state researchers, and a number of fundamental

data which are needed for even a first understanding of the often unexpected properties of the alloy are beginning to appear in the literature. The work

52
on elastic constants of the y, yi, ya-phases and dental amalgam by Grenobie and Katz [36] may be

quoted as an example. On the basis of their meas- urements they attempted to develop a model of the

elastic behavior of dental amalgam which with

further refinement could provide information on

the viscoelastic behavior and on the effect of porosity and mercury content on strength.

4. Chemistry

The constitution and mechanical properties of

amalgams are primarily bulk properties of the

material and can, in general, be considered to be

unaffected by the environment, within practical

limits, at least for a period of a number of years.

Yet all metal or alloy surfaces react with their en- vironment to produce changes in the surfaces.

These changes depend on many variables, among

which are the particular constituents present in

the environment, the composition of the metal it- self, the physical character of the surface or interface with the environment, the flow of material

past the surface, and the temperature.

The many variables involved make the study of

the chemistry of the system of dental amalgam

plus oral environment a difficult one. In vivo

studies are beset with the difficulties that no two

human mouths offer the same environmental constitutents, no two amalgam surfaces placed in

teeth are ever identical, and flow situations are never the same for any two fillings. In vitro

studies, on the other hand, while allowing much

closer control of variables, have the decided disadvantage of being very unlike clinical conditions.

This not only makes factual correlations difficult

to make but also makes it difficult to convince

clinicians that correlations exist even when they

are apparent to the laboratory scientist.

Nevertheless, advances in the understanding of

the corrosion of dental amalgam have been made,

especially in the last decade or so. Prior to about 1940, although numerous studies

related to the corrosion of dental amalgam were

made, the major emphasis was generally placed

on the pathological effect on body tissues of galvanic currents produced by dissimilar metals in

the mouth. Little effort was made to note the effect on the metals themselves.

In 1941 Schoonover and Souder [37] published

the first really extensive study of the corrosion of

dental amalgam. They reported rapid corrosion

of dental amalgam in contact with Au in both

1-percent NaCl solution and in an artificial saliva,

but only gradual corrosion with loss of luster

when it was not in contact with Au. Potential difference between the two metals varied between

446 and 548 mV.

Examination of 40 to 50 freshly extracted teeth

containing amalgam fillings showed that, where

adaptation of the filling to the cavity wall was

poor, corrosion was often so severe that the amalgam had lost most of its strength. This was attributed to the presence of concentration cells. Fusayama, Katayori, and Nomoto [38] disagreed with the results of Schoonover and Souder.

For essentially the same conditions the Japanese

researchers reported no significant corrosion of

the amalgam surfaces except for a slight roughening around the region of contact with the Au.

The amount of corrosion was not considered to be

clinically significant.

Swartz, Phillips, and El Tanir [39] observed

the tarnishing of amalgams in solutions made up

of various concentrations of NajS, NaCl and

H2O2, as well as in air, distilled water, and an

artificial saliva. The degree of tarnish was determined simply by visual observation. They con- cluded that the highest degrees of tarnish occurred

in NasS solutions, the rate and severity of attack

increasing with sulfide concentration. Medium

tarnish occurred in the artificial saliva. Experiments with zinc and nonzinc amalgams indicated

no measurable difference in the susceptibility of

the two types to tarnishing.

Wagner [40] was the first investigator to indicate that one specific phase of dental amalgam

was more susceptible to corrosion than the others.

He pointed out that the ya phase was most subject

to chemical attack and, further, that in amalgams

with a high Sn content, the yz phase could be

continuous and corrosive attack could penetrate

deeply into the amalgam. He also, however,

pointed out the importance of corrosion products

as marginal "sealants."

J0rgensen [41, 42] reported agreement with

Wagner both on the continuity of the yz phase

in dental amalgam and the primary importance

of the corrosion of this phase. He found continu- ous ya networks even when condensation reduced

the Hg content as low as 40 weight percent.

Wagner's and J0rgensen's conclusions led Guthrow, Johnson, and Lawless [43] to an extensive

study of the corrosion of the individual, isolated

phases of dental amalgam in Einger's solution

and in artificial saliva. Potentiostatic measure- ments (figs. 13, 14, and 15) were complemented

with electron micrographs of the surfaces (figs.

16, 17, and 18). This study supplied conclusive

confirmation for the preferential and destructive

corrosion of the ya phase. Only deposition (or

tarnishing) occurred on the yi phase, while the

y phase was essentially neutral.

Otani [44] has summarized much of the re- search in Japan on the corrosion of dental amalgam. In general, polarographic techniques have

been used rather extensively to attempt to determine the effect of manipulative variables on corro- sion in various media. From their work, it appears

that the more Hg an amalgam contains, the

greater the corrosion ; the higher the condensation

pressure, the less the corrosion ; spherical-particle

alloys provide amalgams with higher corrosion

53
resistance; but, in general, dental amalgam is a

highly corrosion-resistant material.

Mueller, Greener, and Crimmins [45], Mueller

Sarkar, and Greener [46] and Mueller and

Greener [47] have made extensive electrochemical

studies of amalgam corrosion both with open circuit and anodic polarization techniques. These

measurements were complemented with optical.

electron microscope, and electron microprobe ex- aminations of the specimen surfaces. They investigated a number of alloys and the effects of

clinical variables such as trituration time and Hg

to alloy ratio and the effect of residual strain

energy on the corrosion rates. In general, practices

which resulted in the formation of more 72 phase

led to more corrosion. They also noted that the

UJd

to

o

0.9

as

0.7

0.6

0.6

0.4

0.3

O.t

0.1

0 0

- 0 1

-0.2

- 0.3

-0.4

Ag^Sn

o Ringer's solution

A Synthetic saliva

JI 100 1000 CURRENT DENSITY (^A/cm^)

10000

Figure 13. Potential-current relations for AgaSn in corrosive media [J/S],

o

o

>

o

0.9 -

O.B -

0.7 -

0.6 -

0.5 -

0.4 -

OS -

0.2 -

0 1 C

0.0 -

- 0.1 -

0.2 -

-0.3 -

- 0.4 -

AggHgj

-1 I I

I 1 1 1 1

° Ringer's solution

4 Synthetic saliva

'

' I 1 1— I

10 CURRENT DENSITY (/xA/cm^)

Figure 14. Potential-current relations for AgzHgs in corrosive media [43].

54
- 0.3

• 0.4

o Ringer's solution

A Synthetic saliva

i ' ' I '

' I

i—

10 too 1000 CURRENT DENSITY (/iA/cm?)

10000

Figure 15. Potential-current relations for SnsHg in corrosive media US'i.

Figure 16. Electron photomicrograph of y phase after

polarization in Ringer's solution for one hr at 0.08 V. (Orig. mag.XSOOO US].

Figure 17. Electron photomicrograph of yi phase after

polarization for 30 min in Ringer's solution at 0.08 V. {Orig. mag.XSOOO) US].

- -J

- *r

- "*

Figure 18. Electron photomicrograph of 72 p'lase after

polarization in Ringer's solution for one hour at —O.Slf

V. (Orig. mag.XSOOO) US].

presence of Zn in the amalgam led to markedly

different initial potentials but about the same as for nonzinc specimens after about 24 hr.

Johnson and Lawless [48] reported that stress

caused a marked increase in the anodic direction

for the corrosion potential of the 72 phase, but only

a slight change for dental amalgam or the y or yi

phases. This effect was believed to be due to surface

film rupture during plastic deformation of the

specimens. They concluded that stress is probably

not an important factor in the corrosion of dental

amalgam under oral conditions.

Mateer and Keitz [49] made a significant contribution toward the understanding of amalgam cor- rosion in vivo. Ground and polished cross-sections

through 50 extracted teeth containing amalgam

55
fillings were examined metallographically. The

authors reported a two-phase corrosion product

deposited as layers in the marginal region between

the filling and the cavity walls, figure 19. These

deposits tended to follow the 72 phase and porous

areas in the amalgam, figi;re 20. Fine, branch-like

penetrations of corrosion product deep into the

amalgam were sometimes observed. It was con- cluded that the corrosive attack consumed the 72

phase, and, indirectly, part of the y phase by formation of new phases due to reaction of Hg re- leased in the original corrosion, with y phase.

Recently, dental alloys containing small

amounts of SnF2 have been placed on the market.

This led Stoner, Senti, and Gileadi [50] to investigate the corrosion of amalgams with inclusions

of such materials. They reported that the inclusion

of SnFa in an amalgam enhanced its corrosion, as did also the addition of NaF into the corroding

solution. The addition of SnF2 into the solution

did not appear to affect the rate of corrosion of

the amalgam.

Many attempts have been made to identify the

products of amalgam corrosion. Techniques involving chemical analysis, spectroscopy, electron

microprobe, and electron and x-ray diffraction

have been used on corroded surfaces, neighboring

areas, and contacting solutions.

Schoonover and Souder [37] and Hyselova,

Zajicek, and Vahl [51] used chemical analyses and

agreed that mainly Sn ions were in the corrosion

products.

Swartz, Phillips, and El Tanir [39] using x-ray

diffraction and Guthrow, Johnson, and Lawless

[43] with x-ray and electron diffraction, identified

AgoS, HgS, and AgCl respectively. The first group

also concluded that a complex (Hg, Ag) Sx was

present while the latter found complex salts which

could not be identified by reference to ASTM files. Mateer and Reitz [49] made x-ray diffraction

analyses of corrosion products scraped from

amalgams in extracted teeth. They reported that

at least SuzSg and /SSnOz were present.

Mueller and Greener [47] usmg an electron microprobe, found chlorine present in the 72 areas of amalgams which had been polarized in a physiological saline solution and suggested that a tin

chloride precipitate might be important in the

passivation process.

It is clear from these studies that a wide variety

of corrosion products may form when dental

Figure 19. Metallographic section through amalgam res- toration in extracted tooth. Cavity wall In enamel (A) and dentin (B) Is at left. Light-gray

corrosion product (C) has deposited against cavity wall. Darker gray corrosion product (D) has deposited against amalgam and has penetrated into regions which were originally

interconnected (72 Sn-Hg and porosity) [49].

5^

FiGUKE 20. Metallographic section showing fatigue crack and bulk corro- sion of amalgam in margin region of extracted tooth. Amalgam fragments have chipped away from cavity wall (A), causing a gutter.

Bullj deposition of the darlj gray corrosion product has taken place in 72 and

porous regions within the amalgam, whereas cracking is visible at the exposed

surface. The two types of attack appear to contribute mutually in destroying

integrity of the restoration [49].

36
amalgam is subjected to an oral environment.

Since the environments to which they are subjected vary widely, this is entirely reasonable.

In general, however, it appears that Sn is the main

constituent which dissolves from the amalgam

and is found in solution, while adhering corrosion

products are probably high in Ag and Hg, are primarily sulfides, and contain some Sn.

The most important conclusion to which nearly

all corrosion studies have pointed is that the 72

phase of dental amalgam is the only phase subject

to destructive type corrosion or dissolution. This

led J0rgensen [42] to suggest that research should

be directed toward elimination, or at least reduction in amount, of the 72 phase, perhaps by re- placing part of the Sn in the original alloy with

Pd. Results of such efforts have not been reported

as yet.

Successful elimination of the 72 phase from

dental amalgam has been reported for two recently

developed alloys.

In 1963 Innes and Youdelis [52] reported studies

on the dispersion strengthening of dental amalgams. They mixed various amounts of metal

powder consisting of the Ag-Cu eutectic (71.9

percent Cu) with powdered AgaSn (78.2 percent

Ag, 26.8 percent Sn). The combination was later

marketed under the trade name Dispersalloy.

Mahler [53] studied amalgams made from the

above alloy. He used an electron microprobe and

reported (1) "the presence of a reaction phase sur- rounding the dispersant particles which was determined to have a composition intermediate between

the intermetallic compounds of CugSn and Cug

Sns," and (2) "the absence of the Sn-Hg phase

(72) in this amalgam."

Johnson [54, 55], considering Ag-Sn-Hg alloys

as electron compounds, showed that it should be

possible to dissolve more Sn in the 71 phase than

had previously been thought possible. Under nonequilibrium conditions, it appears that all of the

matrix Sn in dental amalgam could be incorporated in a single phase with the same structure

as the 71 phase.

Grenoble and Katz [56] showed that the 72

phase disappeared from dental amalgams which

had been subjected to extremely high pressures at

room temperature (about 50 kilobars or 750,000

psi (5,200 MN/m^)).

In order to bring about the elimination of the

72 phase at body temperature, Johnson [57] then

substituted 10 weight percent Au for 10 percent

of the Ag in the original alloy. He reported (fig.

21) that the 71 and 72 phases formed as usual

upon trituration with Hg, but that after 14 days

at body temperature, x-ray diffraction patterns

showed only the presence of the 71 phase. This

material had a very high corrosion resistance.

More recent studies by Mahler [58], however, indicate that part of the Sn may be tied up by Au

Au-CONTAINING AMALGAM

14 DAYS OLD

Au-CONTAINING AMALGAM

I Hr. OLD

g

65 60 55 50 45

DEGREES 29

40 35 30

FiGXJEE 21. X-ray diffraction scans of conventional and

Au-containing amalgams showing absence of 72 phase

in Au-containing amalgam at I4 days 157].

in a "reaction ring" around the alloy particles,

apparently similar to that for Dispersalloy.

The question remains open as to how much cor- rosion of amalgam is desirable. The sealing of

margins and crevices by corrosion products has

been demonstrated and surely may be desirable.

On the other hand, weakening of the amalgam

structure by the destructive corrosion of the 72

phase has been demonstrated and is also surely undesirable. It may be possible to eliminate destructive corrosion effectively by forming an amalgam

without the 72 phase, while at the same time allowing the formation of adherent films of oxides and

sulfides to accomplish the marginal sealing. Only

long-term clinical testing of 72-free amalgams

can give the final answer.

5. Summary

A brief review has been made of the significant

advances made over the past half century or so towards an understanding of the constitution,

strength, and chemistry of dental amalgam.

The conclusion has been reached that an "equilibrium" mixture of components would contain the

phases fii + ji + y-' as shown by Gayler's 60 °C

isotherm. Because dental amalgam is formed clinically at a relatively low temperature, only the 71

and 72 phases are generally found. Recent work

has shown that ^1 does form, however slowly, thus

57
showing that the system is striving toward the

equilibrium state—the higher the temperature the sooner equilibrium is established.

Although numerous measurements have been

made of mechanical properties of amalgams and

a much clearer picture of the microstructure has

evolved, it is still not possible to attain a complete

understanding of the behavior of this material.

The major defect appears to be its multiphase

nature, which results in poor tensile properties.

It does not appear to fit into any of the alloy categories from which the metallurgist can draw on

a theory of strengthening. Nevertheless, recent

improvements in the study of microstructure,

modern advances in the application of electron

fractography, recent measurements of elastic constants of the individual phases, and the possibility

of the elimination of one phase of the amalgam,

all offer considerable hope for a better understanding and improvement of the mechanical properties.

A knowledge of the chemistry of dental amalgam is complicated not only by the multiphase

nature of the material but even more so by the

extreme complexity of the environments in which

tooth fillings are located. Corrosion products and

mechanisms have been difficult to identify. Nevertheless, one outstanding bit of information has

evolved that the ya phase of amalgam is preferentially attacked in a detrimental manner. The possibility of eliminating this phase, therefore, holds

considerable promise toward improving the

lasting qualities of the material in an oral

environment.

We owe it to those colleagues, both past and

present, of the Dental Research Section of the

National Bureau of Standards, in whose honor

this paper has been presented, that the search for

a full understanding of this most useful and intriguing alloy be continued.

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Mueller, H. G., Sarkar, N., and Greener, E. H., Electrochemical properties of commercial dental

amalgams. Annual lADR Meeting New York,

N.Y. (1970).

Mueller, H. G., and Greener, E. H., Electron microprobe analysis of dental amalgam surfaces.

Annual lADR Meeting, Houston, Texas (1969).

Johnson, L. B., and Lawless, K. R., J. Biomed.

Mater. Res. 3, 569 (1969).

Mateer, R. S., and Reitz, C. C, J. Dent. Res., 49,

399 (1970).

58
[50] Stoner, G. E., Senti, S. E., and Gileadi, E., The

effect of NaF and SnFj on the rate of corrosion

of dental amalgams. J. Dent. Res., 50, 1647

(1971).

[51] Kvselova, J., Zajicek, and Vahl, J., Deutsch

Zahnartztl A., 23, 631 (1968)

[52] Inees, D. B. K., and Youdelis, W. V., J. Canadian

Dent. Assoc. 29, 587 (1963).

[53] Mahler. D. B., Microprobe analysis of a dispersant

amalgam, lADR Program of Abstracts and Papers, No. 14 (1971).

[54] Johnson, L. B., J. Biomed. Mater. Res., 4, 269

(1970) . [55] Johnson, L. B., Amount of Sn in the 71 phase of

dental amalgam, J. Biomed. Mater. Res., 5, 239

(1971) . [56] Grenoble, D. E., and Katz, J. L., J. Dent. Res. 50,

109 (1971).

[57] Johnson, L. B., A new dental alloy, lADR Program

of Abstracts and Papers, No. 23 (1971).

[58] Mahler, D. B., personal communication.

452-525 0—72—5 59

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Mateeials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Casting Alloys in Dentistry

Kamal Asgar

School of Dentistry, University of Michigan, Ann Arbor, Mich. 48104

A major difference between the two techniques, hygroscopic and high heat or thermal

expansion, used for casting gold alloys is that the wax pattern can offer some resistance to hygroscopic expansion of the mold while it does not resist thermal expansion. Researchers

have condemned the hygroscopic technique because of nonuniform expansion of the wax

pattern but have not given suflScient attention to other nonuniform dimensional changes

which are present in both techniques and which may combine to affect favorably or unfavorably the accuracy and retention of the casting. Studies in this field should not be

limited to simple measurements of thermal, setting and hygroscopic expansions of the

investment but should be expanded to include strength of the investment along with its roughness, strength and behavior of waxes, and shrinkage patterns of molten metals. A need

for new types of investments is presented by the recent development of many new

nonprecious alloys for dental castings.

Key words : Castings, dental, accuracy ; castings, lost wax technique ; dental materials

gold alloys, dental ; high heat casting technique ; hygroscopic expansion ; investment, dental

thermal expansion casting technique ; wax, inlay casting.

1. Introduction

Casting metals by the lost wax process has been

known in art and industry for many years. Since

the turn of the century, the idea of using the lost

wax procedure in preparing metallic substances

to replace the missing portion of tooth tissue has

been routine practice in dentistry. Yet, the re- quired accuracy of the fit of the cast structure by

the profession has not been completely resolved.

The early castings were grossly short of what is

considered today, an acceptable fit. This was

mainly due to the inferior materials available at

the time. The development of the cristobalite investment by two independent research groups,

Sweeney and his co-workers at the National Bureau of Standards, and Coleman and Weinstein

[1] ^ at the Weinstein Research Laboratories of

New York, probably was the most important single

achievement in the field of dental castings. Dental

gold castings made by using the cristobalite investment were first introduced to the dental profession

at the annual meeting of the American Dental

Association in Buffalo, New York, September,

1932, by Jesrich. At about the same time, Scheu

[2] also presented his hygroscopic casting

technique.

2. High Heat and Hygroscopic

Techniques

Since 1932, over a dozen dental gold casting

procedures and techniques have been introduced

1 Figures in brackets indicate the literature references at the end of this paper.

to the profession by various researchers. As ex- amples, Phillips [3], Hollenback [4], Sweeney

[5], Coleman [6], Crawford [7], Shell [8], Ireland [9], Smyd [10], Gustafsson and Hedegard

[11], J0rgensen [12], Coy [13], Asgar and Mahler

[14], Fusayama [15], and Peyton [16], could be

mentioned. These are modifications and offshoots

of two main techniques, the high heat or thermal

expansion technique and the hygroscopic technique. Theoretically, there is very little difference

between these two main techniques. All of the investments used today in dentistry have some set- ting expansion, some hygroscopic expansion, and

some thermal expansion. The investments are formulated so that if they are properly handled,

the sum of their setting, hygroscopic, and thermal

expansion will be equal to the contraction of dental

gold alloys and wax. The difference between these

two techniques boils down to the fact that in the

high heat technique, a major part of metal shrinkage is compensated by thermal expansion of the

mold. This is obtained by heating the ring up to

about 1200-1300 °F (650-700 °C) . The setting and

hygroscopic expansions of such an investment are rather low. In hygroscopic techniques the major

part of the total expansion is obtained hygroscopically, and the thermal expansion of this type

of investment is low.

Although they are alike theoretically, practically there is a great difference between these two

techniques. Two of the most important differences are

1. In the hygroscopic technique the wax pattern is present as one piece and solid while

hygroscopic expansion is taking place. Thus,

61
the wax pattern could offer some resistance,

whereas in the high heat technique the

major part of expansion is taking place

while the ring is placed inside an oven and

the wax pattern no longer exists as solid or one piece. Thus, it could not offer any

resistance to the expansion forces.

2. In the hygroscopic technique the mold temperature is 700-900 °F (370-480 °C). The

mold temperature in the high heat technique is about 1200-1300 °F (650-700 °C)

or about 400-500 °F (220-280 °C) higher

than that of the hygroscopic technique.

These two points could be considered as the real

differences between these two techniques. If they

were recognized completely and handled properly, many practical problems would be avoided.

The classical woi'k of Docking and his associates

[17], reported in 1948 and 1949, was the first

major research work to clarify the behavior of the

hygroscopic investment. Later, Landgreen and

Peyton [18], Delgado and Peyton [19], Asgarzadeh and Mahler [20], Lyon and Dickson [21],

Ryge and Fairhurst [22], Donnison and Docking

[23], Skinner and Degni [24], Mumford [25],

Fusayama [26], Mahler and Addy [27], Eamshaw [28] , and many others reported their findings

on hygroscopic expansion of investments. As a

result of these findings, the hygroscopic expansion of an investment is no longer a mystery.

3. Effects of High Heat and Hygroscopic

Techniques on the Dimensions of the

Casting

The effect of hygroscopic expansion on wax patterns is somewhat different from that of thermal

expansion. The shape and size of the wax pattern

makes some difference on hygroscopic expansion

and the resultant cast piece. As an example, let us

consider the behavior of two different patterns

such as, one surface inlay (Class I) and three sur- face inlays (MOD) . In the case of one surface inlay, the investment expands away from the wax

pattern and it really does not matter to a great ex- tent what type of wax is used. In the case of an

MOD pattern however, investment located between the two axial walls during setting and hygroscopic expansion of the investment will be

exerting some forces trying to enlarge the size of

the wax pattern in the mesial-distal direction. The

wax pattern, in the meantime, will be offering some

resistance to these expansion forces. Depending on

the type of wax used, hard or soft, more or less

resistance forces can be offered by the pattern. Furthermore, on the pulpal floor area of the axial

walls, the resistance is greater than on the cervical

areas. Some investigators have placed reference marks

on the cervical and pulpal floor areas of an MOD

type of pattern measuring exactly the distances

A and B (fig. 1). Results of these measurements

X A X

Figure 1. Distances A and B were measured precisely

before investing the wax pattern and after expanding

the investment hygroscopically.

have shown that due to the setting and hygroscopic

expansion of an investment, the percent of expansion in cervical areas is more than that in the

pulpal floor area. Therefore, the hygroscopic technique has been condemned. Yet the investigators

have failed to go ahead and complete the casting

procedure. The fact is that the dimensions of dental casting are also changed nonuniformly during

solidifi.cation and shrinkage of cast molten metal.

These studies have clearly indicated that the wax

pattern expands nonuniformly under setting and

hygroscopic expansion forces of the investment.

4. Effects of Nonuniform Shrinkage of the

Alloy and Surface Roughness on the

Dimensions of the Casting

Although the nonuniform expansion of the wax

pattern has made researchers condemn the hygroscopic technique the nonuniform shrinkage during

solidification and cooling of cast alloys has been

completely ignored. To explain this point more

clearly, let us consider two types of patterns, a full

crown type and a Class I type. If a full crown type

of casting were made having the exact dimensions

of its die, obviously such a casting would not fit. In order to have a full crown type of casting fit onto a die, the casting should be slightly oversized

(fig. 2). On the other hand, if a Class I, or one

surface inlay, had exactly the dimensions of its die,

such a casting also would not fit. The casting in

this case should be slightly smaller in order to fit into the die. The question may arise concerning

the fit of an MOD pattern. Mesial-distally, it is

similar to a full crown, which means the casting

should be slightly larger than the dimensions of

the die, whereas in buccal-lingual direction, it behaves like a Class I or one surface inlay which

means it should be slightly smaller than the dimensions of the die. It should be recognized that the

effective shrinkage of metal is also not uniform. In

a Class I, or one surface inlay, the alloy shrinks

to its maximum amount. It could be visualized that

the cast structure, after solidification, may shrink

according to its coefficient of thermal contraction.

That is to say, for every degree that it cools, it

62
Figure 2. Dimensions of a and b on full crown

type castings should be larger than ai and bi. Dimensions of c and d in inlay type castings

should be smaller than Ci and di.

becomes somewhat smaller until it reaches room

temperature. On the other hand, in the case of a

full crown type pattern, the alloy shrinks its least

amount. In this case also, after solidification, metal

should shrink according to its coefficient of thermal

expansion, but due to the shape of the pattern, it should compress the investment inside of the crown. Due to the fact that metals usually do not

possess high strengths at temperatures close to their fusion range, they cannot compress the investment inside of the crown to any extent, and

their effective shrinkage at high temperatures is rather low. As the alloy coojs, its strength increases

and eventually reaches a point where its strength

is higher than that of the investment and can compress the investment inside of the crown. From

such a temperature, down to room temperature, it may shrink according to its coefficient of thermal

contraction. Thus, if all other conditions remained

the same, stronger investments would produce full

crown type castings which seem to have expanded

more. The strength of the investment has little

effect on shrinkage of a one surface inlay.

Not only could the magnitude of shrinkage of

the cast metal vary in these two types of patterns,

the effect of roughness of the cast structure is also quite different. When a wax pattern is properly invested, the investment completely covers all

surfaces of the wax pattern. It should be remembered that the wax pattern is invested in a slurry

which is made by mixing some water with solid

particles of various sizes (fig. 3). To eliminate the

wax pattern, a mold is placed in an oven and

heated to casting temperature. During this process, the excess water in the investment, as well as the water of crystallization of various solids, will

be eliminated. Thus, the smooth surface of the

investment next to the wax pattern becomes some- what rough. Roughness in the case of a Class I, or one surface inlay, acts as an aid to expansion

and its effect would be as if the investment ex- panded more. In the case of full crown roughness.

Figure 3. Roughness of casting causes Oi and bi to get

smaller than a and b. Roughness of casting causes Ct and di to become larger than c and d.

it will prevent the casting from complete seating, as if the investment did not have sufficient

expansion.

To summarize, one may say that in a one surface

inlay, the shrinkage of metal is at its maximum,

but roughness of the cast structure aids the ex- pansion. Whereas, in full crown type patterns,

shrinkage of metal is at its minimum but roughness of the investment prevents the seating of cast

crown or acts against the expansion of the investment. If an investment produces roughness to such

a degree that the shrinkage of cast metal at its

maximum (Class I or one surface inlay), minus

the roughness value of the cast piece, is equal to

the shrinkage of cast metal at its minimum (full

crown type) , plus the roughness of the cast piece,

then such an investment is capable of producing

good MOD castings.

Shrinkage of Metal — Roughness

(Class I) = Shrinkage of Metal + Roughness

(Full Crown)

Usually investments producing smooth casting

surfaces could produce good full crown castings,

but it would be difficult to produce good one sur- face inlays. Similarly, investments producing

somewhat rougher cast surfaces, may produce good

one surface inlays, but it will be difficult to make

a well fitted full crown casting.

5. Retention of the Casting

For dentists, retention of casting is another important point. It is preferred if the casting offers

some retention in the last millimeter before its

complete seating. Questions may arise as to what is responsible for such a retention. If all dimensions

of the die were exactly reproduced on the cast,

theoretically there should not be any retention up

to the last micrometer away from complete seating.

63
All of a sudden, all surfaces of the cast should

contact all surfaces of the die. To analyze the

retention, one should recognize what is responsible

for it in different shape patterns. Such a study can

be done by making castings, seating them on the

die, embedding them in some type of bioplastic

and cross-sectioning the die and the cast. The

space between the cast and the die can be examined

under a microscope. Such a study shows that dental castings are far from a perfect fit. In the case of full crown and MOD type of patterns only on marginal areas are the castings con- tacting the dies (fig. 4) . In the case of Class I and

Class V, due to the fact that shrinkage of metal

is at its maximum, the line A in the cast is slightly

smaller than in the die. Therefore, in Class I some

surfaces of the cast can contact those in the die,

whereas in the case of Class V, such a contact is not

possible (fig. 5). By placing some pins in Class V

preparation, one could get sufficient retention (fig.

6). It should be recognized that the pins during

setting and hygroscopic expansion will be actually

bending slightly and causing the retention. The

marginal adaptation of Class V inlay with pin, in a cast condition, probably is not any better than

the one without a pin. The fact that castings having

some retention can easily be finished and better

marginal adaptation can be obtained at the chair

during the delivery, is subject of another discussion. It is more interesting to observe the retention

of Class III inlays (fig. 7). Under a normal ex- pansion of the investment, the inlay usually has

Figure 4. The casting contacts the die only on the

marginal ridge areas.

Figure 5. In the Class I type of pattern, casting contacts die in some areas. The distance A is responsible for some retention. In Class V

type of pattern, easting does not make any contact usually

with the die.

Figure 6. Class V pattern loith pins.

Castings usually have some retention.

Figure 7. Class III type of inlay.

The retention can be obtained by contacting the two

inner surfaces or by the two outer surfaces.

some retention. By increasing the expansion of the

investment, the inlay would lose its retention; by

further increasing the expansion of the investment,

the inlay regains its retention. The retention first

was obtained probably because the inlays were

very slightly small, and like Class I restorations,

they touched the two inner surfaces of the die.

When the dimensions of the cast inlay were en- larged by increasing the expansion of the investment, the cast no longer could contact the surfaces

of the die, thus it would lose its retention. By further expansion of the investment, the outer sur- faces of the inlay could come in contact.

The retention of many patterns is due to the

setting and hygroscopic expansion of the investment which produce non-uniform expansion of the

wax pattern of very minute amounts. Both Moore

of the Ransom and Randolph Company, and Nieman of the Whip-Mix Corporation have produced

j investments having no setting and hygroscopic '

expansion. Thermal expansion of this investment

64
was sufficiently high to compensate for the shrinkage of gold alloys.

Nieman has even obtained a U.S. Patent for his

investment. Although such an investment sounded

good at first, the castings had no retention and did

not satisfy the needs of the dental profession. The

investment was discontinued.

6. Measurement of the Accuracy

of Castings

It is relatively simple to measure thermal, set- ting, and hygroscopic expansion of an investment.

It is however, more complicated to measure dimensions of cast pieces and compare those with that of

the original die. In this case not only are the various expansions of the investment important, but

also the dimensional changes of the wax pattern

and the shrinkage of the molten metal. If however,

the quality of the fit of casting to a die is to be

considered—the main objective of researchers in

this field, as well as those in practice—then the

roughness of the casting and strength of the investment also should be recognized.

In the past, relatively little research work has

been reported in which the fit of dental gold castings were measured and compared. The early work

of Volland and Paffenbarger [29], Hollenback

and his group [30], Mahler and his associates [31], are among few published articles in this country

concerned with the fit of castings and relating it

to some expansion values. Pomes and Slack [32],

Suffert and Mahler [33], and Barone and Dickson [34] have measured the roughness of dental

gold castings. According to their values, roughness

of dental castings could be as high as 0.4 percent.

When this value is compared with the total shrinkage of the gold alloys, the role of roughness in the

fit of dental castings becomes clear. In figures 8

and 9, the thermal expansion of two investments

are shown. Both investments have identical setting

and hygroscopic expansion. Two castings of the

Bureau of Standards full crown die were made

employing identical techniques. One of the castings fit the die (fig. 8) and the other did not

(fig. 9). Roughnesses produced by these two investments were different. In figure 8, the roughness

measured by the profilometer was 10 microinches

whereas the casting in figure 9 has roughness value

of 25 microinches.

7. Conclusion

The time has come when studies in this field

should not be limited only to measurement of thermal, setting, and hygroscopic expansions of the

investment, but should be expanded to include

strength of the investment along with its roughness, strength and behavior of waxes, and shrinkage pattern of molten metal.

It should also be mentioned that due to the efforts, and hard research work of Nieman [35],

Moore[36], Watts[37], Collins[38], and a few

others, there are good investments available for

dental gold castings. During the last fifteen years,

many new nonprecious alloys have been developed

in industry. The physical properties of some of

these alloys promise their potential use in the field

of crown and bridge. Yet the available investments

are not suitable for their casting. It seems that

there is a great need for some basic research in

this field with the hope of developing a series of

new investments for casting of these new alloys. It may be time to look for the possibility of finding a different binding material than the three

already being used. Perhaps a refractory material

different from those used today in dentistry should

be employed for casting of the newly developed

alloys.

1.4

1.2- 1

^

1.0-

.8

.6

.4-

.2 / 10 Microincries

200 600 1000

TEMPERATURE V

Figure 8. The casting fits the Bureau of Standards die. The Investment had 1.35 percent thermal expansion at a casting

temperature of 1,000°F. with a roughness of 10 microinches.

The setting and hygroscopic expansion of the Investment was

Identical to the one used in figure 9.

TEMPERATURE °F

Figure 9. The casting does not fit the die. The investment had 1.45 percent thermal expansion at a casting

temperature of 1,000°F. Roughness of the Investment was 25

microinches. The setting and hygroscopic expansion of the Investment was identical to the one used in figure 8.

65
8. References

[1] Coleman, R. L., and Weinstein, L. J., Investment,

U.S. Patent 1,932,202.

[2] Scheu, C. H., A new precision casting technique,

J. Am. Dental Assoc., 19:630 (1932).

[3] Phillips, D. W., Present-day precision inlay investing and casting technique, J. Am. Dental Assoc. and Dental Ck)smos, 24:1470 (1937).

[4] HoUenback, G. M., Simple technique for accurate

castings : New and original method of vacuum

investing, J. Am. Dental Assoc., 36:391, 1948.

[5] Sweeney, W. T., Cristobalite for dental investment,

J. Am. Dental Assoc., 20:108 (1933).

[6] Coleman, R. L., Physical properties of dental materials, BS J. Research 1, 867 (1928) RP 32. [7] Cra\vford, W. H., Selection and use of investments,

sprues, casting equipment and gold alloys in making small castings, J. Am. Dental Assoc., 27:1459

(1940).

[8] Shell, J. S., Hodgen-Shell Dental Materials, chapter 8 (The C. V. Mosby Co., St. Louis, Mo., 1938).

[9] Ireland, J., Vacuum investing and its relation to cast surfaces, Brit. Dental J., 86:111 (1949).

[10] Smyd, E. S., Wax, refractory investments and re- lated subjects in dental technology, J. Prosthetic

Dentistry 5:514 (1955).

[11] Gustafsson, C. G., and Hedegard, B., Investing and

casting technique. Acta. Odontol. Scand. 12:233

(1954).

[12] J0rgensen, K. D., Investigations on dental precision

casting technique, Odontol. Tidskr., 62:443

(1954) . [13] Coy, H. D., Casting procedures in dentistry. Intern. Dental J. 5:173 (1955).

[14] Asgar, K., Mahler, D. B., and Peyton, F. A., Hygroscopic technique for inlay casting using controlled

water additions, J. Prosthetic Dentistry, 5:711

(1955) . [15] Fusayama, T., Technical procedure of precision

casting, J. Prosthetic Dentistry, 9:468 (1959) ; 9: 1037 (1959).

[16] Peyton, F. A., Mahler, D. B., and Asgar, K., Controlled water-addition technique for hygroscopic

expansion of dental casting investment, J. Am.

Dental Assoc., 52:155 (1956).

[17] Docking, A. R., Chong, M. P., and Donnison, J. A., The hygroscopic setting expansion of dental cast- ing investments, Australian Dental J. 52:6, 160,

320 (1948) ; 53:261 (1949).

[18] Landgren, N., and Peyton, F. A., Hygroscopic ex- pansion of some casting investments, J. Dental

Res., 29:469 (1950).

[19] Delgado, V. P., and Peyton, F. A., The hygroscopic

setting expansion of a dental casting investment,

J. Prosthetic Dentistry, 3:423 (1953).

[20] Asgarzadeh, K., Mahler, D. B., and Peyton, F. A., The behavior and measurement of hygroscopic

expansion of dental casting investment, J. Dental

Res., 33:519 (1954).

[21] Lyon, H. W., Dickson, G., and Schoonover, I. C, The mechanism of hygroscopic expansion in dental casting investments, J. Dental Res., 34:44

(1955).

[22] Ryge, G., and Fairhurst, C. W., Hygroscopic expansion, J. Dental Res., 35:499 (1956).

[23] Donnison, J. A., Chong, M. P., and Docking, A. R., The effect of surface area on the hygroscopic set- ting expansion and strength of casting investment, J. Dental Res., 36:967 (1957).

[24] Skinner, E. W., and Degni, F., Hygroscopic expansion of dental investments, J. Am. Dental Assoc.,

54:603 (19.57).

[25] Mumford, G., and Phillips, R. W., Dimensional

change in wax pattern during setting of gypsum

investments, J. Dental Res., 37:351 (1958).

[26] Fusayama, T., Hosoda, H., and Kher, V. M., Influ- ences of clinical variables on a cristobalite investment, .T. Prosthetic Dentistry, 11:152 (1961).

[27] Mahler, D. B., and Addy, A. B.. An explanation for

the hygroscopic setting expansion of dental gyp- sum products, J. Dental Res., 39:578 (1960).

[28] Eamshaw, R., The effect of restricted stress on the

setting expansion of gypsum bonded investments,

Australian Dental J., 9:169 (1964).

[29] Holland, R. H., and Paflfenbarger, G. C, Cast inlay

technique, J. Am. Dental A.ssoc, 19:185 (1932).

[30] Hollenback. G. M., and Rhoads, J. E., Comparison

of the expansion of the mold cavity with the

linear casting shrinkage of gold. S. Calif. State

Dental J., 28:73 (1960).

[31] Mahler, D. B., and Addy, A. B., The effect of water

bath in hygroscopic casting technique, J. Prosthetic Dentistry, 15:1115 (1965).

[32] Pomes, C. E., Slack, G. L., and Wise, M. W., Surface

roughness of dental castings, J. Am. Dental

Assoc., 44:545 (1950).

[33] Sliffert, L. W., and Mahler, D. B., Reproducibility

of gold casting made by present-day dental cast- ing technics, J. Am. Dental Assoc., 50:1 (1955).

[34] Barone, J. F., Huff, R. L., and Dickson, G., Surface

roughness of gold castings. Dental Prog., 1:78

(1961).

[35] Neiman, R., Investment composition, U.S. Pat- ent 2,247,395; 2,247,585 ; 2,247,586; 2,247,587;

2,247,588.

[36] Moore, T. E., Dental investment material, U.S. Patent, 1,924,874.

[37] Watts, C. H., Jr., Investment material, U.S. Patent,

2,675,322.

[38] Collins, P. F., Dental investment composition and

process, U.S. Patent, 2,006,733; 1,953,075.

66
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Basic Metallurgy of Dental Casting Alloys

John P. Nielsen

J. F. Jelenko Co., Inc., New York, N.Y. 10801

The complex dental casting alloys have arrived at their present useful state mostly by

trial and error. The three areas of technical significance for these alloys are : inertness in

the oral environment, fabricability (including soldering), and alloy strengthening. Basic

metallurgical studies are being made in this area, and as the gap between the art and

basic metallurgy is closed, improvement in properties and the cutting down of precious metal

content can be expected. There are prospects of new alloy systems and improvement in

casting technique, particularly in automatic casting. Ceramic metal systems need further

study. For the long range, investigations of the prosthetic material-human tissue reactions are necessary. To assist studies in this field a dental materials handbook on properties of

dental alloys and related materials should be compiled and published.

Key words : Alloys, dental casting ; ceramic-metal systems ; chrome-cobalt alloys ; dental

materials ; gold alloys ; metallurgy, dental alloy ; porcelain-gold materials ; solder, gold

alloy.

1. Introduction

Basic metallurgy of dental casting alloys refers

to the established knowledge of interatomic forces,

crystal structures, phase equilibria, solid state diffusion and reaction kinetics, plastic deformation,

crystal strengthening mechanisms, and the like

of such alloys. The assignment to discuss this topic

was given with the suggestion that the future developments be kept in mind more so than a re- view of past accomplishments. In pondering over

this suggestion it became apparent that it was an

appropriate one in that there is not all that much

established basic metallurgy of these alloys to discuss. Dental cast alloys are still in the technological stage in which the art is ahead of their reliance

on basic metallurgy. For example, the dental gold

alloys are generally complexes of four or five alloy

additions involving copper, silver, platinum, palladium, nickel, zinc, tin, indium, and perhaps one

or two others, yet the function of these elements is not well documented in the technical literature.

Apparently the commercial complex alloys have

been arrived at over the years by trial and error. Two particular cases in point are the palladium

and zmc alloying additions. We recently found

the palladium addition as singularly effective in

reducing the tarnisli propensity of silver-containing gold alloys. On checking the silver-containing

dental gold alloys on the market, it was surprising

how many contained the required amount of this

alloy addition for tarnish resistance. Zinc appears

to be a specific element that minimizes the tendency for gold alloy casting porosity. Here again

most dental golds already carry the required zinc

addition. Evidently some unsung heroes in the

past responded to the feedback from the dental

laboratories and the dental profession by continually modifying alloy compositions, involving hundreds of different melts, until a reasonable

optimum was achieved for the various performance characteristics. No doubt many a night was

spent with the melter and perhaps a somewhat

technically oriented gold alloy salesman seeking

the alloys that would be favorably accepted by the

dental profession. Naturally little of such findings

appeared in the literature not so much by default

as by an attempt to preserve secrecy for each little

real or imagined discovery.

The chrome-cobalt dental alloys however were

developed with the advantage of established

metallurgical technology for chemical resistant

alloys, namely, the Stellites. However, here the

diversity of alloy types applicable to dentistry, at

least for the present, is not so wide as in the gold

alloys. Nevertheless, some metallurgical art run- ning ahead of basic metallurgy does exist in the

case of the base metal alloys for baked porcelain

applications.

Another interesting aspect of metallurgical art

in dental alloys is the technique of metal joining

by so-called soldering. Actually some significant

achievements exist in this field, almost completely

developed by technicians. Indeed soldering as the

metallurgical term is known does not apply, but

rather "hard soldering", "brazing", and perhaps

sometimes unfortunately "fusion welding". The

gold alloy solders must be noble and of the same

color as the joined material. Hence the simplest

procedure for making a solder is to take the alloy

to be joined and lower its melting range about

150° C. with zinc and tin additions. In the hands

of a skilled technician, joining gold alloys with

such a solder works quite well. For the dental

67
golds fortttnately the metals to be joined are noble,

1.e., inherently "clean", and hence fluxing what

oxides occur is easily done. The joining of the nongold cast alloys is however a considerably more

serious problem challenging both the metallurgist

and the technician.

Perhaps the most interesting area of metallurgical art versus basic metallurgy is the strengthening of the cast alloys. The research as reported in

the metallurgical (as distinct from dental re- search) literature indicates the logical approach

for studying the strengthening of alloys (chiefly

by age hardening) ; by starting with binai-y systems, then going on to the ternaries, etc. Researchers reporting in the dental literature begin, for

practical reasons, with the complex alloys already

being used by dentists. The complexity of the alloys however limits the attainment of data that re- veal the mechanism of alloy strengthening. Thus,

although the two approaches are heading toward

each other, there is a technological gap.

A brief review of some basic physical metallurgy of cast dental alloys as found mostly in the

metallurgical literature will define this gap more

clearly.

2. The Basic Metallurgy Limits of Cast

Dental Alloys

2.1. Alloy Inertness

The primary nature of the gold alloys is the

retention of some of the nobility inherent in pure

gold. There are two basic criteria for evaluating

the nobility of a metallic element. In one the molar

free energy of the formation of a compound from

the element is measured for standard conditions.

If this free energy value is negative, the compound

will tend to form spontaneously under the specified

conditions. If the free energy is positive, the compound will tend to decompose back to its component elements. The degree of this decomposing

tendency is measured by the positive free energy

value. This degree of instability of the compound

is a measure of the nobility of the metallic component. If the positiveness of the free energy is

maintained at elevated temperatures, where reac- tion rates may be very high, then this tendency for

the compound to decompose at high temperature

is the fire-refining phenomenon characteristic of

the highly noble metals such as gold and platinum.

A second criterion for determining nobility is

the measurement of emf values against a standard electrode in a standard cell under certain specified conditions. Here again positive values of the

emf indicate nobility relative to the standard electrode, with the degree of nobility being indicated

by the positive emf value.

Neither of these criteria serve too well for alloys.

Sometimes a galvanic series is set up for a particular electrolyte, and the metals, which may include alloys, are listed as to their relative anodic

behavior ; those on the upper end of the list will

behave anodic to those below. No one seems to have

made such a list of the different gold alloys for

sulfide-producing environments. The rating of

gold alloys in tarnish tests is the nearest to such a

series. However, such tarnish tests have not been

standardized.

In the case of the chrome-cobalt alloys, their

inertness in certain environments is attributed not

to nobility but to the ability to passivate, i.e., to

produce a protective coating automatically on the

surface. Here again no criterion seems to exist that

would assist the dental profession in classifying

such alloys as to their inertness in the oral

environment.

Thus we have two important classes of cast

dental alloys; the noble and the passive, with no

established evaluating procedure for true inert- ness in the oral environment.

2.2. Fabricability

A significant characteristic of the dental casting

alloys is that they cast readily and give faithful

replication of the detailed contours of the impression pattern. This requires convenient melting and

casting temperature ranges. Also the high density

of the gold alloys yields a low kinematic viscosity, so that the molten metal readily flows and fills the mold in all parts. The chrome-cobalt alloys,

although of lesser density, also rely on the generally low kinematic viscosity of the metals. Measurements of the speed of mold filling indicate fill- ing rates for small castings in fractions of a second.

The solidification takes an additional few seconds.

A part of the castability feature of dental alloys

is that they react very little with the mold material, so that little dressing of the casting is necessary, at least little enough not to disturb the fit of

the casting as a dental restoration, which is a rather

precise requirement.

The gold-base alloys are easily joined by soldering with carefully matched soldering alloys. The

matching is for color and for tarnish resistance.

However the technology seems to be completely

in the hands of the technicians and the dental alloy

manufacturers. There appear to be no criteria as

to surface tension, viscosity, spreading power, or

physical properties of the solder metal. The solder

joint metallographically appears good, although

there is little quantitative data as to the physical

properties. Cui-rently the chrome-cobalt alloys are not soldered in dental applications. The metallurgical art on this problem has as yet not been successful. Here may be an opportunity for basic metallurgy to assist in solving the prolDlem of joining

the base dental alloys.

2.3. Alloy Strengthening

The essential theory for the strengthening of

gold alloys, as reviewed by the metallurgists writing for the dental journals is that an order-dis68
order transformation takes place. This comes from

the discovery of Kurnakov and his associates back

in 1916 that the stoichiometric compositions, AuCu

and AuCug change from random substitutional

solid solution to a superlattice type, all in an FCC

structure, on cooling below about 400 °C. The

superlattice is of coui'se the harder form. The complex dental gold alloys appear to be one-phase

alloys after aging and so the order-disorder theory seems to still apply.

In the high fusing dental gold alloys for baked

porcelain applications, no copper is added (it

stains the porcelain green) , but still age hardening

occurs; accompanied, incidentally, by much loss

of ductility. There appears to be a massive phase

transfoi-mation—what it is, is not clear. When Fe

or Sn are added in small amounts, on the order of one atomic percent, in the presence of Pt in the

alloy, age hardening is substantially increased. Apparently Pt^Fey and Pt«Sn„ compounds of some

kind play a role. It is interesting to point out that for dental applications the age-hardening of gold alloys is designed to occur automatically by simply avoiding

quenching of the casting, by slow cooling in the

mold after solidification. Dental laboratories, or

dentists doing their own metal casting, are not

prone to set up heat treating furnaces scheduled

for appropriate age-hardening heat treatment.

Here is a good example of a kind of metallurgical

art developed between the alloy supplier and the

laboratory technician to achieve acceptable results.

On the other hand, in the metallurgical literature there is little evidence that the convenience

of the dental laboratory technician is considered.

Hence automatic age-hardening by mold coolingis practically never mentioned. Strengthening of

gold alloys in general however has been in part

systematically pursued. The order-disorder transformation is well recognized for gold alloys

containing substantial amounts of copper. The agehardening due to other causes is however also rec- ognized. In the AuCuAg ternaries there is x-ray

evidence of phase decomposition hardening—with

the Cu-rich phase perhaps exhibiting a superlattice structure, and the Ag-rich phase adding substantially to the hardening in certain compositions.

The AuPtPd ternaries show solid state decomposition into two phases on aging, as do the AuPt

alloys, whereas the AuPd alloys do not.

By the time Pt, Pd, Cu, and Ag are added to

gold, the hardening is quite significant. All sorts

of solid state transformations occur, most of which

completely escape the examination with the optical

microscope. Apparently the gold alloy technology

gap exists mostly between the evolution by trial

and error of the complex gold alloys vised in dentistry and the physical metallurgy understanding

of alloy strengthening in these alloys.

As far as the chrome-cobalt alloys are concerned,

these are two-phase alloys at room temperature, one phase being cobalt-rich and one, chromiumrich. Such alloys are generally not ductile, nor are they amenable to age-hardening. However, the

high rigidity of these alloys, twice that of the gold

alloys, plus the inherent hardness of tw^o-phase

mixtures, makes these alloys directly suitable for

stress-bearing applications in dentistry—for ex- ample, for partial restorations. However, as Asgar

has so ably shown, it is not completely necessary

to accept all the loss of ductility usually found in

these alloys. By lowering the interstitial carbon

and molybdenum, and making one or two other

minor modifications, the customary chromiumcobalt alloys can be toughened considerably, without any loss of rigidity, or significant loss of yield

strength. Perhaps there is a sacrifice of stable

carbides, but this is important only for high temperature applications, and certainly of no value in

dentistry. In these chrome-cobalt alloys basic

metallurgy is being intelligently exploited for

dental applications.

3. Future Expectations for Dental

Alloy Technology

Recognizing that a gap exists between metallurgical ait and basic metallurgy in cast dental

alloys, one might well ask what are the future ex- pectations for dental alloy technology. It should be

noted that this gap at present is already being

closed in part. The dental manufacturers are staffing their research departments with metallurgists,

and cui'i'ently there is evidence of good metallurgical reseach being published by them. Furthermore, there is some sponsorship by the National

Institutes of Health at several universities on dental materials research, including cast dental alloys.

It will take a little time for these institutions to become productive, or at least stimulative, of useful

research results, but within 10 years physical

metallurgists and dental alloy experts should be

speaking the same language.

With this in mind, and knowing the motivations

in dentistry, in dental materials manufacturing,

and in government support for dental research,

some technological expectations can be put forth.

This is best done by examining the broad range of

past developments up to the present and attempting to sense the momentums into the future.

The nature of evolving systems is that they

increase in their complexity by a process of successive mutations that continually increases the

adaptability of the system to its surroundings.

Each successful mutation is a quantum jump in

complexity, or a new branch in the existing branch

structure representing the system. Thus we can

trace the tree-like growth of an organization, or of

a science, or of a specific teclmology. Having done

this we are in the position to make some estimates

as to the likely course of future developments in

that system.

In the case of dental alloy technology one can

rather easily construct the evolutionary process of

69
say the last 50 years. Shortly after 1919, due mostly

to the work by the dental research group of the

National Bureau of Standards, it was established

that four types of cast gold alloys could serve the

needs for the overall range of cast dental alloys.

The ancient lost wax process had been applied to

dentistry many years earlier, and by gradual improvement in casting equipment and in investment

materials the technique used with gold-base alloys

became commonplace, available to all dentists.

In the thirties the development of the chromecobalt alloys that were chemically passive lead

naturally to the mutation of using such alloys in

dentistry in place of the costly noble metals where

ductility or burnishability was not important.

In the forties the physical properties of goldbase alloys were sufficiently standardized so that

most gold alloy manufacturers published physical

property charts of their alloys, and the larger companies adopted quality control measures to assure that every melt met the physical properties listed

in their charts.

In the late fifties the porcelain jacket technique

was perfected using a gold-base alloy. The demands on this alloy are exacting. Its thermal ex- pansion coefficient must mat-ch that of the porcelain, it must have high sag resistance at porcelain

baking temperatui^es as liigh as 1050° C, it must

be free of porcelain-staining elements, and it must

contain elements that contribute to the shear

strength of the porcelain-metal bond, which

apparently is chemical in nature.

In the sixties the technique of producing finegrain castings was brought under control, raising

the toughness of the alloys and reducing coring,

thereby minimizing heterogeneity of alloy properties. This period saw also the emergence of white

alloys for porcelain baking.

What may one reasonably expect of the future ? The list below has been developed primarily on

the basis of three motivating factors : ( 1 ) There is

always the pressure to reduce the jewelry aspect

of precious metal alloys, i.e., to cut down metal

costs by reducing gold, platinum, and palladium

content wherever possible. (2) It is always desirable to improve the physical properties such as strength, toughness, and tarnish resistance for the

same metal cost. (3) It is always desirable to cut

down on the need for technicians' skills in the

fabrication of restorations, and obtain results as much as possible automatically and mechanically.

It would seem therefore that in the next few

years the following will emerge

1. Improvement in dental performance of

present-day alloys with minor modifications.

a. improved tarnish resistance of gold

alloys.

b. improved mechanical properties in

both the gold and the chrome-cobalt

alloys.

2. Development of new alloys.

a. lower precious metal-content alloys

but with no sacrifice in dental

performance.

b. new precious metal alloy systems.

c. new non-precious metal alloy systems.

3. Development of new casting equipment

such that good porosity-free restorations are obtained on every casting, requiring

little or no dressing.

4. Development of new soldering alloys and

techniques, particularly for the high heat

alloys.

There are other possibilities that can be projected. Acrylic jacket techniques already in exist- ence might call for new types of alloys. Perhaps

completely new metal fabricating techniques, such

as electroforming or powder metallurgy, might

come into existence in place of casting.

4. Recommended Basic Metallurgy

Programs

Up to this point we have reviewed the dental

casting alloys both as to their metallurgical art

and their metallurgical technology aspects. Based

on judgments concerning past alloy developments,

a projection of the new alloy developments are

given. Now, the motivation for the new developments exist. The question is whether the motivation

relying on the slow growth of the metallurgical

art, i.e., by trial and error, is sufficient to bring

about these improvements in a suitably short time.

The answer is probably no in most cases. The purpose of applying basic technology rather than

simply the trial and error technique is to shorten

the time lag between needs and fulfillment of these

needs. In order to bring this about, it would not be

wise to assign dental alloy development projects

categorically to the metallurgists and anticipate

good results in short order. A young metallurgical

turk would quickly come up with new alloys, but

the chances are that they would not be as good as the ones currently used. Some important but subtle

factors would not be taken into account. For ex- ample, the proposed alloys might be substantially

stronger and have the gold content in what is believed to be the safe range, but after several hundred restorations had been inserted in patients'

mouths, it would be discovered that they discolor

badly. Another likely occurrence in situations

when a young technically trained man is assigned

to make improvements in an old established technology, is that old improvements are rediscovered.

The first order of business is then to close the

gap explicitly between metallurgical art and basic

technology in dental alloys. The young metallurgist with modern sophisticated tools, such as the

electron probe, the scanning electron miscroscope,

and the computer, should first learn why the present alloys work as well as they do. For example,

one of the first j)rojects to assign to the metallur70
gist entering the dental alloy field is the determination of the mechanism of strengthening of the

dental gold alloys by the somewhat automatic agehardening of dental castings by mold cooling. A

second project might be to determine the subtle

resistance to discoloration of the dental golds in

the oral cavity. It is in the course of such a program, where the metallurgist is forced to learn

some of the complicated requirements of dental

castings, that he would develop an appreciation of

the true objectives of his development research

work. It is very probable that in the course of such

a program new questions would be raised whereby

new and more sophisticated improvements could

be conceived in dental technology. For example,

how can the bulk of a crown be cast in some inexpensive high strength metal, then be covered

with a soft burnishable material that is completely

noble?

Another good question for the biomedical metallurgist, for this IS what he would become, would

be the surface reactions on metal-tissue contact.

Many new and interesting avenues would be

opened up for exploration. An enormous amount

of work will eventually be done on this whole

question of prosthetic materials-living tissue

reactions.

It is while the biomedical metallurgist is ranging over these wide problems that he would also

learn to keep his feet on the ground with such

boundary conditions as precision castability, joinability, etc. There is little point in making a superior alloy as to strength and tissue inertness,

if it cannot be fabricated or soldered, or if it is nonburnishable.

One interesting area that a metallurgist could

get into almost immediately is the development of

a technique for producing sound castings. Dental

castings as a rule are reasonably sound. The alloys are sufficiently deoxidized to make them gas free.

Porosity and surface defects, however, remain the

most serious problems in dental castings. Here the

metallurgist can quickly bridge the art versus technology gap, because casting soundness is not one

of the successes of dental metallurgical art. The

reason for this is that except for excessive porosity, or surface defects, the laboratory technician is in

the dark as to the quality of the interior of a

casting.

Perhaps the largest gap between art and technology in dental alloys is the soldering or joining

technique. This technique has been developed

almost completely without metallurgy, between

the trial and error of the gold solder producer and

of the skilled laboratory technician. The metallurgist has a great deal to learn here. However,

once the metallurgist has caught up with the

technician there is the strong possibility for the

development of techniques where manual skill is not required, manual skills being a commodity that

is getting scarcer and scarcer. A good joining

technique of the furnace brazing type would be

most welcome. Incidentally, the high heat type of

dental castings, particularly the base metal alloys are generally not soldered well by the technician.

Here the metallurgist has a clear problem to work

on immediately.

There is need for the eventual development of a simple automatic casting machine, especially for

the high heat alloys. There is no reason why a dental casting cannot be made completely sound,

faithful to the mold details, with little or no finishing required, coming directly from some kind of

automatic casting machine. This development is of

course a little outside the domain of the metallurgist. New heat sources for the melting and the

possibilities of vacuum casting have to be explored,

and perhaps the development of new investment

materials must come about to aid the metallurgist.

One particularly good area for the metallurgist,

together with the ceramist for closing the gap we

are discussing, is the study of the very interesting

and quite successful technique of porcelain bonding to metal restorations. Here, once the metallurgist and ceramist working as a team have caught

up with the technique as it is now practiced, new

improvements would surely be forthcoming.

Finally some long range projects might be described. With the laboratory tools of today it would not be difficult to document all the phase

compositions that occur in the complex gold-base

alloys. Quaternary and higher component phase

diagrams are too difficult to construct. However

a coordination of metallography and rasters available with the electron microprobe (these show

area distribution of selected elements in a microstructure) would furnish the next most useful

information, short of knowing the phase diagrams.

Indeed, such data may actually be more usefiil

than that obtained from phase diagrams. Microprobe analysis can monitor nonequilibrium compositions that occur in castings and on heat

treatment.

Another worthwhile long range project would

be the determination of distortion, dimensional

stability, and residual stresses in dental castings,

for both the gold and the base metal alloys. Along

these lines a study of the stress distribution in

porcelain-on-metal i-estorations could be important. A tempered porcelain, analogous to tempered glass, where the tensile stresses are all

confined to the interior might add to the already

considerable usefulness of porcelain facings in

dentistry.

Perhaps the most important long range program

would be the study of material-tissue reactions.

This would naturally overlap medical materials.

5. A Dental Materials Handbook and

Dental Materials Publications

The most useful reference sources we have to- day on dental materials are the textbooks by Phillips and Skinner, and by Peyton. However, being

71
textbooks, they cannot give space to numerous

phase diagrams, microstructures, and mechanical

properties for a large variety of alloys. The original sources as references are scattered too widely

to be available to all dental materials research

workers. Therefore it seems iustifiable to have compiled a dental materials reference handbook. The

coverage might be in the order of the following:

1. Physical Properties of the (appropriate)

Elements (Obtainable from the ASM

Metals Handbook)

2. Phase Diagrams

This part should include about 50 commonly

used binary phase diagrams involving at least one

of the following components : Au, Ag, Pd, Pt, Cr,

Co, Ni, Fe, Sn, Hg, Cu, Si02 AI2O3, CaO, MgO,

NaoO, and KjO. Discussions of each phase diagram should include crystal structure of phases,

and lattice parameters, where possible. Likewise

there should be about 30 ternary systems described

in isothermal diagram form, involving at least two

of the components of the above. The phase diagrams can be mostly obtained from Hansen]s and

from Guertler's compilations. The nonmetallic systems are obtainable from "Phase Diagrams for

Ceramists," published by the American Ceramic

Society. Quaternary alloy diagrams are not too

common or useful. "However, in due coarse there

should be electron rasters of complex alloys in the

equilibrium state. Wlien these apply to dental material systems, they should be included.

3. Mechanical Properties

The literature should be searched for assembling

as completely as possible the mechanical properties of pertinent alloys, ceramics, and polymeries.

These should include data for cast, wrought, an- nealed and heat-treated conditions.

4. Corrosion Data

The literature should be searched for electrochemical tarnish, and corrosion data. The literature

on electrical contact technology should not be over- looked in this connection.

5. Miscellaneous Properties

Heat conductivity and color data would be use- ful for certain alloy and ceramic systems.

6. Miscellaneous Materials

Standard information on waxes, resins, investment materials, and cements generally used in

dental techniques should be included.

One major advantage of such a handbook would

be the sparing of the dental materials textbook au- thors the chore of including tables of routine data

in their books. A textbook should not have to serve

as a data reference book, but as an expounder of

concepts so that the information in handbooks can

be intelligently used. However, the principal advantage would be to facilitate basic metallurgy

research on dental alloys. Indeed handbooks and

the like are an indispensable tool and no research

laboratory functions well unless the researcher is

surrounded by the bric-a-brac of compilations,

handbooks, wall charts, and data tables.

The appropriate group to issue such a handbook

and renewing it on a five-year basis or so, is the

ADA division at the National Bureau of Standards. The first issue need be only a soft cover photooffset product. The natural demand that would develop will dictate its future development.

To the dental materials researcher it is important that he publish his work and that he follow

the work of other researchers. It will be a luxury

soon to publish dental materials research papers in

general dental research publications. It is a waste

already today to have to subscribe to three or four

dental journals for the 5 percent or so of papers

that apply to dental materials. Furthermore, when

papers are prepared for publication, the authors

may have to strain to connect their paper to dentistry. For example an excellent paper may be unfairly criticized for not having clinical data, when

actually the information without clinical data is

directly useful to dentistry, or may be a prelude

to clinical work. Whether there are enough dental

materials papers to have special issues devoted to

this subject is still uncertain. However, this can

be anticipated in the next few years, particularly

since dental materials research is increasing

significantly.

6. Summary

It appears that cast dental alloys arrived at their

present usefulness mostly by trial and error with

coordination between the dental alloy producers

and the dental laboratories and dentists. The three areas of technical significance for these alloys are

alloy inertness in the oral environment, fabricability (including soldering) , and alloy strengthening. One can find some basic metallurgy in the lit- erature in these areas, but nevertheless a gap exists

between the art and the documented technology.

However, metallurgists are entering the field more

and more, and one can expect advances in the improvement in the properties of the present alloys

and in the cutting down of the precious metal content while maintaining current properties. There is a prospect of new alloy systems, mostly in the nonprecious or low precious metal area. Improvements

in casting technique, mostly in the direction of au- tomatic casting can be expected. Likewise, dependence on the necessary manual skills in soldering,

both of the precious metal and the base metal type,

will be reduced. The trial and error method works

too slowly to bring these improvements to the fore

in the near future. Hence research programs

should be initiated ; first to acquaint the research

metallurgist as to what is already good in the

present-day dental alloys and techniques, and sec- ondly to open avenues as to where improvements

72
can come about. Ceramic metal systems need

further study. Dimensional stability of castings is

also important. A most interesting long range

project, and one that is important for medical

materials in general, is the study of prosthetic material-human tissue reactions. Finally, it seems that

certain research aids are in order to assist the new

metallurgist working on basic metallurgy of dental alloys. A dental materials handbook should be

compiled and revised periodically. Dental materials papers should be given special issue status

in the dental journals.

Considerable assistance in the preparation of

this report came from Joseph Tuccillo of the Jelenko Co., Edmmid M. Wise (formerly of the International Nickel Co.), and Professor Ernest

Levine of New York University. Many references were used and it is difficult to make detailed

credits. The following were the ten most frequently

used.

7. Bibliography

1. Asgar, K., Techow, B. O., Jacobson, J. M., A new

alloy for partial dentures, J. Prosthetic Dentistry

23: 36, February 19(0.

2. ASM Handbook, 8th Ed. vol 1, pp. 1174-1196 (Ameri- can Society for Metals, Metals Park, Ohio, 1961).

3. O'Brien, W. J., Kring, J. E., and Ryge, G., Heat

treatment of alloys to be used for fused porcelain

technique, J. Prosthetic Dent. 14 : 955 Sept-Oct,

1964.

4. Peyton, F. A., Restorative Dental Materials, 3rd Ed.

(The C. V. Mosby Co., St. Louis, Mo. 1968).

5. Shell, J. W., Hogden-Shell Dental Materials (The

C. V. Mosby Co. St. Louis, Mo. 1938).

6. Skinner, E. W., and Phillips, R. W., The Science of

Dental Materials, 6th Ed. (W. B. Saunders Co., Philadelphia, Pa., 1967).

7. Souder, W., and Paftenbarger, G. C, Physical Properties of Dental Materials, NBS Circ. 433, 34-80

(1942).

8. Vines, R. F., and Wise, E. M., Platinum Metals and

Their Alloys (International Nickel Co., 1941).

9. Vines, R. F., and Wise, E. M., Age Hardening Precious Metal Alloys, pp. 190-230 (American Society

for Metals, Metals Park, Ohio, 1940).

10. Wise, E. M., Gold (D. Van Nostrand Co., New York

1964).

73

IV. New Development in Nonmetallic Restorative Materials

452-525 0—72 6

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Dental Porcelain

John W. McLean

Institute of Dental Surgery, Eastman Dental Hospital, London, W.C.T England

Current research on dental porcelain has been concentrated on methods of improving

fracture resistance of porcelain restorations. Enamelling of metals or dispersion strengthening of glasses offer the greatest possibilities for this purpose. The fusion of porcelain to metal copings has proved very successful under clinical conditions, but the necessity for

masking the metal substructure presents difficulties in obtaining the desired aesthetic

characteristics. Replacement of the metal copings by a higher strength ceramic should

improve aesthetics and reduce productive costs. Dispersion strengthening of glass with high

strength alumina crystals has resulted in the production of a new range of aluminous

porcelains for jacket crown and bridge pontic construction.

Key words : Aluminous porcelain ; ceramic materials, dental ; dental materials ; dental

porcelain ; dispersion strengthening of glass ; porcelain, dental ; porcelain-gold techniques

and materials.

1. Introduction

The exacting aesthetic requirements of dentistry

have largely dictated the composition of dental

porcelain. In order to produce highly translucent

enamels, dental porcelain must contain a high proportion of glassy material. Regular dental porcelain has been developed from feldspathic glasses

modified by small amounts of crystalline materials

and coloring pigments, and suffers from the inherent brittleness associated with all glasses.

It is hardly surprising that priority in research

has been given to methods of reinforcing dental

porcelain, since no other incident is more likely

to frustrate the dentist in his work than the

breakage of a porcelain jacket crown. The chipping or complete fracture of porcelain pin teeth or bridge pontics can also occur, involving both dentist and technician in time-consuming repair work.

2. Research Avenues to Improve Dental

Porcelain Strength

In order to improve the strength of dental porcelain a number of avenues are open and future

research would appear to center around five

approaches.

1. Enamelling of metals.

2. Dispersion strengthening of glasses.

3. Enamelling of high strength crystalline

ceramics.

4. Controlled crystallization of glasses.

5. Production of compressed surface layers in

dental porcelain via ion-exchange.

2.1. Enamelling of Metals

The fusion of dental porcelain to metal copings

is now widely used and has proved very successful

under clinical conditions. Providing a strong bond

is achieved between the porcelain veneer and the

metal, there is little chance of leakage at the interface. In addition the porcelain enamel is reinforced

by the metal and is less likely to be placed under

a tensile force which can cause brittle fracture.

The attachment of porcelain to gold in dental

restorations has been variously ascribed to a combination of mechanical, wetting, or Van der Waals

bonding. Vickery and Badinelli [1] ^ have examined the interfacial effects between porcelain and

single crystal and polycrystalline gold surfaces

and showed that, whereas mechanical and compressive forces played significant roles in attachment, Van der Waals forces probably do not make

a meaningful contribution. The only true bonding

effect derived from direct reaction of porcelain

with interstitial compounds developed at grain

boundaries in the metal substrate. No evidence

was found for a chemical bond between porcelain

and gold per se. Industrial research on metalceramic bonding using modern techniques such as electron microscopy or electron probe microanalysis has revealed that a glaze-body interface

is always a region, sometimes very thin indeed,

where diffusion of ions has been going on in both

directions [2]. At the elevated temperatures employed, the ions are quite mobile and will diffuse

from one material into the other. This might be

termed chemical reaction but examination of the

interface revealed no definite compound formation.

It therefore seems probable that the major contribution to effective bonding of dental enamels to

gold is to be found in the compressive stresses developed through accurate adjusting of the thermal

expansions of the gold and porcelain.

1 Figures in brackets indicate the Uterature references at the end of this paper.

77
Whatever the exact nature of this bond the most

important fact is that it has proved very effective

under clinical conditions. Providing the technician

takes care in degassing and cleaning his casting

prior to applying the opaque porcelain the incidence of porcelain actually shearing off the metal

is very low.

The somewhat doubtful aesthetics of the anterior fused porcelain to gold crown is perhaps of

greater clinical significance. In order to ensure

that the appearance of the crown is acceptable, the

dentist is often required to remove more dentin

than is biologically desirable. This heavy destruction of soimd dentin has increased the risk of pulp

mortality and considerable effort is now being

made to reduce the thickness of metal, opaque and

enamel porcelain used in the standard type fused

porcelain to gold crown. In order to mask the metal substructure effectively, the opaque porcelains are heavily loaded

with opacifiers. These opaque layers will also present problems since light reflection from their sur- faces can often be severe. The manufacturers are therefore restricted in the amount of light transmission they can allow through their dentin porcelains which are generally more opaque than the

regular porcelains used in jacket crown work. The

anterior fused porcelain to gold crown will often

lack depth of translucency, and due to the increased opacity, light reflection from the surface

is higher than in an all porcelain restoration fired

in vacuo. This property not only exacerbates any

metameric effects but can result in a deadness in

the mouth, reminiscent of some of the early airfired porcelain production teeth. Vacuum firing of

the enamel and translucent porcelains can improve

the incisal translucency but the cervical two-thirds

of the tooth presents a formidable problem to the

technician if the ultimate in aesthetics is to be

achieved.

There is little doubt that the manufacturers have

made considerable strides in solving some of these

problems. Firstly by increasing the shade range of

the opaque porcelains so that each opaque color is

closely matched in hue to the dentin colors and

secondly by using translucent overlay enamels to

give an illusion of depth to the crown. Built-in

concentrated colors can also improve appearance

and greys and blues can enhance the eifect of incisal translucency. Surface staining will never

achieve the same effect and if natural enamel is to

be simulated to perfection a ceramic crown must

have depth of translucency.

This problem was particularly noticeable during the course of experiments on aluminuous porcelain core materials [3]. Porcelains containing

40 percent very fine calcined alumina (specific sur- face area 6,000 cm^/g) were, for all practical

purposes, totally opaque whereas core porcelains

containing coarser grades of fused alumina (specific surface area 1,150 cm^/g) transmitted up to

20 percent light. The latter porcelains, when

veneered with translucent enamels, produced much

more life-like crowns since some light was still being diffused through the core procelain [4]. By

contrast the totally opaque calcined alumina core porcelain caused much more light reflection from

its surface, and crowns constructed from this material produced very similar effects to the opaque

porcelain used in the fused porcelain to gold technique. It became clear that opaque porcelains must

strike a fine balance between masking out cement

linings or metal but at the same time not reflecting

too much light. It is doubtful whether this fine

balance will ever be struck with the fused porcelain

to gold restoration since it is essential to use very

opaque materials in order to avoid any metallic

greyness showing through. For this reason it is likely that any further improvements in aesthetics

will be achieved through careful control of dentin

translucencies in relation to a wide range of neutral opaque tones. Unfortunately the manufacturers are placed in a dilemma in this situation

since they could very easily produce dentin colors

with higher translucency if these colors were used

in consistent thickness. The variable thicknesses

encountered in crowns made for clinical use will

often rnean that a body dentin color of less than 0.5

mm thickness must be used. In this situation a commercially produced porcelain powder must

have a built-in safety factor, and light transmission through the porcelain must be reduced to a

level where the opaque porcelain does not have

a dominating influence on color. If such an influ- ence occurs, the clinician will face serious metameric problems once the patient experiences

varying artificial lighting conditions. In particular color changes of this type can often be severe

in subdued tungsten light. It is obvious that these

problems could be further reduced if the metal

copings were made thinner, and the search for

higher strength metals which also have a high

rigidity modulus is continuing with accelerated

interest. The stellite-type casting alloys based on

chromium, cobalt, nickel and molybdenum are an

obvious choice but obtaining good casting accuracy

in thin section and a strong bond with dental

porcelain can often prove difficult under average

laboratory conditions. However the manufacturers

are devising improved techniques to simplify these

procedures and acceptable restorations can now

be made.

Successful bonding of porcelain to these nonprecious metal alloys is dependent upon the prevention of a buildup of too thick an oxide layer

at the porcelain-metal interface. A clean and

homogeneous casting is the basic requirement for

successful work. Sprues should be attached in such

a manner that a vertical or horizontal beam serves

as a reservoir with short 2 mm connections to the

casting itself. The casting must be thoroughly

sand-blasted and cleaned with a solvent such as acetone or alcohol before applying the opaque.

78
After the opaque porcelain is applied, the car- dinal point to bear in mind is that vitrification

must be rapid and complete before a thick oxide

separating layer can form and interfere with bonding. The use of metal firing trays to improve the

thermal conduction, and furnace muffles with even

heat distribution will assist the technician. Short

firing cycles of one to two minutes are also recom- mended and opaque porcelains which can vitrify in

this time are now available. Because of the strong

oxide formation in metal areas which are not cov- ered by porcelain it is good practice to extend the

opaque layers into these transition areas and grind

away any excess porcelain after the final bake.

More clinical experience is required before any

assessment of the merits of nonprecious metal alloys versus gold alloys can be made. Even in the

case of the precious metal alloys, the fine balance

that needs to be struck in applying the various

layers of porcelain taxes the skill of the most

experienced ceramist. With a world-wide shortage

in this skill it is hardly surprising that many

fused porcelain to metal crowns do not do justice

to the full potential of these materials.

However, despite these deficiencies, the fused

porcelain to metal restoration has proved of

enormous benefit to dentistry and has widened the

aesthetic scope of both fixed and removable partial

prosthesis.

2.2. Dispersion Strengthening of Glasses

The elimination of metal substructures in crown

and bridge work is desirable, not only for aesthetic reasons but also because of the high cost of

production.

The replacement of metal copings by a higher

strength ceramic seemed to offer possibilities and

a survey of high strength industrial ceramics indicated that increasing use Avas being made of

ceramic oxide crystals as a reinforcing phase in industrial porcelains [3]. It is well known that the sti-ength and elasticity

of glass can be increased by physical interaction

with an included phase of high elasticity [5, 6, Y]

Dispersion strengthening of glass utilizes this principle whereby cei-amic crystals of high strength

and elasticity are fused in a glassy matrix to form

crystal-glass composites. These composites form

a constant strain system and fracture has an equal

chance of starting in either phase. In the absence

of thermal expansion differences the strength and

elasticity will inci-ease, approximately, in proportion to the amount of the crystal phase. The choice

of ceramic crystals is fairly wide but for dental

purposes factors such as fusion temperature, bonding with dental porcelain, color and aesthetic

' values must be taken into account. Alumina

(AI2O3) was found to be the most suitable ceramic

for this purpose [3] and this material is re- ceiving increasing attention from the dental

manufacturers.

Alumina has been used as an additive in regular

dental porcelain for many years [8] but its use has

been mainly confined to making the so-called "hard

core" porcelain which surrounds the pin anchorage in a vacuum-fired tooth. In order to use alumina reinforced porcelain in greater section it became necessary to prepare specialized materials

in which much greater attention had to be paid to

the matching of thermal exjDansions and the sintering characteristics of the glass matrix powders and

alumina crystals.

Research indicated that if the alumina crystals

were dispersed in a borosilicate glass of matched

expansion to the alumina, the strength resulting

from this stress-free matrix was improved [6].

Further work revealed that when fused alumina

crystals of 99.5 percent purity and with a size

range of 20 to 30 /xm (specific surface area 1153

cm-/g) were incorporated in a specially prepared

borosilicate glass containing a high combined

alumina content that "Aluminous Porcelains"

could be produced with transverse strengths of over 20,000 psi (140 MN/m^) and with a light

transmission of up to 20 percent on 1 mm thick

discs [3] . This improvement in strength is approximately double that of regular dental porcelain and

clinical trials with these aluminous porcelains indicated that they could be used in jacket crown

manufacture [9].

It may be generally stated that the strength and

opacity of an alumina reinforced porcelain is a function of its crystal or particle size. The finer

the crystal size the greater the strength and

opacity. Research by the dental manufacturers has

therefore been directed towards optimizing the

crystal-glass particle size relationship to obtain

improved sintering characteristics, strength and

acceptable translucency.

One such study reports the results with a special

borosilicate fusion, high in alumina, in which the

glass was tailored to have good sintering characteristics and high strength in the sintered state [10].

Addition of a closely sized alumina aggregate in

which the preferred alumina grains had fairly

smooth surfaces improved the sintering behaviour

of the alumina-glass composite, and high sintered

densities were obtained. Modulus of rupture values

in the range of 22,000 to 24,000 psi (160 to 165

MN/m-) were obtained and in some cases ceramic

bodies with strength values of over 30,000 psi (210

MN/m^) were produced. Relationships for sintering behaviour and strength were obtained that

were at least qualitatively predictable. These behaviour patterns closely followed Griffith's flow

theory for strength and a viscous flow theory for

sintering. Lowered strengths resulted from solid

inclusions having unfavourably low thermal ex- pansions or by reactions or diffusion resulting in

unfavourable grain boundary stress. Pore structure was dependent upon particle size distribution

of the initial powder and upon the time-temperature firing cycle.

79
The current alumina reinforced porcelains have

benefited from this type of research and new types

of isotropic glasses containing high combined

alumina contents have been developed for veneer- ing the aluminous porcelain cores. It is likely that

further improvements in the strength of these alumina reinforced ceramics is possible but at present

the aluminous porcelain jacket crown cannot be

regarded as a replacement for the fused porcelain

to gold crown. The dentist must still be aware that

he is dealing with a brittle material and aluminous porcelain jackets must have adequate tooth

support and at least 1 mm clearance on the lingual

surface if the alumina reinforcement is to make a

significant contribution to strength [11].

Due to the semitranslucent nature of the aluminous core jDorcelains and the high translucency of

the enamel veneere the anterior aluminous porcelain crown is superior in aesthetics to the fused

porcelain to gold crown.

Clinical experience has indicated that aluminous

porcelain should be used primarily as a replacement for and to extend the use of regular dental

porcelain.

2.3. Dispersion Strengthening of Glass With

Alumina Whiskers

Alumina whiskers are filamentary single crystals having both high surface and crystalline perfection. Their strength at room temperature can

be as high as 2X10« psi (14X10^ MN/m^) with a modulus of elasticity of 70X10« psi (48X10*

MN/m==).

Dispersion of these whiskers in a glass matrix

may be done in several ways. The following two

methods were tried in jacket crown work. The

whiskers were intimately mixed with a finely

ground glass powder used for making aluminous

porcelain and applied to a platinum matrix by

standard technique. It was found that whisker

concentrations above 15 percent created difficulties

with regard to sintering behaviour, and high densities could not be obtained. It became clear that

if sufficient whiskers were to be incorporated in the

glass powder to make a significant contribution

to strength that alternative methods would have

to be used. Vacuum hot pressing techniques allow

high whisker concentrations to be used but such

methods are not commercially viable in dentistry.

An alternative method was tried in which preformed "green" whisker tapes were incorporated

in the jacket crown. These tapes were produced by

using the finely ground glass powder and highly

oriented whiskers held in an organic binder [12].

The tapes were incorporated in aluminous core porcelain on the lingual surface of the crown by

a- lamination technique. Once again problems of

sintering the alumina whisker-glass composite

arose, since the layers of porcelain m between

the tapes tended to be very porous despite firing

in vacuo. It was considered that future research

might make it possible to produce lingual alumina

whisker-reinforced glass shells which could be

lightly ground to fit the jacket preparation. In

this case the shells could be produced by vacuum

hot pressing. However it is doubtful whether this

method could rival the comparative simplicity of

the fused porcelain to gold technique.

The use of preformed "green" whisker-glass

composites in tooth production should not be ignored since it may be possible to tailor-make the

reinforcement to fit exactly in the tooth mould

prior to applying the overlay dentin and enamel

colors.

2.4. Enamelling of High Strength Crystalline

Ceramics

The bonding of aluminous porcelain to high

strength recrystallized alumina provides an alternative method of reinforcing the enamel veneers. Recrystallized aluminas in excess of 85 percent purity are one of the strongest groups of industrial ceramics. Recrystallized or "high alumina" is a particularly attractivB material from the

dental standpoint since it may be colored to dentin

shades by the use of high-temperature-resistant

])igments such as manganese-alumina pink, vana- dium-zircon blue or praesodymium-zircon yellow.

The resultant color backgrounds are very receptive to veneering with dental enamels and in addition a strong ionic bond is formed between the

alumina and aluminous porcelain veneer. This

chemical bonding is fairly tenacious and ion ex- change at the surface of the alumina is such that

diffusion of ions from the porcelain into the alumina can be denionstrated imder ultraviolet light

[3] . High alumina ceramics also exhibit high tensile strengths in excess of 17,000 psi (117 MN/m^),

and their modulus of rupture is in the region of

50,000 psi (345 MN/m^) [3].

More recently methods of firing high purity

alumina containing 0.2 percent magnesium oxide

have been devised in which it is thought that a

spinel is formed at the grain boundaries, slowing

down grain growth, and allowing the diffusions

of porosity along the grain boundaries. The re- sultant materials are almost pore-free which in

turn produces a highly translucent ceramic body.

A completely transparent recrystall ised alumina

has been developed by the General Electric Company ajid marketed under the trade name of "Lucalox". This type of material is particularly useful

for making thin shells for crown facings.

High alumina can be manufactured in fairly

intricate shapes to a high degree of tolerance ( ±2

percent linear) and preformed high alumina reinforcements for crown and bridge construction

are now available. These reinforcements are supplied in the form of rods, sheets, tubes, and dovetail veneers and a number of types of construction are available to the technician [13].

80
Oval alumina tubes provide strong anchorage

areas for custom-built or factory-made bridge

pontics and rival the fused porcelain-to-gold

pontic in strength. Alumina tube pontics may be

used with conventional golds and have the added

advantage of replaceability. They can also ex- tend the use of porcelain in cases of long span

bridges and markedly improve the aesthetics of

the anterior fixed bridge. Similar tubes can also

be used to reinforce post crowns and very good

clinical results have been recorded in close bite cases [14].

Sheets of high alumina, 0.8 mm thick, may also

be incorporated in the lingual surfaces of alumi- nous porcelain jacket crowns to reinforce the critical biting area. These crowns are more aesthetic

than the metal reinforced restorations and clinical

trials have indicated a very comparable strength

performance [4].

All-ceramic fixed bridges can also be constructed

using aluminous porcelain jacket crowns con- nected by high alumina rods. However these

bridges are better confined to single tooth replacement since on long spans there is a risk of the abutment aluminous porcelain crown being placed in

tension and fracturing down the midline. The

alumina reinforced fixed bridge will not replace

fused porcelain to gold, but for the manufacture

of small span anterior bridges, alumina is a material of great aesthetic appeal.

The construction of high alumina copings directly on to platinum matrices has also proved

possible [4], but the control of firing shrinkage is

still a problem. Regular dental porcelain or aluminous porcelain contains sufficient glass phase to

deform and slump over the platinum matrix. By

contrast, high alumina is extremely resistant to

pyroplastic flow and when shrinkage takes place

during sintering, cracks or fissures will open up

in the alumina core powder. In some cases the

platinum will be permanently distorted, as the

alumina crushes it [4]. Experiments are continuing toward producing high content alumina-glass

powders in order to increase pyroplasticity without seriously impairing strength.

2.5. Controlled Crystallization of Glasses

Controlled crystallization of glass was developed

by Stookey [15] in the United States and new and

unique properties were observed in these glassceramics. Not only was the strength of these materials markedly improved but very high thermal

shock resistance was imparted.

Controlled crystallization of glass depends upon

the fact that glass, at ordinary temperatures, is

an undercooled liquid which is in an unstable

state [16]. It can be made to crystallize by heating

to the proper temperature with crystal seed or

nuclei present. The glass is then converted to a dense mass of very tiny interlocking crystals.

Titanium dioxide is an effective nucleating agent

and the starting glass must be homogeneous with

qualities like optical glass. Spodumene is a suitable

glass, compounded from the oxides of lithium,

aluminum, and silicon and has been used extensively to make "Pyroceram" cooking ware. Normally the ware is heated to a temperature where

nuclei are formed from the dissolved titanium

dioxide. This happens at a temperature a little

above the annealing point where the glass shows

the first signs of softening. After myriads of nuclei

have been formed in this way, the glass is slowly

heated to higher temperatures where tiny spodumene crystals grow on the nuclei, converting the

transparent glass to an opaque white mass composed chiefly of spodumene crystals.

The opacity of these glass-ceramics makes them

unsuitable for dental application but more re- cently McCulloch [17] has reported on experiments with a glass-ceramic based on lithia-zinc

oxide-silica and using metal phosphate nucleating

agents. This glass was transparent and amber in

color in the glassy state but became translucent

and tooth-like after crystallization or ceraming

for 1 hr at 600 °C. At this stage modulus of rupture figures in excess of 18,000 psi (124 MN/m^)

were obtained.

This glass-ceramic was used to produce posterior

teeth by moulding in a die and counterdie. These

production teeth were made only in a single shade

but further experiments were performed in which

bars of the vitreous glass were made photosensitive by using silver as a nucleating agent. On cooling, the bars responded to ultraviolet light so that

by differentially irradiating the surface, the glass,

on heating to the ceraming temperature, could be

made to crystallize at different rates, thus creating

a polychromatic effect. It was shown that further

characterisation might be accomplished by applying printed transfers, containing tooth pigments,

to the surface.

These experiments have not vet reached the stage

of commercialization and considerable problems in

fabrication and color control must still present

themselves to the tooth manufacturer. The current

high standard of aesthetics in a vacuum fired anterior porcelain tooth has been created by building

in concentrated colors and multiblended veneers

of dental porcelain by manual application. The

fact that this process has not yet lent itself to automation is indicative of the tremendous problems

encountered in duplicating both the depth of

translucency and color of human enamel and dentin. Dentistry still demands a high degree of

artistic skill even in a mass produced porcelain

tooth.

2.6. Production of Compressed Surface Layers

in Dental Porcelain Via Ion-Exchange

Chemical toughening of glass is widely used in

industry and these techniques may have some application in dentistry.

81
Isard and Lehman [18] have reported on experiments in which specimens of soda glass were heated

in a bath of fused potassium nitrate to effect ionexchange. The compressed surface layer produced

by this treatment imparted considerable strength

to the glass and it was claimed that the high

strength was not removed by abrasion. The treatment could be applied to the complicated shapes

with re-entrants such as jacket crowns and would

be particularly suitable for use with aluminosilicate glasses used in dental porcelain where ionmobilities are relatively high.

The application of this technique to clinical

practice presents certain problems since any oc- clusal adjustments of the crown would remove

the compressed surface layer. However it might

be possible to toughen crowns chemically after

grinding in the mouth and this area of research de- serves further study. Chemical toughening of denture teeth does not appear to be a practical proposition since these teeth are generally adjusted for

occlusion after processing the denture.

3. Additional Areas of Dental

Porcelain Research

3.1. Silane Bonding of Acrylic Resin

to Porcelain

Other areas of research on dental porcelain have

been directed toward improving the bond between

porcelain teeth and the acrylic denture base. It

was shown that when feldspathic porcelain teeth

were treated with gamma-methacryloxypropyltrimethoxysilane that a bond was formed between

the porcelain and acrylic of such strength that

when failure occurred, fracture took place in the

porcelain [19].

Semmelman and Kulp [20] considered that this

bond was not adequate in heat-cured denture bases

when measured from a clinically determined

standpoint and that the pin anchorage was still necessary. This evidence was in agreement with the

work of Myerson [21] who considered that the

unsatisfactory bond strength was produced by excessive shear stress during cooling. He showed that

by reducing the temperature of processing, the

cooling shear stress was reduced and concluded

that the self-cui'e systems produced the strongest

bond with the silane treated porcelain tooth. His

final conclusion, which is probably most pertinent,

stated that the large differential in thermal expansion represented the weak point in the silane

bonded porcelain-acrylic system. This must be

accounted for before the obvious advantage of sil- ane bonding can be used.

3.2. Refractory Die Materials

The development of new types of refractory

die materials for the construction of all-ceramic

alumina reinforced bridgework or for direct bakmg of porcelain jacket crowns has reached a stage

where commercial materials are now available.

Vickei'y [22] has described a technique for using

a castalale, refractory, ceramic die composition in

which it is claimed that accurate reproduction of

impressions taken in silicone or polysulphide elastomers is possible. When coated with a special

polyphase refractory suspension the dies may be

employed for the direct construction of porcelain

jacket crowns and inlays and butt-shoulder

porcelain-on-gold restorations without the use of

platmum foil. The die may then be removed directly from all of these preparations. No details

of the composition of this refractory die material

were given, and its accuracy against the platinum

foil technique has yet to be established both in

the laboratory and clinic.

Miller Yardley [23] has published a clinical re- port on the use of multiunit aluminous porcelain

bridgework constructed on a refractory die material made specially for use with alumina reinforced

ceramics. The ceramic refractory was used to invest the platinum matrices prior to joining the

aluminous porcelain copings with high alumina

I'ods. The fit of these bridges was clinically acceptable and case reports were presented covering an

18 months' postoperative period.

4. Summary

Current research on dental procelain has been

concentrated mainly on methods of improving the

fracture resistance of porcelain i-estorations.

Enamelling of metals or dispersion strengthening of glasses offer the greatest possibilities at the

present time and a review is given of the progress

made in each field.

The significance of the role of opaque porcelains

is discussed in relation to their effect on the

aesthetics of enamel veneers. The use of nonprecious metal alloys as an alternative to gold is also receiving increasing attention

and methods of improving the bond at the metalceramic interface are described.

Dispersion strengthening of glass with high

strength alumina crystals has resulted in the production of a new range of aluminous porcelains for

jacket crown and bridge pontic construction.

Chemical toughening or nucleation of glasses

may also provide otlier means of reinforcement but

at the present time these methods are still in the

experimental stage.

Other areas of research have been concentrated

on impi'oving the bond between acrylic resin and

porcelain denture teeth by the use of silane

coupling agents. The large differential in thermal

expansion between acrylic and porcelain must be

accounted for before these methods become commercially viable.

82
5. References

[1] Vickery, R. C, and Badinelli, L. A., Nature of

attachment forces in porcelain-gold systems, J. Dental Res. 47, 683 (1968).

[2] Batchelor, R. W., Harrison and Son, Stoke-uponTrent, England. Personal communication.

[3] McLean, J. W., and Hughes, T. H., The reinforcement of dental porcelain with ceramic oxides,

Brit. Dental J. 119, 251, (1965).

[4] McLean, J. W., The development of ceramic oxide

reinforced dental porcelains with an appraisal

of their physical and clinical properties, MDS

thesis. University of London, (1966).

[5] Batchelor, R. W., and Dinsdale, A., Some physical

properties of porcelain bodies containing corun- dum, Transactions of the 7th International Ce- ramics Congress, London, England, p. 31, (1960).

[6] Binns, D. B., Some physical properties of two-phase

crystal-glass solids. Science of ceramics Vol. 1, pp. 315-334 (Academic Press, London, England,

1962).

[7] Hasselman, D. P. H., and Fulrath, R. M., Proposed

fracture theory of a dispersion-strengthened glass

matrix, J. Amer. Ceram. Soc. 49, 68, (1966).

[8] Fonvielle, F. P., and Semmelman, J. O., Uses of

alumina in porcelains, lADR Program of Abstracts and Papers, No. 181 (1967).

[9] McLean, J. W., A higher strength porcelain for crown and bridge work, Brit. Dental J. 119, 268,

(1965).

[10] Harman, C. G., and Wiener, I., Sintered glass-basis

composites for iwrcelain prosthesis. lADR Pro- gram of Abstracts and Papers, No. 339 (1969).

[11] McLean, J. W., The alumina reinforced porcelain

jacket crown. J. Am. Dental Assoc. 75, 621 (1967)

[12] Wakelin, R. J., Personal communication.

[13] McLean, J. W., High-alumina ceramics for bridge

pontic construction. Brit. Dental J. 123, 571

(1967).

[14] McLean, J. W., The alumina tube post crown. Brit.

Dental J. 123, 87 (1967).

[15] Stookey, S. D., Catalyzed crystallization of glass in

theory and practice. Ind. and Eng. Chem., 51,

«05 (1959),

[16] Greene, C. H., Pyroceram., The New Scientist, p. 1708. (Dec. 1960).

[17] MacCulloch, W. T., Advances in dental ceramics.

Brit. Dental J. 125, 361 (1968).

[18] Isard, J. O., and Lehman, M. L., Letter to Editor.

Brit. Dental J. 120, 56 (1966).

[19] Paffenbarger, G. C, Sweeney, W. T., and Bowen,

R. L., Bonding porcelain teeth to acrylic resin

denture bases. J. Am. Dental Assoc. 74, 1018

(1967).

[20] Semmelman, J. O., and Kulp, P. R., Silane bonding

porcelain teeth to acrylic. J. Am. Dental Assoc. 76,69 (1968).

[21] Myerson, R. L., Eftects of silane bonding of acrylic

resins to porcelain on porcelain structure. J. Am.

Dental Assoc. 78, 113 (1969).

[22] Vickery, R. C, Badinelli, L. A., and Waltke, R. W.,

The direct fabrication of restorations without

foil on a refractory die. J. Prosthetic Dentistry,

21, 227 (1969).

[23] Yardley, R. M., Multi-unit aluminous porcelain

bridge work. Brit. Dental J. 126, 177 (1969).

83

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseaech, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Dental Silicate Cements

A. D. Wilson

Laboratory of the Government Chemist, London S.E. 1, England

A systematic search for improvement of dental silicate cements requires detailed

knowledge of their formation and structure. Experimental evidence shows the effective

bonding medium in these cements is an amorphous aluminum phosphate. Physicochemical

examination of a number of dental silicate powders showed them to be powdered fluorinecontaining alumino-silicate glasses. The mechanism of the cement-forming reaction was

studied for one cement following extraction of soluble ions by water at various time intervals

after preparation. Infrared spectroscopy was used to record the changing absorption

spectra of the setting cement. The microstructure of a number of fully hardened cements was studied by a variety of techniques : optical and electron microscopy, electron probe

microanalysis infrared spectroscopy and x-ray diffraction. Dissolution in acidic media of

aluminum phosphate bonded silicate cements is an inherent defect stemming from the

fundamental chemistry of the system.

Key words : Aluminum phosphate, amorphous ; dental materials ; dental silicate cements

electron probe microanalysis ; glass, alumino-silicate ; silica gel ; silicate cements, dental

silicate cements, mechanism of hardening, microstructure of, chemical nature ; silicate cements, susceptibility to acid attack.

1. Introduction

In this paper the formation and structure of the

dental silicate cement is described in the light of

recent fundamental work much of which is as yet

unpublished. This work was carried out in the

U.K. at Government research stations by B. E.

Kent, R. J. Mesley (Laboratory of the Government Chemist), R. P. Miller, D. Clinton (National

Physical Laboratory), and K. E. Fletcher (Building Research Station) in collaboration with the

author. Results obtained enable the basic limitations and future prospects of this cement to be discussed from a fundamental viewpoint.

The dental silicate cement has a long history

its successful introduction into dentistry followed

from the Steinbeck patents of 1903^ [1] ^. It re- mains today a popular material despite the introduction of alternative resin systems [2, 3]. What is less certain is whether it has a future ; for its drawbacks—failure by erosion in the mouth, ability to

irritate pulpal tissues etc.—are well Imown [3].

Although some research studies have been re- ported [4, 5, 6] there has been as yet, no basic

change in formulation of commercial materials

since 1907 when Schoenbeck [7] introduced the

use of a fluoride flux in the preparation of the

cement powder. Following this innovation there

have been some 60 years of practical development

which, although it has led to improvements, has

not appreciably changed the charactei- of this material. The question remains whether this cement

1 Figures In brackets Indicate the literature references at the end of this paper.

is capable of significant improvement. This question can only be answered with certainty from

knowledge of the fimdamental nature of the

setting mechanism and the microstructure.

Now, although this material has been known for

a considerable time, little fundamental work has

been published. Undoubtedly the reason for this

is that since the material is apparently amorphous

the principal structural determining technique

x-ray diffraction—is not applicable.

The generally accepted view has been that the

dental silicate cement is bonded by a type of silica

gel [3] . This is not an unreasonable hypothesis ; if

there is an interaction between an acid and a

silicate, silica, often in the form of silica-gel [8]

must result, and indeed there are cements where

the essential bonding medium is known to be

silica-gel [9]. These are the industrial silicate

cements formed by reacting aqueous solutions of

soluble silicates with acids, and, unlike dental

silicate cements, they are acid-resistant—because

silica-gel is insoluble in acids.

They are also weak when compared with dental

silicate cements. The dental silicate cement is one

of the strongest inorganic cements known and

strength values in the region of 3,000 kg/cm^ (300

MN/m-) in compression have been recorded [10]

which are much greater than those reported for the

industrial silicate cements : 150-270 kg/cm^ (15-27

MN/m^) [9].

There are therefore sound reasons for doubting

that silica-gel is the essential bond in dental sili- cate cements, although, of course, it must be present in the cement.

8S
Studies on the eflfect of acids on these materials

have shown that disintegration is accompanied by

loss of phosphates from the cements, an observation which is consistent with the presence of a cementing phosphate bond [11]. The electrical

conductivity of the freshly prepared cements has

been observed to drop sharply during setting [12]

(fig. 1) which suggests that setting is the result

of a type of precipitation process. The increase in

pH which has been observed as cements age is

consistent with this view [13, 14].

These problems have been largely resolved by

recent research where a number of methods of

structural analysis were employed.

2. Physico-Chemical Nature of the

Cement Materials

Part of the explanation of the formation of the

dental silicate cement follows from a consideration of the physico-chemical nature of the cement

forming materials. Qualitative analysis of 16 materials and quantitative analysis of 4 materials

Table 1.

Chemical Composition of Dental Silicate Cement Powders

(given as percent w/w)

Silicate powder

Chemical species

A B C D

SiOj 41. 6 38. 8 34. 5 35. 9

AI2O3 28. 2 29. 1 28. 3 29. 0

CaO 8. 8 7. 7 8. 5 6. 1 NajO 7. 7 8. 2 11. 2 14. 5

ZnO . 3 2. 9 . 1 . 3

P2O5 3. 3 3. 0 3. 3 4. 4

F 13. 3 13. 8 18. 1 15. 2

H2Of550°C) 2. 2 1. 6 1. 7 1. 4

MgO . 1 . 1 . 1 . 1 SrO . 2

105. 7 105. 2 105. 7 106. 8

Less 0 for F _ 5. 6 5. 8 7. 6 6. 4

Total 100. 1 99. 4 98. 1 100. 4

Median particle size Cmicrometers) 8. 6 10. 6 11. 5 15. 5

Chemical Composition of the Liquids

(given as percent w/w)

Chemical species A B C D

PO4'- 47. 3

1. 6

6. 1

47. 8

1. 9

4. 2

53. 8

2. 0

9. 1

63. 8

A15+ 1. 8

Zn2+_.

Mg2+ 1. 3

(table 1) liave shown that these are all of the same

basic type. The liquids are strong aqueous solutions of phosphoric acid-containing metals.

E

60 eo aqe o[ cement, min Figure 1. The changes in electrical conductivity of a dental silicate cement, 20 °C. (Laboratory of the Government Chemist).

86
The powders are unique amongst cements in that

they are ground glassy bodies and not crystalline

clinkers, a physical state which may account for

the translucent nature of the final cement. The dental silicate glass is prepared by fusing a silica and

alumina mixture in a fluoride flux containing

minor amounts of phosphate. The fluoride flux is

based on cryolite ; the addition of fluoride lowers

its fusion point.

The glasses are unusual ; they are silicates, but

unlike most, readily dissolve in mineral acids.

They are also opal glasses of extreme composition

where the fluoride content is higher than that of

any other opal glass. This opacity is caused by the

phase separation of two types of minute droplets

approximately 4000 A and 250 A in diameter

which can be observed under the electron microscope. This phase separation is to be expected as large amounts of fluorine in glasses weaken the

continuity of the 3-dimensional network.

The other feature of interest is that these glasses are aluminosilicates with a mole ratio of Si : Al

approximating unity. The structure of this type of

aluminosilicate plays an important role in cement

formation and requires further discussion.

A simple silicate is a macromolecule based on a continuous series of Si-0 bonds, and structurally

consists of chains or 3-dimensional networks of

linked [SiOJ tetrahedra. Because of its atomic

dimensions aluminum can replace up to half of the

silicon atoms in a network which for valency rea- sons acquires a negative charge. These are the

so-called aluminosilicates. In dental silicate glasses

where the Si : Al approximates to 1 : 1 there are many -[Si04]-[A104] -links; the negative charge

on the network being balanced by the positive

charge on the glass cations, calcium and sodium.

3. Cement Formation

The dental silicate glass can be regarded as a

negatively charged polymer surrounded by a cloud

of cations. This type of structure is susceptible to

attack by acids, since the positively charged protons can easily penetrate into the negatively

charged glass network, a type of acid-base reaction. The principal sites of attack will be at the

aluminum atoms which attract most of the negative charge; when there are a large number of

Al-O-Si links, complete disruption of the network

will result. This is exactly the situation that oc- curs when the dental silicate powder is mixed with

the cement liquid, when infrared spectroscopy

shows (fig. 2) that the alumino-silicate network is

rapidly degraded to a silica or aluminosilica gel,

and that HoPOl ions are formed as protons are

lost from the liquid [15].

Now this process of decomposition will be accom- panied by the liberation of ions locked previously

in the glass lattice. As a result these ions would be

expected to migrate into the cement liquid, that is

I4O0 I2O0 lOOO 900 eOOcm''

1 1

/ n/ \

1 1 1

*. c J / \

\ I

If n

1 X-^'i 1 1 1 1 7 8 9 lO U 12 microns

Figure 2. Changes in the infrared spectrums of a dental

silicate cement as it sets. {Laboratory of the Government Chemist).

Curve I. Freshly prepared cement. Curve II. Set cement. Absorption bands (a) slUca-alumlnophosphate gel

(b) sUica gel

(c) HsPOi Ion

the aqueous phase of the cement, and be rendered

water soluble.

The course of this process is illustrated by re- sults that have been obtained from simple wetchemical experiments. Cements of various ages

were crushed and the concentration of water

soluble ions determined by leaching with water.

Cements were prepared using a powder-liquid

ratio of 4g/ml. The chemical composition of the

powder was : 18.7 percent Si, 15.0 percent Al, 6.3

percent Ca, 5.7 percent Na, 13.3 percent F, 1.0 percent P, 40.0 percent O, and that of the liquid 49

percent H3PO4, 1.25 percent Al, 6.5 percent Zn.

Results are shown as a plot of the concentration of

leachable ions against the age of the cement (fig. 3)

Now it is evident from these data that decomposition and liberation of glass ions (AP^^, Ca^"^, Na"^,

F") from the powder is rapid. By the time mixing

is completed (1 min) considerable amounts of

sodium, calcium, and fluoride have been rendered

water soluble, that is they have been liberated from

the powder as a result of its decomposition.

Indeed at this point, these levels are at a maximum for calcium and fluoride because subsequently

these ions are removed by precipitation and interpretation of results is obscured. For this reason the

initial decomposition process is best illustrated by

the curve for water extractable sodium as sodium

forms no water insoluble product. By the time

mixing has been completed some 10 percent of the

sodium contained in the powder has been liberated ; the amount increasing to a maximum of 20

percent in the following 30 min or so. If this figure

corresponds to the amount of powder decomposed

then the powder is in five-fold excess over the

liquid.

87
aqe of cement hours

Figure 3. Variation of the concentration of water soluble

ions contained in a dental silicate cement as it set and

hardened, illustrating the liberation of ions from the

glass matrix and their subsequent precipitations. (Laboratory of the Government Chemist).

The accumulation of ions in the aqueous phase

of the cement paste is accompanied by a decrease in

its acidity, for this is an acid-base reaction. As a

result AP% Ca-''*, Zn^^ F", and the H2PO4- progressively form insoluble substances causing the cement

to set. This is a type of precipitation process and

its correlation with setting is shown in figure 3. It is best illustrated by the curves for zinc and

phosphate, since if the small amount of phosphate

in the powder is neglected, these are present

initially in a soluble ionic form. There is no prior

extraction process to obscure interpretation of

results. Inspection of this data showed that the

cement sets in 6 min when 50 percent of both of

these species have precipitated. The subsequent

post-set hardening is due to the continuation of

this process and is complete in 48 hr, for the system

described.

This mechanism explains why dental silicate

cements are adversely affected if exposed to moisture shortly after set and before fully hardened.

Ions which would have contributed to the formation of the matrix are removed by leaching [10]

and optimum strength can never be obtained. Any

cement which is dependent on a precipitation set

will be sensitive to moisture in this way. Unfortunately, most rapidly setting cements, and these

are required for dentistry, are based on this type

of mechanism. So the problem of the elimination

of this drawback will not be solved easily. It is

unlikely that it will be remedied at all for the

present type of dental silicate cement.

Even when the cement is fully hardened some

soluble material remains ; the salts of sodium with

dihydrogen phosphate and aluminofluorides. This

is soluble material that is eluted from cements in

the specification tests for solubility and distintegration. What relationship it bears to the clinical

durability of a silicate restoration is uncertain, for

clinical durability will be related to the resistance

of the insoluble part of the matrix towards oral

fluids.

It should be noted that the setting mechanism

of the dental silicate cement is totally different

from that of the Portland cement. In the latter,

interaction between powder and liquid is slow and

is accompanied by formation of the cementing

matrix. The situation for the dental silicate cement

is totally different since setting and hardening are

controlled by increases in pH. The interaction

between powder and liquid is very rapid and most

of it occurs in the initial stages, during and shortly

after mixing. This process is accompanied by a

relatively small change in the nominal pH of the

system which only increases from an initial figure

of 0.8 to 1.7 at the initial set [14]. Subsequent

hardening during the next 48 hr is caused by a much larger change in pH, an increase to 5.0-5.5,

which completes the precipitation process. Little

further interaction between powder and liquid is

required to produce this pH change ; its magnitude

is so small that it cannot be detected by present

methods. This behavior follows from a consideration of the nature of the neutralization curve of

phosphoric acid ; this system has a strong buffering

capacity in the region of its pKa's (2.1, 7.2) but

little in the intermediate regions.

4. Microstructure

The set dental silicate cement has unusual and

interesting features. The best examples are amongst the strongest inorganic cements known

and are unique in being substantially amorphous.

X-ray diffraction of seven examples showed that

they contained only minor amounts of crystallites,

which were however well-defined. These were identified as augelite, AIPO4 A1(0H)3 and fluorite,

CaF2.

The appearance of polished surfaces of dental

silicate cements under incident light is that of a poorly connected mosaic of highly reflecting angular grains, as a micrograph of a typical example

shows (fig. 4). The grains, which appear unattacked, vary in size over a broad range, 1-100

fim in diameter. Porosity shows as black nonreflecting areas, which are more numerous in

weaker cements. Under cross-polarizers and re- flected light a network of birefringence is apparent

in cements with strengths in compression in excess of 1,750 kg/cm2 (170 MN/m^) (fig. 5). This evidence taken together with the amorphous nature

88
Figure 4. Micrograph of a polished surface of a dental

silicate cement, taken by transmitted light, showing

glass grains and porosity. (National Physical Laboratory).

Figure 5. Network of birefringence shown by reflected

light analysed by crossed polarizers. (National Physical

Laboratory)

of the bulk of the c«ment matrix and the isotropic

nature of the crystallites indicates that this birefringence is the result of orientation due to internal stresses. The scanning electron microscope with its high

magnification coupled to a considerable depth of

focus is a powerful tool for determining the physical microstructure of materials. Fractured surfaces

are normally chosen for examination. A micrograph of a typical strong cement is shown (fig. 6).

The dominant feature is a glass particle about 10

fim across, which by its angular appearance is

apparently unattacked. Inhomogeneities on its

surface are indicative of local attack, these also

serving to distinguish it from the surrounding

matrix. The matrix which appears particulate in

strong cements is plate-like in weaker ones (fig. 7)

The appearance of the fractured surfaces suggests

Figure 6. Scanning electron micrograph of a strong

cement. (National Physical Laboratory)

Figure 7. Scanning electron micrograph of a weak

cement. (National Physical Laboratory).

the bonds between particles and matrix are comparatively weak.

Attempts have been made to improve the strongest properties of cements by the inclusion of fibers

and other forms of inert aggregate. In our laboratories it has been found that these are ineffective.

A micrograph of a fiber-containing cement shows

that the matrix has shrunk from the fiber with

crack initiation (fig. 8)

5. Microchemical Nature of the Matrix

Electron-probe microanalysis is an important

technique for determining the chemical nature of

the microstructure of amorphous materials. In this

technique a narrow electron beam, some 2 nm

across, scans a flat polished section and records the

chemical composition of each point on the cement

surface. This enables the distribution of elements

to be mapped as white points on a black

background.

89
FiGTJEE 8. Scanning electron micrograph of a cement

containing glass fliers. {National Physical Laboratory)

In the case of dental silicate cements the element

distribution can be compared with the physical

positions of the particles in the matrix, as shown

by the back scattering of electrons, thus enabling

the chemical composition of both particles and

matrix to be established.

An electron-image photograph and a set of element-distribution photographs of the same area of

study for a dental silicate cement are shown in

figure 9. The cement used was similar to that employed for the reaction studies, metals being

omitted from the liquid to avoid confusing experimental data. From an examination of these it is

apparent that phosphorus is almost entirely in

the matrix. Silicon is associated with the particles,

and there is none in the matrix.

Calcium and aluminum are present in both re- gions. Since originally both were present solely in

the glassy particles, it is clear that there has been

some migration into the aqueous or matrix phase.

No data are shown on the diagram for sodium and

fluorine because of experimental limitations ; however more recent experimental results using improved techniques indicate that some sodium and

fluoride also migrate from the powder grains into

the liquid.

Detailed electron scans show that in fact the

glass cations are displaced from the surface region

of the individual particles, leaving a central unattacked core in the case of the larger particles.

This displacement of cations from the glassy

particles is a result of the penetration of protons

from the liquid which means that water will have

entered the surface layer of each particle. Since

silicon remains in this degraded surface layer it

may be inferred that it is present as a type of

silica gel. Infrared data confirm this deduction. It

is the retention of this siliceous framework which

preserves the geometry of the glassy particles

thus giving them the appearance of remaining

unattacked.

The association of the various cations and anions

in the matrix has, as yet, not been resolved completely. The detection of minor amounts of crystallites, augelite, A1P04A1(0H)3, and fluorite,

CaFa, in the matrix may be indicative of the asso- ciation of ions in the predominant amorphous

phase of the matrix. Infrared spectroscopic data

are consistent with the presence of amorphous

aluminum phosphate which must be considered as the major constituent of the matrix. Aluminum

phosphate is known in ceramic science as a binding

agent [16] and must be regarded as the effective

bonding medium.

The exact nature of the polymer remains a subject for speculation. Results from other studies

indicate that the function of the powder may only

be to supply ions needed for the reaction at an

appropriate rate. Cement strength is far more

dependent on the phosphoric acid concentration of

the liquid than the chemical composition of the

powder (fig. 10). There are therefore good reasons

for supposing that the structure of the phosphoric

acid liquid is important, for it is dependent on acid

concentration [17]. It is known that aluminmn

forms a complex with phosphate in acid solution

and that as the proportion of aluminum increases,

dimers and polymers are formed where both hydroxyl and phosphate act as bridges between the

aluminum atoms [18] . Therefore there are grounds

for supposing that the setting of the cement proceeds with the progressive formation of an aluminum-oxygen-phosphorous polymer as aluminum

enters the liquid from the powder. It is plausible

to suggest that a pH-dependent polymerization

occurs in the cement to give a water-insoluble polynuclear species, which is the effective binding

medium, and is based on phosphate and hydroxjrl

between aluminum ions.

The structure of the dental silicate cement is

more simple than might have been supposed from

the many combinations possible of elements

present. For spatial reasons a combination of sili- con and phosphorus is not possible. Moreover,

even if it were, such a combination is unlikely;

the rare Si-O-P bond is not of a strength comparable with the Si-O-Si, Al-O-Si and Al-O-P

bond, and stable structures do not result [1^].

Another possibility is the metal alumino phosphate of Barrer and Marshall [18] (M20)n_m

( ALOa ) „ ( P2O5 ) m

• xH^O. However, such bodies, at

least in crystalline form, have not been identified

possibly the affinity of calcium for fluoride inhibits their formation. The indications are that the

matrix is a simple mixture of a basic aluminum

phosphate and calcium fluoride, a little of which

has crystallized.

This structure of the dental silicate cement can

be used to predict the clinical behavior of a res- toration. The effective binding medium, basic

aluminum phosphate, is insoluble in neutral solution, but dissolves in acidic and in neutral citrate

media. A silicate restoration would, therefore, be

90
Figure 9. Electron probe microanalysis showing physical microstructure and element distribution {white

spots ona Mack background). (Building Research Station).

(a) Electron image micrograph, (b) Silicon distribution, (c ) Phosphorus distribution, (d) Calcium distribution, (e) Aluminum distribution.

452-525 O—72 7 91
30 'O SO 60 concentration of H^PQ, %

FiGUEE 10. The effect of acid concentration on

the compressive strength of a cement.

To convert psi (pounds per square inch) to N/m-

(newtons per square meter) multiply by 6,895.

expected to give good service under favorable oral

conditions. Saliva itself acts as a near neutral

buffer, and so a silicate restoration washed by

saliva should have a long life. However if sited in

regions where local acidic condition can develop

or if the saliva contains appreciable citrate, early

failure is to be expected. This argument may be

extended to any dental silicate cement as at present

formulated. Whatever variation of proportions of

the basic constituents of both powder and liquid,

it is difficult to see how the present type of dental

silicate cement can be further developed, for its

faults are inherent. It would seem therefore that

future research for an improved translucent

cement will have to follow radically different lines.

If the cement is to be of the precipitation-set type

for rapid setting then it will have to be based on

a more acid resistant binder than aluminum

phosphate.

6. Summary

On mixing the dental silicate powder and liquid,

protons from the liquid penetrate the surface

layer of the glassy particles. The negatively

charged alumino silicate network is disrupted and

glass ions migrate into the aqueous phase. Silicon

remains in the surface as a type of silica gel thus

preserving the initial shape of each particle. With

the exception of Na^, ions which have accumulated

in the aqueous phase (AP"", Ca^"", F", H2P04')

undergo a type of precipitation process as the pH

increases and the cement sets. The effective binding

agent is amorphous aluminum phosphate; the

matrix also contains calcium fluoride. There is

some crystallization of these compounds.

Since these substances are acid-soluble this type

of cement can never be designed to withstand adverse oral conditions. The hardened cement con- •

tains some soluble material, the sodium salts of

dihydrogen phosphate and alumino fluorides. The

specification test only determines these soluble

constituents and therefore does not necessarily

give an indication of the durability of the waterinsoluble phase of the matrix under oral conditions.

The author thanks Mr. J. K. Foreman, Superintendent, Research Division, for his constructive

comments and continuing support and Dr. D. T.

Lewis, C.B., the Government Chemist, for permission to publish.

Crown Copyright. Reproduced by permission

of the Controller of Her Britannic Majesty's

Stationery Office.

7. References

[1] Steenbock, P., German Pat., 174557 (1903)

Brit. Pat. 15,176; 15,181 (1904).

[2] Buonocore, M. G., Int. Dental J. 18, 406 (1968).

[3] Skinner, E. W., and Phillips, R. W., The Science of

Dental Materials, 6th Edition, p. 60 (W. B. Saunders, Philadelphia and London 1967).

[4] Wright, J. W., J. Dental Res. 1, 35 (1919).

[5] Manly, R. S., Baker, C. F., Miller, P. N., and Welch,

F. F., J. Dental Res. 30, 145 (1951).

[6] Rockett, T. J., I.A.D.R. Abstracts #433 (1968).

[7] Schoenbeck, F., U.S. Pat. 897,160 (1907).

[8] Murata, K. J., Bull. U.S. Geol. Surv., 950, 25 (1946).

[9] Vail, J. G., Soluble Silicates Their Properties and

Uses, Vol. 2 (Reinhold Publishing Corporation,

New York, 1952).

[10] Wilson, A. D., and Batchelor, R. F., J. Dental Res.

46, 1425 (1967).

[11] Wilson, A. D., and Batchelor, R. F., J. Dental Rea. 47, 115 (1968).

[12] Wilson, A. D., and Kent, B. E., J. Dental Res. 47,

463 (1968).

[13] Norman, R. D., Swartz, M. L., and Phillips, R. W.,

J. Dental Res. 45, 136 (1966).

[14] Kent, B. E., and Wilson, A. D., J. Dental Res. 48,

412 (1969).

[15] Wilson, A. D., and Mesley, R. J., J. Dental Res. 47,

644 (1968).

[16] Kingery, W. D., J. Amer. Ceramic Soc. 33, 242

(1950).

[17] Van Wazer, J. R., Phosphorus and its Compounds,

Vol. 1, Chemistry (Interscience Publishers, New

York-London, 1958).

[18] Salmon, J. E., and Wall, J. G. L., J. Chem. Soc. 1128 (1958).

[19] Barrer, R. M., and Marshall, D. J., J. Chem. Soc,

6616 (1965).

92
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Composite Restorative Materials ^

R. L. Bowen,* J. A. Barton, Jr.,** and A. L. Mullineaux***

Dental Research Section, Institute for Materials Research, National Bureau of Standards,

Washington, D.C. 20234

This article describes the development of dental composite materials, which have

quickly established a place in the practice of dentistry because of the shortcomings of

alternative esthetic direct filling materials. The goal-directed basic research in which the

composite materials were developed was initiated at the National Bureau of Standards in

1956. Late in 1957, a monomer was synthesized that is suitable for use with reinforcing

fillers. Commercial dental composite materials available in 1969 use this resin together

with particulate fillers and various additives ; they have lower polymerization shrinkage

and coefiicient of thermal expansion, and higher compressive strength and stiffness relative

to unreinforced resins. Compared to silicate cements, the composites have lower solubility,

higher tensile strength, and comparable compressive strength. Composite restorative

materials, when they have been fully developed, should provide the dental profession and

the public with greatly improved restorations for anterior teeth. Key words : Biomaterials ; composites ; dentistry ; fillers ; glass ; methacrylates ; monomers ; prostheses ; resins ; restorative material.

1. Historical Introduction

Although composite dental restorative materials

are yet in their infancy, the roots of their development can be traced back through the scientific lit- erature for many years. In 1843 a German chemist,

Joseph Redtenbacher, reported a new acid to

which he gave the name acrylic acid [1].^ In the

last part of the century, methacrylate esters and

their polymers were discovered [1]. Probably before 1940, Schnebel had found that tertiary amines

(such as Michler's ketone) would activate benzoyl

peroxide to give self-hardening resins [2]. So far

as can be determined, the first self-curing methyl

methacrylate resin formulation for filling teeth

was introduced in the United States in 1948.

There soon appeared on the market a number

of direct filling resins, and many believed that

these materials would be the answer to the shortcomings of the decades-old silicate cement. However, this idea was short-lived [3]. These resin

restorations discolored [4], tended to shrink away

1 This research activit.v was supported in part by research grants to the American Dental Association from the National Institute of Dental Research and is part of the dental research program conducted by the National Bureau of standards, In cooperation with the Council on Dental

Research of the American Dental Association ; the Dental

Research Division of the United States Army Medical

Research and Development Command ; the Dental Sciences

Division of the School of Aerospace Medicine, USAF ; the National Institute of Dental Research ; and the Veterans

Administration.

Associate Director, American Dental Association Research

Unit at the National Bureau of Standards.

**Guest Worker, Dental Research Section, National Bureau of Standards, Lieutenant Colonel, U.S. Air Force. •Research Associate, American Dental Association Research

Unit at the National Bureau of Standards.

2 Figures in brackets indicate the literature references at the end of this paper.

from the cavity walls [5], allowed the exchange

of oral fluids around their margins [6] and were

associated with pulpal inflammation [7-9] and

a high incidence of recurrent decay [10-11].

Furthermore, discrimination between decalcified

dentin and the unreinforced direct filling resin

was difficult because they were both radiolucent.

For these and other reasons, a great many practitioners returned to the use of silicate cements [3].

The shortcomings as well as the merits of sili- cate cements are well known. After nearly a hundred years [12] of development and use, these es- thetic materials with good thermal and anticariogenic properties still have distinct disadvantages

[3]. Most obvious is the susceptibility to disintegration in a decay-like manner [13-15], typically

after about four years in the mouth [3, 14]. Further description is given by Wilson elsewhere in

this publictaion.

Because of the persistent need for a permanent,

esthetic direct filling material, there were continued efforts to improve the methacrylate resins. An

aluminum silicate filler (about 15 percent) was

part of one methyl methacrylate monomer-polymer system [16]. The use of fillers with the proper

index of refraction and a low thermal expansion,

and the development of adhesion in the resin were

advocated [17]. Indeed, when inert fillers were incorporated into poly (methyl methacrylate) , there

was a reduction in the coefficient of thermal expansion and in the water sorption in proportion to the

concentration of fillers [18]. However, there were

limited adhesive characteristics in these and cer- tain other resins investigated [19].

93
Epoxy resins [20], developed early in the present century, had an annual production rate of

about 20 million pounds by 1954. Certain formulations of these resins had intriguing properties including adhesive characteristics and the ability to

harden at moderate temperatures with little

shrinkage. These led numerous investigators to

study epoxy resins as a binder for inorganic fillers

[19, 21]. Primarily with indirect techniques, composite restorations were prepared using powdered

fused silica and porcelain, bonded together with

minor quantities of (heat-cured) epoxy resins. The

good esthetics and other favorable properties of

these composites encouraged the further investigation of this approach to dental restorative

materials [21].

However, attempts to develop composite materials using epoxy resins with various hardening

agents for use in direct techniques met with nu- merous difficulties. Although epoxy resins were

initially adhesive to hard tooth tissues, the

strengths of bonds to enamel or dentin after prolonged exposure to water were disappointing

[19, 21, 22].

2. Early Composite Developments

The goal-directed research in which the composite dental restorative materials were developed

was initiated at the National Bureau of Standards

in 1956. After unsuccessful attempts to use various

epoxy resins and hardening systems, a compromise

between epoxy and methacrylate resins was con- ceived late in 1957 [23, 24]. The reaction sites

(oxirane rings) of the epoxy molecule were re- placed by methacrylate groups. This gave a hybrid

molecule that polymerized through methacrylate

groups [25]. It was suitable for use as a binder for

reinforcing fillers because it was nonvolatile, had

a relatively low polymerization shrinkage, and

hardened rapidly under oral conditions when suitably formulated with an appropriate initiator

system.

This dimethacrylate monomer (BIS-GMA) was

synthesized by the reaction of bisphenol A and

glycidyl methacrylate [26]. The same product is

also produced by the reaction of the diglycidyl

ether of bisphenol A and methacrylic acid [25].

Its structural formula [27] is shown in figure 1. Although BIS-GMA has been referred to as an

epoxy resin, this is not the case ; the original epoxy

(oxirane) groups disappear during the synthesis

and are, in effect, replaced by methacrylate groups.

Unfortunately, this dimethacrylate monomer has

also been called a polyester. The well-established

term "polyester" should be restricted to the polycondensation products essentially of dicarboxylic

acids with dihydroxy alcohols [28].

Since the coefficients of thermal expansion of

organic polymers are many times higher than that

of the dental hard tooth tissues [6, 29], means for

lowering this value were needed. Fillers [17] inHa C=CC0CH2 -C-CHs o/(^\c-/()VoCH2 -C-CH2 OCC^CHs

H "^c^^^ H CH3

H3

BIS-GMA

0

HsC^CCOCHs

CH3

MMA

FiGUKE 1. Structural formula of BIS-GMA.

The asterisks indicate asymmetric carbon atoms that give rise to a number of steroisomers. Methyl methacrylate (MMA is shown for comparison [37].

corporated into the resin could do this [18]. Experimental composites prepared in 1953 utilized

particles of fused silicon dioxide as a reinforcing

filler because of the extremely low coefficient of

thermal expansion of vitreous silica [21]. With

the epoxy resin system, there was relatively good

adhesion between the resin and the surfaces of

these particles.

Later in the investigation, after the more adhesive epoxy resins had been replaced by the less

adhesive dimethacrylate resins (BIS-GMA) [26],

it became apparent that special means for attaining adhesive bonding between the resin matrix

and the filler particle surfaces were required. At

that time vinyl silane coupling agents appeared to

be the most suitable means of improving this bonding [30, 31]. The more reactive y-methacryloxypropyl silane coupling agents [32, 33] had not then

become commercially available. A vinyl silane

coupling agent applied to reinforcing filler gave

composites that had about four times the tensile

strength of composites utilizing the same filler

without its being treated with the vinyl silane

[34]. When the methacryloxypropyl silanes became available, their ability to promote adhesive

bonding between the resin and glass was compared

with that of vinyl silanes ; the methacryloxypropyl

silanes gave significantly stronger and hydrolytically more-stable bonding than did the vinyl

silanes [35-37].

Since the strength and durability of the composites depended on the quality of the bonding between the organic matrix and the reinforcing filler

particles, it was of utmost importance that the best

coupling agents be used in the optimal manner.

The coupling agent most widely used was y-methacryloxypropyltrimethoxy silane, also known as 3-

(trimethoxysilyl) propyl methacrylate. This or- ganofunctional silane was hydrolyzed and con- densed on the surfaces of the filler particles. It

made the particles water re]Dellent, reducing the

water sorption of the composites; it also formed

chemical bonds at the surface of the particles, con- necting the organic polymer and the inorganic

94
filler [38], converting inert fillers to reinforcing

fillers [39].

Although fused quartz or fused silica received

a great deal of attention in experimental work [14,

21, 23, 24, 26, 34], many other filler materials were

also investigated [16, 18, 19, 21, 31-34]. These included the synthetic mineral y8-eucryptite and

closely related materials [35, 40, 41], microcrystalline glasses [42], aluminosilicate glasses, bariunititanium glass, crystalline quartz and pyrogenic

silica [36]. It has been widely held (although not

universally accepted) that condensation between

SiOH groups on the surfaces of these silica-containing fillers and SiOH groups of the hydrolyzed

silane accounted for the bonding between the cou- pling agent of the filler and the filler particle itself. Copolymerization of the methacrylate groups

of the silane with the methacrylate groups of the

monomers comprising the organic continuous

phase has been thought to complete the chemical

linkage between the phases of the composites [38].

The BIS-GMA monomer was too viscous to be

readily mixed with these fillers without first being

thinned with some suitable monomer having a lower viscosity. Among others, methyl methacrylate and ethylene and tetraethylene glycol dimethacrylate, have been used for this purpose

[23, 24].

As with antecedent methacrylates [16], stabiliz- ers such as hydroquinone were first used to inhibit

premature polymerization and give the necessary

storage stability (shelf life) to the BIS-GMA

formulations [23, 26]. On polymerization, less discoloration occurred with the use of the monomethyl ether of hydroquinone. Later, hydroquinone was replaced by BHT (butylated hydroxytoluene; 2,6-di-i5er^-butyl-4-methyiphenol), a stabilizer that appeared to contribute no discoloring

effects [43].

Other additives that contributed to color stability of the restorations were ultraviolet-absorbing

compounds. While these may have contributed to

the stability of the monomers during storage, their

primary purpose was to reduce the amount of

discoloration in the resin of the final composite

material. Typical UV absorbei-s were 2-hydroxy4-methoxy-benzophenone [43] and 2- (2'-hydroxy5'-methylphenyl ) benzotriazole.

Since most of the experimental work on composite materials utilized an amine-peroxide initiating system, another important component has been

the amine accelerator. The most commonly used

of these was N,N-dimethyl-;?-toluidine (DMPT).

This compound interacted with peroxides to produce free radicals which brought about the polymerization of the methacrylate monomer groups

and thus the hardening of the resin binder of the

composite. Investigations of factors that produce

discoloration in these materials led to the introduction of N,N-dimethyl-sym-m-xylidine (DMSX;

also known as DMDA) , which not only gave less

discoloration but also was a more effective accelerator than DMPT when compared on an equimolar basis [43].

In a composite material developed in Great Britain [44], a long-chain mercaptan, together with

an aliphatic peroxide, served as the hardening

initiators.

Although other peroxides have been used [44],

benzoyl peroxide has been the one most used in

composite restorative materials. It has been dispersed either in the reinforcing filler with the aid

of a solvent that is removed by evaporation, or is

dissolved in one of the monomeric liquids of the

composite formulation.

Sulfinic acids or their derivatives were used in

the initiator systems for experimental composite

materials [14].

3. Some Properties of Experimental and

Commercial Composites

Before proceeding with recent and current developments, perhaps it would be well to define

"composite." For this discussion, the term "composite restorative material" refers to a man-made,

three dimensional combination of at least two

chemically different materials with a distinct interface separating the components [39], properties are thus obtained for the restoration of the

form and function of defective teeth which could

not be achieved by any of the components acting

alone. Thus, the use of the term "composite" distinguishes the combination of inorganic aggregates

bonded together with organic polymers from unreinforced direct filling resins and from silicate

cements.

It is primarily the high proportion of reinforcing filler in composite restorative materials that

yields properties different from those of direct

filling resins. Most of the physical properties of

composite restorative materials have been improved by the incorporation of a maximum amount

of reinforcing filler together with a minimum

amount of a binder of cross-linking polymerizable

organic resin.

The "first generation" of commercial dental

composite materials used BIS-GMA resin binder,

described previously, together with major portions

of various particulate fillers and minor portions of

various additives. Data that have been reported

in the literature [45-60] and more recently obtained [61, 62] show that composites have lower

polymerization shrinkages and coefficients of thermal expansion, and higher compressive strengths

and stiffnesses relative to unreinforced resins.

Compared to silicate cements, the composites have

lower solubilities and higher tensile strengths.

More detailed data are given in tables 1 and 2,

including comparisons with dentin and enamel

[24, 63-68]. The quantitative information presented in the tables is incomplete since the methods and conditions of measurement are not described; however, these data will serve to illus95
trate the ranges of values and order of magnitude

to be expected with these materials.

The smoothness of the finished surface is comparable with that of silicate cements [36, 57, 58]

but inferior to that of unreinforced resins [52]

and most of the other restorative materials. The

abrasion resistance of composite materials is su- perior to direct filling resins [36, 47, 48, 51, 56, 58].

Similar to some other dental restorative materials, problems encountered in packaging and

storage include volatility of some ingredients and

premature gelation of resin components [36].

4. Current Research

Current research has the goal of making improvements on this "first generation" of composite

materials now available to the dental practitioner.

For example, there is a degree of pulpal irritation

caused by some of these materials [69, 70], and it has not yet been established which of the various

ingredients contribute most to the inflammatory

reactions of the dental pulp tissues. Other problems that have been encountered are incomplete

color stability [62] and resistance to staining,

difficulties encountered in finishing and polishing,

and lack of x-ray opacity.

The recent investigations at the National Bureau of Standards have led to new dimethacrylate

monomers, reinforcing fillers, accelerators, stabilizers, and adhesion-promoting coupling agents

that will, hopefully, lead to an improved "second

generation" of composite materials. For example,

the BIS-GMA is not completely color stable and

is too viscous for use without being thinned with

a monomer of lower viscosity. Furthermore, it can- not be purified by distillation or by crystallization

since it is inherently a mixture of non-volatile optical isomers. To overcome these problems, three

dimethacrylate monomers were synthesized that can be prepared and purified separately by re- crystallization [27]. These are the condensation

products of 2-hydroxyethyl methacrylate with

the acid chlorides of phthalic, isophthalic and

terephthalic acids, respectively. Upon combining

these three crystalline products, a ternary eutectic

liquid mixture is formed that has a viscosity low

enough to be used without the addition of any

volatile ingredients. Preliminary tests show it to

be non-toxic and non-irritating [27].

Numerous other monomers [71-75] are cur- rently being synthesized and evaluated for use in

dental composite materials.

The research effort to improve polymerization

accelerators [76] has led to the successful synthesis of aromatic amines having unusually high

molecular weights [77] so as to minimize their

solubilities in tissue fluids and thus their freedom

to diffuse into the pulp or other tissues. The structure of these amines is based partly on the fact

that the color stabilities of the composites have

correlated better with the nature of the ring substituents than with the kind of nitrogen substituents in the amine accelerators [76, 77].

Table 1. Some physical properties of esthetic direct-filling materials

Hardening time Polymerization

shrinkage

Water sorption Solubility and

disintegration

Experimental composites

minutes

4 [14]

3-8 [24, 26]

5-7 [34]

7-10 [41] 5 [61]

3. 5-5 [62]

% (by volume)

2 [23]

2. 7-2. 8 [26]

% (by weight)

0. 9 [26]

0. 30-0. 35 [62]

% (by weight)

0. 00-0. 08 [23]

0. 04 [26]

0. 15-0. 22 [61]

Commercial composites

3-4 [50]

1. 5-4. 5 [52] 1-1. 5 [55]

2. 5-5 [59]

3-3. 5 [61]

1. 2-1. 5 [52]

0. 78-2. 46 [53]

1. 60-3. 68 [56]

0. 8-1. 6 [57]

1. 12 [62]

0. 01-0. 08 [53]

0. 12-0. 16 [621

Silicate cements 4-6 [14]

4 [34]

2 [5]

3. 3 [26]

1. 3 [60]

0. 4-1. 1 [14]

0. 4-1. 3 [26]

Unreinforced resin

6 [14]

6 [34]

3-3. 5 [50]

4 [52]

2-4 [59]

6-8 [5]

6. 2 [26]

5. 2 [52]

5-6 [60]

» (1. 0-1. 5 [5])

(2. 1 [26])

b (0. 10 [26])

0. 1 [60]

" Volumetric expansion

Self-curing denture base resins.

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Color stability is also profoundly affected by

the characteristics of the stabilizers that are used.

Significant advances are being made in the synthesis and utilization of stabilizers that are nontoxic and that do not cause discolorization of

composites. [43, 76, 77].

In the instance Avhere there has been evidence

of greater surface staining of composite restorations relative to the adjacent tooth enamel, it has

not yet been determined to what extent this accre- tion of discolored material is due to inadequate

hydrolytic stability of the adhesive bond between

individual particles and the resin matrix, the

chemical nature of the reinforcing filler materials,

the chemical nature and degree of polymerization

of the resin formulations, the roughness of the

surface after the restoration is finished [78], or some combination of these factors. These factors are currently being studied.

Until recently, there has been little or no effort

to prepare special materials to serve as reinforcing

fillers for dental composite materials, rather, materials already available were selected for this application. However, the unique demands on this

kind of composite material warrant the development and use of the most suitable fillers as well

as other ingredients.

One experimental composite [79] contained a mixture of two reinforcing fillers : spherical particles of fused silica made up about two-thirds of

the filler and smaller particles of an x-ray opaque

glass made up about one-third of the filler. The

smaller particles were composed of a special barium fluoride-containing glass that was developed

for this purpose [80]. Radiopacity is desirable so that dentists can distinguish between the filling

material and any carious dentin that underlies the

filling.

The most recent investigations are involved with

the preparation and evaluation of a single glass

filler material containing silica and only enough

barium to give the desired refractive index and

x-ray opacity [81]. Preliminary indications are

that it will be suitable for converting into spherical particles utilizing conventional techniques

while retaining the appropriate refractive index,

and will have a low coefficient of thermal expansion. Chemically, the glass is formulated to have,

at least in theory, hydrolytic stability in its bonding with the silane coupling agents.

Since these composite materials are not intrinsically adhesive to the prepared tooth surface, special coupling agents have been developed which

improve bonding between such composite filler

materials and dentin and enamel. The chemical

design of these special coupling agents was based

on empirical observations of the types of groups

that could displace water from the surfaces of

powdered human enamel and dentin, thereby making the surface organophilic rather than hydrophilic [82].

One of the coupling agents, NPG-GMA (the

reaction product of N-phenylglycine and glycidyl

methacrylate) , has two functions in the same molecule. One function is that of a chelate group that can form multiple bonds with calcium in the tooth

surface. The other function is that of a methacrylate group, which can copolymerize with the

hardening organic matrix of the composite material [83]. A solution of the coupling agent placed

on the prepared tooth surface with the excess solution being wiped away leaves an invisible film.

Following this, the freshly mixed (but not yet

hardened) composite material is placed on top of

this treated surface and the subsequent polymerization apparently includes the methacrylate

groups of the coupling agent. The coupling agent

has presumably also formed ionic and coordinate

bonds with the tooth surface. Laboratory data on

tensile adhesion tests clearly demonstrate that the

use of the coupling agent significantly improved

the adhesion between composite materials and

tooth surfaces [83, 84]. Clinical evaluations are under way [79].

There is a rapidly-expanding wealth of experi- ence and information regarding composite dental

materials, not only of the kind described here, but

also of a wide diversity of materials and techniques [85]. Composite restorative materials, when

they they have been developed to their best potentialities, should eventually provide the dental profession and the public with greatly improved

restorations for anterior teeth.

5. References

[1] Luskin, L. S., Milestones in the History of Acrylic

Products, Or-Chem Topics No. 23, Summer 1967,

Pliiladelphia, Rohm & Haas Co.

[2] Schnebel, Dentistische Reforme 4 and 5, 1942

Blumenthal, L., Recent German developments in

the field of dental resins, fiat final report no. 1185,

Field Information Agency, Technical, Office of

Military Government for Germany (US) (27 May

1947).

[3] FafCenbarger, G. 0., Dental cements, direct filling

resins, composite and adhesive restorative materials : A Resume, Proceedings of the Engineering

Foundation Research Conference on Engineering

in Medicine-Bioceramics, J. Biomed. Mater. Res.

(Special Issue 1972).

[4] Caul, H. J., and Schoonover, I. C, Color stability

of direct filling resins, J. Am. Dental Assoc. 47,

448-452 (Oct 1953).

[5] Smith, D. L., and Schoonover, I. C, Direct filling

resins : Dimensional changes resulting from po- lymerization shrinkage and water sorption, J. Am.

Dental Assoc. 46, 540-544 (May 1953).

[6] Nelsen, R. J.; Wolcott, R. B., and Paffenbarger,

G. C, Fluid exchange at the margins of dental

restorations, J. Am. Dental Assoc. 44, 288-295

(March 1952).

[7] Kramer, I. R. H., and McLean, J. W., The response

of the human pulp to self-polymerizing acrylic

restorations, Brit. Dental J. 92, 255-261, 281-287,

311-315 (May-June 1952).

[8] Grossman, L. I., Pulp reaction to the insertion of

self-curing acrylic resin filling materials, J. Am.

Dental Assoc. 46, 265-269 (March 1953).

98
[9] Kramer, I. R. H., The relationship between pain and

changes in the dental pulp following the insertion

of fillings, Brit. Dental J. 96, 9-13 (Jan 1954).

[10] Hedegard, B., Cold-polymerizing resins as restorative materials, Odontol. Tidskr. 65, 169-212

(June 1957).

[11] Hedegard, B., Co-Report, Synthetic Plastics, Intern. Dental J. 8, 249-250 (June 1958).

[12] Fletcher, T., British Patent No. 3028, 1878, Fletcher,

T., Silicate of lime and alumina, Brit. Dental J. Science 22, 74, (1879).

[13] Henschel, C. J., Observations concerning in vivo

disintegration of silicate cement restorations, J. Dental Res. 28, 528-529 (Oct 1949).

[14] Bowen, R. L., Paffenbarger, G. C, and Mullineaux,

A. L., A laboratory and clinical comi>arison of

silicate cements and a direct filling resin, A

Progress Report, J. Prosthetic Dentistry 20,

426-437 (Nov 1968).

[15] Norman, R. D., Swartz, M. L., Phillips, R. W., and

Virmani, R., A comparison of the intraoral dis- integration of three dental cements, J. Am.

Dental Assoc. 78, 777-782 (April 1969).

[16] Knock, F. E., and Glenn, J. F., Dental Material

and Method, U.S. Patent No. 2,558,139 (June 26,

1951).

[17] Paffenbarger, G. C, Nelsen, R. J., and Sweeney,

W. T., Direct and indirect filling resins : A re- view of some physical and chemical properties,

J. Am. Dental Assoc. 47, 516-524 (Nov 1953).

[18] Rose, B. E., Lai, J., Green, R., and Cornell, J., Direct resin filling materials : Coefficient of

thermal expansion and water sorption of polymethyl methacrylate, J. Dental Res. 34, 589-596

(Aug 1955).

[19] Rose, E. E., Lai, J., Williams, N. B., and Falcetti,

J. P., The screening of materials for adhesion

to human tooth structure, J. Dental Res. 34,

577-588 (Aug 1955).

[20] Lee, H., and Neville, K., Handbook of Epoxy

Resins (McGraw-Hill Book Co., New York,

1967).

[21] Bowen, R. L., Use of epoxy resins in restorative

materials, J. Dental Res. 35, 360-369 (June

1956).

[22] Bowen, R. L., and Mullineaux, A. L., Adhesive

restorative materials, Dental Abstracts 14, 80-82

(Feb 1969).

[23] Bowen, R. L., Dental Filling Material Comprising

Vinyl Silane Treated Fused Silica and a Binder

Consisting of the Reaction Product of Bisphenol

and Glycidyl Acrylate, U.S. Patent No. 3,066,112

(Nov 1962).

[24] Bowen, B. L., and Rodriguez, M. S., Tensile strength

and modulus of elasticity of tooth structure and

several restorative materials, J. Am. Dental

Assoc. 64, 378-387 (March 1962).

[25] Bowen, R. L., A Method of Preparing a Monomer

having Phenoxy and Methacrylate Groups Linked

by Hydroxy Glyceryl Groups, U.S. Patent No.

3,179,623 (April 1965).

[26] Bowen, R. L., Properties of a silica-reinforced poly- mer for dental restorations, J. Am. Dental Assoc. 66, 57-64 (Jan 1963).

[27] Bowen, R. L., Crystalline dimethacrylate monomers,

J. Dental Res. 49, 810-815 (July-Aug 1970).

[28] Bjorksten, J., Tovey, H., Harker, B., and Henning, J., Polyesters and Their Applications, p. 11

(Reinhold Publishing Corp., New York 1956).

[29] Guide to Dental Materials and Devices (5th ed),

pp 51, 98 (Am. Dental Assoc., Chicago, 1970-71).

[30] Vanderbilt, B. M., and Simko, J. P., Jr., Silane

coupling agents in glass-reinforced plastics, Modern Plastics 38, 135-217 (Dec 1960).

[31] Vanderbilt, B. M., and Jaruzelski, J. J., The bonding of fillers to thermosetting resins, Ind. and

Eng. Chem., Prod. Res. and Dev. 1, 188-194

(Sept 1962).

[32] Sterman, S., and Marsden, J. G., Filler-silane interactions. Modern Plastics 41, 254-266 (Oct 1963).

[33] Sterman, S., and Marsden, J. G., Silane coupling

agents as integral blends in resin-flUer systems.

Modem Plastics 40, 125-177 (July 1963).

[34] Bowen, R. L., Effect of particle shape and size dis- tribution in a reinforced polymer, J. Am. Dental

Assoc. 69, 481-495 (Oct 1964).

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Fleischmann, R. M., Evidence for chemical bonding formation at silane coupling agent interfaces, J. Comp. Materials 1, 278-292 (Jan 1967).

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Composite Materials, pp. 501, 7 (Addison-Wesley

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Soc. 34, 87-90 (March 1951).

[43] Bowen, R. L., and Argentar, H., Diminishing dis- coloration in methacrylate accelerator systems,

J. Am. Dental Assoc. 75, 918-923 (Oct 1967).

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filling materials, Brit Dental J. 127, 9-18 (July

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surface treatment of glass fillers on mechanical

properties, Shika Zairyo Kenkyusho Hokoku 2,

629-643, 644-655 (March 1965).

[46] Going, R. E., and Sawinski, V. J., Microleakage of a new restorative material, J. Am. Dental Assoc. 73, 107-115 (July 1966).

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Abrasion of restorative materials, N.Y. State

Dental J. 32, 395-400 (Nov 1966).

[48] Peterson, E. A., Phillips, R. W., and Swartz, M. L., A comparison of the physical properties of four

restorative resins, J. Am. Dental Assoc. 73, 1324-

1336 (Dec 1966).

[49] Bowen, R. L., Adhesive bonding of various materials to hard tooth tissues. VI. Forces developing

in direct-filling materials during hardening,

J. Am. Dental Assoc. 74, 439-445 (Feb 1967).

[50] Matsui, A., Buonocore, M., and Yamaki, M., Heat

of polymerization of certain new and conventional restorative materials, J. Dental Res. 46,

1106 (Sept-Oct 1967).

[51] Gotfredsen, C, Addentd) Investigations of a plastic

filling material, Tandlaegebladet 72, 407-429

(May 1968).

[52] Macchi, R. L., and Craig, R. G., Physical and mechanical properties of composite restorative materials, J. Am. Dental Assoc. 78, 328-334 (Feb

1969).

[53] Freeman, F. H., Composite restorative materials.

Presented before the Dental Materials Group,

lADR, Mar 21, 1969, Houston, Texas.

99
[54] Tani, Y., and Buonocore, M., Marginal leakage and

penetration of basic fuchsin dye in anterior res- torative materials, J. Am. Dental Assoc. 78, 542-

548 (March 1969).

[55] Lee, H. L., Swartz, M. L., and Smith, F. F., Epoxy

resins in dentistry, presented at the ACS National Meeting, April 1969, Minneapolis, Minnesota (in press).

[56] Gotfredsen, C, Physical properties of a plastic fill- ing material (Addent®), Acta Odontol. Scand.

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[57] Lee, H. L., Swartz, M. L., and Smith, F. F., Physical

properties of four thermosetting dental restora- tive resins, J. Dental Res. 48, 526-535 (July-Aug

1969).

[58] Phillips, R. W., Swartz, M. L., and Norman, R. D., Materials for the Practicing Dentist, chapter 59, pp. 182-191 (The C. V. Mosby Co., St. Louis, Mo.,

1969).

[59] Docking, A. R., Modem materials in dental practice, Australian Dental J. 15, 303-309 (Aug 1970).

[60] Schouboe, P. J., Paffenbarger, G. C, and Sweeney,

W. T., Resin cements and posterior-type direct

filling resins, J. Am. Dental Assoc. 52, 584-600

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[62] Barton, J. A., Jr. et al., Unpublished data.

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37, 710-718 (Aug 1958).

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J. W., and Sweeney, W. T., Determination of some

compressive properties of human enamel and

dentin, J. Am. Dental Assoc. 57, 487-495 (Oct

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[65] Stanford, J. W., Weigel, K. V., Paffenbarger, G. C,

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1960).

[66] Craig, R. G., Peyton, F. A., and Johnson, D. W.,

Compressive properties of enamel, dental

cements, and gold, J. Dental Res. 40, 936-945

(Oct 1961).

[67] Souder, W., and Paffenbarger, G. C, Physical

Properties of Dental Materials, NBS Circ. 433,

222 pages (1942).

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J. Am. Dental Assoc. 49, 513-521 (Nov 1954).

[69] Stanley, H. R., Swerdlow, H., and Buonocore, M.

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1967).

[70] Langeland. L. K., Guttuso, J., Jerome, D. R., and

Langeland, K., Histologic and clinical comparison of Addent with silicate cements and cold- curing materials, J. Am. Dental Assoc. 72, 373-

385 (Feb 1966).

[71] French Patent No. 2,008,541 (Jan 23, 1970).

[72] Mihailov, M., and Boudevska H., Synthesis and

polymerization of polyestermethacrylates and

terephthallic and furane-2, 5-diearboxylic acid,

Compt. Rend. Acad. Bulgare Sci. 18, 31-34

(1965).

[73] French Patent No. 2,010,896.

[74] French Patent No. 2,010,905.

[75] Atsuta, M., Nakabayashi, N., and Masuhara, E., Hard methacrylic polymers, II., copolymers of

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400, J. Dental Res. 51, 473-482 (Mar-Apr.

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[78] Chandler, H. H., Bowen, R. L., and Paffenbarger,

G. C, A method for finishing comjxxsite restorative materials, J. Am. Dental Assoc. 83, 344-348

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(Aug 1970).

100
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Cements Containing o-Ethoxybenzoic Acid (EBA)

Gerhard M. Brauer

Dental Research Section, Institute for Materials Research, National Bureau of Standards,

Washington, D.C. 20234

Cements containing o-ethoxybenzoic acid (EBA) are reviewed. Studies of the mechanism of hardening of zinc oxide-eugenol cements indicated the potential usefulness of

other chelating agents in dental cements. Products with greatly enhanced physical and

mechanical properties are obtained by the partial substitution of EBA for eugenol. Similar

to zinc oxide-eugenol cements, these materials are well tolerated by the tissues, but they

also stimulate the formation of reparative dentin. Physical properties of the EBA-containing

cements approach those of the biologically and physiologically less desirable zinc phosphate

cements. The EBA cements have become commercially available and have been well accepted

as luting agents for fixed restorations and as insulating bases. They appear to be the

materials of choice for indirect pulp capping. Resin modified EBA restoratives show good

stress-bearing characteristics and should find applications as an intermediate restorative.

Possible studies to further improve these versatile materials are discussed.

Key words : Crown and bridge cements ; dental cements ; EBA cements ; EBA sedative

bases ; intermediate restorative ; o-ethoxybenzoic acid cements ; pulp capping materials

zinc oxide-EBA-eugenol cements.

1. Introduction

Cements containing o-ethoxybenzoic acid

(EBA) are an outgrowth of studies to improve

zine oxide-eugenol (ZOE) cements which were

first reported in the dental literature about one

hundred years ago [1-3J. ^ Over the years ZOE

cements have found a wide range of applications

in dentistry: as temporary restoratives, sedative

bases, cementing media, for crown and bridge

work, in pulp capping, as soft tissue packs in oral

surgery and periodontics, as root canal sealers in

endodontics and with modifying agents as impression pastes. The ZOE cements possess much better

compatibility than most dental materials with

both the hard and soft tissues of the mouth [4-7].

They have excellent sealing properties [8] and

their sterilizing effectiveness [9] has been demonstrated. ZOE cement also acts as a palliative or anodyne and as a mild non-irritant antiseptic.

Their low strength which may not be large enough

to resist forces of mastication and their lack of

resistance to wear and disintegration deter more

extensive use in temporary restorations.

2. Setting Mechanism of ZOE Cements

A number of studies have shown that the set

mass resulting from mixes containing the proper

proportion of zinc oxide and eugenol consists of

zinc oxide embedded in a matrix of zinc eugenolate

' Figures In brackets Indicate the literature references at the end of this paper.

chelate with any excess eugenol being sorbed by

both zinc eugenolate and the unreacted zinc oxide

[10-11]. More detailed studies of the infrared

spectra have confirmed the 6isdioxachelate structure of zinc eugenolate [12].

/V

Zn

CH3

-CH2—CH=CH!

CHj=CH-ChA^0/ \o\/

ins

The crystal structiu"e of this compound has been

investigated by Douglas [13] and Cartz [14]. X-ray

diffraction of commercial ZOE products or ex- perimentally prepared formulations which also

have a large excess of zinc oxide yield extremely

small values for the relative intensity ratios of the

principal peaks of zinc eugenolate to ZnO. This

suggests that in these cements the zinc eugenolate

reaction product is formed below the 2% level

resolvable in standard preparations [15-16]. High

resolution microscopy also indicates that the zinc

eugenolate product in commercial formulations is small and is confined to the reaction interface [17].

These formulations also contain about 5% free

eugenol after hardening [18].

The setting reaction is speeded up by the

presence of an accelerator such as zinc acetate.

Presence of water or a decrease in the pH of the

reaction mixture also reduces setting time. Water

reacts with zinc oxide to form the hydroxide

which in turn reacts with eugenol [19].

101
(1) ZnO + n H2O > n Zn(0H)2,

(2) n Zn(0H)2 + 2n eugenol > n Zn (eugenol) 2 + 2n H2O.

The formation of the hydroxide is slow, but acid

speeds up the reaction.

(3) n ZnO + 2n CH3COOH > nZn(CH3COO)2 + u H2O,

(4) nZn(CH3COO)2+2?iH20

Zn(OH)2+2?i CH3COOH.

n ZnO+?i HsO-^n Zn(0H)2

Since the zinc hydroxide reacts with eugenol,

reaction (4) goes to the right. Obviously, zinc

acetate can be substituted for acetic acid, but

to start the reaction a trace of water must be

present. Commercial formulations generally contain zinc acetate dihydrate as accelerator which

furnishes the water to initiate the reaction.

3. Modified ZOE and EBA Cements

Low strength is unquestionably the main

disadvantage of ZOE cements. A number of

studies have been undertaken during the last

decade to upgrade the properties of the cements.

The following approaches have been taken:

1. Replacement of zinc oxide by other metal

oxides.

2. Incorporation of fillers, reinforcing or modifying agents.

3. Substitution of eugenol by other chelating

agents.

Only limited studies have been made to substitute

other metal oxides for zinc oxide [11]. Cupric

eugenolate can be prepared by a procedure similar

to that used in the synthesis of zinc eugenolate.

A slight excess of a methanolic solution of cupric

acetate monohydrate is reacted with eugenol at

60° C for one hour {16]. Employing the same

reaction conditions, that is, refluxing methanolic

solutions of eugenol with mercuric-, lead-, nickelous- or calcium-acetate did not yield the respective metal eugenolates. Attempts to synthesize

the cupric or nickel isoeugenolates also were

unsuccessful.

Addition of rosin or hydrogenated rosin improves the- working properties of the mixes. Hydrogenated rosin is commonly used since it is stable

to oxidation and yields cements with good color

lability. Incorporation of up to 20 percent polymer dissolved in eugenol to improve the physical

properties was first suggested by Curtis [20]. An

experimental cement containing surface-treated

zinc oxide (80%) and poly (methyl methacrylate)

(20% ) powder and a liquid consisting of 99 percent eugenol and 1 percent acetic acid had a compressive strength of 64.0 MN/m^ (550 kg/cm^

7,820 psi) , a tensile strength of 4.1 MN/m^ (42 kg/

cm^, 600 psi) and a solubility and disintegration

value of 0.008 g/cm^ [21]. A clinical evaluation of

this cement as an intermediate restorative material

at intervals of 60 days for a period of 12 months

showed this formulation to be superior to three

other experimental cements with regard to the

maintenance of anatomical form and marginal integrity [22]. Products incorporating poly (methyl

methacrylate) in the powder have become commercially available both as luting agents and intermediate restoratives.

The knowledge gained from the characterization

of the reaction products led to extensive investigations with the ultimate aim of obtaining improved

cements by substitution of other complexing

agents for eugenol. Eugenol isomers capable of

forming chelates were synthesized [23] and the effect of the position of substituents on the behavior

of the isomers as evidenced by their ionization constants and reactivity with zinc oxide was determined [24]. In the reaction of eugenol isomers

with zinc oxide, the 1,2,3-trisubstituted isomers do

not react readily compared to the unsymmetrically

1,2,4- or 1,2,5-trisubstituted ones, indicating that

the chelation reaction is greatly influenced by

steric hindrance of the bulky neighboring allyl

groups. The chelation reaction may also depend to

some degree on the ionization constants since the

more acidic chavibetol reacts somewhat faster than

eugenol.

Zinc oxide will react with many chelate-forming

compounds to yield coherent products [25]. Mixes

containing o-ethoxybenzoic acid (EBA) , salicylaldehyde, acetylacetone, o-ethoxyacetic acid or lactic

acid form hard products within one hour at room

temperature. Some of these products disintegrate

in water.

Cements obtained from o-ethoxybenzoic acid

(EBA) and metal oxides of group II of the

periodic table or lead oxide have been studied in

considerable detail [26]. Products formed from

zinc oxide and EBA-eugenol solutions in the ab- sence of an accelerator harden more rapidly and

have higher strength and lower solubility and disintegration vakies than those made with EBA

alone (figs. 1 and 2) . Most useful properties are obtained with liquids containing between 50 to 70

percent EBA. However, these cements have a high

water solubility. Substitution of 2-propoxy-5-

methylbenzoic acid, a higher, more hydrophobic

homologue for EBA, yields cements with unexpectedly high water solubility and disintegration

[26, 27]. On incorporation of rosin or hydrogenated rosin the water solubility is greatly reduced

[26]. Addition of a reinforcing agent such as monocalcium phosphate, heat-treated fused quartz

or alluminum oxide gives products with compressive, shear and tensile strengths that are three to

four times those of conventional ZOE formulations. Thus, these EBA containing products have

mechanical properties similar to those of zinc

phosphate cements.

102
% EBA

0 20 40 60 80 100 % EUGENOL

Figure 1. Effect of composition of the liquid on setting

time and standard consistency powder-liquid ratio of

ZnO-EBA-cugcnol mixes [25].

iqo 80 60 40 20 0

0 20 40 60 80 % EUGENOL

Figure 2. Effect of composition of the liquid on compressive strength and solubility and disintegration of

ZnO-EBA-eugenol cements.

X X solubility and disintegration, o o compressive

strengtn [25].

The properties of cements, based on zinc oxide,

hydrogenated rosin, EBA, and eugenol have been

studied in detail [28] . The strength of the cements

is nearly independent of the particle size of zinc

oxide and fused quartz. The carbon dioxide and

water content of the zinc oxide also have little

effect on the physical properties of the hardened

product [28-30]. Alumina reinforced EBA ce- ments have physical properties superior to those

of cements reinforced with fused quartz [31]. The

preferred composition contained 64 percent zinc

oxide, 30 percent tabular ALOs and 6 percent hydrogenated rosin. A slurry prepared from 1.7 g

of powder per 0.2 ml of liquid can be mixed easily

and will harden in less than 10 minutes. The resulting product has a compressive strength of 93 MN/

m^ (950 kg/cm% 13,500 psi) and a film thickness

of 26 jjm (table 1). With one-surface inlays, these

cements adhere at least as well as commercial zinc

phosphate cements and give much higher tensile

adhesion values than ZOE products. Similar formulations have become commercially available and

have been well accepted as luting agents in crown

and bridge cementation. On incorporation of more

j)owder into the mix, excellent base materials can

be obtained. Especially desirable is their high tenminute compressive strength of 46.1 MN/m^ (470

kg/cm% 6,680 psi) which can easily withstand the

forces encountered in condensing an amalgam.

The brittleness of these luting agents limits their use for temporary restorations of multiple surface

carious lesions in areas subject to heavy masticatory

forces [32]. Stress bearing characteristics of EBA

cements can be improved through the incorporation of powdered polymers of relatively low elastic

moduli [30]. The most suitable resins are methacrylate copolymers, although vinyl copolymers

may also be potentially useful. Other resins, be- cause of their resilient nature are difficult to obtain

Table 1. Physical properties of dental cements ^

Powder-liquid

quantities

Setting

time Tensile strength^ Compressive strength ^ Solubility

and dis- integration

gm/ml min MN/m^ kg/cm 2 kg/cm^ Percent Zinc oxide-eugenol (ZOE) 1. 0-2. 2/. 4 3-8 1. 4-2. 5 14-25 16. 7-38. 3 170-390 0. 02-0. 20

Reinforced ZOE . 6-1. 1/. 3 3-8 1. 5-6. 9 15-70 39. 2-75. 5 400-770 . 05- . 80

EBA (AI3O3 reinforced) 1. 6-2. 0/. 2 7-9 3. 9-7. 4 40-75 58. 9-93. 1 600-950 . 05- . 13

EBA (plastic-modified) 1. 1-1. 3/. 2 7-8 6. 6-9. 8 66-99 50. 0-78. 5 510-800 . 13- . 94

Zinc phosphate cement _ 1. 4/. 5 7-8 3. 2-4. 6 33-46 68. 7-147. 1 700-1, 500 . 10- . 20

' Some of the data given in this table are taken from the results of the collaborative test program—Zinc oxide—Eugenol

Dental Cements ISO Technical Committee 106/ WGl—Filhng Materials.

2 One day.

103
in powdered form. A cement made from powder

containing 58.2 percent ZnO, 27.3 percent AI2O3,

5.4 percent rosin and 9.1 percent methyl methacrylate copolymer and liquid containing 62.5 percent EBA and 37.5 percent eugenol had, after one

week, tensile and compressive strengths of 11.5

MN/m^ (117 kg/cm% 1,660 psi) and 65.1 MN/m==

(664 kg/cm% 9,430 psi) respectively. Thus, the

addition of the polymer greatly increases the tensile strength of the hardened cement. These markedly higher tensile strength values are probably

more important than the somewhat lower compressive strength; the clinical results described later

bear out this point.

Cements containing rosin have somewhat higher

tensile strength, but are more soluble and set more

slowly than those to which hydrogenated rosin has

been added [30]. Improvement in the physical

properties of a formulation containing 58.2 percent ZnO, 27.3 percent AI2O3, 5.4 percent hydrogenated rosin and 9.1 percent methyl methacrylate copolymer in the' powder and 62.5 percent

EBA-37.5 percent eugenol in the liquid was not

obtained on increasing the (1) hydrogenated rosin

content, (2) percentage of eugenol, and (3) ratio

of zinc oxide to aluminum oxide reinforcing agent.

Addition of 0.5 percent aluminum sulfate de- creases very slightly the solubility and disintegration of cements containing rosin or hydrogenated

rosin. A vinyl chloride-vinyl acetate copolymercontaining cement had one-week tensile and compressive strength of 9.8 MN/m^ (100 kg/cmS 1,420

psi) and 91.5 MN/m^ (933 kg/cmS 13,300 psi), re- spectively. Incorporation of an acrylonitrile-butadiene-styrene terpolymer, a polyacetal resin, or various grades of commercial polycarbonate molding powders produced materials with physical

properties somewhat lower than those containing

acrylic copolymer. A summary of the physical

properties of conventional and reinforced ZOE

cements, alumina-reinforced, plastic-modified

EBA cements, and zinc phosphate cements is

given in table 1. A comparison of the first commercially available EBA crown and bridge cements has been

made by Phillips and coworkers [33]. Properties investigated were compressive and tensile

strength, film thickness, solubility and disintegration in both water and acid and retentive characteristics as determined by the amount of tensile

force required to remove one-surface inlays from

prepared cavities following cementation with the

various agents. Physical properties of an experimental alumina-reinforced EBA cement and commercial EBA cements of unknown composition

have also been studied by Custer and Anderson

[34]. The properties of typical commercial EBA

cements have been measured in a collaborative test program which had as its aim the development of

a specification for ZOE and EBA type cements

[35].

4. Mechanism of Setting of EBA Cements

The products formed on hardening of commercial EBA cements have not been completely

characterized [16]. Cements made up of zinc oxide

and a liquid consisting of either EBA or 62.5 percent EBA-37.5 % eugenol were pulverized. The

powder was extracted by shaking with 50 ml of

methanol for 6 hr. Aftei- centrifuging, the methanol layer was decanted and the remaining

powder was dried at 110 °C and weighed. From

cements containing EBA, the original liquid was

removed quantitatively (table 2). Cements con- taining EBA and eugenol lost 60 to 70 percent of

their liquid component on extraction. Probably

all the EBA was removed by the methanol extraction. The results were confirmed by heating the

dried unextracted solid residue in a crucible to re- move any remaining organic matter. There was no

loss in weight on heating the residue of the cement

prepared with EBA liquid. The EBA-eugenol

containing cement showed a 30 to 40 percent loss in

weight which accounted for all the unextracted

liquid. Thus, eugenol is much less readily extracted, and hence more firmly bound than EBA in

the hardened cementa

Table 2. Weight loss of cements on solvent extraction

Cement

containing

zinc oxide

and

Extraction

with

CH3OH

Loss of liquid on heating

CH3OH insoluble residue

Extraction

with

CHCI3

Percent Percent Percent EugenoL 10-12 80-90 ~80

EBA 100 0 "119

EBA and

eugenol 60-70 30-40 "102

"Including some solid extracted with CHCls.

To study the reaction product, zinc oxide (2 g,

0.0246 mole) and o-ethoxybenzoic acid (8 g, 0.048

mole) were stirred together and set aside. After

two months at room temperature, the mixture consisted of a soft layer on top of a hard layer.

X-ray diffraction did not give any peaks. The

product was partially soluble in hot water, insoluble in methanol, ethanol, chloroform, or dimethylformamide.

The reaction products were added to warm

acetone and filtered. On evaporation of the solvent

an amorphous material separated from the filtrate.

The solid residue, insoluble in warm acetone, could

be dissolved by boiling in acetone for 10 min. A

solid (mp 92-100 °C) crystallized on cooling to

0 °C. After drying in a vacuum, analysis for car- bon and hydrogen indicated that the compound

was zinc o-ethoxybenzoate (Anal: Calcd. for

CisHisOeZn : C, 54.63, H, 4.58. Found: C, 54.8; H,

4.6) Absence of an infrared absorption peak

around 1,750 cm-^ indicated that no unreacted

104
COOH is present. The broad absorption band of

o-ethoxybenzoic acid around 1,230 cm-^ which can

be attributed to the ethoxy group [36] is present

in the zinc derivative as a sharp band at 1,240 cm-^.

The spectrum of o-ethoxybenzoic acid has absorptions at 1,745 cm-^ and also carbonyl absorptions at

1,594 and 1,609 cm"\ For the primarily ionicbonded zinc <?-ethoxybenzoate, the absorption

peak of the -COO Zn/2 group falls in the same

region at 1,600 cm'^. Since the uncoordinated

-COO" group also absorbs in the same region [37]

it was not possible to establish from the infrared

spectrum whether the o-ethoxybenzoate possesses a chelate structure.

The same compound was obtained on heating

20 g (0.12 mole) of EBA, 5 g (0.061 mole) of zinc

oxide, and 20 ml of methanol on a steam bath

until most of the solvent had evaporated. On

cooling in a refrigerator, a white solid crystallized

which, on recrystallization from chloroform, had a melting point of 90-103° C. The infrared absorption spectrum was identical with the spectrum of

zinc o-ethoxybenzoate obtained by the procedure

described above. (Anal: Calcd. for CigHigOjZn:

C, 54.63, H, 4.58. Found: C, 54.3; H, 4.6).

The zinc o-ethoxybenzoate could also be synthesized in 20 percent yield from 12.3 g (0.075

mole) EBA dissolved in 20 ml methanol and 8.8 g

(0.04 mole) zinc acetate by heating for 6 hr on a steam bath, filtering and dissolving the residue in

hot chloroform.

Slurries using powder-liquid ratios of 10:1, 5:1,

and 1:1 of zinc oxide powder and EBA or 62.5

percent EBA-37.5 percent eugenol liquid were

prepared and allowed to harden at 37° C. and 100

percent relative humidity. X-ray diffraction patterns showed only minor changes on varying the

powder-liquid ratio with all patterns resembling

closely those of zinc oxide.

On addition of EBA to zinc oxide, either a salt

(I) or chelate complex (II) can form

-coo—Zn-OOC-

-OC2H5 C2H50-

/Axcoo ooc / ^

\ /

Zn

II

Possible formation of a chelate was studied [16]

by titration of an aqueous solution of EBA with

base in the presence of zinc ions and Job's spectrophotometric method of continuous variation [38].

The first procedure depends on the fact that

most metal chelates may be considered as formed

by the displacement of a weak acidic portion of the

chelating agent by a metal ion [39]. Thus, the

addition of metal ion causes a drop in pH and the

greater the tendency for metals to combine with a given chelating agent, the greater the drop in pH.

OH —Me- OCH3 + Me++

CH2-CH=CH2

eugenol

weakly ionized.

OCII3 CllaO-/^

CHj-CH=CH2 CH2=CH-CH

+ 2H+

This simple technique of checking for chelate or complex formation is not very sensitive for EBA

since it is a moderately strong acid. Hence, there is little decrease in pH if chelation occurs. Eesults

of the titration of EBA with base in the presence

and absence of Zn"*""*" were inconclusive.

The composition of chelates can often be

determined by a simple colorimetric technique

introduced by Job [38]. For the chelating reaction:

A+nB^AB„

K= [AB„]

[A][Br

where [A] is the concentration of the* metallic ion

and [B] that of the chelating agent. If [4] + [-5]=

constant, it can be shown that when concentration [AB„] is a maximum

[B]/[A]=n;

that is, for a constant total concentration of metal

and chelating agent, the concentration of chelate

with respect to unreacted metal or chelating agent

is greatest when the metal and chelating agent are brought together in the same ratio in which they

exist in the chelate. If the chelate absorbs light

in the visible or ultraviolet region, a plot of optical

density versus composition of the solution under

the restriction imposed above will give a curve with

a maximum at the composition corresponding to

the formula of the chelate. If substances are present that absorb light in the same region as does the chelate, the difference in optical densities

between the chelate and the estimated amounts of

unchelated absorbing substances may be plotted

as a function of concentration.

The absorption of aqueous EBA and eugenol

solutions as well as of solution of these compounds

containing zinc sulfate were measured in the ultraviolet (250-360 nm). EBA and eugenol solutions follow Beer's law. They show an absorption

peak around 292 nm, whereas ZnSO^ does not

absorb at this wave length. Job's method of

continuous variation was applied to EBA-ZnSO^

solutions having a total concentration of 4X10~*

M. No maximum was observed by plotting optical

density versus composition of the solution. This

result does not necessarily indicate that no chelate

formation takes place since some chelates have

absorption spectra that do not differ significantly

from those of the chelating agent. McKenzie and

105
coworkers [40] have shown that ionic chelates

have nearly the same absorption spectra as their

chelating agent, but that the absorption spectra

for covalent bonded chelates contained strong absorption bands characteristic of the chelate, and

hence of the metal-donor bond. Since EBA is a moderately strong acid, its anion should be a

fairly strong conjugate base. Formation of an

ionic complex with a divalent metal ion such as

Zn^"*" is likely to occur, especially since oxygen

donors favor ionic bonds.

Eugenol forms a five-membered chelate whereas

EBA may give a six-membered ring. Measurement

of the chelate stabiUty constants K as well _ as the formation constants ki and Atj for the reaction

Me+++Chel-'^MChel+

MChel++Chel- ^^MChel2

[MChella

"[Me++][Chel-i]2

would yield valuable information since these constants would show the relative stabihty of the

two ring systems.

5. Clinical Studies

The main purpose of a temporary restoration

is sedation and protection of the tooth from irritants and decay. The ZOE cement is essentially

neutral and, therefore, offers unusual pulpal protection. The EBA-containing cements have the

same mild tissue reaction of unmodified ZOE

cements, which are much superior in this respect

to zinc phosphate cements [41^3].

Unset cements caused some necrosis and a moderately severe inflammatory reaction when they

were first inserted into rat muscle [42]. Granulation tissue formed in the region and healing proceeded rapidly, producing a fibrous capsule to the

implant. When set materials were used as implants, the EBA-containing cement was always less

irritating than zinc oxide-eugenol cements. Bhaskar and coworkers [43] investigated the pulpal

response of four types of restorative materials.

Class V cavities were prepared in 78 teeth of three

miniature swine. The animals were killed after

1, 2, and 3 weeks and the teeth were examined

microscopically. A powder containing 57 percent

ZnO, 28 percent aluminum oxide, 9.5 percent poly

(methyl methacrylate) copolymer and 5.5 percent

rosin and liquid made up from 66.7 percent EBA

and 33.3 percent eugenol was used. The restorative

material appeared to be biologically acceptable to

the dental pulp. No irreversible pulp damage was

observed. Odontoblastic disruj^ti.on and inflammatory infiltrate were not severe. They were least

pronounced in the ZOE restoration, very slightly

more marked in a commercial, resin-modified ZOE

and the EBA formulation, and most prominent in

teeth restored with cyanoacrylate-containing re- storative material. The layer of reparative dentin

formed in response to the filling materials was

proportional to the degree of odontoblastic disruption and the inflammatory infiltrate. It was

much more pronounced in the teeth that were re- stored with cyanoacrylate and EBA than in teeth

filled with ZOE cements. Thus, in cases in which

the therapeutic aim is to require a thick layer of

reparative dentin, a cavity base with either the

EBA or cyanoacrylate is preferable.

It would be of interest to study if the reparative

and secondary dentin formation produced by EBA

cements is as rapid as that formed after the use

of calcium hydroxide. Coleman and Kirk [42]

filled cavities scheduled for extraction for orthodontic reasons with ZOE and EBA cements. The

teeth were extracted after periods ranging from

24 hr to 3 weeks. The teeith were fixed in 10 percent formol saline, decalcified, and examined histologically. Little or no pulpal reaction could be

attributed to either the ZOE or the EBA materials. The odontoblast layer was usually intact.

Although some vacuolation was present in this

region, this was also seen in the control teeth where

no cavities had been prepared.

There has long been a deep interest in an improved ZOE cement that would be suitable for

permanent cementation of cast restorations. The

modified ZOE or reinforced EBA cements have

been employed successfully as crown and bridge

cements and appear to be well suited for this purpose. The absence of irritation on the dentine-pulp

complex and the resulting freedom from postcementation symptoms gives them a big advantage

over zinc phosphate cements. Since their compressive strength is much higher than that of un- modified ZOE cements, their retentive properties

are improved and approximate those of zinc

phosphate cements [33, 44, 45]. A clinical study

using 186 full cast crown bridge retainers and 205

full cast crowns has been reported by Horn [46].

The span of the bridges cemented with this material was limited to a maximum of two consecutive

pontics of bicuspid width. Nineteen of the full cast

crown bridge retainers were of the cantilever type.

Resin veneers were protected from the excess EBA

cement by coating them with silicone grease. No

cavity liners or medicaments were applied. The

crowns were not completely filled with cement,

but a coating was applied to the internal surfaces

and a small amount was allowed to flow into any

crevices or depressions on the teeth prior to insertions. Dryness was not maintained after the res- torations had been seated by applying pressure

for two or three minutes. After cementation, abutment teeth were not painful, and the marginal

relationships were not unduly distorted. In

selected cases, tests with ice-cold water seemed to

indicate excellent insulation. This effect may be

caused by the ability of the cement to obtund pain.

106
A number of crown and bridge cements incorporating EBA have become commercially available in the United States and in Europe. Many

products have film thickness of less than 25 /xm.

They may be used for the final cementation

of metal crowns and bridges retained by metal

crown, porcelain or plastic jackets, porcelain-overmetal restorations, and gold-veneered-with-resin

restorations.

No clinical data are available regarding the re- tention of orthodontic bands cemented with EBA

cements. Bands cemented with an experimental

cement containing 90 percent ZnO and 10 percent

hydrogenated rosin powder and 62.5 percent EBA

and 37.5 percent eugenol required three times as much force to accomplish removal as bands ce- mented with ZOE, but the retention values were

considerably lower than those obtained for

phosphate or silicate cements [47].

Zinc phosphate cement has been the preferred

material for use as an intermediary base beneath

metallic restorations. Zinc phosphate is preferred over conventional ZOE and calcium hydroxide be- cause of its superior strength, despite its inferior

biological and slightly poorer thermal diffusion

characteristics. When ZOE or calcium hydroxide

bases are used, it is often recommended that they

be overlaid with the stronger zinc phosphate

cement [48-50].

The clinical significance of the compressive

strength of a base material has not been defined.

It is obvious, nevertheless, that when amalgam is condensed into the cavity preparation, the base

must have sufficient strength to support the forces

of condensation. If the base does not have sufficient

strength, fracture or displacement of the base

could permit the amalgam to contact the underlying tooth structure and thus negate the thermal

protection afforded by the base.

Furthermore, in deep cavity preparation, the

amalgam could be forced through microscopic

exposures in the floor of the cavity and into the

pulp [51-52]. Chang, Swartz and Phillips [53],

in experiments conducted under laboratoi-y conditions, showed that with ZOE materials a minimum

strength capable of suppoi-ting amalgam condensation ranges from 0.7 to 1.2 MN/m^ (7 to 12 kg/

cm2, 100tol70 psi).

Interest has increased in recent years in using

zinc oxide-eiigenol type cements as bases under

amalgam and inlay restorations. These bases are

nonirritating to the pulp and thus eliminate the

need to protect the pulp Avith subbase materials

when the zinc phosphate cements are used. The

procedure is simplified, time is saved and the

danger of pulp irritation is further controlled. The

now available reinforced ZOE cements and especially the EBA cements, because of their increased

early strength, are capable of withstanding the

forces developed during condensation of amalgam

and those forces which may subsequently be transmitted through the restoration.

EBA cements, employed as cement bases, usually

utilize a higher powder-liquid ratio than when

employed as lutmg agents. It is good practice to

incorporate the maximum amount of powder into

the liquid consistent with a usable consistency so that the powder will be in large excess in the

hardened cement. EBA cements, because of their

greater strength, seem to be well suited as a onestep base in deep cavities for gold, silicate cement

and amalgam fillings [32] . When this cement was

])laced in 32 vital but symptomatic teeth, symptoms siibsided within two days. The cavity preparations were then completed and permanent restorations placed, leaving part of the EBA cement as a base. The teeth remained vital and non- symptomatic.

An alumina reinforced EBA cement was placed

as a base imder a series of amalgam restorations

that were packed with a calibrated spring plugger

at a pressure of 140 kg/cm^. When the teeth were

sectioned after 48 hr, the base was still intact

(fig. 3), whereas ZOE bases fractured at the

pulpal-proximal line angle [31].

ZOE-type cements have been the preferred material for use over recently injured pulps caused by

deep and extensive operating procedures. This is

especially true in the teeth of children in whom

secondai-y dentin has not yet fonned a protecting

barrier within the pulp chamber. The materials are radiopaque [54], seal a cavity better than other

restorative materials [8, 55] and thus prevent

organisms or moisture from the oral cavity gaining access to the cavity floor. This may be the rea- son for the higher percentage of negative cidtures

obtained when lining with ZOE instead of calcium

Figure 3. Section through an amalgam restoration con- densed under HO kg/cnv^ (13.8 MN/ni^) packing pres- sure against an AhOa reinforced EBA iase with a

ten-minute compressive strength of 470 kg/cm' (46.2

MN/m") [31].

452-525 0—72 8 107
hydroxide. Any residual organism probably re- mains in a latent form under sound restorations,

because the environment has been altered and

conditions for growth have become unfavorable.

These organisms could become reactivated if saliva

were to gain access to them through a leak in the

restoration. Such leakage is much less likely if a ZOE or EBA lining is used. Thus, EBA cement,

because of its excellent sealing characteristics [42]

will assist in decreasing the number of organisms

remaining in the dentin and, furthermore, will

promote remiiaeralization of decalcified dentine at

the base of the cavity [43] . These properties should

make EBA cements the material of choice, especially in indirect pulp capping procedures since

calcium hydroxide does not possess the excellent

sealing characteristics, whereas ZOE or modified

ZOE cement does not stimulate the formation of

reparative dentin to any appreciable extent.

Human pulps which had been exposed in the

course o'f normal operative procedures in five non- symptomatic teeth were capped using an EBA con- taining cement and the cavities were filled [32].

Within a week part of the material was removed

and permanent restorations placed, leaving part

of the previous cement filling as a base. None of

the patients reported any symptomatology. None

of the teeth showed any radiographic changes and

all responded normally to vitality tests within the

two to ten months observation per'iod.

Studies of the possible use of EBA cements in

root canal therapy and for gingival dressing are not available. Requirements for an improved root

canal sealer would include (1) a suitable consistency at the time of insertion into the canal, (2)

good dimensional stability to avoid fissures resiilting from shrinkage, (3) good adhesion, and (4)

high degree of insolubility to body fluids [56].

ZOE yields a fairly satisfactory hermetic seal, but

is only slightly adherent to the cavity walls. Furthermore, according to Erausquin and Munizabal,

ZOE can be irritating to the periapical tissues

[57], although the response is reduced by addition

of acrylic polymer spherules [58].

In many patients it is desirable to treat dental

restorative problems for extended periods of time

with a long-term temporary or intermediate restorative material. Such occasion arises in teeth in

highly carious mouths, particularly those of children where immediate excavation of all caries is

indicated. This treatment arrests the caries and

favorably alters the oral flora. Temporary restorations are then placed and the permanent restorations are inserted as scheduling permits. The

Armed Forces also have unique dental requirements such as emergencies at remote sites or in

combat zones that preclude the insertion erf permanent restorations. Criteria for an acceptable "intermediate" restorative include [22] : (1) a satisfactory seal between the cavity preparation and

the material, (2) biologic compatibility with the

pulp, (3) easy removal from the cavity preparation, (4) resistance to abrasion and attrition for

an extended time, (5) ease of manipulation and

placement, and (6) a longer service life than con- ventional ZOE temporary fillings.

Polymer reinforced ZOE cements because of

their greatly improved physical properties [21],

show a high degree of clinical acceptibility after a 12-month observation period [22] . On the other

hand, fused quartz [32] or titanium dioxide [22]

reinforced EBA cements, despite their improved

strength, proved unsuitable as temporary restoratives. Contrary to their low solubility and disintegration in water and dilute acids, the fused

quartz reinforced EBA restoratives disintegrated

rapidly under oral conditions [59]. Thus, a low in

vitro solubility value may be useful in comparing

the relative solubility of products of similar composition, but is not necessarily an indication of the

success of such restorations in the mouth. Comparisons have been made of the in vivo intraoral disintegration of cements using specially designed

partial dentures so that cements are exposed to the

oral environment [60]. Results of these tests showed considerable patient variation in the rate

and amount of cement lost. Abrasion played an

important role in the loss of material with the

greatest loss invariably occurring in those regions

most subject to abrasion by the tongue. Thus, the

in vivo disintegration appears to be an effect of

the interaction of solubility and abrasion.

The resin-modified alumina- reinforced EBA

cement, because of its demonstrated mechanical

and palliative properties, especially its much

higher tensile strength, appears to be very desirable for use as a long-duration temporary

restorative.

In a limited clinical study [30], approximately

50 restorations, including complex restorations

subject to heavy occlusal stresses, were placed using one formulation. Its powder component contained

58.2 percent ZnO, 27.3 percent ALOs, 5.4 percent

rosin and 9.1 percent methyl methacrylate

copolymer.

A powder-liquid ratio of 1.2 g powder per 0.2

ml of liquid was used. The material was usually

mixed on a glass slab, but could also be prepared

by mixmg in a capsule employing a mechanical

mixer. The unreacted eugenol was removed by

blotting or by squeezing the mixed mass in an

amalgam squeeze cloth. The material presented

good manipulative properties and could be readily

adapted to cavity walls and margins. Patients were

recalled and observed periodically during the ninemonth observation jDeriod. The restorative did not

dissolve or disintegrate in the oral fluids. All restorations remained serviceable and showed only minimal signs of wear over the nine-month observation period. All teeth restored with this cement

remained asymptomatic for the entire period of

observation.

The formulation selected had a relatively high

tensile strength, but the in vitro solubility and dis108
integration values were considerably higher (one

week solubility and disintegration 0.94 percent)

than those of other promising mixes. Since all

restorations remained serviceable over the ninemonth observation period, the in vivo solubility

did not appear to be of any significance. Unreacted

liquid in the clinical mixes was removed by blotting. It is conceivable that lower in vitro solubility

and disintegration values would have been obtained if this procedure had been followed in the

preparation of laboratory test specimens.

6. Direction of Future Work

The EBA cements, largely as a result of the ex- tensive studies conducted during the last few years,

have passed from their infancy to a state of incipient maturity. Further enhancement in physical

and mechanical properties of EBA and other

chelate-type cements for use as semipermanent "intermediate" restoratives would be desirable to

make use of their excellent biological properties.

However, progress in improving these materials

may not be as rapid as one has become accustomed

to during the last few years.

Basic investigations that would lead the way to

the development of improved products should include pinpointing the exact mechanism of the set- ting of EBA cements. Determination of the chelate

stability constants of potentially useful chelates as

well as measurement of their hydrolytic stability

would be most useful. Synthesis of prospective

chelating agents and evaluating of the resulting

cements would be desirable.

The existing fundamental knowledge makes it

possible to conduct development of better EBA materials along many lines. Modest improvements

may be obtainable by judicious selection of reinforcing agents such as treated or spherical glass

powders or the partial replacement of zinc oxide

by another reactive metal oxide, such as mercuric

oxide. Upgrading of formulations by whisker re- inforcement does not look promising. Incorporation of slightly soluble fluorides that leach from the

EBA cements at a controlled rate should be studied with the aim of reducing the solubility of the

components of tooth structure and thereby bestowing anticariogenic properties to these cements.

Further studies of novel chelating agents to partially or wholly replace EBA in cementitious compositions, especially those that yield a strong bond

at the tooth-restorative interface, are a potentially

fertile field for developing a greatly superior product. Even if such a breakthrough is realized,

clinical application of such compositions would be

entirely dependent on the results of histological,

pathological and clinical studies in order to prove

beyond a doubt that such a product has the desirable biological properties that have made ZOE

and EBA cements so desirable as dental restorative

materials.

Besides the enhancement of properties of EBA

cements resiilting from successful research efforts,

it is anticii^ated that a major advance in the near

future will be the availability of many new commercial products of this type, tailor-made to the

wide spectrum of specific applications needed by

the practicing dentist.

The product of the future will combine convenience with versatility. It will possess excellent

manipulative properties, will mix with ease to a predetermined, carefully controlled powder-liquid

ratio, will have a setting time that will be relatively unaffected by environmental parameters

such as temperature and humidity, will have a low

film thickness when required, and if desired, can

be color coded to indicate the stage of treatment of

the tooth.

The findings obtained on pulp capping and on

using EBA cements as sedative restorations and

bases point, however, to avenues for further clinical research. Such studies should establish more

clearly any potential advantages of EBA cements

as bases under silicate cement or metallic fillings, as pulp capping oi' cementing media and as intermediate restoratives.

7. Summary

During the last few years considerable interest

has been generated in improving zinc oxideeugenol cements. A better understanding of the

setting mechanism of these cements has become

available which has led to the development of

products Avith enhanced physical and mechanical

properties usually containing o-ethoxybenzoic

acid (EBA). These materials show the mild reac- tions to the tissues including the dental pulp similar to ZOE cements, but also stimulate the formation of reparatiA^e dentin. Physical properties of

the EBA-containing cements approach those of the

biological and physiological less desirable zinc

phosphate cements. The EBA cements have been

well accepted as luting agents for fixed restorations and as insulating bases. They appear to be

the material of choice for indirect pulp capping.

Resin modified EBA restoratives show good

stress-bearing characteristics and should find applications as an "intermediate" restorative.

8. References

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[2] Brauer, G. M., A review of zinc-oxide eugenol type

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109
[7] Massler, M., Effects of filling materials on the pulp. [29]

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[12] Gerner, M. M., Zaidorozhnyi, B. A., Ryabina, L. V., Batovskii, V. N., and Sharchilev, "V. I., Infrared rg^,

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Short, L. N., Light absorption and magnetic

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Phillips, R. W., and Love, D. R., The effect of certain

additive agents on the physical properties of zinc

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Coleman, J. M., and Kirk, E. E. J., An assessment of a modified zinc oxide-eugenol cement. Brit.

Dental J. 118, 482 (1965).

Bhaskar, S. N., Cutright, D. E., Beasley, J. D., and

Boyers, R. C, Pulpal response to four restocative

materials. Oral Surg. Oral Med. and Oral Path.

28, No. 1, 126 (1969).

J0rgensen, K. D., and Hoist, K., The relationship

between the retention of cemented veneer crowns and the crushing strengths of cements. Acta

Odont. Scand. 25, No. 4, 355 (1967).

Oldham, D. F., Swartz, M. L., and Phillips, R. W.,

Retentive properties of dental cements. J. Prosthetic Dentistry 14, 760 (1964).

Horn, H. R., The cementation of crowns and fixed

partial dentures. The Dental Clinics of North

America, pp. 65-81, (W. B. Saunders Co., Philadelphia, Pa., 1965).

Williams, J. D., Swartz, M. L., and Phillips, R. W.,

Retention of orthodontic bands as influenced by

cementing media. Angle Orthodontia 35, No. 4, 278 (1965).

Mosteller, J. H. An evaluation of intermediate base

materials. J. Am. Dental Assoc. 43, 571 (1951).

Messing, J. J., Linings and their manipulation.

Dent. Pract. 8, 336 (1958).

110
[50] Hoppenstand, D. C, and McConnell, D., Mechanical

failure of amalgam restorations with zinc phosphate and zinc oxide-eugenol cement hases. J. Dental Res. 39, 899 (1960).

[51] Lyell, J. S., Base forming materials for restora- tions of silver amalgam. Australian Dental J. 5,

132 (1960).

[52] Phillips, R. W. Cavity varnishes and base materials. The Dental Clinics of North America, p. 159, (1958).

[53] Chong, W. F., Swartz, M. K and Phillips, R. W.,

Displacement of cement bases by amalgam con- densation. J. Am. Dental Assoc. 74, No. 1, 97

(1967).

[54] Sahs, E., Radiopacity of pulp capping materials.

Northwest Dent. 46, No. 2, 113 (1967).

[55] Nichols, E., Endodontics (John Wright and Sons

Ltd. Bristol, England, 1967).

[56] Erausquin, J., and Muruzabal, M., Root canal fill- ings with zinc oxide-eugenol cement in the rat molar. Oral. Surg. 24, No. 4, 547 (1967).

[57] Erausquin, J., and Muruzabal, M., Tissue reaction

to root canal cements in the rat molar. Oral Surg.-

26, No. 2, 360 (1968).

[58] Erausquin, J., and Muruzabal, M., Periapical tissue

response to root canal cements with the addition

of acrylic polymer spherules. Oral Surg. 26, No. 4, 523 (1968).

[59] Brauer, G. M., The relationship between laboratory

tests on solubility of zinc oxide-eugenol type ce- ment and their behavior in the mouth. N.Y. J. Dent. 37, No. 4, 146 (1967).

[60] Norman, R. D., Swartz, M. L., Phillips, R. W., and

Virmani, R., A comparison of the intraoral dis- integration of three dental cements. J. Am. Dental Assoc. 78, No. 4, 777 (1969).

Ill

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Organic Adhesives

Harvey Alter and Abraham Fookson

Gillette Research Institute, Rockville, Md. 20850

The materials and means for improving joint strengths in a wet environment between

dentin or enamel and restoration or cavity liner are reviewed. Critical surface tension,

7c, a descriptive property of surfaces, offers a basis to evaluate capacity of adhesives

to wet tooth surfaces. Used as a pretreatment, an adhesive may function by displacing water. Attachment to tooth surface through hydrogen bonding may also occur. Adhesion

may be improved by use of liners, coupling agents, fillers and tooth surface treatments such

as etching processes. Dental adhesive materials include acrylics, cyanoacrylates, epoxy

resins and polyurethanes, the latter having apparently good promise as a dental adhesive

of the future.

Key words_: Critical surface tension ; dental adhesion improvement through liners, coupling

agents, fillers, and surface treatment ; dental adhesives to include acrylates, cyanoacrylates,

epoxy resins and polyurethanes ; dental materials ; polyurethane.

1 . Introduction

Perhaps this review should have been entitled

"Organic Structural Adhesives," or yet more confining, "Synthetic Organic Structural Dental Adhesives." A more confining title would not only

better indicate the scope of the discussion to follow, but also would help deliver the important

message that dental adhesives are something special; the requirements for their application and

performance probably impose greater constraints

than for any other adhesive application. Certainly,

in no other technology would the designer of an

adhesive joint want to be restricted to a medically

acceptable, low temperature and fast setting, high

strength adhesive, which at the time of application and over the long term, must be resistant to

water. Indeed, some adhesive experts, for example

in the coatings or aerospace fields, may well doubt

that such requirements could ever be met.

But certainly many of the requirements have

been met. Organic polymeric restoratives are now

used in clinical practice, and many truly adhesive

dental polymeric materials are currently under

investigation. This review is intended to cover some of the more promising approaches of research

in new adhesive systems, and to discuss means for

enhancing joint strength with these materials. Eegretfiilly, the scope of this review must be limited

and cannot include all of the pertinent published

work, the recent information on biological adhesives (such as from the barnacle) , nor delve into

differences among dental adhesives, adhesive

restoratives, and cavity liners. This review is re- stricted to materials and means for improving the

joint strength of bonds between dentin or enamel

and another member (which may be the adhesive

113

itself) , in a wet environment, no matter if the material is to be applied as a restoration, adliesive, or

cavity liner.

2. Some Surface Chemical Considerations

The function of an organic dental adhesive is

obviously to bond to the tooth substance, whether

the tooth surface be mineral, protein, or otherwise in nature. So before discussing adhesives,

per se, it is worth considering some aspects of the

chemistry of the tooth adherend surface.

Like other surfaces, tooth—either dentin or enamel—may be described in terms of its critical

surface tension, i.e., the maximum surface tension

of a liquid which will wet or form a zero contact

angle. The now familiar critical surface tension, or yc, concept has been developed by Zisman and

co-workers [1] ^ and has been related to adhesion

and abhesion [2]. The yc value of an adherend

solid then teaches what the surface tension of an

adhesive must be to wet and spread. Also, the yc values are useful for ranking solids as high or low

energy surfaces. For example, table 1 following,

from Zisman [1], ranks some polymer surfaces, a low value of yc indicating a low energy surface.

It is generally believed that metals, minerals,

and certain other inorganic solids have high energy

surfaces, i.e., most liquids will spread on them.

Apparently, this is not true for tooth surfaces;

Uy and Chang [3] measured yc of dentin and

enamel surfaces at 50 and 100 percent relative humidity (RH). Their results are given in table 2. On comparing table 2 with table 1, the yc of

tooth surfaces at 23 °C and 50 percent RH is

1 Figures In brackets Indicate the literature references at the end of this paper.
Table 1. Critical surface tensions of various polymeric

solids [1]

Polymeric solid

Polymethacrylic ester of <^'-octanol Polyhexafluoropropylene

Polytetrafluoroethylene

Polytrifluoroethylene

Poly(vinylidene fluoride)

Poly(vinyl fluoride)

Polyethylene

Polytrifluorochloroetbylene

Polystyrene

Poly (vinyl alcohol)

Poly (methyl methacrylate)-..

Poly (vinyl chloride)

Poly(vinylidene chloride)

Poly (ethylene terephthalate)

Poly(hexamethylene adipamide)

Dynes/cm.

at 20 °C.

10. 6

16. 2

18. 5 22

25

28

31

31

33

37

39

39

40

43

46

Table 2. Critical surface tensions, dynes/cm \S]

Surface 37 °C. (100 23 °C. (50

percent R.H.) percent R.H.)

Human dentin 39. 5 39-49

Human enamel 31. 5 38. 5-40

Bovine dentin 28-31 38-40. 5 Bovine enamel _ 24-49 38-42. 5

about the value of polyvinyl chloride. Looking at

the lower numbers for yc at 37 °C and 100 percent

RH, the tooth surface appears to be like that of

polyethylene or lower surface energy polymer. At

the time of publication, such conclusions were diffi- cult to understand. Although Uy and Chang may

be criticized for using many H-bonding liquids

in making the measurements, the final yc values are nonetheless an experimental result which today,

because of more recent publications by others can

be interpreted.

Other workers have examined supposedly high

energy surfaces at high humidities. Shafrin and

Zisman [4] found that the effect of water on the

spreading of organic liquids on glass was to cause an apparent change in the critical surface tension

from about 46 dynes/cm at 1 percent EH to 30

dynes/cm at 95 percent for the same glass surface.

Essentially similar results were found by Bemett

and Zisman [6] for borosilioate glass, quartz, and

sapphire surfaces (again all supposedly high

energy surfaces) and by Alter and Cook [6] for

keratin fiber surfaces. In the latter study it was

found that the yc of native and oxidized hair fibers

ranged from 34 to 25 dynes/cm from 1 percent to

95 percent R.H, respectively. The higher yc is about

what may be expected for a protein surface [6].

The lower values, 25 to 30 dynes/cm, are interpreted as measures of the critical surface tension

for wetting a condensed water layer, whether the

water layer is on glass, sapphire, keratin, or now,

tooth siurfaces. Thus, a nonmiscible, nonhydrogen

bonding adhesive (or adhesive solution) applied

to tooth substance musit wet and spread on a low

energy surface of condensed water, as measured

by Uy and Chang [3] , and as might be expected

to occur in the high humidity oral environment.

Alternatively, the adliesive may displace the

water from the surface. Such displacement is ac- complished largely by surface-chemical action,

rather than merely d.issolving away the water

layer. The mechanism of water displacement by an

organic liquid has recently been analyzed by Zisman and co-workers [7 J . It may be summarized as

follows.

The "initial spreading coefficient," Sia, of liquid

b displacing liquid a, is related to the surface and

interfacial tensions y by :

Sba =ya~ (ys + yab)

Generailly, when liquid a is water, for any homologous series of h liquids, Sm increases as the solubility in water increases and as the boiling point

decreases. A high value of Si,a is desirable. This is

summarized in figure 1, taken from reference 7. Note the high values, corresponding to effective

water displacement for some familiar liquids such

as ethyl ether and ethyl acetate.

The displacement of water from a surface is

also facilitated by having the liquid partially miscible with water. For example, in figure 2 (also

taken from reference 7), high values of initial

spreading coefficient are associated with the solubility in water for a series of alcohols. The area of

effective water displacement is clearly marked and

corresponds to compounds such as 1-butanol, 2-

metliyl-l-propanol, 1-pentanol, and 2-methyl-2-

butanol.

Tlius, in order to wet the tooth surface, the sur- face tension of the adhesive must be lower than yc, which may be as low as 24 dynes/cm, the yc of a condensed water layer. This is true only if the

60

5 50

40

S 30

20

• n-ALCOHOLS o CELLOSOLVES

• A KETONES

ACETATES o ACETOACETATES

c; ETHERS

i 40 80 120 160 200 240

BOILING POINT (°C)

280 320

Figure 1. Initial spreading coefficient versus toiling point

for various aliphatic %cater-displacing compounds [7].

114
1

1

1

1

1

1

1

1 . 1

1

1

1

1

1 io

1 1+ T — *

>— 1

1

1 EFFECTIVE WATER

{ DISPLACEMENT

^

1 POOR WATER

/ ' DISPUCEMENT

/o NONE

j

1

1

1

1 lo 1

1 1

1

1

1 • NORMAL ALCOHOLS o BRANCHED ALCOHOLS

1

1

1

' 1

1

1

1

1

1

0.01 0.1 1.0 10 100

SOLUBILITY IN WATER AT 20 °C (WEIGHT PER CENT)

Figure 2. Relation of ivator soluMlity to initial spreading

coefficient and loater displacing aMlity of simple nionohydric alcohols.

The vertical lines through points in the region of strong displacement are proportional in length to the areas of a 2-mm water

film displaced by the respective alcohols [T],

adhesive is not hydrogen bonding. Alternatively,

the adhesive (as a pretreatment for the surface)

may displace the water layer. A third alternative

is that the adhesive is hydrogen bonding, such as an epoxy or polyurethane. These types of adhesives are discussed in later sections.

3. Dental Adhesive Materials

3.1. Acrylics

Acrylic polymers, such as poly (methyl methacrylate), have long been used as restoratives [8]

and much careful study has been devoted to these

materials, particularly at the National Bureau of

Standards and by the group this symposium

honors [9, 10]. The attributes and deficiencies of

this class of materials are well known and it is

generally recognized that for all their merit in

clinical practice, they do not adhere to tooth substance. Some improvement in their adhesion can

be obtained, at least qualitatively, by appropriate

copolymerization [11].

The acrylics are usually applied as a mixture of

polymer, monomer, and polymerization catalyst.

One reason for the widespread use of acrylics is that such mixtures can polymerize in place quickly,

particularly when an accelerator is included [12].

The polymerization of such mixtures to polymer is usually accomplished by considerable

shrinkage, 6 percent or more by volume [13], an

intolerable level for adhesive bonding under any

circumstances. Means of avoiding the shrinkage

are the common pracitices of incorporating particulate filler, dissolving polymer in the monomer

before polymerization, and using the "brush"

technique.

Although the conventional aerylate dental materials do not adhere well to tooth substance, they

illustrate a general means of applying adhesive,

viz., permitting the adhesive to polymerize in place

to give monomer and polymer an opportunity to

absorb, and thus produce higher bond strengths.

The importance of polymerizing the adhesive in

place has been demonstrated at least for amine

cured epoxy adhesives [14] ; the importance of

adsorption as an early and necessary step in the

adhesion process is generally accepted.

An improved dental restorative material based

on an acrylic copolymer has been one of the major

contributions from the dental research at the National Bureau of Standards. Bowen [15-18] has

described a composite restorative system based on

acrylic copolymers and their further improvement

by attention to tooth surface preparation, mode of

filling, and adhesion of the restorative to the filler

particles. The resultant materials appear to be improved restoratives, rather than new adliesives

[19].

3.2. Cyanoacrylates

A relatively new class of acrylates, the alkyl

a-cyano-acrylates, is being investigated as a possible restorative [20]. This class of adhesives is of

interest because the monomers polymerize readily

with the aid of a weak base or water as an initiator.

Interestingly, a general precaution in applying

the cyanoacrylates as adhesives is to dry the adherend surfaces, relying on only trace water vapor

as an initiator, and to avoid acidic surfaces. Long

exposure to water, or water vapor, can have a

deleterious effect on the bond strength [21].

The polymerization of the alkyl a-cyanoacrylates is highly exothermic, resulting in a peak

polymerization temperature which may be uncomfortable in vivo. This peak temperature, and the

ability of the monomers to displace blood (and

probably water) , as well as certain of the properties of the polymerized mass, such as mechanical

strength, rate of hydrolysis, and histotoxicity,

depend on the length of the alkyl side chain [20,

22, 23]. The chemistry of this class of adhesive

has been discussed in the context of use as tissue

adhesive in surgical procedures and for hemostasis

[22, 23]. Use in dentistry has been described by

Bhaskar and Frisch [24] and by Collito [25].

3.3, Epoxy Adhesives

The use of epoxy resins as dental restoratives

was described early by Bowen [26]. Later, Lee

[27] reported additional experimental work directed at an epoxy material for clinical dental use. Also, Lee and co-workers [28, 29] have synthesized

new resins with the aim of achieving high bond

strengths. They found that a bulk restorative resin

could be made from the triglycidyl ether of tri115
hydroxybiphenyl cured with cyclohexane bis-

(methylamine) and that the cured material had a good balance of properties for a restorative material. The resin sets moderately slowly and requires

mixing on a hot plate prior to insertion to achieve

clinically feasible set times.

Lee and co-workers [28] have synthesized and

investigated many types of epoxy resins as well

as several different types of curing systems. None

seem feasible at this time for various reasons. Promising results were obtained by blending epoxy

resins with polyurethanes ; the latter are reviewed

in the next section.

The most promising of the epoxy restorative

resins, one based on the triglycidyl ether of trihydroxybiphenyl cured with cyclohexane bis-

(methylamine) , was used in limited clinical trials

[29]. This resin system, filled with alumina, maintained adhesion to human dentin in vivo when

placed in restorations prepared without undercuts for at least three months [29].

The clinical results were obtained with Epoxylite resin NIH-27. If this material must be applied

in a mamier similar to Epoxylite resin NIH-7,

theia prior to inserting the restoration the epoxy

resin and its hardener must be mixed on a temperature-controlled hot plate for 60-90 s, and this

mixing must be well controlled. Then, the finishing should not be attempted for at least 30 mm

after mixing and preferably on a subsequent visit

by the patient to the dentist [29]. No adhesion

test data are given trying to relate the laboratory

procedures with the clinical trials [28, 29].

Epoxy resins have also been used for bonding

orthodontic attachments to teeth [30]. The bond

strengths of commercial epoxy adhesives under

laboratory conditions were reported to be in excess of the forces judged to be exerted on the orthodontic attachment during treatment, and this findingwas verified in limited clinical trials. This report

[30] is brief and qualitative; it is impossible to

tell from it exactly what was done.

Although epoxy resins are useful structural adhesives for many applications, and despite the

large amount of research performed relative to

their use in dentistry, such use seems severely limited. The reason for this is that generally, high

bond strengths are obtained only after high temperature bakes and/or mixing of resin and hardener at elevated temperature. Typical curing

agents are polyfunctional amines, of questionable

utility in contact 'with tissue of any sort [31].

Even the so-called room temperature curing formulations of epoxy adhesives require several hours

to develop their maximum strength, well beyond

reasonable clinical practice times. In our laboratory we experimented briefly with epoxy dental

adhesives which would cure hard in a few minutes

at room temperature, even in the presence of water.

These formulations were catalyzed by BF3 and

its complexes, compounds not to come in contact

with epidermis, let alone other tissue, and the

work was dropped [32]. In short, epoxy resins,

for all of their attributes in adhesives technology,

do not appear to be broadly suited to dental

practice.

3.4. Polyurethanes

As used for dental adhesives, the term polyurethane refers generally to the condensation

polymer between a polyol (derived e.g. from a polyester or polyether), and a polyfunctional

isocyanate. Polyurethane formation may be summarized as in figure 3. The condensation reaction can be rapid at room

temperature and may be catalyzed to take place

in a few minutes by weak bases such as tin ootoate or tertiary amines. The isocyanates are applied

as adhesives similar to acrylics or epoxies in that

mixture of the monomers or prepolymers and the

cross-linking agent (TDI in the example in figure

3) are applied to the joint and permitted to polymerize in place.

An interesting aspect of the chemistry is that

organic isocyanates will react with water to generate CO2. (This is the basis of some urethane

foam production.) The reaction between tolylene

diisocyanate (TDI) and water, is shown in figure

4. This reaction scheme provides for excess isocyanate in a urethane adhesive formulation acting

as a Avater scavenger for water on the tooth surface. In this way, the polyurethane adhesive may

bond to a dry (or temporarily dry) surface. The

CO2 generated should be able to escape from thin

sections, such as a cavity liner. The water scavenging action and any effect it may have on dental

adhesions are speculative at this point.

HO-R-OH+OCNi

polyol

-NCO

CHj

HO-R-O-C-N + Q+ NCO

o H \y

HO - R-O-C-N+OfNCO

O H

polyol

TDI

*- HO-R-O-C-N+Ol-N-C-O-R-OH

II 1 I ^-^ I I II O H

ocN-^OJl'-|r"°~''~°"lr~';' {Of HO OH

to polymer

Figure 3. Polyurethane formation.

CHj CH3

|0)-NCO ^"aO > 0CN-(0)-N-C-0H

TDI substituted carbamic acid

C H 1 C H 1 CH3

X/ H O H \/

substituted urea

Figure 4. Behavior of TDI in the presence of water.

116
The first description of polyurethane adhesives

for dental applications was by Galligan, Schwartz

and Minor [33] who investigated adhesive liners

based on equal parts by weight of TDI and a poly (propylene glycol) of 2,000 nominal molecular weight. This composition contained a tenfold

excess of isocyanate groups; 0.01 percent tripropylamine was in some instances added as a catalyst.

The composition was tested as an adhesive liner

for conventional restorations in freshly extracted,

sound human premolars in conical cavities. (The

cavities were "dried" with a blast of air for 5 s before the liner was applied.) A summary of the

data is given in tables 3 and 4. (The reader is referred to reference 33 for the experimental

methods.) In every case, including when the ure- thane was used as a restoration, an improvement

in properties was obtained with the test composition compared to the results for the conventional

restorative materials. Noteworthy, the joint

strengths of the acrylic restorations were increased

above zero, the value for the acrylic alone. The

liners were adhesive to teeth, and to the restoraTable 3. Adhesion of restorations to teeth [33]

Restoration Strength

Compres- sionThermal

Shock

lb N Cycles Cycles

Acrylic _ - . 0. 0 0 0 1 Urethane-lined* acrylic... 8. 3 37 >400 >40

Amalgam 0. 3 1 0 1 Urethane-lined* amalgam. 8. 6 38 240 >40

Silicate.. 5. 5 24 >400 15

Zinc phosphate 5. 1 23 220 11 Urethane* . .. ._ .. 3, 0 13 >400 >40

*The urethane was the product obtained by curing a 1 :

mixture of tolylene diisocyanate and polypropylene glycol"

Table 4. Tensile strength of urethane-lined acrylic

restorations [33]

Urethane liner Tensile

strength

Iso- cya- Polyglycol

nate*

lb N

TDI Polypropylene glycol ("2010") 8. 3 37

TDI Ethylenediamine-based

polyethylene-propylene

glycol . - 9. 1 40

PAPI Polypropylene glycol ("1010") 4. 3 19

PAPI Polypropylene glycol ("2010") 13. 4 60

PAPI Polyethylene-propylene glycol . 6. 6 29

PAPI Trimethylolpropane based

polypropylene glycol.. _ . .. 8. 9 40

tions, and prevented water from entering the

restoration margin [33].

The adhesion of the polyurethane can be improved by other choices of polyol and isocyanate.

At the present time in our laboratory, we generally obtain higher bond strengths with the

polyurethane from castor oil (triglyceride of

ricinoleic acid) cross-linked with PAPI - (a polyphenylene isocyanate from Upjolin) than with

the polyurethane reported by Galligan et al. [33].

However, the joint strength can be mcreased even more by choice of filler, other additives, coupling

agents, and means of preparing the tooth surface,

than by making small changes in the choice of

reactants in the adhesive. Such means to increase

adhesion are discussed in the next section.

4. Means to Improve Adhesion

Several means have been proposed to improve

dental adhesives by bonding the adhesive chemically to the mineral or protein portion of teeth.

For example, Buonocore [34] investigated the inclusion of glycerophosphoric acid dimethacrylate

in an acrylic restorative with generally encouraging results and obtained evidence to suggest that

there may be some chemical reaction between the

components of the adhesive and the organic matter

of dentin. Similarly, Lee et al. [28] suggested

cross-linking the collagen portion of dentin with

zirconium acetate to render it hydrophobic and

thus improve bond strengths. Brauer et al. have

suggested using chelating agents [35] between the

adhesive and mineral portion of the tooth as well

as the eerie ion induced grafting of synthetic

polymers to the protein portion [36]. Other suggestions have been the use of rubbery liners, cou- pling agents (such as silanes) , proper choice of fill- ers, and acid and enzyme debridement of the prepared tooth surface to improve adhesion or bond

strengtli. These latter suggestions are reviewed in

some detail below.

4.1. Rubbery Liners

The adliesive failure of a joint means that the

joint strength was exceeded by the applied stress. In practice, this stress may be tensile, compressive,

or shear, and usually is a complex mode which

often involves a shear component. (See for ex- ample, reference 37.) Dental adhesive joints are usually tested in tension [38] and there have been

no reports of a stress analysis of any dental adhesive test. Patrick et al. [39, 40] have suggested

that tests of dental adhesive joints include a shear

'PAPI is: NCO NCO

*TDI, tolylene diisocyanate; PAPI, polymethylene

polyphenylisocyanate

Q)--C-H.2-

where n is approximately 1.

117
component, as well, and have devised a punch test for this purpose. Also, they pointed out that a

joint between high modulus dental tissue and a

I'elatively brittle (although having a 20 times

lower modulus) resin produces a mechanical situation which enhances joint failure close to the interface. Therefore, they proposed using a rubbery

liner between dental tissue and restorative, to provide a zone to absorb stress. Obviously, the rubbery

liner must be adherent to the restorative and to the

tissue.

Patrick et al. [39, 40] used a rubbery liner of

partially saponified poly(acrylonitrire-co-butyl

acrylate) and tested composite joints of bovine

dentin (or einamel) -rubbery liner-epoxy resin by

their shear-punch test. These composite joints had

an average test strength of 3415 ± 198 psi (23.6 ±

1.4 MN/m^) without the rubbei-y liner and 4300 ±

254 psi (29.7 ± 1.8 MN/m^) with the liner. In addition, tlae streng'th of the joint Avith liner was less

affected by thermocycling (4 °C, 10s; 65 °C, 10s;

for each cycle) than similar joints without liner

[40].

4.2. Coupling Agents

The term coupling agent usually refers to a compound which can form a bond between the adhesive and adherend. Frequently, such agents are

substituted silanes and are applied to the adherend

surface before applying the adliesive. Such coupling agents are frequently used in glass-polymer

composites; one of their principal functions is to

reduce the moisture sensitivity of the composite

[41]. Presumably, they will have the same function for dental restoratives. Noteworthy, coupling

agents need not be silanes, although, silanes are used most often.

The extension of coupling agent technology to

dental practice is a logical one, and many workers

have done so (e.g., references 27, 39, 40, 42) ; perhaps the earliest use was by Bowen [16, 43]. There

is little question that coupling agents, particularly

silanes, improve the adhesion of joints with dental

materials. Two examples are worth citing.

Patrick et al. [39], using the shear punch test

described earlier, demonstrated the value of a si- lane coupling agent with the rubbery liner. Some

of their data, reproduced in figure 5, illustrate the

higher bond strengths and greater resistance to

water immersion (in this case, synthetic saliva at

37 °C) foT joints prepared with silane coupling

agent, compared to those without. Each point on

this figure represents the average test result of

from 15 to 120 specimens.

In our laboratory, we have demonstrated that

improved bond strengths are obtained (tested after

3 days immersion in water) for acrylic restorations

bonded to either bovine enamel or human dentin

when a selected coupling agent is incorporated in

either the acrylic or the polyurethane liner used

in this work [44] . Some selected results are shown

in table 5. The joint strengths were measured by the butt

test of Lee et al. [38]. The polyurethane liner was

based on PAPI and casitor oil and filled with

titania. The PMMA (poly (methyl methacrylate)

was applied as preformed rod wetted with monomer, to simulate an acrylic restorative. The rationale behind the choice of coupling agent was to

chose compounds with reactive moieties which will

incorporate with both the acrylic and urethane

polymers on polymerization. The compounds were

mixed in the polyurethane before applying and

curing; their effect was to near double the joint

strength of the cured system. (The formic acid

Shear value

(psi)

5, 000

4,000

/\ Silane + liner

Liner

^3 Silane o No treatment

Expoxide restorations with hydroxylapatite filler ^^^^^

I 100 150 Consecutive hours

200

Figure 5. Effect of exposure time in 37 °C saliva on shear values as determined from model bovine restorations [39].

To convert shear value In psi to MN/m» multiply by 0.006895.

118
Table 5. Effect of coupling agents on adhesion of PMMA to formic acid treated bovine enamel [44]

Amount coupling

agent, phr* Joint strength with coupHng agents

a b c d

psi psi psi MN/m^ psi MN/m 2

0 700 4. 8 700 4. 8 700 4. 8 700 4. 8

2 1270 8. 8 1070 7. 4 1160 8. 0 1190 8. 2

5 1125 7. 8 1380 9. 5 1000 6. 9 1340 9. 2

a.2-hydroxyethyl methacrylate; b.2-hydroxypropyl methacrylate

d.l, 3-di(allyloxy)-2-hydroxymethyl propane.

*Parts per hundred parts of resin.

c . 2-t-butylaminoethyl methacrylate

treatment is explained in a later section.) Similar

results have been obtained with other coupling

agents, including silanes, and will be repoi-ted

later [M].

4.3. Fillers

Particulate fillers are usually incorporated in

an adhesive for several reasons, such as to better

match the thermal expansion coefficients of adherend and adhesive and thus improve bond performance. Bowen has shown that the strength of

dental restoratives can be improved by proper at- tention to filler particle size and distribution and,

interestingly, the importance of using a silane

coupling agent to improve the bond between filler

and resin [15]. Kelated to this, Alter has

shown [45] that thermoplastic polymers and rubbers are reinforced by certain particulate mineral

fillers, depending very much on their particle size.

Reinforced thermoplastic polymers, especially us- ing fillers pretreated with silane coupling, are

articles of commerce [46]

The effect of fillers on the adhesion of polyurethanes to dental tissue was demonstrated by

Fookson and Ellison [42] . Table 6 shows some of

their data for the large increase (as much as four-fold) in the force to remove the filled polyurethane restoratives from a conical cavity in

human dentin after 3 days water immersion. Additional data illustrating the importance of proper

choice and amount of filler in polyurethane dental

materials will be reported later [44]

4.4. Surface Preparation

In 1955 Buonocore [47] reported that a phosphoric acid and phosphomolybdate-oxalic acid

treatment of dental enamel increased the adhesion

of acrylic filling materials. More recently, Newman and Sharpe [48] showed that one effect of

such a treatment is to make the tooth surface

more wettable.

In 1956, Bowen [26] reported an improvement

in the adhesive strength and moisture resistance

of epoxy resins to dental tissue by washing the

cut surface of tooth with a solution of ammonium

triacetic acid or ethylenediamine tetra-acetic add,

which are known chelating agents for Ca ion.

Since then, several other workers have reported

the use of acids, sequestering agents, and

enzymes for the etching or debridement of

tooth surfaces prior to applying an adhesive

or restorative (for example, references 28, 34,

36, 42, 44). This appears to be a highly effective general means for improving the adhesion of dental adhesives. Also, the recent widespread use of the scanning electron microscope

(SEM) for examination of the treated dental sur- faces has provided a clear understanding of what

these debridement treatments are achieving. This is

illustrated below with examples from our own

work. The use of the SEM in studying dental

anatomy was recently reported [49].

Figure 6 shows a series of SEM photographs

tracing the appearance of bovine enamel ground

wet with successively finer grit polishing paper.

Tapt.-r 6. Increase in adhesive joint strength of PAPI-castor

oil polyurethane due to addition of fillers [42]

Filler Amount

Average

force to break

phr* lbs N

None _ . _ . 7 31

ZnO, USP 50 12 53

ZnO, USP 100 19 84

Ti02, reagent grade 50 18 80

TiOj, reagent grade - 100 25 111

Super Floss " _ _ _ 50 18 80

Snow Floss ' - . 37 27 120

Adsorption alumina _ . . 50 31 138

Zr02, electronic grade - 100 10 44

Zr02, electronic grade - - 150 9 40

Asbestos, crude - - 12K 14 62

Asbestos, crude _ _ 25 15 67

» A calcined diatomaceous earth—Johns-ManviUe Company, Incorporated.

i> An uncalcined diatomaceous earth—Johns-Manvllle Company, Incor- porated. •Parts per hundred parts of resin.

119
120
and finally with y-alumina (Buhler), a very fine

particle size polishing compound. Although this

successive finer and finer polishing removes much

of the debris from cutting, and although the specimen in the final photo appeared to be highly

polished to the eye, the final specimen can still be considered rough and the enamel anatomy is not visible. Figure 7 shows the effect of treating

the polished enamel with dilute acids for a brief

time. The final polishing debris is removed and the

rod structure of the enamel is clearly evident.

Figure 8 shows a similar effect on dentin where

the sample polished with y-alumina shows a rough

surface and it is difficult to distinguish the farailar

tubular structure of the dentin. Figure 9 shows

that a brief etch with dilute acid again removed

the debris and the tubular structure is clearly evident. Similar results have been obtained with

human tooth tissue. It is possible to etch too far, as shown in figure 10. Here, a good deal of the sur- face tooth tissue was removed, exposing the delicate tubular labyrinth structure. Suoh a surface,

as an adherend, could have la weak boundary layer

(there is insufficient mineral to support the bond)

and the joint shows a low strength.

Surfaces of the types shown in the SEM photographs were used to make adhesive joints with a

polyurethane adhesive and were tested by the butt

joint method. Some selected results are given in

tables 7 and 8. Work currently in progress in our laboratory

extends these observations. There is every indication that bond strengths of polyurethane adhesives

to dental tissue can be greatly increased by short

Figure 7. Bovine enamel etched with 50 percent aqueous

formic acid for one minute {Magnification: 3000X).

treatment with dilute acids and other agents, some

undoubtedly physiologically acceptable.

5. Some Directions for the Future

Some of the state of the art reviewed here leads

to possible future directions for research in organic

dental adhesives. So, we may gaze into a very

Figures. Bovine, dentin polished with y-alumina (Magnification: 3000X).

Figure 9. Bovine dentin treated with 50 percent aqueous

formic acid for one minute {Magnification: 3000X).

121
Figure 10. Excessively etched 'bovine dentin {Magnification: 3000X).

Table 7. Adhesion of filled PAPI-castor oil resin to untreated and formic acid etched bovine tooth tissue

Surface Treatment Joint strength

Dentin.. None ..

psi

173

228

373

>> 705

MNjnfi

1. 2

1. 6

2. 6

4. 9

Enamel.

Formic acid " None - . . Formic acid °.

" 50 percent acid for one minute, then wash.

Cohesive failure.

Table 8. Adhesion of filled PAPI-castor oil resin to citric acid treated bovine enamel

Treatment time

Joint strength

25% aq. acid 50% aq. acid

minutes psi psi

0 410 2. 8 410 2. 8

940 6. 5 1420 9. 8

1 1230 8. 5 1360 9. 4

5 970 6. 7 1410 9. 7

cloudy crystal ball and speculate what an ideal

adhesive system for clinical practice might be some

years from now.

In preparing the restoration, no undercutting

will be necessary, the freshly cut tooth isurface will

be etched with a dilute acid (such as citric) and/or

maybe an enzyme or sequestrant. The etched sur- face will be dried, maybe by surface-chemical displacement, and then a coupling agent, most likely a silane, applied. Alternatively, the coupling agent

will be incorporated in the adhesive.

Today, it appears that the most likely adhesive

to be used will be a polyurethane. The reason for

this is that the polyurethanes have the right bala,nce of properties: low toxicity (judged from the

literature) , controlled and short cure times within

clinical practice, and apparently adequate mechanical and adhesive strength. The polyurethane of

the future will probably be one based on PAPI,

which is the isocyanate of least toxicity [50], so

far, and the polyol will be derived from a polyether, because polyester derived ones have poor

long-term hydrolytic stability in other applications [51]. The polyurethanes we have been using can be cured in clinically feasible time®, controlled

from 5 to 20 min.

The polyurethane will be filled and the filler may

well be pretreated with a silane coupling agent. In

addition, the particle size and size distribution of

the filler will be carefully controlled.

The filled polyurethane may well -be the restorative itself, although, this seems unnecessary.

There now exist many good restorative materials;

it is just that they are not adhesive. We have found

that polyurethanes are adhesive not only to dental

tissue, but also to acrylics (especially with the

right coupling agent) and, in preliminary work,

to dental amalgam.

Much research has yet to be done before this

type of dental restoration can be clinically tested.

Indeed, much of the above speculation is based on

the assumption that in vitro adhesive j,oint testing

is translatable to the in vivo situation. This as- sumption will be tested soon in our laboratory in

animals. Hopefully, other laboratories will do

similar work in the near future.

The work reviewed here, the new results re- ported from our own research, and indeed the

speculation as to the future, are all in large part

possible because of the pioneering and active dental research program carried out these past 50

years at the National Bureau of Standards. Our

thanks to this group, for making it pos'sible, and

best wishes for continued contributions to dental

research.

6. References

[1] Zisman, W. A., Advan. Chem. Ser., 43, 1 (1964).

[2] Zisman, W. A., NRL Report 5699, November 29,

(1961).

[3] Uy, K. C, and Chang, R., Adhesive Restorative

Materials—II, Proc. 2nd Workshop, Bio-materials

Res. Advisory Committee, Nat. Inst. Dental Res.,

(1965).

[4] Shafrin, E. G., and Zisman, W. A., J. Am. Ceramic

Soc, 50, 478 (1967).

122
[5] Bernett, M. K. and Zisman, W. A., J. Colloid and [29

Interf. Sci., 29, 413 (1969).

[6] Alter H., and Cook, H., J. Colloid and Interf. Sci., 29.439 (1969). [30

[7] Baker, H. R., Leach, P. B., Singleterry, C. R., and

Zisman, W. A., Ind. and Eng. Chem., 59 (No. 6), [31

29 (1967).

[8] Skinner, E. W. and Phillips, R. W., The Science of [32

Dental Materials, Chps. 11 and 14 (W. B. Saun- [33

ders Co., Philadelphia, 6th ed., 1967).

[9] PafEenbarger, G. C, Nelson, R. J., and Sweeney, [34

W. T., J. Am. Dental Assoc. 47, 516 (1953).

[10] Coy, H. D., J. Am. Dental Assoc. 47, 532 (1953).

[11] Schwartz, A. M., and Galligan, J. D., Adhesive [35

Restorative Materials—II, Proc. 2nd Workshop

of the Bio-materials Research Advisory Committee, Nat. Inst. Dental Res., (1965). [36

[12] Brauer, G. M., Davenport, R. M., and Hansen,

W. C, Modern Plastics, 34 (No. 8), 153 (1956) ; [37

Brauer, G. M. and Burns, F. R., J. Poly. Sci., 19,

311 (1956). [38

[13] Brauer, G. M., J. Am. Dental Assoc. 72, 1151 (1966)

See also reference 8, p. 225. [39

[14] Alter, H. and SoUer, W., Ind. Eng. Chem., 50, 922

(1958). [40

[15] Bowen, R. L., J. Am. Dental Assoc. 69, 481 (1964).

[16] Bowen, R. L., J. Am. Dental Assoc. 74, 439 (1967).

[17] Bowen, R. L., J. Dental Res., 44, 895, 903, 906

(1965). [41

[18] Bowen, R. L., U.S. Patents 3,194,783 and 3,194,784

(1965).

[19] Lee, H. L. and Swartz, M. L., J. Dental Res., 49, [42

149 (1970).

[20] Civjan, S., Margetis, P. M., and Reddick, R. L., [43

J. Dental Res., 48, 536 (1969).

[21] Coover, H. W., Jr., Ch. 31 in Handbook of Adhesives,

I. Skeist, ed., (Reinhold Pub. Co., New York,

1962). [44

[22] Lehman, R. A., Hayes, G. J. and Leonard, F., Arch.

Surg., 93, 441 (1966).

[23] Woodward, S. C, Herrmann, J. B., Cameron, J. L., [45

Brandes G., Pulaski, E. J., and Leonard, F., Ann. [46

Surg. 162, 113 (1965).

[24] Bhaskar, S. N. and Frisch, J., J. Am. Dental Assoc. [47

77, 831 (1968). [48

[25] CoUito, M. B., U.S. Patent 8,250,002 (1966).

[26] Bowen, R. L., J. Dental Res., 35, 360 (1956). [49

[27] Lee, H. L., Adhesive Restorative Materials—II, Proceedings 2nd Workshop, Bio-materials Res. [50

Advisory Committee, Nat. Inst. Dental Res.

(1965).

[28] Lee, H. L., Cupples, A. L. and Swartz, M. L., Annual

Comprehensive Report No. 68-140 to Nat. Inst. [51]

Dental Res., June (1968).

Lee, H. L., Cupples, A. L., and Swartz, M. L., Comprehensive Report No. 67-112 to Nat. Inst. Dental

Res., June (1967).

Retief, O. H., and Dreyer, D. J., J. Dent. Assoc. South Africa, 22, 338 (1967).

Skeist, I., Ch. 25 in Handbook of Adhesives, I. Skeist, ed., (Reinhold Pub. Co., New York, 1962).

Alter, H., Unpublished.

Galligan, J. D., Schwartz, A. M., and Minor, F. W.,

J. Dental Res., 47, 629 (1968).

Buonoeore, M. J., Adhesive Restorative Dental Materials—I, Proc. 1st Workshop, Bio-Materials Res. Advisory Committee, Nat. Inst. Dental Res. 1961. Driessens, F. C. M., Brauer, G. M., and Termini,

D. J., lADR Program of Abstracts and Papers,

No. 137 (1969).

Brauer, G. M., and Termini, D. J., lADR Program of

Abstracts and Papers, No. 404 (1970).

Patrick, R. L., ed., Treatise on Adhesion and Adhesives, Vol. 1 (M. Dekker, New York, 1967).

Lee, H. L., Swartz, M. L., and Culp, G., J. Dental

Res., 48, 211 (1969).

Patrick, R. L., Kaplan, C. M. and Beaver, E. R., J. Dental Res., 47, 12 (1968).

Patrick, R. L., and Kaplan, C. M., Adhesive Restorative Materials—II, Proc. 2nd Workshop, Biomaterials Res. Advisory Committee, Nat. Inst. Dental Res., (1965).

Brelant, S., in Treatise on Adhesion and Adhesives,

Vol. 2, R. L. Patrick, ed., (M. Dekker, New York,

1969).

Fookson, A., and Ellison, A. H., lADR Program of

Abstracts and Papers No. 136 (1969).

Bowen, R. L., Adhesive Restorative Dental Materials—I, Proc. 1st Workshop, Bio-materials Res. Advisory Committee, Nat. Inst. Dental Res.,

(1961).

Ellison, A. H., and Fookson, A., Annual Comprehensive Report to Nat. Inst. Dental Res. for 1968-

1969.

Alter, H., J. Applied Poly. Sci., 9, 1525 (1965).

Sterman, S., and Marsden, J. G., Modern Plastics,

43, (No. 7), 133 (July, 1966).

Buonoeore, M. G., J. Dental Res., 34, 849 (1955).

Newman, G. V., and Sharpe, L. H., J. New Jersey

State Dental Soc, 37, 289 (1966).

Hoffman, S., McEwan, W. S., and Drew, C. M., J. Dental Res., 48, 242 (1969).

Anon., Tech. Bulletin 105, Engineering, Mechanical

Control and Toxicological Considerations

Urethanes, The Upjohn Co., Kalamazoo, Mich.

(July, 1968).

Gahimer, F. H. and Nieski, F. W., Insulation, p. 89,

August, (1968).

452-525 0—72 9 123
I
V. Mechanical Behavior
1
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseiarch, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Viscoelastic Behavior

Philip L. Oglesby

Dental Research Section, Institute for Materials Research, National Bureau of Standards,

Washington, D.C. 20234

Since the mechanical responses of many dental materials are functions of time as well as of applied stress, viscoelastic theory, and experimental methods should be used in

describing and characterizing these materials. Both static and dynamic methods may be

used. Viscoelastic theory useful for characterization of dental materials and for interrelation of responses under different types of static and dynamic tests includes the Boltzmann

superposition principle for linear materials and as modified and extended for non-linear

materials, the time-temperature superposition principle and approximation methods for

calculation of relaxation and retardation spectra. Methods which have been applied to dental materials such as amalgam, polymeric restorative materials, elastic impression

materials and natural tooth structure include creep, stress relaxation, indentation and forced

vibration methods.

Key words : Amalgam, dental ; creep ; dental materials ; denture base reins ; indentation

test method ; mechanical properties, dental materials ; rocking beam oscillator ; stress relaxation ; torsion pendulum ; viscoelastic methods.

1 . Introduction

Since many restorative as well as natural dental

materials have deformation characteristics which

are time-dependent as well as stress- dependent,

viscoelastic theory and methods olfer the most

advantageous means for description and characterization of their mechanical behavior. However,

even though well established procedures for viscoelastic characterization are available, their application to dental materials has not been extensive.

To encourage further work in this area, theory and

methods described in various publications (refer- ences below) are brought together in this paper

and discussed with particular attention to their

potential utilization in dental research. Several

examples of the application of such methods to

dental materials are described.

The viscoelastic description has numerous advantages: (1) it enables researchers to describe

analytically and to predict the mechanical behavior of these materials both in laboratory ex- periments and in the mouth; (2) it enables one to

separate and quantitatively describe the relative

contribution of the various time-dependent and

non-time-dependent mechanical responses occur- ring simultaneously in a material ; (3) it furnishes

a unified theory whereby the mechanical behavior

of materials with time-dependent responses can

be compared with the behavior of materials with

non-time-dependent responses used for the same

purpose, such as the different types of anterior

restorative materials; (4) it enables one to compare mechanical response of a material under one

test condition to that under another test condition in a quantitative manner by appropriate

transformation equations; (5) it enables one to

select a test or group of mechanical tests that will

fully measure the various mechanical phenomena

occurring in the material and evaluate the mechanical parameters of each; and (6) it isolates

individual mechanical phenomena and their parameters at the macroscale level and together

with microviscoelastic theory, permits better description of the relation of mechanical response

to microstructure.

Both static and dynamic test methods have been

used to investigate the viscoelastic properties of

dental materials. Commonly employed static tests are classified as : (1) a constant stress test, such as a creep test, (2) a constant strain test, such as a

stress relaxation test, or, (3) a test where the

stress and strain are both varied slowly and in

some cases cycled ; ^ an example is the classical

stress-strain test. All of the above types of static

testing have been employed in dental research and

testing. Dynamic test methods may be generally

classified as follows : ( 1

) free vibrational methods,

(2) forced vibrational methods, both resonance and nonresonance, and (3) propagation methods

using either pulses or continuous waves. In many

cases, the same dynamic apparatus may be used

to determine the viscoelastic properties of a material by two or more of the above methods. Whether

the method be static or dynamic, the following factors should be considered when testing a material

(1) The specific mechanical characteristics

of the material to be investigated

1 It becomes difficult to make a distinction between static stress-strain testing and dynamic testing when cycling occurs.

127
(2) hoAv this information on the material

may be interrelated to that obtained

for the material using other methods

and

(3) how the mechanical phenomena and

their parameters obtained for the

material may be directly or indirectly

related to the microstructure of the

material.

2. Creep of Linear Viscoelastic Materials

The creep test as a method of investigation of the

viscoelasticity of a material has the advantage of

simple instrumentation, and is preferred for the

long testing time required for those materials having retardation times that extend over a long time

scale, but has the disadvantage of insensitivity to

the retardation behavior of the material in the

initial short portion of the experimental time scale.

A creep test is normally conducted on the material

in the form of a specimen having a uniform cross

sectional area. A constant load is applied either in

tension, compression, or shear, and the defonnation

is measured in the direction of load application as a function of time. The deformation, detected by

such devices as a cathetometer, strain gage, or

differential transformer, may be measured as a

function of time for periods of less than one

second to many years, if necessary. Creep curves

for the material are obtained for different stresses,

and then deformation behavior of the material as a function of time and stress is extracted from the

family of creep curves. The creep curve of a strain-hardened specimen

consists commonly of one or more of three

phenomena, each of which may or may not be a

linear function of the applied stress; while as a function of time, one is independent, one linear,

and one nonlinear. The three deformation or strain

phenomena are:

(1) Instantaneous elastic strain «/, described

by analogy to a spring having a compliance f/o, or a modulus Gq, where

Jq=\IGq,^

(2) viscous strain which may be described

as analogous to a dashpot when linear,

having a coefficient of viscosity r? equal

to the applied stress divided by the

strain rate, and

(3) retarded elastic strain usually described by analogy to the so-called

Voigt element or series of Voigt elements, the components of which

consist of a spring of compliance Jr,

or modulus Gr in parallel \vith a dashpot having a coefficient of viscosity

f]R, where the retardation time r of

the Voigt element is defined as

r=r)iiJR.

If a material exhibits a linear combination of all three types of strain, €=ej+€r+er, it may be

represented by a spring of compliance Jo in)-

stantaneous behavior) and a dashpot having a

coefficient of viscosity r? (viscous behavior) in

series with one or more Voigt elements (retarded

elastic strain). The graphical behavior of each of

the three types of strain and their combination,

along with its accompanying model and corre- sponding analytical strain function, may be seen

in figure 1 where the strain is assumed to be a

linear function of the stress a. When the strain is a linear function of the stress, the data obtained

at different stresses may be reduced to a single

creep curve by plotting the creep compliance

[J{t) = (.{t)la^ as a function of time or logarithmic

time. The creep compliance J{t) of the combination of the three types of phenomena versus time

and the logarithm of time may be seen in figure 2. When the retarded elastic creep compliance has a continuous distribution of retardation times Tr, the

combination creep compliance J{t)=Ji-{-J^-\-jR

may be described by the following analytical

equation

Jit) =Jo+t/v+ r J{T)[l-e-"^]dT (1)

as shown graphically in figure 2a. Substitution of

L{t)/t for J{t) in eq (1), gives:

(2a)

Time t

(a) Instantaneous Strain

(c) Retarded Elastic Strain

e o£ J„ (1-e "^1)

r 1=1 Bi

Time t ^t = Time of " Stress Removal

(d) Combination of Three Types of Strain

E = Ej + + = J^o + (oe)/n+ oJjd-e"''"')

" E = J o + (ot)/n+ oS J„ (1 - e"'''i)

o 1=1 Ri

" J„(r)(l-e ''^l)dT E = J o + (ct)/n+aS'° J-(t)(1 - e'''''')dT

L O K o Ok

FiGUKE 1.

128
Ln t (b)

FiGUKB 2.

The subtraction of tj-q from both sides of eq

(2a) results in

J{t)-tly)=Jo+ L{r)[\-e-"^]d hi T

(2b)

where

J{t) is the creep compUance

Jo is the instantaneous elastic compliance

tjr) is the viscous response where t is the time after

application of the stress, a, and -q is the coefficient

of viscosity

Jo

are analytical forms of the retarded elastic re- sponse with J{t) and Z(r) being forms of the

retardation spectrum of the material.

The graphical representation of eqs (2a) and

(2b) is seen in figure 2b. It might be mentioned

that J(t) —t/i] represents the elastic portion (instantaneous plus retarded) of the creep compliance

curve in a constant stress experiment and also

represents, after appropriate transformation, the

recovery portion of the curve in figure 2 a when

the stress has been removed. Plots of creep compliance J{t) or {J{t) —t/n) versus In t rather than

versus t for a linear viscoelastic material, have the

advantages of more direct utilization—(1) in calculating the retardation spectrum Z(r) by various

approximation methods as well as giving a better

visual picture of the distribution of retardation

times Tr for the material, and (2) in application of

the time-temperature superposition principle [1,

2, 3]^ to a linear viscoelastic material to obtain its

creep compliance behavior at a given temperature

that othermse would require experimental data

over many decades at that temperature. Therefore, a creep curve may be obtained at a specific

temperature T for times outside the range of

practical observation at this temperature by obtaining creep curves at higher and lower temperatures within the time scale of the creep experiment, and then shifting the higher and lower

temperature creep curves for the linear material

along the axis of the logarithmic time scale until

they all join into a continuous master curve for

the temperature T. Before the experimental creep curves can be shifted to make the master curve at

temperature T, the comphance values should in

theory have a density (or specific volume) correction, but, in practice, the density changes with

temperature are often small enough to be

neglected. The amount the curves are shifted

along the In time axis is described by the equation

(In ^—In to)=\n t/to=ln At,

where the shift is said to be positive when the curve is shifted to shorter times (to the left on the

In time scale) in forming the master curve. The

time-temperature superposition principle is applicable not only to creep data, but also to stress

relaxation data and to dynamic ^ mechanical

testing data. The At values obtained from the

shift on the In time scale, or In l/w (reciprocal

frequency) scale in the case of dynamic measurement, for these different methods of testing are equivalent. The theory and application of the timetemperature superposition principle to a linear

viscoelastic material, as well as the theoretical

significance of At for the material, will be discussed in a later part of this paper.

3. Stress Relaxation of Linear Viscoelastic Materials

Stress relaxation behavior is observed in vis

coelastic materials; that is, the stress in the

material relaxes or decreases with time when the

material is deformed quickly and the deformation

is held constant. Stress-relaxation tests require

slightly more complex instrumentation than creep

tests in that, in addition to requiring a device for

detecting deformation, a load measuring device is required in order to follow the force change with

time. Like the creep test, the stress-relaxation

test is the preferred method for the long test time

required for some materials, but has the disadvantage of the lack of sensitivity to relaxation

2 Figures in bracl^ets indicate the literature references at the end of this paper.

129
behavior exhibited by the material in the initial

short portion of the time scale. Stress-relaxation

data are generally more directly interpreted in

terms of viscoelastic theory than are creep data.

A tensile stress-relaxation device often consists

simply of two clamps between which the specimen

is attached ; the upper clamp is usually attached to a load detecting cell which is rigidly attached to a frame; the lower clamp can be adjusted up or down in respect to the fixed upper clamp to obtain

various deformations in the specimen. Once the

lower clamp has been adjusted to obtain the

desired deformation in the specimen, the clamp is held fixed in respect to the frame. The amount of

deformation in the specimen can be detected by

means of a strain gage, differential transformer

or cathetometer.

The stress-relaxation curves are obtained at

different deformation levels, thus the stress-relaxation as a function of time and deformation is obtained for the material from the family of stress- relaxation curves at different deformations. The

stress is plotted as a function of linear time or of

logarithmic time. If stress is plotted as function

of strain for a common time value from the

family of curves, the stress-strain curves for the

fixed time values will yield the functional relationship between the stress and the strain. With

materials exhibiting a linear stress-strain relationship, dividing the stress values for each curve by

the corresponding fixed strain value gives relaxation modulus G{t) = a{t)/€o and the family of stress- relaxation curves reduces to a single master

modulus relaxation curve. If the material is linear in its stress-strain behavior at a fixed time,

the usual curves plotted are relaxation modulus

curves either as a function of linear time or logarithmic time.

The simplest analog to describe stress-relaxation behavior is the Maxwell model which is a

series combination of a spring of modulus Go and

a linear dashpot having a coefficient of viscosity

17 which has the differential equation of motion:

de (7 1 d(T

Tt^v G'o dt'

(3)

In the normal stress relaxation test as mentioned

previously, the strain is held constant after initial

rapid deformation. Hence, ^ becomes zero and

the solution is:

Got (3a)

A material that can be described by a Maxwell

model is said to have a single relaxation time

defined as r=?j/(?o. Such a material is shown

graphically by a plot of stress as a function of

linear and logarithmic time and also by a plot

of relaxation modulus (stiffness) G{t) = a{t)/e as a function of logarithmic time in figure 3. As

HAXHELL MODEL ' -"MW 3}"^)

G

o

-t/T o(t) = o e

G(t) = o(t)/e = G -t/T

(c)

Figure 3.

previously mentioned, there is the lack of initial

sensitivity to stress-relaxation behavior of certain

materials as a consequence of the finite time

required for application of the fixed strain (rather

than idealized instantaneous application of the

strain). This lack of initial stress-relaxation

sensitivity can be discussed in terms of a material

which can be represented by a single Maxwell

element. If such a material has a short relaxation

time, T, or expressed another way, the stressrelaxation time scale is short compared with the

time required for application of the strain, the

instantaneous maximum stress and corresponding

limiting modulus Go=(Jojt are indeterminate. The

result of this can be seen for a simple Maxwell

material represented by the dotted line in figure

3(a).

Most materials, including dental materials, can- not be characterized by a single Maxwell model

having a single relaxation time. These materials

have more than one relaxation time and, in many

cases, so numerous are the relaxation times that

they can be treated as continuous in their distribution, the continuous function H{t) being called

the relaxation spectrum. When the relaxation modulus G{t) for a linear viscoelastic material is plotted

against logarithmic time (over an extended time

range), most often a decreasing sigmoidal shape

curve is obtained, as illustrated in figure 4. In

figure 4, the limiting short time value of the relaxation modulus is called the glass modulus Gq, while

the value of G{t) at infinite times approaches an

130
equilibrium modulus value or zero, depending

on the microstructure of the material. The differ- ence between the glass modulus Go and the equilibrium modulus Ge is defined as the decay modulus

Gr= Gq— Ge. For example, in the case of amorphous

polymer, the value of Ge would depend on whether

the polymer was crosslinked or not. The cross- linked polymer would exhibit a value G^ at infinite

time, but the noncrosslinked would approach zero

at infinite time. Commonly, even though Ge=0

for noncrosslinked polymers, an intermediate plateau is observed, the length of which is molecular

weight dependent. Since most materials must be

described by a finite number of relaxation times

or a continuous relaxation spectrum, the modulusrelaxation behavior shown in figure 4 may be described by one of the following equations. In the

case of a finite number of relaxation times, the

relaxation modulus equation takes the form

G{t) =^G,e-'^+Ge- (4)

In the case of continuous distribution of relaxation

times, the relaxation modulus equation takes the

form for a linear material

G{t)=j^^ GiT)e-"^dT+Ge (5)

or, upon substituting G{t)^H(t)/t, the equation

becomes

G{t)=p°' H{T)e-"^d In r+Ge- (6)

The monotonic decrease of G{t) from Go to Ge, as shown in figure 4, can be described by eq (6).

It is noted that the sigmoidal relaxation modulus

G{t) curve in figure 4 roughly approximates the

mirror image of the creep compliance J{t) plot

shown in figure 2, both plots being on a logarithmic

time scale. As recalled from earlier discussion, the

glass modulus and glass compliances are related

as G0=1/Jo- Also, the equilibrium modulus

and steady-state compliance Je are reciprocally

related. However, the relaxation modulus G{t)

and the creep compliance function J{t) are reciprocally related only at the limiting values,

and, therefore, their curves on a logarithmic time

scale are not true mirror images for a given linear

viscoelastic material. It has been shown by

Gross [4], and more specifically by Leaderman [5],

that the two functions G{t) and J{t) are related

by a reciprocal relationship between their respective Laplace transforms

pL[J{t)]=l/pL[G{t)] (7)

where the Laplace transform is defined as follows

L[f{t)]= j\-^'Mdt

While equation (7) relates the two functions

J{t) and G{t), there is difficulty in obtaining one

function from the other by means of equation (7)

due to the obstacles involved in the inversion of

the Laplace transform. More often the functions

G{t) or J{t) are given as empuically determined

data and numerical inversion is required. One of

the most severe drawbacks in using the above

equation is that usually one function has been

determined only over a limited time scale and the

entire time scale is requked to make the inversion.

The logarithmic time plot for relaxation modulus

G{t) illustrated in figure 4 has the same advantages

of direct use as in the case of creep compliance

versus In t: (1) in determining the relaxation

spectrum iJ(r) by approximation methods and

giving a qualitative visual picture of the distribution of relaxation times r^, and (2) in applying

the time temperature superposition principle to

the relaxation modulus data for a linear material.

Again, as in the case of creep compliance data, if the relaxation modulus G{t) data are obtained at

various temperatures above and below some

reference temperature T, these higher and lower

temperature modulus curves may be shifted along

the In time axis until they form a master curve at

the reference temperature T. The amount of

shift again being described by (In t — In tg)

=ln tlto=\n At, where the values obtained by

the shifts of the relaxation data are equivalent

to those obtained from the shift of the creep

compliance data on the same linear material^

this result is encompassed in the theory of the

time-temperature superposition principle.

4. Boltzmann Superposition Principle—Relation to Stress- Strain

Behavior

According to Boltzmann's superposition principle, if a series of stresses have been applied to a specimen at various times, the deformation at

any subsequent time is simply the summation

of the deformations which would have been

observed at that time if each of the stresses had

been applied independently.

131
Recalling the definition of creep compliance,

J{t) —6(t)/cr discussed earlier, it has been noted

that in a linear viscoelastic material, when a

single stress <x is applied at time 0 to a specimen

which has no previous stress history, the strain

in the specimen at any time t is related to the

applied stress by the following relationship

e{t)=aJ{t-e). (8)

Next, consider a series of stress increments o-j applied at times df prior to the time t; now the

strain e(t) at time t would be related to the stress

increments o-< by the following equation:

e{t) =-^a,J{t-e,) (9)

CO

Thus, the above finite relationship relates the

strain e(i) at time t to all the previous stress

history by way of the creep compliance J. The

above equations are finite forms of the Boltzmann

superposition principle. These equations are useful

in describing step-function experiments. For ex- ample, consider the creep experiment described by

figure 2(a) where the stress has been introduced

and removed in a stepwise manner and the stress a was maintained over a sufficient time for a steady state condition to be reached. If the time

of removal of the stress c is considered <=0, then

the stress a was applied at a time — Since

t=0 at load removal, the strain at the instant

before load removal is designated e(0) and is related to its elastic, retarded elastic and viscous

compliances as follows:

eiO)=<r{Jo+jR+tJv) (10)

At a time t after stress removal (stress increment

of —a) as shown in figure 2(a), the strain e{t) is related to the two incremental stresses a and — a as follows:

e{t)=a^Jo+J^+^-^^'j-<rJit). (11)

Now, substituting e(0) for (t(Jo+«/r+W'7)

e{0)-e{t)=a[J{t)-tM (12)

which is the transformation mentioned earlier for

the unloaded portion of the curve.

If the stress increments are not introduced in

steps, but in a continuous manner with time, the

strain function becomes a strain integral in accord- ance with the superposition principle taking the

following form:

A<r^O 0 Jo

(13)

or by the use of the chain rule and where the entire

stress history from e^—co is considered, the

above equation takes the following form:

The Boltzmann superposition principle in the

form of eqs (13) and (14) specifies the transformation relationship (for a linear viscoelastic material) between the creep experiment by way of the

creep compliance, and the stress-strain experiment

where the stress-time relation is prescribed, on the

same material. Similarly, the Boltzmann principle can be applied to the determination of stress as a

function of stepwise introduction of strain increments leading to the equation:

a{t)=i:,G{t-e,)e, (15)

CO

where G is the relaxation modulus defined earlier,

thus relating the stress to prior strain history.

When the strain increments are introduced in a continuous manner, the above equation becomes

a stress integral as follows:

<j{t)=^[^G{t-e)^-^de (16)

relating the total stress to the strain history.

The superposition eq (14) is the transformation

equation between the creep function obtained in a creep experiment on a linear viscoelastic material

and the strain function (strain integral) in a

stress-strain experiment on that same material,

where the stress is known as a point by point function of experimental time. Also, the superposition

eq (16) is the transformation equation between a

stress relaxation experiment and a stress-strain

experiment on a given material where the strain

is known in a point by point relation to experimental time. For the case of a constant stress rate,

^^^^=0- =a constant^, substitution of the continuous spectrum form of the creep function given

by eq (2 a),

J{t)=^^'" L{r)[l-e-''^]d In r + tln+J,

in eq (14) followed by differentiation results in

^=i^=J(0. (17) ad (J at

Thus, the slope of the strain-stress curve in a constant stress rate experiment is the creep function

J{i) for a linear material where the time is given

by t =(7/o- since a =a constant. Looking at the con132
stant strain rate case for a stress-strain experiment,

by substituting eq (6) into the superposition eq

(16), and assuming

de

e (a constant) , one obtains the result

-{£: TH{T){l-e-'i')d hi r+Get](18)

which relates the stress integral (j{t) to the relaxation spectrum H{r) and thus to the relaxation

modulus function of the relaxation experiment in a simple manner since for the constant stress rate

experiment, <=e/e; thus stress versus strain is equivalent to a stress-time plot.

5. The Spherical Indentation Test

The problem of the contact of a smooth rigid

sphere of radius R against an initially plane surface of a linear viscoelastic material was analyzed

by Lee and Radok [6]. Their analysis has been the

basis of further study and generalizations by other

workers [7]. The solution developed by Lee and

Radok is

X de

16

where

J{t)

R

m

a{t)

the total force as a function of time or the loading history

the shear creep compliance for the linear

material

the radius of the indenter

the radius of indentation contact

the depth of penetration of the sphere

into the plane surface.

If the total force is applied in the form of instantaneous constant force Po, then P{t) = PoH{t)

where H{t) is the Heaviside step function [H{t) = 0

for t <^ 0, Hit) = 1 for i > 0] and the above solution takes the form in. terms of the shear

compliance as follows:

Jit) =

16 [mf _

16ffl^^[a(0F

3RPoH{t) 3PoH{t)

Using this equation J{t) in. theory could be determined by measuring the central indentation depth

a{t) as a function of time. Practically, however,

this equation has application only when the period

of load application is short compared with the

experimental time scale needed to evaluate J{t)

completely.

The first group to recognize the application of

he spherical indenter as a viscoelastic test for

various dental materials was W. T. Sweeney and

co-workers [8, 9].

6. Dynamic Test Methods

Often a type of test is used where the system

in which the material is a component is excited

into vibration. The stress and strain in the

material vary sinusoidally with time as opposed

to the creep or stress relaxation tests which

are essentially step-function experiments. Such

vibratory tests measure the dynamic response of a material. As in the earlier discussion on creep

and relaxation, this discussion will be restricted

to linear viscoelastic materials where the strain

amplitude is proportional to the stress amplitude.

The vibrational character of a system composed

of a linear viscoelastic material may be described

by a linear time-invariant differential equation

of the form

" d^T

(19)

where y represents the output and x the input.

Such systems described by the above differential

equation may also be described by m^ans of their

weighting or impulse functions [10]. The impulse

response function g{t-T) = W{t-T) associated with

the above differential equation has the form

g{t —r) ='£^ Ctgiit-r)

1 = 1 (20)

where giit-r) is a set of basic solutions to the

homogenous differential equation. The relation

of the input x to the output y for the above

system is expressed by the superposition principle

as follows

y(t)- T)x{T)dl (21)

The ease of determining the weighting function

depends on the form of the differential equation

of the system. A third method of description of

the linear viscoelastic systems is by means of a frequency function [4, 11]. This frequency function bears a close relation in properties to the

system's Aveighting function and is related to the

weighting function by the follo\ving integral

pair:

(22)

W{t-T)=^

J^"

H{ii,)e"'^'-^^dw (23)

The relation of the system's input x to the system's output y in terms of the frequency function

is given by the following

y{t)=^ f^" H{io:)Xiio,)e'"'dw (24)

where X{iw) is the Fourier transform of the input x.

133
The difRculty in determing H{iui) from the differential equation of the system is similar to that of

the weighting function determination.

The terms such as dynamic modulus and compliance, mechanical impedance, logarithmic decrement, and phase angle most often determined

for a vibrating linear, viscoelastic system may be

arrived at by examining the resulting differential

equation of motion or a system with a single

degree of freedom y, having an elastic element as well as viscous character such that the sum of the

forces are zero in accordance with d'Alembert's principle as given

_ Ajy + B,y + Ky^ = /(O

/inertiaX /viscousX / elastic\ /exciting\

\ force / \ force / \ force / \ force /

(25)

In the above system, the viscous and elastic forces

are usually exerted by the viscoelastic material

Avhile the inertia force may be due to the material

sample or may be due to another part of the

system, depending on the system design. The

complete solution of the above differential equation is the sum of the complementary solution

yc and the particular solution yp-.

y^Vc+Vv (26)

The complementary solution yc is the solution

to the above differential equation when the exciting force is zero or what is called the homogenous

differential equation. The complementary solution

yc to the homogenous portion of the differential

equation describes the displacement behavior of

the system upon removal of the exciting force or what is called the free vibrational character of the

system. An important term evaluated for a system

containing a viscoelastic material in free vibration when the system is in the underdamped

condition is the logarithmic decrement. This term

will be discussed in more detail later. The second

part of the solution to the above differential

equation called the particular solution is a specific

solution of the equation when/(i) is not zero, that

is when the exciting force f{t) is acting on the

system. It is a specific solution in that it describes

the displacement behavior of the system under

steady-state conditions after the transient behavior (which is described by yc) has vanished as a component of the displacement y. The terms

commonly evaluated for a system containing a

viscoelastic material, under steady-state vibratory conditions, are the mechanical impedance

and phase angle for the system and, or the material, as a function of exciting frequency. Also, the

mechanical Q factor is determined from the frequency response curve of the mechanical impedance. All of these will be discussed later in

more detail.

First considering the free vibration of the

viscoelastic system after removal of the exciting

force, that is \vhen/(0=O, the resulting homogenous differential equation is Ajy+B,y+Ky=0 (27)

The displacement yc takes the following form when

the system is underdamped

{B,'-AAiK<0):

(28)

where a is the attenuation factor and the

damped oscillation frequency. The attenuation

factor a is related to the logarithmic decrement

5 and the damping frequency and to the coefficients of the differential equation by:

(29)

The logarithmic decrement is defined as the

natural logarithm of the ratio of two successive

maxima y^ and y^+x which occur in the period

Ta=2irlwa and is given by:

5=ln {y„-,/yn)=aT, (30)

Since the antilog of eq (30) is

e^=yi/y2=y2/ys= • =yn/yn+i

then

{e^Y={yilyi+i){yi+ilyi+2) . (yi+n-i/yi+n)=e"''

or 5=-ln (yi/yi+n) (31)

Therefore the logarithmic decrement can be

obtained from two displacement maxima separated

b}^ n periods by use of eq (31). The elastic energy

stored by the system described by eq (27) when

the displacement i/ is at a maximum during the

free oscillation is represented as follows for each

successive maximum:

V,=lKy,^V,=lKy^ . . . V„=l Ky.'

(32)

The energy loss between two successive maxima

Vi and V2 divided by the original energy is

V, KyJ

(25)2 (25)3

2! 3! (33)

When 5 is small, the energy loss per cycle is approximately

V

i2b (33a)

134
Next, looking at the linear system under steadystate conditions when force vibrated, the motion

of which is governed by eq (25) , when the exciting

force is harmonic, the steady-state displacement

yp is a harmonic function. Hence, let the exciting

force be represented by the harmonic function.

f(^t)=Foe*"' (external excitation)

Now the displacement would take the form

y=Yoe'"'. Substitution of these two functions

into (25) leads to:

y j-o

(34)

where G* is defined as the complex modulus or

stiffness. Therefore, a system like that described

by eq (25) has a steady-state modulus G* or compliance J*=l/G*. From eq (34) we note that:

f=G*y

y=J*f

(35a)

(35b)

The complex quantity G* can be written in

rectangular form and polar form as follows:

G*=G'+iG" = \G*\icos e-\-i sin e) = \G*\e''

Fo

gi6

where

\G*\ = (36)

Fo is the absolute value of the

stiffness G* and 9 is the phase angle between the

input / and the output y. Also:

Fo

and

G' = \G*\ cosdG" = \G*\ sine--

Yo

cos 6

sin 6

(37 a)

(37b)

The moduli G' and G" are defined as the storage

and loss moduli of the system. The complex

quantity, G* represents simultaneously the elastic

and damping properties of the system. Since the

complex modulus G* and the complex compliance

J* are reciprocally related as defined by (35a) and

(35b) then from eq (36)

J*=l/G*

Fc

-id J*| {cos d-ismB)=J' -iJ"

(38)

where J' and J" are the storage and loss compliances :

T' — \7* J*\ cos e--

J"^\J*\ sin e--

Yo

Fo

Yo

Fo

cos d

sin 6

(39a)

(39b)

It is noted that the absolute values and their

respective storage and loss values of moduli and

compliances are related as:

\G*\ =^lG'^+G"^ (40a)

(40b)

The ratio of either G"/G' from equations (37 a and b) or J"/J' from (39 a and b) is:

G"/G'=J"/J'=t&n e (41)

where 6 is the phase angle between input and output

and tan 6 is called the loss tangent. If one uses eq (35a) for the harmonic input f=FQe^"', the

output y of the system for steady-state motion is

1. (42)

The output y lags the input / by the phase angle 6

Now, returning to eq (34), it is seen that for a system in steady-state motion governed by eq (25),

the expressions for \G*\, \J*\ and tan 6 are as

foUows

Fo 1 Yo J*

tan d=wB,/iK-AjO)^)

(43a)

(43b)

The two terms above are usually measured as a function of exciting frequency w. The phase angle

6 is measured between the exciting input function

and the output function for a given exciting frequency. Usually, the reciprocal of is measured

at each driving frequency and plotted as a function of frequency. This reciprocal the magnification factor of the system, is determined by

measuring the amplitude of the output in respect

to the input amplitude at a given exciting frequency. The magnification factor is:

1/|6'*| = - 1

where

Kyln-

(44)

=resonance frequency of an undamped system

Resonance occurs when the phase angle 6 is 90°

and w=co„. Substituting u=u„ in eq (44) gives:

or

[V\G*\]res =

i^[l/|6^*|]..,=

1^

K_

(45)

(45 a)

135
The quantity given by equation (45a) is called

the mechanical Q factor of the system described

by eq (25). The Q factor of the system is usually

obtained by taking the frequency difference for

two frequencies on each side of the resonance maximum [12] where

^^2 K

2 BnWn

called the half power points; thus Q is defined as

QK

CO2 —Wi

(46)

If the system contains a viscoelastic material

sample where the damping and elastic responses

of the system are entirely due to the sample and

the inertia of the sample is negligible compared

with that of the system so that when the phase

angle is measured between the input at the sample

and the output displacement, the expression

(43b) becomes

tan B„

COK

(47)

which shows that the phase angle between the

force on the sample and its displacement is directly proportional to the driving frequency co and is equal to given by eq (46) when the driving

frequency is the resonance frequency o)„ of the

total system. It is further noted that the definitions

of G' and G" in (37 a and b) take the following

form using eq (34) for the steady-state response

of the system described by eq (25)

G'=^K-Ajo^^

G"=wBr,

(48a)

(48b)

It is seen that G" is directly proportional to

the damping constant and the exciting frequency w. Now again examining the case where

the damping and elastic response is due entirely

to a viscoelastic sample of negligible inertia

compared to that of the system we see that the

equations (48 a and b) take the following form

for the viscoelastic material

9i

(49a)

(49b)

where G^ and G'J, are the storage and loss

moduli of the material when corrected with the

appropriate shape factor £/^. It is further noted

that the loss modulus of the material G'^ is the

same as that of the system since all the loss is in

the material. Now if the system described by eq

(25) has its damping and elastic forces completely

due to a viscoelastic sample as described above and

the external excitation is of the form Foe*"* (a

harmonic function) then the differential eq (25) can be expressed in terms of the viscoelastic

material moduli G' and G'' as follows

or Ajy + {G:-\-iG'r:)y=Foe*''*

Ajy+Gly=Foe*-' (50)

In the absence of external excitation, /(<)— 0, in

the case of the system described by eq (50),

some investigators [13] have attempted to solve

the free vibration by trying to solve the homogenous differential eq (50).

Ajy+{G',,+iG'r:)y=0 (51)

But it must be pointed out that the form of G'm

and G'J, in (50) occurred only because the form

of the external exciting function fit) was known

and the solution under steady-state conditions is the same form as that of the excitation source. But there is no such analogous situation in the

case of free vibrations. Assuming the solution is of the form expressed by eq (28), then

where

G:r. =A,W-Cc')/9(

G'^=2c,,aAr/ 9(

(52a)

(52b)

c<Jd=w„Vl~/(i^=dahiping frequency

w„=.^^=resonance frequency of the undamped system

f/=B,y^A,9(

a=BJ2Ai

9{ =shape factor

If damping is small enough so that B^^<C<i^Ai9{

hence /d^<<l than equation (52a) and (52b)

take the follomng form

GL={K-B,'/AA,)/9{

G':^<^nB,l9(

(53a)

(53b)

One sees that G'J^ in (53b) is of the same form as

G'rl^ in (49b) and will be equal when the exciting

frequency in (49b) is equal to Wn- Further, it is noted that if

then

B,'«4.Aj

GL-=Kl9{ (54)

which is the same as the G^ in the steady-state

case of (49a)

136
Dynamic methods in which the inertia of the

system is large compared with that of the specimen

of viscoelastic material being characterized are sometimes called dynamic methods ^vith added

inertia. Two examples of this type of method are

(1) the torsion pendulum which is usually used

in free vibration and is very useful for measurement of energy loss in linear systems through the

logarithmic decrement, and (2) the rocking-beam

oscillator. Both of these methods are low-frequency

methods but may be used in free or forced oscillation. The torsion pendulum essentially consists

of a shaft composed of the viscoelastic material

to which a disk of moment of inertia / is attached.

The torsion pendulum as a dynamic test method

and the theory of its free vibration is discussed

in detail in an article by Nielsen [14]. The differential equation of motion for the torsion pendulum

would be that given by eq (25) where Ai=I, the

moment of inertia, y=B, the angular displacement for an external torque j{t)

ie+B,e+Ke=j{t) (55)

Equation (55) holds for small angular displacement. Since the inertia of the viscoelastic specimen

shaft is negligible compared to /, the moment of

inertia of the disk, the equation can be written

in accordance with eq (50a).

or /H«?;+^•6^;')5=/oe-'

(56)

In free vibration /o6*'''=0 and eq (56) takes the

form of eq (51).

or ie+{G:+iG':)e=Q

ie+Gie=o

(57)

where the complex modulus G* in this case is the

complex shear modulus of the viscoelasitc material

with G' and G" being the storage and loss components. The solution to eq (57) would be of the

form of eq (28)

0=0oe(i"d-«)' (58)

expressing the angular rotation 6, as a function of

time for free vibration. Since the above solution is of the form expressed by eq (28) then G' and G"

are related to the attenuation factor according to

eqs (52a) and (52b) as follows:

G'„=IW-o^)l9{ (59a)

(59b)

where S(. is again the shape factor. Now assuming

damping is small enough so that a)d=aj„ and re- calling that the attenuation factor a is related to

the logarithmic decrement 5 as expressed by eq

(29), substitution into (59a) and (59b) leads to

the result:

or

or

G'^=Ij^\^-^-h')ldi

TT

Gl=^irIU5l9i

(60a)

(60b)

where /„ is the resonance frequency of the un- damped vibration of the system recalling that

w„ = 27r/„. Now the ratio of G'^ to G'm using eqs

(60a) and (60b) would be the following for a

viscoelastic material

GL' 4ir6

G' '47r2-52

which for small damping takes the form

Gm /G!n=d/ir

(61)

(62)

The shape factor

^

takes the following forms

[14] for (1) rectangular cross-section specimens

and (2) circular cross-section specimens of the

viscoelastic material:

(1)

(2)

9(=CD'M/16L

9( =TrRy2L

where

C=width of the specimen

Z>= thickness of the specimen

i?= radius of the specimen

Z=length of the specimen

M—Si shape factor obtained from tables

The rocking beam method [15], like the torsion

pendulum, has the restoring force and damping

force provided by the sample of viscoelastic

material. The excited motion of the rocking beam

like that of the torsion pendulum may also be

described by eq (25) , but the displacement of the

sample in the case of the rocking beam is linear,

not angular, the inertial term Ai is the effective

mass (me) of the S3^stem, and the external exciting source is a force /(i), not torque in the equation as shown

m,y+B,y+Ky=f{t) (63)

The rocking beam consists of a beam of large

mass compared to that of the sample, with the

beam resting on a knife edge and having adjustable

weights. The specimen is positioned in the vertical

direction vnth one end clamped to the horizontal

137
beam out from the knife edge and the other end

to a fixed position. The specimen undergoes motion

only in the y coordinate upon excitation of the

beam for small oscillation. Returning to the equation of motion of the rocking beam, it is noted

that since the mass of the viscoelastic specimen

is small compared to the effective mass of the

beam, eq (63) can be written using eq (50 a and

b) in the following form

m,y + {E:+iE':)y=U"^' (64) or mey +E*y=foe'^'

where E*^ is the complex Young's modulus with

E' and E" being the respective storage and loss

components. Now in free vibration again /o^'"' =0

and eq (64) becomes:

m,y-\-E:y=0 (65) or m,y+iE:+iE':)y=0

The solution to eq (65) is again of the form of

eq (28)

y=yQe''''^d-0L)t (gg-)

expressing the linear displacement as a function

of time for free vibration for an underdamped

system; therefore, E^ and E^' are related to the

attentuation factor a according to eqs (52a) and

(52b) as shown

E:=m,W-a')l9( (67a)

¥;' =2mewW^ (67b)

If damping is small enough so that Wd=co„ and

using eq (29) which relates the attenuation factor a to the logarithmic decrement, upon subsititution

in eq (67a) and (67b) the moduli are related to

the logarithmic decrement 5 as follows;

or K=mefn'{^T'-8')/9( (68a)

IT 0

E'^=^Tmefn'8/ 9( (68b)

where w„ =27r/„ with being the undamped

vibrational frequency of the system. Now the

ratio of E'„! to Em is the following:

K7S;=4r6/(47r^-6^) (69)

which if damping is small reduces to

E"/E'm=5/ir

The effective mass of the system is determined

by substituting a spring of negligible mass and

determining free vibrational frequency a)i=27r/i of

the system with the spring in place of the viscoelastic specimen. Now if the natural frequency

a)o =27r/s of the spring is determined using a known

mass mo in free vibration or if the spring constant

Ks is determined by adding weights to the spring

and using Hooke's law, F=~KsX the effective

mass nie is given as follows

Ks o:s' fs' me=—|=mo -^=mo (70)

The shape factor 3^ for the rocking beam is as follows

9(=A/l

where I is the stretched length of the viscoelastic

specimen and A is the corresponding cross-sectional area at such length I. The cross-sectional area Ao of the undistorted specimen is related to A as

follows, assuming constant volume of sample

IqAq— Ia

or Ao=AI/Io=a(i+^^

where Iq is the undistorted length and Al the

change in length upon stretching.

7. Theory of the Time-Temperature

Superposition Principle

Earlier in this discussion it was stated that the

time-temperature superposition principle is applicable to various mechanical test data, creep,

stress relaxation, and various dynamic mechanical tests, and that Leaderman [1] and others

[16-20] have demonstrated that a composite

curve of the mechanical response can be constructed by shifting of data obtained at different

temperatures along a logarithmic time or reciprocal frequency axis. This has also been shown

to be applicable to dielectric or magnetic relaxation phenomena. If this principle is applicable to

the data obtained on a material under different

test methods, a micromechanism occurring in the

material may be confirmed and examined by each

of these methods; for example, transitions in

an amorphous polymer may be explored by j examining the distinct dispersion regions by i mechanical means. For certain materials, in order \ to obtain the composite curve for the reference 5 temperature Tq by reduction of measurements at { various temperatures, the modulus function must .1 be multiplied by a factor Topo/Tp where To is the reference temperature and po is the density

of the material at that temperature, while p is the density at T, the temperature of data meas138
urement. This factor times the modulus function

is plotted against In J- or In (coAt) to give the

composite curve; recalling that In At= (In /—In to) expresses the amount the curves at temperature

T are shifted along the In time axis to form the

composite curve at the reference temperature Tq.

This application of the factor ToPo/Tp to the

mechanical response of a material is in accordance

with the method of reduced variables which

involves two assumptions. The first is that the

modulus function for the material is proportional

to the absolute temperature T and density.

Therefore,

(71)

where Mo and M are the modulus function _ at

temperatures To and T. The second assumption

is that the molecular mobilities of the material at

any temperature T all have the same frequencytemperature dependence which can be described

as a single activation energy of flow AHq which is related to the shift factor At and temperature by

the Arrhenius equation.

AH.

R In An

\l/T-l/To, (72)

where R = the gas constant.

The temperature dependence of the activation

energy of flow AHa for the viscoelastic response of

amorphous polymers at and above the glass

transition temperature Tg of the polymer has

been described by the WLF equation, a semiempirical relation developed by Williams, Landel,

and Ferry [21] and given as:

{51. Q+T-TgY (73)

This equation shows AHa to increase with Tg and

to fall off above Tg. Since AH. measures the

change in molecular motion with temperature,

AHa is a maximum at the glass-transition temperature of an amorphous polymer and is low at

temperatures far above and below the transition.

Using tensile creep data, Bueche [22] calculated

the experimental AHa versus T for polymethylacrylate in the vicinity and above the glass

transition temperature Tg. Bueche gives an ex- tensive discussion of the molecular theory of such

transitions and their relation to AHa.

McCrum and Morris [23] examined the experimental values of AHa for various polymers including poly(methyl methacrylate) and polytetrafluoroethylene as obtained by creep or stress

relaxation experiments and compared them to

those obtained by internal friction measurements.

They observed that the AHa values from creep

and stress relaxation measurements were larger

than those obtained by internal friction measurements. Upon analysis, they concluded that the

true value of AHa was obtained from the tangent

loss (internal friction measurements). They ex- plained the discrepancy in the AHa values from

creep or stress relaxation measurements on the

basis of the temperature dependence of the

relaxed and unrelaxed compliances or moduli

which are the limiting values. As pointed out in

the earlier discussion on creep and stress relations,

the unrelaxed creep compliances and moduli are the instantaneous values, while the relaxed compliance is the instantaneous compliance plus the

limiting or asymptotic value of the retarded

elastic compliance, and the relaxed modulus is just the equilibrium modulus Gg. The inclusion of

the temperature dependence of the limiting moduli and compliances into the time-temperature

superposition principle led McCrum and Morris

[23] to the following relations which define the

McCrum-Morris superposition principle

A.

JRT Jul J,

Ct dn

GuT—GRT

'J uTo ""^0 ^RTo _ Jut _J^^i'q

'T "T p uTn ^uT

JRT ^R^o

JRTn G

(74a)

(74b)

(74c)

(74d)

RT

where J^t and Jrt are the unrelaxed and relaxed

compliances for any temperatm-e T while J^ra

and Jrt^ are values at some reference temjierature

To- 111 the above equation Gut, Grt, and Gu^,, and

Grto are the respective unrelaxed and relaxed

moduli at an}- temperature T and reference temperature To- From eqs (74 a, b, and c) it can be shown that the

retardation spectrum L-r (In t) and the relaxation

spectrum Hj, (In r) at temperature T, for a solid,

obeying the Boltzmann superposition principle, can be related to their respective spectra at a reference temperature Tq hy the following relation:

Lr (In T) = bTLTQ (In r/A^) (75a)

Ht

b (\nT)=-^Hr, (Inr/A^) (75b)

From these results, McCrum and Morris developed

the following general relations for the creep compliance and the relaxation modulus at temperatures T and T.:

J^^(t/Ar)=~ JT{t)+JuTo (76a)

GromT)=-^6r{t)+GRT,(^^'^) (76b)

139

452-525 0—72 10
The above equations are very general and it can

be shown that with appropriate assumptions, such

as bT—dr and Cr= 1 that WLF relations are special

cases. Glass state transitions may be evaluated by

means of the above equation. For example, such

transitions have been usually described by the

following

J^^it/Ar)=JT{t) (77a)

(77b)

which can be obtained from (76a and b) by assuming 6r=Cr=dr=1.0. But McCrum and Morris

assumed for such transitions that br=C7.= d7-5^1

which leads to the following

6roit/Ar) =CrGT{t)

(78a)

(78b)

The above equations, when applied to creep data

and stress relaxation data in the vicinity of a glass

transition [23] gave values of AHa which agreed

with the values obtained by internal friction measurements when applied to data for various

transitions.

8. Approximate Methods for Calculation of the Relaxation and Retardation Spectra of a Material and the

Interrelation of Viscoelastic Response

Functions

Alfrey [24] developed first order approximations

for the calculation of the relaxation spectrum and

the retardation spectrum of a material. The

retardation spectrum using Alfrey's method is simply found by taking the slope of the creep

compliance (J{t)-t/ri) versus the logarithm of

time (t), or, in the case of a dynamic experiment

the slope of the storage compliance versus the

logarithm of frequency (w) as expressed by the

following equations [5, 25]

d[J(t)-th]

d\n t

diJ'ic)]

d\n CO

for creep experiment

(79a)

for dynamic experiment

(79b)

To calculate the relaxation spectrum using this

method one takes the slope of the relaxation

modulus G{t) versus the logarithm of time (t), or, for the dynamic experiment, the slope of the

storage modulus G'{w) versus the logarithm of

frequency (co) given by the following equations

dG{t)

c^ln t

din cj

for relaxation experiment

80(a)

for dynamic experiment

(80b)

Schwarzl and co-workers [5, 26] developed

approximations of the spectra using higher

derivatives of the experimental response function

J{t), J'{o:), J"(w), G{t), G'io:), and G'^ico), but,

while these relations are better approximations

to the spectra, the observed response functions

are not accurate enough to get second derivatives

from the experimental data in most cases. The

approximation relations are expressed as follows:

Retardation spectrum

^('/2'=f^-rf^ for creep data (Sla)

Z'VlM=?[/,(„)-,^{;^,] for dynamic

compliance data

(81b)

where

m=J{t) or J{t)-th

m=J"i.^)-^l^-n

Relaxation spectrum

dG{t) , d'G{t)

dint' d{\n ty

for relaxation data

(82a)

77/(1/0;)=^ [^^"'(co)-^^^^] for dynamic data

(82b)

An interrelation between two different viscoelastic responses was given earlier in the case of

creep and stress relaxation function by eq (7) . The

relation between the dynamic compliance and the

retardation spectrum of a linear viscoelastic material can be shown to be represented [5] by the

following

J'(co)=e/,+ -^^^^^Inr (83a)

^"(-)=r"fx^2^1nr-h- (83b)

The relations between the dynamic moduli

(storage and loss) and the relaxation spectra are

as follows

G'{.^)= f^" ^^_;^f;' ^ln T+G. (84a)

^"(a>)=J_

+ " iJ(T)cOT

dXvL (84b)

140
Since the dynamic and static responses are

interrelated through the spectra for the material, one can be calculated from the other using the

above equations and eqs (2a and b) and (6)

9. Nonlinear Viscoelastic Behavior

Leaderman [1] found in his earlier investigations

of the creep of fibers that the creep and recovery

behavior were nonlinearly related to the stress,

and he proposed a modification of the Boltzmann

superposition principle containing an empirical

function y[o-(T)]. However, Leaderman's modification did not explain nonlinearities later observed

in the mechanical behavior of polymers. In 1960,

Nakada [27, 28] (see also Greene and Rivlin [29])

proposed a theory of nonlinear responses where

the excitation o-(0 and the resulting response are related through a series of normalized orthogonal

functions

cr(i) =Sai0i(O

i = l

i = l

(85)

(86)

where the coefficients aj and 6< are given by

(T{t)<t>i{t)dt (87)

b^= e{t)^iit)dt (88)

^i{t) being the complex conjugate of 4>iit).

If the excitation always precedes the response

and if each of the coefficients, 6/s, of the response

function can be expanded by means of Taylor

series in the coefficients, a/s, of the excitation

function (x{t), then the response function e{t) can be represented by

e{t) =j^y{t,)Jrit,U)dtr

+ f f

<T{ti)<r{t2)J2{t,ti,t2)dtidt2

J — a, J — a>

-co J — CO J — <x>

Mt,t:,t2,t3)dt4t2dt3+ (89)

or

<*) =j^^ <y{t-Ti)J,{t,ri)dT,

c{t —Ti)<x{t —T'^J2{t,Ti,T2)dTidT2 — CO J —CO

<T{t-u)a{t-T2Mt-T3)

-to J — CO J — oa

where JnitMM ....<„) or J„{t,Ti,T2, . . . . Tn) are characteristic weighting functions of the

excitation-response system; where

Jn{t,ti,t2

Jn{t}Ti,T2 rn)=0

ti>t

Ti<0

This general relationship can be applied to static as well as the dynamic response of a nonlinear

viscoelastic material. It will be discussed later in

this paper in respect to the nonlinear behavior of

dental amalgam on which creep data were obtained. If the response of the system is stationary,

the above equation takes the following form

e(t) =j^yit-TOMTr)dT,

J-f-co

/»-|_co

cr{t —Ti)a{t —T2)J2{Ti,T2)dTidT2

- CO J — CO (90)

a{t-TMt-r2Mt-T3)

-co J — CO J — CO

where Jn{ri,r2

J2{Ti,T2,Tz)dTidT2dT3-\- .

T2)=0 Ti<Q

For example, if we assume a dynamic excitation of

the form o-(<) = cro cos wt, then the resulting re- sponse, e {t) , from the above equation is

e{t)=GQ[Ai cos ut-\-A2 sin wt]

-\-(Tq^[Bi cos 2co<+52 sin 2uit-\-B'^

+o-o^[Ci cos 3wi+C'2 sin 3cof +(73 cos w^+C4 sin . . . (91)

where

yli=J/(co), ^2=J/'(co),

B2

j2'(w,-w),

(73 =^ J3 {03,W, —lxi),

C2 =1 Jz"

J3{t,Ti,T2,T^dTidT2dTz-\- .

For step function excitation or creep where

cr=0, ^<0 or (r=o-o, <>0,

the above integral takes the form

eit)=aQKi{t)-]r(7Q^K2{t,t)+<jQ^K3{t,t,t)-\- . . .

(92)

This creep function was used to describe the creep

response of dental amalgam [30].

141
10. Yiscoelastic Methods Applied To

Dental Materials

There are numerous repoi'ts in the literature con- taining data showing viscoelastic characteristics of

dental materials. However, in only a few of these

has there been an attempt to analyze the data in

terms of rheological theory. A few examples are

cited below.

The primary polymeric component of denture

base resins is usually poly (methyl methacrylate)

one of the dental material components to which

viscoelastic methods have been applied. Both static

and dynamic methods have been used to study its

mechanical behavior and the relation to its chemical structure. Creep studies [31-36] and stress re- laxation studies [37], as well as dynamic testing

[38^2], have been reported. Denture base resins

were studied mechanically by the use of stress- strain tests conducted by Barber [43] in 1934 and

by Sweeney and Schoonover [44] in 1936. Their

time dependent characteristics were pointed out by

Sweeney, Paffenbarger and Beall [45] in 1942.

Later Myerson [46, 47] using the dynamic torsion

pendulum method, studied the change in stiffness

and damping capacity as a function of temperature and cross-linking of various methacrylates

used in denture base resins and examined the transitions occurring in various methacrylates at different temperatures. Application of the d.ynamic torsion pendulum was later extended to the study of

denture base viscoelasticity by Braden and Stafford [48] who compared the denture base polymers

of vinyl acrylate with polycarbonate. They ex- amined the shear modulus and loss tangent

(damping capacity) of these polymers as a function of temperature. Static viscoelastic testing of

methyl methacrylate direct filling resin and artificial teeth was carried out by W. T. Sweeny and

co-workers [8, 9] using the indentation testing procedure mentioned previously.

Viscoelastic investigations of impression materials were conducted by Braden using a cylinder

viscometer [49] on alginates and both a cone and

plate, and a parallel plate viscometer [50] on rubber base impression materials.

Much information has been published on the

flow of dental waxes [51-53], but the emphasis has

been on determining temperatures at which large

changes in flow rates take place, rather than in

making comprehensive analyses of the viscoelastic

properties of the materials.

The viscoelastic nature of dental amalgam has

been recognized for many years. Data on the creep

of dental amalgams for a period of up to 8 days

were shown by S6uder and Peters in 1919 [54],

and, in fact, a flow or creep test was included in the

Federal Specification for amalgam in 1926 [55].

This test would be difficult to analyze precisely

since it is used as a measure of the setting time or

setting rate of dental amalgam. Thus, it provides

a summation of the creep of a specimen over a 21-

hour period during which the mechanical characteristics of the specimen are changing to a con- siderable extent.

Stress-strain measurements on dental amalgam

in compression were carried out by Smith, Caul,

and Sweeney [56] prior to 1956, while tensile

stress-strain measurements on amalgam were con- ducted by Rodriquez and Dickson [57] 1960-61.

Because these stress-strain measurements resulted

in a nonlinear relation w^iich depended on the

strain rate, a study of tensile viscoelastic behavior

of dental amalgam was undertaken using static

creep tests on tensile specimens. These creep

studies by Oglesby, Dickson, Rodriquez, Davenport, and Sweeney [30] showed that dental amalgam is nonlinear in its viscoelastic behavior. Strain

hardened amalgam in tension was found to exhibit

three types of mechanical behavior: (1) instantaneous elastic strain, (2) retarded elastic strain

(transient creep), and (3) viscous strain (steady

state creep). Viscoelastic theory was applied to

the creep data obtained for dental amalgam to calculate the tensile stress-strain behavior. Since the

amalgam creep strain was a nonlinear function

of stress, the usual Boltzmann superposition principle could not be used so the nonlinear generalization of the superposition principle developed by

Nakada [27] Avas applied. The experimental tensile stress-strain data were found to be in good

agreement with that obtained by transformation

of the creep data. Since the creep data indicated

that the retarded elastic behavior could not be

clearly separated from instantaneous elastic behavior due to short retardation times, it was neces- sary to employ dynamic tests to obtain the instantaneous elastic modulus.

The first method used for this purpose was the

ultrasonic pulse-echo technique. Young's modulus

as well as shear and bulk modulus for amalgam

were determined by this technique by Dickson and

Oglesby [58]. Later testing on amalgam has been

carried out by Larson [59] using a forced reso- nance technique (50 kHz range), advantageous in

measuring internal friction. Larson also measured

Young's modulus obtaining values in the range of

those reported by Dickson and Oglesby by the

ultrasonic pulse-echo technique. Recently using the

torsion pendulum in free vibration Barton and

Dickson [60] measured the shear modulus as well

as the damping capacity of amalgam over the

temperature range 23-51 °C. They found the shear

modulus to be about 3 X 10" psi, and the internal

friction was found to increase with temperature in

the range covered. Mahler investigated the sec- ondary or nonrecoverable creep of amalgam [61]

and found it to be related to clinical performance

in the mechanical function of amalgam in the

mouth. This secondary creep behavior of amalgam was studied as a function of temperature by

Dickson, Oglesby, and Davenport [62]. The creep

and stress-relaxation responses have been used to

study the setting time of amalgam by Fuse [63]

142
Compressive stress-strain tests were carried out

on dentin, by Peyton, Mahler, and Herslienov [64]

and again later by Craig and Peyton [65]. In this

work Craig and Peyton also conducted compressive

creep tests on human dentin. Dentin was shown to

be a viscoelastic material with a yield point above

which it appears to exhibit (1) instantaneous elastic strain, (2) retarded elastic strain, and (3) vis- cous strain, but below the yield point only the first

two phenomena are observed. AVork on the creep

behavior of dentin should be extended in order to

analytically describe the viscoelastic behavior in

compression and then using the appropriate transformations, the correspondence between the compressive stress-strain and creep behavior shovdd be

demonstrated. These data could then be compared

with that obtained by stress relaxation experiments

and dynamic experiments accounting for anisotropy effects. In 1968, Lugassy and Korostoff [66,

67] rejwi'ted an experiment using sti'ess relaxation

techniques in dentin in whicli they studied the viscoelastic properties in relation to structural anisotropy and compared the residts to polymer

viscoelasticity. Recently Barton and Dickson [60]

have used the torsion pendulum to determine the

moduli and the damping capacity of dentin at

mouth temperature as well as at room temperature.

Little work has been done in the relation of the

viscoelastic behavior of dental materials and tis- sues in relation to their atomic, molecular, and

microstructural properties and processes ; yet there

is a large amount of theory and experimental technology that could be broug'ht to bear. Two Avorks

in the dental field in i-elation to microproperties

are Myerson's work [46, 47] on methacrylate cross- linking ill respect to mechanical properties and

the work of Dickson and Oglesby [62] on the

steady-state ci'eep of amalgam and the detei'mination of the activation energy of the process.

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[37] McLaughlin, J. R., and Tobolsky, A. V., The viscoelastic behavior of polymethyl methacrylate, J. Colloid Sci. 7:555 (1952).

[38] Deutsch, K, Hoff, E. A., and Reddish, W., Relation

between the structure of polymers and their

dynamic mechanical and electrical properties,

Part I. Some alpha-substituted acrylic ester poly- mers, J. Polymer Sci. 13:565 (1954).

[39] Heyboer, J., Dekking, P., and Staverman, A. J., The secondary maximum in the mechanical damping of polymethyl methacrylate : Influence of

temperature and chemical modiflcaltions, Chapter

14. Proceedings of the Second International

Congress on Rheology. Harrison, V. G. W., Ed.

( Butterworths Scientific Pub., London 1954).

[40] Koppelmann, J., Tiber den dynamischen Elastizitiitsmodul von Polymethacrylsauremethylester

bei sehr tiefen Frequenzen. Kolloid Z. 164:31

(1959) . [41] Nielsen, L. W., Dynamic mechanical properties of

high polymers, Soc. Plastics Eng. J. 16:525

(1960) . [42] Schmieder, K., and Wolf, K., Uber die Temperaturund Frequenzabhangigkeit des mechanischen

Verhaltens einiger hochpolymerer Stoffe. Kolloid

Z. 127:65 (1952).

[43] Barber, Ronald, Preliminary tests of some of the newer denture materials, J. Am. Dent. Assoc. 21:1969 (1934).

[44] Sweeney, W. T., and Schoonover, I. C, A progress

report on denture base material, J. Am. Dent.

Assoc. 23:1498 (1936).

[45] Sweeney, W. T., Paffenbarger, G. C, and Beall, J. R., Acrylic resins for dentures, J. Am. Dent. Assoc. 29:7 (1942).

[46] Myerson, R. L., The Internal Friction of Linear and

Crosslinked Phosphate Glasses and Methacrylate

Polymers. M.I.T. Master's Thesis. May 1961.

[47] Myerson, R. L., Effect of crosslinking on mechanical

damping and stiffness of methacrylates, lADR

Abstracts, Fortieth General Meeting, p. 89,

(1962).

[48] Braden, M., and Stafford, G. D., Viscoelastic properties of some denture base materials, J. Dent.

Res. 47:519 (1968).

[49] Fish, S. F., and Braden, M., Characterization of the

setting process in alginate impression materials,

J. Dent. Res. 43:107 (1964).

[50] Braden, M., Viscosity and consistency of impression rubbers, J. Dent. Res. 46:429 (1967).

[51] Craig, R. G., Eick, J. D., and Peyton, F. A., Properties of natural waxes used in dentistry, J. Dent.

Res. 44:1308 (1965).

[52] Ohashi, M., and Paffenbarger, G. C, Melting, flow

and thermal expansion characteristics of some

dental and commercial waxes, J. Am. Dent. Assoc. 72:1141 (1966).

[53] Ohashi, M., and Paffenbarger, G. C, Some flow

characteristics at 37 °C of ternary wax mixtures

that may have possible dental uses, J. Nihon

Univ. Sch. Dent. 11:109 (1969).

[54] Souder, W., and Peters, G. C, Investigation of the

physical properties of dental materials. Dental

Cosmos 62:305 (1920).

[55] Souder, W., Measurement and application of cer- tain physical properties of dental materials, J. Dent. Res. 7:173 (1927).

[56] Smith, D. L., Caul, H. J., and Sweeney, W. T,

Some physical properties of gallium-copper-tin

alloys, J. Am. Dent. Assoc. 53:677 (1956).

[57] Rodriguez, M. S., and Dickson, G., Some tensile

properties of amalgam, J. Dent. Res. 41:840

(1962).

[58] Dickson, G., and Oglesby, P. L., Elastic constants

of dental amalgam, J. Dent. Res. 46:1475 (1967).

[59] Larson, R. V., Internal Friction Damping in Dental

Amalgam. Thesis, Dept. of Mechanical Engineering, University of Utah, August 1967.

[60] Barton, J. A. Jr., Dickson, G., and Oglesby, P. L., Determination of properties of dental materials

'by means of a torsion pendulum, lADR Program

and Abstracts of Papers, p. 172 (March 1968).

[61] Mahler, D. B., and Von Eysden, J., Dynamic creep

of dental amalgam, J. Dent. Res. 48:50 (1969).

[62] Dickson, G., Oglesby, P. L., and Davenport, R. M.,

The steady state creep behavior of dental amalgam, J. Res. Nat. Bur. Stand. (U.S.), 72C (Eng.

and Instr.), No. 3, 215-229 (July-Sept. 1968).

[63] Fuse, N., Flow, stress relaxation and compressive

strength of dental amalgam. Bull. Tokyo Med. and

Dent. Univ. 16:17 (1969).

[64] Peyton, F. A., Mahler, D. B., and Hershenov, B., Physical properties of dentin, J. Dent. Res. 31 :366

(1952).

[65] Craig, R. G., and Peyton, F. A., Elastic and mechanical properties of human dentin, J. Dent. Res. 37:710 (1958).

[66] Lugassy, A. A., and Korostoff, E., Comparative

viscoelastic properties of dentin and bone, lADR

Program and Abstracts, 46th General Meeting,

p. 99 (March 1968).

[67] Lugassy, A. A., Mechanical and Viscoelastic Properties of Cow Bone and Sperm Whale Dentin

Studied under Compression. Thesis, Graduate

School of Arts and Sciences, University of Pennsylvania (1968).

144
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseakch, Proceedings of the oOth Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Low Frequency Determination of Mechanical Properties

Richard L. Myerson

Myerson Tooth Corporation, Cambridge, Mass. 02139

The torsion pendulum is a valuable tool for use in determining the mechanical

properties and molecular structure of dental materials. With the torsion pendulum the

modulus of a material may be determined from the frequency of oscillation ; the internal

friction can be obtained from the rate of damping. Change in internal friction with change

in frequency provides insight into type of structure and relaxation mechanisms. Activation

energy of processes can be determined from shift in peak energy absorption temperature

with change in frequency. A principal advantage of low frequency testing is in the resolution

of dispersions or of internal friction peaks. The torsion pendulum method has been applied

to various polymer materials in studies of the relationship of composition and transitions,

effects of cross-linking, comparison of cast poly (methyl methacrylate) and powder-liquid

molded poly (methyl methacrylate) and the relationship of impiact resistance to peak energy

absorption temperatures. Studies have been made of the relationships between peak temperatures and composition and cross-linking in sodium phosphate glasses. A review of

equipment is presented.

Key words : Activation energy ; dental materials ; glass, sodium phosphate ; internal

friction ; mechanical properties ; polymers, dental ; torsion pendulum.

1. Introduction

The focus of this report is to be on the use of the

low frequency mechanical properties as a tool for

analyzing the molecular structure of the materials

being considered. The emphasis will be on the torsion pendulum. With the torsion pendulum one can determine the modulus of a material by observing the frequency of oscillation. The internal

friction of a material is obtained from the rate of

damping. By noting the change in values of internal friction with changes in frequencies, one can

gain insight into the type of structure and the

relaxation mechanisms. The activation energy of

the processes involved can be determined from the

shift in peak temperature with change in

frequency.

If the internal friction of a material is plotted versus temperature for a fixed frequency of oscillation, there is usually a general rise in the value

with a rapid increase in the area of softening. There

will often be maxima at various lower temperature points. For silicate ceramics the lowest temperature peak has been usually ascribed to an alkali ion diffusion under stress, an intermediate

peak to oxygen ion diffusion, and finally the high

temperature peak to visco-elastic motion. In polymers these peaks at the lowest temperature have

been ascribed to side chain motion and/or very

local motion on the main backbone chain [1].^ The

higher temperature peaks are usually caused by

softening of the material or by the melting of

1 Figures In brackets Indicate the literature references at the end of this paper.

crystallites in the case of a crystalline or semicrystalline material.

Braden and Stafford [2] reviewed the viscoelastic properties of some denture base materials using

a torsion pendulum at less than one cycle per sec- ond. They determined glass transition temperatures which are important in setting a limit of

temperature to which a material can be safely

exposed in cleaning, repair, etc. They determined

the shear modulus with good agreement to calculations made from the tensile modulus. Finally they

determined the internal friction spectrum from

+ 20 °C to +105 °C. They concluded that some

of the materials of high impact resistance gained

their impact resistance by sacrifice of the glass

transition temperature, which limited their higher

temperature usefulness. One impact resistant material also had a higher internal friction. The au- thors stated that this might mean less elastic

behavior.

I suggest that the value of tan 8 (internal friction) as an indication of specific end use properties

might be misleading. First of all, the experimentation is done with very small strains operating

in the linear viscoelastic range. Most stresses that

are damaging will, of course, involve larger strains

and will be nonlinear. Secondly, the loss component in plastics may be due to side chain motion

rather than main chain motion, and this would

probably reflect itself in retarded elasticity rather

than in creep. A significant use of the internal

friction spectrum is as a means of characterizing

a material. It will provide some indication of the

145
likelihood of a build-up of heat during a cyclic

loading. It provides a signal that there may or may not be creep under long term steady state

loading by identification of relaxation processes

and the time-temperature effects.

The energy absorption patterns versus frequency and temperature will yield information

that, in part at least, is attributable to molecular

structure specifics. By controlled experiments—as examples, varying the composition of a copolymer

or the alkali ion I'atio in ceramics—and by use of

other information such as diffusion data, dielectric

losses, etc., the peaks of energy absorption can be

identified. Then the effect of the degree of polymerization or degradation or devitrification, etc., can be better understood by observing the internal

friction spectrum changes.

An important consideration is the background

level of internal friction apart from the peaks.

The background gives an indication of the general

molecular mobility. In the case of a completely

cross-linked glass—i.e., fused quartz—there is virtually zero damping.

2. A Contrast of High and Low

Frequency Testing

It is of interest to contrast the use of high and

low frequency testing. Each has advantages, and

the use of both high and low frequency measure- ments will give ^^ery valuable data relative to the

activation energy of phenomena because of the

large spread in rate of loading. There are also, of

course, some very specific properties of importance

in the end use for which damping properties meas- ured at one end of the spectrum would be more

important. The principal advantage of low frequency testing is in the resolution of dispersions or of internal friction peaks. The lower temperature peaks are usually of lower activation energy,

and as the frequency- is increased, they shift to

Mgher temperatures at a more rapid rate than the

higher frequency peaks. At sufficiently high frequencies this will lead to a blending of peaks.

Nielsen [1] points out that the torsion pendulum

is useful over a very wide range of moduli, and

from very low frequencies to moderately high.

He felt the best frequency range was from a 100th

of a cycle per second to 10 cycles per second.

For determining low frequency damping data

there are three general types of equipment. The

first, and the one which will be emphasized in this

paper, is the freely oscillating pendulum operating

at_ the system resonance. A second type is a driven system involving its natural resonance,

such as a vibrating reed. Here the internal friction

is determined by the shape of the amplitude versus frequency curve in the resonance area. The

third type, a driven nonresonant rotating beam,

developed by Maxwell, will be discussed briefiy

below.

3. Theoretical Aspects and Interpretation of Torsion Pendulum Data

The theoretical aspect of an oscillating stress as applied for a torsion pendulum has been well

covered in the literature, and suitable references

are included with this paper. Because of limitations

of space, only the conclusions, with a minimum

of the background, will be presented here.

In the Voigt solid (fig. 1), a model of a viscoelastic solid is presented. The relationship of

stress and strain for such a solid is as shown in

eq 1.

VOlGT MAXWELL

Figure 1. Voigt and Maxwell models

for, mechanical behavior.

, de (1)

where

ff= stress

elastic modulus

e =strain

77= viscosity

It can be seen that if a force is cyclic (sinusoidal)

the term for the elastic component will be in

phase, whereas the viscous flow term will be

90° out of phase.

In a real solid, the resultant complex shear

modulus of G* will be the resultant of the real

part of the solution, G' (representing the elastic

portion) and the 90° out-of-phase, imaginary

portion, G" as is shown in figure 2.

Figure 2. Relation of tan 8 to G' and O".

The angle 8 represents the lag of the strain to

stress. Tan S is equal to G"/G'.

Zener [3] showed that for a given temperature,

tan 5 is a maximum when the mean relaxation

time (the mean of constant stress relaxation

time and constant strain relaxation time) is equal

146
to jr-^ where F is frequency. Thus, if the temperature is held constant and the frequency of loading

varied, at one frequency there will occur a maximum in the damping. This will yield the relaxation time of the material at this temperature.

Zener also developed the equation for tan 5 as a function of the log decrement of the amplitude.

The equation was arrived at by considering the

rate of energy dissipation per unit volume (in

the range where tan 5 is approximately equal to

sin 5) . He arrived at eq 2

Tan s^IeAM^ (2)

Ao is the initial amplitude; A„ is the amplitude

after n cycles.

3.1. Activation Energies

Since the relaxation phenomena express rate

processes that are temperature dependent, the

activation energy can be determined by testing at

several frequencies. If a viscoelastic solid is tested

at a constant frequency, there will be a temperature showing a maximum in damping. By testing

at several frequencies and determining the peak

temperatures, the activation energy can be

determined for a specific relaxation process by

eq 3.

1 2ttF'

7= 7^6 EIRT

(3)

Where: 2^= frequency

T=relaxation time

activation energy

i?=gas constant

T=absolute temperature

The in-phase component or the elastic component of the complex modulus stores energy as potential recoverable energy. The out-of-phase

imaginary component results in an energy loss,

principally in internal heat.

For real materials there is no single relaxation

time as would be calculated for a simple Voigt

solid. There will exist a distribution of relaxation

times, hence a range of temperatures sho^ving

high damping for a single frequency. Ryder [4] pointed out that the area under a peak of tan 6 versus temperature curve will be related to a number of diffusing species. For the same number

of diffusing elements a narrow distribution of

relaxation times will result in a high narrow peak

compared to a low broad peak for a broad distribution of relaxation times. The effect of changing

compositions can, therefore, be reviewed for this

factor as well as the locations of the peaks.

Fitzgerald [5, 6] listed some of the sources of

energy loss in ceramic materials. He included

1. Flow of heat

2. Diffusion of ionic species

3. Elasticoviscous effects

4. Viscous flow

5. Precipitated crystallites

For the typical alkali silicate ceramic there is a

distinct tan S peak around room temperature that

has been associated with the diffusion under stress

of the sodium ion. A higher peak has been asso- ciated with the nonbridging oxygen ion. The elasticoviscous effects he relates to movements of large

segments of the glassy network as the transformation region is approached. He explained the diffusion of the sodium ion by considering that under

a stress there will be a redistribution of electrical

charges, and in tliis way the equilibrium position

of the sodium ion will shift. In some cases the ion

must get past an obstacle in order to reach its new

equilibrium position. The particles with sufficient

energy will be able to get past an obstacle. The

distribution of particle energy is related to temperature. Thus the diffusion rate will be temperature

dependent and the typical activated process equations will result.

Kirby [7] showed that soda lime glasses and

borosilicates have distinct peaks and different

background values. Of interest was that fused

silica, which has nonbridging oxygen ions and,

of course, no alkali or alkaline earth ions, has

extremely low damping coefficient until the

elasticoviscous effects are reached. Blum [8] in a very carefully controlled experiment showed a

value of 0.000012 for tan 8. Forry [9] worked Avith three compositions of

soda in sodium silicate glasses. As the soda percentage is increased, the low temperature peaks

occurred at lower temperature, were higher, and

were sharper. He found the height above background was almost linear with soda content, ex- trapolating to zero at about 10 percent soda

content.

Hoffman and Weyl [10] reviewed the effect of

alkali and alkaline earth oxide on silicate glasses.

They found that lithium had the highest activation energy (the smallest ion and thus the most

tightly bound), sodium the next, and potassium

the lowest. The alkaline earth oxide additives, be- cause they have network bridging properties,

tighten the structure and suppress the alkali diffusion peak.

Shelby and Day [11] studied the effect of mixed

alkalis and silicate glasses. Figure 3 shows some

of their findings. As the second alkali is added,

the alkali peak shifts to a higher temperature and

becomes smaller. The nonbridging oxygen peak

became slightly higher. A new damping peak,

large and close to the nonbridging ion peak, was

developed and, in fact, can mask it. From their

work they found that it is easy to detect a small

amount of a second alkali.

Fitzgerald [12] showed the effect of annealing

versus chilling of sodium silicate glasses. In the

annealed state—that is, the tighter structure

state—the activation energy for the sodiimi ion

147
I I 1 I I -200 -100 0 +100 +200 +300 +40C

TEMPERATURE "^C

Figure 3. Internal friction of mixed alkali-silicate

glasses.

diffusion was 20 kilocalories per mole versus 16

kilocalories per mole for the chilled glass, thus

giving some indication of the effect of amiealing

on the network.

3.2. Viscoelastic Behavior

Fichter [13] listed a number of possible causes

of viscoelastic relaxation damping for metals.

These include

1. Interfaces and grain boundaries.

2. Changes in lattice order due to stress.

3. Magnetoelasticity, particularly in ferromagnetic materials.

4. Phase changes (a function of local temperature) . 5. Precipitation and recrystallization.

Nowick [14] also reviewed anelastic phenomena

with emphasis on metals. He cited the example of

aluminum where there is a substantial peak in tan

S at low frecjuency at about 300 °F (150 °C) in a polycrystalline material. This peak is absent in a

single crystal. He attributes this to viscoelastic

behavior in the grain boundaries.

Kaelble [15] presented the diagram in figure 4

giving an overview of the phenomena in polymers.

The plot is of modulus versus temperature at con- stant frequency. The relaxations accompanied by

changes in modulus level will also be accompanied

by peaks in the internal friction curve. At the very

low temperature, polymer strain in response to

stress is due to primary bond angle bending and

stretching. Hence there is no diffusion and this

AFTER KAEBLE, 1965

T T

g m

TEMPERATURE (°K)

Figure 4. Modulus of polymers as a function of temperature at constant frequency.

results in very low internal friction and high modulus. As the temperature is increased, side chain

motion or very local chain motion can take place

and the modulus will go down. At the glass transition temperature, segments of the chain will start

in motion and there will be a large drop in modulus. There will be a rubbery plateau above the

glass transition temperature, depending on the

degree of cross-linking and the molecular weight.

The last reduction in modulus above the glass

transition temperature for amorphous materials

comes when the chains themselves actually slip as

in molding. This region is very dependent on

cross-linking, and if the material is cross-linked

the slippage does not occur until decomposition

starts. For linear materials, if there is high molecular weight, entanglements will act as temporary

cross-linking sites and will raise the temperature

of the slip point. In the case of crystalline materials, the glass transition temperature is not ac- companied by a great change in modulus until

the crystallites melt.

Boyer [16], in discussing the plastic transitions

below the glass transition temperature, lists the

following

(a) Side chain group motion such as the ester

groups in polyalkyl methacrylates.

(b) In-chain motions of subgroups as in the

polycarbonates.

(c) "Crank shaft" motion as in polyethylene

when there are about four consecutive

methylene groups.

He describes the glass transition temperature

phenomenon as large "crank shaft" motions involving 30 to 40 carbons in the chain. Above the

glass transition temperature the transitions occur with coordinated motions of entire chains.

McCrum et al. [17] point out that the assignment of mechanisms of relaxation to polymers has

been done qualitatively by experiments rather than

quantitatively. The convention used for polymeric

materials is that the alpha peak is the highest

148
temperature, and beta the next lowest, etc. With

the amorphous materials the alpha peak is usually

the glass transition temperature when there is a modulus change of three orders of magnitude or more. The lower temperature transitions are due

to the local motions in the polymers while they

are still in the glass-like condition; that is, the

larger segments of the chains are frozen in the

glassy state, but the small groupings, are able to

move above their own transition temperatures. In

partially crystalline materials, such as polyethylene, the alpha peak is usually the melting of the

crystallites and the beta the glass transition temperature of the amorphous phase. Thus the magnitude of this peak will be alfected by the degree of

crystallinity, and indeed this can be used as a measure of the degree of crystallinity in a study

of a particular polymer. McCrum mentions that

there is a low temperature absorption peak in

nylons and methacrylates due to absorbed impurities (e.g., water). He also suggests that the

''crank shaft" motion can be applied to other

groupings in a chain other than four methylenes,

provided they allow simultaneous rotation of two

colinear bonds to give the "crank shaft" effect.

Nielsen [1] gives a qualitative picture for the

energy absorption at the transition of a polymer,

wherein as the temperature increases, frozen segments start to move. Below that temperature the

damping is low because deformation is bond angle

bending, and hence highly elastic with little heat

dissipation. At temperatures well above the transition, damping is low because the chain segments are free to move, but the resistance is low

and hence the modulus is low. Thus if the segments

involved are either frozen or very free to move,

damping would be low. The damping peak will occur at the intermediate condition.

Nielsen [18] points out that in polymeric materials, damping losses will be affected by the

following

Crystallinity

Cross-linking

Number of phases

State of aggregation

Various chain motions

Deutsch et al. [19] noted that plasticizers, residual monomer, absorbed water, etc., complicated

the systems.

3.3. Internal Friction and Copolymerization

Internal friction measurements are useful for

the interpretation of copolymerization. If the

starting materials are comonomers of close re- action rates, the end product will be homogenous

with a distribution of effects such as glass transition temperature intermediate between the end

members. The sharpness of the tan S versus temperature peaks will tend to be intermediate between the end members. McCrum, et al. [17] show

in figure 5 the additive nature of a series of

copolymers of methyl and cyclohexyl methacrylate.

The change in the nature of the curves is nearly

quantitatively additive. Poly (methyl methacrylate) has no low tempei'ature peak but a broad beta

peak in the 40 °C region, whereas polycyclohexylmethacrylate has a similar glass transition temperature to poly (methyl methacrylate) and a low

temperature beta peak below 0 °C. The intermediate copolymers are intermediate in both these

respects. The glass transition temperature is relatively unchanged since the end members were

similar.

Nielsen [1, 20] pointed out that if the rates of

reaction are unequal, the initial polymer formed

will be rich in the faster reacting material, and

the final polymer formed rich in the slower material. This will result in broader and lower peaks

than in homogenous copolymers.

With graft or block polymers, where there are two separate phases insoluble in each other present, the resultant tan S versus temperature curve

will show the seperate features of each phase in

proportion to their concentration. Similarly, polyblends made by mechanical mixing will show the

character of both phases.

Deutsch et al. [19] in comparing methyl methacrylate and methyl a-chloro-acrylate, showed that

in replacing the methyl group with the more polar

substituent, the glass transition temperature increased from 110° C to 140° C in the case of the

methyl a-chloro-aorylate. Also, the beta peak temperature was increased and the activation energy

of this peak significantly increased due to the

stronger interchain bonding caused by the polar

group.

1 1 1 COPOLYMERS

METHYL METHACRYLATE CYCLOHEXYL METHACRYLATE

1.00 0.00 —.

0.80 0.20

0.60 0.40

— — 0.40 0.60

0.20 0.80

0.00 1.00 AFTER McCRUM, READ AND WILLIAMS, 1967 SOURCE - HEIJBOER, 1956 ?

A

1/ / O ' J f

K / •

/A y ""S^

1 1 1 1 -100 -50 0 +50 +100 TEMPERATURE °C

Figure 5. Relation of tan 6 to temperature for varying

methacrylate copolymer ratios.

149
3.4. Impact Resistance and Low Temperature

Peaks

Many authors [1, 21-26] have considered the

relationship between the presence of a low temperature peak in a rigid material and impact

strength. In early observations there seemed to be

a correlation between the presence of such a peak

and impact resistance. Later data show that the

low temperature peak is neither a necessary nor

sufficient condition for impact resistance. However,

it is of significance in many cases and can lead to

an interpretation of the mechanisms involved.

Matsuoka [25] in his review of the dynamic

properties of glassy and partly crystalline materials versus impact resistance, points out that in

polymers, ahead of a propagating crack tip, there

IS plastic deformation. The greater this deformation the more energy is required to propagate a crack and the tougher the material will be. The low

temperature transition signifies molecular motion

and should relate to impact resistance. However,

low temperature peaks due to side chain motion

do not relate to the ability of the chains to draw

under stress and, hence, are not related to impact

resistance. Peaks due to local motion in the main

chains are related to impact resistance.

In partially crystalline materials the beta peak

usually signifies the glass transition temperature

of the amorphous phase, and thereby molecular

motion. These materials have good impact resist- ance between the glass transition temperature and

the temperature corresponding to the alpha peak

indicative of the melting of the ciystallites.

The relationship between the secondary low

temperature peak and impact resistance is indirect.

When it signifies side chain motion, as in a cyclohexyl methacrylate, it is of no significance. In mixtures of two materials (or in some block and graft

copoilymers) wherein there are two phases, if one

phase is a rubber there will be a low temperature

peak. This does not necessarily mean impact re- sistance. If the rubbery particles are of the proper

size and distribution and well bonded to the matrix, they will nucleate drawing of the glassy phase

under stress and impart impact resistance.

Heijboer [23] and Turley [22] point out that

the relation between the temperature of the peak

and the tough brittle transition is not clear. Also,

as in the case of poly (2-6 dimethyl-p-phenylene

oxide) , one can have impact resistance with no low

temperature peak. It should be borne in mind in

the correlation of tan 8 and impact resistance,

that the torsion pendulum data is in the linear

viscoelastic range, whereas impact failures involve

large scale molecular movement.

3.5. Composites, Crosslinking, and Orientation

Maeda [27] and Galperin [28] show that in the

case of composites there is a tendency towards a

decrease in the height of the tan 8 peak and an

increase in the temperature of the tan 8 peak relative to the matrix resin.

Reinhardt [29] was concerned with the means

of measuring the progress of the final stages of

cross-linking that would be more meaningful than

some of the indirect physical property tests. The

validity of the chemical tests for unsaturation is doubtful because of the insolubility of the cross- linked polymers. His study was based on dynamic

mechanical tests with specimens of varying cure. During the termination of cure, as the network

lightens there is an asymptotic increase in modulus

and a decrease in tan 8. He showed that curing

for 2 hr at 100° C produces a higher modulus and

lower tan 8 (presumably more cross linking) than

in the self-curing system cured 500 hours at 35 °C.

Malpass [30] showed that high strains (orientation in the polymer) produced a definite peak that

correlated with physical properties (they helped

when in the direction of the stress, but were

detrimental when at 90° to the stress). Annealing

eliminated these peaks and brought the properties

in line with the normal material.

4. Experimental Results

4.1. Glasses

A series of tests on sodium phosphate glasses

were conducted by this author [31]. The glasses

were drawn into fibers and internal friction versus temperature curves made at frequencies around

one cycle per second. Sodium phosphate glasses

will vary in structure depending on the ratio of

sodium oxide to phosphorous pcntoxide. When the

molar ratio is 1, the theoretical structure will be

a long chain. As the ratio becomes greater than 1,

chains become shorter and shorter. If the ratio is

less than 1—that is, the molar proportion of phosphorous pentoxide is greater than the molar proportion of sodium oxide—the structure is cross- linked. The greater the percentage of phosphorous

pentoxide, the greater the degree of cross-linking.

In the sodium phosphate glasses the backbone

structure is an alternating series of oxygen phosphorous atoms. Each phosphorous atom is sur- rounded by four oxygen atoms, three of which are capable of bonding with another phosphorous or sodium atom, and the fourth is a nonbridging ox- ygen atom with a double bond to the phosphorous.

Thus there is present in this structure the sodium

ion and the nonbridging oxygen ion as in the

sodium silicate glasses. In figure 6 it can be seen that there are indeed two peaks below the glass

transition temperature; the lowest temperature

peak is probably the sodium ion diffusion peak,

the intermediate the nonbridging oxygen ion. As

the sodium to phosphorous ratio is dropped, the low

temperature peak drops. Conversely, as the sodium

to phosphorous ratio drojDS, the medium temperature peak increases in height. Both of these

phenomena are in line with the fact that as the

150
TEMPERATURE °C

Figure 6. Internal friction of glasses with varying Na/P

ratios.

sodium to phosphorous ratio decreases, the sodium

ion concentration decreases, but the nonbridging

oxygen ion concentration increases.

It is of interest to note that the glass transition

temperature goes down as the phosphorous pentoxide content goes up and as the cross-linking goes

up, contrary to what is seen in organic glasses. This

suggests that the sodium ion, in coordination with

oxygen ions of other chains, bonds the chains more

strongly than does the actual chain cross-linking.

This is supported by the data of van Wazer and

Hoist [32] who found that when three oxygens

were shared with other PO4 tetrahedra the compound w^as less stable than when only two or one

oxygens were shared with other PO4 tetrahedra.

4.2. Polymers

Work done by this author on the effect of ci'osslinking witli a hexyl methacrylate-ethylene dimethacrylate system was presented to the Dental

Materials Group of the International Association

for Dental Research in 1962 [31-33]. Figure 7

shows the salient features of that series of tests. There is a reduction in background as cross-linking

increases. For the hexyl methacrylate formulations

there is a low temperature dispersion off scale

that is, around —200 °C—probably related to the

hexyl group. There appears to be, in the hexyl

methacrylate, a beta peak at about —100 °C. The

glass transition temperature was raised considerably by addition of 25 percent ethylene dimethacrylate. It should be noted in this case that

there is a very broad peak, possibly similar to the

T

1 1 1 1 1 1 r

TEMPERATURE OC

Figure 7. Internal friction as effected by crossUnking in a hexyl methacrylate-ethylene dimethacrylate system.

beta peak of methyl methacrylate at about 40 °C.

The internal friction in the rubbery region falls

ofi' to a low value, lower than that in the glassy

region.

Ethylene dimethaci-ylate showed a veiy broad

peak at 45 °C. The rate of fall of the modulus

through this transition Avas small, indicating that

it is not the normal glass transition phenomenon.

Again this may be similar to the methyl methaciylate beta peak. Decomposition of the materials

started above 200 °C. The modulus of the ethylene

dimethacrylate polymer at -1-200 °C w^as approximately half that at nearly —200 °C, thus showing

a remarkably small rate of fall with temperature.

A recent series of experiments were conducted

with a number of dental materials. They were

fabricated and machined to specimens approximately 2 in (50 mm) in length between grips, 0.200

in (5 mm) wide and 0.062 m (1.5 mm) thick. The

specimens were maintained free from water by

storage in a desiccator until testing was done.

The tests "were conducted on the Plas-Tech.

Direct Recording Torsion Pendulum. The frequency was in the range of 2 to 10 cycles per sec- ond, and the temperature ranged from —120 °C to

-I- 120 °C. The tests were of a scanning nature. The

chamber was cooled to abou.t —120 °C with liquid

nitrogen, and then the temperature raised about

1 °C per minute. Readings were taken every 10 °C.

Equilibrium was not reached for each temperature

15J
and the results, therefore, are to a certain extent

relative. Only one specimen for most experiments

was tested; for more precise data of certain keypoints the temperature should be controlled and a niunber of runs made.

Samples of methyl methacrylate polymer, ethyl

methacrylate polymer, and an equal-by-weight

copolymer of the two were cast in test tubes and

machined to the specimen geometry. Figure 8

shows tan 8 versus temperature curves for the

three materials. It can be seen that the ethyl methacrylate and methyl methacrylate produce similar

polymers, with, as expected, the ethyl methacrylate having a lower softening temperature. The

copolymer is intermediate although much closer

to the ethyl methacrylate than might be expected.

Figure 9 shows the modulus versus temperature

data. The copolymer is intermediate between the

two end members in this prt>perty.

To test the effect of a mixture rather than a copolymer of ethyl methacrylate and methyl methacrylate, 50 parts by weight of ethyl methacrylate monomer and 50 parts of methyl methacrylate

polymer were rnixed together in a dough and

molded in the usual dental fashion, machined

and tested. Figure 10 shows data for the copolymer and the mixture plotted together. It can be

seen that the mixture has a broader softening

transition point, as is shown by the lower rate of

increase of tan 8 with temperature. Of great interest in this figure is the prominent peak at ap-

.26 -

ZO

M

n

H

«

Em

Zs

.18

.14

.06

.04

.02

0 CAST POLYMEThVL MFTHACRYLATE A CAST POLYETHYL METHACRYLATE a CAST COPOLYMER (.5 MMA - .5 EMA)

+ 100

TEMPERATURE °C Figure 8. Internal friction of methyl methacrylate, ethyl

methacrylate and an equal weight copolymer of the

two methacrylates

4.0

3.0

2.0

1.0

-r -r T"

• CAST POLYMETHYL METHACRYLATE ^ CAST POLYETHYL METHACRYLATE O CAST COPOLYMER (.5 MMA - .5 EMA)

-60 -40 +120

TEMPERATURE °C

Figure &. Effect of temperature on modulus of methyl m,ethacrylate, etlvyl

methacrylate and equal weight copolymer of the two methacrylates.

To convert from psl (pounds per square Inch) to N/m^ (newtons per square meter),

multiply by 6895.

152
I 1 I I I -100 -50 0 +50 +100

TEMPERATURE °C Figure 10. Internal friction versus temperature for

ethyl methacrylate-methyl methacrylate copolymer and

mixture.

proximately —10 °C. This was unexpected and, as

will be shown below, is in some way associated with

the polymer phase. Figure 11 shows the modulus

versus temperature data for the copolymer and the

mixture. Over most of the range the modulus of

the mixture is greater than the copolymer. There

appears to be a change in modulus level at about

the same temperature as the low temperature

maximum.

To determine the effect of the polymer-monomer

fabrication versus casting, a mixture of the above

poly (methyl methacrylate) powder and methyl

methacrylate monomer was made of equal parts

by weight and molded. In addition a compression molded sample of the polymer was also

made. In figure 12, the internal friction versus temperature curves are plotted. The data from the

compression molded sample are remarkably similar to the data of the cast sample with the exception of the pronounced low temperature peak. It

is clear that this phenomenon is related to this

polymer, possibly to the complex surface of the

material. The polymer-monomer mix material

showed a similar low temperature peak and a lower

glass transition temperature. It can be seen in

figure 13 that the cast specimen produced the highest modulus of rigidity presumably because of a

higher molecular weight produced by the slow

cure.

oH

XH

W

a

4.0

3.0

2.0 -

1.0

CAST COPOLYMER (.5 MMA - .5 EMA)

a POWDER-LIQUID MOULDED

.5 EMA MONOMER

.5 MMA POLYMER

-L. -120 -100 -80 -60 -40 -20 -0 +20 +40 +60 +80 +100 +120

TEMPERATURE °C

Figure 11. Modulus versus temperature for ethyl methacrylate-methyl

methacrylate copolymer and mixture.

To convert from psl (pounds per square Inch) to N/m» (newtons per square meter),

multiply by 6895.

153
.26

.22

.06

.04

I T

• CAST POLYMETHYL METHACRYLATE

75 MMA M0N0^4ER

,5 M^4A POLYMER ^ COMPRESSION MOULDED MMA POLYMER

POWDER-LIQUID MOULDED

-100 -50 HOO

TEMPERATURE °C

Figure 12. Internal friction versus temperature for

methyl mcthacrylate polymer prepared hy casting from

liquid, ty molding powder-liquid mixture and hy compression molding powder only.

A comparison of the internal friction curve of

"powder-liquid" denture base material of con- ventional impact resistance with cast poly (methyl

methacrylate) is shown in figure 14. The low temperature properties are similar with the exception

of the polymer associated peak described above.

Since the impact resistance is not enhanced, this

peak is not likely to be an "in the chain" molecular phenomenon. The slightly subdued beta peak

would indicate the presence of a nonmethacrylate

component.

The internal friction curves for two impact re- sistant denture materials compared to cast poly

(methyl methacrylate) are shown in figure 15. Impact resistant material II shows a glass transition

temperature similar to poly (vinyl chloride). Poly

(vinyl chloride) has a broad low temperature peak

at about -25 °C and a minimum at about +20 °C.

There is no evidence of this in this curve ; however,

a blend with poly (methyl methacrylate) would

mask the peak because of the poly (methyl methacrylate) beta peak.

Impact resistant denture base I is obviously

closely related to poly (methyl methacrylate).

There is a slight indication of a low temperature

peak superimposed on a poly (methyl methacrylate) curve between —50 °C and —20 °C. Also,

tliere appears to be a slig'ht low temperature

"polymer" peak near 0 °C. The modulus versus temperature plot of the same three materials

shown in figure 16 shows a dispersion starting

about —60 °C with impact resistant material I

4.0

3.0

o

rH

0< 2.0

1.0

CAST POLYMETHYL METHACRYLATE

D POWDER-LIQUID MOULDED

. 5 MMA MONOMER

.5 MMA POLYMER

COMPRESSION MOULDED MMA POLYMER

-L. -L. -120 -100 -80 .60 -40 -20 -0 +20 +40 +60 +80 +100 +120

TEMPERATURE °C

PiGUEE 13. Modulus versus temperature for methyl methacrylate polymer

prepared by casting from liquid, iy molding powder-liquid mixture and

hy compression molding powder only.

To convert from psl (pounds per square Inch) to N/m' (newtons per square meter),

multiply by 6895.

154
.22

.18

.06

.04

.02 (-

1 "T"

• CAST POLYMETHYL METHACRYLATE D POWDER-LIQUID DENTURE BASE MATERIAL

.26

.22

.14

.10

.04

.02

1 1

• CAST POLYMETHYL METHACRYLATE O IMPACT RESISTANT DENTURE BASE I ^ IMPACT RESISTANT DENTURE BASE II

-50

TEMPERATURE °r TEMPERATURE °C

Figure 14. Internal friction versus temperature for oast poly {methyl methacrylate) and poivdcr-liquid denture

base material.

Figure 15. Internal friction versus temperature for cast poly (methyl methacrylate) and impact resistant denture base materials.

4.0

3.0

o

X

2.0

1.0

T 1 1 1 1 1 1 r—

• CAST POLYMETHYL METHACRYLATE O IMPACT RESISTANT DENTURE BASE I A IMPACT RESISTANT DENTURE BASE II

-120 -100 -60 -40 -20 +20 +40 +60 +80 +100 +120

TEMPERATimE "C

Figure 16. Modulus versus temperature for cast poly (methyl methacrylate)

and impact 7'esista7it denture base materials.

To convert from psl (pounds per square Inch) to N/m^ (newtons per square meter),

multiply by 6895.

452-525 0—72-^ 11 155
that would, support theory that there is some low

temperature transition taking place that might

relate to the toughness of the material.

From the stress strain curves shown in hgure 17

of the two impact resistant materials, it can be

seen that there is very little drawing during the

tensile failure, hence one w^ould not expect evidence

of pronounced low temperature phenomena.

IMPACT RESISTANT DENTURE

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

INCHES PER INCH

FiGUKE 17. Stress strain curves for two impact resistant

denture base materials.

To convert from psl (pounds per square Inch) to N/m^

(newtons per square meter), multiply by 6895.

5. Equipment for Torsion Pendulum

Measurements

Many authors have written about equipment for

measuring low frequency mechanical properties

[8, 11, 17, 22, 26, 31, 34-42]. In March, 1962 a basic

torsion pendulum design was described, before the

Dental Materials Group of the International Association for Dental Research. This device utilized a light beam and a galvanometer scale for visual

data recording. Figui^e 18 is a photograph of the

torsion pendulum enclosed in a Lucite Avail chamber to prevent drafts. Figure 19 shows a cutaway

sketch with the pendulum attached to the top of

the machine and the sample surrounded by a chamber which can be cooled by liquid nitrogen to

about -180 °C or heated to -f-300 °C. Beneath the

temperature control chamber an inertia bar is at- tached to the pendulum via a pin vice. On the

inertia bar there is a mirror to deflect a light beam

and thereby show the amplitude of vibration on

Figure 19. Cutaway sketch of equipment shown in

figure 18.

the galvanometer scale. Figure 20 shows a photograph of the equipment with a zirconimn arc lamp

light soui'ce. The frequency was determined with

a stop watch and the readings on the galvanometer

scale were recorded vocally on a tape recorder.

156
Figure 20. Torsion pendulum equipment with zirconium

arc lamp.

Oscillations were started by the electro-magnetic

coils located near the ends of the inertia rod. Precession in the oscillation was damped out by the

damping oil at the bottom of the pendulum. The

pin vices supporting the specimen were attached to

thin-walled stainless tubing which had the strength

and rigidity to function properly mechanically,

but were poor conductors and minimized thermal

variations.

In designing equipment to operate above two

cycles ]per second, it is necessary to use a recording device.

For measuring very low damping materials, high

sensitivity _ is needed. This can be provided by

operating in a vacuum and thereby eliminating

air resistance. Blum [8] developed a very sensitiye

instrument and recorded with fused quartz a value

for tan S of 0.000013 at a vacuum of 0.5 jum of

mercury. This suggests that fused quartz is an

excellent material for calibrating the internal friction of an instrument itself.

For further improvement in temperature distribution, Douglas et al. [36] recommended the use

of a chromel-alumel thermocouple rather than

copper-constantan because of the high conductivity of the latter. To eliminate residual magnetic

effects from the starting electro magnets, they used

copper wire coils wound on plastic forms and a

piece of silicon iron transformer sheet attached to

the inertia bar.

Nielsen [4r2], in his design to eliminate the effect

of tensile stress, placed the fixed end at the bottom

of the pendulum, with the inertia member at the

top, supported by a fine wire and counter weight.

It is desirable to have all nonaxial rotation

eliminated. This requires good alignment, a good

starting system, and a means of maintaining concentricity. This has been done by bearings (which

have their own friction) and by damping oils. Damping oils will eliminate precession but offer

virtually no resistance to the axial rotation of a smooth cylindrical rod.

The challenge of recording the oscillations of a

torsion pendulum offer the ingenious instrument

designer ample opportunity. For most sensitivity

the optical system recorder seems best. One technique is to send a light beam by a mirror attached

to the inertia bar to a sheet of photographic paper

on a revolving drum. Development of the paper

will then produce a record of the oscillations of

the pendulum. A variation of this used by Turley

[22] employed ultraviolet light reflected 'to ultraviolet sensitive paper. This can eliminate the need

for development of the paper.

Weir [34] described in detail an optical device

for recording the amplitude of oscillation using

two photocells close together near the zero deflection point. The principle of his recording is based

on the fact that for constant frequency in a sinusoidal oscillating system, the velocity at zero deflection is proportional to the amplitude. Thus the

time it takes the light beam to travel between cells

is proportional to the amplitude. The period can

be determined by noting the total elapsed time in one cycle. A digital recorder in conjunction with

a microsecond interval time meter was used to

record the data.

Plajek et al. [37] used a constant light source aimed at the mirror with a shadow device that

would cut off part of the beam in proportion to

the amplitude. Thus the emf that was generated

from the photocell was of a sinusoidal form in

proportion to the frequency and amplitude of the

oscillation of the pendulum.

Gillham [41] utilized an inertia disc of glass

with varying opacity such that a light beam passing through it to a photocell will be of varying

intensity with the angle of rotation.

Klein [40] described a torsional pendulum

utilizing an electromagnetic pickup. This was ac- complished by attaching a permanent magnet to

the pendulum which induced a current in a coil

mounted adjacent to it on the fixed portion of the

instrument. The induced voltage was then

recorded.

Nielsen [42] allowed the "fixed" end of the

pendulum to have a small motion by restraining it with relatively rigid spring rods. The motion is

proportional to the amplitude of the inertia member and is measured by a linear differential variable transformer. He reported recording values

of tan S as low as 0.003.

The Plas-Teoh unit shown in figure 21 was used

for some of the data reported in this paper. The

instrument has a fixed bottom member with a counterweight support system to compensate for

length changes. The inertia member at the top is

supported by a frictionless bearing in a rotary differential variable transformer which is used to

157
Figure 21. Example of commercially availahle torsion

pendulum equipment.

(Courtesy of Plas-Tech Equipment Corporation, Natlck, Mass.)

supply a voltage proportional to the amplitude to a recorder.

Heijboer [26] utilized a recording system

wherein a specially conductive paper on a rotating

drum was marked by an electric spark from the

oscillating inertia member of the pendulum, thus

recording the motion of the pendulum.

Kodi-iguez et al. [35] and Plajeck et al. [37]

presented designs of equipment for recording the

internal friction of weak elastic materials such as

gels. In one case a conical plate type of viscosimeter is used to hold the sample, and an air bearing is used to support the weight and minimize

friction. In the other case a disc-like specimen was

held between two plates and was the elastic member of the pendulum system.

Maxwell [39] described a nonresonant driven

device to allow continuous variation of temperature and frequency at a controlled strain. The use

of a controlled strain instead of a controlled stress

is important in nonlinear viscoelastic materials.

The instrument tests a cantilever beam of circular

cross section, held in a collet and rotated at the

desired speed about its cylindrical axis. A load is

applied at the unsupported end causing a fixed deflection. Each portion of the beam then will be

alternately under tension and then compression.

If the beam is made of a viscoelastic material there

will be a lag in the strain relative to the force,

and a deflection normal to the applied force will

tend to take place. This deflection can be eliminated by applying a force perpendicular to the

initial force direction. The ratio of the force nor- mal to the desired deflection to the deflecting force

itself yields tan 8. These can both be measured

by strain gauges. The advantages of this system

are a wide range of frequencies with a single specimen geometry and with simple recording devices.

Gillham [41] described the torsion braid pendulum. Basically the technique used is a multifilament substrate with the sample disposed

through the strands by melt or solution (or by

other means). This allows a material to be tested

beyond coiiditions wherein it can support its own

weight (i.e., at high temperatures or in its re- action periods) . The use of a multifilament system

allows more material to be tested, better stress

transfer, and minimizes the modulus of the substrate. 6. Future Work

One obvious project from the brief work re- ported in this paper is to further understand the

nature of the low temperature peak in the pearl

polymer used. If this is a surface phenomenon it should be proportional to particle size. A series of

tests with dift'erent sieve fractions from the same

polymer should verify this. If it is a bulk property, then a systematic review of the suspension

agents, stabilizers, catalysts, etc., will have to be

made to determine just Avhat produces this effect.

Of interest to dentistry is energy absorption. It

has often been speculated that the ridges could

be i^rotected if the shock of mastication could be

absorbed. Indeed soft liners have been proposed

for this. With internal friction measurements it would be posible to develop a material of very

high damping capacity at the mouth temperature

region with a broad range of frequency absorption. Whether this is better than rubbery materials that are of low damping capacity is not

known. Clinical tests would be required. However,

recording the internal friction of the denture

materials at a variety of frequencies in the mouth

temperature range Avould allow a characterization

of the material that might profitably be coordinated with clinical data.

The torsional braid pendulum should be of value

in characterizing all types of setting reactions. It

would be possible to get nearly continuous data on

the modulus and the damping capacity of the material versus time at a fixed temperature or at a programmed rate of temperature change. This

should be applicable to dental cements, amalgam,

impression materials, resin, etc. There is evidence that the internal friction of a composite material is affected by the bond between

the resin and the matrix. This would offer a means

of quantitatively evaluating the bond and then

testing again after exposing the composite to

various environments, thermal cycling, etc. The

158
data will give not only the internal friction

changes but also tlie modulus clianges.

High strength materials for denture base use are of obvious interest. A significant amount of work

has been done correlating toughness with internal

friction curves. Experiments might be done developing low temperature peaks of various magnitudes and correlating the data against the toughness of the material, the tensile strength of the

material, and, most important, against the creep

of the material. When there is significant drawing

taking place in absorbing the energy of impact,

there will also be distortion. There is a level of

distortion that is not acceptable, and some brittle- ness would be more suitable.

An intriguing possibility of utilizing the torsion

pendulum is as a means of following the progress

of polymerization in self-cure systems. Following

the shrinkage is unrewarding because the specimen

will shrink by polymerization but also by loss of

monomer either through volatility or leaching in

saliva. The modulus and internal friction curves

will be very sensitive indicators of the degree of

polymerization. It would be interesting to make

self-cure and heat-cure samples and note the

change in internal friction curves of both with

time after storage in 100 °F (38 °C) water. Presumably, in the case of the heat-cure material

where polymerization is virtually complete, the

principle effect will be a plasticizing or the system by absorbed water. In the case of the self -cure material there will be that plasticizing effect, but

coupled with it will be a leaching of the monomer

and a continued polymerization of the monomer

with time.

In analyzing ceramic materials there is a problem of the specimen geometry. With a low frequency pendulum, specimens are needed of the

order of one square millimeter in cross section and

perhaps 5 to 10 cm in length. Most of the work done

in ceramics has been done with drawn glass fibers.

To fire a ceramic to these dimensions is difficult.

To draw a glass fiber from a ceramic melt would

not be satisfactory because the microstructure of a material would be significantly altered. One approach is to fire a specimen oversize and machine

it to the proi^er dimensions. Another approach

would be to use refractory investments to make a mold of about the right dimensions that would

support the ceramic during the firing operation. A

third approach Avould be to use a two component

system—e.g., porcelain on gold. By knowing the

data of the gold system by itself, the information

relative to the porcelain can be approximated. Internal friction measurements should be an excellent

method of determining changes in firing and re- peated firings such as solution, crystal melting,

and devitrification.

There has been a great deal of interest to date

in the nature of the porcelain to gold bond. Evidence has been presented to show that there is ionic

diffusion taking place during the firing reaction

aiding in the bond. The torsion pendulum is quite

sensitive to ionic diffusion in glassy structures as was shown above.

This suggests that a method could be developed

to identify the degree of diffusion and identify

the activation energy for diffusion. Experiments

in metal and ceramic design could then be con- ducted to optimize this diffusion process. The degree of diffusion in the porcelam gold system is

probably small and located near the interface. In

a system of high interface area, it might be possible to get sufficient diffusion to be measurable.

One approach would be to disperse in a ceramic

mix a fine gold alloy powder and then fire at the

appropriate temperature. The electron probe could

be used as before to determine the species of ions

taking part in the diffusion, and the internal friction curves at several frequencies could be used to

give a measurement of the amount of diffusion and

the activation energy of diffusion process.

It should be possible to use internal friction

measurements to help determine the causes of

physical property changes noted in implant materials. For instance, embrittlement through cross- linking would yield a different spectrum than embrittlement through degradation. It would be

worthwhile testing the internal friction of materials as a function of time of implantation.

7. References

[1] Nielsen, L. E., Mechanical Properties of Polymers

(Reinhold Publishing Corp., N.Y., 1962).

[2] Braden, M. and Stafford, G. D. Viscoelastic properties of some denture base materials, J. Dental

Res. 47, 519 (1968).

[3] Zener, C, Elasticity and Anelasticity of Metals

(University of Chicago Press, Chicago, 1948).

[4] Ryder, R. J., The Internal Friction of Simple

Alkali Silicate Glasses Containing Alkaline Earth

Oxides, Ph. D. Thesis, Pennsylvania State University, 1959.

[5] Fitzgerald, J. V., Anelasticity of glass : I. Intro- duction, J. Am. Ceram. Soc. 34, 314 (1951).

[6] Fitzgerald, J. V., Anelasticity of glass II. Internal

friction and sodium ion diffusion in tank plate

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Ceram. Soc. 34, 389 (1951).

[7] Kirby, P. L., Internal friction in glass Part II

Flexural and torsional vibration, J. Soc. Glass

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[8] Blum, S. L., Some physical factors affecting the

internal damping of glass. J. Am. Ceram. Soc. 38, 205 (1955).

[9] Forry, K. E., Two peaks in the internal friction as a function of temperature in sodium silicate glasses, J. Am. Ceram. Soc. 40, 90 (1957).

[10] Hoffman, L. C. and Weyl, W. A., A survey of the

effect of composition on the internal friction of

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[11] Shelby, J. E. and Day, D. E., Mechanical relaxations in mixed alkali silicate glasses, I. Results,

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[12] Fitzgerald, J. V., Anelasticity of glass No. Ill, effect of heat treatment on the internal friction

of tank plate glass, J. Am. Ceram. Soc. 34, 388

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[13] Fiehter, R., Damping in metals. Schweiz. Arch. -Ann.

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159
[14] Nowick, A. S., Anelastic phenomena in metals and [28]

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[16] Boyer, R. F., The relation of transition temperature [31]

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[17] McCrum, M. G., Read, B. G., and Williams, G., Anelastic and dielectric effects in polymeric solids, [32]

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[19] Deutseh., K., Hoff, E., and Reddish, W., Relation

between the structure of polymers and their

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[20] Nielsen, L., Effect of chemical heterogeneity in [35]

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[21] iCuddahy, E. F., and Moacanin, J., Dynamic Mechan- [36]

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Plastic Chemistry Papers, (April 1968). [37]

[22] Turley, S. G., Effect of Polymer Structure on Impact

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[23] Heijboer, J., Dynamic mechanical properties and

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3755-3763 (1968).

[24] Boyer, R. F., Dependence of mechanical properties [39] on molecular motion in polymers. Polymer Eng.

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[25] Matsuoka, S., Aloisio, C. J., and Daane, J. H., Some [40]

aspects of brittle failure in polymers, Applied

Polymer Symp. 5, 103 (1967).

[26] Heijboer, I. J., Modulus and damping of polymers [41]

in relation with their structure, Plastica 19 ii, 489 (1966).

[27] Maeda, M., Tanaka, K., Hibi, S. and Kakei, K., [42]

Mechanical properties of FRP, Part I, Journal of

the textile machinery society of Japan 13, 58-67

(1967).

Galperin, I., Dynamic mechanical properties of a TiOa filled cross-linked epoxy resin from 20-90° C,

Am. Chem. Soc. Div. of Plymer Chem. Preprints

7, 890 (1966).

Reinhardt, J., Dynamic mechanical behavior of a

polyester during cross-linking. Plastics & Poly- mers, 36, 541 (1968).

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performance of impact plastics, Paper presented

at Seventh International Symposium on High

Speed Testing, Boston, Mass. (March 1969).

Myerson, R. L., The Internal Friction of Linear

and Cross-Linked Phosphate Glasses and

Methacrylate Polymers, M.A. Thesis, M.I.T. May

1961. van Wazer, J. R., Phosphorous and its Compounds,

I ( Interscience Publishers, New York, 1958).

Myerson, R. L., Effect of cross-linking on mechanical

damping and stiffness of methacrylates, I.A.D.R. Abstracts of the 40th General Meeting, p. 89

(1962).

Weir, F. E., Use of a new torsion pendulum for

polypropylene property evaluation, SPE Transactions 2, 302 (1962).

Rodriguez, F., Van Brederode, R. A., and Cocks,

G. G., A recording air-ibearing torsion pendulum.

J. Appl. Polymer 'Sci. 22, 2415-2420 (1968).

Douglas, R. W., Duke, P. J., and Magurin, O. V., On

the network contribution to the anelasticity of

glasses, Phys. and Chem. of Glasses 9, 169 (1968)

Plajek, D. J., Vrancken, M. N., and Berge, J. W., A

torsion pendulum for dynamic and creep measure- ments of soft viscoelastic materials. Trans, of the

Soc of Rheology II, 39-51 (1958).

Copley, G. J., A simple photocell counter for the

determination of the logarithmic decrement of the

torsion pendulum, J. Sci. Instr. 43, 845-846

(1966).

Maxwell, B., An apparatus for measuring the re- sponse of polymeric materials to an oscillating

strain, ASTM Bulletin 76-80 (July 1956).

Klein, J., A method for recording torsion or bending oscillations in damping experiments on high

polymers, Plaste Kautschuk 13, 20 (1966).

Gillham, J. K., Thermomechanical properties of

polymer by torsional braid analysis, Appl.

Polymer Symp. 2, 45 (1966)

Nielsen, L. E., A recording torsion pendulum for

the measurement of the dynamic mechanical

properties of plastics and rubbers. Rev. of Sci. Instr. 22, 690 (1951).

160
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings, of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Ultrasonic Methods for Determination of Mechanical Properties

George Dickson

Dental Research Section, Institute for Materials Research, National Bureau of Standards,

Washington, D.C. 20234

Elastic characteristics, shear modulus, Young's modulus, bulk modulus and Poisson's

ratio for materials can be determined by a variety of ultrasonic methods, most of which

involve the measurement of the velocity of sound in the material. The methods are precise,

rapid, nondestructive and applicable to small specimens. As a number of reports in the

literature indicate, ultrasonic methods offer particular advantages to the study of dental

materials and mineralized tissues. Key words : Dental amalgam ; dental materials ; elastic properties ; mechanical properties

nondestructive testing ; ultrasonic techniques.

1 . Introduction

The most commonly reported mechanical property of dental materials is strength, usually compressive or tensile. However, strength is not a measure of the reaction of a material within its

functional range. To obtain some characterization

of the mechanical reaction of a material to the

forces encountered in use, Young's modulus is often determined. Generally, Young's modulus is

calculated from a stress-strain curve obtained by

loading a specimen in a teisting machine and measuring the change in length with increase in load.

While the modulus determined in this way is usually considered a measure of the elastic characteristics of the material, the data often represent a combination of properties : dlastic, retarded elastic,

and viscous with varying magnitudes of time and

stress dependence. This is illustrated in the stress- strain curves for dental amalgam shown in figure

It is evident that even with low stresses, the behavior of dental amalgam is not purely elastic. A

plot of the strain of dental amalgam versus time

when under constant stress (fig. 2) indicates that

at least three types of response to mechanical force

are present, an instantaneous elastic response, a

retarded elastic response, and a viscous response

To determine the magnitude of the instantaneous elastic response, it is necessary to isolate

this particular property of the material.

Time dependent responses such as viscous creep

and retarded elasticity can be eliminated or re- duced to a negligible level if the stress can be applied and removed rapidly enough. One method

1 Figures In brackets Indicate the literature references at the end of this paper.

of accomplishing this in some cases is by introducing sonic or ultrasonic waves into the material.

Essentially this causes small volume elements of

the material to be cyclically strained as a series of

stress waves is propagated through the specimen.

The velocity of the stress wave depends upon the

mass or density of the particles or small volume

elements of the material which are displaced and

upon the elastic characteristics of the material

which provide the forces tending to restore the

particles to their equilibrium positions.

In addition to minimizing time dependent re- sponses, ultrasonic methods have other advantages.

They are rapid and nondestructive, and they permit repeated measurements on the same specimen

STRAIN IN/IN.

Figure 1. Typical stress-strain curves of various amalgams

in tension, using 0.003 in/min head speed and 7 day old

speoimens [1].

To convert psi to MN/ma multiply by 6.895 XIO-^.

161
<

ir 6

1 1 r- 1 1

1 1 1 1 1 1 1 r I

'

3807 pil

-

/ 3458 P«l

2959 p«< /

••*

• ^ 1900 pil ~

-

! 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 200 300 TIME - MINUTES

Figure 2. Creep and recovery of amalgam loaded in

tension [2J.

To convert psl to MN/m= multiply by 6.895X10-'.

as it is aged or subjected to temperature change or

other treatments. Ultrasonic methods can be applied to small and brittle specimens on which

other methods of stress application and strain

measurement may be difficult. Also, ultrasonic

methods are particularly suitable for measuring

elastic coefficients in different directions in anisotropic materials.

For measuring the mechanical properties of

materials, low amplitude or low energy ultrasonic

waves are employed, and the effects of the medium

on the wave are observed. High amplitude waves

which would produce a permanent change in the

specimen are avoided.

There are two types of stress waves which are particularly useful for determination of mechanical properties. These (fig. 3) are the compressional

or longitudinal wave in which particle motion in

the medium is parallel to the direction of propagation of the wave, and the shear or transverse wave

in which the particles vibrate at right angles to the

direction of propagation of the wave.

Direction of Wave Propagation

Particle Vibration

Transverse i-^aves

< >

^ ^ Particle Vibration

Direction of Wave Propagation

Lonpitudinal Waves

Figure 3. Two types of ultrasonic stress waves used for

determination of mechanical properties of materials.

2. Velocity and Modulus

Determinations

For plane waves in an elastic isotropic homogeneous medium [3], the velocities of the trans- verse and longitudinal waves can be shown to be

and

E{l-y)

p(l -1-7) (1-27)

where

p=density

6^= shear modulus

E= Young's modulus

7=Poisson's ratio

When the wave length is large compared to

specimen dimensions [4], the equations for propagation in an infinite elastic medium do not

apply. For example, for wave propagation parallel

to the sides of a thin isotropic plate

V,

E

p(l-7')

For propagation in an isotropic rod mth radius

small compared to wavelength [4], the longitudinal

wave velocity is

Where frequencies employed are high enough so that the wavelength is small compared to specimen

dimensions [3], the shear modulus, Young's

modulus, the bulk modulus and Poisson's ratio can be determined from the velocities of the

longitudinal and transverse waves, and the density

of the material as follows:

Shear modulus G=pVt'

Young s modulus Ji,=pVt

\^ y 2_y 2 J

Bulk modulus K=p\

Poisson's ratio 7=

From these equations, it is evident that determination of elastic constants is essentially a problem

of determination of the velocity of sound in the

material. Various methods can be employed for

determining the velocity of stress waves. One of the

most widely used is the pulse-echo teclmique, which

has been described in numerous papers and books

162
[3, 4, 5, 6]). In this method (fig. 4), a quartz

crystal, or other transducer driven by a pulsed

oscillator, is used to introduce the wave at one of

two parallel faces of the specimen. The same transducer, or a similar one at the opposite face of the

specimen, is used to receive the mechanical vibration and produce an electrical signal which, after

amplification, can be observed on an oscilloscope.

Unless the attenuation of the specimen material is very high, the stress Avave will echo back and forth

through the specimen a number of times, and signals representing each double transit will be

observed.

If a piezoelectric quartz transducer is used, the

crystal may be cut with axes oriented so as to produce either longitudinal vibrations (X cut) or

transverse vibrations (Y or AC cut) in the specimen. The crystal can be driven at its natural frequency or at an odd harmonic by a pulsed oscillator tuned to the required frequency. The time

interval from one pulse to the next is made long

enough to allow the echoes to die out between

pulses. The transducers can be coupled to the specimen with a thin film of oil or a viscous resin.

A waveform generator (fig. 4) can be used to

control the pulse repetition rate and supply a trigger signal to initiate both the pulse and the oscilloscope sweep. The rectified echo signals, after

passage through a broadband amplifier, may be

displayed on an oscilloscope (fig. 5) with a calibrated time scale and the time between echoes

determined. The frequency, pulse length and pulse

repetition rate may vary over a wide range.

Accuracy of the velocity determination will depend on the quality and quantity of echoes which

can be observed, which, in turn, depend upon the

attenuation of the signal by the specimen material, the presence of spurious signals which may

result from reflection from sides of the specimen

with the probability of mode conversion from

longitudinal to transverse waves or vice versa, and

on the precision of specimen preparation. Specimen parallelism to 2 parts in 10* or better is recom- mended, although less accurate parallelism may

produce usable results for some purposes. Under

favorable conditions, velocity determinations with

a sensitivity of 0.1 percent and an accuracy of 1 percent can be obtained [5]

In another pulse method, the sing-around technique (fig. 6), the signal received when the stress

wave has passed through the specimen is used to

trigger the next pulse from the oscillator, and the

pulse repetition frequency is measured. Since time

delays other than that required for the stress wave

to pass through the specimen are ordinarily comparatively small, the period of the system is es- sentially the transit time. For accurate determinations, these other delays must be taken into ac- count. With this simple system, velocity changes

of less than one part in lO'' can be detected [5].

Refinements of the sing-around method have

been made to provide sensitivity to velocity

changes of a few parts in 10^ [7]. These refinements consist of using the signal after a specific

number of echoes rather than after the first transit to refire the transmitter and also using a specific

cycle within the pulse. This method is particularly

Wa V e form Generator

Quartz

Transducers

Spe c i men

Preamplifier

X

Wide Band

Amplifier

Figure. 4. Typical pulse echo system.

Figure t5. Pulse echo pattern, a—input pulse, 6

pulses after passage

through specimen.

163
RF Pulse Generator SpecimeD- Amplifier

Frequency

Counter

Feedback

Figure 6. Simplified diagram of sing-around system.

useful when the interest is in measurements of

small changes in velocity rather than in an absolute value.

Interference methods can be used for measuring

velocities. In optical interferometers, such as the

type used in measuring the setting expansion of

amalgam and in ultrasonic interferometers in a

liquid medium, the number of wave lengths between two reflecting surfaces is changed by a con- tinuous variation of path length. Such a system is

not feasible for ultrasonic measurement of the

properties of solids since path length cannot be

readily varied. A similar effect can be obtained,

however, by a continuous variation of frequency

and consequent change in wave length. For example, sets or two gated, phase-coherent pulses from

a continuoiis wave oscillator may be spaced in time

so that the second pulse which has passed once through the specimen arrives at the receiving

transducer at the same time as the first pulse which

has echoed and passed three times through the

specimen [3]. If the oscillator frequency is then

adjusted, the phase relationship of the pulses will

vary and when the signals are added a series of

nulls will be produced. The velocity can be determined from the frequency difference necessary to

pass from one null to the next. If the phase change

on reflection from the ends of the specimen can

be neglected

where I is the length of the specimen and /i and /a

represent the frequencies at successive null positions. If the specimen material causes relatively

high attenuation of the ultrasonic wave, a continu- ous wave interferometric method can be used [3]

Here, the signal from the receiving transducer is compared with the signal being fed into the transmitting transducer and again phase change is observed as frequency is varied.

Kesonance methods can also be used for velocity

determinations. Such methods generally are used

with frequencies in the lower ultrasonic range. At

the natural frequency of a half-wavelength specimen bar, the longitudinal velocity is

and Young's modulus is E=p my.

One experimental arrangement [8] which has been

used for such resonance determination involves the

use of a three-component system, shown in figure 7,

consisting of a quartz crystal driver with the specimen of closely matched resonance frequency ce- mented to one end and a second quartz bar, also

closely matched in frequency, cemented to the

other end of the driver to act as a piezoelectric

gauge.

Another method of velocity measurement, analogous to optical index of refraction measurement,

is based on the determination of the angle of re- fraction of a stress wave as it passes from a liquid

medium into a solid specimen or on the determination of the focal length of an ultrasonic lens made

from the specimen material [3].

Velocity measurements can also be made by

reflection methods [3]. If ultrasonic pulses are

reflected from the surface of a solid immersed in a liquid medium, the reflection coefficient can be

defined as follows

Oirwhere

Pr and Pi represent the pressure amplitudes of

the reflected and incident pulses,

respectively

Vx and Vi represent longitudinal wave velocities

in the incident (liquid) and reflecting

(solid) media;

Pi and p2 represent densities of the liquid and

solid media.

Instead of making absolute measurements of the

reflection coefficient, the amplitude of the pulse

Specimen

Figure 7. Three component resonance system [8].

164
reflected from the test specimen may be compared

^vith the amphtude of the pulse reflected from a specimen for which the reflection coefficient is known. In this case

where Ai and A2 represent the amplitudes of the

pulses reflected from the known and unknown

materials, respectively. From the reflection coefficient and the density of the test material, the

velocity of the longitudinal wave and the

acoustic impedance, pV, can be calculated. This

method can be used with materials which have a high attenuation. It has the advantage also that

only one flat sm-face is required on the specimen.

The accuracy is reported to be of the order of 5

percent [3].

3. Attenuation of Ultrasonic Waves

The attenuation of ultrasonic waves may be

used to determine the internal friction of a

material. Internal friction may be defined as the

energy loss per cycle in an element of volume

divided by 27r times the maximum energy stored

in the element per cycle or as the tangent of the

angle by which the strain lags behind the stress. When energy losses per cycle are not high [6]

, AE AA 1. A, 5-

Q^

*=tan oL=zr-^=—j=- In -r=- 2TrE ttA t A2 it

where E=energy

.4= amplitude

5=logarithmic decrement

The attenuation of ultrasonic stress waves in

materials and loss interactions are discussed in

detail by Truell, Elbaum, and Chick in a recent

book, Ultrasonic Methods in Solid State Physics

[5]. In brief, energy losses may be divided into two

categories, those that are dependent on the physical characteristics of the specimen material, and

those that are characteristic of the method of

measurement. The latter group includes such

effects as coupling losses, losses due to nonparallelism of specimen faces, phase effects in the

transducers, and diffraction and mode conversion

effects in the specimen. The losses that are of

interest, those that are characteristic of the

material, are of two main types, scattering effects

and absorption effects. Scattering losses are caused

by lattice defects or other discontinuities in the

medium. The relative size of wavelength and

defect and the defect density will determine

whether or not the scattering can be measured.

Absorption losses include dislocation damping

losses, thermoelastic losses, conduction electron

damping losses, phonon-phonon interactions, ferromagnetic resonance effects, paramagnetic reso- nance effects, and nuclear spin energy interactions.

The attenuation of ultrasonic waves in materials can be determined by a number of methods. With

the -i.ili: ""cho technique, attenuation is measured

by comparison of the amplitude of successive

echoes. Losses which occur at each reflection also

must be taken into consideration. Effects of

of coupling losses may be determined by making

measurements with a transducer at only one end of

the specimen and comparing with results obtained

when an identical dummy transducer is coupled

to the opposite end of the specimen [5]. By making

measurements on specimens of different lengths,

effects other than the increased path length can

be kept constant and the attenuation calculated

from measurements on the different specimens.

Reflection of the wave from the sides of the specimen may result in mode conversion from transverse

to longitudinal waves or vice versa, and result in

energy loss and apparent attenuation. Such effects

will depend on wavelength and specimen size.

When the attenuation is small, it can be determined from the decay curve of free vibrations in

the specimen [6]. The resonance apparatus described for determination of velocity has as a primary use the determination of internal friction.

It can be shown [8] that the logarithmic decrement

of the specimen in such a system is proportional

to the ratio of the driver crystal voltage to the

gage crystal voltage

where the constant K is a, characteristic of the

system. After K has been determined from the

free decay of the system, the logarithmic decrement can be determined under different strain

amplitudes or other specimen conditions by measurements of the two voltages.

The internal friction can also be determined

from the amplitude frequency curve in the region

of resonance from the relation

where A/ is the total spread from one side of resonance to the other where the amplitude has

dropped to -j=- times the resonance amplitude [6].

4. Applications of Ultrasonic Methods

to Dental Materials

Ultrasonic methods have been used to measure

the mechanical properties of a number of dental

materials. Dickson and Oglesby [9] reported the

elastic constants of dental amalgams in which the

mercury content was varied by varying the

condensation pressure. Specimens 8mm in diameter

by 6 to 15mm in length were prepared as shown in

165
table 1 . Measurements were made by a pulse-echo

technic with a frequency of 5.5 MHz, a pulse

length of 0.5 to l.O^s and a pulse repetition rate

of 6 KHz. Elastic constants obtained for an

amalgam containing about 48 percent mercur}^

are shown in table 2. As the plot in figure 8 shows,

the relationship between Young's modulus and

mercury content was essentially linear over the

range investigated.

Using a resonance method with the three component system of specimen, driver crystal, and

gage crystal, Larson [10] investigated the Young's

modulus and internal friction of amalgam.' The

frequency was in the 50 KHz range. Young's modulus values of 8.5X10*' to 10.6 X10« psi (5.9X10*

to 7.3 X 10* MN/m^) were found with mercury contents varying from 30 to 60 percent. A peak in

both modulus and internal friction was reported

in the 45 to 50 percent mercury ranges.

As would be expected, the Young's modulus

values obtained for amalgam by the rapid strain

rates inherent in ultrasonic methods are considerably higher than those reported from conventional

stress-strain curve procedures. However, using a

diffraction grating strain gage which permitted

stress-strain curve data to be obtained in 3 to 4

seconds, Gardner, Dickson, and Kumpula [11]

obtained a modulus value of 8X10® psi (5.5X10*

MN/m") Avhich does not differ greatly from those

obtained by ultrasonic methods.

Table 1. Amalgam Specimen Preparation [9]

11 to 8 Hg/Alloy ratio

20 s trituration, Wig-L-Bug

3 to 6 mixes of two pellets

Condensed in steel die

8 mm diameter by 6 to 15 mm length

5,000 to 25,000 psi (35 to 172 MN/m")

35 to 49 percent Hg content

Table 2. Elastic constants of amalgam [9]

Property Value Range*

Hg content . _ . Young's modulusShear modulus. _ . Bulk modulus _ _ _ Poisson's ratio

48. 6%

9. 09X10« psi

3. 41X10« psi

9. 12X108 psi

0. 334

0. 7

0. 06X10«

0. 03X10«

0. 22X10«

0. 005

Range of three values.

To convert psi to MN/m^ multiply by 6.895X lO"'.

10.0-

9.8-

.•

'^9.6- . .

i. • • •

LLJ 9.4 -

9.2 - -

9.0-

S.sl I ! ! I I ! I L I 34 36 38 40 42 44 46 48 50

Hg Content %

Figure 8. Effect of mercury content on Young's modulus

of dental amalgam [9].

To convert psi to MN/m" multiply by 6.895X10-'.

In our laboratory, the pulse-echo method has

been employed to measure the elastic characteristics of experimental composite quartz or glassfilled resins. The atteaiuation of the ultrasonic wave

in these materials is high and often no suitable

echoes are obtained so that the procedure is re- duced to measurement of a single transit time.

Effects of time delays other than transit time are eliminated by measurements on specimens of vari- ous lengths. Data on Young's modulus for an ex- perimental restorative material are given in table

3 [12].

The elastic properties of apatites were investigated by Gilmore and Katz [13] using an ultrasonic interference technic. Powdered specimens

of hydroxyapatite, fluorapatite, chlorapatite, and

sodium chloride-hydroxyapatite composites were

studied when subjected to pressures of up to 50

kbar. Transducers were mounted on the back faces

of tungsten carbide pistons, as sho-wn in figure 9,

and interference was obtained between reflections

from the near and far specimen-piston interfaces.

Table 3. Elastic moduli of experimental composite resin

restorative material [12]

(In 10« psi)

Powder-liquid ratio

(grams powder to Modulus Method 0.4 ml liquid)

1. 10 1. 35 1. 45

E Stress-strain curve.. . 1. 2 1. 3 1. 4

E Ultrasonic. 2. 5 2. 6 2. 4

G Torsion pendulum 0. 7 0. 8 0. 8

G Ultrasonic. . 1. 0 1. 0 0. 9

To convert psi to MN/m^ multiply by 6.895X10-'.

166
X-cut quartz

transducer

Piston

Specimen

Y-cut quartz

transducer

Figure 9. Diagram of transducer, piston and specimen

arrangement for determination of moduli under high

pressure [IS].

Values obtained for some of the apatites, as well

as values for bovine dentin and enamel, are given

in table 4. Moduli of the apatites were found to

increase linearly with pressure in the higher

pressure ranges.

Lees [14] reported values for the specific

acoustic impedance of bovine enamel and dentin

obtained by the reflection coefficient method using

specimens mounted in acrylic resin. A pulse of

60 ns length was reflected from the specimens in

water. The signal amplitude was compared with

that of a pulse reflected from stainless steel under

similar conditions. Values obtained for acoustic

impedance (product of density and sonic velocity)

were in general agreement with values obtained

by other ultrasonic methods.

The elastic coefficients of animal bone have been

investigated by Lang [15] using an ultrasonic

pulse method for measurement of velocities. Analysis of the crystallographic structure of compoTable 4. Elastic moduli of Apatites [iSl

(In 10» psi)

Hydroxyapatite.

Fluorapatite

Chlorapatite

Dentin

Enamel

E

15. 7

3. 0

10. 7

G K

7. 7 12. 5

7. 5 13. 8

6. 3 10. 0

1. 2 2. 6

4. 4 6. 7

nents of bone and the piezoelectric and pyroelectric

characteristics of bone suggested that bone should

behave elastically as a hexagonal single crystal,

and the elastic stillness coefficients were determined

on this basis. Young's and shear moduli found for

axial and transverse bone directions, in dried bovine femur and phalanx and fresh phalanx, are given in table 5.

Table 5. Elastic moduli of bovine bone [16]

(In 10« psi)

Phalanx Femur

Fresh Dried Dried

E (axial) . . - 3. 19 4. 42 3. 77

E (transverse) 1. 64 2. 31 2. 60

G (axial) _ 0. 78 1. 09 1. 18

G (transverse)0. 65 0. 94 1. 07

To convert psi to MN/m^ multiply by 6.895 X 10-^.

To convert psi to MN/m^ multiply by 6.895X10-^.

5. Summary

A variety of ultrasonic methods are available

for determining the elastic characteristics of materials. These methods are precise, rapid, nondestructive, applicable to small specimens, and

they measure a specific property of the material.

As a number of recent reports in the dental literature indicate, they offer particular advantages to

the study of dental materials and mineralized

tissues.

6. References

[1] Rodriguez, M. S., and Dickson, G., Some tensile

properties of amalgam. J. Dental Res. 41, 840

(July-August 1962).

[2] Oglesby, P. L., Dickson, G., Rodriguez, M. S., Davenport, R. M., and Sweeney, W. T., Viscoelastic

behavior of dental amalgam. J. Res. Nat. Bur.

Stand. (U.S.), 72C (3) 203-215 (1968).

[3] Filipczynski, L., Pawlowski, Z., and Wehr, J., Ultrasonic Methods of Testing Materials (Butterworths, London, 1966).

[4] Hueter, T. F., and Bolt, B. H., Sonics (John Wiley &

Sons, Inc., New York, 1955).

[5] Truell, R., Elbaum, C, and Chick, B. B., Ultrasonic

Methods in Solid State Physics (Academic Press,

New York, 1969).

[6] Mason, W. P., Physical Acoustics—Principles and

Methods (Academic Press, New York, 1966).

[7] Forgacs, R. L., Improvements in the sing-around

technique for ultrasonic velocity measurements,

J. Acoust. Soc. Am. 32, 1697 (December 1960).

[8] Marx, J., Use of the piezoelectric gauge for internal

friction measurements. Rev. Sci. Inst. 22, 503

(July 1951).

[9] Dickson, G., and Oglesiby, P. L., Elastic constants of

dental amalgam, J. Dental Res. 46, 1475 (Nov.-

Dec. 1967).

[10] Larson, R. V., Internal Friction Damping in Dental

Amalgam, Thesis, Dept. of Mechanical Engineering, University of Utah (August 1967).

167
[11] Gardner, T. V., Dickson, G., and Kumpula, J. W.,

Application of diffraction gratings to measure- ment of strain of dental materials. J. Dental Res. 47, 1104 (Nov.-Dec. 1968).

[12] Barton, J. A., Jr., Burns, C. L., Chandler, H. H.,

and Bowen, R. L., An experimental intermediaterestorative composite material. (To be published

in J. Dental Res.)

[13] Gilmore, R. S., and Katz, J. L., Elastic properties

of apatites. Proceedings of International Symposium on Structural Properties of Hydroxyapatite and Related Compounds, National Bureau of

Standards, Sept. 1968. To be published.

[14] Lees, S., Specific Acoustic Impedance of Enamel and

(Dentin. Private Communication (1968).

[15] Lang, S. B., Elastic coefficients of animal bone, Sci- ence 165:287 (July 18, 1969).

168
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Stress Analysis of Dental Structures

R. G. Craig

School of Dentistry, University of Michigan, Ann Arbor, Mich. 48104

Because of the complex geometry of dental structures, most stress analysis studies

in dentistry have been experimental. Brittle coatings have provided generally semiquantitative information on the magnitude, direction and sign of surface stresses of fixed bridges,

removable partial dentures, complete dentures and the mandible under various loading

conditions. Data from electrical resistance strain gages attached to the metal surfaces of

pontics on a gold bridge with various load applications are shown. Applications of both

two and three dimensional photoelastie techniques are illustrated. This method, which

provides information on the magnitude, direction and sign of boundary and internal stresses

is dependent upon how well the model represents the real structure. Other stress analysis

methods involve photoelastie coatings, thermophotoelasticity, moire fringe patterns, holography and x-ray diffraction.

Key words : Brittle coatings for stress analysis ; dental materials ; denture, artificial, stress analysis ; photoelasticity, dental applications ; porcelain-gold restorations, stress analysis

stress analysis of dental structures.

1. Introduction

A complete stress analysis of a dental structure

would include the determination of the magnitude

of the principal stresses, their direction and sign

(tension or compression) at all points. Fortunately, from the standpoint of time, a complete

stress analysis is rarely required for evaluation of

design or for fracture analysis.

Theoretical stress analyses of highly simplified

dental structures have been carried out, but simplifying assumptions such as rectangular blocks

for pontics and supporting restorations and ends

of fixed bridges that are free, resulted in unrealistic calculations. The most sophisticated study

to date, considered the roots of anterior teeth to

be parabolas of revolution, the periodontal ligament to have designated physical properties, and

the boundary stresses were calculated as a function

of Poisson's ratio [1].^ As more information becomes available about the properties of human tis- sue and restorative materials, it is expected that

with the use of computer programs, analytical

stress analysis of dental structures will be possible.

At present, however, experimental stress analysis

of dental structures which have complex geometry

is simpler, less time consuming, and more reliable

than analytical stress analysis.

Within the time available, I plan to discuss

experimental stress analysis techniques which have

been applied to dental problems, to point out their

advantages and disadvantages, and to discuss

briefly experimental techniques that as yet have

not been described in the dental literature.

1 Figures In brackets Indicate the literature references at the end of this paper.

2. Experimental Stress Analysis Using

Brittle Coatings

Brittle coatings have been used for many years

by industry to indicate the position and direction

of the maximum tensile strain and stress [2, 3].

The early work involved the study of mill scale

on steel, oxidation layers on bright steel, anodized

layers on aluminum and the application of dried

coatings of lime and Portland cement prepared

from water slurries. Brittle coatings were not used

extensively until the development of resin coatings

which are prepared by spraying a part with a solution of limed wood rosin K, dibutyl phthalate, and

carbon disulfide. The strain sensitivity of the

lacquers is controlled by the amount of plasticizer

and the method is able to detect strains of about

0.0007 in/in (700 /nm/m). The dried coating, if

properly applied, contains uniform finely dispersed

air bubbles and the sensitivity is essentially independent of the thickness between 0.004 and 0.008

in (100-200 /*m). When the coated structure is

loaded, cracks appear in the coating at the point

of maximum tensile strain and proceed perpendicular to the tensile stress. When temperature and

humidity control is used and corrections are made

for creep of the lacquer, quantitative values for the

magnitude of strain can be obtained. These two

quantities may be the only measurements necessary

for the stress analysis, and the method has been

used successfully to predict fatigue failures. It has

the advantages that the tests can be made on the

actual structure and with for practical purposes,

an infinitely large number of strain gages. Compressive strain and stress can be determined by allowing the lacquer to creep under a load less than

169
that required to produce cracks. After sufficient

creep occurs, the load is removed rapidly and the

crack patterns observed. Compressive strain also can be measured by preparing the coating while

the structure is under load; on unloading, the

cracks that appear indicate the compressive strain.

Thus, magnitude, direction, and sign can be determined; the method is limited to surface strain

which may not be too serious since failures are

initiated at surfaces.

A typical example of the use of a brittle lacquer

coating in determining the surface strain on the

metal framework of a mandibular partial denture

when loaded 0}i the left posterior saddle area is shown in figure 1 [4]. The relationship of the load

to the location of the cracks and the direction of

the tensile strain in the saddle, minor connectors

and major connectors can be seen. Of particular

interest is that the maximum tensile strain oc- curred at the left finishing line and that the major

connector is under biaxial tensile strain.

3. Experimental Stress Analysis Using

Resistance Strain Gages

Electrical resistance strain gages, of wire, foil, or semiconductor type, are described in textbooks

[5] and all have been used in dental research [6].

The wire gages are rugged but their large size

limits their use to appliances having large, relatively plane surfaces. The semiconductor gages are noted for their high output but their size again,

presents problems. The foil gages have been the

most practical for small areas on dental bridges

and partial dentures. Unless rosette gages are used,

some prior information is needed about the direction of the stress in the area where the gage is to

be cemented. Of course, the problem of rosette

gages is their rather large size. Strain gages can measure the magnitude, sign, and depending on the

type of gage, the direction of the strain. A large

Figure 1. Mandibular partial denture framework coated

with a brittle lacquer and loaded on the left posterior

portion of the saddle.

number of gages may be necessary in a complex

structure to obtain a reasonably reliable stress

analysis, and thus, an overall idea of the stress

distribution may be difficult to obtain.

A gold bridge with strain gages attached to the

metal surfaces of the pontics is shown in figure 2

[7]. The sign and magnitude of the strain for

various loads and positions of loading are shown

in figure 3 for the gage on the molar pontic. As

the occlusal loading site was moved from the anterior to posterior position, the gage recorded first

Figure 2. Foil strain gages attached to a four-unit gold

alloy posterior bridge.

I 23456789 10 LOADING POSITION

Figure 3. Strain versus loading position for the gage on

the molar pontic of the four-unit gold alloy posterior

bridge.

170
tensile and then compressive strain. Experience

has shown replacement of the gage in as nearly as possible the same position results in strain-position curves of the same general shape, but not identical

magnitude; this condition results from being unable to cement the second gage in exactly the same

location and orientation as the first. Of course, care must be taken in cementation and the gage must

be protected if it is to be used in an adverse

environment.

The strain gage method permits the measure- ment of surface strains of low magnitudes. (Special gages are available for fatigue measurements.) As will be shown later, they are readily

used in impact as well as static loading.

4. Experimental Stress Analysis by

Photoelastic Techniques

Numerous researchers in dentistry have used

photoelastic methods to study stresses in dental

structures [6], however, only the more recent

papers will be referred to in the following

discussion.

The photoelastic method for stress analysis can

be divided into two-dimensional, three-dimensional, reflection, and scattered light techniques

[8-10]. A schematic sketch of a transmission

polariscope is shown in figure 4. In general, the

method involves the use of a plastic model or coating which is birefringent under stress. When plane

polarized light is used to examine the plastic, two

sets of interference lines (fringes) are observed,

one is the loci of constant stress direction (isoclinics) and the second is the loci of constant differ- ence between the principal stresses (isochromatics) . They can be identified using white light since

the isoclinics are black and move as the polarizer

and analyzer are moved. Also, in circularly

polarized light (quarter-wave plates used) only

the isochromatics are observed. The magnitude of

the difference in the principal stresses can be meas- ured by detennining the order of the fringe and

knowing the fringe constant and thiclmess of the

plastic. The sign may be determined by inspection

in white light by noting the order of the color of

the fringes or by using various compensators such

as the Cooker or Babinet. The boundary stress may

be calculated directly, since one or the other of the

principal stresses must be zero. The separation of

the principal stresses in the interior of the model

may be accomplished by additional measurements

of the lateral strain by determining the isochromatics using both normal and oblique incident light, or by the shear difference method.

A dental application of the two-dimensional

method is illustrated in figure 5 which shows a

mesial-distal section of a three-unit bridge [11].

The plastics were selected to have as nearly as

possible the same ratio of the elastic modulii as tooth structure and gold. Only the isochromatics

are shown and the fringes in the areas of the

soldered joint show high tensile stress concentrations and high compressive stresses in the contact

area. Only minimal stress is in the gingival portion of the pontic, although maximum tensile stress

would be predicted from a simple beam in trans- verse bending. The boundary stresses can be obtained simply by noting the fringe order, the sign,

and multiplying the fringe order by the fringe constant and dividing by the thickness of the model.

The separation of the principal stresses has been

plane of polarization

plane- polarized light

circularly polarized light principal stress

directions

plone of polarization

452-525 O—72 12

Figure 4. Schematic of a transmission polariscope.

171
FiGUBE 5. Isochromatic fringes in a tioo dimensional model of a mesial-distal section of a

three-unit bridge.

carried out using the shear difference method of

graphical integration and the results are shown

in figure 6. The advantages of this method are that the

magnitude of boimdary and internal stresses can

be measured as well as their direction and sign.

The disadvantages are related to the model and

how closely or remotely it represents the real

structure.

An additional example of two-dimensional pliotoelasticity is shown in figure 7 [12]. The model

represents a porcelain fused to gold restoration

cemented to dentin; it was constructed of three

plastics having modulii in approximately the same

ratios as the restorative materials. Of particular

0

900-

Figure 6. Separation of principal stresses along a line midivay between A and C for the model in figure 5.

Figure 7. Tico-dhnensional photoelastic model repre- senting a labial-lingual section of porcelain fused to gold restorations; the plastics representing the gold

and porcelain were cemented, with epoxy cement which

in turn was luted to the plastic representing dentin with

dental stone.

interest is the low state of stress in the dentin sec- tion of the model.

The three-dimensional technique takes into ac- covmt the contribution of the third dimension of an

object to the stress distribution. In this technique

a three-dimensional model is prepared by machining from plastic or by casting mto a mold. The

model is subjected to a load and heated slowly under load and finally cooled to room temperature

thus freezing the stresses in the model. The model

may be sliced without release of the frozen stresses as seen in figure 8. Using surface slices and twodimensional slices and sub-slices, the principal

stresses may be determined vising the previously

mentioned methods.

The three-dimensional method may be completely reliable for homogeneous solids, but there

may be serious discrepancies in the model for heterogenous solids or composites. In the study of dentures by Klotzer [13] the problem does not exist

since the properties of denture and photoelastic

plastics are similar. Papirno, Colin, and Kaufman [14] have made progress toward the solution

172
Figure 9. Isochromatic fringes in a photoelastic coating on a gold alloy bridge.

Figure 8. A slice from a three-dimensional model which

has been subjected to a stress freezing cycle; isochromatic fringes are shown.

of the problem by the use of plastics of different

modulii for the restoration and tooth structure.

The use of photoelastic coatings on the actual

structure is a more recent method for studying sur- face stresses. The coating is prepared by p6uring a catalyzed resin into a thin siheet and when it has

jelled it is adapted to the part to be studied. After

the polymerization is complete, the coating is re- moved and then cemented to the structure with a

reflective plastic cement. The structure is loaded

and illuminated with polarized light. In this

method the polarized light passes through the coating, is reflected off the cement surface and observed

with an analyzer. Fringes are observed and interpreted in a manner similar to transmission photoelasticity. A gold bridge coated with photoelastic

plastic and loaded occlusally is shown in figure 9. The first-order fringe is easily visible as the brightest fringe. The highest stress concentration was observed in the area of the soldered joint vsdth the

highest orders being nearest the center. The pontic

and even the area near the site of loading was

under low stress. If the photoelastic coating is thin

and of low modulus compared to the underlying

structure the reinforcing effect of the coating can

be ignored and the stress in the coating will be es- sentially that in the surface of the structure. If

these factors cannot be avoided, corrections must

be made. The magnitude, sign, and direction of the

stresses can be determined as usual, and separation

of the principal stresses is accomplibhed by the

oblique incidence method. The method has the distinct advantage that the actual structure can be

used and questions about the reliability of the

model are avoided.

5. Additional Techniques in Stress Analysis

The three techniques, brittle coatings, strain

gages, and photoelasticity can be used under conditions of impact, as well as static loading. The strain

gage recording system must have a high frequency

response in order to measure the maximum transient strains ; a storage type oscilloscope is particularly useful for these measurements. An example

of the strain on the mesial area of the molar pontic

of a four-unit gold alloy bridge as a result of an

impact on the lingual cusp is shown in figure 10

[15]. The maximum strain did not occur until the

third oscillation and the initial strain was compressive followed by tensile strain. The maximum

strain was 192 microinches/inch ( 192 /xm/m ) and

the maximum stress was 2690 lbs/in^ (18.6

DROPZ. 30«

S.G. I 2 3

Gold

• •: ;. V ^ u

S.G. 2

Figure 10. Strain as a function of time on the

mesial area of the molar -pontic resulting from

an impact blow on the Ungual cusp of the

first molar pontic; total time was 4-5 ms.

173
MN/m^), and the strain decayed with time. Similar measurements showed that torsional, as well as transverse modes of vibration occurred.

The simplest dynamic technique for photoelastic

stress analysis involves a spark light source and a delay circuit connected to the loading device. The

load is applied and a photoelastic stress pattern

is photographed at predetermined delays after

loading.

Thermophotoelasticity also has been used to

study stresses resulting in plastic models from thermal shock [16]. Various arrangements for the application of heat have been used including liquids

and metal resistance strips. This method could be

used to examine the stresses in dental restorations

resulting from thermal shock.

Moire fringe patterns have been used to study

surface deformation. This is simply the comparison between a deformed grid and an undeformed

master grid used as a length standard [17]. The

method is applicable to high temperatures, large

elastic and plastic strains, long term strain

properties of materials, and of two and three

dimensional analysis of transparent models using

embedded grids. The method is best suited to flat

surfaces and the preparation of a grid on curved

surfaces such as dental restorations has limited its

application in dentistry.

Holography has been used in stress analysis to

determine isochromatic, isoclinic and isopachic

(loci of constant sum of the principal stresses)

fringe patterns in two dimensions [18, 19]. Combination isochromatic and isopachic fringe patterns have been obtained which give directly a

solution for the individual principal-stress components. A double exposure method for measuring

surface stresses on dental restorations offers promise as a means of avoiding the use of photoelastic

coatings.

Scattered light photoelasticity has been developed to the point where instruments are available

commercially [20, 21]. Coherent light from a laser

is focused into a thin sheet or beam of light and it

is passed through a plastic model in an immersion

tank under load and the scattered-light fringes are observed. The patterns can be interpreted to

give solutions to three-dimensional problems. This

technique is comparable to locating a polarizer or

analyzer in the interior of the model and stress

information can be obtained without stress-freezing or slicing the model.

The distance between atoms may be used as gage

lengths in the x-ray diffraction of crystalline solids, and strains may be determined by measuring

changes in these interatomic distances. The method

can be used to nifiasure residual surface stresses

without drilling holes to relieve the stress. The

method can give the sum of the principal stresses

in the surface layer and in fine-grained alloys, the

accuracy is about 2,000 lbs/in- (14 MN/m^) and

in coarse-grained alloys it has a much lower

accuracy.

6. Summary

In summary, a number of recently developed

stress analysis techniques, as well as standard

methods offer real promise in the solution of the

numerous related problems in dentistry.

7. References

[1] Haack, D. C, An Analysis of Stresses In a Model

of the Periodontal Ligament, Dissertation

(Kansas State Univ., 1968).

[2] Hetenyl, M., Brittle Models and Brittle Coatings,

636-62, In Hetenyl, M. ed., Handbook of Experimental Stress Analysis (John Wiley & Sons, New

York, 1950).

[3] Dally, J. W., and Riley, W. F., Experimental Stress

Analysis, 91-139 (McGraw-Hill, New York, 1965).

[4] Craig, R. G., and Peyton, F. A., Strain on the

framework of a mandibular free-end partial

denture under load, J. Blomed. Mater. Res. 1:263-74 (Jan. 1967).

[5] Dally, J. W., and Riley, W. F., Experimental Stress Analysis 366-514 (McGraw-Hill, New York,

1965).

[6] Peyton, F. A., Asgar, K., Charbeneau, G. T., Craig, R. G., and Myers, G. E., Restorative Dental Materials 138-9 (Mosiby, St. Louis, 1968).

[7] Tlllltson, E. W., Craig, R. G., and Peyton, F. A., Experimental Stress Analysis of Gold and

Chromium Alloy Bridges, I.A.D.R. Dental Materials Group microfilm, Washington, D.C., March

1967.

[8] Dolan, T. J., and Murray, W. M., Photoelasticity I. Fundamentals and Two-Dlmenslonal Applications

828-923, In Hetenyi, M. ed., Handbook of Experimental Stress Analysis (John Wiley & Sons, New

York, 1950).

[9] Drucker, D. C, Photoelasticity II. Three-Dlmensional Photoelasticity 924-76, In Hetenyi, M. ed.. Handbook of Experimental Stress Analysis (John

Wiley & Sons, New York, 1950).

[10] Dally, J. W., and Riley, W. F., Experimental Stress Analysis, 143-333 (^McG raw-Hill, New York,

1965).

[11] El-Ebrashi, K. M., Craig, R. G., and Peyton, F. A., Photomechanics and Stress Concentrations in

Fixed Dental Restorations, I.A.D.R. Dental Materials Group microfilm, San Francisco, March.

1968.

[12] Craig, R. G., El-Ebrashi, M. K., and Peyton, F. A., Stress Distribution in Porcelain Fused to Gold

Alloy Crowns and Preparations Using Photoelasticity, I.A.D.R. Dental Materials Group

microfilm, San Francisco, March, 1968.

[13] Klotzer, W., tiber polarisationsoptesche Unterisuchen an Prosthesenmodellkorpern, Deut. Zahn.

Zelt. 21:894-901 (1966).

[14] Paprino, R., Colin, L., and Kaufman, E. G., Properties and Preliminary Three Dimensional Photoelastic Results Using Room Temperature Curing

Resins, I.A.D.R. Dental Materials Group microfilm, San Francisco (March 1968).

[15] Tillitson, E. W., Craig, R. G., and Peyton, F. A., Experimental Stress Analysis of Dental Bridges

Using a Dynamic Method, lADR Dental Materials

Group microfilm, San Francisco, March, 1968.

[16] Leven, M. M., and Johnson, R. L., Thermal stresses on the surface of tube-sheet plates of 10 and 33%

percent ligament eflBciency, Exper. Mech., 4, 356

(1964).

[17] Zandman, F., The transfer-grid method, a practical

moire stress-analysis tool, Exper. Mech., 7, 19 A

(1967).

174
[18] Fourney, M. E., Application of holograpliy to photoelasticity, Exper. Mech. 8, 33-8 (1969).

[19] Hovanesian, J. D., Brcic, V., and Powell, R. L., A

new experimental stress-optic method : Stress- holo-interferometry, Exper. Mech., 8, 362-8

(1968).

[20] Srinath, L. S., and Frocht, M. M., The potentialities

of the method of scattered light, International

Symposium on Photoelasticity (Pergamon Press,

New York, 277-93, 1961).

[21] Jenkins, D. R., Analysis of behavior near a cylindrical glass inclusion by scattered-light photoelasticity, Exper. Mech. 8, 467-73 (1968).

[22] Barrett, C. S., X-ray Analysis, In Hetenyi, M., ed., Handbook of Experimental Stress Analysis (John

Wiley & Sons, Inc., New York, 1950).

175

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Relations Between Mechnical Properties and Clinical Behavior

David B. Mahler

University of Oregon Dental School, Portland, Oreg. 97201

A dilemma in the field of dental materials is that in many instances the critical physical properties that are relevant to the failure of restorative materials have not been

identified or when identified the limiting values which separate successful from unsuccessful materials have not been established. To determine the critical property relevant to

clinical marginal fracture of dental amalgam restorations, dynamic creep and other

physical properties including compressive, tensile and transverse strength and transverse

deflection of nine amalgam alloys were determined. Clinical evaluation of restorations

made using alloys widely separated in dynamic creep showed that marginal fracture was

associated with the Theological properties of dynamic creep, static creep and slow compressive strength, but showed little relationship to the other physical properties measured.

Although the relationship between creep and marginal fracture may not be a cause and

effect mechanism, creep appears to be reasonably predictive of clinical marginal fracture.

Key words : Amalgam, dental ; correlation of laboratory and clinical evaluations ; creep of

dental. amalgam ; dental materials; dynamic creep; fracture, marginal of dental amalgam;

mechanical properties of dental materials ; rheological properties of dental amalgam.

If an engineer were to be taken into a clinical

operatory, a tooth which had been restored with a

certain material were pointed out to him, and he

were asked to comment on the efficacy of the restoration, what questions would he be likely to ask ? First he might ask the general question whether

the material was performing satisfactorily. Specifically he might ask the following questions about

the material

1. Is it maintaining its shape, color and

appearance?

2. Is it biologically compatible ? 3. Is it truly restoring the tooth to its original

function ? 4. Is it doing these things over a long time

period ? If the answer was yes to all of these questions, he

might lose interest in the conversation and leave.

If the answer was no, he would probably become

interested and ask how is the material performing

unsatisfactorily. Specifically what are the modes

of failure. Let us assume that we can supply our

engineer with an answer to this question. If so, he

would probably start to think in terms of physical

properties and try to identify in his mind which

property or properties might relate to the mode

of failure outlined.

The dilemma in the field of dental materials

today is that, by and large, we have been unable to

identify these critical physical properties that are

relevant to the failure of restorative materials. In

figure 1, this dilemma is outlined. Furthermore,

where we have identified a pertinent physical

property, we have not been able to assign limiting

values which can separate the successful from the

unsuccessful material.

Once the relevant property is identified, the

procedure for improving the material is fairly

straightforward, although there may be many

difficult and unsolvable problems along the way.

The identification of this relevant property can

function as follows

1. It would allow us to identify the actual

mechanism of failure.

2. It would give us a means of designing this

weakness out of the material.

3. It would allow us to utilize laboratory methods of screening material modifications for

possible improvement.

4. It would allow us to conduct clinical trials

on only those modifications which show

the greatest promise.

I would like to show you an example of how we,

at the Dental Materials Science Department of the

University of Oregon, have attempted to follow

this approach of identifying a relevant physical

property. The example is that concerned with the

marginal fracture ofdental amalgam which is perhaps the most significant failure mode for this

material.

Initially, a laboratory test was devised which

was thought to parallel the situation confronted

by an amalgam restoration in the mouth. Cyclic

compressive loading, which was considered to re- late to the way in which masticatory forces are applied to amalgam restorations under clinical

conditions, was applied to an amalgam test specimen. In addition, water at 37 °C was circulated

around the specimen during the test. The deforma177
MECHANICAL PROPERTIES OF RESTORATIVE MATERIALS

Amalgam

Silicate

Zinc Phosphate

Zinc Oxide Eugenol

Synthetic Resin

Porcelain

Pure Gold

Gold Alloys

Co - Cr Alloys

Stainless Steel Alloys

Base Metal Alloys

\ CLINICAL/ —*- 1 PERFORMANCE

Modulus of Elasticity

Elastic Deformation

Elastic Limit

Modulus of Resilience

Ultimate Strength

Plastic Defonnation

Fatigue Strength

Impact Strength

Creep

Hardness

Wear

Figure 1. Denial restorative materials; clinical performance and mechanical

properties.

tion of the specimen was recorded during the

testing procedure using a linear transducer wired

to a recorder. The parameter measured was the

dynamic creep of the specimen over a fixed time

interval of testing. A scnematic of the test system

is shown in figure 2. The results of testing 9 commercially available amalgam alloys are shown in

figure 3. The first most obvious result was that

amalgam alloys having relatively small differences

in conventional physical properties showed a very

marked difference m dynamic creep properties.

The next step was to determine the clinical significance of these differences. In view of the many

difficulties attendant with clinical testing, it is important to maintain as simple an experimental

design as possible. Therefore, it was considered

appropriate to evaluate the clinical performance

of the two alloys at opposite ends of dynamic

creep behavior, Alloys A and I. If no difference in

clinical performance could be observed, then this

property would have little relevance to clinical

10

DIFFERENTIAL TRANSFORMER

DYNAMIC CREEP TEST STATIC CREEP TEST

FLUCTUATING STRESS CONSTANT STRESS

500-10,000 psi 5250 psi MEAN STRESS = 5250psi

1600 CYCLES/MINUTE

37 C. WATER

AMALGAM SAMPLE FATIGUE OR CREEP MACHINE

n 0 E F G ALLOY

H I

Figure 2. Schematic of systems for dynamic and static

creep testing.

Figure 3. Dynamic creep of nine commercial dental

amalgam alloys.

behavior and there would be little point in testing

alloys in between. Alloy B was also selected for

evaluation in case this property did prove of significance since the dynamic creep value for Alloy B

was close to but significantly different from

Alloy A.

The procedure consisted of placing these three

alloys under conditions which produced amalgam

restorations having physical characteristics close

to those of the laboratory test specimens. This was

accomplished by determining the Hg content of

representative restorations of the three alloys

placed by dentists cooperating on the project. Laboratory test specimens were prepared at these same

Hg contents. After one year of service, occlusal

photographs were taken of these restorations.

Marginal fracture was noted in many of the res- torations and the entire group of photographs

was separated into five groups of marginal frp''-

178
ture as shown in figure 4. The data were subjected

to a Ridit analysis which is shown in table 1. The

table entries are the numbers of restorations as- signed to each category by three evaluators. Alloy

N, which corresponds to Alloy B, having a dynamic creep value slightly higher than Alloy A in

figure 3, was taken as the standard distribution

with a mean value of 0.50. Alloy D, which corre- sponds to Alloy A, having the lowest creep value,

(fig. 3), had a mean value of 0.39 which was

clearly differentiable from Alloy N with a significant value of 3.2. Alloy M having the highest

creep value of all alloys tested and which corre- sponds to Alloy I in figure 3, had a mean value of

0.81 with a significant "z^" value of 10.2.

In simple terms, the results of the clinical evaluation showed that Alloy D, having the lowest

dynamic creep value, had the least amount of

marginal fracture; Alloy N, having a slightly

higher dynamic creep value, had a slightly higher

incidence of marginal fracture ; and Alloy M, having the highest dynamic creep value, had an ex- tremely high incidence of marginal fracture.

In order to test the uniqueness of this relationship of dynamic creep to marginal fracture, other

mechanical property tests were conducted as well.

These included compressive strength, tensile

strength, transverse strength, transverse deflection,

flow as determined in accordance with American

Dental Association specification No. 1, and static

creep. The results of these tests together with the

marginal fracture results are shown in figure 5. Arrows have been drawn in the direction of superior characteristics and the "t" values for comparisons are shown with the arrows. From figure 5,

it is seen that only the rheological properties of

dynamic creep, static creep, and slow compressive

strength relate in the proper direction to marginal

fracture.

Although the relationship between creep and

marginal fracture should not at this time be con- sidered a cause and effect mechanism, this property appears reasonably predictive of the clinical

phenomenon of marginal fracture. Additional

work is being conducted to strengthen this

hypothesis.

Table 1.Ridit analysis of marginal fracture

II III IV Total

Alloy N.

Alloy D.

Alloy M.

Evaluator A

Evaluator B

Evaluator C

Evaluator A

Evaluator B

Evaluator C

Evaluator A

Evaluator B

Evaluator C

8

15

10

18

22

19

10

2

35

15

20

33

19

26

5

15

16

21

6

14

11

18

1119

4

74

13

2

22

13

18

13

1

0

0

0

6

27

56

56

56

58

58

58

52

52

52

X Sx t Alloy D compared to Alloy N 0.39 0.032 3.2 Alloy M compared to Alloy N 0.81 0.026 10.2

179
Alloy D Alloy H Alloy M

Harginal Fracture .39 .50 .81

Compressive Strength (psi) 61U00 ^'^ > 59500 51900

Tensile Strength (psi) 69i»0 ^'^ » 7930 < » 7770

Transverse Strength (psi) 13220 ^'^ » I8030 ^

^''^ 12U50

Transverse Deflection (n) 20 •* -'-'^ » 23 ^'^ * 32

ADA Flow («) 0.50 - ^'^ » 0.65 -,J2iil 3.91

Dynamic Creep {%) 0.86 ^.^j j^ g^ — 8.76

Static Creep (?) 0.76 ^ 2.36 ^ 8.37

Slow Compressive Strength (psi) 38500 *

^'^ 3U6O0 < 231iOO

FiGur.E 5. Comparison of alloys with respect to marginal fracture and mechanical

properties.

(pslX0.0068.9 =MN/mS)

180
Development of Improved Methods for Evaluating

Dental Materials

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50tli Anniversary Symposium,

Held Oct. &-8, 1969, Gaithersburg, Md. (Issued June 1972).

Need for Research To Develop Performance Characteristics

H. P. L. Schoenmakers

Tandheelkundig Instituut, Rijksuniversiteit, Utrecht, The Netherlands

To develop the most important characteristics of restorative materials, an understanding of the processes by which margins of such restorations become damaged is needed.

Bulk and margin properties should be correlated. The mechanical forces developed in

mastication and chemical attack by the oral environment are important factors. Tensile

strength appears a more meaningful data than either shear or compressive strength.

Research for selection of proper tensile test procedures is needed. Cracks and voids in specimens complicate their strength behavior. The role of temperature in laboratory evaluation is important. The mechanisms of corrosion and erosion of dental restorations requires

further research.

Key words : Clinically related strength properties of dental materials ; dental materials

dental materials, correlation of bulk and marginal properties ; dental materials, performance

characteristics ; dental restorations, the role of oral environment factors in stability thereof.

1 . Introduction

The purpose of a dental restoration is to re- establish the patient's ability for chewing and

speaking while also attention must be given to the

esthetic requirements. The quality of the restoration must be on such a level that a long lifetime is

guaranteed. When limited only to the materials

which are used to restore teeth, it may be said that

the lifetime of the filling is restricted by the appearance of recurrent caries in the adjacent tooth

structure. In many cases a correlation can be detected between the recurrent caries and the adaptation of the filling. The adaptation of the filling can be decreased by damage to the margin of the

filling. Such a defect however is seldom discovered

by the patient himself. In most cases the dentist

will trace the defect with the help of a radiographic photograph, which shows the recurrent

caries.

When determining the quality of a filling, at- tention should be directed to the junction between

filling and tooth structure. The adaptation must

be so good that local caries will not occur and the

quality of the margin must be on such a level

that chewing and biting forces, as well as the

chemical attack by the oral environment, can be

resisted.

2. DesirabiHty of Estabhshing a Correlation Between Bulk and Margin Properties

Many authorities have shown that the margins

of amalgam fillings are inferior to the body-part.

Also zinc phosphate cement may show such discrepancies. When cementing a crown or inlay, the

fluid part of the cement runs faster to the margin

than the more viscous material, which may result

in a weaker material at the margin. It seems possible that silicate cement may show such

phenomena also.

The discrepancy between the margin and the

other part of the filling is correlated to the clinical

manipulation of the material. It is well known

that in order to get a better amalgam filling, the

cavity must be filled to excess and then carved or reduced to the right level. Tlie degree of overfilling depends on several factors such as cavity

design, the plasticity of the material, and the skill

of the dentist. Therefore, it may be concluded that

in clinical cases there will not be an exact cor- relation between the properties of the margin and

the body-part of the filling. As the properties of

the filling materials are determined on a rather

large specimen, which most likely shows the characteristics of body-material, it is questionable if

the quality of the filling, particularly of the margin, might be predicted in this way.

Research is needed to determine the correlation

between body and margin properties. If the different brands of a dental material do not show the

same correlation between the body properties and

margin properties, it would be useful to test the

material on a powder-liquid ratio which correlates

with the composition of the margin,

3. Relation of Properties and Forces

Applied Orally

Wlien it must be decided if a material can be

used successfully for a restoration, consideration

must be given to the chemical and physical forces

183
to which the material will be subjected. In general, a distinction can be made between mechanical forces, i.e., chewing and biting forces, which

are applied to the occlusal part of a filling, _and a

chemical attack on the whole filling. Consideration must also be given to the fact that as long

as the filling is not hardened, the adjacent soft

tissue and the pulp may be irritated by the toxic

nature of some ingredients.

The materials which are used for occlusal fill- ings require such strength that fracture or deformation will not occur. It is very difficult to establish the exact requirements because there is a large

diversity in tooth shape and in the forces which

can be developed. Only a qualitative approach

seems possible. In relation to the chewing muscles

average forces can be developed in the magnitude

of 100 to 200 kgf (980 to 1,960N). Also forces of

800 kgf (7,800N) are known, but these are exceptional.

When a force of about 200 kgf (1,960N) is supported by the multi-cusp contact area between the

teeth and their antagonists which is approximately 2 cm% a stress of 100 kgf/cm== (9.8MN/m==)

results. Such a stress is far below the compressive

strength of all filling materials. Amalgam may

show a flow-deformation if the stress remains for a long time. When the force is carried by only one

tooth, a higher stress is possible. Then, the magnitude of the force depends on the distance' to the

temporomandibular joint. The maximum force

might be about 35 kgf (340N). However, it is

questionable if suc!h a force is, in fact, accepted

by the tooth. Normally the force is restricted by a pain reflex.

A more practical approach to assessing the magnitude of the forces, developed on chewing, has

been to utilize electronic measuring devices,

mounted in the elements of a partial prosthesis.

These measurements indicate that chewing forces

on first molars approximate 6 to 7 kgf (60 to 70

N). It seems reasonable to assume that with natural teeth higher forces might be developed.

When the chewing force is doubled, e.g. 14 kgf,

(140N) and the contact area between the tooth

and the antagonist with food in between is established on 0.14 cm^, then a stress is applied of 100

kgf/cm== (9.8MN/m^).

As the stresses ordinarily developed are far

below the compressive strength of the filling materials, it is most likely that fracture will occur only

in exceptional circumstances. When looking for an

explanation of a fracture of a fully supported

margin, it is reasonable to find it in a stress con- centration which is caused by hard substances that

may exist in the food. For example, vegetables

may contain some sandgrit. The stress realized by

biting upon this grit is hard to calculate, because

this depends on the size and shape of the grit and

the magnitude of the developed force. During

chewing, when the hard substance comes between

the teeth, the movement of the jaw will be stopped

by the reflex from the proprioceptive receptors

which are settled on the end of the fibers which

support the teeth. The reflex goes through the

brain and can be suppressed. When a high force

is needed to mill the food the proprioceptive re- ceptors will react at a different level and the force

transmitted by the unexpected hard particles

mig*ht be rather high.

On the other hand, the movement of the jaw

itself is realized by a very small force. In fact the

force is increased as soon as the contact between

the teeth is realized. This means that when biting

on a large particle, the chewing will be interrupted

before a high force is developed. If it is accepted

that hard substances in the food are a main reason

for damage to the margin of fillings, it is clearly

desirable to develop information pertaining to the

magnitude of the developed forces. Especially

when the hard substances are small, a large stress

may develop. By such a phenomenon, fracture will

not occur very often but each incidence may result

in a carious attack.

In many cases the filling and the margins do not

Sossess a perfect shape. A perfect adaptation selom occurs and many times at the margin the fill- ing is not coincident with adjacent tooth structure.

Such a situation is much more dangerous for the occurrence of a fracture, because the filling is no

longer fully supported. It is questionable if this

is a matter of any importance to the specialist in

dental materials. When the filling extrudes because

the dentist incorporated moisture in the zinccontaining amalgam, the inferior result is the re- sponsibility of the dentist.

Whenever there are phenomena which cannot be

controlled by the dentist, they deserve much attention. It is very desirable to be informed in a quantitative way about the influence of the dimensions

of the gap between filling and tooth structure, on

the loss in strength of the margin, and on the

chance for recurrent caries. Such knowledge will

assist in the determination of the requirement for

dental materials.

Fillings are mainly subjected to compressive

forces. However, even in the case of a perfect

adaptation, in addition to the compressive stresses

developed in the filling, tensile and shear stresses

will also be realized. The nature of the stress caus- ing the fracture is difficult to tell because the ratio

between the magnitude of the different stresses is

not known, but experience shows that the resist- ance against compression is much higher than the

resistance to tensile forces. When the margin of the

filling is not perfect, it is likely that greater tensile

and shear stresses are developed. Therefore, it is

difficult to decide which mechanical property is

the criterion that should be used to evaluate the

filling material. Research is needed to solve this

problem. In relation to this, it might be useful to

study the phenomena which initiates the fractures.

184
4. Theory of Fracture in Compression

and Tensile Testing [1-4] '

The compression test has been used most

frequently. It is assumed in this test that fracture

is realized by shear stresses. It is believed an ideal

fracture shows two cones, respectively containing

the bottom and the top surface of the specimen.

This shape of fracture is explained by shear

stresses which are maximal in the planes at an

inclination of 45° with the direction of loading.

This theory, especially when applied to fracture

of brittle materials, is no longer accepted by the

specialists in fracture phenomena. New theories

have been introduced since repeated failures have

been reported of Polaris rocket motor casings at

stresses well below design value. An early tensile

fracture is based upon the existence of cracks,

which cause stress concentrations, according to

the theories of A. A. Griffith [3], published in

1920. The materials that are used in the Polaris

rocket have a high strength but do not possess

much ductility. It seemed that a new property

must be introduced: namely the stress intensity

factor with the formula K=G^fwa, wherein "c" is one half of the length of the crack.

Because most of the dental filling materials

are brittle by nature and contain cracks and

porosities, it might be useful to consider these

newer theories. Comparing the appearance of

the fracture planes respectively caused by tensile

forces and compression forces, it is suggested that

brittle fracture can only be realized by tensile

forces. Further investigations showed that most

'ractures under compressive forces run in an axial

lirection. This was also the case with a single

crystal of sodium chloride which was oriented

in such a direction that oblique fracture planes

with a certain inclination were expected. Although

a scratch on the surface initiated a crack in an

oblique direction, the fracture propagated in the

direction of loading. Thus it may be stated that

the fracture is realized by tensile stresses and

initiated by cracks which cause stress concentrations. However, the influence of the crack is not the same in every case. In the direct tensile

test, the stress concentration depends only on the

longest axis of the assumed elliptical crack, which

increases during fracturing, while in the compressive test the stress concentration depends on

the ratio of both axes of the ellipse and on the

inclination. In the direction of fracturing under

compression, however, only the crack itself is important; and during fracturing the stress is not

increased nor decreased.

For dental purpose a test must be selected which

indicates the strength of the margins. It seems very

probable that a tensile test will offer the best correlation. Besides the direct tensile stress, which is not very favored by reason ofthe difficulty in fab-

' Figures In brackets Indicate the literature references at the end of this paper.

ricating a proper specimen, the cleavage test and

the flexure test have been used. The resulting

strength values, however, are not on the same level,

therefore, research is needed to select the proper

test. Also, the existence of porosities should be

taken into consideration. The appearance of the

fracture surface might give indication of the

source of rupturing.

5. Role of Temperature in Property

Evaluation

In most cases strength measurements are made

at room temperature, although the materials are subjected to forces at mouth temperature. This

may lead to wrong results for materials with a low

fusion temperature, such as the amalgams. It is

known that amalgams which go from room temperature to mouth temperature show a reduction

in compressive strength. The literature gives numbers of 8 percent and 15 percent for the average

reduction. Private observations showed a reduction of about 30 percent. This discrepancy may re- sult from variations in the time which the amalgam is allowed to remain at mouth temperature

and by the difference in the loading speed. The

figures regarding the reduction in strength show

that the different brands did not behave in the

same way. Thus by this effect it might be con- cluded that the testing must be performed at least

at mouth temperature.

In addition, it must be taken into consideration

that higher temperatures are developed while hot

food is swallowed, resulting in a temporary de- crease in amalgam strength. While the oral cavity

temperature decreases rather quickly, it is questionable that the phases of the amalgam come to

equilibrium as rapidly. Where such phenomena

may cause a weakening effect during a longer

time, the chance is increased for damaging of the

margins of the filling. In this paper fatigue

phenomena are not discussed. However, it may be

stated that these are also very important, because

they may introduce the initial defect in the filling.

6. Oral Environmental Effects and Laboratory Testing of Dental Materials

The lifetimes of fillings which are not subjected

to chewing or biting forces will be related to the

chemical attack by the oral environments and to

erosion created by saliva, food, and the soft tissues

which slide over the fillings. The chemical attack

itself can be created by the saliva, the plaque, and

eventually by the food. Because the oral environment varies from person to person, it is hard to

predict the clinical behavior of a filling. A proper

test for measuring the solubility and disintegration of all kinds of cements is desirable. This

means research is needed to determine the phenomena which are responsible for the attack on the

filling materials. Special attention should be given

185
to the plaque which forms on the filling. Often

deep holes are discovered in silicate cement fillings

at places where cleaning is difficult. The basis for

this elfect may be closely related to the theories

which are developed to explain caries in the teeth.

Although the lifetime of silicate cement is limited,

it possesses a special benefit; namely, it decreases

the chance for secondary caries. This is explained

by the freeing of fluorides during the solution of

the cement. If this theory can be proved by proper

experimentation, it could be concluded that a cer- tain solubility of the cement is required. Consequently, then a test is also needed which will show

the solubility of fluorides : when the cements do not

contain fluorides it is senseless to require a certain

solubility.

It may be assumed that solubility phenomena

will tend to create a decrease in the volume of inserted silicate cements. An increase of the volume

of filling materials may also occur such as is evidenced by amalgams which are coated with a cor- rosion product. If a coherent layer on the metal is

not formed, the formation of the corrosion product will go on. The growing forces are large and

could lead to a deformation of the metal, when the

corrosion product has no way to escape. Such

phenomena might occur with the amalgam fillings.

When the flow property indicates that small forces

lead to a deformation of the filling which is followed by a loss of adaptation, then damage of

the margins is more likely to occur. An investigation of the behavior of the corrosion product

might be very useful.

Many times a small inlay that is not subjected

to chewing forces, is dislodged from the cavity. It

is not likely that this dislodgement is a result of

the formation of a corrosion layer upon the gold

alloy ; thus attention must be directed to the zinc

phosphate cement. When this cement is exposed

to acetic acid, a reaction product is noticed which

appeal's on the surface of the specimen. It is diffi- cult to tell if this also happens when the cement

is in contact with lactic acid. However, it seems

useful to research the development of reaction

products on cements and on the forces which are consequently being developed.

Finally, in discussing filling materials, attention

must be directed to ingredients which might ir- ritate the soft tissue and the pulp of the tooth. It

is very important to know if an irritation will be

temporary or if it will lead to a more serious complication. The latter will depend on several factors such as the concentration of the toxic ingredient and the time it is in an active state. According to these aspects research that may lead to the

creation of a safety test would be very useful and

beneficial to dentistry.

7. References

[1] ASTM STP 381, Symposium on Fracture Toughness

Testing and its Application (1965).

[2] ASTM STP 410, Plane Strain Crack Toughness Test- ing of High-Strength Metallic Material (1967).

[3] Griflath, A. A., The phenomena of rupture and flow in

solids. Phil. Trans. Roy Soc. London, Ser. A., 221:

163-198, (1920).

[4] J. Gramberg : The EUipse-with-Notch Theory Ito ex- plain Axia Cleavage Fracturing of Rocks, (to be

published).

186
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseakch, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Need for Correlation Between Laboratory Testing and Clinical Research

Bjorn Hedeg&rd

Odontologiska Kliniken, Tandlakarhogskolan, Goteborg, Sweden

Although priority musit be given to basic and clinical research in preventive dentisitry,

this does not eliminate the need for research on materials and methods for restorative dentistry which is in itself a preventive measure. There is an urgent need for collaborative and

correlative studies involving properties of materials to be studied in the laboratory and

tested in the clinic. Clinical research with materials is slow, and with the present incomplete

information on oral environment and function, it is often difficult to assess properly the

results, but there is no alternative. With sound clinical research on a larger and more

penetrating scale, data and information may be obtained, that will make it possible to set up more meaningful test procedures in the laboratory. And that is the goal : to be able to characterize the dental material in the laboratory and correctly predict its clinical

performance.

Key words : Aging processes, relation to dentistry ; clinical research, dental ; correlation

of laboratory and clinical results ; dental materials ; dentistry, preventative ; dentistry,

restorative ; epidemiological information, dental.

1. Introduction

I regret to have to report a failure. And this on

behalf of the dental profession. Oral health is

obviously our responsibility—^but with the measures we have in our therapeutic arsenal we have

failed to uphold the oral health of the poptdation.

There are many of us in the dental field—about

275,000 qualified dental practitioners spread all over the world. And we will increase our members

to 300,000 within the next decade. In that same

decade the world population will have increased to

and superseded the 3-billion line.

These few figures indicate, of course, the reason

for our failure. In fact, they imply that we as a

profession are facing an impossible task if we, as we mainly do today, limit our activities to those

which we learned and were trained for at the

dental schools ; that is to provide for removal and

healing of pathologic processes and to restore

anatomically—and hopefully also functionally

the damaged tooth or dentition. Within the limitation mentioned we have intricate and sometimes

near-to-impossible problems to face. And still this

is not enough. The responsibility of our profession

goes far beyond that. Its ultimate goal is to find

ways and means to provide oral health to large

populations without relying upon chairside restorative procedures.

Priority should and must be given basic and

clinical research in preventive dentistry. There

can be no objection to this ; obviously a noncarious

tooth is better than a restored one. It should also

be underlined that effective preventive measures

provide the only means with which we can expect

to cope with the situation of the fast-growing

population.

If we thus give highest priority to prevention,

where does that leave us as clinicians and researchers in the field of restorative denistry ? There are

three answers to that

1. It will take a long time to develop effective

and far-reaching preventive methods.

2. Already today we have experience of the

fact that preventive measures are not foolproof ; that is, that a certain degree of

failure has to be expected with any future

method.

3. Traumatizing injuries, malformations, and

congenital deformations will always provide demand for restorative dentistry.

Actually, these answers reach rather far into the

future. The immediate answer is simply : The re- storative procedure is in itself a preventive measure and should consequently be incorporated in the

list. And today and tomorrow the restorative procedures are those, which will occupy most of the

dentist's time. Or, I should rather say conservative

procedures, as preventive measures are always incorporated. I have only to mention scaling.

2. Epidemiological Information

There should also be another foundation laid

down and that is our knowledge today of oral

status, need for restorative treatment, and possibly

also demand for treatment. There are, imfortunately, only a limited number of scientifically sound

epidemiological studies done in this field. Therefore, we can today only give rather rough figures.

It could be outlined this way ; every fiftli person

4&2-525 0—72 13 187
above 16 has no natural teeth. Between one and

two of the remaining four wear full upper and/or

partial removable dentures. At the most, three out

of one thousand adults have noncarious dentitions.

The need for treatment is high—it points to- wards a dentist-population ratio of one to five

hundred. The awareness of need for treatment

seems to be 60-80 percent of the need registered.

But the demand for treatment is still considerably

lower. It is a sad picture we gain from epidemiological studies in so-called well-developed countries. It is also a picture that heavily underlines the

fact that restorative procedures have to be used for a very long period yet—even if the incidence of

caries and periodontal disease is quicMy and drastically reduced. What we actualy lack are more

penetrating epidemiological studies set up on a

larger soale and internationally directed, so that

geographical as well as environmental differences can be traced.

3. Nature and Implication of Current

Restorative Treatment

Now, let us consider the restorative treatment

given. We define restorative in its widest sense,

that is the endeavor for optimally restored function irrespective of degree of dental deficiency.

And here the interesting question to be asked is:

Will the treatment given have to be revised and

if so how long will it last?

Records on dental treatment show consistently

one and the same picture. In the adult patient the

restorations due to primary lesions are in the minority. Today the clinician is mainly concerned

with revision of earlier therapy. Due to secondary

lesions, restoration breakdown and changes in the

properties of remaining tooth substances, revision

of the former treatment will involve more extensive restorative measures, which in due time will

be followed by even more extended therapy. Thus

dental treatment breeds extended dental treatment.

Any process or any measures that can delay the

deterioration of therapy given will in actual fact

increase the productivity of the profession. Just

one example: You have in the United States

around 100,000 dentists. If just one silicate filling

per dentist would have an increased longetivity

from four to five years, you would save 50,000 manhours to be used for dental treatment otherwise

not available. We have to think in terms of productivity irrespective of whether we are considering preventive dentistry or restorative dentistry.

In both instances we are after the same thing:

lasting results.

4. The Deficiency in Our Knowledge

of Oral Function

The problem we deal with in clinical dentistry

is the impaired tooth and the consequent defective

dentition. Here all the questions arise, which have

to be solved in order to perform a correct treatment and restoration. It should be underlined that

all problems derive from the clinic and the answers

finally end up in the clinic.

You may comment that I am saying a mouth

full, and you are right. We cannot today furnish

the laboratory researcher with sufficient clinical

and biological data, so that suitable laboratory

studies under simulated mouth conditions can be

performed. Actually, we know only the gross picture of oral function. It is still debated, whether

tooth contact occurs during the early chewing

cycles, the time period, the direction, and the force

during later contacts. Debate is also going on con- cerning mandibular movements and contact during sleep. All have a bearing on methods and

materials in restorative dentistry.

It is only recently that some understanding has

been reached of tooth deformation and periodontal

performance under load. Studies of the elastic

deformation of the todth, initiated by Korber and

collaborators, have indicated among other findings minor bending by lateral forces. This work,

started in vivo and repeated in the laboratory, has

considerable importance as background for developing materials to be used in the tooth-restoration

system.

Especially in this field of elastic deformation

clinical studies and laboratory investigations of

model systems may close the now existing gap between the basic materials scientist and the clinical

researcher. What is absolutely necessary is a clear

picture of the behavior of the tooth and its various

components in order to define requirements for

the restorative material as well as the restoration.

Here the very close collaboration between clinician, biologist, and physicist may yield the information sought after. Important steps have already

been taken; I may remind you of the work by

Stanford on the physical properties of hard tooth

substances.

5. Proper Assessment of Clinical Performance, A Requirement for Improvement

Under the situation we have at present—^that is incomplete information on oral environment and

its changes with various factors—it is not possible

in laboratory testing to predict the detailed performance of a material in a tooth-restoration system. Obviously certain indications can be made,

but only the clinical test will yield the final answer. It is consequently my firm belief that laboratory testing without evaluation of clinical performance is unsatisfactory.

It should not be necessary to point out that the

same rigid research rules apply in clinical testing

as in laboratory testing, but evidently there is some

confusion among the clinicians as to this. In reality, the mouth is the clinician's laboratory ; he is,

188
however, limited in his approach to evaluation

methods, but there is no limitation to the requirements on the strictness in registration of the

variables.

As to selection of patient material for clinical

studies, the aim of the study is important. We have

in a few studies deliberately selected patients with

high carbohydrate intake with bad oral hygiene

this in comparative studies where two different

restorative procedures were performed intraindividually. The reason for this is obvious ; an acceleration of deteriorating factors decreases the

observation period. For optimal performance a patient group with other characteristics, for instance

low caries incidence and good oral hygiene, may be

the choice.

There are several studies at present where these

circumstances are somewhat combined. In a small

group of patients (50) the treatment is given by

two well-trained operators, and the registrations of

bilateral restorations are done collaboratively. In a

considerably larger group of patients many clinicians (15) perform the treatment according to

their individual methods. In this group only a few variables are registered. All registrations are, however, done by two observers trained for these

studies. With such an organization we hope to obtain information on how the restorations stand up

under different circumstances. Laboratory tests are done according to related specifications and comparisons are to be made later. I am mentioning

these studies merely to underline the fact that the

clinical investigation has to be carefully planned

and that the patient material is one very important

variable.

6. Consideration for Aging Processes in

Clinical Studies

There is another problem that we as clinical

researchers have come to be aware of during the

last 10 to 15 years. About 10 percent of our population in Sweden is 65 or older. I understand that

the percentage of older individuals in the U.S. is similar to that in Scandinavia, that is one out of

ten belongs to the age group above 65. Aging can be

described as decreasing cellular metabolism and

steadily reducing function of the central nervous

system. We as clinicians register this in decreased

function of the stomatogastric system. Saliva production is reduced ; the quality and composition of

saliva changes towards more mucous consistency

and less mineral content and reduced surface tension. The motor activity is decreased, the threshold

sensitivity increases, and function as a whole seems

to be on a more primitive or basic level.

Aging presents certain fundamental psychological factors as the most pronounced problems for

the dental clinician to deal with. However, in re- cent publications on gerodontics, suggestions have

been made that restorative procedures and selection of materials should be viewed in the light of

the properties of aging tissues. This is particularly

important regarding the hard tooth substances,

where changes toward increased brittleness may

provide a basis for specific properties of the re- storative material to be used.

Again, I am actually asking for future collaborative work regarding oral environment changes

with aging, this in order to gain a more sound

basis for the search for suitable materials or for

the selection of procedures in restorative treatment

of the aging patient.

7. Conclusion

It is obvious that there is an urgent need for collaborative and correlative studies involving properties to be studied in the laboratory and given the

final check and test in the clinic. It is, however,

quite obvious that clinical research with dental

materials is time-consuming and slow, but today

there is no way out of that. With sound clinical

research on a larger and more penetrating scale,

data and information may be obtained that will

make it possible to set up more meaningful test procedures in the laboratory in the future. And

that is, of course, the goal : to be able to characterize the dental material and in laboratory testing

correctly predict its clinical performance.

189

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental jVIatebials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Biological Evalvation of Dental Materials

Gunnar Ryge

Dental Health Center, Division of Dental Health

Department of Health, Education, and Welfare, San Francisco, Calif. 94118

Biological evaluation of dental materials includes (1) screening for toxic or other

untoward effects of the materials, and (2) clinical evaluation of materials that pass

the screening tests. Screening tests must take into account the functional requirements of

various types of and service conditions for dental and auxiliary materials. Methodology

for evaluation of clinical performance of materials must include examiner training and

calibration in the use of rating scales. Emphasis is placed on the need for correlation of

laboratory properties with clinical performance.

Key words : Biological evaluation of dental materials ; clinical evaluation of dental materials ; correlation of laboratory and clinical performance ; dental materials ; restorative

materials ; toxicity tests.

1 . Introduction

Biological evaluation of materials used in dentistry includes (1) screening for toxic, allergic, or

other untoward effects of the materials and (2)

clinical evaluation of restorations, appliances, or

devices, the materials for which have passed the

appropriate screening tests. Maybe we should first discuss the definition of a

dental material. About a year ago I sent out a

circular letter to initiate action on the part of the

Subcommittee on Biological Testing which was

formed under the auspices of the FDI Commission

on Dental Materials, Instruments, Equipment and

Therapeutics. Many respondents accepted these

working definitions but as those of you who have

followed Dr. Paflenbarger's excellent and exemplary work for the FDI Commission will know,

the greatest value in balloting of such circular

letters lies in the votes of the dissenters, and, as it has often been demonstrated in Dr. Paffenbarger's

modus operandi, the negative votes and the

thoughtful and thorough objections constituted

the real substance of the discussions on definitions

and classifications.

Although I cannot, in this short presentation,

pay tribute to all of the people who gave of their

time and thought to constructive dissent, I must

mention the excellent suggestions of Dr. Baume

and Dr. Petzold from Switzerland, Alan Grant,

Dr. Thonard, and Alan Docking from Australia,

Dr. Shoemakers and Dr. Van de Woerd from the

Netherlands, Kramer from the United Kingdom,

Fischer from Germany, and several from the Scandinavian countries. Also, several people in the

United States, from military services, from industry, and from universities, came through with excellent conunents and criticisms—and, as you

would expect, George Paffenbarger topped them

all with his closing comment: "Don't shoot me

now—wait until I'm a year older." This letter,

incidentally was dated November 21, 1968, so I

guess Dr. George is safe for another 44 days.

Previous speakers, including Dr. Schoonover and

Dr. Greulich, have made reference to Webster's and

Dorland's dictionaries. I am sure that I should

have done the same before I sent out my working

definitions. Now, however, I'm not going to quote

Webster and Dorland—but Paffenbarger and

Petzold, and I'm not talking about their abridged

version either ! In fact, I don't believe they have

an abridged version ! According to the best au- thority that I can find, this is it

Dental Material : Any substance specially intended

for use in dentistry

(1) to replace tooth structure or tissue lost or missing in part or in whole by injury,

disease, malformation, or by restorative

dental procedures

(2) in the construction of appliances to alleviate malformations and to restore and

improve esthetics and function.

Similarly, the definition of Dental Device has

been clarified, improved, and refined to : "Any res- toration, appliance, instrument, machine, or equipment especially intended for use in dentistry."

Dental instrument is: "Any manual or powered

hand-held tool used in dentistry."

Dental Drug: Any chemical compound or noninfectious biological substance, not used for its

mechanical properties, which may be administered

to or be used on or for patients

(ly as an aid in preventing, diagnosing, and

treating diseases, injuries, malfunctions.

191
and malformation of the teeth, jaws and

mouth

(2) to relieve pain or suffering or to control or improve any physiological or pathological condition.

Finally, Dental Therapeutic is : any dental drug,

dental material, or dental device used for preventing, diagnosing, and treating diseases, injuries, and

malformations of the teeth, jaws, and mouth.

Obviously, there are some overlappings—these

definitions do not segregate all of the categories

that I had hoped to divide up into neat little packages in my so-called working definitions. Some

agents will fall into more than one category, and,

as I have gone through comments from all over

the world in response to that first circular letter,

I can see that, indeed, the dissenters carried the

day and gave us sharper and better definitions

upon which we can build an international system

for biological testing.

The classification of dental materials on the

basis of functional requirements for the various

types and service conditions leads to meaningful

screening tests at various biological leveils. The

classification developed by Ray Bowen in his work

as Secretary of the Subcommittee on Toxicity

Tests under the Specification Committee of the

Dental Materials Group of the lADR does, with

minor modifications, appear to be internationally

acceptable, and the next step, then, is to prescribe

the screening methods that apply to the functional

requirements.

An acute systemic toxicity test, a short duration

inflammatory reaction test, a long duration tumor

production and chronic inflammatory test, an allergic response test, an eye irritation test, a mucous

membrane irritation test, and a pulpal irritation

test can be prescribed selectively in appropriate

order and combinations to meet the needs on a

realistic basis.

In spite of the shortcomings of that first circular

letter on biological testing that was sent out about

a year ago—or perhaps because of these shortcomings—this first effort has produced such a

wealth of good comments that I am indeed optimistic about future progress in this area, and quite a number of those present here today deserve the

credit for it.

2. Motivation for Clinical Evaluation

of Dental Materials

Evaluation of the clinical performance of dental materials has, until a few years ago, been based

upon testimonials. The need for establishment of

sound clinical research methodology has been emphasized by many investigators and I i^ersonally

must confess to ha\dng used the phrase: "The

clinical importance of these findings is not established" in several reports of laboratory studies of

dental materials. Therefore, when the Division of

Dental Health approached me to establish an applied clinical research program on Dental Materials and Technology, I saw the opportunity to do

something about this, and I guess that this is the

reason you have to listen to me today

Most of you are probably aware that there are some inherent advantages in studying dental materials in the laboratory, as compared to clinical

investigations. In the laboratory, one can design

the experiments much neater—one can determine

the number of specimens necessary to obtain statistically significant results, and then proceed to

make the specimens under well-controlled conditions using the advice that one of my physics professors gave me : you make all of the variables constant and make all of the constants zero! Your

specimens stay in their box until they are tested,

and you end up with a fairly well organized piece

of work that you can count on for the next lADR

meeting and a subsequent neat publication. Not so with clinical evaluation ! One of the first objections

I was faced with was the comment : "Are you tell- ing me that you will evaluate dental restorations

placed by a number of operators?" Indeed I

planned to do just that. My scientific friends would

then say : "Don't you realize that you will have

operator variables?" My early answer was: "Yes

sir, I guess I will but has it ever occurred to you

that operator variables do indeed exist in clinical

dentistry ?" The response from not one but several

scientific friends can be summarized in a comment

like : "Don't be smart—what I meant to say was,

all you will find is that there is more difference

between operators than between materials!" I

thought about that and came to the conclusion

that this could very well be so but, on the other

hand, maybe this would be valuable information,

at least if well documented. I don't recall finding

this kind of information in the literature. Perhaps

it would be important to know that differences

between materials were less important than operator variables—particularly if one knew what were

the operator variables that lead to this conclusion.

I am mentioning these comments and discussions

merely to illustrate the philosophy and the viewpoints that motivated me to take on the attempt to

make clinical evaluation of dental materials a legitimate and, hopefully, meaningful discipline.

3. Application of Research Methodology

to Clinical Evaluation of Dental

Materials

How does one evaluate a dental restoration

how does one move from testimony to research

methodology ? Dave Mahler today showed one approach and I believe that my approach will be

useful both by itself and as a supplement to his

method. In clinical practice, every dentist is making evaluations every time he has a patient in the

chair. He looks at each existing restoration and

makes a decision that leads to one of four courses of action : He either decides that the restoration

192
is okay—no action needed ; or else he decides that

there is some doubt about the restoration. He may

decide that he isn't going to do anything now, but

that he would like to see the patient six months

from now for another evaluation ; a third decision

would be that of replacing the restoration for

preventive reasons and, the last course of action

would be that of replacing the restoration because

it is esthetically or functionally unsatisfactory. It

looks bad or damage is occurring around it. How do we, as practicing dentists, arrive at these

judgments? Which are the factors that we consider ? What are the criteria ? A thorough analysis

of the elements that enter into such judgrnents was

precisely the basis for development of rating scales

for clinical evaluation of the performance of re- storative materials, as these rating scales are used

today by the Materials and Technology Branch of

the Division of Dental Health, both in intramural

and extramural or cooperative projects.

The criteria for clinical evaluation, as we used

to call them, were formulated, in their first version,

in the fall of 1964 in a work session that lasted into

the wee hours of the night, and I would like to

mention the names of my three co-workers from

this session, two of whom are here today : Dr. Bjorn

Hedegard who, as most of you know, conducted

one of the first—^and finest—studies on restorative

direct resin materials. Dr. Richard L. Webber and

Dr. R. J. McCune of the Materials and Technology

Branch. Jim McCune wasn't quite satisfied with

the early effort, so he proceeded to take over the

Materials and Technology Branch about 9 months

ago so that he could revise the whole system and

show up the efforts of the previous adminisitration

of the Branch

As a result, the clinical evaluation system now

consists of rating scales for : (1) Color Match; (2)

Cavo-Surface Marginal Discoloration; (3) Anatomic Form; (4) Marginal Adaptation, and (5)

Caries.

To illustrate the type of judgments that are made to measure the clinical performance of

paired anterior and paired posterior restorations

fabricated from contrasting dental restorative materials, reference is made to figures 1 and 2. Figure

1 shows the categories chosen for color match while

figure 2, similarly, describes the categories, or

ratings, used to evaluate marginal adaption.

A similar system is used for arriving at ratings

for Cavo-Surface Marginal Discoloration (fig, 3),

Anatomic Form (fig. 4), and Caries (fig. 5), so that for each restoration, a total of 16 judgments

or choices are made, resulting in 5 ratings. Each

of two examiners carriers out such an evaluation

independently and calls off the ratings to the re- corder. When the two examiners arrive at a different rating for any one category, the recorder will

request a resolution of the disagreement by joint

examination. Such disagreements are usually the

result of one examiner discovering a discrepancy

or defect that was overlooked by the other examiner and are easily resolved by trained examiners.

In addition to the ratings for each pair of restorations, a ranking procedure is used whenever the two

COLOR MATCH

Is ttie restorative Yes

material metallic?

Is the restoration In an anterior tootli? Yes^>Hii_

Call Code

Hotel H

Can you see it witliout using

a mirror?

Call Code

Oscar 0

Test: Visual Inspection at 18"

without mirror on anterior restorations, with mirror on posterior restorations

Is there a mismatch In color, shade and/or translucency between the restoration and the adjacent tooth structure?

"ves^

Is the mismatch between

restoration and adjacent

tooth struchire outside the normal range of tooth color, shade and/or translucency?

"no^

Call Code

Alta A

Call Code

Bravo B

Call Code

Charlie C

FiGtJBE 1. Color match.

193
MARGINAL ADAPTATION

Test: Lightly draw a sharp explorer hack and forth across the margin. If It "catches,"

Inspect for crevice with mirror

if needed

[ Test; }.

I Visually Inspect I

Is there visible evidence of a crevice along the margin

into which the explorer will penetrate?

Is the dentin or base exposed? ""no^

Call Code

Alfa A

Call Code

Bravo B

Test:

Visually inspect or test mobility of restoration with explorer

Is the restoration mobile,

fractured or missing

In part or in toto?

Call Code

Charlie C

Call Code.

Delta D

Figure ;!. Marginal adaptation.

CAVO SURFACE MARGINAL DISCOLORATION

Is the restoration metallic?

Call Code

Hotel H

Test: Visual Inspection of entire margin with

mirror, If needed

Is there discoloration anywhere

on the margin between the restoration and the tooth structure?

Call Code

Alfa A

Has the discoloration penetrated along the margin of the restorative material in a pulpal direction?

^es^

Call Code

Bravo B

Call Code

Charlie C

Figure 3. Cavo-surface marginal discoloration.

paired restorations receive the same rating by

both examiners for any one characteristic.

Clinical evaluatiQn studies are planned according to a set of procedures that include strict adherence to the research protocol and use of specific

procedure records (figs. 6 and 7), evaluation rec- ords (fig. 8), and other forms (figs. 9 & 10) that

permit effective utilization of data processing

eq[uipment. The computer program not only provides tabulation of statistical data but generates

the evaluation records and performs complex editing procedures to guarantee the accuracy of stored

information. The system is based upon the con- cept of a research team consisting of a dentist, a dental assistant, and an observer-recorder, and the

role of each person is defined in the protocol for

the study.

Twice annually, the Materials and Technology

Branch conducts a training, calibration, and test- ing session for its clinical staff to maintain per194
ANATOMIC FORM

Visually inspect with

mirror, if necessary

is the restoration under-contoured

i.e. is the restorative material discontinuous with existing anatomic

lorm?

Call Code

Alfa A

Visually inspect with

mirror, if necessary

Is sufficient restorative material missing so as to expose the dentin

or base?

Call Code

Bravo B

Call Code

Charlie C

FIGUEE4. Anatomic form.

CARIES

Test: Visual Inspection with explorer and mirror

If needed.

Is there evidence of carles contiguous with the margin of

the restoration?

No

Call Code

Alfa A

Yes X Call Code

Bravo B

(1) An area at the restoration margin is carious if an explorer **catches" or resists removal after insertion with moderate

to firm pressure, and is accompanied by one or more of the following:

a. Softness

b. Opacity at the margin, as evidence of undermining or demineralization

c. Etching or a white spot as evidence of demineralization

An area at the margin is also considered carious if the explorer does not "catch," but conditions b or c are present.

Figure 5. Caries.

formance standards. These sessions have also been

attended by personnel from cooperating institutions both from the United States and other

countries.

To supplement the clinical evaluation of paired

restorations fabricated from contrasting restorative materials or of restorations placed with contrasting techniques, an attempt is being made to

design test methodology that will provide meaningful correlation of laboratory data with the clinical findings. Time does not permit a discussion of

this phase of the work but we are in this respect

following similar guidelines as those Dave Mahler

laid down so beautifully this morning.

4. Conclusion

It is not feasible to describe fully in this short

presentation all aspects of the clinical evaluation

scheme that has been developed over the past five

years at the Dental Health Center. I have no hesitation to admit that we have made mistakes along

the way—but we have benefited from some of these

mistakes by being forced to review procedures

and judgments. I hope, however, that I may have

succeeded in arousing your curiosity or even interest in the system and that those of you that might

be inclined to venture into the magic world of

clinical research have obtained some information

that may help you avoid making the same mistakes

that we have made along the way.

I would like to conclude on an optimistic note by

quoting a recent pronouncement by my Division

Director, Dr. Viron L. Diefenbach ; it is particularly apropos to describe the status of clinical re- search : "The best part of our future lies ahead of

us."

195
00 NOT USE

THIS

COLUMN

1-3

i-9

43-44

76

77

78

'9 BLANK

80

PROCEDURE RECORD

POSTERIOR RESTORATIVE STUDY

MATERIALS AND TECHNOLOGY BRANCH, DIVISION OF DENTAL HEALTH, NIH

STUDY name:

10-29 PATIENT'S NAME (type or print)

LAST Fl RST

30 SEX (circle) 31-36 DATE OF BIRTH

M F MONTH DAY YEAR

37-42 DATE

RESTORATIONS

PLACED MONTH DAY YEAR

I NSTI TUTION

PATIENT'S ADDRESS

DENTAL ASSISTANT

OBSERVER/RECORDER

OPERATOR

"Type

OF

TOOTH

NUMBER

SURFACE(S)

(ci rcle)

CLASS

(circle one)

BASE

CODE

LINER

CODE

MATERIAL

CODE

45

T

C

44-47 48-49

M 0 F L 0

50 12 3*5

51 53 54-56 57 60

61-62 63 -64

M D F L 0

65 12 3*5

66 68 69-71 72 75

(T)

(C)

END OF KEYPUNCH

time trituration STARTED

TRITURATION TIME

LAPSE TIME

INSERTION TIME

DELAY TIME

TIME REQUIRED FOR INITIAL

CARV I NO

TIME INITIAL CARVINQ

COMPLETED

DAY FINAL POLISHING

PERFORMED

TIME REQUIRED FOR FINAL

POLI SHI NQ

WAS AN IMPRESSION MADE?

TEST

DO NOT

USE THIS

SPACE CONTROL

HOUR MIN. HOUR MIN.

MIX 1 MIX 2 MIX 1 MIX 2

SEC. SEC. SEC. SEC.

SEC. SEC. SEC. SEC.

SEC. SEC. SEC. SEC.

SEC. 850,

SEC, SEC.

HOUR MIN. HOUR MIN.

MO. DAY YR. MO. DAY YR,

MIN. SEC. MIN. SECYES NOO

M.T. BR. FORM 65 - 9 (rev. 02 29 • 69) PENDING APPROVAL

Figure 6. Procedure record posterior restorative study.

196
00 NOT USE

THIS

COLUMN

I -3

4-S

6-9

43-44

76

77

78

79 BLANK

80

PROCEDURE RECORD

ANTERIOR RESTORATIVE STUDY

MATERIALS AND TECHNOLOGY BRANCH, DIVISION OF DENTAL HEALTH, NIH

6TU0V NAME

10-29 PATIENT'S NAME (type or print)

LAST Fl RST

30 SEX (circle)

M F

31 -36 DATE OF Bl RTH

MONTH DAY YEAR

37-42 DATE

RESTORS^TIONS

PLACED MONTH DAY YEAR

I N6TI TUTI ON

PATIENT'S ADDRESS

DENTAL ASSISTANT

OBSERVER /RECORDER

OP ERATOR

TYPE

OF

PAI R

TOOTH

NUMBER

SURFACE(S)

(ci rcle)

CLASS

(ci RCLE one)

BASE

CODE

LINER

CODE

MATERIAL

CODE

45 T

46 4 7 4 8-49 M 0 F 1 L

12 3^5 51-53 54-56 57-60

C

61 -62 63-64

M D F 1 L

65 12 3^5

66-68 69-71 72-75

(T)

(C)

END OF KEYPUNCH

TIME MIX STARTED

MIXING TIME

LAPSE TIME

INSERTION TIME

TIME MATRIX HELD

DELAY TIME

TIME REQUIRED FOR

INITIAL FINISHING

TIME INITIAL FINI8MINQ

COMPLETED

DATE FINAL FINISHING

PERFORMED

TIME REQUIRED FOR FINAL

Fl Nl SHI NO

WAS AN IMPRESSION MADE?

TEST

00 NOT

USE THIS

SPACE CONTROL

HOUR MIN. HOUR MIN,

SEC . SEC.

SEC. SEC.

SEC. SEC.

SEC. SEC,

SEC. EEC.

SEC. 6EC.

HOUR MIN. HOUR MIN.

MO. DAY YEAR MO. DAY YEAR

MIN. SEC. MIN. SEC.

YES NO

M. T. BR. FORM 65 6 (rev, 02 29 69 ) PENDING APPROVAL

Figure 7. Procedure record anterior restorative study.

197
TYPE OF

PAIR

TOOTH \

NUMBER

SURFACE

CLASS

EVALUATION RECORD

ANTERIOR AND POSTERIOR RESTORATIVE STUDIES

MATERIALS AND TECHNOLOGY BRANCH

DIVISION OF DENTAL HEALTH, U. S. P. H. S.

PATIENT S NAME

STUDY NUMBER

CRITERIA

EXAMINER

FINAL RATING

INSTITUTION NUMBERPATIENT S NUMBER

DATE PLACED

DATE EVALUATED

19

MONTH YEAR

COLOR MATCH

CAVO-SURFACE

MARGINAL

DISCOLORATION

ANATOMIC

FORM

MARGINAL

ADAPTATION

R EXAMINER

A

N EXAMINER

K

0 m s

0 m [u

[o] m [1

0 m @

a m 0

0 m a

0 [2] 0 m 0

0 0 0

0 0 0

REASON

NOT

EVALUATED

TOOTH

MISSING RESTORATION

MISSING RESTORATION REPLACED

PATIENT MOVED

OTHER

(EXPLAINi

REMARKS:

TOOTH \

NUMBER

SURFACE

CLASS

COLUMN 80

EXAMINER

EXAMINER

FINAL RATING

R

A

N

K

EXAMINER 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

EXAMINER 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

CDREASON

NOT

EVALUATED

1

1 TOOTH 1 1 PATIENT REMARKS:

1 1 MISSING 1 1 MOVED

1

1 RESTORATION 1 1 OTHER

1

1 MISSING 1 1 (EXPLAINi

1 1 RESTORATION

1 I REPLACED

Figure 8. Evaluation record.

198
DAILY REPORT OF BATCH NUMBERS AND VARIATIONS OF RESEARCH PROCEDURES

Study Name_ Study Number _____

Institution

Date Work Performed

Day Month Year

PART I: Manufacturer's Batch Numbers

N 1 im Knr Ploced Material Used

Manufacturer's Batch Number

Base (or solid) CaValysf (or liquid)

Test Base

Liner

Restorative

Coritro! Base

Liner

Restorative

PART IK Variations of Research Procedure

D No Variations in Research Procedures Occured on (he Above Date

Signature of nparntn.- Date

Variation of Research Procedure

(Describe Variations and explain)

Check

One

Tooth

Number

Surface & Class Patient' s Name

Test Tooth

Control Tooth

Test

Tooth

Control Tooth

Test ToothD

Control Tooth

COMMENTS: (Include recommendations for the further conduct of this research)

Figure 9. Daily report of batch numbers and variations of research procedures.

199
M.T.Br. Form 69-1

Pending Approval

PATIENT PARTICIPATION AGREEMENT

As port of an effort to develop better materials and procedures for use in dentistry, new and conventional dental materials, techniques, and devices are being investigated in controlled clinical studies.

Since such studies involve considerable expense and require up to five years of periodic examinations,

only persons willing to cooperate fully in this program will be included. Treatment that falls within

the scope of the clinical study will be performed by staff members of

in cooperation with the Materials and Technology Branch, Division of Dental Health, National

Institutes of Health. Your decision regarding participation will in no way affect your eligibility for dental care customarily provided.

If you are interested in participating in this program, please supply the following:

Name Address

(last) (first) (middle)

R,r*i^Anto Sex Telephone

Name and address of your dentist

Date of last dental visit

FLUORIDE HISTORY: In drinking waterD Topically Tablets None

Have you now or have you ever had: YES NO

Rhpiimntir Fpvpt

Heart Trouble _ . _ ,., , ,

High or 1 nw RInnrI Prp«;5iirp

Tuberculosis - . . . Asthma or Hayfever

Allergy to any drug or medicine

Hepatitis (Jaundice) .

Reaction to locc' anesthetic

Prolongea bleeding from an injury

nr Innfl, pittmrtlnn

Are you taking any drugs or medicine?^

Are you under the care of a physician?—

If "yes" to any of the above, please explain

The attending dentist has explained the nature of the study to me and the need to return for periodic examinations. I will do my best to cooperate.

Date Signature (patient, parent or guardian)

FiGUBE 10. Patient participation agreement.

200
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Reseasch, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Corrosion Testing in the Mouth

Kazuo Nagai

School of Dentistry, Nihon University, Tokyo 101, Japan

For many years various base-metal alloys were used as substitutes for gold alloys for

dental restorations in Japan. The adoption of an oflScial requirement that these alloys

should not have a weight loss of more than 3 mg/cm" when immersed in solutions of 0.05 percent hydrochloric acid, 1 percent lactic acid, 1 percent sodium chloride, and 0.1 percent

sodium sulfide for three days, prompted a study of corrosion of a copper-zinc alloy and

dental amalgam in the mouth. Cast copper-zinc specimens had average weight losses in the mouth in the range of 0.022 to 0.034 mg/cm^ per day. Conventional amalgams had losses as small as 0.0063 mg/cm^ per day. AVeight losses in the test solutions ranged up to 150

times those in the mouth while weight losses in artificial saliva in vitro were about twice

those in the mouth. It is believed that the lower corrosion in the mouth (which varies from

place to place) results from a cleaning and inhibiting action by the saliva.

Key words : Base-metal alloys, dental ; copper-zinc dental alloy ; corrosion, clinical tests

corrosion of dental alloys ; dental amalgam ; dental materials.

1. Introduction

Immediately before World War II, and during

the intervening years, the dental profession in

Japan has not been able to use gold for restorative

purposes, the main reason being the critical supply of gold, as well as the economic factor. Consequently, a variety of gold substitute alloys have

been developed. Metallurgically, they include alloys of silver, German silver, copper, stainless steel,

nickel-chromium, chrome-cobalt, and others. Much

research has been accomplished and published on

these alloys, and controversy still exists on the ac- ceptability of the alloys in dentistry. In October

1950, the Ministry of Welfare of the Japanese

Government issued an official test for corrosion of

these alloys. This test required that any gold substitute alloy used in the mouth could not have a weight loss of more than 3 mg/cm^ when immersed

for three days in solutions of: 0.05 percent hydrochloric acid, 1 percent lactic acid, 1 percent sodium

chloride, and 0.1 percent sodium sulfide.

No reason or justification was given to the Profession by the Ministry of Welfare for the composition of the test reagents. No information was

provided as to why a dental restorative material

should be subjected to this kind of test, nor

whether the concentrations of the chemical re- agents were appropriate. This is the basic reason why we initiated a series of studies on dental alloy

corrosion in the mouth, particularly with the copper-zinc alloy and amalgam. We also wished to

confirm the contention that cojDper alloy Avas injurious to the human body both locally and systemically. Consequently, the copper alloy and

amalgam were placed in the mouth to determine

their actual weight losses due to corrosion, as well

as other effects.

2. Corrosion Testing of Copper Alloys

In the first part of my discussion I would like to

introduce an investigation on corrosion testing of

copper-zinc alloy in the mouth which was reported

by Dr. Wakumoto. This test was carried out by

four different subjects using four specimens of

various shapes and sizes. The position of the specimen in each oral cavity was also varied. The specimen was cast and polished in the usual manner in

accordance with the manufacturer's instructions,

and placed in the mouth for 50 days. Weight

change was measured by electric balance every 10

days.

Figure 1 shows the sizes and shapes of the specimens of subjects A to D. The shaded portion in

the figure indicates the copper alloy specimen

having different surface area and shape. The result

of Wakumoto's report is summarized in figure 2. The vertical axis indicates weight loss in mg/cm^,

and the horizontal axis indicates time in days. It

is obvious from this figure that the result was

divided into two groups.

Table 1 shows the weight changes of the copper

alloy specimens in the mouth after 50 days and the

average value per day. The largest weight loss is

2.697 mg/cm^ after 50 days and 0.0539 mg/cm^ per

day in subject B. The least change is 0.730 mg/cm^

after 50 days and 0.0146 mg/cm^ per day in subject

D. This variation may be due to the individual differences among subjects, and a change of alloy

structure of each of the specimens when they were

cast.

201
CORROSION TEST IN THE MOUTH

SHAPE AND SIZE OF Cu ALLOY SPECIMEN

- by WAKUMOTO -

17.22 cm^

FiGTJEE 1. Corrosion test in the mouth by Wakumoto.

The shaded portion in th« drawing Indicates the shape

and size of the copper alloy specimen.

T ^—1 1 —r

O 10 JO 30 40 50 Days

FiGTJBE 2. Weight loss of copper alloy in the moutfi

reported hy Wakumoto.

Table 1. Weight loss of Cu alloy in the mouth for 50 days

(by Wakumoto)

Subject

Weight loss (mg/cm^)

After 50 days For a day

AJx 2. 597 0. 0519

B 2. 697 0. 0539

C 0. 802 0. 0160

D 0. 730 0. 0146

In figure 3 the size and shape of specimens is

given for the in vivo corrosion test of copper alloy

as determined by Dr. Sakimia. This test was performed in order to evaluate the common opinion

that the use of copper alloy for dental purposes

was harmful to the human body. He wore four

specimens which have different shapes and sizes

in his mouth.

Figure 4 is a summary of the results of Dr.

Sakuma. According to his test, the weight loss of

copper alloy in the mouth is between 0.6 and 1.4

mg/cm^ after 50 days.

Although specimen C was broken at 40 days

(table 2) , its weight loss is the largest, being 1.383

mg/cm^ for 40 days, which corresponds to 0.0346

mg/cm^ a day. The least change in Sakuma's re- port is 0.611 mg/cm' after 50 days, and 0.0122

mg/cm^ per day in specimen A.

Figure 3. Corrosion test in the mouth by Sakuma.

The shaded portion in the drawing indicates the shupe and

size of the copper alloy specimen

202
T

1 1 1 r

Weight Loss of Cu Alloy

In the Mouth

2.0 -

- by SAKUMA -

4 Shapvt of Spectman*/

1 Subjact

E» 1.5-

0 10 30 30 40 50

Do y»

CORROSION TEST IN THE MOUTH

SHAPE AND SIZE OF Cu ALLOY SPECIMEN

- by NAGAI-OHASHi

FiQXJBE 5. Corrosion test in the mouth by Nagai and Ohaahi.

The shaded portion in the drawing indicates the shape and size of the copper alloy specimen.

FiGUKE 4. Weight loss of copper alloy in the mouth

reported by Sakuma.

Table 2. Weight loss of Cu alloy in the mouth for 50 days

{by Sakuma)

Specimen

Weight loss (mg/cm')

After 50 days For a day

A 0. 611 0. 0122

B 0. 941 0. 0188

C* 1. 383 0. 0346

D 1. 159 0. 0232

After 40 days

Figures 5 and 6 relate to the in vivo corrosion

test of copper alloy by our laiboratory. We made

three palatal denture specimens using a copper

alloy, so that they have nearly the same size and

shape. The position of the specimen in the mouth

was also designed to be the same. The test was re- peated three times in the same mouth.

As is shown in figure 6, our test results have

less variation among specimens compared with the

two reports described before. This fact proves our

contention that the shape and type of the specimen is of less significance in the corrosion test of

2.0

1.5 n

E

1 .0

O.S

T 1 1 1

Weight Loss of Cu Alloy

in the Mooth

- by NAGAI-OHASHI -

3 Shapes of Sp«clm«n»

I Subject

50

FiGUBE 6. Weight loss of copper alloy in the mouth reported

by Nagai and Ohashi.

203

452-525 0—72 14
dental alloy in vivo than the individual oral cavity

difference among subjects. In our test, average

Aveight loss per day ranges from 0.028 to 0.037

mg/cm- (table 3).

Table 3. Weight loss of Cu alloy in the -mouth for 50

days {by Nagai, Ohashi)

Specimen

Weight (loss mg/cm^)

After 50 days For a day

A 1. 71 0. 0342

B 1. 42 0. 0284

C 1. 86 0. 0372

Figure 7 is a photograph of the copper alloy

specimen when it was placed in the mouth. It has

very nice color and brightness at this stage of

the testing. A slight discoloration was observed

after one month. In figure 8, taken after 50 days,

the copper alloy showed more discoloration and

less luster.

Figure 9 is a typical example of a copper alloy

crown which has been cemented in a patient for

about 1 year. Not much corrosion or discoloration

was observed on the occlusal surface, but quite a few corrosion spots were found on the proximal

surface after it was remoA'rd. Generally speaking,

it is hard to find the corrosion of a copper alloy

crown or inlay by visual inspection during service, but the corrosion is in progress on an unsanitary portion such as the proximal surface or subgingival area. Let us reconsider the main subject of my paper.

When we compare the corrosion value of three

FiGXJEE 7. Copper alloy specimen when it was placed in the

mouth.

FiGtTEE 8. Discoloration of copper alloy specimen after

about 50 days in the mouth.

FiGUKE 9. Typical example of a copper alloy crown which

has been cemented in a patient for about one year.

reports (table 4), the average weight loss for a day is 0.0341 mg/cm- by Wakumoto, 0.0222 mg/

cm= by Sakimia, and 0.0333 mg/cm^ by our laboratory. 'The largest weight loss is 0.0539 mg/cmand the smallest 0.0122 mg/cm? in 11 trials. The

overall average of the three reports is 0.0299 mg/

cm^ per day. According to these results, the copper

alloy which is commonly supposed to have a high

degree of corrosion actually loses weight in the

mouth much less rapidly than was thoug'ht.

Table 4. Comparison of weight loss of Cu alloy in 3 reports

Wakumoto's Report

Sakuma's

Report

Nagai-Ohashi's

Report

Subject (mg/cmV

day)

Specimen(mg/cmV

day)

Specimen

(mg/cmV

day)

A

B

C

D

Average

0. 0519

0. 0539

0. 0160

0. 0146

0. 0341

A

B

C

D

Average

0. 0122

0. 0188

0. 0346

0. 0232

0. 0222

A

B

C

Average

0. 0342

0. 0284

0. 0372

0. 0333

Overall average= 0.0299 (mg/cmVday)

204
3. Corrosion Testing of Conventional

Amalgam

In the next part of this paper, I would like to

discuss tests carried out by our laboratory on the

corrosion of a conventional type amalgam in the

mouth. A cylindrical specimen, 4 mm in diameter,

was made according to the manufacturer's instructions and after 24 hr was polished to be 4 mm

long. The specimen was imbedded in the occlusal

surface of an acrylic resin artificial tooth for an

upper partial denture so as not to contact the opposite teeth. It was taken out and weight change

was measured after 40 days. The test was repeated

three times in one subject.

The weight loss of amalgam after 40 days in

vivo is shown in table 5. A variation of weight loss

between both sides, the right and left, is noted.

Average weight loss is 0.250 mg/cm^ for 40 days,

which is only 0.00625 mg/cm^ per day. This value

includes the weight loss by erosion or abrasion

produced by contact with the tongue and food in

addition to the corrosion, so that actual weight loss

by corrosion must be less than the result we

obtained.

Table 5. Weight loss of amalgam in the mouth after 40 days

(by Nagai, etc.)

Number of

test

Weight loss (mg/cm^)

Average

1 0. 35 0. 21 0. 28

2 0. 28 0. 22 0. 25

3 0. 27 0. 17 0. 22

Average 0. 30 0. 20 0. 25

Average weight loss per day= 0.00625 mg/cm^/day.

Finally, in table 6 the weight changes of human enamel, copi^er alloy, and amalgam in various

environments are compared. In addition to the

specified chemical reagents, an orange juice of pH

3.2 and artificial saliva prescribed by Greenwood

at a pH of 6.8 were used. The weight loss is far

greater in vitro than in vivo. By way of example,

amalgam decreases its weight in 0.05 percent hydrochloric acid solution about 150 times as much

as the decrease in the mouth. Even in 1 percent

sodium chloride solution, it is about 60 times as great as in the mouth. The results indicating that,

greater weight loss was observed in artificial saliva

than in the mouth are probably due to the fact

that human saliva has an inhibitive action to the

metal corrosion.

When we compare the alloy weight loss with that

of human enamel, the weight loss of human enamel

is about 40 times as great as that of copper alloy

and amalgam in 0.05 percent hydrochloric acid,

and more than 100 times as great in 1 percent

lactic acid. Thus the results indicate that the cor- rosion test in the laboratory as proposed by the

Minister of Welfare is far from the actual cor- rosive behavior in vivo.

4. Summary and Conclusions

The purpose of our studies was to determine the

corrosive behavior of dental restorative alloys in

the mouth and, at the same time, to examine them

from a dental health point of view. The salient

conclusions are as follows

1. Even the copper alloy which is commonly

supposed to have a high degi'ee of corrosion,

actually has weight loss m the mouth as small as 0.022 to 0.034 mg/cm^ for a day.

2. The type of amalgam which is widely used in

many countries today has a weight loss in vivo as small as 0.0063 mg/cm^ a day.

3. The basic reason why these alloys are not

corroded as is generally supposed is that the salivary secretion has the benefit of keeping the mouth

clean. It is also due to the fact that the saliva

tends to inhibit the corrosion of alloys.

4. As it became clear by our testing that the

1950 Ministry of Welfare corrosion test was not

realistic as a means of expressing the corrosive

behavior of dental alloys, it was removed from the

specifications.

5. Within the same oral cavity, the degree of

corrosion varies from area to area, that is, anteI'ior teeth, posterior teeth, occlusal, buccal to lingual, or proximal surface. Observation of the

cojoper alloy restorations after they were removed

indicated that the corrosion is most pronounced on

the proximal surface.

Table 6. Comparison of weight loss of permanent tooth, amalgam, and Cu-alloy in various environments (mg/cm^/day]

Env.

Mat. ~ --^ 0.05% HCl 1% Lactic

acid 1% NaCl

Orange

juice

Artificial saliva

Oral

cavity

Teeth 40. 60

0. 94

1. 01

94. 20

0. 83

0. 68

Amalgam. _ _

Cu-alloy _ _ . .

0. 39

0. 073

0. 145

0. 07

0. 0102

0. 059

0. 0063

0. 0299

205

VII. Specifications

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research. Proceedings of the 50th Anniversary Symposium^

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

International Specification Program—Australian Experience

Alan R. Docking

Commonwealth Bureau of Dental Standards, Melbourne, Australia

The Australian dental specifications as a basis for a certification program were

developed through the Standards Association of Australia and not within the dental

association. Products are accredited b.v the Australian Dental Association on the basis of

meeting these specification tests. The Commonwealth Bureau of Dental Standards assists

in preparation of specifications and in accreditation of products. Approximately thirty

Australian dental standards have been issued. Participation in international specification

work has proved of great benefit especially in the raising and maintenance of the quality

of dental goods used by the Australian dental profession.

Key words : Accreditation of dental materials, Australian experience ; Australian dental

specifications ; dental materials ; dental standards ; specifications, dental.

1. Introduction

If imitation is the sincerest form of flattery, the

Australian tribute on this memorable occasion is

of special significance. From the begiimings of

dental materials research in Australia over thirty

years ago an effort has been made to emulate, in a modest way, the highly successful program of

the National Bureau of Standards and subsequently the system of specifications for materials

and of accreditation of dental products employed

by the American Dental Association through the

Fellowship it established at the Bureau. Thanks

to the inspiration and guidance of the NBS Dental

Research Section and the ADA Fellowship there,

the Australian effort has proved to be of immense

value to the dental profession in that country too. The Commonwealth Bureau of Dental Standards

was established in 1947, but the work did not commence then; ten years prior to this, Dr. Howard

K. Worner, with his Materials Research Laboratory at the University of Melbourne School of

Dental Science, had established a close liaison both

technically and personally with members of the

staff of the Dental Research Section of the National Bureau of Standards. It was the knowledge and appreciation of the NBS success story

that largely shaped the destiny of dental materials

research and specifications in Australia.

There are many differences, of course, between

the two systems both quantitatively and qualitatively. In size, the Australian program is very

much smaller. Although Australia is of comparable size to the main body of the U.S.A., its

population is only some twelve million, served by

about four thousand dentists. On this basis one

could expect something of the order of a twentieth

in magiiitude. One would almost imagine that such

a small number is below the critical nucleus size

for the growth, on a national basis, of a systematic

research, standardization, and testing program

for dental materials. All the more credit is due

to the early foresight of members of university

staff and the Australian Dental Association that

such a program was not only initiated, but

flourished.

2. The Bureau of Dental Standards

The staff of the Bureau of Dental Standards

umnbers only eight, none of whom ai'e qualified

dentists, unfortunately. Physicists, chemists, and

metallurgists are included and for clinical opinions

or trials the Bureau relies on the dental schools,

the Services, and individual dentists either independently or through a Panel of Cooperating

Practictioners throughout each State of the

Commonwealth.

There are essential differences in organization

too in that the Bureau of Dental Standards was

established as an activity of the Commonwealth

Department of Health; furthermore, the standard specifications on which the certification program is based, were developed through the Standards Association of Australia and not within the

dental association as it was in America. It is interesting to note that in recent months the cor- i-esponding body in America, The United States

of America Standards Institute, has now become

very actively involved in the preparation of dental specifications and is to become the national or- ganization through which such specifications are

to be promulgated.

In Australia, as in the United States, the national dental association is the body which controls and directs the accreditation program and

209
issues an official List of Certified Products. It has a

special comiriittee for this purpose and for taking

action on other matters concerning the quality and

supply of dental products available on the Australian market. The Standards Association of Australia, through the help of the dental profession,

the industry and the Bureau of Dental Standards,

prepares the specifications; and the Australian

Dental Association accredits products based on

these specifications which they endoree. It is the

Bureau of Dental Standards that does the neces- sary testing, checking, and investigational work

required to assist both in the preparation of specifications and in the accreditation of products

complying with them.

3. International Participation

Because of the limited local demand, Australia

has never been anywhere near self-sufficient in the

manufacture of dental supplies. At the outset,

therefore, cognizance had to be taken of the fact

that a large proi>ortion of dental goods are imported and the accreditation program had to be

adapted accordingly. It is only in recent years

that the American Dental Association program has

included any imported products and these are still very much in the minority. There is a certain

amount of dental manufacturing in Australia,

particularly in relation to gypsum products, amalgam alloys, gold alloys, waxes, modelling compounds, hydrocolloidal impression materials,

orthodontic wires, local anaesthetic solutions, synthetic resin teeth, and a few cements. The large

bulk of equipment, instruments, and other fabricated items are imported ; also, for every product

manufactured in Australia, there are many of the

same type available from overseas, in spite of

various protective tariffs that may be imposed to

foster local production. One of the prime functions

of the Bureau of Dental Standards is to assist

local manufacturers, many of which are not large

enough to support adequate laboratory staff and

facilities. At the same time, the Bureau plays an

important part in advising importers on the quality of their lines and ways in wiiich they could be

improved to meet the specified requirements.

The fact that so much of investigational and

testing Avork is concerned wdth overseas products

and developments has meant that Australia has

maintained a keen interest in the progress on dental materials throughout the world; this has led

to active participation in international committees both through the Federation Dentaire Internationale and the International Organization for

Standarization. This interest has been greatly

stimulated and encouraged by personnel of the National Bureau of Standards and of the American

Dental Association Fellowship to whom we are greatly indebted for their part in initiating international specification programs.

Participation in the international sphere has

proved to be of great benefit to the local specifica- | tion program and to the raising and maintenance j of the high quality of dental goods used by the

Australian dental profession.

4. Australian Specification Program | As to the specification program itself, some

thirty Australian dental standards have been is- sued. These cover a wide range of items including

mercury, amalgam alloy, zinc phosphate and sili- cate cements, impression and laboratory plasters, ; artificial stone, inlay, sticky and modelling waxes,

modelling compound, agar and alginate impression materials, impression paste, inlay and denture

casting gold alloys, cobalt-chromium casting alloy,

casting investment, denture base resin, synthetic

resin teeth, local anesthetic solutions, x-ray films,

hypodermic needles, hand cutting instruments, ! and gold and silver solders. ' Currently, work is in hand to develop Australian standards for elastomeric impression materials, rubber dam, orthodontic cements, matrix j band material, rubber elastics, endodontic instru- ' ments, root canal materials, and toothbrushes.

Some of the existing specifications are under

revision.

The controlling body in the specification program is the Dental Materials Committee of the

Standards Association of Australia. This has equal

representation of profession and trade together ' with representation from the Department of

Health. It decides on policy and priority, but the

drafting work is done through a number of subcommittees appointed for special fields such as gypsum products, impi'ession materials, cements, ! waxes, casting alloys, synthetic resins, instruments,

photographic materials, and, more recently, orthodontic materials, endodontic materials, and toothbrushes. These subcommittees also comprise the

profession, trade, and testing authorities, but also

bring in industrial experts from the respective !

fields.

When the need for a specification is recognized, a preliminary draft is prepared for the consideration of the appropriate subcommittee. This draft

is usually prepared by the Bureau of Dental

Standards and is based on existing overseas specifications and on its own work. When the subject has

been Avell thrashed out in committee and reviewed,

a draft is prepared for public criticism. Such

drafts are widely circulated and are available to

all interested parties.

After consideration of all comments received on

the proof issued for critical review, a new draft

is prepared and a postal ballot is finally taken.

An effort is made to achieve unanimity at this

stage. The specification then is printed as an offi210
cial Australian standard, but is always subject to

amendment or revision in the light of new knowledge and developments.

The system used by the Standards Association

of Australia is modelled largely on that of the

British Standards Institution and it appears to

work very well in British Commonwealth countries particularly.

It now remains for the Australian specifications

to be reconsidered in the light of ISO Recommendations as they appear. An attempt was made on

several occasions to reconcile Australian standards

with the corresponding FDI specifications, but

these have not succeeded because of the expressed

desire of committee members to retain some of the

local differences.

The change in status of FDI specifications from

a professional association backing to a national

basis through ISO members should have a significant influence in dental specification j^hilosophy,

not only in Australia, but elsewhere throughout

the world.

If any counti-y has endeavoured to emulate the

U.S. dental materials program of specification and

certification, it is surely Australia and the fruits are apparent in the standard of dental supplies

currently in use and in the confidence of the profession and trade in the local program.

211

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Development of European Specifications and Testing

Pierre Laplaud

L'Ecole de Chirurgie Denlaire et de Stomatologie de Paris, Paris, France

The development of a European specification program is difficult because of the many

differences of race, nationality, politics, and social systems. One of the first problems is the establishment of a common dental terminology. The lack of sufficient data on the

relationship of properties and clinical results is another difficulty. However, Europe has a long heritage of standardization in other fields and this has made it logical to think of

the ISO as the proper organization to introduce international dental standards into Europe.

Many European countries have already achieved promising results on a national basis. Cooperation on a regional or continental level in the development of a specification and

certification program will help to provide better dental health for the piiblic.

Key words: Certification; dental specification; European dental specifications; FDI ; ISO;

terminology, dental.

1. Introduction

The United. States of America seems to have a natural propensity to be, in a large number of

matters, first in the world. The science of dental

materials was no exception. So, we are here to

celebrate the fiftieth anniversary of the dental re- search at the National Bureau of Standards. The

joint sponsor of this celebration, the Americal Dental Association, has cooperated in this research for

forty-one of the fifty years, and I think we all

agree that this was a fruitful wedding.

Unfortunately, European weddings appear to

be much more dilRcult. We, Europeans differ by

race, nationality, politics, and social systems. And,

when, by accident, we happen to believe in the

same God, we do not worship Him tlie same Avay.

Moreover, I was told, that in some European counties, the dentists themselves were not very strongly

united.

This charming variety has made European dental specification programs interesting, because we

had to meet nearly the same difficulties as to es- tablish a world wide plan. Firstly, we had to think

over a terminology, a philosophy, and a policy. A

terminology for our mutual understanding, a philosophy to ascertain and make our general aims

common, and a policy to enable us to meet our wide range of national and international difficulties. 2. Terminology

Maybe the worst danger of our time is what we

might call "babelization"—using the same word

with different meanings. In Europe, we speak a good number of different languages, more than

twenty. Even when these languages were restricted

to only two, there was no true communication without difficulty. And it appeared dangerous to try to

reduce them to only one; in each language, the

meaning of a word is, by itself, a convention. In

another country, although the term is the same,

its conventional meaning may be different. So we

have to agree, by mutual concessions, upon an international meaning, at least for important dental

terms. Deciding this may help the countries whose

language is neither English nor French, to revise

their own dental terminology, in the same way

they should have revised their political terminology. Problems involving both terminology and

classification belong in a specification program and

are not simple matters either. We are confronted

with just such a problem on the codification of

dental burs and the classification of dental equipment.

Apparently, there is still much work to do as concerns tei-minology, and everyone may have a

share of it. 3. Philosophy

A sound and useful philosophy for international

dental terminology, classification, and specifications is very difficult to establish—we are ignorant

of much of the data we should know. For instance,

experience has taught us that, wlien some physical

properties are within certain limits, restorative materials, such as ones based upon zinc, silver, tin, or pure gold, generally give satisfactory clinical results. Of course, the temptation is great to reduce

every specification to these physical properties.

But we shall have no security whatever about the

clinical behavior of such specified materials. On

the other hand, specifying chemical composition

may, in some cases, hamper progress.

If we agree, for instance, upon a code number

signifying that a bur is an inverted cone, we are

213
bound to specify the ratio between the length of

the head and the mean diameter as well as the ratio

between the two extreme diameters of the head.

The same concerns truncated cones. After that,

should we consider as true inverted or truncated

cones only the burs complying with the adopted

ratios ? Such a classification may come to the same

inconvenience as the specification of a chemical

composition for any dental material. Working out

and agreeing upon a common and prospective

philosophy will certainly need wide cooperation of

experienced specialists and learned scientists.

4. A Policy for Specifications

It must be kept in mind that an international

specification will probably not be so easily revised or amended as a national one. Once upon a time,

in Washington, a senator was writing at his desk.

Suddenly, the door was flung open by a young and

excited secretary shouting that a Mr. Charles

Lindbergh, alone in an airplane, had succeeded

in flying across the Atlantic Ocean, from the

United States to France. The disturbed senator

stared calmly at his secretary and uttered : "Well,

a single man can do anything. Please call again

when a commission has flown across the ocean."

It looks like these days have come. Nqw, dental

commissions fly across the oceans, thus making

clear that we are in the era of efi'ective and

worldwide cooperation in the field of dental

specificatit>ns.

If we provisionally lay aside dental specifications, international cooperation in the field of

standardization is, in Europe, a very old afi^air. Many European national standards associations are more than fifty years old and celebrations of

fifty years of international standardization could

have taken place during the past years. Electrotechnical international standardization began with

the first years of this century. This includes long

established habits and rules that are far too much

implanted in the incredible legal and political

particularisms of Europe to allow easy pacific

changes. And experience proved that, in a good

number of cases, national dental federations had

neither desire, nor power, to discuss these matters

with sufficient force to convince national standards

organizations.

Moreover, these standards organizations might

think that they were very experienced and had

dealt with much more important matters than

dental standardization. They have built many

European organizations : CEN, European Committee for standardization ; COCOR which has the

same function for the European Common Market

CENTRI, organization uniting BSI, DNA, and

AFNOR to speed up the worlc of CEN ; CENEL,

for the coordination of electrotechnical standards,

with CENELCOM the same for the common

market.

With the exception of the electrotechnical

j branch, which is related to CEI, all these organiza- ! tions are very strongly linked with ISO, the International Standards Organization. Wlienever an

ISO Recommendation exists, and is acceptable to

the members of CEN, no further national or re- gional cooperation work is initiated.

Therefore, it was quite logical to think that ISO

was the proper organization to introduce inter-

'

national dental specifications into Europe. And the

International Standards Organization Technical

Committee 106—Dentistry—was established. Its

Secretariat was assigned to the United Kingdom,

which took into account the experience of the British Standards Institution both in dental specifica- , tions and in European standardization. Geographical, historical, and intellectual conditions

have, in the past, made Great Britain able to serve

as an abutment to various kinds of bridges established between America and Europe. Dental standardization was no exception : the nine current EDI j specifications were soon proposed as ISO Recom-

| mendations. The EDI and ISO agreed to inform j each other of their intention to standardize such

and such material, instrument, or eqviipment, thus

avoiding as far as possible the duplication of work.

This agreement between the FDI and ISO, now in

procedure of development, is actually a very promj ising one. The two organizations are, I believe,

j more and more willing to work together. And ! many dentists are related to both, thus insuring a \ good conveyance of our professional ideas and

needs. This might well be that fruitful European

wedding we were searching for.

5. A Policy for Testing and Certification

Another question is the testing of dental materials, instruments, or equipment according to

accepted specifications. Nowadays, many Euroj pean standards organizations have, upon a naj tional basis, their own mark of approval or

j certification. To these are submitted all products,

j national and imported, seeking certification. This [ may work when products are exported to a rather

limited number of countries. But this is not the

| case for many dental materials. And it would be a heavy charge for the manufacturers to seek certification in every country. For Europe only, more

tlian twenty standards organizations might be involved. This will probably not be tolerable for

long.

Europe has to work out a common program for

certification. Perhaps on a CEN and FIDE basis,

perhaps in other ways. Later on, we could sign and

observe agreements with other continental certification programs to obtain cross examination and

cross reference upon a continental basis. This

would avoid a material being sold with the reference: "this material appears on the North

| American—or European—list of certified dental i materials" when, in fact, the exported quality is

214
not the same, the manufacturer knowing well that

his risk is nil because nobody will test his material.

So, I would suggest that within an FDI or ISO/

TC/73/SCI schedule, continents or groups of continents work upon this possibility of cross-testing

and cross-referencing dental products. Anyway,

this question of continental laboratories has to be

solved ; there is little hope that a worldwide laboratory will be convenient. Nor can it be expected

that national laboratories belonging to small or developing nations be efficient or economically feasible. Europe is a good example of a continent

where there is no other possibility. The opinion of

certification specialists—particularly of the Working Group CEN/WG 42-Certification of products—is that such objectives will probably be

reached through a multilateral recognition of national marks indicating conformity with standards. I believe that continental laboratories are of

a paramount importance. Continental dental asso- ciations could probably speed up and improve such

achievements by inducing their members to exert

pressure upon government institutions, or national

standards associations.

6. Research, Backbone of Specification

Underlying the importance of research as the

backbone of a certification program today would

not, I am afraid, be very inventive. In fact, some

very simple specifications could make our work

easier. For instance, we could decide that all the

droppers used for the liquids of our cements

should deliver equal drops of a standard liquid.

And we might specify that every manufacturer

should provide a plastic or metal proportioner for

the powder, with the indication of the corresponding number of drops to obtain the standard or clinical consistency under standard hygrometric and

thermometric conditions. Such a simple indication

might facilitate our work, and the teaching of dentists and dental hygienists.

Other specifications will be much more difficult

to establish. For instance, it was the Swedish Delegation to ISO/TC/106 who called attention to the

need of the biological testing of dental materials.

Here is a very wide and interesting issue about

which we have not much more exploitable knowledge than our forefathers. In fact, we are still "cementing AVooden legs to the teeth of our patients." There is still a lot of work to do. And

Europe is like a mosaic, a puzzle of nations. It

would be very surprising if each one of these nations were able to explore all the fields of science

necessary to improve our specifications. Here

again, European and even worldwide cooperation

is necessary.

7. Conclusion

In conclusion, I am obliged to say that, in the

true meaning of the term, we have then no European specifications and testing program. But, fortunately, many European countries have already

achieved a lot of promising results upon a national

basis, and, concerning the northern countries,

upon a regional basis. In this rather limited paper,

I have cliosen not to speak of these achievements,

although they will certainly prove to be very useful. But, for the future of Europe, as concerns

dental specifications and testing, like many other

matters, there is no better way than cooperation

and coordination. These would be easier to attain

if national dental associations would cooperate

with their national standards organizations and

with government institutions while giving firm

support to the FDI and ISO. Organizations acting

at tlie regional or continental level could also speed

up and improve coordination. There is no doubt

that such cooperation will help provide what we

are all seeking : better dental health for the public.

215

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

Development of South American Specifications and

Testing—Brazilian Experience

Leo Werner SiifTert

Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil

A successful program of specifications for dental materials requires extensive knowledge of and participation in the program on the part of teachers in dental schools. In turn

the dental teachers can involve dental industry and dental societies. In Brazil a specifications program in dental materials was initiated by holding annual meetings to which per- sons recognized as authorities in dental materials were invited to lecture and give assistance

in specification aspects. Certification testing of dental materials has been initiated in only

two dental schools, but it is planned to have dental materials departments at selected

dental schools responsible for three or four materials in a general certification testing

program.

Key words : Brazilian experience, dental materials specifications ; dental materials ; dental

specifications ; dental specifications and testing, South American.

1 . Introduction

During the last 35 years, in many dental meetings throughout South America a great number of

speakers talked about the importance and necessity of a national program of specifications for

dental materials.

Sometimes theses were approved [1, 2] ^ by General Assemblies, concerning this subject. On other

occasions, specific dental materials were tested,

according to existing ADA or FDI specifications

and many articles were published, only a few of

which are referred to here [3, 4, 5].

In Brazil, many theses with titles like "Doutoramento", "Livre-Docente" or "Catedratico", from

teachers belonging to prosthetic dentistry or operative dentistry departments, were completed in

dental materials departments and, in most instances, ADA or FDI specifications were used as a guidance.

The effort of Degni [3] in 1949 should have

been the beginning of a specification program in

Brazil. The effort of Pinto [4] in 1962 should also

have been the beginning of a regular specification

program for dental materials in Argentina.

Why did those two extraordinary leaders in the

field of dental materials in South America fail,

with regard to a regular specification program?

Why did other people fail, each time they intended to start a specification program, in different countiies in South America ? The answer to these questions is precisely what

I intend to analyse and later on, when I give you

1 Figures in brackets indicate the Uterature references at the end of this paper.

a brief report about our "Brazilian Experience"

during the last four years, I hope this could be of

some benefit to other South American countries.

2. Persons and Institutions Involved in a

Specification Program

2.1. The Dentist and the Dental School

A specification program has to involve first of

all, the dentist. What does he think about a specification program ? How much does he care about

a specification program ? How much does he know

about specifications ? I think it is fair to state that the dentist is a product of his dental school. If you want to know

about the quality of teaching of any school of dentistry in South America, you should analyse the

curriculum, you should find out about integration

of teaching programs, you should find out about

teaching facilities, but you should also be able to

evaluate the teachers themselves! How much do

they know about specifications and the complexities of a specification program ? I think that any attempt to start a specification

program, in any South American country, in which

only a minority of the dental teachers has a fair

knowledge about the subject, is going to be a

failure.

The whole teaching staff has to have, at least, a reasonable amount of knowledge about the problem, so that the students and later on, the dentists, as a consequence, will have developed the necessary

mental attitude to understand and accept such a

specification program I This also means, of course, that the dental societies, consequently will easily accept and work

217
toward the program's acceptance. Without preparation of mental attitude there will be no acceptance. As we see, our first vicious circle can only be cut,

by preparing the teaching staff in our dental

schools.

2.2. The Dental Dealer and the Dental Industry

I remember, back in 1954, in Brazil, I was criticized by my colleagues of the teaching staff of my

own dental school, because I was introducing my

students through lectures, visual aids, and demonstrations to the newly developed elastomeric impression materials. They reasoned: "Why teach

it? Those materials are not available from the

dental dealers in Brazil."

My colleagues agreed with me, however, when I answered : "The dental dealers do not carry these

products because there is no demand for them on

the part of the dentist. There is no demand on the

part of the dentist, because we teachers, at the dental school, do hot mention the product !" This is another vicious circle that very often

happens in South American dental schools.

Through the dental schools, the dental dealer,

and especially the dental industry, will get involved in a specification program.

2.3. The Dental Societies

It used to be a policy of past-Presidents of the

Brazilian Dental Association, to nominate a Committee of Dental Materials: a President, a Secretary and usually two or three members.

For many years the by-laws of the Brazilian

Dental Association contained reference to specifications and a specification program. The simple

nomination of a Committee of Dental Materials,

however, used to be the beginning and the end of

a specification program ! I think this is true for

many South American dental societies. The mental

attitude towards acceptance of such a program,

was not prej^a^red. Not even all the dentists realized

its importance.

At the same time as the Committee was nominated, new dental schools were created, with teaching staffs, in most instances, not well prepared for

teaching. Yet those teachers, would somehow prepare dentists who, of course would join a dental

society. I think, around 3,500 dentists graduate

each year from the approximately 70 dental schools

in South America.

To get the dental societies involved in a specification program, we have to start the work, seriously, with all the dental students.

2.4. The Public Health Dental Services

It is unquestionable that dentistry, all over the

world, is seeking new methods, in an effort to ex- tend its service to more and more people. This

occurs through Dental Public Health Programs,

either supported by governments or by private

institutions.

When it comes to restorative dentistry, what

criteria do these institutions use to buy dental j materials? You and I know that the research , program at the National Bureau of Standards , was started in 1919 to answer this question.

Especially with government-supported dental

services, many programs in South America are

restricted by drastic budget cuts, and very often

as a result cheaper dental products are acquired,

unfortunately. i

2.5. The Government

I agree that the government of any country has

to participate in a specification program for dental , materials. But on the other hand, I think it is , unwise to think that the government of any South , American country should start such a program.

In most instances there are not enough funds

available to finance adequately the national standards institutions ! :

, There are still so many other important and

j basic problems to be solved by governments of i South American countries that I think we do not

have the right to expect them to start a specifica- , tion program for dental materials. The government must get involved and cooperate, but is not

supposed to start, the program

As we can see, specifications committees and sub- , committees of national standards institutions need | the active participation of dental schools, dental , materials groups, the dental profession through , state and national dental associations, the associa- j ( tions of dental research, the national and Latin- | | American associations of dental schools, and the \ \ governments. , To be successful, any specification program for

^ dental materials needs the active participation, the j cooperation, of the whole group of persons and , institutions involved !

(

] 3. Brazilian Experience with a Specifica- ( tion Program for Dental Materials |

In Brazil, as in any other South American ( country, we have an excellent starting point : the i tremendous amount of excellent work and experi- s ence accumulated during the 50 years of your ] National Bureau of Standards; the excellence of

postgraduate courses offered by many dental ma- i terials departments in the United States; all the ] information received through your research as- j sociations and institutes ; the American Dental As- : ( sociation; the excellent opportunities offered to ;: ^ Latin American dental teachers, through fellow-

^ ships sponsored by foundations such as the W. K.

Kellogg Foundation. _ ( We also based the starting of a specification pro-

^ gram in Brazil, on the experience of other insti- '

j tutions, such as the Commonwealth Bureau of

|

218
Dental Standards in Australia (22 years), the

BSI, the DNA (Deutscher Normenausschuss) , the

AFNOR (Association Francaise de Normalisation), ISO-TC/106; FDI; the experience of excellent researchers of Switzerland, Sweden, Denmark, and Japan, among others, who were always

very cooperative, sending us information.

3.1. The Brazilian Dental Materials Group

Forty-two out of the approximately seventy dental schools in South America are located in Brazil.

We felt that it was worthwhile to try to start a

specification program through the dental schools

in Brazil. The dental school, we think, is the only

place in which we can prepare the mental attitude

of the futiire dentist toward the acceptance of a

specifications testing program. To achieve this with

the 2,100 students who graduate each year from the

dental schools in Brazil, we need first of all the

understanding, the acceptance, and cooperation of

all the dental teachers. The starting point, of

course, is the teacher of dental materials, participating actively in the Brazilian Dental Materials

Group.

We started the Group (GBMD,Grupo Brasileiro

de Materiais Dentarios) in September, 1965, when

we had our first meeting in Porto Alegre, in the

State of Rio Grande do Sul. Distances are great in

Brazil, and we needed from the beginning the ac- tive participation of as many dental materials

teachers as possible. For our first meeting, we used,

as you might call it, a very good "bait''. To deliver a course in dental materials and to present some

conferences, we were fortunate to have with us at

the time, the Avorld-known authority in dental materials. Dr. Floyd A. Peyton, from the University

of Michigan. Besides Dr. Peyton's course, we had

group discussions on the following subjects: (1)

Dental materials in the dental curriculum : lectures

and laboratory course contents; (2) Integration of

dental materials with clinical departments; (3)

Research in dental materials departments; (4) The

establishment of a Brazilian dental materials

group and the adoption of specifications.

Over one hundred, most of Avhom were teachers

of dental materials, attended the course and the

meeting. Thirty-one dental schools were represented, out of a total of 38 then in existence, in

Brazil

For our second meeting, in 1966, in Belo Horizonte, State of Minas Gerais, we had as our guest

lecturer, Dr. Hector Maddalena, Head of the Dental Materials Department in Buenos Aix-es, Argentina. Again we had a good meeting, with group

discussions related to teaching and research in

dental materials and specifications.

Our third meeting, in 1967, was held in Piracicaba. State of Sao Paulo, and we had over 80 members of our Dental Materials Group present. At

that opportunity Prof. Degni and staff presented

to the group the results of the controlled testing

of amalgam alloys manufactured in Brazil. This

information was sent to all teachers of operative

dentistiy in Brazil, to all the deans of dental

schools, "to all dental associations in Brazil, to most

dental government-supported services and to the

dental industry. Our guest lecturer was Dr. Guillermo INIac Pherson, from the dental school of

Santiago, Universidad de Chile. Thanks to Drs.

Mac Pherson and Maddalena, our group has a good

number of members from Argentina and especially

Chile.

We divided Brazil into five regions, each having a coordinator who is responsible for special

activities related to dental materials in his I'egion.

We have one meeting each year and the meeting

places are chosen two years in advance.

The fourth meeting was in Fortaleza, Ceara and

we had as our special guest lectm^er Dr. George

C. Paffenbarger who presented to us an excellent course in dental materials. Again we had planned

group discussions, related to three main topics:

(1) Philosophy of the teaching of dental materials; (2) Integration; and (3) Teacher training

in the specialized field of dental materials : teaching and research programs.

There were over sixty teachers present, an excellent attendance, if you consider the great distance

of Fortaleza (in the very northern part of the

country) to Central and Southern Brazil, where

most of the dental schools are. Dr. Paffenbarger

has also been a great help to the Brazilian Dental

Materials Group and even though, due to his many

assignments at the time, he could not accept our

invitation to visit the dental schools in the central

and southern parts of the country, we believe he

has a very good idea of what we are trying to

accomplish. Through Dr. Paffenbarger we also

wanted FDI to know a little bit more about our

activities in Brazil.

Our fifth meeting was held this year, in July,

in Bauru, State of Sao Paulo. I think it was an

excellent meeting too, and I am sure Dr. John W.

Stanford could tell you more about it than I ; he

was our special guest lecturer. We appreciated

very much his course and his conferences and I would be tempted to say that he probably returned

with a better impression of our group than our

previous guests: the dental materials teacher is

now, I would say, much better prepared than he

was four years ago. We notice the great difference,

just reading the conclusions of the group discussions ! We make good use of the many excellent articles

published in the Journal of Dental Education,

especially the ones related to teachers and training

programs like the Workshop on Teacher

Education [6].

Our staff of the dental school in Porto Alegi-e

presented to the group the results of the surveys

related to two additional dental materials nmnufactured in Brazil: investments for gold alloys

and alginate impression materials.

452-525 O—72 15 219
I am certain that the dental materials teacher

in Brazil is now much better prepared to exert his

inflvience in preparing the mental attitude of tlie

other teachers of his dental school and, through

them, of the dental students toward acceptance of

a specification program.

We are also noticing the greater interest and

greater cooperation of the dental industry in Brazil toward our specification program.

So far, tests with regard to what you might call a "Certification Program of Materials" in Brazil

were realized only in the Dental School of the

University of Sao Paulo and in the Dental School

of Porto Alegre, University of Rio Grande Do Sul.

The dental materials departments of those two

dental schools are generally considered training

centers for dental materials. In Sao Paulo, especially under the leadership of Dioracy Vieira and

Francisco Degni, we have accomplished quite a lot. Several other dental materials departments

from other dental schools in Brazil are being prepared to participate actively in the specification

program. The idea is to have those chosen dental

materials departments responsible for, say two,

three, or four specific materials, in the general

testing program. The results are distributed

through the Brazilian Dental Materials Group.

Our Group is recognized by the Brazilian Stand- i ards Association (ABNT) as their Committee of

Dental Materials.

We want the Government to enter the picture,

but only later, when we have some concrete accomplishments to show ! '. To finish my presentation, I would like all of f you to know that we of the Brazilian Dental Materials Group are now quite aware of the fact that

the enormous prestige of your National Bureau of

Standards, of many of your excellent dental materials departments, of your dental research institutions, this enormous prestige, I repeat, was

neither easily, nor casually acquired

4. References

[1] Degni, F., Especificacoes e pesquiza, Rev. Bras. Odont. 3, 19-23 (Ago 1945). [ [2] Acuna, R. O., Oficina Chilena de normas para

materiales dentales, Odont. Chilena 15, 11-15

(Mar- Abr 1966).

[3] Degni, F., and Pomes, C, Amalgamas LatinoAmericanas, Rev. APCD 2, 13-24 (,Jan-Feb 1949).

[4] Pinto, F. E., Amalgamas Dentales Argentinas, Rev. Odonto. Arg 50, 253-259 (.Jul 1962).

[5] Vieira, D. F., Pastas Zincoenolicas, Ann. Fac. Farm.

Odont. S. Paulo, 10. 329-347 (1952).

[6] Workshop on Teacher Education. Conclusiones

Teacher training and education programs, J. Dent. Education 30, 54^55 (Mar. 1966).

220
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972).

USA Specification and Evaluation Programs

John W. Stanford

American Dental Association, Chicago, 111. 60611

Reliable and valid laboratory tests which delineate satisfactory materials and rule

out unsatisfactory ones are essential to the development of a satisfactory specifications

program. Factors guiding the American Dental Association in the formulation of specifications are : Relevent tests ; correlation of laboratory, structural, and service tests ; and

quality of product. An important change in organization for tlie development of dental

specifications involves the formation of the USA Standards Committee for Dentistry,

operating under the United States of America Standards Institute. The Certification

Program provides a means for recognition of those materials complying with existing

specifications. A new evaluation program on dental devices is currently issuing pertinent

status reports on the safety and efficiency of such devices. Safety and usefulness are ba.sed primarily upon information established under conditions of clinical use. Key words : Dental materials and devices, specifications for and certification of ; dental

specifications ; specifications ; standards, dental.

1 . Introduction

Programs for the standardization and evalixation of dental materials have been available to the

profession in the United States for many years.

In fact it is well known that the basis for establishing dental research in 1919 at the National

Bureau of Standards was the need for a laboratory

standard for dental amalgam. This was the first

attempt to define a material by means of physical

and chemical properties.

Until 1966 these programs were conducted by

the Association's Council on Dental Research

through its Research Division at the National

Bureau of Standards. The specification and certification programs for materials, along with

other responsibilities, were transferred in 1966 to a new Council on Dental Materials and Devices.

The formation of the new Council elevated programs in dental materials and defaces to a level

within the Association commensurate with their

importance.

The Council has the following By laws : To determine the safety and effectiveness of,

and disseminate information on, materials and

devices which arc ofi'ered to the public or to the

profession.

To encourage the development and improvement of materials and devices for use in dental

practice or to improve the oral health of the

public.

Some of its duties include the following in addition to standardization and evaluation programs

1. Preparation of recommendations in radiation hygiene and practice.

2. Formulation of status reports on materials

and devices.

3. Revision of the Association's publication

Guide to Dental MateHals and Devices.

4. Guidance on advertising and exhibit standards I'elated to materials and devices.

5. Development and evaluation of new materials and devices or the improvement of

existing ones. Before the program was instituted at the National Bureau of Standards, most evaluations of

filling materials Avere based on service tests in

the mouth. Such tests were long draAvn out, ex- pensive, usually poorly controlled and, hence,

frequently inconclusive. Materials must be tested

in the laboratory j^rior to any widespread use. The laboratory tests should be desigiied so that

they will delineate satisfactory materials and rule

out unsatisfactory ones. This terse dictum is difficult to carry out because the design of tests that

will accomplish it requires considerable imagination and the necessary research to prove that the

tests are reliable and valid. By reliable it is meant

that the tests can be repeated by different laboratories with satisfactory agreement in results. By

valid it is meant that the laboratory testing will

predict the behavior of the materials in service.

The American Dental Association believes that

only the highest quality materials obtainable

should be used and does not consider the price at

which a product is sold as any criterion of its

quality. The Association believes that this philosophy is sound and dependable from a public

health standpoint, and the current specifications

221
reflect this policy. Therefore, in designing a specification one has the following in mind : 1. Relevant tests

2. Correlation of laboratory, structural, and

service tests

3. Quality of the product

These factors have gaiided the Specificatioiis Committee of the Dental Materials Gi'oup of the

lADR, which acted as principal consultant to the

American Dental Association in the formulation

of specifications or standards during the period

1953 until 1969. Some of its recent programs have

dealt with formulating specifications or standards

for the following

1. Dental diamond rotary instruments

2. Dental excavating burs

3. Dental radiographic film

4. Direct filling resins

5. Dui^licating materials

6. Endodontic files, reamers, and points

7. Gypsum materials

8. Toxicity tests

9. Zinc silico-phosphate cements

10. Base plate wax

11. Orthodontic wire not containing noble

metals

The Specification for duplicating materials became official on November 1, 1968, and was designated American Dental Association Specification

No. 20. Final approval of s]3ecifications for items

2, 3, and 9 were given in May and June of 1969.

2. Specifications and Biological Toxicity

The formulation of a specification dealing with

toxicity tests deserves discussion. In the past,

specifications have been designed to evaluate materials on the basis of chemical composition and

physical and mechanical properties determined in

the laboratory. These tests have been designed to

simulate actual service conditions as closely as

possible. The degree of the validity of the laboratory test is determined by the degree of correlation

between data obtained from laboratory tests and

the behavior of the materials in service. The safety

of the materials for clinical use has in the past

been largely based on the fact that no serious complications arose from clinical use. In other words,

no screening tests Avere available for new materials before they were placed in clinical use. With the knowledge that new materials were

and are being made available to the profession at

an increasing rate, the Specifications Committee

recommended and the Association agreed that a subcommittee should be established to prepare rec- ommended sci'eening test pi'ocedures to be used in

the laboratory. The subcommittee is progressing

in the development of such recommendations for

guidelines to biological testing. These guidelines

will be of special assistance to both the maniifacturer and the Association in the event of federal

legislation regulating medical devices.

3. U.S.A. Standards Committee for Dental

Materials and Devices

Of special interest here also is the formation of

a USA Standards Committee for Dental Materials

and Devices. The recommendation for such a committee originated in the dental trade. The recom- mendation as presented calls for the phasing out

of the duties of the Specifications Committee of

the Dental Materials Group of the lADR and a

transfer of its duties to a new committee which

would operate under the United States of America

Standards Institute (USASI). During recent

years the United States has become involved in an

international standardization program through

USASI.^ With formation of various committees,

subcommittees, and working groups there has been

some duplication of effort and, in some instances,

mass confusion about the relations of all the organizations involved. The new committee will be

called the USA Standards Committee for Dental

Materials and Devices, and all other committees

will, as such, be disbanded. The American Dental

Association will act as the administrative sponsor

of the committee through the Council on Dental

Materials and Devices and will also handle the

duties of secretary. It is believed that the Association as sponsor of and the dental industry as a

contributor to such a committee will place organized dentistry in a favorable position m the event

federal legislation in the dental device area is

passed.

4. The Certification Program for

Dental Materials

The Certification Program forms the second part

of the program for materials, after specifications

have been adopted. Under this program the manufacturer of a dental material certifies that his product complies with the specifications AVhich have

been approved as official Sjyecifications of the

American Dental Association and that he is in

compliance with the American Dental Association

Advertising and Exhihit Standards. If the product is found to comply Avith the specification, its

name is then placed on the List of Certified Dental

Ma.terials which is maintained by the Council and

published periodically in Tlie Journal of the

American Dental Association.

From time to time, materials on the List are

tested in the laboratories of the American Dental

Association. When a product is found not to comply Avith official specifications, the manufacturer

is notified, and the product is removed from the

List. No manufacturer may claim paii:icii)ation in

the Certification Program except under authorization granted by the Council on Dental Materials

and DeAdces of the Association. The program has

2 Name later changed to American National Standard Institute (ANSI).

222
been increasingly accepted during recent years.

Tables 1 and 2 show the growtli in the number of

specifications and in the number of products on the

List.

Table 1.

List of Certified Dental Materials

Year Number

1929 40

112

145

187

205

225

222

245

270

285

315

305

325

350

345

372

427

475

1932

1934

1936

1938

1940

1942

1945

1948

1951

1954

1956

1958

1960

1962

1964

1967_

1969

Number of Manufacturers or Distributors

Year Number

1969 105

Table 2. New Certifications

During the period of 1952 to 1964 there were approximately 120 new certifications. The following is a breakdown of results:

Materials 120

Complied 110

Failed 10 or about 8%

Compare this to surveys of certified products:

Surveys 955

Failed 5 or about 0.5%

5. Evaluation of Dental Devices

A new program of tlie Association concerns the

evaluation of dental devices, with priority given

to those devices which directly relate to the health

and safety of the patient, the dentist, and his auxiliary personnel. The program, to date, has consisted of initiation of a series of status reports on

the safety and efficacy of devices. These reports

summarize the existing information and may or may not arrive at certain specific conclusions,

depending upon the amount and quality of the

available evidence. At least the reports serve to

acquaint the dentist with the extant Imowledge. It

is planned that other evaluation procedures, such

as specification and certification or acceptance programs, will be utilized in the future. No one type

of program will apply to all devices, and, to be

of greater value to the profession, the program

should include an evaluation of the usefulness and

effectiveness of the devices from both a clinical

and a laboratory standpoint.

The Council has assigned priority for evaluation of the following classes of devices

1. Instruments for surgical cutting

2. Devices for removing calculus

3. X-ray machines

4. Oral hygiene devices

5. Desensitizers

6. Pulp testers

The first application of the acceptance program

is in the oral hygiene device area. The Council

classifies an evaluated device as Acceptahle., as ProvisionctUy Acceftahle or as Unacceptable under

this program. Evaluation of safety and usefulness

of a device is based primarily upon information

established under conditions of clinical use and not

physical standards or specifications. Electric toothbrushes (formerly a device evaluated by the Council on Dental Therapeutics) and oral irrigating

devices are the first devices being rated by the

Council undei' the Classification System which

follows

CLASSIFICATION SYSTEM

1. Acceptance Program: The Acceptance Program applies to devices for which evidence of

safety and usefulness has been primarily established under conditions of use or to devices for

which physical standards or specifications do

not currently exist. Powered toothbrushes are an example of this type of device. Devices ac- cepted under this program shall be classified as described in the following section.

A. Classification of devices evaluated hy the

Council: After consideration of a device has

been completed under the provisions of the

'Acceptance Program", the Council will

classify the device as Acceptahle^ as Provisionally Acceptable or as Unacceptable.

Devices will usually be classified as Acceptable for a period of three years. Acceptance

is renewable and may be reconsidered at any

time. If manufacturing ownership of the

device changes, the period of acceptance expires automatically.

1. Acceptable devices will be listed in Guide

to Dental Materials and Devices and the

manufacturer or distributor may use an

authorized statement as specified in Section V, "Announcement and Maintenance

of Acceptance or Certification", of these

provisions.

223
2. Provisionally Acceptable devices consist

of those which lack sufficient evidence to

justify classification as Acceptable^ but

for which there is reasonable evidence of

safety and usefulness including clinical

feasibility. These devices meet the other

qualifications established by the Council

on Dental Materials and Devices. The

Council may authorize the use of a suitable statement to define specifically the

area of usefulness of a device classified as Provisionally Acceptable. Classification

in this category is reviewed each year and

is not ordinarily continued for more than

three years.

3. Unacceptable devices are those which are dangerous to the health of the user, obsolete, markedly inferior, or useless. These

devices do not meet the standards outlined in the "Provisions for Evaluation

of Dental Devices."

Now, one may ask why the Association has

entered the device field. There are several reasons.

First, no agency outside of the federal government was conducting any type of evaluation of

complex devices in relation to safety of the patient,

the dentist, and auxiliary personnel. The dentist

at present has no information to use in advising

his patients on devices for use at home. The success of the Association's programs in materials indicated that research in improvement and development of new equipment could be hastened by

the new program.

Another factor which should not be dismissed

j lightly is the sale of many quack devices in the } medical field to the elderly of the country. The

Food and Drug Administration currently has au- thority to consider the safety of devices. These are defined as: "Instruments, apparatus, and contrivances, including their components, parts, and '

accessories, intended (1) for use in the diagnosis, . care, mitigation, treatment, or prevention of dis- ! ease in man or other animals; or (2) to affect the

structure or any function of the body of man or other animals."

Even restorative materials used in dentistry are included in this definition of devices. If new legislation similar to that Avliich has been introduced

in the past two sessions of Congress is enacted,

| the Food and Drug Administration will be given j additional authority to consider the safety and | efficacy of devices and to regulate certain marketing practices. The extent to which FDA regulations are applied to dental devices, including mate- ! rials, may depend in part upon the effectiveness

| of an existing professional program.

} One of the main reasons the evaluation program

for materials of the Association has been successful over the last 35 years has been the increasing

cooperation of the manufacturers in standardization and improvement of their materials. The Association especially desires this cooperation and as- sistance from all in the field of therapeutic agents, \ dental materials, and devices in the years ahead.

224
VIII. Appendix. NBS Dental Research Section Personnel
!

I

I

!

!

I
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 354,

Dental Materials Research, Proceedings of the 50th Anniversary Symposium,

Held Oct. 6-8, 1969, Gaithersburg, Md. (Issued June 1972)

Personnel of the Dental Research Section of the National Bureau of Standards

The Dental Kesearch Section of the National

Bureau of Standards has served not only as a re- search laboratory but also as a school for dental

researchers. Ahimni of the Section are widely

scattered in research facilities and educational institutions throughout the United States and in

other countries. In the following tables on personnel of the Dental Research Section an attempt

has been made to include everyone Avho has worked

in the Section for six months or longer. Because

the tables cover a relatively long period of time

and since some of the personnel records are incomplete or unavailable, there are undoubtedly omissions and inaccuracies.

During the past 50 years about 175 people have

been members of the staff of the NBS Dental Research Section, table 1. American Dental Association research associates numbered 62; there were

61 military or Veterans Administration personnel; 31 were Civil Service employees; and 25

others were research associates of Weinstein Research Laboratories, guest workere from various

laboratories, or summer students. A summary is given in table 2. Several members were in more

than one category and because of this the total is

more than 175.

Seventeen members of the staff have been in

the Section ten to 40 years, while 23 others were

in the Section from five to ten years. Nearly 90

staff members were military personnel, Veterans

Administration employees, and other guest

Avorkers, who were stationed in the Section for periods from a few months to as long as six years.

Forty-one of these were dentists and 22 were chemists. Eleven of the guest workers were from outside

the United States.

An indication of the variety of backgrounds is shown in table 3 which lists personnel by profession.

Although the present activities of many of our former personnel are not known, at least ninety

of those listed in table 1 are still in the field of

dentistry. Thirty-five remain in the Dental Research Section; thirty-seven are in the Federal

dental services or in dental schools, and seventeen

are in dental practice, in the American Dental

Association research facility in Chicago or in

other dental activity.

Dr. Wilmer Souder, the founder of our group,

has been retired for some time but is hail and

hardy, living in Landisville, Pemisylvania. Dr.

Greorge C. Paffenbarger who first came to the Dental Research Section over 40 years ago is still working in the Section on a part-time basis, al- though theoretically retired. Dr. Irl C. Schoonover, one of our four Section Chiefs, became

Deputy Director of the National Bureau of Standards, the second in command, before retiring from

Government service in early 1969. Mr. William T.

Sweeney, another former Section Chief and a Government service retiree in 1968, is now working as an associate i^rofessor at the University of

Alabama. Dr. John W. Stanford has become Director of the Division of Biophysics of the American Dental Association. Dr. Harvey W. Lyon is

the Director of the Division of Clinical Studies

for the ADA. Dr. A. F. Forziati is Acting Assistant Director of Physical Sciences, Division of

Water Quality Research, Federal Water Pollution

Control Administration. Dr. Robert J. Nelsen is

the Executive Secretary of the American College

of Dentists. Mr. Denton L. Smith is Director of

Research for The J. M. Ney Company. This position was previously held by Mr. Richard L. Coleman, now retired, one of the early research associates in the Dental Research Section,

227
Table 1. Personnel of the Dental Research Section, NBS, 1919-1969

Name

Adler, Mr. Alfred G

Arevjev, Mr. Vsevolod

Argentar, Mr. Harold

Austin, Mr. Clarence

AvNiMELECH, Dr. Yoram

Balekjian, Dr. Aran

Barber, Dr. Ronald

Barone, Dr. Joseph J

Barton, Dr. John A

Beall, Mr. John R

Beebe, Dr. Douglas M

Berger, Dr. Howard S

Booth, Miss Esther

Bowen, Mrs. Joy S

BowEN, Dr. Rafael- L

Brauer, Dr. Frank J

Brauer, Dr. Gerhard M

Brown, Mr. James J

Brown, Dr. Walter E

Burns, Miss Claire L

Burns, Mr. Francis R

Bush, Mrs. Jesse W

Carlson, Mr. Elmer T

Carlson, Mr. John C

Cassel, Dr. James M

Caul, Mr. Harold J

Chandler, Dr. Harry

Civjan, Dr. Simon

Coleman, Mr. Richard L

Copeland, Dr. Henry I Crist, Dr. Ray F

Davenport, Mrs. Ruth M

Demaree, Dr. Neil C

Denton, Miss Glenna M

Dickens, Dr. Brian

Dickson, Mr. George

DioRio, Mr. Alfred F

Driessens, Dr Ferdinand C. N_

Duffey, Mr. Depue H

Durany, Mr. George

Eden, Dr. George T

EicK, Mr. John D

El Sadr, Mrs. Betty Lee

Epstein, Mr. Earl F

Fanning, Mrs. Rachel J

Farris, Dr. Lovell L

Fee, Mrs. Jean G

Ferguson, Dr. George W

Fischer, Dr. Theodore E

Forsyth, Mr. John W

Affiliation

Army

Army

Army-ADA.

ADA

ADA

ADA.

Navy

Army

Air Force.

ADA

Army_

ADA_

ADA_

ADA.

ADA.

Navy.

NBS._

NBS..

ADA.

NBS.

NBS.

NBS.

ADA.

Navy.

NBS._

ADA.

ADA

Army

Weinstein

Research

Laboratory

Air Force

Summer stu- dent

NBS-.

Navy.

NBS

NBS

NBS

ADA

Guest worker.

ADA

Army-ADA..

Navy.

ADA.

ADA.

ADA.

ADA

Summer

student.

NBS

Navy

Army & Air

Force

NBS

Profession

Metallurgist.

Chemist

Metallurgist.

Chemist

Chemist.

Dentist

Dentist

Dentist

Mechanical

engineer

Dentist

Dentist

Typist..

Chemist.

Dentist.

Dentist.

Chemist.

Student

trainee

Chemist.

Chemist.

Chemist..

Secretary.

Chemist

Technician.

Chemist

Chemical

engineer

Dentist

Dentist

Engineer. _

Dentist

Stud, techn.

Technician.

Dentist

Secretary.

Chemist-.

Physicist.

Chemist-.

Chemist-

ChemistChemistDentist

Mathematician

SecretarJ^

Chemist-.

Chemist

Student

technician.

Chemist

Dentist

Dentist

Technician

Tenure

1954-1956...

1958-1959...

1962-present1960-1961...-

1965-1967- __

1964-1965-

1934-1936-.-

1957-1960---

1965-present1934-1947---

1953-1954.

1928-1929-

1965-1969...

1968-present.

1956-present.

1955-1957---

1950-present1958-1959...

1962-present1951-present.

1954-1956---

1948-1953--.

1968-present1958

1969-present.

1937-present1968-present1961-1963...

1922-1928...

1953-1956-

1962-1963.

1950-present.

1959-1961---

1966-present

1966-present

1940-present

1956-19.59--

1968-1969.-

1961-1963..

1962- 1966..

1964-1965..

1963- 1966.-

1963-1965.

1963-1964-

1953-1955-

1965, 1966-

1951-1954-

1946-1949--

1947-1951-

1953-1955-

Present location or employment

New York.

NBS Dental Research.

Redstone Arsenal, Ala.

Israel Institute of Technology, Haifa.

Naval Medical Research

Institute.

Retired, Boston, Mass.

Army, Fort Sill, Okla.

NBS Dental Research.

Army Med. R&D Command, Washington, D.C.

Deceased.

Private practice. Falls

Church, Va.

Washington, D.C.

NBS Dental Research.

NBS Dental Research.

Private practice Wautoam,

Wisconsin.

NBS Dental Research.

Washington, D.C.

NBS Dental Research.

NBS Dental Research.

Chicago, Illinois. Washington, D.C.

NBS Dental Research.

NBS Dental Research.

NBS Dental Research.

NBS Dental Research.

Walter Reed Hospital.

Retired, West Hartford

Connecticut.

Andrews AFB.

Navy, Garden Grove,

California.

NBS Dental Research.

Navy, Camp Pendelton,

California.

NBS Dental Research.

NBS Dental Research.

NBS Dental Research.

Georgetown Univ. Hosp.

KathoHeke University,

Nijmegen, Netherlands.

Georgetown Univ., Med.

ADA, Chicago, Illinois.

Navy, Univ. of Michigan.

State Univ. of New York

at Buffalo.

Arlington, Virginia.

Univ. of Wisconsin.

University Park, Md.

Private practice, Eatontown, New Jersey.

Philadelphia, Penna.

State Univ. of New York

at Buffalo.

Univ. of Alabama.

Howard University,

Washington, D.C.

228
Table 1. Personnel of the Dental Research Section, NB8, J9i9-JS69—Continued

Name

FoRziATi, Dr. Alfonse F

Gardner, Mr. Alvin F

Gardner, Dr. Thomas V

Glasson, Dr. Gilbert F

Gregory, Mr. Thomas M

Grunewald, Dr. Alvin H

Hansel, Mr. Grant, Jr Hansen, Mr. William C

Hartley, Dr. Jack L

Harvey, Mr. Jack L

Hawkins, Mr. Norman D.,

Hegdahl, Mr. Trond M

HicHO, Mr. George E

Hidnert, Dr. Peter

Hobson, Dr. Robert W

Howe, Mr. Willard B

Hudson, Dr. Donald C

Huff, Mr. Richard L

HuGET, Dr. Eugene F

Isaacs, Mr. Aaron

J0RGENSEN, Prof. Kuud Dreyer_ _

King, Dr. Richard_

Kingsbury, Mrs. Pamela

Krogh-Poulson, Dr. Willy G_

KuMPULA, Mr. John W

KuMPULA, Mrs. Marion P

Laueila, Dr. Riita

Lawson, Mr. Melvin E

Led LEY, Dr. Robert S

Lehmann, Mr. Frank H

Leibfritz, Mr. Walter A

Leussing, Dr. Daniel L

Loebenstein, Dr. William V_.

Longton, Dr. Robert W

Lyon, Dr. Harvey W

Mabie, Mr. Curtis P

Macasaet, Dr. Avelino A

Malmstedt, Mrs. Margaret

Manceamcz, Miss Sandra A. (Mrs. John Hefferren).

Manuszewski, Mr. Richard C

Margetis, Dr. Peter M

McConnell, Mr. Robert M._

McDowell, Dr. Hershel

McLaughlin, Mr. Richard D.

Merchant, Dr. Hubert W

Miller, Dr. William A. C

Mitchell, Dr. James A

Moore, Mr. Clarence

Moore, Mr. Robert E

Moreno, Dr. Edgard C

Affiliation

ADA.

Guest worker.

Army

Air Force &

ADA

ADA

Navy

Army

NBS

Air Force

Army

NBS

Guest worker.

Army

NBS & ADA._.

Army

Army

Air Force.

Army

Army

NBS

Guest worker.

Summer

student

ADA

Guest worker

NBS

ADA

ADA

ADA

Army

Army

Army

ADA

NBS

Navy

Navy

ADA

Guest worker.

ADA

ADA

ADA

Army

Army

ADA

Summer

student

Army

Air Force

Navy

Army

Armv

ADA

Profession

ChemistChemist.

DentistDentistChemistDentistPhysicist.

Dentist -

Chemist-.

Physicist

Dentist - -

Metallurgist -

Physicist.

Dentist - _

ChemistDentistChemistDentist

ChemistDentistStudent

technician

Technician - _ Dentist

Mech. Engr.

technician

Secretary

Chemist

Technician - - Dentist

Chemist

MetallurgistChemist

Chemist

DentistDentist

Petrographer.

Dentist

Secretary

Chemist

Metallurgist

Dentist

Technician - - Chemist

Student technician

Dentist

Dentist

Dentist

Chemist

Chemist

Chemist •-

Tenure

1950-1962-

1958-1959-

1962-1965.

1947-1952.

1963-present.

1947-1949---

1956-

1951-

1951-

1949.

1932.

1965-

1957-

1956.

1955.

1962-1963.

1957-

1952-

1957-

1950-

1957-

1965-

1960-

1953-

1959-

1956-

1959-

1968.

1930-1935-

1957-1958-

1958, 1959-

1967-present1947

1952-present1952-present1958-

1950-

1951-

1959-

1953-

1962.

1964-

1961-

1949-

1960.

1957.

1952-

1960.

1954-

present1963---

1951---

1968-present.

1960-1962---

1968-present.

1958-1965---

1966-present1953-1956--.

1954-1956---

1963-present_

1966, 1967,

1968.

1956-1957---

1955- 1958---

1951-1954-

1959-1961---

1953-1955-.-

1963-presentPresent location or employment

Dept. of the Interior,

Washington, D.C.

Army, Fort Sill, Okla.

Private practice, Waterloo,

Iowa.

NBS Dental Research.

Northwestern University.

Washington, D.C.

Consultant, San Antonio.

NBS (Tire systems).

Deceased.

University of Bergen Oslo,

Norway.

NBS (Engineering

metallurgy)

Deceased.

Army, Fitzsimons Gen.

Hosp., Denver, Colorado.

Univ. of Texas, Houston.

Walter Reed Hospital,

Washington, D.C.

Deceased.

Royal Dental College,

Copenhagen, Denmark.

Private practice, California.

NBS Dental Research.

Royal Dental College,

Copenhagen, Denmark.

NBS Dental Research.

NBS Dental Research.

Paris, France.

Washington, D.C.

Hyattsville, Maryland.

New York, N.Y.'

Chicago, Illinois. Ohio State University.

NBS Dental Research.

Navy, Evanston, Illinois. ADA, Chicago, Illinois. NBS Dental Research.

Univ. of the Philippines,

Manila, Philippines.

NBS Dental Research.

Evanston, Illinois. NBS Dental Research.

Deceased.

ADA, Chicago, Illinois. NBS Dental Research.

Georgetown University,

Washington, D.C.

Emory Univ., Atlanta.

State University of N.Y.

at Buffalo.

Private Practice, Opelousas,

Louisiana.

Cape Kennedy Florida.

NBS Dental Research.

229
Table 1. Personnel of the Dental Research Section, NBS, 1919-1969—Continued

Name Affiliation Profession Tenure Present location or employment

Morris, Mr. Richard W

Morris, Mr. Stephen

MosHONAs, Mr. Manual G.-_

MowERY, Dr. William E

MuLLiNEAUx, Mrs. Anna G_ _ MuLZET, Dr. Alfred P

Nelsen, Dr. Robert J

Nelson, Mrs. Barbara

Ofstead, Mr. Eilert A

Oglesby, Mr. Philip L

Ohashi, Dr. Masayoshi

Oliver, Mr. Jerry A

Oppenheim, Mr. William L

Overberger, Dr. James E

Paffenbarger, Dr. George C

Palcic, Miss Julia C

Parestzkin, Mr. Boris

Patel, Dr. Prafull R

Peiperl, Mrs. Martha D

Perloff, Dr. Alvin

Perkins, Dr. Robert R

Peterson, Dr. Gert Forum. __ Pfeiffer, Dr. Kenneth R

Piermarini, Mr. Gaspar J

PiNcocK, Mr. Douglas G

POPPE, Mr. W. A

PosNER, Dr. Aaron S

Roberts, Mrs. Cora L

Rodriguez, Dr. Mario S

Rupp, Dr. Nelson W

Ryge, Dr. Gunnar

Sacchi, Dr. Hector

Sangermano, Mr. Lawrence D

Schoonover, Dr. Iii C

Schouboe, Mr. Paul J

Serio, Dr. Andrew F

Sheehan, Mr. William D

Simon, Mr. Lester _. Slade, Dr. Phihp E

Smith, Mr. Denton L

Smith, Mr. W. Harold

Snover, Miss Dorothy

Souder, Dr. Wilmer

Stanford, Dr. John W

Stephenson, Mr. S. R

Steinberg, Mr. Harold L

Strassburger, Mr. John

Sullivan, Miss Sandra

Army

Army

Army

Veterans

Administration

ADA

NBS

ChemistADA.

ADA.

ADA

NBS

Guest worker.

Army

Summer

student

Army

ADA.

ADA.

ADA.

ADA.

ADA.

Army

Navy

Guest worker.

Veterans Administration

Army

Summer

student

Weinstein

Res. Lab.

ADA

NBS

Guest worker.

Navy & ADA.

Guest worker.

Guest worker.

Army

NBS

ADA, NBS...

Army

Army.

Army.

Army.

ADA.

ADA

Weinstein

Res. Lab.

NBS

ADA

Army

Army

NBS

ADA.

Chemist.

Dentist

Dental Asst. Mech. Engr.

Dentist

Technician.

Technician

Physicist

Physical Science.

Chemist

Student

Technician

Dentist

Dentist.

Chemist.

Chemist.

Chemist.

ChemistChemist.

Dentist.

DentistDentistChemist

Student technician

Engineer

Chemist.

Secretary.

Dentist. .

Dentist.

Dentist

DentistChemistChemist.

Chemist.

Dentist.

Chem. engr.

Chemist

Chemist

Metallurgist.

Chemist.

ChemistPhysicist.

Chemist-

Chemist-.

Chemist-.

Chemist-.

1960- 1962-

1961- 1962.

1956-1957.

1950-1952.

1958-present1966-1968- __

Secretary 1957-1959

1950-1955.

1957-1958.

1957-1958---

1957-present1962-1965---

1964

1963, 1964,

1965, 1967.

1954-1956--.

1929-present.

1967-present.

1958-1961.--

1965-present.

1957-1958;

1962-1963.

1954-1956...

1957-1959...

1961-1962...

1950

1957-1958-

1966-1967.

192.5-1928.

1950-1961-

1961-1966.

1959-1963.

1953-1955,

1969-present.

1949

1954-1955.

1961-1962.

1935-1953-

19.54-1955.

1950-1952.

1949-1951.

1959-1961-

1956-1958.

1947-1957_

1957-1958.

1925-1926.

1919-1945.

1952-1965-

1950

1958-19.59. 1957-1959-

Florida.

Veterans Administration,

Dayton, Ohio.

NBS Dental Research.

IBM, New York.

American College of Dentists, St. Louis, Mo.

NBS Dental Research.

Nihon University, Tokyo,

Japan.

Georgetown University,

Washington, D.C,

Univ. of West Virginia.

Retired, NBS Dental

Research.

NBS Dental Research.

NBS (Inorganic Mat.).

NBS Dental Research.

Silver Spring, Maryland.

NBS (Crystallography).

Navy, Norfolk, Va.

Copenhagen, Denmark.

Deceased.

NBS (Crystallography)

Temple University,

Philadelphia, Pa. Washington, D.C.

Hospital for Special

Surgery, New York.

Walter Reed Hospital.

Loyola University, New

Orleans, La. NBS Dental Reserach.

U.S.P.H.S., San Francisco,

Calif.

Montevideo, Uruguay.

General Electric Co.

Retired, NBS.

Jacksonville, Florida.

Private practice,

Huntsville, Ala.

New York, N.Y.

ChemStrand, Pensacola,

Florida.

J. M. Ney & Co.,

Bloomfield, Conn.

Deceased.

Retired, Landisville, Pa. ADA, Chicago, Illinois NBS (Radioactivity)

Carrier, Inc. Syracuse, N.Y.

230
Table 1. Personnel of the Dental Research Section, NBS, 1919-1969—Continued

Name

SusA, Mr. Martin E

Sutter, Dr. John R

SwANEY, Mr. Aubrey C

SwANGER, Mr. William H

Sweeney, Mr. W. Timothy. _.

Sweeney, Mr. William T

Taylor, Dr. Duane F

Taylor, Dr. Norris O

Termini, Mr. Dominic J

Turner, Mr. John F

ViOHL, Dr. Jochen

Waldron, Dr. John T

Wallace, Mrs. Betty M

Waterstrat, Mr. Richard M

Webb, Miss Georgia I

Weigel, Mr. Keith V l.. Weiner, Mrs. Helen

Weiss, Mr. Jonas

White, Dr. Eli E

Wiedeman, Mr. William

Williams, Mrs. Billie S

WoELFEL, Dr. Julian B

WoLCOTT, Dr. Robert B

WoRTHiNGTON, Dr. Chas. R-.

Yost, Mr. Ernest L

YuDowiTCH, Dr. Kenneth L_.

Zelenka, Mr. Donald

Affiliation

Army

NBS, ADA_..

ADA

Weinstein

Res. Lab.

Summer

student

NBS, ADA,

NBS

NBS

ADA

NBS

ADA

Guest worker.

Summer student.

ADA.

ADA.

NBSADA

ADA

Army

Summer student.

Army

ADA

ADA

Navy

Guest worker

Army.

ADA.

Army.

Profession

Chemist

Chemist

Chemist

Chemist

Student

Technical!

Physicist

Metallurgist

Chemist

Chemist

Technician

Dentist

Student

Technician

Chemist

Metallurgist.

Secretary

Chemist

Typist

Chemist

Student

Technician

Chemist

Secretary

Dentist

Dentist

Chemist

Physicist

Metallurgist

Tenure

1959-1960.

1966-1967.

1946-1950-

1924-1928.

1965-1966.

1922-1941,

1949-1969.

1954-1961...

1928-1931. _. 1965-present_

1964-1965...

1965-1966.

1959, 1960.

1963-present_

1961-present.

1953-1960...

1957-1959..

1959-1963..

1957-1958..

1955, 1956,

1957.

1962-1963..

1966-1968..

1957, 1958.

1949-1951..

1957-1958..

1952

1957-1958.

1960-1962.

Present location or employment

Howard University

Washington, D.C.

Detroit, Michigan.

Deceased.

Medical College of Virginia.

Retired, University of Alabama.

Univ. of North Carolina

Chapel Hill, N.C.

Deceased.

NBS Dental Research.

Free University of

Berlin, Germany.

Private practice,

Pittsburgh, Penna.

NBS Dental Research.

NBS Dental Research.

State University, Ames,

Iowa.

Washington, D.C.

Washington, D.C.

New Brunswick, N.J. Private practice, Merritt

Island, Fla. Buffalo, New York.

Washington, D.C.

Ohio State Universitv.

UCLA.

Pittsburgh, Pa.

Flushing, Michigan.

Table 2. Afflliation, Table 3. Profession

American Dental Association 62

NBS (Civil Service). 81

U.S. Army 42

U.S. Navy 13

U.S. Air Force ^ 7 Veterans Administration 2

Weinstein Research Laboratory, guest workers,

summer students 25

Total 182

Chemists 65

Dentists 47

Technicians (including summer students) 19

Clerical (administrative) 11

Pliysicists 8

Metallurgists 9

Engineers 7

Mathematician 1 Mineralogist 1

Other 6

Total 174

231


i

II