NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

31 1.05-1 1-3 1 10560

31 1.05-1 1-31 1056 1

NBS REPORT

June 30, 1970 10 29 5

REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30, 1970

The dental research program at the National Bureau of Standards is carried on in cooperation with the Council on Dental Research of the

American Dental Association; the National Institute for Dental Re*

search- the Dental Research Division of the U. S. Army Medical Research and Development Command; the Dental Sciences Division of the

School of Aerospace Medicine, USAF; and the Veterans Administration.

During this report period, funding support was also provided by the

Research and Education Foundation of the American Dental

Association.

IMPORTANT NOTICE NATIONAL BUREAU OF ;

for use within the Governmen

and review. For this reason, whole or in part, is not auti Bureau of Standards, Washin

the Report has been specitica

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

rress accounting documents intended

is subjected to additional evaluation

jre listing of this Report, either in the Office of the Director, National

r, by the Government agency tor which

il copies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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TABLE OF CONTENTS

1. Introduction

2. Reports Issued

3. Papers Published

4. Work in Progress

4.1. Determination of Crystal Structures

4.2. Physicochemical Studies of Apatite and Related

Inorganic Substances

4.2.1. Crystal Growth

4.2.2. Solubilities of Calcium Phosphates

4.2.3. Generalized Least Squares Galculations

4.2.4. Galorimetry and Electro-chemistry

4.2.5. Topical Fluoridation

4.3. Gas and Vapor Adsorption Measurements on Dentin and

Enamel; Dentinal Collagen Solubilization Studies

4.4. Mechanical and Physical Properties of Oral Structures

and Restorative Materials

4.4.1. Dimensional Changes in Teeth Components

4.4.2. Physical Properties of Soft Tissues

4.5. Dental Metallurgical Studies

4.6. Dental Investment Studies

4.7. Grafting of Monomers to Hard and Soft Tissues

4.8. Development of Adhesion to Enamel and Dentin Surfaces

4.9. Dental Tensile Adhesion Test Methods

4.10. Adhesive Direct Filling Materials

4.11. Three-Dimensional Panoramic Dental X-Ray Device

4.12. Bonding Gold-Curing Acrylic to Acrylic Teeth

4.13. Investigation of Dental Amalgams Utilizing Low MercuryAlloy Ratios

4.14. Finishing of Composite Resins

4.15. Hazard of Mercury Poisoning Using an Ultrasonic

Condensing Instrument for Dental Amalgam

4.16. Clinical Research

4.17. Specifications and Related Activities

Appendix

NBS Report 10271

NBS Report 19272

NBS Report 10273

NBS Report 10274

NBS Report 10275

NBS Report 10276

NBS Report 10277

Page

1

1

2

3

36

6

79

12

12

14

16

16

18

19

21

24

27

29

33

35

36

36

37

37

37

38

Aggregation Phenomena of Collagen.

Development of a Radiopaque Denture Base Material.

Physical Properties of a Radiopaque Denture Base Material.

Radiopaque Denture Base Materials - Technic Dentures.

Heats of Adsorption of Ammonia and Carbon Dioxide on Tooth Components.

The Crystal Structure of the Aragonite Phase of CaCO 3'

Tertiary Aromatic Amine Accelerators with Molecular Weight

Above 400.

NBS Report 10278 A Method for Finishing Composite Restorative Materials.

NBS Report 10294 Mechanical Properties - Viscoelastic Methods. Part I - Creep, Relaxation and Stress-Strain Methods.
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REPORT ON DENTAL RESEARCH

at the

NATIONAL BUREAU OF STANDARDS

1, INTRODUCTION

Dental research at the National Bureau of Standards during the

half year ending June 30, 1970 included investigations of a wide range

of materials used in restorative dentistry and of hard and soft natural

tissues. The research program is supported by the Council on Dental

Research of the American Dental Association, the Collaborative Research

Office of the National Institute of Dental Research, the Dental Research Division of the U. S. Army Medical Research and Development

Command, the Dental Sciences Division of the U. S. Air Force School

of Aerospace Medicine, and the Veterans Administration. Support from

the National Institutes of Dental Research is also provided in the

form of grants to Research Associates sponsored by the American Dental

Association. This support is designated NIDR (Grant) where reference

to it is made. In the summaries of work in progress, sponsors of

various projects are identified. However, since most of the projects

are closely related both technically and administratively, reports on

all projects are presented for the Information of all sponsors. Detailed reports issued during the period on some phases of the research

are listed and are appended to this report.

2. REPORTS ISSUED

NBS Report 10271

NBS Report 10272

NBS Report 10273

NBS Report 10274

NBS Report 10275

Aggregation Phenomena of Collagen.

Development of a Radiopaque Denture Base Material.

Physical Properties of a Radiopaque Denture Base

Material.

Radiopaque Denture Base Materials - Technic Dentures.

Heats of Adsorption of Ammonia and Carbon Dioxide on Tooth Components.

1
NBS Report 10276 The Crystal Structure of the Aragonite Phase of

CaCO^

.

NBS Report 10277 Tertiary Aromatic Amine Accelerators with Molecular

Weights Above 400.

NBS Report 10278 A Method for Finishing Composite Restorative

Materials

.

NBS Report 10294 Mechanical Properties - Viscoelastic Methods.

Part I - Creep, Relaxation and Stress-Strain Methods.

3. PAPERS PUBLISHED

Moreno, E. C.; Patel, P. R. ; and Brown, W. E. Isothermal Diffusion in

the Dilute Range of the System Ca(OH) -H„P0 -H 0: Theory. J. Res. NBS

(Physics & Chemistry) 74A, No. 1:55-63, January-February 1970.

Blaugher, R. D. ; Hein, R. E. ; Cox, J. E.; and Waterstrat, R. M. Atomic

Ordering and Superconductivity in A15 Compounds. J. of Low Temp. Physics

1:539-561, No. 6, December 1969.

Dickens, B. and Brown, W. E. The Crystal Structure of Calcium Carbonate Hexahydrate at.,—'-120°. Inorganic Chemistry //3, 9:480-486, March

1970.

Brauer, G. M. ; Termini, D. J.; and Burns, C. L. Characterization of

Components of Dental Materials and Components of Tooth Structure by

Differential Thermal Analysis. J. Dent. Res. 49:100-110, No. 1, JanuaryFebruary 1970.

Brauer, G. M. Pyrolysis-Gas Chromatographic Techniques for Polymer

Identification. Techniques and Methods of Pol5mier Evaluation, Vol. II

entitled Thermal Characterization Techniques, pages 41-105, Ed. P. E.

Slade and L. T. Jenkins, Marcel Dekker, N. Y. , Publisher, 1970.

2
4. WORK IN PROGRESS

4.1. Determination of Crystal Structures

Sponsor: NIDR, NIDR (Grant), ADA

The objectives of the crystallographic studies are to increase

our understanding of ionic arrangements and to provide us with structural experience, so that models can be postulated for (a) groups of

ions present in aqueous solution, (b) phenomena important in nucleation and crystal growth from aqueous solution, and (c) the incorporation of 'defects', impurities, vacancies and nonstoichiometry into

tooth and bone mineral.

Automatic X-ray Diffraction : A magnetic disc for the rapid storage and retrieval of computer

programs and associated data has been added to the computer-automated

Picker X-ray single-crystal diffractometer. Programs are now kept on

the disc and the error-prone and time-consuming process of reading

these programs from paper tape has been by-passed. An improved version

of the operating programs has been received from Dr. Galen Lenhert

of Vanderbilt University and will be tried out soon. This is another

step towards more automatic operation.

Crystal Structure : Aragonite, CaCO

^ The crystal structure of aragonite, an important blomineral, has

been refined to R = 0.024 using new X-ray data collected from an ir- w

regularly shaped fragment. Absorption corrections were applied using

the recently written absorption program. The general features of the

aragonite structure were already well known. The new refinement re- veals that the CO^ group appears to be slightly non-trigonal with

O-C-0 angles of 119.62(4)°, 120.13(8)° and 120.13(8)° and C-0 distances

of 1.288(2) A, and 1.283(2) A and 1.283(2) A.

BaCa(CO ^)^ (alstonite phase)

The crystal structure of the bary tocalcite phase of BaCa(C02)2

was included in the report July-December 1969. Work on the chemically

related phase, alstonite, of BaCa(C02)2 has been started. Thus far,

the progress has been good. The fact that the monoclinic unit cell of

3
alstonite a = 30.163(9)A, b = 17.413(5)A, c = 6.110(1)A, 3 =89.91(1)°

is essentially rectilinear makes twinning very likely. Most samples

of alstonite are twinned; an apparently untwinned crystal was used

to collect the X-ray data. The least squares refinement showed that

3 is significantly different from 90°. The cation positions in alstonite have been located although the identities of one or two have not

been resolved definitely at this time. The cation positions in alstonite, as was suspected, are related to those in aragonite. Work is

still in progress.

C^2—4^2—2-

The structure of CaCH^PO^) ^ * H^O contains corrugated sheets, made

up of Ca...P0, chains, as do the structures of CaHPO ,'2H„0, CaHPO,

,

4 4 2 4

Ca (H^PO^) Cl*

H

2 O and several other related compounds. The existence

of this structural feature in several compounds speaks for its stability.

Interlayering between these compounds could account for variability

in stoichiometry.

Two sets of workers have published papers on the structure of

Ca(H

2PO^)2

*H20 , but the fine details of the structure and the positions

of the hydrogen atoms were not known, although positions for the latter

and a hydrogen bonding scheme had been suggested. We have collected

new X-ray data from a plate-shaped single crystal of CaCH^PO^)^*!^^©

and these data were corrected for absorption. Refinements revealed

the fine details of the structure, and decreased the previously published

standard deviations by a factor of 10. The hydrogen atoms were located,

and their positions were corroborated by the fine structural details.

Part of the hydrogen bonding scheme reported in the literature was

shown to be incorrect.

O The previously reported close approach 0 (1) . . . 0

(1

’ ) , 2.67(5) A

O across a center of symmetry, has now been shown to be 2.855(3) A.

O Since this is greater than the sum, 2.75 A, of the van der Waals radii

of the two oxygens, no attractive force between the two atoms needs be

postulated and, in particular, there is no need for a hydrogen to be

disordered about the center of symmetry in a hydrogen bond 0 (1)-H . . . 0

(1 ' )

.

The hydrogen bonding scheme is now 0(3)-H. . . 0(5) , 0 (4) -H (2) . . . 0 (9)

,

0(6)-H(3). . .0(8) , 0(7)-H(4) . . .0(8) , 0(9)-H(5) . . . 0(8) and 0(9)-H(6) .. .0(6,7)

.

4
Two unusual features in this scheme, which is supported by the P-0

distances, the 0...0 distances and the Fourier electron density difference synthesis, are ( 1 ) the formation of three hydrogen bonds to

one oxygen atom, 0 ( 8 ), and ( 2 ) a bifurcated hydrogen bond from hydrogen

atom H( 6 ), on the water oxygen 0(9), simultaneously to oxygens 0(6) and

0(7) in different phosphate groups. The present hydrogen bonding

scheme is less complex than the previously suggested one.

CaKAsO, *8H_0 4 2—

The crystal structure of CaKAsO, *8H„0 has been determined from

4 2

a plate-shaped single crystal and has been refined to R = 0.045. The w

data were corrected for absorption. The compound was previously thought

to be CaKAsO^* yn^O , but the suggestion by the X-ray analysis of eight

waters of hydration was confirmed in a thermogravimetric experiment.

The crystal was thought by us to be one where it was likely that the

Ca ion would be completely hydrated, and this turned out to be the

case. No other example is known at the present time. The overall

structural arrangement has a strong resemblance to that of the biomineral MgNH^PO^ • bH^O (struvite), although the details are different

in the two cases. The existence of the compounds CaKAsO^ • 5H,^0 and

MgKH(PO^)

2

•

5 H

2

O , which have unit cells related to the unit cells of

CaKAsO^ • 8H,^0 and MgNH^PO^ * 6 H

2 O , suggests that this structural type

may be wide spread and important. The fact that the relatively large

K ion is also completely hydrated in the CaKAsO^* 8 H20 structure gives

some idea of the stability of the overall structural arrangement.

Computer Programs :

A computer program to correct X-ray intensities for the absorption

5
of the X-ray beam by the crystal has been written using subroutines

adapted from a program written by C. W. Burnham of Harvard University.

This program has been used to correct the intensities of the X-ray

reflections of aragonite, CaCH^PO^)

2 *H20 and CaKAsO^

•

8H

2 O . The collection of data from the very important compound CagH^PO ,• 5H„0 , octacalcium phosphate, is now feasible, since it will be necessary to correct these data for considerable absorption. For the absorption cor- rection program, the shape of the crystal must be expressed by giving

the coordinates in three dimensions of three points, usually the

corners of the crystal, for each of the limiting planes (assumed to

be planar) which define the outside of the crystal. A program to

check whether the planes specified are indeed on the outside of the

crystal has been written and is now being used routinely. Some crystal

shapes involve nearly parallel planes which are difficult to separate

by other methods.

4.2. Physicochemical Studies of Apatite

and Related Inorganic Substances

Sponsor: NIDR (Grant), ADA

4.2.1. Crystal Growth

A major goal of research on tooth and bone mineral for many years

has been the growth of larger and purer hydroxyapatite crystals so that more powerful physical methods can be employed in the study of

their properties. A major step was made during the report period when

a new method was developed for controlled growth of hydroxyapatite

crystals

.

The procedure is based on the fact that in the ternary system,

Ca(OH)

2 -H

2PO^-H20 , at a given temperature and pressure, a solution

saturated with respect to two calcium phosphates has a fixed composition. The conversion of an acid calcium phosphate (e.g., CaHPO^) into

a more basic calcium phosphate (e.g., Ca^(P0^) 20H) can be effected by

slowly titrating a mixture of the two solids with water. The yield is

fixed entirely by the Ca/P ratio in the solution. For the two salts

listed above, since the Ca/P ratio for the singular point is about 1/2,

the yield is about 3 moles of PO, , as Ca„(PO,)„OH, for each 7 moles of

4 5 4 3

CaHPO^ used in the hydrolysis reaction. The major advantages of this

6
procedure are its simplicity, good yields, precise control of conditions, avoidence of conditions of local supersaturation and the ability

to continue the crystal growth indefinitely.

The properties of the hydroxyapatite crystals produced by this

technic more nearly resemble those in enamel than do the crystals that

have been obtained heretofore at higher temperatures. They should prove

to be of great value, therefore, in providing information on how the

imperfect crystals in tooth and bone deviate from the more nearly ideal

crystals that are formed at higher temperatures. In particular, the

hydroxyapatite crystals produced thus far are fibrous bundles, often

exceeding 100 mp in length. They apparently are the longest hydroxyapatite crystals that have been grown at atmospheric pressure. These

bundles display anomalous optical properties, appearing optically

positive in water media and optically negative in index of refraction

oils. The thicker crystals lose the anomalous optical character, indicating that the abnormal optical properties relate to form birefringence. Differences in form birefringence, known to be present in

carious vs. non-carious enamel, can be evaluated more readily through

the use of crystals synthesized by the above procedure.

The procedure has also been used to prepare octacalcium phosphate.

Previous efforts to measure the solubility of this salt have failed

because of the instability of the octacalcium phosphate, which caused

it to hydrolyze during equilibrium, and because of our inability to

make a preparation free of impurities. The new procedure appears to

have overcome both of these difficulties, and we are optimistic that our renewed efforts to measure the solubility of this compound will

be successful.

4.2.2. Solubilities of Calcium Phosphates

(a) Solubilities of CaHPO^* 211^0 in the presence of NaCl:

In the previous report, it was mentioned that solubility

studies of CaHPO^- 2H20 were being extended to include sodium chloride

as an additional component to the ternary system Ca (OH) 2~H^PO^-H20 em- ployed previously. The objective is to test the applicability of the

solubility-product equations to more concentrated solutions resembling

biological fluids. A series of solubility data for CaHPO *2H„0 has

4 2

7
been obtained in the pH range of 4.3 to 6.8. Solubility product

constants for CaHP0^'2H20 were calculated with the Debye-Huckel

equation (without a linear correction term) for calculating activity

coefficients. Because this equation is only useful for ionic strength

values below 0.1, we did not expect to obtain values independent

of ionic strength. Instead, a linear relationship was revealed between pK and the ionic strength (for ionic strengths in the range 0.05

sp

to 0.59); the value of pK extrapolated to zero ionic strength was

sp

6.599 and compares well with the thermodynamic value 6.592 reported

previously by this laboratory for CaHPO^* 2H20 in systems which did not

contain NaCl. When a linear correction term containing a parameter

obtained from the least squares regression of this line was included

fl in the Debye-Huckel equation, the new set of calculated values

did not vary with ionic strength. It remains to be determined if the

U Debye-Huckel equation with this linear correction term will be applicable for calculating accurate activity coefficients of other calcium

phosphates and in other solutions, including those of biological origin,

having ionic strength values. in the range 0.1 to 0.5.

(b) Solubility of CaHPO^

•

2 H

2

O in more basic solutions:

CaHPO^* 2 H20 is known to hydrolyze into more basic calcium

phosphates (octacalcium phosphate and hydroxyapatite) above about pH

6.4. It should be possible to avoid this hydrolysis through use of a growth poison which would inhibit formation of the more basic calcium

phosphates. The growth inhibitor (disodium ethane-l-hydroxy-1 , 1-

diphosphonate) for hydroxyapatite reported in the literature is being

tested. If this method proves to be successful in preventing this

hydrolysis, then it should be possible to study the solubility of

CaHPO^'lH^O in the basic pH ranges and thus would allow determination

of stability constant of the ion pair (CaPO^) .

(c) Solubility of human tooth enamel:

The work carried out in the past few years has shown that

it is now possible to measure reliably the solubility of synthetic

hydroxyapatite, Ca^(P0^)20H. We have initiated a project in which we

are trying to determine the suitability of these techniques for the

study of powdered samples of human tooth enamel. If this project is

8
successful, it would be the first time that reliable solubility measurements would have been made on a calcified tissue. The knowledge gained

from this work will be valuable in interpretation of the topical fluoride

experiments and should be of great value in establishing distinguishing characteristics between enamel varying in caries resistance.

Preliminary experiments have been carried out with a powdered enamel

sample which contained some dentin. The ion-activity products derived

from the results indicate that this sample contained appreciable amounts

of calcium phosphate more soluble than the pure, crystalline Ca^(PO^)^OH

used in our previous solubility measurements. In an equilibration with

a pH value of about 4.8, large crystals of CaHPO^'2H20 were observed. In

synthetic mineral systems the invariant point between CaHPO^* 2 H 20 and

(Ca) ^ (PO^) ^OH is at about pH 4.3, again showing that the tooth mineral

contained a fraction more soluble than synthetic Ca_(PO,)_OH.

5 4 3 We are in the process of collecting enamel samples completely free

of dentin to eliminate the possibility that dentin is the cause of the

greater solubility. To achieve this, it is necessary to follow a laborious procedure. Freshly extracted teeth stored in water containing

antibacterial agents are cut to remove the roots and then the dentin

portion is carefully drilled out using dental burrs of various sizes to

reach all the crevices. After enough enamel crowns are collected,

they will be pulverized to a fine powder in a Wiley Mill. Precaution

will be taken to cool the material with liquid nitrogen while pulverizing

so that undue irreversible chemical damage is not done to the enamel

materials. Material derived from carious teeth is kept separate from

that of non-carious teeth. Processing of teeth for this purpose is

nearly completed, and it is expected that the equilibration can be started

soon

4.2.3. Generalized Least Squares Calculations

While collecting solubility data on the compounds Ca^(PO^)^OH,

CaHPO^ , CaHP0^'2H20 and B-Ca^(P0^)2, we have been developing an adjustment procedure for statistically treating the data which would give the

most reliable values for the derived constants, would realistically assess

the errors in these constants, and would explicitly take into account the

chemical model used to describe the system. We believe this procedure

9
has great merit for treating physicochemical data and warrants widespread use. As the solubility data for each of the above compounds

became available, the set of data was processed by this procedure.

Separate treatment of the data does not, however, give a test of possible inconsistencies between sets. We have, therefore, modified the

procedure to permit processing of the combined data. At the same

time, we are re-evaluating the data on the dissociation constants of

H^PO^ since these constants are basic to treatment of the solubility

data. These data on the dissociation constants of H^PO, are considered

3 4

to be highly precise, but the chemical models used in describing the

systems differed somewhat from ours and the data were treated by procedures less powerful than the one discussed above. Recognizing that

any procedure for treating data on ionic solutions involves arbitrary

assumptions and somewhat unrealistic models, we are trying to achieve

maximum internal consistency by treating the combined solubility data

and by re-evaluating the dissociation constants for H^PO^.

Provisional results have been obtained from the combined data on CaHPO^*2H20 (DCPD) , CaHPO^ (DCPA) , B-Ca^CPO^)^ (TCP), and two preparations of hydroxyapatite, Ca^OH(PO^)^, HA-1 and HA-2, the first of which

had been heat-treated prior to the solubility study. The association

constants and for the species [CaHPO^°] and [CaH^PO^"^] were included in the calculation scheme. The input is summarized in the following table:

Compound

No. of Systems and

Condition Functions

Experimental

Temperatures j °C i

Adj . Parameters

in log K

2 C.F. 3 C.F. (A + B/T + CT)

DCPD 35 5, 15, 25, 37. 5i 3

DCPA 11 46 5, 15, 25, 37 3

TCP 35 5, 15, 25, 37 3 HA-1 8 27 5, 15, 25, 37 3

HA2

1 —

10

—

25

—

1

Sub-totals 19 153 13

[CaHPO, °

]

3

[CaH^PO^+l 3

Totals 172 ! 19

1

10
The results may be statedf (1) K exhibits a maximum near 15° , whereas

y

previous calculations on DCPD alone produced a monotonlc decrease; (2)

the maxima in the K 's of DCPD and TCP, and the minimum in K , all of

sp X

which occur between 15 and 25°, were confirmed; (3) the monotonic decrease in K (DCPA) was confirmed; (4) for the heat-treated apatite,

sp

HA-1, K decreased monotonically with temperature; and (5) the second

sp

apatite preparation, HA-2, appears to be slightly more soluble at 25°

than HA-1. It is expected that additional data on DCPA and on a steamheated apatite will be included in the final calculation.

The data on dissociation constants of H^PO^ , obtained by Bates and

Acree (1943 and 1945) with the Ag/AgCl electrode, and those of

Grzybowski (1958) with the calomel electrode, have been re-evaluated.

Although only the customary single condition function per system was

imposed, the present calculations are novel in the following respects:

(1) the relatively small errors in the salt solution concentrations

were incorporated in the treatment; (2) the resulting non-identity co- variance matrix for the EMF measurements made on a single solution at

a series of temperatures was made explicit in the calculations; (3) the

O "distance of closest approach" parameter, a, was treated as an adjustable

parameter for the first time, so that a least-squares estimate could be

obtained. Its value was arbitrarily represented in these calculations

by the equation log S = A + B/T, with A and B adjustable. It should be

mentioned that the "observed" pK^^^ values are actually functions of this

parameter; and (4) the customary smoothing function, pk

2

° = A + B/T + CT

,

has heretofore provided merely a least-squares estimate of the best curve through the final set of pK^°'s. In the present calculations,

the parameters A, B, and C were estimated directly as a part of the adjustment procedure applied to the raw data.

The results to date are as follows: (1) the parameter S has been

satisfactorily determined as a temperature-dependent constant. The

numerical values are comparable to those previously estimated by graphical

means. A more precise estimate of its errors at the several temperatures is now available; and (2) the final smoothed values of pK^° are quite similar to the previous results, but there is a marked difference

in the distribution of the individual points. Whereas the individual

11
values at each temperature, as determined by the previous investigators, tend to cluster uniformly about the average for that temperature, they are markedly skewed in distribution about the smoothed value

In contrast, the present calculations produced a fairly uniform distribution about the values given directly by the equation pK- 2

° = A+B/T+CT.

4.2.4. Calorimetry and Electro-chemistry

Combined with good heat capacity measurements in the literature,

the solubility data collected in these laboratories are a major step

in defining the thermodynamic properties of the biologically important

calcium phosphates. One of the major remaining deficiencies is the

lack of accurate data on the standard heats of solution for these compounds. For example, the preferred value was obtained from a sample

which undoubtedly lacked in purity because it was prepared by calcining the root of a tooth. We have undertaken a program in solution

calorimetry to fill this void in the thermodynamic data on calcium

phosphates

.

The temperature sensing unit (the quartz thermometer) for the

solution calorimeter has been received and extensively tested for ac- curacy and stability. Minor circuit modifications have successfully

reduced short-term temperature drift and improved the precision to a very satisfactory level.

Delays in the shipment of other components of the solution

calorimeter have slowed progress in the program; however, completion

of the calorimeter is anticipated in the near future.

The Bureau's Thermochemistry Section has invited us to participate in a cooperative program. Their interest and cooperation, together with their extensive and highly specialized facilities, are expected to considerably enhance our capabilities and progress in

this area while minimizing equipment redundancy.

Work is continuing on the properties of the mercury-based

phosphate-electrode system Hg (1) /HgH^PO^ referred to in the previous

report. Results to date are incomplete.

4.2.5. Topical Fluoridation

In the last report, it was noted that the reaction of hydroxyapati

(HA) with a fluoride solution to form fluorapatite (FA) proceeds very
slowly with poor yields. As a result, the amount of F uptake after the

topical F treatment is very low and the penetration of F into the enamel

is extremely small (usually less than 5 y). Thermodynamic and kinetic

considerations of the HA-FA system have led to the proposal that FA may

be formed from HA in tooth enamel through the path,

soln

(1) HA ^

DCPD [dicalcium phosphate dihydrate] (I)

(2) DCPD — > FA (II)

* A treatment solution which is saturated with respect to DCPD,

but is undersaturated with respect to HA.

During the past six months, the yields and rates of these two reactions

have been investigated using synthetic HA and DCPD. In accordance with

equation (I) , when HA was treated with a solution having an initial pH

value of 1.8, the following typical results were obtained:

Reaction Time (hr.) DCPD in Product (wt%)

1 19

4 67

7 88

Regarding equation (II) , it was found that DCPD was nearly completely

converted to FA in 5 hours after treatment with a solution containing

only 0.54 wt%. These results on synthetic HA clearly indicate that the

proposed path, as indicated in equations (I) and (II), to form FA from

HA in tooth enamel, is indeed feasible.

Some preliminary studies on enamel were made using extracted human

teeth. Following essentially the same procedure, the teeth were treated

with the acidic solution before being exposed to the F solution. Each

tooth was washed after the F treatment with 100 ml of distilled water

for 24 hours. The F uptake in the outer 25 y layer of the enamel after

this treatment averaged to approximately 6,600 ppm. This value is 2 1/2

times greater than the highest F uptake that has been reported. In

addition, the enamel at the 20 to 25 y region had a sixfold increase in

F as compared to the less than two-fold Increase obtained by previous

methods

Currently, we are proceeding with a more systematic study on tooth

enamel. In order to further decrease the time required for the treatment, a more concentrated F solution may prove beneficial. Another

13
important objective is to further increase the depth of F penetration.

In this regard, several mechanical and chemical techniques are being

investigated.

4.3. Gas dnd Vapor Adsorption Measurements

on Dentin and Enamel; Dentinal Collagen

Solubilization Studies

Sponsor: NIDR

Heats of Adsorption Project

This aspect of the surface chemistry program which dealt with the

adsorption of CO^ and NH^ on dentin and anorganic whole teeth has been

completed. It was reported in a talk given at the New York meeting of

the lADR in March. It has since been written up and submitted for

publication in a paper entitled "Heats of Adsorption of Ammonia and

Carbon Dioxide on Tooth Components". The unusually high capacity for

the adsorption of ammonia by dentin (indicating a surface area of about

2

160 m /g) raised the question as to what might be expected for the adsorption of methylamine, the next higher member of the homologous series.

Accordingly, duplicate adsorption runs were made at zero degrees C. on

2 dentin. The results were 86.2 and 84.7 m /g showing good reproducibility.

2 These values are still very high compared to nitrogen (7.5 m /g), but

slightly more than half that of ammonia.

Water Adsorption

Much fundamental knowledge can be gained from a study of the adsorption of water vapor on bone, dentin, enamel and collagen. Brunauer

,

et al (Hagymassy, Brunauer and Mikhail, J. Colloid and Interface Science

29 , 485, 1969), have recently pointed out that there are many important

adsorbents that have part of their pore systems, often the major part,

inaccessible to nitrogen but accessible to water vapor. The authors go on further to say that certain clays, soils and especially biological

systems, should be Investigated by water adsorption. Inasmuch as the

work just completed on CO

2 and NH^ adsorption demonstrated the profound

influence of polar groups on the capacity for adsorption by dentin, as well as the fact that water is the most abundant component of all

biological systems, the water vapor adsorption project was initiated.

14
Other reasons exist for investigating the water adsorption characteristics of dental materials. Dimensional changes in dentin as a function of temperature and humidity are being investigated by Barton

and Dickson of this laboratory. These studies will be aided by a thorough understanding of the water vapor adsorption-desorption isotherms together with their hysteresis effects.

A careful study was made of the various methods in practice for

measuring water adsorption at room temperature. No method was con- sidered completely satisfactory either because of the complexity of

the system or because of the long time intervals required for attaining equilibrium.

A circulating closed system was finally decided upon utilizing a peristaltic type (''Sigma-Motor”) pump to control the rate of circulation of air saturated with water vapor. The adsorbent was contained in

a weighing tube mounted vertically with openings at each end. Ground

glass caps were fitted over the openings during weighing after attainment of equilibrium at each adsorption point. Either an analytical

balance or a micro balance could be used to measure the water adsorbed

gravimetrically , depending upon the size of the sample and the expected

changes in weight. During the adsorption process, the tube is mounted

up stream from the pump and saturator with the effluent connected back

to the pump to complete the circuit. All connections were made with

Tygon tubing. More than a dozen gas saturators were fabricated according to the design of Bichosky and Storch (J. American Chemical Society

37 , 2695, 1915). Each was filled with a different saturated salt solution and used, one at a time, to obtain the different relative humidity

points that would comprise the abscissas of the resultant adsorption

isotherm. Relative humidities ranged from less than 10% to greater than

97%. Approximately two days of cycling were required for attainment of

equilibrium except at the higher end of the curve. Several experiments

could be performed simultaneously, resulting in further economy of time.

Dead volume of the system is very small compared with other systems. This,

together with a minimum of dependence on diffusion, contribute appreciably

to the superior performance of the apparatus.

A striking observation of the water adsorption characteristics of

15
collagen-containing adsorbents is the large hysteresis loops which

persist over the entire range of humidities. These properties are being studied carefully in the hopes that they may throw light upon

anomolous results of other investigators.

The water adsorption isotherms of dentin, disclosed a surface

2 area of 129 m /g in reasonably good agreement with the ammonia results

2 of 150 m /g; but considerably different from the nitrogen adsorption

2 areas (7.5 m /g) . These findings give credence to the statements of

Brunauer et al referred to earlier regarding biological systems.

Water adsorption measurements have been made on specimens of collagen because collagen comprises approximately 50% by volume and 20%

by weight of dentin. Well characterized linear collagen (rat tall

tendons) was selected for the Initial series of measurements. Surface

2 areas of the order of 400 m /g have been obtained vjhich are in agreement with published values of hide collagen (Kanagy, J. American Leather

Chemists' Assn. 12, 1950). These experiments are still in progress.

Collagen Extraction

Some preliminary experiments for the solubilization of collagen

from tissues in which it is normally Insoluble have been performed based

on a suggested procedure described by Veis and Katz (Biochem. Biophys.

Acta 2^, 96, 1956). It involves the use of NH^SCN saturated solution

in liquid ammonia. This procedure appears to hold promise for the

extraction of dentinal collagen. A fluffy precipitate has been isolated

which resembles other collagen preparations. The procedure may have

further possibilities if the undissolved dentin can be completely rid

of its collagenous component in that preparations of anorganic samples

would then be possible without requiring heating above room temperature.

4.4. Mechanical and Physical Properties of

Oral Structures and Restorative Materials

4.4.1. Dimensional Changes in Teeth Components

Sponsor: NIDR, USAF, VA, REF (ADA)

Studies of the thermal and hygroscopic dimensional changes of

human tooth structure have continued with particular emphasis on the

time dependent dimensional changes in dentin. As indicated previously.

16
when dentin is heated to temperatures above about 35°C, there is an "immediate" expansion followed by a delayed contraction, and on cooling from elevated temperatures an "immediate" contraction is followed

by a. delayed expansion. Results so far are consistent with the

hypothesis that the "immediate" dimensional changes are produced by

the mineral portion of the dentin while the delayed effects are produced by the collagen content. The results suggest that the structure

of dentin is such that the tensile stresses produced by the longitudinal

shrinkages of the collagen on melting produce a bulk shrinkage of the

dentin

The dimensional changes which occur in enamel and dentin equilibrated at different relative humidities were determined using the

interferometer. The dimensional change of dentin when changing the

relative humidity from 10 to 40% is considerably larger than that observed on changing from 40 to 80%. These changes, significantly larger

than those induced thermally are presumed to relate to the moisture

gain by the dentin specimen. Comparison of the dimensional changes of

whole dentin, anorganic dentin and enamel indicates that the hygroscopic

dimensional changes depend on collagen content.

The measurements of the thermal expansion of tooth structure have

been made using an interferometer which presents some difficulties since

the specimen and the interferometer must be in water to prevent drying

out of the tooth specimen. To overcome these difficulties, a miniature

f used-quartz dilatometer has been designed and built. Preliminary testing of small samples of tooth structure using the miniature quartz tube

dilatometer show similar results are obtained with greater ease than

when using the Interferometer technique. No interference due to accumulation of air bubbles has been observed using this procedure. Automatic

recording of the dimensional change and temperature is an improvement

over visually counting fringes with the interferometer. With additional

refinements now being made on the fused-quartz dilatometer, it is hoped

that specimen size can be further reduced so that the effect of enamel

rod and dentinal tubule orientation on the thermal expansion of tooth

structure can be determined.

Investigations of the dimensional changes of extracted dentinal

17
collagen and X-ray diffraction studies of collagen in dentin are planned to determine whether or not melting of the collagen does occur

at the temperatures at which the delayed thermal dimensional changes

are observed. Shear moduli and internal friction measurements of

dentin by means of a torsion pendulum are also being made for comparison with the dimensional change data.

4.4.2. Physical Properties of Soft Tissues

Sponsor: NIDR, VA, REF (ADA)

Investigations of the mechanical behavior of skin were carried

out with a rocking-beam oscillator as proposed on the basis of preliminary studies mentioned in the previous report. An improved rockingbeam oscillator was designed and built. The beam of the oscillator

carries a force transducer from which the specimen of skin is supported

in any medium of interest. To reduce friction, mercury contacts are used to make electrical connections to the transducer. Means of ac- commodating various specimen lengths and for adjusting static (rest

position) force on the specimen are provided. A displacement transducer

(LVDT) attached to the beam support is used to measure the amplitude of

vibration (changes in strain) of the specimen during oscillation. The

rocking beam is driven by the variable frequency, constant amplitude

sine wave output from a function generator, and a curve showing frequency

versus strain amplitude is obtained for the specimen. From these data,

the real and imaginary parts of the modulus, and E

2 are calculated

from the following equations:

X r\. IT ^ ^ A IT where

£ = specimen length

A = specimen cross sectional area m = effective mass of beam

f^ = resonance frequency

Af = band width (difference between frequencies for which the

amplitude has — times its maximum value) JT

The energy loss or internal friction, Q of the specimen is

determined from either the band width or the rate of decay of the

18
amplitude of vibration of the beam when the system is allowed to vibrate

freely by means of the following equations:

niT 1

A

o

where A and A, represent amplitudes of vibration,

o 1

Measurements on several specimens of rat skin over a stress range

of approximately 20,000 to 140,000 N/m^ (3 to 20 psi) indicate that the

real part of the modulus can be represented by an equation of the form,

= ka + 1, where k and I are constants and a is the tensile stress.

Data for the specimens investigated gave values of approximately 140

2 for k and 41,000 N/m (300 psi) for 1. Results so far have not shown

any significant differences in between back and stomach skin for

male rats or between male and female rat skin.

Data for the imaginary part of the modulus have not been studied

in detail, but it appears that they may be represented by an equation

of the form:

2 E^ = Aa + Bo + C.

Although no significant difference in E^ was observed for the different specimens of skin, the data obtained so far suggest that the

value of Q

^ for female rat skin may be 1/3 to 1/2 higher than that for

male skin. This observation is subject to future confirmation since it

is based on a limited number of specimens with a rather high scatter in

Q

^ values.

After sufficient data have been obtained at room temperature,

measurements will be made at various temperatures to determine effects

of temperature on mechanical behavior. Plans for development of a vibrating tuning fork apparatus for observation of shear characteristics

of skin are continuing. Also, the possibility of making measurements on human skin ^ vivo by means of sonic wave propagation is being studied.

4.5. Dental Metallurgical Studies

Sponsor: ADA, NIDR (Grant)

Our previous work on the binary V-Pt phase diagram has confirmed

the existence of an ordered intermediate phase at the composition V^^Pt^^,

19
but the exact origin of this compound at high temperatures was not

established. The crystal structure of this compound was found to be

that of an orthorhombic B19 or AuCd type and it was observed to grow

as thin ribbons in the face-centered cubic platinum solid solution

(our previous report erroneously reported its formation in the bodycentered cubic vanadium solid solution) . The formation of thin ribbons

aligned along certain crystallographic planes in the face-centered

cubic platinum solid solution suggests interesting possibilities of

improving the strength of these alloys by fiber-reinforcement. It is

important, however, to establish the maximum temperature at which the

phase V^^Pt^Q will form and we were unable to do this by conventional

quenching and microscopic or X-ray diffraction studies due to our inability to retain the phase which is stable at higher temperatures.

The phase believed to form by an order-disorder

reaction and we have obtained measurements of its thermal expansion as

a function of temperature which indicate a transformation temperature

of about 1500 - 10°C. A special high temperature dilatometer was necessary for these measurements and this was made available at the Westinghouse Research Laboratories. A volume expansion of about 0.5% was observed as the specimen was heated through the transition temperature.

More work is needed to establish the optimum conditions for possible

fiber-reinforcement in the V-Pt alloys.

The phase diagram for the Cr-Ir alloy system is now essentially

complete and work is proceeding on the Cr-Rh alloy system. These two

systems can be considered as analogues of the Cr-Co alloys since Ir and

Rh are located below Co in the periodic table and may therefore be ex- pected to exhibit similar alloying behavior. In each of these systems

one observes extensive solid solutions based on the face-centered cubic

and hexagonal close-packed crystal structures. The relative amounts of

these two phases in an alloy has a pronounced effect on its strength and

ductility. The composition range of stability for these phases is now

accurately defined in each of these binary systems. We have also observed that additions of Rh to Cr substantially lower the melting temperature to the neighborhood of 1400°C. This observation is encouraging for

the development of casting alloys since one would like to obtain melting

20
temperatures which are equal to or less than those now being used in

the base-metal casting alloys (^1500°C) . Rhodium additions may therefore be considered for imparting desirable properties to Co-Cr casting

alloys since Rh is unlikely to produce undesirable corrosion or toxicity

effects and may in fact yield an alloy having a lower casting temperature and a reduced reaction with the investment mold. Co is known to

react chemically with investment materials during casting and, therefore,

replacing Co with Rh may tend to reduce the extent of such reactions

while at the same time retaining many desirable mechanical properties.

Further work on this interesting alloy system is now in progress.

4,6. Dental Investment Studies

Sponsor : ADA

Two reports have been completed on the petrographic study of investments used for high-fusing cobalt-chromium base alloys. These are en- titled "The Petrography of a High-Fired Phosphate-Bonded Investment" and

"The Petrography of Silica-Bonded Investments". These studies provide,

for the first time, information about the chemical reactions occurring

in various micro-environments during the burnout and casting processes,

which is basic to improving investment-casting technics.

Burnout reactions are promoted by fluxing agents present in the investment such as small amounts of chloride, sulfate sodium, zinc oxide,

etc. In addition, constituents of the quartz-sand component of the investment powder, such as ilmenite, clay and various ferruginous minerals,

aid in the fluxing. Calcined magnesite additions are also important

fluxing agents, particularly in the later stages, and aid in the generation

of investment bonds having hot strength.

Some of the more important burnout reactions in phosphate-bonded

investments with spathic (ferruginous) dead-burned magnesite additions,

with casting-mold temperatures up to 920 to 1000°C, are as follows:

(1) Reactions between silica grains (quartz and cristobalite) and

interstitial phosphate are promoted by the various fluxing agents, and

at casting-mold temperatures yield recrystallized silica bonds of

tridymite, quartz and a densely dendritic vitreous silico-phosphate phase.

(2) Free lime present in the deadburned magnesite probably induces

21
Tnicro-environmental liquefaction in phosphate-bonded investments at

temperatures as low as 500°C.

(3) MgO is removed from encapsulating monticellite (CaMgSiO^)

in the dead-burned magnesite driving reactions toward wollastonite

,

thus forming compounds intermediate in composition between wollastonite

(CaSiO^) and akermanite (Ca2MgSiO20^)

.

(4) Periclase (MgO) in the dead-burned magnesite and MgO removed

by degradation of the monticellite reacts with micro-environmentally

liquefied silica to form forsterite (Mg^SiO^) and, possibly with some alumina associated with the calcined magnesite, to form spinel (MgAl^O^)

.

(5) Magnesioferrite inclusions in the dead-burned magnesite grains

aid in fluxing the periclase cores and apparently back oxidize in places to

form hematite.

(6) Iron oxide mobilized by micro-environmental liquefaction

rapidly diffuses into the Investment matrix and aids in the fluxing of

silica

.

(7) Quartz grains commonly undergo topochemically controlled inversion, forming shallow cristobalite and/or tridymite rims.

Burnout reactions in the silica-bonded investments parallel those

in the phosphate-bonded investments except that liquefaction is not as extensive. Vitreous appearing phases are less evident than in the

phosphate-bonded investments. These reactions are as follows:

(1) The interstitial amorphous silica bond is converted by the

action of mineralizers into cristobalite.

(2) In investments with a low content of mineralizers, and limited

flux action, conversion of interstitial "bonding” silica into cristobalite is highly restricted.

(3) At higher temperatures in more extensively liquefied microregions, tridymite is formed.

(4) On the cooling of more extensively liquefied micro-regions,

quartz is formed interstitial to the tridymite.

(5) Calcined magnesite additions (MgO) react with fluxed silica

to form forsterite (Mg^SiO^)

.

(6) Ferruginous mineral constituents, e.g., llmenite, aid in fluxing and react to form vitreous appearing micro-regions apparently containing disseminated magnetite and hematite.

22
Investment-mold reactions which occur during casting are of two

types — corrosive and thermal. Corrosive action is a result of vapor

diffusion of alloy constituents into the Investment. Oxides of these

vaporized constituents, along with heat emanated from the casted alloy, cause extended liquefaction along the investment-casting Interface.

Electron probe studies have shown that in the typical cobaltchromium phase alloy, chromium diffuses most massively into the investment. Manganese, because of its high vapor pressure, is boiled out of

the alloy near the interface and penetrates very deeply into the investment. Because of its lower vapor pressure, cobalt does not diffuse in

nearly as much as chromium. Iron diffuses in considerably, but nickel,

tungsten, tantalum and molybdenum are not present, mostly because of

their low vapor pressures.

Oxides of manganese, cobalt, and iron tend to push reactions with

silica toward low-melting olivine fields. These oxides thus tend to

extend liquefaction along the casting investment interface. Chromium

forms a refractory chromic oxide and, where sufficient fused magnesium

oxide is present, will form also a refractory magnesium chromium spinel.

Under low oxygen pressures, the spinel is favored over the chromic oxide.

Thus, it would appear that chromium is not responsible for extended

liquefaction along the interface.

Manganese, the most vaporizable alloy constituent, has been found

to have diffused as deep as 60 to 80 microns into the investment mold

interface. Extended liquefaction plugging pores has been found commonl\

up to this depth along the casting interface. Cristobalite and tridyiriite

have formed also in the interface.

Thermal action is due to baking action of heat emanated from the

casted alloy. It results in more extended liquefaction, with reactions

proceeding in the same direction as in the high temperature burnout.

Thermal liquefaction commonly is apparent 100 to 200 microns back of

the interface and is not as laterally extended as is liquefaction in the

corroded layer.

These reactions may effect:

(1) Increases in casting time with concomitant induction of shrinkage porosity by inhibiting mold gas escape due to the obstruction of

casting investment interface pores by a viscous liquid;

23
(2) anisotropic investment-mold deformation by microdensif ication induced by both thermal and corrosive liquefaction during casting, giving rise to a poorly fitting casting; and

(3) production of casting roughness by the particle clumping

caused by the surface tension of liquids formed both during burnout

and in casting.

Based upon the petrographic Insights obtained, the development

of a more refractory investment will be attempted. It is thought that

development of more refractory investment bonds would be the best way

to do this. Under particular consideration is the use of inorganic

polymer bonds generated from solutions of ammonium zirconium carbonate,

zirconium acetate and chloride-complexed aluminum oxides. These will

be used in conjunction with ammonia stabilized silica sols to develop

from -

(1) zirconium solutions, zircon;

(2) aluminum oxide sols, mullite; and

(3) magnesium oxide, forsterite refractory bonds. Also, there

is the possibility that refractory spinel bonds may be developed from

aluminum oxide sols and magnesium oxide.

Also under consideration are finely milled zircon air-setting

cements, calcium aluminate hydraulic cements, and zirconia air-setting

cements. Weak gel bonds with zircon stoichiometry have been developed

by a combination of zirconium acetate and ammonium organic silicates.

Petrographic study of dental porcelains has been launched. Attempts

will be made to find methods of lowering porcelain firing shrinkage by

incorporating various metallic additives (e.g., aluminum and silicon

metal powders) . The firing reactions of various porcelains will be

studied in detail.

4.7. Grafting of Monomers to Hard and Soft Tissues

Sponsor: U.S.A.

The aim of this investigation is to graft monomers to soft and

hard tissues so that after polymerization of the monomer, the proteinaceous surface is bonded chemically to the polymer chains. It is hoped

that the modified collagenous surfaces will have improved physical or biological properties useful for dental (or medical) applications.

24
Since chemical modification of collagen powders had been successful with all monomers studied, including those that had additional

functional or potential cross-linking groups, emphasis was changed

during this period to the grafting of monomers to (1) collagen films

of 0.0005 in. and 0.0002 in. thickness, (2) bone, and (3) dentin. Purified collagen films were used since, similar to soft tissues, they possess

much smaller surface areas than collagen powders. Hence, results of

these studies approximate more closely those one would encounter on grafting to soft tissues. Monomers were grafted onto the collagen films by

the technique described in the previous report. Homopolymer was removed

by extraction of the product for 48 hours with boiling chloroform or by

72-hour extraction with acetone. An increase in weight, as well as the

presence of characteristic bands in the infrared spectra of the respective products, indicated the presence of graft side chains. Thus, the

following monomers, in order of decreasing weight increase, could be

grafted to 0.0002” collagen film: glycidyl methacrylate, methyl methacrylate, isobutyl methacrylate, ethyl acrylate and 1,3-butylene dimethacrylate.

Yields of the graft reaction varied from 121% to 19% and were generally

lower than those obtained on grafting the respective monomer to powdered

collagen. Dimethylaminoethyl methacrylate was the only monomer studied

for which the graft yield was poor and the presence of graft side chains

could not be ascertained by infrared techniques. Exact yields for the

0.0005” films are difficult to determine. Apparently, weight increases on grafting are lower for the thin film (.0002 in.) with the exception of

the monomers capable of crosslinking (1,3-butylene dimethacrylate,

glycidyl methacrylate) . Of practical significance is the fact that on lowering the reaction time from 3 hours to 15 minutes, a significant amount

of methyl methacrylate (10%) was still grafted onto the collagen film.

These results Indicate that grafting of many monomers to collagen films

is possible. This can be accomplished within a few minutes if only a modification of the proteinaceous surface is desired.

On grafting glycidyl methacrylate, dimethylaminoethyl methacrylate,

1,3-butylene dimethacrylate or isobutyl methacrylate to powdered bone, an increase in weight of 60%, 27%, 19% and 11%, respectively, was obtained

after acetone extraction at room temperature. Infrared spectra indicate

25
the presence of the oxirane ring of the glycidyl group as well as the

methacrylate group for the glycidyl methacrylated grafted to bone.

The presence of 1,3-butylene dimethacrylate and dimethylamlnoethyl

methacrylate could also be detected by this technique. Grafting to

bone surface is possible within one hour reaction time, as indicated

by a slight increase in weight. However, a number of monomers (methyl

methacrylate, ethyl acrylate) do not appear to react with bone.

To reduce the time-consuming process of obtaining dentin by pulverizing it with a dental bur, a modification of the technique described

by Battistone and Burnett (J. Dent. Res. 35:255 (1956)) was adopted.

The powdered dentin was then used in a series of grafting experiments.

No appreciable amounts of polymer grafts or increase in weight could

be detected after reacting the dentin with ethyl methacrylate, isobutyl

methacrylate, glycidyl methacrylate, dimethylamlnoethyl methacrylate,

vinyl acetate or 1,3-butylene dimethacrylate. These results seem to

indicate that the dentin surface is not as susceptible to grafting as the surface of powdered bone. Thus, the relative grafting efficiency

of monomers to the solid substrate decreases in the following order:

Powdered collagen > collagen film > powdered bone > powdered dentin.

To determine possible adhesion of bovine graft-treated surfaces

to a commercial restorative resin, six flat tooth sections were subjected to the standard grafting procedures. They were then partially em- bedded in a resin and subjected to the tensile adhesion test of Lee,

Swartz and Culp after placing of a Kadon restoration and storage in water

for 24 hours. No significant adhesion was obtained, indicating that this

particular treatment does not promote adhesion.

Considerable effort was spent on the characterization of the numerous graft polymers previously obtained by Infrared techniques. By the

potassium bromide sample technique, the presence of small amounts of

grafted material is not likely to be detected. However, in nearly all

products containing more than 10% graft pol^mier, new infrared bands

which could be directly attributed to the grafted side chains were observed. The infrared curves of 0.0005" collagen films to which monomers

were grafted were much more suitable for detecting small surface changes

on grafting since the films could be analyzed without any further sample

preparation. For instance, the presence of at least eight additional

26
absorption bands in the treated collagen indicates the presenc of

poly(methyl methacrylate) grafts. Collagen films of 0.0002" thickness absorb too strongly in certain regions of the infrared spectrum

to be useful for definitive characterization. However, typical absorption bands such as ester or glycidyl group on the grafted chain

can be detected.

The Immediate effort during the coming months will be directed

on exploring the usefulness of the grafting techniques to soft tissues

such as skin. The previously developed procedures will be employed

using calf skin or rat skin as substrate. Dehaired skin retaining

the epidermal layer will be employed initially. The presence of this

keratinous layer may necessitate a change in the experimental conditions.

4.8. Development of Adhesion to Enamel and Dentin Surfaces

Sponsor: U.S.A.

The objective of this study is to determine the efficiency of

various enamel pretreatments to develop a rapid clinical feasible surface treatment which will result in efficient bonding between human

enamel tooth surfaces and clinically available acrylic restorative

materials. The commercial acrylic restorative Kadon was used in most

of this experimentation. In a few experiments, the development of adhesion between enamel and the commercial dental resins, Bonfil and

Sevitron, and the composite restorative, Adaptic, was examined.

Further work was conducted to correlate the acid structure-adhesion

efficiency of selected polyfunctional acids. Unfortunately, many acids

that may be potentially useful are only sparingly soluble in water and even a 5% aqueous solution cannot be prepared. By substitution of

ethanol for water as solvent, 5% ethanolic solutions of a number of water

insoluble compounds could be made. Application of these solutions invariably resulted in complete lack of adhesion. This behavior is caused

by the decreased dissociation of the acid in ethanol with the lower con- centration of hydrogen ions reducing the etching effect of the acid.

Thus, selection of acids with the required structural differences to pinpoint structure-adhesive properties relationships was possible only on a very limited scale. For instance, no correlation was found between the

number of carboxylic acid groups attached to the benzene ring and adhesive

27
properties. Especially puzzling is the lack of adhesion of enamel

surfaces treated with benzene hexacarboxylic acid, whereas a bond

2 strength of 56 kg/cm was achieved with the pentacarboxyllc acid.

Addition of 1% methacrylic acid to the monomer to obtain a more

polar polymer, treatment of the enamel with a 2% solution of an acrylic copolymer in chloroform before the lactic acid etch, or treatment with a wetting agent (Aerosol OT) before the etch to possibly in- crease more rapid ingress of the acid into the enamel, actually decreased adhesion. Use of 50% phosphoric acid containing 7% zinc oxide

etch as recommended recently by Buonocore (J. Am. Dent. Assn. 80:324,

1970) gave a significantly lower bond strength than the 20% lactic acid

pretreatment studied in this investigation. Thus, this in vitro test

may be indicative that 20% lactic acid is a potentially very desirable

pretreatment for a clinically useful pit and fissure sealant. Application of a solution of carboxylated polyester of pyromellitic dianhydride

with a hydroxyl-terminated polyester prepared from 2,2-dimenthyl-l,3-

propanediol did not improve adhesion to enamel surfaces despite the

presence of a large number of free acid groups. Similarly, treatment

of the enamel with hydrolyzed silanes did not yield beneficial results.

Roughening the enamel surface by polishing with 240 carborundum paper

lowered greatly the bond strength and increased markedly the coefficient

of variation.

Enamel surfaces pretreated with 20% lactic acid or 5% tetrahydrofuran-tetracarboxylic acid also adhere well to other commercial dental

resins (Bonfil, Sevitron) and to a composite restorative (Adaptic).

Adhesion values of these resins to bovine and human enamel were of the same order of magnitude.

The pH values of the various solutions varied from 0.75 for 50%

citric acid to 3.16 for 5% picolinic acid. Especially noteworthy are

the pH values of 20% lactic and 5% tetrahydrofurantetracarboxylic acid

which were 1.50 and 1.44, respectively. These solutions are much less

acidic than the 50% phosphoric acid containing 7% ZnO which has a pH

less than zero. The latter solution has been used clinically by Buonocore as an efficient pretreatment for pits and fissure sealants without

any apparent deleterious effect on the tissues. Thus, it is likely

that the less acidic lactic acid pretreatment can be tolerated by the

28
oral tissues and may actually be advantageous for this application.

It should be realized, also, that the total amount of acid applied

to the enamel is probably less than 0.1 ml and is partially neutralized within a few seconds on contact with the enamel surface.

Scanning electronmicrographs of the enamel surfaces before and

after the application of the polyf unctional acids proved beyond doubt

that this treatment leads to preferential dissolution of portions of

the enamel surface and to exposure of the enamel rods. The largely increased surface area and greater porosity of the etched enamel facilitates the ingress of the slurry into the crevices and improves wetting

by the uncured resins. This results in a better mechanical interlocking of the restorative resin with the enamel phase.

A detailed report of this investigation is being prepared for

publication.

4.9. Dental Tensile Adhesion Test Methods

Sponsor : NIDR

The objective of this investigation is to evaluate two dental

tensile adhesion tests - one method developed by Lee, Swartz and Culp

of the Epoxylite Corp., and the other method suggested by the Subcommittee on Standard Test Methods for Direct Filling Resins of the ANSC

Committee on Dental Materials and Devices. The evaluation is being con- ducted by employing potential dental adhesive systems and techniques

approximating as closely those which could be used clinically.

The following parameters and their effects on the bond strength

and reproducibility of results as obtained by the two test methods are being investigated:

(1) Substrates (bovine enamel, bovine dentin, human enamel, human

dentin)

(2) Surface treatment of substrates (20% lactic acid (L.A.), 5%

tetrahydrof urantetracarboxylic acid (TTA) , 5% AgNO^ (for dentin only)

,

5% AgNO^ & TTA (for dentin only), 5% AgNO^ & L.A. (for dentin only));

(3) Commercial restorative resins (Kadon, Sevriton, Bonfil, Adaptic,

Durelon)

(4) Instrumental variation

Minor changes were made in the specimen preparation for the ANSC

29
tests. In order to save time, a set screw is used to hold the 2-56

test screw in the penetrometer shaft. By loosening the set screw,

the specimen may be removed from the penetrometer in 6 to 8 minutes,

rather than the suggested 20 minutes. Modifications were also made

in the specimen preparation jig. Rather than using the movable stage

described in the ANSC method, split dies were made with a recess in

the bottom section to hold the specimen and a recess in the top section

to position the spacer. After positioning the specimen and the spacer,

the two sections were held tightly together by 2 screws. All specimens were stored in water at 37°C for 24 hours prior to

testing. The bond strength of the resins was measured also on substrates

which were not etched. No significant bonding was obtained with human

or bovine enamel under these conditions. The only measurable bond

strength for dentin was obtained with Durelon against bovine enamel

with no surface treatment. Acid treatment of bovine enamel before

placing the Durelon did not Improve bonding. The average of five speci2 mens was 11 kg/cm with a coefficient of variation of 26%. All five

specimens gave cohesive failures.

Representative values of the tensile adhesion to enamel surfaces

are given in Table 1. It will be seen that both tests are of a semiquantitative nature with coefficient of variations ranging between 16

and 67%. Such large variations can be expected in this type of measurement using only five specimens and discarding none of the experimental

values. Despite poor reproducibility, both tests clearly indicate the

absence or presence of adhesion to tooth structure. Using a large

number of measurements of tensile adhesion determined by the Epoxyllte

test, it was found that the standard deviation is a function of the level

of the measurement. A reasonably good fit was obtained for the curve:

_ 3/4

S = X

where S is the standard deviation and X is the average value of the tensile adhesion.

Because of the large coefficient of variation, the effect of the

various experimental parameters studied is difficult to evaluate. Wlien a rather fluid mix is used, such as for Bonfil or Sevriton, the slurry

30
flows out of the cylinders on placement in the Epoxylite test and

tends to spread on the tooth surface. This results in poor reproducibility as indicated by the large coefficient of variation. Adhesion

developed with fluid mixes is more suitably evaluated by the ANSC test.

The significantly larger adhesion values of Kadon by the Epoxylite test,

as compared to the ANSC test, are probably the result of the large

powder-liquid ratio (high consistency) of the slurry used. The large

powder-liquid ratio was especially selected because of its improved adhesion to the curved enamel surface as measured by this test.

The use of either human or bovine teeth, because of their non- homogeneous surface characteristics introduces another variable. Substitution of bovine enamel for human enamel does not appear to alter

results significantly. Grinding and polishing a flat surface on the

enamel surface did not give a significant difference in the results

compared to teeth on which no flat had been ground. The following re- sults were obtained using the ANSC test:

Sevriton with lactic acid on bovine enamel -

2 (a) flat area - 36 kg/cm

(b) no flat - 37 kg/cm^

For the ANSC test, no clear-cut trend has been observed on varying

the headspeed of the tensile testing machine from 0.02"/min. to 0.2'Vmin.

For many of the adhesive systems, results by the two tests appear

to be within the range of the experimental error. Specimen preparation

for the ANSC test is much more time consuming, which is a decided disadvantage in routine work with a large number of samples. Both methods,

especially the Epoxylite test, require relatively large (preferably

flat) surface areas to be bonded. This requirement is not particularly

critical in adhesion testing with dentin as a substrate. However, preparing the flat human enamel surfaces necessary for these measurements

without exposing dentin presents serious problems. It is likely that the

effect of slight deviations from planar surfaces is less critical for

the ANSC test.

A comprehensive survey describing studies of the mechanism of adhesion and reactivity of tooth surfaces, potential adhesive systems proposed for bonding to dentin and enamel and test methods used in evaluating

dental adhesives has been prepared. This manuscript will be part of "The

Chemistry of Biosurfaces" (Michael Hare, ed.) to be published at an early

date

31
TABLE 1

Summary of Representative Results

ANSC TEST EPOXYLITE TEST

bond

Powder/Liquid strength* C.V.**

bond

strength* C.V

Etch Resin Ratio kg/cm^ % kg/cm'=^ %

Bovine Enamel:

20% L. A. Kadon .125g/.09 ml 29 30 64 32

Bonf il •llg/3 drops 23 23 33 61

Sevriton .12g/3 drops 40 25 22 67

Adapt ic'^ — — — 18 39

5% TTA Kadon .125g/.09 ml 21 37 54 28

Bonf il .llg/3 drops 25 44 26 51

Sevriton .12g/3 drops 34 42 25 52

Adapt ic"*" — — — 37 51

Human Enamel:

20% L.A. Kadon 27 29 65 19

Bonf il J.lg/3 drops 29 26 — —

Sevriton .12g/3 drops 51 35 — —

Adaptin'^ 31 30 — —

5% TTA Adapt ic"*" 57 16

// All specimens stored under water at 37°C for 24 hours prior to testing

* Average of five specimens

** Coefficient of variation

+ Equal portions of the two pastes

32
4.10. Adhesive Direct Filling Materials

Sponsor: ADA, NIDR (Grant)

Progress has continued in the development of an esthetic dental

material that can be used for both restorative and preventive dentistry.

Work has been centered on the chemical and physical properties of the

material itself, and on means of chemically bonding the material to

hard tooth tissues. The composite material itself consists of (a) re- inforcing filler, (b) dimethacrylate monomers, (c) stabilizers, (d)

tertiary aromatic amine polymerization accelerators, and other minor

additives

(a) A glass formulation and equipment described in the previous

report demonstrated the feasibility of preparing spherical particles

from irregular particles, utilizing an oxygen-acetylene ceramic powder

gun in conjunction with a water-cooled cyclone separator. A necessary

requirement of the reinforcing filler particles is that they match the

refractive index of the polymer matrix rather closely, and a desirable

feature is that they contribute to the composite material sufficient

x-ray opacity for radiographic diagnosis of secondary or underlying decay, should this occur. The last glass formulation investigated contained

BaF^ (8 mole percent) and had minimal but sufficient x-ray opacity;

however, due to the labile nature of fluoride, the commercial glass

powder, obtained by the custom preparation from a commercial source, had

a refractive index that was different from that of the original experimental glass made at NBS. Consequently, during this period, five experimental glass formulations were prepared (in cooperation with the inorganic

glass section) , each containing 16 mole percent of barium oxide as the

constituent contributing both the x-ray opacity and a predicted refractive

index that is desired for this application (n^^ about 1.545). The variables

in this approximated isofract composition line are the proportions of

B20^ and Al20^. All of these formulations theoretically take advantage

of the "alumlnoborate anomally” that should give a further desirable

property of having minimal coefficient of thermal expansion. Furthermore,

these glasses should have good hydrolytic stability, a property that is

most important in terms of the adhesive bonding (by way of the silane

coupling agent) between the glass and the resin phases of the composite.

33
Scanning electron microscope studies of fractured surfaces have indicated that the "weakest link" (the region of fracture propagation)

of the fractured specimens is at or very near to the interface of

resin and reinforcing filler particles. In this series of glass

formulations, the liquidus eutectic region will be sought.

(b) The dimethacrylate monomers studied during this period were

the three condensation products of 2-hydroxyethyl methacrylate and the

corresponding acid chlorides of the phthalic acid position isomers

that had been synthesized originally in the Dental Research Section,

but which have subsequently been obtained by custom synthesis from a commercial source. A manuscript entitled "Crystalline Dimethacrylate

Monomers" is scheduled for publication in the Journal of Dental Research

in its next issue (July-August 1970) . There was some evidence found

that one or more of these monomers might form yellow charge transfer

complexes with tertiary aromatic amines. This could be a problem since

such amines are used as polymerization accelerators, and this possibility

is being studied further.

(c) Sterically hindered phenols are used as antioxidants and as stabilizers for monomers that polymerize by way of free radicals. The

synthesis of 2-methacryloxyethyl-3,5-di-_t-butyl-4-hydroxy hydrocinnamate

was begun. If successful, this compound should provide an effective

stabilizer that contains, in the same molecule, a methacrylate moiety

capable of copolymerization in the methacrylate monomer system, which

would be expected to have advantages both with respect to polymerization

kinetics and also from a toxicological viewpoint, since the compound or

its decomposition products would become part of the polymeric network.

(d) Tertiary aromatic amines such as N ,N-dimethyl-£-toluidine and

N,N-dimethyl-3,5-xylidine are currently used as polymerization accelerators in dental direct filling resins and in the commercial composite

direct filling materials. Since the lower-molecular-weight aromatic

amines are capable of skin penetration and are systematically poisonous,

it is conceivable that they might contribute to the pulpal irritation

that is currently being reported by various investigators . Since the

composite dental materials originated in research here, it behooves the

dental research section to optimize these materials and attempt to solve

any problems arising in their use. Accordingly, synthesis of novel

34
1 ! 1

T

1

I

1

1

tertiary aromatic amine accelerators continued in this period, the

purpose being to determine the highest molecular weight (which would

minimize tissue permeability and thus toxicity) that v/ould be compatible with effective and useful polymerization acceleration. To this

end, the polymeric reaction product of the diglycidyl ether of bisphenol

A and 3,5-xylidine was prepared. This polymer (oligomer) was soluble

in the monomer and gave practical hardening times when it and the

stabilizer were adjusted to suitable concentrations. The product itself is a glassy, brittle solid.

A summer student is being trained in the procedures of testing adhesion between dental materials and hard tooth tissues. A surfaceactive coupling agent, synthesized during the last period (the reaction

product of N-phenylglyclne and £-chlorophenyl glycidyl ether) , will be

compared with NPG-GMA (the reaction product of N-phenylglyclne and

glycidyl methacrylate) for relative efficacy in mediating adhesive bonding between composite materials and hard tooth tissues. The purpose of

this comparison will be to evaluate the relative importance of dispersionforce interactions versus copolymerization between such primers and

polymeric materials.

As a prototype of adhesive varnishes or lacquers, the synthesis of

a compound derived from the diglycidyl ether of bisphenol A and N-phenylglycine was commenced. This compound has two of the monovalent, tridentate chelate moieties on the same molecule (in contrast to the compounds

mentioned in the previous paragraph, each of which has only one such

chelate group) . Subsequent compounds in this series will have more than

two such chelate groups and will have higher molecular weights, increasing until solubility limitations are encountered.

4.11. Three-Dimensional Panoramic Dental X-Ray Device

Sponsor: A.F.

,

ADA

Repair and modification of the "original” panoramic dental x-ray

machine has continued toward the aim of allowing simultaneous exposure

of two films. The initial approach has been with the conventional camera. Variation in positioning, beam width, angulation and exposure time are being examined. The radiographs have changes in the relative positions

of the objects in the pictures between consecutive films. The lack of

35
identity between images seen by the left and right eye is psychologically interpreted as depth in the projected radiographs. Conventional stereoscopes will initially be used to view the radiographs, but

the use of a lenticular screen system permitting group viewing without

use of polarizing glasses or elaborate equipment is to be explored.

4.12. Bonding Cold-Curing Acrylic to Acrylic Teeth

Sponsor : ADA

A solution, equal parts CH^Cl^ and cold-curing methyl methacrylate

monomer, and a procedure for its use have been developed which bond

cold-curing acrylic to acrylic teeth with strengths in excess of 80%

of the cold-curing acrylic. The solution is applied during the processing technique, and does not increase the time or require additional

equipment. Teeth subjected to this treatment pass the American Dental

Association Specification No. 15 test for changes in the surface

characteristics, hardness and resistance to crazing.

Simulated dentures have been made using the new procedure and the

teeth are retained and the denture base strengthened. These results

verify the tensile tests in the laboratory. Each manufacturer of

American Dental Association certified acrylic teeth has been asked to

evaluate the solution and technique. Additionally, four prosthodontists

have requested information concerning this procedure and will provide

information relative to its clinical effectiveness. A manuscript is in

preparation

4.13. Investigation of Dental Amalgams

Utilizing Low Mercury-Alloy Ratios

Sponsor: ADA, NIOR (Grant)

The physical properties of dental amalgams, their resistance to

corrosion, and mercury hygiene in dental practice are generally Improved

wath the use of a minimum amount of mercury in the original mix. However,

one is limited by the need for adequate plasticity to place the mix in

the prepared tooth. Therefore, available amalgam alloys require about

46 to 50 percent mercury. Attempts are being made to reduce the amount

of mercury by a judicious choice of particle sizes of spherical silver

alloy. In this manner it has been possible to reduce the mercury to as low as 35 percent. However, the mix, although quite plastic, is very

36
\

grainy and appears to adapt poorly to the cavity walls. The physical

properties of this amalgam are comparable to the amalgams generally

in use except that the 24-hour static flow is considerably superior.

This may be important clinically because of reports showing a close

correlation between low flow and Improved marginal integrity. Further

efforts will be made to reduce the disadvantages of graininess and

lack of adaptation by using smaller particle sizes.

4.14.

Finishing of Composite Resins

Sponsor: ADA, NIDR (Grant)

One disadvantage of the new anterior composite restorative materials is that the dentist cannot finish them to a smooth surface. To

overcome this deficiency, an instrument was constructed by bonding 1 to 5 pm diamond particles to paper discs using an epoxy adhesive.

Comparisons were made of finishes obtained with this instrument with

those of conventional finishing instruments by means of a scanning

electron microscope. The surfaces formed by the diamond discs were

superior to those obtained by conventional instruments in terms of

smoothness and light reflection.

4.15.

Hazard of Mercury Poisoning Using an Ultrasonic

Condensing Instrument for Dental Amalgam

Sponsor: ADA, NIDR (Grant)

Mercury vapor levels in dental offices are increased during periods

of manipulation of the mercury and amalgam. These manipulations include removal of existing restorations, dispensing and triturating the

alloy and mercury and condensing and carving the alloy itself.

A preliminary investigation in this laboratory (designed to study

the effects of ultrasonic condensation on the properties of dental

amalgam) revealed that when an ultrasonic instrument was used, significant amounts of material were being emitted from the working area in

the form of a vapor-like cloud. This cloud was made up of mercury droplets and alloy particles. Whether or not these materials are present

in sufficient amounts to constitute a health hazard is under investigation.

4.16.

Clinical Research

Sponsor: ADA, NIDR (Grant)

Clinical evaluations of composite restorative materials, the effects

37
of an adhesion-promoting coupling agent (NPG-GMA) and of various

methods of polishing composite materials has continued.

4.17. Specifications and Related Activities

Active participation at the member, committee chairman and subcommittee chairman level in the American National Standards Gommittee

Z156 has continued. Data on the effects of the speed of loading on

the compressive and diametral tensile strength of two types of ZnOeugenol cements were provided Working Group I, ISO/TG-106.

Gontinued progress toward Improved specifications has been made

through chairmanship of the FDI Commission on Dental Materials, Instru

ments. Equipment and Therapeutics.

The physical properties of a series of treated chrome-cobalt

wires for use in partial denture work were determined for Brooks AFB.

In response to an inquiry regarding the quality of certain hand instru

ments, the Knoop Hardness Numbers, % Cr and cutting edge dimensions

were determined for Brooks AFB. Also provided the A.F. was a study of

dimensional stability of a light-bodied polysulfide rubber base Impres

sion material of one manufacturer and a regular-bodied polysulfide

rubber base impression material of another manufacturer, when the two are used in combination.

Evaluation of a hydroclone prototype unit was completed for the

Army.

Site visits, review of project proposals and advisory services on dental materials were provided NIDR.

USCOMM-NBS-DC

38