/^?3 - /

NATIONAL BUREAU OF STANDARDS REPORT

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30, 1953

Dental Research Laboratory

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

2718
NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

cover of this report.

Electricity. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Gage.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenics. Engines and

Lubrication. Engine Fuels. Cryogenic Engineering.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Neutron Measurements. Infrared Spectroscopy.

Nuclear Physics. Radioactivity. X-Rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. Atomic Energy Commission Instruments Branch.

Chemistry. Organic Coatings. Surface Cherpistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Aerodynamics. Engineering Mechanics, Hydraulics. Mass. Capacity, Density, and Fluid Meters. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure. Chemistry of Mineral Products.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Machine

Development.

Electronics. Engineering Electronics. Electron Tubes. Electronic Computers. Electronic

Instrumentation.

Radio Propagation. Upper Atmosphere Research. Ionospheric Research. Regular Propagation Services. Frequency Utilization Research. Tropospheric Propagation Research. High

Frequency Standards. Microwave Standards.

Ordnance Development. These three divisions are engaged in a broad program of research

Electromechanical Ordnance, and development in advanced ordnance. Activities include

Ordnance Electronics. basic and applied research, engineering, pilot production, field testing, and evaluation of a wide variety of ordnance materiel. Special skills and facilities of other

NBS divisions also contribute to this program. The activity is sponsored by the Department of

Defense.

Missile Development. Missile research and development: engineering, dynamics, intelligence,

instrumentation, evaluation. Combustion in jet engines. These activities are sponsored by the

Department of Defense. • Office of Basic Instrumentation • Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-10-0707

0708-10-0724

0708-10-3822

0708-20-3824

July 31, 1953 2718

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30, 1953

Dental Research Laboratory

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 Army Dental Corps, the Air Force

Dental Service, the Navy Dental Corps, and the Veterans Administration.

The publication, repr

unless permission iso

25, D. C. Such perm

cally prepared If the

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

i par t, Is prohibited

idards, Washington

irt has been specifiort for Its own uso.
—
REPORT ON DENTAL RESEARCH

AT THE NATIONAL 1. BUREAU OF STANDARDS

INTRODUCTION

The activities of the Dental Research Laboratory at the National Bureau of

Standards during the half year ending June 30, 1953 have included investigations

of a wide variety of dental materials and equipment and studies of natural tooth

structures. Summaries of the results obtained on work now in progress are given

below. In addition, copies of reports issued on completed phases of the investigations are included.

2.

REPORTS ISSUED

NBS No. 2384

NBS No. 2566

NBS No. 2628

NBS No. 2647

Deterioration of Alginate Hydrocolloidal Dental Impression

Material on Storage

A Transverse Strength Testing Machine

Some Factors Affecting the Dimensional Stability of Ag-Sn-(Cu-Zn)

Amalgams

1 Direct Filling Resins: Mechanical Properties

3.

WORK IN PROGRESS

3.1 Structure of Human Tooth Enamel and Dentin

Investigation of the fundamental structural characteristics of human tooth

enamel and dentin - - -

(a) Fluorescence Investigation . In the investigation of tooth structure by

means of fluorescence two lines of approach were used (l) the application

of the previously developed fluorophotomi orographic method for histological studies (2) the development of equipment for obtaining additional fundamental information on tooth structures through studies of the nature of

the fluorescence emitted.

Fluorophotomicrographs of tooth sections of known history and special histological interest were made in collaboration with Dr. B. M. Bhussry of the

Eastman Dental Clinic. Since work at the Eastman Clinic has indicated that

there may be a relationship between some types of surface pigmentation, age

and caries susceptibility of teeth, the fluorescence studies are being undertaken to obtain information on the characteristics of the pigmentation present.

In response to a request from the Fish and Wildlife Service, sea otter teet

were examined to investigate the possibility of using fluorescence methods,

developed for the study of human teeth, to determine the age of the sea otter. Although definite lines of growth were revealed by ultraviolet excitation, no correlation with age will be possible until teeth of known ages can be obtained.

- 2 -

Since the decay time of the fluorescence emitted by a substance is characteristic of the nature of the material, plans are being drawn up for the

construction of a decay fluorometer capable of measuring extremely short

periods of decay (from 10“° to 10“^ seconds). Construction of a depolarization fluorometer has also been started. From measurements of the decay

time and the depolarization it is hoped that some idea of the molecular

weight of the fluorescing material in tooth structure can be obtained.

(b) Crystallographic Investigation . Through a Fourier analysis of the x-ray

diffraction maxima of the apatite reflections from a sample of steamed bone,

it was demonstrated that the apatite particles ranged from about 10 to 1000A

in size. This indicates that the 50A particles found by low angle scattering were probably apatite rather than carbonate as previously suggested.

An attempt was made to determine whether or not carbonate containing apatites (bone, teeth, etc.) changed in any physically perceptible manner when

the carbonate was leached out with lactic acid. Employing x-ray diffraction as well as electron diffraction, it was impossible to detect any change

in a series of diffraction patterns of samples taken before and after the

acid leach. This points to the probability that the carbonate is not a part

of the apatite structure but is possibly admixed in some manner.

3.2 Surface Grazing of Acrylic Resihs

Determination of the mechanism involved in the surface crazing of acrylic

resins - - -

Investigations of the effect of temperature and molecular weight on the water sorption of methyl methacrylate were completed. Specimens of varying molecular weight showed approximately the same weight increase on storage in water below the second-order transition temperature Tg. Within a few degrees centigrade

of Tg water sorption increased markedly. On storage at 10-20° C above Tg the

disks of highest molecular weight decomposed into the original granules. This

indicates that on molding the specimens at 150°C fusion of the individual particles to a homogeneous material is not accomplished for high molecular weight,

straight chain polymer. On the other hand, molding at higher temperatures will

increase the degree of depolymerization.

Specimens molded at 150°C and used in the water sorption measurements over

a 4° to 85°C range showed a decrease in molecular weight from 1,600,000 to

830,000. Similar degradation was observed for samples of lower molecular weight.'

No hysteresis effect was found for the water sorption of finely powdered

polymethyl methacrylate . Larger granules showed a slight hysteresis effect over

the 10 to 50% relative humidity range.

The waterproofing of SR^ strain gages embedded in resin for measurements of

internal strain has been done effectively by surrounding the gage with £. ;rtj0.001'’

brass shim envelope. There is, however, some doubt as to the adherence of the

brass to the cured methylacrylate. The effectiveness of another method of waterproofing, curing of neoprene around the gage, will be determined.
i . ir: '
- 3 -

3.3 Polymerization of Acrylic Resins

Investigation of the chemical reactions which are responsible for the induction period of the polymerization of methyl methacrylate - - -

Since study of the decomposition rate of cC)CC diphenyl /? pi cry1 hydrazyl

(D.P.P.H.) appears to offer a convenient approach to measurements of rate of

free radical formation, reactivity and efficiency of inhibitors, the apparent

decomposition rate of D.P.P.H. in the presence of 2:2-azo-bis (isobutyronitrile)

(A.I.B.N.) was determined in the presence of over 25 typical inhibitors containing various functional groups. The half time decomposition of inhibitors con- taining phenolic groups is shortened due to the oxidation of the inhibitors.

Diphenylamine, benzidine and phenylene also showed a shortened half time decomposition. The presence of oxygen, sulfur, di-and tri-nitroaromatic compounds, triphenylmethyl chloride, stilbene maleic anhydride and 0.01$ styrene

or methyl methacrylate had no effect on the decomposition rate of D.P.P.H.

Iodine and p-nitroaniline react preferentially with radicals formed from

A.I.B.N. Thus in the presence of 0.007$ iodine the apparent half time decomposition of D.P.P.H. is changed from 138 min to 233 min.

An attempt was made to measure the thermal formation of free radicals from

methyl methacrylate and styrene. The D.P.P.H. decomposition rate is rapidly

increased on increase of the styrene concentration and does not follow any clearcut kinetics.

On the other hand, free radical formation from methyl methacrylate is much

smaller than that for styrene and follows second- order kinetics. This is indicative of the formation of a biradical from monomer, probably through a bimole cular

collision. The reaction order and rate are not appreciably changed over a limited

concentration range. Comparison with previously reported values of the rate constants shows that the results obtained by this method are much too large. This

can be accounted for by side reactions which take place between D.P.P.H. and the

double bond of the monomer.

An investigation was initiated to follow the kinetics of polymerization of

methyl methacrylate in the presence of 2$ benzoyl peroxide and varying concentrations of different accelerators. The course of the polymerization is followed

by means of an ultrasonic viscometer.

3.4 Physical, Chemical and Engineering

Properties of Dental Materials

Determination of the properties and processing characteristics and development of more satisfactory techniques for the use of materials employed in the

practice of restorative dentistry - - -

(a) Metallurgical Investigations . Additional gallium alloys have been made and

investigated as possible replacements for dental amalgam. Gallium-gold in

certain concentrations was found to react exothermically to form AuGa2«

An apparent correlation between the crystal type of the alloying metal and

- 4 -

ease of alloying near room temperature was observed. Gallium alloyed more readily with face centered cubic metals than with body centered cubic metals.

Data so far obtained indicate that the hardness and the strength of certain

gallium alloys are approximately 20 to 30% greater than those of the dental

amalgams now in use.

(b) Chrome Alloys for Dental Castings . Tensile properties of the three most

widely used brands of chrome cobalt casting alloys were determined under a variety of casting conditions and specimen shapes. Threaded specimens appear to be more suitable for specification testing than do the straight rods

now specified for testing of other dental alloys. The data obtained indicate also that it may be desirable for testing purposes to specify a definite

casting procedure. Other properties including chemical composition, hardness

and corrosion resistance of the alloys will be determined to obtain the necessary data for writing a specification for dental chrome alloys.

(c) Rotating Dental Cutting Instruments . Studies were made comparing the behavior of diamond abrasive instruments when cutting pyrex glass with their

behavior when cutting human tooth enamel. It is hoped, thereby, that diamond abrasive instrument tests using pyrex glass, can be related more accu- rately to clinical performance in the practice of dentistry.

A program of high-speed photography is also under way, in which the basic

cutting behavior of dental burs is being studied. Burs of steel and car- bide of conventional design and number of blades, and burs having fewer than

the standard number of blades, will be photographed at speeds of rotation

of from 5000 to 15000 rpm as they cut human tooth enamel and dentin and other

materials. The information obtained from this study should be of great value

in the development of more efficient rotary cutting instruments.

(d) Dental Resin Cements . Modified methyl methacrylate resins are being advocated

as replacements for zinc phosphate cements for fastening appliances and res- torations to teeth. Various properties of these resins were therefore measured for comparison with the properties of zinc phosphate materials. The

standard consistency tests used for silicate and zinc phosphate cements were modified and adapted for testing the resins. It was found that the setting

time of the resin cements is much less sensitive to differences in consistency than is that of the zinc phosphate cements. The compressive strength

of the resins was 12,000± 1000 psi, approximately the same as that of the

zinc phosphate. The opacities of the resin cements varied from Cq^o values

of 0.50 to 1.00 and were thus more satisfactory than the 1.00 (completely

opaque) value for the zinc phosphate cement. Temperature rise on hardening

of the various resin cements varied from 13 to 29°C while that of the zinc

phosphates varied from 4 to 13°C.

(e) Zinc Oxide-Eugenol Impression Paste . Empirically developed zinc oxideeugenol materials are widely used as impression pastes, tooth treatment ce- ments and in certain gingivo-alveolar therapeutic materials. Since there

is very little published information on the mechanism of the hardening of

these materials and since numerous instances of failure are observed, an investigation of the materials was undertaken. Results obtained so far show

- 5 -

that more than 80% of the eugenol in a mixture of the paste can be extracted from the hardening material with chloroform. Thus only 4 mole % of

eugenol could have reacted with the zinc oxide. This suggests that the setting mechanism is either a gel formation or a surface reaction. It was found that the water content of the zinc oxide greatly affects setting time.

Carefully dried zinc oxide mixed with eugenol in a dry atmosphere did not

set within a period of several days.

(f) Porcelain Teeth . A method was developed for determining and specifying

porosity of porcelain teeth. The method developed consists of sectioning,

mounting and polishing the teeth. A cover glass with a square 0.5 mm on a side ruled on its surface is then placed on the polished tooth sample. The

bubbles in the square are counted and the four largest bubbles measured.

This procedure is repeated on five different areas of the tooth. The average number of bubbles per square millimeter is determined and the average

diameter of the four largest bubbles is determined. These two values give

a satisfactory indication of the over-all porosity of the tooth.

(g) Ampules . An instrument for testing the strength of glass anesthetic am- pules was designed for the Armed Forces. Examination of a large number of

ampules for strain under polarized light and subsequent breaking of the

ampules in the test instrument indicated that annealing of the glass was desirable.

(h) Evaluation of Materials . Materials evaluated for the cooperating Federal

dental services and the American Dental Association by specification or special test methods included amalgams, mercuries, gold alloys, silicate

cements, denture base repair resins, alginate impression materials, waxes,

filling lubricants, cavity liners, impression compounds, glass ampules,

plastic teeth and solders.

W. T. Sweeney, Chief

Dental Research Laboratory

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-10-3324 March 26, 1953

NBS REPORTQR'

2334

Progress Report

on

DETERIORATION OF ALGINATE HYDROCOLLOIDAL

DENTAL IMPRESSION MATERIAL ON STORAGE

By

K. R. Pfeiffer*

W. T. Sweeney**

J. L. Harvey***

G. M. Brauer-t

I. C. Schoonover*

*Guest Worker, Veterans Administration Dental Service,

stationed at the Dental Research Laboratory, Division

of Organic and Fibrous Materials.

**Physicist , Dental Research Laboratory

***Guest Worker, U. S. Army, stationed at the Dental Research Laboratory.

lChemist, Dental Research Laboratory.

*Chief , Dental Research Laboratory.

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association,

the Army Dental Corps, the Air Force Dental Service, the Navy

Dental Corps and the Veterans Administration .Dental Service.

The publication, reprinting, or reproduction^ thls!report In anytorm, either In wholelor In part, Is prohibited

unless permission Is obtained In writing from theOffice of the Director, National Bureau of Standards, Washington

25, D, C, Such permission Is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to'reproduce additional copies of that particular report for Its own use,
•

. - .

'
DETERIORATION OF ALGINATE HYDROCOLLO IDAL

DENTAL IMPRESSION MATERIAL ON STORAGE

Abstract

Numerous instances of deterioration of alginate

dental impression materials have been reported# Investigation of samples of these materials stored at

controlled temperatures has shown that deterioration

occurs very rapidly (complete failure in one month)

at 65°C, slowly at 50® C and extremely slowly if at

all at 37*0 or lower. Data obtained indicate that

the deterioration is a result of degradation* probably by depolymerization, of the alginate constituent of the impression material. To prevent deterioration it is recommended that alginate impression

materials be stored at temperatures of 37°C or lower.

1. INTRODUCTION

The alginate type materials for taking dental

impressions were introduced during World War II as

a substitute for the then scarce agar-agars. They

have gained increasing favor because of their ease

of handling and accuracy of reproduction.

The physical properties and clinical behavior

of these materials have been studied by, Skinner

and Pomes /"~1, 2_J7, Cresson /"3 7s Worner / 4_7s>

and Schoonover and Dickson £“

One of the serious

difficulties encountered in their use has been their

tendency to deteriorate during storage. Two distinct

types of aging failure have been observed. In some

cases the material, after storage for a short time at

ordinary temperatures, will not form an insoluble

elastic gel. Schoonover and Dickson /”5_7 showed that

this was caused by the removal of the regarding agent

by premature reaction with the compounds added to

convert the soluble gel into the insoluble form. This

type of difficulty is readily overcome by proper compounding of the mixture and is no longer a serious

factor. The second type of failure is observed most

- 2 -

often when the materials are stored at elevated temperatures

and has been particularly noticeable when these materials

are shipped to and stored in tropical areas. Materials

subjected to these conditions often have inferior physical

properties, the strength of the gels is greatly reduced,

their setting time is altered, and the accuracy of the

reproduction is inadequate for dental use® Experimental

data obtained in this investigation show that this type of

deterioration results from degradation of the alginate

polymer at elevated temperatures

®

2. EFFECT OF TEMPERATURE -ON DETERIORATION OF

ALGINATE IMPRESSION MATERIALS

Samples of five commercial brands of alginate impression

materials were stored in the packages as received, at temperatures of 21°C, 37°C, 50°C and 65°C for one month® At

the end of one month samples of each material were removed

and their setting t imes , permanent deformation, compressive

strength, ease of manipulation and gel-forming characteristics were determined® This procedure was repeated on other

samples stored at 50°C for 3-1/2 months® The method described in American Dental Association Specification No® 11

for hydrocolloidal impression materials /”67 was used to

determine setting time® The permanent deformation was determined by the 'method described by Cresson * The compressive strength was determined on cylindrical specimens approximately 0«5 inch in diameter and 0®75 inch in height loaded

at a head speed of 0®20 inch per minute on the testing machine

until rupture occurred. These specimens prior to crushing

were stored for one hour at 21 ± 1°C under conditions approximating 100$ relative humidity® The gel-forming properties

and ease of manipulation were determined by making mixes of

the material and observing the general characteristics of the

mix® The data obtained are given in Tables 1 and 2® It will

be observed from these data that no appreciable change occurred

in any of the materials stored at 37°C (as compared with those

stored at 21°C) for one month® However, in general the properties of most of the brands were markedly altered after one

month’s storage at 50°C, as indicated in a reduction in their

compressive strengths® The deterioration was more pronounced

after storage for 3-l/2 months at 50°C and resulted in a loss

of strength of from 25 to A5$» Storage at 65°C produced such

rapid deterioration that all the products were unsatisfactory

after one month’s storage at this temperature®

- 3

-

Another series of specimens was stored for 30 months

in an unheated storage room (temperature range approximately ( 5 - 50° C ), Properties of two brands changed

little (Table 2) and these materials were considered

satisfactory for general use after storage. A third impression compound did not set after storage under the same

conditions. This material did not change appreciably on

storage at 21°C for the same length of time.

It was observed that the impression materials stored

at 65°C exhibited an irreversible loss in weight even

after being dried to constant weight over Drierite at room

temperature. The cause of this loss was not clearly

established but is related to the extent of degeneration.

The results are shown in Figure 1 where ultimate compressive strength is plotted against weight loss.

Any decrease in weight on oven drying (of the samples

previously dried over Drierite) was extremely critical^

a loss in weight of as little as 0.25$ of the impression

compound (corresponding to about 1$ of the algin) rendered

the material unsatisfactory for clinical use.

This loss in weight and the resulting loss in strength

do not appear to be caused by oxidation of the organic

alginate 9 since impression materials stored under nitrogen

and heated deteriorated in a similar manner and at approximately the same rate as the materials exposed to air during

heating.

The possibility of microbiological action was rejected

owing to the highly alkaline nature of some ingredients of

the impression compounds and to the fact that no microscopic

evidence of such action was ever apparent in the deteriorated

powders.

Since the inorganic salts and fillers used in, compounding

these materials are usually heat stable at the temperatures

of storage^, loss of the gel-forming properties during storage

is more likely to be associated with a change in the alginate

component of the impression material.

Sodium alginate is a linear polymer of the sodium salt

of anhydro- jB -d-mannuronic acid of colloidal dimensions jj 9 8* 9 S 10s ljjhavlng recurring units of the formula C^HgO^,

and containing one carboxy and two hydroxy groups.

- 4

The structure of alginic acid contains acid groups

which decarboxylate (decompose with loss of carbon dioxide)

almost completely at temperatures above 200°C £~12j . No

evolution of carbon dioxide was observed on storage of

samples at 65 °C, although the samples showed the usual

deterioration. Hence, decarboxylation is not a likely

cause for this change.

Extensive reduction in molecular weight, which is

normally about 15 , 000, has been reported under .a number of

conditions, indicating that the polymer will depolymerize at elevated temperatures. Storage of alginate impression materials at elevated temperature would therefore

account for the deterioration often observed.

To investigate further the depolymerization reaction,

samples of a sodium alginate were carefully dried to constant weight over Drierite and then stored at approximately

50° and 65°C for 7 weeks. At the beginning and periodically throughout the storage period samples of the alginate

powder were removed , dissolved in water and the viscosity

of the solution determined. The solutions were made by

adding 0.25 gram of alginate to 100 ml of distilled water.

The viscosity measurements were made at 37 ± o05°C in an

Ostwald-Fenske viscometer. It was observed that the viscosity decreased with increased time of storage at 65°C

(Fig. 2) . After 3-1/2 months of storage at this temperature,

the viscosity was reduced by ^approximately 30 This decrease in viscosity was attributed to degradation of the

alginate polymer m’

‘ ppears to be a

by phenol, a typical chain terminator. The depolymerization

may take place by an exclusive breaking of terminal links

or a random scission (i.e. all bonds are split with equal

ease). Either mechanism leads to products of lower molecular

weight. It is believed that this depolymerization of the

alginate is the primary cause for the deterioration of

alginate impression materials during storage at elevated

temperatures

.

1. Alginate hydrocolloidal impression materials

deteriorate rapidly at elevated temperatures. Materials

stored for one month at 65°C were unsuitable for dental use

and either failed to set or showed increased setting time

and greatly decreased compressive strength. At 50°C these

materials also deteriorated but at a much slower rate. No

appreciable change was observed in materials stored at 37°C,

chain reaction, that it is inhibited

3. CONCLUSIONS AND RECOMMENDATION

- 5 -

2. Deterioration of the alginate impression materials

is caused by degradation of the alginate constituent, which

probably depolymerizes.

3. Alginate impression materials should not be stored

for prolonged periods at temperatures above 37°C.

Bibliography

1. Skinner, E. W., and Pomes, C. E. Alginate Impression

Materials: Technique for Manipulation and Criteria

for Selection, J. Am. Dent. Assn. 21* 245 (1947) »

2. Skinner, E. W., and Pomes, C. E. Dimensional Stability

of Alginate Impression Materials. J. Am. Dent. Assn.

22, 1253 (1946).

3. Cresson, James. Suggested Revisions for Testing Dental

Elastic Impression Compounds. J. Dent. Res. 28, 573

(1949).

4. Worner, A. K. Impression Materials with an Alginate

Base. Australian J. Dent. 4$, 51<-(1944).

5. Schoonover, I. C., and Dickson, George. Preparation and

Characteristics of Elastic Dental Impression Compounds

with an Alginate Base. J. Am. Dent. Assn. 20, 5o5 (1943)°

6. Paffenbarger , G. C. Hydrocolloidal Impression Materials:

Physical Properties and a Specification. J. Am. Dent.

Assn. 22, 373 (1940).

7. Nelson, William L., and Cretcher, Leonard ft. The Alginic

Acid from Macrocystis Pyrifera. J. Am. Chem. Soc. 51

.

1914 (1929).

3. Nelson, William L., and Cretcher, Leonard H. The Isolation and Identification of d-Mannuronic Acid Lactone

from the Macrpcystis Pyrifera. J. Am. Chem. Soc. 52, 2130

(1930).

9. Nelson, William L., and Cretcher, Leonard H. The Properties of d-Mannuronic Acid Lactone. J. Am. Chem. Soc.

Si, 3409 (1932).

- 6 -

10. Dillon, Thomas, and McGinnis, Annie. On Alginic Acid.

Royal Dublin Soc., Scientific Proc. 20, 129 (1931).

11. Steiner, Arnold B., and McNeeley, William H. Organic

Derivatives of Alginic Acid. Ind. Eng. Chem. 43, 2073

(1951).

12. Perlin, A. S. Thermal Decarboxylation of Uronic Acids.

Canadian J. Res. 22, 27$ (1952).

13. Diaz, E. G., and Aeulle, E. 0. The Viscosity of Sodium

Alginate Solutions. Anales. Real. Soc. Espan. Fis. y

Quim. 46B, 441 (1950).

Table le

Effects

of

Storage

Temperature

on

the

Properties

of

Alginate

Materials

REDUCTION

II

STRENGTH

AFTER

3-1/2

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CO rH t>-

a o3 ca «M

T3 o O g UA a) o •H ua UA UA -4 0 P CO rH a -4 <D o G

A3 T3 P CD •H 0) e

CO CO iH o C G o P y •H O o o CD «4 p *H CD o p (X w T3 CO CO P CO

ctf •H 03

W 03 CO T3

w o rH a) e g CD CD

E-* .p

W P 03 •rH

O « P .H • • 4 1 Ph Cl) fcO o CD rH *H CD 5 cd ch e f-t i—

rH 03 a3 P o 1 M O 0) o CD S> fit O o Q S IS) H E5
.

/it 1 • I
Table 2. Deterioration of Alginate Impression

Materials after Storage for 30 Months

Material

LaboratoryStorage

Conditions

Setting

Time

Permanent

Deformation

(after 12$

strain)

Crushing

Strength

(1 hour

after mix)

min T" psi

D-P Elastic Tested on arrival 3-2/3 2.3 121

Impression 30 mos. at 5-50°Ci 4 2.8 123

Cream

Kalginate Tested on arrival 2-1/2 2.2 80

30 mos. at 5*-50°C! 2-1/2 2.3 86

Zelex Tested on arrival 3-1/2 1.3 61

30 mos. at 5-50°Ci- Did not set

30 mos. at 21°CJi: 3-1/2 1.8 56

•L Kept in unheated storage room.

£ Values considered accurate within 10 percent.

* Kept in constant temperature room.

COMPRESSION

STRENGTH

IN

LB

/IN

FIG. I
STORAGE

TIME

IN

WEEKS
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0703-20-3324 June 15, 1953 2566

A TRANSVERSE STRENGTH TESTING MACHINE

By

W. T. Sweeney*

Harold J. Caul*

Willy Gneug*

*Chief, Dental Research Section, Division of Organic and

Fibrous Materials.

^Research Associate, American Dental Association, Dental

Research Section.

^Instrument Maker, Shops Division.

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association

the Array Dental Corps, the Air Force Dental Corps, the Navy

Dental Corps and the Veterans Administration.

The publication, reprinting, or reproduction of this report In any form, either In whole or In part, Is prohibited

unless permission Is obtained In writing from the Office of the Director, National Bureau o

f Standards, Washington

25. D. C. Such permission Is not needed, however, by an agency for which a report has been specifically

prepared If that agency wishes to reproduce additional copies of that particular report for Its own use.

A TRANSVERSE STRENGTH TESTING MACHINE*

W. T. Sweeney , Harold J. Caul, and Willy Gneug

Abstract

This paper describes a transverse strength t e sting

machine for use in determining the stress-strain relation on small specimens of denture resin. The machine

was designed to replace an older type and has several

added features , such as automatic loading rate, lower

initial load, control of the medium in which the specimen is held during the test (air or liquid), and thermostatic temperature control. The primary objectives

in developing this new machine were to obtain more precise placement of the load and greater efficiency in

operation than were possible with the type of machine

previously used.

1, Introduction

An improved instrument for testing transverse strength has

been developed to replace a machine of older design [1] that

has been in service for some twelve years. The new instrument

provides for applying specific loads to the center of a specimen 2.5 x 10.0 x 65 mm in size, supported on bars 50 mm apart,

and for measuring the deflection produced in the specimen.

* The significance of this type of equipment is evident from

the fact that practically all of the denture base materials used

by the dental profession in this country are evaluated on the

basis of transverse strength. Although the older instrument was

adequate, there were several difficulties encountered in its use.

The specimen holder and specimen had to be positioned by the

operator without the assistance of stops. It was necessary to

align the plunger so that the load contact surface was perpendicular to the sides of the specimen. The loading rate was controlled manually, and it was very difficult for one person to

operate the instrument and record data, since readings were made

at practically the same time as the load increments were added.

Also, this instrument permitted testing in air only, a serious

drawback since the application of stress to acrylic resin produces different amounts of strain, depending upon whether the

specimens are stressed in water or air, making it desirable to

be able to test them in either medium. These difficulties have

been overcome with the new design.

* Detail drawings of this instrument are available and can be

obtained from the Dental Research Laboratory, National Bureau

of Standards, Washington 25, D. C.
!I!iiiii1iI1
- 2 -

2* Description of Instrument

The instrument is constructed of rust-resistant metals

with a base of 9 x 9 in. and an over-all height of 17 in.

(Fig. 1). The design provides for the application of a load in

500 gm increments of lead shot from the 24 cells of drum A.

The load is gradually and uniformly delivered over a period of

30 ± 5 seconds. The operator positions the ring D to which the

lift E is attached in order to open each cell to deliver the load

increment. The ring D is held in the same relative position for

each cell by a spring finger which fits in the grooves on the

ring. Successive units of 500 gm can be applied as desired.

The shot rolls down an inclined plane to symmetrically load the

pan or holder over the piston C through which the load is

applied to the specimen B.

The balancing weight F may be adjusted so that the total

weight of the shot pan, dial pressures and piston is counterbalanced. This makes it possible to read deflections of the

specimen from an initial zero load.

The resin specimen B, 10.00 ± 0.03 by 2.50 ± 0.03 ran and

approximately 65 mm long, is placed over a 50 mm span of the

specimen holder G. The bearing surfaces which contact the specimen are 3*2 mm in diameter. The specimen holder is positioned

by pins in the base so that the load is applied at the middle of

the span.

The sensitive dial gage H has a 25 mm travel and is accurate to 0.01 mm. The gage spring is removed in order to avoid

variable pressure at different positions. The deflection of the

specimen is measured at predetermined time intervals by reading

the dial gage. Thus, by observing the deflection and the load

at the middle of the span, the necessary data for determining the

stress-strain relationship in transverse loading can be obtained.

Stop I, which consists of a metal plug with a rubber top, is

provided to prevent the end of the plunger from contacting the

bottom of the specimen holder when the specimen fails. A closed

spout J is provided for emptying the shot after the test is completed.

Fig. 2 shows a drawing of the instrument with a thermostatically controlled liquid bath in position. The bath fits

inside the base supports and is provided with suitable centering

pins to properly align it with the loading mechanism. The temperature is controlled by a flexible resistance heater and a mercury thermostat to about 0.1°C. This makes it possible to

determine the transverse strength of the resin at any desired

temperature in water or other liquids. For the most precise

control of specimen temperatures in liquids it would probably be desirable to insert a section of heat-insulating material in the

plunger to reduce conduction from or to the specimen at the

point of contact with the plunger.
I
- 3

-

The instrument is designed so that the load is applied

through the square piston normal to the long axis of the specimen and at the mid-point of the span. When a cylindrical plunger was used, the operator adjusted the position of the curved

surface contact visually. This led to uncertainty of the positioning of stress concentration in the specimen under test. On

the new specimen holder lines have been scribed so that the

maximum stress applied will be 25 mm from these lines. Thus, by

placing an index line on the specimen, the position of the

applied load can be accurately determined. This facilitates

the positioning of the specimen at a specific point such as a

repair joint or union of two types of resin in the specimen.

The accuracy of loading the specimen is dependent upon the

low friction roller bearing on the one-inch-square piston and i the low friction roller bearing supports for the counterbalancing

mechanism. The applied load is considered accurate to 25 gm.

A maximum load of 12 kg can be applied without refilling the

loading pan with shot

.

3 . Summary

This instrument is designed to determine the transverse

strength of materials under controlled rates of loading. It

is especially useful for testing resins using the standard

specimen (2.5 x 10.0 x 65 mm) required by the American Dental

Association and Federal specifications for denture base resins.

The gravity feed of lead shot provides a more nearly constant rate of loading than manual loading methods.

The strength of the resin can be determined in air or in a

thermostatically controlled liquid bath.

The instrument provides for accurate positioning of the

specimen and load.

The automatic loading and ease of positioning of the specimen make it possible for one operator to conduct transverse

tests efficiently.

Reference

1. Sweeney, W. T. , Denture Base Material: Acrylic Resins,

J. Am. Dent. Assn. 26, 1363 (1939).



SPECIMEN C\20) b

fcO •H

Pel,

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-20-3824 June 30, 1953

NBS REPORT

2628

Some Factors Affecting the Dimensional Stability

of the Ag”Sri“(Cu~Zn) Amalgams

J 0 Ao Mitchell*

I. Co Schoonover**

George Dickson***

Ho Co Vacher****

* Guest Worker, UoSoNo Dental Corps, Dental Research Section,

Division of Organic and Fibrous Materials

** Chemist, Chief, Polymer Structure Section, National Bureau of

Standards

o

*** Physicist, Dental Research Section, National Bureau of Standards 0 **** Metallurgist, Mechanical Metallurgy Section, National Bureau of

Standards

o

This work is a part of the dental research program conducted at the

National Bureau of Standards in cooperation with the Council on Dental

Research of the American Dental Association, the Army Dental Corps,

the Air Force Dental Corps, the Navy Dental Corps and the Veterans

Administration 0

The publication, reprinting, or reproduction of this report In any form, either In whole or In part, Is prohibited

unless permission Is obtained In wrltlngfrom theOffice of the Director, National Bureau of Standards, Washington

25, D, C. Such permission Is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to reproduce additional copies of that particular report for Its own use,

SOME FACTORS AFFECTING THE DIMENSIONAL STABILITY

OF THE AG-SN-(CU-ZN) AMALGAMS

Abstract

An investigation of the X-ray diffraction

characteristics and of the dimensional changes of

Ag-Sn (Cu-Zn) amalgams during and after their hardening

period indicated that uncombined mercury was present in

the amalgams after the initial solidification and that

this uncombined mercury disappeared during the period

when greatest dimensional changes occur 0 On the basis

of the data obtained it is postulated that the expansion

during the hardening of amalgam results from diffusion

of un combined mercury throughout the mater ialg the subsequent shrinkage then results from combination of this

mercury with existing phases or with residual alloy 0

lo Introduction

Although an enormous amount of experimental work has been done on the

Ag-Sn-(Cu-Zn)* amalgams [l, 2, 3,4,5, 6j , no mechanism has been proposed

which will satisfactorily explain the initial contraction, subsequent expansion, and final contraction which occur during the first twenty-four

hours after amalgamation 0 Various investigators have attempted to explain

these changes in terms of mercury content of the amalgam system,. Their

difficulty may have resulted from the failure to differentiate between

total mercury and combined mercury in this system,. It is probable that

in the hardening of the heterogeneous mass not all the mercury is in the

combined form,, Chemical analysis of the amalgam would not distinguish

between combined and uncombined mercury,, Furthermore, photomicro graphic

evidence £7] indicates that the rate of attack on the alloy particles

by mercury may vary widely depending upon the conditions under which the

specimen was prepared,, The possible diffusion of this mercury if it is

present, and its subsequent reactions with the alloy may account for the

dimensional changes observed in the hardened amalgam,,

* Cu-Zn is placed in parentheses because these elements are not always

present in the above mentioned amalgams,,
4

-
With this in mind this investigation was undertaken to determine

(1) whether uncombined mercury was present after the amalgam had hardened % (2) whether the uneombined mercury* if present* continued to take part in

any reaction! and (3) whether the dimensional changes which occur in

hardened amalgam could be correlated with this reaction 0

2® Experimental .Approach and Procedures

X-ray diffraotion methods can not be used to detect the presence of

free or uncombined mercury in an amalgam at room temperature since the

mercury, being a liquid at this temperature, does not have a crystalline

structure and does not give an X-ray diffraction pattern® However* if

the mercury is chilled below its solidification point* typical lines

representing mercury are detected by diffraction methods® Therefore* if

uncombined mercury is present in amalgam immediately after hardening*

it should be possible to determine its presence by X-ray investigation

of amalgam specimens held below the freezing temperature of mercury®

Furthermore* it should be possible to study the rate of disappearance of

the un combined mercury with time by periodic examination of the specimen

at temperatures below the freezing point of mercury®

There is the possibility that the intermediate phases in the Ag-Sn-

(Cu~2n) amalgams might have a higher mercury solubility at room temperature than at temperatures below the freezing point of mercury® Such

being the case* crystalline mercury at low temperatures might be formed

by rejection from a solid solution which existed at room temperature®

Evidence will be given later in this paper to support the contention

that if this does occur* the amount of mercury involved is insignificant®

In order to investigate the X-ray diffraction patterns of amalgam

at reduced temperatures* specimens were manually condensed employing

standard dental procedures in a stainless steel mold constructed so that it could be cooled with liquid nitrogen during the period of

observation® The amalgam specimens were frozen immediately upon com- pletion of condensation by introduction of liquid nitrogen into the

mold® An iron-constant an thermocouple was used to measure the temperature of the specimens®

The compositions of the three commercial amalgam alloys used in

the investigation are given in Table 1® The amalgams were triturated

in a mechanical amalgamator (Wig-L-Bug) using the mercury-alloy ratio

of 2 to 3 for Alloys 1 and 2* and a ratio of 2 to 3® 6 for Alloy 3®

A Philips high-angle spectrometer goniometer was used to

obtain chart records of the copper K alpha diffraction® The diffraction

unit was operated at 35 KVP and a tube current of 20 milliamperes®

The scanning speed was 1° 20 per minute and the chart speed was l/2

inch per minute®

- 3 -

Dimensional changes of amalgam specimens were measured by means of

dental interferometers [ 9 ] in an air bath at the temperatures

indicatedo

So Results

I® X-ray Diffraction Examination of the Ag-Sn-(Cu-Zn) Amalgams

X-ray diffraction charts of the amalgam alloys at 25®C prior to

amalgamation, of pure mercury at -125®C, and of amalgams at -125°C are shown in Figures 1 to 4 0 The data show that uncombined mercury was

present in the amalgam and that this mercury disappeared with time 8 as indicated by the reduction in the height of the mercury lines 0 Simultaneously with this reduction in uncombined mercury there is also

a disappearance of the lines representing the original alloy particles

and an increase in the ^ 1 and t 2 phase s* 0 This indicates that the

disappearance of the mercury lines was through combination of the

mercury with the alloy to form mercury compound So The disappearance

of the mercury lines also indicates that these lines represent un- combined mercury rather than mercury rejected from a solid solution as

a result of the reduced temperature 0

The data also show that the almost complete disappearance of the

mercury lines within six hours corresponds generally to the period of

time during which expansion and contraction of the amalgams are commonly observed,,

To determine the effect of addition of mercury to fully hardened

amalgam, seven-day -old specimens which had been placed in mercury for

12 hours were cooled to -125 P C and examined by X-ray diffraction

periodically for a period of 24 hours after removal from the mercury,,

Intense mercury lines were observed in the specimens immediately after

removal from the mercury bath, Figure 4 e These mercury lines were not

detected after the specimens had been held at 37 P C for 24 hours or at

21 ®C for 3 days 0

* ^T-original alloy (Ag Sn * Cu,Zn)

o ^ 1 “Ag2Hg3

/

2 ~Sn

7Hg

ft

1-AgHg not observed

- 4 -

2 0 Dimensional Change Determinations of the Ag-Sn-(Cu-Zn) Amalgams

To examine the effect of uncombined mercury on dimensional changes of

hardened amalgam, mercury was added both externally and internally to

hardened amalgam specimens and their dimensional changes were observed by

means of interferometers 0 Hardened specimens of amalgam in iron cups were positioned in interferometers j mercury was added to the cups and the

dimensional changes were observed over a period of 15 days on two series

of specimens held at 21®C and 37®C respectively,, In other experiments a small hole was drilled in the specimen and mercury injected into the hole

in the amalgam by means of a hypodermic syringe after the specimens had

been positioned in the interferometer,, Dimensional changes were observed

as in the above case 0 Large expansions were observed in all cases s Figure 5 0

In two experiments the supply of mercury was removed from contact

with the specimens after a period of 15 days and dimensional changes were observed for an additional period of 26 days. Figure 6 0 No further

expansion occurred in these specimens „ A slight shrinkage was observed

immediately after removal of the mercury supply,.

To obtain information on the effect of temperature on the reactions

occurring during the hardening of dental amalgam, dimensional changes of

amalgams prepared and observed at various temperatures were determined,,

.Amalgams prepared and maintained at 37°C expanded less than specimens

prepared and observed at 21®C (Figures 7, 8, 9 and Table 2) 0 Furthermore,

an overfall shrinkage occurred in all amalgams maintained at 37'*C regardless of the temperature at which they were prepared,. Specimens prepared

at 37®C and maintained at 2l°C shrank less than specimens prepared and

maintained at 37®C„

Discussion of Results

The observations of dimensional changes, along with the X-ray

diffraction evidence that uncombined mercury is present in the amalgam,

tend to substantiate the theory that the mechanism of dimensional change

is associated with the presence of the uncombined mercury

„

The following mechanism is postulated to explain the physical

phenomena observed,, The initial shrinkage always observed results from

the formation of compounds of mercury and alloy having a smaller volume

than the sum of the original volumes of the alloy and mercury,. The subsequent expansion which is observed results from the diffusion into new

areas of uncombined mercury entrapped in spaces throughout the amalgam,,

The reaction of this uncombined mercury with existing phases or with

residual alloy to form compounds of lesser volume would then account for

the contraction which follows the expansion of the amalgam,.
i(1(11111111iiii
- 5 -

This mechanism would account for the dimensional changes which

occurred on the addition of mercury to hardened amalgam0 When mercury

is added to hardened amalgam the diffusion of this mercury throughout

the specimen would produce the large expansion observed* The slight

shrinkage observed immediately after the removal of the supply of

mercury could result from the combination of free mercury in the

specimen with existing phases or with residual alloy*

The greater shrinkage of amalgam mixed and maintained at 37®C rather

than at 21®C may result from a more rapid chemical combination and disappearance of the uncombined mercury at elevated temperatures*

The proposed mechanism would also account for the shrinkage of

amalgam which occurs from over trituration or excessive working during

condensation in that either of these treatments would tend to reduce

the amount of uncombined mercury by either furnishing new surfaces

for reaction with the mercury or by actual mechanical removal of the

mercury* These treatments would also tend to distribute the uncombined

mercury present more uniformly throughout the amalgam so that there

would be fewer mercury-rich areas from which diffusion could take place.

On the basis of this explanation, a reduction in size of the alloy

particle or a heat treatment which would make the particles more

reactive would tend to cause a reduced expansion or a shrinkage of the

amalgam*

The possibility that the dimensional changes could also result from

phase changes which can take place more readily in the presence of un- combined mercury should not be excluded as a possible mechanism.

Summary

1* Residual or uncombined mercury has been demonstrated by

X-ray diffraction studies to be present in amalgams after their

initial solidification*

2* This uncombined mercury disappears during the period when

greatest dimensional changes occur in the hardening of the amalgam system*

3* A possible mechanism for the dimensional changes based on the

presence and disappearance of the uncombined mercury is presented.

- 6 -

BIBLIOGRAPHY

1. Black, G» Vo, Physical Properties of Silver-Tin Amalgams, DoCosmos

£8:965, December 1896.

2. Gray, A* W. , Volume Changes in Amalgams* J. Insto Metals £9*139, 1923 0

3. Gayler, Marie, The Theory of the Setting of Dental Amalgams. J, Brit.

Dent. Assn. 60*11, July 1936.

4. Ward, M. Lo, Peyton, F. A., and Scott, E. Oo, New Concepts of Mercury

Content in AmalgamSo J. Dent. Res* 1£:316, August 1937.

5. Troiano, A. R», An X-ray Study of Dental Amalgams. J. Inst. Metals

58*247, 1938

6 e Ryge, Gj Moffett, Jo Co, and Barlow, A. G. Microstructural

Observations and X-ray Diffraction Studies of Silver-Tin Amalgams.

J. Dent. Res. 3£:152, 1953.

7. Smith, Do L», etal. The Microstructure of Dental Amalgam; To be published.

8o X-ray Diffraction Instrument and Operating Manual, North American

Philips Corporation, Inc.

9. Souder, Wilmer and Paffenbarger , G. C. Physical Properties of Dental

Materials. NoB.S. Circular 433, U. S. Department of Commerce,

National Bureau of Standards, Wash. D. C., U. S. Government Printing

Office, 1942

Table 1

Chemical Composition of Alloys Investigated


Comparison

of

Dimensional

Changes

of

the

Ag-Sn- (

Cu-Zn)

Amalgams

at

21°C

and

37°C

Continued

on

next

page
Continued

co

Q

© ^Is o o

CV CM -4- • « • X NO + 0 3 3 01 v.0 CVl O' • • • » 'CO On -$ -ti- ll l 1 8 S

'JO ON -00 ON 0 • • « Hoo O <r\ + + II

vO vO

O • 0 to -4 -4" 3 B

~4

X cf\ • * • CV CM -4 + 1 or- <p e * 0*2 H O 9 E

vO -4 CM » © ® X, X CM + +

»2o6 Not

observed

X -CO

X

® • ® O 1—| 1—1 ! 3

?>• -4 0_

O I>- H

37 37 changed to

21

Differ

enc

21 37

Difference

C\2 <1CO 0

CO •H&

rGPo

CO

CO

CD ooi—I

c\i pG

COCD X

CDooG

ft

bOG

•H

COG

CD XaoQXGG

bOGX

•H

CO I—I

CTt •HG0PgS3 PG0s • ftX X 0 G P

cr1 o

ft

G

!>> GPOG

CDS1 t>> 0X

pHGG

•H • 0 CO 0 0

r- 0 X G0 p G

ctS 0X

CO0 •H u X X ft 0 O OO 0 g CO ft GG

bQ 0 G 0

«H ft

COG CM 0X X G G O G 0 1—1 XG • G COO

bO ft G

•H 0

>»

•H

O s X »

<*N *aj 0

co s (—1

ft

X

Gp P X •H G £ bO 0 G PGS ®

“d r“1 r'S ft xG S3 G P G X G ft G 0 S G S O

Ph

O

O

•H

X

G P ft cr O G

ft ft

G

Ag-Sn-Zn AMALGAM (ALLOY 1)

Ag-Sn-Cu-Zn AMALGAM (ALLOY 2)

Ag-Sn-Cu AMALGAM (ALLOY 3 )

ft

33* 35* 37* 39* 41* 43* 45* 47* 49* 51 * 53* 55* 57* 59* 61* 63* 65* 67* 69“ 71* 73* 75* 77* 79*

Degrees 20

<NBS>

Figure 1* X-ray diffraction charts (with a scale factor of 4)

made at 25°C.
MERCURY

Degrees 29

<^BS>

Figure 2. X-ray diffraction charts made at -125°C. Scale factor of 32 used for obtaining the tracings of mercury. Scale factor of 4 used for the remainder of the pro- cedures.
Degrees 20

Figure 3. X-ray diffraction charts, a continuation of Figure 2
Degrees 20

Figure 4* X-ray diffraction charts made at -125°C.
100

90

80

70

60

50

40

30

20

10

0

-in DAYS

Figure 5* Dimensional changes in Ag-Sn-Cu-Zn amalgam (Alloy 2)

resulting from addition of mercury.
MICRONS

PER

CENTIMETER

Figure 6. Dimensional changes' of Ag-Sn-Cu-Zn amalgam (Alloy 2) resulting from addition and removal of mercury. 0 days - mercury added internally through hole drilled into

specimen. 15 days - excess mercury removed from hole drilled into specimen.
Figure 7. Comparison of dimensional changes of Ag-Sn-Zn amalgam

(Alloy 1) at 21°C and at 37°C. Equipment, materials,

mixing-condensing procedures at 21°C.
MICRONS

PER

CENTIMETER

Figure S. Comparison of dimensional changes of Ag-Sn-Cu-Zn amalgam

(Alloy 2) at 21 °C and at 37° C. Equipment, materials,

mixing-condensing procedures, and observations were made

at the respective temperatures*


NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 June 30, 1953 2647

DIRECT FILLING RESINS;

MECHANICAL PROPERTIES

By

W . T 0 Sweeney * W. D 0 Sheehan I Eo L, lost L

*Chief, Dental Research Section, Division of Organic and Fibrous

Materials

.

IGuest Worker, U„S. Army, assigned to Dental Research Section

This work is a part of the dental research program conducted at the

National Bureau of Standards in cooperation with the Council on Dental Research of the American Dental Association, the Army Dental

Corps, the Air Force Dental Corps, the Navy Dental Corps and the

Veterans Administration

„

The publication, reprinting, or reproduction of this report In any form, either in whole or In part. Is prohibited

unless permission is obtained In writing from theOlfice of t be Director, National Bureau of Standards, Washington

25, D. C. Such permission Is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to reproduce additional copies of that particular report for Its own use,
I

.1 -
DIRECT FILLING RESINS; MECHANICAL PROPERTIES

Abstract

The use of acrylic type resins for direct filling of cavities

in the teeth has been recently developed to such an extent that a large percentage of the dental profession use this type of material 8 This paper reports the results on the mechanical properties of ten

trade brands of materials , as well as the methods developed for

obtaining data on small specimens ? comparable to dental applications.

A new test is included which is based on rate of indentation of a sphere under constant load.

The obtsti^aed data show that the mechanical properties improve

for approximately 24 hours after mixing. Moduli of elasticity varied

from 1.51 to 3.09 psi and compressive strength from 45 000 to 7 ? 400

psi.

DIRECT FILLING RESINS s MECHANICAL PROPERTIES

1„ INTRODUCTION

The success of-any material used to restore lost tooth structure

depends to a large extent upon its physical and chemical properties.

This applies to all types of filling materials , whether they are metallic, ceramic or resinous. The development of direct filling

resins and their widespread use as restorative materials where

esthestics must be considered, make it desirable to know more about

their mechanical properties. A knowledge of these properties will

aid in determining the type of restorations for which resins are most suitable and in determining the techniques which should be used

to obtain optimum results with these materials.

The mechanical properties reported are of special interest in

the evaluation of resinous filling materials. The modulus of

elasticity of the resin is an index to its stiffness and to the

amount of movement of the restoration at the margin or walls of the

cavity as a result of forces encountered in the mouth. The yield

strength is a measure of the forces required to cause a permanent

deformation in the restoration. The resistance of the resin to

indentation under a continuous load is an indication of its ability

to resist deformation in the mouth in locations where forces are applied repeatedly on occlusal surfaces or constantly at contact

points

„

The investigation of the properties of direct filling resins

was somewhat complicated by the fact that these resins polymerize

in the tooth and that the temperature rise on polymerization £ 1 J,

the rate of hardening and the completeness of polymerization

depend on the size of the restoration. For these reasons measurements

of mechanical properties were made on specimens comparable in size

to restorations used in practice. Since change in mechanical

properties continues for some hours , a series of determinations were

made on specimens of different ages. Determinations of the modulus

of elasticity, yield strength and resistance to indentation of ten

brands of direct resin filling materials (Table l) are reported in

this paper.

2. EXPERIMENTAL PROCEDURE

Modulus of Elasticity and Yield Strength

To determine the modulus of elasticity and yield strength of the

resins , loads were applied to specimens 3 mm in diameter and 6 mm

in length in a Tinius Olsen Testing Machine, and the deformation of

the specimens was measured with Tuckerman strain gages.

All specimens were prepared and tested in a constant temperature room at 22 + 1° C (71 i 2° F) to insure uniformity of specimens

and of testing procedure. Mixing was done according to the manufacturers'
II1HI(3I
-2-

directions insofar as details were available 8 Approximately 0.75 g

of polymer with the recommended ratio of liquid was employed for each

mix. Specimens were molded in a stainless steel split-mold, Fig. 1,

consisting of six cavities , each accurately machined to 3 mm in

diameter and 6 mm in length. The mold was assembled and slightly

overfilled with the resin at the time the material would normally be

placed in the tooth cavity. A steel plate was placed on top of the

mold and pressure was applied by two screw-clamps. Immediately after

it was clamped, the mold was placed in an oven regulated to 37 ± 1° C

(98 ± 2° F). After thirty minutes the mold was removed from the oven

and disassembled. The specimens were removed and allowed to stand

at room temperature (22° C)(72° F) for 3 to 5 days before testing.

The procedure used for measuring the modulus of elasticity was

as follows. Because of the shortness of the specimens, the 1-inch

Tuckerman strain gages used to measure deformations were attached to

steel pins placed above and below the specimens in the testing machine

as shown in Fig. 2. The error due to this pin arrangement was found

to be negligible because of the size of the pins (l/2 inch diameter)

and the relatively low modulus of the plastic.

A continuous load was applied with the testing machine, using a head speed of 0.20 in. per minute, and the strain was read from the

Tuckerman strain gages by means of an Autocollimator [ 2J at 2.5 lb

load increments. Loads were recorded from 5 to approximately 100 lbs

depending upon the strain produced In the material. Six specimens of

each brand were used to obtain an average value. Fig, 3 shows that

the variation obtained in six specimens of a typical material for the

stress-strqin relationship was of the order of 6 percent. The variations are caused mainly by differences in individual specimens.

The average moduli of elasticity of the direct filling materials

investigated varied from 1.51 to 3o09 x 10^ psi. Table 2. The stressstrain curves of the materials are shown in Figs. 4 and 5. The modulus

of elasticity was computed over the stress range of 1,000 to 6,000 psi.

The "yield strength" for the purposes of this paper is defined

as the stress (load per unit cross-sectional area) at which the

deformation produced by a unit stress equals _or exceeds 1.25 times

that observed in the first few increments of stress [3j . The calculated yield strengths (Table 2), which represent an approximation of

the stress that can be applied to the resin without permanent

deformation, ranged from 4? 000 to 7,400 psi.

Resistance to Indentation

The resistance of the resins to indentation was observed by

applying loads to specimens with the Rockwell Superficial Hardness

Tester. By observing the change in the depth of indentation while

the load remained on the resin for ten minutes, flow properties were

determined „ Recovery after flow was determined by measurements made

after removal of the load 0 The relationship between degree of

polymerization and resistance to indentation was determined by loading

specimens at various lengths of time after preparation

,

The specimens were prepared in accordance with the standard

mixing procedure and formed at a temperature of 22 ± 1° C (71 ± 2°F)

in tinfoil lined brass molds 34mm long by 12 mm wide by 1,5 deep,

A load of 2 kg was placed on the top plate of the mold immediately after

the insertion of the resin. Specimens to be tested 10 minutes after

the start of the mix were removed from the mold at 8 l/2 minutes and

stored at room temperature until placed in the testing equipment. All

other specimens were removed from the mold 25 minutes after the start

of the mix and stored at 37 ± 1° C (98 ± 2° F) until five minutes

before testing,, at which time they were allowed to come to room

temperature 0

In measuring* the resistance to indentation in the Rockwell Superficial Hardness Tester the Y-30 scale employing a minor load of 3 kg* a major load of 30 kg and l/2 inch steel ball was used. The minor

load was used for the fiducial reading and the major load applied

for 10 minutes was used to indent the material. Observations were

taken at frequent intervals in order to determine the relationship

between depth of indentation and time of load application. Indentation was observed to ± 0,001 mm by a dial indicator.

The uniformity of the data obtained in measuring indentation is

shown in Fig, 6, The variation obtained when testing five specimens

of an individual brand was within plus or minus 3 percent. The

calculated probable error is of the magnitude of ± 0.004 mm. As in

the stress-strain testing., it is felt that deviations in individual

determinations are caused chiefly by unavoidable differences in

individual specimens rather than by inherent errors in the apparatus

and techniques.

Data for a typical indentation and recovery curve are shown in

Fig, 7,

Table 3 and Figs, 8 and 9 give the indentation values for a 10-minute application of the major load for specimens of various ages

up to 120 hours. As shown in the table,, the average depth of

indentation at 10 minutes was mbre than twice the depth of indentation

at 120 hours 9 and the average depth at 30 minutes was more than 1-1/2

times that at 120 hours. Also included in Table 3 are values for

"indentation strength," These figures ranging from 4100 psi at

10 minutes to 9300 psi at 120 .hours were obtained by dividing the load

on the indenting tool by the projected area of the indentation^ a

- 4 -

method commonly used for calculating hardness as determined by

indentation. In determining the projected area of the indentation

from the depth of indentation a correction was applied to include

the slight indentation produced by the minor load. *

* The corrected equations for projected area were derived as follows?

» L

- = Indentation strength '**" 0 (1)

A

1 A

2

A* = V (2 R y -7 > (2)

A = ^ljR(y + y’) - (y +y')3J (3)

where

L = major load on indenter

j L = —- - minor load on indenter

10

A = projected area of indentaion produced with major load

A

1 A A - = projected area of indentation produced with minor load

10

R = radius of indenter sphere

y* = depth of indentation produced with minor load

y

= instrument reading

. t _ 7 7

- depth of indentation produced with major load

A

If in the above equations the small y^s y* and (y + y j terms are neglected it can be seen that i D (4)

A = ^ // Xt y

or substituting the value 0.25 inch for R and including the factor

for converting y in millimeters to inches.

A

5 = 0.00637y (5)

A more accurate approximation in which fewer terms are neglected

gives

2 h? R 11 2

A 7 7

81

(6)

or substituting as above

A* = 0„00687y - 0.00066y2

(7)

- 5 -

Indentation strength values obtained from equation 7 differ

from those obtained from equation 5 by less than 2.5 percent

if the indentation reading is less than 0.25 mm and by less than

1 percent if the indentation reading is less than 0 o l mm.

The percentage of recovery after indentation of the various

brands of material increased from values of approximately 40 ± 10

percent at 30 minutes to values of 70 ± 10 percent at 120 hours,

Table 4« Recovery was computed by dividing the measured depth caused

by 10 minutes of application of the major load by the amount of . recovery during 10 minutes of application of the minor load.

3. DISCUSSION OF RESULTS

One important characteristic of the direct resin filling materials

demonstrated by the data is their change in properties during the first

few hours after mixing of the monomer and polymer 0 This change is

evident in the results showing the relationship between depth of indentation and age of specimen, Figs. 8 and 9 and Table 3« The resistance

to indentation increases rapidly for approximately six hours. Following this there is slight increase in resistance during the period from

6 to 24 hours, after which this property remains essentially constant.

Table 4 shows that the extent to which the resins recover after being

deformed is also related to age. Recovery under the conditions used

in this test was approximately 30 to 50 percent when the specimens were

l/2 hour old, but was of the order of 75 percent after 24 hours.

That the change in mechanical properties is related to completeness of polymerization is illustrated by Fig. 10. The depth of

indentation obtained at 10 minute application of the major load is

plotted against age of the specimen. Monomer content of the same specimens, determined by a modified form of the method employed by

Lewis and Mayo [V] f is also plotted against age of the specimen.

These data demonstra.te the importance of the presence in the direct

filling resins of the proper catalysts and accelerators to cause

the polymerization of the resins to be as complete as possible.

There is some indication that those resins which cure most

rapidly, as evidenced by early peak temperatures, have higher moduli of

elasticity and yield strengths.

The data on modulus of elasticity point out one of the greatest

weaknesses of the resins. The values obtained were approximately

60 percent of the value obtained for a well cured denture base resin.

Gold alloys used for inlays have a modulus of 15 million psi, while • that of the resins is of the order of 0.3 million psi. This means

that equal forces on similar specimens of resin and gold will produce

approximately 50 times as much deformation in the resins as in the

gold. Clinically this indicates that these resins should be used in

restorations where they will be well supported by tooth structure.

Forces applied to unsupported resin restorations will result in

- 6 -

deformation with resultant movement at the margins of the restoration.

The relatively low yield strength and low resistance to indentation

of the direct filling resins limit their usefulness to some extent.

The resistance to indentation is approximately one-half that of amalgam.

The average yield strength was found to be 6400 psi. Since biting forces

of over 100 lbs are not uncommon, £5j it can be seen that stresses of over 64OO psi could occur on occlusal surfaces and that plastic flow would

occur on direct resins in such locations.

The data presented in the report evaluate the mechanical properties

of the ten brands of direct resin filling material investigated. The

superiority of one brand over another, however, cannot be judged by

mechanical properties alone. To make an overall evaluation it would be

necessary to consider in addition such factors as porosity, curing

shrinkage, dimensional stability, color stability and clinical behavior.

4.

CONCLUSIONS

1. Methods were developed for measuring mechanical properties of

small specimens of resin comparable in size to tooth restorations by

use of two standard testing machines, the Tinius Olsen compression

machine and the Rockwell Superficial Hardness Tester.

2. The direct filling resins tested were found to improve in

mechanical properties during a period of approximately 24 hours after

mixing. This improvement coincided with a decrease in monomer content

during the same period.

3. The moduli of elasticity of the resins tested varied from 1.51 to

3.09 x lCr psi, indicating that, compared with other restorative

materials, the resins are low in stiffness.

4. The yield strength in compression of the direct filling resins

varied from 4>000 to 7,400 psi. This low value would permit plastic

flow to occur under stresses which might be encountered on occlusal

surfaces.

5. The values obtained on the resins for resistance to indentation

were approximately one-half of the values obtained on dental amalgam.

6. In general the mechanical properties of the direct filling resins

are low in comparison with the properties of dental golds, amalgam

or denture base materials.

- 7 -

5. REFERENCES

1. Wolcott, R. B., Paffenbarger, G. C., Schoonover, I, C c , Direct Resinous Filling Materials s Temperature Rise During

Polymerization, J. Am. Dent. Assn. 253 (1951)

2. Tuckerman, L. B., Optical Strain Gages and Extensometers,

Proc. Am. Soc. Test. Mats. 2^, 1017 (1923)

3. Paffenbarger, G. C„, Sweeney, W„ T., and Isaacs, A.,

Wrought Gold Wire Alloys, J. Am. Dent. Assn. 19 . 2061 (1932)

4. Lewis, F. M. and Mayo, F. R., Precise Method for Isolation of

High Polymers, Analytical Edition of Ind, and Eng. Chem. 17 . 134(1945)

5. Howell, A. H„, Manly, R. S., An Electronic Strain Gauge for

Measuring Oral Force, J. Dent. Res. 27, 705 (1948).

Table 1. Direct Filling Resins Investigated

Trade Brand Manufacturer

Ames The W. V-B Ames Co.

A-thermoplast V-10 Athermoplast Products, Inc.

Dentafil Dental Fillings, Ltd.

Duz-all Coralite Dental Products Co.

Fasterown Acralite Co., Inc.

Kadon The L. D. Caulk Co.

Plasto-filling pf3 Plastodent, Inc.

Replica Cosmos Dental Products, Inc.

T exton S„ S 0 White Dental Mfg. Co.

Vitafilling Vitaliner Co.

Table 2. Modulus of Elasticity and Yield Strength

(ages -3-5 days)

Material

Modulus

Elasticity

(average)

Probable

Error

Yield

Strength

psi psi psi

Ames 2.60 x 105 ± 0.10 X 105 5,400

A-thermoplast V-10 1.51 x 105 ± 0.37 X io

5 4,000

Dentafil 2.63 x 105 ± 0.29 X 105 6,600

Duz-all 2.71 x 105H- O o I- 1

F- X 105 6,400

Fastcrown 2.61 x 105 i 0.06 X 105

6,400

Kadon 3.09 x 105 ± 0.23 X 10

5

7,400

Plasto-filling pf3 2.94 x 105 ± 0.17 X 105

7,400

Replica 2.91 x 10

5

± 0.29 X 105 7,200

Texton 2.74 x 10

5 ± 0.12 X 105 —

Vitafilling 2.57 x 105 ± 0.10 X 105 6,400

Ji

Table 3. Indentation and Indentation Strength versus Age of Specimen

Material 10 min 30 min 1 hr 2 hr 4 hr 6 hr 24 hr 120 hr

Depth of Indentation*

mm mm mm mm mm mm mm mm

A-thermoplast 0.277 0.210 0.202 0.181 0.150 0.146 /, 0.137 0.115 1 v-io

Dentafil 0.227 0.173 0.136 0.126 0.117 0.114 J 0.111 0.109

\ Duz-all 0.217 0.139 0.122 0.111 0.104

< 0.101

: 0.095 0.096

i */-*> a Fastcrown 0.264 0.165 0.146 0.136 0.112 0.115 a 0.104 0.101

Kadon 0.219 0.165 0.142. 0.135 0.122 0.115 0.102 0.105 fr

'Plastofidling 0.235 0.146 0.122 0.116 0.112 0.110 0.110 0.102

"

Pf3

Replica 0.228 0.153 0.142 0.131 0.121 0.116 a 0.104 0.106 ^

T exton 0.224 0.167 0.160 0.144 0.127 0.118 - 0.104

"

Vitafilling - - - - - - - 0.103

Average 0.236 0.165 0.146 0.135 0.121 0.117 0.109 0.104

Indentation

i Indentation Strength

psi psi psi psi psi * psi psi psi

Average 4,100 5,900 6,700 7,200 8,000 8,300 8,900 9,300

Indentation

Strength

* Indentations are accurate to approximately 0.005 mm„ Major load

applied for 10 minutes.

Table 4. Recovery of Direct Filling Resins

Age of Specimen

Material 10 min 30 min 1 hr 2 hr 4 hr 6 hr 24 hr 120 hr

% % “T" % % * % *

A-thermoplast * 30 36 45 50 55 55 /o 60

|

V-!0

Dentafil 4 38 46 58 56 64 68 69

Duz-all 21 53 61 61 74 75 81 1 80 (|N

Fastcrown 1 42 49 53 55 64 69 71

Kadon

4 29 41 49 64 63 64 73 79 ( >

Plastofilling 14 45 60 68 73 68 77 76

Pf3 * 7

Replica

V

17 42 49 53 55 64 69 7 71

Texton 27 42 43 50 58 64 - 4 80

Vitafilling - - - - - - - 75

* Too soft to measure.

NOTEs Recovery computed as percent indentation depth recovered after

major load 30 kg was released and minor load 3 kg remained.

CQI ... d r~

t

P* © O

-d I -P 3©

u

. . o

Ef 03

P

H 0) *P p

®i .... 0 03 P 03 42

-rl TJ 6

P 4-

IQ Ci 0) c

Cl ’*vi a o 05 +> £ 0> P3 *H £.< O d fH P W

-r~* •H

CQ •

1 03 mm o 3

o V £0

£-s o 0} P XQ 0)u

r”* p

U r-P O r3 Vi r.~$ a

<y a$

01 d

fj ©

03 "P •a S n a> *H cd »M o £m H 0) P

a-, a! P 10 *H «3 tJ Ti 03 (H d P

O «j

£~ ©

*T +0 O -4 ist +3 r H

•« -P

<—<

ffi P (—

i

03 «S d p CO +3

ta o CO 3 -p p d *> 4 -H <4i P ©

d « oo

>-*-« -$—

o

Ph *H C3 *-4 Vi p

r"i

&0

•rf &4

Fig. 3. Stress-Strain data on six samples of a particular brand of direct

filling resin showing variations caused by unavoidable differences

in the samples.
Fig. 4, Stress-Strain curves for five different brands of direct filling resins.
Fig. 5. Stress-Strain curves for four different brands of direct filling resins.
DIRECT

RESINOUS

FILLING

MATERIALS

TIME —

MINUTES

Fig.

6.

The

variations

of

six

samples

are

shown

when

tested

by

the

Rockwell

Superficial

Hardness

Tester.
DIRECT

RESINOUS

FILLING

MATERIALS

INDENTATION

AND

RECOVERY

TIME

-

MINUTES

Fig.

7.

Indentation

is

plotted

against

time

for

a

typical

indentation

and
DIRECT

RESINOUS

FILLING

MATERIALS

w

IK3c

I isi

g-OJ X SU313WflllW”NOIiVlM3QN!

CO

bi) •H

(it

hardness

increases

with

age

as

shown

by

the

plotting

of

indentation

against

the

age

of

four

different

specimens.
DIRECT

RESINOUS

FILLING

MATERIALS

g-OI X Sd313WmiW-N0llVXN3QNI

m

m

©Z

8 ill 1

9

bL

•H

Hardness

increases

with

age

as

shown

by

plotting

indentation

against

the

age

of

three

different

specimens.
MONOMER

CONTENT

AND

FLOW

cp i0 «$* ro 04

JME-HOURS

Fig.

10.

Relettionship

between

monomer

content

an

<j

flow.
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3, 1901, as amended in Congress in Public Law 619, 1930. J liese include the development and

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