f * — /

Cj^<> . 0.

NATIONAL BUREAU OF STANDARDS REPORT

3495

REPORT ON DENTAL RESEARCH

AT TFS NATIONAL BUREAU CF STAWARDS

Frogress Report

January 1 to June 30, 1954

Dental Research Laboratory

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

cover of this report.

Electricity. Resistance and Reactance Measurements. Electrical Instruments. Magnetic

Measurements. Electrochemistry.

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

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenic Physics. 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-Ray. Betatron. Nucleonic Instrumentation. Radiological Equipment. Atomic Energy Commission Radiation Instruments Branch.

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

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry.

Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass

and Scale. Capacity, Density, and Fluid Meters. Combustion Control.

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.

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.

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

Instrumentation. Process Technology.

Radio Propagation. Upper Atmosphere Research. Ionospheric Research. Regular Propagation

Services. Frequency Utilization Research. Tropospheric Propagation Research. High Frequency

Standards. Microwave Standards.

^Office of Basic Instrumentation ^Office of Weights and Measure*.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-10-0707

0708-20-3824

0708-20-3857

July 30, 1954

NBS REPORT

3495

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU CF STANDARDS

Progress Report

January 1 to June 30, 1954

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

.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr

unless permission Iso!

25, D, C, Such permi

cal iy prepared If tha

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

part, is prohibited

idards, Washington

irt has been specifiort for its own use,

/

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

The work of the Dental Research Laboratory of the National Bureau of Standards has been concerned with several materials used in dentistry, with tooth

structures and with the development of equipment. These researches have covered

a rather wide range of tasks. Summaries of results obtained on work now in progress as well as copies of reports issued on completed tasks during the half

year ending June 30, 1954, are given below:

2, REPORTS ISSUED

Effect of Temperature and Molecular Weight on the Water Sorption

of Polymethyl Methacrylate,

Self-Curing Resins for Repairing Dentures: Some Physical Properties and A Specification.

Physical Properties of Plastic Teeth and A Proposed Specification.

PolY ...... Measurement of Strain in-'M&thyl- Methacrylate Caused by Water

Sorption.

3.

WORK IN PROGRESS

3.1 Structure of Human Tooth Enamel and Dentin , Investigation of tooth structures by means of their fluorescence characterrstr±trs was continued. An electronic depolarization fluorometer t^-fee- -used-4» obtaining data on the molecular weight of fluorescing materials was designed and

calibrated. A special photomultiplier tube selected from a group of approximately

300 was found necessary for satisfactory operation at degrees of polarization below one percent because of the excessive dark current in standard tubes. An ammonium dihydrogen phosphate crystal was obtained for use as the electro-optic

shutter of the decay fluorometer. A 10,000 volt power source and hydrogen thyratron tube will be used to actuate the shutter at speeds of the order of a. billionth of a second. Work on assembling and perfecting the technique for applying

these improvements to fluorescence of tooth and other substances is in progress,

3.2 Method for Measuring Contours of Dental

Impressions, Models and Dent-ores

Work has been started on the development of a quick and accurate method of

measuring and defining the irregular contours of a model or impression of the

oral cavity. The need for such a system is very apparent as it can be used for

quantitatively evaluating different technics and materials used to obtain accurate

reproductions of the surfaces of the mouth.

The instrument is in the preliminary stage of development. Surfaces of

laboratory dentures can be measured to a relative accuracy of the order of 0,001

inch in three dimensions using manual recording and sensitive dial indicators.

NBS Report 3350

NBS Report 3375

NBS Report 3379

NBS Report 3380
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3.3 Low Temperature Alloys

The fundamental investigations of alloys containing gallium which may have

properties comparable or superior to dental amalgams ' c been continued.

Determinations of the compressive strengths of various alloys of galliumnickel, gallium-indium-nickel and gallium-copper-tin showed one of the latter

to be the strongest (53,000 lb/in2

). There are indications that the strength

of the Ga-Gu-Sn alloys increases with an increasing tin content up to 3$% tin

with the gallium content constant. Work is in progress on heat treatments to

stabilize the structure of these alloys as indicated by the micro structure . A

setting expansion of approximately 30 microns per centimeter was observed on a Ga-Cu-Sn alloy with 35% gallium content. An alloy containing J\l% copper and

59% tin was found to have a much lower melting range than do the higher copper

content alloys.

3.4 Resins

(a) Polymer^ation of. Re sins ; Work has been continued on the effect of amine

accelerators on the polymerization of methyl methacrylate. Effects of

variations in catalyst concentration on polymerization kinetics were also

investigated. Setting times of monomer-polymer mixtures were measured with

a penetrometer. The relative efficiency of amine accelerators varies somewhat when monomer-polymer mixtures are employed as compared to the polymerization of monomer only.

A preliminary study was made of sulfinic acid derivatives as accelerators

in methyl methacrylate polymerizations, A series of amine salts of p-toluene sulfinic acid, some of which have not been reported in the literature,

have been synthesized. Most salts of primary and secondary amines gave

colorless solids whereas the salts of tertiary amines are oily liquids.

All amine salts of p-toluene sulfinic acids investigated so far were polymerization accelerators. Colorless polymers were obtained on using sulfinic acid or the sulfinic acid salts of aliphatic amines. Sulfinic acid

salts of aromatic amines are more efficient accelerators but form colored

polymers

,

(b) Water Sorption of Acrylic Resins . Sorption rate at 37° G of methyl methacrylate polymer specimens was investigated. The rate was slow with the

exception of specimens used previously in water sorption-desorption studies

at 100°C. It is likely that swelling of the material in boiling water may

not be completely reversible. On desorption, holes or discontinuities remain at the original water sites. On placing such a sample in water at

37°G sorption becomes abnormally large since water can fill these voids

readily.

The glass temperature of specimens of purified methyl methacrylate polymers of varying molecular weight was determined. Above a molecular weight

of around 500. n00 the increase in glass temperature with molecular weight

is insignificant. The maximum value of the glass temperature was 103°C,

The effect on molecular weight of extended exposure of polymer samples

to boiling water was investigated. The intrinsic viscosity of specimens
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stored at room temperature and in boiling water has been measured*

(c) Dental Resins : A survey of the physical and chemical properties of

dental resin cements and resin filling materials was made. Using, to a large extent, test methods based on Federal and American Dental Association

specifications for silicate and zinc phosphate cements and for denture base

resin, data were obtained on consistency, opacity, color stability, setting

time, compressive strength, film thickness, sorption, solubility, temperature rise on polymerization, refractive index of monomer, composition of

inorganic filler, abrasiveness of filler, and arsenic content* Although

the survey is not yet completed, it appears that some of the ten brands of

resins tested will comply with most of the applicable requirements of the

specifications referred to. In general, color stability was not satisfactory, however

,

The method of determining monomer content of dental resins by infra-red absorption examination of the sublimate from frozen polymer-benzene solutions

was further refined. It is believed that the method can be made accurate

to ±0.1$ monomer in polymer specimens.

Investigation of self-curing denture repair resins has been continued and

a specification has been prepared.

3.5 Zinc Cxide-Eugenol Material

The research on the zinc oxide-eugenol reaction has been very productive of

new information on this material widely used in dental treatment and restorative

processes. The work accomplished during the period covered by this report supports

the previous idea that the reaction produces a chelate structure* A better

understanding of this will assist the producers to supply more uniform products

for the dental applications. Molecular weight determinations of the reaction

products are now being attempted.

Due to the solubility characteristics of zinc eugenolate, attempts to determine its molecular weight by boiling point elevation have thus far been unsuccesful. The solubility of zinc eugenolate in twenty-eight solvents has been determined. Zinc eugenolate was found to be soluble in, and to complex with, ammonium

hydroxide and pyridine and to be very slightly soluble in hot chloroform* It is

soluble in hot gamma butyro lactone and acetamide. Cryoscopic molecular weight

determinations may be possible with these solvents.

Zinc guaiacolate was prepared and its infra-red spectrum compared with the

spectra of guaiacol, 8-quinolinate, eugenol, zinc quinolinolate, and zinc eugenolate. This study gave further confirmation to previous suggestions of a chelate

structure for the zinc oxide-eugenol reaction product.

Several known chelating agents, previously untried, were substituted for

eugenol under conditions of clinical use. Salicylaldehyde was found to set in a hard mass when mixed with Hyperfine zinc oxide under these conditions. A quantitative analysis of the reaction mixture is being made.
I I . ' 1 . • • ’ - . r - ' ’ < . , i V
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Chrome-Cobalt Alloys

The mechanical properties of specimens of chromium-cobalt alloys cast by

various Federal dental laboratories and by manufacturers were determined. Results obtained for the five different brands of alloys tested varied as follows:

modulus of elasticity, 25 ,7-30.8xl0^psi; yield strength*^ 49, 000-65, 700 psi; ultimate

tensile strength, 84,700-117,300 psi; elongation, 1. 2-6.0%; Rockwell 30N hardness,

38.1-60.4.

While there was a large variation from alloy to alloy in the values determined , there was also a considerable difference in the measured properties of a single alloy as cast by various sources.

A study of the grain number and structure was also made. Some of the alloys

proved difficult to etch and revealed only the characteristic dentritic structure.

The data obtained are being used in the preparation of a specification.

3

.7

Rotating Dental Cutting Instruments

The study of methods for determining the cutting efficiency of dental burs

and diamond instruments has been active.

High-speed motion picture studies of steel and c arbide burs and diamond

cutting instruments have been continued. The information gained from this method

of study has been further substantiated by standard laboratory procedures based

on weight loss of human enamel and dentin and other test materials. The data obtained indicate that none of the materials such as bakelite cast resin, glass

filled phenolic resin and various types of glass used as substitutes for dentin

and enamel in cutting tests give results conrparable to those obtained when actual

teet'h are

% cut. --There is- some' indication that' the harder test materials used

give an indication of the wear resistance of the cutting instruments while the

softer materials give an indication of the effectiveness of the design of the

instruments. Results obtained on burs are complicated by the very great variations

in the performance of individual burs of the same manufacturer.

It appears that arbitrary test materials will have to be used for evaluating

and specifying test methods in this field. The accumulation of data under controlled conditions will be necessary before definite standards can be recommended.

3.8

Panographic X-ray Investigation

Excellent progress has been made on this project specifically supported by

the U. S. Air Force for the development of methods and equipment for making satisfactory x-rays of the entire dental arch on a single film.

An experimental device was constructed and used to produce x-ray exposures of

human skulls and their dental structures. An x-ray source and appropriate mechanical apparatus and controls were used to determine proper relationships of subject

and film and to accomplish the various motions required to project a satisfactory

image of the dental structures onto a strip of x-ray film. Radiographs of the
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entire dental arch were produced in approximately 30 seconds of exposure.

Conferences were held with Electronics Instrumentation Section, regarding

progress on the project. A survey of dentists in the various Federal Services

was conducted to obtain opinions as to the practical usefulness of radiographs

obtained and general enthusiasm was expressed with reference to the quality and

scope of these radiographs.

Working drawings are being made and preliminary machine work is being accomplished for construction of a pilot model panoramic x-ray machine.

Upon completion of the pilot model x-ray machine, exposures on human subjects

will be made. Details of design on film carrying mechanism, screening of secondary radiation by the use of appropriate grids, etc. will be studied. Pilot models

will be modified as necessary to improve the quality of the radiographs and improve the efficiency of operation. Technics for the employment of the device

in mass dental surveys will be worked out. Plans for production and use of the

machine by the Armed Services will be made.

3.9 Periapical X-ray Film Survey

Exposed X-ray films and questionnaires have been returned by the 70 dentists

cooperating in the X-ray film survey. Data obtained from the questionnaires and

from densitometer measurements on the films are being used by the X-ray Section

of the Bureau in revising Handbook 41- X-ray Protection. This data will also be

available for the preparation of a specification for dental X-ray film for purchase by government agencies who are at present experiencing considerable trouble

with the X-ray film purchased for dental use.

3.10 Motion Pictures

The motion picture, ’’Hazards of Dental Radiography,” which was given a premier showing at the I.A.D.R. meeting in March was awarded second place in the

medical category at the International Exhibition of Cinematographic Art in Venice 9 Italy. Duplicates are ready for distribution. A script on the 1954 picture.

The Specification and Certification Program on Dental Materials,” has been prepared. The five pictures that have been produced ^»ve had a. wide acceptance,

as shown by the following table. ThSs data j^as of February 23, 1954.

Date

Produced Title

No.

Loans

No . Copies

Sold

1948 Silicate Cement 486 102

1949 Dental Amalgam: Failure Caused by Moisture

Contamination 474 92

1950 Denture Resin 219 27

1951 The Casting of Dental Gold Alloys: Thermal

Expansion Technic 168 22

1952 A Method of Mixing Silicate Cement 4

Total 1362 247
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These data apply only to loans made by the National Bureau of Standards.

Loans made by the A.D.A. film library and by purchasers of the films provide

additional audiences.

Materials evaluated for the cooperating Federal Agencies and the American

Dental Association included many items for which specifications are available

and some items which required the development of special test methods. Included were alginate impression materials, amalgam alloys, amalgamators, abrasive points, denture base resins, denture base repair materials, hydrocolloidal

impression materials, investments, mercury and silicate cement.

3.11 Evaluation of Material

For the Director

by
1 4 ' p ' '
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-20-3824 June 10, 1954

NBS REPORT

3350

Progress Report

on

EFFECT OF TEMPERATURE AND MOLECULAR WEIGHT ON

THE WATER SORPTION OF POLYMETHYL METHACRYLATE

by

G. M. Brauer*

W. T. Sweeney**

* Chemist, Dental Research Section, National Bureau of Standards,

** Chief, Dental Research Section, National Bureau of Standards.

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

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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

unless Dermission 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.

EFFECT OF TEMPERATURE AND MOLECULAR WEIGHT ON

THE WATER SORPTION OF POLYMETHYL METHACRYLATE

Abstract

The effect of temperature and molecular weight on the

water sorption of methyl methacrylate polymers has been investigated . Water sorption is independent of temperature from 0° to

60°C o Above the glass temperature, sorption increases markedly

and the polymer tends to sorb water for an indefinite period.

Molecular weight affects greatly the water sorption at elevated

temperatures o It is believed that the increase in water sorption above the glass temperature is caused by the less rigid

linkage in the polymer chains and an expansion of the . structure which allows water to permeate more readily into the

material.

1. INTRODUCTION

Polymers in contact with water or water vapor will sorb greater or smaller

amounts of water depending on the number of polar groups in the molecule [l, 2,1

[3land others 0 The time necessary to reach sorption equilibrium depends on the

thickness, water permeability and diffusion rate of the material. For many polymeric materials, sorption equilibrium is reached only after prolonged exposure.

At the surface, however, equilibrium conditions are reached within a relatively

short time

.

Recently, a number of investigators have dealt with the kinetics of vapor

sorption by polymer films. Kokes, Long and Hoard |jQ and Long, Bagley and Wilkins

[53 reported a pronounced change in the character of the diffusion which takes

place in the region of the glass temperature of the polymer-penetrant mixture.

With both the polar acetone-polyvinyl acetate and the non-polar benzene-polystyrene system, diffusion was found to obey Ficks law only when the system was above

the glass temperature. A similar behavior was observed for the diffusion of gases

through polyvinyl acetate [6] . Richards [7] showed that high molecular weight polythene s are less soluble

and absorb less organic liquid than low molecular weight polythenes. Samples of

the same molecular weight which contain a higher portion of short chain material

are more soluble and swell more in organic liquids.

The effect of temperature and molecular weight on the water sorption of polymethyl methacrylate was studied in the present investigation.
I
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2. EXPERIMENTAL PROCEDURE

(a) Water Sorption of Molded Specimens.

To make water sorption specimens, polymethyl methacrylate powder was placed

in a mold cavity 5 cm in diameter and 0.05 cm in thickness. The material was

molded into discs in a Carver laboratory press at 10,000 lb/in2 and 150 ± 5°C

for 1 hour. The specimens were dried at 70°C to constant weight, placed in

holders and kept in water at the indicated temperature. At various time intervals the discs were transferred to another water bath kept at room temperature o After 10 minutes, the specimens were dried with a clean towel for 20 seconds, dried in air by waving for 20 seconds and weighed within 20 seconds. The

discs were then placed back in the original water bath until constant weight

was obtained,. After the discs had reached sorption equilibrium, they were dried

to constant weight at 70°C or over sulfuric acid at room temperature. Duplicate

measurements were made for ail determinations.

(b) Water Sorption of Poiymethyl Methacrylate Plates.

Specimens of purified polymethyl methacrylate of approximately 0.63 cm

thickness were obtained from the Rohm and Haas Co.* Table 1 shows the values

for the viscosity molecular weight as reported by the manufacturer, approximate

number average molecular weight determined by the method of Caul and Schoonover

[sD , Knoop Hardness and glass temperature.

The samples were dried to constant weight and placed in boiling water.

After various time intervals, samples were dried and weighed by the procedure

described above 0 Desorption measurements were made by placing the specimens

over anhydrous calcium sulfate (Drierite) in a desiccator first at 21 °C and

finally at 70°C o All measurements were made in duplicate. Another series of

sorption measurements of specimens of the same thickness was made at 37°C.

(c) Effect of Molding on Water Sorption.

Specimens of heat-cured polymethyl methacrylate, 6.0 x 6.0 x 0.7 cm in dimension, were prepared. To avoid any temperature rise, several of these samples were

cut under water into strips 0.05 cm thick, 6.0 cm long and 0.7 cm wide. Another

series of specimens was molded to the same thickness in a Carver laboratory press

at 150°C. Water sorption in boiling water was determined as previously described.

(d) Water Vapor Sorption Measurements.

Attempts to measure water vapor sorption with the McBain balance did not

prove satisfactory. Since the water sorption of polymethyl methacrylate at 30°C

is comparatively small (maximum water sorption less than 2%), a large sample was

required which greatly reduced the sensitivity of this procedure. The weighing

bottle and salt solution method reported in detail by Mellon, Korn and Hoover

[9] was finally adopted. Approximately 4 g samples were placed in weighing

bottles (diameter, 7 cm5 height, 3 cm) and dried to constant weight at

7C°C and a pressure of less than 5 mm Hg e Weighing bottles and samples were kept

over various saturated salt solutions in evacuated dessiccators„at 30.0 ± 0.3°C

in an air bath. Weighing bottles were weighed after a few days using the precau-

* The authors wish to thank W. F. Bartoe of the Rohm and Haas Co. for supplying the samples.

- 3 -

tions given by Mellon et al CO • Weighings at each humidity were repeated until

the change in weight was less than ± 2 mg. The samples were kept a minimum of

10 days at each humidity. Samples were started at the lowest and highest humidities

and on reaching equilibrium were moved to the next higher or lower humidity respectively,, This was continued until the maximum (or minimum) humidity had been reached.

After completion of the last sorption measurement, the samples were dried again by

the procedure described above to check the original weight. An empty weighing

bottle was used as tare for all weighings. This bottle accompanied the bottles containing the samples through all manipulations. Water vapor sorption measurements

by this procedure are considered precise within 0.05$.

(e) Hydrolysis of Polymethyl Methacrylate.

A 100 g sample of polymethyl methacrylate (duPont HG1 powder) and 300 ml water

were placed into a 500 ml round bottom flask. The flask was connected through a ground glass joint to a 35 cm drip tip Friedrichs condenser. To avoid loss of

volatile constituents , the condenser was connected to a mercury seal. The water

was heated to boiling. Continuous agitation was accomplished by means of a magnetic

stirrer. At various time intervals, 1 ml portions of the supernatant liquid were

removed for analysis. Methanol content was determined by mass spectrometry. The

minimum concentration which can be detected by this procedure is less than 0.02

mole percent. The pH of the solution was determined with a Beckman pH meter.

(f) Determination of the Glass Temperatures (Tg).

Glass temperatures (Tg) were determined refractometrically [l0,ll]] using a Zeiss-Abbe refractometer , Specimens of the molded material were placed directly

between the prisms. Strips were cut from the polymethyl methacrylate plates under

water and then placed in the refractometer. Films from the polymer powders were

cast from acetone solutions on the lower prism of the refractometer . The films

were carefully dried at room temperature for at least 24 hours, and at 110°C for l/2

hour.

Refractive index measurements were made by slowly heating the refractometer

to 110° C, a temperature well above Tg, and lowering the temperature to obtain

subsequent readings. Emphasis was placed on relative and not absolute accuracy

of the np readings. Tg values were determined with an accuracy of better than

± 2°C by locating the intersection of the linear portions of the temperature

versus np plot.

(g)

Molecular Weight Determinations.

Viscosity average molecular weights were determined by measuring the intrinsic viscosity [V|^J of chloroform solutions of the polymers at 25°G in a calibrated

Ostwald-Fenske viscometer. Molecular weight was calculated using the relationship

I’ll = 2.52 x 10-3 Mv

0 - 80

£

12 ]

. Viscosity average molecular weights reported for the polymer plates are those

given by the manufacturer.

Approximate number average molecular weights were determined by the method of

Gaul and Schoonover . All determinations were made in duplicate.

3. RESULTS AND DISCUSSION

Effect of

discs (duPont

within 30 days

temperature on the water sorption of molded polymethyl methacrylate

IG 4 fine) is shown in Figure 1. In the 2° to 50°C range, sorntion

•j q ^n^ant an^ L independent of the temperature. The samples reached constant weight

It has been reported £l3j that no further change in weight takes

place even after storage for an additional 5 years. At 70°C, in the neighborhood

of T„, water sorption starts to increase. Two samples stored at 100° C (Table 2)

continued to gain weight after immersion in water for about 200 days. During this

time period, the increase in weight amounted to approximately 10$ (Figure l). This

is nearly 7 times as large as the equilibrium water sorption value at 37°C (1.48$).

The difference in water sorbed by the two specimens stored at 100° C may be caused by

variations in molding conditions.

Polymer discs kept in the boiling water became cloudy. This indicates that

a marked internal inhomogeneity is present giving rise to refractive index gradients, and that presumably water is localized as submicroscopic droplets in the

neighborhood of polar groups [l£-l 5j . On redrying to constant weight, samples immersed at 2° and 37°C showed an insignificant weight loss (less than 0.1$ of the original weight); those stored at

50° and 70°C decreased in weight by less than 0.25$. Samples stored at 100°C gave

a 2.5$ loss in weight. This is probably caused by the increased solubility of low

molecular weight products and plasticizer at elevated temperatures. As can be seen

from Figure 2, molded samples have a much increased water sorption compared to the

unmolded specimens. Apparently degradation takes place during the molding process.

The larger concentration of monomer and low molecular weight material in the molded

specimen will result in an increased sorption. Further evidence of the formation

of water soluble compounds during molding was the increased weight loss on drying.

cUULd to

The formation of polar hydrolysis products such as methacrylic acid, even in

small concentrations, will greatly increase sorption. Hydrolysis of polymer in

boiling water was therefore investigated. Mass spectrometer analysis showed the

absence of methanol in samples removed from water-polymer powder mixtures after

stirring for 3 days at room temperature. When the polymer was kept for 4 days

at 100°C, 0.4$ yield of methanol (based on the theoretical yield of methanol formed

from polymer) was obtained. The water solution was acidic (pH=3) and gave a positive Bayer test for unsaturation. Prolonged (30 days) exposure to boiling water

did not give an appreciable increase in the concentration of the hydrolysis products

formed. This concentration of low molecular weight polar compounds formed is not

sufficient to be considered the primary cause for the increased water sorption.

Mass spectrometer analysis of the final fraction obtained on distillation of the

aqueous solution of the hydrolysis products indicated the presence of methacrylic

and possibly pyruvic acid. The presence of the latter compound would suggest the

formation of an unstable hydroperoxide as an intermediate in the degradation mechanism.

Sorption at 100°C of varying molecular weight samples 0.63 cm in thickness is

shown in Figure 3 and Table 3. The properties of these samples are shown in Table 1,

Lowering of the molecular weight resulted in increased water sorption. The sorption

curves are made up of two distinct parts. After a rapid rise, the curves become

nearly linear. Sorption values are most sensitive to variations in molecular weight

in the lowest molecular weight range. Differences in weight gain of specimens of

molecular weight 393,000 and 1,615,000 are small. Sorption is not significantly

altered by changes in molecular weight above a limiting value of about 500,000. The

logarithmic plot of both water sorption and time gives a nearly linear curve for

the sample of molecular weight 1,615,000 only (Figure 4).

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Placed over anhydrous calcium sulfate, the specimens (thickness 0.63 cm) lose

the sorbed water (Figure 3). Loss in weight after complete desorption is caused

by soluble constituents . It is small and, after drying for 206 days, amounts to a minimum of 0.02$ and a maximum of 0.21$, respectively, for the 1,615,000 and

45.000 molecular weight materials .

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Thin samples (thickness 0.1 cm) of the same materials, when placed in water

at 37°0, reached constant weight in less than 2 months. The sorption amounted to

approximately 2.5$. The water sorption rate of samples of 0.63 cm thickness was

slow. However, samples (thickness 0.63 cm) used previously in the water-sorptiondesorption studies at 100°C, when stored at 37°C, had an increased sorption rate.

After 520 days’ exposure, the lowest molecular weight sample showed a gain in

weight of 12.4$ as compared to 4.6$ for material of the highest degree of polymerization (Figure 5). Plotting the logarithms of time and water sorption gives

a straight line only for the sample of lowest molecular weight. Failure to reach

equilibrium conditions, dependence on molecular weight, and the increased water

sorption rate of these thick specimens may be caused by voids. Swelling of the

sample in boiling water may not be completely reversible. On desorption, the

water diffuses to the surface and evaporates. Holes or discontinuities remain

at the original water sites. On placing such a sample in water at 37°C, sorption

becomes abnormally large since water can fill these voids readily. larger sorption for the lowest molecular weight materials can be expected since more voids

remained after desorption.

Water sorption measurements, in the 4° to 70°C range, of discs (thickness

0.05 cm) molded from f ractionated polymer are reported in Table 4. The substantial lowering of the viscosity molecular weight indicates that degradation takes

place during molding. Constant weight or water sorption equilibrium was obtained

in all determinations. Sorption is independent of molecular weight in the 4° to

37°C range. Between 60° and 70°C, in the vicinity of the glass temperature, sorption values rise with a smaller increase at higher molecular weights. The small

difference between the water sorption values of the specimens of approximately

180.000 molecular weight and those of approximately 390,000 molecular weight is

not consistant with the previous results. The lowest molecular weight specimens

had an abnormally high glass temperature. In the absence of detailed data on

polymerization methods and history of these samples* it is difficult to clarify

this discrepancy.

On raising the water temperature above 70° C, the high molecular weight specimens decomposed into granules similar to those from which they were molded (Figure 6). The fractionated high molecular weight polymer does not contain enough

low molecular weight constituents which can act as plasticizers during the molding process. Although the discs appear homogeneous on visual inspection, the gran*

ules are imperfectly cemented together. On heating the discs in water above the

glass temperature, the adhesive forces at the grain boundaries weaken to such an

extent that the discs fall apart. This phenomenon may also be explained by means

of viscoelastic behavior.

Results of the studies of the sorption of water vapor by methyl methacrylate

polymer at various relative humidities at 30°C are given in Table 5. For most

samples, water uptake is nearly linear from 10 to 50$ relative humidity. In this

range, water probably dissolves in the resin; the quantity dissolved is proportional to the relative humidity in accordance with Henry’s law. Above 80$ relative humidity, there is generally a more rapid increase in the amount of water

sorbed. This upswing may reflect condensation of water on active sites. Within

experimental error, vapor sorption appears to be independent of molecular weight.
-

,
- 6 -

The equilibrium desorption values are slightly higher than the corresponding sorption values in the 10 to 50$ relative humidity range. These variations are not considered significant. From the limited data available, it is not possible to state if

vapor exposure at 100$ relative humidity and liquid i.jmersion sorption procedures

give identical results. Investigations by Kovacs p.6J show that the water immersion procedure gives about 3$ higher weight increase values.

The rapid rise of the water sorption in the neighborhood of Tg of the samples

immersed in water may be explained by an increase in the mobility of the polymer

segments. Above Tg, the chains have a greater degree of freedom. The chain network is expanded as compared to its normal arrangement below the glass temperature.

This allows the water molecules to permeate more easily between the polymer chains.

A larger quantity of water is sorbed. Swelling accompanying the sorption will result in an increase in the number of sorptive sites for the attachment of water

molecules. Equilibrium saturation is not fixed but increases due to the continuous swelling process. Water sorption equilibrium above Tg, if at all obtainable,

will be reached only after many years exposure.

The larger water sorption and diffusion rate of low molecular weight specimens^ especially above Tg,may be caused by increased solubility of water in the

polymer. The larger internal surface of the low molecular weight polymer will

result in an increase in the number of sorptive points exposed within the structure. The probability of attachment of water molecules between ends of polymer

chains is increased. Some form of structural discontinuity is also more likely

to be encountered. Thus, if capillary condensation is involved in the sorption

process, a higher water sorption value will be obtained. The amount of liquid

sorbed is also affected by molecular weight distribution. The presence of monomer

or very short chain polymer is likely to affect greatly the sorption behavior.

4. SUMMARY

The weight of water sorbed by a series of methyl methacrylate polymers has

been studied as a function of temperature and molecular weight. Belov/ the glass

temperature Tg, sorption is nearly independent of temperature. Thin specimens

reach an equilibrium value within a short time. In the vicinity of Tg,the water

sorption value increases rapidly. The polymers show large weight increases on

exposure to water above the glass temperature. The polymer tends to sorb water

for an indefinite period. Sorption equilibrium above Tg was not obtained in

this study. Such values, if experimentally obtainable, will only be reached after

extraordinarily long periods if at all.

Sorption is markedly affected by molecular weight. Samples of larger molecular weight show smaller weight increases. Sorption of the polymers is reversible.

Degradation takes place during molding at 150°C. Slight hydrolysis occurs on exposure to boiling water.

- 7 - REFERENCES

1. Kline, G # M., Martin, A. R. and Krouse, W. A., A.S.T.M. Proceedings

40, 1273 (1940).

2. Baker, D. A., Select. Gov't. Research Rep'ts. (Gt. Britain), Vol. 1,

Plastics, Rep't. Ho. 10, 251 (1952).

3. Sweeney, W. T., Brauer, G. M. and Schoonover, I. C., J. Dent. Research

(in press)

.

4. Kokes, R. J., Long, F. A. and Hoard, J. L., J, Chem. Physics

20, 1711 (1952).

5. Long, F. A., Bagley, E. and Wilkens, J., Abstracts 124th Meeting,

American Chemical Society, Chicago, 111. Sept. 1953, p. 7S.

6. Mears, Patrick, ibid, p. 6S.

7. Richards, R. B., Trans Faraday Soc. £2, 10 (1946).

8. Caul, H. J. and Schoonover, I. C., J. Am. Dent. Assn. 22* 1 (1949)*

9. Mellon, E. F.. Korn, A. H. and Hoover, S, R., J. Am. Chem. Soc.

62, 827 (1947).

10. Wiley, R. H. and Brauer, G. M., J. Polym. Sc. Sc. 2, 455 (1948).

11. Ibid 2, 647 (1948).

12. Baysal , B. and Tobolsky, A. V., J. Polym. Sc. % 171 (1952).

13. Vernonite Workbench 7, (No. 9) p. 2 Sept. 1948, Vernon-Benshoff Co.,

Pittsburgh, Pa.

14. McLaren, A. D. and Rowen, J. W., J. Polym. Sc. 7, 289 (1951).

15. Hauser, P. M. and McLaren, A. D., Ind. Eng. Chem. 40> 112 (1948).

16. Kovacs, A., J. Chim. Phys. 258 (1948).
I

rj .
- 8 -

Table 1

Properties of Folymethyl Methacrylate J2asgaljs€

Average

Sample

No,

Viscosity

Molecular

Weight

No. Avg.

Molecular

Weight

Knoop

Hardness

Glass

Temp.Tg

1 45,000 2000 17.7 79

2 64,000 2900 18.6 86

3 123,000 3500 20.4 91

4A 393,000 4500 21.3 100

5A 1,615,000 5500 21.8 103

Table 2.

Water Sorption of Polymethyl Methacrylate

Material

Temperature

Thickness of discs

gSample 1

HC-4 Fine ^

100° C

0,04 cm

U. L, w

Sample 2

Days in wt. Days in wt.

H20 Increase H20 Increase

% %

3 3.0 4 3.0

27 5.6 32 6.9

73 6.4 61 S.5

102 7.2 99 9.2

213 9.0 172 11.7
I
- 9 -

Table 3

Effect of Molecular Weight on the Water

Sorption of Polymethyl Methacrylate

Thickness of samples =0.63 cm

Temperature = 100°

C

Weight Increased

Time m.w.2 M.W. _ M.W. M.W. M.W

Days 45,000 64,000 123,000 393,000 1,615,<

% % % % %

1 2.25 2.17 2.16 2.11 2.00

2 2.73 2.65 2.61 2.55 2.48

4 3.34 3.10 2.99 2.88 2.81

10 4.56 3.92 3.55 3.25 3.10

15 5.48 4.48 3.94 3.43 3.27

25 6.86 5.43 4.53 3.68 3.47

63 11.90 8.85 6.38 4.61 4.17

108 17.69 12.4

2

8.48 5.40 4.56

1 Average values for 2 specimens,

were less than 4$.

2 M.W. = Molecular weight.

Variations from average

- 10 -

Table 4

Effect of Molecular Weight and Temperature on

the Water Sorption of Polymethyl Methacrylate^

Thickness of molded discs = 0.05 cm.

Molecular

Weight

Glass Temperature, °C

Water Sorption in P

Temperature,

ercent

°C

at

Original Molded

Sample s Sample s Original Molded

Sample Sample

4 30 37 60 70

195,000 180,000 77 93 2.07 1.94 2.00 2.18 2.80

195,000 187,000 77 75 2.05 1.98 2.01 2.22 2.61

600,000 380,000 62 55 2.04 1.94 1.99 2.12 2.77

600,000 398,000 62 59 2.07 1.94 2.00 2.18 2.80

1,560,000 824,000 68 68 2.05 1.97 1.97 2.11 2.35

1,560,000 — — — 2.06 1.95 1.92 2.05 2.26

a Specimens were obtained from H. D. Justi and Son, Inc.

- 11 -

Table 5

Sorption of Water Vapor by Polymethyl Methacrylate

Temperature = 30*0 ± 0.3°C

Weight Increase, %

Saturated Relative B04 Fine Fractionated Polymer

Salt Humidity M.W. = 195,000 M.W. = 1,560,000

Solution % Sorption Desorption Sorption Desorption Sorption Desorption

Li Cl 11.8

MgCl2 31.4

Mg ( NO3 ) 2 50.9

NaCl 75.1

KC1 83.6

KNO3

0.04 0.11

0.27 0.35

0.57 0.64

1.04 0.97

1.18 1.13

0.09 0.12

0.35 0.42

0.53 0.74

1.13 1.04

1.32 1.31

1.56

0.06 0.12

0.32 0.44

0.54 0.70

1.16 1.04

1.35 1.29

93.3 1.33 1.49 1.83 1.67 1.62


Figure 1. Temperature versus water sorption of

polymethyl methacrylate.
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T

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TEMPERATURE VS WATER SORPTION

OF POLYMETHYL METHACRYLATE

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40 60

TEMPERATURE-# C

0 20 00 100


Figure 2. Effect of molding on water sorption of

polymethyl methacrylate

.
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TIME-DAYS

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Figure 3. Water sorption and desorption by polymethyl

methacrylate of varying molecular weight

at 100°C.
20

18

16

14

12

10

8

6

4

2

t r "i r WATER SORPTION AND DESORPTION BY POLYMETHYL

METHACRYLATE OF VARYING MOLECULAR WEIGHT

TEMPER ATURE= 100° C

DESICCANT ANHYDROUS CaS0 4

100 125 150 175

TIME-DAYS

200 225 250 275


Figure 4* Water sorption of polymethyl methacrylate of

varying molecular weight at 100°C.
o

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Figure 5. Water sorption of polymethyl methacrylate

of varying molecular weight at 37°C.
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001


Figure 6. Effect of immersion in hot water on molded

discs.

Front: Disc after storage in water above 70°C.

Rear : Freshly molded disc.
II

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REP0RT

0708-20-3824 June 22, 1954 3375

SELF-CURING RESINS FOR REPAIRING DENTURES:

SOME PHYSICAL PROPERTIES AND A SPECIFICATION

John W. Stanford, B. S.-xClaire Burns #

George C. Paffenbarger , D.D.S. •Jett Research Associate, American Dental Association, National

Bureau of Standards.

# Scientific Aid, Dental Research Section, National Bureau

of Standards.

This work is a part of the Dental Research Program conducted at the

National Bureau of Standards in cooperation with the American

Dental Association, the Army Dental Corps, the Air Force Dental

Service, the Navy Dental Corps and the Veterans Administration.

<NBS>

u. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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

unless permission Is obtained in wrltlngfromtheOfficeoftheDIrector, 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.
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.
SELF-CURING RESINS FOR REPAIRING DENTURES:

SOME PHYSICAL PROPERTIES AND A SPECIFICATION

ABSTRACT

A comparison of some physical properties of self-curing

resins and heat-curing resins, used in repairing artificial

dentures, when tested as outlined in American Dental Association

Specification No. 12, for Denture Base Resins, shows: (1) the

self-curing resins attain a working plasticity in from 1/l\.

to l/5 the time of the heat-curing type; (2) there is no

important difference in water sorption and solubility; (3) the

self-curing resins are less color stable, have less strength, and

are less stiff; (4) repairs effected with the self-curing resins

had approximately 59 percent of the original strength of the

resinj similar repairs with the heat-curing resins had 82 percent;

(5) dentures repaired by the self-curing resins warped the least*

The greater dimensional accuracy of the self-curing resins is

believed to be preferable to the higher strength of the heatcuring materials for repairing dentures.
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SELF-CURING RESINS FOR REPAIRING DENTURES:

SOME PHYSICAL PROPERTIES AND A SPECIFICATION

1. INTRODUCTION

One of the first dental uses of the new self-curing resins

,

based on methyl methacrylate, was as a repair material for artificial

dentures. However, little data on their physical properties have

been published. This paper not only presents data on some physical

properties of the self-curing repair resins but also compares the

strength of repairs effected and the dimensional change during

repair of artificial dentures when the self-curing repair resins

are used with the strength and dimensional changes of repairs

accomplished with the heat-curing resins. Because self-curing

repair resins are being used, a tentative specification is also

offered.

The self-curing resins appear to offer the following advantages over the heat-curing type for use in repairing dentures:

(1) the repair can be made quickly with the self-curing resins,

while the patient waits. In the conventional methods of repairing

a denture with the heat-curing resins, the patient may be without

the denture for many hours; (2) no heating equipment is needed to

process the self-curing resins. On the other hand, controlled

heaters are necessary to maintain the temperature and time of

processing properly the conventional repair; (3) since no external

heat need be applied to polymerize the self-curing resins, the

denture is not warped. When heat-curing resins are used in the

customary repair process, and if the repair is made in boiling

water, warpage invariably results as the high expanding resin
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is confined in the low expanding plaster of Paris mold. The resin

is thereby strained. The strain is partially released when the

repaired denture is removed from the gypsum model. This is evidenced by a change in dimension as the denture will not reseat

properly on the model. Data and observations are presented concerning this point in a later portion of this paper.

2. PHYSICAL PROPERTIES

Materials investigated . For the purpose of determining some

of the physical properties of the self-curing resins, the data

presented were obtained by the tests described for Type II (selfcuring) resins in American Dental Association Specification No. 12

for Denture Base Resin. ^ Five brands of self-curing resins and

three brands of certified heat-curing resins (Table 1) were

investigated. Other brands are available, but these were felt to

be representative of each type.

Mixing and processing technics . The directions of the

respective manufacturers of the brands used had recommended a

variety of processing technics. All specimens made for determining values for the physical properties of the resins were prepared

by compression molding as described by the manufacturer. The

heating schedule was 1-g- hours at 73°C (l63*il0 F) followed by -J- hour

at 100°C (212°F) for the heat-curing resins. The flasks were then

cooled in air for 30 minutes, followed by immersion in water at

23 ± 10®C (73 ± l8°F) for 1$ minutes.
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- -3 - Plasticity , The rate of polymerization of the self-curing

resins governs the available time for trial packing at room

temperature. The working time is controlled by the changes in

the plasticity of the resin. The proper plasticity for starting

the packing was arbitarily taken as the time when the doughy

material separated cleanly from the sides of the mixing jar.

The packing test employed was that described in Specification

No. 12^. The time elapsed between start of mix until it would

separate cleanly from the jar varied from 3.5 to 4.5 minutes

for the five brands of self-curing resins investigated. The

three certified heat-curing resins reached this plasticity in

18 minutes on the average. In accordance with the specification,

the packing test was not started until 5 minutes after this

proper plasticity was reached. All brands investigated were

satisfactory.

Porosity . The porosity test as outlined in the specification consists of a visual inspection of the cured resin.

Examination of the repair resins showed that Biofast, Duz-All,

Fastcure, and Nuweld were not porous. Nu-Densoform had small

a bubbles or voids. None of the heat-curing resins were porous.

Water sorption and solubility . All of the resins tested

met the requirements of the specification for water sorption

and solubility (Table 2). There was no important difference

in solubility or water sorption among the brands of each type.
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- Color stability . On the basis of the existing specification

requirement for color stability, three of the five brands of selfcuring resin (Biofast, Nu-Densoform, Duz-All) failed definitely,

assuming a brownish or yellowish tinge; one (Fastcure) discolored

so very slightly that the change in color was barely perceptible;

the remaining one (Nuweld) complied easily with the requirement,

that is, no change in color was perceptible. Figure 1 shows the

specimens two months after exposure. The heat-curing resins were

color stable. This test is qualitative and relies on a visual

inspection of exposed and unexposed portions of the specimens.

Transverse tests . Data on the strength and stiffness of

resins in transverse bending are given in Table The specification values for deflection at loads of 3*500 g (7.7 lb ) and

5*000 g (11.0 lb) for denture base resins of both self-curing

and heat-curing types are' given in the table also. In addition,

graphs showing the deflection of the resins with respect to load

applied are shown in Figure 2. The data (Table 3 and Fig. 2)

show that none of the brands of self-curing repair resins

complied with the requirements of the specification for a Type II

self-curing denture base resin. Failure of all self-curing

repair resins occurred at or before the 5*000 g (11.0 lb) load;

therefore, no deflection readings could be obtained for this

load (Table 3 )*
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- 5 -

3. STRENGTH AND STIFFNESS OF REPAIRS MADE

WITH SELF-CURING AND HEAT-CURING RESINS

After comparing the strength and stiffness (in transverse

bending) of the self-curing repair resins with heat-curing

resins, it was decided to investigate the strength of the bond

made to the denture base resin by the two types. The method

chosen was based on the transverse test in the specification.

Plates of each of the clear resins of the three heat-curing

brands (Table 1) were made in a gypsum mold as shown in Figure 3*

A J-inch section was cut from the center of these plates. The

two pieces were replaced in the mold and the cut-out section was

filled with pink repair resin (Fig. l\.) . This process resulted

in a butt joint which is not generally used in repairing

dentures; however, it gave the most consistent results among the

various types of joints tested.

To determine the effect of the time of packing on the

strength of the repair, the self-curing repair resins were

packed at three different stages of plasticity. These stages

were dependent upon the elapsed time from the beginning of the

mix to completion of the packing of the repair. The heat-curing

resins were packed when they pulled free from the sides of the

mixing jar. Five specimens were machined from each repaired

plate. A typical specimen is shown at the bottom of Figure Ij.. After the specimens were tested they were examined to determine

the type or condition of break. The two typical breaks obtained

are shown in Figure 5>. The specimen at the top was repaired with
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- 6 -

# The fracture in this case, as in the case of

all the heat-cured repairs, was through the center of the

repaired area where the stress was applied* However, the

specimens repaired with the self-curing resin shown at the

bottom of Figure 5 broke at the bond without exception* There

was a definite bonding between the repaired area and the

original block in the case of the heat-cured repair, whereas

an inferior bond occurred with the self-cured repair* These

results are summarized in Table 1+. Table 5 presents sufficient

data on the self-curing repair resins to permit a statistical

analysis* In these two tables data are given on the self-curing

repair resins packed at three different plasticities depending

upon the amount of time elapsed from the beginning of the mix

to completion of the packing* These times are designated as

stages 1 , 2 and 3 . The average strength of specimens repaired with the heatcuring resins was approximately ^lj.00 g (Table I4.) * This was

approximately 82 percent of the original strength of the clear

heat-cured resins (6/>00 g) . There was no significant difference

in strength of the repaired specimens among the brands of heatcuring resin used in repairing the blocks* The best average

strength of the self-curing repairs was 59 percent of the

original strength as shown by stage 1 in Table 4* For a statistical analysis of the three stages. Table 5 presents the average

load at failure for each of the lj.5 plates constituting this part

of the investigation. From each block five specimens were
. - J ' . • .. - ' 1 - 'S ' j • \
- 7 - prepared so 225 specimens were tested. The variability among the

observed loads for five specimens prepared from any plate is

characterized by a standard deviation of about 200 g.

There appears to be a significant decrease in strength,

approximately lj.00 g, when the self-curing resin is packed at

stage 2, rather than stage 1 (Table 5 ). However, the evidence is

not conclusive that the further decrease between stages 2 and 3

is significant. The specimens repaired with the self-curing

resins in this experiment appear to fall into two groups based

upon the strength in transverse bending. Specimens repaired with

Biofast and Duz-All represent the group of lower strength, while

Pastcure, Nu-Densoform and Nuweld form the group of slightly

higher strength. This follows in order the strength in transverse bending of the foregoing resins (Table 3 )« The three heatcuring resins may not be equally susceptible to repair. The

strength of the repair made on these resins appears to fall into

two groups based upon strength in transverse bending. However,

the evidence is not conclusive except, possibly, in stage 3

(Table 5 ).

In rea<±drig these conclusions an analysis of variance was made

to evaluate the differences due to the nature of the heat-curing

resin, of the self-curing resin, and of the stage of packing.

It was found that, while these conclusions could be made, within

the limits of precision of the experiment, the effects of these

three factors were independent of each other.
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- 8 - In addition to these tables of results two graphs are presented (Pigs. 6 and 7) to show the stiffness of these repairs by

plotting the deflections in millimeters versus the loads applied

in kilograms. The curves in Figure 6 show that the original

resins were stiffer than the heat-cured repairs which were in

turn stiffer than the self-cured repairs. The end point of the

curves is the average load of failure.

The three curves in Figure 7 depict the average deflections

obtained on specimens repaired with the self-curing resins at

three different stages of plasticity. The differences in loads

required for failure can be seen in this graph. There was not

any significant difference in the stiffness of the repairs made

by the three stages of packing.

1|.. CHANGE IN DIMENSION ON REPAIR

The changes in dimension of eleven upper dentures repaired

by different materials and technics were determined by the

following procedure. Stainless steel pins, having cross marks

upon them, were inserted into the second molars. A gypsum model

was made of the tissue-bearing surface of each denture and the

molar-to-molar dimensions were determined with a toolmaker’s

microscope. Each denture was cut through the center from the

central incisors to the posterior margin by a band saw.

Approximately 1/8 inch was removed from each half. The sections

were then seated on the tinfoiled model and the molar-to-molar

distance was again measured.
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- 9 - In the case of the heat-cured repairs, the process was

carried out in the conventional dental manner of investing and

packing. Three of the dentures were repaired by heating for

1-g- hours at 73 - 1

# C (163*14- ± 1*8°F) followed by hour in boiling

water. Three additional dentures were repaired by heating at 73 - 1°C ( 163 - 1.8°F) for 8 hours. At the completion of the curing

cycles the flasks were bench cooled for 30 minutes in air and

then immersed in water^23 ± 10°C (73 - l8°Fj_/r for 1$ minutes.

The flasks were opened and the molar-to-molar dimensions were

measured before and after removing the repaired denture from the

model

.

Five dentures which had been cut in two and replaced on

their respective models were repaired using the five self-curing

resins. These repairs were effected by using a gypsum matrix

of the lingual side of the unbroken denture. The matrix was

held in place by a C -clamp during the curing process. The selfcuring resins were all packed at the optimum time from beginning

of mix so that they would have their greatest possible strength

(stage 1 in Table }±) . After the repairs had been completed, the

dentures were again measured. The dimensional changes in

dentures repaired by the foregoing procedures are shown in

Table 6. The values are the differences between measurements of

the unbroken and the repaired dentures. The average change for

the dentures repaired with the heat-curing resins, which were

boiled during the curing cycle, was approximately 2-§ times the

change taking place in the dentures repaired with the heat-curing

resins which were heated for 8 hours at 73 - 1°C (163*14- - 1«8°F).
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' ' • "

•

' V; •

- ^ v-

.

.

' * ' M ffl j

vv.v
10

This tends to support the general observation made at the beginning

of this paper that the heating of the flask causes a definite

strain in the high- expanding resin during confinement in the lowexpanding plaster of Paris mold. This strain was possibly

released in the form of dimensional change as shown by the data in

Table 6. The changes in the artificial dentures repaired with the

heat-curing resins were in the minus direction, that is, the

molar-to-molar dimensions were smaller than originally. However,

the changes taking place in dimension in dentures repaired with

the self-curing resins were in the plus direction. Here the final

measurement was greater than the original. If the minus changes

in the dentures repaired with the heat-curing resins held at 73

(163.1|- 0 F) for 8 hours are compared with the plus changes in the

dentures repaired with the self-curing resins, it will be noted

that they appear to be of the same order of magnitude. However,

there is considerable difference in the fit of the repaired

dentures when they are reseated upon the gyps-um models (Pig. 8 „ Invariably the dentures repaired by the self-curing resins fit

the model much better than any dentures repaired with heat-curing

technics and resins. It would appear that the distortion of the

denture T s shape is not necessarily reflected in the molar-tomolar measurements. When the eleven dentures which were repaired

were handed to an observer as unknowns he was able to sort them

correctly in the categories A, B and C (Pigs. 8 and 9. The repaired dentures shown in I’igurek, 8... and. 9 are typical.

- 11 -

5. SPECIFICATIONS

The specification for Type II self-curing denture base resin,

as given in the American Dental Association Specification No. 12

for Denture Base Resin1

, is a suitable specification with certain

exceptions for these self-curing repair resins. So the following

specification is suggested*

AMERICAN DENTAL ASSOCIATION

SPECIFICATION NO. 13 FOR SELF-CURING REPAIR RESIN

1. Scope and Classification . 1.1 Scope. This specification is for self-curing resins

used primarily for repairing denture base resins.

1.2 Types and Classes. Repair resins covered by this

specification shall be of the self-curing type, powder and liquid

class, and of two colors, pink and clear,

2 . Applicable Specifications

2.1 Specification. American Dental Association Specification

No. 12 for Denture Base Resin (Acrylic Resins or Mixtures of Acrylic

and Other Resins) Second Revision1 shall be applicable, as It

applies to Type II self-curing resins with the following exceptions

Paragraph Working Qualities, American Dental

Association Specification No. 12, shall be changed to read:
'

.•j - •• 1

*

.

• . . ^

t -

*' /3

.

'

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•

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,

‘

'

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, -it ' \ . S.vjj

’-

VH

- • '

. >|
12

3. 4*1 Working Qualities* The self-curing repair resin when

processed according to instructions furnished by the manufacturer

shall yield a satisfactory repair.

Paragraph » Transverse Deflection, of American Dental

Association Specification No* 12 shall be changed to read:

3*1}.. 7 Transverse Deflection. The deflection of at least three

of the five specimens prepared as outlined in lj_.3«>5> shall meet the

following requirements when tested in water at 37 - l

tf C (98-6 ±

DEFLECTION

for loads-”- 1. 500 g (3.30 lb) to

Class

2500 g

(5.5o 1b)

lj.000 g

( 8/80 lb)

Max. Max.

mm ram SelfCuring

Repair

Resins

1.5

(0.058 in)

4.5

(0.177 in)

* Deflections shall be measured from an

initial load of 1500 g (3*30 lb) to the

load indicated.

There may be some questions raised as to why the stiffness

and strength of the repair resins in transverse bending was used

in the foregoing specification instead of the same properties on

repaired specimens. There were two main reasons: 1) a comparison

of the data in Tables 3 and 5> shows that generally the strength

of the repaired specimens followed the strength of the self-curing

resins used in the repair; 2) the data in Table 5 indicate that

all denture base resins do not repair equally well and it would be

very awkward and extemely difficult to specify a resin which was

to be repaired and which would serve as an unchanging standard.
.

.

ft • 9

t

r.

'

*

: >.

t . *- « V

• :

• -

' 1 i

. . -

'A t
- 13 -

6. SUMMARY AND CONCLUSIONS

Some physical properties of five self-curing resins advocated

for repair of artificial dentures were measured using the test

methods of American Dental Association Specification No. 12 for

Denture Base Resins 1

(Type II, Self-curing Resins). These properties were compared with those of three heat-curing denture base

resins

.

The working plasticity of the self-curing resins was attained

in from 3i to Ip-J- minutes after beginning the mix, while 18 minutes

were required on the average for the three heat-curing resins.

The self-curing resins maintained a suitable working plasticity

for at least 5 minutes. This is sufficient time to trial pack

properly.

One of the self-curing resins had small bubbles or voids.

None of the heat-curing resins were porous.

There was no important difference in water sorption and

solubility between the self-curing and heat-curing types.

Three out of five of the self -curing resins failed to pass

the color stability test of American Dental Association

Specification No. 12

1

. One discolored so very slightly that the

change in color was barely perceptible. The remaining resin

easily complied with the specification.

All of the self-curing resins were weaker and less stiff

than the heat-curing resins in transverse bending.

- Ik - The average strength in transverse bending of specimens

repaired by heat-curing resin was approximately 82 percent of the

average of the original strength of the denture base resins. If

the repair was made with the self-curing resins, the average

strength was approximately 59 percent when the optimum technic

was used. In general it appeared that the optimum technic was

to pack the self-curing repair resin so that the elapsed time

from the beginning of the mix to the completion of the packing

would be from 2 to 3 minutes.

There was a change of -0.8 percent in the molar- to-molar

dimension of full upper dentures repaired by heat-curing resins

which were heated to boiling during processing. If instead of

boiling the repairs were heated at 73°C (I63.l4- C F) for 8 hours,

the shrinkage was reduced to 0.3 percent. There was an increase

of 0.2 percent in the molar -to-molar distance when the repairs

were made with self-curing resins. While there may appear to be

only an academic difference between a -0.3 &nd a +0.2 percent

change in the molar-to-molar distance, the fit of the dentures

repaired by the self-curing resins when reseated on the original

gypsum models was invariably much better than the fit of the

dentures repaired with the heat-curing resins. It is, therefore,

suggested that the self-curing resins be used even though they

are weaker than the heat-curing resins.
,

' • •

•

.

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•

' ->

‘ -i J “ : t

* &§§. m3

. -

,

.

• .

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.

3 <1 d: i

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- 1$ - It is recommended that the American Dental Association

Specification No. 12 for Denture Base Resin (Second Revision)

as it applies to Type II, Self-curing denture base resins be

made applicable to the self-curing repair resins with the

exceptions noted.

REFERENCE

1. Swaney, Aubrey C., Paffenbarger , George C., Caul, H. J.,

and Sweeney, W. T. American Dental Association

Specification No. 12 for Denture Base Resin (Second

Revision). J.A.D.A. Jan. 1953*
...

.

Figure 1

Appearance of resin disks after right side

has been subjected to color stability test

in A.D.A. Specification No. 12^.
NU-DENSOFORM

yMti

BIOFAST

FASTCURE DUZ ALL

WlffiilSfi

NUWELD
/
. . I 1 m i 1 i I : I
Figure 2

Deflection and strength in transverse bending of self-curing repair

resins and heat-curing resins.
1A

vO

Average of

Three

(Table

1)

Nuweld

Nu-Densof

orm

Fastcure

Duz

-All

Biofast

H CNJ rA_d-\A^O

Heat- curing

Resins

Self- Curing

Repair Resins

O

vO

1A

1A

O

1A

1A

-d"

o

-d

1A

<A

O

CA

1A

C\J o

Aj

tA

rH

o

O H OJ CA -d 1A vO

sj:0Q.0nri:xiTM trc uofq.oeTjea;

Load

in

Kilograms

n
Figure 3

Resin plate and its position in

the flask during processing.


.

*

.

.

.
Figure Ij.

Repaired resin plate and example

of specimen machined from it.
I

CLEAR

N CLEAR K £ \ \ ^


Figure 5

Typical breaks of repaired resin

specimens. The specimen at the top

was repaired with heat-curing resin.

The specimen at the bottom with self

curing resin.



Figure 6

Transverse test on original and

repaired specimens.
<H C\J m 1A vQ

Sti9q.9nrrxiTW ut uot^09IJ9Q

o

Load

in

Kilograms

| 8 .

. v I
Figure 7

Transverse test on specimens

repaired with self-curing

resins

.
o rH C\J _=h

Siisq-QnrTiIIW ut uoTq.o©XJ9a

Load

In

-Kilograms


Figure 8

Change in molar- to-molar dimension during heating.

Posterior view. A, Heat-curing repair for lj§- hrs

at 73°C (l63 # [j.°F) plus ^ hr boiling. B. Heatcuring repair for 8 hrs at 73°C (l63*lj.°F).

C. Self-curing repair for \ hr at room temperature,

20-25°C ( 68-77°F)

.

Areas shown in lower half of figure are magnifications

on the order of 8X.
NBS

.

•

.

’
Figure 9

Change in molar- to -molar dimension during heating.

Left buccal view. A. Heat-curing repair for 1^- hrs

at 73°C ( 163 . Ip

0 F ) plus hr boiling. B. Heat-curing

repair for 8 hrs at 73°C (163*4°F). C. Self-curing

repair for \ hr at room temperature, 20-2£ o C (68-77°B).

Areas shown in lower half of figure are magnifications

on the order of 8X.
till

iii®

ilifcifc

NBS

Resins

Investigated

PI Q

CO CO CO CO CO CO CO

to to to to to to to

<J Eq 0s CO CO CO o CO CO

pq

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M

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tD O p 55 P cd 55 <H § •H o Ph 21

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-P •v 1—1 P H P # cd cd • p cd © O P o O p

-p n O P o H p © • a © © © Q Q © CO Q •p P p

•H H 00 • •H • • i—1

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cd cd a cd p P © © P © • o O o rP o ©

PQ O < o EH PI * Q ap

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cd <1 o © rH H •H

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bO bO

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(A31-A62)

Table 2

Water Sorption and Solubility of Self-Curing

Repair Resins and Heat-Curing Resins

TYPE OP RESIN

WATER

SORPTION

WATER

SOLUBILITY

mg/cra^ mg/cm^

Self-curing 0.J+ 0.03

He at -curing 0.6 0.01

A.D.A. Spec.

max. max.

No. 12 for 0.7 o.ol*

Denture Base

Resin
I
Transverse

Tests

of

Self-curing

Repair

and

Heat-curing

Resins P i 1 ft p O o o o O 1 O o O 1 <J &H p bO O © o o o 1 O o o 1 O O P H IA rA O 1 IA C^v vO 1 P -d -d -d“ -d~ IA" vO~ vO vO“ P 1 1 r • • • • i d * 1 d * p CD p d

bC i 3 £ 1 , &H £ o g p i IAIA O -d O 1 O O o g 1 i i 1 • • • • • • • o 1 rAvO CA CA CA 1 AJ IA

'-A 1 1 O 1 1 EH

bfl 1 1 bC o o g p • 1 1 O o 3 l i —-> 1 -d 1 1 1 1 1 O ia i 1 IA

pH

-d i 1 co i 1 Q b£ i 1 <o O g • co o CO CA 1 1 p O rH • • • • 1 1 1 1 1 o -d -d CA AJ 1 1 ft -t O 1 • 1 •

Cx, • X • * • 1 d 1 d s £ £ o bC 1 1 H -d ft IA AJ IA vA -A -d co

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Pdp

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03 P d d 'd • o © •H p Ph • o ©

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•H d d d d 9 ©

© o ©

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CQ p p £5 s i < co ft o p i <

ftp hh ft

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CAC

(1)

Specimens

broke

before

reading

could

be

taken.

Strength

in

Transverse

Bending

of

Repaired

Resin

Specimens 0o

•H

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<d > o O O. vO CO “LA ft co ft CM P < pd CO' CO CO CO" co" CO" co ' CO' co" CM" co

ft • 55

GO ft g ft •H M 0 o O o O o O O O o O O O O

00 O S bO o O o O o O O o o O o O o 0 &q d TA r—1 o CM ft CO q CO CO CM. 00. K ft ft •Pd' Pt Pd -Pf co"' cOT -=f CO CO" CM' CO CO

CO

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55

ft Pd- Pd' pd* pd CO" CO" co CO pd' CO" CM' CO cO

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Id ft 0 0 Ph CO 3

•H s O

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1A go ft • P 55 © 0 H H

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Eh 0 1 ts) o o o o o o o o O o O O o

ft ft d rH_ o H [ft Pd o 0\ rH 00 ov CO o CO

CO ft ft pd* CO CO' CO CO" CO" CM cO CO" CM CM' CO cO

ft ft CO • o

ft -p bO ft 0Q P H cd bO o o o O o o O O O O O O o

•H ft ft o o o O o o O O O o O o o

Tf o C"- co “LA c-v 1A 1A CM “LA pd rH CO CO £ ft cO CO" CO" cO CO co" CM" cO CO" CO' CO' cO cO 0 ft PQ • 0 1 o O O

ra •H ft •H •H

Ph CO o pH rH rH 0 p 0 l>5 > 1 ft ft Ph Ph Ph

oo •P CO CO CO ) O O ) C P cd 0 0 ft <3 ft <d ft <! co

cd 0 ft Ph -p

©

© 0 © 0 © 0

Ph ft o cd -P ft

©© ft •H 0 0 ft •H 0 0 © bD

EH bOft (H ft ft Ph bC •H ft Ph bC •H ft Ph bO bD cd

Ph p ft i—1

•H 3 cd rH ft d cd rH •H 3 cd cd ft p cd ft 03 cd ft -p Ph cd ft ft Ph cd ft ft Ph Ph to •H 0 Ph 0 £ Ph Ph a 0 Ph Ph £ 0 Ph Ph a 0 0

ft d do 0 > O 0 © > O 0 0 > O 0 0 > > ft rP o o ft e < O ft <d O ft <d <! O P <d o

-PbD • £ 0 0 f>s 0

Ph -P CO

-P •pj CO

Plastic Stages

Table

l\.

rH CM CO

CM SoI

Dimensional

Changes

Occurring

in

Artificial

Dentures

During

Repair.

(Degree

of

variation

is

expressed

as

average

deviation.)

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REpORT

0708-20-3824 June 30, 1954 3379

PHYSICAL PROPERTIES OF PLASTIC TEETH

AND A PROPOSED SPECIFICATION

by

W . T . Sweeney

E. L. Yost^

V/. D. SheehanJ. G. Fee^'

1 Chief, Dental Research Section, National Bureau of Standards

2 Guest Worker, U. S. Army, Dental Research Section, National

Bureau of Standards

3 Guest Worker, U . S. Army, Dental Research Section, National

Bureau of Standards

4 J. G. Fee, Physicist, Dental Research Section, National

Bureau of Standards

This work is a part of the Dental Research Program conducted at the National

Bureau of Standards in cooperation with the American Dental Association, the

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

the Veterans Administration.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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

unless permission is obtained in writingfrom 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,

PHYSICAL PROPERTIES OF PLASTIC TEETH

AND A PROPOSED SPECIFICATION

Abstract

The development and extensive use of plastic teeth in recent years has caused the dental profession to desire technical

information on their physical properties. The results on eleven

brands of teeth are reported. Modulus of elasticity, hardness,

stress-strain relationship, crazing, abrasion and recovery data

are included. New methods of tests were developed and procedures

used to evaluate these properties are given. A specification for

plastic teeth is proposed.

1. INTRODUCTION

Plastic teeth have been used by the dental profession for approximately ten

years. During this period, considerable clinical evaluation has occurred. Although dentists are not in total agreement as to the relative value of plastic

as compared to porcelain teeth, the continued widespread sale of plastic teeth

indicates that the dental profession has found them useful in restorative dentistry.

A recent survey of prosthestic specialists indicates that over 90% of them

have used or are using plastic teeth. One-tenth of these men use plastic teeth

exclusively whereas others use them in varying degrees. The majority of the

dentists answering this questionnaire are users of plastic teeth in particular

situations where very short teeth are required.

The replies of this group indicate that plastic teeth are deficient in

some physical properties and that other physical properties are satisfactory and

desirable for dental use. It is also evident that different commercial brands

of plastic teeth vary widely in some properties. Since differences have been

noted, it is necessary to establish laboratory tests and data for the evaluation

of plastic teeth. This rerort includes data obtained on several brands of plasti

teeth.

2. PROPERTIES OF DENTAL SIGNIFICANCE

Plastics have many properties which make them attractive materials for making teeth. The resins which have been most used are methyl methacrylate and copolymers in which it is a major constituent. The chemical properties are very

satisfactory. Methyl methacrylate polymers are non-poisonous, are insoluble in

the fluids found in the oral cavity and the monomer is sufficiently chemically

reactive to other methacrylates under heat and pressure to form a chemical bond.

When mixtures of methacrylate polymer and monomer are subjected to heat and

pressure in processing dentures a chemical union results. The reaction of the

monomer in the uncured denture base with the plastic teeth also forms a chemical bond between the denture base and the tooth. Methyl methacrylate is soluble
•

;

'

'

.

•

: "

\

;

-

’

'

'

’ ? «r, : -

. wp $i y-ii p*

^!

. v ; : ... r^is*vw,»

.

.

.

. -

. ... •:

- ...
- 2 -

more or less in most organic solvents such as ketones, aromatic hydrocarbons,

etc. and it is, therefore, important that the denture not be cleansed in an

organic solvent.

In general, mechanical properties of these resins are low in comparison

with other dental restorative materials such as amalgam, gold alloy, etc.

The low mechanical properties, strength, abrasion resistance, modulus of elasticity and hardness, however, have some advantages. The modulus of elasticity

being lower' than that of porcelain contributes to the elimination of the "clicking” sound when teeth come in contact with each other. The low hardness and abrasion resistance facilitate the adjustment of the occlusion. Also plastic

teeth have a high degree of toughness, therefore, less shattering or breaking

occurs with plastic teeth. Plastic teeth have a very desirable esthetic appearance. The optical properties of these teeth make it possible to match shades

and colors with those of natural teeth.

A serious drawback of some plastic teeth is crazing. The mechanism of

crazing is not completely unders+ood . It is thought to be caused by the relieving of localized strain. Crazing is commonly noticed after repair or rebasing of a denture. Many of the currently manufactured teeth are so formulated that they do not exhibit crazing.

3. MATERIALS INVESTIGATED

The trade names and manufacturers of teeth included in this investigation

are as follows:

Trade Name Manufacturer

Micromold

Vitalon

Trubyte Acrylic

Trubyte Dentron

Denta-Pearl

Denta-Pearl S-R

Luxene

Dura-Blend

Plastic-Five -Phase

Verident

Biocryl

Austenal Laboratories, Inc.

Austenal Laboratories, Inc.

The Dentists’ Supply Co. of New York

The Dentists’ Supply Co. of New York

E. D. Justi and Son, Inc,

H. D. Justi and Son, Inc.

Luxene, Inc.

Myerson Tooth Corporation

Universal Dental Co.

Universal Dental Co.

Biodent Mfg. Co., Heidelberg, Germany

4. EXPERIMENTAL PROCEDURE AND RESULTS

4.1. CRAZING

A study of the crazing of plastic teeth was made on all brands listed above.

This consisted of observing the effect of a solvent on the teeth after giving

them definite treatment. Craze tests were made on plastic teeth in three conditions; (a) as received from the manufacturer, (b) after a curing cycle such

as dentures normally receive in dental laboratories and (c) after a curing

* All teeth procured and tested 1950-52.
t

•

'

,

. . . "

1 ir*

' •• i

•

' ‘

'

'

. '

' ' : " •. • .

'

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'

'

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•

'

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,

'

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.

;

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-
- 3 -

cycle in which special precautions were taken to prevent water from coining

into contact with the tooth.

It was found that it was unnecessary to process the teeth in denture base

material for the curing cycle. The technique used was as follows:

(a) The teeth were invested in gypsum.

(b) The invested teeth were heated in a temperature-controlled water

bath,

(c) The flask was bench cooled to room temperature.

(d) The case was deflasked and the teeth were allowed to dry to room

temperature for several hours or longer.

The craze test consisted of immersing the teeth three times for short intervals (5 sec.) in a solvent (methyl methacrylate monomer or ethyl alcohol).

At first several solvents were used. Methyl methacrylate monomer and alcohol

were finally chosen for test solvents as they would more likely be encountered

in practical use: the monomer, when a denture is repaired or rebased, and the

alcohol when beverages are taken orally.

The crazing was observed by visual inspection and also by the use of a binocular microscope. The specimens were given a second treatment after a day of storage if they did not show crazing immediately.

Table 1 shows 5 brands that did not craze, one brand that exhibited crazing

on some specimens and no crazing on other specimens and 5 brands that showed

very definite crazing by the solvent test.

The types of crazing observed were quite varied as shown by examples in

Figures 1, 2 and 3. It was not possible to judge accurately the degree of crazing because some specimens contained many fine cracks while others contained

fewer but larger cracks.

Examination of sections of crazed teeth, under a microscope, indicated

that the depths of the cracks were quite varied ranging from 0.2 to 1 mm.

Teeth heated to 212°F in water after investing showed more crazing than those

heated to l60°F.

Experiments were conducted to determine if craze-susceptible teeth would

craze when heated under conditions such that no water came in contact with them

during heating. Several procedures were used for this, such as (a) drying out

free water from the gypsum by heating or reduced pressure, (b) surrounding the

teeth with tin foil, and (c) coating the teeth with a thick layer of wax before

investing.

In all cases where water, either in liquid or vapor phase, was prevented

from coming in contact with the teeth, no crazing was produced by the solvent

tests. This leads to the conclusion that water plays an important roll in the

crazing phenomena.
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Practical cases of dentures worn by patients exhibit this same crazing

but usually to a much less degree. Figure 4 shows a practical immediate

denture which was rebased after eight months. The crazing appeared in the

plastic teeth after rebasing, all six of the anterior teeth showed severe

crazing and had to be replaced.

4.2 MECHANICAL PROPERTIES

Data were obtained on the hardness (Knoop), indentation resistance, strength,

modulus of elasticity and elastic recovery of the plastic teeth investigated.

(a) Hardness.

The hardness was observed with the Knoop indentation method at a temperature of 70° ± 2°F, using a 100 gram load with a 10 second descent, and 20 second

contact. The teeth were cut and polished to a smooth flat surface about 2 mm

from the tip of the cusps. The final polish was made with fine polishing paper

(3/0) followed by fine whiting. The teeth were measured in the r, as received”

condition, that is, not dried or stored in water before testing. The Knoop

numbers are given in Table 1,

(b) Indentation Resistance and Recovery.

Flat specimens 2 to 3 mm thick were cut from plastic teeth. The surfaces

were finished with 1 G sand paper. The specimen was placed in a Rockwell

Superficial Hardness Tester and subjected to a 30 KG major load with a l/2

inch ball for ten minutes. At the end of this period the depth of indentation

was observed, the major load was removed and the minor load of 3 KG was applied

for 10 additional minutes. At this time the depth of indentation was again

observed. The recovery value is expressed in per cent of the depth of indentation. The results are shown in Table 1 for the various brands of teeth,

(c) "Indentation Strength"

Since in the procedure described above the indentor sphere will continue

to penetrate the resin until the area of contact is sufficient to resist the

force exerted on the specimen, it is possible to calculate the strength of the

resin from the data obtained. The projected area of the indentation can be

calculated from the depth reading on the Rockwell Superficial Hardness Tester

with a correction for the slight indentation of the minor load. The load divided by this area gives a measure of the strength in simple terms such as pounds per square inch LI The data obtained for indentation strength specimens cut from plastic teeth are shown in Table 1,

(d) Stress-strain Relationship.

Cylindrical specimens 3 mm in diameter by 6 mm long were machined from

the plastic teeth. The specimens were placed in a Tinius Olsen Testing Machine

and loaded in compression with a head speed of 0.20 inches per minute. The

specimens were so mounted that steel pins l/2 inch in diameter contacted the

specimen in such a way that two Tuckerman strain gages could be straddled over

the specimens. Figure 5. The strain was recorded at 2 l/2 pound load increments.
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- 5 -

The modulus of elasticity was calculated from the straight portion of

the stress-strain curve below 7000 psi (Figures 6, 7 and 8). The yield

strength was arbitrarily taken as the stress at which the deformation produced by a unit stress equals or exceeds 1.25 times that observed in the

first few increments of stress. The values for the modulus of elasticity

and yield point are shown in Table 1.

4.3 ABRASION OR WEAR RESISTANCE

The data obtained on abrasion of teeth is of a very preliminary nature

and is reported at the present time for evaluation of the test method. The

method used involves abrading the teeth in a ball mill and observing the loss

of weight. The mill used was a one-quart Abbe porcelain jar mill rotating at

69 revolutions per minute. To compare the wear of different brands of teeth

the mill was charged with the abrasive material and a total of 40 to 50 teeth

of various brands and was run for periods of time ranging from 2 hours to

several hundred hours. The relative effect of several different abrasive

materials and of different tooth shapes is shown in Tables 2 and 3. Relative

wear of eight brands of teeth when placed in the mill with steel balls with

and without additional abrasives is given in Table 4. Most consistent results

were obtained when l/4 inch steel balls with #200 aloxite powder in water was

used as the abrasive materials according to the following technic:

• 1. Wash 1350 grams of l/4-inch carbon steel balls with distilled water

and drain off as well as possible.

2. Place the balls, 20 grams of #200 alox-ite powder, and ICO grams of water

in the one-quart jar; center it on the mill and run it for 2 hours to clean and

condition the jar and balls.

3. Immediately after the above step, clean out the used aloxite powder,

rinse the jar and balls with distilled water, and add 20 grams of fresh #200

aloxite, 100 grams distilled water, and from 40 to 50 weighed teeth. The

actual test should be run immediately after the cleaning charge.

4. The teeth used should be fairly large from 0.3 to 0.5 grams except

in the case of teeth with mechanical retention holes. For these a range of

0.25 to 0.45 grams will give about the same size teeth as the others. These

teeth should be marked clearly. The method used was to bore shallow holes in

recessed parts of the teeth and then fill these holes with drawing inks of various colors.

5. Each tooth should be weighed separately before and after treatment in

the mill, and the percentage loss'of weight calculated.

6. For comparison of brands the average loss for a given brand can be divided by the loss for some brand selected as a standard. This gives a number

that seems to be relatively constant for any run made with the same abrasive and

conditions.
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While the data obtained from the ball mill tests do ’show differences between the various brands of teeth, no data have vet been obtained indicating

whether or not the test results approximate wear which occurs in service. Much

more research will be needed before any statement can be made on the relative

clinical wear resistance of the various brands of teeth.

5 . DISCUSSION OF RESULTS

The data obtained indicate that crazing is affected by the amount of internal strain present in the teeth, Thus, the craze resistance is a function of

the processing conditions. It is evident that the water absorbed by plastic

teeth which are susceptible to crazing plays an important role, since it was

shown that when water is prevented from contacting the tooth all the teeth so treated were craze resistant. Also teeth invested in dental stone and heated

in boiling water exhibit more crazing by the solvent test than do those heated

to l60°F in water. This may be caused by the. greater mutual solubility of resin

and water at the higher temperature. Although the mechanism of crazing is not

established, studies are being made on the theory that the relative solubility

of water and resin changes with temperature and that the dimensional change

caused by curing and cooling is the basic cause of the residual strain in the

resin [2j. So far as the present experimental data are concerned the facts

correlate satisfactorily, but more observations of a precise nature are necessary to prove the validity of this concept. No correlation between complete

craze resistance and the amount of water sorption was found in this investigation. In general, crazing was less for brands of teeth which showed good mechanical properties as evaluated by the methods used in this investigation.

Unfortunately there is no practical method for fabricating dentures under

water-free conditions, as an excess of free water is present in the set dental

stone or plaster used to invest cases for processing. It is considered best,

therefore, to use teeth that do not exhibit crazing if this difficulty is to

be overcome.

The indentation hardness values determined by the Knoop Method show that

the range in hardness is small, only about 4 Knoop numbers. This is to be

expected because the teeth are essentially methyl methacrylate polymers with

the exception of Luxene which contains considerable vinyl polymer.

The ability of the tooth to resist deformation, from pressure applied to

a local area such as biting on a hard particle of food, bone, etc. is important.

To evaluate this property the rate of indentation of a sphere under constant

load was used. The rate of indentation is dependent upon the elastic and

plastic properties. The ball penetrates rapidly for the first few seconds and

then gradually decreases until it comes to rest. For practical procedures a 10-minute load application was found to approach very closely a static condition.

This provided a simple method for observing the flow properties and resistance

to indentation. The probable error of the depth of indentation under the best

condition is of the order of 2 per cent. The depth of indentation for the different brands of teeth studied varied from 0.090 mm to 0.148 mm with the values

for most of the brands between 0.095 and 0.105 mm. This average value 0.103 mm,

is between those of denture base resin 0.096 mm and average acrylic filling resins

(0.108 mm) and is much greater than that of dental amalgam (0.043), Figure 9 .
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One of the fundamental properties of resins is the property of absorbing

energy by deformation and returning closely to its original position. This is

a factor in denture prosthesis as the energy absorbed by oral structures prob-.

ably plays an important role in resorption of tissue. The recovery of the plastic

teeth as measured by the decrease in depth of the indentation of a spherical ball

when the load is removed or reduced, was used to obtain numerical values for the

different teeth. The values are expressed in per cent of the original depth of

the indentation.

The strength of plastic teeth is low. The yield point as calculated from

the stress-strain curves ranges from 6,400 to 8,600 psi. These values are considerably lower than the maximum calculated stress of 17,000 psi on cuspids as reported by Skinner [3j . The fact that clinical cases do not exhibit such deformation is partially explained by the high recovery value. The best mechanical test developed as indicated by precision and repeatability is the application

of the Tuckerman strain gage to small specimens. The data so obtained with constant rate of loading were plotted to show the stress-strain relation. The modulus of elasticity computed over stress ranges of 1,000 to 7,000 psi is shown in

Table 1. The variation of different brancs was from 280,000 to 42G,000 psi.

This property is very significant as it indicates the stiffness of these materials.

The teeth having a low modulus exhibit extreme crazing.

Abrasion or wear resistance of plastic teeth is one of their weakest properties. At present the work on abrasion measurement is only in the developmental

stage. It is considered essential that new and more practical test methods must

be developed to evaluate this factor.

The data reported in this paper in combination with previous data and clinical experience have been utilized for the preparation of a proposed specification

(see below) for plastic teeth. No abrasion test is included in this specification

since none of the tests so far devised have yet been correlated with clinical

wear.

6 . SUMMARY

1. Data is presented on mechanical properties of eleven brands of plastic

teeth,

2. The mechanical orooerties are ouite low in comnari son with other materials used for restorative purposes in the oral cavity.

3. Some brands of teeth were found to be susceptible to crazing after processing as would be done in curing a denture. Five brands crazed consistently,

five brands did not craze and one brand crazed in some instances. Crazing was

not observed on any teeth if water was prevented from contacting them during

heating.

4.

Crazing is caused by relief of internal strain and water is an important
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factor in this mechanism.

5. The resistance to crazing was found to be the most outstanding difference between the commercial brands studied.

6. No definite correlation was found between the physical properties studied

and crazing although, in general, the high physical properties were exhibited by

materials not susceptible to crazing.

*

- 9 -

8. PROPOSED SPECIFICATION FOR PLASTIC TEETH

1. SCOPE AND CLASSIFICATION:

1.1 The teeth described under this specification shall be plastic.

Teeth shall be life-like and so molded and colored that they shall closely

approximate natural teeth in overall appearance. The teeth shall be suitable for the construction of complete and partial dentures by techniques

in general use at t

1 - present time by the dental profession.

1.2 Tyres - This specification covers three types of teeth and one grade:

Type I - Anterior Teeth: Shall consist of molds of teeth approximating

the various forms of human anterior teeth, labially and

lingually.

1 Type II - Anatomic Posterior Teeth: Shall consist of molds of teeth

approximating the forms of human posterior teeth.

Type III - Non-Anatomic Posterior Teeth: Shall consist of molds of

teeth approximating the forms of human posterior teeth in

respect to lateral surfaces but having modified occlusal

surfaces

.

1.2.1 Grade - All types shall be of the highest grade.

2 . APPLICABLE SPECIFICATIONS:

2.1 The following Federal Specification, of the issue in effect on date

of invitation for bids, form a part of this specification:

U-D-226 - Denture-Base -Material (Acrylic resin or mixtures of acrylic and other resins)

2.2 Other Publications : American Dental Association Specification No. 12.

Journal American Dental Association, Vol. 44, 285

March 1952.

3. REQUIREMENTS:

3.1

Material - Shall be a plastic capable of attaining and maintaining a high polish under conditions found in the oral cavity. The material shall

not be adversely affected by processing and reprocessing against Plaster of

Paris or artificial stone (hydrocal) using normal techniques employed in

the repair and rebasing of dentures made from resins which meet American

Dental Association Specification No. 12 or Federal Specification U-D-226,

The material shall be methyl methacrylate polymer, copolymer of methacrylates, or combinations of other resins having suitable properties for the

use intended.
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3.2 Workmanship - The workmanship shall be free of all defects and shall

be first-class in every respect.

3.3 General Requirements :

3.3.1 The plastic teeth shall be life-like in appearance.

3.3.2 The teeth shall contain no ingredients which shall be toxic

or irritating to the oral tissues. «

3.3.3 The teeth shall have a smooth glossy surface and shall be

capable of being cut and repolished to a glossy surface.

3.3.4 The teeth shall be free of any porosity, either on the surface

or when sectioned and observed by low magnification (10X)

.

3.3.5 Translucence: All teeth, anterior and posterior, shall consist

of a gingival, or body, color and a translucent incisal or occlusal,

the two to be blended together in such a manner as to simulate the

color blend of natural teeth, with no line of demarcation between incisal (occlusal) and gingival on the labial (buccal) aspects of the

teeth.

3.3.6 Color: The shades of teeth shall be reproductions of those

found in the human mouth, ranging from the light shades of young

persons to the dark shades of the elderly, including both translucent and semi-translucent incisal tips, as found in natural teeth.

The shades of the teeth shall so color-match the corresponding tooth

on the shade guide that no perceptible difference can be noted by visual

observation in daylight.

3 .4 Detail Requirements :

3.4.1 Bonding : The teeth shall form a chemical bond with acrylic

denture base resin when cured by the heat processing technique. Strength

of the bond shall be not less than 4>500 lbs per sq in.

3.4.2 Hardness : The teeth shall have a Knoop Indentation Hardness

of 17.0 kg/mm^ or more.

,3.4.3 Indentation Herdstamce : The teeth shall not be penetrated more

than 0.11 mm by a l/2" steel ball loaded at 30 kg for 10 minutes.

The recovery of the depth of the indentation shall be at least SO%

in 10 minutes after the 30 kg load is released.

3.4.4 Blushing : The teeth shall not bli^sh, blanch, or change color

when processed in boiling water.

3.4.5 Distortion : The teeth shall not distort or change in shape

when heated in water at 100° C for one hour.

3.4.6 Craze Resistance : The teeth shall be craze resistant to solvents

both before and after being subjected- to- the curing heating cycle in

a water bath.
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4. INSPECTION AND TEST:

Methods of Inspection and Test - Compliance with all detail requirements of this specification shall be determined by the following methods

and procedures. The tests shall be conducted at room temperature (20 to

25° C.) unless otherwise stated.

4.4.1 Bonding : Prepare cylindrical molds, approximately 3 inches long

and 0.3 inch in diameter, in a denture flask using dental stone (hydrocal) as the mold material. Place a posterior tooth in the center with

the occlusal and ridge lap surface normal to the long ads of the mold.

Flush the mold with three applications of hot modern synthetic household detergent solution (one teaspoon to one pint of water) and rinse

with clean boiling water. Pack the mold, with the tooth in the center,

by the normal denture packing technique using clear denture base resin

which complys with American Dental Association Specification No. 12 or Federal Specification U-D-226. Close the flask, process for one hour

at 74 ±1°C, and then place the flask in boiling water for 30 minutes,

remove it and allow it to cool to room temperature in air. Open the

flask, remove the specimen of resin and machine it to 0.25 inch diameter

so that none of the base resin remains on the sides of the tooth and

that only the occlusal and ridge lap surfaces contact the base resin.

Place the resin in a suitable tension machine and pull apart at approximately 0.2 inch per minute head speed. The average of three specimens

shall be used to determine the bonding strength. (Ref. J. Am. Dental'

Assoc., Vol. 44; 235, Mdrch 1952); :r < : .

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4.1.2 Hardness : The hardness shall be determined by the Knoop Indentation Method, using 10 seconds descent and 20 seconds contact with 100

gram load. The specimens used shall be prepared from sections of the

teeth polished flat and parallel using progressively finer polishing

paper. The final finish shall be produced with No. 3/0 emery paper

followed by magnesium oxide powder on a wet polishing wheel. The test

specimens shall be between 2 and 3 mm in thickness and have a smooth

glossy surface. The hardness shall be determined on two anterior and

two posterior teeth, with 10 indentations on each, and the average of

all 40 of the indentations shall be recorded to the nearest 0.1 kg per

sq. mm.

4.1.3

Indentation Resistance : Indentation ’resistance shall be determined

on a Rockwell Superficial Hardness Tester. Specimens of teeth shall

be prepared as described under the section on TT ardness, or the same specimens shall be used. The specimen shall be subjected to a load of 3 Kg

on a l/2 inch diameter steel ball, designated as the minor load, then a load of 30 kg shall be applied, designated as the major load. The major

load shall be allowed to CQntact the specimen for 10 minutes and the

depth of indentation of the ball due to the major load shall be measured

on the dial gage of the instrument. The major load shall b e released,

leaving the minor load on the steel ball. After 10 minutes ^rom the

release, the depth of indentation shall be recorded again. The pene-
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tration shall be the amount of indentation of the ball at the end of

10 minutes of major load application. The recovery shall be the percent of return of the ball in 10 minutes from the release of the major

load with only the minor load on the ball. The depth of the indenataion

shall be recorded to 0.001 mm for individual determinations and the

average of at least three vs lues to the nearest 0.01 mm for the final

result. The recovery shall be the average indentation at 10 minutes

major load application divided into the average difference between

the major load indentation and indentation after the major load has

been released (minor load in contact) for 10 minutes expressed in percent. The same indentations shall be averaged for the penetration and

the recovery.

4.1.4 Blushing Distortion. Craze Resistence : One or more anterior and

posterior teeth of each shade purchased shall be used for this test.

The test specimens shall be submerged in test tubes of water. The tubes

shall be placed in a water bath so that they do not contact the vessel.

The water bath shall be heated from room temperature to 100° G in from

5 to 20 minutes and held at boiling temperature (100°C) for one hour.

The teeth shall be removed and cooled to room temperature in air. Then

the teeth shall be immersed in methyl methacrylate monomer for 5 seconds

and removed. This immersion is repeated after one minute and the teeth

allowed to dry on a towel for two hours at room temperature. If any

blushing, blanching, or change in color, any distortion or any cracks

can be seen, by visual examination or low power magnification, the

teeth fail to meet these requirements.
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- 13 - BIBLIOGRAPHY

1. Sweeney, W. T., Sheehan, W. D., and Yost, E. L., Direct

Filling Resins: Mechanical Properties, NBS Report 2647,

June 30, 1953.

2. Sweeney, V/. T., Brauer, G. M., and Schoonover, I. C.,

Grazing of Acrylic Resins, NBS Report 2738, (To be

published in J. Dent. Res.).

3. Skinner, E. W., The Science of Dental Materials, Third

Ed., p. 17 (W. B. Saunders Co., 1946).

Table

1.

Physical

Properties

of

Plastic

Teeth d -p o o O O O O O O o o o H £ •H o o o O O o o O o O o 0 H 0 SO Pd* H vO M3 H Pd* pd* M3 O- 1—1 H O ft •» •» •k •> •k •k •k •k •k •k >H ft CO xO e- CO CO CO CO 00 M3 CO

ftO -P •H •H 0 CA H H CM CA H pd* CM cA CA 0 o ft • • • • • • • • • • • P H o o O O O O o o o O O H -P +1 +1 +1 +1 +l +1 +1 +i +1 +1 +1 p 00 1A CO CO x0 CO O' CO H eg xO pd* H d cd O • • • • • • • • • • • O H H CA eg CA CA CA CA P± pd* CA CA CA s w X

Xl

-p l P b0 •H O o O O O O O o O O O pop 0 O o O O O O o o O O O 0 H 0 ft O' pd* eg CO 00 CO CO pd* CA O' O' d -P P •* % •» •t * •k •V •k •k •k P cd -p O' xO O' o O' o O' O' O' O' O' M -P OQ H H 1 •» > • HOP • rl O *H 'd P p cd © g cd ©

cd ,Q Pd o >

fc OH o H o eg H A- CO CM 00 t'- O' co P CM • H o O' e- CO O' ao 00 GO CO co CO CO o\ to to 0 H H W P P 0

CP

cd • O -P ca p pn^ i P P H 1 cd p P O CO 00 xO O O' O O' CA p± oo co 0 P d 0 © H p O' -P* o O' O' O' O' O o O' O' d cd {>>d d -p p o H 1—1 O O o o H H o o p o o p p P cd • • • • • • • • • • • H H H 0 3 i H -P o O o o o o o O O o o © Pd* 1A eg sO CA -d- Pd* pd* CA pd* pd* p to eg « • • • • • • • • • • © cd P o O o H O o o o O o o X P 3 +1 +l +i +1 +1 +1 +1 +i +1 +1 +1

[3 0 \ eg eg xO O xO o 1A O' CM t'- pd* > bO • • • • ;• • • • • • • £5 < W CO CO CO H O' o pd co co O' O' H H H eg H eg H H H H H aoo

eg

eg eg o CO eg xO pd* O' O' co p « • • • • # • • • • • W 0 c! 00 O- O' xO O' rH pd* O' co o o tr H H H eg H eg H i—i H CM1 CM p 1 | 1 i 1 i 1 i 1 1 cd to 00 eg CM eg X0 -d- 1A CM O' o oCP] P • « • • • • • • • • • t'- t'- 00 00 O' O' pd* CO o- O'

vU H H H H H H H i—1 H 1—1 H V0N © W 0 O 0 0

cd =*s < © 0 O 0 O o 0 o © p >H >H o «- , O

•H PO KW H P

r*> -P H rH 1 0 P P P p © O 0 cd cd d >

c3 Hd Q

ft©

© P £ ft © -P O P © 0 i—i O P H 0d

cd S O -P -P H H 0 PQI H © 0 l>> o H >5 cd cd P -P 0 d P p cd d X p -p 0 cd 0 cd O o -p P P P p w P cd X P o Hr~ » H H P P 0 0 n P H ft 0 •H EH S > Eh EH ft ft ft ft ft > PQ

Solvents

used:

methyl

methacrylate

monomer

and

ethyl

alcohol.

Some

specimens

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Table 2

Effect of Various Abrasives on Weight Loss

of Teeth in Ball Mill Test

Abrasive Weight loss per hour

drx wet

Oxford crystal feldspar 0.002 — No. 18 glass beads 0.002 — Flint pebbles 0.01 0.04

Steel balls l/4 inch — 0.02

Steel balls l/2 inch — 0.1

Flint pebbles roughened — 0.2

with carborundum

No. 200 aloxite, l/4 inch

steel balls 0.2 0.7

Fine carborundum, l/4 inch

steel balls 0.3

Table 3

Effect of Tooth Shape on Weight Loss

of Teeth in Ball Mill Test

Brand Abrasive Time Cuspid Lat. Incisor Cen. Incisor

hr. Wt.g Los Sj/o Wt. g Loss.$ wt^g Loss,$

Wet 1/2" balls 25 .35 2.34 .23 3.60 .37 2.36

It 25 .39 1.65 .28 2.2Q .43 1.67

tl 25 .33 2.24 .21 2.72 .33 2.55

Wet 1/4" balls 20 .33 .37 .28 .50 .27 .48

n 20 .40 .13 — -

.48 .17

tt 20 .35 .37 .23 .75 .37 .44
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Table 2

Brand

Effect of Various Abrasives on Weight Loss

of Teeth in Ball Mill Test

Abrasive Weight loss per hour,$

dry wet

Oxford crystal feldspar 0.002 — No. 18 glass beads 0.002 — Flint pebbles 0.01 0.04

Steel balls l/4 inch — 0.02

Steel balls l/2 inch — 0,1

Flint pebbles roughened — 0.2

with carborundum

No. 200 aloxite, l/4 inch

steel balls 0.2 0.7

Fine carborundum, l/4 inch

steel balls 0.3

Table 3

Effect of Tooth Shape on Weight Loss

of Teeth in Ball Mill Test

Abrasive Time Cuspid Lat

.

Incisor Cen. Incisor

hr. Wt.g Loss,^ Wt^g Loss.$ Wt^g Loss % %

Wet 1/2" balls 25 .35 2.34 .23 3.60 .37 2.36

It 25 .39 1.65 .28 2.29 .43 1.67

II 25 .33 2.24 .21 2.72 .33 2.55

Wet 1/4" balls 20 .38 -.37 .28 .50 .27 .48

II 20 .40 .18 — -— .48 .17

II 20 .35 .37 .23 .75 .37 .44
i

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Table 4

Relative Wear of Various Brands of

Teeth in Ball Mill test

Brand Wet aloxite powder and Dry Aloxite Dry fine car- Wet 1/2" Wet l/4

1/4” steel balls* 1/4" steel borundum l/4” steel steel

balls steel balls balls balls

1.92 2.12 1 of. — • / 1.95 1.93 .93

1.21 l .17 1.23 1.17 — _ 1.16 1.09 1.04 1.49 1.03 — _ 1.09 1.12 1.13 1.23 1.13 — .95

1.03 1.04 1.07 1.15 1.04 1.09 .89

1.00 1.00 1.00 1.00 1.00 1.00 1.00

.99 .96 .99 1.16 .99 _____ ____ —.52 —.46 .54 —

.36 .53 .75 —

— .86 .75

* Values for three runs are given to show repeatability of results.


Figure 1. Example of crazing produced by immersion

of plastic tooth in solvent.



Figure 2. Example of crazing produced by immersion

of plastic tooth in solvent.
Ill


Figure 3. Example of crazing produced by immersion

of plastic tooth in solvent.


'!
Figure 4. Crazing of teeth on immediate denture after

rebasing.



Figure 5 Specimen and gages in position between the

platens of the testing machine used to record

stress-strain relationships.
INCHES

'

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Figure 6. Stress-strain curves for five brands of plastic

teeth.
oooNOin'tiooJ

Nl OS H3d SSI OOOI— SS3S1S

STRAIN-IN.

PER

IN. X

10“


Figure 7, Stress-strain curves for four brands of plastic

teeth.
PLASTIC

TEETH

STRAIN-IN.

PER

IN. X

10"


Figure 8 Stress-strain curves for two brands of plastic

teeth.
PLASTIC

TEETH

00 h- cd m ro

ni os aad sai oooi-ssaais

STRAIN

-IN;

PER

IN. X

ICT 4


Figure 9. Comparison of indentation and recovery

of plastic teeth (average of ten brands),

denture base resin, direct filling resins,

and amalgam.
0.1 1h

DIRECT

RESINOUS

MATERIAL

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 June 30, 1954 3380

MEASUREMENT OF STRAIN IN METHYL METHACRYLATE

CAUSED BY WATER SORPTION

William C, Hansen*

William T. Sweeney**

* Physicist, Dental Research Section, National Bureau of Standards.

** Chief, Dental Research Section, National Bureau of Standards.

This work is a part of the Dental Research Program conducted at the National

Bureau of Standards in cooperation with the American Dental Association, the

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

the Veterans Administration

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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

unless permission Is obtained In writlngfrom theOfflce 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,

MEASUREMENT OF STRAIN IN METHYL METHACRYLATE

CAUSED BY WATER SORPTION

Abstract

A method of determining the change in the internal residual

strain in polymerized methyl methacrylate during its sorption of

water has been developed. A bonded, wire strain gage embedded

within the polymer is employed to determine the strain increments.

Henry’s and Pick's laws appear to be inapplicable for large specimens totally immersed in water. An assumption is made to the

effect that the change in the strain is in a direct linear relationship to the amount of water sorbed. It is not believed that

this assumption causes so great an error as to result in the contradiction of Henry’s and Fick's laws.

1. INTRODUCTION

The crazing of resins has been attributed mainly to the change in the characteristics of the internal residual strain in the resin during its action as an

absorbent of water. For this reason the development of a method of determining

\ the change in the internal residual strain In polymerized methyl methacrylate was

desirable. Since the primary concern was with the internal effects, such strain

gages as the Tuckerman, acoustical, etc., could not be used. The technique of

photoelasticity was thought to be of practical use, but, since the specimens

were to be placed in a water bath thereby making the determination of the change

in the strain extremely difficult and since each specimen would have to be calibrated for the effective use of this technique, this method was discarded in

favor of using bonded wire strain gages. The advantages of this type of gage

lie in the remote measurement of the change in the strain and in the manufac-

- 2 -

urers calibration of the gage?. There are, however, three major disadvantages

in the use of these gages.

Since the change in the strain is determined by resistance measurements,

any water sorption by the gage causing electrical leakage is detrimental to the

results. Also excessive strains and temperatures will destroy their effectiveness, although the standard gages will withstand strains as high as 1% and temperatures up to BO°C for paper gages and 150°C for phenolic impregnated paper

gage:

(i) Finally .the effective modulus ©f elasticity of the gages must be lower

r-r the material, in which they are to be: used. The gage's modulus of

,:itv ;av be lowered the rddition of lamina of barer and cement to the

surface of the gage

The theory of the gage is based upon the phenomenon that a wire's resistance, to a first approximation, is directly proportional to a distortion along

its longitudinal axis. Hence, any change in strain will result in a corresponding change in the resistance of the gage, both in sign and relative magnitude.

The strain is determined from the gage factor which is determined by the manufacturer. The gage factor is defined by K = dR/R , where K is the gage factor,

dS

R is the resistance of the gage and S is the strain.

METHOD

The gages used were the type designated by the manufacturer, Baldwin-LimaHamilton Corporation, as type A-5 with a gage factor of 1.99±1$ and a resistance

of 120±0.5$ ohms. A photograph of the type of strain gage used is shown in

Figure 1. The Wheatstone bridge employed was the Baldwin-Lima-Hamilton Corporation' s type L strain indicator which gives the readings directly in u in/in.

This was complimented with their six channel switching and balancing unit

(l) Domestic price list, p. 4, July 15, 1953. Balwin-Lima-Hamilton Corp.

Philadelphia 42, Pennsylvania.

- 3 -

which permits the pse of six gages simultaneously and places the strain readings at the same fiducial point if desired. The arrangement of the apparatus

is shown in Figure 2.

The mold used was formed by casting artificial stone around a brass master

form, the gage leads being sealed in one wall. The mold was then linedwith 0.003

foil of pure tin as shown in Figure 3. A mix of methyl methacrylate polymer^)

(?) and methyl methacrylate monomer K J was then placed around the gage, pressure and

heat were then applied to further polymerize the mixture. The specimens were

cured for a period of twenty hours in an air bath at a temperature of 55°C. Since

air has a low specific heat, the heat of polymerization of the resin cannot be re

moved rapidly enough to maintain a constant temperature within the specimen. To

prevent any damage to the gage, the temperature must be kept below S0°G o Hence,

it was necessary to use a low initiating temperature in order to keep the tempeia

ture within the specimen below the desired limit and eliminate any thermal damage

to the gage. Since a flash type mold was used, the true pressure is not known,

although it is estimated to be of the order of 2000 psi.

The positioning of the gage within the resin is shown in Figure 4.

The specimens having been cured and allowed to cool to room temperature,

"were removed from the mold and placed in a water bath at a temperature of 37°

C

±0.1°. The specimens were then allowed 45 minutes to attain thermal equilibrium

and the bridge balanced so that it registered a strain of 1000 u in/in. The

\ 1 changes in the strain were determined in the following manners the first gage

was placed in the bridge circuit by closing the corresponding switch in the

switching and balancing unit and' then given a thirty-second stabilizing period

before the strain change was read from the strain indicator c Having determined

(1) Manufactured by E, I. duPont de Nemours and Co., Polychemicals Department,

Washington, West Virginia. 0olor: H 7500 Composition HO/, 1 (2) Manufactured by the Rohm and Haas Co e , Philadelphia, Pa., with 0„006p

Ilydroquinone added as an inhibitor.
\ - * ' . * • •

.. '
- 5 -

The contradiction of Fick’s and Henry’s laws is probably due to one, or

both, of the following two reasons:

(1) Fick’s and Henry’s laws are not applicable to the type of specimens

studied under the conditions of the experiment.

(2) The assumption that the strain change is in a linear relationship

to the amount of water sorbed is false.

Although it is not likely that the change in strain is in a direct linear

relationship to the water sorbed, it is not believed that the error caused by

this assumption is so great as to cause a contradiction of the predictions of

the general sorption laws. It therefore appears to be far more reasonable that

the predictions of Fick and Henry are inapplicable to the type of specimens

studied

SUMMARY

The change in the internal residual strain of polymerized methyl methacrylate during water absorption was determined by embedding a bonded wire strain

gage in the polymer. Henry’s and Fick’s laws were found to be inapplicable for

the type of specimen under investigation.
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References

1. Houwink, R., Elastomers and Plastomers, General Theory I,

Elsevier Publishing Co., Inc., 1950, pp 321-4.

2. Dobie, W. B., and Isaac, P. C. G., Electric Resistance Strain

Gages, English Universities Press, London, 1948.

3. Yarnell, J., Resistance Strain Gages; Their Construction and

Use, Electronic Engineering, London, 1951.

4. Characteristics and Applications of Resistance Strain Gages,

National Bureau of Standards Circular 523, issued February 15,

1954. (For sale by the Superintendent of Documents, U. S.

Government Printing Office, Washington 25, D. C.) (Price fl.5Q).

5. Circulars 279, 279A. and 279B, Baldwin-Lima-Hamilton CDrporation,

Philadelphia 42, Pennsylvania.

6. Testing Topics, issued quarterly by Baldwin-Lima-Hamilton Corp.,

Philadelphia 42, Pennsylvania.


Figure 1. An enlargement of the type A-5 strain gage.

The length of the measuring oortion of the gage

is l/2 inch, its width 35/64 inch. The overall

dimensions of the gage body are 3/8 inch by

13/l6 inch.



Figure 2. A. Water bath with its thermoregulator, stirrer

heater and immersed gages.

B. Mercury relay switch, 12 volt relay transformer and the gage balancing and switching

unit.

C. Strain indicator.

D. The power supply for the indicator.

E. The constant voltage transformer for the

power supply.
UJ

I
Figure 3. Flash mold used in preparing the specimen

for investigation.
NBS


Figure 4. This figure displays the general position

of the gages in the specimens.


'
Figure 5.

A linear plot of the change in strain vs the

time of immersion.
2000

NI/NI rf Nl Nivais

100

200

300

400

500

600

TIME

IN

HOURS


Figure 6. A semi-logarithmic plot of the logarithm

of the change in strain vs the time.
o 200 400 600

TIME IN HOURS


Figure 7. A semi-logarithmic plot of the change in strain

vs the logarithm of the time.
STRAIN

IN

U

IN/IN

TIME IN HOURS


Figure 8. A semi-logarithmic plot of the change in

strain vs the logarithm of the time.
STRAIN

IN

IN/IN

2000

800 -

1600 -

1400 -

1200

1000

O

A

A

2

3

4

5

10 20 50 100 200 500 1000

TIME IN HOURS

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants

and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

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inside of the front cover. Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

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Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.00). Information on calibration services and fees can be

found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.
V