NATIONAL BUREAU OF STANDARDS REPORT

4548

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

July 1 to December 31* 1955

Dental Research Laboratory

<qjBp>

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

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing

of the divisions and sections engaged in technical work. In general, each section is engaged in

specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instru- ments. Magnetic Measurements. Process Technology. Engineering Electronics. Electronic

Instrumentation. 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.

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

State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays.

Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments.

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

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

Mech&nics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

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. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems.

Analogue Systems.

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

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

Propagation Services.

Radio Propagation Engineering. Frequency Utilization Research. Tropospheric Propagation

Research.

Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-10-0707 January 31 , 1956 4548

0708-20-3824

0708 - 20-3857

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

July 1 to December 31* 1955

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.

(Task 7756-41 BV No. 55-14)

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repi Approved for public release by the . part, is prohibited unless permission is c director of the National Institute of idards, Washington

25, D.C. Such perm Standards and Technology (NIST) >rt has been specifically prepared if thi On October 9 2015 ort for its own use.

REPORT OF DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

1

INTRODUCTION

Investigations covering a wide variety of materials

and equipment used in dentistry and studies of natural tooth

structures have been carried on in the Dental Research Section

of the National Bureau of Standards during the half year ending December 31* 1955- Lists of reports issued and papers

published during this period are given below. Copies of the

reports are appended.

2REPORTS ISSUED

NBS Report 4312

NBS Report 4313

NBS Report 4494

NBS Report 4495

NBS Report 4520

The Polymerization of Methyl Methacrylate in

the Presence of Amine Accelerators.

Alloys of Gallium with Powdered Metals as

Possible Replacement for Dental Amalgam.

Resin Cements and Posterior-Types DirectFilling Resins

Preliminary Study: Tropical Storage of

Dental Materials.

Dimensional Stability of Denture Base Resins.

3

PAPERS PUBLISHED

A Revision of American Dental Association Specification No. 3

for Dental Impression Compound. John W. Stanford, George C.

Paffenbarger and W. T. Sweeney. J.A.D.A. 51 »56 July (1955).

Ionization Chambers for Radiation Data During Dental X-ray

Exposure. Donald C. Hudson and John W. Kumpula. U. S. Armed

Forces Med. J. 6y(No. 8) 1131 August (1955).

Self-curing Resins for Repairing Dentures: Some Physical

Properties. J, W. Stanford, Claire Burns and Geo. C. Paffenbarger. J.A.D.A. 51:307 Sept. (1955).

Setting Reaction of Zinc Oxide and Eugenol. Henry I. Copeland

Jr., Gerhard M. Brauer, W. T. Sweeney and A. F. Forziati.

NBS J. Res. 55:133 Sept. (1955).

American Dental Association Specification No. 13 for Selfcuring Repair Resins (Effective January 1, 1958) . John W.

Stanford, Claire L. Burns and George C. Paffenbarger. J.A.D.A

51:425 Oct. (1955)

Dental Cements. Geo. C. Paffenbarger, V/. T. Sweeney and

P* J. Schouboe. Internat . Dent. J. 5:484 Dec. (1955).
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4. WORK IN PROGRESS

4.1 Human Tooth Enamel and Dentin

(a) Fluorescence Investigation.

In an attempt to correlate Intensity of fluorescence

with organic content in calcified tissues, twelve serial

sections cut from a dog femur were placed in an extractor

and refluxed with ethylenediamine for a total of twelve hours

At the end of each hour one section was removed and preserved

in ethanol. The fluorescence intensities of the twelve specimens were compared photographically and by means of the spectroradiometer. Although there appeared to be a general relationship

between organic content and intensity of fluorescence, interpretation of the results must await a nitrogen analysis of the

bone sections.

Specimens of human teeth which had been exposed in the

artificial mouth constructed at the University of Alabama

were examined by f luorophotomicroscopic methods. The fluorescence of caries-like lesions in these teeth was compared with

that of naturally-occurring caries. The artificial lesions

resembled natural caries to some extent but seemed to differ

in pattern of tooth destruction. These differences may have

been induced by the accelerated production of the artificial

lesions. Further work is being done on this comparison. Fluorophotomicroscopic studies of human enamel and dentin in cooperation with the National Naval Medical Research Institute were

continued

(b) Crystallographic Investigation.

Efforts to produce in the laboratory crystals of nonstoichiometric calcium and lead apatites resembling bone and

tooth mineral were continued. Attempts to leach out calcium

Ions preferentially from apatite crystals by means of organic

chelating agents and to form perfect mixed crystals of calcium

and copper apatites from which copper could be preferentially

removed were unsuccessful. Presence of various foreign ions

(iron, copper, molybdenum, magnesium, etc.) in hydrothermal

bombs in which CaHP04 was hydrolized lead to the formation of

pCa^POij or whitlockite . This unexpected reaction is of interest

since the presence of whitlockite In dental calculi has been

reported. (Formation of whitlockite in the presence of magnesium has also been reported by Trautz) . Emphasis was placed

on the formation of non-stoichiometric lead and calcium apatites

by the hydrolysis of their acid phosphates. CaHP04 refluxed

in water for up to two weeks did not appear to change. However,

the addition of a base such as NaOH, tetramethyl ammonium

hydroxide or triethylamine resulted in the formation of a

material giving the typical apatite x-ray pattern. These solids
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had low Ca to P ratios. This reaction is important since

there is reason to believe that calcification of bone and

teeth takes place with CaHPOjj as an intermediate and apatite

as the final structure

Difference between normal and chalky human enamel were

studied in cooperation with workers at Emory University. Production of chalky enamel by a mild acid leach did not change

the x-ray diffraction pattern but did increase the index of

refraction from 1.615 to 1.625. It is believed that the

leach preferentially removed CaC03 and MgCC>3 from the enamel.

(c) Mechanical Properties.

In a study of methods of determining the mechanical

properties of enamel and dentin an improved method for preparing suitable test specimens by sawing sections from teeth

and grinding cylinders of uniform dimensions from the sections

has been perfected. Preliminary results indicate that enamel

is lower in compressive strength than dentin although it is

approximately six times as hard. The modulus of elasticity

determination presents difficulties in measuring the extremely

small strain obtained with these small specimens (approximately 1x2 mm) . Indirect methods using optical strain gages (Tuckerman) with small specimen between metal plattens have produced

best results. Obtaining better data on the stress-strain relationship of enamel and dentin is of fundamental importance to

the whole subject of the use of tooth restorative procedures.

This work is being continued.

4.2 Low Temperature Alloys

Emphasis was placed on the investigation of the galliumcopper-tin system. The melting point of gallium can be reduced to about 15°C (59°E) by the addition of tin. This forms

a eutectic alloy that is liquid at room temperature and makes

possible mixing and packing without any special equipment. A

number of physical properties of alloys formed by the combination

of the gallium-tin eutectic with powdered copper and copper-tin

alloys have been determined. The dimensional change is 20 /</cm

or less for several alloys with strengths from 43*000 to 55*000

psi. The modulus of elasticity in compression is being determined

at the present time. Indications are that all gallium-coppertin alloys are equal to amalgam and two alloys have a modulus

between 2 and 3 times greater than that of amalgam. This*

combined with adequate strength (45*000 to 55*000 psi) reasonable

dimensional change (20^/cm or less)* good hardness ( 100-130

Brinell) makes it appear that some alloys may be satisfactory

for oral restorations.
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systems present in dental amalgams are in progress as a preliminary means of defining the location of the g phase area

on the Sn-Hg diagram., a series of diffusion experiments have

been run at temperatures between 20°C and 100°C. The specimens were sectioned and mercury concentration measured by

vaporization-weight loss methods . The resulting depth concentration curves indicate a narrow S solid solution range

centering near 22$ Hg. A vapor effusion method will be used

to substantiate these results and more closely define the

limits. It is also hoped to use electrode potential measurements as an independent check, breaks in the curves of electrode

potential vs composition indicating phase boundaries in a

manner similar to the vapor pressure composition curve.

4.3 Resins

Additional studies of the reaction of benzoyl peroxide

with tertiary amines were made. The formation of free radicals

from benzoyl peroxide in the presence of dimethylaniline was

determined in a series of solvents by measuring colorimetrically the disappearance of diphenyl pycrylhydrazyl (D.P.P.H.).

In benzene and most solvents used two moles of D.P.P.H. react

with the free radicals formed on decomposition of benzoyl

peroxide. The kinetics of the reaction in air are complex and

pass initially through an induction period, possibly until a

steady state concentration of an intermediate peroxide-amine

complex is obtained. In the later stages the benzoyl peroxide

decomposition follows first order kinetics. Rate measurements

at temperatures from 25° to 40°C have been made which allow

the calculation of the activation energy in different solvents.

A larger number of moles of D.P.P.H. per mole of benzoyl peroxide is used up when the reaction is conducted in chloroform

instead of benzene. This may indicate a different reaction

mechanism

Apparatus has been assembled to measure the reaction rates

in a vacuum. Results of such determination will be of interest

in determining possible causes of the initial induction period.

The thermodynamic properties of solutions of polymers,

particularly the intrinsic viscosity and volume expansion as

functions of polymer-solvent system and concentration were

studied. A new relation was found for determining the intrinsic

viscosity by determining the relative viscosity at but one

concentration. This relation holds for those systems in which

the curve of log of relative viscosity vs concentration is a

relatively straight line and the curve of specific viscosity

vs concentration is a straight line. For those systems in

which log of relative viscosity vs concentration deviates

considerably from a straight line, plotting intrinsic viscosity

as calculated from the equation vs concentration and extrapolating to zero concentration gives satisfactory results.
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Additional information was obtained on the deterioration

of agar-agar materials. Viscosity, pH, and time required to

fracture specimens under constant load were observed on agaragar and a commercial reversible duplicating material. Specimens were boiled, placed in a water bath at 52 °C and then

allowed to gel at room temperature overnight. This cycle was

followed for a period of weeks . At the end of this time the

specimens were stored at 52 °C for several weeks. Additional

samples were first stored at 52 °C and then subjected to the

boiling gelation cycle. The viscosity decreased during the

boiling-gelation cycle, and increased upon storage at 52°C.

Changes in pH did not seem to be correlated with viscosity

changes. Specimens made from the material maintained at

52 °C fractured at constant load in a shorter time than did

specimens made from material subjected to boiling and gelation.

The affects of both storage at constant temperature and of

boiling gelation cycle on viscosity, and time to fracture

were reversible. A theory of micellar aggregation has been

proposed to explain the change in properties of the materials.

4.5

Rubber Base Impression Materials

Properties of rubber base (thiokol) impression materials

are being determined. Thus far values for strain, set, flow

and setting time (as determined by a penetration test) have

been obtained for 10 brands of material. Measurements of

dimensional stability and temperature rise on setting are being

made. After aging for 30 minutes all of the materials comply

with the requirements fur strain in compression of American

Dental Association Specification No. 11 for agar type impression

materials and most of the materials comply with the requirements

for set after compression.

4.6

Inlay Casting Wax

The physical properties of a selected group of synthetic

waxes and industrial casting waxes were investigated to determine their suitability as bases for dental inlay waxes. The

coefficients of linear thermal expansion between 25 - 45 °C

varied from 212 x 10“^ to 556 x 10"°. The polyethylene waxes

showed fairly low coefficients as low as some of the industrial

casting waxes. Solubility of most waxes was negligible as was

their ash content. Carvability of the majority of waxes was

fairly satisfactory for dental use. However, in all cases the

melting points were too high to permit dental application without plasticization.
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Fused Porcelain

The thermal expansion of various fused porcelains was

determined using the interf erograph . Values for the coefficients ranged from 6.4 x 10-° per °C for specimens made from

a high-fusing porcelain used for making porcelain restorations

to 8.0 x 10”° for specimens made from powdered porcelain teeth.

4.8

Hydraulic Turbine Handpiece

Work on the hydraulic turbine handpiece has been completed.

The instrument has proven to be satisfactory for clinical use

and is now being produced commercially by Bowen and Company*

Inc.* Bethesda* Maryland.

4.9

Panoramic X-ray Investigation

The panoramic dental x-ray machine for use in mass surveys

and examinations has been developed to the point where extensive

clinical tests are needed. The device produces full-mouth

dental radiographs on a single 5 in. x 10 in. film using an

extraoral technic. Patients are positioned either seated or

standing beneath the moving arm of the machine* which carries

an x-ray source and filmholder. Rotation of this arm about

its axis* sweeps the x-ray source and film about the patient's

head* producing a projection of the desired tooth images on the

moving film. Adequate detail is secured to obtain a diagnosis

of general mouth conditions. The panoramic dental x-ray device

is not designed to supplant the conventional dental x-ray machine

in cases where extreme sharpness and detail must be obtained.

A detailed report on the machine is now being prepared.

4.10

Motion Pictures

The motion picture "A Porcelain Jacket Crown Technique"

was completed. The picture presents a simplified method of

making fused porcelain jacket crowns. The essential features

of the method are the employment (l) of a prefabricated porcelain labial veneer made from a stock tooth and (2) of two

porcelains having widely different fusing temperatures. By

this technic the matching of the anatomy* the color and the

translucency are largely determined by the porcelain veneer

which can be tried in the patient ’s mouth when the preparation

of the tooth is completed. Shrinkage on fusion is greatly

reduced and the number of fusions is cut from four or five

needed in the customary technics to two. The film is colorsound with a running time of 23 minutes.
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Materials evaluated for the Federal dental services

and the American Dental Association by specification or special

test methods included amalgam,, denture base resin, impression

compound, inlay casting gold, inlay casting wax, mercury and

plaster of paris.

For the Director

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-10-0707 December 9* 1955

Progress Report

4312

THE POLYMERIZATION OF METHYL METHACRYLATE

IN THE PRESENCE OF AMINE ACCELERATORS

by

G. M. Brauer

Ruth M. Davenport

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.

(Task 7756-41 BV No. 55-14)

<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 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,

THE POLYMERIZATION OF METHYL METHACRYLATE

IN THE PRESENCE OF AMINE ACCELERATORS

The accelerating effect of amines on the benzoyl

peroxide-initiated polymerization of methyl methacrylate was studied. The reaction proceeds fastest

in the presence of tertiary aromatic amines.

An ultrasonic viscometer was used to follow

the course of the polymerization at 37°C in the

presence of the most efficient peroxide -amine systems.

The rate of polymerization often is different than one

would expect from the relative rates of the amineaccelerated peroxide decomposition in non-polymerizable solvents. With many amine -peroxide systems., the

polymerization reaction does not go to completion.

The relative order of efficiency of amines in the

polymerization of monomer -polymer slurries does not

always correlate with results obtained for the polymerization of monomer. Decreasing hardening time improves the indentation and recovery values of the

polymers . Effective amine accelerators invariably

produce colored polymers.

1 . INTRODUCTION

Tertiary amines have been used for a number of years as accelerators for the peroxide-catalyzed polymerization of methyl methacrylate. These accelerators make possible the rapid polymerization of monomer or monomer -polymer slurries at room temperature

[1-8]. Such mixtures have found considerable usefulness in dental

filling resins, cements, and denture base materials.

The increase in the polymerization rate is caused by the more

rapid decomposition of peroxides in the presence of the amines.

The kinetics of the decomposition of solutions of benzoyl peroxide

with a number of amines was studied by Bartlett and Nozaki [9] and

Meltzer and Tobolsky [10]. The latter showed that dimethyl aniline

in the benzoyl peroxide-dimethylaniline initiated systems does not

affect the propagation or termination steps of the chain reaction.

It must therefore change the initiation step. Horner and co-workers

[ 11 - 18 ] proposed the following mechanism:

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the benzoyl peroxide., a transition state (A) occurs in which a

transfer of an electron from the unshared pair of the nitrogen

to the peroxide takes place. The benzoylate anion removes a proton

from the amlnium cation., either in the a position of the side chain

(C) or from the aromatic nucleus - preferably from the para

position (O'). On transfer of an electron from the carbon to the nitrogen. free radicals are formed (D and D’) which may undergo (l)

oxidative dealkylation^ (2) substitution., (3) reaction with solvent or oxygen., (4) act as polymerization initiators in the presence of vinyl compounds. Identification of benzoic acid* carbon

dioxide,, f ormaldehyde* N-methylaniline (E), p-benzoxy-N^ N-dimethylaniline (F), p^p ! -tetramethyl diaminodiphenylme thane and chloride

ion from reaction mixtures of benzoyl peroxide and dimethylaniline

in chloroform [17] support this mechanism. When the reaction occurs

in air,, oxygen is absorbed and secondary peroxides are formed which

decompose with the formation of hydrogen peroxide and N-methylaniline . This can be explained by the following mechanism:

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Goode and Gratch [19] established that one mole of amine is

required for the decomposition of one mole of peroxide In benzene.

A large portion of the amine may be removed unchanged while small

amounts of the amine are converted into f ormaldehyde^ N-methylaniline and p_-b enzoxy-N^N-dime thy1 aniline . The decomposed peroxide

can be accounted for nearly quantitatively as benzoic acid. Imoto

and Choe [20] assume that the reaction of benzoyl peroxide with

dimethylaniline starts with the , formation of an intermediate complex produced by coordination of the unshared electron pair of the

nitrogen atom in the dimethylaniline with the central -0-0- atoms
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This unstable intermediate decomposes into £> o_

! -dime thy1-

aminodiphenyl^ p'- dimethylaminophenyl benzoate and benzoic acid.

The ease with wITich the formation of the intermediate can be

attained determines the decomposition rate of the benzoyl^ peroxide. When an electron-attracting group is introduced into the

benzoyl peroxide molecule^ the polarization of the carbonyl group

decreases and the positive charge on the central oxygen atoms

becomes larger. Formation of the intermediate coordination compound from dime thylani line can take place more readily and the

rate of decomposition of benzoyl peroxide is speeded up.

Some data on the relative efficiency of amines as accelerators in the benzoyl peroxide^-catalyzed polymerization of methyl

methacrylate have been reported [21]. Since the completion of

this investigation^ Lai and Green [22] have reported a qualitative

study of the activating effect of a number of amines on the rate

of polymerization. They recorded the peak temperatures and peak

times for the polymerization of methyl methacrylate slurries.

The results of a qualitative investigation of the polymerization of monomer and a number of monomer-polymer slurries in the

presence of various amines are given. The rate of polymerization

was followed by means of an ultrasonic viscosity apparatus.

2 . EXPERIMENTAL PROCEDURE

2.1 Materials

The methyl methacrylate monomer (Rohm and Haas) was

inhibited with Q o 006% hydroquinone

The benzoyl peroxide (Eastman Kodak) was reagent grade.

Sources of the amines are given in Table 1. These

amines were used without further purification.

The polymer powder* contained 1% by weight of benzoyl

peroxide

* The authors wish to thank the Research Dept, of the L.D. Caulk

Co . for supplying the material
.
- 5

- 2.2 Apparatus

The polymerization rate was determined by means of an ultrasonic viscometer (Ultra-Viscoson) manufactured by Rich-Roth Laboratories [23]. The instrument includes an analog-type computer

which produces an electrical output that is a function of viscosity

and density. Modifications of the instrument increased the sensitivity in the viscosity range below 1 cp . g/cm3. This was accomplished by the inclusion in the system of a microammeter and a

multiplier circuit which was developed by Hansen [24]. Ultrasonic

measurements of the polymerization rate have the advantage of (l)

ease of manipulation (2) speed and (3) simplicity. The UltraViscoson reads in viscosity times density (cp. g/cm3) units. These

readings are affected by the following factors: (l) rise in temperature of the exothermic polymerization reaction; (2) increase in

density on polymerization; and (3) modulus of rigidity of the

solution which may not be negligible in comparison to its coefficient of viscosity.

Some of these variables will compensate each other. The course

of polymerization showed extremely large variations of the viscosity-time curves in the presence of different amines. Because

of this, it is unlikely that the above-mentioned variables (for

which corrections are difficult to make) will greatly affect the

results of this study. Hence, no corrections were made for these

factors. Accurate measurements of the polymerization kinetics are

preferably made by more time-consuming procedures such as dilatometric methods

2.3 Polymerization Studies

2.3,1 Setting Time of Monomer

Preliminary studies were made using 18 x 150 mm test tubes

with 5 nil monomer solutions containing 0.1 g benzoyl peroxide and

2% by volume of the amine under investigation. These solutions

were maintained at 37° ± 1°C in an air bath. The test tubes wereinverted at various time intervals until polymerization was

evidenced by a marked increase in the viscosity. The time after

which no further flow, of the polymerized specimens could be observed was denoted as setting time. The color of the polymeric

product was noted. If the solution did not harden, no further examination was made of that amine

The setting time of monomer containing 2% benzoyl peroxide

and various concentrations of dimethyl -p-toluidine was determined

by the same procedure. The test tubes were stored at 21° ± 1°C.

Polymerization of the amine solutions which set within 2 hours

was followed by means of the Ultra-Viscoson. About 22 ml of the

monomer solution containing known concentrations of benzoyl peroxide and amine were placed in a Pyrex test tube (18 x 150 mm)

in an air bath regulated to 37° ± 1°C. The Ultra-Viscoson probe

was immediately inserted. Care was taken to ensure that the reed

of the probe did not touch the sides of the tube at any time.

- 6 - Reading of the Ultra-Vis coson^ which had been allowed to "warm up"

for at least one hour prior to the insertion of the probe into the

solution, was started at one minute after the immersion of the probe

Readings were taken every minute or two, depending on the speed of

the reaction, for the first hour and then at five minute intervals.

Measurements were usually discontinued when the material reached a

viscosity of about 2000 cp. g/cnP so that the probe could be removed.

2.3.2 Setting Time of Monomer-Polymer Slurries

In order to determine the setting time of monomer -polymer slurries, one ml of monomer solution containing different amine concentrations was mixed at room temperature with 1 g of polymer powder

containing 1% benzoyl peroxide . The rate of hardening was measured

in air at room temperature and at 37 °C ± 1°C using the modification of

the A.S.T.M. penetration test D-5-25 for bituminous materials described by Brauer and Burns [25]. All measurements were made in

duplicate

2.4 Evaluation of Physical Properties

The Rockwell Superficial Hardness Tester was used to determine

the indentation and recovery of polymeric products . One ml of monomer containing or 2% amine and 1 g of polymer containing 1%

benzoyl peroxide were mixed. Indentation and recovery values were

obtained after the mixture had been held at 37 °C for 10 min,, 30 min.,

1 hour, 3 hours, 1 day, 3 days and 5 days by the procedure of

Sweeney, Sheehan and Yost [26], Most of the mixes were too soft to

test at 10 minutes and some at 30 minutes. However, all mixes could

be measured after 1 hour. All tests were made in duplicate at room

temperature „

3. RESULTS AND DISCUSSION

The results of the preliminary experiments on the efficiency

of commercially available amine accelerators are given in Table 1.

It will be noted that only a small number of secondary or tertiary

amines produce rapid polymerization under the experimental conditions . Among the tertiary amines, dimethylaniline is an effective

accelerator. The substitution of two p-hydroxyethyl groups for the

methyl groups in N,

rdime thylani line increases the polymerization

rate. ( 5-Hydroxyethyl)methylaniiine and N-ethyl-N-(^-hydroxyethyl)

aniline show a much reduced efficiency. A methyl group substitution

in the ortho position of dimethylani line reduces the decomposition

rate of benzoyl peroxide to such an extent that polymerization does

not take place readily. An aliphatic group in the meta position

increases peroxide decomposition as compared to the ortho-isomer [ 15 ].

Substitution of electron-repelling groups in the para position

of dime thylani line will increase the accelerating efficiency. In

the presence of dimethyl - jo - toluidine, and N, N - dimethyl - p_ -

- 7

- phenylene -'diamine s the polymerization rate is very rapid. This

is in agreement with Horner’s mechanism since an increase in the

electron density of the unshared pair of electrons of the nitrogen

atoms of substituted dimethylani lines should produce a larger

accelerating effect c. Electron-attracting groups such as aldehydes have a retarding

effect on the polymerization rate. The slow rate of polymerization in the presence of diethyl-p-toluidine and p-diethylaminodiphenyl is unexpected, Horner and Scherf [14] showed that dimethylaniline and diethylaniline decompose benzoyl peroxide at

approximately the same rate . Absorption of oxygen in the benzoyl

peroxide-diethylaniline solutions is much slower and is one-fourth

of that of the equivalent peroxide -dime thylaniline systems [18],

Steric effects of the larger ethyl groups may reduce formation of

a free radical similar to D. Formation of secondary peroxide

would therefore be reduced. Rate of formation of free radicals

by abstraction of a proton from the para position of the aromatic

nucleus to form free radicals similar to D * is little affected by

the presence of N^N-diethyl groups. This would explain the formation* of equivalent amounts of p-benzoxy-N^N-dimethyl- and pbenzoxy-N^N-diethylaniline and the same decomposition rate of

benzoyl peroxide in the presence of the two amines [19]»

Aromatic compounds with amine groups in the side chain,, such

as secondary and tertiary benzylamine derivatives^ are poor polymerization accelerators. None of these compounds produce polymerization in less than 24 hours. Since tribenzylamine rapidly decomposes benzoyl p;eroxide in solution [14]* steric effects must

again be responsible for the lowered reactivity. Dimethyl- 1-

naphthylamine ^ probably for the same reason^ shows little accelera

ting efficiency.

Tr 'h few exceptions^ the aromatic amin-ls form colored products

which range from yellow for dimethyl-p-toluidine to black for

N^N-dimethyl-p-phenyienediamine , Substituents which produce a

deepening of The col"TOs ( bathochromic groups) often increase the

polymerization rate. Introduction of a second dimethylani line

group in P*

? -benzylidenebis (N^N-dimethylaniline) produces a

deepening of the color of the polymeric product.

A Most aliphatic amines have little effect on the rate of polym

erlzation. The decrease In reactivity compared with the aromatic

amines may be due to the formation of a salt of the benzoic acid

(which Is formed from the decomposition of benzoyl peroxide) with

the more basic aliphatic amine. Such a salt does not accelerate

peroxide decomposition. In the presence of secondary amine s^ the

solutions show no appreciable Increase in viscosity^ even on

prolonged standing. Monomer solutions containing diethylenetriamine and triethylenetetramine polymerize after 24 hours at 37 °C.

Tri-n-hexyfamine is the most efficient of the tertiary aliphatic

amines. Further increase in the aliphatic chain length increases

the setting time. Products usually show a lighter shade than

those polymerized with the aromatic amines.
.

I
- 8 - Heterocyclic amines have little effect on the setting time.

An exception is phenylmorpholine which* when added to monomer*

gives a pale yellow polymer in less than 2 hours.

The course of the polymerization in the presence of the more

efficient benzoyl peroxide-amine systems was followed viscometrically. A summary of "the results is given in Table 2 and Figures

1 and 2. After a short induction period* the polymerization rate

increases rapidly. Some curves show an increase in rate after a

considerable amount of polymer has been formed. This effect is

caused by a rise in temperature due to the exothermic polymerization reaction. It can also be attributed to a decrease in the rate

of bimolecular termination of growing chains with increasing viscosity of the monomer -polymer medium [27-29].

Eventually the concentration of free radicals or monomer

becomes very low. Chain growth and termination become small* resulting in a levelling off of the viscosity-time curves. Variations

in the ratio of the benzoyl peroxide -dime thyl-p- to luidine concentration greatly change the location at which the slope of the

curves becomes small (Figure l)

Dime thylanl line is a more efficient accelerator than the diethyl

di(p-hydroxyethyl) * or p-chloro-N*N-diethyl- derivatives. The

substituted toluidines ^Figure 2j show a rapid initial rise in viscosity. Complete polymerization is not reached with dl(^-phenyi~n~

propyl) -p-to luidine . With di(p-hydroxyethyl) -m-toluidine* a 1000

cp 0 g/cm 3 is reached in the shortest period of time (I9i min.).

No levelling off in this viscosity-time curve is observed.

The effect of the different concentrations of dimethyl-ptoluidine on the rate of polymerization is shown in Figure 1. A

2% solution of the amine has the largest initial polymerization

rate but the increase in viscosity with time becomes small in the

neighborhood of 50 cp, g/cm3 e Solutions containing 0.2 5% } 0.50%

and 1% amine have similar increases in viscosity below 1 cp. g/cm3

After 5 minutes* more rapid polymerization takes place for the

0.50^ and 1% solutions. From Figure 3 * showing the effect of the

accelerator concentration on the hardening time of the monomer*

it will be seen that the amine concentration between 0.75^ and

1.0% gives the shortest setting time. Further increase or decrease

in the amount of amine dissolved in the monomer produces a very

rapid increase in the setting time

The setting time of each monomer-polymer slurry is given in

Table 3. Typical penetration-time curves are shown in Figure 4.

Mixtures containing 2% amine concentration in the monomer fifet faster

than those containing 1% amine . With 2% dime thyl-p-to luidine in

the monomer* polymerization is fastest. On increasing the polymerization temperature from 22° to 37 °C* a shorter setting time is

observed. Relative efficiencies of amines in the polymerization

of monomer do not always correlate with those obtained for monomerpolymer slurries. The reason for this lack of correlation cannot

be explained readily. Short setting times are only obtained if

- 9 - complete polymerization takes place rapidly. This may explain the

decrease in the accelerating efficiency of di (p-hydroxyethyl) -mtoluidine and di(^-hydroxyethyl) aniline in monomer-polymer slurries.

Hagger [30] has suggested that the heat of swelling of the polymer

powder particles initiates polymerization. It is conceivable that

uncoiling of the polymer chains and the subsequent expansion (swelling) of the network may vary in the presence of different amines

and amine concentrations. The possibility that the rate of solution of benzoyl peroxide* which is incorporated in the powder,,

varies somewhat in the different amines must also be considered.

The relative rate of reaction of free radicals with monomer and

polymer chains and the chain transfer rate may also be altered by

varying the amine

In order to evaluate the physical properties of the polymer

specimens* indentation and recovery were determined. Results of

these measurements are shown in Tables 4-7. With 1% amine concentration in the monomer (Tables 4 and 5)* most products were too

soft to measure at 30 minutes . After 1 hour* the specimens polymerized with di( (3-hydroxyethyl) -m-toluidine had the lowest indentation and the highest recovery values. Values for dimethyl-ptoluidine followed closely. Otherwise* the results correlated

well with the setting time data; the longer the setting time* the

larger the indentation and the smaller the recovery. Specimens

polymerized with the different amines showed little variation in

indentation at 5 days. Recovery values of products of the slower

setting amines were slightly lower than those for dimethyl-ptoluidine . With 2% amine concentration* (Tables 6 and j) s polymerization occurred more rapidly as evidenced by the lower indentation and higher recovery values after one hour. Five day indentation values for the specimens polymerized with dimethyl -otoluidine were lowest. In the presence of the other amines* indentation and recovery were slightly poorer and showed little

variations for the various amines. Generally* amines which were

polymerized with 2% amine showed improved physical properties

compared to those polymerized with 1% amine. The data are in

agreement with previous studies which show that the molecular weights

of the polymer formed increase with an increase in the polymerization

temperatures [ 31-32 ] , Since the faster setting resins have higher

peak temperatures [ 33 ] s improved indentation and recovery values

should be expected.

4 . SUMMARY

The polymerization of methyl methacrylate has been investigated

in the presence of various benzoyl peroxide -amine systems.

Only a small number of tertiary aromatic amines are efficient

polymerization accelerators.

The course of the polymerization* in the presence of the fastest setting peroxide -amine systems* has been followed with an ultrasonic viscometer. With many amines* the initial reaction rate is
,

• I

:

.

’’’

9 ••
10

fast but the polymerization does not go to completion. The polymerization rate is often different than one would expect from the

relative rate of decomposition of benzoyl peroxide in the presence

of the amine

The setting time of monomer-polymer slurries containing different amines has been determined. The relative order of efficiency

of amines in the polymerization of monomer-polymer slurries is not

always the same as that found in the polymerization of monomer.

Polymerization with dimethyl-p-toluidine or di (p-hydroxyethyl) -mtoluidine give products having the best physical properties.

Improvement of some physical properties, such as indentation

and recovery, is obtained on decreasing the setting time. With

effective amine accelerators, colored products are formed.

The authors wish to thank Mr. Francis R. Burns for assisting in

the experimental work.

11

BIBLIOGRAPHY

1 . Deutsche Gold und Silberscheideanstalt vormals. Roessler

French patent 883., 679 - March 29* 1943

Swiss patent 230,706 - January 31* 1944

Swedish patent 126,006 - July 7* 1949

2. Deutsche Gold und Silberscheideanstalt vormals. Roessler

French patent 895*566 - April 3* 1944

Swiss patent 237*089 - March 31* 1945

3. Kern, W., Makromolek. Chem. _1, 209 (1947).

4. Kern, W., ibid 1* 249 (1947).

5. Noma, K., Nagai, K. and Nishiuro, 0., Chem. High Polymers (Japan)

6

,

479 (1949).

6 . Taylor, P. B. and Frank, S. L., J. Dent. Res. 29 * 486 (1950).

7. Knock, F. E. and Glenn, J. F.* (to L.D. Caulk Co.)

U. S. patent 2,558,139 - June 26, 1951.

8 . Imoto, M. and Choe, S., J. Inst. Polytech., Osaka City Univ. Ser.

C, 4, 115 (1953).

9. Bartlett, P. D. and Nozaki, K., J. Am. Chem. Soc. 69 , 2299 (1947).

10. Meltzer, H. T. and Tobolsky, A. V., J. Am. Chem. Soc., 76 , 5178

(1954). —

11. Horner, L. and Schwenk, E., Angew. Chem. 61 , 4ll (1949).

12. Horner, L., ibid 62 , 359 (1950).

13. -Horner, L. and Schwenk, E., Ann. 586 * 69 (1950).

14. Horner, L. and Scherf, K., Ann. 573 * 35 (1951).

15. Ibid, 574 , 202 (1951).

16. Ibid, 574 , 212 (1951).

17. Horner, L. and Betzel, C., Ann. 579* 175 (1953).

18. Horner, L. and Junkermann, H., Ann. 591 , 53 (1955).

19. Goode, W. E. and Gratsch, S., Abstracts of papers - 126th Annual

Meeting of the American Chemical Society, September 1954* p. 78.

20. Imoto, M. and Choe, S., J. Polym. Sci. 15, 485 (1955).

Feagin, E. C., Abstracts of papers - 30th Meeting of International

Association for Dental Research, Colorado Springs, 1952.

21 .

12

22 . 23.

24.

25-

26 .

27.

28 . 25.

30.

31.

32

33

BIBLIOGRAPHY (cont.)

Lai, J. and Green, R., J. Polym. Sci. 18, 403 (1955).

Roth, W. and Rich, S. R., J. Appl . Physics 24 , 940 (1953).

Hansen, W. C., unpublished data.

Brauer, G. M. and Burns, F. R., J. Polym. Sci. (in press).

Sweeney, W. T., Sheehan, W. D. and Yost, E. L., J. Am. Dent. Assn.

49, 513 (1954).

Norrish, R. G. W. and Smith, R. R., Nature 15£, 330 (1942).

Schulz, G. V. and Blaschke, F., Z. phys . Chen. B 51, 75 (1942).

Trommsdorff, E., Kfthle, II. and Lagally, P., Makromolek. Chem.

1 , 169 (1948).

Hagger, 0., Helv. chim. acta 34, 1872 (1951).

Melville, H. and Watson, V/. F., J. Polym. Sci. 11 , 299 (1953).

Swarc, M., J. Polym. Sci. 13, 317 "(1954)

Caul, K. J., Sweeney, W. T. and Paffenbarger, G. C., J. Am. Dent.

Assn. ( in press)
* . >

». •
- 13

TABLE 1

POLYMERIZATION OF METHYL METHACRYLATE IN THE PRESENCE OF VARIOUS AMINES

Benzoyl peroxide concentration - 2%

Amine concentration- ------ 2^

romatic Amines

N,N-Dimethylaniline

N* N -Di e thy1 ani line

N, N-Di-n-propylaniline

2-Anilinoethanol

N* N-(p -Hydroxyethyl)

methylani line

N. N-Di ( p-hydroxyethyl)-

aniline

N-Ethyl-N- ( p -hydroxyethyl )-

aniline

N-Phenylglycine

Phenylhydrazine

N5 N-Dimethyl-o-toluidine

N_, N-Dime thy1-m-to lui dine

Nj N-Diethyl-m-toluidine

N* N-Di ( p -hydroxyethyl ) -into luidine

2-Aminobenzenethiol

m-Dimethy1 aminopheno

m-Diethylaminophenol

N, N-Dimethyl-p-toluidine

Source** Setting

Time

hrs

Color

E reagent <i amber

F reagent <i dark brown

E reagent

E reagent

<

2

i

np

amber

E reagent np

TE <i gold

TE >24

layered

brown-yellow

E reagent np

yellow-brown

F reagent np amber

E reagent np red-brown

E reagent < 2 pale yellow

E reagent <

2 dark brownyellow

TE <24 amber

C >24 dark (brown

or black)

M technical >24 blue -black

E practical >24 black

E reagent <1 medium yellow
- • •

' ' 'i

‘

.
- 14

TABLE 1

f

romatic Amines (cont .

)

N* N-Diethyl-p-toluidine

| p-Dimethylaminobenzaldehyde

p-Dimethylaminoazobenzene

N, N-Dimethyl-p -phenylenediamine

p-Diethylaminodiphenyl

N, N-Bis (iTphenyl-n-propyl ) - Pptoluidine

p^Chloro-NjN-diethylaniline

P>P f -Tetramethyl diaminodiphenylme thane

P*P'-[p-Hydroxybenzylidenebisdimethylaniline ) ] Sj £

* -Benzylidenebis (N. Ndimethylaniline)

P^ £j P

M -Me thy1i dyne t ri s ( Ndime thylani 1 inej

Dibenzylamine

a Me t hylbenzylmonoethanolamine

Benzyldimethylamine

a-Methylbenzyldiethanolamine

Dimethyl- 1 -naphthylamine

Tribenzylamine

liphatic Amines

Ethylenedi amine

Di -n-propylamine

Di -i sopropano lamine

( cont .

)

Source** Setting

Time

hrs

Color

E reagent < 24 yellow

F reagent > 24 yellow

F reagent < 1 red-brown

E practical > 24 black

E reagent np.

E practical > 24 yellow

E_ reagent < 2 amber - dark

on top

F technical < 1 muddy yellow ,

E reagent np. red (

E reagent < 1 blue-green

E reagent np

purple ; E reagent np. yellow-orange

C+C np. ----- R + H np

£+£ > 24 medium yellow

E reagent > 24 colorless

E reagent > 24 colorless

E reagent np. <s. E practical np. deep, rose

E practical np

dark
'

-

A

. 1

.

‘

.
- 15 -

TABLE 1

Aliphatic Amines ( cont .

)

Di-isobutylamine

Di -sec -butylamine

Di-n-hexylamine

Dicyclohexylamine

Methylcyclohexylamine

Diethylene triamine

Triethylenetetramine

Hexame thylene te tramine

Dime thylaminoe thano

Triethanolamine

2-Di -n-butylamino - 1 -propano

Tri -isopropanolamine

Tri -n-butylamine

Tri -n-hexylamine

Diethyleyclohexylamine

N*N-Dime thyIdodecylamine

N^N-Dimethyl tallowamine

Armeen 2HT

Amine C

Amine S

Sequestrene Na-2 (disodium

ethylenediamine tetraacetate

dihydrate)

Petrolite Wax Amine C

(cont .

)

Source ** Setting

Time

hrs

Color

E reagent np. » E reagent np.

E reagent np.

E practical np

dark

D np. amber

E technical > 2b yellow

E technical > 24 yellow

E reagent np

RH-H > 24 layered* red

to yellow

E reagent < 4 brown-yellow

E practical np. brown

E practical > 24 yellow

E practical > 24

E reagent < 2 pale yellow

E practical np. dark

A > 24 medium yellow

A np

A > 24 mauve to

pale yellow

AT ^ 24 dark amber

A1 > 24 dark amber

A1 > 24 colorless

P > 24 pale yellow

16

TABLE 1 (cont.)

Aliphatic Amines ( cont

) Source ** Setting

Time

hrs

Color

Petrolite Wax Amine B P > 24 pale yellow

Petrol lie tax Amine A P > 24 pale yellow

Jietliy 1 1 i rL o c av b . miy lchloride n > 24 pale yellow

Heteroeyc lie Amines

2- ( 2-Dime thylaminoethylamino )—

pyridine Mo np.

Piperidine F reagent np. dark

Pyrrole F reagent np

black

Pyrrolidine E practical np. dark

Phenylmorpholine E practical < 2 pale yellow

Phthalamide E reagent np. colorless

1 np

- no polymerization took place within 48 hours

A1 Alrose Chemical Co.* M Matheson* Coleman + Bell*

A Armour and Co

* Mo Monsanto Chemical Co.*

C+ C Carbide and Carbon Chemical Co .* P Petrolite Corporation* Ltd'3

C American Cyanamid Co.* R *f H Rohm and Haas Co.*

D E.I. DuPont de Nemours -f Co « * S Sharpies Chemical Co.*

E Eastman-Distillation Products

Industries

TE Tennessee Eastman Co.*

F Fisher Scientific Co. 4

* The authors wish to thank these companies for generously supplying

the samples
1

.

:

'

.

. I -

.
- 17 -

TABLE 2

POLYMERIZATION of METHYL METHACRYLATE with BENZOYL PEROXIDE-AMINE SYSTEMS

Benzoyl peroxide concentration - 2%

Viscosity - cp. g/cm

Amine Concentration - %

Dime thylani line

Diethylaniline

p_-Chloro-N, N-diethylaniline

Di ( p-hydroxyethyl )

aniline

Dimethyl-o-toluidine

Dime thy1-m-toluidine

Diethyl-m-toluidine

Di ( £-hydroxyethyl ) -into luidine

Dimethyl-p-toluidine

Di (r-phenyl-n-propyl )

]D-toluidine

Tri-n-hexylamine

Triethanolamine

5.0 100.0

0.5 1.0 2.0 0.5 1.0

min min min min min

42 18 12 75 26i

45 — 19 70 43

np. np

33 np

np.

np. 30 20 np. 42

np

10| 7 np

21

19 16 14 44 32

11 12f 10 14 17

10 8 5 2l| 18

np. 13 np

np

41

39 29 20 np. 113

25 np. 77 120 np

1000.0

2.0 0.5 1.0 2.0

min min min min

2l£ — 35 43

29 100 63 41

44 np. np

61

25 np. 454 32

23 np

304 100

30 73 524 110

134- np. 194 np

46 34 34 185

• np

np

np. np

76 np. 119 88

200 np

np. 215

np. - No polymerization took place

- 18 - table 3

SETTING TIME of MONOMER-POLYMER SLURRIES

Penetration Test

Mixture: One gram of ground polymer containing benzoyl peroxide

One milliliter of hydroquinone-stabilized monomer

Material Time of Set in min.

Temperature

Amine

37 °C 22 °c

Concentration

in Monomer

2$’ i% 2$ 1

—

Dimethyl-p^-toluidine 7 9 9 9i

Di(p-hydroxyethyl) -m-toluidine 9 12 l8 24|

Diethyl-m-toluidine ll 14 — —

Dime thylani line 12 I7i i9i —

Di(p-hydroxyethyl) aniline 15 2li — —

Diethylaniline l8 <

—

| Commercial monomer and polymer 8 14

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Fig. 1 POLYMERIZATION OF METHYL METHACRYLATE IN THE PRESENCE OF

VARYING CONCENTRATIONS OF DIMETHYL-p -TOLUIDINE

Temperature - 37 °C Benzoyl Peroxide Concentration - 20

Dimethyl-g-toluidine concentration - A = 0.50$, B = 1.000, C = 0.250, D = 2.000
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%
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-10-0707 December 30, 1955 4313

Progress Report

ALLOYS OF GALLIUM WITH POWDERED METALS AS POSSIBLE

REPLACEMENT FOR DENTAL AMALGAM

by

Denton L. Smith*

Harold J. Caul *

* Research Associate, American Dental Association Research

Fellowship, 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.

(Task 7756-41 BV No. 55-14)

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 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 specifi- cally prepared If that agenqf wishes to reproduce additional copies of that particular report for its own use,
' s s

.
ALLOYS OF GALLIUM WITH POWDERED METALS AS POSSIBLE

REPLACEMENT FOR DENTAL AMALGAM

Abstract

The possibility of using gallium as a liquid

metal to replace mercury in dental restorations was

investigated o It was found that gallium would not

combine at room or mouth temperature with bodycentered'*' cubic metals such as tantalum* chromium

and molybdenum.,. Gallium did combine with powdered

face-centered cubic metals such as nickel* gold*

copper* with alloys of copper and tin* and with

cobalt

.

With the exception of gallium-cobalt these

alloys had adequate compressive strength and hardness

but expanded excessively during hardening „ Galliumcobalt- had low compressive strength but exhibited

shrinkage during hardening

.

Gallium alloys have been made that have some

physical properties that are better than dental

amalgam 0 None of the alloys investigated possess

all the desired physical properties for oral

restorations

„

1 . INTRODUCTION

The metallic element gallium was first predicted by Mendeleff

in 1871 in his work on the correlation of the properties of the

elements with their atomic weights [l] a It was also predicated

at about the same* time by de Boisbaudran who concluded* while

studying the spectral lines of the elements* that there was an

element missing between aluminum and indium, in the periodic table.

Gallium was first isolated by de Boisbaudran in 1875 [2].

The amount of gallium present in the earth’s crust is about

the same as lead* roughly 15 g per ton [ 3 ]- Unlike lead which is

concentrated in localized areas* gallium is widely distributed.

The low concentration of the ore and lack of demand for the metal

make gallium relatively expensive at $ 2.50 to $5*00 per gram [ 3 ].

Gallium has several interesting physical properties. Its

melting point is 29.78°C {85*60°F) [4] while its boiling point is

1983 °C (3601°F) [4]* thus giving it one of the longest liquid

ranges of the metals. The metal has a low vapor pressure even

at high temperatures* a pressure of 1 mm of mercury being attained

at 1315°C (2399°F) [4], Gallium expands 3°4$ in volume during

solidification* the solid having a density of 5*90 g/cm3. It has

the property of wetting many materials including tooth structure.
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The true nature of this property has not been determined but the

wetting characteristic is generally attributed to a thin oxide

film on the surface of the metal [6]. It has also been reported

that gallium is essentially nontoxic [7 ]

This combination of physical properties indicates that gallium

instead of mercury might be used as a component in a filling

material . Some alloys of gallium begin to melt at higher temperatures than does dental amalgam. These factors make it desirable

to investigate its possible use in oral restorations. It is expected that gallium, instead of mercury, might be used as a liquid

metal to be combined with a powdered metal or alloy to form such a

filling material . To be successful the combination should readily

mix and condense, preferably using implements similar to those

already familiar to the dentist. The gallium alloy should harden

at mouth temperature within a reasonable length of time and possess

strength, dimensional stability and corrosion resistance equal to

or greater than those of dental amalgam.

2 . MATERIALS AND METHODS

Gallium metal of 99-9+$ purity was used in the experiments

described here. The powdered metals and alloys used were obtained

from laboratory supply houses, powdered metal manufacturers, or

by alloying and comminuting in the Dental Research Section at

the National Bureau of Standards. All metallic powders passed a

No. 325 sieve of the U.S. Standard Sieve Series. For laboratory

control, all mixing and condensing of specimens was at 37 °C

(98 .6°F) which is above the melting point of gallium. Mixing was

accomplished either in a mortar or in a mechanical amalgamator.

All of the triturated mass was condensed. No liquid metal was

expressed prior to or during condensation with one exception as

noted.

Determinations of the dimensional change were made at 37 °C

(98.6°F) using unrestricted specimens 10 mm long and 5 mm in

diameter in a differential interferometer with a fiducial reading

15 minutes after mixing was started. The compressive strength

was determined at 21 °C (69.8°F) using cylinders 8 mm long and

4 mm inddiameter. The strength of one series of gallium-nickelsilicon alloys was determined at 37°C (98.6°F) and at 50°C

(122°F). The rate of loading of the compressive strength specimens was in all cases 80 lbs ( 38.3 Kg) per minute.

3 . RESULTS

Gallium-tantalum mixture . Two mixes of gallium and powdered

tantalum were made, one containing 40$ gallium and the other 25$

gallium. The first was mixed with a mechanical amalgamator, the

second with a mortar and pestle. Both specimens could be easily

packed. These metals did not alloy. At room temperature the

specimens hardened by the freezing of the gallium but readily

softened when reheated to mouth temperature

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- Gallium-chromium mixture . Gallium and finely divided chromium

did not mix well. A specimen containing 35$ gallium hardened by

the solidification of the free gallium present. As in the case of

the gallium-tantalum mixture* the gallium-chromium specimen

softened by the melting of the gallium when heated to body temperature. A mix containing 50$ gallium was too crumbly to pack.

When these mixtures of gallium and chromium were heated over

an open flame the two metals combined with an exothermic reaction

causing the mass suddenly to become bright red. The combination*

after cooling* was a black friable powder.

Gal Hum-molybdenum mixture . Powdered molybdenum was mixed

with 35* 40 and 50% gallium* respectively* in a mortar and in a

mechanical amalgamator. The 50$ gallium mixture was too wet to

pack satisfactorily* while the 35$ gallium mixture was too dry to

pack well. The resultant mixtures did not harden during a 5-week

period at room temperature. Small droplets of what appeared to be

gallium were observed on the surface of the specimens.

Mixtures of gallium and molybdenum also combined exothermically when heated over- an open flame to form a brittle* black mass.

Gallium-silver-indium alloys . Silver alloys containing 22^*

30 and 34$ indium were prepared and chill cast into a steel mold.

These alloys were then comminuted on a lathe. None of these alloys

when combined with gallium hardened during storage at 37 °C (98.6°F)

for 1 month. The combinations remained soft and were easily

deformed in the hand

.

Gallium-gold alloy . A mixture containing 60$ precipitated

gold powder and 40$ gallium hardened. These components react

exothermically to form the intermetallic compound AuGa2 [7].

The rate at which. the reaction occurs is apparently dependent on

the temperature at which the metals are mixed. Mixing temperatures

of about 22°C (73 °F) ^resulted in no appreciable heat generation;

but when the components were mixed at a temperature of approximately 55 °C (131°F) and packed into a 4 x 8 mm cylinder containing

a thermocouple* temperatures as high as 450°C (842°F; were _ recorded

.

The blue color* characteristic of AuGa2 [7]* was noted to

extend throughout the specimen. These alloys had a linear expansion of about 12.5$ during hardening.

Gallium-cobalt alloy . A mixture of 40$ gallium with cobalt

powder contracted approximately 3/^/cm in 24 hours and was stable

for 4 days thereafter. This is the only alloy investigated that

contracted during hardening. The 7-day compressive strength of

this alloy was 12*700 psi with a standard deviation of 1*100 psi.

This low compressive strength makes the gallium-cobalt alloys

unsuitable for oral use* but powdered cobalt may be introduced

into an experimental gallium-powdered metal alloy to control an

excessive expansion.
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Gal lium-nickel alloys. Nickel powder that passed a U.S.

StandardSTeVeTToT^^5~conibined readily with gallium in several

proportions. For practical purposes a gallium content from 20

to 40$ was desirable. Less than 20$ gallium resulted in a dry

granular material that could not be readily packed and more than

40$ gallium made a sloppy alloy that was difficult to handle.

The data on compressive strength and hardness of four galliumnickel alloys are summarized in Table 1. When triturated JQ

revolutions in 30 seconds, the 35$ gallium alloy expanded from

128 to 136x/cm at the end of 24 hours and was about constant

at 140 to l49^v/cm from 2 to 5 days (Fig. 1, Curves 1 and 2).

Mixing this alloy in a mechanical amalgamator for 1 minute instead of a mortar for 30 seconds resulted in a 24-hour expansion

of about 60>^/cm (Fig. 1, Curve 3). This increased to about

75x/cm expansion after 3 days and remained constant for 14 days

thereafter. Increasing the amount of work done on the mix appears

to reduce the expansion and increase the compressive strength of

the 35% gal Hum-nickel alloy.

Either powdered cobalt or powdered silicon reduced the large

setting expansion of gallium-nickel alloys. For example, an

alloy of 35$ gallium,. 55$ nickel, and 10$ cobalt expanded about

55>*/cm at the end of 24 hours (Fig. 1, Curve 4). It continued

to expand for about 6 days until a total expansion of approximately

70 ^/cm had occurred. No additional expansion had occurred at the

end of 16 days.. A specimen containing 35$ gallium, 62$ nickel

and 2$ silicon expanded 37>^/cm in 24 hours. However, this method

of reducing the* high. expansion of the gallium-nickel alloys also

reduced their strength. A comparison of the 24^hour compressive

strengths Is given in Table 2

.

The compressive strength of the 38$ gallium, 60$ nickel and

2$ silicon alloy_remained constant from 20 °C (68°F) to 50°C

(122°F). The strength of amalgam at mouth temperature was reduced

approximately 21$ and at 50 °C (122^F) the strength was decreased

about 43$ (Table 3)

The tarnish resistance of gallium-nickel alloy was superior

to amalgam wherr- exposed to ammonium polysulfide vapor. The sulfide film that .did form on the gallium-nickel alloy was easily

removed by wiping with the finger, whereas the sulfide film on

amalgam was adherent

.

Gallium-copper and galllum-copper-tin alloys . Gallium

combined with copper and copper-tin alloys of several compositions.

Copper powder and a powdered alloy containing 90$ copper and 10$

tin were obtained from a manufacturer of powdered metals. Alloys

with compositions corresponding to the y(73$ copper-27$ tin),

g (68% copper -32$ tin), £(62$ copper-3o$ tin) and >^f4l$

copper-59$ tin) phases of the copper-tin phase diagram (Fig. 2)

[9] were made and comminuted. The compressive strengths of these.

- 5

- when combined with 35$ gallium., are shown in Figure 3» The alloy

made by the combination of gallium and speculum metal (68$ copper32$ tin) was readily mixed and packed and had good strength . This

alloy hardened slowly as shown in the strength-time curve in

Figure 4* and expanded from 46 to 50-^/cm on hardening (Figure 5).

This expansion is less than that of the 35$ gallium-65$ nickel

alloy but is still high when compared to the expansion of dental

amalgam (3 to 13 ^/cm is the range of expansion permitted by

American Dental Association Specification No „ 1 for Dental Amalgam

Alloy) [10]. The hardness of this alloy varied from 100 to 119

Brinell which corresponds to the upper range of a type II (medium)

or the lower range of a type III (hard) inlay casting gold alloy [10],

Gallium-copper-beryllium alloys . Gallium combined with a

powdered copper-beryllium alloy (9^$ copper-2$ beryllium) to form

an alloy that hardened rapidly. The compressive strength (average

of 2 specimens) of this copper-beryllium alloy when combined with

35$ gallium was 19*500 psi at 1 hour and 18*500 psi at 24 hours.

This would indicate that little or no reaction between the gallium

and copper-beryllium alloy occurred after 1 hour. This does not

agree with the data on dimensional change on hardening as shown in

Figure 6. The technique for preparing the compressive strength

specimens was the same as that for Curve 1* Figure 6. Different

trituration times used in preparing this alloy resulted in different

rates at which the expansion occurred (Figure 6) . The discrepancy

in the order of the expansion curves was not investigated because

the alloy had unsatisfactory compressive strength.

When the gallium content was increased to 40$* liquid gallium

squeezed out of a 1 -hour-old specimen during crushing. This specimen had a compressive strength of 14*000 psi.

Gallium and commercial dental amalgam alloys (silver 65$ min.*

tin 25$ min.*' copper 5$ max.* and zinc 2$ max. ) Y* The combination

of gallium with commercial dental amalgam alloys was unsatisfactory.

The compressive strength of these alloys when mixed with gallium

varied from a maximum of 14*000 psi after 1 week for a specimen

containing 20$ gallium* with a powdered alloy containing 69 . 1$

silver* 26 . 8$ tin* 2.8$ copper and 1.4$ zinc* to a minimum of 1*000

psi after 1 day when 35$ gallium was triturated with an alloy containing 74.5$ silver* 25$ tin and 0.5$ zinc. Trituration in all

cases was in a mortar using JO revolutions in 30 seconds with a

3 lb load on the pestle. The 1-day compressive strength of the

74.5$ silver* 25$ tin* 0.5$ zinc alloys* when combined with gallium

in this manner* decreased with an increasing gallium content as

shown in Figure 7» The compressive strength of any of these is

too low for use in the mouth.

* Composition limits for dental amalgam, as per American Dental

Association Specification No. 1 for Dental Amalgam Alloys

(First revision 1934) [10].
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- 6 - 4 . DISCUSSION

This investigation indicates that powdered face-centered

cubic metals such as gold, nickel, copper and some intermetallic

compounds of copper and tin will, when triturated with gallium,

harden at room or mouth temperature. Powdered body-centered cubic

metals such as tantalum, chromium and molybdenum, when triturated

with gallium, if they harden at all, do so too slowly to be of

practical value

.

The strength of alloys containing 3

5

$ gallium-65^ copper,

3 5$ gallium-65^ speculum metal, 35$ gallium-65$ nickel is adequate

for oral use but the expansion on hardening may be excessive as

clinical experience with alloys having a setting expansion of

about 50 ></cm ( 35$ gallium-65$ speculum metal) has not been reported . The wetting action of gallium should give good adaptation

when the alloy is condensed in a cavity. Packing a 35$ galliumr

65$ nickel alloy into a glass tube gave a mirror-like appearance

to- the inside of the tube. This wetting action is also a handicap

when handling gallium alloys. The glass and metal mixing and

condensing instruments become coated with gallium alloy making

cleaning difficult. Since gallium does not wet wax and wax-like

materials and smooth surfaces of some resins such as Teflon and

Nylon, it may be possible to make satisfactory mechanical amalgamator capsules and pluggers from these resins.

The indication that gallium alloys retain their strength at

mouth temperature and above, whereas amalgam does not, may be

significant. Failure of amalgam at the margins of a restoration

may be due, in part, to the reduced strength of the amalgam at

body temperature

.

5 . SUMMARY

The following alloys of gallium with powdered metals and

alloys have been made and found to harden at room or body temperature : 40$ gallium-60$ gold, 35$ gallium-65$ nickel, 38$ gallium60$ nickel-2^ silicon, 35$ gallium-55$ nickel-10^ cobalt, 35$

gallium-65$ copper, 40$ gallium-60$ cobalt; and 35$ gallium with

the following copper-tin alloys; 90$ copper- 10^ tin, 73$ copper27$ tin, 6Q% copper-32$ tin (speculum metal), 62% copper-38$ tin,

and 4l^ copper-59$ tin.

Mixtures of gallium with tantalum, chromium, molydenum and

some commercial amalgam alloys would not harden or were too weak

to be of practical value.

The 40^ gallium-gold alloy reacted exothermically under certain

conditions and was therefore impractical for oral restorations.

- 7

- The 35$ gallium-nickel alloy had good strength and hardness

but expanded 140 to 149 ^/cm in 2 days. The expansion was reduced

to about 75 z^/cm by increasing the amount of work done on the mix

during trituration.

Cobalt powder or silicon powder added to gallium-nickel alloys

also reduced the expansion but the 1-day strength of the alloy

was decreased by this treatment.

The 35$ gallium-speculum metal alloy had good strength and

hardness but set slowly. Lt continued to gain strength during 10

days storage at body temperature. This alloy had a setting expansion of 46 to 50 >v/cm.

Gallium alloys have been made with some physical properties

that are adequate for oral restorations. However, sufficient data

on the physical properties of gallium-powdered metal alloys are not

available to recommend any so far investigated as possible replacements for dental amalgam. Prior to any such recommendations the

alloy or alloys must be assessed biologically.
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8

BIBLIOGRAPHY

1. Mendeleeff, D. K. Die. Periodische Gesetzmassigkeit

der Chemischen Elmente. Analen der Chemie und

Pharmacies Supplementband 8 Heft 2y 10, 1871.

2. de Boisbaudran, L. Sur Un Nouveau, Le Gallium.

Ann. Chim. et Phys . 10:100 Jan. 1877.

3. Thompson, A. P. and Harner, H. R. Ga: A Byproduct

Metal. J. of Metals 191:91 Feb. 1951.

4. Roeser, W. F. and Hoffman, J. I. Freezing Point of

Gallium. NBS J. Res. 13:673 Nov. 1934.

5. Atomic Energy Commission. Liquid Metals Handbook,

Ed. 2. U. S. Government Printing Office, Washington,

D. C,, June 1952.

6. Wilkinson, W. D. Properties of Gallium. Argonne

National Laboratory Report 4109* 1948.

7. Meek, S. F., Harrold, G. C. and McCord, C. P.

Toxicity of Gallium. Industrial Med. .12:7 Jan. 1943.

8. Zintl, E., Harder, A. and Haucke, W. Legierungphasen

Mit Fluoritstruktur . Z. Phys. Chemie. B. 35:354

March 1937.

9. Raynor, G. V. Annotated Equilibrium Diagram Series

No. 2. Inst, of Metals, London 1944.

Paffenbarger, G. C. and Sweeney, W. T. American Dental

Association Specifications for Dental Materials, Ed. 1.

American Dental Association, Chicago, 1955.

10 .
V
- 9

- TABLE 1

Compressive Strength and Brinell Hardness of Some

Gallium-nickel Alloys

Compres- Baby Brinell

Composition sive hardness

Gallium-Nickel Strength 15 Kg load

wt jo wrjo psi —

l'/l TaTr Remarks

23 77 14,000 48 Triturated 2 min.

3 lb load on pestle.

Specimen 3 days old

33 67 32,000 90 Triturated 2 min.

3 lb load on pestle.

Specimen 1 day old.

35 65 49,000 90 Triturated l\ min.

3 lb load on pestle.

Specimen 1 day old

44 56 33,000 70 Triturated 2 min.

3 lb load on pestle.

Specimen 1 day old

62,000 100 As above except excess

gallium was expressed

during condensation.

Above values are individual or average of two determinations

.

10 -

TABLE 2

Effects of Addition of Cobalt and Silicon to the Compressive

Strength of Gallium-nickel Alloys

Compressive Strength

Composition Average Standard Deviation

Wt % psi psi

35 gallium

65 nickel

39,600 5,500

35 gallium

58^- nickel

cobalt

31,100 2*300

38 gallium

60 nickel

2 silicon

25,500 800

All specimens made by triuration in a mortar* 70 revolutions

in 30 seconds under a 3 lt> load.
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Compressive strength (l day) of amalgam and an alloy of

38$ gallium* 60$ nickel* and 2$ silicon at 21°C* 37°C

and 50°C (70% 99° and 122°F)

Temperature Amalgam*

38$ gallium* 60$ nickel* 2$ silicon

Average Standard Deviation

°C psi psi psi

21 49,500** 25,500 800

37 39* 000** 26,700 1*300

50 28,600** 25*200 2*000

* Alloy composition - Silver 70.3$* Copper 2.4$* Tin 25-5$

and Zinc 1 . 8$

** Average of two determinations

SETTING EXPANSION

GALLIUM-NICKEL ALLOYS

Figure 1. Dimensional Change on Hardening of Gallium-Nickel

and Gallium-Nickel-Cobalt Alloys at 37°c (98.6°F)

Curves 1 and 2.

Curve 3.

Curve 4

.

35# gallium-65# nickel triturated 70 revolutions

In 30 seconds at 3 lb load.

Same as 1 and 2 except triturated 1 minute

in a mechanical amalgamator.

35# galllum-55^ nickel -10# cobalt triturated

75 revolutions in 30 seconds at 3 lb load.

Cu-Sn Copper-Tin

°C Atomic Percentage Tin °P

Figure 2 . Copper-tin Constitution Diagram .

COMPRESSIVE

STRENGTH

-

PSI

COMPRESSIVE STRENGTH

GALLIUM-COPPER-TIN ALLOYS

0 Sn 10 20 30 40 50 60 Sn

POWOEREO METAL COMPOSITION - WEIGHT %

Figure 3. Three-day Compressive Strength of Copper and Copper-^Pln

AlloyB when Combined with 35^ Gallium.

Trituration in a mortar, 70 revolutions with a 3 lb load.

Center line is average; upper and lower lines are

standard deviations

.

COMPRESSIVE

STRENGTHPSI

COMPRESSIVE STRENGTH WITH TIME

GALLIUM- SPECULUM METAL

SPECIMEN AGE

Figure 4. Compressive Strength of 35# Gallium, 65# Speculum Metal

(68# copper-32# tin).

Triturated 70 revolutions in 45 seconds with a 3 lb load.

Center line is average; upper and lower lines are

standard deviations

.
.
EXPANSION

-MICRONS

PER

CENTIMETER

SETTING EXPANSION

GALLIUM-COPPER-TIN ALLOY

TIME-HOURS

Figure 5- Dimensional Change on Hardening of Three Specimens of

35# C&LLlium-65# Speculum Metal (68# copper-32# tin) .

EXPANSION-MICRONS

PER

CENTIMETER

SETTING EXPANSION

Figure 6. Dimensional Change on Hardening 35# Gallium65# Copper -Beryllium Alloy ( 98# copper-2# beryllium)

.

Trituration in a mortar with 3 lb load.

Curve 1 - 50 revolutions in 30 seconds.

Curve 2-70 revolutions in 45 seconds.

Curve 3 - 100 revolutions in 60 seconds.

COMPRESSIVE

STRENGTH

PSI

COMPRESSIVE STRENGTH

GALLIUM-DENTAL SILVER ALLOYS

Figure 7. Compressive Strength at 24 hours of

Gallium-Silver-Tin-Zinc Mixtures

(powdered alloy composition, silver 74 . 5#,

tin 25#, zinc 0.5#).

Triturated 70 revolutions in 30 seconds

with 3 lb load.
'
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3324 December 31* 1955 4494

Progress Report

RESIN CEMENTS AND POSTERIOR-TYPE DIRECT-FILLING RESINS

by

P. J. Schouboe*

Geo . C . Paffenbarger**

W. T. Sweeney***

Research Associate, American Dental Association

Research Fellowship, Dental Research Section,

National Bureau of Standards

.

Senior Research Associate, American Dental Association Research Fellowship, 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.

(Task 7756-41 BV No. 55-14)

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 '^obtained in wrltingfrom 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.
'

.

•
RESIN CEMENTS AND POSTERIOR -TYPE DIRECT -FILLING RESINS

Abstract

A physical property survey of nine resin

cements and two posterior-type direct-filling

resins used in dentistry showed (a) times of setting ranging from 4 to 12 minutes; (b) a temperature rise of from 10 to 4l°C above 37°C on 200

mm^ specimens; (c) a polymerization shrinkage

of 3 to 8 percent by volume; (d) an expansion on

sorption of water of 0.2 to 0.6 percent linearly;

(e) slight solubility in distilled water; (f)

some with optical opacities that approximate

enamel and dentin; (g) one with a radio-opacity

equal to that of zinc phosphate cement; (h) a

compressive strength about the same as that of

zinc phosphate cement; (i) no adhesion to the

hard tooth tissues under oral conditions; and

(j) color instability under ultra-violet light.

At the present stage of development the resin

cements should be considered as experimental materials and not as a complete substitute for

zinc phosphate cement.

1

.

INTRODUCTION

Dentistry has long awaited the development of a cement

with such superior properties that it would displace zinc

phosphate cement. Since 1952 some synthetic resin . cements

have been available commercially for dental use* but little

informative literature about them is available. It is the

purpose of this report to supply some data on the physical

and chemical properties by which these resin cements may be

compared with each other and with zinc phosphate cements.

2

.

MATERIALS INVESTIGATED

The materials surveyed are listed in Table 1 according

to type^ batch or control numbers of the manufacturer and

date of procurement. Standard deviations are given in the

tables presenting test results if more than two specimens

were tested. Otherwise the data are the average of duplicate

tests .

3.

EXPERIMENTAL PROCEDURE AND RESULTS

3.1 Composition

Powder. The proportion of organic and inorganic constiuents ofTHe powders was determined quantitatively (Table 2)

by extraction with chloroform. The resins in P . F. and Replica
.

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powders would not dissolve in chloroform; hence., these two

resins are omitted from Table 2. The resins were not identified.

The chloroform-insoluble portions of the powders referred to in

Table 2 did not contain organic materials (as no charring

occurred on heating) and were., therefore* considered inorganic.

Some of these inorganic fillers were identified by spectrographic* petrographic and X-ray diffraction analyses (Table 3)

.

Liquid . The refractive indices of the various resin

liquids ranged from 1.413 to 1.424 at 25°C. The index of refraction of methyl methacrylate monomer is 1.412 at 25 °C.

The density of the liquids ranged from 0.938 to 0.944 g/ml

at 25 °C. The density of methyl methacrylate monomer is O .938

at 25 °C. The liquids had an odor characteristic of methyl

methacrylate monomer but no chemical identification was attempted.

All of the monomers except P.F. and Replica were apparently

free of cross-linking agents* as all of the powders and all of

the hardened resins except P. F. and Replica dissolved in chloroform.

Cement c The water-soluble arsenic content of all of the

resins was determined by the modified Gutzeit Method [1] . The

specimens were prepared according to the method outlined in

the American Dental Association Specification No. 9 for Dental

Silicate Cement [2] except that the resin specimen was not

powdered. Instead* films of resin approximately 300 microns

thick were used. All of the resins were within the maximum

permissible limit of 0.0002 percent (2 parts per million) of

arsenic [ 2 * 3 ] required by the silicate cement and zinc phosphate

cement specifications.

3.2 Consistency of Mixed Unset Resin Cement

The method for determining the consistency of the unset

cement was patterned after those in the American Dental Association specifications for silicate cement [ 2 ] and zinc phosphate cement [ 3 ]

.

Apparatus . A diagram of the apparatus used in determining

the consistency is shown in Figure 1. The glass measuring tube

(A) is employed in the test for consistency for silicate cements.

The method of forming the disk (B) is used in the test for

consistency of zinc phosphate cements

.

Method . The following technic was used in preparing

all specimens unless otherwise stated. The resin powder was

placed in a heap on one end of a glass slab (3 in. wide* 6 in.

long and 1 in. thick) . A crater was made in the powder with

a flexible stainless steel spatula (S.S. White No. 324). Then

0.5 ml of monomer was ejected into the crater in 5 seconds with

a luer-type* tuberculin* hypodermic syringe having a capacity

of 1 milliliter. The monomer was allowed to soak into the powder
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- 3

- for 5 seconds, after which the mixture was spatulated vigorourly for 20 seconds, using approximately 1 square inch of

the surface of the mixing slab in order to obtain as homogeneous a mixture as possible. The glass measuring tube (A in

Fig. 1) was then slightly overfilled with the mix and struck

off flush on the end of the tube with the spatula.

These operations were completed in 20 seconds, after

which 0.5 ml of the mix was ejected on the glass plate in 10

seconds. Fifteen seconds later the glass plate weighing approximately 20 ± 2 grams and the 100 gram weight (total weight,

120 grams) were placed on the cement as shown in B, Figure 1.

The weight was removed 10 minutes after the mixing was started.

The average of the major and minor diameters of three disks

was recorded to the nearest 0.5 mm. The tests were conducted at

21 °C (70 °F) and at a relative humidity of 60 ± 10 percent.

Test Results . The diameter of the disk is a measure of

the consistency of the resin under test conditions. The effect,

of powder-liquid ratio on consistency is shown in Figure 2.

In a former investigation of the zinc phosphate cements [4]

a consistency that produced a disk 30 mm in diameter was the

average cementing consistency preferred by dentists. With the

exception of Justi, Tyton, Vitacril and Grip, the resins had

a suitable cementing consistency when a 30 millimeter disk

was produced. The consistencies of the four foregoing materials

were too heavy for use as a cement at a 30 millimeter disk.

For these four materials, a disk diameter of 40 millimeters

was judged to coincide with a proper cementing consistency.

The slope of the lines in the graph (Fig. 2) indicates

the degree of change in consistency with the powder-liquid

ratio. As the slope of the lines approaches the horizontal,

the more critical is the effect of slight changes in the

powder-liquid proportions on consistency. In other words,

the slope of the lines indicates the rate of polymerization

which in turn is governed by a number. of factors, the principal ones being the proportions of catalyst and promoter present

and the solubility of the polymer (powder) in the monomer

(liquid)

.

The data fell on a straight line when testing many of the

resins. Considerable scatter from a straight line occurred

with some other resins . Repeated check tests confirmed this

experience and indicated that the reproducibility of the data

is connected with inherent ..characteristics of the individual

products. The maximum standard deviation for a sample of any

color (Table l) was less than 1 millimeter, except Tyton and

Surgident, in which cases the standard deviations were 2 and

1.5 millimeters, respectively.
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the shrinkage during polymerization* there should be some

correlation between the powder-liquid ratios and the polymerization shrinkage. The resins in the upper portion of the

graph (Fig. 2) should have less shrinkage than those in the

lower part. As is shown in Figure 3* this relationship is

generally present.

The test for consistency provides a simple and rapid

method of detecting small differences among lots or batches

and colors of the resin and could serve as a simple production

control test.

3.3 Time of Setting

In this report the time of setting is the elapsed period

between the beginning of the mix and the time at which a

standard 1 lb. Gillmore needle does not leave a perceptible

circle on the surface of the cement specimen.

Apparatus and Method . The test was conducted as described

in American Dental Association Specification No. 8 for Zinc

Phosphate Cement [3] except that the specimen was transferred

to an atmosphere of 100 percent relative humidity at 37 °C

(98.6°F) in 1 minute* instead of 3 minutes after starting the

mix. This change was made because a proper mix of the resin

may be made in 30 seconds while at least 1-| minutes are required

for making a proper mix of zinc phosphate cement. Also if the

resin cement sets unprotected in the air* the monomer evaporates

quickly causing the formation of a crust. This was especially

true when thin* slow-setting mixes having a high monomer content

were tested. In this case the crust became thick enough to

support the weight of the needle even though the resin cement

was not polymerized in the center of the specimen.

Test Results '. The times of setting determined at three

consistencies on all of the resins show a much narrower spread

(with the exception of Surgident) than either zinc phosphate

or silicate cement (Table 5). In some instances (P.F.* Tyton

and Vitacril) there was as little as 30 seconds difference in

the time of setting of very thin and very thick mixes. Therefore*

the resin cements are far less critical than the zinc phosphate

cements in variation of time of setting among different consistencies of mix.

As the Gillmore needle test for setting time uses a cylindrical specimen 6 mm high and 10 mm in diameter* it departs

from clinical practice where a thin film of cement is used*

Consequently data obtained by a rough test for setting time

that employs thin films were compared with data obtained by

the standard Gillmore needle test. The film setting-time test

- 5

- consisted of cementing two glass microscope slides together

under a load of 100 grams * with an overlap of approximately

25 mm2 on the ends and applying a 1 kilogram load axially.

This force would shear the unset but not the set film of cement.

A new specimen was tested every 30 seconds until one was found

that would sustain the load. Times of setting by this film

test were approximately 2 minutes longer than the times of

setting obtained by the Gillmore needle method.

Substances like eugenol interfere with or prevent the

polymerization of the self-curing resins* hence prolong the

time of setting indefinitely.

3.4 Temperature Rise During Polymerization

Some dentists may feel that sufficient heat is generated

during the polymerization of these resins to be of consequence clinically. It was decided* therefore* to measure the

temperature rise during hardening of the resins. The method

employed was that used in other investigations [4, 7* 8], The

data show that the temperature rise of resin cements on setting

of a cylinder having a volume of 200 mm3 ranged from 10

to 4l°C above 37 °C which is comparable with the direct filling

resins. As the cements are used in much smaller volumes than

200 mm3 * no danger of overheating the pulp i*s anticipated.

3.5 Shrinkage on Polymerization

The volume shrinkage of the resins* of a zinc phosphate

cement and of a silicate cement as they pass from a fluid to

a solid state was measured by a displacement method previously

described [ 9 ]. The data (Fig. 3) show that at the end of

30 minutes* the resins shrink from 3-4 to 8.3 percent by volume;

the silicate cement* 1.3 percent; while the zinc phosphate

cement had an expansion of 0.3 percent. However* the slope

of the terminal portion of the curves shows that the shrinkage

of the resins is almost over at 30 minutes but that the shrinkage

of the zinc phosphate cement persists. The silicate cement

continued to shrink for 24 hours after which there was little

change from the final value of 2 percent by volume [ 9 ] . The

zinc phosphate cement shrinkage was not followed beyond 30

minutes. A final volume shrinkage for a zinc phosphate cement

in water was computed from data on the linear shrinkage in another report [5] as 0.15 percent at -the end of 30 days* after

which no additional change was noted. Thus* the resins have

considerably more shrinkage on hardening than do either the

silicate or the zinc phosphate cements

.

Shrinkage may not be as critical when the resin is used

as a cement as when it is used as a filling. The film of cement

pressed between two rough surfaces (those of an inlay and cavity)

is restrained and stretched so that the shrinkage is localized.
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- 6 - The amount of shrinkage of a thin cement film is of little

practical significance. If a cement film 100 microns thick

shrank 8 percent by volume on hardening, the linear shrinkage

would be 2.7 percent. On a film 100 microns thick this would

amount to about 3 microns

.

3.6 Expansion on Sorption of Water

In common with denture base resins [10] and the direct

filling resins [9]* the resin cements expand on sorption of

water. The data in Table 5* obtained on single specimens

by a previously described method [ 9 ] 9 show the expansion at

1 day and at 3 months . These data are of the same order as

those obtained on direct filling resins [ 9 ]. The clinical

significance of expansion caused by water sorption is not

definitely known. It may be responsible for the resin cements

not adhering to tooth structures under mouth conditions. As

in the case of the shrinkage on polymerization, the actual

dimensional change in the thickness of the film of a cement

between an inlay and the tooth would be very small.

All of the resins and a zinc phosphate cement were

placed in small cylindrical cavities prepared in stainless

steel and in aluminum blocks, which were kept submerged in

water at body temperature for 9 months. None of the materials

expanded visibly above the surfaces of the blocks except Tyton.

This behavior is not consistent with the data in Table

3.7 Solubility and Disintegration

Fully polymerized pure methyl methacrylate resin should

be highly insoluble in water. However, the resin cements

and filling materials contain catalysts, promoters, mineral

fillers, and often considerable quantities of residual monomer.

Therefore, it was decided to determine the solubilities by the

method outlined in the American Dental Association Specification

No. 8 for Zinc Phosphate Cement [3] with some modifications.

These modifications were: (a) the two flat plates were covered

with 0.003 inch tin foil to facilitate removal of the specimens;

(b) the plates and cement were placed in the 37°C (98.6°F) - 100 percent relative humidity atmosphere at 2 instead of 3

minutes after start of mix; (c) after the water was evaporated,

the weighing bottles were dried to constant weight at 120 °C

(248°F) instead of l49°C (300°F).

The residue obtained by this method is the amount of

material dissolved and eroded by the distilled water from the

specimens and is reported in Table 6 as the percentage of their

original weight. The average solubility and disintegration

of the resins is approximately the same as the maximum of 0.3

percent permitted by the American Dental Association Specification for Zinc Phosphate Cement [3]» However, the zinc phos-
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- phate cements are attacked ^readily by sulfides while the resins

are not . Since sulfides are present in the mouth* they may

be at least partially responsible for the failure of zinc

phosphate cements in service. The silicate cements are not

attacked by sulfides. This* too* may account for the silicate

cements lasting longer in the mouth than zinc phosphate cements*

even though the silicate cements are more soluble in water [8],

Thus this solubility test does not appear to represent some

mouth conditions and should not be used to intercompare different

types of dental materials . However* experience has shown the

test to be useful in comparing cements of the same type.

A 1 percent solution of glucose was mixed with saliva

in a ratio of 3 to 1 and allowed to ferment until it reached

a ph of 3.4. When films of resins* 0.5 mm thick* were immersed

in this glucose-saliva mixture, bubbles developed on the surfaces

of Acrynamel and Dura Cement in 24 hours . The color of the

surfaces of Acrynamel* Dura Cement* Dura Kore* and Smith’s

Resin Cement in contact with the mixture were bleached. The

surface of the zinc phosphate cement darkened* The gjoss was

not removed from the surface of the resins after a week's immersion. The surface of the zinc phosphate cement was so severely attacked that it could be rubbed off easily.

When the resin films were immersed in 5 percent solutions

of acetic acid (pH 2.4)* citric acid (pH 1.9)* and lactic acid

(pH 1.8) all of the cements which contained carbonate fillers - namely, Acrynamel* Dura Cement* Dura Kore* Smith's and Tyton - had gas bubbles form on the surface and were bleached severely.

The other resins were not visibly affected. A 5 percent solution of ammonium hydroxide had little or no visible effect on

any of the resins

.

3.8 Opacity

The dentist is interested in the opacity of cement to

both visible light and X-rays; the former for esthetic purposes

and the latter for oral diagnosis.

Optical Opacity. The optical opacities of enamel* dentin

and silicate cements were given in a previous report [8] wherein

the limits of opacity for silicate cements wpre set at a range

of 35 to 55 percent on specimens 1 mm thick. This range was

based largely on the average opacity of enamel - 39 percent,

and of dentin - 70 percent . It would therefore be desirable

for any filling material to have an opacity in the range of 35

to 55 percent. This opacity range would also be desirable in

a cement used with translucent materials such as fused porcelain

and synthetic resins

.

The optical opacity of the resins was determined by the

method outlined in American Dental Association Specification
) -
No. 9 for Dental Silicate Cement [2], except that the glass

plates, between which the 1 mm thick specimen was molded,

were covered with tin foil ( 0.003 in. thick) to facilitate

removal of the specimen. Also the time of placing the mix

between the plates was 1 minute instead of 3 minutes after

beginning the mix.

Opacities for the various resins are given in Table 7.

The color of some of the resin cements has a pronounced

effect upon the opacity, the lighter colors being less opaque.

However, it is possible, as the data in Table 7 show, to

produce a resin cement with the proper optical opacity for

cementing translucent filling materials. The red and black

copper phosphate cements, the zinc phosphate cements and the

zinc oxide -eugenol cements are all 100 percent optically

opaque on specimens 1 mm thick [11].

Radio -opacity . When using cements as a base under other

restorations or for cementing opaque materials, the cements

should be as radio-opaque as possible in thin sections so

that there will be no confusion on the X-ray picture between

the cement and decayed areas

.

The radio-opacity was determined by X-raying a six-step

wedge of cement (Fig. 4) under a set of standard conditions.

As each step was approximately 0.5 mm thick, the six-step

wedge ranged in thickness from 0.5 to 3.0 mm. The exposures

were made at normal incidence with a film-target distance of

16 inches at 10 milliamp-eres ^and 65 kilovolt peak. Kodak

Radiatized film was exposed for 1 second. The solutions were

Eastman Kodak liquid developer and fixer made up according to

the manufacturer’s directions. The film was agitated constantly during processing, developed 4^ minutes at 20 °C,

rinsed for 2 minutes in running water, shaken as dry as possible

fixed for 10 minutes and washed for 20 minutes in running water.

Film densities for various thickness of the cement wedges

were read on the zero to three scale of an Ansco densitometer.

The data on the 2 mm thick step in the wedge are presented in

Table 8. The lower readings indicate the greater radio -gp&city.

The resin cements should be as radio -opaque as zinc phosphate

cements, if used as a base or as a cement for opaque filling

materials. Only Grip had such a radio-opacity. Most of the

resins were less radio-opaque than silicate cements.

3.9 .Film Thickness

If a cast restoration of lost tooth structures is to be

seated accurately, the cement film between the casting and

the tooth should be as thin as possible. Thus, the cementing

medium should have only small particles on it. One method

for determing the effect of particle size of the cement powder

- 9

- on the cement film is to measure the thickness of a film between

optically flat plates. Such a test which is a part of American

Dental Association Specification No. 8 for Zinc Phosphate

Cement [3] was employed except that the load was applied to the

plates 1 minute instead of 3 minutes after mixing was started.

The data (Table 9) show that a film thickness as low as 10

microns may be obtained with one resin cement (Grip). Six of

the cements would comply with the film thickness requirement

of the American Dental Association Specification for Zinc Phosphate Cement [3] as they had a film thickness of 40 microns

(0.0016 in.) or less at the standard consistencies v (Fig . 2).

Thinner than standard consistencies produced films less thick

and thicker than standard consistencies produced films of greater

thickness. This is probably so because monomer in excess softens

the polymer grains in the powder which permits them to be flattened

before the cements harden. Evidently the particle size of the

inorganic fillers was less than that of the polymer particles

in the cement

.

The data on film thickness in Table 9 show that resin

cements can be formulated thab have thinner films than any of

the currently certified zinc phosphate cements.

3.10 Compressive Strength

The compressive strength of a dental filling material

should give some indication of its resistance to masticatory

forces. The compressive strength of a cement should be useful

in evaluating the ability of a cement to hold restorations in

place. As will be subsequently shown., the retention is not due

to the true adhesion of the cements to tooth structure or to

the restorations being cemented, but to a mechanical interlocking when the cement flows into the irregularities of the surfaces

of the cavity and the restoration. To displace the restoration

it is necessary to shear off these minute projections; the higher

the compressive strength, the higher the force required to shear

them. The strength of the individual particles of powder in

the set cement would influence the amount of shear required to

dislodge a cemented restoration if the particle was large enough

to wedge between the surfaces of the cavity and the restoration.

The compressive strength of the resins was determined on

specimens 1 hour and 1 week old by the procedure used in determining the compressive strength of silicate cement [ 2 ] except

as follows; (a) the test cylinders were 6 mm ( 0.236 in.) in

diameter and 10 mm (0.394 in.) high; (b) tin foil [O.O76 mm

( 0,003 in.) thick] covered the glass plates used in forming the

ends of the molds in which the test cylinders were prepared;

(c) these plates and the stainless steel molds were lubricated

sparingly with a silicone (Dow-Corning high vacuum grease)

;

(d) the ends of the test cylinders were made plane and parallel

by facing the specimens on a lathe. The aluminum oxide filler
Iiiii
10

present In P. F. dulled the cutting tool so badly that it had

to be resharpened after cutting each specimen. (This dulling

of instruments by the aluminum oxide abrasive has been observed

by dentists in cutting P. F. with a dental bur and should not

be interpreted as indicating that this resin is extremely hard)

.

It was difficult to obtain smooth-sided cylinders with

some of the cements having high shrinkage values (Fig. 3)

.

Better specimens of these cements were obtained by filling . a retaining ring approximately half full of the resin and by

inserting the mold inside this ring causing the powder-liquid

mixture to rise from the bottom to the top surface of the mold,

in much the same manner as specimens used in the test for compression of the hydrocolloidal impression materials are formed

[15].

The resins did not fracture on failure in compression in

the same manner as the various dental cements and amalgam

which, being brittle, suddenly go to pieces at the ultimate

load. Instead the test cylinders of the resin barrel-out and

sometimes shear diagonally.

The compressive strengths of the resins and various other

dental restorative materials are given in Table 10. As is

common with most restorative materials formed by a chemical

reaction between a powder and a liquid, the resin cements generally increase in strength with time. The strength at 1 week

is usually more than that at 1 hour. These data also show that

the compressive strength of the resin cements at 1 week is on

the average roughly two to five times that of zinc oxide-eugenol

cement; seven-eights of the minimum value (12,000 lb/in^) for

zinc phosphate cement in American Dental Association Specification No. 8 [3]; less than one-half the 24-hour minimum value

(23*000 lb/in2) for silicate cements in American Dental Association Specification No. 9 [2]; and from one-third to onefifth that of amalgam. The clinical use of zinc oxide-eugenol

cements, zinc phosphate cements, silicate cements and dental

amalgam has placed their lasting qualities against masticatory

stresses in the mouth in the same order as their compressive

strength (Table 10)

.

Since the resins have about the same compressive strength as the zinc phosphate cements which do not

hold up well in the mouth, it is reasonable*to assume that

the direct filling resins cannot be used successfully for restorations subject to masticatory forces*- In fact, experience

with the indirect filling resins proved this. It is believed

that the resin cements would retain an inlay or other appliance in place to about the same degree as zinc phosphate cements

on the basis of their comparative compressive strength.

3.11 Indentation Hardness

The hardness of the resins, as determined by the Knoop

indenter, fell within a range of 9 to 16 kg/mm^ indicating
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11

that these resins are no more resistive to indentation than

polymethyl methacrylate denture base resin. The incorporation

of an abrasive such as alumina in P. F. does not make the

resin significantly harder, but the alumina does make the

compound abrasive

.

3.12 Adhesion

A rough screening test for adhesion was employed to find

out if any of the resin cements or the direct filling resins

(posterior type) were actually adhesive to enamel and dentin

under conditions of use . A flat surface was ground on one

side of the crowns of freshly extracted teeth which were not

allowed to dry out. The flat surface of enamel and dentin

was swabbed with cotton pellets and dried with warm air as

is done in practice. The various resins were then applied to

the clean, dried surface. All of the resins showed an initial

adhesion to the tooth tissues and if the tooth and resin were

kept dry the adhesion was so strong that usually the tooth would

be broken if an attempt was made to remove the resin. However,

after soaking the tooth with the resin on it in water, this adhesion was lost. There were no exceptions, and zinc phosphate

cement adhered as long as the resin cements. After 24 hours

soaking in water most of the cements could be easily dislodged;

after 1 week soaking, all of them could. Similar tests were

performed in the mouth using the buccal surfaces of the lower

posterior teeth except, of course, that the surfaces of the

teeth were not ground. The teeth were isolated under a rubber

dam and the buccal surfaces were cleaned and dried with cotton

and warm air blasts and a small amount of resin was applied to

every other tooth; zinc phosphate cement was used on the alternate teeth. At the end of 1 week, all were off. The zinc phosphate cement persisted as long as the resin cements.

3.13 Color Stability

Experience over a number of years has shown that the results of laboratory tests using the color stability procedure

outlined in American Dental Association Specification No. 12

for Denture Base Resin [16] correlate with service conditions.

The denture base resins that discolored under this test discolored in use. The same held for the denture repair resins

and the direct filling resins. The procedure was therefore

employed to test the color stability of the resin cements.

None of the materials (Table l) complied with the requirements.

Some discolored more than others. Methods for making the

self -curing resins color stable to ultraviolet light are now

available; hence these resin cements will probably be made color

stable soon.

As pointed out previously in the discussion of the solubility of the resin cements, those containing carbonate fillers
:

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12

bleach in the presence of acids such as those present in an

incubated glucose-saliva mixture.

3.14 Biological Effects

The authors have not located any literature on the

effect of the resin cements upon the tooth pulp. Nor are

they acquainted with reports of gross pulp damage after

clinical use. The similarity of the resin cements to the

direct filling resins (anterior type) would suggest about the

same degree of pulp irritation with resin cements as occurs

with the direct filling resins. As is well known* there is

considerable controversy in the literature dealing with both

laboratory and clinical experience on the degree of irritation

caused by the self-curing resins. An extensive study by

Johnson [17] with one resin cement was on the whole favorable.

4. SUMMARY AND CONCLUSIONS

Data presented deal with the physical and chemical evaluation of nine resin cements and two posterior-type direct-filling

resins. All of these resins are somewhat similar to the anterior-type direct^ftiling resins. None of the resins contained

significant amounts of arsenic. The major difference between

the resins and the anterior-type direct-filling resins appears

to be the usfe of mineral fillers in the cements which run from

5 to 67 percent by weight of the powders

.

All tests on physical properties were conducted on specimens made from standard consistencies using modifications of

tests outlined in American Dental Association Specifications

for Zinc Phosphate Cement [3]* Silicate Cement [2]* and Denture

Base Resin [16 ]

.

The times of setting ranged from 4 to 12 minutes at the

standard consistencies. Large deviations from the standard

consistencies usually had a surprisingly smaJJL,^effect on the

time of setting. The resins were much less sensitive than the

zinc phosphate or silicate cements to consistency variations.

The temperature rise on setting of a cylinder having a

volume of 200 mm3 ranged from 10 to 4l°C above 37 °C which is

comparable with the direct filling resins. As the cements

are used in much smaller volumes than 200 mm3* no danger of

overheating the pulp is anticipated.

The shrinkage on polymerization ranged from roughly 3 to

8 percent by volume which is far more than reported shrinkages

for zinc phosphate cement ( 0.15 percent) and silicate cement

(2 percent) . The shrinkage* while being a very objectionable

property* is less critical in a cement than in a filling material because the cement is generally used as a thin film in

which the shrinkage in its length is localized by the restraint

of the surfaces being cemented. The shrinkage of the cement

over even a comparatively wide cement margin is not large.

Even with a cement having a shrinkage of 8 percent by volume*
;

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- 13

the linear shrinkage of a film 100 microns thick would he only

about 3 microns

.

The resins expanded linearly on sorption of water from

0.2 to 0.6 percent on disks 0.5 mm thick in 1 day. In five

resins the expansion decreased with time up to 3 months immersion. In two there was a slight increase, while in four

there was no significant change in the 1-day and 3~month values.

The expansion caused by water sorption may be responsible for

failure of the initial adhesion of the cement to hard tooth

tissues

.

The resin materials were slightly soluble in distilled

water. When subjected to the solubility test used in the

American Dental Association Specification for Zinc Phosphate

Cements [3]* the cements and direct filling resins ranged in

solubility from almost zero to 0.4 percent by weight. The

resin cements having carbonate fillers were attacked by a

glucose-saliva mixture.

Resin cements may be made with optical opacities in the

range of human enamel or dentin. These cements would be useful

in cementing translucent filling materials such as fused porcelain. The resin cements can be made as radio-opaque as

zinc phosphate cement. Such opacities are needed when cementing opaque materials like the gold alloys or when using the

cement as a base for other filling materials. The high radioopacity is needed to distinguish the cement from recurrent or

unexcavated decay in interpreting X-ray pictures.

The film thickness of one of the resin cements was as low

as 10 microns; this is considerably thinner than the minimum

thicknesses obtained with current zinc phosphate cements, which

range from 30 to 40 microns . Therefore, resin cements can be

formulated that should permit very close seating of inlays and

other dental appliances.

The compressive strength of the resins was from two to

five times that of zinc oxide-eugenol cement, near that of

the zinc phosphate cement, less than half that of silicate

cement and from one-third to one-fifth that of amalgam. Clinical experiences based upon the resistance to mastication

arrange the foregoing materials in the same order as the compressive strength values.

The indentation hardness (Knoop) number was of the same

order as that of other dental resins

.

All of the cements and posterior-type direct-filling resins

adhered to the tooth tissues if kept dry. No adhesion was retained under oral conditions

.
I IS

;
None of the materials was color stable to ultraviolet

light as determined by the color stability test employed in

the American Dental Association Specification No. 12 for Denture Base Resin [ 16 ]. Since self -curing acrylic resins can

now be made color stable, undoubtedly the resin cements and

direct filling resins presently available will all be made

color stable or relatively so to ultraviolet light.

No bioassay of the resin cements has been found in the

literature nor are the authors acquainted with reports of

gross pulp damage after clinical use. The similarity of the

resin cements to the anterior-type direct-filling resins would

suggest about the same degree of pulp reaction with the resin

cements as occurs with the direct filling resins.

At the present stage of development it might be well to

consider the resin cements as experimental materials and not

as outright unlimited substitutes for zinc phosphate cement.

- 15 -

5

.

BIBLIOGRAPHY

1. Hillebrand, W. F . , Lundell, G. E. F . , Bright, H. A . , and

Hoffman., J. I. Applied inorganic analysis, ed. 2.

New York, John Wiley and Sons, 1953^ p. 269 .

2. Paff enbarger. Geo. C., Swaney, A. C., Schoonover, I. C.,

and Dickson, George. American Dental Association

Specification No. 9 for dental silicate cement (first

revision effective - July 1, 1950) . JADA 40:186

Feb. (1950).

3. Paff enbarger. Geo. C., and Beall, John R. American Dental Association Specification No. 8 for dental zinc phosphate cement (first revision effective July 1, 1938)

.

JADA 24:2019 Dec. (1937).

4. Paff enbarger. Geo. C., Sweeney, W. T., and Isaacs, Aaron.

Zinc phosphate cements: physical properties and a

specification. JADA 21:907 Nov. (1934).

5. Paff enbarger. Geo. C., Sweeney, W. T., and Isaacs, Aaron.

A preliminary report on the zinc phosphate cements.

JADA 20:1960 Nov. (1933).

6. Paff enbarger. Geo. C. Silicate cement: an investigation

by a group of practicing dentists under the direction of

the American Dental Association Research Fellowship at the

National Bureau of Standards. JADA 27:l6ll Oct. (1940).

7. Wolcott, Robert B., Paff enbarger. Geo. C., and Schoonover,

I. C. Direct resinous filling materials: temperature

rise during polymerization. JADA 42:253 March (1951) . 8. Paff enbarger. Geo. C., Schoonover, I. C., and Sourer,

Wilmer. Dental silicate cements: physical and chemical

properties and a specification. JADA £5:32 Jan. (1938).

9. Smith, D. L., and Schoonover, I. C. Direct resin filling

materials: dimensional changes resulting from polymerization shrinkage and water sorption. JADA 4-6:540 May (1953) . 10. Sweeney. W. T. Denture base materials: acrylic resins.

JADA 281 I 863 Nov. (1939).

11. Paff enbarger. Geo. C., and Caul, H. J. Dental cements.

Proceedings of the Dental Centenary Celebration.

Baltimore, Maryland, The Waverly Press. March (1940).

12. Council on Dental Research. List of Certified Dental

Materials Revised to December 15* 1954. JADA 50:199

Feb. (1955)*

- 16

13. Worner, Howard K. The physical and mechanical properties

of 'copper* cements. Austral. J. Dent. 44:411 Dec. (1940)

and 45:1 Jan. (1941).

14. Souder, Wilmer, and Peters, Chauncey, G. An investigation

of the physical properties of dental materials. Dent.

Cosmos .62:305 March (1920).

15. Paffenbarger, Geo. C. Hydrocolloidal impression materials

physical properties and a specification. JADA 27 : 373

March (1940).

16. Swaney, Aubrey C., Paffenbarger, Geo. C . , Caul, Harold,

and Sweeney, W. T. American Dental Association specification No. 12 for denture base resin, second revision.

JADA 46:54 Jan. (1953).

17. Johnston, J. F. A one year evaluation of an acrylic

cement. J. Pros. Dent. 4:689 Sept. (1954).

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- 19 - TABLE 2

Proportion of Inorganic and Organic Constituents of Resin

Powders

Chloroform Chloroform

\ Soluble Insoluble

Type Material (organic) (inorganic)

fo 'by wt °/o by wt

Acrynamel 50 50

Resin Dura 41 59

Cements Grip __ 33 67

Justi 73 27

Smith’s 42 58

Surgident 75 25

Tyton 4l 59

Vitacril 95 5

Posterior -type

direct -filling

Dura Kore 4l 59

resins

•

A66-57

20

TABLE 3

Some Inorganic Fillers from the Resin Powders

Type Materials

Major elements

determined by

spectrographic

analysis#

Compounds

identified by

petrographic

analysis

Acrynamel Calcium

Silicon

Aluminum

Calcium carbonate

Quartz

Mica

Dura Calcium

Barium

Silicon

Calcium carbonate

Barium carbonate

Resin

Grip Calcium

Tungsten

* tungstate* Calcium

**

Cements Justi Barium

Silicon

Titanium

Quartz

***

Smith 1 s Calcium

Sodium

Calcium carbonate

*** Surgident Sodium

Zinc

*

Tyton Calcium

Barium

Silicon

Calcium carbonate

Barium carbonate

Quartz

Vitacril Titanium

Silicon

Zincr:

*

Posteriortype Direct

filling

Resins

Dura Kore.

P.F.

Calcium

Barium

Silicon

Aluminum

Sodium

Silicon

Calcium carbonate

Barium carbonate

Aluminum trioxide

*** X56-Y35 A82-TO/ ' k8T=nr

# More than 10 percent

* Particle size too small for petrographic identification.

** Identified by X-ray diffraction.

*** Two unidentified materials.

21

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- 23

TABLE 5

Linear Expansion on Sorption of Water

by Resin Disks 0.5 mm Thick

Type Material

Expansion

1 Day 3 Months

Jo %

Resin Cements Acrynamel 0.2 0.1

Dura Cement 0.3 0.2

Grip 0.3 0.5

Justi 0.5 0.5

Replica 0.6 0.7

Smith 1 s 0.3 0.3

Surgident 0.3 0.2

Tyton 0.6 0.5

Vitacril 0.3 0.3

Posterior-type Dura Kore 0.4 0.4

Direct -filling P. F. 0 .

6

0.4

Resins

A63-58

TABLE 6

Solubility and Disintegration of Resin Cements

in Distilled Water

Type Material Weight Loss

Resin Cements Acrynamel

$

0.4

Dura 0.3

Grip 0.1

Justi 0.0

Replica 0.1

Smith's 0.4

Surgident 0.1

Tyton 0.4

Vitacril 0.2

Posterior-type . Dura Kore 0.4

Direct -filling

P. F. 0.3

Resins

Other Zinc phosphate cement 0.3*

dental materials Silicate cement 1 .4**

a66— l8o

* Maximum permitted by American Dental Association

Specification No. 8 [3]. ** Maximum permitted by American Dental Association

Specification No

.

q [2].

25

TABLE 7

Optical Opacity of Resin Cements and

Posterior-Type Direct-Filling Resins

Variations

Type Material with color

c 0. 70*

Resin cements Acrynamel 0.45 -1.00

Dura 0.55 - 0.70

Grip 1.00

Justi 0.60 -1.00

Replica 0.40 - 0.45

Smith f s 0.50 - 0.60

Surgident >0.70 - 1.00

Tyton 70.70 - 1.00

Vitacril 0.70 ->0.70

Posterior -type

Dura Kore 0.60 - 1.00

Direct -filling resins

P. F. 0.45 - 0.60

Zinc Phosphate Cements [11

]

1.00

Silicate Cements [2] 0.35 - 0.55

A66-35

* The Co . 70 value represents the daylight apparent reflectance

(Rq) of the cement specimen (l mm thick) when placed over a

black glass backing divided by the daylight apparent reflectance (Rq ^q) of the cement specimen when placed against a

white backing with an apparent daylight reflectance relative

to MgO of 70 percent. When the reflectances are the same,

the Cq jq value is 1.00 and the cement is 100 percent opaque.

If Co . 70 i- s equal to 0.50, the cement is said to be 50 percent

opaque ^ etc.
• :

•-
TABLE 8

Radio-opacity of Some Resins,, Enamel and Dentin

Type Material Film density*

Resin Cements Acrynamel 2.7

Dura 1.9

Grip 0.8

Justi 2.7

Replica 2.9

Smith 1 s 2.4

Surgident 2.2

Tyton 1.8

Vitacril . _ _ .2.9

Posterior-type Dura Kore 1.9

Direct-filling P. F. 2.7

Resins

Other Dental Silicate Cement 1.9

Cements Zinc Oxide -Eugenol Cement 0.6

Zinc Phosphate 0.8

Hard Tooth Dentin++ 1.3

Tissues Enamel ++ 1.1

* Average readings

A82-113

on the 2 mm thick specimens recorded as

observed on the zero to three scale. The lower the film

dentisty readings the greater the radio -opacity

.

++ Data from curve drawn from readings of five molars.
. *
TABLE 9

Film Thickness of Resin Cements

Material Film Thickness

microns

Acrynamel 60

Dura 40

Grip 10

Justi 25

Replica 120

Smith '

s

25

Surgident 30

Tyton 25

Vitacr.il 85

Zinc phosphate cements on List of

Certified Dental Materials [12] 30 to 40

A66-89

- 28

TABLE 10

Compressive Strength of Some Dental Restorative Materials

Age of Specimens

Type Material 1 hr 1 wk Consistency

lb/in* lb/in2

Acrynamel 9,500 + 500 10,000 ± 700 Cementing

Dura 9,000 ± 300 11,500 ± 200 11

Grip 10,000 ± CO 00 12,000 ± 500 1! Resin

Justi 8,000 ± 00 10,500 ± 300 It Cements

Replica 8,500 + 200 10,000 ± 200 1! Smith ' s 11,000 ± 200 12,500 ± 300 It Surgident 6,000 ± 200 7,500 ± 100 It Tytun 10,500 ± 100 10,500 ± 300 tl Vitacril 9,000 ± 300 10*500 ± 200 tt Posteric^-

type Direo t Dura Kore 8,500 ± 200 11,000 ± 600 (Filling or

Filling

Resins P. F. 10,500 ± 100 11,000 ± 100 ( cementing

Zinc oxide — Other eugenol 2,000 to

cement 5,000 [in Filling

dental

Red copper 14,000 [13] Cementing

cements phosphate

cement 21,000 [11] Filling

and

Black copper 6,000 to

filling phosphate 22,000 [ 13 ] Filling

cement 9,000 [ 11 ]

tt materials

Zinc 2,000 to

Phosphate 2,300 to 13,800 [4] Cementing

cement 9,200 [7] 19,000 [11] Filling

Zinc 22,000 to Filling

Silicate

cement

25,000 [11]

Silicate 10,500 to

cement 12,000* 21,500 [8] Filling

Amalgam 30,000 to

51,550 [14] Filling

A66-71 * Unpublished data collected In a former

investigation [8],
.

.

'

r

- re

...

1 V /

-

^

'

. tt . m.;

-

-

: '

; T

•

}qc 1 t'&T

.

-

; no -

_•
apparatus for consistency tests

ON RESINS

B

SPECIMEN

Figure 1. Apparatus used for measuring the consistency of unset resin cements and

direct filling resins for posterior teeth:

A. Glass tube used to measure volume of the soft cement.

B. Weight and glass plates which flattened soft cement into

a disk.

The diameter of the disk was used as a measure of the consistency of

the mix.
.

.
GRAMS

OF

POWDER

IN

0.5

ML

OF

LIQUID

DIAMETER OF DISK IN MM

Figure 2. The effect of the powder-liquid ratio upon consistency of some mixed but unhardened dental resins . The 30 ± 1 mm and the

40 ± 1 mm areas as indicated by the extended lines on the bottom

of the diagram are the consistencies used in preparing the

specimens for the different tests.

8 12

TIME-MINUTES

16 20 24 28

I LU0

<

1

4

<rx

CD

8

i—I—

r

i—

r

i_i I L

CURING SHRINKAGE

J I I 1 I I

32 36

ZINC PHOSPHATE

CEMENT

SILICATE CEMENT

P.F.

TYTON

VITACRIL

ACRYNAMEL

DURA CEMENT

SMITH'S

REPLICA

DURA KORE

JUSTI

GRIP

SURGIDENT

I I

Figure 3 . The volumetric shrinkage occurring as various dental resins and

cements change from the fluid to the solid state.

t» 0 G E

O o LA

I G *H • x th x: o

-p §

0 -P £ 0 ro Eh XI 0

Oh -P X2 O bO

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 December 31 , 1955 4495

Progress Report

PRELIMINARY STUDY: TROPICAL STORAGE

OF

DENTAL MATERIALS

by

Henry I. Copeland, Jr.*

W. T. Sweeney**

* Major, USAF (DC), Guest Worker, 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 .

(Task 7756-41 BV No. 55-14)

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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

unless perm ssion is obtained in writing from theOffice of the Director, National Bureau of Standards, Wash ngton

25, D. C. Such permiss : on 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.

PRELIMINARY STUDY: TROPICAL STORAGE

OF

DENTAL MATERIALS

Abstract

A preliminary survey of some dental materials

stored for four years in Puerto Rico by the Armed

Services was made to determine changes in properties

of dental significance. Comparison was made with

duplicates stored at the National Bureau of Standards. Alginate impression materials and investments were found to have deteriorated to an unusable

state. Methyl methacrylate monomer (liquid) containing dissolved polymer had polymerized. Some

items which had volatile and low melting constituents were shown to be improperly packed and melted.

The tests substantiated the present practice of

phase buying and preferential storage. The survey

indicates the desirability for further study of

the effect of storage on many dental items.

1 . INTRODUCTION

Deterioration of dental materials during transport and

storage is of particular interest to the Armed Services. Long

periods of storage* often under extremely adverse conditions*

can be a military necessity, Materials which are unusable or

in an inferior condition on arrival at the operational level

cause a loss of time and money as well as an interruption in
InSi
2

dental service.

In 1951* while a guest worker at the Dental Research Section of the National Bureau of Standards, Col. T. E. Fischer,

USAF (DC), planned an investigation to determine critical

time limits for the storage of certain dental materials in

tropical climates. Although circumstances prevented carrying out the investigation as originally planned, a number of

materiel items have been returned to the National Bureau of

Standards after four years of storage, and their condition examined by visual observation and applicable specification tests,

2. CONDITIONS OF STORAGE

To avoid variations in composition, date of manufacture,

and packaging, samples for storage at three locations were selected with the same batch number.

The dental materials listed under Column A* Table 1 were

stored in a constant temperature and humidity room in the

Dental Research Laboratory of the National Bureau of Standards

.

The samples were sealed in a packing case with a moistureproof liner. The room is maintained at approximately a temperature of 70 °F and a relative humidity of 60%

.

The items listed in Column B, Table 1 were stored on open

shelves in the penthouse of the Rodriguez Army Hospital, San

Juan, Puerto Rico. The prevailing temperature at this site was

the highest of the three locations

.

The materials listed under Column C, Table 1 were stored

on open shelves at the Tropical Research Laboratory, San Juan,

- 3

- Puerto Rico, where they were exposed to year round relative

humidities of 75 to 79$.

3. RESULTS AND DISCUSSION

3.1 General Effects of Storage

The appearance of the packaging of individual samples

reflected storage conditions. Metal parts of containers

showed varying amounts of rusting. Containers stored in the

moisture-proof case at the National Bureau of Standards were

clean and relatively free of rust; the penthouse containers

were spotted with rust; and the seaside samples were very

rusty (as may be seen in Figure l) . Cardboard cartons containing materials with volatile or low melting point ingredients

were stained by these ingredients . Figure 2 shows cartons of

a protective coating material. The penthouse sample, "B'

! , which had been subjected to higher temperatures, is badly

discolored

.

Materials which were obviously affected by storage or

which had become unusable are marked by an asterisk in Table

1. They are discussed in detail below.

3.2 Investment

The investment samples showed no caking or lumping and

appeared in usable condition; however, consistency tests,

performed according to Federal Specification U-I-5^6, proved

samples from the two tropical locations to be unsatisfactory.

Figure 3 shows typical results of the consistency tests.

Samples stored under "A" conditions show normal slumping.

Samples stored under "B" and "C" conditions set so rapidly

that the test specimens supported their own weight in less

than two minutes* a setting rate too fast for dental use.

Since such behavior is most likely caused by partial

hydration [1], samples of investments from "A", "B"* and

"C" were dried to constant weight at 110°C. The results*

as shown in Table 2* show the varying water content and

setting times.

Assuming that all water in the sample is in the calcium

sulfate and is removed completely by the drying process* calculations on "A" sample of investment indicate that there

wosoe approximately 30 grams of calcium sulfate hemihydrate

per 100 grams of the investment. This is a reasonable proportion for investments of this type. Comparing the percent

weight lost by "B" and "C" samples to that lost by "A" samples

it is readily apparent that partial hydration of the material

stored at "B" and "C" had occurred. Further calculations

suggest that in "B" samples approximately 9% of the hemihydrate might be considered to be hydrated to the dihydrate form

and in "C"* approximately 15$.

The setting rate of the hydrated investment may be slowed

and acceptable consistency obtained* by increasing the ratio

of water to investment powder when mixing. This procedure is

not recommended because it weakens the investment [2] and

changes both the setting and thermal expansion [3]. An inlay

cast into a mold made of this partially hydrated material is

not likely to fit satisfactorily.
• , : v

. . /•

s

. . - ....

..

.

* '

•

*
- 5 - 3.3 Resin Materials

Dental resin materials are procured for the Armed Services

in powder/liquid form. In addition to samples of the standard

supply items* samples of several brands of "filling" resin

kits were also stored.

Past experience with resin liquids has indicated the

necessity for phase buying and preferential storage of these

materials to avoid premature polymerization induced by heat.

In this study* "A" samples of denture base resin liquids were

found to be still liquid while "B" and "C" samples had polymerized into clear solids* which* of course* are useless in

the dental laboratory. Figure 4 shows the effect of four

years of storage at "C" on a methyl methacrylate liquid.

It is interesting to note that all of the filling resin

liquids were still in the liquid phase. The liquid of one

brand had changed in appearance from optically clear to a

cloudy pink. The available sample was too small for analysis.

» However* it is suggested that the change might be due to some

chemical reaction involving the tertiary amine initiator. Another brand had not polymerized but the plastic seals on the

bottle caps were broken and a large portion of the liquid

had evaporated. The remaining samples were unaffected by

storage

.

These observations substantiate the practice of preferential storage (below 37 °C) and phase buying of denture base

resin liquids to avoid loss due to short shelf-life. It also

- 6 - appeared that the self-cured (filling) resin liquids are more

resistant to polymerization when stored at elevated temperatures than are the heat-cured resin liquids.

Tests for polymer content were positive for the unpolymerized heat-cured resin liquids and negative for the selfcured resin liquids. Samples giving positive tests for polymer

content solidified in less than 23 hours when stored in a

constant temperature oven at 85°C. Samples giving a negative

test resisted polymerization for more than 168 hours when

stored under these same conditions.

Tests for traces of polymer were negative for twelve of

fourteen ADA certified denture resin liquids currently on the

market. The two brands of resin liquids* which showed polymer

content* polymerized into an unusable hard mass in less than

17 hours when stored in the constant temperature oven at 85 °C.

Two brands of resin liquids* picked at random from the twelve

brands without traces of polymers* showed no evidence toward

solidification (increased viscosity) after 72 hours in the

85°C oven.

From these results it appears that resin liquids without

polymer content can be expected to have a longer shelf -life.

A more definitive study of this type might prove the desirability of a change in Federal Specification U-R-179 for Resin*

Acrylic* Dental which permits the resin liquid to be "methyl

methacrylate monomer or methyl methacrylate monomer which

has been partially polymerized."
-

,
- 7

- 3.4 Alginate Impression Material

All samples stored had deteriorated to an unusable condition. On mixing at standard temperature and humidity, a

gummy, grainy mass was formed in less than one and one-half

minutes

.

Pfeiffer [4] demonstrated that alginate impression material

deteriorates rapidly when stored at temperatures exceeding 37°C.

The fact that samples stored under standard temperature and

humidity conditions were unusable suggests some factor in

addition to temperature affects this material. Procurement

and supply agencies now avoid loss of these materials by both

phase buying and preferential storage.

3.5 Materials with Volatile Solvents and

Low Melting Point Ingredients

This group of materials would be expected to be adversely

affected by high temperatures during storage. Some items were

stored only in the roof-top location, while others were stored

in two or all three locations

.

A protective coating material of the theobroma type was

stored at all locations, but only the samples stored at "B"

were affected to any extent . The metal tube containers had

lost approximately one-third of their contents. The remaining

portion was easily expressed from the tubes but appeared drier

and more grainy than the samples stored at "A" and n C". The

material was still usable for protecting restorations. The

loss in volume was due to the effect of elevated temperatures
} .1
- 8 - which caused this low melting point material to liquefy and

evaporate

.

Cellulose acetate cast lacquer samples stored at "A" and

"C" had separated into a liquid phase and a gel phase.

Samples of this material from "B" had completely lost their

solvent { 95% chloroform, 5% alcohol, by volume) through evaporation

.

Wax shapes and a brand of sheet casting wax from "B"

were found to have lost a part of their more volatile ingredients, making the materials unusable for the purposes intended.

This deterioration occurred only to the samples where higher

temperatures were encountered. Another brand of sheet casting

wax and samples of a baseplate wax were unaffected by the

higher temperatures

.

Two of three brands of preformed plastic patterns were

also affected by the higher temperatures met in the roof-top

location, but the two brands were affected in different ways

.

One brand had lost its plasticizer and had become very brittle,

whereas samples of the other brand had flowed together and were

so sticky and gummy that they could be separated neither from

themselves nor their cardboard mounts.

3.6 Cement Filling Materials

Silicate cement stored at "B" and red copper cement stored

at "A", "B", and "C" gave no evidence of deterioration.

Fischer [ 5 ] has shown that deterioration of these materials is

due to loss of water from the cement liquid. At the present

time, procurement specifications require that cement liquids
.
- 9

- be packaged in sealed glass ampoules. This improvement in

packaging has been effective in preventing deterioration of

these materials during storage.

4 . SUMMARY

Deterioration of some materials is avoided only through

phase buying and preferential storage, measures which cannot

always be utilized. Improved packaging appears to be all that

is necessary to insure long shelf -life for other materials.

The following recommendations are the result of inspecting and

testing dental materials after storage for four years:

1. The condition of investments stored in humid climates

shows that the investments are inadequately protected from

moisture by the snap-in-lid metal containers in which these

materials sire now supplied. Better containers should be used.

2. Low melting point materials packaged in screw-cap metal

tubes need to be more adequately protected from high

temperature by puncture -to-open seals.

3. Specifications for the procurement of preformed plastic

patterns should contain a temperature stability requirement.

4. An improvement in the storage stability of denture base

resin liquids is desirable.

5. There is a need for a more storage-stable elastic impression material

.
>d
-10

BIBLIOGRAPHY

1. Farmer, G. J . , and Skinner, E. W., Effect of relative

humidity on the setting time of plaster of paris.

Northwestern Univ . Bull. 43 , 12-16 Dec. (1942).

2. Souder, W., and Paffenbarger, G. C., Physical properties of dental materials. Circular of National Bureau

of Standards C433, page 88. U. S. Govt. Printing Office,

Washington, D. C. (1942).

3. Volland, R. H., and Paffenharger, G. C., Cast gold inlay

technic as worked out in the cooperative research at the

National Bureau of Standards. J.A.D.A. 19 , 185 Feb.

(1932)

.

4. Pfeiffer, K. R., Harvey, J. L., and Brauer, G. M.,

Deterioration during storage of alginate hydrocolloidal

dental impression material. U.S. Armed Forces Med. Jnl

.

6 , 1315-1320 Sept. (1954).

5. Fischer, T. E., and Schoonover, I. C., Deterioration of

silicate cements in the tropics. U.S. Armed Forces Med.

Jnl. 2, 907-913 June (1951).

11 ABC Table 1. Materials Stored

NBS at 70 °F, 60% R.H. Puerto Rico, Roof-top Puerto Rico^ Seaside

Investment

Denture Resin Liquid

Acrylic Resin Powder

Clear Resin Kit P/L

Alginate Material*

Protective Coating

Baseplate Wax

Wax Shapes

3 Brands Casting Wax

Cast Lacquer*

Cavity Varnish

Copper Cement P/L

Investment*

Denture Resin Liquid*

Acrylic Resin Powder

Clear Resin Kit P/L*

3 Brands "Filling”

Resin Kits P/L*

Alginate Material*

Protective Coating*

Baseplate Wax

Wax Shapes*

2 Brands Casting Wax

3 Brands- Preformed

Plastic Patterns*

Cast Lacquer*

Cavity Varnish

Silicate Cement P/L

Copper Cement P/L

Investment*

Denture Resin Liquid*

Self-cure Resin P/L

Acrylic Resin Powder

Clear Resin Kit P/L*

Alginate Material*

Protective Coating

Baseplate Wax

Wax Shapes

3 Brands Casting Wax

Cast Lacquer*

Cavity Varnish

Copper Cement P/L

*Items marked by an asterisk were found to have been adversely

affected by storage.
.

'

„
12

Table 2. Water Content and Setting Time of Investment

Sample Water Content3, Setting Tirne^

w

' (min)

A 1.8 20

B 2.2 2

C 2.5 1.5

a Average of two determinations of weight loss on

heating to constant weight at 110 °C.

b On undried samples

.

Is

clean

and

relatively

free

of

rust.

Containers

B

!

and

,!C"

,

stored

on

open

shelves

In

the

hospital

penthouse

and

pavilion

of

the

Tropical

Research

Laboratory,

are

progressively

more

rusty

Indicating

the

effect

of

the

increased

humidity

at

the

site

of

storage.

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 December 31* 1955 4520

Progress Report

DIMENSIONAL STABILITY OF

DENTURE BASE RESINS

by

W. E. Mowery*, Claire L. Burns’ , George

Dickson", and W. T. Sweeney**

* Guest Worker, Dentist, Veterans Administration, Dental

Research Section, National Bureau of Standards.

’ Chemist, Dental Research Section, National Bureau of

Standards

.

" Physicist, Dental Research Section, National Bureau of

Standards

.

** Chief, Dental Research Section, National Bureau of

Standards

.

This work is 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.

(Task 7756-41 BV No. 55-14)

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 wrltingfromtheOffice 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,

DIMENSIONAL STABILITY OF

DENTURE BASE RESINS

Abstract

Dimensional changes during processing and

during a period of two years of clinical service

were determined on dentures made from heatcuring and self -curing materials. The magnitude

of dimensional changes of both heat-cured and

self-cured dentures was small, seldom exceeding

0.2 mm. The average processing shrinkage was

less and the average expansion during service was

slightly greater for self-cured than for heatcured dentures. During service upper dentures

changed less in dimension than did lower dentures

and on the average the flange to flange change

of upper dentures was negative wnile all other

changes were positive. In general, the greatest

service change on all dentures occurred during the

First month. After two months very little change

occurred

.

1 . INTRODUCTION

Self-curing denture base resins which polymerize at room

temperature are now being used to a considerable extent by the

dental profession. These resins are generally similar to the

methyl methacrylate denture base materials which have been used

for many years with the exception that polymerization of the

liquid in the mixture is initiated by an activator present In

the polymer powder rather than by the application of heat.

Data on some of the properties of self-curing denture resins

have been reported previously [1,2], The present investigation

was undertaken to determine the relative dimensional stability

of self -cured and heat-cured dentures. For this purpose measurements of the dimensional changes during processing and during

a period of two years of clinical service were made on a series

of dentures.

In order to supplement the number of dentures which could

be made in the National Bureau of Standards laboratory a cooperative program was established with the Veterans Administration. Under this program dentures were processed at the

following installations: Veterans Administration Center,

Kecoughton, Virginia; Veterans Administration Center, Martins

burg, Virginia; and Veterans Administration Hospital, Perry

Point, Maryland.

Materials used in this project were purchased on the open

market from the latter part of 1950 to the early part of 1952

and are listed in Table I.

- 2 -

2 . PROCEDURE

A total of 91 full dentures of self-curing materials and

28 full dentures of heat-curing materials were processed using

routine acceptable techniques. To provide reference points

for determination of dimensional changes stainless steel pins*

l/8 M long and l/l6 n in diameter polished on one end and ruled

with fine cross marks, were cemented in the central fossae of

the last molars (designated hereafter as the A and B positions)

and in the disto-buccal peripheral flanges (designated hereafter

as the C and D positions) of the dentures. All measurements

were made at 70 °E ± 2°P on a toolmaker’s microscope and recorded

to the nearest 0.0001 in. The average of five readings was

taken as the measurement

.

On the dentures made at the National Bureau of Standards

readings were made on the waxed-up model prior to processing;

after removal from the cast; on the denture just prior to insertion; at 30-day intervals for six months after delivery to the

patient; and then at 6-month intervals. The dentures fabricated

at the Veterans Administration hospitals were first measured just

prior to insertion and then on the same schedule as above. Curing

shrinkage, therefore, could be determined only on the dentures

made at the National Bureau of Standards. The change from the

waxed-up measurement to the measurement after removal from the

cast of the processed denture was taken as the curing shrinkage.

A series of technique denture blanks with only the last

molars in place was made in order to supplement the number of

dentures on which curing shrinkage could be determined. Two

heat-processing methods were used. One group of the technique

dentures was processed for 9 hours at l60°F and another group

for lj> hours at l60°F and \ hour at 212 °F. The self-curing

materials were processed according to the directions supplied by

the manufacturer. The technique dentures were made of the same

materials used for the clinical study.

3 . RESULTS

3 . 1 Curing Shrinkage

The study of curing shrinkage of the clinical and the

technique dentures yielded the results given in Table 2.

In both groups of dentures the heat-cured materials had

a greater average curing shrinkage but there is overlapping of

the ranges. Results obtained on the technique dentures indicate

that the type of heat processing did not significantly effect

the amount of shrinkage

.
,

-

; r : v-
- 3

- 3.2 Dimensional Stability

In the study of the dimensional changes of the dentures

in use the measurement made just prior to insertion (after

immersion in water for at least two days) was taken as the zero

reading and all values were calculated with that as a reference

point

.

Many of the dentures were lost to the project in the course

of testing as a result of the unavailability of patients or of

wearing away of the cross marks on the pins inserted in the teeth

The number of dentures measured at each interval over the two

year period is shown in Table 3.

Dimensional changes of the dentures while in service are

shown in Figures 1 to 5 and Table 4. Curves showing the changes

in dimension AB (molar to molar) for each of the dentures are

plotted in Figures 1 to 4 in order to give an overall picture of

the range in dimensional change of the different dentures and an

indication of variation with time for individual dentures.

Average curves for both the AB (molar to molar) and CD

(flange to flange) dimensions are shown in Figure 5- The values

plotted are averages of all values obtained at the time indicated

Since many of the dentures were not available for measurement at

later periods, the averages plotted at two years are based on

only one-tenth to one-sixth the number of dentures included in

the one-month averages (see Table 3). It is obvious from examina

tion of the individual curves in Figures 1 to 4 that deviations

from the averages are large compared to the differences between

average curves

.

A statistical analysis was made of 6 heat-cured dentures and

28 self-cured dentures on which data were complete through the

first 180 days. Table 4. The results are summarized as follows:

The standard deviation characterizing the variability of

replicate measurements made on the same denture is about 0.00015

inch. However, the experimental errors involved in the study

are higher than indicated by the precision of measurement. This

is due to variability in behaviour between different dentures

and variations with time of individual dentures (possibly resulting from uncontrollable factors in the clinical use of the

dentures). The differences between dentures are of two types:

(l) systematic, in which one rate of change of dimensions of

one denture is consistently higher or lower than that of

another denture and (2) random, in which the rate of change

of one denture is sometimes higher and sometimes lower than

that of another. The latter variability is characterized by

a standard deviation of approximately 0.0014 inch or about 10

times the standard deviation of replicate measurements . The

standard deviations of the systematic differences between selfcuring dentures varied between 0.003 and 0.0017 inch. No

- 4 - systematic variations between heat-cured dentures were observed.

The upper heat -cured dentures as a group showed no significant changes in dimensions throughout the 180 first days of

test. Lower heat-cured dentures increased, on the average,

by about 0.00044 inch per 30 days over the AB dimension and

about 0.00073 inch over the CD dimension.

Both upper and lower dentures made with self -curing resins

showed significant dimensional changes during the first 180

days. A gradual slowing down in the rate of change occurred

during this period. The observed rates of change are shown in

Table 4.

3.3 Clinical Evaluation

According to patient reaction all but one of the dentures,

regardless of the type of resin used, have given comfortable

and satisfactory service. The one report of loss of fit occurred

after the denture had been in service a little over a year.

Measurements made after the complaint revealed no excessive

changes. The case history of the patient showed that he had

previously worn another denture for one year but that the oral

tissue was still not firm. It seems probable, therefore, that

the failure In this case was a result of tissue change rather

than change in dimension of the denture.

4. DISCUSSION

The results obtained show that with the precision of measurement employed in this study variations in dimensional changes

during processing and during service can be demonstrated for

both heat-cured and self -cured dentures. However, it should

be pointed out that the magnitude of these variations is not

large; in most cases the changes are within the range of -0.1

to 0.4$ (approximately - 0.05 to +0.2 mm). According to patient

reaction these changes do not significantly affect the servicability of the dentures.

On the average the processing shrinkage of self -cured

dentures was less than that of heat-cured dentures. Such an

effect would be expected since the lower polymerization temperature of the self-curing materials should tend to give less

thermal contraction after polymerization.

The average expansion of the self-cured resins during service was slightly greater than that of heat-cured materials.

This might be the result of a lower sorption of water by the

self -cured denture during processing and a subsequent higher

sorption and expansion during service.
. .

' V

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.

.

.

. ’

.

.

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- 5 - There appears to be a greater increase in dimension during

service in the lower than in the upper dentures. Also the change

in dimension from molar to molar (A to B) is greater percentagewise in most cases than is the change from flange to flange

(C to D) indicating that the change is not a uniform enlargement but involves some distortion of the dentures. The average

flange to flange change during service on upper dentures was

negative while all other average changes during service were

positive

.

In general* individual dentures showed the greatest dimensional changes during the first month and after two months

almost no significant changes took place . It has been demonstrated [3] that dimensional changes of approximately 1% can

be produced in dentures by changes in water content. It is

possible* therefore* that the variations observed during service

may be dependent to a considerable extent upon changes in water

content resulting from conditions under which the dentures are

used in service .

5 . SUMMARY

The magnitude of dimensional changes of both heat-cured

and self -cured resins was small* seldom exceeding 0.2 mm.

Patient reaction indicated that changes of this amount did not

significantly affect the fit of the dentures.

The average processing shrinkage of self-cured dentures

was less than that of heat-cured dentures and the average of

expansion of self-cured dentures during service was slightly

greater than that of the heat-cured dentures.

During service upper dentures changed less in dimension

than did lower dentures and on the average the flange to flange

change on upper dentures was negative while all other changes

were positive

.

On the average the greatest change on all dentures occurred

during the first month. After two months very little significant change occurred.

b

6 . BIBLIOGRAPHY

1. Caul* H. J.* Stanford* J. W. and Serio* A. F.

Properties of self-curing denture base resins*

J.A.D.A. 44:295 (1952)

.

2. Swaney* Aubrey C.* Paff enbarger* G. C.* Caul* H. J.*

and Sweeney* ¥. T. American Dental Association

Specification No, 12 for denture base resin: second

revision. J.A.D.A. 46:54 (1953).

3. Sweeney, W. T., Paffenbarger, G. C. and Beall, John

R. Acrylic resins for dentures. J.A.D.A. 29: 7

(1942). —
>
7

Table 1

DENTURE BASE MATERIALS USED

Brand Name Manufacturer Date Received

at NBS

Self-Curing Resins

Acralite 88 Acralite Co., Inc. 6/50

Isodent Precision Dental Mfg.

Co

.

12/50

Duz-All Coralite Dental

Products Co.

12/50

Cold Cure Perfected Dental

Products Co.

1/51

Cur-O-Matic Acri-Lux Dental

Mfg. Co., Inc.

l/5l

Coldpac The Motloid Co., Inc. 12/50

1 Bio -Therm B. L. Dental Co. Inc. 12/50

Nu-Set J. Yates Dental Mfg.

Co . 2/51

Heat-Curing Resins

Certified

Acrylic

Lee S. Smith and Son

Mfg. Co.

3/52

Crystolex

Formula 102

Kerr Mfg. Co. 8/52

Densene 33 Cosmos Dental

Products, Inc.

2/52

Table 2

CURING SHRINKAGE OF DENTURE BASE RESINS

Specimen Material Cure No . of

Dentures

Shrinkage

%

Dentures

Self -curing Mfr »s directions21 0.27 ± 0.15*

Heat-curing Mfr ’ s directions8 0.43 ± 0.12

Technique

Self -curing Mfr ’s

directions

5 0.25 ± 0.03

Dentures Heat-curing 9 hrs at l60°F 5 0.35 ± 0.09

Heat -curing 1^ hrs at l60°F

\ hr at 212 °F

5 0.34 ± 0.07

* Plus or minus values indicate average deviation
—

.

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;

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• :

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: J •

-

- :
~ 9

-

Table 3

NUMBER OF DENTURES MEASURED AT EACH TIME PERIOD

Denture Dimen- Time - Days

sion

30 60 90 120 150 180 360 540 720

Selfcured

Upper AB 3o 36 35 35 30 30 23 10 4

Upper CD 35 35 35 35 30 24 15 11 6

Lower AB 32 32 32 32 25 19 12 8 5

Lower CD 30 30 30 30 25 21 11 8 4

Heatcured

Upper AB 10 10 9 9 8

8

6 5 3 1

Upper CD 9 9 8 8 5 4 3 1

Lower AB 10 10 9 9 8 6 4 2 1

Lower CD 9 9 8 3 8 6 4 2 1
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10

Table 4

RATE OF DIMENSIONAL CHANGE OF DENTURES

Type of Period Dimensional Change

Material (Days) AB CD

Upper Lower Upper Lower

in. in

.

in. in.

Selfcured

0-30

30-60

60-90

90-120

120-150

150-180

0.0021

0.0012

0.0006

0.0002

0.0000

-0.0001

0.0022

0.0015

0.0009

0.0004

-0.0001

0.0001

0.0009

0.0000

-0.0005

-0.0003

-0.0003

-0.0003

0.0017

0.0014

0.0004

0.0000

-0.0005

-0.0004

0-30 0.0007 0.0003 -0.0002 0.0013

30-60 0 . 0004 0.0008 +0.0002 0.0010

Heat- 60-90 0.0003 0.0006 -0.0001 0.0012

90-120 0.0002 0.0005 +0.0002 0.0006

cured 120-150 -0.0005 0.0004 -0.0003 0.0003

150-180 -0.0001 -0.0000 -0 . 0004 0.0000

CHANGE

for all dentures made from same brand of material are

designated with same number.
.

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.

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CO

>-<

Q

u0 CD ft CQ C-i ft 0) Ctf 3 >

!3a} 0 O ft h *

p 0

O *-P

« C Cr} P o S

ccJ ft 0 CQft

XI £j O

0

ft d'd o *h c T5 cc5 0 U

0'~'&

ft Ch > cd 0

^ H S

0 Q cd w £ m « ^ Ch 50 o S 0

X-P o X

ft X £

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I

rC3 i—10 0^0

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50 0

X CP 0

as < 0 x: X C-i -P a U P ft 0-p £

ft > X

cti o 0 'd X -d 0

O 0 -P

ft 0 ft 0

to Ci ft c5 X 3 C(j 50 0-P ft a x x 0

ft 0 O 0 rr-< ft i ftni

CM0

%

50

•H

Figure

3.

Dimensional

changes

during

service

of

self

-cured

lower

dentures

over

AB

(molar

to

molar)

dimensionCu:

ves

for

all

dentures

made

from

same

brand

of

material

are

designated

with

same

number.
'
HEAT

CURED

LOWER

AB

t

1 1 1 1 1 1 r

O 00 CD OJ o OJ CD ” o O d o o o o o

39NVH0 %

OJ

c/)

<

O Q

CDro

J o

CvJ

02 E 02 • MO 0 0 CP Ofl

•H Li E

Li Li Lh d

d d C

TO H 0

O TO 0

ra E d E 0^ E d

bO CQ qcg 02 d < 0 jo

jC

y

Li >P

Li 3 «H 0 -P £

iH > C

S

O O TO TO 0

O 02 -P

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02 Li H £

£ d d hO 0 -P «H E C U 0

•H 0 O 0

TO L-i TO

0

Lid

bO

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39NVH0 %

Figure

5*

Average

dimensional

change

during

service

of

self

-cured

and

heat

-cured

dentures

over

AB

(molar

to

molar)

and

CD

(flange

to

flange)

dimensions.
' '

•
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