NATIONAL BUREAU OF STANDARDS REPORT

4255

REEORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30, 1955

Dental Research Laboratory

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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 Initruments. 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. AEG Radiation Instruments.

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

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

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

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

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 to June 30 * 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.

NBS PROJECT NBS REPORT

0708-10-0707

0708 -20-3824

0708-20-3857

July 28, 1955 4255

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

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repri

unless permission Is ot

25, D.C. Such permi

caily prepared if tha

Approved for public release by the

director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

part, is prohibited

dards, Washington

rt has been specifiirt for its own use.

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

1

.

INTRODUCTION

Investigations of materials and equipment used in dentistry

and of natural tooth structures have been carried on in the

Dental Research Section of the National Bureau of Standards during the half year ending June 30, 1955* Lists of reports issued

and papers published during this period and summaries of work

now in progress are given below. Copies of the reports are

appended

.

2

.

REPORTS ISSUED

NBS Report 4129

NBS Report .4155

NBS Report 4225

NBS Report 4226

NBS Report 4227

Sulfinic Acid Derivatives as Accelerators in

the Polymerization of Methyl Methacrylate

.

Preliminary Investigation of Color in Dental

Resins

.

The Relationship Between Residual Monomer and

Some Properties of Self -Curing Dental Resins.

Development of Methods for Evaluation of Rotating Dental Diamond Abrasive Instruments.

A Method for Measuring the Mucosal Surface Contours of Impressions, Casts, and Dentures.

3

.

PAPERS PUBLISHED

The Mechanism of Hygroscopic Expansion in Dental Casting Investments. I. C. Schoonover, H. W. Lyon, George Dickson. J. Den.

Res. 34:44 Feb (1955)

.

£-ray Diffraction Studies of Tooth Structure. Aaron S. Posner.

Norelco Reporter Vol. II, No. 2, Page 26, March-April (1955).

Factors Influencing the Cutting Characteristics of Rotating

Dental Instruments. D. C. Hudson* J. L. Hartley, Robert Moore,

William T. Sweeney. J.A.D.A. 50:377 April (1955).

Effects of Temperature and Degree of Polymerization on Sorption

of Water by Polymethyl Methacrylate. G. M. Brauer and W. T.

Sweeney. Mod. Plastics 3j2 (No. 9) :138 May (1955).

Crazing of Acrylic Resins. W. T. Sweeney, G. M. Brauer and I.

C. Schoonover. J. Den. Res. 34:306, June (1955).

4.

WORK IN PROGRESS

4.1 Structure of Human Tooth Enamel and Dentin

The f luorophotomicroscopic studies of human enamel and dentin

were continued. Specimens of tooth sections which had been demineralized with chelating agents and specimens which had been

deorganized with potassium hydroxide -glycol and with ethylene
.

;

•

"

, ... /,

•

•

'
2

diamine were obtained from the National Naval Medical Research

Institute. Fluorophotomicrographs of the deorganized specimens

showed several interesting structual details. Series of lamellalike filamentous processes extending from the dentine-enamel

junction to the outer surface of the enamel were present in many

cases. Very prominent developmental lines (about 100 microns

wide) in the enamel were observed in rare instances. The very

low intensity of fluorescence in these lines suggests that organic

matrix formation was interrupted during the period of their formation.

Ordinarily dentin, high in organic content, fluoresces brighter

than enamel. However, in tooth sections which had been treated

with ethylene diamine, fluorescence of the dentin was less intense

than that of the enamel, indicating that more organic matrix had

been removed from the dentin. This observation is consistent with

the idea that the organic matrix of dentin is a collagen, whereas

the organic matrix of enamel resembles the keratins and, therefore,

is more resistant to hydrolysis.

Detailed studies were continued on the preparation of calcium

phosphates . A need for a standard hydroxyapatite was finally

filled by the growth of very pure, single crystals of this substance. This will serve as an X-ray diffraction standard to compare with the non-stoichiometric synthetic hydroxyapatites which

resemble bone and tooth mineral. The study of these precipitated

hydroxyapatites not having the hydroxyapatite chemical analysis

but yielding the X-ray diffraction pattern of apatite, was continued. X-ray methods are being devised to characterize their

structure. Single crystals of lead apatite [^^(PO^) g(0H) 2 ] were prepared in a hydrothermal system. This is part of a program

in which many lead apatites of differing stoichiometry are being

studied crystallographically . It is felt that the apatite structure with lead ions missing to lower the Pb/PO/j. ratio would offer

a more radical change in the X-ray diffraction data and lend itself

to solving the existence of a low Ca/PO^. ratio in the (calcium)

hydroxyapatites

.

4.2 Low Temperature Alloys

Additional gallium alloys were investigated for possible use

as tooth restorative materials. Alloys of gallium, copper and

beryllium attained their maximum compressive strength rapidly

(one hour) but the strength, 20,000 lb/in2

, is low compared to

dental amalgams. The setting expansion of these alloys like that

of almost all of the gallium alloys studied is also high, approximately 50 /JL/cm. Alloys of gallium and cobalt exhibited a slight

contraction, about 3 JUL/cm and a low strength 13*000 lb/in2 . Addiction of 10% cobalt to a gallium-nickel alloy reduced the setting

expansion from 130 to 50 jn/cm. The low strength of gallium-
k i

,

,

: *

sir'

*

.

' • '

• ki

,

•

• ;

)fL t'

-•

. :

.

' :

'*
- 3

- cobalt alloys, however, may limit the usefulness *of cobalt as a

modifying agent to reduce expansion. Gallium-nickel -silicon

alloys retained their compressive strength on elevation from

room temperature to 50 °C whereas the strength of ordinary dental

amalgams was reduced approximately 40% at the higher temperature.

4.3 Resins

(a) Polymerization of Resins.

Studies of the kinetics of decomposition of benzoyl peroxide

in the presence of dimethyl aniline were initiated. The rate of

free radical formation in benzene was determined colorimetrically

with diphenyl picrylhydrazyl . The reaction is of the 0.4th order

with respect to dime thy1 ani 1 ine . Determination of the order of

reaction with respect to benzoyl peroxide and diphenyl picrylhydrazyl have been started.

The Ultraviscoson instrument which uses a vibrating reed

to determine the viscosity-density product of solutions was investigated to determine the possibility of using it for determination of the molecular weight of polymethyl methacrylate. The

observations so far obtained indicate that the viscosity-density

relation is concentration-dependent but may not be sufficiently

sensitive for molecular weight determinations with the specimen

cell ordinarily used. A new specimen cell is being built which

will allow more accurate positioning and better temperature control to increase the precision. This method, if successful, will

make a simple and fast technic available for following changes

in molecular weight of resins of the methyl methacrylate type.

(b) Properties of Dental Resin Cements.

Many of the dental resin cements were found to have about

the same solubility in water as do the zinc phosphate cements.

However, the resin cements are not attacked by sulfides as are

the zinc phosphate cements. The effects of acetic, citric and

lactic acids and ammonium hydroxide on the cements was determined.

The most notable effect was observed in the materials using carbonate fillers. Gas bubbles formed on the surfaces of the specimens and the color and optical opacity of the specimens were

changed .as the reaction took place between the carbonate and the

acidic solutions.

4.4 Chromium-Cobalt Alloys

The chemical compositions and melting points were determined

for the series of chromium-cobalt alloys previously tested for

mechanical properties. On the basis of the results of these

tests a proposed tentative specification has been drafted and

circulated for comment to all manufacturers, agencies, and groups
.

'

.

,

'

• \ff:

.

'

. /

' :

. U. .V

•

.
- 4 - known to have an interest in the dental application of these

materials. The final report on the project and the specification are being prepared for publication.

4.5 High-Speed Handpieces

A testing procedure for dental handpieces was devised. On

the basis of clinical observation a running period of one -half

hour was selected as an approximation of one day of clinical

usage. A one-eighth inch round-ball burnisher was chucked in

the handpiece in place of a bur and a load of 100 grams was

applied to the burnisher. During the first 30 hours of running

the handpieces were immersed and run forward and backward in

carbon tetrachloride and in oil for 30 seconds after each half

hour and were disassembled and lubricated after each 2\ hours.

Between 30 and 100 hours the periods before immersion oiling

and disassembly were increased to one and five hours respectively.

After 100 hours the handpieces were run without further cleaning

or lubrication until a total of 200 hours running time had

elapsed or failure had occurred. Temperature rise, backlash,

runout and lateral play were measured during the testing period.

In general the results obtained indicated that temperature

rise was higher in new than in worn instruments, that contraangle handpieces were more likely to fail at high speeds than

were straight handpieces, that handpieces operated at speeds

above those recommended by the manufacturer gave consistently

poor service and that care and attention devoted to maintenance

were more significant factors in the failure of the handpieces

than were differences in design.

4.6 Panoramic X-ray Investigation

Several improvements have been made in the prototype panoramic X-ray machine for taking exposures of the entire dental

arch on a single film. Changes have been made in the film-drive

mechanism to secure smoother film movement. Gears and racks

have been replaced by a cable -drive system which is simple and

smooth in operation. Film loading has been simplified so that

the cassette is now positioned from beneath the film carrier

rather than from the front where it interfered with patient

positioning. A simplified master-drive mechanism has been designed and is being installed to increase the flexibility of

operation of the machine and at the same time improve the overall appearance. Further improvements in devices for positioning

the patient have been made

.

X-ray tube improvement has been secured through cooperation

with a tube manufacturer. It appears that a satisfactory X-ray

source is now available with a small 0.8 mm focal spot which

gives better definition in radiographs produced with the machine.
if

-

.

ev;

'

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I m I
- 5

- 4.7

Motion Pictures

The motion picture* "Dental Materials: Specification and

Certification* " showing how a specification is formulated and

used was completed. The photography has been completed and

the narration written for the picture* "Porcelain Jacket Crowns:

A Rapid Precise Technic for the Dentist." A script was prepared

for a motion picture on the hydraulic contra-angle handpiece.

4.8 Hydraulic Turbine Handpiece

Designs have been made for diamond cutting tools suitable

for use with the hydraulic turbine contra-angle handpiece. Because of the high rotary speed and unique cutting characteristics of this instrument fewer tools are needed than with conventional handpieces. However* the tools must be more accurately

balanced and more precisely centered in the chuck than is required

for conventional instruments. Clinical evaluation of the hydraulic

turbine handpiece is being carried on in five private offices.

4.9 Zinc Oxide -Eugenol Materials

Setting times of zinc oxide -eugenol mixtures using different

particle size zinc oxide and varying powder-liquid ratios were

determined. No correlation was found between particle size and

setting time

.

Difficulties were encountered in water sorption studies of

Hyperfine zinc oxide due to the unexpected presence of ammonia in

the material . Analysis showed that the material contains about

1.2% of ammonia. This ammonia is incorporated during the manufacture of zinc oxide from zinc carbonate in an ammonia atmosphere.

4.10

Agar-Agar Materials

A study of the causes of deterioration of agar-agar duplicating materials was initiated. These materials* on continued reuse* show changes in physical properties which make them unsuitable

for further service. Heating a 2^ aqueous agar-agar solution to

100°C caused a considerable decrease in pH (from 5*7 to 3-0) within

10 days. Solutions stored at this elevated temperature turned dark

and did not form gels at room temperature.

4.11

Inlay Casting Wax

An investigation of the basic properties of waxes especially

those of the synthetic type has been undertaken with the objective

of developing an improved inlay casting wax because it is considered

that the wax is the source of greatest error in precision casting

process at present.
I

,

’

\
4.12 Evaluation of Materials

Materials evaluated for the Federal dental services and

the American Dental Association by specification or special

test methods included denture base resins, impression compounds,

inlay casting gold alloys, inlay casting investments, inlay

casting waxes, pliers, separating mediums, and wrought gold wire

alloys

.
.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-10-0707 June 6, 1955

NBS REPORT

4129

Progress Report

SULFINIC ACID DERIVATIVES AS ACCELERATORS IN

THE POLYMERIZATION OF METHYL METHACRYLATE

by

G. M. Brauer—

and 1 F . R . Burns—

1 Chemist, Dental Research Section.

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association

the Army Dental Corps, the Air Force Dental 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 specifically prepared If that agency wishes to reproduce additional copies of that particular report for its own use,

SULFINIC ACID DERIVATIVES AS ACCELERATORS IN

THE POLYMERIZATION OF METHYL METHACRYLATE

Abstract

A study was made of the efficiency of amine

salts of p-toluene sulfinic acid as accelerators

in the polymerization of methyl methacrylate.

These compounds are readily synthesized by addition

of amine to the ether solution of the sulfinic

acid. Most of the salts are stable in air. Salts

of secondary and tertiary aromatic amines such as

the methyl aniline or dimethyl -p-toluidine salts

are very efficient accelerators in the benzoyl

peroxide initiated polymerization. However,, colored

products are formed. Aliphatic amine salts as well

as the free acid give colorless polymers, but the

setting time is increased considerably. Heterocyclic amines show reduced activity as polymerization

accelerators

.

The setting times of monomer -polymer mixtures

containing some of these accelerators have been determined . Physical properties of the polymers have

been evaluated by measuring their resistance to

indentation and recovery.

1 . INTRODUCTION

In 1948 the use of organic sulfinic acids to promote

rapid polymerization was advocated for a number of reasons,

one of which was to decrease the color of the polymeric

product [1-9]. These compounds proved to be excellent accelerators for the emulsion or bulk polymerization of methyl

methacrylate and other vinyl monomers. Colorless polymerization products were readily formed. This is a big advantage

over the colored products which are usually formed when methyl

methacrylate is polymerized with a benzoyl peroxide -amine system* Furthermore, sulfinic acid shows no induction period.

The polymerization starts immediately even when hydroquinone

stabilized monomer is used. The sulfinic acid does not oxidize the hydroquinone inhibitor or react with any quinone

present

.

Unfortunately, sulfinic acids are rather unstable in air.

In the presence of air they readily oxidize to sulfonic acids

and other oxidation products which do not act as polymerization

activators . One method of improving the storage life of these

materials consists of dissolving the sulfinic acid in a silicone oil in a capsule. Before polymerization the capsule is

2

added to the monomer in which it dissolves and the sulfinic

acid is liberated. Commercial products of this type have met

some success. Obviously, monomer to which sulfinic acid has

been added must be used within a short period of time.

The sulfinic acids most commonly suggested as accelerators

are p-toluene sulfinic acid (i), chlorobenzene sulfinic acid

(II), 2-naphthalene sulfinic acid (ill) or anthracene sulfinic

acid (IV).

The presence of an aromatic or condensed ring system increases the stability towards oxidation.

M Recently Bredereck and Bader [10-11] have synthesized

derivatives of sulfinic acid such as oxysulfones, a-aminosulfones and sulfinic acid salts. Some of these compounds had

good storage stability and were efficient accelerators for

the polymerization of monomer -polymer mixtures at room temperature., especially after the addition of primary alcohols

[12-13]. In the present investigation a more detailed study

of the use of amine salts of p-toluene sulfinic acid as

possible accelerators in the polymerization of methyl methacrylate was undertaken.

2. EXPERIMENTAL PROCEDURE AND RESULTS

2.1 SYNTHESIS

The sodium salt of p-toluene sulfinic acid was used as

starting material. This salt is stable and can be obtained

commercially

.

In order to prepare various p-toluene sulfinic acid salts

which might have long storage life and good accelerating properties, the sodium salt was acidified in water . CH3

0

+hci

SOzNa.

ch3

^ + £JaCl

SOzH

The free sulfinic acid was difficult to dry without partial conversion into the sulfonic acid and thiosulfonic ester.

The free acid was dissolved in ether solution according to the

method suggested by Bredereck [10] and an ether solution of

a very slight excess of the respective amine was added. The

following reaction takes place:

SOzli

+

p-toluene sulfinic

acid

p-toluene sulfinic

acid salt

R = CnH2n+i = aliphatic

= = aromatic

R 1 = aliphatic, aromatic or H group

R

n = aliphatic or H group

If a salt crystallized out, it was filtered off. Sometimes, on partial evaporation of the solution, an oily layer

separated. This oil was removed and extracted with ether.

If the material did not crystallize, the remaining solvent

was removed at room temperature at approximately 60 mm pressure.

The material was then heated to 60°C at 2 to 4 mm pressure,

cooled and stored at 2 to 4°C until crystallization occurred.

The yield of impure solid or oil was nearly quantitative.

Solids were purified by repeated recrystallization,

usually from ether to which small quantities of methanol had

been added. Recrystallization was continued until a colorless

crystalline material with a sharp melting point was obtained.

Those compounds which had been reported in the literature were

identified by their melting points. The composition of all

other solids was determined by analysis for carbon, hydrogen,

sulfur or nitrogen. Considerable difficulty was encountered

in purifying oily products, which had high boiling points,

even in high vacuum. These compounds were tested for their

accelerator efficiency without further purification.

Table 1 gives the melting points obtained in this study

and those found in the literature. Analysis of the crystalline

compounds not previously reported is also given. It will be

seen that most salts of tertiary amines are oils. Salts of

most primary and many secondary aliphatic amines are solids.

The p-toluene sulfinic acid salts of di-iso-butylqmine, N-methylcyclohexylamine, N, N-diethylcyclohexylamine and piperidine were

unstable in ether solution and the corresponding sulfonic acids

crystallized on standing. On repeated recrystallization, the

dicyclohexyl and dibenzylamines also oxidized to the sulfonic

acid

.
; * '

'

'

.T
- 4 -

2 . 2 POLYMERIZATION

The effectiveness of these salts as accelerators of the

peroxide catalyzed polymerization of methyl methacrylate was

determined. Monomer containing 2% benzoyl peroxide and 0.006%

hydroquinone inhibitor was prepared and varying concentrations

of sulfinic acids were added. The polymerization reaction

at 37°C was followed by means of an Ultra-Viscoson ultrasonic

viscosity apparatus in the manner previously reported [14].

Results of the polymerization studies are given in Table

2 and Figure 1 . The salts of the aromatic amines are the

most efficient accelerators encountered in this study. In

the presence of N-methyl aniline salt, a 1000 cps value was

reached in less than 12 minutes. In comparison, the time necessary to reach 1000 cps for the commercially-used benzoyl

peroxide -di-f3-hydroxyethyl-m- to luidine and benzoyl peroxidedime thyl-p-toluidine systems as determined in a previous study

were 21 and 36 minutes respectively. Salts of naphthyl amines

showed a much reduced activity. No attempt was made to synthesize the N-methyl -p-toluidine salt which, based on experiments in the peroxide -amine system, should show an even greater

activity than the N-methyl aniline salt. The polymerization

products of all aromatic amines were yellowish or red. It

may be possible to decrease somewhat the color of these products by careful purification of the raw materials and possibly by distillation of the amine salts in a molecular still.

In agreement with Bredereck [10], it was found that the storage stability of the aromatic amines was much better than that

of the free p-toluene sulfinic acid. The N-methyl aniline

salt showed no decrease in activity after storage at room

temperature for 5 months. The crystalline m-toluidine salt

showed a faint yellow color within 10 months.

Salts of aliphatic amines showed a much decreased activity.

The oily tertiary amine salts were somewhat faster acting

than the crystalline secondary amines. Tri-n -hexyl amine

appeared to be the best accelerator in this series. Increase

in chain length of the hydrocarbon radical decreased the accelerating activity. The isomeric di-n-butyl and di-iso- butylamines had approximately the same accelerating efficiency. The

di- sec -butyl amine was a less effective accelerator. Amines of

cyclic hydrocarbons such as dicyclohexylamine are only slightly

soluble in monomer and do not accelerate the polymerization

appreciably. Salts of heterocyclic amines show only slightly

improved accelerating efficiency as compared to the aliphatic

amine salts.

Bredereck, in his paper [12], shows the high efficiency

of a-oxy and a-amino sulfones . He reported that the latter

compound is stable in air. Derivatives from p-toluene sulfinic acid were prepared according to his synthesis [10].

2

- 5

- SOiVL

£) + CHxO

ch3

SOtCHzOK

O + CaHfJfH,

CHa

^0»CH,PH

^0ACH£l-NCaH^

ch3 a

Monomer containing 2% of the a-oxysulfone or a-aminosulfone and 2% benzoyl peroxide polymerized rather slowly.

After 1 hour, the solutions showed a viscosity of 6.5 and

24 cps, respectively.

Phosphinic acids have also been suggested as accelerators and their use has been patented by Loritsch [15] • The

di-n-propylamine and methyl aniline salts of phenyl phosphinic

acid were prepared by methods similar to those used for sulfinic acid salts. The salts are oily liquids. The di-npropylamine salt gave an analysis of 0=59*1* H=9-2$ (calculated values 0=59.2$, H=9.0$). Neither phenyl phosphinic

acid nor its di-n-propylamine or methyl aniline salts showed

any appreciable effects on the polymerization rate . Use of

a hydroperoxide such as tertiary-butyl hydroperoxide instead

of benzoyl peroxide as suggested by Loritsch did not decrease

the setting time

.

2.3 HARDENING OF MONOMER_POLYMER MIXTURES

In order to determine the setting time of monomerpolymer mixtures containing various amines, a penetration

test was employed. The test method chosen was a modification

of the A.S.T.M. penetration test for bituminous materials [ 16 ]

.

This method employs a standard needle mounted on a dial gage

graduated in 1/1000 in. •• One gram of ground polymer powder containing 1% benzoyl

peroxide was mixed according to the established dental procedure with 1 ml of hydroquinone-inhibited monomer containing

the various amine salts. Additional peroxide, methanol or

hydrogen peroxide were also added to the monomer. The mix

was placed in a brass ring 0.5 cm high and 1.1 cm in diameter,

which was on a glass plate* A piece of polyethylene was placed

on top of the ring before a second plate and a 100 g weight

were positioned on it. At 2-1/2 minutes from the start of

mix, the top plate and the weight were removed and the ring

was placed in a 37 U C air bath. The needle attached to the

dial gage and under 100 g load, was lowered into the ring at

3 minutes from the start of the mix and left for 5 seconds.

A reading was made and the needle was removed. The procedure

was repeated every minute until the material could no longer be

- 6 - penetrated. Penetration was begun in the center of the ring

and continued in a clockwise direction around the ring with

the final readings being made around the outside edge. Pinal

readings made near the center of the specimen would not give

accurate results.

It will be seen in Table 3 that mixtures containing the

methyl aniline salt as well as the N-ethyl-bis-(p-tolylsulfonemethyl ) -amine showed a very short setting time. The mixture

hardened much slower when di-n-hexyl amine salt was used as

accelerator. Under the experimental condition no appreciable

decrease in setting time on addition of methanol or a trace

of hydrogen peroxide as reported by Bredereck was observed.

Setting times as short as 2 to 4 minutes as reported by

Bredereck and co-workers [12] for the polymerization of methyl

methacrylate in the presence of the a-aminosulfone, benzoyl

peroxide, 1 6% methanol and a trace of hydrogen peroxide were

not obtained in these experiments. The preparation of the

polymer^ particle size and the exact monomer -polymer ratio

which were employed are not given by Bredereck. In the absence of more complete data it is not possible to explain

these variations in setting time. Since Bredereck followed

the course of the polymerization by the rise in temperature

only, his results do not make it possible to draw any conclusions concerning the physical properties of his polymeric

products

.

In order to obtain information on the physical characteristics such as hardness and elastic properties of the polymerized materials their indentation and recovery were measured

using the Rockwell superficial hardness tester. Details of

the procedure have been described by Sweeney, Sheehan and

Yost [17]. Results of the indentation tests and percent

recovery are given in Table 3-

A good dental restorative material should have low indentation values and high percent recovery. Polymers prepared

with the methyl aniline salt of p-toluene sulfinic acid showed

such behavior. The salt of tri-n -hexylamine sets much slower.

Addition of methanol considerably decreased the setting time,

but the resulting polymer was too soft for most applications.

Polymers prepared in the presence of this accelerator have

physical characteristics similar to those of materials polymerized with amine accelerators such as di-P-hydroxyethyl-mtoluidine or dimethyl -p -toluidine . N-ethyl-bis-(p-tolylsulf one -methyl ) -amine produced a softer material. Indentation

and hardness were improved somewhat by the addition of 1 6%

methanol and a little hydrogen peroxide. Substituting tbutyl hydroperoxide for a portion of the benzoyl peroxide

catalyst brought about a slight improvement of physical properties .

- 7

-

3

.

SUMMARY

1. The use of amine salts of p-toluene sulfinic acid

and a -amino sulfones as accelerators in the polymerization of

methyl methacrylate has been investigated.

2. Salts of aromatic amines are very efficient polymerization accelerators. The polymerization products are usually

colored

.

3. Salts of aliphatic and heterocyclic amines show reduced activity as polymerization accelerators. Most of the

products formed are colorless.

4. The stability of the amine salts and a-amino sulfones

appears to be better than that of the free sulfinic acid.

5. The setting times of a number of monomer -polymer

mixtures containing the amine salts have been measured.

Resistance to indentation and recovery were used to evaluate the physical properties of the polymeric products

formed in the presence of the various accelerators.

- 8 - BIBLIOGRAPHY

1. Hagger, 0. New catalysts for the polymerization of

ethylenes at room temperature. Helv. chim. acta

31:1624-1630, (1948).

2. Ibid. Utilization of the energy of swelling for the

acceleration of the sulfinic acid promoted polymerization of methacrylate esters at room temperature.

Helv. chim. acta 34:1872-1876, (1951).

3. Castan, P. and Hagger, 0. Polymerization of vinyl compounds in the presence of a sulfinic acid catalyst.

U. S. patent #2,567 .>803., Sept. 11, 1951 (to Gebr

.

deTrey), Swiss patent #255* 978* Feb. 1, 1949.

4. Imperial Chemical Industries Ltd. Polymerization catalysts for ethylenically unsaturated compounds. Brit,

patent #586 , 796, April 1, 1947*

5. Farbwerke Hoechst . Method for the preparation of polymers or mixed polymers of unsaturated compounds.

German patent #861 , 156 , December 29., 1952.

6 . Ynfiesta, J. L., Yanes, J. F. and Sanchez, F. R.

Sulfinic acids as catalysts in vinyl polymerization:

I. Study of the velocity of polymerization of methyl

methacrylate. Anales real soc . espan fis y. quim.

4TB:53-60, (1951).

7. Hashihama, Y., Sumitomo, H. and Okabe, J. Acrylic

polymers II. J. Chem. Soc. Japan, Ind . Chem. Sec.

56 : 25 -28 , (1953).

8 . Taylor, P. B. and Frank, S. L. Low temperature polymer!

zation of acrylic resins. J. D. Res. 29:486-493., (1950)

9. Schroder, A. and Santorik, St. New investigations on

so-called self -polymerizing resins. Schweizerische

Monatszeitschrif t fur Zahriheilkunde 60:937^ (1950).

M _ 10. Bredereck, H. and Bader, E. Preparation of a-oxy and

a-aminosulfones and sulfinic acid salts of organic

bases. Chem. Ber. 87:129-140, (1954).

M 11. Bader, E. and Hermann, H. D. New methods for the synthe

sis of sulfonamines . Chem. Ber. 88:41-49, ( 1955 )

.

I! 12. Bredereck, H., Bader, E. and Wohnhas, A. a-oxy, a-amino

sulfones and sulfinic acid salts as polymerization catalysts. Makromolek . Chem. 12:100-109, (1954).

- 9

- BIBLIOGRAPHY (Cont'd)

13. W. C. Heraeus G.M.B.H. and Deutsche Gold und Silberscheideanstalt vorm. Roessler, British patents #710,534 and

#710, 535* June 1 6 , 1954; British patent #708 , 077 j April

28, 1954; German patent #913^477^ June 14, 1954; and German patent #9l6,733.> August 16, 1954.

14. Davenport, R. M. and Brauer, G. M. Amine accelerators

for the polymerization of methyl methacrylate.

(unpublished data)

.

15. Loritsch, J. A. Polymerization in the presence of an

organo -phosphorous compound as an activator. U. S. patent

#2,5^3,635, February 27, 1951 . (to G.E.)

16. American Society for Testing Materials. Standard method

of test for penetration for bituminous materials.

A.S.T.M. Designation D5-53T.

17. Sweeney, W. T., Sheehan, W. D. and Yost, E. L. Mechanical properties of direct filling resins. J.A.D.A. 49:

513 - 521 , ( 1954 ).

TABLE 1

PROPERTIES OF AMINE SALTS OF p -TOLUENE SULFINIC ACID

MELTING POINT COMPOSITIONAMINE

found reported found calculated

°C °C % %

Aliphatic

di-n-propyl 60 S=12 .

N=5.4 S=12 .4 N=5 .4

di-n-butyl 105 104-6

di-iso-butyl— 120 C=59 -7 H=9 .

0 =60.1 H=9 . C&.

di-sec -butyl oil

di-n-hexl 119 119

dicyclohexyl— 153 0=67.5 H=9 • 3 0=67.5 H=9.2

N-methyl cy1 cohexyl— 138 0=58.7

N= 4.9

H=8 .0=58.9

N= 4.9

h=8 . ik

diethanol oil

tri-n-butyl oil 40-43

tri-n-hexyl oil

cl N,N-diethylcyclohexyl— 100 0=62.5 H=8 .7 C=6l .9 H=8 . 8^

Aromatic

aniline 116 118

N -methyl aniline oil

m-toluidine 117 120 0=63.4

N= 5.2

H=6.5 0=63.8

N= 5-3

H=6.5

dimethyl -p -toluidine oil

dibenzyl amine

dimethyl -a-naphthyl amine

132-

oil

0=70.8

N= 4.0

H=6 .

0=71.2

N= 4.0

H=6.5

phenyl hydrazine 158 158

11

TABLE 1 (Cont'd)

MELTING POINT COMPOSITION—

AMINE

found reported found calculated

°C °C % %

Heterocyclic

pyridine oilmorpholine 124-5 C=54 . 1 H=7 .

C=54 . 3 H=7 .

o piperidine— 130 130 C=56.1 H=7 .4 C=55 .9 H=7 . 4—

pyrrole solid—

pyrrolidine oil

a Sulfonic acid salt.

b Calculated for sulfonic acid salt.

c On recrystallization oxidized to the sulfonic acid salt.

M.P, = 179°C composition % , C=64.0 H=8.7 (found).

C=64.5 H=8.7 (calculated).

d On recrystallization oxidized to the sulfonic acid salt.

M.P. = l62°C composition % , S=7-7 N=3.8 (found).

S=7 .7 N=3 .7 (calculated).

£ Oxidizes to hygroscopic solid.

£ No sharp melting point.

g Analyses were performed by R. A. Paulson of the Analytical

Chemistry Section of the National Bureau of Standards.

12

TABLE 2

ACCELERATORS OF THE POLYMERIZATION OF METHYL METHACRYLATE

AMINE SALTS OF p -TOLUENE SULFINIC ACID

Benzoyl peroxide concentration: 2%

SOLUBIL- AMINE POLYMER

AMINE SALT ITY IN CONCEN- TIME VISCOSITY COLOR

MONOMER TRATION

% min cps

iphatic

di-n-propyl s

2.0 60 10 faint

2.0 163 1000 yellow

di-n-butyl si . s

<2.0 60 12 colorless

0.5 60 19

0.05 60 1

di -iso -butyl si . s . <2.0 60 18 slight

yellow

di-sec-butyl s

2.0 60 1.5 yellow

di-n-hexyl si . s

<2.0 60 9 colorless

0.2 60 no change

dicyclohexyl si . s

<0.5 60 no change colorless

N-methyl cyclohexyl ins

<0.2 60 no change colorless

diethanol ins

<2.0 60 no change colorless

tri-n-butyl s

2.0 60 5 faint

yellow

tri-n-hexyl s

2.0 60 105 colorless

2.0 101 1000

1.0 60 50

N, N-diethyl cyclohexyl s

2.0 60 8 slight

yellow

- 13 - TABLE 2 (Cont'd)

AMINE SALT SOLUBIL- AMINE POLYMER

ITY IN CONCEN- TIME VISCOSITY COLOR

MONOMER TRATION

% min cps

Aromatic

aniline s

2.0 36 3600 yellowish

0.5 ll6 1000 red

N -methyl aniline s . 2.0 12 1300 reddish

1.0 14 1000 yellow

0.5 60 72

m-toluidine s

2.0 • 28 1000 reddish

0.5 60 475 brown

dimethyl -p-toluidine s . 2.0 17 1000 brownish

yellow

dibenzyl amine si . s

<0.5 60 6 colorless

dimethyl -a-napthylamine ins

<0.5 60 no change reddish

orange

phenyl hydrazine si . s . <2.0 60 21 yellow

0.2 60 11

0.05 60 32

Heterocyclic

pyridine s . 2.0 60 13.3 colorless

morpholine ins

<2.0 60 28 colorless

0.2 60 no change

pyrrolidine s

2.0 60. 24 colorless

piperidine s . 2.0 60 12 colorless

Other accelerators

(Not amine salts)

p-toluene sulfinic acid s . 2.0 130 1000 colorless

1.0 205 1000

di-fe-hydroxyethy]^-mtoluidine

s

1.0 21 1000 yellow

dimethyl -p-toluidine s . 2.0 140 1000 yellow

0.5 36 1000

14

TABLE 3

SETTING TIME AND PHYSICAL PROPERTIES OF

MONOMER -POLYMER SLURRIES

Mixture : 1 g ground polymer containing 1$ benzoyl

peroxide and 1 ml of hydroquinone stabilized

monomer

.

SETEXP . ADDITIVES IN MONOMER TING INDENTATION RECOVERY

NO. TIME

30 -min 1-hr 5 -days 30-min 1-hr 5 -days

min mm mm mm % % %

1 2% A§ 6-8 .24 .15 .10 29 46 73

2 2fo >30

3 Same as 2 + 13 soft soft

16$ methanol

4 Same as 2 + 8 soft soft

30$ methanol

5 2$ C£ 8-9 soft .24 soft 26

6 4$ C +4$ benzoyl 6-7 .25 .17 16 49

peroxide

7 Same as 6 + - .26 .15 12 54

6$ methanol

8 Same as 6 + 7 .17 .13 49 68

16$ methanol

9 Same as 8 + 7 soft .20 soft 65

1 drop 30$ H2 O2

10 Same as 8 + 7

1 drop 3$ H2 O2

11 4$ C +4$ t- 7 .23 .12 30 63

butylhydroperoxide +

16$ methanol

12 Same as 11 4

- - soft .27 soft 2

2 drops 30$ H2O 2

13 Same as 11 + 7 .19 .14 4l 66

14

1 drop 3$ H2O 2

2$ di-((3-hydroxyethyl ) - 10 .13 .10 46 78

m-toluidine

15 2$ dimethyl -p-toluidine 8 .18 .13 .09 39 52 79

16 1$ dimethyl -p-toluidine 12 .23 .15 .10 38 48 76

a A = N-methyl aniline salt of p-toluene sulfinic acid

.

b B = di-n-hexyl amine salt of p-toluene sulfinic acid.

c_ C = N-ethyl-bis- [p-tolylsulfone-methyl ] -amine

.

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.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 June 15, 1955 4155

Progress Report

PRELIMINARY INVESTIGATION OF COLOR IN DENTAL RESINS

by

Robert B. Moore*

' PFC, USA, Guest Worker, 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 publication, reprinting, or reproduction of this report in any form, either in whole or in part, is prohibited

unless permission is obtained in writing from the Office of the Director, National Bureau 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,
.

*
PRELIMINARY INVESTIGATION OF COLOR IN DENTAL RESINS

Abstract

The color control of dental materials Is of major

importance to the dental profession. In this study several

of the factors affecting the color of dental resins were

investigated. The color of a translucent resin is the

function of three light components. One component is the

light reflected from the surface of the resin, another is

the light diffused internally by the pigment, and finally

the light which passes through the specimen, is reflected

off a background and returns through :the specimen. The

color and intensity of the latter two components are affected by a change in thickness. A decrease in monomer

to polymer ratio of these resins causes a small increase

in opacity. In the normal range of monomer to polymer

ratios this effect is relatively unimportant. With the

color difference meter used in this study the precision

of measurements of opacity and color is greater than is

the precision of duplication of specimens.

1 . INTRODUCTION

This report is a summary of initial findings in the field of

color measurements of dental materials. A great amount of work

has been done in the past in the field of color. Various systems

of color specification [1,2] have been devised and a number of

systems of measuring color [1,2] have been developed. However,

little work has been done in applying the science of color to the

field of dental materials.

2

In measuring the color of various dental materials a number

of problems are encountered . All human structures exhibit the

property of translucency (the ability to transmit light). If a

dental material to be placed in the mouth is to appear life-like

and match the substance which it is to represents it must not only

have the same color but must also have the same opacity. Opacity

is used to designate different degrees of absorption of light.

Another factor involved in the color measurement of these materials is that of thickness. The thicker a material is* the greater

is its opacity. In substances such as teeth the thickness is of

such importance that minor variations will appreciably affect the

color. In the tooth the thickness is not uniform,, thus causing color

variations throughout

.

The background of any translucent material also has its effect

4 on the color. The translucent material will usually tend to approach

the color of the background. The more translucent the material the

closer it will approach the color of the background,

A further complication is that structures such as teeth consist

of two or more components of different colors and opacities. The

observed color of a tooth is a function of both materials. Since

each of these materials varies in thickness the composition of the

ultimate observed color becomes complex.

Another factor involved is the thin layer of liquid which coats

the oral cavity. The effect of this layer is to give a high gloss

to the surface and also to intensify the color.

- 3

-

2. EXPERIMENTAL WORK

In this preliminary study* all efforts were concentrated on

the evaluation of the effect of thickness and monomer -polymer ratio

on the observed color and opacity of translucent heat-curing resin

materials

.

Various methods were attempted to determine the best method

for producing color specimens. Dental flasks were used at first*

but these were discarded because in the thicker specimens it was

impossible to provide a sufficient reservoir to fill the voids created

by the contraction of the monomer -polymer mix during polymerization

.

The best method developed was curing in a Carver press with

the mix under a constant positive pressure. It was felt that variations in curing time* temperature* and pressure would have negligible

effect on the color and opacity of the specimens. However* in spite

of this* great effort was made to reduce these variations to a minimum. The specimens were cured for one hour at 100 ± 10°C. About

ten minutes of this hour were used to heat the specimen from room

temperature to curing temperature. The pressure was held constant

at 2*000 ± 200 psi

.

Using the Carver press method* specimens of various thicknesses

between 1/4 ram and 8 mm were prepared. Specimens were also prepared

using different ratios of monomer to polymer. The range covered

by these ratios was from O .38 ml/gm to 0.68 ml/gm. The manufacturer^

recommendation was for O .56 milliliter of monomer per gram of polymer.

The specimens were measured by a tristimulus colorimeter

(Gardner Automatic Color Difference Meter) to determine the color

-irrelative to a standard. For each color a reflectance value ri Rd' !

alid two color coordinate values ! 'a

n and "b" are given. ,T Rd rT is defined [3] as 100 times the amount of light reflected by a sample

divided by the amount of light reflected by a perfectly diffusing

sample when the light is incident upon the sample at an angle of

45° and the measuring device records the light diffused perpendicularly from the sample. A completely absorbing specimen would have

an "Rd" value of zero^ and a perfectly diffusing white would have

a value of 100. "a" and "b" are [3] the rectangular coordinates

of chromaticity in any plane intersecting the color solid perpendicularly to the white-black axis. Zero values of "a" and "b" indicate

the color specimen is on the white-black axis, thus some shade of gray.

A positive value of "a" indicates the color is on the red side while

a negative value places it on the green side. A positive value of

"b n indicates the color is on the yellow side and a negative value

locates it on the blue side. Units of color difference for these

three values are equivalent to NBS units of color difference. The

NBS unit is approximately five times as large as the smallest difference perceptable to a skilled observer under the best experimental

conditions

.

All specimens were measured twice for color, once while using

a white background and once while using a black background. The

white background had the color coordinates, "Rd n 8l.l, "an -4.0,

and "b” +2.9. For a black background no backing was used. The

light was allowed to dissipate itself into a darkened room. Thus

the black background was essentially a black body with the coordinates "Rd" 0, "a" 0, and "b" 0. A yellow color standard ("Rd"

60.3* "a" -0.2, and "b" +22.5) was used to standardize the colorimeter before each set of readings. The instrument was also checked
'

X '

'

.

-
- 5

- with the standard after the readings to determine any drift. The

aperture of the colorimeter was inches in diameter. By measuring this large an area, any small local color inconsistancies were

made relatively unimportant

.

3 . RESULTS

Figures 1, 2 and 3 show the effect of thickness on the color

coordinates "Rd", "a

! ’, and "b n

, respectively, for #69 Hue-Ion acrylic

resin mixed with a ratio of O .56 milliliter of monomer per gram of

polymer. The results obtained for this resin were chosen as a representative of the group of ten resins tested.

An examination of the graphs shows that in the thicker specimens the "Rd", "a", and "b” readings for the white background approach

those for the black background. With thick specimens (5 mm or more)

there was no measureable difference in the readings with a change in

background. The color of the thick specimens is the sum of two

different light components. One component is the light which is

reflected off the surface of the specimen and the other component

is that one which is composed of the light which enters the specimen

and is returned by diffusion from the individual pigment particles.

These two components are not necessarily the same color.

In the thin specimens (1.5 nim and less) with a white backing

the color approaches that of the background. The color of the

specimen will never have the same color as that of the background

because the main color component will always be diluted by a secondary color component . In this range of thickness the main color

component is the color of the light passing through the specimen

and being reflected by the background. This component will approach

r the color of the background. The secondary color component is that

light which is reflected from the surface of the specimen. This
.

3d '

'

.

.
- 6 - color component will have its own color. This component will not

vary in color or intensity for different thickness specimens. In

the case of a black background the main component is that light

which enters the specimen and is reflected by the pigment. Its

effect decreases with a decrease in thickness until it finally

becomes negligible in relation to the surface-reflected light.

With a black background the color of the specimen will approach

the color of the surface-reflected light, rather than the background color.

In the range of 0.5 mm to 4 mm with the white background there

is a deviation from the approach towards the background color. As

the specimens decrease in thickness the intensities of the colors

(both the "a" and "b" values) increase to a maximum, then reverse

and begin to approach the color of the backing. The color of a specimen in this region is a function of three components. The first and

least important component is the light which is reflected from the

surface of the specimen. The second component, whose color intensity

decreases with a decrease in thickness, is that light which is reflected internally from the pigment of the specimen. This component with increasing thickness approaches the color of an opaque

specimen. The third component is the light which passes through

the specimen and is reflected off the background and returns through

the specimen. In very thin specimens this component is the dominant factor. Its color approaches that of the background as the

specimens become thinner. As the specimens become thicker the light
* r

|

•:

• PI

. .

: 4

i

.
- 7

- of this component must pass through increasing amounts of pigment.

The pigment acts as an absorber,, removing certain portions of the

light spectrum. Thus as the amount of pigment increases the greater

is the absorbing action and the more intense in color is this light.

As the thickness of the specimen continues to increase the magnitude

of this component rapidly decreases. The rate of increase in color

intensity of this component is much greater than that of the internally-reflected component. The combination of these two components

causes the characteristic peak in ,! a'

! and ! 'b

!

' coordinates.

The colors at infinite thickness of the ten Hue-Ion toothcolored resins and one Vitacrylic pink-colored resin which were used

in this study are listed in Table 1. The colors of the resins are

averages of the prepared specimens. These coordinates should not

be taken as the absolute colors of the resins because of the inability

to accurately reproduce specimens. These figures indicate* however*

the range of colors which were measured.

Table 1

Shade "Rd" V "b"

No. 61 Hue -Ion ......45.... +1 +18

No. 62 Hue -Ion 45 +1

No. 65 Hue -Ion 45 +3 — +19

No. 66 Hue -Ion 45 +1 +19

No. 67 Hue-Ion 38 +1 +20

No. 69 Hue -Ion 38 0 +18

No. 78 Hue -Ion 34 0. . .

.

+19

No. 87 Hue -Ion 32 +2 ....+19

No. C Hue -Ion 49 0 +18

No. D Hue -Ion 47 0. . .

.

+20

Vitacrylic 24 ....+33.... . . . .+15
> I

.
Figures 4, 5 and 6 show the results of the color measurements

of a series of specimens which were prepared using two different

ratios of liquid to powder. The 0.68 ml/gm mix is a very thin mix

while the 0.44 ml/gm is a very thick mix. Normally a mix would

never be made that fell outside of these limits. For the preparation of these specimens No. 87 Hue-Ion acrylic resin was used. Since

opacity is the. ratio of the black background "Rd n value to the white

background "Rd" value., it can be seen from Figure 4 that there is

a slight increase in opacity with a decrease in ratio of monomer to

polymer. Such a change would be expected. A decrease in monomer

per unit volume would necessitate an increase in pigment (contained

in the polymer) per unit volume, thus increasing the opacity. The

values for "a" and "b" shown in Figures 5 and 6 indicate that the

ratio of monomer to polymer cause little change in color. In the

range of normal variations of the ratio, the effect on color and

opacity is negligible.

The reproducibility of readings on the colorimeter was checked

by taking four sets of readings on 18 different specimens. The

standard deviation for duplicating readings was 0.16 NBS units of

color difference. Out of 144 measurements one value deviated between 0.6 and 0.5 NBS units, one between 0.5 and 0.4, three between

0.4 and 0.3 and all the rest by less than 0.3.

The ability to reproduce specimens is not nearly as good as

the ability to measure their color. The standard deviation of

prepared specimens was 1 .06 NBS units of color difference.
;

-

:f

’
- 9 - 4 . SUMMARY

The color of translucent acrylic resins can be explained as

the sum of three components. The first component is that light

which is reflected from the surface of the specimen. The magnitude

of this component is constant and is usually small compared to the

sum of the three components. The second component is that light

which enters the specimen, is diffused off the pigment and is returned . This component is always present and its importance along

with its color intensity increases with an increase in thickness.

The third component is the light which passes through the specimen

and reflects off the background and is returned through the specimen.

This component increases in color intensity with an increase in

specimen thickness. In the case of thick specimens or those with a

black background this component is absent.

A decrease in monomer to polymer ratio of these resins causes

a small increase in opacity. In the normal range of monomer -polymer

ratios this effect is relatively unimportant.

With the color difference meter used in this study the precision

of measurements of opacity and color is greater than is the precision

of duplication of specimens

.
...

»

-

.
10

BIBLIOGRAPHY

1. Evans , Ralph M. An Introduction to Color , John Wiley and Sons, Inc., New York (1948).

2. Judd, Deane B. Color in Business, Science and

Industry , John Wiley and Sons, Inc., New York

(1952)

.

3 . The Gardner Automatic Color Difference Meter

(instruction Pamphlet). Gardner Laboratory,

Inc., Bethesda 14, Md.

INDIRECT

FILLING

RESINS

EFFECT

OF

THICKNESS

ON

"Rd"

COMPONENT

H < IUJ

a:e>

4
-
INDIRECT

FILLING

RESINS

EFFECT

OF

THICKNESS

ON

“a"

COMPONENT

JQ

INDIRECT

FILLING

RESINS

EFFECT

OF

MONOMERPOLYMER

RATIO

ON

COLOR £ E E E IO

10I

UJ 2 O O 3 O c/> < 9

CSJ 0)X

UJ

IUJ 9 2

h-<X <x x

UJ < 2

>-0 ui

Q. H 1 <

<r z

uj

— 2 ° °Z oO o O 2 o

u. K o o

-I H O O o

U. QC

UJ 3 . 9O

UJ UJ C/) X X UJ H I- X *o J I I i L O O o

<0 •O Rt o

CSJ

UIX3o X

INDIRECT

FILLING

RESINS

EFFECT

OF

MONOMERPOLYMER

RATIO

ON

COLOR

UJ 111 </> X X lil I- H OC md

UJa

15O
,
INDIRECT

FILLING

RESINS

EFFECT

OF

MONOMER-POLYMER

RATIO

ON

COLOR oo

UJO3

if) o o

ro+ O

CM+ O+ <

tr z<

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 June 30, 1955 4225

Progress Report

RELATIONSHIP BETWEEN RESIDUAL MONOMER AND

SOME PROPTERTIES OF SELF -CURING DENTAL RESINS

by

Harold J, Caul*

William T. Sweeney**

George C . Paffenbarger*

* Research Associate of the American Dental Association

at the National Bureau of Standards,

** Chiefs 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 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,
'

- -
RELATIONSHIP BETWEEN RESIDUAL MONOMER AND

SOME PROPERTIES OF SELF-CURING DENTAL RESINS

Abstract

The maximum temperature and the time to reach

it during polymerization are reported for 15 brands

of self -curing dental acrylic resins and for one

of these resins to which known amounts of promoter

were added. The promoter, dimethyl -para-toluidine,

decomposes the benzoyl peroxide catalyst at room

temperature and initiates the polymerization reaction of methyl methacrylate. The rate of polymerization and the maximum or peak temperature are

dependent on the amount of promoter. Within the

limits of promoter concentration used, increased

promoter results in higher maximum temperature

and reaction rate. When other factors are held

constant the higher maximum temperatures, which

exist for a matter of seconds, result in less residual monomer after the polymerization. Data are

presented showing that as the residual monomer

becomes less, physical properties such as strength

and modulus of elasticity increase.

1 . INTRODUCTION

The self-curing acrylic resins are now offered to the dental profession for several purposes including uses as cements,

denture bases, denture base liner and repair materials, and

direct filling materials. These compounds contain various

catalysts and promoters that have a pronounced effect on the

polymerization reaction and the amount of monomer remaining in

the cured resin.

It is generally accepted in polymer chemistry that larger

molecules are produced as the temperature of polymerization is

decreased. On the other hand, as the temperature of polymerization is decreased, the amount of monomer converted to polymer

is decreased. Figure 1 shows that as the temperature starts

rising due to the polymerization reaction, the maximum or peak

temperature occurs in a matter of minutes. As the peak temperature increases, the time to reach it decreases

This investigation was undertaken to determine the relationships between residual monomer content and the physical properties

of the self -curing resins.
:

.

.

.
2

2 . MATERIALS AND TEST PROCEDURES

The materials studied in this investigation grouped according to their use are listed in Table 1.

The peak temperature during polymerization and its time of

Occurrence were determined on cylindrical specimens (200 mm

in volume) according to the procedure outlined by Wolcott, Paffenbarger and Schoonover [1]

.

The residual monomer content was determined by the method

of Caul, Wall and Acquista [2], using the frozen benzene infrared method on specimens at least 2 days old.

The yield strength. Indentation strength and modulus of

elasticity were determined according to the methods outlined by

Sweeney, Sheehan, and Yost [3]

.

Values of indentation strength

are included in this report for two reasons . First, it Is a very

convenient method of measuring the strength of small specimens

and, second, the value for indentation strength should be comparable to yield strength.

3. RESULTS AND DISCUSSION

Figure 2 shows the relationship between peak temperature

and time to reach it for the materials investigated. This graph

includes data from the Wolcott report as well as more recent

data obtained by their technique . A single smooth curve cannot

be used to cover all points since the results are too scattered.

As many of these materials contain different promoters, crosslinking agents, fillers, polymers and monomers, a single smooth

curve would not be expected to fit the data.

The behavior of one material with various amounts of a

promoter was investigated. Specimens were prepared with Duz-All

polymer (a direct filling resin), methyl methacrylate monomer

and dimethyl-p-toluidine (a promoter); the compositions are shown

in Table 2. The peak temperature during polymerization and the

time to reach it are shown as open circles in Figure 2, A smooth

curve can be drawn through these data.

The monomer content, modulus of elasticity, yield strength,

and indentation strength were determined for the specimens made

with the various concentrations of promoter. These data are

presented in Table 2. The interrelationships are shown graphically in Figures 3 and 4.

- 3

- Figure 3 shows the relationships between the amount of promoter and (1) and the peak temperature , (2) time to reach peak

temperature, and (3) the amount of residual monomer. Increased

amount of promoter results in higher peak temperature, but shortens

the time to reach it. The higher temperature peaks force the

polymerization more nearly to completion and reduce the residual

monomer content.

Figure 4 shows that as the amount of residual monomer increases the modulus of elasticity and yield and indentation

strengths decrease

.

4. SUMMARY AND CONCLUSIONS

When a direct filling resin polymer is mixed with methyl

methacrylate monomer containing dimethyl -para- toluidine promoter,

polymerization will occur at room temperature in a relatively

short time.

The peak temperature during polymerization and the time to

reach this temperature are reported for 15 brands of selfcuring dental resins and for a self -curing resin containing

known amounts of dimethyl -p-toluidine

.

Values for residual monomer, peak temperature, time to

reach peak temperature, modulus of elasticity, yield strength,

and indentation strength were determined on samples of resin

prepared from one polymer with monomer containing different

amounts of promoter. It was shown that all of these factors and

properties are interrelated.

As the amount of promoter is decreased, the temperature

rise during polymerization decreases while the residual monomer

and time to reach peak temperature increase . As the residual

monomer increases, such physical properties as modulus of elasticity and yield and Indentation strengths decrease.

~ 4 -

BIBLIOGRAPHY

1. Wolcott, Robert B., Paffenbarger, G. C., and

Schoonover, I. C. Direct resinous filling materials

Temperature rise during polymerization. J.A.D.A.

42:253, 1951.

2. Caul, Harold J. , Wall, Leo A., and Acquista,

Nicolo. The determination of monomer content

of polymethyl methacrylate. Manuscript submitted to J.A.D.A.

3- Sweeney, W. T., Sheehan, W. D., and Yose, E. L.

Mechanical properties of direct filling resins.

J.A.D.A. 49:513, 1954.

- 5

- TABLE 1

Identifying

Number Brand Type

1 A-Thermoplast V-10

2 Dentafil

3 Duz-All

4 Fast Crown

5 Kadon

6 Plasto-filling pf 3 Direct filling resins

7 Replica

8 Sevriton

9 Sweden

10 Texton

11 P.P.

12 Dura Base Denture base reliner

13 Dura Cement Resin cements

14 Acrynamel

15 Staweld Denture base repair

Amount of

Duz-All polymer* dimethyl -pmethyl methac- toluidine

rylate monomer based on

monomer

16

17

18

19

20

0.50$

0,25$

0.20$ Experimental resins

0.15#

0 . 10$
i .

' L Q

•• - . - • -•*•••• ... -

.
Properties

of

Acrylic

Resins

Prepared

with

Different

Amounts

of

Promoter 6

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( 194)

7 8 9 10 II MINUTES

12 13 14 15 16 <^B^>

Figure 1. Relationship between temperature rise and curing time

of self-curing dental filling resins.
- '
SELF-CURING

RESINS

PEAK

TEMPERATURE

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'
RESIDUAL

MONOMER—

%

BY

WEIGHT

SELF- CURING RESINS

EFFECT OF PROMOTER CONCENTRATION

ON POLYMERIZATION

Figure 3 Effect of promoter concentration on the amount of residual monomer, peak

temperature time, and temperature rise.

PEAK

TEMPERATURE

TIMEMINUTES

TEMPERATURE

RISE

-°C
.

-

'
STRESS

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ON PHYSICAL PROPERTIES

— 4.0X10*

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MODULUS OF ELASTICITY

YIELD STRENGTH

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5

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Figure 4 Residual monomer effect on the modulus of elasticity and

indentation and yield strengths of self-curing resins.

MODULUS

OF

ELASTICITY

-

PSI

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0708-20-3824

NBS REPORT

June 30, 1955 4226

Progress Report

DEVELOPMENT OF METHODS FOR EVALUATION OF

ROTATING DENTAL DIAMOND ABRASIVE INSTRUMENTS

by

J. L. Hartley*

D. C. Hudson 1 W. T. Sweeney"

George Dickson**

* Major USAF (DC), Guest Worker , Dental Research Section,

National Bureau of Standards.

1 Colonel USAF (DC), Guest Worker, Dental Research Section,

National Bureau of Standards.

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

** Physicist, Dental Research Section, National Bureau of

Standards

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

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. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr.nt ng, or reproduction of this report in any form, either n whole or n part, s prohibited

unless permission isonta ned in writing from theOffice of the Director, National Bureau of Standards. Washington

25, D. C. Such perm ss on 15 not needed, however, by a Government agency for wh.ch a report nas been specifically prepared if that agency wishes to reproduce additional cop es of that part cular report for its own use,
.

r

... .....
DEVELOPMENT OP METHODS FOR EVALUATION OP

ROTATING DENTAL DIAMOND ABRASIVE INSTRUMENTS

Abstract

To determine the cutting effectiveness of

rotating diamond instruments measurements were

made of the rate of removal of material by these

instruments from pyrex glass rods, common window

glass and human enamel cut under controlled loads

and speeds of rotation. Results obtained on enamel

could not be closely correlated with those ob-*

tained on glass although it was observed that

those instruments which were the most effective

and those which were the least effective in cutting glass were also the most and least effective,

respectively, in cutting enamel. Cutting in glass

resulted in complete breakdown of the instruments

through loss of diamonds in fifteen minutes or less,

while the breakdown point was not attained in enamel

although tests as long as one hundred and forty

minutes were made. Examination of diamond points

which had been discarded after clinical use indicated, however, that there is breakdown of critical areas of the instruments when used clinically

In enamel. To evaluate diamond Instruments it is

essential to have a breakdown test capable of

measuring the ability of bonding material to retain
' f

;•
2

the diamonds. The results obtained indicate

that such a test can be based on cutting effectiveness in glass.

1 . INTRODUCTION

A continuing program of research on the cutting characteristics of rotating dental instruments has been conducted at the

Dental Research Laboratory of the National Bureau of Standards,

This project has included studies of the cutting effectiveness

of diamond instruments. There has been a widespread tendency

toward the increased use of these instruments as a result of the

higher handpiece rotational speeds now available to the dental

profession. Recent work [1, 2], has indicated that diamond

abrasive instruments operate more efficiently than steel or

carbide burs in the removal of tooth enamel. Ingraham’s work

[3L provides a more efficient method for the rapid removal of

hard tooth structures utilizing modern instrumentation. There

have been, however, many reports from the field of unsatisfactory

and inconsistant performance of these instruments. The work

reported here was therefor undertaken for the purpose of developing laboratory methods for evaluating diamond abrasive instruments,

2, EXPERIMENTAL PROCEDURE AND RESULTS

In this study a weight -loss method was utilized to compare

the cutting effectiveness of various brands of diamond instruments, A machine with accessory timing and controlling features

which has been designed and constructed at the School of Aviation

Medicine, Randolph Air Force Base, Texas, was modified for the

diamond evaluation. A moveable platform which held the material
- ' ; o |

a. 1 1 >< to wsm * . .
- 3 - to be cut could be brought automatically under predetermined

loads to a precision spindle and draw-collet chuck for the

diamond instrument . Cutting application and release were accurately controlled by an electronic interval timer through an

electro-magnet and spring mechanism.

Pyrex glass rods, one-quarter of an inch in diameter

(Figure l) were selected as the test material for the 3/32

inch cylindrical diamond points first evaluated. The measure

of cutting rate was obtained by determining weight loss of the

pyrex rod after a fixed interval of cutting time at a specific

load. Speeds of 3* 5* 10 and 15 thousand revolutions per minute

were employed.

The cutting cycle consisted of making a series of three

five-second cuts in the same point on the rod, then shifting to

a new area, where the cycle of three cuts was repeated. After

four such cutting cycles or a total of one minute, the specimen

(Figure 2) was removed and weighed. This procedure was repeated

at the end of each minute of cutting for a total of five minutes

Beyond this point and up to a total of twenty-five minutes,

weight loss was determined at the end of each five minutes of

cutting. The weight loss during cutting time will hereafter be

referred to as the rate of cut

Five each of ten brands of 3/32 of an inch cylindrical

diamond instruments were subjected to the test procedure. The

load was 400 grams. The averages of the values for the five

instruments of each brand, shown in bar graph form (Figure 3)*

are plotted in order of decreasing effectiveness
m

.

.

s, *

.

.
- 4 -

of the instruments and show the rate of cut per minute at five

minute intervals for fifteen minutes. In all cases beyond this

time, the instruments had deteriorated to the point where they

were no longer effective in pyrex, These same data are plotted

showing the total material removed in fifteen minutes of cutting

at the four speeds (Figure 4) . There is considerable variation

In the performance of these instruments. The instrument group

brand M I

n >for example^ cut more material at the lowest speed

(3>000 rpm), than all but one of the other brands at any speed.

Those instruments in the higher performance group showed a general increase in cutting rate with the Increase in speed; those

in the low performance group did not, due to their more rapid

deterioration

A series of experiments was conducted with the intention

of correlating the results of cutting human enamel and glass.

The instruments were diamond wheels approximately one-quarter

of an inch in diameter by one -sixteenth of an inch in thickness.

The teeth were mounted so that they were at a 45 degree angle

to the long axis of the grinding instrument (Figure 5) * The

load was 200 grams. These instruments cut more effectively than

the cylindrical diamonds, because of their greater diameter.

Reducing the load from 400 to 200 grams prevented cutting through

the enamel into the dentin of the teeth during the cutting period.

The diameter of the instruments varied somewhat from brand to

brand. In order to provide a common basis for comparative results, rotational speed was adjusted to provide a constant
. ' ' , ‘
- 5 * peripheral rate of travel for all brands. This was 450 feet per

minute at approximately 5*000 rpm and 900 feet per minute at approximately 10*000 rpm. A different tooth was used for each minute of

cutting* Twelve* five-second cuts were made* each in a new location, Each tooth was weighed before and after the cutting cycle

of one minute.

There is no apparent breakdown of the bonding of the abrasive

under the conditions of this experiment (Figure 6) « The cutting

rate remained essentially the same for a total of one hundred and

forty minutes of cutting by two brands of instruments, brand "I",

chosen from the outstanding performance group* and brand "B n * from

the low performance group, as determined by the performance of

these instruments in pyrex glass.

In order to compare the relative cutting rates of several

brands of diamond instruments, a series of experiments was performed on enamel. The total cutting time was ten minutes. Ten

teeth were used for each instrument* the average weight of material

i removed per tooth was considered the cutting rate of that instrument . The load was 20Q grams; the angle of cut was forty-five

degrees. Twelve* five -second cuts were made in each tooth* each

cut on a npw area of the tooth surface. Results obtained at

5*000 rpm (450 feet per minute) and 10*000 rpm (900 feet per minute)

are arranged in order of decreasing effectiveness of the instruments in Figures 7 and 8.

These same instruments were then re-run at 450 and 900 feet

per minute in common one-eighth of an inch thick window glass.
'

*

.
- 6 - The angle of cutting was forty-five degrees (Figure 9) - The procedure consisted of making a series of three five-second cuts at

a 200 gram load in each new location on the specimen. The glass

was weighed before and at the end of each minute of cutting during

the first five minutes* then every five minutes for a total of

twenty minutes. The data were plotted for the first five minutes

only because breakdown of all instruments occurred within this

period (Figures 7 and- 8) . For purposes of comparison* the data

for these instruments while cutting glass are also arranged in

the order of relative cutting effectiveness in enamel

3. DISCUSSION

It is believed that the peripheral speed attained by these

instruments,, because they are very small in diameter, is too low

for optimum performance. Commercial grinding practice with industrial wheels [4], calls for a peripheral speed of from five

to seven thousand feet per minute. Dental instruments only 3/32

of an inch in diameter have a peripheral speed of 360 feet per

minute at 15*000 rpm. In order to obtain the recommended speed

for commercial grinding practice* dental diamond abrasive instruments of this size would require rotational speeds of approximately

245*000 rpm.

There appeared to be very little correlation between the

performances of diamond instruments In enamel and In glass. The

cutting procedures in glass resulted in a rapid breakdown of the

instruments* while this point was not obtained* under the conditions

of this investigation* in enamel. Close correlation would not

be expected unless this breakdown also occurred in enamel
' * . .
- 7 - However, instrument brand "I", which was outstanding in pyrex glass

when evaluated as a 3/32 of an inch cylindrical point at speeds of

3 X 5, 10 and 15 thousand rpm, with a 400 gram load, was one of the

best instruments in enamel when evaluated as a 1/4 inch wheel* with

a 200 gram load. This was also noted when the instrument brand

"I", 1/4 inch wheel cut common window glass. Under the same conditions instruments of brand "A ,; and brand ’B" performed very

poorly in both glass and enamel. All diamond instruments varied

considerably in performance. Inconsistency of performance by

individual instruments of the same brand was noted throughout

the study.

It is to be emphasized that the cutting of glass results In

an evaluation made under severe conditions of heavy loading upon

a material that is unlike enamel in structure. Pyrex is homogenous, and is considerably harder than human enamel. Knoop hardness

for this glass is 430, human enamel hardness was found to be Knoop

300 to 340. Analysis of the glass-cutting test indicates that

it is, in effect, an accelerated breakdown test, wherein the material cut is considerably more destructive to the diamond instrument than is human enamel; this causes a loss of the diamond abrasive, and evaluates the strength of the bond between the diamonds

and the head of the instrument

.

Breakdown of the bonding medium, with resultant loss of the

diamond abrasive, does occur in clinical use. Diamond instruments

which had been discarded after clinical operative procedures were

collected and examined. It was determined that loss of the abrasive occurred consistantly at the relatively an all area or point
.
- 8 - of the instrument which received the greatest loading in use.

The other surfaces of the instruments showed little or no loss

of the abrasive, A study of operative techniques in which diamond instruments were used, indicated areas on the instruments

which would be repeatedly subjected to the greatest pressure

per unit area (Figure 10) . This breakdown in a limited area of

the instrument occurs more frequently with diamond than with

other abrasive instruments because the diamond instruments do not

change shape, as long as they cut, and are used to cut exacting

angles* grooves and bevels in operative or crown-and -bridge dentistry. These areas, A and B, (Figure 10) corresponded to the

areas of greatest diamond loss noted on those instruments which

had been discarded as being no longer of clinical value . The

necessary use of small* specifically-shaped instruments results

in low peripheral speeds* even at the higher rotational speeds

available today. During the preparation of an exacting cut at

these low peripheral speeds* the instrument is subjected to

repeated high pressures in certain areas: thus* breakdown of the

instrument in enamel occurs in clinical usage even though it did

not occur under controlled laboratory conditions. Because of

this breakdown* an instrument must be evaluated for its ability

to resist loss of the abrasive. An accelerated test, using a

c. material such as glass* should determine the ability of a diamond

instrument to retain its abrasive particles under the harsh conditions necessarily encountered in clinical use. A specification

for diamond instruments should also include a requirement for the

removal of an established amount of a test material* possibly one

which does not cause breakdown. This would eliminate an instrument

- 9 ~ which cut ineffectively even though it did not lose its abrasive,

4. CONCLUSIONS

1 . There is a wide variation in the cutting ability of

diamond instruments under the conditions of this experiment.

2. Those instruments that performed in an inferior manner

in glass, were those that deteriorated rapidly, losing a greater

part of their abrasive during the first few cuts.

3. The more durable instruments removed a greater amount

of material as rotational speed was increased. The less durable

diamond instruments demonstrated little or no improvement In their

performance at the higher speeds

4. Close correlation of the relative cutting efficiencies

of these diamond instruments in glass and enamel could not be

obtained under the conditions of this experiment. The breakdown which occurred in glass was not produced in up to one

hundred and forty minutes of cutting in enamel. Those Instruments that were outstanding In cutting glass were in most cases

also outstanding in enamel. The poorest instruments in glass

were also relatively Ineffective in enamel. All instruments were

erratic in behavior,

5. Breakdown of these instruments does occur under clinical

use possibly because of their small size which results in low

peripheral speeds and high pressures on specific areas of the

instruments. This breakdown is due to failure of the bonding

medium with resultant loss of the diamond abrasive.

10

6. In the evaluation of the relative cutting effectiveness

of dental diamond instruments^ a test of the bonding medium should

be included. Such a test can be based on cutting effectiveness

in glass.

11

BIBLIOGRAPHY

1. Peyton, F. A.* Temperature rise and cutting efficiency of

of rotating instruments. New York Dent. J. 3^8:439 Nov.*

1952.

2. Hudson, D. C., and Hartley, Jack L., et al., Some factors

influencing the cutting characteristics of rotating dental

cutting instruments. J.A.D.A. 50:377 April 1955.

3. Ingraham, Re;x, and Tanner, H. M., The adaptation of modem

instruments and increased operating speeds to restorative

procedures. J.A.D.A. 47:311 Sept., 1953-

4. Kent, William, Mechanical Engineers Handbook, 12th ed.,

Vol. 2, Sec. 23. John Wiley and Sons, Inc., New York, N. Y.

1952.
,, .

"

.

<
Figure

1.

Specimen

holder

of

bur-evaluating

machine

with

pyrex

rod

specimen

in

place.

A

movable

platform

is

electronically

controlled

to

bring

the

specimen

into

contact

with

the

rotating

diamond

Instrument

for

a

predetermined

period

with

a

known

load.

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Figure 3. Cutting rates of ten brands of dental diamond abrasive instruments. Five

instruments of each brand were evaluated. Note the high cutting rate of

instruments of brand "I" at all four speeds, and the low rate of instrumentsof brands "A" and "B".
.
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Figure

4

,

Cutting

rates

of

the

same

instruments

as

in

Figure

3

,

showing

total

material

removed.

Note

that

the

high-cutting-rate

diamond

points

at

the

left

removed

more

material

as

the

speed

of

rotation

was

increased,

but

that

those

instruments

at

the

right

did

not.

Figure

5.

Apparatus

on

movable

platform

for

holding

and

shifting

human

tooth

specimen;

in

weight-loss

determination

for

relative

cutting

rates

of

one

-quarter-inch

diamond

wheels.

Controls

adjust

locking

and

horizontal

and

vertical

moveme:

of

the

tooth.

ROTATING

DIAMOND

INSTRUMENTS

CUTTING

RATES

HUMAN

ENAMEL

200

GRAM

LOAD

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INSTRUMENT

Figure

7

•

Comparison

of

cutting

rates

of

one-quarter-inch

diamond

wheels

in

enamel

at

5,

000

rpm

with

the

cutting

rates

of

'

the

same

instruments

in

common

window

glass.

Letters

indicate

brands,

the

subscripts

Indicate

individual

instruments

of

those

brands

1

CO

2

e*CO UJ

cr < z

“ o ° 3? < < •- * 0Hi

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INSTRUMENT

Figure

8.

Comparison

of

cutting

rates

of

one

-quarter-inch

diamond

wheels

in

enamel

at

10,000

rpm

with

the

cutting

rates

of

the

same

instruments

in

common

window

glass.

Letters

indicate

brands,

the

subscripts

indicate

individual

instruments

of

those

brands.

Figure

9.

Specimen

holder

for

one-by-three

inch

common

window

glass

specimens,

mounted

on

movable

platform

of

rotating-instrument

evaluating

machine.

Note

45-degree

angle

of

cut.

Figure

10.

Clinical

use

of

diamond

wheels.

Points

"A"

and

"B"

Indicate

areas

of

this

diamond

wheel

which

would

receive

the

highest

pressures

per

unit

area

resulting

in

breakdown

of

the

bonding

of

the

abrasive

to

the

instrument.

NATIONAL BUREAU OF STANDARDS REPORT

A METHOD FOR MEASURING THE MUCOSAL SURFACE CONTOURS

OF IMPRESSIONS, CASTS, AND DENTURES

N. W . Rupp*, George Dickson’,

M. E. Lawson, Jr.", and W. T. Sweeney**

* LCDR DC USN, Guest Worker, Dental Research Section, Na-

- tional Bureau of Standards

’ Physicist, Dental Research Section, National Bureau of

Standards

" 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.

The data in this report was presented in a thesis by N.

W. Rupp in partial fulfillment of the requirements of the

Graduate School, Georgetown University, Washington, D. C.

for the degree of Master of Science.

NBS PROJECT

0708-20-3824 June 30, 1955

NBS REPORT

4227

Progress Report

(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 specifically prepared if that agency wishes to reproduce additional copies of that particular report for its own use,
• •
A METHOD FOR MEASURING THE MUCOSAL SURFACE

CONTOURS OF IMPRESSIONS, CASTS AND DENTURES

Abstract

A practical technique based on comparative

measurements of identical areas of two precisely

positioned surfaces has been developed for measuring the mucosal surface contour of impressions,

casts and dentures. Using a pantographic type of

comparator the dimensional differences between

two surfaces are determined. These differences

can be measured over an entire area to an accuracy

of ±0.002 of an inch in less than an hour’s time.

With this method it is possible to make the follow*-

ing determinations: (l) relative accuracy of impressions; (2) growth or recession in the mouth

as recorded on casts; and (3) effect of different

techniques or materials on the dimensional accuracy

of dentures

1 . INTRODUCTION

The accuracy with which dentures fit the oral mucosa has

been the subject of many articles and discussions. The opinions expressed are usually based on clinical experience, or

observations made on how well a denture fits a stone cast or

on measurements in a horizontal plane between marks on the

denture [1], The method presented here for measuring the
.

,

*

.

.
2

mucosal contour provides quantitative data on the comparison

of two surfaces whether they be of soft impression material*

stone or denture base resin. Therefore* it is possible to determine the accuracy with which a denture reproduces the original

impression* the patterns of growth or recession as reflected

in casts* and the dimensional stability of different materials

used in impressions and dentures.

A survey of the literature revealed only the Krogh-Poulsen

[2] technique for measuring mucosal contour. In this method

the positioning is difficult to reproduce accurately and the

measurements tedious to make. A more accurate and convenient

method was* therefore, sought. Thus this method using a panto4 graphic type of comparator was developed [3].

2. DESCRIPTION OF THE COMPARATOR

The comparator* j, Figure 1* a modification of the Dentagraph [4]** consists of a framework in the form of a parallelogram having one side fixed and the four corners hinged. When

one of the two corners away from the fixed side is moved the

other follows a similar path [5]. The framework is mounted on

pivots so that it can be rotated with the fixed side as the axis.

The whole unit is mounted on ways* or ball bearing tracks* so

it can be moved forward and backward. Ball-shaped pointers

with a diameter of 0.14-7 inch are attached to the corners of

the free side. The pointer on the left is fixed and acts as

* Designed and Manufactured by G. R. Kern* 5525 23rd North*

Arlington* Virginia.
-

.

'

»

'
- 3 - a guide. Figure 2, b, while the pointer on the right moves up

and down indicating the amount of movement in a vertical direction on a dial gage. Figure 2, y. These readings of the gage

are recorded as the Y coordinates . At the mid point of the

sweep of the gage the indicator point is in the same horizontal

plane as the guide pointer. When the indicator is touching an

area higher than the guide the gage will read plus; if lower,

minus. Thus the mucosal contours on the two impressions,

casts or dentures can be compared as to difference in height

providing the pointers are touching corresponding points on the

two objects.

In order to determine the distance moved laterally a dial

gage is mounted on a telescoping rod across the diagonal of the

parallelogram. Figure 2, X, this gage indicates the change in

shape of the parallelogram as the guide is moved laterally.

Multiplying the reading by a factor, 2.3, gives' an approximation

of the distance traveled in the lateral, or X axis. The pointers

may be locked at predetermined points on the lateral, or X axis,

by a set screw on the telescoping rod while readings are taken

on the Y axis.

A lock plate, containing a series of fifteen holes, is

mounted on the frame of the comparator. Figure 2, Z. The whole

unit may be fixed on the ways at definite forward-backward, or

Z axis, positions by dropping a pin thru a hole in a plate on

the side of the parallelogram mounting into one of the holes on

the lock plate.
. ' • V • "
- 4 - The objects to be measured are mounted on two separate platforms on the comparator. Figure 3 . The left platform has the

following three adjustments: (l) four leveling screw legs.

Figure 3> d, to make the plane of the left platform parallel

to that on the right platform, (2) one set screw. Figure 3* e,

on the base to lock the left platform after rotation into alignment with the right platform, (3) two screw-thread drives.

Figure 3> f, for anterior-posterior or forward-backward, and

lateral positioning.

The right platform has only one adjustment, a lock-nut

arrangement. Figure 3> g, on a jack screw for raising and lowering the platform. By using all the adjustments with care it

is possible to position precisely the objects to be measured.

3. PROCEDURE FOR USE OF THE COMPARATOR

3.1 Positioning of the Objects to be Measured

Before any measurements can be made the two objects to be

compared must be positioned similarly on the platforms of the

comparator. In principle the only requirements for positioning,

if the objects are identical, are: (l) the objects must be so

located that distances between corresponding points on the two

objects are the same as the distance between the indicator points,

and (2) the lines connecting corresponding points must be parallel

to the line connecting the two indicator points when the Y, or

vertical axis, dial indicator is in the zero position. In a

practical case these conditions cannot be obtained since the

objects to be compared are similar but not identical. Therefore,

the objects are positioned so their planes of symmetry are
'

•

.

. H

-
- 5 - approximately perpendicular to the X axis. Corresponding points

in these planes are placed so that the distance between them is

the same as the distance between the indicator points on the

comparator and lines connecting these and other corresponding

points are made as nearly as possible parallel to the line

connecting the indicator points when the Y axis dial indicator

is in a zero position.

In positioning dentures, corresponding anterior and posterior reference points near the midline are first adjusted

so that they fall below the indicator points, A bench mark

placed near the midline at the posterior of the impression and

reproduced in the denture serves as a posterior reference point.

The posterior reference point is adjusted to a zero position

in the Y axis and further adjustments are made until all points

in the ridge area give as nearly the same reading as can be obtained. Simultaneous with this, opposing slopes are also adjusted

so that Y axis readings as nearly similar as possible are obtained . Considerable care must be exercised in making the positioning adjustments. Coarse adjustments of the planes of the dentures

are made by using as am indicator a spirit level mounted on a

tripod which may be placed on the dentures. Further adjustments

are made using the indicator points of the comparator on flat

surfaces of the denture. Final adjustments are made by using the

Y axis dial indicator to determine when readings on opposite

slopes of the denture are similar. Since a slight horizontal

displacement of the dentures will result in a large difference

- 6 - in the Y or vertical axis dial indicator readings on the slopes

of the denture^ this procedure makes possible a very precise

location of the denture, or other object being measured.

3.2 Measurement of Differences

The bench mark is used as the fiducial point when measuring

If it is not practical to make bench marks, there are landmarks

which can be used. After the pointers are placed in the marks

on the dentures the Z axis lock plate. Figure 2, z k is moved so

the pin will drop into hole number three, four, or five and the

number of the hole recorded. The plate is locked in position.

Now, as the pin is moved from hole to hole the distance from the

posterior bench mark forward is known. A reading of the X axis

dial indicator. Figure 2, x, is recorded while the points are in

the posterior bench mark. The pin is then removed from the lock

plate, the pointers are moved to an anterior bench mark ar landmark, and another X axis reading is made. These readings are

recorded as shown on Table 1 so future mountings can be checked

for alignment. If the alignment of each successive mounting in

a series of measurements of dentures or impressions is not consis tant, the arcs scanned will not be over the same paths. The

method for alignment is described later in this section under

remounting

The pointers are moved back to the posterior bench mark and

the pin is reinserted in the lockplate. The parallelogram Is

now free to move in only two directions, vertical and lateral.

The movement of the points in these directions Is in the paths

of arcs which have a nine-inch radius. The pointers are moved
.
- 7 - to the left buccal flange and the set screw tightened to prevent

movement along the X axis. Now the pointers are brought carefully

to the surface of the dentures and the readings on both the X and

Y axes are recorded. An electronic device to indicate the moment

the pointer touches the surface is attached to the right denture.

The device registers the first contact of the pointer by completing a circuit which activates a relay to turn off a light, Figure

2, a. The next reading is made by loosening the X axis set screw

and moving the pointers to the right and again securing the set

screw. The pointers are lowered to the surfaces and another

reading is made on both the X and Y indicators . As many readings

as desired can be made at each setting of the Z axis. A series

of eleven readings made at Z, number is shown on Table 1.

After this arc is completed the pin is removed from the lock

plate and the parallelogram is moved along the Z axis to the point

desired to make the next readings, such as Z at number 8 and Z

at number 11 on Table 1. In this way the differences between

the two surfaces in the vertical direction, or Y axis, are determined at definite points on the X and Z axes. Measurements at

these points can. be repeated since the X and z readings at the

posterior bench mark are known, and the positions of all the other

readings are known relative to that point.

A device for recording the path of the guide point along the

X and Y axes is mounted on the left platform. Figure 4 . The

stylus marks the different points where readings are made . This

profile is no-t an accurate reproduction of the contour, because

the pointers are ball-shaped and different sides of the ball touch

- 8 - the surface when readings are made on different sides of the

ridge. The tracing will, however* give an approximate profile

of the surface and indicate on this profile the points at which

differences in the surface height are recorded.

When a case is remounted* or another case compared^ the

case on the right platform must be replaced as closely as possible

to the original position for a slight rotation will throw the arc

scanned over a different area and the readings will be inaccurate.

This repetition of position is done by the use of a key consisting of a plaster or compound impression made of the case while

mounted

A series of five remountings with one repositioning for each

mount was analyzed statistically. The standard deviation of experimental error within a single mount was 1 thousandth of an inch.

The standard deviation when differences due to remounting were

included was 1.8 thousandths of an inch. It is recommended that

one reset series of readings be made for each mounting. A comparison of the reset readings to the original series shows whether

positioning is accurate

A direct calibration of the Y axis indicator was made by

using a series of millimeter gage blocks on the right platform.

At each addition or subtraction of a gage block a reading was

made on the Y indicator. Each comparator reading was within

5 ten-thousandths of an inch of the calculated value for that

reading

The X and Z axes readings are used to reproduce the points

at which Y axis measurements are recorded. Readings along the
"I

X
- 9 - X and Z axes, therefore, although they are reproducible and make

possible precise positioning, provide only approximate measurements of the distances on the dentures or other objects compared.

The distance the points travel on the X axis was determined by

measuring on a measuring microscope the distance between marks

made by the pointers at different readings on the dial indicator.

From these findings the factor, 2.3, to convert the dial readings

to the distance moved by the points was determined to be accurate

to ±0.01 inch. The distance from hole to hole on the Z axis was

measured on the measuring microscope.

4. EXAMPLES OF RESULTS OBTAINED ON DENTURES AND IMPRESSIONS

4.1 Dimensional Accuracy of Two Dentures Cured by Different

Techniques

Comparative measurements were made showing the effect of

different curing cycles on the dimensional accuracy of dentures.

The dentures were processed by different curing cycles and then

compared to the common original impression. A metal impression

was used in place of the usual impression material to eliminate

as much as possible any change in the impression. The changes

measured are those occurring from the impression to the completion

of the curing cycle. One of the dentures was cured by the recommended slow cure. After placing the packed flask in a cold tank

the temperature was raised slowly to l65°F and held for eight

hours . The tank: was then allowed to return to room temperature

before the flask was removed and opened. The other case was

placed in boiling water for one hour then placed under cold

running water. The flask was opened as soon as it was cool

- 10

enough to handle . Figure 5 shows graphically the results obtained. The lower part is a profile of a portion of the denture

measured and the upper part shows the plus and minus variation

of the denture surface heights from the original impression.

The plotted points can be projected down to the profile to see

where the measurements were made. A pictoral representation of

the changes occurring from the original impression to the cured

denture processed by the slow cure is shown on Figure 6 where the

differences are plotted from the profile curve.

4.2 Dimensional Accuracy of an Alginate Impression

An alginate impression taken of a stone case poured in a

metal impression was compared to the original metal impression.

The alginate impression was mounted within a plioform bag in the

mounting ring to maintain one hundred percent relative humidity

and minimize dimensional changes while the measurements were being

made. The data are given in Table 2. The readings of 30 thousandths

and 27 thousandths of an inch were on the buccal surface of the left

alveolar ridge. When the impression material was removed from the

tray a space, not noticeable previously, was found between the

alginate and the tray at the point where this reading was taken.

5. CONCLUSIONS

A method for making comparative measurements of contoured surfaces has been developed. These measurements give the dimensional

difference between two surfaces in the vertical direction, or Y

axis, at different points in the horizontal plane, or X and Z

axes. The precision of the measurements depends on the ability to

position accurately the objects being compared. The components of
'

,

\ i 141 ;t <

.

. : ... 4 . j

j • g

•: •' ft

• ''•$

•

•; •

.

' ,

*
11

the comparator used make this positioning possible. A statistical

analysis of the readings from a series of five remountings* with one

resetting for each remount, shows the standard deviation to be

1.8 thousandth of an inch.

Two examples for the use of this method are presented:

(l) measuring the changes due to the effects of different curing

cycles on the dimensional accuracy of dentures, and (2) measuring

the dimensional accuracy of an alginate impression.

Additional possible uses for this type of measurement are:

determining the rate of growth or recession of oral tissues by

measurements on a series of casts made over a period of time;

evaluating the dimensional stability of different materials used

in making impressions, casts, or dentures; and studying the resulting tissue displacement from different impression techniques.
•

.

--

-

.

'

'
12

BIBLIOGRAPHY

1. Sweeney, W. T., Denture base material. J.A.D.A. 26: 1863

(1939).

2. Krogh-Poulsen, W. G., Paffenbarger, G. C., and Schoonover,

I. C., Two methods of graphing the "mucosal’' surface of

plaster models. J.A.D.A. 3,6:397 (1948).

3. Whitehead, T. N., The design and use of instruments and

accurate meachanism. Cover Publications Inc. (1954).

4. Lllley, G. A. Dental Laboratory Review, 36 May (1948).,

5. Rolt, F. H., Gauges and fine measurements. McMillan and

Co. 3H-333 (1929).

- 13 -

TABLE 2

COMPARATIVE MEASUREMENTS OF ALGINATE IMPRESSION

Posterior Bench Mark .49-4 (Z at 5)

Anterior Bench Mark .500

Z at 7

X y

a* b

in. in.

1 .035 + .007 + .004

2 .105 + .030 + .027

3 .156 + .005 + .004

4 .241 + .002 .000

5 .500 .000 .000

6 • 753 -.004 -.002

7 .867 + .004 + .006

8 .926 .000 + .002

9 .999 + .006 + .004

Vy

* a Original positioning,

b Reset positioning.

A*67-lll
'
1

2

3

4

5

6

7

8

9

10

11

14 - TABLE 1

MEASUREMENTS OP CHANGES PROM IMPRESSION TO DENTURE

Three different arcs scanned on the Z axis.

a b

Posterior Bench Mark .497 .497 Z at 5

Anterior Bench Mark .508 .508

Z at 5 Z at -8

X* y X* y

a b a b

inin. in. in.

.025 + .007 + .006 1 .086 + .002 + .001

.047 .000 .000 2 .153 -.015 -.016

.091 -.016 -.014 3 .183 -.007 -.008

.162 -.002 -.002 4 .270 + .005 + .004

.270 + .007 + .005 5 .503 -.004 -.005

.538 + .002 .000 6 .771 + .008 + .010

.780 + .006 + .005 7 .844 -.005 -.006

.885 -.002 .000 8 .906 -.015 -.016

.973 -.010 -.014 9 .908 + .004 + .003

.160 + .002 .000

.224 + .010 + .009

Z at 11

X* y

a b

in. in.

1 .234 + .003 + .003

2 .380 -.019 -.018

3 .512 -.005 -.005

4 .668 -.018 -.018

5 .781 +.003 + .003

* These readings times 2.3 equal x dimension in

inche s

A-67-113

Figure 1. COMPARATOR FOR MEASUREMENTS OF MUCOSAL CONTOURS

OF IMPRESSIONS, CASTS, AND DENTURES.

s

Figure 2. PARALLELOGRAM OF COMPARATOR.

a. Light, to indicate instant pointer touches

surface being measured,

b. Guide pointer, permanently positioned.

x. Mai indicator, registers change in- shape

of parallelogram.

y. Dial indicator, registers change in vertical

direction of the surface the indicator pointer

touches in relation to the surface the guide

touches

z. Lock plate , holds parallelogram in a definite

forward -b ackward position while measurements

in. the X and Y axes are being made

Figure 3. PLATFORMS OF COMPARATOR.

d. Tilting adjustment , four screw legs to adjust

the plane of the left platform.

e . Rotating adjustment, set screw to control

rotation of the left platform.

‘ f . Screw thread travel , two lead screws to control

lateral and forward -backward position of the

left platform.

g. Lock nut, controls the vertical height of the

right platform
I

/
.

Figure 4. PROFILER IN PLACE.

INCHES

INCH

(THOUSANDTH)

Figure 5. COMPARISON OF DIMENSIONAL CHANGES RESULTING FROM

DIFFERENT CURING CYCLES.

Both dentures were compared to the original metal

impression.

Upper portion of the graph shows the plus and minus

deviation of the surface heights as indicated by

the c omparat or

Lower portion of the graph is a profile of part

of the denture area measured. The plotted points

on the upper portion can be projected down to the

profile to show where the changes took place.

INCHES

Figure 6. DIMENSIONAL CHANGE FROM IMPRESSION TO DENTURE.

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Aet of Congress, Mareh

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants

and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.
/