O

.v •

NATIONAL BUREAU OF STANDARDS REPORT

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Dental Research Laboratory

For the Half Year Ending

December 31, 1952

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

2286
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. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

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

Technology. Length. Gage.

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

Lubrication. Engine Fuels. Cryogenic Engineering.

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

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

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

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

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

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

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

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

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

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

Development.

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

Instrumentation.

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

Frequency Standards. Microwave Standards.

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

Electromechanical Ordnance, and development in advanced ordnance. Activities include

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

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

Defense.

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

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

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

NBS PROJECT NBS REPORT

0708-10-0707 February 15, 1953 2286

0708-10-0724

0708-10-3822

0708-10-3824

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

Dental Research Laboratory

For the Half Tear Ending

December Jl 9 1952

The dental research program at the National Bureau of Standards

is carried on in cooperation with the Council on Dental Research

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

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

The publication, re. Approved for public release by the

unless permission is Director of the National Institute of

25 ,d,c. Such pen Standards and Technology (NIST)

cal ly prepared If tt on October 9, 2015

In part, Is prohibited

indards, Washington

sort has been speclfiport for Its own use,

REPORT ON DENTAL RESEARCH

AT THE NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

The activities of the Dental Research Laboratory at the

National Bureau of Standards for the six-month period ending

December 31, 1952 are covered in this report. As various

phases of the work were completed, detailed reports have been

issued to the agencies cooperating in the program. Copies of

reports on work completed during this period are attached.

In addition, the results obtained on work now in progress

are briefly summarized.

2 . REPORTS ISSUED

NBS No, 2032

NBS No. 2039

NBS No. 2242

NBS No. 2233

The Color Stability of Direct Filling Resins

Thermal Expansion of Silica-Gypsum Investments: Effect of a New Factor, Preheating

Mechanism of Hygroscopic Expansion in Dental

Casting Investments

Temperatures Developed in Rotating Dental

Cutting Instruments

3 „ WORK IN PROGRESS

3.1 Structure of Human Tooth Enamel and Dentin

Investigation of the fundamental structural characteristics of human tooth enamel and dentin.

(a) Fluorescence Investigation . The spectra of decalcified

specimens of enamel and dentin were examined to obtain

information on the source of fluorescence of tooth

structures. Although the fluorescence spectra of these

specimens of enamel protein and dentin protein were

found to be very similar to those of undecalcified

enamel and dentin, the energy maximum, in each case, was

displaced towards the blue end of the spectrum by approximately 500 reciprocal centimeters. This suggests

that the observed fluorescence spectrum of normal teeth

is the weighted average of the protein (organic) and the

apatite (inorganic) constituents of the teeth. To test

this hypothesis samples of enamel and dentin free from

organic material are now being prepared.

)

- 2 - Observations on tooth sections from teeth frozen immediately

after extraction showed that carious and tf pre-carious ft lesions frequently fluoresced a bright red at their edges 0 The intensity of the red fluorescence decreased rapidly if

the tooth section was kept at room temperature but could be

preserved for several days in frozen specimens 0 Even at

this low temperature, however, the red fluorescence almost

disappeared after one or two weeks*

A number of strains of bacteria known to be associated with

tooth decay were examined, but none exhibited fluorescence

similar to that observed in the carious areas. Attempts to

culture the red fluorescing material at the National Institutes of Health have so far been unsuccessful*

(b) Crystallographic Investigation * The study of particle size

distribution in teeth, bone and apatite minerals was continued* The method of low angle scattering of x-rays was

employed* All carbonate-containing basic calcium phosphates (bone, teeth, etc*) examined seem to have a particle

size distribution which extends almost into the molecular

range, i*e* 50 angstroms*

A new method being pursued makes possible the determination

of particle size distribution from 10,000 A to 10 A through

a detailed study of the broadening of the Bragg (high angle)

x-ray reflections of the material* The method involves the

study of the corrected shape of the Bragg reflection which

is represented by a cosine Fourier series* The set of

Fourier coefficients obtained will then give information

as to the spread of particle size in the range mentioned*

By c oupling the results of the above x-ray methods with

other approaches, such as low temperature nitrogen adsorption and preferential solubility studies, it is hoped that

a better understanding may be had of the placement of the

trace elements in teeth, bone and allied apatites*

3*2 Surface Crazing of Acrylic Resins

Determination of the mechanism involved in the surface

crazing of acrylic resins*

Water sorption measurements of various methyl methacrylate

polymers were continued* Sorption as well as craze susceptibility of commercial samples is dependent on the presence and

concentration of copolymer and plasticizers* It was established

that remolding of specimens at 150°C increases markedly the

water sorption at 100°C* Polymer disks of varying molecular

weight showed little difference in sorption values on storage

at 4°C* Measurements at other temperatures are now being conducted* For plastic teeth no correlation could be found
'

.

SM- Vrl.V,'

1
- 3 - between water sorption at 100°C and craze resistance., All samples investigated gave a continuous weight increase even after

storage for 37 days 0 In general the rate of sorption decreased

with time c A few samples, however, showed a considerable increase in the sorption rate after a definite time interval*

Specimens which had been heated at 100°C in water for over 200

days and then dried to constant weight and stored in water at

37°C showed increased water sorption with a decrease in molecular weight*

The internal, strain dm resins i

:s being studied by imbedding

SR-4 resistance strain gages in samples of methyl methacrylate* The change in strain caused by the sorption of water at

various temperatures will be used to determine why the resin,

when cured in contact with a water-containing surface, shows

poor craze resistance*

Data on the water vapor sorption_ of 'specimens of varying molecular weight were obtained to determine the effect of relative

humidity and to correlate the results withthose obtained for

disks immersed in water* Below the transition temperature

vapor sorption appears to be independent of molecular weight

below a relative humidity of SO percent* Water uptake is

nearly linear from 10 to 50 percent relative humidity* Above

70 percent relative humidity there is a rapid increase in the

amount of water sorbed.

3 *3 Polymerization of Acrylic Resins

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

methacrylate * The hydrolysis of polymethyl methacrylate specimens exposed to boiling water for A0 days was investigated* Reaction

products identified were methyl alcohol, methacrylic acid and

traces of carbonyl compounds, possibly pyruvic acid or formaldehyde * Residual catalyst was found in all commercial polymer

samples analyzed* This explains the lower degradation temperature of peroxide catalyzed polymer*

The usefulness of the free radical

,

c* -diphenyl^ picryl

hydrazyl (D»P*P*H.) in the study of radical formation and

reaction was investigated* Preliminary results obtained with

2;2-azo-bis ( isobutyronitrile ) (A.I.B.N*) in benzene solutions

of D„P*P*H* indicate that half-time decomposition of D.P.P.H*

may offer a new and convenient approach to measure rate of

free radical formation, reactivity and efficiency of inhibitors*

- 4 -

3 oh Physical, Chemical and Engineering

Properties of Dental Materials

Determination of the properties and processing characteristics and development of more satisfactory technics for the

use of materials employed in the practice of restorative dentistry o

(a) Metallurgical Investigations » The investigations of amalgams for dental use have been extended to include a study

of the possibility of using gallium as a substitute for

mercury. Gallium has the advantages of being nontoxic and

of wetting tooth structure 0 Experimental alloys which set

at room temperature have been made of gallium. Alloys of

copper-tin, nickel, nickel-silicon, and silver-tin, with

gallium are being investigated to determine whether properties suitable for dental restorations can be obtainedo

An alloy which can give better adaptation to the cavity

walls of the tooth is greatly needed,,

(b) Chrome Alloys for Dental Castings , Chrome alloys from a number of commercial sources are being studied for determination of such mechanical properties as modulus of

elasticity, strength, elongation and hardness. This

project will be extended to include a wider group of

alloys and accessory materials to provide the Federal

dental services with sufficient data to specify a simple

and satisfactory material for clinical use,

(c) Revolving Dental Cutting Instruments , The work on revolving cutting instruments has "been progressing very satisfactorily, using the standardized procedure for evaluating

dental burs previously developed. Study of the diamond

abrasive instruments will be completed shortly. Diamond

instruments of various manufacturers appear to vary widely

in their effectiveness in cutting pyrex glass. Instrument

wear is accelerated when eccentricity is excessive, and

vibrations amplitude rises as eccentricity increases,

(d) Abrasion or Wear Resistance of Plastic Teeth , An accelerated test was developed to show differences in resistance

to abrasion of various brands of plastic teeth. The teeth

previously soaked in water to equilibrium are placed in a

ball mill with l/4-inch steel balls and number 200 Aloxite

powder and tumbled for 2 hours,

(e) Development of Ultrasoni c T est Methods , Ultrasonic technics for evaluating the "elastic constants of polymethyl

methacrylate are being studied. The construction of an

experimental instrument which employs the diffraction of

monochromatic light by ultrasonic waves has been completed.

- 5 -

This method is an outgrowth of the theory of Fues and

Ludoffo A nondestructive test procedure would be very

useful in following the change in properties produced by

processing variables in this and other resins.

(f) Clinical Research on Self-Curing Denture Base Resins . The

clinical study of self-curing denture base resins in cooperation with the Veterans Administration has been progressing satisfactorily,, More than 100 practical dentures are

being observed and measured for dimensional changes periodically,, Observation of 30 dentures made at the Bureau

and 91 made in VA hospitals indicates a very high percentage of satisfactory service,,

(g) Periapical X-ray Film Survey ,, A preliminary survey of

periapical x-ray film has been initiated to determine the

practitioners 5 requirements „ Properties under investigation include emulsion speeds, light and water resistance

of the film wrapping, and the shelf life,, Common x-ray

practice procedures are to be determined through a ques- • tionnaire which will be sent to general practitioners and

specialists . If warranted, a specification will be developed .

(h) Hydraulic Turbine Contra-Angle Handpiece „ A hydraulic

handpiece which develops sufficient torque at speeds

above 14 s 500 r.p.m. to cut dental enamel with diamond

instruments has been designed „ Two experimental contraangle handpieces have been constructed of different turbine

designs and with several types of bearings . The driving

mechanism is a closed system; water flows to the turbine

in the handpiece through nylon tubing from a hydraulic

pump driven by l/2 H.P„ motor . This provides a power

system relatively free of vibration for revolving cutting

instruments

„

(i) Clinical Camera,, A 35 mm clinical camera, which is uniquely

suitable for the clinical testing program, has been completed o Magnifications up to four diameters with adequate

depth of focus can be obtained „ The photo-flash bulbs used

as a light source simplify the exposure factors „ Focusing

is done through the lens using a reflex-type camera.

(j) Evaluation of Materials . Materials evaluated for the cooperating agencies by specification or special test methods

included plastic teeth, porcelain teeth, direct filling

resins, silicate cement, amalgam alloys, mercuries, cavity

liners, elastic impression materials, impression pastes,

impression compounds, duplicating material, denture base

resin, base plate waxes, inlay waxes, wax solvents, gold

alloys, chromium-cobalt alloys and pickling salts.
o
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708- 20-3824 - 2032

Progress Report

on

THE COLOR STABILITY OF DIRECT FILLING RESINS

by

Harold Jo Caul^

Io Co Schoonover2

American Dental Association Research Associate at the National

Bureau of Standards

i 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 Ameri can Dental Association, the Army

Dental Corps, the Air Force^De^^w^xQorps, the Navy Dental Corps

and the Veterans A.dministrav!^^*^^*-^

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

unless permission is obtained inwrltingfromtheOffice of the Director, National Bureau of Standards, Washington

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

The Color Stability of Direct Filling Resins

Abstract

Data are presented showing the amount of color change

occurring in direct filling resins during storage at room

temperature in the dark and when exposed to the radiation of

a sunlampo These data were collected with a Hunter Multipurpose Refleetometer* Most of the changes taking place

during storage occurred within one day after formation of

the specimens and amounted to approximately 2 NBS units of

color difference o A color change of about 5 NBS units

resulted from exposure to the sunlampo The value of the

instrument for work on dental materials was greatly increased

by the addition of a special adapter which makes possible

the determination of color and color changes of small

specimens or of small areas of specimens*

1* Introduction

The test commonly employed for determining the color stability of

resins used in dentistry is based upon a visual comparison of the color

of a specimen exposed to the radiation of a sunlamp and the color of an unexposed sample £1^} This is not a satisfactory test with the newer

acrylic resins which polymerize at room temperature as a result of the

addition of accelerators and of a higher catalyst concentration,

because of the instability of these materials and their tendency to

form colored decomposition products at room temperature (Taylor and

Frank DG )* These chemical reactions, which produce discolorations

after polymerization, occur slowly on unexposed specimens even though

they are carefully protected and stored in the dark. This may cause

the standard or unexposed specimen to change during the period required

for the exposure test* The use of permanent color standards is not

feasible because of the varied color shades used by the manufacturers

and because of the mottled appearance used for good reproduction of the

oral tissues*

The visual comparison test is wholly unsatisfactory for determining

the amount of color change, because no quantitative results are obtained*

To overcome these difficulties of measuring color stability the Hunter

Multipurpose Reflectometer, which supplies a numerical value for color, was employed* This paper describes the method and results obtained with

direct filling resins*
.

... X:

•

•• .rv :•

'

' •

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r
- 2

The use of an instrument which measures color numerically will

be very helpful in solving many of the problems in dentistry where

color is an important factor© For example p accurate color determinations would make it possible to prepare a single shad© guide for

silicate and direct filling resins and artificial teeth instead of the

many different shade guides used by the various manufacturers 0 Measurement of mouth tissue colors can be expected to result in the

production of more natu.r&l colors for artificial dentures

2 0 Procedure

The Hunter Multipurpose Refleetometen Jj£] was used in this

investigation0 Because of the necessity of preparing small specimens

in order to approximate the polymerization conditions occurring in a filling in the mouth s an adapter was developed to be used in con- junction with the condensing lens which may be used with the

refieetometer©

Specimens (39 x 12 x l-l/2 mm) were prepared from each brand of

material using a tin-foil lined brass mold© Mixing of the material

was don© according to the instructions of the manufacturer © Color

measurements were made one hour after each specimen was prepared and

at intervals during seven months of storage in air in the dark at

room temperature o At the end of this time the specimens were subjected to the color stability test of American Dental Association

Specification No© 12 for Denture Base Resin fl which consists of exposing

the specimens to the radiation of an S-l bulb in a General Electric

sunlamp at a distance of seven inches for 24 hours© Color measurements

were mad© immediately before and after this testo These specimens were

again stored in air at room temperature in the dark and color measurements were made about 1-1/2 months later©

3© Results

The materials listed in Table 1 were used in this study©

The results of the color measurements are shown in Figures l s 2 P 3 and 4© In all cases the first measurement was made when th©

specimen was on© hour old and this measurement was used as the

standard to which all color comparison® ware made© S@vrit.on and

Sweden color comparisons were mad© on exposed and unexposed portions

of specimens subjected to the American Dental Association color

stability test© Sevriton changed in color 4 KBS units while Swedon

changed 8 KBS units©

Figures 1 and 3 show average values of color changes and

yellowness index change© Values for Atha moplast were omitted from

these averages because of the extremely large color change in this

resin during storage and because th© material is no longer on the

market©

Figures 2 and 4 show the cumulative effect of each treatment on

the various brands of material,.

4® Discussion

The magnitude of any color difference between two colors may be

found in NBS units by calculation from the CIE tri stimulus values or from colorimetric comparison of the two colors (see £43 and £53 )•

Color and color difference determinations may be made ’with the Hunter

Multipurpose Re fie ctom© ter , and may be determined to approximately

1 NBS unit by the use of this instrument® A trained observer is able

to see ’with difficulty a color difference of 1 NBS unit® Less than

about 10 NBS units is usually seen as a variation of the same color,

while more than 10 NBS units of color difference is usually seen as change to another color®

Hie plots of color difference (Figures 1 and 2) show that the

specimens changed in color about 2 NBS units of color difference

during storage at room temperature in the dark® Most of this change

occurred during the first 24 hours (Figure 1)®

Mien the specimens were exposed to the American Dental Association color stability test, which is designed to accelerate the

color changes that would occur on exposure of the resin to light

during normal use, the average change in color ’was approximately

5 NBS units®

It should be noted (Figure 1) that the color change during the

period of storage at room temperature in the dark after the exposure

test amounted to 1 to 3 NBS units for all of the materials with the

exception of Athermoplast V10® It thus appears that the reactions

initiated by exposure of the resin specimens to the radiation of the

test lamp may continue for some time after the completion of the

exposure test®

Lightness index changes (variation in gray) which were very small

occurred in six of the brands of resin® The lightness index has been

defined as the square root of the reflectance® The seventh brand,

Athermoplast Y10, darkened materially®

The changes in the yellowness index (Figures 3 and 4) show that

the specimens become yellower® The similarity of Figures 1 and 3

and of Figures 2 and 4 indicates that the color changes are, to a large extent, changes in yellowness and analysis of the original data

shows this to be true®

- 4 -

This investigation deals only with the color changes occur; ’ng

during storage of the direct filling resins in air at room temperature

in the dark and by exposure to light* Other types of discoloration

caused by the staining of foods and other materials in the mouth are not considered in this report*

5o Summary

By the use of the Hunter Multipurpose Reflectometer the color

and the color changes occurring in direct filling resins have been

quantitatively determined*

Data have been presented showing that the direct filling resins

change color with age in the absence of light* The average value of

color difference is about 2 NBS units* Most of the color change occurs during the first 24 hours*

The effect of the American Dental Association color stability

test (exposure to the radiation of a sunlamp) on the several brands

of material tested varies from approximately 1 to 11 NBS units of

color difference* The average value of about 5 NBS units is an easily noticeable color change*
'

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'

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References

1. Research Commission, Revised American Dental Association

Specification No. 12 for Denture Base Material, Acrylic Resin

or Mixture of Acrylic and Other Resins (effective Jan. 1, 1942),

J. A. D. A., 29*127, (Jan.) 1942.

2. Taylor, P. B., and Frank, S. L. , Low temperature polymerization

of acrylic resins, J. Dent. Research, 29*486-492, (Aug*) 1950.

3. Hunter, Riohard S., A Multipurpose Photoelectric Reflectometer,

J. Research NBS, 25*596 (Nov.) 1940.

4. Judd, Deane B., Colorimetry, National Bureau of Standards

Circular 478, page 29, issued March 1, 1950. (For sale by the

Superintendent of Documents, U. S. Government Printing Office,

Ifashington 25, D. C.) (Price 30 cents)

5. Colors of Polystyrene Plastics, Commercial Standards, CS 156-49

(For 6ale by the Superintendent of Documents, U. S. Government

Printing Office, Washington 25, D. C. ) (Price 5 cents)
' . - V 1

•- ! - '

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'*

J' l • : ; ' V • • " '. !• •
Table 1

Direct Filling He sins

Brand Manufacturer

Athermoplast V10 Athermoplast Products Inc.

Dentafil Dental Fillings, Ltd*

Duz All Coralite Dental Products Company

Fastorown Acrylite Company, Inc.

Kadon The L. D. Caulk Company

Plastofilling pf3 Plastodent, Inc.

Replica Cosmos Dental Products, Inc.

Sevriton* Amalgamated Dental Co., Ltd.

Swedon* Svedia Dental Indus tri AB.

* Color comparison were made only on exposed and unexposed portions of

specimens after American Dental Association color stability test.
t

SI '

.. ? i * • i ! v ! .. i ' ' * r v *. :
;• /. ' ‘i ;• * !

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.;" 4:;;nr .. ; .
Figure 1

Average difference in color stability of the direct

filling resins.
DIRECT

RESINOUS

FILLING

MATERIALS

o

30N3y 3 djia aonoo jo siinh ssn

TIME

IN

DAYS

-

'' Si t ft Vj? j * ?.4** ? l ?; | r i X. .* £, -:

\
Figure 2

Difference in eolor stability of the direet filling resins*
X

OJ OJ

CD

O

<

30N3d333IQ dOlOO 30 SlINfl SQN

V-
Figure 3

Average yellowness index of the direct filling resins*
YELLOWNESS

INDEX

DIRECT

RESINOUS

FILLING

MATERIALS

o

X3QNI SS3NM0T13A

TIME

IN

DAYS

$

• •
figure 4

Yellowness index changes of the direct filling resins.
CHANGES

IN

YELLOWNESS

INDEX

iD

IUo

QG

000

NATIONAL BUREAU OF STANDARDS REPORT

mS PROJECT NBS REPORT

0703-10-3324 December 2, 1952 2039

PROGRESS REPORT

ON

THERMAL EXPANSION OF SILICA-GYPSUM

INVESTMENTS: EFFECT OF A NEW FACTOR, PREHEATING

Ge F. Glasson*

W. T. Sweeney**

I. C. Schoonover***

*Guest Worker, U. S. Air Force Dental Service,

Dental Research Section, Division of Organic

and Fibrous Materials.

**Physicist, Dental Research Section

***Chief , Dental Research Section

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association, the Army Dental Corps, the Air Force Dental Corps, the

Navy Dental Corps and the Veterans Administration.

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

unless permission Is obtained In writing from 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,

Abstract

It has been observed that the thermal

expansion properties of some silica-gypsum

investments are changed by a pre-heat

treatment (three hours at 120° C) which

apparently causes a partial loss of water

of crystallization of the gypsumo The presence of boric acid and sodium chloride

seems to be associated with the increased

thermal expansion produced in some investments and decreased thermal expansion produced in other investments » X-ray diffraction and petrographic analysis gave no

evidence of the reason for this change 0 The altered thermal expansion affects both

the dimensional accuracy and the formation

of fins on castings

lo Introduction

Ever since the adoption of an inlay casting investment

specification by the American Dental Association in 1930?

the thermal expansion compensation technique has enjoyed

widespread usage in the casting of dental restorations 0 The addition of boric acid by Weinstein in 1929 LX]

and sodium chloride by Moore in 1933 r2J to quartz-plaster

investments „ and the development of an investment containing cristobalite in 1933 [3 §, V] have done away with investments that shrink when they are heatedo Various techniques for precision dental casting have been introduced

from time to times, but none have displaced the thermal

expansion compensation technique from being the most usedo

Yetj despite its favor and usage over a period of twentyodd years , and despite the fact that it has been investigated for a still longer period of time;, the technique contai

some undiscovered variables 0 The purpose of this investiga

tion has been to obtain information on one such variable in

the alteration of the thermal expansion of plaster-silica

investments; namely, the effect produced by drying at elevated temperatures and then cooling prior to the normal

continuous burnout procedure o In some investments the

subsequent thermal expansion is increased, and in other

- 2

investments the thermal expansion is decreased,, Such changes

can critically affect the results in the field of precision

casting, and it is therefore important to recognize and

evaluate these changes

,

The changes in thermal expansion may result from dehydration of the calcium sulphate. It has been demonstrated

by Weiser et al. J31 that the dehydration of CaSC^°2H o 0

follows definite steps; above 100° C the CaSO^ is in the

hemihydrate form,

2, Procedure

The following investments were used in this study.

Material Manufacturer

Cristobalite Model Investment Kerr Manufacturing Company

Pvromold No, 2 Investment Wm„ Getz Corporation

R & R Gray Investment Ransom & Randolph Company

All investment specimens and molds used in the investigation

were mixed according to the manufacturers recommendation for

water-powder ratio, using room-temperature tap water, and

were mechanically spatulated and vibrated.

The thermal expansions of the investments and the changes

in length resulting from the preheating and cooling cycle

were determined using the fused quartz tube apparatus as described by Hidnert and Sweeney Go] , The temperature of the

specimen was measured with an iron-eonstantan thermocouple,

the junction being placed in the center of the specimen.

Specimens were heated at a rate of approximately 10° G per

minute

,

Setting expansions of the three investments were determined by the comparator microscope method described by Coleman [7 3 ° In addition to the investigation of commercial investments, thermal expansion data were obtained on plaster ana

hydro cal as received, and with the addition of two common

constituents of dental investments, sodium chloride and

boric acid. These were studied by x-ray diffraction and

petrographic methods before and after heating.

The partial denture and M,0,D, inlay castings were made

by conventional dental procedures using various combinations

of the investments studied. The partial denture castings were

examined for fin formation. The dimensional accuracy wa;. evaluated by placing the castings on M,0,D, steel dies.

- 3 -

3o Results and Discussion

Thermal Expansion

The dimensional changes caused by heating are represented

graphically in Figures 1, 2 and 3 (curves marked: As Cast ? 1) 0 In all cases the values are the average of several tests „ By

drying the specimens at 120° C for three hours and then cooling them to room temperature before heating in the thermal

expansion apparatus , different amounts of thermal expansion

resulted (curves marked: Preheated, 2) 0 This treatment simulates one used by prosthetic laboratories for drying investment casts prior to waxing up a partial denture framework

pattern 0

It is also accepted procedure by some prosthetic laboratories to dip investment casts into a mixture of rosin and

wax at 150-165° C after drying, to facilitate the adhesion

of patterns to the casto A series of specimens treated in a

like manner exhibited thermal expansions as represented in

curves marked: Rosin-wax dipped , 3 S in Figures 1, 2 and 3°

It will be noted that in Figures 2 and 3 the expansion curves

of both the preheated and rosin-wax dipped specimens are

relatively the same, making it evident that the change in

thermal expansion properties is a result of the preheating 0 The fact that the thermal expansion of the rosin-wax

dipped specimen in Figure 1 is unlike that of the preheated

specimen cannot be explained at this time c

Change in Length as a Result of Preheating

It has been suggested that the change in thermal expansion of these investments is due to the loss of water during

the preheat treatment 0 Sweeney and Taylor jT$j have shown

that the loss of water of crystallization of gypsum products

is accompanied by a loss in linear dimension 0 Specimens of

the three investments heated at 120° C for three hours and

cooled to room temperature exhibited the following changes

in length:

Material

Cristobalite Model Investment

Pyromold No c 2 Investment

R & R Gray Investment

These are the average values of four specimens on which the

measurements are accurate to 0 o 02 percent o There is no

apparent correlation between the dimensional change caused

by preheating and the differential thermal expansion 0

Change in L ength

— 0 o O/4. -OolO

4

Setting Expansion

Setting expansions of the three investments determined by

the comparator microscope method described by Coleman [7jj were

as follows:

Whether or not the full setting expansion is effective

in partially compensating for casting shrinkage of gold is

controversial o However , with these materials this will not

appreciably affect the total expansion obtained because all

the setting expansion values are very nearly the same 0 Effects of Composition

A chemical analysis , Table 1, shows the amounts of boric

acid and sodium chloride used in these investments 0 Inasmuch

as silica is a very inert material , it is thought that the

boric acid or sodium chloride reacts with the calcium sulphate in these investments 0 Therefore, the effect of these

materials on the thermal expansion of plaster and hydrocal

was studied o Approximately the same ratio of boric acid or

sodium chloride to gypsum present in the investments was

added to the plaster and hydrocal. Figure 4 shows the thermal expansions of French ? s plaster and R & R Castone as received, with the addition of l c 5 percent sodium chloride,

and with the addition of 5°2 percent boric acido In plotting the curves the fiducial reading of the expansion was

taken as the original length of the specimen 0 The loss in

length in the preheated specimens is indicated by these

fiducial readings of the respective curves 0 The plaster had less thermal expansion than hydrocal

when given the same treatment » For example, curve 1-a is

less than 2a; curve 3b is less than 4b; etc Q The expansions of both plaster and hydrocal were

increased very slightly by the addition of 1<>5 percent

sodium chloride o However, the preheating did not affect

these expansion curves significantly, although a shrinkage

did occur during the preheating cycle

„

The effect of adding 5 °2 percent boric acid on the thermal expansions of plaster and hydrocal is quite evident o The

loss in length of the specimens due to preheating is less

and the specimens withstood a much higher temperature without deterioration o The expansion of plaster and hydrocal is

generally greater at all temperatures measured (Figure 4)

Material

Cristobalite Model Investment

Pyromold No 0 2 Investment

R & R Gray Investment

0 0 21

0 o 26

0 o 21

5

The most significant effect of the addition of boric acid was

to increase the temperature to which the specimens that had

been preheated could be heated before loss of strength

occurred o This is in agreement with the work of Coleman [’7’j

who noted that the addition of boric acid to silica-gypsum

investments caused an increase in strength at elevated temperatures o

In an attempt to correlate the expansion data obtained

with changes in crystallographic structure , the specimens

described in Figure 4 were subjected to x-ray diffraction

and petrographic analysis 0 Probably because of the relative

impurity of the samples , only minor differences could be

detected, and these were not significant 0 Experimental Castings

In order to determine the immediate practical significance of these changes in thermal expansion properties produced by preheating, technic partial denture castings were

made Q A basic set of eighteen castings were made using the

three investments 0 Six casts of each investment were poured,

three of which were dried at 120° C for three hours and

dipped in a rosin-wax mixture, consisting of 75 percent

commercial lump rosin and 25 percent paraffin, at 155° C

for thirty seconds 0 After waxing up the patterns, one cast

without treatment was invested in each of the three investments, and one cast with the drying and dipping treatment

was invested in each of the three investments 0 In this

manner, different combinations of mold and cast thermal expansions were compared 0 From these and many additional

technic castings it was shown that if the thermal expansion

of the cast exceeds the thermal expansion of the mold, even

by as little as 0 o 2 percent, the mold will crack, allowing a

fin to form on the castingo Conversely, and contrary to a common theory caa the cause of fins, if the thermal expansion of

the mold exceeds that of the cast by as much as 0 o 5 percent,

no fins will be formed 0 It is therefore important to maintain

like thermal expansions of the cast and the mold in casting

partial denture frameworks to prevent casting failures due go

fin formation o Figure 5 shows a cross-sectioned mold and

partial denture cast that has been heated to casting tempei

ture, where the thermal expansion of the cast exceeds that

the moldo The cracking of the mold is evident at the corruw

of the casto Figure 6 shows two technic castings made from

identical wax patterns 0 The casting on the right is satisfactory; the casting on the left exhibits the typical fin

formation resulting from a high-expanding cast and low-expanding moldo

Castings made on M o 0oD o steel dies demonstrate that the

dimensional accuracy of a casting can be affected by preheating alone c Figure 7 illustrates three Mo 0 o D o inlays ir.
-
- 6 - place on steel dies G The molds in this case were given the

regular burnout procedure „ It can be seen that castings C

( Cri stobaiite Model Investment) and R (R & R Gray Investment)

fit the dies wello The thermal expansion of these investments is 1 percent at 700° Co Casting P (Pyromold No 0 2

Investment) does not fit the moldo The thermal expansion

of this investment is 0o7 percent at 700° C 0 Figure 8 shows

three M o 0 0 D o inlays in place on the same steel dies, the

molds of which were subjected to a 120° C preheat treatment

followed by cooling to room temperature prior to the regular

burnout procedure o The fit of casting C remains unchanged;

that of casting P is Improved s and that of casting R is worse 9 as can be predicted from the differential thermal expansions

shown in Figure 9o From these experiments it must be pointed

out that the burnout procedure should be a continuous one to

prevent changes in the thermal expansion properties of the

mold o

4o Summary

lo It has been observed that the thermal expansion of

some casting investments is markedly changed by a preheat

treatment consisting of heating the specimens of investment

for three hours at 120° C and then cooling them to room

temperature 0

2 0 Of the three investments studied , one showed an

increase in the thermal expansion , another practically no

change j and the thirds a decrease in thermal expansion 0

3o The actual loss in dimension caused by the preheating of investments cannot be correlated with resulting

changes in thermal expansion 0

4° The thermal expansion changes due to preheating

have been demonstrated to be one cause of fin formation on

partial denture castings

„

5 o X-ray diffraction patterns and petrographic analysis fail to show appreciable changes in gypsum crystals due

to preheating

6 0 It is suggested that the difference in thermal expansion may be caused by unknown crystalline phases of the

gypsum, or by mineralization of the calcium sulphate crystals

by impurities or minor constituents

„

- 7 -

5. Bibliography1 . U. S. Patent No. 1,703,436.

2. U. S. Patent No. 1,924,374.

3 . U. S. Patent No. 1,932,202.

4. Sweeney, W. T., Cristobalite for Dental Investment,

J. Am. Dent. Assn. 20, 103 (1933)*

5. Weiser , Harry B. ; Milligan, W. 0.; and Ekholm, W. C.,

Mechanism of the Dehydration of Calcium Sulphate

Hemihydrate, J. Am. Chem. Soc. £3, 1261 (1936).

6. Hidnert, Peter; and Sweeney, W. T., Thermal Expansion

of Magnesium and Some of its Alloys, NBS J. Res. 1, 771

(Nov. 1928).

7. Coleman, R. L., Physical Properties of Dental Materials,

NBS J. Res. 1, 367 (Dec. 1923).

3. Sweeney, W . T.; and Taylor, D. F., Dimensional Changes

in Dental Stone and Plaster, J. Dent. Res. 29, 749

(1950).

Chemical

Analysis I 'O rH CV 0 0 o o 03 p vD

03 • 4J o

35 o o 0 O

35

p

ca . 1 73 O cv 0 0 m o o 03 p

oa • • P o

ffi o C4

O 0 a p

03go -4- VO t>- S TM • • ©

C\) C\J <A2 oo

Cd OA cv bo • e © S o o o

-4 UA t>- c^- o • • • CO ua UA 0 cv cv cv

vO r- OA 0 • • • o o o CN2 rH rH 1 1 0 0

OA o UA

O o 3 • a

OJ 3 o 3

CCJ P p

© (0 rH £ P O 3 QJ 3

<H „ 0 O <-A vO O .h P O • © M H p rH to -4 3 *h o5 vD UA UA H COS 0 P P 3 •H >, 0 rH 03 E 03 0 T3 3 P 3 P H O 0 05 o O 0 3 P H E CV P5 > CQ 0 0 o 3 •H 03 3 • H 3 O O 1 o g Ph s C4 0

03O3s3

•H3OEE

03 P

T30

+3

05 P

•Ha

•HO03

cp 3 0 o3 P P

•H

-P

03S T33

.3 03 o

•H 6

x: 3 5 3

•H

to E P 3 3 rH 0 Co E

<D ~ H 3 0 o3

£h -H O

to to 03 0

03 -3

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X 3 o CO v


Figure 1

Thermal expansion of dental investment.
THERMAL

EXPANSION

2

UL

o

100

200

300

400

500

600

700

TEMPERATURE

°C


Figure 2

Thermal expansion of dental investment.
THERMAL

EXPANSION

±N30 U3d Nl N0ISNVdX3

0

100

200

300

400

500

600

700

FIG. 2

TEMPERATURE

°C


Figure 3

Thermal expansion of dental investment.
1N30 d3d Nl N0ISNVdX3

ro

O

Ul

o

300

200

300

400

500

600

700

TEMPERATURE

°C


Figure 4

Thermal expansion of gypsum.
THERMAL EXPANSION OF GYPSUM

FIG. 4


Figure 5

Cross-section of partial denture mold which

was heated to casting temperature. Note

cracks at corners.


.
Figure 6

Technic castings made from identical wax

patterns. Right shows satisfactory casting; left, typical fin formation resulting

from use of high-expanding cast and lowexpanding mold.
NBS


Figure 7

Fit of M.O.D. inlays on steel dies; molds

given regular burnout. C, Cristobalite

P, Pyromold; R, R & R Gray.



Figure 3

Fit of M.O.D. inlays on steel dies; molds

given preheating treatment at 120° C and

cooled to room temperature before regular

burnout. C , Cristobalite ; P, Pyromold;

R, R & R Gray.


• ' ' - : ‘ ' • . ‘T';v : v ' • • - - r: i-
Figure 9

Effect of preheating on thermal expansion

of three dental investments.
THERMAL

EXPANSION

AT

700

PREHEATING

- 3

HOURS

AT

I20°C

1N30 d3d Nl N0ISNVdX3

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

070S-10-3S24 February 3, 1953 221+2

Progress Report

on

THE MECHANISM OF HYGROSCOPIC EXPANSION

IN DENTAL CASTING INVESTMENTS

by

H„ W. Lyon*

George Dickson**

I. C. Schoonover***

* Lt . , Dental Corps, U.S.N. Guest Worker assigned to

the Dental Research Section, Division of Organic

and Fibrous Materials,

** Physicist, Dental Research Section.

*** Chief, Dental Research Section.

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

the National Bureau of Standards in cooperation with the American

Dental Association, the Array Dental Corps, the Air Force Dental

Service, the Navy Dental Corps and the Veterans Administration.

The publication, reprinting, or reproduction^ thls'report In anylform, either In wholelor In part, Is prohibited

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

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

THE MECHANISM OF HYGROSCOPIC EXPANSION

IN DENTAL CASTING INVESTMENTS

Abstract

One of the methods -used to produce the expansion of dental investments necessary to compensate

for the casting shrinkage of gold alloys consists

of placing the mold in water during the setting

period „ The mechanism of this hygroscopic expansion

has not been definitely established 0 On the basis

of the data presented it is believed that hygroscopic

expansion of dental investments is a continuation of

the normal setting expansion during the period when

the investment is in a semisolid state 0 The Introduction of additional water causes hygroscopic expansion by permitting further hydration of the calcium sulphate during this time 0 The inert filler

present in the investment serves to weaken its

structure and thus permit the precipitating hydrate

crystals to expand the mass of material 0

lo INTRODUCTION

One of the fundamental problems in restorative dentistry

is the fabrication of cast gold appliances which accurately

fit the cavity prepared in a tooth or the space between teeth

located anywhere in the dental arches „ Two important factors

which must be considered in making such castings are the

dimensional change of the wax pattern , and the contraction or

casting shrinkage of gold on cooling from the liquid state to

room temperature,, At the present time there are two widely

used technics that compensate for these shrinkages,, Both

depend upon expansion of the casting investment

„

The older, more universal technic, often referred to as

the "thermal expansion" method, depends upon the presence in

the investment of a material or materials which, when heated,

will expand in a known manner to enlarge the moldo

The more recently introduced method for increasing the

mold size is based on the observation that some types of

investment materials, when placed in water during the setting

period, expand three to four times their normal setting expan^

sion 0 The mechanism of this enlargement, commonly termed

"hygroscopic expansion," will be discussed,,

2

/ ,1 The phenomenon of hygroscopic expansion was reported by

Wilson in 1920 [ij and was called to the attention of the

dental profession by Scheu [2j in 1933 » Scheu’ s original

observations on the expansion of setting inlay investment

upon the addition of water are the basis for the present hygroscopic technic o Scheu noted that all investments expanded

hygroscopically and that the amount of expansion depended upon

the water-powder ratio and time of immersion 0 Scheu defined

hygroscopic expansion as that expansion occurring when water

comes in contact with a mass of setting investment 0 Numerous investigators [3, 4,5,6] have published the re»

sults of their findings following Scheu’ s original observations,, Degni [7] stated that the hygroscopic expansion was

extremely sensitive to confinement of any kind. Degni also

used the colloidal theory for the setting of plaster of Paris

to explain the hygroscopic expansion process

In a recent paper by Landgren [S] an excellent report on

some of the factors affecting hygroscopic expansion was presented o Landgren noted that the water-powder ratio played an

important role in determining the amount of expansion. He

showed that the thicker the mix, the greater the amount of

hygroscopic expansion. Landgren also showed that hygroscopic

expansion decreases with an increase in time between spatulation and immersion when immersion is made after the initial

set o Generally speaking, the investigations mentioned above

have been primarily concerned with measuring the amount of expansion which can be obtained hygroscopically and with determining the factors which affect such expansion. The mechanism

of expansion has not been definitely established. It is the

purpose of this paper to present a possible explanation of this

phenomenon and to report some data supporting the explanation.

2. MECHANISM OF SETTING AND HYGROSCOPIC EXPANSION

2.1 Reactions Involved During the

Normal Setting Expansion

Typical inlay investments contain approximately 30 percent

gypsum (plaster of Paris or hydrocal) and 65 percent silica

,

the gypsum serving as a bonding medium for the silica. Plaster

of Paris and the closely allied compound, dental stone or hydrocal, are essentially hemihydrates of calcium sulphate, CaS0^°

l/2 HpO. When the hemihydrate is mixed with water, the dihydrate

CaSOji "2 HpO, or gypsum is formed. The solubility of the hemihydrate is O .65 grams per 100 milliliters of water and of the

dihydrate 0.21 grams per 100 milliliters at 23»5°C [9J <= Thus,

when plaster is mixed with water, a nearly saturated solution of

the hemihydrate and a supersaturated solution of the dihydrate

are quickly obtained. As the dihydrate is rapidly precipitated

from solution, more hemihydrate is dissolved and crystallization

continues. This process of solution and crystallization continues until the precipitated crystals have intermeshed so

tightly that solidification occurs.

The hydrated gypsum formed in this process has a density

of 2.32 [10] and a molecular weight of 172, giving a molar

volume of 74.2 milliliters. Values varying from 2.55 to 2.75

have been reported for the density of the hemihydrate [9,10] . Since the hemihydrate has a molecular weight of 145, this gives

a molar volume of approximately 55 milliliters. Thus, a volume

increase of approximately 35 percent occurs on hydration. However, if the 27 milliliters of water required for this reaction are considered, a net shrinkage of 10 percent occurs. This

shrinkage is not observed in a specimen of setting plaster. A

slight expansion always occurs. The explanation of this apparent anomaly is based on the crystalline growth of the precipitated gypsum, which results in a porous structure with a greater

apparent volume. Thus, the setting expansion observed in plasters results from crystal growth after and during the period

when the mass becomes sufficiently solid to retain a definite

form and just prior to the time at which the material attains

sufficient strength to resist further deformation.

Apparently the hygroscopic expansion observed in investment materials results from an increased crystal growth over

the period of time in which the mass is in a semisolid state

and is a continuation of the normal setting expansion. The

observations made during this investigation of the effect

that inert substances and added water have on the amount of

hygroscopic expansion appear to have a direct bearing on this

proposed mechanism of continued expansion.

2.2 Effect of Inert Substances

The expansion observed when hydrocal is mixed with water

is of the order of .04 to .06 percent. Hydrocal cannot be made

to exhibit hygroscopic expansion. It was observed, however,

that if an inert material was added to the hydrocal prior to

mixing with water, the resulting mass invariably expanded

hygroscopically. In these experiments the following inert

materials were employed: silica, zinc dust, iron powder,

acrylic resin powder, pumice, graphite, silica gel, sulfur,

lamp black and talc. All the resulting mixes exhibited hygroscopic expansion. Some of the materials, particularly those

difficult to wet, exhibited less expansion than others. Typical

results obtained are shown in Table 1.

4

Table 1

Hygroscopic Expansion of Mixtures of

Hydrocal and Inert Substances

Substance Added

Setting Expansion

Inert substance

20 percent' by weight

Inert substance

60 percent by weight

percent percent

Pumice o ^ o75

Graphite °7 1 06

Talc o 8 2o3

Silica Gel r

o 1— Sol

It appears that the effect of the inert material was to

weaken the structure of the mass to such an extent that expansion resulting from crystalline growth could continue for

a longer period of time before the mass gained sufficient

strength to resist further deformation 0 Such crystalline

thrust or growth is not uncommon in the solidification of

metals from the liquid state [11]

„

In addition to loosening the structure of the setting

investment, the presence of inert materials may allow more

efficient penetration of water into the structure of the investment masso

2o3 Effect of Water

To make an investment expand hygroscopicaily , additional

water must be made available to the investment during a critical period in the hardening of the mass 0 This need for

additional water is difficult to explain, particularly because

of the fact that water in excess of that required for complete

hydration is always added in making the initial mix 0 The

following explanation for this need of additional water is

proposedo

In the setting reaction of investments crystallization

does not occur uniformly throughout the masso During this

nonuniform crystallization, powder particles begin to dissolve

and crystals of the hydrate grow on the surface of the undissolved hemihydrate 0 Further crystalline growth causes water

to be retained in the voids in and around the crystallites,,

Thus, although sufficient water is present initially to

hydrate all the hemihydrate , it is temporarily prevented from

doing so by this entrapment „ Eventually, by a process of
.

-
- 5 - diffusion of the water or water vapor , complete hydration

will occur o Hoxvever, the process is slow and the mass will

have gained sufficient strength to resist deformation before

hydration is completed

„

If the investment is placed in water prior to such hardening, the water rapidly penetrates the mass as a result of

capillary action*, Additional hemihydrate is dissolved, dihydrate is precipitated, and further crystallization continues with sufficient rapidity to expand the semisolid mass.

That some such mechanism is in operation is made evident by

the rapidity with which water is imbibed by the mass during

this critical period and its immediate effect in expanding

the masso This was demonstrated by placing a cylindrical

specimen of investment 1 0 25 inches in diameter and 1„5

inches high under a dial gage „ At the critical period during

which hygroscopic expansion occurs most readily, drops

of water were applied to various areas over the surface of

the specimen by means of a medicine dropper 0 An immediate

increase in length was observed as each drop of water was

added

„

In other experiments a small amount of water was placed

around the base of a specimen 0 When the setting reaction

reached the critical period for hygroscopic expansion to occur,

the water was rapidly pulled into the specimen and expansion

followed immediatelyo

2 0 i+ Volume Changes During Hydration

If the proposed mechanism of further crystallization is

valid, some of the hemihydrate must remain unhydrated during

the period in which the setting mass is most susceptible to

hygroscopic expansion. That hemihydrate is present in the

mix during this period was demonstrated by an experiment in

which the rate of hydration was observed by measuring the

decrease in volume of a mixture of investment and water containing a large excess of water 0 The investment powder was

inserted into an apparatus so designed that all air could be

pumped out of the powder mass and water added as shown in

Figure 1.

Sufficient water was added to cover the specimen and rise

in a graduated capillary tube joined to the specimen chamber.

The apparatus, sample, and water were in an air bath maintained at a constant temperature to within ± 0 o l°C o The

height of the water in the capillary tube was recorded at intervals from eight minutes to three hours after the water was

admitted to the specimen chamber

„

6 -

The results obtained on several brands of investment and

one dental artificial stone are shown in Figure 2„ The data show

that shrinkage of the inyestment-water mixtures continued at a rapid rate for approximately one hour after the investment and

water were mixed 0 This indicates that hemihydrate is present

in the mix and that hydration continues during the time that

hygroscopic expansion occurs. It is reasonable to believe,

therefore, that water added to the investment causes hygro

scopic expansion to occur by inciting a more complete hydration while the material is in a semisolid state 0 It is possible that the water added weakens the structure of the setting

investment and thus permits a greater expansion of the investment by the growth or thrust of the precipitating hydrate

crystals 0

2 0 5 Effect of Retarders and Accelerators

If hygroscopic expansion is only an extension of setting

expansion as a result of sorption of water during a critical

period in the setting of gypsum, as proposed, materials which

retard the hydration and hardening of gypsum would be expected

to lengthen this critical period, thus permitting more sorption of water and causing an increase in the hygroscopic expansion of investmentSo To test this, cylindrical specimens

of an investment to which a retarder had been added were prepared o These cylinders were placed under a dial gage as soon

as they possessed sufficient strength to support themselves

„

Water was then poured into the container surrounding the investment and the amount of expansion was measured on the dial

gage Q Similar experiments were made on specimens containing

no retarder „ The increased amount of expansion in specimens

containing retarders is shown in Table 2 0 The retarders

apparently prevented rapid hydration and allowed greater

crystalline growth in a weakened structure

„

The experiment was repeated substituting accelerators for

retarders. The extreme rapidity of set resulted in a much

lower expansion., Probably because of the increased speed of

hydration and the rapid interlocking of the crystals into an

extremely hard mass in a short period of time, the amount of

expansion was not affected by immersion in water.,

2„6 Production of Hygroscopic Expansion

by Silica Gel

Based on the principles demonstrated in the experiments

on the effect of inert substances and water on hygroscopic

expansion, an experimental investment was prepared using

silica gel previously saturated with water and air dried,

mixed with hydrocalo Specimens of this investment containing

50 percent hydrocal and 50 percent silica gel expanded

7

-

approximately 3 percent without the addition of water . The

amount of expansion of other specimens containing different

ratios of hydrocal to silica gel was roughly proportional to

the amount of silica gel added. It is postulated that the

silica gel, capable of holding up to 90 times its own weight

of water, acted both as an inert material and as a source of

water for the continued hydration of the hemihydrate.

Although these experimental investments behaved in

accordance with the proposed mechanism for hygroscopic expansion, they were not satisfactory for practical use in an

inlay technic.* However, this preparation of an investment

which will expand hygroscopic-ally without the addition of

water suggests the possibility of the development of a

satisfactory dental investment based on this principle.

Table 2

Effect of Accelerators and Retarders on Hygroscopic

Setting Expansion of Cristobalite Investment

Material Used Action Setting

Time Expansion

min percent

Crystobalite investment + ferrous sulphate, 2 percent;

wet asbestos liner; not immersed Accelerator 10 .6

Crystobalite investment + ferrous sulphate, 2 percent;

wet asbestos liner; immersed Accelerator 10 .6

Cristobalite investment, wet

asbestos liner; not immersed None 25 i—

o o

Cristobalite investment; freely expanding; immersed None 25 1 o 5

Sodium acetate, A percent; wet

asbestos liner; not immersed Retarder 90 1.2

Sodium acetate; freely expanding; immersed Retarder 120 2.2

Heating these investments caused shrinkage and reduced

strength. Castings made in unheated molds were unsatisfactory, apparently because of the vaporization of water

in the investment by the molten gold.

3 o CONCLUSIONS

On the basis of the data presented it is believed that

hygroscopic expansion of dental investment is a continuation

of the normal setting expansion during the period when the

investment is in a semisolid state» This continued expansion is produced by the introduction of additional water

which permits further hydration of calcium sulphate at this

time 0 The inert filler present in the investment weakens

its structure and thus permits the precipitating hydrate

crystals to expand the mass of material

Bibliography

1. Wilson, G, Ho Manual of Prosthetic Dentistry , p<. 7$

(Lee and Febiger, Philadelphia, 1920)

2o Scheu, Co Ho T, A New Precision Casting Technic." J» Am c Dento Assn. 19: 63 O (1933)

3. Smyd, Eo S e "Factors Which Influence Casting Accuracy:

A Universal Casting Technic.” J. Am. Dent. Assn. 36 : 160-172 (1943).

4o Docking, H. R. "The Hygroscopic Setting Expansion of

Dental Casting Investments, Parts I, II and III."

Austral. J. Dent. 52: 6-11; 160-166; 320-329 (1943).

5. Hollenback, G. M 0 "Precision Gold Inlays Made by a Simple Technic." J. Am. Dent. Assn. 22° 99-109 ( 1943 )

6 . Coy, H. Do "Important Factors in Making Satisfactory

Small Castings." N.Y„ J. Dent. Vol. XX, No. 3, P° 105-109.

7. Degni, F. "Hygroscopic Expansion of Dental Investments."

Thesis, Northwestern University Dental School.

3. Landgren, N. "Hygroscopic Expansion of Some Casting Investments." Thesis, University of Michigan School of

Denti stry 0

9. Kelly, K. K., Southard, J. C., and Anderson, C. T.

"Thermodynamic Properties of Gypsum and Its Dehydration

Products." Bureau of Mines Technical Paper 655 (1941)

10. Winchell, A. N, Microscopic Character of Artificial Minerals

p. 226-229 (John Wiley & Sons, New York, 1931)

11 o "Method for Obtaining High Pressures at Low Temperatures."

J. Physics (USSR) 3: 193-200; J. Exper. & Theor. Physics

(USSR) 14: 439-47 T1944)
.

'

\

Figure 1

Apparatus used to investigate rate of hydration of gypsum in water by measurement of

decrease in volume resulting from hydration

The gypsum is placed in the specimen chamber

at the bottom of the apparatus. After this

chamber has been evacuated, water from the

spherical reservoir is allowed to flow around

the specimen. The reservoir is then sealed

off, the appropriate capillary tube is opened,

and the change in volume is observed during

the period of hydration of the specimen.



Figure 2

Rate of hydration of dental investments

and stone as indicated by change in volume of water and investment or stone

mixture o Change in volume of the mixture

is indicated as milliliters of water,,
H31VM dO 1IAI o i

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

oo- o to i 10-3 $24 February 15 , 1953 22&3

Progress Report

on

TEMPERATURES DEVELOPED IN

ROTATING DENTAL CUTTING INSTRUMENTS

by

Donald C, Hudson*

Wo To Sweeney**

*Colonel, Dental Service, U« So Air Force, stationed

at the National Bureau of Standards Dental Research

Laboratory , Division of Organic and Fibrous Materials,

**Physicist, Dental Research Laboratory,

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association, the Army Dental Corps, the Air Force Dental Service,

the Navy Dental Corps and the Veterans Administration,

The publication, reprinting, or reproduct|on!of thls'report In anytform, either In wholeJor In part, Is prohibited

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

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

ROTATING DENTAL CUTTING INSTRUMENTS

Abstract

The Air Force has been interested in the problem

of heat trauma in dental operating as a possible cause

of pain occurring in teeth at high altitudes . A

direct approach to the problem of bur temperature measurements has been undertaken, placing a thermocouple

in the head of the bur 0 Behavior of dental burs of

steel and carbide has been studied at speeds up to

24,000 r 0 p.m 0 Temperatures ranging from 149 to 275°F

were recorded when cutting natural dentin and a synthetic substitute o Cutting rates were also determined

over the same speed range, and were found to be proportional to the speed of rotation of the bur. Results of the work indicate that temperatures well above

the limit of tolerance of tooth structures to heat are

attained by the cutting bur. A coolant would appear

to be necessary if unfavorable reactions are to be prevented when operating dental burs at high speeds.

An Air Force survey conducted by some 200 dental officers

in 1946 revealed that large numbers of flying personnel were

afflicted with dental pain when subjected to low atmospheric

pressures [1] ° This pain was of such severity as to interfere

with the performance of duty as members of an air crew. Most

of these painful conditions were found to occur in recently

filled teeth, and it was suspected that the trauma of cutting,

and the heating of tooth structure which occurs during cavity

preparation, were responsible for the ill effects.

A project was started at the School of Aviation Medicine

at Randolph Field, Texas, at that time, to determine some of

the factors causing trauma to dental pulps when preparing

cavities by means of rotating cutting instruments. This project was resumed at the National Bureau of Standards in 1950

under a grant from the School of Aviation Medicine.

A review of the dental literature bearing on the subject

of heat production and cutting efficiency of burs, and the

traumatic effects of cavity preparation and pulp reactions to

heat was conducted. Work by Jeserich [2] , Bodecker [3] , Walsh and Simmons [4] , Henschel [5,6,7] , Anderson and Van

Praagh [8], Lisanti and Zander [9], and Peyton and Vaughn [10J , led to several conclusions which formed the basis for the

study carried out at the National Bureau of Standards.

- 2 - These conclusions were: (a) The upper physiological tolerance

of normal dentin to heat lies in the range $5° to 130°F [5]

(b) Application of heated instruments in the range 125° to

600°F to the floor of cavity preparations in dogs 5 teeth for

varying lengths of time caused definite alterations in the

odontoblastic layer,, blister formation and inflammatory

changes [9]

(c) Heat production by revolving burs increases

with increasing speed of rotation [10] » (d) Cutting rate

increases with an increase 'in speed and pressure [10] a Previous workers used indirect methods, e 0 g 0 calorimetric

determinations of heat production during burring, to determine

bur temperature [7] » Others measured temperatures by means of

thermocouples inserted in teeth at points adjacent to that

being cut by the revolving bur [10] . Estimates of bur temperature were also made, based upon such things as observed

changes in color of the steel, and the melting of tin-lead

alloys when cut by dental burs [7]°

A more direct approach to the problem of determining bur

temperature was attempted in the study reported here* Efforts

were directed toward the measurement of the temperature of the

head of the bur itself during cutting operations

„

APPARATUS AND METHODS

A machine designed to rotate dental burs at speeds up to

25,000 rop 0 m 0

, with accessory timing and controlling features,

constructed at the School of Aviation Medicine, was shipped to

the National Bureau of Standards at the time of the project

transfer,, It was modified so that the following conditions

could be accurately controlled: (a) speed of rotation up to

25,000 rcDom,, to ± 200 rop 0 m.; (b) depth of cut to <>15 ±0«001

inch; (c) cutting load to 600 ± 1 gram 0 Provision for accurate recording of time to 1/1000 minute was included in the

apparatus, as was control of the starting and stopping of the

cutting cycle by means of electromagnetic devices., All burring

was done with 2 <,3 5 mm, #3 eight-bladed round burs cutting

axially, mounted in a vertical precision chuck 0 Specimens to

be cut were lowered from above by gravity and weights were

placed on the spindle to exert the desired pressure on the bur

(Fig, l)o

A constantan wire, #36, 0o 005 inch in diameter, was soldered in the groove between the blades of each test bur

(Fig, 2)o With the aid of a binocular microscope, the excess

solder was cut away with a fine instrument to prevent its

interfering with the operation of the bur„ The wire was

secured to the bur shank with an insulating adhesive until it

reached the slip ring commutator to which it was connected

(Fig»3)o The circuit was completed through the bur itself

and another commutator ring at the base of the chuck 0

- 3 -

The e 0 m o fo generated in the bur-constantan combination was

picked up by brushes connected to a Brown Instrument recording

potentiometer calibrated in degrees centigrade 0 The burconstantan combination was calibrated and a curve obtained

which was used to correct the readings of the recording thermometer o RESULTS

Human tooth enamel and dentin were first cut holding the

tooth by hand and exerting sufficient force to achieve normal

cutting as in operatingo The temperatures recorded varied

from 150°F at 2,000 roPom 0 to 275°F at 7*500 r 0 pom* when cutting dentin alone 0 When the bur was moved laterally to cut

both enamel and dentin, as in the extension of a cavity

preparation, temperatures as high as 350°F were observed,,

An effort was made to penetrate the occlusal enamel of a

molar tooth at 7*500 r 0 pom 0 The bur was engaged in a noncarious occlusal pit, and force such as would be exerted in

normal operating was applied „ Cutting was very slight and the

temperature of the bur mounted rapidly, reaching an indication

of over 600°F o Examination of the bur revealed dulling of

the blade edges (Fig c 4-A), and marked color changes were

notedo The blue color developed corresponds to that indicated

for steel temper colors at temperatures near 600°F £llj * Reproducibility of results was unsatisfactory due to the

wide variations in properties of natural tooth substance from

tooth to tootho Therefore, it was decided that a synthetic

substance that was cut by the bur at approximately the same

rate as was dentin, would be used for all temperature and

cutting rate measurements „ The material chosen was a cast,

filled Bakelite resin (P-l$3) from which conveniently sized

specimens were prepared 0 The curve shown in Fig* 5 is plotted from data obtained

in a series of ten test cuts in this material at a load of

200 grams and at speeds from 2,000 to 24*000 r*p*m* (Table 1) . All cuts in each run were made to a depth of 0 o 15 inch, and

the temperatures attained by the bur are shown to approach a maximum at a speed of 6,000 r 0 pom 0 As the speed increased

beyond this point, there was little change in temperature* A

series of five cuts was made at a load of 200 grams, with the

constantan wire soldered on the slope of the bur blade rather

than in the groove between blades* It will be noted from

Table 1 and Fig* 6 that the temperatures recorded at points

on the blade were higher than those taken at points farther

from the edge (in the groove)*

- 4 -

The time required to complete a fixed depth of cut, plotted

against rotational speed is shown in Fig 0 7» The data are shown

in Table 1 0 It is interesting to note that the time necessary

to complete the fixed depth cut falls rapidly as the speed increases, until the region 8,000 to 12,000 rop 0 m 0 is reachedo Beyond this point the curve flattens and the time required to complete the cut at 24,000 rop 0 m 0 is only about 3 seconds less than

that required at 10,000 rop s m OJ whereas the difference in time

required to make the cut between 2,000 and 12,000 r 0 p s m. is as

much as 24 seconds 0 These data indicate within the limits of

experimental error that steel and carbide burs cut faster as

speed of rotation increases up to about 20,000 r 0 pom 0 Practically, there would not appear to be any great advantage as far

as rate of cutting is concerned, in increasing speeds of rotation above approximately 12,000 r 0 p 0 m 0 The time required for

cutting at this speed is so low that further reduction would

probably not decrease significantly the time required for the

preparation of a cavity under actual operating conditions

„

The above experiments were repeated, using steel-shanked,

carbide-tipped burs of the same size and blade arrangement as

those of the all-steel burs, A curve of temperature versus

revolutions per minute, plotted from data given in Table 1, is

shown in Fig 0 8 0 The time required to reach a fixed depth in

cutting is given in Table 1, and the curve is shown in Fig 0 9«

It will be noted that the rate of cut compares closely with

that of the all-steel bur Q Carbide-tipped burs penetrated tooth enamel easily without

damage to the bur blades (Fig 0 4-B) 0 Lisanti and Zander have shown that instruments heated to

temperatures in the range 125-600°F and applied to the pulpal

floors of cavities in normal dogs ? teeth produced definite

pulpal reactions « It would be reasonable to assume, therefore, that dental operative procedures performed with burs

that reach temperatures of from 140° to 250°F may be responsible

for unfavorable reactions in the human tooth „ Postoperative

pain occurring at high altitudes in recently restored teeth of

flying personnel may be due to volume changes in blisters or

areas of separation of the odontoblastic layer from the pulpal

wall with variations in barometric pressure 0 The use of a

coolant would appear to be indicated, especially when operating

at high speeds

CONCLUSIONS

1» The temperatures reached by the dental bur in cutting

natural dentin ranged from 150° to 275°Fo Temperatures attained

when using a synthetic substitute ranged from 140° to 250°F

measured on the slope of the bur blade » Since these values are

- 5 -

above those said to be tolerated by normal human dentin » it

would seem advisable to use some form of coolant when burring

,

especially at high speeds 0

2 0 Steel and carbide burs cut more rapidly as their speed

of rotation increases up to about 20,000 r 0 pom 0 under the conditions used in this investigation

3o Steel burs are not suited to the cutting of tooth

enamel because of their relative softness, whereas carbide burs

will cut readily 0

REFERENCES

lo Preliminary Report, School of Aviation Medicine,

31 December 1949.

2 0 Jeserich, P 0 H 0 Factors Necessary to Minimize Thermal

Changes in Tooth Structure from Operative Procedures, N 0 Y c Jo Dentistry V, 275-2? 3 (1935).

3o Bodeeker, C 0 F» Demonstration of Possible 111 Effects

of Heat on the Pulp, Caused by Rapid Operating Technic,

Jo Am c Dent „ Assn 0 26: 527-532 (1939).

4o Walsh, Jo P 0

, and Simmons, H 0 F 0 Comparison of the

Heat Production and Mechanical Efficiency of Diamond Instruments, Stones and Burs at 3 $.000 and 60,000 R o PoM c , New Zealand

Dento Jo 45: 23-32 (1949)

5o Henschel, C 0 Jo Development of Thermal Control? Jo

Am 0 Dento Assn 0 3j3:194-=200 (1946) o

60 Henschel, C 0 Jo Heat Impact of Revolving Instruments

on Vital Dentin Tubules, J 0 Dento Res 0 £2:323 (1943)

7o Henschel, C D Jo The Friction of Revolving Steel Burs,

Jo Am c Dento Assn c 31,° 395 (1944).

3 C Anderson, Do Jo, and Van Praagh, G 0 Preliminary Investigation of the Temperatures Produced in Burring, Brito Dento

Jo 73 : 62-64 (1942)

9o Lisanti, V 0 F», and Zander, H. A 0 Thermal Injury to

Normal Dog Teeth: In Vivo Measurements of Pulp Temperature Increases and Their Effect on the Pulp Tissues, Jo Dento Res 0 31 « 543-558 (1952).
.

.
- 6

10. Peyton, F. A„, and Vaughn, A® B 0 Thermal Changes Developed During

the Cutting of Tooth Tissue® Paper presented at the Mid-Winter Meeting of

the Chicago Dental Society, February 1950

11. Young, C. Bo F. Coloring Steel, Metals Handbook, p. 730 (1948

Edition).

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Figure 1

Bur evaluating machine » Vertical spindle

with chuck, driven by variable speed motor,

along with accessory timing and limiting

controls
\

BS


Figure 2

Thermocouple wire soldered in groove between

bur blades



Figure 3

Bur-holding chuck, showing commutator and

brush assembly.



Figure 4

A. Steel bur after attempting to cut human

tooth enamel at 7,500 r.p.m. Note turned

edge of blades.

B. Carbide bur after cutting enamel at 7,500

r.p.m. No apparent damage.



Figure 5

Temperature in groove between bur blades,

plotted against speed of rotation. Force

of 200 grams applied, cutting dentin substitute with steel bur. Shaded areas

indicate limits of probable error.
275

h.

250

225

200

175

ki

§ 150

1

£ 125

2

Ui

100

75

50 |~

25

J L

<NBS>

1 1 i i 4 6 8 10 12 14 16 18 20 22 24

SPEED IN RPM X 1000


Figure 6

Temperature on slope of bur blade plotted

against speed of rotation „ Force of 200

grams applied, cutting dentin substitute

with #3 steel bur. .Shaded areas indicate

limits of probable error.
TEMPERATURE

IN

*F

1 i i i i l —l—l i 1 1 1—

2 4 6 8 10 12 14 16 18 20 22 24

SPEED IN RPM X 1000


Figure 7

Time required to complete fixed depth of cut

to 0*15 inch, plotted against speed of

rotation. Force of 200 grams applied, #3

steel bur cutting dentin substitute.
SPEED IN RPM X 1000


Figure 8

Temperature in groove between bur blades

on #8 carbide bur plotted against speed

of rotation. Force of 200 grams applied,

cutting dentin substitute.
TEMPERATURE

IN

*F

275

2 4 6 8 10 12 14 16 18 20 22 24

SPEED IN RPM X 1000


Figure 9

Time required to complete fixed depth of

cut to 0.15 inch, plotted against speed

of rotation. Force of 200 grams applied

to #8 carbide bur, cutting dentin substitute.
50

45

2 4 6 8 10 12 14 16 18 20 22 24

SPEED IN RPM X 1000

THE NATIONAL BUREAU OF STANDARDS

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of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950.

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