NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

31 1.05-11-31 10260 December 3 1, 1968 10 0 27 31 1.05- 11-31 10560

31 1.05-1 1-311056 1

REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

Progress Report

July 1 to December 31, 1968

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

National Institute for Dental Research; the Dental Research

Division of the U. S. Army Medical Research and Development

Command; the Dental Sciences Division of the School of Aerospace Medicine, USAF; and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF ST/

for use within the Government, and review. For this reason, the whole or in part, is not authori Bureau of Standards, Washingto

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

1 the Government agency for which

ipies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
•• - U
TABLE OF CONTENTS

Page

1. Introduction 1

2. Reports Issued 1

3. Papers Published 2

4. Work in Progress 2

4.1 Crystallography of Calcium Carbonates

and Calcium Phosphates 2

4.2 Solubilities of Calcium Phosphates 7

4.3 Study of Dentinal and Enamel Surfaces

from Sorption Isotherms 12

4.4 Mechanical Behavior of Skin 14

4.5 Thermal Expansion of Tooth Structure 15

4.6 Metallurgy 16

4.7 Adhesion to Dentine and Enamel Surfaces l 8

4.8 Development of an Adhesive Restorative

Material 20

4.9 Composite Resin Material for Temporary

Posterior Restorations 22

4.10 Zinc Oxide-Eugenol Type Restorative

Materials 23

4.11 Rapidly Fabricated Surgical Splint 24

4.12 Grafting of Polymers to Hard and Soft

Tissues 25

4.13 Clinical Research 26

4.14 Specification and Advisory Activities 26

Appendix:

NBS Report 9879 An Evaluation of a Gal 11m- Palladium-Tin

Alloy for Restorative Dentistry.

NBS Report 9983 A Filled Cold-Curing Acrylic Resin as a

Splinting Material.

NBS Report 9984 The Particle Size and Shape of Silver

Alloys for Dental Amalgam.

NBS Report 9985 The Crystal Structure of CaC0^*6Hp0 at

-120°C. ^ NBS Report 9986 Crystal Structure Transformations in VPt

Produced by Plastic Deformation at Room

Temperature

.

NBS Report 10024 Adsorption Theory Interpretation of Ca^

and P ^2 Exchange with Hydroxyapatite.

REPORT ON DENTAL RESEARCH

at the

NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

Dental research at the National Bureau of Standards during

the half year ending December 31, 1968 included investigations

of a wide range of materials used in restorative dentistry as

well as studies of mineralized tissues and related compounds.

The research program is supported by the Council on Dental

Research of the American Dental Association, the Collaborative

Research Office of the National Institute of Dental Research,

the Dental Research Division of the U, S. Army Medical Research

and Development Command, the Dental Sciences Division of the

U. S. Air Force School of Aerospace Medicine, and the Veterans

Administration. In the summaries below of work in progress,

sponsors of various projects are identified. However, since

most of the projects are closely related both technically and

administratively, reports on all projects are presented for the

information of all sponsors. Detailed reports issued during

the period on some phases of the research are listed below and

are appended to this report,,

2. REPORTS ISSUED

NBS Report 9879

NBS Report 9983

NBS Report 9984

NBS Report 9985

NBS Report 9986

NBS Report 10024

An Evaluation of a Gallium -Palladium-Tin Alloy

for Restorative Dentistry.

A Filled Cold-Curing Acrylic Resin as a Splinting Material.

The Particle Size and Shape of Silver Alloys

for Dental Amalgem.

The Crystal Structure of CaC03 *6H2 0 at -120°C.

Crystal Structure Transformations in VPt

Produced by Plastic Deformation at Room Temperature.

Adsorption Theory Interpretation of Ca45 and P" 2 Exchange with Hydroxyapatite.

1
3. PAPERS PUBLISHED

Brauer, G. M„; McLaughlin, R., and Huget, E. F. Aluminum Oxide

as a reinforcing agent for zinc oxide-eugenol-O-ethoxybenzoic

acid cements. J. Dent, Res., 47:622 July-Aug. 1968.

Oglesby, P. L.; Dickson, G. ; Rodriquez, M. L.; Davenport, R.M.,

and Sweeney, W. T. Viscoelastic behavior of dental amalgam.

J. Res. NBS (Chem. Eng. and Instr.) 72C:203 July-Sept. 1968.

Dickson, G.; Oglesby, P . , and Davenport, R. The steady-state

creep behavior of dental amalgam, J. Res. NBS (Chem. Eng. and

Instr.) 72C:215 July-Sept. 1968.

Huget, E. F., and Brauer, G. M. Reactivity of the components

of tooth structure. J. Colloid 8 Interface Sci., 27:714 Aug.

1968 „

Bowen, R. L.; Paff enbarger, G. C., and Mullineaux, A. L.

A laboratory and clinical comparison of silicate cements and

a direct-filling resin with and without fused silica:

A progress report. J. Pros. Dent. 20:426 Nov. 1968.

Moreno, E. C.; Gregory, T. M., and Brown, W. E. Preparation

and solubility of hydroxyapatite. J. Res. NBS (Physics and

Chem.) 72A:773-782, Nov. -Dec. 1968.

Gardner, Jr., T. V./ Dickson, G», and Kumpula, J. W.

Application of diffraction gratings to measurement of strain of

dental materials. J. Dent. Res. 47:1104-1110, Nov. -Dec. 1968.

Eick, J. D. and Hegdahl, T. Segregation in dental gold casting

alloys. J. Dent. Res. 47:1118-1127, Nov. -Dec. 1968.

4. WORK IN PROGRESS

4.1. Crystallography of Calcium

Carbonates and Calcium Phosphates

Sponsor: NIDR, ADA

In general, two classes of compounds are of primary

interest in mineralization phenomena - the calcium phosphates

and the calcium carbonates. Our objectives are to increase the

understanding of how the ions are arranged in ideal structures

so as to provide a basis for models describing chemical

reactions such as dissolution, crystallization and the incorporation of carbonate, fluoride and other impurities in bone mineral.

2
Ca^Mg, (P04 )b

:

The structure has been determined up to the

point of final refinement. Because of the unusually large

number of data collected to solve this structure, the computer

programs had to be modified and tested. The structure is known

well enough at this stage (agreement between calculated and

measured structure factors is quite good, R = 0.08) to draw the

following conclusions:

1. The composition has been reported both as

Ca4.5 Mg4.5 ^ P04^6 and aS Ca4 Mg

5

and has been indicated to have a small

range of variability. The structure shows

that one of the cation sites can contain

either Ca or Mg, apparently in a random

distribution. The formula should therefore be written Ca^

^ ^4 ^(Cci, Mg)(P0

4 )g.

In our crystal it appears that the variable

site was about equally filled with Ca and

Mg, thus yielding an empirical formula

Ca^ I- Mg^ c- (PO^)^. After the final refinement, the characteristics of this site will

be compared with those of the other Ca and

Mg sites in an attempt to understand why one

site behaves so differently.

2. When the model of the structure is viewed

along [ 201 ], an unexpected degree of pseudosymmetry is observed. When the difference

between Ca and Mg and relatively small shifts

in the positions of the atoms are ignored, an

approximate three -fold inversion axis can be

seen parallel to [ 201 ] and a basic pseudo-cell

can be defined with the dimensions a* = 8.68

(along [101]), b

y = 9.99 (along [010]), and

c

' = 14.88 A (along [201]). This cell has

pronounced orthohexagonal character.

3
When this cell is compared with that of

glaserite, NaK^SO^^, a definite relationship between the two structures can be seen.

The structure of glaserite consists of two

kinds of columns, one entirely made up of

cations and one with cations and anions

alternating. In the Ca^

5 Mg^ ^(Ca,

structure, similar columns are present

parallel to c

'

. The cation columns are

essentially the same as in glaserite, but

every other cation site is vacant in the

anion-cation columns. This unexpected

relationship between these two compounds

provides new insight into structural

relationships among calcium phosphates.

Glaserite is known to be closely related to

CaK^lUPO^)^ and CaKPO^, and has a formal

structural relationship to hydroxyapatite.

Glaserite has a cation- toanion ratio of 12/6;

the cation-vacancy mechanism in the calcium

magnesium phosphate makes it possible to

extend the structural relationship between

glaserite and hydroxyapatite to a salt with

a cation-to-anion ratio (9/6) lower than

that ( 10/6 ) in hydroxyapatite.

3. One of the main objectives in our study of

Ca^

^ Mg^ j_(Ca,Mg) (P0^)^ is to find relationships between it and the two better known

tricalcium phosphates, (3 -Ca^ (PO^) 2, and

$-Cag(P0^)2 , neither of which has a known

structure. The dimensions of the pseudoorthohexagonal cell strongly suggest that

there are relationships between it and the

Mg)(r°

4 ) 6

4
two tricalcium phosphates. For example,

whitlockite is known to have a prominent

pseudo-repeat in its structure of c_' /4

(i.e., 3.72 A). The a unit cell dimension

o of whitlockite, 10.43 A, is quite similar

in length to the b

‘ axis of Ca^

^ Mg^

(Ca,Mg) (PO^)^, 9.99 A. The dimensions of

the pseudo -ortho rhombic cell of a-Ca^PO^)^

a = 15.22, b = 20.71, and c = 9.11 A, are

also similar to those of the pseudoorthohexagonal cell of Ca^ c Mg^ ^(Ca,Mg)

(PO^)^. It now appears that no two of

these compounds are iosstructural , but it

is likely that they will share important

structural features.

Computer Programming for Crystal -Structure Analysis:

Extensive use of computers is necessary in all stages of crystal

structure analysis. The computer programs are constantly being

improved and adapted to new situations. For this reason, a considerable amount of time is spent in developing and revising

programs

.

The results of a crystal-structure determination are most

conveniently expressed in the form of a diagram which shows both

the positions of the atoms and their thermal vibrations in the

form of ellipsoids. Such a program, "The Oak Ridge Thermal

Ellipsoid Program," has been adapted to the output of programs

used at the NBS for the earlier stages of calculation, and is

now being used to draw diagrams to accompany manuscripts submitted for publication.

A computer program recently written by W. R. Busing of

Oak Ridge National Laboratory has been tested in the calculation

of the lattice energies and concomitant parameters of Na

2

C02 *H20

5
The program uses least squares methods to calculate new

parameters from initial estimates. The present conclusion is

that least squares refinements in which the parameters are all

equally weighted do not refine to the minimum energy. A new

version with provision for weighting has been received and

will be used in further tests.

One of the difficulties with the x-ray analysis of the

structures of crystals containing hydrogen is that the hydrogen

positions are systematically too close to their bonded atoms.

Computer programs have been written to impose an idealized

water geometry on the approximate water geometry obtained in

the determinations of crystal structures of hydrate salts and

to place a hydrogen in a realistic position with a reasonable

X-O-H bond angle and 0-H distance for hydrogens on such groups

as PO^ groups. In this way it is possible to obtain a more

realistic set of hydrogen positions and to test the rationality

of a proposed hydrogen bonding scheme.

Petrographic Study of Pineal Calcifications . Concentrically micro banded spherite calcifications, calcospherites,

are a common occurrence in the pineal gland, early dentine,

aortic walls and elsewhere. Since the calcospherite is

essentially the sole form of calcification in pineal gland

parenchyma,, these calcifications are good subjects for the

characterization of the calcospherite form.

The mineral phase present in pineal calcifications has

been identified with x-rays by earlier workers to be hydroxyapitite. Acid treatment evolves CO

2

and the possibility of

a carbonate apatite has been suggested. The present study

employed optical and electron microscopic techniques, x-ray,

thermogravimetry, electron probe and chemical analysis.

A carbonate hydroxyapatite is confirmed as being the predominant mineral phase present. While this work will be subsequently submitted as a detailed report, the following

6
conclusions can be given at this time: (1) Bovine and human

calcifications occur as concentrically microlaminate bodies in

which protein-rich (collagen or keratin) zones alternate with

apatite-rich zones. (2) Apatite crystallites commonly are

approximately 200-220 A wide. These crystallites have preferred

orientation of the C-axis perpendicular to the micro laminae.

(3) These calcifications appear to be mineralogically similar to

enamel

.

4.2. Solubilities of Calcium Phosphates

Sponsor : ADA, NXDR

Previous reports have described the results of solubility studies

on the dicalcium phosphates, CaHPO^ and CaHPO^ ’ 21^0, which are now

completed, and similar studies on hydroxyapatite, Ca^0H(P04 )^,

and whitlockite, (3-Ca

3 (P0 4

) 2

, which are still in progress.

Solubility measurements on hydroxyapatite at 37 °C were described

in the previous report. Similar measurements (from undersaturation) at 5°, 15° and 25°C have now been completed. Some of these

data remain to be processed. The present values for the solubility constants are 3.20 x 10” 59

, 2.74 x 10” 5 ' , 2.60 x

10”'" and

2.12 x 10“ 69 for 5°, 15°, 25° and 37°C» A log vs 1/T plot of

these data defines a straight line but the standard enthalpy and

entropy of dissolution derived from this plot differed from those

obtained from calorimetric data by more than the expected experimental errors. The reason for this difference is being investigated.

During the present report period processing of the solubility data on whitlockite at 25°C was completed and measurements

were extended to 5°C and 15°C. The solubility constant at 25°C

is 1.38 x 1CT 29

/ the data at 5° and 15°C are still being processed

but they already indicate that whitlockite has a negative thermal

coefficient of solubility as predicted in the previous report.

The data at 25° C permitted calculation of the formation constant

7
for the ion pair [CaHPO^] 0 which was found to be in good agreement with the constants derived from the solubility data on

CaHPQ^ and CaHPO^ * 2^0 . The metastability of whitlockite is

such that a similar test of the formation constant for the ion

pair [CaH

2

P0

4

] + could not be made.

We have now made solubility studies on four preparations

of hydroxyapatite.

1. Hydroxyapatite prepared by titrating Ca(0H)2 with

HgPO^, and stabilizing by extraction and by boiling

for a period of 10 days; These are the results

described above, and comprise the most extensive

series of solubility measurements made on a single

preparation of hydroxyapatite.

2. Hydroxyapatite prepared as above by Dr. Avnimelech in

connection with his surface studies: The solubility

product of this material was subject to a relatively

large error, within which the two sets of measurements are in agreement.

3. Hydroxyapatite precipitated slowly and then heated in

steam at 1000°C: Contrary to expectations based on

the fact that the particle size of this material was

significantly larger than either of the above two,

its solubility product was greater by almost an order

of magnitude (K (25°C) = 2.60 x 10~ 59 vs 3.73 x 10“ 58

sp

for the steam-heated material). The difference was

attributed to the heat treatment which may have

introduced extra free energy into the crystals through

lattice defects or some other mechanism.

4. Hydroxyapatite heated in air at 1000°C: This material

had an apparent solubility product 103 times as great

as the steam heated material. This difference was

attributed to the fact that under the conditions of

heating, the hydroxyapatite is known to be unstable and

8
will tend to form whitlockite and tetracalcium

phosphate, Ca^C^PO^^. It is believed that when

the air heated material is returned to an aqueous

solution for the solubility measurements, these

products reform hydroxyapatite, but in such

a poorly crystallized form that it yields an

unusually large solubility product.

The ability to detect differences in the free energies of

various hydroxyapatite preparations through solubility measurements, if valid, constitutes a valuable means for the study of

one of the most basic properties. We cannot, however, rule out

the possibility that some of the differences must be attributed

to systematic errors arising from variations in the experimental

conditions of the varioussets of solubility measurements. Two

types of experiments will be used to study this phenomenon:

(1) The solubilities of hydroxyapatites prepared in different

ways, or exposed to various thermal treatments, will be measured

simultaneously. (2) Solubility measurements will be made in

which equilibrium is approached from conditions of supersaturation, In each of the four studies on hydroxyapatite described

above, equilibrium was approached only from undersaturation.

Equilibrium was approached from both directions in the studies

with the dicalcium phosphates, thus added considerably to the

veracity of the results.

Our initial attempt to approach equilibrium by passing a supersaturated solution through a column of hydroxyapatite

defined an isotherm which we now know corresponds to that of

whitlockite. Apparently, the precipitation reaction within the

column resulted in the formation of whitlockite instead of

hydroxyapatite. More recent experiments, in which batches of

hydroxyapatite were rotated for longer times in a thermostatic

bath, yielded points which fell fairly close to the hydroxyapatite

isotherm, but they represented solubilities which are believed

to be significantly different from those obtained from under9
saturation. We are investigating whether this apparently

greater solubility relates to a property of the hydroxyapatite

formed in the equilibration process or to the formation during

this process of a small amount of a metastable calcium phosphat e

To further the studies on hydroxyapatite, a new method of

preparation was developed which is based on the hydrolysis of

a-Ca

3

(P04

) 2

. The hydroxyapatite crystals obtained in this way

appear to be the largest (up to 50pm) yet obtained in a

synthesis carried out at atmospheric pressure. One of the

advantages of this method is its simplicity since it involves

direct hydrolysis of an easily prepared material, a-Ca^ (P0

4

) 2

, in boiling water. Although the crystals derived in this way

are well formed, voids paralleling the £ axis are common. In

this respect they resemble crystals grown in hydrothermal bombs.

This suggests that the growth mechanisms in the two procedures

are similar. In common with so many other methods for the

preparation of hydroxyapatite, the infrared spectra of these

crystals show some contamination by acid calcium phosphates.

It is believed that, under the conditions of preparation,

octacalcium phosphate is the only acid calcium phosphate more

insoluble than a-Ca

3 (P0 4

) 2

. Preliminary solubility measurements on hydroxyapatite

prepared in this way indicate that it has an unusually high

solubility. Investigations are continuing to learn if this

relates to an inherent instability of the hydroxyapatite prepared in this way, or whether it is caused by a contaminant.

The same technique is being used to synthesize f luorapatite,

CA

3F(PC»4

) 3

which is to be used in solubility studies. This

material should have advantages over material prepared at high

temperatures, the method most commonly used up to now, because

of the possibility that fluorapatite prepared at a high temperature may not be representative of material formed under

physiological conditions.

10
Thermodynamic Properties of Calcium Phosphates , The solubility constants for the various calcium phosphates can be used

to calctilate their standard free energies of dissolution. When

these are combined with cryogenic and thermal data, it then

should be possible to calculate standard free energies, enthalpies

and entropies of formation. There are, however, inconsistencies

in the data in the literature caused by experimental errors,

variations in the experimental conditions, and the use of illdefined materials. Attempts are being made to reduce the errors

in the standard thermodynamic quantities and to make a self

consistent set of data for the calcium phosphates by using

solubility data. Such a set was reported at the NBS Symposium on

Structural Properties of Hydroxyapatite and Related Compounds,

The need for more precise heats of dissolution data is recognized,

however, and we are making plans to obtain such data experimentally.

The effect of temperature on the solubilities of the dicalcium phosphates showed an unexpected behavior. While the

solubility product constant of CaHPO^ decreased monotonically with

temperature (i.e., increased linearly with 1/T), the constant for

CaHPO^^^O showed a maximum at about 25°C, Initially, we thought

the behavior of CaHPO^^^O related to phenomena occurring in the

solution, but the contrasting behavior of the two salts pointed

to the properties of the solid phases as the source of difference

in temperature dependence. Calculations were made, therefore, of

the temperature dependence of solubility for the two salts using

heat capacity data. It was found that CaHPO^ * 2^0 should have

a maximum in the solubility product constant near where it was

found experimentally; CaHPO^ also shows a theoretical maximum,

but at a very low temperature (~ -70°C), thus explaining why

the solubility constant decreases monotonically in the range

5° to 37° C

.

Ionization of HF ; In a test of the reliability of the

fluoride electrode for use in solubility measurements, a discrepancy was found between calculated and measured activities of the

11
fluoride ion. The experimental conditions were such that this

could not be explained by inadequacies in the calculation of

ionic activities. After a series of experiments, it was concluded that the discrepancies had to be attributed to errors

in the reported values for the dissociation constants of HF

and HF^ . In view of the importance of the fluoride ion in

the prevention of caries, a study of these dissociation constants was initiated. The experimental work on about 100

solutions has been completed, the data are being processed,

and the indications are that they will yield improved values

for the dissociation constants.

Diffusion in Calcium Phosphate Solutions : In a previous

report, we described the plans to use a diaphragm-type diffusion cell containing a thin cellulose membrane. Trials with

this cell, however, showed that the thickness of the unstirred

solution layer adjacent to the membrane was comparable in magnitudeto the thickness of the membrane, so that relatively

small variations in solution compositions or experimental conditions would have large effects on the diffusion rates. Metal

membranes should be unaffected by formation of electrostatic

charges on surfaces, but trials with a thin silver membrane

indicated that it was chemically unstable as shown by its

accumulation of P

32 and Ca45 radioactivity during the experiments. A new, thicker, diaphragm of sintored platinum has been

obtained and it appears to be free of the above difficulties.

4.3. Study of Dentinal and Enamel Surfaces

from Sorption Isotherms

Sponsor: NIDR

Little information is available regarding the presence and

relative abundance of acidic and basic functional groups at the

surface of dentin and enamel. During this period a study was

initiated to determine quantitatively the presence of those

sites that are accessible to CO

2

or NH^ . These gases were

12
selected since they are likely to react chemically with acid

or basic groups present in dental collagen. Initially, the

adsorption isotherms at temperatures in the vicinity of the

respective boiling points of the adsorbates will be determined

and from these data the isosterlc heats of adsorption will

be obtained. It Is hoped that results of this investigation

will yield valuable information with respect to the surface

composition of odontic powders.

Because of the molecular interaction and finite size of

the molecules at the temperature of the experiment, correction

terms for the non-ideal behavior of the gases must be applied

to obtain the exact amount of gas adsorbed on the surface of

the substrate. A literature search indicated that tables containing these correction terms are out-dated and incomplete.

Therefore, more accurate methods for calculating and comparing

non-ideal gas correction terms for use in this type of work

were developed and a manuscript summarizing the results of this

investigation was prepared for publication.

A cryostat was designed and built to obtain adsorption

data at different temperatures within ± 0.5°C. The final design consisted of a liquid nitrogen heat sink to which excess

heat was conducted thermally. A solid copper rod bent into an

inverted U-tube with appropriate radiating vanes cooled the

adsorption vessel sufficiently so that additional heat was required to sustain a desired temperature. The other arm of

the thermal conductor was Immersed in a Dewar surrounding the

sample. The Dewar contained a low freezing pt liquid such as

trichlorethylene (or toluene) in the stirrer, thermocouple,

heater, etc. For controlling the temperature at the desired

cryogenic setting, a hand-wound heater enclosed in a glass

tube was used. The temperature regulator was responsive to

changes in resistance of a temperature-sensing probe. A 5000-

ohm carbon composition resistor had a sufficiently high

13
temperature coefficient of resistance to make it sufficiently

sensitive to function satisfactorily for this purpose.

The first series of experiments were performed on anorganic teeth in order to obtain a control (free of collagen)

with which to compare the results that will later be obtained

with dentin and enamel. Using CO2 as the adsorbate at dry

ice temperature, it was found necessary to pre-condition the

sample by purging in helium at 110°C for several hours. Before

this pre-treatment had been done, the isotherms of samples degassed at room temperature showed significantly lower adsorption

and were not reproducible. Subsequent evacuations at room

temperature between successive adsorption experiments gave

reasonably reproducible isotherms.

Preliminary findings indicated isosteric heats of adsorption decreasing to a constant vlue of about 4 kilocalories per

mole as surface coverage increased to about 0.7 of a monolayer.

Besides this investigation, work on two papers "Adsorption

Theory Interpretation of Ca v and Y Exchange with Hydroxyapatite" and an "Empirical Method for Optimizing the Number of

Terms of a Fitted Power Series" was completed. The manuscripts

of these studies were written up and are being reviewed.

4.4. Mechanical Behavior of Skin

Sponsor: NIDR

Using rat skin as a model system for the mechanical behavior of the mucous membrane of the oral cavity, preliminary

stress-strain determinations have been carried out on adult

rat skin (500 gm male rats). The elastic behavior was found

to be the predominant phenomenon over short time intervals

being in the approximate ratio of 5/1 in relation to the time

dependent phenomenon for time intervals less than 5 minutes.

The elastic behavior was found to be nonlinear in its behavior

which is in agreement with other workers who ran conventional

stress-strain curves at a given strain rate. However, their

14
stress-strain data, unlike ours, contained some viscoelastic

contribution, as well as elastic. This viscoelastic contribution has nearly been eliminated from our data, but with

its elimination, nonlinearity is still observed. The work of

R. D. Harkness and coworkers described the stress-strain behavior of skin by a power function, while V. Wright and coworkers described their stress-strain data by means of an exponential function. With the nearly complete elimination of

the viscoelastic contribution in our determination, we found

that exponential description is more closely representative

than the power function description, but the data obtained

thus far indicate that neither description is representative.

The elastic behavior of stomach skin was found to be readily

distinguishable from that of back skin on the same animal.

4.5. Thermal Expansion of Tooth Structure

Sponsor: USAF, VA, NiDR

Investigations of the thermal expansion of human tooth

enamel and dentin are being undertaken because of the importance of this property in the stability of restorations in

teeth. This is of particular importance in regard to the adhesive restorative materials which are the objective of numerous

studies in this and other laboratories. Differences between

the thermal expansion of tooth and restorative materials will

tend to separate the restoration from the tooth surface. If

adhesion is maintained, thermal expansion differences may tend

to cause fracture in tooth or restoration if the viscoelastic

characteristics of the materials do not accommodate the thermally produced dimensional changes.

The determination of accurate thermal expansion values

is complicated by the small size of the specimens that can be

obtained from natural teeth. Results obtained in the present

study have verified the wide variability previously found in

15
the expansion of dentin. Present objectives are to determine if these variations can be related to differences between teeth, effects of orientation, the viscoelastic characteristics of dentin, variations in the contributions of

the organic and inorganic portion of the material to the expansion of the composite, or to other factors.

4 . 6 . MetallurgySponsor: ADA, USAF, VA^.MlOR

The objective of our metallurgical research program is

to provide information which can be used either directly or

indirectly to develop new alloys for dentistry or to modify

present alloys in order to obtain an enhancement of such

properties as strength, ductility, and ease of fabrication.

In developing new or Improved alloys, the metallurgist is

greatly assisted by a knowledge of the relevant phase diagrams

which summarize graphically the structure of a given alloy

system as a function of temperature and composition.. Phase

diagrams are readily available for many base-metal alloy systems

since many of these alloys are widely used in industrial applications. Alloys of the noble metals, however, find only

limited industrial use although they are widely used in dentistry where cost is not a major deterrent. Phase diagrams for

noble metal alloys are, therefore, frequently unavailable or

unreliable, thus severely retarding the use of noble metals

in alloy combinations with each other or in combination with

base metals.

In order to promote a more favorable consideration of

the many desirable properties imparted by the use of the noble

metals, we have undertaken a systematic exploration of binary

phase diagrams in which a noble metal is combined with various

base metals. During the past six months, we have continued to

prepare alloys spanning the entire range of composition in

the binary systems Cr-Pt, V-Pt, Cr-Ir, and Cr-Rh. These alloys

16
are being subjected to extensive heat-treatment at various

temperatures, followed by a metallographic examination of

their microstructures and analysis by x-ray diffraction and

electron microprobe methods.

Despite experimental difficulties in maintaining high

purity in these alloys, we have succeeded in establishing the

general outline of the binary phase diagrams for the systems

Cr-Pt and V-Pt and are now engaged in determining the exact

temperatures and compositions at which critical structure

transformations occur.

The Cr-Pt system contains only one intermediate phase

having the approximate formula Cr^gPt2Q and a composition range

of several percent. Chromium is extensively soluble in platinum

and initially Increases its melting temperature to a maximum

in the vicinity of J>0 atomic percent chromium followed by a gradual decrease in liquidus and solidus temperatures at higher

chromium contents. Atomic ordering analagous to that which

occurs in the copper-gold system is observed over a wide range

of compositions and is associated with ferromagnetic behavior.

The V-Pt system contains four intermediate phases at the

approximate compositions VPt^, VPtg, VPt, and V^Pt with the

latter phase being stable over a rather extensive composition

range on both sides of the “ideal" composition. The phase VPt

is of particular interest since its behavior may lead to important developments in regard to desirable mechanical properties. Its crystal structure can be viewed as an orthorhombic

distortion of the ordered tetragonal CuAu type structure which

is responsible for the hardening of dental gold alloys. The

phase VPt forms by means of an ordering reaction from the

platinum solid solution at a rather high temperature (not yet

determined, but believed to be about 1500°C). When the phase

VPt is subjected to severe deformation at room temperature,

its crystal structure changes to that of either the CuAu type

or to that of a disordered face-centered cubic structure

17
depending on the extent of cold-working. These structures

are apparently metastable since strain-annealing results in

a transformation to the original orthorhombic structure.

When alloys such as V^Pt^ are annealed in the two-phase

region between the face-centered cubic platinum solid solution and the phase VPt, one obtains ribbons of the phase VPt in

the face-centered cubic solid solution matrix phase. Such a structure suggests possible strengthening by means of the

well-known "whisker" or "fibre-reinforcement" mechanisms. I n

addition, since the phase VPt transforms as a result of coldwork to the usually ductile face-centered cubic structure,

one might expect a possible enhancement of ductility as a result of cold-working contrary to conventional behavior. Confirmation of such behavior must await a comprehensive investigation of the mechanical properties of these alloys, but such

an investigation is beyond the scope of our present phasediagram studies.

In order to complete our work on these alloy systems, we

will need to study the observed transformations using dilatometric (thermal expansion) and thermal analysis techniques.

This requires specialized equipment for measuring transformation

temperatures while maintaining sample composition and purity.

It appears that it may be necessary to contract for such work

in other laboratories since the necessary equipment is not now

available at the National Bureau of Standards.

4.7. Adhesion to Dentine and Enamel Surfaces

Sponsor: USA

A study of potential adhesive liners that bond to tooth

structure and plastic restorative materials has been initiated.

One aspect of this work is the ^mthesis of tailor-made molecules that may be capable of bonding to tooth surface and at

the same time are compatible with the well-accepted dental

resins. The bonding between bovine enamel or dentin and a

18
commercial cold-curing poly(methyl methacrylate) restorative

resin was determined for a series of these newly synthesized

compounds. Treatment of enamel with 10$ solutions of (l) equimolar condensation product of 2-hydroxyethyl methacrylate and

tetrahydrofurane-2,3,4, 5-tetracarboxylic acid (HEM-THFTC),

(2) partially polymerized 4-hydroxy-3-methoxystyrene, (3) Nphenylglycine-glycidyl methacrylate (NPG-GMA), and (4) methacrylate chromic chloride (Volan L) in acetone, increased

significantly the adhesive properties when the materials were

stored in a 100$ relative humidity environment. However, when

stored in water at 3l°C for 24 hours, the bond strength was

decreased markedly. Failures -were always of an adhesive

nature. Pretreatment of the surface with transition metal

chloride did not improve bonding. None of the compounds

studied adhered to dentinal surfaces. No bonding was obtained

with trans -isoeugenol , 4-hydroxy-5-methoxystyrene and its

methacrylate ester, 4-hydroxy- 3

,

5-dinitr os tyrene, allylphosphonic

acid and its hydroxyethyl methacrylate ester and N-allyliminoN,N-dimethylene diphosphoric acid.

Another study dealt with treating enamel and dentinal surfaces with inorganic compounds, especially halides, and determining the bond strength of the tooth surface acrylic resin

interphase. In this test, the enamel surface was cleaned with

a 5$ aqueous solution of tetrahydrofurane, 2, 3, 4, 5-tetracarboxylic

acid (THFTC) by swabbing for 15 seconds. After rinsing with

water, the surface was dried and wetted with 4 n NaOH and excess

base was wiped off after 15 seconds. A 5 to 10$ aqueous solution of the salt was applied and excess was removed after 15

seconds. The cold curing resin was then applied and the

samples immersed In water at 57°C for 20 hours. Treatment of

the enamel surfaces with many halide solutions increased the

adhesion. For instance, treatment with FcCl^^f^O^ A1C1|*6H20

and ammonium vanadate gave tensile strength of 2Q- Kg/cm ,

19
2 3 4o ± 3 kg/cm and 29 ± 12 kg/cm , respectively, compared to

2

5 ± 2 kg/cm for untreated teeth. The tensile adhesion of

AlCl-j treated enamel surfaces was dependent on the restorative used, decreased on substituting IN NaOH or 10% Na2

C0-j

for the 4n NaOH, and decreased with decreasing pH of the

aqueous solution in which the assembly was stored prior to

the adhesion test. Unfortunately, the bond strength became

negligible on storage in water for 30 days. Dentinal surfaces

did not adhere to the acrylic restorative. With 10% ammonium

vanadate solutions, pretreatment of the enamel with 1.5 N NaOH,

wiping off excess precipitate, and acidification with 0.5 N

HC1 gave optimum results. Tensile adhesion values varied

little with the pH of the aqueous solution (4.3 to 9.5) in which

the test specimens we re stored.

4.3. Development of an Adhesive Restorative Material

Sponsor: ADA, USAF, VA. V'-HOR

With the objective of improving the quality and sophistication of dental materials, the development of an adhesive

restorative material was further pursued. The material under

investigation, depending on the physical, chemical and biological properties that can be realized, will have applications in

preventative as well as restorative dentistry.

Thi3 composite material utilizes (l) a maximal amount

of particulate reinforcing filler together with (2) a minimal

amount of cross-linking, polymerizable organic resin. An

organofunctional silane coupling agent (5) promotes bonding

at the filler-polymer interfaces. A surface-active comonomer

(4) improves bonding at the interface between the composite

material and the tooth surface.

With regard to the reinforcing fillers for the restorative material, a manuscript has been accepted for publication

(February 1969) by the Journal of Dental Research covering

our development of a special barium fluoride glass. It has

20
been Informally reported that three dental manufacturers

have sought this glass from an industrial glass manufacturer, one company placing an order for 1000 pounds of it. We

have just developed a similar x-ray-opaque glass that is free

of fluoride, since fluoride undesirably raises the thermal

expansion of the glass (from about 3.3 to about 7.3 ppm/°C )

.

The crosslinking, dimethacrylate monomer system has

been described in a manuscript that is now being prepared

for submission to a journal. The rapid polymerization of

this resin is accomplished with an amine-peroxide initiator

system. In the past six months, work was continued in investigating the effect of moderately high molecular weight

alkyl substituents on the properties of aromatic amine accelerators. Compounds under investigation which have been

synthesized are N-dodecyl-N-methyl-p-toluidine, N-dodecylN -methyl

sym-m-xyll dine and N,N-dimethyl-p-dodecyl aniline

.

These compounds were compared for ability to accelerate

polymerization and remain color stable (in specimens incorporating them) during exposure to a sunlamp for 2k hours.

They were compared also to their corresponding lowermolecular-weight homologs N, N-dimethyl-p-toluidine and N,Ndlmethyl- sym-m-xylidine. N,N-dimethyl-p-t-butylaniline

containing a tertiary alkyl substituent on the aromatic ring

was also synthesized. This compound was needed to investigate the effect of alkyl ring substituents not possessing

hydrogen atoms alpha to the benzene ring. The initial color

and the color stability of specimens prepared with this amine

compared favorably with those prepared with the others. The

results suggest that a higher-molecular- weight homolog of

this compound should be synthesized and evaluated.

The corranerci ally- avail able organofunctional silane

coupling agents appear to be satisfactory for this application. However, there was suggestive evidence that silica

surfaces formed at high temperature need to be "re-hydroxylated" to get the best silane surface treatment. Since the

2l
filler-resin interface is of such great importance to the

properties of particulate composites, this should be studied

further.

The most emphasis has been placed on perfecting the

surface-active comonomer or adhesion-promoting primer system.

The major problem under attack is the lack of understanding

of why the stability of the formulations during storage is

unpredictable. Efforts were made to understand the mechanism

of instability and to formulate stable solutions of the adhesion promoting coupling agent, NPG-GMA (the adduct of Nphenylglycine and glycidyl methacrylate). Special emphasis

was given to the synthesis and evaluation of surface-active

methacrylate monomers analogous to NPG-GMA, but with modifications that (l) lower the solubility parameter (allowing

solubility in liquid dimethacrylate monomers), and (2) elucidate the factors involved in storage stability in the liquid

state. Approximately one-third of the 15 syntheses to these

ends were considered successful.

During the last six months, three additional composite

restorative materials, n Adaptic" (Johnson & Johnson),

"Blendant" (Kerr) and !, DFR" (Surgident), have been placed on

the market by dental manufacturers. These, as were two

previously available materials, "Addent" (3M Co.) and "Dakor"

(Caulk), have been based on, and result from, the research

done in this project. A very modest clinical comparison is

planned to evaluate the value of X-ray opacity for diagnostic

and prognostic procedures.

4.9. Composite Resin Material for

Temporary Posterior Restorations

Sponsor: USA

The physical properties of a resin-silica composite

material developed for use as a temporary posterior restorative material have been investigated. It is intended that

this material withstand the forces of mastication for at least

22
two years, be radio opaque so that it is possible to distinguish between the material and remaining or recurring

caries and that it exhibit minimal pupal irritations. Ease

of manipulation and minimal cavity preparation are also desired. Esthetic properties are of secondary importance.

The formulation is based on a liquid ternary eutectic of

isomeric phthalate esters of 2 hydroxyethyl methacrylates

and a resin filler consisting of irregular particles of

vitreous silica and an x-ray opaque glass.

The material can be spatulated on a slab in one and onequarter minutes at a ratio of 1.35g of powder to 0.4 ml of

liquid. Setting time is five minutes, and a maximum temperature rise of 5 to 7°C is observed between 4 and 6 minutes

after the beginning of mix. Shrinkage on setting was found

to be approximately 0.4^.

Tensile strength of the composite at 5°00 to 6000 psi

is near that of dental amalgam, while the compressive strength

at 25,000 to 30,000 psi is approximately one-half that of

amalgam. Thermal expansion is approximately 30 to 35 parts

per million per °C between 25 and 60 °C

.

Water sorption of the composite is considerably below

the maximum ADA specification limit for denture base resin.

Water solubility is low in comparison to silicate cement.

Radio opacity is sufficient for distinguishing between

the cement and caries. Color stability is good. Optical

opacity is near or above the maximum specification limit for

silicate cement.

Properties of the material appear to be adequate for

temporary posterior restorations. Clinical evaluation is

needed before routine use can be recommended.

4.10. Zinc Oxide-Eugenol Type Restorative Materials

Sponsor: USAF, VA, USA

The investigation to develop clinically improved

23
temporary restorative materials with good tissue tolerance

was continued. During this period, emphasis was placed on

reducing the in vitro solubility of these materials and

studying the effect of different polymeric fillers as reinforcing agents. Formulations containing bentonite-coated

rosin have higher compressive strength, but higher solubility,

than those containing hydrogenated rosin. Substitution of

poly(ethyl methacrylate), polyacetal or butadiene-styreneacrylonitrlle terpolymer for the methyl methacrylate copolymer

powder lowered the tensile strength. However, formulations

containing polycarbonate powders as reinforcing agents look

promising. Coating the surface of aluminum oxide with a

polymer film instead of the incorporation of polymer powder

in the formulation does not improve the physical properties

of the resulting cement. A nine-month clinical study of a

limited number of restorations, including those subject to

heavy occlusal stresses, showed no sign of wear during this

period of observation.

4.11. Rapidly Fabricated Surgical Splint

Sponsor: USA

The investigation of suitable compositions for development of a rapid splint for fractures of the mandible and

alveolar fractures has been completed. Best results were obtained using 50% methyl methacrylate copolymer, 50% CaCO^

powder and a liquid consisting of methyl methacrylate containing 0.2% N,N-dimethyl-p-toluidine . If a hardening time of

less than 10 minutes is desired, 0.2% of benzoyl peroxide can

be added to the powder. The material has been used successfully in a variety of applications by the U. S. Army here and

abroad. A detailed report describing the results of this investigation Is appended.

24
b.12. Grafting of Polymers to Hard and Soft Tissues

Sponsor: USA

A graft copolymer is comprised of a high molecular

weight backbone to which a second polymer is attached at

intervals along the chain. This chemical modification will

alter the properties of the substrate according to the type

of monomer unit selected as a graft. Grafting procedures

produce surface modifications of natural and synthetic polymers and yield tailor-made products with preselected physical

and chemical properties. If successful, grafting may offer

an attractive technique of bonding chemically to collageneous

surfaces such as dentin.

An extensive review of the literature indicates that

much progress has been made during the last two years in the

grafting of long chain polymeric compounds to natural products,

including fibrous proteins such as wool or corium collagen.

Thus, it appears feasible that chemical pretreatment of hard

and soft tissues may facilitate the grafting of polymerizable

monomers to such tissues.

Detailed information of the optimum conditions under which

vinyl monomers such as acrylonitrile or methacrylate esters

can be grafted to collagenous surfaces in an environment which

could be duplicated clinically are not available. Thus,

initial investigations utilize a model system: finely divided

purified collagen suspended in water, acrylonitrile monomer

and ceric ammonium nitrate as redox Initiator. Since collagen

contains alcoholic groups such as hydroxyproline, hydroxyl ysine, serine and threonine, the presence of free radicals on

the backbone Initiates polymerization at these sites to produce a graft copolymer.

TV v Ce +

^y'RCHoOH < Ceric alcohol complex

^ TTT _i7 Ce±xx

+ H RCHOH

free radical

25
The resulting product is then washed with water to remove

acrylonitrile monomer and extracted with dimethylf ormamlde

to dissolve any polyacrylonitrile. Definitive identification of grafts onto the collagen surface using multiple total

reflectance infra-red techniques awaits the receipt of a 25

reflection attenuated total reflectance equipment ordered

recently. If the graft sites cannot be characterized by this

technique, "labelled1

’ monomers, such as chloroacrylonitrile

or 2-chloroethyl methacrylate, will be employed as grafting

agents

4.13. Clinical Research

Sponsor: ADA, USAF, Vk } NlDft

Methods were formulated for clinical evaluation of direct

filling materials developed in the laboratory. Scales of

reference for grading surface texture or roughness, lack of

marginal flushness, marginal notch or crevice and other qualities of restorations were prepared. Composite restorations,

50 with and 50 without the NPG-GMA coupling agent, have been

placed and will be observed to see if there is a discernible

difference in serviceability.

Investigations of the effectiveness of radio opacifying

agents In direct filling and denture base resins are in

progress. The effects of the radio opacifying agents on

mechanical and optical properties and other characteristics

of the materials are also being studied.

4.14. Specification and Advisory Activities

Sponsor: ADA, NIDR, USAF, VA

Although the Dental Research Section conducts very little

research oriented directly toward specification development,

members of the staff, through activity on various committees

and otherwise, contribute significantly to the formulation of

26
national and international specifications for dental materials. Members of the staff are active on 11 of the l4 subcommittees formulating new or revised specifications for

the American Dental Association, serving as chairmen of 3

and secretaries of 7 of the committees. In addition, members of the staff hold the chairmanships of the Specifications

Committee of the Dental Materials Group of the International

Association for Dental Research; the United States of America

Standards Institute Committee for the International Organization for Standardization Committee for Dentistry; and the

Commission on Dental Materials, Instruments, Equipment and

Therapeutics of the Federation Dentaire Internationale.

Advisory and consultative activities included lectures

to dental groups in this country and abroad, review of numerous papers for publication in research Journals and reviews

of grant applications including site visits for the National

Institute of Dental Research.

27

USCOMM-NBS-DC