NATIONAL BUREAU OF STANDARDS REPORT

311.05— NBS PROJECT NBS REPORT

1 L— 3110560 June 30, 1971 10 600

311.05-

11-3110561

REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

Progress Report

January 1 - June 30, 1971

The dental research program at the National Bureau of

Standards is carried on in cooperation with the American

Dental Association- 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 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.

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

5 Office of the Director, National

iy the Government agency for which

opies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
.
TABLE OF CONTENTS

Page

1. Introduction 1

2. Reports Issued 1

3. Papers Published 2

4. Work in Progress 3

4.1. Determination of Crystal Structures 3

4.2. Physicochemical Studies of Apatite and Related 7

Inorganic Substances

4.2.1. Crystal Chemistry 7 4.2.2. Solubility Studies 9

4.2.3. Topical Fluoridation 12

4.2.4. Calorimetry 13

4.3. Surface Characterization Through Adsorption Studies 14

4.4. Mechanical and Physical Properties of Oral Structures 15

and Restorative Materials

4.4.1. Dimensional Changes in Teeth Components 15

4.4.2. Mechanical and Electrical Properties of Hard 16

and Soft Tissues

4.5. Collagen-Water Interaction 18

4.6. Dental Metallurgy 21

4.7. Dental Ceramics 23

4.8. Dental Investment Studies 24

4.9. Grafting of Monomers to Collagen and Soft and Hard 25

Tissues

4.10. Centrifugal Tensile Strength Testing 27

4.11. Adhesive Direct Filling Materials 28

4.12. Synthesis of Monomers with n-Base Characteristics 31

4.13. Three-Dimensional Panoramic Dental X-Ray 32

4.14. Correlation of Dimensional Change on Hardening in Amalgam 32

Restorations with Marginal Leakage

4.15. Clinical Research 33

4.16. Specifications and Related Activities 33

4.17. Fiftieth Anniversary Symposium Proceedings 34

Appendix:

NBS Report 10421 Research Technics Used in Evaluating Dental Materials.

NBS Report 10422 Determination of the Degree of Polymerization of a Polymeric Aromatic Amine from NMR Data Using a "LeastSquares" Approach.

NBS Report 10441 The Crystal Structure of CaKAsO. *8Ho 0.

4 2

NBS Report 10442 A Stabilizing Comonomer: I. Synthesis and Confirmation

of Structure.

NBS Report 10521 Influence of Alloy Particle Size and Shape.

i
NBS Report 10522 Dimensional Changes in Amalgam.

NBS Report 10578 Confirmation of the Structure of Previously Reported

Tertiary Aromatic Amine Accelerators with Molecular

Weights Above 400.

NBS Report 10579 A Refinement of the Crystal Structure of CaHPO^

(Synthetic Monetite).

NBS Report 10580 A New Series of X-Ray-Opaque Reinforcing Fillers for

Composite Materials.

NBS Report 10582 A Review of Recent Studies of In Vitro Reactivity of

Surfaces of Calcified Tissues.

Also to be included as part of this semi-annual report, but not bound, are

NBS Report 10533 Dental Cements, Direct Filling Resins, Composite and

Adhesive Restorative Materials: A Resume.

NBS Report 10571 Noble Metal Constitution Diagrams.

NBS Report 10581 Intermediary Bases and Cementation.

NBS Report 10598 Adhesive Restorative Materials.

NBS Report 10599 Solubilities of Phosphates and Other Sparingly Soluble

Compounds

.

ii
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 June 30, 1971 included investigations of a wide range

of materials used in restorative dentistry and of hard and soft natural

tissues. 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. Support from the National

Institute of Dental Research is also provided in the form of grants to

Research Associates sponsored by the American Dental Association. This

support is designated NIDR (Grant) where reference to it is made. In

the summaries 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 and are appended to this report.

2. REPORTS ISSUED

NBS Report 10421

NBS Report 10422

NBS Report 10441

NBS Report 10442

NBS Report 10521

NBS Report 10522

Research Technics Used in Evaluating Dental Materials.

Determination of the Degree of Polymerization of a Polymeric Aromatic Amine from NMR Data Using a "LeastSquares" Approach.

The Crystal Structure of CaKAsO, *8H_0.

4 2

A Stabilizing Comonomer: I. Synthesis and Confirmation

of Structure.

Influence of Alloy Particle Size and Shape.

Dimensional Changes in Amalgam.

1
NBS Report 10533

NBS Report 10571

NBS Report 10578

NBS Report 10579

NBS Report 10580

NBS Report 10581

NBS Report 10582

NBS Report 10598

NBS Report 10599

Dental Cements, Direct Filling Resins, Composite

and Adhesive Restorative Materials: A Resume.

Noble Metal Constitution Diagrams.

Confirmation of the Structure of Previously Reported

Tertiary Aromatic Amine Accelerators with Molecular

Weights Above 400.

A Refinement of the Crystal Structure of CaHPO^

(Synthetic Monetite)

.

A New Series of X-Ray-Opaque Reinforcing Fillers for

Composite Materials.

Intermediary Bases and Cementation.

A Review of Recent Studies of JLn Vitro Reactivity of

Surfaces of Calcified Tissues.

Adhesive Restorative Materials.

Solubilities of Phosphates and Other Sparingly Soluble

Compounds

.

3. PAPERS PUBLISHED

Woelfel, J. B. and Paf fenbarger , G. C. Expanding and Shrinking 7-Year Old

Dentures: Report of Cases. JADA 81:1342-1348, No. 6, December 1970.

Bardos, D. I.; Waterstrat, R. M. ; Rowland, F. J. ; and Darby, J. B., Jr.

The Magnetic Susceptibility of Transition-Metal A15-Type Phases. J. Low

Temp. Phys. 3:509-518, November 1970.

Dickens, B. and Bowen, J. S. Refinement of the Crystal Structure of the

Aragonite Phase of CaCO^. J. Res. NBS 75A:27-32, Jan. -Feb. 1971.

Chandler, H. H. ; Rupp, N. W. ; and Paf fenbarger , G. C. Poor Mercury Hygiene

from Ultrasonic Amalgam Condensation. JADA 82:553-557, March 1971.

Wallace, Betty M. and Brown, W. E. Stoichiometric Composition of Whitlockite. J. Dent. Res. 50:343-346, March-April 1971.

Dickens, B. ; Hyman, A.; and Brown, W. E. Crystal Structure of Ca^Na^ (C0„)

(Shortite). J. Res. NBS 75A:129-135, March-April 1971.

1 5 5

Chandler, H. H. ; Bowen, R. L. ; and Paf fenbarger , G. C. The Need for Radiopaque Denture Base Materials: A Review of the Literature. J. Biomed. Mat.

Res. 5:245-252, May 1971.

Chandler, H. H. ; Bowen, R. L. ; and Paf fenbarger , G. C. Development of a Radiopaque Denture Base Material. J. Biomed. Mat. Res. 5:253-265, May 1971.

Feagin, F. ; Patel, P. ; Koulourides , T. ; and Pigman, W. Study of the Effect

of Calcium, Phosphate, Fluoride and Hydrogen Concentrations on the Remineralization of Partially Demineralized Enamel Surfaces. Arch. Oral. Biol.

16:535- , May 1971.

2
Huget, E. F. and Brauer, G. M. Heats of Immersion of the Components

of Tooth Structure (Research Annotation). J. Dent. Res. 50:776, MayJune 1971.

Means, C. R. ; Rupp, N. W. ; and Paf f enbarger , G. C. Clinical Evaluation

of Two Types of Resilient Liners on Dentures. JADA 82:1376-1380, June 1971.

Dickens, B. and Brown, W. E. The Crystal Structure of Ca^ (PO , ) 2^10^

(S-Phase or Silicocarnotite) . Tschermaks Mineralogische und Petrographische Mitteilungen (TMPM) 16:1-27, June 1971.

Dickens, B. and Brown, W. E. The Crystal Structure of Ca Mg (Ca,Mg) (P0 , ) ., 2

• TMPM 16:79-104, June 1971.

7 y 1

Rupp, N. W. and Paf f enbarger , G. C. Significance to Health of Mercury Used

in Dental Practice: A Review. JADA 82:1401-1407, June 1971.

45 32 Loe benstein, W. F. The Exchange of Ca and P with Hydroxyapatite as Interpreted by Adsorption from Solution. J. Colloid Interface Sci. 36:234-

246, June 1971.

4. WORK IN PROGRESS

4.1. Determination of Crystal Structures

Sponsor: NIDR, NIDR (Grant), ADA

The objective of our research program is to secure structural data on

biologically important inorganic substances so as to evolve a better understanding of processes such as mineralization and demineralization and to

assist in a systematic approach to the development of adhesion between restorative materials and tooth structure.

Crystals are grown of pure compounds or of compounds with predetermined

amounts of selected impurities. Their crystal structures are determined

by X-ray and/or neutron diffraction. The structural parameters give the

overall arrangement in the solid phase. The sites of the impurities are

usually obvious from these parameters. The reason for the choice of a particular atomic site by an impurity is often apparent. From the structural

parameters, much crystal chemistry can be inferred.

Papers have been written on the crystal structures of CaHPO^ (monetite)

and CaKAsO^ • 8^0 . Papers on Ca(H

2

PO^)

2

’H^O , CaCO^ (aragonite), Ca

2 Na

2

(CO^)^

(shortite) , BaCa(C0

3

) 2 (barytocalcite) , Ca^ (PO^)

2

S i°4 ( s il icocarnot ite) and

Ca^Mgg (Ca,Mg)

2

(PO^) have been published in journals during the report

period, or are in press.

3
3-Ca

3

(P0

4

) 2

$-Ca

3

(P0

4

) 2

is found in salivary ducts and glands in areas where

the drinking water contains magnesium ions, and is also found in carious

materials. It is stabilized by small cations, and in the presence of

such cations is produced In vitro under conditions where the pure compound decomposes into OHAp and monetite. The material found in vivo

invariably contains magnesium.

To investigate the stabilizing role of the magnesium impurity, we

have prepared at about 1350° C samples of 3-Ca

3

(P0

4

) 2

which contain various

amounts of magnesium as an impurity. The crystal structures of two of

these samples have been determined and the locations of the impurity have

been found. Presently, we are trying to obtain a good electron-microprobe

analysis to use as an overall constraint on the amount of impurity in the

specific crystals which were used in the structural analysis. We will

then attempt to obtain, by least squares treatment of the diffraction

data, the distribution of these amounts of impurities over the possible

places in each structure. At this stage, we know the approximate distributions of impurity in samples of 3~Ca

3

(P0

4

) 2

containing approximately 10%

and 4%, respectively, of the cations as magnesium. In the 10% Mg sample,

the Mg impurity is concentrated into one crystallographic site (the total

amount of the impurity present was constrained in the least squares analysis to that introduced in the preparation). One cation site in the crystal

structures appeared to be only partially occupied; actually, only one-half

of these sites in the crystal structure were occupied. In the 10% Mg

sample of 3-Ca

3

(P0

4

) 2

, no magnesium was found in this site which, therefore,

had a total occupancy of 0.5 Ca. A second sample of 3-Ca

3

(P0

4

) 2

came from

a preparation in which 4% of the cations were magnesium. In the second

sample, the occupancy by calcium was about twice as high as by magnesium

in both these sites.

The occupancy parameters are only approximate at this time and will

be revised after we have determined the composition of the crystals by

electron microprobe analysis and after we have evaluated our data and

processing procedure using the error analysis program which is being written and which will be described later. An earlier microprobe analysis was

4
unsuccessful because flat surfaces were not ground on the specimens.

The specimens have been remounted and flat surfaces have been ground.

Comparison with other recent research now suggests that

0-Ca„ (PO . ) _ exists in two slightly different arrangements, one when

j 4 2

formed at higher temperatures (^1000° C) and the other formed from an

aqueous system (i.e., in vivo or hydrothermally) . The material

formed rn vivo is too fine grained for single crystal work, but it probably can best be simulated by materials synthesized hydrothermally in

crystallization chambers at about 200°C. We have, therefore, been preparing to grow samples of B-Ca^CPO^)^ hydrothermally with different amounts

of magnesium present. Their crystal structures will then be determined.

We have recently repaired the inert liner in our crystallization chamber.

The old liner was cracked and introduced impurities from the chamber wall,

and the seal leaked.

Ca(H

2

P0

4

) 2

The previously unknown crystal structure of CaC^PO^^ has been determined by X-ray and neutron diffraction. The X-ray structural determination

proceeded reasonably well, but as is unavoidable in X-ray crystallography,

gave fairly imprecise locations for the hydrogen atoms. Neutron diffraction

work was undertaken because it provides structural parameters for hydrogen

atoms which are essentially as precise as those of heavier atoms. However,

the structural analysis did not proceed as well as the X-ray analysis because the neutron data were more widely affected by extreme secondary

extinction. This extinction must be corrected before fine details of the

hydrogen atoms may be discerned. Isotropic secondary extinction corrections did not prove satisfactory. As a preliminary to more complicated

extinction corrections, an error checking program is being written.

The results of the neutron structural determination at this stage

show that at least two of the five crystallographically distinct hydrogen

atoms in the structure of CaO^PO^^ may be centrosymmetrically disposed

between the two oxygen atoms coordinated to each hydrogen atom. Centrosymmetric hydrogen bonds are very rare and poorly understood. We hope to

elucidate the hydrogen bonds in CaC^PO^^ by (i) further refinement after

a good extinction correction has been applied to the neutron diffraction,

and (ii) use of data from spectral techniques such as IR.

5
a-Ca^ (PO^)

£

The structure of a-Ca

2

(PO^)

2

is probably related to those of

Ca

5

(PO^)

2 SiO^, Ca

7

Mg

9

(Ca,Mg)

2

(PO^)

12 , and K^NaCSO^, with systematic

cation vacancies present in the first two materials and in a-Ca

2

(PO^)

2

. a-Ca

2

(PO^)

2

contains a pseudo-cell which is expanded by a factor of

three into the real cell, presumably by slight differences in the positions of atoms in different pseudo-cells. These differences, and hence

the real structure, may be difficult to find but an attempt to determine

the crystal structure of a-Ca

2

(PO^)

2

will be made. The structure is

needed to shed light on several anomalies in the B-Ca^(PO^)

2

structure

and to help complete the correlation of the roles played by systematic

cation vacancies in calcium phosphates. Over 12,000 X-ray diffraction

intensities have been measured from a single crystal of a-Ca

2

(PO^)

2

. About 3,000 more are needed to complete the measurement of the unique

set of data (i.e., because of the large amount of data, no data have been

measured twice or more, as is our usual custom to track down measurement

error). Work on the solution of the crystal structure of a-Ca

2

(PO^)

2

will

commence when the measurement of the data is complete.

Computer Programs

The site occupancy parameters, which lead directly to the position

and extent of impurity substitution in crystal structures, are the least

precisely determined structural parameters. The parameters representing

apparent atomic motion are likewise among the more poorly determined quant

ties, yet provide important indications of the forces in the solid state

and of the mechanisms by which substitution of impurities which differ in

size from the constituent atoms is achieved. Thus, we have a general

concern about improving the already high quality of our crystallographic

data.

The X-ray diffractometer programs have been modified to allow output

of the X-ray intensity data directly onto magnetic tape. Another modifica

tion allows automatic realignment of the crystal at well-definsl intervals

under local computer control. The Univac 1108 computer program which was

used to process the data as output on paper tape has been rewritten to

process the magnetic tape and has been further extensively modified in

6
several ways, which include (i) better corrections for unavoidable

fluctuations in intensity of the X-ray beam due to air scattering, (ii)

an estimate of the importance of short term and long term fluctuation

in beam intensity, and (iii) greatly improved computing speed.

Because of the need to elucidate the nature of the severe secondary

extinction in the neutron data obtained from a crystal of CaCH^PO^)^

and because of its general applicability, a program has been written to

evaluate the standard deviations attached to measured intensities and

refined structural parameters. This program is currently being tested.

Basically, it computes the ratio of the actual difference between two

equivalent but independent quantities to the uncertainties in these

quantities as estimated from statistical procedures and compares the

distribution of these ratios with the normal probability distribution.

The fit should be good if the errors present in the quantities are random

and if the scales of the standard deviations are approximately correct.

Misfits will occur if systematic error is present or if the standard

deviations are all too high or too low.

4.2. Physicochemical Studies of Apatite

and Related Inorganic Substances

Sponsor: NIDR (Grant), ADA

4.2.1. Crystal Chemistry

A new method was described in the previous report for the preparation

of hydroxyapatite and other calcium phosphates. The hydroxyapatite

crystals produced by this method are relatively large and are especially

suitable for use in studies in which synthetic crystals that mimic those

in enamel are needed. One feature of the product formed by this process

is an apparent low Ca/P ratio. Since the crystals formed by this process

are unusually well formed for a "non-stoichiometric" hydroxyapatite, their

physical and chemical properties are being investigated in the hope that

an answer can be found to the old question of what causes the apparent

non-stoichiometry in apatites. The cause may have an important bearing

on two other important questions: (1) What is the reason that the carious

lesion forms under the surface of the enamel rather than forming an eroded

spot, and (2) Why do individuals differ in their caries susceptibility?

It was found that by continued washing of the non-stoichiometric

7
product, the composition would slowly approach the ideal composition

without the crystals changing significantly in morphology or optical

properties. A series of products derived from the original precipitates

have now been prepared; these vary in Ca/P ratio from about 1.59 to 1.67.

Pyrolysis of some of these products show that they behave in a way that

is normal for Mnon-stoichiometric M apatites — the greater the deviations

4_

in Ca/P from the ideal value, the greater the amount of ^ 2^7 that is

formed. On the assumption that the low Ca/P ratio was due to the presence

of CaHPO^ which decomposes under pyrolysis according to the reaction

2 CaHP0

4 > Ca2

P

2

0

?

+ H

2

0 , it was found that the formation could be used to show that the apatite

portion of the sample had the theoretical Ca/P ratio of 1.67. This view

is in accord with (i) the X-ray diffraction and infrared absorption results that show no significant change although the gross Ca/P ratios of

the preparations varied considerably, and (ii) evidence obtained subsequently

with the polarizing microscope showing the presence of significant amounts

of divided birefringent crystallites of CaHPO^; the latter were not given

adequate recognition in the initial examinations. Thus, although we

initially thought we had prepared a material that might have bearing on

"non-stoichiometric apatites", careful investigation revealed that the

cause of the apparent anomaly was the presence of a secondary phase of

acid calcium phosphate. We believe this has bearing on many other reported

cases of non-stoichiometric apatites in which adequate efforts were not

made to detect the presence of such phases.

Modifications of the crystal-growth procedure have been used to produce several other products.

(1) Fairly large amounts of octacalcium phosphate were prepared for

use in a solubility study.

(2) Chlorine-containing hydroxyapatites were prepared for study by

X-ray diffraction and infrared spectroscopy.

(3) Chlorapatite , Ca^(PO^)^Cl, was prepared, possibly for the first

time, from an aqueous system.

Relatively small amounts of Cl were found to disrupt the structure

to the extent that the 638 cm ^ peak caused by the librational motion of

the OH hydrogen was not detectable, and the OH stretching peak was

8
considerably broadened. This effect may signify that Cl in the structure

may alter the lattice energy and thereby significantly increase the solubility of the hydroxyapatite. This may have an important bearing on how

Cl” in enamel may affect its solubility and on the formation of caries.

The major drawback of our procedure for growing hydroxyapatite is

the difficulty in removing the last traces of CaHPO^ . Continued washing

for several weeks, although successful, precludes preparation of large

amounts. We have tried several methods to accelerate removal of CaHPO.

4

without significant success. This phase of the work is being given special

attention; neutral ammonium citrate solution, which is used for this purpose

in the fertilizer industry, is being tried under various conditions.

4.2.2. Solubility Studies

Solubility of tooth enamel

A primary objective of this part of the project was to explore solubility behavior of important biominerals by utilizing the techniques applied

to the synthetic calcium phosphates in our previous studies. We have made

considerable progress with pooled samples of powdered human tooth enamel.

The method of approach involved successive batchwise extractions of a powdered enamel sample with a given phosphoric acid solution in the con-

-2 -4

centration range 1.7x10 -1.5x10 molar. Each extraction was carried out until apparent equilibrium between the solid and liquid phase was

reached. By choosing phosphoric acid as the extracting solvent, we were

able to restrict the foreign ions to only those coming from the enamel

mineral. This simplified the interpretation of data.

The equilibrium measurements (pH, calcium concentration and phosphorus

concentration) were interpreted through the use of considerations such as ion-activity products for the various calcium phosphates, the stoichiometry

of the dissolution reaction, the electroneutrality unbalance, cumulative

amounts (wt%) of calcium dissolved, and "potential diagrams". The ionO

j ^

O — activity products (Ca ; (PO. ; (OH ), (anI1A ), derived from the results

of extractions indicate that the enamel sample contained appreciable amounts

of calcium phosphate more soluble than the pure, crystalline Ca,. (PO^)^OH,

(OHAp). In an equilibration with a pH value of about 4.8, large crystals

of CaHPO^^H^O (DCPD) were observed, whereas in synthetic mineral systems

the invariant point between DCPD and OHAp is about pH 4.3, again suggesting

9
that the tooth mineral contained a fraction that is more soluble than

synthetic OHAp. The ion-activity products, aQj^p> showed a strong dependence on the cumulative amount of solid extracted; the greater the

amount of solid extracted, the smaller was the ion-activity product.

The smallest value of the activity product after about 30% extraction

of the solid was about lx 10 ; this value is still significantly

—58

larger than the solubility product constant of synthetic OHAp, 3.7 x 10

The electroneutrality unbalance contributed by the dissolution of foreign

ions from the enamel mineral was large and cationic in nature in the

first extractions; it dropped off to an insignificant amount in the subsequent extractions when about 5% of the solid phase had dissolved. This

suggests that the more soluble foreign cations are dissolving in the

initial extractions.

From the plots of quantities proportional to the chemical potentials

of CaCOH)^ and H^PO^ for enamel solubility data and the previous synthetic

calcium phosphate solubility data, it was apparent that the enamel samples

were more soluble than hydroxyapatite but less soluble than other calcium

phosphates. The negative reciprocal of slopes of the lines for synthetic

minerals in such plots represent the Ca/P ratios of the respective solid

phases; we cannot apply this criterion to the enamel data because in each

group of points for enamel solubility we find varying ion-activity products

due to changing solubility. Potential diagrams have limited use for

establishing the stoichiometry of the equilibrating solid phase in tooth

and bone mineral because the slope of the line may be affected by how much

of the mineral has dissolved.

The results obtained in the batch-wise equilibrations require considerable care in their interpretation because the presence of impurity ions

and sometimes the precipitation of acidic calcium phosphates altered the

apparent stoichiometry of the dissolution reaction. The results obtained

in the continuous, titration-type equilibrations, which were initiated

near the end of this report period, were more consistent and less affected

by the above factors. Although the equilibration times were shorter than

those used in the batch-wise experiments, the indicated solubilities were

greater, suggesting that reprecipitation was minimized and that the results

are more indicative of the solubility characteristics of the original enamel

mineral

.

10
Solubility of dicalcium phosphate dihydrate

Solubility measurements on CaHPO^* 2H^0 in the presence of various

concentrations of NaCl, described in the report for the previous periods,

were completed. A manuscript for publication is in preparation.

Efforts were made to extend the solubility measurements on CaHPO^* 2^0

into the range of pH’s above 7 with the objective of collecting information

on the stability of the ion pair CaPO^”. The procedure involved the use

of disodium ethane-l-hydroxy-1 ,1-diphosphonate to prevent the precipitation

of hydroxyapatite. Although this method has prevented the crystallization

of hydroxyapatite in many other situations, it was not successful in this

instance. The possible reason for this is that the times required for

our equilibrations were much longer than for the other studies in which

the diphosphonate has been used.

Future work on solubility of calcium phosphates relates to the effects

of NaCl on the solubility of hydroxyapatite and CaHPO^. The measurements

will be made on solutions in equilibrium with both solids (i.e., at the

singular points) and at the temperatures, 25°C and 37° C. In this way, we

will be able to use the hydroxyapatites prepared by the method described

above without being concerned about the presence of unreacted CaHPO^.

Measurements made at the singular points are governed by an additional

constraint on the system that should be of considerable value in the

theoretical treatment of the data. These equilibrations should also give

us a unique means of testing the reliability of the calcium and hydrogen

electrodes.

Solubility of octacalcium phosphate

From the stability point of view, in physiological pH ranges (6.4 to

7.5), octacalcium phosphate (OCP) , Ca 0 H0 (PO. )

• 5H 0 , is in the center among

o Z 4 6 Z various calcium orthophosphate compounds; hydroxyapatite, Ca^(P0^)^0H, and

g-tricalcium phosphate, ft-Ca^PO^^, are more stable, and anhydrous dicalcium phosphate, CaHPO^, and dicalcium phosphate dihydrate, CaHPO^^H^O,

are less stable. Although it has not been proven that OCP participates

either as a precursor in tooth and bone mineralization or as a more or less permanent component (as it is in calculus) , there is considerable

evidence to support this view. Thus, its solubility is important because

it would define minimum levels at which it can form.

11
A large batch of OCP was prepared by utilizing a new method which

was described in a previous report. We are presently continuing solubility measurements on the new preparation of OCP over a range of solution compositions. Initial results indicate that the measurements are

satisfactory; however, the conversion of OCP to CaHPO^* 2H^0 is quite

rapid in the pH range below that of the singular point for these two

compounds. This will restrict the range of compositions that can be

studied, thereby precluding an independent check of the ion-pair formation constant for CaHo P0,

+

derived in studies of the other salts. How2 4

ever, we do not anticipate that this difficulty would prevent us from

improving the existing value of the solubility constant at 25° C and

establishing values for the constant at 5, 15, and 37°C.

Additional experiments are being carried out on aqueous mixtures

of OCP and CaHPO^*2H20 to check the singular point composition.

4.2.3. Topical Fluoridation

Thermodynamic and kinetic considerations have led us to propose

that fluorapatite (FAp) might be formed from hydroxyapatite (OHAp) in

tooth enamel through the following path.

DCPD so In*

OHAp — DCPD [I]

DCPD —

f”

FAp [in

* A pretreatment solution that is saturated with respect

to DCPD but is undersaturated with respect to OHAp. [DCPD = CaHP0^*2H 0]. These solutions range in composition from

those saturated with respect to DCPD and OHAp (pH 4.3)

to those saturated with respect to DCPD and Ca(H PO,

)

9 '2H 0

(pH < 2).

2422

Preliminary studies on synthetic OHAp and DCPD indicated that this proposed path is feasible. Studies were also made on extracted human teeth;

some representative results were presented in the last report.

Although fluoride is known to react rapidly with DCPD in aqueous

solutions, there is great uncertainty as to the nature of the product

formed. CaF

2

, OHAp, FAp and unreacted DCPD are among the compounds that

have been observed as the product of such a reaction. Because of its

complexity, it is necessary to study reaction [II] in more detail.

12
When DCPD was immersed in 0.1 wt% F solution, near complete conversion of DCPD to FAp was obtained after 24 hours reaction time. This indicates that the rate and yield of this reaction are many orders of magnitude greater than those for the reaction of OHAp with F to form FAp. The

optimum F concentration for this reaction appears to be somewhere around

0.2 wt%. Too high an F concentration resulted in large amounts of CaF^,

while too low an F concentration did not provide enough F for the reaction

to proceed to the right. DCPD did not hydrolyze in significant amounts

to form OHAp (or an OHAp-FAp solid solution) with F concentrations below

0.05 wt%.

In line with the study of the reaction of DCPD with F, it was also

found that DCPD would react with CaF^ to form FAp. The rate of this reaction was greatly enhanced in the presence of OHAp, indicating that the

OHAp acted as a template for the formation of FAp. This result encourages

us to believe that we may have found a way to overcome one of the major

limitations (the slowness of solid state reactions) to the efficacy of

the FAp reaction. By forming CaF^ and DCPD within the enamel, we hope

that these will then react to form FAp layers on the OHAp crystals. The

thickness of these layers would not be limited by the slowness of a solid

state reaction and the acid produced in the reaction would tend to facilitate its rate.

4.2.4. Calorimetry

We have been setting up calorimetry facilities for measuring the heat

given off when calcium phosphates dissolve. This information is needed to

characterize fully the thermodynamic properties of these compounds. Since

this work is being carried out in close cooperation with the Thermochemistry

Section of the National Bureau of Standards, it has been possible to develop our capability in this field to a very high level of accuracy. During

calibration and testing of our calorimeter, measurements have been made on

the heat of dissolution of tris (hydroxymethyl) aminomethane (tris) in

hydrochloric acid. This is the standard reference material distributed by

the National Bureau of Standards for calibration of solution calorimeters.

As an indication of the quality of calorimetry we are now able to perform,

these measurements on the heat of solution of tris are to be reported at

the international calorimetry conference, July 1971, and will be incorporated

13
into the value that will be used to support the certification of this

material as a standard reference. As part of this development, we participated in a thorough analysis of the sources of errors involved in

calorimetric measurements and contributed to improving the automation

of data collection.

Work on the heats of dissolution of calcium phosphates has been

initiated; a reference material for these studies will be CaO. This

material was chosen because it will be used to adjust the final solution

compositions so that they will be the same irrespective of the calcium

phosphate salt which is being measured. Preliminary measurements have

been made on the heat of dissolution of hydroxyapatite; these will permit

us to establish the most propitious conditions for the final measurements.

A National Bureau of Standards report, "Digital Data Acquisition

and Computer Computation Applied to Calorimetric Experiments", has been

written.

4.3. Surface Characterization Through Adsorption Studies

Sponsor: NIDR

Water Adsorption

The work involved in the water vapor adsorption and desorption on

collagen, dentin, enamel, anorganic whole teeth, polymethyl methacrylate,

and investment compound is being prepared for publication.

Additional work along these lines is being extended to include dental

cements and other restorative materials inasmuch as they come in direct

contact with the teeth. These materials will include zinc phosphate

cement, polycarboxylate cement, zinc oxide-eugenol , a composite (filled

resin), and an amalgam. Also, a sample of anorganic dentin is included

to complement the earlier series.

High Temperature Water Adsorption

A preliminary experiment has been performed to determine the feasibility of higher than room temperature water adsorption at humidities

approaching saturation. This was carried out with dentin using a conventional "desiccator method". It was found that at 96% relative humidity

and 50° C (where the vapor pressure of water is 92.5 torr) , dentin contained

24% water. This may be compared with 13.5% water when saturated at 23°C

14
where the vapor pressure is 21.1 torr. It is significant that while

the vapor pressure of water is more than four times greater at the

higher temperature, the water content of dentin is barely double.

Hysteresis Theory

Some progress has been made in this laboratory in advancing the

theory of adsorption hysteresis. These developments are being tested

by applying them to water-on-dentin data in an attempt to account for

the course of adsorption within the region contained in the hysteresis

loop

.

Indirect Determination of Dry Wt.

Data points (as fast as they are obtained) are being checked against

this original theory to compare them with actual drying experiments. An

understanding of adsorption hysteresis is necessary in order to eliminate

anomalous results. Preparation for publication will be deferred until

after a Hysteresis Theory paper has been submitted, in order to follow a logical sequence.

4.4. Mechanical and Physical Properties of Oral

Structures and Restorative Materials

4.4.1. Dimensional Changes in Teeth Components

Sponsor: NIDR, USAF, VA

Studies of the dimensional changes of tooth structure were concentrated on dentin, particularly on the time dependence of the "thermal

expansion" of dentin. All evidence obtained so far is consistent with

the hypothesis that in addition to the thermal dimensional change when

dentin is heated or cooled in water, there is also a dimensional change

due to water movement in and out of the material. Water content, which

comes to equilibrium slowly, is apparently reduced as temperature is

raised. Thus, the heating or cooling of dentin in water is accompanied

by two dimensional change phenomena, opposite in direction, one of which

is time dependent. The range of dimensional change when a specimen at

equilibrium at a temperature, t^, is raised to a higher temperature, t^,

and then brought back to t^ appears to be limited by two parallel curves - one representing the "instantaneous" thermal expansion from t^ to

with t^-equiiibrium water content, and the other representing the

15
"instantaneous" thermal contraction from t^ to t^ with t 2

~equilibrium

water content. The experimental temperature-AL curve may fall anywhere

within these limits depending upon the time-temperature history of the

specimen.

In addition to the time dependence of the "thermal expansion" of

dentin, there also appear to be anisotropic variations as well as toothto-tooth and location-in-tooth variations. Preliminary data suggest that

the anisotropic effects are not present in anorganic dentin, but further

verification of this observation is needed.

Measurements of the rapid heating or cooling coefficient on about 30

—6

dentin specimens gave an average value of 7.5 x 10 /°C (S.D. = 1.0) for

specimens in which the direction of the dentinal tubules was generally

perpendicular to the direction of measurement. The coefficient with

—6

tubules more nearly parallel to the direction of measurement was 5.4 x 10 /°C

(S.D. = 0.7). Measurements made in three directions on two rectangularprism specimens of root dentin gave the following results:

Direction A

Specimen

B

Perpendicular to tubules 7.2 x 10

-6

7.6 x 1(T

6

Parallel and perpendicular (1) 6.4 5.3

Parallel and perpendicular (2) 4.9 4.9

Volume 18.5 17.8

The overlapping of ranges may result from difficulty in obtaining specimens

with tubules aligned uniformly in the desired direction, as well as from

other specimen-to-specimen variations.

4.4.2. Mechanical and Electrical Properties

of Hard and Soft Tissues

Sponsor: NIDR, VA

A new experimental program is being developed to study the physical

properties of hard and soft tissues. A varied experimental approach involving piezoelectric, dielectric, mechanical, x-ray, dilatometric , NMR

and other o tservations will be employed in accordance with the demands

of the problem. The phenomena to be investigated include viscoelastic

effects, relaxation times, crystalline and amorphous phase transitions

16
and molecular structure. In order to obtain a broad view of these

phenomena, observations as a function of temperature will be emphasized.

Information about molecular structure and the molecular origins of the

observed phenomena will be sought.

Initial attention will be given to the electrical and mechanical

measurements of collagen and bone as a function of temperature. Most

published data in this area have been confined to observations at a constant temperature, so that a limited physical picture exists. For

example, the molecular changes which occur in collagen near 40° C can be

studied in detail with dynamic mechanical, piezoelectric, and x-ray

measurements using temperature as a variable. The piezoelectric effect

has its origin in the oriented crystalline regions of the sample. With

x-rays, the oriented and unoriented crystalline regions are observed.

The dynamic mechanical measurements are used to examine the viscoelastic

properties involving both amorphous and crystalline regions. With these

experiments, it will be possible to isolate the origins of the observed

phenomena in the oriented crystal, randomly oriented crystal, or amorphous

phase. A molecular description of the observed phenomena will be pursued,

and may require data from other experiments.

The physical properties of the dentin and enamel components of tooth

structure will be studied using mechanical and viscoelastic measurements.

These materials can be viewed as a composite, and the interpretation of

the experimental data will rely on recent advances in the understanding

of the mechanical behavior of composite materials. This work will be correlated with thermal expansion measurements now being made.

In the area of dental materials, consideration will be given to the

problem of stress distribution, stress corrosion, and fatigue in dentures.

A study will be made of the feasibility of using thin piezoelectricallyactive polymer films as stress-strain detectors. This work would rely on

an already existing piezoelectric program in the polymers and electronic

technology divisions of NBS.

An immediate undertaking will be the construction of appropriate

experimental equipment. A piezoelectric measurement system will be developed and the possibility of making viscoelastic measurements with same

system will be considered. Temperature control over a suitable temperature

17
range and the capability of iri vivo measurements will be incorporated

in the apparatus.

A rough draft of the second part of a paper on viscoelastic properties has been completed and is being edited for inclusion in the Proceedings of the 50th Anniversary Symposium.

4.5. Collagen-Water Interaction

Sponsor: NIDR

Calorimetry

The effort to definitively characterize the dimensional changes of

tooth structure which occur as the temperature of a bathing medium is

varied (e.g., see 4.4.) has resulted in an intensive study of possible

changes in dentin which are responsible for its anomalous behavior at

temperatures as low as seven degrees above normal mouth temperature. To

ascertain whether the anomalous behavior involved a phase transition in

the collagenous component of dentin, experiments were conducted with a differential scanning calorimeter. Typical endotherms were observed in

the expected temperature range with commercial Perkin-Elmer equipment

when specimens of collagenous tissues such as bovine hide, rat-tail tendon,

kangaroo-tail tendon and demineralized bovine dentin were examined in

water. However, the noise level of the instrument was not sufficiently

low to permit investigation of mineralized dentin, the collagenous component of which accounted for only 20% of its weight. The microcalorimetry

section of NBS was approached and with equipment utilizing 500 thermocouples around the reaction vessel rather than the usual single thermocouple

and into which numerous additional features have been incorporated to give

an overall sensitivity of 60 microvolts, samples of collagen and dentin in

water were again examined for heat absorption or evolution. In addition

to some intriguing additional information on tendon collagen, the instrumentation has clearly shown the absence of an endothermic transition in

dentin over a temperature range 35°C - 90° C. The collagen component of

dentin, when not confined by the mineral hydroxyapatite, however, shows

a normal first order melting phenomena, melting a few degrees less than

60° C. The microcalorimetric measurement of dentin provides very strong

evidence for the true composite nature of dentin and clearly indicates that

18
its mechanical properties must be analyzed from this point of view.

NMR and IR Studies

Further experimental studies of the water of hydration in collagenous

tissue were obtained using near-infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, in combination with differential scanning calorimetry (DSC). The results can be roughly grouped into two categories, as follows

A. Nature of Bound Water in Helical vs. Denatured Collagen.

When wet animal tendon is heated above about 55°C, the hydrogenbonded helical molecular conformation of the collagen breaks down into a random-coil structure, with some reforming of hydrogen bonds between the

chains. However, one would expect a net change (probably loss) in the

number of intramolecular hydrogen bonds after the helix-coil transition.

This should lead to a like change in the number of hydrogen-bonding sites

on the protein available to water, and therefore might be epxected to

produce a change in the amount of unfreezable water in the system. In

order to test this hypothesis, the amounts of unfrozen water in native

and denatured collagen were measured by DSC. Samples of wet KTT (kangarootail tendon) were each sealed in small aluminum pans to prevent evaporation

during the experiment. Each was frozen below -40° C and scanned through the

melt. From the calibrated melt endotherm, the weight of ice in the

"frozen" collagen was calculated. Each sample was then heated above 60°C,

through the helix-coil endotherm. The samples were then refrozen and

the ice melt endotherm was again recorded. The observed changes in the

amount of unfrozen water after vs. before denaturation for the three

samples were -1.2%, -6.7%, and +0.8%. Two of these values are within the

replicate reproducibility of this type of experiment, and the third is only

outside this value by a small amount. It would, therefore, appear that

any changes in the availability of hydrogen-bonding sites to water before

and after denaturation are so small as to be impossible to detect with

this technique.

A full discussion of the NMR experiment is beyond the scope of this

report. Briefly, it consisted in recording the shape and width of the

NMR spectrum of D^O in a sample composed of small segments of randomly

oriented KTT, before and after heat denaturation of the collagen. The

19
observed narrowing of the NMR line after heat denaturation is consistent

with rotational diffusion of the water in the collagen matrix, and not

with ijn situ rotation of the water molecules at their hydrogen-bonded

sites on the collagen. The latter effect would likely have produced a spectrum very much like that of undenatured collagen. Although the result is only qualitative, it is a further indication that the water in

the collagen matrix is not at all "icelike", but is rather a fluid with

considerable translational mobility. (It should be possible to measure

the self-diffusion coefficient directly by another NMR technique, which

is not presently available here).

B. IR Spectra of Water in KTT.

Infrared spectroscopy offers a few additional advantages in studying

the nature of water and ice in collagen. Briefly, they may be summarized

as follows

(1) Very high sensitivity. Two water bands in the near-IR spectrum

of hydrated collagen at 1.45y and 1.94y are almost completely unobstructed

by the collagen spectrum. They are the same wavelengths as the bands of

pure water, and do not indicate the high-frequency shifts observed for

ice, again indicating the liquid-like nature of the water. We have observed concentrations of water as low as about 0.1 water molecule per

amino acid, which is well below the concentration observable by the

present wideline NMR spectrometer.

(2) Very fast measurement. The timescale of the IR measurement is

-12

so short (^10 sec) that the water molecules are not able to diffuse

measurably or rotate by more than a fraction of a complete revolution during the time in which they are absorbing IR energy. This is in contrast

with the very long NMR timescale, in which the molecules diffuse at least

several hundred molecular diameters during the measurement. The IR bands

are dichroic, indicating anisotropic distribution of water molecule

orientations

.

(3) Direct observation of all the water in "frozen" collagen. NMR

spectroscopy sees only the unfrozen water in collagen, since the ice

spectrum is too broad to detect. Calorimetry only detects the melting of

the frozen water component. The IR bands, which arise from all the water

in the frozen collagen, show frequency shifts which must correspond to an

overlap of water and ice bands.

20
4.6. Dental Metallurgy

Sponsor: NIDR (Grant)

Binary Noble Metal Constitution Diagrams

The existing metallurgical literature concerning binary or ternary

constitution diagrams contains only a relatively small percentage of

alloy combinations involving the noble metals (i.e., those metals which

are exceptionally resistant to chemical corrosion or oxidation, such as gold and metals of the platinum group). To help fill this gap, a report

(see Appendix) has been published which presents four new binary constitution diagrams. These diagrams include the Cr-Pt, Cr-Ir, Cr-Rh and V-Pt

alloy systems. In this research, all experimental alloys were prepared

from starting materials having a nominal purity of at least 99.9% and precautions were taken to insure that no significant contamination was introduced during alloy preparation and heat-treatment.

The program is being continued with the current effort centered on

the vanadium-ruthenium and vanadium-rhodium systems.

Gallium-Palladium Alloy

Previous research in this section pointed up the potential of a gallium-palladium-tin alloy in dental restorative work. At the most recent

IADR meeting, some German researchers presented data confirming and supplementing these studies. The need for clinical examination of the alloy has

long been stressed by members of this section and recent clinical studies

at the University of Alabama now suggest that there are some serious stress

corrosion problems with these materials which must be taken into account

in the future. Corrosion studies are, therefore, being undertaken in cooperation with other laboratories. These studies will investigate both

the gallium-palladium-tin alloy and also a gallium-copper-tin alloy for

which satisfactory clinical results have been claimed in some recent Russian

publications

.

Amalgam Research

A development in amalgam research at the University of Virginia, and

thought by some to be a major breakthrough, involves incorporation of gold

into the alloy. This is believed to cause elimination of the so-called

y

2 phase (tin-mercury phase) which is considered to be the weakest mechanically and least corrosion resistant of the phases that are formed in amalgam.

21
A cooperative program to test if indeed the phase has been

eliminated has been initiated between this section, the reactor group

with their neutron diffraction capability at NBS , and the University of

Virginia group. The indications are that the phase is absent, although

some questions remain to be answered.

Nitinol Alloys

A research proposal has been prepared to explore the possible uses

of Nitinol alloys in dentistry. These alloys are quite unique in their

ability to "remember" a pre-determined shape. Any arbitrary shape may

be pre-selected simply by shaping and constraining the metal mechanically

and then heating it to a dull red heat. The metal may then be cooled to

room temperature (i.e. , below mouth temperature), and if it is now rather

severely bent or deformed, it can be restored to its pre-selected shape

merely by heating it to a temperature which lies between mouth temperature and boiling water temperature, depending on the alloy composition.

In returning to its pre-selected shape, the metal exerts considerable

force, comparable to its tensile strength of about 100,000 psi. It is

quite pliable below the transition temperature, however, and this suggests a possible dental application as clasp wires in cases where a large

undercut is needed. The clasp could, for example, be rendered pliable

by placing it in ice water and subsequently replacing it in the mouth.

Upon warming to mouth temperature, it would restore itself to a preselected shape and grasp securely to a tooth. This method is impossible

with present alloys since they are incapable of undergoing such large

strains without being permanently deformed.

The Nitinol alloys are composed mainly of nickel and titanium in

equi-atomic proportions. It is expected that they will be quite corrosionresistant in an oral environment and will be biologically inert. They

were discovered at the Naval Ordnance Laboratory at White Oak, Maryland

several years ago, but only recently have the "memory transition temperatures" been adjusted so as to be sufficiently low to enable its use in the

human body.

Borazon

The use of cubic boron nitride, Borazon, as a possible dental grinding material to replace diamonds is being considered. The General Electric

22
Company has recently begun to market this material. It possesses the

same atomic structure as diamond, and is nearly as hard, but unlike

diamond, it can be heated to high temperatures without decomposing.

Furthermore, it appears to cut hard material more efficiently than

diamond. Its possible use in dentistry for cutting or grinding tooth

structure ought to be explored and preliminary negotiations are now

under way to investigate this possibility in cooperation with the General

Electric Company.

4.7. Dental Ceramics

Sponsor: ADA

In cooperation with the Navy Bureau of Ships, several graphite crucibles were vacuum- impregnated with zirconium-acetate solutions, heat cured

and dried. Titanium alloy was then melted in them. Carbon pickup from

both the unimpregnated and impregnated crucibles was large. Tests suggest

that the zirconium-acetate impregnated crucibles might prove to have

slightly greater stability upon prolonged use. However, carbon pickup

by the molten metal was so high that no further work appears merited.

Work continues on the development of porcelain "frits" prepared by

the gelling of inorganic-polymer solutions. By this procedure, all constituents of the porcelain are mixed as liquid solutions and gelled. The

air-cured and hardened gels are then calcined at a suitable temperature.

Calcination temperature controls "frit" porosity and density. A nearly

pure mullite porcelain has been prepared by this procedure. However, the

firing shrinkage was greater than 50%.

Work is now being directed toward developing "frits" with suitable

densities and wetting properties. If this is successful, attempts will

be made to spherulitize these gels and thus control "frit" particle size

and shape. It is hoped that porcelains made out of these "frits" may

show reduced firing shrinkage due to better control of particle size and

more localized and restricted flux action.

Dilute chloride-stabilized-alumina and ammonium-stabilized-silica

water solutions were substituted for distilled water in the fabrication

of low-and-high-fusing straight-dental-porcelain bodies. Alumina solution additions had too refractory an effect on the porcelains and severely

retarded their maturation. Preliminary tests using silica solutions appear

23
to hold promise wtih regard to greatly reducing balling during firing

and increasing green porcelain strength.

Extensive tests are being performed on small cylindrical pellets

of a low fusing (Apco) porcelain. Pellets were prepared using 1.0 to

15% SiO^ water solutions. These were fired with identically prepared

controls in which only distilled water was added. Results show that

the silica solution additions greatly reduced balling during firing,

thereby increasing the sharpness of prepared edges and, due to restricted

flow, decreased disproportionation in the porcelain body form. Increase

in the green strength of the dried porcelain body was noted. In the 1.0

to 5.0% SiC^ solution additions, translucency appeared unaffected and

only a slight change in the Gardner tristimulus color values was noted.

Gloss was reduced, but, in all cases, was adequately compensated for by

the addition of Superglaze enamel coats. Firing shrinkage was appreciably

reduced only in bodies in which solution additions with greater than 5%

SiO^ were used. Pronounced decrease in edge strength was noted when

using solution additions having SiO^ concentrations in excess of 8%.

Extensive tests are now underway to assess the significance of any strength

decline.

The mapping out of firing shrinkage, firing color and tensile strength

stability diagrams for the various porcelains is underway. The effects

of firing rate and temperature are being assessed. Detailed petrographic

studies are to be made of reaction mechanisms.

4.8. Dental Investment Studies

The thermal expansion compensation technique has been used extensively in the casting of dental restorations. That additives such as NaCl

and H^BO^ influence the thermal expansion of dental investments has been

known for a long time. However, the nature of the phenomenon by which

these and other additives influence thermal expansion has not been adequately

explained, although the evidence points to reaction with the gypsum binder

component of the investment. As a part of a one-year guest worker program

at NBS, Dr. S. Nomoto, Professor of Dental Materials at Tokyo Medical and

Dental University, has begun a study of the thermal expansion (ADA Specification No. 2 for casting investment for dental gold alloy) of investment

24
materials with various amounts of additives and of the effect of wax

dipping on the thermal expansion of such materials. Petrographic analysis

will be employed to correlate structure and the physical properties

observed

.

4.9. Grafting of Monomers to Collagen and Soft and Hard Tissues

Sponsor: U. S. Army, NIDR

The aim of this study is to modify the surface of the proteinaceous

material by chemical attachment (grafting) of polymeric side chains to soft

and hard mineralized tissues containing a collagenous or keratinous component. Previous studies in this laboratory have shown that a variety of

monomers can be grafted to these substrates. It therefore appears possible

that surfaces with the desired degree of hydrophilic to hydrophobic balance

to suit specific applications can be obtained.

During this period, the main emphasis was placed on (a) grafting of

monomers to ratskin, and (b) studying the mechanism of the grafting of

methyl methacrylate onto an insoluble collagen substrate.

(a) Grafting of Monomers to Soft Tissues (U. S. Army)

The ratskins were prepared by the procedure previously developed. In

some experiments anionic and non-ionic emulsifiers were added to the reaction mixture. Soluble homopolymers in the specimens were removed from

the products by subsequent extraction with acetone and ethanol for at least

24 hours. The treated skins were then dried in a vacuum and weighed. Significant weight increases were obtained with a variety of monomers such as methyl-, isodecyl- and glycidyl methacrylate, cyanoethyl acrylate, ethylene

dimethacrylate and ethylene glycol monomethacrylate. Furthermore, use of

comonomers such as ethylene dimethacrylate-methyl methacrylate or maleic

anhydride-methyl methacrylate increased the yield as compared to the use

of a single monomer. Increase in weight (based on the weight of the

original material) of up to 48% was obtained on grafting monomers to ratskin, although the average increase in weight ranged from 8 to 20%, as compared to an increase of 4% for a blank run in which addition of monomer

was omitted. Visual inspection of some of the graft products showed profound surface changes. For instance, the isodecyl acrylate graft yielded

a highly tacky surface.

25
Additional studies with collagen films gare considerable weight

increases, indicative of successful grafting, with the following monomers:

butenediol, isodecyl-, cyanoethyl- and cellusolve acrylate, diallyl phosphite and triallyl phosphate. Further investigations using fluorine containing monomers to obtain surfaces that are not readily wetted in an

aqueous environment are being initiated. Another aspect of assessing the

efficiency of certain co-initiators to increase the rate of grafting will

be explored.

(b) Characterization of the Bonding Site of the Grafted

Side Chain (NIDR)

The previous results indicate that a chemical bond is formed between

the collagen backbone and the graft polymer. Since the strength of this

bond should be high, it may be possible to utilize the grafting technique

to obtain improved adhesion. This grafting reaction may, therefore, be

of clinical usefulness in a variety of applications. Little is known,

however, of the type of attachment and location of the bonding site of

the side chain polymer on the substrate. An investigation has been initiated to obtain a better understanding of the mechanism of grafting to a collagen backbone and the mode of attachment of the side chain.

Methyl methacrylate was grafted in nearly quantitative yield (94.6%

after removal of soluble homopolymer) onto steer hide collagen powder.

The product was refluxed with 6N HC1 for 24 hours to hydrolyze the protein backbone. The insoluble residue was washed with dilute acid, base

and water to remove any aminoacid. Most of the grafted polymethyl methacrylate (70%) was recovered.

The water insoluble hydrolyzate showed a solubility behavior (soluble

in acetone, chloroform, methylene dichloride, benzene and dioxone) similar

to that of poly (methyl methacrylate). Its infrared spectrum is also

nearly identical with this polymer.

The insoluble hydrolyzate gave a positive protein test with ninhydrin.

It gave a negative test with o-phthaldehyde for glycine residues, a negative test with 1 ,2-naphthoquinone-4-sulfonate , vanillin and isatin-ninhydrin

reagents for proline or hydroxyproline , a negative test for tyrosine with

l-nitroso-2-naphthol . It gave a positive test for serine or threonine

(no ninhydrin color formation after treatment of the spot with NalO^).

Serine is the only amino acid present in collagen containing a primary

26
alcohol group. This group is much more likely to react with ceric initiator than the other amino acids containing secondary alcohol groups

(threonine, hydroxyproline , hydroxy lysine) . Furthermore, serine residues

are more abundant in the collagen backbone (3.1% of total residues)

than those of threonine (1.9%). Based on these preliminary studies, it

is postulated that the insoluble hydrolyzate has the following structure:

CH -

HO - CH

H

2

N - CH

COOH

C

cooch

3

CH - C

cooch

3 _ n

Further confirmation of this structure for the isolated graft is now

being sought by (1) elementary analysis, (2) dinitrophenylation , (3) additional spot tests, and (4) positive identification of glyoxylic acids

which should be one of the oxidation products when this serine derivative

is treated with periodate. The polymer chain will also be characterized.

This will include determination of the viscosity molecular weight and

presence of any stereoregular polymer by NMR techniques. Preliminary

data indicate that characteristic 60 Mc/sec NMR spectra of the polymer

cannot be obtained at room temperature because of its relatively low solubility. However, it is anticipated that spectra run in suitable solvents

at 150° C or above will increase resolution by minimizing dipolar broadening.

Another approach to establish the structure of the polymer will be

explored. Methyl methacrylate or other suitable monomer will be grafted

to the amino acids or poly (amino acids) present in collagent. From the

rate of grafting and the yields obtained in these reactions, the site of

the attachment of the monomer to the amino acid residue in collagen should

be established.

4.10. Centrifugal Tensile Strength Testing

Sponsor: NIDR

A collaborative study employing the centrifugal tensile strength

testing procedure was initiated with Dr. Forsten, School of Dentistry,

27
S.U.N.Y. at Buffalo, and Institute of Dentistry, University of Turku,

Finland. The measurements are designed to aid in a study of the effects

of manipulation variables on the strength of amalgam. Centrifugal tensile strength testing data is to be compared with a transverse specimen

testing procedure. Particular emphasis is being placed on determining

the decrease in final transverse strength of amalgams caused by delayed

condensation and the relation of this decrease in strength to the onehour strength and manipulation technique. Preliminary measurements indicate that the two strength tests similarly evaluate or correlate manipulation variables.

4.11. Adhesive Direct Filling Materials

Sponsor: ADA, NIDR (Grant)

Work has continued in the development of an esthetic, composite

dental material that can be used for both restorative and preventive

dentistry. The endeavors have been centered on optimizing the chemical

and physical properties of the material itself, and on improving means

of chemically bonding the material to hard tooth tissues. The composite

material itself consists of (a) reinforcing filler, (b) polymethacrylate

monomers, (c) stabilizers, (d) tertiary aromatic amine polymerization

accelerators, and other necessary additives.

Improvement, if not perfection, of composite dental materials is

incumbent upon the Dental Research Section since it was primarily responsible for the invention and development of this kind of restorative material. According to the latest estimate, something like 250 million of

these restorations have been placed, about 50% of all esthetic restorations.

(a) A necessary requirement of the reinforcing filler particles

is that they match the refractive index of the polymeric matrix rather

closely, and a desirable feature is that they contribute to the composite

material sufficient x-ray opacity for radiographic diagnosis of secondary

or underlying decay should this occur. Consequently, during this period,

a glass formulation containing 16.5 mole percent (31.9 wt.%) barium oxide

as the constituent contributing both the x-ray opacity and a predicted

refractive index that is desired for this application (n^ about 1.548)

was ordered and obtained from a commercial company. The formulation.

28
developed here and described in previous reports, theoretically takes

advantage of the "aluminoborate anomaly" that should give minimal coefficient of thermal expansion. Preliminary studies of methods of silane

treatment and composite preparation were made.

A manuscript describing "A New Series of X-Ray-Opaque Reinforcing

Fillers for Composite Materials" was submitted to the Journal of Dental

Research.

A project was initiated to determine the feasibility of making a composite material with a metal powder reinforcement. It is possible

that the malleability of the metal reinforcing filler would result in a composite with greater resistance to wear and attrition than the present

glass-filled composites. Various metals will be used and initial testing

will be to determine if the metals can be bonded to the resin matrix with

appropriate coupling agents. If such bonding is successful, physical

properties testing will be completed.

(b) The phthalate dimethacrylate monomers reported during previous

periods were the subject of U. S. Patent No. 3,539,526: "Ternary Eutectic

Dimethacrylate Monomer System and Restorative Dental Material Prepared

Therefrom", Rafael L. Bowen, Nov. 10, 1970. Dental manufacturers have

commenced to utilize these resins in some of their products.

The phthalate monomers have II-acid characteristics; work was commenced this period on the synthesis of monomers with n-base characteristics.

(See Synthesis of Monomers with n-base characteristics).

(c) Sterically hindered phenols are used as antioxidants and as stabilizers for monomers that polymerize by way of free radicals. 3,5-

Di-_t-butyl-4-hydroxybenzyl methacrylate (BHM) was synthesized. This compound

was demonstrated to be an effective stabilizer that contains, in the same

molecule, a methacrylate moiety capable of copolymerization in the methacrylate monomer system. This would be expected to have advantages both

with respect to polymerization kinetics and also from a toxicological

viewpoint since BHM or its decomposition products would become part of

the polymeric network.

The manuscript describing the synthesis of BHM was approved for publication by the National Bureau of Standards (WERB)

.

29
(d) Tertiary aromatic amines such as N,N-dimethyl-_p-toluidine

and N ,N-dimethyl-3 ,5-xylidine are currently used as polymerization accelerators in dental direct filling resins and in the commercial composite

direct filling materials. Since the lower-molecular-weight aromatic

amines are capable of skin penetration and are systemically poisonous

,

it is conceivable that they might contribute to the pulpal irritation that

is currently being reported by various investigators.

Hence, novel tertiary aromatic amines were synthesized, having high

molecular weights. The latest of these N,N-Bis (3-_p-n-butylphenoxy-2-

hydroxypropyl)-]3-n-butylaniline , a white crystalline solid (m.p. 98-103°C)

was prepared by custom synthesis by a chemical supply company. This and

other similar amines, prepared in the Dental Research Section previously

are currently being evaluated.

A manuscript entitled, "Tertiary Aromatic Amine Accelerators with

Molecular Weights Above 400", describing these compounds has been accepted

for publication in the Journal of Dental Research.

These amines are examples of a more general scheme to provide the

required ingredients with optimum functionality, utilizing maximum feasible

molecular weights ("bulkiness") of the organic compounds so as to minimize

undesired tissue penetration and untoward side effects in biomaterials

formulations

.

A summer student was trained in the procedures of testing adhesion

between dental materials and hard tooth tissues. He is helping evaluate

surface-active coupling agents, synthesized during previous periods. One

compound, NPG-CGE (the addition product of N-phenylglycine and p-chlorophenyl glycidyl ether) was compared with NPG-GMA (the reaction product of

N-phenylglycine and glycidyl methacrylate) for relative efficacy in

mediating adhesive bonding. The purpose of this comparison was to evaluate

the relative importance of dispersion-force interactions versus copolymerization between such primers and polymeric materials. These two compounds

were comparable (within the precision of the test method) and appeared to

give significantly stronger bonds compared with the controls (no coupling

agents)

.

As a prototype of adhesive varnishes or lacquers, a compound derived

from the diglycidyl ether of bisphenol A and N-phenylglycine was prepared.

30
This compound has two of the monovalent, tridentate chelate moieties on

the same molecule (in contrast to the compounds mentioned in the previous

paragraph which have only one such chelate group). Preliminary adhesion

tests were encouraging. Subsequent compounds in this series will have

more than two such chelate groups and will have higher molecular weights,

increasing until solubility limitations are encountered.

To obtain increased information on adhesive bonding mechanisms so as to obtain improved adhesion of the restorative and preventive materials to

the hard tooth tissues, preliminary or exploratory attempts were made to

synthesize eight new organic compounds. These included a methacrylate

monomer containing a urethane moiety, as a model compound to study the

mechanism of adhesion in polyurethanes, and various methacrylate monomers,

each containing a free carboxyl group, for the continuing study of surface

active comonomers. One methacrylate monomer contains an amine-BF^ complex,

for possible use in preventive dentistry.

4.12. Synthesis of Monomers with II-Base Characteristics

Sponsor: NIDR

Previously, it had been shown that the isomeric crystalline dimethacrylate monomers derived from 2-hydroxyethyl methacrylate and the corresponding acid chlorides of phthalic, isophthalic and terephthalic acid, respectively, formed a ternary eutectic that is a liquid at room temperature.

Although this colorless, slightly viscous liquid appears to offer many

advantages as a binder material for composites, it does suffer from the

fact that one or more of the isomeric monomers forms a yellow charge

transfer complex with tertiary amines (frequently used as polymerization

accelerators for such systems).

A way to circumvent the charge transfer complex problem is to employ

similar isomeric crystalline monomers that possess diminished n-acid

character. Three such monomers should be the bis (2-methacryloxyethyl)

ethers of hydroquinone , resorcinol and catechol, respectively. These may

be prepared by the following reaction scheme:

)ch

2

ch

2

oh

CHq

I 3

2CH

2

=C C0C1

Acetone/PYR

-PYR-HC1

CH0

I 3

CH

2

=CC0

2

CH

2

CH

2

chq

I 3

CH CH 0

2

CC=CH

(o,m,p)

31
Thus far, both the para isomer, bis (2-methacryloxyethyl) ether of hydroquinone and the meta isomer, bis (2-methacryloxyethyl) ether of resorcinol

have been successfully synthesized by this procedure. The para isomer

appears to have a melting point of 69-70° , whereas the meta isomer has

a melting point just above room temperature (m.p. 31-33°C). In view of

the favorable melting point relationship of the para/meta isomers, it

may be possible to prepare a liquid binary eutectic by the proper admixture of the two isomers. If this is not possible, then the ortho isomer

derived from catechol will be synthesized and, hopefully, a liquid ternary

eutectic of the three isomers may be prepared.

4.13. Three-Dimensional Panoramic Dental X-Ray

Sponsor: ADA, NIDR

Various modifications to the original panoramic x-ray unit are being

made to make the best possible stereo pair. An improved viewing device

is being developed to observe the results of the changes on the depth of

field in individual roentgenographs and the effect this has on the stereo

viewing

.

Associated with the viewing device is the design and construction of

the camera (the film carrier and image former). This will begin as soon

as the best system of optical grid or lenticular plate is selected. This

system will form the image on the film emulsion from the intensifying

screen. The thin lines formed on the film will be viewed through the

lenticular grid or plate to give some spatial relationship or stereo

effect

.

4.14. Correlation of Dimensional Change on

Hardening in Amalgam Restorations with

Marginal Leakage

Sponsor: ADA

The laboratory phase of the project continues as the quantity of

space is being determined between the tooth and the various dental amalgams

being used. Clinical restoratives are being placed to correlate laboratory

findings and long term clinical experience (3-5 yrs.). The various alloys

being used include those with extreme (60 ym/cm) dimensional change during hardening and two commonly used alloys, Spheralloy and Dispersalloy

.

32
Also, we have begun laboratory testing of a recently developed alloy

with 10% gold as a replacement for a portion of the silver in dental

amalgams. Should this material give satisfactory tensile and compressive strengths and acceptable dimensional changes during hardening, then

it will be included in the clinical testing program.

4.15. Clinical Research

Sponsor: ADA, NIDR (Grant)

Clinical evaluations of the composite restorative materials, the

effects of the adhesion-promoting coupling agent (NPG-GMA) , and of various

methods of polishing composite materials have been continuing.

Sixty amalgam restorations have been placed in the investigation

of the correlation between dimensional change on hardening of five amalgam

types and clinical and laboratory (air pressure) observation of marginal

leakage.

4.16. Specifications and Related Activities

Sponsor: ADA

A re-examination was made of the physical property tests currently

prescribed in ADA Specification No. 1 for alloy for dental amalgam. Five

currently approved alloys were subjected to the tests in order to evaluate the test methods. This is part of a series of tests on which to

base recommendations for future revisions.

A draft of the revised Denture Base specification was re-written,

incorporating the comments received by "all interested parties". Completion of this specification will require acquisition of data on the properties of pour-type resins and a study of the substitution of alginates

for tin foil in processing.

G. C. Paffenbarger attended meetings of two Joint FDI-ISO Working

Groups (Secretary), two sessions of ISO/TC 106 Working Group 3, and the

Plenary Session of the International Standards Organization Technical

Committee 106 for Dentistry, June 6-11, Munich, Germany.

G. C. Paffenbarger presided as chairman of the FDI Commission on

Dental Materials, Instruments, Equipment and Therapeutics, June 16.

33
4.17. Fiftieth Anniversary Symposium Proceedings

Sponsor: ADA

Twenty-four of the papers presented at the Symposium have been

edited and turned in to the NBS publications section for preparation

of the bound copy of the written proceedings of the symposium.

34