NBSIR 77-1397

Properties and Interactions of

Oral Structures and

Restorative Materials

James M. Cassel

Dental and Medical Materials Section

Polymers Division

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Annual Report for Period

July 1, 1976 thru September 30, 1977

November 1977

Issued: December 1977

Prepared for National Institute of Dental Research

Bethesda, Maryland 20014

1

NBSIR 77-1397

PROPERTIES AND INTERACTIONS OF

ORAL STRUCTURES AND

RESTORATIVE MATERIALS

James M. Cassel

Dental and Medical Materials Section

Polymers Division

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Annual Report for Period

July 1, 1976 thru September 30, 1977

November 1977

Issued: December 1977

Certain commercial materials and equipment are identified

in this report to specify the experimental procedure. In no instance does such identification imply recommendation

or endorsement by the National Bureau of Standards or that

the materials and equipment identified are necessarily the

best available for the purpose.

Prepared for National Institute of Dental Research

Bethesda, Maryland 20014

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

ABSTRACT i

SUMMARY ii

INTRODUCTION 1

TECHNICAL REPORT 3

I. Adhesion of Restorative and Caries-Preventive 3

Materials to Tooth Surfaces

A. Surface Characterization of Dental Materials 3

by Water Adsorption

B. Technique for Assessing Coupling Agents 4

C. Research for Improved Composite Restorations 7 and Sealants

D. Techniques for Bonding to Mineralized Collagen 31

Surfaces

II. Physical Properties of Hard Tissue and Dental Materials 41

A. Dimensional Change Measurements 41

B. Wear Resistance of Restorative Materials 51

C. Base Metal Alloy Research 59

REFERENCES 80

FIGURES

Page

continued

Figure 15: Casting size as a function of alloy type - manufacturer’s results

70

Figure 16: Casting size as a function of alloy type - NBS laboratory results

70

Figure 17: Casting size as a function of melting

temperature

71

Figure 18: Castability test results for one Ni-Cr alloy

as a function of casting temperature

78

Figure 19: Castability test results for one gold based

type III alloy as a function of casting

temperature

79
FIGURES

Page

Figure 1: Hardening times of composites containing varying

concentrations of tertiary amine accelerators

23

Figure 2: Tensile strength of composites as a function of

the concentration of tertiary amine accelerator

24

Figure 3: Tensile strength of cyanoacrylate to bone bond

after storage in water

36

Figure 4: Vitreous silica dilatometer used for measurement

of the thermal expansion of amalgam

43

Figure 5: Thermal expansion of amalgams - "conventional"

alloys

45

Figure 6: Thermal expansion of amalgams - "9% copper"

alloys

46

Figure 7: Thermal expansion of amalgams -"high copperlow silver alloys"

47

Figure 8: New multispecimen wear generating device and data

acquisition system for determining the wear

resistance of dental materials

53

Figure 9: Wear generating device with three independently

loaded wear heads

54

Figure 10: Human tooth enamel slider resting on dental

amalgam wear specimen. Sapphire tipped measuring

probes are beneath the LVDT on the left side of

the specimen. During operation the nozzle on the

right directs a jet of water on to the specimen

55

Figure 11: Example of problem encountered in preliminary

trials. The low points on the "wear" curves at

260 and 340 minutes are artifacts produced by

the build up of wear debris on the probes

58

Figure 12: Problems encountered during fabrication with

various ceramic alloys

60

Figure 13: Die used for wax pattern construction and

casting size determination

63

Figure 14: Pressed wax pattern, sprue and crucible former 65
ABSTRACT

Previous encouraging results with titanium and silicone based

coating agents for coupling dental resins to tooth surfaces have been

reinforced with accelerated aging data. Accelerated aging data for

bone bonded to bone specimens indicates that a viscous isoamyl cement

may be worthy of clinical evaluation. Procedures for synthesis and

purification of crystallizable materials offer promise as adhesion

promoters through reaction with collagen and may have utility as diluent

comonomers with Bis-GMA in composites or sealants. The feasibility of

generating free radical induced polymerization of dental resins by using

more thermally stable organic peresters or hydroperoxides and an ascorbic

acid ester has been demonstrated. Design, assembly and debugging of a

new multispecimen wear instrument has been completed. Judging by their

thermal expansion characteristics, high copper type amalgams show little

or no evidence of phase changes at temperatures less than 9 °C. The

technique sensitivity of the higher melting non-precious bonding alloys

has been demonstrated by examining quantitatively the goodness of fit

attained by manufacturers of such alloys who made castings to a standard

die and wax pattern that we supplied. A technique to fabricate oversize Ni-Cr crown castings is described.

i
SUMMARY

The research on dental materials that is described in this report

is a part of a larger total effort that is carried out in collaboration

with the American Dental Association Health Foundation. Also part of

this collaborative effort through support funding and/or assignment of

research personnel are the Armed Services Dental Corps. In addition,

the National Association of Dental Laboratories, representing some

3,000 prosthetic laboratories, is a participant via the research associate program of the National Bureau of Standards.

While the bulk of the research progress in this report represents

the efforts of persons supported under the Interagency Agreement with

NIDR, there will of necessity be a degree of contribution that originates

through the intentionally strong interaction among all persons participating in this collaborative dental program.

The objective is to effect improved dental health through development of new knowledge and better understanding of mechanical, chemical,

and physical properties of restorative and caries-preventive materials

and their interaction or relationship with the natural components of

the oral structure.

A wide variety of techniques has been brought to bear to generate

approaches by which new and improved adhesive restorative dental

materials might be developed. These include surface characterization,

methods for evaluating coupling agents, synthesis to improve todays

composite restorative materials and pit and fissure sealants, investiga11
tion of new approaches to bonding restorations to tooth surfaces and

measurements of critical dimensional changes in newer type amalgams. A

wear-generating and recording system, redesigned and built to allow

multiple specimen examination has been debugged and is in initial stages

of evaluation. Procedures for quantitative assessment of the goodness

of fit of dental alloy castings, gold or non-precious types, have been

developed.

From analysis of water adsorption data including calculation of heats

of adsorption, we conclude that these types of data do not aid in predicting the potential of a coating agent as a coupling agent between the

resin matrix of a composite and the hydroxyapatite of a tooth surface.

Previous encouraging results with two coating agents assessed as promising

coupling agents by an indirect mode of tensile adhesion testing, have been

reinforced with accelerated aging data.

We have made several advances in our efforts to develop highly

purifiable, less viscous methacrylate monomer systems as substitutes for

the currently used Bis-GMA resin which is excessively viscous and inherently

difficult to purify. Using a fast amine accelerator we have shown that

both the hardening rate and diametral tensile strength of composites prepared from resin systems Bis-GMA/Triethyleneglycol dimethacrylate and a

highly purifiable ether-linked aromatic dimethacrylate system requiring no

diluent are comparable. We have synthesized but have not purified

a "star" methacrylate monomer that appears to be less viscous than

Bis-GMA despite an expected two-fold increase in molecular weight and

iii
I

1

should undergo less volume change on polymerization. We have

demonstrated procedures for synthesis and purification of an isomeric

system of aromatic aldehyde dimethacrylates. The system is considered

primarily as promising for evaluation as adhesion-promoting agents but

since the component isomers are soluble in Bis-GMA, they offer utility

as diluent comonomers for composites or fissure sealants. Five methacrylate monomers with isocyanate functional groups capable of reaction

with the collagen of dentin have been synthesized and isolated in varying

degrees of purity. These monomers, miscible and copolymerizable with

Bis-GMA will be examined as adhesion promoters and as components of

composite resin systems.

To improve the thermal stability of composite systems, we are examining

potential substitute peroxides for benzoyl peroxide. Using ascorbyl

palmitate as the means of generating the free radicals from more stable

organic peresters or hydroperoxides, we have shown cure times comparable

to those normally observed with the conventionally cured Bis-GMA system

that employs benzoyl peroxide and dimethyl-p-toluidine.

Based on amine structure-property relations that we have established,

we were encouraged to synthesize several new amines as potential polymerization accelerators in the anticipation that they would be more efficient

and less toxic than those currently employed with dental resins. Dimethylaminophenyl acetic acid, based on results achieved to date, appears particularly to be of promise. Effect of amine concentration on physical

properties of composites has been determined for both currently used and

experimental amines. Tensile strength is a more sensitive measurement to

IV
the type and concentration of amine than is compressive strength with

the maximum in tensile strength of the composite being achieved only

over a narrow concentration range of amine accelerator.

A study of methods for evaluation of pit and fissure sealants has

been concluded. A series of relatively simple test methods has been

proposed

An investigation of the potential application of cyanoacrylates as

adhesion promoters for dental application has been extended to include

long term body temperature as well as accelerated aging. After 6 months

storage in water at 37°C, n-amyl, isoamyl and a viscous isoamyl cyanoacrylate bonding to bone was reduced 30-35%, a reduction much less than

that shown with the ethyl derivative. Accelerated aging for one month

at temperatures cycled between 5° and 55°c gave comparable results. The

viscous isoamyl cyanoacrylate with the highest initial bond strength to

bone appears to be the least degradable in water and hence may be worthy

of clinical evaluation, provided its toxicity is acceptable. However, the

results to date indicate that the presently available non-filled 2-cyanoacrylates are of limited use for dental application as adhesives due to

their degradability and their inability to serve as a gap filling adhesive.

As a part of a general investigation of the physical and clinical

properties of amalgams made from some of the recently introduced alloys

with high copper content, the thermal expansion characteristics of the

amalgams have been determined. Information on the stability of the

amalgams at elevated temperatures was sought rather than coefficient of

v
expansion data. In addition to a "normal" reversible thermal expansion

of about 25 ppm per °C, "conventional" amalgams exhibit large dimensional

changes that are time dependent as well as temperature dependent. The

high copper alloy amalgams show little or no evidence of phase changes

within the range investigated. It appears that the general features of

the three types of thermal expansion curves are related to the peritectic

transformation ^

(Ag-Hg) 3 (Ag-Hg) + Hg and the diffusion reaction

$ (Ag-Sn) + Hg •**"—^ (Ag-Hg)

Design and assembly of a new multispecimen wear instrument and data

collection system has been accomplished. Debugging of the equipment has

not been excessively difficult and wear data on dental materials is

anticipated to show an even more significant difference between classes

of direct restorative materials and at a higher confidence level than was

shown with the prototype single specimen device.

A significant problem with non-precious alloy crown and bridge fabrication as identified by a survey undertaken of prosthetic dental laboratories

is the difficulty in attaining the required goodness of fit in such

castings. The degree of problems with fitting was shown to be in inverse

order to the noble metal content of the alloy employed. In like manner

the problems with porcelain fused to metal restorations were seen to be

major with non-precious alloys and minimal with high noble metal alloys.

We have verified the technique sensitivity of the higher melting nonprecious, e.g. Ni-Cr, porcelain bonding alloys exhibited in our early

laboratory experimentation, by examining quantitatively the goodness of

vi
fit attained by manufacturers of such alloys who made castings to the

standard die and wax pattern which we supplied. A gold based alloy as

well as a non-precious alloy offered by the manufacturer were cast at

the same time. In every case, the high nobel metal alloys were cast

oversize whereas all non-precious alloys were cast undersize. This

consistent difference in casting size between the two alloy types under

a wide variety of laboratory techniques could discourage use of Ni-Cr

alloys for operators unwilling to use shims or die coats (of questionable

accuracy) or to machine out the interior of undersized castings to permit

seating and cementation.

We have reported herein a technique modification involving hand

spatulation, dry asbestos, and hygroscopic expansion that provides

sufficient expansion for one phosphate bonded investment at least, to

permit fabrication of oversize Ni-Cr castings.

We have shown that casting size is dependent on melting point of

the alloy with casting diameter decreasing linearly with increasing

melting point under fixed mold conditions.

Cooperatively with the Armed Forces Defense Personnel Support Center

and the U. S. Navy, we have developed a castability test applicable to

either Ni-Cr or gold alloys.

vn

INTRODUCTION

The National Bureau of Standards is primarily a measurement organization. Paths from measurement to improved dental service lead from

measurement and characterization to development of new techniques for

using materials, to laboratory prototypes of improved materials and

devices, to clinical evaluation and, finally, commercial production

by industry.

The dental research program at NBS is a cooperative effort both

in personnel and funding, involving the American Dental Association,

the Armed Forces Dental Corps, and the National Association of Dental

Laboratories. The research reported in the following pages is only

that portion of the dental program which is supported by interagency

agreement with NIDR.

The objective of the research effort discussed briefly in the

following pages is to effect improved dental health through development

of new knowledge and better understanding of the mechanical, chemical

and physical properties of materials used or potentially of use in

restorative and preventive dental practices. An outline of the

research as approved in the interagency agreement is given below:

I. Adhesion of Restorative and Caries-Preventive Materials to Tooth

Surfaces

A. Surface Characterization of Dental Materials by Water

Adsorption*

B. Technique for Assessing Coupling Agents*

*Areas of work supported by interagency agreement only for first half

of year.

1
C. Research for Improved Composite Restorations and Sealant

Materials

D. Techniques for Bonding to Mineralized Collagen Surfaces

II. Physical Properties of Hard Tissue and Dental Materials

A. Dimensional Change Measurements

B. Wear Resistance of Restorative Materials

C. Base Metal Alloy Research

2
TECHNICAL REPORT

I. Adhesion of Restorative and Caries-Preventive Materials to Tooth

Surfaces

A. Surface Characterization of Dental Materials by Water

Adsorption

For several years we have used adsorption measurements from vapor

phase as an analytical tool for characterizing dental materials, both

natural and synthetic [1,2,3]. Relative to the development of improved

coupling agents between restoration and tooth surface, we have shown

that the surface area available to water molecules on a "coupling"agent -

coated hydroxyapatite (HA) surface is reduced by the coating, irrespective

of whether the coating agent could be shown by any other means to represent a poor or good coupling agent. Therefore surface area, as determined by adsorption measurements, is not a viable means of distinguishing

between coupling agents.

Since the heat of adsorption defines the intensity of interaction

on the surface, this aspect of vapor adsorption has now been examined.

Employing BET equation [4], isosteric heats of adsorption (AE) were

obtained from the c-parameter for uncoated and silane* coated HA (Table 1)

.

AE represents the difference between the average heat of adsorption for

the first monolayer of water vapor and the heat of liquif ication of

water at 23°C.

Coating with the silane coupling agent shown by other means to be a

potential coupling agent (IB of this report) , is seen to nearly double

the value for AE. Since the lower concentration of silane

* 3-methacryloxypropyltrimethoxy silane

3
was more than sufficient to form a monolayer, the four-fold concentration

increase, corresponding to the high concentration of Table 1; did not

further increase the heat of adsorption. These observations also suggest

that the water vapor did not penetrate the silane coating.

TABLE 1

Comparison of Heats of Adsorption (water) of Uncoated and Silane-Coated

Hydroxyapatite.

AE Cal/mol

Sample 1 Sample 2

Uncoated Hydroxyapatite 850 890

Silane-Coated Hydroxyapatite

(low concentration) 1600 1560

Silane-Coated Hydroxyapatite

(high concentration) 1620

B. Technique for Assessing Coupling Agents

We have described in several Annual Reports [5,6] a novel indirect

testing procedure designed to minimize some of the disadvantages that are

inherent in routine butt-joint adhesion testing to enamel surfaces. We

have utilized this test procedure to evaluate the capabilities of coupling

agents for adhering resin restorative systems to tooth surfaces. The

test employs tensile strength measurements on composite specimens that

are prepared by dispersing varying amounts of hydroxyapatite powder that

has been coated with a coupling agent of interest in a resin matrix

4
(Bis GMA)* and then polymerizing. Specimen tensile strength is plotted

as a function of the particulate weight fraction for each series of tests

involving the same coupling agent. The more negative the slope of such

plots, the poorer is the coupling agent. The choice of synthetic amorphous

HA (as opposed to a fibrous state) precluded any structural reinforcement

which would have interfered with the interpretation of the effect of the

coupling agent being evaluated. A description of the test procedure and

some results obtained with it is scheduled for publication in a current

issue of the Journal of Dental Research [7],

In the previous Annual Report [6] we have described results obtained

by the indirect testing procedure that indicated good coupling between

HA and Bis-GMA was achieved by using either of two coating agents, isopropyltrimethacryl titanate (TI) or 3-methacryloxypropyltrimethoxy silane

(SI).

During the current reporting period, a means of accelerated aging

was adopted to assess the stability of the coupling effect achieved with

the two promising coating agents. The composite specimens were directly

exposed in a water bath for one month to temperatures that cycled between

7°C and 60°C with a two hour duration at each temperature. This rather

severe aging procedure produced a weakening of the adhesion developed

with either of the promising coating agents TI or SI. For example, when

tensile strength of the non-aged composite specimens containing varying

amounts of coated HA powder suspended in Bis-GMA was plotted against the

* Abbreviation used to describe addition product of biophenol A and

glycidyl methacrylate.

5
2/3 power of the particulate weight fraction, the tensile strength fell

off from about 30 MPa at zero weight fraction (i.e., resin only) in a

reasonably linear fashion giving slopes of -18 MPa and -19 MPa respectively for the SI and TI specimens as opposed to -94 for uncoated HA

specimens (Table 2). Upon accelerated aging for one month a decrease in

strength of the coated specimens was observed as indicated by increased

slope values for both TI and SI specimens of -69 MPa.

Table 2 2/3

Slopes of Tensile Strength vs. (Particulate Concentration)

Non-Aged After Accelerated Aging

(MPa) (MPa)

SI -18 -69

TI -19 -69

NPG-GMA* -66 —

Uncoated -94

* N-phenylglycine glycidyl methacrylate, a coupling agent developed by

R. Bowen previously in this laboratory [8].

That the silane and titanate coupling agents do have potential is

shown by the fact that the slope values of the severely aged specimens

were not essentially different from that found with non-aged NPG-GMA

coated specimens. NPG-GMA has been considered the best coupling agent

for its purpose developed to date although it has found only limited

applications

These results are being prepared for publication as an annotation

in the Journal of Dental Research.

6
C. Research for Improved Composite Restorations and Sealants

In spite of the many merits of dental composite restorative and

sealant materials, there is still need for further improvements, especially in the two critical areas of adhesion to dental tissues and overall

stability.

An important constituent of these materials is the dental resin

system. The current most widely used resin systems are based on the

monomer, Bis-GMA, developed by Dr. R. L. Bowen [9] of this laboratory.

This work represented a major breakthrough in the area of dental composite

and sealant materials. However, in spite of its many excellent properties,

it does suffer from several deficiencies, e.g. a color problem which may

be inherent in its chemical structure or due to impurities, and a highly

viscous nature which necessitates the use of a diluent monomer. The use

of the usual thinner comonomers (e.g. triethyleneglycol dimethacrylate,

methyl methacrylate, etc.) increases the polymerization shrinkage of the

system and may make it less biocompatible (e.g. due to volatility, tissue

penetration)

.

1. New Resin Systems

In this laboratory we have pursued several possible solutions

to these problems. A major emphasis has been placed on the evolvement

of a resin system capable of being highly purified and not requiring a

diluent to achieve the workable viscosity necessary for clinical application.

To achieve high purity in the resin component we have developed systems of crystallizable isomeric dimethacrylate monomers. We have

shown that when mixed in proper proportion, eutectics with melting point

7
less than room temperature are obtained and the viscosity of the resulting

liquids is low enough that dilution is not required.

The first of such isomeric monomer systems was that developed

by Bowen [10] in this laboratory on an NIDR supported grant. This system

consisted of three aromatic diester dimethacrylates derived from the condensation reaction of the appropriate acid chlorides of the phthalic acid

isomers with 2-hydroxyethyl methacrylate. The phthalic acid derived

monomer system unfortunately developed a color instability when polymerization was induced by the usual benzoyl peroxide-tertiary amine initiator

system. The source of the color instability was traced to the formation

of colored charge transfer complexes involving the pi electron deficient

aromatic diester dimethacrylates and the pi electron rich tertiary aromatic amine accelerator [11].

To circumvent the formation of a colored charge transfer

complex, we synthesized two analagous crystalline dimethacrylate monomer

systems with greater pi electron densities [12,13]. One system is based

on the dimethacrylate derivatives of the isomeric dihydroxybenzenes

(e.g. hydroquinane, resorcinol and catechol), the other on dimethacrylate

derivatives of the three corresponding hydroxybenxoic acids. These systems,

the one a diether, the other an ether-ester, proved to be color free in

the presence of tertiary aromatic amines, and were readily purified by

recrystallization. On proper admixture they formed ternary liquid

eutectics of moderate viscosities at room temperature.

a. Some Property Data on the Diether Methacrylate

System

In this report period we have examined some of the

8
properties of composite systems prepared from these monomer systems. With

a slow amine accelerator such as N ,N-bis (2-hydroxyethyl) p-toluidine , the

hardening time of a system comprised of the eutectic diether monomer system and glass powder coated with benzoyl peroxide was more than twice that

observed with 70% Bis-GMA/30% TEGDMA*. With DMSX**, which is a faster amine

accelerator, hardening rates were comparable as were the diametral tensile

strengths, i.e. 37 and 43 MPa respectively. This result is similar to

that reported earlier for the eutectic diester monomer system [14].

b . "Star" Monomer Synthesis

Even though the Bis-GMA based composite restorative

systems significantly reduce the volume change that occurs on in situ

hardening relative to that which occured with direct filling resin systems,

it has been recognized that further reduction may be required if adhesion

to the cavity wall is to be possible. To this end we have prepared a

tetra star monomer from pentarythritol and an isocyanato urethane

methacrylate as shown below:

4 CH

2 -C(CH2

) CC>2 (CH2>20

2 CNH CH 3 + C(CH2 0H)^

NCO

ch

2 =c(ch

3 )co

2 (ch

2

) 2 o

2 cnh CH

3

NH

CO CH

4

* Triethyleneglycol dimethacrylate

** Dimethyl symmetrical xylidine

9
While we have improved the color of the reaction product through use

of decoloring charcoal, we have not, as yet, isolated the monomer as a

crystalline solid. The material is challenging in that the pale yellow

liquid appears to have a viscosity less than that of Bis-GMA despite an

expected two-fold increase in molecular weight.

c . Synthesis of Functional Monomers

Research to develop adhesive bonding to the dentin, the

collagen-containing component of tooth structure, has progressed in several

areas. (1) Aldehyde Methacrylates

The natural reactivity of aldehydes toward

collagen suggested the synthesis of aldehyde methacrylates as a potential

means of coupling dental resins to dentin. We have previously [6] reported on the synthesis, purification, and characterization of the monomers

p- (2 -methacryloxyethoxy) benzaldehyde and 0-( 2 -methacry loxy ethoxy)benzaldehyde. These were derived as crystalline solids (hence in high purity)

which melted respectively at 47-48°C and 52-53°C. The meta isomer derived

from m-hydroxybenzaldehyde melted below room temperature and was difficult

to purify.

In the present report period we completed the

purification and characterization of meta(2-methacryloxyethoxy)benzaldehyde (MMB) . The isomer was isolated as a white crystalline solid, m.p.

21-22°C, after treatment with decolorizing charcoal in ethanol and after

several recrystallizations from a mixed solvent of ethanol and m-hexane

(1:1 v/v) at 20°C. Elemental analysis calculated for C-j^H-^O^ was

C 66.7%; H 6.02%; found was C 66.8%; H 5.95%. In comparison with the

para (PMB) and ortho (0MB) isomers, MMB is the lowest melting of the

10
(2-methacryloxyethoxy)benxaldehydes. Not only are ternary liquid mixtures

of these isomers easily prepared, but binary liquid mixtures of MMB with

the other two isomers are easily prepared. MMB itself is easily liquified

by only slight warming (e.g. exposure to body heat).

As expected, MMB undergoes free radical polymerization under a variety of conditions (e.g., by chemical initiation with

benzoyl peroxide and N ,N-dimethyl-p-toluidine , by modest heating in the

absence of air, and by photo initiation, e.g., exposure to ultraviolet

light in the presence of methyl ether of benzoin.)

Photopolymerization can occur without the use of

a photoinitiator (e.g. methyl ether of benzoin) but at a much slower rate.

This system of aldehyde methacrylates is considered primarily as a

promising candidate for evaluation as an adhesion-promoting agent

although it may have further utility as diluent comonomer for dental

composites and pit and fissure sealants since the monomers are miscible with Bis-GMA.

This work on aldehyde methacrylates was presented

at the 1976 IADR Meeting in Copenhagen and a manuscript has been accepted

by the J. Dent. Research.

(2) Monomers with Isocyanate Functional Groups

A second type of methacrylate monomer that we

have synthesized to contain a functional group capable of reaction with

the organic (collagenous) phase of bone or dentin is the isocyanate

methacrylate. We have completed synthesis (as per schemes below) and

isolation of five such functional monomers.

11
0 CH-j

II I 3 + HOCH2CH2OC-C-CH2

ipdi HEMA IPKI-1IEMA (MW -v 352 )

TDI

0 cu_

+ HOCH2 CU2 OC-C^Cl^

25 °C

n-hcxane

HEMA

0 0 CH 3

II li

| HHC-OCIUCH-OC- C--CH,

l_„

TDI—HEMA (MW ^ 30 '.) m.p. 67 -6 C°C

OCNRNCO +

0 CII3

il 1 J HOCH

2CH

2OC-C=CH2

25 *

n-licxanc

(Et

3N)

HDI[H- (CIIy

) 6

) HEMA

CII3 CII3 TMDJ (K- CII 2-C"Cn

2-CH-CH2

-Cll2

] ^H3 PD7(R"C3() Hydrocarbon]

MW GOO

0 0 pH,

D II 1 J OCN'L\1 IC-OC!I2 CH

2OC-C-CI! 2

HDI-HEMA

(MW ~ 298 )

TMD1 -1 IEMA (MW ^ 340) nm-HEMA . (MW ^ ? 30 >

12
The physical state in which the three monomers were isolated is as shown

below:

DDI-HEMA

IPDI-HEMA

TDI-HEMA

HDI-HEMA

TMDI-HEMA

Pale yellow liquid

Yellow viscous liquid

White crystalline solid

White solids

Pale yellow liquid

Elemental and NMR analyses were used to confirm the structures of TDI-HEMA

and IPDI-HEMA. Infrared spectra on all five isocyanate methacrylates was

consistent with assigned structures. Each was shown to polymerize in 2-8

minutes via chemical initiation by the benzoyl peroxide-amine catalyst

system and to polymerize rapidly by photoinitiation in the presence of a

photosensitizer. These monomers were also shown to be miscible and

copolymerizable with the Bis-GMA system. In addition to evaluation as

an adhesion promoter, DDI-HEMA deserves consideration as a component in

a composite resin system since DDI-HEMA has very large bulk that can be

expected to give low polymerization shrinkage while at the same time it

possesses a workable viscosity.

posite formulation depends to a large degree on the thermal and chemical

stability of the peroxide component of the initiator system. Further,

there is the relative color instability inherent in the tertiary aromatic

amines used as accelerators of the polymerization. The discoloration

observed when an otherwise acceptable composite resin system, i.e. the

This work was presented at the 1977 AADR

meeting in Las Vegas.

2 . New Initiator/Accelerator Systems

The storage stability of a typical paste-paste dental comtertiary eutectic of the three isomeric dimethacrylate derivatives of

phthalic acid, employed the benzoyl peroxide-tertiary amine catalyst has
spurred a search for accelerators other than aromatic amines. Because

many of the usual ambient temperature free radical initiations are based

on some kind of oxidation-reduction reaction, we have begun an investigation of several types of organic peroxides and the natural non-toxic

reducing agent, L-ascorbic acid. Progress in these areas is summarized

in the following pages.

a. New Initiator Systems

We have begun an investigation of the feasibility of

substituting for benzoyl peroxide, peroxides that have improved thermal

(and hence storage) stability such as the organic peresters, e.g. t-butyl

perbenzoate, and hydroperoxides such as cumene hydroperoxide. These

chemical structures are given below:

The chemical activation of these types of peroxides to yield initiating

radicals usually requires the presence of a redox metal compound such as

iron, copper, cobalt, etc., salts. These metal salts are reductants in

+1 +2 their lowest oxidation state (e.g. Cu , Fe , etc.) and can transfer an

electron to the peroxide (oxidant) bringing about its decomposition to

initiating radicals. They are considered extremely potent activators

for peresters and hydroperoxides. However, there are several problems

associated with the use of these redox metal activators in a dental

monomer system. If the metals are used in the concentrations comparable

to those employed with the amine type activators, both color and toxicity

14
become problems. In addition, the metal activators which need to be in

I I their lower oxidation states (e.g. Cu , Fe ) to be effective, are susceptible to oxidation on storage to their higher valance state (Cu+2,

Fe ) which makes them less effective.

As a way of circumventing these problems we are investigating the natural relatively mild reducing agent, L-ascorbic acid

(Vitamin C) , as a possible activator for these peroxides. The chemical

structure of ascorbic acid is

C

Ic

I HO-C (

ll- HO-C

i K-C

l 110- Cil

ICH

2

0H

Ascorbic Acid

(Vitamin C)

Because of the limited solubility of ascorbic acid in most monomers, we

also investigated ascorbyl palmitate, which has a better solubility in

most monomers, as an activator for peresters and hydroperoxides. For the

same Bis-GMA type monomer systems, we obtained cure times comparable to

those obtained with ascorbic acid.

A comparison of the two activators is shown below for

a monomer system consisting of 70% Bis-GMA and 30% triethylene glycol dimethacrylate. We used a powder/liquid formulation with the ascorbic acid

and palmitate deposited on the silanized glass beads. The monomer contained the perester, t-butyl perbenzoate. By means of a cross slide

15
immobilization technique, the working times were determined. Working times

of 2-1/2 minutes are considered clinically acceptable. Table 3 and Table

4 summarize some of our results. As can be seen, the lowest working

times were 4 minutes using ascorbyl palmitate as the activator. By using

extremely small amounts of cupric and ferric salts the working time can

be further reduced to the 2 to 3 minute range. Table 5 shows the typical

effect of a redox metal compound such as cupric acetate on the working

time with the same monomer system with 1.5% TBPB and glass beads (0.45%

ascorbic acid). For composites prepared with glass beads (0.3 % ascorbyl

palmitate) the working times were extremely short (under 2 min.). Cupric

and ferric salicylate in catalytic amounts also were effective in reducing

the working times to under 3 minutes. Without the ascorbic acid or

palmitate being present, no polymerization occurred in these systems.

To check for effects of other metal impurities, we

obtained a standard reference material for copper, SRM-1080A, which is

cupric benzoylacetonate . This copper compound is slightly soluble in the

monomer and we prepared several solutions having from 0.001 to 0.0001%

+2

Cu . A slight light blue coloration was noted in the more concentrated

solutions but no coloration was visible at 0.0001% level.

In addition, it appears possible to combine the perester

TBPB, with this cupric compound without encountering any storage problems.

As indicated earlier, the redox metals are effective activators for these

peroxides only in their lower oxidation states. The ascorbic acid or

palmitate apparently reduces the cupric or ferric cation to the corresponding cuprous or ferrous state which will immediately activate the

16
decomposition of the perester. Simultaneously ferric ions and cupric

ions are reformed, which because of the presence of the ascorbic acid,

are reduced again, thereby creating a chain activation mechanism for

the decomposition of the peroxide. Composites prepared using these

minute quantities of copper have shown no signs of extraneous coloration

and appear similar to those prepared without these catalytic amounts of

redox metal.

Using a liquid monomer formulation consisting of 70%

Bis-GMA and 30% TEGDMA (triethylene glycol dimethacrylate) and a glass

powder properly treated to enhance reinforcement we have prepared composite tensile strength specimens using various initiating systems. Comparison has been made with the conventionally cured material, e.g. benzoyl

peroxide-dimethyl-p-toluidine. Representative values of the diametral

tensile strength measured after 24 hours are given in Table 6. The

highest values were obtained with monomer containing minute quantities

of copper (as copper benzoyl acetonate) which may be indicative of a

more efficient initiator system.

These new types of initiator systems, by employing

more stable peroxides, e.g. peresters, and more "natural" reducing

agents or accelerators (e.g. ascorbic acid) offer promise for improving

the storage and color stability, the biocompatibility and other properties not only of dental composite and sealant materials but also other

medical materials based on the concept of "cold-curing" resin systems.

17
Table 3

Working Tines for Class Reads (0.452 Ascorbic Acid)*

and Liquid Monomer (0. 52 TBPB)**

Trial • Li quid /Powder Ratio Consistency of Paste

'Jor't

1 1/3 Thick 7

2 1/3 Thick 8 •

3 1/3 Thick 6

4 1/2 Fluid 6

5 1/2 Fluid 9

6 1/1 Very Fluid 8

7 1/1 Very Fluid 10.

* Class 1beads (silanlzcd) prepared by depositing ascorbic acid from 32 Ethanol jc solution using a rotary evaporator.

** Liquid monomer consisted o£ 70Z Hit:- CHA and 302 TIXUXA and contained 0.52 TBPB Bis-CUA The adduct of Lisphenol A and glycidyl methacrylate

TEGDi li *= Triethylcne glycol dimi-thacrylate TBrB - t-butyl rerbenzoalc

Table 4

Working Times for Class Beads (0.302 Ascorbyl l'laiaitatc)* and Liquid Monomer (1.52 TOPE)**

Trial Liquid l'owder Ratio r. . . , _ Working-Time Consistency of Paste . .

. (min)

1 1/1.7 Fluid 11

2 1/0.84 Very Fluid 6

3 1/2.5 Medium 4

4 1/1.96 Medium 8 5 1/1.91 Medium 5

6 1/2.7 Thick 5

7 1/3.2 Very Thick 7 8 1/33 Very Thick 4

9 1/3.4 Very Thick 4

10 1/36 Very Thick 4 11 1/3.6 Very Thick 4

* Class beads (sllanizcJ) ptepared by

32 ctbanolic solution us 1 ng a rotar deposit ini; ascorbyl pa)nJt*lc

y pv.ipoiotor.

from

** Sane nonniaev as In Table 3 except concentration of TBPB is 1.52.

Table 5

Effect of Cupric Acetate |Cij(OAC) ) on Working Time

Trlol Liquid/Powder Ratio Percent (Cu(OAC)j) in Working-Tine

Composite (min)

1 1/3 0.057 2

2 1/3 0.029 3

3 1/3 0.0)9 2 >or compos J i os ptc;orcil vllh glass IhmiIs (0.3/1 asroibyl plami laic) the working

timer; uiii- extremely short (mulcr 2 mJn). Cupric and ferric salicylate in calaylic aiM'uuti. also wore. iffictive in’ rrdudili; the workinj; limes to under

llitec 11 tliules.

18
Table 6

Comparative 24-hour Diametral Tensile Strength of a Typical Dental

Composite Prepared with Various Initiating Systems

Sample

Number Formulation TS (psi)

1 A. (0.45% A. A.) 3 parts 5832

B. (1% TBHP) 1 part

2 A. (0.45% A. A.) 3 parts 5784

B. (1% CHP) 1 part

3 A. (0.45% A. A.) 3 parts 6631

B. (1% TBPB) 1 part

4 A. (0.45% A. A.) 2-1/2 parts 8277

B. (1% TBPB + 0.0001 Cu+2

) 1 part

5 A. (0.75 BP) 3 parts 6337

B. (0.23% DlfPT) 1 part

A = glass powder consisting of 67% fused silica plus 33% of an x-ray opaque

BAF2 containing glass, silanized with -Y-mcthacry] oxypropyltriethoxy silane

and sieved (100 mesh). This glass was coated with ascorbic acid (A. A.) or benzoyl peroxide (BP).

B •= liquid monomer consisting of 70% Bis-GMA [bis (3-mcthacryloxy-2-hydroxypropyl) bisphcnol A] and 30% TECDMA (triethylencgycol dimethacrylate)

Peroxides used:

TBHP « Tert-butyl hydroperoxide

CHP c Cumene hydroperoxide

TBPB *= t -butyl porbenzoate

BP = benzoyl peroxide

Cu+2r: ion form of copper benzoyl acetonate

Typical auine accelerator *= DMPT (dimethyl-p-toluidine)

19
b. Improved Amine Accelerators

A number of amines have been suggested as accelerators

for dental resins but esthetic and biocompatibility requriements have

greatly limited the number of compounds that can be used for this application. The currently used tertiary dialkyl aromatic amines that are

used in the benzoyl peroxide initiated polymerization of methacrylates

suffer from a basic color instability and relatively high toxicity index.

Part of the discoloration problem of dental composites probably is

traceable to the color-prone amine accelerators.

The amines investigated in this report period were

either those used in current composite restorative materials, e.g.

dimethyl-p-toluidine (DMPT) , dihydroxyethyl-p-toluidine (DHEPT) dimethylsymxylidine (DMSX) ; those predicted to be highly reactive based on

structure-property relationships, (i.e. free energy relationships) e.g.

4-N ,N-dimethylaminophenyl acetic acid (DAPAA) or its methyl ester

(MDMAPAA) , methyl 4-N ,N-dimethylaminoglutethimide (DMAG) ; or some amines

previously synthesized in this laboratory which had not been evaluated,

e.g. di-2-hydroxypropyl-p- toluidine (DHPPT) , low and high melting isomers

of bis(3- p - tolyloxy-2-hydroxypropyl)m-xylidine (BTX) , dimethylaminobenzaldehyde (4-DMAB) , tetramethylammonium 4-dimethylaminobenzaldehyde (TMADMAB)

lithium 4-methylaminobenzoate (LDMAB) , 3-dimethylaminobenzoic acid ( 3-DMABA)

and dimethylaminopyridine (DMAP)

The structure-property relationships found in this

laboratory [15] for correlating the reactivity of tertiary amines with

benzoyl peroxide in vinyl monomers indicated that amines with aryl(ring)

substituents have a a+

value [16] close to -0.20 would be the most reactive.

20
A library survey indicated that p-Cl^-COC^H^ has a o+ value of -0.16,

thus recommending the potential of phenylacetic acid or ester derivatives

The a+

for the aryl substituent of aminoglutethimide is not known but

is expected to be near that of p-tert-butyl (-0.26).

The several amine syntheses carried out in this report

period are shown below:

DAPAA (and MDMAPAA)

h

2n

CH I ch

2 cooh >

k

2 co

3 DMSO, 100 °C

(ch

3

) 2 n ch

2 coor

R = H Crystalline solid, m.p. 110-112°C

R CH

3 Pale Yellow liquid

DMAG

\ /

c-ch9 H

0

toluene

reflux

(-ch 3 oac

)

+

OAC

T

HN CH

? \ /

C -CH

2

// 0

Beige crystalline solid

m.p. 147-149°C

21
The specification for Direct Filling Resins [17] requires

a minimum working time of 1.5 min and a maximum hardening time of 8 min.

Formulations containing DHPPT , BTX (low melting and high melting isomers)

DMAB , 3-DMABA and MAP at the concentrations studied did not harden within

the time limit. The effect of amine concentration on hardening time of

the composites is shown in Fig. 1. Increasing the amine concentration to

approximately 30 mm (millimolal) decreases the hardening time. At this

molality the most efficient amines (DMPT, DMSX and DMAPAA) yield a polymer

within 1 minute. As will be shown below composites with high concentrations

of an amine accelerator have poor color stability. Thus, liquids with concentration greater than 50 mm amine were generally not employed in this

investigation. It is likely that on incorporating larger concentrations

of amine into the resin, the hardening time, after passing through a

minimum, will increase.

Inspection of Fig. 1 indicates that shortest hardening

times (1 min) are obtained with DMAPAA, DMSX and DMPT. Among these, resins

containing DMAPAA give minimum hardening times at a lower molal concentration than those into which DMSX or DMPT are incorporated. Based on the

results of this investingation it can be concluded that the approximate ordei

of the accelerating ability of the respective amines is DMAPAA > DMSX >

MDMAPAA, DMPT>DMAG>DHEPT.

The effect of amine concentration on the tensile strength

of the composite is shown in Fig. 2. Values of the tensile strength for

DMAPAA could not be determined above 16.8 mm amine concentration because

of the insufficient working time for the preparation of test specimens.

Similarly, the hardening time of materials with 13.3 mm DHEPT was considered too slow for practical use. Therefore, the tensile strength of

this formulation was not determined.

22
s

E!E

03O

Coo

03c

E<

23

Figure

1:

Hardening

times

of

composites

containing

varying

concentrations

of

tertiary

amine

accelerators
50 -

45

40

s.I 35

30

25

20 _L

10 20 30 Amine Concentration, m m40

Figure 2: Tensile strength of composites as a function of

the concentration of tertiary amine accelerator

24
A maximum tensile strength is observed in the 16.5 to

17 mm concentration range except for DMAG which yields a maximum at

approximately 23.5 mm. The peaks are sharpest for DMSX and DMPT and

appear to be much broader for DMAG and DHEPT. Maximum tensile strength

of about 50 MPa is obtained with DMAPAA, MDMAPAA and DMSX. The tensile

strength values obtained on incorporating DMPT, DHEPT and DMEG ranging

from 43.6 to 44.8 MPa are slightly lower.

The compressive strength values of the composites are

much less sensitive to changes in amine concentration than the tensile

strength values. Thus between 10 and 35 mm amine in the liquid component

the tensile strength of 6 amines shown in Fig. 2 varies from 23.4 to 50.4

MPa, a 115% increase whereas the compressive strength ranges from 147 to

221 MPa, i.e. a 50% increase. The compressive and tensile strength curve

versus amine concentration in the liquid component are very similar.

Compressive strength increases with amine concentration and reaches a

maximum between 17 and 23 mm.

The DMAPAA proved to be the most reactive accelerator,

yielding products of good mechanical strength and excellent color stability.

The overall properties of composites with MDMAPAA and DMAG also proved

to be very satisfactory. The 4-dimethylaminobenzoic acid and its tetrame thy 1 ammonium and lithium salts cured fairly rapidly and had good strength,

but their color stability and water sorption were only fair.

For unfilled resins, the rate of curing proceeds most

rapidly using a molar peroxide-to-amine ratio between 1.10 and 1.50

[18, 19]. A composite formulation prepared from powder coated with 1%

peroxide and 17.0 mm amine in the liquid has a molar peroxide-amine ratio

25
of 6.5. Thus, for composites, a much larger molar excess of peroxide is

required to obtain optimum hardening time. This much larger excess of

peroxide is to be expected since only a small portion of the peroxide is

accessible to the amine before the composite is cured.

A minimum of 34 MPa is required for the tensile strength

of composite resins [17]. Practically all the composites prepared with the

amines used in this study pass this specification test. The range of amine

concentrations that yield cured resins with high strength is quite critical.

Compressive strength is somewhat less dependent on the type and concentration of amine used. Maximum compressive and tensile strength are obtained

in the same amine concentration range. Some of the amines, first evaluated

in this study, especially DMAG, DMAPAA and BTX, show excellent color stability. Thus, proper quality control is required in the manufacture of

these materials to assure that the liquid component contains the exact

accelerator concentration.

Although the evaluation of some of these accelerators

would indicate their potential usefulness, their biocompatibility will

have to be established. In selecting tertiary amines to be synthesized

consideration was given to compounds with promising toxicological characteristics, hopefully better than those of the presently used accelerators.

The parent compounds, 4-aminophenylacetic acid, its derivatives and aminoglutethimide have been suggested respectively as anti-inflammatory agents

[20, 21] and anticonvulsant agent [20]

The 4-aminobenzoic acid (Vitamin

B, anticanitic agent) and its derivatives are employed as sunscreen agents,

local anesthetics and to raise blood salicylate level [20]. The N,N26
dimethylated derivatives of aniline or p-toluidine have toxicological properties similar or better than the parent compound [20]. Similarly, examination of a homologous series of 4-aminophenylacetamide and n,N-dialkylaminophenylacetamides shows that the toxicity (expressed in gm/kg of test

animal) steadily decreases as the amine hydrogen is replaced by increasingly

longer alkyl substituents [22]. Analogously, one would expect that the tertiary amine derivatives under investigation (DMAG, DMAPAA, MDMAPAA, 4-DMBA,

TMADMBA, 4DMBA) will have the same order of biocompatibility as their

unsubstituted primary amine homologues; but this awaits further investigation. Based on the results of this study we conclude:

1. The overall characteristics of composites (hardening time, strength and color stability) containing N ,N-dimethylaminophenylacetic acid (DMAPAA) , its methyl ester (MDMAPAA) or dimethylaminoglutethimide (DHAG) compare favorably to restorative resins cured with

commonly used tertiary amines or other accelerators evaluated in this

study.

2. Maximum tensile strength for the composite is

obtained only over a narrow concentration range of accelerator.

3.

Study of Properties of Sealants

A study of the properties of experimental sealants which

had been started previously was completed. It was shown that on substitution of other monomers for methyl methacrylate as diluent for

Bis-GMA-based sealant, properties of the sealant can be varied widely.

Thin films of sealant on storage in water lose weight for many weeks

after an initial weight increase. Unless a hydrophilic diluent such as

glycidyl methacrylate is used the decrease is generally larger than the

11
initial gain resulting from water sorption. Incorporation of a photocrosslinking agent, e.g. (l-azo-5-acryloxymethyl-3 , 7-dioxabicyclo [3,3,0]

octane) (AADO) into the formulations lowers the water solubility of the

UV-cured sealant, thus improving the dimensional stability of the

adhesive. Measurement of the apparent water sorption on storage of

specimens for 12 to 27 months showed a continuous loss in weight. These

changes were much smaller in formulations containing the AADO crosslinking agent; e.g. the apparent water sorption of a formulation containing 66% Bis-GMA, 28% methyl methacrylate and 5% AADO was 0.2% after

the 25 month observation period. This formulation would be expected to

possess excellent dimensional stability in an aqueous environment.

In the preceeding study and in the follow-up investigation to

develop and evaluate physical test methods to characterize sealants, a

number of new tests were suggested which were shown to be valid as specification tests. Some of these tests are described in the following paragraphs

A gravimetric test which measures the amount of uncured

monomer that remained at the sealant-air interface was developed. The

sealant is cured in a brass ring on aluminum foil at 37°C and 100% RH.

The ring and foil which have previously been weighed are removed from

the bath. Foil, ring and cured and uncured material are weighed to the

nearest 0.1 g two minutes after curing. The surface is then patted dry

with filter paper and the weight of the disk is redetermined. The

difference between the two weights gives the weight of residual monomer

which can be expressed as percent of total weight of cured and uncured

sealant

Results for 5 determinations for a chemically cured sealant

28
gave a mean of 1.76 ^ with a standard variation of 0.76 %. The mean value

for uncured surface monomer of 6 commercial sealants ranged from 1.8%

to 14.0%.

We have developed two simple tests for semi-quantitative

evaluation of the penetration ability of sealants into fissures. The

first test measure's the flow of sealant into a capillary and is mainly

dependent on the viscosity of the sealant. The exact bore of capillary

tubing is measured with a measuring microscope. The tubing used is

approximately 0.005 cm and is readily obtainable. A 4 cm length of this

tubing is positioned vertically. By means of a micropipette with disposable tips, exactly 10 yl of sealant, mixed according to manufacturer's

instructions, are placed immediately after mixing on top of the tubing

before curing. The time between placing the sealant on the tubing and

its hardening and the distance the sealant traveled are determined. The

distance traveled varied from 0.5 cm to 4.0 cm.

A similar test is conducted for capillary rise. A vial (3 cm

high, 0.8 cm in diameter) is filled with freshly mixed sealant up to a

mark placed 0.2 cm from the bottom of the vial. Immediately, a 4 cm long

capillary tube (0.05 cm in diameter) is placed through a plastic cap

fitting the vial so that it penetrates 0.01 cm below the surface of the

sealant. The maximum rise of sealant in the capillary tube above the level

of the material in the vial is measured after curing of the material

which usually occurs within 1 minute. Values ranged from no capillary

rise to 2.0 cm. Thses simple tests were found to be fast and easily

reproducible. They should correlate quite well with the penetration of

sealant under clinical conditions which is greatly dependent on the

29
viscosity and surface tension of the sealant.

We have also studied an indentation and recovery test for

sealants as a measure of the gross mechanical properties of the sealant.

This test is similar to the one described in the American Dental Association Specification No. 15 for Acrylic Resin Teeth [23]. Specimens are

prepared by the procedure described for the residual monomer test. After

removal of the monomer by blotting with filter paper and storage of the

specimens in water at 37°C for 24 hours, the indentation resistance is

measured with a Rockwell Hardness Tester with a steel ball 0.635 cm (0.25 in)

or 0.158 cm (0.062 in) in diameter. The center of the top surface in contact with air during polymerization is subjected to a minor load of 3 kg

by lowering the steel ball and the dial is adjusted to zero. Then a major

load of 15 kg, which includes the minor 3 kg load, is applied for 5 min.

After this time, the depth of the indentation is read from the dial gauge

and recorded to the nearest 0.001 mm as Reading A. The major load is

then released leaving the minor load on the specimen. Five minutes later

the gauge is read and the value recorded as Reading B. Percent recovery

is calculated as follows:

Percent Recovery = Avg. Reading A -Avg. Reading B

Avg. Reading A

The use of the smaller (0.158 cm) ball cannot be recommended since many

of the specimens broke on application of the load. Smaller indentation

resistance values were obtained with the larger diameter ball indenter.

Even with the 0.64 cm ball indenter, different values for the various

brands investigated were obtained. The results of the measurements of

different specimens of the same brand were quite reproducible. Such

30
tests measuring gross mechanical properties may be useful for predicting

the clinical performance of a sealant.

D. Techniques for Bonding to Mineralized Collagen Surfaces

The goal of developing dentally applicable adhesive procedures has

long been considered a worthy one [24]. Such procedures and materials

would have ready application in restorative and preventive dentistry as

well as in orthodontics.

The objective of the current effort has been to determine the

feasibility of covalent or ionic bonding of polymer chains to the components of mineralized tissue such as bone or dentin. Where true chemical

bonding cannot be confirmed we are interested in evaluation of the

"adhesive" effect achieved through interpenetrating networks of natural

and synthetic polymer. Additionally we are examining the stability of any

attachment where the initial adhesive strength is promising.

While acid etch pretreatment of enamel has proven a successful means

of adhering resins or composites to enamel surfaces, it has not been

effective with collagen containing substrates such as dentin or bone. One

approach, initiated in this laboratory, has been to generate grafts of

polymer chains to such surfaces using ceric ammonium nitrate or persulfatebisulfate initiated polymerization [25-27]. The formation of a true graft

was shown to depend on the selection of monomer and the type of initiator

employed

.

1 . Cyanoacrylates

In the Annual Report for FY 76 [6] we reported the following

results of our initial research on cyanoacrylates:

1. With a commercially available methyl-2-cyanoacrylate

31
curing times under UV on dentinal or bone surfaces pretreated with dimethyl-p-toluidine ranged from 15 sec. to

1 minute.

2. Tensile strengths observed between bovine dentin treated

with methy 1-2-cyanoacrylates and an in situ curing composite

were found to be less than those reported by others in the

literature.

3. Evidence that the methyl-2-cyanoacrylate lacked gapfilling ability was demonstrated. This would be of more

concern in restoration applications than in preventive

techniques

4. A bone/cyanoacrylate/bone tensile strength measurement

technique was developed for rapid screening.

5. Using commercially available cyanoacrylates and several

from the Army Medical R&D Laboratory more encouraging initial

bond strengths to bone were obtained.

6. A modification of the Lee, Swartz and Culp [28] test

procedure was developed for measuring adhesive strength between

cyanoacrylate-treated dentin and an acrylic resin system.

We have continued the research on cyanoacrylates. NMR

analysis was used to identify the nature of the alkyl groups in the commercial cyanoacrylates that were used. Room temperature curing times on

bone of cyanoacrylates with 2-5 carbons in the ester side chain varied

from a few seconds to less than 3 minutes.

a. Isothermal Aqueous Storage at 37°C

We measured the deterioration in bond strengths devel32
oped by cyanoacrylates In bond-cyanoacrylate-bone specimens as a function

of storage time in water at 37°C. The data for up to one month storage

periods is given in Table 7.

TABLE 7

Bond Strength of Various Materials to Bone

Temperature: 37°C

h2o Tensile Strength

Material Surface Pretreatment Storag e Mean S.D. Days MPa MPa

Composite (control) — 1 0.03 .07 Bonding resin (control)

2-Cyanoacrylates

acid etch (a) 1 0.06 .12

Ethyl (product A) —- (a) 1 0.92 .26

" (product B) (a) 1 1.45 .61 " (dental adhesive) —- (a) 1 1.82 .75 " (product C) —- (a) 1 3.34 .69

• 1 (t —- (b) 1 5.06 1.45

It II —- (b) 7 4.21 1.21

II II — 1 6.60 1.13

II II — 7 3.88 1.36

II II — 30 0.16 0.19

" " presence of arterioles 1 8.25 1.21

• 1 II ethanol 1 5.64 1.80

• 1 It It ale DMPT (c) 1 6.38 1.95

II II 1% ale DMPT (c) 30 0.17 0. 17

" " modified aminofunctional silane 1 0.29 .10 n-Butyl 1% ale DMPT (c) 1 2.86 1.75

Isobutyl 1% ale DMPT (c) 1 6.04 2.20

• 1 1% ale DMPT (d) 1 6.44 2.79

II It ale DMPT (d) 30 3.87 1.27

" (commercial) — 1 6.62 1.73

" (completely dry bone) — 1 0 0

• I It ale DMPT (c) 1 4.79 1.06

II — 30 4.65 1.51 n-Amyl — 1 6.62 1.7 5

II It ale DMPT (c) 1 5.94 2.23

" (arterioles) It ale DMPT (c) 1 7.84 1.42

• 1 It ale DMPT (d) 30 2.53 .80 Isoamyl — 1 6.27 0. 94

II It ale DMPT (c) 1 5.57 1. 15

II It ale DMPT (d) 1 5.43 .86

II It ale DMPT (c) 30 4.48 2.15

Isoamyl (viscous) — 1 7. 18 1.47

II II (b) 1 4.72 1.27

• 1 II It ale DMPT (c) 1 5.53 2.02

II II It ale DMPT (c) 30 4.70 2.04

n-Hcxyl It ale DMPT (d) did not cure in 3 min.

Ethylcellosolve -—(b) 1 0 0 Fluorinated

• 1 IX ale DMPT

1

1 J:3&

1 . 04

0:75

(a) Test specimens kept for 30 min. at 37°C and 10C o RH. prior to storage in water.

(b) Test specimens kept for 3 min. at 22°C and 50°i RH prior to storage in water.

(c) DMPT = N,N-dimethyl-p-toluidine.

(d) Only one mating surface treated.

33
The adhesive is seen to be nil for controls in which the

bone cylinders were joined by a composite resin or a dental bonding resin

even after acid etching of the bone surfaces. A dry bone surface resulted

in no bonding. The ethyl cellosolve 2-cyanoacrylate treated specimens

showed no improvement over the control. All cyanoacrylates with 2 to 5

carbon atoms in the alkyl group produced significant tensile adhesive

values after storage of one day. A maximum bond strength of 7.2 MPa after

one day water storage was observed with viscous isoamyl 2-cyanoacrylate.

This tensile adhesion value is of the order of magnitude reported for

etched enamel-composite bond strength.

After one months storage the strength of all specimens

decreased. Over this time span it appeared that hydrolytic stability

increased with the alkyl side chain length. Some bonds were completely

disrupted in this time interval, e.g. ethyl-2-cyanoacrylate.

To explore further the relation of side chain alkyl

group and hydrolytic stability, specimens were stored in water at 37°C

for up to six months. This data is given in Table 8 and Fig. 3. It is

apparent that the isobutyl, n-amyl and isoamyl cyanoacrylates, while losing

strength as a function of time, do hold up much better than does ethyl

cyanoacrylate. After the six month storage the n-amyl, isoamyl and

viscous isoamyl 2-cyanoacrylate retained respectively 67, 70%, and 71%

of their first day bond strength.

Pretreatment of the bond surfaces with 1% alcoholic

dimethyl-p-toluidine, to speed up curing of the adhesive, slightly reduced

the one-day bond strength. These treated surfaces gave much lower tensile

adhesion values than the untreated substrate after one and three month

34
water storage and retained only a small percentage of the original strength

after a 6 month observation period. Again the n-amyl and the viscous isoamyl homologues exhibited the largest 6 month adhesive strength.

Table 8

TENSILE STRENGTH OF THE CYANOACRYLATE-BONE BOND

No Pretreatment

2 cyanoacrylate Tensile Strength in

1 day 1 month

MPa after ^0 Storage

3 month 6 month

Ethyl (Alpha Ace) 6 . 60 0.16 — —

n-Butyl 2.86 — — —

Isobutyl 6.62 4.65 4.62 4.12

n-Amyl 6.62 — 3.80 4.84

Isoamyl 6.27 — 4.98 4.39

Isoamyl 7.18 — 6.33 5.14

Ethyl cellosolve 0 — — —

Bone pretreated with 1% ale.

Ethyl (Alpha Ace)

dimethyl-p6.38-toluidine

0 — —

Isobutyl 4.79 — 2.87 0.15

n-Amyl 5.94 2.53 1.12 1.57

Isoamyl 5.50 4.48 0.62 0.51

Isoamyl, viscous 5.53 4.70 4.21 2.06

35
TENSILE

STRENGTH

IN

P/IP

8

CO

CODE:

«- ft ISOBUTYL

•—o N-AMYL

ISOAMYL

ISOAMYL, VISCOUS « * ISOBUTYL. BONE

FRETFiEATED

D R ISOAMYL. VISCOUS,

BONE PRETREATEO

1 2 3 4 5

STORAGE IN WATER (MONTHS)

Figure 3: Tensile strength of cyanoacrylate to bone bond

after storage in water

36
b . Accelerated Aging Test

With equipment built in the laboratory, bonecyanoacrylate-bone specimens were subjected to a cyclic exposure to

water at 5° and 55°C, the specimens being 70 s at each temperature

with 10 s taken for the transfer. These results are given in Table 9.

Isobutyl and viscous isoamyl 2-cyanoacrylates gave the highest one day

tensile values. However all values were much lower than those obtained

with the same specimen types when aged isothermally at 37°C. As was

true in the isothermal aging, pretreatment to achieve rapid cure times

significantly encouraged deterioration of bond strengths. The bond

established by viscous isoamyl cyanoacrylate which appears to be the

most hydrolytically stable is greatly deteriorated, e.g., by 67% of

the 1 day 37° value of 7.2 MPa, in 30 days of cycle testing. The test

correlates well with the long-term isothermal storage studies and

should be of value in predicting lifetime behavior.

The viscous isoamyl cyanoacrylate with the highest

initial bond strength to bone also shows the least bond strength deterioration in water. It has the further advantage that it should

possess greater gap filling potential than the less viscous materials

and is considered to be less toxic than the lower homologues of the

2-cyanoacrylate series. It may prove useful clinically where an intermediate term bone adhesive is indicated.

37
Table 9

TENSILE STRENGTH OF ADHESIVE-BONE BOND AFTER TEMPERATURE CYCLING

Temperature of Water bath: 5° & 55°C; Number of cycles per day: 540

Cyanoacrylate Tensile Strength in MPa after cycling

1 day 12 days 30 days

Ethyl 3.90 0

-

Isobutyl 4.52 4.06 1.61

Isobutyl (a) 3.36 3.19 -

n-Amyl 4.59 2.92 0.59

Isoamyl - 1.64 -

Isoamyl, viscous 4.40 2.73 2.32

Isoamyl, viscous (a) 3.97 2.37 -

(a) Bone pretreated with 1% ale. dimethyl-p-toluidine

c . Nature of Adherence of Cyanoacrylate to Bone

Ethyl 2-cyanoacrylates were spread on the surface of a

bone slab primed with a hexane solution of tritheylamine . Infrared

spectra taken 2 hours later clearly identified the cyanoacrylate.

Upon extraction with acetone and dimethylsulfoxide an I.R. spectra of

a fresh bone surface was found. Similar results were observed when

the most hydrolytically stable cyanoacrylate, viscous isoamyl, was

used. It appears that contrary to the reaction of amino acids with

cyanoacrylates [28], covalent grafting of bone to cyanoacrylate does

not occur.

38
Table 10

TENSILE STRENGTH OF THE DENTIN-ACRYLIC JOINT BONDED WITH

VARIOUS CYANOACRYLATES

Temperature of water: 37°C

2-Cyanoa crylate Treatment Days in H^O Tensile Strength, MPa

Ethyl -- 1 2.80

Ethyl DMPT 1 1.67

n-butyl -- did not cure -- Isobutyl DMPT 1 3.50

Isobutyl -- 1 5.54

Isobutyl 37% H

3

P0

4

1 7.23

Isobutyl 37% H

3 P0

4

90 1.89

n-Amyl -- did not cure —

n-Amyl DMPT did not cure --

Isoamyl -- did not cure --

Isoamyl DMPT 1 3.22

Isoamyl

,

viscous DMPT 1 2.29

Isoarayl

,

viscous DMPT 1 2.65

Isoamyl

,

viscous 37% H

3

P0

4

1 3.00

Isoamyl

,

viscous 37% H

3

P0

4

90 0.25

Temperature Cycling

Temperature of water: 5°C and 55°C, Number of cycles: 540

2 -Cyanoacrylate Surface

Treatment

Tensile Strength in MPa after cycling

1 day 7 days 30 days

Isobutyl -- 0.12 -- Isobutyl 37% H

3

P)

4

5.65 3.32 0.25

Isoamyl 37% H

3

P0

4 -- -- 0.00

Isoamyl

,

viscous -- 0.00 --

Isoamyl

,

viscous 37% H

3

P0

4

2.34 2.53 0.00

39
!

d. Cyanoacrylate Bonding of Acrylic Resin to Dentin

Results of the bond strength of the dentin joined to acrylic

rod through cyanoacrylates are given in Table 10. Only the ethyl-, isobutyland viscous isoamyl 2-cyanoacrylate are cured within the 3 min. period that

a force is applied to the assembly. By far the best bond strength (5.54 MPA)

is obtained with the commercial isobutyl 2-cyanoacrylate. The bond

strength for the 2-cyanoacrylates is higher for the bone to bone than

for the dentin-poly (methyl methacrylate) joint. Pretreatment of

dentin with dimethyl-p-toluidine lowered the bond strength. Acid-etch

pretreatraent of the dentin surface with 37% phosphoric acid surprisingly

increased adhesion. Thus the one-day tensile strength of the dentinacrylic rod is 7.23 MPa (1080 psi). However, as with bone, bond

strength decreases greatly on storage in water. Thus after 90 days at

37°C tensile adhesion with the isobutyl- and viscous isoamyl2-cyanoacrylate is reduced to 1.89 and 0.25 MPa, respectively. Thermocycling greatly reduced the bond strength. Acid-etched specimens showed

minimal adhesion and the bond was destroyed completely on 30 days of

thermocycling. These results indicate that the presently available

non-filled 2-cyanoacrylates studied in this investigation could find

only very limited usefulness, if any, as adhesives for cementing

dentin to acrylic restoratives since (1) the bond degrades quite

rapidly in an aqueous environment even after acid etching of the

substrate and (2) the two mating surfaces must fit together closely

without gaps or voids since these adhesives do not act as gap fillers,

a quality usually required for clinical application.

40
Further studies to improve adhesion to dentin by incorporating (a) polymerizable crosslinking agents into the cyanoacrylate

monomer and (b) by pretreatment of dentin surfaces with copolymerizable

compounds are currently under investigation.

II. Physical, Mechanical and Physicochemical Properties of Dental

Materials

A. Dimensional Charge Measurements

As part of a general investigation of the physical and clinical

properties of amalgams made from some of the recently introduced

alloys with high copper content, a study was concluded of the thermal

expansion characteristics of the amalgams. The objective was to

obtain information on the stability of amalgams at elevated temperatures

rather than to determine coefficients of thermal expansion. The

thermal expansion curves demonstrate that in addition to a "normal"

reversible thermal expansion of about 25 parts per million per degree

Celsius, some of the amalgams exhibit large dimensional changes that

are time dependent as well as temperature dependent.

The thermal expansion apparatus used is a miniature vitreous

silica dilatometer shown schematically in Fig. 4. The end of the

silica tube containing the specimen was immersed in an ethylene

glycol bath which provided heating or cooling rates between 10 and 19 C

of approximately 0.35°C per minute. Openings in the tube permit the bath

liquid to contact the specimen.

The compositions of the alloys on a weight percent are given in

Table II. The alloys can be classified into three groups: first, those

listed as "conventional" which fall into the composition limits given in

41
Table 11

COMPOSITION OF ALLOYS FOR AMALGAMS

Alloys Silver Tin Copper Zinc Classification

% % % %

ADA Spec. 1(1970) >65 <29 <6 <2

A 69.8 26.3 3.7 0.3 Conventional

B 69.0 27.4 3.4 1.2 Conventional

C 70.1 19.2 9.6* 1.1 9% Copper

D 70.0 20.1 9.5* 0.4 9% Copper

E 70.0 20.4 9.4* 0.2 9% Copper

F 42.2* 30.0* 27.8* 0.0 High Copper,

Low Silver

G 58.5* 28.4 12.9* 0.1 High Copper,

Low Silver

H** 60.2* 24.4 12.1* - High Copper,

Low Silver

I 58.5* 28.4 12.9* 0.3 High Copper,

Low Silver

* Differs from specification composition limit ** Contains 3.4% Indium
Specimen

Figure 4. Vitreous silica dilatometer

used for measurement of the

thermal expansion of amalgam.

43
American Dental Association Specification No. 1 (1970); second, those

listed as "9% copper" which contain more than the 6% maximum copper

listed in the specification but comply with the other composition

limits although they contain less tin than do the conventional alloys;

and third, those listed as "high copper-low silver" which contain 12%

or more copper and less than the 65% minimum silver listed in the

specifications. The thermal expansion curves for amalgams within any

one of the three composition classes are similar. There are, however,

distinct differences between the thermal expansion curves for different

classes

The curves for the two examples of conventional amalgams (Fig. 5)

are alike in that each shows a sharp shrinkage on heating at near 75°C

and an expansion on cooling in the 50 to 40°C range. There is a

difference above 75°C with an expansion occurring in amalgam A while a

shrinkage is observed for amalgam B.

In Fig. 6, all three amalgams made from the "9% copper" alloys

show irreversible expansion beginning near 55 or 60°C on heating and

continuing, with some deviation near 75°C, until cooled to near 40°C.

The decreased rate of expansion in amalgams D and E and the shrinkage

in amalgam C near 75°C on heating, correspond to the sharp shrinkage

observed in the conventional composition amalgams at this temperature.

The specimen of amalgam E shows a growth of about 100 pra/cm or 1%

during the heating and cooling cycle.

The four amalgams with high copper and low silver content (Fig.

7) are more stable dimensionally over the temperature range investigated

than are the other amalgams. With the exception of amalgam I, which

44
TEMPERATURE,°C

Figure 5: Thermal expansion of amalgams - "conventional"

alloys

45
TEMPERATURE, °C

Figure 6: Thermal expansion of amalgams - "9% copper

alloys

46
TEMPERATURE, °C

Figure 7: Thermal expansion of amalgams -"high copperlow silver alloys"

47
exhibited some shrinkage between 60 and 85°C, they show very little

irreversible dimensional change. There is no evidence of the abrupt

shrinkage at 75°C on heating or of the large expansion on cooling

observed near 50°C with the conventional amalgams.

Although differences between the three classes of alloys investigated were readily observed, evaluation of the significance of the

results and explanation of the differences in irreversible or time

dependent dimensional changes are not so readily accomplished. In regard

to those amalgams which begin to show growth near 60°C, the "9% copper"

amalgams, it seems reasonable to expect that the same phenomena may

occur to some extent in the mouth. On the other hand, dimensional changes

that were observed only after heating to near 75°C would appear to be of

limited clinical significance. As to the reasons for the differences

between the "thermal expansion" curves for the three classes of amalgams

investigated, the data were not decisive but do provide some guidance for

speculation

In conventional amalgams reacted at 37°C one generally observes

residual particles of the y phase (Ag^Sn) in a matrix consisting

mainly of the y^ phase (Ag^Hg^) and some y^ phase (Sn_,Hg) . The y phase

and the y^ phase are reported not to undergo any phase transformations

over the temperature range of these experiments and the observed sharp

dimensional changes must therefore be accounted for mainly through

phase transformations which occur in the y^ phase. If the y^ phase

has the composition Ag^Hg^ in the amalgam, then the binary phase

diagram predicts the transformation y^

-> p + Liquid Hg would occur at

about 127°C in heating. The y^ phase in conventional dental amalgams,

48
however, is known to contain a few percent tin in solution, and this

dissolved tin may lower the transformation temperature. The liquid

mercury released by this transformation could react with residual y

phase particles in at least two ways;(l) it could dissolve in the y phase

and (2) it could also react with the y phase to form the p phase

(Ag,. <-Hg^ ) which is stable at this temperature, and once formed would

remain stable to room temperature unless sufficient liquid mercury

remained to produce the reverse reaction P + Liquid Hg -* y^

The appearance of the curves for conventional amalgams (Fig. 5)

suggests that all of these mechanisms are operating to produce the

observed behavior. The sharp decrease in volume occurring at about

75°C on heating is attributed to the y^

-* p + Liquid Hg transformation

occurring at this reduced temperature because of the presence of tin

in the y^ phase. At this point droplets of mercury can often be seen

to form on the surface of the specimen. The newly released mercury

dissolving in the y phase would be expected to produce an expansion

above 75°C as seen in the curve of amalgam A. Howqver, if both of

these reactions and also the reactions of the liquid mercury with the

y phase to produce the p phase are going on simultaneously, the result

might be the shrinkage seen in amalgam B. On cooling the reverse

reaction P + Liquid Hg -> y^ may be responsible for the rather sharp

expansion at about 50°C. The liquid mercury droplets disappear from

the surface shortly before this expansion occurs (and crystals of

y^ (Sn^Hg) are left on the surface).

In the amalgams made from the "9% copper" alloys (Fig. 6) which

are low in tin content compared to the conventional alloys, the y^

phase may contain sufficient copper to lower the temperature at which

49
the solid state diffusion reaction, y^

-> p + Hg, begins to near 60°C

tending to produce shrinkage. If however, at the same time the released

mercury is dissolving in the y phase, this may produce sufficient

expansion so that expansion is observed in the curves. The slight

shrinkage or reduced rate of expansion at 75°C may result from the

equilibrium transformation y^

-* p Liquid Hg as in the conventional

amalgams. On cooling, these reactions continue until the temperature

falls to about 60°C. The p + Hg -* y^ transformation cited as responsible for the expansion near 50°C in the "conventional" amalgams,

perhaps does not occur in the "9% copper" amalgams because there is

not sufficient liquid mercury. In fact, the droplets of

liquid observed on the surface of the "conventional" amalgam were not

observed on the "9% copper" amalgams.

In the amalgam made from "high copper - low silver" alloys (Fig.

7) the strong tendency toward formation of the compound Cu^Sn^ may

mean that there 'is little incorporation of either tin or copper in the

y^ phase. This phase would then be essentially a a binary Ag^Hg^

compound, and the transformation y^

-* P + Hg would not occur below the

equilibrium peritectic reaction temperature of about 127°C. The

result would be the retention of the original structure throughout the

temperature range of these investigations, and the thermal expansion

curves should show no irreversible dimensional changes. This is

essentially what is observed in three of the four amalgams of this

class

This work has been reported at the 1977 AADR meeting in Las Vegas.

n 50
B. Wear Resistance of Dental Materials

The objective is to assemble and perfect instrumentation and

methods for the generation and precise measurement of relative wear of

dental materials. Of major concern is the simulation of human masticatory conditions to such an extent that the in vitro wear data will

be similar to that in the mouth, while reducing the period of years

required for clinical evaluation to days or hours in the laboratory.

To further the attainment of this objective, the design and

construction of a new device for measuring the wear resistance of

dental materials was completed. The earlier wear machine designed and

used in our laboratory demonstrated highly satisfactory wear generating

characteristics [29], but had two disadvantages. First, it was a

single-specimen machine on which not more than one specimen could be

run each day; and thus an extensive period of time was required to

obtain data on a sufficient number of specimens. Second, the data

acquisition system assembled from components then available was subject

to frequent malfunction with attendant loss of data necessitating

preparation and running of additional specimens. The new wear apparatus

was designed to overcome these difficulties and at the same time

retain the wear generating characteristics of the previous device. In

both the old and the new device, the design is of the classic pin-ondisk configuration. It incorporates a stabilized turntable which

carries the specimen mounting cup. The pin, or enamel slider, is a

round cylinder of enamel and underlying dentin and is suspended in a

mounting cup from a vertically displaceable arm. The flat face of the

slider rests on the flat of the restorative disk, with the center of

51
the slider 6 mm off center of the disk. The arm is loaded to establish a

stress of 1 kg/mm (approx. 10 MPa) at contact between the members of the

wear couple. A continuous stream of water lubricates the specimen. The

rotation of the specimen produces an annular wear tract with a width of 1.

to 2.0 mm and diameter of approximately 12 mm. The specimen rotates at 27

rpm, which gives a sliding speed of 1.7 cm/s. Wear is determined by inter

mittent measurement of the depth of the wear track.

To overcome one of the disadvantages of the previous apparatus,

the new machine (Figs. 8,9,10) is a multispecimen device with three

wear stations thus greatly reducing the time required for evaluation

of a material. The mechanical features include higher tolerance bearing systems, greater horizontal rigidity in slider carrier arms and a

direct gear drive to all turntables so aligned as to use gear thrust

to maintain a good bearing seating force. Preliminary tests of the

new device indicate it to be very stable and vibration free.

To overcome problems in data collection, the new device has a

completely redesigned and much more sophisticated and versatile

control system combined with a data acquisition system of high reliability. Data are automatically printed out at regular intervals

during a run and are also recorded on magnetic tape for processing by

computer

All ancillary functions for the wear generator are electrically

powered (LVDT lift, water solenoid, measurement stepping, etc.) as

opposed to pneumatic and mechanical means used on the earlier testing

device. As a result, all of these functions operate reliably under

full program control. The new control circuitry has been designed so

52
mmstsg"
Figure

9:

Wear

generating

device

with

three

independently

loaded

wear

heads
55

Figure

10:

Human

tooth

enamel

slider

resting

on

dental

amalgam

wear

specimen.

Sapphire

tipped

measuring

probes

are

beneath

the

LVDT

on

the

left

side

of

the

specimen.

During

operation

the

nozzle

on

the

right

directs

a

jet

of

water

on

to

the

specimen
that all LVDT scanning functions are handled by solid-state switching,

reducing the noise experienced previously. The digital voltmeter has

been factory-modified so that measured values shown in the display

register are passed directly through the BCD output to the printer

instead of running one measurement behind as is common in many of

these systems. This feature greatly reduces complexity of program

assembly.

The data acquisition and control system has responded well to

debugging. Programming is at present being done by repetitive magnetic

tape command while awaiting a program module from the manufacturer.

This module is the final piece of equipment needed for full utilization

of the system. Until delivery of the programmer, work will continue

by means of tape program control.

From preliminary reliability tests, the improved design of the

fully automated instrument appears to have overcome all of the problems

experienced in this area with the single specimen prototype device.

Repeated measurements on non-wearing surfaces show that the instrument

is capable of measuring to a level of ±0.7-0. 8 micrometers when

operated in circulating air whose temperature varies approximately

±2°C. It is anticipated that the instrument accuracy will be improved

I to ±0.3-0. 4 rpicrometers in a more stabilized environment.

It is anticipated that newly acquired wear data will show an even

more significant difference between classes of direct restorative

materials and at a higher confidence level than was shown with the

prototype single specimen device. Early results obtained with the new

apparatus indicate that one source of variability in this recorded

56
data previously obtained may be the adhesion of small amounts of wear

debris to the surface of the contact probes used to measure wear track

depth. During the time it is on the probe, the debris causes a false

indication of reduced or negative wear as shown in Fig. 11 which is a

plot of data for a short trial run on three amalgam specimens. The

low points in two of the curves (at 260 and 340 minutes respectively)

are apparently the result of gradual buildup of wear debris on the

surface of the sapphire probes followed by rapid release of the debris.

In order to prevent such irregularities, the measurement sequence is

being changed to eliminate rubbing contact between measurement probes

and specimens. Such changes are relatively easy on the new apparatus

since they can be accomplished to a large extent by programming changes

with a minimum of hardware alternations. The programmer now on order

will further improve the versatility of the apparatus in this respect.

The first phase of testing with the new wear appartus will be

directed to all types of amalgam alloys and composite restorative

materials available to the practicing dentist. This phase of testing

commercially available materials will be undertaken before the wear

evaluation of experimental materials in order to build sufficient

background data against which future developments can be compared.

Such background data will serve another important purpose: that of

test method simplification where needed. Also comparison of the wear

resistance of these materials to their individual component makeup

should provide highly pertinent data to dental manufacturers as a

guide in formulation of improved materials.

57
o*3OSOroO

CMcno00

CMO*3CMOo

CMo

vOO

CMo

00o

*3o I

58

Figure

11:

Example

of

problem

encountered

in

preliminary

trials.

The

low

points

on

the

"wear

curves

at

260

and

340

minutes

are

artifacts

produced

by

the

build

up

of

wear

debris
As has been stated in earlier reports, perhaps the single most

difficult phase of the wear measurement procedure is the preparation

of human tooth enamel pins or sliders. Some priority must therefore

be given to investigation of an enamel substitute in the wear couple.

In their work on characterization of surface failure of nonmetallic single crystals under sliding, Powers and Craig [30] used

large crystals of natural fluorapetite to attempt to identify the wear

phenomenon of enamel. Both natural and synthetic fluorapatite crystals

are available commercially and if comparable to enamel in wear response

and effect would be a readily obtainable and easily managed substitute

for slider heads used in this test method. Consideration should also

be given to the sintered hydroxyapatite recently proposed by Rootare,

Powers and Craig [31] for use in wear studies.

Other materials that may need to be considered are th ultra-high

tensile strength fibers or whiskers of boron, carbon and glass, as a

high pack density filler in a matrix of Bis-GMA, epoxy or other high

strength resin. Any of the aforementioned substitute materials

which produce wear values on the background data restorative materials

comparable to that obtained with human enamel on level of significance

of 95% or better could be considered acceptable.

C. Base Metal Alloy and Investment Research

The objectives of the research on base metal alloys and investments

have been to determine the characteristics of these materials relating

to the serviceability of the alloys for use in cast crowns and bridges,

to determine the effect of fabrication techniques on the properties of

the castings, and to utilize this information for the development of

59
Ni-Cr crown and bridge alloys. Each manufacturer, using the investment and technique of his choice prepared castings for our evaluation.

These castings were prepared from Ni-Cr based alloys and also from

various noble metal alloys; nine castings were made for each alloy

examined

:

Jelenko

:

Jelenko-O* 98% Noble

Cameo* 80% Noble

Midas** 52% Noble

Jel-Bon* Ni-Cr

Howmedica

:

Microbond #2* 98% Noble

A-6** 78% Noble

Microbond NP^* Ni-Cr

J. M. Ney Co. SMG-W* 81% Noble

Ney-Oro B-2** 78% Noble

Neydium* Ni-Cr

Johnson & Johnson Bak-on-Yellow* 98% Noble

Dental Products Div. Ceramalloy* Ni-Cr * Crown and bridge alloy for porcelain bonding

** Crown and bridge alloy not for porcelain bonding, Type III

Gold or Type III Substitutes

HIGH NOBLE MEDIUM LOW NOBLE NON-PRECIOUS

NOBLE

Problems encountered during fabrication with

various ceramic alloys

60

Figure 12:
improved techniques and to provide guidance for the development of

improved alloys.

A comprehensive survey of the 3044 member laboratories of the

National Association of Dental Laboratories (NADL) conducted by a

Research Associate in this laboratory sponsored by NADL has revealed

that 25% of all crown and bridge units fabricated by these laboratories

are cast from Ni-Cr alloys. Although the Ni-Cr alloys have gained

wide acceptance because of their lower cost, reporting laboratories

list significant problem areas:

22% of Reporting Labs

21% of Reporting Labs

39% of Reporting Labs

45% of Reporting Labs

38% of Reporting Labs

Miscastings

Poor Fit

Porcelain Problems

Extended Finishing Times

Soldering and Joining

Problems

As seen in Fig. 12 there appears to be a strong negative correlation between total noble metal content and the degree of difficulties

experienced during fabrication of porcelain bonded to metal restorations.

1. Fit of Porcelain Fused to Metal Crown and Bridge Castings

Our most extensive work has been in the area of casting accuracy

and castability. As noted in last year's report, the phosphate

bonded investments necessary for use with the high melting porcelain

bonding alloys are "technique sensitive." This technique sensitivity

leads to castings which are generally undersize when made by us from

Ni-Cr alloys using the instructions supplied by the investment manufacturer or the metal manufacturer. We have verified these "in

house" experimental results through visits to major manufacturers of

61
Each manufacturer was asked to prepare castings from at least one

gold based alloy, and a non-precious Ni-Cr alloy. Both categories

have the same end use in the dental profession: metal substructures to

which porcelain is bonded to produce functional and esthetic extracoronal restorations.

a. Methods and Materials

(1) Die

A hardened stainless steel die with dimensions

approximating an upper anterior crown preparation (Fig. 13, Part 1)

was used for wax pattern construction and casting size determinations.

The surface of the die which contacted the pattern was a truncated

cone with base diameter of 5.0 mm, height of 6.0 mm and an included

angle of 5.7° (taper is 10%). This taper is within the critical range

recommended for retention of clinical castings.

(2) Patterns and Sprue

Nearly invariant wax patterns 1 with a wall

thickness of 1.0 mm were constructed using the total die assembly

(Fig. 13). An amount of Kerr's 2 regular inlay wax slightly in excess

of that needed for the pattern was warmed for 5 minutes along with the

1

2

std. dev, of wax pattern diameter (n = 9)

5» 00 mm design diameter

Kerr Manufacturing Co., Romulus, Mich.

x 100 = 0.02%

62
Figure 13: Die used for wax pattern construct ion and

casting size determination

i 63
lightly lubricated 3 die assembly in an oven held at 50°C. The wax was

placed in the former (Part 3) and all parts fitted into the sleeve.

The sleeve aligned assembly was then compressed and held in a tightly

closed bench vise at ambient room temperature (23 ± 1.5°C) for one

minute. The vise pressure on the softened wax caused extrusion of

excess wax through the 1.4 mm diameter vent in the cap (Part 1) and

created a wax flash between the former (Part 3) and the spacer (Part

2). The assembly was removed from the vise, placed in room temperature water for two minutes, dried and the pattern removed from the

die. The pattern was trimmed of flash, replaced on the die, rewarmed,

repressed, and brought to room temperature in a water bath as described

above. The second pressing frequently allowed metal to metal contact

of the die parts, produced minimal flash at the pattern base and

increased uniformity in wall thickness of the pattern. The stress

relieved, annealed pattern was examined for defects and a 10 gauge

hollow plastic sprue with a 6 mm diameter reservoir 4 located 1.5 mm

from the point of attachment was luted to the center of the occusal

surface of the pattern (Fig. 14).

(3) Mold Lining Material

The effects of restrictive stresses on setting,

hygroscopic, and thermal expansion of typical phosphate and gypsum

3 # 4 compound, Dow Corning Corp., Midland, Mich.

4 E-S Sprue Pins, Surgident Ltd., Los Angeles, CA

64

investments have been well reported. Minor restraint suppresses mold

expansion markedly during hygroscopic set and to a lesser extent

during thermal expansion of both investments. To minimize restraint

and provide for greater mold expansion, the standard liner condition

employed for castings prepared at NBS utilized two layers of asbestos

(Table 12).

To further increase mold expansion by reducing restraint against

the setting phosphate investment, 40 mil. (1.02 mm) or 80 mil. (2.03

mm) thick strips of highly resilient spun kaolin refractory furnace

liner 5 were substituted for the stiffer and less yielding asbestos

strips as mold liners. The Kaowool, cut in strips 4.1 cm wide, was

used in the same fashion as asbestos. One strip of 40 mil. (1.02 mm)

thick Kaowool was dimensionally equivalent to a double thickness (2 x

0.6 mm = 1.2 mm) of asbestos.

(4) Investing, Burnout and Casting

Manufacturers' technical personnel prepared

several series of castings from patterns made on site by us. Each

manufacturer was instructed to choose the conditions and techniques used

with his alloys so that full size castings would result. Die assemblies

had been supplied in advance to allow practice with the particular

pattern configuration used in the test series. Using the same alloys

5 Kaowool, Babcock and Wilcox, Refractories Div., Augusta, GA.

66
Table 12

CASTING DETAILS

Casting Ring

Asbestos

Whipmix 31.8 mm x 47.6 mm (l-l/4"xl-7/8") Pyronel

Whipmix 41.3 mm x 0.8 mm (1-5/8” x 1/32”)

2 layers, dry

Investment Whipmix Ceramigold

Liquid/Powder Ratio 16cc/100 g Undiluted Investment Liquid

Spatulation

Setting time

before

By hand-1 min. (120 spatulations/min.

)

45 min. 38°C (100°F) Hygrobath followed by overnight

casting bench set

Burnout 1 h to 816°C (1500°F), 1 h heat soak at

816°C (1500°F)

Placement of Ring 5 to 10 s before metal filled the mold cavity

irk in Casting Machine

JU Whipmix Corp., Louisville, KY.

JUL Electromat-Howmedica Corp., Chicago, IL.

67
at NBS, we prepared comparable series of castings but used uniform mold

conditions (Table 12) so that possible differences in casting size

between alloys might be detected.

(5) Devesting and Surface Preparation

The hot casting rings were bench cooled to

room temperature and the cylinder of investment pressed from the ring.

The investment cylinder was cleaved along its axis and the casting

lifted free. Adherent investment was removed from the casting with 50 pm

alumina using an air abrasive unit with an operating pressure of 80

psi (0.55 MPa).

Bubbles and other irregularities characteristic of the surface of the phosphate investment mold were removed

before measuring the casting. The casting was placed on the hardened

steel die with moderate finger pressure and rotated. This produced

shiny spots on the bearing surface of the casting which were easily

identified under 5x magnification and removed with a #2 round bur.

The process was repeated several times or until smeared areas of metal

to metal contact indicated that isolated nodules or bubbles were not

interfering with further seating of the casting. The bubbles and

nodules were removed individually without wholesale grinding or

machining of the casting surface. The hardened steel die surface was

not abraded by this procedure.

(6) Measurement of Patterns and Castings

Completed patterns were placed on the die

with the spacer (Fig. 13, Part 2) removed. Using a measuring microscope

68
graduated to .00254 mm (0.0001 inch) the distance between the pattern base

and the die base was measured at 120° intervals around the circumference of

the pattern. The mean of the three measurements established the

reference position of the seated pattern. The completed casting, with

the top of any oversize casting removed to permit seating, was placed

on the same die with 5 Kg of force and the distance between the casting base

and die base measured as above. Because of the 10% taper of the die

the displacement of the casting relative to the reference position of

the pattern was equal to ten times the diametral change in an undistorted

casting necessary to produce the measured displacement. Percent

change in diameter of the base of the casting relative to the 5.0 mm

diameter of the pattern from which it was made is given as the value

of percent oversize or undersize of the casting.

All six series of precious PFM alloy castings

were oversize with the four high noble metal content (>90%) alloys

slightly more oversize on average (+0.51%) than the two medium noble

metal content (80%) alloys (+0.33%). All five series of Ni-Cr alloy

castings were undersize (-0.31%) on average) and were smaller by 0.76%

than the average of the six precious PFM alloys (+0.45%). Of the

three Type Ill/Type III substitutes, all of which were cast into the

same brand of gypsum investment, one series was oversize (+0.47%) and

two 0 Figure 15) were undersize (-0.41% and -0.76%). The -

0.76% undersize Howmedica A-6 series had an extremely high standard

deviation with individual casting size extremes of -2.67% and +0.91%.

/\ Porcelain fused to metal.

69
CASTING

DIAMETER

Figure 15: Casting size as a function of alloy type - manufacturer's results

CASTING SIZE - NBS LABORATORY RESULTS

Figure 16: Casting size as a function of alloy type - NBS laboratory results

70
CASTING

SIZE

VS.

MELTING

POINT

STANDARDIZED

MOLD

CONDITION

(3ZISU3AO %) U313IAIVIQ 0NI1SV3

71

Figure

17:

Casting

size

as

a

function

of

melting
All six casting series made in our laboratory

with the standardized mold technique (Fig. 16) were oversize with a range

of +0.02% oversize for Howmedica NP-2 to 1.00% oversize for Jelenko 0.

The significant differences in degree of oversize were related to the

melting points (liquidus) of the alloys. The least squares line in

Fig. 17 shows the relationship of casting size to melting temperature

for this mold condition. Degree of oversize decreased linearly with

increasing melting point (liquidus) nearly 1.0% over a temperature

range of 222°C (400°F).

Substituting Kaowool for the asbestos liner

and with all other mold and melting conditions held constant resulted

in increase in the mean size of NP-2 castings from +0.02% for a dry,

double asbestos liner to +0.31% for a 40 mil. thickness Kaowool liner

and to +0.52% for an 80 mil thickness liner (Fig. 16). Bench setting

of the investment ring at ambient room temperature (23 ± 1.5°C) resulted

in a reduction in mean size of NP-2 castings to -0.19% as compared to a

mean size of +0.02% when the investment rings were allowed to set in

the 38°C (100°F) waterbath (Fig. 16).

Ceramalloy castings made using the standardized

method for this study in which the casting ring was removed from the

816°C burnout even just 5 to 10 seconds before molten metal entered

the mold cavity were on average 0.95% oversize. Preliminary data show

that increasing the casting delay time to 20 seconds was accompanied

by an increase in mean size of Ceramalloy castings to +1.34%. Further

increase in the casting delay time to 45 seconds was accompanied by a

decrease in mean size of Ceramalloy castings to +0.72%.

12
c. Discussion

The inability to produce full size base metal

(NiCr) castings is apparently a universal problem. Twenty percent of

the laboratories responding to the survey on C & B alloys reported

problems of fit when using Ni-Cr alloys while only 5% reported problems

of fit when using precious PFM alloys (including alloys with total

precious metal content of 70 to 90% and Pd-Ag alloys). Cooperating

manufacturers in this study, without exception, produced undersize NiCr PFM castings when they used laboratory techniques and investments

of their choice to optimize results. The same manufacturers, without

exception, produced oversize precious PFM castings again using laboratory techniques and investments of their choice. This consistent

difference in casting size between the two alloy types under a wide

variety of laboratory techniques could discourage use of Ni-Cr alloys

for those operators unwilling to use shims or die coats (of questionable accuracy) or to machine out the interior of undersize castings to

permit seating and cementation.

The technique modification involving hand spatulation, dry asbestos, and hygroscopic expansion described here (Table

13) provides sufficient expansion for one phosphate bonded investment

at least, to permit fabrication of oversize Ni-Cr castings. The

results are for one size, shape and composition of wax pattern but the

general trends observed here can be expected to occur although influenced

by these variables in castings made from patterns of other sizes and

shapes. The reasons for the greater size mold cavity using this

technique are not obvious, however it is felt that the use of relatively

high energy mixing, such as results from using the blade

73
Table 13

MODIFIED TECHNIQUE FOR INCREASED EXPANSION (CERAMIGOLD INVESTMENT)

Requirements

:

1 . L/P ratio: 16/100

2. Dry asbestos: 2 layers

3. Hand spatulation: 1 minl. (120 spat. /min.)

4. Vibrate mix, flex bowl: 30 sec

5. Invest pattern

6. Hygroscopic set: 38°C, 45 min.

7. Overnight bench set

Mix Vacuspat, may interfere with the full expansion potential

of the bonded investment by accelerating the setting reaction. Additionally, the use of dry asbestos which neither perfectly adapts to

the casting ring nor becomes reduced in thickness (as does wet asbestos

when light finger pressure is used) in adapting the liner to the ring

may be an important factor in increasing mold expansion. Our measurements indicated that the thickness of two layers of dry asbestos as

used in this study was approximately 20% greater than that of two

layers of the same asbestos wetted and lightly adapted to the casting

ring.

The linear relationship between the melting

temperature (liquidus) and the degree of oversize of castings is well

supported by this study. This relationship would be expected. The

coefficients of thermal expansion of the various PFM alloys must be

approximately equivalent to provide compatibility with bonded porcelain

veneers. It follows that high melting alloys which have a greater

74

1
thermal differential between solidification and room temperature must

exhibit greater casting shrinkages than alloys with lower melting

points. A more complete explanation of the correlation between melting

temperature and casting size would take account of the facts that:

1. None of the alloys examined is either a pure metal or of

eutectic composition so that each of the alloys has a melting range

(the temperature interval between the liquidus and solidus which was

26°C to 111°C for PFM alloys in the study).

2. The initial shape of the casting is determined somewhere

between the liquidus and solidus temperatures where the growing

dentrites interlock, become fixed in relation to one another and

prevent further feeding from the sprue (riser) which still remains

liquid

.

3. Plastic deformation of the metal casting will occur until

that temperature is reached at which the yield strength of the cooling

metal is great enough to deform the investment.

The internal restraint to investment expansion in

the casting ring may be reduced by the use of more resilient liners

such as Kaowool, with consequent increase in casting size. This is

critically important for high melting alloys such as Howmedical NP^

which produced significantly smaller castings under the constant mold

conditions employed than those Ni-Cr alloys and precious metal alloys

with substantially lower melting temperatures.

The preliminary data for casting time delay

indicate that allowing the hot casting ring to rest in the cradle of

the centrifugal casting machine for 20 seconds before filling the mold

75
cavity may further increase casting size. This paradoxical result

which seems to be at variance with sensible physical reasoning is

reported primarily for completeness sake but also to stimulate interest

in a possibly important variable of casting production. The casting

time delay of 45 seconds resulted in a decrease in casting size of

0.23% (Ceramalloy + .95% oversize for 5-10 second delay versus Ceramalloy + 0.72% oversize for 45 second delay). The investment mold

cavity would be expected to be somewhat smaller after 45 seconds

exposure to room temperature environment. Also the metal casting ring

which is approximately 1.2% greater in diameter at the burnout temperature of 816°C than at room temperature (22°C) decreases about 0.1% in

JUJC

/> rt diameter for each 56°C drop in temperature . The highly conductive

metal casting ring would chill relatively more rapidly and might

possibly exert a constrictive effect on the investment.

The non-uniformity of casting size determinations for Type Ill/Type

III substitutes was not expected but reflects the variability in

casting accuracy possible with accepted reliable materials which

reportedly may be as great as 1% within a given technique for a single

operator. The emphasis in this study was on fit of PFM alloys with

the Type III alloys included only for comparison since casting accuracy

for this type of alloy had already been well reported. It is probable

that all three of the Type III alloys, which have equivalent melting

temperatures, could have been cast equally oversize with available

investing techniques that take account of both wax strength and pattern

shape

.

-Jrk Manufacturers Data. 76

i
d. Summary 1.

The results of this study show that Ni-Cr

castings produced in manufacturers' laboratories were consistently

undersize and corroborate dental laboratory reports which identify fit

of Ni-Cr PFM castings as a significant problem area.

2. A modified investing technique which involves

hand mixing of investment and the use of more resilient and thicker

ring liners permitted routine fabrication of oversize castings from a

wide range of Ni-Cr alloys.

3. Casting size was dependent on melting point of

the alloy with casting diameter decreasing lineraly with increasing melting

point under fixed mold conditions.

2. Test for Castability of Dental Alloys

In a project carried out largely with Armed Forces Defense

Personnel Support Center funding, U.S. Navy Research Associates and an

NBS physicist and technician supported on NIDR funding, a procedure for

determining the castability of either Ni-Cr or gold alloys has been

developed

.

Castings are made from a pattern constructed from No. 18 mesh

(1.00 mm) polyester sieve cloth. This provides a grid of 100 open

squares and 210 segments where a segment is defined as the part of a

sieve filament connecting two adjacent parallel filaments. Determination of the percentage of complete squares or segments in a metal

specimen cast from this pattern alloys a numerical value for "castability" to be assigned to the casting, and permits a quantitative

definition of castability for a particular alloy under specified

11
casting conditions. Preliminary castability data show Ni-Cr alloys to

be more sensitive to the amount of superheat (temperature elevation

above the melting temperature) at which the alloy is cast than are the

gold based Type III alloys. Far greater amounts of superheat were

necessary for the Ni-Cr alloy (Fig. 18) than for the gold based Type

III alloy (Fig. 19) to obtain nearly equal castability values. Castability values for Type III gold alloys were significantly higher when

these alloys were cast into gypsum molds burned out at 650°C (thermal

expansion technique) than when cast into molds burned out at 480°C

(hygroscopic technique).

%

COMPLETENESS

OF MESH PATTERN

CASTABILITY

Biobond Ni-Cr

Figure 18: Castability test results

as a function of casting

for one Ni-Cr alloy

temperature

78
%

COMPLETENESS

OF

MESH PATTERN

Figure 19:

CAST ABILITY

Hey B-2 Type M

CASTING TEMPERATURE °C

Castability test results for one gold based

type III alloy as a function of casting

temperature
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related materials, J. Dent. Res. 52, 271-80 (1973).

2. Loebenstein, W. V. , The surface area of aggregates applied to

dental materials, J. Biomed. Matls. Res. 9, 35-53 (1975).

3. Loebenstein, W. V. , Unbound water content from application of

adsorption theory, J. of Res., NBS 79A , 565-76 (1975).

4. Brunauer, S., Emmett, P. H. and Teller, E., Adsorption of gases

in multimolecular layers, J. Am. Chem. Soc. 60, 309-13 (1938).

5. NBSIR 75-766 Properties and interactions of oral structures and

restoractive materials.

6. NBSIR 76-1095 Properties and Interactions of Oral Structures and

Restorative materials.

7. Loebenstein, W. V. and Kumpula, J. , New method for evaluating

coupling agent bonding of polymer to tooth mineral, J. Dent. Res.

July- Aug. (1977).

8. Bowen, R. L. , Surface-active comonomer and method of preparation,

U. S. Patent No. 3, 200, 142 August 1965.

9. Bowen, R. L. , Dental filling materials comprising vinyl silane

treated fused silica and a binder consisting of a reactive product

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(1962)

.

10. Bowen, R. L, Crystalline dimethacrylate monomers, J. Dent. Res.

49, 810-15 (1970).

80
11 . Argentar, H. and Bowen, R. L. , Colored charge-transfer complexes

from N,N-dimethyl-p-toluidene.

12. Bowen, R. L. and Antonucci, J. , Dimethacrylate monomers of

aromatic diethers, J. Dent. Res. 54, 594-604 (1975).

13. Antonucci, J. and Bowen, R. L. , Dimethacrylates derived from

hydroxylbenzoic acids, J. Dent. Res. 55, 8-15 (1976).

14. Barton, J. , Burns, C., Chandler, H. and Bowen, R. L. , An experimental radiopaque composite material, J. Dent. Res. 52, 731-37

(1973).

15. Argentar, H. , Personal communication.

16. Brown, H. and Okamoto, Y. , Electrophylic substituent constants,

J. Am. Chem. Soc. 80, 4979-87 (1958).

17. Council on Dental Materials and Devices, New American Dental

Association Spec. No. 27 for Direct Filling Resins, JADA 94 ,

1191-94 (1977).

18. Brauer, G. , Davenport, R. and Hansen, W. , Accelerating effect of

amines on polymerization of methyl methacrylate, Mod. Plastics

34, 153-68 (1956).

19. Bowen, R. and Argentar, H. , A method for determining the optimum

peroxides to amine ratio for self-curing resins, J. Appl. Poly.

Sci. 17, 2213-22 (1973).

20. The Merck Index, Stecher, P. Ed. 8th ed. (1968).

21. Carney, R. and DeStevens, G. , a-(tert aminophenyl) aliphatic

acids, U. S. Patent 3, 657, 230 April 18, 1972.

81
22 . Lapkin, V. , Pharmacology of alkylated derivatives of phenylisopropylamide of p-aminophenylacetic acid, Russ. Pharmacol. Tox.

37, 256-59 (19740.

23. Guide to Dental Materials and Services, 7th ed. American Dental

Association, Chicago, 1974, p 212.

24. Phillips, R. W. , Research needed fro dental education and practice

Symposium on Dental Materials, NBS Special Publication 354 (1972).

25. Brauer, C. M. and Termini, D. J. , Grafting of acrylates and vinyl

chains onto collagan with ceric initiator, J. Appl. Poly. Sci.

17, 2557-68 (1973).

26. Brauer, C. M. and Termini, D. J. , Chemical modification of bone

surfaces, 4th Bratislava IUPAC Sponsored International Conference

on Modified Polymers and Their Preparation and Properties, Reprints

of Short Communications, Bratislava, 1975, Vol. 1, p 10.

27. Brauer, C. M. , Termini, D. J. and Levy, S., Surface chemical

modification of hard tissue, J. Dent. Res. 56, 646-59 (1977).

28. Lee, H. L. , Swartz, M. L. and Culp, G. , Static load testing of

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Poly. Sci. Al, 9, 2977- (1971).

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82
32. Roatare, H. M. , Powers, J. M. and Craig, R. C., Snythetic enamel

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83
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-1397

2. Gov’t Accession

No.

3. Recipient’s Accession No. J

4. TITLE AND SUBTITLE

Properties and Interactions of Oral Structures and

Restorative Materials

5. Publication Date

6. Performing Organization Codti

7. AUTHOR(S)

James M. Cassel

8. Performing Organ. Report Notl

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.ji 3110561

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NIDR, NIH

Bethesda, Md.

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Previous encouraging results with titanium and silicone based coating agents for

coupling dental resins to tooth surfaces have been reinforced with accelerated aging

data. Accelerated aging data for bone bonded to bone specimens indicates that a viscc

isoamyl cement may be worthy of clinical evaluation. Procedures for synthesis and

purification of crystallizable aromatic aldehyde dimethacrylates are given. These

highly purifiable materials offer promise as adhesion promoters through reaction with

collagen and may have utility as diluent comonomers with Bis-GMA in composites or sealants. The feasibility of generating free radical induced polymerization of dental

resins by using more thermally stable organic peresters or hydroperoxides and an ascoi

bic acid ester has been demonstrated. Design, assembly and debugging of a new multispecimen wear instrument has been completed. Judging by their thermal expansion

characteristics, high copper type amalgams show little or no evidence of phase changes

at temperatures less than 90°C. The technique sensitivity of the higher melting non- precious procelain bonding alloys has been demonstrated by examining quantitatively

the goodness of fit attained by manufacturers of such alloys who made castings to a standard die and wax pattern that we supplied. A technique to fabricate oversize Ni-( crown castings is described.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons ) Adhesive; base metal; composites; dental materials; dental

alloy; polymerization; resins; restorative materials; wear.

18. AVAILABILITY ESI Unlimited

E_! For Official Distribution. Do Not Release to NTIS

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Springfield, Virginia 22151

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21. NO. OF PAG

107

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