NATL INST. OF STAND & TECH NBS

Aiilab i7ibb3 ! PUBLICATI0NS

NBSIR 82-2623 (NIH)

Properties and Interactions of

Oral Structures and Restorative

Materials

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Polymer Science and Standards Division

Dental and Medical Materials

Washington, DC 20234

Annual Report for Period

October 1, 1981 thru September 30, 1982

Issued December 1982

Interagency Agreement

YOI-DE-4001 5-11

Prepared for uc

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itional Institute of Dental Research

ethesda, MD 20014

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NBSIR 82-2623 (NIH)

DEC 15 1982

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^/** . PROPERTIES AND INTERACTIONS OF

ORAL STRUCTURES AND RESTORATIVE

MATERIALS

James M. Cassel, J. A. Tesk, G. M. Brauer, J. M. Antonucci,

W. Wu, and J. E. McKinney

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Polymer Science and Standards Division

Dental and Medical Materials

Washington, DC 20234

Annual Report for Period

October 1, 1981 thru September 30, 1982

Issued December 1982

Interagency Agreement

YOI-DE-4001 5-1

1

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, MD 20014

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

ABSTRACT i SUMMARY ii

INTRODUCTION VII

I. Dental Composites, Resins, Cements and Adhesion: Synthesis and

Evaluation 1

A. Monomer Systems Designed to Reduce Polymerization Shrinkage,

Residual Unsaturation, and Hydrophi 1 icity 1

1. Resin Formulations of Bulky Dimethacrylates, BIS-GMA

+ UDM 1

2. Resin Formulations of PFMA and a Bulky Monomethacrylate 3

3. Resin Formulations of PFMA and Crosslinking Diluent

Monomers 6

4. Polythiols for Reducing Vinyl Unsaturation and

Polymerization Shrinkage 10

B. Initiator-Accelerator System Improvement: Efficiency and

Biocompatibility 16

1. Initiator Research 16

2. Accelerator Research 17

C. Composite Filler and Filler/Resin Coupling Improvement . . 37

D. Dental Cement Innovations and Adhesion Research 39

1. Vani Hate-Based Cements 39

2. Intermediate Restorative Resins Based on VanillateEBA Cements 53

3. Dimer Acid Polycarboxyl ate Cements 59

4. Thiol/ene Cements 68

5. Development of Adhesive Bonding Techniques 69

II. Wear Resistance and Related Physical Properties of Dental

Composite Restorative Materials 72

A. Solvent Diffusion Rates in Dental Composites 73

B. Influence of Chemical Softening on Hardness and Wear ... 82

1. Wear Measurements on Chemically Soaked Composites . . 88

C. Degree of Cure in Dental Composites as Function of Cure

Temperature and Time 96

D. Influence of Degree of Cure on Hardness and Wear 101

E. Erosion Tester 106
III. Dental Alloys and Ceramics

A. Alloy Castability

B. Porcelain-Alloy Stress Compatibility. . .

C. Porcelain-to-Metal Bond Strength

D. Fit and Soldering of Dental Alloy Castings

REFERENCES

INVITED TALKS (1982)

ABSTRACTS IADR/AADR MEETING, MARCH 82, NEW ORLEANS.

ABSTRACTS SUBMITTED FOR 1983 AADR MEETING

PAPERS PUBLISHED 91982)

MANUSCRIPTS IN REVIEW OR ACCEPTED FOR PUBLICATION .
ABSTRACT

Significant accomplishments in the field of dental materials research for the

past year include:

(1) New polymeric formulations designed to reduce curing shrinkage, residual

unsaturation and hydrophilicity in dental composite restorations have been developed.

A high molecular weight, moderately viscous polythiol, pentaerythritol tetra(3-mercaptopropionate) , not only contributes toward this goal but improves both color and

color stability.

(2) The recently reported dental cements based on esters of vanillic acid have

now been shown to have excellent long term solubility characteristics. Modification of

formulations yields uniquely high strength properties. Their potential for development of higher strength intermediate restorative resins has been demonstrated.

(3) Deterioration of composite restorative materials exposed to solvents that

simulate food debris was shown to be maximal in 75% ethanol-water solutions as judged

by silver stain examination of the microdefect regions, changes induced in surface

hardness and wear loss as measured by a pin on disc apparatus.

(4) Supporting the hypothesis that environmental factors, i.e. food-derived oral

components, are the key antagonists to improved durability of composite restorations

is the knowledge that the resin matrix as cured at 37°C has a low degree of polymerization (DP) and hence is more susceptible to softening by penetrating molecules.

Infrared spectroscopy measurements have now shown that the DP in a given composite is

more sensitive to monomer composition than to initiator type or concentration. The

rate of solvent induced softening of composites has been demonstrated to depend on the

temperature at which the composite is cured.

(5) To overcome some of the dissatisfaction with existing porcelain-metal bond

strength (PMBS) tests, a bending beam composite specimen was designed and initial four

point bending results obtained in collaboration with several dental manufacturers

indicate the potential for a simple, reproducible, and informative test method.

(6) Using a recently developed alloy castability test method, it has been shown

that castability values are strongly dependent on mold and alloy temperatures and that

observations of the castability value standard deviation can be used to optimize

casting conditions.

i
SUMMARY

New polymeric formulations designed to reduce curing shrinkage,

residual unsaturation and hydrophi 1 icity in dental composite restorations

have been developed and evaluated. To reduce polymerization shrinkage

emphasis has been placed on high molecular weight bulky monomers such as

blends of BIS-GMA and diurea dimethacrylate types. While typically

moderate to fast-acting accelerators such as bis(N ,N-2-hydroxypropyl )ptoluidine or p -tert-butyl -N ,N-dimethylani 1 ine could not be used with

such formulations, substitution of slower acting accelerators such as

ethyl -4-dimethyl ami nobenzoate or ascorbyl palmitate yielded formulations

that set in clinically acceptable times. Powder/liquid, paste/paste or

single paste/visible light cured formulations produced composites with

excellent physical and esthetic properties.

To achieve a lower residual unsaturation in composite resin matrices

without increasing the polymerization shrinkage as occurs when methyl

methacrylate serves as monomer diluent, combinations of bi cyclic mono

methacrylates such as isobornyl methacrylate with a highly fluorinated,

multifunctional prepolymeric methacrylate (PFMA) were examined. The

latter, having a molecular weight of approximately 10,300 and a high

fluorine content, helps to reduce polymerization shrinkage and impart

hydrophobic character to the cured composite. Rather low strength

properties in these composites were improved only slightly by using a

glass filler that had been not only silanized but also treated with a

fluorinated silane agent. Substitution of crosslinking dimethacrylates

for the isobornyl methacrylate and retention of the highly fluorinated

PFMA along with a fluorinated monomer diluent yielded composites with

very high fluorine content and with adequate strength properties . A

i i
new application to dental materials of Fourier Transform Infrared Spectroscopy showed the desirably low water sorption properties of these composites.

For reduction of both residual unsaturation and polymerization

shrinkage in the composite resin matrix high molecular weight polythiols

such as pentaerythri tol tetra(3-mercaptopropionate) , PETMP, have been

shown to have promise as potential non-vinyl comonomers as well as

synergistic polymerization accelerators. Modest amounts of PETMP (<5%

by weight) reduced unsaturation of a typical composite resin system by

as much as 20 percent. Studies of PETMP with slow, moderate and fast

amine accelerators demonstrated the effectiveness of the polythiol in

enhancing esthetics as well as color stability. PETMP offers the means

for fashioning dental resin systems with a wide range of properties,

e.g. tough elastomers to rigid glasses.

Despite many inferior properties dental cements are estimated to

find application in 50 percent of all dental restorative procedures. In

the FY81 report (NBS IR 81-2431; PB82-195579 A08) the development of

improved cements based on reaction of zinc oxide, ethoxybenzoic acid and

hexyl vanillate was described. While biocompatibility data is being

accumulated on such cements, new vanillate derivatives such as decamethylene divanillate and methacryloyl ethyl vanillate and esters of

syringic acid have been synthesized and their incorporation into cements

studied. The latter compounds were included since syringic acid had

earlier been shown to have caries inhibiting effects in rats. Strength,

adhesion and solubility properties of some of these cements are equally

or more promising than those obtained with the hexyl vanillate. The

hexyl vanillate type cements have the unique property of not inhibiting

polymerization of dental resins (in contrast to the eugenol cements they
are designed to replace). Therefore investigations have been conducted

to modify these cements by inclusion of a polymerizable monomer system

as a means for developing improved intermediate restorative materials.

By this technique cements with strengths three to six times that of

comparable eugenol cement formulations were achieved. Particularly

promising as an intermediate restorative liquid component is a formulation

consisting of hexyl vanillate, ethoxybenzoic acid, dicyclopentenyloxyethyl methacrylate and methyl methacrylate.

Our recent demonstration (1) that a subsurface damaged region

exists in clinically worn composite restorations , (2) that the microvoids

generated in this region are isotropic and not of a stress-induced

preferred orientation, (3) that microvoids exist throughout the resin

matrix as well as in the filler/resin interface region, and (4) that

immersion in food simulating chemical solvents covering a rather wide

range of solubility parameters will induce a form of damage similar to

that observed in the clinical specimens has shifted the emphasis in

"wear" research to the resin component of composites. A classical pin

on disc wear test (described in previous reports) and an erosion technique

provide accelerated testing of specimens degraded in a manner believed

to simulate in vivo degradation processes. Fourier Transform Infrared

Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Scanning

Electron Microscopy (SEM), Small Angle X-ray Scattering (SAXS), and the

silver staining procedure have all been brought to bear in an analysis

of the mechanisms governing the durability of composite restorative

materials.

Supporting the hypothesis that environmental factors, i.e. foodderived oral components, are the key antagonists to improved durability

iv
of composite restorations is the knowledge that the resin matrix as

cured at 37°C has a low degree of polymerization and hence is more

susceptible to softening by penetrating molecules. FTIR measurements

have now shown that the degree of polymerization, DP, in a given composite is more sensitive to monomer composition than to initiator type

or concentration. Such measurements have also been used to project the

maximum DP that can be expected to be attained in a year at 37°C as well

as the extent of increase in DP that is achievable at higher initial

curing temperatures. Knoop Hardness measurement of composites immersed

in food simulating solvents showed that the rate of solvent-induced

softening of the composites depends on the temperature at which the

composites are cured. In vitro wear results on solvent immersed composites corroborate the hardness measurement data and reinforce the need

to improve resin resistance to such attack.

A critical factor in the clinical performance of porcelain fused to

metal restorations is assumed to be the strength of the bonding. Existing

tests for bond strength have deficiencies that have inhibited their

application universally. In an attempt to overcome some of the principal

difficulties with these tests, a bending beam composite specimen was

designed for evaluation. The specimen is prepared to provide all smooth

surfaces at porcelain-metal junctions and when subjected to the proper

bending moment, creates tension at the bond and within the porcelain.

Theoretical beam model stress calculations were checked by the finite

element method and showed smooth transition in surface stress across the

interface. Initial experimental data obtained in collaboration with

three dental manufacturers indicate the potential for a simple, reproducible

and informative test method.

v
A castability test for dental alloys that evolves a castability

value, C , by determination of the percentage of cast alloy segments

that comprise a rectangular grid, is shown to be useful in optimizing

casting conditions. The castability value defines what can be expected

for an alloy used under controlled conditions in a dental laboratory.

In a study of the effects of mold and alloy temperatures, the dependence

of C

v on temperature was clearly indicated. For each alloy examined an

optimal mold and casting temperature could be discerned beyond which

castability values changed very little. The degree of "superheat"

required to produce the optimal casting temperature is unique to each

alloy. As the limiting value for castability is approached (C = 100

percent) the standard deviation (SD) from the mean determination of C

v appears to decrease. Hence, larger SD 1 s for some alloys may be indicative of potential for further optimizing conditions for casting of that

alloy.
c

INTRODUCTION

The National Bureau of Standards is primarily a measurement organization.

Paths of 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, to commercial production by industry.

The dental research program at NBS has been a long standing cooperative

effort both in personnel and funding, with the American Dental Association.

Specific projects are carried out for the U. S. Army and a Guest Worker

program with the U. S. Navy exists. With the exception of the research

on porcelain fused to base metal alloy - research also participated in

and partially funded by the U. S. Navy Medical R&D Command - the research

described herein is only that portion of the dental program supported by

interagency agreement with the National Institute of Dental Research.

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. The major areas of research

are given below:

I. Dental Composites, Resins, Cements and Adhesion: Synthesis

and Evaluation

II. Wear Resistance and Related Physical Properties of Dental

Composite Restorative Materials

III. Dental Alloys and Ceramics

VI 1

I. Dental Composites, Resins, Cements and Adhesion: Synthesis and

Evaluation

A. Monomer Systems Designed to Reduce Polymerization Shrinkage,

Residual Unsaturation, and Hydrophil icity

1. Resin Formulations of Bulky Dimethacrylates BIS-GMA + UDM

A commercial aliphatic diurethane dimethacrylate (Vivadent)

prepared from trimethyl hexamethylene di isocyanate (approximately an

equimolar mixture of the 2,2,4- and 2,4,4-isomers) and 2-hydroxyethyl

methacrylate (HEMA) - designated by us as UDM - was shown in earlier

reports to be an excellent resin for dental composites [1,2]. Composite

formulations (e.g. powder/liquid, paste/paste) had excel lent working

,

setting time, mechanical and esthetic characteristics . The molecular

weight (470) and bulk of UDM are somewhat less than that of BIS-GMA and

since UDM also contains some residual HEMA, it would be expected to have

a higher polymerization shrinkage than BIS-GMA. Since the commercially

available UDM does have a workable viscosity at ambient temperatures, in

contrast to BIS-GMA, we decided to investigate a 1:1 blend of UDM and

BIS-GMA as a means of obtaining a low-shrinking resin system of workable

viscosity. Preliminary results in a powder/liquid formulation (powder/

liquid ratio = 3) using low concentrations of moderate and fast-acting

amine polymerization accelerators did not yield workable mixes. The

moderate- and fast-acting accelerators were bis(N,N-2-hydroxypropyl )-ptoluidine (DHPPT) and p- tert -butyl -N ,N-dimethyl ani 1 i ne (BDMA), respectively.

With slow-acting accelerators such as ethyl -4-dimethylaminobenzoate(4-

EDMAB) and ascorbyl palmitate (AP) workable composite mixes were obtainable

with clinically acceptable setting times [3]. The viscous resin system s BIS-GMA/UDM, also was adaptable for use in paste/paste (peroxide-amine)

1
and single paste/visible light formulations (powder/liquid = 3).

Composites of excellent mechanical and esthetic properties resulted from

the use of these techniques. For example, the paste/paste formulation

using 0.3% DHPPT in the amine paste and 1% benzoyl peroxide silanized

glass in the peroxide paste gave, with equal parts of each paste, is a

composite with good handling properties, a setting time of 1.5 min.,

very good esthetics and color stability and a diametral tensile strength

of 52.2 ± 2.0 MPa (5 specimens.) From a one minute visible light photo

polymerization of a single paste formulation containing 0.2% camphoroquinone and 0.2% ethyl 4-dimethyl aminobenzoate (4-EDMAB) by weight

(based on the resin), composite specimens of similar mechanical properties

(diametral tensile strength = 47.5 ± 1.8 MPa, 6 specimens) and excellent

esthetics and color stability were obtained.

The use of high molecular weight, bulky conventional dental monomers

(e.g. BIS-GMA, UDM, etc.) in resin formulations is an obvious method of

reducing polymerization contraction and, thereby, minimizing residual

stresses in composites and sealants. However, the highly reactive

nature of such viscous dental resins precludes their use in powder/

liquid or paste/paste formulations with peroxides and fast-acting polymerization accelerators without the addition of relatively large amounts of

inhibitor (e.g. BHT). With moderate- and slow-acting accelerators

suitable working and setting times can be obtained with these reactive,

viscous resins in paste/paste and powder/liquid formulations employing

peroxide initiators and low levels of inhibitor. Since photo-polymerizable composites have virtually unlimited working times, the visible

light initiation systems are well suited for use with this type of

dental resin systems.

2
2. Resin Formulations of PFMA and a Bulky Monomethacrylate

For conventional dental resin systems, such as those based on

BIS-GMA or UDM as the major monomeric constituent, the degree of polymerization (DP) is determined to a great extent by the nature or quantity of

the diluent monomer(s) used in the monomer system [4,5]. With a photopolymerized monomer system consisting of approximately 40 mol percent

BIS-GMA and 60 mole percent methyl methacrylate (MMA), Ruyter and Gyorosi

detected a relatively low content of unreacted vinyl groups [4]. This

high degree of conversion to polymeric material was caused by the large

number of readily diffusible MMA molecules present in the resin. When

the resin was exposed to shortwave ultraviolet irradiation, the residual

unreacted methacrylate groups was only 5%. This high degree of conversion

was attributed to higher yield of radicals produced by the short wave UV

exposure. This last result indicates some dependency of degree of conversion on the initiation system. With visible light we have seen

similar decreases in residual vinyl unsaturation as a result of increasing

time of irradiation. Unfortunately, the use of MMA as a major diluent

monomer to achieve a high DP leads to a relatively high degree of polymerization shrinkage and poor marginal adaptation.

A commercially available reactive, bulky (MW=222), alicyclic

monomethacrylate, isobornyl methacrylate-iBM (see structure below) was

shown in previous reports to yield composites with acceptable working

and setting characteristics, very good esthetics and color stability,

and good mechanical properties when used as a diluent monomer for BISGMA or the urea dimethacrylate (shown below), TBAEM-HDI [2].

3
CH. 0

H

2

C CH—OC-C=CH

ch

3-c-ch

3 H,C CH.

CH, 0 0 0 CH

=1 _ i 3 Ji h u i ^

; 2 CH o =C-C-0-CH 9 CH 9 -N-C-NHCH-CH,CH ? CH

7 CH CH -NHC-N-CH? CH2 0C-C=CH

2 CH

2

|| L L

\ c c. c L l c ,4

3 o CH-j-C-CH, tbaem-hdi

1 I J CH

3

-C-CH

3 CH. CH.

iBM

Unlike the bulky, aliphatic, long-chain monomethacrylates (e.g. lauryl and

stearyl methacrylates ) which have sluggish rates of polymerization compared to

that of MMA at ambient temperatures , iBM seems to be even more reactive than MMA

(see later section.) Another advantage of using iBM and similar alicyclic monomers

over long-chain alkyl methacrylates is the stronger copolymeric matrix that

results from the use of this bicyclic methacrylate . This would be expected since

the homopolymer of iBM has a glass transition temperature (T ) comparable to that

of PMMA (110°C vs 104°C) whereas the homopolymer of stearyl methacrylate has a

much lower (-100°C). For the same mol percent of diluent monomer and assuming

a similar extent of polymerization, iBM copolymers should exhibit lower contraction

on polymerization than the corresponding MMA copolymers.

The highly fluorinated prepolymer mul tifunctional methacrylate, PFMA

(see structure below), that was synthesized in these laboratories is a novel

resin that should yield dental polymeric matrices that have low surface energies

and that are extremely hydrophobic and resistant to the sorption of chemical

fluids derived from the mastication of food in the oral environment [6-10].

CF,3-OOF H-CHy-OC-CH=CH-CH

CF„

H-CHj-C

0

;-CH.

1CH. PFMA

=0

C-CH.

iCH.

HN~ 10

MW OF REPEATING UNIT = 103Z AVG MW = 10,326?

4
As is discussed at greater length in section II of this report,

a likely, significant factor influencing the wear behavior of dental

composites and sealants is the water-assisted diffusion of certain

chemical moities derived from the presence of food in the oral environment which cause the plasticization of these polymeric materials. This

chemical softening makes composites and sealants more prone to various

degradative processes (e.g. chemical, mechanical, etc.) which lead

ultimately to loss of anatomic form. By virtue of their high fluorine

content resins based on PFMA should yield polymeric matrices having solubility parameters far removed from those of food generated oral fluids

and are, therefore, less likely to be plasticized.

A problem with the viscous, polyfluorinated thermosetting

resin, PFMA, has been its somewhat lower reactivity to free radical

vinyl polymerization compared to conventional resins (e.g. BIS-GMA)

.

This lower reactivity is mainly due to the lower vinyl content of PFMA

but may also arise from a somewhat higher solubility of oxygen in highly

fluorinated resins. By suitable adjustments in the initiator system

(e.g. more active accelerators) and the type and concentration of the

diluent monomer(s), PFMA-based resins with acceptable setting times

(2. 0-8.0 min) have been formulated. Due to its somewhat more flexible

chemical nature, PFMA does not yield polymers or copolymers of as high

moduli as the corresponding BIS-GMA derived polymers. However, PFMA

based composites with acceptable mechanical strength have been made with

the aid of the following types of diluent monomers: neopentyl glycol

dimethacrylate (NPDMA), 1 ,6-hexamethyl ene dimethacrylate (HMDMA), 1,10-

decamethylene dimethacrylate (DMDMA) and methyl methacrylate (MMA.)

5
The last formulation would be expected to yield composites

with low residual vinyl unsaturation but, because of the relatively high

content of MMA (33% by weight), also with a high degree of polymerization

contraction. The diametral tensile strength obtained with this PFMA/MMA

resin system using a powder/liquid ratio of three was 45 MPa. In view

of the bulkier nature of iBM compared to MMA and the positive results

obtained with this bicyclic diluent monomer and BIS-GMA or TBAEM-HDI , we

investigated several formulations of PFMA with iBM. In contrast to the

tensile strength results found with the PFMA/MMA formulation, none of

the PFMA/ iBM formulations passed the minimum ADA specification for DTS

[11]. The moderate-acting accelerator, N,N-Bis(2-hydroxypropyl )-ptoluidine (DHPPT), when used without the tetrathiol, PETMP, gave at the

0.2% concentration a sluggish setting composite (>8 min). PETMP reduced

the setting time significantly even at the low concentrations used in

these formulations (Table 1). Higher concentrations of PETMP with

PFMA/iBM resulted in the formation of emulsions indicative of the lack

of compatibility of PETMP and PFMA. The relatively low mechanical

strength of these composites may be enhanced by the addition of some

crosslinking diluent (e.g. NPDMA) and/or the use of an initiator system

that results in better cured composites. The use of F-Glass (dual

silanized glass) which was prepared by a technique described in a previous

report [2] improved the ease of mixing of these powder/liquid formulations

and seemed to improve their mechanical strength slightly (compare formulations A and A' and C and C ' , Table 1.)

3. Resin Formulations of PFMA and Crosslinking Diluent

In Table 2 are summarized the results obtained with PFMA resin

6

Monomers
formulations employing cross linking diluent monomers and fast-acting

amine accelerators. As can be seen DTS values are enhanced significantly

by the use of aliphatic hydrocarbon dimethacrylate diluents such as

HMDMA, NPDMA, DMDMA and 2,2-bis[p-$-methacryloxyethoxy)phenyl]propane

(BIS-EMA). The use of fast-acting amine polymerization accelerators such as

p- tertb

utyl -N,N-di methyl aniline (BDMA) , 3,5-dimethyl-N,N-dimethylanil ine

(DMSX) , p-N,N-diethylaminophenylacetic acid (DEAPAA) and p-N ,N-dimethylaminophenethanol (DMAPE) also may contribute to the attainment of higher

strength composites. Highest DTS values were obtained with Formulations

2, 2' and 3. After 7 days storage at 37°C and 100% relative humidity,

composites based on formulation 3 showed a significant increase in DTS

(42.4 MPa) which may indicate an increasing DP with time. The use of FGlass did not significantly affect mechanical properties but did improve

the mixing characteristics of these powder-liquid formulations. Formulation

1, which had the highly fluorinated long chain monoacrylate, pentadecafluorooctyl acrylate (PDFOA), as a co-diluent, gave composites with acceptable

DTS values. This PFMA based resin had the highest weight per cent of

covalently bound F (approximately 36%) which is only slightly less than

that of the hydrophobic resin based on octafluoropentylmethacrylate

(OFPMA) and BIS-EMA (38% F) developed by Craig et al [7,8]. Formulations

3 (31% F) and 4 (33% F), had the lowest water sorption after 7 days of

2 2

any composites we have studied, 0.15 mg/cm , and 0.13 mg/cm respectively

which compare favorably with that reported for the OFPMA/BIS-EMA composite

(0.14 mg/cm ). Use of fluorinated glass (4') did not enhance hydrophobicity.

These hydrophobic composites based on fluorine-containing resins reached

equilibrium with water much more rapidly (2-3 days) than conventional

composites. Similar behavior was noted for the OFPMA/BIS-EMA composites

7
TABLE 1

Setting Time and Strength Properties of PFMA-IBM-Derived Composites

( Powder/ 1 i quid formulations = 3)

Formulation

of Resin Composition

Setting Time

(min.

)

Diametral Tensile

Strength

(MPa Std Dev)

A PFMA 62.57% 3.5 26.6 1.8

IBM 36.17% ( 5)

3 PETMP 1 1.06%

DHPPT

2

0.20%

A

1 Same as A but using dual 3.25 27.2 0.4

silanized glass or (5)

F-Gl ass 4

B PFMA 74.60% 3.0 30.1 1.1

IBM 23.94% (5)

PETMP 1.27%

DHPPT 0.19%

C PFMA 62.45% 3.0 23.1 2.5

IBM 36.10% (4)

PETMP 1.05%

DHPPT 0.40%

C' Same as C but using 3.0 26.8 3.2

F-Glass 4 (5)

1 Pentarythri tol tetramercaptopropionate

2 N,N-Bis(2-hydroxypropyl )-p-toluidine

3 Number of specimens

4 Dual silanized glass (F-Glass) was prepared by a second silanization of A- 174

glass using the fluoroorganic silane agent TDFOS-f-1 ,1 ,2 ,2-tetrahydrotridecaf 1 uorooctyl dimethyl chi orosi 1 ane) 2

8
[8]. Unlike the relatively mechanically weak OFPMA/BIS-EMA derived

composites, the PFMA based composites have acceptable mechanical strength

properties along with low water sorption. In addition, the polymerization

shrinkage of the PFMA composites should be much lower than that of

OFPMA/BIS-EMA composites.

a. Water Sorption Studies by Fourier Transform Infrared

Spectroscopy (FTIR)

Since the water-related properties of dental composite

and sealant materials may be critical factors in determining their

performance in the oral environment, we have initiated a study to better

understand the mechanism of the diffusion of water and related fluids

into these materials. Water sorption measurements by use of nearinfrared spectroscopic techniques can be a very sensitive method for

assessing water uptake by composite and sealant specimens. Absorbance

due to water occurs in a very transparent region of the near-infrared

spectrum at 5203-5220 cm~^ (2. 0-1. 9 um). By standard methods of quantitative IR analysis, the absorbance peak due to water is directly proportional to the amount of water present in the specimen. In contrast

to the determination of water sorption by the method outlined in ADA

Specification No. 27 which requires immersion of a relatively large

specimen in water, correction for solubility effects, long equilibration

times and the measurement of small changes in large numbers, the near-IR

method has the following advantages: (1) thin films which have short

equilibration times can be used, (2) measurements after immersion in

water versus exposure to atmospheres of 100% relative humidity (which

eliminates leaching or solubulity effects) can be made, and (3) the time

dependent water-sorption behavior of the specimen may be easily monitored.

9
The following spectra indicate the sensitivity of this

method. Figure 1A shows the near-IR spectrum of a dry commercial composite (Silar) after storage in a desiccator and Figure IB is the near

IR spectrum of the same sample after exposure to an atmosphere of 100%

relative humidity for 24 hours. The extreme reluctance to water uptake

by composite films derived from the fluorinated resin, PFMA, (Formulation 3, Table 2) is made strikingly evident by the absence of any

absorbance peak for water after the dry film is exposed to the humid

atmosphere (compare Figure 2A and 2B) for 24 hours.

4. Polythiols for Reducing Vinyl Unsaturation and Polymerization Shrinkage

For reduction of residual unsaturation as well as polymerization shrinkage we have been investigating high molecular weight,

moderately viscous polythiols such as pentaerythri to! tetra(3-mercaptopropionate), PETMP, as a potential non-vinyl comonomer as well as synergistic

polymerization accelerator.

The rationale for the use of polythiols such as PETMP to

reduce the number of unpolymerized vinyl groups is based on the well

known thiol /ene reaction which occurs by a free radical chain mechanism

[12,13]

1 )

2 )

3)

RSH + ^

C = C

I

I RS-C-C* + RSH

RS* + NC = C(f / \

*

RSC-C* + H

1 r1

1 RS-C-C-H +

I 1.

RS-C-C*

\ \

X

RSWhere >70 is an initiating radical

With olefins or polyolefins that are reluctant to propagate a

10
TABLE 2

Setting Time and Diametral Tensile Strengths of PFMA

Composites Using Crosslinking Diluent Monomers

Formulation Composition of

Resin (w/o)

Powder/Liquid

Ratio

Setting Time

(min.)

Diametral Tensile

Strength MPa [Std.

Dev. ](No. of spec.

)

1 PFMA, 59.0%

HMDMA 15.2

BIS-EMA 10.0

PDFOA 15.2

DMSX 0.6

3 4.0 36.4 [3.0]

(5)

2 PFMA 69.5

NPDMA 28.6

PETMP 1.6

DMAPE 0.3

3.5 3.0 38.7 [0.9]

(5)

2' Same as 2 but

with F-Glass

3.5 3.0 38.8 [1.2]

(5)

3 PFMA 69.2

NPDMA 29.2

PETMP 1.0

DEAPAA 0.6

3.5 3.0 39.2 [2.3]

(5)

42.4 [1.0*]

(4)

4 PFMA 74.3

DMDMA 24.9

PETMP 0.5

BDMA 0.3

3.5 4.0 35.8 [2.9]

(5)

4' Same as 4 but

with F-Glass

3.5 4.25 35.3 [2.0]

(5)

* After 7 day storage at 37°C and 100% Relative Humidity

11
ABSORBANCE

-

:

,•

---ABSORBANCE

o

SILRR FILM IN HUM IB ATMOSPHERE 21 H 5/5/82 (3)

IB

Figure 1 (A & B)

12
2A

o

O HYDROPHOBIC HRTERIRL SR/9 2<iH IN HUH I D RTM.

WRVENUMBERS

Figure 2 (A & B)

13
polymer chain by a free radical addition mechanism, e.g.

ch

2

=ch-ch

2 -r-ch2

ch=ch

2

,

polythiols can yield polymers by a free radical step addition polymerization mechanism. When used with multifunctional vinyl monomers that

readily undergo free radical self-addition reactions of vinyl groups to

create a propagating polymeric chain, e.g.

CH o =C(CHj-C-0-R-0C-C(CHj=CHo , c II ll Sc.

0 0

the use of polythiols such as PETMP leads to a hybrid polymer

composed of polymethacrylate and polythioether elements in the network

structure, that is a poly(methacrylate-thioether) is formed by a dual

mechanism involving free radical self-addition and free radical stepaddition as shown in Figure 3. Which mechanism will predominate in the

polymerization will depend, among other factors, on the relative reactivity of the vinyl monomer and the polythiol and their relative concentrations. Also, the chemical, physical and mechanical properties of the

resulting polymer will be determined by the molecular structures of the

vinyl monomers and the polythiol and their relative concentrations. For

example, using a relatively high content of PETMP (e.g. 33% by weight)

with BIS-GMA yields an elastomeric copolymer.

Preliminary results using infrared spectroscopic methods indicate

that there is indeed a significant reduction in vinyl unsaturation on

using modest amounts of PETMP (<5% by weight) in a typical dental resin

formulation. IR spectra of the two resin formulations A and B shown

below were recorded, the formulations were photopolymerized using the

same conditions of irradiation and the spectra obtained on the polymer

fi 1 ms

.

14
POLYTH I QL/PQLYENE REACTION

CROSS LINKED POLYMER

0 CH.

R— (0-C-C=CH

2

) 2

X1 R('wwSH)

CH. CH. CH.

~~~~C —CH„ Svw^vwn,s—CH 0 —C —C- 2

I II H 0

0—R—0 —C-C-CH0

{ f

2

6 CH.

CH.

0 CH.

ch

3—C— C- 0 - R- 0— C—0 =CH,

CH.

CH^—c^

II

o=l: l

CH_—C—:CH 0

- o , s 2

o=c f

0 l

!, | R } d LCH.

l

L

I , c —

0

5

I i o !

ICH.

1

CK„

i °

•CH

u{0

18

SvuVo

Hhere X = an initiative radical

Figure 3.

15
A B

BIS-GMA 84.45 82.02

DMDMA* 14.90 14.48

N ,N-dimethylaminoethanol 0.45 0.00

Camphoroquinone 0.20 0.20

PETMP 0.00 3.30

*DMDMA = 1 ,10-decamethylene dimethacryl ate

Formulations A and B showed a conversion of vinyl groups to polymer

respectively of 52% and 63%.

The addition of about 3% PETMP to a commercial amine-containing

paste (ADAPTIC) also showed a significant decrease (8-14%) in residual

vinyl unsaturation of the cured composite compared to a similar composite

prepared with the usual amine paste. Adding 4% PETMP to Durafill resulted

in about a 20% increase in DP after 7 days storage at 37°C. Further

studies on both experimental and PETMP modified commercial dental composites are in progress.

B. Initiator-Accelerator System Improvement: Efficiency and

Biocompatibility

1 . Initiator Research

The benzoyl peroxide present as polymerization initiator in the

paste or powder component is primarily responsible for the poor storage

stability of dental composite formulations. Therefore we have endeavored

to develop initiator-accelerator systems that do not utilize benzoyl

peroxide as initiator. Reactivity of such systems has been determined by

measurements of the setting time of composite formulations containing

peroxide initiators more thermally stable than benzoyl peroxide in the

16
tertiary amine accelerated polymerization of dental composite resins.

During this reporting period composite formulations containing

t-butyl perbenzoate (an initiator with longer thermal half-life than

benzoyl peroxide) and N ,N-diethylaminophenethanol were prepared. The

effect of the addition of acids such as sorbic, benzoic or methacrylic

acid on the reactivity of the initiating system was determined. It was

hoped that rate of curing of the perester-initiating system would be

speeded up in the presence of acids. However, with the specific compositions studied no clinically useful polymerization rates, i.e. those

with cure times of less than 10 minutes were obtained.

2. Accelerator Research

a. Update on Previously Reported Efficient Amine Accelerators

During FY81 we had synthesized and evaluated a series of

new tertiary aromatic amine accelerators based on derivatives of 4-N,Ndial kyl ami nophenyl acetic acids and 4-N ,N-dial kylaminophethanol s [14].

These compounds proved to be very reactive in the peroxide initiated

polymerization of composites. Even very low concentrations of these

accelerators in the monomer lead to rapid cures. The resulting experimental composites exhibited light shades and excellent color stability.

The 4-N,N-dialkylaminophenethanol has since been made available commercially. Subject to results of biocompatibility tests this amine is

expected to become an ingredient of commercial dental composites.

An invited paper "Initiator-Accelerator Systems for

Dental Appl ications" was presented at the symposium on "Initiations of

Polymerization" at the 1982 Semi-Annual meeting of the American Chemical

Society. The manuscript has been accepted for publication in the ACS

Symposium Series.

17
b. PETMP as Accelerator and Reactive Diluent

PETMP*, in addition to reacting by the thiol /ene addition

reaction, also can function as an accelerator for the ambient decompositi

of BP. With exception of Formulation 1 (Table 5) it appears that PETMP

is not an especially active promotor for the polymerization of acrylic

monomers containing BP. With a fast polymerizing monomer such as BIS-GMA

and in high concentrations, PETMP is an effective accelerator which also

acts as a diluent comonomer. The relatively low diametral tensile

strengths of the BIS-GMA (2) / PETMP (1) composites is indicative of a

high content of flexible polythioether elements incorporated into the

structure of the polymeric matrix. This type of composite, as expected,

exhibited excellent esthetics and color stability.

PETMP as a^ Synergistic Agent for SI ow-Acting Amine

Polymerization Accelerators

The ethyl ester of 4-dimethylaminobenzoate (4-EDMAB) was

shown in a previous study to be a slow-acting accelerator for composites

prepared with the dental resin, BIS-GMA (7)/TEGDMA (3) especially when

stabilized with 0.1% BHT. The effect of adding PETMP to such composites

is to reduce the setting time as shown in Table 6. Another effect of

PETMP is to ameliorate the effects of air inhibition on the surface cure

of these composites.

Effect of PETMP on Composite Formulations Using Moderate

Acti ng Amine Polymerization Accelerators

DHPPT (Table 3) may be classified as an amine accelerator

of intermediate activity, i.e. more reactive than 4-EDMAB but less

reactive than BDMA or DMSX. The results of adding PETMP to powder/liquid

* Tables 3 and 4 list the names and abbreviations pertinent to this

investigation.

18
Table 3. Polymerization Accelerators

Name Abbreviation Source

Ethyl 4-dimethylaminobenzoate 4-EDMAB Aldrich Chemical Co.

Milwaukee, V/I

N,N-Bis(2-hydroxypropyl )-p-toluidine DHPPT Ivoclar AG

(Schaan, Liechtenstein)

< N,N-Dimethyl-sym-xyl idi ne DMSX Aldrich Chemical Co.

Milwaukee, W1

p-tert-Butyl-N,N-dimethylanil ine BDMA Aldrich Chemical Co.

Milwaukee, HI

Ascorbyl Palmitate AP Pfizer, Inc.

New York, NY

Per.taerythritol tetra(3-mercaptopropionatePETMP Evans Chemetics,

W. R. Grace & Co.

Darien, CT

Table 4.. Monomers

Name Abbreviation Source

2 ,2-BIS[p- (y-methocryl oxy3-hydroxypropoxy) phenyl] propane

BIS-GMA Freeman Chemical Co.

Milwaukee, WI

NCO Monomer NCO The L. D. Caulk Co.

Philadelphia, PA

Tri ethyleneglycol dimethacrylate TEGDMA Esschcm Co.

Essington, PA

1 ,10-decamethylene dimethacrylate DMDMA II

1 H, 1H- Pen ta decaf luoroctyl methacrylate PD FOMA Columbia Organic

Chemicals Co. Inc.

Columbia, SC

Isobornyl methacrylate iBM Polysciences, Inc.

Warrington, PA

Methyl Methacrylate MMA Aldrich Chemical Co.

Milwaukee, WI

19
Table 5

SETTING TIMES OF EXPERIMENTAL COMPOSITES* USING PETMP

AS THE ACCELERATOR AND REACTIVE DILUENT

Formulation Monomer System Setting DTS in MPa

(wt. %) Time [Std. Dev.]

(min) (no. spec.)

1 BIS-GMA (67)

PETMP (33)

4.0 24.6 [0.9]

2 NCO (93.72)

PETMP (6.28)

15.0 - —

3 BIS-GMA (64.17)

TEGDMA (27.50)

PETMP ( 8.33)

14.0

4 BIS-GMA (43.10)

TEGDMA (20.60)

PETMP (31.30)

14.0 — — —

5 BIS-GMA (82.18)

DMDMA (14.51)

PETMP ( 3.31)

13.0 —*

( 6 )

* Powder/ liquid formulation (P/L = 3)

Powder is silanizcd glass (1% BP)

20
Table 6

EFFECT OF PETMP ON THE SETTING TIME OF AN EXPERIMENTAL

COMPOSITE* USING A SLOW-ACTING POLYMERIZATION ACCELERATOR (4-EDMAB)

Formulation Cone, of 4-EDMAB Cone, of PETMP Setting Time (min)

6 0.50 0 8.5

7 0.50 1.17 8.0

8 0.49 2.16 4.5

9 0.49 3.54 6.0

10 0.49 4.86 5.0

* Powder/liquid formulations employing P/L = 3

Powder is silanized and coated with 1% 3P.

Liquid consists of BIS-GMA (7)/TEGDMA (3) with 0.1% BHT

and the indicated weight percent of 4-EDMAB and PETMP

21
composite formulations using BIS-GMA (7)/TEGDMA (3) containing diminishing

amounts of DHPPT are summarized in Table 7. A synergistic accelerative

effect due to the addition of PETMP is clearly evident in the reduction

of setting time. Formulations D, D‘ and D" clearly demonstrate the

benefit of PETMP as a synergistic activator for the BP/amine initiator

system. The extremely small amount of DHPPT used in D" (0.0122%) along

with 2.4% PETMP resulted in an acceptable composite formulation with

respect to setting time and tensile strength. Without PETMP, composites

prepared with this amount of DHPPT fail to meet ADA specifications

(Formulation D) . A doubling of the DHPPT concentration was necessary to

obtain a composite formulated without PETMP that passed ADA specifications

(Formulation D'). The composites prepared with PETMP all had superior

esthetics compared to those formulated only with the amine accelerator.

Effect of PETMP on Composite Formulations Using FastActi ng Ami ne Accelerators

The use of BDMA in powder/liquid formulations based on

BIS-GMA (7)/TEGDMA (3) containing 0.1% BHT classified this amine accelerator

as highly reactive. Thus, with 0.23% BDMA and a powder/liquid ratio of

3 a very fast setting time of 1 minute was observed (diametral tensile

strength = 50 MPa). In Table 8 are summarized the results of using this

highly reactive accelerator with a relatively slow-polymerizing monomer

system in the presence and absence of PETMP. The NC0 monomer system

itself (and when modified with most conventional diluent monomersjis

fast-polymerizing. However, we observed that the addition of the long

chain, highly fluorinated monomethacrylate, PDF0MA, moderates the rate

of polymerization considerably . Thus, with this moderate-polymerizing

monomer system it is necessary to use relatively high concentrations of

22
Table 7

N

I

I

EFFECT OF PETMP ON THE SETTING TIME AND DTS OF AN EXPERIMENTAL COMPOSITE*

USING A MODERATELY ACTIVE AMINE POLYMERIZATION ACCELERATOR (DHPPT)

ftrmulation Monomer Content Cone, of Cone, of Setting DTS in MPa

BIS-GMA/TEGDMA DHPPT PETMP Time [Std. Dev.'

(wt. %) (wt. %) (wt. %) (min) (no. spec/

1 , 69.90/29.94 0.16 0 2.0 51.8 [2.4] (4)

1

A

' 67.60/29.00 0.16 3.24 40 sec —

B 69.97/29.93, 0.10 0 2.5 50.4 [1.6] (5)

1 B' 68.43/29.30 0.11 2.16 1.5 52.2 [0.4] (5)

1

c 69.97/29.98 0.05 0 3.0 47.7 [1.3] (4)

1

C' 69.20/29.66 0.056 1.08 2.5 48.8 [1.7] (5)

j C" 68.42/29.31 0.055

' 2.20 2.0 47.3 [1.3] (6)

1 D 69.99/30.00 0.0125 0. 8.0+ 30.0 [3.5] (5)

* D' 69.98/29.99 0.025 0

' 5.0 42.3 [1-0] (5)-

|

D" 68.32/29.27 0.0122 2.40 1.75 42.0 [3.9] (5)

In these powder/liquid formulations the powder/liquid ratio = 3

23
BDMA in order to obtain acceptable setting times and diametral tensile

strengths. Because of the increased amine concentration, the esthetics

of the cured composites were less than satisfactory (Formulations E and

G, Table 8). The addition of PETMP to these formulations had the effect

of reducing the setting times and/or the amine concentration and of

significantly improving the diametral tensile strengths of the cured

composites (Formulations F and H). In addition, the esthetic qualities

of these composites were strikingly enhanced.

Effect of PETMP on Composite Formulations Using Ascorbic

Acid Type Accelerators

Powder/liquid formulations based on BIS-GMA (7)/TEGDMA

(3) containing dissolved ascorbyl palmitate, AP, did not show any

significant change in setting time as the concentration of PETMP was

varied over a wide range (3.5 to 32.8%). Formulation J (Table 9) gave

virtually the same setting time and diametral tensile strength as that

obtained using PETMP as both reactive diluent and accelerator (Formulation

I, Table 3). As the concentration of PETMP decreases, the tensile

strength of the composite increases. The most salient effect of PETMP

on this type of composite material is the effective stabilization against

loss of accelerative activity of AP dissolved in monomer systems. After

8 months, formulations K and L, by setting time and DTS measurements,

exhibited virtually no loss of accelerative potency. The esthetic

qualities and color stabilities of the cured composites were excellent.

Tertiary Accelerator Systems Based on PETMP , AP and Ami ne

Accelerators

The effect of the addition of minute amounts of a fastacting amine accelerator such as DMSX to a monomer system containing

24
Table 8

EFFECT OF PETMP ON THE SETTING TIMES AND DTS OF AN EXPERIMENTAL

COMPOSITE* USING A FAST-ACTING AMINE ACCELERATOR (BOMA)

Formulation Monomer Content

(wt. %)

Cone, of

BDMA

(wt. %)

Cone, of

PETMT

(wt. %)

Setting

Time

(min)

DTS in MPa

[Std. Dev.]

(no. spec.)

E NCO (50.49)

DMDMA (39.12)

PDFOMA ( 9.91) BHT ( 0.09)

0.39 0 5.0 39.0 [3.0] (5)

F NCO (49.66)

DMDMA (38.47)

PDFOMA ( 9.75) BHT ( 0.09)

0.38 1.65 2.5 48.0 [2.5] (4)

G NCO (58.55)

DMDMA (31.53)

PDFOMA ( 9.02) BHT ( 0.18)

0.57 0 4.25 40.3 [3.3] (5)

H NCO (57.70)

DMDMA (31.06)

PDFOMA ( 8.37) BHT ( 0.13)

0.20 .1.99 4.25 46.1 [3.3] (5)

* Powder/Liquid Ratio = 4

Table 9

SETTING TIMES AND DTS OF AN EXPERIMENTAL COMPOSITE* USING PETMP AND AP

Formulation BIS-GMA/TEGDMA

(wt. %)

Cone, of

PETMP

(wt. %)

Cone, of

AP

(wt. %)

Setting

Time

(min)

DTS in MPa

[Std. Dev.]

(no. SDec.)

J 66.6/0.0 32.8 0.60 4.0 23.3 [0.9] (5)

K 59.5/25.5 14.3 0.66 4.0 36.1 [1.3] (4)

L 67.1/28.8 3.5 0.64 4.0 41.7 [2.2] (5)

M 69.5/29.8 0 0.70 4.0 44.1 [1.6] (5)

P/L = 3, powder Is silanlzed glass (12 SP)

25
PETMP and AP is to both reduce the setting time and increase the diametral

tensile strength of these composites. The results of this study utilizing

the ternary accelerator system PETMP-AP-DMSX are shown in Table 10.

PETMP as a_ Photoreductant for Visible Light Curable Composi tes

The results of this study assessing the photoreducing

capabilities of PETMP are summarized in Table 11 and indicate that this

polythiol is an excellent reducing agent for camphoroquinone (CQ) . Preliminary

experiments indicate that the photoinitiator system CQ/PETMP gives depths

of cure very similar to those obtained with CQ and a tertiary aliphatic

amine reducing agent such as N,N-dimethylethanolamine. The esthetics of

these photocured composites is excellent. The color stability of the

CQ/PETMP composites appears to be even better than that of the already

excellent CQ/tertiary aliphatic amine composites.

Storage Stabi 1 i ty

Most of the solutions of PETMP stored well at room temperatures

in glass or polyethylene vials. The BIS-GMA solution containing 33%

PETMP did gel after several weeks but this could be stabilized by the

addition of AP, BHT or both. The pastes containing CQ and PETMP stored

well at room temperature if kept in opaque containers.

Advantages of Us i ng Polythiols in Dental Resins

The use of monofunctional thiols as chain transfer agents

or polymerization modifiers in free radical vinyl polymerizations is

well known. In essence, the chain transfer reaction involves the abstraction

of the thiol hydrogen (-SH) from the mercaptan (RSH) by the radical end

of a chain-growing polymer, thereby terminating the polymer chain, but

creating simultaneously a new radical species, the thiyl radical (RS-),

which is capable of initiating, by addition to a vinyl monomer, a new

26
Xto

2:Q

O.

ZT<

00c .—_ ^—„

VO LO VC vO

*o 1—1 • 1—1

r—1

(—1 1—1

Cl. • CJ m- to 00 VO s > CJ • • • •

CL) Cl Osi cvj r— c a to 1ZI1 l I 1—i 1 1 •r— • 0 CVJ CO r—

to 2 0 • • • •

*—1 1 1 c CO r— CO

ta c uo

O

O in

o_ 4-> E — • • • • <: •4-1 *t— E

CJ 1—

CM

r0 CO

HLU

Cl. O

to*

toUJ

to

to4-o ^ X• to U 21 4-> c 0 oo O 4- -

0 - o_ O ^ 2:

O uo 0 0 0 • o_ • CO CO 0 u< ^ • • • • cu c 2 O 0 0 0

_J 0 —-1 JD < c_>

<T3 f—

t— XTUJ

2:—4

O' 4-

UJ

O

Cl. O- 0 X • 21 • O LO

UJ Ol

— *-> • • 0 0 c uj 2 CO r—

u_

O Cl.

—

0 O

to»—oQ<

CO

LlJ o

UJto oC3

UJ< +J M 2 O -—

1 toCO1 c 2 0 C 4-> O <0

r>» Oo

uo

COo

00

COCM

r-* VO

1— co

00 CO

CO

CM

CO

VO

CT. CM

CO

r-« 00

VO

00

Col CM

Co

VO

Q.

CO

Sr*

S_

aj *0>6

cx -O

CJ

fMc

fO

to

•l COn_J

o_*

xc cr to

27
Table 11

24-HOUR DIAMETRAL TENSILE STRENGTH OF EXPERIMENTAL

VISIBLE LIGHT CURED COMPOSITES* USING PETMP AS THE PHOTOREDUCTANT

Formulation Monomer Content

(wt. %)

DTS [Std. Dev.]

(No. of Spec .

)

X BIS-GMA (80.2)

DMDMA (14.2)

PETMP (5.4)

CQ ( 0.2)

43.5 [1.2] (6)

Y NCO (96.07)

PETMP ( 3.73)

CQ ( 0.20)

46.5 [0.5] (5)

Filler is A-174 silanized 325 mesh glass; P/L = 3, time of

irradiation = 60 seconds.
polymer chain. The net result is that there is a decrease in the average

molecular weight of the polymer but no essential change in the rate of

polymerization.

If a difunctional mercaptan is used as a chain transfer

agent, the molecular weight of the polymer should be twice that obtained

with a monothiol, assuming the same chain transfer constant for each

thiol group. This is the case since both ends of the dithiol, after

conversion to thiyl radicals (• S-R-S • ) , can initiate new polymer chains.

Polythiols, R(SH)

n where n > 2 not only yield polymers

with still higher molecular weights when polymerized with olefins such as

methyl methacrylate, but with polyolefins (polyenes) crossl inked polymers

are formed (Fig. 3). Thus PETMP and BIS-GMA undergo both chain extension

and crosslinking to yield a hybrid poly(methacrylate-thioether) network

structure. The inclusion of relatively large amounts of PETMP (e.g.

33%) in the dental resin yields composites having relatively low tensile

strengths due to the increased flexibility introduced into the organic

matrix by the high thioether content. Lowering the PETMP content of

dental resins (e.g. 1- 5%) results in composites with acceptable mechanical properties. It is evident that polythiols such as PETMP can

effectively replace part of the vinyl monomer content of conventional

dental resin systems. By virtue of multiple free radical thiol/ene

reactions, polymeric matrices having reduced residual vinyl unsaturation

and, perhaps, greater oxidative stability should result.

PETMP and similar polythiols also can replace a considerable

part of the amine accelerator component of dental restorative, sealant

and other acrylic based materials which are polymerized by the BP-amine

initiator system. Synergistic accelerator systems based on PETMP/amines

29
lead to cured materials having enhanced esthetics and color stabilities.

The mechanism of synergistic acceleration by thiols is not known but may

involve regeneration of the tertiary aromatic amine by the action of

PETMP on one or more primary products that result from the initial

electron-transfer reaction with BP. A possible mechanism is suggested

in Figure 4.

The improved esthetics and color stabilities of the cured

resins are probably due in part to the prevention of the usual oxidative

attack on these primary products which result in colored byproducts.

With dental resins containing the non-amine accelerator,

ascorbyl palmitate, the effect of PETMP appears to be to improve the

storage stability of the oxygen sensitive ascorbic acid type promotor.

Conversely, AP itself acts as an effective stabilizer for monomer systems

containing relatively high contents of PETMP. With extremely small

amounts of tertiary aromatic amine accelerators PETMP and AP form a very

effective ternary accelerator system for BP initiated polymerizations.

Similar ternary accelerator systems using catalytic amounts of redox

metals (e.g. copper, iron) probably can be devised to accelerate the

decomposition of other peroxides, such as peresters and hydroperoxides

PETMP and other polythiols have been shown to be effective

reducing agents for ultraviolet photosensitizers such as benzophenone.

The mechanism involves abstraction of a hydrogen atom from the thiol

group by the excited benzophenone in the triplet state to form ketyl and

thiyl radicals (Figure 5). The thiyl radicals are believed to be the

principal initiators of polymerization. Presumably, a similar mechanism

prevails in the case of the visible light polymerization of dental

resins formulated with the diketone, camphoroqui none , and PETMP.

30
SYNERGISTIC ACCELERATOR - INITIATOR SYSTEMS

BASED ON PETttP AND TERTIARY AROMATIC AMINES

Possible Mechanism:.

electron

rearrong.

Ami ni urn Radical

Cation

Radical

Complex

Decomposition

Products

Figure 4.

31
Mechanism of Photoinilialion.

R2 C=0 + + R3 C=0 1 (n, n*) single! 0)

RjCO ’(n, tt*) R2 C=0 3 (n, n*) triple! (2)

R,C=0 J (n, 77*) + R'SH R2 COH + R'S- (3)

kctyl lhiyl

radical radical

2R2 COH R2 C(OH)C(OH)R2 pinacol

(4)

R'S- + CH2 =€HCH2 R" =* R'SCH 2 CHCH2 R" (5)

R'SCH 2 CHCH 2 R" + R'SH R'SCH 2 CH2 CH 2 R" + R'S- (6)

Figure 5.

32
From this study it appears that PETMP is an effective

color-stable photoreductant for di ketones and, therefore, can replace or

supplement the tertiary aliphatic amine reducing agents currently used

in visible light curable dental composites.

Since polythiols such as PETMP can be used with a variety

of vinyl monomers, by proper selection of chemical structural features

in both the polythiol and the polyene components and adjustment of their

relative concentrations, it should be feasible to fashion dental resin

systems with a wide range of properties (e.g. tough elastomers to rigid

glasses .

)

Potential Benefits of Us i ng PETMP with Denture Base and

Bone Cement Materials

Preliminary experiments were conducted to assess the

effect of PETMP on the ambient polymerization of MMA and i BM using the

initiator system BP/DHPPT.

In several 4 ml vials containing 1 g of the monomer was

dissolved the requisite amount of BP. To these solutions were added the

requisite quantities of PETMP (optional) and DHPPT in the order given.

The vials were sealed with teflon lined caps and stored at the indicated

temperatures (22° ± 3°C or 37° ± 1°C). The progress of the polymerization

was determined by visually noting the increase in viscosity by simple

inversion of each vial at 3-minute intervals. The time at which no

further flow of the polymerized specimen occurred was taken as the

setting time. The nature of the surface cure was noted by means of a

metal probe and the coloration of each specimen also was noted (See Table 12).

33
Table

12

I

CJ 4-

i- O *- 3 a

CJ> i- E

o

CJ r— r—

CJ O O

O O CC

s_ *o 3 c

CO <o

>>

CJ 5

rO O

4-> r—

>, CJ u >>

CJ

in

*" in

CJ .M <

o C- o o

I o

c u

o

o

fO in

4-J 0* in

QJ (U

CJ r—

r— O S_

4—) 4— o

JC •

1 cn CJ o

•«- >J O CJ to 1—

o_

l—UJ

Cl.

Li_ O

I

oUJ

cn C

a. -r4-> CJ CJ co E

oC\J o

CO CO Lf> V

I

[
Similar experiments were conducted with the MMA solutions and a denture

base powder (PMMA with 3% BP, Esschem Co., Essington, PA) using a powder

/liquid ratio of 2.

Setting Time :

The setting times for the various composite formulations

used in this study were measured as outlined in ADA Specification No. 8

(Gilmore Needle Test.) The results of this preliminary study are summarized

in Table 12. It is evident that the addition of relatively small amounts

of PETMP (ca 6%) to a self-curing acrylic resin can have beneficial

effects on the curing process and the final polymer. With the moderately

reactive amine accelerator, DHPPT, and PETMP there was a marked reduction

in setting time and an improvement in the nature of the surface cure of

the polymer (non-tacky) resulting from the polymerization of MMA at

22°C. A noticeable exotherm occurred after about 25 minutes from the

start of the polymerization. In the polymerization of i BM at 37°C, the

changes in setting time and surface character of the polymer cured with

PETMP, though not as dramatic as was the case for MMA, were still significant.

In the polymerization of both monomers, the pronounced color formation

that is a characteristic of the BP-amine initiator system in the presence

of oxygen was obviated by the use of PETMP.

Both polymers prepared with PETMP appear to be uncrosslinked (soluble in methylene chloride, monomer). When these polymers

were dissolved in their respective monomer (e.g. MMA, i BM) containing BP

and DHPPT, to form doughs, colorless polymers resulted, indicating that

the PETMP moities in the original polymers also are effective in color

prevention.

35
A preliminary study to ascertain the effects of PETMP on

a typical denture base material gave results similar to those reported

in Table 12 for the monomers. Thus, the addition of MMA (0.73% DHPPT) to

PMMA (3% BP)gave a dough that set in 8 minutes at 37°C. With MMA (0.69%

DHPPT) containing 5.8% PETMP and PMMA (3% BP), the setting time was 6

minutes. The most striking effects of PETMP again are in the excellent

esthetics and enhanced color stability of the cured acrylic. Even after

7 months at ambient temperatures and lighting conditions the specimens

prepared with PETMP retain their original white appearance whereas those

without PETMP have darkened considerably from their original pale yellow

coloration. Specimens kept at 0°C for four months have shown a similar

behavior except that there has been a slight gradual yellowing in the

PETMP specimens whereas the specimens without PETMP are severely discolored.

Summary of PETMP Research

Polythiols such as pentaerythri tol tetra(3-mercaptopropionate), PETMP, can be used in the formulation of novel initiator systems

that result in dental polymeric materials with enhanced esthetics and color

stabilities. PETMP, as a synergistic activator for benzoyl peroxide-amine

Initiators, drastically reduces setting times of dental resin formulations,

and thereby permits the use of lower amine concentrations. In addition,

PETMP enhances the storage stabilities in the dissolved state of the highly

biocompatible promotors of the ascorbic acid class. Also, PETMP is an excellent color-stable photoreductant for visible light curable composites. Finally,

the use of polythiols as comonomers with dental resins has the potential for

reducing residual vinyl unsaturation and further enhancing the esthetics and

color stabilities of polymerized acrylic materials.

36
C. Composite Filler and Filler/Resin Coupling Improvement

The interfacial bonding between resin and filler is crucial to the

performance of inorganic reinforced polymeric composites such as dental

composite restorations [15]. This bonding is greatly improved by the

addition of a coupling agent, e.g. an organic silane, that is usually

applied to the surface of the filler. A systematic study of silanization

procedures has been conducted in our laboratory [2]. One of the most

promising silanization procedures reported in the literature [16-22] was

that of Kaas and coworkers [16]. To enable us to follow the silanization

process and the stability of the interfacial bonding, we employed colloidal

fumed silica as a reasonable substitute for glass. Primary amines, especially n-propyl ami ne dissolved in normal, aliphatic hydrocarbons or cyclo

hexane enhanced the silanization of silica by y-methacryloxypropyl trimethoxysilane and yielded greater and more stable silica-silane bonding.

In the present year we evaluated the effect of various procedures employed

for silanizing the reinforcing agent on the aging of cured composites containing this ingredient. Aging of specimens was established from the loss

of tensile strength on storage in water at 37°C. Results of these investigations are given in Table 13. Composites with filler silanized

with 0.5% y-methacryloxypropyl trimethoxy si lane in cyclohexane to which 2%

n-propyl ami ne had been added had somewhat higher initial tensile strength

which decreased less on storage in water at 37°C than those silanized in

37
Table 13

Effect of Silanizing Treatment of Filler on

Aging of Cured Composite

Liquid: BIS-GMA: 84.7%

Tetraethyleneglycol dimethacrylate 14.9%

N,N-dimethyl-£-toluidi ne 0.23%

Butyl ated hydroxytoluene 0.2%

Powder: Corning Glass 7724, 325 mesh, silanized band coated with

1% benzoyl peroxide

Si lanizi ng

Treatment of Glass

Tensile Strength, MPa

Storage of Cured Composite in H

2 O

% Decrease in

Tensile Strenath

Initial 54 days 184 days 54 days 184 days

0.5% silane in acetone 54.5[1.7]

a 45.7 [0.8] 34.6 [0.1] 16 36.

5

0.5% silane in cyclohexane 54. 9[0 . 5] 43.2 [1.2] 37.0 [1 .0] 21.3 32.6

0.5% silane in cyclohexane

plus

2.0% n-propyl amine 59 . 9[ 1 .9] 48.5 [1.0] 41.9 [0.9] 19 30

a Standard variation of 5 measurements

b y-methacryl oxypropyl trimethoxysi 1 ane

38
cyclohexane or acetone in the absence of amine. The dependence of the

tensile strength of the cured resin on the concentration of n-propylamine

in the silanizing solution is given in Table 14. A 2% solution of n-propylamine and 0.5% of the silane reagent in cyclohexane gives composites with

optimum tensile strength.

D. Dental Cement Innovations and Adhesion Research

It has been said that "dental cements are, perhaps, the weakest and

least durable of all restorative materials, and have a brittle or even

friable nature" [23]. Despite many inferior properties dental cements

are estimated to be used in 50 percent of all dental restorations as luting

agents for fixed restorations and orthodontic appliances, as a base under

permanent restorations to insulate against thermal and chemical shock,

as root canal fillings, and pulp capping agents and as temporary or semipermanent restoratives . Cements in current use are far from ideal with

respect to certain critical properties that determine durability in the

oral cavity. They lack toughness (i.e. have a strong tendency to brittle

failure), hydrolytic stability (i.e. have a strong tendency towards excessive

erosion under the action of oral fluids), and (with the exception of the

zinc carboxyl ate and ionomer glass cements) adequate adhesion to tooth

structure.

1. Vani 1 late-Based Cements

a. Cements from Hexyl Vani 1 late, o-Ethoxybenzoic Acid

and Zinc Oxide

While eugenol , a 2-methoxyphenol , reacts with zinc oxide

(ZnO) to yield dental cements having excellent tissue tolerance, the cements

39
Table 14

Effect of Concentration of n-Propylamine in Silanization

of Filler on the Tensile Strength of the Cured Composite

Liquid: BIS-GMA: 84.7%

Tetraethyl eneglycol dimethacrylate 14.9%

N ,N-dimethyl -£-tol uidine 0.23% . Butylated hydroxytol uene 0.2%

Powder: Corning glass 7724, 325 mesh, si lanized in presence of

n-propyl amine in cyclohexane and coated with

1% benzoyl peroxide

Concentration of n-PropylAmine in Silanization - %

Tensile Strength of Cured Composites,

MPa

Unsi lanized 26.3 (0.3)

d

0.5 54.9 (0.5)

1.0 53.1 (0.7)

2.0 59.9 (1.9)

3.0 52.8 (1.5)

a Standard variation of 5 measurements
would be much more useful if their solubility in oral fluids were lower,

strength properties were increased and they did not inhibit the hardening

of resins in dental composites with which they come in contact [24].

While much research has been conducted to replace eugenol, the use of

eugenol substitutes has usually produced cements with poor physical

properties. The strength and excellent biocompatibility of cements

based on reaction of ZnO with o_-ethoxybenzoic acid (EBA) have made them

especially useful as luting agents and bases but their disintegration is

too rapid to allow their employment as more permanent restorations [25-

27].

During the previous year it was reported that zinc oxide

mixed with vanillate esters (Table 15) such as n-hexyl vanillate (HV)

dissolved in o^-ethoxybenzoic acid yielded non-eugenol containing cements

[2], These cements do not inhibit free radical polymerization and adhere

strongly even in water to non-precious metals, amalgams and composites.

They do not disintegrate in water (6 month F^O uptake of 0.8% versus

weight loss of 12.4% for a ZOE cement) and considerably exceed the

requirements of the ANSI/ADA Specification No. 30 for Types II, III and

IV ZOE type Restoratives.

Thus, a portion of the cement, when used as a temporary

filling, can be retained as a base for acrylic resin restorations. The

material with suitable modifications looks most promising as a base,

temporary restorative and endodontic cement. Cementitious restoratives

with drastically improved mechanical properties and which therefore could

find applications as long-lasting temporary fillings were prepared by

incorporating a dimethacrylate monomer, peroxide initiator, amine accelerator and glass reinforcing fillers into the cement formulations.

41
cnu

<u

4-> W

UJ

•o

CJ <o

ID

CO

<*: c

«3

vn

CJ

4-> U

QJCLO

s- OT3c

«3

vn

cn

Cl JZ

4-> cz>

to]

-oVO

X vo ’ 0 0 CO VO <T% >-

C7» 0 to

c: ro • C O in • 0 IO

*»-*-© CM CO ^r CM

•— o V • • V

CJ o. a>

O 2; CM vj*

CJ -O

s- CM CM Cl CM

4~> t- 3 0 O cz

O CZ Cl m r— •r— 0—

TZ) i/»

X

X c=X Odd CO ro c ro

Q.

D

L CJ O- CO ro •L> ro

cr» • 03 CO CJ CJ\ c *0 -0 r— TO #—

«*—1 Q_ Cl 1 1 1 r— \ O ro ,

4-> CM -*-0 3 CO CO O O* O O T3 r— »— cz CO Cl CC

CJ>

4J XCJ <u*4 0 ro

*G *3

C CM 1 1 w~

i- c CO 10

<*- »—« • • <3 r— r-* o:X T3 CO r-%. ! c r—

GO • 3 • • • c 0 CO 0% 00 0 u»- 3 «/» O *- C- •O

v-i "O "O Cl ro to ro

>% >%

O c ro vo

r— ~T~r

— «*— • • • «3 ro E CO o\ GO c 0 u C 01

4-> r— O

ra -0 "O CM cr% CM

4-1 c CJ\ to C X 3 • • • CJ 0 0 cr» CO £ -u. c VO VO VO

Cl c

*— 0

UJ -O

i- -a CO 0 to

r3 Cl vO r— CO O *— • • • <C r-v O CO O CO r*. vo • c c? C O CJ CM I 0 cz JO 31

•r— c c 4-1 r3 41 CZ 4J 0 0 <T3 •*- “O 0 i td ra «*— ISI d 3: <= O Cl 4J 4J -r- CJ <0 4J

-CZ rj «3 r— Q.C ro ! 4-> *4- r— 1 O O r—

Cl

— r— r—

OC

— C3 r— 2: t- •v— 4J 4-1 31 •r— 3 4-» 4J VI S CJ <T3 4J | Q. i/% >% O «3 Z </> •*— i_ u U •r—

<4- *3 "3

O V4- 4-1 to j O CJ x cr W 0 <0

CJ >%

«-»

X

*0 i- CJ #— CJ sz r— +J r— >%

•*— VI >» *-» >s >% j C UJ X CL 0 JZ a CJ CJ 4-* > 3: 3: a UJ • . 1 1 1 c| cl c CM IIIIIII
During the current reporting period larger quantities of

hexyl vanillate were prepared. Samples of the powder and liquid were

then sent to a number of institutions for biological evaluation of the

basic formulation. Work to determine tissue compatibility of the materials

has been initiated at the Dental School, University of Indiana. Another

study of the mechanism of the hardening reaction and the possible use of

these materials as soft tissue packs is being conducted at Northwestern

University. Properties of several other monovanillate esters are summarized

in Table 15. If these toxicity and biocompatibility studies show that

the materials are suitable, a number of dental schools here and abroad

have expressed a willingness to conduct clinical studies. Such investigations initially should establish the potential of the materials as a

base. Are the materials a palliative similar to ZOE? Do they allow the

formation of reparative dentin? Can they be used clinically as a temporary

restoration with final placement of a composite without total removal of

the base? Can they be used successfully for cementing crowns or shortspan bridges? How do the sealing characteristics of these cements

compare with those of established materials and what are the possibilities

as a root canal sealer? Because of the generic nature of the compositions,

improvement of specific properties may be possible by minimal changes in

the formulation of the cements.

During this reporting period efforts have been directed

into two areas, (1) synthesis of vanillate derivatives such as purified

decamethylene and other divani Hates, methacryloylethyl vanillate and

esters of syringic acid and their evaluation as ingredients of dental

cements and (2) development of improved intermediate restoratives based

on composite formulations made from hexyl vanillate (HV) -EBA-acryl ic

43
monomer and ZnO-glass powders.

(1) Related Synthesis

(a) Methacryloyl ethyl Vanillate

Hydroxyethyl vanillate was prepared by reaction

of 10.0 g (0.06 mol) vanillic acid with 6.0 g (0.06 mol) KHCO^ in water.

After CO

2

evolution the solution was filtered, the water evaporated and

the potassium vanillate was dried in a vacuum oven. The salt (13.2 g,

0.06 mol) was reacted under reflux with 5.6 g (0.07 mol) of chloroethanol

and 0.25 g KI in 200 ml cyclohexanone for 24 hours. The cooled solution

was extracted with water and the solvent was distilled off. The residue

was recrystal 1 ized from chloroform. The purified solid melted at 85-

87°C (literature value 85°C).

Hydroxyethyl vanillate (8.6 g, 0.04 mol) and

tri ethyl amine (4.1 g, 0.04 mol) were added to 100 ml CH^C^ and 50 ml

acetone. Methacryloyl chloride (4.2 g, 0.04 mol) was diluted with

CH^Cl

2

and added dropwise with stirring to the solution, cooled to 5°C.

After addition of the reagent the solution was allowed to rise to room

temperature and stirring continued for four hours. The amine hydrochloride

was filtered and the solution was concentrated. The remaining material

was taken up in CH^Cl

2

and extracted with 15% aquous NaCl . The dried

organic layer was concentrated and put through a silica gel column.

Last traces of solvent were removed in a vacuum. (Yield 8.63 g, 77%).

Gas chromatographic analysis indicated the product to be more than 95%

pure. Since the compound polymerized on standing at room temperature,

butylated hydroxytol uene (BHT) was added as an inhibitor.

Cements prepared from 3% methacryloyl ethyl

vanillate (MEV) - 10.0% n-hexyl vanillate (HV) or 5% (MEV) - 7.5% HV,

44
87% EBA, 1% dihydroxyethyl-£-toluidine and 0.02% BHT inhibitor liquid

as well as 1% benzoyl peroxide in the powder (P/L ratio 1.3 g/0.23 ml)

cured in 6.5 min and had good strength properties (compressive strength

of 67 MPa (SD 4) and 52 MPa (SD 14) and tensile strength of 6.4 MPa (SD 0.2)

respect! vely ) . Some shrinkage occurred on curing of specimens which may

be the result of polymerization through the methacrylate groups present

in the composition.

(b) Butanediol divanillate - A mixture of

redistilled butanediol (0.02 mole), vanillic acid (0.05 mole) dissolved

in toluene, and tetrahydrofuran were refluxed for 42 hours in the presence

of 0.25 g p-toluenesul fonic acid. No reaction took place.

In a second attempt potassium vanillate was

prepared and refluxed with butanediol in cyclohexanone for 3 days. No

reaction occurred.

(c) Additional Di and Trivanillate Syntheses -

The diglycidyl ether of Bisphenol A (3.4 g, 0.1 mol) was reacted under

nitrogen with vanillic acid (4.2 g, 0.025 mol), and 5 drops triethyl amine

in 10 ml tetrahydrofuran at 100°C for 18 hours. The solvent was distilled

off in a vacuum around 50°C. The residue was dissolved in methylene

chloride and extracted with water and 5% aq. sodium bicarbonate. On

evaporation of the solvent an extremely viscous amber oil remained which

proved difficult to purify.

A mix of 65% ZnO - 30% A^Og - 6% hydrogenated

rosin powder and 17% of the diglycidyl ether divanillate - 83% EBA (P/L

ratio = 1.3 g/0.2 ml) was difficult to handle. It hardened in 5 min,

but had a low compressive strength 19.6 MPa (SD 1.3) and a tensile strength

of 7.36 MPa (SD 1.49).

45
Synthesis of a trivanillate from trimethylol propane

(3.35 g, 0.025 mol) and vanillic acid (16.8 g, 0.10 mol) and 0.25 g

stannous oxalate was also attempted by the procedure described above.

However, purification of the resulting product proved difficult. A 10%

solution of the impure product in EBA when mixed with the zinc oxide

powder (P/L =1.3 g/0.2 ml) hardened in 5.5 min yielding a cement with

compressive strength 33.6 MPa (SD 11.6), tensile strength 5.09 MPa (SD 0.65).

Addition of a divanillate to HV-EBA cement

compositions should make it possible to obtain crosslinking. This may

result in improved mechanical properties and greater insolubility of the

cured cement. In previous work we have shown that the decamethyl ene

divanillate (DDV) is more readily prepared than similar divanillates.

To purify the reaction product of vanillic acid and decamethylene diol

the crude dark-brown viscous divanillate, dissolved in CH^Cl

2

> was

subjected to liquid gradient chromatography on an activated Florisil

column. On successive washing with CH^C^, CHCl^ and methanol the

divanillate was eluted (MP % 50°C). From its IR and NMR spectra and its

analysis for carbon and hydrogen the identity of the compound was

established. The high molecular weight of the divanillate made efforts

to establish its exact purity by gas chromatography unsuccessful.

Cements prepared from 64% ZnO, 30% Al^O^ and

6% hydr. rosin powder and 11% DDV - 89% EBA at an approximate luting

consistency (P/L ratio 1.3 g/0.2 ml) set in 5 min, yielding cements with

compressive strength 60-70 MPa and tensile strength 7.5 MPa. The

increased brittleness and somewhat higher strength properties of these

cements as compared to HV-EBA cements may indicate that the DDV has

crosslinking capability.

46
(d) Salicylic Acid Derivatives- Synthesis of a

methacrylated salicylic acid capable of chelation as well as polymerization

was attempted. j COOH COOH

OH

k

2 C0

3

+ ki

CH,

I 3 BrCH

2CH

20CC = CH

2 6

HC1

9

0CHo CH o 0C-C-CH9 2 2 i 2 CH

3

On refluxing the reactants a viscous dark oil formed which, after purification,

could not be recrystal 1 ized from various solvents nor could it be distilled

at 2 mm pressure.

b. Cements from n-Hexyl Syringate

Syringic acid (3 ,5-dimethoxy-4-hydroxybenzoic acid)

COOH

CH 3° (j^) 0CH

3

OH

has an additional methoxy group in the 5 position compared to vanillic

acid. It inhibits caries of the cotton rat by approximately 39% when

0.1% of syringic acid is incorporated into the 802 cariogenic diet [28].

On the other hand eugenol in the diet did not inhibit caries at the

level tested. Although caution must be exercised when extending results

on caries inhibition beyond the experimental conditions under which they

were obtained, it appeared of interest to synthesize liquid or low

melting esters of syringic acid. Such compounds, because of their

structural similarity with vanillic acid, should form useful cements

with zinc oxide. Incorporation of syringic acid into such composition

could, it is hoped, make such cements cariostatic. The initial objective

47
therefore, was to synthesize suitable derivatives of syringic acid and

to evaluate derived dental cements.

To 9.91 g (0.05 mol) of syringic acid was added 10.22 g

(0.10 mol) of dried hexanol and 0.1 g p-toluenesulfonic acid. To dissolve

the acid 25 ml cyclohexanone was added. The mixture was refluxed for 10

hours. Since considerable unreacted ,syringic acid remained, more hexanol

(0.1 mol) was added along with 0.1 g p-toluenesulfonic acid and cyclohexanone

and refluxing was continued for an additional 24 hours. Solvent was

removed by distillation. The residue was taken up in CH^C

1

2 and extracted

successively with aqueous NaHCO^ and b^O. The organic phase was vacuum

distilled. A red liquid distilling at 190-220°C/3-4 mm was obtained

which crystallized on cooling. Infrared spectrum of the solid indicated

that this was a mixture of syringic acid with hexyl syringate. Further

purification of the CH^C^ solution by repeated extraction with 5%

NaHCO^, H^O, each saturated with CaClg, removal of the solvent and

recrystall ization from n_-hexane yielded very pale yellow crystals MP=62.0-

62 . 5°C (yield 4.2 g, 30%).

n-Hexyl Syringate-Based Cements

n_- Hexyl syringate easily dissolved in EBA. Thus, liquids

containing 7-14% hexyl syringate in EBA could readily be prepared.

Cements at luting consistency (1.2 g powder 0.2 mL liquid) harden

rapidly (4 min) and had compressive strengths from 54 to 62 MPa (Table

16) respectively. These values are similar to those of hexyl vanillateEBA cements. Cements with 3 to 5% n-hexyl syringate, 8 to 10% n-hexyl

vanillate and 87% EBA as liquid have a lower compressive strength (38-43

MPa), but higher tensile strength (6.1 to 7.5 MPa). Their solubility is

lower than that of Z0E cements. The hexyl syringate cements were quite

brittle. Modification of the formulations used may improve this property.

48
TABLE 16

Properties of Cements Containing ji-Hexyl Syringate

Powder: 64% ZnO - 30% Al^O^ - 6% Hydrogenated Rosin

Powder/Liquid Ratio: 1.3 b/0.2 mL

Composition of

n-Hexyl n-Hexyl

Syringate Vanillate

Liquid, %

EBA (%)

Setti ng

Time (min.)

Strength,

Compressive

MPa

aTensile

7 - 93 4 54.4 (12) -

10.5 - 89.5 4 62.2 (10.8) 5.5 (1.4)

14 - 86 4 59.2 (5.7) -

5 8 87 4.5 42.9 (6.0) 7.5 (1.2)

3 10 87 8 37.7 (12.0) 6.1 (0.3)

a Average of 5 determinations ( standard deviation.)

49
c. Bonding Measurements with Vanillate Cements

The bond strength of vanillate cements to composites and

non-precious metals was measured quantitatively by a modification of the

procedure of Lee, et al . [29].

The thoroughly mixed cement was placed in the recessed

tip of a stainless steel cylinder and was covered with a polyethylene

sheet. To smoothen the surface a glass slab was pressed against the

sheet. Three minutes after starting the mix the covered cylinder was

placed in a 37°C - 100% relative humidity bath. Eight minutes after

starting the mix a typical composite (Adaptic) was mixed and packed into

a tapered mold (smaller diameter facing up) that had been coated with 3%

polycrystall ine wax in toluene (release agent). The cylinder containing

the cement was removed from the bath 9.5 minutes after initiating the

mix. The polyethylene sheet and any remaining flash were removed.

After 10 minutes the cement end of the cylinder was placed against the

composite. A 3 kg load was placed on the cylinder for 15 seconds. The

tapered mold was stored in a 37°C - 100% relative humidity bath for 1

hour and then in water for 23 hours prior to placing the assembly into a

universal testing machine. The tensile strength was determined for 5

specimens at a loading rate of 0.2 in/min.

Results of the measurements are given in Table 17. All

failures were cohesive within the cement. The tensile strength of the

bond was 5.5 MPa (SD 2.6) which was statistically equivalent to that obtained

for the diametral tensile strength of the cement 5.0 MPa (SD 0.1). Thus,

the interfacial cement-composite bond is stronger than the tensile

strength of the cement. In comparison, composite bonded to a commercial

Z0E cement (Fynal) had a bond strength of 0.3 MPa (SD 0.5) with all failures

occurring adhesively at the composite-cement interface.

50
TABLE 17

Bond Strength of Luting Cements

Storage in water for 24 hours at 37°C

Bond Bond Strength, MPa

a Bond Failure

Adaptic to HV-EBA 5.5 (2.6) Cohesive

Adaptic to Fynal 0.3 (0.5) Adhesive

Pentillium bonded to HV-EBA 4.8 (1.0) Cohesive

Penti Ilium bonded to Fynal 0.6 (0.8)

b Adhesive

a Mean of all five measurements and standard deviation

13 Stored at 37°C and 100% RH for 24 hours

51
For measurements of bonding to non-precious alloy the

cement was coated onto flat stainless steel (Pentillium) disks. Three

minutes after start of the mix the specimens were transfered to a 37°C -

100% RH bath and treated by a procedure similar to that described for

the composite. The bond was ruptured cohesively. The bond strength was

4.8 MPa (SD 1.0). Specimens cemented with ZOE cement (Fynal) broke adhesively

with much lower bond strengths.

d. Heat of Setting of Vanil late-EBA-Zinc Oxide Cement

A study was conducted to determine the heat of reaction

of the setting of HV-EBA-ZnO cement. The measurement was conducted in

a very sensitive isothermal calorimeter having special compartments to

compensate for the heat of mixing of the powder and the liquid. At

41 °C a sharp peak occurred soon after mixing the components. The

reaction mixture had cooled down to near its equilibrium temperature in

less than 30 min. Preliminary results gave a aH= 0.02 cal/g. Thus, the

reaction is rapid. Any rise in temperature on hardening is very moderate

and would be of little significance clinically.

52
2 . Intermediate Restorative Resins Based on Vanillate-EBA

Cements

The development of high strength intermediate restorative

resins takes advantage of the unique behavior of HV-EBA cements in not

inhibiting polymerization. Monomeric dimethacrylates and monomethacrylates

containing an amine accelerator added to the HV-EBA liquid and mixed

with zinc oxide-silanized glass-benzoyl peroxide initiator powder very

significantly improve the properties of the resulting cements. Mixes of

these ingredients have good working properties, and a satisfactory

hardening time. These HV-EBA cements, subject to tissue tolerance

studies, should be useful as intermediate-term restoratives.

An immediate aim of this study was to determine the minimum

concentration of monomer that must be added to the cement to obtain

these high strength restoratives (Table 18). Cured cements with good

working properties could be obtained that have compressive and tensile

strengths respectively of 100 MPa - 200 MPa and 11 MPa - 18 MPa. The

strengths of these composite cements are three to five and four to six

times that of ZOE materials. Even higher strength properties can most

likely be obtained by incorporating larger concentrations of dimethacrylates into the mix. It was felt that compositions with a higher percentage of monomer may not possess the desirable tissue tolerance

characteristics associated with ZOE and EBA restorative materials. Thus

emphasis was placed on mixes containing equal proportions of ZnO and

silanized glass and HV-EBA and monomer respectively in the powder and

liquid. Inspection of the results given in Table 18 also leads to the

following conclusions: (1) increasing the concentration of monomer in

the mix increases mechanical properties of the cement; (2) silanization

53
Table 18

Properties of n-Hexyl Vani 1 late-EBA Polymer Composite Cements

Ingredients

:

Powder: : 64% Zn0-30% Al-CL^ hydrogenated rosin

I 2: Silanized glass coated with 1% benzoyl peroxide

Liquid: I 3

: 12.5% ji-hexyl vanillate - 87.5% EBA

I 4 : 1 ,10-decamethylene dimethacryl ate (DMDMA) + 1% N ,N-diethyl ami nophenyl acetic acid (DEAPAA) + 0.02% butylated hydroxytol uene (BHT)

Powder/Liquid ratio: 1.8 g/ 0.2 ml

Composition Setting Time Strength

Powder**

Ii I 2

Liquid**

I 3I 4

(min) Compressive

MPa

Tensile

MPa

2 1 1 l a - 137.6 (5.8)

h

2 1 1 lb 9 127.9 (7.1) 15.7 (1.4)

2 1 2 . 1 7.5 109.9 (8.0) 10.7 (2.6)

2 ic Id - 9 49.9 (2.9) 8.0 (1.3)

1 1 2 1 7 118.1 (6.3) 11.8 (1 .8)

1 1 1 ie 7 127.0 (4.4) 18.0 (1.9)

1 1 1 1^ 10 131.9 (7.2) 13.0 (0.8)

1 1 1 2g6 177.9 (1.0) 15.9 (0.8)

1 1.5 1 19 9.5 150.1 (4.8) 14.3 (3.8)

1 1.5 2 1 5.0 132.8 (6.9) 15.6 (0.9)

1 2 1 2 4.5 ^199 -

1 2 2 19 5.0 136.3 (4.8) 17.1 (3.0)

* Strength of Z0E cement: compressive: 16-38 MPa; tensile: 1. 4-2.5 MPa

**Parts by weight

a DMDMA + 0.5% N ,N-dimethylpenylacetic acid

b DMDMA + 0.5% DEAPAA + 0.05% BHT

c Unsilanized glass

d Powder-liquid ratio 1.3 g/0.2 ml

e DMDMA + 0.5% DEAPAA + 0.04% BHT

f 5% BIS-GMA added

g Slurry difficulty to mix

h Standard deviation of 5 determinations

54
of the glass powder used greatly improves strength; (3) incorporation of

5% BIS-GMA with the dimethacrylate does not improve the characteristics

of the cement; (4) too high a concentration of glass decreases the

working properties of the mix.

The storage stability of the combined hexyl vanillate-EBAdimethacrylate and amine accelerator liquid is not very good at elevated

temperatures . Efforts were directed (1) to study the reason for premature

polymerization of the combined liquids, and (2) to study the use of more

storage-stable liquid systems.

Accelerated aging tests were conducted at 45°C with liquids of

various compositions containing mono- and dimethacrylate monomers,

different accelerators, hexyl vanillate and EBA or other acids. The

formation of solid polymer on storage was observed visually. Results

given in Table 19 indicate that the n-hexyl vanil late-EBA solution is

very stable. Dimethacrylates can be stored with tertiary amines in the

presence of inhibitors such as BHT. However, acids such as EBA when

dissolved in dimethacryl ates in the presence of tertiary amines lead

rapidly to the partial polymerization of the monomer. The neutral

diethylaminophenylacetic acid ethyl ester imparts considerably longer

storage stability to the dimethacrylate-HV-EBA solution than the corresponding free acid or even N,N-dihydroxyethyl-£-toluidine.

Solutions containing methyl methacrylate monomer with HV-EBA

and tertiary amine accelerators can be stored much longer at elevated

temperatures than the more readily polymerizable dimethacrylates . An

HV-EBA solution mixed with methyl methacrylate containing 1% dihydroxyethyl-jD-toluidine and 0.02% BHT inhibitor did not polymerize within a

270 day observation period. Substitution of monomethacryl ates for

55
Table 19

Storage Stability of Cement Liquids at 45°C

Liquids were prepared from equal parts, by weight.

Composition of Liquids Storage Stability

of combined liquid

at 45° C Part A Part k B

HV

a DMDMA

C + 1% DEAPAA

e 60 days

EBA

b II II 1.5 hr

II II 17 days

12.5% HV - 87.5% EBA 600+ days

II II DMDMA + DEAPAA + 0.02% BHT9 3 hr

II II " +

11 + 0.05% BHT 3 hr

II II " + " + 0.08% BHT 3 hr

II II 11 + 1% DEAPE

f + 0.02% BHT 1 hr

II II " +1% DMABA9 + 2.5 days

II II 11 + 1% DHEPT

h + 0.1% BHT 10 days

II II 11 + 1% DEAPAA-EE

1 +

" 15 days

ii it MMA

d + 1% DEAPAA-EE + 0.02% BHT %180 days

II II " +1% DHEPT + 270+ days^

II II cyclohexyl methacrylate 200+ days

QM 657m

+ 1% DHEPT + 0.02% BHT ^200 days

II II 70% QM 657

m

+ 30% MMA - 1% DHEPT +

| 0.02% BHTi

^1 80 days

1

a n-Hexyl vanillate

b o-ethoxybenzoic acid

c 1,10 decamethylene dimethacrylate

d methyl methacrylate

e 4-N ,N-diethylaminophenylacetic acid

^ 4-N ,N-diethylaminophenethanol

9 4-N,N-dimethylaminobenzoic acid

b

4-N ,N-dihydroxyethyl tolui di ne

1 4-N,N-diethylaminophenylacetic acid ethyl ester

9 butyl ated hydroxytoluene

1/ time after which solid polymer had precipitated

1 liquid had darkened

m dicyclopentenyloxyethyl methacrylate

56
dimethacrylates into the appropriate compositions yielded materials

suitable as restoratives. Properties of the resulting composites are

given in Table 20. Using methyl methacrylate in the liquid ingredient,

cured resins with compressive strength of 130 MPa and tensile strength

of 10.3 MPa were obtained. Substituting higher molecular weight, less

volatile monomethacrylates yields restoratives with similar properties.

However, addition of cyclohexyl methacrylate increases the curing time

to 13.5 min. A composition containing isobornyl methacrylate had poor

storage stability at 45°C.

By far the most satisfactory properties were obtained with

solutions containing commercially available dicyclopentenyloxyethyl

methacrylate (QM-657) [30]

ch2 * V

CH

;c-c-o-ch2 ch 2 -o

and methyl methacrylate as monomer. The former monomer according to the

manufacturer's fact sheet has low volatility, low odor, low viscosity,

low curing shrinkage and is non-mutagenic. Its acute oral toxicity in

rats and acute dermal toxicity in rabbits are both greater than 5 g/kg.

Acute toxicity screening tests on rabbits are reported to have shown

only slight irritation of the eyes and a primary skin irritation index

of 2.5. Guinea pigs were not sensitized on exposure. A formulation

made from a liquid containing 1 part HV-EBA and 1 part 70% dicyclopentyl

oxyethyl methacrylate and 30% methyl methacrylate hardens in 6.5 min. to

a material having a compressive strength of 122 MPa and a tensile

strength of 13.4 MPa. Furthermore, the liquid component when stored for

2 months at 45°C showed no signs of premature polymerization. A somewhat

57
TABLE 20

Properties of Intermediate Restorative Materials

Powder: 1 part: 64% ZnO - 30% Al^Og - 6% hydrogenated rosin

1 part: silanized glass + 1% benzoyl peroxide

Liquid: 1 part: 12.5% hexyl vanillate - 87.5% EBA

1 part monomer + 1% N ,N-dihydroxyethyl -p-tol uidine + 0.02% BHT

Powder/Liquid ratio: 1 . 8g/0 . 2 mL

Monomer in Liquid (%) Setting Time Strength, MPa

a (min) Compressive Tensile

Methyl methacrylate (MMA)' 9.5 129.6 (3.8) 10.4 (0.7)

II .. b

6 107.9 (5.2) 12.3 (0.3)

QM-657 C

8 104.3 (8.6) 13.2 (1.7)

70% QM - 6 57 + 30% MMA - 6.5 122.3 (9.3) 13.4 (0.5)

II +

.. d

6 107.2 (10.3) 12.1 (2.3)

65% QM- 6 57 + 25% MMA + 10% MAA 5 114.3 (4.0) 12.6 (1.6)

70% " + 30% " + 1% MMA e

5 98.2 (5.2) 12.3 (1.9)

70% 11 + 30% " +0.1% MMA 5 103.8 (6.2) 10.2 (1.4)

65% diclopentenyl methacrylate + 35% MMA

1" 7 100.8 (10.3) 12.5 (2.1)

95% ' " + 5% MMA 6.5 99.1 (17.3) -

Isobornyl Methacryl ate 8 105.3 (6.0) 15.2 (0.2)

Cyclohexyl methacrylate 13.5 111.3 (22.5) 15.8 (2.0)

70% Cyclohexyl methacrylate + 30% MMA 11.5 — - --

Commercial intermediate restorative (IRM) 7.5 62 -•--

a Standard deviation of 5 measurements

b Substituted 1% N ,N-dimethyl ami nophenethanol for the 1% dihydroxyethyl

p-tol uidi ne

c Dicyclopentenyloxyethyl methacrylate

d Powder composition, 1 part 64% Zn0-30% Al^Og - 6% hydrogenated rosin;

1.5 part silanized glass + 1% benzoyl peroxide

e Water solubility of the cement: 0.58%

f Powder-liquid ratio 1.8/0.22 mL

gg
faster cure is obtained with the addition of 10% methacrylic acid, but the

strength properties of the composite are not improved. We plan to study

the effect of addition of polymerizable vanillates and cross-linking

divanillate as well as the effect of incorporating a silanized silica or

zinc oxide powder on properties of the resulting composite.

3. Dimer Acid Polycarboxylate Cements

The conventional polycarboxylate dental cements are based on the

reaction of a liquid form of a polymeric polyacid (e.g. polyacrylic acid in

an aqueous solution) with a solid basic inorganic phase (e.g. zinc oxide,

etc.) to give a polycarboxyl ate polymer. In this case, the polycarboxyl ate

reaction crosslinks the polymer and aids in its adhesion to tooth structure.

A drawback of this dental material is its hydrophi 1 ici ty

.

Dimer acid shown below as a complex isomeric mixture of acyclic

monocyclic and bi cyclic carboxylic acids, is a unique polybasic acid in its

hydrophobic character and room- temperature liquidity. The latter property

suggested it should be examined for dental cement applications.

CHj[CK2 ]iCK{CH2 ],C00H

CK 2 [CH 2 JjCH— cick2 ] 2 ccoh

Acyclic

COOK

I |ch 2 ),

ICHjJjCOOH

CH=CH[CH 2 ]<CKj

|CH 2 ]<

CHj

Monocyclic

59
Polycarboxylates that are formed from dimer acids and divalent

metals are linear polymers having carboxyl ate linkages in their backbone as

depicted in a simplified fashion below;

y'O , . 0V /0 _j_, <x G H 0 'C 0 H © >

^

0 0 J. xo 0

Hydrophobic Backbone

Such polymers have been prepared by fusion or metathesis [31-33].

The goal of this study was to apply the fundamental acid-base

reaction mechanism of conventional nonaqueous dental cements (e.g. EBA

cements) to development of tough, hydrophobic cements by the use of the

bulky, flexible, hydrophobic dimer acid as the liquid component (proton

donor) and various basic oxides and hydroxides as the powder component

(proton acceptors)

.

Dimer acid (DA) is a viscous liquid produced by the dimerization of C-j g

fatty acids (e.g. linoleic, oleic acids, etc.). It is

a aliphatic, dibasic acid whose structure is essentially that of

a long-chain di carboxylic acid with 2 alkyl side chains (R^ and R^). The

structure is somewhat uncertain but appears to contain at least one ethylenic

bond and another linkage (X) resulting from some type of dimerization

of the two olefinic fatty acid molecules that form DA. The~exact nature

of this linkage has not been completely defined; it may be as simple as

a single carbon-to-carbon bond or as complex as a cyclic structure

depending on factors such as the type of unsaturated C-| 8 fatty acid used

and reaction conditions, such as temperature and catalyst type (see

schematic below).

60
Schematic Structure of Dimer Molecule

Dimer

Acid

No. of

carboxyl groups

No. of

carbon atoms

Approx.

molecular weight

Approx.

equivalent weight

2

36

565

283

Table 21

Dimer Acid Cement Materials

Components Function Source

A. Liquid - a high

molecular weight

liquid dibasic acid

1 . Dimer Acid

Proton Donor

(or acid)

Emery Industry, Inc.

B. Powder - finely divided

basic or amphoteric

sol id

1. ZnO

Proton Acceptor

(or base)

PROCOL-Sol

2. MgO

3. CaO

4. Ca(0H)

2

J. T. Baker Chem. Co.

J. T. Baker Chem. Co.

J. T. Baker Chem. Co.

C. Fi Hers Improve Dimensional

Stabi 1 i ty

1. CaJPOjp

2. CaC0

3

3. Si0

2 (325 mesh)

61

Fisher Scientific Co.

Fisher Scientific Co.

Corning Glass Co.
As can be seen from the above schematic structure of dimer acid, the

molecule has a bulky, flexible hydrophobic structure between the terminal

hydrophilic carboxyl groups. Empol 1010 Dimer Acid (Emery Industries,

Inc.) is the purest grade of commercially available DA and is a polymer

grade acid suitable for use as a reactant in high polymers of the polyamide,

polyester, and urethane types. This acid typically contains 97% dibasic,

3% tribasic, essentially no monobasic acids, no metallic content, and an

extremely low residual unsaturation. High purity and low unsaturation

contribute greatly to the excellent oxidation stability exhibited by

this grade of DA and its derivatives.

The high quality of this DA is exemplified by its light color

and excellent color stability, two properties which are most important

to manufacturers of light colored end products such as dental restorative materials and cements. Table 21 summarizes the materials used in

this study and indicates their function in dental cements and their

commercial sources.

The results of this study are summarized in Tables 22-24.

Table 22 is a summary of the properties of DA cements formed using ZnO

as the major basic component of the powder. From the standpoint of the

highest strength, formulations 1, 3, and 4 are the most promising. On

breaking under compression or diametral tensile stress, cylindrical

specimens prepared with these formulations fractured but did not crumble

into powdery debris. Some of the samples subjected to compressive

stress fractured but without serious failure (that is, except for

apparently minor cracks, the specimens remained intact).

62
TABLE 22

MECHANICAL PROPERTIES OF DIMER ACID CEMENTS BASED ON ZINC OXIDE

AS THE MAJOR BASIC COMPONENT

24-Hour Mechanical Strength

Compressive Diametral Tensile

Setting [Std. Dev.] [Std. Dev.]

P/L Time (No. of (No. of

Formulation Powder Ratio (min.) Specimens) Specimens)

ZnO 7.0

8.3

49.1 [0.9] (5) 6.2 [0.8] (5)

49.7 [3.3] (4) 6.8 [0.2] (3)

2

3 Zn0(5)

Si0*(2)

9.5

9.8 33.5 [0.5] (5) 5.1 [0.3] (5)

Zn0(6)

HgO(l)

7.0 46.6 [1.0] (5) 5.8 [0.8] (4)

Zn0(9)

CaO(l)

3.5 7.0 18.4 [0.7] (5)

Zn0(7)

CaO(l)

6.0 28.7 [0.9] (2)

Zn0(4)

CaO(l)

7.0 21.4 [1.9] (4)

8 Zn0(3)

CaO(l)

5.5 18.6 [1.1] (3)

SiOo* = glass powder (325 mesh) silanized with 2% B-carboethoxypropyl triethoxysi lane

.

63
Table 23 summarizes the properties of some cements derived

from MgO and DA. Although not as strong as those prepared with ZnO,

probably due to relatively low powder/liquid ratio (P/L = 1-3), these

cements are strong enough to pass the ADA specification for cements.

Table 24 summarizes DA cements prepared with Ca(0H)

2 as the

main basic ingredient. The salient feature of the Ca(0H)2 cements (e.g.

formulation 1) is their change in dimensions under a compressive load

resulting in elastic and plastic deformation. Many of these specimens

displayed only minor cracks and usually did not fracture. There was a

slight recovery in dimensions on storage at 37°C for one month. The

specimens formulated with fillers such as SiC^ and Ca^PO^^ showed only

slight deformation under compressive load.

The water solubility or disintegration of the ZnO - DA cement,

P/L = 7 (Formulation 1, Table 12) is only 0.1% for storage in distilled

water at 37°C for 24 hours. The Ca(0H)

c -DA cement, P/L = 1.5 had a

higher solubility in distilled water for 24 hours storage at 37°C as

expected from the greater solubility of CalOH)^ compared to ZnO. The

value obtained was 1.5% which is rather good for a Ca(0H)2 cement and

much lower than that of a commercial cavity liner material (DYCAL). The

compressive strengths of many of the DA-Ca(0H),p cements also exceed that

of this widely used liner material (DYCAL).

64
The likely mechanism for the formation of polymerized (i.e.

hardened) dimer acid cements is shown below:

Polycarboxyl ate cement, a linear metal

carboxyl ate polymer M = Zn; Mg; Ca; etc

Figure 6

The dicarboxyl ic acid and the divalent metal oxide or hydroxide undergo a

step type acid base reaction leading to a metaloorganic polymer - specifically

a linear divalent metal dicarboxyl ate polymer. FTIR analysis supports the

above mechanism.

65
TABLE 23

MECHANICAL PROPERTIES OF DIMER ACID CEMENTS BASED ON MAGNESIUM

OXIDE AS THE MAJOR BASIC COMPONENT

. 24-Hour Mechanical Strength

Powder

P/L

Ratio

Setti ng

Time

(min)

Compressive

[Std. Dev.]

(No. of

Specimens)

Diametral Tensile

[Std. Dev.]

(No. of

Specimens)

1 MgO 1 6.1 35.2 [1.0] (4) 3.4 [0.6] (5)

2 MgO(l)

SiOg(l)

2 6.1 34.1 [0.4] (5) 4.4 [0.6] (5)

3

a KgO(l)

Si0

2

**(2)

3 6.0 37.6 [1.21] (5) 4.8 [0.7] (5)

a

a

i

i

i

i

i

a

i

i

a

a

Si O^* = glass powder (325 mesh) silanized with 2% 2-carboethoxypropyl triethoxysi lane |

SiO

? **= glass powder (325 mesh) coated with 1% PEAA -15 from a xylene soultion.

PEAA - 15 = copolymer of ethylene and 15% by weight of acrylic acid. I

a - 7 day storage at 37°C instead of 24 hour at 37°C in 100% relative

humidity chamber £

I

f

66
TABLE 24

MECHANICAL PROPERTIES OF DIMER ACID CEMENTS BASED ON CALCIUM HYDROXIDE

AS THE MAJOR BASIC COMPONENT

24-Hour Mechanical Strength

Formulation Powder

P/L

Ratio

Setting

Time

(min.

)

Compressive

[Std.Dev.]

(Number of

Soecimens

)

Diametral

Tensile

[Std.Dev.]

(Number of

Specimens)

1 Ca(0H)

2 1.5 3.0 24.6 [0. 5] (5) 4.2 [0 - 7] ( 5)

2 Ca(OH) (2) 2 Si0

2*(l)

6.0 23.5 [0.9](4)

3 Ca(0H)

2 (l) 3 Si0

2* (2)

5.0 23.5 [0.23(2)

4 Ca(0H)

2 (l) 2 Si0

2***(l)

4.3 23.0 [0 . 7] (3

)

3.4 [0.5](5)

5

•? Ca(0H)

2 (l) 2 CaCo

3 (1)

6.5 22.5 [1 - 0] ( 4) 3.4 [0 . 3] ( 5)

6 Ca(OH)

2

( 1 ) 3 Ti0

2

(1)

7.0 29.7 [1.01(4) 5.8 [0.8](3)

7 Ca(OH)

2

(2) 2 Ca

3 (P0

4

) 2 (1)

6.7 32.0 ._[0.6](5) 5.7 [0.6](5)

8 Ca ( OH

)

2

( 2 ) 2

a Ca

3 (P0

4

) 2 (l)

10.0 31.9 [0.31C3J 4.8 Cl_.2](4)

9 Ca(OH)

2

( 2) 2 bCa

3 (P04

) 2

7.3 • 33.4 [0.7] 4.8 [0.5](5)

Si0

2 *= glass powder silanized with 2% 2-carboethoxypropyl trietloxysi lane

Si0

2***= same glass but heated for 2 hours at 200°C

a Ca

3 (P0

4

) 2

= Ca

3 (P0

4

) 2P retreated with DA

bCa

3 (P0

4

) 2

= Ca

3 (P0

4

) 2 pretreated with Ti

F

4

67
4. Thiol /ene Cements

We are exploring the synthesis of high molecular weight

polybasic acids (for cement applications) by means of the thiol/ene

reaction. Initial experiments involved the reaction of undecylenic

acid with 3-mercaptoproprionic acid and PETMP (the polythiol described

in I. A. 4. of this report) as shown below:

0 fi 1 ) H0-C-CH

2

CH

2

SH+CH

2

=CH-CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

C-0H

n Sunlight

°

*

i>

H0C-CH

2

CH

2

-S-CH

2

-CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

C-0H

white solid, m.p 95-7°C

2) 4 CH

2

=CH-CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

CH

2

C-0H + PETMP

|

n-hexane/CH

2

Cl

2

Sun! ight

white solid, m.p. 63-4°C

The first reaction occurred within minutes on exposure to

sunlight to give a white solid believed to be the addition product

shown. The second reaction required a mixed solvent of n-hexane and

methylene chloride (1:4, v/v) to obtain a homogeneous mixture. A

longer reaction time at room temperature was required (about 7 hours).

The work up phase yielded a white waxy solid, m.p. 63-64°C.

Another potential cement forming organic acid that we are

currently synthesizing involves the reaction of 3-mercaptoprop

68
acid with eugenol as shown below. The product is a viscous liquid.

HSCH

2

CH

2

C - OH + CH

2

=CH-CH

2

OH

0CH

3

p

HO-C-CH,CH,-S-CH,CH,CH- CL C C 2

EUGENOL

Sunlight, Photosensitizer

0CH

3

5. Development of Adhesive Bonding Techniques

The objective of this research has been (1) to develop a basis

for clinically applicable adhesion techniques suitable for bonding

polymeric materials to mineralized tissues, e.g. dentin and bone, and

(2) to evaluate potential adhesive systems for their ability to serve as

protective coatings or surface sealants, as liners, or as adhesives for

composites and other restoratives.

Alkyl cyanoacrylates polymerize rapidly at room temperature.

They form the strongest known bonds between dentin and acrylic polymers

[34]. Unfortunately , weakening of these bonds takes place on exposure

of the joint to water or high humidity. Monomers incorporating cyanoacrylate groups or with an ally! and a cyanoacryl ate group in the molecule

have been synthesized [35, 36]. These compounds polymerize anionically

and crosslink when heated or when exposed to certain polymerization

initiators. The resulting highly cross-linked polymer should be resistant

to hydrolytic fission and possess increased hardness. Recent studies have

shown that interfaces cemented with ally 1 cyanoacrylate have improved heat

stability compared to less cross-linked cyanoacrylate joints [36]. Formulations incorporating allyl cyanoacrylates and higher alkyl 2-cyanoacryl ates

,

69
difunctional aromatic diol dimethacrylate esters as well as tri(allyloxy)-s-triazine and peroxide free radical initiators have been suggested

as pit and fissure sealants [37]. Adhesives containing these ingredients

and suitable crosslinking agents such as p-xylene glycol bisacrylate,

bisphenol A dimethacrylate have been proposed as orthodontic bracket

compositions [38].

Since ally 1 2-cyanoacrylate is at present not available commercially its synthesis was initiated. Allyl cyanoacetate was obtained by

direct esterification of cyanoacetic acid (170.1 g, 2 mol) with allyl

alcohol (162.6 g, 2.8 mol) in the presence of 3.4 g concentrated sulfuric

acid and 0.17 g jD-toluenesulfonic acid as catalysts and 250 ml toluene

for azeotropic removal of the water formed during the reaction. After

cooling the reaction mixture, a small quantity of precipitate was removed

by filtration. The filtrate was extracted with 30% aqueous NaCl and 5%

Na^CO^ until the aqueous extract had a pH of 7. After removing the

residual solvent by mild heating, the allyl cyanoacetate was distilled

at 95-96°C/5 mm. The liquid was purified by redistillation. BP=83-

84°C/2. 5 mm, njp = 1.4417, yield = 93.2%.

Paraformaldehyde (4.2 g) was added to 25 ml benzene and 0.05 g

piperidine. This mixture was heated to reflux and then the allyl cyanoacetate (21.2 g) was added dropwise. The reaction mixture was kept

basic by dropwise addition of 50% aqueous NaOH. Unreacted formaldehyde

was distilled off and the solution was refluxed with excess benzene to

remove the remaining formaldehyde. Finally, the solvent was distilled

off.

70
(CH

2°)n

C00CH

2

-CH=CH

2

I nCH

ICN

Piperidine

cooch

2

-ch=ch

2

-C-CH + nH

2

0

Paraformaldehyde Polymer

The polymer was dried in the presence of 1 g phosphorus pentoxide, 1 g

phosphoric acid and 0.1 g hydroquinone.

A number of procedures were studied to obtain optimum conditions

for the depolymerization. In the preferred method the viscous polymer

containing the drying agents was distilled into a receiving flask containing 0.05 g P^Oj. and hydroquinone which was cooled in a dry iceacetone bath. The major portion of the distillate (BP 140-1 50°C/4mm)

was collected. The crude monomer was purified by vacuum distillation.

All distillation equipment was rinsed with hydrochloric acid prior to

use and was then dried thoroughly. A flow of S0

2 was bubbled through

the monomer in the flask and through the receiver. The distilled nonviscous 2-allylcyanoacrylate was stored in a bottle which had been

flushed with S0

2

. Gas chromatographic and IR analysis indicated a

monomer purity of 99%. The monomer when bonded to glass had excellent

adhesive properties. The availability of this monomer will allow us to

formulate adhesive compositions containing this cross-linking agents as

an ingredient.

71
II. Wear Resistance and Related Physical Properties of Dental Composite

Restorative Materials

In connection with our overall effort in developing improved composite

restoratives the objective here is to develop, or make use of, appropriate

techniques to measure wear resistance and mechanical properties of

dental materials and to use the forthcoming data as performance indicators

to predict in vivo durability of these materials. The completion of the

development of these methods to a satisfactory state is particularly

important because of the long times and efforts required for clinical

evaluations. In order to accomplish this task, some fundamental research

on wear and related mechanical properties is being done to obtain a

better understanding of the relevant mechanical and chemical degradation

processes. This information will be of. use in determining the principal variables of influence for which appropriate modifications can be incorporated

in the wear apparatus to obtain meaningful results. In addition, the

improved understanding of ways in which specific mechanical properties

of composite systems correlate with wear behavior is expected to be

instrumental in developing more durable systems.

Two types of wear tests have been, or are being, developed here

which comprise a classical pin and disc apparatus and an erosion technique.

(The latter was constructed during this fiscal year.) Although these

particular methods are not necessarily expected to be used as final

performance indicators, they do give basic information relevant and

possibly essential to the development of an appropriate durability test,

as will be apparent in later sections of this report. Complementary

efforts involve Fourier Transform Infrared (FTIR), Differential Scanning

Calorimetry (DSC), Scanning Electron Microscopy (SEM) and a microdefect

72
analysis using x-ray and optical microscopic observation of damaged

regions made resolute by a silver staining technique. During this

fiscal year a Mass Absorption/Desorption Analyzer was developed to

measure diffusion rates of chemical solvents into polymers and dental

composites

.

From examination of the subsurface damage structures of in vivo

restorations and from comparison of these structures with those which

occur within in vitro damaged or worn composite specimens, we conclude

that the chemical softening of the matrix plays an important role in the

in vivo wear process [2, 39]. We have demonstrated experimental ly that

a large variety of chemicals covering a wide solubility parameter range

can soften the BIS-GMA based polymer matrix [40]. Consequently similar

oral substances can be expected to have a detrimental effect on the wear

resistance of the composites.

The diffusion rate of chemicals in polymeric systems depends strongly

on the degree of polymerization and cross-link density, both of which

are known to increase with higher curing temperature in thermosetting

plastics. We have therefore explored the effect of the curing temperature on the diffusion rate of chemicals within dental restorative composites.

A. Solvent Diffusion rates in Dental Composites

The effect of four chemicals was studied on two commercial composites

designated A* and B**. The specimens were cast between two glass plates

and cured at either 37°C and 100°C for 24 hours. After 2, 4, and 14

days, the penetrating depth of the chemicals into the composite was

measured using Ag

+ ions as labeling agent. The penetration depth data

* Adaptic - Johnson & Johnson, East Windsor, NJ 08520

** Prisma - L. D. Caulk Co., Milford, DE 19963

73
M

!>- «3 TJC

•Hw

-P

0)u

Q) xz

CJ

4-1 Q

ft>1 X

ftCoH4

-> (0u

-P

<uc

d)

(ft o

-p

T3

<U 4-> n3 rH

0)P 1—1 cn

ttJ -P o C —'

o3 -Pm

H c0 P u

<D -P 03 e

CU P 3. £ PP C p •H

ta c a 0 m H •H

(/} -p P ft •H 4-1 i—1

4-1 Tl •P *rH C

TSm P

rp

M 0 u

C/3

— * Q

74

RIRRRRRRRR88IIPPPii
were fitted to the equation P = Dt

c

, where P is the depth, t is the

elapsed time and D and c are adjustable parameters. The resultant

2

values of D, c and corresponding correlation coefficients r are given

3 1/2

in Table 25. The solubility parameter in (cal/cm ) ' of each chemical

was also included in the Table.

The values of the time exponent c in Table 25 are much nearer to

unity than 0.5 in almost all the cases; this result supports a Class II

diffusion mechanism. The only exceptions are those data of composite B

specimens cured at 100°C. The rate constant D was also dramatically

reduced in those cases. The above data indicated that the diffusion

rate of the chemicals in these composites and especially in composite B

can be reduced significantly by increasing the curing temperature. The

wear resistance of the high temperature cured composites exposed to

different chemicals is being studied using the pin-disc wear equipment.

The above experimental technique is useful to identify the type of

diffusion process; however, for more quantitative information regarding

the saturation content of the penetrants and diffusion, an automatic Mass

Absorption/Desorption Analyser was developed which operates as follows:

The changes in weight of the specimens situated in a constant temperature chamber are monitored after the vapor of the penetrants is introduced. The increase in weight with time or the absorption isotherm

provide the base to calculate the diffusion coefficients and the permeability parameters. The experimental equipment has been completed

(Figs. 7 and 8) and the sensitivity of the weight measuring device is

_5

10 gram. The output of the device is digitalized and stored on tape

for further calculations.

75

Fig.

8.

Photograph

of

the

constant

temperature

chamber

(shown

in

Fig.

7)

containing

the

automatic

electrobalance.
78

Fig.

9.

Absorption

isotherm

of

water

into

dental

resins

cured

at

37°C

(70%

BIS-GMA,

30%

TEGDMA)

.
Absorption

of

Water

in

Dental

Resin

Cured

at

60

C

70%

BIS-GMA

30%

TEG

DMA

ClM eiduiBs jo %) juejuoo juejjeued

79

Fig.

10.

Absorption

isotherm

of

water

into

dental

resins

cured

at

60°C

(70%

BIS-GMA,

30%

TEGDMA)
Absorption

of

Ethanol

in

Dental

Resin

Cured

at

37C

70%

BIS-GMA

30%

TEG

DMA

1X5

CO

LO

OJ c o i

CVJ

w0E

1X5 -h1- cc

D“

</) o

LOo

+-> ro -a

a>

s-3O

toc

'r-to

<D

rO

+->

a; -ao

+->oc

to

+-><u M—O

S-<v SZ

+-> o

toco

+-> CL

s_o

to XI

<c

cn

Hi

liP oCJ3 UJoCO

CJ3I GO

b—I cao «pp oo

r'- co

80
Absorption

of

Ethanol

in

Dental

Resin

Cured

at

60C

70%

BIS-GMA

30%

TEGDMA

81

Fig.

12.

Absorption

isotherm

of

ethanol

into

dental

resins

cured

at

60°C

(70%

BIS-GMA,

30%

TEGDMA).
A set of absorption isotherms is shown in figs. 9 thru 12. The

specimens were made of 70% BIS-GMA and 30% TEGDMA cured at 37°C and

60°C, respectively. The water absorption process into this dental resin

is clearly a Fickian (Class I) type. The values of the amount of water

_g

to saturate and the diffusion coefficient are 3.35% and 9.8 x 10

2 -9 2 cm /sec for the 37°C cured specimens and 1.25% and 4.4 x 10 cm /sec

for the high temperature cured one. The diffusion process of ethanol

into this dental resin is clearly a class II type as demonstrated by the

absorption isotherms which indicate saturation values of 18% and 10% for

the resins cured at 37°C and 60°C respectively. The physical significance of this type diffusion behavior is twofold: (1) the molecules of

the resin are partially dissolved by the penetrant, consequently, the

resin changes from a glassy state to a rubbery state. Little wear

resistance can be expected with composites composed of a rubbery matrix.

This point will be elucidated in a later part of this report. (2) The

desorption process of penetrants causing a class II diffusion is abrupt

at the very beginning until a hard surface skin is formed. Hereafter,

the desorption process is extremely slow with penetrator trapped within

the once swollen resin for an extended time. Such slow desorption

characteristics of dental resin are likely to be one of the main causes

of the subsurface damage layer observed in all the in vivo worn specimens.

B. Influence of Chemical Softening on Hardness and Wear

As stated in the last section, chemical softening of dental composite restoratives is believed to be very relevant to in vivo durability.

Hence, the influence of food simulating chemical solvents on hardness

and in vitro wear is being studied. The chemicals used are included in

Table 26, along with their solubility parameter values in SI Units and

examples of food they simulate. These chemicals include those recommended

82
by FDA CFR 177.2420 (1980) (also see Ref. 41). Although the acetic acid

solution is not expected to play a role as a polymer solvent, it is

conceivable that this solution may degrade interfacial bonding (between

matrix and filler) of some composites.

Three commercial composites, Adaptic, Profile, and Prisma-fil were

evaluated. Adaptic was chosen as a conventional type and hence served

as a control. Profile is unique in that it employs a strontium glass.

Prisma-fil is a current light activated system. The composites were

mixed according to the manufacturer's specification and shaped into thin

discs suitable for wear tests. After initial curing they were stored in

distilled water at 37°C for one day after which they were immersed in

each of the solvents included in Table 26 for one week. Although 0.1%

acetic acid was included in this study, its solubility parameter value

is not included in the Table. We believe that the polymer matrix is

not soluble in this chemical as with some of the others. However, there

is a possibility that the filler, or interface, may be damaged from

extended immersion of the composite in this chemical.

Earlier results of this work were included in the last report [2].

As a consequence of the large specimen-to-specimen variation in the

hardness and wear measurements we found it necessary to obtain more

data

Knoop Hardness Number ( KNH) Measurements

During the immersion of the specimens in the various chemicals the

Knoop Hardness of these specimens was monitored at 0, 3, and 7 days. The

averages taken over five values on each specimen was given in Tables 27,

28, and 29 for Adaptic, Profile, and Prisma-fil. Although these tables

contain considerably more detail than usually presented in an annual

83
TABLE 26

Chemicals Used and

Solubility Parameter Values*

Substance

6 x 10' 4

(J 1/2 m

- 3/2

) Examples of Foods Simulated

Acetic Acid 0.1% NA Citrus fruits, vinegar.

tomatoes

Ethanol in Water:

0% 4.79

25% 4.24

50% 3.70 Water, light beverages,

75% 3.15 alcohol, candy, syrups.

100% 2.60 wine

Heptane 1.51 Vegetabl e oil, fats

NA=Not applicable

* Ref. 42

84
TABLE 27

Knoop Hardness of Adaptic as Affected by

Solvent Immersion

1 KNH

) Specimen Sol vent Day 0

(SD)

Day 3

(SD)

Day 7

(SD)

P./P

f

< P

i /Pf>Av

1 Water 56.5 (H.4) 67.9 (11.3*) 61.1 (7.2**) 0.92

2 92.4 (12.5) 78.5 (2.2) 65.4 (1.7) 1.41

3 34.7 (3.4) 31.5 (1.7) 29.4 (1.7) 1.18 1.17

4 2 5% Eth 87.3 (16.8) 36.7 (6.0) 36.4 (3.4) 2.40

5 44.2 (2.9) 31.5 (3.3) 23.1 ( 1 . 6 ) 1.91 2.15

6 50% Eth 57.8 <9.7 ) 38.4 (2.7) 35.6 (6.8) 1.62

7 56.7 (4.8) 36.2 (4.5) 18.8 ( 1 . 5 ) 3.02 2.32

8 75 % Eth 70.6 (3.2) 38.7* 53.5** 1.32

9 97.4 47.7* 49.6** 1.96

10 59.8 (3.9) 17.2 0-5) 17.6 0.4) 3.40

n 52.3 ( 7.6) 15.5 (0.8) 15.7 (1.0) 3.33 2.50

12 100% Eth 51.5 82.2* 45.5** 1.13

13 57.7 ( 6.2) 31.1 (4.0) 19.3 0.5) 2.99 2.06

14 Heptane 110.9 70.0* 80.1 ** 1.38

15 78.8 ( 8.9) 95.1 (4.4) 87.0 (7.7) 0.91 1.15

16 Aceti

c

Acid(0. 1%)

55.1 ( 5.5) 61.8 (5.2) 64.1 (2.6) 0.86

17 47.3 ( 2.9) 48.2 (5.1) 27.8 (3.1) 1.70

18 63.1 ( 4.4 ) 29.1 (1.9) 33.1 (2.0) 1.91 1.49

Average 65.3

St. Dev. 20.3

* Day 1 ** Day 4

85
TABLE 28

Knoop Hardness of Profile as Affected by

Solvent Immersion

Specimen Sol vent

I

M

KNH

1 0

9)

Day 3

(SD)

Day 7

(SD)

p./ Pf <VP

f>Av

(13.4) 87.3 (6.2) 70.8 (2.9) 1.17

(9. 5 ) 82.6 (6.9) 66.1 (3.1) 1.31

(3.8 ) 49.1 (3.4) 44.3 (4.6) 1.11

0 - 5 ) 27.9 (1.8) 26.0 (1.4) 1.16 1.19

(1.6 ) 38.8 (6.3) 42.1 (4.9) 1 .70

(6.1) 24.3 (2.2) 24.4 (3.6) 1.76

(3.4) 27.5 (1.9) 26.8 (2.5) 1.50 1.65 1 (5.8) 33.3 (5.6) 33.2 (3.7) 1.47

(7.1) 38.3 (3.3) 27.7 (2.8) 2.29

(4.1 ) 21.6 (2.2) 20.1 (0.6) 2.30

(4.3) 32.1 (1.0) 26.0 (5.4) 2.27 2.08 | (5.6) 38.4' * 42.0** 1.48

40.4' * 52.0** 1.67

(3.1 ) 19.7 (2.8) 19.6 (2.4) 2.70

(4.1 ) 22.8 (3.7) 16.7 (1.6) 2.65

(4.4) 26.5 (0.9) 19.3 (1.8) 2.82 2.26 j 42.

4

: k 43.1** 1.19

(3.4 ) 36.1 (3.2) 18.6 (1.9) 2.77

(2.0) 34.9 (3.7) 31.4 (1.6) 1.77 1.91 J

90.

3

: k 113.4** 0.64

(5.2 ) 77.3 (7.0) 83.6 (9.5) 0.95 0.79 1

(4.5) 45.6 (4 . 1 ) 38.4 (1.0) 1.77

6.3 ) 41.5 (3 . 3 ) 18,8 (1.0) 2.80

(2.7) 22.2 (2.7) 22.1 0-9) 2.21 2.25 "

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Water

25/o Eth

Average

St, Dev.

Day 1

50% Eth

75% Eth

100% Eth

Heptane

Acetic

Acid(0. 1%)

82.8

86.3

48.7

30.1

71.7

42.8

40.3

48.8

63.3

46.2

58.9

62.0

86.9

53.0

44.3

54.5

51.4

51.6

55.5

72.9

79.4

68.0

52.7

48.8

58.4

15.2

Day 4 86
TABLE 29

Knoop Hardness of Prisma Fil as

Affected by Solvent Immersion

KNH

Specimen Solvent Day 0

(SD)

Day 3

(SO)

Day 7 (SO)P /P

i f 'YV«*

1 Water 53.8 (5.6) 52.6 (2.2) 54.4 (4.9) 0.99

2 88.3 (9.8) 82.9 (3.1) 65.5 (2.2) 1.35

3 30.8 (4.7) 30.0 (2.8) 28.7 (1.0) 1.07 1.14

4 25% Eth 51.7 (2.8) 36.3 (2.7) 37.3 (1.6) 1.39

5 32.8 (0.7) 28.9 (0.9) 25.1 (0.7 ) 1.31 1.35

6 50% Eth 50.2 (2.0) 31.7 (2.9) 29.7 (0.9 ) 1.69

7 48.8 (1.3) 34.5 (1.4) 29.0 (0.8 ) 1.68 1.69

8 75% Eth 58.6 (3.3) 28.7* 37.7** 1.55

9 57.6 40.2* 38.2** 1.51

10 43

.' 5 (2.7) 18.7 (1.7) 17.2 (0.8 ) 2.53

11 37.5 (2.3) 22.2 (1.2) 25.5 (1.0 ) 1.47 1.77

12 100% Eth 45.0 40. 2'* 37.4** 1.20

13 49.4 (1.7) 32.6 (1.5) 31.5 (1.3.) 1.57 1.39

14 Heptane 64.6 67.3'* 72.0** 0.90

15 59.0 (2.3) 59.8 (3.3) 52.7 (3.0 ) 1.12 1.01

16 Acetic 48.7 (1.5) 42.5 (1.6) 36.4 (7.9 ) 1.34

17 Acid(0. 1%) 40.2 (0.7) 35.6 (1.1) 28.0 (2.8 ) 1.44

18 37.3 (1.2) 33.6 d.i) 26.7 (1.3 ) 1.40 1.39

Average 49.0

St. Dev. 13.4

* Day 2

•'* Day 4

87
report, they are included here to portray the difficulties in obtaining

reliable KNH values on composites. In the tables the numbers to the

right of each of the KNH values are the standard deviations obtained

from five measurements at different positions on each specimen, p^/p^

is the ratio of the initial hardness value (day 0) to the final day (day

7). This ratio is taken to be a measure of chemical softening. The

hardness ratios are followed by their average values for each chemical.

Ideally all of the KHN values for each composite at day 0 should be

identical, since these measurements were obtained before immersion in

the chemicals. The average KHN (day 0) value is given at the bottom of

each table. The accompanying large standard deviation represents both

specimen to specimen variation and that within each specimen. In Fig.

13, p_j/p.p, which is a measure of chemical softening, is plotted against

the solubility parameters of the chemical solvents. The ethanol concentrations and H=heptane are indicated as parameters. Although acetic

acid is not included here for the reason given above, it produces a

significant degree of softening on all composites. In the figure the

largest effect is seen on Adaptic, and the least, on Prisma-fil. The

-4 1/2 -3/2 hardness ratios reach a maximum at about 5 = 3.15 x 10 J m

' which

corresponds to an ethanol concentration of 75%. We believe that this

value of 5 approximates that for BIS-GMA.

1. Wear Measurements on Chemically Soaked Composites

The pin and disc wear apparatus [6, 43] is completely automated

comprising three identical rotating platforms containing the disc shaped

specimens of the material to be investigated. A stationary slider (pin)

of stainless steel, type 303, impinges on each of the rotating specimens

producing a wear track of 12 mm mean diameter. Stainless steel pins are

88
IIj

89

Fig.

13.

Hardness

ratio,

p^/p^»

plotted

against

solubility

parameter, 6

for

three

composites.
used because they produce subsurface damage similar to that observed on

in vivo worn restorations [44]. The slider is loaded with weights to

obtain the desired normal stress which is usually 10 MPa. The depth of

the wear track is measured periodically at 10 equally spaced positions

along the track circumference using linear variable differential transformers

with ruby tipped probes as shown in the figure. Distilled water at 37°C

flows over the specimens to control temperature, lubricate and wash away

debris

.

Immediately after completion of the chemical soaking period,

pin and disc wear measurements were taken on the three classes of specimens.

The wear measurements comprise two parts. The first is a set of measurements

for which the average track depth (from a set of 10 values around the

track ci rcumference) is taken at each of 5 sequential disc revolutions.

The track depth at the end of the fifth revolution is referred to here

as the initial wear, s_. Reliable initial wear data is very difficult to

obtain because of the requirement that pin and disc surfaces must be very

smooth and parallel. Subsequently track depth measurements were taken

every hour for 30 hours where each hour corresponds to about 1500 cycles.

In most cases for which the wear was apparently linear with time (or

track length) the steady state wear rate m

$ = ds/dt, where s^ is the track

depth, was evaluated by a linear regression.

Since we do not have wear data on each specimen before immersion,

we cannot treat the results in a manner parallel to that for the hardness

employing the ratio p^/p^. In figure 14 the initial wear for the three

composites is plotted against the solubility parameter of the solvents.

The trends are similar to those of p./p^ shown in figure 13, except the

90
I

I

I

I

I

I

I

I

I

1

I

I

I

I

I

I

I

1

12 3 4 5

SxlCT4

(J

l/2 m~3/2

)

Fig. 14. Initial wear s plotted against solubility parameter,

6 , for three composites.

91
values of s for Profile tend to maximize at the larger value 6 which

corresponds to a 59% ethanol solution.

The corresponding steady state wear rates, m , obtained subsequently are shown in fig. 15 for Adaptic and Prisma Fil. At most values

of 6 there is little distinction between the m

$

values of the two composites.

The wear behavior of Profile is more complicated and had to be analyzed

in a different manner. In figure 16 the wear track depth s is plotted against

time for a specimen preimmersed in water at 37°C for one week and for a specimen stored in air at the same temperature. The distinction between the

amounts of wear in the two cases is considerable. Also note the large

extent of nonlinearity for the water soaked specimen. With the other two

composites the response is linear, and there is little or no distinction

between wear rates of specimens stored in water or air. The value m

$ = 0.86

ym/h of the air stored Profile specimen approximates those obtained for the

other two composites stored in either water or air.

A plausible explanation for the difference between the amounts of

wear of water and air-stored Profile specimens is that the glass filler or

sanilizer used may undergo some hydrolysis when stored in water. This could

weaken the interfacial bonding manifested by a larger wear rate. Possible

explanations for the nonlinear wear observed for water stored Profile are

decreasing penetrant concentration and/or time for damage infliction decreasing

with depth.

The general wear behavior of dental composites (except for Profile

and possibly others affected by water) may be summarized as depicted schematically by figure 17. As indicated in the figure the initial wear may

be severe. As much as 180 ym of material has been removed over the first

five revolutions. This high wear rate is attributed to the removal of a 92
4

0

I

J ! L

2 3 4

SxlO" 4

(J l/2 m‘3/2

)

5

Fig. 15. Steady state wear, m . plotted against solubility parameter,

6, for three composites.

93
250r

200

Profile

37° C Cure

150

£

CO 100

° Stored in Water

50

o

o o o o

0

G °

0 0 0 0 0 0 0 0 0 0 0 0 0 0 °

tored jn Air

P--PQ-J 1 1

I

I

o O O O O O

0 10 20

t (hr)

30

Fig. 16. Wear track depth, s, of Profile with time for specimens stored in

air and water prior to tests.

94
I

I

I

I

I

I

I

I

I

f

I

i

I

I

I

I

J

1

I

Fig. 17. Schematic comparison between the wear of a chemically

softened and unsoftened composite.

95
a severely damaged layer induced by the solvent. After this surface

layer is removed the wear reaches an apparent steady state, which approximates 3 m/h at 1550 rev/h; with for example, Adaptic preimmersed in a 75%

ethanol solution. In the steady state region the composite is softened

by the diffusion of solvent, but no damage is apparent from the microdefect

analyses. Beyond one week of immersion the solvent may penetrate 200 urn

of material. The removal of this amount would require nearly 3 days of

wear at 3 ym/h. After this period the wear rate is expected to reach

that corresponding to the unsoftened composite. For Adaptic this value

is about 1 ym/h.

C. Degree of Cure in Dental Composites as Function of Cure Temperature

and Time

The extent of -polymerization as depicted by the residual double

bond concentration of the dental composites cured at 37°C has been

assumed to be relatively low. This low degree of the double bond conversion

may be a consequence of either the high viscosity of the partially cured

macromolecules or the unsati sfactory performance of the initiators. The

purpose of the work described in the following section is to determine

the factors which limit the degree of polymerization of the dental

composites and result in a vul nerabi 1 i ty to chemical softening.

The concentration of the unreacted double bond concentration was

measured with a Fourier Transform Infrared (FTIR) technique. The specimens

were thin films cured for up to seven days under conditions of 37°C and

100% RH. The specimens were exposed to room temperature and lower

humidity only during the IR measurements which required one minute on

the average. The results of these FTIR measurements are summarized in

figure 18. The most striking feature of the results is that the degree

96
LOO

COO

CO0

3

C

£

0

E

h

0

k_

3

o

*—

</) O

Q_

CMo

o s-s

• • CM

r~* v—/ • 2 « o

''"N (d o pa O CJ) CO

1 S'-? i—i c/a (—

m <c

« pa > t « CJ • Q -—

o CJ o

r'- W r~ 00 O

H

u , <3

n) o s • Q

4-1 O

0» Cd w

oi pa <d H

5-2 CJ »

CO i-t 1 d cn o

0) ». M CO s ^?n o

>-( CO o ^

CUo <c

P3

3

cj

l cn

Q

o

CdH

3

CJ

I C/3 M

pa

pa o

O

fd

pa O

o'-S CM ^

oo s

ca o

cno

CO

cn

*r— U_

O O

co fd w X

<uc

QJe

a

CJ

O

CD

O

CO

O O

CO

oCNI (%) spuog aiqnoQ jenpisay

97
of polymerization depends essentially on the viscosity of the monomer or

the resultant polymer. The concentration of the initiator and the type

of initiator have only a minor effect on the degree of curing once an

adequate amount is used. Furthermore, the copolymer with 70% BIS-GMA

and 30% TEGDMA has a limited extent of postcuring after the first 30

minutes. Extrapolation at 37°C/100% RH data to one year yields double

bond concentrations far in excess of that obtained by curing the same

system at elevated temperature, i.e. 150°C. The monomer system with 7/3

BIS-G MA/TEGDMA will still have 36% double bond unreacted after one year

versus 10% left for the same polymer cured at 150°C.

The degree of cure of a dental resin can be improved by raising the

curing temperature as shown in fig. 19; however, the time interval

between mixing the resin to start the polymerization and heating

needs to be kept at a minimum. A subsequent heating of specimens cured

at lower temperature, e.g. 37°C, can improve the degree of cure only to

a limited extent depending on the elapsed time. Table 30 demonstrates

this with an unfilled resin composed of 70% BIS-GMA and 30% TEGDMA. The

amount of post-curing due to subsequent heating was calculated from the

exothermal peak area of the DSC result. These results clearly indicate

that heat needs to be applied to the dental composite directly after

mixing to achieve a high degree of cure. However, this immediate heating

process is clinically unfavorable. This suggests a new direction for

improving the current dental composition, namely, development of a

composite that can be post-cured to high completion within a broad time

interval after the initial curing.

98
Additional

Polymerization

During

DSC

Scan

(296-450°K)

of

BIS-GMA

Resins

Cured

and

Conditioned

at

37°C,

100

Rll

i

i

/ 3 ^

I

!

99
percentage

of

residjal

double

bonds

RESIDUAL DOUBLE BOND CONCENTRATION OF

BIS-GMAC73 TEGDMAC33 CURED AT DIFFERENT TEMPERATURES

Fig. 19. Residual double bond concentration of 7/3 BIS-GMA/TEGDMA cured

at different temperatures

.

100
D. Influence of Degree of Cure on Hardness and Wear

As stated earlier in this report, chemical softening by food simulating liquids plays an important role on hardness and subsequent wear.

It was assumed that chemical softening could be reduced by increasing

the extent of crosslinking, or polymerization , which may be accomplished

by increasing the cure temperature. Although elevating the cure temperature

is not practical for in situ restorations , the resulting information is

expected to give valuable insight on the nature of these degradation

mechanisms

Data were obtained on three commercial composites: Adaptic (macrofilled), Silar (microfilled) and Prisma Fil (visible light activated).

Specimens were cured isothermal ly at three temperatures: 37, 60 and

80°C. For the chemically cured composites curing was initiated immediately

after mixing. With Prisma Fil, the specimens were allowed to reach

thermal equilibrium at the cure temperatures before the light source was

applied. After curing, the specimens were soaked in distilled water and

75% ethanol-25% water solution at 37°C for one week prior to tests. The

75% ethanol solution was chosen because it produced the most damage on

all composites investigated here as shown earlier in this report.

In figure 20 the hardness ratio,

p.j/p.f

is plotted against the cure

temperature T

c for the three composites. The upper set comprising data

for the three specimens cured at three temperatures pertains to preimmersion in the 75% ethanol solution. Except at 37°C the extent of

softening (measured by p^/p^) is about the same for the three composites,

and the softening decreases with increasing T . With the water soaked

specimens a slight amount of softening is apparent. The reason for the

apparent increase of P^/P^ between 37 and 60°C for Silar is not understood

101
102

Fig.

20.

Hardness

ratio,

P.j/Pf,

plotted

against

cure

temperature,

T

,

for

three

composites.
unless it is attributed to the imprecision Ap/p^ 30% based on standard

deviation.

The initial wear over five revolutions is depicted on figure 21.

All data pertain to the 75% ethanol soaked specimens. In all cases the

initial wear essentially vanished at T = 80°C. With Prisma Fil the

initial wear is insignificant except at T

c

= 37°C. The most dramatic

result was obtained on Silar for which the wear changed from about 180

to 0 urn from T

c = 37 to 60°C. This result indicates that this composite

is very vulnerable to chemical attack when cured in situ ; however, this

deficiency may be removed by curing at 60°C. The initial wear on the

water soaked specimens is not shown because it was insignificant.

The corresponding steady state wear rates are shown in figure 22.

Except for one case, the value of m decreases with increasing T . Why

the rate for Silar is essentially constant between 37 and 60°C is not

understood. This is the region where the corresponding initial wear

decreased dramatical ly

As stated earlier the initial wear pertains to the removal of an

extensively damaged surface layer. The damage is visible through the

silver staining microdefect analysis, which reveals the removal of

polymer matrix. The steady state wear is accelerated from preimmersion

of the specimen in an appropriate solvent. The damage- which is apparent

from the accelerated rate is not visible from the microdefect analysis.

In both cases the extent of damage is decreased by increasing the cure

temperature. This produces a more durable material as seen from the

increase in hardness and wear resistance of the composites even when

they are preimmersed in the solvent.

103
200

Initial

wear,

s,

plotted

against

cure

temperature,

composites.
Adaptic

o

00

o

o

co o

h-°

o

to

o<3

-

ro CVJ

(q/iii*^) s

uu

o

.ro

105

Fig.

22.

Steady

state

wear,

m

plotted

against

cure

temperature,

T

,

for

^

c

three

composites.
E. Erosion Tester

During this fiscal year the development of a new test procedure was

initiated and completed, except for the temperature control. The purpose

of this effort is to obtain an expedient and reliable test to evaluate

the erosion of composite restoratives resulting from the impingement of

high velocity liquid jets of various food simulating chemical solvents.

With composite restorative materials, this device was expected to compliment

the pin on disc wear apparatus, which is operationally complicated and

expensive.

The principle of operation of the erosion tester is depicted schematically in fig. 23. A pressure approximating 46 psi is generated by a

pump which forces the liquid through a tapered nozzle. The resulting

jet, which is constant velocity and non divergent, impinges upon the

restoration causing the erosion of material. The food simulating liquid

is collected at the bottom of the container after which it circulates

through a temperature bath and finally back to the pump. The amount of

erosion, measured by material loss, is to be determined by weighing the

tooth (with restoration), or the specimen and cup, at appropriate intervals.

In the original concept of this device the amount of wear was to be

determined by the weight loss of the composite specimen during the

erosion process. Using a 75% ethanol solution no detectable weight loss

was observed on an Adaptic specimen after testing over a period of one

week. It is conceivable that any detectable weight loss is counterbalanced by the absorption of solvent into the specimen.

106
Pressure Gage

Jet Stream

Restoration

Fig. 23. Schematic drawing of the erosion tester.
From electron scanning microscopy some damage is apparent over

extended periods of erosion. Figure 24 is a micrograph showing the

surface of an Adaptic specimen prior to tests. The surface appears to

be smooth except for voids and faint horizontal cutting marks. Using

about the same magnification figure 25 shows the surface at the same

specimen after being subjected to a 46 psi jet of 75% ethanol solution

for 6 days. The rough appearance results from the exposure of filler

particles laid bear from the removal of polymer matrix. Figure 26 is

eqivalent to figure 25 except at higher magnification. Using this

magnification the filler particles are clearly visible.

In future work an effort will be made to quantify the erosion

tester, possibly by producing more severe erosion in order that a significant weight loss may be detected over a reasonable time.

108
Specimen 8b

Control (Adaptic)

Fig. 24. Micrograph of the surface of an uneroded specimen of Adaptic

109
Specimen 8a - Erosion Tested

with 75% Ethanol - 6 days;

Jet Pressure 46 PSi

(Adaptic)

Fig. 25. Micrograph of the surface of a specimen of Adaptic eroded by

a 75% ethanol solution for 6 days.

110
i

Specimen 8a - Erosion Tested

with 75% Ethanol - 6 days;

Jet Pressure 46 PSi

(Adaptic)

Fig. 26. Micrograph of the surface shown in figure 25 but at higher

magnification.

Ill
III. Dental Alloys and Ceramics

The continually changing economic conditions, both domestic and

foreign, have resulted in steady interest in new dental alloy and ceramic

products which are less expensive or more price-stable, supply stable

and of satisfactorily predictable performance. The latter requires the

development of more useful reliable techniques for evaluation of critical

performance parameters for materials than those currently in existence.

Often the evaluation methods are not well defined or are inefficient

t and/or expensive to conduct. The objective of the present investigation

is directed toward development of standard test methodologies which will

meet the needs of the dental industry/profession in development and

control of alloy and ceramic products, both as starting materials and in

processing through to final systems as dental prostheses.

A. Alloy Castability

A technique for determining a castability value, C , for a dental

V alloy has been described in previous reports [2]. In essence, C

y

is

the percentage of cast alloy segments which comprise a rectangular grid

(fig. 27). Castability in this case is considered as the ability of an

alloy to fill a mold under specific conditions including mold and alloy

temperature.

The evaluation technique has been applied to a number of dental

alloys to determine the ability to distinguish between alloys under a

set of prescribed conditions (Table 31). Results of this examination

are shown in fig. 28 and clearly indicate that significant differences

can be shown to exist between many of the alloys, i.e. the method can

distinguish between the castability values of different alloys. It

112
Pattern

Test Portion:

Runner Bars:

Sprue

Sprue Length:

1 mm opening (No. 18) polyester sieve

cloth, 10 x 10 sq.

10 gage round wax

6 gage round

10 mm

Mold

Ring: Whip-Mix 1.5" x 2"

Liner: 1 layer, damp asbestos

Mixing Conditions: Full strength liquid (H^O - Ceramigold II)

investment manufacturers' recommended L/P

ratios vacuum mix, 400 rpm, for 1 min, vibrate 30 sec.

Setting Conditions: Hygroscopic technique, 100°F water bath, 45 min.

Burnout: 1 hour to alloy manufacturers' recommended,

temperature, 1 hour heat soak

Casti ng

Weight of Alloy: Base metal 11.5 - 13 g, Olympia, Cameo and

W-l, 12.4 g (8 DWT) Triumph 10.9 g, Jelenko "0"

10.1 g (6.5 DWT)

Machine:

Temperature:

Melt Soak Time:

Speed:

Howmedica El ectromatic , induction melting

Alloy manufacturers' recommended temperature

Alloy manufacturers' recommended time or

10 sec. if not specified

400 rpm at maximum acceleration

Crucible: Heat soaked at mold temperature for 30 min.

Pattern Orientation: Vertical

Table 31, Standard Evaluation Procedure

113
38mm

Figure 27

114
CASTAB1LITY

USING STANDARD EVALUATION PROCEDURE

WITH CERAMIGOLD INVESTMENT

u-b r

C-ll

C-M

L-B

T-C

N-P

T-R

O-C

W-1

N-D

L-C

Figure 28.

115
should be noted, however, that the results do not define what an acceptable

castability value must be; it defines the value which should be expected

for a consistently produced alloy or for an alloy used under controlled

conditions in a dental laboratory. The investigation was extended to

examine effects of different mold temperatures and alloy temperatures on

castability (figs 29 and 30). This has been reported (see Abstracts

1982, Use of a Castabil ity. . .etc. ) and a manuscript is in review for

publication. In total, the results show the dependence of the castability value C

v on temperatures and that the standard deviation (SD) is

itself a function of the castability value, being small under conditions

which render C

v near the extremes of its possible range for each alloy.

This variation of SD could be interpreted as being indicative of the

ability to consistently obtain good castings under a given set of conditions.

Because SD itself appears to be somewhat an indicator of the consistency

with which castings may be repeatedly made to the same level of quality

it can be a useful tool in conjunction with C

y for optimizing casting

condi tions

.

To determine, by regression analysis, the optimum conditions for

castability of an alloy, it is necessary to have SD as a known function

of C . An alternative approach is to find a transformation, C

y

, of C

y which will render the transform of SD, SD, independent of C . This

matter has been pursued with Mr. Harold Argentar (formerly with the

American Dental Association Health Research Foundation, then a Guest

Worker with NBS, and currently an independent consultant) who found that

standard transformations are not adequate and the transform

116
X = Jlx\

2/3- /c;

2/3-

appears to work well. An abstract has been prepared for presentation of

this at the AADR, 1983. Assuming separation of variables, one may write

1) C

v = f(TA

)g(T

m

)h(C

i

) where T

ft = (Talloy - V,^), T

m

= mold temperature and are ith concentrations of various alloy elements. Similarly,

2) C^" = F(T^)G(T

m

)H(C.) where F, G and H are independent functions.

Functions will be assumed for F, G and H, and regression analysis conducted for representation of the overall effects of the three variables,

l'A’T"[

Tr

and . A more complete discussion is planned for presentation at

the AADR, 1983.

If one assumes that the major effect of repeated castings of dental

alloys is change in C. , then effects on C

v of repeated castings should

be calculable from the functional representation of C

v (or C ) and using

before and after chemical analysis of various elements in an alloy. A

program has been devised for leading to determination of C

y (C.) via

multivariant analysis of C

y for alloys of several compositions. This

will require additional determinations of to cover effects of Si, Ga,

Be, Bo, Mm, Sn, etc. Casting studies of additional alloys are in progress

and results for Ceramalloy II will be presented. If one assumes

2 f = a+bT^+cT^ , g = d+eT

m

; and for h = constant requires that C

v = 0

__ 9

at = 0, then an expression for C

v is given by

C

v A TA

+ B T

A

2 + C Vm + D '

Other functions, of course, can be attempted for obtaining the best fits

to the data.

B. Porcelain-Alloy Stress Compatibility

Failure of porcelain fused-to-metal restorations is often attributed

117
BQ

<3,

l—

CO<o oo

h>- coO

CM

CO

CD

hCM

CD

r^- CO

LO

T—coo

00

I".

"cr o

I-'- o3*-• CO

l_

QJCLE

CD h-O

118

Figure

29.
USING

STANDARD

EVALUATION

PROCEDURE

WITH

RECOMMENDED

MOLD

TEMPERATURE

AND

INVESTMENT

ooo

CN

CD

<3

-

co

a;

hco

119

!

emperature

Degrees

Superheat

Above

Liquidus
to residual stress in porcelain following the porcelain firing cycle.

This stress has its origin in the differences in thermal expansion which

exist between porcelain and alloy throughout the firing temperature

range. However, other properties also affect the final level of stress

retention. These include thermo-viscoelastic properties (primarily of

the porcelain), thermal conductivity and temperature dependent elastic

moduli. Geometric variations in restorations are also a contributing

factor. The objective of this phase of study was to develop a technique

which can be used to experimentally confirm or indicate stress compatibility

and to back it with a firm theoretical analysis.

The experimental portion of this work utilizes the change in gap of

a porcelain veneered split metal ring to reflect the strain generated by

residual stresses. Theoretical analysis via the finite element method

is to be used to verify strain predictions by comparison with experimental

gap change calculations and to then calculate residual stresses. The

computer simulation requires the use of a number of experimentally

determined input properties as previously mentioned. Progress on this

project has proceeded in several items, 1) theoretical considerations of

the dependence of stress on mechanical properties, 2) calculation of the

extent to which experimental measurements may be affected by diffusion

bonding of porcelain and alloy (and also various porcelain layers), 3)

experimental determination of the temperature dependence of elastic

moduli, 4) construction of final FEM's (finite element models).

Item 1 :

Theoretical studies of the dependence of stress and strain (gap

changes) on mechanical properties showed that neither stress nor strain

were affected significantly (less than one percent) by choice of Poisson's

120
ratio for porcelain or alloy. Stress was greatly affected by the values

for elastic moduli which were chosen but strain was not affected to a

significant degree. A manuscript for publication has been prepared and

has cleared NBS review.

Item 2 :

The effects of diffusion bonding between different layers of porcelain and porcelain and alloy have been investigated theoretically.

Calculations were extended to investigate effects of opaque layers

having various thicknesses and thermal expansion properties and also the

affects of diffusion bonding on thin layers of glaze. A manuscript has

been accepted for publication in the Journal of Dental Research and

results are summarized as follows:

1. The strain measured on multimaterial layered split rings or flat

strips is unaffected by changes in thermal expansion when the

thickness of porcelain (t-j) + diffusion zone ^J/alloy thickness

(t^), (t-j + t^/t^ £0.8 m/0.5 mm (note that 0.025 mm is the

experimental limit of detection.)

2. Total strain decreases as t

orce i ai

- n /t

a i -j increases above t-j/tg « 1.

3. Total strain is maximized with noble alloys for t-j/tg »1. This

would also indicate that distortion of clinical porcelain-fused-tometal fixed partial denture restorations is similarly reduced by

either higher or lower ratios, t-j/tg.

4. For nonprecious alloys, total strain is always lower for t-j/t^ =** 1

.

A ratio t-j/tg ~ 0.8 mm/0.5 mm significantly reduces total strain.

This indicates that PFM restorations might also be distorted by the

condition t-j/tg »1

.

121
5. For low porcelain/alloy ratios, t-|/tg Z .2/. 5 detectable interfacially

induced strain can be produced from interfacial layers as thin as 1

to 6 if the thermal expansion of the interface is radically

changed (by a factor of two for example.)

6. For porcelain layers much thinner than alloy, t-|/tg Z 0.2/0. 5,

nonprecious alloys generate lower interfacial ly induced strain than

gold alloys.

7. For noble al loy-porcelain interfaces less than 8/xm thick, interfacially

induced strain is always lowest for equal thicknesses of porcelain

and alloy.

8. By proper choice of porcelain/alloy ratios, experimental information

on the properties of different layers might be obtained. For

example, for noble alloys, choosing t-j/t^ « 0.5/0. 5 = 1, total strain

is maximized and interfacial effects are minimized. For low porcelain/

alloy thickness ratios, interfacial effects become detectable. An

example of this would be the application of a glaze to a body

porcelain. Diffusion could change the thermal expansion coefficient

of the glaze relative to that obtained from bulk measurements of

the glaze.

9. Although proper choice of t-j/t^ may minimize interfacial ly induced

strain effects and strain correlations between theory and experiment

obtained via use of bulk properties, stress at the interface might

be significantly altered by the changes in the interface. Knowledge of the interfacial stresses would be important in the overall

determination of compatibility.

122
10. Changes in elastic moduli. Eg* of interfacial zones are expected to

produce insignificant strain effects over the range 0.3 < 3.3

(here is the modulus of the alloy).

Item 3

Because the amount of stress retention (or relaxation) which can

take place at high temperatures will depend on the elastic constants at

those temperatures it is of value to know the temperature dependence of

the elastic moduli. A sonic technique with the necessary apparatus has

been borrowed from the Fracture and Deformation Group of NBS for this

purpose. Calculations have shown that specimens ~ 5 cm x 1 cm x 0.2 cm

should provide the best response for both alloy and porcelain for measuring

shear and longitudinal resonance frequencies. Specimens have been

prepared and experiments are just beginning. Preliminary results for

Ceramco and Vita porcelains have been obtained which show the Young's

modulus at 68.2 GPa for Vita VMK68 porcelain and 69.2 GPa for Ceramco

porcelain. Increasing temperatures result in a decreasing modulus for

each. An abstract has been submitted on this for presentation at the

AADR meeting in 1983.

Item 4

A number of finite element models have been constructed for making

preliminary studies, for example: on gap change without incorporating

temperature gradients or stress relaxation and for stress distributions

under those conditions or on the effects of elastic properties on stress

and strain. None of the programs devised thus far have been able to

carry out a complete analysis with all variables accounted for.

One of the prime difficulties lay in finding (or developing) a

computer program which allowed for temperature dependent stress relaxation

123
(thermo-visco-elastic behavior) while at the same time calculating

time dependent temperature gradients. A major obstacle was the lack of

high symmetry and the need to maintain aspect ratios for each element;

the thinness of the ring resulted in an unwieldy number of elements

( ig. 31). These problems were overcome by noting that the small gap,~

1 mm, in a circumference of ~ 32 mm resulted in nearly axial symmetry.

This was utilized for calculation of radial temperature gradients for

most of the ring. Detailed element calculations were then carried out on

small sections which were required to meet temperature and equilibrium

conditions for the rest of the ring (for which temperature profiles had

been obtained from the axial symmetry calculations). A trial run has

been made with the new model and program and the program runs to completion.

The next step will be to input experimentally determined properties for

a rough comparison of theoretical and experimental gap changes. The

final steps will involve using experimentally determined boundary cooling

rates for fine tuning of the system for coincidence of calculated and

experimental gap changes.

C. Porcelain-to-Metal Bond Strength.

A simple, bending-beam-composite (BBC) was proposed late in 1981

for evaluation of the porcelain-to-metal bond strength (Fig. 32) and two

papers were presented on this at the 1982 meeting of the AADR. With the

specimen shown, up to four fractures can be obtained, there will be

either porcelain-metal bond strength (PMBS) fractures or porcelain

rupture strength fractures (PRS). According to simple beam theory, the

failure stress a RMBS Qr pRSis given by

124
125
to

<D

-oc

coE

COCD JD

>0O

-O

CD

i-CLEOo

c:

CJo

cucl

in

COo

ClEOoE

cOa

CDCDc

“Oc

CD CO

cj 00 3 •r— *o JC oo o h- •r— S- to CL 00 3: • CO o

00 00 sz

CD CD to to CD CD C

*a

T3 o c

CD <D •r— o

4-J •r- CD CD CJ 4J 4- 4- CD <J

•r— •r— 00 CD c c 00 -X LX c

•f— 4-> CD t- CO -C

sz CD i

CD

-M

C CD c CJ SO •r— S4-> o <D

CD CL JC

CSJ JC 1—*

4-> 4-

Ll_

O

C •

** CD CD

C

CD CD

U o U 3: fO

S- 4-> 4— 4-> o CD L. CJ 4— _Q 3 o

oo 00 CO 4-> C a 4—3 4- JC 4— O 4-> 4-> a m r— c c CD o o c CD * 1

CJ o JC

4-> i

4-> ro 00 CO CD •*— C •r— to • r— r— JC to CL u CD 00 CL •r— S- S~ JC 4-> o

00 +J

CD JC

CD4

4-> toE

13X

CO

-C

4->

CDCJ

S-o

G_ • •* 4-J 0J 4—3 00 C > C C cO CD O 4-> 4—3 CJ

•r—

C CO JZ *3

4-> CD c 4-> cO U E o S- CJ o CJ cn

to E JO

cO 00 c CD TJ <o

•r— c to CD CD

#-—cO •r- cu 4-5 4 a o CO 4—3 CD C 3 CJ to CJ <D T3

L- JO

O T3 CD o S-. C JC CL CO CL •*— 4-J

GJ

CJ

cO U—

L.3

CM

00

CD

S-3

CD

126
°PMBS or PRS = 6Fi( t-y)

w

where

b

3

+nc

3

+12 (b[(c+b/2)-y]

2 +nc[c/2-y] 2

)

/ t

2

+c

2

( n-1 ) \

y

= l/2i I and

t+c( n-1

)

F = 2x the applied force for the bending moments

L = the moment arm

n = Modulus of Metal/Modulus of Porcelain

Terms are according to figure 32.

The beam model stress calculations were checked by the finite

element method (FEM) and showed smooth transition in surface stress

across the interface. These calculations also showed some curvature

in the stress field in porcelain away from the interface at a few mm

from the interface. FEM calculations also confirmed that for the dimensions of specimens for which c/t ~ .3 - .5 mm, the stress is virtually

independent of c/t (fig. 33). For n > 1 the stress transition across

the interface is not only smooth, but the surface stress difference

between the maximum in metal and the minimum in porcelain is almost

equally divided by two at the interface and the transition is gradual

for several mm in the porcelain. Hence, for example, the 14% difference

(fig. 33) translates into a 7% difference at the interface and the

gradual nature of the stress transition results in an uncertainty of

only a few percent in PMBS calculations by simple beam theory.

The effects of first fracture cracks on subsequent failures is the

subject of recent, ongoing FEM calculations. The results indicate that

under totally elastic conditions, St. Venant's principle is valid and

127
VARIATION OF NORMALIZED MAXIMUM TENSILE STRESS,

d WITH THICKNESS OF PORCELAIN, b, WITH CONSTANT

BENDING MOMENT, M

Figure 33. a

f (PMBS or PFS) as a function of load, F, applied to

fixture (which provides load F/2 at each knife edge (fig. 32)

and specimen properties and goemetry. All symbols have been

defined in text.

128
for specimen dimensions which have approximately 1 cm slabs of porcelain

the fracture at one interface does not influence fracture stresses at

other interfaces. The key point, however, is that the conditions must

be elastic. Experimentally, it has been observed that stress relaxation

in the metal will cause fractures in other interfaces at lower applied

loads, F. This is interpreted as being due to higher stress being borne

by the porcelain than would be calculated by elastic theory.

The new test method has been received well by industry and three

major dental companies* have cooperated in further investigations by

supplying "visitor" workers for one week each at NBS. The results of

studies on specimens of these materials have shown the test method

capable of distinguishing (at the .05 level), between experimental

systems known to produce weaker bonding and those producing stronger

bonding. Experimental fractures near the interfaces also showed the

types of curvature predicted from the FEM. Two abstracts on this work

were submitted for the 1983 meeting of the AADR.

D. Fit and Soldering of Dental Alloy Castings

No work was done on this phase of the project due to concentration

of efforts on the preceeding sections.

* Dentsply International, York PA

Williams Gold and Refining Co., Buffalo, NY

Howmedica, Inc., Dental Div., Chicago, IL

129
REFERENCES

1. Antonucci , J. M.; Grams, C. L. and Termini, D. J.: 0. Dent. Res.

58: 1887-1899, 1979.

2. Cassel , J. M., et al . : Properties and Interactions of Oral Structures

and Restorative Materials, NBSIR 81-2431 (1981) Available from NTIS

as PB82-1955 79.

3. Antonucci, J. M.; Peckoo, R. J.; Schrull , C.; and Toth, E. E.: J.

Dent. Res. 60:1325-1361 , 1981

.

4. Ruyter, E.; and Gyorosi, P.: Scan. J. Dent. Res. 84:396-400, 1976.

5. Ruyter, E.; and Svendsen, S.: Acta. Odont. Scand. 36 75-82, 1978.

6. Craig, R. G.; and Wang. T. K.: J. Dent. Res. Spec. Issue B, 56^

B216, 1977.

7. Douglas, W. H.; Craig, R. G.; and Chen, C. J.: J. Dent. Res. Spec.

Issue A 57, 250, 1978.

8. Douglas, W. H.; Craig, R. G.; and Chen, C. J.: J. Dent. Res. 58^,

1981-1986, 1979.

9. Craig, R. G.: J. Dent. Res. 58, 1544-50, 1979.

10. Craig, R. G.; and Wang, T. K. : J. Oral Rehabil. 7^(5), 361-368, 1980.

11. Council on Dental Materials, Instruments and Equipment, New American

Dental Association Specification No. 27, for Direct Filling Resins

JADA 94, 1191-1194, 1977.

12. Kehr, C. L.; and Wszolek, W.: Organic Coatings and Plastic Chemistry,

165th National Meeting, American Chemical Society, Dallas, TX, 295-302,

1973.

13. Morgan, C. R.; Magnotta, F. and Ketley, A. D.: J. Poly Sci, 1_5,

627-645, 1977.

130
14. Brauer, G. M.; Stansbury, J. W.; and Antonucci , J. M. : 4-N,NDialkylaminophenethanols, Alkanoic Acids and Esters: New Accelerators

for Dental Composites: J. Dent. Res. 60, 1343-1348, (1981).

15. Pleuddemann, E. P., ed.: Composite Materials, Vol . 6, Interfaces in

Polymer Matrix Composites, Academic Press, New York, 1974, p. 18.

16. Kaas, R. L. and Kordos, J. L: The Interaction of Alkoxy Silane

Coupling Agents with Silica Surfaces, Poly. Eng. and Sci. 11 11-18,

1971.

17. Bowen, R. L.: Silica Resin Direct Filling Material and Method of

Preparation, U.S. Pat. 3,194,784, 1965.

18. Bascom, W. D. : Structure of Silane Adhesion Promoter Films on Glass

and Metal Surfaces, Macromol . 5_, 792-798, 1972.

19. Inoue, H.; and Kohama, S.: Effect of Polymer Silane Coupling Agents

on Adhesion of Polypropylene and Polystyrene to Glass, J. Appl . Poly.

Sci. , 19, 1939-1954, 1975.

20. Ishida, H.; and Koenig, J. L.: Fourier Transform Infrared Spectroscopic

Study of the Silane Coupling Agent/Porous Silica Interface,

J. Colloid and Interface Sci., 64_, 555-564, 1978.

21. Koelling, J. G. ; and Kolb, K. G.: Infrared Study of Reaction Between

A1 koxysi lanes and Silica, Chem. Commun. (London), Nov. 6-7, 1965.

22. Erickson, P. W.: Silane Treatment of Glass Fabric from Nonaqueous

Solvents, Annual Tec. Conf. Soc. Plast. Ind., Reinf. Plast/Campus

Div., Proc. 24th 19-B-1-10, 1969.

23. Wilson, A. D.: "Dental Cements - General" in Scientific Aspects of

Dental Materials" ed. von Fraunhofer, J. A., Butterworths , London

and Boston, p. 131 (1975).

131
24. Brauer, G. M.: A Review of Zinc Oxide Eugenol Type Filling Materials

and Cements, Rev. Beige. Med. Dent. - Belg. Tijds, vr Tandheelk,

20, 323, Nov. 1965.

25. Brauer, G. M. : Cements from Metal Oxides and Chelating Agents, U.S.

Patent No. 2,936,242, May 1960.

26. Civjan, S., and Brauer, G. M. : Physical Properties of Cements,

Based on Zinc Oxide, Hydrogenated Rosin, o-Ethoxybenzoic acid, and

Eugenol. J. Dent. Res. 43_, 281, March-April 1964.

27. Brauer, G. M. : Cements Containing o-ethoxybenzoic acid (EBA). In:

Dental Materials Research (ed. Dickson, G. and Cassel , J. M.) Proc.

50th Anniversary Symp., Nat. Bur. Stds, 1969, NBS Spec. Pub. 354,

1972, pp 101-111.

28. Thompson, D. T.; Vogel, J. and Phillips, P. H.: Certain Organic

Substances and Their Incidence of Dental Caries in the Cotton Rat.

J. Dent. Res. 44, 596-599 (1965).

29. Lee, H. L.; Swartz, M. L.; and Culp, G. : Static Load Testing of

Dental Adhesives, J. Dent. Res. 48:211-216, 1969.

30. Rohm and Haas, Pamphlet CM-46, Reactive Diluent QM-657.

31. Cowan, J. C.; and Teeter, H. M.: Ind. Eng. Chem. 36, 146-152, 1944

32. Paquot, C.; Parron, R. ; and Vassi 1 1 ieres , C.: Bull. Soc. Chem.

France, 317-319, 1959.

33. Economy, J.; Mason, J. H.; and Wohrer, L. C.: J. Poly. Sci . Part A1, 8; 2231-2244, 1970.

34. Brauer, G. M.; Jackson, J. A.; and Termini, D. J.: J. Dent. Res.

58, 1900-1907, 1979.

35. Buck, C. J.: U.S. patent 3,975,422, 1976; 4,003,942, 1977; 4,012,402,

1977; 4, 013, 703, 1977; 4,041,062, 1977; 4,041,063, 1977.

132
36. Kotev, D. M.; Novakov, P. C.; and Kabaivanov, V. W.; Angew Makromol . Chem. 92: 41-52, 1980.

37. Dombroski, J. R.; and Stoakley, D. M. , U.S. patent 4,136,138, January

23, 1979.

38. Stoakley, D. M. et al . : U.S. patent 4,134,929 (1979).

39. Wu. W.; Toth, E. E. ; Moffa, J.; and Ellison, J.: Subsurface Damage Layer

of in vivo Worn Dental Composite Restorations, submitted to J. Dent. Res.

40. Wu. W.; Pestaner, J.; and Bowen, R. : Softening of BIS-GMA Base Polymer

by Chemicals with Different Solubility Parameters, Proc. of Sym. on

Composite Resins in Posterior Teeth, Oct. 1982.

41. FDA Guidelines for Chemistry and Technology Requirements of Indirect

Food Additive Petitions, March 1976.

42. Burrell, H.: Solubility Parameter Values, in Polymer Handbood, 2nd Ed.,

Brandrup, J. and Immergut, E. H., eds, Wiley and Sons, New York, 1975,

Section IV-337.

43. McKinney, J. E. : Apparatus for Wear of Dental Restorative Materials,

Wear 76, 337-347 (1982).

44. McKinney, J. E.; and Wu, W.: Relationship Between Subsurface Damage

and Wear of Dental Restorative Composites. J. Dent. Res. 61 -9

,

1083-1088 (1982).

133
Invited Talks (1982)

Initiator-Accelerator Systems for Dental Application. G. M. Brauer,

American Cehmical Society Symposium, Las Vegas, NV

A Dental and Medical Materials Program at NBS. J. M. Cassel , Johns

Hopkins University, Baltimore, MD

Wear Mechanisms in Dental Composites. W. Wu, Symposium on Materials,

American Association of Dental Schools, New Orleans, LA.

A Small Angle X-ray Study of Particulate Reinforced Composites. W. Wu,

International Union of Pure and Applied Chemistry, Macromolecules Session,

Amherst, MA

Castability and Fit of Nonprecious Alloys. J. A. Tesk, Eastern Maryland

Dental Society, Ocean City, MD

Recent Developments in Metals as Dental Bio/Alloys. J. A. Tesk,

R. W. Hinman1

, R. M. Waterstrat2 and W. deRijk2

, Symposium on Recent

Developments in Biological Alloys, AIME, Dallas, TX

1 USN/NBS Research Associate

2 ADA/NBS Research Associate

134
ABSTRACTS IADR/AADR MEETING, MARCH 82 , NEW ORLEANS (ON MICROFILM):

A Quantitative/Expedient Porcelain-to-Metal Bond Test, Part I: Theoretical

Basis. J. A. Tesk, G.E.O.Widera

1

, R. W. Hinmarf, W. deRijk3

.

Cements Containing Vanillate Esters, EBA and Zinc Oxide. G. M. Brauer

and J. W. Stansbury.

Use of a Castability Test for Optimizing Mold and Casting Temperature.

R. W. Hinman, J. A. Tesk, R. P. Whitlock, E. E. Parr^, and J. S. Durkowski

4

,

Dental Resin and Initiator Systems Based on Polythiols. J. M. Antonucci

,

J. W. Stansbury and D. J. Dudderar.

Subsurface Wear Defects Analysis of Both In Vivo and In Vitro Dental

Restorative Composites. W. Wu, E. E. Toth and J. P. Moffat

Influence of Chemical Softening on Wear of Three Types of Dental Composites.

J. E. McKinney and W. Wu.

A Quantitative/Expedient Porcelain-To-Metal Bond Test, Part II. Preliminary

Results. W. G. deRijk, J. A. Tesk, R. W. Himnan, E. E. Parry, M. Conner6

, G. E. 0. Widera.

Effect of Thermal Parameters on Compatibility in Porcelain Veneered Split

Metal Rings. G. E. 0. Widera, N. Munir7

, J. A. Tesk, R. W. Hinman.

1 University of Illinois

2 USN/NBS Research Associate

3 ADA/NBS Research Associate

4 USN

5 USPHS Hospital, San Francisco, CA

6 Guest Worker

7 Private Consultant

135
ABSTRACTS SUBMITTED FOR 1983 AADR MEETING

Castability of Dental Alloys, Repeated Casting, Mold and Alloy Temeprature

Effect. S. Hirano1

, J. A. Tesk, and R. W. Hinman^

Dental Cements Containing Hexyl Syringates. G. M. Brauer and J. W. Stansbury.

Divanil lates and Polymerizable Vanillates as Ingredients of Dental Cements.

J. W. Stansbury, G. M. Brauer and J. M. Antonucci

.

Effect of Degree of Cure on Hardness and Wear of Three Commercial Dental

Composites. J. E. McKinney and W. Wu.

Effect of Elastic Modulus on Stresses in Ceramo-metal Systems. J. A. Tesk,

R. W. Hinman, W. deRijk3

, H. R. Kase4

.

Elastic Constants of Two Dental Porcelains, H. R. KSse, E. Case5

, and

J. A. Tesk.

Factors Controlling the Degree of Polymerization of Dental Resins. W. Wu.

Formulation and Evaluation of Fluorine-Containing Composite Resins.

J. M. Antonucci, S. Venz6

, G. M. Brauer, J. W. Stansbury and D. J. Dudderar.

Functional Representation of a Castability Value, C , for Dental Alloys.

R. W. Hinman, J. A. Tesk, H. Argentar7 and S. Hirano:

Microleakage and Water Sorption of Hydrophilic and Hydrophobic Dental

Composites. S. Venz, N. W. Rupp3 and J. M. Antonucci.

Porcelain to Metal Bond Strength Measurements as Determined by the Four

Point Bending of a Composite Beam. W. G. deRijk3

, J. A. Tesk, M. ConnorSand

K. A. Jennings8.

Stress Analysis of Bond Test Specimen. G. E. 0. Widera8

, J. A. Tesk,

W. J. deRijk, and R. W. Hinman.

ADDITIONAL ABSTRACTS (FY83)

Acrylic Resin Compatible Adhesive Cements. G. M. Brauer and J. W. Stansbury,

Table Clinic, Maryland State Dental Association Annual Meeting, Baltimore,

MD, Oct. 7-10, 1982

A SANS Investigation Into the Plastic Deformation Mechanisms of Polyethylene. W. Wu. American Physical Society, March 1983.

Beryllium, The Need for and Uses in Dental Alloys. J. A. Tesk, Invited for

Symposium on Non Precious Alloys, American Association of Dental Schools,

March 1983.

Tsurumi Univ., Japan

NBS/USN Research Associate

ADA/NBS Research Associate

5 Univ. Gottingen, Germany

NBS, Frac. & Defor. Div.

6 Freie Univ. Berlin

7 Guest Worker, former ADA

8 Guest Worker (NIH)

9 Univ. of Illinois

136
PAPERS PUBLISHED (FY82)

1. Hardening Shrinkage of Hydroscopic Expansion of Composite Resins.

R. L. Bowen, J. Rapson and G. Dickson. J. Dent. Res. 61 (5) , 654-60

(1982).

2. Relationship Between Subsurface Damage and Wear of Dental Restorative

Composites. J. E. McKinney and W. Wu. J. Dent. Res. 6J_( 9 ) , 1083-88

(1982).

3. Apparatus for Measuring Wear of Dental Restorative Materials.

J. E. McKinney, Wear 76_, 337-47 (1982).

4. Influence of Filling Material and Silanization Method on Strength and

Marginal Adaptation of Experimental Composites. K. Dermann, G. M. Brauer,

and N. W. Rupp. Deutsch Zahnarztl . A. 37^, 160-163 (1982).

MANUSCRIPTS IN REVIEW OR ACCEPTED FOR PUBLICATION

1. Small Angle Neutron Scattering of Partially Segregated Polymer Blends.

W. Wu, accepted for publication in Polymer.

2. Application of Small Angle X-ray Technique in Damaged Particulate

Reinforced Composites - A Theoretical Treatment, W. Wu, accepted for

publication in Polymer.

3. Effects of Porcelain/Al loy Interfacial Diffusion Zones on Thermomechanical Strain. J. A. Tesk, R. W. Hinman, G. E. 0. Widera,

A. D. Holmes and J. M. Cassel . Accepted for publication in J. Dental

Research.

4. Physical Metallurgy of Dental Casting Alloys, Parts C. Cr-Co-Mo

System, D. Ni-Cr System, E. Fe and Austenitic Stainless Steel, and

F. Titanium Alloys. J. A. Tesk and R. M. Waterstrat. Accepted

for publication in Encyclopedia of Materials Science and Engineering.

5. Solders and Fluxes. 1st Revision, J. A. Tesk. Accepted for 2nd

edition of Dentists Desk Reference.

6. Subsurface Damage Layer of In Vivo Worn Dental Composite Restorations.

W. Wu, E. E. Toth, J. F. Moffa, J. A. Ellison, to be submitted to

J. Dent. Res.

7. Post Curing of Dental Restorative Resins, W. Wu and B. M. Fanconi

,

to be submitted to Polymer Science & Engineering.

3. Development of High Strength Acrylic Resin-Composite Adhesive Cements

G. M. Brauer, J. W. Stansbury, accepted for publication in J. Dent. Res.

137
9.

Influence of Chemicals In vitro Wear of Dental Restorative Materials.

W. Wu and J. E. McKinney, accepted for publication in J. Dent. Res.

10. Wear Mechanisms of Dental Composite Restorations. W. Wu. Submitted

to Polymer.

11. Solvent Effects on Bonding Organosilane to Silica Surfaces. T. M. Chen

and G. M. Brauer. Submitted to J. Dent. Res.

12. Effect of Hygrophilic Diluents on the Properties of Cured Composites.

K. Dermann, N. W. Rupp and G. M. Brauer. Accepted for publication

in J. Dent. Res.

13. Intraoral Secondary Radiation Dose from Neutron Activated Dental

Materials. J. A. Tesk, R. P. Whitlock and A. D. Holmes. To be

submitted to Medical Physics.

14. Theoretical /Experimental Studies of Stress Compatibility in Porcelain

Veneered Split Metal Rings. G. E. 0. Widera, J. A. Tesk, R. W. Hinman,

and N. Munir. To be submitted to J. Biomedical Materials Research.

15. Fit of Multiple Unit Fixed Partial Denture Castings. R. W. Hinman,

J. A. Tesk, E. E. Parry and G. T. Eden. To be submitted to Journal

of Prosthetic Dentistry.

16. A Practical Method for Evaluating the Castability of Dental Alloys.

R. W. Hinman, J. A. Tesk, R. P. Whitlock, E. E. Parry and J. J. Durkowski

to be submitted to J. Dental Research.

138
NBS-1 14A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2623

2. Performing Organ. Report No. 3. Publication Date

4. TITLE AND SUBTITLE

Properties and Interactions of Oral Structures and Restorative Materials

5. AUTHOR(S)

J. M. Cassel , J. A. Tesk, 6. M. Brauer, J. M. Antonucci , W. Wu, and J. M. McKinney

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. Y01 -DE-4001

5

8. Type of Report & Period Covered

Annual

10/1/81-9/30/82

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

National Institute for Dental Research

National Institutes of Health

Westwood Building, Room 506

Bethesda, MD 20205

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

Dental cements based on esters of vanillic acid display excellent long term solubility characteristics . _ Modification of formulations yield uniquely high strength

properties and the potential for high strength intermediate restorative resins. Pol - meric formulations designed to reduce curing shrinkage, residual unsaturation and

hydrophil icity in composites have been developed. A high molecular weight polythiol,

pentaerythritol tetra(3-mercaptopropionate) , contributes toward this goal and improves

color and color stability. Deterioration of composite restorative materials exposed to

solvents simulating food debris is maximal in 75% ethanol-water solutions as judged by

silver stain examination, changes in surface hardness and wear loss. The resin matrix

as cured at 37°C has a low degree of polymerization (DP) and is susceptible to softening

by penetrating molecules. Infrared spectroscopy revealed that the DP in a composite is

more sensitive to monomer composition than to initiator type or concentration. The rate

of solvent-induced softening of composites depends on the cure temperature. A bending

beam porcelain-metal specimen was designed. Initial four point bending results obtained

in collaboration with dental manufacturers indicate the potential for a simple, reproducible, informative test. Alloy castability values generated in a new test method are strongly dependent on mold and alloy temperatures. Data variance can be used to optimize casting conditions.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Castability; cements; compatibility; composites; polymerization;

and wear resistance.

13. AVAILABILITY

porcelain/alloy;

14. NO. OF

PRINTED PAGES

1x3 Unlimited For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

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