NBSIR 81-2431

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

Octocer 1, 1980 thru September 30, 1981

Issued December 1981

Interagency Agreement

YOl-DE-4001 5-1

1

Qt

ICO

• Sl'2451 W• i i ed for National Institute of Dental Research

Bethesda, MD 20014

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NBSIR 81 -2431

PROPERTIES AND INTERACTIONS OF

ORAL STRUCTURES AND RESTORATIVE

MATERIALS

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

W. Wu, J. E. McKinney, and R. W. Hinman

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 , 1 980 thru September 30, 1 981

Issued December 1981

Interagency Agreement

YOI-DE-4001 5-11

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 i i

INTRODUCTION x

TECHNICAL REPORT

I. Dental Composite, Resin, Sealant and Cement Chemistry and

Development 1

A. New Monomers for Reducing Polymerization Shrinkage 1

1. Diurea Dimethacryl ate Monomer 1

2. Bulky Monofunctional Diluent Monomers 3

3. Hydrophobic BIS-GMA Type Monomers 5

4. Non-Vinyl (Polythiol) Reactive Types 7

5. Expanding Monomer Types 10

B. Development of Ifoproved Initiator/Accelerator 10

Systems

1. Initiator Research 10

2. Amine Accelerators 11

3. Non-Amine Accelerators 15

4. Vacuum Polymerization of Composite Resins 18

C. Analysis for Degree of Polymerization of Dental 19

Resi ns

D. Resin/Filler Coupling Improvement 31

E. Microporous Glass as Reinforcing Filler for 36

Composites and Denture Base Systems

F. Improving Dental Cements 38

1. Vani 1 1 ate-Contai ni ng Cements 38

2. New Polybasic Acid-Derived Cements 56

3. Polyvinyl Phosphoric Acid Cements 58
Page

II. Development of Adhesive Bonding Techniques 61

A. Mineralization of Dentin as an Adhesive- 61

Developing Technique

B. Cyanoacrylates 74

C. "Reactive Adhesive" Formulations 82

III. Wear Resistance and Mechanical Properties of Dental 84

Composites

A. Pin and Disc Wear and Related Knoop Hardness 86

Number (KHN) Measurements

1. Instrumentation 86

2. Influence of Silanization Treatment on Wear 93

3. Mechanically Induced Damage and Wear on 93

Composite Restorations

4. Chemical Softening of Dental Composite 98

B. Environmental Effect on Physical Properties of 111

Dental Composite Restoratives

1. Changes (Compressive Strength and KHN) in

Composites Induced by Immersion in Chemical

Media and by Thermocycling 111

2. Investigation of in vivo Wear Mechanisms of

Dental Composite Restorations 117

IV. Dental Alloys and Ceramics 121

A. Porcelain/Alloy Compatibility: Residual Stress 122

B. Porcelain-to-Al loy Bend Strengths 126

C. Castability Evaluation 127

D. Fit and Soldering 128

E. Fluorescing Agents for Dental Porcelain 129

REFERENCES 130

INVITED TALKS (FY 81) 136

ABSTRACTS IADR/AADR MEETING, MARCH 81, CHICAGO, ON MICROFILM 137
PAPERS PUBLISHED (FY 81)

MANUSCRIPTS IN REVIEW

ABSTRACTS SUBMITTED FOR 1982 MEETINGS

Page

138

139

140

ABSTRACT

Significant accomplishments in the field of dental materials research

for the past year include:

(1) New dental cements, based on esters of vanillic acid or its

isomers, that overcome major shortcomings of zinc oxide-eugenol cements have

been formulated. Noninhibition of the polymerization of dental methacrylate

resins that they contact, along with improved strength and solubility

properties are advantages gained with the new materials.

(2) A Fourier Transform IR spectroscopic examination of the cyanoacrylatepurified collagen (dry) interaction indicates that the high initial bond

strength of cyanoacrylates to dentin is accomplished through secondary bond

formation which by nature will tend in time to be broken by water.

(3) A very close relationship was demonstrated between the softening of

composites by chemical solvents selected for food simulation and composite wear

performance as measured with a classical pin (stainless steel) on disc (composite) wear apparatus. A correlation was demonstrated between wear, after

immersion of the composites in ethanol/water solutions, and the change in

hardness of the composite resin during its immersion.

(4) Thin section, optical, transmission microscopy, applied to clinically worn composites after silver staining for contrast, revealed that

defects within the damaged sublayer beneath the wearing surface were distributed throughout the polymer matrix rather than confined to the filler/matrix

i nterface.

(5) A theoretical development of small angle x-ray scattering (SAXS) as applied to three phase (filler, matrix, microcrack) systems permitted the

volume concentration and characteristic length of microcracks in both i_n vi tro

and j_n vivo worn composite specimens to be determined.

(6) Theoretical calculations showed that for the porcelain/alloy

thickness ratios (i.e., ^ 1) being employed experimentally, interfacial

effects could be neglected for the split ring gap change analysis and a knowledge of a precise value for Poisson's ratio was not required.
SUMMARY

Efforts to provide resin systems for composites that shrink minimally

on polymerization demonstrated that composites with good properties could

be obtained using the very bulky diurea dimethacrylate reaction product of

N-t-butyl -2-ami noethyl methacrylate and hexamethyl ene di isocyanate. Similarly

we have demonstrated useful composite formulations employing a bulky monofunctional tricylic aliphatic methacrylate. To produce hydrophobic BIS-GMA

type monomers we have reacted highly fluorinated alcohols with di epoxy

compounds to produce diols that in turn are convertible to diacrylate

monomers as desired. Initial results with bulky polythiols, i.e., R(CI-L-SH)

u X

with x ^ 2 indicate that these compounds may be useful with non-amine

accelerated systems as well as with the conventional amine-peroxide type.

Three amines, p-(dimethyl amino) phenethanol , p-(diethylamino) phenyl

acetic acid and its ethyl ester, shown previously to be promising fast

acting accelerators for dental composites, were synthesized in greater

quantities and offered to cooperating laboratories to determine their

toxicological and biocompatible characteristics. The p- (dimethyl ami no)

phenethanol, synthesized and evaluated in this laboratory is now available

from a chemical manufacturer.

Some promising leads have developed for improving the storage stability

of i ni tiator-accel erator components of composite formulations where the

non-amine accelerator, ascorbyl palmitate (AP) is employed. This promising

accelerator creates no storage stability problems in powder- 1 i quid formulations when it is coated on the surface of the powder. By employing a

combination of bisulfite and di hydroxymal eic acid we have retained the

original setting time after three months in formulations where AP is dissolved

in the monomer for paste/paste application.
A non-spectroscopic method, differential scanning calorimetry (DSC),

has been demonstrated to be a useful additional tool for assessing the

extent of polymerization (EP) in dental resin systems. Since the EP is a

major controlling factor in developing the properties of the polymerizing

system and because traditional IR analytical methods may be hampered by

strong interference in the vinyl absorption region, the demonstration that

DSC offers a comparatively facile and rapid measurement for EP as well as a

technique for determining the effectiveness of inhibitors should be of

interest to composite resin developers.

Primary amines were shown to be the most effective, aromatic and

heterocyclic amines the least effective, as catalysts for the silane coupling

agent treatment of composite filler particles. Incorporation of one fluorinated silane agent into the normal silane coupling treatment of filler

particles did not reduce water sorption of the resulting composites to the

extent expected but indicated a potential means for gaining a whiter composite

with better color stability.

A new dental cement has been formulated that overcomes a major shortcoming of zinc oxide-eugenol cements. The cements, based on esters of

vanillic acid or its isomers, do not inhibit the polymerization of composite

or resin formulations placed in contact as do zinc oxide-eugenol cements.

Significant further advantages are evidenced by the strength and decreased

rate of disintegration in water. Because of the potential usefulness of

these new cement formulations a "patent disclosure" has been prepared.

To capitalize on the Guest Worker participation of Dr. Derrick Beech,

Director of the Australian Dental Standards Laboratory and internationally

recognized for his work on development of adhesive techniques for dental

application, various mineralization pretreatments of dentin and the mechanisms

-ii-i
governing the interaction of cyanoacrylates with dentin surfaces have been

intensively examined. Results point to the importance of the role of cementcontaining cations in the bonding of polycarboxyl ate cements to dentin.

Calcium when precipitated on dentin as calcium phosphate (Ca^PO^)^) was

shown to give significant improvement to the bonding achieved with either of

the commercial polycarboxyl ate cements that were examined. Other precipitated metal phosphates generally produced a slight decrease in bond strength

with a cement containing Sn++

or Zn

+^" ions. The latter ion as the cation

of precipitated phospate is as effective as Ca++ while Sn++

has little

effect. If Sn++

is not present in the polycarboxyl ate cement, Sn ++

on the

tooth surface many diffuse into the cement with concomitant loss of beneficial

effects.

Fourier Transform infrared spectroscopy did not reveal evidence of any

new bond formation when a cyanoacrylates were mixed with purified dry

collagen. This evidence for a lack of primary bonding, combined with the

instability to water of the initially strongly adhering cyanoacrylate-dentin

interface, suggested the following explanation for this interaction:

cyanoacrylates polymerize rapidly by initiation at the tooth surface (good

adaptation, intimate contact); polymerize only in thin films (minimal polymerization stresses) and are very polar polymers (hydrogen bonding and dipole

interactions with the collagen of dentin). Further, if adhesion is via secondary bonding only, then the deterioration of this bonding by water will

not be affected by improving the resistance of the polymer to degradation.

Significant progress has been made toward the objective to develop, or

make use of, appropriate techniques to measure wear resistance and mechanical

properties of dental materials and to use the data obtained as performance

indicators to predict i_n vi vo durability of composite restorations. A

iv
classical pin on disc apparatus and an erosion technique provided basic

information relevant and possibly essential to the development of an appropriate durability test. Complementary efforts involved microdefect analysis

using x-ray and optical microscopic observation of damaged regions made

resolute by a silver staining technique. It is asserted that for development

of a relevant wear test, the subsurface damage resulting from i_n vi tro wear

should resemble that obtained from i_n vivo worn specimens.

A procedure for very careful calibration of the pin-on-disc apparatus

has been developed that allows more precise interpretation of the data. A

similarity in the results with dental composites to that experienced elsewhere with ultra high molecular weight polyethylene (the very durable plastic

component of orthopedic prostheses) has been noted in that distinct regions

of wear phenomena were evidenced by significant changes in the slopes of

track depths (volume losses) vs track length or time of test. The initial

relatively slow wear rate period, interpreted as being dominated by adhesion

and/or abrasion mechanisms, was followed by an increase in subsurface damage

to the point where severe breakdown occurs from fatigue. The occurence of

the onset of a fatigue mode in the wear process has occurred at earlier

times as the stress levels applied in the test were increased. Silver

staining microdefect analysis of specimens removed from the wear process at

the different stages of wear revealed essentially no subsurface damage in

the initial (adhesive and/or abrasive) wear region but a considerable damage

in the region where the fatigue mode of deterioration has been added to the

wear process.

A very close relationship was shown between the softening of composites

by chemical solvents selected for food simulation and their wear performance.

The solubility parameters (Hildebrand theory) of various solvents

v
and BIS-GMA have been determined. The unfilled polymer is shown to have a

solubility parameter with components 7 < 6^ ^ 9 and 2 ^ 6

a ^ 16, the wide

spread range of 5 being attributed to the complexity of the chemical

Q

structure of this monomer molecule. The parameters 5^ and 6 are the

dispersion and the association components of the solubility parameter

3 \ respectively, the units of these parameters are in (cal/cm )

. Measurements

of Knoop Hardness and initial wear data e.g., track depths after only five

cycles, showed that significant attack of BIS-GMA occurred in ethanol/water

solutions as well as in pure ethanol. In fact the degradative effect as

judged by initial wear data was significantly greater for a concentration

of 75 percent ethanol in water than for 100 percent ethanol. The reversal

in the order of the rates of wear at much longer time periods is attributed

to differences in rates of diffusion of the immersion media. A correlation

was obtained between wear, after immersion of the composite specimens in

different ethanol/water sol utions

, “and the change in hardness (KHN) of

BIS-GMA as a result of similar immersion. Although the dependence of both

wear and hardness rates were not monotonic with concentration, the dependence

of wear on the hardness ratio is monotonic. Preliminary results with a

visible light cured composite indicate that while its surface hardness

decreased as the concentration of ethanol in water increased, the wear

resistance remained good in contrast to the conventionally cured composites

used as control

.

A strontium glass containing composite was found to be much more

susceptible to surface deterioration by acids than composites with conventional quartz or glass filler. Thermocycling did not significantly influence

the physical properties of any of the composites.
Thin section, optical transmission microscopy of clinically worn

composites, silver stained for contrast, revealed that defects within the

damaged sublayer beneath the wearing surfaces were located throughout the

polymer matrix rather than being confined at the filler/matrix interfaces.

A theoretical treatment of small angle x-ray scattering (SAXS) applied

to three phase (filler, matrix and microcrack) composite systems was completed

for presentation to an American Physical Society and IUPAC meetings in

1982. By SAXS analysis the volume concentration and characteristic length

of the microcracks in the damaged sublayer regions of compression-fatigued,

i n vitro specimens differed from that of specimens recovered after clinical

use.

The lack of fundamental knowledge regarding nonprecious and low precious

metal alloys as alternatives to gold alloys for fixed partial dentures, has

resulted in problems encountered clinically and in the dental laboratory.

These problems have dictated the need for examination of the many factors

involved, particularly in the porcelain fused to metal application. Residual

stress in porcelain from cooling after firing is considered a primary

factor leading to failures in the porcelain to metal bonding; precise

universally agreed- to techniques for assessing porcelain/alloy compatibility

do not yet exist. As a first step toward assessing compatibility and as a

technique which might find acceptance as a standard method of evaluation,

the analysis of gap changes in a porcelain veneered split metal ring subjected

to laboratory firing schedules has continued. Theoretical calculations

showed that the knowledge of a precise value for Poisson's ratio is not

critical to either evaluation of stress or correlation of theoretically

predicted gap changes with those determined experimentally. Calculated

residual stress is, however, very dependent on the elastic modulus value
and its determination for different porcelains is being planned for. Analysis

showed quite clearly that no one-to-one correspondence in magnitudes or locations of maximum shear, tensile (compressive), tangential and radial stresses

exist; this is important in consideration of stress relaxation and changes of

the glass transition temperature as affected by cooling and stress.

The sensitivity of experimental measurements of split ring gap change

to changes in properties of the procel ai n-metal interface was addressed. A

theoretical study showed that while the porcelain/alloy thickness ratios

being employed experimentally, i.e., t

p 0rce -| ai- p /t

a

-| ] 0y

= interfacial

effects could be neglected, there are other conditions where this is not so.

A prime property needed for theoretical analysis of the split ring gap

change is the temperature dependence of the shear viscosity (for stress

relaxation/retention near Tg). This property was determined for several

dental body porcelains. Although the viscosity itself was not measured

directly, a close correlation was found between the strain point (found by

others) and the temperature (obtained upon heating) of maximum rate of

recovery of quenched excess volume for previously, rapidly cooled expansion

specimens.

Studies were initiated to determine how precisely heat transfer properties

are needed and to investigate other properties that may be deemed necessary

for the complete analysis of split ring gap changes.

Much proposed testing of porcelain/alloy bond strengths utilizes testing

of shear properties whereas failure is best reflected in tensile properties.

Further, preparation of specimens for shear testing is often time consuming.

A test is under development that utilizes the principle of developing

uniform tensile stresses in layers of a uniform cross-section beam subjected

V'i'i'i
to pure bending. The ease of specimen preparation and the non-sensitivity

of results to porcelain thickness render the test design and methodology

potentially attractive as a generalized adhesive bonding test.

Further development of castability test procedure employing a polyester

grid pattern for determining an alloy castability value, CV, occurred (the

percentage of completed cast segments is referred to as the castability value,

CV). Preliminary results indicate that the optimum casting temperature

(superheat) and mold temperature obtained by this method can differ from the

manufacturer' s instructions. Optimum casting conditions are accompanied by

higher precision (predictability i.e., smaller standard deviation) for the

CV.

A prime consideration in casting of dental alloys involves obtaining a

satisfactory clinical fit of multiple unit, fixed, partial dentures (FPD's).

This is of particular concern for nonprecious alloys which as a class are more

difficult to join single units together by soldering. A study on factors

affecting fit showed quite clearly that the use of an all wax sprui ng-runner

bar system produced less distortion (better fit) than a pattern which is

comprised of mixed materials, in wax and plastic.
INTRODUCTION

The National Bureau of Standards is primarily a measurement organization.

Paths for 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 commerical production by industry.

The dental research program at NBS is a cooperative effort both in

personnel and funding, involving the American Dental Association and the U.

S. Army and Navy Medical R&D Commands. 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--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 four major areas of research as approved in the

interagency agreement are given below:

I. Dental Composite, Resin, Sealant and Cement Chemistry and

Development

II. Development of Adhesive Bonding Techniques

III. Wear Resistance and Mechanical Properties of Dental

Composite Restorative Materials

IV. Dental Alloys and Ceramics
I. Dental Composite, Resin, Sealant, and Cement Chemistry and Development

A. New Monomers for Reducing Polymerization Shrinkage

1. Diurea Dimethacryl ate Monomer

The synthesis of the diurea dimethacryl ate , DTBAEM-HDI, from

the reaction of N-t-butyl -2-ami noethyl methacrylate (TBAEM) and hexamethyl ene

di isocyanate (HDI) was described in a previous report [1]. This very bulky

white, waxy solid monomer has a molecular weight of 538 vs 512 for BIS-GMA.

It has two tri-substituted urea groups, each with a bulky t-butyl substituent.

Employing diluent monomers commonly used with BIS-GMA, formulations

giving adequate setting times and yielding composites with very good color

stability and diametral tensile strengths well above the ADA specification

minimum (34 MPa) were produced (Table 1).

Water sorption measurements of comparable formulations (5A, 5B of

2

Table 1) indicated that the DTBAEM-HDI composite sorbed 0.77 ± 0.01 mg/cm

2 vs 0.55 ± 0.01 mg/cm for the BIS-GMA composite. The water sorption could

likely be lowered by increasing the fraction of DMGDMA employed or by incorporating a more hydrophobic diluent monomer.

A second diurea dimethacrylate with a MW of 592 was synthesized from

TBAEM and the alicyclic di i socyanate , isophorone diisocyanate ( I PD I ) . This

extremely bulky monomer

C

o

CH

3

0

H

2

C=C— c-o—ch

2—ch

2—n—c— nh -CH

2 — NH-C-N-CH

2 —CH

2

-0-C-C=CH

2

CH

3

(CH

3

) 3

-c 0 c (CH

3

) 3

CH

3

(CH

3

) 2

DTBAEM-IPDI

1
TABLE 1

Setting times and 24 hour diametral tensile strengths of

experimental composites* based on DTBAEM-HDI

Diametral Tensile Strength

Formulation

Monomer

System Wt.%

Setting Time

(min.

)

± Std. Dev. in MPa

(No. of Specimens)

1 TBAEM-HDI 77.0 3.0 43.7 ± 2.5 (5)

NPGDMA 22.7

DHPPT

3

2 TBAEM-HDI 77.0 2.5 42.6 ± 3.1 (4)

NPGDMA 22.6

DHPPT 0.37

3** TBAEM-HDI 75.0 2.0 39.6 ± 3.0 (5)

HMGDMA

C 24.7

DHPPT 0.3

4 TBAEM-HDI 75.0 2.0 44.6 ± 3.9 (4)

HMGDMA 24.7

DHPPT 0.3

5A TBAEM-HDI 85.0 3.5 45.2 ± 3.5 (5)

DMGDMA

0

14.7

DHPPT 0.3

5B BIS-GMA 85.0 2.0 . 44.9 ± 2.9 (6)

DMGDMA 14.7

DHPPT 0.3

6 TBAEM-HDI 82.0 3.5 37.6 ± 2.2 (5)

TEGDMA 17.9

DMSX

e 0.13

7 *** Same as above

8 TBAEM-HDI 70.0 1.5 44.8 ± 1.4 (5)

TEGDMA

1 29.8

DMSX 0.23

9 TBAEM-HDI 80.0 2.2 42.2 ± 3.4 (5)

TEGDMA 19.7

DHPPT 0.3

10 TBAEM-HDI 80.0 1.5 46.4 ± 1.5 (5)

TEGDMA 19.6

DHPPT 0.4

* Powder/liquid formulations used with P/L = 3 except where noted. Powder is

usual glass filler silanized and coated with 1% benzoyl peroxide (BP). ** P/L =2.5

*** P/L = 2.5 and blended glass filler used (equal parts 1% BP and 0.3% M glass)

3

DHPPT = N ,N-di-(2-hydroxypropyl )-p-tol uidi ne °NPGDMA = Neopentyl glycol dimethacrylate

^MMGDMA = Hexamethyl eneglycol di methacrylate

a DMGDMA = Decamethyl eneglycol dimethacrylate

3

DMSX = N,N-dimethyl-sym-xyl idine

r TEGDMA = Triethyl eneglycol dimethacrylate

2
is a viscous liquid (rather than a solid as DTAEM-HDI) and should, therefore,

yield stable resin formulations with more types of diluent monomers. On

mixing DTBAEM-IPDI with triethylene glycol dimethacrylate to give a 4:1

solution, we observed no evidence of phase separation over a period of several

weeks. Using 0.2% N,N-dimethyl-3,5-xylidine (DMSX) as the amine accelerator,

a powder liquid formulation (P/L = 3) employing silanized glass filler

containing 1% benzoyl peroxide evidenced a setting time of three minutes and

resulted in a composite with a very white appearance. The diametral tensile

and compressive strengths were only marginal, however, e.g., 34 to 204 MPa,

respectively.

2. Bulky Monofunctional Diluent Monomers

The quantity of unreacted vinyl groups or, conversely, the

degree of polymerization of a hardened dental resin, is influenced by the

nature and amount of the diluent monomer employed [2]. While the highest

degree of polymerization was reported [3] for the monofunctional monomer

diluent methyl methacrylate (MMA), its use as a major diluent monomer leads

to increased polymerization shrinkage and poor marginal adaptation.

For MMA we have substituted bulky monofunctional methacrylates. Previously we described the synthesis of a monofunctional tricyclic aliphatic

methacrylate, MAC (MW = 292). Table 2 demonstrates use of this unique

monomer in several formulations where it comprises all or a major portion

of the diluent monomer.

3
Table 2

Setting Time and Diametral Tensile Strength of an Experimental Composite** Employing MAC as the Primary Diluent

Setting Diametral

Experimental Formulations P/L Time (min.) Tensile Strength

BIS-GMA 64.8% 3 3.0 45.2 ± 1.4 (4)

MAC 26.4%

CDMDMA

3 8.6%

DMAPE

b 0.2%

BIS-GMA 71.0% 2 2.5 40.8 ± 2.1 (4)

MAC 28.8%

DMAPE 0.2%

^Cyclohexane dimethanol dimethacrylate

d

4-N , N-di methyl ami nophenethanol

*Powder is same as used in table 1

As can be seen from these results, both the curing characteristics and

mechanical properties are in the acceptable range for a dental composite

material. The initial color and color stability of these composites were

excel lent.

A less bulky alicyclic methacrylate is also undergoing evaluation as

diluent monomer.

CH.

h

2 c

h

2

c

CH-0C-C=CH,

CPL-C-CH3

| 3 —c

H

CH,

CH.

Isobornyl methacrylate (IBM)

4
As can be seen by the results summarized in Table 3, the use of IBM

as a diluent monomer gave composites having acceptable setting times and

diametral tensile strengths. Both the initial color and the color stability

of these composites were excellent.

Table 3

Setting time and diametral tensile strength of experimental

composites employing IBM as the diluent monomer

Formul ati on Monomer System, Wt. % P/L

Setti ng

Time (min).

Diametral Tensile Strength

± Std. Dev. in MPa

(No. of Specimens)

1 BIS-GMA 79.8 3 2.0 46.9 ± 2.3

IBM 19.9 (4)

DHPPT 0.3

2 BIS-GMA 69.8 3 2.5 44.9 ± 2.4

IBM 29.9 (6)

DHPPT 0.3

3 DTBAEM-HDI 85.0 3 3.0 42.2 ± 0.9

IBM 14.7 (5)

DHPPT 0.3 .

3. Hydrophobic BIS-GMA Type Monomers

Previous studies [4-6] on the use of highly fluorinated dental

monomer systems for enhancing the hydrophobici ty of composite and sealant

materials indicated the need for thermosetting or crosslinking fluorinated

monomers for achieving adequate mechanical strength. Unfortunately, at

present only highly fluorinated monomethacrylates and one fluorinated precursor of a dimethacrylate, the very expensive hexafl uoropentane-l,5-diol

,

are currently commercially available.

The least expensive of the commercially available, highly fluorinated

alcohols are those based on the telomerization of tetraf 1 uoroethyl ene with

methanol. These telomer alcohols are much more acidic than their hydrocarbon

analogs having dissociation constants almost four powers of ten higher than,

5
for example, ethanol. Their general structure is shown below

H-f-CF

2

-CF

2

-}-

n CH

2

0H

where n = 1.5.

We have explored the reaction of the higher members of this series,

i.e., n = 3.5, with diepoxy compounds such as the diglycidyl ether of bisphenol A (DGEBA) as a means of synthesizing highly fluorinated, bulky diols

which may then be converted by standard chemical procedures to the corresponding di acrylate or di methacrylate monomers. The first member of this

series of diols was prepared from DGEBA and H-(CH

2

-CF

2

) 4

CH

2

0H by heating two

parts of the fluorinated alcohol with one part of di epoxy compound at 150°

for 17 hours in the presence of one percent tri ethyl ami ne as the catalyst.

DGEBA

+-2H — (CH

2

-CF

2

) 4

CH

2

)H

0 CH

1 CH (CF CF ) H

0

ch

2

(cf

2

cf

2

) 4

h

MW = 1204

After isolation, the product was a pale yellow highly viscous liquid.

While IR analysis supported the assigned structure we now postulate that the

product is probably a mixture of oligomers of various molecular weights.

Solubility studies using n-heptane and methylene chloride resulted in three

fractions.

6
Fraction I (about 10 percent of the product) was soluble in n-heptane.

Fraction II (about 45 percent) was reasily soluble in methylene chloride.

Fraction III (about 45 percent), a very viscous material, was soluble in

methylene chloride after several hours at room temperature. IR analysis

failed to reveal significant chemical structural differences among the

three fractions. The disappearance of the epoxy absorption band of DGEBA

and the appearance of a strong OH band was a positive indication that the

reaction had occurred.

Part of the original unfractionated product was dissolved in methylene

chloride and allowed to react with methacryoyl chloride using tri ethyl amine

as the hydrogen chloride accepter. After the usual isolation procedure a

yellow, highly viscous oil was obtained. IR analysis did reveal the presence

of both ester and vinyl structural features, but some OH units remained

unchanged. The product could be polymerized by conventional ambient free

radical techniques, but the polymerization was rather sluggish. Purification

of the fluorinated polyol is probably needed to obtain a purer monomer which

should give faster and more complete cures.

4. Non-Vinyl Agents (Polythiol) Reactive Types

Another approach that holds promise not only toward reducing

residual vinyl unsaturation but also of reducing polymerization shrinkage

and of enhancing the oxidative stability of the polymer matrix is the use

of bulky polythiols.

The free radical addition reaction of monothiols (RSH^SH) to olefins,

the well-known chain transfer reaction, is a classical method of regulating

the molecular weight of vinyl polymers. With multifunctional thiols, i.e.,

R(CHpSH) where x = 2, and polyenes, i.e., R(CH=CH

?

) , where y ^ 2, chain

extension and crosslinking occur to give vinyl polymers having polythioether

elements in their structure [7].

7
A simplified representation of this type of thiol /ene polymerization

is illustrated below.

0 CH.

R— (0 —C —C=CH

2

) 2

CH.

w\aC—CH,-

<? 2

c

5 c

3

<?

CH.

R(<wv\.SH)

CH.

SvwvRvmS—CH, —C —C—0 —R —0—C —C —CH.

H 0

CH.

0 CH.

CH,— C— C-O-R-O-C—0=CH,

CH.

CH —

3

i o=c

0

fr0 o=c

I CH —C-JCH

3

I o=c

I 0

CH.

C=0

I 0

I R

I

<?

CH.

CH,

I 3

CH

I c=o

I 0

0 CH.

^.rA/vC CHg

0—

C

0

Ic=o

I vw%C- CH.

i

;

|

H

2 e

Free Radical

Initiator

8
Adding relatively small amounts of the polythiol, pentaerythri tol

3-mercapto propionate-(PETMP) , to a BIS-GMA monomer system gave a resin

system that was less subject to air inhibition than conventional dental

methacrylate resins. In addition, it was observed in curing by the usual

benzoyl peroxide-amine accelerator type initiator system that composite

and sealant materials set faster with the addition of the polythiol. In

order to obtain a workable setting time, the amount of amine accelerator

that was customarily used had to be reduced drastically. The cured composite

specimens possessed excellent esthetics because of the reduced amine concentration and the presence of the polythiol elements and enhanced color stability.

Depending on the concentration of PETMP both the setting time and the tensile

strength could be varied. With high concentrations of PETMP (< 20 percent)

composites with relatively low diametral tensile strength (23 to 34 MPa) were

obtained due to the increased flexibility of the polymeric matrix. With

lower concentrations of PETMP (e.g. , 1 to 5 percent) composites of a harder

nature were obtained (DTS 34 to 52 MPa).

With the non-amine initiator system, benzoyl peroxide/ascorbyl

palmitate, PETMP in concentrations up to 14 percent did not significantly

reduce the setting time of the composite material. However, PETMP does

appear to improve the storage stability of monomer solutions containing

ascorbyl palmitate. After six weeks the same setting time of 4-1/2 minutes

was obtained for a formulation of 64.4 percent BIS-GMA, 27.6 percent DMGDMA

(decamethylene glycol dimethacrylate), 7.3 percent PETMP, and 0.66 percent

ascorbyl palmitate when cured with silinized glass coated with one percent

benzoyl peroxide (powder/liquid ratio = 3).

Several typical formulations with PETMP are shown below in table 4.

9
-'

•:

j
Table 4

Setting time and 24-hour diametral tensile strength (DTS)

of experimental composites formulated with PETMP

Formulation Compos i ti on Setting Time(min.

)

A BIS-GMA (54%) Too fast to

measure

DMGDMA (20%)

PETMP (26%)

DHPPT (0.24%)

BHT (0.16%)

DTS + STD. DEV.

(MPa) No. of Spec.

B BIS-GMS (68.4%)

TEGDMA (29.3%)

PETMP (2.2%)

DHPPT (0.11%)

1.5 52.2 ± 4 (5)

C BIS-GMA (68.8%) 2.5 48.8 ± 1.7 (5)

TEGDMA (29.5%)

PETMP (1.1%)

DHPPT (0.056%)

D BIS-GMA (67.1%) 4.5 41.7 ±2.2 (5)

TEGDMA (28.9%)

PETMP (3.5%)

AP (0.64%)

E BIS-GMA (53.3%) 4.5 3.3 ± 1.3 (5)

DMGDMA (27.6%)

PETMP (7.3%)

AP (0.66%)

Polythiols such as PETMP have been shown to be effective reducing agents

for UV photosensitizers such as benzophenone [7]. We have also shown that

they can be used in visible light photocuring systems that employ di ketones

such as camphorqui none. With the following resin formulation of 80.2 percent

BIS-GMA, 14.2 percent DMGDMA, 5.4 percent PETMP, and 0.2 percent camphorquinone a visible light source (Caulk) gave cures in 20 seconds to a depth

of 2 mm in composites formulated with 75 percent by weight of silanized glass.

Samples of this same composite when cured for one minute gave specimens of

9a
excellent esthetics and color stability with DTS values of 43.5 ± 1.3 MPa.

Again the curing of these specimens in air showed less air sensitivity than

those cured without PETMP.

5. Expanding Monomer Types

Effort in this fiscal year has been restricted to developing

a cooperative education (graduate degree) program wth Maryland University

Chemistry Department. Mr. Jeff Stansbury has begun this program with

Dr. William Bailey who has been a pioneer in the area of expanding monomer

types [8]. We anticipate the work will emphasize monomers that are compatible

with methacrylate diluents and have appropriate solubility parameters.

B. Development of Improved Ini tiator/Accel erator Systems

1. Initiator Research

The results of our investigation into more stable peroxyesters and

hydroperoxides as initiators for the polymerization of dental resins were

‘summarized in the preceding annual report [4] and were presented at the

IADR/AADR meeting in Chicago.

We have begun examination of new photochemical polymerization initiating

systems that could be predicted to be biocompatible on the basis of structure.

Several tertiary aromatic amines which we have established to have widely

different reactivity toward benzoyl peroxide have been looked at for their

capacity to accelerate resin polymerization initiated by camphorqui none.

Little difference in rates of cure was noted whether accelerators known to

be fast acting with benzoyl peroxide, e.g., 4-N,N-dimethylaminophenylacetic

acid or slow acting such as 4-N ,N-dimethyl aminobenzoic acid, were exposed

to a quartz line projection lamp (19 V, 80 W), employed in a fiber optic

1

i ght.

One especially reactive composite formulation consisted of BIS-GMA (7)/

TEGDMA (3) and 70 percent by weight of silanized glass filler combined with

10
a photoinitiator system of 0.2 percent 4-EDMAB* and 0.1 percent camphorquinone. This formulation cured to a depth of 9 to 10 mm with a three minute

exposure to the light source.

We have found that the combination of riboflavin (0.01 percent)

(Vitamin B^) and ascorbic acid (0.7 percent) (Vitamin C) in an aqueous ethanol

system is capable o polymerizing methyl methacrylate in a few hours. A

problem to be dealt with is that riboflavin has only modest solubility in

water and is virtually insoluble in the usual dental monomer systems. However,

a yellow paste BIS-GMA based composite formulation with a minute quantity of

riboflavin and 0.7 percent ascorbyl palmitate (AP) cured to a white product

in two minutes between glass slides when exposed to sunlight. After three

minutes of sunlight exposure the same formulation when placed in a Teflon

mold and covered with glass slides, cured to a depth of 3 mm. We intend to

prepare an organophilic derivative of riboflavin and to examine this approach

to photoinitiation further.

2. Amine Accelerators

a. Development of Fast-Acting Biocompatible Amine Accelerators

The development of fast-acting amine accelerators that impart

maximum color stability to composites and should be much more biocompatible

than the presently used amine accelerators was completed. Three amines,

^-(dimethyl amino)phenethanol , p-(diethylamino)phenyl acetic acid and its

ethyl ester, shown previously to be most promising as accelerators for dental

composites, have been synthesized in larger quantities and offered to cooperating laboratories to determine toxicological and biocompatibility characteristics.

The accelerator, p-(dimethylamino)phenethanol , synthesized and evaluated pre-

*Ethyl , 4-N,N-dimethylaminobenzoate

11
viously in this laboratory has now been made available in research quantities

by a chemical manufacturer.

Synthesis of high-molecular weight ester derivatives of p-(dial kylamino)-

phenethanol containing free carboxylic acid groups were attempted since

these types of organic structure appear to provide great accelerative

reactivity for the curing of composites.

After several unsuccessful attempts mono-p-(diethyl ami no)phenethanol

succinate was prepared from 5 g (0.026 mol) of p-(diethylamino)phenethanol

and 7.81 g (0.078 mol) succinic anhydride in 25 ml acetone. After evapora20 5 tion of solvent and purification a thick amber-col ored oil (n^ ' = 1.5230)

remained. This compound had an infrared spectrum consistent wth its chemical

structure, and a purity of 94 to 95 percent as determined from gas chromatographic analysis at 160 °C. It appears to decompose at higher temperature.

Similarly, the mono-p-(diethylamino)phenethanolmal eate was synthesized

since it was expected that this compound would be an excellent accelerator

that might have the added advantage of copolymerizing in the curing of the

22 composite. A viscous dark brown oil (n^ = 1.5416) was obtained. The

oil was an effective accelerator for the curing of composites, but the

purification of the compound was difficult. The mono-p- (di ethyl ami no)-

phenethanol methacrylate was obtained from p-(diethylamino)phenethanol

and methacryl chloride in the presence of tri ethyl ami ne. The compound

distilled at 160 to 165 °C/4 mm, = 1.5380, but had a deep dark-red

coloration which made it unsuitable as an accelerator for dental composites.

The mono-p-(diethyl ami no)phenethanol succinate was incorporated as

accelerator into typical powder-liquid composite formulations. The bulk

of material hardened, but a soft layer remained at the surface, probably

caused by air inhibition. The cause for this air inhibition is unknown.

12
Two papers [9,10] resulting from these investigations were published

in 1981.

b. Amine Accelerators with Intermediate Accelerative Reactivity

As a means to correlating amine structures with their accelerative reactivity, when used with benzoyl peroxide as the initiator, we have

investigated slow, moderate, and fast acting amines.

Representatives of these moderately reactive accelerators are

N,N-bis(2-hydroxyethyl)-p-toluidine (DHEPT), which is widely used in chemically activated dental restorative resins, and N , N-b i s(2-hydroxypropyl )-

p-toluidine (DHPPT). Although both amines are solids, DHPPT is more easily

purified than DHEPT since it has a higher melting point, 109 to 110 °C.

We produced a highly purfied sample of DHPPT and are using this white,

crystalline solid amine to study the effect of concentration on the setting

time and diametral tensile strength of dental composite materials. In

table 5 are summarized the results obtained for powder/liquid formulations

based on BIS-GMA (7)/TEGBMA (3) as the resin component. Even at extremely

low concentrations (e.g. , 1.1 mmol/kg) composite specimens having adequate

setting time and diametral tensile strengths (DTS) were obtainable. The

initial color and color stability of these composites were excellent. As

expected, as the amine concentration increased, both the initial color and

the color stability of composites were adversely affected. For concentrations higher than 12.6 mmol /kg, it was necessary to add BHT to obtain workable

setting characteristics. A significant decrease in DTS (about 15 percent)

was noticed for the 12.7 mmol/kg formulation when 0.09 percent BHT was

added.

13
Table 5

Effect of DHPPT concentration on the setting time and 24-hour diametral

tensile strength of an experimental composite*

formulated with BIS-GMA (7)/TEGDMA (3)

DHPPT

Wt.%

Concentration

mol /kq

Setting Time

(min.)

Diametral Tensile Strength

± Std. Dev. in MPa

(No. of Specimens)

0. 42

a 18.8 3.4 52.0 ± 2.7 (5)

0. 32

a 14.4 4.0 51.3 ± 1.8 (5)

0. 28

a 12.6 5.5 51.0 ± 2.2 (4)

0.28 12.6 1.5 58.4 ± 2.4 (5)

0.16 7.2 2.0 51.8 ± 2.4 (4)

0.10 4.5 2.5 50.4 ± 1.6 (5)

0.05 2.2 3.0 47.7 ± 1.3 (4)

0.025 1.1 5.0 42.3 ± 1.0 (5)

* Corning Glass 7724 (325 mesh) silanized with 3-methacryloxypropyltrimethoxysi lane and coated with one percent benzoyl perozide. A

powder/liquid ratio (P/L) of 3 was used.

a In these formulations, 0.09 percent of 2,6-t-butyl-p-cresol (BHT) was added to the liquid components.

A similar study was performed using a commercial diurethane dimethacrylate resin system in place of the BIS-GMA resin system. The results shown in

Table 6 paralleled those observed in Table 5 showing the dependence of

setting time and DTS on the concentration of DHPPT. The initial color and

color stability of these composites was somewhat better than those based on

BIS-GMA for the same DHPPT concentration.

Excellent results also were obtained from a paste/paste system that was

formulated using BIS-GMA (7)/TEGDMA (3) as the resin and DHPPT as the

accelerator. The catalyst paste (P/L = 3) contained one percent BP stabilized

with 0.7 percent tyrosine and the accelerator paste (P/L = 3) contained

0.4 percent DHPPT and no added stabilizer. Equal parts of the two pastes

14
Table 6

Effect of DHPPT concentration on the setting time and 24-hour diametral

tensile strength of an experimental composite* formulated

with a di urethane dimethacryl ate resin**

DHPPT

Wt. %

Concentration

mmol /kg

Setting Time

(min.

)

Diametral Tensile Strength

± Std. Dev. in MPa

(No. of Specimens)

0.25 11.2 1.5 57.6 ± 4.7 (4)

0.16 7.2 2.0 56.9 ± 3.2 (5)

0.10 4.5 2.5 47.5 ± 3.7 (5)

0.05 2.2 3.5 49.8 ± 1.1 (5)

0.025

*

1.1 5.0 40.4 ± 3.2 (5)

**Same powder and powder/liquid ratio as used in Table 4.

From Vivadent

gave composite specimens setting in four minutes and having a DTS of

50.2 ± 3.1 MPa.

As a moderately reactive amine accelerator of high purity, DHPPT is

expected to be useful in our work to elucidate the mechanisms of the complex

amine-peroxide reactions.

3. Non-Amine Accelerators

We have shown previously that ascorbic acid (AA) or its

palmitate ester (AP) functioned best as a polymerization accelerator if

used in powder/liquid formulations. That is, the storage stability of the

ascorbic acid accelerator is maximized by keeping it in the solid state

coated on the glass filler component. This necessitates the dissolution of

the peroxide initiator in the monomer system (liquid component), which,

especially in the case of benzoyl peroxide, makes it difficult to improve

the storage stability of the formulation.

We have been investigating how AP behaves in the dissolved state by

studying the effect of various concentrations of dissolved AP on the properties

15
of a normal powder/liquid dental composite material. A series of solutions

of AP in BIS-GMA (7)/TEGDMA (3) were prepared and used with a typical

silanized glass powder coated either with one percent benzoyl peroxide

(Table 7) or one percent t-butyperoxymaleic acid (Table 8). The variation

of setting times and DTS with AP concentration for composites prepared

with one percent BP powder are shown in Table 7. For freshly prepared

solutions there seems to be little variation in setting times on increasing

the concentration of AP almost tenfold(0.1 mmol/kg to 0.9 mmol/kg). This

is in contrast to the behavior observed with amine accelerators. The effect

of AP concentration on DTS more nearly parallels the results found with

amine accelerators.

Table 7

Effects of ascorbyl palmitate concentration on setting time and

diametral tensile strength of an experimental composite material*

using benzoyl peroxide as an initiator

Ascorbyl Palmitate

Concentrati on

Wt. % mmol /kg

Setting Time

(min.)

Diametral Tensile Strength

MPa = Std. Dev.

(No. of Specimens)

0.1 2.4 4.0 35.1 + 1.6 (6)

0.2 4.8 3.0 37.4 + 1.3 (6)

0.3 7.3 4.0 37.8 + 2.3 (6)

0.4 9.7 3.5 41.1 + 1.6 (6)

0.5 12.1 3.75 41.1 + 2.5 (5)

0.6 14.5 4.0 42.7 + 2.0 (6)

0.7 16.9 3.75 44.1 + 1.6 (5)

0.8 19.3 4.25 45.8 + 1.4 (6)

0.9 21.8 4.0 43.9 + 2.2 (6)

Powder/liquid formulations employing powder/liquid ratio of 3. Powder is

a silanized glass filler and coated with one percent BP. Liquids are monomer mixtures consisting of seven parts BIS-GMA and three parts TEGDMA

containing the indicated concentrations of ascorbyl palmitate. No BHT was added.

16
Table 8

Effects of ascorbyl palmitate concentration on setting time and

diametral strength of an experimental composite material*

using TBPM as an initiator

Ascorbyl Palmitate

Concentration Setting Time

Diametral Tensile Strength

MPa = Std. Dev.

wt. % mmol/kq (min.) (No. of Specimens)

0.1 2.4 6.25 35.1 + 3.3 (6)

0.2 4.8 5.5 36.6 + 2.9 (4)

0.3 7.3 4.25 47.2 + 2.0 (7)

0.4 9.7 6.5 45.0 + 1.8 (8)

0.5 12.1 4.0 47.9 + 1.4 (4)

0.6 14.5 6.0 44.4 + 2.2 (6)

0.7 16.9 4.25 48.3 + 1.0 (5)

0.8 19.3 4.25 46.5 ± 1.7 (6)

0.9 21.8 4.25 45.2 + 3.5 (4)

* Same as Table 6 except one percent TBPM was used as an initiator instead of one percent BP.

TBPM = t-butylperoxymaleic acid

The effect of AP concentration on DTS more nearly parallels the results found

with amine accelerators.

With composites prepared using the one percent TBPM powder (Table 8), a

greater but somewhat erratic variation of setting time with the concentration

of AP was observed. The setting times, in some cases, are somewhat longer

for TBPM than for BP formulations. However, on the average, DTS values tend

to be somewhat higher for the TBPM formulations.

The initial color of both types of composites was excellent. The color

stability of the AP/TBPM composites was superior to those prepared with the

AP/BP initiator system, although the latter were excellent compared to that

of the usual amine/BP composites.

17
All the AP solutions in BIS-GMA (7)/TEGDMA (3) showed loss of accelerative efficacy on storage in the dark in glass vials at 22 °C after 30 days.

However, the more concentrated solutions (e.g., 0.6 to 0.9 percent) still

gave acceptable setting times (7 to 8 minutes vs the original 3 to 4 minutes)

when measured using one percent BP powder (P/L = 3) at 37 °C.

Several potential stabilizing agents have been added to new AP solutions.

The most promising stabilizers used thus far appear to be the combination of

sodium bisulfite (NaHSO^) and dihydroxymaleic acid. A solution of 0.7 percent

AP in BIS-GMA (7)/TEGDMA (3) containing 0.04 percent dihydroxymaleic acid

and 0.07 percent NaHSO^ has retained its original setting time of 4.0 minutes

after three months using one percent BP powder.

4. Vacuum Polymerization of Composite Resins

A recent article reports that the use of a relatively short

application of vacuum during the preparation of a dental composite had

several beneficial effects on the finished Composite [11]. A significant

reduction in the porosity and surface roughness was noted. The reduced

porosity apparently increased the diametral tensile strength by about

11 percent. Since the removal of oxygen supply by vacuum application

shortened the initiation of polymerization time drastically, it was

necessary to add a retarder to the composite mix in order to obtain

workable consistencies.

We have tried a variant of this technique with composite materials

based on the AP/BP initiator system which, because of relatively long

setting characteristics should not require the presence of a retarder.

Our initial experiments indicate that the vacuum technique is quite feasible

with ascorbic acid type composite materials and a material of workable consistency is obtained after exposure to house vacuum for 30 to 60 seconds.

18
The diametral tensile strengths of these composites were ten percent higher

than similar composites prepared without exposure to vacuum. This technique

needs to be examined with other relatively slow, chemically activated and

light cured dental resin composite materials.

C. Analysis for Extent of Polymerization of Dental Resin

The final properties of a dental sealant or composite restorative

material are directly related both to the chemical composition of the dental

monomer system and to the degree of conversion that has occurred in this

monomeric component. It would be helpful in the development of improved

dental resin materials to be able to assess rapidly and conveniently the

extent of polymerization (EP) of any dental monomer system that can be

hardened under control conditions that are clinically relevant.

Several workers have used infrared (IR) spectroscopy to measure the

EP of several commercial sealant and composite materials [2,12-14]. These

IR techniques measure EP by monitoring the decrease in the carbon-carbon

double bond (^C=C^) absorbance (1639 cm of the methacrylate monomer(s)

that occurs concomitantly with vinyl addition polymerization. In the case

of dental composite materials, especially, the traditional IR analytical

methods are often hampered by difficulties in sample preparation and by a

strong interference in the critical vinyl absorption region by the siliceous

filler component. In addition, there may be similar problems of interfering

absorption bands with certain monomer systems.

These difficulties may be circumvented to some extent by means of an

alternate technique based on differential scanning calorimetry (DSC). Unlike

the traditional calorimetric methods, DSC experiments are far less arduous

and time consuming and require only milligram quantities of material. Other

workers have successfully utilized DSC to study the polymerization of both

monofunctional and multifunctional vinyl monomers [15-17].

19
The monomers used in the present study, their abbreviations, and commercial sources, are given in Table 9. A similar listing of the accelerators

used to formulate the various initiator systems with benzoyl peroxide (BP)

is given in Table 9a. The materials were used as received. For the

composite studies, the filled component was prepared from a barium oxide

containing silica glass (No. 7724, 325 mesh) obtained from Corning Glass

Works (Corning, NY). The glass powder was silanized with 3-methacryloxypropyltrimethoxysilane (A- 174, Union Carbide Corp. , New York, NY), by a

previously described procedure [18] to give a reinforcing filler with

0.5 percent silane content. A portion of this silanized glass was coated

with one percent by weight BP as previously described [19].

Samples were placed in DSC aluminum pans which were hermetically sealed

and quickly transferred to the sample holder of a Perkin-Elmer DSC-2. Pans

plus seals were weighed prior to the addition of samples. A similar weighed,

sealed, but empty DSC pan was used in the reference holder. All polymerization experiments were conducted under nitrogen in either the temperaturescanning or the isothermal mode [20].

The instrument was calibrated using indium, AH,* . . = 680 cal g \

MP 156.4 °C. For the thermally initiated polymerizations of specimens containing only BP as the initiator the usual sampling technique of obtaining an

initial (unpolymerized) and a final (polymerized) weight for the specimen

was used (Tables 11 and 12). The comparison of the two weighings indicated

that essentially no material loss occurred during the scan.

With two component formulations (Tables 13-16), the peroxide and

accelerator portions were weighed onto separate sections of a small aluminum

dish. The separated components were then combined and mixed thoroughly for

one minute before transfer of an aliquot to a DSC pan which was then sealed.

20
Table 9

Monomers Used in DSC Study

Name Abbrevi ati on Source

2 ,2-BIS[p-(y-methacryloxy-p-hydroxypropoxy) BIS-GMA Freeman Chemical Co.

phenyl Jpropane (Milwaukee, WI)

Triethylene glycol dimethacrylate TEGDMA Esschem Co.

(Essington, PA)

2 ,2-BIS[p-(p-methacryloxyethoxy)phenyl Jpropane BIS-EDMA Espe GmbH

(Seefeld, Germany)

Isobornyl methacrylate IBM Polysci ences (Warrington, PA)

Table 9a

Accelerators Used ini DSC Study

Name Abbrevi ati on Source

p-tert-Butyl-N-N-dimethylani 1 i ne DMBA Aldrich Chemical Co.

(Milwaukee, WI)

4-Dimethyl ami nobenzoic acid 4-DMAB II

Ethyl -4- dimethyl ami nobenzoate 4-EDMAB II

N,N-BIS(2-hydroxypropyl )-p-tol uidine DHPPT Ivoclar, AG (Shaan,

Li echtenstei n)

Ascorbyl palmitate AP Hoffman-LaRoche , Inc.

(Nutley, NJ)

Molar Heats of Polymerization of Methacrylate Monomers

Monomer AHp kcal/mol Reference

Methyl methacrylate 13.8 23

Ethyl methacrylate 13.8 23

n- Propyl methacrylate 13.7 23

n- Butyl methacrylate 14.3 23

n- Hexyl methacrylate 14.4 23

Lauryl methacrylate 13.6 17

21
Table 11

Extent of Polymerization (EP) of Thermally^ Initiated Polymerizations by DSC

? AH (cal/g)

T ± Std. Dev. EP

P

Monomer BP Concentration (°C) (No. of Specimen) (% JL—CTT reacted)

IBM 1.0 115 55.3 + 1.9 (3) 90.3

BIS-EDMA 1.0 72 28.6 + 1.0 (3) 47.7

BIS-GMA (7)

TEGDMA (3)

1.0 90 34.2 + 1.2 (3) 52.0

II 0.39 95 28.6 (1) 43.5

II 2.2 90 40.7 (1) 61.9

TEGDMA 1.0 92 82.3 + 1.5 (3) 86.3

II 2.0 86 81.3 ± 1.2 (3) 85.5

^Thermal scan rate =2.5 deg/mi n.

i = peak temperature of DSC curve.

Table 12

EP of Poly[BIS-GMA(7)/TEGDMA(3)] as a Function of BP Concentration

' and Thermal Scan Rate

Thermal Scan

Rate (deg/mi n)

0.39 1.0 2.2

AH

cal/g

EP

%

T

P °C

AH

cal/q

EP

%

T

P °C

AH

cal/q

EP

%

0-0

1—o

0.62 25.5 38.8 85 34.8 52.9 86 32.1 48.8 84

1.25 27.6 42.0 90 36.5 55.4 90 39.1 59.4 89

2.5 28.6 43.5 95 35.2 53.4 90 40.7 61.9 90

5.0 32.2 48.9 102 37.3 56.7 102 40.4 61.4 100

10.0 49.1 74.6 110 41.0 62.3 109 42.6 64.8 107

20.0 36.0 54.8 119 40.9 62.2 118 42.1 63.9 115

22
The sample weight was obtained after the run by subtracting the weight of

the pan plus seal from the final weight, assuming no significant loss of

materi al

.

The observed heat of polymerization, AH^^cal g ^), is based on the

planimeter measurement of the total area under the recorded curve or curves.

In several representative DSC experiments, a base line for the thermal scans

was obtained by a second thermal scan of the polymerized material over the

same temperature range [21,22].

The exothermic heat of reaction (enthalpy) arising from the free radical

polymerization of methacrylate monomers is directly proportional to the

number of vinyl groups undergoing this self addition reaction, assuming the

absence of any significant side reactions. The molar heat of polymerization,

AHp, is the heat evolved during the complete polymerization of one mole of

monomer and is a primary thermodynamic quantity determined only by the initial

and final states of the monomer and polymer, respectively, and independent of

the mechanism of polymerization. AHp's have been determined by various

methods for a series of methacrylate monomers. The AHp values of several

representative monofunctional methacrylates are given in Table 10.

As a basis for calculating the degree of conversion of monomer to

polymer or the EP of the dental monomer systems used in this study, the AHp

value of lauryl methacrylate (LMA), a bulky (molecular weight = 254),

monofunctional methacrylate was chosen as a standard. The AHp value for

LMA, as determined in a similar DSC study, is 13.6 kcal/mol [17]. A basic

assumption of this method is that heat of reaction for the conversion of a

vinyl group to saturated carbon-carbon bond is the same for all methacrylate

monomers and only proportional to the number of reacted vinyl groups. Thus,

for 100 percent conversion, the AHp of LMA should be the same as that of

23
methyl methacrylate (MMA) or isobornyl methacrylate (IBM) but only one-half

that of BIS-GMA or TEDDMA which are difunctional. Conversely, for a monomer

mixture of 7 parts BIS-GMA and 3 parts TEGDMA, the AHp of this monomer system

is twice that of LMA or 2 x 13.6 kcal/mol or 27.2 kcal/mol. For this monomer

mixture, the calculated heat of polymerization in calories per gram, assuming

100 percent conversion, is derived as follows:

For 100% Conversion of 1 g of BIS-GMA (7)/TEGDMA (3)

CALC

AH = (number of moles BIS-GMA + number of moles TEGDMA) x 27,200 cal/mol

7,3

CALC

0.

7

+

0.

3

27,200 cal; where 512 and 286 are the molecular

512 286 weights of BIS-GMA and TEGDMA, respectively.

CALC

AH = (0.00242 x 27,200 cal

7,3

= 65.8 cal

Therefore,

EP = r- x 100; where AH , is the observed heat of polymerization for a

. UC3 1C OD S A

/ , 3 BIS-GMA (7)/TEGDMA (3) specimen

Thus, if = 33.0 cal/g,

EP = 33.0/65.8 x 100 = 50.2 percent

Thermally initiated polymerizations using BP as the initiator were conducted by DSC for IBM, a bulky, monofunctional methacrylate, BIS-EDMA, a

solid, crystalline dimethacrylate (mp = 44° by DSC), the monomer mixture,

BIS-GMA (7)/TEGDMA (3) and TEDGMA alone at the same thermal scan rate of

2.5 degrees per minute. The observed heats of polymerization (AH) and the

calculated extents of polymerization (EP) for these monomers are given in

Table 11. In the case of the solid monomer, BIS-EDMA, the solid initiator,

24
BP, was mechanically dispersed into the monomer first by gentle spatulation

using a plastic spatula, and then by gentle shaking in a plastic vial.

Samples from this mix were used in the DSC runs. The liquid monomers containing dissolved BP also contained 0.1 percent dissolved BHT (2,6-di-tertbutyl-4-methlphenol ) as a stabilizer. For the polymerization experiments

employing 1 percent and 2 percent BP, the AH values are averages obtained

from three runs using specimens of approximately equal size (e.g., 5 to 7 mg).

The other results represent single runs. The results in Table 11 illustrate

the much higher conversions possible in the case of a monofunctional bulky

monomer (IBM) or a flexible dimethacrylate such as TEDDMA compared to that

achieved with a rigid functional methacrylate such as BIS-EDMA. The temperature range for these experiments was the same, 27 to 140 °C. Table 11 also

illustrates the increase in EP with increasing concentration of BP for

BIS-GMA (7)/TEGDMA (3). The temperature, Tp, was the temperature at the

peak of the major exothermic curve of the thermogram.

Table 12 summarized the effect of the thermal scan rate and BP content

on the EP of BIS-GMA (7)/TEGDMA (3).

For a given thermal scan rate, Tp and the conversion of monomeric units

to polymer (EP) usually increased with an increase in the BP concentration.

With an increasing thermal scan rate, Tp increased and the conversion values

(EP) also tended toward maximum values. The high conversion (EP = 74.6 percent)

recorded for the 0.39 percent BP solution at a thermal scan rate of 10 deg/mi

n

appears to be an anomalous result. The EP value (54.8 percent) at a thermal

scan rate of 20 deg/min is probably more indicative of the maximum conversion

for this formulation.

The results shown in Tables 13-16 are for polymerization of the same

monomer system using various types of chemical activators for the free

radical decomposition of BP. With fast-acting chemical activators, such as

25
Table 13

EP by DSC"*" for Poly[BIS - GMA(7)/TEGDMA(3)] Chemically Activated

Polymerizations —BP/ami ne accelerator

L ^ BP

la/l

bp

T

P AH EP

Amine Acc.

& Cone. BP Cone. Ratio

(Wt. %) (Wt. %) (°C) (cal/q) (%)

DMBA 0.39 0.45 49 31.0 47.1

(fast) 1 0.96 44 32.3 49.1

0.23 1 1.12 41 36.8 56.0

DHPPT 1.0 0.80 61 35.2 53.5

(mod.

)

1 1.01 61 34.3 52.1

0.32 I 1.35 64 36.7 55.7

0.42 1.0 0.54 57 37.8 57.4

0.42 2.2 1.30 49 31.4 47.8

4-DMAB 1.0 1.11 58 33.8 51.4

(slow)

0.50 2.2 0.94 56 32.5 49.4

1

2

3

Thermal scan rate =2.5 deg/mi n.

LA

: liquid monomer consisting of BIS-GMA(7)/TEGDMA(3)

with indicated concentration of amine accelerator

=same liquid monomer as in but with indicated

concentration of BP and 0.1 percent BHT.

Table 14

EP by DSC* for Poly[BIS-GMA(7)/TEGDMA(3)] Chemically

Acti vated Polymeri zati on--BP/4-EDMAB

la Q_ CQ _l LA/L

BP

T

P AH EP

4-EDMAB

cone.

(Wt. %)

BP Cone.

(Wt. %)

Rati o

(°C) (cal/q) (%)

0.5 1.0 0.84 78 31.2 47.4

1.0 1.0 0.96 66 32.4 49.3

1.3 1.0 0.92 62 34.5 52.4

0.5 2.2 1.04 65 34.6 52.5

1.0 2.2 0.81 57 33.6 51.1

1.3 2.2 0.42 55 33.0 50.2

1.3 2.2 0.52 53 34.0 51.7

1.3 2.2 0.72 52 35.7 54.2

1.3 2.2 1.02 51 31.4 47.7

1.3 2.2 1.72 51 34.4 52.3

1.3 2.2 2.57 52 33.8 50.2

^Thermal scan rate =2.5 deg/mi n.26
the amine accelerator DMBA, it proved difficult to obtain satisfactory DSC

thermograms by normal sampling techniques when BIS-GMA (7)/TEGDMA (3)

containing 2.2 or 1 percent BP was used. Special sampling and mixing techniques,

such as prior cooling of the two components, may be needed in these cases.

However, satisfactory DSC curves were obtained with the 0.39 percent BP solution

in BIS-GMA (7)/TEGDMA (3). Moderately reactive amine accelerators (e.g. , DHPPT)

or those of low reactivity (e.g., 4-DMAB, 4-EDMAB) are better suited to this

type of DSC experiment and permit wider ranges of accel erator-i ni tiator

concentrations to be explored (see Tables 13 and 14).

It was observed that in polymerizations employing the fast-acting amine

accelerator, DMBA, that the Tp's were lower (in some cases unrecordabl e) than

the Tp's obtained with the slower-acting activators. For the chemicallyactivated polymerizations, the Tp's correlated well with the observed setting

times of the formulations. Thus, the formulations that had fast setting

times had low Tp ' s.

The relatively low reactivity of 4-EDMAB toward BP compared to the

other amine accelerators permitted a wide activator concentration range

to be studied. As shown in Table 14, one significant effect of increasing

the concentration of 4-EDMAB relative to BP is to lower Tp to what appears

to be a limiting temperature (51 to 52 °C). These results parallel our results

on the effect of 4-EDMAB concentration on the setting time of composites [24].

The correlation of 4-EDMAB concentration with EP, however, does not appear

to be as straightforward. Both the concentration of the amine and BP as well

as the ratio of these two components of the initiator system are important

parameters affecting EP [25].

The use of the moderately reactive non-amine accelerator, AP, also is

well suited to DSC studies of chemically-activated polymerizations (Table 15).

27
Table 15

EP by DSC

1 for Poly[BIS-GMA(7)/TEGDMA(3)] Chemically Activated

Polymeri zati on —BP/AP

la

la/l

bp

T

P AH EP

AP Cone.

(Wt. %) Ratio (°C) (cal/g) (%)

0.54 1.10 43 34.6 52.6

0.54 1.40 49 35.0 53.4

0.56 0.80 40 35.2 53.5

0.56 0.94 45 34.4 52.3

0.59 0.91 41 35.9 54.6

0.59 1.33 44 35.1 53.4

0.59 1.59 49 36.4 55.3

^Thermal scan rate =2.5 deg/mi n. ^L

rp = BIS-GMA(7)/TEGDMA(3) containing one percent BP and

0.1 percent BHT.

Table 16

i

EP by DSC* for Poly[BIS-GMA (7)/TEGDMA (3)] Liquid/Liquid

Paste/Paste and Powder/Liquid Formulations

|

T

p

AH EP

1

Run Accelerator Part Peroxide Part Ratio (°C) (cal/q) %|

1 La=0.5% DHPPT LBp

=1.0% BP, 0.1% BHT 1 42 30.5 46.^

L/L In BIS-GMA(7)/TEGDMA(3)

1

2

P/P

Paste: 75%, Paste: 75%,

silanized glass silanized glass

fi Her +L

a

fi Her +Lg

p

1 45 28.2 42.8

3

J

Pow/L

Same as in run 1 Silanized glass filler Pow/L=

2.9

28 29.7 45.2

1

L = Liquid; P = Paste; Pow = Powder

* Thermal scan rate 2.5 deg/mi n.

i

28
The effect on Tp of increasing the concentration of AP relative to BP by

increasing the l^/L^ ratio does not seem to be the same as that observed

for 4-EDMAB, but instead shows an opposite trend. Also, there seems to be

less fluctuation in these EP values than was observed with the BP-amine

initiator systems. All the EP values for the chemically-activated polymerizations were in the same general range (43 to 57 percent).

The EP values obtained with the chemically-activated polymerizations

for a liquid/liquid, a similar paste/paste, and a powder/liquid formulation

are compared in Table 16. The somewhat lower EP of the composite from the

P/P formulation is probably due to absorption by the glass filler of some

of the exothermic heat of polymerization. The low Tp and relatively high

EP value for the Pow/L run probably is due to the high BP/amine concentration.

The high EP values observed in the case of the thermal polymerization

with BP alone for TEGDMA (see Table 11) were also observed when this monomer

is polymerized by chemically activating the decomposition of BP with an

amine accelerator. However, on contrast to the single exotherm obtained

in the thermal polymerization thermograms, the chemically-activated polymerizations of TEGDMA have thermograms with two distinct exotherms, and

therefore, two peak temperatures (T

^ and T^). The results of two different

formulations are summarized in Table 17. The effect of doubling the BP and

DHPPT concentrations is to lower the peak temperatures without significantly

affecting the EP values.

The same type of behavior also was observed on doubling the BP concentration in the thermal polymerization experiment (Table 11).

The organic component of dental composite restorative and sealant

materials primarily consists of several methacrylate monomers whose overall

functionality is greater than one. The copolymerization of such multifunc29
Table 17

EP by DSC* for Poly(TEGDMA)**

Chemically-activated Polymerizations (BP/DHPPT)

Ka

Ami ne Cone.Acc.

(wt.%)

l

BP

BP cone. (wt.%)

LA/L

Bp

Ratio

Ifil

°C

Ie2

°C

AH

(cal/g)

(± Std. Dev.

)

EP

0.50 1.0 1 50 50 79.5

(± 0.9)

83.5

1.0 2.0 1 28 58 78.1

(± 1.0)

82.1

*Thermal scan rate =2.5 °C/min

**AH represents average of three experimental runs for each formulation of

LA/L

BP*

tional vinyl monomers, under ambient conditions, leads to the formation of a

crosslinked, three-dimensional network polymer having residual unsaturation

in the form of pendant vinyl groups and, perhaps, unreacted monomer or

oligomeric species as well. Therefore, the properties of this complex network

copolymer will be determined not only by the chemical structure and composition

of the monomer system, but also by the degree of conversion or extent of polymerization (EP) of the vinyl groups. The presence of unreacted monomer and

oligomers can have a plasticizing effect (i.e., lowering the glass transition

temperature) on the polymer, thereby altering the physical and mechanical

properties of dental materials fabricated from the monomer system. In addition, the presence of pendant residual carbon-carbon unsaturation can make

the polymeric matrix more susceptible to oxidative degradative reactions

[26,27]. In dental composite materials such chemical degradative reactions,

if unchecked by stabilizers, can lead to poor esthetics (i.e., color instability) and poor durability (i.e., loss of anatomic form). It is important,

therefore, to be able to assess, at least on a relative basis, the degree

30
of residual unsaturation remaining after the polymerization of various

dental monomer systems.

DSC offers a comparatively facile and rapid method for determining

the degree of conversion of vinyl monomers to polymer. The EP values of

a variety of polymerized dental monomer systems can be compared under the

same conditions as a means of selecting those having the least residual

unsaturation. Thus, for a given principal monomer (e.g. , BIS-GMA), the

effect of the diluent monomer's structure and composition can be evaluated.

For example, the use of a bulky nonfunctional methacrylate and a smaller

quantity of a flexible di methacrylate may result in a more complete polymerization of a rigid dimetacryl ate such as BIS-GMA or BIS-EDMA.

Another potential area of application for DSC is in the evaluation of

inhibitors for dental monomers with a view toward improving the storage life

of dental materials based on vinyl monomers. It has been shown that DSC

can be a rapid, quantitative method for determining the effectiveness of

various inhibitors of vinyl polymerization [28].

D. Resin/Filler Coupling Improvement

Previous results showed that addition of n-propyl ami ne to

3-methacryl oxypropyl trimethoxysi 1 ane (A-174) dissolved in cyclohexane enhanced

the silanization of silica. This treatment also yielded a more stable SiO^-

silane bond on storage of the silanized silica in water at elevated temperature (Table 18). On thermocycling in water for one week the high initial

tensile strength of the n-propyl ami ne-si 1 anized silica composite dropped

39.7 percent, but leveled off on further thermocycling. The lower tensile

strength of composite specimens containing silica not treated with n-propyl ami ne

decreased 24.5 percent after one-week cycling but decreased continuously

on further exposure to water. The rather low strength properties of the

31
Table 18

Effect of Thermocycling Time on Resin-Filler Bonding as Judged by

Measured Tensile Strength of Composites

Tensile Strength, MPa

0 1 week 2 weeks 3 weeks

Composite* containing fumed

silica treated with A174**

and tri propyl amine in

cyclohexane

18.1±1.0 10.910.6 9.310.7 9.510.9

Composite* containing fumed 9.410.5 7.110.4 6.310.5 5.410.1

silica treated with A174

in cyclohexane

Thermocycling at 5 to 55i °C

* Composition of the composite

Benzoyl peroxide paste, percent by weight

BIS-GMA 53.62

TEGDMA 22.99

* BHT 0.154

Benzoyl peroxide • 0.766

Filler 22.47

Amine Paste, percent by wei ght

BIS-GMA 53.88

TEGDMA 23.11

BHT 0.154

Dimethyl -ptol ui di ne 0.386

Filler 22.47

** 3-methacryl oxypropyl trimethoxysi lane

composites prepared with the colloidal silica are probably the result of

incomplete copolymerization on curing of the methacrylate present on the

treated silica and the methacrylate groups present in BIS-GMA and diluent

monomer.

Further results of the dependence of the effectiveness of the silanization on the specific amine is shown in Table 19. Aromatic and heterocyclic

32
Table 19

The Effect of Amines on the Silanization of Silica Surfaces

Ami nes Basicity Bonding, %*

Primary amines

n-propyl ami ne 3.84 x 10" 1

100

sec-butyl ami ne -- 100

-4

tert-butyl ami ne 4.79 x 10

* 100

Secondary amines

di ethyl ami ne 9.5 x 10“ 4

100

di propyl ami ne 1.0 x 10“ 3

100

di-n- hexyl ami ne -- 100

di i sopropyl ami ne 1.09 x 10" 3

72.9

di i sobutyl ami ne 1.02 x 10‘ 3 58.0

Tertiary amines

tri ethyl amine 5.5 x 10“ 4

65.5

tri-n-butylami ne 8.52 x 10“ 5 68.0

N , N-di methyl benzyl ami ne 8.14 x 10“ 6 52.7

qui nucl i di ne 3.8 x 10“ 4 74.8

Aromatic amine

N , N-di methyl ani 1 i ne 1.1 x 10" 9

4.7

Fumed silica pellet was treated with A174 and a respective amine (molar

ratio = 1 /0.0042 moles) in cyclohexane for 1/2 hour, washed with acetone

for ten minutes, and air dried.

* -1

Calculated from relative intensity ratio of IR peaks of 3740 cm (free OH

bond) and 1850 cm ^ (siloxane bond in the silica).

amines are poor catalysts. Primary amines are most efficient probably because

of the absence of steric hindrance. On the other hand, di i sopropyl ami ne and

di-i sobutyl ami ne containing bulky side groups attached to the nitrogen atom

are much less efficient than diethyl amine and di -n-propyl ami ne , two compounds

which have about the same basicity constant. For tertiary amines, the

efficiency does not appear to be related to the basicity of the amine which

33
may indicate that another reaction mechanism might be involved in the

silanization than in the reaction with primary or secondary amines.

Glass particles (Corning 7724) silanized in two different solvents

(acetone and cyclohexane) with and without n-propylamine and then stored

in water for various lengths of time have been prepared. The treated

reinforcing fillers coated with one percent benxoyl peroxide will be

incorporated into a composite formulation and the effect of the different

silanizing and aging treatments on the physical properties of the cured

composites will be determined.

An extension of this work was initiated to investigate the j_n vi tro

wear performance of experimental composites silanized with and without

n-propylamine (see III, this report).

An oral presentation, "Application of Silane Coupling Agent to Inorganic

Components of Composites," was presented at the annual meeting of the International Association for Dental Research, Chicago, March 1981.

While improvement in the hydrolytic stability of the coupling between

resin and filler has been shown as a result of silanizing the filler in

the presence of N-propylamine, it would be desirable to improve this aspect

further. One approach in this laboratory and elsewhere has been to explore

the use of highly fluorinated dental monomers. A potential problem that

may arise from the use of such a highly fluorinated resin matrix with a

conventionally silanized glass filler is the possible lack of sufficient

compatibility between the two components to give optimum bonding of the

polymeric matrix to the glass. To this end, we have initiated a study to

explore the efficacy of fluorinated silane coupling agents (i.e., those

with polymerizable vinyl groups) commercially available. However, on the

possibility that through synergistic interaction we could derive the hydro34
phobic benefits of a highly fluorinated silane agent that was not a coupling

agent if employed with the usual silane coupling agent, A- 174, we examined

tri decaf 1 uoro-1,1,2 ,2- tetrahydrooctyl dimethyl chi orosi lane (TDFOS) whose

structure is shown below:

CH,

i CF

3

CF

2

CF

2

CF

2

CF

2

CF

2

-CH

2

CH

2 -Si-Cl

CFL

TDFOS

The silane agent proved very reactive with unsilanized glass filler

as evidenced by a mildly exothermic reaction during a silanization procedure

using cyclohexane as th carrier solvent.

Initially we applied 0.5 percent of TDFOS to the glass filler by usual

silanization techniques but without an amine catalyst (cyclohexane solvent

plus 2 hours at 60 °C, followed., by rotary evaporation of the solvent and

vacuum drying for 24 hours). The second step involved applying 0.5 percent

of A-174 to the TDFOS treated glass by the usual procedure (cyclohexane

plus 2 percent n-propylamine as the catalyst, etc.). The resulting dual

silanized glass was then coated with 1 percent benzoyl peroxide (BP) and

used to make composite specimens from a BIS-GMA (7)/TEGDMA (3) resin

mixture for diametral tensile strength determination. The DTS values

were significantly lower (~ 30 MPa) than those obtained using the conventional A-174 silanized glass filler coated with 1 percent BP (DTS ^ 45 MPa).

However, by reversing the silanization procedure (i.e., applying A-174

first and TDFOS second) a glass filler was obtained which, when used to

prepare composite specimens, gave diametral tensile strengths of better

than 40 MPa.

35
Table 20 summarizes the effect of the dual silanization treatment on

the tensile strength of various experimental composite materials. All the

specimens were prepared by powder/liquid formulation techniques employing

filler coated with 1 percent BP. In addition, the composite specimens

containing either of the fluorinated silanized glasses had a much whiter

appearance than those prepared with the A- 174 silanized glass. Also, they

have somewhat better color stability. However, water sorption studies

carried out on composites using the new dual silanized glass filler revealed

little difference from the water sorption of composites prepared with only

A- 174 silanized glass.

E. Microporous Glass as Reinforcing Filler for Composites and Denture

Base Systems

The objective was to use a barium-free microporous glass to

achieve a reinforced but radiopaque composite with improved finishing

qualities and to examine potential application to denture base systems.'

A prototype porous gel-route-prepared glass containing 40 percent

SiO^, 30 percent ZrC^, and 30 percent A^Og was obtained from Owens-Illinois.

In a typical composite formulation the glass filler proved highly prorous.

Working properties of the mix were poor. An extremely low powder/liquid

ratio was required apparently because of the high porosity of the glass.

Curing of the composite was very slow or sometimes did not occur. Furthermore, on addition of peroxide initiator the glass turned grayish. This

apparently resulted from reaction with organic residues remaining in the

glass after manufacture.

36
Table 20

Effect of Fluorosilane (TDOFS) Treatment of Glass Filler on 24 Hour

Diametral Tensile Strength of Experimental Composites

Silane Treatment DTS + Std. Dev.

Liquid Formulation of Glass Filler P/L Ratio (No. of Specimens

BIS-GMA (8)

A IBM (2)

0.3% DHPPT

1st TDF0S

2nd A- 174 3 30.0 ± 1.5 (6)

B Same as A 1st A- 174

2nd TDF0S 3

41.7 ± 2.3 (5)

C Same as A Only A- 174 3 46.9 ± 2.3 (4)

BIS-GMA (85%)

D DMGDMA (14.7%)

DHPPT (0.3%)

1st TDF0S

2nd A- 174

3 29.2 ± 5.0 (5)

E Same as D 1st A-174

2nd TDF0S

3 48.5 ± 3.9 (6)

F Same as D Only A-174 3 53.1 ± 2.2 (6)

BIS-GMA (83.7%)

G DMGDMA (14.8%)

PDFOMA (1.2%)

DHPPT (0.3%)

1st A-174

2nd TDF0S

3 47.4 ± 4.0 (6)

H Same as G Only A-174 3 48.4 ± 2.0 (4)

BIS-EDMA (49.3%)

I DMGDMA (41.3%)

PDFOMA (9.2%)

BDMA (0.2%)

1st A-174 4 48.9 ± 3.5 (5)

J Same as I Only A-174 4 43.2 ± 1.2 (5)

Monomers

BIS-EDMA = BIS-(2-methacryloxyethoxyl ) ether of Bis phenol A

IBM = Isobornyl methacrylate

DMGDMA = decamethyl ene glycol dimethacrylate

PDFOMA = pentadecaf 1 uoroctyl methacrylate amine

DHPPT = N ,N-bi s(2-hydroxypropyl )-p-tol uidi ne BDMA = t-butyl -dimethyl ani 1 ine

37
Incorporation of microporous glass to obtain a radiopaque easily

polishable denture base was also investigated. From 20 to 40 percent of

y-methacryloxypropyltrimethoxysi lane-treated microporous glass was incorporated into a typical denture base resin. Rectangular denture base specimens

of the different materials were obtained after a 9 h cure in a denture

flask at 74 °C and 0.5 h at 100 °C. The working properties of the mix

were not altered by incorporation of the glass. The experimental materials

were somewhat lighter in color, but all other properties were similar to

current commercial material. Finish and polish of the experimental materials

were very satisfactory. However, when the specimens were placed between a

wedge and subjected to x-rays, the denture base showed no radi opacity on the

exposed film, despite the radi opacity of the filler.

We anticipate being supplied with other microporous fillers which will

be examined for these applications.

F. Improving Dental Cements _ -

1. Vani 11 ate-Contai ni ng Cements

Zinc oxide-eugenol (Z0E) cements, because of the excellent biocompatibility of the hardened material, have found a wide range of

applications in dentistry [29,30]. They have excellent sealing characteristics and possess bacteriocidal qualities. Their limited strength and

poor resistance to wear and disintegration, partially because of their

relatively high solubility in oral fluids, deter their more extensive use

as temporary restorations. Because the electron-rich phenolic hydroxyl

group in the eugenol inhibits free radical polymerization, acrylic resins

and to a lesser extent, dental composites, in contact with a Z0E cement do

not polymerize completely. This incomplete cure results in polymer surface

38
regions with poor physical properties such as low surface hardness. Furthermore, ZOE cements adhere only weakly to acrylic restorations and poorly to

bone or dental tissues.

Although the eugenol incredient is relatively nontoxic, free eugenol

has some inflammatory characteristics [31-35]. It has a penetrating,

long-lasting odor and lingering taste which can be unpleasant to many

patients. Incompletely hardened cements containing much residual eugenol

can produce irritation and toxic cell reactions [36,37].

To overcome some of these deficiencies, especially to improve the

mechanical strength of the ZOE cement, much research has been conducted to

replace eugenol, but the use of eugenol substitutes usually results in

cements possessing poor physical properties.

Zinc oxide will react with many chelate forming compounds, especially

those containing o-methoxyphenol (guaiacol) groups, to yield cementitious

products. Cements obtained from o-ethoxybenzoi c acid (EBA) and zinc oxide

have found a considerable number of applications in dentistry because of

their strength and excellent biocompatibility, especially as luting agents

and as bases [38,39]. However, the materials disintegrate too rapidly

under clinical condition to be employed for more permanent restorations.

The initial objectives of this study were to (1) synthesize 2-methoxyphenol (guaiacol) derivatives such as 2,5-dimethoxyphenol (DMP) and esters

of vanillic (4-hydroxy-3-methoxybenzoic acid) or its isomers which are

liquids at room temperature, (2) study their reaction with metal oxides,

and (3) evaluate the properties of the resulting cements.

*

39
Results of the investigation of the usefulness of DMP cements were

described in the previous report [1]. DMP containing cements may be useful

where materials with properties intermediate to those of ZOE or EBA are

desired provided DMP is as biocompatible as the structurally related eugenol

The second phase of this effort was concerned with cements in which

the liquid ingredients are esters of vanillic acids or its isomers containing

o-al koxyphenol or other groups capable of forming chelates.

Neither vanillic acid nor its common isomers are liquids at room

temperature, which is a prerequisite for conveniently preparing a cement by

mixing the chelator with a metal oxide. However, the hexyl ester of vanillic

acid is a liquid at room temperature. Hexyl 4-hydroxy-3-methoxybenzoate

(hexyl vaniH ate, HV) was synthesized from vanillic acid and n-hexanol [40],

refluxed for' 26 h and distilled under reduced pressure.

On mixing hexyl vanillate (HV) with zinc oxide (reagent grade, sieved

through a No. 70 sieve) a c-ohesive cement is formed slowly, hardening only

after several hours. More rapid curing is obtained on preparing solutions

of the vanillate in another liquid chelating agent such as o-ethoxybenzoic

acid. Typical results obtained with mixes containing solutions of these

chelating agents with 64 percent zinc oxide, 30 percent aluminum oxide,

6 percent hydrogenated rosin powder and a powder/liquid (P/L) ratio of

1.7 g/0.2 ml are given in Table 21 and Fig. 1. The best physical properties

are obtained in the 5 to 25 percent vanillate region. The cement containing

12.5 percent hexyl vanillate hardened in 6.5 min to give a one-day compressive

strength of 84 MPa and a diametral tensile strength of 6.2 MPa. This

compares with a one week compressive strength range of 16 to 38 MPa and a

tensile strength of 1.4 to 2.5 MPa for ZOE cements [39].

40
3WJJ.

OMIIJL3S

PROPERTIES OF n-HER YL VAiXilLLATE - o- • ETHOXYCENZOIC ACID-ZINC OXIDE CEMENTS

o-Efhoxypenzoic Acid, %

Figure 1

41
Table 21

Properties of Hexyl Vanillate-o-Ethoxybenzoic Acid

Cements with Zinc Oxide-Al umi num Oxide-Hydrogenated

Rosin Powders

Composition of Liquid

n-Hexyl Vanillate EBA

% %

Setting Time

mi n

Properties of Cement

3

Strength

Compressive Tensile

MPa MPa

100 -- > 180 -- -- 50 50 9.5 — —

37.5 62.5 7.25 57.9 ± 12.7 —

25 75 7.0 56.4 ± 10.6 —

17 83 7.25 70.6 ± 10.6 5.6 ± 0.3

12.5 87.5 6.5 83.6 ± 11.0 6.2 ± 0.8

10 90 6.5 81.3 ± 12.1 —

5 95 6.5 78.4 ± 5.4 6.0 ± 1.1

0 100 6.25 62.6 ± 9.0 “ —

Powder-liquid ratio: 1.7 g/0.2 ml

Powder: 64 percent Zn0--30 percent AlgOg— 6 percent Hydrogenated Rosin

42
With different P/L ratios, compositions having varying physical properties, useful for different dental applications, can be prepared (Table 22).

Depending on the consistency, mixing characteristics and film thickness

desired for specific applications, the materials are useful as intermediate

restoratives, insulating bases, root canal restorations, and as pulp capping

and luting agents.

Generally, the larger the amount of powder incorporated per unit

volume of liquid the greater was the strength and the lower the water

solubility (Table 22). Too high a P/L ratio yields mixes of high consistency, which, because of poor handling characteri sties , would not be suitable

for clinical applications.

The powder- 1 i quid ratio to be employed depends on the ultimate use of

the cement. Thus, a material prepared by mixing 1,1 g to 1.3 g of powder

with 0.2 ml liquid is suitable as a luting agent for cementation of crowns

and bridges where a material having a thin film thickness is preferred.

Mixes of heavier consistency (see Table 22) are useful as insulating bases,

and with or without modifications, as materials for temporary fillings.

Other uses of these materials are as pulp capping agents, root canal restorations, and with modifications, as impression pastes. The material properties

clearly surpassed the requirements of American Dental Association Specification

No. 30 for Dental Zinc Oxide-Eugenol Type Restorative Materials, Type III,

Class 1 (filling materials and bases), and Type IV (cavity liners) [41].

If the powder components are sieved to give a proper film thickness in the

hardened cement, these materials will significantly exceed the requirements

of Type II, Class 1 cements (for permanent cementing purposes).

Comparison of data given in Table 22 shows that hexyl vanillate HV-EBA

disks solubilize considerably less than disks prepared from EBA cements

43
n O' 0) r~

s: i— r~

—i. QJ o —i* JD r

•- QJ JCl

C

fD Q. c —)• 3 -J* I-1 H-* i-1 ro ro Q.

QJ Q. • • • • • cn 33 • • I—1 CO —i o o CO o 33 QJ i _j c —i. CD

O' fD ro

CL • —> on —!• c+ << m

03 CT) cn O') O) 3 —1 O 3> • • • 1 • Ja —Ja —h » • o cn cn 1 o 3 3

fD

QJ CO n • o on 3 <n O') CO CO CO r> 3 fD CO i—1 CO M CO o

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h-*

M cn "J M M co ro —<•

M • • • • < O O') ro O ro fD mc

<-+ cn cn O') -O _i* • • • • • 3 CO o ro CO CO

—\

CO fD 1+ i+ l+ 1+ 1+ 3

QJ 2 cn CO o o o o o 33 —la fD • • • • • 0) -J 3 cn (-> co CD -e» fD <+

a-: SC

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roro

44
employing the same P/L ratio of 2.0 g/0.2 ml. Similarly, at a luting

consistency, HV-EBA disks lose much less weight than those prepared from a

commercial ZOE cement.

Specimens with the composition given in Table 23 were prepared according

to the ADA Specification No. 30, immersed in water that was changed weekly

and weighed after various time periods. The table compares solubility data

for the cement specimens containing hexyl vani 1 1 ate-EBA and those prepared

from a commercial ZOE cement.

TABLE 23

Solubility and Disintegration of Hexyl Vani 1 1 ate-EBA Cements (HV-EBA)

Powder: 64% ZnO; 30% A^O^; 6% Hydrogenated Rosin

Liquid: 12.5% Hexyl Vanillate; 87.5% EBA

Powder/Liquid Ratio: 1.3 g/0.2 ml for the experimental cement;

0.67 g/0.2 ml for commercial material as recommended in the manufacturers instructions

Average Change in Weight Percent

Material 1 day 1 month 6 months

HV-EBA -0.27 0.48 0.80

ZOE 1

" -1.85 -3.38 -12.40

When the water in which the specimens were stored

during the last week prior to the 6-month storage

period was evaporated, the weight of the solid

residue was, respectively, 0.5 percent of the

HV-EBA specimens and 0.8 percent of the ZOE

materi al

.

t Commonly used commercial ZOE cement.

If desirable, the setting time of the HV-EBA cements can be reduced by

addition of small amounts of acid to the liquid. Table 24 shows the rapid

decrease in the setting time (from 6.5 min to 20 s) of cements to which

0.1 to 10 percent acetic or acrylic acid was added. An acid capable of

forming a chelate may be most suitable to reduce setting time. Thus,

45
addition of 0.8 percent dimethylolpropionic acid to the liquid decreased

setting time and improved mechanical properties of the cement.

Because of the electron poor nature of the hydroxyl group of hexyl

vanill ate neither this compound nor HV-EBA cements inhibit polymerization

of acrylic or composites. On the other hand, the inhibiting action of

eugenol or 2,5-dimethoxyphenol when added in small amounts to the catalyst

paste of a commercial composite is given by Table 25. Thus, the presence

of even 0.1 percent of these compounds in the composite mix yields a soft

polymer even after allowing 15 min to cure. Addition of 1 or 5 percent of

these phenolic compounds further retards the polymerization. In comparison

neither EBA or HV in up to 5 percent concentration appreciably affect the

polymeri zati on.

TABLE 24

Effect of Acid Addition on the Properties of

HV-EBA Cements

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

Liquid: 12.5% Hexyl Vanill ate; 87.5% EBA

Powder/Liquid Ratio: 1.3 g/0.2 ml

Strength

Acid Added to Liquid

%

Setting Time

mi n Compressi ve MPa

Tensi 1

e

MPa

0. 1 Aceti

c

5.5 61.4 ± 11.2 5.0 ± 0.1

0. 1 Acetic 4.5 55.1 ± 17.5 --

1 Acetic 2 to 2.5 — -- 0.1 Ac ry 1 i c 5 54.3 ± 14.5 --

1 Acrylic 3 40.3 ± 6.4 -- 10 Acrylic 0.3 -- --

1 Benzoic 5.0 -- --

1 Polyacrylic (MW=20,000) 4

-- -- 1 Polycarboxyl ated

Polystyrene 4.5 -- —

0.8 Dimethylolpropionic 4 71.7

i

—

i

+1 6.4 ± 0.8

3 Hydroci nnami

c

6

“ —

46
TABLE 25

Inhibiting Effect of Phenolic Derivatives on the Cure of a Commercial Composite

Cure Time in Min of Composite Containing

Liquid added o-Ethoxy- 2,5 Dimethoxy- n-Hexyl

to Composite, %

a Eugenol benzoic Acid Phenol Vanillate

0 4 to 4. 5 4 to 4.5 4 to 4. 5 4 to 4.5

0.1 “V 30 (soft) 4 to 4.5 15 (soft) 4 to 4.5

1 > 180 4 90 (soft) 4.5 to 5

5 > 1400 4 to 4.5 > 1400 6

a Phenolic derivatives were mixed into the catalyst paste. Concentration

based on total weight of pastes used to prepare composite (Adaptic).

Not only is the rate of polymerization of resin that is contacted

unchanged when using the newly developed cement but also the surface properties of composites cured against the newly hardened cement remain unaltered.

Table 26 gives the results of indentation and recovery tests of the surface

of a composite cured against HV-EBA cement which had been mixed 10 min before

placing the composite. The surface hardness of composite specimens cured

against the HV-ZOE cements (57.6 pm) is similar to that of composite specimens

(blanks) which were not in contact with cement. However, composites cured

against ZOE cement had higher indentation (63.1 pm) and lower percent

recovery values indicating an inhibited (lower degree) cure (polymerization)

resulting from the presence of eugenol at the resin-cement interface.

47
TABLE 26

Indentation and Percent Recovery of Composites Cured

in Contact with Dental Cements

Cement was cured for 10

Composite was stored in

min prior to placing

100% RH for 24 h.

the composite.

Composite Cured Against Indentation3

pm

Recovery, — 56.9 ± 3.5 76.3 ± 4.5

Z0E Cement 63.3 ± 3.1 70.3 ± 3.9

HV-EBA Cement 57.6 ± 2.1 78.1 ± 2.8

^Using a 12.7 mm steel ball loaded at 30 kg for 10 min.

u 10 min after release of the 30 kg load.

The HV-EBA cements adhered strongly to nonprecious dental alloys and

to amalgams, dental resins, and composites. Thus, nonprecious alloys or

amalgam disks or rods, when cemented together with the cements and placed

in water, adhered strongly. Even after 10 months storage of the specimens

in water this bonding was still extremely strong. Similarly, cement disks

cured against dental composite adhered strongly to the composite after

10 months storage in water. Cements based on Z0E or EBA do not bond to

such substrates to any appreciable extent. A weaker bond is formed between

the HV-EBA cement and gold. The strong adhesion of the cement, especially

to resins and nonprecious alloys, should prove most useful for clinical

applications where a strong cement-resin bond is desired.

The complete absence of inhibition of HV-Z0E cements coupled with

their excellent adhesion to resins and metals is advantageous and should

make it possible to place acrylic resins or composites over bases employing

this cement. Furthermore, a portion of the cement when used clinically as

48
a temporary restoration can be retained as a base for an acrylic resin

restoration. A complete reexposure of deep cavities is avoided, reducing

irritation of the dental tissues during this procedure.

HV-EBA cements have no odor. The presence of the vanillate esters

should yield cements with bacteriocidal properties. Although hexyl vanillate

has been considered as a food preservative, the toxicological and biocompatible properties of the newly developed cements must be studied further

before its dental usefulness can be ascertained.

It is possible to improve the properties of the HV-EBA cements in many

ways. Investigations conducted so far have been directed toward (1) determining the scope of the reaction of vani 1 late-EBA with ZnO, and (2) modifying

the properties by incorporation of polymeric reinforcing agents and addition

of polymerizable compounds and reinforcing fillers to the formulations.

Replacing hexyl vanillate with other esters of vanillic acid yielded

cements with properties similar to those employing hexyl vanillate. However,

these cements may possess specific properties such as increased hydrophobicity

which makes the hardened material more desirable for specific applications

in dentistry where water repel lancy and minimum solubility of the material

in the mouth's fluids is of prime importance. The n-heptyl , n-decyl, and

2-ethyl hexyl vanillate and hexyl isovanillate were synthesized. The heptyl

vanillate, synthesized by a procedure similar to that described for the

hexyl compound, has a boiling point of 181 to 183 °C at 2 mm pressure and a

melting point at 24.5 to 30 °C. (Yield: 70 percent.)

The n-decyl vanillate was synthesized by refluxing decyl alcohol with

vanillic acid in the presence of jD-tol uenesul fonic acid. After removal of

the unreacted alcohol, purification, and recrystall i zation from n-pentane,

the n-decyl vanillate had a melting point of 40 to 40.5 °C. (Yield: about

50 percent.

)

49
2-Ethyl hexyl vanillate was prepared from the correspondi ng , dry alcohol

(13.0 g, 0.10 mol), vanillic acid (8.4 g, 0.05 mol), and 0.1 g £-tol uenesul fonic

acid. After refluxing for 27 h and extraction with NaHCO^ and H^O the

product distilled at 192 to 193 °C at 2 mm. The slightly yellow viscous

liquid was 98 to 98.5 percent (gas chromatography) pure. (Yield: 97 percent.)

n-Hexyl isovanillate was obtained on refluxing n-hexanol (dried over

molecular sieve), 10.2 g (0.1 mol), isovanillic acid (3-hydroxy-4-methoxybenzoic acid, 8.4 g, 0.05 mol), 0.1 g p-tol uenesul fonic acid, and 100 ml

toluene for 24 h. Most of the solvent was distilled off, another 0.1 mol

alcohol and xylene were added, and the mixture was refluxed an additional

10 h. The unreacted acid was filtered off. After addition of methylene

chloride, and extraction with NaHCO^, the organic layer was dried and

distilled. The n-hexyl isovanillate distilled at 173 °C/2.5 mm. The

slightly viscous, yellow liquid proved 97 to 98 percent pure by g-as chromatographic analysis. (Yield: 59 percent.)

Properties of cements containing 1.3 g of powder mixed with 0.2 ml

liquid having the same mole percentage of the respective vanillate and EBA

are given in Table 27.

Another vanillyl containing compound, vanillyl methacrylate, was

synthesized by the method shown below:

I ° ch

? ci2 + CEL = C— C— Cl ^

1

{! (OLCH KN

0

J

Vanillyl Alcohol

0

l QL0C— C= OL

( 2

! 2

OH

Vanillyl Methacrylate

50
TABLE 27

Properties of Cements Containing Various Vanillate

Esters and EBA in the Liquid

Powder: 64% ZnO; 30% A^O^; 6% Hydrogenated Rosin

Powder/Liquid Ratio: 1. 3 g/0.2 ml

Liquid Setting Time Strength

,

MPa

Vani 1 late EBA mi n Compressive Tensi 1

e

12. 5%-n- Hexyl 87.5% 5.5 61.4 ± 11.2 5.0

i

+1 o t—

13. 2 n-Heptyl 5.5 58.3 ± 11.3 6.3 ± 0.8

12.3% n-Decyl 87.7% 5.0 41.5 ± 11.9 -- 15.3% n-Decyl* 84.7% 4.5 50.1 ± 5.2 5.6 ± 0.4

18.3% n-Decyl

13.9% 2- Ethyl81.7% 5.0 42.6 ± 7.0 “ “

hexyl* 86.1% 5.5 48.5 ± 4.2 5.6 ± 0.5

*t

12.5% n-Hexyl T 87.5% 5.0 55.1 ± 9.4 6.5 C\J I

+1 1—

* Equimolar composition

^ Isovanillate

of vanillate in the 1 iquid •

The colorless liquid product, b.p. 160 to 170°/4 mm,n^ = 1.5313 was

characterized by gas chromatographic (GLC) and infrared (IR) analyses. GLC

analysis indicated the presence of some impurities (~ 5 percent) in this

monomer. Because of the greater electron densities of the aromatic rings

of both vanillyl alcohol and its methacrylate derivative compared to vanilli

acid or its hexyl ester (HV), greater inhibition of the resin polymerization

was anticipated. Neat vanillyl methacrylate did not polymerize within one

hour using an initiator system consisting of 1 percent benzoyl peroxide and

0.5 percent N,N-dimethyl-3,5-xyl idine. However, vanillyl methacrylate did

photocure in less than 2 min with 0.5 percent benzoin ethyl ether and UV

radiation (Nuvalite). Therefore, it may be possible to employ vanillyl

methacrylate in photocurable cement.

51
Polymeric reinforcing agents can be incorporated in either the liquid

or the powder. A number of rubbery polymers were dissolved in the liquid.

Alternatively, small particle size polymers were incorporated in the powder

resulting in cements having properties given in Table 28. Thus, less

brittle materials with improved tensile and compressive strength especially

useful as temporary restoratives can be prepared by addition of various

polymeric reinforcing agents to the powder or liquid.

Investigations were initiated to take advantage of the unique behavior

of these cements in not inhibiting polymerization. Thus, monomeric liquids

such as 1,10-decamethylene glycol dimethacrylate containing an amine accelerator were prepared and mixed with powder containing peroxide initiator.

This addition of monomer improved the physical properties of the cement.

TABLE 28

Composition—Powder: 64% ZnO; 30% A^O^; 6% Hydrogenated Rosin — Liquid: 12.5% Hexyl Vanillate; 87.5% EBA plus polymer

additive dissolved in liquid.

P/L Ratio: 1.3 g/0.2 ml liquid.

Polymer

Addi tive

Setting Time

mi n Strength, MPaCompressive Tensile

None

2% Polyurethan (Estane 5712,

5.5 61 5.0

B. F. Goodrich)

3% Butadiene-acryl oni tri 1 e

,

vinyl terminated (Hycar 1300

5.0 65 5.7

X-22, B. F. Goodrich)

10% Methacrylate copolymer

(Acryloid K 120N, Rohm

6.0 to 6.5 61 5.7

and Haas)f 4.0 31 6.6

10% Vinylite VYHH (Union Carbide)!

f Added to the polymer powder

6.0 to 6.6 76 8.2

52
As an example the powder was thoroughly mixed with 0.1 percent finely

powdered benzoyl peroxide.

To the liquid containing 12.5% hexyl vanillate—87.5 percent EBA was

added 10 percent of 1,10-decamethylene glycol dimethacrylate containing

0.5 percent £-(dimethyl ami no)phenthanol accelerator.

The powder and liquid were mixed employing 1.3 g of powder per 0.2 ml

liquid. The material hardened in 6 min giving a cement with compressive

strength of 79.4 ± 1.8 MPa and a tensile strength of 6.3 ± 0.8 MPa.

Cementitious restoratives with drastically improved mechanical properties which could find applications as long- lasting temporary restoratives

were prepared by incorporating glass or silica fillers along with the metal

oxides into these formulations. Thus, in preliminary studies a powder was

prepared from 64 percent zinc oxide, 30 percent aluminum oxide, and 6 percent

hydrogenated rosin. Two parts of this powder were mixed with 1 part glass

(Corning Glass 7724) silanized with 3-methacryloxypropyl-trimethoxysi lane

which had been coated with 1 percent benzoyl peroxide. The liquid consisted

of a mixture of 1 part of 12.5 percent hexyl vanillate; 87.5 percent EBA,

and 1 part 1,10-decamethylene glycol dimethacrylate containing 0.5 percent

£- (dimethyl ami no) phenyl aceti c acid.

The powder and liquid were mixed in a ratio of 1.8 g powder per 0.2 ml

liquid. The material hardened in 6 min to give a cement with a compressive

strength of 137.6 ± 5.8 MPa, and a tensile strength of 14.1 MPa. Thus, the

mechanical strength of these materials was about three times that of Z0E

cements.

Future investigations will use different monomers and monomer concentrations as well as different amounts of glass fillers to optimize properties

of these promising materials. Furthermore, it will be necessary to establish

53

if formulations containing monomeric methacrylates can be used in deep

cavities without causing inflammatory responses.

An attempt has been made to synthesize a number of divani 1 lates.

These compounds are capable of forming chelates with metal oxides at two

different sites within the molecule. Thus, such compounds are potential

crosslinking agents which may strengthen the phyical properties of dental

cements based on the reaction of metal oxides with chelating agents.

Divani llates can be prepared from the corresponding diols and vanillic

acid using methods described previously involving the synthesis of vanillic

esters. Thus, 1,10-decamethylene divanillate was prepared from 1,10-decanediol

(0.025 mol) and vanillic acid (0.063 mol) in the presence of 0.25 g

£-tol uenesul fonic acid. The mixture, dissolved in 50 ml toluene and 40 ml

tetrahydrofurane, was refluxed for 30 h and stripped of solvent. The

residue was purified by extraction with base. The dark brown oil was

dissolved in methanol and boiled with activated carbon to remove impurities.

The diester was characterized by infrared and GLC analyses and appeared to

be 95 percent pure.

1.3 g of powder of the composition given previously was mixed with

0.2 ml of liquid containing 11 percent 1,10-decamethylene divanillate and

89 percent EBA (equal to the molar composition of the liquid used previously).

The mix hardened in 4.5 min to give a cement with compressive strength of

70.6 MPa and a tensile strength of 7.5 MPa.

Similarly, 1,6-hexamethylene divanillate was synthesized and incorporated

into the liquid. The resulting mix hardened in 5 min and had a compressive

strength of 47.8 ± 3.8 MPa and tensile strength of 6.2 ± 0.8 MPa. Polymeg 2000,

an oligomeric diol was used in the esterification reaction of this alcohol

with vanillic acid as shown below:

53a
2

CO„H

O

i och

3

OH

+ HO- CH

2

CH

2

CH

2

CH

2 - 0 -H

L SnClp, toluene

0

C— 0— ( CH-CH-CH CH 0 — 0)

c c l c

OH

0

n

n = 28

The product obtained by this reaction was a waxy solid, m.p. = 28 °C,

yield 65 percent, and was characterized by IR analysis.

A cement formulation was prepared by dissolving 12.5 percent polymeg

divanillate in EBA. Cement specimens had approximately 2/3 the compressive

strength of HV-EBA cement specimens. A cement formulation using only a

small amount of the polymeg divanillate in the HV-EBA liquid gave a compressive strength value of 57.1 ± 5.9 MPa.

In the reaction of 1,2-propanediol and vanillic acid the primary

hydroxyl group of 1,2-propanediol reacted preferentially (IR analysis) so

that the actual product, a viscous yellow oil, is the monovanillate

An attempt was also made to synthesize the trivanillate ester of trimethyl ol propane , but the desired product proved difficult to purify.

Two other compounds which could react with metal oxides to yield

chelates were also synthesized:

54
1 . 1,8-Bi s(o-carboxy phenoxy)octane was prepared.

Synthesis of 1,8-Bi s(o-carboxy phenoxy)octane

The compound was synthesized from 0.2 mole salicylic acid, 0.1 mole

1,8-dibromo-octane , 0.4 mol NaOH and potassium iodide. The mixture was

refluxed in ethanol. After removal of solvent and purification an impure

product was obtained, MP = 132 to 142 °C.

The compound, dissolved in EBA and mixing with the ZnO powder formed a

rather brittle cement in 5 min.

2. Sebacyl di (dimethylolpropionic acid) was synthesized from 0.025 mole

sebacyl chloride, 0.05 mole dimethylolpropionic acid and 0.055 mole triethylamine at room temperature. Addition of 5 percent of the purified acid to

12.5 percent HV; 87.5 percent EBA liquid and addition of the ZnO powder gave

55
a cement in 5 min which had properties (compressive strength 55.4 ±6.5 MPa,

tensile strength 5.1 ± 0.4 MPa) very similar to HV-EBA cement not containing

the sebacyl di (dimethyl ol propionic acid).

Because of the potential usefulness of these noneugenol , nonpolymerizationinhibiting, high strength cements developed during the year, a "patent

disclosure" has been prepared. A talk "Cements from 2,5-Dimethoxyphenol

and Zinc Oxide" by J. W. Stansbury, H. Argentar, and G. M. Brauer was

presented at the annual meeting of the International Association for Dental

Research, Chicago, 1981.

2. New Polybasic Acid-Derived Cements

The resinous nature of certain salts derived from both monocarboxylic (e.g. abietic acid) and dicarboxylic (e.g., dimer sebacic acid) acids

has been known for some time [42-44]. The preparation of well defined

polyvalent metallic di carboxyl ates such as calcium sebacate usually requires

either high temperature (fusion reaction) or solution (metathesis reaction)

techniques which are probably not suitable to the usual dental clinical

situation for the preparation of cements.

Since the solid phase reaction at ambient temperature between the

relatively high melting typical organic diacids (e.g., sebacic acid,

m.p. 133 °C) and the still higher melting basic polyvalent metallic compounds

(e.g., Ca^H)^, m.p. 580 °C) is likely to be extremely slow, this route to

polycarboxyl ate cements is precluded. A solution technique, while feasible,

would have to contend with the presence of solvent (e.g., b^O) molecules

and other byproducts in the structure which would tend to weaken the polycarboxyl ate cement.

56
Dimer acids, a complex isomeric mixture of 36-carbon diacids with smaller

amounts of 54-carbon tribasic acids and trace amounts of 18-carbon monobasic

monomer acid, is a unique polybasic acid in its hydrophobic character and

room-temperature liquidity. The principal chemical structural feature of

dimer acids is that of a tetrasubstituted cyclohexene dicarboxyl ic acid. A

simplified representation of dimer acids is as follows:

where the R groups are long chain alkyl groups, two of which are terminated

The conventional polycarboxyl ate dental cements are based on the

reaction of a liquid form of a polymeric poly acid (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 polycarboxylate reaction crosslinks the polymer and aids in its adhesion to tooth

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

Polycarboxyl ates 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:

R

R

4

by carboxyl groups (e.g., R-j = (CH

2 )g CC^H, = CH=CH(CH2 )g COgH,

R

3 = CH3

(CH

2

) 4

-, and R4 = CH

3

(CH

2

) ?

- etc.).

0

0

© M++ ©

0

0 I

-u 0

Hydrophobic Backbone

57
In the past, such polymers have been prepared by fusion or methathesis

[42].

We have initiated a study to determine the feasibility of preparing

such polycarboxyl ate polymers, termed hal atopolymers , by direct reaction of

the liquid dimer acid with basic polyvalent salts such as CaCOH^. Preliminary results using powdered CaCOH^ and a highly pure dimer acid have been

encouraging. A hard, white cementious material was obtained in about 2 min

at 37 °C and 100 percent relative humidity from mixing 2 parts by weight of

CaCOH)^ with 1 part by weight of dimer acid. A 1:1 powder/liquid ratio

gave a somewhat more flexible cement in about in about 4 min at 37 °C and

100 percent relative humidity. Other salts of calcium such as the carbonate

and the various phosphates did not react under these conditions. However,

mixtures of these salts with various amounts of CaCOH^ did form hard

masses in reasonable setting times.

Since the conventional polycarboxyl ate cements display good adhesive

properties, it will be interesting to see if the dimer acid cements have

similar properties with respect to hard tooth tissue. The dimer acid

cements may not be as hygroscopic as the conventional polycarboxyl ate

cements due to the relatively low concentration of carboxylate groups in a

basically hydrophobic hydrocarbon structure.

3. Polyvinyl Phosphonic Acid Cements

There is extensive literature concerning the ability of polyphosphonic acids to strongly complex with tooth structure. [45-49]

Addition of a polymeric phosphate acid to a zinc phosphate and a polycarboxylate cement improved their mechanical properties and adhesion to

tooth structure [50].

58
A review of possible alternatives to polycarboxyl ic acids for use in

dental cements reveals that polyphosphonates are feasible ones. Since

polyvinyl phosphonic acid is a strong acid and forms strong complexes with

apatitic surfaces, cements made with it should be adhesive and more insoluble

than polycarboxyl ic acid cements. Polyvinyl phosphonic acid greatly reduces

the solubility of enamel and dentin in acid and the cement will therefore

be caries inhibiting. It will also be biologically bland in dental applications because complexation with metal ions will prevent diffusion of acid

into the pulp, similar to polycarboxyl ate cements.

The objective of this work was to produce a cement using aqueous

polyvinyl phosphonic acid as the liquid and examine its properties.

Initially, polymerizations of vinyl phosphonic acid using 0.5 percent

azo bis i sobutyroni tri 1 e (AIBN) initiator at 70 °C for 5 h resulted in a

polymer whose molecular weight was too great to give a workable mix.

Therefore, it Was polymerized with 2 percent AIBN at 65 °C for 2 h, and a

lower molecular weight was obtained. A 75 percent aqueous solution of this

polymer reacted very rapidly with available zinc oxide or silicate powders.

An attempt to deactivate zinc oxide by heating at 1000 °C for 5 h did not

reduce its reactivity significantly. However, after washing silicate

powder for 5 min with 10 percent phosphoric acid, filtering, washing with

distilled water, refiltering and drying, the product gave a workable cement.

At a powder/liquid ratio of 1.5/1 the cement had a compressive strength

of 55 MPa. With further work this could undoubtedly be bettered. Unfortunately, the cement showed no adhesion to tooth structure, even when the

tooth surface was primed with the cement liquid, or the cement was mixed at

a powder/liquid ratio of 1.2/1 to provide more available polymer for bonding.

59
The bond strength of a cement may depend on the polyacid used. For

example, co-polymerization of polyacrylic acid with itaconic acid for use

in gl ass-ionomer cements reduces the bond strength. Polyitaconic acid will

form a satisfactory cement but it is not adhesive to tooth structure [51].

Investigation of the bonding ability of cements made from different

polyacids should give valuable information on the mechanism of bonding and

factors which affect it. Polyvinyl phosphonic acid cements need to be

studied further.

60
II. Development of Adhesive Bonding Techniques*

A. Mineralization of Dentin as an Adhesive - Developing Technique

Since the introduction of hypermineralization as a method of modifying

the dentin surface to improve the adhesion of polycarboxyl ate cement

[52] the approach has been found to benefit the bonding of composites

alone [53], or in conjunction with surface active co-monomers [54] or

cyanoacrylates [55]. Clinically feasible mineralizing agents have been

developed which are effective with polycarboxyl ate cements after application

for less than 3 mins. [56,57].

Commercially available cements containing a polycarboxyl ic acid

(polycarboxyl ate and glass ionomer) are continually changing and,

unfortunately, the effect of a given mineralizing agent varies with the

particular formulation of each product. [58,59] This suggests that the

cations in the cement play a major role. in adhesion. [59] Since the

adhesion of polycarboxyl ic acid cements is essentially via ionic interaction

of carboxylate anions to metal cations on the tooth surface, the adsorption,

exchange or deposition of metal ions other than calcium or their salts

on dentin may be beneficial. A study by Beech [60] found that pretreatment

of dentin with a range of metal salt solutions gave no improvement in

the bond strengths of polycarboxyl ic acid cements. However, sequential

application of 1 M CaCl^ and 0.5 M Na^PO^ (Ca

3

(P0

4

) 2

precipitation) for

3 min. increased the bond strength of polycarboxyl ate cement but not

gl ass-i onomer.

* Acknowledgement is given for the contribution of Dr. Derrick R. Beech,

Director, Australian Dental Standards Laboratory who participated as a Guest Worker at NBS in FY 81.

61
The overall object of the current work was to enhance the bonding of

polycarboxyl ate and glass ionomer cements and of self-curing monomers such

as cyanoacrylates and methacrylates, by increasing the mineral content of

the dentin surface or by changing its ionic character. Specific objectives

were, to further investigate the j_n situ precipitation technique using a

range of metal ions, as well as utilizing other methods of depositing

cations to find a way of improving the bond strength of all polycarboxyl ic

acid cements.

Bond strengths for the materials examined (Table 28) were determined

by a method first reported by Beech (52). Briefly, a circular area (3 mm

in diameter) of flat dentin or enamel surfaces of human teeth mounted in

self-curing resin was masked off. An orthodontic button was cemented

thereon in place and, after 24 hrs. storage in water, the load required to

pull the button free was determined. The results are summarized in Tables

29-31 and in the SEM pictures of Fig. 2-13. For the latter, treated dentin

sections were coated with gold/palladium and examined in a Cambridge or

ETEL SEM.

The original formulation of polycarboxyl ate cement containing zinc

oxide and perhaps some magnesium oxide in the powder typically had bond

strengths to enamel and dentin of 5 and 2.5 MPa, respectively (Table 30,

Poly C). Incorporation of 8% stannous fluoride in Durelon gave bond strengths

for enamel and dentine of 5.8 and 4.5 MPa, respectively, in previous work

reported by Beech [60]. The present work has found 6.3 and 5.2 MPa for

Durelon (Table 29). The tensile strength of the cement is only around 6

MPa so these bond strengths are approaching the upper limit. Nevertheless,

the calcium phosphate precipitation technique does produce a worthwhile

improvement with Durelon and Poly C. It is effective after application for

only 1 min. Other precipitated metal phosphates generally produced a

62
slight decrease in bond strength with Durelon, except Sn^CPO^)^ and Zn^CPO^^

(Table 29). The latter leaves the bond strengths unaffected, whereas the former

appears as effective as Ca^PO^^ precipitation. Zn++

and Sn++are present in

Durelon powder and these may assist in bonding to like ions. Sn++ also is

known to have a special affinity for hydroxyapatite. Sn^CPO^^ did not

improve the bond strength of Poly C (Table 30). Presumably Sn++ions must

• +*+* be present in the cement to equilibrate with those on the surface. If Sn ++

is not in the cement, the Sn on the tooth surface will diffuse into the

cement and any benefit will be lost.

Methods of depositing mineral suggested by Drs. Chow and Brown

(Table 29) probably could be made to work in a 1-3 min. time period.

Ferric oxalate, as suggested by Dr. Bowen, has some potential with glass

ionomer cement, particularly when mixed with polyacrylic acid (Table 30).

The SEM of a ferric oxalate treated surface, Fig. 3, was notably different

from the control (Fig. 2). It is difficult to improve glass ionomer cement

bond strengths [60]. Glass-ionomer cements experience considerable ionic

migration during their 24 hr. setting period, which may nullify the effect

of most surface treatments by removing ions deposited on the dentin surface.

Contrary to the claims of Shalaby et al_. [61], 50% ferric chloride

application after citric acid pretreatment was not beneficial to the bonding

of Fuji glass-ionomer.

It was generally found that j_n si tu precipitations (Fig. 5) did not

alter surface morphology compared to the controls (Fig. 2 and Fig. 4)

although residual precipitate was sometimes found. The treatments examined

in Figs. 6-13 gave varied surface appearances, with occlusion of the ends

of the tubules in many cases. Crystal growth was evident after ferric

chloride treatment (Fig. 13), despite washing.

63
TABLE 28

Pol ycarboxyl a te and Glass - Ionomer Material^

Type Name Manufacturer Powder/Li qui

d

Ra t i o

FI uori dated

Polycarboxyl ate

Durel on ESPA 1 .5:1

Polycarboxyl ate Poly C De Trey 1.5:1

G1 ass-Ionomer Fuji Ionomer

Type I

G. C. Chenr.'cal 1.4:1

64
TABLE 29

Durelon Pol ycarboxylate Cement

Mean

Treatment Substrate No. of Specimens Bond Strength

+ SD (MPa)

None Dentin 7 5.3 + 1.61)

4

6 5.64.37 ± 0.82) r ,, 0.89)

5J6 ± 1 .23

6 5.49 + 1.15)

[4.5 ± 1 .1]**

None Enamel 4 4.3 + 1.09*

6 3.02 * 0.6 *

6 5.04 4- 1 .73)

6 6.60 ± 1.84) 6.24 ± 1.93

6 7.20 4 1.97)

[5.8 ± 1.5]**

Slurry of Ca Phosphates

- 3 min (W. Brown)

+ OHAP Dentin 6 3.44 + 0.47

+ NaF

11 6 4.14 ± 0.78

CaHP0.-2H

? 0 forming solution

- 3 min (L. Chow)

Ferric oxalate - 1 min

II 6 2.99 + 1 .15

(R. Bowen)

A6D0-33 (62)

II 6 4.74 ± 1 .01

A600-38 (42)

11 6 2.06 ± 0.48

A600-34 (12.62)

U 6 4.87 ± 1 .64

6% Ferric oxalate + poly2-50) acrylic acid 5 „

- 1 min n 5 5.54 ± 4- 2.^0

IK Ca Cl

2 + 0.5M Na

3 ?0

4

n 6 5.26 ± 2.47)

3 min it i 6 6.65 ± 1.9 )

ii 6 5.44 ± 1 .04) 6 . e 7 + 2.05

• H 6 7.92 ± 2.36)

1 min II 6 6.1 ± 2.33

30 sec

H 6 5.85 ± 1 .28

1M CuCl

^ + 0.5M Na

3 P0

4 (3 min) II 6 3.86 ± 0.94

1H ZnCl

^ +--0.5M Na

3 ?04 (3 min)

•

II

II 6

-

6 5.54.88

±

± 1.57) , 2.16)

5 ' 4 1.83

1M Mg Cl

2 + 0.5M Na

3 P04 (3 min) II a 3.63 ± 0.90

1M SnCl

2 + 0.5M Na

3

P0

4 (3 min) II

II 6 - 6 6.19

6.91

+

± 1.69)1.96)

b -" 4- 1.78

1H BaCl

2 + 0.5M Na

3 P0

4 C min) II 6 1.58 4- 0,6

1H FeCl 0 + 0.5H Na..P0,

2 o 4 C- mi n

)

U 6 3.89 + 0.65

0.5M CrOHSO. (3 min) Fresh 11 6 1 .44 ± 0.55

IK CrOHSO. (3 min) Old II 6 2.39 4 0.83

15% FeCl

3 + 0.5M Na3 P0

4 (3 min) 11 6 1 .89 4 0.46

* Results for enamel anomalously low ccmpargsj to previous work

** Values from previous work
TABLE 30

Poly C Pol ycarboxyl ate Cemen t

Mean

Treatment Substrate No. of Specimens Bond Strength

+ SD (MPa)

None Enamel 6 4.19 4 1 ' 11

> 4.5f * 0.97

II 5 5.08 4 0.53)

None Denti

n

6 1.79 4 1 .01)

II 6 2.76 4 1.03)

2,23 1 1 .10

IK SnCl

2 + 0.5M Na

3 P0

4 min

)

II 6 1.51 4 0.97

1M ZnCl

2 + 0.5M Na

3 P04 (3 min) II e 1.76 4 0.68

1M CaCl, + 0.5M Na

3 P°4

(•; min) II 5 3.4 4 1 .07)

L

If

II 6 3.09 4. 1 .04)

' 3/23 ± 1 .00

II P min) II 5 3.20 4 0.58

it e 3.61 ± 1 ,30

6* Ferric oxalat

acrylic acid

e + poly-

(3 min)

II c 2.19 4 1.58

H II t; 1 ,94 4 1 .89

66
TABLE 31

Fuji Glass-Ionomer Cement

Treatment Substrate No. of Specimens

Mean

Bond Strength

+ SD (MPa)

None Enamel 6 5.58 ± 1.3) * •

6 4.85,1.5) 5 - 22 51 - 62

[4.5 ± 1.5]*

None Dentin 6 2.73 ± 0.83)

5 2.0 ± 0.72)

2 ' 40 " °' 83

[2.5 ± 1.6]*

Ferric Oxalate (R. Bowen)

6% : 1 min It 6 . 2.93 ± 0.4

12.6%:3 min n 5 3.0 ± 0.24

'

6% -Ferric oxalate + poly- u 6 3.41 ± 1.0 ) acrylic acid (3 min)* ) 3.37 ± 1.51

II u 6 3.34 ± 1.93)

1M A1(N0

3

) 3 + 0.5M Na-jPO^ (3 min) ii 6 2.28 ± 1.18

1M SnCl

^ + 0.5M Na

3 P0

4 (3 min) it 6 2.61 ± 0.16

'I'M' ZnCl^ + 0.5M Na

3

P0

4

(3 min) ii 5 1.54 ± 0.79

15% FeCl

3

(1 min) ii 5 1.31 ± 0.3

50% citric acid (1 min) then

15% FeCT

3

(1 min) ii 6 1.72 ± 1.18

15% FeCl

3 + 0.5M Na

3 P0

4 13 min) ii 6 1.50 ± 0.89

67
68
Fig. 4 Dentin. 1 min citric acid (control)

•Fig. 5 Dentin. 1 min citric acid then 1M CaCi_

2 +0.5M Na

3 P0

4 (3 min)

69
Fig. 6 Dentin. 1 min citric acid then 6.7% ferric oxalate (1 min,

70
Fig. 8 Dentin. 1 min citric acid then 6.7% ferric

oxalate + polyacrylic acid (3 min)

Dentin. 1 min citric acid then 15% FeCl

3

(1 mi

Fig. 9
Fig. 10 Dentin. 1M SnCl^ (3 min) - blown off

I

I

I

I

I

I

I

I

i

off

72 fl

3
Fig. 12 Dentin. 6.7% ferric oxalate + polyacrylic acid (3 min)

min

73

3 Fig. 13 Denti

n

1 citric acid then 15% FeCl (1 min)
B. Cyanoacrylates

Some cyanoacrylates show a much greater bond strength to dentin

than other types of self-curing monomers, after a short period of water

immersion. [62,63] This is strongly suggestive of some type of specific

interaction, but the nature of this interaction has not been determined.

Most previous workers have examined the effect of varying the ester

group of the cyanoacrylate on the bonding to dentin. However, the

results of Brauer et al . [63,64] clearly show that the bonding behavior

of cyanoacrylates, ostensibly of the same chemical type but from different

sources, varies widely. Thus a study of the bonding of the homologous

series of alkyl 2-cyanoacrylates is only meaningful when each contains the

same amount and type of additives, such as polymerization inhibitors:

these additives play a dominant role.

Causton and Johnson [55] have claimed improved adhesion after deposition

of mineral 1 ayers ,

• together with improved resistance of the bond to water.

An objective of the present work was to investigate the bonding of

cyanoacrylates to dentin, with particular reference to the bonding

mechanism and ways of improving the strength of the bond and its durability.

The materials examined and some gas chromatographic analyses are

indicated in Table 32. All products investigated by gas chromatography

were found to consist of one main component, with the exception of Loctite

Superbonder 420 which appears to be a 60:40 mix of ethyl and isobutyl

2-cyanoacrylate. All the cyanoacrylates listed in Table 32 were found to

have several minor constituents which could not be identified and which

differed widely between products.

74
TABLE 32

Cyanoacrylate Materials

Ester Group Name Source

G. C. Retenti on Times

(mm from injection)

BBS tri fl uoroisopropyl MBR 4197 3M 32, 122*, 143, 170

Ethyl (60%) + isobutyl (40%)

Superbonder 420 Loctit? 32, 125, 132* (60%),

150, 164* (40%), 1 £6

n-Butyl IS413, medium

viscosity

Loctite 130, 141 , 146, 1 72 . 177*

Isobutyl Tennes.'.ee Eastman

133, 141 , 148, 152,

158, 164*

Ethyl Permabond

Type 101

Pearl Chem.

Co,, Jipan

57, 130*, 168

* Major component (s).

75
The cyanoacrylate was applied to a circular area of enamel or

dentin 4 mm in diameter exposed by masking tape. Then an acrylic rod

(rod test) or an orthodontic button with a mesh base coated in freshly

mixed Adaptic composite (button test) was pressed firmly on to the

cyanoacrylate. After 24 hours storage in water the tensile force required

to dislodge the rod or button was determined.

The button test was employed because, by application of freshly

mixed composite on the cyanoacrylate, it duplicated the potential use of

cyanoacrylates as adhesive cavity liners for composite restorations.

[65] From Table 33, only Eastman isobutyl and Permabond ethyl gave substantial

bond strengths. The bond strength for Permabond was almost double that

for Eastman with the button test but they were essentially the same with

the rod test. In the rod test (Table 34) it was noted that Permabond

polymerized much more quickly than Eastman. Permabond showed a greater

spread of results in the rod test with some approaching 11 MPa. The

bond strength to Adaptic was measured (Table 33) to allay fears that

bond failure was occurring at the interface with Adaptic rather than the

tooth surface.

The button test results (Table 33) using human dentin as substrate and

citric acid pretreatment were in contrast to those of Brauer [63,64] who

used bone-bone interface bonding to investigate adhesion with cyanoacryates.

As a result of the strong initial bonding that he achieved, Brauer solicited

the cooperation of Mclnnes and Cl ayton- Jones of the University of

Wi twaterstrand , So. Africa to conduct a study of the pulp response induced

by pretreatment of unlined labial Class V cavities. While the treatment

(0.1 or 1.0% citric acid) removed the dental bur-created smear layer, severe

pulpal responses were noted. To circumvent the serious effects of citric

76
Table 33

Cyanoacrylates: Button Test Results

•Materi al Substrate Treatment

No. of

Specimens

_ Kean

Bond itrerotn

± SD (MPa J

MBR 4197, 3M

(trifluoroisoprcpyl )

• Dentin None 5 1.17 ± 0.41

Superbonder 420, • Loctite (ethyl + isobutyl 1

II II 6 1.27 ± 0.46

IS 413 medium : viscosity, Loctite 1 (n-butyl) II l« 5

• 2.51 ± 0.9

! Eastman (isobutyl) fl ll 6 . 2.45 ± 0.58

tl II 6 2.52 ± 1.1

n 1% citric

acid 1 min 6 1.89 ± 0.3

•t II 6 2.43 ± 0.57

Enumel 2.5% PolySAM 6 1.4 ± 0.33

% n None 6 1.18 ± 0.13

Adaptic II 6 4.90 ± 1.04

Permabond (ethyl) Dentin II 6 4.48 ± 1 .29

* U

U

CaC^+Na^PO^

(3 min)

H 6

6 0.54 ± 0.22

0.77 ± 0.24

77
Table 34

Cyanoacrylates (rod tesr )

Material Substrate Treatment

No of

Specimens

Mean Bend

Strength ±SD

MPa

Range

Permabond (ethyl) Dentin None 6 5.9 ± 3.75 2.68-10.94

Eastman (isobutyl) n II 5 6.29 ± 1.25 5.1 - 7.65

- 5 5.82 ± 0.92 5.0 - 6.55
acid, the utility of sodium citrate and the di sodium salt of ethylene

diamine tetracetic acid (Na

2

EDTA) has been examined. As will be seen from

Table 35, at least for the bone substrate, the 1% sodium citrate or 1%

Na^EDTA pretreatment do not lower the bond strength of the bone to bone

joints (bonded with isobutyl 2-cyanoacrylate) that had been stored in

water for 24 hours or thermocycled for this length of time between water

baths at 5°C and 55°C. However, a pretreatment using 17% Na

2

EDTA solution

which is an excellent medium to remove the smear layer reduced the strength

of the bond. Similar results were observed in pretreatments with Dical,

a calcium hydroxide based material. Since very satisfactory bone-bone

adhesion for time periods useful for temporary cementation were obtained

with sodium citrate or Na

2

EDTA it would be desirable to encourage a

cooperative study of the effect on the underlying pulp of using them in

pretreatment of exposed dentin surfaces.

No direct proof of covalent bonding between protein and cyanoacrylate

via initiation of cyanoacrylate polymerization by the basic groups in

protein, has been published. Leonard et aj_ [66] have shown that amino

acids in aqueous solution will attach as end groups on polycyanoacrylate

chains. This, in conjunction with the obvious ability of cyanoacrylate

to bond to skin and other proteinaceous surfaces, has resulted in the

assumption that covalent bonding to protein occurs. One technique for

examining the mechanism of bonding is infrared spectroscopy (IR).

Fourier Transform IR spectra (Nicolet FT-IR) of dentin and Permabond

cyanoacrylate were subtracted from the spectrum of a mixture of both. The

IR spectra of dentin, steerhide collagen and mixtures of Permabond with

dentin and with steerhide collagen after extraction of cyanoacrylate with

dimethyl formamide were recorded. FT IR showed no evidence for new bond

79
Table 35

Effect of Pretreatment of Bone on Tensile Strength

of Bone Joined with Cyanoacrylate Adhesive

Treatment

Tensile Strength of Bond

Stored 24 hr

in 37° water

MPa

Yhermocycl ed 24 hr

5-55°C water

MPa

Control 4.8 ± 0.5 3.3 ±0.8

1% citric acid in 30% EtOH 6.3 ± 0.5 4.0 ± 0.3

1% Na citrate in 30% LtOH 5.8 ± 0.6 4.4 ± 0.8

17% Na

2 EDTA Aq. Sol. 2.6 ± 0.6 — -

1% Na

2 EDTA in 30% EtOH 4.7 ± 0.4 5.1 ± 1.5

80
formation as a result of any chemical reaction which may occur between

cyanoacrylate and dentinal protein. Likewise, no residual cyanoacrylate

could be detected in the IR spectrum of denti n-cyanaoacryl ate or steerhide

collagen-cyanoacrylate mixtures after washing with excess dimethyl formamide.

Further, a mixture of Permabond with basically dry steerhide collagen did

not initiate polymerisation after 15 min. In contact with a moist powder

Permabond will polymerise in less than 20 sec. The use of dry collagen

eliminates competition of water with the basic groups in the protein for

reaction with cyanoacrylate; dry collagen should promote the

reaction of cyanoacrylate with collagen. If cyanoacrylate will not react

with pure collagen, the chances of chemical reaction with the tightly

coiled cross-linked collagen embedded in hydroxyapatite in the presence of

large amounts of water, proteinaceous fluid and dentin debris, appears

remote. Amino acids with amine or hydroxyl side groups are a minor

proportion of the protein chain and are frequently involved in inter or

intra molecular hydrogen bonding. In contrast, the great amount of water

present in physiological systems may overwhelm other potential initiators.

The decrease in bond strength of cyanoacrylates to dentinal surfaces on

exposure to moisture has been attributed to degradation of the polymer.

However, the decrease in bond strength appears much too rapid to arise

from degradation. If primary bonds, covalent or ionic, are not formed, then

penetration of water along the bond interface would rapidly break down secondary bonds. Self curing methacrylates will bond well to enamel etched with

2% phosphoric acid by secondary bonds but without tag formation. [67] Such

bonds resist water penetration until stressed, when rapid failure occurs. [68]

Why do cyanoacrylates adhere better to dentin than other self curing

resins? The answer probably is in the unique characteristics of

cyanoacrylates: they polymerize rapidly by initiation at the tooth surface

81
(good adaptation, intimate contact); polymerize only in thin films

(minimal polymerization stresses) and are very polar polymers (hydrogen

bonding and dipole interaction with protein). It is certainly reasonable

to expect that crystal growth on the dentin surface will improve bonding

by mechanical interlocking. Treatment of dentin with the calcium phosphate

i n si tu precipitation technique virtually eliminated bonding (Table 33).

This technique will deposit amorphous calcium phosphate rather than

crystals. The decrease is presumably caused by covering the protein with

such a mineral layer.

Finally, if adhesion of cyanoacrylate to dentin is by secondary bonds

only, then the breakdown of the bond in water will not be affected by

improving the resistance of the polymer to degradation.

C. "Reactive Adhesive" Formulations

A series of "reactive adhesives" were formulated and their bond

strength to bone was determined. These reactive adhesfves were formulated

with various components to produce a chemical cure within a few minutes.

The adhesives are usually prepared by dissolving an elastomer in a monomer

or oligomer solution. Polymerization takes place through a free radical

mechanism. The cured system is toughened by the elastomeric domains which

are incorporated into the structure by grafting.

Adhesive formulations were prepared from elastomers that were soluble in

the monomers used (acrylic or methacrylic acid and methacrylate ester monomers)

benzoyl peroxide initiators and inhibitor (butyl ated hydroxytol uene , BHT).

The second component was a solution of a tertiary amine accelerator with

which the substrate surfaces (bone) are treated prior to application of the

elastomer-monomer component. The bond strength of the joint between two

pieces of bone substrate was evaluated after storage in water.

82
A commercial polyurethane (Estane 5712), reported to be a reaction

product of tetramethyleneglycol ethers of varying molecular weight and

methylene di isocyanate was found to be soluble in the monomers we were

using. Joints prepared from a reactive adhesive formulation composed of

33% Estane, 33% decamethylene glycol dimethacrylate (DMDMA), 17% acrylic

acid, 17% methyl methacrylate, 0.04% BHT and 3.6% benzoyl peroxide in which

the substrate had been treated with 10% alcoholic dimethyl -p-tolui dine

did not debond on extended storage in water. The highest bond strength

following storage of the test specimens in water for 3 months was about

1 MPa which is much lower than the strength that can be attained with

alkyl 2-cyanoacrylates.

Replacement of methacrylic acid for acrylic acid decreased adhesion

to bone. Substitution of methacrylate esters such as 2-ethyl hexyl acrylate

for acrylic acid resulted in mixtures in which the polyurethane was

insoluble. The rubbery component did not dissolve completely on using

1 , 1 0-decamethyl ene glycol dimethacrylate. Formulations with Estane 5713

and 5740 were also employed, but the elastomeric component did not

dissolve in the monomer system. Compositions containing other rubbery

toughening agents such as vinyl terminated butadiene-acrylonitrile

(reactive liquid polymer Type 1300 X- 22 and 1300 X-23) or chlorosulfonated polyethylene (Hypalon H-30) did not yield satisfactory adhesives

Pretreatment of the bone with 1% citric acid or 1% sodium citrate did not

improve bonding. Thus, the reactive adhesives investigated did not look

very promising for bonding mineralized tissues and their study was

di sconti nued.

83
III. Wear Resistance and Mechanical 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

i n vi vo 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 from which appropriate

modifications may 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. 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

microdefect analysis using x-ray and optical microscopic observation of

damaged regions made resolute by a silver staining technique. In order to

obtain a relevant wear test, we assert that the subsurface damage resulting

from in vi tro wear should resemble that obtained from in vivo worn specimens.

84
L VDT 9

/ 8

Fig. 14. Schematic illustration of principle of pin and

disc measurement.

85
Other aspects examined in these studies are the couplings between chemical

solvents and wear and between subsurface damage generated during wear and

wear itself.

The following discussions include the work performed during this fiscal

year on wear and mechanical properties of dental composites and their component materials.

A. Pin and Disc Wear and Related Knoop Hardness Number (KHN)

Measurements

1. Instrumentation

The pin and disc wear apparatus is completely automated comprising three identical rotating platforms containing the disc shaped specimens

of the material to be investigated. The principle of operation is depicted by

Fig. 14. A stationary slider (pin) of human enamel, or appropriate substitute, impinges on each of the rotating specimens producing a wear track

of 12 mm mean diameter. 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. Figure 15 is

a close-up photograph showing the specimen and cup, wear pin and mounting

collet, probes and transformer, and the thermocouple junction.

A portion of activity during this period was dedicated to instrumentation maintenance. Some of the maintenance was deemed necessary by a gradual

increase in unreliability which was eventually traced to a power supply instability in the microcomputer controller. Eventually the computer became

inoperative. The power supply failed completely and inflicted damage on

86
87

Fig.

15.

Close-up

photograph

showing

wear

pin

and

mounting

assembly,

transformer

assembly,

and

water

jet

nozzle.
some other components. After the computer was repaired and returned to us,

its performance was excellent, but only for several weeks after which the

power supply failed for the second time. In order to complete the set of

wear measurements being undertaken at that time, an external power supply was

connceted. After these were completed the computer was again returned for

repair. It is our opinion that the present version of the computer's power

supply has insufficient capacity for its application. Unfortunately, space

limitations make it difficult to install a larger one. A separate external

power supply may be the answer. We have informed the manufacturer that this

is a problem they will have to solve.

Periodically, about once a year, it is appropriate to re-evaluate the

experimental imprecision and LVDT (linear variable differential transformer)

and related electronic drift rate. This is accomplished by simulating a

wear run using polished brass discs as specimens. Since the wear pins are

always maintained at a distance from the specimens, no wear is generated.

Ideally, the apparent track depth would be zero at all times. In Fig. 16

the apparent track depths are plotted against time for the No. 1 position

(out of 10 possible ones around the wear track circumference). Since the

drift rates are not known during actual wear runs, the standard deviation

values included in the figure apply to the residuals about the average in

lieu of those about the statistically fitted straight lines shown. For a

wear rate of 1 pm/hr a drift rate of 15 Nm/hr will produce an error of 1.5%.

This error is small in comparison to that caused by specimen to specimen variation of composites or that attributed to inhomogeneity within a composite

specimen. In some cases, the standard deviation of the residuals about statistically fitted straight lines on actual wear data are often less than those

given in Fig. 16. The lower variances result from the drift rates being

unknown for an actual wear run.

88
I 1

'

1 1 ! 1 1 0 5 10 15 20 25 30 35

Time (hrs)

Figurel6. Apparent track depth s versus time for a "no-wear" run. The

drift rates shown are obtained from linear regressions on the

data

.

89
During this reporting period, some improvements were made on the method

of calibrating the track depth measurement system. A calibration procedure is

conducted periodically about every three months. In this process, 1 pm of

differential displacement is made to produce 10 mV DC differential from the

LVDT conditioning amplifier. The LVDT core displacement is determined by using

a step shim comprising five 150 pm glass shims glued together as shown

schematically by the inset at the top of Fig. 17. During calibration the

core probe rests against position 0 which extends over the entire length

of the shim as revealed by the inset. The other probe, which is attached to

the LVDT housing, rests against any one of the other four positions. The

thickness of the shim at each position is measured by a micrometer. Although

the micrometer is read to 1 pm, readings are not expected to be better than

±2.5 pm.

The results from the most recent calibration are included in Fig. 17.

The LVDT readout in terms of displacement is plotted against the shim height

relative to the number 0 position. The straight line shown is obtained from

a linear regression on these data. The position numbers indicated at the

data points correspond to the positions shown in the inset at the top of the

figure. Departures from linearity and unity slope are not visible on this

plot for the data obtained at each of the three wear stations.

Figure 18 gives the residuals about the straight lines plotted against

shim height for the three wear stations. In this plot distinctions between

the three sets of data and apparent departures from linearity are clearly

visible. The LVDT outputs are read to 1 mv which corresponds to an ordinate

value of 0.1 pm on the figure. It is clear that the data, are not entirely

randomly distributed. The fact that the data tend to aggregate at each

position indicates the existence of systematic error in the shim thickness

90
Step Shim

e>

/

Figurel7. Illustration of calibration procedure. For explanation,

see text.

91
O

Station

#1

©

Station

#2 oo

CDO

inoo

ro E

cp

’cd

O

92

Fig.

18

Displacement

residuals

about

the

fitted

straight

line

shown

on

Fig.

17

for

the

three

wear

stations.
measurement. These are attributed to nonlinearity (screw displacement

versus rotation) of the micrometer. The errors shown are commensurate

with those expected from nonlinear micrometer response.

The track depth measurement system is calibrated over 600 pm. The

largest track depth measured so far on a wear specimen is 150 pm.

A manuscript which includes descriptions of the wear instrumentation

and the calibration procedure has been submitted for publication [69].

2. Influence of Silanization Treatment on Wear

Research by Brauer and Chen* of our Group has pointed to the possibility of catalyzing the silanization treatment of glass filler particles with

an amine as a means of improving the silane coupling action between

filler and resin matrix. Improvement in this bonding would presumably

be reflected in wear resistance. The specimens were prepared with the

compositions given in Table 36. The 1.9% silane catalyst concentration

produced the highest value in diametrical tensile strength. Although

wear measurements were attempted, the results were unsatisfactory. The

principal difficulty was that the wear specimens were not sufficiently

homogeneous to obtain meaningful data. Considerable improvement is

anticipated if times for equivalent cures are extended. This investigation

was interrupted by Dr. Brauer 1 s recent illness. As a result of recent

termination of Chen's visit to NBS, it is not clear at this time when

this activity will be resumed.

3. Mechanically Induced Damage and Wear on Composite Restoratives

In the FY80 Annual Report [4], we included some preliminary data on

the wear of Adaptic at various normal stress levels ranging between 2.5

and 20 MPa. In most cases the data reveal abrupt changes in the wear

rate by roughly a factor of two during the course of wear as depicted

*Guest Worker, Food and Drug Administration

93
TABLE 36

Composite Composition Used to Study Influence of

Silanization Catalyst on Wear and Mechnical Properties

Inqredi ent Composition %

Powder #1 #2 #3

Corning Glass #7724 98.5 98.0 96.6

A- 174 Silane = 3-methacryl oxypropyl

trimethoxysi 1 ane

0.5 0.5 0.5

Benzoyl peroxide 1.0 1.0 1.0

q-propyl amine (silanization catalyst) 0 0.5 1.9

Liquid

BIS-GMA 72. 1

1 ,6-Hexamethylene glycol

di methacrylate 27.5

Dimethyl -p-tol ui di ne 0.23

Butylated hydroxytol uene (inhibitor) 0.2

Powder-Liquid Ratio = 3:1

93a
symbolically by Figure 19. The occurrence of these breaks (at the onset of B)

is usually earlier with increasing stress. This phenomena is interpreted as

the initial wear (in region A) being dominated by an adhesion and/or abrasion

process. As wear progresses subsurface damage increases to a point where

severe breakdown occurs from fatigue. At this point a steady state is

obtained for which wear is enhanced through the addition of a fatigue mode

(Region B). The occurrence of the onset of B at earlier times with increasing

stress results from more extensive damage generated at the larger stresses.

This behavior is more general in that it is not restricted to composite

restoratives. Similar results were obtained by Prof. Dowson and coworkers

[70] at the University of Leeds on ultra high molecular weight polyethylene

(UHMWPE). The principal distinction between their techniques and ours is

that their disc is the abrader, and their pin is the specimen. Their configuration, accordingly, leads to a constant normal stress on the specimen,

whereas our produces an oscillating stress with periods correspondi ng to

revolutions.

The results of their research on UHMWPE are in many ways similar to

our on Adaptic . In their work the wear rates in regions A and B differ

by about a factor of two, and the time for the onset of B decreases with

increasing stress. The principal difference is in the time required for

the onset of B. In our work these times range between 5 and 20 hours,

whereas theirs, 100 to 600 hours. Their stress range, 3.5 to 22 MPa, is

similar to ours (2.5 to 20 MPa). Whether this distinction in times for

the onset of B may be attributed to the differences between techniques

or material difference (UHMWPE vs Adaptic ) is not clear at this time.

Intuitively, cycling stress is expected to produce failure at an earlier

time, which is evident from comparison of their and our results.

93b
Volume

Loss

Jig. 19. Symbolic vetr response resultiag from damage generated duriag vear.

94
(UJT/) s

QJ

IS)

is. s_0'

l/l

_r. 4 > CL

-D *o) tv

s.- +3 C )

•5c

.r:ooU4-> CL| fO "Uco

i.a

a'

95

Figure

20.

Extended
The silver staining microdefect analysis [71] was used on specimens

on which wear was terminated intermediately in both regions A and B. The

specimen with wear terminated in A showed essentially no subsurface damage,

whereas the one in B revealed considerable damage similar to that observed

on occlusal surfaces of j_n vi vo worn specimens. Since this analysis is

destructive, we were not able to obtain damage assessments as wear progresses.

Very recently we obtained some results on Adaptic using a stainless

steel pin at 10 MPa for 80 hours, which is a longer time than any used

previously by us. The results are shown on Figure 20, where the wear track

depth is plotted against time. In addition to the aforementioned break

at about 15 hours, a larger change occurs at 60 hours, where the wear

becomes essentially catastrophic, being nearly ten times as great as the

initial value.

A plausible i nterpretation for the second break is revealed by the

silver staining micro-defect analysis for which the results are displayed

on Fig. 21. From the micrograph a fault line is apparent which emerges

downward from the corner of the wear track and eventually turns parallel

to the wear surface at a depth of about 300 pm. The fault line demarcates

a damaged layer which extends to the surface and wears at the high rate

displayed on the right hand side of Fig. 20.

A somewhat general observation in clinical tests on composites is

that anatomic form deterioration seems to accelerate with time (see, for

example, Ref. [72]). The increasing wear rate resulting from wear mode

conversion, or addition, and severe subsurface damage may provide partial

explanations for this phenomenon.

A manuscript including the results of this activity has been submitted

for publication [73],

96
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97

SPECIMEN

10

MPA

Fig.

21.

Micrograph

showing

worn

and

unworn

surfaces

of

an

Adaptic

specimen

over

83

hours
4. Chemical Softening of Dental Composites

Another process which may cause deterioration in dental composites is the interaction with chemical solvents. The solvents selected are

among those recommended in Reference [4] for food simulation. These solvents

are water, ethanol-water solutions, acetic acid, and heptane. The greatest

interactive effects are expected when the solvent has a solubility parameter

value which approximates that of the polymer base, which usually comprises

BIS-GMA. Preliminary tests involved immersing specimens of Adaptic in

ethanol, cyclohexanone, and distilled water at 37 °C for two weeks.

Immediately afterwards pin and disc measurements using stainless steel pins

were made. Cyclohexanone was selected before we were aware of the FDA

Report [4] recommending food simulators to be used in connection with the

evaluation of food container lining materials.

The results are shown on Figure 22, where the wear track depth at

10 MPa is plotted against number of cycles or disc revolutions. Ethanol

produces the largest effect for which 11 microns are removed in the

first cycle. With water a slight decrease is observed, which may be

attributed to roughening of the initially polished surface. Subsequent

measurements with data taken every hour (about 1500 rev/h) are depicted

by Figure 23. Over the first hour the wear of the chemically-soaked

specimens is severe in contrast to that for the water-soaked one which

is negative, presumably because of the "break in" process mentioned

above. Through the remaining 44 hours, the wear of the chemically

soaked specimens exceeded that for the water soaked one by about a

factor of three. The total wear did not exceed the depth of the damaged

region which is about 200 pm as revealed by the silver staining technique.

98
Figure 22. Track depth s over first five revolutions for Adapti

c

specimens soaked

previously in ethanol, cyclohexanone, and water.

99

(jno
The resulting loss of hardness of the dental composite due to various

chemicals has been reported in a previous report [4].

.

All the chemical solvents used in the work mentioned previously are

in their pure form. One question which remains to be studied is the

effect of concentration of various chemicals on the degree of softening

of BIS-GMA polymer. Chemicals existing in the intraoral environment are

believed to be diluted with water. Therefore a set of tests using

ethanol/water mixtures was conducted; the change in surface hardness and

in wear resistance of chemical soaked composites were measured.

Unfilled BIS-GMA polymer samples were immersed in 100, 75, 50, and

25% V/V ethanol/water solution and the drop in surface hardness was

monitored as a function of immersion time. The data are shown in Figure 24.

A severe decrease in hardness was observed in all these immersed samples

especially that in 75% ethanol. By locating the solubility parameter

components of these chemicals used so far in the softening study on a

5 vs 5 , (association and dispersion part of the solubility parameter)

a plot (Fig. 25), it becomes apparent that BIS-GMA has a solubility parameter

with 7 ^ 6 da, ^ 9 and 2^6 ^16. The wide spread of the value of S„

a component of BIS-GMA molecule is believed to be a consequence of the

monomer size of BIS-GMA. According to the Hildebrand theory [75] for

mixed solvents, any mixture of chemicals with solubility parameters in

the range mentioned above will be a good solvent for BIS-GMA. Such a

result was indeed observed in our recent work on methanol and DMSO

mixture (Fig. 26).

Immediately after soaking Adaptic specimens in 25, 50, and 75%

solutions of ethanol in water, pin and disc measurements were made on

these three specimens. The results including those using the 0 and 100%

ethanol solutions from previous work are shown in Fig. 26. The wear track

100
140

120

o

o • o •

o

o°6 *

100

Ethonol

2.92 ym/hr °. e

o • o

o ©

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to

80

60

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3.19 /xm/hr

o O •

40

20

0

o ° • o • Waler

0.88^.m/hr

do

,oo u°

o0 ooooo ,Q 20 30 40

t (hr)

Figure 23. Extended wear for specimens used in Figure 3. Track depth s is

plotted against time. (1 hr = 1500 revolutions)

101

o* O

O
Figure

24.

Effects

of

various

concentrations

of

ethanol

T3

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Ui

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102

Time

(days)

0

5

12

17

0

5

12

17

0

5

12

17

0

5

12

17

VOl.

cone,

of

_i

nn

n

/

-rr-n/

ethanol

in

100/s

75

%

50%

25%

water
Solubility

Parameters

of

Various

Solvents

and

Bls"GMA

coCM

inCM

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103

Solubility

Parameter

(Association)
104

Figured.

Effect

on

Knoop

Hardness

of

unfilled

BIS-GMA

(30%

TEGDMA)

from

immersion

in

mixtures

of

methanol

and

DMSO.
Revolutions

Fi gure 27 . Wear track depth s versus revolutions in indicated

ethanol -water solutions.

105
depths are plotted for each of the first five disc revolutions. The

concentrations of ethanol in water are included as parameters. The

initial wear rate does not increase monotonical ly with increasing ethanol

concentration. The largest wear shown occurred at 75% ethanol for which

60 pm of composite were removed during the first pass.

The continuation of the wear shown on Fig. 27 extended to 40 hours

is illustrated in Fig. 28. The abscissa interval between data points is

one hour which corresponds to approximately 1500 specimen disc revolutions.

The initial values shown in Fig. 28 are the final ones in Fig. 27.

There is a change in the ordering of the wear rates with concentration

as the wear progresses. In Fig. 28 the highest wear rate occurs at 100%

ethanol in contrast to 75% for the first pass as shown in Fig. 27. This

change in ordering is attributed to the higher diffusion rate in BIS-GMA

of pure ethanol over that of lower ethanol in water concentrations.

A correlation is obtained between wear after soaking Adapti

c

specimens

in different ethanol solutions and change in hardness of pure BIS-GMA,

induced by soaking. Since Knoop hardness involves penetration over only

about the first 20 microns of substance, it is appropriate to consider

wear over the first five passes for this application. Figure 29 shows

the wear (after five passes) plotted against the ratio P./P^ where P..

and P^ are the initial average (before soaking) and final (average after

soaking) Knoop hardness values as shown in Figure 24. The alcohol-water

concentrations are included as parameters. Although the dependences of

both wear and hardness ratio are not monotonic with concentration, the

deppendence of wear on the hardness ratio is monotonic. In fact, except

for the 75% value, nearly linear response is obtained.

A manuscript which includes the results of this activity has been

submitted for publication [76].

106
160

140

120

o o°75%

.°° „°I00%

2.2 /xm/hr 0 o°° 0 °°

^ ° O

° 3.2 /xm/hr

o50%

100-

E

V)

o 2.5 jMn/hr

80

60

40

20

o o

o o

O o 0-8/xm/hr 0 o25%

O 0

- o

,oO oO oo 0-0 o o°

oooooo

OOO oOOO° OOO 1

o o o o 0%

0 0 o 0 0 0.9 /xm/hr

il8o0 0 o o

O O o o 0

J ! I 1 J 1 ! I 0 10 20

t (hr)

30 40

Figure 28. Wear track depth s versus time after extended immersion

in ethanol -water solutions. One hour corresponds to 1500

revolutions. The initial data point is the final one in

Figure 14.

107
Figure 29. Wear track depth s at five cycles versus hardness ratio P./P f at

different indicated concentrations of ethanol in water.

1

108
The work on the influence of chemical solvents has been extended to

include additional commercial dental composites, in an ongoing activity

at the time of this writing. The restoratives being investigated are

Profile, which uses a strontium glass filler, Prisma Fil, which is vi sable

light activated, and Adaptic, which is a conventional composite and is

used here as a control.

The specimens were stored in the solvents at 37°C for one week prior

to the wear tests. Knoop hardness values were obtained during this time.

Since we do not completely know the polymer composition of all of these

composites, we were not able to do hardness measurements on the pure

polymer base as with the previous work. Knoop Hardness Numbers on these

composites are difficult to obtain because of the uncertainties which

result from whether the indentor strikes a region of filler or polymer.

For this reason we obtained the average of 10 hardness velues on each

specimen.

Wear measurements were made on specimens previously stored in 0, 25,

50, 75, and 100% ethanol in water solutions, and heptane. Maximum

initial wear (after five cycles) was obtained at either 75 or 100%

ethanol storing conditions with Adaptic being the largest (38 pm) and

Prisma-Fil smallest (5 pm). At all other storing conditions the initial

wear of Prisma-Fil was essentially non-existent.

The preliminary results for the three composites are summarized in

Fig. 30. The initial wear track depth is plotted against P^ the final

Knoop Hardness Number (KHN), obtained after soaking. Hardness values were

not obtained on the 25 and 50% ethanol soaked specimens. A common threshold

value of surface hardness is approximated for all three composites regardless

of the storing conditions. Below this value the initial wear increases

109
40

O Adap4 ic

G Prof i i s 0 Prisma Fil

Concentrations:

% Ethanol in Water

H = heptane

O

100

30

E

3.

CO 20 r

O

75

>75

10

0

Q

75

G75

075

O

,00 CD75

100

c° o

0

of ' o

H

/r'.H H

0 20 40 60 80

p f (KHN)

100 120

Fig. 30. Initial wear track depths (after 5 cycles) versus final KHN, P^.

Prior to wear test the specimens were stored for one week at

37°C in the indicated solvents.

110
dramatically with decreasing hardness. Although the Prisma Fil specimens

are softened as seen by an apparent decrease in surface hardness with

increasing ethanol concentration, its wear resistance remains good in

contrast to that for Profile and, in particular, Adaptic.

B. Environmental Effect on Physical Properties of Dental

Composite Restorative

1. Changes (Compressive Strength and KHN) in Composites Induced by

Thermocycl i ng

The physical properties in terms of the compressive strength and the

surface hardness (as reported in III A4) of dental restorative composites

were found to be weakened [77] by certain food simulating chemicals chosen in

one of the FDA guidelines [74]. All but one of the composites tested

herein were immune to attack by acids. Furthermore, thermocycling seemed

to cause no negative effect on the physical properties of all the composites

tested.

A close examination using scanning electron microscope [78,79] and

subsurface damage layer analysis [80] of the worn surfaces of composites

pre-immersed in chemicals reveal that stress may not be the only dominant

cause of excessive wear observed.

The materials tested included two conventional composites —Adaptic,

Concise, one microfill composite--Si lar, one strontium glass filled

composite—Profile and a visible light cured composite—Prisma. The

surface hardness of the specimens were measured using a Knoop hardness

tester, each data point reported herein is an average of eight measurements

from two specimens. The compressive strength was determined using an

Instron machine and each data point represents an average of 2-4 specimens.

Ill
Lactic acid [81,82], acetic acid, and phosphoric acid with their

pH values maintained at 4.5 were used. The specimens were immersed in

these acids at 37°C and were removed at the end of 1, 3, 7, and 15 days to

test their physical properties. No significant trend of change in

compressive strength was observed in all the acid immersed specimens. All

the specimens except Profile showed no change in Knoop hardness measurement

results. The data are shown as follows:

TABLE 37. Change of Knoop Hardness Number (KHN)

and Compressive Strength (C.S.) in MPa of

Dental Composites Caused by Immersion in Acids

%

I

I

I

I

Days of Immersion ( Adapti

c

)

0 1 3 7

Acetic C.S. 172 110.0 288 114.1 273.5133.2 245 119.1

Acid KHN 66.31 5.5 57.41 6.3 52.31 7.0 57.11 4.8

Lactic C.S. 272 no 283 1 0 289.5124.8 261 111.3

Aci d KHN 67.51 4.2 49.61 3.4 61.61 4.4 59.71 4.6

Phosphoric C.S. 272 110 271 1 5.66 273.0146.5 235.5+21.9

Aci d KHN 62.31 4.6 56.51 9.5 54.7+ 5.0 61.5+10.0

Days of Immersion (Profile)

0 1 3 7

Acetic C.S. 317 134.2 302.5+16.3 300 124.0 271 117.0

Aci d KHN 78.3110.0 76.01 8.6 65.21 5.5 57.5124.7

Lactic C.S. 317 134.2 291 1 0 259.0177.8 289.01 8.5

Aci d KHN 71.051 8.2 62.31 6.2 59.21 5.0 56.91 9.6

Phosphoric C.S. 317 134.2 315 123.6 312.0119.8 253.5+13.4

Aci d KHN 71.81 8.0 77.61 8.4 65.41 3.8 77.21 4.7

, s 219.91230

56. 5± 6 .PI

219.9118.^

59. 5± 3.0

243. 5±1

7

59. 0± 4 a

15 I

290.6121

53.11 7

314.2118 8

54.91 7®

314.2125.9

66.61 9M
TABLE 37. (cont'd)

Days of Immersion (Prisma)

0 1 3 7 15

Acetic C.S. 336. 5±39.

3

369 ±22.6 342.5129.0 349.5127.6 342 ±29.5

Acid KHN 59. 6± 3.5 57. 3± 5.8 43.01 6.1 56.11 8.2 53.21 6.7

Lactic C.S. 336. 5±8.

3

310 ± 5.7 333 ±21.5 298.5110.6 353 ±32.0

Acid KHN 55. 5± 4.6 57. 0± 8.1 56.71 9.4 51.51 6.5 48.31 5.7

Phosphoric C.S. 336. 5±39.

3

278.5138.9 342.5175.7 318 ±17.0 385 ±37.6

Acid KHN 47. 5± 3.9 62. 3± 3.4 49.61 2.8 48.41 5.6 46.81 3.4

Days of Immersion (Si lar)

0 1 3 7 15

Acetic C.S. 290. 3±20.

9

324 ±2.8 310.5127.6 306.3115.

1

287.51 8.8

Acid KHN 28 ±4.8 27. 9± 4.0 26.61 7.8 26.31 3.7 26.51 4.6

Lactic C.S. 290. 3±20.

9

339.5119.1 307.5113.4 270.91 5.5 322. 1124.2

Acid KHN 28. 6± 4.5 32.0110.2 30.71 5.4 28.11 5.2 30.81 4.9

Phosphoric C.S. 290. 3±20.

9

328 ±14.1 316 ±8.5 318.1122.2 322.6114.3

Aci d KHN 27. 1± 6.2 31.41 8.8 33.6118.3 32.41 5.6 27.9110.5

Days of Immersion (Conci se)

0 1 3 7 15

Acetic C.S. 270. 5± 8.8 268.5124.8 270 ±26.9 304.5113.4 322. 1120.5

Acid KHN 85. 9±17.

4

75.8115.2 78.5117.3 67.2119.6 69.0115.8

Lactic C.S. 270. 5± 8.8 270 1 4.2 286.5116.3 278 ±15.6 298.5118.4

Aci d KHN 78.2114.4 81.3116.3 106 ±33.6 71 116.4 76.7116.2

Phosphoric C.S. 270.51 8.8 276 1 5.7 273 ±41.0 290.5110.6 306.4127.2

Aci d KHN 77 ±11.4 66.5113.9 102 ±26.2 79.7110.0 79.2114.8

113
The major difference between Profile and the other composites tested

is the filler composition; strontium glass was used in Profile but not in the

other ones. Therefore, it is reasonable to believe that reactions among

strontium glass fillers and the acids gave rise to the weakening of the

surface hardness of Profile."

The thermocycling test was carried out by immersing specimens in 5°C

and 55°C water baths and cycling at a rate of 540 cycles per day. Specimens

were removed at the ends of 1 , 3, 6, and 9 days for measuring the physical

properties. For all the specimens tested, no significant change of both

the compressive strength and the surface hardness was observed. Only the data

of Concise will be presented in Table 38, and these data are believed to

be representative for all other composites tested:

TABLE 38. Changes of Compressive Strength

and KHN of Conci se Resulting From

Thermocycling (5°C-55°C)

Cycling Date

Compressive 279 ±17.8 268.8±18.0 295. 8±14. 1 299.8±23.9 284.8±11.5

strength

(MPa)

0 1369

KHN 77.3110.9 78.81 9.4 79.31 5.6 81.5± 4.7 82.71 9.2

According to one of the FDA guidelines, the following chemicals were

chosen for food simulating purposes: acetic acid with pH < 5.0 for acidic

food, n-heptane for fatty food, ethanol for beverage and pure water. Three

additional chemicals were used in the present study to fill the gap of

solubility parameters between n-heptane and ethanol; they were methanol,

acetone, and ethyl acetate.

114
The composite specimens were immersed in these chemicals at 37°C

and removed at the end of 1 , 3, 7, and 10 days for the physical properties

measurements. The results of Adaptic only will be shown in the following

table, and all other composites showed similar behaviors.

TABLE 39. Effects of Different Chemicals

on the Compressive Strength and KHN of Adapti

c

*

Immersion Date

0 1 3 7 11

n-heptane C.S. (MPa) 270. 8± 8.4 269.3114.5 278.3120.5 291.3120.5 283.0113.5

(7.4) KHN 73 ±21.4 70.0115.9 80.1111.0 75.3117.4 86.7124.6

Ethyl acetate C.S. 280. 0±20.

1

247.0126.2 236.01 2.7 233.0120.5 230.0119.6

(9.1) KHN 67. 2±14.

1

43.41 5.7 43.01 7.9 48.41 9.5 41.11 2.8

Acetone C.S. 230. 0±26.

5

203.01 5.0 178.0130.

1

142.0115.6 134.01 9.9

(9.9) KHN 56. 5± 7.6 39.21 2.7 31.31 2.8 30.01 2.1 30.61 2.5

Ethanol C.S. 258.0120.3 246.0133.9 225.01 5.0 250.0114.9 208.0133.9

(12.7) KHN 60.5114.6 48.2113.4 44.01 8.9 35.01 4.8 35.11 5.9

Methanol C.S. 230.0128.6 202.01 3.5 213.01 5.7 179.01 6.4 152.0118.0

(14.5) KHN 63.51 5.1 36.71 5.3 36.61 2.2 32.71 5.4 32.01 2.7

Water C.S. 277.0114.1 288.0114.2 282.0123.2 264.01 7.1 207. 1117.6

(23.4) . KHN 62.41 4.5 57.41 9.8 54.71 6.0 60.6110.

1

58.91 5.° 3

*The number listed under each solvent is the value , " of its solubility parameter in terms of (cal/cm3 )9.

| These data indicate that chemicals with solubility parameters between

those of n-heptane and water will soften Adaptic. Furthermore, the Knoop

hardness test seems to be a more sensitive one in detecting the property

change than the compressive strength test.

V

I

X

I

f

115
A more complete study on the softening of BIS-GMA based dental composites

by chemicals with different solubility parameters has been finished, the results

have been submitted for publication [77]. The effect of the chemical

softening on the wear resistance of some dental composites was discussed in

the foregoing section (III A4).

The softening rate of dental composites by any given chemicals was

found to depend on the curing temperature, hence the degree of the Knoop

hardners number of any given composite by chemical softening could be

fitted by a simple equation relating the KHN with the immersion time t as

KHN = (KHN)o exp (-Dt)

The values of (KHN)

q and D for both Adaptic and Prisma cured at 37°C and

100°C are listed in Table 40.

TABLE 40. Initial Knoop Hardness Number (KHN) and Time

Constant D. of Dental Composites as A Function of Curing Temperature T

c T

c (°C) (KHN)q D(Day

_1

) Chemi cal

Adaptic 37 53.9 0.051 Ethanol

Adaptic 100 79.5 0.025 Ethanol

Adaptic 37 58.2 0.29 Methanol

Adapti

c

100 68.4 0.051 Methanol

Adaptic 37 59.4 0.12 Ethyl Acetate

Adaptic 100 83.9 0.031 Ethyl Acetate

Adaptic 37 44.7 0.050 Acetone

Adapti

c

100 80.2 0.044 Acetone

Pri sma 37 46.

1

0.030 Ethanol

Pri sma 100 61.0 0.028 Ethanol

Pri sma 37 48.0 0.113 Methanol

Pri sma 100 59.9 0.039 Methanol

Pri sma 37 49.5 0.039 Ethyl Acetate

Pri sma 100 66.3 0.018 Ethyl Acetate

Pri sma 37 44.4 0.035 Acetone

Pri sma 100 57.8 0.029 Acetone

116
The data shown above indicate that the softening rate changes not

only with the curing temperature as expected, but also changes with the

composites used. Prisma seems to have a lower softening rate for all the

chemicals tested.

2. Investigation of Iji Vivo Wear Mechanism of Dental Composite

Restorations

Nine biopsy specimens have been received from Dr. S. Moffa and Ellison

of U.S.P.H.S hospital at San Francisco. A special technique of obtaining

thin sections from dental composites has been developed. Transmission

optical micrographs were obtained from the silver stained thin sections of

these biopsy specimens. The results clearly demonstrate that the defects

within the damaged sublayer beneath the wear surface are located throughout

the polymer matrix instead of being confined at the filler matrix

interfaces. An example is given in Fig. 31 which is a transmission optical

micrograph from a 2+ year old Adaptic restoration. The polymeric matrix

within the damaged sublayer was uniformly stained by silver particle

indicating creation of defects throughout the matrix.

A theoretical development of the small angle x-ray scattering (SAXS)

in three phase (filler, matrix and microcrack) systems has been completed

and this work will be presented at both '81 APS meeting in New Orleans and

'82 IUPAC meeting in Amherst, Mass. Both volume concentration and the

characteristic length of the microcracks within the damaged sublayer can

be determined from SAXS data using this theoretical development.

A quantitative study of the microdefects within the damage layer using

a small angle x-ray scattering technique (SAXS) is in progress. One example

of the results will be given.

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A profile restoration removed from a second molar after 13 months

clinical service was examined. The x-ray scattering intensity was

calibrated with a 1 . 5% vol. LUDOX solution as a standard reference. The

main beam intensity was monitored and integrated throughout the whole

measurement. The average volume concentration of the microdefects

within a 200 pm layer beneath the occlusal surface was determined to be

0.092% using the density of the strontium glass filler within this

3 specimen as 3.5 g/cm . The corresponding values of the volume concentration

of the microdefects are 0.113% and 0.072% if the strontium glass density

3

were assumed to be 3 and 4 g/cm , respectively.

The linear dimensions of the microdefects within the specimen were

found to be isotropic judged from the uniform azimuthal distribution of

the scattered x-ray intensity. Furthermore, by using a modified Porod's

scheme the average value of the dimension of the microdefects was determined

o to approximate 40 A. Both the theoretical and the experimental work of

SAXS in composites will be described in manuscript form in the near

future.

A microdefect analysis by small angle x-ray scattering technique

was carried out on compression fatigued composite restorative material

(Profile). The specimen was fatigued with 5000 cycles half sine compressive

stress at 60% of its compressison strength. The specimen was then

sectioned and examined with SAXS technique. The results can be summarized

as follows:

volume concentration of microdefects: 0.32% (p . . . , =3.5) 'strontium glass

average linear dimensions:

o parallel to compressive stress: 226 A

perpendicul ar to compressive

o stress: 181 A

119
The data shown above on the in vitro specimen suggest that the

damage accumulated inside the _i_n vivo specimen is not likely caused by a

compression fatigue process. Similar work using SAXS was also conducted

on a Profile specimen worn by pin-disc equipment with 10 MPa normal

load. The volume concentration of the microdefects within this specimen

was found to be too low to be detected by SAXS.

120
IV. Dental Alloys and Ceramics

Stable prices, lower costs, and often superior properties have resulted

in an almost monotonical ly increasing use of nonprecious and low precious

metal alloys at the expense of gold alloys for fixed partial dentures. Experience with these alloys for porcelain veneered prostheses has indicated [83]

that due to the lack of fundamental knowledge regarding these systems, problems

have been encountered clinically and in the laboratory. Residual stress in

porcelain from cooling after firing is thought to be a primary factor leading

to failures; precise universally agreed- to techniques for assessing compatibility do not yet exist. A simple, convenient porcelain metal bond test needs

to be developed for product development, industrial quality control, and use

by standards committees. Methodology for optimizing and controlling casting

conditions in the laboratory are also needed, as are techniques to assess the

ability of solders to produce acceptable joints for dental applications. The

question of how to prepare patterns of fixed partial dentures to obtain multi

unit castings which give the best obtainable fit has only recently begun to be

addressed in depth; continued work in this area is needed. The following

sections summarize progress which has been made toward addressing these

problems and indicates directions for future studies.

In addition to dissemination of research results at IADR and Biomaterials

Society meetings (as is indicated in the following material), invited papers

will be presented at the AIME Annual Meeting (Recent Developments in Metals as

Bio/Dental Alloys: Tesk, Waterstrat, Hinman, and DeRijk), and a Symposium on

Finite Element Methods (FEM) and Analysis Techniques (Applications of FEM in

Dentistry: Widera, Tesk, Munir, and Hinman) in conjunction with the International Congress on Technology and Technology Exchange in Pittsburgh, February 1982. A chapter on Porcel ai n/All oy Fixed Partial Dentures has also been

121
invited for inclusion in a revised edition of Advanced Restorative Dentistry

by Lloyd Baum and Richard McCoy.

A. Porcelain/Alloy Stress Compatibility; Residual Stress:

As previously described [84,85], many factors are involved here.

Ultimately, a combined materials approach is indicated (for example, see

Bertolotti [86] and Fairhurst [87]). The program at NBS is using the

analysis of gap changes in a porcelain veneered split metal ring subjected

to laboratory firing schedules as the first step toward assessing compatibility

and as a technique which might find acceptance as a standard method of

evaluation. Both experimental and theoretical methodologies are employed

and considered as necessary compliments. For the mathematical modeling,

knowledge of numerous physical and mechanical properties are required; some

may not be critical to the problem at hand, others very definitely are.

Theoretical calculations completed during the past year have shown

that knowledge of the precise value of Poisson's ratio is not critical to

either evaluation of stress or of correlation of theoretical and experimental

values for gap changes. The calculation of residual stress is, however,

very dependent on accurate measurement of the elastic modulus. The determination

of this value for different porcelains is planned as part of future studies.

(This was originally scheduled for FY81 ; before proceeding, however, theoretical

assessment of the contribution of all measurable parameters was deemed

appropriate to provide guidance for experiments). This analysis showed

quite clearly that no direct correspondence in magnitudes or locations of

maximum shear, tensile (compressive), tangential and radial stresses exists;

this is important in consideration of stress relaxation and changes of the

glass transition temperature (Tg) as affected by cooling and stress. The

finite element technique is the only method known which is capable of handling

122
these affects in a pointwise manner with the complexities involved. (A

manuscript [88] on this was presented at the IADR March 1981 meeting).

Another question pertaining to sensitivity of experimental measurements

of gap change to changes in properties of the porcelain-metal interface was

addressed. The results of this theoretical study were presented at the IADR

March 1981 meeting [89] and showed that for the porcelain/alloy thickness ratios

being employed experimentally, i.e., t , . /t ,, >1, interfacial

effects may be neglected. This i_s not the case under some conditions as was

shown.

One of the prime properties needed for theoretical analysis of the gap

change is the temperature dependence of the shear viscosity (for stress relaxation/retention near Tg). This property was determined for several dental

body porcelains (Table #1). Although the viscosity, q> was not measured

directly, it was found from the results of others [90] and expansion measurements

which were conducted that a close correlation exists between the strain point

and the temperature, obtained upon heating, of maximum rate of recovery of

quenched excess volume for previously, rapidly cooled expansion specimens.

This was reported at the IADR Meeting, March 1981 [91].

Efforts are under way to develop knowledge of other properties which may

be deemed necessary for the complete experimental, theoretical analysis of

the gap changes. A sensitivity study is being conducted to determine how

precisely heat transfer properties are needed. Measurements of elastic

modulus are planned (1982-83). Finite element computor models and programs

are being developed (1982-83). Ultimately, correlations between experiment

and theory will be sought (1983-84). Additional viscoelastic properties are

to be determined (1982-83). To render the ring work more germaine as a

123
these affects in a pointwise manner with the complexities involved. (A

manuscript [88] on this was presented at the IADR 1981 March meeting).

The results of another set of calculations were presented at the IADR

March 1981 meeting [89] and showed that for the porcelain/alloy thickness

ratios being employed experimentally, i.e., tporceia-jn/tai -] oy -

1 » interfacial effects may be neglected. This is not the case under some conditions

as was shown.

One of the prime properties needed for theoretical analysis of the

gap change is the temperature dependence of the shear viscosity (for

stress relaxation/retention near T . This property was determined for

y several dental body porcelains (Table 41). The viscosity, n, was not

measured directly. The results of others [90] and expansion measurements

which we conducted showed that a close correlation exists between the

strain point and the temperature corresponding to the maximum rate of

recovery of quenched excess volume for previously, rapidly cooled expansion

specimens. This was reported at the IADR Meeting, March 1981 [91].

Efforts are under way to develop knowledge of other properties

which may be deemed necessary for the complete experimental and theoretical analysis of the gap changes. A study is being conducted to determine

the required levels of accuracy and precision that are needed in the

values for the heat transfer properties. Measurements of elastic modulus

are planned (1982 - 1983). Finite element computer models and programs

are being developed (1982 - 1983). Ultimately, correlations between

experimental measurements and theoretical determinations will be sought

(1983-84). Additional viscoelastic properties are to be determined

(1982-83). To render the ring work more germane as a standardized

124
Table #41

Activation Energy, Q, for Deformation of 1 st Fire-Body Porcelains

Porcelain Q KJ/mol Kcal/mol

a. Vita VMK-68 354 ± 43 84.5

b. Biobond 336 ± 36 80.3

c. Micro Bond 322 ± 10 77.0

d. Will -Ceram 362 ± 13 86.5

e. Ceramco 318 ± 5 75.9

f. Neydium 216 ± 150 51.7

125
compatibility evaluation, finite element models of crowns, bridges, etc.,

will be developed, using the properties and techniques which will have been

shown to provide agreement between theoretical and experimental studies of

the gapped rings (1984-86). One abstract [92] of a sensitivity study of

the behavior of a split metal ring with variable heat transfer properties

was submitted for the 1982 annual meeting of the IADR.

B. Porcelain-to-Al loy Bond Strength

Numerous porcelai n-to-metal bond tests have been proposed over the year,

yet a simple, expedient, quantitative test has not been devised. A major

drawback is the fact that most proposed test methods utilize testing under

shear conditions. This has a number of detriments but perhaps the most

stringent one is that brittle materials (porcelain) fail in tension. Other

problems are that the preparation of specimens is often time consuming. A

test methodology is under development which utilizes the principle of

development of uniform tensile stresses in layers of a uniform cross-section

beam subjected to pure bending. Along an alloy beam, (elastic modulus, E ),

longitudinal sections have layers replaced by a different material (porcelain)

of elastic modulus, E . The neutral layer of these sections shifts with

changes in E . Calculations show for 1 ^ n ~ 2 (n = E /E ) porcelain can

p m p

r

comprise from 50 to 80% of beam thickness, t, without altering the stress

distribution significantly. Upon application of a force, F, fracture occurs

either at the porcelain fracture stress (PFS) in porcelain or at the porcelainmetal bond strength (PMBS) at the interface. The fracture stress,

a

f

(

Q

f = a

PMBS

or a

PFS^

is given by

_ 6F£(t~y)

CT f w[b3 + nc3 + 1 2(b(c+b/2)-y) z + nc(c/2-y) 3 )]

where y = j t^cCrrT)

’ b = sectlon thl

" ckness °f porcelain, c = section

thickness of metal, w = beam width. For the dimensions of specimens tested

126
(62 x 11 x 3 mm), and for 1 . 5 mm thick porcelain sections, = 72. 5F

for n=l and = 62. OF for n=2.

The ease of specimen preparation and the non-sensitivity of results

to porcelain thickness render the test specimen design and methodology

as potentially attractive for use as a generalized adhesive bonding

test. Detailed stress analysis on general systems will be explored via

simple engineering mechanics as well as finite element analysis (1982-

83), with consideration for determination bonding of hard tissue (enamel

or bone) to adhesives or alloplastic surgical implants (1982-84).

Experimental studies and statistical analysis of several dental porcelain/alloy

combinations are planned with examination of potential usefulness of

3 point loading (also possible with the specimen design under consideration,

symmetrical about the mid-section) (1982-84). Development of a master

mold pattern for casting of alloy specimens should be completed in 1982.

Two abstracts [93, 94] on this methodology were submitted for the 1982

annual meeting of the IADR.

C. Castability Evaluation

Studies continue on the use of the castability value as determined by

the use of a polyester grid pattern in cast alloy (presented at the 1981

annual meeting of the IADR) [95]. (Completed cast segments are counted and the

percentage of such segments is referred to as the castability value, cv).

The effects of superheat (over alloy liquidus) and mold temperature were

investigated. Preliminary results indicated optimum casting temperature

(superheat) and mold temperature may be determined and that these are not

always coincidental with manufacturers' instructions. Optimum castability

is seen to be accompanied by higher predictability, i.e., smaller standard

deviation for the cv.

127
The cv is also dependent on the investment brand which is used. Studies

will continue, extended to other alloy systems and investments to determine if

the behavior observed thus far is universal in nature (1982-83). Effects of

cast grid size variations on the cv are also planned (1982-85 to assess

effects and the benefits from using certain grids to delineate certain features

of castability for correlation with observed practical results. For example,

is a finer grid useful for predicting sharpness of cast margins?; is a

heavier grid useful for hot tearing and so on. Determination of castability

phase diagrams will be considered. A manuscript [96] on progress of this

work was submitted for the 1982 annual meeting of the IADR.

D. Fit and Soldering

Among the most complex phenomena involved are the fit of cast multiple

unit fixed partial dentures (MUFPD'S) and the joining of multiple sections

before and after porcelain application, and the effects of porcelain application itself on fit. Preliminary results [97] (presented at the 1981 annual

meeting of the IADR) showed that variability could be reduced (predictability

increased) for MUFPD'S if an all wax sprue and runner bar system were used

relative to using wax sprues and gates and a plastic runner bar. Moreover,

a bench setting technique was found to be desirable relative to a hygroscopic technique (the latter is consistent with numerous observations reported

in the dental literature). This study is only in the initial stages, and

effects of ring sizes and types, investments, and techniques are planned

with effects of sectioning and joining and effects of porcelain

application are planned for the next several years (1982-86). The objective

is to determine optimal conditions/techniques for producing sound MUFPD'S.

The effects of the porcelain application phase on fit will be examined for
correlation with opening and closing of gaps of split metal ring porcelain/alloy

combi nati ons.

E. Fluorescing Agents and Master Frits for Dental Porcelains

Studies had been proposed to examine the need for new fluorescing

agents for dental porcelains, in particular with regard to combined

esthetics (transl ucency plus color plus intensity) and preparation of

master fluorescing compounds. Techniques for preparation of master

frits which may be used in place of mixed feldspar are also under consideration.

Work in these areas is projected in cooperation with the ADAHF unit at

NBS. Progress in these areas has been temporarily suspended to allow a

more concentrated effort on commercial porcelain/alloy studies described

throughout the preceeding pages of this report.

129
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131
[40] Pearl, I. A., and McCoy, J. F.: J. Am. Chem. Soc. 69:3071-3072, 1947.

[41] American National Standards Institute/American Dental Association Specification No. 30 for Dental Zinc Oxide-Eugenol Type Restorative Materials,

J. Am. Dent. Assn. 95:991-995, 1977.

[42] Cowan, J. C. , and Teeter, H. M. : Ind. Eng. Chem. 36:148-152, 1944.

[43] Paquot, C. , Perron, R. , and Vassi 1 1 ieres , C. : Salts of Highly Saturated

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[44] Economy, J. , Mason, J. H. , and Wohrer, L. C.: J. Poly. Sci., Part A-l,

8:2231-2244, 1970.

[45] Bartels, T. , Schuthof, J. , and Arends, J.: J. Dent. Res. 7:221-229, 1979.

[46] Bartels, T. , and Arends, J.: Caries Res. 13:218-226, 1979.

[47] Voegel , J. C. , and Frank, R. M. : In Tooth Surface Interactions and

Preventive Dentistry , IRI Press, Ltd. (London) 1981, pp. 147-156.

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1974.

[49] Anbar, M. , St. John, G. A., and Elward, T. E. : J. Dent. Res. 53:1240-1244,

1974.

[50] DeGroot, K. , DeVisser, A. C. , Driessen, A. A., and Wolke, J. G. C. :

*

J. Dent. Res. 9:1493-1496, 1980.

[51] Purrman, R. : Personal Communication.

[52] Beech, D. R. : Brit. Dent. J. 135:442, 1973.

i [53] Causton, B. E. , Samara-Wichrama, D. Y. D. , and Johnson, N. W. : Brit.

Dent. J. 140:339, 1976.

[54] Causton, B. E. , Daymon, M. , and Johnson, N. W. : J. Dent. Res. 60:1321,

1981.

[55] Causton, B. E. , and Johnson, N. W. : J. Dent. Res. 60:1315, 1981.

[56] Levine, R. S. , Beech, D. R. , and Garton, B. : Brit. Dent. J. 143:275,

1977.

[57] Causton, B. E. , and Johnson, N. W. : Improvement of Polycarboxyl ate

Adhesion to Dentin by the Use of a New Calcifying Solution: An i_n vitro

Study. Brit. Dent. J. , in press.

[58] Beech, D. R. : Int. Dent. J. 28:338, 1978.

I [59] Causton, B. E. , and Johnson, N. W. : J. Dent. Res. 58:1383, 1979.
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to Dentin: Effect of Surface Modification and Time after Extraction.

Report to Australian Dental Research and Education Trust, March 1981.

[61] Shalaby, H. S. , Asmussen, E. , and Jorgensen, K. D. : Scand. J. Dent. Res.

To be published.

[62] Beech, D. R. : J. Dent. Res. 51:1438-1442, 1972.

[63] Brauer, G. M. , Jackson, J. A., and Termini, D. J.: J. Dent. Res. 58:

1900-1907, 1979.

[64] Brauer, G. M. , Kumpula, J. W. , Termini, D. J. , and Davidson, K. M.

J. Biomed. Mat. Res. 13:593-606, 1979.

[65] Beech, D. R. , and Kurer, H. G. : IADR Prog. & Abstracts, 54:1356, 1974.

[66] Leonard, F. , Kulkarni, R. K. , Brandes, G. , Nelson, J. , and Cameron, J. J.:

Poly. Sci. 10:259-272, 1966.

[67] Soetopo, . ., Beech, D. R. , and Hardwick, J. L. : J. Oral Rehab. 5: 69-80, 1978.

[68] Beech, D. R. , and Soetopo, . .: Unpublished work.

[69] McKinney, J. E. : Apparatus for Measuring Wear of Dental Restorative

Materials, submitted for publication in Wear.

[70] Atkinson, J. R. , Brown, K. J. , and Dowson, D. : Trans. Am. Soc. Mech. Eng.

100:208-218, 1978.

[71] Wu, W. , and Cobb, E. N.: J. Biomed. Mat! . Res. 15:343-348, 1981.

[72] Phillips, R. W. , Avery, D. R. , Mehra, R. , and Swartz, M. L. : J. Prosth.

Dent. 30:891-897, 1973.

[73] McKinney, J. E. , and Wu, W. : Relationship Between Subsurface Damage and

Wear of Dental Restorative Composites, submitted for publication in

J. Dent. Res.

[74] FDA Guidelines for Chemistry and Technology Requirements of Indirect

Food Additive Petitions, March 1976.

[75] Smith, E. B. , Walkey, J. , and Hildebrand, J. M. : J. Phys. Chem. 63:

703-4, 1959.

[76] Wu, W. , and McKinney, J. E. : Influence of Chemical on i_n vi tro Wear of

Dental Restorative Composites, submitted to J. Dent. Res.

[77] Wu, W. , Pestaner, J. , and Bowen, R. L. : The Softening of BIS-GMA Base

Polymer by Chemicals with Different Solubility Parameters, submitted

to J. Biomed. Mat. Res.

133
[78] Kusy, R, P. , and Leinfelder, K. F. : J. Dent. Res. 56(5):544, 1977.

[79] O'Brien, W. J. , and Yee, J. , Jr.: J. Dent. Res. 58(A):942, 1979.

[80] Wu, W. , Toth, E. E. , and Moffa, J. P. : Work submitted to 1981 IADR

Meeting, New Orleans, LA.

[81] Cole, M. F. , Bowden, G. H. , Korts, D. C. , and Bowen, W. H. : Caries Res.

12:190, 1978.

[82] Geddes, D. A. M. : Caries Res. 9:98, 1975.

[83] Bauer, R. W. : "Survey on the Use of Casting Alloys in Commercial Dental

Laboratories, Part II, Ceramic Alloys," NADL Journal, September 1977.

[84] Whitlock, R. P. , Tesk, J. A., Widera, G. E. , Holmes, A., and Parry, E. E.

Consideration of Some Factors Influencing Compatibility of Dental Porcelain and Alloys, Part I: Thermo-physical Properties," Precious Metals,

Proc. 4th Inti. Precious Metals Inst. Conf. , Toronto, Ontario, Canada,

June 1980, Pergamon Press.

[85] Tesk, J. A., Whitlock, R. P. , Widera, G. E. 0., Holmes, A., and Parry, E. E. Ibid . Part II, Stress.

[86] Bertol otti , R. L. : J. Dent. Res. 59:1972-1977, 1980.

[87] Fairhurst, C. W. , Anusavice, K. J. , Ringle, R. D. , and Twiggs, S. W.

J. Dent. Res. 60:815-819, April 1981.

[88] Widera, G. E. 0., Tesk, J. A., Whitlock, R. P. , Hinman, R. W. , and

Parry, E. E. : J. Dent. Res. 60(A):838, 1981.

[89] Tesk, J. A., Hinman, R. W. , Widera, G. E. 0., Cassel , J. M. , and

Holmes, A. D. : J. Dent. Res. 60(A):508, 1981.

[90] Bertol otti , R. L. , and Shelby, J. E. : J. Dent. Res. 58:2001-2004, 1979,

and Bertolotti, R. L. , private communication.

[91] Tesk, J. A., Hinman, R. L. , Whitlock, R. P. , Holmes, A., and Parry, E. E.

J. Dent. Res. 60(A):839, 1981.

[92] Widera, G. E. 0., Munio, N. , Tesk, J. A., and Hinman, R. W. : Effect of

Thermal Parameters in Compatibility in Porcelain Veneered Split Metal

Rings, in preparation.

[93] Tesk, J. A., Widera, G. E. 0., Hinman, R. W. , and deRijk, W. G. : A Quantitative/Expedient Porcelain-to-Metal Bond Test, Part I, Theoretical

Basic, in preparation.

[94] deRijk, W. G. , Tesk, J. A. , Hinman, R. W. , Parry, E. E. , Conner, M. , and

Widera, G. E. 0.: A Quantitative/Expedient Porcelain-to-Metal Bond Test,

Part II, Preliminary Results, in preparation.

134
[95] Whitlock, R. P. , Hinman, R. W. , Eden, G. T. , Tesk, J. A., Dickson, G.

,

and Parry, E. E. : J. Dent. Res. 60(A): 374, 1981.

[96] Hinman, R. W. , Tesk, J. A., Whitlock, R. P. , Parry, E. E. , and

Durkowski , J. S. : Use of a Castability Test for Optimizing Mold and

Casting Temperature, in preparation.

[97] Hinman, R. W. , Tesk, J. A., and Parry, E. E. : J. Dent. Res. 60(A):262,

1981.

135
Invited Talks (1981)

Adhesion to Tooth Structure. G. M. Brauer. Session on Biomedical

Adhesion, Adhesion Society, Savannah, Ga.

Dental Materials Research at NBS. J. Cassel. Georgetown Dental

School Special Topics, Seminar Program, Washington, D. C.

Evaluation of Dental Porcelain and Alloy Characterization. John A. Tesk.

Johnson and Johnson Dental Research Center, East Windsor, New Jersey.

Factors Influencing Selection of F.P.D. Alloys. J. A. Tesk and

R. W. Hinman. American Association of Dental Schools, Annual Meeting,

Biomaterials Group.

Current Concepts in Metal Porcelain Systems. R. A. Hinman and J. A. Tesk.

The University of Texas, Teleconference Network of Texas, Washington,

D. C.

136
Abstracts IADR/AADR Meeting, March 81, Chicago, on Microfilm

Effects of Porcel ai n/Alloy Interfacial Diffusion Zones on Thermomechanical

Strain. Abstract 508, J. A. Tesk, R. W. Hinman 1

, G. E. 0. Widera2, J. M. Cassel , and A. D. Holmes.

Fit of Multiple Unit Fixed Partial Denture Castings. Abstract 262, R. W. Hinman 1

, J. A. Tesk, E. E. Parry 1

.

Cements from 2,5Dimethoxyphenol and Zinc Oxide. Abstract 249, J. W. Stansbury,

H. Argentar3 and G. M. Brauer.

Degree of Polymerization of Dental Resins by Differential Scanning Calorimetry.

Abstract 234, J. M. Antonucci and E. E. Toth.

Influence of Chemicals on In Vitro Wear of Dental Restorative Composites.

Abstract 243, W. Wu and J. E. McKinney.

Theoretical /Experimental Studies of Stress Compatibility in PorcelainVeneered Split Metal Rings. Abstract 838, G. E. 0. Widera2

, J. A. Tesk, R.

P. Whitlock1

, R. W. Hinman 1

, E. E. Parry 1

.

Application of Silane Coupling Agent to Inorganic Components of Composites.

Abstract 235, T. M. Chen

4

and G. M. Brauer.

Peroxy Esters and Hydroperoxides as Initiators for Composite Resins.

Abstract 49, G. M. Brauer and J. W. Stansbury.

Relationship Between Subsurface Damage and Wear of Dental Restorative

Composites. Abstract 1095, J. E. McKinney and W. Wu.

Effect of Hygroscopic Diluents on the Properties of Cured Composites.

Abstract 236, K. Dermann, N. W. Rupp 3 and G. M. Brauer.

A Practical Test to Evaluate the Castability of Dental Alloys. Abstract

374, R. P. Whitlock, R. W. Hinman 1

, G. T. Eden 1

, J. A. Tesk, G. Dickson,

and E. E. Parry 1

.

Temperature Dependence of Shear Viscosity for Several Dental Porcelains.

Abstract 839, J. A. Tesk, R. W. Hinman 1

, R. P. Whitlock1

, A. D. Holmes, E.

E. Parry 1

.

Research Associate, U. S. Navy

2 University of Illinois, Chicago

3 Research Associate, American Dental Association

4 Guest Worker, Food & Drug Administration

5 Guest Worker, Free University of Berlin, Germany

137
Papers Published (FY81)

Effect of Diluent Monomer on the Physical Properties of BISGMAbased

Composites. D. Dulik 1

, R. Bernier 1, and G. Brauer. J. Dent. Res. 60, 983-

89 (1981).

SlowActing Amine Polymerization Accelerators. Paradimethylaminobenzoic

Acid and its Ethyl Ester. J. Antonucci , R. Peckoo 1

, C. Schruhl 1

, and E. Toth. J. Dent. Res. 60, 132531 (1981).

The Accelerative and Adhesive Bonding Capabilities of Surface Active

Accelerators. J. Antonucci, D. Misra2

, and R. Peckoo 1

. J. Dent. Res. 60,

133242 (1981).

4N ,NDial kyl ami nophenethanol s , Alkanoic Acids and Esters: New Accelerators

for Dental Composites. G. Brauer, J. Stansbury, and J. Antonucci. J. Dent. Res. 60, 134348 (1981).

A Silver Staining Technique for Investigating Wear of Restorative Dental

Composites. W. Wu and E. Cobb3

, J. Biomed. Matls. Res. 15, 34348 (1981).

Consideration of Some Factor Influencing Compatibility of Dental Porcelain

and Alloys, Part I, Thermophysical Properties. R. P. Whitloch4

, J. A. Tesk, G. E. 0. Widera 5

, A. Holmes, and E. E. Parry4

, Precious Metals,

Proceedings of the 4th International Precious Metals Institute Conference,

Toronto, Ontario, Canada, June 1980, Pergamon Press (1981):

Consideration of Some Factors Influencing Compatibility of Dental Porcelain/

Alloy, Part II, Porcelain/Alloy Strain. J. A. Tesk, R. P. Whitlock4

, G. E. 0. Widera5

, A. Holmes, and E. E. Parry4

, Precious Metals, Proceedings of

the 4th International Precious Metals Institute Conference, Toronto, Ontario,

Canada, June 1980, Pergamon Press (1981).

Solder and Fluxes. J. A. Tesk. Dentists Desk Reference, Materials, Instruments and Equipment, First Edition, American Dental Association Publications

(1981).

An Amine Accelerator for Color Free Curing of ColdCuring Methyl Methacrylate

Denture System. H. Argentar2

, J. A. Tesk, and E. E. Parry4, JADA, 102,

664665 (1981).

1 C00P Student, U. of Notre Dame, Baltimore, MD

2 Research Associate, American Dental Association

3 Research Associate, Georgetown Univ. Dental School, Washington, DC

4 Research Associate, U. S. Navy

5 University of Illinois, Chicago, IL

138
The following manuscripts are under NBS editorial review or have been

submitted for publication:

Intraoral Secondary Radiation Dose from Neutron Activated Dental Materials.

J. A. Tesk, R. P. Whitlock1

, and A. Holmes.

Theoretical/Experimental Studies of Stress Compatibility in PorcelainVeneered Split Metal Rings. G. E. 0. Widera2

, J. A. Tesk, R. W. Hinman 1

, N. Munir3

.

Physical Metallurgy of Dental Casting Alloys. J. A. Tesk and R. M. Waterstrat

(Parts C; CoCoMo System

D; NiCr System

E; Fe and Austenitic Steels

F; Titanium Alloys.

Effects of Porcelain/Alloy Interfacial Diffusion Zones on ThermoMechanical

Strain. J. A. Tesk, R. W. Hinman 1

, G. E. 0. Widera2, A. D. Holmes and J.

M. Cassel.

Application of Small Angle Xray Technique to Damaged Particulate Reinforced

CompositesA Theoretical Treatment. W. Wu.

Apparatus for Measuring Wear of Dental Restorative Materials. J. McKinney.

Effect of Hydrophilic Diluents on the Properties of Cured Dental Composites.

K. Dermann4

, N. Rupp 5, and G. Brauer.

Extent of Polymerization of Dental Resins by Differential Scanning Calorimetry

J. Antonucci and E. Toth.

Relationship Between Subsurface Damage and Wear of Dental Restorative

Composites. J. McKinney and W. Wu.

Influence of Chemicals on j_n vitro Wear of Dental Restorative Composites.

W. Wu and J. McKinney.

Cements from 2,5Dimethoxyphenol and from Hexyl VanillateOEthoxyBenzoic Acid

and Zinc Oxide. J. Stansbury, H. Argentar5

, and G. Brauer.

Peroxy Esters and Hydroperoxides as Initiators. G. Brauer and J. Stansbury.

The Softening of BISGMA Base Polymer by Chemicals with Different Solubility

Parameters. W. Wu, J. Pestaner6

, and R. Bowen 5

. Research Associate, U.S. Navy

2 University of Illinois, Chicago

3 Consul tant

4 Guest Worker, Frei University of Berlin

5 Research Associate, American Dental Association

6 C00P Student, Johns Hopkins Univ. , Baltimore

139
Abstracts Submitted for 1982 AADR/IADR Meeting

Cements Containing VaniTI ate Esters, EBA and Zinc Oxide. G. M. Brauer and

J. W. Stansbury 1.

Dental Resin and Initiator Systems Based on Polythiols. J. M. Antonucci

,

J. W. Stansbury 1

, and D. J. Dudderor2

.

Subsurface Wear Defects Analysis of Both I_n Vivo and I_n Vitro Dental Restorative Composites. W. Wu, E. Toth, and J. P. Moffa3

.

Influence of Chemical Softening on Wear of Three Types of Dental Composites.

J. E. McKinney and W. Wu.

Use of a Castability Test for Optimizing Mold and Casting Temperature. R. W. Hinman4

, J. A. Tesk, R. P. Whitlock4, E. E. Parry4 and J. S. Durkowski 4

.

A Quantitative/Expedient PorcelaintoMetal Bond Test; Part I, Theoretical

Basis. J. A. Tesk, G. E. 0. Widera 5

, R. W. Hinman, and W. Derijk6

.

A Quantitative/Expedient PorcelaintoMetal Bond Test; Part II, Preliminary

Results, W. G. Derijk6

, J. A. Tesk, R. W. Hinman4, E. E. Parry4, M. Conner8

, and G. E. 0. Widera5

.

Effect of Thermal Parameters on Compatibility in Porcelain Veneered Split

Metal Rings. G. E. 0. Widera, N. Munir7

, J. A. Tesk, and R. W. Hinman4

.

Converted to COOP Student, U. of Maryland as result of RIF at NBS 2 C00P

Student Notre Dame College, Baltimore

3 U.S. Public Health Service Hospital, San Francisco

4 U.S. Navy Research Associate

5 U. of Illinois

6 Amer. Dent. Assoc. Research Associate

7 Consul tant

8 Guest Worker NIDR

140
Abstracts Submitted for Biomaterials Society Meeting, 1982, Orlando, FL

On the Compatibility of Dental Porcelain Metal Alloy Systems.

G. E. 0. idera, J. A. Tesk, N. Munir, R. W. Hinman, and W. Derijk.

Environmental Effect on Physical Properties of Dental Composite Restoratives

W. Wu and J. Pestaner

Additional Abstracts, FY82

Recent Developments in Metals as Dental Bio/Alloys. J. A. Tesk, R. W. Hinman,

R. Waterstrate, and W. Derijk. Symposium on Recent Developments in Biological

Alloys, AIME

Small Angle X-ray Analysis Applied to Three Phase Composite Systems.

W. Wu. IUPAC and Am. Phys. Soc.

141

NBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 81-2431

2. Performing Organ. Report No. 3. Publication Date

December 1981

4. TITLE AND SUBTITLE

properties and Interactions of Oral Structures and Restorative

Materials

5. AUTHOR(S) j. m. Cassel, J. A. Tesk , G. M. Brauer, J. M. Antonucci, W. Wu,

J. E. McKinney, R. W. Hinman

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. Y01-DE-40015

8. Type of Report & Period Covered

Annual Report

10/1/80 - 9/30/81

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

bibliography or literature survey, mention it here)

New dental cements that overcome major shortcomings of zinc-oxide-eugenol type

materials were formulated. The new cements, based, on esters of vanillic acid or its

isomers, do not inhibit polymerization of methacrylate resins that they contact and

have improved strength and solubility properties over previous cements. The softening

of dental composites by chemical solvents selected for food simulation was demonstrated

to directly affect their wear performance as measured with a classical pin (stainless

steel) or disc (composite) wear apparatus. Wear resistance following immersion of the

composites in ethanol/water solutions correlated with changes in hardness as a result

of the immersion. Small angle x-ray scattering (SAXS) Analysis has yielded information

on the volume concentration and characteristic length of microcracks in both in vitro

and in vivo worn composite specimens. Theoretical calculations showed that for the

porcelain/alloy thickness ratios (i.e. £ 1) being employed experimentally, inter facial

effects could be neglected for the split ring gap change (compatibility) analysis

and a knowledge of a precise value for Poisson's ratio was not required for accurate

assessment of residual stresses. (A castability test was refined to the point that a castability value can be determined for any given dental alloy.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Castability; cements; compatibility; composites; polymerization; porcelain/alloy; and wear resistance.

13. AVAILABILITY 14. NO. OF

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