NBSIR 79-1943

Properties and Interactions of

Oral Structures and Restorative

Materials

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

J. E. McKinney, W. P. Whitlock, G. B. McKenna

Dental and Medical Materials

Polymer Science and Standards Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Annual Report for Period

October 1 , 1 978 thru September 30, 1 979

Novemb||r 1979

Issued December 1979

Interagency Agreement

YOI=DE=40015-11

Pripared for National Inatitute of Dental Research

iethesda, MD 20014

NBSIR 79-1943

PROPERTIES AND INTERACTIONS OF

ORAL STRUCTURES AND RESTORATIVE

MATERIALS

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

J. E. McKinney, W. P. Whitlock, G. B. McKenna

Dental and Medical Materials

Polymer Science and Standards Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Annual Report for Period

October 1 , 1 978 thru September 30, 1 979

November 1979

Issued December 1979

Interagency Agreement

YOI-DE-40015-11

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, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr.. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

ABSTRACT i

SUMMARY iit

INTRODUCTION 1

TECHNICAL REPORT 2

I. Dental Composite, Resin, Sealant and Cement 2

Chemistry and Development

A. Prepolymer Monomers for Dental Resins 2

B. Synthesis of Urea Di methacrylates 8

C. Bulky Monofunctional Monomers for Dental Resins 10

D. New Initiator/Accel erator Systems 11

E. New Stabilizers for Dental Monomers 33

F. Reaction of Silane Coupling Agent with Quartz 35

G. Autocurable Blends as Routes to Improving Chemical 39

and Mechanical Properties of Denture Base and

Other Dental Materials

II. Development of Adhesive Bonding Techniques 41

A. Cyanoacrylates 41

B. Urethane (Isocyanate) Adhesives 42

C. Multifunctional Adhesion Promoting Surface-Active 46

Amine Accelerators

III. Wear Resistance and Mechanical Properties of Dental 56

Materials

A. Wear Instrumentation and Data 56

B. Mechanical Properties Measurements 58

C. Materials 82
TABLE OF CONTENTS (cont.)

Page

IV. Dental Alloy, Ceramic and Implant Research 89

A. Porcelain - Alloy Characterization 90

B. Alloy Corrosion Study 94

C. Dental Porcelain: Processing Methods ana i;aw 98

Materials

D. Castability and Fabrication Research 98

E. Irradiation Effects from Neutron Activated 99

Dental Materials

F. Implant Research 101

REFERENCES i02

PUBLICATIONS AND TALKS 108

ABSTRACTS PREPARED FOR PRESENTATION IN FY1980 .. Ill
ABSTRACT

Promising new dental resins that have been synthesized include

surface energy reducing polymethacrylates (ave. M.W. = 10,320; each

repeating unit of 1032 M.W.), a diurea dimethacrylate to lower polymerization shrinkage and methacryloxyethyl-l-adamantyl carboxyl ate. The

latter is a tri -cyclic saturated monofunctional monomer that should

also aid in reducing polymerization shrinkage and residual vinyl unsaturation. Initial attempts to improve durability used a rubber toughening approach with BIS-GMA and indicate a less brittle but tougher matrix

can be achieved. Acetyl thiourea and ascorbic acid have been shown to

be promising non-amine type of accelerators for the BIS-GMA system.

New amine accelerators such as N,N-dialkylaminophenethanol and N,Ndimethylaminophenyl acetic acid are highly reactive and yield composites

with high compressive strengths and good initial color. The amine,

N,N-dimethyl-p-t-butyl ami line, originally synthesized in this laboratory but now commercially available, gives cured denture base

materials with excellent color stability and offers potential for

solving some current problems in achieving matching shades in such

materials. Urethane methacrylates containing residual isocyanate groups

confer much lower bond strengths than are achievable with the 2-cyanoacrylate esters reported on last year. Wear data indicate that sintered

hydroxyapatite is a suitable model for human tooth enamel. Fatigue as

a mechanism contributing to wear is being examined by measuring wear

resistance on pre-fatiguing materials.

Dimensional changes induced thermally in commercial dental porcelains are indicative of differences such as the crystalline forms that
are present. Be appears to reduce corrosion resistance of crown and

bridge Ni-Cr alloys subjected to anodic polarization measurement

following casting or casting and fire-cycling to' s.mulate procelain

bake on. Calculation of secondary radiation effeccs impartea to oral

tissues from neutron irradiated gold crowns indicates the desirability

for developing more information in this area.
SUMMARY

The research on dental materials that is described in this report

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

with the American Dental Association Health Foundation. The U.S. Army

and Navy Medical R&D Command are also part of this collaborative effort

through support funding and/or assignment of research personnel.

While the bulk of the research progress in this report represents

only the efforts of NBS staff supported under the Interagency Agreement

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

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

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

chemical, and physical properties of restorative and caries-preventive

materials and the mechanisms by which clinical performance is governed.

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

approaches by which new and improved adhesive restorative dental

materials might be developed. These include syntheses to improve

today's composite restorative materials and pit and fissure sealants,

investigation of new approaches to bonding restorations to tooth surfaces and exploration of rubber toughening of the composite's resin

matrix as a means of generating increased durability in these systems.

The precision of an automatic wear-generating and recording system is

being improved to permit valid laboratory assessment of the wear resistance of dental materials. Thermal expansion and contraction data are Hi
determined on base metal alloys and dental porcelains in order to assess

the influence of this factor in the bonding of porcelain fused to metal.

The need for more durable dental composite resin systems has suggested that the use of elastomeric prepolymer motioniers be considered as

a means for reducing the brittleness in the glassy resin matrix while

at the same time toughening the composite to allow better finishing

by the dentists as well as greater durability in service. Preliminary

formulations using 44% BIS-GMA, 49% triethyleneglycoldimethacrylate and.

7% of a resin that is basically butadiene acrylonitrile copolymer modified with about 3% of acrylate groups produced polymers by the usual

cold-curing (benzoyl peroxide-amine) and photocuring (benzoin methyl

ether + UV) techniques that were less brittle than the control material.

Dynamic mechanical testing of these and other similar materials is being

delayed until adequate temperature control is achieved with the servohydraulic mechanical test machine.

Several promising new dental resins have been synthesized. These

include: Polyfl uorinated methacrylates (average M.W. = 10,320; each

repeating unit of 1032 M.W. and having two methacrylate groups as side

chains and 24 fluorines substituted in isopropyl idine units

of the prepolymer chain to lower the surface energy and gain greater

resistance to hydrolytic degradative processes); a diurea dimethacrylate

which should be useful in gaining reduced polymerization shrinkage; and

methacryloxyethyl-l-adamantyl carboxyl ate (a tri -cyclic saturated monofunctional monomer which because of its bulk and relative paucity of

reactive double bonds should also aid in reducing polymerization shrinkage

and residual unsaturation.

Efforts to improve the catalyst system employed with dental resins
has continued with particular emphasis on development of more efficient,

less toxic accelerators. Particularly promising are two non-amine type

accelerators, acetyl thiourea and ascorbic acid (or its palmitic acid ester)

while newly synthesized amines such as N,M-dial kylaminophenethanol and

N, N-dimethyl ami nophenyl acetic acid appear to be very efficient and capable of yielding corrtposites with high compressive strengths, good initial

color and color stability and low water adsorption characteristics. Blending glass filler particles with a solid peroxide initiator with glass

filler coated with ascorbic acid or ascorbyl palmitate seems to produce

a storage^stable mixed glass filler. By employing these blended

fillers in powder/liquid formulations with the liquid consisting only

of dental resin or dental resin plus very small amounts of a redox metal

or an amine accelerator, we have obtained composites with excellent

esthetics, superior to comparable composites prepared using amine

accelerators.

One amine synthesized several years earlier by Research Associates

of the American Dental Association for application in composite restorative systems, has now been shown to have particular merit for denture

base application. This amine, N,N-dimethyl-p-t-butylaniline or DMBA

(now commercially available) was shown to produce denture base specimens

with no evidence of discoloration as compared to the usually very

evident discoloration observed with the commonly employed N ,N-dimethyl

p-toluidine. Subsequent exposures to UV light showed specimens with

O. 5 and 0.25X UV absorber to be virtually free of discoloration. DMBA

appears to have much promise for achieving matching shades with denture

base systems.

V
Development of an adhesive restorative dental material has been a

long sought goal of researchers in numerous laboratories. A truly adhesive technique would conceivably eliminate the marginal leakage that

induces secondary caries and attacks restorations but it would also

allow modified tooth cavity preparations that minimize removal of sound

tooth structure. One of the more promising approaches to such adhesion

has been demonstrated with 2-cyanoacrylates which, when a proper

acidic cleansing of the tooth surface is utilized, yield the highest bond

strengths to dentin heretofore determined. A detailed summary of this

research-including quantitative data on the discouragingly rapid rates of

debonding that occur in such systems in the presence of water and under

thermal cycling conditions - has been prepared and published as a part of

an ACS symposium on the durability of macromolecular materials.

Based on knowledge that organic isocyanates react readily with active

hydrogen compounds such as the amino acid hydroxy and amino-containing

side chains, methacrylate monomers with pendant isocyanate groups have

been synthesized and examined for adhesiveness to bone and dentin surfaces.

Optimum one-day bond strengths, obtained after an alkaline rinse of the

substrate surface, were relatively low, indicating that these compounds as

we have used them to date do not appear to have as much promise as the

cyanoacrylate adhesives.

The synthesis of a multifunctional surface-active amine accelerator

Poly SAM-1 capable of complexing Ca'*"*' was reported earlier. Currently

the efficacy of this and other surface active amines as polymerization

accelerators and as coupling agents for mediating bonding to hard tooth

tissues is being evaluated. Measurements of diametral tensile strengths
of hydroxyapati te powder-filled composites with and without surface-active

amine accelerators adsorbed to the hydroxyapatite indicate that this is

promising route to be continued.

Improvements in both the wear-generating and data acquisition components of equipment being developed to measure the wear resistance of

dental materials have significantly increased the reliability of this

test method. To negate the inhomogeneity and inconvenience of preparation experienced with human tooth enamel pins and to investigate the

influence of different opposing materials on the wear process, measurements have been made on one composite and one amalgam with a sintered

hydroxyapati te (durapatite) , a mild steel and a stainless steel. The

greater wear resistance of amalgam in the initial stages of wear generation relative to that observed as the wear process continues is tentatively attributed to the presence of an oxide layer, typical of what

would be present continually in the clinical situation. The usefulness

of softer pin materials to simulate opposing body effects generated by

food particles is being considered.

As a part of the program to determine mechanisms of wear, work has

been initiated to investigate the relationship between wear resistance of

composite materials, their fatigue behavior, their fracture toughness

and their crack propagation characteristics.

In a porcelain-fused to base metal alloy study, the dimensional

change resulting from thermal cycling has been completed for nearly all

ceramic specimens. The considerable variation in the expansion/cono traction characteristics noted among different ceramic products appears

related directly to the thermal history of the specimens, i.e., the
number of firing cycles to which the material has been exposed during

fabrication. Qualitative and/or quantitative changes in crystallinity

of the ceramic materials appear to develop as a function of thermal

history. Raman spectroscopy and x-ray analysis are being applied in

structural determinations.

Specific combinations of alloys and bonding porcelains have been

fused together and analyzed for void (no contact) formation at the interface (i.e. for completeness of "wetting") • Variations in porcelain alloy

combinations as well as techniques of porcelain application appear to

influence the area of contact. The physical and mathematical design for

a porcelain-dental alloy composite structure to be used for evaluating

the net effects of the factors studied on residual interfacial and tensil

stresses (primary contributions to porcelain-metal failure) has been completed. An initial stress analysis has indicated the desirability of

developing appropriate theory for the change in gap of a multi-layered

split metal ring to allow for stress analysis within and between layers.

Anodic polarization studies of three Ni-Cr alloys indicates that

fire cycling and/or conditioning and the presence of Be may influence

corrosion resistance.

Calculation of secondary radiation effects from neutron irradiated

gold crowns indicates the need for further efforts in this area.

v-lii
INTRODUCTION

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

measurement and characterization to development of new techniques for

using materials, to laboratory prototypes of improved materials and

devices, to clinical evaluation, and finally, to commercial 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 and Sealant Chemistry and Development

II. Development of Adhesive Bonding Techniques

III. Wear Resistance and Mechanical Properties of Dental Materials

IV. Dental Alloy, Ceramic and Implant Research

1
TECHNICAL REPORT

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

Dental composite restorations have essentially replaced silicate

cements as the preferred technique for an aesthetic restoration procedure.

With the development of filled and unfilled resins , means for

fractured incisal repair, pit and fissure sealant, anticaries treatments

and orthodontic bracket attachment are being realized. Despite improved

dental health as a result of the use of composite restorations, their

color stability, storage stability, durability and finishabil ity are

still in need of improvement.

The objective of this part of the research is to provide direction

for improved composite restoratives and sealants with enhanced durability, color and storage stability and lower toxicity levels.

A. Prepolymer Monomers for Dental Resins

Prepolymer monomers are oligomers or loW MW polymers which

have reactive functional groups (e.g. acrylate, methacrylate, isocyaI nate, etc.) capable of further polymerization.

1. Polyfluorinated Prepolymer Monomers

a. Synthesis of Polyfluorinated Oligo (Ether Acrylates)

Direct esterification of the polyfluorinated oligo

(ether-ol), as shown below, with methacryloyl chloride (Method A) or

acrylic anhydride (Method B) gives the corresponding polyfluorinated

prepolymer monomers in excellent yield. A paper entitled "Synthesis of

Polyfluorinated Polymethacrylates for Dental Applications," presented

at the (March 1979) lADR meeting, gives details of this work (1). That

2
study demonstrated the feasibility of using prepolymer monomers as the

major constituent of dental resin systems. The polyfl uorinated polymethacrylate, PFMA, can be formulated to give low surface energy resin

binders or sealants that have excellent mechanical strength. The use of

low surface energy dental resins, such as those derived from polyfl uorinated monomers, should lead to polymeric matrices that are highly resistant to degradative processes (e.g., hydrolysis, oxidation, etc.) that

occur in the oral environment. Not only should these dental polymers

resist chemical or biochemical modification, such as may be evidenced

by discoloration and material loss through chemical degradation, but

they should also resist physical modifications such as staining or

excessive mechanical wear. Moreover, as demonstrated by Craig et al .

,

(2-4), dental composites utilizing fl uorinated resin binders exhibit

extremely low water sorption properties and marginal leakage. These

new fl uorinated monomer systems, which have much higher fluorine

contents than those of Craig et al . , would be expected to exhibit

similar hydrophobic characteristics.

b. Synthesis of Polyfl uorinated Dimethacrylates

The synthesis of dimethacrylates from the corresponding polyfl uorinated diols as outlined below (Method C and D) represents another approach to forming dental polymeric matrices of low

surface energies and good mechnaical properties (1). These monomers will

be formulated into dental resin systems and their properties evaluated.

2. Rubber Toughened Dental Composites and Sealants

We have expanded our studies on the use of prepolymer

monomers to include elastomeric prepolymer monomers. It is hoped that

3
METHOD A

SYNTHESIS OF FLUORINATED PQLYnETHACRYLATE (PFMA)

CF,

:-0CH2CH‘CHt-

[f, in

oc

CF,

;-ch=ch-ch2'

CF,

-OCH 2 CH-CHJ-0

-

CF, in

25*C

N, CHjCLj

7 HRS.

MW OF REPEATING UNIT = 1032 AV6 M’W = 10.32^?

VISCOUS LIQUID ^ BIS-6MA

4
METHOD B

SYNTHESIS OF FLUORINATED POLYACRYLATE (PFA)

fF3 :f3

;-ochtCH-ch7-o

2

;-CH=CH-CHjlt-OCHjCH-CHrO- i

1 4.

[f, ly -^3 <JH ^^3 :f3 6h

N

55“C

CieH33(CH3)3l^R®

. 42-72 HRS.

p

CH,= CH-Cx

^ 0

CH2= CH-C^ ^

ii 0

MW OF REPEATING UNIT = 1004; AV6 MW = 10.040

VISCOUS LIQUID ^ BIS-6MA

5
NETHOD C

SYNTHESIS OF HEXAFLU0R0-1,5-PENTANEDI0L

D I METHACRYLATE (HFPDMA)

HOCH2 CF2 CF2 CF2 CH2 OH

CH

I CH2=C-

(ET)3 N

CH2CL2

CH

25°Cj 1 HRS

CH.

I ^ T"3

CH2=C-C-0CH2Cp2CF2CF2CH20C-C=CH;

0 0

HFPDMA

MW = 3A8

VISCOSITY = MOBILE LIQUID MMA

6
METHOD D

SYNTHESIS OF BIS-(2-METHACRYL0XYETH0XYHEXAFLUORO-2-PROPYL)BENZENE [bIS(MEHFP)0]

K2CO3

DMSO

90»C

4 HRS

0 CH 3

BRCH2CH^0C-C=CH2

CH

3

CF

3

CH=C-COCH;CH;0-C—

0 CF

3

CF, CH,

-C-OCH-CH-OC-C=CH,

I 2 2 II 2 CF. 0

BIS(mMEHP)0

MW = 63A

MODERATELY VISCOUS LIQUID (LESS THAN BIS-GMA)

7
their use in relatively minor amounts with conventional dental resins

will result in dental composites and sealants of enhanced wear resistance without adversely affecting the other properties of these materials.

Preliminary experiments have been with a commercial viscous liquid copolymer of butadiene and acrylonitrile that has been modified with a few

percent terminal acrylate groups (Hycar Resins, B. F. Goodrich Co.).

Formulations of conventional dental resins with Hycar resins were easily

cured at ambient temperatures. The resulting copolymers containing

between 2.4 and 10% Hycar resin appear to be less brittle than the

normal BIS-GMA resin and diametral tensile strengths of the corresponding

composites were acceptable (Table 1).

B. Synthesis of Urea Dimethacrylates

A diurea dimethacrylate was synthesized in excellent yield from

N-t-butyl -2-ami noethyl methacrylate (TBAEM) and hexamethylene di isocyanate (HDI) by the reaction shown below:

CHo

I 2CH2»C-C-0-CH,CH,-NH + 0CNCH,CH.,CH,CH,CH,CH,NC0 ^li 22i 222222

0 HDI

TBAEM

CH,-C-CH,

3 I 3

CH.

CH,

1

0

11

CH2C12.

25»C

ET3N

0 CH.

II I

' CH,'C-C-0-CH,CH,-N-C-NHCH,CH,CH,CH,CH-CH,-NHC-N-CH,CH,OC-C«CH, Z|| zzi z z z z z z ( z z ^ z CH

3

-C-CH

3

C»3

TBAEM-HOI CH

3

-C-CH

3

CH3

Preliminary structural confirmation of this new monomer was obtained by

infrared spectral analysis. The diurea dimethacrylate (TBAEM-HDI) is a

white, waxy low melting solid, (m.p.50*c). This monomer which

8
Table 1. Setting Times and Diametral Tensile Strength of BIS-GMA/TEGDMA

Composites* Toughened with HYCAR

Formulation Monomer System

Setting

Time

(min)

Diametral Tensile Strength

MPa ± std. dev.

(No. of specimens)

1 BIS-GMA = 44.4%

TEGDMA = 48.4%

HYCAR = 7.0%

PMSX = 0.2%

2.0 41.1 ± 3.8 (3)

2 BIS-GMA = 44.9%

TEGDMA = 44.9%

HYCAR = 9.9%

DMSX = .025%

BHT = .08%

2.0 40.1 ± 1.9 (3)

3 BIS-GMA = 66.4%

TEGDMA = 28.4%

HYCAR = 4.8%

DHPPT = 0.32%

BHT =0.10%

1.45 57.3 ± 2.8 (5)

4 BIS-GMA = 68.1%

TEGDMA = 29.2%

HYCAR = 2.4%

DMSX = .18%

BHT = .1%

1.0 49.4 ± 3.0 (6)

*• Composites were prepared using usual powder/liquid formulation

technique; powder- liquid ratio = 3

9
can be expected to exhibit low polymerization shrinkage, will be evaluated

as an organic resin for dental composites.

C. Bulky Monofunctional Monomers for Dental Resins

We have synthesized several types of relatively high molecular

weight (bulky) monomethacrylates for use as comonomers with the usual

di methacrylate dental monomers (e.g. BIS-GMA). The usual diluent comonomers are relatively low molecular weight monomethacrylates (e.g. methyl

methacrylate) or higher molecular weight dimethacrylates of relatively

low viscosity (e.g. TEGDMA). Infrared analytical techniques have shown

that cured dental composites have a considerable quantity of unreacted

methacrylate groups (5-7). Dental resins containing methyl methacrylate

had the highest degree of polymerization; i.e. least residual methacrylate groups (5). Unfortunately, the use of MMA as a diluent monomer leads

to increased polymerization shrinkage of dental resin systems. The

relatively bulky monofunctional methacrylates described below are

designed to reduce both polymerization shrinkage and residual unsaturation.

1. Synthesis of Methacryloxyethyl-l-Adamantanecarboxylate (MAC)

The synthesis of this tri -cyclic saturated bulky monofunctional monomer is shown below:

i

AD-C-Cl + HOCH fw OC-C = CH^

CH2C12. ET3N

0 0 CH.

1 11 1 ^

• ADC-OCH2CH2OC-C = CH2

where AD »

10
This monomer, a colorless liquid, is currently undergoing characterization

and will be examined in dental resin formulations for its potential in

reducing polymerization shrinkage. Infrared spectral analysis confirms

the assigned structure.

2. 2-Naphthyl ethyl Methacrylate (NEM-2)

The monomethacrylate, 2-naphthylethyl methacrylate (NEM-2),

was synthesized from 2-naphthyl ethanol and methacryloyl chloride as

shown

CH2CH2OH

0 CH,

It I 3 CH2CH2OC-C = CHg

NEM-2 is a low melting solid, m.p. 35-6°C, which gives a

liquid of moderate viscosity on melting. The assigned structure was

confirmed by IR analysis. This liquid polymerizes quite readily by the

usual cold-curing methods (BP/amine, UV + photoinitiator).

D. New Initiator/Accelerator Systems

1. Non-amine Accelerators

a. Thiourea Accelerators

Certain thiocarbonyl compounds such as thiourea,

allyl thiourea and acetyl thiourea have been used to accelerate the free

radical decomposition of hydroperoxide initiators (8,9).

(1) Acetyl thiourea (ACTU)

Acetyl thiourea, CH

2

-C-NH-C-NH

2

, was dissolved

in a typical dental monomer system, e.g. BIS-GMA (7)/TEGDMA (3).

Solutions of ACTU ranging in concentration from 0.25% to 1% by weight

were evaluated for their ability to activate the decomposition of 1 and

11
2 percent solutions in BIS-GMA (7)/TEGDMA (3) of the following initiators:

Benzoyl peroxide, t-butyl perbenzoate, t-butyl hydroperoxide and cumene

hydroperoxide. Rotational hardening times (e.g. crossed slide technique)

indicated that ACTU did not accelerate the decomposition of benzoyl

peroxide at ambient temperatures; with t-butyl perbenzoate, the rate

of cure was too slow (> 1 hour) for dental applications. With the

hydroperoxide (CHP), the rotational hardening times were considerably

less. However, in contrast to most amine/benzoyl peroxide initiator

systems which require relatively small quantities (0.2 - 0.3%) of the

amine accelerator (e.g. DMSX, DMAPAA), the hydroperoxide/ACTU initiator

system requires relatively large amounts of ACTU (e.g. 2% CHP requires

1% ACTU for rotational hardening time of 6h min., or a setting time

(Gilmore test) of 6 min). One percent solutions of allylthiourea (ALTU)

also gave a rotational hardening time of 6h min. with the 2% CHP

solution. Other thiocarbonyls such as 2-thiohydantoin and 2-thiobarbituric acid resulted in much longer rotational hardening times (> 1

hour) and soft sets.

Composites prepared with a paste (P/L = 3) containing 1% ACTU and a similar paste containing 2% CHP exhibited no

coloration (similar to those obtained with ascorbic acid) and had excellent diametral tensile strengths (47.4 MPa ± 2.4 MPa, 4 specimens).

Color stability of these composites also was excellent, showing little

change after 24 hours exposure to UV. In addition, the storage stability

of the peroxide component (e.g. CHP) in BIS-GMA/TEGDMA is superior to

that of a 2% bonzoyl peroxide solution in the same monomer system at

ambient temperatures. However, the biocompatibility of ACTU needs to

12
be evaluated before it can be recommended for clinical application

b. Triacetylmethane

largely as the enol

Triacety

, CHo-?

methane (TAM)

,

pH

CH 3 C -CH which exists

-C=C-CH

3 did not prove effective as an

accelerator for hydroperoxides, (e.g. CHP), peresters (e.g. TBPB) or

benzoyl peroxide (BP). BP solutions in BIS-GMA (7)/TEGDMA (3) cured

the fastest with TAM but the setting times are excessive for dental

applications. In fact, under aerobic conditions, TAM is an effective

stabilizer for BP solutions. Only 0.1% TAM stabilized a 1% BP solution

in BIS-GMA (7)/TEGDMA (3) without loss of its cold-curing effectiveness

after 6 months storage at room temperature in a polyethylene container,

c. Ascorbic Acid Systems - Blended Glass Filler

Formulations

We have shown previously (10) that it is possible

to blend a glass filler coated with a solid peroxide (e.g. t-butyl

peroxymaleic acid, benzoyl peroxide, etc.) with glass filler coated with

ascorbic acid (AA) or ascorbyl palmitate (AP) and obtain storage

(thermally) stable mixed glass filler. These blended fillers are then

employed in powder/liquid formulations with the liquid consisting of

only dental resin or dental resin plus small amounts of redox metal or

amine accelerator, depending on the nature of the blended glass employed.

Some results obtained using a blended filler of BP glass and AA or AP

glass are shown in Table 2. All of these composites exhibit excellent

esthetics, superior to comparable composites prepared using only amine

accelerators. A synergistic mechanism of acceleration may be operative

here, as indicated in our earlier reports (compare Formulations 1-3).

13
Sattlng TfMS and 24 EiparlmanUl Compos Itts

Table 2

Hour Diametral Tensile Strength of Glass*Based on Blended * Formulations

Setting Diametral Tensile Monomer 1 :1 Blend Glass Time Strength In MPa Formulation System InItlator/ACC fNIn.l (No of Soeclmenl

1 lIS-GNAl 70S It BP/0.3X AP 4 42.9 t 2.8 (S)

TEGONA*- 30t

DMAPE> 0.028t

BHT^ 0.02t

2 Same as above Same as above 4 1/2 38.4 t 2.3 (4) but no ONAPC

3 Same as above Same as above 2 1/2 44.8 t 2.S (3) but DMAPE-O.OaS

4 THUOHA’ 67X Same as above 2 46.6 s 3.4 (3) 81S*GMA 23X nGDHA 10X

CNSX> 0.077X

8HT • 0.05X

S Same as above IX BP/0.3X AA

8 THUOHA 83X Same as above

81S-GMA 12X

TE60MA SX

ONSX 0.039X

BHT 0.02X

7 Same as above 1X'8P/0.3X AP 2 46.8 t 3.3 (3) 8 THUDHA SOX Same as above 1 44.7 t 1.8 (3) 81S-GNA 3SX

TEGOHA ISX

DHSX 0.115X

8HT 0.07X

2 S0.1 t 1.1 (3)

2 1/2 42.7 s 2.3 (4)

9 Same as above IX BP/0.3X AA 1 46.7 t 3.0 (6)

10 Ternary Ether IX 6P/0.3X AP 4 43.2 ± 0.3 (2)

DHSX 0.1 25X

BHT O.IX

*Powder/11qu1d ratio 3; Powder sllanized 32S mesh glass beads. *BIS-CMA Bis-[p-(3-methacryloxy-2-hydroxypropo*ypheny1)] Isopropylldene. *TEG0MA • triethyleneglycol dimethacrylate.

*0MAPE • p-N.N-d1methylain1nophenethanol

.

••BHT 2,6-d1-t-butyl-p-cresol.

*THU0MA trimethyl hexamethylene diurethane dimethacrylate (Vlyadent). •OMSX N,N-d1methy1-syn-xyl1dlne.

’Ternary Ether Eutectic 14
2. Amine Accelerators

Amines are crucially important to the hardening of dental

resins since in situ pol.ymerization requires chemical means rather than

thermal to generate free radicals from the peroxide initiator. Since

amines can be a source of discoloration, of toxic response and can

through the manner in which they influence the rate of free radical

formation affect the molecular weight and molecular weight distribution

of the polymeric matrix, it is very important to develop information

that can yield efficient, biocompatible and non-color inducing amines.

a. Structure-Property Studies Employing Slower Acting

Amine Types Including Surface Active or AdhesiveGenerating Amines

The critical properties (e.g. reactivity, color

stability, biocompatibility, etc.) of tertiary aromatic amine polymerization accelerators are governed by aromatic ring substituents as well

as substituents on the amine nitrogen. We have studied the effects of

such substituents by examining setting times (Gilmore needle test) and

diametral tensile strengths (DTS) of composites prepared from an experimental P/L formulation consisting of silanized glass filler coated with

1% BP and the resin system BIS-GMA (7)/TEGDMA (3). The resin component,

in some cases, had in addition to the amine accelerator, 0.09% by weight

of BHT ( butyl ated hydroxytoluene) as an inhibitor.

The amines selected for this study with their

abbreviations are listed in Table 3. Composites were prepared in all

cases by using a 3:1 powder-liquid ratio.

Some indication of how chemical structural variations

15
Table 3

Amine Accelerators

Name Abbreviation Source

N, N-Di methyl -3, 5-Xyl idine DMSX Commercial

N,N-Dimethyl-p-t-butylanil ine DMBA Commercial

N ,N-Di sopropyl -p-tol ui di ne DIPPT Commercial

4- Dimethyl ami nobenzoic Acid 4-DMAB Commercial

N,N -Di-(2-hydroxypropyl ) -p-tol ui dine DHPPT Commercial

3-Di methyl ami nobenzoic Acid 3-DMAB Commercial

Ethyl 4-dimethyl aminobenzoate 4-EDMAB Commercial

4-Dimethyl ami nosal icyl ic Acid 4-DMAS Commercial

Acetyl 4-dimethyl aminosalicylic Acid ADMAS Synthesized

4-Dimethylaminocinnamic Acid 4-DMAC Synthesized

4-Dimethyl ami nocinnamic Acid 4-DEAC Synthesized

Polyfunctional Surface-Active

Amine Polymerization Accelerator

Poly SAM-1 Synthesized

16
in the amine accelerator can effect such properties as setting time and

diametral tensile strength of a composite material are shown in Table 4.

The monomer system used in this study was formulated without added BHT

but contained inhibitors added by the manufacturer.

Because of the extremely short working times (< 1 min)

of composite materials containing DMSX and DMBA, it was necessary to employ paste/paste formulations rather than the usual powder/liquid technique. Even then it was difficult to prepare specimens for DTS measurements. Of the two amines, DMBA appeared to be the more reactive

accelerator as measured by the setting time of equimolar mixtures. For

practical applications these extremely reactive accelerators require the

addition of more inhibitor or a slower polymerizing monomer system. For

example, better composites were prepared from DMBA when 0.09% BHT was

added to the monomer system (Table 5). On the other hand, longer setting

times and somewhat lower DTS values result from adding BHT to monomer

containing DHPPT or DMAB. The setting time of the composite formulated

with the sterically hindered amine, DIPPT, increases from 2 3/4 min to

> 8 min on adding 0.09% BHT. A clinically acceptable setting material

now requires higher concentration of DIPPT (0.25% vs 0.41%.)

DHPPT, a moderately fast-acting accelerator, gave

composites with the highest DTS values in the absence of added BHT

(Table 4). The para and meta forms of DMAB at 0.25% gave almost identical DTS values but the setting time with the meta isomer was significantly

shorter. At higher concentrations (0.50%), 3-DMAB appears to give composites with higher DTS values. The study of the newly synthesized

DMAC and DEAC was limited because of the relatively low solubility of

17
Table 4

Setting Time and Diametral Tensile Strength of an Experimental

Composite Material* Using Various Types of Amine Accelerators

Amine Concentration

wt. % mMolal

Setting

Time

(min)

Diametral Tensile Strength

(No. of specimens)

MPa ± Std. Dev.

DMSX** 0.20 13.4 2/3 41.9 ± 1.8 (2)

DMBA** 0.23 13.0 1/2 43.9 ± 2.2 (3)

DIPPT 0.25 13.1 2 3/4 46.9 ± 3.6 (6)

DHPPT 0.28 12.6 1 1/2 58.4 ± 2.4 (5)

4-DMAB 0.25 15.2 6 1/2 51.5 ± 2.2 (4)

3-DMAB 0.25 15.2 2 1/2 51.5 ± 3.8 (6)

4-DMAB 0.50 30.3 4 51.4 ± 3.7 (5)

3-DMAB 0.50 30.3 2 56.8 ± 4.9 (4)

DEAC 0.22 10.0 2 1/2 47.6 ± 3.3 (5)

DMAS 0.36 21.0 3 53.3 ± 2.5 (4)

ADMAS 0.50 23.5 4 49.6 ± 4.1 (5)

Poly SAM-1 0.50 2.5 1 1/2 49.7 ± 2.3 (4)

* Powder/Liquid formulations employed powder/liquid ratio = 3

Powder is glass filler (325 mesh) silanized with y-methacryloxypropyltrimethoxysilane and coated with 1% benxoyl peroxide. Liquids are monomer mixtures consisting of 7 parts of BIS-6MA and 3 parts of

TEGDMA containing the various concentrations of amine but no added BHT .

**Paste/paste rather than powder/liquid formulations. Powder/liquid

ratio remains 3:1.

18
these amines in the monomer system. These amines, which are bright

yellow solids, give solutions of similar hue. In the presence of sunlight, polymerization occurs with the fading of color to yield colorless

polymers and composites. However, with BP initiated polymerizations the

color is only partially diminished which is probably indicative of the

lack of participation by the vinyl groups of these amines in the

polymerizations involving BP.

The fact that DMAS, a derivative of salicylic acid,

accelerates the polymerization of BIS-GMA/TEGDMA as well as it does is

surprising in view of its chemical structure which contains not only the

electron withdrawing carboxyl group para to the amine function but also

a built-in inhibitor in the phenol ic-OH group. Composites prepared with

this amine had high DTS but were pinkish. The acetyl derivative, ADMASA,

a white solid, m.p. lU-lB^C, was synthesized as shown below:

DMAS ADMAS

The acetyl derivative, ADMAS, also gave glass particle reinforced composites with excellent strength (50 MPa) but with only slightly less

coloration than observed with the parent amine, DMAS.

In spite of its bulky nature (M.W.> 2000), the

polyfunctional surface active amine polymerization accelerator. Poly

SAM-1 , previously prepared (11) proved to be an effective activator

19
even at low concentrations as shown in Table 4.

Most of the composites had good to excellent initial

colors except those prepared with DMAC, DEAC, DMAS, and ADMAS. However,

during storage at 37° C in a 100% relative humidity chamber overnight

the initially white composites prepared with 4-DMAB and 3-DMAB darken

to a light tan hue, with the para isomer giving the darker shade. DMAC

and DEAC composites which are already colored appear to behave similarly.

The origin of this post coloration reaction is not known. The other

light composites darken only slightly on 24 hour storage in this chamber.

Color stability tests indicate that all composites

prepared with BP-amine accelerator undergo a color change on exposure

to UV. Color changes are difficult to quantify, but subjectively it

appears that the following amines gave the least color change for the

same equimolar concentrations, DMSX, DMBA, DHPPT, and 4-EDMAB. With

4-EDMAB at moderate concentrations composites with the best initial color

(white) were obtained. In addition, 4-EDMAB, by analogy with its parent,

the local anesthetic, ethyl -p-ami nobenzoate (Benzocaine) , fias a

high probability of exhibiting excellent biocompatibility.

From theoretical considerations, 4-EDMAB would be

expected to be a relatively slow-acting promotor for the decomposition

of BP. With a slow-polymerizing monomer such as methyl methacrylate,

for example, polymerization proceeds much slower by many hours than with

a fast-acting accelerator such as DMBA or DMS-X. However, with fastpolymerizing monomers, such as BIS-GMA, TEDGMA, ETC., clinically acceptable hardening times (< 8 min.) are possible using the usual powder/

liquid formulation technique (see Table 4 and 5). The mechanical

20
Table 5

Setting Time and Diametral Tensile Strength of an Experimental Composite Material* With Added BHT

Using Various Amine Accelerators.

Amine

Concentration

wt. % mMolal

Setting

Time

(min.

)

Diametral Tensile Strength

MPa ± Std. Dev.

(No. of specimens)

DMBA 0.23 13.0 1.0 50.1 :t 2.6 (5)

DIPPT 0.41 21.5 8.0 50.1 :t 2.0 (5)

DHPPT 0.28 12.6 5.5 51.0 :t 2.2 (4)

DHPPT 0.32 14.4 4.0 51.3 ;t 1.8 (5)

DHPPT 0.42 18.8 3.5 52.0 :t 2.7 (5)

4-DMAB 0.28 17.0 6.5 40.9 :t 6.4 (5)

4-DMAB 0.33 20.0 5.0 44.0 : o

J.J C\J (5)

DMAC 0.20 10.3 5.0 41.1 :t 3.0 (5)

* Same as in Table 4 ; P/L = 3, Monomer system contains 0.09 BHT

in addition to the amine accelerator

21
properties of composites prepared with this amine can be quite acceptable (Table 6 and 7). Compressive strengths for 0.5% and 0.7% EDMABwere

285.6±13.1 (4) and 301.4±11.2 (3) MPa. Higher DTS values and shorter

setting times are obtained when the monomer system is free of added BHT

(Table 6). Significant reductions in DTS values and somewhat longer setting

times often result from the addition of 0.09% BHT. Slow-acting promoters, such as 4-EDMAB and 4-DMAB,are more susceptible to the inhibitory

effects of BHT than are fast-acting accelerators such as DMSX and DMBA

(see Table 4 and 5 as well as Tables 6 and 7).

In Table 8 are shown some composites prepared by

using ascorbyl palmitate-BP blended powder/liquid formulations with

4-EDMABA as a synergistic promotor. The initial colors of these composites are the best we obtain when using amines as part of the initiator

system. In addition, color stability is also greatly improved while the

setting times are shortened considerably. Mechanical properties are

quite acceptable (see Table 8).

Another advantage of the use of a relatively slow

promotor as 4-EDMAB is that it permits a wider amine concentration range

to be studied with fast polymerizing monomers than is possible with the

fast acting amines such as DMSX, DMBA, DMAPAA and DMAPE. This should

prove helpful in not only leading to a better understanding of the

amine/peroxide initiator system but also in determining the extent of

polymerization of monomer systems by techniques such as differential

scanning calorimetry.

22
Table 6

Effect of EDMAB Concentration on the Setting

Time and Diametral Tensile Strength of an Experimental Composite Material* With No

Added BHT

EDMAB

Concentration

wt.% mMolar

Setting

Time (min.)

Diametral Tensile Strength

MPa ± Std. Dev.

(no. of specimens)

0.20 10.4 11.0 44.3 ± 1.6 (5)

0.26 13.5 8.0 50.6 ± 1.0 (3)

0.30 15.6 8.0 51.2 ± 2.2 (5)

0.40 20.7 7.5 47.7 ± 2.6 (7)

0.50 25.9 6.0 54.2 ± 1.1 (3)

0.60 31.1 5.5 50.9 ± 1.4 (4)

0.70 36.3 6.0 54.9 ± 1.5 (4)

0.80 41.4 5.5 51.7 ± 1.9 (5)

0.90 46.6 5.0 52.0 ± 1.5 (4)

1.30 67.4 4.5 53.8 ± 2.3 (7)

Same as in Table 4 except for amine

23
Table 7

Effect of EDMAB Concentration on the Setting

Time and Diametral Tensile Strength of an Experimental Composite Material* With Added BHT

EDMAB Diametral Tensile Strength

Concentration Setting MPa ± Std. Dev.

wt. % mMolar Time (min.) (no. of specimens)

0.20** 10.4 17.5 35.6 ± 1.7 (7)

0.30** 15.6 13.5 42.1 ± 1.2 (8)

- --*** 0.40 20.7 12.0 45.1 ± 1.4 (4)

0.50 25.9 7.0 45.4 ± 1.1 (4)

0.58 30.0 8.5 42.8 ± 2.4 (5)

0.69 35.8 7.0 44.6 ± 2.7 (5)

0.80 41.4 6.5 48.0 ± 1.8 (5)

0.90 46.6 6.5 47.4 ± 3.5 (5)

1.00 51.8 . 7.5 47.6 ± 3.3 (5)

1.50 77.7 6.0 48.2 ± 2.5 (8)

2.00 103.6 4.5 47.0 ± 2.3 (4)

* Same as in Table 4 except monomer system contains 0.09% BHT

in addition to EDMAB

Cured in mold for 30 minutes instead of customary 15 minutes

Cured in mold for 20 minutes

24
Table 8

Effect of EDMAB Concentration on the Setting

Time and Diametral Tensile Strength of An

Experimental Blended Powder*/Li gui d*Composi te

Material

EDMAB Diametral Tensile Strength

Concentration Setting MPa ± Std. Dev.

wt. % mMolar Time (min) (no. of specimen)

0.05 2.6 5.5 45.8 + 2.1 (5)

0.10 5.2 5.5 43.0 + 1.5 (4)

0.20 10.4 3.5 43.4 + 2.4 (4)

0.30 15.6 3.5 51.2 + 1.8 (4)

0.40 20.7 4.0 51.4 + 0.5 (3)

0.50 25.9 3.5 51.7 + 1.7 (3)

0.60 31.1 3.5 51.9 + 2.0 (3)

0.70 36.3 3.0 49.6 + 0.9 (5)

0.80 41.4 3.0 48.0 + 2.1 (4)

0.90 46.6 4.0 51.2 + 3.4 (7)

* Blended powder consists of equal parts of silanized glass coated

with 1% benzoyl peroxide and 0.3% ascorbyl palmitate, respectively,

P/L ratio = 3

+ Liquid = same monomer system as in Table 7

25
b. Fast Acting Amine Homologues of N,N-Dimethylaminophenyl acetic Acid and N,N-Dimethylaminophenethanol

We have previously reported (12) dimethyl ami nophenyl

acetic acid (DMAPAA) to be a very reactive accelerator for the diacyl

peroxide initiated polymerization of composite resins. The overall

characteristics of composites derived with this amine or its methyl ester

compared favorably with composites cured with commonly used commercially

available tertiary amines. An important additional characteristic of

DMAPAA is that it has been shown to be non-mutagenic and non-cytotoxic

(13). A preliminary evaluation of a related amine p-methylaminophenethanol (DMAPE), also synthesized in this laboratory, indicated very

high reactivity and excellent properties in derived composites

of these amines (both of which are highly reactive but likely to be

non- toxic.) We have prepared the following series of new amines.

Most of the compounds were synthesized from primary amines. Their

structures were confirmed by IR and NMR. Products were purified care- fully since secondary amine by-products act as inhibitors in the curing

of composites. Purity was established by gas chromatographic analysis

to be >99%. Physical properties are given in Table 9. A brief description of those amines synthesized during this year follows:

We have therefore continued to synthesize homologues

R'= H,CH3,C2Hg

n = 1 or 2

26
Table 9

Tertiary Aromatic Amines Synthesized

Melting or Amine Abbreviation Boiling Point, *C

1. Aminophenethanol

N,N-dimethyl® DMAPE MP « 52-53

N,N-diethyl° DEAPE BP 153-154/4 mm

2. Ami nophenyl acetic Acid

N,N-dimethyl‘' DMAPAA MP « 108-112

N,N-diethyl«l DEAPAA MP - 148

3. Ami nophenyl acetic Acid

N,N-dimethyl -methyl ester® DMAPAA-ME BP = 107/1 mm

N,N-d1ethyl -ethyl ester^ DEAPAA- EE BP * 135-137/2 mm

4. Aminophenyl propionic Acid

N,N-dimethyl9 DMAPPA MP » 103-106

Method of synthesis: primary amine and ^CHoI through quaternary ammonium

salt, “CoHsI in aq. MeOH, ^C 2 H 5

l, ^CHtI In OMSO, ^C2 H

5 Br in OMSO

QdimethyTaminobenzaldehyde + malonic acid. The resulting cinnannic acid

was reduced with zinc amalgam.

27
p-N,N-Dimethyl and Diethyl ami nocinnamic Acids

These compounds were first synthesized as intermediates using a modification of the Shoppe procedure (14) for the diethyl

homologue. The general reaction is as follows:

- CHO+CH

^COOH

^"COOH 100 CH=CH-C00H+C02+H20

pyridine,

piperidine

Yields of 44% and 22% were obtained respectively for the methyl and

ethyl derivatives.

Both the p-N,N-dimethyl and -diethyl derivatives act

as photo-initiators for composite resins in sunlight. The photocuring

process occurs with the disappearance of the yellow coloration of the

initiator in the cured resin.

p-N ,N-Dimethyl ami nophenyl propi oni c Acid

This compound, mp = 104-106“ C, was prepared by

reduction of the corresponding cinnamic acid (see above) using a zinc

amalgam prepared from mossy zinc, mercuric chloride, water and concentrated HCl . Although the procedure we followed (based on the Clemmensen

reduction reaction) is rather easy, the yield proved to be poor (approx.

7%). This however, was sufficient to evaluate the usefulness of this

compound as a polymerization accelerator for composite resins.

p-N ,N-Di ethyl ami nophenyl aceti c Aci

d

Preparation of this compound was accomplished as

follows: 39.3 g (0.40 mol) of potassium acetate and 150 ml of ethanol

were added, with stirring, to 15.1 g (0.10 mol) of p-ami nophenyl acetic

acid. Then, dropwise, 18.4 ml (0.23 mol) iodoethanane were added, the

28
reaction mixture was gradually heated to 78 C and refluxed 6 hours. The

insoluble material was filtered off. Most of the ethanol was evaporated and the solution was made strongly acidic with concentrated HCl

.

The resulting precipitate was filtered. The filtrate was diluted and

brought to pH 4.5 with aq. NaOH. A precipitate formed which was filtered.

The brown solid (9 g.) on recrystallization yielded nearly colorless

crystals. MP = 148.5-149° C. It is probably the quanternary ammonium

salt. Its structure is being established from IR and NMR spectra.

p-N ,N-Di ethyl ami nophenethanol

This compound was prepared by the following procedure: 12.0 g (0.087 mol) potassium carbonate and 100 ml 25% aq.

methanol were added, with stirring to 9.0 g (0.057 mol) p-ami nophenethanol . Then 10.8 ml (0.135 mol) ethyl iodide was added dropwise. The

mixture was gradually heated and refluxed for 24 hours. The impure

product formed a brown top layer which was separated. It distilled at

123-125 C/2mm. yield: 9 g; 79%.

The product was purified by refluxing 8.8 g. (0.046

mol) for 6 hours with 5 ml (0.063 mol) iodoethane and 8 g. K

2 CO3 in 40 ml

25% ethanol. The two layers were extracted with dichloromethane. The

extract was combined, the solvent evaporated and the impure product

distilled. BP: 127-129 C/2mm . Yield: 6.4 g; 52%.

Further purification through the amine hydrochloride

neutralization and subsequent extraction with dichloromethane yielded

a product BP: 153-154 C/4mm, n^^ = 1.5503.

29
p-N,N-Diethylaminophenylacetic acid-ethyl ester

The reaction of p-ami nophenyl acetic acid (22.7 g),

anhydrous K2CO3 (80 g), dimethyl sulfoxide (75 ml), bromoethane (25 ml)

and KI (75 mg) was carried out for 2 hours with stirring in a flask

equipped with reflux condenser, thermometer and dropping funnel. An

equivalent second portion of bromoethane was added and stirring continued for an hour . The mixture was heated overnight at 55 - 60“C,

cooled and extracted with dichloromethane. After evaporation of the

solid the product was obtained by vacuum distillation. Further purification was achieved by refluxing the ester 10 min. with acetic anhydride

before pouring the reaction mixture into 50 ml of water. After 30 min.

the pH of the solution was adjusted to 8 with 10% NaOH and the oil layer

was separated. Vacuum distillation yielded a product with a boiling

point of 135-137“ C/2iTM and n^^= 1.5262.

N,N-Trimethylammoniumphenyl butyric acid halide

Reaction of p-ami nophenyl butyric acid (16.6 g),

aqueous 25% (by vol.) methanol (300 ml) and K2CO3 (28 g) was carried out

in an appropriate flask. lodemethane (21.8 ml) was added dropwise with

stirring and the reaction mixture refluxed for 8 hours. On evaporation

of the solvent KI separated and was filtered off. The filtrate was neutralized with 20% HCl and the resulting precipitate recovered by filtration.

NMR analysis shows that refluxing this compound with

ethanol amine or refluxing it with silver oxide in aq. methanol and heating the resulting solid at 140“ C in an inert atmosphere did not decompose

the quan ternary salt. The quanternary ammonium salt, purified from

n-amyl alcohol had a MP = 165 to 166“ C.

30
(1) Evaluation of DMAPAA.and DMAPE Homologues

To determine the reactivity of the amines and

the properties of the resulting materials, composites were prepared

using a 3 to 1 powder/liquid ratio. The powder was Corning glass 7724

treated with gamma-methacryloxypropyl trimethoxysilane and coated with

1% benzoyl peroxide. The liquid contained 72.4% BIS-GMA, 27.6% 1,6-

hexamethylene glycol dimethacrylate (diluent) and 0.2% butyl ated hydroxytoluene inhibitor. This diluent was substituted for the tri ethyleneglycol dimethacrylate (TEGDMA) since there is some evidence (15) that

diluents having only (CH

2 )^ may be preferable to those having

(CH2-CH20)^ groups in the chain. The properties of the composites are

given in Table 10. They were determined by procedures described in

ADA Specification No. 27 (16). All amines studied gave adequate setting

times with dimethyl ami nophenyl acetic acid (DMAPAA) yielding the shortest

and the higher homologue dijjiethyl ami nophenyl propionic acid (DMApPA) the

longest setting time. For unfilled resins the rate of curing proceeds

most rapidly for molar peroxide to amine ratios between 1.1 to 1.5.

For composites containing initially undissolved peroxide a much larger

excess of peroxide is required to obtain a minimum setting time. This

should be expected since only a small portion of the peroxide is accessible to the amine before the composite is cured. The mechanical properties are not as dependent on the amine concentration as was observed

previously with TEGDMA. Optimum properties (tensile strength: 53 MPa,

compressive strength: 302 MPa, water sorption: 0.55 gm/cm^) were obtained

in the 15-19 millimolal concentration range. The propionic acid or esters

of N,N-diethyl derivatives did not yield composites with better mechanical

31
properties than those containing the corresponding acetic acid or

dimethyl derivatives. For all formulations tensile strength is much

above the minimum specification requirement of 34 MPa (16). The initial

shades of many composites are very light. Color stability, even without incorporation of an ultraviolet stabilizer into the material, and

water sorption are very satisfactory. Best overall physical properties

were observed for the di alkyl ami nophenethanols and DMAPAA. The low

volatility and reduced diffusion rates of the higher molecular weight

amines, especially those containing alcohol or ester groups, should

confer good biocompatibility.

c. A New Amine Accelerator for Denture Base Resins

Denture base resin systems that are currently on the

market employ N,N-dimethyl-p-toluidine (DMPT) as the amine for accelerating the polymerization of the methacrylate monomer. The amount needed

varies from zero in heat cured systems to 1/2 - 1% for low temperature

autopolymerizing materials such as pour resins and the reline and repair

materials. Due to the differing amounts of amines used, and subsequent

discoloration from the presence of amine, it is currently not possible to

match all shades of heat cured denture bases with pour or reline and

repair materials; however, it is desirable to be able to accomplish

this for economic and esthetic reasons.

In cooperation with the terican Dental Association

Research Institute at the National Bureau of Standards, a newly commercially available amine (N,N-dimethyl-p-t-butylaniline, of DMBA) was

evaluated for color effects in denture materials.

Specimens of cured denture base were prepared using

32
commercial poly (methyl methacrylate) powder and methyl methacrylate

monomer with DMPT as were specimens prepared from the same powder and a solution of 0.5 DMBA in methacrylate monomer; various weight percentages

of UV absorber were added to monomer portions. Curing times were approximately equal to room temperature. The specimens prepared using DMBA

displayed no evidence of discoloration upon curing whereas those prepared

with DMPT displayed the usual high degree of discoloration. Subsequent

exposures to UV light showed specimens with 0.5 and 0.25% of UV absorber

and DMBA to be virtually free of discoloration due to UV light.

From the standpoint of color it is concluded that

DMBA is more suitable for use in denture base systems than is DMPT. If

the application of DMBA for this purpose results in the use of

sufficient quantities to reduce its price, some of the current problems

in achieving and matching shades within denture base systems could be

eliminated.

E. New Stabilizers for Dental Monomers

One of the requirements of a dental composite restorative or

sealant materials is a reasonable shelf-life, preferably at ambient

temperatures or at least under simple refrigeration. The paste or

liquid component which contains the initiator (e.g. "catalyst"), usually

benzoyl peroxide (BP), exhibits the poorest shelf life. The use of

stabilizers which are antioxidants and polymerization inhibitors

stabilizes the monomer system against autoxidation and premature

polymerization. Common stabilizers for dental monomers are the methyl

ether of hydroquinone (MEH) and 2 ,6-di

tert- butyl -p-cresol (BHT) shown

bel ow

33
An ideal stabilizer for dental monomers should be biocompatible,

highly effective as an inhibitor, but without adverse affects on the

"cold-curing" process.

We have investigated the feasibility of using two naturally

occurring amino acids, tyrosine and tryptophan, as potential stabilizers

for dental monomers. The structures of these a-amino acids are shown

below:

HO- CH,-CH-C0,H

2 I 2

• NHg Oq

CH..-CH-CO 0H / 2 I X NH.

TYROSINE

. IH

TRYPTOPHANE

We prepared solutions of these amino acids in BIS-GMA (7)/

TEGDMA (3) in the concentration range of 0.05 to less than 0.1%. To

these solutions were added various amounts of BP to give several concentrations of the initiator (0.25%, 0.5%, 1% and 2.1%). These solutions

were stored in air permeable polyethylene bottles and have remained

unpolymerized after 7 months. In addition, several solutions

34
stored in partly filled glass vials have also not gelled. The

tryptophane solutions have taken on more color (yellow) than those containing tyrosine or a 0.1% BHT control solution. All the solutions have

retained their "cold curing" effectiveness as measured against a standard

monomer solution containing 0.2% p-dimethyl ami nophenyl acetic acid.

Composites prepared from the tyrosine/BP solutions utilized in

paste/paste formulation have shown excellent diametral tensile strength,

comparable to those obtained with BHT/BP systems.

F. Reaction of Silane Coupling Agent with Quartz

To improve dental composite restoratives attention must be directed not only to the resin but also to the filler and its bonding to the

matrix. The interface between fused quartz or glass particles and the

polymeric matrix plays a profound role in the behavior of composite dental

materials. The basic load transfer mechanisms between the substrate and

matrix is dependent on a strong interfacial bond, either chemical or mechan

ical. The interface furthermore plays an important role in the fracture

behavior of composites. An understanding of the reactions at the interface and the ability to control them would yield mechanically improved,

more homogeneous materials.

Coupling agents applied on the surface of Si02 substrates

greatly improve the interfacial and water susceptible properties of the

composite. The mechanical strength of glass reinforced composites is

significantly increased by pretreatment of the glass particles with

silanes. Glass, or fused quartz particles employed as reinforcing

agents in BIS-GMA based composites are usually pretreated with a dilute

solution of Y-methacryloxypropyl t rime thoxysi lane (MPTS) coupling agent

35
Table 10

Properties of Con’.posites with Various Amine Accelerators

Amine

mMolal Cone.

Type in Liquid

Setting

Time

Min.

Tensile

Strength

MPa

Compressive

Strength

MPa

Color 5/ Stabi 1 ity“

H

2 O

Sorptij

mg /cm'

DMPT 15 4.5 45.9 276 3.5

U 17 3 43.0 246 3.5 .58

H 19 4.5 47.6 246 3.5 .56

DMAPE 15 3.5 46.1 262 3.5 .53

17 3 47.7 271 3 .55 N 19 3 50.1 302 3

M 21 2.5 39.6 * 286 3 - OEAPE 15 6 45.6 277

" a/ 17 3 53.4 302 • *

U 17 4 49.1 294 3

H 19 4.5 48.0 279 3 -

DMAPAA 13 2 49.8 288

M 15 2 52.2 276 3 .56

M 17 2.5 51.6 279 3

N 19 2 50.2 296 3 .61

DMAPPA 15 6 41.4 241 3 .55

M 17 4 43.1 263 4 .55

M 19 5 45.8 258 3 .58

OMAPAA-ME 17 3.5 49.8 273 - -

DEAPAA-EE 13 4.5 51.2 278

M 15 4 48.0 276 . •

N 17 4 46.5 276 3 .

N 19 5 47.0 283 2.5 - ^ prepared with O.IS BHT. ^ rating scale by visual observation: 1 very large change to 4 no change.

36
which appears to be the most compatible silane for the acrylic monomers

used as the dental matrix resin. In a recently started study we are

seeking the most effective treatment to obtain a permanent bond to

glass or fused quartz. We are investigating the affects of different

applications of the silane to the glass or fused quartz surface on the

permanency of the bond and on the mechanical properties of the cured

composite. Initially, the presence and permanency of the bond is

being studied by infrared spectra of the treated powders. Changes in

the application of the silane will include the following parameters:

(1) type, volatility, and polarity of solvent to be used, (2) silane

concentration, (3) reaction time, (4) drying procedure (rate of drying,

maximum temperature employed), (5) pH of solvent media, and (6) effect

of impurities such as water in the solvent and effect of substrate

particle size and distribution on the effectiveness to obtain a permanent

bond.

Wafers were molded from Cabosil colloidal silica and subjected

to various silanizing treatments for 2 hours. Formation of chemical

bonding between the silica and the silane was established from the

disappearance of the free hydroxyl band of silica at 3740 cm~^ . The

siloxane peak of the bulk silica at 1850 cm'^ was used as internal

standard. Only incomplete bonding takes place when the wafers are

treated with MPTS dissolved in acetone, n-pentane or cyclohexane (Table

11). Furthermore, the MPTS solution in acetone decomposes rapidly in

the presence of light. With 2% n-propyl amine catalyst added to the

silanizing solution the free hydroxyl band disappears indicating that

the silane has reacted with the silica. This bond is not destroyed

37
for the treated wafer stored for 280 hours in water at 68 C if the silane

treatment was conducted with cyclohexane as solvent using n-propyl amine

as catalyst (Table 12). Other treatments - even those employing the

catalyst - result in a partial hydrolysis of the bond and a reappearance

of free hydroxyl groups in the silica. This hydrolysis of silanized

silica will be studied further.

Table 11

EFFECT OF N-PROPYL AMINE ON CHEMICAL BONDING OF SILANES TO SILICA SURFACES

Reaction Time: 2 hr at room temperature

Bonding,

Silica Treatment With no catalyst 2% n-propyl amine

1% A-174 in acetone

(solution decomposes in light

0 100

2%A-174in 90% acetone -• 10% H

2 O 23.0 100

2% A-174 in ii-pentane 33.5 100

2% A-174 in cyclohexane 26.5 100

® Calculated from relative intensity ratio of infrared absorption peak at 3740 cm"l (free OH band) and around 1850 cm"^ (siloxane linkage in the

bulk of the silica). Samples were washed with acetone for 15 minutes

and air-dried prior to infrared measurements.

Table 12

STABILITY OF SILICA - SILANE BOND

Wafer stored in water at 680C for 280 hrs.

Silane Treatment (2 hr) of Silica Wafer

Bonding, %®

Before Storage After Storage

2% A-174, 2% n-propyl amine in cyclohexane 100 100

It II II II II in pentane 100 71.5

II II II II II 90% acetone10% H2O 100 41.1

II II II II II 100% acetone 100 26.5

II II II in cyclohexane 26.5 0

II II II in pentane 33.5 33.5

II II II in 90% acetone - 10% H

2 O 23 20

^Calculated from relative intensities ratio of IR peaks at 3740 cm"^ and

1850 cm' "I

38
G. Autocurable Blends as Route to Improving Chemical and Mechanical Properties of Denture Base Materials (and Other Dental

Materials)

Denture base materials as well as direct filling resins and

bone cements are based on use of a monomer/polymer system such as

methyl methacrylate (MMA) in poly (methyl methacrylate )(PMMA). There are

limitations to the improvement in properties that can be gained by this

route and alternative approaches are needed. The use of an elastomeric

polymer or copolymer in place of or to supplement PMMA with MMA or similar monomer systems can produce polymer blends or alloys with greater

toughness and ductibility and less brittleness. The polymer blend

approach to achieving new or improved properties in polymeric-based

systems is one being given forefront consideration today. In such

polymer blends component polymers are bound together by forces which

are relatively weak when compared with covalent bonding in copolymers.

Because of compatibility properties, a major generic problem is to control phase separation which unless on a micro scale can be very

deleterious.

Purely physical polymer blends are commonly prepared by mechanical mixing of polymer components in the melt or by dissolution in a common solvent followed by casting and solvent removal. Our route will

be to form polymer blends by autocuring bulk polymerization techniques.

1 . Sol ubil i ty Studies

Various copolymers of polyvinyl idiene fl uoride, PVF

2

,

will dissolve in methyl methacrylate (MMA) to varying degrees depending

on such factors as the molecular weight of the copolymer, degree of

39
1

crystal 1 ini ty, etc. A copolymer of tetrafluoroethylene and vinyl idene fluoride is quite soluble in MMA ( 30%). Similarly Kel-F

elastomer consisting of 70% PVF

2 and 30% (CF2 -CFCI) is also soluble in

MMA. Vi ton A, an elastomeric copolymer vinylidene fluoride and perfluoropropylene is also soluble in MMA. However, the homopolymer PVF

2

is only swelled by MMA at room temperature.

Heating PVF2 in MMA in a sealed tube (25-140 C) resulted

first in solution, followed by polymerization. The concentration of PVF2 was of the order of 10% and the acrylic rod appears to have a tougher

character than that of pure PMMA. At this point it is not known whether

a blend of PMMA and PVF2 or a graft or copolymer of the two was formed.

The polymer is soluble in MMA so crosslinking apparently did not occur.

We plan to study the autocuring behavior of the MMA-PVF

2 copolymer

solutions and evaluate the properties of these polymers.

40
II. Development of Adhesive Bonding Techniques

A clinically applicable technique which establishes and maintains

adhesive bonding to the dentin tooth surface would offer an important

means of reducing the recurrent caries that is encouraged by the microleakage of saliva and bacteria between restoration and tooth. Perhaps

even more important is the potential for significant modification of

cavity preparation and technique of placing restorations that would allow

for greater preservation of sound tooth structure than is now possible.

A. Cyanoacrylates

A summary of research in which we have been investigating the

potential of various cyanoacrylate monomers as a means for generating adhesive bonding between the tooth surface (dentin) and a composite restorative material was prepared for review by the NBS editorial process and has

now been published as a part of the ACS Symposium Series, No. 95 - 1979

Durability of Macromolecular Materials (17).

We showed in the previous Annual Report (10) that the pretreatment of dentin with dilute acids such as 0.1 or 1% aq. ale. (1:2)

citric acid prior to application of isobutyl cyanoacrylate yields strong

bonding between acrylic rods and dentin even after thermocycl ing the assembly for 24 hours. Stanley et al . (18) and others have shown that etching

of dentin with rather concentrated solutions, such as 37% phosphoric acid,

can produce an undesirable pulp response. The proposed dilute acid pretreatment may be applicable clinically where a strong, short-term attachment

to dentin is desired provided that the very dilute (0.1 to 1%) citric acid

solution can be tolerated by the tissues. To evaluate the possible clinical

use of dilute citric acid solutions, the effect of citric acid on the surfaces

41
of dental cavity preparation and the underlying pulpal tissues has been

initiated by Dr. P. M. Mclnnes and Dr. Clayton Jones of the Dental

Research Institute of the University of the Witwaterstrand, South Africa.

In this study 48 vervet monkeys (cercopithecus aethiops

pygerthrus) are being used as the experimental model. Restorations have

been placed after the following pretreatments: no etching and filling

with negative and positive control material (ZOE and silica cement) and

etching with 0.1% and 1% aqueous and 30% alcoholic citric acid and subsequent placement of an amalgam filling. The animals are being sacrificed

after 2 days, 28 days and 56 days. The teeth and tissues will be examined

histologically and histopathological ly using the criteria suggested by

Stanley (19) following details outlined by the present investigators.

Results of the 2-day interval study will reveal the initial response to

the acids and to the trauma of the operative procedure. The 4-week interval observations will be more severe than those of the 2-day interval and

should show the full-blown inflammatory response to the pulp. After 8

weeks it is expected that the pulpal lesions will have undergone resolution.

B. Urethane (Isocyanate) Adhesives

The well-known reactions involving organic isocyanates and

active hydrogen compounds and the potential availability of a significant

number of such active sites in the collagenous component of dentin suggested the synthesis of methacrylate monomers with pendant isocyanate

groups. This synthesis was reported in an earlier report (10) and the

galley proofs of an accepted manuscript have been returned to the Journal

of Dental Research. Laboratory investigation of some of these urethane

derivatives is described in what follows.

42
Bone specimens were cemented together with formulations containing the urethane-methacrylate, a polymerizable diluent (such as methyl

methacrylate or triethylene glycol dimethacrylate), and suitable redox

initiating systems (benzoyl peroxide initiator and dimethyl -£-tolui dine

accelerator)

The urethane derivatives used were:

0 CH3

K I ^

and

9 9 9H3

OCNRNHC-OCH CH OC-C « CHo

2 2 ^ where

R : -CH

2

- C - CH^

I 2 2 2

CH3

^ TMDI-HEMA

and R: C^g hydrocarbon DDI-HEMA

In the initial studies, 1 ittle or no adhesion was indicated with

formulations containing TMDI-HEMA or DDI-HEMA after thermocycling the bone

specimens for one day at 5° and 55°C (540 thermocycles per day). Some

results for bonding an IDPI-HEMA based adhesive to bone are given in

Table 13. Significant adhesion was obtained only on pretreatment of the

bone with 1% NaOH to expedite urethane or urea formation, followed* by

storage of the bonded specimens in air from 5 to 30 min. before 24 hr.

storage in water. Optimum one-day adhesion of 2.6 MPa was found on keeping

43
BONDING

OF

EXPERIMENTAL

URETHANE-METHACRYLATE

CONTAINING

ADHESIVES

TO

BONE
the sample at room temperature for 30 min. before immersion in water.

Specimens treated in the same manner and thermocycled for 7 days had

bond strength of 1.9 ± 0.7 MPa, but the specimens fell apart on thermocycling for 30 days. Storage of the bone in air, or pretreatment of

the substrate with ^% NaHC03 , 1% Na 4 EDTA, 1% citric acid, 0.1 N CaC03

or 1% H

2 O

2 did not improve adhesion. Application of several commercial

urethane curing agents containing organometal 1 ic compounds which

should .speed up urethane formation to the bone did not improve joint

strength even if the substrate had previously been treated with

1% EDTA. Bond strength of specimens of 1% NaOH pretreated bone

varying from 0.1 to 1.0 MPa were obtained on curing of compositions

containing 48.6% trimethyl ol propane trimethacrylate, 38.1% of a

tri methyl hexamethylene urethane dimethacrylate and 14.3% of either

IPDI, DDI, or IPDI-HEMA and the previously employed concentrations of

initiator and accelerator. On decreasing the concentration of

polymerizable dlmethacrylate diluent so that the HEMA-IPDI concentration

increased to 60%, a bond strength of 1.5 MPa was obtained.

Although statistically significant adhesion was obtained with

these isocyanate urethane methacrylate-containing adhesives, the values

for the strength of the bonded joints was considerably lower than for

those cemented with 2-cyanoacrylate esters.

45
C. Multifunctional Adhesion Promoting Surface-Active Amine

Accelerators

1. POLYSAM-1 and Other Amine Accelerators

We have previously (20) reported the synthesis of a polyfunctional surface active amine accelerator designated POLYSAM-1, the

structure of which is illustrated as follows:

The proposed mechanism for adhesion involves a bi functional action:

1) attachment to the apatite surface via chelation bonding between

the multiple N-phenyl glycine groups and surface calcium or calciumsubstituted atoms (e.g., Fe, Cu), and 2) formation of a radical cation

on the N-methyl-p-toluidine portion of the structure which then forms a

free radical that may participate by covalent bonding in the

polymerization process.

46
In spite of its MW (> 2000), POLYSAM-1 can

be an effective amine accelerator for the cold-curing of BIS-GMA monomer

systems containing benzoyl peroxide (BP). For example, one part of a

0.5% solution of POLYSAM-1 in BIS-GMA(7)/TEGDMA(3) was mixed with three

parts of silanized glass coated with 1% BP to give composites having excellent diametral tensile strengths [49.7 ± 2.3 MPa (4); setting time -

1 1/2 min.]. Earlier we had shown that POLYSAM-1 can be irreversibly

adsorbed onto powdered synthetic hydroxyapatite (HA). In this adsorbed

state, POLYSAM-1 is also an effective polymerization accelerator for

BIS-GMA monomer systems, thereby making it feasible to prepare HA filled

composites from HA having various quantities of chemically adsorbed

POLYSAM-1. The maximum amount of chemisorbed POLYSAM-1 is 4.2%.

We had previously shown that HA with 4.2% adsorbed

POLYSAM-1 accelerated the polymerization of methyl methacrylate containing 1% BP if the bulk polymerization was conducted under aneorobic conditions (e.g., vacuum). The polymer obtained from this polymerization

was not completely soluble in good solvents (CHCl^, CH

2 CI 2

) for

poly(methyl methacrylate) . About 15-20% of the polymer resisted dis-

_3

solution. Infrared analysis revealed the presence of PO^ , and

therefore, HA. Treatment of this polymer fraction with aqueous HCl

followed by extraction with CHCl^, resulted in the solubilization of

most of this fraction in that solvent. The IR spectra of the CHCl^ soluble material no longer had adsorption bands for HA (e.g., PO^ ) but was

essentially that of poly (methyl methacrylate) with some adsorption bands

indicative of the presence of POLYSAM-1 moieties.

47
A similar experiment was performed with a new monomethacrylate, 2-naphthyl ethyl methacrylate (NEM-2). (Synthesis described

in Section I .

)

^ NEM-2 is a low melting solid, m.p. 35-36°C, which gives a

liquid of moderate viscosity on melting. This liquid polymerizes quite

readily by the usual cold-curing methods (BP/amine, UV + photosensitizer)

and the polymer of NEM-2, PNEM, is soluble in CHCI

3

. A thin composite

disc was prepared using liquefied NEM-2 containing ^% BP and HA coated

with POLYSAM-1 . Only 50% of the PNEM appeared to be susceptible to

dissolution. A control experiment with uncoated HA gave only soluble

polymer.

These preliminary results suggest that POLYSAM-1 has

potential as an adhesion promoting agent for dental composites and

sealant materials.

Several additional surface-active amines have been examined for adhesion promoting ability. Dr. Misra of this laboratory has

demonstrated that p-N,N-dimethylaminophenyl acetic acid (DMAPAA) is irre- .

versibly adsorbed on synthetic HA from methylene chloride solutions.

However, unlike the irreversible adsorption of POLYSAM-1 onto HA from

both protic solvents (e.g., C2

H 5 OH) and aprotic solvents (e.g., CH2

CI 2 ),

the adsorption of' DMAPAA is reversible from protoic solvents (e.g.,

washing with excess solvent will eventually remove the amine in the case

of protic solvents). Using powder liquid formulations with an HA to which

DMAPAA was adsorbed and BIS-GMA(7)/TEGDMA(3) containing various concentrations of BP, extremely fast cures were observed (< 1 min.). This

48
amine appears to be more reactive in the adsorbed state than when

dissolved. Preliminary experiments with a dilute solution of dimethylami nobenzoic acid (DMABA) in methylene chloride and hydroxyapatite indicate a certain amount of solute is adsorbed irreversibly . However, protic

solvents will remove the DMABA from hydroxyapatite. The amine p-N,Ndi methyl ami nosali cyclic acid (DMASA) is also adsorbed irreversibly to

HA from aprotic solvents but reversibly from protic solvent. HA-filled

composites with the adsorbed DMASA were readily prepared also.

The diametral tensile strength determination (DTS) has

been shown to be a valid and useful method for assessing the relative

effectiveness of different silane coupling agents used in glass-filled

composite resin systems (21). Table 14 compares the DTS obtained with

specimens prepared with unsilanized and silanized glass filler, respectively. The results are clearly indicative of the existence of some

coupling action between the glass filler and dental resin via the agency

of the silane. It seemed reasonable to expect that DTS measurements of

HA-filled composite specimens could reveal the presence or absence of

coupling between resin and HA via POLYSAM-1 and other surface active

amines and therefore serve as an indicator of adhesive potential.

Table 15 summarizes the setting times and DTS values

obtained using HA with various amounts of adsorbed POLYSAM-1. Table 16

lists similar data for HA containing adsorbed p-N,N-dimethylaminosal icycl ic

acid (DMASA). The highest DTS for a composite prepared from HA containing an adsorbed surface-active amine accelerator was that of

Formulation 2, Table 15.

49
The results summarized in Table 17 are for uncoated HAfilled composites with the various surface-active amine accelerators

initially dissolved in the accelerator paste. Again the highest DTS value

is for a POLYSAM-1 formulation (B). Significant DTS values were observed

for a DMAPAA formulation (D) and for formulation (F) which utilized the

acetyl derivative of DMASA, ADMASA, as the accelerator. The latter DTS

value was for 72 hours rather than 24 hours.

Table 18 represents an attempt to devise a control for the

preceding experiments. The accelerator, DMSX, is an effective but nonsurface-active amine accelerator. The DTS values of HA-filled composites

using DMSX were significantly lower than those for some surface active

amine formulations (e.g.. Formulations 2, Table 5 and B, Table 17).

Since the diametral tensile strength of a composite is

affected not only by the coupling agent, but also by

the degree of polymerization of the dental resin, which in turn is dependent on the nature and concentration of the amine accelerator (22),

it is not surprising that the results of Tables 15-18 are difficult to

interpret compared to the results shown in Table 14 on the unsilanized

versus silanized glass composites. The surface-active agents used in

HA-filled composites have a dual function, acting as polymerization

accelerators as well as potential coupling agents. The nature of the

amine-peroxide reaction is still not well understood and may not be the

same in every respect with all tertiary aromatic amine accelerators.

However, the results of this study seem to indicate that at least a

modest coupling action may accrue from the proper use of POLYSAM-1 and

other surface-active amine accelerators.

50
TABLE 14

DIAMETRAL TENSILE STRENGTHS OF EXPERIMENTAL COMPOSITES*

FORMULATED WITH UNSILANIZED [U] AND S I LAN I ZED*’ [S] GLASS FILLER (325 mesh)

Formulation Peroxide Paste Acc. Paste P/L

Setting

Time

(min.)

Diametral

Tensile

Strength in MPa

(no of spec)

U BIS-GMA (7)

TEGDMA (3)

BP » n

TYR'« O.U

BIS-GMA (7) 3

TEGDMA (3)

DMAPAA^= 0.1 855 TYR = 0.05%

5s 28.3 ± 1.5

S Same as above Same as above 3 3 49.8 ± 1.9

(4)

*

1

2

t

Composites were prepared from equal parts of peroxide and accelerator paste

TYR Tyrosine

DMAPAA » p-N,N-d1methyl ami nophenyl acetic acid

Silane agent * gamma methacryloxypropyl trimethoxy silane

51
Table 15

Setting Times and Diametral Tensile Strengths (24 hr.)

of HA-FIlled Composites* Using Adsorbed POLYSAM-1

Formulation Monomer System HA^ P/L

Setting

Time

(min.)

DTS In MPa

(no of spec)

1 BIS-GMA (7)

TEGDMA (3)

BP • U

TYR • 0.2*

4.U

POLYSAM-1

5/6 21s 23.4 ± 1.9

(7)

2 Same as above 4.2S

POLYSAM -1 2/3 2 29.9 ± 2.6

(4)

3 Same as above 2 2t

POLYSAM-1

5/6 4 18.8 ± 2.5

(5)

4 Same as above Same as above - 2/3 2 25.5 ± 1.3

(7)

5 Same as above Same as above

5/6 .. 6 22.4 ± 6.0

.(7)

6 Same as above Same as above

2/3 6 21.2 ± 0.9

(4)

7 Same as above'

but BP • 0.5X

4 24

POLYSAM-1

2/3 3*s 23.2 i 2.2

Composites prepared using powder/liquid technique

t POLYSAM-1 adsorbed on HA from methylene chloride solution

52
Table 16

Setting Times and Diametral Tensile

Strengths of HA-Filled Composites*

Using Adsorbed Surface-Active Amines

Setting Diametral

Formulation Monomer System HA P/L Time Tensile

(min.) Strength in MPa

(no of spec)

I BIS-6MA (7)

TEGDMA (3)

BP - n

TYR = O.U

1.9%

DMAPAA

5/6 very fast 19.9 ± 4.9

(4)

II Same as above same as above2/3 1^5 24.5 ± 6.6

(4)

III Same as above

but BP = 1%

and TYR = O.U

2.4% . DMASA^

2/3 5 24.1 ± 5.4

(5)

IV Same as above

but BP = 2.U

and TYR = O.U

2.4%

OMASA

2/3 3 23.*2 ± 2.9

(4)

* Composites prepared by powder/liquid technique

^ OMASA = p-N,N-dimethylaminosal icyl ic acid

53
Table 17

Setting Times and Diametral Tensile Strengths of

* HA-FIlled Composites Using Dissolved SurfaceActive Amine Accelerators

Formulation Peroxide- Paste Acc. Paste P/L

Setting

Time

(min.

)

Diametral

Tensile

Strength in MPa

(no of spec)

A BIS-GMA (7)

TEGDMA (3)

BP » 1%

TYR = 0.1%

BIS-GMA (7)

TEGDMA (3)

POLYSAM-1 1%

5/6 •v* 1 min

very fast

20.9 ± 1 .5 (4)

B Same as above Same as above

POLYSAM-1 = 0.5%

2/3 -V 10 36.5 ± 4.4

(4)

C Same as above Same as above

DMAPAA = 0.18%

2/3 . 3 22.3 ± 5.2

(5)

D Same as above

BP = 0.25%

TYR = 0.05%

Same as above

DMAPAA = 0.18%

2/3 8 31.9 ± 3.7

(4)

E Same as above

BP = 1%

TYR = 0.1%

Same as above

4-DMASA = 0.36%

2/3 15 16.9 ± 2.7

' (5)

F Same as above

BP = 1%

TYR = 0.1%

4-ADMASA^

0.5%

2/3 17 34.0 ± 8.4

(5)

* Composites were prepared from equal parts of peroxide and accelerator paste

' Formulation F was stored for 72 hours in 100% humidity bath at 37°C

2 ADMASA = Acetyl derivative of OMASA

54
Table 18

Setting times and Diametral Tensile Strengths of

HA-Fllled Composites Using OMSX (Control)

Setting Diametral

Formulation Peroxide Paste Acc. Paste P/L Time

(min)

Tensil

e

Strength (MPa)

(no of spec)

1 BIS-GMA (7)

TEGDMA (3)

BP > U

TYR * 0.1

BIS-GMA (7) 5/6

TEGDMA (3)

DMSX"^* 0.23%

4 23.7 ± 1.7

(5)

2 Same as above

but BP = 0.5%

TYR « 0.05%

Same as 2/3

above but

DMSX = 0.23%

2 23.2 ± 2.6

(5)

Composites were prepared using equal parts of peroxide and accelerator paste

DMSX = N,N-d1methyl-sym-xylid1ne, a non-surface-active accelerator.

55
III. Wear Resistance and Mechanical Properties of Dental Materials

The principal objective is to develop, or make use of, appropriate

techniques to measure wear resistance and mechanical properties of dental

materials and to use the forthcoming data as performance indicators to

predict in vivo durability of these materials. The completion of the

development of these methods to a satisfactory state is particularly important because of the long times and efforts required for clinical evaluations. In order to accomplish this task, some fundamental research

on wear and related mechanical properties is being done to obtain a

better understanding of the relevant degradation processes. This information will be of use in determining the principal variables of influ- ,

ence from which appropriate modifications may be incorporated in

the wear apparatus to obtain meaningful results. In addition, the improved understanding of the ways in which specific mechanical properties

of the composite systems correlate with wear behavior is expected to be

instrumental in developing improved systems. The following paragraphs

describe the work performed during this fiscal year on wear and mechanical

properties of dental materials.

A. Wear Instrumentation and Data

The wear apparatus consists of three identical rotating platforms containing specimens of the material to be investigated. A stationary slider of about 2 mm diameter containing human enamel, or appropriate substitute, makes contact with each of the rotating specimens

producing a wear track of about 12 mm in diameter. The slider is loaded

to obtain a normal stress of 10 MPa on the wearing surface of the

56
specimen. The depth of each track is measured automatically at arbitrarily selected intervals (depending on the wear rate) at 10 equally

spaced positions along the track circumference using linear variable

differential transformers with suitable probes. Distilled water at 37°C

flows over the platforms to control temperature, lubricate, and wash

away debris. Figure 1 is a photograph of the entire wear apparatus.

The mechanical part of the apparatus is shown in Fig. 2 depicting the

three wear stations. A closeup of one of these is given by Fig. 3 revealing its specimen cup, wear pin and mounting collet, and track depth

measurement assembly.

The assembly of the data acquisition equipment has now

been completed and the apparatus made operational. As mentioned in the

previous annual report (10), the data acquisition equipment used previously was owned by the MSAF and was repossessed by them las;t fiscal

year. Although replacement imposed considerable inconvenience upon us

and delayed our research effort, we were able to incorporate substantial

improvements and a much higher degree of reliability in the 'new system.

The apparatus is now controlled by a computer, which actually simplifies

the measurement procedure and makes it more flexible. The computer also

has sufficient memory (36k bytes) to carry out the wear statistical

calculations using a program written in the programming language BASIC.

The measurement system has been modified to determine the wear

track length by counting cycles rather than from the product of the

assumed angular velocity and time interval. Counting cycles is a more

direct approach and also removes a small uncertainty in the track length

resulting from fluctuations in angular velocity.

57
'Vkm1«(49

General view of the wear apparatus

05Et«il

II

1 V jfBk^dt ^r riT

1

i S>. ‘'"«**4, — *' •=35* Mioo
5^

View

of

the

mechanical

portion

of

the

wear

apparatus.
Close-up view of single wear station showing specimen cup, wear

pin and mounting collet, and track depth measurement assembly.

The thermocouple junction shown in contact with the specimen is

immersed in the lubricating fluid (distilled water).

60 '
In our preliminary measurements the wear pins (approximating

2 mm diam.) were secured in their cup-like holders using a poly (methyl

methacrylate) denture base resin. After completing long wear runs we

occasionally observed a slight tilt of the pins with respect to their

holders which resulted from a small amount of creep in the resin. In

'Order to correct this problem the pin mounting assemblies were modified

to accommodate jewelers' lathe collets and draw bars as depicted in

Fig. 3. The pins are secured by applying appropriate manual torque to

the weight support table in a manner similar to securing a drill in a

jeweler's lathe. This technique is particularly suitable for the

accommodation of the durapatite wear pins, which we are evaluating as

an alternative to enamel.

61
The definition of the wear rate used in this work is

where V is the volume loss from wear, L is the total sliding distance,

and W is the normal force. The units are expressed in terms of compliance (reciprocal modulus) or, alternatively, volume/energy. Although

the wear rate as defined above may appear to be a material property,

it is not because wear always involves at least a wear couple, and the

results depend considerably on the properties of the opposing material

^he influence of different pin materials will be shown later in this

report.

In terms of the experimental variables, r and s, where r is

the number of revolutions and s is the track depth, Eq. (1) takes the

form

Q

= ^ ds

Traa dr ( 2 )

where a is the pin diameter (assumed to be equal to the track width)

and a is the normal stress. An advantage of using Eq. (2) over other

possible forms is that the track diameter does not appear (as it does

implicity in Eq. (1)), thus eliminating a small source of systematic

error. This redundancy is removed in a cancellation of V and L in

Eq. (1). Both of these quantities are linearly dependent on track diameter.

Two dental materials, an amalgam Velvalloy and composite

Adaptic . have been selected to evaluate the performance of this technique.

The particular materials were chosen because they represent two different

classes of dental materials and were used by Phillips and coworkers (27)

in their clinical tests for durability taken over three years.

62
In the previous annual report (10) we described our measurements

on these materials using wear pins made from human enamel. Although

enamel is taken to be a component of the oral wear process, there are

several serious objections to its use in our wear tests. First, enamel

is anisotropic and inhomogeneous. The effect on the wear on the opposing

material may depend upon the orientations of the crystal structure with

respect to the pin axis and the location in the tooth from which the pin

is made. Second, there is considerable variation in properties

between specimens from different subjects and even from the same subject. In evaluating restorative materials which are nearly alike,

we do not wish to test them against a reference material whose

properties may vary considerably. Third, there is no acceptable way of

knowing when the enamel layer is depleted during the tests and the underlying dentine becomes effective in the wear couple. Fourth, enamel is

chemically unstable. It is very difficult to store the enamel pins for

long periods of time and to be sure that significant degradation has not

occurred. Finally, fabrication of enamel pins is extremely difficult

and time consuming.

In view of the objections to human enamel in this application,

we have been evaluating other materials to determine a satisfactory

substitute. The forthcoming results are expected to be useful

in evaluating the influence of different opposing materials on the

wear process. To date, in addition to human enamel, we have made wear

^measurements on Velvalloy and Adaptic using durapatite, mild steel, and

stainless steel.

63
Table 19 includes a summary of what we take to be our best

values of G for the wear couples measured to date. The first three were

measured by the old acquisition system, where the values of G are taken

from averages of those at number 1 and 6 positions (180® apart) on each

rotor. The statistical quantities for these measurements have not yet

been completed because the old system was taken from us before we could

change the tape format to evaluate the data on the new system. We are

planning to do this at a later time in order to evaluate all of the

data consistently. The remaining couples were measured with the new

acquisition system where the values of G are averaged over 10 positions

on each of the three rotors.

Two different standard deviations for the last four couples

are included in the table. The first, Average Standard Deviation, is

the usual one where the standard deviations of the wear rates are calculated from the residuals about a linear regression on the track depth/

number of revolutions data at each position and then averaged over the

ten positions. The second. Position Standard Deviation’, is that obtained

from the 10 wear residuals (over position) about their average.

The difference between the two standard deviations is illustrated in Fig. 4, where the values of m = ds/dt (proportional to the

wear rate G) are plotted against the track position number (1 thru 10).

These values pertain to Velvalloy using stainless steel pins for which

the corresponding wear rate and deviations are included in Table 19.

The average value (over position) of m is shown on the right hand side.

The standard deviation of each slope is indicated by the vertical

64
TABLE 19

WEAR DATA SUMMARY

Couple

Wear Rate G

TPa-1

Average

Std. Dev. G

TPa-1

Position

Std. Dev. G

TPa-l

Pin Wear

Rate

nm/rev

PMMA-En 0.141 - - . -

Vel-En 0.0632 - - 1.4

Ad-En 0.0106 - - 4.2

Vel -Dur 0.0533 0.00049 0.0058 -

Ad-Dur 0.0084 0.00012 0.0032 -

Vel-SS 0.0488 0.00036 0.0055 0.16

Ad-SS 0.0615 0.00070 0.0036 4.7

PMMA = poly(methyl methacrylate)

Vel = Velvalloy

Ad = Adapt!

c

En = enamel

Dur = durapatite

SS = stainless steel, type 18-8, 303

65
f-

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66

Position

Number

Dependence

of

the

amount

of

wear

on

rotor

position

for

Vel

val 1

oy

and

illustration

of

the

two

types

of

standard

deviations

used

to
thickness of each solid rectangle, and the average of these deviations

is designated by the thickness of the rectangle on the right. The large

bracket on the right gives the standard deviation of the residuals

(over position) of m about their average.

The average standard deviation, which is the average (over

position) of the standard deviations of the slopes m, is a measure of

experimental imprecision and irregular wear behavior of each position.

The position standard deviation, which is the standard deviation of the

residuals of m about their average, is a measure of the variation of the

wear rate with position and is assumed to be related to inhomogeneity.

With the dental materials examined, the position standard deviation

is much larger than the average standard deviation as indicated by

Table 19.

Filling materials are expected to be inhomogeneous because of

the manner in which they are prepared. Since our specimens are large

(in comparison to the amount of material used in a restoration) they are

prepared in several stages. With the short condensation, or cure

times, it is essentially impossible to pack these specimens with no

voids, discontinuities, or density gradients. Accordingly,

it is necessary to obtain many measurements along the track circumference

(as is done with our apparatus) to obtain a reliable average value of

the wear rate.

Wear rate data on the pins are included in Table 19 for the

few cases where they are available. These were obtained from two point

micrometer readings before and after each run. They are not considered

to be particularly reliable, since each value in the table is based on

67
only two points, and the precision of the micrometer is about ± 10 ym.

However, they are an indication of how these dental materials degrade when

opposed by enamel (or other dental materials which may oppose them).

Adaptic wears enamel three times as fast as Velvalloy and wears stainless steel 30 times as fast as Velvalloy . The high wear rate with

Adaptic against stainless steel indicates that Adaptic may inflict

serious damage when contacting, for example, a gold or base metal restoration. On the other hand the same result would apply to enamel since

the wear of enamel and stainless steel are about the same against

Adaptic . We hope to obtain more extensive and reliable data on pin wear

and may wish to incorporate this facility in the apparatus. This additional information could be of considerable value to assess the relative

damage inflicted on proximate opposing Class 2 restorations of the same

or different materials.

Dr. W. Bailey of the University of Connecticut has also made

wear measurements (unpublished data) on the Adapti c-durapati te couple

using a pin and disc machine similar to ours. The principal differences

are that his machine has seven rotors instead of three and makes two

measurements along the circumference of each track instead of ten. Under

-4

similar conditions he .obtained an average value of m = 0.81 x 10 ym/rev

-4

which compares with our value of 1.32 x 10 on the same couple.

-4

Considering that our minimum value at position 2 is 0.51 x 10

and our position standard deviation is 0.42 x lO"^, we and Dr. Bailey

consider these results to be in reasonable agreement in that the

difference may be essentially attributed to sample variation and

68
inhomogeneity. His corresponding value of G for the Adaptic-durapatite

couple is 0.0051 TPa”^ which compares with our value of 0.0084 given in

Table 19.

The following is a more detailed account of our wear activity

during FY 1 979 .

Immediately after the new acquisition system was completed in

January* we made wear measurements on Velvalloy and Adaptic using

durapatite wear pins. Durapatite is a form of sintered hydroxyl

apatite, supplied by Sterling-WInthrop Institute. The name durapatite

is used to distinguish this form from other forms of hydroxyl apatite.

Machining wear pins from durapatite is very tedious and time consuming.

It is harder and much more brittle than enamel. ,

The results using these couples are shown in Fig. 5, where

the wear track depths are plotted against the number of revolutions.

The track depths shown are the average values taken over the ten positions along the track circumference. The separations of adjacent data

points along the abscissa correspond to approximately one hour. These

measurements were taken after the wear track profile was assumed to have

been established (in this case 20 hours at fulT load). As usual Velvalloy

reveals a more irregular response than Adaptic .

For both materials there are initial responses which are distinct

from the apparent steady state. Discontinuities in rates are commonly

observed in wear measurements and are often attributed to wearing through

a surface layer which has different properties from the bulk. This

behavior is typical of metals, which usually form oxide surfaces. It is

69
70
not clear why such a low initial rate, including an apparent negative

one, should exist for a composite unless the wear track profile was not

completely established.

The control values shown on Fig. 5 were obtained using a brass

disc (in lieu of a dental material specimen) which was not subjected to

wear. These values are obtained to determine the imprecision and possible drifts which could bias the results. Since these measurements were

taken, improvements which result in higher precision have been made to

further stabilize the measurements.

The values of m are obtained from linear regressions over the

regions included by the calculated straight lines. Using Fq. (2) for

which m = ds/dt, the wear rates and deviations given in Table 19 for

these couples are obtained.

At about 37,000 revolutions Adaptic reveals an anomaly which

apparently arose from a spurious electronic instability in the signal

conditioner for the linear variable differential transformers. This

kind of a result is not apparent in either Velvalloy or the control.

As indicated on Fig. 5, the points including and above the spurious

region are excluded from the linear regression analysis.

Values given in Table 19 for the durapatite couples may be

compared with our earlier values (10), 0.0632 and 0.0106, on the same

materials using enamel wear pins. The durapatite values are slightly

smaller, but considering the amount of variance mentioned earlier they

are considered to be in good agreement and consistent in that the ratios

of the former values to the latter approximate 0.82.

71
In the previous annual report (10) we stated that our values on

Velval loy and Adaptic are not in agreement with those obtained by different workers, Powell and Dickson (23), on a prototype apparatus for

which the values 0.013 and 0.022 TPa”\ respectively , were obtained.

Their values would reverse the order of v;ear resistance ranking. These

results are based on unpublished data and, apparently, on a very limited

amount which has not been repeated. The same ranking was obtained by

Powell, et al . (24), using a pin and disc device; however, the authors

noted with Velval loy that amalgam debris deposited on the enamel pins

during wear. This could result in some effective lubrication reducing

the amount of wear. We did not find such deposits in our work, which

could account for this discrepancy.

A motor control failure on our jeweler's lathe has temporarily

prevented us from making the durapatite pins necessary to continue measurements on dental materials using durapatite pins. Our shops are to

supply some additional durapatite pins. To obtain some insight on the

influence of a vastly different material on the wear process, we made

some wear pins from drill rod (mild steel), which is available in 2 mm

diameter. The mild steel turned out to be a bad choice

because of the rapid accumulation of rust from the continuous supply of

37°C distilled water. To circumvent this problem we obtained from our

shops some pins of type 18-8, 303 stainless steel which revealed no

significant amount of rust during the measurements.

The results on Velvalloy and Adaptic using stainless steel

pins are depicted in Fig. 6, where the average values of the track depth

are plotted against the number of revolutions. This is one of our longer

72
Velvalloy Adaptic Control ooo o o o ooooo o •om

cm

cm

cm

cm

cym

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omo

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lO • O 0

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ro oooo o ooo o o o o

om or- o

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Rev X

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Fig.

6.

Track

depth/revolution

response

for

Velvalloy

and

Adaptic

using

stainless

steel

wear

pins.
runs, taken over 48 hours. With stainless steel pins, the wear on Adaptic

is more irregular and increases by a factor of six over that obtained using

durapatite pins. On the other hand, wear on Velvalloy using stainless

steel is more regular than that with durapatite, but the rates are

essentially identical.

With these data, distinct initial responses are again observed.

Adaptic reveals accelerated wear nearly in the initial region; however,

this may be an irregularity similar to that observed near the end of the

run. With Velvalloy there appears to be a gradually increasing rate as

the steady state is approached. This’ kind of initial response seems to

be typical of Velvalloy .

In order to determine whether or not these initial responses

are artifacts resulting from track break-in only, measurements on these

couples through and slightly beyond the initial region were repeated

one week later. As shown in Fig. 7, wear on Adaptic is regular but the

gradually increasing wear on Velvalloy persists as the steady state is

approached. Composites sometimes display air inhibition during curing.

This could lead to a surface effect revealed initially by a faster rate such

as that seen in Fig. 6. This effect is not seen in Fig. 7, presumably

because the undercured surface would have been removed. With the amalgam our tentative conclusion is that a protective layer reforms (assumed

to be oxidation) during periods of mechanical inactivity. According to

our data, the reformed surface has higher wear resistance than the bulk

material

.

The values of m shown on Fig. 7 for the steady state regions

are used to obtain the corresponding values of G in Table 19. The initial

74
i(j. 7. Repeat of initial regions of Fig. 6 used to assess the meaning

of these regions.
-5

value 2.9 x 10 for Velvalloy is taken from a linear regression on the

first three points. The corresponding value of G is’O.0018 TPa"\ which

is far below that for the Adaptic-durapatite couple.

Although the above results are considered to be somewhat preliminary in our research effort, they do indicate certain factors

which may be relative to the oral wear process and indicative of the

direction of research to follow in the future.

The clinical results of Phillips and coworkers (25-27) on

these materials indicate very slight deterioration in anatomic form of

Velvalloy compared with Adaptic , whereas our steady state wear data

indicate that Adaptic is superior, which is in apparent contradiction.

On the other hand the oral wear process is only physically active for a

very small portion of time (say 1 to 4%) relative to the total, whereas

in our tests wear is continuous. Thus at least for the amalgams and

base metals, we should not rely entirely on the steady state wear,

but should pay more attention to the initial response. It is the wear

region immediately after physical dormancy which is probably the most

relevant to the oral process.

The other possibility is that we are conducting our tests on

an opposing material which may not be dominant in the oral wear process.

As pointed out by Dr. R. L. Bowen of the ADA Research Institute at NBS,

the enamel -restorative wear couple may not be appropriate since enamel

rarely comes in contact with Class 2 restorations during mastication.

Alternatively, different kinds of food of varying degrees of hardness,

much below that of enamel, may be more effective in the in vivo wear

process. Our results, using wear pins made from several different

76
materials, demonstrate considerable differences on the amount of wear,

and in one case the ranking on the wear is reversed. Although stainless

steel is softer than enamel, it is still much harder than the average

constituent of food. Accordingly, it appears to be appropriate that we

should investigate the influence of much softer materials used as wear

pins, such as a plastic or rubber, for which the physical properties,

in particular hardness, are much closer to that of food.

Another argument relevant to the apparent contradiction in the

wear ranking between clinical and laboratory tests mentioned above is

the following. Phillips and coworkers (25) also made -in vitro abrasion

tests with a mechanical tooth brushing machine. Using a slurry of calcium carbonate the abrasive wear on Velvalloy and Adaptic was about the

same. However, when pumice, a much harder material, was used the weight

loss of the Velvalloy exceeded that of Adaptic by a factor of ten. As

pointed out earlier, their observations on changes in anatomic form

indicate Velvalloy to be superior to Adaptic . A possible explanation

for the differences in these rankings is that both enamel and pumice

are hard materials which appear to wear Velvalloy more than Adaptic . On

the other hand softer materials as indicated from our work, and from

the clinical tests of Phillips and coworkers (assuming that food is

effectively the opposing material in mastication), produce more wear

on Adaptic . Our planned tests using the softer materials mentioned

above should reveal whether or not this supposition is valid.

77
B. Mechanical Properties Measurements

In the course of initiating work on mechanical properties,

we have adopted two approaches which we feel will provide information

which is essential to an understanding of the wear processes which lead

to the degradation of dental composites. Part of this work also focuses

on the development of new types of resin systems for use in dental composites and will be discussed under the section on Materials.

In the first instance, we have decided to study the relationship betv/een wear resistance and the fatigue behavior of the composite

systems. The approach is based upon the concept of damage accumulation*

which has been elucidated by work performed in the NBS Polymer Science

and Standards Division (28,29) and elsewhere (30). The basic premise in

this approach is that clinical wear processes occur over time and are

superimposed, on a composite system which is undergoing continual thermal

and mechanical cycling. Thus, the wear observed clinically occurs on a

restoration which undergoes progressive thermal and mechanical fatigue

damage with time.

Experimentally, we take samples of material and determine the

fatigue lifetime. By cycling the material to various fractions of that

lifetime (various states of accumulated damage) and then conducting

wear studies on the fatigued sample, we determine how the wear rate is

affected by the state of damage.

*In general, time and cyclic failure can be modeled as a process in

which the damage in a material accumulates as a function of time and/or

cycles. Although the material has not failed it has accumulated a certain amount of damage which has changed the residual durability of the

material. This could affect observed wear data which are generally

obtained on virgin or undamaged material.

78
A preliminary attempt to begin this sort of experiment was

unseccessful . We wanted to perform compressive fatigue tests on samples

which could be sectioned and placed directly into the wear apparatus.

As it turned out, the sample size required was a cylinder approximately

15 mm in diameter by 50 mm in length. This size is too large to test in a

reasonable time on our MTS testing machine, which has a 10 kM capacity.

Thus, the maximum stresses attainable on the machine are below the endurance limits reported by Draughn (31) for composite restorative systems. In order to avoid extensive modification of the wear apparatus

to accept smaller specimen sizes, we are currently planning on performing fatigue tests on thin sheets which have been cut into dumbbell shaped

specimens and then conducting the wear tests on the face of the specimens.

These specimens should be adequate to assess the effects of fatigue and

damage resistance on wear performance.

The second approach which we are taking to correlate the

mechanical properties of dental composites with wear behavior is the

study of the resistance of these materials to crack propagation. By

studying fracture toughness and fatigue crack propagation in dental composites and correlating them with the fatigue/wear study discussed above,

we plan to determine the contribution of these parameters to the wear of

dental composites.

The fracture toughness of a material is essentially its resistance to unstable propagation of a crack. Strictly speaking, the

fracture mechanics analyses are valid only for linearly elastic materials

and not for viscoelastic materials. However, even with this limitation.

79
fracture mechanics does serve as a useful empirical tool for the study

of the resistance of materials to crack propagation.

For our studies we have chosen to use the compact tension

specimen (32) depicted in Fig. 8. For this specimen geometry the stress

intensity factor K-j is determined from the following equation

(see ref. (32)):

Y P/F

BW (3)

where

P = load

a = crack length

B = specimen thickness

W = effective width of specimen

Y = geometry factor

2 Th6 geometry factor Y in units of in is given by ASTM

E399-74 (32)

2 3 4

Y = 29.6 - 185.5 |+ 655.7 (|) - 1017 (|) + 638.

9(^)

(4)

and the conversion of Kj from psi /TTT given by equation (3) to

MPA /nr is:

Kj(psi /fn)x 1 .099 X lO"^ = Kj(MPA /iT) (5)

Thus, if one has a known crack geometry, it is possible to

obtain one of two pieces of information. The fracture toughness,

(critical stress intensity factor), can be determined by increasing the

load on the specimen until unstable propagation occurs. Kjc IS determined from the maximum load and the initial crack length. The other

piece of information which can be obtained is the rate of stable crack

80
EE

LT)

CO

CD

II

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CM

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$-u

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—

COMPACT

TENSION

SPECIMEN
propagation under an oscillating load. It is conventional to obtain

the incremental growth per cycle, da/dN, and correlate it with the range

of the stress intensity factor aK. It is often found (33-35) that a

power law relationship describes the data quite well;

da/dN = Aak" (6)

In our experimental program we have built adapters for the MTS

universal testing machine to hold the compact tension specimen. In

addition, we have set up a traveling microscope to follow crack propa-

•gation. Figure 9 shows the general setup and Fig. 10 a closeup of a

specimen and the adapter. To gain experience with this type of specimen

and the problems with initiating a stable fatigue crack, we have conducted some preliminary work on commercial sheets of poly(methyl

methacrylate) machined into compact tension specimens. Figure 11 shows

the results obtained using a sinusoidal load at a frequency of 0.50 Hz.

The data fit an equation of the form of Eq. (6) and agree reasonably %

well with data from the literature.

Because varying the temperature of polymer and composite speci

mens can significantly influence mechanical response of such materials

and provide clues to the understanding of the mechanical properties per

se, we have ordered a temperature controlling unit for the MTS universal

testing equipment. .

C. Materials

Part of this program is involved with tests and measurements

on commercial composite restorative materials. However, in order to

gain the experience needed in the molding techniques required for

82
Fig. 9. General view of MTS servo-hydraulic

specimen and travelling microscope

equipment showing fracture

for measuring crack length.

83
1

F1.; Close-up view of grips used to mount compact tension specimen in

servo-hydraul ic machine

84
CRACK

GROWTH

RATE,

da/dN,

m/cycle

STRESS INTENSITY FACTOR, Kj, MPavfm

Fig. 11. Fatigue crack growth rate versus maximum stress intensity factor,

Kj for poly(methyl methacryl ate) . Tests were run as half-sines

in load control at 0.5 Hz.
specimen preparation, we have begun working with bone cement and

BIS-GMA systems. In addition, because of our observation that the

BIS-GMA system so commonly used in the composite restoi^atives is extremely brittle we have started working with a method for toughening

the system which we think has the potential for improving the performance

of composite restorative systems. (See also Section I, part A, 2 of

this report.)

The formulations for our successful attempts in molding are

included in Table 20. One of the more serious problems we encountered

is the inclusion or generation of air bubbles. In some cases these may

be alleviated by reducing the viscosity, or extending the cure time. In

formulations 1 and 2 for bone cement the cure time was extended over

several hours by deleting the amine accelerator. Suitable cylinders

were obtained for compression tests in both cases.

The remainder of the samples involve BIS-GMA. Formulation 3

is photo-cured using ultraviolet radiation. The difficulty with this

technique is that air bubbles are often generated after the curing is

initiated, apparently from air formerly in solution. Since the radiation was applied from the top, the top surface cured first, trapping

the bubbles after they appeared. A possible solution to this problem

is to use a glass plate and apply the radiation from the bottom.

Formulation 4 produced a good sample which will be tested

later. Although we would prefer a 70 to 30% ratio of polymer to diluent,

the viscosity is too high for air bubbles to diffuse out during the

curing process.

86
TABLE 20

Formulations for Mechanical Test Specimens

II Bone Cement (Heat Cure)

Foly(methy1 methacrylate)w1th IX* benzoyl peroxide

V ethyl methacrylate (no accelerator)

#2 Bone Cement (Heat Cure) .

poly(methyl methacrylate) with IX* benzoyl peroxide

ethyl methacrylate (no accelerator)

#3 Experimental Dental Resin (Photo Cure)

bis-gha’

TEGDMA*

B2X .

38X

SOX

sox

68X

29.2X

0.9X

1.9X

14 Experimental Dental Resin (Cold Cure)

BIS-6MA

H- TEGDMA

BHT

N 0.23X OMSX®

H 0.7X BP®

SOX

sox

0.09X

SOX

sox

IS Experimental Dental Resin (Cold Cure)

''bIS-GHA 4SX

TEGDHA 48X

Ml Hycar (Prepolymer Acrylate) , 7X

IbHT . 0.05X

M 0.2X DMSX SOX

M BP * SOX

' BIS-GHA B1s-[p-(3-methacryloxy-2-hydroxypropoxyphenyl )] Isopropyl idene

9

TEGOMA • tri ethyleneglycol dimethacrylate

^ BKT • 2,6-d1-t-butyl-p-cresol

® BEE benzoin ethyl ether (auto Initiator)

® DMSX N,N-d1methyl-sym-xyl Idine

® BP • benzoyl peroxide 87
Finally, a sample using a prepolymer according to

formulation 5 was molded in an attempt to obtain a specimen with higher

impact strength but with little or no decrease in tensile strength. A1

though no testing has been performed on this material, the initial obser

vation that the material machines in a less brittle fashion than does

formulation 4 and has a rougher fracture surface when broken are encouraging. Further work with other rubber contents, etc., will help to

determine how much potential this approach has for improving the

performance of the composite restorative materials.
IV. Dental Alloy, Ceramic and Implant Research

Chromium-cobalt-nickel based alloys developed for removable partial

denture prostheses have demonstrated properties and clinical serviceability surpassing those of the previously used gold alloys. The increased casting precision required in crown and bridge application has

been a prime factor in efforts to expand the use of such alloys to fixed

appliances. A previous annual report (10) has addressed this concern.

An additional concern for such application has been with the excessive

stresses developed interfacially in fusing porcelain to such substrates;

such stresses can result ultimately in failure of the prostheses.

In the area of dental ceramics the fabrication of dental porcelain

from feldspar is of increasing concern as a result of diminished availability of inexpensive high quality feldspar with low iron content and

high potash concentration. New methods of production and sources for

new materials are needed. In a related field it has become desirable

to develop nonradioactive fluorescing agents for dental porcelain to

avoid unnecessary radiation exposure to technicians or patients. An

additional concern with radiation effects involves the neutron activation

of dental materials following their exposure to sublethal doses of

neutron radiation.

Much of the interest in dental implants centers around theoretical

analysis to provide a clearer understanding of the effects of stress on

alveolar bone with anticipation that implants may be more effectively

designed than has been possible up to now. There is need for purely

theoretical studies to help point the direction for combined theoretical

clinical and purely clinical efforts.

89
A. Porcelain-Alloy Characterization

The characterization of ceramic materials available for bonding to dental casting alloys has progressed well. The measurements of

dimensional change resulting from thermal cycling has been completed for

nearly all ceramic specimens. The degree of expansion/contraction of

the ceramic materials relative to the same properties in the substrate

alloys during recommended firing schedules must be compatible since interfacial stress can be increased markedly through a "mismatch" of these

properties. The data collected thus far identify considerable variation in the expansion/contraction characteristics among different manufacturers' ceramic products. Further, these changes appear to be linked

directly to the thermal history of the specimen, i.e., the number of

firing cycles to which the material is exposed during fabrication processes. Generally, it would appear on early examination of the data that

the expansion coefficient (a) increases with repeated thermal cycling.

However, this phenomenon does not manifest in the same way for all materials.

Further observations suggest that qualitative and/or quantitative changes

in crystallinity of the ceramic materials may develop as a function of

thermal history. Preliminary results from x-ray diffraction analyses indicate

that, indeed, diffraction patterns are different for these specimens. Another

technique well suited for a study of crystall inity is Raman spectroscopy.

Analysis of discrete regions with a lateral spatial resolution of 6 ym

is possible with the Raman microprobe. This technique was applied to

porcelains subjected to a variety of defined firing schedules. Raman

spectra were obtained from 10-4000 cm’^ using the 514.5 nm line of an

argon-krypton laser at 60 mW. Sequential analyses were made at 20 ym
intervals on the surface of discs cut from 4 mm rods. The spectra

obtained from the various specimens differed significantly. Specimen

A exhibited a glassy silicate spectrum with a very broad band at 810 cm"

and shoulders at 1600 and 2350 cm’\ Specimen B presented a typically

crystalline spectrum with sharp bands at 615, 450 and 250 cm"\and weak

bands at 925, 875, and 335 cm"^ . Variations in spectra occurred from

microregion to microregion, but identical spectra repeated every 60 ym

This indicates that regions of heterogeneity repeat spatially. Work is

in progress to examine larger areas of numerous specimens to determine

the overall internal structure. An abstract has been prepared on this

work for submission for the 1980 meeting of the AADR. Further work is

underway to correlate these differences with changes in the refractory

nature of the materials. These observations may have major operational

significance inasmuch as some ceramic materials can, apparently, withstand more (repeated) exposure to high temperatures during laboratory

fabrication processes without showing appreciable changes in either

coefficient of expansion or phase structure.

The ability of ceramic powders to "wet" substructure alloy

surfaces during the fusion process is a critical element in promoting

chemical and mechanical interaction at the interface which is necessary

for accomplishing optimum bonding of the materials. Factors which may

influence this "wetting" phenomena include: 1) chemical composition of

the materials, 2) particle size (and configuration) of the ceramic

powder, 3) nature of oxides formed on the metal surface, 4) substrate

alloy surface preparation, and 5) techniques employed in applying the

(opaque layer) porcelain. Analyses of particle size distributions among

91
T1

the various manufacturers' products have been completed and the data is

being examined at this time. Some differences in this regard have been

found to exist among the products examined. Specific combinations of

alloys and bonding porcelains have been fused together and are presently

being analyzed via void (no contact) formation at the interface for

completeness of "wetting." Variations in alloy surface preparation,

preconditioning of the substrate .(oxidation) and (opaque) porcelain

application techniques are being evaluated to ascertain the effects of

these variables on the wetting process. Preliminary analysis of the data

collected in this phase of the investigation suggest that:

1) variations in porcelain alloy combinations influence the area of contact between the materials after fusion, and 2) different techniques used

for applying (opaque) porcelain powders to the alloy substrate may alter

the completeness of the "wetting" process. Knowledge gained from the

foregoing experiments should assist greatly in more appropriate selection or matching of compatible alloy/porcelain combinations as well as

fabrication techniques.

Correlation of the above scientific data with the recognized

clinical problem being studied is essential. The physical and mathematical design for a porcelain-dental casting alloy composite structure

to be used for evaluating the net effects of the factors studied on

residual interfacial and tensile stresses (primary contributors to

porcelain-metal failure) has been completed. Although the specimen has

been designed, further investigation into optimization of experimental

procedures and methods of measurement for the specimen is required.

92
A porcelain veneered split metal ring is the specimen of

choice. The gap or opening of the split provides a self-referencing

measurement of residual stress within porcelain and metal. Timoshenio

(36) developed the following expression relating the curvature, p, of

a bimaterial strip to its original curvature R, the coefficients of expansion a^, the thickness, t, of the strip and the temperature

range through which it is heated:

1 l\ _

( 1 )

v/here K is a function of layer thicknesses and their elastic moduli,

For a split metal ring, and for small changes in p, we can write

I

P R

“ R+4R R

’ w'’®''® P '

^ 1 - M > 1

R 1^2 R ^

( 2 )

(2a)

Since the change in gap is very nearly equal to the change in circumference for a narrow split, if Ad = change in gap, then

so that

and

Ad = 2ttAR and

" • '-4 i)

- 6.28 K(a.-a.)AT

Ad = r-^--

A more refined analysis yields

Ad = - 6.22 K

Equation 6 differs from Eq. 5 by 1%.

93

(3)

(4)

(5)
Experimentally, the change in gap of one porcelain veneered

metal ring was measured to be - 0.538 mm, the calculated value based on

Eq. 5 predicted a change of - 1.09 mm, in good agreement considering

the following factors: 1) Timoshenio's work considers only a onedimensional stress analysis and does not allow for stress gradients,

and 2) the analysis does not provide for multiple layers of material

with differing coefficients of expansion.

As a consequence of the preceding, it has been decided to

develop, with cooperation from the U. of 111., Chicago, the appropriate

theory for change in gap of a multi-layered split metal ring to allow

for stress analysis within and between each layer. It has also been

decided to pursue the use of finite element analysis to check the analytic approach. Abstracts have been prepared for submission of

1) theoretical, and 2) experimental results at the 1980 meeting of the

AADR, as well as submission of implications of measured differences of

porcelain expansion curves at the 1980 meeting of the lADR. Specimen

fabrication methods are undergoing development at this time.

B. Alloy Corrosion Study

The extent to which pitting corrosion might be predicted in a

clinical situation for a given alloy of a given composition and microstructure is of importance for at least two reasons: 1) the greater the

degree of pit formation, the more the tendency for plaque formation with

its accompanying undesired organic acid release, and 2) the pit formation

process must of itself release ions to the surrounding medium that may

be deleterious in one way or another. Availability at NBS of extensive

expertise and instrumentation in the area of corrosion and alloy

94
microstructure determination and the presence on the staff of a guest

worker from the University of Ddsseldorf for a year with a strong interest in this area allowed an immediate attack on this phase of our base

metal alloy effort.

Anodic polarization curves of as cast and fire-cycled samples

were generated and breakdown potentials determined. Essentially the

'

procedure of Meyer and Nally (37) was employed. The electrolyte used was

the artificial saliva solution of Fusayama but modified through an adjustment in pH to 3.2 by addition of hydrochloric acid. The solution

pH was lowered to reproduce plaque conditions and/or crevice environments which occur in the mouth. The pH adjustment assured that each

polarization curve was measured under constant conditions since we have

noted some variation in the pH of the Fusayama solution with time.

Three Ni-Cr alloys being used currently for porcelain veneering and representing typical materials of this kind have been investigated: NP2 (no Be but 8% Ga) Ultratec (Be), Wiron 77 (no Be). These

materials not only consist of different chemical compositions but their

microstructures which were observed by light micrographs and SEM exhibit

major differences. NP2 appears to be almost single phase with only a

few precipitates within the grain whereas Ultratec and Wiron 77 show a

eutectic constituent in the microstructure.

Samples were either cast or cast and fire-cycled to simulate

porcelain bake-on procedures. Judged by these test procedures NP2

appears to be the most corrosion resistant alloy (highest breakdown potential) followed in turn by Wiron 77 and Ultratec. This sequence

remained the same in the as-cast as well as the fire-cycled condition.

95
The values for pitting potential were lowered by fire-cycling, nitratec

was the only alloy to show an active peak under the chosen test

condi tions

SEM pictures of samples after anodic polarization runs revealed

differences in surface film and pit formation. On NP? samples (Figs. 12

and 13) pittings occurred within the grain. Pits were not as numerous

as the precipitates or irregularities mentioned earlier. This alloy was

not covered by a continuous oxide layer. Such a layer was only generated in a small area around the pits. Wiron 77 exhibited a thick continuous oxide film showing considerable evidence of cracks. This film

appeared to be thicker on the eutectic microstructure and could not be

penetrated by back scattered electrons (Figs. 14 and 15). Ultratec was

covered by a thin oxide film which could be penetrated by back-scattered

electrons (Figs. 16 and 17). The preferential attack of the eutectic

microstructure was quite evident since one phase of these areas had been

completely removed from the surface. According to the literature (38),

these parts possess a high percentage of the total beryllium content.

The following conclusions can be drawn to this point:

1) in the as-cast and fire-cycled forms NP2 evidence greater corrosion

resistance by the tests applied followed in turn by Wiron 77 and Ultratec,

2) fire-cycling and/or conditioning lowers the corrosion resistance of

each casting alloy examined, 3) Ultratec undergoes active corrosion

under the test conditions, has a low breakdown potential and appears to

be subject to crevice or other corrosion attack, and 4) the eutectic

component of Ultratec microstructure appears to be preferentially

attacked while that of Wiron 77 is not.

96
^<r V

•>’ ••

•s ,

•v'i.,.

» «

hMfci

jpm I, SOt-im

Fig. 12 NP2, SEM Micrograph Fig. 13 NP2, Light Micrograph

^ ^ * *•• ..‘V.- . I • Lw< --Ti V.. 'V‘ « • 0/

'

’ '; • * • • - • -. •) !C \ V / \

"w— * «!** '’‘A, -f

. '* H. > Sr/ ’A

|» , .L * ‘ ' •

* ' *j> -“V '

. /

* - *•

^ ».' j' '^' . •» ^

'•

• '. : ' s* ^ • *

t : '.: ^ . C-. • -*.*^ V >50Mm ? p' .^-«x50 pm ; f .V • .-> '*•

. . .'. '^.—^-Qpnii _ j.

Fig. 16 Ultratec, SEM Micrograph Fig. 17 Ultratec, SEM Micrograph
Further investigation is needed with other Be-containing

alloys to ascertain whether their in vitro corrosion resistance may

generally be lower. The sites on Wiron 77 as well as on Ultratec causing

the breakdown of the protective surface film in terms of pitting should

be chemically analyzed. Results obtained to date have been summarized

and reported in part at the 37th Annual ESMA meeting in August 1979.

C. Dental Porcelain: Processing Methods and Raw Materials

Porcelain Manufacturing Processes: The American Dental

Association Health Foundation Research Unit at the NBS has been involved

in research on production of dental porcelain utilizing a gel technique. In cooperation with this effort, a modification of this approach

has been suggested and is being explored. Experimental batches of

porcelain are being prepared for evaluation against commercially available porcelain regarding properties and esthetic consideration, new

avenues for introduction of fluorescence into porcelain without the use

of uranium will be explored. Potential fluorescing agents have been

identified and generally involve the use of rare earth elements; these

are also being considered by others. The methods of introducing the

fluorescing agents into porcelain will utilize some approaches which

have not been followed by others. Fluorescing agents have been procured.

D. Castability and Fabrication Research

A review of casting, finishing and production processes for

fabrication of metal crowns has been initiated but is currently only

in the early stages.

98
E. Irradiation Effects from Neutron Activated Dental Materials

As a result of the very limited information available in the

literature, calculations have been made to estimate the effects of

secondary radiation from gold crowns. Gold is the only element examined

to date and was chosen because of its high neutron activation cross

section, relatively short half life (2.7 days), its extensive use as a

dental restorative material, and the fact that information in reference 39

provided some concept of the maximum neutron energies which could be

expected to be encountered. The latter is important due to the energy

dependence of the activation cross section. Gold crowns were

approximated by 2.3 grams equivalent, 0.5 cm radius discs located

at buccal proximal positions.

Estimates on y radiation doses delivered to soft tissue were

made for single crowns and a mouth full of crowns using activation

cross sections for neutrons of the following energies: thermal neutrons,

1 ev neutrons and 4.91 ev neutrons; (4.9 ev corresponds to a neutron

resonance energy for gold (40)). An approximate expected epithermal

neutron distribution was calculated and used for determining the

fraction of neutrons which might exist near the resonance activation

energy and thereby estimate the additional secondary radiation. A

50 REM neutron exposure was assumed as this is clearly a sublethal exposure (41), higher exposures cause proportional ly higher activation and

1 97 secondary radiation. Activated crowns consist of gold and radioactive gold which has a r constant of 2.3 R-cm /hr-millicurie (42).

The results are that a single gold crown is likely to provide an 99
additional 2 REM y dose to local tissue and six or more adjacent crowns

provide an additional dose between 3 and 5 REM.

1 98

The second type of radiation emitted from gold is 6 radia-

> tion which consists of electrons having a distribution of energy. Exact

calculation of the dose delivered to tissue is complex due to the energy

distribution and the energy dependent absorption and shielding within

the crown itself. Beta rays have a relatively short range relative to

the diameter of the disc and therefore the estimate of dose to local

tissue may be well approximated by considering the radiation as emanating from a semi-infinite slab. Several approximate formulations are

available for estimating the dose (43,44). These consider B rays emanating from an infinite 'area of variable source strength per unit -area.

Some calculations were also made using modifications which rendered

approximate corrections for the attenuation of energy and absorption of

particles within an emitter of finite thickness. A range-energy re- Too •• LI X lation R = .407 E * (45) and an exponential absorption law, N = No e

(46) were combined to estimate the dose in basal cells at 10 ym and 56 ym

(36) from the surface of epidermal tissue. Stopping power in tissue

tabulations by Berger and Seltzer (47) was also employed. The results

show that local tissue can receive a B dose of 'v. lOO REM following a

50 REM neutron dose.

Combined effects of b plus y radiation can be at least

100 REM or higher, which is 200% of the initial dose. Further, if an

individual were to receive a neutron dose of 200 REM, which allows for

a high level of survival (41), local tissues would receive an additional

dose of 'V 400 REM, which certainly provides increased concern over long

100
term complications from the additional exposure, especially as the

effects of partial irradiation of an organ are not known and 8 damage

may be irreversible.

It has been calculated that temporary shielding of local

tissues from maximum energy g radiation from gold is possible by using

.40 cm of ordinary mouth guard material. The use of higher density

materials decreases the thickness of shielding required according to the

2

relationship tp = .40 g/cm , where t is the thickness and p the density.

However, for subgingival restorations, shielding is virtually impossible.

The results of these calculations have been prepared for submission as

an abstract for the 1980 meeting of the lAOR.

F. Implant Research

A feasibility study for the development of finite element

models of teeth and tooth shaped implants, with inclusion of directional,

non-uniform response to the periodontal ligament has been initiated.

Use of such properties has not been included in previous studies by

researchers in this field. Finite element analysis studies have progressed to the state that it is important to know how results may be

affected by this refinement. The feasibility study is being conducted

in cooperation with the University of Illinois, Chicago, where the

Materials Engineering Department has been involved with bioengineering

studies for several years using finite element techniques (48-53).

101
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(49) Tesk, J. A. and Widera, 0. E., "Consideration of Mechanical

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107
Publ i cations and Talks

1. New Amine Accelerators for Composite Resins, G. M. Brauer, D. Dulik,

J. Antonucci , and H. Argentar, Polymer Preprints, 1_9, 585-90 (1978).

2. Evaluation of Commercial and Newly-Synthesized Amine Accelerators

for Dental Composites, D. M. Dulik, J. Dent. Res.,^, 1308-16 (1978)

3. New Amine Accelerators for Composite Restorative Systems, G. M.

Brauer, D. Dulik, J. Antonucci, D. Termini, and H. Argentar,

(Accepted for publication in J. Dent. Res.)

4. New Initiator Systems for Dental Resins Based on Ascorbic Acid,

J. Antonucci, C. Grams, and D. Termini (Proofs returned to .3. Dent.

Res. 6/20/79). Abstract No. 704, Program and Abstracts, lADR (1978)

5. Synthesis of Polyfluorinated Polymethacrylates for Dental Applications, J. Antonucci, J. Griffith, R. J. Peckoo, and D. J. Termini,

Abstract No. 599, lADR Abstracts (1979).

6. An Amine Accelerator for Color-free Curing of Autopolymerizabl

e

Methyl -Methacrylate Denture Systems, H. Argentar, J. A. Tesk, and

E. E. Parry; in NBS review to be submitted to J. Dent. Res. for

publication.

7. Durability of the Bond Between Mineralized Tissues and Potential

Adhesives, G. M. Brauer, D. J. Termini, and J. A. Jackson; in

"Durability of Macromol ecular Materials," R. K. Eby (Ed.),

Advances in Chemistry , 95 , 411-420 (1979).

8. Durability of the Rond Between Bone and Various 2-Cyanoacrylates in

an Aqueous Environment, G. M. Brauer, J. Kumpula, D. j. Termini, and

K. Davidson, J. Biomed. Mater. Res., 13, 593-606 (1979).

108
9.

Bonding of Acrylic Resins to Dentin with 2-Cyanoacrylate Esters,

G. M. Brauer, J. A. Jackson, and D. J. Termini. (Accepted for

publication in J. Dent. Res.)

10. A Study of Surface Active Amines, R. J. Peckoo. MD/DC Regional

ACS Student Affiliate Meeting, March 10, 1979. (Miss Peckoo is

summer-time employee working with J. Antonucci.)

11. Adsorption of a Polyfunctional Surface-Active Amine Accelerator on

Hydroxyapatite, 178th ACS National Meeting, Washington, DC,

Sept. 1979; Colloid and Surface Chemistry, Abstract No. 33;

D. N. Misra, R. L. Bowen, J. M. Antonucci and W. F. Cuthrell.

(Submitted for publication in Journal of Colloid and Interface

Science.)

12. Physical and Chemical Properties of Wear, G. Dickson, J. Dent. Res.,

1535-1543 (1979).

13. Development of Fiber Reinforced Polymer Composites for Orthopedic

Applications, G. B. McKenna; 3M Company, Central Research Auditorium, 3M Center, St. Paul, MM (June 25, 1979).

14. Time Dependent Behavior in Polymers--Viscoelasticity and Failure,

G. B. McKenna; Dept, of Chemical Engineering & Materials Science,

U. of Minnesota, Minneapolis, MN (June 29, 1979).

15. Fit of Porcelain Fused to Metal Crown and Bridge Castings, G. Eden,

0. Franklin, J. Powell, Y. Ohta, and G. Dickson. (Publication now

promised by J. Dent. Res., Nov. 1979.)

16. Casting Roughness of Dental Alloys, R. Bauer*. (Invited paper at

Third International Precious Metal Conference, Chicago, IL, May 1979;

based on work done in NBS alloy program.)

*Research Associate, Nat. Assoc. Dental Labs., 1976-77.

109
17 . Solders and Fluxes, J. A. Tesk. (A chapter accepted for publication in the to be published Dentists Desk Reference by the American

Dental Association.)

18. Conceptualization and Development of New Dental Biomaterial s--A

Market! ng-Manufacturing-R&D Perspective, J. A. Tesk; Johns Hopkins

University, Baltimore, MD (February 6, 1979).

19. Fit of Non-precious Alloys for Ceramic Restorations and Surface

Roughness of Dental Castings, R. Bauer*; Mid American Dental Laboratory Conference, Chicago, IL, September 13-15, 1979; based on

work done in NBS alloy program.

20. SEM Studies of Dental Casting Alloy Corrosion Behavior, H. Weber**

and A. Fraker***; Thirty-Seventh Annual Electron Microscopy of

America Meeting, San Antonio, TX, August 13-18, 1979; (to be

published in proceedings).

21. The Casting of Non-precious Porcelain Fused-to-Metal Alloys,

R. P. Whitlock; Tri Services Dental Laboratory Symposium, San

Antonio, TX, September 24-26, 1979.

*Research Associate, Nat. Assoc. Dental Labs., 1976-77.

**Guest Worker from University of Dtlsseldorf, West Germany, July 1978-

June 1979.

***Member, Metal Science and Standards Division, NBS.

no
Abstracts Prepared for Presentation in FY1980

1. Theoretical Stress Analysis of Porcelain Veneered Split Metal Ring

for Evaluation of Compatibility of Porcelain-Alloy Systems,

G. E. 0. Widera, J. A. Tesk, and R. P. Whitlock; AADR Meeting,

Los Angeles, March 1980.

2. Biocompatible Amine Polymerization Accelerators: Para- Dimethyl -

aminobenzoic Acid and its Ethyl Ester, J. M. Antonucci , R. J. Peckoo,

C. Schruhl , and E. Toth; AADR Meeting, Los Angeles, March 1980.

3. A Porcelain Veneered Split Metal Ring for Evaluation of Compatibility

of Dental Porcelain-Alloy Systems, R. P. Whitlock, J. A. Tesk,

E. E. Parry, and G. E. 0. Widera; AADR Meeting, Los Angeles,

March 1980.

4. Intraoral Secondary Radiation Dose from Neutron Activated Dental

Materials, J. A. Tesk and R. P. Whitlock; lADR Meeting, Japan,

June 1980.

5. The Accelerative and Adhesive Bonding Capabilities of SurfaceActive Accelerators, J. M. Antonucci, D. N. Misra, and R. J. Peckoo; •

I ADR Meeting, Japan, June 1980.

6. Observations of Significant Differences in Thermal Expansion

Characteristics of Dental Porcelains, R. P. Whitlock, J. A. Tesk,

E. Parry, and G. Dickson; lADR Meeting, Japan, June 1980.

7. In Vitro Wear Measurements on Dental Materials, J. E. McKinney;

lADR Meeting, Japan, June 1980.

8. Derivatives of £-N,N-Dial kylaminophenylalkanoic Acid: New Accelerators for Dental Composites, G. M. Brauer and J. W. Stans bury;

lADR Meeting, Japan, June 1980.

Ill
9. Differences in the Crystallinity and Spatial Heterogeneity of

Dental Porcelains, F. S. Casciani, R. P, Whitlock, J. A. Tesk,

and E. Parry; AADR Meeting, Los Angeles, March, 1980.

10. Consideration of Directional Properties in the Stress Analysis of

Teeth and Dental Implants, G. E. 0. Widera, J. A. Tesk, and

R. P. Whitlock; International Biomaterials Society Meeting,

April 8-12, 1980.

112
NBS-114A (REV. 9-76)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. IR 79-1943 . -V

Z, $ Acce$$:iQn No.

4. TITLE AND SUBTITLE

Properties and Interactions of Oral Structures and

Restorative Materials

5. Publication Date

7. AUTHOR(S)

James M. Cassel

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. YOl-DE-40015-11

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fStreer, City, state, ziP)

National Institute of Dental Research

National Institutes of Health

Bethesda, MD 20205

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Sofbware Summary, is attached.

16. ABSTRACT (A 200-word or Jeaa factual aummary of moat aigfiificant information. If document includes a significant bibliography or

literature aurvey, mention it here.) Promising new polymerization shrinkage-reducing dental resins that have been synthesized include prepolymer fluorinated polymethacrylate monomer (ave. MW = 10,320);

diurea dimethacrylate; and methacryloxyethyl-l-adamantyl carboxyl ate, a tri -cyclic

saturated monofunctional monomer. Initial experiments to gain greater durability

using a rubber toughening approach with BIS-GMA indicates a less brittle but

tougher matrix can be achieved. Acetyl thiourea and ascorbic acid have been shown

to be promising non-amine type of accelerators for the BIS-GMA system while new

highly reactive amine types such as N,N-dimethyl-p-t-butylanil ine, gave cured denture

base materials with excellent color stability and may solve current problems in

achieving and matching shades in such materials. Research to develop adhesive

restorative materials indicates that urethane methacrylates with residual isocyanate

groups yield less adhesion than is achievable with 2-cyanoacrylate esters. Pin-ondisc wear data indicated sintered hydroxyapatite pins may be useful substitutes for

tooth enamel. A plan to evaluate fatigue as a contributing wear mechanism calls for

pre-fatiguing materials before determining the laboratory wear resistance.

Dimentional change data that results from thermal cycling of different porcelains

indicate considerable variation, some of which appears relatable to the crystalline

forms that are present. Be appears to lower corrosion resistance in crown and bridge

Ni-Cr alloys subjected to anodic polarization measurement following casting, or

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

casting and fire-cycling, to simulate porcelain bake on.

Key Words: rator; alloy; base metal; casting; composite; cyanoacrylate; dental

alloy; initiator; resin; wear.

18. AVAILABILITY 6^ Unlimited

I I For Official Distribution. Do Not Release to NTIS OOrder From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- QOrder From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

124

22. Price

$ 6.50

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