NATIONAL BUREAU OF STANDARDS REPORT

10 598

Progress Report

on ADHESIVE RESTORATIVE MATERIALS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards

1

was established by an act of Congress March 3, 1901. Today,

in addition to serving as the Nation’s central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

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standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

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Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

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uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

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1 Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Located at Boulder. Colorado 80302.

2 Located at 5285 Port Royal Road. Springfield, Virginia 22151.
NATIONAL BUREAU OF STANDARDS REPORT

ADHESIVE RESTORATIVE MATERIALS

R. L. Bowen*, DDS, J. A. Barton**, DOS and A. L. Mullineaux***

* Associate Director, American Dental Assn. Research Unit,

National Bureau of Standards, Washington, D. C. 20234.

** Guest Worker, Dental Research Section, National Bureau of

Standards, Washington, D. C. 20234, LT-C from the U. S. Air Force.

*** Research Associate, American Dental Assn. Research Unit,

National Bureau of Standards, Washington, D. C. 20234.

This research activity was supported in part by research grant

DE02494 to the American Dental Association from the National

Institute of Dental Research and is part of the dental

research program conducted by the National Bureau of Standards,

in cooperation with the American Dental Association; the

United States Army Medical Research and Development Command;

the Dental Sciences Division of the School of Aerospace

Medicine, USAF; the National Institute of Dental Research; and

the Veterans Administration.

NBS PROJECT

311.05-11-3110561 May 27, 1971

NBS REPORT

10 598

Progress Report

on

IMPORTANT NOTICE NATIONAL BUREAU OF ST/

for use within the Government. and review. For this reason, the whole or in part, is not authori Bureau of Standards, Washingto

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

! the Government agency for which

ipies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
'

A
COMPOSITE RESTORATIVE MATERIALS

R. L. BOWEN, DDS* J. A. BARTON, JR., DDSt

and A. L. MULLINEAUX*

DENTAL RESEARCH SECTION; NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C. 20234

This research activity was supported in part by research

grants to the American Dental Association from the National Institute of Dental Research and is part of the

dental research program conducted by the National Bureau

of Standards, in cooperation with the Council on Dental

Research of the American Dental Association; the Dental

Research Division of the United States Army Medical Research and Development Command; the Dental Sciences Division

of the School of Aerospace Medicine, USAF; the National

Institute of Dental Research; and the Veterans Administration.

* Associate Director, American Dental Association Research

Unit at the National Bureau of Standards.

t Guest Worker, Dental Research Section, National Bureau

of Standards, Lieutenant Colonel, U. S. Air Force.

+ Research Associate, American Dental Association Research

Unit at the National Bureau of Standards.
•V 4 ' - - M ‘ •'

*. ^ •

•i
1

COMPOSITE RESTORATIVE MATERIALS

I Hi^toricaJ^Irvt^

Although composite dental restorative materials are yet in

their infancy, the roots of their development can be traced

back through the scientific literature for many years. In

1843 a German chemist, Joseph Redtenbacher , reported a new

acid to which he gave the name acrylic acid. 1 In the last

part of the century, methacrylate esters and their polymers

were discovered. 1 Probably before 1940, Schnebel had diound

that tertiary amines (such as Michler ' s ketone) would

activate benzoyl peroxide to give self-hardening resins. 2

So far as can be determined, the first self-curing methyl

methacrylate resin formulation for filling teeth was introduced in the United States in 1948.

There soon appeared on the market a number of direct

filling resins, and many believed that these materials would

be the answer to the shortcomings of the decades-old silicate

cement. However, this idea was short-lived. 3 These resin

restorations discolored, 4 tended to shrink away from the

cavity walls, 5 allowed the exchange of oral fluids around

their margins, 6 and were associated with pulpal inflammation 7 ” 9
2

and a high incidence of recurrent decay. 10-11 Furthermore,

discrimination between decalcified dentin and the unreinforced

direct filling resin was difficult because they were both

radiolucent. For these and other reasons, a great many practi

tioners returned to the use of silicate cements. 3

The shortcomings as well as the merits of silicate

cements are well known. After nearly a hundred years

12 of

development and use, these esthetic materials with good

thermal and anticariogenic properties still have distinct

disadvantages. 3 Most obvious is the susceptibility to disintegration in a decay-like manner,

13-15 typically after

about four years in the mouth. 3 ’ 14 Further description is

given by Wilson elsewhere in this publication.

Because of the persistent need for a permanent, esthetic

direct filling material, there were continued efforts to

improve the methacrylate resins. An aluminum silicate filler

(about 15%) was part of one methyl methacrylate monomerpolymer system. 16 The use of fillers with the proper index

of refraction and a low thermal expansion, and the develop-
3

ment of adhesion in the resin were advocated. 17 Indeed, when

inert fillers were incorporated into poly (methyl methacrylate)

there was a reduction in the coefficient of thermal expansion

and in the water sorption in proportion to the concentration

of fillers. 18 However, there was limited adhesive characteris

tics in these and certain other resins investigated. 19

Epoxy resins, 2 ° developed early in the present century,

had an annual production rate of about 20 million

pounds by 1954. Certain formulations of these resins had

intriguing properties including adhesive characteristics and

the ability to harden at moderate temperatures with little

shrinkage. These led numerous investigators to study epoxy

resins as a binder for inorganic fillers. 19 ’ 21 Primarily

with indirect techniques, composite restorations were prepared

using powdered fused silica and porcelain, bonded together

with minor quantities of (heat-cured) epoxy resins. The

good esthetics and other favorable properties of these composites encouraged the further investigation of this approach

to dental restorative materials. 21
4

However, attempts to develop composite materials using

epoxy resins with various hardening agents for use in direct

techniques met with numerous difficulties. Although initially

adhesive to hard tooth tissues, the strengths of bonds between

epoxy resins and enamel or dentin after prolonged exposure

to water were disappointing. 1

' 2

’ 2

The goal-directed research in which the composite

dental restorative materials were developed was initiated

at the National Bureau of Standards in 1956. After unsuccessful attempts to use various epoxy resins and hardening

systems, a compromise between epoxy and methacrylate resins

was concieved late in 1957. 2 3,24 The reaction sites (oxirane

rings) of the epoxy molecule were replaced by methacrylate

groups. This gave a hybrid molecule that polymerized through

methacrylate groups.

25 It was suitable for use as a binder

for reinforcing fillers because it was nonvolatile, had a

relatively low polymerization shrinkage, and hardened rapidly

under oral conditions when suitably formulated with an

appropriate initiator system.
5

This dimethacrylate monomer (BIS-GMA) was synthesized

by the reaction of bisphenol A and glycidyl methacrylate.

The same product is also produced by the reaction of the

diglycidyl ether of bisphenol A and methacrylic acid. 25

Its structural formula27 is shown in Figure 1.

Although BIS-GMA has been referred to as an epoxy

resin, this is not the case; the original epoxy (oxirane)

groups disappear during the synthesis and are, in effect,

replaced by methacrylate groups. Unfortunately, this

dimethacrylate monomer has also been called a polyester.

The well-established term "polyester" should be restricted

to the polycondensation products essentially of dicarboxylic

acids with dihydroxy alcohols. 28

Since the coefficients of thermal expansion of organic

polymers are many times higher than that of the dental hard

tooth tissues, 6 ’ 29means for lowering this value were needed.

Fillers 17 incorporated into the resin could do this. 18 Experimental composites prepared in 1953 utilized particles

of fused silicon dioxide as a reinforcing filler because of
6

the extremely low coefficient of thermal expansion of

vitreous silica . 21 With the epoxy resin system, there was

relatively good adhesion between the resin and the surfaces

of these particles.

Later in the investigation, after the more-adhesive

epoxy resins had been replaced by the less-adhesive dimethacrylate resins (BIS-GMA )

26 it became apparent that

special means for attaining adhesive bonding between the

resin matrix and the filler particle surfaces were required.

At that time vinyl silane coupling agents appeared to be

the most suitable means of improving this bonding . 30 '*31

The more reactive y-methacryloxypropyl silane coupling

agents 32 ' 33 had not then become commercially available. A

vinyl silane coupling agent applied to reinforcing filler

gave composites that had about four times the tensile strength

of composites utilizing the same filler without its being

treated with the vinyl silane . 34 When the me thacryloxypropyl

silanes became available, their ability to promote adhesive

bonding between the resin and glass was compared with that

of vinyl silanes; the me thacryloxypropyl silanes gave signi-
7 -

ficantly stronger and hydrolytically more-stable bonding

than did the vinyl silanes. 35-37

Since the strength and durability of the composites

depended on the quality of the bonding between the organic

matrix and the reinforcing filler particles, it was of

utmost importance that the best coupling agents be used

in the optimal manner. The coupling agent most widely used

was y-me thacryloxypropyltrimethoxy silane, also known as

3- ( trimethoxysilyl) propyl methacrylate. This organofunctional

silane was hydrolyzed and condensed on the surfaces of the

filler particles. It made the particles water repellent,

reducing the water sorption of the composites; it also

formed chemical bonds at the surface of the particles,

connecting the organic polymer and the inorganic filler, 38

converting inert fillers to reinforcing fillers. 39

Although fused quartz or fused silica received a great

deal of attention in experimental work, 1

’ 2

' 2

' 2

' 2

* 3

many

other filler materials were also investigated. 1

’ 1

’ 1 9 * 2 1 ’ 31-34

These included the synthetic mineral 3-eucryptite and closely
8

related materials , 3

’ 40 ' 4

microcrystalline glasses , 42 aluminosilicate glasses, barium- titanium glass, crystalline quartz

and pyrogenic silica . 36 It has been widely held (although not

universally accepted) that condensation between SiOH groups

on the surfaces of these silica-containing fillers and SiOH

groups of the hydrolyzed silane accounted for the bonding

between the coupling agent of the filler and the filler

particle itself. Copolymerization of the methacrylate groups

of the silane with the methacrylate groups of the monomers

comprising the organic continuous phase has been thought to

complete the chemical linkage between the phases of the com-

. , 3 8 posites

The BIS-GMA monomer was too viscous to be readily mixed

with these fillers without first being thinned with some

suitable monomer having a lower viscosity. Among others,

methyl methacrylate and ethylene and te trae thylene glycol

dimethacrylate, have been used for this purpose . 2

* 2

As with antecedent methacrylates , 16 stabilizers such as

hydroquinone were first used to inhibit premature polymerization and give the necessary storage stability (shelf life)
9

to the BIS-GMA formulations

2

' 2

On polymerization, less discoloration occurred with the use of the monomethyl ether of

hydroquinone . Later, hydroquinone was replaced by BHT (butylated

hydroxytoluene ; 2 , 6-di- tert-butyl-4-methylphenol) , a stabilizer

that appeared to contribute no discoloring effects. 43

Other additives ’ that contributed to color stability

of the restorations wer-e ultraviolet- absorbing compounds. While

these may have contributed to the stability of the monomers

during storage, their primary purpose was to reduce the amount

of discoloration in the resin of the final composite material.

Typical UV absorbers were 2-hydroxy-4-methoxy-benzophenone 4

and 2- (2

7 -hydroxy- 5 Methylphenyl) benzotriazole

Since most of the experimental work on composite materials

utilized an amine-peroxide initiating system, another important

component has been the amine accelerator. The most commonly

used of these was N,N-dimethyl-p>-toluidine (DMPT) . This compound interacted with peroxides to produce free radicals which

brought about the polymerization of the methacrylate monomer

groups and thus the hardening of the resin binder of the composite. Investigations of factors that produce discoloration
10

in these materials led to the introduction of N, N-dimethylsym-m-xylidine (DMSX; also known as DMDA) , which not only

gave less discoloration but also was a more effective accelerator than DMPT when compared on an equimolar basis . 43

In a composite material developed in Great Britain , 44

a long-chain mercaptan, together with an aliphatic peroxide,

served as the hardening initiators.

Although other peroxides have been used , 44 benzoyl

peroxide has been the one most used in composite restorative

materials. It has been dispersed either in the reinforcing

filler with the aid of a solvent that is removed by evaporation, or is dissolved in one of the monomeric liquids of the

composite formulation.

Sulfinic acids or their derivatives were used in the

initiator systems for experimental composite materials . 14

Ill

Before proceeding with recent and current developments,

perhaps it would be well to define "composite.” For this

discussion, the term "composite restorative material" refers
11

to a man-made, three dimensional combination of at least

two chemically different materials with a distinct interface separating the components ; 39 properties are thus obtained

for the restoration of the form and function of defective

teeth which could not be achieved by any of the components

acting alone. Thus, the use of the term “composite" distinguishes the combination of inorganic aggregates bonded

together with organic polymers from unreinforced direct

filling resins and from silicate cements.

It is primarily the high proportion of reinforcing

filler in composite restorative materials that yields

properties different from those of direct filling resins.

Most of the physical properties of composite restorative

materials have been improved by the incorporation of a

maximum amount of reinforcing filler together with a minimum

amount of a binder of cross-linking polymerizable organic

resin.

The "first generation" of commercial dental composite

materials used BIS-GMA resin binder, described previously.
12

together with major portions of various particulate fillers

and minor portions of various additives. Data that have

been reported in the literature 45-60 and more recently obtained61 ’ 62 show that composites have lower polymerization

shrinkages and coefficients of thermal expansion, and

higher compressive strengths and stiffnesses relative to

unreinforced resins. Compared to silicate cements, the

composites have lower solubilities and higher tensile

strengths. More detailed data are given in Tables 1 and

2, including comparisons with dentin and enamel. 24 ’ 63 ' 68

The quantitative information presented in the tables is

incomplete since the methods and conditions of measurement are not described? however, these data will serve

to illustrate the ranges of values and order of magnitude

to be expected with these materials.

The smoothness of the finished surface is comparable

with that of silicate cements 36 ' 57 ’ 58 but inferior to

that of unreinforced resins52 and most of the other restorative materials. The abrasion resistance of composite materials

is superior to direct filling resins. ’
13

Similar to some other dental restorative materials,

problems encountered in packaging and storage include

volatility of some ingredients and premature gelation

of resin components . 36

IV Curr^rvtJR^seaxch

Current research has the goal of making improvements

on this "first generation" of composite materials now

available to the dental practitioner. For example, there

is a degree of pulpal irritation caused by some of these

materials , 69 ’ 70 and it has not yet been established which

of the various ingredients contribute most to the inflammatory reactions of the dental pulp tissues. Other

problems that have been encountered are incomplete color

stability62 and resistance to staining, difficulties encountered in finishing and polishing, and lack of x-ray

opacity.

The recent investigations at the National Bureau of

Standards have led to new dimethacrylate monomers, reinforcing fillers, accelerators, stabilizers, and adhesionpromoting coupling agents that will, hopefully, lead to an
14

improved "second generation" of composite materials. For

example, the BIS-GMA is not completely color stable and is

too viscous for use without being thinned with a monomer of

lower viscosity. Furthermore, it cannot be purified by

distillation or by crystallization since it is inherently

a mixture of non-volatile optical isomers. To overcome these

problems, three dimethacrylate monomers were synthesized that

can be prepared and purified separately by recrystallization. 27

These are the condensation products of 2 -hydroxyethyl methacrylate with the acid chlorides of phthalic, isophthalic

and terephthalic acids, respectively. Upon combining these

three crystalline products, a ternary eutectic liquid mixture is formed that has a viscosity low enough to be used

without the addition of any volatile ingredients. Preliminary tests show it to be non- toxic and non-irritating. 27

Numerous other monomers

71-75

are currently being

synthesized and evaluated for use in dental composite

materials

The research effort to improve polymerization accelerators 76 has led to the successful synthesis of aromatic amines
15

having unusually high molecular weights 77so as to minimize

their solubilities in tissue fluids and thus their freedom

to diffuse into the pulp or other tissues. The structure

of these amines is based partly on the fact that the color

stabilities of the composites have correlated better with

the nature of the ring substituents than with the kind of

nitrogen substituents in the amine accelerators . 76 ’ 77

Color stability is also profoundly affected by the

characteristics of the stabilizers that are used. Significant advances are being made in the synthesis and utilization of stabilizers that are non-toxic and that do not

cause discoloration of composites . 413 ’ 76 ’ 77

In the instance where there has been evidence of greater

surface staining of composite restorations relative to the

adjacent tooth enamel, it has not yet been determined to

what extent this accretion of discolored material is due

to inadequate hydrolytic stability of the adhesive bond

between individual particles and the resin matrix, the

chemical nature of the reinforcing filler materials, the

chemical nature and degree of polymerization of the resin
16

formulations, the roughness of the surface after the restoration is finished , 78

or some combination of these factors.

These factors are currently being studied.

Until recently, these has been little or no effort to

prepare special materials to serve as reinforcing fillers

for dental composite materials, rather, materials already

available were selected for this application. However, the

unique demands on this kind of composite material warrant the

development and use of the most suitable fillers as well as

other ingredients.

One experimental composite 79 contained a mixture of

two reinforcing fillers: spherical particles of fused silica

made up about two-thirds of the filler and smaller particles

of an x-ray opaque glass made up about one-third of the

filler. The smaller particles were composed of a special

barium fluoride-containing glass that was developed for this

purpose . 80 Radiopacity is desirable so that dentists can

distinguish between the filling material and any carious

dentin that underlies the filling.
17

The most recent investigations are involved with the

preparation and evaluation of a single glass filler material containing silica and only enough barium to give the

desired refractive index and x-ray opacity . 81 Preliminary

indications are that it will be suitable for converting

into spherical particles utilizing conventional techniques

while retaining the appropriate refractive index, and

will have a low coefficient of thermal expansion. Chemically, the glass is formulated to have, at least in theory,

hydrolytic stability in its bonding with the silane

coupling agents.

Since these composite materials are not intrinsically

adhesive to the prepared tooth surface, special coupling

agents have been developed which improve bonding between

such composite filler materials and dentin and enamel.

The chemical design of these special coupling agents was

based on empirical observations of the types of groups that

could displace water from the surfaces of powdered human

enamel and dentin, thereby making the surface organophilic

rather than hydrophilic . 82
18

One of the coupling agents, NPG-GMA (the reaction

product of N-phenylglycine and glycidyl methacrylate)

has two functions in the same molecule. One function is

that of a chelate group that can form multiple bonds with

calcium in the tooth surface. The other function is that

of a methacrylate group, which can copolymerize with the

hardening organic matrix of the composite material . 83 A

solution of the coupling agent placed on the prepared tooth

surface with the excess solution being wiped away leaves

an invisible film. Following this, the freshly mixed

(but not yet hardened) composite material is placed on

top of this treated surface and the subsequent polymerization apparently includes the methacrylate groups of the

coupling agent. The coupling agent has presumably also

formed ionic and coordinate bonds with the tooth surface.

Laboratory data on tensile adhesion tests clearly demonstrate that the use of the coupling agent significantly

improved the adhesion between composite materials and

tooth surfaces . 83 ’ 84 Clinical evaluations are under way . 79
19

There is a rapidly-expanding wealth of experience and

information regarding composite dental materials, not only

of the kind described here, but also of a wide diversity of

materials and techniques . 85 Composite restorative materials,

when they have been developed to their best potentialities,

should eventually provide the dental profession and the public

with greatly improved restorations for anterior teeth.
20

H

0

ii CHH

0 0

OCH2 -C-CH2 0CC=CH2

H CH.

BIS-GMA

O

H 2 C=CC0CH3

ch3

MMA

FIG 1. Structural formula of BIS-GMA. The asterisks

indicate asymmetric carbon atoms that give rise

to a number of steroisomers . Methyl methacrylate

(MMA) is shown for comparison. 2
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