NBSIR Ib-lbd

Properties and Interactions of Oral

Structures and Restorative Materials

James M. Cassel

Dental and Medical Materials Section

Polymers Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1975

Annual Report for Period:

July 1 , 1974 - June 30. 1975

Identification of coiranercial materials or equipment in

this report does not imply recommendation or endorsement

by the National Bureau of Standards

Prepared for National Institute of Dental Research

Bethesda, Maryland 20014

NBSIR 15-166

PROPERTIES AMD INTERACTIONS OF ORAL

STRUCTURES AND RESTORATIVE MATERIALS

James M. Cassel

Dental and Medical Materials Section

Polymers Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1975

Annual Report for Period:

July 1, 1974 - June 30, 1975

Identification of commercial materials or equipment in

this report does not imply recommendation or endorsement

by the National Bureau of Standards

Prepared for National Institute of Dental Research

Bethesda, Maryland 20014

U.S. DEPARTMENT OF COMMERCE. Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

ABSTRACT i

SUMMARY ii

INTRODUCTION ' 1

TECHNICAL REPORT 2

I. Adhesion of Restorative and Caries-Preventive

Materials to Tooth Surfaces 2

A. Surface Characterization of Dental Materials

by Water Adsorption 2

B. Novel Approach to Development and Assessment of

Adhesion in Dental Materials 10

C. Synthesis for Improved Adhesion and Stability

of Composite Restorative and Sealant Materials 20

D. Polymeric Grafting, A Technique for Bonding to

Mineralized Collagen Surfaces 25

II. Physical, Mechanical and Physicochemical Properties

of Hard and Soft Tissues and of Dental Materials ^ 27

A. Dimensional Changes in Tooth Structure 27

B. Wear Resistance of Dental Restorative Materials 29

C. High Modulus Reinforcement, Techniques for the

Improvement of Dental Cements and Composites 41

REFERENCES 43
V

FIGURES

Page

.Figure IA-1: Apparatus for gravimetric measurement of

water adsorption 14

Figure IB-1: Influence of storage time of peroxide-containing

and amine-containing monomer preparations on the

tensile strengths of freshly polymerized Bis-GMA ... 15

Figure IB-2 : Verification of 2/3 power weight fraction

relationship with tensile strength 16

Figure IB-3: Effect on tensile strength of a one-month exposure

to an aqueous environment at 23°C for composites

containing different surface concentrations of

silane-coated hydroxvapat i te 17

Figure IB-4: Tensile strength deterioration of composites

with different surface concentrations of oleic

acid-coated hydroxvapat ite filler after one- and

two-month's exposure to water 18

Figure IB-5: Correlation of Instron tensile strength data on composites with that determined on experimental

centrifugal apparatus . 19

Figure IIB-1: Instrumentation for measuring depth of v;ear track. . . 36

Figure IIB-2: Data acquisition system for wear resistance

measurement 37

Figure IIB-3: Average wear deptli vs. time for an amalgam and a composite dental restorative material 38

Figure IIB-A: Wear depth vs. track measurement position for

amalgam restorative material 39

Figure IIB-5: Wear depth vs. track measurement position for

composite restorative material 40
ABSTRACT

A diversity of experimental approaches is being investigated to

develop techniques which can be applied in formulation of adhesive

restorative and caries-preventive dental materials. A newly developed

testing procedure, still in the evaluation stage, has demonstrated ex- pected ranking of adhesive bond strengths within a group of selected

coating agents, both coupling and release types. Deterioration on exposure to water of cohesive bond strength in the Bis-GMA matrix has

been indicated without concurrent deterioration of the coupling agent.

A monomer system with potential as a composite or pit and fissure

sealant resin and giving a polymerizable liquid of desired viscosity

at room temperature was derived from three low-melting, crystalline

isomers of ether-ester linked aromatic dimethacry lates . A series of monomers of varying chemical structure was evaluated for possible application in photopolymerization experiments designed to investigate

modification of hard tissue by grafting. Modifications to extend the

running time of a dental restorative wear-generating experiment threefold and which allow the computerized recording of ten, instead of two,

depth of wear measurements around the circular wear track have significantly reduced the standard error of measurement. Surface coating

techniques being developed to strengthen interfacial bonding in dental

cements and other dental materials show more promise with titanate than

with carbon reinforcing fibers.

i
SUMMARY

A diversity of experimental approaches has been pursued to develop

techniques which can be applied in the formulation of truly adhesive re- storative and caries-preventive materials, lack of which remains a major

handicap to improved dental health. The approaches include surface

characterization, exploration of novel techniques for comparing adhesive

strength, synthesis of monomers having selected functional groups, and

the modification of tooth and bone surfaces by grafting. Measurements of

the wear resistance of restorative materials and dimensional change in

tooth specimens as a function of temperature have been made.

Water vapor adsorption data have been determined for a number of

dental materials and a procedure developed by which the dry weight of a moisture-adsorbing specimen may be calculated from the data alone. Application of both the Brunauer, Emmett and Teller (BET), as well as the Harkins,

Jura (H-J) adsorption equations has been demonstrated. Development of an indirect method of measuring adhesive strength has continued. The method,

hydroxyapatite

in which increasing content of variously coated"/ f iller particles is incorporated into composite test specimens, is being examined for its utility

in the comparison of the relative merits of selected candidate coupling

agents for use between dental hard tissue and restorative materials. The

testing procedure demonstrated aging effects in Bis-GMA, but not in a series

of coating agents, on storage in water for one month. With this novel testing procedure, the bond strengths between selected coating agents and

synthetic hydroxyapatite were as expected.

In research directed to improving the resin component of today's

dental composite restorative system, a monomer system consisting of the

three isomeric 2-methacry loxy ethoxybenzoates , each in high purity, has

been investigated. From melting point data and certain theoretical re- lationships, the composition of three binary eutectics was determined

preliminary to formation of a ternary eutectic. The system polymerizes

to a hard clear resin either with peroxide-amine initiation, or by ultraviolet photo-polymerization. It is a reasonable expectation that resins

derived from such ether-ester monomer systems will exhibit enhanced hydrolytic, chemical and UV oxidative stability and will have potential

for both restorative and preventive dental application. Using a condensation scheme, several functional monomers (monomers with pendant groups

ii
capable of reaction with dentin or enamel) liave been synthesized as crystalline solids. These monomers may have application as adhesivepromoting agents in dental application.

The poly f luoroaromat ic methacry late , 2-methacryloxyethoxypentaf luorobenzene , was polymerized by chemical initiator (benzoyl peroxide)

and is being investigated for its fluoride-releasing capability by fluoride electrode analysis. This monomer, used as a comonomer with Bis-GMA,

may offer potential as a fluoride-releasing pit and fissure sealant system.

A series of monomers of different chemical structure was evaluated

for possible application in photo-initiated polymerization experiments

designed to investigate modification of hard tissue by grafting. In these

experiments, the methyl ether of benzoin was used as the photo initiator.

The monoester derivative of 2-hydroxyethyl methacrylate and phthalic acid,

as well as a mixture of the monofunctional monomers methacrylic acid and

2-dimethylaminoethyl methacrylate, formed polymer films under UV exposure

in 30-40 seconds. A wide range of monofunctional monomers, however, could

not be polymerized under these conditions. Methyl methacrylate was co- polymerized by photo initiation with several methacrylate monomers having

aldehydic pendant groups for increased adhesion to tooth or bone surfaces.

The laboratory evaluation of these materials as adhesive-promoting co- monomers is in progress.

Coefficient of expansion has been determined on human tooth enamel,

dentin and restorative materials. No correlation between direction of

the enamel rods and the coefficient of expansion has been observed.

The initial testing of a very limited number of specimens on a wearsimulating apparatus indicated a marked difference between the wear re- sistance of an amalgam alloy and a composite restorative material analogous to that found by prior clinical service testing. The standard error

of the measurement was high and steps have been taken to reduce the

variance. By extending the running time of the experiment threefold and

allowing for the recording of an increase of fivefold in the number of

depth of wear track measurements that could be made around the circular

wear track, the scatter of the data has been significantly reduced.

Average volume losses per minute per square millimeter of wear couple

-3 -3 3 2 contact under water at 37°C were 0.23 x 10 and 0.13 x 10 mm /min(mm ) for composite and amalgam.

iii
To strengthen bonding between the filler particles and the resin

matrix of polycarboxylate-type dental cements, several techniques are under evaluation as a means of most efficiently coating both potassium

titanate and graphite fibers with a poly (acrylic acid) surface. The

techniques involve (1) vapor phase deposition of monomer, and subsequent

in situ polymerization, (2) deposition of monomer from a dilute solution

of the monomer, followed by in situ polymerization, and (3) deposition

of polymer directly from a dilute solution of the poljrmer. A comparison

of the techniques has indicated that the titanate fibers are more sus- ceptible than the graphite to the procedures that have been investigated

to date.

iv
INTRODUCTION

The National Bureau of Standards is primarily a measurement organization. Paths from measurement to improved dental service lead from measurement and characterization to development of new techniques for using materials, to laboratory prototypes of improved materials and devices, to

clinical evaluation and, finally, commercial production by industry.

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

personnel and funding, involving several agencies of government and the

American Dental Association. The bulk of other agency support for the total

program derives from the National Institute of Dental Research.

The research reported in the following pages is only that portion of

the NBS dental program which is supported by interagency agreement.

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 the components of teeth and bone, as well as m.aterials used

or potentially of use in restorative and preventive dental practices.

An outline of the research as approved in the interagency agreement is

given below, along with participating personnel.

I. Adhesion of Restorative and Caries-Preventive Materials

to Tooth Surfaces

A. Surface Characterization of Dental Materials by Water

Adsorption — Dr. W. V. Loebenstein

B. Novel Approach to -Development and Assessment of Adhesion in

Dental Materials — Dr. W. V. Loebenstein and Mr. J. W. Kumpula"

C. Synthesis for Improved Adhesion and Stability of Composite Restorative and Sealant Materials — Dr. J. M. Antonucci

D. Polymeric Grafting, A Technique for Bonding to Mineralized

Collagen Surfaces — Dr. C. M. Brauer, Dr. J. M. Antonucci, and

Mr. D. J. Termini

II. Physical, Mechanical and Physicochemical Properties of Hard

and Soft Tissues and of Dental Materials

A. Dimensional Changes in Tooth Structure — Mr. G. Dickson and

Dr. J. M. Cassel

B. Wear Resistance of Dental Restorative Materials — Dr. J. M. Powell

and Mr. G. Dickson

C. High Modulus Reinforcement, Techniques for the Improvement of

Dental Cements and Composites — Dr. J. M. Antonucci, Dr. G. M.

Brauer, and Mr. D. J. Termini

1
TECHNICAL REPORT

I. Adhesion of Restorative and Caries-Preventive Materials to

Tooth Surfaces

A. Surface Characterization of Dental Materials by

Water Adsorption

There are many facets associated with the characterization of

dental materials by the application of adsorption methods. Probably the

most widely known characterization is the specific surface of the adsorbent, which is logical since the adsorption process is a surface-related

phenomenon. In the case of water vapor adsorption, it is not immediately

apparent that the moisture content of an insoluble material (which is

certainly a bulk phenomenon) might also be subject to interpretation as a surface property. This is a consequence of the multi-layer aspect of

adsorption

.

Two well-known adsorption theories, the Brunauer, Emmett and

Teller^"''^ (BET) and the Harkins and Jura^^^ (H-J), have been widely used over the past three decades for determination of surface area. The BET

equation appears to be somewhat more sensitive, but has the disadvantage

of being confined to a relatively small relative pressure (humidity, in

the case of water vapor) range; namely, from 0.1 to 0.3. The H-J isotherm equation, while perhaps less sensitive, is applicable over the entire

pressure axis. For these reasons, the two theories tend to complement one another. Both isotherm equations have an important property in common. Each is capable of being expressed mathematically as a straight line from

which the numerical values of the slope and/or intercept are used to

calculate surface area. Mathematically, two points are required to establish a straight

line. Thus, only two adsorption measurements would be needed, providing

(a) the experimental points were error-free, and (b) a straight line did,

in fact, exist. Since condition (a) is never true, several adsorption

points are generally measured. This increases the reliability of the

results by affording an estimate of goodness of fit and permitting a leastsquares treatment to be made of the data. Since there are now more than

sufficient points for the determination of the two required parameters,

the data may be called upon to furnish an additional piece of information -

2
namely, the dry weight of the adsorbent. Since the amount of water adsorbed per g at every humidity point above zero is an implicit function

of the dry weight, this latter quantity mav be looked Lipon as an additional

unknown constant to be determined simultaneously by an extension of the

least squares process. It is significant that no additional experimental

measurements are required for this than would normally be obtained in determining only the surface area. It is also significant that the method is

capable of predicting the correct dry weipjit during the desorption (drying)

portion of the adsorption-desorption cycle. This is especially significant

because an incorrect surface area value would result from desorption data

when adsorption-desorption hysteresis was in evidence.

Dental materials, for the most part, were used in collecting the

data since their characterization is a primary objective of this work, but

the method is perfectly general and not restripted to this specialized

field. Apparatus used in collecting data is illustrated in Figure IA-1

(3 and has been discussed in detail in previous reports and in a publication.

It may be seen in some of the results shown here (Table 5) that

the dry weight may be calculated by using humidified weights from saturation down to about 50% r.h. This would be especially useful in biological

applications where viability of the organism (or tissue) might be destroyed

by drying completely.

The amount of water adsorbed per gram at some particular humidity

is not an experimental measurement. It is customarily calculated from two

measurements: (a) the weight of the sample equilibrated at that humidity,

and (b) the weight of the sample equilibrated at zero-humidity - devoid of

any adsorbed water - namely, the dry weight. An experimental error in (a) causes an error only in that particular adsorption point. It can frequently

be identified and discarded if it departs appreciably from a smooth curve constructed through the remaini-ng points of the isotherm. An experimental

error in (b) , on the other hand, adversely affects all of the adsorption

points and sometimes to such an extent as to make the entire experiment

invalid. The present method utilizes all of the (a)-type measurements to

calculate the least-square values for (b)

.

Table 1 lists the surface areas and compares the dry weights calculated by the method of this investigation with experimentally measured

values for a number of pulverized dental cements. Also included are the re- sults for barium glass spheres used in the formulation of dental composites.

The latter material was measured before and after subjecting the spheres to

a 24-hour Soxhlet water extraction. (The resultant increase in surface area

3
Table 1. ADSORPTION

Low Humidity Range (0.082 -4 0.334)

Comparison of BET with H-J for the same data

Surface

Area

Expt. // Material (m^/g)

A444161 Polycarboxylate cement (4 points)

Calculated BET 60.4

H-J 61.3

Experimental ________

A444143 Zinc Phosphate Cement (6 points)

Calculated BET 41.8

" H-J 32.4

Experimental ________

A444164 Zinc Oxide-Eugenol Cement (4 points)

Calculated BET (no convergence) -

H-J 5.2

Experimental ________

A444155 Barium Glass —as received (6 points)

Calculated BET 3.22

H-J 3.15

Experimental _________

A444156 Barium Glass —water extracted (4 points)

Calculated BET 8.7

" H-J 4.6

Experimental ________

A444175 Barium Glass —water extracted (6 points)

Calculated H-J 9.5

Experimental ________

Dry Weight

8

1.255

1.241

1.249

1.5646

1.5703

1.5779

3.1274

3.1297

1.2886

1.2889

1.2895

0.9636

0.9648

0.9649

0.9646

0.9657

4
is caused by irregularities in the surface formed by the dissolving of

(4) surface ions ,

.

)

Table 2 llliiistrates that valid estimates of surface area as well as dry weights are obtainable from desorption (drying) measurements for certain

dental materials. This situation arises when the adsorption-desorption

hysteresis loop is virtually absent (at least, in the range of humidities

within which the experimental measurements are made).

The results of water vapor adsorbed on human dentin are shown in

i Table 3. Reasonably good agreement in surface area was obtained using

both the BET and the H-J isotherm equations. Also, 'both extremities of

the humidity scale were involved in the comparison. The numerical values

2 obtained ranging from 143 to 150 m /g are bracketed by values reported in

2 (5) 2 (3) recent publications of 128 m /g and 152 m /g, respectively.

2 WJien the surface area is low, i.e. less than, say, 1 m /g, the experimental error associated with the low humidity range becomes relatively

high - so high, in fact, that meaningful results may be unattainable. This

difficulty can be eliminated in many instances by confining the calculations

to the high humidity range. An example of this is illustrated in Table 4

where the material under investigation was a pulverized sample of dental

amalgam.

In the final example are included the dry weights predicted, as compared

with experimentally measured values for representative dental materials as

a result of desorption. Again, the BET equation is not applicable in this

range, and the surface areas are shown in parentheses because of the

hysteresis effect.

The information contained in the results presented here will be included

(6) in a manuscript which has been accepted for publication.

5
Table 2. ADSORPTION VS. DESORPTION

Surface Dry

Expt. // Material Area Weight

(m^/g) (g)

Poly(inethyl methacrylate

)

(Low humidity

range 0.082 > 0.334)

A444065 Adsorption (5 points)

Calculated : BET _________ 30.4 3.5029

H-J _________ 24.9 3.4-878

A444064 Desorption (6 points)

Calculated : BET _________ (31.6)* 3.5022

H-J _________ (29.4) 3.4847

Experimental ____________ 3.5039

"BIO GEL" (commercial hydroxyapatite

)

A444136 Adsorption (Intermediate humidity

range 0.312 ^ 0.804)

Calculated (4 points)

H-J _________ 43.3 0.3788

A444138 Desorption (High humidity

range 0.926 > 0.542)

Calculated (5 points)

H-J _________ (47.2) 0.3792

Experimental ____________ 0.3802

* Surface area calculations obtained from desorption data are enclosed

in parentheses, since they are often not valid.

** These humidities are above the range where BET is valid.

6
Table 3. ADSORPTION ON DENTIN

Low Humidity vs. High Humidity

Surface

Area

Expt. No. (m^/g)

A503140 Calculated (6 points 0.082 0.33A)

BET _________

H-J (poor convergence)

Experimental - -- -- -- -- --

•A503078 Calculated (5 points 0.082 —^ 0.312)

i BET (no convergence)

H-J _________ 3

A503079 Calculated (4 points 0.334 1.000)

H-J _________ 150

Experimental ___________

Dry

Weight

(g)

2.0202

2.0671

0.2847

0.2848

0.2898

7
Table 4. ADSORPTION - DESORPTION

High humidity range (0.435 - 0.971)

Surface

Expt. No. Material Area Dry Weight

2/ g m /g ^

Pulverized Dental Amalgam

A444124 Adsorption (7 points)

Calculated H-J ----- o.44 5.6135

A444126 Desorption (4 points)

Calculated H-J ----- (0.74) 5.6132

Experimental ________ 5.6145

8
Table 5. HIGH HUMIDITY DRYING (DESORPTION)

CURVES FOR PREDICTING V^ATER CONTENT

Surface

Exp t . # Material Area Dry Weight

(m^/g)

A444153 Barium Glass - as received

(Humidity range 0.926 > 0.435)

Calculated (6 points) K-.J - - - - - (2.5) 1.28897

Experimental 1.28947

A444158 liarium Glass - vater extracted

(Humidity range 0.971 1 0.435)

Calculated (7 points) H-J _ _ _ _ _ (3.4) 0.9663

Experimental _________ 0.9657

A503091 Commercial Synthetic Hydrox\"apatite

(Humidity range 1.000 > 0.~5 7;) Calculated (5 points) H-J _____ (117) 0.2612

Experimental _________ 0.2733

A503136 NPG-GMA Coated Hydroxyapatite

(Humidity range 1.000 > 0.542)

Calculated (7 points) H-.T _____ (25) 0.4788

Experimental _________ 0.4805

9
B. Novel Approach to Development and Assessment

of Adhesion in Dental Materials

An indirect tensile strength testing ])rocedure is under development for assessing and comparing the relative merits of candidate coupling

agents for use between dental hard tissue and either resin or resincontaining composite restorations. A detailed description of the testing

method was given in the Annual Report for July 1, 1973 - June 30, 1974.

Briefly, the indirect method consists of dispersing measured amounts of

a finely divided synthetic commercial hydroxyapat ite (HA) uniformly

throughout the monomer prior to polymerization. Before this is done,

however, the HA powder is precoated with the candidate adhesive agent to

be tested. Tensile strength measurements are then performed on the

polymerized specimens. The advantages of this indirect method may be:

(1) better interface reproducibility, (2) ability to make replicates,

(3) randomization of forces, (4) speed of specimen preparation, and (5)

reproducible application of bonding agent. Perhaps its most important

advantage is the introduction of an additional dimension (or degree of

freedom). This is manifested by a two-dimensional plot of specimen

tensile strength as a function of weight fraction of the particulate

phase for each series of tests using the same coupling agent.

Each series would always include some specimens containing resin,

only, i.e. zero weight fraction of particulate. The choice of amorphous

HA (as opposed to a fibrous state) minimized any geometric enhancement

which might have masked the interpretation of the effect of the coupling

agent being tested.

Replicate specimen preparation provided a mechanism for estimating

experimental error under various conditions. This could be used to assess

by standard statistical methods,, conditions under which polymerization was initiated, tensile testing techniques employed, etc., in order to obtain

the best reproducibility possible.

One of the results obtained was the effect of shelf life of monomer

formulations on the tensile strength of the polymerized Bis-GMA specimens.

The monomer formulations were prepared to contain either aromatic amine ac- celerator or benzoyl peroxide initiator, and polymerization resulted on mixing equal amounts of the two formulations. The tensile strength of the

polymer specimens was observed to increase slightly up to about 7 months of

10
monomer formulation shelf time and decreased thereafter to approximately

one-half that attained with non-aged formulat ion? . This is portrnved in

Figure IB-1. The explanation for these formulation aging effects lias not

been determined. The effects observed may possibly relate to an optimum

in the ratio of amine to peroxide, this ratio for undetermined reasons changing as a function of the age of the monomer formulations em.ployed.

Studies have sho^vTi that the speed of polym.erization of this monomer

system may be optimized by control of the amine/peroxide ratio.

It has been pointed out in the preceding annual report why the

weight fraction of particulate raised to the 2/3 power should result in a more linear plot of tensile strength than the weight fraction itself. This

has been confirmed time and again by exneriments, although in some extreme

instances (when the so-called coating agent was, in fact, a "release agent")

some curvature still remained. The reasonableness of the 2/3 power re- lationship can be demonstrated by superimposing on the zero-weight fraction

values of Figure IB-1 the corresponding least-square intercepts (shown in

(+) signs) in Figure IB-2. Where only a single point appears in the printout, the dual points are in virtual coincidence.

Determination of the effect of aging specimens in an aqueous en- vironment is not influenced by the aforementioned monomer aging. This is

because the pair of polymerized specimens to be compared were prepared at

the same point in time. In every series tested, the result of a one-month

soaking in distilled water at room ter.perature reduced the cohesive bond

strength of the Bis-GMA polym.er, while having virtually no effect upon the

coating agent. This is discernible from Figure IB-3 corresponding to averag

tensile strength values for specimens containing a Silane coating agent

(SI-1) described by its manufacturer as "polymeric silicone fluid containing some unhydrolyzed chlorine silicon bonds." The asterisks in Figure IB-3

correspond to tensile strength of the originally polymerized composites,

while the dots represent the strength of samples after exposure to water

for a month. Note that the lower points do not fall off more rapidly (than

the upper group) v:ith increasing particulate composition as they would if

the coating agent had been weakened.

It cannot be assumed that the weakening of the cohesive bonds of

the polymer from a one-month's water immersion was virtually complete within

that period of time. In some of the later series, a sufficient number of

11
one-month sample test specimens were water-soaked for an additional month.

Figure IB-4 shows that the specimen tensile strength for composites con- taining HA with an aliphatic! acid coating agent (AF-1) , after soaking for

two months (asterisks) was consistently weaker than for those which had

been water-soaked for only one month. The relative positioning of the

points in an almost parallel manner again confirms that these results are

a consequence of the weakening of the cohesive polymer bonds and not of

the interfacial bonding involving the coating agent. Improved polymer

casting experience has enabled a sufficient number of specimens to be prepared for comprehensive comparisons to be made between the centrifugal

tensile strength apparatus developed in this laboratory and the conventional INSTRON testing machine. Both type test specimens were polymerized at

the same time from each mix and samples were prepared in duplicates for

different composite compositions of sample AF-1 and all samples tested

after one-month exposure to distilled water. The results plotted in

Figure IB-5 disclose a one-to-one correspondence between the two tensile

test methods.

It is clear that the rate of decrease in tensile strength, when

expressed as a function of the surface concentration of particulate phase

2/3

(i.e. f ) , is a measure of the weakness of the adhesive bonds of the

coating agent: the steeper the slope, the poorer the adhesive. Of the

various coatings selected, two were chosen with the expectation of performing well. They were NPG-GMA, the addition reaction product of N-phenyl

( 8

)

glycine and glycidyl methacrylate developed by R. L. Bowen , and TI-1

,

a new commercial coupling agent consisting of isopropyl-triisostearic

titanate.

In addition, three coating agents were chosen to be representative of poor or mediocre adhesive properties. They were AF-1, an aliphatic

fatty acid (oleic) characterized by a long straight chain with a weak link

(double bond) and a single carboxylic acid group on the end, and two com- mercial preparations designated by their manufacturers to be non-adhesive;

FL-1, a straight-chain highly fluorinated primary alcohol, dihydro per

fluor octanol; and SI-1, a commercial silane-containing mold-release agent,

described earlier. In addition to these five coating agents, one series

of experiments was run with no agent whatsoever.

From a comparison of the steepness at which the tensile strength

12
fell off as a function of weight fraction of particulate, the order of

adhesive bond strengths would be:

(NPG-GMA) > (TI-1) > (uncoated) > (AF-1) > (FL-1) > (SI-1)

which is entirely consistent with the expected performance. Only minor

changes in the order of decreasing adhesion are found for the series

after one-month soaking in water.

It should be pointed out that additional work will be necessary

in order to increase the reliance on this method before it can be adopted

as a standard procedure. The scatter of the points, in particular, must

be diminished appreciably. This problem seems to hinge on the polymerization process, probably to a greater extent than on other factors. A different method of initiating polymerization is being evaluated at the

present time.

Some of the conclusions arrived at by interpretation of the

tensile strength dependence on particulate weight fraction are subject

to confirmation by independent methods. Wherever possible, this will be

done

.

13
14
60

+

+

++

+ 40

+

+

+

+ *

+

20 -+

+

+ *

+

+

+

+ 0» - No. of months before polymerization

+ -- -- __ + + + + -_-_-__-- + 0« 4 8 12 16 20

Figure IB-1

The Influence of storage time of peroxide-containing and aminecontaining monomer preparations on the tensile strengths of " freshly polymerized Bis-GMA. (The comparison covers a total

span of sixteen months.)

15
80

60.

40

20

0.

++

+

+

++

+

+

+ *

+ +

+

+

+

+

+

+

+

+

+ Pre-polymerization timo, (monthc^^

W. 4 8 12 16 20

Figure IB-2

SiTiilar to Figure iB-1 , but showing the comparison between experimental polymer-only points (Asterisks) and values extrapolated

from least-square fitted coated -hydroxyapatite composites (Plus

Signs). Each extrapolated point was obtained from a different

coating agent series. (V/here asterisk-only appears, the 'two values are in virtual coincidence.)

16
+

+

+

++

40

u

bOC

(UH

4-1 C/3

COC

0) H

30

20

10

4-++

+

+

+

+

+++

+ .

+

+

+

+

++

+

+

++

*

- + - - 0.06- + —

0. 12 0.18 0.24

- - + 0.30

2/3 * ^„ - Surface Concentration of Particulate Phase, (f ) Figure IB-3 . Effect on tensile strength of a one-month exposure to an aqueous environment at

23°C for composites containing different surface concentrations of a silanecoated hydroxyapatite (SI-1). (Dots correspond to the water-exposed specimens)

*Surface concentration is defined here as the weight fraction (a dimensionless

quantity) of the particulate phase raised to the two-thirds power.

17
+

+

+

+

30

+

+

e

^ +

w) 20

(3

a) u +*

iH +

•H

CO -

10

+

+ + +••

+ +

+ + +

+ +

+

+

0 + + + + + 0. 0.10 0.20 0.30 0.40 0.50

2/3 # Surface Concentration of Particulate Phase, (f ) Figure IB-4 . Comparison similar to Figure IB-3, but made between a one-month water exposed

(Plus Signs) and a two-month water exposed composite sefies containing different surface concentrations of an oleic acid-coated hydroxyapatite (AF-1)

.

Except for the zero concentration, all t^^7o-month water exposed data (*) represent single values. One-month water exposed data (+) represent averages of

two or more specimens.

// Surface concentration is defined here as the weight fraction (a

dimensionless quantity) of the particulate phase raised to the

two-thirds power.

18
30

+

+

+

+

++

20

+

10

+ o

+ M *

* + *

+ * + * *

+ *

+ *

* CENTRIFUGAL APPARATUS

K« - + + + + + +

0. 10 20 30 40 50

Tensile Strength (10 N/m ) Figure IB-5

.

A one-to-one correspondence is foun:] betvreen the commercial Instron

tensile strength apparatus and the experimental Centrifugal Instru

ment for specimen samples pol;>'7r!eri zed from the sam.e mixes.

19
C. Synthesis for Improved Adhesion and Stability

of Composite Restorative and Sealant Materials

The resin, Bis-GMA, the polymeric basis of all commercially

produced dental composite restorative materials, has certain deficiencies,

the most important being its excessive viscosity, which requires addition

of a dilutent for application in the dental office, and its inability to

be produced commercially in pure form.

, (9) Previous research by Bowen of this laboratory resulted in

development of methods for synthesizing three crystalline and highly purified dimethacrylate esters of the isomeric phthalic acids. Mixed in proper

proportion, a ternary eutectic liquid at room temperature, was obtained.

High purity and an acceptable viscosity were factors which encouraged the use of this system. However, a color instability, traceable to formation

of a charge transfer complex with the tertiary aromatic amine employed as an accelerator in the polymerizing system made the system unacceptable

for dental application.

To circumvent the charge transfer complex formation, a system

consisting of bis (2-methacryloxyethyl) ethers of the three isomeric aro- matic diols (hydroquinone , resorcinol and catechol) was developed. The

system has promise and it is currently being investigated for its potential as a field composite system for the Army Dental Corps.

Ether-Ester Dimethacrylate

In the past year, efforts have been concentrated on the aromatic

ether-ester dimethacrylate system. The three crystalline monomers are conveniently synthesized by the facile condensation reactions involving

hydroxybenzoic acids and 2-bromoethyl methacrylate . As predicted, the

isomeric monomers have relatively low melting points and may be highly

purified by recrystallization techniques.

Highly purified samples of the three isomeric 2-methacryloxyethyl

2-methacryloxyethoxybenzoates were characterized by NMR and IR analyses.

Both analyses support the proposed structures. A carbon-hydrogen analysis

was in agreement with the stoichiometric formulae. Melting points of

the individual monomers are listed below:

20
Monomer M.P . (C)

MoM 34-35

MmM 49.5-51

MpM 50-51.5

MoM = 2-Methacryloxyethyl-o- (2-methacryloxyethoxy)benzoate

MmM = 2-Methacryloxyethyl-n- (2-methacryloxyethoxy )benzoate

MpM = 2-Methacryloxyethyl-p- (2-methacryloxyethoxy )benzoate

From these melting points and the theoretical relationship shown below,

the composition of the three binary eutectics was estimated:

* T (C) Binary Proport ions e MoM/MmM 76/24 27

MoM/MpM 77/23 27

MmM/MpM 49/51 33

* Theoretical proportions

100 (T^ - T ) V = 2 e

1 T 12+ - 2T

e

where is the predicted mole % of the lower melting component in the

eutectic, T is the m.p. of the lower melting component, is the m.p. of

the higher melting component, and is the eutectic temperature (first

sign of melting of the binary mixture).

The three binary diagrams were incorporated into a triangular

ternary diagram to arrive at the ternary eutectic. The following three

ternary mixtures gave liquids of moderate viscosity at 25°C.

1. 50% MoM, 25% MmM, 25% MpM - solidified at 5°C.

2. 60% MoM, 20% MmT'l, 20% MpM - liquified at 5° after several months.

-3. 68% MoM, 16% MmM, 16% MpM - solidified at -15°C, to be tested

at 5°C.

It is interesting that this monomer system may exist in the liquid

state at room temperature for a range of compositions. This property may

make it possible to obtain improved storage stability of this monomer system

by proper choice of composition. For example, a mixture that is liquid near room temperature, but frozen solid when refrigerated, probably would

have less tendency toward inadvertent polymerization or oxidation on prolonged

storage than a similar mixture that exists as a liquid at the storage temperature as well as at room temperature.

21
Ternary Mixtures with Amine Accelerators

In contrast to the aromatic dlester dimethacry lates (phthalate

monomer system) which form yellov; charge-transfer complexes with the usual

tertiary aromatic amines commonly employed as polymerization accelerators,

the aromatic ether-ester dimethacry lates do not discolor on exposure to

such amines.

Polymerization

These liquid monomeric mixtures undergo facile polymerization to

hard, clear, glossy resins in less than five minutes when initiated with

benzoyl peroxide plus N ,N-dimethyl-p-toluidine . The same monomer mixtures,

when activated by the methyl ether of benzoin, undergo ultraviolet photopolymerization in about 30 seconds to yield a clear, tough sealant film.

This monomer system, with its already workable viscosity, may have advantages over the widely used sealant system of Bls-GMA thinned with methyl

methacrylate (MMA) . No increase in viscosity should occur using the etherester system since it is non-volatile. The Bis-GMA system can change

in viscosity if care is not taken to guard against volatilization of MMA.

It is encouraging to note that poly (ethyleneoxybenzoate) , a linear

ether-ester polymer derived from p-hydroxybenzoic acid and ethylene oxide,

is reported to have excellent resistance to sunlight, weather and chemicals - comparable with, and possibly better than, that of polyethylene terephthalate

,

the analogous ester-ester linear polymer.

It is a reasonable expectation that resins derived from the etherester monomer system will exhibit enhanced hydrolytic, chemical and UV

oxidative stability. To understand what effect, if any, electron donating

and electron withdrawing ring substituents have on such polymeric properties

as hydroxylitic , oxidative and color stability, a comparative study of the

resins derived from the aromatic dlester, diether and ether-esters dimethacry lates should be made.

Functional Monomers

Monomers having pendant functional groups capable of reacting or interacting with enamel and/or dentin are classified as functional monomers

and may serve as adhesion-promoting agents in dental applications. For ex- ample, a monomer or polymer carrying the hydrophilic carboxylate group is

functionally capable of reacting with enamel (e.g. polycarboxylate cements).

22
Unlike the polycarboxylate cement, the functional monomer also would react with the resin monomer system to give polymers that are not water

soluble or swellable to any great extent. Consequently, such monomers

should lead to more hydrolytically resistant bonding between resin and

enamel.

It has been previously shown in this laboratory that the methacryloxyethyl ester of p-hydroxybenzoic acid significantly improved the adhesion between a restorative resin and dentin . ^"'"'^^ Originally, this ester

methacrylate was synthesized by the acid catalyzed condensation of hydroxyethyl methacrylate with p-hydroxybenzoic acid. The monomer obtained by

this route, however, has proved difficult to purify by recrystallization

.

An alternate synthesis of this functional monomer by the condensation of the potassium salt of p-hydroxybenzoic acid and 2-bromoethyl

methacrylate has yielded a crystalline solid, m.p. 82-84°C. The monomer

has been characterized by NMR and IR analysis.

By a similar condensation scheme, but employing a double salt of

p-hydroxybenzoic acid with 2-bromoethyl methacrylate and using mild reaction

conditions, p-methacryloxyethoxybenzoic acid, m.p. 141-142°, was formed.

The IR spectrum was consistent with the proposed structure.

, o An interesting functional monomer having a pendant carbonyl (C^ H)

group was synthesized also by the condensation of 2-bromoethyl methacrylate

and p-hydroxybenzoyldehyde in the presence of potassium carbonate in an aprotic solvent. The solid monomer, m.p. 47-48°, has been characterized

by IR and NMR. It undergoes polymerization on heating somewhat above its

m.p. to give a hard, glassy polymer. Some of the polymer appears to be

crosslinked, as most of it fails to dissolve in the common organic solvents

(e.g. acetone, methylene chloride, tetrahydrofuran) . Apparently, the

carbonyl group participates in the polymerization to som.e extent. The IR

analysis of a polymeric film, however, indicated the presence of some free

carbonyl groups. The monomer in its molten form undergoes rapid photopolymerization in the presence of the methyl ether of benzoin. Benzoyl

peroxide and N,N-dimethyl-p-toluidine , also initiate polymerization in a few minutes at 50°C.

Thermal polymerization of this monomer on a bone substrate re- sulted in, at first, a colorless film. However, on standing, the film

became slightly yellow. A control experiment on a glass slide gave only

a colorless film, which did not yellow with age. The development of the

23
yellow color on bone may be due to reaction with free amino groups in the

collagenous substrate via a Schiff base type of reaction. The film has

resisted removal by soaking in acetone or water. A corresponding film on

a glass slide was removable by solvent action. If these preliminary ex- periments are valid, this type of monomer may prove useful in improving

adhesion to dentin and bone. We plan to investigate this monomer further

and to synthesize liquid monomers with free aldehyde groups to augment

this reaction with the collagenous substrate. Moreover, since aldehydes

are oxidizable to carboxylic acids, the further possibility exists of improved adhesion through interaction with the mineral phase of the substrate, as well as through reaction with the collagenous phase.

A liquid aldehyde monomer of the aliphatic class has now been

synthesized. It is derived from 5-hydroxypentanal by the appropriate con- densation reaction with methacryloyl chloride.

I 3

„ n Et N

CH^ = C - C - CI + HOCH^CH^CH^CH^CH ^j, )

CH„ 0 0

( II l\ CH^ = C - C - OCH^CH^CH^CH^CH

This monomer is a colorless liquid of moderate viscosity. Characterization

of this functional monomer is in progress.

The polyf luoroaromatic methacrylate , 2-methacryloxyethoxypentaf luorobenzene , has been successfully polymerized by chemical initiation

(benzoyl peroxide) . A sample of the homopolymer has been immersed in

distilled water at room temperature in order to determine if this polymer

has any fluoride ion releasing capability. The fluoride ion concentration

will be measured periodically by means of a fluoride electrode.

Another interesting functional monomer which may have the ability

to slowly release F after polymerization is that derived from the reaction

of pentaf luorophenol and 2-bromoethyl methacrylate in the presence of

potassium carbonate in an aprotic solvent. The monomer is fluid

despite its relatively high molecular weight (296). We hope to study the

polymerization and copolymer! zation of this monomer and the behavior of

the corresponding resins in water. The presence of any F will be determined, using the fluoride electrode method previously reported for. the

model compounds.

24
This, and similar f luoroaromatic monomers, may also react with

free amino groups as well as H^O present in collagenous substrate and

thereby form covalent attachments while F is being released into the

mineral phase.

D. Polymeric Grafting, A Technique for Bonding

to Mineralized Collagen Surfaces

Grafting of polymeric side chains to hard tissues such as bone

or dentin offers an attractive technique of altering the surface properties,

especially the surface characteristics. It is a potential means to obtain

good adhesion to hard tissue surfaces. Such techniques may find applications

in improving adhesion of restorative materials to dentinal surfaces and as bone cements in oral surgical procedures.

Grafting of polymers to bone surfaces was continued using the

persulfate-bisulf ite redox system as initiator. As judged by weight gains

of bone, 2-cyanoethyl acrylate , cellosolve, glycidyl m.ethacrylates , and

1,3-butylene dimethacry late were the most effective grafting agents.

Moderate grafting yields were achieved with solutions of zinc or calcium

acrylates , hydroxyethyl methacrylate , ethylene dimethacrylate and divinyl

benzene. Little or no reaction occurred with a number of monomers such

as 2-ethylhexyl acrylate, hexaf luoroisopropyl acrylate, isobutyl methacrylate and pentadecafluorooctyl methacrylate.

A manuscript entitled "Chemical Modification of Hard Tissues - I. Bone" was prepared and submitted for NBS review. Additional experiments

have been conducted to provide further evidence for some of the conclusions

drawn in this paper.

A series of monomers of different chemical structure was evaluated

for possible application in photopolymerization experiments designed to

modify hard tissue either by grafting, interpenetration or adsorption

onto the substrate.

In these experiments, a Nuva Seal light was employed. A number

of photoinitiators were examined, but in most of the polymerizations,

the methyl ether of benzoin was employed. Except in a few cases, most

monofunctional monomers (e.g. methyl methacrylate, 2 -hydroxyethyl methacrylate, 2-hydroxyethyl methacrylate, styrene, benzyl acrylate, tetrahydrofurfuryl methacrylate, pentadecaf luorooctyl methacrylate, methacrylic

25
acid and 2-dimethylaminoethyl methacrylate) gave little, if any, polymer

on glass microscope slides, even after prolonged irradiation (2 minutes).

However, an equimolar mixture of the last 2 monomers, methacrylic acid

and 2-dimethylaminoethyl methacrylate, produced a polymeric film in 30-

40 seconds. Apparently, the two monomers act as a more viscous difunction

al monomer because of H-bonding.

A second monofunctional monomer that formed a polymer in 30-40

seconds is the monoester derivative of 2-hydroxyethyl methacrylate and

phthalic acid. Again, the monofunctional monomer acts as a difunctional

one, probably due to H-bonding of the free carboxyl group. The polymer

formed was a clear, hard film. This monomer will be evaluated for any

adhesion-promoting ability due to the presence of the free carboxyl group

which may interact with mineralized tissue or enamel. Preliminary ex- periments have indicated there is greater adhesion to enamel with this

monomer system than with Bis-GMA.

Except for the exceptions noted above, most monofunctional

monomers do not photopolymerize at a fast enough rate to be clinically

applicable. It seems that the photopolymerizability of methacrylate

monomers may be related to their functionality and viscosity. The more viscous difunctional methacry lates set up faster than the less viscous

(e.g. Bis-GMA versus ethylene glycol dimethacrylate) . The aromatic diester dimethacrylates , aromatic diether dimethacrylates and the aromatic

ether-ester dimethacrylates, all polymerize nicely to tough, clear coatings. Since these monomers are not as viscous as Bis-GMA, they do not

require the addition of a diluent comonomer (e.g. MMA)

.

In an attempt to obtain a more hydrophobic and oleophobic coating, equivalent amounts of triethylene glycol dimethacrylate and pentadecaf luorooctyl methacrylate were photocopolymerized . Most of the polymerization occurred in the triethyleneglycol dimethacrylate layer and the

polymer was not particularly hydrophobic. However, using chemical initiation, with N-N-dimethyl-p-toluidine in the highly fluorinated monomer

and benzoyl peroxide in the triethyleneglycol dimethacrylate, a polymer

was obtained with a more hydrophobic outer coating. We hope to apply

this technique to make more water and oil-resistant sealants.

In addition to studying the photograf ting behavior of acrylate

26
and methacrylate monomers, which may graft to collagenous substrates

by free radical coupling reactions, we have explored the possibility of

grafting via other chemical reactions by means of functional monomers.

For example, it is well-known that collagen is capable of undergoing the

Schiff base reaction with aldehydes. There are indications that the

interesting functional monomer, 2-methacryloxyethoxybenzaldehyde , may be

capable of forming covalent links with collagenous surfaces via the

Schiff base reaction. Subsequent polymerization or, more likely, co- polymerization with another methacrylate monomer should result in success

ful polymeric grafting.

Previously, we had shown that the solid monomer (m.p. 47-48°C) , 2-methacryloxyethoxybenzaldehyde , could be polymerized in the molten stat

(approx. 50°C) by chemical or photoinitiation . On a collagenous surface

such as bone, a tough, slightly yellowish, film was formed by thermal

polymerization of this monomer, which was resistant to rem.oval by organic

solvents and water.

We have shown that it is possible to copolymerize a solution of

this aldehyde methacrylate in methyl methacrylate by UV irradiation at room temperature. Work is in progress to evaluate the adhesive strength

of such copolymers on collagenous substrates compared to similar polymers

without the aldehyde monomer. A liquid aldehyde methacrylate of the aliphatic class has been

synthesized and will also be evaluated as an adhesion-promoting comonomer

II. Physical, Mechanical and Physicochemical Properties

of Hard and Soft Tissues and of Dental Materials

A. Dimensional Changes in Tooth Structure

The property of dimensional change of dental restorative materials relative to host tooth components is of obvious clinical importance.

Presumably, differences in capacity for dimensional change should be

minimized if problems such as marginal leakage are to be avoided. Differ

ences may impose severe strains on interfacial bonding, thus severely inhibiting development and/or retention of adhesive bonding.

The dimensional changes of tooth enamel and dentin on heating

and cooling are to a considerable extent governed by the thermal expansion of the mineral component of the tooth. However, particularly with

27
dentin, the dimensional changes are modified by the organic component.

Although collagen has a high coefficient of tliermal expansion, its presence

reduces the thermal expansion of dentin. Thus, it is evident that although the dimensional changes are induced by temperature change, other

factors (such as changes in water content) are involved. Similarly,

factors other than thermal expansion of components may be involved in

the temperature-induced dimensional changes of restorative materials.

The objective is to determine the temperature-related dimensional changes of human tooth enamel and dentin and of restorative materials, to explain these changes in terms of various factors such as water

content, micro and macrostructural aspects, phase changes, or degree of

bonding of components which may be involved, and to point out possible

relationships between thermally-induced dimensional changes and clinical

performance of restorative materials.

Data obtained on the thermal expansion of enamel, whole dentin,

anorganic dentin, bovine bone and bovine dentinal collagen and on the

temperature-dependent changes in water content of bovine Achilles tendon,

kangaroo tail tendon, ivory and seal tooth, provide a consistent picture

of the thermally-dependent dimensional changes of tooth structures. In

brief, the data indicate that the collagen component is responsible for

the time-dependence of the thermal expansion of dentin. The dimensional

changes of collagen resulting from its loss of water on heating (in water)

,

and its gain of water on cooling, do apparently greatly affect the dimensions of dentin. Neither the thermal expansion of collagen nor its phase

transformation dimensional changes appear to be major factors in the effect of collagen on the dimensional changes of dentin.

In dentin and bone, large anisotropic variations in the coefficient of linear thermal expansion are observed. (In bovine femur, for

example, the coefficient was found to vary from near zero in the radial

—6

direction to approximately 2.5 x 10 /°C in the circumferential direction,

—6

and approximately 8 x 10 /°C in the axial direction. Measurements made

along three mutually perpendicular axes in human tooth enamel indicate

anisotropy. However, the directional variations are much less prominent

than in dentin or bone and the data obtained so far do not relate the

direction of minimum expansion to any specific direction within the

tooth. The magnitude of the variations observed in enamel are shown in

the table below:

28
Thermal Expansion of Tooth tlnamel

10 to 60°C

Coefficient X 10

Relative to Specc A Spec B Tooth Surface Coef

.

S D. Coef

.

S D.

Perpendicular 13 .0 0 3 11.7 1 0

Parallel 11 .8 0 5 13.4 0 2 Parallel 13 .1 0 5 13.4 0 2 Volume Expansion 37 .9 38.5

If it is assumed that in the specimens used for these measurements, the enamel rods are approximately perpendicular to the tooth surface,

no relationship between rod direction and coefficient of thermal expansion

is apparent. However, a precise determination (not yet made) of rod orienta

tion within the specimens may show more correlation than is now evident.

B. Wear Resistance of Dental Restorative Materials

The property of resistance to wear can be assumed to be a critical

one for all dental materials subject to any of the various types of wearing

forces in the mouth. It is, of course, crucial for materials placed at oc-,

clusal surfaces where stress is imposed. Development of adequately wearresistant materials, without degrading other properties, would be greatly

assisted by design of a laboratory system which could simulate clinical

wear performance. To be of practical value, the laboratory system should

reduce by several orders of magnitude the time required to observe measurable (and reproducible) quantities of wear. The problem is thus the selection and incorporation of the optimum combination of mouth-simulation, ac- celerating and variable-reducing characteristics. Once this has been ac- complished, the system can be used to determine wear characteristics of

materials before they are subjected to extensive clinical evaluation.

The initial design and construction of a simulated physiological

wear generating and measuring device (Figure IIB-1) were given in detail

in the FY 1975 Annual Report. The wear generating portion of the device

has posed no problem; however, modifications to the measuring, and later

to the data collecting, systems have been necessary. Measurements have

been restricted to one commercial amalgam and one commercial composite,

29
each of which have been the subject of one, two, and three-year clinical

(11-13)

obse rvations

.

Due to the apparently erratic nature of some of the wear depth

measurements, improvements were made toward stability. These include:

(1) a solenoid operated water shutoff valve to prevent flow of water over

the measuring points and beam ends while they are in their down (measuring)

position; (2) blunting of the wear track measuring pointer to prevent its

falling into small voids or grooves in the bottom of the wear track; (3) an improved relay coil power supply to the drive motor braking system to

provide greater reliability in placing the measuring points in the same spot for each measuring cycle; (4) factory reconditioning and updating of

the recorder for wear depth and fluid environment temperature; and (5)

addition of a voltage regulating power supply to the LVDT signal conditioning module to eliminate the possible effect of line voltage surges.

Initial testing of a very limited number of specimens showed a marked difference between the wear resistance of an amalgam alloy and a composite restorative material in the same order of wear shown by prior

clinical service testing.

(1 ^) Referring to the volumetric wear equation:

3

V = Wear volume (in mm ) k = Wear constant

V = -— W = System load (1) 3p L = Length of slide (total)

p = Penetration hardness of softer wear couple member

the wear constant k has been described as a statistical probability that

a fragment of wear debris will form between surface asperities of materials

in sliding ' contact . ^"^ It is analogous to coefficient of friction and is

generally referred to as the coefficient of wear. Consequently, it is a very convenient term used to describe the wear performance of materials in

a sliding mode. It must be remembered, however, that materials for which

these concepts were developed may have vastly different properties from

those of concern in this investigation. Smaller coefficients of wear (k)

^'alues indicate greater resistance to wear.

To arrive at the wear coefficients, a straight line was fitted to

the recorded wear track depth data to give a least-square estimated slope

30
on a wear vs. time plot for the two materials tested. Calculated depth of

V7ear values for each lO-ninute running period between TTieasurenents were .0304;

for the analgam, and .0743ijin for composite restorative material with standard

errors of the slopes of .0109 and .0238, based on 34 and 25 measuring points,

respectively

.

As observed in the statistical inf orm.ation , the standard errors

of slope left something to be desired in that the slopes were different only

at a 90% level of confidence, using the significance test described by

Bowen and Argentar . ^ "^^ Further examination, however, indicated that, with

a capability of testing for a 24-hour run, the precision of slopes could be

improved by approximately 2.5 tir.es:

degrees of freedom. 24-hour run = 142

degrees of freedom, present run = 23 \ -ft

= 2.5

In order to test for the longer period, it was necessary to improve the fluid supply to the testing apparatus.

A 75— liter shaker bath was Drocured, modified and installed with

the testing apparatus. The bath was connected directly to the facility

distilled water supply through a regulating float valve and provides a continuous supply of 37°C (i 1°C) distilled water to the test specimen.

The water temperature has proved sufficiently stable that the thermocouple

output recording was discontinued.

A modification was also r.ade regarding the electrical circuit re- sponsible for stopping the restorative specim.en under the depth measuring

tips. Additional circuitry allowed two points on the wear track (180°

apart) to be m.easured instead of one, and, in effect, doubled the original

data output tor e ach restorative SDecir.en. The two points were measured

alternately after each 20-m.inute wear period.

In an effort to expedite testing tim.e, five specimens each of an amalgam and a composite restorative material were tested under a load of

2 2

1.5 kg/imn of enariel face contact instead of the original 1 kg/mm . Un2 fortunately, the 1.5 kg/mm load proved too high for the amalgam-enamel

couples, causing edge fracture of the enam.el sliders. The sharp fractured

edges exercised a milling effect under sliding and removed far more restorative material than norm.al from, the wear track, falsifying the wear data.

As a result of these findings, all future tests of this configuration were

2 limited to 1 kg/mm loads.

31
Wear coefficients were calculated from test volumes using the

Archard equation (Eq . 1). Coefficients were also calculated using the

equation used by McGrew^"^ ^ (Eq . 2):

V = KmVJL Km = Wear coefficient without hardness influence

These K values differ from the original Archard equation values in that

3 these are given in units of mm /mm kg, based on wear volume divided by

sliding distance and by load. The difference between the two methods of

calculation is in the use of hardness values. In the Archard equation,

the quotient derived by the division of the product of load and sliding

distance into the wear volume is multiplied by three times the hardness

value for the material tested. The McGrew equation omits any consideration of hardness.

Observation of alternately recorded points on the analog strip

chart showed a depth differential between the two sets of measured points.

While some differential (5-6 ym) may be expected from loss of perpendicularity to the vertical mounting position due to the pretest preparation

and polishing of specimen surfaces, the amount observed ran as high as 18

ym. Also observed was the fact that the rate of wear measured at the two

positions was seldom similar. From the observations, it was concluded

that the 360° wear track may not be of uniform depth; probably a consequence

of dissimilar wear rates at various locations around the wear track. It

is possible that the nonhomogeneity of restorative materials throughout

their bulk could account for dissimilar wear rates at their wearing surfaces.

Such an occurrence has not been previously reported in dental materials

literature

.

The non-uniformity of track depth was confirmed by multiple surface profile measurements made with a Talysurf profilometer moving from

the unworn material inside the wear track circle, across the track to un- worn material outside the circle (Figure IIB-2). Comparison of the various

track crossings on the same specimen indicated track depth differences as high as 15 ym. Considering that the previously indicated wear for the

total test run was based on the average of the two measured depth points,

it was concluded that only two measuring points on the wear track would

not be sufficient for precise total wear track volume measurements.

32
It was decided that a minimum of ten measured points, equally

spaced around the 360° wear track, would produce the desired precision.

While the two measuring periods per hour could be handled easily, the

greatly increased data acquisition rate from 10 points would render the

data recording method highly impractical.

In order to efficiently collect and prepare for computer processing the large amount of data generated, an integrated data acquisition

system was procured on loan from the Microcalorimetry Section and the ADA.

The system consists of a digital -voltmeter with binary code digital output

to receive the calibrated depth measurements from the measuring tip-LVDT

system. The voltmeter is interfaced through a coupler-controller to a teletype equipped with a paper tape punch. The teletype provides a direct printout of data as well as punched data-containing tape in American

Standard Code II for direct input to computer processing (Figure IIB-2).

In order to provide ten equally spaced depth readings taken at

the same relative point on the specimen for each measuring cycle, a triggering device was required. A ten-slot chopper wheel was constructed and

attached to the specimen turntable shaft. A light source-photocell

receiver-amplifier device was designed and constructed and saddled over

the edge of the chopper wheel. The purpose of this device is to command

the data acquisition system to take a single measurement each time light

is admitted by a chopper wheel slot.

After each thirty-minute wearing period, the specimen turntable

is placed into one slow (15-second) revolution during which time the sapphire tipped measuring points are in contact v/ith the wear track and

reference area. During the 15-second revolution, the ten-slot wheel

initiates the digital voltmeter to effect the recording of ten equally

spaced wear track depth measurements and the current time of day on the

teletype printout and placement of the data on punched tape.

Data from the ten-point measuring system has verified the suspecte

inaccuracy in the old one and two-point system. The average wear coefficient for the ten measured positions of the latest measuring system gives

-4 -4

values of 0.45 x 10 and 0.46 x 10 for the same materials above. These

last two figures do not actually suggest the same wear resistance for both

materials as it may appear. It must be remembered that the values were

derived from the Archard equation (Eq. 1) in which K (wear coefficient) is

33
directly proportional to liardness. The wear coefficient values for the same two materials as calculated from the equation as taken from McGrew

(Eq. 2) are not influenced by hardness. These k values were found to be

-6 -6 3 ^

0.22 X 10 and 0.13 x 10 mm /mm kg for composite and amalgam, respective

ly. The validity of these latter k^ values is reinforced by the average

volume loss per minute per square millimeter of wear couple contact which

-3 -3 3 2 were 0.23 x 10 and 0.13 x 10 mm /min(mm ) for composite and amalgam.

For data treatment, two statistical models were used. In the

first model, the errors were assumed to accumulate with the passage of

time as wear occurs. The second, or independent mode, has been shown to

be useful in a number of experimental instances where a specimen undergoes

change with time and a property affected by the change in the material is

intermittently measured.

Results so far obtained are consistent with the assumption that

the true error model lies somewhere between the independent and the cumu- lative with composites favoring the independent and amalgam the cumulative

model. Techniques for handling such cases have not heretofore been

published.

Some of the sources of variability in the data are shown in the

next few Figures. Figure IIB-3 shows a plot of wear-track depth versus

time. Each point is an average of the ten depth readings taken at the end

of a 30-minute wear cycle. The data for the composite show a fairly uniform wear pattern. There are some apparent reversals in depth, but they

seldom represent more than a few tenths of a micrometer. The curve for

amalgam, while it shows a lower average wear rate, is also more Irregular.

The pattern of alternating slow and rapid wear seems to be typical of

amalgam. Transfer of amalgam to the surface of the enamel slider occurs during the wear period. Whether or not the fluctuations in wear rate cor- respond to fluctuations in the amount of amalgam on the slider has not

been determined, but that would appear to be a possibility. The data indicate the necessity for testing over a long wear period since wear rates

based on short periods could vary greatly.

Figure IIB-4 shows for amalgam the variation of depth of wear at

the ten measurement positions around the approximately 40 mm circumference

track. Each of the curves connects ten depth measurements made in the course of one measurement cycle. It is obvious that some areas of the

34
specimen show more wear than do others. Also, as indicated by the similarity

of the curves obtained at different times, the wear patterns established

during the first few hours of the test period tend to continue throughout

the period. Thus, it appears that there are variations in wear resistance

throughout the specimen, and the average value for a particular specimen

will depend on the mix of low and high resistance areas in that specimen.

Figure IIB-5 shows that the variation in wear resistance from one position to another occurs in the composite as it does in amalgam.

One anticipated source of scatter in the wear results was the

variability of sliders made from human enamel. It was thought, however,

that the use of enamel with its inherent variability offers advantages over other materials which might be more uniform, but would not duplicate the wear characteristics of enamel. It is difficult to determine the contribution that differences in the enamel make to the variability of the results

since sliders often cannot be used for more than one test period. Even when

a slider can be re-used, there is the possibility of different wear character'

istics at different depths in the enamel. Examination of several sliders

after use indicated that there were significant deviations from the intended

orientation of the enamel rods perpendicular to the wear surface. More

uniform preparations may reduce the scatter in future measurements. We would

like to substitute another material for the enamel, but at present we do not

know of a material which satisfactorily duplicates the characteristics of

enamel as the antagonist in the wear couple.

Although this study has not included measurements of the volume

loss of enamel, it is obvious that enamel loss is much greater when sliding

in contact with the composite material than when against amalgam. This observation, which is in agreement with results reported by Powell, Phillips

and Norman , ^''"^^ may have clinical significance no less important than that

of the wear rates of the restorative materials.

The results reported herein were presented before the American

(iS) Association for Dental Research meeting in New York, April 1975.

35

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C. High Modulus Reinforcement, Techniques for the

Improvement of Dental Cements and Composites

The objective of this research is to develop techniques for improving the interfacial bonding between the reinforcing filler and the

polymeric matrix of dental cements and composites. Initially, our efforts

have been directed toward improving the properties of dental cements of

the polycarboxylate type. If successful, these studies would be extended

to other dental composite systems.

Since the polymeric matrix of the polycarboxylate cement is

basically poly(acrylic acid), we have attempted to make the reinforcing

filler component more compatible with the matrix by depositing a thin

coating of the same or a similar polymer on the filler particles. Ideally,

to achieve strong interfacial bonding, the polymeric coating should be

firmly attached to the filler particles as well as being chemically and

physically compatible with the polymeric matrix.

In the present study, a commercially available polycarboxylate

(PGA) provides the polymeric matrix of the cement. Two fibrous materials,

potassium titanate and graphite, have been selected as reinforcing fillers.

Several techniques for the deposition of thin polymeric coatings

onto the filler particles are under evaluation.

One technique involves allowing monomer in the vapor state to

permeate the filler in a vacuum. The monomer is subsequently polymerized

in situ. Our initial attempts at using this technique have involved the

use of acrylic acid as the monomer, and gairana irradiation to initiate

polymerization. Some problems were encountered using this technique,

primarily having to do with polymerization of the monomer prior to deposition on the filler particles. However, a crude burning test of each

treated fiber indicated the presence of some polymeric deposit. The

acidity of aqueous suspensions of the two treated fibrous fillers was

found to range from pH 3.3 - 3.9. An equilibrated aqueous slurry of un- treated potassium titanate had a somewhat basic pH of 8.5. The untreated

graphite suspension was virtually neutral. Extraction of the treated

fibers with water for 24 hours (Soxhlet extraction) removed some of the

poly(acrylic acid) deposit (pH of extract reached 4.0 - 4.6). A further

extraction with dioxane (a good solvent for poly (acrylic acid)) removed

virtually all of the coating from the graphite (pH 5.3), but had little

41
effect on the potassium titanate fibers (pH 4.6). It would appear that some acrylic acid and/or poly(acrylic acid) has become ionically attached to the potassium titanate, perhaps because of the somewhat basic

character of this filler.

A second technique for polymer deposition onto filler is to suspend the filler in a dilute solution of monomer and evaporate the solvent

with constant agitation. The monomer, coated on the filler particles,

may be polymerized by thermal and/or chemical initiation. We have used

this technique to prepare potassium titanate fibers coated with the

polymer of the acryloxyethyl ester of phthalic acid (CH2 = CHCO^CH^CH^O^CC^H, 2222642CO^H) . Polymerization of the monomer was achieved by

simple thermal polymerization under vacuum. A related technique involves the deposition of polymer from a polymer solution onto the filler. For example, potassium titanate fibers

were agitated in a dilute aqueous solution of polyacrylic acid while the

solvent was gradually stripped away under vacuum. A comparison of these

treated fibers with those obtained from the gamma radiation polymerization of vaporized acrylic will be made.

42
REFERENCES

1. Brunauer, S. ; Emmett, P. H. ; and Teller, E. : Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc. 60:309 (1938).

2. Harkins , W. D. and Jura, G. : Surfaces of Solids XIII. A Vapor Adsorption

Method for the Detemination of the Area of a Solid Without the Assumption

of a Molecular Area, and the Areas Occupied by Nitrogen and Other Molecules

on the Surface of a Solid, J. Am. Chem. Soc. 66:1366 (1944).

3. Loebenstein. W. V. : The Surface Area of Aggregates Applied to Dental Materials, J. Biomed. Materials Res. 9:35 (1975).

4. Razouk, R. I. and Salem, A. S. : The Adsorption of Water Vapor on Glass

Surfaces, J. Phys. Colloid Chem. 52:1208 (1948).

5. Loebenstein, W. V. : Adsorption of Water on Tooth Components and Related

Materials, J. Dent. Res. 52:271 (1973).

6. Loebenstein, W. V. : Unbound Water Content from Application of Adsorption

Theory, to be published in the July-August 1975 Issue, Section A, Physics

and Cheittistr\^ J. Res. NBS.

7. Bowen, R. L. and Argentar, H. : A Method for Determining the Optimum Peroxideto Amine Ratio for Self-Curing Resins, J. Appl. Pol\Tner Sci. 17:2213 (1973).

8. Bowen, R. L. : Adhesive Bonding of Various Materials to Hard Tooth Tissues.

II. Bonding to Dentin Promoted by a Surface-Active Comcnomer, J. Dent. Res.

44:895 (1965).

9. Bowen, R. L. : Crystalline Dimethacrylate Monomers, J. Dent. Res. 49:810

(1970).

10. Bowen, R. L. : Adhesion Promoting Dental Materials, U.S. Patent 3,635,889

(1972) . 11. Phillips, R. W. ; Avery, D. R. ; Mehra, R. and Swartz , M. L.: One-Year Observations on a Composite Resin for Class II Restorations, J. Prosth. Dent.

26:68 (1971).

12. Phillips, et al. : Two-Year Observations on a Composite Resin for Class II

Restorations, J. Prosth. Dent. 28:164 (1972).

13. Phillips, et al. : Three-Year Observations on a Composite Resin for Class II

Restorations, J. Prosth. Dent. 30(#6):891 (1973).

14. Rabinowicz , E. : Predict Wear of Metal Parts, Product Engineering :71 (1958).

15. Archard, J. F. : Contact and Rubbing of Flat Surfaces, J. Appl. Phys. (1953).

16. McGrew, J.: ASTM, Standardization News, Sept. 1974.

17. Powell, J. M. ; Phillips, R. W. and Norman, R. D. : In Vitro Wear Response

of Composite Resin and Amalgam and Enamel, J. Dent. Res., in press.

18. Powell, J. M. and Dickson, G. : In Vitro Wear Testing of Restorative Materials, AADR Abstract No. 356 (1975).

USCOMM-NBS-DC

43
NBS-n4A (REV. 7-73)

L'.S. DEPT. OF COMM.

BI3LiOGRAPHIC DATA

SHEET

L PUHLK ATION OR REi^ORT NO.

NBSIR 75-766

2. Gov't Accession

No.

3. Rec ipient's Accession No.

4. TITLK AND SUBTITLt:

Properties and Interactions of Oral Structures

and Restorative Materials.

5. l-'ubl ication Date July 31, 1975

6. Performing Organization Code

7. AUTF:OR(S)

J. M. Cassel

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 3110561

11. C^on t r act /Gran t No. NIDR YOl-DE-40015

12. Sponsoring Organization Name and Complete Addrcs.s (Street, City, State, ZIP)

Biomaterials Program

National Institute of Dental Research, NIH

506 Westwood Building

Bethesda, Maryland 20014

13. Type of Report & Period

Annuaf Report

14. Sponsoring Agency Code

NIDR

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant irj formation. If document includes a significant

bibliography or literature survey, mention it here.)

A diversity of experimental approaches is being investigated to develop techniques which can be applied in formulation of adhesive restorative and cariespreventive dental materials. A newly developed testing procedure, still in the

evaluation stage, has demonstrated expected ranking of adhesive bond strengths

within a group of selected coating agents, both coupling and release types.

Deterioration on exposure to water of cohesive bond strength in the Bis-GMA matrix

has been indicated without concurrent deterioration of the coupling agent. A

monomer system with potential as a composite or pit and fissure sealant resin and

giving a polymerizable liquid of desired viscosity at room temperature was derived

from three low-melting, crystalline isomers of ether-ester linked aromatic dimethacrylates. A series of monomers of varying chemical structure was evaluated for

possible application in photopolymerization experiments designed to investigate

modification of hard tissue by grafting. Modifications to extend the running time

of a dental restorative wear-generating experiment three-fold and which allow the

computerized recording of ten, instead of two, depth of wear measurements around

the circular wear track have significantly reduced the standard error of measurement. Surface coating techniques being developed to strengthen interfacial bonding

in dental cements and other dental materials show more promise with titanate than

with carbon reinforcing fibers.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Adhesive materials; composites; dental materials; polymer grafting; resins;

restorative materials; sealants; wear resistance.

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