Journal of Oral Science, Vol. 41, No. 1, 15-18, 1999

A study on impact and tensile strength of acrylic resin filled

with short ultra-high molecular weight polyethylene fibers

Babur Taner, Arife Dogan, Teoman Tincer•˜ and All Erkan Akinay•˜

Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey§Department of Chemistry, Middle East Technical University, Ankara, Turkey

(Received 25 May 1998 and accepted 8 February 1999)

Abstract: In dentistry, acrylates have been used for

preparing denture bases for 50 years. Although polymethylmethacrylates (PMMA) are known to be an ideal base material, they possess some undesirable mechanical properties, especially their impact strength and tensile strength, which appear to be unsatisfactory for some applications. Additives and fibers have therefore been used to enhance and improve these properties over the last two decades. The present article describes the mechanical properties, impact and tensile strength of PMMA reinforced with chopped ultra-high molecular weight polyethylene fiber (6 mm long). It was found that, although the processing involved for high loading of fibers into the PMMA was difficult, the resulting improvement of impact strength was substantial. (J. Oral Sci. 41, 15-18, 1999)

Key words: short UHMWPE fibers; PMMA; tensile strength; impact strength.

Introduction Acrylic resins, especially polymethylmethacrylates, have

been widely used as base resins in dentistry over the last five decades. These resins have a number of advantages, including widespread use, ease of application and repair, esthetic appearance with addition of colorant, and low cost. However, some of their mechanical properties, particularly impact strength, are still unsatisfactory. In order to improve the mechanical properties of acrylic resins, various methods have been proposed. These have generally been aimed at strengthening the acrylic by addition of reinforcing materials.

Among these materials, the fibers of synthetic polymers have become very popular since the 1950s, as described in detail by MacGregor(1) and Stafford(2). Attempts to enhance the

properties of denture base resin escalated with the development of high-modulus and high-strength fibers, such as aramid, carbon and ultra-high molecular weight polyethylene

(UHMWPE). Besides the fiber materials used for this purpose, sapphire whiskers(3), metallic fibers such as steel

strengtheners(4) and also metal cages(5) have been applied in dentures. Among these, ultra-high modulus UHMWPE fibers

have drawn the most attention because of their advantageous

physical and chemical properties in addition to biological stability.

The addition of high-modulus UHMWPE fibers in different

forms, such as continuous filaments, woven or chopped into short fibers, for improving the impact strength of denture base

polymethylmethacrylate has been reported in a number of

papers(6-14). Two major problems were encountered in these studies: weakness of polymer fiber-base polymer adhesion and difficulty in mixing the acrylic dough in the presence of the

fiber material.

To overcome the weakness of the interfacial adhesion

between UHMWPE fibers of all kinds short, continuous or woven various types of modifications have been carried out,

including plasma treatment(7,12), incorporation of fibers

through a peroxide initiator(9), coupling agents(9), and strong acid oxidation(6,7). These modifications improved the impact

and flexural properties of UHMWPE-acrylic denture

composites to a certain extent. However, in some cases

conflicting results have been reported. Although plasma-

treated UHMWPE fibers were claimed to enhance both impact and flexural strength(7), they appeared to have no effect on

impact strength or flexural modulus due to a change in the

morphology of the fibers because of plasma etching during the

plasma treatment(12). Compared to short-cut and continuous fibers, a marked increase in the impact resistance of multilayer-

woven, highly drawn linear polyethylene fibers has been

reported(13,14). Although the second problem has been rarely mentioned, the dispersal of short fibers of UHMWPE higher

than 4% by weight has been reported to render acrylic dough

unprocessable(12). On the other hand, woven and continuous UHMWPE appears to be easy to introduce into denture base

material.

In the present study, we attempted to evaluate and compare

the impact and tensile strength of denture base polymethyl-

methacrylate which was reinforced with various amounts of chopped UHMWPE fiber, ranging from 1% to 10% by weight,

without any chemical treatment or modification. To over-

come the difficulty in corporating the fibers into the acrylic

material, we added some extra liquid MMA.Correspondence to Dr. Arife Dokan, Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey

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Materials and Methods

UHMWPE fiber (Dyneema SK 66, DSM High-performance

Fibers, B.V. Netherlands) and acrylic resin (QC 20 De Trey,

Dentsply, UK) were used in this work. The relevant properties

of UHMWPE fiber, Dyneema SK 66 supplied by the

manufacturer were as follows; Modulus: 95 GPa, tensile

strength: 3000 MPa, and fiber density: 0.97 g/cm3. The polymer

had a viscosity average molecular weight of 3,000,000.

Long UHMWPE fibers chopped to a length of about 6mm

were applied to acrylic resins saturated with methylmethacrylate

(MMA) for 24 h. The acrylic resins containing 1 %, 2%, 3%,

4%, 5%, 7% and 10% (by weight) chopped UHMWPE fibers

were prepared by mixing thoroughly at a powder-liquid ratio

of 3:1 in an agate mortar by hand. During this hand-mixing,

particularly after 4% addition of chopped UHMWPE, we

added some extra liquid MMA to ease the mixing procedure.

The addition of the extra MMA liquid did not change the

powder-liquid ratio to more than 3:1.2 in the final dough, and

therefore it resulted in easy processibility and only a slight

change in composition which, we assumed, did not affect the

final measured properties. These mixtures were then placed

into specially prepared brass molds and pressed at 100 atm for

5 min to remove any voids.

The polymerization was carried out in two steps in a water

bath according to the manufacturer; initially at 70•Ž for 24 h

and finally in boiling water (in Ankara this corresponded to

96•Ž) for 20 min to achieve complete polymerization. Then,

the molds were left to cool to ambient temperature before

being opened. The samples were polished with P360A grade

waterproof silicon carbide paper (English Abrasives Ltd.,

London, UK) for impact and tensile testing.

Impact testing (Plastic Impact Machine 414, Hounsfield,

USA) was carried out using a specimen measuring 60 x 7.5 x

4.0 mm. For every filler loading and blank PMMA, at least

ten tests were carried out. The results were tabulated in N/m

and compared with the control group.

The tensile test specimens were 60 mm long and 2 •~ 2 mm

in cross-section. An Instron 1102 tensile testing machine

(Instron, USA) was used at a draw rate of 5 mm/min with an

initial gauge length of 35 mm. The tensile tests were done at

ambient temperature (23•Ž). The results of tensile testing at

sample failure of were evaluated in terms of MPa units.

Fractured surfaces of the specimens were investigated by

scanning electron microscopy (SEM, JSM 6400, Japan) at

different magnifications after the specimens had been gold-

coated.

Student's t test was applied for the statistical analysis of both

impact and tensile test results.

Results

The average and standard deviation of the impact strength

tests of acrylic resin filled with UHMWPE fiber samples are

presented in Table 1 and Fig. 1 . These results represent ten

measurements. According to these results, a linear relationship

exists between fiber concentration and impact resistance,

while no statistical difference is seen in 1% and 2% UHMWPE

fiber concentration, the highest impact resistance being at a

10% UHMWPE fiber concentration. Average values of ten tensile strength measurements and

their standard deviation are given in Table 2 and Fig. 2. It can

be seen that after an initial drop in tensile strength, there was

a slow recovery up to a 5% UHMWPE fiber content. A further increase in UHMWPE fiber content drastically decreased the

tensile strength.

SEM fractographs with different magnifications for the

impact and tensile fractured surfaces are shown in Figs 3, 4, 5 and 6. Figure 3 shows the fractured surface in the control

group (unfilled PMMA), and the other fractographs show the fractured surfaces of UHMWPE fiber-containing PMMA.

Polyethylene fibers were apparently pulled out from the main

matrix, PMMA, indicating weak adhesion between the acrylic

resin and UHMWPE. However, samples studied by SEM

showed the presence of weak adhesion between fibers and PMMA (Figs 4 and 5), and also the fracture of fibers

(fibrillation) in a transverse direction as a result of impact force (Fig. 6).

Discussion As mentioned previously, the difficulty in incorporating a

high amount of UHMWPE into acrylic paste was partly

Table 1 Results of impact strength of UHMWPE-filled PMMA

and control PMMA

Fig. 1 Variation of impact strength with UHMWPE

fiber content (% weight) in PMMA resin.

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overcome by addition of liquid MMA during blending of

fibers and powder. The possibility of air bubbles and voids in the dough during moulding was minimized by cold

compression before the polymerization. Indeed, the standard

deviation even at a high concentration of UHMWPE fibers was

found to be within the limits of experimental error of both static

tests, and was generally lower than some previousy published data(1,13), with the exception of the 5% UHMWPE impact

test results. A large scatter in impact measurements was

obtained at 5% UHMWPE fiber content. One possible reason

may have been lack of proper mixing in this sample. However,

the mean impact strength of the 5% UHMWPE fiber-acrylic

composite did not alter the increasing trend of the sample

impact strength, as shown in Fig. 1.

The variation of the tensile strength with the fiber

Table 2 Results of tensile strength of UHMWPE-filled PMMA

and control PMMA

Fig. 2 Variation of tensile strength with UHMWPE

fiber content (% weight) in PMMA resin.

Fig. 3 Pure acrylic resin fractured (tensile fracture) surface

under SEM (magnification •~ 200).

Fig. 4 Tensile fractured surface of 5% UHMWPE fiber-

containing sample under SEM (magnification •~

700). The main appearance of PMMA remained

unchanged while UHMWPE fibers were pulled-

out of the main matrix.

Fig. 5 Tensile strength test of 10% UHMWPE fiber-

containing sample observed by SEM (magnifica-

tion •~ 900).

Fig. 6 Impact fractured test sample of 7% UHMWPE

fiber-containing sample observed by SEM (mag-

nification •~ 1000).

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concentration (Fig. 2) showed a sudden decrease with the initial addition of UHMWPE, and then a slow recovery up to 5% UHMWPE, reaching a maximum value for filled acrylic material. The last two concentrations of UHMWPE-filled PMMA, 7% and 10%, resulted in the lowest values of tensile strength, the tensile strength of 10% UHMWPE-filled acrylic being almost one third that of the original unfilled PMMA. In tensile tests, the results were found to be more reproducible in comparison with the impact strength.

With the addition of UHMWPE fibers, the main matrix continuity is immediately disturbed, and thus the stress transfer between these two materials, UHMWPE and PMMA, and also within the same materials, is apparently interrupted. Upon application of tensile force, the reflection of this discontinuity and weak adhesion between PMMA and fibers, ends with a decrease in tensile strength at 1% UHMWPE loading. The slight improvement in the tensile strength after this initial drop may be attributed directly to the UHMWPE fibers which may transfer the applied stress to a certain extent within themselves. However, a decrease in tensile strength becomes inevitable at high loading. SEM fractographs (Figs 4 and 5) strongly indicated both weak adhesion and pull-out of fibers from the main matrix.

On the other hand, the impact strength seems to reflect another property of the material upon addition of a large amount of UHMWPE fibers. When impact testing is compared with tensile testing, the measured impact values show a shock energy-absorbing capacity rather than a uniaxial forced drawing condition on the material in a tensile test. It appears that the shock-absorbing capacity increases almost linearly with UHMWPE fiber load. The present results are in good agreement at low fiber loading when compared with earlier published studies(8,9,12,14). Highly flexible and high modulus UHMWPE fibers acting as shock absorbers caused a further significant increase in the impact strength when their content in PMMA was increased to a high amount, even in the absence of any chemical treatment. In fact, as shown in Fig. 6, some fibers were fibrillated along their longitudinal axis and bore some polymer on their surfaces before they were pulled out of the PMMA matrix. Hence, the impact force, concentrated within a very short time in a very small volume, is absorbed mainly by the randomly oriented UHMWPE fibers. It seems that even untreated UHMWPE may act effectively as a substantial shock absorber, even though the tensile strength falls to a very low value.

Conclusion Although in dental applications tensile elongation does not

reach the limit of failure strain, the results indicate that loss of the tensile strength of UHMWPE fiber-filled PMMA becomes an important issue. The main clinical issue, besides the fatigue problem in dental protheses, is that the impact

property seems to be substantially improved by increasing the

concentration of UHMWPE fibers in the PMMA composite. Furthermore, it is worth mentioning that the incorporation of a high amount of UHMWPE fibers into the dough by addition of extra liquid MMA during mixing appears to be a very effective approach.

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