A Study on Impact and Tensile Strength of Acrylic Resin Filled with Short Ultra-High Molecular Weight Polyethylene Fibers

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 strengtheners(4) and also metal cages(5) have been applied in preparing denture bases for 50 years. Although dentures. Among these, ultra-high modulus UHMWPE fibers polymethylmethacrylates (PMMA) are known to be an have drawn the most attention because of their advantageous ideal base material, they possess some undesirable physical and chemical properties in addition to biological mechanical properties, especially their impact strength stability. and tensile strength, which appear to be unsatisfactory for The addition of high-modulus UHMWPE fibers in different some applications. Additives and fibers have therefore forms, such as continuous filaments, woven or chopped into been used to enhance and improve these properties over short fibers, for improving the impact strength of denture base the last two decades. The present article describes the polymethylmethacrylate has been reported in a number of mechanical properties, impact and tensile strength of papers(6-14). Two major problems were encountered in these PMMA reinforced with chopped ultra-high molecular studies: weakness of polymer fiber-base polymer adhesion and weight polyethylene fiber (6 mm long). It was found that, difficulty in mixing the acrylic dough in the presence of the although the processing involved for high loading of fibers fiber material. into the PMMA was difficult, the resulting improvement To overcome the weakness of the interfacial adhesion of impact strength was substantial. (J. Oral Sci. 41, 15-18, between UHMWPE fibers of all kinds short, continuous or 1999) woven various types of modifications have been carried out, including plasma treatment(7,12), incorporation of fibers Key words: short UHMWPE fibers; PMMA; tensile strength; through a peroxide initiator(9), coupling agents(9), and strong impact strength. 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 Introduction conflicting results have been reported. Although plasma- Acrylic resins, especially polymethylmethacrylates, have treated UHMWPE fibers were claimed to enhance both impact been widely used as base resins in dentistry over the last five and flexural strength(7), they appeared to have no effect on decades. These resins have a number of advantages, including impact strength or flexural modulus due to a change in the widespread use, ease of application and repair, esthetic morphology of the fibers because of plasma etching during the appearance with addition of colorant, and low cost. However, plasma treatment(12). Compared to short-cut and continuous some of their mechanical properties, particularly impact fibers, a marked increase in the impact resistance of multilayer- strength, are still unsatisfactory. In order to improve the woven, highly drawn linear polyethylene fibers has been mechanical properties of acrylic resins, various methods have reported(13,14). Although the second problem has been rarely been proposed. These have generally been aimed at mentioned, the dispersal of short fibers of UHMWPE higher strengthening the acrylic by addition of reinforcing materials. than 4% by weight has been reported to render acrylic dough Among these materials,the fibers of syntheticpolymers have unprocessable(12). On the other hand, woven and continuous become very popular since the 1950s, as described in detail UHMWPE appears to be easy to introduce into denture base by MacGregor(1) and Stafford(2). Attempts to enhance the material. properties of denture base resin escalatedwith the development In the present study, we attempted to evaluate and compare of high-modulus and high-strength fibers, such as aramid, the impact and tensile strength of denture base polymethyl- carbon and ultra-high molecular weight polyethylene methacrylate which was reinforced with various amounts of (UHMWPE). Besides the fiber materials used for this purpose, chopped UHMWPE fiber, ranging from 1% to 10% by weight, sapphire whiskers(3), metallic fibers such as steel without any chemical treatment or modification. To over- come the difficulty in corporating the fibers into the acrylic Correspondence to Dr. Arife Dokan, Department of Prosthodontics, material, we added some extra liquid MMA. Faculty of Dentistry, Gazi University, Ankara, Turkey 16

Materials and Methods fiber concentration, the highest impact resistance being at a UHMWPE fiber (Dyneema SK 66, DSM High-performance 10% UHMWPE fiber concentration. Fibers, B.V. Netherlands) and acrylic resin (QC 20 De Trey, Average values of ten tensile strength measurements and Dentsply, UK) were used in this work. The relevant properties their standard deviation are given in Table 2 and Fig. 2. It can of UHMWPE fiber, Dyneema SK 66 supplied by the be seen that after an initial drop in tensile strength, there was manufacturer were as follows; Modulus: 95 GPa, tensile a slow recovery up to a 5% UHMWPE fiber content. A further strength: 3000 MPa, and fiber density: 0.97 g/cm3. The polymer increase in UHMWPE fiber content drastically decreased the had a viscosity average molecular weight of 3,000,000. tensile strength. Long UHMWPE fibers chopped to a length of about 6mm SEM fractographs with different magnifications for the were applied to acrylic resins saturated with methylmethacrylate impact and tensile fractured surfaces are shown in Figs 3, 4,

(MMA) for 24 h. The acrylic resins containing 1 %, 2%, 3%, 5 and 6. Figure 3 shows the fractured surface in the control 4%, 5%, 7% and 10% (by weight) chopped UHMWPE fibers group (unfilled PMMA), and the other fractographs show the were prepared by mixing thoroughly at a powder-liquid ratio fractured surfaces of UHMWPE fiber-containing PMMA. of 3:1 in an agate mortar by hand. During this hand-mixing, Polyethylene fibers were apparently pulled out from the main

particularly after 4% addition of chopped UHMWPE, we matrix, PMMA, indicating weak adhesion between the acrylic added some extra liquid MMA to ease the mixing procedure. resin and UHMWPE. However, samples studied by SEM The addition of the extra MMA liquid did not change the showed the presence of weak adhesion between fibers and

powder-liquid ratio to more than 3:1.2 in the final dough, and PMMA (Figs 4 and 5), and also the fracture of fibers therefore it resulted in easy processibility and only a slight (fibrillation) in a transverse direction as a result of impact force change in composition which, we assumed, did not affect the (Fig. 6). final measured properties. These mixtures were then placed into specially prepared brass molds and pressed at 100 atm for Discussion 5 min to remove any voids. As mentioned previously, the difficulty in incorporating a The polymerization was carried out in two steps in a water high amount of UHMWPE into acrylic paste was partly bath according to the manufacturer; initially at 70•Ž for 24 h and finally in boiling water (in Ankara this corresponded to Table 1 Results of impact strength of UHMWPE-filled PMMA 96•Ž) for 20 min to achieve complete polymerization. Then, and control PMMA 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, Fig. 1 Variation of impact strength with UHMWPE while no statistical difference is seen in 1% and 2% UHMWPE fiber content (% weight) in PMMA resin. 17

Table 2 Results of tensile strength of UHMWPE-filled PMMA and control PMMA

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. 2 Variation of tensile strength with UHMWPE fiber content (% weight) in PMMA resin. Fig. 5 Tensile strength test of 10% UHMWPE fiber- containing sample observed by SEM (magnifica- tion •~ 900).

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

under SEM (magnification •~ 200).

Fig. 6 Impact fractured test sample of 7% UHMWPE overcome by addition of liquid MMA during blending of fiber-containing sample observed by SEM (mag- fibers and powder. The possibility of air bubbles and voids nification •~ 1000). in the dough during moulding was minimized by cold compression before the polymerization. Indeed, the standard obtained at 5% UHMWPE fiber content. One possible reason deviation even at a high concentration of UHMWPE fibers was may have been lack of proper mixing in this sample. However, found to be within the limits of experimental error of both static the mean impact strength of the 5% UHMWPE fiber-acrylic tests, and was generally lower than some previousy published composite did not alter the increasing trend of the sample data(1,13), with the exception of the 5% UHMWPE impact impact strength, as shown in Fig. 1. test results. A large scatter in impact measurements was The variation of the tensile strength with the fiber 18

concentration (Fig. 2) showed a sudden decrease with the concentration of UHMWPE fibers in the PMMA composite. initial addition of UHMWPE, and then a slow recovery up to Furthermore, it is worth mentioning that the incorporation of 5% UHMWPE, reaching a maximum value for filled acrylic a high amount of UHMWPE fibers into the dough by addition material. The last two concentrations of UHMWPE-filled of extra liquid MMA during mixing appears to be a very PMMA, 7% and 10%, resulted in the lowest values of tensile effective approach. strength, the tensile strength of 10% UHMWPE-filled acrylic being almost one third that of the original unfilled PMMA. In References tensile tests, the results were found to be more reproducible 1. MacGregor, A.R., Graham, J., Stafford, G.D. and in comparison with the impact strength. Huggett, R.(1984) Recent experiences with denture With the addition of UHMWPE fibers, the main matrix polymers. J.Dent. 12,146-157 continuityis immediately disturbed, and thus the stress transfer 2. Stafford, G.D., Huggett, R., MacGregor, A.R. and between these two materials, UHMWPE and PMMA, and Graham, J.(1986) The use of nylon as a denture-base also within the same materials, is apparently interrupted.Upon material. J.Dent. 14,18-22 application of tensile force, the reflection of this discontinuity 3. Grant, A.A. and Greener, E.H. (1967) Whisker and weak adhesion between PMMA and fibers, ends with a reinforcement of polymethyl methacrylate denture base decreasein tensilestrength at 1%UHMWPE loading.The slight resins. Aust.Dent.J. 12,29-33 improvement in the tensile strength after this initial drop may 4. Ruffino, A.R. (1985) Effect of steel strengtheners on be attributed directly to the UHMWPE fibers which may fracture resistance of the acrylic resin complete denture transfer the applied stress to a certain extent within themselves. base. J.Prosthet.Dent. 54,75-78 However, a decrease in tensile strength becomes inevitable at 5. Belfiglio, E.J. (1987) Using metal bases in making high loading. SEM fractographs (Figs 4 and 5) strongly complete dentures. J.Prosthet.Dent. 58,314-317 indicated both weak adhesion and pull-out of fibers from the 6. Ladizesky,N.H. and Ward, I.M. (1983) A study of the main matrix. adhesion of drawn polyethylene fibre/polymeric resin On the other hand, the impact strength seems to reflect systems. J.Mater.Sci. 18,533-544 another property of the material upon addition of a large 7. Braden, M., Davy, K.W.M., Parker, S., Ladizesky, amount of UHMWPE fibers. When impact testing is compared N.H. and Ward,I.M.(1988) Denture base poly (methyl with tensile testing, the measured impact values show a shock methacrylate) reinforced with ultra-high modulus energy-absorbingcapacity rather than a uniaxial forced drawing polyethylene fibres. Br.Dent.J. 164,109-113 condition on the material in a tensile test. It appears that the 8. Gutteridge,D.L. (1988) The effect of including ultra- shock-absorbing capacity increases almost linearly with high-modulus polyethylene fibre on the impact strength UHMWPE fiber load. The present resultsare in good agreement of acrylic resin. Br.Dent.J.164,177-180 at low fiber loading when compared with earlier published 9. Andreopoulos, A.G., Papaspyrides, C.D. and studies(8,9,12,14). Highly flexible and high modulus Tsilibounidis, S. (1991) Surface treated polyethylene UHMWPE fibers acting as shock absorbers caused a further fibres as reinforcement for acrylic resins. Biomaterials significant increase in the impact strength when their content 12,83-87 in PMMA was increased to a high amount, even in the absence 10. Clarke, D.A., Ladizesky, N.H. and Chow, T.W. (1992) of any chemical treatment. In fact, as shown in Fig. 6, some Acrylic resins reinforced with highly drawn linear fibers were fibrillated along their longitudinal axis and bore polyethylene woven fibres. 1. Construction of upper some polymer on their surfaces before they were pulled out denture bases. Aust.Dent.J. 37,394-399 of the PMMA matrix. Hence, the impact force, concentrated 11. Dixon, D.L. and Breeding, L.C. (1992) The transverse within a very short time in a very small volume, is absorbed strengths of three denture base resins reinforced with mainly by the randomly oriented UHMWPE fibers. It seems polyethylene fibers. J.Prosthet.Dent. 67,417-419 that even untreated UHMWPE may act effectively as a 12. Gutteridge, D.L. (1992) Reinforcement of poly (methyl substantial shock absorber, even though the tensile strength methacrylate) with ultra-high-modulus polyethylene falls to a very low value. fibre. J.Dent. 20,50-54 13. Ladizesky, N.H., Pang, M.K.M.,Chow, T.W. and Ward, Conclusion I.M. (1993) Acrylic resins reinforcedwith woven highly Although in dental applications tensile elongation does not drawn linear polyethylene fibres. 3. Mechanical reach the limit of failure strain, the results indicate that loss properties and further aspects of denture construction. of the tensile strength of UHMWPE fiber-filled PMMA Aust.Dent.J. 38,28-38 becomes an important issue. The main clinical issue, besides 14. Ladizesky, N.H., Chow, T.W. and Cheng, Y.Y. (1994) the fatigue problem in dental protheses, is that the impact Denture base reinforcement using woven polyethylene property seems to be substantially improved by increasing the fiber. Int.J.Prosthodont. 7,307-314