Increasing the Interfacial Adhesion in Poly(methyl methacrylate)/carbon Fibre Composites by Laser Surface Treatment

Transcription

1 Increasing the Interfacial Adhesion in Poly(methyl methacrylate)/carbon Fibre Composites by Laser Surface Treatment A. Nematollahzadeh 1, S.A. Mousavi S. 1, R.M. Tilaki 2 and M. Frounchi 1 * 1 Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran 2 Department of Physics, Sharif University of Technology, Tehran, Iran Received: 4 October 2005 Accepted: 6 January 2006 SUMMARY The impact strength of poly(methyl methacrylate)/carbon (long) fibre composites for denture prosthesis applications was improved by fibre surface treatment. The carbon fibre surfaces were modified by Nd:YAG laser irradiation at 1064 nm wavelength. Laser light intensity was adjusted at 100 mj per pulse that only changed the fibre surface roughness and did not lead to fibre rupture, as verified by scanning electron microscopy. Increased surface roughness of the fibres improved the adhesion of poly(methyl methacrylate) to the fibre surface. Adhesion between the fibres and poly(methyl methacrylate) was evaluated by a tear-off method and by scanning electron microscopy. The results also suggest that laser irradiation enhances the wettability of the fibres by introducing active sites onto the fibre surfaces which react with the resin and impart stronger interfacial adhesion. INTRODUCTION Poly(methyl methacrylate) resin, PMMA, is the most widely used denture base material because of its good appearance and ease of processing. Nevertheless it may fail in use because of its unsatisfactory impact and fatigue strength 1-3. Various fibres such as glass, carbon 4-5, aramid 6-7 and ultra high molecular weight polyethylene fibres 8-10 have been used as reinforcements and it has been shown that these fibres increase the impact strength of the denture base polymer. An essential requirement for obtaining fibre-reinforced polymers with desired mechanical properties is good adhesion between the matrix polymer and the reinforcing fibres. It has been reported that chemical bonding or chemical affinity increases the impact strength of carbon fibre composites 11. Other reports have investigated the effects of the silane treatment of glass fibres on PMMA-glass *Corresponding author. frounchi@sharif.edu fibre composites intended for denture prosthesis applications They showed that silane-treatment was effective in bonding each fibre to the polymer matrix and in improving the composite mechanical properties. There has been a need to develop techniques to alter the fibre surfaces to overcome their poor wettability by PMMA resin and to improve fibrematrix adhesion. Significant improvements in interfacial bonding can be realised by the plasma surface treatment of fibres. Plasmas generally introduce active sites onto the fibre surfaces that are able to react with the resin or that essentially result in etching and oxidation of the surfaces. Nevertheless, controlling the surface temperature of the fibres during plasma treatments is difficult. In the present work the effect of Nd:YAG pulse laser irradiation on the surface of carbon fibres was investigated in a search for better adhesion between the carbon fibres and PMMA resin so as to achieve higher impact strength. Rapra Technology,

2 A. Nematollahzadeh, S.A. Mousavi S., R.M. Tilaki and M. Frounchi EXPERIMENTAL A conventional heat-curing powder-liquid (i.e. polymer/monomer) mixture type of denture base PMMA having a mass ratio of PMMA powder to liquid methyl methacrylate monomer (MMA) of 2.5 was used. Benzoyl peroxide at 0.5 wt% was added to the MMA as initiator. The mixture was cured in a stainless steel mould at 70 C for 30 min and then post-cured by heating in an oven at 100 C for 2 h. The dimensions of the samples were 127 mm 12.5 mm 6.25 mm. As for the reinforced samples, untreated or laser-treated carbon fibres were first impregnated with MMA monomer and then incorporated into the bulk of the resin mixture. The fibres were aligned parallel to the long axis of the test specimen. The fiber content was 1 volume percent equivalent to about 1.49 weight percent. For laser treated samples, carbon fibre strands were irradiated by a Nd:YAG pulse laser. The laser light wavelength was 1064 nm and each fibre strand was irradiated by five pulses with 100 mj energy per pulse. Test specimens were stored in distilled water at 22 C for 24 h before testing. The impact strength of the un-notched samples ( mm) was determined on a Charpy-type Zwick 5102 pendulum impact tester. The test was carried out with a pendulum rated at 5 J. Although the Charpy impact test is usually conducted on notched specimens, these specimens were not notched since this would have cut the superficial fibres. Ten specimens were used for each sample and the average of the ten results was reported. Adhesion of the carbon fibres to the heat-cured PMMA was determined by placing an untreated or laser treated carbon fibre bundle, which had been wetted with methyl methacrylate monomer, on the surface of the uncured PMMA resin (Figure 1). After complete curing of the PMMA in an oven, the carbon fibre bundle was torn off the PMMA specimen. A gold-sputtered fibre bundle was then examined with a scanning electron microscope (SEM) to investigate the interfacial adhesion between the fibres and PMMA. RESULTS AND DISCUSSION One of the factors that affects the interfacial adhesion in the composites is the fibre surface roughness. Surface irregularities can improve the fibre - polymer interaction by increasing the effective contact surface between polymer and fibres and consequently improving the mechanical properties of the composite. SEM micrographs of the commercial carbon fibres, as shown in Figure 2a, indicate some irregularities on the fibre surfaces. Laser ablation can further increase the surface irregularities for better interfacial adhesion, as shown in Figure 2b. The ablation process is usually accompanied by the creation of chemically active sites on the fibre surface which could form chemical bonds with the resin at the interface, contributing to stronger adhesion. Nevertheless, laser ablation should be controlled to prevent excessive damage to the fibres that might lead to premature rupture. Figure 1. Schematic diagram of the method used to determine the adhesion between carbon fibres and PMMA. After polymerisation of the monomer, the carbon fibre bundle was torn off and the surface analysed by SEM 586

3 Figure 2. SEM micrographs of carbon fibre surface a) before laser treatment; b) after laser treatment Figure 3. SEM micrographs of interface in PMMA/carbon fibre composite a) before laser treatment of fibres; b) after laser treatment The laser ablation mechanism can be explained as follows. The incident electromagnetic energy is rapidly converted into thermal energy on the fibre surface and the surface temperature rises sharply so that melting takes place. Then the molten carbon fibre absorbs further laser light and vaporises 14. Apart from physical surface modification, this kind of laser treatment is expected to activate the carbon fibre surface to provide good wettability of the fibre surface and thus increase polymer adhesion to the fibre. Figure 3 shows SEM micrographs of the PMMA/carbon fibre interface in reinforced samples before and after laser treatment. The presence of stronger interfacial adhesion between resin and fibres and less debonding of the fibres is shown by a comparison of Figure 3a with Figure 3b. Also improved fibre surface wettability after the fibre surface treatment by laser irradiation is verified by comparing Figure 4a with Figure 4b where it is clearly shown that the PMMA has spread effectively on the laser treated fibres. It is very important that the dental composite impact strength is sufficient to withstand sudden mechanical loads as the impact forces can be caused by an accident or chewing. The impact test results have been summarized in Table 1. As shown, the mean impact strength in those test groups reinforced by only 1 volume % laser treated carbon fibres was higher than that of the test groups reinforced by untreated fibres. 587

4 A. Nematollahzadeh, S.A. Mousavi S., R.M. Tilaki and M. Frounchi Figure 4. SEM micrographs of carbon fibres wetted by PMMA resin a) before laser treatment of fibre surface; b) after laser treatment Table 1. Impact strength of specimens reinforced with treated and untreated carbon fibres Samples Mean value ± Standard deviation (kj/m 2 ) Neat PMMA 24 ± 2.1 PMMA composite with laser treated fibres PMMA composite with untreated fibres 70 ± ± 6.3 This is further evidence of the effectiveness of the laser surface treatment in achieving stronger interfacial adhesion. In fact, laser ablation increases the number of fibre surface threads (the morphology of the laser treated surface is a thread-like morphology. The surface of the treated fibers in Figure 2b shows grooves and threads whilst the surface of untreated ones is quite smooth in Figure 2a.), and hence the frictional force between the fibres and polymer matrix. Another parameter affected by fibre surface modification is the impregnation of the fibres by the polymer. As mentioned earlier, after laser ablation good impregnation could enhance fibre wettability and improve the adhesion of the polymer particles to the fibre. In the present study, the impact strength of PMMA-carbon fibre composites was improved by increasing the fibre surface roughness by laser irradiation. Compared with other surface treatment methods, the advantages of the laser treatment method are the simplicity of composite manufacture, the high controllability of the treatment and the avoidance of chemicals, which is important in dental applications. CONCLUSIONS The impact strength of carbon fibre composite for denture prosthesis applications was improved by fibre surface treatment. Fibre surface was modified by Nd:YAG laser irradiation at 1064 nm wavelength. Laser light intensity was adjusted to change only the fibre surface roughness, avoiding fibre rupture. SEM microphotographs showed that laser light intensity at 100 mj per pulse only affected the fibre surface morphology. Fibre surface roughness increased after laser ablation and improved the polymer s adhesion to the fibre surface. Adhesion between the fibres and the PMMA was examined by a fibre tear-off method followed by SEM microscopy. SEM results showed that laser irradiation increased the fibre wettability and polymer particle adhesion. REFERENCES 1. Yazdanie, N. and Mahood, M., Carbon Fibre Acrylic Resin Composite: An Investigation of Transverse Strength, J Prosthet Dent, 54(4), (1985), Vallittu, P.K., Comparison of the In Vitro Fatigue Resistance of an Acrylic Resin Removable Partial Denture Reinforced with Continuous Glass Fibres or Metal Wires, J Prosthodont, 5, (1996),

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