Advanced Materials Research Vol. 812 (2013) pp 100-106 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.812.100
The Effect of Zeolite on the Crystallization Behaviour and Tribological Properties of UHMWPE Composite BOON PENG Chang1,a, HAZIZAN Md Akil1,3,b and RAMDZIAH Bt Md Nasir2,3,c 1
School of Materials and Mineral Resources Engineering, Engineering Campus Universiti Sains Malaysia, Nibong Tebal, Penang, 14300, Malaysia 2
School of Mechanical Engineering, Engineering Campus Universiti Sains Malaysia, Nibong Tebal, Penang, 14300, Malaysia 3
Cluster of Polymer Composite (CPC), Science and Engineering Research Centre (SERC), Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, 14300, Malaysia a
, [email protected]
(corresponding author), c [email protected]
Keywords: Zeolite, UHMWPE, Polymer Composite, Crystallinity, Tribology
Abstract. In this work, the effects of adding different filler loadings (5–20 wt%) of zeolite to the ultra-high molecular weight polyethylene (UHMWPE) matrix on the crystallinity behaviour and tribological properties were studied. The zeolite/UHMWPE composites were fabricated using hot compression moulding. The crystallization behaviour was investigated using differential scanning calorimetry (DSC). The tribological properties were monitored using a Ducom TR-20 pin-on-disc tester under different sliding speeds of 0.209 ms-1 and 0.419 ms-1 and with various applied loads of 5, 10, 15, 20, 25, 30 and 35 N. The worn surfaces of the zeolite/UHMWPE composites were observed under the scanning electron microscope (SEM). The results showed that the addition of zeolite into UHMWPE matrix can effectively enhance the percentage crystallinity of the UHMWPE. 15 wt% zeolite-reinforced UHMWPE composites show the increase of 47% in percentage crystallinity as compared to pure UHMWPE. The wear mass loss of the composites was found to be reduced by the incorporation of zeolite in UHMWPE. In addition, the average coefficient of friction (COF) was also found to decrease with the addition of zeolite. The lowest average COF was obtained by 20 wt% zeolite reinforcements into UHMWPE. Shallower grooves and smoother worn surfaces were observed for zeolite/UHMWPE as compared to pure UHMWPE. Introduction The popularity of ultra-high molecular weight polyethylene (UHMWPE) has raised when it was first used as potential materials for artificial joint implant applications in early 1960s. UHMWPE is a semicrystalline thermoplastic possess exceptional properties due to its extremely long chain entanglement. Among the prominent properties are its high toughness , chemical inertness and the highest wear resistance compared to other thermoplastics . To date, it has been widely used in engineering applications such as total joint replacements bearing , lining for dump trucks, bumpers and siding for ships  plus many more. Despite its outstanding properties, the long term wear problem occurs after certain service period still remain a challenge, especially for total joint replacements. The modification of UHMWPE to enhance their properties is currently a hot research topic. The regularly applied approach is by incorporation reinforcement filler into the UHMWPE matrix. In literature, many studies using various reinforcement incorporated with UHMWPE matrix in order to enhance its tribological properties. This includes the addition of carbon fiber , carbon nanotube , zirconium , hydroxyapatite , alumina , ZnO  and others. Inorganic materials, such as metal oxide, ceramic, and mineral, have attracted significant research interest as polymer filler due to their excellent mechanical properties, ability to induce electrical and optical properties in polymer. Zeolites are an inorganic material based on tetrahedral AlO4 and SiO4 aluminosilicate with micro-porous structure. Its unique micro-porous structure makes it important in applications such as catalysts, separation process and cation-exchange materials in the last decade. Recently, there is a significant amount of research using different types All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 184.108.40.206-27/08/13,03:47:09)
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of zeolite and clay minerals as particulate fillers, either natural (clinoptilolite, mordenite, chabazite), or synthetic (A-type, X-type, Y-type) reinforced into the polymer matrix [10-13]. Zeolite-reinforced polymer composite is able to improve the mechanical properties and crystallinity behaviour. Aksoy et al.  reported the mechanical properties of polyurethane films can be enhanced by adding zeolite beta particles. Zeolite-filled epoxy possesses significant improvements in mechanical properties have been reported by Lee et al. . In particular, zeolite also have potential as a nucleating agent in promoting nucleation in polymer [16, 17]. Zeolites are ceramic based materials which consist of silica and alumina elements. Ceramic materials are well known as high mechanical strength and anti-wear materials which often are the best choices as polymer fillers. Surprisingly, the use of zeolite in improving the tribological properties of polymer has not been reported before. Moreover, the non-toxic nature of zeolite is one of the advantages for it to potentially become a choice of materials that is widely employed in medical applications that has no hazardous side effects on human health . In this study, the response of different filler loadings (5-20 wt%) of zeolite-reinforced UHMWPE matrix on the crystalline and tribological behaviour were attempted. The tribological properties were investigated using pin-on-disc test rig with a variety of applied loads (5-35 N) and sliding speeds (0.209, 0.419 ms-1). The worn surfaces of pure UHMWPE and zeolite/UHMWPE composites were observed under scanning electron microscope (SEM). Experimental Materials. The UHMWPE grade GUR 4120 in powdered form with an average molecular weight of 5 × 106 gmol-1 and density of 0.93 gcm-3 was purchased from Ticona Engineering Polymer, China. Commercial Zeolite powder of less than 45 µm 99.9% was purchased from Sigma Aldrich, USA. Composite Preparation. The composites were produced by using hot compression molding machine (GT 7014-A30C, GOTECH Inc., Taiwan) with the compression pressure of 1000 psi (~6.90 MPa). Different filler loading of zeolite was mixed homogeneously with UHMWPE using a dry mechanical ball mill. The mixing process took 4 h to complete: 2 h each for both clockwise and anti-clockwise direction with the rotation speed of 30 Hz. After mixing, the compound was preheated at a temperature of 160°C for 10 min and hot pressed for 7 min. The composite sheet was obtained after cool pressing at 15ºC for 5 min. The composite sheets were then cut into an appropriate dimension for further testing. The composites designation was shown in Table 1. Differential Scanning Calorimetry (DSC). Thermal analysis of zeolite/UHMWPE was performed using differential scanning calorimeter DSC 6, Perkin Elmer, USA, under a nitrogen atmosphere. The sample weight was measured around 8-10 mg. The samples were heated from 30ºC to 200ºC with a heating rate of 10°C·min-1. Then hold for 1 minutes and subsequently cooled down to 30 ºC with the same rate of 10°C·min-1. The melting temperature (Tm) and enthalpy of fusion (H) of the samples was obtained from the DSC thermograms. The percentage crystallinity (PC) of zeolite/UHMWPE was derived from the ratio of area of melting peak under DSC heating curves to the enthalpy of melting of 100% crystalline of UHMWPE as shown in the Eq. 1 below: (1) where Hm is the enthalpy of melting peak of the sample, H100 is the enthalpy of melting for 100% crystalline of UHMWPE i.e. 291 J·g-1  and Wf is the weight percent of the filler loadings. Tribological Properties. The wear and frictional properties of the zeolite/UHMWPE were performed using Ducom TR-20 pin-on-disc tester under dry sliding conditions. The test samples were cut into a rectangular shape with dimensions of 9 × 9 × 30 mm. The abrasion area of the samples was 9 mm × 9 mm. Silicon carbide (SiC) abrasive paper of grade 1200 (grit size ~5 µm) was pasted on the rotating stainless steel disc surface using double-sided adhesive tape. The
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samples were abraded against the abrasive paper in dry sliding conditions for 600 s using different variations of applied loads (5, 10, 15, 20, 25, 30, 35 N) and sliding speeds (0.209, 0.419 ms-1). The weight loss of each sample after a wear test was measured using Shimadzu (AUW 220D) digital electronic balance with the decimal place of ± 0.01 mg accuracy. Worn Surface Analysis. The worn surfaces of the composites were observed using the Hitachi TM-3000 Tabletop Scanning Electron Microscope (SEM). All the samples’ surfaces were prior coated with gold/palladium (Au-Pd) layer by using a vacuum sputter chamber before scanning. Results and Discussion Effect of Zeolite on Thermal Properties. The DSC thermogram of the heating curves of pure UHMWPE and zeolite/UHMWPE are shown in Fig.1. It was observed that the melting temperature of the UHMWPE shows no significant changes upon the addition of zeolite. The total heat of fusion (H) for melting of zeolite/UHMWPE was found to increase with the addition of 5, 10 and 15 wt% of zeolite compared to pure UHMWPE as shown in Table 1. The H shows reduction for 20Z/UHMWPE composite. The percentage crystallinity (PC) of zeolite/UHMWPE increased by 28.3, 36.4 and 47.0% for 5, 10, 15 wt% respectively as compared to pure UHMWPE. The PC was reduced with the addition of 20 wt% of zeolite. In the studies conducted by Zhiping et al. , they found that the increase in degree of crystallinity for zeolite/PP is much more significant as compared to CaCO3/PP composites. The results show that the addition of zeolite has a significant role on promoting the crystallinity of UHMWPE. On this matter, the high surface polarity of zeolite acts as heterogeneous nucleating agents during the crystallization of UHMWPE, hence increasing H of zeolite/UHMWPE in return. However, the results took an opposite turn when the filler-filler interaction becomes more dominant in high zeolite loadings. Aggregation of filler will interfere the mobility of polymer chain during crystallization process and restrict the crystalline ability of the polymer. Therefore, the H and PC decreased under high filler loading conditions. Table 1: Melting temperature, enthalpy of fusion, and percentage crystallinity of pure UHMWPE and zeolite/UHMWPE composites. UHMWPE composites Pure UHMWPE 5 wt% Zeolite/UHMWPE 10 wt% Zeolite/UHMWPE 15 wt% Zeolite/UHMWPE 20 wt% Zeolite/UHMWPE
Tm (ºC) 0Z/UHMWPE 134.4 5Z/UHMWPE 134.3 10Z/UHMWPE 133.8 15Z/UHMWPE 133.8 20Z/UHMWPE 133.4
H (Jg-1) 92.7 113.9 114.7 116.8 92.4
PC (%) 32.1 41.2 43.8 47.2 39.7
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Fig.1: Heating curve of DSC thermograms for pure UHMWPE and zeolite/UHMWPE composites at different filler loadings. Effect of Zeolite on Tribological Properties. Fig.2 shows the weight loss due to wear and the average coefficient of friction (COF) of the zeolite/UHMWPE in comparison to pure UHMWPE as a function of applied load at different sliding speeds. A lower weight loss would directly translate into a higher wear resistance. In general, zeolite/UHMWPE show lower weight loss as compared to pure UHMWPE under both sliding speeds. Under sliding speed of 0.209 ms-1, 10Z/UHMWPE exhibits the lowest weight loss. On the other hand, at the sliding speed of 0.419 ms-1, the weight loss showed a minimum at 20Z/UHMWPE. The reduction in weight loss upon the reinforcement of zeolite into UHMWPE may be due to the increase of load-carrying capacity of the samples. Since the pure UHMWPE is relatively soft in nature, the incorporation of zeolite will make it become harder and the zeolite filler will support part of the stress from being directly exerted on the UHMWPE. Therefore, the wear resistance was increased.
Fig.2: Weight loss and average coefficient of friction as a function of applied load for pure UHMWPE and zeolite/UHMWPE under sliding speed of (a), (c), 0.209 ms-1 and (b), (d), 0.419 ms-1.
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Under both sliding speeds, the average COF of the zeolite/UHMWPE was lower as compared to pure UHMWPE. It can be seen that the average COF value was reduced with the increasing zeolite loading in UHMWPE. The lowest average COF is obtained for 20Z/UHMWPE for both sliding speeds. This may be due to the better transfer film forming by the deposited wear debris of the zeolite/UHMWPE on the counterface as compared to pure UHMWPE. A good transfer film will effectively cushion the counterface asperities from being directly contacted with the sample surfaces and reduce the frictional contact with the counterface. Hence, the average COF value was decreased after the addition of zeolite. The worn surfaces of pure UHMWPE, 10Z/UHMWPE and 20Z/UHMWPE under both 0.209 and 0.419 ms-1 sliding speeds at 35 N are shown in Fig.3. Grooves, furrow and plastic deformation caused by the micro-cutting and micro-ploughing of the sliding against SiC counterface were observed for all samples. This shows that abrasive wear and adhesive wear are the main wear mechanisms for all the samples. For UHMWPE, deep grooves and rough surfaces were observed (Fig.3 (a) and (d)). The severity of grooves and plastic deformation on the worn surface of UHMWPE was reduced by reinforcing it with zeolite. Shallower grooves and smoother surfaces are observed for both 5Z/UHMWPE and 10Z/UHMWPE as compared to pure UHMWPE under both sliding speeds (Fig.3 (b),(c),(e) and (f)). During the sliding wear test, the samples were subjected to sliding shear stress exerted by the applied load and hard asperities of the counterface. This will cause plastic deformation, grooves and furrow on the samples contacting surfaces and subsequently materials removed from the sample surface as wear debris deposited on the counterface. The addition of zeolite in UHMWPE could enhance the load-carrying capacity and led to the increase in micro-cutting and micro-ploughing resistance. Therefore, the zeolite-filled UHMWPE would prevent materials removal from the contacting surfaces and led to reduction in weight loss. The wear mechanism was transformed from adhesive wear and abrasive wear to mild adhesive wear and abrasive wear upon reinforcing zeolite into UHMWPE.
Fig.3: SEM micrographs of worn surfaces under 35 N applied load at 0.209 ms-1 for (a) 0Z/UHMWPE, (b) 10Z/UHMWPE, (c) 20Z/UHMWPE and at 0.419 ms-1 for (d) 0Z/UHMWPE, (e) 10Z/UHMWPE, (f) 20Z/UHMWPE. The arrow indicates the sliding direction. Conclusion The percentage crystallinity of UHMWPE was increased with the addition of zeolite. The 15 wt% zeolite/UHMWPE showed highest percentage crystallinity with the increment of 47% as compared to pure UHMWPE. The melting temperature of UHMWPE remained the same after the addition of zeolite. The tribological properties of UHMWPE were improved by the incorporation of zeolite. The average COF of the UHMWPE was reduced with the increase of zeolite loading. The
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addition of zeolite in UHMWE reduces the severity of the adhesive wear and abrasive wear. The improved crystallinity and tribological properties of zeolite/UHMWPE will be beneficial as a primary material alternative in the medical component application for years to come. Acknowledgements The authors gratefully acknowledge the Universiti Sains Malaysia Exploratory Research Grant Scheme (ERGS) and Polymer Composites Research Cluster Fund for the funding of this entire research. The grant numbers are 203/PBAHAN/673007, 1001/PKT/8640013 and 203/PMEKANIK/6071192. The first author would also like to thank Universiti Sains Malaysia (USM) fellowship program for the financial support. References  M.K. Steven, The UHMWPE Handbook Ultra-High Molecular Weight Polyethylene in Total Joint Replacement, Elsevier Academic Press, London, 2004.  H.L. Stein, Engineered Materials Handbook, Ultra High Molecular Weight Polyethylene (UHMWPE). Vol. 2, 1999. pp. 167-171.  G. Guofang, Y. Huayong, F. Xin, Tribological properties of kaolin filled UHMWPE composites in unlubricated sliding, Wear 256 (2004) 88-94.  X. Dangsheng, Friction and wear properties of UHMWPE composites reinforced with carbon fiber, Materials Letters 59 (2005) 175-179.  Y.S. Zoo, J.W. An, D.P. Lim, D.S. Lim, Effect of Carbon Nanotube Addition on Tribological Behavior of UHMWPE, Tribology Letters 16 (2004) 305-309.  K. Plumlee, C.J. Schwartz, Improved wear resistance of orthopaedic UHMWPE by reinforcement with zirconium particles, Wear 267 (2009) 710-717.  L. Fang, Y. Leng, P. Gao, Processing and mechanical properties of HA/UHMWPE nanocomposites, Biomaterials 27 (2006) 3701-3707.  D.S. Xiong, Wear properties of nano-Al2O3/UHMWPE composites irradiated by gamma ray against a CoCrMo alloy, Biomedical Materials 1 (2006) 175-179.  B.P. Chang, H. M. Akil, R. B. M. Nasir, Comparative study of micro- and nano-ZnO reinforced UHMWPE composites under dry sliding wear, Wear 297 (2013) 1120-1127.  J. Wen, J.E. Mark, Mechanical properties and structural characterization of poly(dimethylsiloxane) elastomers reinforced with zeolite fillers, Journal of Materials Science 29 (1994) 499-503.  F. Özmihçi, D. Balköse, S. Ülkü, Natural zeolite polypropylene composite film preparation and characterization, Journal of Applied Polymer Science 82 (2001) 2913-2921.  J.L. Acosta, E. Morales, M.C. Ojeda, A. Linares, The effect of interfacial adhesion and morphology on the mechanical properties of polypropylene composites containing different acid surface treated sepiolites, Journal of Materials Science 21 (1986) 725-728.  H. Pehlivan, D. Balköse, S. Ülkü, F. Tihminliogˇlu, Characterization of pure and silver exchanged natural zeolite filled polypropylene composite films, Composites Science and Technology 65 (2005) 2049-2058.  E.A. Aksoy, B. Akata, N. Bac, N. Hasirci, Preparation and characterization of zeolite beta– polyurethane composite membranes, Journal of Applied Polymer Science 104 (2007) 3378-3387.
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