Short Aramid Fibre Reinforced Ultra-High Molecular Weight Polyethylene Composite. A new hip-prosthesis Hofsté, Joanna Maria

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1 Short Aramid Fibre Reinforced Ultra-High Molecular Weight Polyethylene Composite. A new hip-prosthesis Hofsté, Joanna Maria IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 1997 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Hofsté, J. M. (1997). Short Aramid Fibre Reinforced Ultra-High Molecular Weight Polyethylene Composite. A new hip-prosthesis s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Summary Total joint replacements have improved the quality of life of thousands of people over the last quarter of this century. Debilitating disease such as osteoarthritis (disorder due to excessive wear and tear of joint surfaces) and rheumatoid arthritis (subacute or chronic form of inflammatory of the joint), avascular necrosis (bloodless death of tissue), bone cancer, and trauma can be treated by using prostheses. The surgery eliminates pain and patients regain mobility and functionalíty of their joints. The first total hip replacement (THR), made of stainlessteel, was performed in 1938 by P.W. Wiles. About 25 years later, in 1962, the first THR was implanted using Ultra-High Molecular Polyethylene (UHMWPE) for the acetabular cup of the hip joint and is nowaday still used as bearing material in joint replacements. The combination of materials used in THR is very crucial for its long-term performance. Many combinations have been investigated and used during the last years. The acetabular cup can be made from UHMWPE, cobalt chromium alloy, alumina ceramic or carbon fibre reinforced polyethylene. The femoral head can be made from stainlessteel, cobalt chromium alloy, alumina ceramic, coated metal, or zirconia ceramic. lt is very important that the material combination gives a low friction coefficient and extremely low wear, since the wear debris can cause a lot of problems, like chronic inflammatory reaction, bone resorption, and loosening. Although UHMWPE is considered biocompatible, the wear debris that is released into the tissue surrounding the jornt can cause a chronic inflammatory reaction. lt seems that the polyethylene wear debris retrieved from tissues surrounding the joint have many similarities with the particle structures observed in the virgin powder from which polyethylene components are made by extrusion or moulding processes. UHMWPE has an extremely high viscosity caused by the high molecular weight of the material. This high melt viscosity prevents welding, even for compression moulding. The result of the incomplete welding of the powder are boundaries which can act as crack initiatorso that wear is accelerated. More prostheses are being implanted in younger and more active patients. This implies that the requirements of UHMWPE are becoming more severe. Due to this'new' patience group, the required revision surgery is nowadays more than 30 percent (!). lt is becoming evidenthat, the surgical procedure is only successful if the prosthesis correctly located without any infection of the tissue surrounding the implant and the wear properties are limited. To extend the life-time of the artificial joint, it is necessary to decrease the wear rate, which in turn reduces the number and

3 106 Summary volume of debris generated, so that the prostheses can be used in younger patients more successfully. It is known that the incorporation of fibres into a polymer may improve its wear resistance depending on the test conditions. Fibre reinforcement is most effective in the range of lower speeds, higher contact pressure and smooth counterparts, conditions that are the most favourable for joint prostheses. In this explorative study a few factors which may influence the wear behaviour of a composite have been investigated. These factors are the kind of fibre, fibre content, fibre distribution, fibrematrix adhesion, and processing temperature and pressure. In Chapter 1 a general introduction is given about THR, like the history, materials used in the THR, and the problems that nowadaystill occcur with THR. In Chapter 2 the mechanical properties, like the modulus, ultimate tensile strength, and strain at break of short aramid fibre reinforced UHMWPE with respect to the fibre content, aspect ratio, and processing conditions are discussed. The ultimate tensile strength, ou, and strain at break, e' are the most important material properties that influence the volume wear, ÀV, according to the experimental Ratner- Lancaster correlation, AV cc (o, e,)-t. The tensile strength and strain at break of the aramid fibre reinforced UHMWPE composite decrease with fibre content, but are hardly influenced by the aspect ratio, processing pressure and temperature. A problem of the aramid fibre reinforced UHMWPE composite is the difference in thermal expansion coefficient. Because of this microcavities are observed around the fibres and the fibres act as network defects, leading to a decrease in strength and strain at break of the composite with fibre content. The modulus of the aramid fibre reinforced UHMWPE composite increases with fibre content, aspect ratio, and processing pressure and temperature. Although the two most important properties which influence the volumetric wear rate, the ultimate tensile strength and strain at break, are hardly influenced by the fibre aspect ratio, processing pressure and temperature, the wear behaviour of UHMWPE improves by the incorporation oí the aramid fibres. Obviously, there are more important factors which influence the wear behaviour of a composite. This is discussed in Chapter 3. It seems that besides the ultimate tensile stress and strain at break, the hardness and the friction coefficient are important factors that affect the wear behaviour of a polymer. lt is found that the wear rate of UHMWPE decreases by the incorporation of aramid fibres. Since the wear behaviour of the composite is determined by competing effects, (i) a positiv effect due the probability of stress transfer, increase in hardness, and decrease of the friction coefficient, and (ii) a negativ effect due to the decrease of (orer), a minimum in wear rate is observed at

4 Summary volume percent aramid fibres. The wear resistance of UHMWPE increases by a factor 2.5 for the composites tested at a contact pressure of 6 MPa. Since it was found that the processing temperature and pressure have a marked influence on the mechanical properties of the composite, is likely that the wear behaviour of the composite is also influenced by the processing conditions. This is described in Chapter 4. The processing pressure and temperature are crucial for the final properties due to (i) the reduction of microcavities, (ii) the welding of the UHMWPE granules, and (iii) the mrnimization of the volume change on cooling. lt is observed that the wear resistance of the composite increases with processing pressure and temperature. A composite pressed at a pressure of 25 MPa and a temperature of 225 oc has a wear resistance of 2.5 x 108 Nm/mm3, while a composite pressed at a pressure of 900 MPa and a temperature of 300 oc has a wear resistance of 7.7 x 108 Nm/mm3. Thus, the wear resistance increases with a factor 3. In Chapter 5 the influence of triboelectrification on the formation of short fibre composites is discussed. lt is found that an attraction between the fibres and powder particles is important in order to achieve a composite with a proper fibre distribution. In the case of UHMWPE and aramid fibres enough surface charges are generated by triboelectrification. Since UHMWPE powder mixed with Poly(ethylene terephthalate) (PET) fibres hardly gives rise to electrical charge, no composite with a proper fibre distribution can be made. This problem can be solved by charging the PET fibres and UHMWPE particles with corona before the mixing step or even better by using oxidized UHMWPE. Due to surface oxidation polar groups are generated on the surface which can form hydrogen bonds between the fibres and UHMWPE particles. Chaoter 6 describes the ínfluence of oxidized UHMWPE on the mechanical and tribological of short aramid Íibre reinforced UHMWPE composites. lt is found thai both properties are enhance due to the surface modification. The modulus, yield stress, and stress at break are enhanced by 33 %o, 17 o/o, and 9 %, respectively. The wear resistance increases with 117 %. Thus, although a respectively small mechanical improvement is observed, the wear resistance increases by a factor 2. lt is thoughthat these improvements are caused by a better fibre/matrix adhesion and a more homogeneous fibre distribution. ln Chapter 7 the mechanical and tribological behaviour of short UHMWPE fibre reinforced UHMWPE composites are discussed. The fibres and matrix are of the same chemical nature which probably promote good bonding at the interface, which is essential for its properties. The mechanical properties are indeed improved enormously. The modulus and ultimate tensile strength of a 60 volume percentage composite show an improvement of 600 % and 160 %, respectively, in comparison

5 108 Summary with pure UHMWPE. The wear rate decreaseslightly by the incorporation of UHMWPE fibres. The wear behaviour of this composite is determined by two competing effects; (i) a positive effect due to stress transfer to the fibres and (ii) a negative effect due to the reduction in toughness (orer) with fibre content. Since the enormous decrease in toughness overbalances the positive effect of stress transfer, the wear rate of UHMWPE is hardly iníluenced by the UHMWPE fibres. Thus, although the fibre-matrix bonding of the UHMWPE fibre reinforced UHMWPE composite is much better than in the aramid fibre reinforced UHMWPE composite, the wear rate of the latter is much lower than the former. The kind of fibres used in a composite is very crucial for its wear behaviour. In Chapter 8 the wear behaviour of short Poly(ethylene terephthalate) (PET) fibre reinforced UHMWPE composite is determined. lt has been claimed that the PET fibre has a better wear resistance than the aramid fibre. However, in thrs study it has been found that the wear rate of the PET fibre reinforced UHMWPE composites is higher than that of the aramid fibre reinforced UHMWPE composites. In a composite the fibre-matrix adhesion is very crucial for its properties. At the interface stress is transferred from the matrix to the fibres. The Appendix describes the chemical modification of the aramid fibres with isocyanates, like hexanediisocyanate and oleylisocyanate, to improve the adhesion between the aramid fibres and the UHMWPE matrix. The hexanadiisocyanate modified fibres are further modified with oleylamine. The aliphatichains chemically bonded to the fibre surface may react with the UHMWPE matrix with dicumylperoxide. On account of the modification, the charge generation on the surface, which is important to achieve a composite wrth a proper fibre distribution, altered, and so no homogeneous composite could be made. On account of the inhomogeniteity the standar deviation of the mechanical properties is large. lt is known that there is a large difference in thermalexpansion coefficient, leading to microcavities around the fibres. Thus, if there is bonding between the fibres and matrix, caused by the chemical modification of the aramid fibres, debonding will occur on cooling due to the large shrinkage. Altogether, it can be said that the incorporation of 5 volume percent aramid fibres improves the wear resistance of UHMWPE by a factor 2.5. With higher processing temperature and pressure and oxidized UHMWPE the wear resistance improve still further by a factor 3 and 2, respectively. Thus, it may be fair to say that if the lifetime of a hip prosthesis only determined by the wear behaviour of UHMWPE, the life{ime can be extended from 15 to 225 years.