University of Groningen Short Aramid Fibre Reinforced Ultra-High Molecular Weight Polyethylene Composite. a New Hip-Prosthesis H

View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by University of Groningen

University of Groningen

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): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.

Download date: 12-11-2019 Summary

Totaljoint replacements have improved the qualityof lifeof thousandsof peopleover the lastquarter of thiscentury. Debilitating diseases such as osteoarthritis(disorder due to excessivewear and tear of jointsurfaces) and rheumatoidarthritis (subacute or chronicform of inflammatoryof thejoint), avascular necrosis(bloodless death of tissue),bone cancer, and trauma can be treatedby usingprostheses. The surgeryeliminates pain and patientsregain mobility and functionalítyof theirjoints. The firsttotal hip replacement(THR), made of stainlesssteel, was performed in 1938by P.W.Wiles. About 25 yearslater, in 1962,the firstTHR was implanted usingUltra-High Molecular Polyethylene (UHMWPE) for the acetabularcup of the hip jointand is nowadaysstill used as bearingmaterial in jointreplacements. The combinationof materialsused in THR is verycrucial for its long-termperformance. Manycombinations have been investigated and usedduring the lastyears. The acetabularcup can be madefrom UHMWPE, cobalt chromium alloy, alumina ceramicor carbonfibre reinforced polyethylene. The femoral head can be madefrom stainlesssteel, cobalt chromium alloy, alumina ceramic, coated metal, or zirconia ceramic.lt is veryimportant that the materialcombination gives a lowfriction coefficientand extremelylow wear, since the weardebris can cause a lot of problems,like chronic inflammatory reaction, bone resorption, and loosening. AlthoughUHMWPE is consideredbiocompatible, the weardebris that is released intothe tissuesurrounding the jornt can cause a chronicinflammatory reaction. lt seemsthat the polyethylenewear debris retrieved from tissues surrounding the joint havemany similarities with the particlestructures observed in the virginpowder from whichpolyethylene components are madeby extrusionor mouldingprocesses. UHMWPEhas an extremelyhigh viscosity caused by the highmolecular weight of the material.This high melt viscosity prevents welding, even for compression moulding.The resultof the incompletewelding of the powderare boundarieswhich canact as crackinitiators so thatwear is accelerated. Moreprostheses are beingimplanted in youngerand moreactive patients. Thisimplies that the requirementsof UHMWPEare becomingmore severe. Due to this'new'patience group, the requiredrevision surgery is nowadaysmore than 30 percent(!). lt is becomingevident that, the surgicalprocedure is onlysuccessful if the prosthesisis correctlylocated without any infectionof the tissuesurrounding the implantand the wearproperties are limited.To extendthe life-timeof the artificial joint,it is necessaryto decreasethe wearrate, which in turnreduces the numberand 106 Summary volumeof debrisgenerated, so thatthe prosthesescan be usedin youngerpatients moresuccessfully. It is knownthat the incorporationof fibresinto a polymermay improveits wear resistancedepending on the testconditions. Fibre reinforcement is mosteffective in the rangeof lowerspeeds, higher contact pressure and smoothcounterparts, conditionsthat are the mostfavourable for joint prostheses. In thisexplorative study a few factorswhich may influence the wearbehaviour of a compositehave been investigated.These factors are the kindof fibre,fibre content, fibre distribution, fibre- matrixadhesion, and processingtemperature and pressure. In Chapter1 a generalintroduction is givenabout THR, like the history, materialsused in the THR,and the problemsthat nowadays still occcur with THR. In Chapter2 the mechanicalproperties, like the modulus,ultimate tensile strength,and strainat breakof shortaramid fibre reinforced UHMWPE with respect to the fibrecontent, aspect ratio, and processingconditions are discussed. The ultimatetensile strength, ou, and strain at break,e' arethe mostimportant material propertiesthat influence the volumewear, ÀV, according to the experimentalRatner- Lancastercorrelation, AV cc(o, e,)-t.The tensile strength and strainat breakof the aramidfibre reinforced UHMWPE composite decrease with fibre content, but are hardlyinfluenced by the aspectratio, processing pressure and temperature. A problemof the aramidfibre reinforced UHMWPE composite is the differencein thermalexpansion coefficient. Because of thismicrocavities are observedaround the fibresand the fibresact as networkdefects, leading to a decreasein strengthand strainat breakof the compositewith fibre content. The modulusof the aramidfibre reinforcedUHMWPE composite increases with fibre content, aspect ratio, and processingpressure and temperature. Although the two mostimportant properties whichinfluence the volumetricwear rate, the ultimatetensile strength and strainat break,are hardlyinfluenced by the fibreaspect ratio, processing pressure and temperature,the wearbehaviour of UHMWPEimproves by the incorporationoí the aramidfibres. Obviously, there are moreimportant factors which influence the wear behaviourof a composite.This is discussedin Chapter3. It seemsthat besides the ultimatetensile stress and strainat break,the hardnessand the frictioncoefficient are importantfactors that affectthe wear behaviourof a polymer.lt is foundthat the wearrate of UHMWPEdecreases by the incorporationof aramidfibres. Since the wearbehaviour of the compositeis determinedby competingeffects, (i) a positiveeffect due the probabilityof stress transfer,increase in hardness,and decrease of the frictioncoefficient, and (ii)a negativeeffect due to the decreaseof (orer),a minimumin wearrate is observedat Summary 107

5 volumepercent aramid fibres. The wear resistance of UHMWPEincreases by a factor2.5 for the compositestested at a contactpressure of 6 MPa. Sinceit was foundthat the processingtemperature and pressurehave a markedinfluence on the mechanicalproperties of the composite,it is likelythat the wearbehaviour of the compositeis alsoinfluenced by the processingconditions. Thisis describedin Chapter4. The processingpressure and temperature are crucial for the finalproperties due to (i)the reductionof microcavities,(ii) the weldingof the UHMWPEgranules, and (iii) the mrnimization of the volume change on cooling.lt is observedthat the wearresistance of the compositeincreases with processing pressureand temperature.A compositepressed at a pressureof 25 MPaand a oC temperatureof 225 hasa wearresistance of 2.5 x 108Nm/mm3, while a oC compositepressed at a pressureof 900MPa and a temperatureof 300 hasa wearresistance of 7.7 x 108Nm/mm3. Thus, the wearresistance increases with a factor3. In Chapter5 the influenceof triboelectrificationon the formationof shortfibre compositesis discussed.lt is foundthat an attractionbetween the fibresand powder particlesis importantin orderto achievea compositewith a properfibre distribution. In the caseof UHMWPEand aramid fibres enough surface charges are generated by triboelectrification.Since UHMWPE powder mixed with Poly(ethylene terephthalate) (PET)fibres hardly gives rise to electricalcharge, no compositewith a properfibre distributioncan be made.This problem can be solvedby chargingthe PETfibres and UHMWPEparticles with corona before the mixingstep or evenbetter by using oxidizedUHMWPE. Due to surfaceoxidation polar groups are generated on the surfacewhich can form hydrogen bonds between the fibresand UHMWPEparticles. Chaoter6 describesthe ínfluenceof oxidizedUHMWPE on the mechanical andtribological of shortaramid Íibre reinforced UHMWPE composites. lt is foundthai bothproperties are enhanceddue to the surfacemodification. The modulus,yield stress,and stressat breakare enhanced by 33 %o,17 o/o,and 9 %, respectively.The wearresistance increases with 117 %. Thus,although a respectivelysmall mechanicalimprovement is observed,the wear resistance increases by a factor2. lt is thoughtthat these improvements are caused by a betterfibre/matrix adhesion and a morehomogeneous fibre distribution. ln Chapter7 the mechanicaland tribological behaviour of shortUHMWPE fibrereinforced UHMWPE composites are discussed. The fibres and matrixare of the samechemical nature which probably promote good bonding at the interface,which is essentialfor its properties.The mechanicalproperties are indeedimproved enormously.The modulusand ultimatetensile strength of a 60 volumepercentage compositeshow an improvementof 600 % and 160%, respectively,in comparison 108 Summary withpure UHMWPE. The wearrate decreases slightly by the incorporationof UHMWPEfibres. The wearbehaviour of thiscomposite is determinedby two competingeffects; (i) a positiveeffect due to stresstransfer to the fibresand (ii) a negativeeffect due to the reductionin toughness(orer) with fibre content. Since the enormousdecrease in toughnessoverbalances the positiveeffect of stresstransfer, the wearrate of UHMWPEis hardlyinÍluenced by the UHMWPEfibres. Thus, althoughthe fibre-matrixbonding of the UHMWPEfibre reinforced UHMWPE compositeis muchbetter than in the aramidfibre reinforced UHMWPE composite, the wearrate of the latteris muchlower than the former. The kindof fibresused in a compositeis verycrucial for itswear behaviour. In Chapter8 the wearbehaviour of shortPoly(ethylene terephthalate) (PET) fibre reinforcedUHMWPE composite is determined.lt hasbeen claimed that the PETfibre hasa betterwear resistance than the aramidfibre. However, in thrsstudy it has been foundthat the wearrate of the PETfibre reinforced UHMWPE composites is higher thanthat of the aramidfibre reinforced UHMWPE composites. In a compositethe fibre-matrixadhesion is verycrucial for its properties.At the interfacestress is transferredfrom the matrixto the fibres.The Appendix describes the chemicalmodification of the aramidfibres with isocyanates, like hexanediisocyanateand oleylisocyanate,to improve the adhesionbetween the aramidfibres and the UHMWPEmatrix. The hexanadiisocyanatemodified fibres are furthermodified with oleylamine. The aliphaticchains chemically bonded to the fibre surfacemay reactwith the UHMWPEmatrix with dicumylperoxide. On accountof the modification,the chargegeneration on the surface,which is importantto achievea compositewrth a properfibre distribution, is altered,and so no homogeneous compositecould be made.On accountof the inhomogeniteitythe standarddeviation of the mechanicalproperties is large.lt is knownthat there is a largedifference in thermalexpansioncoefficient, leading to microcavitiesaround the fibres.Thus, if thereis bondingbetween the fibresand matrix,caused by the chemicalmodification of the aramidfibres, debonding will occur on coolingdue to the largeshrinkage. Altogether,it can be saidthat the incorporationof 5 volumepercent aramid fibresimproves the wearresistance of UHMWPEby a factor2.5. With higher processingtemperature and pressureand oxidized UHMWPE the wearresistance improvesstill further by a factor3 and2, respectively.Thus, it maybe fairto saythat if the lifetimeof a hip prosthesisis onlydetermined by the wearbehaviour of UHMWPE,the life{imecan be extendedfrom 15 to 225years.