Carbon_fiber_reinforced_composite6.pdf

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CHAPTER6Polymer-MatrixCompositesPolymersPolymer-matrixcompositesaremucheasiertofabricatethanmetal-matrix,carbon-matrix,andceramic-matrixcomposites,whetherthepolymerisathermosetorathermoplast.Thisisbecauseoftherelativelylowprocessingtemperaturesrequiredforfabricatingpolymer-matrixcomposites.Forthermo-sets,suchasepoxy,phenolic,andfurfurylresin,theprocessingtemperaturetypicallyrangesfromroomtemperaturetoabout200°C;forthermoplasts,suchaspolyimide(PI),polyethersulfone(PES),polyetheretherketone(PEEK),polyetherimide(PEI),andpolyphenylsulfide(PPS),theprocessingtempera-turetypicallyrangesfrom300to400°C.Thermosets(especiallyepoxy)havelongbeenusedaspolymermatricesforcarbonfibercomposites.Duringcuring,usuallyperformedinthepresenceofheatandpressure,athermosetresinhardensgraduallyduetothecompletionofpolymerizationandthecross-linkingofthepolymermolecules.Thermoplastshaverecentlybecomeimportantbecauseoftheirgreaterductilityandprocessingspeedcomparedtothermosets,andtherecentavailabilityofthermoplaststhatcanwithstandhightemperatures.Thehigherprocessingspeedofthermoplastsisduetothefactthatthermoplastssoftenimmediatelyuponheatingabovetheglasstransitiontemperature(T,)andthesoftenedmaterialcanbeshapedeasily.Subsequentcoolingcompletestheprocessing.Incontrast,thecuringofathermosetresinisareactionwhichoccursgradually.Epoxyisbyfarthemostwidelyusedpolymermatrixforcarbonfibers.TradenamesofepoxyincludeEpon,Epi-rez,andAraldite.Epoxyhasanexcellentcombinationofmechanicalpropertiesandcorrosionresistance,isdimensionallystable,exhibitsgoodadhesion,andisrelativelyinexpensive.Moreover,thelowmolecularweightofuncuredepoxideresinsintheliquidstateresultsinexceptionallyhighmolecularmobilityduringprocessing.Thismobilityenablestheresintoquicklywetthesurfaceofcarbonfiber,forexample.85 86CARBONFIBERCOMPOSITESEpoxyresinsarecharacterizedbyhavingtwoormoreepoxidegroupspermolecule.Thechemicalstructureofanepoxidegroupis:whereBe=benzenering.Forliquids,nisusuallylessthan1;forsolidresins,nis2orgreater.Thecuringofanepoxyresinrequiresacross-linkingagentand/oracatalyst.Theepoxyandhydroxylgroups(-OH)arethereactionsitesforcross-linking.Cross-linkingagentsincludeamines,anhydrides,andaldehydecondensationproducts.Inthecuringreaction,theepoxideringisopened(calledringscission)andadonorhydrogenfrom,say,anamineorhydroxylgroupbondswiththeoxygenatomoftheepoxidegroup.Ethylenediamineisanaminewhichservesasacross-linkingagent.EpoxidegroupsatEthyleneCross-linkformedtheendsoftwodiaminebetweentwolinearlinearepoxymoleculesepoxymoleculesAsnoby-productisgivenoffduringcuring,shrinkageislow.Themers(repeatingunits)oftypicalthermoplastsusedforcarbonfibersareshownbelow,whereBe=benzenering. Polymer-MatrixComposites87ThepropertiesoftheabovethermoplastsarelistedinTable6.1.Incontrast,epoxieshavetensilestrengthsof30-100MPa,moduliofelasticityof2.8-3.4GPa,ductilitiesof04%andadensityof1.25g/cm3[3].Thus,epoxiesaremuchmorebrittlethanPES,PEEK,andPEI.Ingeneral,theductilityofasemicrystallinethermoplastdecreaseswithincreasingcrystallinity.Forexample,theductilityofPPScanrangefrom2to20%,dependingontheTable6.1Propertiesofthermoplastsforcarbonfiberpolymer-matrixcomposites.PESPEEKPEIPPSPITg("C)230"170"225"86"256bDecompositiontemperature("C)550"590"555"527"550bProcessingtemperature("C)350"380"350a316"304bTensilestrength(MPa)84'70'105'66'138bModulusofelasticity(GPa)2.4'3.8'3.0"3.3"3.4bDuctility(%elongation)30-80'50-150'5M5'2'5bIzodimpact(ftlbhn.)1.6'1.6'1'