Chapter8FiberPropertiesandIdentificationFiberPropertiesandIdentificationMechanicalpropertiesTensilepropertiesTensiledeformationFiberPropertiesandIdentificationTensiledeformation:someconceptsElongation

StrainFiberPropertiesandIdentificationTensiledeformationLoad:ForceappliedtoextendthefiberUnfairtocompareforceappliedtofiberswithdifferentsizes.

StressEngineeringstressUnit:Pa=N/m2ordyne/cm2psi=pound/in2FiberPropertiesandIdentificationTensiledeformationFortextileyarnsandfibers,hardtodeterminecross-sectionareauselineardensity:gf/denierorN/texFiberPropertiesandIdentificationTensiletestingoffibersTestingconditions:ASTMstandardfortextiletesting:70°F(21°C)and65%relativehumidity(RH)Stress-straincurvesFiberPropertiesandIdentificationStress-straincurvesInitialmodulusTheslopeofthefirstsectionofthestress-straincurve(ratioofthestresstostrain)alsocalledYoung’smoduluscloselyfollowsHook’sLawsmallermodulus,easiertoelongateundersmallstresses.Fordifferentapplications,differentrequirements:ordinaryapparels:highinitialmoduluswomen’ssheerhosiery:lowinitialmoduluscomposites:extremelyhighinitialmodulusFiberPropertiesandIdentificationStress-straincurvesInitialmodulusFiberPropertiesandIdentificationStress-straincurvesYieldpointThepointatwhichthestress-straincurveflattensorchangesitsslopesignificantlyFiberPropertiesandIdentificationStress-straincurvesYieldpointThepolymers“yield”ormolecularchainsstarttomoveandpassoneanother.Thedeformationispermanent.Theslopeofthestress-straincurveafteryieldpointreliesonthestrengthofintermolecularforces.Mostfibershaveyieldpointbutnotcottonandflaxduetostrongintermolecularforces.FiberPropertiesandIdentificationStress-straincurvesHardeningpointThepolymermoleculesbecomemoreorientedasstrainincreasesandthefiberreachesadeformationlimit,wherethemoleculeshavethehighestorientationand(stress)hardeningoccurs.FiberPropertiesandIdentificationStress-straincurvesFailurepointWherethefiberfails.Sometimesnotnecessarythemaximumstresspoint.Dependsondefinition.FiberPropertiesandIdentificationStress-straincurvesTenacityStressatthepointofrupture.Stressatthemaximumload.Unit:N/tex,gf/denierFiberPropertiesandIdentificationStress-straincurvesStrainorelongationatbreak%ofstrainatthepointofrupture.Strainatthemaximumload.FiberPropertiesandIdentificationStress-straincurvesWorkofrupturePhysics:Work=Force′Distance=F

′

sIfF=f(s)isnotaconstantWhenchangeofdistanceDsisverysmall,Fisalmostaconstant,thusDw=FDsorwhenDs?

0,dw=f(s)dstherefore:

FiberPropertiesandIdentificationStress-straincurvesWorkofruptureAreaunderload-displacementcurve.SpecificworkofruptureAreaunderstress-straincurveFiberPropertiesandIdentificationElasticrecoverypercentofstrainbeingrecoveredwhenloadisreleased.Occursattheinitialpartofthestress-straincurvewheredeformationissmallandalmostnorelativemovementofmolecules.Energyabsorbedcanberecoveredcompletely.Elasticrecovery-ascross-link-

ˉasH-bondandionicbond-

ˉasstrainorstress-FiberPropertiesandIdentificationResilience(Workofrecovery)TheratioofenergyrecoveredtoenergyabsorbedMaybeextensional,flexural,compressionalortorsional.Isafunctionofdeformation.-ascrimpandcross-link-

ˉasH-bondandionicbond-ˉasstrain-Resilience-

?wrinkleˉe.g.PETvsCottonandRayonFiberPropertiesandIdentificationFlexuralrigidity(stiffness)HoweasyafibercanbebentDefinedasEIwhereE=Young’smodulusofthefiberI=MomentofinertiaofthefiberMathematically,FiberPropertiesandIdentificationFlexuralrigidity(stiffness)Thus:forafiberwitharoundcross-sectionFiberPropertiesandIdentificationAbrasionresistanceVerydifficulttomeasureDeterminedlargelybystructureofyarnandfabric.Affectedbytemperatureandmoisturecontente.g.NylonhasthehighestandacetatelowestFiberPropertiesandIdentificationFlexibilityDefinedasnumberoftimesafibercanbebent180°beforeitfails.Crystallinity

-,flexibilityˉFiberdiameter-,flexibilityˉFiberPropertiesandIdentificationTimerelatedmechanicalpropertiesCreep:underaconstantloadorstress,deformationorstrainincreaseswithtime.FiberPropertiesandIdentificationForceElongationTimeTimeelasticelongationcreepdeformationinstantstrainrecoverycreeprecoveryPermanentdeformationFiberPropertiesandIdentificationTimerelatedmechanicalpropertiesCreepA=elasticelongation(initial)B=creepdeformationC=instantstrainrecoveryD=creeprecoveryE=permanentdeformationFiberPropertiesandIdentificationTimerelatedmechanicalpropertiesStressrelaxationAfiberisloadedataconstantdeformation,observethestresschangewithtime.elongationTimeTimestressFiberPropertiesandIdentificationTimerelatedmechanicalpropertiesStressrelaxationAtaconstantdeformation,stressdecreaseswithtime.Decreaseofstressisduetothemolecularmovementinthestressedfibersuchasunentanglementofmolecules.Therateofrelaxationisdependentonfactorsliketemperature,intermolecularforces,stiffnessofmolecules.FiberPropertiesandIdentificationSorptionpropertiesTermsHydrophilic:waterlovingorlikeswaterHydrophobic:wateravoidingHygroscopic:hydrophilicfibersthatabsorbssignificantamountofwaterormoisturewithoutfeelingwettotouch.Oleophilic:howtenaciouslyfibersholdabsorbedoradsorbedoil,notamountofoilabsorbed.Oleophobic:doesnothaveastrongaffinityforoil,butreadilyreleasesoilinadetergentsolution.FiberPropertiesandIdentificationSorptionpropertiesVaporouswaterabsorption%MoistureregainFiberPropertiesandIdentificationSorptionpropertiesVaporouswaterabsorption%MoisturecontentFiberPropertiesandIdentificationSorptionpropertiesVaporouswaterabsorptiondependsontemperatureandrelativehumidityaswellasatmosphericpressure.FiberPropertiesandIdentificationSorptionpropertiesrelativehumiditytheamountofmoisture(watervapor)beingheldinairdividedbythemaximumamountofwatervaporcanbeheldinair.leth=massofwater/unitvolumeofairandhs=massofwaterwhenairissaturatedatthesametemperatureandpressureFiberPropertiesandIdentificationSorptionpropertiesImportanceofmoistureregainweightoftextilematerialschangeofmechanicalpropertiescomfortconsiderationsWhyafibrousmaterialabsorbsmoremoisture?morepolargroupsorH-bondsinamorphousregions.FiberPropertiesandIdentificationSorptionisothermRHMoistureregain0ABCFiberPropertiesandIdentificationSorptionisothermSegmentA:H2Oattachestopolargroupsinamorphousphase.Fastabsorptionduetoavailabilityoflargenumberofpolargroups.SegmentB:H2Ostillattachestopolargroupsonlybutlessofthemavailableslowerabsorption.SegmentC:H2OattachestopolargroupsaswellasH2Obeingabsorbedpreviouslyinthecapillaryporesofthefiber,alsobreakingupofH-bondsopensthespaceunavailablebeforefasterabsorption.FiberPropertiesandIdentificationSorptionisothermNote:WaterabsorptioninamorphousregionsonlySignificantamountofmoistureabsorptionwiththeexistenceofH-bondonlyPropertiesofafiberchangewithitsmoisturecontent.Absorption-desorptioncurvesaredifferent:sorptionhysteresisexistsFiberPropertiesandIdentificationSorptionisothermRelativeHumidityMoistureregain0desorptionabsorptionFiberPropertiesandIdentificationAbsorptionofliquidwaterSwellingUponabsorptionofwater,fibersswelllargerdiameterandlongerwhichdirectionswellsmoredependsonmolecularorientationFiberPropertiesandIdentificationAbsorptionofliquidwaterMechanicalpropertychangeduetowaterabsorptionTenacityingeneral,withH-bonds,tenacityasmoistureregainbutforcottonandflaxorsomeothernaturalcellulosefibers,theoppositeistrueduetostructuralcharacteristics.Noeffectonfibersthatdonotabsorbwater.FiberPropertiesandIdentificationAbsorptionofliquidwaterMechanicalpropertychangeduetowaterabsorptionModulusingeneral,withH-bonds,modulusasmoistureregainbecausewaterbreaksH-bonds(workslikeaplasticizer).Nissan’sequation:E=E0exp(a-bM).Wetinitialmodulus:importantforlaundry,e.g.rayonNoeffectonhydrophobicfibers.HeatofwettingWettingofafiberisanexothermalreaction:heatreleasedwhenfibersabsorbwater.Expressedascaloriespergram(cal/g)orJoulespergramofdryfiber.Amountofheatisproportionalto#ofH-bondsandnatureofH-bonds.Highforhygroscopicfibers;lowfornon-hygroscopicfibersHeatofwettingInfluencescomfortRHincreases,absorbmorewater;increasetemperature;feelwarmer.RHdecreases;releasewater;decreasetemperature;feelcooler.Oilabsorptionorrelease(Oleophilicity)Fibersabsorbthemostoilarenotnecessarythosethatholditmosttenaciously.Ingeneral,fibersabsorbmoreoilalsotendtoreleaseiteasily:

e.g.FiberswithH-bondsabsorbmoreoilbutreleaseiteasilyMostsyntheticfibersdonotabsorbasmuchoilbutdoholdittenaciouslyduetotheirlowsurfaceenergysuchthatwatercannotwettheirsurfaceeasily.ThermalPropertiesSpecificheatandthermalconductivitySpecificheat:ameasurementoftheamountofheatrequiredtochangethetemperature1°Cofaunitmassofthefiber.AcharacteristicofthematerialNotasimportantasheatofwettingthatinvolvesalotmoreheat.Thermalconductivity:ameasureofrateofheatflowthroughfibers(notasinglefiberbutamassoffibers)Unit:Calofheat/cm2offibrousloftHeatResistanceHeatresistanttemperature:atwhichafiberstartstodegrade,evidencedasdecreaseofmechanicalproperties(weakerandbrittle)SofteningandmeltingGlasstransitiontemperature:(Tg)polymericfibersallhaveatemperatureabovethatafiberchangesfromaglassystatetoarubberystate(amorphousphasestartto“melt”).Meltingtemperature:(Tm)atwhichthecrystallinephasemeltsormoleculesincrystallinephasestarttomovearound.Decompositiontemperature:atwhichpolymermoleculesdecomposeintosmallmolecules.SofteningandmeltingNoteTgisextremelyimportantforperformanceofafiber.TmandTgdependonintermolecularforces,structure,configurationandstiffnessofthemolecule.MeltingtemperatureexistsonlywhenthepolymerhascrystallineregionandadecompositiontemperaturehigherthanTm.FiberswithTmorasofteningtemperaturearethermoplasticfibers.CombustibilityConditionsforcombustion:availabilityoffuel(combustiblevolatiles)availabilityofO2heatprovided:ignitiontemperatureformostofthefibers:780-1000F,415°C–538°CCombustibilityNoncombustiblefibersnofuelorcompletelyoxidizedorextremelystablestructuresCombustiblefibersnormalflameresistanteitherinherentlyormodified,containingstablestructuresorCl,P,BrorFthatchangesdecompositionprocessesCombustibilityMeasurementofflammabilityLimitingoxygenindex%