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AstudyonusabilityofmagnesiumoxidewithtitaniumdioxideinPVCdoorandwindowprofiles
1.Introduction
Polyvinylchlorideisamongthemostwidelyusedsyntheticorganicpolymermaterials.Plasticizedpolyvinylchloridecompositionsarewidelyencounteredas,forinstance,vinylsheetgoodsandasobjectsformedfromplastisols.Polyvinylchlorideiscommerciallyavailableinavarietyofgrades,someofwhicharesuitableforpreparingrigid,plasticizer-freecompositionsforextrusion.
Forplastics,prolongedexposuretothesun’selectromagneticradiationintheultraviolet(UV)regioncanleadtophotooxdiationanddegradationofphysicalproperties,oftenmanifestedbycolorchangeandembrittlement.Similarly,theUVcomponentofordinaryfluorescentlightingcandegradepolymersandmanyoftheadditivesusedwiththem.
TheeffectiveUVradiationthatdoesreachtheearth’ssurfaceextendsfromabout290–400nm.ThisrangehappenstoincludethehighestenergycomponentUVband,andthesegmentaround300nm,whichisthemostdistractivetoplastics.Someman-madehigh-energyradiationsourcesmercuryarclamps,xenonarcs,carbonarcs,andvarioussun-lampscanemitradiationatwavelengthsbelow290nmandthesecandegradeplasticsevenmoreseverelythannaturalsunlight.Hence,theyareoftenusedforacceleratedtestingofplastics.
TheenergycontentofUVradiationinthe290–400nmcanrupturemostofthechemicalbondspresentinpolymerstructures.NotallthepolymersareequallyaffectedbyUVradiation,andsomehaveadegreeofresistance,otablypolymethylmethacrylatesandfluorocarbons.Others,thatintheirpureformscouldbeexpectedtoberesistanttoUV,aredegradedbecauseofcontaminantspresentthatactassitesforUVenergyabsorption.
Absorptionofradiationenergybypolymerproducesmolecularexcitations:
ifthelevelofabsorbedenergyishighenough,itcanactivateachemicalreactionwherebyinternalbonds(carbontocarbon,carbontohydrogen,carbontohalogen,etc.)arebrokensothatpolymerdegradationresults.PVCisdamagedbydehydrochlorination(releaseofhydrogenchloride),autooxidationandechanochemicalchainscission.Thisdegradationiscausedbythesimultaneoussequenceofthesereactions.
Dehydrochlorination,prevailingreactionduringprocessing,leadstoincreasingdiscoloration.Inthecourseoftheproceedingdegradationthephysicalpropertiesarealsochangedinthedirectionofincreasingembrittlement.PVCofidealconstitutionshouldbethermallystable,whichwasconcludedfrominvestigationswithmodelsubstances.Therefore,ithastobeassumedthatthedamage,articularlythedehydrochlorinations,startsfromsitesofthemacromoleculewithlabilechlorine–carbonbonds.PVCcanbedegradedbyheatandsunlights.Thereleaseofhydrogenchloride,whichistheindicationofPVCdegradationinprolongedexposuretothesun’selectromagneticradiationintheUVregion,isoccurredaccordingtothefollowingreactions:
ThecolorofPVC-basedarticleischangedfromyellowtoblackaccordingtodegreesofthedegradation.Oncethereactionhasstarted,polymersquicklyandprogressivelyexperiencechangesinappearance:
surfacequalities,gloss,chalking,color,electricalproperties,tensilestrengthandelongation;
andcanreachtheendpointsofembrittlementandtotaldisintegration.
ThedegradationofpolymersexposedtoUV,oftendescribedasphotodegradationandfrequentlyidentifiedasphotooxidation,canfollowvariousroutes.ByabsorbingUVradiationdirectly,apolymermoleculecanreachahigh-energyexcitedstatewhereitbecomesunstable.Iftheexcessenergycanbedissipatedinafashionthatdoesnotaffectthemoleculebymakingitphosphoresceorfluoresce,orbyconvertingtheenergytoheatthatcanbecarriedaway,orbytransferringtheenergytoanothermolecule,photochemicalreactiondoesnotstartedandthus,polymerdegradationwillnothappen.However,suchactionsoccuronlyrarely,sincemostpolymerscannotdisposeoftheexcitationenergywithoutundergoingachemicalreactionthatsetsoffadegradativeprocess.
Intheory,manypurepolymersshouldnotabsorbUVradiation,andthus,notbesubjecttophotodegradation.However,inpracticethemostpolymerscontainimpuritiessuchascarbonylorcarboxygroupsorhydroperoxidesthatreadilyabsorbradiationinthe290–400nmrangecausingthemtobreakdown.Thus,generatingsiteswithinthepolymerstructurewherechemicalreactionscanbeinitiatedandpropagatedbyfreeradicals.Theactivegroupsmaybeunavoidablypresentasaresultofreactionsthatoccurduringpolymerization.Similarly,metallicionsarepresentinmostpolymersasresiduesfrompolymerizationcatalysts,orasconstituentsofcompoundingadditivessuchasheatstabilizers,antioxidants,colorants,fillersandothers.ThemetalionsarehighlyreceptivetotheabsorptionofUVradiation,andareefficientintransferringtheabsorbedenergytothepolymermoleculesaroundthem,thus,theyactasphoto-sensitizersandcanpromotedegradationatthesametimethattheyperformtheirdesiredfunctions.
Anothercontributortophotodegradationofpolymersisoxygen,whichhelpsanyfreeradicalsthatmaybeliberatedbytheUVtoinitiateandpropagateoxidationofthepolymer,hence,thetermphotooxidation.
Polyvinylchloridesuffersfrompoorheatstability.Itsdegradationoccursbyautocatalyticdehydrochlorinationinitiatedatthelabilesitesinthepolymerchains.Thisleadstoseverediscolorationandlossofmechanicalproperties.Thedehydrochlorinationmostprobablyproceedsbyachainmechanisminvolvingradicalintermediates.VariousdefectsitesinPVCarebranching.
Inorganicandorganicthermalstabilizersarecommonlyaddedtoprotectthepolymerfromheatdegradation.Amongthemostwidelyusedultravioletstabilizersistitaniumdioxidepigment.Fillingapolyvinylchloridecompositionwiththispigmentsubstantiallyreducestheeffectivedepthofpenetrationofultravioletlightintothesurfaceofanarticleformedfromsuchacomposition.
Mohamedetal.pointedoutthatbarbituricacidandthiobarbituricacidarenontoxicorganics,thermallystablematerialsofhighmeltingpoint.Bothcontainactivemethylenegroups,andcanactasH-donorthroughtheirenolichydrogengroups,whichcanintervenewiththeradicalspeciesderivedfromthethermaldegradationofPVC.TheyinvestigatedthepossibilityofusingbarbituricacidanditsthioanalogueasthermalstabilizersforrigidPVC.
Theeffectivestabilizationoftenrequiresacombinationofantioxidantsysteminwhichcomplementaryoverlapofdifferentmechanisticpathwaysinvolved.Thisactoftenreferredtoassynergism,isthemotivationfortheuseofadmixingcompositionofdibutyltinmaleateandtrinitroanditsesterhomologues.ThestabilizationagentsofdibutyltinmaleateandtrinitroesterscouldretardsomewhatthephotodegradationofPVC.ItishopedthatthetotalstabilizingeffectofthisadmixedsystemshouldbegreaterthanthesumoftheindividualeffectswhenPVCissubjectedtoanenvironmentwheretheeffectsofheatandUVarecombined.Turotietal.investigatedtheeffectofthestabilizingactionofadmixedmixturesofdibutyltinmaleateandtrinitroanditsesterhomologuesonpolyvinylchlorideexposedtonaturalatmosphere.Intheirstudy,thedegradationandstabilizationreactionsweremonitoredbycolorformation,tensilestrengthandelongationatbreak,reducedviscosityaswellasdeterminationoftimetoembrittlement.ItisobservedthatthestabilizedPVCsamplehasaneffectivereductionindegradationreactions.
Titaniumdioxideisbyfarthemostimportantofwhiteinorganicpigmentsandpossessesall-roundsuitability.Whilerutiletitaniumdioxideishighlyreflectiveatvisiblewavelengths,itisalsohighlyabsorptiveatultravioletwavelengths.However,althoughtitaniumdioxideisahighlyeffectiveultravioletlightstabilizerforpolyvinylchloridecompositions,itdoeshaveseveralseriousdrawbacks.Animportantdisadvantageisthecostoftitaniumdioxidewhichhashistoricallytendedtobehighcomparedwithfillerorextenderpigmentssuchascalciumcarbonateandtalc.Anothersignificantdisadvantageofusingtitaniumdioxideasanultravioletstabilizerinunplasticizedpolyvinylchloridecompositionsisthathistoricallytitaniumdioxidehasbeenperiodicallyinshortsupply.
Therelativelyhighcostoftitaniumdioxideisanespeciallysignificantdisadvantageforthemanufactureofarticlesforexteriorusefromunplasticizedpolyvinylchloridecompositionsbecausesucharticlesmustoftenhavesubstantiallygreaterdimensions,forstructuralreasonsthantheeffectivepenetrationdepthofultravioletlightinthearticles.Thus,itishighlydesirabletoabletoreducetheleveloftitaniumdioxideinsuchacompositionwithoutexperiencinganaccompanyingincreaseintherateofdegradationandreductioninservicelife.AlthoughitseemstodecreasetheleveloftitaniumdioxideinthePVCcompositionwilltendtoincreasetheeffectivepenetrationdepthofultravioletlengthandwillconsequentlyacceleratethedegradationofthePVC,theexperimentalobservationsdonotsupportsuchanexpectation.SincePVCcompositionsconsistgenerallyoffromabout0.5–5partsbyweightofrutiletitaniumdioxideperhundredpartsbyweightofthepolyvinylchloride,thereisnoguaranteeforthebulkoftitaniumdioxidetolocateneartheexternalsurfacesofarticlesexposedtosunlights.
Inthisstudy,usabilityofmagnesiumoxidewithtitaniumdioxideinthePVCcompositionsforformingoftheexteriorarticlessuchasdoorandwindowprofileisinvestigatedintermsofdeterminingdi