外文翻译关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究.docx

外文翻译关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究.docx

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外文翻译关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究

外文翻译---关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究

外文资料翻译

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,usabilityofmagnesiumoxidewithtitaniumdioxideinthePVCcompositionsforformingoftheexteriorarticlessuchasdoorandwindowprofileisinvestigat