The Role of Additives in Engineering Plastics

TheroleofadditivesinengineeringplasticsINTRODUCTIONThemostimportantrequirementofanyadditivesisthatitshouldbeeffective,forthepurposeforwhichithasbeendesigned,ataneconomiclevel.Improvementsinonepropertycan,however,leadtodeteriorationinothersand,consequentlyitistheoverallperformanceofanadditiveinagivenformulationthatdeterminesthefinalchoice.Theeffectivenessofcompoundingadditivesdependsonthecorrectprocedureofincorporationintothepolymermatrix.Theappropriatephysicalformoftheadditiveinthepolymermatrixdependsinturnonthemechanismbywhichitexertsitsfunction.Completecompatibility(i.e.mutualmiscibilityatmolecularlevel)andmobilityordiffusibilityoftheadditivemoleculeswithinthepolymermatrixareessentialiftheactionoftheadditiveissuchthatanyorallthemoleculesofthesystemaretointeractwitheachother.Totalincompatibilityandimmobilityoftheadditivemoleculesaredesirablewhentheadditiveexertsitsfunctionatasupermolecularlevel,i.e.theactiontheadditiveisderivedfromitsintrinsicphysicalpropertiesinthebulkormacroscopicform.Partialcompatibilityisrequiredwhenastrongaffinitybetweenpolymerandadditiveistobeexertedattheinterface.Thecompatibilityanddiffusibilityofadditivesinpolymericcompositionsisnormallyassessedby“trialanderror”methods;apracticewhichisstilllikelytobeusedforsometimeinthefuture.Theprincipalreasonforthissituationistobeattributedtothelackofsoundscientificalternatives.Theadditivemustneithervolatilizeoutofthematrixduringprocessingnorexudetothesurfaceduringservice.Thisimpliesthattheadditivemusthavealowvapourpressureathightemperaturesandmustnotaggregate,i.e.precipitateorcrystallizeoutofthepolymermatrix,onageing,andleavebehindafinefilmofadditivedeposits.Thisphenomenoniscalled“chalking”.Theadditivemustnotbeextractablebyliquidswithwhichthehostpolymericcompositionmaycomeintocontactduringfinishingoperations,neithermustitexudeoutduringitslifeinservice.Thetwolatterphenomenaareknownas“bleeding”and“blooming”respectively.Notonlywouldthesephenomenaproduceaestheticallyobjectionableeffectsandcontaminateliquidsandotherproductsincontactwiththeplasticscomponent,butthelossofadditivesfromthesystemwouldinevitablyreduceitsefficiency.Bleedingandbloomingphenomenaareobviouslyrelatedtothekineticsofdiffusionandconsequentlyaredependentonparameterssuchascompatibilityoftheadditivewiththepolymer,molecularsizeoftheadditive,physic-chemicalinteractionsbetweenadditiveandpolymermolecules,configurationofpolymerchainsandintermolecularvoidsetc.Henceinsolubleinorganicadditives,suchaspigments,fillersetc.areunlikelytobleedorbloom,whereassolublelowmolecularweightplasticizersaremorelikelytoexudetothesurfaceduringprocessingandsubsequentlyageingandmayevenconstituteavehicleforthemigrationofothersolubleadditivessuchasprocessingstabilizers.Anadditivemustnothaveanydamagingeffectsonthehealthofthepersonnelengagedincompoundingandprocessingoperationnoronthatoftheconsumer,especiallywhentheplasticscomponentisintendedforpackagingoffoodproductsorfortoys.Legislationisparticularlystringentwithrespecttoprotectionofconsumersagainsttoxiceffectsderivedfromextractedadditives.Additivesusedinplasticsarenormallyclassifiedaccordingtotheirspecificfunction.Itmaybeconvenienttoclassifythemintogroupsandtosubdividethemfurtheraccordingtotheirmoreprecisefunction:1.Additiveswhichassistprocessinga)Processingaidsb)Lubricants2.Additiveswhichmodifythebulkmechanicalpropertiesa)Plasticizersb)Fibrousreinforcementc)Non-fibrouspropertyenhancersd)Fillers/extenderse)Couplingagents3.Additivesusedtoreduceformulationcostsa)Particulatefillersb)Diluentsandextenders4.Surfacepropertiesmodifiersa)Anti-staticadditivesb)Slipadditivesc)Anti-wearadditivesd)Anti-blockingadditivese)Adhesionpromoters5.Opticalpropertiesmodifiersa)Colorants(Pigmentsanddyes)b)Nucleatingagents6.Anti-ageingadditivesa)Anti-oxidantsb)Stabilizers(Heat/Ultraviolet)c)Preservatives7.Othersa)Foaming(blowing)agentsb)Flameretardantsc)Smokesuppressants1.AntioxidantsMostpolymersaresubjecttoattackbyoxygenfromair.Therefore,mostrequiretheadditionofsomeantioxidanttominimizedegradationduringprocessing,storageandservice.Polymersdeterioratethroughthesenumerouscomplexchemicalreactions.Theinterrelationshipsamongheat,light,oxygenandpolymerstabilityarestillnotwellunderstood.Molecularweightischangedbymostofthesereactions,viachainscissionand/orcross-linking.Bothprocessesarethoughttobetakingplacesimultaneouslyinpolymers,butatdifferentratesdependingonthetemperatureandnatureofthepolymeritself.Chainscissionresultsinthelossofmolecularweight,increasedmeltflow,decreasedtoughness,reducedtensilestrengthandeventualpowderformation.Cross-linkingincreasemolecularweight,butnotnecessarilytoughness,exceptintheinitialstages;gellation,embrittlement,decreasedelongationanddecreasedmeltflowresult.Theresultisabroadeningofmolecular-weightdistributionandtheintroductionoflargequantitiesofoxygenintheformofhydroxyl,carbonyl,ester,andacidgroups.However,significantdeteriorationcanoccurwithlittlechangeinthesizeofthepolymerchain(s)and/orthemolecularweight.Polargroupsandcolorproducingunsaturatedsequencescanalsoformi