Natural-Latex天然胶乳-胶乳英文ppt-橡胶

NaturalLatexControlandIndustrialProceduresEmilyLiuMaterialScienceNaturalLatexA.IntroductionB.ControlProceduresIntroductionCentrifugedlatexisprocessedintofinalarticlesbydipping,extruding,andcoating.Differentarticleshavedifferentphysicalandchemicalrequirements,whichmeansthatthelatexusedintheirmanufacturemustbemodifiedbytheadditionofagreatvarietyofsubstancesthatgiveeachformulationitsuniquecharacteristics.Thesecharacteristicsarewell-specifiedandmustbecarefullycontrolled.ExampleofaControlSheetforLATZLatexAnexampleofacontrolsheetforthetypeofnaturalrubberlatexthatiscommonlyusedinmakingsurgicalglovesisgivenintheTable1.ThepurposeofthisPPT：Weexplainbrieflyhowsomeofthesecharacteristicsarecontrolled.ControlProcedures3456DryRubberContent(DRC)SurfaceTensionpHMeasurementMechanicalStabilityKOHNumberTotalSolidContent(TSC)Viscosity271ControlProcedures1.DeterminationoftheTotalSolidContent(TSC)ofLatex•AddyourtitleinhereTheTSCoflatexisobtainedbydryingasampleoftheliquidlatextobeassayedandweighingtheresidue.Thevalueobtainedisthesumofthedryrubbercontentplusthesolidnonrubbercontents,whichincludestheprotein.Latexthatisusedintheproductionofgloves,catheters,andsoonhasaTSCof60~62%.TheDRCoflatexisobtainedbycoagulatingtherubberinthesampleoflatextobeassayedwithaceticacid,thendrying,washingandweighingtheresultingmassofrubber.ThedifferencebetweentheTRCandtheDRCisameasureoftheofnaturallatex.ThedifferencebetweentheTRCandtheDRCisameasureofthetotalnonrubbercontentsofthelatex.Theselatterconstituentsareprincipallylatexproteins,lipids,andmineralsalts.InthecaseofHAandLATZlatexes,thosearecommonlyusedintheproductionofgloves,thenonrubberconstituentsmakeupbetween1%and2%ofnaturallatex.2.DeterminationoftheDryRubberContent(DRC)ofLatex3.MeasurementofSurfaceTensionDeterminationofthesurfacetensionoflatexisaveryusefulmeasure,assurfacetensionisafunctionof:1.Theamountofsoapaddedtostabilizethelatex;2.Thenaturalsoapsformedafterconcentrationfromfattyacidsinthelipidphaseofthelatex;3.Themineralsalts(mg/Na)thatarepresentinthelatex.Alowsurfacetensionvalueindicatesgoodstability,althoughforuseindippingsystemsthiscouldleadtoatendencytoformwebsofrubber.ThepHoffreshlatexisclosetoneutrality.Itwillchangemoreorlessrapidlyowingtothepresenceofmicroorganismsandenzymes,leadingtoprogressiveacidificationofthelatexandeventuallytospontaneouscoagulation.Becauseammoniaisaddedtopreservethelatex,thatis,topreventcoagulation,pHdeterminationisimportant,asthestabilityoftheliquidlatexdependstoagreatdegreeonitspH.4.pHMeasurementInaddition,ammoniahasanindirectinfluenceonstability,increasingcolloidalstabilitythroughtheformationoffattyacidsurfactants.WhenthepHofalatexislessthan9.5,whichmayoccur,especiallyinthecaseofLAlatex,thenthepHmustbewatchedveryclosely,frequentlyhavingtobeincreasedchemicallywhenitisreceivedandsiphonedatitsfinaldestination.TheKOHnumberisameasureoftheconcentrationofacidradicalsthatwouldhavebeenpresentasfreeacidswereitnotforthepresenceofammoniathathasconvertedthemtoammoniumsalts.Sincesomeoftheseacidsareproducedbymicroorganisms,apoorlypreservedlatex—onewithtoomanymicroorganisms—willtendtohaveahighKOHnumber.AhighKOHnumberisnot,however,necessarilyindicativeofapoorlypreservedlatex,becausethistestdoesnotdistinguishbetweentheammoniumsaltsoftheacidswhoseanionsdestabilizelatexandthosethatenhancestability.5.KOHNumberTheKOHnumberis,nevertheless,anindexofthequalityofthelatex.AhighKOHnumberisusuallytakenasevidenceofdegradationofthelatexowingtohydrolysisofproteinsoccurringaftertoolongaperiodinstorage.Forexample,averyfreshlatexarrivinginEuropeafter6-10wkintransportfromtheFarEastmighthaveaKOHnumberof0.04-0.05,whereasanagedlatexwouldhaveaKOHnumberof0.70-0.8onarrival.TheKOHnumberofcentrifugedorcreamedlatexesmustnotexceed1.0.Mechanicalstabilityisdefinedastheresistanceofthelatextoflocculationduringstirringoragitation.Itismeasuredbythemechanicalstabilitytime(MST)testinwhichlatexisstirredrapidly(14,000rpm)understandardizedconditionsandthetimerequiredforvisualflocculationtooccurisnoted.6.MeasurementofMechanicalStabilityExpressedinseconds,thistestallowsthebehaviorofthelatextobeevaluatedduringthecourseofthedifferentoperationsitundergoesduringitstransformationintoafinishedproduct.Themechanicalstabilityofnaturallatexshouldbe≥650s.Mechanicalstabilitytendstodecreaseduringlongperiodsoftransportationandstorageowingtotheslowhydrolysisofthestabilizersbytheammonia.Itisthereforenecessarytobeveryprudentwhenanaturallatexisreceivedandfoundtohavelowmechanicalstability,forexample,600s.Suchalatexissusceptibletotheformationofgrainsandwillbefilterablewithdifficulty.Manufacturersgenerallyavoidthisproblembyaddingstabilizerssuchasananionicfattyacidalcohol,forexample,laurylsulfate.Amongthephysico-chemicalpropertiesoflatex,viscosityisoneofthemostimportantwithrespecttotheproductionandprocessingoflatexproducts.Theviscosityofafluidistheratiooftheshearstresstotheshearrateduringitsflow.7.ViscosityMostlatexesexhibitdecreasingviscositywithincreasingshearrateandforthisreasonanyreferencetolatexviscosityshouldincludeadescriptionofth