聚合物和添加剂

8.POLYMERSAND

THEIRADDITIVESThetermmacromolecule,orpolymer,isappliedtosubstancesofhighmolecularweightthatarecomposedofalargenumber(usuallyatleast100)ofunitsoflowmolecularweightjoinedbycovalentbonds.Ifthelowmolecularweightunitsmakingupthemacromoleculearebondedend-to-endinalongchainandnocovalentchemicalbondsexistbetweenthechains,themacromoleculesarecalledlinearpolymers.Suchpolymers,unlessofextremelyhighmolecularweight(1,000,000),canusuallybedissolvedand,whenheated,theysoftenormeltsothattheycanbeextrudedintofibersormouldedintodesiredshapes.Thesepolymersaresaidtobethermoplastic.Ontheotherhand,ifthepolymerchainsarelinkedtogetheratnumerouspoints,thepolymerisonelargethree-dimensionalmolecule,infusibleandinsoluble.Suchpolymersarecalledcross-links.Certainlinearpolymers,referredtoasthermosetting,containgroupswhich,whenheated,reacttogivecross-linkedpolymers.1Theprocessbywhichsmallmoleculesundergomultiplecombinationtoformmacromoleculesispolymerization.Smallmoleculesfromwhichamacromoleculeorpolymercanbemadearecalledmonomers.Twotypesofpolymerizationarerecognized:(1)condensationpolymerizationand(2)additionpolymerization.Apolymer-formingreactioninvolvingeliminationofasmallmoleculesuchaswateroralcoholbetweenmonomerunitsisdescribedascondensationpolymerization.2Inadditionpolymerization,unsaturatedorcyclicmoleculesaddtoeachotherwithouteliminationofanyportionofthemonomermolecule.Theempiricalformulaofthepolymeristhen,ofcourse,thesameasthatofthemonomer.Reactionscapableofformingmacromoleculesbyeitheradditionorcondensationpolymerizationmustbefunctionallycapableofproceedingindefinitely.Whenevertwomonomermoleculesreact,theproductmustcontainafunctionalgroupcapableofreactingwithanothermoleculeofmonomer.Incondensationpolymerization,eachmonomerunitmusthaveatleasttwofunctionalgroups.Inadditionpolymerization,themonomerneedhaveonlyonefunctionalgroup-thepresenceoftwoormorefunctionalgroupsusuallyleadstotheproductionofcross-linkedadditionpolymers.Themodulusandotherphysicalpropertiesofpolymercanbeimprovedbytheadditionoffillersandotheradditives,suchasplasticizers,antioxidants,heatstabilizers,ultravioletstabilizers,andflameretardants.Additivesareessentialfunctionalingredientsofpolymers,andwheneverpossible,eachshouldbeusedinoptimumamountsfortheattainmentofhigh-qualityproducts.

Plasticizer:AccordingtotheASTM-D-883definition,aplasticizerisamaterialincorporatedinaplastictoincreaseitsworkabilityandflexibilityordistensibility.Theadditionofaplasticizermaylowerthemeltviscosity,elasticmodulus,andglasstransitiontemperature(Tg)ofaplastic.Antioxidants:Polymerssuchaspolypropylene(PP)arenotusableoutdoorswithoutappropriatestabilizers,becauseofthepresenceofreadilyremovablehydrogenatomsonthetertiarycarbonatoms.PPandmanyotherpolymers(RH)areattackedduringprocessingoroutdooruseintheabsenceofstabilizersbecauseofachaindegradationreaction.Therateofchain-reactiondegradationisretardedbythepresenceofsmallamountsofantioxidants.HeatStabilizers:Inadditiontothefree-radicalchaindegradationdescribedforpolyolefins,anothertypeofdegradation(dehydrohalogenation)alsooccurswithchlorine-containingpolymerssuchasPVC.Whenheated,PVCmaylosehydrogenchlorideandformachromophoricconjugatedpolyenestructure.Thistypeofdegradationcanbedecreasedbythepresenceoflead,bariumandcadmiumsaltswhichactasscavengersforhydrogenchlorideandthereforecanbeusedasheatstabilizers.UltravioletStabilizers:Whilemuchofthe-esnuenr'gysrahdiiagthionisabsorbedbytheatmosphere,someradiationinthe280to400nmradiationis100to72kcal,itissufficientlystrongtocleavecovalentbondsandcauseyellowingandembrittlementoforganicpolymers.Sincetheeffectofultravioletradiationonsyntheticpolymersissimilartoitseffectonthehumanskin,itisnotsurprisingthatultravioletstabilizerssuchasphenylsalicylatehavebeenusedformanyyearsinsuntanninglotions.FlameRetardants:Sincesomepolymersareusedasshelterandclothingandinhouseholdfurnishing,itisessentialthattheyhavegoodflameresistance.Combustionisachainreactionthatmaybeinitiatedandpropagatedbyfreeradicalslikethehydroxylfreeradical.Hydroxylradicalsmaybeproducedbythereactionofoxygenwithmacroalkylradicals.Halogenradicalsproducedbythereactionofhydroxylradicalswithhalides,suchasHX,mayserveasterminatorsforthechainreaction.Otherpolymeradditivesarecolorants,antistaticagents,foamingagents,preservatives,andprocessingaidorlubricants.8聚合物及其添加剂大分子或聚合物这个名称，被用来表示一些高分子量的物质。这些物质由很多（通长至少为100）个低分子量的单元由共价键连接而成。如果组成大分子的这些低分子量的单元以尾-尾相接的方式彼此连接成一个长链，并且链间无共价的化学键存在，这样的大分子被称为线型聚合物。这样的聚合物，除分子量特别高的（1,000,000以上）外，通常都能被溶解。当被加热时，它们就会软化或者熔化，以致于它们能被挤压成纤维或者注塑成所需的形状，这类聚合物被称为热塑型聚合物。另一方面，如果这些聚合物链在许多的结点处交联在一起，这样的聚合物就是一个巨大的三维分子，是不可熔化及不可溶解的。这样的聚合物被称为交联聚合物。一些被称为热固型聚合物的线型聚合物，含有一些基团，当被加热时就反应得到交联聚合物。这种小分子进行多次组合形成大分子的过程被称为聚合，组成大分子或聚合物的小分子被称为单体。人们已知有两种聚合方式：（1）缩聚，（2）加聚。一个涉及在两个单体单元之间消除一个小分子（例如水或醇）形成聚合物的反应被称为缩聚反应。在加聚反应中，不饱和的或环状的分子之间彼此加成，没有消除任何分子，这样，聚合物的实验式就当然与单体的实验式相同。一个形成高分子化合物的反应，不管是通过加聚还是通过缩聚来进行的，都必须有许多官能团的参与才能完成。当两个单体分子发生反应时，其产物中必须具有可与另一单体反应的官能团。在缩聚反应中，

每个单体至少要有两个官能团；而在加聚反应中，每个单体只要有一个官能团即可，如果一个单体有两个或更多的官能团，通常将导致交联的加聚物的形成。聚合物的模数及其它物理性质可以通过加入填料或其它添加剂如增塑剂、抗氧化剂、热稳定剂、紫外线稳定剂及阻燃剂等而得以改善。添加剂是聚合物中起重要作用的组分，而且，每种添加剂都应尽可能在最佳用量下使用，以得到高品质的产品。增塑剂：按照美国试验材料协会的定义（ASTM-D-883），增塑剂是一种可增加塑料的可加工性、延展性及膨胀性的物质。同时，加入增塑剂，还可以降低塑料在熔融态时的粘度、弹性模