塑料阻燃机理最精辟

塑料阻燃机理最精辟之总结编辑廖家志原著：SERGEIV.LEVCHIK1.1引言1.2聚合物燃烧与测试1.2.1可燃性的实验室测试1、 Flammabilityofpolymersisassessedprimarilythroughignitability,flamespread,andheatrelease。聚合物的燃烧性主要从易燃性、火焰传播性、释热性三个方面进行评估。2、 Numerousflammabilitytestsareknownandareperformedeitheronrepresentativesamplesoronanassembledproduct.Testscanbesmall,intermediate,orfullscale。许多可燃性测试可用试样或最终产品进行，在规模上分为小型、中型及大型测试。3、 AV-0classificationisgiventomaterialthatisextinguishedinlessthan10safteranyflameapplication.Themeancombustiontimeforthefivespecimenstested(10flameapplications)shouldnotexceed5s,andnocombustibledripscanbeobserved.AV-1classificationisreceivedbyasamplewithmaximumcombustiontime<50sandmeancombustiontimeforfivespecimens< 25s.Nocombustibledripsshouldbeobserved.ThesampleisclassifiedV-2ifitsatisfiesthecombustiontimecriteriaofV-1butflammabledripsignitingthecottonareallowed。UL94V0级评定标准：每次点燃后10s内即发生自熄，5个试样(10次点燃)的平均燃烧时间不超过5s,无具有引燃性的熔融滴落。V-1级评定标准：试样最长的燃烧时间小于30s，平均燃烧时间小于25s，无具有引燃性的熔融滴落。V-2级评定标准：符合V-1级标准的燃烧时间限制，允许具有引燃性的熔融滴落。4、 TheLOItestdoesnotrepresentarealfirescenario,butitisgoodasascreeningtoolbecauseitgivesanumericalvalueinsteadofadiscreteclassification(e.g.,V-0,V-1,V-2).虽然LOI法仅能测试实验室条件而非真实火情下材料的易燃性，但其可提供较详实的数据而非不连贯的等级划分(例如,V-0,V-1,V-2)。5、 Theconecalorimetertestisabench-scale(medium-sized)testdevelopedatNISTwhichquicklygainedpopularityintheacademiccommunityaswellasforstandardizationpurposes(e.g.,ISO5660-1,ASTME-1354).Itisalsousedasatoolforfireprotectionengineeringbecauseitallowspredictionoflarge-scaletestresults.Aconecalorimetermeasuresconsumptionofoxygenfromaburningsample100x100mminareaandupto50mmthick.Theheatreleaseiscalculatedfromtheoxygenconsumptiondata.锥形量热仪测试法是美国国家标准及技术研究院(NIST)制定与规范的一个中等规模实验室测试法，它很快得到了学术界的普遍认可，并为其制定了相关标准(例如ISO5660-1,ASTME-1354)。因该法所得数据与大规模测试所得数据具有一定的相关性，故它也是研究火安全防护工程的重要方法。Cone法可测试面积为100mmx100mm、厚度不超过50mm试样的燃烧耗氧量，并由耗氧量计得样品的释热量。1.2.2聚合物的燃烧6、 Ignitabilitydependstoalargeextentonhowquicklythesurfacecanberaisedtotheignitiontemperature.Specialconsiderationhastobegiventopolymersthatmeltbeforethermaldecomposition.Usually,atalowheatexposure,meltingprecedesignitionandthepolymercan or ,removingheatfromthesurface.Thisphenomenonisbeneficialforflameretardancyofuncharrablepolymers.Ontheotherhand,atahigherheatexposure,ignitionmayoccurbeforethesurfaceisheatedtosufficientdepthforthemeltedmaterialtoflow,andsuchpolymersmayigniterelativelyeasy.材料的易燃性在很大程度上取决于材料表面温度上升到燃点的速度。人们异常关注熔融温度低于热降解温度的聚合物。通常暴露于较小热源时，如果材料在被点燃之前为熔融态，则可通过流动和熔滴来降低表面热量。这十分有益于非成炭类聚合物的阻燃。相反，暴露于较大热源时，材料在可为熔融流动态之前即被点燃，该类聚合物则相对易燃。7、Thepossibilityofextinguishingapolymerflamedependsonthemechanismofthermaldecompositionofthepolymer.Whereasignitionofapolymercorrelatesprimarilywiththeinitialtemperatureofdecomposition,steadycombustionisrelatedtothetendencyofthepolymertoyieldachar,whichisproducedattheexpenseofcombustiblevolatilefragments.聚合物的自熄性取决于其热降解机理，其易燃性则相关于其热降解的初始温度，稳定燃烧则与其成炭性相关，成炭性越好，可燃性挥发物则越少。8、Fourgeneralmechanismsareimportantforthermaldecompositionofpolymers:(1)randomchainscission,inwhichthepolymerbackboneisrandomlysplitintosmallerfragments:(2)chain-endscission,inwhichthepolymerdepolymerizesfromthechainends:(3)eliminationofpendantgroupswithoutbreakingofthebackbone:and(4)cross-linking.Onlyafewpolymersdecomposepre-dominantlythroughonemechanism:inmanycasesacombinationoftwoormoremechanismsisineffect.Forexample,polyethyleneandpolypropylenetendprimarilytodecomposeviarandomchainscission,whichinthecaseofpolyethyleneisalsoaccompaniedbysomecross-linking.Poly(methylmethacrylate)andpolystyrenetendtodepolymerize,poly(vinylchloride)primarilyundergoeseliminationofpendantgroups(dehydrochlorination),andpolyacrylonitrilecrosslinks.Intermsofflammability,randomscissionanddepolymerizationpolymersareusuallymoreflammablethanpolymersthatcrosslinkorremovependantgroups.Cross-linkingleadstoprecursorsofcharandasaresult,tolowerflammability.Eliminationofpendantgroupsresultsindoublebonds,whichcanalsogivecrosslinksorleadtoaromatization.对干聚合物而言，以下四个机理十分重要：①无规断链，聚合物主链随机断裂为链段碎片：②链端断裂，聚合物从链端开始裂解：③无主链破坏的端基消除：④交联。大多数情况下，聚合物的热降解遵循两种或两种以上上述机理，极少数只遵循其中单个机理。比如，PE和PP主要通过无规断链的方式裂解，但PE的裂解还伴随一些交联反应的发生；PMMA和PS受热时将发生解聚反应；PVC主要发生端基消除反应(脱HCI)；聚丙烯腈(PAN)热分解时则发生交联。对可燃性而言，热分解时发生无规断链和解聚的聚合物通常比发生交联或端基消除的聚合物更加易燃。交联可促进成炭，从而降低可燃性。端基消除导致形成双键，可促进发生交联或芳构化。9、 Ingeneral,polymerswitharomaticorheterocyclicgroupsinthemainchainarelesscombustiblethanpolymerswithanaliphaticbackbone.Polymerswithshortflexiblelinkagesbetweenaromaticringstendtocrosslinkandchar.Thesepolymersarethermallystableandshowrelativelygoodflameretardancy.Forexample,bisphenolA一basedpolycarbonate,phenolformaldehyderesins,andpolyimidesareself-extinguishingandshoweitheraV-2orV-1ratingintheUL-94test.Ontheotherhand,polymerswithrelativelylongflexible(aliphatic)linkagesarestillrelativelycombustibledespitearomaticsinthebackbone.Examplesofthesepolymersarepoly(ethyleneterephthalate),poly(butyleneterephthalate),polyurethanes,andbisphenolA一basedepoxyresin.一般而言,主链上含芳环或杂环基团的聚合物比脂肪族聚合物更为难燃c芳环间较短的柔性键可发牛交联并成炭,此类聚合物具有较好的热稳定性和阻燃性。例如在UL94测试中，双酚A型聚碳酸酯、苯酚甲醛树脂、聚酰亚胺都可以自熄并达到V-2或V-1级。但是，含有较长柔性键(脂肪链)的聚合物的聚合物，虽主链上有芳环却仍相对易燃；例如PET、PBT、PU、双酚A型环氧树脂。10、 Charringofpolymersproceedsthroughvariousstages:(1)cross-linking,(2)aromatization,(3)fusionofaromatics,and(4)graphitization.Theabilityofapolymertoperforminoneorseveralofthesestagesleadingtocharformationdependsprimarilyonthepolymerstructure.However,thisperformaneecanbeimprovedsignificantlybytheuseofflameretardants.Althoughmanypolymerstendtocross-linkatearlystagesofthermaldecomposition,thisdoesnotnecessarilyresultincharformation.Charisformedonlyifthecross-linkedpolymercontainsaromaticfragmentsand/orconjugateddoublebondsandispronetoaromatizationduringthermaldecomposition.聚合物的成炭分为多个步骤，依次为：①交联；②芳构化；③芳族稠环化；④石墨化。聚合物的成炭性主要取决于其结构，但是同时也可以通过使用阻燃剂来获得提高。尽管多种聚合物在热分解初始阶段就趋干交联,但这并不足以导致成炭。炭层的形成条件是：热分解过程中，交联聚合物含有芳环碎片及或共轭双键，并趋于芳构化。11、Fusedaromaticringsinthechartendtoassembleintosmallstacks,whichareprecursorsofgraphite.Thesepregraphiticdomainsareembeddedintheamorphouschar.Thistypeofchar,calledturbostraticchar,isusuallyformedat600to900C,temperaturestypicallyfoundonthesurfaceofburningpolymers.Charthatcontainsmorepregraphiticdomainsismorestabletothermaloxidationandthereforelesslikelytoburnawayandexposethepolymersurfacetotheheatoftheflame.Ontheotherhand,highlygraphitizedcharsarerigidandmayhavecracks,whichdonotretarddiffusionofcombustiblematerialstotheflame.Thebest-performingcharwouldbeamorphousuncrackedcharwitharequisitepregraphiticdomaincontent.炭层中的芳香族稠环趋于形成小型堆叠结构，称为石墨化前体。这些前体镶嵌于无定形化炭层中形成“无规炭层”，该结构通常于聚合物表面温度为600-900°C间时形成。炭层中含有的石墨前体越多，其热稳定性就越好，因此似乎也较难燃尽，并降低聚合物表面暴露于火焰热流中的概率。但另一方面，高度石墨化的炭层十分坚硬且可能存在裂缝，导致无法有效阻止可燃材料暴露于火焰。因此，性能最为优异的炭层应是含有适量石墨化前体的无定形无裂缝炭层。1.3阻燃1.3.1一般阻燃作用机理12、Althoughflameretardantsmaydifferfromoneanotherintermsofchemicalstructure,certaingeneralmechanismsofactionareapplicabletovariousclassesofflameretardants.Thefirstlineofseparationnormallydistinguishesgas-phase-activeandcondensed-phase-activeflameretardants.Gas-phase-activeflameretardantsactprimarilythroughscavengingfreeradicalsresponsibleforthebranchingofradicalchainreactionsintheflame.Thisisthechemicalmechanismofactioninthegasphase.Otherflameretardantsgeneratelargeamountsofnoncombustiblegases,whichdiluteflammablegases,sometimesdissociateendothermically,anddecreasethetemperaturebyabsorbingheat.Thisslowscombustionandmayeventuallyresultinextinguishmentoftheflame.Thisisthephysicalmechanismofactioninthegasphase.尽管各种阻燃剂的化学结构不同，但通用的作用机理是可适用于多种阻燃剂的。通常根据作用机理，可将其分为气相作用阻燃剂和凝聚相作用阻燃剂两大类■气相作用阻燃剂主要诵过捕捉燃烧的链支化反应所需的活性自由基发挥作用，这是其化学作用的机理。另一类阻燃剂可产生大量的不可燃气体以稀释可燃气体,有时也可以通过吸热来降低材料表面温度。这有效降低了材料的可燃性，也可致其自熄。这是阻燃剂于气相中的物理作用机理。13、 Condensed-phasemechanismsofactionaremorenumerousthanthegas-phasemechanisms.Charring,discussedbrieflyabove,isthemostcommoncondensed-phasemodeofaction.Again,charringcouldbepromotedeitherbychemicalinteractionoftheflameretardantandthepolymerorbyphysicalretentionofthepolymerinthecondensedphase.Charringcouldalsobepromotedbycatalysisoroxidativedehydrogenation.凝聚相作用机理比气相作用机理更为常见。前面简要讨论过的成炭是最为常见的凝聚相作用机理。提高材料的成炭性有多种途径：聚合物和阻燃剂的反应型成炭、聚合物在凝聚相滞留成炭或氧化脱氢成炭。14、Someflameretardantsshowalmostexclusivelyaphysicalmodeofaction.Examplesarealuminumhydroxideandmagnesiumhydroxide.Ontheotherhand,thereisnosingleflameretardantthatwilloperateexclusivelythroughachemicalmodeofaction.Chemicalmechanismsarealwaysaccompaniedbyoneorseveralphysicalmechanisms,mostcommonlyendothermicdissociationordilutionoffuel.Combinationsofseveralmechanismscanoftenbesynergistic.有些阻燃剂几乎是通过单一的物理作用机理阻燃的，例如氢氧化铝（ATH）和氢氧化镁（MH）•然而，没有一种阻燃剂是通过单一的化学作用机理而阻燃的。化学作用机理通常伴随一种或多种物理机理（如分解导致的吸收热量或稀释可燃气体）发挥阻燃作用。多种阻燃机理配合发挥作用，称之为协效阻燃。1.3.2各类阻燃机理卤系阻燃剂15、 Halogen-containingflameretardantsrepresentthemostdiversifiedclassofretardants.Tobeeffective,halogen-containingflameretardantsneedtoreleasehalogenintheformofradicalorhalogenhalideatthesametemperaturerangeorbelowthetemperatureofdecompositionofthepolymer.Theoretically,fourclassesofchemicalcompoundscanbeusedashalogenatedflameretardants:thosecontainingfluorine,chlorine,bromine,oriodine.Fluorinatedorganicsarenormallymorestablethananyotherpolymersanddonotreleasefluorineradicalsorhydrogenfluoride.Nevertheless,thereareafewexamplesofthecommercialuseoffluorinatedflameretardantsoperatingdifferentlyfromallotherhalogenatedflameretardants,andtheywillbediscussedlater.Bycontrast,iodinatedorganicshaveverylowthermalstabilityandcannotbeprocessedwithmostcommercialpolymers.Inaddition,fluorineandiodinearemoreexpensivethanchlorineorbromine,whichalsolimitsdevelopmentofflameretardantsbasedonthesetwohalogens.卤系阻燃剂代表品种繁多的一类阻燃剂。简言之，卤系阻燃剂即是可在聚合物分解温度范围内或低于降解温度时释放出卤素自由基或卤化物的阻燃剂。理论上，含氟、氯、溴、或碘的化合物都可以作为卤素阻燃剂使用。含氟有机物通常比任何聚合物都稳定，极难释放出氟自由基或氟化氢。不过，现已有一些与所有其它卤系阻燃剂的作用机理截然不同的工业化氟系阻燃剂。与氟化物不同的是,含碘有机物的热稳定性极差，不能与大多数工业聚合物一起加工。再者，氟或碘比溴或氯价格昂贵，这也极大限制了氟系与碘系阻燃剂的发展。16、 Chlorinatedaromaticproductsarerelativelystableandthereforenotveryefficient,butchlorinatedaliphaticandcycloaliphaticflameretardantsarewellknown.Thechlorinecontentinsomechlorinatedparaffinscanreach70%,andsomeimprovedgradescanbeusedinpolyolefinsandinhigh-impactpolystyrene（HIPS）.Abroadrangeofbrominatedflameretardantsarecommerciallyavail-able.Brominatedflameretardantshelpmaintainagoodbalanceofphysicalproperties,suchasgoodimpactandtensilestrengthandahighheatdistor-tiontemperature.Theseflameretardantsaregenerallysuitableformanyplastics;however,theirprincipaluseisinengineeringplasticsandepoxyresins.Inthiscasetheemphasisisonaromaticproducts.Althoughaliphaticbrominatedflameretardantsareoftenmoreefficientthanaromatics,theirusehasbeenlimitedtocertainpolymers.Forsimilarstructuresthereisusuallyacorrelationbetweendegreeofbrominationandthermalstability.Fullybrominatedaromaticshavelowvolatilityandareusedinengineeringresinswitharelativelyhighprocessingtemperature.Polymericandoligomericbrominatedaromaticflameretardantsarealsowidelyused.Inadditiontogoodthermalstability,theyshowbetterphysicalproperties.Oneofthemaindisadvantagesofmanybrominatedaromaticflameretardantsistheirlowresistancetoultraviolet（UV）light;how-ever,therearespeciallydesignedcommercialflameretardantsthatshowgoodUVstability.氯系芳香族化合物的热稳定性相对较好，因此并不十分高效，但氯系脂肪族和脂环族阻燃剂则是效果相对较好的两种阻燃剂。一些氯化石蜡的含氯量达70%,可用于聚烯烃和HIPS的阻燃。很多溴系阻燃剂都是可工业化应用的。溴系阻燃剂可保持材料较好的物理性能，例如良好的抗冲击性、拉伸强度与较高的热变形温度。这些阻燃剂通常可用于许多塑料，现主要用于工程塑料和环氧树脂。在这方面，研究重点是芳香族卤系阻燃剂。尽管溴系脂肪族阻燃剂通常比溴系芳香族阻燃剂更为高效，但是它们仅适用于特定的聚合物。溴系阻燃剂结构相近时，含溴量越高其热稳定性越好。完全溴化的芳环化合物具有较好的热稳定性，可用于加工温度相对较高的工程塑料。溴系芳香族低聚物阻燃剂也应用广泛，除具备优良的热稳定性外，还表现出良好的物理性能。许多溴系芳香族阻燃剂的缺点是抗紫外性能较差，不过现在已经开发出了经特殊设计的抗紫外线性能优良的工业化溴系阻燃剂。17、 Itisgenerallyacceptedthatthemainmechanismofflameretardantactionofhalogenatedflameretardantsisinthegasphase,anditisprimarilythechemicalmodeofaction.Thereactionbeginswiththeabstractionofhalogenradicalfromtheflameretardant.Thishalogenimmediatelyabstractshydrogenfromeithertheflameretardantadditiveorthepolymer.人们普遍认为，卤系阻燃剂的主要作用方式为气相作用，且主要为化学作用。首先，作用始于阻燃剂受热分解放出卤自由基，自由基可迅速捕获阻燃剂或聚合物热分解放出的H自由基。18、 Intheabsenceofasynergist,hydrogenhalidesvolatilizeandentertheflame.Hydrogenhalideswillquicklyreactwithhydrogenorhydroxylradicalsandregeneratethehalogen.ExamplesofsuchreactionswithHBrareshownbelowinreactions(1.4)and(1.5).FurtherbromineradicalswillreactwithhydrocarbonsinthegasphaseandregenerateHBrasshowninreaction(1.6),withtheprocessrepeatinguntilbromineleavestheflame.若不添加协效剂，HX(卤化氢)将挥发至火焰中，并迅速与H自由基或0H自由基反应重新生成卤素自由基X。此后，在气相中，卤素自由基X将与烯烃类聚合物反应生成HX,该反应循环往复地进行，直至耗尽火焰中的X自由基。19、 Atomichydrogenandhydroxylradicalsareveryimportantforsustainingcombustion.Thehydrogenradicalisresponsibleforthechain-branchingfree-radicalreactionsintheflame, whereasthehydroxylradicalisresponsiblefortheoxidationofCOtoC02,whichisahighlyexothermicreactionandisresponsibleforthelargerpartoftheheatgenerationintheflame.H自由基与OH自由基对维持燃烧起至关重要的作用。研究表明，H自由基主导了火焰中的链支化自由基反应;而OH自由基则可氧化CO生成C02,这是一个放热量极大的反应，燃烧产生的大部分热即来源于此。20、 Insomeotherreactions,themorereactiveradicals(H•，OH・，CH3•)arereplacedbythelessactiveBr・radicals.IfBr•meetsH•inthepresenceofaneutralmolecule(thirdbody),HBrisregenerated.lthasbeenfoundbyspectroscopythattheintroductionofhalogen-containinginhibitorsintotheflameclearlyreducestheconcentrationofH・，OH・andHCO・radicals,whereasthereisanincreaseinthecontentofthediradicalsC2"andsoot.Astheconcentrationofinhibitorisincreased,theflametemperaturedecreases.Smalladditionsofhalogeninhibitors(ontheorderofafewmol%)canreducetherateofflamepropagationupto10-foldandhaveamarkedeffectontheignitionlimits.Ontheotherhand,halogensacceleratetheformationofsootintheflame.在一些其它反应中，活性较强的自由基(H・，OH・，CH3・)可被活性较弱的自由基Br•取代，而Br•又可与其它分子中的H•中心生成HBr。光谱学研究表明，添加卤系阻燃剂可明显降低火焰中H・，OH・和HCO•自由基的数量，但却增加了c2-(碳双自由基)和烟炱的数量。火焰温度降随阻燃剂的添加量增大而降低，添加少量的卤系阻燃剂即可使材料的火焰传播速度降至原来的1/10，并有效降低易燃性，但卤系阻燃剂的不足之处是提高了烟炱的量。21、 ItiswellestablishedthatSb2O3issynergisticwithhalogen-containingflameretardantsbecauseitfacilitatesdeliveryofhalogenatomsinthegasphaseandprolongsresidenceofthehalogensintheflamezonesothatmore“hot"radicalscanbescavenged.ItisalsowellestablishedthatSb2O3catalysesdehalogenationoftheflameretardant,sohalogenscanbemovedintotheflameatalowertemperature.Intheflame,antimonytrihalide(e.g.,SbBr3)isreducedstepbysteptometallicantimony,whichcouldbefurtheroxidizedbytheoxygenorhydroxylradical.研究表明，Sb203可与卤系阻燃剂产生协同作用，因其可以促进气相中卤自由基生成，并可延长其在火焰中的作用时间，从而可使其捕获更多燃烧所需要的活性自由基。研究表明，Sb2O3还可催化卤系阻燃剂脱卤，使卤素自由基在较低温度下即可参与阻燃。22、 Interferencewiththeantimony-halogenreactionwillaffecttheflameretardancyofthepolymer.Forexample,metalcationsfromcolorpigmentsoraninertfillersuchascalciumcarbonateortalcmayleadtotheformationofstablemetalhalides,renderingthehalogenunavailableforreactionwithantimonyoxide.Theresultisthatneitherthehalogennortheantimonyistransportedintothevaporzone.Siliconeshavealsobeenshowntointerferewiththeflameretardantactionofhalogenatedflameretardants.妨碍卤-锑反应将影响聚合物的阻燃性。例如，着色剂或惰性填料(如碳酸钙或滑石粉)中的金属离子可与卤素离子形成稳定的金属卤化物，妨碍卤素与锑氧化物的反应，结果导致卤和锑不能进入气相发挥阻燃作用。另外，含硅化合物对卤素阻燃剂的阻燃作用也存在负面影响。23、Itisalsobelievedthatthelargeheatcapacityofhydrogenhalidesandtheirdilutionoftheflameresultsinadecreaseinthemassconcentrationofcom-bustiblegasesandthetemperatureoftheflame.人们认为，卤化氢较大的热容和对火焰的稀释作用可有效降低可燃气体的浓度与火焰温度。24、 Asmentionedearlier,thehalogenradicalsevolvedfromtheflameretardantinthecondensedphaseabstractthehydrogenfromthepolymerandproduceunsaturation.Thedoublebondsareknowntobeprecursorsofcharformationthrougheithercross-linkingoraromatization.Ifhydrogenabstractedfromthearomaticring,thisringhasachancetocouplewithanotherringandstartformingpolyaromaticstructures,whichareprecursorsofgraphiticdomainsinthechar.Thischarformationisanimportantcondensed-phasecontributionofhalogen-basedflameretardants,whichisoftenoverlooked.如前所述，在凝聚相中卤系阻燃剂释放的卤自由基与聚合物中的氢原子反应生成卤化氢，并形成不饱和键，不饱和键可通过交联或芳构化成炭。如果卤自由基与芳环上的氢原子发生反应，所形成的不饱和芳环就可与其它芳环形成多芳环结构，这就是炭层中重要的石墨化前体的来源。该成炭作用是常被人们所忽视的卤系阻燃剂在凝聚相中的阻燃作用。25、 Thereisanothercondensed-phasemodeofactionthatisspecificforaliphaticbromine,anditistheoppositeofcharformation.BromineradicalsgeneratedthermallyatlowtemperatureinthepolymermeltcancausechainscissionattertiaryCatoms.Examplesofpolymerswherethismechanismisoperationalarepolystyrene(foams)andpolypropylene(preferablythinparts,films,orfibers).Thedecreasedmolecularweightcausesfastdrippingofthehotpolymer,whichcoolstheflameandeventuallyextinguishesit.脂肪族溴系阻燃剂具备一种特有的凝聚相作用方式，该作用与成炭作用原理相反。较低温度时，聚合物熔体中的溴自由基可诱发碳链上叔碳原子处的链断裂；此机理适用的聚合物基体包括PS和PP(尤其为片材、薄膜及纤维)。分子量大的降低导致热聚合物快速的熔融滴落，降低了火焰的温度并可致其自熄。26、 Poly(tetrafluoroethylene)(PTFE)isusedataverylowlevel(0.01to0.5wt%)incombinationwithotherflameretardantstosuppressflamingdrips.TheflameretardantactionofPTFEisnotrelatedtoanychemicalreactionoffluorineorhalogenfluoride.Duringpolymerprocessingat200to300C,PTFEparticlessoften,theshearforceofextrusionelongatestheparticlesupto500%,andmicrofibrilsareformed.Uponcombustionthemicrofibrilsshrinkbackwhenthepolymermeltsandanetworkthatpreventsdrippingisformed.ThisflameretardantactionofPTFEisaphysicalphenomenon.极少量(0.01to0.5wt%)的PTFE复配其它阻燃剂可以有效抑制聚合物的熔融滴落。PTFE的阻燃作用机理与氟或氟卤化物的所有化学作用都无关。在聚合物加工温度(200°C~300°C)下，PTFE微粒会发生软化，挤出成型时的熔体中的微纤维将发生回缩,形成一个可抑制熔融滴落的类网络结构。PTFE的这种阻燃作用为物理作用机理。27、 Potassiumperfluorobutanesulfonateisaddedtopolycarbonateatalowloadingof0.05to0.2wt%whichallowspreservationoftransparencyandclarityofthepolymer.Evenatsuchalowconcentration,perfluorobutanesulfonateprovidesaV-0ratingtothepolycarbonate.Inthiscasethesulfonategroupisprimarilyresponsiblefortheflameretardanteffectoftheproduct,whereastheperfluorobutanegroup,duetoitsstrongelectron-withdrawingeffect,increasestheacidityofthesulfonategroup.Nocontributionofhydrogenfluorideisknownforpotassiumperfluorobutanesulfonate聚碳酸酯（PC）中添加少量（0.05-0.2%）全氟丁基磺酸钾时，材料仍可保持良好的透明性，并可使PC的阻燃性达到UL94V-0级。磺酸盐发挥了主要的阻燃作用，电负性较强的全氟丁基基团可增强磺酸盐的酸性。对全氟丁基磺酸钾而言，氟化氢并没有参与阻燃作用。1.3.2．2磷系阻燃剂28、 Phosphorus-basedflameretardantsarethesecondmostwidelyusedclassofflameretardants.Recenteffortsinthedevelopmentofnewflameretardantshaveshiftedstronglytowardphosphorusandotherhalogen-freesystems.Amongphosphorus-basedflameretardants,oneshoulddistinguish(1)elementalredphosphorus,(2)inorganicphosphates,(3)numerousorganicphosphorus-basedproducts,and(4)chlororganophosphates.Althoughmanyphosphorusflameretardantsexhibitgeneralmodesofaction,therearespecificsforeachclassmentionedabove.现在，磷系阻燃剂是仅次于卤系阻燃剂的、应用最为广泛的一类阻燃剂。目前，新型阻燃剂的研发重点已转向磷系阻燃剂与其它无卤体系。磷系阻燃剂分为：（1）红磷；（2）无机磷系；（3）许多有机磷系化合物；（4）氯化有机磷酯。尽管许多磷系阻燃剂表现出通用的阻燃作用机理，但是，上述四种是各有其特性的。29、 Itisgenerallyacceptedthatphosphorusflameretardantsaresignificantlymoreeffectiveinoxygen-ornitrogen-containingpolymers,whichcouldbeeitherheterochainpolymersorpolymerswiththeseelementsinpendantgroups.Effectivephosphorusflameretardantsaremorespecificthanhalogen-basedproductstocertainpolymers.Thisrelatestothecondensed-phasemechanismofaction,wherethephosphorusflameretardantreactswiththepolymerandisinvolvedinitscharring.研究表明，磷系阻燃剂应用于含氧或含氮聚合物（可为杂链或端基基团中含有氧或者氮元素的聚合物）时特别有效。对于某些聚合物，磷系阻燃剂比卤系阻燃剂更为有效专用。这与磷系阻燃剂的凝聚相作用方式有关，它可以与聚合物发生反应并参与成炭。30、Theflameretardancyofcellulosehasbeenstudiedingreatdetail,whichgavegoodinsightforunderstandingtheinteractionofphosphorusflameretardantswithpolymerscontaininghydroxylgroups.Phosphorusflameretardants,intheformofeitheracidsderivedfromdecompositionofammoniumphosphatesaltsorofphosphateesters,react(esterifyortransesterify)withthehydroxylgroupsofthecellulose.Uponfurtherheating,phosphorylatedcelluloseunder-goesthermaldecompositionandasignificantamountofcharisformedattheexpenseofcombustiblevolatileproductsthatwouldbeproducedbyvirgincel-lulose.Somenitrogen-containingcompounds,suchasurea,dicyandiamide,andmelamine,willacceleratephosphorylationofcellulosethroughformationofaphosphorus-nitrogenintermediate,andthussynergizetheflameretardantactionofphosphorus.Phosphorus-nitrogensynergismisnotageneralphenomenonbutdependsonthestruetureofthephosphorusandnitrogenflameretardantsaswellasthepolymerstrueture.有关纤维素阻燃的研究已经十分详尽，较好地解释了磷系阻燃剂与含羟基聚合物的相互作用。磷系阻燃剂可通过磷酸铵盐或磷酸酯分解产生酸源，后者可与纤维素的羟基发生酯化或者酯交换反应。通过进一步加热，磷酸化纤维素将发生热分解，未被磷酸化的纤维素分解生成的可燃性挥发产物减少，大量的炭层形成。一些含氮化合物(如尿素、双氰胺、三聚氰胺等)可与含磷化合物反应生成P-N中间体，后者可促进纤维素磷酸化。研究表明,P-N阻燃体系的协效作用取决于磷系及氮系阻燃剂和聚合物的结构。31、 Similartocellulose,phosphateesterscantransesterifyotherpolymers.Forexample,polycarbonatescanundergorearrangementduringthermaldecomposition,wherephenolicOHgroupsareformedwhichthenbecomethetargetforattackbyaromaticphosphateestersThus,phosphorusisgraftedonthepolymerchain.Charwillbeformeduponthermaldecompositionofthisgraftedpolymer.Similarphosphorylationchemistrywasfoundforpolyphenyleneether(PPE;acomponentofaPPE/HIPSblend),whichalsotendstorearrangeuponheatingandformphenolicOHgroups.类似于纤维素，磷酸酯也可以与其它聚合物发生酯交换反应。例如，热分解过程中PC可发生重排形成酚羟基，后者可与芳环磷酸酯发生反应，进而磷可接枝到聚合物链上，该磷接枝聚合物发生热分解即可成炭。该磷酰化同样可适用于聚苯醚(PPE或PPE/HIPS),因其在加热时也可发生重排而形成酚羟基。32、fthepolymercannotbeinvolvedinthecharringbecauseoftheabsenceofreactivegroups,ahighlycharringcoadditiveisusedincombinationwiththephosphorusflameretardant.Thecoadditiveisusuallyapolyol,whichcanundergophosphorylationsimilartothatofcellulose;pentaerythritolisatypicalexampleofsuchapolyol.Melaminecanbeusedinconjunctionwiththissystemaswell.Thesecombinationsofflameretardantsarecalledintumescentsystemsbecausetheyformaviscousswollencharonthesurfaceoftheburningpolymer.Thecharimpedestheheatfluxtothepolymersurfaceandretardsdiffusionofvolatilepyrolysisproductstotheflame.Thismechanismofactionismostlyphysicalbecausethepolymeritselfisnotnecessarilyinvolvedinthecharringprocess,butitsvolatilizationisretardedsignificantly.若聚合物因缺乏反应型基团而难于成炭，则可使用高效成炭剂与磷系阻燃剂复配。该成炭剂通常为多元醇，它们可发生类似于纤维素磷酸化的反应。季戊四醇就是一种典型的此类多元醇。添加三聚氰胺可组成协效的阻燃系统，该系统因可在聚合物表面形成粘性膨胀熔体炭层而得名“膨胀阻燃系统”该炭层可在聚合物表面形成隔热层，并有效阻止挥发性可燃气体扩散至火焰。该系统主要为物理作用阻燃，因聚合物本身并未参与成炭，但其分解产生的挥发性可燃产物则可被有效“阻挡”33、 Phosphorusflameretardantscanremaininthesolidphaseandpromotecharringorvolatilizeintothegasphase,wheretheyactaspotentscavengersofH•or0H・radicals.Volatilephosphoruscompoundsareamongthemosteffectiveinhibitorsofcombustion.Arecentstudyshowedthatphosphorusatthesamemolarconcentrationis,onaveragef,ivetimesmoreeffectivethanbromineand10timesmoreeffectivethanchlorine.磷系阻燃剂可以通过两种方式发挥阻燃作用：第一种是在凝聚相中促进成炭，第二种是在气相中作为H•或OH•自由基的强力捕获剂。气相作用磷系阻燃剂是一种十分高效的阻燃剂。近来有研究表明，在相同添加量时，磷系阻燃剂的阻燃效率平均是溴系阻燃剂的5倍以及氮系阻燃剂的10倍。34、 Ifconditionsareright,phosphorus-basedmoleculescanvolatilizeandareoxidized,producingactiveradicalsintheflame.Ontheotherhand,phosphorusflameretardantstendtoreactwiththepolymerortooxidizetophosphoricacidinthecondensedphase.Thisfavorsmostlycondensed-phasemechanisms.Itischallengingtodesignaphosphorus-basedflameretardantthatwillvolatilizeintotheflameatrelativelylowtemperaturesbutwillnotbelostduringpolymerprocessing.特定条件下，在气相中磷系阻燃剂可挥发或被氧化生成活性自由基，或者可在凝聚相中与聚合物发生反应或被氧化生成磷酸。后者即为磷系阻燃剂主要的凝聚相作用机理。现在，研发在较低温度下即可挥发生成自由基捕捉剂，但同时在聚合物加工过程中不发生降解的磷系阻燃剂仍是一个难题。35、 Redphosphorusisthemostconcentratedsourceofphosphorusforflameretardancy.Infact,itisveryeffectiveinsomepolymers,suchasthermoplasticpolyestersorpolyamides,whereself-extinguishingUL-94V-0performancecanbeachievedatloadingsoflessthan10wt%.Despitetheapparentchemicalsimplicityofthisadditive,itsmechanismofactionisnotcompletelyunderstood.Mostresearchersagreethatinoxygen-ornitrogen-containingpolymers,redphosphorusreactswiththepolymerandinducescharformation.AlthoughthereisabeliefthatredphosphorusisoxidizedandhydrolyzedbywaterbeforeitFormationoftheresidueisconsideredtobeacondensed-phasecontributionofmelamine,whereasammoniaevolutiondilutestheflamewithnoncombustiblegases.红磷是一种极为重要的磷系阻燃剂（上海北京顺德注：翻译错误，themostconcentrated是浓度最大的意思）。实际上，在一些聚合物（如热塑性聚酯或尼龙）中它是十分高效的，添加量少于10%即可使聚合物达到UL94V-0级。尽管红磷化学成分单一，但人们仍然尚未完全了解其阻燃机理。多数学者较为赞同的观点是，在含氧或者含氮聚合物中，红磷可与聚合物发生反应促进成炭。尽管有学者认为，红磷在与聚合物反应前会发生氧化或水解，但同时也有强有力的证据表明红磷可在干燥的惰性气氛中直接与聚酯或PA发生反应。也有一些研究表明，PA6中的红磷为自由基作用机理的阻燃。红磷在烯烃类聚合物（如聚烯烃、PS）中的阻燃效果不佳。学者们认为，这是因为红磷在烯烃类聚合物中将降解为白磷（P4）,后者可挥发而作用于气相。三聚氰胺阻燃剂36、Melamineisauniqueproductwith67wt%nitrogeninthemoleculeandfairlyhighthermalstability.Melaminealsoformsthermallystablesaltswithstrongacids.Melamineitself,melaminecyanurate,melaminephosphate,melaminepyrophosphate,andmelaminepolyphosphatearecommerciallyavailableforvariousflameretardantapplications.Themechanismofflameretardantactionofmelamineisdifferentfromthemechanismofmelami