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AnalysisofFlame Retardancyin PolymerScience
Editedby HenriVahabi
UniversityofLorraine,CentraleSupelec,LMOPS,Metz,France
MohammadRezaSaeb DepartmentofPolymerTechnology,FacultyofChemistry,Gdansk UniversityofTechnology,Gdansk,Poland
GiulioMalucelli DepartmentofAppliedScienceandTechnology, PolitecnicodiTorino,andLocalINSTMUnit,Alessandria,Italy
Elsevier
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ArthurRichardHorrocks 1Introduction................................................................................1
1.1Polymersandthefiretriangle.. ................. .......................1
1.2Glossaryofterms..............................................................3
2Thermaltransitions,thermoplasticity,andgeometric effects.........................................................................................5
2.1Thermophysicaleffects...... ...............7
2.2Thermallythinversusthermallythickmaterials.... .........8
2.3Effectofsamplegeometry,orientation,and physicalstructure..............................................................8
3Fuel-formingreactions:Polymerpyrolysisandignition.........10
3.1Thermaldegradationorpyrolysis...................................10
3.2Pyrolysisofindividualpolymertypes..............................12
4Oxidativedegradation...............................................................27
5Combustionandfirespread:Effectofincidentheatflux.......28
5.1Ignition.............................................................................30
5.2Effectofheatflux.............................................................31
5.3Smoke..............................................................................36
6Flameretardance:Effectofflameretardantson ignition,combustion,andsmokegeneration..........................37
6.1Flame-retardanttypesandcharacteristics.......... ..........39
6.2Synergism,additivity,andantagonism............................41
6.3Environmentalchallengesandthepotentialfor nanotechnologyFRdevelopments..................................44
4.1IncompletecombustioninPCFCbycontrolling
5.2Predictingthetemperatureofsolidsurfaceat ignitionfromPCFC.........................................................108
5.3Correlationswithfiretests............................................110
5.4Milligram-scaleflamecalorimetry (MFC).... 112
6Concludingremarksandfutureperspectives.......................113 Acknowledgments........................................................................114 References...................................................................................114
CHAPTER4Evaluationofgasphase:Mechanismsandanalyses....117
SabyasachiGaan
1Introduction............................................................................117
2Typesofgas-phasemechanism... ...119
3Commonanalyticaltoolsforgas-phasemechanism evaluation...............................................................................124
3.1Thermogravimetry-infraredspectroscopy (TG-FTIR)/massspectrometry(MS)coupled analysis..........................................................................124
3.2Directinsertionprobe-massspectrometry(DIP-MS)...129
3.3Pyrolysis-gaschromatographycoupledtechnique.......131
3.4Microscalecombustioncalorimeter(MCC)andits variations........................................................................136
3.5Conecalorimeter...........................................................142
3.6Detectionofphosphorus-basedgas-phasereactive species...........................................................................143
4Concludingremarksandfutureperspectives.......................152 Acknowledgment..........................................................................153 References...................................................................................153
CHAPTER5Evaluationofgasphase:Smokeandtoxicityanalysis...161
EricGuillaume
1Introduction............................................................................161
2Smokecontents......................................................................161
2.1Gaseousfireeffluents....................................................161
2.2Solidandliquidfire effluents... 165
3Analysisofsmoke...... .............166
3.1Smokeopacity................................................................166
3.2Smokegases’concentrations........................................170
4Impactsofsmoke...................................................................175
4.1Visibilitythroughsmoke.... ...........................175
4.2Smokeinhalation...........................................................176
4.3Environmentaleffects....................................................185
5Conclusionsandperspectives................................................186
2Fundamentalsofcharandresidueformation.......................193
2.1Ceramization..................................................................193
2.2Intumescence.................................................................193
2.3Physicalbarrier(nanocomposite)..................................194
2.4Charring.........................................................................195
3Chemicalcharacterization:Chemicalcomposition...............195
3.1Fouriertransforminfraredspectroscopy(FTIR)...........196
3.2Ramanspectroscopy......................................................199
3.3X-rayphotoelectronspectroscopy(XPS). .........201
3.4X-raydiffraction(XRD)...................................................205
3.5Solid-statenuclearmagneticresonance(ssNMR).......207
3.6Electronspinresonance(ESR)......................................213
4Microscopy:Morphologyoftheresidue.................................215
4.1Scanningelectronmicroscopy(SEM)............................215
4.2Electronprobemicro-analysis(EPMA).................. .......216
4.3Transmissionelectronmicroscopy(TEM).....................217
4.4X-raycomputedtomography(CT)..................................220 5Dynamicsofchar/residueformation.. .......223
5.1Viscosity..........................................................................223
5.2Deformationandexpansion...........................................224 6Conclusionsandfuturetrends.....
2.1Conceptoffireresistance......
2.2Fireresistanceevaluation—Experimentalandmodeling characterization...... ...........239
2.3Influenceoffireresistanceobjectivesonhuman behavior..........................................................................241
3Materialapplicationsoffireresistance.................................242
3.1Non-combustiblematerials...........................................244
3.2Combustiblematerials... .......245 4Conventionalapproachoffireresistance..............................248
4.1Generalprinciples..........................................................248
4.2Buildingapplication....... ............249
4.3Transportapplication.....................................................261
4.4OutsideEurope...............................................................265
4.5Limitsoftheconventional approach............................. 266 5Performanceapproach...........................................................266
5.1Firedynamicsforfire resistance....................... ...........267
5.2Fireanalysisappliedtostructuralanalysis..................280
5.3Thermalanalysis............................................................283
5.4Experimentalapproachusinglarge-scaleand real-scaletests..............................................................284
5.5Structuralfireengineering............................................286 6Conclusionsandperspectives................................................292 Acknowledgments........................................................................293 References...................................................................................293
CHAPTER8Characterizationofhigh-temperaturepolymers forextremeenvironments...........................................299
HaoWuandJosephH.Koo
1Introduction............................................................................299
2High-temperaturepolymers...... ............303
2.1High-temperaturethermosets.... .303
2.2High-temperaturethermoplastics................................308 3Aerothermalablationtestingforhigh-temperature applications.............................................................................311
3.1Oxyacetylenetestbed(OTB)..........................................311
3.2Simulatedsolidrocketmotor (SSRM)...........................313
3.3Subscalesolidrocketmotor(charmotor)....................315
3.4LHMELtestfacilities......................................................316
3.5ICPtestfacilities............................................................318
3.6Arcjettestfacilities.......................................................321 4Concludingremarks...............................................................325 References...................................................................................326
CHAPTER9Correlationbetweenlaboratory-andreal-scale fireanalyses.................................................................333
LaurentAprin,LaurentFerry,FredericHeymes, RodolpheSonnier,andPascalZavaleta
1Introduction............................................................................333
4Cables.....................................................................................401
4.1Europe............................................................................401
4.2Others.............................................................................405
5Electrotechnicalproducts......................................................406
5.1Principle.........................................................................406
5.2Maintestmethods.........................................................406 6Others.....................................................................................407
6.1UL94...............................................................................407
6.2Heatreleaseratemeasurements.. ............408
6.3Smokecorrosivity..........................................................410 7Transportation........................................................................410
7.1Roadtransportationfield...............................................410
7.2Railtransportationfield.................................................413
7.3Marinefield....................................................................425
7.4Aeronauticalfield...........................................................431 8Conclusionsandperspectives................................................435 References...................................................................................437 INDEX...................................................................................................................449
Contributors
LaurentAprin LaboratoryfortheScienceofRisks(LSR),IMTMinesAles,Ales,France
VytenisBabrauskas FireScienceandTechnologyInc;JohnJayCollegeofCriminalJustice,NewYork, NY,UnitedStates
BenjaminBatiot InstitutPprime(UPR3346CNRS),UniversitedePoitiers,Poitiers,France
SergeBourbigot Univ.Lille,CNRS,INRAE,CentraleLille,UMR8207-UMET-UniteMateriauxet Transformations,Lille;InstitutUniversitairedeFrance(IUF),Paris,France
LaurentFerry IMTMinesAles,PolymersCompositesandHybrids(PCH),AlesCedex,France
SabyasachiGaan LaboratoryofAdvancedFibers,Empa,SwissFederalLaboratoriesforMaterials ScienceandTechnology,St.Gallen,Switzerland
EricGuillaume EfectisFrance,Saint-Aubin,France
FredericHeymes LaboratoryfortheScienceofRisks(LSR),IMTMinesAles,Ales,France
ArthurRichardHorrocks IMRI—UniversityofBolton,Bolton,UnitedKingdom
JosephH.Koo KAI,LLC;WalkerDepartmentofMechanicalEngineering,TheUniversityofTexasat Austin,Austin,TX,UnitedStates
ThomasRogaume InstitutPprime(UPR3346CNRS),UniversitedePoitiers,Poitiers,France
RodolpheSonnier IMTMinesAles,PolymersCompositesandHybrids(PCH),AlesCedex,France
HaoWu KAI,LLC,Austin,TX,UnitedStates
PascalZavaleta InstitutdeRadioprotectionetdeSu ˆ reteNucleaire(IRSN),StPaul-Lez-Durance Cedex,France
Preface
Anydevelopmentinthefieldofpolymerscienceandtechnologyhasalwaysbeen accompaniedbythequestfordesignandmanufactureofhigh-performancepolymericmaterials,particularlypolymercompositesandnanocomposites.Inthe meantime,preciseanalysisofpolymershasadvancedprovidingresearcherswith usefulinsightsintotheprocessing-microstructure-properties-performanceinterrelationships.Flame-retardantpolymericmaterialshavebeenatthecoreofattentionoverthepastfewdecades,asevidencedbythecommercializationofmany flame-retardedsystems.However,thereisacontinuedneedforfireanalysismakingitpossibletoevaluatethefireperformanceandalsotoidentifythemechanismsunderlyingtheflameretardancyofpolymersinbothgasandcondensed phases.Typically,firebehavioranalysiscanbeviewedintermsofflammability, ignitability,heatreleaseduringcombustion,flamespread,amountandintensity ofsmoke,andtoxicity.Severalstandardtestmethodsareavailablefortheanalysis offireincludingASTM,ISO,andEN,afewtomention.Firetestscanbeviewed fromthescaleperspectiveassmall-scale,bench-scale,andlarge-scalemeasurements.Nevertheless,researchersarequiteoftenlookingfornewwaysfortheanalysisoffirebycorrelatingtheoutcomesofstandardizedtestsand/ordefiningnew measures/indices,aimedatadeeperunderstandingofthemechanismscontrollingtheflameretardancyofpolymers.
Exploringthemechanismsofactionofflameretardantsinpolymersisanessentialrequirement.Besidesstandardmethodsthatmainlyfocusonthephysicsof fire,therewillalwaysbetheneedforsupplementaryteststounraveltheroleof chemistry,whichincludetheanalysisofchar(intumescentorcompact/dense) andmineralresidue.Sincenewflame-retardantsystemsaremainlycomplex/ hybridsystems,inwhichtwoormoreflameretardantswithsimilarordifferent natureareinvolved,theanalysisofflameretardancysometimesbecomesaseriouschallengeforresearchers.Suchhybridmaterialsareaimedatcoveringa widevarietyofapplicationsrangingfromcablesandprotectivecoatingsto theconstructionandbuildingmaterialsaswellasthoseinrailwaysandaviation.Thisbookprovidesusefulinformationaboutfireanalysisinpolymerscience.Startingfromgeneralconceptsanddefinitions/terms,severaladvanced
featuresoffireanalysisarecoveredbyanumberofexpertsfromallaroundthe worldwithlong-standingexperienceinthefield.In Chapter1,fundamentalsof fireanalysisincludingtheflammability,ignition,andfirespreadareintroduced,followedby Chapter2 onforcedcombustion,viewedfromtheperspectiveofconecalorimetrytests.In Chapter3,forcedcombustionisdiscussedin termsofmicroscalei.e.,pyrolysis-combustionflowcalorimetry(PCFC)tests.In Chapter4,thegasphaseanalysisispresented,andmechanismsaredescribed, followedbyfurtherevaluationofsmokeanalysisoverviewedin Chapter5. Evaluationofthecondensedphaseisthensurveyedbychar/residueanalysis in Chapter6.Analysisoffireresistanceofmaterialsisalsodiscussedin Chapter7,followedbycharacterizationoffireinhigh-performancepolymers usedforadvancedapplications(Chapter8).Thelasttwochaptersaredevoted tothecorrelationbetweenlaboratory-scaleandreal-scalefireanalyses (Chapter9)andfireanalysisfromanindustrialperspective(Chapter10).
Theeditorsofthisbookhopethatthechapterscancontributetodeepeningthe knowledgeofstudents,technicians,engineers,researchers,andpolicymakers ofindustryworkinginthefieldfrombothacademicandindustrialsectors.The evolutioninmaterials,processing,andstandardtestmethodswillnecessitate furtheranalysesonthefireretardancyofpolymermaterials.Webrought togetherwell-knownscientiststowritethisbookinthehopethattheoutcome couldpersuadescientistsforcontinuedinnovation,adaptationand/orestablishmentofnewtestmethodsinamorerobustandreliablemanner.
HenriVahabi MohammadRezaSaeb GiulioMalucelli
Fundamentals:Flammability,ignition, andfirespreadinpolymers
ArthurRichardHorrocks IMRI—UniversityofBolton,Bolton,UnitedKingdom
1Introduction
Thefiresafetyofmaterialsandproductsisdeterminedbyanunderstandingof howtheymayreacttoanignitingsourceintheenvironmentsinwhichtheyare used,suchasdomesticdwellings,publicbuildings,industriallocations,and transport.Theuseofpolymericmaterialsforstructuralitems,interiordecoration,furnishings,etc.,introducestheproblemoftheirpotentialflammability andincreasedfirehazard,whichtheirpresenceintroducesintermsofincreasingtheriskoffireinjuryandpotentiallossoflife.
Thischapterisintendedtoprovideanoverviewofthesignificantaspectsthat determinetheeffectofheatonthepolymericmaterials,theirsubsequentignitionandcombustion,andtherelatedfactorsinfluencingflamespreadrates. Theelementsofstrategiesthatincreasetheflameretardancyorresistanceof polymersareparticularlystressed.Inaddition,ageneralappraisalofthese andhowtheyrelatetofiretestingmethodologiesareprovidedasthebasis forsubsequentmorein-depthstudieslaterinthisbook.
Thetextisprovidedwithanumberofsignificantreferencestoenablethereader topursueeachtopicinfargreaterdepththanispossiblehere.Thereferencesare followedbyanextensivebibliographycomprisingthemostsignificanttexts publishedduringthelast20yearsorso.
1.1Polymersandthefiretriangle
Tounderstandthecombustionprocess,anappreciationoftheso-calledFire Triangleisanessentialfirstrequirement.Inessence,anycombustionrequires thethreeelements:afuel,heat,andoxygenactingtogetherinconcert. Fig.1 illustratesthisprocess,andthetriangleis“broken”ifanyoneofthesefactors isremoved.Ideally,thefuelmustbeeitheragasorvapor.
Thesimpleviewofthepolymerpyrolysis/combustionprocess. 2 CHAPTER1:Fundamentals:Flammability,i
Thefiretriangle.
Sincepolymersaresolids,theirconversionintoflammablefuels,asaresultof theapplicationofheatasaflame,ahotsurface,orinradiantform,mustfirstbe understood.Thismayberepresentedbythegeneralizedreactionsin Scheme1
Hc ΔHc heat
Volatiles + flammable gases + Polymer
Oxygen non-flammable gases + char
CO + CO2 + H2O (+ NOx + HCN) + smoke
SCHEME1
Theheatrequiredtothermallydegradeorpyrolyzethepolymerisdefinedas theheatofpyrolysis, ΔHp,andthereactionsinvolvedareusuallyendothermic. Theevolvedvolatilesandflammablegasesrepresentthe“fuels”in Fig.1,andat thetemperatures,atwhichpyrolysisoccurs,usuallyoxidizeandigniteinthe presenceofoxygen.However,inrealfires,perfectoxidationofallvolatiles tocarbondioxideandwaterdoesnotoccur;hence,ifthereisaninclusion ofcarbonmonoxideandifnitrogenispresentinthepolymer(e.g.,polyamides, polyacrylics,wool,silk),thereistheprobabilitythatnitrogen-containinggases, NOx,andhydrogencyanide,HCN,mayalsobepresent.Polymerscontaining sulfursuchaswool,keratin,vulcanizedrubber,andpolysulfonesmayinclude gasessuchashydrogensulfideandsulfurdioxideinthefiregases.Smokeagain
FIG.1
isaproductofimperfectcombustion.Oxidationisanexothermicreactionand soheatisevolved, ΔHc,theheatofcombustion,whichfeedsbacktopyrolyze morepolymers.
1.2Glossaryofterms
Thewholeareaofpolymer,fiberandtextile,film/laminate,andcomposite materialburningpropertiesandtheirrelatedrisk-reducingmethodologies useanumberofterms,whichinsomecasesmaybequiteconfusing,especially tothenon-specialist.Thefollowingisalistinanalphabeticalorderofthemore commonterms,originallytakenfromthoseidentifiedbyLewin [1],andit includesanumberofadditionsandvariationsrelatingtothewholeareaof flame-retardantpolymersanditsliterature:
Additive(flameretardant): A(flameretardant)compoundaddedafterthe polymerhasbeensynthesized,butbeforeorduringitsconversiontothe finalform(e.g.,fiber,plastic),notcovalentlyboundtopolymer substrate.
Afterglow: Glowingcombustioninamaterialaftercessation(naturalor induced)offlaming.
Afterglowtime: Thetimetheflamecontinuestoburnaftertheignitionflame isremoved.
Antagonism: Theobservedeffectivenessofcombinationsofcompoundsis lessthanthesumoftheeffectsofindividualcomponents.
Autoignition:Spontaneousignitionofamaterialwhenheatedinair.
Back-coating: Acoatingappliedtothereversefaceofafabricinamannerthat doesnotaffecttheaestheticsorotherpropertiesoftheface.
Char: Thecarbonaceousresidueorcharformedduringorremainingafter pyrolysisorcombustion.
Charlength: Thedifferencebetweenoriginallengthandremainingunburned lengthofmaterialaftertestingaspecimenbyexposuretoaflame.
Coating(flameretardant): Alayerofsecondarymaterialcomprisingaflame retardantandabinderoraflame-retardantresindepositedonthepolymer materialsurfaceorwithinthepolymersurface.
Combustion:Self-catalyzedexothermicreactioninvolvingfuelandoxidizer. Condensed-phaseflameretardant:Aflameretardantthatmodifiesthepolymer pyrolysisprocesstoreduceflammablevolatileformationandusually increaseinchar.
Damagedlength: Theextentofdamageproducedoverthespecimenbyan ignitionsourceandthesubsequentsubstrateignition.Itmayincludechar, formationofahole,discoloredregion,orzonehavingreducedtensile propertiesoracombinationthereof.
Finish(flameretardant): Acompoundorcombinationofcompoundsadded afterconversiontotheendproduct(e.g.,fiber,yarn,fabric),whichmaybe chemicallybondedordepositedonfiber,yarn,fabric,orthin-filmsurfaces.
Fireresistance:Thecapacityofamaterialorstructuretowithstandfire withoutlosingitsfunctionalproperties.
Flame:Combustionprocessinthegasphaseaccompaniedbytheemission ofvisiblelight.
Flameresistance:Thepropertyinamaterialofexhibitingresistanceto ignitionand/orminimalflammability;thetermisoftensynonymouswith flameretardancy,butmaybeconsideredtorelatetomaterials,whichdonot igniteunderaflamebutmaybedamagedbyit.
Flameretardanceorretardancy:Thepropertyinamaterialofexhibiting resistancetoignitionandreducedflammability;thetermisoften synonymouswith flameresistance butmaybeconsideredtorelateto materials,whichwilligniteunderaflameandwhichonremovalwilleither self-extinguishorburnveryslowly.
Flameretardant:Chemicalcompoundcapableofimpartingflameresistance to(orreducingtheflammabilityof)amaterial,towhichitisaddedor combinedwith.
Flamepropagation:Spreadofflamefromregiontoregioninacombustible material(burningvelocity ¼ rateofflamepropagation).Intextilefabrics,the timetoburnaspecifiedlengthoffabricismoreeasilydefined.
Flamespread: Theextentofpropagationofflameinspaceoroveraspecimen surfaceunderspecifiedtestconditions.
Flamingdebris: Materialsseparatingfromthespecimenduringthetest procedureandfallingbelowtheinitialloweredgeofthespecimenand continuingtoflameasitfalls.Thesemayincludemoltendrips,bothflaming andnon-flaming.
Flammability:Thetendencyofamaterialtoburnwithaflame.
Gas-orvapor-phaseflameretardant: Aflameretardantthatinteractswiththe flamechemistry,duringwhichvolatilesareoxidizedbyoxygeninair.
Heatflux: Theintensityofaheatorignitingsourcewithrespecttoadefined areaormassofmaterial.
Ignition: Initiationofcombustion.Infiretesting,analternativedefinition mightbe,“Thestage,atwhichthetestspecimensustainsaflameforaperiod of1sormoreafterremovaloftheignitingflame.”
Ignitiontimeortimetoignite: Thetimetakenforasampletoignitewhen subjectedtoanignitionsource,adirectheatfluxorboth.
Inherentflameorfireresistance: Thepropertyofapolymer,inwhichthe chemicalstructureresistspyrolysisandgivesofffewflammablevolatiles, therebyreducingitseasecombustionandincreasingitsabilitytomaintain itsfunctionalpropertiesathighertemperatures.Suchorganicpolymers usuallyhavearomaticstructures.
Limitingoxygenindex(LOI): Minimumoxygenpercentintheenvironment thatsustainsburningunderspecifiedtestconditions.
Nanocoating,includingsol-gelandlayer-by-layer: Coatingsonpolymersurfaces of >100nmorevenofmolecularthickness.Sol-gelcoatingsgenerally compriseacross-linkedsurfacecoatingbasedonsilicanetworks;layer-bylayercostingscomprisemultilayersandwich-typestructures,inwhicheach nanolayerhasanequalandoppositeelectricalcharge.
Peakheatreleaserate:Themaximumrateofheatreleasefollowingthe ignitionofasample.
Pyrolysis: Irreversiblechemicaldecompositionduetonon-oxidativeheating. Rateofheatrelease: Theamountofheatreleasedperunittimeatagiventime byspecimenburningunderspecifiedtestconditions.
Residualflametime: Thetimetakenforburningfragments(e.g.,molten drips)fallingfromthesampleandwhichburnonthebottomofthetest cabinettoextinguish.
Self-extinguishing: Theincapabilitytosustaincombustioninairunderthe specifiedtestconditionsaftertheremovalofexternalheatsource.
Smoke: Finedispersioninairofparticles,usuallyindividuallyinvisible,of carbonandothersolidsandliquidsresultingfromincompletecombustion. Itsopacityisduetoscatteringand/orabsorptionofvisiblelight.
Smoldering: Combustionwithoutflameandwithoutpriorflaming combustion,butusuallywithincandescenceandsmoke.
Surfaceflash: Therapidspreadofflameoverthesurfaceofamaterialwithout ignitionofitsbasicstructure.
Synergism: Theobservedeffectivenessofcombinationsofcompoundsis greaterthanthesumoftheeffectsgivenbyindividualcomponents.
Thermaltransitiontemperature:Thetemperatureatwhichaphysicalor chemicalchangeoccurswhenapolymerisheated.
Vertical,horizontal,30°,45°,orotherdefinedangle(striporcoupon)test: Orientationofthetestspecimenwithrespecttothehorizontalduring flammabilitytestingunderspecifiedconditions.
2Thermaltransitions,thermoplasticity, andgeometriceffects
Theburningbehavioroforganicpolymersisinfluencedandoftendetermined byanumberofthermaltransitiontemperaturesandthermodynamicparameters. Table1 liststhecommonlyavailablepolymerswiththeirtypicalorindicativephysicalglassorsecond-order, Tg,andmelting, Tm,transitions,if appropriate,whichmaybecomparedwiththeirchemicallyrelatedtransitions ofpyrolysis(underaninertatmosphere), Tp,andignitionandtheonsetof flamingcombustion, Tc [2,3].Alltransitiontemperaturesin Table1 should
Table1 Indicativethermaltransitionsofthemorecommonlyusedpolymers [2,3]
Naturalpolymers
Cellulose––35035018.419
Keratin(e.g.,wool)––2456002527
Thermoplasticpolymers
Nylon65021543145020–21.539
Nylon6.65026540353020–21.532
Poly(ethylene terephthalate) 80–90255420–44748020–2124
Polyacrylic100 >220a 290 >25018.232
Polyethylene,LDPE 125105–115 >450 >45018–1945
Polyethylene,HDPE135–140 >450 >45018–1945
Polypropylene 20165 >400 >40018.644
Poly(methyl methacrylate),PMMA 110200b >300 3801825
Poly(lacticacid),PLA60–65150–160 300–21–24–PVC <80 >180 >18045037–3921
Polystyrene 100240c >2801942
Thermosetpolymers
Vinylester 110– >250–20–23–
Unsaturatedpolyester 90–130– >250–20–2229–30
Phenol-formaldehyde270–300–440–520– 2528–29
Epoxy120–220–360–430–22–2330–31
Inherentlyflame-andheat-resistantpolymers
Meta-aramid (e.g.,Nomex) 275375a 410 >50029–3026
Para-aramid (e.g.,Kevlar) 340560a >590 >5502926
Polyimide(resin) >300– >450– 3526
aWithdecomposition. bSyndiotactic. cIsotactic.
beconsideredtobeindicativeonlysincetheyhavebeenobtainedfromvarious literaturesources,insomecasesaveraged,andinanycase,theyaredependent onpolymerhistoryandthemethodusedtorecordthem.Inaddition,typical heatsofcombustionaregivenas ΔHc.Generally,thelowertherespective Tc (andusually Tp)temperatureandthehottertheflame,themoreflammable
isthepolymer. Table1 listsalsotypicallimitingoxygenindex(LOI)values, whicharemeasuresoftheinherentburningcharacterofamaterialandmay beexpressedasapercentageordecimal [4].TheLOItestwasdevelopedbyFenimoreandMartinin1966 [5] andiswellestablishedinanumberofstandards includingASTMD2863andISOEN4589-2 [6].
PolymershavingLOIvaluesof21vol%or0.21orbelowigniteeasilyandburn rapidlyinair(containing20.8%oxygen).ThosewithLOIvaluesabove21vol% igniteandburnmoreslowly,andgenerallywhenLOIvaluesriseabovevalues ofabout26–28vol%,polymersusuallymaybeconsideredtobeflameretardantandwillpassmostsmallignitiontestswhensubjectedtoasmallflame inthehorizontalandverticalorientations.
2.1Thermophysicaleffects
Thermoplasticity,determinedbythemagnitudeofthesecond-ordertemperature, Tg,whichmayormaynotprecedemelting,isanimportantfactorthatinfluencesthefirebehaviorofanypolymer.Thistransitionisassociatedwiththe temperaturerequiredforpolymersegmentalmotiontooccurwithinthenoncrystallineregionofapolymer.Inamorphouspolymers,suchasatacticpolystyreneandpoly(methylmethacrylate),onlyasecond-ordertransitiontemperature mayberecordedasisthecaseinallthermosetpolymers(sometimesreferredto asresins),inwhichcovalentcross-linksexistbetweenthepolymericchains.
Semi-crystallinepolymers,whichareoftenfiber-forming,mayalsohavean associatedmeltingpoint, Tm (e.g.,polyamidesornylons,polyesters,polyolefins),unlessthedegreesofcrystallinityareexceptionallyhigh(e.g.,cellulose, poly(para-aramid)),whichresultinsecond-orderand/ormeltingtransition temperaturesbeingsohighthatthermaldecompositionoccursfirst.
Whensubjectedtoheat,athermoplasticpolymermayshrinkawayfromany ignitingflame,therebygivingtheillusionofflameretardancyinthattheshrinkageoccurswellbelowtheignitiontemperature.Infact,inthinmaterialslike textilefabricsandfilms,theshrinkagemaybesufficienttoenableanotherwise quiteflammablepolymernottoigniteandsopassagivenflammabilitystandard.Notallthermoplasticpolymersmelt,especiallythosewithhigh Tg values, sincethermaldecompositionmayprecedeorpreventmelting.Ifapolymer melts,thenmoltendripsmayformwhich—whileremovingthermalenergy fromtheheatedzone,andthuspreventingorreducingthetendencyto ignite—mayalsoaddtothefirehazardifthedripsdoigniteandsospread theflameviaflamingdripstounderlyingoradjacentmaterials.In Table1,most ofthepolymerswithwell-definedmeltingpointswillsufferfromthisphenomenon.Somethermoplasticpolymerslikepolyacrylicswillappeartomelt,while decomposingatthesametime;thisisespeciallynoticeablewithsomeinherentlyflame-andheat-resistantpolymerslikepoly(meta-andpara-)aramids.
2.2Thermallythinversusthermallythickmaterials
Itisimportantalsowhileconsideringthesethermophysicaleffectstoconsider thedifferencebetweenthermallythinandthermallythickmaterials [7].Polymersaregoodthermalinsulators,andsowheninthebulkform,ifexposedto heatononesurface,willbesubjectedtoonlythelocalizedeffectsthattheheat sourcegenerates,withtheinteriorexperiencinglittleincreaseintemperaturefor sometime.Inotherwords,athermalgradientwillbecreated,whichcouldbe closeto1000°Conthesurfaceandambienttemperaturewithinoratthereverse surfaceofthematerial.Suchamaterialisdefinedasbeing thermallythick.This willmeanthatthermaldegradationorpyrolysisreactionsgeneratingfuelwill occuratdifferentratesacrossthisgradient.Shouldthepolymerbecharforming,thenitmayformalayerofcharimmediatelybelowtheflamezone, whichwillactasafurtherbarriertoheattransfer.
Ontheotherhand,ifthematerialisverythin,thentherateofheattransfer acrossitsthicknessmaybesofastthatnothermalgradientisactuallymeasurable.Materialsthatbehaveinthismanneraretypicallyfilms,textilefabrics,and thinlaminatesandaresaidtobe thermallythin.Experienceshowsthatthermally thinbehaviorisobservedwhenmaterialthicknessesarelessthan3or4mm, andsoforthicknessesabove5mm,thermallythickbehaviorisobserved.Ifheat fluxesaresignificantlyhigh( 50kW/m2,avaluetypicallyequivalenttoan averageroomfireaboveatflashover—see Section5),thenthethermallythin tothickboundarymaybecloserto10mm,especiallyinmaterialslike wood [8]
Thus,thinfilms,laminates,andtextilesareusuallyconsideredtobethermally thin.Inthecaseoftextilestructures,singlelayersmaybethermallythinbutas layersareadded,theymaybeabletoshowthermallythickbehavior [9].This maybeusefullyexploitedwhendesigningheat-protectiveclothingandespeciallyfirefighters’garments [10,11]
Inconclusion,itisgenerallythecasethatthermallythinmaterialswilltherefore ignitesoonerandburnmuchmoreintenselythanthermallythicksamples.
2.3Effectofsamplegeometry,orientation,andphysical structure
Inadditiontotheeffectsofthickness,anumberofothergeometricalfactors mayinfluencetheignitionandburningpropertiesofmaterials,someofwhich aresummarizedbelow.
Ignitionsourcelocationandsampleorientation:Materialssubjectedtoan ignitionsourceappliedtothetopofthesampleusuallyshowthelongest ignitiontimesinceheatfromanygeneratedflamewillbedissipatedupward, awayfromthesample.Conversely,ignitionatthebottomofasamplewill
producetheshortesttimesinceheatfromflamesgeneratedwillbe transferredtothesampleimmediatelyinadvanceoftheflame.Onceignited, theadvancingflamewillpreheatthematerialandtheflamewillaccelerate rapidlyasaconsequenceofthisso-calledchimneyeffect.Thedegreeof accelerationwilldecreaseastheangleofadvancingflamereducesfrom90° withrespecttothehorizontal.Standardtestmethodsusuallyreflectthe orientationthatagivenmaterialwillbeusedduringitsservicelifeandthe firehazardposed.Thus,forexample:
(i)Buildingwallpanels,curtainsandblinds,andprotectiveclothing willbetestedinverticalorientations(90°);
(ii)Flooring,includingcarpets,andcarinteriorfabricsaretestedinthe horizontalmode(0°);
(iii)Tentingandinternalstrappingsforaircraftluggagecontainment maybetestedatotheranglessuchas30°,45°,or60° tothe horizontal.
Faceversusedgeignition :Sampleedgeignitionpresentsamoresevereignition conditionthanfaceignition,sinceintheformertheflamewillcontactnot onlythesampleface,butalsotheloweredgesurfaceand,inthecaseofathin laminate,filmortextile,thereverseface.Thus,testingmethodsreflectthe applicationhazardsothattextilesmayoftenbetestedattheedge,whereas wallpanelsmaybeexposedtoafaceignitionsource.
Effectofheterogeneityandstructure :Inmultiphasematerialscontaininga polymerasabinderresininacompositeforexample,ordispersed reinforcingshortglassfibersinanextrudedormoldeditem,theactionof heatmaycausethestructuretodelaminateordeconstruct,thereby exposinginnersurfacestobothheatandoxygen.Inaddition,exposed reinforcingelementsmaypreventotherwisemoltenpolymerfrom drippingawayfromtheignitionzone.Inbothcases,theburning propertiesofthematerialwillbesignificantlydifferentfromthoseofthe polymeritself.Agoodexamplehereistheverycommonpolyester/cottonblendedtextilefabric,whichwhenheatedthepolyestermeltsandwetsthe surfacesoftheadjacentcharringcottonfibers,whichpreventtheformer fromdrippingaway.Thisso-calledscaffoldingeffectexplainswhy polyester/cotton-blendedfabricsoftenposeagreaterfirehazardthanthose comprisingcottonalone.
Foamsandtextilesalsocontainairorothergaseswithintheirstructures,which willsignificantlyaffecttheirburningproperties.Thus,lowfabricareadensity valuesandopenstructuresaggravatetheburningrateandsoincreasethehazardsofburnseveritymorethanheavierandmultilayeredconstructions [12].
CorrelationsbetweenLOIlinearlywithrespecttoareadensityandlogarithmicallywithairpermeabilityhavebeenshownforaseriesofcottonfabrics, althoughcorrelationcoefficientswerelow [13].
