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EuropeanPolymerJournal42(2006)1362–1369

EUROPEANPOLYMERJOURNAL

www.elsevier.com/locate/europolj

Thermalstabilityand?ameretardante?ectsof

halloysitenanotubesonpoly(propylene)

MingliangDu,BaochunGuo*,DeminJia

DepartmentofPolymerMaterialsandEngineering,SouthChinaUniversityofTechnology,Guangzhou510640,China

Received18August2005;receivedinrevisedform2November2005;accepted6December2005

Availableonline18January2006

Abstract

Naturallyoccurredhalloysitenanotubes(HNTs)withhollownanotubularstructureswereusedasanewtype?llerforpoly(propylene)(PP).NanocompositesbasedonPPandHNTswerepreparedbymeltblending.Scanningelectronicmicroscopy(SEM)resultssuggestedHNTsweredispersedinPPmatrixevenlyatnanoscaleafterfacilemodi?cation.Ther-malstabilityofthenanocompositeswasfoundremarkablyenhancedbytheincorporationofHNTs.Conecalorimetricdataalsoshowedthedecreaseof?ammabilityofthenanocomposites.Entrapmentmechanismofthedecompositionprod-uctsinHNTswasproposedtoexplaintheenhancementofthermalstabilityofthenanocomposites.Thebarriersforheatandmasstransport,thepresenceofironinHNTs,areallresponsiblefortheimprovementinthermalstabilityanddecreasein?ammability.Thoseresultssuggestedpotentialpromising?ameretardantapplicationofHNTsinPP.ó2005ElsevierLtd.Allrightsreserved.

Keywords:Halloysitenanotubes;Nanocomposites;Thermalstability;Flameretardant;Poly(propylene)

1.Introduction

SinceBlumstein?rstreportedtheenhancedther-malstabilityofpoly(methylmethacrylate)(PMMA)/montmorillonite(MMT)claynanocom-positesin1965[1],thermalstabilityand?ameretar-dantpropertiesofnanoparticlereinforcedpolymercompositeshaveattractedconsiderableattentioninpolymerarea.In1976,FujiwaraandSakamoto[2]showedthepotential?ameretardantpropertiesofnylon-6/claynanocomposites.Giannelis[3],Gilmanetal.[4,5],havedonemuchworkonthethermalsta-bilityand?ameretardantofpolymernanocompos-*Correspondingauthor.

E-mailaddress:psbcguo@scut.edu.cn(B.Guo).

ites.Themostcommonthermalstabilityand?ameretardant?llersatnanoscaleincludenaturalinorganicclayminerals,especiallythosewithlayeredstructures,magnesiumhydroxide,aluminiumhydroxideandsoon.Morerecently,followedbythediscoveryofmulti-wallandsingle-wallcarbonnanotubes(CNTs),theunusualthermalstabilityand?ameretardantpropertiesofCNTs?llednanocompositeshaveinspiredmanyscientists’interests[6–8].ReportedbyKashiwagietal.withalittleincorporationofCNTs,thermalstabilityand?ammabilitypropertiesofpoly(propylene)increasedsigni?cantly[6].

However,itiswellknownthatthepreparationprocessofnanoparticlesisrelativelycomplexandcostly.Andforthenaturallayeredclayminerals,thegenerallyrequiredorganictreatmentisnot

0014-3057/$-seefrontmatteró2005ElsevierLtd.Allrightsreserved.doi:10.1016/j.eurpolymj.2005.12.006

M.Duetal./EuropeanPolymerJournal42(2006)1362–13691363

su?cienttogeneratewell-intercalatedand/orexfoli-atedmorphologiesinpolymermatrix.Toachievesatisfactory?ameretardantpropertiesofpolymer/claycomposites,intercalativepolymerizationormoresophisticatedtreatmentofclayisnecessary.AsfortheCNTs,theirrelativelyhigherpriceandpigmentationabilityforpolymers,however,willrestricttheirapplicationas?ameretardant?llersinpolymers[6].Asaconsequence,easilyavailableande?ective?ameretardantnano?llersarestilllackingandhighlydesired.

Halloysitenanotubes(HNTs)isakindofalumi-nosilicateclayswithhollownanotubularstructureminedfromnaturaldepositsincountriessuchasChina,America,Brazil,Franceandsoon.HNTsarechemicallysimilartokaolinite.Mainlyofhollowmicrotubuleshavetypicaldimensionsofnanoscale[9].Typically,HNTsareusedinthemanufactureofhighqualityceramicwhite-ware[10].Inrecentyears,HNTsareusedasnanotemplatesornano-scalereactionvesselsinsteadofCNTsorboronnitridenanotubes(BNNTs)[11,12].

Recently,theauthorsattemptedtoutilizeHNTsasnano?llerpolymers,suchasnaturalrubber,nitrilerubberandpolypropylene[13].ItisfoundthatwiththeincorporationofHNTsinPP,thethermalstabil-ityand?ameretardantofthePP/HNTsnanocom-positesareincreasedsigni?cantly.Inthiswork,wereportthee?ectsofHNTloadingandsurfacemod-i?cationonthethermalstabilityand?ameretardantpropertiesofPP/HNTsnanocomposites.2.Experimentalsection2.1.Materials

TheisotacticPPwasmanufacturedbyGuangz-houPetro-chemicalincorporation,withamelt?owindexof2.84g/10min(afterISO-1133:1997(E)).TheHNTs,gradeofUltra?ne,wereprovidedbyImerysTablewareAsiaLimited,NewZealand.Brightnessof98.9%asmeasuredbyaMinoltaCR300usingD65lightsource.Theelementalcom-positionisasfollows(wt%):SiO2,49.5;Al2O3,35.5;Fe2O3,0.29;TiO2,0.09[14].c-methacryloxypropyl-trimethoxysilane,withtradenameofZ6030,wasmanufacturedbyDowCorning.2.2.Modi?cationofHNTs

Twohundredmilliliterof95%ethanolwasaddedintoa?ask.ThePHvalueoftheethanolsolution

wasadjustedtoabout5.0bydroppingalittleoface-ticacid.Thesilanewasthenaddedandmildlystir-redfor5min.About100goftheHNTswereaddedtothesolutionandstirredfor15min.Mostofthesolventwasevaporatedatambienttemperatureandthesolventresiduewasremovedundervacuumat70°C.

2.3.PreparationofPP/HNTsnanocompositesPPandHNTsormodi?edHNTswereblendedusingatwo-screwextruder.Thetemperaturesettingfromthehoppertothediewas180/190/195/200/200/190°C,andthescrewspeedwas100rpm.Thepelletizedgranulesweredriedfor5hunder80°Candtheninjectionmouldedunderthetemperatureof200°C.

2.4.X-raydi?raction(XRD)

TheXRDwasperformedwithaRagukuModelD/maxIIIdi?ractometer.TheX-raybeamwasnickel-?lterCuKa(k=0.1542nm)radiationoper-atedat40kVand30mA.Correspondingdatawerecollectedfrom1°to50°atascanningrateof1°C/min.

2.5.Scanningelectronmicroscopy(SEM)

Thefracturesurfaceofimpactsamplesandtheresiduesoftheconetestswereplatedwithathinlayerofgoldbeforetheobservations.TheSEMobservationswerethendoneusingLEO1530VPSEMmachine.

2.6.Thermalgravimetricanalysis(TGA)

TGAanalysiswasconductedusingaTAinstru-mentsTGA2950from50°Cto600°Cataheatingrateof20°C/minundernitrogenandairatmo-sphere.TheTGAdatareportedherearereproduc-iblewithvariationlessthan10%.2.7.Flammability

Evaluationsof?ammabilityofPP/HNTsnano-compositeswereperformedusingaFireTestingTechnologyconecalorimeter(Standard)(afterASTME-1354-90a).Thetestswereperformedatanincidentheat?uxof50kW/m2.Theconedatareportedherearereproduciblewithvariationlessthan10%.

1364M.Duetal./EuropeanPolymerJournal42(2006)1362–1369

3.Resultsanddiscussion

3.1.MorphologyofPP/HNTsnanocompositesThemorphologyoftheHNTswithalumenstructureisshowninFig.1.However,apartfromthenanotubes,afewparticlesandagglomeratesalsocouldbefound.Fig.2showstheX-raydi?raction(XRD)spectraofhalloysitenanotubes.Thebasalspacere?ectionsindicateasharppeakat12.05°,correspondingtoa001basalspacingof0.73nm.Theseresultscon?rmthemultiwallnanotubularstructuresatnanoscaleofhalloysite.

Fig.3showsthee?ectsofHNTsloadingandsur-facemodi?cationonthedispersionofHNTsinPPmatrix.ItcanbeseenthatmostoftheHNTsaredispersedinPPmatrixatnanoscale.Themodi?edHNTs,however,aredispersedmoreuniformly.AlthoughsomeaggregatesarefoundintheHNTs?llednanocomposites,theparticlesofhalloysiteinthemodi?edsystemareveryuniform.IntheimpactfracturedsurfaceofPP/unmodi?edHNTsnano-composites,especiallythat?lledwith30phrunmodi?edHNTs,manycavitiesarefoundandtheinterfaceisverysharp.ThecavitiesinthatofPP/modi?edHNTssystems,however,arereducedremarkablyandtheinterfacebecomesmuchblur-rier.Thissuggeststhatsurfacemodi?cationHNTshassigni?cante?ectonthedispersionofHNTsinthePPmatrix.

Fig.1.TEMphotooftheHNTs.

).u.a( ytinsetnI010203040502 theta (oC)Fig.2.XRDspectraoftheHNTs.Fig.4showsthemorphologyofthecombustionresiduesofthenanocompositeswith10phrmodi-?edHNTsloading.ThephotoindicatesthattheHNTskeeptubularstructuresevenafterthecombustion.

3.2.ThermalstabilityofPP/HNTsnanocompositesTheTGAcurvesforthePP/HNTsnanocompos-itesinnitrogenareshowninFig.5andthecharac-teristicweightlosstemperaturesaresummarizedinTable1.ThedatarevealsthattheloadingofHNTsandthesurfacemodi?cationofHNTshavedrastice?ectsonthethermalstabilityofthenanocompos-ites.AsshowninTable1,thetemperatureat5%weightlossforneatPPis384°C.However,thistemperatureforthenanocomposite?lledwith10phrunmodi?edHNTsisincreasedto414°C,whichis30°ChigherthanthatofneatPP.Thistemperatureisfurtherincreasedto444°Cforthenanocomposite?lledwith10phrmodi?edHNTs,whichis60°ChigherthanthatofneatPP.WithhigherHNTsloading(30phr),thenanocompositesshowmuchinferiorthermalstability,asindicatedbythetemperatureat5%weightloss.Thetempera-tureat10%weightlossandthatatmaximumweightlossrateshowasimilartrend.

Somepreviousinvestigationssuggestedthatthebarrierpropertiesofthenanoscale?llerswereresponsiblefortheenhancementofthermalstabil-ityofnancomposites.Gilmanbelievesthatthebar-rierpropertiescouldincludeboththethermalbarrier,[4,5]whichprotectsthepolymerfromcon-tactingwith?re,andthemasstransportbarrier,whichslowsdowntheescapeofthevolatileproductsduringtheprocessofdegradation,and

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Fig.3.SEMphotosofimpactfracturedsurfaceofPP/HNTscomposites:(a)HNTs10phr,(b)modi?edHNTs10phr,(c)HNTs30phrand(d)modi?edHNTs30phr.

Fig.4.Morphologyofcombustionresidueofthenanocompos-iteswith10phrmodi?edHNTsloading.

theinvestigationsalsoshowedthattheintercalatedlayeredsilicatenano?llerhadmuchbettere?ects

onthermalstabilityofpolymermatrixthanexfoli-atedlayeredsilicatenano?ller[4].Itisreasonabletopostulatethatthebarriere?ectsofHNTswithtubularstructuremaysomewhatbeinferiortothoseoflayeredsilicatenano?llers,especiallytheinterca-latedlayeredsilicatenano?llers.However,thether-malstabilityofthePP/HNTsnanocompositesreportedhereisnotinferiortotheliteraturedataforthePP/layeredsilicatenanocomposites.There-foreitcouldbeconcludedthatthebarriere?ectsofHNTsisnottheleadingfactorindeterminingthethermalstabilityofPP/HNTsnanocomposites.Someotherinvestigationsindicatetheironoxidesinthesilicate?llersandcarbonnanotubescouldactas?ameretardantadditivesandleadtosomeradicaltrappingduringtheprocessofdegradation,thusenhancethethermalstabilityofnanocompos-ites[6,15].Chemicalanalysisshowsthatthere’sabout0.29wtt2O3intheHNTs[14].Thepres-enceofironintheHNTsmay,tosomeextent,be