w w w . j m r t . c o m . b r
Availableonlineatwww.sciencedirect.com
Original
Article
Effect
of
multi
walled
carbon
nanotube
on
mechanical,
thermal
and
rheological
properties
of
polypropylene
Salih
Hakan
Yetgin
∗KütahyaDumlupinarUniversity,SimavTechnologyFaculty,DepartmentofMechanicalEngineering,Simav,Kütahya,Turkey
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r
t
i
c
l
e
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n
f
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Articlehistory:
Received6June2019 Accepted13August2019 Availableonline27August2019
Keywords: Polypropylene MWCNT Nanocomposites Mechanicalproperties Rheologicalproperties
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Inthisstudy,multi-walledcarbonnanotube(MWCNT)filledpolypropylene(PP) nanocom-positespreparedbymeltprocessingmethodsbyemployingextruderandinjectionmolding techniqueswereexaminedwithvariouscharacterizationmethodsandtestprocedures,in detail.Aimandnoveltyoftheworkweretomerelyinvestigatetheeffectsofamountand dispersionofMWCNTsonmechanical,thermalandrheologicalpropertiesofPP includ-ingnocompatibilizerandthuschemicalinteractionand/orinterfacialadhesioneffect.The mechanicaltestresultsshowedthattheincorporationofMWCNTsincreasedthetensile strength(18.4%),flexuralstrength(35.2%)andmodulusofelasticity(45%)whileitdecreased theimpactstrength(18%)andelongationatbreak(690%)valuesofPP/MWCNT nanocom-posites.ThermalanalysisdatarevealedthattheMWCNTadditionslightlyincreasedthe crystallizationpeakonsetandpeakmaximumtemperaturesofPPundernon-isothermal conditions.Frequency-dependentmeltrheologicalbehaviorsofnanocompositesinlinear viscoelasticregimepointedoutthatthestoragemodulus(G’),lossmodulus(G”),complex viscosity(*),andrelaxationtimeofPPincreasedwiththeincreasingamountofMWCNT. Non-linearrheologicaltestssuchascreepandstressrelaxationalsodepictedthat nanocom-positesexhibitedlowercreepstrainandrelaxationratethanPP.Basedonthethermaland mechanicaltestresults,0.3wt%ofMWCNTcouldbeconsideredasthecriticalfilleramount alsocalledas“percolationthreshold”forimprovingthesolid-statephysicalpropertiesof PP/MWCNTnanocompositesunderthecircumstancesofnocompatibilizer.
©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
Polymers and polymer composites are extensively used in the industry. In machine design, polymer-based materials are specifically and preferentially employed due to their
∗ Correspondingauthor.
E-mail:hakan.yetgin@dpu.edu.tr
manyadvantagessuchaslightness,chemicalresistance,easy processing, andrecyclingcomparedto metalsandmetallic alloys.However,unreinforcedpolymerspossesssome disad-vantagesintermsoftheirmechanicalandthermalproperties. In order to enhance some physicalproperties ofpolymers various types of fibers such asglass fiber [1], carbon fiber
[2], and aramid [3] and particles such as talc [4], calcium carbonate [5], and carbon black [6] having different parti-cleshapeandsizesareintroducedintopolymers.Inrecent
https://doi.org/10.1016/j.jmrt.2019.08.018
2238-7854/©2019 The Author. Published by Elsevier B.V. This is an open access article underthe CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
years, carbon-based nanomaterials (e.g. carbon nanotube-CNT,carbonnanofiber-CNF,andgraphene derivatives)have becomethemostimportantfillersforimprovingthephysical propertiesofpolymers[7,8].Superior structuraland physi-calpropertiesofcarbonnanotubessuchasone-dimensional (1D)geometry,extremelyhighaspectratio,modulusof elas-ticity (200–1000GPa), and strength (200–900GPa), electrical and thermal conductivity makethem excellent candidates as nanofillers to produce advanced polymer nanocompos-ites [9–11]. Single-walled or multi-walled carbon nanotube (SWCNTorMWCNT) filledpolymercompositescanbe pro-duced by in-situ polymerization, solution mixture or melt mixing (extrusion and injection)techniques. Among these productionmethods,meltprocessingparticularlyprovidesa cost-effectiveprocesswhichenablestoensurebothfast pro-ductionand environmentalbenefitsas asolvent-freeroute
[7]. Zouet al.[12]investigatedthe izod impactstrength of MWCNT filled high density polyethylene (HDPE) compos-itesproducedbytwin-screwextrusionandinjectionmolding methods.Kanagarietal.[13]reportedthatmechanical prop-erties of HDPE were improved with the addition of CNT in the HDPE/CNT composites produced byinjection mold-ing.This wasexplained withthe good load transfereffect andthe interfaciallinkingbetweenCNTandHDPE. Onthe otherhand,Xiaoetal.[14]statedthatwhentheamountof MWCNTwaskeptatabout10wt%,theelasticmodulusvalue increasedby89%andthetensilestrengthincreasedby56% intheMWCNTfilledLDPEcomposites.Ogasawaraetal.[15]
manufacturedpolyimide(PI)/MWCNTcompositesbyin-situ polymerizationmethodand foundthattheglass transition temperature(Tg),elasticmodulusandyieldstrengthvaluesof
compositesincreasedwiththeadditionofMWCNT.Liuetal.
[16] prepared polyetherimide (PEI)/MWCNT nanocomposite filmsbysolutioncastingandthermalimidizationmethods. Theyreportedthat1wt%ofMWCNTimprovedtheTgofPEI
about10◦CandtheelasticmodulusofPEIabout250%.Phang etal.[17]investigatedmechanicalpropertiesofMWCNTfilled polyamides(PA6)producedbymelt-compounding.Theyfound thattheelasticmodulusandyieldstrengthvaluesofPA6were improvedbyabout214%and162%,respectivelywiththe addi-tionof2wt%ofMWCNTs.
Polypropylene(PP)isamemberofpolyolefinsandoneofthe mostwidelyusedsemi-crystallinethermoplasticsexistingin variouscrystallineformssuchasthemonoclinic␣-formand thehexagonal-form.PP-basedpartsarecommonlyusedin differentindustriessuchasmilitary,marinevehicles, pack-aging,automobilesand constructions duetotheir physical andchemical propertiesaswell astheirrelativelylow cost advantage[18].However,PPexhibitslowthermal,electrical, andmechanicalpropertiesandalsoahighcoefficientof fric-tion underdry sliding conditions comparedto engineering plastics.Bettinaetal.[19]investigatedtheinfluenceof carbon-based additives such as thermally reduced graphite oxide (TRGO), multi-layer graphene, carbon black (CB), MWCNT, andexpandedgraphite(EG)withdifferentparticlesizesand shapesonthe flameretardancy andmechanical properties of isotactic polypropylene (iPP). Novais et al. [20] investi-gatedtheinfluenceofchemicalfunctionalizationandpolymer meltblendingconditionsoncarbon nanotubedispersionin polypropylene,aswellasitsinfluenceontheelectricaland
mechanicalproperties.Seoetal.[21]statedthatthestorage modulusofPP(547MPa)increasedby52.3%withtheaddition of7.5phrofCNT.Thiebaudetal.[22]determinedthatthe mod-ulus ofelasticityofPPcompositeincreasedwithinarange of26.9–40.2%, and theyieldstrength improved from 17.3% to 30.3% depending on the increased amount of MWCNT. Xiao et al. [23] studied the impact and tensile behaviors ofPP/MWCNTnanocompositesatlowspeed.Theyreported thatbothtensile strengthandimpactstrength valueswere increasedwhentheamountofMWCNTwasabout0.6wt%. Dimitriosetal.[24]alsostudiedthemechanicaland rheologi-calpropertiesofCNTfilledPPnanocomposite.Theynotedthat duetothereinforcementeffectofCNTs,thetensilemodulus, tensilestrengthandstoragemodulusofPP/CNTs nanocom-positeincreased,however,aboveacertainCNTscontent,the mechanicalpropertiesare reducedduetoagglomerationof theCNTs.
The rheological properties of composite or nanocom-posite can provide quantitative information to access the microstructuralfeaturesofthesystemrevealinginformation ontheformationofpercolatednetworkstructure,filler disper-sionoragglomeration,orientation,andinterfacialinteraction between filler and polymermatrix. Furthermore, it is also veryimportanttoappraiserheologicalbehaviorsof compos-itesamplessoastounderstandtheeffectofnanotubeson innerstructuresandoperatingpropertiesofpolymer/MWCNT nanocomposites [21,25]. Several studies have reported the influence of MWCNTs and polypropylene grafted maleic anhydride (PP-g-MA) on the rheological behavior of PP nanocomposites[25–31].Po-Hsiangetal.[30]wereprepared thefunctionalizedMWCNTs(f-MWCNT)/PP,pristineMWCNTs (p-MWCNT)/PP and MA-g-PP/f-MWCNT/PP nanocomposites. TheyinvestigatedtheeffectofMWCNTandPP-g-MAonthe crystallization,rheology,andmechanicalbehaviorproperties ofnanocomposites.Irenaetal.[31]werepreparedPP/MWCNT nanocompositeswithnanofillerconcentrationsintherange of0.05–1wt%MWCNTandthepolypropylenegraftedmaleic anhydride (PP-g-MA)amount from 0to7.5wt%. The study wasconductedtoexaminetheeffectofMWCNTandPP-g-MA contentsonthethermal,mechanical,andviscoelastic prop-ertiesofPP/MWCNTnanocomposites.Prashanthaetal.[32]
investigated the rheological properties ofMWCNT-filled PP nanocompositespreparedviameltblendingmethod. Accord-ing to the results of the rheological tests, the fluid-solid transitionwasobservedin2%MWCNTfillednanocomposites. WhentheamountofMWCNTwasincreased,theMWCNT net-workformedinthePPmatrixandsupportedtheincreaseof mechanicalpropertiesbyexhibitingmoresolid-like(i.e.more elastic)behavior.Prashanthaetal.[33]alsostudiedtheeffect ofpolypropylenegraftedmaleicanhydride(PP-g-MA)onthe mechanicalandthermalpropertiesofMWCNT/PP nanocom-positesproducedbythe extrusionmethod.Theyconcluded thatPP-g-MAincreaseddynamicmoduliandviscosityofthe PP/MWCNTnanocomposites.Tensileandflexuralmoduliand Charpy impact resistanceof the PP/MWCNT nanocompos-itesalsoincreasesbytheadditionofPP-g-MA.Pawanetal.
[25] investigatedthe effectof MWCNTsonthe viscoelastic parameters suchascomplex viscosity(*),elasticmodulus andlossmodulus.IncorporationofMWCNTsinthepolymer matrixresultedinhighercomplexviscosity(*),storage(G’)
andlossmodulus(G”)ascomparedtoneatpolymer,especially inthelow-frequencyregion,suggestingachangefromliquid tosolid-likebehaviorinthenanocomposites.Leeetal.[34]also wereexaminedtheeffectofMWCNTandcompatibilizer (PP-g-MA)loadingontherheologicalandelectricalpropertiesof MWCNT/PPnanocomposites.Shearviscosityandyieldstress valueswereincreasedwiththeMWCNTloadingatlowshear rate region due to increased interaction between MWCNT particles. However, rheological and electrical properties of highlyconcentrated MWCNTcomposites withthePP-g-MA werenotimprovedcomparedwithPP/MWCNT nanocompos-ites.TheyareconcludedthatthePP-g-MAdoesnotplayan importantrole in improvementofthe interaction between PP matrix and MWCNT particles. Achmad et al. [27] also examinedtherheologicalandelectricalpropertiesofPP com-positesfilledwithvariousamountsofMWCNTs.Theyshowed thatthestorage(G’),lossmodulus(G”)andcomplex viscos-ity (*) increasedwiththe increasing amountofMWCNTs. Zadhoushetal.[35]werepreparedthePP/MWCNT nanocom-positesusingpolypropyleneandmodifiedcarbonnanotubes (CNTs) via melt-extrusion process. Pure PP was compared withmaleicanhydridegraftedpolypropylene(PP-g-MA), rein-forced with 0.5, 1, and 1.5wt% COOH-modified nanotube (PPgCNT)and polypropylene/unmodifiedCNT nanocompos-ite(PPCNT)in1wt%content.Theywereexaminedtheeffect ofmodifiedCNTandcompatibilizer(PP-g-MA)loadingonthe rheologicalandmechanicalpropertiesofCNT/PP nanocom-posite. Asa result of study, nanocompositewith modified nanotubeshowed highertensile strength,tensile modulus, and lower elongation at break compare with unmodified CNT/PPnanocomposite.Accordingtotheoscillatory rheome-tryresults,storagemodulus,lossmodulus,complexmodulus, andcomplexviscosityofPPgCNTswerehigherthanPPand PP/CNT.
As briefly reviewed above, melt-compounded polyolefin composite and nanocomposites are generally prepared by usingaconsiderableamountofcompatibilizer,atleast two-foldoffilleramountfornanofillers.Compatibilizersconsist ofvariousfunctionalgroups.Compatibilizerusagehasbeen consideredasaformulationnecessityforpolyolefinsystems due to (i) non-polar structures of polyolefins, (ii) improve theinterfacialinteractionbetweenfillerandmatrix,and(iii) resultingfillerdispersion,mechanical,and physical proper-tiesofthecomposite.Thepresentworkhasaimedtoquantify onlytheeffectofMWCNTamount onthephysical proper-tiesofPP/MWCNTcompositeswithoutapossibleimpactof chemicalinteraction, asadifferent pointofview from the literature. Besides the mechanical and thermal properties, rheologicalmodelinghasbeen performedtounderstanding themicrostructuralpropertiesofPP/MWCNTcomposites.In order to investigate the effect of MWCNT addition on the mechanical,thermalandrheologicalpropertiesof polypropy-lenecompositesaseriesofPP-MWCNTnanocompositesthat haveMWCNTloadingswithintherangeof0.1–2.0wt%were producedusingmeltprocessingmethodemployingextruder andinjectionmoldingtechniques.Mechanical,thermaland rheologicalpropertiesofnanocompositesampleswere char-acterized.
2.
Materials
and
methods
Polypropylene was used as matrix material(PP, Exxonmo-bilChemical Company, commercial code: PP3374E3)witha specific gravityof0.9g/cm3 and amelt flowindex (MFI)of
1.3g/10min.(at230◦Cand2.16kg).Multiwalledcarbon nan-otubes(MWCNTs)werepurchasedfromDetsanA.S¸.(Turkey). ThepurityofMWCNTwas declaredtobehigherthan 97% bythemanufacturer.Average outer-diameterand lengthof MWCNTs are about 10–20nm and 10–30m, respectively and theirsurfacearea ishigher than200m2/g.Before melt
processing, MWCNT powder was driedat 80◦C for 24h to eliminate moisture. PP/MWCNT nanocomposite specimens wereproducedwithtwo-stepmixingprocess.Inthefirststep, PP-MWCNTpelletswerepreparedbyusingaco-rotating twin-screwextruderoperatedwithascrewspeedof350rpmand barrel temperature of 160–200◦C from main feeder to die. Inthesecondstep,nanocompositepelletsweremoldedinto 4mmthick-platesformechanicaltestbyusinganinjection molding machine. Thebarrel temperature rangedbetween 215–230◦Candthemouldtemperaturewaskeptat30◦C.The injectionpressureandspeedwere88barand44rpm, respec-tively. MWCNT content inthe nanocomposite composition variedwithintherangeof0.1–2wt%.
Mechanical properties of injection molded PP/MWCNT specimensweremeasuredwithaZwick-Roell-ZR100 univer-saltension/compressiontestmachineatambientconditions (roomtemperatureandhumidity)withacrossheadspeedof 10mm/minaccordingtotheASTM D638standard.Flexural propertiesofnanocompositesweredeterminedwiththesame instrument byemploying athree point bendingtest appa-ratus with a crosshead displacement rate of5mm/min as per theASTMD790 standard.Charpy notchedimpacttests were carried out according tothe ISO 179-1 standardon a Zwick-Roell-HIT5-5Pimpactpendulum.Vtypenotch speci-menssized4×10x40mmwereprepared.Atleastfivesamples weretestedineachnanocompositecompositionandaverage resultswerereported.
Melting and crystallization behaviors of PP and the nanocomposites were analyzedina heatfluxtypeDSC (SII Nanotechnology,ExStar6200).Temperatureandheatflow cali-brationoftheinstrumentwerecarriedoutbyusinghighpurity calibrationstandards,indium(In),tin(Sn),andzinc(Zn) met-als.InDSCanalyses,samplesweighingabout9–10mginan aluminumcruciblewereheatedfrom0to220◦Cwitha heat-ingrateof10◦C/minandkeptatthistemperaturefor2min toremovethethermalhistorythencooledfrom 220to0◦C withacoolingrateof10◦C/minbyusinganelectricalcooling device,ThermoScientificEK90C/SIIintracooler.After complet-ingthemelt-crystallizationprocess,sampleswerekeptat0◦C for2min.thenheatedagainfrom0to220◦Cwithaheating rate of10◦C/min.Allrunswere carriedout undernitrogen (N2)atmosphere(flowrateof50mL/min)topreventthermal
degradationofsamples.Degreeofcrystallinity(Xc%)valuesof nanocompositeswerecalculatedbyusingthefollowingEq.1; Xc(%)= HHo m
whereHm istheexperimentallyobtainedsecond melting
enthalpyvalueofsample(J/g),(1-␣)istheweightpercentof PPinsample,Ho
mistheenthalpyvalueofmeltingofa100%
crystallineformofPP(209J/g)[3,20].
RheologicalbehaviorsofPPandPP/MWCNT nanocompos-iteswerecharacterizedbyperformingseveraltestprocedures in melt statein a rotationalrheometer, AR-G2 (TA Instru-ments).Strainsweeptestswerecarriedoutfrominitialstrain valueof0.1%tofinalstrainvalueof100%withafrequency(f) of1Hz(=6.283rad/s)at190◦Ctodeterminelinear viscoelas-tic(LVE)regionofsamples.Shearmoduli(storagemodulus,G’ andlossmodulus,G”)valueswererecordedasafunctionof shearstrain(%).Then,atimesweeptestwasperformedforPP andPP/1MWCNTnanocompositebyapplyingashearstrainof 10%withaconstantfrequencyof1Hzfor10minat190◦C.In timesweeptest,G’andG”valueswerefollowedasafunction oftime.Frequencysweeptestswereperformedbyoscillating samplemeltwithinafrequencyrangeof0.1–100rad/switha particularstrainvalue(5%)inLVEregionat190◦C. Viscoelas-ticmoduli(G’andG”),dynamic(orcomplex)viscosity(*),and lossfactor(tanı)valueswererecordedasafunctionofangular frequency(ω).Then,creeptestswereimplementedby apply-ingastepstressvalueof100Pa tosamplemeltedat190◦C for3minand resultingshearstrain (%) and compliance(J) responsesofsampleswererecordedasafunctionoftime(s). Instressrelaxationtest,astepstrainof10%wasappliedto thesamplemeltedinequilibriumstateat190◦Candresulting time-dependentdecayinshearstress[(t)]wasobserved.In eachrheologytest,newsamplewasloadedtorheometerand equilibratedinordertoavoidpossibleeffectsof thermome-chanicalconditionsand deformationhistoryonrheological responseofthesample.
Scanningelectron microscopic(SEM)images weretaken on thefractured surfacesof PP/MWCNTnanocomposite. A thinlayerofgoldwassputterdepositedontosamples. Elec-tronmicroscopy imagingofthePP/MWCNTnanocomposite was performed under high vacuum with a NanoSEM 650 SEM instrument operating at 5kV. X-ray diffraction (XRD) methodwasalsousedformicrostructuralcharacterizationof nanocomposites.XRDtestswerecarriedoutbyusinga Pan-alytical(Empyrean) modelwide angle X-rayDiffractometer (XRD)withaCuK␣,runningat45kVand40mA,scanningfrom 10◦to40◦withthestepof2◦/min.
3.
Results
and
discussions
Thereinforcingeffectsofnanofillersaremainlygovernedby theiramountand degreeofdispersioninto thematrix.For thisreason,morphologicalcharacterizationisveryimportant forprobinglevelofnanotubedispersionintopolymermatrix
[10]. SEMimages ofMWCNTand fracturedstructureof PP polymer,0.3wt%and1wt%MWCNTfilledPPnanocomposites wereindicatedinFig.1.ItcanbeseeninFig.1(a)that MWC-NTsarelongandhighlyentangledtubeshavinganaverage diameterof30–50nmandlengthofseveralmicrometers.
Table1representstheresultsofthemechanicalproperties ofPPandPP/MWCNTnanocomposites.Thetensilestrength is increased from 29.7MPa for PP polymer to 35.2MPa for 2%MWCNTfilledPPnanocomposite.Therateofincreasein
tensile strength is 18.4% compared tothe unreinforcedPP polymer. Thebettertensilestrengthobtainedmightbedue toabetterdispersionofMWCNTinpolymer,thehigher sur-faceareaofMWCNTandgoodadhesionbetweenMWCNTand polymer[36,37].ThetensilemodulusofPP/MWCNT nanocom-positesalsoincreaseswiththeincreasingcontentofMWCNT. This indicates that the addition ofMWCNT into PP phase improves stiffnessandrigidity ofcomposites. The percent-ageincreaseinelasticmoduluswasdeterminedas45.0%with respecttothe PP.Similarresultwere obtainedby Prashan-thaetal.[32].TheywereobservedthattheYoung’smodulus increased from 26.9% to 40.2% and yield stress increased 17% to30.3% relativetothe unfilledpolypropylenewithan increase of MWNT concentration. The increase in elastic moduluscanberelevanttothestiffnessfactor ofMWCNT. This factor restrictsthe mobility ofPPchains,reduces the flexibility and results in increased stiffness [37]. A similar relationship was also observed by Ansari et al. [38]. They reported that the elastic modulus of PP composites rein-forcedwithMWCNTandfeldsparincreasedwiththeMWCNT contentbecauseofhigheraspectratioofMWCNTand forma-tionofC CbondingbetweenPPandMWCNT.Dimitrios[24]
reported thattensilestrength and modulusofPP/MWCNTs were increasedwithincreasingMWCNTsduetonanotubes act as reinforcement agents at low CNTs (2–2.5wt%) con-tent while with further increase of MWCNTs, the tensile strengthofPP/MWCNTsnanocompositesdecreasesduetothe formed aggregatesactasmechanical failureconcentrators. Basedonthe previouslyreportedworksandrelevant litera-tureonthesubject,increaseintensilepropertiesofCNTfilled PPcompositesmightbeattributedtoseveralfactors.Firstly, CNTsgenerallypromotethecrystallizationofPPandimprove thedegreeofcrystallinityvaluesthusnanocompositesmay exhibit highermechanicalproperties. Increaseindegreeof crystallinity is alsorelated to the fact thatcompatibilizers dramaticallyimprovetheinterfacialadhesionbetweenCNT surfacestopolymerchainsthereforeinducethenucleation byalsoreducing thesegmentalmobilityofmatrix polymer. Secondly, in the caseofwell-dispersed CNTsinto polymer matrix,fillersallowmoreuniformloaddistributionin com-positestructure.Thirdly,stronginteractionsbetweenpolymer chainsandCNTsenableamoreefficientloadtransferfrom matrixtonanotubes[18,24,39].
Inadditiontotensilestrengthandelasticmodulus,impact propertiesare alsoimportantforpolymernanocomposites. Impact strength ofPPandPP/MWCNTnanocomposites are listedinTable1.Impactstrengthdecreaseswiththe increas-ingcontentofMWCNT.Thisdecreaseisabout18%,depending ontheMWCNTcontent.Reduceinimpactstrengthismore noticeablefortheCNTcontentupto0.3wt%.Thismightbe attributedtotheexistenceofnanotubeagglomerationsinPP thatactasstressconcentrationsandledtocrackinitiation
[32]. Prashanthaet al. [32]haveobtainedsimilar resultsin theirstudyofthemechanicalpropertiesofMWCNTandclay filled PPcomposites. Prashanthaet al.[33]alsostated that the notch impact resistance values of PP nanocomposites arereducedbytheadditionofMWCNTandPP-g-MAintheir another study.Zadhoush et al.[35] stated thatunmodified CNT filledPPnanocomposite showweakerproperties than PP-g-MA and modified CNT filled PP nanocomposite due
Fig.1–SEMimagesofa)MWCNT,b)PPpolymer,c)0.3wt%andd)1wt%filledMWCNT/PPnanocomposites.
Table1–ResultsofmechanicalpropertiesofPPandPP/MWCNTnanocomposites.
Samples Tensile strength(MPa) Tensile modulus(MPa) Elongationat break(%) Impactstrength (kJ/m2) Flexural strength(MPa) Flexural modulus(MPa) PP 29.76 950 150 9.23 26.4 771 PP/0.1MWCNT 30.27 1020 100 9.05 27.8 815 PP/0.2MWCNT 32.64 1168 82 8.71 30.2 886 PP/0.3MWCNT 33.35 1214 39 8.24 32.2 924 PP/1MWCNT 33.74 1289 28 7.85 33.6 1023 PP/2MWCNT 35.26 1378 19 7.41 35.7 1148
tothe lower distribution and crystallizationin unmodified CNTs, resulting in less load transmission. Khodabandelou and Aghjeh [40] represented that the impact strength of PP/CNTnanocompositesisslightlyhigherthanthatofneat PP.However,theadditionofPP-g-MAhasanegativeeffecton theimpactresistanceofPP,becauseofits poormechanical propertiesrelativetotheneatPP.Generally,additionofrigid particlesintoPPreducedtheelongationatbreak.Elongation atbreakresultsofPPand PP/MWCNT nanocompositesare alsogiveninTable1.ItisseenthatneatPPexhibitsductile behaviorspecifiedwiththeelongationatbreakof150%.But, incorporationofMWCNTintoPPyieldedasignificant reduc-tionintheelongationatbreak.Elongationatbreakvalueof PP/2MWCNTsamplewasfoundtobe19%.Juan[41]reported thatreductioninelongationatbreakwasdependentonthe segmentalflexibilityofthe polymerchains.The conforma-tionchangeofpolymerchainisrestrictedbytheMWCNTs, whichinducesadeclineinelongationatbreak.Mertensand Senthilvelan[42]alsoreportedthatCNTsrestrictthemobility ofpolymer chains, and thus the percentage elongation of PPreducedwithincreasingCNTscontentfrom0.5to5wt%. Dimitriosetal.[18]statedthatelongationatbreakvaluesof nanocompositesare reduced becauseofthe aggregation of MWCNTandhighcrystallizationrates.Prashanthaetal.[33]
statedthat inspiteofthe betterdispersionofMWCNTsto PPmatrixwithPP-g-MA,elongationatbreakdecreaseswith
increaseinMWCNTcontent.Intheirstudy,similar elonga-tion breakvalueswereobtainedwithand withoutPP-g-MA becauseofthepresenceoffewmasterbatchaggregates.
AsshowninTable1,bothflexuralstrengthandmodulus significantlyincreasewiththeincreasingamountofMWCNT. FlexuralstrengthofPPincreasedfrom26.4MPato35.7MPaby introducing%2ofMWCNT.FlexuralmodulusofPP/MWCNT nanocompositeswereimprovedsignificantlyastheMWCNT contentwasincreasedfrom0.1to2wt%.Theflexuralmodulus ofPP,771MPa,issteadilyincreasedto815,886,924,1023,and 1148MPaforthenanocompositeshavingMWCNTcontentof 0.1,0.2,0.3,1.0and2.0wt%,respectively.Thisresultsuggests thattheinterfacialinteractionsallowaneffectivestress trans-ferbetweenPPandMWCNT.Atthesametime,theMWCNT canraisethefractureenergyandprovideastronginterfacial shearstress,andforthisreasoncrackpropagationcanbe pre-vented[38].Theflexuralmodulusincreasesapproximatelyup to17.4% and 36.3%inpresenceof0.5%byvolumeof non-functionalizedandamino-functionalizedcarbonnanotubes, respectively.However,furtherincreasesofthefillercontent alwaysinducesslightreductionsofthesameparameter prob-ablyduetotheformationofclustersoragglomerates,resulting inlowerpropertiesattheinterfacewiththehostingmatrix andareducedinterfaceareabetweenCNTsandthematrix[7].
Fig. 2(a) and (b) illustratethe crystallization exotherms and melting endotherms of PP and its nanocomposites,
Fig.2–CrystallizationexothermsandmeltingendothermsofPP/MWCNTnanocomposites.
Table2–CrystallizationandmeltingparametersofPPandPP/MWCNTnanocompositesdeterminedbyDSCanalysis.
Samples Cooling Secondheating
Tc,o(oC) Tc,p(oC) Hc(J/g) Tm(oC) Hm(J/g) Xc(%) PP 112.5 107.6 62.5 152.13 66.1 31.6 PP/0.1MWCNT 112.7 108.2 71.4 150.81 74.8 35.8 PP/0.2MWCNT 113.4 108.4 68.6 151.11 75.9 36.4 PP/0.3MWCNT 113.9 108.6 69.1 151.46 75.6 36.3 PP/1MWCNT 114.6 109.5 65.8 151.08 69.4 33.5 PP/2MWCNT 115.5 109.6 61.6 152.25 68.5 33.4
Tc,o,crystallizationpeakonsettemperature;Tc,p,crystallizationpeakmaximumtemperature;Hc,crystallizationenthalpy;Tm,melting
temperature;Hm,meltingenthalpy(heatoffusion);Xc,degreeofcrystallinity.
respectively. The melting temperature (Tm), crystallization temperature(Tc)anddegreeofcrystallization(Xc%)ofPPand its nanocomposites values are given in Table 2. As repre-sented in Table 2, the crystallization peak onset (Tc,o) and peakmaximum(Tc,p)temperatures increasewiththe
addi-tion ofMWCNTs. Thisnucleating effectof CNTsisknown andhasbeendescribedbyBeate[43].Fortheincorporation of0.05wt%MWCNTscausedanincreaseintheonset crys-tallizationtemperatureTc,o of11◦Cand anincreaseinthe
maximumcrystallizationtemperatureTc,mof7◦Cin
compar-isontothecomparablyprocessedPA66.Additionally,addition of2wt% ofMWCNTincreasedTc ofPPnanocompositesby 11◦C[39],andadditionof1wt%longandshortlengthMWCNT increasedTc valueby13◦C [44].Itwas alsofoundthatthe crystallizationenthalpyvaluesofnanocomposites, normal-izedwithrespecttothePPamount,werehigherthanthatofPP exceptthePP/2MWCNTsample.Itisknownthatthe crystal-lizationpropertiessuchasdegreeofcrystallinity,spherulite size,and crystalliteform (␣ or  crystallites) influencethe mechanicalproperties ofsemi-crystallinepolymers. There-fore,understandingtheeffectofnanoparticlesonnucleation andcrystallinitydevelopmentofsuchmaterialscanbe consid-eredascrucialinformationforquantifyingstructure-property relationships[24,44].TheeffectsofCNTsonthecrystallization behaviorofpolymershavebeen extensivelystudiedby dif-ferentresearchers[24,42,43].AllofthemindicatedthatCNT actedasnucleating agents,which induceeasierand faster crystallizationunderisothermalandnon-isothermal condi-tions.Owingtotheverysimilarcrystallinityandnumerous nucleatingsites,spherulitesizeinnanotubefilledcomposites isexpectedtobemuchsmallerthanthatofunfilledPP.Itwas obtainedthatthemeltingbehaviorandmeltingtemperature
(Tm)ofPPdidnotchangedmuchwiththeadditionofMWCNT. Themelting temperaturerange ofPPand its nanocompos-iteswasinrangeof150–152◦C.Thisfindingisinagreement withtheresultsofotherauthors[43].But,thedegreeof crys-tallinity (Xc%) values of nanocomposites varied depending onthe MWCNTamount.AsseeninTable2,Xc valueofPP increasedabout5%withtheMWCNTamountincreasedupto 0.3wt%thenitslightlyreducedwiththefurtheradditionof MWCNT.TherearedifferentstudiesontheeffectofMWCNT on the crystallinity ofthe PPpolymer. In these studies, it wasdeterminedthattheXc ofPPcouldremainunchanged, declineorincreaseslightlyastheMWCNTloadingincreased. Qiuetal.[45]describedanincreaseintheoverallcrystallinity from28%to33–34%withtheMWCNTincorporation(1wt%) dependingontheMWCNTfunctionalization.Gradyetal.[46]
foundabouta2%increaseintheXcofPPbyaddingof1.8wt% ofSWNTsundernon-isothermalcrystallizationrun.Mertens andSenthilvelan[42]reportedthattheXcofPPincreasedfrom 47.7%to52.5%whentheCNTamountincreasefrom0to1wt% becauseofnucleatingsitesbut,theXcvaluereducedto43.7%
inthecaseof5%CNTbecauseCNTsrestrictedthemobilityof PPchainsandactedasbarrierstothecrystalgrowth.Ersoyand Onder[47]alsoreportedtheXcvaluesofPP/CNTcomposites (0,1.8,4.6,and8wt%)determinedbytheDSCanalysis.The
XcvalueofPPraisedfrom39.7%to44.7%fortheCNTamount of4.6wt%.ButtheyalsofoundthatafurtherincreaseinCNT amountupto8wt%reducedtheXcvaluedownto42%.Irena etal.[31]reportedtheXcvalueofPP/MWCNTnanocomposites increasedfrom43.4%to46.2%byincreasingoftheMWCNT contentfrom0%to5wt%.Theyalsostatedthattheaddition of PP-g-MAin the range of2.5–7.5% does nothave signifi-cantinfluenceonthethermalandcrystallizationbehaviorof
Fig.3–XRDpatternsofPPanditsnanocomposites.
PP/MWCNTnanocomposites.Themeltingandcrystallization peaksbecomebroaderbyincreasingtheamountofPP-g-MA duetotheformationofvariouslysizedcrystals.
MWCNTs can alter the crystallizationprocess and also crystalstructureofnanocomposites[27]. TheX-ray diffrac-tionpatternsofPPandPP/MWCNTsaregiveninFig.3.The characteristiccrystallinereflectionof␣-formofPPwereclearly observedat2of13.9◦(110),16.7◦(040),18.4◦(130),20.9◦(111), 21.0◦(041),and25.3◦(060)inthesepatterns.Accordingtothese results,PPdidnotform-structurecrystalsbut,MWCNT addi-tioninducedtheformationof-crystals(2=15.9◦)(300)under identicalcrystallizationconditions.Asitcanbeseen,the sam-pleswhichcontainsmallconcentrationsofMWCNTs(upto 2.0wt%),includethepeakwhichcorrespondstothereflections from-crystals,whileforhigherconcentrationsofMWCNTs, thepeakdisappearsduetothestrong␣-nucleatingeffectof MWCNTs[18].Althoughmoststudies[27,42,48]pointoutthat MWCNTscould onlyleadtothe formationof␣-PPcrystals, onlyafewstudies[18,39]declaredthatMWCNTsmaycreate -phasePP.
Fig.4(a)showsG’valuesofnanocompositesasafunction ofshearstrain.ItisseenthatallsamplesshowaNewtonian behavioratlowshearstrainregionuptoabout30%.Then,they exhibitatransitionregionfromNewtoniantonon-Newtonian behavior.Thistransitionregionspecifies linear viscoelastic (LVE)rangeforasampleandthestoragemodulus(G’)valueat thisNewtonianplateauiscalledplateaumodulus(G’p).Itwas
foundthatincreasingMWCNTamountimprovestheG’p,as
expected.TheG’pvaluesofPPandPP/2MWCNTsampleswere
foundtobe4050and14,700Pa,respectively.Fig.4(b)compares G’curvesofPPandPP/1MWCNTnanocompositerecorded dur-ingtimesweeptestinlinearscale.Timesweeptestisafast, practical,andpreciseindicatorforobservingeffectof ther-malissueson rheologicalbehaviorsofapolymermelt.If a thermaldegradationoccurs inapolymermelt,modulusor viscosityvaluesreduceasaresultofdecreaseinmolecular weight(Mw)whilethesevaluescouldalsoshowanincrease incaseofcrosslinkingandevaporationofsolventorvolatile compounds.AvariationinG’(orshearviscosity, *)within ±5%ofG’pisgenerallyacceptedasthestabilitylimitfora
par-ticulartesttime.ItwasfoundthatchangesinG’pwereabout
1.0%and2.5%forPPandPP/1MWCNTsamples,respectively. Thisresultindicatesthatallspecimensarestableat190◦Cfor 10minandfurthertestprocedurescanbeappliedsafely.
ViscoelasticpropertiesofPP/MWCNTsnanocompositesas a function of MWCNT contents are presented in Fig. 5(a)
and(b).Itisseenthatbothstorageandlossmodulusofthe PP/MWCNTcompositesincreasedsignificantlyrelativetothe PPmatrixwithadditionofMWCNT,especiallyinthelow fre-quencyregion.Inthelowfrequencyregion,thisincreaseis more pronouncedforG’ values.This behavioris a charac-teristicresponseofthermoplasticcompositesinLVEregime whichgenerallyleadstosolid-likeorpseudosolid-like behav-ior. This phenomenon can be explained by the fact that, withincreasingMWCNTcontent,thenanotube-nanotubeand polymer-nanotube interactions increased[49] which led to theformationofinterconnectedornetwork-likestructureof MWCNT,whichrestrictedthesegmentalmobilityofmatrix chains[50].SimilarresultswerereportedbyPrashanthaetal.
[33]forthePP/MWCNTnanocompositespreparedwithand withoutPP-g-MAascompatibilizer.Atlowfrequencies, typi-calhomopolymer-liketerminalbehaviorwasobservedinthe caseofPPandPP/PP-g-MA/1%MWCNT,whileatransitionfrom liquid-liketosolid-likebehaviorwasobservedfortheMWVNT content of2wt%. This behaviour can be attributed to the formationofananotubenetworkinthepolymermatrix(as also called percolation threshold). Therefore, the solid-like (orpseudosolid-like)behaviorcanbeattributedtothe exis-tenceoffrictionalinteractionsbetweenthehighlyanisotropic particles.Lee etal. [34]statedthat the G’and *valuesof PP/MWCNTcompositespreparedwithPP-g-MA(forevenmore than anamount of3wt%)showed similar values to those ofPP/MWCNTnanocompositeswithoutPP-g-MA.Itcouldbe concluded that the rheological properties of thermoplastic compositesaremoreMWCNTamountthanPP-g-MAloading inMWCNT/PPcompositesystem.ImprovementinG’at low-frequencyregioncould beusedtoquantifymicrostructural features of thermoplastic nanocomposites and determine someimportantparameters suchasaspectratio, filler dis-persion and agglomeration issues, formation of physical network(percolationthreshold)and/orpolymer-filler interfa-cialinteractions basedonseveralmathematicalapproaches
[25,27,34].Theeffectofnanotubesontherheologicalbehavior ismarginalathighfrequencyregion.Thisbehaviorsuggests thattheCNTsactasareinforcingagentorstiffeningagentat lowerfrequency,whereasathigherfrequencieschainslippage mayberesponsibleforthesimilarrheology[25].Inthisstudy, furtherquantitativeestimations arenotattemptedbecause thegeometricalfeaturesofMWCNTarealreadywell-known.
Fig. 6 demonstrates complex viscosity (*) curves of PP and nanocompositesasa functionofshearratewhichare alsoknownasflowcurves.Flowcurvesofsampleswere con-stitutedbyusingdynamicshearviscositydataoffrequency sweeptest,accordingtoempiricalCox-Merzrule[51].The Cox-Merzrulesimplysuggestsaconnectionbetweenthecomplex viscosity*(ω)measuredinfrequencysweep(atafixedstrain amplitude withinLVEregime)and thesteadyshear viscos-ity()measuredasafunctionofshearrate(),definedas followingEq.2;
()∼=∗(ω)=ω (2)
The Cox-Merz rule has been extensively tested and acceptedtobeapplicableasapracticalwayformany poly-meric systems since 1958. Flowcurves ofnanocomposites
Fig.4–Storagemodulus(G’)ofPPandPP/1MWCNTnanocompositeasfunctionsofshearstrainandtime.
Fig.5–G’andG”ofPPandPP/MWCNTnanocompositesasafunctionoffrequency.
Fig.6–Complexviscosity(*)ofPPandPP/MWCNT nanocompositesasafunctionofshearrate.
werefittedbytheWilliamsonmodel[52].Williamsonmodel isdefinedasEq.3;
= o
1+(K). m (3) whereo isthezeroshearrateviscosity(Pa.s),is. theshear rate(s−1),Kistheconsistencytimeconstantrelatedtothe relaxationtime(s),andmistherateindex.Williamsonmodel parametersaregiveninTable3.
ItwasfoundthattheoandKvaluesofnanocomposites
readilyincreasedwiththeincreasingamountofMWCNT.This resultindicatesthat introducingofMWCNTimprovesmelt viscosityatlowshearrateregionandrelaxationtimeofPP duetothe reducingchainmobility. But,the flowcurves of PPandnanocompositesalsoimplythattheviscositycurves ofallsamplescouldpossiblymergeathighshearratesand
Table3–WilliamsonmodelparametersofPPandits nanocomposites. Sample o(Pa.s) K(s) m PP 2877 0.1546 0.6198 PP/0.1MWCNT 2978 0.1612 0.6067 PP/0.2MWCNT 4366 0.3477 0.5916 PP/0.3MWCNT 5392 0.6171 0.5583 PP/1MWCNT 8161 0.7663 0.6419 PP/2MWCNT 15060 0.9785 0.6483
becomecomposition-independent.Itcanbeconcludedthat thesePP/MWCNTnanocompositeswouldnotinducea pro-cessingdifficultyinprocessingoperationsconductedathigh shearratessuchasinjectionmolding,fiberspinningetc.Effect ofMWCNTadditiononthe relaxationtimesofPPwasalso determined byemployingcrossoverpoint. Fig.7(a)displays representative shear moduli (G’ and G”) curves of PP and PP/2MWCNTsample.Cross-overmodulus(Gx)andfrequency
(fx)whereG’equalstoG”,canbesimplyusedtocomparemelt elasticityandrelaxationtimesofsamples.Thecrossoverpoint isalsorelatedtomeltstrengthofapolymer.Therelaxation time(,ins)isexpressedasthereciprocaloffx(inHz=s−1).Gx
andvaluesofnanocompositesasafunctionMWCNTamount isgiveninFig.7(b).ItwasfoundthattheGxvaluesreducedbut
valuesincreasedwiththeMWCNTamount.Improvementin valuescanbequantifiedwithasimplepower-lawequation giveninFig.7(b).
Fig.8(a)representssemi-logarithmiccreepcompliance(J) graphsofPPnanocompositesunderastressvalueof100Pa at190◦C.Complianceisknownastheinversemodulus(1/G, Pa−1)mathematicallyand generallyused tocalculate long-term deformationofapolymericsystemunderaparticular thermomechanicalcondition.Lowercompliancevaluerefers
Fig.7–a)Arepresentativeshearmodulus(G’andG”)curvesofPPandPP/2MWCNTsampleandb)Gxandvaluesof nanocompositesasafunctionofCNTamount.
Fig.8–a)Creepcompliance(J)graphsofPPandPP/MWCNTnanocompositesandb)ovaluesofnanocompositesasa
functionofCNTamount.
Fig.9–Time-dependentdecayinshearstressvaluesofPPanditsnanocompositesinarealandrelativey-scales.
tohighermodulusandthuslowerdeformation(strain).Creep compliancecurveswerefittedwiththeequationofdiscrete retardationspectrum(DRS)and◦valuesofnanocomposites
wereestimated.TheequationofDSRcanbedefinedasfollows Eq.4; J(t)=Jo+
(Jk(1−exp(−t k)))+ t o (4)whereJo istheinstantaneouscomplianceando isthezero
shear viscosity. The parameters Jk and k are referred to
the compliance and retardation time of the kth element, respectively.Thisequationisastandardrheologicalmodelfor modelingLVEbehavior[53].Modelfitwassucceededwithsix elements(k=6),butJkandkvalueswerenotreportedhere.
Fig.8(b)comparesovaluesofnanocompositesasafunction
ofMWCNTamountbytwodifferentapproaches;Williamson modelandDSR.Itisclearlyseenthatdifferenttestprocedures andmathematicalapproaches yieldalmost thesame trend inviscosityincrease.Introducing2wt%ofMMWCNTintoPP matrixincreasestheovalueofPPmorethanfive-fold(from
3000Pa.sto16,000Pa.s).
StressrelaxationcurvesofPPanditsnanocompositesare representedinFig.9(a).Thesecurvesshowthetime depen-dentstressdecayofspecimens.Itwasobtainedthatincreasing MWCNTamountincreasedtheinitialstress(o)level.Itwas
foundthatMWCNTdecreasedtherelaxationrateofPP.This effect can bemoreprecisely seen inFig. 9(b)which repre-sents the semi-logarithmic stress decay in the initial part of test. Slope of the given lines can be used to compare the initial rate of relaxation. It is obviously seen that the introduction of MWCNTdecreases the slope which points out a significantdecrease in relaxationrate.As aresult, it can be concluded that time-dependent rheological behav-iorsofPP/MWCNTnanocompositesconfirmtheirstrainand frequency-dependentbehaviorsmentionedbefore.
4.
Conclusion
In this study,a seriesofPP/MWCNT nanocompositeshave been preparedaccordingtomeltprocessingmethodsusing atwinscrewextruderandaninjectionmolding.Theeffects ofMWCNTsloadinginarangeof0.1–2wt%onthethermal,
rheological, and mechanical properties of MWCNTs filled PPnanocompositeshavebeen measureddepending onthe MWCNTamount.
It hasbeen found that the tensile and flexural proper-ties of PP/MWCNT nanocomposites increased in line with the increment of MWCNT amount. The impact strength of nanocomposites decreased with the MWCNT addition. Meltrheologytestsdemonstratedthatthestoragemodulus (G’),loss modulus (G”),and complex viscosity (*) of sam-plesraisedwiththeincrementoftheamountofMWCNTs. Basedonthethermalandmechanicaltestresults,0.3wt%of MWCNTcouldbeconsideredasthecriticalfilleramountalso calledas“percolationthreshold”forimprovingthesolid-state physicalpropertiesofPP/MWCNTnanocomposites.However rheological measurements did not yieldsuch a distinctive composition. Melt rheology tests showed that PP/MWCNT nanocompositescouldbeeasilyprocessedathighshearrates toproducenanocompositeshavingimprovedlong-termtime dependentphysicalproperties.Consequently,itcanbe con-cludedthatthesecompositescanbepromisingcandidatesfor beingusedindifferentengineeringapplications.
Declaration
of
conflicting
interests
Theauthor declared no potentialconflicts ofinterest with respecttotheresearch,authorship,and/orpublicationofthis article.
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