Morphology, mechanical, cross-linking, thermal, and tribological properties of nitrile and

Applied Surface Science 257 (2010) 565–573Contents lists available at ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scMorphology,mechanical,cross-linking,thermal,and tribological properties of nitrile and hydrogenated nitrile rubber/multi-walled carbon nanotubescomposites prepared by melt compounding:The effect of acrylonitrile content and hydrogenationBlaˇz Likozar ∗,Zoltan Major 1Polymer Competence Center Leoben GmbH,Montanuniversität Leoben,Roseggerstrasse 12,A-8700Leoben,Austriaa r t i c l e i n f o Article history:Received 15February 2010Received in revised form 7July 2010Accepted 13July 2010Available online 17 July 2010Keywords:Multi-walled carbon nanotubes NanocompositesMechanical properties Elastic propertiesDynamic mechanical thermal analysis (DMTA)Hydrogenated nitrile rubbera b s t r a c tThe purpose of this work was to prepare nanocomposites by mixing multi-walled carbon nanotubes(MWCNT)with nitrile and hydrogenated nitrile elastomers (NBR and HNBR).Utilization of transmission electronic microscopy (TEM),scanning electron microscopy (SEM),and small-and wide-angle X-ray scat-tering techniques (SAXS and WAXS)for advanced morphology observation of conducting filler-reinforced nitrile and hydrogenated nitrile rubber composites is reported.Principal results were increases in hard-ness (maximally 97Shore,type A),elastic modulus (maximally 981MPa),tensile strength (maximally 27.7MPa),elongation at break (maximally 216%),cross-link density (maximally 7.94×1028m −3),density (maximally 1.16g cm −3),and tear strength (11.2kN m −1),which were clearly visible at particular acry-lonitrile contents both for unhydrogenated and hydrogenated polymers due to enhanced distribution of carbon nanotubes (CNT)and their aggregated particles in the applied rubber matrix.Conclusion was that multi-walled carbon nanotubes improved the performance of nitrile and hydrogenated nitrile rubber nanocomposites prepared by melt compounding.© 2010 Elsevier B.V. All rights reserved.1.IntroductionNanostructured materials gained great importance in the past decade on account of their wide range of potential applications in many areas.Multi-walled carbon nanotubes (MWCNT)are one of the most promising reinforcing fillers utilized in such nanos-tructured materials and their application ranges from general fields,due to almost universally improved properties of their nanocomposites,to specific fields such as microcontact printing [1],functional carbon nanotube devices [2],polymer artificial muscles [3],nanofluidic devices [4],microwave absorbers [5],etc.More than a few reviews of generally improved properties of polymer nanocomposites have been published in the recent years,some of them researching elastomer nanocomposites,specifically.Schaefer and Justice [6]overviewed the factual nanoscaled phe-∗Corresponding author.Present address:Faculty of Chemistry and Chemical Technology,University of Ljubljana,Aˇs kerˇc eva cesta 5,1000Ljubljana,Slovenia.Tel.:+38612419500;fax:+38612419530.E-mail addresses:blaz.likozar@fkkt.uni-lj.si (B.Likozar),zoltan.major@jku.at (Z.Major).1Present address:Institute of Polymer Product Engineering,Johannes Kepler Uni-versität,Altenberger Straße 69,A-4040Linz,Austria.nomena in nanocomposites,Maiti et al.[7]analyzed elastomers containing different nanofillers,whereas Bokobza [8]examined multi-walled carbon nanotubes/elastomer composites,respec-tively.Nitrile and hydrogenated nitrile rubbers (NBR and HNBR)are high-performance rubbers with combination of excellent mechan-ical properties,abrasion resistance,adhesion to fabrics and hot air aging resistance,which can be used in many diverse appli-cations.Such a high-performance elastomeric material may thus,should it be combined with the filler as versatile as MWCNT,yield utterly applicable nanocomposites.Research of the latter,however,is scarce.Yang et al.[9]and Guo et al.[10,11]considered relatively inexpensive alternative fillers for (H)NBR,that is graphite and hal-loysite,respectively.The price of MWCNT,however,is decreasing steadfastly.The only three studies were published by Kader et al.[12],who analyzed the morphology of conducting filler-reinforced nitrile rubber composites by electrostatic force microscopy,Felhos et al.[13],who examined the tribological testing of the perox-ide cured HNBR with different MWCNT and silica contents under dry sliding and rolling conditions against steel,and Lu et al.[14],who studied hydrogenated carboxylated nitrile rubber reinforced by multi-walled carbon nanotubes.Up to date,there has not been a study to the best of our knowl-edge,which would critically examine morphology,mechanical and0169-4332/$–see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2010.07.034566 B.Likozar,Z.Major/Applied Surface Science257 (2010) 565–573Table1Formulation of the peroxide-curing system for(H)NBR/MWCNT compounding. Materials Amounts(phr a)Nitrile and hydrogenated nitrile rubber(NBR and HNBR)100Cross-linking agent:dicumyl peroxide(DCP)b15Filler:multi-walled carbon nanotubes(MWCNT)30a Parts per hundred rubber parts in weight.b PEROXAN DC-P+.curing properties of both unhydrogenated and hydrogenated nitrile rubbers with varying acrylonitrile contents/MWCNT composites,so the latter was consequently the aim of this study.2.Materials and methods2.1.MaterialsNitrile rubber(NBR)bulk(Intene50,Europrene N1945GRN, Nitrilast-26M,KER N-29,Europrene N3345,and Europrene N3945 of acrylonitrile(AN)content of0.0–39.0wt%)was obtained from Semperit Technische Produkte GmbH(Austria).Hydrogenated nitrile rubber(HNBR)bulk(Therban LT2007,Therban A3407, Zetpol2020L,and Therban A3907of acrylonitrile(AN)content of21.0–39.0wt%)was obtained from Erwin Mach Gummitechnik GmbH(Austria).Multi-walled carbon nanotubes(MWCNT)powder (MWCNT content of90wt%and the aspect ratio of approximately 1000)was provided by Cheap Tubes Inc.(USA).Other chemicals and compound curing additives were purchased from stores and used as received.2.2.Preparation procedureMWCNT were mixed with rubbers for0.5h at100◦C using melt compounding(MC),which is the commonest rubber mixing technique,so that they were added directly into(H)NBR in Braben-der Plasti-Corder PLD-Type651(Germany)instrument equipped with W50C measuring mixer.In a typical procedure,the rub-ber/MWCNT ratio of100g/30g was used.For regular tests,a peroxide curing system was added(15g dicumyl peroxide(DCP)/130g compound).(H)NBR/MWCNT com-pounds were thus further mixed in Buzuluk(Czech Republic) two-roll mill with the curing ingredient for8min at45◦C according to the recipe in Table1.To prepare samples for all tests,curing time (t90)was determined using Monsanto Alpha Technologies MDR 2000(USA)moving die rheometer(MDR)for12min at170◦C.Prior to testing,the compounds were vulcanized in a press for2–4min at the temperature and pressure of170◦C and20MPa,respec-tively,using the optimum cure time(t90),determined from the MDR results.2.3.Characterization and testingThe morphologies of the cut surfaces of the compounds and MWCNT were determined by scanning electron microscopy (SEM)(Zeiss DSM962,Germany)at3kV.Transmission electron microscopy(TEM)observations were performed with FEI Tecnai 12(USA)microscope equipped with the embedded CCD camera at the acceleration voltage of120kV(the room temperature-and the cryo-cut samples with the thickness of85nm and120–150nm, respectively).Wide-and small-angle X-ray scattering(WAXS and SAXS)data(compounds and MWCNT),that is intensity(I)ver-sus scattered angle(2Â),was acquired with Bruker AXS Nanostar (USA)diffractometer and Cu K␣1radiation(8047.9eV)was used as X-ray source(the resolution of0.005◦for SAXS and0.1◦for WAXS).Fourier transform infrared spectroscopy(FT-IR)spectra of the samples,that is transmittance(T)versus wavenumber( ),were recorded by PerkinElmer FT-IR System Spectrum GX(USA)spec-trometer using the attenuated total reflectance(ATR)technique with the resolution of2cm−1.All mentioned measurements were conducted at the room temperature.Shore A hardness of the vulcanizates was measured according to ASTM D2240and DIN53505using type Shore A durometer(Zwick Härteprüfgerät,Germany)equipped with the Shore A-type inden-ter at three different locations of a sample(>6mm in thickness) to obtain the average value.IRHD hardness of the vulcanizates was measured using IRHD durometer(Zwick,USA)equipped with the steel ball-type indenter at three different locations of a sam-ple(>6mm in thickness)to obtain the average value.Tensile tests of the dumbbell specimens of the vulcanizates were carried out on MTS Polymer Test System(USA)testing machine at the speed of500mm min−1(displacement and force were measured every 0.001s,each specimen’s dimensions were measured three times, each test was repeated three times)to construct stress( )ver-sus strain(ε)diagrams.Storage modulus,E ,and loss tangent, tan(ı),were measured as a function of temperature(T)on Bose (USA)dynamic mechanical analyzer(DMA)under tension mode at 1Hz and10Hz,and3K min−1(each specimen’s dimensions were measured three times).For density determination,the Archimedes principle(the buoyancy method with water)was adopted.The test rig,that is the density measurement kit,was adjusted to Mettler Toledo DeltaRange(Switzerland)microbalance(each specimen’s dimensions were measured three times,each test was repeated three times).To determine the tear strength,the right angle-type specimen with cut was subjected to500mm min−1deformation rate on MTS Polymer Test System(USA)(displacement and force were measured every0.001s,each specimen’s dimensions were measured three times).The cross-link density,i.e.the elastically active network chain density,n(mol cm−3),was determined by the Flory–Rehner equation[15]through equilibrium swelling experi-ments using toluene as a solvent at the ambient temperature:n=−n(1− 2)+ 2+ 1 22V1( 1/32− 2/2)(1)where 2is the polymer volume fraction in the vulcanizate, swollen to equilibrium,and V1is the solvent molar volume (106.6cm3mol−1for toluene at23◦C). 1is the rubber–toluene interaction parameter and is taken between6.49×10−5and0.797 for(H)NBR depending on acrylonitrile content and hydrogenation [16–18](each specimen’s dimensions were measured three times). All mentioned measurements except for DMA were conducted at the room temperature.The thermal conductivity of nanocomposites was measured by hot disk thermal constants analyzer TPS2500(Sweden)using the transient plane source technique(each test was repeated three times).Tribological properties were evaluated using the pin-on-plate(POP)(the velocities of2.5mm s−1,25mm s−1,and 250mm s−1),the fretting(the velocity of20mm s−1),and the ball-on-plate(BOP)(the velocities of2.5mm s−1,25mm s−1,and 250mm s−1)testers(UMT–Universal micro-tribometer manufac-tured by Center for Tribology,USA).All mentioned measurements were conducted at the room temperature.3.Results and discussion3.1.Morphology of rubber/nanotubes compositesCarbon nanotubes,obtained by catalytic chemical vapor depo-sition,were used as received(see Fig.1a–c).When the carbon nanotubes were directly mixed with(H)NBR in a brabender mixer, their dispersion was good as evidenced by the absence of anyB.Likozar,Z.Major /Applied Surface Science 257 (2010) 565–573567Fig.1.Characterization of morphology –SEM micrographs showing (500–10,000×magnification):(a–c)MWCNT powder;(d–f)BR/MWCNT nanocomposite prepared by MC;(g–i)HNBR/MWCNT nanocomposite with 36.2wt%AN content prepared by MC.large MWCNT aggregates and any large isolated amount of rub-ber matrix without carbon nanotubes as a filler in Fig.1d–i.That is,melt compounding gives a fine and uniform dispersion of carbon nanotubes in (H)NBR as shown in Fig.1d–i.30phr of MWCNT is a high content,and thus a poor dispersion by melt mixing would be ually,the homogeneity of the MWCNT-reinforced polymer nanocomposites may be achieved by solution mixing [19],but in this case a great excess of solvent would be needed per rubber amount.In our case,the high shear and temperature dur-ing compounding proved to grant excellent homogeneity.It can be seen that the cross-section dimensions of the dispersed carbon nanotubes’aggregates are quite small (tens of nanometers)with a few tens of micrometers scale length.This causes some difficul-ties in clearly observing the diameter and the length of nanotubes with TEM at once,especially in distinguishing the length of indi-vidual carbon tubes.By comparing Fig.1d–f with g–i,it can clearly be seen that the interface between the carbon tubes and (H)NBR is greatly improved in the case of HNBR/MWCNT nanocomposite with 36.2wt%AN content prepared using MC.This definitely enhanced the tensile strength of this carbon nanotubes composite.As shown in Fig.2,the dispersion quality of carbon nanotubes in these rubber matrices is truly quite satisfactory.MWCNT of different diame-ters and lengths seem to be completely exfoliated that is separated one from another.MWCNT apparently break during processing as the aspect ratio is quite lower than the initial 1000.Nonetheless,the latter and the exfoliation definitively should have a positive effect on mechanical and other properties.As dimensions reach the nanometer level,interactions at phase interfaces become largely improved,and this is important to enhance material properties.The MC method can be used with MWCNT,with the possibility to attach the polymer to the surface through van der Waals or forces or hydrogen bonds,which optimize the interface bonding and,conse-quently,the mechanical properties.In addition,in the TEM images (Fig.2),elongated voids in the amorphous phase are clearly vis-ible being of a typically random orientation,where the interface between the amorphous and the MWCNT phase was before cryo-cutting.Moreover,MWCNT synthesis and processing often provide functional groups that can react with the polymer matrix or initiate the cross-linking of polymers to improve strength of the interface between the inorganic component and the polymer matrix.The latter improves the nanocomposite properties in addition to the weak interactions at the unhydrogenated and hydrogenated nitrile rubbers–nanotubes interface to which a portion of the improve-ment may be attributed.568 B.Likozar,Z.Major/Applied Surface Science257 (2010) 565–573Fig.2.Characterization of morphology–TEM micrographs showing(110,000–150,000×magnification):(a and b)BR/MWCNT nanocomposite prepared by MC;(c and d) HNBR/MWCNT nanocomposite with36.2wt%AN content prepared by MC.Fig.3.Characterization of morphology–X-ray diffractograms(intensity vs.scattered angle)showing:(a)wide-angle X-ray scattering(5–30◦)and(b)small-angle X-ray scattering(1–5◦)of MWCNT powder,BR/MWCNT nanocomposite and HNBR/MWCNT nanocomposite with36.2wt%AN content prepared by MC.B.Likozar,Z.Major/Applied Surface Science257 (2010) 565–573569Table2Properties of(H)NBR/MWCNT nanocomposites prepared by MC.AN content(wt%)0.019.028.529.033.039.021.0(H)34.0(H)36.2(H)39.0(H)Hardness(shore,type A)97928787898776798080 Hardness(IHRD)99919187919184838583Elastic modulus at0.1%elongation (MPa)980.8a90.9a48.9a43.1a65.4a71.8a13.7a20.1a24.6a16.5a 1074.2b131.5b73.6b61.1b107.0b125.1b14.3b22.9b27.1b17.9bElastic modulus at100%and200% elongation(MPa)–––––––11.8–15.098.3––––––––12.9–Tenstile strength(MPa)17.317.213.519.220.414.77.021.027.79.6 Elongation at break(%)17394877577981187216125 Permament set(%)2111101101Average molecular weight between cross-links(g/mol)9c93c174c194c132c118c572c408c331c485c 57d219d663d911d676d373d384d319d255d265dCross-link density(×1026m−3)794.2c73.6c39.6c34.9c53.0c58.2c11.1c16.3c19.9c13.3c118.8d31.2d10.4d7.4d10.3d18.4d16.6d20.8d25.8d24.5dLoss tangent at the glass transition temperature 0.067e0.342e0.529e0.545e0.497e0.535e0.591e0.633e0.656e0.651e 0.074f0.367f0.536f0.551f0.486f0.587f0.623f0.617f0.684f0.653fDensity(g cm−3) 1.131 1.134 1.144 1.126 1.158 1.142 1.057 1.102 1.095 1.075 Tear strength(nK m−1) 4.18.28.17.410.511.27.18.611.29.0a Determined by dynamic mechanical analysis at25◦C and1Hz.b Determined by dynamic mechanical analysis at25◦C and10Hz.c Determined by dynamic mechanical analysis at25◦C.d Determined by equilibrium swelling in toluene at23◦C.e Determined by dynamic mechanical analysis at1Hz.f Determined by dynamic mechanical analysis at10Hz.3.2.XRD analysis of nanocomposite structuresThe microstructure of the(H)NBR/MWCNT nanocompositesprepared by MC was further studied by XRD,which revealed thenon-existence of ordered intercalated and exfoliated microstruc-tures(see Fig.3),as indicated by the absence of any well-definedreflections in the small-angle range.Fig.3shows the XRD patterns of MWCNT powder.MWCNTdisplay only one diffraction peak at2Â=25.4◦,corresponding toa basal spacing of0.350nm,which is the lamellar characteristicof multi-walled carbon nanotubes.After melt compounding withrubber,the XRD pattern of MWCNT changed.That is,after meltcompounding with rubber,a new(002)diffraction peak emergedat2Â=26.7–26.8◦,corresponding to a basal spacing of0.335nm,which is due to the graphitization of the MWCNTfiller.However,the peak corresponding to the one at2Â=25.4◦for MWCNT powderstill remained,and even had approximately the same intensity.The reason is that not all of the carbon nanotubes were in theirinitial state after the melt compounding.However,with the right melt compounding conditions,the initial,a few tens of nanome-ters thick carbon nanotubes,could be retained in the rubbery dispersion,and those nanotubes were from all sides surrounded by rubber molecules.The XRD pattern in thesmall-angle range(1–5◦)of the MWCNT powder is shown in Fig.3,and did not display any well-defined diffraction patterns for the ordered structure.After the addition of MWCNT to(H)NBR,the rubber chains surrounded the nanotubes and formed a network in bulk phase and carbon nanotubes were separated by strong mechanical forces.During subsequent mix-ing and heating,stresses and high temperature caused the(H)NBR macromolecules and carbon nanotubes to exfoliate simultaneously and a nanoscale dispersion of carbon nanotubes was hence formed in the(H)NBR matrix.This is similar to the process of preparing rub-ber/clay[6,7,10],rubber/silica[7],rubber/nanotubes[6–8,12–14], rubber/graphite[7],rubber/metal carbonate,oxide,sulfate,or methylacrylate[7]nanocomposites by MC.It is noted that there is competition between the separation of carbon nanotubes by bulk rubber and the re-aggregation of the nanotubes,which depends on many factors,such as nanotubes’loading,acrylonitrile content in rubber,type of nanotubes,rubber type and its hydrogenation.After melt compounding,the primary diffraction peak of nan-otubes at wide-angles has shifted to a lower angle of24.1–24.3◦(Fig.3a),indicating that a majority of MWCNT became indi-vidually intercalated by(H)NBR macromolecules.Another peak at17.4–18.1◦is thought to be due to the(H)NBR structure, e.g.the structure of(H)NBR macromolecules between carbon nanotubes.In the wide-angle range,the diffraction of nan-otubes at2Â=24.1–24.3◦,corresponding to a basal spacing of 0.336–0.339nm,is a bit more weak in the HNBR/MWCNT nanocom-posite with36.2wt%AN content prepared by MC than that of polybutadiene with the same loading of MWCNT,as shown in Fig.3a.The lower intensity of the peak at2Â=24.1–24.3◦for HNBR/MWCNT composite with36.2wt%AN prepared by MC can be ascribed to both the cracking of nanotubes during the thermal expansion and consequent exfoliation,and the overlapping of car-bon nanotubes and rubber peaks.Disregarding broken MWCNT,Fig. 4.Characterization of mechanical properties–tensile properties(stress vs. strain)of(H)NBR/MWCNT nanocomposites with different AN contents prepared by MC.570 B.Likozar,Z.Major /Applied Surface Science257 (2010) 565–573Fig.5.Characterization of mechanical properties–dynamic mechanical properties (storage modulus (a)and loss tangent (b)at 1Hz vs.temperature (−100to 100◦C))of (H)NBR/MWCNT nanocomposites with different AN contents prepared by MC.however,the thickness of any remaining aggregate is generally nano-scale due to the separation effect of the rubber chains.These observed results indicate that MC gives a great extent of exfoliation,intercalation and separation of nanotubes in the (H)NBR matrix.3.3.Mechanical propertiesThe effects of AN content on both mechanical and physical properties are summarized in Table 2.Fig.4shows typical ten-sile stress–strain curves of the vulcanizates filled with the same amounts of MWCNT by MC,and comparisons of their reinforcement efficiency in different rubbers (i.e.hydrogenated and unhydro-genated)and with different acrylonitrile content (i.e.0.0–39.0wt%)at the same loading of 30phr,respectively.For nanocomposites prepared with HNBR,the tensile strength increases substantially with AN content in the range 21.0–36.2wt%.This is probably due to optimally cross-linked structure at 36.2wt%,which may be seen from the highest cross-link density of this nanocomposite in Table 2.Generally,hydrogen atom is abstracted easier from tertiary carbon atoms in acrylonitrile units than from secondary or vinyl carbon atoms in methylene and butadiene units,respectively,and the first should in turn produce more cross-links.One would there-fore expect the tensile strength to increase even from 36.2wt%to 39.0wt%AN content.Nonetheless,thus formed tertiary carbon rad-icals also tend to be more stable and less reactive,and apparently there is an optimum in the ratio between hydrogen abstraction and cross-link formation reactions at 36.2wt%AN content.Also,the stress at small strain (under 20%)increases more remarkably with increasing AN,when compared to NBR/MWCNT (30phr)com-posites by melt compounding (in this case the stress at small strain is actually decreasing with AN content)(Fig.4).The different ten-sile properties of (H)NBR/MWCNT vulcanizates prepared by MC can be attributed to the different cross-linking quality of nanotubes’compounds.Nonetheless,the fine dispersion of carbon nanotubes achieved by MC generates more interfacial interaction,resulting in good reinforcement.It can be seen from Fig.4that HNBR/MWCNT (36.2wt%AN content)nanocomposite prepared by MC exhibits the highest tensile strength and elongation at break compared to those of other reinforced composites,such as all presented NBR/MWCNT nanocomposites.The high aspect ratio and large surface areas of carbon nanotubes offer a strong reinforcement.In addition,the polar groups on the MWCNT surface,e.g.–OH and –COOH,might interact physically and chemically with rubber macromolecules,leading to strong interfacial interactions,which are further sub-stantiated by the increased glass transition temperatures of the nanocomposites,which will be discussed in one of our future papers.3.4.Dynamic mechanical propertiesFig.5shows the temperature dependence of the storage mod-ulus (E )and tan(ı)of (H)NBR/MWCNT nanocomposites.It can be seen that the storage modulus for NBR/MWCNT nanocomposites at a given temperature above T g decreases with increasing AN con-tent up to 29.0wt%when it starts increasing again,resulting in a decreased E compared with polybutadiene (BR)with no acryloni-trile content.In this case,the same applies as in the case of tensile mechanical properties,that is that the lowest cross-link density of the nanocomposite with 29.0wt%AN content in NBR,presented in Table 2,results in the lowest glass transition temperature.At this particular AN content the hydrogen abstraction and addition reactions of primary radical seem to be the least favourable as com-pared to consequent cross-link formation reactions.In addition,the glass transition is strongly shifted to lower temperature as AN content increases up to 29.0wt%when it starts increasing again.T g of HNBR/MWCNT composites were higher when compared to the nanotubes-and silica-reinforced hydrogenated nitrile rubber nanocomposites reported elsewhere [13],which is attributed to the markedly reduced mobility of the HNBR chains around the MWCNT.Also,the area of the tan(ı)peak is greatly enlarged with increasing AN content up to 29.0wt%when it starts decreasing again,indicat-ing significantly increased damping.Conversely,for HNBR/MWCNT composites,T g changes without a trend with increasing AN con-tent.Nonetheless,the T g of NBR/MWCNT (21.0wt%AN content)composite prepared by melt compounding,would be ∼31–38◦C higher than that of hydrogenated NBR,which had the same AN content.Similarly,the storage modulus in HNBR/MWCNT (30phr)composites prepared by melt compounding is inferior to that of NBR/MWCNT nanocomposites prepared by MC.With 30phr MWCNT,the storage modulus at 25◦C is enhanced by 6.0–8.3times in the latter composite compared to that of hydrogenated NBR,but the enhancement for 34.0wt%AN content nanocomposite is only 3.3–4.8times.3.5.Cross-linking of nanocompositesAs shown in Fig.6,the initial torque (e.g.within the first 1min)of the NBR/30phr MWCNT nanocomposites is much higher than the torques of melt compounded hydrogenated NBR/30phr MWCNTB.Likozar,Z.Major/Applied Surface Science257 (2010) 565–573571Fig.6.Characterization of curing properties–cure curves(elastic torque vs.time (0–12min))of(H)NBR/MWCNT nanocomposites with different AN contents pre-pared by MC.composites.Thisfinding is attributed to the high contents of unsat-urated groups of NBR macromolecular chains,in addition to thefine dispersion of MWCNT in rubber matrix.Interestingly,the torque of the33.0wt%AN content NBR/30phr MWCNT nanocomposite with curing reagent increases more rapidly with time at170◦C than in the case of some NBR with lower AN content,clearly con-firming that a chemical cross-linking reaction of NBR,which was not in the same extent present with these lower AN content NBR, has occurred.More evidence of the cross-linking phenomenon is provided by the strong shift of the glass transition of the nanocom-pounds compared with uncured rubber,as shown in Table3.The IR spectra of(H)NBR/MWCNT(30phr)nanocomposites before and after cross-linking are shown in Fig.7.It is reasonable to assume that the–C N group on(H)NBR macromolecular chain does not take part in the curing reactions,and the IR peak is at =2236cm−1. For theuncured nanocomposite,the peak at =1639cm−1corre-sponds to C C stretch pared to the change in peakat =2236cm−1after cross-linking,theintensity of the peak at=1639cm−1is distinctly reduced.This indicates that part of theC C participates in the curing reaction,which is consistent withthe radical curing mechanisms.This was verified,as the intensity ofTable3Glass transition temperatures of(H)NBR/MWCNT nanocomposites determined byDSC.AN content(wt%)Uncured(◦C)Cured(◦C)Curing process duration(170◦C,curing agent:DCP)(min)0.0–– 3.019.0−43.4−27.4 2.028.5−33.8−16.4 2.029.0−41.0−14.9 2.033.0−33.4−20.6 2.039.0−26.3−17.3 2.521.0(H)−44.9−41.9 3.034.0(H)−33.5−28.5 3.536.2(H)−33.4−32.9 2.539.0(H)−29.6−25.6 3.5the peak at =964cm−1,corresponding to C–H stretch vibration,is distinctly reduced after cross-linking,as well.Generally,15phrof DCP is a high concentration in view of general compounding andDCP originating radicals may react with the surface of MWCNT,leading to a change in cure characteristics.In fact,the results ofn th order and autocatalytic kinetic model show that any contentof functionalized MWCNT in the range between1.2and30phrincreases effective thermal conductivity,but decreases the vul-canization rate of functionalized MWCNT/(H)NBR nanocomposites[20].3.6.Tribological and thermal propertiesWith afine dispersion of MWCNT in(H)NBR and a stronginterface between MWCNT/(H)NBR by MC,MWCNTfiller shouldconfer good tribological properties on the rubber nanocompos-ites.Similar to inorganic nanotubes,MWCNT composites also showsuperior contact-mechanical properties.As given in Table4,theaverage Hertzian contact stress for both fretting and BOP config-uration ofHNBR/30phr MWCNT nanocomposites is reduced by40–70%compared to that offilled NBR vulcanizates;however,thestress on the melt compounding prepared composite/steel con-tact is only maximally decreased by17%/wt%AN content.MWCNTalso offer the advantage over inorganic nanotubes,that they make(H)NBR more thermally conductive.NBR nanocomposites becomethe most conductive when the AN content reaches28.5wt%.Thethermal conductivity of the nanocomposites with30phr MWCNTis higher than the rubber compositesfilled with0–10phr con-Fig.7.Characterization of curing properties–FT-IR spectra(transmittance vs.wavenumber(500–3500cm−1))showing:(a)uncured and(b)cured(at170◦C for2–4min) (H)NBR/MWCNT nanocomposites with different AN contents prepared by MC.。