Thermal conductivity of natural rubber nanocompositeswith hybrid fillers☆

Junping Song *,Xiteng LiKaiyan Tian Lianxiang Ma Wei Li ,Shichune Yao

1 Key Laboratory of Rubber-plastics,Ministry of Education,Shandong Provincial Key Laboratory of Rubber-plastics,Chineseand German School of Scienceand Technology,College of Electromechanical Engineering,Qingdao University of Science and Technology,Qingdao 266061,China

2 Department of Energy Engineering,Zhejiang University,Hangzhou 310027,China

3 Department of Mechanical Engineering,Carnegie Mellon University,Pittsburgh,PA 15213-3890,USA

Keywords:Modi fi ed carbon nanotube Carbon black Hybrid fi ller Natural rubber Thermal conductivity

ABSTRACT Natural rubber nanocomposites fi lled with hybrid fi llers of multi-walled carbon nanotubes(CNTs)and carbon black(CB)were prepared.CNTs w ere ultrasonically modi fi ed in mixture of hydrogen peroxide(H2O2)and distilled w ater(H2O).The functional groupson the surface of CNTs,changesin nanotube structure and morphology w ere characterized by Fourier transform infrared spectroscopy(FT-IR),Raman Spectroscopy,and transmission electron microscopy(TEM).It shows that hydroxyl(OH·)is successfully introduced.The surface defects of modi fi ed CNTs w ere obviously higher than those of original CNTs,and the degree of agglomeration w as greatly reduced.Thermal conductivity of the composites w as tested by protection heat fl ow meter method.Compared with unmodi fi ed CNTs/CB fi lling system,the thermal conductivity of hybrid composites is improved by an average of 5.8%w ith 1.5 phr(phr is parts per hundred rubber)of hydroxyl CNTs and 40 phr of CB fi lled.A three-dimensional heat conduction netw ork composed of hydroxyl CNTs and CB,as observed by TEM,contributes to the good properties.Thermal conductivity of the hybrid composites increases as temperature rises.The mechanical propertiesof hybrid compositesare also good with hydroxyl CNTs fi lled nanocomposites;the tensile strength,100%and 300%tensile stress are improved by 10.1%,22.4%and 26.2%respectively.

1.Introduction

Thermally conductive polymer composites have attracted considerable attention because of their comprehensive application in many areas.Theaddition of high thermally conductive fi llersisthemost effective w ay to improve the thermal conductivity of polymer composites.Carbon black(CB)is w idely used in industry to form the thermally conductive polymer because of its low cost and abundant supply.Chia-Ken Leong et al.[1]indicated that carbon black is superior to other materials,which are more thermally conductive(graphite,diamond and nickel particles,and carbon fi laments),in providing thermal pastes of high performance.Junping Song et al.[2]found that natural rubber(NR)fi lled with 26.2 vol%CBgives a thermal conductivity of 0.42 W·m-1·K-1.

High content of CBmust be used to fabricate the thermally conductive polymer;how ever,low er content of carbon nanotubes(CNTs)could meet the same goal.Since their fi rst study nearly tw o decades ago by Iijima[3],CNTs have been the focus of considerable research efforts.CNTs share numerous fascinating performance,such as gigantic thermal conductivity[4,5],extremely large surface area[6],exceptional mechanical strength[7-10]and fl exibility[11,12].Numerous investigators have reported remarkable physical and mechanical properties of this new form of carbon,among w hich the thermal conductivity of CNTs has received considerable attention[3,13].As one of the allotropes of carbon material,CNTs have been proved to be more ef fi cient in improving the thermally conductive polymers due to their large aspect ratio and good intrinsic thermal conductivity.The higher thermal conductivity when using CNTs at 1.4 vol%instead of CB is about 0.506 W·m-1·K-1[14-16].Namely,the percolation threshold of CBis much higher than CNTs in thermal conductive polymers[17].

Many reported experimental resultsabout thermal conductivities of polymer/functional CNTs composites are lower than that of polymer/pristine CNT composites[18-20].Florian et al.[21]found that w hen theamount of CNTsw aslessthan 0.5%,thenon-functionalized CNTssigni fi cantly improved the thermal conductivity of the epoxy resin matrix,w hile theamino functionalized CNTsfailed to improvesigni fi cantly.The reason for the failure in thermal conductivity improvement w ith functionalized CNTs fi lling is due to the non-uniform dispersion of CNTs and bad interface betw een CNTs and matrix.On the one hand,CNTs commonly exhibit the large agglomerates in polymer matrix because of their high van der Waals force betw een CNTs[22,23].It leadsto theincrease of percolation threshold to a certain extent.And on the other hand,the low thermal conductivity is mainly caused by the high contact resistance betw een the CNTs and high interfacial thermal resistance betw een the CNTs and surrounding polymer matrix[24].Therefore,further experimental treatment is needed to achieve the bene fi tsof thermal conductive CNT-based nanocomposites w ith homogeneously dispersed nanotubes[25].Many research efforts have been directed tow ard producing CNTs/polymer composites for functional and structural applications[26-28].The oxidation of CNTs by w et chemical methods,photo-oxidation,or oxygen plasma has gained much attention as an attempt to purify and enhance the chemical reactivity of the graphitic netw ork.Typically,the oxygen-containing groups on the surface of CNTs mainly contain carboxyl and hydroxyl groups after chemical reagent treatment.And through such harsh treatments,CNTs can be effectively puri fi ed.The effects of the commonly used acid on CNTssurface morphology have been w ell documented in previous studies.The detailed observation and analysis of Zhang et al.[29]show ed that the defects on CNTs,original or new ly created,played a crucial role in the oxidation process.

In addition,dispersion of CNTs is not only a geometrical problem of dealing w ith the length and size of the CNTs,but also relates to the method on how to separate individual CNTs from CNTs agglomerates and stabilizethem in polymer matrix to avoid secondary agglomeration.

Although CNTs can provide polymer with such good properties,but its application is limited because of the high cost,and dif fi culty in dispersion w hen fi lled alot.Therefore,to apply it together w ith other fi llers such as CBhas been tried in industry.Some w orks have been done to study the mechanical and electrical properties of the hybrid polymer systems.How ever,little w ork has addressed thermal conductivity,w hich is a very important property for applications of the materials.

In this paper,we attempted to incorporate fewer CNTs and some amounts of CB into natural rubber(NR)and explore the thermal conductivities of the fi nal materials.To achieve an enhanced thermal conductivity and reasonable mechanical properties,there are at least tw o problems to be resolved.The fi rst one is to modify the CNTsurface by peroxide to achieve a good dispersion in rubber,and the second one is to understand the difference on the thermal conductivity improvements of rubbers by adding modi fi ed CNTs and unmodi fi ed CNTs into composites,w hich already contains CB.

2.Experimental

2.1.Materials

Multi-w alled carbon nanotubes(MWCNTs)w ere prepared by a chemical vapor decomposition(CVD)technique[30].The CVD-CNTs have diameters of 5-10 nm,an average purity of 95%and a typical tube length of 100 um.DBPvalue of CB(N234)is 121 × 10-5m3·kg-1.DBP value re fl ects the volume of dibutyl phthalate absorbed by 100 g of CB.Surface area of CB(N234)tested by the statistical thickness method(STSA)is 107 × 103m2·kg-1and p H value of CB(N234)is 7.2.Standard Malaysian Rubber(SMRCV 60),hydrogen peroxide,distilled water and other reagents are commercially available.

2.2.Synthesisof hydroxyl CNTs

CNT(2.0 g)w as dispersed in a 200 ml:200 ml mixture of H2O2and H2O.Then the mixture w as placed into w ater bath of 80°Cand sonicated for 1 h.After sonicating,thesurfaceof CNTsw ill generate carboxyl and hydroxyl groups.The mixture w as cleaned for about 5 times by distilled w ater and dried for about 48 h at 50°Cin a vacuum oven.

The mechanism of oxidative reaction w asshow n in Fig.1.Becauseof the CVDmethod,CNTshave ahighly complex structureand the reaction process is full of possibilities.Hydrogen peroxide decomposes to produce hydroxyl radicals(HO·),which have strong electrophilic addition and oxidation properties.When HO·comes into contacting with CNTs,it w ill attack the unsaturated double bond on the surface of the CNTs and the electrophilic addition reaction w ill occur.Then the C--OH groups are produced.

Fig.1.Mechanism of hydrogen peroxide oxidation of carbon nanotubes.

2.3.Preparation of composites

The recipe used in the formulation of the NR compounds was given in Table 1.The compounds w ere prepared by HAAKETorque Rheometer(a product of Thermo Electron Corporation)w ith mixing speed of 45 r·min-1and initial temperature of 60 °C.Rubber w as masticated for 2 min fi rstly,then stearic acid,zinc oxide,antioxidant and microcrystalline wax w ere added sequentially and continued mixing for 2 min.The fi llers CB as w ell as CNTs w ere incorporated into the compound and continued mixing for 6 min.Finally,accelerators agent and sulfur w ere incorporated into the compound at BL-6175-type tw o-roll mill(Bao-round precision detection equipment Co.,Ltd.Products).

Table 1 Formulation of Mixes in phr①

The compounds produced w ere eventually cured in a rectangular shape and mold measuring 150 mm×145 mm×2 mm in a standard hot press machinemodel(VC150T-FTMO-3RT-909-type vacuum curing Products)under a pressure of 16 MPa at 150°Cfor about 10 min to produce a rectangular sheet w ith thickness of 2 mm.After curing,samples w ere cut and set for property test.

2.4.Characterization

Fourier transform infrared spectroscopy(FTIR)absorbance spectra w ere recorded on a Nicolet FTIRspectrophotometer,the measurement of w hich is from 4000 cm-1to 400 cm-1.

A Renishaw In Via micro Raman system spectrophotometer w ith a 514 nm argon-ion laser source w as used to evaluate the disruption of theπsystems in the nanotube structure.

The dispersion of the pristine CNTsand modi fi ed CNTsw asobserved w ith transmission electron microscopy(TEM,JEOLJEM-2100,Japan)at an acceleratingvoltageof 200 kV,asw ell asthedispersion of the CNTsin the NRmatrix,using ultrathin specimens(200 nm)cut by a microtome(UC7,Germany)w ith a diamond knife.

An electron tensile tester(Zw ick/Roell Z005,Germany)w as used to measure the tensile property at a uniform speed of 500 mm·min-1according to GB/T-529-2009(tensile property).

Thermal conductivities of the nanocomposites were measured with thermal conductivity meter(DTC-300,America).Thesamplesw ereprepared in round-shaped forms,w ith a diameter of 12.7 mm and a thickness of about 1.9 mm.

3.Results and Discussion

3.1.Functional characterization of CNTs

3.1.1.FT-IRmeasurement

The FT-IR spectra of the pristine and hydrogen peroxide treated CNTsare shown in Fig.2.The characteristic bands indicating generation of functional groupswere observed in spectrum of surfacetreated CNTs.The biggest difference is the size of O--H peak at 3430 cm-1.That peak of H2O2treated CNTsisbigger than that of pristine CNTs.In addition,the peak at about 1100 cm-1is oxygen groups because of oxidation of hydrogen peroxide.It indicates that hydroxyl groups are produced successfully on the hydrogen peroxide treated CNTs.

Fig.2.FI-IRspectra of pristine MWCNTs and H2O2 treated MWCNTs.

3.1.2.Raman measurement

Raman spectroscopy is a valuable tool for characterization of the defect on CNTs.The Raman spectra of pristine CNTs and hydrogen peroxide treated CNTs are shown in Fig.3.Typically,the ratio of intensities between the Dand Gbands(ID/IG)of the CNTs in Raman spectroscopy is taken as a measure of defects in CNTs[31].Table 2 indicated the defects on the hydrogen peroxide treated CNTs.The increase of defects on the hydrogen peroxide treated CNTswascon fi rmed by the upshift of the D band and the increased ID/IGratio from 1.03 to 1.13.Bigger increase in the comparative ID/IGveri fi es that the sp2hybridized carbon atoms in the graphitic structure of CNTs have been converted to sp3hybridized atoms.Lots of w ork indicated that defects like junctions on CNTs usually gave rise to an increase of the local thermal resistance and reduced the thermal conductivity due to lattice defects[32,33].

3.1.3.TEM measurement

Fig.3.Raman spectra of pristine and hydrogen peroxide treated.

Table 2 ID/IGintensity of CNTs

The effect of oxidation by H2O2under neutral conditions on morphologies and structures of CNTs w ere show ed in Fig.4.According to Fig.4(a),(b),(c)and(d),because of the process of oxidation of carbon nanotubes,the interaction betw een CNTs is w eakened and the dispersion is more uniform,but the length of the modi fi ed CNTs is reduced in some extent.Though parts of the w all structures are eroded in oxidation,most of the CNTsurface retained good graphite structures[Fig.4(e)and(f)].

3.2.Characterization of nanocomposites

3.2.1.TEM measurement of nanocomposites

The dispersion morphology of 1.5 phr modi fi ed CNTs and pristine CNTs fi lled nanocomposites w ere characterized by TEM,as show n in Fig.5.It show s that the unmodi fi ed CNTs(a)and(c)are obviously longer than the modi fi ed(b)and(d)and in the pristine CNTs fi lled nanocomposites,CBaggregates into big agglomerates,as circled w ith A and B.How ever,in the modi fi ed CNTs fi lled nanocomposites,CB is mostly homogenously distributed in rubber matrix.There show s a very good dispersion of both CNTs and CBin the modi fi ed CNTs fi lled nanocomposites(b)and(d).It can be concluded from the images of TEM that thegood dispersion of modi fi ed CNTspromotesthe dispersion of CBin NRmatrix to some extent.Thisismaybe because the HO·on the surface of CNTs attracts CB fi ller,w hich has the same hydrophilic surface.

3.2.2.Vulcanization characteristics

Table3 show sthevulcanization characteristicsof modi fi ed CNTsand pristine CNTs fi lled nanocomposites.The t10,t90and torque increase gradually w ith increasing content of modi fi ed CNTs.Usually,the presence of C--OH can in fl uence the curing process and additional heating time is required to cure nanocomposites due to its higher active energy.As show n in Fig.2,there are some C--OH groupson thesurface of modi fi ed CNTs,so w ith the increasing fi lling amount,the content of C--OH in thecompoundsincreases.It leadsto the increase of t10and t90.Therefore,the more hydroxyl content with the increasing of fi lling,the more heating time is needed to fi nish the process of Vulcanization.The increasing of minimum torque value(ML)and maximum torque value(MH)may be due to the movement of NR molecular chains being restricted by the modi fi ed CNTs and CBuniformly dispersed,and ML and MH increase w ith the fi lling increasing.

Fig.4.Pristine CNTs(a),(c)and(e);modi fi ed CNTs(b),(d)and(f).

3.2.3.Mechanical properties

The chemical bonding of CNTs is composed entirely of sp2carboncarbon bonds.This bonding structure stronger than the sp3bonds found in diamond provides CNTs w ith extremely high mechanical property.

Table 4 and Fig.6 show the mechanical properties of modi fi ed CNTs and pristine CNTs fi lled nanocomposites.It show s that w ith modi fi ed CNTs fi lled,the modulus(stress at 100%and 300%strain)are enhanced signi fi cantly by about 22.4%and 26.2%respectively.The tensile strength is improved by 10.1%,however,the elongation at break decreases to a great extent.Fig.6 shows the typical strain-stress curves.It can be seen that the tensile modulus for modi fi ed CNTs fi lled nanocomposites is higher than that of the pristine CNTs.The free space betw een polymer chains are smaller and the movement of chains is restricted much more w hen the amount of fi llers is increasing and evenly dispersed,thus the probability of straightening for rubber chains is reduced.CNTs and CB are mostly homogenously distributed in rubber matrix under unstretched state,exhibit random orientation and form three-dimensional network structure.During the stretching process,CNTs,CBand rubber connect each other and form “string bag”structure.The “string bag”structure is formed by the well dispersed CNTs,CBand the rubber matrix.Rubber matrix is recognized as framew ork and bears stress.Therefore,as fi ller content increases,the mechanical strength increases signi fi cantly and the elongation at break decreases.

Fig.5.Pristined CNTs fi lled nanocomposites(a)and(c);modi fi ed CNTs fi lled nanocomposites(b)and(d).

Table 3 Curing characteristic of modi fi ed CNTs/CB/NRand pristine CNTs/CB/NRnanocomposites

Table 4 Mechanical properties of polymer composites

3.2.4.Thermal conductivity measurementsand reinforcement mechanism It is w ell know n that the transport of heat is of great importance in the service life of rubber[34].Fig.7 and Table 5 show the thermal conductivity of modi fi ed CNTs and pristine CNTs fi lled nanocomposites.It can be seen that modi fi ed CNTsenhancethe thermal conductivity much more than the pristine one.The same increasing trend in thermal conductivity is observed at testing temperature of 40 °C,60 °Cand 80 °C.When the content of modi fi ed CNTsis 1.5 phr,the thermal conductivity isincreased by 6.9%,4.4%and 6.1%at 40 °C,60 °Cand 80 °Crespectively,compared with the pristine one.And the thermal conductivity increases as temperature rises.

Material properties are closely related to its microstructure.As show n in Fig.5,the pristine CNTs(a)and(c)are longer than the modi fi ed(b)and(d),and in the pristine CNTs fi lled nanocomposites,CBaggregates into big agglomerates.How ever,in the modi fi ed CNTs fi lled nanocomposites(b)and(d),there show s a very good dispersion of both CNTs and CB.Therefore the much better thermal conductivity can be attributed to the shorter length of modi fi ed CNTs w hich means more numbers of CNTs at the same fi lling amount,and the much better dispersion of hybrid fi llers,w hich results in a good 3-Dheat conduction path.How ever,the decreased thermal conductivity can also be seen w hen the content of CNTs is above 1.5 phr,especially that of 4#.This is probably because that under the experimental process,too much CNTs cannot be fully dispersed.It will cause fi ller aggregates,and that w ill cause more phonon scattering.In the future study,w e shall adjust appropriate process to make more fi llers homogenously dispersed,and a better performance can be expected.

4.Conclusions

In this w ork,properties of NRnanocomposites fi lled w ith CNTs and CBhybrid fi ller w ere investigated,and CNTs w ere modi fi ed by H2O2.The C--OH introduced on the surface of CNTs makes a good dispersion of CNTs both in alcohol and rubber matrix.And the good dispersion of CNTsin rubber also improvesthedispersion of CB.Therefore,w ith modi fi ed CNTs fi lled,agood 3-Dheat conduction path madeup of the hybrid fi ller wasformed.Asaresult,the thermal conductivity of modi fi ed CNTs fi lled nanocomposites is much better than that of the unmodi fi ed one.With the same fi ller content,the thermal conductivity increase rate reaches about 6.9%,4.4%and 6.1%respectively at testing temperature of 40 °C,60 °C and 80 °C.The mechanical properties of modi fi ed CNTs/CB/NR nanocomposites are also clearly improved.Compared w ith pristine CNTs fi lled nanocomposites,the modulus(stress at 100%and 300%strain)and tensile strength of the modi fi ed CNTs fi lled nanocomposites are improved by 22.4%,26.2%and 10.1%,respectively.It also can beconcluded that with theincreaseof modi fi ed CNTs,thetension set at 100%and 300%is obviously increased.And this systematic investigation on themechanicaland thermal conductivity propertiesof modi fi ed CNTs fi lled hybrid nanocomposites w ill provide a new perspective to other CNTs fi lled polymer systems.In the future,a kind of better modi fi cation method w ill be explored on the basis of hydroxylated CNTs and a better process will be tried to make CNTs evenly dispersed as much as possible,even w hen fi lling is of a larger amount.

Fig.6.Stress-strain curves of modi fi ed CNTs/NRand pristine CNTs/NRnanocomposites.

Fig.7.Thermal conductivity of modi fi ed CNTs/NRand pristine CNTs/NRnanocomposites.

Table 5 Composition and thermal conductivity of the prepared materials