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Reinforcement of hydrogenated carboxylated nitrile-butadiene rubber by multi-walled carbon nanotubes

Reinforcement of hydrogenated carboxylated nitrile-butadiene rubber by multi-walled carbon nanotubes
Reinforcement of hydrogenated carboxylated nitrile-butadiene rubber by multi-walled carbon nanotubes

Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber by multi-walled carbon nanotubes

Lan Lu,Yinghao Zhai,Yong Zhang *,Christopher Ong,Sharon Guo

State Key Laboratory of Metal Matrix Composites,School of Chemistry and Chemical Technology,Shanghai Jiao Tong University,200240Shanghai,PR China

1.Introduction

Since the study of carbon nanotubes (CNT)published by Iijima [1],they have been intensively researched [2–4].Because of their unique structure,CNT possess excellent electrical,thermal and mechanical properties.Many studies have focused on the reinforcing effect of CNT on polymers.There are two important issues for effective reinforcement by CNT,i.e.adequate dispersion of CNT in polymers and strong interfacial bonding between CNT and https://www.wendangku.net/doc/866704602.html,T tend to aggregate together owing to the strong van der Waals forces between individual carbon nanotubes and their large aspect ratio (above 1000).Moreover,due to their inert surface,the interfacial bonding between CNT and polymers is not strong enough to have the load transfer from matrix to ?llers effectively.The poor dispersibility and high cost of CNT continue to hinder their use as reinforcing ?llers for polymers.One solution to the poor dispersibility is to chemically functionalize CNT.Such chemical modi?cation can improve their solubility in solvents and enhance their compatibility with polymers [5–7].If it leads to excellent property improvement at low CNT loadings,the cost of polymer/CNT composites can also be reduced.

Many studies concerning polymer/CNT composites have been largely performed [8–10],but only a few of them were devoted to the rubber composites [11–17].Yue et al.studied the effect of multi-walled carbon nanotubes (MWCNT)on curing and mechanical

properties of hydrogenated nitrile rubber [11].Lopez-Manchado et al.showed that the incorporation of low concentration of single-walled carbon nanotubes (SWCNT)into natural rubber (NR)increased the storage modulus of the composites [12].Frogley et al.observed ‘‘pseudo-yield’’phenomenon during stretching for SWCNT-?lled silicon rubber [13].They believed it was due to trapping and release of rubber within SWCNT clusters.

In the previous work,we have shown that MWCNT could provide high reinforcement for the elastomeric matrix [18].We used MWCNT to reinforce hydrogenated carboxylated acryloni-trile–butadiene rubber (HXNBR)in the present work.HXNBR is a high performance rubber with combination of excellent mechan-ical properties,abrasion resistance,adhesion to fabrics and hot air aging resistance,which can be used in many diverse applications [19].Incorporation of MWCNT into HXNBR would lead to the development of a new rubber composite with commercial importance.In this work,morphology,curing properties,mechan-ical properties and dynamic mechanical properties of HXNBR/MWCNT composites were investigated.2.Experimental 2.1.Materials

HXNBR (Therban 1XT VP KA 8889),containing 33%acrylonitrile,5%carboxylic acid and 3.5%residual double bonds,was produced by LANXESS Deutschland GmbH.Its Mooney viscosity ML (1+4)1008C is 77?7.MWCNT was purchased from Chengdu Organic Chemicals Co.,Ltd.,China.Their average diameter is between 8and

Applied Surface Science 255(2008)2162–2166

A R T I C L E I N F O Article history:

Received 21June 2008

Received in revised form 30June 2008Accepted 1July 2008

Available online 10July 2008Keywords:

Carbon nanotubes Rubber

Mechanical properties

A B S T R A C T

Hydrogenated carboxylated acrylonitrile–butadiene rubber (HXNBR)and multi-walled carbon nanotubes (MWCNT)composites were prepared.The dispersion of MWCNT in HXNBR matrix was evaluated by ?eld emission scanning electron microscopy.HXNBR/MWCNT composite had shorter scorch time and optimum curing time compared with that of un?lled HXNBR.The tensile strength and modulus of HXNBR/MWCNT composites increased with increasing MWCNT content.Mooney–Rivlin equation was used to describe the stress–strain behavior of un?lled HXNBR and the strain ampli?cation factor was taken into account for HXNBR/MWCNT composites.The Mullins effect and dynamic mechanical properties of HXNBR/MWCNT composite were also investigated.

?2008Elsevier B.V.All rights reserved.

*Corresponding author.Tel.:+862154741297;fax:+862154741297.E-mail address:yong_zhang@https://www.wendangku.net/doc/866704602.html, (Y.Zhang).Contents lists available at ScienceDirect

Applied Surface Science

j o u r n a l h o m e p a g e :w w w.e l se v i e r.c om /l oc a t e /a p s us c

0169-4332/$–see front matter ?2008Elsevier B.V.All rights reserved.doi:10.1016/j.apsusc.2008.07.052

25nm and the purity is higher than95%.Dicumyl peroxide(DCP)with reagent purity grade was purchased from Sinopharm Group Chemical Reagent Co.,Ltd.,China.

2.2.Sample preparation

The HXNBR compounds with0,1,2and4phr MWCNT(parts per hundred rubber by weight)were prepared in a two-roll mill at room temperature.The DCP content was3phr for each compound. The compounds were cured at1708C for20min under pressure.

2.3.Characterization of HXNBR/MWCNT composites

Microstructure of MWCNT was observed by JEM2100F transi-tion electron microscope(TEM;JEOL Co.,Japan).MWCNT were dissolved

in tetrahydrofuran and the suspension was sonicated for 30min.The sample was prepared by dropping the MWCNT suspension in a copper TEM grid and subsequent drying.Field emission scanning electron microscopy(FESEM;JSM-7401F,JEOL Co.,Japan)was used to observe the morphology of the frozen fracture surface of HXNBR/MWCNT composites.

Curing characteristics were measured over a40-min period at 1708C using a moving-die rheometer(UCAN2030from Taiwan) according to ISO3417.The curing rate index(C R)was used to represent the curing rate of the compound,which was determined according to ISO Standard3417:

C R?

100

90s2

where t90is the optimum curing time and t s2is the scorch time.

Tensile tests were carried out in an4465tensile machine(Instron Co.,UK)at a crosshead speed of500mm/min. The dumbbell shape samples were75mm in length,1mm in thickness and4mm in width.The curves for Mullins effect were obtained also by using tensile test sample on a tensile machine at a crosshead speed of500mm/min.A sample was extended to200% elongation and retraced,and then the operation repeated for twice after resting1min.The fourth operation was performed on the same sample after it had rested for24h at room temperature to ensure full recovery.

Shore A hardness was measured by using a hand-held Shore A durometer according to Chinese Standard GB531-83(similar to ISO 868and ASTM D2240).Permanent set was determined by the value of the length of unstrained sample divided by the net elongated value of strained sample after resting for3min.

Dynamic mechanical analysis(DMA)was performed with DMTA IV(Rheometric Scienti?c Inc.,USA)under a nitrogen atmosphere at a heating rate of38C/min fromà808C to208C and a frequency of1Hz.

3.Results and discussion

The TEM images of pristine MWCNT are shown in Fig.1.High purity,uniform diameter distribution and long MWCNT can be observed in Fig.1.MWCNT is hollow and concentrically tubular in shape.However,because of the high aspect ratio(up to1000),they are highly entangled and exhibit a strong tendency to agglomerate into bundles,which may reduce the effective aspect ratio when incorporating into rubber.

The FESEM images of the frozen fracture surface of HXNBR/ MWCNT(100/4)composite reveal the dispersion of MWCNT in HXNBR,as shown in Fig.2.All the white spots and small?brils must be the end or pulled-out part of MWCNT on the fracture surface.Although the dispersion of MWCNT is uniform in some areas,there are still several aggregates existing in the composite (Fig.2a),which indicates that the processing method was not powerful enough to disentangle the aggregations of MWCNT. Fig.2b is a high magni?cation image of the HXNBR/MWCNT composite.The average diameter of MWCNT calculated from the scale bar is about50nm which is roughly twice larger than the pristine MWCNT.This is because MWCNT appear larger when they are observed by SEM[20].

The curing curves of HXNBR and HXNBR/MWCNT are shown in Fig.3and some parameters of curing properties are reported in Table1.Three regions are observed in Fig.3.The?rst region is the scorch delay or induction period where the torque of compounds decreased.The second region is where the curing reaction occurred.The network structure was formed in this period, leading to the sharp increment of the torque.In the third region, curing curves reached to a plateau when the network matured by equilibrium[12].The scorch time(t s2)and optimum curing time (t90)were shortened after adding MWCNT into HXNBR.As reported by several authors[11,21],CNT would absorb some curing agent or accelerator in sulfur vulcanized system,leading to an increase of scorch time.In the case of a DCP curing system,the curing properties of rubber are not as in?uenced by the CNT.The reason for the shorter scorch time of the HXNBR/MWCNT composite is probably due to the increase of thermal conductivity of HXNBR in the presence of MWCNT which could promote the ful?llment of vulcanization[17].

M L is the lowest torque and M H the highest torque at curing curves.M HàM L represents the crosslink density of vulcanizates. M HàM L slightly increases after loading MWCNT,indicating that the addition of MWCNT has little effect on the crosslink density of HXNBR vulcanizate.C R increase from8.7minà1to14.7minà1after adding MWCNT in HXNBR,indicating that MWCNT could increase the curing rate of HXNBR.

Fig.1.TEM image of pristine MWCNT.

Table1

Curing characteristics of HXNBR and HXNBR/MWCNT composites

Sample M HàM L(dN m)t S2(min)t90(min)C R(minà1) HXNBR7.0 2.113.68.7 HXNBR/MWCNT7.1 1.58.314.7

L.Lu et al./Applied Surface Science255(2008)2162–21662163

Representative stress–strain curves for pure HXNBR and HXNBR/MWCNT composites are shown in Fig.4.The ultimate tensile properties re?ect the expected trend considering the presence of MWCNT in HXNBR.It is obvious that MWCNT have a great reinforcing effect on HXNBR with respect to small ?ller loadings.Incorporation of MWCNT in HXNBR leads to the increase in both tensile strength and the elongation at break.The mechanical properties of HXNBR/MWCNT composites are listed in Table 2.Shore A hardness of the composites is slightly increased,indicating that small additions of MWCNT do not largely in?uence the rubbery nature of the HXNBR matrix.The tensile strength of the HXNBR/MWCNT (100/4)composite increased roughly 60%and the modulus at 300%increased approximately ?ve times compared with that of pure HXNBR.

The stress–strain behavior of un?lled rubber can be described by Mooney–Rivlin equation [22,23]:

s red ?

s l à1=l

?2C 1t2C 2l

à1

The reduced stress s red is a linear function of the reciprocal extension ratio 1/l .s is the nominal tensile stress (force divided by the undeformed cross-sectional area of sample),the extension ratio l =L /L 0is the ratio of deformed and undeformed length,C 1and C 2are adjustable parameters.From the theory of rubber elasticity,C 1is related to crosslink density which can be obtained from the intercept.This equation can provide a satisfactory description of stress–strain curves in the low strain region.However,in the case of ?lled rubber,the strain ampli?cation factor V should be used to take into account both the disturbance of strain distribution and the absence of the deformation of ?llers.This factor V is derived from Guth equation [24]:V ?1t2:5c t14:1c 2

where c is the ?ller volume concentration.

The modi?ed extension ratio l *of ?lled rubber should be calculated by following equation:

l ??1tV e

where e is the ratio of extension length D L and undeformed length L 0.As seen in Fig.5,stress–strain curves are plotted as s /(l *à1/l *)against l *à1of all samples.C 1values of ?lled HXNBR are higher than that of un?lled one.It is probably because the addition of MWCNT could introduce additional ?ller–?ller and

?ller–matrix

Fig.2.FESEM images of the frozen fracture surface of HXNBR/MWCNT (100/4)composite at low (a)and high (b)

magni?cation.

Fig.3.Curing curves of HXNBR and HXNBR/MWCNT (100/2)compound at 1708

C.

Fig.4.Stress–strain curves of HXNBR/MWCNT composites.

L.Lu et al./Applied Surface Science 255(2008)2162–2166

2164

networks.Furthermore,in the case of HXNBR/MWCNT composites, the upturns occurred at a low strain is more signi?cant than that of HXNBR/MWCNT composite,which is ascribed to the increase of the modulus after loading MWCNT into HXNBR.

The stress-softening phenomenon or the Mullins effect generally occurs in rubber-like materials during cyclic loading where the stress–strain behavior is observed to be softer(more compliant)during reloading after an initial loading excursion[25].Different mechanisms have been proposed to explain the Mullins effect including that there is structure existing in rubber which is formed by chemical and physical crosslinks of?ller–?ller and?ller–polymer interactions[26].

The Mullins effect is more pronounced in?lled rubbers than un?lled ones.And generally the Mullins effect of?lled rubbers is much more signi?cant in the case of strong matrix–?ller interaction than that of the poor one.Fig.6displays the Mullins effect of HXNBR/MWCNT(100/2)composite.The stress softening of the MWCNT-?lled HXNBR vulcanizate is derived mainly from

Table2

Mechanical properties of HXNBR/MWCNT composites

Sample

1234

MWCNT content(phr)0124 Hardness,Shore A43475051

Tensile strength(MPa)13.3?0.215.7?0.516.1?1.121.6?1.2 Elongation at break(%)600?3622?6659?8627?7 Modulus at100%(MPa)0.90?0.01 1.10?0.02 1.22?0.02 2.41?0.02 Modulus at300%(MPa) 1.30?0.01 2.20?0.05 2.99?0.07 6.63?0.06 Permanent set(%)15172020

Fig.6.Mullins effect displayed by HXNBR/MWCNT(100/2)composite.

the breakage of aggregates of MWCNT and the chain slippage of the attached polymer segments along the surface.After resting for24h at room temperature,the vulcanizate gained part of its elastic recovery owing to the thermal motion of the molecule.Bokobza et al.reported that there was a pronounced‘‘stress-softening effect’’for MWCNT-?lled NR or SBR[27,28].With combination of other experimental results,they concluded that the interactions between MWCNT and rubber would be the occlusion of rubber into the MWCNT aggregates[16].

DMA was used to characterize the dynamic mechanical properties of HXNBR and HXNBR composites.The temperature dependence of the storage modulus G0,loss modulus G00and loss factor tan d are given in Fig.7.The addition of MWCNT increases G0 in glassy region and hardly affects G0in rubber region.The increment of G0is due to the hydrodynamic reinforcement by introducing?llers.For conventional?llers,the interfacial interac-tions between rubber and?llers introduce additional crosslinks into the network,giving rise to an increase in the modulus in polymer matrix or increase in the viscosity in liquids[29].The peak value of G00also increased after the addition of MWCNT.The peak of tan d slightly shifts towards higher temperature,indicating that the small loading of MWCNT(2phr)has a little effect on the glass transition temperature(T g)of the composite.As reported by Kolodziej et al.[28],T g of NR/MWCNT composites did not change signi?cantly even with high loading of MWCNT and high degree of reinforcement.Lopez-Manchado et al.observed a shift of around 48C of T g values between un?lled NR and NR?lled with10phr SWCNT and they concluded that there are strong interfacial interactions between?ller and matrix[12].

4.Conclusions

HXNBR/MWCNT composites were prepared with different MWCNT contents.The scorch time and optimum curing time of HXNBR/MWCNT compound were shorter while curing rate index was larger than that of pure HXNBR.MWCNT had a remarkable reinforcing effect on HXNBR with respect to small?ller loading. The tensile strength of the HXNBR/MWCNT(100/4)composite was 21.6MPa,approximately60%higher than that of pure HXNBR. DMA showed that MWCNT could increase the storage modulus of HXNBR composite and had a little effect on the T g.

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