Applied Surface Science 255 (2008) 2162–2166

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Applied Surface Science

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Reinforcement of hydrogenated carboxylated nitrile–butadiene rubberby 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, 200240 Shanghai, PR China

A R T I C L E I N F O

Article history:

Received 21 June 2008

Received in revised form 30 June 2008

Accepted 1 July 2008

Available online 10 July 2008

Keywords:

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 field emission scanning electron microscopy. HXNBR/MWCNT composite had shorter

scorch time and optimum curing time compared with that of unfilled 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 unfilled HXNBR and the strain amplification

factor was taken into account for HXNBR/MWCNT composites. The Mullins effect and dynamic

mechanical properties of HXNBR/MWCNT composite were also investigated.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Since the study of carbon nanotubes (CNT) published by Iijima[1], they have been intensively researched [2–4]. Because of theirunique structure, CNT possess excellent electrical, thermal andmechanical properties. Many studies have focused on thereinforcing effect of CNT on polymers. There are two importantissues for effective reinforcement by CNT, i.e. adequate dispersionof CNT in polymers and strong interfacial bonding between CNTand polymers. CNT tend to aggregate together owing to the strongvan der Waals forces between individual carbon nanotubes andtheir large aspect ratio (above 1000). Moreover, due to their inertsurface, the interfacial bonding between CNT and polymers is notstrong enough to have the load transfer from matrix to fillerseffectively. The poor dispersibility and high cost of CNT continue tohinder their use as reinforcing fillers for polymers. One solution tothe poor dispersibility is to chemically functionalize CNT. Suchchemical modification can improve their solubility in solvents andenhance their compatibility with polymers [5–7]. If it leads toexcellent property improvement at low CNT loadings, the cost ofpolymer/CNT composites can also be reduced.

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

* Corresponding author. Tel.: +86 21 54741297; fax: +86 21 54741297.

E-mail address: yong_zhang@sjtu.edu.cn (Y. Zhang).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.07.052

properties of hydrogenated nitrile rubber [11]. Lopez-Manchadoet 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]. Frogleyet al. observed ‘‘pseudo-yield’’ phenomenon during stretching forSWCNT-filled silicon rubber [13]. They believed it was due totrapping and release of rubber within SWCNT clusters.

In the previous work, we have shown that MWCNT couldprovide high reinforcement for the elastomeric matrix [18]. Weused MWCNT to reinforce hydrogenated carboxylated acryloni-trile–butadiene rubber (HXNBR) in the present work. HXNBR is ahigh performance rubber with combination of excellent mechan-ical properties, abrasion resistance, adhesion to fabrics and hot airaging resistance, which can be used in many diverse applications[19]. Incorporation of MWCNT into HXNBR would lead to thedevelopment of a new rubber composite with commercialimportance. 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 (Therban1XT VP KA 8889), containing 33% acrylonitrile,5% carboxylic acid and 3.5% residual double bonds, was producedby LANXESS Deutschland GmbH. Its Mooney viscosity ML (1 + 4)100 8C is 77 � 7. MWCNT was purchased from Chengdu OrganicChemicals Co., Ltd., China. Their average diameter is between 8 and

Fig. 1. TEM image of pristine MWCNT.

Table 1Curing characteristics of HXNBR and HXNBR/MWCNT composites

Sample MH �ML (dN m) tS2 (min) t90 (min) CR (min�1)

HXNBR 7.0 2.1 13.6 8.7

HXNBR/MWCNT 7.1 1.5 8.3 14.7

L. Lu et al. / Applied Surface Science 255 (2008) 2162–2166 2163

25 nm and the purity is higher than 95%. Dicumyl peroxide (DCP) withreagent purity grade was purchased from Sinopharm Group ChemicalReagent Co., Ltd., China.

2.2. Sample preparation

The HXNBR compounds with 0, 1, 2 and 4 phr MWCNT (partsper hundred rubber by weight) were prepared in a two-roll mill atroom temperature. The DCP content was 3 phr for each compound.The compounds were cured at 170 8C for 20 min 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 weredissolved in tetrahydrofuran and the suspension was sonicated for30 min. The sample was prepared by dropping the MWCNTsuspension in a copper TEM grid and subsequent drying. Fieldemission scanning electron microscopy (FESEM; JSM-7401F, JEOLCo., Japan) was used to observe the morphology of the frozenfracture surface of HXNBR/MWCNT composites.

Curing characteristics were measured over a 40-min period at170 8C using a moving-die rheometer (UCAN 2030 from Taiwan)according to ISO 3417. The curing rate index (CR) was used torepresent the curing rate of the compound, which was determinedaccording to ISO Standard 3417:

CR ¼100

t90 � ts2

where t90 is the optimum curing time and ts2 is the scorch time.Tensile tests were carried out in an Instron 4465 tensile

machine (Instron Co., UK) at a crosshead speed of 500 mm/min.The dumbbell shape samples were 75 mm in length, 1 mm inthickness and 4 mm in width. The curves for Mullins effect wereobtained also by using tensile test sample on a tensile machine at acrosshead speed of 500 mm/min. A sample was extended to 200%elongation and retraced, and then the operation repeated for twiceafter resting 1 min. The fourth operation was performed on thesame sample after it had rested for 24 h at room temperature toensure full recovery.

Shore A hardness was measured by using a hand-held Shore Adurometer according to Chinese Standard GB531-83 (similar to ISO868 and ASTM D2240). Permanent set was determined by the valueof the length of unstrained sample divided by the net elongatedvalue of strained sample after resting for 3 min.

Dynamic mechanical analysis (DMA) was performed withDMTA IV (Rheometric Scientific Inc., USA) under a nitrogenatmosphere at a heating rate of 3 8C/min from�80 8C to 20 8C and afrequency of 1 Hz.

3. Results and discussion

The TEM images of pristine MWCNT are shown in Fig. 1. Highpurity, uniform diameter distribution and long MWCNT can beobserved in Fig. 1. MWCNT is hollow and concentrically tubular inshape. However, because of the high aspect ratio (up to 1000), theyare highly entangled and exhibit a strong tendency to agglomerateinto bundles, which may reduce the effective aspect ratio whenincorporating into rubber.

The FESEM images of the frozen fracture surface of HXNBR/MWCNT (100/4) composite reveal the dispersion of MWCNT inHXNBR, as shown in Fig. 2. All the white spots and small fibrilsmust be the end or pulled-out part of MWCNT on the fracturesurface. Although the dispersion of MWCNT is uniform in someareas, there are still several aggregates existing in the composite

(Fig. 2a), which indicates that the processing method was notpowerful enough to disentangle the aggregations of MWCNT.Fig. 2b is a high magnification image of the HXNBR/MWCNTcomposite. The average diameter of MWCNT calculated from thescale bar is about 50 nm which is roughly twice larger than thepristine MWCNT. This is because MWCNT appear larger when theyare observed by SEM [20].

The curing curves of HXNBR and HXNBR/MWCNT are shown inFig. 3 and some parameters of curing properties are reported inTable 1. Three regions are observed in Fig. 3. The first region is thescorch delay or induction period where the torque of compoundsdecreased. The second region is where the curing reactionoccurred. 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 byequilibrium [12]. The scorch time (ts2) and optimum curing time(t90) were shortened after adding MWCNT into HXNBR. As reportedby several authors [11,21], CNT would absorb some curing agent oraccelerator in sulfur vulcanized system, leading to an increase ofscorch time. In the case of a DCP curing system, the curingproperties of rubber are not as influenced by the CNT. The reasonfor the shorter scorch time of the HXNBR/MWCNT composite isprobably due to the increase of thermal conductivity of HXNBR inthe presence of MWCNT which could promote the fulfillment ofvulcanization [17].

ML is the lowest torque and MH the highest torque at curingcurves. MH �ML represents the crosslink density of vulcanizates.MH �ML slightly increases after loading MWCNT, indicating thatthe addition of MWCNT has little effect on the crosslink density ofHXNBR vulcanizate. CR increase from 8.7 min�1 to 14.7 min�1 afteradding MWCNT in HXNBR, indicating that MWCNT could increasethe curing rate of HXNBR.

Fig. 2. FESEM images of the frozen fracture surface of HXNBR/MWCNT (100/4)

composite at low (a) and high (b) magnification.

Fig. 3. Curing curves of HXNBR and HXNBR/MWCNT (100/2) compound at 170 8C.

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

L. Lu et al. / Applied Surface Science 255 (2008) 2162–21662164

Representative stress–strain curves for pure HXNBR andHXNBR/MWCNT composites are shown in Fig. 4. The ultimatetensile properties reflect the expected trend considering thepresence of MWCNT in HXNBR. It is obvious that MWCNT have agreat reinforcing effect on HXNBR with respect to small fillerloadings. Incorporation of MWCNT in HXNBR leads to the increasein both tensile strength and the elongation at break. Themechanical properties of HXNBR/MWCNT composites are listedin Table 2. Shore A hardness of the composites is slightly increased,indicating that small additions of MWCNT do not largely influencethe rubbery nature of the HXNBR matrix. The tensile strength ofthe HXNBR/MWCNT (100/4) composite increased roughly 60% andthe modulus at 300% increased approximately five times comparedwith that of pure HXNBR.

The stress–strain behavior of unfilled rubber can be describedby Mooney–Rivlin equation [22,23]:

sred ¼s

l� 1=l2¼ 2C1 þ 2C2l

�1

The reduced stress sred is a linear function of the reciprocalextension ratio 1/l. s is the nominal tensile stress (force divided bythe undeformed cross-sectional area of sample), the extensionratio l = L/L0 is the ratio of deformed and undeformed length, C1

and C2 are adjustable parameters. From the theory of rubberelasticity, C1 is related to crosslink density which can be obtainedfrom the intercept. This equation can provide a satisfactorydescription of stress–strain curves in the low strain region.However, in the case of filled rubber, the strain amplificationfactor V should be used to take into account both the disturbance ofstrain distribution and the absence of the deformation of fillers.This factor V is derived from Guth equation [24]:

V ¼ 1þ 2:5c þ 14:1c2

where c is the filler volume concentration.The modified extension ratio l* of filled rubber should be

calculated by following equation:

l� ¼ 1þ Ve

where e is the ratio of extension length DL and undeformed lengthL0. As seen in Fig. 5, stress–strain curves are plotted as s/(l* � 1/l*)against l*�1 of all samples. C1 values of filled HXNBR are higherthan that of unfilled one. It is probably because the addition ofMWCNT could introduce additional filler–filler and filler–matrix

Table 2Mechanical properties of HXNBR/MWCNT composites

Sample

1 2 3 4

MWCNT content (phr) 0 1 2 4

Hardness, Shore A 43 47 50 51

Tensile strength (MPa) 13.3 � 0.2 15.7 � 0.5 16.1 � 1.1 21.6 � 1.2

Elongation at break (%) 600 � 3 622 � 6 659 � 8 627 � 7

Modulus at 100% (MPa) 0.90 � 0.01 1.10 � 0.02 1.22 � 0.02 2.41 � 0.02

Modulus at 300% (MPa) 1.30 � 0.01 2.20 � 0.05 2.99 � 0.07 6.63 � 0.06

Permanent set (%) 15 17 20 20

L. Lu et al. / Applied Surface Science 255 (2008) 2162–2166 2165

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

The stress-softening phenomenon or the Mullins effect generallyoccurs in rubber-like materials during cyclic loading where thestress–strain behavior is observed to be softer (more compliant)

Fig. 5. Mooney–Rivlin plots of HXNBR/MWCNT composites.

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

during reloading after an initial loading excursion [25]. Differentmechanisms have been proposed to explain the Mullins effectincluding that there is structure existing in rubber which is formedby chemical and physical crosslinks of filler–filler and filler–polymerinteractions [26].

The Mullins effect is more pronounced in filled rubbers thanunfilled ones. And generally the Mullins effect of filled rubbers ismuch more significant in the case of strong matrix–fillerinteraction than that of the poor one. Fig. 6 displays the Mullinseffect of HXNBR/MWCNT (100/2) composite. The stress softeningof the MWCNT-filled HXNBR vulcanizate is derived mainly from

Fig. 7. Storage modulus, loss modulus (a) and tan d (b) as a function of temperature

for HXNBR and HXNBR/MWCNT (100/2) composites.

L. Lu et al. / Applied Surface Science 255 (2008) 2162–21662166

the breakage of aggregates of MWCNT and the chain slippage of theattached polymer segments along the surface. After resting for 24 hat room temperature, the vulcanizate gained part of its elasticrecovery owing to the thermal motion of the molecule. Bokobzaet al. reported that there was a pronounced ‘‘stress-softeningeffect’’ for MWCNT-filled NR or SBR [27,28]. With combination ofother experimental results, they concluded that the interactionsbetween MWCNT and rubber would be the occlusion of rubber intothe MWCNT aggregates [16].

DMA was used to characterize the dynamic mechanicalproperties of HXNBR and HXNBR composites. The temperaturedependence of the storage modulus G0, loss modulus G00 and lossfactor tan d are given in Fig. 7. The addition of MWCNT increases G0

in glassy region and hardly affects G0 in rubber region. Theincrement of G0 is due to the hydrodynamic reinforcement byintroducing fillers. For conventional fillers, the interfacial interac-tions between rubber and fillers introduce additional crosslinksinto the network, giving rise to an increase in the modulus inpolymer matrix or increase in the viscosity in liquids [29]. The peakvalue of G00 also increased after the addition of MWCNT. The peak oftan d slightly shifts towards higher temperature, indicating thatthe small loading of MWCNT (2 phr) has a little effect on the glasstransition temperature (Tg) of the composite. As reported byKolodziej et al. [28], Tg of NR/MWCNT composites did not changesignificantly even with high loading of MWCNT and high degree ofreinforcement. Lopez-Manchado et al. observed a shift of around4 8C of Tg values between unfilled NR and NR filled with 10 phrSWCNT and they concluded that there are strong interfacialinteractions between filler and matrix [12].

4. Conclusions

HXNBR/MWCNT composites were prepared with differentMWCNT contents. The scorch time and optimum curing time ofHXNBR/MWCNT compound were shorter while curing rate indexwas larger than that of pure HXNBR. MWCNT had a remarkable

reinforcing effect on HXNBR with respect to small filler loading.The tensile strength of the HXNBR/MWCNT (100/4) composite was21.6 MPa, approximately 60% higher than that of pure HXNBR.DMA showed that MWCNT could increase the storage modulus ofHXNBR composite and had a little effect on the Tg.

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