Multi-Wall Carbon Nanotubes/Styrene Butadiene Rubber (SBR) Nanocomposite

Fullerenes, Nanotubes, and Carbon Nanostructures, 15: 207 214, 2007 Copyright # Taylor & Francis Group, LLC ISSN 1536-383X print/1536-4046 online DOI: 10.1080/15363830701236449 Multi-Wall Carbon Nanotubes/Styrene Butadiene Rubber (SBR) Nanocomposite Nazlia Girun, Fakhrul-Razi Ahmadun, and Suraya Abndul Rashid Department of Chemical and Environmental Engineering, University Putra Malaysia, Serdang, Malaysia Muataz Ali Atieh Department of Biotechnology Engineering, International Islamic University Malaysia, Kuala Lumpur, Malaysia Abstract: A floating catalyst chemical vapor deposition (FC-CVD) method was designed and fabricated to produce high-quality and -quantity carbon nanotubes. The design parameters like the hydrogen flow rate; reaction time and reaction temperature were optimized to produce high yield and purity of Multi-Wall Carbon Nanotubes (MWCNTs). Multi-Walled Carbon Nanotubes (MWNTs) were used to prepare natural rubber (NR) nanocomposites. Our first efforts to achieve nanostructures in MWNTs/styrene butadiene rubber (SBR) nanocomposites were formed by incorporating carbon nanotubes in a polymer solution and subsequently evaporating the solvent. Using this technique, nanotubes can be dispersed homogeneously in the NR matrix in an attempt to increase the mechanical properties of these nanocomposites. The properties of the nanocomposites such as tensile strength, tensile modulus, elongation at break and hardness were studied. Using different percentages of carbon nanotubes from 1 wt% to 10 wt%, several nanocomposites samples were fabricated. Significant improvements in the mechanical properties of the resulting nanocomposites showed almost 10% increase in the Young s modulus for 1 wt% of CNTs and up to around 200% increase for 10 wt% of CNTs. Keywords: Multi-wall carbon nanotubes, styrene butadiene rubber, nanocomposites, Young s modulus Address correspondence to Muataz Ali Atieh, Department of Biotechnology Engineering, International Islamic University Malaysia, Kuala Lumpur 50728, Malaysia. E-mail: motazali@iiu.edu.my 207

208 N. Girun et al. INTRODUCTION Since their first observation by Iijima in 1991 (1), CNTs have been the focus of considerable research. From unique electronic properties and a thermal conductivity higher than that of diamond to mechanical properties where the stiffness, strength and resilience exceed those of any current material, CNTs offer tremendous opportunities for the development of fundamental new material systems (2 5). It has been reported that CNTs are extremely strong, with the strength of tens of Gpa, and exceptionally stiff, with Young s modulus in TPa range. Various methods to grow CNTs have been developed, including laser ablation (6), arc discharge (7) and chemical vapor deposition, (CVD) (8). However, CVD is the only process with which CNTs can be produced with high yields and high purity. For multi-walled carbon nanotubes, a number of graphene layers are co-axially rolled together to form a cylindrical tube. The spacing between graphene layers is about 0.34 nm. Theoretically, the tensile modulus and strength of a graphene layer can reach up to 1 TPa and 200 GPa (9), respectively. CNTs may provide the ultimate reinforcing materials for the development of a new class of nanocomposites (10, 11). EXPERIMENTAL The Chemical Vapor Deposition (CVD) method was employed for the synthesis of CNT and CNF. The method is cheap and requires a relatively low deposition temperature compared to other techniques used to synthesize CNT and CNF. A CVD Setup has been designed and fabricated to synthesize CNF and CNT (Figure 1). The synthesis of CNTs and CNFs in the present experiment was conducted in a horizontal tubular reactor. The horizontal reactor is a quartz glass tube of 50 mm in diameter and 900 mm in length and heated by a silicon carbide Figure 1. Experimental setup for CNT synthesis.

Producing MWCNTs and SBR Nanocomposites 209 heating element. In this experiment catalyst ferrocene, was heated in a heating flask up to 1208C to produce ferrocene vapour. At this instant, hydrogen gas was then bubbled into a benzene flask to realize a mixed vapour of hydrogen and benzene. This mixture was, in turn, passed into the vapour of the heated ferrocene before the feed gas was then passed into the reactor. The reactor tube was heated up to 8008C at a speed of 108C/min; hydrogen gas reduced ferrocene into Fe ions, which then aggregated into nanoscaled Fe catalyst particles to grow CNTs and CNFs. The variation of flow rate was used to study the effect of H 2 on the growth of CNTs and CNFs. The flow rate may influence the concentration of benzene and its absorption on the catalyst particles. The conditions for reaction were also fixed as: reaction temperature (RT ¼ 8508C), reaction time (Rt ¼ 45 minutes), hydrogen flow rate (H 2 ¼ 300 ml/min) and ferrocene evaporating temperature (FT ¼ 1208C). Argon (Ar) gas was flown into the CVD reactor to prevent the oxidation of catalytic metal while raising the temperature. At the end of the growth, the reactor was allowed to cool down to room temperature. The morphological features CNTs were observed by SEM (scanning electron microscope) and TEM (transmission electron microscope) (12). The experiment was conducted by varying the amount of SBR and CNTs. Here, the CNTs were added into SBR as filler. The total amount of dry rubber and dry CNTs is 10 grams. The amount of CNTs that was used varying 1%, 3%, 5%, 7% and 10% of 10 grams. In this study the achievement of nanostructures of MWNTs/styrene butadiene rubber (SBR) nanocomposites were formed by incorporating carbon nanotubes in a polymer solution and subsequently evaporating the solvent that has been reported previously (12). RESULTS AND DISCUSSION SEM Observations The resultant carbon nanotubes were characterized extensively using SEM. Figure 2 shows typical SEM images of carbon nanotubes. High purity, carbon nanotube films were observed in the image. The SEM observation shows that these carbon nanotubes are tens of microns long (up to 50 microns) with uniform diameters. The bulk morphology of the long carbon nanotubes are film-like and oriented. The purity of these carbon nanotubes are more than 95%. The surfaces of these are smooth and less deflective as shown in the image. TEM Observations TEM was carried out to characterize the structure of nanotubes, grown at a temperature of 8508C, reaction time at 45 minutes and hydrogen flow rate 300 ml/min. To prepare TEM samples, some alcohol was dropped on the

210 N. Girun et al. Figure 2. SEM images of carbon nanotubes, reaction temperature 8508C, hydrogen flow rate 300 ml/min, and reaction time 45 minutes. nanotube films. Then, these films were transferred with a pair of tweezers to a carbon-coated copper grid. The TEM images of nanotubes are presented in Figure 3. It is obvious from the images that all the nanotubes are hollow and tubular in shape. In some of the images, catalyst particles can be seen inside the nanotubes. High purity (.90%), with uniform diameter distribution and long CNTs were observed in the image. Figure 3. TEM images of CNTs: (a) Low resolution; (b) High resolution.

Producing MWCNTs and SBR Nanocomposites 211 Figure 3b shows the High Resolution Transmission Electron Microscope (HRTEM) of carbon nanotubes, showing that a highly ordered crystalline structure of CNT is observed. The clear fringes of graphitic sheets are well separated by 0.34 nm and aligned with a tilted angle of about 28 toward the tube axis. Effect of CNTs on the Stress-Strain of CNTs/SBR Nanocomposites Figure 4 shows the stress-strain curve. It shows that the stress was increased while the strain was decreased, as expected when the amounts of CNTs were increased. The increment of stress level was due to the interaction between the CNTs and the rubber. A good interface between the CNTs and the rubber is very important for a material to withstand the stress. As described above, CNTs are highly strong materials compared to other types of fillers, thus making them good candidate as nanofillers. Under loading, the matrix distributes the force to the CNTs, which carry most of the applied load. When CNTs/SBR nanocomposites are subjected to loads, the CNTs act as carriers of the loads. Stresses are transferred from SBR along the CNTs, leading to effective and uniform stress distribution, which result in a composite with good mechanical properties. At low-level amounts of CNTs, the CNTs are not capable of transferring loads to one another and stresses accumulate at certain points of the nanocomposites, leading to low tensile strength. The value of strain was decreased with increasing amount of CNTs. The decrease in elongation at break with increasing the amount of CNTs that is rigid arises from the fact that the actual elongation experienced by the polymer matrix is much greater than the measured elongation of the Figure 4. Maximum stress and strain of CNTs/SBR nanocomposites for different percentages of CNTs.

212 N. Girun et al. nanocomposite specimen. Although the specimen is part of filler that is CNTs and part of matrix that is SBR, all the elongation comes from the polymer if the filler is rigid. As the amount of CNTs is increased, the amount of SBR should be decreased. So, the elongations of nanocomposites at elongation break are supposed to be decreased. An elongation break decrease indicates that the incorporation of CNTs into SBR can improve the stiffness of the composite. Effect of CNTs on the Young s Modulus of CNTs/SBR Nanocomposites The Young s modulus of the composites normalized with that of the pure matrix is presented in Figure 5. The results indicated that the Young s modulus increased with increase in the amount of the CNTs. However, at 1 and 3 wt% of CNTs, the increment of the modulus is not as high as that of the tensile strength. The same value of the modulus and the tensile strength were observed at 5 wt% of CNTs. While at 7 and 10 wt%, the modulus was higher than the tensile strength. This is because at high CNTs loading, the nanocomposites were able to withstand more loads. The initial slope of the stress-strain curve gives the modulus, which is independent of the speed of deformation, since the first part of the curve corresponds to the stretching of the sample. At higher elongation, the slope of the curves decrease and their magnitude depend on the speed of testing. At 1 wt% of CNTs, the Young s modulus increased by 11.36% compared to the pure SBR. At 3 wt% CNTs, the increase in the Young s modulus was 19.04%. Further increases in the amount of CNTs from 5, 7 to 10 wt% increased the Young s modulus by 28.51%, 65.27%, and 193.91%, respectively, compared to the SBR without CNTs. Figure 5. of CNTs. Young s modulus of CNTs/SBR nanocomposites for different percentage

Producing MWCNTs and SBR Nanocomposites 213 CONCLUSION In summary, we have demonstrated the successful fabrication of nanocomposites consisting of Styrene Butadiene Rubber (SBR) matrix with 1 10 wt% multi-walled carbon nanotubes (MWCNTs). The exceptional mechanical and physical properties of CNTs make this new form of carbon an excellent candidate for composite reinforcement. CNTs/SBR nanocomposites that we have prepared show improvement in their mechanical properties compared to pure SBR (SBR without CNTs). This has been proven by the increment of stress and Young s modulus as the amount of CNTs increased. The stress value or normally known as tensile strength has been increased to 21.0% for 1 wt% of CNTs up to 70.26% for 10 wt% of CNTs, while the Young s modulus or modulus of elasticity has been increased to 11.36 for 1 wt% of CNTs up to 193.91% for 10 wt% of CNTs compared to SBR without CNTs. ACKNOWLEDGMENT The authors gratefully acknowledge International Islamic Universe Malaysia and Universiti Putra Malaysia for their support. REFERENCES 1. Iijima, S. (1991) Helical microtubules of graphitic carbon. Nature, 354: 56. 2. Collins, P.G. and Avouris, P. (2000) Nanotubes for electronics. Scien. Amer., 283: 62. 3. Treacy, M.M.J., Ebbesen, T.W., and Gibson, J.M. (1996) Exceptionally high Young s modulus observed for individual carbon nanotubes. Nature, 381: 678. 4. Falvo, M.R. and Clary, G.J. (1997) Bending and buckling of carbon nanotubes under large strain. Nature, 389: 582. 5. Flahaut, E., Peigney, A., Laurent, C., Marlière, C., Chastel, F., and Rousset, A. (2000) Carbon nanotube metal oxide nanocomposites: microstructure, electrical conductivity and mechanical properties. Acta Mater., 48: 3803. 6. Guo, T., Nikolaev, P., Thess, A., Colbert, D.T., and Smalley, R.E. (1995) Catalytic growth of single-walled manotubes by laser vaporization. Chem. Phys. Lett., 243: 49. 7. Journet, C., Maser, W.K., Bernier, P., Loiseau, A., de la Chapelle, M.L., Lefrant, S., Deniard, P., Lee, R., and Fisher, J.E. (1997) Large-scale production of single-walled carbon nanotubes by the electric-arc technique. Nature, 388: 756. 8. Nikolaev, P., Bronikowski, M.J., Bradley, R.K., Fohmund, F., Colbert, D.T., Smith, K.A., and Smalley, R.E. (1999) Gas-phase catalytic growth of singlewalled carbon nanotubes from carbon monoxide. Chem. Phys. Lett., 313: 91. 9. Qian, D., Dickey, E.C., Andrews, R., and Rantell, T. (2000) Load transfer and deformation mechanisms in carbon nanotubes polystyrene composites. Appl. Phys. Lett., 76 (20): 2868.

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