Dispersion of entangled carbon nanotube by melt extrusion

Transcription

1 Korea-Australia Rheology Journal Vol. 22, No. 2, June 2010 pp Dispersion of entangled carbon nanotube by melt extrusion Joo Seok Oh 1, Kyung Hyun Ahn 1 and Joung Sook Hong 2, * 1 School of Chemical and Biological Engineering, Seoul National University 2 Department of Chemical and Environmental Engineering, Soongsil University (Received January 29, 2010; final revision received February 18, 2010; accepted March 3, 2010) Abstract This paper investigates the dispersion of carbon nanotube (CNT) in a polymer melt during iterative extrusion by measuring electrical and rheological properties. 2 wt% CNT as received was mixed with polymer (low density polyethylene) through twin screw extruder. The extrusion was iteratively performed at the same process condition. At the same time, the rheological and electrical properties were measured. We expected the mixing energy applied on entangled CNT increases with process time, which improves the CNT dispersion. The electrical property of intermediate composite was effectively improved by iterative extrusion. After the fifth extrusion, CNT/LDPE composite reached to the conductive electric level (surface resistance E+5 Ω/ sq). Also, the rheological properties of composite were increased according to CNT dispersion. Especially the rheological properties over lower frequency region were significantly increased by the dispersed nanotube. This paper suggests that the dispersion by only iterative mixing process results in a disentanglement of CNT and they forms an electrically useful structure. The rheological and electrical measurements indicate that the CNT disentangled by the iterative mixing method forms a percolation structure. Keywords : carbon nanotube, melt extrusion, surface resistance, rheology, dispersion 1. Introduction CNT has been regarded as attractive as a reinforcing agent by its mechanical strength and good conductivity. CNT with no defect has a high mechanical strength as much as 1000 GPa which is comparable to that of metal, glass fiber, and graphite fiber. In addition, the electrical properties of CNT can be used to induce conductivity to non-conducting materials(chen et al. 2006; Harris 1999; Lau and Hui 2002). As an electrical filler in polymer, carbon black has been used in industry due to its low cost and easy process. Practically, it needs to be contained over 25 wt% to realize the conductivity in an insulating polymer, which makes much difficult to maintain the mechanical property of polymer by high filler content. Also they come out from the fractured surface during usage as well. Instead of carbon black, CNT has been studied because CNT can provide an electrical property with its low content by its high aspect ratio and high electrical property(harris 1999). However, previous studies on mechanical and electrical performance in several CNT applications did not obtain a consistency in properties of CNT-composite because of the CNT dispersion is much different from the dispersion of spherical particles such as carbon black. A *Corresponding author: polymer@ssu.ac.kr 2010 by The Korean Society of Rheology nanoscale diameter of CNT and high aspect ratio gives much larger surface area, which induces significant entanglement between tubes even from synthesis step by a van der Waals force. Then a disentanglement of entangled CNT must be worked out to realize its high potential in practical applications. To improve the dispersion of CNT, a number of methods have been introduced to produce CNT/polymer composites by using a chemical modification of incorporated CNT (Chen et al. 2006; Harris 1999; Lau and Hui 2002; Krause et al. 2009; Bauhofer and Kovacs 2009; Li and Shimizu 2007; Chen et al. 2007; Zhu et al. 2003; Lee et al. 2008), in-situ polymerization(kumar et al. 2002; Deng et al. 2002; Funck and Kaminsky 2007), surfactant-assisted dispersion(vigolo et al. 2002; Rastogi et al. 2008; Huang et al. 2009), interfacial polymerization(haggenmueller et al. 2006), electro-spinning(wang et al. 2005), and solution mixing(qian et al. 2000). Those methods provide successful progress in performance with the incorporation of small amount of CNT (<1% CNT). For example, solution mixing of CNT and molten polystyrene(ps) led to 30~40% increase in elastic stiffness and 25% increase in tensile strength with 1% CNT addition(qian et al. 2000). But those dispersion methods demand a usage of solvent and are not practically applicable due to the formation of severe aggregation of nanotubes as the content of CNT increases. All methods are based on the chemical treatment to modify surface functionality or to remove the impurity of CNT. Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 89

2 Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong Fig. 2. SEM picture of CNT powder (Nanocyl NC7000). Fig. 1. Schematic diagram of iterative mixing process: extrusion- pelletizing-dry mixing. The CNT/LDPE extrudate is pelletized and dry-mixed. Mixing process is repetitively performed at the same process condition. As the iteration goes on, the entangled CNT disperse more, which increase rheological property of composite and decreases electrical resistance. Without any chemical modification on CNT, the direct mixing of CNT and matrix did not result in any progress in the CNT dispersion so far. In this study, we investigated the dispersion of CNT by means of polymer melt compounding. Polymer melt compounding with no CNT-chemical treatment is useful, especially in industry because it does not demand additional process step. The melt compounding studied so far (Chen et al. 2006; Harris 1999; Lau and Hui 2002; Krause et al. 2009; Bauhofer and Kovacs 2009; Li and Shimizu 2007; Chen et al 2007; Zhu et al. 2003) has been performed in micro-compounder of lab-scale, but we increased the scale of compounding extruder (L/D~38.7). Furthermore, the composites were subjected to the extrusion iteratively. Then, the applied mixing energy on CNT dispersion would be further increased. The rheological and electrical properties of polymer composite were measured after each extrusion. Electrical, rheological and morphological approaches have shown how CNT dispersion is accomplished depending on iterative process. 2. Experimental η*, storage modulus G ) of LDPE depending on the num- ber of extrusion. produced by the chemical vapor decomposition method and the purity is higher than 87.5%. Fig. 2 presents the SEM image of CNT powders (NC7000) Preparation of CNT/LDPE composites CNT/LDPE composites were prepared by melt compounding of LDPE and CNT. Melt compounding was performed by using a twin screw extruder (APV Co. D=19 mm, L/D=38.7) with two kneading zones. Extruder has seven heating zones and its temperature profile was set to 170, 180, 190, 200, 210, 210, 210 C from feeding zone to die, respectively. The rotation speed of screw was fixed to 200 rpm. The composition of CNT was fixed to 2 wt% in this study. As shown in Fig. 1, extrusion was repetitively performed under the same extrusion condition. After each extrusion, the extrudate was pelletized and dry-mixed by mild-tumbler. To investigate the effect of extrusion on CNT dispersion, the extrudates were then compressionmolded for rheological and electrical measurement using a o 2.1. Materials Low density polyethylene (LDPE 5322) used in this study was provided by Hanwha Co. Ltd (Korea). The density of LDPE is g/cm (ASTM D1505) and it has a melt flow index of 3.2 g/10 min (ASTM D1238, 190 C/ 2.16 kg). CNT used in this study is multiwalled carbon nanotube purchased from Nanocyl Co. Ltd (NC7000). It is 3 o 90 Fig. 3. Comparison of rheological properties(complex viscosity Korea-Australia Rheology Journal

3 Dispersion of entangled carbon nanotube by melt extrusion hot press molder (PHI Pasadena Hydraulics, Inc.). It was molded at 190 o C for 10 min and then annealed at room temperature. Because the iterative extrusion of polymer may cause degradation by shear and heat, the rheological properties of LDPE were also checked out as shown in Fig. 3. The rheological properties of LDPE maintains constant for fifth extrusion, which means there is no change in molecular structure by iterative extrusion Characterization The composite morphology was examined by scanning electron microscopy (SEM) using a JEOL model Hitachi- S4800 apparatus operating at an accelerating voltage of 7 kv to 20 kv. The samples for SEM were fractured in liquid nitrogen after each extrusion and then sputtered with palladium to avoid charging on the fractured surface. The thermal analysis was measured by differential scanning calorimeter(dsc) using a DSC Q2000 (TA instruments). DSC was performed with sample of about 2 to 5.0 mg between 20 to 150 o C by 10 o C/min. The rheological properties were measured at 190 o C by means of RDSII (Rheometrics Inc.) with a parallel plate fixture (25 mm diameter). The complex viscosity (η*(pas)), storage modulus (G (Pa)), and loss modulus (G (Pa)) were measured as a function of frequency(ω(rad/s)) using the dynamic oscillatory mode. As an electrical property, the surface resistance was measured by Hiresta-Up (Mitsubishi Chemical Co.) with bar-type probe. Fig. 4. Comparison of thermal analysis (DSC) of LDPE, 2 wt%cnt/ldpe depending on the number of extrusion. For the DSC measurement, heat scan was performed twice between 20 o C and 150 o C by 10 o C/min. The DSC data shown above is on second round. 3. Results The addition of 2 wt% CNT to polymer melt leads to a significant increase in viscosity by a physical networking of the dispersed CNTs (Huang et al. 2006). However, this increase in viscosity of composite is able to be expected when 2wt% CNT is dispersed sufficiently. From a hydrodynamical point of view to accomplish the CNT dispersion, the composite needs to be under process for several times under simple shear flow to apply more energy against whole van der Waals force between entangled CNTs (Huang et al. 2006). In fact, it is difficult to expect the dispersion of CNT by simple extrusion of short processing time. If the extrusion time is longer by iterative extrusion, the CNT dispersion is able to be expected as shown in Fig. 1. First of all, 2 wt% CNT/LDPE composite and LDPE were checked their degradation by iterative extrusion based on rheology and thermal analysis (DSC). From Fig. 3 and Fig. 4, it is known that iterative extrusion of short processing time does not modify LDPE and composite. Fig. 4 compares the thermal analysis of composite and LDPE with the number of extrusion. As expected from Fig. 3, the thermal analysis of LDPE is not changed during repetitive extrusion. In the case of 2 wt%cnt/ldpe, the slope of Fig. 5. Comparison of rheological properties(complex viscosity η*, storage modulus G ) of 2 wt%cnt/ldpe depending on the number of extrusion. beginning stage of endotherm peak looks changed a little by iterative extrusion but the maximum peaks are the same at 108 o C, which means the molecular weight of PE is not changed during iterative extrusion. Fig. 5 represents the variation of rheological properties of 2 wt% CNT/LDPE composite depending on the number of extrusion. Because LDPE was confirmed its consistency of rheological properties over repetitive extrusions as shown in Fig. 3, it can be considered the increment of rheological properties is purely caused by the dispersion of CNT. The storage modulus reflects the structural change of composite by the influence of relaxation time Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 91

4 Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong Table 1. Slope of G of 2 wt%cnt/ldpe Materials Slope of G /G ω at lower frequency LDPE wt%cnt/ldpe(# iteration=1) wt%cnt/ldpe(# iteration=3) wt%cnt/ldpe(# iteration=5) 4.6 =0.1 Fig. 7. Comparison of morphologies of 2 wt%cnt/ldpe after the first (a-c) and the fifth extrusion (d-f). Fig. 6. Variation of surface resistance of 2 wt%cnt/ldpe depending on the number of extrusion. ( λ)ω 2λ-2 d( ln λ), here λ is relaxation time and ( G (ω ) = H ω λ H(λ) is relaxation time spectrum)(larson 1999). The storage modulus of the first extrudate simply increases all over the observation frequency region compared to that of LDPE, which is expected by the filling effect in composite. However, when extruded three times, the storage modulus increases while the slope at lower frequency region starts to decrease. Even after the fifth extrusion, the storage modulus of composite increases further and they present the plateau at lower frequency region. We believe that this plateau of storage modulus is originated from the network structure of the disentangled CNTs (Huang et al. 2006). The disentangled CNT initially increases the relaxation times like an ultra molecule, which is reflected in the increases in storage modulus at low frequency region (Larson 1999). Table 1 lists the slope of G at lower frequency depending on the number of iteration. The slope of 2 wt% CNT/LDPE shows non-terminal behavior as the number of extrusion increases. The slope of composite after fifth extrusion significantly decreases from to 4.6. As CNT disentangles further over several extrusions, they 92 form a physical network structure. Then, it dominates the rheological behavior of the composite, as can be seen with the plateau at low frequency region. Meanwhile, the structural change of CNT/LDPE composite is confirmed through the surface resistance. Fig. 6 shows the surface resistance of extrudates with extrusion time. The surface resistance is simply decreased over the extrusion time, which means the number of CNT interconnection effectively increases as the extrusion goes on. The surface resistance is much lower when CNT in polymer forms an electric path homogeneously. With the fifth extrusion, the composite reaches to the conducting electric level (surface resistance E+5 Ω/sq). Above fifth extrusion of composite, the decrease of surface resistance becomes slow. Fig. 7 compares the morphology of CNT/LDPE composite. The morphology of the first extrudate is not so different from that of composite after the fifth extrusion. Over iterative extrusions, the change of rheological and electrical properties of the composite can be strong evidence to the change of the CNT dispersion, while the morphological observations are difficult to discriminate the effect of multiple extrusions on CNT dispersion. To ensure those observations, the iterative extrusions were performed with other commercial multi-walled CNTs (JEIO (purity 94.1%), CM95 (purity, 95%)). All CNTs show the same effect of multiple extrusions on rheological properties and surface resistance of CNT composite. Table 2 lists the surface resistance and storage modulus of composite of different CNTs depending on extrusion time. The storage modulus at frequency 0.1 rad/s increases as much as 300% Korea-Australia Rheology Journal

5 Dispersion of entangled carbon nanotube by melt extrusion Table 2. Complex viscosity, storage modulus and surface resistance of composite with different CNT CNT # iteration Viscosity (Pas) at shear rate 0.1 rad/s Storage Modulus (Pa) at shear rate 0.1 rad/s Surface resistance (Ω/sq) K 0.1 K 1.0E+12 NC K 0.4 K 2.7E+08 JEIO CM K 1.4 K 4.3E K 0.1 K 1.0E K 0.3 K 6.3E K 0.8 K 2.2E K 0.1 K 1.0E K 0.2 K 1.0E K 0.3 K 1.0E+10 Fig. 8. Schematic diagram of CNT dispersion during mixing: long range dispersion occurs during dry mixing and extrusion, short range dispersion is an individual CNT diffusion. A long- and short range dispersion simultaneously occurs during extrusion and a dry mixing helps long range dispersion of CNT. in case of NC7000. Also, the surface resistance decreases significantly with extrusion time. Those observations show that the change of electrical and rheological property of composite depending on the number of extrusion is not so different with CNT though CNT has different morphology or geometry according to the synthesis of CNT. Rather the dispersion of CNT influences the property of composite more significantly. 4. Discussion Based on the observations in this study, it can be known that the iterative extrusion of CNT composite results in the increase in storage modulus and viscosity and the improvement of electric conductivity by the enhanced CNT dispersion. We believe that the repetitive hybrid mixing of dry mixing and melt compounding improves effectively the dispersion of CNT. After extrusion, CNT composite extrudate solidifies under air and it is continuously pelletized as small as 3~4 mm of diameter, and then it is dry-mixed again. The pelletizing and dry-mixing enhance the long range dispersion of CNT particles as shown in Fig. 8. The following melt compounding induces the long and short range dispersion of CNT. To obtain CNT dispersion especially under viscous medium, it requires large energy enough to unravel the individual CNT out of the entangled CNT clumps because CNT has a significant van der Waals force by large surface area. As can be known from Fig. 3, LDPE is viscous as much as 100 to 1000 Pas at high shear. CNT with the aspect ratio of 1000 will take a long time to Fig. 9. Variation of surface resistance and complex viscosity of 2 wt%cnt/ldpe depending on the number of extrusion. diffuse under the given flow condition. In fact, the residence time of extrusion at 200 rpm (apparent shear rate at extruder clearance of 2 mm gap > s -1 ) ranges only 2 to 5 minutes. Then, it is difficult to expect the complete disentanglement of CNT clumps in a single extrusion though the complex flow in an extruder is able to induce dispersion effectively. Therefore, the iterative process of dry-mixing and melt-compounding effectively induces the long and short range dispersion of CNT and the property of the composite changes significantly. Fig. 9 presents the surface resistance and rheological property of CNT composite over iterative extrusion. The surface resistance linearly decreases with iteration. After the fifth iteration, CNT forms an electrical percolation over observation area and Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 93

6 Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong the surface resistance almost reaches to a saturation value. However, the rheological properties of the composite change continuously over extrusions. Especially, the rheological properties at low shear rate increase significantly, while those at higher shear rate increase a little. It means that CNT maintains the diffusion over multiple extrusions and the multiple extrusions enhance the dispersion continuously. The dispersed CNTs increase the complex viscosity of CNT composite further. Then, we believe that the rheological properties indicated the degree of CNT dispersion more precisely compared to the electrical property. 5. Conclusions The iterative mixing process of dry mixing and melt compounding was found to enhance CNT dispersion. Iterative extrusion was found to apply more mixing energy on CNT particles to induce CNT dispersion. During iterative extrusions, the surface resistance of composite decreased and their rheological properties were increased. The reduction of surface resistance and the increases in rheological properties were caused by the disentangled CNT. This paper shows that the intensive mixing process can induce a disentanglement of CNT dispersion effectively. References Bauhofer, W. and J. Z. Kovacs, 2007, A review and analysis of electrical percolation in carbon nanotube polymer composites, Composites Science and Technology 69, Chen, G.-X., H.-S. Kim, B. H. Park and J. S. Yoon, 2006, Multiwalled carbon nanotubes reinforced nylon 6 composites, Polymer 47, Chen, G.-X., Y. Li and H. Shimizu, 2007, Ultrahigh-shear processing for the preparation of polymer/carbon nanotube composites, Carbon 45, Deng, J., X. Ding, W. Zhang, Y. Peng, J. Wang, X. Long, P. Li, and A. S. Chan, 2002, Carbon nanotube-polyaniline hybrid materials, Eur Polym J. 38, Funck, A. and W. Kaminsky, 2007, Polypropylene carbon nanotube composites by in situ polymerization, Composites Science and Technology 67, Haggenmueller, R., F. Du, J. E. Fischer, and K. I. Winey, 2006, Interfacial in situ polymerization of single wall carbon nanotube/nylon 6,6 nanocomposites, Polymer 47, Huang, Y. Y., S. V. Ahir, and E. M. Terentjev, 2006, Dispersion rheology of carbon nanotubes in a polymer matrix, Phys. Rev. B 73, Ji,Y., Y. Y. Huang, A.R. Tajbakhsh, and E. M. Terentjev, 2009, Polysiloxane surfactants for the dispersion of carbon nanotubes in nonpolar organic solvents, Langmuir 25(20), Krause, B., P. Potschke, and L. Haußler, Influence of small scale melt mixing conditions on electrical resistivity of carbon nanotube-polyamide composites, Composites Science and Technology 69, Kumar, S., T. D. Dang, F. E. Arnold, A. R. Bhattacharyya, B. G. Min, X. Zhang, R. A. Vaia, C. Park, W.W. Adams, R. H. Hauge, R. E. Smalley, S. Ramesh, and P. A. Willis, 2002, Synthesis, Structure, and Properties of PBO/SWNT Composites, Macromolecules 35, (2002). Larson, R., 1999, The structure and rheology of complex fluids, Oxford University Press, New York. Lau, K.-T. and D. Hui, 2002, The revolutionary creation of new advanced materials Carbon nanotube composites, Composites Part B 33, Lee, G.-W., S. Jagannathan, H. G. Chae, M. L. Minus, and S. Kumar, 2008, Carbon nanotube dispersion and exfoliation in polypropylene and structure and properties of the resulting composites, Polymer 49, Li, Y. and H. Shimizu, 2007, High-shear processing induced homogenous dispersion of pristine multiwalled carbon nanotubes in a thermoplastic elastomer, Polymer 48, Liu, J., T. Wang, T. Uchida, and S. Kumar, 2005, Carbon nanotube core-polymer shell nanofibers, J App. Poly. Sci. 96(5), Peter J. F. Harris, 1999, Carbon nanotubes and related structures : new materials for the 21 st century, Cambridge University Press. Qian, D., E.C. Dickey, R. Andrews, and T. Rantell, 2000, Load transfer and deformation mechanisms in carbon nanotubepolystyrene composites, Appl Phys Lett. 76, Rastogi,R., R. Kaushal, S. K. Tripathi, A. L. Sharma, I. Kaur, and L. Bharadwaj, 2008, Comparative study of carbon nanotube dispersion using surfactants, J. Colloid Int. Sci. 328, Vigolo, B., P. Poulin, M. Lucas, P. Launois, and P. Bernier, 2002, Improved structure and properties of single-wall carbon nanotube spun fibers, Appl Phys Lett. 81, Zhu, J., J. Kim, H. Peng, J. L. Margrave, V.N. Khabashesku, and E.V. Barrera, 2003, Improving the dispersion and integration of single-walled carbon nanotubes in epoxy composites through functionalization, Nano Lett. 3, Korea-Australia Rheology Journal