Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites

Transcription

1 Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites YASIN KANBUR* Department of Polymer Science and Technology Middle East Technical University, Turkey ZUHAL KU C ÜKYAVUZ Chemistry Middle East Technical University, Turkey ABSTRACT: Polypropylene(PP)/carbon black(cb) composites at different compositions were prepared via melt blending of PP with CB. The effect of CB content on mechanical and electrical properties was studied. Test samples were prepared by injection molding and compression molding techniques. The effect of processing type on mechanical and electrical properties was also investigated. Composites become semi-conductive by addition of 2 wt% CB. The relation between mechanical and electrical properties was discussed. The influence of CB content on morphology and crystallinity was also studied. KEY WORDS: mechanical properties, injection molding, compression molding, polymer matrix composites (PMCs). INTRODUCTION COMPOSITE MATERIALS BASED on conductive fillers dispersed within insulating thermoplastic matrices have a wide range of application. For very many years it has been common practice to improve the electrical conductivity of plastics and rubbers by the incorporation of certain additives like special grades of carbon black [1]. Carbon black is a very important filler, especially in the rubber industry. Their fine particle size, high particle porosity, and compatibility with organic materials made them obvious candidates for use as fillers [2]. The strong interaction between polymer and carbon black particles improve the mechanical properties of the composites [3,4]. The conductivity of a polymer CB composite depends upon the factors such as carbon black content, the physical and chemical properties of the carbon black chosen, the chemical structure and crystallinity of the polymer and process conditions [5,6]. In this article, electrical, mechanical and thermal properties of polypropylene/carbon black composites were investigated. The main goal was to analyse the effect of different processing types and varying carbon black content on mechanical properties and electrical conductivity. The effect of the CB content on morphology and thermal stability was also investigated. *Author to whom correspondence should be addressed. kyasin@metu.edu.tr Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 28, No. 18/ /9/ $1./ DOI: / ß SAGE Publications 29 Los Angeles, London, New Delhi and Singapore

2 2252 Y. KANBUR AND Z. KU C ÜKYAVUZ Materials EXPERIMENTAL Vulcan XC-72 type carbon black was supplied from Cabot Corporation. Polypropylene (EH 251, MFI ¼ 28 g/min, melting temperature ¼ 1738C) was a product of Petkim, Turkey. Composite preparation Polypropylene and carbon black were mixed by using Brabender Plastic Coder, PLV- 151 at 218C and 75 rpm for 1 min. Composites were compressed in a mold for 8 min at 218C then fast cooled. Injection molded samples were prepared at 218C, mold temperature was at room temperature. In the experiments, micro-injection molding which is produced by Daca Instruments was used. For compression molding 6 and for injection molding 8, different compositions were prepared. Analytical Methods Melt flow property measurements were performed by using the Coesfield Material Test, Meltfixer LT. The measurements were done at 218C. Materials were allowed to melt for 5 min, then standard weight placed on the piston of the instrument was used to compress the sample. After that, the weight of the flow sample were reported for 1 min. A tensile test was done by using a Lloyd LS computer controlled tensile machine. Dog bone-shaped molded samples were used and measurements were done at 248C. A 5 kn load cell was used in the measurements. Charpy impact tests (unnotched) were done by using Pendulum Impact Tester of Coesfield Material Test machine. The test specimens were prepared by injection molding. Bar shaped specimens having size 5.5 cm were used in the experiments. Thermal properties of composites were studied with Dupont Thermal Analyst 2 DSC 91S instrument at the temperature range between 25 and 2258C under N 2 atmosphere. Heating rate was 58C/min. Percent crystallinities were calculated from the peak areas. JEOL, JSM 64 was used to obtain SEM micrographs. Fracture surface of the samples were analysed. Conductivity measurements were done by using a four probe technique. For the electrical conductivity measurements, composites were compressed at 218C to produce a film of about.1 mm thickness. To measure the conductivity of the composites prepared by injection, molding test samples were cut from the injection molded samples of about 2. mm thickness. Thermal gravimetric analysis of pure CB, pure PP, and PP/CB composites were done by using Perkin Elmer Pyris 1 TGA. RESULTS AND DISCUSSION Figure 1 shows melt flow properties of the samples. Melt flow index values of the composites decrease with increasing carbon black content since increasing carbon black content increases the viscosity of the composites. A boundary layer, the amount of which

3 Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites 2253 depends on filler concentration, of the polymer is formed because of the strong interaction between polymer matrix and carbon black. Polymer molecules are adsorbed on the surface of carbon black particles and this decreases the mobility of the polymer chains. Increasing boundary layer density further increases the viscosity of the samples [7]. Effect of CB on percent crystallinity of PP was investigated by DSC. Percent crystallinity, calculated from DSC termograms are shown in Figure 2. Increasing CB content up to 3 wt% decreases the percent crystallinity from 41 to 33%. Since adding carbon black affects the crystal structure of the polypropylene, deformation occurs in the crystalline structure. The effects of carbon black content on impact properties of composites are shown in Figure 3. Impact strength of composites decrease with increasing filler content Melt flow index (g/1 min) Figure 1. Melt flow properties of samples Percent crystallinity Figure 2. Percent crystallinity.

4 2254 Y. KANBUR AND Z. KU C ÜKYAVUZ 45 4 Impact strength (kj/m 2 ) Figure 3. Impact test. The samples which have carbon black lower than 1 wt% do not break with the applied force since the applied energy is absorbed by the polypropylene matrix. Increasing filler content made the composites more brittle. It can also be seen from SEM micrographs of fracture surface of composites that increasing carbon black increases the brittleness of the samples (Figure 4(a d)). As can be seen in Figure 4(a d) the estimated size of the carbon black particles are in the range of 1 nm. Dispersion of carbon black in polypropylene matrix becomes more uniform as CB content increases. The effect of carbon black content on tensile strength and Young s modulus was represented in Figure 5(a) and (b), respectively. For compression molded and injection molded samples, an increase was observed in Young s modulus and tensile strength. Young s modulus of the composite increases with increasing carbon black content because carbon black is stiffer than polypropylene. The increase of Young s modulus and tensile strength of the samples prepared by using injection molding is higher than the samples prepared by using compression molding. This result can be explained by the orientation of the polymer chains in the direction of applied force for the samples prepared by injection molding. In compression molded samples there is no orientation of the polymer chains, and mechanical strength of the samples are lower than the injection molded samples. Figure 5(c) represents the percent deformation at break with increasing filler content. Percent deformation at break of the samples decrease with the increasing filler content since addition of carbon black restricts the motion of the polymer chains. Addition of conductive carbon black particles in an insulative polymer matrix causes its electrical resistance to decrease. At a concentration known as electrical percolation threshold (P c ) an infinite cluster of conductive carbon black particles extends throughout the insulative matrix and the resistivity drops several orders of magnitude [8 1]. PP is an insulative polymer. The conductivity of pure PP is S/cm [1]. As can be seen in Figure 6, electrical conductivity increases up to 1 6 S/cm upon

5 Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites 2255 (a) (b) (c) (d) Figure 4. Fracture surface of blend containing: (a) pure polypropylene, (b) 5% carbon black, (c) 15% carbon black, and (d) 3% carbon black.

6 2256 Y. KANBUR AND Z. KU C ÜKYAVUZ (a) Tensile strength (MPa) Compression molding Injection molding (b) 12 Young's modulus (MPa) Compression molding Injection molding (c) 1 Percent deformation at break (%) Compression molding Injection molding Figure 5. Mechanical properties of testing materials: (a) tensile strength, (b) Young s modulus, and (c) percent deformation at break. addition of 2 wt% CB. Electrical properties of the samples increases with increasing filler content as shown in this figure. There is an abrupt increase in conductivity when CB content exceeds 2%. Electrical conductivity of composites is very close to the value of pure matrix up to a value of filler content. This value is called a percolation

7 Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites Log conductivity (siemens/cm) Compression molding Injection molding Figure 6. Electrical conductivity of the composites prepared by compression and injection molding. threshold. After that point, conductivity increases rapidly because the filler starts to form a conductive layer through the polymer matrix [11]. After the threshold point, carbon black particles agglomerate and this causes the formation of the conducting network [12]. Effect of the process type on the electrical conductivity is illustrated in Figure 6. The increase in electrical conductivity with increasing filler content is more pronounced in the samples prepared by compression molding. Since injection molding causes orientation of fillers and this causes an increase the distance between the CB particles. And as result of this, CB particles cannot form agglomerate and this decreases the electrical conductivity. In the article by Chodak et al. [13] a relationship between percent deformation and electrical conductivity was proposed. We demonstrate this relationship in Figure 7(a) for compression molded and in Figure 7(b) for injection molded samples. Percent deformation at break of the samples decreased with increasing filler content, because polymer chains are absorbed on the surface of the carbon black and the force applied cannot be transported to each point equally and materials become more brittle and hard. The decrease at percent deformation at break is more obvious for the compression molded samples. Addition of the filler content more than the percolation threshold value causes a decrease in the percent deformation at break. After the percolation threshold point, material become more brittle [13,14]. The influence of the amount of carbon black on thermal stability of the composites is given in Figure 8. Increasing amount of carbon black increases the thermal stability of the composites. The temperature at which thermal decomposition starts, shifts to higher temperatures [15]. Pure PP decomposes at about 48C. Adding 3% CB increases the decomposition temperature to 538C.

8 2258 Y. KANBUR AND Z. KU C ÜKYAVUZ (a) 1 1 Percent deformation at break (%) Log conductivity (siemens/cm) Percent deformation at break 3 Log conductivity 6 4 (b) 1 Percent deformation at break (%) Log conductivity (siemens/cm) Percent deformation at break(%) Log conductivity Figure 7. (a) The relationship between electrical conductivity and percent deformation at break as a function of carbon black content for composites prepared by compression molding. (b) The relationship between electrical conductivity and percent deformation at break as a function of carbon black content for composites prepared by injection molding. CONCLUSIONS PP/CB composites were prepared via melt blending PP with CB. The effect of processing type on mechanical and physical properties was investigated. Injection molded samples

9 Electrical and Mechanical Properties of Polypropylene/Carbon Black Composites Pure CB Weight% 6 4 3% CB 2 Pure PP 5% CB 15% CB Temperature ( C) Figure 8. Thermal properties of testing materials. had better mechanical properties when compared with compression molded samples due to the orientation of the polymer chain in the direction of the applied force. Electrical conductivity increased more in compression molded samples with added carbon black. Percent deformation at break values of the samples decreased with increasing filler content. The relation between percent deformation at break and electrical conductivity was studied. After percolation threshold a sudden increase in electrical conductivity was observed. Increasing filler content after threshold point show negative effect on percent deformation at break. Addition of carbon black improved the thermal properties of the composites. REFERENCES 1. Brydson, J. A. (1999). Plastic Materials, 7th edn, p. 12, Reed Educational and Professional Publishing Ltd. 2. Rothon, R. N. (22). Particulate Fillers for Polymers, Rapra Technology Ltd., Volume 12, Number 9, pp Chung, D. D. L. (1994). Carbon Fiber Composites, p. 113, Butterworth-Heinemann. 4. Pramanik, P. K., Khastgir, D. and Saha, T. N. (1992). Conductive Nitrile Rubber Composite Containing Carbon Fillers: Studies on Mechanical Properties and Electrical Conductivity, Composites, 23(3): Xanthos, M. (25). Functional Fillers for Plastics, p. 324, Wiley-VCH Verlag. 6. Yui, H., Wu, G., Sano, H., Sumita, M. and Kino, K. (26). Morphology and Electrical Conductivity of Injection-molded Polypropylene/Carbon Black Composites with Addition of High-density Polyethylene, Polymer, 47(1): Petrovic, Z. S., Martinovic, B., Divjakovic, V. and Budinski- Simendic, J. (1993). Polypropylene-Carbon Black Interaction in Conductive Composites, J. Appl. Polym. Sci., 49(9): Ranjbar, Z. and Rastegar, S. (26). Morphology and Electrical Conductivity behavior of Electro-deposited Conductive Carbon Black-filled Epoxy Dispersions Near the Insulator-conductor Transition Point, Colloids and Surfaces A: Physicochem. Eng. Aspects, 29: Stauffer, D. and Aharony, A. (1992). Introduction to Percolation Theory, 1st edn, Taylor and Francis, London. 1. Adler, J., Meir, Y., Aharony, A., Harris, A. B. and Klein, L. (199). Low-concentration Series in General Dimension, J. Statist. Phys., 58:

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