The Influence of Morphology on the Electrical Properties of 10MeV Electron Beam Irradiated Polyethylene/ ethylene-vinyl-acetate Blend (PE/EVA)

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1 Journal of Applied Chemical Research, 14, (2010) ISSN : The Influence of Morphology on the Electrical Properties of 10MeV Electron Beam Irradiated Polyethylene/ ethylene-vinyl-acetate Blend (PE/EVA) A. Noori* 1, F. Ziaie 2 1 Islamic Azad University, Taft Branch, P. O. Box , Taft, Yazd, Iran. 2 Agricultural, Medical and Industrial Research School (AMIRS), P. O. Box , Karaj, Iran. * noori@taftiau.ac.ir, (Received 21 Mar. 2010; Final version received 14 Apr. 2010) Abstract In this work a series of low-density polyethylene (LDPE) blends with different percentages of poly (ethylene-vinyl-acetate) (EVA) were prepared. The samples were irradiated under the 10 MeV electron beam in the range of kgy. We observed by scanning electron microscope (SEM) that the morphological pattern of the blended sample after and before irradiation. It was concluded that the homogeneous pattern can be obtain in at least 30% of EVA in the blend and cross-linking makes the pattern more homogeneous especially at ~170 kgy. The variations of surface and volume resistance of the irradiated samples were also studied as a function of EVA content and the radiation dose. It was revealed that the surface and volume resistance of the blends are maximum at ~170 kgy of radiation dose and 30 wt % of EVA. Keywords: Morphology, Polyethylene/ethylene-vinyl-acetate blend (PE/EVA), Electron beam, Surface resistance, Volume resistance. Introduction Mixing poly(ethylene-co-vinyl acetate) EVA and semi-crystalline polymers such as low density polyethylene (LDPE) gives composite having better flexibility, toughness and high resistance to environmental stress cracking due to increased adhesive strength at the matrix-rubber particle interface [1]. The relation between morphology of LDPE/EVA blends and their mechanical and electrical properties have been reported in several publications [2-9] as well. Several studies have indicated that the gel content and the cross linking density of EVA/LDPE blends at optimum radiation dose increase with increasing EVA content, and the highest gel content is observed when the amount of EVA in the blended samples reaches 30 wt% [10, 11].In this work, attention has been focused on the role of cross-linking on surface structure of polyethylene/ ethylene-vinyl-acetate (PE/EVA) blends with different percentages, and how the morphology affect the electrical properties of this polymer. In fact, the aim is to investigate the effect of 10 MeV electron beam radiation upon the morphological pattern of LDPE/EVA blends, and its

2 38 / ISSN : A. Noori et al., J. Appl. Chem. Res., 14, (2010) effects on electrical properties of the blends. Experimental Materials Low density polyethylene (LDPE 0075, MFI = 0.75 g/10 min) supplied by Bandar Imam Petrochemical Company of Iran, with density of 0.92 g/cm 3, and Ethylene-vinyl-acetate (EVA18%, MFI = 2.2 g/10 min), grade 8430 supplied by Hyundai Company, Korea, were used. Sample preparation Low density polyethylene granules were melted in a cam type internal mixer Plasti-Corder, BRABENDER Company, made in Germany, with speed of 50 rpm at 145 o C for 5 min, and then EVA were added gradually and allowed to mix for 10 min. The EVA percentages in the blends were 10%, 20% and 30%. The samples were prepared in a sheet form with 2+0.1mm thickness using the warm press system at 160 o C. DSC system A differential scanning calorimeter system, DSC-50 made by Shimadzu Company, Japan, was used at a heating rate of 10 o C/min to study the thermo-diagram of the samples. The required temperatures using in warm press system were found from this diagram (Figure 1). Sample irradiation The samples were irradiated with doses of 70, 120 and 170 kgy with a constant dose rate. The irradiation was performed using the Rhodotron type electron accelerator machine, TT200 model, using 10 MeV electron beam with a maximum of 8 ma beam current Heat flow rate (mw) EVA Melting point (~87 C) EVA Melting point (~111 C) Temp. ( C) PE/EVA blend EVA Figure 1. DSC diagram for EVA and PE/EVA blend.

3 A. Noori et al., J. Appl. Chem. Res., 14, (2010) 39 ISSN : / Microscopic technique A scanning electron microscopy system (SEM) XL30 series, made by Philips Company, Holland, was used to investigate the morphology of the samples. The magnification of the system can be obtained from 25x to x, using the LaB6 filament. The surfaces of the samples were covered with thin gold layer prior to the SEM investigations. Resistance measurement Surface and volume resistance were measured at room temperature by Teraohmmeter, made by CEAST Company, Italy. Results and Discussion Figure 2 compares the surface resistance of EVA/LDPE blends containing different percent of EVA as a function of radiation dose. It can be seen that as the content of EVA in the blend increases, the lower surface resistance is reached. All samples, except unblended LDPE, show an increase of surface resistance up to 170 kgy of radiation dose. For the radiation doses higher than 170 kgy, the surface resistance of the blended samples decreased abruptly and then converged to each other. On the other hand, the maximum of the trace was observed at 170 kgy for the EVA content of 30 wt%. However with increasing the radiation dose, the surface resistance of the LDPE remain unchanged [12]. Surface Resistance (10 12 ohm) EVA/PE 10/90 20/80 30/70 0/ Dose (kgy) Figure 2. Surface resistance vs. absorbed dose of 10 MeV electron beam irradiated EVA/LDPE blends.

4 40 / ISSN : A. Noori et al., J. Appl. Chem. Res., 14, (2010) Similar results is obtained for volume resistance of the blend containing 30 wt% of EVA which is higher than the other samples around the 170 kgy radiation dose (Figure 3). The noticeable peaks in the surface and volume resistance curves around 170 kgy may be related to higher cross linking density at this dose of radiation which causes increased average molecular weight of polymer. It is shown that influence of molecular weight on morphological and electrical properties of PE is significant and any increase in molecular weight leads to an increase in the volume resistivity [13, 14]. 100 Volume Resistance (10 12 ohm) Dose (kgy) EVA/PE 10/90 20/80 30/70 0/100 Figure 3. Volume resistivity vs. absorbed dose of 10 MeV electron beam irradiated EVA/LDPE blends. An increase in cross linking points can be considered as barrels (growth of the number of traps in material) which prevent the charge movement between polymer chains and thus increases the electrical resistance [15]. At higher doses, the cross linking process occurs while chain scission reaction is also happenes [16]. The polar and ionic products which are introduced into the polymer matrix due to irradiation can probably reduce the resistance of samples [17, 18]. Figure 4 shows the SEM micrograph of the PE/EVA samples with the different weight percent of EVA in the blends. The bright spots of various size and shape are due to the presence of EVA copolymer distributed into the dark areas which are the semi-crystalline PE matrix. In the case of 80/20 wt% of PE/EVA samples, larger EVA particles are present compared to the 90/10 wt% of the samples. The image of the samples with the weight percent of 70/30 shows more

5 A. Noori et al., J. Appl. Chem. Res., 14, (2010) 41 ISSN : / homogenous structure compared to the others. It means that increasing the EVA content in the blend causes the formation of an interpenetrating polymer network and makes the samples more flexible. Therefore samples can deform and relax better under stress. In general, rubber content, rubber particle size, and the inter-particle distance are important factors in the deformation and fracture of toughened plastics [19]. Figure 4. SEM micrograph of PE/EVA samples with different weight percent of EVA, a) 90/10; b) 80/20; c) 70/30 as PE(%)/EVA(%), for un-irradiated samples. The decisive role of the inter-particle distance in contrast to the particle diameter has been mentioned by several authors [20, 21]. This means that a critical inter-particle distance exists, which describes the brittle-tough transition of the material. Therefore, we have chosen the weight percent of 70/30 for the later experiment which is the radiation effect on the microstructure of the blends. Figure 5 shows the SEM micrograph of PE/EVA samples which were irradiated under the 10 MeV electron beam with different doses. In this case, the images are visually smoother in contrast, with addition of absorbed dose. Irradiation changes the polymer network; therefore, it changes the morphological state of the polymer. An increase of the radiation dose leads to an

6 42 / ISSN : A. Noori et al., J. Appl. Chem. Res., 14, (2010) increase in cross-linking degree in amorphous area and leads to increased molecular weight. Therefore, one of the important factors which affect the properties of polymers especially the electrical properties is the morphological state of the polymer. Figure 5. SEM micrograph of PE/EVA samples with the weight percent 70/30, irradiated under the 10 MeV electron beam with different doses of 0, 70, 120, and 170 kgy. Conclusion The higher content of EVA in the blend causes lower surface and volume resistance in the PE/ EVA blend. The maximum values for surface and volume resistances were observed at 170 kgy for all PE/EVA blends, and this value was highest for the sample with EVA content of 30 wt%. These results indicate that at higher cross linking density observed at this dose of radiation, the average molecular weight of the polymer was increased. The SEM image of the samples with the weight percent of 70/30 shows more homogenous structure in comparison with the 80/20 and 90/10 blends. The irradiation by 10 MeV electron beam make the pattern of the blends more homogeneous resembling a single polymer. Therefore, one of the important factors which affect the properties of polymers especially the electrical properties is the morphological state of the polymer.

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