INORGANIC NANOFILLER EFFECTS ON RESISTIVITY AND ABSORPTION CURRENTS IN LOW DENSITY POLYETHYLENE NANOCOMPOSITES

INORGANIC NANOFILLER EFFECTS ON RESISTIVITY AND ABSORPTION CURRENTS IN LOW DENSITY POLYETHYLENE NANOCOMPOSITES ILONA PLESA 1, FLORIN CIUPRINA 1, PETRU V. NOTINGHER 1, DENIS PANAITESCU 2 The study of the influence of the nanosized particles on the electrical properties of the polymer composites has became of a real interest in last years for most of the research teams working in the field of dielectrics all over the world. In the present work the effects of three different types of nanofillers SiO 2, Al 2 O 3 and TiO 2 on the resistivity and on the absorption currents in low density polyethylene (LDPE) based nanocomposites are analyzed. The influence of the electric field and of the shape of nanoparticles (spherical or rod-like) on the results is also discussed. Key words: Nanocomposite, Polyethylene, Nanofiller, Absorption current, Volume resistivity. 1. INTRODUCTION Polymer nanocomposites with better electrical, thermal and mechanical properties than the traditional polymer microcomposites are emerging as excellent materials for dielectric and electrical insulation applications [1 6]. The term nanodielectrics for such materials has become popular in the last years of researches. Nanodielectrics are dielectric materials made of polymers in which are added nanofillers of diameter of 1 to 100 nm, usually from 1 to 10 wt. % in content and homogeneously dispersed in the polymer matrix [1]. Many studies have reported an improvement of the electrical, mechanical and/or thermal properties of polymers as a result of the addition of different nanofillers [2 7]. In our previous studies we have analyzed the dielectric behaviour of polymer nanocomposites made of low density polyethylene (LDPE) filled with nanoparticles of alumina (Al 2 O 3 ), silica (SiO 2 ) and titania (TiO 2 ). The results obtained at room temperature have emphasized different dielectric behaviours of 1 University Politehnica of Bucharest, Faculty of Electrical Engineering, ELMAT Laboratory, Romania, E-mail: iplesa@elmat.pub.ro 2 National Institute of Research and Development in Chemistry and Petrochemistry, ICECHIM, Bucharest, Romania Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 56, 3, p. 277 284, Bucarest, 2011

278 Ilona Plesa et al. 2 the nanocomposites, depending on the frequency and on the filler nature and concentration. In the case of the permittivity, values smaller than those for the base polymer are noticed for the LDPE nanocomposites with a low content of Al 2 O 3 and of TiO 2 fillers at very low frequencies. The tan δ values are smaller than for the unfilled polymer only in the case of TiO 2 and Al 2 O 3 fillers at very low frequencies. A reduced chain movement is thought to be responsible for the lowering of the nanocomposites permittivity and loss tangent with respect to the unfilled polymer [8, 9]. The alternate polarization current values are smaller for the filled material than for the unfilled one, in agreement with dielectric spectroscopy observations, this effect being more evident for 2wt. % concentration of Al 2 O 3 particles. A low content of nanofiller of Al 2 O 3 leads also to an improvement of mechanical properties [10]. The influence of temperature on the dielectric behaviour of nano-sio 2 filled low density polyethylene was investigated over a frequency range of 10 mhz 10 MHz and for temperatures from 250 K to 350 K. The frequency variations of ε r ' and tan δ in the LDPE-SiO 2 tested nanodielectrics indicate an α-relaxation process which is enhanced by the filler concentration and shifted to higher frequencies with the increasing temperature [11]. Polymer nanocomposites properties (electrical, thermal, mechanical) revealed until now by several studies recommend these materials for future use as electrical insulation for power apparatus, power cables, outdoor insulators, and insulated wires for electric power technologies as well as printed circuit boards for electronics [12]. The technology of addition of nanofillers in the polymer matrix is not yet completely controlled, whereas the effect of the filler on the dielectric properties of the polymer nanocomposites is far to be fully understood. Nevertheless, the properties of nanocomposites were found to depend on the type of nanoparticles, physical and chemical conditions of their surfaces, the kind of coupling agents to bridge inorganic and organic substances chemically and physically, the kind and content of compatibilisers and/or dispersants, and the polymer matrices [1, 6, 11]. The goal of this paper is to analyze the effects of the type, concentration and shape of the particles used as nanofillers on the absorption current and resistivity values. 2. EXPERIMENTAL 2.1. SYSTEMS INVESTIGATED The nanocomposites tested in this study were polyethylene-sio 2, polyethylene-tio 2 and polyethylene-al 2 O 3. The polymer matrix for the polyethylene nanocomposites used in this study was a low density polyethylene

3 Inorganic nanofiller in low density polyethylene nanocomposites 279 (LDPE) commercially available at Arpechim Pitesti (Romania). It had a density of 0.916 g/cm 3 and a melt index (at 190 C) of 0.3 g/10 min. A maleic anhydride graft polyethylene (MA-PE) from Aldrich Co, with the density 0.925 g/cm 3 and having 3% maleic anhydride, was used as compatibilizing agent for a better compatibility between the nanofiller and the polymer matrix, and for a better dispersion of the nanoparticles [11]. The content of nanofillers was 2 and 5 wt%. Two types of nanocomposites were prepared with spherical SiO 2 nanoparticles, one with particles of an average diameter of 15 nm and the other one with particles of an average diameter of 10 nm. The nanoparticules of TiO 2 were either of spherical shape with 15 nm average diameters for one of the LDPE-TiO 2 nanocomposites, or of rod-like shape (10 nm diameter, 40 nm length), for another LDPE-TiO 2 nanocomposite. The nanoparticules of Al 2 O 3 were sperical with an average diameter of 40 nm. Fig. 1 Installation used for nanocomposite manufacturing [9]. The nanocomposites were manufactured by direct mixing method shown in Figure 1, resulting materials with homogeneously dispersed particles as discussed in [10]. The nanocomposite samples for electrical tests performed in this study were plaques of square shape (10 10 cm 2 ) having the thickness of 0.5 mm. 2.2. ELECTRICAL CHARACTERIZATION The absorption currents were recorded with an electrometer Keithley 6517, connected with a Keithley 8009 test fixture and with a computer. The sample was placed between the two plane electrodes of the test fixture and a voltage step of 10 V or of 500 V was applied to each sample during 70 minutes, while the current variations were recorded for 95 minutes starting from the voltage application. For each sample one thousand values of the absorption current were recorded, each at

280 Ilona Plesa et al. 4 every 5.6 seconds. The current values presented in this paper for each formulation represent a mean between the values recorded on two samples of each type. All the measurements were performed at ambient temperature (27 ºC) and humidity ( 50 %). 3. RESULTS AND DISCUSSION Figure 2 shows the absorption current variations as a function of time for different nanocomposites: LDPE Al 2 O 3, LDPE SiO 2 and LDPE TiO 2, for 5 wt% filler content. 1E-10 I 1E-11 1E-12 Unfilled 5 wt% nano-sio 2 5 wt% nano-al 2 O 3 5 wt% nano-tio 2 I [A] 1E-13 absorption 1E-14 1E-15 0 1000 2000 3000 4000 t [s] conduction t Fig. 2 Absorption currents variations for different LDPE nanocomposites as a function of time. Fig. 3 Typical time variation of the absorption current in an electrical insulating material. To explain these variations we consider the structure of the absorption current i a (Fig. 3): i a (t) = i i (t) + i p (t) + i ss (t) + i c (t), (1) where i i (t) is the charging current of the capacitor with vacuum dielectric, i p (t) the polarization current, i ss (t) the space charge current and i c (t) the conduction current [13]. The component i p (t) is given by the electric polarization phenomena that consist in very small motions of a large number of attached charges. The current i ss (t) corresponds to the movement of the space charge which usually develop in polymer samples during their fabrication (by molecule fracturing, etc.), by charge injection from the electrodes and due to the degrading agents (heat, electric field, high energy radiation, etc). In the polymer nanocomposites, the presence of nanoparticles determines, on the one hand, an increase of the concentration of electric dipoles especially inside

5 Inorganic nanofiller in low density polyethylene nanocomposites 281 nanoparticles and/or inside the polymer-nanoparticle interface layers, and on the other hand, an increase of the space charge mainly placed at the huge nanofillerpolymer interface area. Consequently, i p (t) and i ss (t) components of the absorption current increase with the increase of filler concentration, these leading to a slower decrease of the absorption current than in the unfilled polymer, as remarked from Fig. 2. Figure 4 shows the influence of the nanoparticle s shape on the absorption currents, in the case of titania (TiO 2 ). For the same filler content (2 wt%), the absorption current values are higher for rod-like than for spherical nanoparticles. Since the surface of a spherical TiO 2 particle of 15 nm diameter is 716 nm 2, and the surface of a rod-like particle is 1413 nm 2, the results in Fig. 4 could be due to the larger area of the interfaces for rod-like nanofillers. In Figs. 5, 6 and 7 it can be observed the influence of the voltage on the absorption currents in the samples of unfilled LDPE, of LDPE SiO 2 nanocomposites with 5 wt% filler content and of LDPE Al 2 O 3 nanocomposites with 5 wt% filler content, respectively. 1x10-1 0 1x10-10 LDPE-TiO2 2 wt% filler content Unfilled LDPE 1x10-11 spherical TiO 2-15 nm rod-like TiO 2-10x40 nm 1x10-1 1 1x10-1 2 10 V 500 V I [A] 1x10-12 1x10-13 I[A] 1x10-1 3 1x10-14 1x10-1 4 10-15 0 500 1000 1500 2000 2500 3000 3500 4000 t [s] 10-1 5 0 50 100 150 200 250 300 t[s] Fig. 4 Absorption currents variations in LDPE- TiO 2 nanocomposites with different shape of nanoparticles, for a test voltage of 500 V. Fig. 5 Absorption currents variations in unfilled LDPE, for 10 V and for 500 V test voltages. While in the unfilled LDPE samples the current values are not very much influenced by the voltage, in the nanocomposite samples, the current values for 10 V are significantly smaller than for 500 V, the highest difference being noticed for the SiO 2 nanofiller (Fig. 6). This behaviour could indicate that in nanocomposites, unlike in the unfilled polymer, a higher number of charges are detrapped from the shallow traps presents in the forbidden band, when a higher electric field corresponding to the voltage of 500 V is applied. Under a low electric field (as for a voltage of 10 V), these charges, which are thought to be placed mainly in the

282 Ilona Plesa et al. 6 nanofiller-matrix interface area, could form a space charge which can reduce the carrier movement from one nanoparticle to another, through the polymer matrix. However, the authors think that other experimental techniques (e.g. dielectric spectroscopy) and different levels of physical approach are needed in order to check the controlling factors in charge transport, to fully understand the conduction mechanism and to develop a consistent model for the behaviour observed here. From the engineering point of view, a smaller conduction current under a low electric field means a higher resistivity, as can be seen in Table 1, which recommend these nanocomposites for a number of applications in electrical engineering and especially in electronics. 1x10-1 0 LDPE-SiO 2 5 wt% filler content 1x10-1 1 10 V 500 V 1x10-1 0 LDPE-Al 2 O 3 5 wt% filler content 1x10-1 1 10 V 500 V 1x10-1 2 1x10-1 2 I[A] 1x10-1 3 I[A] 1x10-1 3 1x10-1 4 1x10-1 4 10-1 5 0 50 100 150 200 250 300 t[s] 10-1 5 0 50 100 150 200 250 300 t[s] Fig. 6 Absorption currents variations in LDPE- SiO 2 nanocomposites, for 10 V and for 500 V test voltages. Fig. 7 Absorption currents variations in LDPE- Al 2 O 3 nanocomposites, for 10 V and for 500 V test voltages. In Table 1 the volume resistivity values of the tested samples, relative to the resistivity of the unfilled polyethylene are shown for the two voltages used in this study. Table 1 Relative volume resistivity for polyethylene nanocomposites with different fillers Material Relative volume resistivity at 10 V Relative volume resistivity at 500 V Unfilled LDPE 1 1 LDPE with 5 wt% nano-sio 2 39.39 0.54 LDPE with 5 wt% nano-al 2 O 3 6.08 0.19 LDPE with 5 wt% nano-tio 2 4.09 0.72

7 Inorganic nanofiller in low density polyethylene nanocomposites 283 The resistivities were calculated with the following equation: S U ρ v =, d I where ρ v is the volume resistivity, S electrode surface, U applied voltage [V], I average current after 300 s starting from the voltage application and d sample thickness. The average current was calculated by using the last 10 current values measured before reaching 300 s from the voltage application. The resistivity values shown in Table 1 for the tested LDPE nanocomposite samples are strongly influenced by the test voltage. When the test voltage of 500 V was applied, all the nanocomposites have showed a smaller resistivity, but in the same order of magnitude, with respect to the unfilled LDPE. On the contrary, the resistivity of all the tested nanocomposites is higher than that of the unfilled polymer when using a voltage of 10 V. The highest relative resistivity was obtained for the LDPE-SiO 2 nanocomposite at a level of nanofillers of 5 wt%. The electrical resistivities determined in the present study recommend the tested nanocomposites to be used as electrical insulations with an improved resistivity for low voltage applications. (2) 4. CONCLUSIONS The results presented in this paper emphasize different electrical behaviour of the nanocomposites with polyethylene matrix and inorganic filler, depending on the type and the content of nanoparticules. The absorption current values for LDPE nanocomposites with Al 2 O 3, SiO 2 or TiO 2 nanofillers depend on the nanoparticle type and shape. The values of the absorption currents for the tested LDPE nanocomposites are higher and have a slower decrease than for the unfilled LDPE, and this is mainly due to the higher values of the polarization and space charge components of the current, determined by the new dipoles and charge carriers introduced by nanostructuration. The resistivity of the tested LDPE nanocomposite samples is strongly influenced by the test voltage. Thus, when using a voltage of 10 V, the resistivity of all the tested nanocomposites is higher than that of the unfilled polymer, the highest increase being noticed for the LDPE-SiO 2 nanocomposite at a level of nanofillers of 5 wt%. On the contrary, when the test voltage was 500 V, all the nanocomposites have showed a smaller resistivity, but in the same order of magnitude, with respect to the unfilled LDPE. This behaviour, which recommend the tested nanocomposites to be used as electrical insulations with an improved resistivity for low voltage applications, could be explained by the presence in

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