Polarization Processes of Nanocomposite Silicate-EVA and PP Materials

Abstract

Recent works indicate that polypropylene (PP) and ethylene-vinylacetate (EVA) filled by nanosilicates may present low content of space charge and high electric strength. It has been proved that the dispersion of an inorganic phase in the polymeric matrix may improve voltage endurance, dielectric strength, thermal stability and mechanical properties in relation to particle size and arrangement on nanometric scale. Two polymeric materials, widely employed as electrical insulation for apparatus involved in energy transport, that is, ethylene vinylacetate (EVA) and isotactic polypropylene (PP) are examined. The nanofiller consists of an organophilic layered silicate (MEE), the purified MEE will be named MEE-w.Dielectric spectroscopy in time and frequency domain constitutes a useful tool for electrical insulation evaluation and diagnosis, and it can contribute to the understanding of electrical behaviour of complex solid polymer systems. In this paper, the results of dielectric spectroscopy analysis in time and frequency domain, for purified and unpurified filler nanocomposites, in a wide temperature range, are presented.Frequency-domain dielectric measurements were performed in order to appreciate the relaxation processes taking place at lower temperatures (-20 to 20 °C) over the 10 2 -10 6 Hz frequency range.The charging-discharging current measurements, through Fourier time to frequency data transformation, gave information about the slowest polarisation processes, that are usually hidden by the conduction current contribution. Space charge measurement data are also considered in order to understand the effect of nanostructuration and purification on charge carriers. While the relaxation process of EVA, α process, associated with glass transition of the material amorphous phase, results unchanged from base to nanostructured material (i.e., the nanofiller particles do not affect the chain mobility of the polymer), nanocomposites EVA and PP have shown the rise of a new process at higher temperatures respect to the typical host material processes, as well as a different distribution of relaxation processes. In Fig. 1, that displays the imaginary permittivity of EVA+MEE-w nanocomposite, the loss peaks, due to the α and α' process, can be clearly seen. The α' process can be ascribed to the interaction between the polymer matrix and MEE nanolayers.The temperature location of the process, which approaches the melting region of EVA, and its absence in the pure polymer let us argue that the α' relaxation process can be ascribed to a charge polarization of the Maxwell-Wagner-Sillars type, which is due likely to charge trapping at the interfaces between the nanofiller particles and the polymer. All the presented relaxation processes are thermally-activated and their activation energies, reported in Table 1, were estimated reporting the loss peak frequency and temperature of the isothermal measurements in the Arrhenius plot.The EVA nanocomposite obtained by an unpurified filler has shown a huge increment of real and imaginary...

Effect of aspect ratio and water contamination on the electric properties of nanostructured insulating materials

“…Focusing on dielectric spectroscopy, a peak of the imaginary permittivity ( Figures 9B and 10B) can be observed in the frequency range 10-20 Hz for nanofilled nanocomposites. Since this peak is present only in the nanocomposite materials (as it can be seen by the comparison with Figure 8B), it has been associated with the polymer-filler interface [27]. A large peak increase with respect to the dry material, together with a broadening of the loss spectrum from 10 -2 Hz to 10 4 Hz, is observed in the wet fluorohectorite specimens.…”

“…It is noteworthy that a washing treatment, as described in [25][26][27], is applied to the filler with the purpose of removing the by-products (ionic species) of the organic modification process. Moreover the EVA matrix does not need any compatibilization process, because the high polarity of the acetoxy groups allows a good dispersion of nanofillers.…”

“…For instance, contaminants left by the filler compatibilization process can worsen noticeably electrical properties of a nanostructured material with respect to the base material. In particular, a significant decrease of dc electric strength and increase of space charge accumulation is observed often in nanostructured materials, if contaminants of the manufacturing process are not properly removed by intense washing treatments [25][26][27]. This may occur particularly for those fillers that need a compatibilization process which can introduce extra ionic species in the matrix.…”

Space charge dynamics in silica-based polyethylene nanocomposites

“…The magnitude of the internal charge is often found to be much less and the dynamics of charge decay are much faster in the case of nanocomposites [5][6]. The promising space charge behavior of nanocomposites was also supported by many other studies [7][8][9] While reduced space charge formation in nanocomposites can be explained through the introduction of a shallow trap band [3,8] at interfaces and the resulting increase in charge carrier mobility, it has also been suggested that deep traps may occur in certain nanocomposites [8]. With the presence of deep traps, space charge accumulation could be worsened.…”

Polyethylene/silica nanocomposites: absorption current and the interpretation of SCLC

“…Indeed, thanks to the presence of the nanofiller/polymer interfaces [5][6][7][8][9], such material systems have been envisioned as an answer to many high voltage insulation problems [10]. For example, the partial discharge resistance, electrical treeing growth, space charge formation and dielectric breakdown performance of nanodielectrics have been compared with the respective unfilled and microfilled counterparts and worthwhile improvements in such properties have been reported [11][12][13][14][15][16].…”