Abstract
The effect of an organoclay on the electrical properties of a polymeric host is described. The matrix was composed of a blend of high- and low-density polyethylene, to which an ethylene/(vinyl acetate) copolymer was added, to increase compatibility between the organoclay and the matrix and, thereby, improve the organoclay dispersion. The ratio between the compatibiliser and organoclay was found to be important in forming a well-dispersed system, as evidenced by thermogravimetric analysis, X-ray diffraction and scanning electron microscopy, indicating the effectiveness of the chosen compatibilisation strategy. DC conductivity was found to be determined by the precise distribution of the organoclay throughout the system; changes in morphology and phase structure of the matrix polymer resulting from changes in imposed thermal history had little effect per se, but varying degrees of self-assembly of the organoclay facilitated by different residence times within the quiescent melt could result in changes in overall DC conductivity of several orders of magnitude. Consequent increases in DC conductivity led to reductions in DC breakdown strength, implying failure through some avalanche or thermal process. However, a monotonic increase in observed AC breakdown strength implies that the associated underlying failure process is then very different.
Highlights
Polyethylene (PE) is a versatile polymer that can be synthesised with many different molecular architectures
For PE/ethylene and vinyl acetate (EVA)/5/Q, this occurs over the same temperature range for both air and nitrogen, in contrast to the behaviour of the unfilled PE/EVA/0/Q—this difference in behaviour is a result of the organoclay leading to a barrier effect, so increasing the temperature for thermo-oxidative degradation [22,23,24]
The decomposition curve of PE/EVA/5/Q contains a shoulder that is absent in the case of PE/EVA/0/Q, which can be attributed to the initial decomposition of the quaternary ammonium salts in the organoclay [23]
Summary
Polyethylene (PE) is a versatile polymer that can be synthesised with many different molecular architectures. The matrix was chosen to be a blend of high-density PE (HDPE) and low-density PE (LDPE), since such systems can be used to generate a wide range of different lamellar textures without changing the molecular composition of the system To this base material, EVA was added to enhance compatibility between the polymer and the organoclay filler. Samples were held isothermally at 140 °C in the melt for 3 h before being quenched (as above for Q), in order to combine liquid/liquid phase separation in the melt with rapid subsequent crystallisation (designated ISO) These samples will hereafter be referred to using the following nomenclature: PE/EVA/X/Y where X is the organoclay loading (either 0, 1, 5, 9 or 13 wt%) and Y is the thermal profile (either Q, SC or ISO). Quoted conductivity values were derived from the average of the last ten current measurements
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