Masaki Kawahigashi

The importance of morphology on electrical strength of LDPE/XLPE insulations

An attempt was made to change the morphology of LDPE/XLPE (low-density polyethylene/cross-linked polyethylene) by several methods, and the impulse breakdown strength IBDS of the samples was investigated. It was found that the thermal procedure which effectively changed the morphology of polypropylene did not alter the morphology of LDPE/XLPE significantly; consequently, IBDS was not influenced in this way. Monoaxial and biaxial mechanical stretches of XLPE resulted in a certain alteration of the morphologies of these samples. IBDS was enhanced during these stretches. Removal of the lower-molecular portion from the LDPE matrix produced an increase in density and enhanced IBDS for the resultant LDPE. This suggests that this removal would give a compactly arranged amorphous region for LDPE, and that the amorphous part in LDPE may play the predominant role in electrical breakdown phenomena.<<ETX>>

Influence of morphology on electrical breakdown strength for polypropylene

To elucidate the mechanism of electrical breakdown for an insulating polymer, the authors investigated the correlation of impulse breakdown behavior and morphological structure for polyolefins. Since polypropylene (PP) has a morphology which can easily be observed under a microscope with polarized light, the behavior of this polymer was studied. The morphological structure of PP was controlled by the following methods: (1) incorporation of a variety of additives or polymers having different molecular weights, (2) addition of a nucleating agent, and (3) control of crystallizing temperature and period. As a result of a series of experiments, it was confirmed that the impulse breakdown property of PP was closely related to its morphology, and these methods were effective in improving the impulse breakdown property of polyolefins by changing the morphology.<<ETX>>

Morphology and dielectric strength of some new blended polymeric materials

Enhancement of the dielectrical strength of low density polyethylene(LDPE) by means of blending of some new elastic polymers was studied. The di- and tri-block styrene-isoprene copolymers were found to be most effective for improving the impulse breakdown strength of LDPE. The most optimum blending ratio for individual copolymers, for example, di-block copolymer with styrene content of 20 wt% gave the most significant improvement of Imp. BDS of LDPE with 35 to 40 wt% blending ratio. In this copolymer, styrene block offers a hard segment, and the isoprene unit behaves as a soft segment. This structure can be considered to give direct influence on the final morphology of the elastomer/LDPE blends. TEM observation on several blended polymers which gave excellent Imp. BDS revealed that the dispersion and morphological nature of these samples were closely related to the processing temperature. Certain relation of improvement in Imp. BDS of blended LDPE and its morphological nature are discussed.<<ETX>>