The influence of the polymer matrix on the PTC properties polyethylene/carbon black composite

Xi, B. and Chen, G. (2001) The influence of the polymer matrix on the PTC properties polyethylene/carbon black composite. pp. 109-112 .

Abstract

Carbon black/polymer composites can be used in a variety of industrial applications, ranging from temperature sensors to fault current limiters. This is mainly due to their positive temperature coefficient of resistivity (PTC). As carbon black particles are loaded in a polymer matrix over a critical volume fraction, the composite becomes an electrical conductor at room temperature and exhibits a strong PTC effect on approaching its melting point. A model based on phase changes in the polymer matrix has been proposed by authors [1]. In this model, carbon black particles mainly reside in the amorphous phase in the composite, thus forming a conductive channel below the melting point. When temperature rises near to the melting point of the polyethylene, the crystalline phase starts to melt leading to formation of new amorphous regions. The volume of the new amorphous regions is larger than that of the previous crystalline region. Some portion of the new amorphous phase will extend into the previous amorphous regions. As the newly formed amorphous phase contains no carbon black, this extension can reduce and finally break up the conducting channel in the previous amorphous phase, resulting in rapid increase in resistivity of the composite. Thus this model can be used to explain the PTC effect in semicrystalline polymeric composite. Obviously the PTC properties can be affected by polymer matrix. In this paper the influence of the polymer matrix (LDPE, LLDPE and HDPE) on the PTC properties has been investigated, with a particular emphasis on the thermal stability of the PTC properties.

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