Influence of Anisotropy on the PTC Effect in Injection Moulded Samples of CB-filled Polyethylene and Polystyrene

Abstract

This paper reports on measurements of the temperature dependence of resistivity, ρ, in injection moulded samples of LDPE, HDPE, and PS filled with two grades of carbon black (CB). The CB concentration was adjusted to give a relatively small PTC-effect in isotropic (annealed) samples. It was found that the orientation of the matrix polymer that occurred during the injection moulding process highly enhanced the PTC-efficiency of all the compounds used. The magnitude of this effect and the level of RT resistivity were orientation dependent. Upon annealing (second heating run) the behaviour of the isotropic samples was recovered. In general, ρ increased with the degree of orientation in a fashion similar to the effects associated with lowering the CB content. One of the effects observed in highly oriented samples was a pronounced frequency-dependence of the resistivity (ρ decreasing at higher frequencies), and a non-ohmic I-V-behaviour. The ρ-data relate to d.c. and 1 kHz. Samples with higher and lower orientation were obtained by microtoming the surface and core portions of the injection moulded test bars.