Improvement of thermo-switch properties in polyolefin/carbon black composites through the addition of wax

[摘要] Conductive polymer composites (CPCs) based on LDPE, HDPE and iPP filled with carbon black (CB) and Zn micro-particles and blended with medium-soft wax were studied. The aim of this study was to investigate the effect of CB content, the presence of wax, γ-irradiation, and Zn metal powder as second filler on the thermal and mechanical properties, and thermo-electrical behaviour, of the composites. CB was used as the main filler to impart electrical conductivity to the composites. CB was generally not homogeneously dispersed in all the composites, while Zn was randomly dispersed with clear areas that were void of Zn, and agglomeration of Zn particles in other areas. It also seemed as if the dispersions did not depend on the type of polymer or the presence of wax, and as if there were a weaker interaction between CB and Zn than between CB particles. The crystallinities of the polymers generally decreased with an increase in CB loading, while the presence of Zn and/or wax gave rise to higher crystallinities. The presence of wax, however, had little influence on the melting and crystallization temperatures of the polymers. Irradiation reduced the crystallinities of the polymers in all the samples, as expected because of radiation-induced crosslinking. The storage modulus values of all the composites were higher than those of neat LDPE, although the presence of wax reduced these values, and the intensities of the �?transition depended on the type and amount of filler and on the presence of wax. All the composites show significantly lower resistivity than the neat polymers, and the resistivity decreased with increasing filler content. The 22 vol.% CB containing composites showed lower resistivities than the 12 vol.% CB + 10 vol.% Zn containing composites. The presence of wax caused a slight decrease in resistivity, while the irradiated samples generally showed higher resistivity values than the non-irradiated samples. The switching temperature shifted to higher values with increasing CB content, while it remained the same when part of the CB was replaced with Zn. Wax did not significantly influence the PTC intensity in the low CB content composites, but it caused a slight increase in PTC intensity in the Zn-containing samples. The irradiated samples generally showed an increased PTC intensity, but a reduced NTC intensity. All the samples showed a drop in resistivity after thermal ageing and good electrical stability during thermal cycling. The HDPE and iPP composites showed a fairly stable resistivity behaviour in the temperature range of investigation. Both the presence of filler and wax reduced the impact strengths of the composites.

[发布机构] University of the Free State