Stability of thermo-sensitive properties of carbon-black/styrene-butadiene-rubber composite membranes

Conducting composite membranes were prepared by using styrene butadiene rubber as matrices and incorporating carbon blacks and some organic crystals (2,6-di- tert -butyl-4-methyl phenol, BMP). A number of composite membranes exhibited a thermo-sensitive property which corresponded to a positive temperature coefficient (PTC) effect. Their DSC thermograms and x-ray patterns indicated that there were no significant changes in the structure of membranes although these membranes were subjected to discontinuous and repeated thermal cycle measurements of more than 100 runs. The switching characteristic and PTC intensity of membranes showed a satisfyingly reproducible stability. It was also found that the thermal ageing treatment could remarkably influence conductive and PTC properties of composite membranes. When these composite membranes were thermally aged at a temperature of 90 or 100 °C for various times up to 480 h, no essential changes in the resistivity were observed and their PTC intensity and switching characteristic were still effectively maintained. However, when these composite membranes were exposed to an elevated thermal ageing temperature higher than 100 °C over various time intervals from 24 to 480 h, significant changes in resistivity were detected and their original PTC properties could almost totally disappear depending on the thermal ageing temperature and time. The obtained results revealed that these composite membranes could maintain their thermo-sensitive stability for a long term in a thermal environment in which the temperature was lower than 100 °C.