Process for making a positive temperature coefficient conductive polymer from a thermosetting epoxy resin and conductive fillers

Abstract

A process for making an improved current limiting composition comprising mixing an epoxy thermosetting resin containing more than one 1,2 epoxy groups per molecule with an acid anhydride curing agent, an epoxy reactive diluent, first and second co-accelerators and conductive additive particles to form a liquid mixture. The liquid mixture is heated at a first temperature range below gelation temperature while drawing a vacuum to drive off volatiles and gases. Continued heating at a second temperature range promotes gelation and heating a third temperature range effects a final cure. A first co-accelerator becomes effective during the gelation temperature range and a second co-accelerator does not become effective until the final cure temperature range. Gelation takes about 2 to 4 hours at about 130° C. to 140° C. and final cure takes about 13 to 18 hours at about 140° C. to 160° C. The epoxy resin is preferably a diglycidyl ether of bisphenol A, the epoxy reactive diluent is preferably a diglycidyl ether of neopentyl glycol, the acid anhydride hardener is preferably 1-methyl tetrahydrophthalic anhydride and the conductive additive particles are selected from the group consisting of carbon black, nickel fiber, nickel flake, nickel beads and copper flake. An effective amount of said alumina trihydrate is employed in the mixture to prevent dielectric breakdown, arcing and carbon tracking under high voltage conditions in current limiting compositions. The first co-accelerator is 1-methyl imidazole and the second co-accelerator is chromium acetylacetonate.