Tracking-resistant electrical insulators containing silica and alumina filler in a polyester resin matrix

摘要

Electrical insulators having a high tracking resistance are obtained from a highly filled polymerizable composition, by injection molding said composition within a short period of time as low as 5 minutes, said composition containing an amount of up to 96% by weight of an improved filler which comprises, based on the total composition, from about 40 to about 52% by weight of silica sand having a particle size such that no less than about 70% by weight of the particles are retained on a 60 mesh U.S. standard sieve and no more than about 30% by weight of the particles are retained on a 40 mesh U.S. standard sieve, an amount of from about 23 to about 36% by weight of silica sand having a particle size such that no less than about 80% by weight of the particles are retained on a 325 mesh U.S. standard sieve and no more than about 11% by weight of the particles are retained on a 200 mesh U.S. standard sieve, and a smaller proportion of finely comminuted alumina; and the remainder of the composition comprising a relatively high viscosity unsaturated polyester resin precursor incorporated in said filler. Fibrous materials may be added to the filler for increases strength and dimensional stability.