Method for increasing resistance to treeing in polymeric insulating layer

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The present invention is a method for enhancing the purity of a plurality of small, discrete pellets of polymeric material having the purpose of forming a dielectric layer on metal electrical conductor cable.
Various polyolefin compounds are the most commonly used principal ingredients of these pellets.
Preferred polyolefin materials are polyethylene and poly(ethylene-propylene).
Copolymer of ethylene with another unsaturated monomer such as 1-butene or other .
alpha.
-olefins, ethylacrylate, butylacrylate or vinylacetate may also be used.
In such copolymers the weight of ethylene is preferably at least 60% of the total weight of ethylene and other unsaturated compounds.
In a particularly preferred embodiment, the polymeric material comprises a low-density, high molecular weight polyethylene.
Various additives are included in the pellets to improve their service life.
Organo silane compounds are added, usually in an amount of less than 10 parts by weight per 100 parts by weight of the polymer, for improved resistance to water treeing.
Various formulations of organo silane additives in polymeric dielectric compositions are set forth in U.
S.
Pat.
Nos.
4,144,202 and 4,212,756, for example, which patents are incorporated herein by reference.
The pellets of the invention may contain other additives in addition to organo silanes.
The most commonly used other additives are organic peroxides or similar substances with the ability to effect cross-linking.
Examples of some suitable peroxides include dicumyl peroxide, di-t-butyl peroxide and 2,5-dimethyl-2,5-di(t-butylperoxy)-hexyne-3.
A preferred peroxide is a mixture of meta and para isomers of .
alpha.
,.
alpha.
'-bis(t-butylperoxy) diisopropyl benzene.
This mixture is sold commercially by Hercules under the trade name VULCUP.
The quantity of peroxide is generally about 0.
1 to 5 parts by weight per 100 parts by weight of the polymer.
The polymer may also be provided with an antioxidant, suitably in a quantity of 0