Tree-resistant ethylene polymer compositions

Abstract

Propagation of electrical trees and water trees in electrical insulation made of ethylene homopolymers or copolymers with an unsaturated monomer is inhibited by the addition to the insulating composition of an organic carboxylic ester having at least one aromatic ring and at least three carboxylic ester groups, the ester being liquid at the operating temperature of the electrical equipment in which the insulation is used. The main utility of the inhibitors of this invention is in the primary insulation for power transmission cables, especially those used in high voltage applications.

We claim: 
     
       1. A tree-resistant composition for use in .[.electrical equipment.]. .Iadd.a power transmission cable.Iaddend., said composition consisting essentially of an ethylene polymer selected from the group consisting of ethylene homopolymers and ethylene copolymers with at least one other ethylenically unsaturated monomer, ethylene being present in such copolymers in an amount of at least 85 weight percent, and, as a tree-growth inhibitor, an effective amount of at least one organic carboxylic ester having at least one .Iadd.but no more than two .Iaddend.aromatic ring.Iadd.s .Iaddend.and at least three .Iadd.but no more than four .Iaddend.carboxylic ester groups, .Iadd.at least one carboxylic ester group being attached to one aromatic ring and the remaining carboxylic ester groups being attached to the other aromatic ring, if present, which is fused to the first aromatic ring or joined to it by a single bond, an alkylene group, a carbonyl group, or a hetero atom, .Iaddend.said inhibitor melting below about 50° C. and being liquid at the power transmission cable's intended operating temperature. 
     
     
       2. A composition of claim 1 wherein the amount of the tree-growth inhibitor is about 0.25-5% based on the weight of the ethylene polymer. 
     
     
       3. A composition of claim 2 wherein the amount of the tree-growth inhibitor is 1-3% based on the weight of the ethylene polymer. 
     
     
       4. A composition of claim 1 wherein the ethylene polymer has a density of about 0.92 g/cm 3  or less. 
     
     
       5. A composition of claim 4 wherein the ethylene polymer is a homopolymer. 
     
     
       6. A composition of claim 1 wherein there is present, in addition to the ethylene polymer, also a polypropylene or ethylene/propylene copolymer, the amount of such additional polymer or copolymer being up to about 10 weight percent of the ethylene polymer. 
     
     
       7. A composition of claim 1 wherein the tree-growth inhibitor is selected from the group consisting of esters of mellitic acid, trimesic acid, hemimellitic acid, trimellitic acid, and pyromellitic acid with a C 4  -C 12  aliphatic or cycloaliphatic alcohol. 
     
     
       8. A composition of claim 7 wherein the ester is tetraoctyl pyromellitate. 
     
     
       9. A composition of claim 1 which also contains an organic peroxide. 
     
     
       10. A composition of claim 1 wherein the inhibitor is a mixed ester of a polycarboxylic acid with two or more alcohols. 
     
     
       11. A composition of claim 1 which also contains a form of elemental carbon as a filler.