Typical insulated electric power cables generally comprise one or more high potential conductors in a cable core that is surrounded by several layers of polymeric materials including a first semi-conductive shield layer (conductor or strand shield), an insulating layer (typically a non-conducting layer), a second semi-conductive shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket (which may or may not be semi-conductive). Additional layers within this construction, such as moisture impervious layer, are often included.
In general, semi-conducting shield layers can be classified into two distinct types. The first type is a shied layer that is securely bonded to the insulation layer so that stripping the shield layer is only possible by using a cutting tool that removes the shield layer with some of the insulation layer. The second type is a shield layer that is “strippable” from the insulation layer, i.e., the shield layer has a defined, limited adhesion to the insulation layer so that the former can be peeled away from the latter without removing latter. Stripability has conventionally been controlled through the use of highly polar polymer blends comprising ethylene vinyl acetate (EVA), nitrile rubber and chlorinated polyethylene (CPE).
Strippable shield formulations of EVA and nitrile rubbers are described in U.S. Pat. Nos. 4,286,023 and 4,246,142. One problem with the strippable shield formulations of EVA and nitrile rubber is that the formulations require relatively high vinyl acetate content to achieve the desired adhesion level, which results in the formulations being more rubbery then is desired for commercial, high speed extrusion of electric cable. Furthermore, nitrile rubber is expensive and has poor thermal stability, which negatively impacts properties after thermal aging.
Alternatively, adhesion-adjusting additives have been proposed for use with EVA, including, for example, waxy aliphatic hydrocarbons (U.S. Pat. No. 4,933,107); low-molecular weight polyethylene (U.S. Pat. No. 4,150,193); and silicone oils, rubbers and block copolymers that are liquid at room temperature (U.S. Pat. No. 4,493,787). However, these compositions have experienced only limited commercial success.
The need exists for a blend that will allow the insulation shield to be strippable while retaining sufficient adhesion so that the insulation shield layer does not strip away from the insulation layer during use. Furthermore, the blend should preferably exhibit crosslinkability and better thermal stability than EVA/polypropylene blends and EVA/nitrile-butadiene rubber (NBR) blends.