Propylene Polymers for Insulation of Communication Cable

摘要

Polyolefins have long been recognized as a very fertile field in which to search for plastic dielectrics. Some of the reasons for this fact include the availability of these materials in large quantities from numerous suppliers; their relatively low price, which makes them attractive for large-volume applications; and their excellent electrical insulating properties, resulting in part from their high degree of nonpolarity. Originally, low-density polyethylene was used exclusively among the polyolefins for this application. The promise of better mechanical properties led to the introduction of high-density polyethylene, sometimes called linear polyethylene, for both insulation and jacketing of wire and cable. Both high- and low-density polyethylene are currently in wide use for these applications. Low-density polyethylene is most frequently used for conductor insulation while high-density polyethylene is more widely used as a jacketing compound, reflecting the relative strong points of the 2 materials. Polypropylene has better electrical properties than either high- or low-density polyethylene. It also has a much higher glass-transition temperature, however, which results in more brittleness than is found in polyethylene. Initially this brittleness caused the rejection of polypropylene in most electrical applications. Further work has largely overcome the brittleness problem through the development of impact-modified polypropylenes and propylene-ethylene copolymers, a new class of materials also included under the general heading of propylene polymers. (Author)