Abstract :
High-density polyethylenes can offer an advantage of 20degC, and polypropylenes an advantage of 55 degC, in the upper temperature limit, over the currently used low-density-polyethylene insulation. None of the 16 high-density polyethylenes examined, however, combines good mechanical properties with the desirable dielectric characteristics at high temperatures. Many are relatively tough when prepared by quenching the melt but embrittle on subsequent exposure to 100° C; the majority have low impact strength and inadequate resistance to thermal cracking. The commercial polymers with the best mechanical performance have relatively poor dielectric properties. Commercial polypropylenes are all very brittle, and, if cooled slowly from melt, crystallise with visible voids between spherulites. With few exceptions their loss tangent is high at high temperatures and mains frequency. One type of modified polypropylene, made in America, shows a moderate improvement in mechanical properties coupled with excellent dielectric characteristics. Another type, made in Britain, shows outstandingly good mechanical properties, which include high impact strength and high extensibility, even after slow cooling from melt.
