V. Properties of different polyethylenes

AT THE PRESENT TIME the manufacturers of polyethylene in the United States are producing but one grade of the material for radio-frequency dielectric purposes, and this standard grade is covered by a Bureau of Ships specification. In England on the other hand the material produced by the Imperial Chemical Industries under the trade name Alkathene has been commercially available in various grades for the cable industry. The British classify their polyethylene according to a plasticity (melt-viscosity) index based on the amount of material at 190 degrees centigrade which flows through an orifice of arbitrary size under a definite pressure. The smaller the ICI plasticity index, the higher is the average molecular weight of the polyethylene. The British have commonly used for their radio-frequency cable a moderately low-molecular-weight grade known as Alkathene 20, and in order to improve its extrudability and low-temperature mechanical properties it has been their practice to modify the polyethylene slightly by the addition of some high-molecular-weight polyisobutylene. The highest — molecular-weight grades of polyethylene are the toughest forms of the material, but they are also not entirely satisfactory from the standpoint of smooth uniform diameter extrusion on wire. The standard American product represents a compromise between practical extrudability and good mechanical properties over the widest possible temperature range. It is superior to the polyisobutylene-modified British cable insulation chiefly in respect to very low-temperature impact and bend resistance and to deformation resistance at relatively high service temperatures, approaching 100 degrees centigrade.