The preparation and properties of fractions from high density polyethylenes

A number of fractions have been prepared on a large scale from Ziegler 0·945‐density polyethylene and Phillips 0·96‐density polyethylene. Their physical properties have been compared with those of whole polymers having much broader distributions of molecular weight. It is shown that the main effects of narrowing the molecular‐weight distribution of polyethylene are to lower its density, to reduce its tensile yield strength, probably to increase its impact strength, and (with some reservations) to increase its ultimate elongation. Little change is evident in melt index or folding endurance. The effects on stress‐cracking resistance depend upon the type of polymer being considered. The relationship between yield strength and density is independent of intrinsic viscosity and of intrinsic viscosity distribution.