The effect of molecular weight distribution on polyethylene film properties

Abstract Polymer molecular weight heterogeneity affects the rheological properties of polymer melts such as melt viscosity, fracture and die swell. These rheological properties affect the conversion of the polymer from the bulk resin state to its final usable form. In this particular study, the effect of molecular weight distribution on polyethylene blown film characteristics was studied. The effect of the molecular weight heterogeneity on the rheological characteristics of the polymer in the molten state and its effect on the film properties is presented. The properties studied included film gloss, haze, tear resistance and film impact strength. This study shows that broadening the molecular weight distribution increases haze and reduces film gloss. Further, it was shown that a linear relationship exists between film gloss and external haze. Both values are measures of surface irregularities in the film which are affected by the drawing characteristics of the polymer. A broader molecular weight distribution results in increased impact strength as measured by the Dart Drop Impact Test. This is, it is believed, a result of the increase in long chain branching of the higher molecular weight fractions of the polymer which cause a higher degree of molecular weight entanglement at the branch sites. In contrast the tear strength is reduced as the molecular weight distribution broadens because of the low molecular weight fraction in the broad spectrum material which tend to decrease resistance to tear.