Mechanisms of ductile tear in blown film from blends of polyethylene and high melt strength polypropylene

Abstract

Deformation processes associated with ductile tear in blown films of polyethylene blended with up to 30 wt% high melt strength polypropylene (hmsPP) were studied. The tear resistance was determined with a reinforced trouser tear test. During stable crack growth, a crack-tip damage zone was transformed into a continuous yielded zone at the fractured edge. The relationship to lamellar morphology was probed with atomic force microscopy. Balanced tear characteristics of polyethylene film reflected the nearly isotropic lamellar morphology. In contrast, the highly oriented shish-kebab morphology of hmsPP domains in the blend films resulted in increasingly anisotropic behavior as the amount of hmsPP increased. The most important manifestation was a significant reduction in machine direction (MD) tear. Good adhesion of polyethylene and hmsPP in blend films prevented interfacial failure and provided stress transfer to the dispersed phase at high strains. In MD tear, extension of the matrix by the normal processes of lamellar breakup and fibrillation caused rotation of hmsPP domains into the loading direction and in a later stage shear displacement of reoriented hmsPP lamellae. Locally, hmsPP domains constrained deformation of polyethylene lamellae. Factors that increased constraint on the polyethylene matrix such as increasing the amount of hmsPP or increasing the aspect ratio of hmsPP domains reduced the MD tear resistance. In transverse direction (TD) tear, the oriented hmsPP domains deformed by lamellar shear processes concurrently with lamellar breakup and fibrillation of the polyethylene matrix. As a result, the blend film preserved good TD tear resistance.