Influence of the molecular architecture of low-density polyethylene on the texture and mechanical properties of blown films

Three types of low-density polyethylene materials were investigated with respect to the influence of the molecular architecture on the mechanical and use properties of blown films. The materials were a branched polyethylene synthesized by free-radical polymerization under high-pressure (HP-LDPE), a linear ethylene-hexene copolymer (ZN-LLDPE) produced by low-pressure Ziegler-Natta catalysis, and an ethylene-hexene copolymer (M-LLDPE) from metallocene catalysis. The extrusion and blowing conditions were identical for the three materials, with a take-up ratio of 12 and a blow-up ratio of 2.5. The blown films displayed a decreasing puncture resistance in the order M-LLDPE, ZN-LLDPE, and HP-LDPE. In parallel, the tear resistance of the films became increasingly unbalanced in the same order of the polymers. The morphological study showed an increased anisotropy of the films in the same polymer order, the crystalline lamellae being increasingly oriented normal to the take-up direction. This texturing caused a detrimental effect on the mechanical properties of the films, notably increasing the capacity for crack propagation. The phenomenon was ascribed to the kinetics of chain relaxation in the melt that governed the ability of the chains to recover an isotropic state from the flow-induced stretching before crystallization. The puncture resistance was examined in terms of both texture and strain-hardening capabilities. (C) 2003 Wiley Periodicals, Inc.