Mechanical properties, stress-relaxation, and orientation of double bubble biaxially oriented polyethylene films

Biaxially oriented linear low density polyethylene (LLDPE) films were produced using the double bubble process with different machine direction (MD) orientation levels and the same transverse direction (TD) blow-up ratio. Their mechanical behavior was characterized in terms of the tensile strength and tear resistance. The viscoelastic behavior of oriented films was studied using dynamic-mechanical thermal analysis (DMTA). The microstructure and orientation were characterized using microscopy, X-ray diffraction pole figures, and birefringence. The results indicate that MD ultimate tensile strength increases and the TD one decreases with MD stretching ratio. Tear propagation resistance, in general, remained mainly constant in TD and decreased in MD, as the draw ratio was increased. The morphology analyses exhibit a typical biaxial lamellar structure for all samples with different lamellar dimensions. Orientation of c-axis in crystalline phase, molecular chain in amorphous phase along MD increased with draw ratio. In most crystals, a-axis was located in the normal direction (ND) and the b-axis in the ND-TD plane. A good correlation was observed between c-axis orientation factor and MD mechanical properties. (c) 2006 Wiley Periodicals, Inc.