Crystalline‐state extrusion of low density polyethylenes

Abstract

AbstractThree low density polyethylenes, one long branched (A) and two linear (B and C), have been solid‐state‐extruded at several constant temperatures from ambient to 80°C and to draw ratios ⩽ 8. The initial densities and melt indices of A, B, and C are 0.920, 0.920, and 0.935 g/cm3, and 1.9, 0.8, and 1.2, respectively. Melt‐crystallized cylindrical billets were extruded through conical dies in an Instron Capillary Rheometer. The linear polymers were found to draw by extrusion more readily than the branched; all three strain‐harden. Density, birefringence, tensile, and thermal properties have been evaluated as functions of extrusion temperature and draw ratio. Despite a measured loss via die swell, substantial orientation takes place during solid‐state extrusion as evidenced by increases in transparency, birefringence, and tensile modulus (up to 4.5 times that of the original isotropic polymer). Depending on the polymer and the draw temperature, density does go through a minimum or shows a monotonic increase with draw by extrusion. A minimum in modulus is also observed at low draw and at all draw temperatures for all three polymers. The highest tensile moduli achieved are 0.73, 0.46, and 1.5 GPa for A, B, and C, respectively, at their highest draw ratio. The melting point for polymer B decreases with extrusion draw ratio, whereas it remains constant after a small initial drop, for the two others. For all three low density polyethylenes, birefringence increases rapidly with extrusion draw and then levels off at high draw. The birefringence limit is similar for A and B, i.e., 0.046 ± 0.004, but higher for C, i.e., 0.068 ± 0.009. This work extends beyond others in that it studies the effect of short as well as long branches in solid‐state extrusion by comparing the linear and long branched LDPE polymers and LDPE with prior evaluations of HDPE.