Comparative study of structure and tensile properties of melt-pressed and extruded-blown films of high-density polyethylene

摘要

Bulk high-density polyethylene (HDPE) films crystallized from isotropic melts show a spherulite structure; the tensile stress-strain (S-S) curves of such melt-pressed (M-P) films show a characteristic shape caused by necking. On the other hand, extruded-blown (E-B) HDPE films show an anisotropic structure in which lamellae are stacked normal to the extrusion direction. The S-S curves of E-B films showed various shapes for various elongation directions relative to the lamellar orientation direction. When drawn perpendicular to the extrusion direction (parallel to the lamellar orientation direction), the S-S curve showed typical necking behavior resulting from unfolding from one part of the lamellae. When drawn diagonal to the extrusion direction (diagonal to the lamellar orientation direction), the yield stress was smaller due to slippage between the lamellae. When drawn parallel to the extrusion direction (perpendicular to the lamellar orientation direction), the S-S curve was smooth, but it was shown that micronecking occurred repeatedly in various parts of the specimen. The characteristic shape of the S-S curve of the M-P films was reconstructed as the sum of all S-S curves of the E-B films in various directions. From a structural viewpoint, the deformation behavior of the bulk film, which has a spherulite structure, can be simulated at least qualitatively by the deformation behavior of anisotropic E-B films in various directions. [References: 19]