Mechanical anisotropy of self-reinforced polyethylene crystallized during continuous-melt extrusion

Abstract

Oriented, extended chain structures are formed in self-reinforced polymeric materials. Once molecular chains of polymers have been oriented, virtually all properties, such as mechanical and thermal properties, become anisotropic due to morphology changes. The mechanical anisotropy of highly oriented thermoplastics prepared by several methods, including cold-drawing [1], hydrostatically annealed orientation [2], solid-state extrusion [3], die-drawing [4] and elongational ow injection molding [5], has been investigated. However, to the author's knowledge, no work has been reported on the mechanical anisotropy for the self-reinforced polymers prepared from ow-induced crystallization employing a conventional extruder and a conical die by controlling the extrusion pressure and melt temperature. This has been shown to be an economical method to prepare bulk polyole®n materials with high properties [6±9]. The aim of this letter is, hence, to study the mechanical anisotropy for the self-reinforced high-density polyethylene (HDPE) sheets crystallized from continuous-melt extrusion, by microhardness and tribological property measurements. The microhardness is a property sensitive to the molecular orientation and morphological changes in polymeric materials and is, therefore, a useful method for the investigation of the mechanical anisotropy developed within oriented semicrystalline polymers [1±5]. The experimental apparatus is composed of a 20 mm conventional single-screw extruder and a conical slit die having a tapered entrance half-angle of 308 and width and height at the lip of 20 3 1:5 mm. A general-grade HDPE 5000S, produced by Yangzi Petrochemical Corp., was used. Its melt index was 0:9 g=10 min. When preparing self-reinforced sheets 1.5 mm thick, the die lip was cooled with air jets from a blower to lower the die outlet temperature to about 128 8C and hence to increase the extrusion pressure. In the following, the sheets prepared under extrusion pressures of 30 and 40 MPa are, respectively, called low self-reinforced and highly self-reinforced sheets. For comparison purposes, the normal sheet was prepared at a 160 8C die outlet temperature and under a 3 MPa extrusion pressure. Both the self-reinforced and normal sheets were extruded without any downstream tension. For the microhardness (MH) measurements, the specimen surface was ®nely polished to improve focusing. The MH value at the specimen surface was determined at room temperature (23 8C) using a microhardness tester equipped with a Vickers square pyramidal diamond with included angles a ˆ 1368 between the non-adjacent faces of the pyramid. The specimen was positioned to obtain indentations with one diagonal parallel and one perpendicular to the extrusion direction (i.e., the orientational direction of molecular chains) of sheets. The length of the diagonals of the residual indentation was measured immediately after load removal with a microscope equipped with a micrometer eyepiece. The MH value (in MPa) was then calculated using the following expression: