Self-reinforced polypropylene (PP) sheets have been prepared from melt flow-induced crystallization through a conical slit die fed by a conventional extruder. Their structure and properties, influenced by the die pressure ranging from 20 to 50 MPa and die outlet temperature, are studied by scanning electron microscopy observation, differential scanning calorimetry analyses, tensile strength, and light transmittance measurements. At a die outlet temperature of 1627C and a pressure above 30 MPa, conspicuous increases in the melting peak, tensile strength, and light transmittance (they can be used to characterize the self-reinforcement degree of sheet) are observed. The self-reinforcement degree, however, increases only slightly with increasing pressure as it exceeds 40 MPa. Raising the die outlet temperature from 162 to 1727C results in a further increase in the self-reinforcement degree (for example, a highest tensile strength of 288 MPa) while keeping the pressure at 40 MPa, so bulk PP materials with high properties can be produced from continuous melt extrusion under pressures lower than 40 MPa. Furthermore, the melt temperature plays an important role in determining the properties of self-reinforced polymeric materials. Q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 2111–2118, 1998

Continuous extrusion was studied of self-reinforced high density polyethylene (HDPE) sheets from flow-induced crystallization at die pressures varying from 30 to 60 MPa. Their morphology, thermal behavior, tensile strength, and light transmit-tance were tested. Flow fields of a polymer melt through a converging wedge channel were also investigated by direct visual observations in conjunction with a theoretical analysis. The extensional strain rate increased abruptly as the melt approached the exit of the converging channel, this resulting in a higher crystallization rate. So, achieving the crystallization of molecular chains just in front of the exit of the converging channel may favor to extrude the bulk polymeric materials having high properties under lower pressures (e.g., 40 MPa or lower), this having been realized in the present work. The tensile strength of the self-reinforced HDPE sheet prepared at a 40 MPa pressure was enhanced by a factor of 8.

Studies on the melting in a single-screw extruder have mainly concentrated on the Maddock melting mechanism. The dispersive melting mechanism, named by us, is of great practical utility. A six-block physical model is first proposed for the dispersive melting mechanism in the present study. Then nonisothermal non-Newtonian mathematical models for the dispersive and Maddock melting mechanisms are developed. It has been demonstrated that the dispersive melting model has a number of advantages over the Maddock melting model: the mechanical power consumption can be decreased; the polymer temperature profile is more uniform and its average temperature is lower. Furthermore, melting can be accelerated and hence the total melting length shortened.

The morphology of high-density polyethylene (HDPE)/polyamide-6 (PA-6) blends with different melt-shear-viscosity ratios (VRs), prepared by combining a single-screw extruder with a convergent die was studied. Two different screw geometries, metering and mixing screws, and three screw speeds, 20, 40 and 60 rpm, were evaluated to investigate their effects on the morphology of extruded ribbons. Two mixing screws with low and high shear intensity, respectively, were used. Both the geometry and speed of screw were found to have an important role in the morphological changes of the blends. In contrast to previous studies, the results shown in this work reveal that it is possible to develop a laminar structure of PA-6 in an HDPE matrix with a VR larger than one by controlling the flow fields, through appropriately combining the type and shear intensity of the screw with its speed. A well-developed laminar PA-6 phase with an aspect ratio of about 100 was obtained under the optimum combination.