PROCESSING AND PHYSICAL PROPERTY RELATIONSHIPS IN INJECTION-MOLDED ISOTACTIC POLYPROPYLENE .1. MECHANICAL-PROPERTIES

The aim of the research reported in these two articles was to explore the relationship between processing conditions and the physical proper ties of different grades of isotactic polypropylene injection moldings and propylene/ethylene copolymers. This first article describes the m ethods and processing conditions used for molding, together with mecha nical test results. Both conventional and shear-controlled orientation injection molding (SCORIM) have been employed for the production of m oldings. SCORIM is based on the application of specific macroscopic sh ears to a solidifying melt, which in turn, facilitates enhanced molecu lar alignment. SCORIM results in more pronounced molecular orientation than conventional injection molding, which is consistent with the sub stantial increase in Young's modulus of moldings produced by SCORIM. B y controlling the processing parameters it is possible to control and enhance the stiffness without loss of tensile strength. An increase of up to four times in impact strength has been achieved with SCORIM as well as a substantial increase in Young's modulus. The conventional in jection moldings containing pronounced molecular orientation exhibited impact resistance well below that for the SCORIM moldings. The mechan ical tests carried out at 80 degrees C showed that the high-temperatur e mechanical properties of all the materials, converted into moldings using SCORIM, exhibited substantial enhancement when compared with mol dings of the same material converted by conventional injection molding . (C) 1997 John Wiley & Sons, Inc.