THE MANAGEMENT OF MICROSTRUCTURE IN SEMICRYSTALLINE POLYMERS

The microstructure of semi-crystalline thermoplastics has an important influence on the physical properties of extruded and moulded compound s. Plastics processing methods develop different morphologies in polym ers depending on the process and processing conditions applied. This p aper features the control of morphology in isotactic polypropylene and polyethylene processed by injection moulding. Both conventional injec tion moulding and shear controlled orientation injection moulding (SCO RIM) were used in the production of bars for the purpose of characteri zation of microstructure and physical properties. SCORIM provides for substantial control over the morphology of moulded thermoplastics. A h igh degree of molecular alignment can be achieved via SCORIM processin g which contributes to enhanced stiffness and impact strength. It is p ossible to enhance stiffness without sacrificing impact resistance. Th e enhancement in stiffness is mainly attributed to the 'shish-kebab' m orphology developed in SCORIM, and both polypropylene and polyethylene exhibit shish-kebab microstructures. The principles of the developmen t of molecular alignment using SCORIM as explained in this paper can b e extended to other thermoplastics.