Relationships between micromorphology and impact properties of injection moulded isotactic polypropylene

Abstract

The micromorphology and properties of injection moulded semi-crystalline polymers can be significantly modified by changes in the injection moulding parameters and the design of mould. Further modifications to the microstructure and properties occur following the incorporation of additives into a thermoplastic compound, such as stabilisers, pigments, nucleants, etc. Many published papers refer to these changes in microstructure, but very little attention has been devoted to the detailed characterisation of the outer surfaces of injection mouldings, which play an important part in determining impact properties.

The main objective of this research project was to carry out a systematic examination of the micromorphology of a range of isotactic polypropylene injection mouldings, paying particular attention to the outer layers which are difficult to characterise and to then relate the micromorphology revealed to the impact properties. Models which relate the mechanism of impact failure to microstructure and processing route have been developed as a result. Particular attention was given to the careful production of injection mouldings to ensure reproducible micromorphologies and mechanical properties for set processing conditions. This was carried out prior to very comprehensive characterisation of micromorphology using a range of analytical techniques including, straightforward microtomy and optical microscopy, x-ray diffraction, Differential Scanning Calorimetry and the application of a new etching and replication method for transmission electron microscopy studies. The impact properties of mouldings were determined by instrumented falling-weight and instrumented Izod impact test methods.

Injection mouldings revealed a complex skin-core morphology, the width, degree of orientation, crystal form (α or β) and spherulite size of the various layers were found to be strongly dependent on the moulding parameters and additives incorporated. The role of such factors in determining the impact properties was identified providing valuable information to commercial moulders of polypropylene.