Characterisation of the mechanical and oxygen barrier properties of microfibril reinforced composites

Abstract

A relatively new type of reinforced composite material, derived from immiscible blends of thermoplastic homopolymers, is characterised in this doctoral research. Microfibril Reinforced Composites (MFCs) utilise common engineering and commodity polymers to create high strength and stiffness microfibrils dispersed in an isotropic matrix. Unlike traditional polymer composites, MFCs use the dispersed component of a blend to create an even distribution of in situ reinforcing microfibrils via a simple extrusion, drawing and processing technique.

This research quantifies the mechanical and oxygen gas barrier properties of polyolefin-based MFCs containing polyethylene terephthalate (PET) microfibrils. It is concerned not only with identifying MFCs with the best properties, but also with how manufacturing parameters influence those properties.

Characterisation is split into several parts. Initial investigations into blend development during extrusion and drawing were conducted. The main purpose of this was to gain a better understanding of the factors influencing the morphological changes that occur during production. Blend viscosity ratio and capillary number were identified as key factors in determining the onset of coalescence, deformation and break up of the dispersed polymer. The effects on microfibril formation of several important manufacturing parameters were highlighted, with die diameter and extrusion speed the most influential of these. A significant skin-core microstructure was observed. Formation of elongated microfibres (with negligible molecular chain alignment) was shown to occur during extrusion, which was subsequently justified via modelling of the shear stress flow fields in the die.

Drawn blends gave very high tensile strengths and stiffnesses due to highly oriented molecular chains. A threshold draw ratio of 3.5, at which properties change considerably, was identified. Mechanical properties of injection moulded MFCs from polypropylene were not considerably better than the neat matrix polymer. However, those from polyethylene (PE) showed significant improvement via injection moulding and directional compression moulding. MFCs with just 30% microfibril content displayed tensile properties up to six times greater than neat PE.

Measurements of oxygen gas permeability highlighted improvements of up to 65%. Processing and cooling conditions were shown to significantly influence permeability via a Taguchi experimental design analysis. MFC storage containers from PE/PET were injection moulded as proof-of-concept on completion of the research.