Flow, expansion and processing behaviours of gas-containing filled polyethylene
The utilization of fillers in polymeric materials has exhibited a substantial growth during the last decades. Chalk (calcium carbonate) and glass fibres are two common examples of such fillers. These fillers/reinforcing elements are used in order to reduce the cost of material and change its physical (mechanical) properties. Foaming of the polymer using a dispersed gas can also change the cost as well as the properties. In the present work, the combined effect of hard fillers (chalk and glass fibres) and foaming the material (using a blowing agent) on its properties and flow behaviour (processability) has been investigated.
Firstly, an experimental characterisation of expansion kinetics of polyethylene containing the above fillers, that have different shapes (spherical and short fibres), was performed. The experiments were performed without any superimposed polymer flow. The expansion kinetics was described in terms of relationships between the equilibrium expansion ratio, the foaming time, the compressibility and the critical pressure for dissolving the gas in the polymer melt. The addition of the filler to the gas-containing polymer dramatically changed the character of the free gas phase formed during the expansion. The expansion kinetics was independent of the filler type and content and the amount of blowing agent. The equilibrium expansion ratio decreased as the filler content increased, and it was smaller with the glass fibres than with the spherically shaped chalk particles, provided that the filler content was the same.
Secondly, the effect of the gas generation on the rheological properties of unfilled and filled polyethylene melts was studied. The foaming was noted to have a pronounced influence on the flow behaviour of the melt. For flowing filled thermoplastics, two radically different expansion mechanisms are possible and were assessed formation of new gas cells or expansion of already formed cells. In general, the expanded structure after cessation of the flow was more uniform in the case of the filled melts than noted for the unfilled counterparts. There were indications that the expansion was more rapid during flow than when the melt was in rest.
Thirdly, it was investigated how to produce foamed and filled thermoplastic products by injection moulding. Parameters of interest in this context are e. g. the injection speed, the processing temperature, the residence time in the mould, the thickness of the product and the concentration of the blowing agent. The influence of the foaming and the filler content on the mechanical properties of polyethylene samples is reported. To some extent, the experimental data can be used to adjust the processing parameters and to optimize the content of the filler content in order to achieve foamed details with different (or desired) mechanical properties.