Abstract
Crosslinked polyethylene and ethylene-vinyl acetate copolymer foams were produced by single stage and two-stage heat and chill compression moulding techniques. At a fixed blowing agent concentration, crosslinking agent concentrations were adjusted to give similar gel contents independent of base polymer.
Relative densities of fully expanded foams produced by either the single stage or heat and chill process did not show significant variation. However, relative densities of partially expanded material after the first stage of the heat and chill process showed a considerable decrease at higher vinyl acetate contents resulting in a consequent reduction in further expansion in the second stage.
Cell structures of foams from single stage expansion and after each stage of the heat and chill process were observed by scanning electron microscopy. Micrographs were analysed to give mean cell sizes and cell size distributions. The studies showed reasonable regularity of cell structure for all foams expanded by the single stage process and for high vinyl acetate content foams produced by the heat and chill process. However heat and chill processed foams from polyethylene and low vinyl acetate content base polymers showed a pronounced bimodal cell size distribution. This was attributed to possible crystallinity, solubility and diffusion effects. It was concluded that cell structure regularity was adversely affected if the cooling cycle after the first stage allowed crystallisation of the polymer.
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