Compression Moulded Ethylene Homo and Copolymer Foams I. Effect of Formulation

Various foam formulations based on polyethylene and ethylene-vinyl acetate copolymers were investigated. Interactions between crosslinking and blowing agent concentrations were systematically studied for each base polymer using a two-stage compression moulding technique to manufacture the foams and the effects on physical properties were determined.

Initial studies progressively increased blowing agent concentration, whilst maintaining a fixed crosslinking agent level, resulting in density and relative density reduction as expected. However, it was observed that at any specific blowing agent concentration, foam density increased with increasing chain branching in polyethylene and with increasing vinyl acetate content in the copolymers.

This phenomenon was satisfactorily explained in terms of increasing crosslinking efficiency with increasing tertiary hydrogen concentration in the base polymer by comprehensive studies on gel content, melt strength at the expansion temperature and cell size trends. Surprisingly, cell size increased with vinyl acetate content at any particular gel content suggesting absolute cell size is a function of nucleation in the first stage of the process.