Low-density rotomoulded polymer foams

Abstract

Solid polymer foams are well-known materials used to provide insulation, packaging impact protection, low slip shoe soling and so on. This paper examines the nature of the foams produced when combined with the process of rotomoulding, long established as the means by which hollow polymer shapes are made. Rotomoulding refers to the fact that a mould with a meltable or sinterable powder inside it is heated and rotated about two axes at right angles to distribute the powder over the inside of the mould to form a skin. This heating phase is followed by a cooling phase. The aim of the research reported here is to determine the conditions under which a hollow moulding with a skin made from one polymer powder, in this case low density polyethylene, can be made at the same time as a foam made from another polymer is formed to fill the cavity but not to penetrate through the skin. The foam in this case is polystyrene with around 6% (w/w) n-pentane pre-absorbed. The whole system is referred to as the Rotofoam© process. Experiments on both the laboratory and the full industrial scales are reported. The Rotofoam© laboratory kinetics rig allows the foam development to be seen by eye and by camera as a glass mould undergoes the two axes rotations. Temperatures inside the foam and in the mould are monitored via a system of slip rings and hollow axles. Examination by SEM allows the micro-development of the foam to be seen and linked to a simple shoebox-like model of a foam cell which correlates well with overall foam density measurements. The model also ties together the heat flow needed to expand the foam and heat the polystyrene and polyethylene, with the heat transfer rates calculated from the material conductivities, the material path lengths and the imposed temperature difference between mould and foam. Finally, the paper reports the results obtained by the use of foam control agents — hydrated salts in this case — which by release of steam during the heating phase act to retard the pentane-driven foam expansion until the polyethylene skin is formed. The diffusion of the steam through the cell walls into the foam cavities is briefly discussed.