The thermal-expansion behaviour of hot-compacted polypropylene and polyethylene composites

In this paper we describe a study of the thermal-expansion behaviour of hot-compacted woven polypropylene and polyethylene composites. The thermal expansion of the compacted sheets is shown to relate to both the molecular structure which is produced after the hot-compaction procedure has been completed and the architecture of the woven material, which includes the effects of thermal and elastic anisotropy. The annealing effect of the hot-compaction process on the structure of hot-compacted polypropylene tapes, was investigated by measurements of shrinkage force, elastic modulus and thermal expansion measurements. The hot-compaction process is seen to have a profound effect on the structure of the original polypropylene tapes, owing to the combined effects of shrinkage of the oriented amorphous phase and melting and recrystallisation of the crystalline fraction. The influence of the woven architecture of the composites has been investigated through the development of a simple model, which shows that the measured composite values can be understood in terms of the anisotropic thermal expansion of the individual fibres or tapes of which the compacted composites are composed, together with their anisotropic thermal and elastic properties: account is also taken of the woven architecture which comprises the compacted woven materials. The effect of the mechanical and thermal anisotropy was, as expected, found to be most pronounced in the hot-compacted polyethylene composites.