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 exp ansion of the compacted sheets is shown to relate to both the molecular str ucture which is produced after the hot-compaction procedure has been comple ted and the architecture of the woven material, which includes the effects of thermal and elastic anisotropy. The annealing effect of the hot-compacti on process on the structure of hot-compacted polypropylene tapes, was inves tigated by measurements of shrinkage force, elastic modulus and thermal exp ansion measurements. The hot-compaction process is seen to have a profound effect on the structure of the original polypropylene tapes, owing to the c ombined effects of shrinkage of the oriented amorphous phase and melting an d recrystallisation of the crystalline fraction. The influence of the woven architecture of the composites has been investigated through the developme nt 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 w ith their anisotropic thermal and elastic properties: account is also taken of the woven architecture which comprises the compacted woven materials. T he effect of the mechanical and thermal anisotropy was, as expected, found to be most pronounced in the hot-compacted polyethylene composites. (C) 200 0 Elsevier Science Ltd. All rights reserved.