Investigation into thermoformability of hot compacted polypropylene sheet W. Prosser, P. J. Hine, and I. M. Ward This paper describes an investigation into the thermoformability of a new class of oriented polymeric material recently developed, namely hot compacted polypropylene sheet. Exploitation of any new material requires an intimate understanding of a whole range of factors, amongst which thermoformability is pre-eminent. This is particularly true for oriented polymeric materials, for while the preferred molecular alignment gives enhanced properties such as stiffness, strength, and resistance to impact, the downside is that the stretched molecular chains tend to limit further flow under stress, making thermoforming difficult. The aim of the present study was to establish the critical parameters for successful thermoforming of hot compacted polypropylene sheet. Elevated temperature tensile tests were used to investigate the stress–strain behaviour of the compacted materials. The crucial parameters were found to be the post-yield modulus, which gives a measure of the resistance of the material to large scale deformation, and the strain to failure, which gives the upper limit on deformation. The post-yield modulus was found to be significantly affected by the test temperature and the high strain hardening behaviour of the material confirmed that significant force is required to thermoform the compacted polypropylene sheets. A hemispherical mould, with built-in gripping plate, was used to carry out a study of the thermoforming behaviour of the compacted sheets, and the results were found broadly to confirm the conclusions of the tensile tests. A linear relationship was found between the tensile force and the postforming force, reinforcing the synergy between the two tests. In addition the forming tests showed that the best temperatures to use were either side of the melting point of the melted and recrystallised phase, depending on the amount of postforming deformation required. Different gripping arrangements were investigated both in which the sheet was fully gripped and in which the sheet was allowed to flow into the mould during forming. The different schemes were found to control whether a successful component could be produced under different conditions and at different ultimate strains. Finally, the tests with the hemispherical mould showed that thermoforming this shape requires significant interlaminar shear deformation, and above 15% strain this resulted in destruction of the interlayer bond. For strains greater than this, successful thermoforming could only be achieved by allowing the material to flow into the mould. PRC/1638 © 2000 IoM Communications Ltd. Mr Prosser is at the Institute of Materials Science and Testing of Plastics, University of Loeben, Austria and Dr Hine and Professor Ward are at the IRC in Polymer Science and Technology, University of Leeds, Leeds LS2 9JT, UK. Manuscript received 8 May 2000; accepted in final form 11 July 2000. INTRODUCTION with a suitable matrix material (e.g. epoxy resin), to produce a fibre reinforced composite.7 The choice is The last 20 years has seen the development of a large number of routes for the production of oriented therefore between a single phase oriented material of medium stiffness, or a two phase material incorporat- polymeric materials by cold drawing below their melting points. The attraction of these materials, ing high stiffness fibres. Recent research has seen the development of a new compared with isotropic polymers, is their enhanced stiffness and strength by virtue of preferred molecular class of oriented polymeric sheet material, designed to bridge between these two classes of material. The alignment. Production technologies such as ram and hydrostatic extrusion1,2 as well as the more recently process used, termed ‘hot compaction’, takes as its starting point the highly oriented fibres or tapes described die drawing and roller drawing,3,4 in which the isotropic material is pulled or pushed through a described above. Under suitable conditions of temper- ature and pressure the surface of the oriented material suitable die, enable the manufacture of large cross- section products, such as rods, tubes, and sheets, melts, and on cooling this molten material freezes to form a matrix, which binds the structure together. A to be achieved. These large scale products tend to show intermediate levels of molecular orientation. usable temperature window has been found to exist in which sufficient material is melted to form a strong Alternative routes, such as melt spinning of low molecular weight polyethylene5 and gel spinning of bond between the oriented tapes or fibres, while retaining a substantial percentage ( >70%) of the high molecular weight polyethylene,6 produce small diameter, but much more highly oriented, fibres. original oriented phase. The result is a single type ‘self- reinforced’ material with an interesting combination These highly oriented fibres can be used, together Plastics, Rubber and Composites 2000 Vol. 29 No. 8 401 ISSN 1465–8011