Extrusion processing for manufacture of low-density, fine-celled polypropylene foams

A continuous extrusion process for the manufacture of low-density, fine-celled polypropylene foams is presented. Due to its outstanding functional characteristics and low material cost, polypropylene foams have been considered as a substitute for other thermoplastic foams in industrial applications. However, only limited research has been conducted on the production of polypropylene foams because of the weak melt strength, and no research has been conducted to investigate the mechanisms that govern the expandability of polypropylene foams. This thesis presents the effective strategies for increasing the volume expansion ratio as well as the mechanisms governing the foam density of polypropylene foams. The basic strategies taken in this study for the promotion of a large volume expansion ratio of polypropylene foams are: (a) to use a branched material for preventing cell coalescence; (b) to use a long-chain blowing agent with low diffusivity; (c) to lower the melt temperature for decreasing gas loss during expansion; and (d) to optimize the processing conditions in the die for avoiding premature crystallization. The effects of processing and materials parameters on the foam morphologies of polypropylene materials were thoroughly studied using a single-screw tandem foam extrusion system. A careful analysis of extended experimental results obtained at various processing conditions indicates that the final volume expansion ratio of the extruded polypropylene foams blown with butane is governed either by loss of blowing agent or by crystallization of the polymer matrix. By tailoring the processing conditions in the die, ultra low-density, fine-celled polypropylene foams with very high expansion ratio up to 90-fold were successfully produced from the branched polypropylene resins. Fundamental studies have also been conducted to investigate the effect of various processing and materials parameters on the thermodynamic, thermal and melt fracture behaviors of polypropylene melts with foaming additives that influence the cell morphology of polypropylene foams.