Property opportunities with polyolefins: a review. Preparations and applications of high stiffness and strength by uniaxial draw

Abstract

Abstract Major progress has been made in preparation of high-modulus and high-strength fibres and films by uniaxial drawing of semicrystalline polyolefins. Solid-state coextrusion followed by tensile drawing on ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (UHMWPP), and poly(4-methyl-1-pentene) starting from single-crystal mats, prepared from dilute solutions, produces superdrawn films with tensile moduli approaching the theoretical values reported for the crystal for each of these three polyolefins. For PE, a modulus of 220 GPa and strength of 6 GPa have been achieved. For PP, the corresponding maximum values are 37 and 2.2 GPa. This two-stage drawing technique has also been successfully applied for ultradrawing PE reactor powders. Films of compacted powder, prepared below the melting point of the powder, have been solid-state coextruded at 110°C to a draw ratio of 6, followed by tensile drawing at 120–135°C. The drawability and the uniformity of the resultant fibres and films are sensitively affected by the drawing temperatures and rates, and also by the choice of reactor powder. The maximum draw ratio achieved directly for reactor powder is ∼85, with a corresponding tensile modulus of ⩽ 130 GPa. For special conditions, high uniaxial draw and high modulus for selected reactor powders have been achieved simply by direct compaction plus calendering. In none of these processes is the polyolefin melting point exceeded. Applications of the oriented polyolefins are cited in terms of use as tapes, yarn and fabrics.