Process for producing stretched articles of ultrahigh-molecular-weight polyethylene

Claim 1

A process for producing a stretched article of ultrahigh-molecular-weight polyethylene, which comprises, (1) melt-kneading a mixture composed of (A) 15 to 80 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of ultrahigh-molecular-weight polyethylene having an inherent viscosity (η), determined at 135° C. in decalin, of at least 5 dl/g and (B) 85 to 20 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of a paraffinic wax having a melting point, determined by the DSC method, of 40° to 120° C. and a weight average molecular weight (Mw), determined by the GPC method, of at least 230 but less than 2,000 in a screw extruder while maintaining the temperature of the mixture at 180° to 280° C., (2) melt-extruding the molten mixture through a die kept at a temperature of 180° to 300° C., under a draft condition of a draft ratio of less than 2 or in the absence of a draft, (3) cooling the resulting unstretched extrudate to solidify it, and (4) subjecting the unstretched solidified extrudate to a stretching treatment at a temperature of 60° to 140° C. at a stretch ratio of at least about 3:1 when the step (2) is carried out while a draft is applied to the unstretched extrudate or at a stretch ratio of at least about 10:1 when step (2) is carried out in the absence of a draft, said stretching treatment is carried out in a heated liquid medium which is a solvent for the paraffinic wax (B) whereby the paraffinic wax (B) is extracted from the extrudate simultaneously with the stretching treatment, such that the amount of the wax (B) remaining in the stretched articles is not more than about 10% by weight.