Process for preparing surface-modified biaxially oriented film of high

Abstract

The biaxially oriented films of high molecular weight polyethylene is formed and its surface modified to provide a hydrophilic property to the resulting film. The film is prepared by melt-kneading the high molecular weight polyethylene with an ordinarily solid hydrocarbon plasticizer and a raw fabric sheet is formed from the resulting molten mixture. After the hydrocarbon plasticizer is extracted the resulting unstretched sheet is stretched under conditions providing a stretched film having a specific surface area of 70 m.sup.2 /g or more and a fibril structure. This stretched film is then heated under standard length constraint conditions to reduce the specific surface area of the stretched film by at least 20 m.sup.2 /g to obtain a biaxially oriented film of high molecular weight polyethylene. The surface of the film is thereafter subjected to surface modification by polymerizing a vinyl monomer having a hydrophilic group on the surface of the film whereby the film surface becomes hydrophilic, as measured by a contact angle of the film surface to a water drop at a level of not more than 90.degree.. The biaxially oriented films are further characterized by their excellent gas permeability, surface smoothness, tensile modulus and tensile strength.