GB822672A - Improvements in or relating to the treatment of polyoxymethylene film - Google Patents

Abstract

Translucent to opaque crystalline polyoxymethylene film, containing spherulites detectable with a visible light microscope using crossed Nicols, is pressure-rolled at a roll temperature not exceeding 120 DEG C., and preferably at a temperature between room temperature and 100 DEG C., until the film is clear and substantially no spherulites are detectable with a visible light microscope using crossed Nicols. To secure increased tensile strength, the rolled film may thereafter be stretched while maintained at a temperature within the range between the optical melting-point of the polyoxymethylene and a temperature not lower than 65 DEG C. below the optical melting-point, preferably within the range 120-180 DEG C. The optical melting-point is defined as the temperature range from the point at which crystallites are seen to start to disappear to the point at which they are no longer evident, the film being heated at about 2 DEG F. per minute and observed under a polarized light microscope. The optical melting-point of polyoxymethylene films is stated to be within the range 183-185 DEG C. The films are preferably those described in Specification 753,299 and prepared from polyoxymethylenes of p 45,000-75,000 molecular weight and above. The film may be stretched (in one or two directions) to 1.5 to 3 times or over its original length and/or breadth, and is preferably stretched after the rolling step in a direction perpendicular to the rolling direction, or in two mutually perpendicular directions. In a typical example, a film, formed by melt-pressing powdered polyoxymethylene, having an average molecular weight of about 50,000, between chrome plates at 200 DEG C. for 2 minutes under a pressure of about 200 tons/sq. ft. and quenching the formed film in water at about 4 DEG C., is rolled in the longitudinal direction between two stainless steel rolls maintained at about 21 DEG C. until it is doubled in length, the film being at room temperature before entering the nip of the rolls. The film is then heated to about 180 DEG C. and can be stretched by about 100% of its original dimension without breaking.