Toughened biaxially oriented film

#1. Field of the Invention

#The invention relates to biaxially oriented polyamide films. More particularly, it describes a method for producing toughened biaxially oriented polyamide films having superior strength and resistance to pinholes, cracks, and other structural defects.

#2. Description of the Related Art

#A dry mixture of 95% (w/w) nylon 6 (Capron® from Honeywell) and 5% (w/w) ethylene/ethyl acrylate/maleic anhydride terpolymer (Lotader 4720 from AutoFina) was prepared and charged to the feed throat of a single screw extruder fitted with a mixing screw. To this dry mixture was also added 1.3% of an antiblock masterbatch containing approximately 7% silica. The dry mixture was conveyed along the length of the barrel, and melt blended at a melt temperature of approximately 266° C. (510° F.) to thereby form a polymer blend. After conveyance from the extruder to a T-die the polymer blend was cast onto a polished roll maintained at a temperature of approximately 7° C. (45° F.). Die temperatures were maintained at such temperatures as to achieve the desired cast polymer film gauge profile. The thickness of the initially cast polymer film was approximately 10 times the desired thickness of the final polymer film, after biaxial orientation. The edges of the cast polymer film were immediately placed into contact with the casting roll surface with air jets. Additional pinning was facilitated through the use of electrostatic pinners, which were operated with a power between about 18 kW and about 25 kW. After quenching the polymer film on the casting roll the film was passed through a series of hot water baths. The first water bath was maintained at a temperature of about 43° C. (110° F.), while the second bath was maintained at a temperature of about 52° C. (125° F.). The passage pathway was controlled so as to achieve a moisture content in the film of about 3.5% (w/w) before orientation. After plasticization the film was simultaneously oriented in both the machine and transverse directions, with the final orientation being 3×3. The film passed through eleven heating zones, which facilitated both orientation and controlled shrinkage properties. The preheating zones before orientation were approximately 185° C. (365° F.), the stretching zones were approximately 168° C. (335° F.), and the heat setting zones were approximately 185° C. (365° F.). After orientation, the film was corona treated and wound onto a collection bobbin.

#COMPARATIVE

#Example 1 was repeated except that electrostatic pinning was not used, the second bath was maintained at a temperature of about 60° C. (140° F.), the moisture content of the film was about 2.0%., the preheating zones before orientation were approximately 177° C. (350° F.) and the stretching zones were approximately 149° C. (300° F.). The orientation was unsuccessful as the film was not uniformly oriented across the finished film width.