Method of manufacturing polyvinyl alcohol films oriented in two orthogonal directions

Claim 1

In a method of manufacturing a polyvinyl alcohol film successively oriented in two orthogonal directions by using a roller type longitudinal elongation machine and a clip type lateral elongation machine, the improvement which comprises the steps of longitudinally elongating a polyvinyl alcohol film by more than 200% while maintaining the initial water content (Z × 100)% of the film at a value of from 10 to 35%, where Z represents a value obtained by dividing the weight of the water contained in the film by the weight of the dry film, and (Z × 100)% represents a proportion in percent of the weight of the water contained in the film with respect to the weight of the dry film, and maintaining the temperature in the film in a range of from a lower limit of (1.50/Z + 20)° C. to an upper limit of 75° C., preheating the longitudinally elongated film to a temperature lower than the temperature at which the film is elongated laterally while maintaining the water content thereof at at least 7%, elongating laterally the preheated film by more than 200% at a temperature of from 45° to 110° C. while preventing the film from shrinking longitudinally, drying and heat treating the film at a temperature of 110° to 210° C. while permitting the film to cause it to shrink by less than 15% in the lateral direction, and finally cooling the film to room temperature under tension.