MANUFACTURE OF POLARIZING FILM

PURPOSE:To improve the heat and moisture resistances by stretching a polyvinyl alcohol film dyed with a dichroic dye under heating in a dry state, heat treating the stretched film at a specified temp. under tension, and treating it with boric acid. CONSTITUTION:A dichroic dye is allowed to be adsorbed on an unstretched film with 99.8% degree of saponification, and the film is dried and stretched at about 90 deg.C by >= three times in one direction. While keeping the tension state, the stretched film is held at 150-200 deg.C for 1-20min to accelerate crystallization. By the heat treatment the heat and moisture resistances are remarkably improved. The film is then immersed in an aqueous soln. contg. 3wt% boric acid at 50 deg.C under tension for about 10min, and it is washed in water at room temp. and dried with hot air at about 80 deg.C. Cellulose triacetate films are pasted on both sides of the resulting polarizing film with an isocyanate adhesive to obtain a polarizing plate with superior heat and moisture resistances.