REINFORCED FILM AND ITS PREPARATION

PURPOSE:To obtain a reinforced film, capable of reducing the residual strain, and having a long duration of use, by changing the sulfonyl groups on the side opposite to the side in which a reinforcing cloth is embedded in a fluorinated polymeric film having the side chain containing sulfonyl groups into an amine salt of a sulfonic acid. CONSTITUTION:A fluorinated polymer having sulfonyl groups in the side chain thereof in the form of sulfonyl fluorides is formed into a film having a thickness of 50-1,000mu. Only one side thereof is then hydrolyzed with an aqueous solution of a compound expressed by the formula Me(OH)n (Me is metallic cation; n is the valence of the Me) to convert the sulfonyl fluorides into groups -(SO3)nMe and then treated with a compound expressed by the formula NR1R2 R3 (R1 is 1-5C alkyl; R2 and R3 are H or R1) to form a layer in which the sulfonyl groups are -SO3NHR1R2R3, preferably having a thickness of 5-25mu. A reinforcing cloth is embedded from a side in which the sulfonyl groups are sulfonyl fluoride groups, and the whole layer is hydrolyzed with an aqueous solution of a compound expressed by the formula Me(OH)n to convert the sulfonyl groups into the groups -(SO3)nMe.