Chlorine-resistant membrane for reverse osmosis. I. Correlation between chemical structures and chlorine resistance of polyamides

Correlation between the chemical structure of polyamides and the reactivity to hypochlorous acid was investigated to evaluate their chlorine resistance as a membrane material for reverse osmosis. Modes of interaction of polyamides with hypochlorous acid were classified into three; no reaction, reversible chlorination at the amide nitrogen, and irreversible chlorination at the aromatic nucleus. These modes of interaction were dependent upon the amine component of polyamide. Secondary amines such as piperazine and N-methylaniline gave amides which were quite stable against hypochlorous acid. Thus, poly(isophthaloyl piperazine) was most stable among the polyamides investigated in this study. Polyamide derived from aliphatic primary diamines such as polyethyleneisophthalamide gave an N-chlorinated amide, which could be reversibly dechlorinated by the treatment with a reducing agent such as sodium hydrosulfite. Polyamides comprised of aromatic primary diamines such as poly(m-phenyle-neisophthalamide), on the other hand, gave a product with chlorinated m-phenylene nucleus which could not be dechlorinated by a reducing agent.