Novel polymer membranes for reverse osmosis

Claim 1

A semi-permeable membrane for use in the treatment of feed solutions by reverse osmosis, ultrafiltration or related techniques consisting essentially of a polyvinyl coacetal resin having a backbone chain of the general formula -X.sup.1 -X.sup.2 -X.sup.3 -X.sup.4 -X.sup.5 . . . . X.sup.n - wherein each X of X 1 , X 2 . . . X n is selected from the group consisting of hydroxy-bis-methylene --CH 2 --CH(OH)--, acetoxy-bis-methylene --CH 2 --CH(OCO.CH 3 )--, a first acetal group --CH.sub.2 --CH--CH.sub.2 --CH-- || OO ∠ ∠ R.sup.1 H and a second acetal group --CH.sub.2 --CH--CH.sub.2 --CH-- || OO ∠ C ∠ R.sup.2 H provided that said first and second acetal groups are both present and together comprise 25 to 100% of the chain, said groups R 1 and R 2 being different and each being selected from the group consisting of a hydrogen atom, straight or branched alkyl or alkenyl groups containing up to 6 carbon atoms and phenylalkyl or phenylalkenyl groups wherein the alkyl or alkenyl group is straight or branched and contains up to 6 carbon atoms.