Preparation and Characterization of Cationexchange Starches Containing Carboxyl Groups

Rice starch was first crosslinked using epichlorohydrin. The crosslinked starch was then subjected independently, to graft polymerization with acrylonitrile (AN) and cyanoethylation reactions. The poly (AN)-starch graft copolymer and the cyanoethyl starch so obtained were treated with sodium hydroxide to effect conversion of the cyanide groups of the modified starches into carboxyl groups via alkaline hydrolysis. It was found that the alkaline treatment results in decrement in the nitrogen content and increment in the carboxyl content. The magnitude of both contents is determined by sodium hydroxide concentration as well as duration and temperature of the treatment. It was also found that beside conversion of the cyanide groups to carboxyl groups, alkaline hydrolysis brings about cleavage of ether linkage of the copolymer and the cyanoethyl starch. This was evidenced by large difference observed between the sum of the substituents before and after hydrolysis. In this regard, cleavage of the ether linkage is greater with cyanoethyl starch than the poly (AN)-starch graft copolymer. The potential value of the hydrolyzed modified starches to serve as cation exchanger was assessed through measurements of critical properties such as durability, swellability, pK_a values and ion exchange capacity.