Change of anion exchange membranes in an aqueous sodium hydroxide solution at high temperature

Abstract

Properties of commercial anion exchange membranes were examined after the membranes had been immersed in an aqueous sodium hydroxide solution, 3.0 to 9.0 N, of high temperature, up to 75°C. The anion exchange membranes used had N-methyl-pyridinium groups or benzyl trimethylammonium groups bonded to a styrene-based copolymer as anion exchange groups, with backing polymers. Alkali deterioration of the membranes was classified: decomposition of backing polymers (added inert polymer and backing fabric) and decomposition of anion exchange groups. N-Methyl-pyridinium groups were especially easy to decompose compared with benzyl trimethylammonium groups, and lost ion-exchange ability. Benzyl trimethylammonium groups also lost ion-exchange ability and changed partially in weakly basic anion exchange groups. The anion exchange membranes with various anion exchange groups, benzyl trimethylammonium groups, triethyl-, tri- n-propyl-, tri- n-butyl-, and 1-azonia-4-azabicyclo-[2,2,2]-octane (different hydrophobicity) were prepared from base membranes of chloromethylated polysulfone, and copolymer of chloromethylstyrene and divinylvenzene with polyethylene fabric and powder. Though the membrane based on polysulfone weakened mechanically by immersion the membrane in the alkali solution, polyethylene reinforced-membrane did not change mechanically. However, with increasing chain length of alkyl groups bonded to ammonium groups, loss of the anion exchange capacity was remarkable and 1-benzyl-azonia-4-azabicyclo-[2,2,2]-octane groups were also not stable. The most stable anion exchange groups among these were benzyl trimethylammonium groups.