Fluoropolymer Membranes *

Abstract

The development of perfluoroionomers, fluoropolymers containing cation exchange groups, has been central to the introduction of membrane cell technology for the electrolysis of aqueous sodium chloride, now being introduced worldwide for chloralkali production. Only fully fluorinated polymers can withstand the extreme conditions found in the cell chlorinated brine at 80°C on one side of the membrane and strong caustic soda at 80°C on the other.

The first perfluoroionomer membranes, introduced by Du Pont in the early 1970's as Nafion®, were based on copolymers of tetrafluoroethylene with a perfluorovinyl ether containing a sulphonyl fluoride group, which in the final membrane is hydrolysed to the sulphonic acid to introduce the ion selective properties. However, these early membranes were only suitable for the production of dilute caustic, as at high caustic concentration, the back migration of OH- ions reduced the cell efficiency.

Much of the subsequent development has been targeted at improving the ion selectivity of membranes, to enable strong caustic to be produced at high efficiency.

The development of asymmetric membranes has been key to achieving these improvements, with a thin highly ion selective barrier layer on the cathode side of the membrane and a thicker, highly conductive support layer on the anode side. Amongst the various weakly acidic groups evaluated, the carboxylate group has provided the most effective ion barrier properties and all current commercial membranes for electrolysis of sodium chloride have such a cathode layer. This paper reviews the requirements for a chloralkali membrane and covers each stage in membrane production, from monomer synthesis and copolymerisation to membrane fabrication and post treatment. It also includes the more important developments introduced to improve membrane performance.