Perfluorocarboxylic acid membrane

The Asahi Chemical Company of Japan has developed a perfluorocarboxylic acid membrane (54) (55) (56). It is reported to be formed from Nafion films wherein the SO3H groups on the cathode surface are split off and the adjacent CF2 groups thereafter oxidized to carboxylic acid groups. 

Perfluorocarboxylic Acid Membrane and Membrane Chlor-Alkali Process Developed by Asahi Chemical Industry... 

In April 1975, Asahi Chemical started operation of a membrane chlor-alkali plant with a capacity of 40,000 MT/Y of caustic soda using Nafion perfluorosulfonic acid membrane. In 1976, this membrane was replaced by perfluorocarboxylic acid membrane developed by Asahi Chemical. The total caustic production capacity of plants based on Asahi Chemical s membrane chlor-alkali technology using perfluorocarboxylic acid membrane will reach 520,000 MT/Y in 1982, at seven locations in various countries. 

The monolayer perfluorocarboxylic acid membrane can be prepared, if necessary, by chemical treatment which causes the chemical reaction throughout the membrane. 

Type of membrane Monolayer perfluorocarboxylic acid membrane (COOH) Multilayer membrane of perfluorocarboxylic and sulfonic acid by lamination or coating (C00H S03H) Multilayer membrane of perfluorocarboxylic and sulfonic acid by chemical treatment (COOH/SO3H)... 

Water Content. Figure 4 shows the relation between the water content of perfluorocarboxylic acid membrane prepared by chemical treatment and the ion exchange capacity with varying external solution concentration. As the concentration of the external solution increases, the membrane shrinkage increases and the water content is therefore decreased. The influence of... 

Water content in Figure 4 can be expressed as a function of ion exchange capacity and external solution concentration by the following empirical equation, which is similar to that proposed for perfluorosulfonic acid membranes by W.G.F. Grot in 1972 (44). The water content of perfluorocarboxylic acid membrane is much lower than that of perfluorosulfonic acid membrane. 

Figure 9. Electric resistance, ion exchange capacity (meq/g dry resin) and concentration of NaOH, for perfluorocarboxylic acid membrane prepared by chemical...

Table III shows the mechanical properties of perfluorocarboxylic acid membrane prepared by chemical treatment and perfluorosulfonic acid membrane.

Perfluorocarboxylic acid membrane prepared by chemical treatment... 

Table V. PVEX Monomer Appropriate for Perfluorocarboxylic Acid Membrane.

Influence of Electrolysis Conditions. Among the various electrolysis conditions, brine purity has the most significant effect on the life of the membranes. The presence of a small amount of multivalent cations leads to formation of metal hydroxide deposits in the membrane, and thus causes a decrease in current efficiency, an increase in cell voltage, and damage to the polymer structure of the membrane. With perfluorocarboxylic acid membrane, the presence of more than 1 ppm of calcium ion will begin to cause these problems in a very short period (1 - 8). To obtain stable current efficiency and cell voltage, it is therefore essential to establish effective brine purification methods. 

Perfluorocarboxylic acid membrane is prepared by hydrolyzing the film of copolymer of PVEX monomer with carboxylic acid group or its precursor and TFE by conventional method. 

The typical perfluorocarboxylic acid membrane developed by Asahi Chemical is a multilayer membrane prepared by chemical treatment. The structure of the membrane is optimized for high current efficiency and low electric resistance. The thickness of the carboxylic acid layer is in the range of 2 to 10 microns. The chemical structure of the membrane is as follows (72). 

Seko, M., S. Ogawa, K. Kimoto. 1988. Perfluorocarboxylic acid membrane and membrane chlor-alkali process developed by Asaki Chemical Industry. In "Perfluorinated lonomer Membranes". Edts. A. Eisenberg, H.L. Yeager. ASC-Symposium Series 180. American Chemical Society, Washington. 

The water content of a perfluoro sulfonic acid membrane and a perfluorocarboxylic acid membrane illustrated in Fig. 18 clearly... 

In this process, a cation-exchange membrane such as perfluorocarboxylic acid membrane is used to partition the anode from the cathode assembly. The cation-exchange membrane allows only the hydrated positive ions, the sodium and hydronium ions, into the cathode compartment. The membrane also prevents the hydroxide ions generated in the cathode or chlorine ions in the anode from migrating across. 

Possibly one of the most complex deliberately designed of all polymer products is the family of membranes described by Seko for the selective passage of sodium (Na+) ions and repulsion of hydroxide ions in a chlorine cell. These perfluorocarboxylic acid membranes are claimed to represent an improvement on the Dupont Nafion class of perfluorosulphonic membranes designed for the same purpose, in that CF/OH" segregation is practically complete. Industrially, the implications for chlorine and caustic soda technology are profound. 

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