Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
1998-12-04
2002-10-22
Le, Hoa T. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C427S002110, C427S002140, C427S217000, C427S220000, C428S404000, C428S407000, C428S699000, C428S701000, C428S702000, C435S007100, C435S007700, C435S007800, C435S007900, C435S287900, C436S072000, C436S073000, C436S080000, C436S120000, C436S127000, C436S501000, C514S495000, C514S499000, C514S501000, C514S706000, C514S770000
Reexamination Certificate
active
06468657
ABSTRACT:
BACKGROUND OF THE INVENTION
Ion-exchange membranes play an important role in separation and purification processes. These membranes generally consist of either highly swollen gels or microporous structures with fixed charges derived from ionizable functional groups localized at the pore walls (Strathmann, H., In
Synthetic Membranes: Science, Engineering and Applications;
Bungay, P. M.; Lonsdale, H. K.; de Pinho, M. N., Eds.; NATO ASI Series C: Mathematical and Physical Sciences Vol. 181; D. Reidel Publishing Company: Dordrecht, Holland, (1986), pp 1-37). A membrane that contains fixed positive charges is called an anion-exchange membrane, and a membrane bearing fixed negative charges is called a cation-exchange membrane. The general purpose of ion-exchange membranes is not to exchange ions but to transmit them in a controlled way (Meares, P., In
Mass Transfer and Kinetics of Ion Exchange;
Liberti, L.; Helffefich, F. G., Eds.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366). Co-ions (i.e., ions with the same charges as the fixed charges) are excluded from the pores, whereas counterions (i.e., ions with opposite charges to the fixed charges) selectively transport across the membrane. Ion-exchange membranes have been used in the following processes, classified by the driving forces of transport of ions (Meares, P., In
Mass Transfer and Kinetics of Ion Exchange;
Liberti, L.; Helffefich, F. G., Eds.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366): (a) for electrical driving forces, desalination, demineralization, concentration of solutions, exchange of ions, and oxidation-reduction (e.g., chlor-alkali processes); (b) for driving forces of concentration gradient, diffusion dialysis, solid electrolytes in batteries, and ion-selective electrodes, (c) for driving forces of pressure, reverse osmosis and piezodialysis.
The most common functional groups in cation-exchange membranes are sulfonic acid (SO
3
H) and carboxylic acid (—COOH) groups. The Nafion brand perfluorosulfonated polymer membranes (Meares, P., In
Mass Transfer and Kinetics of Ion Exchange;
Liberti, L.; Helffefich, F. G., Ed.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366; Yeager, H. L. et al., In
Perfluorinated Ionomer Membranes;
Yeager, H. L.; Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, D.C., (1982); pp 1-6) are an example of the first type. These membranes were developed by E. I. du Pont de Nemours & CO. during the 1960's and are still extensively used in industry. The perfluoro-sulfonic acid is ionized at normal pH values because of their low pK
a
values (<1) (Seko, M. et al., ibid.; pp 365-410). In the seventies some Japanese companies (Asahi Chemical Co., Asahi Glass Co., and Tokuyama Soda Co.) developed perfluorocarboxylated membranes that contain carboxylic acid groups (Seko, M. et al., ibid.; pp 365-410; Sata, T. et al., ibid.; pp 411-425; Ukihastfi, H. et al., ibid.; pp 427-451; Sato, K. et al.,
PolymerJ.,
23, 1991, 531-540). Although the perfluorocarboxylic acid has a higher pK
a
(2-3), it can be nearly completely ionized even in weakly acidic environments. Other functional groups such as phosphonic acid (—PO
3
H
2
) and sulfonamide (—SO
2
NH
2
) are sometimes used but are less practical.
The functional groups in anion-exchange membranes are usually quaternary ammonium [—N
+
(CH
3
)
3
] and to a lesser extent quaternary phosphonium [—P
+
(CH
3
)
3
] and tertiary sulfonium [—S
+
(CH
3
)
2
]. Anion-exchange membranes are frequently less stable than cation-exchange membranes because the basic groups are inherently less stable than the acidic groups (Strathmann, H. In
Synthetic Membranes: Science, Engineering and Applications;
Bungay, P. M.; Lonsdale, H. K.; de Pinho, M. N., Eds.; NATO ASI Series C: Mathematical and Physical Sciences Vol. 181; D. Reidel Publishing Company: Dordrecht, Holland, (1986); pp 1-37).
Conventional ion-exchange membranes are generally produced by: (a) polymerization (condensation or addition) of ionogenic monomers; (b) introduction of ionizable groups into a polymer film by grafting and/or chemical treatment; or (c) heterogeneously dispersing an ion exchange material into a binder polymer matrix.
The Nafion perfluorosulfonated membranes are copolymers of tetrafluoroethylene and perfluorovinyl ethers containing sulfonic acid groups (Yeager, H. L. et al., In
Perfluorinated Ionomer Membranes;
Yeager, H. L.; Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, (1982); pp 1-6). Synthesis of perfluorocarboxylated membranes is similar, but the sulfonic acid groups in perfluorovinyl ethers are replaced by carboxylic acid groups (Seko, M. et al., ibid.; pp 365-410; Sata, T. et al., ibid.; pp 411-425; Ukihastfi, H. et al., ibid.; pp 427451; Sato, K. et al.,
PolymerJ.,
23, 1991, 531-540).
Perfluorinated ionomer (i.e., ion-containing polymer) membranes constitute a significant portion of ion-exchange membranes and are extensively used in the chlor-alkali industry (Dotson, R. et al., In
Perfluorinated Ionomer Membranes;
Yeager, H. L. Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, D.C., 1982, pp 311-364). Composite membranes are sometimes utilized to improve the separation performance. An exemplary composite membrane has layers of both sulfonic acid and carboxylic acid polymers bound together to improve permselectivity (Sato, K. et al.,
PolymerJ.,
23, 1991, 531-540). Composite layers having a layer of polyanion adsorbed onto a cation-exchange membrane have been prepared. Such membranes could prevent the precipitation of hydroxides and simplify the control of membrane fouling (Meares, P., In
Mass Transfer and Kinetics of Ion Exchange;
Liberti, L.; Helffefich, F. G., Eds,; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366). Recent development in synthesis of ion-exchange membranes focus on new polymerization processes, such as radiation induced grafting (Chakravorty, B. et al.,
Membr. Sci.
1989, 41, 155-161; Gineste, J.-L. et al.,
Polym. Sci.:
Part A:
Polym. Chem.
1993, 31, 2969-2975) and copolymerization, (,G. K. et al.,
Membr. Sci.
1992, 68, 133-140) plasma polymerization of ionomer films (Brumlik, C. J. et al.,
Electrochem. Soc.
1994, 141, 2273-2279), deposition of thin polymer film by plasma polymerization (Osada, Y. In
Membrane Science and Technology;
Osada, Y.; Nakagawa, T., Eds.; Marcel Dekker, Inc.: New York, 1992; pp 167-201), and grafting of functional groups on polymers treated by ozonization Elmidaoui, A. et al.,
Appl. Polym. Sci.
1991, 42, 2551-2561).
In conventional ion-exchange membranes prepared by polymer chemistry, ion transport operates in a gel phase formed by sorption of water and swelling of the membrane due to the hydrophilic functional groups on the polymer backbone (Leddy, J. J. In
Synthetic Membranes;
Chenowetb, M. B., Ed.; MMI Press Symposium Series; Harwood Academic Publishers: London, 1986; pp 119-128). The size of pores, however, is difficult to control and there can be undesired transport of water and co-ions across the membrane, leading to poor perm-selectivity. Perfluorocarboxylated membranes are believed to have higher permselectivity than the Nafion membranes due to less uptake of water by the carboxylic groups than by the sulfonic groups (Sato, K. et al.,
PolymerJ.,
23, 1991, 531-540).
Ion-exchange membranes with a porous structure were recently prepared by several techniques to offer membranes with both suitable pore sizes and good ion exchange capacity. These techniques include oxidative etching of gel-like ion-exchange membranes (Mizutani, Y. et al.,
J. Appl. Polym. Sci.
1990, 39, 1087-1100), chemical modification of preformed ultrafiltration membranes (Breitbach, L. et al.,
Angew. Ma
Abbott Nicholas
Hou Zhizhong
Stroeve Pieter
Le Hoa T.
The Regents of the University of California
Townsend and Townsend / and Crew LLP
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