Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-10-13
2001-06-26
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S200000, C162S164100, C162S164200, C526S242000, C526S243000, C526S249000, C427S154000
Reexamination Certificate
active
06252000
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to blends of highly fluorinated ion exchange polymers and highly fluorinated nonfunctional polymer which are useful in the manufacture of membranes for electrochemical cells.
BACKGROUND OF THE INVENTION
Highly fluorinated ion exchange polymers are well known for use in making membrane separators for electrochemical cells. Such cells typically employ a membrane of ion exchange polymer which serves as a physical separator between the anode and cathode while also serving as an electrolyte. Cells with membrane separators can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells for the production of electrical energy.
Highly fluorinated, and particularly perfluorinated, ion exchange polymer membranes are well suited for use in electrochemical cells due to excellent chemical resistance, long life, and high conductivity. However, membranes of such polymers do not have a high tensile strength. Moreover, the membranes during use must have a certain water content to be conductive and, in some applications, are in contact with aqueous solutions. The membranes typically swell under such conditions which further decreases tensile strength. Accordingly, for some applications, such as chloralkali electrolytic cells, it is necessary for the membranes to be reinforced with fabrics to provide the required strength. For other applications such as fuel cells, unreinforced polymer is adequate to provide the required strength when the membranes have a thickness of at least about 50 &mgr;m. However, increased strength is desired, especially to facilitate handling during manufacture and in end use processing operations. Membranes with increased tensile strength but without woven reinforcements are desired, particularly very thin, high performance membranes.
Highly fluorinated nonfunctional polymers are particularly useful for applications which experience weathering, harsh chemicals or other severe conditions. However, since they also have a very low surface energy, it is difficult to get such polymers to adhere to substrates. Highly fluorinated polymers with improved adhesion properties are desired for some applications.
SUMMARY OF THE INVENTION
The invention provides a polymeric composition comprising a blend of highly fluorinated ion exchange polymer and highly fluorinated nonfunctional polymer, the blend having interpenetrating continuous phases of the highly fluorinated nonfunctional polymer and the highly fluorinated ion exchange polymer. Preferably, the highly fluorinated nonfunctional polymer has a glass transition temperature of at least about 80° C. It is also preferable for the nonfunctional polymer to be perfluorinated. Most preferred nonfunctional polymers are selected from the group consisting of amorphous polymers of perfluoro-2,2-dimethyl-1,3,-dioxole.
In another preferred form of the invention, the highly fluorinated nonfunctional polymer comprises at least about 10 weight % of the composition, more preferably, at least about 15 weight % of the composition and, most preferably, about 20 weight % to about 40 weight % of the composition.
In a preferred composition, the highly fluorinated ion exchange polymer is selected from the group consisting of ion exchange polymers with sulfonate, carboxylate, phosphonate, imide, sulfonimide and sulfonamide functional groups, most preferably, the highly fluorinated ion exchange polymer is perfluorinated and has sulfonate ion exchange groups.
It is preferred for the ion exchange polymer of the composition to have an ion exchange ratio (IXR) of about 9 to about 14, most preferably, about 10 to about 13.
The invention is advantageously employed in a membrane made of the composition. Such membranes have a higher tensile strength than membranes of the ion exchange polymer alone. The invention is especially useful for very thin membranes, preferably those with a thickness of less than about 50 &mgr;m.
The invention also provides a liquid composition comprising a highly fluorinated solvent, a highly fluorinated ion exchange polymer precursor and a highly fluorinated nonfunctional polymer with the polymers being miscible with each other in the solvent. Preferably, the highly fluorinated ion exchange polymer precursor has sulfonyl fluoride groups.
The invention also provides a method for making an ion change membrane comprising:
forming a liquid composition of a highly fluorinated solvent, a highly fluorinated ion exchange polymer precursor and a highly fluorinated nonfunctional polymer, the polymers being miscible with each other in the solvent;
depositing the liquid composition on a substrate;
removing the highly fluorinated solvent from the composition to form a membrane; and
removing the membrane from the substrate.
Preferably, in the method of the invention, the ion exchange polymer precursor has sulfonyl fluoride groups and the method includes hydrolyzing the polymer to convert the sulfonyl fluoride groups to sulfonate groups.
The compositions are also useful for protective and other coating applications where the properties of highly fluorinated polymers are desired but with improved adhesion to substrates.
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E. I. Du Pont de Nemours and Company
Nutter Nathan M.
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