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Abstract

Covers processes for making microporous membranes and molecular filtration membranes from vinylidene fluoride polymers, and the products so produced. A continuous version of the process consists of applying a layer of a solution of a polyvinylidene fluoride polymer to a rigidly supported surface of a backing belt to form a film, then passing the belt through a formation bath, in which the membrane is formed. To form the polymer solution, the polymer is dissolved in a liquid vehicle that will dissolve up to about 25% by weight of the polymer at a temperature of at least 50.degree. C. but that dissolves substantially less of the polymer at about 20.degree. C. (room temperature). The polymer solution is formed at 50.degree. C. or higher, and the formation bath is maintained at about room temperature. The formation bath is a mixture of the liquid vehicle solvent with a second liquid that is miscible with the solvent but that is not itself a solvent for the polymer. The film is maintained immersed in the formation bath until it has been converted to a porous membrane whose pore structure is essentially fully formed, by leaching of the solvent from the film and its replacement with the non-solvent liquid. Thereafter the residual solvent is extracted from the porous membrane, and after separation of the membrane from the backing belt, the membrane is dried. The preferred solvent - non-solvent system for the formation bath is acetone-water. A preferred formation bath composition is from about 70% to about 80% acetone by volume.

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