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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

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°C. but that dissolves substantially less of the polymer at about 20°C. (room temperature). The polymer solution is formed at 50°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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A continuous process for forming a microporous membrane from a polymer that is at least 90% by weight of polyvinylidene fluoride, the membrane having at least 60% of its volume taken up by intercommunicating capillary pores of average size below about 14 micrometers, comprising: applying a layer of

A continuous process for forming a microporous membrane from a polymer that is at least 90% by weight of polyvinylidene fluoride, the membrane having at least 60% of its volume taken up by intercommunicating capillary pores of average size below about 14 micrometers, comprising: applying a layer of a solution consisting essentially of the polymer in acetone at a polymer concentration of from about 15% to about 25% by weight of the solution as a thin layer to a rigidly supported surface of a travelling backing belt, immersing the belt carrying the layer of polymer solution in a formation bath consisting essentially of a mixture of acetone and water, with the acetone forming from about 55% to about 85% by volume of the bath, the balance being water, maintaining the layer immersed in and passing it through an elongate path in the formation bath between the point of entry of the belt and layer into the formation bath and the point of departure of the belt and layer from the bath, to permit replacement of the acetone in the layer with the formation bath liquid, until the layer has been converted to a microporous membrane having an average pore size in the range from about 25 nanometers to about 14 micrometers, wherein the pores intercommunicate, form at least 60% by volume of the membrane, and are of substantially uniform size, and the porous structure is essentially fully formed, continuously withdrawing formation bath liquid from the bath adjacent the entry point of the belt into the bath, continuously monitoring the composition of the withdrawn formation bath liquid, and adding the necessary amount of water to maintain the liquid at a composition within the specified range of composition for the bath, so that it is recirculable liquid, returning the recirculable liquid to the formation bath adjacent the departure point of the belt from the bath, separating the formed membrane from the belt, washing the membrane with water, apart from the formation bath, to extract any residual acetone or formation bath liquid from the membrane, and then drying the membrane to remove water. A microporous membrane of polyvinylidene fluoride prepared in accordance with claim 1.