Processes of making a porous membrane material from polyvinylidene fluoride, and products

A continuous process for forming a microporous membrane from a homopolymer of vinylidene 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 16% to about 19% by weight of the solution as a thin layer to a rigidly supported surface of a travelling backing belt from a supply reservoir, while maintaining a substantially constant head on the solution in the supply reservoir as it is applied, to deposit the layer on the belt at a uniform thickness,
• immersing the belt carrying the layer of polymer solution in, and carrying it through a shallow, elongate formation bath that consists essentially of a mixture of acetone and water, with the acetone forming from about 70% to about 80% by volume of the bath, the balance being water,
• maintaining the layer immersed in the formation bath to permit replacement of the acetone in the layer with 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• continuously monitoring the composition of the withdrawn formation bath liquid and adding the necessary amount of water to maintain the liquid at a composition within the specified range of composition for the bath,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 formation bath liquid from the membrane, and then
• drying the membrane to remove water.

In a process for making either a microporous or an asymmetric membrane that has at least 60% of its volume taken up by pores, from a polymer of vinylidene fluoride, by casting a thin layer of a 15% to 25% by weight solution of the polymer on a substrate, then passing the substrate into a formation bath comprising a liquid that is not a solvent for the polymer but that is miscible with the solvent for the polymer to form a porous membrane, removing the solvent and the non-solvent from the porous membrane, and drying, the improvement wherein the casting solution is formed by dissolving the polymer in hot acetone above 50.degree. C., and including the steps of:

• immersing the cast layer in a formation bath consisting essentially of a mixture of acetone and water, with the acetone forming at least a major portion, up to 80% by volume, of the bath, the balance being water,
• maintaining the layer immersed in the formation bath until the layer has been converted to a porous membrane whose pore structure is essentially fully formed,
• separating the porous membrane from its substrate, then
• washing the membrane with water to remove residual liquid from the membrane.