Process for producing reinforced microporous membrane

Claim 1

A process for producing an integral single layer polymeric microporous filter membrane within which a reinforcing web is embedded, comprising: (a) preparing a first and second reservoir of a casting solution of a polymer in a solvent system comprising a mixture of at least one solvent and one non-solvent for the polymer, the amount of non-solvent being no greater than an amount required to induce nucleation of the solution to obtain a visible precipitate; (b) passing a reinforcing web through the first reservoir, completely surrounding the web with casting solution and depositing a sufficient amount of the solution onto the web to completely coat the web; (c) directly passing the coated web from step (b) through a calendering means to simultaneously calender both sides of the coated web under conditions sufficient to provide for the penetration of the solution into the web and to remove substantially all entrapped air from the web; and (d) passing the calendered reinforcing web from step (c) through the second reservoir, completely surrounding the web with casting solution and depositing a sufficient amount of the solution onto the web to again completely coat the web; and (e) passing the coated web from step (d) through a quenching bath comprising a non-solvent system for the polymer for a time sufficient to form micropores throughout the coatings to thereby form a single layer microporous filter membrane within which a reinforcing web is embedded.