Multilayer Porous Membrane and Process of Manufacture

Abstract

A multilayer porous membrane formed of an extrudable polymer is provided. The membrane is formed by co-extruding at least two compositions, each of which comprises a heated porogen and polymer while in contact with each other under conditions to minimize or prevent unstable interfacial flow between extruded layers. In a preferred embodiment, the two compositions are different such that the layers have a different average pore size.

What is claimed is: 
     
         1 . A method comprising:
 coextruding two or more polymeric compositions to forth an integral sheet, each of the polymeric compositions comprising a polymer and porogen, wherein the polymer does not form an extrudable polymeric composition with the porogen at about room temperature;   phase separating the polymer and porogen in the sheet to form a polymeric gel sheet having two or more regions with differing pore size; and   removing the porogen from each region of the gel sheet to form a microporous polymeric membrane having two or more regions with differing pore size.   
     
     
         2 . The method of  claim 1  further comprising restraining the polymeric gel sheet as the porogen is removed. 
     
     
         3 . A method for producing a multilayer microporous polymeric membrane, comprising the steps of:
 forming at least two mixtures, each mixture comprising a polymer composition and a porogen composition;   separately heating each of said mixtures to an elevated temperature to produce a corresponding solution for each of said mixtures;   co-extruding each said solution in contact with at least another of said solutions through a forming dire under moderate shear to form a multilayer sheet, wherein adjacent layers correspond to said contacted solutions;   cooling the multilayer sheet sufficiently to cause phase separation resulting in a polymer-rich, porogen-poor phase and a polymer-poor, porogen-rich phase in each layer of said multilayer sheet;   creating a microporous structure in the multilayer sheet by selectively removing said porogen composition from said layers of said multilayer sheet to yield a microporous multilayer membrane having two or more regions or layers of different porosity; and   drying the multilayer membrane.   
     
     
         4 . A method of  claim 3 , wherein each of said polymer compositions comprise a polymer selected from the group consisting of polyethylene, ultrahigh molecular weight polyethylene (UHMW-PE); polypropylene; poly (tetrafluoroethylene-co-perfiuoroalkylvinyl ether) wherein alkyl is propyl, methyl or mixtures thereof, (PFA); poly(tetrafiuoroethylene-co-hexafluoropropylene) (FEP); and mixtures thereof. 
     
     
         5 . A method of  claim 3 , wherein each of said porogen compositions comprises a porogen selected from the group consisting of hydrocarbons, decane and higher homologues, mixed hydrocarbons, mineral oil, mineral oi-dibutyl sebacate mixtures, paraffin wax, di(2-ethylehexyl) phthalate, di(2-ethylehexyl) adipate, dibutylphathalate, dibutylsebacate, tetralin, n-decanol, 1-dodecanol, and diphenylmethane, and mixtures thereof. 
     
     
         6 . A method of  claim 3  wherein said porogen compositions comprises a mixture of mineral oil and dibutyl sebacate. 
     
     
         7 . A method of  claim 3 , wherein said porogen comprises mineral oil. 
     
     
         8 . A method of  claim 3 , wherein each said mixture comprises from about 2% to about 25% polymer composition by weight of said mixture. 
     
     
         9 . A method of  claim 3 , wherein each said mixture comprises about 10% to about 20% polymer composition by weight of said mixture. 
     
     
         10 . A method of  claim 3 , wherein polymer content of mixtures producing adjacent layers differs by about 0 to about 10% by weight. 
     
     
         11 . A method of  claim 3 , wherein polymer content of mixtures producing adjacent layers differs by about 0 to about 8% by weight. 
     
     
         12 . A method of  claim 3 , wherein the elevated temperatures employed to produce a solution from each mixture, differs by about 0 to about 100° C. in solutions producing adjacent layers. 
     
     
         13 . A method of  claim 3 , wherein the elevated temperatures employed to produce a solution from each mixture, differs by about 0 to about 50° C. in solutions producing adjacent layers. 
     
     
         14 . A method of  claim 3 , wherein the ratio of extrusion flow rates of solutions producing adjacent layers is about 10:1 to about 1:10. 
     
     
         15 . A method  claim 3 , wherein the ratio of extrusion flow rates of solutions producing adjacent layers is about 4:1 to about 1:4. 
     
     
         16 . A method of  claim 3 , wherein the membrane is cooled by bringing it into contact with a solid surface. 
     
     
         17 . A method of  claim 3 , wherein porogen is removed by extraction. 
     
     
         18 . A method of  claim 17 , wherein the membrane is restrained during extraction. 
     
     
         19 . A method of  claim 3 , wherein the membrane is restrained during drying. 
     
     
         20 . A method of  claim 3 , wherein said multilayer membrane has two layers. 
     
     
         21 . A method of  claim 3 , wherein said multilayer membrane has three layers. 
     
     
         22 . A method of  claim 3 , wherein said multilayer membrane has 4 or more layers. 
     
     
         23 . A method for producing an integral dual layer microporous polymeric membrane, including the steps of:
 forming at a first mixture comprising a polymer composition and a porogen composition, and a second mixture comprising a polymer composition and a porogen composition, wherein each said mixture can be the same or different;   heating said first mixture and said second mixture, separately and independently, to an elevated temperature to produce a first solution and a second solution, respectively;   co-extruding the first solution and the second solution in contact with each other through a forming die under moderate shear to faun a dual layer sheet, wherein a first layer corresponds to the first solution and a second layer corresponds to the second solution;   cooling the dual layer sheet sufficiently to cause phase separation resulting in a polymer-rich, porogen-poor phase and a polymer-poor, porogen-rich phase in each layer of said dual layer sheet;   creating a microporous structure in the dual layer sheet by selectively removing said porogen composition from said dual layer sheet to yield a dual layer microporous membrane; and   drying the dual layer membrane.   
     
     
         24 . A method of  claim 23 , wherein each of said polymer compositions comprises ultrahigh molecular weight polyethylene (UHMW-PE); and each of said porogen compositions comprises mineral oil, dibutyl sebacate, or a mineral oil-dibutyl sebacate mixture. 
     
     
         25 . A method of  claim 23 , wherein each of said polymer compositions comprises ultrahigh molecular weight polyethylene (UHMW-PE); and each of said porogen compositions comprises a mineral oil-dibutyl sebacate mixture.