Porous Membrane

Abstract

A membrane comprises a sheet of expanded polytetrafluoroethylene. The sheet includes a first extrudate made from a first PTFE fine powder resin mixed with processing lubricant at a first lube rate. A second extrudate is made from the first PTFE fine powder resin mixed with processing lubricant at the first lube rate. The first and second extrudates combined into an integrated tape structure that is stretched in first and second directions. The sheet is porous and has a gas permeability of at least 0.1 CFM according to ASTM D737. The sheet has an average Mullen Hydrostatic Burst pressure in the range of 135 psi to 175 psi according to ASTM D751.

What is claimed is: 
     
         1 - 11 . (canceled) 
     
     
         12 . A method of making a membrane comprising the steps of:
 providing a first extrudate made from a first PTFE fine powder resin mixed with processing lubricant at a first lube rate;   providing a second extrudate made from the first PTFE fine powder resin mixed with processing lubricant at the first lube rate;   overlying the first and second extrudates;   calendering the overlying first and second extrudates into an integrated tape;   stretching the integrated tape in the machine direction; and   stretching the integrated tape in the transverse direction to provide a porous sheet that has a gas permeability of at least 0.1 CFM according to ASTM D737 and an average Mullen Hydrostatic Burst pressure in the range of 135 psi to 175 psi according to ASTM D751.   
     
     
         13 . The method of  claim 12  wherein the calendering step includes the step of combining the first and second extrudates into the integrated tape structure by a mechanically interlocking nodes and fibrils making up the first and second extrudates. 
     
     
         14 . The method of  claim 12  wherein the first lube rate is in the range of 15 wt-% to 17 wt-% of lubricant to PTFE fine powder resin. 
     
     
         15 . The method of  claim 12  wherein the first lube rate is in the range of 14 wt-% to 22 wt-% of lubricant to PTFE fine powder resin. 
     
     
         16 . The method of  claim 12 , wherein the steps of stretching the integrated tape in the machine direction and stretching the integrated tape in the transverse direction further provide a pore size in the range of 0.01 micron to 1.0 micron. 
     
     
         17 . The method of  claim 12 , wherein the steps of stretching the integrated tape in the machine direction and stretching the integrated tape in the transverse direction further provide a pore size in the range of 0.01 micron to 0.25 micron. 
     
     
         18 . The method of  claim 12 , wherein the steps of stretching the integrated tape in the machine direction and stretching the integrated tape in the transverse direction further provide an average Mullen Hydrostatic Burst pressure in the range of 150 psi to 175 psi according to ASTM D751. 
     
     
         19 . A method of forming a PTFE membrane comprising the steps of:
 providing a first PTFE extrudate;   providing a second PTFE extrudate;   overlying the first PTFE extrudate and the second PTFE extrudate;   calendering the overlying first PTFE extrudate and second PTFE extrudate into an integrated tape, thereby mechanically interlocking nodes and fibrils of the first PTFE extrudate to nodes and fibrils of the second PTFE extrudate;   biaxially stretching the to integrated tape provide a microporous PTFE membrane.   
     
     
         20 . The method of  claim 19 , wherein the step of biaxially stretching the integrated tape provides the PTFE membrane with a pore size in the range of 0.01 micron to 1.0 micron. 
     
     
         21 . The method of  claim 19 , wherein the step of biaxially stretching the integrated tape provides the PTFE membrane with a pore size in the range of 0.01 micron to 0.25 micron. 
     
     
         22 . The method of  claim 19 , wherein the step of biaxially stretching the integrated tape provides the PTFE membrane with an average Mullen Hydrostatic Burst pressure in the range of 135 psi to 175 psi according to ASTM D751. 
     
     
         23 . The method of  claim 19 , wherein the step of biaxially stretching the integrated tape provides the PTFE membrane with an average Mullen Hydrostatic Burst pressure in the range of 150 psi to 175 psi according to ASTM D751. 
     
     
         24 . The method of  claim 19 , wherein the first PTFE extrudate and the second PTFE extrudate are substantially identical in thickness. 
     
     
         25 . The method of  claim 19 , wherein the first PTFE extrudate and the second PTFE extrudate are between about 0.035 inches and 0.045 inches in thickness. 
     
     
         26 . The method of  claim 19 , wherein the first PTFE extrudate and the second PTFE extrudate each include a lubricant and are substantially identical in lube rate. 
     
     
         27 . The method of  claim 19 , wherein the first PTFE extrudate and the second PTFE extrudate each include a lubricant between about 14 wt-% and 22 wt-%. 
     
     
         28 . The method of  claim 19 , further comprising the step of sintering the PTFE membrane after the step of biaxially stretching.