Biaxially oriented microporous membrane
A microporous membrane is made by a dry-stretch process and has substantially round shaped pores and a ratio of machine direction tensile strength to transverse direction tensile strength in the range of 0.5 to 5.0. The method of making the foregoing microporous membrane includes the steps of: extruding a polymer into a nonporous precursor, and biaxially stretching the nonporous precursor, the biaxial stretching including a machine direction stretching and a transverse direction stretching, the transverse direction stretching including a simultaneous controlled machine direction relax.
1. A method of making a microporous membrane comprising the steps of:
extruding a polymer into a nonporous precursor, and
biaxially stretching the nonporous precursor, the biaxial stretching including a machine direction stretching and a transverse direction stretching, the transverse direction stretching including a simultaneous controlled machine direction relax in the range of 20 to 75%;
wherein the total transverse direction stretch being in the range of 400 to 1200%.
2. The method of claim 1 wherein the polymer excludes any oils for subsequent removal to form pores or any pore-forming particulate to facilitate pore formation.
3. The method of claim 1 wherein the polymer being a semi-crystalline polymer.
4. The method of claim 1 wherein the polymer being selected from the group consisting of polyolefins, fluorocarbons, polyamides, polyesters, polyacetals, polyoxymethylenes, polysulfides, polyvinyl alcohols, co-polymers thereof, and combinations thereof.
5. The method of claim 1 further comprising the step of:
annealing the nonporous precursor after extruding and before
biaxially stretching.
6. The method of claim 5 wherein annealing being conducted at a temperature in the range of Tm-80° C. to Tm-10° C.
7. The method of claim 1 wherein biaxially stretching comprising the steps of:
machine direction stretching, and
thereafter transverse direction stretching including a simultaneous controlled machine direction relax.
8. The method of claim 7 wherein machine direction stretching being conducted hot or cold or both.
9. The method of claim 8 wherein cold machine direction stretching being conducted at a temperature <Tm-50° C.
10. The method of claim 8 wherein hot machine direction stretching being conducted at a temperature <Tm-10° C.
11. The method of claim 1 wherein the total machine direction stretch being in the range of 50-500%.
12. The method of claim 1 wherein the total transverse direction stretch being in the range of 100-1200%.
13. The method of claim 1 wherein the machine direction relax being in the range of 30-80%.