Biaxially oriented microporous membrane
View Patent ↗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 including a simultaneous controlled machine direction relax.
1. A method of making a microporous membrane comprising the steps of:
extruding a semi-crystalline 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 restrained machine direction relax in the range of 5 to 80%, to form substantially round shaped pores having an aspect ratio in the range of 0.75 to 1.25,
wherein the method is a dry stretch process and the polymer excludes any oils for subsequent removal to form pores or any pore-forming particulate to facilitate pore formation.
2. The method of claim 1 wherein the polymer being selected from the group consisting of polyolefins, fluorocarbons, polyamides, polyesters, polyacetals (or polyoxymethylenes), polysulfides, polyvinyl alcohols, co-polymers thereof, and combinations thereof.
3. The method of claim 1 further comprising the step of:
annealing the non-porous precursor after extruding and before biaxially stretching.
4. The method of claim 3 wherein annealing being conducted at a temperature in the range of T m −80° C. to T m −10° C.
5. The method of claim 1 wherein biaxially stretching comprising the steps of:
machine direction stretching, and
thereafter transverse direction stretching including a simultaneous restrained machine direction relax.
6. The method of claim 5 wherein machine direction stretching being conducted either hot or cold or both.
7. The method of claim 6 wherein cold machine direction stretching being conducted at a temperature <T m −50° C.
8. The method of claim 6 wherein hot machine direction stretching being conducted at a temperature <T m −10° C.
9. The method of claim 1 wherein the total machine direction stretch being in the range of 50-500%.
10. The method of claim 1 wherein the total transverse direction stretch being in the range of 100-1200%.
11. The method of claim 1 wherein the machine direction relax being in the range of 15 to 65%.
12. The method of claim 1 wherein the machine direction relax being in the range of 50-75%.
13. The method of claim 1 wherein the polymer being one or more polyolefins.
14. The method of claim 1 wherein the microporous membrane having a ratio of machine direction tensile strength to transverse direction tensile strength in the range of 0.5 to 5.0.
15. A method of making a microporous membrane comprising the steps of:
extruding a semi-crystalline polymer into a nonporous precursor, and the semi-crystalline polymer being one or more polyolefins, 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 restrained machine direction relax in the range of 5 to 80%, to form substantially round shaped pores having an aspect ratio in the range of 0.75 to 1.25,
wherein the method is a dry stretch process and the polymer excludes any oils for subsequent removal to form pores or any pore-forming particulate to facilitate pore formation.