Multi-layer polyolefin porous membrane, battery separator using same, and process for producing multi-layer polyolefin porous membrane

1. A multi-layer polyolefin porous membrane comprising:

at least two layers,

wherein the multi-layer polyolefin porous membrane has a shut-down temperature in a range of 128 to 135° C., a rate of increase in air permeation resistance from 30° C. to 105° C. scaled to 20 μm thickness less than 1.5 sec/100 cc Air/° C., and a thickness not more than 20 μm; and

a plurality of protrusions of polyolefin having a size W within a range of 5 μm≦W≦50 μm and a height H within a range of 0.5 μm≦H, and wherein the protrusions are randomly disposed on one side of the multi-layer polyolefin porous membrane in a density not less than 3/cm 2 and not more than 200/cm 2 .

2. The multi-layer polyolefin porous membrane according to claim 1 , wherein the air permeation resistance scaled to 20 μm thickness is in a range of 50 to 300 sec/100 cc Air.

3. The multi-layer polyolefin porous membrane according to claim 1 , wherein a maximum shrinkage of the multi-layer polyolefin porous membrane in a transverse direction is less than 5%.

4. The multi-layer polyolefin porous membrane according to claim 1 , wherein at least one of the at least two layers comprises a resin with a melt flow rate in a range of 25 to 150 g/10 min and a melting point not less than 120° C. and less than 130° C.

5. A battery separator, comprising:

a multi-layer polyolefin porous membrane, comprising:

at least two layers, and

a plurality of protrusions on a side, wherein the protrusions have a size W within a range of 5 μm≦W≦50 μm and a height H within a range of 0.5 μm≦H, and wherein the protrusions are randomly disposed on one side of the multi-layer polyolefin porous membrane in a density not less than 3/cm 2 and not more than 200/cm 2 ,

wherein the multi-layer polyolefin porous membrane has a shut-down temperature in a range of 128 to 135° C., a rate of increase in air permeation resistance from 30° C. to 105° C. scaled to 20 μm thickness less than 1.5 sec/100 cc Air/° C., and a thickness not more than 20 μm; and

a modified porous layer laminated on at least on the side of the multi-layer polyolefin porous membrane having the protrusions.

6. A method of manufacturing a multi-layer polyolefin porous membrane, the method comprising:

adding a forming solvent to polyolefin resin to form a polyolefin resin solution;

melting and kneading the polyolefin resin solution;

extruding the melted and kneaded polyolefin solution through a die;

chilling at least a portion of the extruded polyolefin solutions to form a multi-layer gel-like product with a chill-roll having a surface, wherein, during the chilling, the forming solvent on the chill-roll is removed;

stretching the gel-like product in MD (the machine direction) and in TD (the transverse direction) to obtain a stretched and molded material;

extracting the stretched and molded material;

removing the forming solvent from the stretched and molded material;

drying the stretched and molded material to obtain a multi-layer porous molded material; and

treating the multi-layer porous molded material with heat to obtain a multi-layer polyolefin porous membrane, wherein the multi-layer polyolefin porous membrane comprises at least two layers, and has a shut-down temperature in a range of 128 to 135° C., a rate of increase in air permeation resistance from 30° C. to 105° C. scaled to 20 μm thickness less than 1.5 sec/100 cc Air/° C., and a thickness not more than 20 μm.

7. The method of manufacturing according to claim 6 , wherein the forming solvent is removed from the chill-roll with a doctor blade.