High-strength separator

1 . A method for producing a polyolefin microporous membrane separator for a lithium ion secondary battery, wherein the method comprises the following steps:

(1) a step of using a continuous mixer under conditions with a temperature of 20° C. to 70° C., a shear rate of 100 seconds −1 to 400,000 seconds −1 and a residence time of 1.0 seconds to 60 seconds, for mixing of a plasticizer with a polyolefin powder containing polyethylene (PE), to obtain a mixed slurry;

(2) a step of loading the mixed slurry into a twin-screw extruder and extruding the mixed slurry to produce a resin composition;

(3) a step of extruding the resin composition into a sheet and cooling the sheet to solidification to process the solidification into a cast sheet;

(4) a step of biaxially stretching the cast sheet in an area increase factor of 20 times to 200 times, to form a stretched sheet;

(5) a step of extracting the plasticizer from the stretched sheet to form a porous body; and

(6) a step of heat treating the porous body at a temperature below the melting point of the porous body, and stretching the porous body,

wherein the mixed slurry consists of the polyolefin powder and the plasticizer, and one or more selected from a group consisting of a dehydrating condensation catalyst, a metal soap, an ultraviolet absorbers, a light stabilizer, an antistatic agent, an anti-fogging agent, a color pigment, and an antioxidant, and

wherein a weight ratio of the polyolefin powder in the mixed slurry is greater than 0 weight % and is 50 weight % or lower based on the weight of the mixed slurry.

2 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein swelling, melting and/or kneading of the polyolefin powder is carried out in the twin-screw extruder in step ( 2 ).

3 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the polyethylene is ultrahigh molecular weight polyethylene (UHMWPE), and the content of UHMWPE in the polyolefin powder is 2 weight % to 90 weight % based on the weight of the polyolefin powder.

4 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the polyethylene is ultrahigh molecular weight polyethylene (UHMWPE), and the UHMWPE has a viscosity-average molecular weight (Mv) of 300,000 to 9,700,000 and a molecular weight distribution (Mw/Mn), represented as the ratio of the weight-average molecular weight (Mw) with respect to the number-average molecular weight (Mn), of 3 to 15.

5 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the mixed slurry is fed to the twin-screw extruder at a temperature of 25° C. to 80° C. in step (2) to produce the resin composition.

6 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the porous body is stretched in the transverse direction and/or the lengthwise direction in step (6).

7 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the degree of crystallinity of the polyethylene of the polyolefin microporous membrane is 80 to 99% as measured by X-ray diffraction (XRD), and the crystallite size of the polyethylene of the polyolefin microporous membrane is 14.2 to 40.0 nm as measured by XRD, and wherein the cross-sectional crystal orientation of the polyolefin microporous membrane is 0.80 to 0.99.

8 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the equivalent mean pore size calculated according to Porod's law is 50 nm to 150 nm, for small angle X-ray scattering (SAXS) measurement of the polyolefin microporous membrane.

9 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the polyethylene is ultrahigh molecular weight polyethylene (UHMWPE), and the content of UHMWPE in the polyolefin powder is 2 weight % to 90 weight % based on the weight of the polyolefin powder.

10 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the polyethylene is ultrahigh molecular weight polyethylene (UHMWPE), and the UHMWPE has a viscosity-average molecular weight (Mv) of 300,000 to 9,700,000 and a molecular weight distribution (Mw/Mn), represented as the ratio of the weight-average molecular weight (Mw) with respect to the number-average molecular weight (Mn), of 3 to 15.

11 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the mixed slurry is fed to the twin-screw extruder at a temperature of 25° C. to 80° C. in step (2) to produce the resin composition.

12 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the porous body is stretched in the transverse direction and/or the lengthwise direction in step (6).

13 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the degree of crystallinity of the polyethylene of the polyolefin microporous membrane is 80 to 99% as measured by X-ray diffraction (XRD), and the crystallite size of the polyethylene of the polyolefin microporous membrane is 14.2 to 40.0 nm as measured by XRD, and wherein the cross-sectional crystal orientation of the polyolefin microporous membrane is 0.80 to 0.99.

14 . The method for producing a polyolefin microporous membrane separator according to claim 2 , wherein the equivalent mean pore size calculated according to Porod's law is 50 nm to 150 nm, for small angle X-ray scattering (SAXS) measurement of the polyolefin microporous membrane.

15 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the weight ratio of the polyolefin powder in the mixed slurry is greater than 0 weight % and is 40 weight % or lower based on the weight of the mixed slurry.

16 . The method for producing a polyolefin microporous membrane separator according to claim 1 , wherein the weight ratio of the polyolefin powder in the mixed slurry is greater than 0 weight % and is 30 weight % or lower based on the weight of the mixed slurry.

17 . A method for producing a polyolefin microporous membrane separator for a lithium ion secondary battery, wherein the method comprises the following steps:

(1) a step of using a continuous mixer under conditions with a temperature of 20° C. to 70° C., a shear rate of 100 seconds −1 to 400,000 seconds −1 and a residence time of 1.0 seconds to 60 seconds, for mixing of a plasticizer with a polyolefin powder containing polyethylene (PE), to obtain a mixed slurry;

(2) a step of loading the mixed slurry into a twin-screw extruder and extruding the mixed slurry to produce a resin composition;

(3) a step of extruding the resin composition into a sheet and cooling the sheet to solidification to process the solidification into a cast sheet;

(4) a step of biaxially stretching the cast sheet in an area increase factor of 20 times to 200 times, to form a stretched sheet;

(5) a step of extracting the plasticizer from the stretched sheet to form a porous body; and

(6) a step of heat treating the porous body at a temperature below the melting point of the porous body, and stretching the porous body,

wherein the mixed slurry consists essentially of the polyolefin powder and the plasticizer, and

wherein a weight ratio of the polyolefin powder in the mixed slurry is greater than 0 weight % and is 50 weight % or lower.

18 . A method for producing a polyolefin microporous membrane separator for a lithium ion secondary battery, wherein the method comprises the following steps:

(1) a step of using a continuous mixer under conditions with a temperature of 20° C. to 70° C., a shear rate of 100 seconds −1 to 400,000 seconds −1 and a residence time of 1.0 seconds to 60 seconds, for mixing of a plasticizer with a polyolefin powder containing polyethylene (PE), to obtain a mixed slurry;

(2) a step of loading the mixed slurry into a twin-screw extruder and extruding the mixed slurry to produce a resin composition;

(3) a step of extruding the resin composition into a sheet and cooling the sheet to solidification to process the solidification into a cast sheet;

(4) a step of biaxially stretching the cast sheet in an area increase factor of 20 times to 200 times, to form a stretched sheet;

(5) a step of extracting the plasticizer from the stretched sheet to form a porous body; and

(6) a step of heat treating the porous body at a temperature below the melting point of the porous body, and stretching the porous body,

wherein the mixed slurry consists essentially of the polyolefin powder and the plasticizer, and one or more selected from a group consisting of a dehydrating condensation catalyst, a metal soap, an ultraviolet absorbers, a light stabilizer, an antistatic agent, an anti-fogging agent, a color pigment, and an antioxidant, and

wherein a weight ratio of the polyolefin powder in the mixed slurry is greater than 0 weight % and is 50 weight % or lower.