KR20120113647A - Porous separator and its preparing method - Google Patents

Abstract

PURPOSE: A porous separation membrane with same pore characteristics in both skin layers and a manufacturing method thereof are provided to enhance electrical performance and mechanical strength. CONSTITUTION: A porous separation membrane with same pore characteristics in both skin layers comprises the following steps: manufacturing a raw material resin mixture by mixing polyethylene resin having the average molecular weight of 500,000 or less, solid paraffin-based oil having the average molecular weight of 3000-5000, and liquid paraffin based oil having the average molecular weight of 900-1500; extruding and cooling the raw material resin compound; drawing the extruded raw material resin compound; and extracting the solid paraffin oil and liquid paraffin oil by depositing the raw material resin compound into an organic solvent.

Description

Porous separator and its preparing method

The present invention relates to a porous membrane and a method for manufacturing the same, and more specifically, it is made of polyethylene resin having a weight average molecular weight of less than 500,000, micropores are oriented in a multilayer structure, and both surface layers (core layers) Compared to the skin layer, pores are relatively large in size, and both surface layers have the same pore characteristics, and thus the present invention relates to a porous membrane and a method of manufacturing the same having high mechanical strength and excellent electrical performance.

Recently, with the rapid development of portable electronic devices related industries such as smart phones, the demand for lithium ion batteries and lithium polymer batteries, which are representative secondary batteries, is greatly increased. In particular, with the high oil price era, electric vehicles such as hybrid cars and plug-in cars are becoming practical, and the demand for lithium secondary batteries is expected to explode in the future. In accordance with such industrial demand, weight reduction, miniaturization and high capacity of lithium secondary batteries are required as new technical challenges.

The separator is a main component that determines the performance of the secondary battery, and is inserted between the positive electrode and the negative electrode to prevent a short circuit phenomenon in which the negative electrode and the positive electrode contact each other. In addition, the separator has a myriad of micro pores (micro pores) are formed, the ion material is moved between the positive electrode and the negative electrode through the pore to repeat the discharge and charging.

The separator mainly consists of a polyolefin resin having excellent chemical stability and electrical properties, and the performance of the separator, that is, mechanical strength and electrical performance, varies depending on the size, distribution ratio, and orientation structure of the pores serving as a passage for the ionic material. Therefore, various techniques have been developed for the pore structure of the separator.

For example, Korean Patent No. 373204 (registration date; February 10, 2003) has an active layer on both surfaces and a support layer inside thereof, and is composed of 20,000 seconds. A multicomponent composite membrane for polymer electrolytes having an air permeability of less than / 100 cc is introduced.

In Korea Patent No. 577371 (the date of registration, May 01, 2006), polyolefin resin is used as a main material, and an amorphous layer having a predetermined thickness is formed on both surfaces, and a crystal layer is formed inside. ), Wherein the pore size of the amorphous layer is 1 or less, the pore size of the internal crystal layer is 5 or less, and the total porosity is 50% or more.

In addition, Korean Patent No. 776029 (Registration date November 06, 2007) has a distribution ratio of 80 to 2% pores with a size of 80 nm to 2 and 3 to 10% of pores with a size of less than 80 nm on the cathode side surface. On the positive electrode side surface, a polyolefin separator for secondary batteries is introduced, wherein the distribution ratio of pores having a size of 30 nm to 1 is 90 to 97%, and the distribution ratio of pores having a size of less than 30 nm is 3 to 10%. .

However, such a separator is passed between a casting roll and a nip roll to cool the sheet extruded through a T-die during the manufacturing process, wherein the casting roll Since the and nip rolls have different radii, the cooling effect of the two rolls respectively contacting both surfaces of the sheet cannot be the same.

As such, the conventional separation membrane has a slight difference in the cooling rate of both surface layers in the manufacturing process, and thus there is a problem that it is not easy to control the pore characteristics, that is, the size and distribution of the pores of both surface layers. In addition, when using a raw material resin having a low molecular weight in order to improve the porosity of the separator, the ion permeability is increased to improve the electrical performance, but there is a problem that the mechanical strength such as tensile strength is lowered.

The problem to be solved by the present invention is to provide an excellent electrical performance and excellent mechanical strength at the same time, and further to provide a porous separator and a method for manufacturing the same surface properties of both surface layers.

The porous membrane according to the present invention comprises i) a polyethylene resin having a weight average molecular weight of less than 500,000, ii) micropores are oriented in a multilayer structure, and iii) both skin layers have a size of 0.01 nm to 0.05. The porosity distribution rate is 90-97%, and the inner layer (core layer) is characterized in that the distribution ratio of pores having a size of 0.05-1 is 90-97%.

The method for producing a porous separator according to the present invention includes: A) 70 to 90 parts by weight of a solid paraffinic oil having a weight average molecular weight of 3000 to 5000 and a weight average molecular weight of 900 to 100 parts by weight of polyethylene resin having a weight average molecular weight of less than 500,000. Preparing a raw material mixture including 70 to 90 parts by weight of liquid paraffinic oil which is 1500; B) extruding and cooling the raw material mixture; C) stretching the raw resin mixture extruded in the step B); D) extracting the solid paraffinic oil and the liquid paraffinic oil by dipping the raw material resin mixture drawn in the step C) in an organic solvent.

In addition, the method for producing a porous membrane according to the present invention includes a) a weight average molecular weight of 3000 to 5000 as a pore-forming additive with respect to 100 parts by weight of a polyethylene resin having a melt index of 0.01 to 0.6 g / 10 minutes and a weight average molecular weight of less than 500,000. Raw material resin containing 70 to 90 parts by weight of phosphorus solid type paraffinic oil, 70 to 90 parts by weight of liquid type paraffinic oil having a weight average molecular weight of 900 to 1500, and 6 to 10 parts by weight of antioxidant Preparing a mixture; b) mixing the raw material resin mixture into the screw for extrusion and melting at a temperature of 180 to 250 to extrude a gel sheet having a thickness of 1000 to 6000, and casting the gel sheet to a surface temperature of 30 to 60 pass between a roll and a nip roll, wherein the nip roll has an arc-shaped inverse gradient in the axial direction on its surface because the diameter D1 of the central portion is smaller than the diameter D2 of both edges. Iii) cooling said gelled sheet using what is formed; c) The cooled sheets are gradually drawn in the direction of the machine direction in the transverse direction, followed by the machine direction by about 5 to 15 times. Preparing a stretched film that is 6 to 50; d) immersing the stretched film in an extraction solvent to remove the pore-forming additive, and heat fixing at a temperature of 110 to 150; And a control unit.

The porous separator according to the present invention has a high mechanical strength under the same conditions as the conventional separator, and at the same time has low electrical resistance and has excellent electrical performance. Therefore, it is expected that the present invention can greatly contribute to realizing light weight, miniaturization and high capacity of secondary batteries such as lithium ion batteries and lithium polymer batteries.

In addition, by using a solid paraffinic oil and a liquid paraffinic oil together as a pore-forming additive, it is possible to increase the draw ratio, and as a result, the strength is excellent and thinning is possible.

1 is a scanning electron microscope photograph taken in a cross-sectional view of the separator according to the present invention,
2 is a scanning electron microscope photograph of the surface of both surface layers (A, C) and the inner layer (B) of the separator according to an embodiment of the present invention,
3 is a scanning electron microscope photograph of the surface of both surface layers (A, C) and the inner layer (B) of the separator according to another embodiment of the present invention.
4 is a view showing the structure of a nip roll (nip roll) used to cool the gel sheet.

Hereinafter, the present invention will be described in detail. However, terms used to describe the present invention may be used as a concept specifically defined for the purpose of the present invention.

The porous separator according to the present invention consists of a polyethylene resin having a weight average molecular weight of less than 500,000. In this case, when the polyethylene resin having a weight average molecular weight of less than 300,000 is used, the stretchability of the separator is improved, but the mechanical strength is weakened. On the contrary, when the polyethylene resin having a weight average molecular weight of 500,000 or more is used, the mechanical strength of the separator is improved, but it is soft. There is a problem in that the productivity is lowered due to poor elongation and kneading, and it is not easy to control the pore size.

For reference, conventional separators for secondary batteries are poor in productivity and pore characteristics, but in order to maintain desirable mechanical strength, a polyolefin resin having a weight average molecular weight of 500,000 or more, preferably 1,000,000 to 15,000,000 is mainly used.

In addition, the separator of the present invention has a cross-sectional structure in which micropores are oriented in multiple layers. 1 is a scanning electron microscope (SEM) photograph of a 20,000-fold magnification of a cross-sectional view of a separator according to an embodiment of the present invention, in which fine fibers are arranged side by side in the horizontal direction of FIG. In the meantime, a number of fine pores are arranged in the layered layer, and in particular, the size of the pores disposed in the inner layer of the membrane (middle portion of FIG. 1) compared to the pores disposed on both surface layers of the membrane (the upper and lower portions of FIG. 1). It can be seen visually that is greater.

Since the separator has a multi-layered pore structure, it is possible to maintain excellent mechanical strength while using polyethylene resin having a much lower weight average molecular weight than conventional separators, and to directly open air cells directly related to breathability. The specific gravity of is formed high. Here, the open cells are micropores connected to each other in the width direction of the separator, and the ionic material moves more smoothly between the anode and the cathode through these open cells.

2 and 3 are SEM images of 20,000 times magnification of both the surface layer and the inner layer of the separator prepared according to the embodiment of the present invention. The left photograph A is a photograph of the upper surface of the separator, and the right photograph C is a photograph of the lower surface of the separator, respectively, and the middle photograph B is a peeling of the surface layer from the separator to show the inner layer. The picture taken.

2 and 3 confirm that the small pores are distributed in both surface layers (photographs A and C) of the separator, and the relatively large pores are distributed in the inner layer (photograph B) of the separator. Can be. In the same way, several SEM photographs were taken of the separation membrane of the present invention, the pore sizes distributed in the surface layer and the inner layer were measured, and the distribution thereof was calculated. As a result, the distribution ratio of pores having a size of 0.01 nm to 0.05 in both surface layers was obtained. 90-97% of the pore size was 0.05 ~ 1 in the inner layer, and the pore size and distribution of both surface layers were found to be the same.

In addition, it can be seen that thick fiber bundles are distributed in the inner layer of the separator (photograph B), as if leaf veins are distributed, and the size of pores distributed in the inner layer is enlarged due to these fiber bundles, and at the same time, It is estimated that a polyethylene resin having a much smaller weight average molecular weight can be used, while maintaining excellent mechanical strength.

Hereinafter, the method for producing a porous membrane according to the present invention A) 70 to 90 parts by weight of solid phase paraffinic oil having a weight average molecular weight of 3000 to 5000 and a weight average molecular weight of 900 to 100 parts by weight of polyethylene resin having a weight average molecular weight of less than 500,000 Preparing a raw resin mixture comprising 70 to 90 parts by weight of liquid paraffinic oil of 1,500; B) melting and extruding the raw resin mixture; C) stretching the raw resin mixture extruded in the step B); D) extracting the solid paraffinic oil and the liquid paraffinic oil by dipping the raw material resin mixture drawn in the step C) in an organic solvent.

A) Raw material resin  Mixing process of the mixture

First, 70 to 90 parts by weight of a solid type paraffinic oil having a weight average molecular weight of 3000 to 5000 as a pore forming additive with respect to 100 parts by weight of polyethylene resin having a melt index of 0.01 to 0.6 g / 10 minutes and a weight average molecular weight of less than 500,000. And, a raw resin mixture comprising 70 to 90 parts by weight of a liquid paraffinic oil having a weight average molecular weight of 900 to 1500, and 6 to 10 parts by weight of an antioxidant is prepared.

The raw material resin mixture is first dissolved by heating the solid paraffinic oil, and then mixed with the liquid paraffinic oil to prepare a pore-forming additive. In this way, the pore-forming additive maintains the gel state, and it is preferable to mix the gel-based pore-forming additive with the polyethylene resin and the antioxidant again.

In this case, when the polyethylene resin having a melt index of less than 0.01 g / 10 minutes is low in flowability, it is difficult to mix with the pore-forming additive, and it is difficult to obtain a uniform sheet in the stretching process. In addition, when a polyethylene resin having a melt index of 0.6 g / 10 minutes or more is used, the flowability is too high, so that the resin may flow in the sheet extrusion step, and the mechanical strength of the finished separator is lowered.

And the ratio of the solid paraffinic oil and the liquid paraffinic oil in the pore-forming additive is 1: 0.8 ~ 1.2, preferably 1: 1 ratio. If the amount of the solid paraffinic oil and the liquid paraffinic oil is less than 70 parts by weight, respectively, the porosity of the separator may be lowered, resulting in poor charging capacity of the secondary battery. Phase separation may occur between the polyethylene resin and the pore-forming additive to cause breakage of the sheet.

It is preferable to use a wax as the paraffinic oil.

Conventional antioxidants may be used as the antioxidant, but in particular, phosphate-based additives such as phosphite esters may be used.

In the present invention, in addition to the above antioxidants, various additives such as ultraviolet absorbers, anti blocking agents, pigments, dyes and inorganic fillers may be added as necessary.

B) Raw material resin  Extrusion and cooling process of the mixture

Next, the raw resin mixture is mixed into the extrusion screw, melted at a temperature of 180 to 250, and sufficiently mixed, and then the raw resin mixture is extruded through a T-die to have a thickness of 1000 to 6000. A gel sheet is produced.

The gel sheet is then passed between a casting roll and a nip roll whose surface temperature is adjusted to 30 to 60. In this way, the sheet surface layer in direct contact with the casting roll and the nip hole cools and solidifies relatively quickly, and the sheet inner layer cools and solidifies slowly compared to the sheet surface layer. In this case, the pore-forming additive is also solidified with the polyethylene resin to form relatively bulky particles in the surface layer that is rapidly cooled, and form relatively small particles in the inner layer that is gradually cooled.

However, the casting roll has a radius of 1.4 to 1.6 times larger than that of the nip roll. Therefore, when the gel sheet is passed between the casting roll and the nip roll, the nip roll rotates much faster than the casting roll, so the sheet cooling ability of both rolls is different, and thus the pore structures formed on both surface layers of the sheet are different from each other. This happens.

In the present invention, in order to solve such a problem, a 'reverse gradient' is formed on the surface of the nip roll in the axial direction. In the present invention, the "reverse gradient" refers to the diameter D1 of the center portion of the nip roll is smaller than the diameter D2 of both edges, as shown in Figure 4, so that the outline is formed in an arc shape. At this time, the nip roll has a length (L) of 800 ~ 1000mm in the axial direction, it is preferable that the radius (R) of the circular arc to form the reverse gradient, that is, the circle circumscribed in the axial direction is 500,000 ~ 2,000,000mm. For reference, in FIG. 4, the radius R of the arc is relatively small compared to the length L of the nip roll. However, it should be understood that the radius R is much larger.

As such, when the inverse gradient is formed on the surface of the nip roll, the surface area of the nip roll is increased so that the cooling capacity is the same as that of the casting roll even though the rotational speed is faster than that of the casting roll. If the radius R of the arc forming the inverse gradient is less than 500,000 mm, the difference between the central diameter D1 and both edge diameters D2 is so large that the thickness deviation between the central portion and both edges of the sheet is too large. There exists a problem of becoming large, and when it is 2,000,000 mm or more, there exists a problem which the cooling effect of a nip roll runs short.

On the other hand, when the surface temperature of the casting roll and the nip roll is less than 30, the pore-forming additive is rapidly cooled and fixed to the surface of the roll to cause irregularities on the surface of the gel-like sheet or to obtain a sheet having a uniform thickness. On the contrary, when the pore-forming additive is not solidified, it is difficult to form pores having a large size, and liquid paraffinic oil is buried on the surface of the casting roll, and a sliding phenomenon occurs between the sheet and the roll, and the sheet can be stretched at a constant ratio. No problem occurs.

C) sheet Stretch  fair

Next, the cooled sheet is successively stretched by about 5 to 15 times in the machine direction and then in the transeverse direction to produce a film having a thickness of 6 to 50. In other words, the film is first stretched 5 to 15 times in the longitudinal direction, and then stretched 5 to 15 times in the horizontal direction. In this way, the thickness variation formed by the inverse gradient of the nip roll is eliminated, and the pore-forming additives distributed in the surface layer and the inner layer have a multi-layered alignment structure in which the layer layers are arranged at the same time.

In general, when manufacturing a porous separator, uniaxial stretching is performed in one of the longitudinal and transverse directions, or simultaneous biaxial stretching is performed in both directions. However, in the case of the uniaxial stretching, the productivity is lowered because the machine must be stretched only in the direction in which the sheet is ejected, that is, in the longitudinal direction. In addition, in the case of simultaneous biaxial stretching, since the stretching force applied to the sheet is reduced, there is a problem that high speed or wide stretching is difficult.

However, in the present invention, by performing the sequential biaxial stretching sequentially in the horizontal direction after the longitudinal direction, high magnification stretching with excellent productivity is possible, and in the future, fine pores inside the separator are formed in a multilayer structure, and have excellent mechanical strength. do.

D) Pore Forming Additive Extraction Process

Finally, the stretched sheet is immersed in the extraction solvent to remove the pore-forming additives to form micropores, and heat-fixed to 110 to 150 in the heat fixing chamber to remove residual stress. At this time, examples of the organic solvent that can be used include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons, ethers such as diethyl ether and dioxane, and the like.

When the heat setting temperature is less than 110, a problem of deterioration in heat resistance of the separator may occur.

The porous separator according to the present invention may be widely used as a separator of an electronic component such as a lithium ion cover sheet in addition to the separator for a secondary battery.

Hereinafter, examples of the present invention will be described. However, the scope of the present invention is not limited by these examples.

A pore-forming additive was prepared by mixing 9.9 kg of solid wax with a weight average molecular weight of 3746 and 9.9 kg of liquid wax with a weight average molecular weight of 1304. A raw material resin mixture was prepared by adding 12.3 kg of this 380,000 polyethylene resin and 1.0 kg of phosphite ester as an antioxidant.

The raw resin mixture was introduced into an extrusion screw, and extruded through a T-die while maintaining the rotational speed of the screw at 400 rpm to form a gel sheet having a thickness of 1800.

The gel sheet was cooled while passing between a casting roll and a nip roll where the surface temperature was maintained at 40 respectively. In this case, the casting roll and the nip roll using a ratio of 1.5: 1.0, the nip roll is 900mm in length, the radius (R) of the arc to form a reverse gradient on the surface in the axial direction is 1,700,000mm Used.

The sheet is first stretched 10 times in the machine direction in a lab stretching machine, and then sequentially stretched 10 times in the transeverse direction, and then the stretched sheet is immersed in a methylene chloride solution. The pore-forming additive was eluted and removed.

Finally, the stretched sheet was heat-fixed for 4 minutes in a thermal chamber at a temperature of 130 to prepare a polyethylene separator having a thickness of 10.1.

Except for maintaining the rotational speed of the extrusion screw in 376rpm in Example 1 to the thickness of the gel sheet 2800 was carried out in the same manner as in Example 1 to prepare a polyethylene membrane having a thickness of 16.2.

The polyethylene separation membrane having a thickness of 19.9 was prepared in the same manner as in Example 1, except that the thickness of the gel sheet was 3400 by maintaining the rotational speed of the extrusion screw at 360 rpm.

[Checking Pore Characteristics]

A cross-sectional view of the separator prepared according to Example 1 was magnified 20,000 times with a scanning electron microscope (SEM), and a photograph thereof was attached to FIG. 1. 1, it can be seen that the separator of the present invention has a cross-sectional structure in which micropores are oriented in a multilayer manner.

In addition, the upper and lower surface layers and the inner layer of the separator prepared according to Examples 1 and 2 were taken 20,000 times magnification by SEM, and the photographs were attached to FIGS. 2 and 3, respectively. As shown in FIG. 2 and FIG. 3, the upper and lower surface layers (photographs A and C) of the separator of the present invention have a high distribution of relatively small pores, and the inner layer (photograph B) of relatively large pores. It can be seen that the distribution ratio is high. In addition, unlike the conventional separation membrane in the inner layer (picture B), it can be seen that thick fiber bundles are formed side by side like a leaf vein.

[Physical Test]

The electrical performance and mechanical properties of the separators prepared in Examples 1, 2, and 3 and Comparative Examples commercially available (Gumhui, China) were respectively measured, and the results are compared to the following Table 1. It was.

Test Items Example Comparative example One 2 3 Thickness (㎛) 10.1 16.2 19.9 21.6 Ion Conductivity (10-⁴S / cm) 8.9 8.3 7.3 6.8 Breathability (sec / 100ml) 161.5 229.8 230.1 415.6 Tensile strength (kgf / cm2) MD 1948 2140 2102 977 TD 2395 1564 1403 926 Tensile Elongation (%) MD 82 51 62 156.8 Stone breaking strength (Kgf) 529 698.2 697.5 408.9

As shown in Table 1, the separator prepared according to the embodiment of the present invention shows a much higher level of ion conductivity than conventional commercial products. For reference, the high ionic conductivity contributes to the increase of the charging and discharging efficiency and the cycle of the secondary battery, thereby improving the durability of the secondary battery.

In addition, the separator prepared according to the embodiment of the present invention was relatively thin compared to the commercially available products and excellent in breathability, it was confirmed that both mechanical properties, that is, excellent tensile strength and puncture strength (Puncture strength).

[Test Methods]

The test method for the test item of Table 1 is as follows.

1) Ionic Conductivity (10-⁴S / cm): After fixing the separator impregnated with electrolyte between Ni metals of the same area and sealing it with a pouch, measure the ion conductivity using an impedance measuring instrument.

2) Breathability (sec / 100ml): Measure the time that 100ml of air passes through the sample of 30 X 30mm using Toyoseiki breathability meter.

3) Tensile strength (kgf / ㎠): Measure the applied force until breaking occurs in the longitudinal (MD) and transverse (TD) directions for a sample of size 20 X 200mm using an Instron tensile strength tester.

4) Tensile Elongation (%): Using an Instron tensile strength tester, measure the increased rate applied until the fracture occurs in the longitudinal direction (MD) for a sample of 20 X 200mm in size.

5) Penetration Strength (Kgf): Using Katotech Penetration Strength Tester, measure the force applied until the sample is punctured by applying a force to the sample with a stick of 100 X 50mm in size.

Claims (8)

i) Polyethylene resin having a weight average molecular weight of less than 500,000, ii) Fine pores are oriented in a multilayer structure, and iii) The distribution ratio of pores having a size of 0.01 nm to 0.05 in both skin layers is 90-97%. The inner layer (core layer) is a porous separator, characterized in that the distribution ratio of the pores having a size of 0.05 ~ 1 is 90 ~ 97%.
The porous membrane of claim 1, wherein each of the surface layers has the same porosity.
A) 70 to 90 parts by weight of solid paraffinic oil having a weight average molecular weight of 3000 to 5000 and 70 to 90 parts by weight of liquid paraffinic oil having a weight average molecular weight of 900 to 1500 with respect to 100 parts by weight of polyethylene resin having a weight average molecular weight of less than 500,000. Preparing a raw resin mixture comprising;
B) extruding and cooling the raw material mixture;
C) stretching the raw resin mixture extruded in the step B);
And d) extracting the solid paraffinic oil and the liquid paraffinic oil by immersing the raw resin mixture stretched in the step C) in an organic solvent.
a) 70 to 90 weight of solid type paraffinic oil having a weight average molecular weight of 3000 to 5000 as pore-forming additive with respect to 100 parts by weight of polyethylene resin having a melt index of 0.01 to 0.6 g / 10 min and a weight average molecular weight of less than 500,000. Preparing a raw resin mixture comprising 70 to 90 parts by weight of a liquid paraffinic oil having a weight average molecular weight of 900 to 1500, and 6 to 10 parts by weight of an antioxidant;
b) mixing the raw material resin mixture into the screw for extrusion and melting at a temperature of 180 to 250 to extrude a gel sheet having a thickness of 1000 to 6000, and casting the gel sheet to a surface temperature of 30 to 60 pass between a roll and a nip roll, wherein the nip roll has an arc-shaped inverse gradient in the axial direction on its surface because the diameter D1 of the central portion is smaller than the diameter D2 of both edges. Iii) cooling the gel sheet using the one formed;
c) successively stretching the cooled sheet about 5 to 15 times in the machine direction and then in the transeverse direction to produce a stretched film having a thickness of 6 to 50;
d) depositing the stretched film in an extraction solvent to remove the pore-forming additive, and then thermally fixing the film to a temperature of 110 to 150; Method for producing a porous membrane, characterized in that comprises a.
The method of claim 4, wherein the amount of the solid type paraffinic oil and the liquid type paraffinic oil is 1: 0.8 to 1.2.
The method of claim 4, wherein the nip roll (nip roll) has a length (L) of 800 ~ 1000mm in the axial direction, the radius (R) of the arc to form a reverse gradient on the surface is used 500,000 ~ 2,000,000mm Method for producing a porous membrane characterized in that.
According to any one of claims 3 to 6, wherein the raw material mixture is first mixed with the polyethylene resin and antioxidant in the state of mixing the solid (para type) oil and liquid (para-type) paraffin-based oil Method for producing a porous membrane, characterized in that mixing.
a) a pore-forming additive is prepared by mixing 80-82 parts by weight of a solid wax having a weight average molecular weight of 3746 and 80-82 parts by weight of a liquid wax having a weight-average molecular weight of 1304, and the melt index is 0.4-0.5 g. Preparing a raw material mixture by mixing 100 parts by weight of polyethylene resin having a weight average molecular weight (MW) of 38 minutes per 10 minutes and 8 to 9 parts by weight of a phosphite ester which is an antioxidant;
b) mixing the raw material resin mixture into a screw for extrusion, melting and extruding at a temperature of 200 to produce a gel sheet having a thickness of 1100 to 2200, and casting the gel sheet to a surface temperature of 40 ) And a nip roll, wherein the nip roll has a length (L) of 900 mm, and a reverse gradient (R) having a radius (R) of circumferentially circumferential arc (R) of 1,700,000 mm is formed on the surface thereof. Cooling the gel sheet using the same;
c) producing a stretched film having a thickness of 10 to 20 by stretching the cooled sheet 10 times in the machine direction and then successively stretching the film 10 times in the transeverse direction;
d) immersing the stretched film in methylene chloride solution to remove the pore-forming additive and heat fix for 4 minutes at a temperature of 130; Method for producing a porous membrane, characterized in that comprises a.

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