KR20150079114A - Manufacturing method of excellent thickness uniformity biaxially streched PTFE membrane having porosity and using thereof - Google Patents

Abstract

The present invention relates to a manufacturing method of a biaxially stretched porous PTFE separation membrane, which is allowed to improve thickness uniformity of the PTFE separation membrane through simple processes without using an extra reinforcing film, thereby minimizing defects on the PTFE separation membrane and having outstanding improved thickness uniformity while improving difference pressure in a biaxial direction.

Description

TECHNICAL FIELD The present invention relates to a biaxially oriented porous PTFE separator having improved thickness uniformity and a biaxially oriented porous PTFE separator prepared by the method.

The present invention relates to a method for producing a biaxially oriented porous PTFE separation membrane having improved thickness uniformity and a biaxially oriented porous PTFE separation membrane produced thereby. More specifically, the present invention relates to a biaxially oriented porous PTFE separation membrane, And a biaxially stretched porous PTFE separator prepared by the method. The present invention relates to a biaxially oriented porous PTFE separator, and a biaxially oriented porous PTFE separator.

Conventionally, a porous body made of polytetrafluoroethylene (hereinafter referred to as "PTFE ") not only has excellent chemical resistance, heat resistance, weather resistance, and nonflammability but also has properties such as non-stickiness and low friction coefficient. Moreover, since it is a porous structure, it is also excellent in transparency, flexibility, flexibility, trapping and filtering of fine particles, and the like. Therefore, the material made of PTFE is used in a wide range of fields such as filtration of fine chemicals and filters for wastewater treatment.

Among them, the PTFE filter medium mainly consists of a paste composed of a mixture of PTFE fine powder and a lubricant, which is rolled through a rolling process between the two rollers, formed into a sheet state, and after the lubricant is removed, Technology, that is, a method for producing a PTFE flat membrane, is well known.

Specifically, as an example of a method for producing a PTFE flat membrane, Japanese Patent Laid-Open Nos. 1980-075433, 1985-104319, and 1991-017136 disclose a method for manufacturing a PTFE porous body by improving the pore size of a PTFE porous body will be. In addition, the PTFE porous body produced by the stretching of PTFE disclosed in the above patent documents has a fine structure composed of a plurality of fine fibrils (fine fibers) and a plurality of nodes (nodules) connected to each other by the fibrils , And this fine structure forms a continuous porous porous structure. At this time, the porous PTFE porous body can arbitrarily set the porous structure such as the pore diameter and porosity by controlling the stretching conditions.

The PTFE sheet type separation membrane can be utilized as a gas separation membrane or fiber. However, in the P TFE sheet type separation membrane, biaxial orientation must be performed in order to form pores. In the final biaxial orientation, the side faces of the uniaxially oriented sheet are elongated earlier than the central portion. Specifically, FIG. 1 is a cross-sectional view of a conventional biaxially stretched PTFE separation membrane, which shows that the height a of the central portion is higher than the height b of both ends. The removal rate and the air permeability may vary depending on the position of the membrane in the uniaxial direction and the biaxial direction, particularly in the biaxial direction due to the thickness deviation, and the permeability and the removal rate may vary in the liquid filter application. Also, when the biaxially stretched separator film is rolled up in a roll shape due to the thickness deviation, a thicker portion of the separator may be wound around the thinner portion than the relatively thin portion, which may make winding difficult.

In order to improve this, Japanese Patent Application No. 2003-416008 discloses an attempt to secure a uniform thickness by attaching a polymer reinforcing membrane to a uniaxially stretched PTFE sheet and biaxially stretching it. However, in this case, it is difficult to remove the bonded polymer sheet, and there is a fear that the sheet may be scratched or damaged even if it is separated. Also in this case, the thickness variation coefficient value exceeds 15%, and the variation coefficient of the differential pressure exceeds 5%.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a method of manufacturing a semiconductor device, which can remarkably improve thickness uniformity while minimizing defects of a separator through a simple process without using a separate reinforcing film Also disclosed is a method for producing a biaxially oriented porous PTFE separation membrane having improved thickness uniformity and improved deviation of differential pressure in a biaxial direction, and a biaxially oriented porous PTFE separation membrane produced thereby.

A method for producing a biaxially oriented PTFE porous separation membrane for solving the above problems, comprising the steps of: (1) uniaxially stretching a PTFE separation membrane; (2) imparting a thickness variation such that a center portion of the separator is thinner than an end portion thereof with respect to a stretched axial direction of the uniaxially stretched PTFE separator; And (3) biaxially elongating the PTFE separation membrane to which the thickness variation is imparted. The present invention also provides a method for producing a biaxially oriented porous PTFE separation membrane having improved thickness uniformity.

According to a preferred embodiment of the present invention, in the step (2), the center portion of the separation membrane may be thinner by 10 to 30% than the end portion.

According to another preferred embodiment of the present invention, in the step (2), a thickness variation may be imparted through the process of rolling the center portion.

According to another preferred embodiment of the present invention, the step (2) may be performed by biaxial stretching in a direction perpendicular to the uniaxial stretching direction.

According to another preferred embodiment of the present invention, the stretching ratio of the monoaxial stretch may be 10 to 25 times.

According to another preferred embodiment of the present invention, the stretching ratio of the biaxial stretch may be 15 to 25 times.

According to another preferred embodiment of the present invention, the average thickness of the biaxially oriented porous PTFE separation membrane may be 5 to 15 μm and the width may be 1200 to 1800 mm.

According to another preferred embodiment of the present invention, the thickness deviation in the step (2) may be such that the thickness gradually decreases from the end portion to the center portion.

According to another preferred embodiment of the present invention, the biaxially stretched porous PTFE separator after the step (3) has a thickness variation coefficient CV1 of the following relational expression 1 of not more than 15% The coefficient (CV2) value may be less than 5%.

[Relation 1]

Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100

[Relation 2]

Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100

According to another preferred embodiment of the present invention, the step (2) may provide a thickness variation so that the center portion is thinner than both end portions.

According to another preferred embodiment of the present invention, there is provided a uniaxially oriented porous PTFE separation membrane having a thickness thinner than a center portion of a separation membrane with respect to a stretched axial direction of a uniaxially stretched PTFE separation membrane.

According to another preferred embodiment of the present invention, the central portion of the separation membrane may be 10 to 30% thinner than the end portion.

According to another preferred embodiment of the present invention, a thickness gradient may be formed such that the thickness gradually decreases from the end portion to the center portion.

According to another preferred embodiment of the present invention, the thickness variation coefficient CV1 of the following relational expression 1 is 15% or less, the thickness variation uniformity of the differential pressure variation coefficient CV2 of the following relational expression 2 is 5% A biaxially oriented porous PTFE separator is provided.

[Relation 1]

Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100

[Relation 2]

Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100

According to another preferred embodiment of the present invention, a thickness gradient may gradually be formed from the end portion to the central portion. The thickness gradient may gradually decrease or increase in thickness from the end portion to the center portion.

According to another preferred embodiment of the present invention, there is provided an air filter including the biaxially oriented porous PTFE separation membrane of the present invention.

The method of manufacturing a biaxially oriented porous PTFE separation membrane of the present invention can remarkably improve thickness uniformity through a simple process without using a separate reinforcing film. Thus, the thickness uniformity can be remarkably improved while minimizing the defects of the PTFE separator, and the uniformity of the differential pressure in the biaxial direction can be remarkably improved.

1 is a cross-sectional view of a conventional biaxially oriented PTFE separation membrane.
2 is a schematic diagram of a compressor that can be used in the present invention.
3 is a schematic diagram of an extruder that can be used in the present invention.
4 is a schematic diagram of a calender roll that can be used in the present invention.
FIG. 5 is a schematic view of a rolling apparatus according to a preferred embodiment of the present invention, and FIG. 6 is a cross-sectional view of a monoaxially stretched PTFE separation membrane having a thickness gradient produced thereby.
FIG. 7 is a cross-sectional view of a biaxially stretched PTFE separation membrane according to a preferred embodiment of the present invention, and FIGS. 8 and 9 are enlarged views of a surface portion of the separation membrane.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail with reference to the accompanying drawings.

As described above, there has been an attempt to secure a uniform thickness by biaxially stretching a polymeric reinforcing membrane attached to a conventional uniaxially stretched PTFE sheet. However, in this case, it is difficult to remove the bonded polymer sheet, and there is a fear that the sheet may be scratched or damaged even if it is separated. Also in this case, the thickness difference between the central part and the both ends still has a problem that the variation coefficient value of the average thickness exceeds 15% and the variation coefficient of the differential pressure exceeds 10%.

Accordingly, the present invention provides a method for producing a biaxially oriented PTFE porous separation membrane, comprising: (1) uniaxially stretching a PTFE separation membrane; (2) imparting a thickness variation such that a center portion of the separator is thinner than an end portion thereof with respect to a stretched axial direction of the uniaxially stretched PTFE separator; And (3) biaxially elongating the PTFE separation membrane to which the thickness deviation is imparted. The present invention has been made to solve the above-mentioned problems by providing a biaxially oriented porous PTFE separation membrane having improved thickness uniformity. As a result, the method of manufacturing a biaxially oriented porous PTFE separation membrane of the present invention can remarkably improve thickness uniformity through a simple process without using a separate reinforcing film. This allows for significantly improved thickness uniformity while minimizing defects in the PTFE membrane.

Hereinafter, a method of manufacturing a PTFE separator according to a preferred embodiment of the present invention will be described. Specifically, prior to the step (1) described above, it can be produced according to a conventional method for producing a PTFE separator. Therefore, the following description is intended to assist the understanding of the present invention, but the present invention is not limited thereto.

First, a paste is prepared by mixing polytetrafluoroethylene (PTFE) powder and a liquid lubricant. The PTFE powder contained in the paste of the present invention can be used without limitation as long as it is conventionally used in the PTFE separation membrane. Preferably, the average particle diameter of the PTFE powder may be 300 to 500 탆, but is not limited thereto. The molecular weight and the like are not particularly limited, and commercially available products may be used. Examples thereof include Polyflon F-104 (Daikin Industries) and Fluon CD-123 (Asahi ICI Fluoropolymers Co., Ltd.).

The liquid lubricant contained in the paste of the present invention is for performing smooth extrusion and preform formation while wetting the surface of the PTFE fine powder and is not particularly limited as long as it is a material that can be removed by means such as evaporation extraction by heat after forming into a sheet . For example, as the liquid lubricant, various alcohols, ketones, esters, and the like may be used in addition to hydrocarbon oils such as liquid paraffin, naphtha, white oil, toluene, and xylene. According to a preferred embodiment of the present invention, the paste may include 10 to 50 parts by weight of a liquid lubricant based on 100 parts by weight of polytetrafluoroethylene (PTFE) powder.

Next, the paste can be compressed and preformed in a compressor. Specifically, a compressor which can be used in the present invention can be used without limitation as long as it is usually used for compressing a polymer separator. FIG. 2 is a schematic view of a compressor that can be used in the present invention. When the paste is introduced into the compressor for producing a separator 20, the paste can be molded into the preform 21 by applying pressure. In this case, a preform having various cross sections can be manufactured according to the shape of the compressor, and the size of the preform can also be set according to conditions.

On the other hand, the temperature and the pressure inside the compressor can be set according to the condition of the compressor applied in the production of the conventional PTFE separator, and preferably the pressure can be performed at 18 to 25 ° C and 1 to 3 MPa.

Next, the preform may be extruded from an extruder. Specifically, the extruder which can be used in the present invention can be used without limitation as long as it is usually used for the production of a polymer membrane, and can be preferably a ram extruder. FIG. 3 is a schematic diagram of an extruder that can be used in the present invention. The preform is put into the extruder 30, and the extrudate 31 is extruded through the extruder 30. In this case, the extrudate 31 having various cross sections may be manufactured according to the shape of the extruder 30, and preferably it may have a rod shape. The size of the extrudate can also be set according to the conditions. The extrusion conditions may be carried out under the conditions of an extrusion process of a conventional PTFE separation membrane, and preferably the extrusion step may be performed at a pressure of 5 to 15 MPa at 60 to 85 ° C.

In the next step, the extruded extrudate may be calendered in a sheet form. Fig. 4 is a schematic view of a calender roll that can be used in the present invention. When the extrudate extruded through step (3) passes through a calender roll 40, it is spread into a sheet shape 41. Fig. In this case, the sheet thickness may be 200 to 300 mu m.

As a next step, the calendered sheet can be heated to remove the liquid lubricant. Specifically, the heating temperature of the sheet may be in the range of the temperature at which the liquid lubricant is removed, preferably 200 to 250 ° C. The heating time can be performed for 10 seconds to 10 minutes.

Next, the PTFE separation membrane produced by the conventional method described above is uniaxially stretched as the PTFE separation membrane in the step (1). Specifically, a conventional PTFE sheet is conveyed through a roller. In this case, the PTFE sheet can be stretched in the longitudinal direction using the speed difference between the rollers, but not limited thereto, and can be stretched according to the stretching method used in ordinary sheet production. The stretching temperature may also be 250 to 320 ° C, but is not limited thereto, and the stretching ratio may be 10 to 25 times. If the stretching ratio is less than 10 times, the thickness of the final separator may be reduced. If the stretching ratio exceeds 25 times, the fibril and node structure of the longitudinally stretched sheet may be destroyed. A problem that is difficult to be given may arise.

Next, in step (2), a thickness variation is imparted so that the center portion of the separation membrane is thinner than the end portion with respect to the stretched axial direction of the uniaxially stretched PTFE separation membrane. 5 is a schematic view of a rolling apparatus according to a preferred embodiment of the present invention, in which the surface of the lower roll 51 is flat, but the upper roll 50 is designed to gradually increase in thickness from both ends to the center. Through this, the uniaxially stretched PTFE separating film passing between the upper roll 50 and the lower roll 51 becomes thinner at the center portion of the separating film than the end portion with respect to the uniaxially stretched axial direction. Specifically, FIG. 6 is a cross-sectional view of a uniaxially stretched PTFE separation membrane provided with a thickness variation according to a preferred embodiment of the present invention, in which the thickness of both ends b is larger than the thickness a of the central portion of the separation membrane 90. If the biaxial stretching is performed afterwards, it is possible to prevent the side portion of the uniaxially stretched sheet from being stretched earlier than the center portion of the uniaxially stretched sheet, which will inevitably cause the biaxial stretching, to become thicker. Also in this step, the separation membrane can pass through the rolling device in a direction parallel to the primary stretched axial direction.

In the meantime, the first stretched separator in the present invention may include a thickness gradient in which both ends of the separator gradually increase in thickness from the center through the rolling step. Thus, the thickness deviation can be minimized naturally in the second elongation step. In the present invention, both end portions are explained to be thicker than the central portion, but it is also possible that only one end portion is gradually thickened.

Furthermore, it is also within the scope of the present invention to design the bottom roll 51 to have a concave shape on both sides of the primary stretched membrane with a thickness gradient.

On the other hand, more preferably, the center portion of the separation membrane may be 10 to 30% thinner than both ends. If a thickness gradient of less than 10% occurs, the thickness deviation of the final biaxial oriented diaphragm may increase. If it exceeds 30%, the thickness gradient may be given preferentially to the quasi-elongated sheet portion. There may arise a problem that the thickness deviation occurs or the middle part of the separation membrane is broken in the biaxial stretching process (see Table 1).

Next, (3) the PTFE separating film to which the thickness deviation is given is biaxially stretched. In this case, preferably, the biaxial stretching can proceed in the vertical direction of the uniaxial stretching. Specifically, the PTFE separator forms pores composed of a node and a fibril through a stretching process. Preferably, the biaxial stretching temperature is 150 to 250 ° C and the stretching ratio is 15 to 25 times. If the stretching ratio is less than 15 times, the pore size of the membrane is formed to be smaller than both ends and the porosity of the membrane may be lower than that of both ends. If the stretching ratio exceeds 25 times, the particle removal rate may be lowered to 99.99% or less .

On the other hand, in the present invention, it is assumed that the film is uniaxially stretched in the longitudinal direction and biaxially stretched in the transverse direction. However, the present invention is not limited thereto but may be stretched in the opposite direction.

7 is a cross-sectional view of a biaxially oriented porous PTFE separation membrane according to a preferred embodiment of the present invention. The average thickness of the biaxially oriented porous PTFE separation membrane of the present invention may be an average thickness of 5 to 15 μm and a width of 1200 to 1800 mm. Further, a rectangular or square sheet shape is possible in a range that satisfies the above-mentioned width range.

 On the other hand, in the case where the thickness variation coefficient (CV1) of the following relation 1 is 15% or less and the coefficient of variation (CV2) of the differential pressure of the following formula 2 is 5% or less A biaxially oriented porous PTFE membrane having improved uniformity is provided. If the value of the thickness variation coefficient exceeds 15%, the differential pressure on both end faces is relatively thicker than in the center part, and the pressure difference may occur. In addition, when the differential pressure difference of the separation membrane exceeds 5%, the cross section having a higher differential pressure than the central portion must be cut, so that the effective area of the separation membrane can be reduced.

[Relation 1]

Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100

[Relation 2]

Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100

Figs. 8 and 9 are enlarged cross-sectional views of the surface of Fig. 7. Fig. In FIG. 8, a thickness gradient is formed so as to gradually decrease in thickness toward both ends with respect to the center, and in FIG. 9, a thickness gradient is formed so as to gradually increase the thickness. Such a difference may occur depending on the degree of the uniaxial stretching and the biaxial stretching ratio. Particularly, when the stretching ratio of the biaxial stretching is large, it can have the structure shown in Fig. 8, and when the stretching ratio of the biaxial stretching is relatively small, And the like. On the other hand, it is advantageous to have a thickness gradient so that the thickness gradually increases toward both ends with respect to the central part, as compared with the opposite case in the same thickness deviation (see Table 1).

Preferably, the biaxially stretched PTFE separation membrane may be fired to prevent heat shrinkage. In this case, the firing temperature may be performed at 340 to 400 ° C for 10 seconds to 10 minutes.

Hereinafter, the structure and effect of the present invention will be described in more detail with reference to examples and comparative examples. However, this embodiment is only an example for explaining the present invention in more detail, and the scope of the present invention is not limited to these embodiments.

<Examples>

A PTFE paste was prepared by mixing PTFE polymer (DF-204, Shangdong dongyue) and liquid paraffin (Exopon Mobil product, trade name: Isopar-H) as a liquid lubricant at a weight ratio of 8: 2. The PTFE paste was compressed at 20 占 폚 under a pressure of 2 MPa to form a preform. The preform was introduced into the ram extruder. The conditions inside the ram extruder were extrusion molded at a pressure of 9 MPa (90 kg / cm 2) and an extrusion process at 60 캜 to form a rod having an outer diameter of 15 mm. Thereafter, the extruded rod was passed through a rolling device (calender roll) to form a sheet having a thickness of 300 탆 and a width of 155 mm. The formed PTFE sheet was heated at 220 캜 for 5 minutes to remove liquid paraffin. The sheet thus prepared was stretched 20 times in the uniaxial direction at 300 DEG C to prepare a sheet having a thickness of 120 mu m and a width of 120 mm. Thereafter, the PTFE sheet was re-rolled using the rolling apparatus shown in Fig. At this time, the lower roll is flat and the upper roll has a thickness variation so that the thickness gradually decreases from the center to both ends. At this time, the thickness deviation between the center and the side of the roll is 0.1 mm. The gap between the upper roll and the lower roll was adjusted to 80 mu m based on the center portion. After the rolling treatment, the thickness of the center portion of the sheet was 85 μm and the thickness of the side portions was 100 μm. The sheet thus prepared was stretched 25 times in the biaxial direction at 250 ° C. After stretching, heat setting was performed at 360 캜 for 15 seconds. As a result of measuring the thickness in the biaxial direction, a stretched film having an average thickness of 11 μm and a thickness deviation of 1.2 μm and a width of 1500 mm was obtained. The air filter performance of the stretched film was measured. As a result, the differential pressure was 23 mmH ₂ O and the removal rate was 99.997%.

&Lt; Examples 2 to 4 >

A biaxially stretched PTFE separation membrane was prepared in the same manner as in Example 1, except that the process was performed under the conditions shown in Table 1 below.

&Lt; Comparative Example 1 &

The biaxially stretched PTFE separation membrane was produced in the same manner as in Example 1 except that the re-rolling process was not performed.

&Lt; Comparative Example 2 &

A biaxially stretched PTFE separation membrane was prepared in the same manner as in Example 1, except that the process was performed under the conditions shown in Table 1 below.

Experimental Example

1. Removal rate (%)

The collection efficiency (%) for dust particles is measured by an automated efficiency tester (TSI, Modle 8130). Specifically, an aerosol containing sodium chloride was applied to the filter at an air flow rate of 32 LPM (surface wind speed of 5.33 cm / sec) to measure the sodium chloride concentration before and after the passage, and the amount of sodium alumina collected by the filter was calculated This was divided by the amount of sodium chloride prior to the call and the percentages were determined to determine the collection efficiency. The removal rate is expressed as a percentage of the initial amount of sodium chloride (100%) minus the collection efficiency. At this time, the aerosol was prepared by dissolving sodium chloride in distilled water to make an aqueous 20% by weight sodium chloride solution, and then evaporating moisture by a heater to make sodium chloride to have a size of about 0.3 탆.

2. Thickness variation coefficient

The thickness variation coefficient was measured according to the following relational expression (1). At this time, the standard deviation of the thickness and the average thickness were measured at five points in the vertical 9 points on the basis of the center.

[Relation 1]

Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100

3. Measurement of differential pressure (mmH ₂ O)

The differential pressure (mmH ₂ O) was measured from the pressure loss before and after passage of the filter sample at an air flow rate of 32 LPM, similar to the collection efficiency measurement method.

4, differential pressure coefficient of variation

The differential pressure variation coefficient was measured according to the following equation (2). At this time, the standard deviation of the differential pressure and the average differential pressure were measured at five points in the vertical 9 points on the basis of the center.

[Relation 2]

Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100

Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 1 Condensed Mystery 20 20 20 20 20 20 (1) Thickness in the center (占 퐉) 85 111 48
105 115 119 (2) End thickness
(탆) 100 116 102 80 116 120 (3) Thickness deviation (%) 15 4.3 53 24 0.9 0.8
2 Condensation Mystery 25 25 25 25 25 30 (4) Average thickness
(탆) 11 18 33 9.6 14.3 8.7 (5) Thickness in the center portion (占 퐉) 10
17
11 20 13 5 (6) End thickness
(탆) 13 20 42 6 10 15 (7) Thickness standard deviation
(탆) 1.5 3.1 13.4 4.6 3.0 3.1
(8) Coefficient of thickness variation (%) 13.1 18.2 40.6 47.9 20.9 35.1 (9) Mean differential pressure
(mmH ₂ O) 23 28 50 22 30
24 (10) Coefficient of differential pressure variation (%) 4.3 8.6 13.3 19.1 15.4 12.0 (11) Removal rate (%) > 99.999 > 99.997 > 99.997 > 99.997 > 99.992 > 99.99

In Table 1, (1) is the central portion thickness (μm) in the re-rolling step, (2) is the end portion thickness (μm) in the re-rolling step, (탆) after biaxial stretching, (4) is the average thickness (占 퐉) of the separator after biaxial stretching, (5) (7) is the thickness standard deviation (탆) of (5) and (6).

As can be seen from Table 1, the embodiments of the present invention exhibited excellent removal rates as compared with the comparative examples. Furthermore, the first embodiment satisfying the thickness variation coefficient of the present invention has a very small fluctuation ratio of the pressure difference as compared with the other embodiments in which it can not be satisfied.

The biaxially oriented PTFE porous separation membrane produced by the production method of the present invention has improved thickness uniformity and can be widely used in the fields of fibers, filtration membranes, and particularly air filters.

Claims (17)

A method for producing a biaxially stretched PTFE porous separator,
(1) uniaxially stretching the PTFE separation membrane;
(2) imparting a thickness variation such that a center portion of the separator is thinner than an end portion thereof with respect to a stretched axial direction of the uniaxially stretched PTFE separator; And
(3) biaxially elongating the PTFE separation membrane to which the thickness variation is imparted, the biaxially oriented porous PTFE separation membrane having improved thickness uniformity. The method according to claim 1,
The method of claim 2, wherein the middle portion of the separation membrane is 10 to 30% thinner than the end portion of the separation membrane. The method according to claim 1,
Wherein the step (2) includes the step of rolling the center portion to give a thickness variation, wherein the thickness uniformity of the biaxially oriented porous PTFE separation membrane is improved. The method according to claim 1,
Wherein the biaxial stretching in the direction perpendicular to the uniaxial stretching direction is performed in the step (2). The method according to claim 1,
Wherein the stretching ratio of the uniaxial stretching is 10 to 25 times. The method for producing a biaxially oriented porous PTFE separation membrane according to claim 1, The method according to claim 1,
Wherein the stretching ratio of the biaxially stretched polypropylene film is 15 to 25 times the thickness of the biaxially oriented polypropylene film. The method according to claim 1,
Wherein the biaxially oriented porous PTFE separation membrane has an average thickness of 5 to 15 μm and a width of 1200 to 1800 mm. The method according to claim 1,
Wherein the thickness deviation of step (2) is such that a thickness gradient is gradually decreased from the end portion to the center portion. The method according to claim 1,
The biaxially stretched porous PTFE separator after step (3) has a thickness variation coefficient (CV1) value of 15% or less and a thickness variation coefficient (CV2) value of a differential pressure differential value (CV2) (EN) METHOD FOR MANUFACTURING THE SAME.
[Relation 1]
Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100
[Relation 2]
Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100 The method according to claim 1,
Wherein the step (2) includes the step of imparting a thickness variation so that the center portion is thinner than the both end portions of the porous PTFE separation membrane. A uniaxially stretched porous PTFE membrane having a thickness thinner at the center portion of the separator than at an end thereof with respect to a stretched axial direction of the uniaxially stretched PTFE separator. 12. The method of claim 11,
Wherein a center portion of the separation membrane is 10-30% thinner than an end portion thereof. 12. The method of claim 11,
Wherein the membrane has a thickness gradient so that the thickness gradually decreases from the end to the center. According to another preferred embodiment of the present invention, the thickness variation coefficient CV1 of the following relational expression 1 is 15% or less, the thickness variation uniformity of the differential pressure variation coefficient CV2 of the following relational expression 2 is 5% Biaxially Oriented Porous PTFE Membrane.
[Relation 1]
Coefficient of variation (CV1,%) = (standard deviation of thickness / average thickness) * 100
[Relation 2]
Coefficient of variation (CV2,%) = (standard deviation of differential pressure / mean differential pressure) * 100 15. The method of claim 14,
And a thickness gradient is progressively formed from the end to the center of the porous PTFE membrane. 16. The method of claim 15,
Wherein the thickness gradient gradually decreases in thickness from the end portion toward the center portion. The biaxially oriented porous PTFE membrane having improved thickness uniformity. An air filter comprising the biaxially oriented porous PTFE separation membrane of any one of claims 14 to 16.

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