JP2011178970A - Method of manufacturing polytetrafluoroethylene porous film and air-permeable member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a PTFE porous film for manufacturing the polytetrafluoroethylene (PTFE) porous film capable of satisfying both of air flow quantity and strength on high level. <P>SOLUTION: A non-calcinated PTFE sheet is extended in a predetermined direction at a temperature under a melting point of the PTFE, and the extended sheet is heat-treated at a temperature equal to or above the melting point, then the sheet after the heat treatment is further extended to a direction orthogonal to the predetermined direction. The PTFE porous film obtained by the method has a high air flow quantity and strength. The PTFE porous film is suitable for an application as an air-permeable member which is fixed at an opening part of the housing and assures the air permeability of the housing while preventing the inversion of a foreign substance to the inside of the housing. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a polytetrafluoroethylene porous membrane. Moreover, this invention relates to a ventilation member provided with the polytetrafluoroethylene porous membrane obtained by the said manufacturing method.

  Ensures ventilation inside and outside the housing of electrical components for vehicles such as lamps, pressure sensors, ECUs (Electrical Control Units), and electrical products such as mobile phones and cameras, as well as water and dust inside the housing. A ventilation member for preventing intrusion of foreign matter is attached. Such a ventilation member is realized by a gas permeable membrane that allows gas to pass therethrough but does not allow foreign matter to pass therethrough. As the gas permeable membrane, a polytetrafluoroethylene (PTFE) porous membrane is suitable.

  As a method for producing a PTFE porous membrane, a method of sequentially biaxially stretching an unfired PTFE sheet is widely adopted. The PTFE porous membrane obtained by this method has a porous structure composed of innumerable PTFE fibrils generated by stretching and voids between the fibrils.

  As a specific method for obtaining a PTFE porous membrane by sequential biaxial stretching, Japanese Patent Laid-Open No. 7-196831 discloses that after unsintered PTFE sheet is sequentially biaxially stretched at a temperature below the melting point of PTFE, A method of heat treatment at a temperature above the melting point is disclosed. In JP-A-2007-260547, the first step of sequential biaxial stretching is performed on an unfired PTFE sheet at a temperature equal to or higher than the melting point of PTFE, and then the second step of stretching is performed on the melting point of PTFE. A method (baking stretching method) is disclosed in which a heat treatment is performed at a temperature lower than the melting point of PTFE after performing in a direction different from the first-stage stretching at a temperature lower than that. In Japanese National Publication No. 11-515036, the unsintered PTFE sheet is subjected to successive biaxial stretching at the first stage at a temperature lower than the melting point of PTFE, and then heat-treated at a temperature higher than the melting point of PTFE. Thereafter, a method is disclosed in which the second-stage stretching is performed at a temperature equal to or higher than the melting point of PTFE.

Japanese Laid-Open Patent Publication No.7-196831 JP 2007-260547 Japanese National Patent Publication No. 11-515036

  In the methods disclosed in Japanese Patent Application Laid-Open Nos. 7-196831 and 11-515036, a PTFE porous membrane having a large air permeability can be obtained by setting a high ratio of each stretching in sequential biaxial stretching. The strength of the obtained PTFE porous membrane is reduced. In the firing stretching method, a high-strength PTFE porous membrane can be obtained, but it is difficult to obtain a PTFE porous membrane having a large air flow rate.

  An object of this invention is to provide the manufacturing method of the PTFE porous membrane which can manufacture the PTFE porous membrane which made high air flow rate and high intensity | strength compatible.

  In the method for producing a porous PTFE membrane of the present invention, an unsintered PTFE sheet is stretched in a predetermined direction at a temperature lower than the melting point of PTFE (stretching A), and the stretched sheet is heat treated at a temperature equal to or higher than the melting point. Then, the sheet after the heat treatment is further stretched in a direction orthogonal to the predetermined direction at a temperature lower than the melting point (stretching B).

  The ventilation member of the present invention includes a ventilation film that allows gas passing through the opening to pass through and a support that supports the ventilation film in a state of being fixed to the opening of the housing. The gas permeable membrane has a PTFE porous membrane obtained by the method for producing a PTFE porous membrane of the present invention.

  According to the method for producing a porous PTFE membrane of the present invention, a porous PTFE membrane having both high air permeability and high strength can be obtained.

  A PTFE porous membrane having such characteristics is suitable for use as a ventilation membrane of a ventilation member. The ventilation member of the present invention including the ventilation membrane having the porous PTFE membrane realizes high air permeability inside and outside the housing by attaching to the opening of the housing, and also provides water and dust to the inside of the housing. More reliably suppress the entry of foreign matter.

It is sectional drawing which shows an example of the ventilation member of this invention typically. It is a schematic diagram for demonstrating the method of evaluating the intensity | strength of the thickness direction of the PTFE porous membrane implemented in the Example.

[Extension A]
In stretching A, an unsintered PTFE sheet is stretched at a temperature below the melting point of PTFE. As long as such stretching of the unfired PTFE sheet is possible, the specific method of stretching A is not particularly limited. For example, an unfired PTFE sheet may be stretched using a heating furnace maintained at a temperature lower than the melting point of PTFE (zone stretching method). In the zone stretching method, the PTFE sheet can be stretched at a temperature lower than the melting point by appropriately setting the time during which the PTFE sheet is positioned in the heating furnace. Stretching A may be performed by a hot roll method using a heated roll heated to a temperature lower than the melting point of PTFE.

  The drawing direction of the drawing A (predetermined direction) is any direction in the plane of the PTFE sheet, and the specific direction is not particularly limited, but the unfired PTFE sheet is subjected to an extrusion process and / or a rolling process. In the case of the sheet obtained through the process, for example, it is the MD direction (Machine Direction: extrusion direction, rolling direction) in each step. When the unsintered PTFE sheet has a strip shape, for example, it is the longitudinal direction thereof.

  The stretching temperature of stretching A is less than 340 ° C, preferably less than 327 ° C. The lower limit of the stretching temperature is, for example, 150 ° C., and below this, it becomes difficult to obtain the PTFE porous membrane itself. When the stretching temperature of the stretching A is equal to or higher than the melting point of PTFE, the strength of the PTFE porous membrane finally obtained after the stretching B is lowered. The stretching temperature of the stretching A is preferably 150 ° C. or higher and lower than 327 ° C.

  The upper limit of the draw ratio of the stretch A is preferably 40 times or less, and more preferably 20 times or less. The lower limit of the draw ratio is, for example, 1.2 times, and preferably 2 times or more. The stretch ratio of the stretch A is preferably 2 to 40 times. When the stretch ratio of the stretch A is excessively large, the strength of the PTFE porous membrane finally obtained after the stretch B is lowered.

  In the stretching A, it is considered that in the unfired PTFE sheet, fibrils extending in the stretching direction are formed and nodes in which PTFE is partially aggregated are formed.

[Heat treatment]
In the production method of the present invention, the PTFE sheet after stretching A is heated to a temperature equal to or higher than the melting point of PTFE. The specific method of the heat treatment is not particularly limited, and for example, the PTFE sheet after stretching A may be accommodated in a heating furnace maintained at a temperature equal to or higher than the melting point of PTFE. By appropriately setting the time during which the PTFE sheet is accommodated in the heating furnace (usually 5 seconds or more, preferably 10 seconds or more and 200 seconds or less), the PTFE sheet can be heat-treated at a temperature equal to or higher than the melting point of PTFE. You may heat-process using the heating roll heated to the temperature more than melting | fusing point of PTFE.

  The heat treatment temperature is 340 ° C. or higher, and preferably 350 ° C. or higher and 400 ° C. or lower.

  The heat treatment may be performed continuously following the stretching A.

  In the heat treatment, it is considered that the integration of fibrils in the PTFE sheet proceeds. Thereby, the diameter of the PTFE fibril formed by the stretching A is increased, and a high-strength porous PTFE membrane is finally obtained.

  If necessary, the PTFE sheet after stretching A may be further stretched in the same direction as the stretching A (predetermined direction) while heat-treating. The draw ratio of the draw accompanying this heat treatment is at most 2 times.

[Stretching B]
In the production method of the present invention, the PTFE sheet after heat treatment is further stretched in a direction orthogonal to the direction of stretch A at a temperature lower than the melting point of PTFE. As long as such stretching of the PTFE sheet after heat treatment is possible, the specific method of stretching B is not particularly limited. For example, the zone stretching method described in stretching A can be applied.

  The stretching direction of stretching B is not particularly limited as long as it is an in-plane direction of the PTFE sheet and is a direction orthogonal to the direction of stretching A. When the unsintered PTFE sheet before the stretching A is a sheet obtained through an extrusion process and / or a rolling process, for example, it is a TD direction (Transverse Direction) in each process. When the unsintered PTFE sheet before stretching A has a strip shape, for example, the width direction. In order to stretch the belt-like sheet in the width direction, for example, a tenter method may be used. If the sheet is rectangular, a biaxial stretching machine can be used.

  In the production method of the present invention, the unsintered PTFE sheet before stretching A is in a strip shape, the direction of stretching A (predetermined direction) is the longitudinal direction of the strip-shaped sheet, and the direction of stretching B (predetermined The direction perpendicular to the direction) may be the width direction of the belt-like sheet.

  The stretching temperature of stretching B is less than 340 ° C, preferably less than 327 ° C. The lower limit of the stretching temperature is, for example, 50 ° C. Below this, it is difficult to obtain a PTFE porous membrane having a high air flow rate by increasing the interval between the integrated fibrils by heat treatment. When the stretching temperature of the stretching B is equal to or higher than the melting point of PTFE, the strength of the obtained PTFE porous membrane is lowered. The stretching temperature of stretching B is preferably 50 ° C. or higher and lower than 327 ° C.

  The upper limit of the draw ratio of the stretch B is preferably 40 times or less, and more preferably 10 times or less. The lower limit of the draw ratio is, for example, 1.2 times, and preferably 2 times or more. The draw ratio of the stretch B is preferably 2 to 40 times, more preferably 2 to 10 times, and even more preferably 2 to 6 times. If the stretch ratio of stretch B is excessively large, the film may break. If the stretch ratio of stretch B is excessively small, the strength of the finally obtained PTFE porous membrane is lowered.

  That is, in the production method of the present invention, the stretch ratio of the stretch A (in a predetermined direction) is 2 to 40 times, and the stretch ratio of the stretch B (in a direction orthogonal to the predetermined direction) is 2. It is preferable that they are 2 times or more and 40 times or less.

  Stretching B may be performed continuously following the heat treatment.

  In the stretch B, in the PTFE sheet after the heat treatment, the interval between the fibrils integrated by the heat treatment is increased, and the air permeability is improved while the strength of the sheet is ensured.

[Unfired PTFE sheet]
The method for forming the unfired PTFE sheet used in the production method of the present invention is not particularly limited. For example, a mixture of PTFE fine powder (fine powder) and a liquid lubricant may be formed into a sheet by at least one method selected from extrusion and rolling.

  The kind of PTFE fine powder is not particularly limited, and a commercially available product can be used. Commercially available PTFE fine powders are, for example, Polyflon F-104 (manufactured by Daikin Industries), Fullon CD-123 (manufactured by Asahi ICI Fluoropolymers), and Teflon 6J (manufactured by Mitsui / DuPont Fluorochemicals).

  The type of the liquid lubricant is not particularly limited as long as it can wet the surface of the PTFE fine powder and can be removed by means such as evaporation or extraction after the mixture is formed into a sheet. The liquid lubricant is, for example, a hydrocarbon oil such as liquid paraffin, naphtha, white oil, toluene, xylene, and various alcohols, ketones, esters, and the like can also be used.

  The mixing ratio of the PTFE fine powder and the liquid lubricant can be appropriately adjusted according to the type of the PTFE fine powder and the liquid lubricant or the method of forming the PTFE sheet. Usually, the liquid lubricant is 100 parts by weight of the PTFE fine powder. Is about 5 to 50 parts by weight.

  The specific method of extrusion and / or rolling is not particularly limited. For example, after the mixture is extruded into a rod shape, the obtained rod-shaped formed body may be rolled with a pair of rolls to be formed into a sheet. Alternatively, the mixture may be extruded into a sheet as it is, or may be further rolled after being extruded into a sheet.

  What is necessary is just to adjust the thickness of a non-baking PTFE sheet | seat suitably according to the thickness of the PTFE porous membrane to obtain, and it is about 0.05-0.5 mm normally.

  The liquid lubricant is preferably removed from the PTFE sheet by a technique such as heating or extraction before the stretching A is performed.

  In the production method of the present invention, the PTFE porous material having the following characteristics is adjusted by adjusting the temperature of stretch A, stretch B and heat treatment, the stretch ratio of stretch A and B, the thickness of the unfired PTFE sheet, and the like. A membrane is obtained.

[Air flow rate]
In the production method of the present invention, a PTFE porous membrane having a Gurley permeability measured in accordance with JIS P8117 can be formed at 3 seconds / 100 mL or less, preferably 2 seconds / 100 mL or less.

[Strength]
In the production method of the present invention, a PTFE porous membrane having an in-plane strength measured by the method described in the Examples can be formed at 20 MPa or more, preferably 24 MPa or more. Also, a PTFE porous membrane having a strength in the thickness direction measured by the method described in the Examples and having a thickness of 10 N / 25 mm or more, preferably 11 N / 25 mm or more can be formed.

[Ventilation member]
The ventilation member of the present invention includes a ventilation film having a PTFE porous film obtained by the production method of the present invention described above. The PTFE porous membrane obtained by the production method of the present invention has high air permeability and high strength. The gas permeable membrane having such a porous PTFE membrane has high air permeability and more reliably prevents the passage of foreign matters such as water and dust, that is, exhibits high water resistance, for example. The ventilation member of the present invention realizes high air permeability inside and outside the casing by attaching to the opening of the casing based on such characteristics of the PTFE porous membrane and the ventilation membrane included in the member. Prevent foreign objects from entering inside more reliably.

  The gas permeable membrane provided in the gas permeable member of the present invention may have a layer other than the PTFE porous membrane obtained by the production method of the present invention. The layer is, for example, a breathable support material that supports the PTFE porous membrane. In this case, the strength of the gas permeable membrane is improved, and the life of the gas permeable member including the gas permeable membrane is extended.

  The PTFE porous membrane and the gas permeable membrane having the gas permeable support material can be manufactured by superimposing the two or by integrating them with a technique such as adhesive lamination or heat lamination.

  The material and structure of the breathable support material are not particularly limited as long as the breathable support material is more breathable than the PTFE porous membrane obtained by the production method of the present invention.

  The structure of the breathable support material is, for example, felt, non-woven fabric, woven fabric, or mesh (mesh-like sheet). From the viewpoint of strength, flexibility, and workability in the manufacturing process, a breathable support material made of a nonwoven fabric is preferable. In this case, at least some of the fibers constituting the nonwoven fabric are composite fibers having a so-called core-sheath structure. Is preferred. When the core component has a core-sheath structure in which the melting point of the core component is higher than the melting point of the sheath component, thermocompression bonding between the breathable support material and the PTFE porous membrane in manufacturing the breathable membrane is facilitated.

  The material of the breathable support material is polyolefin such as polyethylene and polypropylene; polyester; polyamide such as nylon; aromatic polyamide; and a composite material thereof. As the breathable support material, a porous film of a fluorine resin, for example, PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer) or FEP (tetrafluoroethylene / hexafluoropropylene copolymer) may be used.

  FIG. 1 shows an example of the ventilation member of the present invention. A ventilation member 1 shown in FIG. 1 includes a ventilation membrane 2 having a PTFE porous membrane obtained by the above-described production method of the present invention, and a support 3 that supports the ventilation membrane 2. The gas permeable membrane 2 is joined to the support 3 so as to cover one opening of the cylindrical support 3. The ventilation member 1 is fixed to the opening 12 of the housing 11 by inserting the opening 12 of the housing 11 into the other opening of the support 3. In a state where the ventilation member 1 is fixed to the opening 12, the gas passes from the inside of the housing 11 to the outside or from the outside to the inside through the ventilation film 2, the support 3 and the opening 12. Flowing. Since the gas permeable membrane 2 exists in the gas path, entry of foreign matter such as water and dust into the housing 11 is suppressed.

  The ventilation member 1 shown in FIG. 1 further includes a protective cover 4. The protective cover 4 is fixed to the support 3 so that a gas path is secured between the outside of the housing 11 and the gas permeable membrane 2. The protective cover 4 prevents a relatively large foreign object from colliding with the gas permeable membrane 2 and damaging the gas permeable membrane 2.

  The support 3 and the protective cover 4 are typically made of resin. The support 3 is preferably made of a resin or elastomer having flexibility at room temperature. In this case, the housing 11 can be easily and reliably fixed to the opening 12.

  The configuration of the ventilation member of the present invention is not limited to the example shown in FIG. As long as the effect of the present invention is obtained, the ventilation member of the present invention can have any configuration.

  Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples.

  In this example, four types of PTFE porous membranes were produced by changing the production method (one type of Example and three types of Comparative Examples), and the air permeability and strength of the produced PTFE porous membranes were evaluated.

  The evaluation method of the characteristic in a PTFE porous membrane is shown.

[Air flow rate]
The Gurley air permeability (second / 100 mL) of the PTFE porous membrane was evaluated using an automatic Gurley type densometer in accordance with JIS P8117.

[Strength]
As the strength of the PTFE porous membrane, the strengths in the in-plane direction and the thickness direction were evaluated as follows. In addition, the in-plane direction which evaluated the intensity | strength is a rolling direction (MD direction) in the PTFE sheet before extending | stretching obtained by rolling. Further, the strength in the in-plane direction is tensile strength, and the strength in the thickness direction is peeling strength.

-In-plane direction (MD direction) strength In-plane direction strength (MPa) was evaluated by conducting a tensile test using Tensilon (Orientec, UTM-III-100) on a PTFE porous membrane. . The conditions of the tensile test are as follows: the sample shape is a strip having a width of 10 mm, the distance between chucks is 20 mm, the tensile speed is 200 mm / min, and the measurement temperature is 25 ° C.

-Strength in the thickness direction The strength in the thickness direction (N / 25 mm) was determined as follows. First, the PTFE porous membrane to be evaluated was processed into a strip shape having a width of 25 mm so that the TD direction was the width direction. Separately, a 25 mm wide strip-shaped adhesive film (manufactured by Tosero, Admer QE-060) and an aluminum sheet are prepared, and the PTFE porous membrane is sandwiched between the pair of the aluminum sheets via the adhesive film. did. At this time, the width direction and the longitudinal direction of the PTFE porous film, the adhesive film, and the aluminum sheet were aligned. In addition, by making the length of the aluminum sheet longer than the length of the PTFE porous membrane, each aluminum sheet was provided with a portion that did not contact the PTFE porous membrane via an adhesive film. Next, each layer was thermally welded by heating at 200 ° C. for 2 seconds, and a laminate of aluminum sheet 103 / adhesive film 102 / PTFE porous film 101 / adhesive film 102 / aluminum sheet 103 (a strip shape having a width of 25 mm, FIG. 2 (a)).

  Next, as shown in FIG. 2B, the aluminum sheets 103 were pulled in directions opposite to each other using the portion 103 a of the aluminum sheet 103 not contacting the PTFE porous membrane 101 as a handle. When the pulling force is increased, the PTFE porous membrane 101 starts to tear at a certain point as shown in FIG. 2C. At this point, the force applied to the aluminum sheet 103 is applied to the thickness of the PTFE porous membrane. The strength in the vertical direction (N / 25 mm) was used.

Example 1
100 parts by weight of PTFE fine powder (manufactured by Daikin Industries, Polyflon F104) and 19 parts by weight of dodecane as a liquid lubricant were uniformly mixed to form a PTFE paste which is a mixture of PTFE fine powder and liquid lubricant. Next, the formed PTFE paste was extruded into a rod shape at a pressure of ² MPa (20 kg / cm ² ) and further rolled with a pair of metal rolls to form a strip-shaped PTFE sheet. Next, the liquid lubricant was removed from the formed PTFE sheet by heat drying at 120 ° C. for 3 minutes.

  The unsintered and unstretched PTFE sheet thus obtained was stretched in the MD direction (longitudinal direction) by a zone stretching method at a stretching temperature of 300 ° C. and a stretching ratio of 4 times, and then at 380 ° C. for 30 seconds. Heat treated. Next, the PTFE sheet after the heat treatment was stretched in the TD direction (width direction) by a zone stretching method at a stretching temperature of 100 ° C. and a stretching ratio of 5 times to obtain a PTFE porous membrane. In addition, a pair of rolls having different rotation speeds were used for stretching in the MD direction. In this case, the draw ratio is substantially indicated by the ratio of the rotational speeds of both rolls. Stretching in the TD direction was performed by a tenter method. The stretching method is the same in the following comparative examples.

(Comparative Example 1)
An unsintered and unstretched PTFE sheet obtained in the same manner as in Example 1 is stretched in the MD direction (longitudinal direction) by a zone stretching method at a stretching temperature of 300 ° C. and a stretching ratio of 4 times, and subsequently the TD. The film was stretched in the direction (width direction) at a stretching temperature of 200 ° C. and a stretching ratio of 5 times. Next, the PTFE sheet after stretching was heat-treated at 380 ° C. for 60 seconds to obtain a PTFE porous membrane based on the method disclosed in JP-A-7-96831.

(Comparative Example 2)
An unfired and unstretched PTFE sheet obtained in the same manner as in Example 1 was stretched in the MD direction (longitudinal direction) at a stretching temperature of 380 ° C. and a stretching ratio of 4 times by a zone stretching method. Next, the stretched PTFE sheet was heat-treated at 380 ° C. for 60 seconds. Next, the heat-treated PTFE sheet is stretched in the TD direction (width direction) at a stretching temperature of 300 ° C. and a stretching ratio of 5 times, and the method disclosed in JP 2007-260547 A (firing stretching method) A PTFE porous membrane based was obtained.

(Comparative Example 3)
An unfired and unstretched PTFE sheet obtained in the same manner as in Example 1 was stretched in the MD direction (longitudinal direction) at a stretching temperature of 300 ° C. and a stretching ratio of 4 times by a zone stretching method at 380 ° C. Heat treatment was performed for 30 seconds. Next, the heat-treated PTFE sheet is stretched in the TD direction (width direction) by a zone stretching method at a stretching temperature of 380 ° C. and a stretching ratio of 5 times, and the method disclosed in Japanese Patent Publication No. 11-515036 A PTFE porous membrane based on was obtained.

  Table 1 below shows the evaluation results of the air flow rate and strength of each PTFE porous membrane produced in Examples and Comparative Examples. Moreover, about the Example and the comparative example 3, after extending | stretching to MD direction and heat processing, the evaluation result of the air flow rate and in-plane intensity | strength of the PTFE sheet in the time before extending | stretching to TD direction is shown in an intermediate. Table 1 shows the air flow rate and the in-plane strength. Further, with respect to Comparative Example 2, the evaluation results of the air permeability and in-plane strength of the PTFE sheet at the time point after the stretching in the MD direction at a temperature equal to or higher than the melting point of PTFE and before the stretching in the TD direction are shown as intermediates Table 1 also shows the air flow rate and the in-plane strength.

  As shown in Table 1, a PTFE porous membrane having a higher air flow rate and higher strength was obtained in the examples than in the comparative examples.

  According to the method for producing a PTFE porous membrane of the present invention, a PTFE porous membrane having both high air flow rate and strength can be obtained. Such a PTFE porous membrane can be used as a gas permeable membrane for a ventilation member. Suitable for use.

DESCRIPTION OF SYMBOLS 1 Ventilation member 2 Ventilation film 3 Support body 4 Protective cover 11 Case 12 Opening part 101 PTFE porous film 102 Adhesive film 103 Aluminum sheet 103a Handle

Claims (5)

An unsintered polytetrafluoroethylene sheet is stretched in a predetermined direction at a temperature below the melting point of polytetrafluoroethylene,
After heat treating the stretched sheet at a temperature equal to or higher than the melting point,
The method for producing a polytetrafluoroethylene porous membrane, wherein the heat-treated sheet is further stretched in a direction orthogonal to the predetermined direction at a temperature lower than the melting point. The unsintered polytetrafluoroethylene sheet is strip-shaped,
The predetermined direction is a longitudinal direction of the sheet;
The method for producing a polytetrafluoroethylene porous membrane according to claim 1, wherein the orthogonal direction is a width direction of the sheet. The draw ratio in the predetermined direction is 2 to 40 times,
The method for producing a polytetrafluoroethylene porous membrane according to claim 1, wherein a draw ratio in the orthogonal direction is 2 to 40 times.   The mixture of the polytetrafluoroethylene fine powder and the liquid lubricant is formed into a sheet by at least one method selected from extrusion and rolling to obtain the unfired polytetrafluoroethylene sheet. A method for producing a polytetrafluoroethylene porous membrane. A gas permeable membrane that allows gas passing through the opening to pass through while being fixed to the opening of the housing;
A ventilation member that supports the gas permeable membrane,
The ventilation member in which the said ventilation film has the polytetrafluoroethylene porous film obtained by the manufacturing method in any one of Claims 1-4.

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