JP2792354B2 - Polytetrafluoroethylene porous membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous film of polytetrafluoroethylene (hereinafter referred to as "PTFE") and a method for producing the same, and more particularly, to a method of floating in air and gas used in a clean room such as a semiconductor industry. The present invention relates to a novel porous PTFE membrane excellent as an air filter suitable for capturing fine particles and having a small pressure loss of air and gas, and a method for producing the same.

[0002]

2. Description of the Related Art As a material for an air filter used in a clean room, a filter medium formed by adding a binder to glass fiber and making paper is often used. However, such filter media have several disadvantages. For example, the presence of attached fibrils in the filter medium, the occurrence of self-dusting at the time of bending due to processing, or the increase in the binder to prevent self-dusting increases the pressure loss (Japanese Patent Application Laid-Open No. 63-163). No. 16019). In addition, this filter media
There is also a problem that, when contacted with a certain chemical such as hydrofluoric acid, dust is generated due to deterioration of the glass and the binder.

In order to solve these problems, it has been proposed to use an electret filter medium made of synthetic fiber (see Japanese Patent Application Laid-Open No. 54-53365), but it has been shown that electret attenuation occurs.

Therefore, it has been proposed to use a stretched porous PTFE membrane as an auxiliary means to prevent these drawbacks and to obtain a clean space (Japanese Patent Laid-Open No. 63-16 / 1988).
019 and JP-A-2-284614). However, this proposal also requires a hole diameter of 1 μm to prevent an increase in pressure loss.
m or more porous membrane is used. The reason that it is possible to collect suspended particles smaller than the pore size found in this proposal is based on the following theory.

[0005] It is said that there are three main mechanisms for removing particles in a fluid (Domnick Hunter F. Limited).
ilters Limited) Catalog: 1) Direct blockage: A mechanism in which relatively large particles are blocked by microfibers and removed as if sieved. 2) Inertial collision: A mechanism by which particles cannot turn as quickly as gas when passing along a winding path between microfibers, and eventually strike and adhere to the microfibers. 3) Diffusion / Brownian motion: Very small particles are governed by intermolecular forces and static electricity, and rotate spirally in gas, resulting in a large apparent diameter and sticking to microfibers, similar to inertial collisions Mechanism to do. In addition, a method of removing electret fibers by a mechanism of collecting electric charge (JP-A-54-53365) has been proposed. However, as shown in the data described in JP-A-2-284614, 1 μm or less is proposed. It can be seen that particles cannot be completely removed.

PTFE used as a filter medium
A typical example of the porous membrane is disclosed in Japanese Patent Publication No. 56-17216. In the present invention, it is necessary to increase the draw ratio and increase the porosity in order to obtain a filter membrane having a small pressure loss. This results in a larger pore size. Conversely, if the pore size is reduced, the stretching ratio cannot be increased, resulting in a porous film having a large pressure loss.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a PTFE porous membrane having a small pore size and a small pressure loss. Still another object of the present invention is to provide a filter medium having an improved collection performance of ultrafine particles.

[0008]

In order to solve the above-mentioned problems,
In the present invention, the porous membrane is predominantly made of fibrils by stretching the semi-fired PTFE biaxially at least 50 times in the stretch area ratio and heat-treating it at a temperature not lower than the melting point of PTFE. The area ratio of fibrils and nodules by microscopic image processing is 99: 1 to 75:25, the average fibril diameter is 0.05 μm to 0.2 μm, the maximum area of the nodules is 2 μm ² or less, and the average A porous PTFE membrane having a pore size of 0.2 to 0.5 μm is provided.

Further, the present invention provides a semi-sintered body having a thickness of about 20%.
1 / or less (if the original thickness of the semi-sintered body is, for example, 100 μm, it becomes 5 μm or less after stretching and firing) and the average pore diameter is 0.2
A ～0.5Myuemu, provides a PTFE porous membrane pressure loss with air permeated at a flow rate of 5.3 cm / sec, characterized in that a 10~100mmH ₂ O.

Although the PTFE porous membrane of the present invention can be used as it is, it can also be reinforced by laminating it with another low-pressure-loss porous material (reinforcing material). Laminated PTF
The E-porous membrane has improved handleability. Laminated P
The TFE porous membrane can be folded in a pleated shape and used as a filter for collecting ultrafine particles.

As the reinforcing material, non-woven fabric, woven fabric, mesh, and other porous membranes can be used. As the material of the reinforcing material, olefin (for example, polyethylene, polypropylene, etc.), nylon, polyester, aramid, or a composite thereof (for example, non-woven fabric composed of core / sheath fiber, low melting point material and high melting point material) A two-layer nonwoven fabric, etc., and a fluorine-based porous membrane (eg, PFA (tetrafluoroethylene / perfluoroalkylvinyl ether copolymer), FEP (tetrafluoroethylene / hexafluoropropylene copolymer), PTFE porous membrane, etc.)
Can be exemplified.

In particular, a nonwoven fabric made of fibers having a core / sheath structure, a two-layer nonwoven fabric of a low-melting material and a high-melting material, and the like are preferable. Such reinforcements do not shrink during lamination.
In addition, a laminate film with such a reinforcing material is made of HEPA.
It is easy to process as a filter, and the insertion pitch can be increased when making it into a filter element.

In the laminating mode, the porous PTFE membrane of the present invention may be laminated on one or both sides of the reinforcing material, or the porous PTFE membrane of the present invention may be sandwiched between two reinforcing materials.

The laminating method may be appropriately selected from known methods, such as thermocompression bonding by melting a part of the reinforcing material, thermocompression bonding using a powder of polyethylene, polyester, PFA or the like as an adhesive, hot lamination. Thermocompression bonding using a melt resin is preferred.

Although the particle collecting mechanism has been described above, in order to reliably collect the particles, it is necessary to prevent the particles adhering to the fibers of the filter from being detached again and to shield the penetrating particles. For that purpose, a filter material having a pore size smaller than the size of the particles to be surely collected should be used. Therefore, PTF
It is needless to say that a porous membrane having a small average pore diameter is preferred.

If the pore size and the porosity of the filter material are the same, the pressure loss is proportional to the film thickness.

Even if the filter material has the same pressure loss, pore size, porosity, and film thickness, the particle collecting performance is different. In theory, fine fibers of 0.5 μm or less can be used and a binder can be used. It is said that it is preferable to keep the amount as small as possible, that is, to reduce the portion other than the fiber (see the Abstracts of the 52nd Annual Meeting of the Society of Chemical Engineers, Emi). P of the present invention
The TFE porous membrane satisfies these conditions.

The present invention will be described in more detail including the production method. The material before stretching of the PTFE porous membrane used in the present invention is:
This is based on the PTFE semi-fired body defined in JP-A-59-152825. This semi-sintered PTFE is stretched at least 50 times, preferably at least 100 times, more preferably at least 2 times in the biaxial direction.
The structure of the stretched porous body stretched and fired 50 times has a unique membrane structure composed of fine fibers having almost no nodes.

Moreover, the PTFE thus produced is
The average pore size of the porous membrane is extremely small, usually from 0.5 μm
0.2 μm, and the thickness of the film is also reduced from 1/20 before stretching to about 1/100.

These requirements are suitable for an air filter material for maintaining a high clean space for processing a semiconductor fine pattern.

A PTFE porous membrane having such a structure cannot be obtained by a conventional manufacturing method. For example, Tokiko 56
According to No. 17216, page 11, left column, line 23 et seq., "FIG. 1 shows the stretching effect in the uniaxial direction, but the stretching effect in the biaxial direction and the stretching in all directions are the same. Such fibril formation occurs in that direction, creating a web-like or cross-linked shape and concomitantly increasing the strength. Porosity also increases as voids increase in number and size. "Increasing the draw ratio only increased the pore size.

The pressure loss decreases as the pore diameter increases and as the film thickness decreases. In order to manufacture an air filter having a small pore size and a low pressure loss, a thin PTFE porous membrane may be used.
In JP-A-17216), the width and the thickness hardly decrease even when the stretching ratio is increased. However, if the stretching ratio is extremely increased, the pore size becomes large, and as a result, the film thickness before stretching must be reduced, and the film must be stretched at a low ratio.
However, the thickness of an industrially usable film before stretching is at most 30 μm to 50 μm. In consideration of quality and yield, a film before stretching having a thickness of about 100 μm is usually used.

One feature of the present invention is that the object can be achieved using a film before stretching having a thickness of about 100 μm which does not hinder industrial productivity.

The general ranges and preferred ranges of each parameter in the present invention are shown together. General range Preferred range Firing degree: 0.30 to 0.80 0.35 to 0.70 Stretching ratio: MD 4 to 30 times MD 5 to 25 times TD 10 to 100 times TD 15 to 70 times Total 50 to 1000 75 times 850 times When the total stretching ratio is 250 times or more, the degree of firing is 0.1 times.
It is preferably from 35 to 0.48. Average pore size: 0.2-0.5 μm 0.2-0.4 μm Film thickness: 0.5-15 μm 0.5-10 μm Fibril / knot area ratio: 99 / 1-75 / 25 99 / 1-85 / 15 Average fibril diameter: 0.05-0.2 μm 0.05-0.2 μm Maximum area of nodule: 2 μm ² or less 0.05-1 μm ² Pressure loss: 10-100 mmH ₂ O 10-70 mmH ₂ O

The porous PTFE membrane of the present invention can be used not only as an air filter but also for example, of the present invention.
When the liquid is vaporized using the E porous membrane as a partition, a clear gas body from which fine particles in the liquid have been removed can be obtained. One example of such a specific application is a clean humidifier diaphragm.

Further according to the present invention, a very thin PTF
E porous membrane can be industrially produced, and the PT
The FE porous membrane can be used for applications requiring water repellency and applications requiring air permeability.

[0027]

Example 1 An unstretched unfired film having a thickness of 100 μm manufactured from PTFE fine powder (“Polyflon Fine Powder F-104” manufactured by Daikin Industries, Ltd.) was heated in an oven at 339 ° C. for 50 seconds. A continuous semi-baked film having a baking degree of 0.50 was obtained.

Next, this semi-baked film is cut into about 9 cm squares, and the four sides of the cut film are sandwiched between the clips of the apparatus using an apparatus (manufactured by Iwamoto Seisakusho) capable of simultaneously and sequentially stretching in two directions. After heating at a temperature of 320 ° C. for 15 minutes, 100
The film was stretched 5 times at a stretching speed of% / sec. Next, the film was continuously stretched 15 times while fixing the length in the MD direction in the width direction (referred to as the TD direction) of the film to obtain a porous film stretched 75 times (area ratio) in total.

This porous membrane is fixed by a frame so as not to shrink,
The heat set was performed by placing it in an oven at an ambient temperature of 350 ° C. for 3 minutes.

Example 2 A semi-baked film having the same degree of baking of 0.5 as in Example 1 was used.
8 times in the MD direction and 25 times in the TD direction as in the first embodiment.
The film was stretched twice and a porous film stretched 200 times in total was obtained. This porous membrane was heat-set at 350 ° C. for 3 minutes as in Example 1.

Example 3 Unstretched and unsintered with a thickness of 100 μm, which was produced from PTFE fine powder ("Polyflon Fine Powder F104" manufactured by Daikin Industries, Ltd.) by a usual processing method of paste extrusion, rolling rolls, and auxiliary agent drying. The film was heat-treated in an oven at 338 ° C. for 45 seconds to obtain a continuous semi-baked film having a baking degree of 0.40. In this step, the film before heat treatment has a width of 215 mm and a specific gravity of 1.5.
A 5 g / cm ^3, after the heat treatment, the width 200 mm, changing the specific gravity of 2.25 g / cm ^3, the thickness hardly changed.

Next, this semi-baked film was first stretched 20 times in the longitudinal direction using the apparatus of the reference example. The stretching conditions in the longitudinal direction were as follows. Rolls 3 and 4: unwinding speed 0.5 m / min, room temperature, film width 200 mm Roll 6: peripheral speed 4 m / min, temperature 300 ° C. Roll 7: peripheral speed 10 m / min, temperature 300 ° C. Roll 10: peripheral speed 10 m / Min, temperature 25 ° C Roll 2: winding speed 10m / min, room temperature, film width 1
45 mm Distance between the outer diameters of the rolls 6 and 7: 5 mm As a result, the stretching area magnification in the longitudinal direction becomes 14.5 times by calculation.

Subsequently, the film was stretched about 34 times in the width direction and subsequently heat-set by an apparatus shown in FIG. 25 which can continuously clip both ends of the longitudinally stretched film. The stretching and heat setting conditions in the width direction were as follows.・ Film running speed 3m / min ・ Preheating oven temperature 305 ℃ ・ Width stretching oven temperature 320 ℃ ・ Heat setting oven temperature 350 ℃ As a result, the total area magnification of stretching in the longitudinal direction and width direction is almost calculated. 490 times.

Example 4 A nonwoven fabric was laminated on both sides of the stretched film in Example 3 using the laminating apparatus shown in FIG. The lamination conditions were as follows. - upper nonwoven ELEVES T1003WDO (Unitika Ltd. products) and lower side nonwoven Mel fit BT030E (UniCel® Ltd. products) Temperature 0.99 ° C. of the heating roll was measured the pressure loss of the film laminated with the nonwoven fabric mean 25mmH ₂ O met Was. (This measurement is an average value of four places obtained by equally cutting the both ends to obtain a width of 800 mm and dividing it into four equal parts in the width direction. The maximum value is 27 mmH ₂ O,
Min 23mmH was ₂ O. )

Reference Example A semi-baked film having the same degree of baking of 0.5 as used in Example 1 was used.
The film was stretched by the apparatus shown in FIG. That is, the semi-baked film was sent from the film unwinding roll 1 to the rolls 6 and 7 via the rolls 3, 4, and 5, and was stretched 6 times in the MD direction. The stretched film was wound around a winding roll 2 via rolls 8 and 9, a heat set roll 10, a cooling roll 11 and a roll 12.

The stretching conditions at this time were as follows. Roll 6: Roll surface temperature 300 ° C., peripheral speed 1 m / min Roll 7: Roll surface temperature 300 ° C., peripheral speed 6 m / min Distance between the outer diameters of the roll 6 and the roll 7: 5 mm Roll 10: Roll surface temperature 300 ° C. Peripheral speed tuned to roll 7

Next, the stretched film is 1 m long and 1 width wide.
It was cut to 5 cm, stretched 4 times in the TD direction at room temperature without fixing the width, and heat-set at 350 ° C. for 3 minutes in the same manner as in Example 1. The stretched film did not have the nodules referred to in the present invention.

Examples 1, 2 and 3, a reference example, and two kinds of commercially available 0.1 μm porous membranes as comparative examples were prepared. The average pore diameter, the film thickness, the fibril / nodule area ratio, the average fibril diameter, and the maximum nodule were obtained. The area and pressure drop were measured. The measuring method is as described below. Table 1 shows the results.

[0039]

[Table 1] Note: Commercial product A: A porous membrane used for 0.1 μm Fluorogart TP cartridge manufactured by Millipore. Commercial product B: Advantech Toyo T300A293-
D PTFE membrane filter In Example 3, the both ends of the stretched film were equally cut and the width was 800 m.
The average value was obtained by measuring the four points after dividing the width into four equal parts in the width direction.

From the results shown in Table 1, the average pore size of the PTFE porous membrane of the present invention is almost the same as that of the commercial product A and the reference example, but the pressure loss is very small. The pressure loss of the porous PTFE membranes of Examples 1 and 2
It can be seen that the average pore size of the commercial product B is considerably large, although the pressure loss is almost the same as that of the commercial product B. Further, it can be seen that the pressure loss can be further reduced even when the average pore diameter is hardly changed by stretching the area magnification to nearly 500 times as in Example 3.

From the fibril / node area ratio, the commercial product A
The example is larger than the example. The average fibril diameter is smaller in the example than in the reference example. Also,
The maximum nodule area of the present invention is considerably smaller than that of the commercial product A.

A method for measuring each characteristic described in this specification will be described. Mean flow pore size measured according to the description of the average pore size of ASTM F-316-86 with (MFP) was defined as the average pore diameter. The actual measurement was made using a Coulter Porometer.
ometer) [Coulter Ele
ctronics) (UK)].

[0043] Using the film thickness Mitutoyo Corporation Ltd. 1D-110MH type film thickness meter, the thickness of the entire measured repeatedly five porous membrane dividing the thickness at 5, one obtained value Film thickness.

The pressure loss porous membrane was cut into a circle having a diameter of 47 mm, and the effective transmission area was 1
The filter was set in a 2.6 cm ² filter holder, the inlet side of the filter was pressurized to 0.4 kg / cm ² , the flow rate of the air flowing out of the outlet side was adjusted by a flow meter manufactured by Ueshima Seisakusho, and the permeation rate of the porous membrane was adjusted to 5. Adjusted to 3 cm / sec. The pressure loss at that time was measured with a manometer.

The firing of the PTFE semi-fired body of the firing of the present invention is determined in the following manner. First, 3.0 ± 0.1 mg from the unfired PTFE
Is weighed and cut out, and a crystal melting curve is first determined using this sample. Similarly, from the semi-fired PTFE, 3.0 ±
A 0.1 mg sample is weighed and cut out, and a crystal melting curve is obtained using this sample.

The crystal melting curve was measured by a differential scanning calorimeter (hereinafter referred to as
It is called "DSC". For example, DSC-50 manufactured by Shimadzu Corporation
(Type). First, a sample of the unfired PTFE is charged into a DSC aluminum pan, and the heat of fusion of the unfired body and the heat of fusion of the fired body are measured in the following procedure. (1) The sample is heated to 250 ° C. at a heating rate of 50 ° C./min and then from 250 ° C. to 380 at a heating rate of 10 ° C./min.
Heat to ° C. One example of the crystal melting curve recorded in this heating step is shown as curve A in FIG. The peak position of the endothermic curve appearing in this step is defined as “the melting point of the unfired PTFE” or “the melting point of the PTFE fine powder”. (2) Immediately after heating to 380 ° C, the sample is cooled to 250 ° C at a cooling rate of 10 ° C / min. (3) The sample is heated again to 380 ° C. at a heating rate of 10 ° C./min. One example of the crystal melting curve recorded in the heating step (3) is shown as curve B in FIG. The peak position of the endothermic curve appearing in the heating step (3) is defined as "the melting point of the fired PTFE".

Subsequently, the crystal melting curve of the semi-baked PTFE is recorded according to the step (1). Curve 1 in this case
An example is shown in FIG. The heat of fusion of the unfired, fired, and semi-fired PTFE bodies is proportional to the area between the endothermic curve and the baseline, and is automatically calculated by setting the analysis temperature for DSC-50, manufactured by Shimadzu Corporation. .

Therefore, the degree of firing is calculated by the following equation. Firing degree = (ΔH ₁ −ΔH ₃ ) / (ΔH ₁ −ΔH ₂ ) where ΔH ₁ is the heat of fusion of the unfired PTFE and ΔH ₂ is P
The heat of fusion of the fired TFE, ΔH _3, is the heat of fusion of the semi-fired PTFE. Japanese Patent Application Laid-Open No.
A detailed description is given in -152825.

Image analysis The area ratio between fibrils and nodules, average fibril diameter, and maximum nodule area were measured by the following methods. A photograph of the surface of the porous film is taken with a scanning electron microscope (Hitachi S-4000 type is Hitachi E
1030 type) (SEM photograph. Magnification 1000 times to 50 times)
00 times). This photograph is processed by an image processing device (main unit name: Nippon Avionics Co., Ltd.
00II, control software name: Ratok System Engineering Co., Ltd. TV image processor Image command 4198), separates into nodules and fibrils, and obtains an image consisting only of nodules and an image consisting only of fibers. The maximum nodule area was determined by processing the image consisting of nodules only, and the average diameter of fibrils was obtained by processing the image consisting only of fibrils (the total area was divided by 1/2 of the total circumference). The area ratio between fibrils and nodules was determined from the ratio of the sum of the areas of the fibril images to the area of the nodules.

FIGS. 4 and 5 show SEM photographs of the fiber structure of the porous PTFE membranes produced in Examples 1 and 2, respectively. FIG. 6 and FIG. 7 are views showing image processing of FIG. 4 and FIG. 5, respectively. 8 and 9 are views of the fibrils separated from FIGS. 6 and 7, respectively. 10 and 11
FIG. 8 is a view of a nodule separated from each of FIGS. 6 and 7.

FIGS. 12 and 13 show SEM photographs of the fiber structures of the porous membranes of the commercial products A and B, respectively. FIGS. 14 and 15 are diagrams of fibrils separated from the image-processed diagrams of FIGS. 12 and 13, respectively. FIG. 16 and FIG.
FIG. 7 is a diagram of a nodule separated from the image obtained by processing each of FIGS. 12 and 13.

Definition of a nodule A nodule is one that satisfies any of the following. (1) Multiple fibrils are connected together (Fig. 1
8: Part filled with dots. (2) Connected lump is thicker than fibril diameter
(FIG. 21 and FIG. 22: shaded portion) (3) Primary particles and primary particles are gathered, and fibrils extend radially from there (FIGS. 19, 20, and 23: shaded portion). Is an example that is not considered a nodule. That is, if the fibrils are branched, but the fibrils and the bifurcation have the same diameter, the bifurcation is not considered a nodule.

[Brief description of the drawings]

FIG. 1 is a schematic view of a stretching apparatus used in a reference example.

FIG. 2 is a view showing an example of a crystal melting curve of unfired PTFE and fired PTFE measured by DSC when measuring the degree of firing.

FIG. 3 is a view showing an example of a crystal melting curve of semi-baked PTFE measured by DSC when measuring a degree of firing.

FIG. 4 is an SEM photograph of a fiber shape of the PTFE porous membrane produced in Example 1.

FIG. 5 is an SEM photograph of a fiber shape of the PTFE porous membrane produced in Example 2.

6 is a photograph showing a fiber shape obtained by performing image processing on FIG.

FIG. 7 is a photograph showing a fiber shape obtained by performing image processing on FIG. 5;

FIG. 8 is a photograph of the fiber separated from FIG.

FIG. 9 is a photograph of the fiber separated from FIG.

FIG. 10 is a photograph of a nodule particle separated from FIG.

FIG. 11 is a photograph of a nodule particle separated from FIG.

FIG. 12 is an SEM photograph of a fiber shape of a porous membrane of a commercial product A.

FIG. 13 is an SEM photograph of a fiber shape of a porous membrane of a commercial product B.

FIG. 14 is a photograph of a fiber shape separated from the image obtained by processing the image of FIG.

FIG. 15 is a photograph of a fiber shape separated from the image obtained by image-processing FIG.

FIG. 16 is a photograph of a nodule particle separated from the image obtained by image-processing FIG.

FIG. 17 is a photograph of a nodule particle separated from the image obtained by processing the image of FIG.

FIG. 18 is a schematic view of an example of a fibril-knot structure.

FIG. 19 is a schematic view of an example of a fibril-knot structure.

FIG. 20 is a schematic view of an example of a fibril-knot structure.

FIG. 21 is a schematic view of an example of a fibril-knot structure.

FIG. 22 is a schematic view of an example of a fibril-knot structure.

FIG. 23 is a schematic view of an example of a fibril-knot structure.

FIG. 24 is a schematic view of an example of a fibril-knot structure.

FIG. 25 is a schematic view of a stretching device and a laminating device used in Examples 3 and 4.

[Explanation of Signs] 1: Film unwinding roll, 2: Winding roll, 3, 4, 5, 6,
7, 8, 9: roll, 10: heat set roll, 11: cooling roll, 12: roll, 13: film unwinding drum,
14: unwinding control mechanism, 15: preheating oven, 16: width stretching oven, 17: heat fixing oven, 18, 19:
Laminating roll, 19: heating roll, 20: winding control mechanism, 21: film width direction stretched film winding drum,
22, 23: Non-woven fabric mounting drum

Continued on the front page (51) Int.Cl. ⁶ Identification code FI B29K 105: 04 B29L 7:00 C08L 27:18 (72) Inventor Osamu Inoue 1-1, Nishiichitsuya, Settsu-shi, Osaka Daikin Industry Co., Ltd. Yodogawa Inside the factory (72) Inventor Katsunori Yamamoto 1-1, Nishiichitsuya, Settsu-shi, Osaka Daikin Industries, Ltd. Inside Yodogawa Factory (72) Inventor Tomoo Kusumi 1-1, Nishiichitsuya, Settsu-shi, Osaka Daikin Industries, Ltd. (56) References JP-A-59-152825 (JP, A) JP-A-2-6832 (JP, A) JP-A-3-221541 (JP, A) (58) Fields studied (Int. Cl ^6, DB name) C08J 9/00 -. 9/42

Claims (6)

(57) [Claims] 1. A polytetrafluoroethylene porous film obtained by stretching a semi-fired polytetrafluoroethylene body and then heat-setting it at a temperature equal to or higher than the melting point of the fired polytetrafluoroethylene body. Image ratio of fibrils to nodules is 99: 1
~ 75: 25, average fibril diameter is 0.05 μm ~
A polytetrafluoroethylene porous membrane having a thickness of 0.2 μm, a maximum nodule area of 2 μm ² or less, and an average pore diameter of 0.2 μm to 0.5 μm. And wherein the wherein the mean pore diameter of 0.2Myuemu～0.5Myuemu, and pressure loss with air permeated at a flow rate of 5.3 cm / sec is 10mmH ₂ O~100mmH ₂ O Polytetrafluoroethylene porous membrane. 3. A polytetrafluoroethylene semi-baked article is stretched in a biaxial direction at an extension area magnification of at least 50 times,
A polytetrafluoroethylene porous membrane which is heat-set at a temperature not lower than the melting point of the polytetrafluoroethylene fired body. 4. The polytetrafluoroethylene porous membrane according to claim 1, which is laminated on an olefin porous material or a fluorine porous material with or without an adhesive. 5. A polytetrafluoroethylene semi-fired body is stretched in a biaxial direction at an extension area ratio of at least 50 times,
The method for producing a polytetrafluoroethylene porous membrane according to any one of claims 1 to 3, wherein heat setting is performed at a temperature equal to or higher than the melting point of the polytetrafluoroethylene fired body. 6. An air filter comprising the polytetrafluoroethylene porous membrane according to claim 1.