WO2015170674A1

WO2015170674A1 - Porous film manufacturing method

Info

Publication number: WO2015170674A1
Application number: PCT/JP2015/063052
Authority: WO
Inventors: 永井　雅之; 宏樹上原; 健山延; 拓映坂村
Original assignee: 国立大学法人群馬大学
Priority date: 2014-05-07
Filing date: 2015-05-01
Publication date: 2015-11-12
Also published as: JP6351358B2; JP2015214598A

Abstract

Disclosed herein is a porous film manufacturing method including: a heating step in which at least a part of a non-porous precursor film that includes a polyacetal resin is heated as a heated part; and a stretching step in which a porous film comprising a stretched part that was made porous by biaxially stretching the heated part of the precursor film is manufactured. In this manufacturing method: the heating step is at least performed in the stretching step; in the heating step, at least 5 mol of oxyalkylene units having two or more carbon atoms is added with respect to 100 mol of oxymethylene units in the polyacetal resin; the melt flow rate of the polyacetal resin is 0.1-30 g/10 minutes; and the heated part is heated at a temperature less than the melting point of the precursor film, and in the stretching step, the real biaxial stretch ratio, as defined in formula (1) below, is a value greater than 25 times in the stretch part of the precursor film. Real biaxial stretch ratio=d_before/d_after… (1) (In formula (1), d_before represents the thickness of the precursor film before the biaxial stretching, and d_after represents the minimum thickness of the stretched part of the precursor film after the biaxial stretching)

Description

Method for producing porous film

The present invention relates to a method for producing a porous film.

Polyacetal resin is sometimes used as a filter such as a fuel filter because of its excellent mechanical properties and solvent resistance. In this case, the filter is made of a nonwoven fabric obtained by fiberizing a polyacetal resin by melt spinning (see, for example, Patent Document 1 below).

JP 2008-138326 A

However, the filter described in Patent Document 1 has the following problems.

That is, since the filter described in Patent Document 1 is composed of a nonwoven fabric obtained by fiberizing a polyacetal resin by melt spinning, it is difficult to produce a filter having fine pores that may be required in the future. there were.

This invention is made | formed in view of the said situation, and aims at providing the manufacturing method of the porous film which can manufacture the porous film which has a fine opening easily.

The present inventors have repeatedly studied to solve the above problems. For example, the present inventors have examined whether a polyacetal resin can be stretched to form openings. As a result, it has been clarified that no aperture is formed only by uniaxially stretching the polyacetal resin. It has also been found that there is a case where no opening can be formed even if biaxial stretching is performed. Thus, as a result of further earnest studies, the present inventors have found that the above problems can be solved only by setting the characteristics of the polyacetal resin, the temperature during stretching, and the stretching ratio within a specific range, and the present invention has been completed. It came to do.

That is, in the present invention, a heating step of heating at least a part of a non-porous precursor film containing a polyacetal resin as a heated portion, and the heated portion of the precursor film are made porous by biaxial stretching. And a heating step in which the heating step is performed at least in the stretching step, and in the heating step, an oxyalkylene having 2 or more carbon atoms per 100 moles of oxymethylene units in the polyacetal resin. The amount of units introduced is 5 mol or less, the melt flow rate of the polyacetal resin is 0.1 to 30 g / 10 min, the heated portion is heated at a temperature lower than the melting point of the precursor film, and the stretching In the process, it is defined by the following formula (1) with respect to the stretched portion of the precursor film. Quality biaxial draw ratio to a value of more than 25 times, a method for producing a porous film.
Real biaxial stretching ratio = d _before / d _after (1)
(In the above formula (1), d _before represents the thickness of the precursor film before biaxial stretching, and d _after represents the minimum thickness of the stretched portion of the precursor film after biaxial stretching)
Here, the substantial biaxial stretching ratio means the product of the stretching ratios of two orthogonal stretching axes calculated on the assumption of the same volume change.

According to this manufacturing method, a porous film having fine pores can be easily manufactured. Moreover, according to the said manufacturing method, since a porous film is formed by extending | stretching the to-be-heated part of a precursor film biaxially, it also becomes possible to make a porous film high intensity | strength.

In the above production method, it is preferable that in the heating step, the amount of the oxyalkylene unit having 2 or more carbon atoms introduced into the polyacetal resin is 0.5 mol or more per 100 mol of the oxymethylene unit.

In this case, it is possible to increase the strength of the porous film as compared with the case where the introduction amount of the oxyalkylene unit having 2 or more carbon atoms relative to 100 mol of the oxymethylene unit is less than 0.5 mol.

In the manufacturing method, in the stretching step, it is preferable that the substantial biaxial stretching ratio with respect to the stretched portion of the precursor film is 500 times or less. That is, it is preferable to perform biaxial stretching so that the substantially biaxial stretching ratio of the precursor film with respect to the heated portion is 500 times or less.

In this case, the precursor film is more difficult to break as compared with the case where the substantially biaxial stretch ratio with respect to the stretched portion of the precursor film exceeds 500 times.

In the manufacturing method, in the heating step, the difference between the heating temperature of the heated portion of the precursor film and the melting point of the precursor film is preferably 50 ° C. or less.

In this case, compared with the case where the difference between the heating temperature of the heated portion of the precursor film and the melting point of the precursor film exceeds 50 ° C., biaxial stretching can be easily performed, and the porous film can be produced more efficiently. it can.

In the manufacturing method, in the heating step, the difference between the heating temperature of the heated portion of the precursor film and the melting point of the precursor film is preferably 1 ° C. or more.

In this case, the stretchability of the heated portion of the precursor film can be further improved.

In the manufacturing method, in the stretching step, it is preferable that a stretching speed of the heated portion is 1 mm / min or more.

In this case, sufficient productivity can be secured.

In the manufacturing method, in the stretching step, it is preferable that a stretching speed of the heated portion is 500 mm / min or less.

In this case, the heated portion of the precursor film can be uniformly biaxially stretched without breaking.

In the present invention, the “melting point of the precursor film” refers to a value measured by DSC for the material constituting the precursor film, and specifically, at a heating rate of 10 ° C./min in a nitrogen atmosphere. The peak temperature of the melting peak measured in the temperature range of 30 to 200 ° C.

According to the present invention, there is provided a method for producing a porous film that can easily produce a porous film having fine pores.

It is a top view which shows 1 process of the manufacturing method of the porous film of this invention. It is a top view which shows the porous film obtained with the manufacturing method of the porous film of this invention. It is a figure which shows the observation result by the atomic force microscope about the stretched film obtained in Example 1. FIG. It is a figure which shows the observation result by the atomic force microscope about the stretched film obtained by the comparative example 3. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 1 and 2. FIG. 1 is a plan view showing one step of the method for producing a porous film of the present invention, and FIG. 2 is a plan view showing the porous film obtained by the method for producing a porous film of the present invention.

As shown in FIGS. 1 and 2, the production method of the present invention includes a heating step of heating at least a part of a non-porous precursor film 10 containing a polyacetal resin as a heated portion 20, and a coating of the precursor film. And a stretching step of manufacturing a porous film 30 including a stretched portion that has been made porous by stretching the heating portion biaxially. And in this manufacturing method, a heating process is performed in an extending process at least. That is, at least in the stretching step, biaxial stretching of the heated portion 20 is performed while heating the heated portion 20.

In the heating step, the introduction amount of the oxyalkylene unit having 2 or more carbon atoms per 100 moles of the oxymethylene unit in the polyacetal resin is 5 moles or less, and the melt flow rate of the polyacetal resin is 0.1 to 30 g / 10. The heated portion 20 is heated at a temperature lower than the melting point of the precursor film 10. In the stretching step, the substantially biaxial stretching ratio defined by the following formula (1) with respect to the stretched portion of the precursor film 10 is set to a value exceeding 25 times.
Real biaxial stretching ratio = d _before / d _after (1)
(In the above formula (1), d _before represents the thickness of the precursor film _before biaxial stretching, and d _after represents the minimum thickness of the stretched portion of the precursor film 10 after biaxial stretching)

According to the production method of the present invention, it is possible to easily produce a porous film 30 having fine pores. In addition, according to the manufacturing method of the present invention, the porous film 30 is formed by biaxially stretching the heated portion 20 of the precursor film 10, so that the strength of the porous film 30 can be further increased. Become.

Hereinafter, the production method of the present invention will be described in more detail. Specifically, the heating step and the stretching step will be described in detail.

<Heating process>
In the heating step, as described above, at least a part of the precursor film 10 is heated as the heated portion 20.

(Precursor film)
The precursor film 10 is non-porous and contains a polyacetal resin. Here, in the polyacetal resin, the introduction amount of oxyalkylene units having 2 or more carbon atoms per 100 moles of oxymethylene units (hereinafter simply referred to as “introduction amount of oxyalkylene units”) is 5 moles or less. Here, “the number of carbon atoms is 2 or more” means that the number of carbon atoms is plural. Examples of the oxyalkylene unit having 2 or more carbon atoms include an oxyethylene unit, an oxypropylene unit, and an oxybutylene unit.

When the introduction amount of the oxyalkylene unit is 5 mol or less, biaxial stretching can be performed more easily than in the case where the amount exceeds 5 mol, and the porous film 30 can be easily obtained.

The introduction amount of the oxyalkylene unit is preferably 3 mol or less, more preferably 2 mol or less. The introduced amount of the oxyalkylene unit may be 0 mol. That is, the polyacetal resin may be a homopolyacetal resin that does not contain an oxyalkylene unit and has only an oxymethylene unit.

However, the introduction amount of the oxyalkylene unit is preferably 0.5 mol or more. In this case, the strength of the porous film 30 can be further increased as compared with the case where the amount of oxyalkylene units introduced is less than 0.5 mol.

The melt flow rate (hereinafter referred to as “MFR”) of the polyacetal resin is 0.1 to 30 g / 10 minutes. In this case, the productivity of the film is further improved as compared with the case where the MFR of the polyacetal resin is less than 0.1 g / 10 minutes. On the other hand, when the MFR of the polyacetal resin is 0.1 to 30 g / 10 min, it becomes easier to stretch compared to the case where the MFR of the polyacetal resin exceeds 30 g / 10 min.

The MFR of the polyacetal resin is preferably 1 to 20 g / 10 minutes, more preferably 2 to 10 g / 10 minutes, from the viewpoint of improving stretchability.

The thickness of the precursor film 10 is not particularly limited, but is preferably 1000 μm or less, and more preferably 500 μm or less. In this case, when the thickness of the precursor film 10 is 1000 μm or less, it becomes easier to stretch compared to the case where the thickness exceeds 1000 μm.

The precursor film 10 may further include a thermoplastic resin other than the polyacetal resin. Examples of such thermoplastic resins include polyester resins and polyether resins. Examples of the polyester resin include polylactic acid, polyhydroxybutyric acid, and polyglycolic acid. Examples of the polyether resin include polydioxolane, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. The thermoplastic resin is preferably blended at a ratio of 1 to 200 parts by mass with respect to 100 parts by mass of the polyacetal resin.

The precursor film 10 includes an antioxidant, a heat stabilizer, a colorant, a nucleating agent, a fluorescent brightening agent, or a release agent such as a fatty acid ester-based or silicon-based compound such as pentaerythritol tetrastearate, a sliding agent, Antistatic agents such as polyethylene glycol and glycerin, higher fatty acid salts, UV absorbers such as benzotriazole or benzophenone compounds, or light stabilizers such as hindered amines, solvents necessary for porosity, plasticizers, An additive such as an inorganic filler may further be included as necessary.

The precursor film 10 can be obtained by, for example, melt press molding a material constituting the precursor film 10. Melt press molding is obtained, for example, by sandwiching the above material with a pressure plate through a release film, pressing the obtained structure with a press while melting the material, and then gradually cooling to room temperature. Can do. The melt press molding may be performed only once or a plurality of times.

(Heating temperature during stretching)
The heating temperature of the heated portion 20 of the precursor film 10 is set to a temperature lower than the melting point of the precursor film 10. When the heating temperature of the heated portion 20 of the precursor film 10 is set to a temperature equal to or higher than the melting point of the precursor film 10, the heated portion 20 is in a molten state and is not sufficiently stretched, and the porous film 30 having fine pores. It is because you cannot get.

The heating temperature of the heated portion 20 of the precursor film 10 may be a temperature lower than the melting point of the precursor film 10. The difference between the heating temperature of the heated portion 20 of the precursor film 10 and the melting point of the precursor film 10 is not particularly limited, but is preferably 50 ° C. or less. In this case, biaxial stretching can be easily performed compared to the case where the difference between the heating temperature of the heated portion 20 of the precursor film 10 and the melting point of the precursor film 10 exceeds 50 ° C. It can be manufactured more efficiently.

The difference between the heating temperature of the heated portion 20 of the precursor film 10 and the melting point of the precursor film 10 is more preferably 30 ° C. or less from the viewpoint of improving the transparency of the film, and 5 ° C. or less. More preferably.

However, from the viewpoint of improving the stretchability of the heated portion 20 of the precursor film 10, the difference between the heating temperature of the heated portion 20 of the precursor film 10 and the melting point of the precursor film 10 is 1 ° C. or more. More preferably.

Here, the melting point of the precursor film 10 is usually 140 to 170 ° C.
The heating step is performed at least in the stretching step, but may be further performed before the stretching step.

<Extension process>
In the stretching step, as described above, the porous film 30 including the stretched portion made porous by biaxially stretching the heated portion 20 of the precursor film 10 is manufactured. In the stretching step, the substantially biaxial stretch ratio defined by the above formula (1) with respect to the stretched portion of the precursor film 10 is a value exceeding 25 times.

(Substantially biaxial stretching ratio)
The substantially biaxial stretching ratio with respect to the stretched portion (porous film 30) of the precursor film 10 is defined by the above formula (1). Here, the biaxial stretching ratio is set to a value exceeding 25 times. In this case, it becomes easier to obtain the porous film 30 having fine pores as compared with the case where the substantially biaxial stretching ratio is 25 times or less.

The substantial biaxial stretching ratio is more preferably 40 times or more, still more preferably 50 times or more, and particularly preferably 60 times or more.

However, it is preferable that the substantially biaxial stretching ratio is 500 times or less. In this case, the precursor film 10 becomes more difficult to break. The substantial biaxial stretching ratio is more preferably 200 times or less, and even more preferably 150 times or less.

In this specification, the apparent draw ratio calculated based on the distance between chucks along the two draw axes A and B according to the following formula (2) is expressed as an apparent biaxial draw ratio.
Apparent biaxial draw ratio = (L1 / L2) × (L3 / L4) (2)
(In the above formula (2), L1 and L3 represent distances between two chucks after biaxial stretching along two stretching axes A and B, respectively, and L2 and L4 represent two stretching axes A and B, respectively. Represents the distance between two chucks before biaxial stretching along
Here, the two extending axes A and B are orthogonal to each other. Further, (L1 / L2) and (L3 / L4) in the apparent biaxial stretching ratio may be 1 or more, and may be the same or different.

When only a part of the precursor film 10 is the heated part 20, only the stretched part of the precursor film 10 after biaxial stretching is cut out, and the cut-out stretched part is used as the porous film 30. The In this case, the substantial biaxial stretching ratio in the above formula (1) is larger than the apparent biaxial stretching ratio in the above formula (2). This means that the heated portion 20 is sufficiently stretched and thinned by being heated, and the portions other than the heated portion 20 are not sufficiently heated and thus are not sufficiently stretched and do not become thin.

Further, the precursor film 10 may be entirely the heated part 20 or only a part thereof may be the heated part 20. Here, when the whole of the precursor film 10 after biaxial stretching is a stretched portion, the entire precursor film 10 is used as the porous film 30 as it is. When the entire biaxially stretched precursor film 10 is a stretched portion, the substantial biaxial stretch ratio in the above formula (1) is equal to the apparent biaxial stretch ratio in the above formula (2).

Although the extending | stretching speed | rate of the to-be-heated part 20 is not restrict | limited in particular, It is preferable that it is 1 mm / min or more from a viewpoint of ensuring sufficient productivity. However, the stretching speed is preferably 500 mm / min or less and more preferably 20 mm / min or less from the viewpoint of uniformly biaxially stretching without breaking the heated portion 20 of the precursor film 10. preferable. The stretching speed in the biaxial stretching according to the present invention is a value calculated based on the distance between chucks.

Since the porous film obtained by the production method of the present invention has fine openings and high strength, it can be suitably used as a reinforcing material for reinforcing a gas separation membrane, for example. This is due to the following reason. That is, since the porous film has fine pores, the gas that has permeated through the gas separation membrane can be permeated, and gas can be supplied to the gas separation membrane through the porous film. Moreover, since the porous film has high strength, it is possible to support a gas separation membrane having poor strength, and the gas separation membrane can be made thinner.

Moreover, since the porous film obtained by the production method of the present invention has fine pores, it can itself be suitably used as a filter for restricting the flow of fine particles. In particular, since a polyacetal resin has solvent resistance, the porous film obtained by the production method of the present invention is suitable as a filter used in a solvent. Examples of such a filter include a battery separator.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to the following examples.

The polyacetal resin (hereinafter referred to as “POM”) used in Examples and Comparative Examples was as follows. Hereinafter, “oxymethylene unit” is abbreviated as “OM”, and “oxyalkylene unit” is abbreviated as “OA”. Hereinafter, MFR refers to a value measured under conditions of 190 ° C. and 2.16 kg based on the ASTM-D1238 standard.
(1) POM1
Amount of OA introduced per 100 mol of acetal copolymer OM having the following characteristics: 1.4 mol MFR: 2.5 g / 10 min Melting point: 165 ° C.
(2) POM2
Amount of OA introduced per 100 mol of acetal copolymer OM having the following characteristics: 1.4 mol MFR: 9.0 g / 10 min Melting point: 165 ° C.
(3) POM3
Amount of OA introduced per 100 mol of acetal copolymer OM having the following characteristics: 0.6 mol MFR: 9.0 g / 10 min Melting point: 165 ° C.
(4) POM4
Amount of OA introduced per 100 mol of acetal copolymer OM having the following characteristics: 5.1 mol MFR: 9.0 g / 10 min Melting point: 155 ° C.

(Examples 1 to 3)
First, a polyimide film for mold release of 125 μm is placed on a 120 mm × 120 mm × 2 mm thick square stainless steel plate, and then a 105 mm × 105 mm window is placed on a 120 mm × 120 mm × 0.50 mm thick square stainless steel plate. About 10 g of pellets made of POM (POM1 to 3) shown in Table 1 were placed in the window. A mold release polyimide film having a thickness of 125 μm was placed thereon, and a square stainless steel plate having a thickness of 120 mm × 120 mm × thickness 2 mm was further placed thereon. Thus, the first structure was obtained.

The first structure thus obtained was sandwiched between upper and lower press mechanisms installed in a desktop press device, held at 160 ° C. for 5 minutes, and then melted at a pressure of 2.5 MPa (cylinder pressure 30 MPa) for 5 minutes. The product was press-molded, slowly cooled to room temperature, and taken out. Thus, a first film having a thickness of 500 μm was formed.

From this first film, a 52 mm × 58 mm × 500 μm portion was cut out as a second film.

Next, a polyimide film for mold release having a thickness of 125 μm is placed on a disk-shaped stainless steel plate having a diameter of 110 mm × thickness of 2 mm, and then a disk-shaped stainless steel plate having a diameter of 110 mm × thickness of 0.30 mm has a thickness of 70 mm × 70 mm. A window was cut out and the second film was placed in the window. A mold release polyimide film having a thickness of 125 μm was placed thereon, and a disc-shaped stainless steel plate having a diameter of 110 mm and a thickness of 2 mm was further placed thereon. A second structure was thus obtained. And after hold | maintaining this 2nd structure at 200 degreeC for 5 minute (s), it melt-molded for 5 minutes at the pressure of 2.5MPa (cylinder pressure 30MPa), and took out, after cooling gradually to room temperature. Thus, a press film having a thickness of 300 μm was produced. And from this press film, a 35 mm × 35 mm × 300 μm portion was cut out and used as a precursor film.

Next, the precursor film was biaxially stretched as follows using a biaxial stretching machine (product name “BIX704S” manufactured by Island Kogyo Co., Ltd.) to obtain a stretched film. That is, first, a precursor film is sandwiched and set at room temperature by four chucks of a biaxial stretching machine, and then a part to be heated is used as a part to be heated to blow heated air on the part to be heated. The temperature is raised to the stretching temperature shown in Table 1, and after maintaining at this temperature for 5 minutes, the substantial stretching ratio (substantially biaxial stretching ratio) becomes the value shown in Table 1 with respect to the heated portion of the precursor film. Thus, simultaneous biaxial stretching was performed at a stretching speed of 20 mm / min. And the extending | stretching part obtained by extending | stretching a to-be-heated part was cut out from the precursor film after biaxial stretching. Thus, a stretched film comprising stretched portions having the film thicknesses shown in Table 1 was obtained. Table 1 also shows the apparent biaxial stretching ratio ((L1 / L2) × (L3 / L4)) of the entire precursor film.

(Comparative Example 1)
The precursor film of Example 1 was used as the film of Comparative Example 1 (unstretched film).

(Comparative Example 2)
A thickness of 12 μm was obtained in the same manner as in Example 1 except that the biaxial stretching was performed so that the substantial stretching ratio (substantially biaxial stretching ratio) and the apparent stretching ratio (apparent biaxial stretching ratio) were the values shown in Table 1. A stretched film was obtained.

(Comparative Example 3)
First, a precursor film was prepared in the same manner as in Example 1.

Next, this precursor film was uniaxially stretched as follows using a uniaxial stretching machine (product name “Tensilon Universal Testing Machine RTC-1325A” manufactured by Orientec Co., Ltd.) to obtain a stretched film. It was. The thickness of the precursor film used for uniaxial stretching was 500 μm, and the precursor film was cut into a dumbbell shape having an initial sample length of 50 mm (length of stretched portion: 3 mm) and a width of 4 mm. The precursor film was set on a chuck at a predetermined temperature in a thermostatic chamber, held for 5 minutes, and then uniaxially stretched at a stretching speed of 20 mm / min. Thus, a stretched film comprising stretched portions having the film thicknesses shown in Table 1 was obtained. The substantially uniaxial stretching ratio is defined by the following formula (3).
Real uniaxial stretching ratio = S _before / S _after (3)
(In the above formula (3), S _before represents the cross-sectional area before uniaxial stretching in the precursor film, and S _after represents the cross-sectional area of the stretched portion after uniaxial stretching)
Table 1 also shows the apparent stretch ratio (apparent uniaxial stretch ratio) (L1 / L2) of the entire precursor film.

(Comparative Example 4)
A precursor film was produced in the same manner as in Example 1 except that POM4 was used instead of POM1 as a constituent material. Then, biaxial stretching was attempted in the same manner as in Example 1 except that the stretching temperature was set as shown in Table 1 for the heated portion of the precursor film. However, the precursor film was broken during the stretching, and the biaxial stretching could not be performed.

The properties of the stretched films or unstretched films obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were evaluated as follows.

[Characteristic evaluation]
The stretched films or unstretched films obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were checked for the presence or absence of openings. The presence or absence of the opening was confirmed based on the result of observation with an atomic force microscope (AFM) and the result of the nitrogen gas permeability coefficient.

(1) Observation Results by AFM First, the stretched films or the unstretched films obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were observed with AFM (E-sweep manufactured by SIINT). The results are shown in Table 1. In Table 1, when there was an opening, “present” was displayed, and when no opening was found, “no” was displayed. In Comparative Example 4, since the precursor film could not be stretched, “−” was displayed in Table 1. Moreover, about the stretched film obtained in Example 1, the observation result by AFM is shown in FIG. 3, and about the stretched film obtained in Comparative Example 3, the observation result by AFM is shown in FIG.

(2) Nitrogen gas permeability coefficient The nitrogen gas permeability coefficient was measured using a differential pressure method based on JIS K7126. More specifically, the nitrogen gas permeability coefficient is a differential pressure type gas / vapor permeability measuring device (product name “GTR-30XAD, G6800T • F (S)”, GTR Tech Co., Ltd./Yanaco Technical) Science Co., Ltd.). The test differential pressure was 1 atm, and nitrogen gas in a dry state was used. The test temperature was 23 ± 2 ° C., and the transmission area was 1.52 × 10 ⁻³ m ² (φ4.4 × 10 ⁻² m)). The results are shown in Table 1. In Comparative Example 4, since the precursor film could not be stretched, “−” was displayed in Table 1.

In addition, the criteria for judging whether fine pores are formed in the stretched film or the unstretched film were as follows. That is, when an opening is observed in the observation result by AFM and the nitrogen gas permeability coefficient exceeds 1.0 × 10 ⁻¹⁴ (mol · m / m ² · s · Pa), a fine opening is not obtained. It was judged that it was formed.

(3) Mechanical strength A 5 mm × 30 mm portion was cut out from the stretched film or the unstretched film obtained in Examples 1 to 3 and Comparative Examples 1 and 2 to obtain test pieces. Moreover, about the stretched film obtained by the comparative example 3, since the width | variety was narrower than 5 mm, the part of length 30mm was cut out and it was set as the test piece. These test pieces were subjected to a tensile test at 23 ± 2 ° C. using an RTC-1325A manufactured by Baldwin Co., Ltd., and the maximum stress on the stress chart recorded at a tensile speed of 20 mm / min was divided by the film cross-sectional area. The value was taken as the tensile strength. The results are shown in Table 1. In Comparative Example 4, since the precursor film could not be stretched, “−” was displayed in Table 1.

From the results shown in Table 1, it was found that the stretched films of Examples 1 to 3 had pores. Moreover, since the nitrogen gas permeability coefficient was high, it was also found that the opening was a through hole. On the other hand, it was found that the stretched films or the unstretched films of Comparative Examples 1 to 3 did not have pores. In Comparative Example 4, a stretched film could not be obtained.

Therefore, according to the method for producing a porous film of the present invention, it was confirmed that a porous film having fine pores can be easily obtained.

DESCRIPTION OF SYMBOLS 10 ... Precursor film 20 ... Heated part 30 ... Stretched part or porous film A ... Stretch axis B ... Stretch axis

Claims

A heating step of heating at least a part of a non-porous precursor film containing a polyacetal resin as a heated portion;
Including a stretching step of producing a porous film composed of a stretched portion made porous by stretching the heated portion of the precursor film biaxially,
The heating step is performed at least in the stretching step,
In the heating step, the polyacetal resin has an introduction amount of oxyalkylene units having 2 or more carbon atoms to 100 mol of oxymethylene units of 5 mol or less, and a melt flow rate of the polyacetal resin of 0.1 to 30 g / 10 min. And heating the heated part at a temperature below the melting point of the precursor film,
The method for producing a porous film, wherein, in the stretching step, a substantially biaxial stretching ratio defined by the following formula (1) is set to a value exceeding 25 times with respect to the stretching portion of the precursor film.
Real biaxial stretching ratio = d before / d after (1)
(In the above formula (1), d before represents the thickness of the precursor film before biaxial stretching, and d after represents the minimum thickness of the stretched portion of the precursor film after biaxial stretching)
2. The method for producing a porous film according to claim 1, wherein, in the heating step, an introduction amount of an oxyalkylene unit having 2 or more carbon atoms per 100 mol of oxymethylene units in the polyacetal resin is 0.5 mol or more.
The method for producing a porous film according to claim 1 or 2, wherein, in the stretching step, the substantially biaxial stretching ratio with respect to the stretched portion of the precursor film is set to 500 times or less.
The porous film according to any one of claims 1 to 3, wherein, in the heating step, a difference between a heating temperature of the heated portion of the precursor film and a melting point of the precursor film is 50 ° C or less. Production method.
The method for producing a porous film according to claim 4, wherein, in the heating step, a difference between a heating temperature of the heated portion of the precursor film and a melting point of the precursor film is 1 ° C or more.
The method for producing a porous film according to any one of claims 1 to 5, wherein, in the stretching step, a stretching speed of the heated portion is 1 mm / min or more.
The method for producing a porous film according to claim 6, wherein in the stretching step, the stretched speed of the heated portion is 500 mm / min or less.