阅读说明：本技术 延伸多孔性膜及其制造方法 (Stretched porous film and method for producing same ) 是由 田中伸幸 井村哲也 镰田邦男 于 2018-11-30 设计创作，主要内容包括：实现了兼备通气性、耐水性和柔软性的延伸多孔性膜。根据本发明的一个实施方式的延伸多孔性膜包括以特定的质量比含有特定的聚乙烯系树脂和热塑性弹性体的树脂组合物,并且该延伸多孔性膜的透湿度为1400g/m<Sup>2</Sup>·24h以上。(An extended porous film having air permeability, water resistance and flexibility is realized. An extended porous film according to an embodiment of the present invention includes a resin composition containing a specific polyethylene resin and a thermoplastic elastomer at a specific mass ratio, and the extended porous film has a moisture permeability of 1400g/m 2 24h or more.)

1. An elongated porous film characterized by comprising a polymer having a density of 0.900g/cm³Above and 0.940g/cm³The following polyethylene resin,

1.0 to 16 parts by mass of a thermoplastic elastomer per 100 parts by mass of the polyethylene resin, and

a resin composition of an inorganic filler,

moisture permeability of 1400g/m measured at 40 ℃ and 60% relative humidity according to ASTM E96²24h or more.

2. The extended porous film according to claim 1, wherein the thermoplastic elastomer is an olefin-based elastomer and/or a styrene-based elastomer.

3. The stretched porous film according to claim 1 or 2, wherein the stretched porous film is stretched in the machine direction at a distance between chucks of 50mm and a stretching speed of 200mm/min in accordance with JIS K7127, and the strength in the machine direction when the distance between chucks is stretched by 5% is 0.3N/25mm or more and 2.5N/25mm or less.

4. The extended porous film according to any one of claims 1 to 3, wherein the melt mass flow rate of the resin composition measured at 190 ℃ in accordance with JIS K7210 is 2.0g/10min or more.

5. The stretched porous film according to any one of claims 1 to 4, wherein the air permeability measured by the Wang-Mill test method based on JIS P8117 is 300 seconds/100 m L or more and 2000 seconds/100 m L or less.

6. The extended porous film according to any one of claims 1 to 5, wherein the resin composition further comprises a paraffin-based oil.

7. A method for producing an extended porous film, comprising:

the density is 0.900g/cm³Above and 0.940g/cm³A mixing step of mixing a thermoplastic elastomer and an inorganic filler in an amount of 1.0 to 16 parts by mass based on 100 parts by mass of the polyethylene resin to obtain a resin composition;

a molding step of molding the resin composition into a film shape; and

and a porosification step of expanding the film obtained in the molding step at least in the machine direction to make the film porous.

8. The method for producing an elongated porous film according to claim 7, wherein the mechanical-direction draw ratio in the porous step is represented by the following formula II:

y is more than or equal to 1.4 and less than or equal to 0.075X +2.5 … … (formula II),

in the formula, X represents the blending ratio (parts by mass) of the thermoplastic elastomer to 100 parts by mass of the polyethylene resin, and Y represents the draw ratio (times).

Technical Field

The present invention relates to an elongated porous film and a method for producing the same.

Background

In general, personal care products such as diapers need to allow air, steam, and the like to pass therethrough, and to prevent stuffiness and the like by not allowing liquid to pass therethrough. Therefore, air permeability and water resistance are required for personal care products such as diapers. In order to meet this demand, a porous film having fine pores is used by molding a resin having water repellency such as a polyolefin resin into a film. Such a porous membrane has a configuration that allows air or the like to pass through but does not allow liquid to pass through.

Patent document 1 discloses a breathable film composed of a resin composition containing a polyethylene resin having a specific density, melting point and MFR, an olefin thermoplastic elastomer, an inorganic filler and a plasticizer, and having a strength when stretched by 20% in the transverse direction and a residual strain after stretching by 50% within specific ranges.

Patent document 2 discloses a breathable elastic membrane including a high-performance elastomer such as a styrene-based block copolymer and a low-performance elastomer such as polyolefin in which a plurality of particles suitable for forming pores are filled in a membrane in a state where the membrane is stretched and made into a thin film.

Disclosure of Invention

Problems to be solved by the invention

However, the breathable film has room for improvement in air permeability, water resistance and flexibility.

One aspect of the present invention has been made in view of the above problems, and an object thereof is to realize an elongated porous film having air permeability, water resistance and flexibility in combination suitable for personal care products such as diapers.

Means for solving the problems

The present inventors have conducted intensive studies in order to solve the above problems, and as a result, have found that: by using a resin composition comprising a specific polyethylene resin and a thermoplastic elastomer at a specific mass ratio and adjusting the moisture permeability to a specific range, an elongated porous film having air permeability, water resistance and flexibility can be realized. That is, the present invention includes the following configurations.

A kind of delayAn expanded porous film characterized by comprising a polymer having a density of 0.900g/cm³Above and 0.940g/cm³A polyethylene resin having a moisture permeability of 1400g/m as measured at 40 ℃ and a relative humidity of 60% in accordance with ASTM E96, and a resin composition comprising a thermoplastic elastomer and an inorganic filler in an amount of 1.0 to 16 parts by mass based on 100 parts by mass of the polyethylene resin²24h or more.

A method for producing an elongated porous film, comprising bringing a density of 0.900g/cm³Above and 0.940g/cm³A mixing step of mixing a thermoplastic elastomer and an inorganic filler in an amount of 1.0 to 16 parts by mass based on 100 parts by mass of the polyethylene resin to obtain a resin composition; a molding step of molding the resin composition into a film shape; and a porosification step of elongating the film obtained by the forming step at least in a machine direction to make the film porous.

Effects of the invention

According to one aspect of the present invention, an effect is obtained that an extended porous film having air permeability, moisture permeability, and flexibility can be obtained.

Detailed Description

An embodiment of the present invention will be described below, but the present invention is not limited thereto.

The present inventors have made intensive studies and found that the above-mentioned prior art has the following problems, for example, the technique disclosed in patent document 1 is described as having flexibility and stretchability, but the technique disclosed in patent document 1 uses a large amount of a thermoplastic elastomer, and therefore, exhibits a very high value of air permeability of 15000 seconds/100 m L, and a high value of 8000 seconds/100 m L even when stretched at a high ratio.

In addition, in the technique disclosed in patent document 2, a density of less than 0.900g/cm is used³As a low performance elastomer, a polyethylene plastomer or a polyolefin plastomer. For this reason, it is considered that the melting point of the resin composition is loweredAnd the following problems occur at the time of heat setting. First, when the film is not heat-set, the film is wound into a roll and then the film is gradually wound up, which tends to cause blocking. When heat-setting is performed at an optimum temperature, the melting point and the heat-setting temperature are close to each other, and the resin composition may be melted again. Further, the film is remelted, and the formed holes are clogged, thereby reducing the air permeability. On the other hand, when the heat-setting temperature is lowered, the film is gradually wound up after being wound in a roll form, and blocking is likely to occur, as in the case where heat-setting is not performed.

Therefore, the extended porous membrane according to one embodiment of the present invention solves the above-described problems of the prior art, and has air permeability, moisture permeability, and flexibility in combination. The details will be described below.

1. Extended porous film

An extended porous film according to an embodiment of the present invention comprises a film having a density of 0.900g/cm³Above and 0.940g/cm³A polyethylene resin having a moisture permeability of 1400g/m as measured at 40 ℃ and a relative humidity of 60% in accordance with ASTM E96, and a resin composition comprising a thermoplastic elastomer and an inorganic filler in an amount of 1.0 to 16 parts by mass based on 100 parts by mass of the polyethylene resin²24h or more. By combining a polyethylene resin having such specific physical properties with a thermoplastic elastomer at a specific mass ratio, desired flexibility can be obtained in addition to water resistance. In addition, by setting the moisture permeability to a specific range, desired air permeability can be obtained. Therefore, an extended porous film having air permeability, water resistance and flexibility can be realized.

The stretched porous film may be a film composed of a resin composition containing a polyethylene resin, a thermoplastic elastomer, and an inorganic filler, or may be a film formed by laminating sheets or the like made of a material other than the resin composition.

1.1 polyethylene-based resin

The polyethylene resin has a density of 0.900g/cm³Above and 0.940g/cm³Hereinafter, more preferably 0.905g/cm³Above and 0.935g/cm³The following. When the density is within the above range, an extended porous film having desired flexibility can be obtained by combining with a thermoplastic elastomer described later. In addition, density and melting point are related to some extent. When the density is within the above range, the heat-setting temperature and the melting point are separated to some extent, and therefore, the polyethylene resin can be prevented from melting and clogging the pores of the stretched porous film at the same time as the heat-setting. Therefore, the reduction of the air permeability can be prevented.

The polyethylene resin includes, for example, a linear low density polyethylene (LL DPE), a branched low density polyethylene (L DPE) and an ultra-low density polyethylene (V L DPE). furthermore, when a plurality of polyethylenes are used, the melt mass flow rate can be easily adjusted, and therefore, it is preferable³The polyethylene resin of (2), for example, High Density Polyethylene (HDPE). In this case, the polyethylene resin used had an overall density (density of a mixture of a plurality of polyethylene resins) of 0.940g/cm³The following may be used. More preferably, the density of the polyethylene resin used is all within the above range.

1-2 thermoplastic elastomer

The thermoplastic elastomer is added for the purpose of improving flexibility. The content ratio of the thermoplastic elastomer is preferably 1.0 part by mass or more and 16 parts by mass or less, more preferably 1.5 parts by mass or more and 14 parts by mass or less, and further preferably 2.0 parts by mass or more and 12 parts by mass or less, with respect to 100 parts by mass of the polyethylene resin. When the content of the thermoplastic elastomer is 1.0 part by mass or more, greater flexibility can be imparted to the stretched porous film. When the content of the thermoplastic elastomer is 16 parts by mass or less, the strength of the stretched porous film can be improved. In addition, if the content of the thermoplastic elastomer is 16 parts by mass or less, the occurrence of the tensile resonance phenomenon is possible, and therefore, the productivity can be improved.

The thermoplastic elastomer is preferably an olefin elastomer and/or a styrene elastomer.

The olefin-based elastomer includes a mixture of a polymer composed of a hard segment and a polymer composed of a soft segment, and a copolymer of a polymer composed of a hard segment and a polymer composed of a soft segment. Examples of the hard segment include a segment composed of polypropylene. Examples of the soft segment include a segment composed of polyethylene, a segment composed of a copolymer of ethylene and a small amount of a diene component, and the like. Specifically describing the soft segment, there can be exemplified ethylene-propylene-copolymer (EPM), ethylene-propylene-diene copolymer (EPDM), and partially crosslinked products obtained by adding an organic peroxide to EPDM, and the like.

The mixture of the copolymer of the olefin elastomer and the copolymer may be obtained by graft modification of an unsaturated hydroxyl monomer and a derivative thereof, an unsaturated carboxylic acid monomer and a derivative thereof, and the like.

Examples of the olefin-based elastomer include "Thermorun" manufactured by Mitsubishi Chemical, "EXCE L INK manufactured by JSR," ESPO L EX TPE manufactured by Sumitomo Chemical, "Milastomer" manufactured by Mitsui Chemical, "Sarlink" manufactured by Teknor Apex (テクノエイペック), "Shanduping" manufactured by Exxon Chemical (エクソンケミカル), and "ACTYMER-G" manufactured by Riken Technos.

Examples of the styrene-based elastomer include an elastomer having a polystyrene block as a hard segment and each block of polybutadiene, polyisoprene, polyethylene-polybutylene, polyethylene-polypropylene, or the like as a soft segment. That is, as the styrene-based elastomer, a styrene-butadiene block copolymer, a styrene-isoprene block copolymer, a styrene-ethylene-butylene block copolymer, a styrene-ethylene-propylene block copolymer, and the like can be cited.

Examples of the styrene-based elastomer include "RABARON" manufactured by mitsubishi chemical corporation, "ESPO L EX SB" manufactured by sumitomo chemical corporation, "Tuftec" manufactured by asahi chemical corporation, "elastomer Ar" manufactured by aronkasi corporation (アロン chemical corporation), "SEPTON" manufactured by Kuraray (クラレ), and "EARNESTON" manufactured by Kuraray Plastics.

Further, the above-mentioned commercially available thermoplastic elastomer product may be a mixture comprising a thermoplastic elastomer and other components (for example, polypropylene, paraffin oil, etc.). Such a product can be used so that the amount of the thermoplastic elastomer contained in the product is in the above-described ratio with respect to 100 parts by mass of the polyethylene-based resin.

That is, the resin composition may include polypropylene, paraffin oil, and the like. Further, as for the resin composition, the paraffin-based oil may be contained by using the thermoplastic elastomer including the paraffin-based oil as described above, and the thermoplastic elastomer not including the paraffin-based oil and another paraffin-based oil may also be included. By including the paraffin oil in the resin composition, the flexibility of the stretched porous film can be further improved. The content of the paraffin oil is preferably 2 to 18 parts by mass per 100 parts by mass of the polyethylene resin.

1-3 inorganic filler

The inorganic filler is added to make the membrane porous. As the inorganic filler, known inorganic fillers can be used without limitation, and examples thereof include inorganic salts such as calcium carbonate, barium sulfate, calcium sulfate, barium carbonate, magnesium hydroxide and aluminum hydroxide, inorganic oxides such as zinc oxide, magnesium oxide and silica, silicates such as mica, vermiculite and talc, and organic metal salts. Among the inorganic fillers, calcium carbonate is preferable from the viewpoint of cost performance and dissociability with the polyethylene-based resin.

The blending ratio of the inorganic filler in the resin composition is preferably 80 parts by mass or more and 200 parts by mass or less, and more preferably 85 parts by mass or more and 150 parts by mass or less, with respect to 100 parts by mass in total of the polyethylene resin and the thermoplastic elastomer. When the blending ratio of the inorganic filler is 80 parts by mass or more, the frequency of occurrence of a gap per unit area, which occurs when the polyethylene resin and the inorganic filler are separated from each other, can be increased. Therefore, the adjacent gaps are easily communicated with each other, and the air permeability becomes good. When the blending ratio of the inorganic filler is 200 parts by mass or less, the elongation at the time of film stretching is good, and the stretching is easy.

1-4, other ingredients

The resin composition may further include additives used in general resin compositions. Examples of the additives include antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, neutralizers, lubricants, antifogging agents, antiblocking agents, antistatic agents, slip agents, colorants, plasticizers, and the like. In addition, the resin composition may contain a small amount of resin components other than the polyethylene resin and the thermoplastic elastomer, within a range not impairing the effects of the present invention. Specifically, if the amount is 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, based on 100 parts by mass of the total of the polyethylene resin and the thermoplastic elastomer, it is acceptable to add other resin components.

1-5 Properties of stretched porous film

The moisture permeability of the stretched porous film is preferably 1400g/m²24h or more, more preferably 1600g/m²24h or more. Since the moisture permeability is within the above range, air permeability and moisture permeability are excellent. For example, when the stretched porous film is used as a backsheet of a diaper, stuffiness can be prevented when the diaper is worn. Although the upper limit of the moisture permeability is not particularly limited, it is preferably 10000g/m from the viewpoints of mechanical properties, water resistance and liquid leakage resistance²24h or less, more preferably 5000g/m²24h or less.

The moisture permeability was measured under the conditions of 40 ℃, a relative humidity of 60%, a measurement time of 24 hours and a pure water method using ASTM E96 as a standard, and further, in the present specification, the moisture permeability was an average value of 10 pieces of 10cm × 10cm samples taken from an extended porous film.

The 5% tensile strength of the stretched porous film is preferably 0.3N/25mm or more and 2.5N/25mm or less, and more preferably 0.5N/25mm or more and 2.3N/25mm or less. The lower the 5% tensile strength, the softer. If the 5% tensile strength is 2.5N/25mm or less, more flexibility can be imparted. If the 5% tensile strength is 0.3N/25mm or more, the elongation of the film against the linear tension applied in the machine direction at the time of secondary processing can be suppressed.

The 5% tensile strength was measured as the mechanical strength when the specimen was stretched in the mechanical direction at a distance between chucks of 50mm and a stretching speed of 200mm/min in accordance with JIS K7127 to stretch the specimen by 5%. That is, the 5% tensile strength was measured as the stress in the machine direction when the distance between chucks was elongated by 2.5 mm. In the present specification, the 5% tensile strength is a value measured for a sample having a width of 25mm and a mechanical direction length of 150mm taken from an elongated porous film.

The melt mass flow rate of the resin composition is preferably 2.0g/10min or more, more preferably 2.0g/10min to 5.0g/10min or less, and still more preferably 2.0g/10min to 4.0g/10min or less. If the melt mass flow rate is within the above range, film formation can be performed more stably. If the melt mass flow rate is 2.0g/10min or more, the pressure of the extruder on the resin during film formation can be suppressed, and adverse effects on film formation can be prevented. Further, if the melt mass flow rate is 5.0g/10min or less, the inward bending of the edge can be more effectively suppressed when the film is formed by T-molding. Thus, a desired product width can be easily obtained. Further, the smaller the melt mass flow rate, the greater the 5% elongation strength tends to be. The melt mass flow rate of the resin composition was measured by the A method at 190 ℃ in accordance with JIS K7210.

The air permeability of the stretched porous film is preferably 300 seconds/100 m L or more and 2000 seconds/100 m L or less, more preferably 400 seconds/100 m L or more and 1600 seconds/100 m L or less, and further preferably 400 seconds/100 m L or more and 1100 seconds/100 m L or less, the smaller the value of the air permeability, the easier the gas passes through the film, and if the air permeability is within the above range, the stuffiness at the time of wearing can be prevented when the stretched porous film is used as a backsheet of a diaper, and the air permeability is measured by the wang's test machine method in accordance with JIS P8117.

The heat shrinkage rate of the stretched porous film in the machine direction is preferably 5.0% or less, and more preferably 3.5% or less. If the 5% tensile strength is large and the heat shrinkage rate in the machine direction is 5.0% or less, the film elongation against the line tension applied in the machine direction at the secondary processing can be more suppressed. The thermal shrinkage in the machine direction is preferably as close to 0%, but practically 0.5% or more.

The heat shrinkage in the machine direction was measured by taking a sample of 15cm × 15cm from an elongated porous film, marking the sample with a mark line so that the distance between the mark lines in the machine direction was 10cm, allowing the sample to stand at 50 ℃ for 24 hours, cooling to room temperature, and measuring the length between the mark lines.

Formula I { (10 cm-length between mark lines after cooling (cm)/10 cm) } × 100.

The weight per unit area is preferably 10g/m²Above and 35g/m²Hereinafter, more preferably 11g/m²Above and 32g/m²Hereinafter, it is more preferably 12g/m²Above and 30g/m²The following. When the weight per unit area is within the above range, an elongated porous film having excellent air permeability, moisture permeability and mechanical strength can be obtained. If the weight per unit area is 10g/m²As described above, the mechanical strength of the film can be improved. In addition, if the weight per unit area is 35g/m²Hereinafter, sufficient moisture permeability can be obtained.

The blocking strength (also referred to as peel strength) is preferably 1.0N/1000mm²The following. If the adhesive strength is 1.0N/1000mm²The blocking strength was measured by collecting 2 specimens of 25mm × 80mm in thickness from an elongated porous film, superposing the specimens 40mm each as a test piece, placing the test piece in a constant temperature and humidity chamber at 40 ℃ and 70% relative humidity in a state where 10kg is added to the repeated portion of the test piece for 24 hours, and passing the test piece through 24 hoursAfter the lapse of time, the mixture was cooled to room temperature and the adhesion strength was measured by a tensile tester.

2. Method for producing stretched porous film

A method for producing an extended porous film according to an embodiment of the present invention includes forming a film having a density of 0.900g/cm³Above and 0.940g/cm³The method for producing the polyethylene resin film comprises a mixing step of mixing a polyethylene resin, 1.0 to 16 parts by mass of a thermoplastic elastomer and an inorganic filler per 100 parts by mass of the polyethylene resin to obtain a resin composition, a molding step of molding the resin composition into a film, and a porosification step of elongating the film obtained in the molding step at least in a machine direction to porosify the film. By combining a polyethylene resin having such specific physical properties with a thermoplastic elastomer at a specific mass ratio, an elongated porous film having desired flexibility in addition to water resistance can be obtained. Further, by stretching and making porous a film comprising a resin composition having a specific composition, an extended porous film having a desired air permeability can be obtained. Therefore, an extended porous film having air permeability, water resistance and flexibility can be realized. Further, as for the matters already explained in "1, extending porous film", the explanation will be omitted below, and the above description will be appropriately referred to.

2-1, mixing step

The mixing procedure is to set the density at 0.900g/cm³Above and 0.940g/cm³A mixing step of mixing a thermoplastic elastomer and an inorganic filler in an amount of 1.0 to 16 parts by mass based on 100 parts by mass of the polyethylene resin to obtain a resin composition. First, a polyethylene resin, a thermoplastic elastomer, an inorganic filler, and if necessary, additives are mixed. The mixing method is not particularly limited, and a known method can be used. For example, it is preferable to mix for about 5 minutes to 1 hour using a mixer such as a Henschel mixer, a super mixer, or a tumbler mixer. In this case, the polyethylene resin and the thermoplastic elastomer can be stably granulated by setting the melt flow rates to be substantially the same. Due to the fact thatIn this case, it is preferable that the melt mass flow rates of the polyethylene resin and the thermoplastic elastomer are kept uniform.

The obtained mixture can be generally kneaded and pelletized by a method such as wire-cutting, thermal-cutting or underwater-cutting using a kneader such as a high-kneading type twin-screw extruder or a tandem type kneader. It is preferable to mix, knead and granulate in advance because uniform dispersion of the resin composition can be promoted. Further, depending on the blending of the resin composition, the resin composition may be directly fed into a kneader without mixing and pelletized.

2-2, Molding Process

The molding step is a step of molding the resin composition into a film shape. The pellets obtained as described above are preferably formed into a film shape by a circular die or a T-die attached to the front end of an extruder. In this case, the cooling method in the case of using the T-die method is not particularly limited, and a known method such as a roll method (ニップロール method), an air knife method, or an air cell method can be used. Further, depending on the order of blending the resin composition, the resin composition may be directly fed into an extruder without mixing or kneading, and a film may be formed.

2-3, a step of forming a porous body

The step of forming pores is a step of stretching the film obtained in the step of forming pores at least in the machine direction. By stretching the film obtained in the molding step, the interface between the resin component (the polyolefin resin and the thermoplastic elastomer) and the inorganic filler is peeled off. Further, minute voids are generated at the interface by the peeling, and the voids form communication holes penetrating in the film thickness direction, thereby forming an extended porous film. The stretching can be performed by a known method such as a roll stretching method or a tenter stretching method. The stretching may be uniaxial stretching or biaxial stretching.

In addition, the mechanical-direction draw ratio in the porosification step is preferably represented by the following formula II:

y is more than or equal to 1.4 and less than or equal to 0.075X +2.5 … … (formula II),

in the formula, X represents the blending ratio (parts by mass) of the thermoplastic elastomer to 100 parts by mass of the polyethylene resin, and Y represents the draw ratio (times).

By carrying out the stretching under the condition that the above formula II is established, since the film is sufficiently stretched, the thickness unevenness is not easily generated, and the tensile strength is increased, so that a sufficient number and size of pores are formed. Therefore, if the draw ratio is set to such a specific draw ratio, a stretched porous film having air permeability, moisture permeability and flexibility can be more easily obtained. The extension may be a single extension or a multiple extension.

The elongation temperature is preferably in a temperature range of not lower than ordinary temperature and lower than the softening point of the resin composition. When the stretching temperature is not lower than room temperature, stretching unevenness is less likely to occur, and the thickness is more likely to be uniform. In addition, if the stretching temperature is lower than the softening point, the stretched porous film can be prevented from melting. Therefore, the stretched porous film can be prevented from collapsing in pores and from deteriorating in air permeability and moisture permeability. The stretching temperature can be appropriately adjusted depending on the combination of the physical properties and the stretching ratio of the resin composition to be used.

2-4, Heat-setting Process

The manufacturing method may include a heat-setting process. The heat-setting step is a step of heat-setting the stretched porous film after stretching in order to suppress heat shrinkage in the stretching direction. The heat setting refers to a heat treatment performed in an environment without changing the dimension while maintaining a state of tension due to stretching on the stretched film. As a result, by heat setting, elastic recovery during storage, shrinkage and rollback due to heat, and the like can be suppressed.

As the heat setting method in the case of using the roll stretching method as the stretching method, there is a method of heating the stretched film by a heated roll (annealing roll). In addition, as a heat setting method in the case of adopting a tenter stretching method as the stretching method, there is a method of heating the stretched film in the vicinity of the outlet of the tenter.

The temperature for heat setting is preferably 70 ℃ to 95 ℃, more preferably 80 ℃ to 95 ℃. If the heat-setting temperature is 70 ℃ or higher, heat shrinkage can be suppressed by sufficient heat-setting. In addition, if the heat-setting temperature is 95 ℃ or less, the collapse of the pores of the extended porous film due to heat can be more effectively prevented.

The heat-setting time is preferably 0.2 seconds or more, more preferably 0.5 seconds or more, and further preferably 1.0 second or more. If the heat-setting time is 0.2 seconds or more, heat shrinkage can be suppressed by sufficient heat-setting. The heat-setting time is preferably 20 seconds or less, and more preferably 15 seconds or less. Although this cannot be said to depend on the combination with the heat-setting temperature, if the heat-setting time is 20 seconds or less, the collapse of the pores due to the melting of the extended porous film can be prevented better. Thus, the reduction of air permeability and moisture permeability can be prevented.

The heat-set time is the time to maintain the extended porous film at the heat-set temperature. For example, in the case of the roll extension method, it means a time during which the film is in contact with an annealing roll. The number of annealing rolls is not particularly limited, but in the case of two or more annealing rolls, the heat-setting time is the sum of the times during which the stretched porous film is brought into contact with the respective annealing rolls. In addition, in the case of the tenter extension method, the heat-setting time means a time during which the heat-setting temperature is heated and maintained at the tenter exit. When the heat setting is divided into a plurality of times and the heating is performed, the sum of the respective heating times is obtained.

The present invention is not limited to the various embodiments described above, and various modifications may be made within the scope shown in the claims, and embodiments obtained by appropriately combining the technical means respectively disclosed in the different embodiments are also included in the technical scope of the present invention.