JPH04335043A - Production of porous film - Google Patents

Abstract

PURPOSE:To obtain a porous film having high strength in the direction of thickness, high rigidity, good gas permeability and good balance among these properties and having a nonsticky surface. CONSTITUTION:A process for producing a porous film by forming a resin composition comprising 100 pts.wt. polypropylene resin or polyethylene resin having a density of 0.940g/cc or above and 100-400 pts.wt. filler into a film, wherein the resin composition in the form of a film extruded from the extruder die is drawn at a draw ratio of 20-1000, solidified by cooling, and unidirectionally stretched in the longitudinal direction at a stretch ratio of 1.3-6.0.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention provides a method for producing a porous film suitable for uses such as packaging materials, filtration materials, leak prevention materials, battery separators, electric double layer capacitor separators, etc. Regarding. More specifically, it is possible to stably produce a porous film that has excellent rigidity and strength in the thickness direction, and maintains sufficient air permeability without collapsing the pores even under pressure. Regarding the manufacturing method. [0003] Conventionally, various methods have been proposed for producing many porous films from thermoplastic resins for use as packaging materials, filtration materials, leakage prevention materials, and the like. Japanese Patent Publication No. 46-40119 discloses that a polymer with a degree of crystallinity of 50% or more is formed into a film at a tensile rate of 20 or more, crystallization is promoted by heating, then cold-stretched, and then , a method for producing a porous film that is heat-treated under tension has been proposed. However, this method requires a step of promoting crystallization after forming a film, which not only lengthens the process, but also requires a long residence time in this step, resulting in poor productivity. . [0006] In addition, since this method generates micropores between polymer molecules in a highly crystallized film due to the stress during stretching without adding fillers, the degree of crystallinity and the degree of stretching are slightly affects the porosity of the film,
Not only is it difficult to uniformly control the porosity and provide a certain level of air permeability, but the film is often torn during stretching, resulting in poor stretching workability. This film tearing during stretching occurs particularly when the strain rate during stretching is high, and this method is problematic. Furthermore, since this method produces resin alone without adding fillers, the rigidity, elastic modulus, and strength in the thickness direction are not sufficient. Therefore, when used under pressure, the pores of the film collapse and a certain level of air permeability cannot be maintained.For example, when used under pressure as a material for battery separators and electric double layer capacitor separators, The pores are collapsed and ion permeability is reduced. [0008] In JP-A-57-47334 and JP-A-60-199037, a resin composition comprising a polyolefin resin and a filler incompatible with the resin is formed into a film by melt extrusion. A method for producing a porous film is disclosed in which the porous film is then subjected to a stretching treatment. [0009] The method disclosed in JP-A No. 57-47334 involves adding liquid rubber (for example,
This method is characterized by adding liquid polybutene, liquid polybutadiene). When a large amount of a third component such as liquid rubber is added to a polyolefin resin, the cohesive force of the polymer molecules of the polyolefin resin becomes small, and the polymer molecular chains become slippery and difficult to align. Therefore,
Although the anisotropy of the film is reduced, the mechanical strength, rigidity, and strength in the thickness direction are reduced. Therefore, when used under pressure as a material for a battery separator or an electric double layer capacitor separator, for example, the pores of the film are crushed and the ion permeability is reduced. Furthermore, since the mechanical strength is low, when producing a thin film, the film tends to tear more frequently during the stretching process, which poses a problem in productivity. [0011] Furthermore, the third component such as liquid rubber bleeds out onto the film surface, resulting in a sticky feel and impairing the surface condition. Therefore, when the film is used as, for example, a battery separator or a filter material, the third component has disadvantages such as elution, adversely affecting the electrode reaction, or being mixed into the filtrate. Furthermore, JP-A-60-199037 discloses a porous film having excellent flexibility made from a resin composition consisting of a polyethylene resin having a specific melt index and density and barium sulfate having a specific particle size. A method of manufacturing is disclosed. However, the porous film obtained by this method has poor rigidity and
The strength in the thickness direction is low. Therefore, when used under pressure, the pores of the film are crushed and air permeability cannot be maintained. For example, when used under pressure as a material for battery separators and electric double layer capacitor separators,
The pores of the film are crushed and the permeability of ions is reduced. As a countermeasure, if high-density polyethylene or polypropylene is used instead of the resin disclosed in the publication and a porous film is produced according to the method disclosed in the publication, the film may tear during stretching and become unstable. Stretching operation is not possible. Therefore, the method disclosed in this publication cannot be said to be sufficient as a method for producing a porous film with good rigidity and strength in the thickness direction. [0014] An object of the present invention is to improve the above-mentioned problems. That is, the object of the present invention is to have a material that has excellent rigidity and strength in the thickness direction, maintains sufficient air permeability without collapsing the pores even under pressure, and has a good surface condition without a sticky feel. An object of the present invention is to provide a method for manufacturing a porous film. Another object of the present invention is to provide a method for producing a porous film with good stretchability, which prevents the film from tearing when pores are generated in the film by stretching. Means for Solving the Problems As a result of extensive studies, the present inventors melt-extruded a resin composition consisting of a polypropylene resin or a polyethylene resin having a specific density and a filler, and It was discovered that the above problem could be solved by stretching the resin composition at a specific tensile rate, cooling and solidifying it, and further uniaxially stretching it in the longitudinal direction at a specific magnification, thereby completing the present invention. That is, the present invention provides 100 parts by weight of a polypropylene resin or a polyethylene resin having a density of 0.940 g/cm3 or more, and 100 to 100 parts of a filler.
When forming a resin composition consisting of 400 parts by weight into a film by melt extrusion, the resin composition in the form of a film discharged from an extruder die is stretched at a tensile rate of 20 to 1000, cooled and solidified, and then vertically 1.3 to 6 in one axis direction
．． This is a method for producing a porous film characterized by stretching 0 times. The tensile rate in the present invention is the reciprocal of the rate at which a film-like resin composition discharged from a die of a melt extruder is stretched in the longitudinal direction and the thickness of the film is reduced until it is cooled and solidified. Specifically, it is the quotient obtained by dividing the lip opening degree of the die by the thickness of the cooled and solidified film. (Hereinafter, it will be simply referred to as tensile rate.) Also, the longitudinal direction in the present invention is the length direction of the film. Further, the density of the polyethylene resin in the present invention is a value measured by a method specified in JIS K 6760. The polypropylene resin used in the present invention is a homopolymer of propylene or a copolymer of propylene and α-olefin. These resins may be used alone or in a mixture of two or more. Examples of α-olefins include ethene, butene, hexene, octene, and the like. When copolymerizing these α-olefins,
Preferably it is less than 8% by weight, based on propylene. It is not preferable to use a copolymer copolymerized in an amount of 8% by weight or more because the resulting film will have lower rigidity and strength in the thickness direction. If the rigidity and strength in the thickness direction decrease, for example, when the film is used under pressure as a material for a battery separator or an electric double layer capacitor separator, the pores of the film will collapse and the ion permeability will decrease. The polyethylene resin is a homopolymer of ethylene or a copolymer of ethylene and α-olefin having a density of at least 0.940 g/cm 3 . Examples of the α-olefin include butene, hexene, octene, and the like. When copolymerizing ethylene and α-olefin, the density of the resulting copolymer generally decreases as the addition ratio of α-olefin increases, but the density is 0.940.
It is preferable to carry out copolymerization within a range that does not result in less than g/cm3. These resins may be used alone or in a mixture of two or more. Among polyethylene resins, the density is 0.9.
If it is less than 40 g/cm3, the stretchability is good, but the resulting film has low rigidity, longitudinal elastic modulus, and other mechanical strengths, especially strength in the thickness direction, so it is not preferable. If the rigidity and strength in the thickness direction are low, for example, when the film is used under pressure as a material for a battery separator or an electric double layer capacitor separator, the pores of the film are crushed and the ion permeability is reduced. Both the polypropylene and polyethylene resins mentioned above are melt intex (hereinafter referred to as MI).
is preferably in the range of 0.1 to 15.0 g/10 min. More preferably, it is in the range of 0.5 to 8.0 g/10 min. If the MI is less than 0.1/10 min, the melt viscosity is high and the resin pressure increases during the extrusion process, resulting in a decrease in productivity and the resulting film has poor air permeability, which is not preferable. If it exceeds 15.0 g/10 min, a film with good air permeability can be obtained, but the melt viscosity is low, so the thickness accuracy of the film decreases, and film tearing occurs during stretching. This is not preferable because the workability becomes worse and the mechanical strength of the resulting film is also reduced. [0023] For polypropylene resin, MI is based on JIS K 6758, and the load is 2.1.
6 kg at a temperature of 230° C. For polyethylene resins, the value was measured at a load of 2.16 kg and a temperature of 190° C. in accordance with JIS K 6760. The filler used in the present invention may be inorganic or organic. Examples of inorganic fillers include barium sulfate, calcium sulfate, magnesium sulfate, barium carbonate, calcium carbonate, magnesium carbonate, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, silica, alumina, talc, glass powder, etc. Ru. Among these, barium sulfate, calcium carbonate,
Magnesium hydroxide, zinc oxide, and silica can be particularly preferably used. Nylon, polystyrene, etc. can be used as the organic filler. These inorganic and organic fillers may be used alone or in a mixture of two or more. From the viewpoint of dispersibility in the resin and physical properties such as pore size and air permeability of the resulting film, the average particle size of the filler is preferably 0.05 to 10 μm, more preferably 0.1 to 5 μm. It is 0 μm. If the average particle size exceeds 10 μm, the pore size of the porous film obtained by stretching becomes too large, and the film is likely to break during stretching, especially when producing a thin film. Stretching stability decreases. In addition, if it is less than 0.05 μm, the filler will have poor dispersibility in the resin and will tend to aggregate, making it easy for the film to break during stretching.
Stretching stability decreases. As a result, aggregates of the filler are formed in a fish-eye shape on the surface of the obtained film, resulting in poor surface condition of the film. In order to improve the dispersion of the filler into the resin, it is preferable to coat the surface of the filler with a fatty acid such as stearic acid or its metal soap, a silane-based or titanium-based coupling agent, or the like. The blending amount of the filler is 100 to 400 parts by weight per 100 parts by weight of the resin. If the blending amount is less than 100 parts by weight, the filling amount is too small and sufficient air permeability cannot be obtained even when stretched at low temperature and at a high magnification. If it exceeds 400 parts by weight, the filling amount is too large to allow stable stretching, and the film is likely to break during stretching, resulting in decreased stretching stability. [0028] As long as the purpose of the present invention is not impaired,
In addition to the above-mentioned fillers, known lubricants, dispersants, infrared absorbers, dyes, pigments, crystal nucleating agents, etc. used in the molding process of polyolefin resins may be added. There are no particular restrictions on the method of mixing the resin, filler, and various additives added as necessary in the present invention. Preferably, the resin composition is prepared by mixing for 1 to 30 minutes at a temperature ranging from room temperature to 90° C. using a conventional mixer such as a Henschel mixer, a super mixer, or a tumbler type mixer. The obtained resin composition may be directly subjected to the film forming process, but from the viewpoint of improving the dispersibility of the filler in the film forming process, it is preferable to use a single or twin screw extruder, preferably in the resin composition. From the viewpoint of reducing moisture content, it is preferable to melt-extrude at a temperature range of 160 to 260° C. using an extruder having a vent hole, process the pellets into cylindrical or prismatic pellets, and use the pellets for film-forming and stretching. There are no particular restrictions on the method of processing into pellets. Usually used strand cut type, hot cut type, etc. are preferably used. [0031] Furthermore, before the resin composition is subjected to a film forming process, it is preferable to reduce the water content in the resin composition to 500 ppm or less. If the water content is 500 ppm or more, air bubbles may be generated in the resulting film or the film may be torn, making it impossible to perform a stable stretching operation, which is not preferable. [0032] The above resin composition is subjected to a film forming process and formed into a film. The extruder may be a single-screw extruder or a twin-screw extruder, but a single-screw extruder is preferred from the standpoint of quantitative extrusion. Further, either the T-die method or the inflation method may be used. [0033] The extrusion temperature is the melting point of the resin used + 30°C ~
below the decomposition temperature of the resin. Preferably, the temperature range is from the melting point of the resin +40°C to less than 260°C. If the extrusion temperature is less than the melting point + 30°C, melt fracture will occur, and if it exceeds the decomposition temperature of the resin, the resin will thermally decompose.
It becomes impossible to form it into a film. [0034] When the resin composition is extruded into a film form from the die of an extruder by the above method and solidified by cooling, the tensile modulus is 2.
The lip opening degree and take-up speed of the die are adjusted so that it becomes 0 to 1000. The preferred tensile modulus is 35 to 1000,
More preferably, it is in the range of 50 to 800. The lip opening of the die is adjusted using the opening adjustment bolt. If the tensile modulus is less than 20, the molecular orientation in the longitudinal direction of the resulting unstretched film will be small. Therefore, a necking phenomenon occurs when the film is stretched in the next step, and stretching unevenness remains at low stretching ratios, making it impossible to obtain a uniformly stretched film. This stretching unevenness can be prevented by stretching to a high magnification of 7.0 times or more. However, if the film is stretched to a high magnification of 7.0 times or more, the frequency of tearing of the film during stretching becomes extremely high. In addition, a porous film obtained by stretching at a high magnification has too large a molecular orientation in the longitudinal direction, and although the mechanical strength in the longitudinal direction is good, the rigidity and the strength in the thickness direction are reduced. Furthermore, if the strain rate during stretching is high, the film will tear before it is evenly stretched. [0036] If the tensile rate exceeds 1000, the molecular orientation in the longitudinal direction of the obtained unstretched film will become too large, so that when the film is stretched in the next step, it will not be possible to stretch the film until the pores are sufficiently opened. Otherwise, the film will be torn. As mentioned above, when the resin composition is discharged in the form of a film from the extruder die and solidified by cooling, the polymer molecular chains are properly oriented in the longitudinal direction by setting the tensile modulus to 20 to 1000. . Therefore, during the stretching process in the next step, the film does not cause necking phenomenon.
Even with relatively low stretching, a uniformly stretched film can be obtained. As a result, a porous film with a uniform degree of pore opening is obtained. Cooling the resin composition discharged from the die with an air knife, air ring, etc. and reducing the distance from the die until it cools and solidifies, that is, the air gap, greatly increases the vertical orientation of the molecular chains. It is preferably used in this respect. The stretching process is carried out at a predetermined temperature in the longitudinal uniaxial direction at a stretching ratio of 1.3 to 6.0 times. The preferred stretching ratio is 1.5 to 5.5 times, more preferably 1.
It is 7 to 5.0 times. [0040] If the stretching ratio is less than 1.3 times, the degree of stretching will vary and a film with uniform pore size cannot be obtained. If it exceeds 6.0 times, the air permeability is sufficient, but the film will break more frequently during the stretching process, and the resulting porous film will have too much longitudinal molecular orientation and will not be easily machined in the longitudinal direction. The mechanical strength is good, but the rigidity and strength in the thickness direction are reduced. Therefore, when used under pressure as a material for a battery separator or an electric double layer capacitor separator, for example, it is not preferable because the pores of the film are crushed and the ion permeability is reduced. [0041]Also, transverse uniaxial stretching or longitudinal/horizontal sequential biaxial stretching can be considered as the stretching treatment, but transverse uniaxial stretching causes a necking phenomenon, and a uniform film cannot be obtained unless stretched at a high magnification. Tears occur and the stretching operation cannot be performed stably. Similarly, even in the case of longitudinal/horizontal sequential biaxial stretching, a necking phenomenon occurs during the transverse stretching process, making it impossible to stably perform the stretching operation. This is considered to be due to the orientation of molecular chains in the unstretched film. [0042] The temperature of the stretching treatment is in the range from the glass transition temperature of the resin used +20°C to the melting point of the resin -10°C. The temperature range is preferably from 10°C to the melting point of the resin -20°C from the viewpoint of the workability of stretching, the degree of opening of the obtained film, the ventilation characteristics, etc. After the stretching process, it is preferable to heat set the film in order to improve the shape of the pores formed in the film and the dimensional stability of the film. The heat setting temperature is generally in the range of 60°C to -5°C below the melting point of the resin. Further, after heat setting, the film may be relaxed by less than 20% in the longitudinal direction. EXAMPLES The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to the following Examples unless it departs from the gist thereof. Evaluations in Examples were performed by the following method. 1. Thickness (μm) Using a dial gauge specified in JIS B 7509, 10 samples were stacked and measured under the following measurement conditions, and the thickness was converted into the thickness per sample. Measurement temperature: 23°C
Measurement pressure 0.4kg/cm2
2. Tensile properties were measured in accordance with the method specified in JIS P 8113, and the measurement conditions were as follows. [0048] Test piece size: 100mm length x 25m
m width measurement temperature 23℃ tensile speed 200mm/min ■5% mod
The stress at 5% elongation (kg/mm2) was measured in the machine direction of the film (hereinafter referred to as MD) and the direction perpendicular to the machine direction of the film (hereinafter referred to as TD), and the stress per 1 mm2 cross-sectional area was calculated. It was converted into ■ Breaking strength (kg/mm2) The stress at which the test piece broke was measured in MD and TD, respectively, and was converted into stress per 1 mm2 cross-sectional area. ■Elongation at break (%) The elongation when the test piece breaks is measured in MD and TD, respectively. 3. Air permeability (sec/100ml) JIS
It was measured in accordance with the method specified in P.8117. The lower the air permeability, the better the air permeability. 4. Stretching Stability The stretching stability was evaluated as follows based on observation during stretching. Very good──────────1 Good─
────────────2 There is uneven stretching──────
──────3 Stretching tear occurs ─────────
4 The frequency of stretching tearing is extremely high──────50052
]5. Thickness change rate (%) This is an evaluation of the strength in the thickness direction, and the smaller the thickness change rate, the
Indicates that the strength in the thickness direction is large. [0053]Thickness change rate=(1-t1/t0)×100
t0: 1. above. Thickness (μm) t1 measured by the method of
: 1 above, except that the measurement pressure was 10 kg/cm2
．． Thickness (μm) measured in the same manner as in 0054
]6. Rigidity (ring crush value) (g/mm) A stainless steel cylindrical jig (A) with a wall thickness of 5 mm, an inner diameter of 50 mm, and a height of 5 mm is stood vertically. A stainless steel cylindrical jig (B) with a wall thickness of 5 mm, an outer diameter of 49 mm, and a height of 5 mm is inserted into the jig (A). Jig (A) (B)
Place a sample piece of the following size in the cylindrical gap between the film T.
Insert the sample piece so that D is in the vertical direction to make the sample piece cylindrical. Using a load plate of 80 x 80 mm, a load was applied in the vertical direction at a loading rate of 20 g/min to the cylindrical sample piece placed at a height of 5 mm on the jigs (A) and (B). Find the load at which the film will collapse. The ring crush value is the quotient of this load divided by the thickness of the test piece. The larger this value is, the higher the rigidity is. [0055] Test piece size: MD 160 mm x TD 10 mm Measurement temperature: 23°C [0056] Example 1 High-density polyethylene (A-1) having the MI and density shown in [Table 1], filler (B-1) and additives ( C-1) was used at the blending ratio shown in [Table 2] and mixed using a Henschel mixer to obtain a resin composition. The resin composition was extruded using a vented twin-screw extruder at a molding temperature of 200°C and processed into pellets. [Table 1] Next, using a single screw extruder having a T-die with a lip interval (opening degree) adjusted to 5.0 mm, molding temperature was 200°C.
It was melt extruded. The resin composition discharged from the T-die,
It was pulled while being cooled using an air knife, further cooled with a casting roll at a temperature of 80°C, solidified, and wound up with a winding roll. The thickness of the obtained unstretched film was 50 μm. (Tensile rate: 100) The unstretched film was stretched in the longitudinal uniaxial direction at a stretching temperature of 50°C and a stretching ratio of 3 times using a roll stretching machine, and then the film was relaxed by 5% and heat-set at 115°C. A porous film with a thickness of 23 μm was obtained. The above-mentioned various properties of the obtained porous film were evaluated, and the results are shown in [Table 2]. [Table 2] Examples 2 to 5 Porous films were obtained in the same manner as in Example 1, except that the lip opening degree of the T-die was adjusted and the tensile rate was set as shown in [Table 2]. Ta. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 2]. As the tensile modulus increases, the MD 5% modulus and breaking strength become slightly larger, and the MD orientation becomes larger. Comparative Examples 1 to 2 Stretching was carried out in the same manner as in Example 1, except that the lip opening of the T-die was adjusted to obtain the tensile ratio shown in [Table 3] and the stretching ratio shown in [Table 3]. processed. In Comparative Example 1, when stretched 3 times in the longitudinal direction, clear necking deformation occurred.
Stretching unevenness remained, and a film with uniform thickness and openings could not be obtained. In Comparative Example 2, an attempt was made to stretch the film up to 7 times, but some stretching unevenness remained and an extremely large number of stretching tears occurred. The results are shown in [Table 3]. [Table 3] Comparative Example 3 Stretching treatment was attempted in the same manner as in Example 1 except that the tensile rate was 1200. However, the degree of molecular orientation in the longitudinal direction of the unstretched film was too high and the stretching ratio could not be increased to more than 3 times. The results are shown in [Table 3]. Examples 6 to 9 Polyolefin resins (A-2 to A-5) shown in [Table 1]
) A porous film was obtained in the same manner as in Example 1, except that a porous film was used. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 4]. [Table 4] Comparative Examples 4-5 Polyolefin resins (A-6 and A-6) shown in [Table 1]
A porous film was obtained in the same manner as in Example 1 except that -7) was used. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 5]. In both cases, the stretching stability was good, but because the density of the resin used was low, the strength in the thickness direction and the longitudinal direction was low.
Air permeability was also slightly low. [Table 5] Examples 10 to 12 Porous films were obtained in the same manner as in Example 1, except that the stretching treatment was carried out at the stretching ratio shown in [Table 6]. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 6]. [Table 6] Comparative Examples 6 to 7 Stretching was carried out in the same manner as in Example 1 except that the stretching ratios shown in [Table 6] were used. In Comparative Example 6, the stretching ratio was too large, and stretching blur occurred, making stable processing impossible. In Comparative Example 7, the stretching ratio was too small, and stretching unevenness remained. The obtained results are shown in [Table 6]. Examples 13 to 15 The same procedure as in Example 1 was carried out except that the resin compositions had the blending ratios shown in [Table 7] and the tensile ratios shown in [Table 7].
A porous film was obtained. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 7]. [Table 7] Comparative Examples 8 to 9 Stretching treatment was carried out in the same manner as in Example 1 except that the resin compositions having the blending ratios shown in [Table 7] were used. In Comparative Example 8, the amount of filler was too large, and stretching and blurring occurred, making it impossible to stably process the film. In Comparative Example 9, the amount of filler was too small, resulting in low air permeability. The obtained results [
Table 7]. Examples 16-17 The fillers (B-2 and B-3) shown in [Table 1] were mixed with the fillers (B-2 and B-3) shown in [Table 8].
] A porous film was obtained in the same manner as in Example 1, except that the blending ratios shown were used. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 8]. [Table 8] Comparative Examples 10 to 11 An unstretched film was formed in the same manner as in Example 1, and stretched using a film stretcher manufactured by Toyo Seiki Co., Ltd. In Comparative Example 10, horizontal uniaxial stretching was attempted, but stretching breakage occurred and it was not possible to obtain a film with uniform thickness and openings. In Comparative Example 11, 2.0 in the vertical direction
An attempt was made to perform sequential biaxial stretching in which the film was stretched twice and then in the transverse direction, but the film broke during stretching and it was not possible to obtain a film with uniform thickness and openings. Comparative Example 12 A porous film was obtained in the same manner as in Example 1, except that the resin composition shown in Table 8 was used. The properties of the obtained film were evaluated in the same manner as in Example 1, and the obtained results are shown in [Table 8]
Shown below. Although the resulting film had good air permeability, its mechanical strength and strength in the thickness direction were low. Also,
The surface had a sticky feel compared to the porous films obtained in Examples and other comparative examples. Effects of the Invention: The film obtained by the method of the present invention has a good balance of high strength in the thickness direction, high rigidity, good air permeability, etc., and also has a surface condition with no sticky feeling on the surface. It is a good porous film. [0078] Therefore, in addition to applications such as packaging materials, filter media, and leak prevention materials, it is suitable for applications such as battery separators and electric double layer capacitor separators that require sufficient air permeability without crushing pores even under pressure. . Further, when a porous film is produced by the method of the present invention, the stretching process can be stably performed without tearing the film when generating pores in the film by the stretching process.

Claims (3)

[Claims] [Claim 1] Polypropylene resin or 0.94
Polyethylene resin 1 having a density of 0 g/cm3 or more
When forming a resin composition consisting of 00 parts by weight and 100 to 400 parts by weight of a filler into a film by melt extrusion, the resin composition in the form of a film discharged from an extruder die is subjected to a tensile strength of 20 to 1000. 1. A method for producing a porous film, which comprises stretching the film at a constant speed, cooling and solidifying the film, and further stretching the film 1.3 to 6.0 times in the longitudinal uniaxial direction. [Claim 2] The thickness change rate of the porous film is 7.0.
% or less. % or less. 3. The method for producing a porous film according to claim 1, wherein the porous film has a ring crush value of 3000 g/mm or more.