JP2001191403A - Porous film and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film showing improve shrinkage resistance in a stretching direction and its manufacturing method. SOLUTION: A porous film has a shrinkage in a stretching direction of 2% or less when it is heated at 60 deg.C for an hour. Its manufacturing method comprises a porous of stretching at least in a uniaxial direction a film containing 100 pts.wt. of a mixture consisting of a polyolefin-based resin of 25 to 55 wt.% and an inorganic filler of 75 to 45 wt.%, a cured castor oil of 0.5 to 5 pts.wt. and a dehydrated castor oil of 0.5 to 5 pts.wt.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous resin film and a method for producing the same. More specifically, the present invention relates to a porous resin film having excellent flexibility, adhesiveness, moisture permeability, etc., and excellent shrinkage resistance in the stretching direction, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a film in which an inorganic filler, various oils, waxes, etc. are added to a polyolefin resin is stretched in at least one axial direction, and voids are generated by causing interfacial separation between the inorganic filler and the polyolefin resin. A number of methods have been proposed to generate and produce porous films. Since these porous films have a large number of minute voids inside, they do not allow liquid to permeate while having high moisture permeability. Taking advantage of this property, it is widely used in sanitary materials such as disposable diapers, building materials, reflective films, battery separators, and the like.

In a disposable diaper, a porous film is required to have flexibility in order to give a cloth-like soft feeling.
In addition, when the diaper is attached to a human body, the diaper is fixed with a fastening tape or the like. Further, in disposable diapers, films printed in various ways have been used in recent years. Printing is preferably performed on a specific portion of a diaper, and for this purpose, it is necessary that the printing pitch shift is small. The pitch deviation of printing is related to the heat shrinkage of the film in the machine direction (MD direction), and a film having a small heat shrinkage is required.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is to
In view of the above problems, it is an object of the present invention to provide a porous resin film in which flexibility, adhesiveness, moisture permeability and the like are maintained at the same level as conventional products, and have improved shrinkage resistance in the stretching direction, and a method for producing the same.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that a resin composition containing a specific amount of a polyolefin resin, an inorganic filler, a hardened castor oil, and a dehydrated castor The product film is stretched at least in a uniaxial direction at a specific magnification, and then (A-25) to (A-5) ° C based on the melting point (A ° C) of the resin composition.
It has been found that the above problem can be solved by heat setting at least in the temperature range of 0.004 minutes and controlling the heat shrinkage during heat setting to a range of 15 to 25%, and arrived at the present invention.

That is, the present invention provides 100 parts by weight of a mixture containing 25 to 55% by weight of a polyolefin resin and 75 to 45% by weight of an inorganic filler, 0.5 to 5 parts by weight of hardened castor oil, and 0.5 to 5 parts by weight of dehydrated castor oil. A porous film obtained by stretching a film of the resin composition containing 5 parts by weight at least uniaxially, and having a shrinkage in the stretching direction of 2% or less when heated at 60 ° C. for 1 hour.

The porous film according to the present invention comprises 25 to 55% by weight of a polyolefin resin and an inorganic filler 7
A method for producing a porous film comprising 100 parts by weight of a mixture containing 5 to 45% by weight, 0.5 to 5 parts by weight of hardened castor oil, and 0.5 to 5 parts by weight of dehydrated castor oil, wherein the film of the resin composition is used. Is stretched at least 1.5 to 3 times in a uniaxial direction, and then at least 0.1 mm in a temperature range of (A-25) to (A-5) ° C. based on the melting point (A ° C.) of the resin composition. It is manufactured by heat setting for 004 minutes and controlling the heat shrinkage during heat setting within the range of 15 to 25%.

[0008] Another preferred characteristic of the porous film according to the present invention is that the moisture permeability is 1000 to 4000 g / m ^2.
・ 24hr, adhesive strength is at least 600g / 25m
m, the characteristic that the rigidity is 80 mm or less.

[0009] The polyolefin resin in the present invention comprises:
Linear low-density polyethylene, or, for 100 parts by weight of linear low-density polyethylene, branched low-density polyethylene 50
Mixtures with less than parts by weight are preferred. Dehydrated castor oil is
Comprising at least 30% by weight of 9,11-octadecadienoic acid and at least 30% by weight of 9,12-octadecadienoic acid,
Further, an ester of a fatty acid and glycerin containing both acids in a total amount of 70% by weight or more is preferable. Also, hardened castor oil is
Esters of glycerin with fatty acids containing 70% by weight or more of 12-hydroxyoctadecanoic acid are preferred.

The porous film according to the present invention has excellent flexibility, adhesiveness, moisture permeability, etc., and also has excellent shrink resistance in the stretching direction as compared with conventional products. That is,
The shrinkage in the stretching direction is small. Therefore, it can be used in a wide variety of applications such as sanitary materials such as disposable paper diapers, building materials such as roof waterproof materials, reflective films, battery separators, and the like.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The porous film of the present invention firstly produces a film of a resin composition containing a specific amount of a polyolefin-based resin, an inorganic filler, a hardened castor oil, and a dehydrated castor, and the obtained film is at least uniaxially specified. Stretching at a specific magnification, and then, at a specific temperature, a heat shrinkage of 15 to 2
It is manufactured by controlling and heat setting in the range of 5%.

The polyolefin resin used in the present invention includes high-density polyethylene, medium-density polyethylene, and linear low-density polyethylene. These are used alone or as a mixture of two or more. Further, a branched low-density polyethylene produced by a high-pressure method may be mixed. Further, polypropylene can also be used. Examples of the polypropylene include a homopolymer of propylene or a copolymer of propylene and another α-olefin. The density of the polypropylene is 0.90-0.
91 g / cm ³ , melt index 0.1 to 10 g
/ 10 minutes is preferred.

These polyolefin resins may be resins produced using a multi-site catalyst such as a Ziegler catalyst, or resins produced using a single-site catalyst such as a metallocene catalyst. .

[0014] Of the above polyolefin resins, ethylene-α- is considered in consideration of moldability into a film, moldability, flexibility, shrinkage resistance, and stretchability of the obtained film.
Linear low density polyethylene which is an olefin copolymer is preferred. The density of linear low density polyethylene is 0.91-
It is about 0.93 g / cm ³ . Considering the moldability and stretchability of the film, it is preferable that the linear low-density polyethylene has a melt index of about 0.5 to 20 g / 10 minutes.

The linear low-density polyethylene may be mixed with a branched low-density polyethylene produced by a high-pressure method as long as the above properties of the film are not impaired. The maximum mixing amount of the branched low-density polyethylene is less than 50 parts by weight of the branched low-density polyethylene based on 100 parts by weight of the linear low-density polyethylene. Preferably it is 20 parts by weight or less. Commercial products of the linear low-density polyethylene used in the present invention include those manufactured by Mitsui Chemicals, Inc., trade names: Ultzex UZ2021L, Evolue SP2040, and the like. As a commercially available product of branched low-density polyethylene, Mitsui Chemicals, Inc., trade name: Mirason F9
67 and the like.

Here, the melt index is based on ASTM.
Based on the method specified in D-1238, in the case of polyethylene resin, the temperature is 190 ° C. and the load is 2.16 k.
g, in the case of a polypropylene-based resin, measured at a temperature of 230 ° C. and a load of 2.16 kg.

The inorganic filler is, for example, calcium carbonate,
Examples include barium sulfate, calcium sulfate, barium carbonate, magnesium hydroxide, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, silica, and talc. Of these, calcium carbonate and barium sulfate are preferred. The average particle size of the inorganic filler is preferably 20 μm or less, more preferably 10 μm or less.
Most preferably 5 to 5 μm. Further, it is more preferable that the inorganic filler has been subjected to a surface treatment in order to improve the dispersibility with the resin. As the surface treatment agent, those capable of making the surface hydrophobic by coating the surface of the inorganic filler are preferable. For example, higher fatty acids such as stearic acid and lauric acid, and metal salts thereof can be mentioned.

The composition ratio of the polyolefin resin to the inorganic filler is 25 to 55% by weight of the polyolefin resin and 75 to 45% by weight of the inorganic filler. Preferably, the content of the polyolefin resin is 30 to 50% by weight, and the content of the inorganic filler is 70 to 50% by weight. 4 inorganic fillers
When the content is less than 5% by weight, adjacent voids formed by peeling off the interface between the polyolefin-based resin and the inorganic filler become difficult to communicate with each other. On the other hand, if it exceeds 75% by weight, elongation during stretching of the film is lost and stretching becomes difficult.

The porous film of the present invention comprises a resin composition containing a polyolefin-based resin and an inorganic filler, and further contains, as a third component, specific amounts of both components of dehydrated castor oil and hardened castor oil. The amounts of the dehydrated castor oil and the hardened castor oil affect the dispersibility of the polyolefin resin and the inorganic filler, the moldability of the film, the stretchability, the flexibility, the adhesive strength to the adhesive tape, and the like.

If the amount of the dehydrated castor oil is too small, good dispersibility between the polyolefin resin and the inorganic filler cannot be obtained, and the resulting film has poor flexibility.
On the other hand, if the amount is too large, the adhesiveness to the adhesive tape deteriorates. If the addition amount of the hardened castor oil is too small, good dispersibility between the polyolefin resin and the inorganic filler cannot be obtained,
It is difficult to obtain a uniformly stretched film. On the other hand, if the amount is too large, the flexibility of the obtained film decreases.

The dehydrated castor oil in the present invention comprises 9,
30% by weight or more of 11-octadecadienoic acid, and 9.1
Containing 30% by weight or more of 2-octadecadienoic acid, and
It is an ester of a fatty acid and glycerin containing both acids in a total amount of 70% by weight or more. Commercial products include Ito Oil Co., Ltd.
Product name: DCO. The hydrogenated castor oil contains 12-hydroxyoctadecanoic acid at 70% by weight.
It is an ester of the fatty acid and glycerin contained above. Examples of commercially available products include Caster Wax manufactured by Ito Oil Co., Ltd.

Further, additives used in the resin composition may be blended. Such additives include, for example, antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers,
Examples include a neutralizing agent, a lubricant, an anti-fogging agent, an anti-blocking agent, an antistatic agent, a slip agent, and a coloring agent.

The porous film of the present invention is produced by the following method. Polyolefin resin, inorganic filler, and dehydrated castor oil, and hardened castor oil, if necessary with other additives Henschel mixer, super mixer,
Mix using a mixer such as a tumbler type, and then knead with a normal single or twin screw extruder,
Pelletize. Next, the pellets are molded at a temperature equal to or higher than the melting point of the polyolefin resin, preferably equal to or higher than the melting point + 20 ° C. and lower than the decomposition temperature, by a known molding method such as an extruder equipped with a T-die or the like or an inflation molding machine equipped with a circular die. Kneading, melting, and film formation using a machine. In some cases, the resin composition can be directly formed into a film by a molding machine without pelletizing.

The temperature of the formed film is not lower than room temperature and lower than the softening point of the resin (value measured by a method specified in JIS K-6760-1995) by a known roll stretching method, tenter stretching method or the like. In the above, stretching is performed at least 1.5 to 3 times in a uniaxial direction to form a porous film. Stretching may be performed in one stage or may be performed in multiple stages. When the stretching ratio is lower than 1.5 times, the moisture permeability of the porous film decreases. When the stretching ratio exceeds 3 times, the contraction rate of the porous film increases.

In the present invention, the stretched film produced as described above is heat-set by a known roll method or the like. The heat setting is performed in order to stretch the film into a porous film and then apply heat to the film in advance to forcibly shrink the film, thereby suppressing shrinkage of the completed porous film. The roll method heat setting is performed by heating a stretched porous film using a heated roll (annealing roll). The temperature of the annealing roll is controlled in a specific temperature range based on the melting point of the resin composition. If the temperature is too low, the shrinkage of the porous film becomes large, and it is difficult to obtain the low shrinkage aimed at by the present invention. If the temperature is too high, the film will stick to the roll, causing troubles of winding.

The temperature of the heat setting, that is, the temperature of the annealing roll, is based on the melting point (A ° C.) of the resin composition.
(A-25) ° C to (A-5) ° C. The time for heat setting is at least 0.004 minutes. If the time is less than 0.004 minutes, the heat fixation becomes insufficient and the shrinkage rate increases. In consideration of productivity and the like, the upper limit of the heat fixing time is about 1 minute (60 seconds). The number of annealing rolls for heat setting may be one or two or more. Here, the heat setting time is a time required for the heat setting, that is, a time during which the porous film contacts a heating source such as an annealing roll. In the case of the roll method, it means the time of contact with the annealing roll. When two or more rolls are used, the heat setting time is the sum of the time during which the film is in contact with each annealing roll. Further, it is preferable to control the shrinkage rate (hereinafter referred to as an annealing rate) at the time of heat setting to be in a range of 15 to 25%.

The method of controlling the annealing rate is preferably a method of controlling the peripheral speed of the annealing roll (S ₁ ) and the subsequent speed of the peripheral speed of the roll (S ₂ ). Annealing rate formula: it can be represented by _{[{1- (S 2 / S 1} )} × 100 ]. If the annealing rate is too small, the contraction rate of the porous film increases, and if the annealing rate is too large,
The film sags during heat setting, making it impossible to produce it stably. The annealing roll is desirably made of metal to efficiently transmit heat to the film.

The porous film of the present invention produced as described above preferably has a thickness of about 10 to 100 μm. As another characteristic, the moisture permeability is 1000 to 40
00g / m ² · 24hr, adhesion strength of at least 600
g / 25 mm, and the stiffness is preferably 80 mm or less.

[0029]

The present invention will be described in more detail with reference to the following examples. Note that the present invention is not limited to these examples. The moisture permeability, heat shrinkage, and the like shown in the examples are values measured by the following methods.

(1) Moisture Permeability (g / m ² · 24 hr) 10 samples (10 cm × 10 cm) were taken from a porous film.
Samples are taken and measured under the conditions of a temperature of 40 ° C., a relative humidity of 60% and a pure water method based on the method specified in ASTME-96, and the average value is calculated. The measurement time is 24 hours.

(2) Heat shrinkage (%) A sample (MD: 20 cm, TD: 1c) from a porous film
m) are sampled, and marked lines are inserted so that the distance between the marked lines is 10 cm. After leaving for 1 hour at a temperature of 60 ° C. and cooling to room temperature, the distance between the marked lines is measured with a steel measure and determined by the following equation. Shrinkage = {(L ₁ −L ₂ ) / L ₁ } × 100 Here, L ₁ indicates the distance between marked lines before heating, and L ₂ indicates the distance between marked lines after heating.

(3) Melting point (° C.) A raw material (including a predetermined amount of polyolefin resin, inorganic filler, hardened castor oil, and dehydrated castor oil) of a porous film is sampled, and PERKIN-ELMER, 7 series heat Using an analysis system, the temperature was raised at 5 ° C./min, and the melting peak was taken as the melting point. If there are multiple melting peaks,
The hottest peak is considered the peak of the feed.

(4) Adhesive strength (g / 25 mm) A 25 mm wide pressure-sensitive adhesive polypropylene tape (manufactured by Toyo Chemical Co., Ltd., trade name: Karalyan PP, with an adhesive layer) is provided on one surface of the porous film. The film is attached so that the stretching direction of the conductive film is the peeling direction to obtain a sample. After leaving this sample in an oven at 60 ° C. for 48 hours, a Tensilon tensile tester (manufactured by Toyo Baldwin Co., trade name:
Tensileon) is used to peel off the polypropylene tape from one surface of the porous film according to the 180-degree peeling method specified in JIS-Z0237, and the peeling stress at that time is measured.

(5) Rigidity (mm) Measured according to the method (cantilever method) specified in JIS-L1096. <Sample adjustment> 200m width
After winding a porous film having a length of 300 mm, a length of 300 mm and a thickness of 40 μm around a 25 mm wide gold scale, the gold scale is removed. The obtained flat roll (width 25 mm, length 300 mm) is reciprocated one time by a 1 kg roller to obtain a sample.

(6) Uniformity of film thickness Three samples (machine direction: 101 cm, direction perpendicular to the machine direction: 5 cm) were sampled from the porous film, and 1 c in the longitudinal direction.
For a total of 300 measurement points at m intervals, a thickness measuring machine (UPRIGHT DIAL G, manufactured by PEACOCK)
AUGE NO. 25), the thickness is measured, and the average thickness (X), the maximum thickness (MAX), and the minimum thickness (MIN) are measured.
Is calculated, and [(MAX)-(MIN)] / (X) is calculated, and this is defined as the film thickness uniformity.

Example 1 Linear low density polyethylene (manufactured by Mitsui Chemicals, Inc., trade name:
39% by weight (Ultzex UZ2021L) (97.5% by weight in polyolefin resin), branched low-density polyethylene (Mitsui Chemicals Co., Ltd., trade name: Mirason F96)
7) 1 part by weight (2.5% by weight in the polyolefin resin), 60 parts by weight of calcium carbonate (trade name: SST-40, manufactured by Dowa Calfine Co., Ltd.), and hardened castor oil (Ito Oil Co., Ltd.) as a dispersant 1) parts by weight and dehydrated castor oil (Ito Oil Co., Ltd.)
(Trade name: DCO), 1 part by weight in a tumbler mixer, and then, using a tandem-type kneading extruder, 230
The mixture was uniformly kneaded at a temperature of ° C. and processed into pellets. The pellets were extruded using an extruder equipped with a T-die.
After the melt film formation at 40 ° C., a stretching ratio of 2.5 times between a preheating roll and a stretching roll heated to 70 ° C.,
Stretching roll speed 200 m / min (preheating roll speed 80
m / min) in the machine direction. Subsequently, using two heat fixing rolls (diameter 40 cm),
Heat fixing was performed at an annealing rate of 20% to obtain a porous film having a thickness of 40 μm. In the heat setting, the contact of the film with each heat setting roll was a half circumference of each roll. The main production conditions are shown in [Table 1], and the obtained results are shown in [Table 2].

Example 2 A porous film was obtained in the same manner as in Example 1 except that the contents of the hardened castor oil and the dehydrated castor oil were changed to 3 parts by weight and 2 parts by weight, respectively. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Example 3 The polyolefin resin of Example 1 was replaced with a linear low-density polyethylene (manufactured by Mitsui Chemicals, Inc., trade name: Evolu SP2).
040) A porous film was obtained in the same manner as in Example 1 except that the amount was changed to 40 parts by weight. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Example 4 A porous film was obtained in the same manner as in Example 1 except that the stretching roll speed in Example 1 was changed to 100 m / min. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Example 5 Example 1 was repeated except that the annealing rate of Example 1 was changed to 15%.
A porous film was obtained in the same manner as described above. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Example 6 A porous film was obtained in the same manner as in Example 1 except that the annealing temperature in Example 1 was changed to 95 ° C. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 1 A porous film was obtained in the same manner as in Example 1 except that the stretching roll speed in Example 1 was changed to 300 m / min. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 2 Example 1 was repeated except that the annealing rate in Example 1 was changed to 10%.
A porous film was obtained in the same manner as described above. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 3 Example 1 was repeated except that the draw ratio in Example 1 was changed to 3.5 times.
A porous film was obtained in the same manner as described above. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 4 Example 1 was repeated except that the annealing rate of Example 1 was changed to 30%.
A porous film was produced in the same manner as in Example 1, but the film could not be formed. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 5 A porous film was obtained in the same manner as in Example 1 except that the annealing temperature in Example 1 was changed to 80 ° C. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 6 A porous film was obtained in the same manner as in Example 1 except that the hardened castor oil and the dehydrated castor oil were removed. The production conditions and the obtained results are shown in [Table 1] and [Table 2].

Comparative Example 7 A porous film was produced in the same manner as in Example 1 except that the hardened castor oil and the dehydrated castor oil of Example 1 were replaced with 3 parts by weight of castor oil (manufactured by Ito Oil Co., Ltd., trade name: refined castor oil). I got The production conditions and the obtained results are shown in Table 1 and
It is shown in [Table 2].

[0049]

[Table 1]

[0050]

[Table 2]

<Explanation of Symbols in Table> A: manufactured by Mitsui Chemicals, Inc., trade name: Ultzex UZ2
021L, B: manufactured by Mitsui Chemicals, Inc., trade name: Evolu SP2040, C: manufactured by Mitsui Chemicals, Inc., trade name: Mirason F967, D: manufactured by Ito Oil Co., Ltd., trade name: Custer wax, E: Ito Oil refinery Co., Ltd., trade name; DCO,
F: manufactured by Ito Oil Co., Ltd., trade name; refined castor oil.

[0052]

The porous film according to the present invention has excellent flexibility, adhesiveness, moisture permeability and the like, and also has excellent shrink resistance in the stretching direction as compared with conventional products. That is,
The shrinkage in the stretching direction is small. Therefore, it can be used in a wide variety of applications such as sanitary materials such as disposable paper diapers, building materials such as roof waterproof materials, reflective films, battery separators, and the like.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C08L 23/06 C08L 23/06 91/00 91/00 // B29K 23:00 B29K 23:00 105: 04 105: 04 B29L 7:00 B29L 7:00 (72) Inventor Toshiyuki Enomoto 2-1, Tango-dori, Minami-ku, Nagoya-shi, Aichi F-term (reference) 4F074 AA16 AA20 AA20D AA65 AB03 AD11 AG01 CA02 CA03 CC02Y CC04Z CC32Z DA10 DA22 DA49 DA53 DA59 4F210 AA03 AA07 AA08 AB01 AB11 AB16 AB19 AG01 AG20 QC01 QC02 QC05 QW07 4J002 AE05X AE05Y BB001 BB031 EH057 FD016 GB01 GD00 GL00 GP00

Claims (10)

[Claims] 1. A polyolefin resin 25 to 55% by weight.
And a resin composition film containing 100 parts by weight of a mixture containing 75 to 45% by weight of an inorganic filler, 0.5 to 5 parts by weight of hardened castor oil, and 0.5 to 5 parts by weight of dehydrated castor oil were stretched in at least one axial direction. A porous film comprising 6
A porous film having a shrinkage in the stretching direction of 2% or less when heated at 0 ° C. for 1 hour. 2. The porous film according to claim 1, wherein the polyolefin resin is a linear low-density polyethylene or a mixture of linear low-density polyethylene of 100 parts by weight and branched low-density polyethylene of less than 50 parts by weight. 3. The dehydrated castor oil contains at least 30% by weight of 9,11-octadecadienoic acid and at least 30% by weight of 9,12-octadecadienoic acid, and the total amount of both acids is 70%.
The porous film according to claim 1, wherein the porous film is an ester of a fatty acid and glycerin, which are contained in an amount of not less than weight%. 4. The porous film according to claim 1, wherein the hydrogenated castor oil is an ester of a fatty acid containing 70% by weight or more of 12-hydroxyoctadecanoic acid and glycerin. 5. A water vapor transmission rate of 1000 to 4000 g / m ² ···
2. The porous film according to claim 1, which is 24 hours. 6. An adhesive strength of at least 600 g / 25 m
The porous film according to claim 1, wherein m and the rigidity are 80 mm or less. 7. Polyolefin resin 25 to 55% by weight
And 100 to 100 parts by weight of a mixture containing 75 to 45% by weight of an inorganic filler, 0.5 to 5 parts by weight of hardened castor oil, and 0.5 to 5 parts by weight of dehydrated castor oil. A film of the composition is stretched at least 1.5 to 3 times in a uniaxial direction, and then in a temperature range of (A-25) to (A-5) ° C based on the melting point (A ° C) of the resin composition. A method for producing a porous film, comprising: heat setting for at least 0.004 minutes; and controlling a shrinkage ratio during heat setting to a range of 15 to 25%. 8. The polyolefin resin is a linear low-density polyethylene or a mixture of 100 parts by weight of a linear low-density polyethylene and less than 50 parts by weight of a branched low-density polyethylene. A method for producing a porous film. 9. The dehydrated castor oil contains at least 30% by weight of 9,11-octadecadienoic acid and at least 30% by weight of 9,12-octadecadienoic acid, and the total amount of both acids is 70%.
The method for producing a porous film according to claim 7, which is an ester of a fatty acid and glycerin containing not less than% by weight. 10. The method for producing a porous film according to claim 7, wherein the hydrogenated castor oil is an ester of a fatty acid containing 70% by weight or more of 12-hydroxyoctadecanoic acid and glycerin.