JP2008094911A - Porous film and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film having high gas permeability and strength, especially a porous film having a strength and a gas permeability sufficient for the use as a separator for a battery. <P>SOLUTION: A porous film having a gas permeability of 10-1,000 sec/100 ml per 25 μm thick and a pin puncture strength of 2-8 N per 25 μm thick is produced by preparing a raw film from a resin composition produced by compounding 100 pts.mass of a polyolefin resin (melting point; Tm°C) with 2-25 pts.mass of a filler, drawing the raw film by a single or multiple stage low-temperature drawing at -20 to +50°C at a draw ratio of 1.1-3.0, and further drawing the film by a ≥2-stage high-temperature drawing at (Tm-70) to (Tm-5)°C at a draw ratio of 1.1-3.0. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous film and a method for producing the same, and the porous film is used as a packaging product, a sanitary product, a livestock product, an agricultural product, a building product, a medical product, a separation film, a light diffusion plate, a reflective sheet, or a battery separator. In particular, it can be suitably used as a separator for a non-aqueous electrolyte battery such as a lithium ion secondary battery used as a power source for various electronic devices.

  The polymer porous body with many fine communication holes is used for separation membranes used for the production of ultrapure water, generation of chemicals, water treatment, waterproof and moisture permeable films used for clothing and sanitary materials, batteries, etc. It is used in various fields such as separators.

Various techniques as described below have been proposed as a technique for providing a large number of fine communication holes in this type of polymer.
For example, in Japanese Patent Application Laid-Open No. 5-009332 (Patent Document 1), a mixture of ultrahigh molecular weight polyethylene, inorganic fine powder and plasticizer is molded into a sheet shape while kneading and heating and melting, and then inorganic fine powder and plastic It has been proposed that the agent is extracted and dried, and stretched to obtain a microporous membrane.
This method has a problem that the cost required for the extraction process is large. Furthermore, since the plasticizer is extracted and removed using an organic solvent, a large amount of the organic solvent is required, which is not preferable in terms of the environment.

In Japanese Patent Laid-Open No. 10-50286 (Patent Document 2), a high melting point polyolefin film and a low melting point polyolefin film are heat-treated to adjust birefringence and elastic recovery, respectively, and then subjected to thermocompression bonding to three or more layers. It is proposed to produce a porous film to be used as a battery separator by stretching the porous film in two stages, making it porous, and then heat setting.
After forming a crystalline polymer film in this way, the pores are formed by peeling the interface between the amorphous and the crystalline, which have different mechanical properties by stretching. In the method, conditions for obtaining a preferred porous structure under stretching conditions such as stretching temperature, stretch ratio, and multi-stage stretching are very narrow, and control by extrusion and stretching is not easy, and the process required for porosity is complicated. Because it takes a long time, it is difficult to increase productivity. Specifically, when the draft ratio is low, it becomes difficult to make the pores, while when the draft ratio is high, molding becomes difficult and special equipment is required.
As described above, it is not easy to increase the productivity of the porous film by the aperture stretching method, and therefore it is not preferable in view of production on an industrial scale.
The draft ratio refers to the original thickness / the thickness after stretching, that is, the stretching ratio.

In JP 2004-95550 A (Patent Document 3), a porous film used as a separator for a lithium secondary battery is stretched at least in a uniaxial direction from a sheet formed from a resin composition containing a thermoplastic resin and a filler. Is gained by.
Thus, after forming a film by adding a filler to the polymer, the so-called filler addition method in which the interface between the polymer and the filler is peeled off by stretching to form pores, the process is simple and the productivity increases. However, there is room for improvement in terms of increasing the strength of the porous film, particularly the pin puncture strength applied locally. The strength is improved by simply lowering the filler content, but the porosity decreases when the filler content is lowered, and as a result, when used as, for example, a battery separator, ionic conduction deteriorates. Problems such as failure to satisfy battery performance may occur.

JP-A-5-009332 Japanese Patent Laid-Open No. 10-50286 JP 2004-95550 A

  The present invention has been made in view of the above problems, and has sufficient strength when used as a porous film having air permeability and strength, particularly pin puncture strength, particularly as a separator for a lithium secondary battery. It is an object to provide a porous film having air permeability and a method for producing the same.

In order to solve the above problems, the present invention provides a first invention as follows:
A raw film is prepared from a resin composition in which 2 to 25 parts by mass of a filler is blended with respect to 100 parts by mass of a polyolefin resin having a melting point Tm ° C.
After stretching the original film at a draw ratio of 1.1 to 3.0 times by one-step or more low temperature stretching at -20 to + 50 ° C,
(Tm-70) to (Tm-5) Provided is a method for producing a porous film, characterized by being stretched at a draw ratio of 1.1 to 3.0 times by high-temperature stretching at two or more stages at a temperature of ing.

The inventors diligently studied a method for producing a porous film that can improve the pin puncture strength of the film while maintaining an appropriate air permeability of about 10 to 1000 seconds / 100 ml. As a result, it has been found that by producing a porous film by the above production method, it is possible to produce a porous film having low air permeability and high strength, and the present invention has been achieved.
In the method for producing a porous film of the present invention, in addition to pore formation by interfacial separation between a polyolefin resin and a filler by adding a filler (filler), non-crystalline and crystalline interfacial separation of the polyolefin resin by opening stretching. Micropores are formed.
Thus, the present invention uses a filler addition method and an aperture stretching method in combination, and further provides a porous film having excellent air permeability and strength by using an optimum filler blending amount and stretching conditions. To manufacture. Therefore, the air permeability of the film can be maintained by the fine pores formed by opening stretching, while the amount is much smaller than the amount of filler used in the conventional filler addition method. And since the addition amount of a filler can be restrained, film strength, especially pin puncture strength is not reduced.

That is, as described above, after producing a raw film from the resin composition, the raw film is subjected to low temperature stretching at a temperature of -20 to + 50 ° C, and (Tm-70) to (Tm-5) ° C. High-temperature stretching performed at a temperature of
Peeling occurs between the polyolefin lamellar crystals by the low-temperature stretching, and the resulting microspace is expanded by high-temperature stretching. In addition, interfacial separation between the polyolefin resin and the filler is also generated during the low temperature stretching and the high temperature stretching.
Since sufficient porosity cannot be obtained by only one of low temperature stretching and high temperature stretching, it is essential to perform both low temperature stretching and high temperature stretching.

As described above, the temperature during the low-temperature stretching is set to -20 to 50 ° C, and particularly preferably 0 to 35 ° C. This is not preferable because it may not be stretched at a temperature lower than −20 ° C. and may break, and when it exceeds 50 ° C., the porosity becomes insufficient even when stretched and porous in the next high-temperature stretching step. Because there is a fear.
Further, the low-temperature stretching may be performed in one stage or may be performed in a plurality of stages of two or more stages, but it is preferable that the number of stretching stages is 1 to 3.
The draw ratio in the low temperature drawing is preferably 1.05 to 2.0 times per step, and the overall low temperature drawing is 1.1 to 3.0 times, and 1.2 to 2.0 times. Is preferable. This is because only a film having a low porosity can be obtained when the stretching ratio of the low-temperature stretching is less than 1.1 times as a whole, and when it exceeds 3.0 times, a predetermined porosity and pore diameter cannot be obtained.

After the low-temperature stretching, the low-temperature stretched film is stretched at a high temperature.
The temperature of the high temperature stretching is (Tm-70) to (Tm-5) ° C, and more preferably (Tm-60) to (Tm-10) ° C.
The Tm value is determined from the temperature at the peak top of the DSC melting peak.
The high-temperature stretching is performed by multi-stage stretching of 2 or more stages, and the number of stretching stages is preferably 2 to 20 stages, and further, 4 to 10 stages are used in order to balance air permeability and pin penetration strength appropriately. Is more preferable.
The reason why the number of steps is two or more is that the fine pores formed between the lamellar crystals cannot be expanded by one-step stretching, and the porosity becomes insufficient. This is because the strength tends to decrease due to excessive expansion.
The draw ratio of the high temperature drawing is preferably 1.05 to 1.5 times per step, and more preferably 1.1 to 1.2 times. As described above, by stretching a plurality of stages at a low stretch ratio, it is possible to develop pores of an appropriate size without cutting the amorphous part.
In the entire high-temperature stretching, the stretching ratio is 1.1 to 3.0 times, and preferably 1.4 to 2.8 times. This is because the porosity is low when the draw ratio of the whole high-temperature drawing is less than 1.1 times, whereas when the draw ratio exceeds 3.0 times, reduction due to deformation of the pores occurs, so that the porosity is not good. This is because it is not preferable because it is sufficient.

In the method for producing a porous film of the present invention, it is preferable to heat-treat the formed raw film before stretching.
By this heat treatment, it is possible to partially crystallize the amorphous part of the polyolefin resin and increase the crystal part. The heat treatment can be performed by any known method such as a heated air circulation oven or contact heating with a heating roll. Moreover, it is also possible to divert the above-mentioned extending | stretching apparatus.

In addition, after the low-temperature stretching, during the high-temperature stretching, the low-temperature stretched film may be heat-set at a temperature equivalent to that during the high-temperature stretching.
This heat setting is performed by heating in a heated air circulation oven or the like with the film stretching direction fixed so that the dimensions do not change. The heat setting time is usually 1 to 5 minutes and may be 2 to 3 minutes.

Furthermore, it is preferable to perform heat treatment after the high-temperature stretching.
The heat treatment is preferably performed at a temperature of (Tm-55) to (Tm-35) ° C, and more preferably at a temperature of (Tm-50) to (Tm-35) ° C. This is because the film is not sufficiently porous at a temperature lower than (Tm-55) ° C., and the film melts at a temperature higher than (Tm-35) ° C.
The reason why the heat treatment is performed after the high-temperature stretching is to prevent shrinkage in the stretching direction due to residual stress during stretching, and this treatment results in a porous film that can satisfy dimensional stability.
Examples of the heat treatment include heat fixing for fixing the stretching direction so that the dimensions do not change and heat shrinking to such an extent that the film width after stretching is reduced by 5 to 50%.

  The low-temperature stretching and the high-temperature stretching are performed by performing a stretching process in at least one axial direction by roll stretching, tenter stretching, rolling, or the like. Stretching is preferably uniaxial stretching in the film flow direction (MD), and roll stretching is particularly preferred.

As a method for producing the raw film stretched by the low-temperature stretching and the high-temperature stretching, first, a melt-kneaded product of the resin composition of the porous film of the present invention is produced.
Specifically, the polyolefin resin constituting the resin composition, the filler, and other components as required are mixed with a powder mixer such as a Henschel mixer, and heated and kneaded with a uniaxial or biaxial kneader, mixing roll, kneader, etc. To do. In view of the dispersibility of the filler, it is preferable to use a biaxial kneader.
Next, after heating or kneading the resin composition as it is or after granulation, a raw film is obtained by melting and molding using an extruder or the like under a temperature condition not lower than the melting point of the polyolefin resin and lower than the decomposition temperature. Yes.
Examples of the film molding method include T-die molding, inflation molding, calendar molding, press molding, and the like. Any of these may be used in the present invention, but T-die molding or inflation molding is preferably used.

When a raw film is obtained by a melt molding method using a T die, the draft ratio, which is the thickness after extrusion molding with respect to the thickness of the discharge port of the melted resin composition, is preferably 10 to 300, more preferably 20 to 200. It is.
When the draft ratio is less than 10, the film is not sufficiently oriented in the flow direction (MD), and it is difficult to form pores by stretching. Further, when the draft ratio exceeds 300, the lip gap of the T die is widened and the film is taken up, so that thickness unevenness is likely to occur, and molding is difficult unless special equipment is used.
The take-up speed of the raw film is not particularly limited, but is usually 10 to 50 m / min.

The resin composition constituting the porous film of the present invention contains a polyolefin resin as a main component.
Examples of the olefin resin include monoolefin polymers such as ethylene, propylene, 1-butene, 1-hexene and 1-octene; and other olefin polymers such as ethylene, propylene, 1-butene, 1-hexene, 1-octene and vinyl acetate. The main component is a copolymer with a monomer.
Examples include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, polypropylene, ethylene-propylene copolymer, polybutene, ethylene-vinyl acetate copolymer, and the like. The above can be mixed and used. Among these, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene, and ethylene-propylene copolymer are preferably used.
As the polyolefin resin of the present invention, it is particularly preferable to use high density polyethylene and / or polypropylene.

The polyolefin resin may be either a homopolymer polyolefin or a copolymer polyolefin, but is preferably a homopolymer polyolefin.
When it is a copolymer polyolefin, a copolymer polyolefin having an α-olefin comonomer content of 2 mol% or less is preferred. There is no restriction | limiting in particular in the kind of alpha-olefin comonomer, For example, ethylene, propylene, 1-butene, 1-hexene, 1-octene, 1-decene etc. are mentioned.

The polyolefin is not particularly limited with respect to the polymerization method, and polyolefins produced by any polymerization method such as one-stage polymerization, two-stage polymerization or higher multi-stage polymerization can be used.
The polymerization catalyst for polyolefin is not particularly limited, and any catalyst such as a Ziegler type catalyst, a Philips type catalyst, or a Kaminsky type catalyst can be used.

  The melt flow rate of the polyolefin resin is preferably 0.01 or more and 20 or less. More preferably, it is 0.03 or more and 10 or less. A melt flow rate of less than 0.01 is not preferable because film formation becomes difficult. On the other hand, if the melt flow rate exceeds 20, the strength decreases, which is not preferable.

  The weight average molecular weight of the polyolefin resin is preferably 50,000 or more and less than 1,000,000. More preferably, it is 80,000 or more and less than 900,000. This is because if the weight average molecular weight is less than 50,000, the mechanical strength decreases, which is not preferable. On the other hand, if it is 1 million or more, extrusion tends to be difficult. About the measurement of a weight average molecular weight, it measures by the method as described in an Example using GPC (gel permeation chromatography).

The compounding quantity of the filler in the polyolefin resin composition used for this invention is 2-25 mass parts with respect to 100 mass parts of polyolefin resin. Preferably it is 5-20 mass parts. When the blending amount of the filler is less than 2 parts by mass with respect to 100 parts by mass of the polyolefin resin composition, since pores derived from the filler are reduced when the film is stretched, the communication is poor and the air permeability is low. Since it becomes high, it is not preferable. Moreover, when it exceeds 25 mass parts, the void | hole from a filler will increase and air permeability will fall, but pin puncture intensity | strength will fall.
In the filler addition method, in order to satisfy the air permeability, the filler is blended at least 50 parts by mass, usually about 100 to 300 parts by mass with respect to 100 parts by mass of the resin component. For this reason, a reduction in pin puncture strength is inevitable. However, in the present invention, not only pores due to the addition of the filler but also micropores formed by the above-described aperture stretching method are formed, so that the air permeability and strength of the film can be maintained with a small amount of filler added.

As said filler, any filler of an inorganic filler and an organic filler can be used, and it can be used 1 type or in combination of 2 or more types.
Examples of inorganic fillers include carbonates such as calcium carbonate, magnesium carbonate and barium carbonate; sulfates such as calcium sulfate, magnesium sulfate and barium sulfate; chlorides such as sodium chloride, calcium chloride and magnesium chloride; calcium oxide and magnesium oxide. And oxides such as zinc oxide, titanium oxide and silica; and silicates such as talc, clay and mica. Of these, calcium carbonate and barium sulfate are preferably used.

  In order to improve the dispersibility in the polyolefin resin, the inorganic filler may be hydrophobized by coating the surface of the inorganic filler with a surface treatment agent. Examples of the surface treatment agent include higher fatty acids such as stearic acid or lauric acid, or metal salts thereof.

As the organic filler, resin particles having a melting point higher than the melting point of the polyolefin resin are preferable so that the filler does not melt at the stretching temperature, and crosslinked resin particles having a gel content of about 4 to 10% are more preferable.
Examples of the organic filler include polystyrene, polymethyl methacrylate, polycarbonate, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, polytetrafluoroethylene, polyimide, polyetherimide, melamine, and benzoguanamine. And other thermoplastic resins and thermosetting resins. Among these, cross-linked polystyrene and the like are particularly preferable.

  In the present invention, among the fillers described above, inorganic fillers are preferable. In particular, when used as a battery separator, it is preferable to use barium sulfate because of its reactivity with the electrolytic solution.

  The average particle size of the filler is preferably 0.01 to 3 μm, more preferably 0.05 to 1 μm. When the average particle size is less than 0.01 μm, voids are difficult to be formed during stretching, and thus often cannot play a role of assisting in the formation of porosity. Further, the dispersibility of the fillers decreases due to aggregation between the fillers, causing unevenness of stretching, and the contact area of the interface between the polyolefin resin and the filler is increased, so that interfacial peeling due to stretching is difficult and porosity is often difficult. On the other hand, if the average particle diameter exceeds 3 μm, the diameter of the pores formed by stretching becomes too large, which is not preferable because the strength is reduced.

Further, other thermoplastic resins may be blended with the polyolefin resin as long as the object of the present invention is not impaired.
For example, when a battery separator is used, a thermoplastic resin other than a polyolefin resin may be blended for the purpose of imparting functions such as a shutdown temperature (temperature at which pores close) or a heat-resistant temperature. Specific examples of the thermoplastic resin include fluorine resin, polystyrene, vinyl acetate resin, acrylic resin, polyamide resin, acetal resin, and polycarbonate.
As for content of thermoplastic resins other than the said polyolefin resin, 5-40 mass parts is preferable with respect to 100 mass parts of polyolefin resin, More preferably, it is 5-30 mass parts. When the content of the thermoplastic resin is less than 5 parts by mass, the effect of the added thermoplastic resin does not appear. On the other hand, when the content exceeds 40 parts by mass, the target pin penetration strength cannot be satisfied.

  In addition, polyolefin resins have low molecular weight compounds that are compatible with thermoplastic resins as required, and additives that are generally blended into resin compositions, such as antioxidants, heat stabilizers, light stabilizers, and UV absorbers. An agent, a neutralizing agent, an antifogging agent, an antiblocking agent, an antistatic agent, a slipping agent or a coloring agent may be blended. When blending these additives and the like, it is preferable to blend 0.05 to 1 part by mass in 100 parts by mass of the resin composition.

As a second invention of the present invention, a porous film produced by the method for producing a porous film of the first invention,
There is provided a porous film having an air permeability of 10 to 1000 seconds / 100 ml per 25 μm thickness and a pin puncture strength of 2 to 8 N per 25 μm thickness.
Since the porous film of 2nd invention is obtained by the said manufacturing method which used both the aperture stretch method and the filler addition method, two types of void | holes from which a size and a shape differ are formed.

Furthermore, as a third invention, a porous film obtained by stretching a raw film made of a resin composition containing a polyolefin resin and a filler,
The content of the filler is 2 to 25 parts by mass with respect to 100 parts by mass of the polyolefin resin.
There is provided a porous film having an air permeability of 10 to 1000 seconds / 100 ml per 25 μm thickness and a pin puncture strength of 2 to 8 N per 25 μm thickness.

The porous film of the third invention is characterized in that it contains a polyolefin resin and a small amount of filler, and has a required air permeability and pin puncture strength. If it is, it will not be limited to what was manufactured by the said manufacturing method.
As the polyolefin resin, filler, and other components to be blended, the same components as those used in the method for producing the porous film can be used.

The porous films of the second and third inventions have an air permeability of 10 to 1000 seconds / 100 ml, preferably 50 to 900 seconds / ml, more preferably 100 to 500 seconds / 100 ml per 25 μm thickness. is there.
When the air permeability per 25 μm thickness is less than 10 seconds / 100 ml, when used as a battery separator, the retention of the electrolyte is lowered, the capacity of the secondary battery is lowered, and the cycle performance is lowered. This is because there is a risk of being lost. On the other hand, if the air permeability exceeds 1000 seconds / 100 ml, the ionic conductivity becomes low, and sufficient battery performance cannot be obtained when used as a battery separator.

The porous film has a pin penetration strength of 2 to 8 N per 25 μm thickness in addition to the air permeability. Preferably, it is 2-5N.
If the pin puncture strength is less than 2N, when used as a battery separator, it may break during product assembly and the production yield may decrease. If it exceeds 8N, other physical properties will be adversely affected. In particular, the air permeability is not preferable because it does not reach the target range.

The air permeability is measured with a B-type Gurley densometer in accordance with JIS P 8117, and the pin puncture strength is the maximum load when a hole having a hole with a radius of curvature of 0.5 mm is pierced at a piercing speed of 300 mm / min. The maximum load obtained by measurement is obtained as a converted value per 25 μm thickness of the porous film using the following formulas.
Conversion value = actual value × (25 / film thickness (μm))
The film thickness is measured with a dial gauge of 1/1000 mm.

In the porous film of the present invention, the porosity is also an important factor for limiting the porous structure. The porosity of the porous film of the present invention is preferably in the range of 5 to 80%. This is because, if the porosity is less than 5%, it is difficult to substantially obtain communication. Moreover, if the porosity is higher than 80%, handling becomes difficult from the viewpoint of strength, which is not preferable. The porosity is more preferably 20 to 70%, and particularly preferably 25 to 60%.
The porosity is calculated by measuring the substantial amount W1 of the porous film, calculating the mass W0 when the porosity is 0% from the density of each component, the blending amount of each component, the thickness of the film, etc. The porosity is calculated based on the following formula from the difference from the actual amount of the quality film.
Porosity (%) = {(W0−W1) / W0} × 100

Since the range required for the air permeability and the porosity differs depending on the application, the air permeability and the porosity are appropriately adjusted according to the application. For example, when used for sanitary products such as diapers and sanitary products, the air permeability is preferably 10 to 1,000 seconds / 100 ml.
The air permeability and porosity can be controlled by adjusting the type of polyolefin resin, the content of filler, and the stretching conditions. For example, if the filler content is increased or the draw ratio is increased, the porosity increases, so the air permeability decreases.

The porous films of the second and third inventions can be applied to various uses that require air permeability.
For example, battery separators, disposable paper diapers, sanitary materials such as pads or bed sheets for absorbing body fluids such as sanitary products, medical materials such as surgical clothing or base materials for hot compresses, clothing such as jumpers, sportswear or rainwear Material, building materials such as wallpaper, roof waterproofing material, heat insulating material, sound absorbing material, desiccant; moisture-proofing agent; oxygen scavenger; disposable body warmer; Can be used.

  Among them, since the porous film of the present invention has optimum performance as a battery separator, the fourth invention provides a battery separator characterized by comprising the porous film, According to a fifth aspect of the present invention, there is provided a battery characterized by using the battery separator.

The porous film of the present invention is particularly suitably used as a separator for a non-aqueous electrolyte battery such as a lithium ion secondary battery used as a power source for various electronic devices.
When used as the battery separator, the air permeability is preferably 50 to 900 seconds / 100 ml, and more preferably 100 to 500 seconds / 100 ml. When the air permeability is less than 50 seconds / 100 ml, there is a possibility that the electrolyte retention will be reduced, the capacity of the secondary battery will be lowered, and the cycle performance will be reduced. On the other hand, if the air permeability exceeds 900 seconds / 100 ml, the ion conductivity becomes low and sufficient battery characteristics cannot be obtained.
Further, when the porous film of the present invention is used as a battery separator, the porosity is preferably 25 to 70%, and more preferably 25 to 65%. If the porosity is less than 25%, the ion permeability is low and it is difficult to obtain sufficient battery performance. Further, if the porosity exceeds 70%, it is not preferable from the viewpoint of battery safety.

  As described above, according to the method for producing a porous film of the present invention, a decrease in pin puncture strength is suppressed by suppressing the blending amount of the filler to a small amount. Since the micropores formed in (1) can be added, a porous film having an appropriate air permeability and excellent strength can be produced.

  Furthermore, since the manufacturing method of the porous film of the present invention can easily control the manufacturing conditions, the process management can be easily performed, and the productivity of the porous film can be increased. In addition, since a large amount of organic solvent is not used in the manufacturing process, the burden on the environment can be reduced.

  In particular, the present invention can provide a porous film having air permeability and strength suitable for use as a separator for a lithium secondary battery.

Hereinafter, embodiments of the present invention will be described.
The porous film of the embodiment is a raw material composed of a resin composition in which 2 to 25 parts by mass of an inorganic filler composed of barium sulfate or the like is blended with 100 parts by mass of a polyolefin resin composed of high-density polyethylene and / or polypropylene. The anti-film is drawn into a porous film.

The porous film has an average thickness of 5 to 100 μm, an air permeability of 10 to 1000 seconds / 100 ml per thickness of 25 μm, a pin penetration strength of 2 to 8 N per thickness of 25 μm, and a porosity of 5 to 80%.
The air permeability, the porosity, and the pin penetration strength are measured by the methods described in the examples described later.

An example of the method for producing the porous film will be described in detail below.
A resin composition containing a polyolefin resin, a filler, and optionally other components is mixed with a powder mixer such as a Henschel mixer and heated and kneaded with a uniaxial or biaxial kneader, a mixing roll, a kneader, etc. Adjust.

The polyolefin resin has a weight average molecular weight of 50,000 to less than 1,000,000, a density of 0.89 to 0.92 g / cm ³ , a melt flow rate of 2.0 to 4.0 g / 10 min, and a melting point of 150 to 180 ° C. Homopolymer polypropylene is used.
In addition, about the said physical property of polyolefin, it measures by the method as described in an Example.
As the filler, barium sulfate or calcium carbonate having an average particle diameter of 0.1 to 1 μm is used. The filler content is 2 to 25 parts by mass with respect to 100 parts by mass of the polyolefin resin.
In addition, polyolefin resins generally include additives that are blended in resin compositions, such as plasticizers, antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, neutralizers, antifogging agents, antiblocking agents, You may mix | blend an antistatic agent, a slip agent, or a coloring agent.
When mix | blending these additives etc., the compounding quantity is mix | blended so that it may become 0.05-1 mass part in 100 mass parts of resin compositions.

The kneaded resin composition is extruded into a film shape under a temperature condition of 190 to 220 ° C. using a T-die for multilayer molding to produce a raw film.
The draft ratio at this time is 10 to 300, preferably 20 to 200. In this embodiment, the pellets are not formed in this way, but are used as they are in the film forming process.

Next, the obtained raw film is subjected to heat treatment.
In this embodiment, it is performed by heating at a temperature of 110 to 135 ° C. for 10 to 30 hours with a hot air circulating oven. By setting the temperature, crystallization can be promoted without melting the film.

Next, the heat-treated raw film is made porous by stretching.
Stretching is performed at a low temperature in one or more stages and then at a high temperature in two or more stages.
The low-temperature stretching is performed at −20 to + 50 ° C., and in this embodiment, the temperature of the stretching roll is adjusted to 0 to 35 ° C., and stretching is performed at the difference in peripheral speed.
The number of draw stages in the low-temperature drawing of this embodiment is one or two, and the draw ratio is 1.05 to 2.0 times per stage, and the draw ratio is 1.1 to 3.0 in the whole low-temperature drawing. I try to double it.

Subsequently, high temperature stretching is performed after low temperature stretching.
Similarly to the low temperature stretching, the high temperature stretching is performed by adjusting the temperature of the stretching roll and by the difference in peripheral speed.
High temperature stretching is performed in at least two stages, and in this embodiment, it is performed by multistage stretching of 4 to 10 stages. The temperature of the high temperature stretching is from (Tm-70) to (Tm-5) ° C. when the melting point of the polyolefin resin is Tm ° C. In the present embodiment, polypropylene having a melting point of 160 to 170 ° C. is used, so the temperature of the drawing roll is 90 to 165 ° C.
The draw ratio of high-temperature drawing is 1.05 to 1.5 times per stage, and the draw ratio of the whole high-temperature drawing is 1.1 to 3.0 times.

  Finally, in order to ensure the dimensional stability of the film, a heat-treated film is subjected to low-temperature stretching and high-temperature stretching. In this embodiment, the heat treatment is performed for 30 to 90 seconds in a heating circulation oven adjusted to 105 to 145 ° C. while fixing the stretching direction so that the dimensions do not change.

As described above, the porous film of the present invention obtained by the manufacturing method described above has an average thickness of 5 to 100 μm, an air permeability of 100 to 1000 seconds / 100 ml per 25 μm thickness, and a porosity of 5 ˜80%, pin puncture strength is 2-8 N per 25 μm thickness.
The porous film of the present invention having the above-mentioned physical properties is appropriately adjusted for air permeability and porosity according to the use, and various kinds of air permeability are required such as battery separators, sanitary products such as diapers and sanitary products. Can be used for applications. In particular, it is preferably used as a battery separator.

Next, a nonaqueous electrolyte battery containing the porous film of the present invention as a battery separator will be described with reference to FIG.
Both electrodes of the positive electrode plate 21 and the negative electrode plate 22 are wound in a spiral shape so as to overlap each other via the separator 10, and the outside is stopped with a winding tape to form a wound body. When winding in this spiral shape, the separator 10 preferably has a thickness of 5 to 40 μm. If the thickness is less than 5 μm, the separator may be easily broken. If the thickness exceeds 40 μm, the battery area may be reduced when wound into a predetermined battery can as a battery separator, and the battery capacity may be reduced. Because there is.

  A wound body in which the positive electrode plate 21, the separator 10 and the negative electrode plate 22 are integrally wound is housed in a bottomed cylindrical battery case and welded to the positive and negative electrode lead bodies 24 and 25. Next, the electrolyte is injected into the battery can, and after the electrolyte has sufficiently penetrated into the separator 10 or the like, the positive electrode lid 27 is sealed through the gasket 26 around the opening periphery of the battery can, and precharging and aging are performed. Type non-aqueous electrolyte battery.

As the electrolytic solution, an electrolytic solution in which a lithium salt is used as an electrolytic solution and is dissolved in an organic solvent is used. The organic solvent is not particularly limited, but for example, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone, esters such as dimethyl carbonate, methyl propionate or butyl acetate, and nitriles such as acetonitrile. , 1,2-dimethoxyethane, 1,2-dimethoxymethane, dimethoxypropane, 1,3-dioxolane, ethers such as tetrahydrofuran, 2-methyltetrahydrofuran or 4-methyl-1,3-dioxolane, or sulfolane These may be used alone or in combination of two or more.
Among them, an electrolyte obtained by dissolving lithium hexafluorophosphate (LiPF ₆₎ at a rate of 1.4 mol / L in a solvent obtained by mixing 2 parts by mass of methyl ethyl carbonate relative to ethylene carbonate 1 part by weight is preferred.

As the negative electrode, an alkali metal or a compound containing an alkali metal integrated with a current collecting material such as a stainless steel net is used. Examples of the alkali metal include lithium, sodium, and potassium. Examples of the compound containing an alkali metal include an alloy of an alkali metal and aluminum, lead, indium, potassium, cadmium, tin or magnesium, a compound of an alkali metal and a carbon material, a low potential alkali metal and a metal oxide, and the like. Or a compound with a sulfide or the like.
When a carbon material is used for the negative electrode, the carbon material may be any material that can be doped and dedoped with lithium ions, such as graphite, pyrolytic carbons, cokes, glassy carbons, a fired body of an organic polymer compound, Mesocarbon microbeads, carbon fibers, activated carbon and the like can be used.

  In this embodiment, as a negative electrode, a carbon material having an average particle size of 10 μm is mixed with a solution in which vinylidene fluoride is dissolved in N-methylpyrrolidone to form a slurry, and this negative electrode mixture slurry is passed through a mesh of 70 mesh. After removing the large particles, uniformly apply to both sides of the negative electrode current collector made of a strip-shaped copper foil having a thickness of 18 μm and dry, and then compression-molded with a roll press machine, cut, and strip-shaped negative electrode plate What is used.

  As the positive electrode, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, manganese dioxide, metal oxide such as vanadium pentoxide or chromium oxide, metal sulfide such as molybdenum disulfide, etc. are used as active materials. , These positive electrode active materials are combined with conductive additives and binders such as polytetrafluoroethylene as appropriate, and finished with a current collector material such as a stainless steel mesh as a core material. It is done.

In the present embodiment, a strip-like positive electrode plate produced as follows is used as the positive electrode. In other words, lithium graphite oxide (LiCoO ₂ ) was added with phosphorous graphite as a conductive aid at a mass ratio of 90: 5 and mixed, and this mixture was mixed with a solution in which polyvinylidene fluoride was dissolved in N-methylpyrrolidone. To make a slurry. The positive electrode mixture slurry is passed through a 70 mesh net to remove large particles, and then uniformly applied to both sides of a positive electrode current collector made of an aluminum foil having a thickness of 20 μm and dried. After compression molding, it is cut into a strip-like positive electrode plate.

  Examples of the porous film of the present invention will be described below. In addition, this invention is not limited to an Example.

(Example 1)
Homopolypropylene (“F104A” manufactured by Prime Polymer Co., Ltd., density: 0.90 g / cm ³ , melt flow rate: 3.2 g / 10 min, melting point 165 ° C.) with respect to 100 parts by mass as a filler barium sulfate ( 20 parts by mass of “B-55” manufactured by Sakai Chemical Industry Co., Ltd., particle size: 0.66 μm) was blended and compounded, and melt extrusion was performed at 200 ° C. Molding was performed with a T die having a lip opening of 2 mm, and the film was guided to a cast roll at 90 ° C. to obtain a raw film. The thickness of the raw film was 40 μm on average and the draft ratio was 50.
Next, heat treatment was performed for 24 hours in a hot air circulating oven heated to 120 ° C.
The heat-treated raw fabric was stretched in a single step with a roll held at 25 ° C. as a low-temperature stretch, and stretched 1.2 times. At this time, the roll speed on the supply side was 1.6 m / min.
The film stretched at a low temperature was stretched at a high temperature. The high-temperature stretching was a roll maintained at a temperature of 120 ° C. in the same direction as the low-temperature stretching, and 1.1-fold stretching was stretched 6 stages, that is, 1.7 times per stage.
The stretched film was heat-set at 120 ° C. for 60 seconds to obtain a porous film.

(Example 2)
11.1 parts by mass of barium sulfate was added as a filler to 100 parts by mass of polypropylene. Otherwise, the porous film was obtained in the same manner as in Example 1.

(Example 3)
5 parts by mass of barium sulfate was added as a filler to 100 parts by mass of polypropylene. Otherwise in the same manner as in Example 1, a porous film was obtained.

(Comparative Example 1)
No filler was added, the temperature of the low-temperature stretching was set to 35 ° C., and in order to further reduce the air permeability, the conditions of the high-temperature stretching were set to a stretching ratio of 2.0 times with eight stretching stages. Otherwise in the same manner as in Example 1, a porous film of Comparative Example 1 was obtained.
(Comparative Example 2)
Barium sulfate was used as 50 parts by mass with respect to 100 parts by mass of polypropylene. Otherwise in the same manner as in Example 1, a porous film was obtained.
(Comparative Example 3)
1 part by mass of barium sulfate was used as a filler with respect to 100 parts by mass of polypropylene. Otherwise in the same manner as in Example 1, a porous film was obtained.

About the porous film obtained by the Example and the comparative example, the thickness (film thickness), the air permeability, and the pin penetration strength of the film were measured with the following method.
(1) Film thickness (film thickness)
The measurement was performed using a 1/1000 mm dial gauge.
(2) Air permeability According to JIS P 8117, the air permeability was measured with a B-type Gurley densometer (manufactured by Toyo Seiki Seisakusho).
In addition, the air permeability was calculated | required as a conversion value per thickness 25 micrometers using the following formula | equation.
Conversion value = actual value × (25 / film thickness (μm))
(3) Pin puncture strength A needle having a curvature radius of 0.5 mm was pierced at a puncture speed of 300 mm / min, and the maximum load when a hole was formed was measured. The obtained maximum load was defined as pin puncture strength.
The pin puncture strength was determined as a converted value per 25 μm thickness using the same formula as the above air permeability.

Moreover, the measuring method of the weight average molecular weight of the resin used by the Example and the comparative example, a density, and a melt flow rate is shown below.
(4) Weight average molecular weight It measured by GPC (gel permeation chromatography). The measurement conditions are shown below.
Equipment: WATERS 150-GPC
Temperature: 140 ° C
Solvent: 1,2,4-trichlorobenzene Sample concentration: 0.05% (injection amount: 500 μl)
Column: One Shodex GPC AT-807 / S
Two Tosoh TSK-GEL GMH8-HT Dissolution conditions: 160 ° C., 2.5 hours Calibration curve: A standard sample of polystyrene was measured, and third-order calculation using polyethylene conversion constant (0.48) (5) Density The density gradient tube method was used according to JIS K7112.
(6) Melt flow rate Measured according to JIS K 7120.

  The results are shown in the table below.

When the filler is not blended or the blending amount is small as in the porous films of Comparative Examples 1 and 3, the pin puncture strength is 2 to 8 N which is the target, but the air permeability is the target 10 to 1000. It was much higher than sec / 100ml.
On the other hand, when the blending amount of the filler was increased to 50 parts by mass with respect to 100 parts by mass of the polyolefin resin as in the porous film of Comparative Example 2, the target air permeability was obtained. The stab strength was lower than intended.
Thus, the porous films of Comparative Examples 1 to 3 did not have the desired air permeability and pin penetration strength.

  On the other hand, the porous films of the present invention of Examples 1 to 3 satisfy the target air permeability of 10 to 1000 seconds / 100 ml, and at the same time the pin puncture strength is in the range of 2 to 8 N. It has been confirmed that it has pin stab strength.

It is a partially broken perspective view of the nonaqueous electrolyte battery which has accommodated the porous film of the present invention as a nonaqueous electrolyte battery separator.

Explanation of symbols

10 Separator 20 Non-aqueous electrolyte battery 21 Positive electrode plate 22 Negative electrode plate

Claims (8)

A raw film is produced from a resin composition containing 2 to 25 parts by mass of a filler with respect to 100 parts by mass of a polyolefin resin having a melting point of Tm °
After stretching the original film at a draw ratio of 1.1 to 3.0 times by one-step or more low temperature stretching at -20 to + 50 ° C,
(Tm-70) to (Tm-5) A method for producing a porous film, wherein the film is stretched at a draw ratio of 1.1 to 3.0 times by high-temperature drawing at two or more stages at a temperature. A porous film produced by the production method according to claim 1,
A porous film having an air permeability of 10 to 1000 seconds / 100 ml per 25 μm thickness and a pin puncture strength of 2 to 8 N per 25 μm thickness. A porous film obtained by stretching a raw film comprising a resin composition containing a polyolefin resin and a filler,
The content of the filler is 2 to 25 parts by mass with respect to 100 parts by mass of the polyolefin resin.
A porous film having an air permeability of 10 to 1000 seconds / 100 ml per 25 μm thickness and a pin puncture strength of 2 to 8 N per 25 μm thickness.   The porous film according to claim 2 or 3, wherein the polyolefin resin is made of high-density polyethylene and / or polypropylene.   The porous film according to any one of claims 2 to 4, wherein the filler is an inorganic filler.   The porous film according to claim 5, wherein the inorganic filler is barium sulfate.   A battery separator comprising the porous film according to any one of claims 2 to 6.   A battery comprising the battery separator according to claim 7.

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