JP2011074214A - Microporous film, laminated microporous film, battery separator and method for producing microporous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microporous film that has a high film-breaking temperature and has a good balance between porosity and air permeability. <P>SOLUTION: The microporous film is formed from a thermoplastic resin composition containing 100 pts.mass of (a) a polypropylene resin and 5-90 pts.mass of (b) a polyamide resin, has a sea-island structure composed of the polypropylene resin constituting the sea part and the polyamide resin constituting the island part and has an air permeability of 10 to 5,000 sec/100 cc. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a microporous film, a laminated microporous film, a battery separator, and a method for producing a microporous film.

  Microporous films, particularly polyolefin microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like, and are particularly used as lithium ion battery separators. In recent years, lithium ion batteries have been used for small electronic devices such as mobile phones and notebook personal computers, and also applied to hybrid electric vehicles and the like.

Here, a lithium ion battery used for a hybrid electric vehicle is required to have higher output characteristics for taking out a lot of energy in a short time. Moreover, since lithium ion batteries used for hybrid electric vehicle applications are generally large and have a high energy capacity, it is required to ensure higher safety.
The safety described here refers to safety against battery short-circuiting (short-circuiting) due to melting of the resin used in the separator, particularly in a high temperature state that occurs when the battery is used.
One of the means for improving the safety of the battery is to set the temperature at which the short circuit occurs inside the battery as the film breaking temperature of the separator and to raise the film breaking temperature.

And for the purpose of providing the microporous film used as the battery separator which can respond to such a situation, for example, in patent document 1, polypropylene microporous as a heat insulation layer is added to the conventional polyethylene microporous film. A composite microporous film (battery separator) having a laminated structure in which films are laminated has been proposed. Polypropylene is used to increase the short-circuit temperature.
Patent Document 2 proposes a battery separator having a structure in which a specific resin porous powder polymer is coated on a synthetic resin microporous film made of polyethylene, and stability at high temperatures is improved.

Japanese Patent Laid-Open No. 05-251069 Japanese Patent Laid-Open No. 03-291848

  However, the composite microporous film (battery separator) disclosed in Patent Document 1 and the synthetic resin microporous film made of polyethylene disclosed in Patent Document 2 are coated with a resin porous powder polymer. All battery separators have improved stability at high temperatures, but when they are tested for heat resistance under harsh conditions such as battery oven tests, they still have insufficient properties, and are even more heat resistant. Improvement of the property is desired.

  Therefore, in the present invention, an object is to provide a microporous film that is not easily torn even at a high temperature and has good heat resistance. The microporous film has a high film breaking temperature and a good balance between porosity and air permeability. It aims at providing an adhesive film.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a microporous film having a sea-island structure with a polypropylene resin as a sea part and a polyamide-based resin as an island part has no micro-island structure. It was found to be superior in heat resistance and hard to break even at high temperatures as compared with a conductive film.
Furthermore, since the microporous film having such a sea-island structure has a high film breaking temperature and sufficient porosity and air permeability, it is suitable for use as a separator for a lithium ion battery. The headline and the present invention were completed.
The present invention is as follows.

[1]
(A) It is composed of a thermoplastic resin composition containing 5 to 90 parts by mass of polyamide resin with respect to 100 parts by mass of polypropylene resin, and has a sea-island structure in which the polypropylene resin is a sea part and the polyamide resin is an island part. And a microporous film having an air permeability of 10 seconds / 100 cc to 5000 seconds / 100 cc.

[2]
The microporous film according to [1], wherein the porosity is 20% to 70%.

[3]
The microporous film according to [1] or [2], wherein the thermoplastic resin composition further contains (c) an admixture in a proportion of 1 to 20% by mass in the thermoplastic resin composition. .

[4]
The microporous film according to [3], wherein the admixture (c) is an epoxy group-containing ethylene copolymer.

[5]
The microporous film according to any one of [1] to [4], wherein the island part has a particle size of 0.01 μm to 10 μm.

[6]
A laminated microporous film comprising the microporous film according to any one of [1] to [5] and a microporous film formed of a thermoplastic resin having a melting point of 100 ° C to 150 ° C.

[7]
A battery separator comprising the microporous film according to any one of [1] to [5] or the laminated microporous film according to [6].

[8]
A method for producing a microporous film according to any one of [1] to [5] or a microporous film constituting the laminated microporous film according to [6], wherein: (A) The manufacturing method of the microporous film containing each process of (B).
(A) Cold stretching in which a film made of a thermoplastic resin composition containing (b) 5 to 90 parts by mass of a polyamide resin is stretched at a temperature of −20 ° C. or more and less than 100 ° C. with respect to 100 parts by mass of the polypropylene resin Process.
(B) A hot stretching step of stretching the cold-stretched film at a temperature of 100 ° C. or higher and 170 ° C. or lower after the cold stretching step.

  According to the present invention, it is possible to obtain a microporous film suitable as a battery separator that is difficult to break even at high temperatures, that is, has a high film breaking temperature and a good balance between porosity and air permeability.

The scanning electron micrograph which measured the microporous film obtained in Example 1 by 1000-times multiplication factor is shown.

  DESCRIPTION OF EMBODIMENTS Hereinafter, modes for carrying out the present invention (hereinafter referred to as the present embodiment) will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and is within the scope of the gist of the present invention. Various modifications can be made.

[Microporous film]
The microporous film of this embodiment is
(A) Polypropylene resin: For 100 parts by mass,
(B) Polyamide resin: It consists of a thermoplastic resin composition containing 5 to 90 parts by mass.
It has a sea-island structure in which the polypropylene resin is a sea part and the polyamide resin is an island part.
The air permeability is 10 seconds / 100 cc to 5000 seconds / 100 cc.

((A) polypropylene resin)
(A) A polypropylene resin (hereinafter sometimes abbreviated as PP) is a polymer containing polypropylene as a monomer component, and may be a homopolymer or a copolymer.
When it is a copolymer, it may be a random copolymer or a block copolymer.
Moreover, when it is a copolymer, there is no limitation in particular about the copolymerization component copolymerized with a propylene monomer, For example, ethylene, butene, hexene, etc. are mentioned.
When the polypropylene resin is a copolymer, the copolymerization ratio of propylene is preferably 50% by mass or more, more preferably 70% by mass or more, and further preferably 90% by mass or more.

A polypropylene resin can be used 1 type or in mixture of 2 or more types.
Polypropylene resin can be prepared using a predetermined polymerization catalyst, but the polymerization catalyst is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst.
Further, there is no particular limitation on the stereoregularity, and isotactic or syndiotactic can be used.
In addition, the polypropylene resin to be used can be used alone in the present embodiment even if it has any crystallinity or melting point, but has different properties depending on the physical properties and applications of the resulting microporous film. You may use the polypropylene resin composition which mix | blended two types of polypropylene resins in the specific range.

  As a polypropylene resin, the melt flow rate (MFR) is preferably 0.1 to 100 g / 10 min when measured under a load of 2.16 kg at 230 ° C. in accordance with ASTM D1238. More preferably, it is -80 g / 10 min.

In addition to the above-mentioned polypropylene resin, the polypropylene resin is disclosed in JP-A-44-15422, JP-A-52-30545, JP-A-6-313078, and JP-A-2006-83294. Known modified polypropylene resins can also be used.
Further, it may be a mixture of the above-mentioned polypropylene resin and the modified polypropylene resin in an arbitrary ratio.

((B) Polyamide resin)
The polyamide resin is a polymer having an amide bond (—NHCO—) in the main chain.
Specific examples of the polyamide resin include polycaprolactam (nylon 6), polytetramethylene adipamide (nylon 46), polyhexamethylene adipamide (nylon 66), polyhexamethylene sebamide (nylon 610), polyhexa Methylene dodecamide (nylon 612), polyundecamethylene adipamide (nylon 116), polyundecalactam (nylon 11), polydodecalactam (nylon 12), polytrimethylhexamethylene terephthalamide (nylon TMHT), polyhexa Methylene isophthalamide (nylon 6I), polynonanemethylene terephthalamide (9T), polyhexamethylene terephthalamide (6T), polybis (4-aminocyclohexyl) methane dodecamide (nylon PACM12), poly (3-methyl-aminocyclohexyl) methane dodecamide (nylon dimethyl PACM12), polymetaxylylene adipamide (nylon MXD6), polyundecamethylene hexahydroterephthalamide (nylon 11T (H)), and at least of these Examples thereof include polyamide copolymers containing two different polyamide components, and mixtures thereof.
Among the polyamides, polyhexamethylene adipamide (nylon 66) is preferable from the viewpoint of heat resistance.

((C) Admixture)
As shown in the scanning electron micrograph of FIG. 1, the microporous film of the present embodiment has a sea-island structure composed of a sea part of polypropylene resin and an island part of polyamide resin.
The particle diameter of the island part is preferably in the range of 0.01 μm to 10 μm.
The particle size of the island portion is defined as the major axis diameter when the maximum length of each particle is measured as the major axis diameter and the minimum length is measured as the minor axis diameter.
A method for measuring the particle size of the island will be described later.

In order to control the size (particle size) of the island part in the sea-island structure, the microporous film of this embodiment includes (c) an admixture in addition to (a) polypropylene resin and (b) polyamide resin. It is preferable to manufacture using a thermoplastic resin composition.
(C) The admixture refers to (a) a polypropylene resin matrix that acts as a dispersant for dispersing (b) the polyamide resin in the matrix of the polypropylene resin, and (c) by using the admixture, When molding into a microporous film, good porosity and good air permeability are obtained.

(C) As an admixture, an epoxy group-containing ethylene copolymer is preferable from the viewpoint of dispersibility of the polyamide-based resin.
The epoxy group-containing ethylene copolymer is usually 50 to 99% by mass of ethylene units, 0.1 to 30% by mass of unsaturated carboxylic acid glycidyl ester units or unsaturated glycidyl ether units, and ethylenically unsaturated ester compound unit 0. It is a copolymer consisting of ˜50 mass%.

  The unsaturated carboxylic acid glycidyl ester unit is a structural unit generally represented by the following chemical formula (1).

In the above formula (1), R ¹ represents a hydrocarbon group having 2 to 18 carbon atoms having an ethylenically unsaturated bond.

  Specific examples of the compound having the structural unit of the above formula (1) include glycidyl acrylate, glycidyl methacrylate, itaconic acid glycidyl ester, and the like.

  Moreover, the said unsaturated glycidyl ether unit is a structural unit normally represented by following Chemical formula (2).

In the above formula (2), R ² represents a hydrocarbon group having 2 to 18 carbon atoms having an ethylenic unsaturated bond, X represents a methylene group or a phenoxy group.

  Specific examples of the compound having the structural unit of the above formula (2) include allyl glycidyl ether, 2-methylallyl glycidyl ether, and styrene-p-glycidyl ether.

Furthermore, examples of the ethylenically unsaturated ester compound unit include, for example, saturated carboxylic acid vinyl esters such as vinyl acetate and vinyl propionate, methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, An α, β-unsaturated carboxylic acid alkyl ester such as butyl acid may be contained.
In particular, vinyl acetate, methyl acrylate, and ethyl acrylate are preferable from the viewpoints of productivity and mechanical properties of the microporous film.
The ethylenically unsaturated ester compound unit is a constituent component that can be appropriately used in the epoxy group-containing ethylene copolymer, and is not an essential component.

  (C) As a manufacturing method of the epoxy group containing ethylene copolymer which is a component, it is a well-known method, for example, the monomer attached | subjected to copolymerization is 500-4000 atmospheres and 100-300 degreeC in presence of a radical generator. , A method of copolymerizing in the presence or absence of a suitable solvent or chain transfer agent, or a method of mixing an unsaturated epoxy compound and a radical generator in polyethylene and melt graft copolymerizing in an extruder. be able to.

The proportion of the (c) admixture in the thermoplastic resin composition is preferably 1 to 20% by mass, more preferably 1 to 15% by mass.
Setting such a ratio is suitable from the viewpoint of (b) the dispersibility of the polyamide resin and the porosity and air permeability of the microporous film of the present embodiment resulting from this dispersion.

((D) additional component)
In the thermoplastic resin composition constituting the microporous film of the present embodiment, in addition to the above-described components, other additional components such as olefins may be added as necessary without departing from the characteristics and effects of the invention. Elastomers, antioxidants, metal deactivators, heat stabilizers, flame retardants (organic phosphate ester compounds, ammonium polyphosphate compounds, aromatic halogen flame retardants, silicone flame retardants, etc.), fluorine polymers , Plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weathering (light) improvers, polyolefin nucleating agents, slip agents, inorganic Or organic fillers and reinforcements (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive car Down black), various colorants, mold release agents, etc., may be added.

[Method for producing microporous film]
The manufacturing method of the microporous film of this embodiment shall include the following steps (A) and (B).
According to this method, a microporous film having good permeability and a sea-island structure can be obtained.
(A) (a) A cold stretch of a film made of a thermoplastic resin composition containing 5 to 90 parts by mass of a polyamide resin with respect to 100 parts by mass of a polypropylene resin at a temperature of −20 ° C. or more and less than 100 ° C. Stretching process.
(B) A hot stretching step of stretching the thermoplastic resin composition at a temperature of 100 ° C. or higher and lower than 170 ° C. after the cold stretching step.
The manufacturing method of the microporous film of this embodiment includes a film forming step and a stretching step, and it is preferable to use the steps (A) and (B) as the stretching step.

A film made of the above-described thermoplastic resin composition to be the microporous film of the present embodiment can be produced by a sheet molding method such as T-die extrusion molding, inflation molding, calendar molding, Skyf method or the like. In particular, T-die extrusion molding is preferable from the viewpoints of physical properties and applications required for the microporous film of the present embodiment.
On the other hand, in the stretching step, a method of stretching in a uniaxial direction and / or a biaxial direction by a roll, a tenter, an autograph or the like can be adopted. In particular, from the viewpoint of physical properties and applications required for the microporous film obtained in the present embodiment, two or more stages of uniaxial stretching with a roll are preferable. Moreover, when carrying out uniaxial stretching in the (A) process and the (B) process, although there is no limitation in the extending direction, it is preferable that it is the same direction, The direction is an extrusion direction (henceforth MD direction). Preferably there is.

Although the manufacturing method of the microporous film of this embodiment is demonstrated concretely below, this embodiment is not limited to the following manufacturing method.
(Process for producing a film comprising a thermoplastic resin composition)
(A) Polypropylene resin, (b) Polyamide resin, and, if necessary, (c) Admixture in a specific range is supplied to the extruder, and is 200 ° C to 300 ° C, preferably 260 ° C to 280 ° C. By melt-kneading at a temperature, pellets of a thermoplastic resin composition in which a polyamide resin is dispersed in a polypropylene resin are obtained.
Next, the pellets of the thermoplastic resin composition are supplied to an extruder and extruded from a T-shaped die at a temperature of 200 ° C. to 280 ° C., preferably 220 ° C. to 250 ° C., and this film is 20 to 150 ° C. Preferably, it is cast on a roll at 90 ° C. to 130 ° C. and solidified by cooling.

The draw ratio after extrusion into a film as described above, that is, the film winding speed (unit: m / min) is the extrusion speed of the thermoplastic resin composition (the line in the flow direction of the molten resin passing through the die lip). The value divided by the speed (unit: m / min) is preferably 10 to 500, more preferably 50 to 300, and still more preferably 100 to 250.
The film winding speed is preferably about 2 to 400 m / min, more preferably 10 to 200 m / min. Setting the draw ratio in the above range is preferable from the viewpoint of improving the air permeability of the target microporous film.

  The film forming is performed by supplying a specific range of (a) polypropylene resin, (b) polyamide resin, and, if necessary, (c) admixture to the extruder, and 200 ° C to 300 ° C, preferably By melting and kneading at a temperature of 260 ° C. to 280 ° C., the polyamide-based resin is dispersed in the polypropylene resin, and the kneaded product is directly extruded into a film form from a T-die without being once formed into pellets. It may be carried out by casting on a roll of 20 to 150 ° C., preferably 90 to 120 ° C. and cooling and solidifying.

(Heat treatment process)
The thermoplastic resin composition film produced as described above is preferably subjected to heat treatment (annealing) as necessary.
As a method of annealing, for example, a method of contacting a film on a heating roll, a method of exposing to a heated gas phase before winding, a method of winding a film on a core body and exposing to a heated gas phase or heated liquid phase, In addition, a method of combining these may be mentioned.
As annealing conditions, for example, it is preferable to anneal at a heating temperature of 100 ° C. to 150 ° C. for 10 seconds to 100 hours.
If the heating temperature is 100 ° C. or higher, the air permeability of the target microporous film is further improved. If the heating temperature is 150 ° C. or lower, the films may be bonded to each other even if the film is annealed on the core. It becomes difficult to fuse. A more preferable heating temperature range is 130 ° C to 150 ° C.

(Cold drawing process)
Next, the cold drawing process will be described.
After the heat treatment as described above, it is preferably cold-drawn at least 1.05 times to 2.0 times in at least one direction while being kept at -20 ° C or higher and lower than 100 ° C.

The stretching temperature in the cold stretching step is preferably -20 ° C or higher and lower than 100 ° C, more preferably 0 ° C or higher and 50 ° C or lower.
By stretching at −20 ° C. or higher, the microporous film is difficult to break, and when stretched at less than 100 ° C., the air permeability of the resulting microporous film becomes better.
Here, the stretching temperature is the surface temperature of the film in the cold stretching step.

The draw ratio of cold drawing in the cold drawing step is preferably 1.05 times or more and 2.0 times or less, more preferably 1.2 times or more and 1.7 times or less.
When the draw ratio is 1.05 times or more, a microporous film having good air permeability is obtained, and when it is 2.0 times or less, a microporous film having a uniform film thickness is obtained.
The cold stretching of the microporous film may be performed in at least one direction and may be performed in two directions, but preferably, the uniaxial stretching is performed only in the film extrusion direction (hereinafter referred to as “MD direction”).
In the present embodiment, in the cold stretching step, the microporous film is preferably uniaxially stretched 1.1 to 2.0 times in the MD direction at a temperature of 0 ° C. or more and 50 ° C. or less.

(Heat drawing process)
Next, the heat stretching process will be described.
After cold-drawing as described above, the film is hot-stretched at least 1.05 times to 5.0 times in at least one direction with the film held at 100 ° C. or more and 170 ° C. or less.

The stretching temperature of the heat stretching is preferably 100 ° C. or higher and 170 ° C. or lower, more preferably 120 ° C. or higher and 150 ° C. or lower.
By heat stretching at 100 ° C. or higher, the film is less likely to break, and if it is heat stretched at 170 ° C. or lower, the air permeability of the intended microporous film is improved.
Here, the stretching temperature of the heat stretching is the surface temperature of the film in the heat stretching process.

The draw ratio of the hot drawing in the hot drawing step is preferably 1.05 to 5.0, more preferably 1.1 to 5.0, and still more preferably 2.0 to 4 0.0 or less.
When the draw ratio in the heat stretching step is 1.05 times or more, a microporous film having good air permeability is obtained, and when it is 5.0 times or less, a microporous film having a uniform film thickness is obtained. .
The hot stretching may be performed in at least one direction and may be performed in two directions, but is preferably performed in the same direction as the cold stretching direction, more preferably uniaxial stretching only in the same direction as the cold stretching direction. Do.
In the present embodiment, in the hot stretching process, the film that has been cold-drawn through the cold-drawing process is uniaxially stretched 2.0 to 5.0 times in the MD direction at a temperature of 100 ° C. or higher and 150 ° C. or lower. Is preferred.

  In addition, in addition to each above-mentioned extending process, the manufacturing method of the microporous film of this embodiment may further perform a predetermined extending process.

(Heat setting process)
In the manufacturing method of the microporous film of this embodiment, it is preferable to include the heat setting process which heat-fixes with respect to the film obtained through the heat | fever extending process preferably at 120 degreeC or more and 170 degrees C or less.
As this heat setting method, a method of heat shrinking to such an extent that the length of the film after heat setting is reduced by 3 to 50% with respect to the length of the microporous film before heat setting (hereinafter, this method is referred to as “ A method of heat setting so that the dimension in the stretching direction does not change.

The heat setting temperature is preferably 120 ° C. or higher and 170 ° C. or lower, and more preferably 130 ° C. or higher and 160 ° C. or lower.
Here, the heat setting temperature is the surface temperature of the microporous film in the heat setting process.

  As described above, the production process of the film made of the thermoplastic resin composition, the heat treatment step if necessary, then the cold drawing step, the hot drawing step, and further through the heat setting step if necessary, the purpose A microporous film is obtained.

[Structure of microporous film]
The microporous film of the present embodiment obtained by the production method described above has a sea-island structure in which the polypropylene resin is a sea part and the polyamide resin is an island part.
(Sea-island structure)
The “sea-island structure” is a structure in which a polypropylene resin skeleton that is a sea part is formed between island components made of polyamide resin particles, as shown in the electron micrograph of FIG.
That is, a structure in which a polyamide resin is dispersed in a plurality of islands in a matrix (matrix) made of polypropylene resin.
The sea-island structure (dispersed state) as described above can be easily measured and observed using an electron microscope or the like.
As an example of electron microscope observation, after placing a microporous film to be measured on a sample stage, an osmium coating of about 3 nm is applied, and an acceleration voltage is set to 1 kV using a scanning electron microscope (HITACHI S-4700). For example, it can be observed at a magnification of 1,000 times.

The particle size of the island is preferably 0.01 μm to 10 μm, more preferably 0.1 μm to 5 μm.
By setting the particle size within the above range, the pore structure (pores) of the microporous film can be uniformly adjusted with respect to the film thickness direction and the film surface direction.

A microporous film having a uniform pore portion is suitable as a battery separator.
In the present embodiment, the particle size can be measured as follows.
First, a scanning electron micrograph (magnification: 5,000 times) is obtained in the same manner as the measurement method for observing the sea-island structure for the microporous film to be measured.
Next, 100 polyamide resin particles dispersed in a polypropylene resin as a matrix are arbitrarily selected, and the maximum length of each particle is measured as the major axis diameter and the minimum length is measured as the minor axis diameter. The particle diameter is defined as the major axis diameter.

The ratio of the major axis diameter / minor axis diameter is preferably 1 to 5, more preferably 1 to 3.
Setting the ratio of the major axis diameter / minor axis diameter to such a range is preferable from the viewpoint of openability.
In addition, by appropriately selecting the polypropylene resin, polyamide resin, and admixture to be used, the polypropylene resin particles having such a particle diameter and major axis / minor axis ratio can be dispersed in the matrix polypropylene resin. Can do.

[Physical properties of microporous film]
(Porosity)
The microporous film of this embodiment preferably has a porosity of 20% to 70%, more preferably 35% to 65%, and still more preferably 45% to 60%.
When the porosity is set to 20% or more, sufficient ion permeability can be secured when used for battery applications.
On the other hand, if it is set to 70% or less, sufficient mechanical strength can be ensured.
Moreover, when the porosity of a microporous film exists in such a range, the heat resistance improvement effect by sea island structure will be more notably exhibited.

5-40 micrometers is preferable and, as for the film thickness of the microporous film of this embodiment, 10-30 micrometers is more preferable.
In addition, the porosity of the microporous film of this embodiment can be adjusted by appropriately setting the composition ratio, the stretching temperature, the stretching ratio, and the like of the thermoplastic resin composition.
In addition, the porosity can be calculated from the volume and mass of a 10 cm square sample using the following formula.
Here, the volume is an apparent volume (including the volume of pores in the film). For example, the volume of a 10 cm square sample is 10 cm × 10 cm × film thickness (cm).
Porosity (%) = (Volume (cm ³ ) −Mass (g) / Density of resin composition constituting film) / Volume (cm ³ ) × 100

(Air permeability)
The microporous film of the present embodiment has an air permeability of 10 seconds / 100 cc to 5000 seconds / 100 cc, preferably 50 seconds / 100 cc to 1000 seconds / 100 cc, more preferably 100 seconds / 100 cc to 500 seconds / 100cc.
By setting the air permeability to 5000 seconds or less, sufficient ion permeability can be secured.
On the other hand, setting it to 10 seconds or more is preferable from the viewpoint of obtaining a homogeneous microporous film having no defects.
Moreover, when the air permeability of the microporous film is in such a range, the effect of improving the heat resistance by the sea-island structure is more remarkably exhibited.
The air permeability in the microporous film of the present embodiment can be adjusted by appropriately setting the composition ratio of the thermoplastic resin composition, the stretching temperature, the stretching ratio, and the like.
The air permeability can be measured using a Gurley type air permeability meter according to JIS P-8117.

[Laminated microporous film]
The microporous film of the present embodiment may be configured to be laminated with another predetermined resin layer.
Examples of the resin layer include a microporous film made of a polyolefin resin such as polyethylene resin and polypropylene resin, and a microporous film made of a saturated polyester resin such as polyethylene terephthalate resin and polybutylene terephthalate resin.
From the viewpoint of physical properties and applications where a microporous film is required, a melting point measured by a method according to JIS K-7121, a microporous film composed of a resin having a temperature of 100 ° C. to 150 ° C. or less, and the present embodiment It is preferable to make a laminated body with a microporous film.
As described above, when the microporous film composed of a resin having a melting point of 100 ° C. to 150 ° C. or less and the microporous film of the present embodiment are used as a laminate, when used as a battery separator, Battery safety is dramatically improved.
Examples of the resin having a melting point of 100 ° C. to 150 ° C. or less include a polyethylene resin, and examples of the polyethylene resin to be used include so-called high density polyethylene, medium density polyethylene, and low density polyethylene.

The resin layer may contain a filler such as calcium carbonate, barium sulfate, alumina, talc, and the like. Furthermore, the structure of a laminated body should just be two or more layers, and both the microporous film and resin layer of this embodiment can exist as a laminated body surface layer.
As a manufacturing method of a laminated body, for example, a co-extrusion method using a T die or a circular die, a laminating method in which each layer is separately extruded and pasted, a laminating method in which separately porous films are pasted, etc. Can be mentioned.

[Use of microporous film and laminated microporous film]
The microporous film and laminated microporous film of this embodiment can be used as a separator for a lithium ion battery.
The microporous film of the present embodiment is a microporous film having excellent porosity and air permeability, and is a microporous film having heat resistance.
That is, for example, although it is a thermoplastic resin composition film using a polypropylene resin as a base material (matrix), the film form can be maintained even at a temperature exceeding 200 ° C. of the polypropylene resin melting point. This improvement in heat resistance is surprising, which could never have been predicted from the prior art.
Regarding the improvement of heat resistance, when the microporous film of the present embodiment is applied to a battery separator, 200 ° C. or higher can be realized at a film breaking temperature shown in a battery oven test or the like that has been carried out in recent years.
The microporous film having a membrane breaking temperature of 200 ° C. or higher obtained in the present embodiment can dramatically improve the heat resistance as a battery separator.
In the microporous film of the present embodiment, the mechanism that achieves such a high film breaking temperature is that the morphology with polypropylene resin as the sea part and polyamide resin as the island part improves the heat resistance (film breaking temperature) of the film. It is thought that it contributed.

Next, although an example and a comparative example are given and explained more concretely, the present invention is not limited to the following examples.
In addition, about the used raw material and the evaluation method of various characteristics, it is as follows.

(1) Dispersion particle diameter of polyamide resin (μm)
The particle size of the polyamide resin dispersed in the microporous film was measured, and the measured particle size range (maximum particle size to minimum particle size) was shown.
The fact that the dispersed particle diameter of the polyamide resin can be measured means that a sea-island structure is formed.
The particle size was measured as follows.
First, after loading a microporous film to be measured on a sample stage, an osmium coating of about 3 nm is applied, and a scanning electron microscope (HITACHI S-4700) is used with an acceleration voltage of 1 kV and a magnification of 5,000 times. Observed.
Next, 100 polyamide resin particles dispersed in a polypropylene resin as a matrix were arbitrarily selected, and the maximum length of each particle was measured as the major axis diameter, and the minimum length was measured as the minor axis diameter.
The particle size was defined as the major axis diameter.

(2) Film thickness (μm)
It measured with the dial gauge (Ozaki Seisakusho: "PEACOCK No.25" (trademark)).

(3) Porosity (%)
A 10 cm square sample was taken and calculated from the volume and mass according to the following formula.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of thermoplastic resin composition constituting film) / volume (cm ³ ) × 100

(4) Air permeability (sec / 100cc)
It measured using the Gurley type air permeability meter based on JIS P-8117.

(5) Film breakage temperature A microporous film was attached to a 40 mm square holder in a constrained state around the circumference, and left in a hot air circulation oven set at 170 ° C. for 30 minutes.
The sample oven was looked into and it was judged by visual observation whether the film was torn.
If the film was not torn, the temperature of the oven was raised by 10 ° C. and the same test was performed. The temperature at which the film was broken was defined as the film breaking temperature.

The components (a) to (c) used in the following examples and comparative examples are shown.
((A) component polypropylene resin)
(A-1) Propylene homopolymer, MFR = 0.4
((B) component polyamide resin)
(B-1) Leona 1300S manufactured by Asahi Kasei Chemicals Corporation
(B-2) Leona 1500 manufactured by Asahi Kasei Chemicals Corporation
(Epoxy group-containing ethylene copolymer of component (c))
(C-1) Sumitomo Chemical Co., Ltd. Bond Fast E

[Example 1]
(A-1) 90 parts by mass of component, (b-1) 10 parts by mass of component, and (c-1) 3 parts by mass of admixture at a temperature of 240 to 280 ° C. and a screw rotation speed of 300 rpm. Used and melt-kneaded to obtain a thermoplastic resin composition as pellets.
The thermoplastic resin composition pellets obtained as described above were introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, and 240 ° C. via a feeder, and the lip thickness installed at the tip of the extruder. After extruding from a 3 mm T-die, the molten resin was immediately applied with 25 ° C. cold air and drawn on a cast roll cooled to 120 ° C. at a draw ratio of 150 to form a precursor film.
This precursor film was heat-treated at 130 ° C. for 1 hour, and uniaxially stretched 1.2 times at the temperature of 25 ° C. (MD direction, the same applies hereinafter), and then the stretched film was further 2.0 times at the temperature of 140 ° C. The film was uniaxially stretched (MD direction, the same applies hereinafter) and heat-fixed at 150 ° C. to obtain a microporous film.
As described above, the film thickness, porosity, air permeability, and membrane breaking temperature were measured for the obtained microporous film.
The measurement results are shown in Table 1 below.

[Example 2]
(A-1) A twin screw extruder in which 80 parts by mass of component, 20 parts by mass of (b-1), and 5 parts by mass of (c-1) admixture were set at a temperature of 240 to 280 ° C. and a screw rotation speed of 300 rpm. Used and melt-kneaded to obtain a thermoplastic resin composition as pellets.
The thermoplastic resin composition pellets obtained as described above were introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, and 240 ° C. via a feeder, and the lip thickness installed at the tip of the extruder. After extruding from a 3 mm T-die, the molten resin was immediately applied with cold air of 25 ° C. and drawn on a cast roll cooled to 120 ° C. at a draw ratio of 150 to form a precursor film.
A microporous film was produced in the same manner as in Example 1 except that the precursor film obtained as described above was used, and measurement was carried out in the same manner as in Example 1.
The measurement results are shown in Table 1 below.

Example 3
The above (b-2) was used instead of the above (b-1). For other conditions, a microporous film was prepared in the same manner as in Example 1, and the measurement was performed in the same manner as in Example 1.
The measurement results are shown in Table 1 below.

Example 4
As a polyethylene resin, pellets of the product name “Suntech S160S” manufactured by Asahi Kasei Chemicals Co., Ltd. are introduced through a feeder into a single screw extruder set at 20 mm in diameter, L / D = 30, and 180 ° C. After extruding from a T-die having a lip thickness of 3.0 mm, the molten resin was immediately drawn into a cast roll that had been cooled to 95 ° C. by applying cold air at 25 ° C. to draw a precursor film.
This precursor film was heat-treated at 130 ° C. for 1 hour and uniaxially stretched 1.5 times at a temperature of 25 ° C., and then the stretched film was further uniaxially stretched 2.0 times at a temperature of 120 ° C. Heat setting was performed at 130 ° C. to obtain a polyethylene microporous film A.
The polyethylene microporous film A obtained as described above (melting point 135 ° C., film thickness 8 μm, porosity 50%, air permeability 100 seconds / 100 cc) and the same method as in Example 1 described above, A microporous film B (thickness 9 μm, porosity 45%, air permeability 150 seconds / 100 cc) having a draw ratio changed to 280 and a film thickness changed to 9 μm becomes B / A / B A three-layer microporous film was prepared by laminating in a layer configuration using a laminator.
The obtained three-layer microporous film was measured by the same method as in Example 1.
The measurement results are shown in Table 1 below.

[Comparative Example 1]
The component (a-1) was introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, and 240 ° C. through a feeder, and extruded from a T die having a lip thickness of 3.0 mm installed at the tip of the extruder. After that, a precursor film was formed by immediately drawing 25 ° C. cold air on the melted resin and drawing it on a cast roll cooled to 95 ° C. at a draw ratio of 150.
A microporous film was produced in the same manner as in Example 1 except that the precursor film obtained as described above was used, and measurement was carried out in the same manner as in Example 1.
The measurement results are shown in Table 1 below.

As shown in Table 1, each of the microporous films of Examples 1 to 4 having a sea-island structure has a higher membrane breaking temperature than the microporous film of Comparative Example 1 that does not have a sea-island structure. And had excellent heat resistance.
These have a film-breaking temperature of 200 ° C. or higher, and the heat resistance has been drastically improved, so it has been found that when used as a battery separator, the safety against battery short-circuit (short) is greatly improved. .

  The microporous film of the present invention has industrial applicability as a battery separator and other various separation membranes.

Claims (8)

(A) For 100 parts by mass of polypropylene resin,
(B) a thermoplastic resin composition containing 5 to 90 parts by mass of a polyamide resin,
It has a sea-island structure in which the polypropylene resin is a sea part and the polyamide resin is an island part,
A microporous film having an air permeability of 10 seconds / 100 cc to 5000 seconds / 100 cc.   The microporous film according to claim 1, wherein the porosity is 20% to 70%.   The microporous film according to claim 1 or 2, wherein the thermoplastic resin composition further contains (c) an admixture in a proportion of 1 to 20% by mass in the thermoplastic resin composition.   The microporous film according to claim 3, wherein the admixture (c) is an epoxy group-containing ethylene copolymer.   The microporous film according to any one of claims 1 to 4, wherein the island portion has a particle size of 0.01 µm to 10 µm. A microporous film according to any one of claims 1 to 5,
A microporous film formed of a thermoplastic resin having a melting point of 100 ° C. to 150 ° C .;
A laminated microporous film.   A battery separator comprising the microporous film according to any one of claims 1 to 5 or the laminated microporous film according to claim 6. A microporous film according to any one of claims 1 to 5, or a method for producing a microporous film constituting the laminated microporous film according to claim 6,
The manufacturing method of the microporous film containing each process of the following (A) and (B).
(A) Cold stretching in which a film made of a thermoplastic resin composition containing (b) 5 to 90 parts by mass of a polyamide resin is stretched at a temperature of −20 ° C. or more and less than 100 ° C. with respect to 100 parts by mass of the (a) polypropylene resin. Process.
(B) A hot stretching step of stretching the cold-stretched film at a temperature of 100 ° C. or higher and 170 ° C. or lower after the cold stretching step.