JP2016060061A - Laminated microporous film and method for producing the same and cell separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated microporous film having excellent thermal dimensional stability and electric resistance in addition to heat resistance, shutdown function or the like and a method for producing the same, and to provide a cell separator.SOLUTION: There is provided a laminated microporous film which comprises a microporous film including a polyolefin, a porous film including inorganic particles and a resin binder which is laminated on one surface or both surfaces of the microporous film, where the microporous film has a thickness of 1.0 to 4.0 μm and a basis weight of 1.0 to 5.0 g/m.SELECTED DRAWING: None

Description

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

  Nonaqueous electrolyte batteries, particularly nonaqueous secondary batteries typified by lithium ion secondary batteries, have a high energy density and are widely used as main power sources for portable electronic devices such as mobile phones and laptop computers. This lithium ion secondary battery is required to have a higher energy density, but ensuring safety is a technical issue. The role of the separator is important in ensuring the safety of the lithium ion secondary battery, and from the viewpoint of having a shutdown function, polyolefins, particularly polyethylene microporous membranes are currently used. Here, the shutdown function refers to a function of blocking pores in the microporous membrane when the temperature of the battery rises, and blocking the current, and has a function of preventing thermal runaway of the battery.

  On the other hand, lithium ion secondary batteries have been increased in energy density year by year, and heat resistance has been required in addition to a shutdown function to ensure safety. However, the shutdown function is contrary to heat resistance because the operating principle is to close the hole by melting polyethylene. For this reason, after the shutdown function is activated, the battery may continue to be exposed to a temperature higher than the temperature at which the shutdown function is activated, whereby the separator may be melted (so-called meltdown). As a result of this meltdown, a short circuit occurs inside the battery, and as a result, a large amount of heat is generated, and the battery is exposed to dangers such as smoke, ignition, and explosion. For this reason, in addition to the shutdown function, the separator is required to have sufficient heat resistance that does not cause meltdown in the vicinity of the temperature at which the shutdown function operates.

  In this regard, conventionally, in order to achieve both heat resistance and a shutdown function, one surface or both surfaces (front and back surfaces) of the polyolefin microporous film are coated with a heat resistant porous layer, or a nonwoven fabric made of heat resistant fibers is laminated. The technology is proposed. For example, a nonaqueous electrolyte battery separator is known in which a heat-resistant porous layer made of a heat-resistant polymer such as aromatic aramid is laminated on one side or both sides of a polyethylene microporous membrane by a wet coating method (Patent Documents 1 to 3). 4). Such a non-aqueous electrolyte battery separator operates with a shutdown function near the melting point of polyethylene (about 140 ° C), and the heat-resistant porous layer exhibits sufficient heat resistance, so that meltdown occurs even at 200 ° C or higher. Therefore, it exhibits excellent heat resistance and shutdown function.

  On the other hand, for the purpose of improving the short-circuit prevention effect and heat resistance, it has also been attempted to add an inorganic filler to the heat-resistant porous layer of the separator. It has also been reported that the effect of preventing short circuit and thermal shrinkage due to the resulting separator punch-through is improved (Patent Documents 5 to 9). Patent Documents 10 and 11 below propose battery separators in which a composition composed of a plate-like filler and an organic binder is applied to a sheet-like material.

JP 2002-355938 A JP 2005-209570 A JP 2005-285385 A Japanese Patent No. 3175730 JP 2007-157723 A JP 2007-31151 A JP 2008-4442 A JP 2008-66094 A JP 2008-210784A JP 2008-123988 A JP 2008-123996 A

  However, in Patent Documents 5 to 9, a woven fabric and a non-woven fabric are used as the separator base material, and the problem of non-uniformity derived from the base material has not been solved, and further short circuit suppression and uniform ion permeability are not solved. Improvement is also necessary.

  In Patent Documents 10 and 11 below, the porosity of the heat-resistant porous layer is insufficient, and further improvement in ion permeability is desired.

  Furthermore, in order to improve the discharge performance at a large current and the discharge performance at a low temperature of the lithium ion battery, it is necessary to reduce the resistance of ions in a state where the separator holds the electrolyte solution as much as possible. A separator having a low electric resistance in the state of being present is desired.

  As described above, a practical separator that satisfies all the functions of heat resistance, shutdown function, thermal dimensional stability, ion permeability, and electrical resistance has not been obtained.

  The present invention has been made in view of the above problems, and in addition to heat resistance, shutdown function and the like, a laminated microporous film excellent in thermal dimensional stability and electrical resistance, a method for producing the same, and a battery separator The purpose is to provide.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by a laminated microporous film having a predetermined configuration, and have completed the present invention.

That is, the present invention is as follows.
[1]
A microporous membrane containing polyolefin;
A porous layer containing inorganic particles and a resin binder, laminated on one or both sides of the microporous membrane,
The porous layer has a thickness of 1.0 to 4.0 μm,
Basis weight of the porous layer is 1.0 to 5.0 g / m ^2, laminated microporous film.
[2]
The laminated microporous film according to [1], wherein the inorganic particles include an aluminum silicate compound.
[3]
The laminated microporous film according to [2] above, wherein the aluminum silicate compound contains calcined kaolin.
[4]
The microporous membrane is
A first microporous film composed of a first resin composition having a melting point T _mA ;
Wherein a laminated film and a second microporous film composed of a second resin composition having a low melting point T _mB than the melting point T _mA, one of the items [1] to [3] 1 The laminated microporous film according to Item.
[5]
The laminated microporous film according to [4] above, wherein the second resin composition is polyethylene.
[6]
The laminated microporous film according to any one of [1] to [5], wherein the film thickness is 16 μm or less.
[7]
The laminated microporous film according to any one of [1] to [6], wherein the air permeability is 10 to 5000 seconds / 100 cc.
[8]
A first resin film composed of a first resin composition having a melting point T _mA, and a second resin film composed of a second resin composition having a melting point T _mB lower than the melting point T _mA A film forming step of forming a laminated film containing polyolefin,
An opening step of opening the film by a dry method to form a microporous film;
A porous layer forming step of forming a porous layer containing inorganic particles and a resin binder on the surface of the microporous membrane to form the laminated microporous film according to any one of [1] to [7] above When,
The manufacturing method of the lamination | stacking microporous film which has these in this order.
[9]
In the film forming step, it consists second resin composition having a first resin film made from a first resin composition having a melting point T _mA, a low melting point T _mB than the melting point T _mA The method for producing a laminated microporous film according to [8] above, wherein a laminated film containing polyolefin and laminated with a second resin film is formed by a coextrusion method.
[10]
A battery separator comprising the laminated microporous film according to any one of [1] to [7].

  According to the present invention, in addition to heat resistance, a shutdown function, etc., a laminated microporous film excellent in thermal dimensional stability and electrical resistance is provided, and the obtained separator is a non-ionic material such as a lithium ion secondary battery. It is effective to improve various performances and safety of water-based secondary batteries.

It is a schematic sectional drawing which shows the cell for an electrical resistance measurement of a lamination | stacking microporous film.

  Hereinafter, a form for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail with reference to the drawings as necessary. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the positional relationship such as up, down, left and right is based on the positional relationship shown in the drawings unless otherwise specified. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

  Unless otherwise specified in this specification, “including as a main component” and “mainly including” are preferable as a ratio in which a specific component is contained in a composition (matrix component) including the specific component. Means 50% by mass or more, more preferably 80% by mass or more, and may contain 100% by mass.

[Laminated microporous film]
The laminated microporous film of the present embodiment includes a microporous film containing polyolefin, and a porous layer containing inorganic particles and a resin binder laminated on one or both sides of the microporous film, The thickness is 1.0 to 4.0 μm, and the basis weight of the porous layer is 1.0 to 5.0 g / m ² .

  The laminated microporous film of the present embodiment has excellent thermal dimensional stability and electrical resistance in addition to heat resistance and a shutdown function by having the above-described configuration. Surprisingly, since the weight per unit area of the porous layer can at least exhibit the above performance, a laminated microporous film with good cost performance is obtained.

  The film thickness of the laminated microporous film is preferably 16 μm or less, more preferably 15 μm or less, and even more preferably 12 μm or less. When the film thickness is 16 μm or less, it tends to be more effective in reducing the size of the battery. Further, when the film thickness is 5 μm or more, the mechanical strength tends to be excellent, and the film thickness of the laminated microporous film is within the above range by appropriately setting the thickness of each resin film, the draw ratio, etc. Can be adjusted. Moreover, the film thickness of a lamination | stacking microporous film can be measured by the method as described in an Example.

  The air permeability of the laminated microporous film is preferably 10 to 5000 seconds / 100 cc, more preferably 50 to 1000 seconds / 100 cc, and further preferably 100 to 500 seconds / 100 cc. When the air permeability is 10 seconds / 100 cc or more, the film tends to be more uniform without defects. Further, when the air permeability is 5000 seconds / 100 cc or less, the ion permeability tends to be further improved. In addition, the air permeability of the laminated microporous film of this embodiment can be adjusted to the above-mentioned range by appropriately setting the composition of the resin composition constituting each layer, the stretching temperature, the stretching ratio, and the like. The air permeability is measured using a Gurley type air permeability meter according to JIS P-8117.

  The porosity of the laminated microporous film is preferably 50% to 70%, more preferably 53% to 65%, and further preferably 56% to 60%. When the porosity is 50% or more, the ion permeability tends to be further improved. On the other hand, when the porosity is 70% or less, the mechanical strength tends to be further improved. The porosity of the laminated microporous film can be adjusted by appropriately setting the composition of the resin composition constituting each layer, the stretching temperature, the stretching ratio, and the like. For example, the porosity can be increased by increasing the density of the resin composition, increasing the thermal stretching temperature, or increasing the stretching ratio.

The porosity of the laminated microporous film is calculated by cutting out a 10 cm × 10 cm square sample from the film and using the following formula from the volume and mass of the sample.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) × 100

[Microporous membrane]
The microporous membrane contains polyolefin. Although it does not specifically limit as polyolefin, For example, the polymer of polyolefins, such as polyethylene (PE), polypropylene (PP), polybutene-1, poly-4-methylpentene, an ethylene propylene copolymer, and ethylene -Copolymers of olefin hydrocarbons such as tetrafluoroethylene copolymer and other monomers. Polyolefin may be used individually by 1 type, or may use 2 or more types together.

  The polyolefin is a polymer containing olefin 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 a copolymerization component, For example, olefins, such as ethylene, propylene, butene, and hexene, or the other monomer copolymerizable with these is mentioned. When the polyolefin is a copolymer, the copolymerization ratio of the olefin is preferably 50% by mass or more, more preferably 70% by mass or more, and further preferably 90% by mass or more.

  The stereoregularity of the polyolefin is not particularly limited, and examples thereof include isotactic or syndiotactic.

  The polyolefin may have any crystallinity or melting point. Further, two or more polyolefins having different crystallinity and melting point may be blended at a specific blending ratio depending on the physical properties and applications of the obtained microporous film.

  The melt flow rate (MFR) of polyolefin (based on ASTM D1238, measured under a load of 230 ° C. and 2.16 kg. The same applies hereinafter) is preferably 0.1 to 100 g / 10 from the viewpoint of the moldability of the film. Min, more preferably 0.1 to 80 g / 10 min.

  The polyolefin is a known modified polypropylene as described in JP-A-44-15422, JP-A-52-30545, JP-A-6-313078, and JP-A-2006-83294. Also good. Further, the polyolefin may be a mixture of the above-mentioned polypropylene and the modified polypropylene in an arbitrary ratio.

  The polymerization catalyst used for obtaining polypropylene is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst.

  In addition to polyolefin, the microporous membrane (first resin composition and second resin composition to be described later) may contain additional components such as antioxidants, metal deactivators, Heat stabilizer, flame retardant (organophosphate ester compound, ammonium polyphosphate compound, aromatic halogen flame retardant, silicone flame retardant, etc.), fluorine polymer, plasticizer (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 reinforcing materials (polyacrylonitrile fiber, carbon Black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various colorants, release agents and the like.

The microporous film may be a single film or a laminated film in which two or more kinds of films are laminated. When the microporous film is a laminated film, microporous film, the first and the microporous film first formed from the resin composition, low melting point T _mB than the melting point T _mA with a melting point T _mA And a second microporous film composed of a second resin composition having the following. By using such a microporous membrane, when the internal temperature of the battery rises due to an abnormal current, the high melting point resin layer tends to be held without melting even if the low melting point resin layer melts. . As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be further improved.

The melting point T _mA of the first resin composition and the melting point T _mB of the second resin composition can be measured by a method based on JIS K-7121. The materials of the first resin composition and the second resin composition may be the same or different.

The difference between the melting point T _mA and the melting point T _mB is preferably 5.0 ° C. or more, more preferably 10 ° C. or more, and further preferably 15 ° C. or more. When the difference in melting point is 5.0 ° C. or more, when the internal temperature of the battery rises due to abnormal current, the first microporous film will melt even if the second microporous film melts. Easy to hold. As a result, the film shape or sheet shape of the battery separator is maintained, and the safety tends to be improved. The upper limit of the difference between the melting point T _mA and the melting point T _mB is not particularly limited, but is preferably 150 ° C. or lower.

  When the microporous film is a laminated film, the mode is not particularly limited. For example, a laminated body composed of one first microporous film and one second microporous film, A laminate comprising one microporous film and a second microporous film laminated on both sides thereof, one second microporous film and a first microporous film laminated on both sides thereof Each of the resin films was alternately arranged such that a laminate comprising: a first microporous film-a second microporous film-a first microporous film-a second microporous film A laminated body is mentioned. Among these, from the viewpoint of more effectively and reliably exhibiting the effects of the present invention, a laminate comprising the one second microporous film and the first microporous film laminated on both sides thereof. Embodiments are preferred.

(First microporous film)
The first microporous film is composed of a polyolefin first resin composition having a melting point _TmA . The first microporous film can be obtained by stretching the first resin composition to make it porous.

The first resin composition has a higher melting point than the second resin composition. Its melting point T _mA is preferably 150 ° C. to 280 ° C. When the melting point T _mA is within the above range, the balance between the film breaking temperature and the film formability tends to be good. In order to obtain such a first resin composition, a resin having a melting point of 150 ° C. to 280 ° C. may be included in the resin composition.

(First resin composition)
The resin that can be included in the first resin composition is not particularly limited, and examples thereof include polyethylene (PE), polypropylene (PP), polybutene-1, poly-4-methylpentene, and an ethylene-propylene copolymer. And a polymer containing an olefin hydrocarbon such as an ethylene-tetrafluoroethylene copolymer as a monomer component. Among these, polypropylene is preferable.

(Second microporous film)
The second resin film is composed of a second resin composition having a low melting point T _mB than the melting point T _mA. The second microporous film can be obtained by stretching the second resin composition to make it porous.

The second resin composition has a higher melting point than the first resin composition. The melting point T _mB is preferably 100 ° C to 150 ° C, more preferably 100 ° C to 145 ° C, and further preferably 100 ° C to 140 ° C. When the melting point T _mB is within the above range, the safety of the battery tends to be dramatically improved. In order to obtain such a second resin composition, a resin having a melting point of 100 ° C. to 150 ° C. may be included in the resin composition.

(Second resin composition)
The resin that can be contained in the second resin composition is not particularly limited. For example, polyolefin, polyethylene terephthalate, polybutylene terephthalate, or the like, which is a polymer containing an olefin hydrocarbon such as polyethylene or polypropylene as a monomer component. Of saturated polyester. Among these, polyethylene is preferable, and more specifically, so-called high density polyethylene, medium density polyethylene, and low density polyethylene are exemplified. High density polyethylene is more preferred. By using such a resin, the safety of the battery tends to be dramatically improved.

  The MFR of the second resin composition is preferably 0.010 to 10 g / 10 minutes, more preferably 0.10 to 3.0 g / 10 minutes, and further preferably 0.10 to 2.0 g / minute. 10 minutes, most preferably 0.10 to 1.0 g / 10 minutes. When the MFR of the second resin composition is 0.010 g / 10 min or more, fish eyes tend not to occur in the second microporous film. Further, when the MFR of the second resin composition is 10 g / 10 min or less, drawdown is unlikely to occur and the film formability tends to be good.

The density of the second resin composition is preferably 945 to 970 kg / m ³ , more preferably 955 to 970 kg / m ³ , still more preferably 960 to 967 kg / m ³ , most preferably 963 to 967 kg / m ³ . When the density of the second resin composition is 945 kg / m ³ or more, a microporous film with better air permeability tends to be obtained. Moreover, when the density of the second resin composition is 970 kg / m ³ or less, the film tends to be difficult to break when stretched.

(Porous layer)
The porous layer is a layer laminated on one side or both sides of the microporous film, and includes inorganic particles and a resin binder. Although not particularly limited, the porous layer is specifically a layer in which inorganic particles or inorganic particles and a microporous film are bound by a resin binder.

  The thickness of the porous layer is 1.0 to 4.0 μm, preferably 1.0 to 3.5 μm, and more preferably 1.0 to 3.0 μm. When the thickness of the porous layer is 1.0 μm or more, the heat resistance is further improved. Moreover, an electrical resistance improves more because the thickness of a porous layer is 4.0 micrometers or less.

The basis weight of the porous layer is 1.0 to 5.0 g / m ² , preferably 1.0 to 4.5 g / m ² , and more preferably 1.0 to 4.0 g / m ² . . When the basis weight is 1.0 g / m ² or more, the heat resistance is further improved. Moreover, when the basis weight is 5.0 g / m ² or less, the electrical resistance is further improved.

[Inorganic particles]
As the inorganic particles, any of synthetic products and natural products can be used without particular limitation. Examples of natural products include, but are not limited to, for example, kaolinite, dickite, nacrite, halloysite, pyrophyllite, audinite, montmorillonite, beidellite, nontronite, bolcon score, saponite, sauconite, swinoldite, vermiculite, Burcellin, Amesite, Keriaite, Freponite, Blindriaite, biotite, phlogopite, iron mica, eastite, siderophyllite tetraferri iron mica, scale mica, polylithionite, muscovite, celadon stone, iron celadon Stone, Iron Alumino Ceradon Stone, Alumino Ceradon Stone, Tobe Mica, Soda Mica, Clint Knight, Kinoshita, Bite Mica, Ananda Stone, Pearl Mica, Clinochlore, Chamosite, Penantite, Nimite, Baylicroa, Dongbasite Cookeite, and the like Sudoaito.

  Among these, aluminum silicate compounds such as kaolinite, dickite, nacrite, halloysite, and pyrophyllite are preferable. By using such inorganic particles that do not have ion exchange capacity, lithium ion migration is less likely to be inhibited, and since electrochemical stability is higher, a higher quality laminated microporous film tends to be obtained. .

  Furthermore, kaolin mainly composed of kaolin minerals such as kaolinite and wax stone mainly composed of pyrophyllite are more preferable because they are inexpensive and easily available.

  The kaolin is not particularly limited, and examples thereof include wet kaolin and calcined kaolin obtained by calcining this. The calcined kaolin is obtained by calcining a kaolin mainly composed of kaolin minerals such as kaolinite or a pyroxene composed mainly of pyrophyllite. In addition to the release of crystal water during the firing treatment, impurities are removed, so that the chemical stability in the battery, particularly the electrochemical stability, is higher than other kaolins and the like. Therefore, there is a tendency that a higher quality laminated microporous film can be obtained by using calcined kaolin. In addition, by using calcined kaolin, it is possible to realize a very lightweight porous layer while maintaining high permeability. In addition, even with a thinner porous layer thickness, thermal contraction of the porous film at high temperatures is suppressed, and excellent heat resistance is achieved. It tends to develop sex. By using calcined kaolin, it is possible to realize a separator that is excellent in lightness and has a suppressed rate of increase in air permeability.

  The synthetic product is not particularly limited, and examples thereof include synthetic zeolite.

  The content of the aluminum silicate compound is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and still more preferably 80% by mass with respect to the total amount of the inorganic particles. It is 100 mass% or less. When the content of the aluminum silicate compound is within the above range, the heat resistance tends to be further improved.

  The content of the inorganic particles having a particle size of more than 0.20 μm and not more than 1.4 μm is preferably not less than 2.0% by volume, more preferably not less than 3.0% by volume with respect to the total amount of the inorganic particles. More preferably 5.0% by volume or more. Further, the content of the inorganic particles having a particle size of more than 0.2 μm and not more than 1.4 μm is preferably 90% by volume or less, more preferably 80% by volume or less based on the total amount of the inorganic particles. When the content of the inorganic particles having a particle size larger than 0.2 μm and not larger than 1.4 μm is within the above range, even when the porous layer thickness is thinner (for example, 7.0 μm or smaller), Thermal shrinkage is suppressed and excellent heat resistance tends to be exhibited.

  The content of inorganic particles having a particle size of more than 0.20 μm and not more than 1.0 μm is preferably 1% by volume or more, more preferably 2% by volume or more with respect to the total amount of inorganic particles. In addition, the content of inorganic particles having a particle size greater than 0.20 μm and 1.0 μm or less is preferably 80% by volume or less, and more preferably 70% by volume or less with respect to the total amount of inorganic particles. When the content of the inorganic particles having a particle size larger than 0.20 μm and not larger than 1.0 μm is within the above range, even when the porous layer thickness is thinner (for example, not larger than 7.0 μm), Thermal shrinkage is suppressed and excellent heat resistance tends to be exhibited.

  Further, the content of inorganic particles having a particle size of more than 0.50 μm and not more than 2.0 μm is preferably 8.0% by volume or more, more preferably 10% by volume or more with respect to the total amount of inorganic particles. In addition, the content of inorganic particles having a particle size of more than 0.50 μm and not more than 2.0 μm is preferably 60% by volume or less, more preferably 50% by volume or less based on the total amount of inorganic particles. When the content of the inorganic particles having a particle size larger than 0.50 μm and not larger than 2.0 μm is within the above range, even when the porous layer thickness is thinner (for example, not larger than 7.0 μm), Thermal shrinkage is suppressed and excellent heat resistance tends to be exhibited.

  Furthermore, the content of the inorganic particles having a particle size of more than 0.60 μm and not more than 1.4 μm is preferably 1.0% by volume or more, more preferably 3.0% by volume or more with respect to the total amount of the inorganic particles. is there. Further, the content of the inorganic particles having a particle size of more than 0.60 μm and not more than 1.4 μm is preferably 40% by volume or less, more preferably 30% by volume or less based on the total amount of the inorganic particles. When the content of the inorganic particles having a particle size larger than 0.60 μm and smaller than 1.4 μm is within the above range, even when the porous layer thickness is thinner (for example, 7.0 μm or smaller), Thermal shrinkage is suppressed and excellent heat resistance tends to be exhibited.

  Examples of the method for adjusting the ratio of the particle size include a method of pulverizing inorganic particles using a ball mill, a bead mill, a jet mill or the like to reduce the particle size.

  The average particle size of the inorganic particles is preferably 0.10 μm to 15 μm, more preferably 0.10 μm to 5.0 μm, still more preferably 0.10 μm to 4.0 μm, and even more preferably 0.00. It is 14 micrometers-3.9 micrometers, Most preferably, it is 0.14 micrometers-3.0 micrometers or less. When the average particle diameter of the inorganic particles is within the above range, even when the thickness of the porous layer is thinner (for example, 7 μm or less), thermal shrinkage of the porous film at a high temperature is suppressed, and excellent heat resistance tends to be exhibited. The “average particle size” refers to the arithmetic average particle size of the particle size distribution.

  When calcined kaolin is used as at least a part of the inorganic particles, the average particle diameter of the calcined kaolin is preferably 0.10 μm to 4.0 μm, more preferably 0.20 μm to 3.5 μm, still more preferably Is 0.40 μm to 3.0 μm. When the average particle diameter of the calcined kaolin is within the above range, even when the porous layer is thin (for example, 7 μm or less), thermal shrinkage at high temperature is suppressed, and excellent heat resistance is exhibited. There is a tendency.

  The content of the inorganic particles is preferably 50% by mass or more and less than 100% by mass, more preferably 70% by mass or more and 99.99% by mass or less, and further preferably 90% by mass with respect to the total amount of the porous layer. The content is 99.9% by mass or less, and particularly preferably 95% by mass or more and 99% by mass or less. When the content of the inorganic particles is within the above range, the binding properties of the inorganic particles, the permeability of the laminated microporous film, and the heat resistance tend to be further improved.

[Resin binder]
Although it does not specifically limit as a resin binder, When using a laminated microporous film as a separator for lithium ion secondary batteries, it is insoluble with respect to the electrolyte solution of a lithium ion secondary battery, and a lithium ion secondary battery It is preferable to use one that is electrochemically stable within the range of use.

  Such a resin binder is not particularly limited. For example, polyolefins such as polyethylene and polypropylene; fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer Fluorine-containing rubber such as ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, Methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate Rubbers such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, etc .; melting point and / or glass transition temperature of polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, polyester, etc. Resin having a temperature of 180 ° C. or higher. Among these, polyvinyl alcohol is preferable from the viewpoint of binding properties.

  When polyvinyl alcohol is used as the resin binder, the saponification degree is preferably 85% or more and 100% or less, more preferably 90% or more and 100% or less, and further preferably 95% or more and 100% or less. Particularly preferably, it is 99% or more and 100% or less. A saponification degree of 85% or more is preferable because the temperature at which a short circuit (short circuit temperature) is further improved and good safety performance tends to be obtained.

  Moreover, the polymerization degree of polyvinyl alcohol is preferably 200 or more and 5000 or less, more preferably 300 or more and 4000 or less, and further preferably 500 or more and 3500 or less. When the polymerization degree is 200 or more, inorganic particles can be firmly bound with a small amount of polyvinyl alcohol, and the increase in the air permeability of the laminated microporous film due to the formation of the porous layer is further suppressed while maintaining the mechanical strength of the porous layer. Tend to be. Moreover, it exists in the tendency which can prevent the gelatinization at the time of preparing a coating liquid because a polymerization degree is 5000 or less.

[Method for producing laminated microporous film]
The method for producing the laminated microporous film of this embodiment is as follows:
A first resin film composed of a first resin composition having a melting point TmA and a second resin film composed of a second resin composition having a melting point TmB lower than the melting point TmA are laminated. A film forming step of forming a laminated film containing polyolefin,
An opening step of opening the film by a dry method to form a microporous film;
Forming a porous layer containing inorganic particles and a resin binder on the surface of the microporous film to form the laminated microporous film;
In this order.

[Film formation process]
The film forming step includes a first resin film composed of a first resin composition having a melting point TmA, and a second resin composed of a second resin composition having a melting point TmB lower than the melting point TmA. This is a step of forming a laminated film containing polyolefin and laminated with a film.

  As a method for forming a laminated film, a method of forming a laminated film in which each resin film is laminated by an extrusion method using a T die or a circular die and then stretching the laminated film to make it porous (a); T die Or a circular die, each resin film is extruded separately and then laminated to form a laminated film by laminating each resin film, and then the laminated film is stretched to make it porous ( b): A method in which each resin film is separately extruded using a T die or a circular die, and further stretched to obtain microporous films that have been made porous, respectively, and then the microporous films are bonded together ( c).

  Among these, in the manufacturing method of the laminated microporous film of the present embodiment, the method (a) is preferable from the viewpoint of physical properties required for the obtained laminated microporous film and initial / running cost. On the other hand, although the air permeability is slightly inferior to the method (a), the method (c) is also preferable from the viewpoint that the heat shrinkage rate of the laminated microporous film can be reduced.

  In the present specification, “resin film” refers to a resin composition formed into a film shape, and a microporous film can be obtained by stretching the resin composition to make it porous.

  In any of the production methods (a) to (c), the draw ratio after extrusion (film winding speed (unit is m / min) / resin composition extrusion speed (molten resin passing through the die lip) The linear velocity in the direction of the flow is in m / min.)) Is preferably 10 to 500, more preferably 100 to 400, and even more preferably 150 to 350.

  Moreover, the winding speed of the film is preferably 2.0 to 400 m / min, more preferably 10 to 200 m / min.

[Annealing process]
It is preferable that the manufacturing method of the laminated microporous film of this embodiment has an annealing process which performs heat processing (annealing) with respect to the obtained laminated film. As an annealing method, for example, a method in which a laminated film is brought into contact with a heating roll; a method in which the laminated film is exposed to a heated gas phase; a method in which the laminated film is wound on a core body and exposed to a heated gas phase or a heated liquid phase And a method of combining them; The conditions for these heat treatments are appropriately determined depending on the type of material constituting the film.

The heating temperature when annealing the laminated film is preferably (T _mB −30) ° C. or higher and (T _mB −2) ° C. or lower, more preferably, from the viewpoint of the balance of porosity, air permeability and heat shrinkage rate. Is (T _mB −15) ° C. or higher and (T _mB −2) ° C. or lower. The heating time is preferably 10 seconds to 100 hours, more preferably 1 minute to 10 hours.

[Opening process]
The opening step is a step of forming a microporous film by opening the laminated film by a dry method. The “dry method” refers to a stretch opening method that does not use a solvent. By using the dry method, the obtained hole diameter tends to be relatively small, and a direct through hole structure tends to be easily formed.

  The hole opening step is not particularly limited, but a cold stretching step of cold-stretching the laminated film at least 1.05 times to 2.0 times in at least one direction, and a laminated film at least 1.05 times to 5.0 in one direction. It is preferable to have a heat stretching step of heat stretching twice. Either the cold stretching step or the hot stretching step may be performed first, but it is preferable to perform the hot stretching step after performing the cold stretching step. There exists a tendency for the air permeability of the laminated microporous film obtained by this to improve more.

  In the case of forming a laminated film in which the first microporous film and the second microporous film are laminated in advance as in the methods (a) and (b) above, It is preferable to include a cold stretching step of performing stretching of 1 to obtain a stretched laminated film. In addition, when the second microporous film and the first microporous film are separately made porous and then laminated as in the method (c) above, the first microporous film is laminated to each film. It is preferable to include a cold stretching step of obtaining a stretched laminated film by performing stretching.

(Cold drawing process)
When cold-stretching is performed on the laminated film, the temperature is preferably −20 ° C. or more (T _mB −60) ° C. and more preferably 0 ° C. or more and 50 ° C. or less. It exists in the tendency which can suppress a fracture | rupture more because the extending | stretching temperature of cold extending | stretching is -20 degreeC or more. Moreover, when the drawing temperature of cold drawing is (T _mB -60) ° C. or lower, the porosity and the air permeability tend to be further improved. Here, the drawing temperature of cold drawing means the surface temperature of the film in the cold drawing process. The surface temperature of the film can be measured with a contact thermometer.

  The draw ratio in the cold drawing step is preferably 1.05 to 2.0 times, more preferably 1.1 to 2.0 times. The cold stretching is performed in at least one direction, but may be performed in both the film extrusion direction (hereinafter referred to as “MD direction”) and the film width direction (hereinafter referred to as “TD direction”). Preferably, uniaxial stretching is performed only in the film extrusion direction.

(Heat drawing process)
When heat-stretching is performed on the laminated film, the stretching temperature in the heat stretching step is preferably (T _mB −60) ° C. or higher and (T _mB −2.0) ° C. or lower, more preferably (T _mB −30). ) ° C. or more and (T _mB −2.0) ° C. or less. It exists in the tendency which can suppress a fracture _| rupture more because the extending _| stretching temperature of hot stretching is ( _TmB- 60) _degreeC or more. Moreover, it exists in the tendency for a porosity and air permeability to improve more because the extending _| stretching temperature of heat _| fever stretching is ( _TmB- 2.0) degree C or less. Here, the stretching temperature in the heat stretching process means the surface temperature of the film.

  The draw ratio in the heat drawing step is preferably 1.05 times to 5.0 times, more preferably 1.1 times to 5.0 times, and still more preferably 2.0 times to 5.0 times. is there. The hot stretching may be performed in at least one direction and may be performed in both the MD and TD directions, but is preferably performed in the same direction as the cold stretching direction, and more preferably only in the same direction as the cold stretching direction. It is to perform uniaxial stretching.

[Heat setting process]
It is preferable that the manufacturing method of the laminated microporous film of this embodiment has the heat setting process which heat-sets a microporous film after a hole opening process. By carrying out the heat setting process, shrinkage in the stretching direction of the laminated microporous film due to residual stress acting during stretching in the opening process can be suppressed, and the delamination strength of the resulting laminated microporous film tends to be further improved. .

  Although it does not specifically limit as a heat setting method, For example, the method of carrying out heat contraction to the extent that the length of the lamination | stacking microporous film after heat setting reduces 3 to 50% (henceforth this method is called "relaxation"), Examples include a method of fixing so that the dimension in the stretching direction does not change.

When heat setting is performed on the microporous membrane, the heat setting temperature is preferably (T _mB −30) ° C. or more and (T _mB −2) ° C. or less, more preferably (T _mB −15) ° C. or more ( T _mB -2) It is not higher than ° C. Here, the heat setting temperature means the surface temperature of the film.

  In the cold drawing step, the hot drawing step, other drawing steps and the heat setting step, the drawing is carried out in one or two or more stages in a uniaxial direction and / or biaxial direction by a roll, a tenter, an autograph or the like. A heat fixing method may be employed. Among these, from the viewpoint of physical properties and applications required for the laminated microporous film obtained in the present embodiment, it is preferable to perform two or more stages of uniaxial stretching and heat setting with a roll.

[Porous layer forming step]
The porous layer forming step is a step of forming the laminated microporous film by forming a porous layer containing inorganic particles and a resin binder on the surface of the microporous film.

  The method for forming the porous layer is not particularly limited, and examples thereof include a method of applying a coating liquid containing inorganic particles and a resin binder on the surface of the microporous film.

(Coating solution)
Examples of the inorganic particles and the resin binder contained in the coating solution are the same as those described above. The coating liquid may contain other components such as a solvent.

  Although it does not specifically limit as a solvent which can be used in a coating liquid, For example, what can disperse | distribute or melt | dissolve an inorganic particle and a resin binder uniformly and stably is preferable. Such a solvent is not particularly limited, and examples thereof include N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, and hexane. .

  Various additives such as surfactants, thickeners, wetting agents, antifoaming agents, PH preparations containing acids and alkalis are added to the coating solution in order to stabilize dispersion and improve coating properties. May be added. These additives are preferably those that can be removed when the solvent is removed, but are electrochemically stable in the range of use of the lithium ion secondary battery, do not inhibit the battery reaction, and are stable up to about 200 ° C. If present, it may remain in the porous layer.

  The method for dissolving or dispersing the inorganic particles and the resin binder in the solvent of the coating solution is not particularly limited as long as it can achieve the dissolution or dispersion characteristics of the coating solution necessary for the coating step. Examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade.

  The method for applying the coating solution to the porous film is not particularly limited as long as it can achieve the required layer thickness and coating area. For example, gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast Examples include a coater method, a die coater method, a screen printing method, and a spray coating method.

  Furthermore, it is preferable to perform a surface treatment on the surface of the microporous film prior to coating, because it facilitates the coating of the coating liquid and improves the adhesion between the porous layer and the surface of the microporous film. The surface treatment method is not particularly limited as long as it does not significantly impair the porous structure of the microporous membrane. For example, corona discharge treatment method, mechanical surface roughening method, solvent treatment method, acid treatment method, ultraviolet oxidation Law.

  The method for removing the solvent from the coating film after coating is not particularly limited as long as it does not adversely affect the microporous film. For example, a method of drying at a temperature below its melting point while fixing the microporous film, a method of drying under reduced pressure at a low temperature, a method of immersing in a poor solvent for the resin binder to solidify the resin binder and extracting the solvent at the same time, etc. Can be mentioned.

[Use]
The laminated microporous film of the present embodiment is suitably used as a battery separator such as a non-aqueous secondary battery separator, more specifically as a lithium secondary battery separator. In addition, it is also suitably used as various separation membranes.

In addition, each physical property in this specification can be measured according to the method described in the following Examples, unless otherwise specified.

Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. The evaluation methods of various properties in the examples and comparative examples are as follows.

(1) Melting point The melting point of the resin composition was measured by a method based on JIS K-7121.

(2) Melt flow rate (MFR)
The MFR of the resin composition was measured according to JIS K7210. Polypropylene was measured under conditions of 230 ° C. and 2.16 kg, and polyethylene was measured under conditions of 190 ° C. and 2.16 kg. The unit of MFR is g / 10 minutes.

(3) Density The density of the resin composition was measured according to JIS K7112. The unit of density is kg / m ³ .

(4) Film thickness (μm)
The film thickness of the laminated microporous film was measured with a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(5) Porosity (%)
A 10 cm × 10 cm square sample was cut out from the laminated microporous film, and the porosity was calculated from the volume and mass of the sample using the following formula.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of resin composition (g / cm ³ )) / volume (cm ³ ) × 100

(6) Air permeability (sec / 100cc)
The air permeability was measured with a Gurley type air permeability meter based on JIS P-8117. In addition, the value which converted the film thickness of the lamination | stacking microporous film into 20 micrometers was made into air permeability.

(7) 120 ° C. heat shrinkage (%)
The laminated microporous film was cut to 100 mm in the MD direction and 100 mm in the TD direction, and allowed to stand in an oven at 150 ° C. for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the hot air would not directly hit the sample. After the sample was taken out of the oven and cooled, the length (mm) was measured, and the thermal contraction rate of MD and TD was calculated by the following formula.
MD thermal shrinkage (%) = (100−MD length after heating) / 100 × 100
TD heat shrinkage rate (%) = (100−length of TD after heating) / 100 × 100

(8) Puncture strength (N)
A handy compression tester KES-G5 manufactured by Kato Tech Co., Ltd. is attached with a needle having a diameter of 1 mm and a radius of curvature of 0.5 mm at the tip, at a temperature of 23 ± 2 ° C., and a needle moving speed of 0.2 cm / sec. A piercing test was conducted. This was converted to a thickness of 20 μm based on the measured value of the film thickness, and was defined as the puncture strength.
Puncture strength (N) = Measured puncture strength × 20 / film thickness

(9) Electric resistance (Ω · cm ² )
A cell made of SUS shown in FIG. 1 was prepared. Here, reference numeral 1 in FIG. 1 denotes a cell body, reference numeral 2 denotes a polytetrafluoroethylene seal, reference numeral 3 denotes a spring, and reference numeral 4 denotes a laminated microporous film impregnated with an electrolytic solution.

  A sample of a laminated microporous film cut out in a circular shape is impregnated with an electrolytic solution, placed in the cell 1 shown in FIG. 1, and this cell 1 is accommodated in an oven set at −30 ° C. for sufficient time. After the temperature in the oven was stabilized at −30 ° C., the electrical resistance (Rs1) per film sample was first measured.

Next, the cell 1 is taken out from the oven, and five more film samples impregnated with the electrolytic solution are stacked in the cell from the bottom to the top in FIG. 1, and the cell is stored in the oven at −30 ° C. Then, after sufficient time had elapsed, the temperature in the oven was stabilized at −30 ° C., and then the electrical resistance (Rs6) per six film samples was measured.
The electrical resistance of the film sample was calculated from the above Rs1 and Rs6 according to the following equation.
Electrical resistance (Ω · cm ² ) = {[Rs 6 (Ω) −Rs 1 (Ω)] / 5} × 2.00 (cm ² )

This measurement was performed at least 5 times, and the average value was taken as the value of electrical resistance. Note that LIPASTE-EP2BL / FSI1T (trade name) manufactured by Toyama Pharmaceutical Co., Ltd. was used as the electrolyte. The electrical resistance was measured using a Hioki 3532-80 chemical impedance meter (trade name) manufactured by Hioki Electric Co., Ltd., and the real part (resistance) of the impedance at 100 kHz was taken as the value of electrical resistance. The effective area of the electrode shown in FIG. 1 was 2.00 cm ² .

(10) Shutdown temperature, short-circuit temperature a. Production of positive electrode 92.2 parts by mass of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, 2.3 parts by mass of flake graphite and acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) as a resin binder Was prepared and dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of a 20 μm-thick aluminum foil serving as a positive electrode current collector using a die coater so that the positive electrode active material application amount was 250 g / m ² . After drying at 130 ° C. for 3 minutes, using a roll press machine, compression molding was performed so that the bulk density of the positive electrode active material was 3.00 g / cm ³ to obtain a positive electrode.

b. Preparation of Negative Electrode 96.6 parts by mass of artificial graphite as a negative electrode active material, 1.4 parts by mass of ammonium salt of carboxymethylcellulose and 1.7 parts by mass of styrene-butadiene copolymer latex as a resin binder were prepared, and these were purified water. To prepare a slurry. This slurry was applied to one side of a 12 μm-thick copper foil serving as a negative electrode current collector using a die coater so that the negative electrode active material application amount was 106 g / m ² . After drying at 120 ° C. for 3 minutes, using a roll press machine, the negative electrode active material was compression-molded so that the bulk density of the negative electrode active material was 1.35 g / cm ³ to obtain a negative electrode.

c. Preparation of Nonaqueous Electrolytic Solution LiBF ₄ as a solute was dissolved in a mixed solvent of propylene carbonate: ethylene carbonate: γ-butyllactone = 1: 1: 2 (volume ratio) to a concentration of 1.0 mol / L, A water electrolyte was prepared.

d. Measurement of shutdown temperature and short-circuit temperature A negative electrode cut into 65 mm × 20 mm and immersed in a non-aqueous electrolyte for 1 minute or longer, 9 μm (thickness) × 50 mm × 50 mm aramid film with a 16 mm diameter hole in the center, 65 mm × Prepare a laminated microporous film cut into 20 mm and immersed in a non-aqueous electrolyte for 1 hour or more, a positive electrode cut into 65 mm × 20 mm and immersed in a non-aqueous electrolyte for 1 minute or more, a Kapton film, and a silicon rubber with a thickness of about 4 mm. Then, they were laminated in this order on a ceramic plate to which a thermocouple was connected. This laminate is set on a hot plate, heated at a rate of 15 ° C./min with a pressure of 4.1 MPa applied by a hydraulic press, and the impedance change between the positive and negative electrodes is 1 V AC and 1 kHz. The measurement was performed up to 200 ° C.

  The temperature at the time when the impedance reached 1000Ω was taken as the shutdown temperature, and the temperature at which the impedance again fell below 1000Ω after the shutdown was taken as the short-circuit temperature.

(11) Coating basis weight A sample of MD100 mm × TD100 mm was cut from each of three locations from a microporous membrane and a laminated microporous film, and each weight was measured using an electronic balance. And the basis weight (g / m ² ) of the laminated microporous film. Further, the difference in basis weight between the microporous film and the laminated microporous film measured in this way was defined as a coating basis weight (g / m ² ).

In addition, the used resin is as follows.
Polypropylene (a-1): melting point 165 ° C., MFR 0.5 g / 10 minutes Polypropylene (a-2): melting point 165 ° C., MFR 0.5 g / 10 minutes Polyethylene (b-1): melting point 136 ° C., MFR 0.25 g / 10 Minute Polyethylene (b-2): Melting point 137 ° C., MFR 0.22 g / 10 min Polyethylene (b-3): Melting point 135 ° C., MFR 0.76 g / 10 min

The inorganic particles are aluminosilicate minerals mainly composed of kaolinite (Al ₂ Si ₂ O ₅ (OH) ₄ ), and kaolin mainly composed of kaolinite is calcined at a high temperature. The kaolin was wet-ground with a bead mill.

[Example 1]
Polypropylene (a-1) having a diameter of 20 mm, L / D (L: distance (m) from the raw material supply port to the discharge port of the extruder, D: inner diameter (m) of the extruder, the same applies hereinafter) = 30 , To a single screw extruder set at 220 ° C. via a feeder, and polyethylene (b-1) is charged to a single screw extruder set at 20 mm diameter, L / D = 30, 200 ° C. via a feeder. Then, it was extruded from a co-pressed T die having a lip thickness of 3.0 mm installed at the tip of the extruder. Immediately thereafter, 25 ° C. cold air was applied to the melted resin, and the cast layer was cooled to 95 ° C. and wound at a draw ratio of 200 times and a winding speed of 15 m / min. The outer layer was the first microporous film (A -1) A three-layer laminated film (Af-1) having an inner layer having the structure of the second microporous film (B-1) was formed (coextrusion step). The laminated film (Af-1) was annealed for 6 hours in a hot air circulating oven heated to 130 ° C. (annealing step).

  Next, the annealed laminated film was uniaxially stretched 1.3 times in the machine direction at a temperature of 25 ° C. to obtain a stretched laminated film (cold stretching process). Subsequently, the stretched laminated film was uniaxially stretched 3.0 times in the longitudinal direction at a temperature of 120 ° C. (strain rate: 0.5 / second) to obtain a microporous film (thermal stretching step). Then, it was heat-fixed by relaxing 0.8 times at a temperature of 130 ° C. The obtained microporous membrane was measured for film thickness, porosity, air permeability, puncture strength, thermal contraction rate, electrical resistance, and shutdown / short-circuit temperature. The results are shown in Table 1 (dry microporous membrane).

  Next, 97.0 parts by mass of calcined kaolin and 3.0 parts by mass of polyvinyl alcohol (hereinafter sometimes abbreviated as “PVA”. Average polymerization degree 1700, saponification degree 99% or more) are 150 parts by mass. A coating solution was prepared by uniformly dispersing in water, and coated on the surface of the microporous film using a gravure coater. Water was removed by drying at 60 ° C. to obtain a laminated microporous film in which a porous layer having a thickness of 1.6 μm was formed on the microporous membrane. The resulting laminated microporous film was measured for film thickness, air permeability, puncture strength, thermal shrinkage, electrical resistance, and shutdown / short temperature. The results are shown in Table 1.

[Example 2]
A laminated microporous film was obtained in the same manner as in Example 1 except that a porous layer having a thickness of 3.0 μm was formed on the microporous film.

[Comparative Example 1]
A laminated microporous film was obtained in the same manner as in Example 1 except that a porous layer having a thickness of 4.6 μm was formed on the microporous film.

[Comparative Example 2]
A laminated microporous film was obtained in the same manner as in Example 1 except that a porous layer having a thickness of 5.9 μm was formed on the microporous film.

[Comparative Example 3]
47.5 parts by mass of polyethylene resin (b-2), 47.5 parts by mass of polyethylene resin (b-3) and 5 parts by mass of homopolymer polypropylene (a-2) were dry blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is added to 99% by mass of the obtained pure polymer mixture, and again A mixture such as a polymer was obtained by dry blending using a tumbler blender. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 −5 m ² / s) was injected into the extruder cylinder by a plunger pump as a plasticizer. The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65% by mass. The melt-kneading conditions were set at a preset temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die to obtain a sheet-like polyolefin composition having a thickness of 1300 μm.

  Next, it led to the simultaneous biaxial tenter stretching machine, and simultaneous biaxial stretching was performed 7 times in the MD direction and 6.4 times in the TD direction. At this time, the set temperature of the simultaneous biaxial tenter was 118 ° C. Next, the solution was introduced into a methyl ethyl ketone tank, and liquid paraffin was extracted and removed, and then methyl ethyl ketone was removed by drying.

  Furthermore, it was led to a TD tenter heat fixing machine and heat fixed. The heat setting temperature was 122 ° C., the TD relaxation rate was 0.80, and a polyolefin resin porous membrane was obtained. The resulting polyolefin resin porous membrane was measured for film thickness, porosity, air permeability, puncture strength, thermal shrinkage, electrical resistance, and shutdown / short temperature. The results are shown in Table 1 (wet microporous membrane).

  Next, 97.0 parts by mass of calcined kaolin and 3.0 parts by mass of polyvinyl alcohol (hereinafter sometimes abbreviated as “PVA”. Average polymerization degree 1700, saponification degree 99% or more) are 150 parts by mass. A coating solution was prepared by uniformly dispersing in water, and coated on the surface of the polyolefin resin porous film using a gravure coater. Water was removed by drying at 60 ° C. to obtain a laminated microporous film in which a porous layer having a thickness of 5.5 μm was formed on the polyolefin resin porous film. The resulting laminated microporous film was measured for film thickness, air permeability, puncture strength, thermal shrinkage, electrical resistance, and shutdown / short temperature. The results are shown in Table 1.

  The laminated microporous film of Examples 1 and 2 produced with a specific porous layer thickness and basis weight has excellent thermal dimensional stability and electrical resistance in addition to heat resistance, shutdown function, etc. It has been found that the microporous film of Example 1 having a thin porous layer and a small basis weight has very good thermal dimensional stability and electrical resistance.

  Surprisingly, although the reason is not clear, it was possible to obtain a laminated microporous film having a coating weight per unit area smaller than that of a wet film, that is, having good cost performance.

  Since the laminated microporous film of this embodiment is excellent in heat resistance, it can be suitably used for separation and purification of various substances at high temperatures.

  The laminated microporous film of the present embodiment has industrial applicability as a battery separator such as a non-aqueous secondary battery separator, particularly as a lithium ion secondary battery separator.

  DESCRIPTION OF SYMBOLS 1 ... Cell, 2 ... Sealing material, 3 ... Spring, 4 ... Separator for non-aqueous secondary batteries impregnated with electrolyte

Claims (10)

A microporous membrane containing polyolefin;
A porous layer containing inorganic particles and a resin binder, laminated on one or both sides of the microporous membrane,
The porous layer has a thickness of 1.0 to 4.0 μm,
Basis weight of the porous layer is 1.0 to 5.0 g / m ^2, laminated microporous film.   The laminated microporous film according to claim 1, wherein the inorganic particles include an aluminum silicate compound.   The laminated microporous film according to claim 2, wherein the aluminum silicate compound contains calcined kaolin. The microporous membrane is
A first microporous film composed of a first resin composition having a melting point T _mA ;
Wherein a laminated film and a second microporous film composed of a second resin composition having a low melting point T _mB than the melting point T _mA, according to any one of claims 1 to 3 Laminated microporous film.   The laminated microporous film according to claim 4, wherein the second resin composition is polyethylene.   The laminated microporous film according to any one of claims 1 to 5, wherein the film thickness is 16 µm or less.   The laminated microporous film according to any one of claims 1 to 6, wherein the air permeability is 10 to 5000 seconds / 100 cc. A first resin film composed of a first resin composition having a melting point T _mA, and a second resin film composed of a second resin composition having a melting point T _mB lower than the melting point T _mA A film forming step of forming a laminated film containing polyolefin,
An opening step of opening the film by a dry method to form a microporous film;
A porous layer forming step of forming a laminated microporous film according to any one of claims 1 to 7, by forming a porous layer containing inorganic particles and a resin binder on the surface of the microporous membrane,
The manufacturing method of the lamination | stacking microporous film which has these in this order. In the film forming step, it consists second resin composition having a first resin film made from a first resin composition having a melting point T _mA, a low melting point T _mB than the melting point T _mA The method for producing a laminated microporous film according to claim 8, wherein a laminated film containing polyolefin and laminated with a second resin film is formed by a coextrusion method.   A battery separator comprising the laminated microporous film according to claim 1.

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