JP2005129435A - Battery separator made of polyolefin resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a battery separator made of polyolefin resin easily realizing uniform dispersion in a production process by simplifying composition of the resin, having a shutdown function in spite of the simplified composition, with a small hole diameter and a high void ratio, and further, having excellent productivity. <P>SOLUTION: The battery separator made of polyolefin resin is formed by melting and kneading a resin composition containing polyolefin resin (C) consisting of crystalline polypropylene (A) and a polypropylene-α-olefin copolymer (B) to get a membranous molten matter, molding the molten matter into a membranous molding, and then, drawing the membranous molding in one direction, of which, the polyolefin resin (C) consists of 30 to 90 wt.% of the crystalline polypropylene (A) and 10 to 70 wt.% of the propylene-α-olefin copolymer (B), and is provided with pores communicated to an area of the propylene-α-olefin copolymer (B). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a battery separator made of polyolefin resin. Specifically, the present invention relates to a highly safe battery separator having a low hole closing temperature and a high film breaking temperature.

For organic solvent-based battery separators, a porous film mainly composed of polyethylene resin or polypropylene resin is used from the viewpoint of chemical resistance, and the following methods are generally known as manufacturing methods thereof.
(a) A method of obtaining a porous film by heating a crystalline polypropylene sheet formed at a high draft ratio as necessary and stretching it in at least one direction to fibrillate between crystal lamellae (hereinafter referred to as “single component stretching”). Law ").
(b) A method of extracting the organic liquid material, the inorganic filler and the like from a sheet formed by mixing an organic liquid material, an inorganic filler and the like with a polyolefin resin, and performing stretching before and after the extraction as necessary (hereinafter referred to as the following) "Mixed extraction method").
(c) A method in which a sheet formed by mixing polyethylene and polypropylene is stretched in at least one direction to form voids (pores) at the interface between different polymers (hereinafter referred to as “multi-component stretching method”).

  In the multi-component stretching method, it is difficult to finely disperse a different polymer in a matrix polymer, and since the stretchability is also inferior, it is difficult to obtain a battery separator with a small pore diameter or a battery separator with a large porosity. Battery separators using a mixed extraction method or a single component stretching method are the mainstream.

On the other hand, in the case of an organic solvent battery, in order to prevent the temperature inside the battery from rising abnormally due to misuse of the battery, etc. and causing an accident such as ignition, the separator does not break when the temperature reaches a certain level. There is a demand for a function (hereinafter referred to as “shutdown function”) that shuts down the current by closing the hole. A sufficient shutdown function can be obtained by this shutdown function to only the single component stretching method or a solvent extraction method technology, film tear temperature (hereinafter "film break temperature" referred to) T _b and the temperature to occlude the pores ( It is desired to reduce the hole closing temperature in order to increase the difference in T _s (hereinafter referred to as “hole closing temperature”) ΔT = T _b −T _s and stop the abnormal reaction at an earlier stage to suppress the temperature rise. Yes.

  In response to this requirement, using the characteristics of the melting point difference between polypropylene and polyethylene, the polypropylene microporous membrane and the polyethylene microporous membrane are multi-layered, and the difference between the pore clogging temperature and the membrane breaking temperature is increased. Has been disclosed (see, for example, Patent Document 1). However, in this case, not only is the manufacturing process complicated, but it is difficult to reduce the thickness of the separator, which is insufficient as a battery separator that is required to be reduced in size and weight.

  In addition, a technique for improving the hole closing temperature and the film breaking temperature of a battery separator by mixing polypropylene and polyethylene at a specific ratio to form a film and then stretching the film is disclosed (for example, see Patent Document 2). ). Battery separators need to have small pore diameters in order to prevent the detached positive electrode material from passing through the separator, but the pores obtained by this technique are the interface between polypropylene and polyethylene. Since it is formed by peeling, it has been difficult to use for a battery separator having a large pore diameter and requiring fine pores.

  On the other hand, component A composed of ethylene-propylene block copolymer, component B composed of propylene homopolymer or random copolymer, and component C composed of low molecular weight polypropylene, and optionally composed of component D composed of calcium carbonate and beta spherulite nucleating agent A porous membrane comprising a polymer composition to which component E has been added and a method for producing the same are disclosed (for example, see Patent Document 3).

In these technologies, sufficient porosity and air permeability cannot be obtained by using only the ethylene-propylene block copolymer, so that improvement is attempted by a multi-component system. However, the diameter of the formed pores is large, and the positive electrode material not only short circuit between the positive and negative electrodes is concerned with a transmission, since the difference [Delta] T = T _b -T _s of film breakage temperature T _b and pore closing temperature T _s is small, there is a problem in the shutdown function. In addition, since the pore diameter is large, it is difficult to reduce the thickness of the membrane and increase the porosity, and it is difficult to improve the electric capacity of the battery. Using this porous membrane as a battery separator difficult.

JP-A-6-98395 JP-A-4-206257 JP-A-4-309546

  The present invention has been made to solve the above-described problems related to conventional polyolefin resin battery separators, and has a shutdown function despite a simple resin composition, and has a small pore diameter and a high porosity. It is an object to provide a separator with excellent productivity.

  As a result of intensive studies, the inventors of the present invention contain a polyolefin resin (C) composed of crystalline polypropylene (A) and a propylene-α-olefin copolymer (B) dispersed in the crystalline polypropylene (A). A battery separator formed by melt-kneading a resin composition to form a film-shaped melt, forming the film-shaped melt into a film-shaped molding, and then stretching the film-shaped molding in at least one direction The polyolefin resin (C) comprises 30 to 90% by weight of the crystalline polypropylene (A) and 10 to 70% by weight of the propylene-α-olefin copolymer (B), and the propylene-α-olefin copolymer ( B) The present invention was completed on the basis of the finding that this problem was solved by a polyolefin resin battery separator having pores communicating with the region. In the present invention, the continuous pores refer to pores that are continuously formed in the copolymer (B) region and consequently become a path connecting both surfaces of the battery separator.

The present invention is constituted by the following.
1. A resin composition containing a polyolefin resin (C) composed of crystalline polypropylene (A) and a propylene-α-olefin copolymer (B) dispersed in the crystalline polypropylene (A) is melt-kneaded to form a film. A battery separator formed by forming a melt and forming the film-shaped melt into a film-shaped molded product, and then stretching the film-shaped molded product in at least one direction, wherein the polyolefin resin (C) is crystalline. Polyolefin having 30 to 90% by weight of polypropylene (A) and 10 to 70% by weight of propylene-α-olefin copolymer (B) and having pores communicating with the region of propylene-α-olefin copolymer (B) Resin battery separator.

2. The melt flow rate ratio MFR _PP / MFR _RC of the melt flow rate MFR _PP of the crystalline polypropylene (A) and the melt flow rate MFR _RC of the propylene-α-olefin copolymer (B) is 0.1 to 10. 2. The polyolefin resin battery separator according to 1 above.

3. 3. The polyolefin resin battery separator as described in 2 above, wherein the melt flow rate ratio MFR _PP / MFR _RC is 0.2 to 5.

4). 4. The polyolefin resin battery separator as described in any one of 1 to 3 above, wherein a draft ratio when the film-shaped melt is formed into a film-shaped molding is in the range of 1 to 10.

5). 4. The polyolefin resin battery separator according to any one of items 1 to 3, wherein a draft ratio when the film-shaped melt is formed into a film-shaped product is in the range of 1 to 3.

6). Any one of said 1-5 characterized by polyolefin resin (C) consisting of crystalline polypropylene (A) 40 to 70 weight% and polypropylene-alpha-olefin copolymer (B) 30 to 60 weight%. A battery separator made of polyolefin resin according to item 1.

7). The battery separator made of polyolefin resin according to any one of 1 to 6 above, wherein the propylene content of the propylene-α-olefin copolymer (B) is 30 to 80% by weight.

8). The battery separator made of polyolefin resin according to any one of 1 to 7 above, wherein the propylene content of the propylene-α-olefin copolymer (B) is 40 to 70% by weight.

9. The polyolefin resin (C) is obtained by a multistage polymerization method including the steps of producing a crystalline polypropylene (A) in the first stage and continuously producing a propylene-α-olefin copolymer (B) in the second stage. The polyolefin resin battery separator according to any one of 1 to 8 above, wherein the battery separator is made of a polyolefin resin.

10. The air resistance (Gurley) is 1 to 2,000 seconds / 100 ml, film breakage temperature _{T b} is not more 0.99 ° C. or higher, film breakage temperature _{T b} and pore closing temperature _{T s} the difference [Delta] T _(T b -T _s of 10) The polyolefin resin battery separator according to any one of 1 to 9 above, which is 20 ° C. or higher.

  The battery separator made of polyolefin resin of the present invention has a propylene-α-olefin copolymer (B) (hereinafter simply referred to as “copolymer (B)” in crystalline polypropylene (A) under a specific processing method. Obtained by improving the stretchability at low temperature by using a specific polypropylene resin finely dispersed and forming pores by cleavage of the copolymer (B) in the copolymer (B) region. Battery separator. In addition, the polyolefin resin battery separator of the present invention is an economical battery separator obtained without using a complicated manufacturing process as in the prior art. High battery separator. Furthermore, since the pores are fine and excellent in stretchability and high porosity, it is an excellent battery separator suitable not only for safety but also for the purpose of high battery capacity and thinning.

Hereinafter, embodiments of the present invention will be described.
(1) Polyolefin resin (C)
The battery separator made of polyolefin resin of the present invention comprises crystalline polypropylene (A) and copolymer (B), and the copolymer (B) is finely dispersed as a region in the matrix of crystalline polypropylene (A). The polyolefin resin (C) used is used.

(I) Crystalline polypropylene (A)
The crystalline polypropylene (A) is a crystalline polymer mainly composed of propylene polymerized units, preferably polypropylene having 90% by weight or more of propylene polymerized units. Specifically, it may be a propylene homopolymer, or may be a random or block copolymer of 90% by weight or more of propylene polymer units and 10% by weight or less of α-olefin. As the α-olefin used when the crystalline polypropylene (A) is a copolymer, ethylene (included in the α-olefin in the present invention), 1-butene, 1-pentene, 1-hexene, 1-octene, Examples include 1-decene, 1-dodecene, 4-methyl-1-pentene, and 3-methyl-1-pentene. Among these, it is preferable from the viewpoint of production cost to use a propylene homopolymer or a propylene-ethylene copolymer having a propylene polymer unit content of 90% by weight or more.

Further, the melt flow rate MFR _PP of the crystalline polypropylene (A) is preferably in the range of 0.1 to 50 g / 10 min from the stability of film formation.

(ii) Propylene-α-olefin copolymer (B)
The copolymer (B) is a random copolymer of propylene and an α-olefin other than propylene. The content of propylene polymerized units is preferably in the range of 30 to 80% by weight, more preferably 35 to 75% by weight, still more preferably 40 to 70% by weight, based on the weight of the entire copolymer (B). is there. When the content of the propylene polymerized unit is more than the above range, pores are hardly formed in the copolymer (B) region present in the matrix of the crystalline polypropylene (A), and the content of the propylene polymerized unit is When the amount is less than the above range, interfacial peeling between the crystalline polypropylene (A) and the copolymer (B) is likely to occur, so that the low-temperature stretchability is lowered and the pore diameter is likely to be large.

  Examples of the α-olefin other than propylene used in the copolymer (B) include ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and 4-methyl-1. -Pentene, 3-methyl-1-pentene, etc. are mentioned. Among these, a propylene-ethylene copolymer using ethylene as an α-olefin is preferably used from the viewpoint of production cost.

The melt flow rate MFR _RC of the copolymer (B) is not particularly limited, but a range of 0.1 to 20 g / 10 min is preferable because of excellent molding processability.

(iii) Polyolefin resin (C)
The polyolefin resin (C) is composed of crystalline polypropylene (A) and a copolymer (B). Melt flow rate ratio of melt flow rate MFR _PP of crystalline polypropylene (A) and melt flow rate MFR _{RC of} propylene-α-olefin copolymer (B) MFR _PP / MFR _RC (hereinafter referred to as “MFR ratio”) Is not limited, but the MFR ratio is preferably from 0.1 to 10, and more preferably from 0.2 to 5. When the MFR ratio is within the above range, since the copolymer (B) is finely dispersed in the crystalline polypropylene (A), fine and continuous pores are obtained by low-temperature stretching. As a result, a battery separator having high air permeability is obtained because the contact points between the fine pores are increased. Further, the membrane breaking temperature _Tb is 150 ° C. or more, and the membrane breaking temperature _Tb and the pore closing temperature are obtained. difference ΔT ₌ T b -T _s of T _s is that 20 ° C. or more cell separator obtained. Further, since the polyolefin resin (C) is excellent in stretchability, it is easy to obtain a battery separator having a high porosity and a large battery capacity, and the air permeability is further increased. In the battery separator, according to cross-sectional observation with a scanning electron microscope (SEM), a large number of fine pores having a size of 1 to 2 μm are connected, and a fine pore having a maximum major axis of the pore diameter of 7 μm or less. This battery separator is recognized and excellent in terms of safety.

When the MFR ratio is larger than 10, the pore diameter of the pores formed by stretching is larger than the case where the MFR ratio is 0.1 to 10, and the proportion of the connected pores tends to decrease. Further, a battery separator having a small difference ΔT = T _b −T _s between the film breaking temperature T _b and the hole closing temperature T _s is obtained. In the battery separator, according to cross-sectional observation by SEM, a large number of pores having a diameter of about 5 μm are connected, and pores having a maximum major axis of the pore diameter of 12 μm or less are recognized.

  In the polyolefin resin (C), the content of the crystalline polypropylene (A) is 30 to 90% by weight, preferably 40 to 70% by weight, and the content of the copolymer (B) is 10 to 70% by weight, preferably 30%. ~ 60% by weight. When the content of the copolymer (B) is less than 10% by weight, the continuous pores formed in the copolymer (B) region are reduced, and it is difficult to obtain continuous pores, which exceeds 70% by weight. In this case, it is difficult to obtain a finely dispersed structure of the copolymer (B) present in the crystalline polypropylene (A).

  The production method of the polyolefin resin (C) is not particularly limited, and any production method may be used as long as the above conditions are satisfied. For example, the polyolefin resin (C) may be produced by mixing the crystalline polypropylene (A) and the copolymer (B) obtained by polymerization separately, by melting, kneading or the like. Specifically, a copolymer (B) polymerized using a Ziegler-Natta catalyst such as a titanium-supported catalyst or a commercially available ethylene-propylene rubber corresponding to the copolymer (B) and crystalline polypropylene (A) are melted. The method of mixing can be illustrated.

  Alternatively, the polyolefin resin (C) may be produced by continuously polymerizing the crystalline polypropylene (A) and the copolymer (B) by multistage polymerization. For example, by using a plurality of polymerization vessels, a crystalline polypropylene (A) is produced in the first stage, and then a copolymer (B) is produced in the presence of the crystalline polypropylene (A) in the second stage. A method for continuously producing the resin (C) can be exemplified. This continuous polymerization method is lower in production cost than the melt mixing method described above, and a polyolefin resin (C) in which the copolymer (B) is uniformly dispersed in the crystalline polypropylene (A) can be stably obtained. Therefore, it is preferable.

  In the present invention, a particularly preferred polyolefin resin (C) is produced by the above continuous polymerization method and adjusted so that the MFR ratio is 10 or less, more preferably in the range of 0.2 to 5.

  In the battery separator made of polyolefin resin of the present invention, many fine cleavages are observed in the copolymer (B) region finely dispersed in the crystalline polypropylene (A). Since the copolymer (B) contains a certain amount or more of a propylene component, the copolymer (B) is compatible with crystalline polypropylene, and the copolymer (B) having compatibility with the crystalline polypropylene (A) is crystallized. Since the strength is lower than that of the conductive polypropylene (A), it is presumed that cleavage occurred in the copolymer (B) region due to the stretching stress. This mechanism is fundamentally different from conventional multi-component stretching methods in which inorganic fillers and different polymers are mixed and stretched. As a result, the obtained battery separator is excellent in safety because the pore diameter is small, and the electric capacity can be increased because the porosity is high.

  The porosity of the battery separator of the present invention is not particularly limited, but is preferably 10 to 90%, more preferably 30 to 80%, and more preferably 50 to 80%. If the porosity is in the above range, a function as a battery separator can be obtained, and there is no fear that the strength is lowered. In particular, when the porosity is 50 to 80%, it is possible to increase the capacity and size of the battery.

  Furthermore, taking the case where the α-olefin of the copolymer (B) is ethylene as an example, the polyolefin resin battery separator of the present invention has pores formed in the copolymer (B) having a low melting point at a temperature. It is recognized that the crystalline polypropylene (A), which is a matrix polymer, does not break up to around 170 ° C. due to its high melting point, and has an excellent shutdown function, while closing with increasing.

  The pore clogging due to this temperature rise is greatly influenced by the MFR ratio, but is also influenced by the propylene content in the copolymer (B), and the propylene content in the copolymer (B) is 30% by weight. When the ratio is less than 1, pore clogging due to temperature rise is less likely to occur. This is because the compatibility between the crystalline polypropylene (A) and the copolymer (B), which is a matrix polymer, decreases due to the decrease in the propylene content, and pores are formed at the interface between the crystalline polypropylene (A) and the copolymer (B). Even if the temperature rises, for example, it is presumed that the pores are not easily blocked because the shrinkage of the copolymer (B) is strong.

  In the present invention, the copolymer (B) region means a region occupied by the copolymer (B) itself and a boundary region between the copolymer (B) and a substance adjacent thereto. Therefore, the pores generated in the copolymer (B) region include pores due to cleavage generated in the region occupied by the copolymer (B) itself, and the crystalline polypropylene (A) and the copolymer (B). And pores due to interfacial delamination that occur in the boundary region.

Specifically, the polyolefin resin (C) having the MFR ratio as described above can be produced by a method described in International Publication No. 97/19135, JP-A-8-27238, and the like.
In addition, the polyolefin resin (C) can be produced by the above-described method, and one having a desired specification may be selected from commercially available products.

The MFR ratio is usually determined by measuring MFR _PP of crystalline polypropylene (A) and MFR _RC of propylene-α-olefin copolymer (B). However, when the polypropylene resin is continuously produced by multistage polymerization (when the crystalline polypropylene (A) is first polymerized and then the copolymer (B) is polymerized), the MFR _{RC of the} copolymer (B) is used. Of MFR _PP of crystalline polypropylene (A) that can be directly measured, melt flow rate MFR _{WHOLE of} the resulting polyolefin resin (C), and content of copolymer (B) in polyolefin resin (C) The MFR ratio can be obtained by calculating MFR _RC from W _RC (weight%) by the following formula.
log (MFR _RC ) = {log (MFR _WHOLE ) − (1−W _RC / 100) log (MFR _PP )} / (W _RC / 100)

(2) Resin Composition for Forming a Polyolefin Resin Battery Separator The resin composition that is a molding material for the film-shaped molded product for forming the polyolefin resin battery separator of the present invention is usually in addition to the polyolefin resin (C). An antioxidant, a neutralizing agent, an α crystal nucleating agent, a β crystal nucleating agent, a surfactant, and the like used in the polyolefin can be blended as necessary.

  Antioxidants include tetrakis [methylene-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate] methane, 2,6-di-t-butyl-4-methylphenol, Phenolic compounds such as n-octadecyl-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) propionate and tris (3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate Antioxidant, or tris (2,4-di-t-butylphenyl) phosphite, tris (nonylphenyl) phosphite, distearyl pentaerythritol diphosphite, tetrakis (2,4-di-t-butylphenyl) Examples thereof include phosphorus-based antioxidants such as -4,4'-biphenylene-diphosphonite.

  Examples of neutralizing agents include higher fatty acid salts such as calcium stearate. Examples of inorganic fillers and anti-blocking agents include calcium carbonate, silica, hydrotalcite, zeolite, aluminum silicate, magnesium silicate, and the like. Can be exemplified by higher fatty acid amides such as stearic acid amide, and the antistatic agent can be exemplified by fatty acid esters such as glycerol monostearate.

  Alpha crystal nucleating agents include talc, aluminum hydroxy-bis (4-t-butylbenzoate), 1,3,2,4-dibenzylidene sorbitol, 1,3,2,4-bis (p-methylbenzylidene) Sorbitol, 1,3,2,4-bis (p-ethylbenzylidene) sorbitol, 1,3,2,4-bis (2 ′, 4′-dimethylbenzylidene) sorbitol, 1,3,2,4-bis ( 3 ', 4'-dimethylbenzylidene) sorbitol, 1,3-p-chlorobenzylidene-2,4-p-methylbenzylidenesorbitol, 1,3,2,4-bis (p-chlorobenzylidene) sorbitol, sodium-bis (4-t-Butylphenyl) phosphate, sodium-2,2′-methylene-bis (4,6-di-t-butylphenyl) phosphate Calcium-2,2′-methylene-bis (4,6-di-t-butylphenyl) phosphate, aluminum dihydroxy-2,2′-methylene-bis (4,6-di-t-butylphenyl) Known α-crystal nucleating agents such as phosphate can be used. These may be used alone or in combination of two or more.

  The amount of these additives can be appropriately selected depending on the required characteristics of the polyolefin resin battery separator, but it is usually preferably about 0.001 to 5% by weight based on the total amount of the resin composition.

  Further, the resin composition for forming the polyolefin resin battery separator of the present invention includes a propylene homopolymer and a monomer other than propylene containing propylene as a main component within a range not impairing the effects of the present invention. Two or more random polymers, and other olefin resins such as polyethylene resin, polybutene resin, and polymethylpentene resin may be used in combination.

  Furthermore, an ethylene-diene elastic copolymer, an ethylene-propylene elastic copolymer polymerized with a single site catalyst or a known multisite catalyst in order to reduce the plugging temperature of the pores of the battery separator or to impart flexibility. An elastic copolymer such as a styrene-butadiene elastic copolymer may be added.

  The method for blending the polyolefin resin (C) and the above additives is not particularly limited, and is blended by a usual blending device such as a mixer with a high-speed stirrer such as a Henschel mixer (trade name), a ribbon blender, and a tumbler mixer. A method (dry blending) can be exemplified, and further a pelletizing method using a normal single screw extruder or twin screw extruder can be exemplified.

(3) Formation of polyolefin resin battery separator The polyolefin resin battery separator of the present invention is obtained by melting and kneading the resin composition containing the polyolefin resin (C) as a main component to form a film-like melt. After forming into a film-shaped molded product within a draft ratio of 1 to 5, the film-shaped molded product can be formed by stretching in at least one direction at a temperature of 100 ° C. or lower. The process consists of a film forming process and a stretching process.

(i) Film-forming process In the film-forming process for obtaining a film-shaped molded product from the resin composition, methods such as a known inflation film molding method, T-die film molding method, and calendar molding method are used. A T-die film forming method with high thickness accuracy and easy multilayering is preferably used.

  The resin composition can be formed at an extrusion temperature of 180 ° C. or higher. The purpose is to reduce the pressure inside the die and reduce the draft ratio described later, and the rigidity of the crystalline polypropylene (A) as the matrix polymer. An extrusion temperature of 220 to 300 [deg.] C. is preferably used in order to improve the viscosity and make it easy to produce uniform and fine pores in the copolymer (B) region dispersed in the crystalline polypropylene (A).

The melt-kneaded resin composition is extruded from the die lip. At this time, the linear velocity V _{CL in} the flow direction (MD) of the resin composition passing through the die lip and the flow direction (MD) line of the film-shaped molded product The draft ratio (V _CL / V _f ) defined by the ratio of the speed V _f is an important factor for achieving the present invention. Generally, the draft ratio is about 10 to 50 when a thermoplastic resin film is formed. In the present invention, the draft ratio at the time of forming the resin composition is 1 to 5, preferably 1 to 3, and the film-shaped product obtained thereby has excellent stretchability and is finely communicated by stretching. Fine pores are easily formed.

  Further, the rigidity of the crystalline polypropylene (A) which is the matrix polymer is improved, and uniform and fine pores are generated in the propylene-α-olefin copolymer (B) region dispersed in the crystalline polypropylene (A). In order to facilitate the cooling of the film-like molded product extruded from the die lip, it is desirable to cool slowly, and it is desirable to cool the temperature of the cooling roll in the range of 60 to 120 ° C., more preferably 70 to 110 ° C. . When the roll temperature is less than 60 ° C., it is difficult to obtain the desired porosity, and when it exceeds 120 ° C., the molten resin tends to adhere to the roll, resulting in poor productivity.

  The thickness of the film-like molded product obtained in the film forming process is not particularly limited, but is determined by the stretching and heat treatment conditions in the next stretching process and the required characteristics of the battery separator, and is 20 μm to 2 mm, preferably 50 μm. The film forming speed is preferably in the range of 1 to 100 m / min. The film-shaped moldings having these thicknesses include a combination of an inflation molding apparatus, an air knife having the cooling roll and an air outlet, the cooling roll and a pair of metal rolls, the cooling roll and a stainless steel belt, and the like. It can be obtained by various film forming apparatuses such as a T-die film forming apparatus and a calendar forming apparatus.

  Furthermore, the polyolefin resin battery separator of the present invention is formed into a film by coextruding a resin composition containing a known inorganic filler, organic filler, etc. with the polyolefin resin battery separator forming resin composition of the present invention. It doesn't matter as a thing.

  In addition, you may heat-process in order to further improve a crystallinity degree before using for the obtained film-form molding to the next extending process. The heat treatment is performed, for example, by heating at a temperature of about 80 to 150 ° C. for about 1 to 30 minutes with a heated air circulation oven or a heating roll.

(ii) Stretching process The film-shaped molded product formed in the film-forming process is then stretched in at least one of the machine direction (MD) direction or the transverse (TD) direction, and into the crystalline polypropylene (A). Fine pores communicating with the finely dispersed propylene-α-olefin copolymer (B) region are formed. In this respect, the production method of the present invention is fundamentally different from the conventional single-component stretching method, multi-component stretching method, mixed extraction method, and the like. Thus, the production method of the present invention is a single component stretching method in which pores are expressed by fibrillation between lamellar crystals of a crystalline polyolefin (A), such as complicated extraction and drying steps such as a mixed extraction method. In addition to eliminating the need for a crystallization step by heat treatment after film formation as seen in Fig. 1, the stretchability is poor and the average pore size is large in the case of the multicomponent stretching method that creates voids at the interface between the matrix polymer and the filler. Problems such as easy and low porosity are greatly improved, and battery separators having an arbitrary average pore diameter and porosity can be provided with excellent productivity.

  In addition to the uniaxial stretching method of stretching in one direction, the stretching method includes a sequential biaxial stretching method of stretching in the other direction after stretching in one direction, a simultaneous biaxial stretching method of stretching simultaneously in the longitudinal and transverse directions, and There are a method of performing multistage stretching in a uniaxial direction, a method of further stretching after sequential biaxial stretching and simultaneous biaxial stretching, and any method may be used. Incidentally, since the film-shaped molded product is drafted in the film-forming process, even a film-shaped molded product formed at a low draft ratio is a finely dispersed copolymer (in the crystalline polypropylene (A)). B) is oriented along the resin flow direction, that is, the longitudinal (MD) direction, and the first stage of stretching is preferably performed by a uniaxial stretching method in the transverse direction or a simultaneous biaxial stretching method in the longitudinal and transverse directions. A sequential biaxial stretching method in which stretching in the longitudinal direction in the first stage and stretching in the lateral direction in the second stage may be performed.

The first stage stretching temperature is preferably lower than the melting point T _mα of the copolymer (B), and a temperature range of 10 to 100 ° C. is preferably used. In the present invention, the polyolefin resin (C) is specified. It was found that by using the composition, the stretchability in these low temperature regions was excellent. The draw ratio is not particularly limited and is determined from the second-stage drawing conditions performed as necessary and the required characteristics of the battery separator, and is usually in the range of 1.5 to 7 times.

  If the draw ratio is in the above range, a battery separator having excellent characteristics can be obtained, and there is no risk of a decrease in productivity due to frequent draw breaks. In the case of simultaneous biaxial stretching, the area ratio (= longitudinal stretching ratio × lateral stretching ratio) is preferably 2 to 50 times, and more preferably 4 to 40 times. If the area magnification is in this range, a battery separator having excellent characteristics can be obtained, and there is no fear of a decrease in productivity due to frequent stretching.

The battery separator of the present invention performs second-stage stretching as necessary, and the second-stage stretching temperature is preferably 10 ° C. or lower than the melting point T _mc of the crystalline polypropylene (A). Moreover, when this _{extending | stretching} temperature is higher than melting _| fusing point Tm ( _alpha ) of a copolymer (B), there exists a tendency for the porosity not to increase so much and to reduce the thickness of the battery separator obtained. Furthermore, when the _stretching temperature is lower than T _mα , the porosity increases, but the thickness tends not to decrease much.

  The stretch ratio of the second stage is determined by the required characteristics of the battery separator, but is preferably in the range of 1.5 to 7 times. If the draw ratio is within the above range, the drawing effect will be sufficient, and there is no possibility that the productivity will be lowered due to the drawing being cut.

  It is preferable that the membrane-shaped molded product that has been formed into pores by the above stretching step and then becomes porous is then heat-treated. This heat treatment is mainly intended for heat fixation for maintaining the formed pores, and is usually carried out on heating rolls, between heating rolls or through a hot air circulating furnace.

In this heat treatment (heat setting), the film-like molded product that has become porous while maintaining the stretched state is heated to a temperature 5 to 60 ° C. lower than the melting point T _mc of the crystalline polypropylene (A). It is carried out by setting it to 50%. When the heating temperature is higher than the above upper limit temperature, the formed pores may be clogged, and when the temperature is lower than the above lower limit temperature, heat fixation tends to be insufficient, and the pores are clogged later, Further, when used as a battery separator made of polyolefin resin, thermal shrinkage easily occurs due to temperature change.

  The thickness of the polyolefin resin battery separator of the present invention is not particularly limited, but is preferably 5 to 100 μm, more preferably 10 to 50 μm from the viewpoint of productivity.

  The polyolefin resin battery separator of the present invention can be subjected to a hydrophilic treatment such as a surfactant treatment, a corona discharge treatment, a low temperature plasma treatment, a sulfonation treatment, an ultraviolet treatment, and a radiation graft treatment, if necessary. Various coating films can be formed.

In addition, the battery separator made of polyolefin resin of the present invention may be used by laminating a nonwoven fabric, a woven fabric, a porous film made of another material, or the like for the purpose of imparting known properties such as reinforcing the separator itself or improving battery capacity. Absent.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these.
The polyolefin resin (C) used in Examples and Comparative Examples polymerizes crystalline polypropylene (A) at the first stage by a continuous polymerization method, and propylene-α-olefin copolymer (B) at the second stage. It was obtained by polymerizing (propylene-ethylene copolymer).
In addition, the measuring method and evaluation method used by the Example and the comparative example are as follows.

(1) Porosity: The bulk specific gravity was determined from the battery separator sample 100 × 100 mm, and from the non-porous membrane-shaped molded product sample 100 × 100 mm before stretching, an automatic specific gravity meter DENSIMETER manufactured by Toyo Seiki Seisakusho, The true specific gravity was calculated | required by DS and the porosity was calculated | required from the following formula.
Porosity (%) = (1-bulk specific gravity / true specific gravity) × 100

(2) Maximum pore size: The maximum value of the length in the major axis direction of the pore was determined as the maximum pore size by observation with a scanning electron microscope (SEM) of the longitudinal (MD) and lateral (TD) cross sections.

(3) Melt flow rate (MFR): Measured in accordance with JIS K 7210 under conditions of a temperature of 230 ° C. and a load of 21.18N.

(4) Air permeability resistance (Gurley): an index indicating air permeability, and the time required for 100 ml of air to pass through was measured with a B-type Gurley densometer (manufactured by Tester Sangyo Co., Ltd.) in accordance with JIS P 8117. .

(5) Hole closing temperature T _s and film breaking temperature T _b : A sample fixed to a 3-inch circular holder is left in a constant temperature bath at a temperature range of 120 ° C. to 170 ° C. for 5 hours and heat-treated. , air resistance (the Gurley) was measured to the temperature at which 10,000 seconds / 100ml and pore blocking temperature _{T s.} Further, the temperature was equally heat treated film tearing occurs with the membrane broken temperature T _b.

(6) Stretchability: A test piece having a width of 40 mm and a length of 100 mm and having a length direction of the longitudinal direction (MD) or a transverse direction (TD) was prepared from a film-shaped molded article. The test piece was stretched uniaxially every 0.5 times in the length direction under the conditions of a stretching temperature of 23 ° C. and a deformation rate of 200% / second, and the stretch ratio not to stretch and break was defined as the stretchable ratio, and the stretchability was evaluated. . The higher the stretchable ratio, the better the stretchability, and the easier it is to form a film-like molded product, the higher the porosity is.

1) Preparation of resin composition for battery separator formation Tetrakis [methylene-3- (3 ′, 5′-di-t-butyl) was added as a phenolic antioxidant to the polyolefin resin (C) shown in Example 1 of Table 1. -4'-hydroxyphenyl) propionate] 0.1% by weight of methane, 0.1% by weight of tris (2,4-di-t-butylphenyl) phosphite as a phosphorus antioxidant, and steer as a neutralizing agent After blending 0.1 wt% of calcium phosphate and mixing with a Henschel mixer (trade name), the mixture was melt kneaded and pelletized using a twin screw extruder (caliber 50 mm) to obtain a resin composition for forming a battery separator.

2) Creation of battery separator [Film forming process / Creation of unstretched film-like molded product]
Using a 20 mm extruder equipped with a T die having a lip width of 120 mm, the pellet-shaped resin composition was melted at an extrusion temperature of 280 ° C. and a discharge rate of 4 kg / h, and the clearance was adjusted to 0.20 mm. The film was extruded from the lip into a film and cooled and solidified on a cooling roll at 80 ° C. to prepare a film-shaped molded product having a width of 100 mm and a thickness of 200 μm. When the film-like molded product in a molten state was cooled and solidified, the non-contact surface with the cooling roll was air-cooled with an air knife.
Table 1 shows the evaluation results of the stretchability of the obtained film-like molded product.

3) [Stretching process / Creation of battery separator]
The film-shaped molded product is stretched in the transverse direction (TD direction) under the conditions of a stretching temperature of 23 ° C., a deformation rate of 200% / second, and a stretching ratio of 3 times while constraining the machine direction (MD direction). A polyolefin resin battery separator was obtained by stretching in the machine direction (MD direction) under conditions of a stretching temperature of 100 ° C., a deformation rate of 1,000% / second, and a stretching ratio of 3 times. Table 1 shows the characteristics of the obtained battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 1 except that the polyolefin resin (C) shown in Example 2 of Table 1 was used. Table 1 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A battery separator made of polyolefin resin was obtained in the same manner as in Example 1 except that the polyolefin resin (C) shown in Example 3 of Table 1 was used. Table 1 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 1 except that the polyolefin resin (C) shown in Example 4 of Table 1 was used. Table 1 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

Using a polyolefin resin (C) shown in Example 5 of Table 1, a polyolefin resin battery separator was obtained in accordance with Example 1. In Example 5, during the stretching in the transverse direction, there were frequent breaks under the condition of a stretching ratio of 3 times, so that the cell was made of a polyolefin resin battery separator by stretching at a stretching ratio of 2.5.
Table 1 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 1)
A polyolefin resin battery separator was obtained in accordance with Example 1, using the polyolefin resin shown in Comparative Example 1 of Table 1 in place of the polyolefin resin (C). Table 1 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 2)
Using the polyolefin resin (C) shown in Comparative Example 2 in Table 1, a polyolefin resin battery separator was prepared according to Example 1. In Comparative Example 2, when stretched in the transverse direction, the stretch breakage occurred at a stretch ratio of less than 1.5 times, resulting in poor stretchability. With a slight stretch of about 1.2 times the transverse stretch ratio, the characteristics as a battery separator were It was not obtained.

(Comparative Example 3)
Instead of the polyolefin resin (C), 50% by weight of a propylene homopolymer (crystalline polypropylene having an MFR of 2.5 g / 10 min) and an ethylene homopolymer (MFR of 0.75 g / min (temperature 190 ° C., load 21. 18N) HDPE) A polyolefin resin battery separator was prepared according to Example 1 except that 50% by weight was used. However, when stretched in the transverse direction, stretch breakage occurred at a stretch ratio of less than 1.5 times. The stretchability was inferior. The characteristics as a battery separator could not be obtained by a slight stretching at a lateral stretching ratio of about 1.2 times.

  A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the die lip clearance was adjusted to 0.6 mm in the film forming step. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the die lip clearance was adjusted to 1.2 mm in the film forming step. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 4)
A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the die lip clearance was adjusted to 2.0 mm in the film forming step. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the transverse draw ratio was 5 times and the longitudinal draw ratio was 6 times. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the transverse stretching temperature was 80 ° C. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 5)
A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the transverse stretching temperature was 120 ° C. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 4 except that the stretching in the vertical direction was not performed and only the stretching in the horizontal direction was performed. Table 2 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

    Performed in the same manner as in Example 1 except that the polyolefin resin (C) shown in Example 11 of Table 3 was used to adjust the lip clearance of the T die to 0.4 mm and the lateral stretching temperature was set to 80 ° C. did. In Example 11, when the film was stretched in the transverse direction, there were many stretch breaks under the condition of a stretch ratio of 3 times, so that the battery separator made of polyolefin resin was stretched at a stretch ratio of 2.5 times. Table 3 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A battery separator made of polyolefin resin was obtained in the same manner as in Example 11 except that the polyolefin resin (C) shown in Example 12 of Table 3 was used. Table 3 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A battery separator made of polyolefin resin was obtained according to Example 11 except that the polyolefin resin (C) shown in Example 13 of Table 3 was used. Table 3 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 11 except that the polyolefin resin (C) shown in Example 14 of Table 3 was used. Table 3 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 12 except that the lip clearance of the T die was 0.2 mm in the film forming step. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 12 except that the lip clearance of the T die was 1.2 mm in the film forming step. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  A polyolefin resin battery separator was obtained in the same manner as in Example 12, except that the stretching in the longitudinal direction was not carried out and only the stretching in the transverse direction was carried out. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

  According to Example 12, except that in the stretching step, the first stage stretching was stretched 3 times in the longitudinal direction at a stretching temperature of 23 ° C., and the second stage stretching was stretched 3 times in the transverse direction at a stretching temperature of 80 ° C. Thus, a battery separator made of polyolefin resin was obtained. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 6)
A polyolefin resin battery separator was obtained in the same manner as in Example 12 except that the lip clearance of the T die was set to 2.0 mm in the film forming step. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

(Comparative Example 7)
In the film forming step, a polyolefin resin battery separator was obtained in the same manner as in Example 12 except that the temperature of the cooling roll was set to 30 ° C. Table 4 shows the stretchability of the film-like molded product and the characteristics of the battery separator.

For polyolefin resin battery separator obtained in Example 6, the results of measurement of the film tearing temperature _{T b} and the pore closing temperature _{T s, T} b 170 ° C., _{respectively,} were T s 140 ° C.. ΔT was 30 ° C. and had an excellent shutdown function as a battery separator.

For polyolefin resin battery separator obtained in Example 16, the results of measuring the film tearing temperature _{T b} and the pore closing temperature _{T s, T} b 170 ° C., _{respectively,} were T s 160 ° C.. ΔT was 10 ° C. and had a slight shutdown function as a battery separator.

(Comparative Example 8)
With respect to a commercially available battery separator (trade name Celgard 2400, manufactured by Celgard Co., Ltd., thickness 27 μm, porosity 38%, air permeability 600 sec / 100 ml), the film breaking temperature T _b and the pore closing temperature T _s were measured. result, _{T b} is a was a 165 ° C., _{T s} because it did not become a 160 ° C. but there air resistance is at 710 seconds / 100ml 10,000 seconds / 100ml or higher is not measurable, as a battery separator The shutdown function was not recognized.

Using the polyolefin resin battery separator obtained in Example 6, the positive electrode was LiCoO ₂ , the negative electrode was lithium metal, and the electrolyte was an ethylene carbonate / dimethyl carbonate (1: 2) mixed solution containing 1 mol / L of LiPF ₆ . A coin-type secondary battery was created, and the charging condition was changed from 3.0 V to 4.2 V (0.8 mA) and the discharging condition from 4.2 V to 3.0 V (0.8 mA) in an environment of 30 ° C. and 100 cycles. The charge / discharge test was conducted. The discharge capacity at the 100th cycle was 119 mAh / g with respect to the discharge capacity at the first cycle of 140 mAh / g, and the characteristics as a secondary battery were in a range where there was no problem.

(Comparative Example 9)
Instead of using the polyolefin resin battery separator obtained in Example 6, a commercially available battery separator (trade name Cellguard 2400, manufactured by Celgard, thickness 27 μm, porosity 38%, air resistance 600 sec / 100 ml) A 100-cycle charge / discharge test was conducted in the same manner as in Example 21 except that. The discharge capacity at the 100th cycle was 117 mAh / g with respect to the discharge capacity at the first cycle of 138 mAh / g.

(Table 1)

(Table 2)

(Table 3)

(Table 4)

  The polyolefin resin battery separator of the present invention is a battery separator that can increase the capacity of the battery because of its high porosity, and has an excellent shutdown function, including a lithium primary battery, a lithium ion secondary battery, and It is suitably used for organic solvent battery separators such as lithium secondary batteries.

It is a conceptual diagram which shows the observation surface of the battery separator made from polyolefin resin of this invention. It is a scanning electron micrograph (magnification: 5000 times) of MD cross section (Edge View) of the polyolefin resin battery separator obtained in Example 4. It is a scanning electron micrograph (magnification: 5000 times) of the TD cross section (End View) of the polyolefin resin battery separator obtained in Example 4. 4 is a transmission electron micrograph of a TD cross section (End View) in the vicinity of a polypropylene-α-olefin copolymer region of a polyolefin resin battery separator obtained in Example 4. FIG. The conceptual diagram explaining FIG. It is a scanning electron micrograph (magnification: 5000 times) of MD cross section (Edge View) of the polyolefin resin battery separator obtained by the evaluation of stretchability in Example 1. 2 is a scanning electron micrograph (magnification: 5000 times) of a TD cross section (End View) of a battery separator made of polyolefin resin obtained by evaluation of stretchability in Example 1. FIG.

Explanation of symbols

A: Crystalline polyolefin (A)
B: Propylene-α-olefin copolymer (B)
D: pores formed in the copolymer (B) region

Claims (10)

A resin composition containing a polyolefin resin (C) composed of crystalline polypropylene (A) and a propylene-α-olefin copolymer (B) dispersed in the crystalline polypropylene (A) is melt-kneaded to form a film. A battery separator formed by forming a melt and forming the film-shaped melt into a film-shaped molded product, and then stretching the film-shaped molded product in at least one direction, wherein the polyolefin resin (C) is crystalline. Polyolefin having 30 to 90% by weight of polypropylene (A) and 10 to 70% by weight of propylene-α-olefin copolymer (B) and having pores communicating with the region of propylene-α-olefin copolymer (B) Resin battery separator. The melt flow rate ratio MFR _PP / MFR _RC of the melt flow rate MFR _PP of the crystalline polypropylene (A) and the melt flow rate MFR _RC of the propylene-α-olefin copolymer (B) is 0.1 to 10. The polyolefin resin battery separator according to claim 1. The polyolefin resin battery separator according to claim 2, wherein the melt flow rate ratio MFR _PP / MFR _RC is 0.2 to 5. The polyolefin resin battery separator according to any one of claims 1 to 3, wherein a draft ratio when the film-shaped melt is formed into a film-shaped molded product is in the range of 1 to 10. The polyolefin resin battery separator according to any one of claims 1 to 3, wherein a draft ratio when the film-shaped melt is formed into a film-shaped molded product is in a range of 1 to 3. The polyolefin resin (C) is composed of 40 to 70% by weight of crystalline polypropylene (A) and 30 to 60% by weight of a polypropylene-α-olefin copolymer (B). A battery separator made of polyolefin resin according to item 1. 7. The polyolefin resin battery separator according to claim 1, wherein the propylene-α-olefin copolymer (B) has a propylene content of 30 to 80 wt%. The battery separator made of polyolefin resin according to any one of claims 1 to 7, wherein the propylene content of the propylene-α-olefin copolymer (B) is 40 to 70% by weight. The polyolefin resin (C) is obtained by a multistage polymerization method including the steps of producing a crystalline polypropylene (A) in the first stage and continuously producing a propylene-α-olefin copolymer (B) in the second stage. 9. The polyolefin resin battery separator according to claim 1, wherein the battery separator is made of polyolefin resin. The air permeability resistance (Gurley) is 1 to 2,000 seconds / 100 ml, the membrane breaking temperature Tb is 150 ° C. or higher, and the difference ΔT (T _b −T _s ) between the membrane breaking temperature T _b and the hole closing temperature T _s The polyolefin resin battery separator according to claim 1, wherein the temperature is 20 ° C. or higher.

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