JP2019196470A - Low heat shrinking polyolefin microporous film and its production method - Google Patents

Abstract

To provide a low heat shrinking polyolefin microporous film having excellent low shrinking property during high temperature as a separator for improving safety of a lithium ion secondary battery and combining a shut-down property and a melt-down property, and furthermore having useful characteristics such as air permeability, compressive resistance, electrolyte impregnation property, mechanical characteristics, appropriate surface roughness and high productivity and its production method.SOLUTION: A polyolefin mixture is made of 25 to 50 mass% of ultrahigh molecular weight polyethylene, 1 to 15 mass% of polyethylene, 35 to 65 mass% of copolymer resin of 4-methyl-1-pentene and α-olefin having 3 or more carbon atoms, 0.1 to 2 mass% of ethylene-ethylene/butylene-ethylene block copolymer, and 0.5 to 5 mass% of propylene-based elastomer having a special micro-structure covering an entire amorphous part by taking a network structure by mutually joining "islands" that are spiral crystalline parts of nano-order level of 10 nm to 50 nm, or, a gel-like sheet molded material obtained by adding, as needs arise, fine particles of an inorganic filler, by melting and kneading a plasticizer for film forming, followed by extruding from a die, after a gel-like sheet molded object obtained by cooling is elongated, a solvent for film formation is removed, furthermore followed by heat fixing. A product has a shut-down temperature in the neighborhood of 135°C, non-meltdown characteristics of not breaking a film during a high temperature of 195°C or higher, low-shrinking property during high temperature, relatively large surface roughness, and excellent electrolyte impregnation property. A single layer low heat shrinking polyolefin microporous film having excellent also in other characteristics, that is, a balance of air permeability, compression resistance, and mechanical characteristics, and its production method are provided.SELECTED DRAWING: None

Description

The present invention relates to a low heat shrinkable polyolefin microporous membrane and a method for producing the same.
Specifically, it has a shutdown temperature of 140 ° C. or lower, which is useful as a separator for enhancing the safety of lithium ion secondary batteries, and is not melted (non-melt down) until the liquid electrolyte is thermally decomposed and deactivated by 190 ° C. or higher. The present invention relates to a low heat-shrinkable polyolefin microporous membrane having both of the above properties at the same time and excellent in low shrinkage at high temperatures, mechanical properties, and air permeability, and a method for producing the same.

Polyolefin microporous membranes are used in various applications such as battery separators, electrolytic capacitor membranes, various filters, moisture permeable and waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When the polyolefin microporous membrane is used as a battery separator, particularly as a separator for a lithium ion secondary battery, its performance is deeply related to battery characteristics, battery productivity, and battery safety. Therefore, excellent air permeability, mechanical characteristics, heat resistance, low shrinkage, shutdown characteristics, non-meltdown characteristics, and the like are required. For example, when the mechanical strength is low, when used as a battery separator, the voltage of the battery may decrease due to a short circuit of the electrode. If metal foreign matter is mixed in the battery, if the puncture strength of the battery separator is low, the electrode is short-circuited, leading to abnormal heat generation of the battery.
In recent years, lithium ion secondary batteries have become widespread as main power sources for portable electronic devices such as notebook personal computers, mobile phones, and integrated camcorders. Due to the demand for higher performance and longer driving time of these portable electronic devices, technological development for further increase in energy density, higher capacity, and higher output is being promoted in lithium ion secondary batteries.
On the other hand, from the viewpoint of the safety of large lithium-ion secondary batteries as power sources for hybrid vehicles, electric vehicles (EV), and aircraft, the separator shrinks and melts, causing a short circuit due to membrane breakage. You must avoid going to. In addition to shutdown characteristics, it is also required to have sufficient heat resistance from the viewpoint of non-meltdown (non-meltdown) characteristics.

The current lithium-ion secondary battery separator uses a polyethylene microporous membrane mainly composed of polyethylene, which has shutdown characteristics as a function to ensure the safety of the battery. It is difficult to devise not to go down, judging from the melting point of polyethylene. Recently, in order to improve heat resistance, a heat resistant resin containing ceramic particles is further applied on a polyethylene microporous film or polypropylene microporous film and then dried. However, it requires additional equipment for application, and it is necessary to apply as thin a layer as possible so that the pores thus formed are easily closed in the application process and the air permeability related to lithium ion conductivity is well maintained. This is contrary to the heat resistance and increases the product cost. In addition, the ceramic coating film impairs the flexibility of the polyolefin microporous film, and may cause cracks, especially in the vicinity of the winding shaft of the cylindrical battery, and powder generation of the coating film. On the other hand, the polyolefin microporous membrane has an advantage that there is almost no deterioration due to the electrolytic solution. Recently, regarding the characteristics of the separator, whether lithium ion conductivity is good or not can be easily judged from the measurement result of the porosity and the test result of the air permeability.
In addition to mechanical strength, characteristics related to battery life such as cycle characteristics and characteristics related to battery productivity such as electrolyte injection properties are also emphasized. In particular, the electrode of the lithium ion secondary battery repeats expansion / contraction associated with charge / discharge. Therefore, loading / releasing of the force applied to the separator in the thickness direction is repeated. In order to obtain a long-life battery, it is necessary to improve the adhesion at the interface between the separator and the electrode. Due to the increase in electrode size and electrode density accompanying the increase in capacity of batteries in recent years, the pressure on the separator tends to become stronger during battery assembly. In order to maintain the battery characteristics in such a situation, it is required that the change in permeability of the separator due to the compression is small. If the separator is easily compressed, there is a high risk of battery capacity reduction (deterioration of cycle characteristics). Moreover, due to the increase in the electrode size accompanying the increase in capacity of the battery as described above, the electrolyte injection property to the battery is reduced, which causes a decrease in battery productivity. The electrolyte injection property must also be taken into account when improving the separator.

JP 2011-184671 A Patent No. 5766291 JP 2017-88836 A

"Lithium ion secondary battery second edition", Nikkan Kogyo Shimbun 1996, p. 107-p. 120 “Unknown battery development secret story”, KEE Co., Ltd. 2015, p. 101-p. 105

Patent Documents 1 and 2 propose that a microporous film is obtained by mixing an olefin-based elastomer with a poly (4-methylpentene-1) resin (PMP) and a polyethylene-based resin.
Patent Document 3 proposes mixing an olefin block copolymer containing ethylene as a constituent component.
Non-Patent Document 1 shows the characteristics of a single-layer polyethylene microporous membrane, a single-layer polypropylene microporous membrane, and a three-layer polyethylene micropolypropylene membrane made of polyethylene / polypropylene / polyethylene. The above shows that the characteristics cannot be maintained thermally. Non-Patent Document 2 shows that there is a heat-resistant polyolefin microporous membrane having shutdown characteristics and low shrinkage at high temperatures, but details are not disclosed.

  It has a shutdown temperature of 140 ° C or lower, which is useful as a separator that enhances the safety and life of lithium ion secondary batteries, and is non-melted (non-melt down) until the liquid electrolyte is thermally decomposed and deactivated to 190 ° C or higher It is to provide a low heat-shrinkable polyolefin microporous membrane having both properties at the same time and excellent in low shrinkage at high temperatures, mechanical properties, and air permeability, and a method for producing the same.

  The present inventors tried to improve the heat resistance of a microporous polyethylene film by paying attention to a copolymer resin of 4-methyl-1-pentene and an α-olefin having 3 or more carbon atoms as a heat-resistant polyolefin. Polypropylene-based elastomer having “network” specific microstructure in which “islands” which are helical crystal parts of nano-order level of 10 nm to 50 nm are connected to each other to form a network structure and cover the entire amorphous part as an elastomer resin By interposing an ethylene-ethylene-butylene-ethylene block copolymer having a resin and a polyethylene block at both ends of the copolymer, a melted and kneaded resin mixture can be obtained in the extrusion to obtain a uniform nano-order melt-dispersed resin mixture. did. Even after 1 hour exposure in a circulating hot air dryer at 190 ° C. by finding a resin composition that can be stretched higher, can form a thin film with high tensile strength, and can maintain good air permeability. The present invention has reached the present invention of a microporous polyolefin membrane exhibiting a heat shrinkability of not more than% and a method for producing the same.

  Since the polyolefin microporous film having low heat shrinkability of the present invention has low heat shrinkability up to 190 ° C., it is not necessary to apply ceramic particles for further improving heat resistance. Moreover, it is a polyolefin microporous membrane having a single layer structure, and since flexibility and flexibility are maintained as a separator in the battery assembly process, workability is not impaired. It can be easily manufactured by utilizing the existing wet separator equipment, changing to the resin composition of the present invention, and making a strong kneading screw unit and finely adjusting the processing temperature such as the extruder temperature and the film stretching temperature.

  Further, in the present invention, the life of the capacity retention in the pouch type battery, the prism type battery and the cylindrical can battery in which the surface layer of the polyolefin microporous membrane having a single layer structure is provided with an ultrathin film using an aqueous solution of a crosslinkable polymer as a raw material And a surface-modified polyolefin microporous membrane can be provided.

The microporous membrane of the present invention is suitably produced by a production method including the following steps. Each process will be described in detail below.
(Step 1) Ultrahigh molecular weight polyethylene 25-50% by mass, polyethylene 1-15% by mass, copolymer resin 35-65% by mass of 4-methyl-1-pentene and α-olefin having 3 or more carbon atoms, ethylene -Ethylene / Butylene-Ethylene Block Copolymer 0.1-2% by mass “Islands”, which are helical crystal parts of nano-order level of 10 nm to 50 nm, are connected to each other to form a network structure and the whole amorphous part A polyolefin resin mixture comprising 0.5-5% by mass of a propylene-based elastomer resin having a special microstructure covering the surface, an antioxidant, and if necessary, an inorganic filler powder is supplied to a twin screw extruder, and the plasticizer is heated. Injecting the mixed solution and melt-kneading;
(Step 2) A step of extruding the melt-kneaded material prepared in Step 1 from a T-die, cooling and rolling with a roll, and forming into a gel-like sheet molding;
(Step 3) A step of stretching the gel-like sheet molded product obtained in Step 2 in a biaxial direction (MD direction, TD direction) by a sequential method or a simultaneous biaxial stretching method;
(Step 4) Step of extracting and removing the plasticizer from the thin film obtained in Step 3 with a solvent, and drying;
(Step 5) A step of heat-treating, re-stretching and heat-setting the microporous membrane obtained in Step 4 as necessary;
(Step 6) Applying an aqueous solution of a crosslinkable polymer to the surface layer of the microporous membrane obtained in Step 4 or 5 and drying to form an ultrathin film;

  If necessary, various additives such as an ultraviolet absorber, an antiblocking agent, a nucleating agent, a pigment, a dye, and an inorganic filler are added to the melt-kneaded product of the polyolefin resin mixture as long as the effects of the present invention are not impaired. Can do. For example, fine silica, alumina silica or the like can be added as a pore-forming agent. Alternatively, a so-called ceramic-containing surface layer film may be formed by applying an organic solvent solution or an aqueous solution of a ceramic such as alumina or silica and a binder to the microporous film.

Although the method of melt kneading is not particularly limited, it is usually carried out by uniformly kneading in a twin screw extruder. This method is suitable for preparing a melt of the polyolefin mixture. The melting temperature is preferably in the range of the average melting point of the polyolefin resin + 70 ° C. to + 100 ° C. Specifically, the melting temperature is preferably set in the range of 200 to 300 ° C in the extrusion temperature zone, and more preferably in the range of 210 to 280 ° C. The plasticizer may be added before the start of kneading or may be added in the middle of the twin screw extruder during kneading.
In melt-kneading, it is preferable to add an antioxidant in order to prevent oxidative deterioration of the polyolefin resin mixture.

The screw length (L) and diameter (D) ratio (L / D) of the twin screw extruder is preferably in the range of 30 to 100, more preferably in the range of 40 to 60. If L / D is less than 30, melt kneading may be insufficient. In particular, it becomes difficult to perform micro-complete melt kneading of ultrahigh molecular weight polyethylene, 4-methyl-1-pentene, and a copolymer resin of α-olefin having 3 or more carbon atoms.
When L / D is more than 100, the residence time of the polyolefin mixture melt and the plasticizer is excessively increased, leading to thermal degradation. The shape of the screw is not particularly limited, but it is preferable to select a screw from the viewpoint of melting and kneading components having greatly different resin melting points. The cylinder inner diameter of the twin screw extruder is preferably 26 to 150 mm. When the polyolefin resin mixture is put into the twin screw extruder, the ratio Q / Ns of the amount Q (kg / h) of the polyolefin resin mixture and the plasticizer to the screw rotation speed Ns (rpm) is 0.1 to 0.55 kg. / H / rpm is preferable.
When Q / Ns is less than 0.1 kg / h / rpm, the polyolefin resin resin is excessively sheared, leading to a decrease in strength and shutdown temperature. On the other hand, when Q / Ns is more than 0.55 kg / h / rpm, it may not be uniformly kneaded. The ratio Q / Ns is more preferably 0.2 to 0.5 kg / h / rpm. The screw rotation speed Ns is preferably 60 rpm or more. The upper limit of the screw rotation speed Ns is not particularly limited, but is in the range of 150-300 rpm.

  As a method for forming a gel-like sheet molding, a resin and a plasticizer are melt-kneaded with a twin-screw extruder, and a melt of the polyolefin resin mixture is directly from the twin-screw extruder or from another extruder via a gear pump. It is obtained by extruding from a T-die and cooling. Cooling is preferably performed to at least the gelation temperature or lower. Cooling is preferably performed to 40 ° C. or lower. By cooling to a gelling temperature or lower, the phase separation structure in which the polyolefin resin mixture phase is microphase-separated by the plasticizer can be fixed. In general, when the cooling rate is low, the higher-order structure of the obtained gel-like molded product becomes coarse, and the pseudo cell units forming the gel structure tend to be large. When the cooling rate is high, the cells become dense cell units. When the cooling rate is less than 50 ° C./min, the degree of crystallinity increases, and it is difficult to obtain a gel-like product suitable for stretching. As a cooling method, a method of directly contacting cold air, cooling water, or other cooling medium, a method of contacting a roll cooled by a refrigerant, or the like can be used. In addition, after melt-kneading the resin and the plasticizer with a batch-type kneader, a method of cooling into a sheet / film using a compression molding machine can be used.

  As the die lip, a sheet die lip having a rectangular base shape is usually used, but a double cylindrical hollow die lip, an inflation die lip, or the like can also be used. In the case of a sheet die lip, the die lip gap is usually in the range of 0.2 to 8 mm, and is heated to a temperature of 190 to 250 ° C. during extrusion. The extrusion rate of the melt is preferably in the range of 0.2 to 80 m / min.

  As the stretching method, flat stretching, tubular stretching, roll rolling and the like can be used. Of these, flat stretching is preferred from the viewpoint of stretching uniformity. The stretching temperature is preferably selected within the range of about 120 ° C. to the average melting point of the polyolefin mixture. When the stretching temperature is less than 110 ° C., film breakage due to excessive stretching stress is caused and the melt shrinkage resistance is deteriorated. The upper limit of the stretching temperature is preferably not more than the average melting point of the polyolefin resin mixture constituting the microporous membrane. If it is below the average melting point temperature of the resin mixture, it becomes possible to prevent film breakage due to melting of the resin. When the stretching temperature exceeds the average melting point temperature, the polyolefin resin mixture is melted and molecular chains cannot be oriented by stretching. The stretching temperature is desirably as high as possible in order to reduce the thermal shrinkage of the microporous membrane.

  Biaxial stretching is performed by heating a gel-like sheet molded article and then using a normal tenter method, roll method, inflation method, rolling method, or a combination of these methods. Any of simultaneous biaxial stretching, sequential stretching or multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used. Although the order of biaxial stretching and plasticizer removal can be arbitrarily set, it is preferable to remove the plasticizer after biaxially stretching the gel-like sheet molding. However, it is not intended to limit in this order. For example, after removing the plasticizer from the gel-like sheet molded product, biaxial stretching is performed. The gel-like sheet molded product is biaxially stretched, and then the plasticizer is removed and further biaxially stretched. The plasticizer may be removed during each uniaxial stretching when the gel sheet is sequentially biaxially stretched. Stretching the film before removing the plasticizer at least once in at least one direction (referred to as stretching before removal) and further stretching the film after removing the plasticizer at least once in at least one direction (stretching after removing the plasticizer) Can be implemented). The stretching at least once means that one-stage stretching, multi-stage stretching, or multi-stage stretching is performed. The stretching in at least one direction means longitudinal uniaxial stretching, transverse biaxial stretching, simultaneous biaxial stretching, and sequential biaxial stretching.

  When biaxial stretching is performed after forming a gel-like sheet molded product, the stretching speed in the longitudinal direction (MD) and the transverse direction (TD) is the longitudinal direction and transverse direction when unstretched in the region where the unstretched sheet is stretched. When the length of each is defined as the ratio of the length stretched per minute, the stretching speeds in the machine direction and the transverse direction of biaxial stretching are both 20 times / min or less. When the stretching speed in the longitudinal direction or the transverse direction is more than 20 times / minute, the melt shrinkage resistance is deteriorated. The stretching speed in each direction is preferably 15 times / minute or less, and more preferably 10 times / minute or less.

The stretch ratio is 50 times or less in terms of the total area ratio. The total area magnification is the product of the total draw ratio in TD and the total draw ratio in TD. If the draw ratio is more than 50 times in terms of the total area ratio, the high temperature shrinkage resistance may deteriorate. The stretching ratio is preferably 40 times or less in terms of the total area ratio. The magnification in the uniaxial direction in the machine direction (MD) and the transverse direction (TD) is preferably 3 to 20 times, more preferably 4 to 15 times, and further preferably 6 to 10 times. If the stretching ratio is 4 times or more, sufficient stretching orientation is provided, so that the strength of the microporous film can be improved. If the stretching ratio is 20 times or less, the structure of the microporous film is destroyed due to excessive stretching. It is possible to prevent. Although there is no restriction | limiting in particular in the extending | stretching minimum of each direction, It is preferable that it is 4 times or more from a viewpoint of productivity. From the viewpoint of improving mechanical strength, it is preferable that the total area magnification is 9 times or more by making it 4 times or more in MD and TD, and it is 16 times or more in total area magnification by making it 4 times or more in MD and TD. Is more preferable. The ratio of the total stretch ratio in MD and the total stretch ratio in TD (total stretch ratio in MD) / (total stretch ratio in TD) is not particularly limited, but in any case of simultaneous biaxial stretching, sequential stretching or single-stage stretching 0.5 to 2 is preferable, 0.7 to 1.3 is more preferable, and 1 is most preferable. As long as the stretching speed in each direction is 20 times or less, MD and TD may be different from each other, but are preferably the same.

  Depending on the desired physical properties, the film may be stretched by providing a temperature distribution in the film thickness direction. By providing a temperature distribution in the film thickness direction and stretching, a microporous film generally excellent in mechanical strength can be obtained. You may perform the process (hot roll process) which makes a hot roll contact at least one surface of the stretched gel-like molding before washing | cleaning. The stretched gel-like molded product is brought into contact with the heating roll. The contact time between the heating roll and the stretched gel-like molded product may be 0.1 second to 1 minute. You may make it contact in the state which hold | maintained heating oil on the roll surface. The heating roll may be either a smooth roll or an uneven roll that may have a suction function.

  Since the polyolefin mixture resin phase is phase-separated from the plasticizer for film formation, a porous film can be obtained by removing the plasticizer for film formation using a cleaning solvent. As a method for removing the cleaning solvent and the plasticizer for film formation using the cleaning solvent, known methods can be used.

  Heat treatment (heat drawing treatment and / or heat setting treatment and / or heat shrinking treatment) may be performed at least once after the drawing step or after removal of the plasticizer. The heat treatment temperature is determined in consideration of a change in strength, a change in air permeability and the like due to the heat treatment.

In order to adjust the physical properties such as porosity in the hot stretching process, the tenter method, roll method or rolling method that is usually used is used, and the draw ratio in one direction is at least once and at least uniaxial direction is 1.01-2. It is preferable to carry out at a draw ratio of 0.0, more preferably at a draw ratio of 1.02 to 1.5. The heat setting treatment is performed by a tenter method, a roll method, or a rolling method to stabilize the crystallization of the polyolefin resin in the low heat shrinkable polyolefin microporous film. The heat shrinking treatment may be performed by a tenter method, a roll method or a rolling method, or may be performed using a belt conveyor or a floating roll.
The heat shrink treatment is preferably performed in a range of 50% or less in at least one direction, 20% or less, more preferably 10% or less.

  You may perform combining the above-mentioned heat | fever extending process, heat setting process, and heat shrink process many. In the hot roll treatment, a treatment (hot roll treatment) in which a hot roll is brought into contact with at least one surface of the stretched gel-like molded product before washing may be performed. The stretched gel-like molded product is brought into contact with a heating roll whose temperature is adjusted to a crystal temperature of the polyolefin resin mixture + 10 ° C. or higher and lower than the average melting point of the polyolefin resin mixture. The contact time between the heating roll and the stretched gel-like molded product is preferably 0.5 seconds to 1 minute. You may make it contact in the state which hold | maintained heating oil on the roll surface. The heating roll may be either a smooth roll or an uneven roll that may have a suction function. In particular, it is preferable to perform a heat shrinking treatment after the heat stretching treatment because a microporous membrane having a low shrinkage rate and a high strength can be obtained.

  The film thickness of the low heat-shrinkable polyolefin microporous membrane according to a preferred embodiment of the present invention is 5 to 100 μm, preferably 6 to 40 μm, more preferably 10 to 25 μm. If the film thickness is 5 μm or more, the film has sufficient strength as a microporous film, and if it is 100 μm or less, the film has sufficient permeability and has a high electrolyte solution impregnation property. In addition, although the thickness of a microporous film can be suitably selected according to a use, when using as a separator for small lithium ion secondary batteries, 5-20 micrometers is preferable and 5-12 micrometers is more preferable. The film thickness is measured according to JIS standard K7130. When used as a separator for a large lithium ion secondary battery of an electric vehicle driving power source, 20 to 40 μm is preferable.

  The distribution of the micropore structure region and the coarse pore structure region is not particularly limited. Usually, in both cross sections in the MD direction and TD direction of the microporous membrane, the lamella structure of the fibril fiber of PMP and UHMWPE is also expressed. As a result, the micropore structure region and the coarse pore structure region are irregularly interlaced. The size of each region is also irregular. This structure is presumed to have both shutdown characteristics and non-meltdown characteristics. Such a structure can be observed, for example, with a transmission electron microscope (TEM) or the like, and can be measured as a maximum height difference with an atomic force microscope (AFM).

  Since it has a relatively large space and a relatively large surface roughness due to the coarse pore structure as described above, it has excellent air permeability and electrolyte absorption, and changes in air permeability when pressurized are small. Therefore, when used as a separator for a lithium ion secondary battery, excellent battery productivity and battery cycle characteristics can be realized.

  When the polyolefin microporous membrane of the present invention is used as a lithium ion secondary battery separator, it is possible to maintain high productivity of the battery and to prolong the life of the battery due to excellent cycle characteristics. The polyolefin microporous membrane of the present invention obtained by uniform melting does not break up to 190 ° C. or higher while ensuring low heat shrinkage while being a monolayer membrane that is economically advantageous from the viewpoint of production cost and production equipment. The polyethylene microporous membrane that has so-called non-melt-down characteristics and is not found in other heat-resistant separators also maintains the shutdown temperature that has contributed to battery safety. Further, the fluorine-based electrolyte complex salt used in the lithium ion secondary battery is decomposed at around 190 ° C. and loses lithium ion conductivity and becomes inactive as a battery, so that the runaway of the battery can be suppressed or prevented. Since the separator can prevent a short circuit between the edge portions of the positive electrode and the negative electrode due to the low heat shrinkage characteristic, it is also excellent from the viewpoint of battery safety.

  Next, the low heat-shrinkable polyolefin microporous membrane of the present invention and materials used in the production method will be described.

[1] Ultra high molecular weight polyethylene The ultra high molecular weight polyethylene constituting the polyolefin microporous membrane of the present invention has a viscosity average molecular weight determined from the intrinsic viscosity in the range of 500,000 to 10 million, and the ultra high molecular weight polyethylene is used alone or Two or more types are used in combination. Not only an ethylene homopolymer, but also a copolymer containing a small amount of other α-olefin may be used in combination. Other α-olefins other than ethylene include copolymers with α-olefins having 3 to 30 carbon atoms such as propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. It is done.
Conventionally, ultra-high molecular weight polyethylene of 3.5 million or more was considered to be difficult to extrude and knead, and was rarely used. However, ultra high molecular weight polyethylene with a viscosity average molecular weight of about 1 million was mixed with a plasticizer. By adding ultra-high molecular weight polyethylene so that the difference in viscosity average molecular weight is within 1 to 2.5 million, it is also possible to use ultra high molecular weight polyethylene with a viscosity average molecular weight of over 3.5 million and exceeding 6 million to 10 million. Can be. Furthermore, it has been found that even with ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 7 million to 10 million, even if it is a thin film of 15 μm or less, the tensile strength and the piercing strength can be maintained satisfactorily.

[2] Polyethylene The polyethylene of the present invention has a viscosity average molecular weight of 150,000 to less than 500,000, preferably 200,000 to 450,000, high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene ( LDPE) and linear low density polyethylene (L-LDPE), which are used alone or in combination of two or more. Not only an ethylene homopolymer, but also a copolymer containing a small amount of other α-olefin may be used in combination. Other α-olefins other than ethylene include copolymers with α-olefins having 3 to 30 carbon atoms such as propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. It is done.
The polyethylene mixing ratio is in the range of 1 to 20% by mass.

  For the viscosity average molecular weight, decalin is used as a solvent, the intrinsic viscosity [η] is measured at a measurement temperature of 135 ° C., and the viscosity average molecular weight (Mv) is calculated by the following formula.

[3] Copolymer resin of 4-methyl-1-pentene and α-olefin having 3 or more carbon atoms The present invention uses methyl 4 instead of poly (4-methyl-1-pentene) homopolymer. -Copolymer resin polymer resin of methyl-1-pentene and α-olefin having 3 or more carbon atoms. 4-methyl-1-pentene and at least one α-olefin such as propylene, 1-butene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1- It is a copolymer with an α-olefin having 3 to 30 carbon atoms such as octadecene. A powder is preferable from the viewpoint of miscibility with the ultrahigh molecular weight polyethylene of the present invention, and preferably from 80% by mole or more of 4-methyl-1-pentene because of the miscibility and the heat resistance retained after mixing. It is a copolymer mainly composed of methyl-1-pentene, and the composition ratio of 4-methyl-1-pentene and α-olefin is a melting point (Tm) measured based on a DSC (differential scanning calorimeter) test. ) Is adjusted in the range of 200 to 250 ° C, preferably 220 to 240 ° C. Further, the melt flow rate (MFR) measured under conditions of a load of 5 kg and a temperature of 260 ° C. according to ASTM D1238 is 0.05 to 250 [g / 10 min]. It is preferably in the range of 1 to 100 g / 10 minutes. When the melt flow rate is less than 0.05 g / 10 min, the melt viscosity is high and the moldability is poor, and when the melt flow rate exceeds 250 g / 10 min, the melt viscosity is low and the film formability is poor, and the mechanical strength is also low. In the present invention, a copolymer resin of 4-methyl-1-pentene and an α-olefin having 3 or more carbon atoms is used in the range of 17 to 65% by mass. If it is less than 17% by mass, the degree of improvement in heat resistance of the polyethylene microporous membrane is insufficient, and after heat exposure in a circulating hot air dryer at 190 ° C. for 1 hour, low heat shrinkage is not exhibited, which is not preferable. Moreover, even if it exceeds 65 mass%, the further low heat shrinkability cannot be anticipated, and cost becomes high and is not preferable.

[5] Ethylene-ethylene / butylene-ethylene block copolymer The ethylene-ethylene / butylene-ethylene block copolymer is saturated by hydrogenating the double bond of the butadiene portion of the block copolymer obtained with the living catalyst. This is a modified elastomer. There is Dynaron CEBC (manufactured by JSR Corp. (Dynalon 6200P, 6100P, 6201B)).

[6] Propylene-based elastomer resin A propylene-based elastomer resin and a structural unit derived from an α-olefin having 2 to 30 carbon atoms (excluding propylene) is a nano-order spiral crystal part of 10 nm to 50 nm. The islands are connected to each other to form a network structure and have a microstructure that covers the entire amorphous part. For example, “Notio SN0285”, “Notio PN3560”, “Notio PN2060”, “Notio PN2070”, etc., manufactured by Mitsui Chemicals, Inc.) are available. Use in combination with a copolymer resin of 4-methyl-1-pentene and α-olefin and the ethylene-ethylene-butylene-ethylene block copolymer can minimize the amount of addition. Is optimal. In addition, the flexibility generally decreases when the crystallinity is increased to increase the heat resistance, but when the propylene-based elastomer resin is mixed, the amorphous part is incorporated at the nano level inside the crystal part, Even if the heat resistance is increased, the flexibility does not decrease because it is connected to the surrounding amorphous part.

[7] Plasticizer As the plasticizer that can be used in producing the microporous membrane of the present invention, either a liquid plasticizer or a solid plasticizer can be used. Examples of the liquid plasticizer include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions having boiling points corresponding to these. In order to obtain a gel-like molded article having a stable solvent content, it is preferable to use a non-volatile liquid plasticizer such as liquid paraffin or mineral oil. The viscosity of the liquid plasticizer is preferably in the range of 30 to 500 cSt at 25 ° C., and more preferably in the range of 50 to 200 cSt. When the viscosity of the liquid plasticizer at 25 ° C. is less than 30 cSt, the discharge of the melt of the polyolefin mixture from the die lip is non-uniform and kneading is difficult. On the other hand, if it exceeds 500 cSt, it is difficult to remove the plasticizer.

  The solid plasticizer preferably has a melting point of 80 ° C. or lower. As such a solid plasticizer, waxes such as paraffin wax and microcrystalline wax, higher alcohols such as seryl alcohol and stearyl alcohol, polyoxyethylene stearyl ether, poly Polyoxyethylene alkyl ethers such as oxyethylene isostearyl ether, polyoxypropylene stearyl ether, polyoxypropylene alkyl ether such as polyoxypropylisostearyl ether, polyoxyethylene alkyl esters such as polyoxyethylene stearyl ester, polyoxy And polyoxypropylene alkyl esters such as propylene stearyl ester. You may use it, mixing a liquid solvent and a solid solvent suitably. In particular, it is useful because the mixture is solidified at room temperature by mixing polyoxyethylene alkyl ether such as polyoxyethylene steryl ether and polyoxyethylene isostearyl ether with liquid paraffin. Forming a soft gel-like sheet that can be stretched when the resin and plasticizer are uniformly melt-kneaded at a temperature equal to or higher than their melting points, and then the kneaded product is cooled to below the solidification temperature of the mixed resin at an arbitrary cooling rate. You can get things.

  The amount of the plasticizer that can be used can be changed depending on the compatibility of the resin and the plasticizer in the phase separation structure and the weight ratio of the resin to the total weight of the resin and the plasticizer (polymer mass fraction). For example, the blending ratio of the polyolefin mixture and the plasticizer is 10 to 55 parts by weight of the polyolefin resin mixture and 90 to 45 parts by weight of the plasticizer, preferably 100 parts by weight of the total of both, preferably the polyolefin resin mixture Is 30 to 50 parts by mass and the plasticizer is 70 to 50 parts by mass. When the ratio of the polyolefin resin mixture is less than 10 parts by mass, when extruding the polyolefin mixture melt, swell and neck-in are increased at the die outlet, and the moldability and self-supporting property of the gel-like molded product are lowered. On the other hand, when the proportion of the polyolefin mixture exceeds 55 parts by mass, the film formability of the gel-like sheet molding is lowered, and the pores are small and the gas permeability is poor.

[8] Solvent The solvent for removing the plasticizer is preferably a poor solvent for the resin used, a good solvent for the plasticizer, and a boiling point lower than the melting point of the film. . For example, hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1-trichloroethane, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, acetone and 2- Ketones such as butanone, chain fluorocarbons such as C ₆ F ₁₄ and C ₇ F _16, hydrofluoroethers such as C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ perfluoroether or the like can be used. In order to prevent shrinkage of the film during removal of the plasticizer / solvent, the film is immersed in a solvent while constraining the film in at least one direction, and after removal of the plasticizer, the solvent is dried and removed by a heat drying method, an air drying method or the like below the melting point of the film. This method can be suitably used.

Other polyolefins may be added to such an extent that the characteristics of the low heat shrinkable polyolefin microporous membrane of the present invention are not significantly impaired. Block polymer of polypropylene and ethylene-propylene copolymer is made from propylene and ethylene as raw materials. First, propylene is used as a raw material, using metallocene catalyst, titanium-based catalyst, etc., and flowing in a reactor such as a pipe for several seconds to several minutes. 10-40 mol% of polypropylene is synthesized, 90-60 mol% of a mixture of ethylene and propylene is introduced, and ethylene-propylene copolymer is continuously synthesized in a similar time. At this time, the ratio of ethylene and propylene can be changed, and the block chain length is changed by changing the polymerization time. Further, by repeating this step or changing the polymerization time at the time of repeating, the material containing the multi-block body and the present block copolymer can have any ethylene content. The block copolymer can have compatibility with a copolymer resin of 4-methyl-1-pentene and an α-olefin having 3 or more carbon atoms and polyethylene. For example, R110E, R110MP, T310E, M142E etc. are mentioned as Prime TPO by Prime Polymer Co., Ltd. Further, there are propylene block polymer, qualia manufactured by Sun Allomer Co., Ltd., polypropylene impact copolymer manufactured by Nippon Polypro Co., Ltd., Newcon, tufselen, exelen manufactured by Sumitomo Chemical Co., Ltd., and the like. It is a block polymer of polypropylene, ethylene, and propylene copolymer, which is made of a living catalyst whose active sites are capable of polymerizing both ethylene and propylene, and the active sites are sufficiently stable during the polymerization time. A high one is preferred.
The molecular weight of the polypropylene resin that may be added is not particularly limited, but polypropylene having a viscosity average molecular weight of 100,000 or more and a viscosity average molecular weight of about 400,000 can be used.

  Examples of inorganic fillers that can be added include nano-sized boehmite / alumina having an aspect ratio of alumina and aluminum hydroxide, silica (silicon oxide), sodium aluminosilicate, sodium calcium aluminosilicate titania, zirconia, magnesia, Oxides such as ceria, yttria, zinc oxide, iron oxide, nitrides such as silicon carbide, silicon nitride, titanium nitride, boron nitride, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin Ceramics such as clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand Such as glass fiber and the like. One of these can be used alone, or two or more can be used in combination. Preferred are electrochemically stable silica, alumina and titanium, and silica and aluminosilicate are particularly preferred. The average particle size of the inorganic filler powder particles is preferably 1 nm or more, more preferably 10 nm or more, and the upper limit is preferably 100 nm or less. When the average particle size is 100 nm or less, peeling between the polyolefin resin and the inorganic filler tends not to occur even when stretching or the like is performed, which is preferable from the viewpoint of suppressing the generation of macrovoids. On the other hand, setting the average particle size to 1 nm or more is preferable for ensuring the dispersibility of the inorganic filler particles during melting. By adding inorganic filler particles, the tensile strength of the microporous membrane can be increased and the thermal shrinkage rate can be further reduced.

For example, when n or more types of polyolefin resins having different melting points are used (here, n represents an integer of 3 or more), the melting point T ₁ ° C. of the ultrahigh molecular polyethylene and the melting point T ₂ ° C. or T ₃ ° C. of the polyolefin resin used. ... The average melting point T of the resin mixture consisting of T _n ° C is defined as the following equation.

式中、χ_１ ^＋χ_２ ^＋χ_３・・・χ_ｎ＝１ χ_１；超高分子ポリエチレンの質量分率、χ_２〜χ_ｎ；融点Ｔ_２℃、Ｔ_３℃、・・・Ｔ_ｎ℃の他に使用されるポリオレフィン樹脂のそれぞれの質量分率である。ここで融点とはＪＩＳＫ７１２１に基づいて示差走査熱量測定（ＤＳＣ）により求められた数値を言う。

_{^{Wherein, χ 1 + χ 2 + χ}} 3 ··· χ n = 1 χ 1; mass fraction of ultra high molecular polyethylene, χ ₂ ~χ _n; mp _{T 2 ℃, T 3 ℃,} ··· T n It is the mass fraction of each polyolefin resin used in addition to ° C. Here, the melting point refers to a numerical value obtained by differential scanning calorimetry (DSC) based on JISK7121.

  As a water-soluble crosslinkable polymer to be applied to both or one side of the low heat-shrinkable polyolefin microporous membrane of the present invention, an aqueous solution of a fluoropolymer having an alternating copolymer of fluoroethylene / vinyl ether in the main chain, acrylic resin / polyamide / imide There is one or a mixture of an aqueous resin solution, an acrylic resin / polyimide aqueous solution, an acrylic-modified silicone resin resin aqueous solution, and an aqueous solution composed of polyvinylidene fluoride-hexafluoropropylene and a crosslinkable acrylic resin. Since no organic solvent is used as a solvent, it is environmentally friendly. A surface layer film of 0.1-5 microns, preferably 0.1-2 microns, formed by applying or immersing an aqueous solution and then drying and crosslinking is formed.

  In the surface layer film, the evaporated passages in the micro region occupied by moisture become communication pores. Moreover, the liquid electrolyte solution is required to have a characteristic that it is easily wettable as a surface layer. The liquid electrolyte preferably swells but does not dissolve. Further, it is preferable that the resin applied by biting into the micro-order irregularities on the surface of the separator exhibits an anchor effect.

  The low heat shrinkable polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties.

(1) The air permeability (Gurley value) is 20 to 400 seconds / 100 ml. When the air permeability is within this range, the battery capacity is large when the microporous membrane is used as a battery separator, and the cycle characteristics of the battery are also good. When the air permeability is less than 20 seconds / 100 ml, the shutdown is not sufficiently performed when the temperature inside the battery rises. The air permeability is measured using a Gurley type air permeability meter according to JIS P8117.

(2) The porosity is 20 to 60%, preferably 25 to 40%. When the porosity is 20% or more, sufficient lithium ion conductivity is exhibited, and when the porosity is 60% or less, sufficient mechanical strength is obtained. When used as a battery separator, the risk of short-circuiting the electrodes is small. The porosity is measured by a gravimetric method. A sample is cut into a square of 5.0 cm square, and the volume (cm 3) and weight (g) are measured. The resin density (g / cm ³ ) used is measured according to ASTM D1505. Calculate by the following formula.

(3) The maximum pore size of the microporous membrane is preferably 0.1 to 5 μm, more preferably 0.3 to 3 μm, and still more preferably 0.3 to 1.5 μm. If the maximum pore diameter is 0.1 μm or more, the required permeability as a separator is obtained, and if it is 5 μm or less, it is possible to prevent a short circuit due to an electrode desorption component. The surface of the low heat shrinkable polyolefin microporous membrane is observed with a scanning electron microscope (SEM). Arbitrarily 20 holes are selected, and the diameters of these holes are calculated and averaged.

(4) As for the piercing strength, a piercing test is performed with a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec, and the piercing strength is indicated by N from the maximum piercing load g. It is desirable that it is 1.5N or more. If the piercing strength is less than 1.5 N, there is a possibility that a short circuit of the electrode may occur when the microporous membrane is incorporated into a battery as a battery separator. The puncture strength is 2.0 N or more, more preferably 2.5 or more. A short circuit due to a desorbed component from the electrode does not occur, and the separator has sufficient strength.

(5) The tensile strength at break is measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.
The tensile strength at break is preferably 10 MPa or more in both the MD direction and the TD direction in order to prevent film breakage.

(6) The tensile breaking elongation is measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.
It is preferable that the tensile elongation at break is 5% or more in both the MD direction and the TD direction without fear of film breakage.

(7) The thermal shrinkage after exposure for 1 hour in a circulating hot air dryer at a temperature of 190 ° C. marks the MD and TD directions of a sample cut out to 50 mm × 50 mm. The upper surface is sandwiched between a 451 g glass plate having a size of 210 mm × 297 mm and the lower portion between stainless steel 304 plates for 1 hour. At the first hour, the sample was cooled to room temperature and the dimensional changes in the vertical and horizontal directions were measured. The heat shrinkage ratio was the difference between the area of the first sample piece and the area of the first sample piece after exposure. Divide by to get the percentage. The longitudinal (MD) direction is the extrusion direction during the production of the microporous membrane, and the transverse (TD) direction is the direction orthogonal to the extrusion direction during the production of the microporous membrane. The heat shrinkage rate is preferably 20% or less. Preferably it is 10%, More preferably, it is 5% or less. When the thermal shrinkage rate exceeds 20%, when the microporous membrane is used as a separator for a lithium ion secondary battery, the end of the separator shrinks during abnormal heat generation, and the possibility of occurrence of a short circuit of the electrode increases.

(8) Film thickness change rate after a film is sandwiched between a pair of stainless steel plates having a high smooth surface and heated and compressed at 90 ° C. for 5 minutes under a pressure of 2.2 MPa (22 kgf / cm ² ) using a press machine. Is preferably 20% or less, assuming that the film thickness before compression is 100%. When the film thickness change rate is 20% or less, when the microporous membrane is used as a lithium ion secondary battery separator, the battery capacity is large and the cycle characteristics of the battery are also good. The film thickness is measured with a contact thickness meter (manufactured by Mitutoyo Corporation). The ultimate air permeability (Gurley value) after heat compression under the above conditions is 600 seconds / 100 ml / 20 μm or less. When the ultimate air permeability is 500 seconds / 100 ml / 20 μm or less, the air permeability change and the film thickness change are small even when pressurized, and the battery capacity is large when used as a lithium ion secondary battery separator, Excellent permeability, mechanical properties and heat shrinkage.

(9) The surface roughness is measured with an atomic force microscope (AFM). A preferable range of the maximum height difference is 100 nm or more and 600 nm. When used as a battery separator, the contact area with the electrolyte is large, and the electrolyte injection property is excellent. More preferably, it is the range of 150 nm to 600 nm. If the thickness is less than 100 nm, the wetness of the electrolytic solution is also deteriorated, and the separator is hardly detached from the separator winding roll. If it exceeds 600 nm, the mechanical strength of the separator is lowered, which is not preferable.

(10) The evaluation of the meltdown resistance was carried out by using an electrolyte (1M LiBF ₄ / polypropylene carbonate (PC): Gamma-butyrolactone (γ-BL) (1/1 volume ratio)) is impregnated and sealed. It is sandwiched between 6 mmφ electrodes and heated at a rate of 5 ° C./min. The temperature and impedance value of the sample are measured with an impedance analyzer 6430B manufactured by Toyo Technica. The impedance value is an alternating current method with an amplitude of 10 mV and an alternating current of 1 kHz applied within a range of 10 mA. The impedance value is plotted against the temperature, and the ON degree that becomes 1000 ohms or more in the process of increasing the impedance value is the shutdown temperature. And Further, the battery container is continuously heated at a temperature rising rate of 5 ° C./minute, the temperature of the battery container and the impedance value are measured, and the temperature when the impedance value becomes less than 300 ohms again is defined as the meltdown temperature. The evaluation of the non-melt-down characteristic indicates that when the temperature reaches 195 ° C. and does not break, it retains a high impedance value as 195 ° C., which means that it has excellent non-melt-down characteristics.

(11) In battery evaluation, the separator made of the heat-resistant polyolefin microporous membrane is not particularly limited in the type of battery using the separator, but is particularly suitable for lithium ion secondary battery applications. For the lithium ion secondary battery using the separator made of the microporous membrane of the present invention, a known electrode and electrolyte may be used. The structure of the lithium ion secondary battery using the separator made of the microporous membrane of the present invention may also be a known one.
For example, as a positive electrode active material, 92 parts by weight of lithium cobalt oxide (LiCoO ₂ ; 10 micron) powder, ₂ parts by weight of acetylene black (manufactured by Denki Kagaku Kogyo), 2 parts by weight of fine graphite (manufactured by Nippon Graphite), polyvinylidene fluoride A positive electrode agent paste is prepared using 6% by mass of an N-methylpyrrolidone solution of polyvinylidene fluoride so that the dry weight of Kureha Chemical Industry Co., Ltd. is 4 parts by mass. The obtained paste is applied onto an aluminum foil having a thickness of 15 μm, dried and pressed to produce a positive electrode. It consists of a graphitized carbon (manufactured by Hitachi Chemical) powder 97 as a negative electrode active material, 1 part of CMC (carboxymethylcellulose) (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and 2 parts of a solid content of a carboxy-modified butadiene latex (manufactured by Nippon Zeon). A negative electrode paste is prepared using an aqueous solution. The obtained paste is applied onto a copper foil having a thickness of 12 μm, dried and pressed to produce a negative electrode.

(12) The positive electrode was cut into a size of 20 mm × 50 mm and attached with a tab. The negative electrode was cut into a size of 22 mm × 52 mm and attached with a tab. The separator was cut into a size of 26 mm × 56 mm. These positive electrode / separator / negative electrode were joined, an electrolytic solution was injected, and sealed in an aluminum laminate film to produce an aluminum laminate exterior cell.
Here, a 1M solution of LiPF ₆ dissolved in ethylene carbonate / ethyl methyl carbonate (3/7 weight ratio) is used as the electrolytic solution. In this cell, the amount of discharge electricity at 0.2C and 2C was measured, and the battery performance was defined as (discharge amount of electricity at 2C) / (discharge amount of electricity at 0.2C) × 100. 95% or more is regarded as good battery performance. Here, the charging condition is 0.2 C 4.2 V CC / CV 8 hours, and the discharging condition is CC discharge with a 2.75 V cutoff.

  Among the low heat shrinkable polyolefin microporous membrane of the present invention, in the case of an aluminum laminate exterior cell using a separator formed by applying the above polymer aqueous solution, 3 hours after injecting the electrolyte, the cell is pressed and sealed, Measure battery internal resistance. A potentio / galvanostat with a built-in impedance analyzer was used. An alternating voltage with an amplitude of 10 mV superimposed on OCV (open circuit voltage) as OV was applied from 300 KHz to 0.1 Hz, and the impedance was obtained from the response current. The impedance value at the end of the first discharge is 1, and the increase rate (%) of the 30th impedance is shown. Make the bottom chi. The charge / discharge cycle under the above conditions was repeated to determine the retention rate of the 50th discharge capacity based on the second discharge capacity.

Thus, the low heat shrinkable polyolefin microporous membrane of the present invention can be suitably used as a battery separator, a capacitor separator, a filter, and the like. In particular, it is optimal as a separator for lithium ion secondary batteries. The low heat-shrinkable polyolefin microporous membrane of the present invention is bonded to a polyethylene microporous membrane having a shutdown temperature in the range of 125 to 142 ° C. by taking advantage of the excellent non-meltdown characteristics of the present invention. It can also be used as a multilayer separator. It can also be used in combination with a polyethylene microporous membrane during battery assembly.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

7.1% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 1.15 million, 16.4% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 2 million, and 16.4% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 39.50 million , 2.9% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 5,810,000, 1.2% of ultra high molecular weight polyethylene having a viscosity average molecular weight of 631,000, and 5.2% by mass of high density polyethylene having an average molecular weight of 334,000. 4-methyl-1-pentene-1-decene-1 copolymer (MFR 260 ° C., 5 kg load, 9 g / 10 min) having an average molecular weight of 184,000, medium density polyethylene of 2.0% by mass and a melting point peak of 232 ° C. 4-methyl-1-pentene-1-decene-1 copolymer having 38.7% by mass and a melting point peak of 224 ° C. (MFR 260 ° C., 5 kg load, 25 g / 10 min) 7.7% by mass and ethylene-ethylene / butylene-ethylene block copolymer (Dynalon CEBC manufactured by JSR) [grade name 6200P] 0.12% by mass [grade name 6100P] 0.09% by mass, [grade name 6201B] Tetrakis as an antioxidant with respect to 100.0 parts by mass of a polyolefin resin composed of 0.09% by mass and 2.1% by mass of Noio [grade name SN0285] manufactured by Mitsui Chemicals, Inc. as a polypropylene elastomer resin [Methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] 0.5 part by mass of methane and 3,9-bis (2,6-ditertiarybutyl-4-methylphenoxy) -2 , 4,8,10-tetraoxa-3,9-diphosphapi [5,5] undecanlonyl Rokishifeniru) - propionate] and the methane 0.05 part by weight to prepare a polyolefin resin mixture having an average melting point of 179 ° C. a mixed resin parts by dry blending calculated. 37 parts by mass of the resin mixture was charged into a twin-screw extruder (cylinder diameter: 52 mm, screw length (L) to diameter (D) ratio L / D: 48, using a strong kneading type screw). 63 parts by mass of liquid paraffin [68 cst (40 ° C.)] 65% by mass is supplied from the side feeder of the shaft extruder, and a polyolefin / plasticizer mixed melt is prepared under the conditions of a temperature of 245 to 200 ° C. and a screw rotation speed of 300 rpm. . This is extruded from a T-die through a gear pump installed at the tip of a twin-screw extruder, and is taken out with a cooling roll to form a gel-like sheet molding. Biaxial stretching is performed on the resulting gel-like sheet molding using a biaxial simultaneous tenter stretching machine. Subsequently, the film is passed through a continuous plasticizer extraction apparatus using methylene chloride as a solvent, and a low-shrinkage polyolefin microporous film heat-treated at 130 ° C. by a heated roll press as a heat stretching treatment step is obtained. The characteristics are shown in Table 1. As a result of the evaluation test of the non-melt down characteristics, the shutdown temperature showed a rapid increase in impedance value around 135 ° C., showed 5000 ohms or more, and maintained a high impedance value until it exceeded 195 ° C.

4.4% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 1.09 million, 7.9% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 2 million, and 16.4% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 39.50 million , 5.1% by mass of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 5,810,000, 5.1% of ultra high molecular weight polyethylene having a viscosity average molecular weight of 631,000, and 7.2% by mass of high density polyethylene having an average molecular weight of 334,000 4-methyl-1-pentene-1-decene-1 copolymer (MFR 260 ° C., 5 kg load, 9 g / 10 min) having an average molecular weight of 184,000, medium density polyethylene of 2.0% by mass and a melting point peak of 232 ° C. 4-Methyl-1-pentene-1-decene-1 copolymer having 36.4% by mass and a melting point peak of 224 ° C. (MFR 260 ° C., 5 kg load, 25 g / 10 minutes) 2.1% by mass and ethylene-ethylene / butylene-ethylene block copolymer (Dynalon CEBC manufactured by JSR) [grade name 6200P] 0.10% by mass [grade name 6100P] 0.10% by mass, [grade name 6201B] Tetrakis as an antioxidant with respect to 100.0 parts by mass of a polyolefin resin consisting of 0.10% by mass and 3.1% by mass of Miteo Chemical Co., Ltd.'s Notio [grade name SN0285] as a polypropylene elastomer resin. [Methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] 0.5 part by mass of methane and 3,9-bis (2,6-ditertiarybutyl-4-methylphenoxy) -2 , 4,8,10-tetraoxa-3,9-diphosphapi [5,5] undecanlonyl Rokishifeniru) - propionate] and the methane 0.05 part by weight to prepare a polyolefin resin mixture having an average melting point of 181 ° C. a mixed resin parts by dry blending calculated. 40 parts by mass of the resin mixture was put into a twin screw extruder (cylinder diameter: 52 mm, screw length (L) to diameter (D) ratio L / D: 48, using a strong kneading type screw). 60 parts by mass of liquid paraffin [68 cst (40 ° C.)] 65% by mass is supplied from the side feeder of the axial extruder, and a polyolefin / plasticizer mixed melt is prepared under the conditions of a temperature of 245 to 200 ° C. and a screw rotation speed of 300 rpm. . This is extruded from a T-die through a gear pump installed at the tip of a twin-screw extruder, and is taken out with a cooling roll to form a gel-like sheet molding. Biaxial stretching is performed on the resulting gel-like sheet molding using a biaxial simultaneous tenter stretching machine. Next, the film is passed through a continuous plasticizer extraction apparatus using methylene chloride as a solvent, and a low-shrinkage polyolefin microporous membrane that has been heat-set at 125 ° C. by a heated roll press as a heat stretching treatment step is obtained. The characteristics are shown in Table 1. As a result of the evaluation test of the non-melt down characteristics, the shutdown temperature showed a rapid increase in impedance value around 135 ° C., showed 5000 ohms or more, and maintained a high impedance value until it exceeded 195 ° C.

7.7% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 1.09 million, 24.1% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 2 million, and 10.7% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 39.50 million , 4.3% by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 5,810,000, 4.3% ultra high molecular weight polyethylene having a viscosity average molecular weight of 631,000, and 5.7% by mass of high density polyethylene having an average molecular weight of 334,000. 4-methyl-1-pentene-1-decene-1 copolymer having a melting point peak of 232 ° C. (MFR 260 ° C., 5 kg load, 9 g / 10 min) and 32.9% by mass and 4-methyl-1 having a melting point peak of 224 ° C. -Pentene-1-decene-1 copolymer (MFR260 ° C., 5 kg load, 25 g / 10 min), 6.6% by mass, ethylene-ethylene-butylene-ethylenebromine Copolymer (Dynalon CEBC manufactured by JSR) [grade name 6200P] 0.14% by mass [grade name 6100P] 0.11% by mass, [grade name 6201B] 0.11% by mass and Mitsui as a polypropylene elastomer resin Tetrakis [methylene-3- (3,5-ditertiary) as an antioxidant with respect to 100.0 parts by mass of a polyolefin resin mixture comprising 3.4% by mass of Notio [grade name SN0285] manufactured by Chemical Co., Ltd. Butyl-4-hydroxyphenyl) -propionate] 0.35 parts by weight of methane and 3,9-bis (2,6-ditertiarybutyl-4-methylphenoxy) -2,4,8,10-tetraoxa-3,9 -Diphosphapi [5,5] undecanlonylhydroxyphenyl) -propionate] methane 0.035 mass Preparative to prepare a polyolefin resin mixture having an average melting point of 173 ° C. a mixed resin parts by dry blending calculated. 40 parts by mass of the resin mixture was charged into a twin-screw extruder (cylinder diameter: 52 mm, screw length (L) to diameter (D) ratio L / D: 48, using a strong kneading type screw). 60 parts by mass of liquid paraffin [68 cst (40 ° C.)] is supplied from the side feeder of the screw extruder to prepare a polyolefin / plasticizer mixed melt under the conditions of a temperature of 245 to 200 ° C. and a screw rotation speed of 300 rpm. This is extruded from a T-die through a gear pump installed at the tip of a twin-screw extruder, and is taken out with a cooling roll to form a gel-like sheet molding. Biaxial stretching is performed on the resulting gel-like sheet molding using a biaxial simultaneous tenter stretching machine. Subsequently, the film is passed through a continuous plasticizer extraction apparatus using methylene chloride as a solvent, and a low-shrinkage polyolefin microporous film heat-treated at 130 ° C. by a heated roll press as a heat stretching treatment step is obtained. The characteristics are shown in Table 1.
As a result of the evaluation test of the non-melt down characteristics, the shutdown temperature showed a sudden increase in impedance value at around 134 ° C., showed 5000 ohms or more, and maintained a high impedance value until it exceeded 195 ° C.

固形分５０％を精製水で希釈して固形分１５％のエマルジョンを２ミクロンのクリアランスのバーコートで塗布した。室温から１０５℃まで昇温しながら減圧下に水分を蒸発させ乾燥・架橋した。これを上記の電池に組み込んだ。その結果を表２に示す。The low-shrinkage polyolefin microporous membrane obtained in Example 1 is made of FEVE manufactured by Asahi Glass Co., Ltd.

50% solids were diluted with purified water and a 15% solids emulsion was applied in a 2 micron clearance bar coat. While raising the temperature from room temperature to 105 ° C., water was evaporated under reduced pressure to dry and crosslink. This was incorporated into the above battery. The results are shown in Table 2.

ースルホコハク酸ナトリウム）の固形を精製水に溶かした乳化剤液を一緒に混合して、含有アルコールを減圧下に除去する。精製水で希釈して固形分１０％のエマルジョンとする。２ミクロンのクリアランスのバーコートで塗布した。室温から徐々に水分を蒸発させながら昇温し１０５℃まで減圧下に乾燥した。これを上記の電池に組み込んだ。その結果を表２に示す。

-Sodium emulsifier solution in which the solid of sodium sulfosuccinate) is dissolved in purified water is mixed together and the alcohol contained is removed under reduced pressure. Dilute with purified water to make an emulsion with 10% solids. It was applied with a 2 micron clearance bar coat. The temperature was raised while gradually evaporating water from room temperature, and drying was performed under reduced pressure to 105 ° C. This was incorporated into the above battery. The results are shown in Table 2.

とする。実施例１で得られる低収縮性ポリオレフィン微多孔膜をこのエマルジョンの中を通して、ＰＥＴ製のドクターブレードで膜両表面をかきとった。室温から１０５℃まで昇温しながらで水分を蒸発し、減圧下に乾燥した。これを上記の電池に組み込んだ。その結果を表２に示す。A varnish solution of polyamide / imide (A) manufactured by Hitachi Chemical Co., Ltd. and Miflon manufactured by Asahi Glass Co., Ltd.

And The low-shrinkage polyolefin microporous membrane obtained in Example 1 was passed through this emulsion, and both surfaces of the membrane were scraped with a PET doctor blade. The water was evaporated while raising the temperature from room temperature to 105 ° C. and dried under reduced pressure. This was incorporated into the above battery. The results are shown in Table 2.

製水に溶かした乳化剤液と一緒に混合して固形分４０％の乳濁液を作る。実施例１で得られる低収縮性ポリオレフィン微多孔膜をこの乳濁液を通して、ＰＥＴ製のドクターブレードで膜両面をかきとった。室温で水分を蒸発し、膜状になってから１０５℃で減圧下に乾燥した。これを上記の電池に組み込んだ。その結果を表２に示すPMMA (A) obtained from Sigma-Aldrich and PVDF-HFP (weight ratio 92: 8) copolymer (B) are dissolved in N-methyl-2-pyrrolidone (NMP). Emulsification of NOF Corporation

Mix with emulsifier solution dissolved in water to make an emulsion with 40% solids. The low-shrinkage polyolefin microporous membrane obtained in Example 1 was passed through this emulsion, and both surfaces of the membrane were scraped with a PET doctor blade. Water was evaporated at room temperature, and after film formation, the film was dried at 105 ° C. under reduced pressure. This was incorporated into the above battery. The results are shown in Table 2.

The low-shrinkage polyolefin microporous membrane obtained in Example 1 is a polyimide aqueous solution obtained by adding a water-soluble organic amine to polyamic acid and a PMMA polymer obtained by emulsion polymerization (partially crosslinked by dimethacrylate and triacrylate). Was formed at a solid content ratio of 1: 1 to form a 1 micron dry film on both sides. This was incorporated into the above battery. The results are shown in Table 2.

  Silicone emulsion KM-9703 (solid content 45%) manufactured by Shin-Etsu Chemical Co., Ltd. was coated on the low-shrinkage polyolefin microporous film obtained in Example 1 with a bar coat having a clearance of 2 microns. Water was evaporated while the temperature was raised from room temperature to 105 ° C., followed by drying and crosslinking under reduced pressure. This was incorporated into the above battery. The results are shown in Table 2.

Comparative Example 1

を作製した。実施例１で得られる低収縮性ポリオレフィン微多孔膜の両面にそれぞれ２ミクロンのバーコート塗工し、室温から徐々に加熱しながら１０５℃まで昇温しながら減圧乾燥した。これを上記の電池に組み込んだ。その結果を表２に示す。

Was made. Each of the low-shrinkage polyolefin microporous membranes obtained in Example 1 was coated with a 2 micron bar coat, and dried under reduced pressure while gradually heating from room temperature to 105 ° C. This was incorporated into the above battery. The results are shown in Table 2.

Comparative Example 2

  Through the low-shrinkage polyolefin microporous membrane obtained in Example 1 through a polyimide resin aqueous solution manufactured by Toray Industries, Inc., the membrane surface was scraped with a PET doctor blade. Water was evaporated at room temperature to form a film, and then dried at 110 ° C. under reduced pressure. This was incorporated into the above battery. The results are shown in Table 2.

Comparative Example 3

  Nothing was applied to the low-shrinkage polyolefin microporous membrane obtained in Example 1, and the battery was assembled as is.

  According to the present invention, high-performance lithium ion secondary batteries with high energy density, high output, and large size have high safety shutdown characteristics, non-meltdown characteristics, and low shrinkage when exposed to high temperatures. It is possible to provide a low-heat-shrinkable polyolefin microporous membrane that has other good separator characteristics, is easy to handle as a separator, and is an excellent lithium-ion secondary battery separator, and a method for producing the same.

Claims (6)

Ultrahigh molecular weight polyethylene 25-50% by mass, polyethylene 1-15% by mass, copolymer resin of 4-methyl-1-pentene and α-olefin having 3 or more carbon atoms, ethylene-ethylene butylene -Ethylene block copolymer 0.1-2% by mass "islands" that are nano-order level helical crystal parts of 10 nm to 50 nm are connected to each other to form a network structure and cover the entire amorphous part A polyolefin microporous membrane having low thermal shrinkage at high temperatures and having both shutdown characteristics and non-meltdown characteristics, characterized by containing 0.5-5 mass of a propylene-based elastomer resin having a microstructure. In a twin screw extruder, ultrahigh molecular weight polyethylene 25-50% by mass and polyethylene 1-15% by mass, copolymer resin of 4-methyl-1-pentene and α-olefin having 3 or more carbon atoms, 35-65% by mass , Ethylene-ethylene-butylene-ethylene block copolymer 0.1-2% by mass, “islands”, which are helical crystal parts of nano-order level of 10 nm to 50 nm, are connected to each other to form a network structure and amorphous A polyolefin resin mixture comprising 0.5-5 mass of a propylene-based elastomer resin having a special microstructure covering the entire part, an antioxidant, and if necessary, an inorganic filler powder is supplied to a twin screw extruder, and a plasticizer After injecting the liquid, melt-kneading, extruding from a die, and cooling, the gel-like sheet molded product is stretched by a sequential method or a simultaneous biaxial stretching method, and then the solvent for film formation is removed. A method for manufacturing a low-thermal shrinkage polyolefin microporous film according to claim 1, wherein the door. A low heat-shrinkable polyolefin microporous membrane according to claim 1 having a crosslinkable polymer (1) fluoroethylene vinyl ether) alternating copolymer in the main chain, a fluoropolymer aqueous solution, a polyamide-imide resin aqueous solution, a polyimide aqueous solution, silicon Low heat-shrinkable polyolefin with a 0.3-5 micron-thick surface layer that is dried or crosslinked after applying or dipping an aqueous solution of resin, polyvinylidene fluoride-hexafluoropropylene and an aqueous solution of a crosslinkable acrylic resin or a mixture thereof A surface-modified microporous membrane formed on both sides or one side of a microporous membrane. The copolymer resin of 4-methyl-1-pentene and α-olefin having 2 or more carbon atoms contains 4-methyl-1-pentene containing 80 mol% or more of 4-methyl-1-pentene and 3 to 30 carbon atoms. The melting point (Tm) measured on the basis of a DSC (Differential Scanning Calorimeter) test is in the range of 220 to 240 ° C. 3. The low heat shrinkable polyolefin microporous membrane according to 3. Propylene elastomer resin is a copolymer of propylene and α-olefin having a specific structure, and consists of a structural unit derived from propylene and a structural unit derived from α-olefin having 2 to 30 carbon atoms (excluding propylene). The “islands”, which are nano-order helical crystal parts of 10 nm to 50 nm, are connected to each other to form a network structure and have a microstructure that covers the entire amorphous part. Low heat shrinkable polyolefin microporous membrane. A separator for a lithium ion secondary battery, which is the low heat shrinkable polyolefin microporous membrane according to claim 1.