JP2021038379A - Porous polyolefin film - Google Patents

Abstract

To provide a porous polyolefin film that has excellent output properties, safety and process transportability when used as a battery separator.SOLUTION: A porous polyolefin film has a porosity of 50% or more, a piercing strength of 3.7 N or more in terms of 10 μm, and a tensile elasticity of 980 MPa or more in MD direction.SELECTED DRAWING: None

Description

The present invention relates to a porous polyolefin film.

The porous polyolefin film is used as a filter, a separator for a fuel cell, a separator for a condenser, and the like. In particular, it is suitably used as a separator for lithium-ion batteries widely used in notebook personal computers, mobile phones, digital cameras, and the like. The reason is that the polyolefin microporous membrane has excellent mechanical strength, shutdown characteristics, and lithium ion permeation performance.

With the widespread use and expansion of applications of lithium-ion secondary batteries, the quality level required for separators is improving year by year, and various characteristics such as strength and transparency are required at a high level. For example, since the separator is wound under tension, it is preferable that the tensile strength and the tensile elastic modulus are high in order to prevent film rupture during winding. Further, since the winding direction of the separator is usually the MD direction, the tensile elastic modulus and the tensile strength in the MD direction (hereinafter, may be simply referred to as "MD tensile elastic modulus" and "MD tensile strength") are TD. It is preferably moderately higher than the tensile modulus and tensile strength in the direction (hereinafter, may be simply referred to as "TD tensile modulus" and "TD tensile strength").

On the other hand, with the increase in size of secondary batteries such as automobile batteries, ensuring safety is becoming more and more important. Therefore, from the viewpoint of preventing film rupture due to foreign matter or impact in the battery, it is preferable that not only the tensile elastic modulus and tensile strength but also the piercing strength is high.

In particular, since lithium-ion secondary batteries have been used in vehicles in recent years, it is necessary to shorten the charging time and improve acceleration, and the required characteristics of batteries include rapid charging (large current charging) and increased power consumption (large current). Discharge) is required. Along with this, the improvement of output characteristics has become even higher as a requirement for separators.

Further, as the cruising range of automobiles increases, the capacity of batteries is increasing, and there is a further demand for thinner separators. However, in general, if the film thickness is reduced in order to increase the transparency, the mechanical strength is lowered. Further, if the porosity is increased in order to increase the permeability, the mechanical strength is lowered. When the mechanical strength of the separator is lowered, a short circuit (foreign matter resistance) due to electrodes or foreign matter or a film rupture (impact resistance) is likely to occur when the battery is impacted, and the safety of the battery is lowered. Therefore, higher strength than before is required.

As a method for improving the strength of the porous film, a method of stretching a sheet, a method of laminating a porous sheet, and the like are widely known. However, with the prior art, it has been difficult to obtain a porous film having an excellent balance between strength and permeability.

For example, in Patent Document 1, ultra-high molecular weight polyethylene and high-density polyethylene are melt-kneaded and simultaneously biaxially stretched, the solvent is removed from the obtained stretched product, and then stretching is performed again in the TD direction. The porous film produced by the above is described. However, although this porous film has a high porosity and high puncture strength, the MD tensile elastic modulus is not always sufficient, and there is room for improvement in process transferability. In addition, there is a problem that it is difficult to make a thin film because of the laminated structure.

In Patent Document 2, ceramic particles are added to a polyolefin resin, melt-kneaded, and simultaneously biaxially stretched, then the solvent is removed from the obtained stretched product, and then the stretching is performed again in the TD direction. The ceramic composites produced are listed. Although this ceramic composite has a high porosity and a high piercing strength, it has a problem that the MD tensile elastic modulus is not always sufficient and the transportability is inferior. Further, since it contains ceramic particles, there is a problem that it is difficult to make a thin film.

In response to the diversifying customer needs associated with higher energy density, higher capacity, higher output, and faster charging as described above, improvements have been made to the development of separators with excellent output characteristics, safety, and process transportability. There is room.

Patent No. 4794098 Patent No. 5288736

An object of the present invention is to solve the above-mentioned problems. That is, it is an object of the present invention to provide a porous polyolefin film having excellent output characteristics, safety and process transportability when used as a battery separator.

That is, the present invention has porosity of 50% or more, the puncture strength of the film thickness 10μm conversion is not less than 3.7 N, porosity longitudinal tensile modulus _{E MD} is characterized in that at least 980MPa It is a polyolefin film.

Further, the present invention is a battery separator characterized by using the porous polyolefin film of the present invention.

The present invention is a secondary battery characterized by using the battery separator of the present invention.

Since the porous polyolefin film of the present invention is excellent in output characteristics, safety and process transportability when used as a battery separator, it is required to increase the energy density, capacity and output of electric vehicles and the like. It can be suitably used as a battery separator for a secondary battery.

The porous polyolefin film of the present invention has a porosity of 50% or more. The porosity is more preferably 52% or more, still more preferably 55% or more, and most preferably 58% or more. By setting the vacancy rate to 50% or more, the ion permeability can be improved and the output characteristics of the battery can be improved, especially when used as a battery separator for quick charging applications that will be required in the future. .. The higher the porosity is, the more preferable it is from the viewpoint of output characteristics, but if it is too high, the strength may decrease, so the upper limit is about 70%. In order to set the porosity within the above range, it is preferable that the raw material composition of the film is within the range described later, and the stretching conditions at the time of film formation are within the range described later.

The porous polyolefin film of the present invention has a piercing strength of 3.7 N or more when converted to a film thickness of 10 μm. It is more preferably 4.0 N or more, further preferably 4.4 N or more, still more preferably 4.7 N or more. By setting the puncture strength to 3.7 N or more, it is possible to suppress the occurrence of a short circuit due to turning or foreign matter in the battery when the thin film is formed, and to improve the safety of the battery. From the viewpoint of improving safety, the higher the piercing strength is, the more preferable it is, but the improvement of the pore ratio and the improvement of the piercing strength are often trade-offs, and the upper limit is about 10N. In order to set the puncture strength in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions at the time of film formation are in the range described later.

Porous polyolefin film of the present invention, the longitudinal tensile modulus _{E MD} (unit, MPa) of the film is not less than 980 MPa. The tensile elastic modulus is more preferably 1100 MPa or more, further preferably 1300 MPa or more, still more preferably 1500 MPa or more. By _{setting the E MD} to 980 MPa or more, the transportability of the film can be improved in the coat layer coating step and the battery winding step. Is preferably as high E _MD in view of the film transportability improver in coating layer coating process and cell winding process, the improvement of improvement and E _MD of porosity often a trade-off, about 2500MPa is at the upper limit is there. In order to set the E _MD in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions at the time of film formation are in the range described later.

In the present invention, the direction parallel to the film forming direction of the film is referred to as the film forming direction, the longitudinal direction, or the MD direction, and the direction orthogonal to the film forming direction in the film surface is referred to as the width direction or the TD direction. ..

In order to improve the output characteristics of the battery, it is necessary to increase the porosity of the porous film. However, when the porosity increases, the amount of resin in the porous film decreases, so that the puncture strength decreases and the safety of the battery may decrease. Therefore, there is a trade-off relationship between the porosity and the puncture strength, and there is a problem in achieving both output characteristics and safety.

In the present invention, the ultra-high molecular weight polyethylene content of the porous polyolefin film is in the range described later, and the film forming conditions are in the range described later, and in particular, the porous film after solvent extraction is in the MD direction and the TD direction under the conditions described later. It was found that both high porosity and high puncture strength can be achieved by performing dry re-stretching on the film. That is, by adding ultra-high molecular weight polyethylene, a porous film having excellent puncture strength and tensile elastic modulus can be obtained. Furthermore, the dry re-stretching progresses the cleavage of the fibrils and increases the porosity, and at the same time, the orientation of the polyolefin crystals progresses, the puncture strength and tensile modulus increase, and the porosity, puncture strength and tensile modulus in the MD direction It has been found that a porous film having a modulus satisfying the above range and excellent in output characteristics, safety and process transferability can be obtained.

Porous polyolefin film of the present invention, the ratio of the longitudinal tensile when the elastic modulus E _MD, the tensile modulus in the width direction is E _TD (unit, MPa), MD and TD directions of the tensile modulus of the film The E _MD / E _TD is preferably 1.0 or more. The E _MD / E _TD is more preferably 1.0 or more and 6.0 or less, still more preferably 1.0 or more and 3.0 or less, and further preferably 1.0 or more and 2.0 or less. By _{setting the E MD} / E _TD to 1.0 or more, the transportability of the film can be improved in the coating layer coating step and the battery winding step. Is preferably as high _E MD _{/ E TD} in terms of the film transportability improver in coating layer coating process and cell winding _process, E MD _{/ E TD} is 6.0 greater than the MD and TD directions of the anisotropy May occur, and wrinkles may occur in the MD direction or tears may occur in the MD direction during film transfer. In order to set E _MD / E _TD in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions at the time of film formation are in the range described later.

Porous polyolefin film of the present invention, when the tensile strength in a longitudinal direction of the film was _{M MD} (unit, _MPa), that _{M MD} is not less than 180MPa preferred. The tensile strength in the MD direction is more preferably 200 MPa or more, further preferably 240 MPa or more, still more preferably 290 MPa or more. By setting the tensile strength to 180 MPa or more, it is possible to suppress the occurrence of film rupture in the coat layer coating step and the battery winding step, and improve the transportability of the film. Further, when the thin film is formed, it is possible to suppress the occurrence of a short circuit due to turning or foreign matter in the battery, and improve the safety of the battery. From the viewpoint of improving film transportability and safety, the higher the tensile strength is, the more preferable it is, but the improvement of the porosity and the improvement of the tensile strength are often trade-offs, and the upper limit is about 500 MPa. In order to set the tensile strength in the above range, it is preferable that the raw material composition of the film is in the range described later and the stretching conditions at the time of film formation are in the range described later.

The porous polyolefin film of the present invention preferably has an average pore diameter of 10 nm or more and 140 nm or less determined by a palm poromometer. By setting the average pore diameter to 10 nm or more, the ion permeability becomes sufficient when used as a battery separator, and the output characteristics of the battery can be improved. By setting the average pore diameter to 140 nm or less, more preferably 45 nm or less, still more preferably 40 nm or less, still more preferably 35 nm or less, it is possible to prevent short circuits due to winding or foreign matter in the battery when the thin film is formed, and the battery. The safety of the battery can be improved. In order to set the average pore size in the above range, it is preferable that the raw material composition of the film is in the range described later, and the stretching conditions at the time of film formation are in the range described later.

In each of the porous polyolefin films of the present invention, the ratio of the average pore diameter to the maximum pore diameter, (average pore diameter) / (maximum pore diameter), which is determined by a palm poromometer, is preferably 0.70 or more. When (average pore diameter) / (maximum pore diameter) is 0.70 or more, the pore diameter distribution is sufficiently uniform, so that good battery output can be obtained when used as a battery separator, and short circuits due to dendrite formation can be suppressed. .. (Average pore diameter) / (maximum pore diameter) is more preferably 0.75 or more, still more preferably 0.78 or more, still more preferably 0.80 or more. The upper limit of (average pore diameter) / (maximum pore diameter) is 1.0.

The thickness of the porous polyolefin film of the present invention is preferably 15 μm or less. The thickness is more preferably 10 μm or less, further preferably 7 μm or less, and further preferably 6 μm or less. By setting the thickness to 15 μm or less, the ion permeability becomes sufficient and the output characteristics of the battery are improved, especially when used as a battery separator for quick charging. From the viewpoint of improving battery characteristics, it is preferable that the thickness is thin, but if the thickness is thin, short circuits are likely to occur during winding or due to foreign matter in the battery, which may reduce the safety of the battery. Therefore, the lower limit is about 3 μm. Is. The thickness can be adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched sheet, the film forming speed, the stretching ratio, and the like within a range that does not deteriorate other physical properties.

Next, the raw material of the porous polyolefin film of the present invention will be described, but the present invention is not necessarily limited thereto.

The porous polyolefin film of the present invention is a film containing polyolefin as a main component. Here, in the present invention, the "main component" means that the ratio of the specific component to all the components is 50% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more. , More preferably 99% by mass or more.

The polyolefin resin used in the present invention is preferably polyolefin, and may be a polyolefin composition. Examples of the polyolefin include polyethylene and polypropylene, and two or more kinds of these may be blended and used.

As for the polyolefin constituting the porous polyolefin film, it is preferable to use polyethylene. The content of polyethylene is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 99% by mass or more, when the total amount of polyolefin is 100% by mass. By setting the composition ratio of polyethylene within the above range, it is possible to obtain a highly uniform microporous membrane with little or no phase separation of polymer species.

The above porous polyolefin film has a weight average molecular weight as a raw material 1.0 × 10 ⁶ or more ultra-high molecular weight polyethylene (hereinafter, simply referred to as "UHMWPE".) Preferably contains a. The content of UHMWPE is preferably 40% by mass or more, more preferably 50% by mass or more, still more preferably 60% by mass or more, assuming that the total mass of the polyolefin is 100% by mass. By containing the ultra-high molecular weight polyolefin resin as a raw material, the pores can be made finer and the heat resistance can be improved, and the strength and elongation can be further improved.

Porous polyolefin film of the present invention, after a film by deposition conditions which will be described later, the film by gel permeation chromatography (GPC) method, when measuring the molecular weight distribution by weight, molecular weight 1.0 × 10 ⁶ The content of the above polyolefin component is preferably 35% by mass or more. Molecular weight in the film 1.0 × 10 ⁶ or more polyolefin component 35 wt% or more, more preferably 40 mass% or more, more preferably 45 wt% or more, by more preferably 50 mass% or more, piercing strength A porous polyolefin film having an excellent tensile elastic modulus can be obtained, and safety can be improved.

In addition, various additions such as antioxidants, heat stabilizers and antistatic agents, ultraviolet absorbers, and blocking inhibitors and fillers are added to the porous polyolefin film of the present invention as long as the effects of the present invention are not impaired. The agent may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyethylene resin. Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris (3,5-di). -T-Butyl-4-hydroxybenzyl) Benzene (for example, BASF "Irganox" (registered trademark) 1330: molecular weight 775.2), tetrakis [methylene-3 (3,5-di-t-butyl-4-hydroxy) It is preferable to use one or more selected from [phenyl) propionate] methane (for example, "Irganox" (registered trademark) 1010: molecular weight 1177.7 manufactured by BASF). Appropriate selection of the type and amount of antioxidant and heat stabilizer is important for adjusting or enhancing the characteristics of the porous film.

The porous polyolefin film of the present invention is obtained by biaxial stretching using the above-mentioned raw materials. The biaxial stretching method can be obtained by any of the inflation method, the simultaneous biaxial stretching method, and the sequential biaxial stretching method. Among them, film forming stability, thickness uniformity, high rigidity and dimensional stability of the film are obtained. It is preferable to adopt the simultaneous biaxial stretching method or the sequential biaxial stretching method in terms of controlling the above.

Next, the method for producing the porous polyolefin film of the present invention will be described, but the present invention is not necessarily limited thereto. The method for producing a porous polyolefin film of the present invention comprises the following steps (a) to (e).
(A) A polyolefin solution is prepared by kneading and dissolving a polymer material containing a single polyolefin, a polyolefin mixture, a polyolefin solvent mixture (plasticizer), an additive, and a polyolefin kneaded product (preparation of a polyolefin solution).
(B) The melt is extruded, molded into a sheet and cooled and solidified (formation of extruded material and formation of gel-like sheet).
(C) The obtained sheet is wet-stretched by a roll method or a tenter method (wet stretching step).
(D) Then, the plasticizer is extracted from the obtained stretched film and the film is dried (washing / drying step).
(E) Subsequently, dry re-stretching / heat treatment is performed (dry re-stretching / heat treatment step).

Hereinafter, each step will be described.

(A) Preparation of polyolefin solution Prepare a polyolefin solution in which the polyolefin resin is heated and dissolved in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving polyethylene, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification. Solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane and liquid paraffin, mineral oil distillates having corresponding boiling points, and dibutylphthalates. Examples thereof include phthalates that are liquid at room temperature, such as dioctyl phthalates. In order to obtain a gel-like sheet having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the melt-kneaded state, it is miscible with polyethylene, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The blending ratio of the polyolefin resin and the plasticizer may be 100% by mass based on the total of the polyolefin resin and the plasticizer, and the content of the polyolefin resin may be appropriately selected within a range that does not impair the moldability, but is 5 to 50% by mass. Is preferable. By making the polyolefin resin 5% by mass or more (that is, the plasticizer is 95% by mass or less), when molding into a sheet, the swell and neck-in at the outlet of the base are suppressed to a small size, and the moldability of the sheet deteriorates. It is possible to prevent deterioration of film forming property. On the other hand, when the polyolefin resin is 50% by mass or less (that is, the plasticizer is 50% by mass or more), it is possible to suppress the shrinkage in the thickness direction from becoming large and improve the molding processability.

The viscosity of the liquid solvent is preferably 2 × 10 ^-5 to 2 × 10 ^-4 m ² / s at 40 ° C. When the viscosity at 40 ° C. is 2 × ^10-5 m ² / s or more, the sheet obtained by extruding the polyolefin solution from the die is unlikely to become non-uniform. On the other hand, ^{if it is 2 × 10 -4} m ² / s or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

(B) Formation of Extruded Product and Formation of Gel-like Sheet The uniform melt-kneading of the polyolefin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin solution, it is preferably performed in a twin-screw extruder. If necessary, various additives such as antioxidants may be added as long as the effects of the present invention are not impaired. In particular, it is preferable to add an antioxidant in order to prevent the oxidation of polyethylene.

In the extruder, the polyolefin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of polyolefin resin + 10 ° C.) to (melting point of polyolefin resin + 120 ° C.). More preferably, it is (melting point of polyolefin resin + 20 ° C.) to (melting point of polyolefin resin + 100 ° C.). Here, the melting point means a value measured by DSC based on JIS K7121 (1987) (hereinafter, the same applies). For example, in the case of polyethylene, the melt-kneading temperature is preferably in the range of 140 to 250 ° C. More preferably, it is 160 to 230 ° C, still more preferably 170 to 200 ° C. Specifically, since the polyethylene composition has a melting point of about 130 to 140 ° C., the melt-kneading temperature is preferably 140 to 250 ° C., more preferably 180 to 230 ° C.

It is preferable that the melt-kneading temperature is low from the viewpoint of suppressing the deterioration of the resin, but if it is lower than the above-mentioned temperature, unmelted matter is generated in the extruded product extruded from the die, which causes film rupture or the like in the subsequent stretching step. If the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin becomes intense, and the physical properties of the obtained porous film, for example, the strength and the porosity may be inferior. In addition, the decomposed product precipitates on a chill roll or a roll in the stretching process and adheres to the sheet, which leads to deterioration of the appearance. Therefore, it is preferable to knead within the above range.

Next, the obtained extrude is cooled to obtain a gel-like sheet, and the cooling can immobilize the microphase of polyethylene separated by the solvent. It is preferable to cool to 10 to 50 ° C. in the cooling step. This is because the final cooling temperature is preferably set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower. If the cooling rate is less than 30 ° C./min, the crystallinity increases and it is difficult to obtain a gel-like sheet suitable for stretching. Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure forming the gel structure also becomes large. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to high toughness of the film in addition to uniform stretching.

Examples of the cooling method include a method of directly contacting with cold air, cooling water, and other cooling media, a method of contacting with a roll cooled with a refrigerant, a method of using a casting drum, and the like.

Further, the polyolefin porous film of the present invention is not limited to a single layer, and may be a laminated body. The number of layers is not particularly limited, and may be two layers or three or more layers. As described above, the laminated portion may contain a desired resin in addition to polyethylene to the extent that the effects of the present invention are not impaired. As a method of forming the polyolefin porous film into a laminate, a conventional method can be used. For example, desired resins are prepared as required, and these resins are separately supplied to an extruder to obtain a desired resin. There is a method of forming a laminated body by melting at a temperature, merging in a polymer tube or a die, and extruding from a slit-shaped die at each desired laminated thickness.

(C) Wet stretching step The obtained gel-like (including laminated sheet) sheet is stretched. As the stretching method used, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching by a simultaneous biaxial tenter, etc. Can be mentioned. The stretching ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, but it is preferable to stretch 5 times or more in any direction. The area magnification is preferably 25 times or more, more preferably 36 times or more, still more preferably 49 times or more. By setting the area magnification to 25 times or more, it is possible to obtain sufficient stretching and sufficient film uniformity, and it is possible to obtain an excellent porous film from the viewpoint of strength.

The stretching temperature is preferably in the range of (melting point of the gel-like sheet + 10 ° C. or lower _{) to (crystal dispersion temperature of polyolefin resin Tcd} ) to (melting point of the gel-like sheet + 5 ° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 100 ° C., the stretching temperature is preferably 90 to 125 ° C., more preferably 90 to 120 ° C. Crystal dispersion temperature _{T cd} is obtained from the temperature characteristics of dynamic viscoelasticity was measured according to ASTM D 4065. Alternatively, it may be obtained from NMR. When the temperature is 90 ° C. or higher, the pores are sufficiently opened, the film thickness is easily made uniform, and the pore ratio is also improved. By setting the temperature to 125 ° C. or lower, melting of the sheet can be prevented and blockage of the holes can be prevented.

Due to the above stretching, the higher-order structure formed on the gel sheet is cleaved, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Stretching improves the mechanical strength and expands the pores, making it suitable as a battery separator.

(D) Cleaning / drying step Next, the solvent remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyethylene phase and the solvent phase are separated, a porous film can be obtained by removing the solvent. Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like. These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent having a low surface tension, shrinkage is suppressed by the surface tension of the gas-liquid interface when the network structure forming microporous is washed and dried, and a porous film having porosity and permeability can be obtained. .. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

The cleaning method can be carried out by a method of immersing the gel-like sheet in a cleaning solvent and extracting it, a method of showering the gel-like sheet with the cleaning solvent, a method using a combination thereof, or the like. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the gel-like sheet. The washing temperature may be 15 to 30 ° C., and if necessary, heat to 80 ° C. or lower. At this time, from the viewpoint of enhancing the cleaning effect of the solvent, from the viewpoint of preventing the physical characteristics of the obtained porous film from becoming non-uniform in the TD direction and / or the MD direction, the mechanical properties and electricity of the porous film. From the viewpoint of improving the physical characteristics, the longer the gel-like sheet is immersed in the cleaning solvent, the better. The above-mentioned washing is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the porous film is less than 1% by mass.

Then, in the drying step, the solvent in the porous film is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. Sufficient drying can prevent the porosity of the porous film from decreasing and the permeability from deteriorating in the subsequent heat treatment.

(E) Dry re-stretching / heat treatment step The dried porous film is stretched at least in the uniaxial direction (dry re-stretching). Dry re-stretching promotes fibril cleavage, increases porosity and average pore size, and provides a porous film with excellent permeability. Further, the orientation of the polyolefin crystal is promoted, the puncture strength and the tensile elastic modulus / tensile strength are increased, and a porous film having excellent safety and process transportability can be obtained.

The dry re-stretching is the same as the above-mentioned stretching while heating the porous film, MD uniaxial stretching by a roll stretching machine, TD uniaxial stretching by a tenter, a roll stretching machine and a tenter, or a sequential biaxial by a combination of a tenter and a tenter. It can be performed by stretching, simultaneous biaxial stretching with a simultaneous biaxial tenter, or the like.

The re-stretching temperature is preferably equal to or lower than the melting point of the polyethylene composition, and _{more preferably within the range of (Tcd-} 20 ° C.) to the melting point. Specifically, 70 to 135 ° C. is preferable, and 110 to 132 ° C. is more preferable. More preferably, it is 120 to 130 ° C.

The dry re-stretching ratio is preferably 1.40 times or more in the MD direction. By setting the MD re-stretching ratio to 1.40 times or more, more preferably 2.00 times or more, further preferably 2.50 times or more, still more preferably 3.00 times or more, the orientation of the polyolefin crystal in the MD direction can be adjusted. A film that progresses, has excellent tensile elastic modulus and tensile strength in the MD direction, and has excellent process transferability can be obtained.

The upper limit of the re-stretching ratio in the MD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 5.00 times or less. By setting the re-stretching ratio in the MD direction to 5.00 times or less, crushing in the thickness direction can be suppressed and a decrease in the porosity can be suppressed. Further, it is possible to suppress excessive anisotropy in the MD direction and the TD direction, and to prevent wrinkles in the MD direction and tears in the MD direction during film transfer.

The final re-stretching ratio in the TD direction is preferably selected within a range in which the product (area ratio) of the re-stretching ratio in the MD direction and the final re-stretching ratio in the dry-type re-stretching satisfies the conditions described later. It is preferably 40 times or more, more preferably 2.00 times or more, further preferably 2.50 times or more, and further preferably 3.00 times or more. The upper limit of the final re-stretching magnification in the TD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 5.00 times or less. By setting the final re-stretching ratio in the TD direction to 5.00 times or less, the occurrence of crushing in the thickness direction can be suppressed, and a decrease in the porosity can be prevented. In addition, it suppresses film rupture and enables stable film formation.

The product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction is preferably 2.00 times or more, more preferably 4.00 times or more, and 6.00 times or more. Is more preferable, and it is more preferable that the value is 8.00 times or more. The upper limit of the product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 15.00 times or less. By setting the product of the dry re-stretching ratio in the MD direction and the final dry re-stretching ratio in the TD direction to 15.00 times or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in the porosity. In addition, it suppresses film rupture and enables stable film formation.

The ratio of the dry re-stretching ratio to the wet stretching ratio (dry re-stretching ratio / wet stretching ratio) is preferably 0.16 or more, more preferably 0.20 or more, and preferably 0.25 or more. It is more preferably 0.30 or more, and further preferably 0.30 or more. The upper limit of the dry re-stretching ratio / wet stretching ratio varies depending on the resin composition of the film and the wet stretching ratio, but is preferably 0.60 or less. By setting the dry re-stretching ratio / wet stretching ratio to 0.60 or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in the porosity. In addition, it suppresses film rupture and enables stable film formation.

Further, it is preferable that the product (total area magnification) of the area magnification of wet stretching and the area magnification of dry re-stretching is 100 times or more. By setting the total area magnification to 100 times or more, more preferably 125 times or more, further preferably 150 times or more, still more preferably 200 times or more, the cleavage of the fibrils is promoted, the porosity is increased, and the permeability is increased. An excellent film can be obtained.

The upper limit of the total area magnification varies depending on the resin composition of the film, but is preferably 400 times or less. By setting the total area magnification to 400 times or less, it is possible to suppress the occurrence of crushing in the thickness direction and prevent a decrease in the porosity ratio. In addition, it suppresses film rupture and enables stable film formation.

(F) Other Steps Further, depending on other uses, the porous film can be hydrophilized. The hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge, or the like. The monomer graft is preferably carried out after the cross-linking treatment. It is preferable that the porous polyolefin film is crosslinked by irradiation with ionizing radiation such as α-ray, β-ray, γ-ray, and electron beam. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the porous polyolefin film.

In the case of surfactant treatment, any of nonionic surfactant, cationic surfactant, anionic surfactant or amphoteric surfactant can be used, but nonionic surfactant is preferable. The multilayer porous film is immersed in water or a solution prepared by dissolving a surfactant in a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the multilayer porous film by the doctor blade method.

The porous polyolefin film is made of a fluororesin porous body such as polyvinylidene fluoride or polytetrafluoroethylene, or a polyimide or polyphenylene sulfide for the purpose of improving meltdown characteristics and heat resistance when used as a battery separator. A surface coating such as a porous body or an inorganic coating such as ceramic may be applied.

The porous polyolefin film obtained as described above can be used for various purposes such as a filter, a separator for a fuel cell, and a separator for a capacitor. Especially when used as a separator for a battery, the output characteristics, safety and process Since it is excellent in transportability, it can be preferably used as a battery separator for a secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.

Hereinafter, the present invention will be described in detail with reference to Examples. The characteristics were measured and evaluated by the following methods.

1. 1. Weight average molecular weight (Mw), molecular weight distribution of film The weight average molecular weight of ultra-high molecular weight polyethylene and high-density polyethylene raw materials, and the molecular weight distribution of porous polyolefin film were determined by gel permeation chromatography (GPC) under the following conditions. ..
-Measuring device: GPC-150C manufactured by WATERS CORPORATION
-Column: SHOWEX UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
-Solvent (mobile phase): O-dichlorobenzene-Solvent flow rate: 1.0 mL / min-Sample concentration: 0.1 wt% (Dissolution condition: 135 ° C / 1H)
・ Injection amount: 500 μL
-Detector: WATERS CORPORATION differential refractometer (RI detector)
-Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a predetermined conversion constant.

2. Film thickness The film thickness of 5 points within the range of 50 mm x 50 mm of the polyolefin microporous film is measured by a contact thickness meter, Lightmatic VL-50 manufactured by Mitsutoyo Co., Ltd. (10.5 mmφ super hard spherical stylus, measuring load 0.01 N). The measurement was performed, and the average value was defined as the film thickness (μm).

3. 3. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), a needle with a spherical surface (radius of curvature R: 0.5 mm) at the tip and a diameter of 1 mm is applied to _{a porous film with an average film thickness of T 1} (μm) by 2 mm. _{Puncture at a speed of / sec, otherwise measure the maximum load L 1} (load immediately before penetration, unit: N) in accordance with JIS Z 1707 (2019), _{and L 2} = (L ₁ x 10). _{The puncture strength L 2} (N / 10 μm) when the film thickness was 10 μm was calculated by the formula of / T _1.

4. Pore ratio A sample was cut into a square of 50 mm × 50 mm square from the microporous polyolefin membrane, the thickness was measured by the above method, and the volume of the sample was determined from the thickness and area. In addition, the mass (g) of the sample was determined using an electronic balance, and ^{it was calculated from them and the polymer density (g / cm 3} ) using the following equation. The above measurement was performed at three different points in the same porous film, and the average value of the porosity was obtained.
Pore ratio = [(volume-mass / polymer density) / volume] x 100.
The polymer density was calculated assuming a constant value of 0.99 g / cm ^3.

5. Tensile strength / tensile elastic modulus The tensile strength / tensile elastic modulus in the MD direction and the TD direction was measured by a method based on ASTM D882 using a strip-shaped test piece having a width of 10 mm. The test piece was fixed using a double-sided tape (NW-15 manufactured by Nichiban Co., Ltd.), and the measurement was performed under the conditions of a distance between gripping tools of 20 mm and a test speed of 100 mm / min. The tensile elastic modulus (MPa) was determined by dividing the slope of the curve between the tensile tensions of 2 to 3 N by the cross-sectional area before the test.

6. Maximum pore diameter and average pore diameter Using a palm poromometer (manufactured by PMI, CFP-1500A), the maximum pore diameter and average pore diameter were measured in the order of Dry-up and Wet-up. Wet-Stay up-pressured porous polyolefin film surface tension sufficiently soaked in 1.59 × ¹⁰ -2 N / m of PMI Co. Galwick (trade name), the air is converted from pressure begins to penetrate The maximum pore diameter was used.

For the mean diameter, the pore diameter was converted from the pressure at the intersection of the pressure and the half slope of the flow rate curve in the Dry-up measurement and the pressure at the intersection of the Wet-up measurement curve. The following formula was used to convert the pressure and pore diameter.
d = C · γ / P
In the above formula, "d (μm)" is the pore size of the porous polyolefin film, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is the wetting tension of the immersion liquid. , A constant determined by the contact angle, etc.

Hereinafter, examples will be described in detail.

[Example 1]
Consists of 40 wt% ultrahigh molecular weight polyethylene (UHMWPE), weight average molecular weight Mw2 is 60 mass% high density polyethylene (HDPE) is 0.3 × 10 ⁶ in weight average molecular weight Mw1 is in 2.0 × 10 ⁶ 75% by mass of liquid paraffin was added to 25% by mass of the polyethylene composition, and 0.7% by mass of 2,6-G t-butyl-p-cresol and 1.1% by mass based on the mass of polyethylene in the mixture. Tetrax [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane was added as an antioxidant and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extruded product was taken up with a cooling roll at 15 ° C. to form a gel-like molded product. The obtained gel-like molded product was subjected to wet simultaneous biaxial stretching at 115 ° C. so as to be 5 × 5 times, and then the stretched sheet was immersed in methylene chloride at 25 ° C. to remove liquid paraffin. And air dried at room temperature. Then, the dried sheet is subjected to MD uniaxial stretching by a roll stretching machine under the conditions of a temperature of 113 ° C. and a stretching ratio of 2.00 times in the MD direction, and then a temperature of 130 ° C. and a stretching ratio of 3.00 times in the TD direction. Tenter stretching was carried out under the conditions of a relaxation rate of 8.0% and a final stretching ratio of 2.76 times to prepare a porous polyolefin film.

Table 1 shows the raw material characteristics, film forming conditions, and evaluation results of the obtained porous polyolefin film. This porous polyolefin film was excellent in porosity, puncture strength and MD tensile elastic modulus, and had characteristics suitable for a separator for a secondary battery.

[Examples 2 to 4]
A porous polyolefin film was prepared in the same manner as in Example 1 except that the raw materials shown in Table 1 were used as the raw material of the porous polyolefin film and the film forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are as shown in Table 3.

[Example 5]
The weight average molecular weight Mw1 is 2.0 × 10 ultra high molecular weight polyethylene (UHMWPE) 10 wt% liquid paraffin 90 wt% was added in ^6, 2 of 0.7 wt% as a further based on the weight of polyethylene in mixture, 6-di-t-butyl-p-cresol and 1.1% by mass tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane were added as antioxidants. It was melt-kneaded and a polyethylene resin solution was prepared in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extruded product was taken up with a cooling roll at 15 ° C. to form a gel-like molded product. The obtained gel-like molded product was subjected to wet simultaneous biaxial stretching at 115 ° C. so as to be 5 × 5 times, and then the stretched sheet was immersed in methylene chloride at 25 ° C. to remove liquid paraffin. And air dried at room temperature. Then, the dried sheet is sequentially tenter-stretched by a biaxial method under operating conditions of a temperature of 125 ° C., a stretching ratio of 2.00 times in the MD direction, a stretching ratio of 2.00 times in the TD direction, and a heat fixing temperature of 125 ° C. This was carried out to prepare a porous polyolefin film.

[Example 6]
A porous polyolefin film was prepared in the same manner as in Example 5 except that the raw materials shown in Table 1 were used as the raw material of the porous polyolefin film and the film forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are as shown in Table 3.

[Example 7]
The weight average molecular weight Mw1 is 1.5 × 10 80 wt% liquid paraffin ultra high molecular weight polyethylene (UHMWPE) 20 wt% was added in ^6, 2 of 0.7 wt% as a further based on the weight of polyethylene in mixture, 6-di-t-butyl-p-cresol and 1.1% by mass tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane were added as antioxidants. It was melt-kneaded and a polyethylene resin solution was prepared in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extruded product was taken up with a cooling roll at 15 ° C. to form a gel-like molded product. The obtained gel-like molded product was roll-stretched at 113 ° C. so as to be 8 times in the MD direction, and tenter-stretched at 124 ° C. so as to be 7.4 times in the TD direction (wet sequential two). Axial stretching). The stretched sheet was immersed in methylene chloride at 25 ° C. to remove liquid paraffin and air-dried at room temperature. Then, the dried sheet was tenter-stretched at 131 ° C. in the TD direction under the conditions of a draw ratio of 1.39 times, a relaxation rate of 6.5%, and a final draw ratio of 1.30 times to obtain a porous polyolefin film. Made.

[Examples 8 and 9]
A porous polyolefin film was prepared in the same manner as in Example 7 except that the raw materials shown in Table 1 were used as the raw material of the porous polyolefin film and the film forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are as shown in Table 3.

[Comparative example 1]
Consists of 40 wt% ultrahigh molecular weight polyethylene (UHMWPE), weight average molecular weight Mw2 is 60 mass% high density polyethylene (HDPE) is 0.3 × 10 ⁶ in weight average molecular weight Mw1 is in 2.0 × 10 ⁶ 75% by mass of liquid paraffin was added to 25% by mass of the polyethylene composition, and 0.7% by mass of 2,6-di-t-butyl-p-cresol and 1.1% by mass based on the mass of polyethylene in the mixture. Tetrakiss [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane was added as an antioxidant and melt-kneaded to prepare a polyethylene resin solution in a twin-screw extruder. Subsequently, this polyethylene resin solution was extruded from a sheet forming die installed at the tip of the extruder, and the obtained sheet-like extruded product was taken up with a cooling roll at 15 ° C. to form a gel-like molded product. The obtained gel-like molded product was subjected to wet simultaneous biaxial stretching at 115 ° C. so as to be 5 × 5 times, and then the stretched sheet was immersed in methylene chloride at 25 ° C. to remove liquid paraffin. Then, it was air-dried at room temperature to prepare a porous polyolefin film.

[Comparative Examples 2 to 5]
A porous polyolefin film was prepared in the same manner as in Comparative Example 1 except that the raw materials shown in Table 2 were used as the raw material of the porous polyolefin film and the film forming conditions were changed as shown in Table 1. The evaluation results of the obtained porous polyolefin film are as shown in Table 4.

In Example 1, wet simultaneous biaxial stretching was performed to remove liquid paraffin, and then dry re-stretching was performed. Compared to Comparative Example 1 in which dry re-stretching was not performed, the porosity and puncture strength were higher. Has been obtained. Further, by performing the dry re-stretching in the MD direction, the orientation in the MD direction progresses, a high tensile elastic modulus in the MD direction is obtained, and the transportability is excellent.

In Examples 2 to 4, high porosity and high puncture strength are obtained by performing dry re-stretching at a higher stretching ratio than in Example 1. Further, by increasing the dry re-stretching ratio in the MD direction, a higher tensile elastic modulus in the MD direction than in Example 1 is obtained, and the elastic modulus ratio E _MD / E _TD is large and the transportability is excellent. A film has been obtained.

In Examples 5 to 9, 100% by mass of UHMWPE is used as the raw material polyolefin resin, and high porosity, high puncture strength, and high tensile elastic modulus in the MD direction are obtained.

The porous polyolefin films of Examples 1 to 9 are excellent in output characteristics, safety, and transportability when used as a battery separator.

Comparative Example 2 is a porous polyolefin film obtained by performing dry re-stretching only in the TD direction as compared with Example 1, and good piercing strength was obtained, but the porosity was less than 50%, and in the future. There is room for improvement in terms of ion permeability when used as a battery separator for quick charging applications, which is required for the above.

Comparative Example 3 was subjected to dry sequential stretching under the conditions of 3.50 times in the MD direction and 5.43 times in the TD direction as compared with Example 1, and as a result, the film was ruptured.

Since Comparative Example 4 was not subjected to dry re-stretching as compared with Example 5, it was insufficient in all of the porosity, puncture strength, and tensile elastic modulus in the MD direction.

Compared with Example 5, 100% by mass of HDPE was used as a raw material in Comparative Example 5, and a good porosity was obtained. However, since it did not contain UHMWPE, the piercing strength was insufficient and safety was achieved. Is inferior.

Claims (10)

Porosity is 50% or more, the puncture strength of the film thickness 10μm conversion is not less than 3.7 N, porous polyolefin film longitudinal tensile modulus _{E MD} is characterized in that at least 980 MPa. The porous polyolefin film according to claim 1, wherein the E _MD / E _TD is 1.0 or more with respect to the E _MD and the tensile elastic modulus E _{TD in the width direction.} Tensile strength in a longitudinal direction _{M MD} is not less than 180 MPa, the porous polyolefin film according to claim 1 or 2. The porous polyolefin film according to any one of claims 1 to 3, wherein the average pore size is 10 nm or more and 140 nm or less. The porous polyolefin film according to any one of claims 1 to 4, wherein the ratio of the average pore diameter to the maximum pore diameter (average pore diameter) / (maximum pore diameter) is 0.70 or more. The porous polyolefin film according to any one of claims 1 to 5, wherein the film thickness is 15 μm or less. The porous polyolefin film according to any one of claims 1 to 6, which contains polyethylene as a main component. The film, in the molecular weight distribution by weight, the content of molecular weight 1.0 × 10 ⁶ or more polyolefin component is not less than 35 mass%, the porous polyolefin film according to claim 1. A battery separator using the porous polyolefin film according to any one of claims 1 to 8. A secondary battery using the battery separator according to claim 9.