Polyolefin microporous membrane and method for producing the same

Abstract

To provide a polyolefin microporous membrane that, when used as a battery separator, can impart high output characteristics and high safety and contribute to miniaturization and high output of batteries, and a method for producing the same.SOLUTION: A method for producing a polyolefin microporous membrane with a thickness of 10 μm or less includes: an extrusion step (step (a)) in which a resin composition containing a polyolefin resin and a pore-forming material is melt-kneaded and extruded; a sheet forming step (step (b)) in which the extrudate obtained in the step (a) is formed into a sheet; a primary stretching step (step (c)) in which a sheet-like molded product obtained in the step (b) is stretched at least once in at least one axial direction; a replacement step (step (d)) of replacing the pore-forming material from a stretched sheet obtained in the step (c); a step (step (e)) of drying a solvent-displaced membrane obtained in the step (d); a secondary stretching step (step (f)) in which the sheet obtained in the step (e) is stretched in a film flow direction (MD); and a step (step (g)) of stretching the film obtained in the step (f) in a film width direction (TD) and then heat-setting and / or heat-relaxing the film. In the step (f), a ratio (εmax / εmin) of a most downstream strain rate εmax (% / s) to a most upstream strain rate εmin (% / s) is 1.1 or more and 2.5 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a separation membrane used for separation, selective permeation, etc. of substances, and a polyolefin microporous membrane widely used as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and capacitors, and a method for producing the same.

Background Art

[0002] Polyolefin microporous membranes are widely used as separation membranes for separation and selective permeation of substances, and as separators for electrochemical elements such as alkaline secondary batteries and lithium secondary batteries. In particular, it is preferably used as a battery separator film for lithium secondary batteries. The reasons include that the polyolefin microporous membrane has excellent mechanical strength, shutdown temperature, and ion permeation performance.

[0003] Among them, lithium-ion secondary batteries are used in a wide range of applications such as notebook personal computers and smartphones. In particular, with the expansion of the electric vehicle market, higher output and higher safety of the battery are required. Therefore, as the performance of the polyolefin microporous membrane used as a separator film, thinning and low resistance are required. However, since the strength decreases accordingly, short circuits (foreign matter resistance) due to electrodes or foreign matters and film breakage (decrease in impact resistance) are likely to occur when the battery is impacted, and the safety of the battery decreases. Therefore, it is necessary to increase the strength of the resin constituting the polyolefin microporous membrane, or to increase the strength by manufacturing methods such as increasing the draw ratio and introducing secondary drawing.

[0004] In addition, in lithium-ion secondary batteries, since lithium precipitation is likely to occur, it is necessary to improve the dendrite resistance of the polyolefin microporous membrane used as a separator film. The dendrite of a lithium-ion secondary battery is a needle-like crystal generated near the interface between the negative electrode and the separator during charge and discharge, and when it grows and penetrates the separator, it may cause a short circuit. Therefore, a polyolefin microporous membrane with a small pore diameter is required to improve the dendrite resistance.

[0005] As described above, with the miniaturization and high output of lithium-ion secondary batteries, the polyolefin microporous membrane used for the separator film is required to achieve both high strength and small pore size in order to reduce the film resistance and ensure high safety.

[0006] As a method for manufacturing a polyolefin microporous membrane used for a separator film, a wet stretching (primary stretching) in which a sheet obtained by melt-kneading a resin portion constituting the microporous membrane and a plasticizer for forming pores is stretched before extracting the plasticizer, and a secondary stretching in which a film obtained after extracting and drying the plasticizer is stretched. In the wet method consisting of the above configuration, a method of setting the strain rate and the number of stretching steps in the secondary stretching process within a specific range is disclosed (Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in Patent Document 1, it is stated that the discharge characteristics are improved by setting the strain rate within a specific range. However, due to the safety requirements in recent years, the strength per unit film thickness and the reduction of the small hole diameter cannot be achieved. In Patent Document 2, the strain rate in the secondary stretching is deliberately increased towards the latter stage to generate high stress and suppress the curling property. However, if the ratio of the strain rate and the ratio of the tension are large, web floating is likely to occur in the stretching of thin film or high-speed feeding, and it is difficult to uniformly control the physical properties of the film. In Patent Document 3, it is stated that the stretching unevenness in the film flow direction (MD) can be improved by increasing the number of stretching stages from 3 to 5. However, when the speed and width are increased to increase the production capacity, the stretching stress increases. Therefore, in practice, if the ratio of the strain rate and the tension ratio are not manufactured within a specific range, the physical property difference in the MD and the film width direction (TD) becomes large. In particular, since the difference between the maximum and minimum air permeability resistance in the TD becomes large, when the polyolefin microporous membrane obtained by such a method is used as a separator in a non-aqueous secondary battery, the battery characteristics such as the rate characteristics and the cycle characteristics deteriorate.

Means for Solving the Problems

[0009] In order to solve the above problems and achieve the object, the present invention has the following configuration. (1) A method for manufacturing a polyolefin microporous membrane with a film thickness of 10 μm or less, comprising an extrusion step ((a) step) of melt-kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, a sheet forming step ((b) step) of forming the extrudate obtained in the (a) step into a sheet shape, a primary stretching step ((c) step) of stretching the sheet-shaped formed product obtained in the (b) step at least once in at least one axial direction, a substitution step ((d) step) of substituting the pore-forming material from the stretched sheet obtained in the (c) step, a drying step ((e) step) of drying the solvent-substituted membrane obtained in the (d) step, a secondary stretching step ((f) step) of stretching the sheet obtained in the (e) step in the film flow direction (MD), and a step ((g) step) of stretching the film obtained in the (f) step in the film width direction (TD) and then performing heat setting and / or heat relaxation, wherein the ratio (εmax / εmin) of the minimum strain rate εmin (% / s) to the maximum strain rate in the (g) step is 1.1 or more and 2.5 or less. (2) The method for manufacturing a polyolefin microporous membrane according to (1), wherein the (f) step has a step of performing multi-stage stretching using a plurality of rolls, and the ratio (Tn+1 / Tn) of the tension Tn (N) upstream of each roll to the tension Tn+1 (N) downstream thereof is 1.1 or more and 7.0 or less in all stretching stages of the multi-stage stretching. (3) The method for manufacturing a polyolefin microporous membrane according to (1) or (2), wherein the stretching temperature in the (f) step is (Tm - 60) °C or more and (Tm - 30) °C or less, where Tm is the melting point of the polyolefin resin. (4) A polyolefin microporous membrane having a film thickness of 10 μm or less, an average pore diameter determined by a porometer of 30 nm or less, a piercing strength of 50 gf / μm or more, and a difference R between the minimum and maximum air permeability resistances with respect to the average value of the air permeability in the film width direction (TD) of 10% or less. (5) The polyolefin microporous membrane according to (4), wherein the thickness deviation in the film width direction (TD) is 3% or less. [Advantages of the Invention]

[0010] According to the present invention, when used as a separator for a lithium-ion secondary battery, a polyolefin microporous membrane capable of imparting high output performance and high safety and contributing to miniaturization and high output of the battery, and a method for producing the same can be provided. Further, by setting the strain rate and tension in the secondary stretching step within a specific range, a polyolefin microporous membrane with small variation in in-plane air permeability and a method for producing the same can be provided. A secondary battery including such a polyolefin microporous membrane as a separator has a uniform in-plane electrochemical reaction and good battery characteristics such as rate characteristics and cycle characteristics.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments for carrying out the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments and can be variously modified and implemented within the scope of the gist thereof.

[0012] The method for producing a polyolefin microporous membrane of the present invention includes an extrusion step ((a) step) of melt-kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, a sheet forming step ((b) step) of forming the extrudate obtained in the (a) step into a sheet shape, a primary stretching step ((c) step) of stretching the sheet-shaped formed product obtained in the (b) step at least once in at least one axial direction, a substitution step ((d) step) of substituting the pore-forming material from the stretched sheet obtained in the (c) step, a step ((e) step) of drying the solvent-substituted membrane obtained in the (d) step, a secondary stretching step ((f) step) of stretching the sheet obtained in the (e) step in the film flow direction (TD), and a step ((g) step) of heat-fixing and / or heat-relaxing the film obtained in the (f) step after stretching it in the film width direction (TD). It is a method for producing a polyolefin microporous membrane with a film thickness of 10 μm or less, wherein the ratio (εmax / εmin) of the minimum strain rate εmin (% / s) to the maximum strain rate εmax (% / s) in the (f) step is 1.1 or more and 2.5 or less. Each step will be described below.

[0013] (a) Step: Extrusion Step (a) In the process, a resin composition containing a polyolefin resin and a pore-forming material is melt-kneaded and extruded. As the polyolefin resin used here, various polyethylene-based resins can be used, such as ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, linear low-density polyethylene, etc. Further, the polyethylene-based resin may be a homopolymer of ethylene or a copolymer of ethylene and other α-olefins. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, etc. Here, the polyethylene-based resin shall contain ethylene in an amount of 50 mol% or more based on all raw material monomer components.

[0014] The polyolefin microporous membrane according to the embodiment of the present invention preferably contains ultra-high molecular weight polyethylene (hereinafter described as resin A) and high-density polyethylene (hereinafter described as resin B) among the above-mentioned polyethylenes.

[0015] The ultra-high molecular weight polyethylene used as resin A preferably has a weight average molecular weight (Mw) of 800,000 or more, more preferably 900,000 or more, and even more preferably 1,000,000 or more. Also, the weight average molecular weight (Mw) is preferably 2,500,000 or less, more preferably 2,000,000 or less, and even more preferably 1,500,000 or less. By setting the weight average molecular weight of resin A within the above range, the strength of the polyolefin microporous membrane can be increased and the pore structure can be refined.

[0016] The melting point of resin A is preferably 135°C or lower, more preferably 134°C or lower. By setting the melting point of resin A within the above range, it becomes easy to refine the pore structure of the polyolefin microporous membrane, and the shutdown characteristics are also good. Also, from the viewpoint of compatibility with permeability and strength, the melting point of resin A is preferably 125°C or higher, more preferably 127°C or higher, and even more preferably 130°C or higher.

[0017] Incidentally, by controlling the molecular structure (monomer species) constituting Resin A and adjusting the number and length of side chains, it is possible to set the melting point of Resin A within the above range. For example, in the case of a polyethylene-based resin, a method of copolymerizing ethylene with another α-olefin and controlling the copolymerization ratio and the type of α-olefin used can be mentioned.

[0018] The content of Resin A in the polyolefin microporous membrane is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more. Also, it is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less. By setting the content of Resin A in the polyolefin microporous membrane within the above range, it becomes easy to increase the membrane strength and to refine the pore structure.

[0019] The high-density polyethylene (density: 0.940 g / m 3 or more and 0.970 g / m 3 or less) used as Resin B preferably has a weight-average molecular weight (Mw) of 10,000 or more, more preferably 20,000 or more, and even more preferably 50,000 or more. Also, the weight-average molecular weight (Mw) is preferably 200,000 or less, more preferably 150,000 or less, and even more preferably 100,000 or less. By setting the weight-average molecular weight of Resin B within the above range, a structure in which fibrils are evenly oriented in the film plane after stretching can be obtained, and it becomes easy to adjust the physical property deviation in the plane to a preferable range.

[0020] The melting point of Resin B is more preferably 128°C or higher, and even more preferably 130°C or higher. Also, it is preferably 135°C or lower, and more preferably 134°C or lower. By setting the melting point of Resin B within the above range, in addition to the pore structure being refined, excellent shutdown characteristics can be obtained.

[0021] The heat of crystal fusion (ΔH) of resin B measured by differential scanning calorimetry (DSC) is preferably 200 J / g or more, more preferably 210 J / g or more, and even more preferably 220 J / g or more. By setting the heat of crystal fusion (ΔH) of resin B within the above range, the film strength can be increased while suppressing an increase in the shutdown temperature. Although no upper limit is particularly provided for the heat of crystal fusion (ΔH) of resin B from the above viewpoints, it is preferably 280 J / g or less from the viewpoint of film-forming properties.

[0022] The content of resin B in the polyolefin microporous membrane is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more. Also, it is preferably 70% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. By setting the content of resin B in the polyolefin microporous membrane within the above range, a structure in which fibrils are evenly oriented in the film plane after stretching can be obtained, and it becomes easy to adjust the physical property deviation in the plane to a preferable range.

[0023] The polyolefin microporous membrane according to an embodiment of the present invention may contain a resin other than a polyethylene-based resin. For example, adding a polypropylene-based resin is preferable from the viewpoint of improving the heat resistance of the microporous membrane. As the type of the polypropylene-based resin, in addition to homopolypropylene, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer can contain a copolymer component with an α-olefin other than propylene. Examples of the α-olefin include ethylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. Here, the polypropylene-based resin is defined as containing propylene in an amount exceeding 50 mol% with respect to all raw material monomer components.

[0024] The addition amount of the polypropylene-based resin is preferably 20% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less based on the total mass of the polyolefin microporous membrane. By setting it within the above range, a polyolefin microporous membrane excellent in productivity, quality, and strength can be obtained.

[0025] The polyolefin microporous membrane can contain resin components other than the polyethylene-based resin and the polypropylene-based resin, if necessary. Further, various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent, a filler, a crystal nucleating agent, and a crystallization retarder may be contained within a range that does not impair the effects of the present invention.

[0026] The above polyolefin resin is heated and dissolved in a pore-forming material to prepare a resin composition containing the polyolefin resin and the pore-forming material. The pore-forming material is not particularly limited, and examples thereof include a plasticizer (film-forming solvent). The plasticizer is not particularly limited as long as it is a solvent capable of uniformly dispersing the polyolefin resin, but it is preferably a liquid at room temperature in order to enable relatively high magnification stretching. In the present embodiment, as the pore-forming material (plasticizer) added to the polyethylene resin, aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin are preferred because they have high compatibility when the polyolefin resin is polyethylene or polypropylene and the solution becomes uniform. Further, from the viewpoint of stabilizing the content during film formation, it is more preferable to use a non-volatile solvent as the pore-forming material. These plasticizers may be recovered and reused by operations such as extraction, distillation, and reflux.

[0027] The ratio of the polyolefin resin to the pore-forming material is not particularly limited, but the content of the polyolefin resin is preferably 10 to 50% by mass based on the total mass of the resin composition. By setting the content of the polyolefin resin within the above range, the dispersion state of the polyolefin resin and the plasticizer becomes good, and the resulting microporous membrane has excellent strength, permeability, and heat resistance. Also, the formability of the sheet and the film-forming property become good. When the weight ratio of the resin is 50% by mass or less, the formability of the sheet is improved and the strength is likely to increase. When it is 10% by mass or more, it is easy to form the pore structure required for the separator.

[0028] The method of kneading the polyolefin resin and the pore-forming material is not particularly limited, but the polyolefin resin and the plasticizer are melt-kneaded by a screw extruder such as a single-screw extruder or a twin-screw extruder with a predetermined formulation. The resin temperature during kneading is preferably 150°C or higher, more preferably 160°C or higher, still more preferably 180°C or higher, and the upper limit is preferably 250°C or lower, more preferably 240°C or lower, still more preferably 230°C or lower. By setting the temperature of the polyolefin resin composition during kneading within the above range, a decrease in strength due to resin deterioration can be prevented, and the polyolefin resin and the plasticizer can be uniformly melt-kneaded.

[0029] (b) Step: Sheet forming step Next, in the sheet forming step (b), the melt of the polyolefin resin composition obtained in step (a) is supplied from an extruder to a die and extruded into a sheet. The extrusion method may be either the T-die method or the inflation method. Further, a plurality of polyolefin resin compositions of the same or different compositions may be supplied from a plurality of extruders to a single multi-manifold type composite T-die and laminated in layers, and then extruded into a laminated sheet. After the melt-kneaded product is formed into a sheet by the die, it is cooled and solidified by contacting it with a temperature-controlled metal roll. At this time, the temperature of the metal roll to be contacted is preferably 10-50°C. In the cooling step, it is preferable to cool it to 10-50°C before the first stretching step described later. Generally, when the cooling rate is slow, relatively large crystals are formed, so the higher-order structure of the gel-like sheet becomes rough, and the gel structure forming it also becomes large. On the other hand, when the cooling rate is fast, relatively small crystals are formed, so the higher-order structure of the gel-like sheet becomes dense and uniform stretching becomes possible, so that the strength of the polyolefin microporous membrane can be increased or the pore structure can be refined.

[0030] (c) Step: Primary stretching step Next, in step (c), the sheet-shaped molded product obtained through the sheet forming step is stretched at least once in at least a uniaxial direction. The stretching method is not particularly limited, but after heating, it is preferably stretched at a predetermined magnification by the tenter method, the roll method, or a combination thereof. The stretching step performed before the extraction step (d) is referred to as "primary stretching". The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used, but simultaneous biaxial stretching is preferred.

[0031] The area magnification of the single-stage biaxial stretching is preferably 16 times or more and 100 times or less, more preferably 25 times or more and 50 times or less. Also, the stretching ratios in the MD and TD directions may be the same or different from each other, but are preferably 4 times or more and 10 times or less, more preferably 5 times or more and 9 times or less. By setting the area stretching ratio to 16 times or more, an improvement in mechanical strength and permeability can be expected. Here, the stretching ratio is the area stretching ratio based on the sheet before stretching.

[0032] When a polyethylene resin is used as the polyolefin resin in the single-stage stretching, the stretching temperature is preferably in the temperature range of the melting point (Tm - 40°C) to (Tm - 5°C) of ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene resin compositions. Since the polyethylene resin usually has a crystal dispersion temperature of about 125 to 135°C, the stretching temperature is preferably 90 to 130°C, more preferably 105°C or higher, even more preferably 110°C or higher, and also more preferably 120°C or lower, even more preferably 117°C or lower.

[0033] (d) Step: Plasticizer substitution step (d) In the process, the pore-forming material is replaced from the stretched sheet obtained in the (c) process to obtain a solvent-substituted membrane. The method for replacing the pore-forming material is not particularly limited. For example, a method of removing (washing) the pore-forming material (plasticizer, film-forming solvent) using a washing solvent can be mentioned. In the stretched sheet obtained in the (c) process, the polyolefin resin phase is phase-separated from the pore-forming material (film-forming solvent) phase. Therefore, when the pore-forming material (film-forming solvent) is removed, a porous membrane composed of fibrils forming a fine three-dimensional network structure and having three-dimensionally irregularly communicating pores (voids) is obtained. The obtained stretched product is washed with a solvent to remove the remaining solvent. As the washing solvent, volatile ones such as hydrocarbons such as pentane, hexane, and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as trifluoroethane, and ethers such as diethyl ether and dioxane can be used. These solvents can be appropriately selected according to the solvent used for dissolving the polyolefin composition and used alone or in combination. The washing method can be carried out by a method of immersing in a solvent and extracting, a method of showering the solvent on both sides of the sheet, or a method by a combination of these. The plasticizer extracted by these methods and the washing solvent used for extraction may be recovered and reused by operations such as extraction, distillation, and reflux.

[0034] (e) Process: Drying process (e) In the process, the solvent-substituted membrane from which the pore-forming material has been removed obtained in the (d) process is dried. As the drying method, a heat drying method or an air drying method can be mentioned. The drying temperature is preferably 10°C or higher and 80°C or lower, and more preferably 20°C or higher and 60°C or lower. It is more preferable to carry out the drying until the washing solvent becomes 3 parts by mass or less with the total mass of the solvent-substituted membrane being 100 parts by mass (dry mass).

[0035] (f) Process: Second stretching process In the second stretching step, the porous membrane sheet obtained through step (e) is heated at a predetermined temperature and stretched in the film flow direction (MD). This stretching step (the stretching step performed after the (e) drying step) is called "secondary stretching", and the membrane obtained by secondary stretching is called a "secondary stretched membrane". In secondary stretching, the porous membrane sheet obtained through the extraction step of (e) is stretched in the MD direction. From the viewpoint of easily controlling the strain rate at each stage, it is preferable to use a multi-stage roll stretching machine as the stretching method. In secondary stretching, multi-stage stretching in which stretching is performed by distributing the stretching ratio at each stage so as to reach the final ratio is preferable. When performing multi-stage stretching, stretching is performed such that the strain rate in the stretching stage after the n-th stage is greater than the strain rate in the previous stage. For example, it is preferable to control the strain rate in the (n + 1)-th stretching stage to be greater than the strain rate in the n-th stage. By stretching such that the strain rate is greater than that in the previous stretching stage, the torque in roll stretching can be stably stretched.

[0036] When the polyolefin microporous membrane is made into a thin film of 10 μm or less, the uniformity in the film width direction (TD) deteriorates significantly. The inventors of the present invention intensively studied the cause of such a problem and found that when a polyolefin microporous membrane of a thin film of 10 μm or less is roll stretched, the process tension in the roll stretching process becomes small, so web floating occurs, and the peeling point and landing point on the roll are difficult to determine, and stress is difficult to propagate uniformly. When this web floating occurs, the uniformity in the film width direction (TD) of the obtained polyolefin microporous membrane is impaired, and the air permeability resistance deviation in TD or the TD thickness deviation becomes large.

[0037] As a result of intensive studies by the inventors to solve the above problems, in the method for producing a polyolefin microporous membrane with a thin film thickness of 10 μm or less, it has been found that by controlling the stretching conditions in the film flow direction (MD) of the secondary stretching step, the uniformity in the film width direction (TD) can be improved. That is, it is preferable to control the strain rate ratio of stretching in the film flow direction (MD) in the secondary stretching step. When the strain rate in the first stage, which is the most upstream stage of the secondary stretching step, is εmin (% / s) and the strain rate at the most downstream in the secondary stretching step is εmax (% / s), it is preferable that εmax (% / s) / εmin (% / s) is within 2.5 or less, and more preferably within the range of 2.0 or less. If the strain rate ratio is in a range larger than the above, the speed ratio and the tension ratio become large, and it becomes impossible to maintain a film structure with small in-plane variations, which is not preferable. Also, the lower limit of the strain rate ratio is preferably 1.1 or more, and more preferably 1.3 or more. If it is less than 1.1, the film stretched upstream contracts due to elastic recovery force, etc., the roll torque becomes unstable, and the uniformity of the TD of the film is impaired, which is not preferable.

[0038] It is presumed that the stress is uniformly dispersed, and as the stress propagates, a more uniform and dense pore structure can be formed. The calculation of the strain rate at each stage in the secondary stretching is performed as follows.

[0039] Strain rate (% / second) = (Stretching ratio - 1) × 100 ÷ Stretching time (seconds) Stretching time (seconds) = Stretching section (m) ÷ Average velocity between two points (m / second) Here, as the stretching section, the tangential distance between the upstream roll and the downstream roll when the film peels off from the upstream roll and lands on the downstream roll in each stretching stage is adopted. In the multi-stage roll stretching machine in the secondary stretching step, the roll diameter and the distance between rolls in each stretching stage may be constant or may not be unified.

[0040] Also, in multi-stage stretching, it is similarly preferable to set the tension ratio between the upstream and downstream of the roll within a specific range. Starting from the top of the roll in the nth stage, the upstream tension, that is, the tension difference generated between the rolls in the (n - 1)th stage and the nth stage, is calculated as follows. Here, the free torque represents the torque when the roll is rotated at the same speed and temperature as the stretching conditions in the no-load state. tn(N)=|Torque(%) - Free torque(%)|×Rated torque(N·m)÷Roll radius(m)÷Reduction ratio(-) Therefore, in the roll of the nth stage, the tension Tn(N) generated upstream starting from the top of the tower is Tn=t0 + t1+…+tn, and the tension generated downstream is Tn+1=t0 + t1+…tn+1. Here, t0 uses the process tension of the film at the entrance of the secondary stretching process. In roll stretching, if the tension ratio on the upstream side of the roll with respect to the downstream side of the roll starting from the top of the roll is large, the film cannot be properly gripped on the roll, the landing point and peeling point of the film are not determined, and the strain rate cannot be controlled. Therefore, in the process of performing multi-stage stretching using a plurality of rolls in the (f) process, it is preferable that the tension ratio Tn+1 / Tn in each stretching stage is 1.1 or more and 7.0 or less, and more preferably 2.5 or more and 6.0 or less.

[0041] The stretching ratio in the MD of the secondary stretching process is preferably 1.2 times or more, more preferably 1.4 times or more, still more preferably 1.6 times or more, and preferably 4 times or less. By setting the stretching ratio in the MD of the secondary stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within a desired range.

[0042] In this project, the stretching temperature of MD is preferably in the temperature range of (Tm - 60°C) to (Tm - 30°C) of polyethylene and the polyethylene resin composition. Since the polyethylene resin usually has a crystalline dispersion temperature of about 125 to 135°C, the stretching temperature of MD is preferably 80°C or higher, preferably 120°C or lower, and more preferably 105°C or lower. By setting the stretching temperature in the MD direction within the above range, the obtained polyolefin microporous membrane has excellent permeability and strength, can be stretched uniformly, and can be stretched while maintaining a small pore diameter.

[0043] If the roll stretching in the secondary stretching process is continuously used, the roll surface will become dirty and the friction coefficient between the roll and the film will decrease, which will affect the quality. Therefore, a cleaning mechanism may be provided to maintain the friction coefficient between the roll and the film. The cleaning method is not particularly limited, but it is preferable to clean by pressing a cleaning cloth or a scraper against the roll.

[0044] The number of stretching stages when performing multi-stage stretching in the secondary stretching process is not particularly limited, but it is preferably 5 or more, more preferably 6 or more, and preferably 7 or more and 15 or less.

[0045] (g) Step: Heat fixing and / or heat relaxation step In the (g) step, after stretching the film obtained in the (f) step in the film width direction (TD), heat fixing and / or heat relaxation is performed. The stretching ratio of TD at this time is preferably 1.2 times or more, more preferably 1.4 times or more, still more preferably 1.6 times or more, particularly preferably 1.7 times or more, and preferably 4 times or less. By setting the stretching ratio in the TD direction of dry stretching within the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within a desired range.

[0046] The stretching method is not particularly limited, but after heating, it is preferably stretched at a predetermined ratio by the tenter method, the roll method, the inflation method, or a combination thereof.

[0047] In this process, the stretching temperature in the TD direction is preferably 80 °C or higher, more preferably 100 °C or higher. Also, it is preferably 145 °C or lower, more preferably 135 °C or lower. By setting the stretching temperature in the TD direction within the above range, the resulting polyolefin microporous membrane has excellent permeability and strength and can be stretched uniformly.

[0048] Also, in this process, after obtaining the above stretching, the dried polyolefin microporous membrane can be heat-treated. The heat treatment stabilizes the crystals and homogenizes the lamellae. As the heat treatment method, heat fixation treatment and / or heat relaxation treatment can be used. Heat fixation treatment is a heat treatment in which heating is performed while maintaining the dimensions of the membrane unchanged. Heat relaxation treatment is a heat treatment in which the membrane is thermally shrunk in the MD direction and / or TD direction during heating. The heat fixation treatment is preferably performed by a tenter method or a roll method.

[0049] The relaxation rate in the heat relaxation treatment is the value obtained by dividing the dimensions of the membrane after the relaxation treatment by the dimensions of the membrane before the relaxation treatment. The relaxation rates in the MD and TD directions of the membrane are both preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. Also, from the viewpoint of the flatness of the microporous membrane, it is preferably 0.80 or more, more preferably 0.90 or more.

[0050] The polyolefin microporous film obtained through the above steps (a) to (g) is wound up after cutting and removing both ends held by clips or the like as necessary to obtain an intermediate product roll. In addition, during winding, it is preferable to wind while oscillating with an amplitude of 100 to 200 mm in the width direction in order to make the outer diameter of the intermediate product roll uniform. The winding width used in this evaluation was the width of this intermediate product roll as the winding width.

[0051] The method for producing a polyolefin microporous membrane of the present invention can provide a polyolefin microporous membrane excellent in uniformity in the film width direction (TD) even if it is a thin film.

[0052] As one aspect of the present invention, a polyolefin microporous membrane can be mentioned in which the difference R between the minimum and maximum of the air permeability resistance with respect to the average value of the air permeability in the film width direction (TD) is 10% or less. By setting the difference R between the minimum and maximum of the air permeability resistance with respect to the average value of the air permeability in the film width direction (TD) within the above range, a polyolefin microporous membrane having a film structure with small variation in air permeability in the plane can be obtained. When such a polyolefin microporous membrane is used as a separator in a non-aqueous secondary battery, the electrochemical reaction in the plane occurs uniformly, and the battery characteristics such as rate characteristics and cycle characteristics tend to be improved. In addition, the air permeability of the polyolefin microporous membrane is preferably 1000 seconds or less, more preferably 500 seconds or less, and even more preferably 300 seconds or less.

[0053] As one aspect of the present invention, a polyolefin microporous membrane in which the thickness deviation in the film width direction (TD) is 3% or less can be mentioned. When the thickness deviation in the film width direction (TD) is within the above range, a polyolefin microporous membrane with small variation in air permeability in the plane can be obtained.

[0054] As one aspect of the present invention, a polyolefin microporous membrane in which the thickness deviation in the machine direction (MD) of the film is 3% or less can be mentioned. When the thickness deviation in the machine direction (MD) of the film is within the above range, a polyolefin microporous membrane with small variation in air permeability in the plane can be obtained.

[0055] As one aspect of the present invention, a polyolefin microporous membrane having a film thickness of 10 μm or less, an average pore diameter determined by a porometer of 30 nm or less, and a piercing strength of 50 gf / μm or more can be mentioned. The piercing strength is preferably 53 gf / μm or more and 80 gf / μm or less. The average pore diameter determined by a porometer is more preferably 10 nm or more and 25 nm or less. By setting the piercing strength and the average pore diameter determined by a porometer within the above ranges, it is possible to achieve both mechanical strength and high safety. Further, when the bubble point pore diameter (maximum pore diameter) is 40 nm or less, it is preferable because the strength and safety can be further improved.

[0056] As one aspect of the present invention, a polyolefin microporous membrane having a porosity of 30% or more can be mentioned. The porosity is more preferably 35% or more. The upper limit of the porosity is not particularly limited, but is preferably 80% or less because a decrease in film strength can be suppressed. When the porosity is within the above range, the microporous membrane has excellent output characteristics when used as a separator for a secondary battery. The porosity can be set within the above range by adjusting the raw material formulation, draw ratio, heat setting conditions, etc. in the manufacturing process.

Examples

[0057] The present invention will be described in more detail with reference to examples, but the embodiments of the present invention are not limited to these examples. In addition, the evaluation in this application was carried out in an environment of a temperature of 23°C and a humidity of 65% unless otherwise specified. The evaluation methods and analysis methods used in the examples are as follows.

[0058] (Measurement method) (Film thickness) The film thicknesses at any five points within a 50 mm × 50 mm range of the polyolefin microporous membrane were measured with a contact thickness gauge, "Lightmatic" (registered trademark) VL-50 manufactured by Mitutoyo Corporation (10.5 mmφ carbide spherical measuring head, measuring load 0.01 N), and the average value was taken as the thickness (μm).

[0059] (Basis weight) The basis weight of the polyolefin microporous membrane was calculated by the following formula after cutting a 50 mm × 50 mm square sample from the polyolefin microporous membrane and measuring the mass (g) at room temperature of 25°C. Basis weight (g / m 2 ) = mass (g) / (50 (mm) × 50 (mm)) × 10 6 (Porosity) A 50 mm × 50 mm square sample was cut from the polyolefin microporous membrane, and its volume (cm 3 ) and mass (g) were measured. From these values and the PE bulk density (g / cm3), the porosity of the polyolefin microporous membrane was calculated by the following formula. The PE bulk density was calculated assuming a constant value of 0.99 g / cm 3 . This measurement was performed by cutting samples from three arbitrary positions on the polyolefin microporous membrane and calculating the average value of the measured porosities.

[0060] Porosity (%) = [(volume - mass / PE bulk density) / volume] × 100.

[0061] (Air permeability) For the polyolefin microporous membrane, in accordance with JIS P-8117:2009, the air permeability (seconds / 100 cm 3 ) was measured at 25°C in an atmosphere using a Wang Research air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T).

[0062] (Puncture strength) The puncture strength was measured in accordance with JIS Z 1707 (2019), except that the test speed was 2 mm / second. Using a force gauge (DS2-20N manufactured by Imada Co., Ltd.), the maximum load (mN) when a polyolefin microporous membrane was punctured with a needle having a diameter of 1.0 mm and a spherical tip (curvature radius R: 0.5 mm) was measured, and the value obtained by dividing this by the film thickness was defined as the puncture strength.

[0063] (Measurement of TD air permeability deviation) In the sample wound up according to the above embodiment, in the range of 10% inside the full width from both ends to the center in the TD direction, the air permeability resistance was measured at intervals of 5% of the full width, and the average value, the difference R between the largest value and the smallest value, and were obtained. The measurement method was the same as the above air permeability measurement. Further, the following was used as an index representing the difference in air permeability resistance in the TD direction. Air permeability resistance deviation = R ÷ Ave. × 100 (%).

[0064] (Bubble point pore diameter and average pore diameter) Using a palm porometer (trade name, model: CFP-1500A) manufactured by PMI, measurements were taken in the order of Dry-up and Wet-up. For Wet-up, pressure was applied to a microporous membrane sufficiently immersed in Galwick (trade name) with a known surface tension, and the pore diameter converted from the pressure at which air began to penetrate was defined as the bubble point pore diameter (maximum pore diameter). For the average pore diameter, the pore diameter was converted from the pressure at the point where the curve showing half the slope of the pressure-flow rate curve in the Dry-up measurement intersects the curve in the Wet-up measurement. The following mathematical formula was used for the conversion between pressure and pore diameter.

[0065] d = C·γ / P In the formula, "d (μm)" is the pore diameter of the microporous membrane, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" was taken as a constant. Three points, two points 25% inside from both ends toward the center and one point at the center, were measured with respect to the winding width, and the average value was obtained.

[0066] (Measurement of MD thickness deviation) For the film thickness measurement in the MD direction, a transmittance diameter was installed at the outlet of the (f) process, and the film thickness deviation in-line was measured. Keyence's IB-30 (wavelength 660 mm) was used for the apparatus, and the average value of 1024 times was measured at a measurement period of 100 μs. Further, for the calculation of the evaluation index, the coefficient of variation obtained from the following formula using the average transmittance of the transmitted light intensity and the deviation of the transmitted light intensity obtained from the transmitted light intensity meter was used as an index of the film thickness deviation for evaluation. MD thickness deviation (%) = (transmitted light intensity deviation (standard deviation) / average transmittance) × 100 (Measurement of TD thickness deviation) Using a continuous film thickness measuring instrument (manufactured by Meisan Co., Ltd., RC-1), the range from a point 10% inward in the central direction to the other end 10% inward in the central direction with respect to one end of the wound sample width was measured at a measurement pitch of 0.048 mm. Also, for calculating the evaluation index, the average film thickness and the standard deviation of the film thickness were used as follows.

[0067] TD thickness deviation (%) = (standard deviation of film thickness / average film thickness) × 100.

[0068] (Example 1) To the polyolefin raw material, 70% by mass of ultra-high molecular weight polyethylene with an Mw of 1.2×10 6 and a melting point of 134°C was used as resin A, and 30% by mass of high-density polyethylene with an Mw of 6.0×10 4 and a melting point of 132°C was used as resin B. 75% by mass of liquid paraffin was added to 25% by mass of the above polyolefin raw material, and further 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis(methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate)methane were added as antioxidants based on the mass of the ultra-high molecular weight polyethylene and mixed to prepare a polyolefin resin composition. The obtained polyolefin resin composition was put into a twin-screw extruder and kneaded at 180°C to prepare a polyolefin solution. The obtained polyolefin solution was supplied to a T-die adjusted to 200°C, formed into a sheet through the die, and then cooled with a casting drum controlled at 35°C to form a gel sheet. The gel sheet was first stretched at 110°C in both the MD direction and the TD direction with a stretching ratio of 5 times using a simultaneous biaxial tenter stretching machine. After the wet-stretched gel sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then air-dried. Subsequently, as the second stretching, the microporous membrane was set on a roll set at 80°C with a roll-type stretching machine, the number of stretching steps was set to 10 so that the stretching ratio became 2.0 times, and the strain rate ratio εmax / εmin After stretching in the MD direction so that it became 2.0, it was further stretched 1.7 times in the TD direction at 130 °C using a tenter stretching machine to obtain a polyolefin microporous membrane. The film formation conditions of the polyolefin microporous membrane and the evaluation results of the obtained polyolefin microporous membrane are shown in Table 1.

[0069] (Examples 2 to 7, Comparative Examples 1 to 5) It was carried out under the same conditions as in Example 1, except that the stretching ratio, stretching temperature, number of stretching steps, and strain rate ratio (εmax / εmin) in the secondary stretching process were changed.

[0070] [Table 1]

Claims

1. An extrusion step ((a) step) of melt-kneading and extruding a resin composition containing a polyolefin resin and a pore-forming material, A sheet forming step ((b) step) of forming the extrudate obtained in the step (a) into a sheet, A primary stretching step ((c) step) of stretching the sheet-like formed product obtained in the step (b) at least once in at least one axial direction, A substitution step ((d) step) of substituting the pore-forming material from the stretched sheet obtained in the step (c), A step ((e) step) of drying the solvent-substituted membrane obtained in the step (d), A secondary stretching step ((f) step) of stretching the sheet obtained in the step (e) in the film flow direction (MD), A method for producing a polyolefin microporous membrane having a film thickness of 10 μm or less, comprising a step ((g) step) of stretching the film obtained in the step (f) in the film width direction (TD) and then thermally fixing and / or thermally relaxing it, wherein the ratio (εmax / εmin) of the minimum strain rate εmin (% / s) to the maximum strain rate in the step (g) is 1.1 or more and 2.5 or less.

2. The method for producing a polyolefin microporous membrane according to claim 1, wherein the step (f) has a step of performing multi-stage stretching using a plurality of rolls, and the ratio (Tn+1 / Tn) of the tension Tn (N) upstream of each roll to the tension Tn+1 (N) downstream thereof is 1.1 or more and 7.0 or less in all stretching stages of the multi-stage stretching.

3. The method for producing a polyolefin microporous membrane according to claim 1 or 2, wherein the stretching temperature in the step (f) is (Tm - 60) °C or higher and (Tm - 30) °C or lower, where Tm is the melting point of the polyolefin resin.

4. A polyolefin microporous membrane having a film thickness of 10 μm or less, an average pore diameter determined by a porometer of 30 nm or less, a puncture strength of 50 gf / μm or more, and a difference R between the minimum and maximum air permeability resistances with respect to the average value of the air permeability in the film width direction (TD) of 10% or less.

5. The polyolefin microporous membrane according to claim 4, wherein the thickness deviation in the film width direction (TD) is 3% or less. ​

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