Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B27/00—Layered products comprising a layer of synthetic resin
  • B32B27/32—Layered products comprising a layer of synthetic resin comprising polyolefins
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B5/00—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts
  • B32B5/22—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed
  • B32B5/32—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed at least two layers being foamed and next to each other
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
  • C08J9/26—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a solid phase from a macromolecular composition or article, e.g. leaching out
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/409—Separators, membranes or diaphragms characterised by the material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
  • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
  • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
  • H01M50/491—Porosity
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to a polyolefin microporous membrane, a multilayer polyolefin microporous membrane, and a battery.
• Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors.
• filters such as filtration membranes and dialysis membranes
• separators for batteries and separators for electrolytic capacitors.
• the microporous film made of polyolefin as a resin material is widely used as a secondary battery separator in recent years because it has excellent chemical resistance, insulating properties, mechanical strength, and the like and has shutdown characteristics.
• the secondary battery separator has a current that flows between the electrodes due to blockage of the separator holes at high temperatures to prevent mechanical strength and to prevent thermal runaway of the battery in order to prevent short circuits due to foreign matter entering the wound body or battery.
• Shutdown characteristics for shutting off are required. The shutdown characteristics are generally considered to be better as the temperature at which the holes are closed (shutdown temperature) is lower.
• a characteristic viscosity of 5 ⁇ 10 5 Pa ⁇ s or more and 5 ⁇ 10 6 Pa ⁇ s or less and a polyolefin having a non-Newtonian fluidity of 0.15 or more and 0.4 or less and a crystallinity of Disclosed is a separator for a non-aqueous electrolyte battery, which comprises a porous substrate having a content of 65% or more and 85% or less, and a heat-resistant porous layer provided on at least one surface of the porous substrate and containing a heat-resistant resin.
• a separator for a non-aqueous electrolyte battery is provided that has excellent mechanical strength and shutdown characteristics and is also excellent in short circuit resistance at high temperatures.
• Patent Document 2 when measured by DSC, the crystallinity is 65 to 85%, the proportion of lamella crystals in the crystal is 30 to 85%, the crystal length is 5 nm to 50 nm, and A microporous polyolefin membrane having a crystal length of 3 nm to 30 nm is disclosed. It is described that a polyolefin microporous membrane that provides excellent mechanical strength and shutdown characteristics even when combined with a heat-resistant porous layer and that can prevent the electrolyte solution from running out is provided.
• the weight average molecular weight of 1 ⁇ 10 6 or more (a) is 21 to 60% by weight, the weight average molecular weight of 1 ⁇ 10 4 or more and less than 1 ⁇ 10 6 (b) and the weight average molecular weight.
• the total of the fraction (c) of less than 1 ⁇ 10 4 is 40 to 79% by weight, the fraction (c) is 30% by weight or less, and the weight average molecular weight / number average molecular weight is 7 to 50
• a polyolefin microporous membrane having a bubble point value of more than 980 kPa is disclosed. It is described that a microporous polyolefin membrane having excellent balance of air permeability, porosity, pore diameter, bubble point value, mechanical strength, dimensional stability, shutdown characteristics and meltdown characteristics and a method for producing the same are provided. .
• Secondary batteries such as lithium-ion secondary batteries, are widely used as batteries for personal computers, mobile phones, etc. because of their high energy density.
• the secondary battery is also expected as a power source for driving a motor of an electric vehicle or a hybrid vehicle and a stationary storage battery.
• the present inventors have conducted extensive studies to solve the above problems, the highly stretched microporous film is excellent in mechanical strength, but the structure in the in-plane direction and the film thickness direction is anisotropic, It has been found that the structural change leading to shutdown is less likely to occur at a lower temperature as compared with a microporous film that is not highly stretched, and as a result, it is difficult to lower the shutdown temperature. Then, by adjusting the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction to a predetermined value, the structural balance between the in-plane direction of the film and the film thickness direction is adjusted, and a polyolefin fine particle that can solve the above problems is obtained. The inventors have found that a porous membrane can be provided and completed the present invention.
• the porous film of the present invention has the following constitution.
• a polyolefin microporous membrane having a crystallinity higher than the crystallinity and a difference between the crystallinity in the in-plane direction at a shutdown temperature measured by wide-angle X-ray measurement and the crystallinity in the in-plane direction at room temperature is 58% or more.
• a battery separator comprising the polyolefin microporous membrane according to any one of (1) to (6) or the multilayer microporous membrane according to (7).
• the polyolefin microporous membrane of the present invention has a low shutdown temperature, and when used as a battery separator, it can provide a battery with excellent shutdown performance.
• the difference between the crystallinity in the film thickness direction at room temperature and the crystallinity in the in-plane direction measured by wide-angle X-ray diffraction is 25% or less. Is larger than the crystallinity in the in-plane direction, and the difference between the crystallinity in the in-plane direction and the crystallinity in the in-plane direction at the shutdown temperature measured by the wide-angle X-ray measurement is 58% or more.
• the film thickness direction means a direction perpendicular to the surface of the polyolefin microporous film.
• the in-plane direction means a direction perpendicular to the film thickness surface of the polyolefin microporous film.
• the crystallinity is determined by fitting the amorphous halo and crystalline peaks from the diffraction spectrum obtained by wide-angle X-ray diffraction measurement in which the measurement sample is heated from room temperature to X-ray irradiation, and the peak area of the crystal and the peak of the amorphous The area is obtained and can be calculated from the following formula.
• Crystallinity (%) (Crystal peak area) / (Crystal and amorphous peak area) x 100
• the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature are the differences between the crystallinities obtained from the diffraction spectra at room temperature when X-ray irradiation is performed in the film thickness direction and the in-plane direction, respectively.
• the crystallinity in the in-plane direction at the shutdown temperature is the difference between the crystallinities obtained from the diffraction spectrum at the shutdown temperature when X-ray irradiation is performed in the in-plane direction.
• the shutdown temperature is a permeability measured with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) while heating the microporous membrane from room temperature (25 ° C.) at a temperature rising rate of 5 ° C./min.
• the structure balance in the film thickness direction and the structure in the in-plane direction can be known from the difference in crystallinity in each direction.
• the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature measured by wide-angle X-ray diffraction is 25% or less, and the crystallinity in the film thickness direction is the crystallinity in the in-plane direction. If it is larger than this, the difference in structure between the film thickness direction and the in-plane direction becomes small, the structure in the in-plane direction changes at a temperature lower than the conventional shutdown temperature, and the shutdown characteristic is improved.
• the upper limit of the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25 ° C) is preferably 20% or less.
• the lower limit of the difference between the crystallinity in the film thickness direction and the in-plane crystallinity is not particularly limited, but is preferably 5% or more, and more preferably 8% or more.
• the thermally stable structure in the in-plane direction becomes smaller than that in the film thickness direction, and the in-plane direction important for controlling the shutdown characteristics is Is promoted, the shutdown characteristic is improved.
• the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature is, for example, in the production of the polyolefin microporous film, for example, the content of the nucleating agent, the weight average molecular weight of the resin composition as a raw material ( The above range can be obtained by adjusting Mw), molecular weight distribution (MwD), stretching conditions, particularly the stretching temperature of the film after drying described below.
• the structural change in the in-plane direction up to shutdown is due to the difference between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at room temperature obtained by wide-angle X-ray measurement while raising the temperature described later. I can know.
• the difference between the in-plane crystallinity at shutdown and the in-plane crystallinity at room temperature is 58% or more. When this difference is less than 58%, the change in the in-plane direction (crystal melting) is suppressed, and the shutdown characteristic is deteriorated.
• the lower limit of the in-plane crystallinity at the shutdown temperature and the in-plane crystallinity at room temperature is preferably 60% or more, and more preferably 65% or more.
• the upper limit is not particularly limited, but is preferably 90% or less, more preferably 85% or less. Within the above range, it indicates that crystal melting in the in-plane direction is preferentially progressing, and the shutdown characteristic is improved. If this difference exceeds 90%, there are too many thermally unstable structures in the in-plane direction, the mechanical strength is reduced, and the thermal stability of the entire film is significantly reduced. When placed in, there is concern about thermal stability.
• the difference between the crystallinity in the in-plane direction at the shutdown temperature and the crystallinity in the in-plane direction at the room temperature is, for example, in the production of the microporous polyolefin film, for example, the content of the nucleating agent, the resin used as the raw material
• the above range can be adjusted by adjusting the weight average molecular weight (Mw) of the composition, the molecular weight distribution (MwD), the stretching conditions, particularly the stretching temperature of the film after drying described below.
• the melting peak temperature of the polyolefin microporous membrane is set to a measurement temperature range of 0 ° C. to 200 ° C. in a nitrogen atmosphere (20 mL / min), and a holding time of 10 minutes after reaching 200 ° C. at a temperature rising rate of 10 ° C./minute. (First temperature increase), then, under a nitrogen atmosphere (20 mL / min), the temperature is decreased at a temperature decrease rate of 30 ° C./min. Further, in a nitrogen atmosphere (20 mL / min), the second temperature rise is carried out after the measurement temperature range reaches 0 ° C.
• the temperature rise rate reaches 10 ° C./min to 200 ° C. and the holding time is 5 minutes. .
• the straight line connecting 0 ° C. and 200 ° C. was used as the baseline, and the temperature at the melting endothermic peak was read, which was observed by the second heating by DSC of the microporous polyolefin membrane.
• the melting peak temperature is used. When two or more peaks are observed, the melting point peak temperature of the microporous membrane is read from the most endothermic peak.
• the melting peak temperature of the polyolefin microporous film observed at the second heat of DSC is preferably 115 ° C or higher and 132 ° C or lower, more preferably 120 or higher and 130 ° C or lower, and further preferably 122 ° C or higher and 130 ° C or lower.
• the melting peak temperature of polyolefin can be determined by the differential scanning calorimetry (DSC) method described later.
• the molecular weight distribution of the polyolefin microporous membrane is preferably 5 or more and 30 or less, more preferably 16 or more and 25 or less, and still more preferably 17 or more and 23 or less.
• the molecular weight distribution of the polyolefin microporous membrane can be determined by the gel permeation chromatography (GPC) method described later.
• the step of producing the polyolefin microporous membrane for example, at least one of branched polyethylene and an antioxidant is contained, and kneading conditions (particularly temperature and screw rotation speed) are adjusted. By doing so, the above range can be obtained.
• the upper limit of the film thickness of the polyolefin microporous film is not particularly limited, but is, for example, preferably 20 ⁇ m or less, more preferably 12 ⁇ m or less, still more preferably 9 ⁇ m or less.
• the lower limit of the film thickness is not particularly limited, but is preferably 1 ⁇ m or more, more preferably 3 ⁇ m or more. When the film thickness is within the above range, the battery capacity is improved when the polyolefin microporous film is used as a battery separator.
• the lower limit of the porosity of the polyolefin microporous membrane is not particularly limited, but is, for example, 20% or more, more preferably 30% or more, and further preferably 40% or more.
• the upper limit of the porosity is not particularly limited, for example, it is preferably 70% or less, more preferably 60% or less, and further preferably 50% or less.
• the porosity is in the above range, the amount of electrolyte retained can be increased and high ion permeability can be secured. Further, even when the porosity is high, the battery safety can be ensured due to the excellent shutdown characteristic.
• the lower limit of the porosity is preferably 20% or more from the viewpoint of further enhancing the ion permeability and the rate characteristics.
• the porosity can be set within the above range by adjusting the blending ratio of the constituent components of the polyolefin, the draw ratio, the heat setting conditions, and the like in the manufacturing process.
• the shutdown temperature of the microporous polyolefin membrane was the temperature at which the air permeation resistance obtained from the elevated temperature air permeation resistance measurement reached the detection limit of 1 ⁇ 10 5 seconds / 100 cm 3 Air.
• the shutdown temperature is 110 ° C. or higher and lower than 135 ° C., preferably 110 ° C. or higher and 134 ° C. or lower, more preferably 110 ° C. or higher and 133 ° C. or lower, and even more preferably 110 ° C. or higher and 132 ° C. or lower.
• the shutdown temperature is lower than 135 ° C., the thermal runaway of the battery at high temperature can be suppressed and the safety is improved as compared with the conventional microporous membrane.
• the shutdown temperature is lower than 110 ° C., for example, there is a concern about thermal stability in the case of being arranged in the vicinity of a vehicle-mounted high temperature section and a concern that the air permeation resistance may sharply deteriorate during heat fixation.
• the lower limit of the 9 ⁇ m equivalent puncture strength of the polyolefin microporous membrane is preferably 1.35 N or more, more preferably 1.80 N or more, and further preferably 2.25 N.
• the upper limit of the 9 ⁇ m-converted puncture strength is not particularly limited, but is 3.6 N or less, for example.
• the film strength of the polyolefin microporous film is excellent.
• the secondary battery using the polyolefin microporous film as a separator can suppress the occurrence of short circuit of electrodes.
• the puncture strength in terms of 9 ⁇ m can be adjusted by adding a nucleating agent, adjusting the weight average molecular weight Mw, the molecular weight distribution MwD, or the stretching conditions (particularly, the stretching temperature of the film after drying, which will be described later) when manufacturing the microporous polyolefin membrane. By doing so, the above range can be obtained.
• the heat shrinkage ratio of the polyolefin microporous film in the machine direction and the width direction at 105 ° C. for 8 hours is preferably 12% or less, more preferably 6% or less, and further preferably 4% or less.
• the lower limit of the heat shrinkage ratio in the machine direction and the width direction is not particularly limited, but is preferably 0.5% or more.
• the heat shrinkage in the machine direction and the width direction is 12% or less, the risk of deformation inside the battery and short circuit at the end when used in the battery can be reduced.
• the heat shrinkage ratios in the machine direction and the width direction are 0.5% or more, pore closure due to heat shrinkage easily occurs, and it is possible to prevent deterioration of the shutdown characteristic.
• Examples of the method for producing a polyolefin microporous membrane include a dry film-forming method and a wet film-forming method.
• a method of producing a film by combining wet and dry methods is preferable from the viewpoint of controlling the structure and physical properties of the membrane.
• melt-knead the polyolefin and the film-forming solvent to prepare a resin solution.
• the melt-kneading method for example, the method using a twin-screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.
• polyolefin examples include polyethylene and polypropylene.
• polyethylene various polyethylenes can be used, and examples thereof include ultra high molecular weight polyethylene, high density polyethylene, branched high density polyethylene, medium density polyethylene, branched low density polyethylene, and linear low density polyethylene. From the viewpoint of improving the mechanical strength and the shutdown property of the microporous membrane, it is more preferable to use branched high density polyethylene.
• Polyethylene may be an ethylene homopolymer or a copolymer of ethylene and another ⁇ -olefin.
• ⁇ -olefin examples include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate and styrene.
• Polyethylene may be contained in an amount of 50 mass% or more based on 100 mass% of the microporous polyolefin membrane.
• high density polyethylene When it contains high-density polyethylene, it has excellent melt extrusion characteristics and uniform stretch processing characteristics.
• high-density polyethylene (density: 0.920 g / m 3 or more and 0.970 g / m 3 or less) has excellent melt extrusion characteristics and uniform drawing processing characteristics.
• the weight average molecular weight (Mw) of the high-density polyethylene is preferably 1 ⁇ 10 4 or more and less than 1 ⁇ 10 6 .
• the content of the high-density polyethylene is preferably 50% by mass or more as the lower limit and 100% by mass or less as the upper limit with respect to 100% by mass of the microporous polyolefin membrane.
• the branched high-density polyethylene preferably has a weight average molecular weight of 1 ⁇ 10 4 or more and less than 1 ⁇ 10 6 , a molecular weight distribution of 10 or more and 30 or less, and a melting point of 115 ° C. or more and 132 ° C. or less.
• polyethylene in this range is used, the crystallinity is easily adjusted, the molecular weight distribution of the polyolefin microporous film and the difference in crystallinity between the film thickness direction and the in-plane direction are easily adjusted to an appropriate range, and the shutdown characteristics are improved.
• the case where the branched high-density polyethylene is contained is more preferable because the in-plane orientation is difficult to proceed and the shutdown temperature can be lowered.
• Ultra high molecular weight polyethylene has a weight average molecular weight (Mw) is preferably 1 ⁇ 10 6 or more, more preferably 8 ⁇ 10 6 or less 1 ⁇ 10 6 or more. When the Mw of the ultra high molecular weight polyethylene is within the above range, the moldability becomes good.
• the ultra high molecular weight polyethylene can include at least one. For example, two or more kinds of ultra high molecular weight polyethylene having different Mw may be mixed and used as a raw material.
• ultra high molecular weight polyethylene when ultra high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the microporous polyolefin membrane is made thin.
• the ultrahigh molecular weight polyethylene content may be 0% by mass or more and 70% by mass or less based on 100% by mass of the polyolefin microporous film.
• the content of the ultra high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the molecular weight distribution (MwD) of the microporous polyolefin membrane can be easily controlled within a specific range described below, and the extrudability and kneading property can be easily controlled. Tends to be more productive.
• the molecular weight distribution of the polyolefin is preferably 5 or more and 30 or less, more preferably 16 or more and 25 or less, and further preferably 17 or more and 23 or less.
• the molecular weight distribution of the polyolefin can be determined by the gel permeation chromatography (GPC) method described later.
• the molecular weight distribution of the polyolefin can be within the above range.
• the polyolefin may contain other resin components other than polyethylene and polypropylene, if necessary.
• the other resin component for example, a resin imparting heat resistance can be used.
• various additives such as an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent and a filler, a crystal nucleating agent, and a crystallization retarder. It may be contained.
• the resin solution may contain components other than the above-mentioned polyolefin and the film-forming solvent, for example, a crystal nucleating agent (nucleating agent), an antioxidant and the like.
• a crystal nucleating agent nucleating agent
• the nucleating agent is not particularly limited, and a known compound-based or fine particle-based crystal nucleating agent can be used.
• the nucleating agent may be a masterbatch obtained by previously mixing and dispersing the nucleating agent in the polyolefin.
• the polyolefin preferably contains the above branched polyethylene and high density polyethylene.
• the polyolefin microporous membrane may also contain high density polyethylene, branched polyethylene and a nucleating agent. By including these, the puncture strength can be further improved.
• the molten resin is extruded and cooled to form a gel-like sheet.
• the resin solution prepared above is fed from an extruder to one die and extruded into a sheet to obtain a formed body.
• a gel-like sheet is formed by cooling the obtained molded body.
• Cooling is preferably performed at a rate of 50 ° C./min or more up to at least the gelation temperature. Cooling is preferably performed to 25 ° C. or lower. When the cooling rate is within the above range, the crystallinity is kept in an appropriate range, and the gel-like sheet suitable for stretching is obtained.
• a cooling method a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but it is preferable to contact with a roll cooled with a cooling medium for cooling.
• the stretching (first stretching) of the gel-like sheet is also called wet stretching.
• Wet stretching is performed in at least a uniaxial direction. Since the gel-like sheet contains a solvent, it can be stretched uniformly.
• the gel-like sheet is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof.
• 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 stretching and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.
• the final area stretching ratio (area magnification) in wet stretching is, for example, preferably 3 times or more, more preferably 4 times or more and 30 times or less in the case of uniaxial stretching.
• the final area stretching ratio is preferably 9 times or more, more preferably 16 times or more, further preferably 25 times or more.
• the upper limit of the final area stretching ratio is preferably 100 times or less, more preferably 64 times or less.
• the final areal draw ratio is preferably 3 times or more in both the longitudinal direction and the width direction, and the draw ratios in the machine direction and the width direction may be the same or different from each other. When the stretching ratio is 5 times or more, improvement in puncture strength can be expected.
• the wet stretch ratio refers to the stretch ratio of the gel-like sheet after the wet stretch ratio, based on the gel-like sheet before the wet stretch.
• the stretching temperature is preferably within the range of the crystal dispersion temperature (Tcd) to Tcd + 30 ° C. of the polyolefin, and more preferably within the range of the crystal dispersion temperature (Tcd) + 10 ° C. to the crystal dispersion temperature (Tcd) + 28 ° C. , Tcd + 15 ° C. to Tcd + 26 ° C. is particularly preferable.
• Tcd crystal dispersion temperature
• Tcd crystal dispersion temperature
• the crystal dispersion temperature (Tcd) means a value obtained by measuring the temperature characteristic of dynamic viscoelasticity based on ASTM D4065.
• the above ultra-high molecular weight polyethylene, polyethylenes other than ultra-high molecular weight polyethylene, and polyethylene compositions have a crystal dispersion temperature of about 90-100 ° C. Therefore, the stretching temperature when polyethylene is used as the raw material can be, for example, 90 ° C. or higher and 130 ° C. or lower.
• the film-forming solvent is removed from the stretched gel-like sheet to form a microporous film.
• the removal of the film-forming solvent is performed by cleaning with a cleaning solvent. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Laid-Open No. 2002-256099 can be used.
• the microporous membrane from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method.
• the dried microporous membrane is stretched.
• the stretching of the microporous membrane after drying consists of the second stretching and the third stretching, and is also referred to as dry stretching.
• the dried microporous membrane film is stretched in at least a uniaxial direction.
• biaxial stretching either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferred.
• sequential stretching it is preferable to stretch in the machine direction (second stretching) and then continuously stretch in the width direction (third stretching).
• the dry stretching method can be performed with a tenter stretching machine, a roll stretching machine, or the like while heating.
• the area stretching ratio (area magnification) of dry stretching is preferably 1.1 times or more, and more preferably 1.2 times or more and 9.0 times or less.
• the puncture strength, the shutdown temperature, etc. can be easily controlled within a desired range.
• uniaxial stretching for example, it is 1.2 times or more, preferably 1.2 times or more and 3.0 times or less in the machine direction or the width direction.
• the stretching ratio in the machine direction and the width direction is 1.0 times or more and 3.0 times or less, respectively, and the stretching ratios in the machine direction and the width direction may be the same or different from each other. It is preferable that the stretching ratio in the width direction is substantially the same.
• the dry stretching after stretching in the machine direction by more than 1.0 times and 3.0 times or less (second stretching), it is continuously stretched in the width direction by more than 1.0 times and 3.0 times or less (third stretching). Stretching) is preferable. Particularly when the roll stretching is used, the stretching in the machine direction is preferably less than 1.4 times, more preferably less than 1.3 times. Within the above range, the stress during polyolefin stretching is adjusted, the number of thermally unstable structures increases in the in-plane direction and the film thickness direction, and in-plane changes that are important for controlling the shutdown characteristics occur preferentially. It becomes a structure.
• the dry stretching ratio refers to the stretching ratio of the microporous membrane after dry stretching, based on the microporous membrane before dry stretching.
• the second stretching is preferably a three-stage stretching or more, more preferably a four-stage stretching or more, and further preferably a five-stage stretching or more. By doing so, the stress around one step is dispersed, and it is possible to suppress uneven drawing.
• the dry stretching temperature is usually 90 to 135 ° C., more preferably 100 to 135 ° C., and further preferably 105 ° C. to 135 ° C. from the viewpoint of stretching stress control.
• the second stretching temperature is preferably 100 ° C or higher and 120 ° C or lower, more preferably 110 ° C or higher and 120 ° C or lower.
• the stress during polyolefin stretching is adjusted, the number of thermally unstable structures increases in the in-plane direction and the film thickness direction, and in-plane changes that are important for controlling the shutdown characteristics occur preferentially. It becomes a structure.
• the second stretching temperature By setting the second stretching temperature to 120 ° C. or lower, it is possible to prevent partial melting of the film and the like, and to prevent uneven appearance of the microporous film after the second stretching.
• the dried microporous membrane may be subjected to heat treatment.
• the heat treatment method at least one of heat fixation treatment and heat relaxation treatment can be used.
• the heat setting treatment is a heat treatment of heating while holding both ends in the width direction so that the dimension of the film in the width direction does not change.
• the heat setting treatment is preferably performed by a tenter method or a roll method.
• the thermal relaxation treatment is a heat treatment for thermally shrinking the film in the machine direction and the width direction during heating.
• a thermal relaxation treatment method the method disclosed in JP-A-2002-256099 can be mentioned.
• the heat treatment temperature is preferably within the range of (Tcd) to (Tm: melting point) of the second polyolefin, more preferably within the stretching temperature of the microporous membrane of ⁇ 5 ° C., and within the second stretching temperature of the microporous membrane of ⁇ 3.
• the range of ° C is particularly preferred.
• heat treatment and thermal relaxation treatment may be performed after the third stretching.
• the relaxation temperature is, for example, 80 ° C. or higher and 135 ° C. or lower, preferably 90 ° C. or higher and 133 ° C. or lower.
• the final dry stretching ratio is, for example, 1.0 time or more and 9.0 times or less, preferably 1.2 times or more and 4.0 times or less.
• the relaxation rate can be 0% or more and 70% or less.
• the polyolefin microporous membrane of the present invention may be a single layer, but may also be formed by laminating resin compositions having different compositions such as different polymer species, molecular weights and their compounding ratios.
• another porous layer may be laminated on the polyolefin microporous membrane of the present invention to form a multilayer porous membrane.
• the other porous layer is not particularly limited, but for example, a porous layer such as an inorganic particle layer containing a binder and inorganic particles may be laminated.
• the binder component constituting the inorganic particle layer is not particularly limited, and known components can be used, and examples thereof include acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, and polyimide resin. Can be used.
• the inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used, and for example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon and the like can be used. it can.
• the multi-layer polyolefin porous film may be one in which the porous binder resin is laminated on at least one surface of the polyolefin microporous film.
• Measurement method and evaluation method From the center of the polyolefin microporous film in the width direction, 5 pieces were cut out in 3 cm x 3 cm, and the overlapped pieces were used as a measurement sample, and the temperature was raised from room temperature (25 ° C) to about 160 ° C at 5 ° C / min in a synchrotron radiation facility. Meanwhile, wide-angle X-ray diffraction measurement was performed. X-rays were perpendicularly incident on the surface of the measurement sample (the surface of the polyolefin microporous film), and measurement was performed to obtain a diffraction spectrum in the film thickness direction.
• X-rays were incident perpendicularly to the film thickness of the measurement sample (film thickness of the polyolefin microporous film), and measurement was performed to obtain a diffraction spectrum in the in-plane direction. In each direction, measurement was performed once near the center of the sample at room temperature and shutdown temperature. An amorphous halo and a crystalline peak were fitted from the obtained diffraction spectrum to determine the crystalline peak area and the amorphous peak area. The crystallinity in each direction was calculated using the following formula.
• Crystallinity (%) (Crystal peak area) / (Crystal and amorphous peak area) x 100
• the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature were defined as the difference between the crystallinities obtained from the diffraction spectra at room temperature when X-ray irradiation was performed in the film thickness direction and the in-plane direction, respectively.
• the crystallinity in the in-plane direction at the shutdown temperature was defined as the difference in crystallinity obtained from the diffraction spectrum at the shutdown temperature when X-ray irradiation was performed in the in-plane direction.
• the shutdown temperature is a temperature measured by the method described later.
• the temperature was lowered at a temperature lowering rate of 30 ° C./min.
• the second temperature increase was performed under a nitrogen atmosphere (20 mL / min) in a measurement temperature range of 0 ° C. to 200 ° C., a temperature increase rate of 10 ° C./min, and when the temperature reached 200 ° C.
• the holding time was 5 minutes. From the DSC curve obtained by the second heating, the straight line connecting 0 ° C. and 200 ° C.
• the temperature at the peak of the endothermic melting was read, and the melting observed by the DSC reheating of the microporous polyolefin membrane was observed.
• the peak temperature was used. (When two or more peaks are observed, the melting point peak temperature of the microporous film is read from the most endothermic peak.)
• Weight average molecular weight / molecular weight distribution of polyolefin The weight average molecular weight (Mw) and molecular weight distribution (MwD) of the polyolefin were determined by the gel permeation chromatography (GPC) method under the following conditions.
• ⁇ Measuring device GPC-150C manufactured by Waters Corporation ⁇ Column: Shodex 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 / 1 h) ⁇ Injection volume: 500 ⁇ l ⁇ Detector: Differential Refractometer (RI detector) manufactured by Waters Corporation Calibration curve: It was created from the calibration curve obtained using a monodisperse polystyrene standard sample using polyethylene or a conversion factor (0.46).
• RI detector Differential Refractometer
• Weight average molecular weight / molecular weight distribution of polyolefin microporous membrane The weight average molecular weight (Mw) and molecular weight distribution (MwD) of the polyolefin microporous membrane were determined by gel permeation chromatography (GPC) method under the following conditions.
• ⁇ Measuring device GPC-150C manufactured by Waters Corporation ⁇ Column: Shodex 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 / 1 h) ⁇ Injection volume: 500 ⁇ l ⁇ Detector: Differential Refractometer (RI detector) manufactured by Waters Corporation Calibration curve: It was created from the calibration curve obtained using a monodisperse polystyrene standard sample using polyethylene or a conversion factor (0.46).
• RI detector Differential Refractometer
• the measurement cell was made of an aluminum block and had a structure with a thermocouple directly under the microporous membrane.
• the sample was cut into a 5 cm ⁇ 5 cm square, and the temperature was measured while fixing the periphery with an O-ring.
• Porosity (%) [1- (10000 ⁇ W / ⁇ ) / (X ⁇ Y ⁇ T 0 )] ⁇ 100 W: sample weight (g) ⁇ : polyethylene resin density 0.95 (g / m 2 ). T 0 : sample thickness ( ⁇ m) The density of the resin used as the raw material is used as ⁇ .
• Puncture strength (N) Using a puncture tester NDG5 (manufactured by KatoTech), the maximum load when piercing a polyolefin microporous film at a speed of 2 mm / sec with a needle having a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm ( N) was measured.
• the puncture strength is a value calculated by the following formula in which the maximum load (N) is converted to 9 ⁇ m.
• Puncture strength maximum load (N) ⁇ 9 ( ⁇ m) / thickness of polyolefin microporous film T 0 ( ⁇ m) [Heat shrink]
• the polyolefin resin solution was supplied to a T-die from a twin-screw extruder, extruded, and cooled while being taken by a cooling roll to form a gel-like sheet.
• the gel-like sheet was simultaneously biaxially stretched (first stretching) by a tenter stretching machine at 110 ° C. in both the machine direction and the width direction by 5 times.
• the stretched gel-like sheet was immersed in a methylene chloride bath to remove liquid paraffin, and then dried.
• the dried film was stretched 1.1 times in the width direction at 110 ° C. (second stretching) using a batch-type stretching machine. Then, heat setting was performed at 115 ° C. for 10 minutes to obtain a polyolefin microporous film.
• Table 1 shows the production conditions and measurement results of the polyolefin microporous membrane.
• Example 2 A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that simultaneous biaxial stretching was performed in both the machine direction and the width direction by 8 times, and the second stretching was performed in the machine direction by 1.2 times in the machine direction.
• Example 3 Liquid paraffin was added as a film-forming solvent to high-density polyethylene (polyethylene 1) having a Mw of 5 ⁇ 10 5 and a molecular weight distribution of 5 so that the resin concentration shown in Table 1 was obtained, and the mixture was melt-kneaded by a twin-screw extruder and machine direction was applied. Simultaneously biaxially stretching in the width direction and 5 times in the width direction, and second stretching in the machine direction at a temperature of 115 ° C. in the machine direction by 1.2 times, and then heat setting at 125 ° C. for 10 minutes. A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the above was performed.
• Table 1 shows a polyethylene resin composition consisting of high-density polyethylene (polyethylene 1) having Mw 5 ⁇ 10 5 and molecular weight distribution 5 shown in Table 1 and ultrahigh molecular weight polyethylene (polyethylene 2) having Mw of 1 ⁇ 10 6 and molecular weight distribution 8 shown in Table 1.
• Liquid paraffin was added as a film-forming solvent to the resin concentration shown, and the mixture was melt-kneaded with a twin-screw extruder, and the first stretching was performed simultaneously with a tenter stretching machine at 90 ° C. in both the machine direction and the width direction at 5 times.
• Polyolefin was obtained in the same manner as in Example 1 except that the film was biaxially stretched, the second stretch was performed at 90 ° C in the machine direction by 1.2 times, and then the heat setting was performed at 125 ° C for 10 minutes. A microporous membrane was obtained.
• Example 2 The first stretching is simultaneously biaxially stretched by a tenter stretching machine at 90 ° C. in both the machine direction and the width direction at 9 times, and the second stretching is performed at 90 ° C. at 1.2 times in the machine direction, and then, A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that heat setting was performed at 125 ° C for 10 minutes.
• the difference between the crystallinity in the film thickness direction and the crystallinity in the in-plane direction at room temperature (25 ° C.) is 25% or less, and the crystallinity in the film thickness direction is more than that in the in-plane direction. It is also clear that the microporous film having a difference in crystallinity in the in-plane direction at the shutdown temperature by room-angle X-ray measurement and the crystallinity in the in-plane direction at room temperature of 58% or more has excellent shutdown characteristics. became.
• the polyolefin microporous membrane of the present invention has excellent shutdown characteristics when incorporated into a secondary battery as a separator. Therefore, it can be suitably used for a secondary battery separator that is required to have a high capacity.

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