Classifications

• • 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
  • H01M50/411—Organic material
  • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
  • H01M50/417—Polyolefins
• • 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

Definitions

• the present invention relates to a separation membrane used for material separation, selective permeation, etc., and a polyolefin microporous membrane widely used as a separator for electrochemical reaction devices such as alkaline batteries, lithium secondary batteries, fuel cells, and condensers. .
• the present invention also relates to a battery separator and a secondary battery.
• Polyolefin microporous membranes are mainly used as filters, separators for fuel cells, and separators for condensers.
• it is suitably used as a separator for non-aqueous electrolyte secondary batteries such as lithium ion batteries widely used in notebook personal computers, mobile phones and the like.
• non-aqueous electrolyte secondary batteries such as lithium ion batteries widely used in notebook personal computers, mobile phones and the like.
• the polyolefin microporous membrane has excellent mechanical strength, shutdown temperature, and ion permeation performance.
• the separator is also required to have a function to ensure safety when the battery overheats.
• the shutdown temperature described above is a function of blocking the current by melting the polyolefin microporous membrane and clogging the pores, and the lower the shutdown temperature, the better.
• the temperature inside the battery continues to rise for a certain period of time, but if the temperature is higher than the shutdown temperature, the separator will perforate and the insulation will not be maintained, and the meltdown phenomenon may occur.
• Meltdown temperature is preferably as high as possible.
• Patent Document 1 a microporous film containing polyethylene and polypropylene as essential components and a polyethylene microporous film are laminated to achieve both the shutdown characteristics of the polyethylene microporous film and the heat resistance of the polypropylene-containing layer.
• a battery separator is described.
• the microporous membrane described in Patent Document 1 improves heat resistance by blending polypropylene with a higher melting point than polyethylene, but the phase separation structure of polyethylene and polypropylene impairs other basic characteristics as a separator such as membrane strength. there is a possibility. Further, there is no mention of the heat resistance when the heat resistant porous layer is provided on the microporous membrane.
• the object of the present invention is to solve the above. That is, when a heat-resistant porous layer is provided and used as a battery separator as a laminated film, it is possible to provide a high level of safety against abnormal heat generation of the battery, and the excellent film strength reduces the battery resistance and increases the capacity.
• An object of the present invention is to provide a polyolefin microporous membrane that can be
• a numerical range represented using "-" means a range including the numerical values described before and after "-" as lower and upper limits.
• the main component is a polyethylene resin, and after heat treatment at 160 ° C., the arithmetic average roughness Sa (A) measured by scanning white interference microscopy is 1.0 ⁇ m or less, and the piercing strength in terms of basis weight is 500 mN / (g / m 2 ) or more.
• a heat-resistant porous layer when a heat-resistant porous layer is provided and used as a battery separator, it is possible to provide high safety against abnormal heat generation of the battery, and excellent permeability and film strength lead to low battery performance. It is possible to provide a polyolefin microporous membrane capable of increasing resistance and capacity.
• the polyolefin microporous film according to the embodiment of the present invention contains a polyethylene resin as a main component, and has an arithmetic mean roughness Sa (A) of 1.0 ⁇ m or less measured by a scanning white light interference microscope after heat treatment at 160 ° C. It has a piercing strength in terms of basis weight of 500 mN/(g/m 2 ) or more.
• the polyolefin microporous membrane according to the embodiment of the present invention has an arithmetic mean roughness Sa (A) of 1.0 ⁇ m or less, preferably 0.5 ⁇ m or less, measured by scanning white light interference microscopy after heat treatment at 160 ° C. , more preferably 0.2 ⁇ m or less, still more preferably 0.1 ⁇ m or less, and particularly preferably 0.06 ⁇ m or less.
• Sa (A) of the polyolefin microporous membrane after heat treatment at 160 ° C. to the above range, the membrane shape retention performance when the heat resistant porous layer is provided to form a laminated membrane, that is, the meltdown characteristic is excellent, and the laminated membrane can be obtained. Excellent safety when used as a battery separator.
• the lower limit of Sa(A) after heat treatment at 160° C. is not particularly limited from the above viewpoint, it is, for example, 0.01 ⁇ m or more from the viewpoint of compatibility with productivity. Specifically, Sa (A) and Sa (B), which will be described later, can be measured by the method described in Examples. In order to make the Sa(A) after the heat treatment at 160° C. within the above range, it is preferable to set the raw material composition and the film forming conditions of the microporous membrane within the range described below.
• the polyolefin microporous membrane according to the embodiment of the present invention has Sa(A)/ Sa(B) is preferably 20 or less.
• Sa(A)/Sa(B) is more preferably 10 or less, still more preferably 5 or less, particularly preferably 2 or less, and most preferably 1 or less.
• the film shape retention performance at high temperatures is excellent, and a heat-resistant porous layer is provided on the polyolefin microporous film to form a laminated film, which is used as a battery separator. In this case, the adhesion to the heat-resistant porous layer is excellent, so that a battery with excellent safety can be obtained.
• the lower limit of Sa(A)/Sa(B) is not particularly limited from the above viewpoint, it is, for example, 0.1 or more from the viewpoint of compatibility with productivity.
• the above Sa(B) is preferably 0.05 ⁇ m or more, more preferably 0.06 ⁇ m or more, still more preferably 0.08 ⁇ m or more, and particularly preferably 0 .10 ⁇ m or more.
• the upper limit of Sa(B) is not particularly limited from the above viewpoint, it is, for example, 0.5 ⁇ m or less from the viewpoint of compatibility with the strength and film formability of the microporous membrane. In order to make Sa(B) within the above range, it is preferable to set the raw material composition and film-forming conditions of the polyolefin microporous membrane within the range described later.
• the polyolefin microporous membrane according to the embodiment of the present invention has a piercing strength in terms of basis weight of 500 mN/(g/m 2 ) or more, preferably 600 mN/(g/m 2 ) or more, more preferably 800 mN/(g /m 2 ) or more, more preferably 1000 mN/(g/m 2 ) or more.
• a piercing strength in terms of basis weight 500 mN/(g/m 2 ) or more, preferably 600 mN/(g/m 2 ) or more, more preferably 800 mN/(g /m 2 ) or more, more preferably 1000 mN/(g/m 2 ) or more.
• the upper limit of the puncture strength in terms of basis weight is not particularly limited, since it becomes easy to control Sa (A) after heat treatment at 160 ° C. in an appropriate range, for example, it is 2000 mN / (g / m 2 ) or less. Preferably.
• the puncture strength in terms of basis weight can be measured by the method described in Examples.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a surface porosity of 15% or more, more preferably 17% or more, still more preferably 20% or more, and particularly preferably 22% or more.
• a surface porosity of the polyolefin microporous membrane By setting the surface porosity of the polyolefin microporous membrane within the above range, the resistance of the battery separator can be reduced, and in addition, the polyolefin microporous membrane is provided with a heat-resistant porous layer to form a laminated film, which is used as a battery separator. In this case, high safety can be imparted due to excellent adhesion to the heat-resistant porous layer.
• the upper limit of the surface porosity is not particularly limited from the above viewpoint, it is, for example, 50% or less from the viewpoint of compatibility with the strength of the microporous membrane.
• the surface porosity can be evaluated and calculated by the method described later. In order to keep the surface porosity within the above range, it is preferable to set the raw material composition of the microporous membrane and the film forming conditions within the range described below.
• the sum of the maximum contraction forces in terms of basis weight in the longitudinal direction and the width direction measured by thermomechanical analysis (TMA) is 12.0 mN/(g/m 2 ) or less. It is preferably 10 mN/(g/m 2 ) or less, and still more preferably 8 mN/(g/m 2 ) or less.
• TMA thermomechanical analysis
• the lower limit of the sum of the maximum shrinkage forces in terms of basis weight in the longitudinal direction and the width direction is not particularly limited, but from the viewpoint of compatibility with the strength and permeability of the microporous membrane, it is, for example, 1 mN/(g/m 2 ) or more. is.
• the sum of the maximum contractile forces in terms of basis weight in the longitudinal direction and the width direction can be measured by the method described in Examples.
• the polyolefin microporous membrane according to the embodiment of the present invention has a molecular weight of 50,000 or less with respect to the peak area of all molecular weight components in the differential molecular weight distribution curve of the polyethylene resin measured by the gel permeation chromatography (GPC) method described later. It is preferable that the area ratio of the polyethylene-based resin component is 10% or more and the area ratio of the polyethylene-based resin component having a molecular weight of 1,000,000 or more is 10% or more.
• the area ratio of the polyethylene-based resin component having a molecular weight of 50,000 or less is more preferably 15% or more, still more preferably 20% or more.
• the area ratio of the polyethylene-based resin component having a molecular weight of 50,000 or less is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less.
• the area ratio of the polyethylene-based resin component having a molecular weight of 1,000,000 or more is more preferably 15% or more, and still more preferably 20% or more.
• the area ratio of the polyethylene-based resin component having a molecular weight of 1,000,000 or more is preferably 35% or less, more preferably 30% or less, and still more preferably 25% or less.
• the polyolefin microporous membrane can be strengthened while suppressing heat shrinkage. When this is used as a battery separator, the meltdown property is excellent.
• the raw material composition and kneading conditions of the microporous film should be within the ranges described later. It is preferable to
• the area ratio of the polyethylene resin component having a molecular weight of 2 million or more is preferably 10% or less in the molecular weight distribution of the polyethylene resin measured by the GPC method, and more It is preferably 8% or less, more preferably 6% or less. Further, the area ratio of the polyethylene-based resin component having a molecular weight of 2,000,000 or more is preferably 1% or more, more preferably 3% or more.
• the amount of the polyethylene-based resin component having a molecular weight of 2,000,000 or more in the polyolefin microporous membrane is set to the above range, it is possible to increase the strength while suppressing the heat shrinkage of the polyolefin microporous membrane. Excellent meltdown properties when used as a separator.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has an air permeability in terms of thickness of 30 seconds/100 cm 3 / ⁇ m or less, more preferably 20 seconds/100 cm 3 / ⁇ m or less, and still more preferably 15 seconds/100 cm. 3 / ⁇ m or less.
• the air permeability in terms of thickness it is preferably 1 second/100 cm 3 / ⁇ m or more because compatibility with film strength is easily achieved.
• the thickness-equivalent air permeability can be set within the above range by adjusting the mixing ratio of the raw materials, the draw ratio, the heat setting conditions, and the like in the manufacturing process.
• the thickness of the polyolefin microporous membrane according to the embodiment of the present invention can be appropriately adjusted depending on the application, but is preferably 20 ⁇ m or less, more preferably 15 ⁇ m or less, still more preferably 10 ⁇ m or less, and particularly preferably 8 ⁇ m or less. . Moreover, it is preferably 2 ⁇ m or more.
• the thickness can be within the above range by appropriately adjusting film-forming conditions such as extrusion conditions.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a porosity of 30% or more, more preferably 35% or more, and still more preferably 40% or more.
• the upper limit of the porosity is not particularly set, it is preferably 80% or less because a decrease in film strength can be suppressed.
• the porosity can be set within the above range by adjusting the raw material formulation, draw ratio, heat setting conditions, and the like in the manufacturing process.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a thermal shrinkage rate of 15% or less in the MD and TD directions after the polyolefin microporous membrane is stored at 120° C. for 1 hour, and preferably 10% or less. is more preferable, and 8% or less is even more preferable. If the heat shrinkage ratio of the polyolefin microporous membrane is within the above range, the polyolefin microporous membrane is excellent in safety during abnormal heat generation when used as a battery separator. In addition, the lower limit of the thermal shrinkage rate in the MD direction and the TD direction after storage at 120 ° C.
• the main component of the polyolefin microporous membrane according to the embodiment of the present invention is polyethylene resin.
• the term "main component" as used herein refers to the component with the highest content in terms of % by mass among the components constituting the polyolefin microporous membrane.
• the polyethylene-based resin component occupying the polyolefin microporous film is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 96% by mass or more, and 99% by mass or more. is particularly preferred.
• the microporous membrane has excellent film forming properties and uniformity, and at the same time, the performance balance as a battery separator, such as membrane strength and permeability, is achieved.
• the polyolefin microporous membrane may contain two or more types of polyethylene resins, in which case the total amount of the polyethylene resins may be the amount of the polyethylene resin component constituting the polyolefin microporous membrane.
• the content of the polyethylene-based resin in the polyolefin microporous membrane can be measured by the method described below.
• polyethylene-based resins can be used for the polyolefin microporous membrane according to the embodiment of the present invention.
• the polyethylene-based resin may be an ethylene homopolymer or a copolymer of ethylene and other ⁇ -olefin.
• ⁇ -olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene.
• the polyethylene-based resin shall contain ethylene in an amount exceeding 50 mol % with respect to all raw material monomer components.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably contains ultrahigh molecular weight polyethylene (hereinafter referred to as resin A) among the polyethylenes described above, and resin A and high-density polyethylene (hereinafter referred to as resin B). It is more preferable to include
• the weight average molecular weight (Mw) of the ultra-high molecular weight polyethylene used as resin A is preferably 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 1,400,000 or less.
• the melting point of Resin A is preferably 133°C or higher, more preferably 135°C or higher. By setting the melting point of the resin A within the above range, a microporous membrane having excellent permeability and strength can be obtained. Also, the melting point can be measured by the DSC method described later.
• 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.
• 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, preferably 20,000 or more. More preferably, it is 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.
• the melting point of Resin B is more preferably 128°C or higher, more preferably 130°C or higher. Also, it is preferably 135° C. or lower, more preferably 134° C. or lower.
• the heat of crystal fusion ( ⁇ H) of Resin B measured by differential scanning calorimetry (DSC) is preferably 200 J/g or more, more preferably 210 J/g or more, and preferably 220 J/g or more. More preferred.
• DSC differential scanning calorimetry
• the polyolefin microporous membrane is used as a battery separator, it is possible to increase the membrane strength while suppressing deformation of the membrane due to crystal melting by setting the heat of crystal melting ( ⁇ H) of the resin B within the above range. It is possible to give the battery safety.
• the amount of heat of crystal fusion ( ⁇ H) of Resin B from the above point of view it is preferably 280 J/g or less from the point of view of film formability.
• the half width of the crystal melting peak is preferably 6°C or less, more preferably 5°C or less. Also, it is preferably 1° C. or higher, more preferably 2° C. or higher.
• the content is within the above range, deformation of the polyolefin microporous membrane due to melting of crystals is suppressed, and safety is excellent when used as a battery separator.
• the content of resin B in the polyolefin microporous film 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.
• the polyolefin microporous membrane according to the embodiment of the present invention may contain a resin other than a polyethylene-based resin.
• 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.
• block copolymers and random copolymers can be used as polypropylene resins.
• Block copolymers and random copolymers can contain copolymer components with ⁇ -olefins other than propylene. , 4-methylpentene-1, octene, and the like.
• the polypropylene-based resin shall contain propylene in an amount exceeding 50 mol % with respect to all raw material monomer components.
• the amount of polypropylene resin added is preferably 20% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to the total mass of the polyolefin microporous membrane. By setting it as the said range, it becomes the polyolefin microporous membrane excellent in productivity and quality, and excellent in intensity
• the polyolefin microporous membrane can contain resin components other than polyethylene-based resins and polypropylene-based resins, if necessary.
• various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retardants are added to the extent that they do not impair the effects of the present invention. may be included.
• the polyolefin microporous membrane of the present invention is preferably used as a laminated membrane having one or more heat resistant porous layers on at least one surface.
• the heat-resistant porous layer is not particularly limited, it preferably contains, for example, a resin binder and inorganic particles.
• binder components include acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, polyvinyl alcohol resins, and cellulose ether resins.
• inorganic particles constituting the heat-resistant porous layer include alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, zeolite, glass filler, kaolin, talc, mica, titanium dioxide, calcium fluoride, Lithium fluoride or the like can be used.
• the average particle size of the inorganic particles is preferably 0.3 ⁇ m or more and 1.8 ⁇ m or less, more preferably 0.5 ⁇ m or more and 1.5 ⁇ m or less.
• the average particle size of particles can be measured using a laser diffraction type or dynamic light scattering type measuring device. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe were measured with a particle size distribution analyzer (Microtrac HRA manufactured by Nikkiso Co., Ltd.), and 50% of the particles were accumulated from the small particle side in terms of volume. The value of the particle diameter (D50) at time may be taken as the average particle diameter.
• the shape of the particles is not particularly limited and may be a spherical shape, a substantially spherical shape, a plate shape, or a needle shape.
• the heat-resistant porous layer may contain a component for adjusting wettability, such as a surfactant, in order to improve coatability.
• the proportion of inorganic particles in the heat-resistant porous layer is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 95% by mass or more. Moreover, it is preferable that it is 99 mass % or less.
• the thickness of the heat-resistant porous layer is preferably 0.5 to 5 ⁇ m, more preferably 1 to 4 ⁇ m, from the viewpoint of achieving both heat resistance and high battery capacity when used as a battery separator.
• the method for forming the heat-resistant porous layer is not particularly limited, but for example, reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, pipe doctor method, A blade coating method, a die coating method, and the like can be mentioned. Further, after coating the coating liquid on the polyolefin microporous film by the above method, the solvent is dried under the conditions of a drying temperature of 40 to 100° C. and a drying time of 3 to 120 seconds to form a heat resistant porous layer. be able to.
• the solid content concentration of the coating liquid for forming the heat-resistant porous layer is not particularly limited as long as it can be applied uniformly, but is preferably 20% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 80% by mass or less.
• the solvent used in the coating liquid is not particularly limited as long as it can uniformly disperse the binder and inorganic particles, and examples thereof include water, alcohols, and acetone.
• Method for producing polyolefin microporous membrane Next, a method for producing a polyolefin microporous membrane according to an embodiment of the present invention will be described.
• the method for producing a polyolefin microporous membrane include a dry film-forming method and a wet film-forming method.
• a wet film forming method is preferable from the viewpoint of controlling the structure and physical properties of the film.
• the method for producing a polyolefin microporous membrane in an embodiment of the present invention preferably includes the following steps (1) to (5) in order, and may further include the following step (6). After or instead of step (6), the following step (7) may be included.
• a step of melt-kneading the polyolefin resin and a membrane-forming solvent to prepare a polyolefin resin composition (2) A step of extruding and cooling the polyolefin resin composition to form a gel-like sheet (3) The gel-like A first stretching step of preheating and stretching the sheet (4) A step of removing the film forming solvent from the stretched gel sheet (5) A step of drying the sheet after removing the film forming solvent (6) A second stretching step of preheating and stretching the dried sheet (7) Heat-treating the dried sheet
• a polyolefin resin composition is prepared by heating and dissolving a polyolefin resin in 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 the solvent is preferably liquid at room temperature in order to enable stretching at a relatively high magnification.
• Solvents include aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate, Examples include phthalate esters such as dioctyl phthalate which are liquid at room temperature. In order to obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.
• the mixing ratio of the polyolefin resin and the plasticizer is preferably such that the content of the polyolefin resin is 10 to 50% by mass with respect to the total mass of the polyolefin resin composition.
• the melt-kneading of the polyolefin resin and the plasticizer is preferably carried out in a twin-screw extruder from the viewpoint of obtaining a uniform kneading state.
• adjusting the distance between the tip of the screw and the vent hole of the twin-screw extruder, the distance between the vent holes, and adjusting the screw structure suppresses the torque fluctuation during extrusion, and the polyolefin resin and plasticizer consisting of multiple types can be suppressed. It is preferable from the viewpoint of uniform dispersion.
• the outermost diameter of the screw is D
• the distance between the tip of the screw and the vent hole is 1.0D to 15.0D
• the raw material conveying direction is between 1.0D and 15.0D. It is preferable to use at least one screw piece with a length of 0.2D to 0.9D, and not to use two or more screw pieces with a length of 1.5D or more in the raw material conveying direction.
• 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, and preferably 240° C. or lower. More preferably, it is 230° C. or lower.
• Q/Ns calculated from the ratio of the extrusion mass Q (kg/hr) and the screw rotation speed Ns (rpm) is preferably 0.01 or more, more preferably. is 0.05 or more, more preferably 0.1 or more. This makes it possible to prevent a decrease in strength due to deterioration of the resin during kneading.
• the upper limit is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less, so that sufficient shear can be applied to the polyolefin resin composition. It is possible to obtain a uniform dispersion state.
• a melt of the polyolefin resin composition is supplied from an extruder to a die and extruded into a sheet.
• the extrusion method may be either a flat die method or an inflation method.
• a plurality of polyolefin resin compositions having the same or different composition may be supplied from a plurality of extruders to one multi-manifold type composite T-die, laminated in layers, and extruded into a sheet having a laminated structure.
• the extrusion temperature is preferably 140-250° C.
• the extrusion speed is preferably 0.2-15 m/min.
• the sheet-shaped melt-extruded resin composition is cooled and solidified to form a gel-like sheet. It is preferable to cool to 10 to 50° C. in the cooling step. This is because it is preferable that the final cooling temperature is equal to or lower than the crystallization finish temperature, and by making the higher-order structure finer, it becomes easier to perform uniform stretching in subsequent stretching.
• the cooling rate at this time is preferably 50° C./min or more, more preferably 100° C./min or more, still more preferably 150° C./min or more. In general, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarser, and the gel structure forming it also becomes larger.
• cooling rate when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes denser, leading to uniform stretching as well as improvement in the strength and elongation of the film.
• a cooling method at this time there are a method of direct contact with cold air, cooling water, or other cooling medium, a method of contact with a roll cooled with a refrigerant, a method of using a casting drum, and the like.
• the neck-in rate of the resin melt-extruded into a sheet is preferably 70% or more, more preferably 80% or more, and even more preferably 85% or more.
• the neck-in rate is a value calculated by the formula: A/B ⁇ 100, where A is the width of the gel-like sheet after cooling and solidification, and B is the width of the outlet of the die.
• the upper limit of the neck-in rate is not particularly set from the above viewpoint, it is preferably 99% or less from the viewpoint of film forming stability.
• the formulation of the polyolefin resin composition is adjusted, the resin temperature during extrusion is adjusted, the spacing between the casting drum and the die lip is adjusted, and casting is performed with an air knife or air chamber. It can be adjusted by assisting the close contact between the drum and the gel-like sheet.
• the preheating temperature is preferably 90 to 130°C, more preferably 105°C or higher, still more preferably 110°C or higher, and more preferably 120°C or lower, still more preferably 117°C or lower.
• the gel-like sheet after preheating is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. 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 multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.
• the draw ratio (area draw ratio) in this step is preferably 16 times or more, more preferably 25 times or more.
• the draw ratio is preferably 4 times or more, more preferably 5 times or more, in both the machine longitudinal direction (MD direction) and the machine width direction (TD direction).
• the draw ratios in the MD direction and the TD direction may be the same or different. Mechanical strength and permeability can be improved by setting the areal draw ratio within the above range.
• the area draw ratio in this step is preferably 100 times or less, more preferably 64 times or less.
• the draw ratio in this step refers to the area draw ratio of the polyolefin microporous film immediately before being subjected to the next step, based on the polyolefin microporous film immediately before this step.
• the stretching temperature in this step is preferably in the range of the crystal dispersion temperature (TCD) of the polyethylene resin to (TCD + 30) ° C., more preferably (TCD + 5) ° C. or higher, particularly preferably (TCD + 10) ° C. or higher. , (TCD+28)° C. or less is more preferable, and (TCD+26)° C. or less is particularly preferable.
• TCD crystal dispersion temperature
• the crystal dispersion temperature is obtained by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065.
• TCD crystal dispersion temperature
• a polyethylene-based resin is used as the polyolefin resin
• ultra-high molecular weight polyethylene, polyethylene other than ultra-high molecular weight polyethylene, and polyethylene resin compositions have a crystal dispersion temperature of about 100 to 110°C, so the stretching temperature is set to 90 to 130°C. , more preferably 105° C. or higher, still more preferably 110° C. or higher, and more preferably 120° C. or lower, further preferably 117° C. or lower.
• the above-described stretching causes cleavage between the polyethylene and the lamellae, miniaturizing the polyethylene-based resin phase and forming a large number of fibrils. Fibrils form a three-dimensionally irregularly connected network structure.
• the film-forming solvent is removed (washed) using a washing solvent.
• the polyolefin resin phase is phase-separated from the film-forming solvent phase. Therefore, when the film-forming solvent is removed, a porous film composed of fibrils that form a fine three-dimensional network structure and having pores (voids) that are irregularly communicated three-dimensionally can be obtained. 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 methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.
• the polyolefin microporous membrane from which the membrane-forming solvent has been removed is dried by a heat drying method or an air drying method.
• the drying temperature is preferably lower than the crystal dispersion temperature (TCD) of the polyolefin resin, and particularly preferably lower than the TCD by 5°C or more. Drying is preferably carried out until the residual washing solvent is 5 parts by mass or less, more preferably 3 parts by mass or less, with the total mass of the polyolefin microporous membrane being 100 parts by mass (dry mass).
• the dried polyolefin microporous membrane may be stretched at least uniaxially (second stretching step).
• the polyolefin microporous membrane may be preheated before the second stretching step.
• the preheating temperature is preferably 90 to 140°C, more preferably 95°C or higher, still more preferably 100°C or higher, and more preferably 150°C or lower, still more preferably 140°C or lower.
• the stretching of the polyolefin microporous membrane can be performed by a tenter method, a roll method, an inflation method, or the like in the same manner as described above while heating. Stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential biaxial stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential biaxial stretching) may be used.
• the area draw ratio in this step is preferably 4.0 times or less, more preferably 2.0 times or less, further preferably 1.7 times or less, and 1.5 times or less. is particularly preferred.
• the stretching ratios in the MD direction and the TD direction may be the same or different.
• the draw ratio in this step refers to the draw ratio of the polyolefin microporous film immediately before being subjected to the next step, based on the polyolefin microporous film immediately before this step.
• the dried polyolefin microporous membrane can be subjected to a heat treatment.
• the heat treatment stabilizes the crystals and homogenizes the lamellae.
• heat setting treatment and/or heat relaxation treatment can be used as a heat treatment method.
• the heat setting treatment is a heat treatment in which the film is heated while maintaining the dimensions of the film.
• the thermal relaxation treatment is a heat treatment that thermally shrinks the film in the MD direction or the TD direction during heating.
• the heat setting treatment is preferably performed by a tenter method or a roll method.
• the relaxation rate in the relaxation treatment is a value obtained by dividing the dimension of the film after the relaxation treatment by the dimension of the film before the relaxation treatment.
• the heat treatment temperature is preferably within the range from the TCD to the melting point of the polyolefin resin.
• the melting point can be measured with a differential scanning calorimeter (DSC) according to JIS K7121 (1987).
• the polyolefin microporous membrane obtained as described above can be used in various applications such as filters, separators for secondary batteries, separators for fuel cells, and separators for condensers.
• the present invention also relates to a battery separator using the polyolefin microporous membrane described above, and in particular to a battery separator using a laminated film having one or more heat-resistant porous layers on at least one surface of the polyolefin microporous membrane. is preferred.
• a heat-resistant porous layer is provided and used as a battery separator, it is possible to provide a high level of safety against abnormal heat generation in the battery. becomes possible.
• the details of the heat-resistant porous layer are as described above.
• the present invention also relates to a secondary battery using the above battery separator.
• Puncture strength in terms of basis weight (mN/(g/m 2 )) maximum load (mN) / basis weight of polyolefin microporous membrane (g/m 2 )
• the basis weight of the polyolefin microporous membrane was calculated by the following formula by cutting a 50 mm ⁇ 50 mm square sample from the polyolefin microporous membrane, measuring the mass (g) at room temperature of 25° C.
• basis weight (g/m 2 ) mass (g)/(50 (mm) x 50 (mm)) x 10 6
• GPC Global permeation chromatography
• Mw Weight average molecular weight of polyolefin resin
• area ratio of polyethylene component with molecular weight of 50,000 or less in polyolefin microporous membrane to peak area of all molecular weight components area of polyethylene component with molecular weight of 1 million or more to peak area of all molecular weight components. The ratio, the area ratio of the polyethylene component having a molecular weight of 2,000,000 or more to the peak area of all molecular weight components, was determined by GPC under the following conditions.
• the area ratio of each molecular weight component was obtained from the ratio of the area of each molecular weight region to the peak area of all molecular weight components.
• ⁇ Measuring device GPC-150C manufactured by Waters Corporation
• ⁇ Column Shodex UT806M manufactured by Showa Denko K.K.
• ⁇ Column temperature 135°C
• Solvent (mobile phase): o-dichlorobenzene ⁇
• Solvent flow rate 1.0 ml / min ⁇
• Sample concentration 0.1 mass% (dissolution condition: 135 ° C.
• ⁇ Injection amount 500 ⁇ l ⁇ Detector: Waters Corporation differential refractometer (RI detector) • Calibration curve: Created using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.
• DSC Different scanning calorimetry
• 6.0 mg of the sample is enclosed in an aluminum pan, and the temperature is raised from 30 ° C. to 230 ° C. at 10 ° C./min using a PYRIS Diamond DSC manufactured by Parking Elmer (first temperature rise), and then held at 230 ° C. for 5 minutes. Then, it was cooled at a rate of 10° C./min, and the temperature was again raised from 30° C. to 230° C. at a rate of temperature rise of 10° C./min (second temperature rise).
• the heat of crystal fusion, the half width of the crystal melting peak, and the melting point of the polyolefin resin used as the raw material are between 60° C. and 200° C.
• the polyethylene-based resin content of the polyolefin microporous membrane is obtained by drawing a baseline between 60 ° C. and 155 ° C. in the temperature distribution curve of the endothermic amount measured at the second temperature rise in the above DSC measurement. and the area of the crystalline melting peak ( ⁇ H2) obtained by drawing a baseline between 155°C and 200°C, according to the following formula.
• Polyethylene-based resin content (%) of polyolefin microporous membrane 100 ⁇ ⁇ H1 / ( ⁇ H1 + ⁇ H2)
• the polyolefin microporous film was cut into 15 mm squares and attached to the center of a polyimide tape (API-114AFR manufactured by Chuko Kasei Co., Ltd., tape width 19 mm) cut into 20 mm lengths so as not to create wrinkles.
• the polyolefin microporous film attached to the polyimide tape is placed on an aluminum plate of 5 cm square and 2 mm thick with the polyimide tape surface facing down, and wrinkles are generated with the polyimide tape (API-114AFR manufactured by Chuko Kasei Co., Ltd., tape width 19 mm). The four sides were fixed to the aluminum plate so that it would not be exposed.
• the polyolefin tape was attached so that the outer peripheral width of about 2 mm of the polyolefin microporous membrane was fixed.
• the above sample was placed in an oven with an internal temperature of 160° C., and taken out 15 minutes after being placed.
• a sample was also prepared with the surface of the polyolefin microporous membrane reversed at the time of initial tape attachment, to prepare a total of two samples for evaluation.
• noise is removed from the surface SEM image by averaging 3 pixels x 3 pixels, and then dynamic binary processing is performed using a threshold of -30 gradation from an image obtained by averaging 21 pixels x 21 pixels.
• Dark areas were extracted by performing value processing. All independent dark areas were counted as pores, and the area of the apertures within the SEM observation area was calculated by summing up all the areas of the counted pores . ), the surface porosity was defined as the ratio of the area of the open pores. The above measurements were made at 5 points each on both surfaces of the polyolefin microporous membrane, totaling 10 points, and averaged.
• the maximum shrinkage force in the longitudinal direction was obtained by sampling the above measurement so that the longitudinal direction of the polyolefin microporous membrane was aligned with the longitudinal direction of the polyolefin microporous membrane, and calculating the average value of three measurements in the same procedure.
• the maximum shrinkage force in the width direction was obtained by sampling the above measurement so that the length direction of the test piece was aligned with the width direction of the polyolefin microporous membrane, and calculating the average value of three measurements in the same procedure. From the maximum shrinkage forces in the longitudinal direction and the width direction obtained by the above method, the sum of the maximum shrinkage forces in the basis weight in the longitudinal direction and the width direction was calculated according to the following formula.
• Thermal shrinkage A square sample of 100 mm in the MD direction and the TD direction was cut out from the polyolefin microporous membrane. Next, the sample was placed in an oven with an internal temperature of 120° C. and heated. One hour after the sample was placed, the sample was taken out, and then the lengths of the sample in the MD and TD directions were measured. Assuming that the length in the MD direction after being placed in the oven is L MD (mm) and the length in the TD direction is L TD (mm), the thermal shrinkage after storage at 120° C. for 1 hour was calculated by the following formula. Further, this measurement was performed at arbitrary three points in the plane of the polyolefin microporous membrane, and the average value was calculated. Formula 1: 120 ° C.
• a measurement sample and a gasket were placed in this order from the lower lid side on the inner bottom of the lower lid of the 2032 type coin cell member.
• a solution was prepared by adding 0.3% by mass of a surfactant F-444 (manufactured by DIC) to the above-mentioned coin cell, and 0.1 mL of the solution was injected into the coin cell.
• the sample was allowed to stand for 1 minute under a gauge pressure of -50 kPa twice to impregnate the polyolefin microporous membrane with the electrolytic solution. Thereafter, a wave washer and an upper lid were placed on the spacer in this order from the spacer side, and sealed with a coin cell caulking machine (manufactured by Hosen Co., Ltd.) to prepare an evaluation cell.
• a coin cell caulking machine manufactured by Hosen Co., Ltd.
• the cell for evaluation was sandwiched between coaxial contact probes placed in an oven, and the resistance of the cell was measured at an amplitude of 50 mV and a frequency of 1 kHz using an LCR meter (manufactured by Hioki Denki).
• the temperature of the coin cell was monitored by attaching a temperature sensor to the upper lid of the cell. The resistance was measured while The temperature at which the resistance of the evaluation cell first exceeded 1 k ⁇ cm was taken as the shutdown temperature of the polyolefin microporous membrane, and the temperature at which the temperature was continued to rise from the shutdown temperature and the resistance became 1 k ⁇ cm again was taken as the meltdown temperature.
• A, B, and C were regarded as acceptable.
• B Meltdown temperature of 180°C or higher and resistance value at 180°C of 1 k ⁇ cm 2 or higher and lower than 10 k ⁇ cm 2
• C Meltdown temperature of 170 ° C. or more and less than 180 ° C.
• D Meltdown temperature of 160 ° C. or more and less than 170 ° C.
• the coating liquid was applied onto the polyolefin microporous film using a wire bar and dried for 1 minute in a hot air oven set at 50° C. to obtain a laminated film having a heat resistant porous layer.
• a wire bar was selected and applied so that the coating thickness of the heat-resistant porous layer after drying was 3 ⁇ m.
• the meltdown temperature of the laminated film obtained by the above procedure was measured according to the method described above.
• Example 1 As polyolefin raw materials, 70% by mass of ultra-high molecular weight polyethylene having an Mw of 1.0 ⁇ 10 6 and a melting point of 136° C. as resin A, and an Mw of 6 ⁇ 10 4 , a melting point of 132° C. and ⁇ H of 220 J/g as resin B. , 30% by mass of high-density polyethylene having a half width of the crystalline melting peak of 4°C was used. Add 75% by mass of liquid paraffin to 25% by mass of the polyolefin raw material, and further add 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass based on the mass of ultra-high molecular weight polyethylene.
• tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane was added as an antioxidant and mixed to prepare a polyolefin resin composition.
• the obtained polyolefin resin composition was charged into a twin-screw extruder having a screw structure shown in Table 1 and kneaded at 180° C. to prepare a polyethylene solution.
• the resulting polyethylene solution was supplied to a T-die controlled to 200°C, and the extrudate was cooled by a casting drum controlled to 35°C to form a gel sheet.
• the sheet conveying direction at the time of forming the gel-like sheet was defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction within the film plane was defined as the width direction.
• the neck-in rate of the sheet was adjusted to 85% by adjusting the distance between the top of the casting drum and the die lip.
• the resulting gel-like sheet was cut into 80 mm squares, set in a batch-type biaxial stretching machine, preheated at 115° C. for 300 seconds, and stretched at a stretching temperature of 115° C. and a stretching speed of 1000 mm/min. Simultaneous biaxial stretching was carried out so as to stretch the sheet 8 times in the longitudinal direction and 8 times in the width direction.
• the stretched membrane is washed in a methylene chloride washing tank to remove liquid paraffin, the washed membrane is dried in a drying oven adjusted to 20°C, and heated in an electric oven at 125°C for 10 minutes.
• a polyolefin microporous membrane was obtained by fixing treatment (heat treatment).
• Example 2 As polyolefin raw materials, 90% by mass of ultra-high molecular weight polyethylene having Mw of 1.0 ⁇ 10 6 and a melting point of 136° C. as resin A, and Mw of 6 ⁇ 10 4 , melting point of 132° C. and ⁇ H of 220 J/g as resin B. , was carried out in the same manner as in Example 1, except that 10% by mass of high-density polyethylene having a half-value width of the crystalline melting peak of 4°C was used.
• Example 3 80% by mass of ultra-high molecular weight polyethylene having an Mw of 1.5 ⁇ 10 6 and a melting point of 136° C. as the resin A, and a resin B having an Mw of 6 ⁇ 10 4 , a melting point of 132° C. and a ⁇ H of 220 J/g as polyolefin raw materials. , was carried out in the same manner as in Example 1, except that 20% by mass of high-density polyethylene having a half width of the crystalline melting peak of 4°C was used.
• Example 4 The procedure was carried out in the same manner as in Example 1, except that the neck-in rate of the sheet was adjusted to 91% by adjusting the distance between the top of the casting drum and the die lip when forming the gel sheet.
• Example 5 The gel-like sheet is simultaneously biaxially stretched 7 times in the longitudinal direction and 7 times in the width direction, then the liquid paraffin is removed, and the washed film is dried for 10 minutes in a drying oven adjusted to 100°C. After that, it is cut into a square of 80 mm square, set in a batch-type biaxial stretching machine, preheated at 130° C. for 10 minutes, heat-set, and stretched at a stretching temperature of 130° C. and a stretching speed of 1000 mm/min in the width direction. The same procedure as in Example 1 was carried out, except that the film was stretched 2.0 times in the width direction, and then subjected to thermal relaxation treatment so as to be 0.90 times in the width direction.
• Example 1 As polyolefin raw materials, 40% by mass of ultra-high molecular weight polyethylene having Mw of 2.5 ⁇ 10 6 and a melting point of 133° C. as resin A, and Mw of 3.5 ⁇ 10 5 as resin B, melting point of 135° C., and ⁇ H of 190 J. /g, using 60% by mass of high-density polyethylene having a half-value width of the crystalline melting peak of 6°C, a gel-like sheet was cut into a square of 80 mm square, preheated at 115°C for 300 seconds, and stretched at a temperature of 115°C. was carried out in the same manner as in Example 1, except that the sheet cut into a square was stretched 7 times in the longitudinal direction and 7 times in the width direction at a stretching speed of 1000 mm/min.
• Example 2 The procedure was carried out in the same manner as in Example 1, except that 100% by mass of ultra-high molecular weight polyethylene having an Mw of 1.0 ⁇ 10 6 and a melting point of 136° C. was used as the resin A as the polyolefin raw material.
• Second preheating is performed, simultaneous biaxial stretching is performed at a stretching temperature of 110°C, liquid paraffin is removed, the washed film is dried in a drying oven adjusted to 20°C, and is placed in an electric oven at 120°C for 10 minutes. It was carried out in the same manner as in Example 1, except that heat setting treatment (heat treatment) was performed.
• Example 8 The procedure was carried out in the same manner as in Example 1, except that the neck-in rate of the sheet was adjusted to 69% by adjusting the distance between the top of the casting drum and the die lip when forming the gel sheet.
• Examples 1 to 5 are mainly composed of polyethylene, have an arithmetic mean roughness Sa (A) of 1.0 ⁇ m or less measured by scanning white interference microscopy after heat treatment at 160 ° C., and have a piercing strength in terms of basis weight. It is 500 mN/(g/m 2 ) or more. Therefore, good results are shown in meltdown property evaluation after providing the heat-resistant porous layer. On the other hand, in Comparative Examples 1, 2, 7 and 8, Sa (A) and puncture strength in terms of basis weight did not satisfy the above ranges, and the meltdown properties after providing the heat resistant porous layer were poor.
• Sa (A) and puncture strength in terms of basis weight did not satisfy the above ranges, and the meltdown properties after providing the heat resistant porous layer were poor.
• Comparative Examples 3 to 6 uniform and high-quality samples could not be obtained, and in particular in Comparative Examples 4 and 5, samples worthy of evaluation as microporous membranes could not be obtained. Also in Comparative Examples 3 and 6, the heat-resistant porous layer could not be uniformly formed on the polyolefin microporous membrane due to unevenness in the polyolefin microporous membrane, and the meltdown characteristics were poor.
• the polyolefin microporous film of the present invention is provided with a heat-resistant porous layer and used as a laminated film as a battery separator, it is possible to impart high safety against abnormal heat generation of the battery, and it is excellent in film strength. It can be suitably used as a separator for secondary batteries that require high capacity.

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