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
• • 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
• • 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
  • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
  • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
  • C08J2323/04—Homopolymers or copolymers of ethene
  • C08J2323/06—Polyethene
• • 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
• • 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
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to polyolefin microporous membranes, battery separators, and secondary batteries.
• Polyolefin microporous membranes are used as filters, separators for fuel cells, separators for condensers, etc.
• Polyolefin microporous membranes are particularly suitable for use as separators for lithium-ion batteries that are widely used in laptop personal computers, smart phones, electric vehicles, and the like.
• the reason for this is that the polyolefin microporous membrane has excellent mechanical strength and shutdown characteristics, and has characteristics suitable for ensuring the safety of batteries.
• lithium-ion secondary batteries in particular, have been developed with the aim of increasing the size of the batteries, increasing the energy density, increasing the capacity, and increasing the output, mainly for in-vehicle applications. Along with this, the demand for the safety of separators has become even higher.
• Shutdown characteristics and meltdown characteristics can be mentioned as the characteristics responsible for battery safety.
• Shutdown characteristics refer to the ability of the separator to block pores when the temperature inside the battery becomes abnormally high, such as when the inside of the battery is overcharged and overheated. By blocking the pores of the separator, the resistance is increased and the battery reaction is interrupted, so that the safety of the battery can be ensured.
• the lower the shutdown temperature the higher the safety effect.
• the polymer in the microporous membrane melts and the pores cannot be kept closed due to shrinkage or the like, and the resistance decreases. This phenomenon is called meltdown.
• the meltdown temperature is defined as the temperature at which the resistance value falls below a certain value. The higher the meltdown temperature and the higher the resistance value at high temperature, the better the meltdown characteristics, and the higher the effect on battery safety.
• Patent Document 1 describes that a microporous membrane having excellent meltdown properties can be provided by mixing polyethylene with polypropylene having a weight average molecular weight of 500,000 or more.
• Patent Document 2 describes that a microporous membrane capable of achieving both shutdown characteristics and meltdown characteristics can be provided by laminating polypropylene-containing layers.
• the present invention achieves a high level of both puncture strength and permeability in addition to shutdown characteristics and meltdown characteristics without impairing film forming properties, and when used as a battery separator, exhibits excellent safety and output.
• An object of the present invention is to provide a polyolefin microporous membrane having properties.
• a polyolefin microporous membrane that satisfies predetermined requirements for ⁇ Tm and ⁇ S, which will be described later, can be manufactured without containing a large amount of a heat-resistant raw material such as polypropylene.
• the inventors have found that it is possible to achieve compatibility with meltdown properties and other properties without impairing film properties, and have completed the present invention.
• the present invention includes the following configurations 1-9. 1.
• TmB the temperature at the point where the intensity is the highest below 145 ° C.
• ⁇ Tm the temperature at the point where the intensity is the highest below 145 ° C.
• Tm A the temperature showing the minimum value in the range of 145° C. or more and less than 155° C. in the differential melting endothermic curve.
• a polyolefin microporous membrane that is less than 5.
• the polyolefin microporous membrane according to 1 above which has a tan ⁇ at 130° C. of 0.35 or more obtained from dynamic viscoelasticity measurement.
• the polyolefin microporous membrane according to 1 or 2 above which has a puncture strength of 400 mN/(g/m 2 ) or more in terms of basis weight. 4. 4.
• the polyolefin microporous membrane according to any one of 1 to 3 above which has an air permeability of 50 seconds/100 cm 3 /(g/m 2 ) or less in terms of basis weight. 5. 4. The polyolefin microporous membrane according to any one of 1 to 4 above, which contains polyethylene as a main component. 6. 6. The polyolefin microporous membrane according to any one of 1 to 5 above, which does not have a peak at 155° C. or higher in the melting endothermic curve. 7.
• any one of 1 to 6 above, wherein the molecular weight distribution of polyethylene measured by GPC method contains 10% by mass or more of a polyethylene component having a molecular weight of 50,000 or less and 15% by mass or more of a polyethylene component having a molecular weight of 1,000,000 or more.
• 9. 9 A secondary battery using the battery separator described in 8 above.
• polyolefin microporous membrane of the present invention it is possible to achieve both high levels of puncture strength and permeability in addition to shutdown characteristics and meltdown characteristics without impairing film-forming properties.
• a polyolefin microporous membrane can be suitably used as a battery separator while maintaining safety and output characteristics.
• ⁇ Tm is 8.0 or more and 12.0 or less.
• ⁇ Tm is 8.0 or more, preferably 8.5 or more, and more preferably 9.0 or more.
• ⁇ Tm is 12.0 or less, preferably 11.5 or less, and more preferably 11.0 or less. That ⁇ Tm is within the above range means that the polyolefin microporous membrane has crystal structures with different properties in terms of thermal stability. That is, when ⁇ Tm is within the above range, it is easy to achieve both shutdown characteristics and meltdown characteristics within a range in which mechanical strength and permeability are not deteriorated, which is preferable.
• ⁇ S is greater than 0.1 and less than 0.5.
• ⁇ S exceeds 0.1, preferably exceeds 0.15, and more preferably exceeds 0.2.
• ⁇ S is less than 0.5, preferably less than 0.4, more preferably less than 0.35, even more preferably less than 0.3.
• the ratio of the structure having the characteristic of melting at a relatively low temperature, which contributes to the shutdown characteristic, and the structure having the characteristic of maintaining the structure up to a relatively high temperature, which contributes to the meltdown characteristic is It tends to be suitable, and it is easy to achieve both shutdown characteristics and meltdown characteristics.
• the raw material composition of the polyolefin microporous membrane should be within the ranges described later, and the stretching conditions and heat setting conditions during the formation of the polyolefin microporous membrane should be within the ranges described below. is preferred.
• the polyolefin microporous membrane according to the embodiment of the present invention contains structures that contribute to shutdown characteristics and meltdown characteristics in a well-balanced manner because the ⁇ Tm and ⁇ S are within the above ranges.
• the polyolefin microporous membrane of the present invention is used as a battery separator, it has high safety and can be suitably used as a battery separator for secondary batteries.
• the tan ⁇ at 130°C of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 0.35 or more.
• tan ⁇ at 130° C. refers to a value obtained from dynamic viscoelasticity measurement under conditions of a heating rate of 10° C./min and a frequency of 10 Hz.
• the tan ⁇ at 130° C. is more preferably 0.37 or more, more preferably 0.40 or more.
• the upper limit of tan ⁇ at 130° C. is not particularly limited, it is preferably 1.0 or less considering that polyolefin is the main component.
• the raw material composition of the polyolefin microporous membrane should be as described below, and the stretching conditions and heat setting conditions during the polyolefin microporous membrane formation should be within the ranges described below. is preferred.
• the microporous polyolefin membrane according to the embodiment of the present invention has a puncture strength converted to a basis weight of 400 mN/(g/m 2 ) or more.
• the lower limit of the puncture strength in terms of basis weight is more preferably 450 mN/(g/m 2 ) or more, more preferably 500 mN/(g/m 2 ) or more, even more preferably 600 mN/(g/m 2 ) or more, and 650 mN.
• /(g/m 2 ) or more is particularly preferable, and 700 mN/(g/m 2 ) or more is more particularly preferable.
• the upper limit of the puncture strength converted to basis weight is not particularly limited, it is preferably 1800 mN/(g/m 2 ) or less in consideration of the balance with various physical properties such as heat shrinkage of the polyolefin microporous membrane.
• the membrane rupture resistance against foreign matter is high when used as a battery separator, and the short-circuit resistance is likely to be improved. This makes it possible to provide a battery with higher safety.
• the raw material composition of the polyolefin microporous membrane should be as described below, and the stretching conditions and heat setting conditions at the time of forming the polyolefin microporous membrane should be adjusted. It is preferable to make it within the range described later.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably contains polyethylene as a main component. Specifically, it is preferable that the proportion of polyethylene in the polyolefin in the polyolefin microporous membrane is 93% by mass or more.
• the proportion of polyethylene in the polyolefin in the polyolefin microporous membrane is more preferably 96% by mass or more, still more preferably 99% by mass or more, and particularly preferably 100% by mass. If the proportion of polyethylene in the polyolefin is within the above range, it is easy to improve the film-forming properties, and it is easy to obtain a polyolefin microporous film excellent in puncture strength and shutdown properties.
• the polyolefin microporous membrane according to the embodiment of the present invention does not have a peak at 155° C. or higher in a melting endothermic curve obtained by measuring with a differential scanning calorimeter at a heating rate of 10° C./min. preferable.
• a peak at 155° C. or higher it is easy to improve the film formability, and to easily obtain a polyolefin microporous film excellent in puncture resistance and shutdown properties.
• the polyolefin microporous film according to the embodiment of the present invention contains 10% by mass or more of a polyethylene component having a molecular weight of 50,000 or less and 15 mass% of a polyethylene component having a molecular weight of 1,000,000 or more in the molecular weight distribution of polyethylene measured by the GPC method. % or more is preferable.
• the content of the polyethylene component having a molecular weight of 50,000 or less is preferably 10% by mass or more, more preferably 13% by mass or more, and even more preferably 15% by mass or more.
• the polyethylene component having a molecular weight of 50,000 or less is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less.
• the content of the polyethylene component having a molecular weight of 1,000,000 or more is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 23% by mass or more.
• the content of the polyethylene component having a molecular weight of 1,000,000 or more is preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 25% by mass or less.
• the polyethylene component having a molecular weight of 2,000,000 or more is 5 mass % or more and 20 mass % or less in the polyethylene molecular weight distribution measured by the GPC method.
• the content of the polyethylene component having a molecular weight of 2,000,000 or more is preferably 5% by mass or more, more preferably 7% by mass or more, and even more preferably 9% by mass or more.
• the content of the polyethylene component having a molecular weight of 2,000,000 or more is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less.
• the molecular weight distribution of polyethylene By setting the molecular weight distribution of polyethylene within the above range, it is easy to appropriately control the relaxation behavior at high temperatures while maintaining the puncture strength and shutdown characteristics of the polyolefin microporous membrane. This makes it possible to improve meltdown characteristics.
• the raw material composition of the polyolefin microporous membrane In order to set the molecular weight distribution of polyethylene measured by the GPC method of the polyolefin microporous membrane to the above range, the raw material composition of the polyolefin microporous membrane should be within the range described later, and the film forming conditions of the polyolefin microporous membrane should be described later. It is preferable to be within the range.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has an air permeability converted in terms of basis weight of 50 seconds/100 cm 3 /(g/m 2 ) or less, more preferably 40 seconds/100 cm 3 /( g/m 2 ) or less, more preferably 30 sec/100 cm 3 /(g/m 2 ) or less, and particularly preferably 25 sec/100 cm 3 /(g/m 2 ) or less.
• the air permeability in terms of basis weight is 50 seconds/100 cm 3 /(g/m 2 ) or less, the ion permeability can be easily maintained, and deterioration of output characteristics when used as a battery separator can be suppressed.
• the raw material composition and lamination structure of the microporous membrane should be within the range described later, and the stretching conditions and heat setting conditions during the formation of the polyolefin microporous membrane will be described later. It is preferable to be within the range.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a porosity of 25% or more.
• the porosity is more preferably 30% or higher, still more preferably 35% or higher, and particularly preferably 37% or higher.
• the upper limit of the porosity is not particularly limited, considering the mechanical strength of the microporous membrane, 60% or less is substantially the upper limit. When the porosity is within the above range, it is easy to maintain mechanical strength and ion permeability when used as a battery separator. As a result, it is possible to maintain output characteristics and safety when used in batteries.
• the raw material composition of the polyolefin microporous membrane may be set within the range described later, and the stretching conditions and heat setting conditions during the formation of the polyolefin microporous membrane may be set within the range described below. preferable.
• the film thickness of the polyolefin microporous membrane according to the embodiment of the present invention is appropriately adjusted depending on the application, and is not particularly limited.
• the film thickness is preferably 1 ⁇ m or more and 25 ⁇ m or less.
• the film thickness is more preferably 2 ⁇ m or more and 15 ⁇ m or less, still more preferably 2 ⁇ m or more and 12 ⁇ m or less, particularly preferably 2 ⁇ m or more and 10 ⁇ m or less, and most preferably 2 ⁇ m or more and 9 ⁇ m or less.
• the film thickness can be adjusted by adjusting the extruder screw rotation speed, the width of the unstretched sheet, the film-forming speed, the stretching ratio, etc., within the range that does not deteriorate other physical properties.
• the basis weight of the polyolefin microporous membrane according to the embodiment of the present invention is appropriately adjusted depending on the application, and is not particularly limited.
• the basis weight is preferably 1.0 g/m 2 or more and 10.0 g/m 2 or less.
• the basis weight is more preferably 1.5 g/m 2 or more and 7.0 g/m 2 or less, and even more preferably 2.0 g/m 2 or more and 6.5 g/m 2 or less.
• the basis weight can be adjusted by adjusting the number of screw revolutions of the extruder, the draw ratio, etc., within a range that does not deteriorate other physical properties.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a shutdown temperature of 140°C or less according to the temperature-programmed impedance method.
• the shutdown temperature is more preferably 139° C. or lower, still more preferably 137° C. or lower, and particularly preferably 135° C. or lower. If the shutdown temperature is 140° C. or less, a battery with high safety can be obtained when used as a battery separator for a secondary battery that requires high energy density, high capacity, and high output, such as an electric vehicle. can provide. If the shutdown temperature is 100° C. or less, the holes may close even in a normal use environment or in the battery manufacturing process, and the output characteristics may deteriorate.
• the raw material composition constituting the polyolefin microporous membrane should be within the range described later, and the stretching conditions and heat setting conditions during the formation of the polyolefin microporous membrane should be within the ranges described below. is preferred.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a resistance value of 1.0 ⁇ 10 3 ⁇ cm 2 or more at 160° C. by the temperature-programmed impedance method, and more preferably 1.2 ⁇ 10 3 ⁇ cm 2 . cm 2 or more, more preferably 1.5 ⁇ 10 3 ⁇ cm 2 or more. If the resistance value at 160° C. by the temperature-programmed impedance method is 1.0 ⁇ 10 3 ⁇ cm 2 or more, the secondary battery needs high energy density, high capacity, and high output for electric vehicles and the like. It is possible to provide a battery with high safety when used as a battery separator for a battery. It is said that the higher the resistance value at 160° C.
• the raw material composition constituting the polyolefin microporous membrane should be within the range described later, and the stretching conditions and heat setting at the time of forming the polyolefin microporous membrane should be set. It is preferable to set the conditions within the range described later.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably has a resistance value of 1.0 ⁇ 10 2 ⁇ cm 2 or more at 180° C. measured by the temperature-programmed impedance method, and more preferably 2.0 ⁇ 10 2 ⁇ cm 2 . cm 2 or more, more preferably 4.0 ⁇ 10 2 ⁇ cm 2 or more, and particularly preferably 6.0 ⁇ 10 2 ⁇ cm 2 or more. If the resistance value at 180° C. by the temperature-programmed impedance method is 1.0 ⁇ 10 2 ⁇ cm 2 or more, it is a secondary battery that requires high energy density, high capacity, and high output for electric vehicles and the like.
• the raw material composition constituting the polyolefin microporous membrane should be within the range described later, and the stretching conditions and heat setting at the time of forming the polyolefin microporous membrane should be set. It is preferable to set the conditions within the range described later.
• the polyolefin microporous membrane according to the embodiment of the present invention preferably contains high-molecular-weight polyethylene and low-molecular-weight polyethylene as described below.
• a mixture of two or more types of high-molecular-weight polyethylene and low-molecular-weight polyethylene may be used, but from the viewpoint of structural uniformity and physical properties of the polyolefin microporous membrane, it is more preferable that each of them is composed of one type of polyethylene raw material.
• the proportion of high molecular weight polyethylene in the polyolefin microporous membrane according to the embodiment of the present invention is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more.
• ⁇ -olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. Also, the ⁇ -olefin can be confirmed by measuring with C 13 -NMR.
• the high-molecular-weight polyethylene used in the embodiment of the present invention preferably has a weight-average molecular weight (hereinafter referred to as Mw) obtained by high-temperature GPC measurement or the like of 8.0 ⁇ 10 5 or more, preferably 9.0 ⁇ 10 5 or more. is more preferable, and 1.0 ⁇ 10 6 or more is even more preferable.
• Mw of the high molecular weight polyethylene is preferably 2.0 ⁇ 10 6 or less, more preferably 1.5 ⁇ 10 6 or less, and even more preferably 1.2 ⁇ 10 6 or less. When Mw is within the above range, stretching stress is likely to be transmitted efficiently. This makes it possible to improve the mechanical strength while suppressing an increase in shrinkage force of the polyolefin microporous membrane.
• the high molecular weight polyethylene used in the embodiment of the present invention preferably has a melting point of 135°C or higher, more preferably 135.5°C or higher, as measured by a differential scanning calorimeter (DSC). Also, the melting point is preferably 138° C. or lower. When the melting point is within the above range, the polyolefin microporous membrane is likely to have high thermal stability and the meltdown property can be improved.
• DSC differential scanning calorimeter
• the heat of fusion ⁇ H (J/g) obtained from a differential scanning calorimeter (DSC) of the high-molecular-weight polyethylene used in the embodiment of the present invention is preferably 160 J/g or more, more preferably 170 J/g or more, and 180 J/g. The above is more preferable.
• the upper limit of ⁇ H is not particularly limited, it is typically 250 J/g or less due to the characteristics of high molecular weight polyethylene.
• ⁇ H is within the above range, when used in a polyolefin microporous membrane, the structure tends to be stabilized and deterioration of permeability can be suppressed, which is preferable.
• the high-molecular-weight polyethylene used in the embodiment of the present invention preferably has a high recrystallization ability in order to improve meltdown characteristics in addition to having a melting point within the above range.
• This recrystallization ability can be observed from a differential scanning calorimeter (DSC) under high-speed cooling conditions, which will be described later.
• DSC differential scanning calorimeter
• a peak or shoulder attributed to recrystallization may be observed at a temperature higher than the peak top temperature.
• the recrystallization ability of polyethylene-based resin the crystal melting rate at 135 ° C.
• high-temperature crystal melting rate when measuring with a differential scanning calorimeter (DSC) under the above high-speed cooling conditions is simple. can be used as a useful index.
• the high-temperature crystal melting ratio (H) of the high-molecular-weight polyethylene used in the embodiment of the present invention is preferably 5.0% or more, more preferably 10.0% or more, and even more preferably 13.0% or more. Although the upper limit is not particularly limited, it is typically 30% or less due to the characteristics of polyethylene.
• the high-temperature crystal melting ratio (H) is within the above range, crystals having a higher melting point than the raw material are likely to be formed during or after stretching. This makes it possible to control the structure that contributes to the meltdown properties of the polyolefin microporous membrane within an appropriate range. Examples of specific conditions for high-speed cooling conditions will be described in detail in Examples.
• the proportion of low molecular weight polyethylene in the polyolefin microporous membrane according to the embodiment of the present invention is preferably 10% by mass or more, more preferably 15% by mass or more, still more preferably 20% by mass or more, and 30% by mass or more. It is particularly preferred to have
• the low molecular weight polyethylene used in the embodiment of the present invention may be an ethylene homopolymer, or a copolymer containing other ⁇ -olefins to lower the melting point of the raw material as described later. There may be.
• Other ⁇ -olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene, and the like. Also, the ⁇ -olefin can be confirmed by measuring with C 13 -NMR.
• the weight-average molecular weight (hereinafter referred to as Mw) of the low-molecular-weight polyethylene used in the embodiment of the present invention obtained by high-temperature GPC measurement or the like is preferably 12 ⁇ 10 4 or less, more preferably 9.0 ⁇ 10 4 or less. 0 ⁇ 10 4 or less is more preferable.
• Mw of the low-molecular-weight polyethylene is preferably 1.0 ⁇ 10 4 or more, more preferably 3.0 ⁇ 10 4 or more, and even more preferably 5.0 ⁇ 10 4 or more.
• Mw is within the above range, the structure formed by the high-molecular-weight polyethylene is less likely to be disturbed, and low-melting-point crystal formation and reduction in shrinkage force during melting are likely to be possible. As a result, it is possible to achieve compatibility between mechanical strength, shutdown characteristics, and meltdown characteristics.
• the melting point (°C) of the low-molecular-weight polyethylene used in the embodiment of the present invention obtained from a differential scanning calorimeter (DSC) is preferably 134°C or lower, more preferably 133°C or lower, and even more preferably 132°C or lower.
• the melting point of the low-molecular-weight polyethylene is preferably 125°C or higher, more preferably 127°C or higher, still more preferably 130°C or higher, and particularly preferably 131°C or higher.
• the melting point of the low-molecular-weight polyethylene is within the above range, the melting point of the structure before stretching can be lowered within an appropriate range, and when it is made into a polyolefin microporous membrane, the structure formed by the high-molecular-weight polyethylene is less likely to be inhibited, Crystal formation with a low melting point is likely to be possible. This makes it possible to improve the shutdown characteristics of the polyolefin microporous membrane.
• the low molecular weight polyethylene used in the embodiment of the present invention preferably has a heat of fusion ⁇ H (J / g) obtained from a differential scanning calorimeter (DSC) of 200 J / g or more, more preferably 210 J / g or more, 220 J/g or more is more preferable.
• ⁇ H heat of fusion
• the upper limit of ⁇ H is not particularly limited, it is typically 260 J/g or less due to the properties of polyethylene.
• ⁇ H is within the above range, low-melting-point crystals can easily be formed without excessively reducing the amount of crystals in the polyolefin microporous membrane. This makes it possible to achieve both shutdown characteristics and transparency.
• the half width of the maximum peak on the melting endothermic curve obtained from a differential scanning calorimeter (DSC) of the low-molecular-weight polyethylene used in the embodiment of the present invention is preferably 6.0°C or less, more preferably 5.5°C or less. , is more preferably 5.0° C. or less, and more preferably 4.5° C. or less.
• the lower limit of the half-value width is preferably 1.0° C. or higher, more preferably 3.0° C. or higher. When the half-value width is within the above range, it is likely to be possible to suppress an excessively low melting point of the polyolefin microporous membrane. This makes it possible to achieve both shutdown characteristics and transparency.
• the low-molecular-weight polyethylene used in the embodiment of the present invention preferably has a low recrystallization ability in order to improve shutdown characteristics, in addition to having a melting point within the above range.
• the crystal melting ratio at 135°C or higher hereinafter also referred to as high temperature crystal melting ratio: H
• the high-temperature crystal melting ratio (H) of the low-molecular-weight polyethylene used in the embodiment of the present invention is preferably 3.0% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.
• the lower limit of the high-temperature crystal melting ratio (H) is 0% or more. If the high-temperature crystal melting ratio (H) of the low-molecular-weight polyethylene is within the above range, the excessively high melting point of the crystals that contributes to the shutdown characteristics shown mainly by peak B during crystal stabilization during stretching or after stretching. quenching can be suppressed, and the shutdown characteristics of the polyolefin microporous membrane can be improved.
• the polyolefin microporous membrane according to the embodiment of the present invention contains the above high-molecular-weight polyethylene and low-molecular-weight polyethylene, it may further contain other polyolefin-based resin raw materials such as polypropylene for the purpose of improving meltdown characteristics. good.
• the ratio of polyolefin resin raw materials other than high-molecular-weight polyethylene and low-molecular-weight polyethylene is 7.5 in the polyolefin microporous membrane. It is preferably less than 5% by mass, more preferably less than 1% by mass, and particularly preferably 0% by mass.
• the proportion of polypropylene in the polyolefin microporous membrane is preferably less than 7.5% by mass, more preferably less than 5% by mass, even more preferably less than 1% by mass, and particularly preferably 0% by mass.
• the polyolefin-based resin used in the polyolefin microporous membrane according to the embodiment of the present invention contains an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, and an antiblocking agent, as long as the effects of the present invention are not impaired.
• Various additives such as agents and fillers may be contained.
• an antioxidant for the purpose of suppressing oxidative deterioration due to heat history of the polyethylene resin.
• antioxidants examples include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di -t-butyl-4-hydroxybenzyl)benzene (for example, BASF "Irganox” (registered trademark) 1330: molecular weight 775.2), tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxy It is preferable to use one or more selected from phenyl)propionate]methane (for example, “Irganox” (registered trademark) 1010 manufactured by BASF, molecular weight 1177.7). Appropriately selecting the types and amounts of antioxidants and heat stabilizers to be added is important for adjusting or enhancing the properties of the polyolefin microporous membrane.
• the polyolefin microporous membrane according to the embodiment of the present invention can be obtained, for example, by biaxially stretching the raw materials described above.
• the biaxial stretching method may be an inflation method, a simultaneous biaxial stretching method, or a sequential biaxial stretching method. Simultaneous biaxial stretching or sequential biaxial stretching is preferably employed in terms of controlling properties.
• the polyolefin microporous membrane is preferably produced by a method including the following steps (a) to (e).
• An example of a method for producing a polyolefin microporous membrane using the raw material described above will be described below, but the method is not necessarily limited to this.
• the film forming direction is defined as the MD direction
• the film width direction perpendicular to the MD direction is defined as the TD direction.
• step (a) Step of preparing a polyolefin-based resin solution
• a polyolefin-based resin solution is obtained by melt-kneading a material containing a polyolefin-based resin and a plasticizer (solvent) to heat and dissolve the polyolefin-based resin in the plasticizer.
• the plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but the solvent is preferably liquid at room temperature in order to enable stretching at a relatively high magnification.
• Solvents include, for example, aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding thereto, and dibutyl phthalate. and dioctyl phthalate, which are liquid at room temperature.
• a non-volatile liquid solvent such as liquid paraffin.
• the solvent that is solid at room temperature may be mixed with the liquid solvent.
• solid solvents include stearyl alcohol, ceryl alcohol, paraffin wax, and the like. However, if only a solid solvent is used, uneven stretching may occur.
• the viscosity of the liquid solvent is preferably 20-200 cSt at 40°C. If the viscosity at 40° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin resin solution from the die is less likely to be uneven. On the other hand, if it is 200 cSt or less, the removal of the liquid solvent is easy.
• the viscosity of the liquid solvent is measured at 40° C. using an Ubbelohde viscometer.
• the mixing ratio of the polyolefin resin and the plasticizer is preferably in the following range, with the total of the polyolefin resin and the plasticizer being 100% by mass.
• the content of the polyolefin resin may be appropriately selected within a range that does not impair the moldability, but it is preferably 10 to 50% by mass, more preferably 15 to 30% by mass. If the polyolefin resin content is less than 10% by mass, or if the plasticizer content is 90% by mass or more, swell or neck-in tends to increase at the exit of the die during sheet molding, resulting in poor sheet moldability. tends to deteriorate and the film formability tends to decrease. On the other hand, when the polyolefin resin exceeds 50% by mass, or when the plasticizer is 50% by mass or less, shrinkage in the thickness direction tends to increase, and moldability tends to decrease.
• a specific method for uniformly melt-kneading the polyolefin resin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin resin solution, it is preferable to use a twin-screw extruder. If necessary, various additives such as antioxidants may be added to the polyolefin resin solution within a range that does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.
• the screw diameter and screw in the twin-screw extruder It is preferable to set the shape and the number of pieces as shown in (1) to (3) below. By setting the content within the following range, it is possible to reduce unmelted substances in the polyolefin resin solution after melting, easily suppress outflow from the vent hole, and improve quality such as discharge accuracy.
• non-uniformity of physical properties such as film thickness and basis weight can be suppressed when a polyolefin microporous film is formed, and quality and physical properties can be improved.
• 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 polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted.
• the melt-kneading temperature varies depending on the polyolefin resin used, but is preferably from (the melting point of the polyolefin resin +10° C.) to (the melting point of the polyolefin resin +120° C.).
• the melt-kneading temperature is more preferably (melting point of polyolefin resin +20° C.) to (melting point of polyolefin resin +100° C.).
• the melting point refers to a value measured by DSC based on JIS K7121 (1987) (hereinafter the same).
• the melt-kneading temperature is preferably in the range of 140 to 250°C.
• the melt-kneading temperature is more preferably 160-230°C, most preferably 170-200°C.
• the melt-kneading temperature is preferably 140 to 250°C, more preferably 150 to 210°C.
• the melt-kneading temperature is preferably lower, but if the temperature is lower than the above temperature, unmelted substances are generated in the extruded product extruded from the die, causing film breakage etc. in the subsequent stretching process. It may be the cause. Further, if the melt-kneading temperature is higher than the above temperature, thermal decomposition of the polyolefin will be intense, and the physical properties of the resulting microporous membrane, such as strength and porosity, may be inferior. In addition, the decomposed product may deposit on chill rolls, rolls in the stretching process, etc., and may adhere to the sheet, leading to deterioration of the appearance. Therefore, it is preferable to set the melt-kneading temperature within the above range. Then, preferably, after that, a filter is used to remove foreign matter and denatured polymer.
• step (b) Step of forming a gel sheet
• a gel sheet is formed by molding the polyolefin-based resin solution and solidifying it by cooling.
• a gel-like sheet is obtained by extruding a polyolefin-based resin solution to form an extrudate and cooling the obtained extrudate. Cooling can fix the microphase of the polyethylene-based resin separated by the solvent. It is preferable to cool to 10 to 50°C in the cooling step. This is because the final cooling temperature is preferably set to the crystallization finish temperature or lower. That is, by making the higher-order structure finer, it becomes easier to perform uniform stretching in subsequent stretching.
• the cooling is preferably carried out at a rate of 30° C./min or more until at least the gelation temperature or lower. If the cooling rate is less than 30° C./min, the degree of crystallinity tends to increase, making it difficult to obtain a gel-like sheet suitable for stretching. 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. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes denser, leading to uniform stretching as well as improvement in the strength and elongation of the film.
• the melting point of the gel-like sheet observed with a differential scanning calorimeter (DSC) is preferably 128°C or lower, more preferably 127°C or lower.
• the lower limit of the melting point is preferably 115° C. or higher considering the effect on heat setting after stretching.
• the melting point of the gel sheet can be adjusted by adjusting the melting point of the polyolefin resin, the ratio of the plasticizer, cooling conditions, and the like.
• Cooling methods include direct contact with cold air, cooling water, and other cooling media, contact with rolls cooled with a refrigerant, and the use of casting drums.
• step (c) Step of stretching the gel-like sheet
• the gel-like sheet obtained in step (b) is stretched.
• the stretching method used include MD uniaxial stretching with a roll stretching machine, TD uniaxial stretching with a tenter, sequential biaxial stretching with a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching with a simultaneous biaxial tenter, and the like. is mentioned.
• the draw ratio varies depending on the thickness of the gel-like sheet, but from the viewpoint of uniformity of the film thickness, it is preferable to draw the sheet by 5 times or more in any direction.
• the area magnification is preferably 25 times or more, more preferably 49 times or more, and even more preferably 64 times or more.
• the area magnification is preferably 150 times or less.
• the area magnification is 150 times or less, breakage during production of the microporous membrane can be easily suppressed, and productivity can be improved.
• the area magnification is 150 times or less, it is possible to prevent excessive orientation and increase in the melting point of the microporous membrane and increase in the shutdown temperature.
• the stretching temperature is preferably set to the melting point of the gel sheet + 10°C or less, more preferably in the range of (the crystal dispersion temperature Tcd of the polyolefin resin) to (the melting point of the gel sheet + 5°C).
• the stretching temperature is preferably 90 to 125°C, more preferably 90 to 120°C.
• the crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D4065. Alternatively, the crystal dispersion temperature Tcd may be obtained from NMR.
• the stretching temperature is 90° C. or higher, the pores are easily formed sufficiently. Thereby, the uniformity of the film thickness can be easily obtained, and the porosity can be appropriately increased.
• the stretching temperature is 125° C. or lower, the sheet is less likely to melt and clogging of pores can be suppressed.
• the stretching stress increase rate in the region of 1 to 5 times in each direction exceeds 0.05 MPa / times and is 0.1 MPa / times. It is preferably less than
• the higher-order structure formed in the gel-like sheet is cleaved, the crystal phase is refined, and many fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure.
• Such stretching improves the mechanical strength and enlarges the pores, making it suitable for battery separators.
• the polyolefin resin is in a sufficiently plasticized and softened state, so that the high-order structure is smoothly cleaved and the crystal phase is uniformly refined. can be done.
• strain during stretching is less likely to remain, and the heat shrinkage rate can be made lower than in the case where the film is stretched after removing the plasticizer.
• step (d) a step of extracting (washing) and drying the plasticizer
• step (d) extracting (washing) and drying the plasticizer from the stretched gel-like sheet stretched in step (c) .
• the plasticizer (solvent) remaining in the gel sheet is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a microporous membrane can be obtained by removing the solvent.
• washing solvents include saturated hydrocarbons such as pentane, hexane and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain fluorocarbons.
• These cleaning solvents have a relatively low surface tension (eg, 24 mN/m or less at 25°C). By using a cleaning solvent with a relatively low surface tension, the shrinkage of the network structure forming the micropores due to the surface tension of the air-liquid interface is suppressed during drying after cleaning, resulting in a suitable porosity and permeability.
• a microporous membrane having These cleaning solvents can be appropriately selected according to the plasticizer and used alone or in combination.
• Examples of the cleaning method include a method of immersing the gel-like sheet in a cleaning solvent for extraction, a method of showering the gel-like sheet with a cleaning solvent, and a combination of these methods.
• the amount of the cleaning solvent used varies depending on the cleaning method, but generally it is preferably 300 parts by mass or more per 100 parts by mass of the gel-like sheet.
• the washing temperature may be, for example, 15 to 30°C, and heating to 80°C or less if necessary.
• the viewpoint of enhancing the cleaning effect of the solvent the viewpoint of preventing the physical properties of the resulting polyolefin microporous membrane from becoming uneven in the TD direction and/or the MD direction, and the mechanical properties of the polyolefin microporous membrane And from the viewpoint of improving electrical properties, the longer the gel-like sheet is immersed in the cleaning solvent, the better.
• the washing as described above is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the polyolefin microporous membrane is less than 1% by mass.
• the solvent in the polyolefin microporous membrane is dried and removed in the drying process.
• the drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. Insufficient drying may reduce the porosity of the polyolefin microporous membrane in the subsequent heat treatment, resulting in poor permeability.
• step (e) Step of performing at least one of heat treatment and re-stretching
• at least one of heat treatment and re-stretching is performed as necessary.
• the dried microporous polyolefin membrane may be stretched (re-stretched) at least uniaxially.
• the re-stretching can be performed by a tenter method or the like while heating the polyolefin microporous membrane in the same manner as the stretching described above.
• Re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial stretching and/or sequential stretching are combined.
• the re-stretching temperature is preferably below the melting point of the polyethylene composition, more preferably within the range of (Tcd-20°C) to the melting point. Specifically, the temperature is preferably 70 to 140°C, more preferably 110 to 138°C. Most preferably, it is 120-135°C.
• the re-stretching ratio is preferably 1.01 to 3.0 times, particularly preferably 1.01 to 2.0 times, more preferably 1.2 to 1.8 times in the TD direction. 0.3 to 1.6 times is particularly preferred.
• it is preferably 1.01 to 1.6 times each in the MD and TD directions.
• the re-stretching ratio may be different between the MD direction and the TD direction.
• the relaxation rate from the maximum re-stretching ratio is preferably 0.9 or less, more preferably 0.85 or less. If the relaxation rate is too low, the occurrence of wrinkles and the deterioration of permeability may be affected, so the relaxation rate is preferably 0.7 or more.
• the polyolefin microporous membrane after re-stretching may be subjected to heat treatment (heat setting treatment) in order to adjust the thermal shrinkage rate and shrinkage stress.
• heat treatment heat setting treatment
• the polyolefin microporous membrane after drying may be subjected to a heat setting treatment.
• the heat setting temperature is preferably, for example, 125 to 140° C. in the case of a structure containing a polyethylene resin as a main component.
• a second heat setting treatment may be performed in a direction different from the direction in which the relaxation adjustment was performed in the first heat setting treatment.
• the polyolefin microporous membrane may be subjected to hydrophilization treatment.
• Hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably carried out after the cross-linking treatment.
• ionizing radiation such as ⁇ -rays, ⁇ -rays, ⁇ -rays and electron beams.
• electron beam irradiation an electron dose of 0.1 to 100 Mrad is preferred, and an acceleration voltage of 100 to 300 kV is preferred.
• the cross-linking treatment increases the meltdown temperature of the polyolefin microporous membrane.
• nonionic surfactants In the case of surfactant treatment, nonionic surfactants, cationic surfactants, anionic surfactants or amphoteric surfactants can all be used, but nonionic surfactants are preferred.
• a polyolefin microporous membrane is immersed in a solution of a surfactant dissolved in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyolefin microporous membrane by a doctor blade method.
• a porous layer containing a resin other than a polyolefin resin is formed by coating or vapor deposition for the purpose of imparting functions such as meltdown properties, heat resistance, and adhesiveness. It may be laminated into a multi-layer polyolefin porous membrane.
• porous layers include, but are not particularly limited to, porous layers such as inorganic particle layers containing a binder and inorganic particles.
• the binder component constituting the inorganic particle layer is not particularly limited, and known components can be used.
• Examples include acrylic resins, polyvinylidene fluoride resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, and the like. can be used.
• the inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used. For example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, and the like can be used. can.
• the porous binder resin may be laminated on at least one surface of the polyolefin microporous membrane.
• the polyolefin microporous membrane obtained as described above can be used in various applications such as filters, separators for fuel cells, and separators for condensers.
• it when used as a battery separator, it exhibits excellent safety and output characteristics, so it is preferably used as a battery separator for secondary batteries that require high energy density, high capacity, and high output, such as electric vehicles. be able to.
• the thickness of the polyolefin microporous membrane at 5 points within the range of 50 mm ⁇ 50 mm was measured with a contact thickness gauge (Mitutoyo Co., Ltd. Lightmatic VL-50, 10.5 mm ⁇ carbide spherical probe, measurement load 0.01 N). , and the average value was taken as the film thickness ( ⁇ m).
• Porosity (%) ⁇ (volume-mass/membrane density)/volume ⁇ x 100
• the molecular weight of the polyolefin resin was determined by gel permeation chromatography (GPC) under the following conditions.
• GPC gel permeation chromatography
• ⁇ Measuring device GPC-150C manufactured by Waters Corporation
• ⁇ Column Shodex UT806M manufactured by Showa Denko K.K.
• ⁇ Column temperature 160°C -
• Solvent mobile phase
• 1,2,4-trichlorochlorobenzene - Solvent flow rate 1.0 mL / min -
• Sample concentration 0.1 wt% (dissolution conditions: 160 ° 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.
• the melting point (°C) of the gel-like sheet before stretching of the polyolefin microporous membrane was measured by the differential scanning calorimetry (DSC) method based on JIS K7121. A 20 mg sample was sealed in an aluminum pan, and the temperature was raised from 30° C. to 230° C. at 10° C./min using a PYRIS Diamond DSC manufactured by Parking Elmer to obtain a melting endothermic curve. The peak top temperature on the obtained melting endothermic curve was defined as the melting point (°C) of the gel sheet (before stretching).
• Proportion (% by mass) of polyethylene component with a molecular weight of 50,000 or less in the polyolefin microporous membrane (component amount of polyethylene with a molecular weight of 50,000 or less) ⁇ (component amount of polyethylene with a total molecular weight) ⁇ 100 ⁇ Measuring device: GPC-150C manufactured by Waters Corporation ⁇ Column: Shodex UT806M manufactured by Showa Denko K.K. ⁇ Column temperature: 160°C - Solvent (mobile phase): 1,2,4-trichlorochlorobenzene - Solvent flow rate: 1.0 mL / min - Sample concentration: 0.1 wt% (dissolution conditions: 160 ° C.
• ⁇ Injection volume 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.
• Proportion (% by mass) of polyethylene having a molecular weight of 1,000,000 or more in the polyolefin microporous membrane (component amount of polyethylene having a molecular weight of 1,000,000 or more) ⁇ (component amount of polyethylene having a total molecular weight) ⁇ 100
• Proportion (% by mass) of polyethylene having a molecular weight of 2,000,000 or more in the polyolefin microporous membrane (component amount of polyethylene having a molecular weight of 2,000,000 or more) ⁇ (component amount of polyethylene having a total molecular weight) ⁇ 100
• a polyolefin microporous membrane was enclosed in an aluminum pan, and a PYRIS Diamond DSC made by Parking Elmer was used as a differential scanning calorimeter to measure 10°C/min from 30°C to 230°C (number of data: 30 pieces/°C). to obtain a melting endothermic curve. Furthermore, the melting endothermic curve was corrected by setting a linear baseline in the range of 60°C to 160°C. Further, after squaring the value obtained by differentiating the obtained melting endothermic curve with respect to temperature, the data for five consecutive temperature points was averaged (smoothed) to obtain a differential melting endothermic curve.
• the peak temperature A was defined as the minimum temperature within the temperature range of 145° C. or more and less than 155° C. in the obtained temperature-differential melting endothermic curve.
• Tm A was scored as nil if there were no % higher values. If there are two or more samples that meet the above Tm A conditions, the one with the lower temperature is taken as the Tm A.
• the peak B is the point with the highest intensity below 145 ° C in the melting endothermic curve corrected by the baseline, that temperature is taken as Tm B , and by subtracting Tm B from Tm A , ⁇ Tm (° C., Tm A ⁇ Tm B ). Furthermore, in the melting endothermic curve corrected by the baseline, the intensity ratio (S A /S B ) when the intensity at Tm A was S A and the intensity at Tm B was S B was defined as ⁇ S. Moreover, the presence or absence of a peak at 155° C. or higher in the obtained melting endothermic curve was also confirmed.
• the melting point of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) in accordance with JIS K7121. 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 under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer (first temperature rise), Hold at 230 ° C. for 5 minutes, cool at a rate of 10 ° C./min, heat again from 30 ° C. to 230 ° C. at a rate of 10 ° C./min (second temperature rise), and each melting endothermic curve Obtained. The temperature of the peak top on the melting endothermic curve obtained in the second heating was taken as the melting point of the polyolefin resin raw material.
• the melting point of the raw material polyolefin resin was measured by differential scanning calorimetry (DSC) in accordance with JIS K7121. 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 under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer (first temperature rise), Hold at 230 ° C. for 5 minutes, cool at a rate of 10 ° C./min, heat again from 30 ° C. to 230 ° C. at a rate of 10 ° C./min (second temperature rise), and each melting endothermic curve Obtained. The heat of fusion on the melting endothermic curve obtained in the second temperature rise was integrated from 60° C. to 160° C. to obtain ⁇ H (J/g) of the polyolefin resin raw material.
• High-temperature crystal melting ratio of polyolefin resin H
• the high-temperature crystal melting ratio (H) of the polyolefin resin was measured by differential scanning calorimetry (DSC) under the following high-speed cooling conditions. 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 under a nitrogen atmosphere using a PYRIS Diamond DSC manufactured by Parking Elmer (first temperature rise), Hold at 230 ° C. for 5 minutes, cool at a rate of 300 ° C./min, heat again from 30 ° C. to 230 ° C. at a rate of 10 ° C./min (second temperature rise), and each melting endothermic curve Obtained.
• DSC differential scanning calorimetry
• a linear baseline was set in the range of 60 ° C. to 160 ° C., and the heat quantity was calculated from the area surrounded by the linear baseline and the melting endothermic curve. This was converted into the mass of the sample to calculate the total heat of fusion H all . Also, after 135°C, the amount of heat was calculated from the area of the portion surrounded by the linear baseline and the melting endothermic curve, and converted to the mass of the sample to calculate the high-temperature crystal melting heat H 135°Cover .
• LiPF 6 lithium hexafluorophosphate
• EC ethylene carbonate
• EMC ethylmethyl carbonate
• Example 1 100 parts by mass of a polyolefin resin containing 80% by mass of PEa as high-molecular-weight polyethylene and 20% by mass of PEd as low-molecular-weight polyethylene, an antioxidant tetrakis[methylene-3-(3,5-ditertiarybutyl-4 -Hydroxyphenyl)-propionate]methane 0.5 parts by mass was blended to prepare a mixture. 20 parts by mass of the resulting mixture was charged into a twin-screw extruder (inner diameter 58 mm, screw specification A), and 80 parts by mass of liquid paraffin [35 cst (40 ° C.)] was supplied from the side feeder of the twin-screw extruder. C. and 200 rpm to prepare a polyolefin resin solution.
• the polyolefin resin solution is passed through a filter from each twin-screw extruder to remove foreign matter, then supplied to a T-die, and the extruded body is cooled by a cooling roll controlled to 15°C at a take-up speed of 5 m/min while taking it. , to form a gel-like sheet.
• the gel-like sheet was simultaneously biaxially stretched by a tenter stretching machine at 115°C so as to stretch 8 times in both the MD and TD directions.
• the stretching stress increase rates in the 1- to 5-fold regions in the MD and TD directions during this stretching were 0.080 MPa/fold and 0.075 MPa/fold, respectively.
• the stretched gel-like sheet was fixed to an aluminum frame plate of 30 cm ⁇ 30 cm, immersed in a methylene chloride bath controlled at 25° C., shaken at 100 rpm for 10 minutes to remove liquid paraffin, and air-dried at room temperature. .
• the resulting dry film was heat-set at 125°C for 10 minutes.
• the thickness of the resulting polyolefin microporous membrane was 8.4 ⁇ m, and Table 4 shows the blending ratio of each constituent component, manufacturing conditions, evaluation results, and the like.
• Examples 2-6 A polyolefin microporous film was obtained by forming a film in the same manner as in Example 1 except that the raw material composition and process conditions were changed to those shown in Table 4.
• the polyolefin resin solution is passed through a filter from each twin-screw extruder to remove foreign matter, then supplied to a T-die, and the extruded body is cooled by a cooling roll controlled to 15°C at a take-up speed of 5 m/min while taking it. , to form a gel-like sheet.
• the gel-like sheet was simultaneously biaxially stretched by a tenter stretching machine at 115°C so as to stretch 7 times in both the MD and TD directions.
• the stretching stress increase rates in the 1- to 5-fold regions in the MD and TD directions during this stretching were 0.150 MPa/fold and 0.080 MPa/fold, respectively.
• the stretched gel-like sheet was fixed to an aluminum frame plate of 30 cm ⁇ 30 cm, immersed in a methylene chloride bath controlled at 25° C., shaken at 100 rpm for 10 minutes to remove liquid paraffin, and air-dried at room temperature. .
• the resulting dried film was heat-set at 125°C for 10 minutes.
• the thickness of the obtained polyolefin microporous membrane was 8.6 ⁇ m, and spots were found here and there.
• Table 5 shows the compounding ratio of each constituent component, manufacturing conditions, evaluation results, and the like. In the table, regarding the item of film-forming property, if there was no problem in film-forming property, it was indicated as " ⁇ ", if there was a problem in film-forming property, " ⁇ " A case where film formation was difficult was marked with "x”.
• Comparative Example 6 A film was formed in the same manner as in Comparative Example 1 except that 23 parts by mass of PEc as high molecular weight polyethylene, 55 parts by mass of PEe as low molecular weight polyethylene, and 22 parts by mass of PP as other polyolefin were used.
• the resulting polyolefin microporous membrane had a thickness of 8.7 ⁇ m, many spots were found here and there, and the appearance was remarkably deteriorated compared to Examples 1-4 and Comparative Examples 1-4.
• Table 4 shows the mixing ratio of each constituent component, manufacturing conditions, evaluation results, and the like.
• Example 7 A film was formed in the same manner as in Example 1 except that the screw specification of the twin-screw extruder was set to C, but the vent-up during extrusion was severe and a uniform gel-like sheet could not be obtained, so the evaluation after stretching was abandoned. .
• Example 8 A film was formed in the same manner as in Example 3 except that the screw specification of the twin-screw extruder was set to D, but the vent-up during extrusion was severe and a uniform gel-like sheet could not be obtained, so the evaluation after stretching was abandoned. .
• microporous polyolefin membranes of Examples 1 to 6 achieved a low shutdown temperature and high resistance at high temperatures without impairing the film-forming properties such as deterioration of appearance.
• Examples 1 to 4 are superior in appearance to Examples 5 and 6, and maintain other physical properties such as puncture strength and permeability at high levels.
• Comparative Examples 1 to 4 the resistance at high temperature was remarkably deteriorated.
• Comparative Examples 5 and 6 the addition of polypropylene improved the resistance at high temperatures compared to Comparative Examples 1 to 4, but poor appearance due to spotting was confirmed, and the film formability deteriorated.
• the polyolefin microporous membrane of the present invention does not impair the film formability, is excellent in shutdown characteristics and meltdown characteristics, and is excellent in safety when used as a battery separator. Therefore, it can be suitably used as a secondary battery separator that requires a high battery capacity and a thin film.

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