Polyolefin microporous membrane and air filter

Abstract

Provided is a polyolefin microporous membrane containing a polyolefin, having a bubble point of 0.04-0.30 MPa, and having a value obtained by multiplying the membrane thickness (μm) and the bubble point (Mpa) of 2.0-4.5.

Description

Polyolefin microporous membrane and air filter

[0001] The present disclosure relates to microporous polyolefin membranes and air filters.

[0002] Conventionally, polyolefin microporous membranes have been known as porous membranes used in filter media. For example, JP 2023-025511 A, JP 2023-025510 A, JP 2023-025509 A, JP 2023-025508 A, and JP 2018-176097 A disclose filter media and filters containing polyolefin microporous membranes.

[0003] Japanese Patent Application Laid-Open Nos. 2023-025511, 2023-025510, 2023-025509, and 2023-025508 disclose laminated membranes having a microporous membrane containing polyolefin and a porous support layer, in which the microporous membrane and the porous support layer are bonded together with scattered adhesive parts containing a thermoplastic resin.

[0004] JP 2018-176097 A describes a polyethylene-containing film having a bacterial capture performance of 10 or more when Brevundimonas diminuta ATCC 19146 is used as a test bacterium in accordance with JIS K3835:2006, and a water permeability coefficient of 40 m ３ / m ２ / h / MPa~80m ３ / m ２ / h / MPa porous membranes are disclosed.

[0005] Polyolefin does not contain halogen elements, and therefore, a filter whose filter material is a polyolefin microporous membrane is advantageous in that there are fewer restrictions on production and use, and fewer restrictions on disposal after use.Furthermore, in recent years, the ecotoxicity and human toxicity of organic fluorine compounds have been reported, and the restrictions on production and use of organic fluorine compounds have been tightened worldwide, which increases market demand for polyolefin microporous membranes and calls for further improvements in the performance of polyolefin microporous membranes.

[0006] The present disclosure has been made under the above circumstances, and an object of the present disclosure is to provide a microporous polyolefin membrane that has excellent particle separation performance and fluid treatment efficiency.

[0007] Specific means for solving the above problems include the following: <1> A polyolefin microporous membrane comprising a polyolefin, having a bubble point of 0.04 MPa to 0.30 MPa, and wherein the product of the membrane thickness (μm) and the bubble point (MPa) is 2.0 to 4.5. <2> The polyolefin microporous membrane according to <1>, which has not been subjected to a hydrophilization treatment. <3> The polyolefin microporous membrane according to <1> or <2>, wherein the polyolefin comprises polyethylene. <4> The polyolefin microporous membrane according to any one of <1> to <3>, wherein the polyolefin comprises ultra-high molecular weight polyethylene, and wherein the proportion of the ultra-high molecular weight polyethylene in the polyolefin is 1% by mass to 50% by mass. <5> The polyolefin microporous membrane according to any one of <1> to <4>, having a Gurley value of 1 second / 100 mL to 20 seconds / 100 mL. <6> The polyolefin microporous membrane according to any one of <1> to <5>, having a porosity of 60% to 90%. <7> The polyolefin microporous membrane according to any one of <1> to <6>, for use as a filter material for an air filter. <8> An air filter comprising a filter material comprising the polyolefin microporous membrane according to any one of <1> to <6>.

[0008] According to the present disclosure, a microporous polyolefin membrane is provided that has excellent particle separation performance and fluid treatment efficiency.

[0009]

[0023] The following describes embodiments of the present disclosure. These descriptions and examples are intended to illustrate the embodiments and are not intended to limit the scope of the embodiments.

[0010] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in an example.

[0011] In the present disclosure, "A and / or B" is synonymous with "at least one of A and B." In other words, "A and / or B" means that it may be only A, only B, or a combination of A and B.

[0012] In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0013] In the present disclosure, when referring to the amount of each component in a composition, if multiple substances corresponding to each component are present in the composition, the total amount of the multiple substances present in the composition is meant unless otherwise specified.

[0014] In this disclosure, MD (Machine Direction) refers to the longitudinal direction of a polyolefin microporous membrane produced in a continuous shape, and TD (Transverse Direction) refers to the plane direction of the polyolefin microporous membrane perpendicular to the MD. In this disclosure, TD is also referred to as the "width direction."

[0015] In the present disclosure, the side of a polyolefin microporous membrane from which gas or liquid flows in is referred to as the "upstream" side, and the side from which gas or liquid flows out is referred to as the "downstream" side.

[0016] <Polyolefin microporous membrane> In the present disclosure, a polyolefin microporous membrane refers to a microporous membrane containing a polyolefin, which has a structure in which numerous micropores are connected inside and allows gas or liquid to pass from one surface to the other.

[0017] The polyolefin microporous membrane of the present disclosure has a bubble point of 0.04 MPa to 0.30 MPa, and the value obtained by multiplying the membrane thickness (μm) and the bubble point (MPa) is 2.0 to 4.5. The polyolefin microporous membrane of the present disclosure has excellent particle separation performance and fluid treatment efficiency. An index of fluid treatment efficiency is the flow rate per unit time.

[0018] The polyolefin microporous membrane of the present disclosure has a bubble point of 0.04 MPa or more. A polyolefin microporous membrane with a bubble point of less than 0.04 MPa has a pore size that is too large and has poor particle separation performance. From the viewpoint of excellent particle separation performance, the polyolefin microporous membrane of the present disclosure has a bubble point of 0.04 MPa or more, preferably 0.05 MPa or more.

[0019] The polyolefin microporous membrane of the present disclosure has a bubble point of 0.30 MPa or less. A polyolefin microporous membrane with a bubble point of more than 0.30 MPa has a pore size that is too small and has poor fluid treatment efficiency. From the viewpoint of excellent fluid treatment efficiency, the polyolefin microporous membrane of the present disclosure has a bubble point of 0.30 MPa or less, preferably 0.28 MPa or less, more preferably 0.26 MPa or less.

[0020] The bubble point of a polyolefin microporous membrane is determined by a bubble point test in accordance with JIS K3832:1990 "Bubble Point Test Method for Microfiltration Membrane Elements and Modules." Ethanol is used as the test liquid. The liquid temperature during the test is 24±2°C, and the applied pressure is increased at a rate of 2 kPa / sec.

[0021] The polyolefin microporous membrane of the present disclosure has a value obtained by multiplying the membrane thickness (μm) and the bubble point (MPa) of 2.0 or more. Polyolefin microporous membranes with a multiplication value of less than 2.0 tend to have poor particle separation performance. From the viewpoint of excellent particle separation performance, the polyolefin microporous membrane of the present disclosure has a multiplication value of 2.0 or more, preferably 2.1 or more, more preferably 2.2 or more.

[0022] The polyolefin microporous membrane of the present disclosure has a value obtained by multiplying the membrane thickness (μm) and the bubble point (MPa) of 4.5 or less. Polyolefin microporous membranes with a multiplication value exceeding 4.5 tend to have poor fluid treatment efficiency. From the viewpoint of excellent fluid treatment efficiency, the polyolefin microporous membrane of the present disclosure has a multiplication value of 4.5 or less, preferably 4.4 or less, more preferably 4.2 or less.

[0023] The polyolefin microporous membrane of the present disclosure preferably has a membrane thickness of 7 μm to 110 μm. From the viewpoints of mechanical strength and particle separation performance, the membrane thickness of the polyolefin microporous membrane is preferably 7 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. From the viewpoints of relatively low filtration pressure and ease of processing to increase the filtration area, the membrane thickness of the polyolefin microporous membrane is preferably 110 μm or less, more preferably 105 μm or less, and even more preferably 100 μm or less.

[0024] The thickness of the polyolefin microporous membrane is the arithmetic average of measurements taken at 20 points using a contact-type measuring instrument. Specifically, the following measurement (1) or (2) is performed. (1) Measurement is performed using a contact-type film thickness gauge. An example of a contact-type film thickness gauge is an ABS Digimatic Indicator, model number: ID-S112X (Mitutoyo Corporation), and measurement is performed using a cylindrical contact probe with a bottom diameter of 6.5 mm. (2) Measurement is performed using a contact length measuring instrument. An example of a contact length measuring instrument is LITEMATIC (Mitutoyo Corporation), and measurement is performed using a cylindrical contact probe with a diameter of 5 mm, applying a load of 7 g.

[0025] The polyolefin microporous membrane of the present disclosure preferably has a Gurley value of 1 second / 100 mL to 20 seconds / 100 mL. From the viewpoints of mechanical strength and particle separation performance, the Gurley value of the polyolefin microporous membrane is preferably 1 second / 100 mL or more, more preferably 3 seconds / 100 mL or more, and even more preferably 5 seconds / 100 mL or more. From the viewpoint of requiring a relatively low filtration pressure, the Gurley value of the polyolefin microporous membrane is preferably 20 seconds / 100 mL or less, more preferably 18 seconds / 100 mL or less, and even more preferably 15 seconds / 100 mL or less.

[0026] The Gurley value of the polyolefin microporous membrane is a value measured in accordance with JIS P8117:2009 "Paper and paperboard -- Air permeability and air resistance test method (intermediate range) -- Gurley method."

[0027] The polyolefin microporous membrane of the present disclosure preferably has a porosity of 60% to 90%. From the viewpoint of relatively low filtration pressure, the porosity of the polyolefin microporous membrane is preferably 60% or more, more preferably 65% ​​or more, and even more preferably 70% or more. From the viewpoint of mechanical strength and particle separation performance, the porosity of the polyolefin microporous membrane is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less.

[0028] The porosity of the polyolefin microporous membrane is calculated by the following formula.

[0029]

[0030] where ε is the porosity (%) of the polyolefin microporous membrane, and the mass of each of constituent materials 1, 2, 3, ..., n of the polyolefin microporous membrane is W １ , W ２、 W ３ , ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ , d ２ , d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the polyolefin microporous membrane is t (cm).

[0031] When used as a filter material for an air filter, the polyolefin microporous membrane of the present disclosure is preferably hydrophobic from the viewpoint of maintaining breathability without getting wet. Since polyolefin is a hydrophobic resin, the polyolefin microporous membrane itself is hydrophobic. Therefore, the polyolefin microporous membrane of the present disclosure is preferably a polyolefin microporous membrane that has not been subjected to a hydrophilization treatment. Examples of methods for hydrophilizing a polyolefin microporous membrane include coating with a hydrophilic material (such as polyvinyl alcohol or cellulose), graft polymerization of a hydrophilic monomer, and physical hydrophilization treatment (such as plasma treatment, corona discharge treatment, ultraviolet irradiation, or electron beam irradiation).

[0032] The polyolefin microporous membrane of the present disclosure preferably has a mesh structure. A mesh structure refers to a structure in which the resin is continuously connected in a mesh shape and has a large number of pores. The mesh structure of the polyolefin microporous membrane may be a planar mesh structure in the in-plane direction of the polyolefin microporous membrane, or a three-dimensional mesh structure in the in-plane direction and thickness direction of the polyolefin microporous membrane. The polyolefin microporous membrane preferably has a three-dimensional mesh structure. The mesh structure of the polyolefin microporous membrane can be confirmed by observing the polyolefin microporous membrane with a scanning electron microscope (SEM).

[0033] The polyolefin microporous membrane of the present disclosure may be a microporous membrane made of only polyolefin, or may be a microporous membrane made of polyolefin and a material other than polyolefin. Examples of the material other than polyolefin include surfactants. The polyolefin microporous membrane may contain a surfactant to the extent that it does not affect the effects of the present disclosure.

[0034] In the polyolefin microporous membrane of the present disclosure, the polyolefin preferably accounts for 90% by mass or more of the total mass of the polyolefin microporous membrane, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

[0035] The polyolefin microporous membrane of the present disclosure may contain a resin other than polyolefin. Examples of the other resin include an acrylic resin, a styrene resin, and a butadiene rubber. The mass proportion of the other resin relative to the total mass of the polyolefin microporous membrane of the present disclosure is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0036] The polyolefin microporous membrane of the present disclosure preferably does not substantially contain fluorine-containing resin.Examples of fluorine-containing resins include polyvinylidene fluoride resins and fluorine-containing rubbers.Examples of polyvinylidene fluoride resins include homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride and halogen-containing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene; copolymers of vinylidene fluoride and other monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and other monomers other than halogen-containing monomers; and mixtures thereof.

[0037] The polyolefin microporous membrane of the present disclosure being substantially free of a fluorine-containing resin means that the mass proportion of the fluorine-containing resin in the total mass of the polyolefin microporous membrane is 1 mass% or less. The mass proportion of the fluorine-containing resin in the total mass of the polyolefin microporous membrane of the present disclosure is preferably as low as possible, preferably 0.5 mass% or less, more preferably 0.1 mass% or less, and particularly preferably 0 mass%. That is, it is particularly preferable that the polyolefin microporous membrane of the present disclosure is free of a fluorine-containing resin.

[0038] [Polyolefin] Examples of polyolefins constituting the polyolefin microporous membrane of the present disclosure include homopolymers (i.e., polyethylene, polypropylene, polybutylene, polymethylpentene, etc.) or copolymers of ethylene, propylene, butylene, methylpentene, etc., and mixtures thereof.

[0039] The polyolefin microporous membrane of the present disclosure is preferably a microporous membrane formed using two or more polyolefins that differ from each other in at least one of the type of monomer, degree of polymerization, degree of branching, crystallinity, stretchability, and molecular orientation. By using two or more polyolefins, a three-dimensional network structure is easily formed in the polyolefin microporous membrane by fibrillation during stretching.

[0040] The weight-average molecular weight of all polyolefins constituting the polyolefin microporous membrane of the present disclosure is preferably 500,000 or more, more preferably 800,000 or more, and even more preferably 1,000,000 or more, from the viewpoint of densifying the porous structure of the polyolefin microporous membrane.The weight-average molecular weight of all polyolefins constituting the polyolefin microporous membrane of the present disclosure is preferably 5,000,000 or less, more preferably 4,000,000 or less, and even more preferably 3,000,000 or less, from the viewpoint of membrane-formability of the polyolefin microporous membrane.

[0041] The weight average molecular weight of the entire polyolefin constituting the microporous polyolefin membrane is determined by dissolving the microporous polyolefin membrane in o-dichlorobenzene under heating and measuring the molecular weight by gel permeation chromatography. Monodisperse polystyrene is used for molecular weight calibration.

[0042] The polyolefin constituting the polyolefin microporous membrane of the present disclosure preferably contains polyethylene. The polyolefin microporous membrane of the present disclosure is preferably a polyethylene microporous membrane. In the present disclosure, a polyethylene microporous membrane refers to a microporous membrane in which polyethylene is the resin that accounts for the largest proportion by mass of all resins.

[0043] In the polyethylene microporous membrane, polyethylene preferably accounts for 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more of the total mass of the polyethylene microporous membrane. The polyethylene microporous membrane may contain a surfactant to the extent that it does not affect the effects of the present disclosure.

[0044] The polyethylene microporous membrane is preferably a microporous membrane formed using two or more polyethylenes that differ from each other in at least one of degree of polymerization, degree of branching, crystallinity, stretchability, and molecular orientation. Using two or more polyethylenes facilitates the formation of a three-dimensional network structure in the polyethylene microporous membrane by fibrillation during stretching.

[0045] Examples of polyethylene constituting the polyolefin microporous film and the polyethylene microporous film include ultra-high molecular weight polyethylene, high-density polyethylene, and a mixture of ultra-high molecular weight polyethylene and high-density polyethylene.

[0046] The polyolefin microporous membrane and the polyethylene microporous membrane preferably contain ultra-high molecular weight polyethylene (UHMWPE) from the viewpoint of increasing the mechanical strength of the microporous membrane and densifying the porous structure of the microporous membrane. In the present disclosure, ultra-high molecular weight polyethylene (UHMWPE) means polyethylene having a weight-average molecular weight of 1 million to 6 million. From the viewpoint of the mechanical strength of the microporous membrane, the weight-average molecular weight of UHMWPE is preferably 2 million or more, more preferably 3 million or more, and even more preferably 4 million or more. From the viewpoint of the membrane-formability of the microporous membrane, the weight-average molecular weight of UHMWPE is preferably 5.5 million or less, more preferably 5 million or less, and even more preferably 4.8 million or less.

[0047] When the polyolefin microporous membrane and the polyethylene microporous membrane contain UHMWPE, the proportion of UHMWPE in the polyolefin is preferably 1% to 50% by mass. When the proportion of UHMWPE in the polyolefin is 50% by mass or less, the pore size of the microporous membrane is not too small, and fluid treatment efficiency is excellent. From this viewpoint, the proportion of UHMWPE in the polyolefin is more preferably 48% by mass or less, and even more preferably 45% by mass or less. When the proportion of UHMWPE in the polyolefin is 1% by mass or more, the mechanical strength of the microporous membrane is easily increased. From this viewpoint, the proportion of UHMWPE in the polyolefin is more preferably 5% by mass or more, and even more preferably 10% by mass or more.

[0048] The microporous polyolefin membrane and the microporous polyethylene membrane preferably contain UHMWPE and high density polyethylene (HDPE). In the present disclosure, high density polyethylene (HDPE) refers to a polyethylene having a density of 920 kg / m ３ The density of HDPE is 960 kg / m from the viewpoint of film-forming properties of a microporous film. ３ The weight average molecular weight of HDPE is preferably 200,000 to 800,000 from the viewpoint of the mechanical strength of the microporous membrane.

[0049] The mass ratio of UHMWPE to HDPE (UHMWPE:HDPE) in the polyolefin microporous membrane and the polyethylene microporous membrane is preferably 1:99 to 50:50, more preferably 5:95 to 50:50, and even more preferably 10:90 to 50:50, from the viewpoint of balancing particle separation performance and fluid treatment efficiency.

[0050] The weight-average molecular weight of the entire polyethylene contained in the polyolefin microporous membrane and the polyethylene microporous membrane is preferably at least 500,000, more preferably at least 800,000, and even more preferably at least 1,000,000, from the viewpoint of densifying the porous structure of the microporous membrane. The weight-average molecular weight of the entire polyethylene contained in the polyolefin microporous membrane and the polyethylene microporous membrane is preferably at most 3,000,000, more preferably at most 2,800,000, and even more preferably at most 2,500,000, from the viewpoint of ensuring that the pore size of the microporous membrane is not too small and that fluid treatment efficiency is excellent.

[0051] [Method for Producing Polyolefin Microporous Membrane] The polyolefin microporous membrane of the present disclosure can be produced, for example, by a production method including the following steps (1) to (4).

[0052] Step (1): A step of preparing a polyolefin solution containing a polyolefin and a solvent. Step (2): A step of melt-kneading the polyolefin solution, extruding the melt-kneaded mixture through a die, and cooling and solidifying the extrudate to obtain a first gel-like molded product. Step (3): A step of primarily stretching the first gel-like molded product and drying the solvent to obtain a second gel-like molded product. Step (4): A step of secondarily stretching the second gel-like molded product.

[0053] By controlling the conditions in steps (1) to (4), it is possible to control the thickness, bubble point, and porosity of the polyolefin microporous membrane.

[0054] -Step (1)- Step (1) is a step of preparing a polyolefin solution containing a polyolefin and a solvent.

[0055] The polyolefin used in step (1) may be one kind or two or more kinds. The polyolefin preferably contains polyethylene, more preferably UHMWPE and HDPE.

[0056] The solvent used in step (1) is not limited as long as it can swell or dissolve polyolefin. Solvents are broadly classified into non-volatile solvents with a boiling point of 210°C or higher at atmospheric pressure and volatile solvents with a boiling point of less than 210°C at atmospheric pressure.

[0057] Examples of non-volatile solvents include liquid paraffin, paraffin oil, mineral oil, and castor oil. The non-volatile solvents may be used alone or in combination of two or more. Liquid paraffin is preferred as the non-volatile solvent.

[0058] Examples of volatile solvents include tetralin, ethylene glycol, decalin (also known as decahydronaphthalene), toluene, xylene, diethyltriamine, ethylenediamine, dimethyl sulfoxide, and hexane. One type of volatile solvent may be used alone, or two or more types may be used in combination. Decalin or xylene is preferred as the volatile solvent.

[0059] The solvent used in step (1) is preferably a volatile solvent, more preferably decalin or xylene, and even more preferably decalin.

[0060] From the viewpoint of forming a porous structure with mechanical strength, the polyolefin concentration of the polyolefin solution is preferably 10% by mass to 40% by mass, more preferably 15% by mass to 35% by mass, and even more preferably 20% by mass to 30% by mass. When the polyolefin concentration of the polyolefin solution is 10% by mass or more, the mechanical strength of the polyolefin microporous membrane is ensured. When the polyolefin concentration of the polyolefin solution is 40% by mass or less, pores are likely to form in the polyolefin microporous membrane.

[0061] Step (2) is a step of melt-kneading the polyolefin solution, extruding the melt-kneaded mixture through a die, and cooling and solidifying the extrudate to obtain a first gel-like molded product. The first gel-like molded product is preferably formed into a sheet shape.

[0062] The melt-kneading of the polyolefin solution is preferably carried out using a kneading extruder. A kneading extruder is a device that applies pressure and heat to a material to be treated while continuously conveying the material. The structure of a kneading extruder is generally broadly divided into a material inlet, a barrel, and a die, from upstream to downstream. A screw is provided inside the barrel. A heater that heats the inside of the barrel is provided around the barrel. The screw may be a single-screw type or a twin-screw type, with the twin-screw type being preferred.

[0063] The temperature of the polyolefin solution in the die of the kneading extruder is preferably in the range of MP°C to MP+100°C, where MP°C is the melting point of the polyolefin (when two or more polyolefins are used, the highest melting point among the melting points of those polyolefins is taken as MP°C).

[0064] Methods for cooling the extrudates include, for example, immersing the extrudates in water or an organic solvent, or contacting the extrudates with a cooled metal roll. The cooling temperature is preferably 10° C. to 40° C. When immersing the extrudates in water, it is preferable to create a water flow on the surface of the water bath to prevent the solvent released from the extrudates from adhering to the extrudates.

[0065] Step (3) is a step of primarily stretching the first gel-like molded product and then drying the solvent to obtain a second gel-like molded product. The primary stretching involves stretching the first gel-like molded product in at least one direction.

[0066] The primary stretching is preferably uniaxial stretching in which the first gel-like molded product is stretched in the MD. The stretching ratio in the primary stretching is preferably 1.1 to 3 times, more preferably 1.1 to 2 times, from the viewpoint of forming a porous structure with mechanical strength. The stretching temperature in the primary stretching is preferably 75°C or lower.

[0067] The drying in step (3) is preferably carried out at a temperature at which the gel-like molding does not deform, and is preferably carried out at 60°C or lower.

[0068] In step (3), the primary stretching and drying may be carried out simultaneously or stepwise. For example, the primary stretching may be carried out while pre-drying and then main drying, or the primary stretching may be carried out between pre-drying and main drying.

[0069] Step (4) Step (4) is a step of secondarily stretching the second gel-like molded product in at least one direction.

[0070] The secondary stretching is preferably biaxial stretching. The biaxial stretching may be any of sequential biaxial stretching in which longitudinal stretching and transverse stretching are carried out separately, simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are carried out simultaneously, a process of stretching in the longitudinal direction multiple times and then stretching in the transverse direction, a process of stretching in the longitudinal direction and then stretching in the transverse direction multiple times, or a process of sequential biaxial stretching and then further stretching in the longitudinal direction and / or transverse direction once or multiple times.

[0071] From the viewpoint of imparting a good balance between particle separation performance and fluid treatment efficiency to the polyolefin microporous membrane, the stretching ratio in the second stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 5 to 90, more preferably 10 to 60. The stretching temperature in the second stretching is preferably 80°C to 135°C.

[0072] Step (4) may be followed by heat setting, preferably at a temperature of 120°C to 145°C to control the porous structure of the polyolefin microporous membrane.

[0073] After the heat setting treatment, the polyolefin microporous membrane may be subjected to an extraction treatment of the solvent remaining therein and an annealing treatment. The extraction treatment of the remaining solvent is carried out, for example, by immersing the heat-set sheet in a methylene chloride bath to dissolve the remaining solvent in the methylene chloride. After the polyolefin microporous membrane immersed in the methylene chloride bath is removed from the methylene chloride bath, the methylene chloride is preferably removed by drying. The annealing treatment is preferably carried out after the extraction treatment of the remaining solvent. The annealing treatment is carried out, for example, by transporting the polyolefin microporous membrane on rollers with a surface temperature of 80°C to 120°C or through a thermostatic bath with a temperature of 80°C to 120°C.

[0074] [Uses of the microporous polyolefin membrane] The microporous polyolefin membrane of the present disclosure is suitable for a filter medium that allows gas or liquid to pass through and separates fine particles. The present disclosure provides a filter medium comprising the microporous polyolefin membrane of the present disclosure.

[0075] The filter medium of the present disclosure may be a filter medium consisting only of the polyolefin microporous membrane of the present disclosure, or may be a filter medium consisting of the polyolefin microporous membrane of the present disclosure and other members. The other members include a sheet-like reinforcing member arranged in contact with part or all of the main surface or side surface of the polyolefin microporous membrane; a guide member for installing the polyolefin microporous membrane in a device; etc.

[0076] Particles to be separated by the polyolefin microporous membrane and filter medium of the present disclosure include biological particles, resin particles, metal particles, mineral particles, ceramic particles, etc. The size of the particles to be separated is, for example, 1 nm to 100 μm.

[0077] Biological particles include particles contained in or released by living organisms, particles parasitic on living organisms, microorganisms, lipid-membrane vesicles, and fragments thereof. Biological particles include viruses, virus parts (e.g., de-enveloped particles of enveloped viruses), bacteriophages, bacteria, spores, fungi, molds, yeast, pollen, cysts, protozoa, unicellular algae, plant cells, animal cells, cultured cells, hybridomas, tumor cells, red blood cells, white blood cells (e.g., lymphocytes, monocytes, granulocytes), platelets, organelles (e.g., cell nuclei, mitochondria, vesicles), exosomes, apoptotic bodies, lipid bilayer particles, lipid monolayer particles, liposomes, enzymes, enzyme aggregates, proteins, protein aggregates, and fragments thereof. Biological particles also include artificial objects.

[0078] There is no limit to the size of biological particles to be separated by the polyolefin microporous membrane and filter medium of the present disclosure. The diameter or major axis length of the biological particles is, for example, 1 nm or more, 5 nm or more, 10 nm or more, or 20 nm or more, and, for example, 100 μm or less, 50 μm or less, 10 μm or less, or 5 μm or less.

[0079] The polyolefin microporous membrane and filter medium of the present disclosure are suitable for use as filter mediums for separating bacteria through gas passage. The bacteria to be separated are preferably of nano- or micro-order size. In this case, the diameter or major axis length of the bacteria is preferably 100 nm to 10 μm.

[0080] The polyolefin microporous membrane of the present disclosure is suitable as a filter medium for an air filter. Details of the air filter and the filter medium for an air filter will be described later.

[0081] Another application of the polyolefin microporous membrane of the present disclosure is a pouch for capturing functional particles. Examples of functional particles to be captured include biological particles, resin particles, metal particles, mineral particles, ceramic particles, pharmaceuticals, foods, enzymes, catalysts, microorganisms, gas absorbents, dehumidifiers, deodorizers, and heat-generating agents. The pouch is produced, for example, by folding or stacking polyolefin microporous membranes cut to a predetermined shape and size, and then bonding part or all of the outer edges of the stacked polyolefin microporous membranes.

[0082] <Air Filter> The air filter of the present disclosure is a device equipped with a filter medium including the polyolefin microporous membrane of the present disclosure and used to remove fine particles from gas containing the particles. Examples of particles to be removed include viruses, bacteria, spores, fungi, mold, pollen, dust, and soot. The size of the particles to be removed is, for example, 1 nm to 100 μm.

[0083] Examples of air filters of the present disclosure include dust masks, medical masks, coarse dust air filters, medium efficiency air filters, high efficiency air filters, and ultra-high efficiency air filters.

[0084] The filter medium included in the air filter of the present disclosure may be in the form of, for example, a single polyolefin microporous membrane of the present disclosure, a stack of multiple polyolefin microporous membranes of the present disclosure, a stack of the polyolefin microporous membrane of the present disclosure and another filter medium, or a pleated polyolefin microporous membrane of the present disclosure.

[0085] An example of an embodiment of the air filter of the present disclosure is a cartridge that can be attached to a device, a pipe, a vent, etc. The air filter of this embodiment includes, for example, a filter medium and a housing, with the filter medium housed inside the housing.

[0086] The polyolefin microporous membrane of the present disclosure will be described in more detail below with reference to examples. The materials, amounts used, ratios, processing procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present disclosure. Therefore, the scope of the polyolefin microporous membrane of the present disclosure should not be construed as being limited by the specific examples shown below.

[0087] In the following description, syntheses, treatments, manufacturing, etc. were carried out at room temperature (25° C.±3° C.) unless otherwise specified.

[0088] <Measurement Methods and Evaluation Methods> The measurement methods and evaluation methods used in the examples and comparative examples are as follows.

[0089] [Thickness of Polyolefin Microporous Membrane] Using a contact-type film thickness meter (ABS Digimatic Indicator, model number: ID-S112X, Mitutoyo Corporation) and a cylindrical contact probe with a bottom diameter of 6.5 mm, measurements were taken at 10 uniformly spaced points in the width direction, and then at another location in the same manner, and the film thickness was calculated by arithmetically averaging the 20 points in total.

[0090] [Porosity of polyolefin microporous membrane] The porosity ε (%) of the polyolefin microporous membrane was calculated using the following formula: where W is the mass of each of constituent materials 1, 2, 3, ..., n of the polyolefin microporous membrane. １ , W ２、 W ３ , ..., W ｎ (g / cm ２ ) and the true density of each constituent material is d １ , d ２ , d ３ , ..., d ｎ (g / cm ３ ) and the thickness of the polyolefin microporous membrane is t (cm).

[0091]

[0092] [Gurley value of polyolefin microporous membrane] Measurement was performed using a Gurley densometer (Toyo Seiki Seisaku-sho, Ltd., model number: G-B3C) in accordance with JIS P8117:2009. A polyolefin microporous membrane with a diameter of 28.6 mm was used as a sample, and the time (seconds) required for 200 mL of air to pass through was measured. Half of this time was defined as the time (seconds / 100 mL) required for 100 mL of air to pass through.

[0093] [Bubble Point of Polyolefin Microporous Membrane] A bubble point test was performed according to JIS K3832:1990. A measurement device was configured according to JIS K3832:1990 "Figure 1: Example of the configuration of a filter disc bubble point test device." Ethanol was used as the test liquid. A polyolefin microporous membrane was cut into a circle, immersed in ethanol, and then placed in the test device. The liquid temperature during the test was set to 24±2°C, and the test was performed while increasing the applied pressure at a rate of 2 kPa / sec, to measure the bubble point (BP) of the polyolefin microporous membrane.

[0094] [LRV] In order to evaluate the particle separation performance of a microporous polyolefin membrane, the following test was carried out to determine the LRV (Logarithmic Reduction Value), which is an index of bacterial separation performance.

[0095] The following equipment was prepared: - Oil-free air compressor, model number: ACP-10A, Takagi Co., Ltd. (hereinafter referred to as "air compressor"); - Baby tank for lab testing, model number: BT-700S, Advantech (hereinafter referred to as "pressurized tank"); - Stainless steel line holder, model number: KS-47, Advantech (hereinafter referred to as "holder"); - Membrane filter with a pore size of 0.22 μm, model number: A020B025A, Advantech (hereinafter referred to as "membrane filter").

[0096] The polyolefin microporous membrane was cut into a circle with a diameter of 47 mm, immersed in ethanol, and then placed inside a holder.

[0097] The LRV was determined by performing the following steps (1) to (5). (1) Preparation of Test Bacterial Solution: The test bacteria were inoculated onto TSA medium and cultured at 30°C for 24 hours. The grown colonies were suspended in 10 mL of TSB medium and cultured at 30°C for 24 hours. 2 mL of this culture was added dropwise to 1,000 mL of salted lactose broth medium and cultured at 30°C for 24 hours. This culture was diluted 10-fold with physiological saline and mixed well to prepare a test bacterial solution. (2) Measurement of Bacterial Count in Test Bacterial Solution: The test bacterial solution was serially diluted 10-fold with physiological saline. 0.1 mL of the test bacterial solution or the diluted solution was smeared onto SA medium and cultured at 30°C for 48 hours, and the number of grown colonies was counted. The number of bacteria per 500 mL of test bacterial solution was determined from the number of colonies counted. (3) Bacteria Isolation Procedure An air compressor was connected to a pressure tank containing approximately 550 mL of test bacterial solution, and the valve was closed. Compressed air was sent from the air compressor to pressurize the pressure tank to 0.21 MPa. The valve was opened, and the entire test bacterial solution was passed through the holder containing the sample and collected in a water collection container. After the entire test bacterial solution had passed through the holder, the pressurization by the air compressor was stopped, and the pressure inside the pressure tank was returned to atmospheric pressure. Hereinafter, the liquid collected in the water collection container will be referred to as the "treated liquid." (4) Measurement of the Number of Bacteria in the Treated Liquid 50 mL and 450 mL of the treated liquid were filtered through membrane filters. Since the test bacteria were too large to pass through the membrane filter, most remained on the membrane filter. After filtering the treated liquid, the membrane filter was attached to SA medium, and after 3 days of incubation at 30°C, the number of developed colonies was counted. The number of colonies counted was used to determine the number of bacteria per 500 mL of treated liquid. (5) Calculation of LRV LRV was calculated using the following formula: LRV = log 10 (number of bacteria per 500 mL of test bacterial solution / number of bacteria per 500 mL of treatment solution) An LRV of 8 or more was determined to be excellent in bacteria separation performance.

[0098] [Air flow rate] The following measurements were carried out to evaluate the fluid treatment efficiency of a polyolefin microporous membrane. A measurement device was constructed according to "Appendix Figure 1: Example of the configuration of a pressure loss measurement device" described in JIS B9927:1999 "Clean room air filter performance test method" and "Appendix: Clean room air filter media performance test method." A polyolefin microporous membrane was cut into a circle and placed in a holder (effective area 20 cm) inside the measurement device. ２ The flow rate of clean air flowing through the measuring device was gradually increased, and the air flow rate (L / min) was determined when the pressure loss reached 500 Pa.

[0099] [Assessment] The polyolefin microporous membranes were classified according to LRV and air flow rate as follows, and the filter performance of the polyolefin microporous membranes was judged. A: LRV is 8 or more and air flow rate is 0.80 L / min or more. B: LRV is 8 or more and air flow rate is 0.40 L / min or more. C: LRV is less than 8, or air flow rate is less than 0.40 L / min.

[0100] <Production of polyolefin microporous membrane> [Example 1] UHMWPE with a weight average molecular weight of 4,600,000 and a microporous polyolefin membrane with a weight average molecular weight of 560,000 and a density of 950 kg / m ３ and HDPE in a mass ratio of 40:60 to prepare a polyethylene composition. The polyethylene composition and decalin were mixed so that the resin concentration was 25 mass % to prepare a polyethylene solution.

[0101] The polyethylene solution was charged into a kneading extruder and extruded into a sheet form from a T-die at a die temperature of 166°C, and the extrudate was cooled in a water bath at a water temperature of 15°C to obtain a first gel-like sheet.

[0102] The first gel-like sheet was pre-dried for 5.8 minutes in an atmosphere at 30°C, then stretched in the MD at 1.1 times its original length, and then dried for 4.0 minutes in an atmosphere at 60°C to obtain a second gel-like sheet (base tape). The residual amount of solvent in the second gel-like sheet was less than 1% by mass.

[0103] Next, as the second stretching, the second gel-like sheet (base tape) was stretched in the MD at a stretching ratio of 3.5 at a temperature of 90° C., and then stretched in the TD at a stretching ratio of 12.0 at a temperature of 125° C. Immediately thereafter, it was heat-treated at 128° C. for heat setting.

[0104] The heat-set sheet was continuously immersed in three separate methylene chloride baths for 48 seconds each to extract decalin from the sheet. After removing the sheet from the methylene chloride bath, the sheet was brought into contact with a heated roll with a surface temperature of 38.9°C to dry and remove the methylene chloride. The sheet was then transported in a heated atmosphere at 110°C and annealed. A polyethylene microporous membrane was thus obtained. The physical properties of the polyethylene microporous membrane are shown in Table 3.

[0105] Examples 2 to 6, Comparative Examples 1 to 4 A polyethylene microporous membrane of each example was produced in the same manner as in Example 1, except that the composition of the polyethylene solution was changed as shown in Table 1 and the production process conditions were changed as shown in Tables 1 and 2. The physical properties of each polyethylene microporous membrane are shown in Table 3.

[0106] When the polyethylene microporous membranes of Examples 1 to 6 were observed with a scanning electron microscope, they had a three-dimensional network structure in which fibril-like resin was continuously connected in a network shape and had numerous micropores.

[0107]

[0108]

[0109]

[0110] The disclosure of Japanese Patent Application No. 2024-031529, filed on March 1, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A microporous polyolefin membrane comprising a polyolefin, having a bubble point of 0.04 MPa to 0.30 MPa, and a value obtained by multiplying the membrane thickness (μm) by the bubble point (MPa) of 2.0 to 4.

5.

2. The microporous polyolefin membrane according to claim 1, which has not been subjected to a hydrophilization treatment.

3. The microporous polyolefin membrane according to claim 1 or claim 2, wherein the polyolefin comprises polyethylene.

4. The microporous polyolefin membrane according to any one of claims 1 to 3, wherein the polyolefin comprises ultra-high molecular weight polyethylene, and the proportion of the ultra-high molecular weight polyethylene in the polyolefin is 1% by mass to 50% by mass.

5. The microporous polyolefin membrane according to any one of claims 1 to 4, which has a Gurley value of 1 second / 100 mL to 20 seconds / 100 mL.

6. The microporous polyolefin membrane according to any one of claims 1 to 5, having a porosity of 60% to 90%.

7. The microporous polyolefin membrane according to any one of claims 1 to 6, for use as a filter medium for an air filter.

8. An air filter comprising a filter medium comprising the polyolefin microporous membrane according to any one of claims 1 to 6.

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