Polyolefin microporous membrane and air filter

Abstract

To provide a polyolefin microporous membrane that exhibits superior particle separation capability and fluid treatment efficiency.SOLUTION: 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.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Polyolefin microporous membranes have been known as porous membranes used in filter media. For example, Patent Documents 1 to 5 disclose filter media and filters containing polyolefin microporous membranes.

[0003] Patent Documents 1 to 4 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] Patent Document 5 describes a polyethylene-containing membrane that has 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 40m 3 / m 2 / h / MPa~80m 3 / m 2 / h / MPa. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-025511 [Patent Document 2] Japanese Patent Publication No. 2023-025510 [Patent Document 3] Japanese Patent Publication No. 2023-025509 [Patent Document 4] Japanese Patent Publication No. 2023-025508 [Patent Document 5] Japanese Patent Application Publication No. 2018-176097 Summary of the Invention [Problem to be solved by the invention]

[0006] Polyolefins do not contain halogen elements, and therefore filters whose filter material is a microporous polyolefin membrane are advantageous in that there are fewer restrictions on their production and use, and fewer limitations on their disposal after use. Furthermore, in recent years, reports have been made of the ecotoxicity and human toxicity of organic fluorine compounds, and restrictions on the production and use of organic fluorine compounds have been tightened worldwide. This has led to increased market demand for polyolefin microporous membranes, and further improvements in the performance of polyolefin microporous membranes are required.

[0007] It is against this background that the present disclosure has been made. An object of the present disclosure is to provide a microporous polyolefin membrane that has excellent particle separation performance and fluid treatment efficiency. [Means for solving the problem]

[0008] Specific means for solving the above problems include the following aspects. <1> Contains polyolefin, The bubble point is 0.04MPa to 0.30MPa. the value obtained by multiplying the film thickness (μm) and the bubble point (MPa) is 2.0 to 4.5; Polyolefin microporous membrane. <2> No hydrophilic treatment has been applied. <1> The polyolefin microporous membrane according to claim 1. <3> The polyolefin comprises polyethylene. <1> or <2> The polyolefin microporous membrane according to claim 1. <4> The polyolefin contains 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. <1> ~ <3> The polyolefin microporous membrane according to any one of the above. <5> Gurley value is 1 second / 100 mL to 20 seconds / 100 mL. <1> ~ <4> The polyolefin microporous membrane according to any one of the above. <6> Porosity is 60% to 90%. <1> ~ <5> The polyolefin microporous membrane according to any one of the above. <7> For use as a filter medium in air filters, <1> ~ <6> The polyolefin microporous membrane according to any one of the above. <8> <1> ~ <6> An air filter comprising a filter medium comprising the polyolefin microporous membrane according to any one of the above items. [Effects of the Invention]

[0009] According to the present disclosure, a microporous polyolefin membrane is provided that has excellent particle separation performance and fluid treatment efficiency. DETAILED DESCRIPTION OF THE INVENTION

[0010]

[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.

[0011] 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 this 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 this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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 MD. In this disclosure, TD is also referred to as the "width direction."

[0016] 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.

[0017] <Polyolefin microporous membrane> In the present disclosure, a microporous polyolefin membrane refers to a microporous membrane containing a polyolefin, which has a structure in which numerous micropores are interconnected inside and allows gas or liquid to pass from one side to the other.

[0018] 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.

[0019] 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.

[0020] 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, and more preferably 0.26 MPa or less.

[0021] The bubble point of a polyolefin microporous membrane is determined by a bubble point test in accordance with JIS K3832:1990, "Test method for bubble point of 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.

[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 2.0 or more. Polyolefin microporous membranes with a multiplication value of less than 2.0 tend to have poor particle separation performance. To achieve 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, and more preferably 2.2 or more.

[0023] 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. To achieve 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, and more preferably 4.2 or less.

[0024] The microporous polyolefin membrane of the present disclosure preferably has a membrane thickness of 7 μm to 110 μm. The 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 mechanical strength and particle separation performance. The 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, from the viewpoints of requiring a relatively small filtration pressure and being easy to process to increase the filtration area.

[0025] The thickness of the polyolefin microporous membrane is determined by measuring 20 points with a contact type measuring device and calculating the arithmetic mean of the measurements. Specifically, the following measurement (1) or (2) is carried out. (1) Measurement is performed using a contact-type film thickness gauge. An example of a contact-type film thickness gauge is the 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 machine. An example of a contact length measuring machine is LITEMATIC (Mitutoyo Corporation), which uses a cylindrical contact measuring probe with a diameter of 5 mm and applies a load of 7 g to the measurement.

[0026] 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 viewpoint 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. 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, from the viewpoint of requiring a relatively small filtration pressure.

[0027] The Gurley value of the polyolefin microporous membrane is a value measured in accordance with JIS P8117:2009 "Paper and paperboard - Test method for air permeability and air resistance (intermediate range) - Gurley method."

[0028] The microporous polyolefin membrane of the present disclosure preferably has a porosity of 60% to 90%. 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 requiring a relatively small filtration pressure. The porosity of the polyolefin microporous membrane is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, from the viewpoints of mechanical strength and particle separation performance.

[0029] The porosity of the polyolefin microporous membrane is calculated by the following formula.

[0030]

number

[0031] where ε is the porosity (%) of the polyolefin microporous membrane, and the masses of the constituent materials 1, 2, 3, ..., n of the polyolefin microporous membrane are W1, W 2、 W3, …, W n (g / cm 2 ) and the true densities of the constituent materials are d1, d2, d3, ..., d n (g / cm 3 ) and the thickness of the microporous polyolefin membrane is t (cm).

[0032] 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 hydrophilization treatment. Examples of treatment 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).

[0033] The polyolefin microporous membrane of the present disclosure preferably has a mesh structure. Mesh structure refers to a structure in which the resin is continuously connected in a mesh shape and has numerous 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 network structure of the microporous polyolefin membrane can be confirmed by observing the microporous polyolefin membrane with a scanning electron microscope (SEM).

[0034] 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.

[0035] 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.

[0036] The polyolefin microporous membrane of the present disclosure may contain other resins besides polyolefins. Examples of other resins include acrylic resins, styrene resins, butadiene rubber, etc. The mass proportion of other resins in 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.

[0037] The polyolefin microporous membrane of the present disclosure preferably does not substantially contain a 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 monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and monomers other than halogen-containing monomers; and mixtures thereof.

[0038] 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, and is 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 does not contain a fluorine-containing resin.

[0039] [Polyolefin] Examples of polyolefins constituting the polyolefin microporous membrane of the present disclosure include homopolymers of ethylene, propylene, butylene, methylpentene, etc. (i.e., polyethylene, polypropylene, polybutylene, polymethylpentene, etc.), copolymers, and mixtures thereof.

[0040] 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. Using two or more polyolefins facilitates the formation of a three-dimensional network structure in the polyolefin microporous membrane through fibrillation during stretching.

[0041] The weight average molecular weight of the entire polyolefin 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 the entire polyolefin constituting the polyolefin microporous membrane of the present disclosure is preferably 5 million or less, more preferably 4 million or less, and even more preferably 3 million or less, from the viewpoint of membrane-formability of the polyolefin microporous membrane.

[0042] 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 it by gel permeation chromatography. Molecular weight calibration is performed using monodisperse polystyrene.

[0043] The polyolefin constituting the microporous polyolefin membrane of the present disclosure preferably includes 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.

[0044] In the polyethylene microporous membrane, polyethylene preferably accounts for 90% by mass or more of the total mass of the polyethylene microporous membrane, more preferably 95% by mass or more, and even more preferably 99% by mass or more. The polyethylene microporous membrane may contain a surfactant as long as it does not affect the effects of the present disclosure.

[0045] 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.

[0046] 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.

[0047] The polyolefin microporous membrane and the polyethylene microporous membrane preferably contain ultra-high molecular weight polyethylene (UHMWPE) from the viewpoints of increasing the mechanical strength of the microporous membrane and densifying the porous structure of the microporous membrane. In this disclosure, ultra-high molecular weight polyethylene (UHMWPE) refers to polyethylene with 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.

[0048] When the polyolefin microporous membrane and the polyethylene microporous membrane contain UHMWPE, the proportion of UHMWPE in the polyolefin is preferably 1% by mass 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.

[0049] The microporous polyolefin membrane and the microporous polyethylene membrane preferably include UHMWPE and high density polyethylene (HDPE). In this disclosure, high density polyethylene (HDPE) has a density of 920 kg / m 3The density of HDPE is 960 kg / m3 from the viewpoint of film-forming properties of microporous membranes. 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.

[0050] The mass ratio of UHMWPE to HDPE (UHMWPE:HDPE) contained 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.

[0051] The weight average molecular weight of the entire polyethylene contained in the polyolefin microporous film and the polyethylene microporous film 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 microporous film. The weight average molecular weight of the entire polyethylene contained in the polyolefin microporous membrane and the polyethylene microporous membrane is preferably 3,000,000 or less, more preferably 2,800,000 or less, and even more preferably 2,500,000 or less, so that the pore size of the microporous membrane is not too small and fluid treatment efficiency is excellent.

[0052] [Method for producing polyolefin microporous membrane] The microporous polyolefin membrane of the present disclosure can be produced, for example, by a production method including the following steps (1) to (4).

[0053] 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 molding and drying the solvent to obtain a second gel-like molding. Step (4): A step of secondarily stretching the second gel-like molding.

[0054] By controlling the conditions in steps (1) to (4), it is possible to control the thickness, bubble point, and porosity of the microporous polyolefin membrane.

[0055] -Process (1)- Step (1) is a step of preparing a polyolefin solution containing a polyolefin and a solvent.

[0056] 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.

[0057] The solvent used in step (1) is not limited as long as it can swell or dissolve the 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.

[0058] 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.

[0059] 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.

[0060] The solvent used in step (1) is preferably a volatile solvent, more preferably decalin or xylene, and even more preferably decalin.

[0061] 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 in the polyolefin solution is 10% by mass or more, the mechanical strength of the microporous polyolefin membrane is ensured. If the polyolefin concentration in the polyolefin solution is 40% by mass or less, pores are likely to be formed in the microporous polyolefin membrane.

[0062] -Process (2)- In step (2), the polyolefin solution is melt-kneaded, the melt-kneaded mixture is extruded through a die, and the extrudate is cooled and solidified to obtain a first gel-like molded product. The first gel-like molded product is preferably formed into a sheet.

[0063] 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.

[0064] 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 those polyolefins is taken as MP°C).

[0065] 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.

[0066] -Process (3)- In step (3), the first gel-like molded product is subjected to a primary stretching and the solvent is dried to obtain a second gel-like molded product. In the primary stretching, the first gel-like molded product is stretched in at least one direction.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] -Process (4)- Step (4) is a step of secondarily stretching the second gel-like molded article, in which the second gel-like molded article is stretched in at least one direction.

[0071] The secondary stretching is preferably biaxial stretching. The biaxial stretching may be any of the following: 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 followed by further stretching in the longitudinal and / or transverse directions once or multiple times.

[0072] 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 secondary 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 secondary stretching is preferably 80°C to 135°C.

[0073] Step (4) may be followed by a heat setting treatment, which is preferably carried out at a heat setting temperature of 120°C to 145°C in order to control the porous structure of the polyolefin microporous membrane.

[0074] After the heat setting, 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.

[0075] [Uses of microporous polyolefin membranes] The microporous polyolefin membrane of the present disclosure is suitable for use in 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.

[0076] The filter medium of the present disclosure may be a filter medium consisting solely of the polyolefin microporous membrane of the present disclosure, or a filter medium consisting of the polyolefin microporous membrane of the present disclosure and other members, such as 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.

[0077] 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.

[0078] Biological particles include particles contained in living organisms, particles released by living organisms, particles parasitic on living organisms, microscopic organisms, lipid-membrane vesicles, and fragments thereof. Biological particles include viruses, virus parts (e.g., particles obtained by de-enveloping 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] Another application of the polyolefin microporous membrane of the present disclosure is as a pouch for capturing functional particles, such as biological particles, resin particles, metal particles, mineral particles, ceramic particles, pharmaceuticals, foods, enzymes, catalysts, microorganisms, gas absorbents, dehumidifiers, deodorizers, and heat generating agents. The bag-shaped body is produced, for example, by folding or overlapping a polyolefin microporous membrane cut to a predetermined shape and size, and then bonding part or all of the outer edges of the overlapped polyolefin microporous membranes.

[0083] <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.

[0084] 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.

[0085] 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.

[0086] 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, and the filter medium is housed inside the housing. [Example]

[0087] 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.

[0088] In the following description, syntheses, treatments, manufacturing, etc. were carried out at room temperature (25°C ± 3°C) unless otherwise specified.

[0089] <Measurement and evaluation methods> The measurement and evaluation methods used in the examples and comparative examples are as follows.

[0090] [Polyolefin microporous membrane thickness] Twenty points were measured using a contact 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, and the film thickness was calculated by arithmetic averaging.

[0091] [Porosity of polyolefin microporous membrane] The porosity ε (%) of the polyolefin microporous membrane was calculated using the following formula: where W1, W2, W3, W4, W5, W6, W7, W8, W9, W10, W11, W12, W13, W14, W15, W16, W17, W18, W19 ...9, W20, W21, W22, W23, W24, W25, W26, W27, W28, W29, W30, W31, W32, W33, W34, W35, W36, W37, W38, W39, W40, W41, W42, W43, W44, W45, W46 2、 W3, …, W n (g / cm 2 ) and the true densities of the constituent materials are d1, d2, d3, ..., d n (g / cm 3 ) and the thickness of the microporous polyolefin membrane is t (cm).

[0092]

number

[0093] [Gurley value of polyolefin microporous membrane] Measurements were performed using a Gurley densometer (Toyo Seiki Seisakusho, Model G-B3C) in accordance with JIS P8117:2009. Using a 28.6 mm diameter polyolefin microporous membrane as a sample, the time (seconds) required for 200 mL of air to pass through was measured. Half of this time was taken as the time (seconds / 100 mL) required for 100 mL of air to pass through.

[0094] [Bubble point of microporous polyolefin membrane] A bubble point test was conducted in accordance with JIS K3832:1990. The measurement device was configured in accordance with JIS K3832:1990 "Figure 1 Example of configuration of filter disc bubble point test device." Ethanol was used as the test liquid. A circular piece of polyolefin microporous membrane was cut out, immersed in ethanol, and placed in a test apparatus. The test was performed at a liquid temperature of 24±2°C and while applying a pressure increasing rate of 2 kPa / sec, to measure the bubble point (BP) of the polyolefin microporous membrane.

[0095] [LRV] In order to evaluate the particle separation performance of the polyolefin microporous membrane, the following test was carried out to determine the LRV (Logarithmic Reduction Value), which is an index of bacterial separation performance.

[0096] The following tools were prepared: Oil-free air compressor, model number: ACP-10A, Takagi Co., Ltd. (hereinafter referred to as "air compressor"). Lab test baby tank, model number: BT-700S, Advantech. hereinafter referred to as the "pressure tank." Stainless steel line holder, model number: KS-47, Advantech. Hereinafter referred to as "the holder." 0.22μm pore size membrane filter, model number: A020B025A, Advantech. Hereinafter referred to as "membrane filter".

[0097] A polyolefin microporous membrane was cut into a circle with a diameter of 47 mm, immersed in ethanol, and then placed inside a holder.

[0098] The LRV was calculated 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 the test bacteria solution. (2) Measurement of the number of bacteria in the test solution The test bacterial solution was serially diluted 10-fold with physiological saline. 0.1 mL of the test bacterial solution or diluted solution was smeared on SA medium and cultured at 30°C for 48 hours, and the number of colonies that developed was counted. The number of bacteria per 500 mL of test bacterial solution was calculated from the number of colonies counted. (3) Bacterial 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, and the pressure inside the pressure tank was increased to 0.21 MPa. The valve was opened, and the entire test bacterial solution was passed through a holder containing a 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 treatment liquid 50 mL and 450 mL of the treatment solution were filtered through a membrane filter. Because the test bacteria were too large to pass through the membrane filter, most of them remained on the membrane filter. After filtering the treatment solution, the membrane filter was attached to SA medium and cultured at 30°C for 3 days, after which the number of colonies that grew was counted. The number of bacteria per 500 mL of treatment solution was calculated from the number of colonies counted. (5) Calculation of LRV The LRV was calculated using the following formula: LRV = log10 (number of bacteria per 500 mL of test solution / number of bacteria per 500 mL of treatment solution) LRV8 or higher was judged to be excellent for bacterial isolation.

[0099] [Air flow rate] In order to evaluate the fluid treatment efficiency of the polyolefin microporous membrane, the following measurements were carried out. The measurement device was configured according to "Appendix Figure 1: Example of configuration of pressure loss measurement device" described in JIS B9927:1999 "Clean room air filters - 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 into the measuring device was gradually increased, and the air flow rate (L / min) was determined when the pressure loss reached 500 Pa.

[0100] [judgement] 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 determined. A: The LRV is 8 or more and the 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.

[0101] <Production of polyolefin microporous membrane> [Example 1] UHMWPE with a weight average molecular weight of 4.6 million and UHMWPE with a weight average molecular weight of 560,000 and a density of 950 kg / m 3 A polyethylene composition was prepared by mixing the above HDPE with decalin at a mass ratio of 40:60. The polyethylene composition and decalin were mixed so that the resin concentration was 25 mass % to prepare a polyethylene solution.

[0102] 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.

[0103] The first gel-like sheet was pre-dried for 5.8 minutes in an atmosphere at 30°C, then stretched 1.1 times in the MD, 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.

[0104] 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 in the TD at a stretching ratio of 12.0 at a temperature of 125°C, followed immediately by heat treatment at 128°C for heat setting.

[0105] The heat-set sheet was immersed in three separate methylene chloride baths for 48 seconds each to extract the 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 to 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.

[0106] [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.

[0107] When the microporous polyethylene 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.

[0108] [Table 1]

[0109] [Table 2]

[0110] [Table 3]

[0111] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. Contains polyolefin, The bubble point is 0.04 MPa to 0.30 MPa, The value obtained by multiplying the film thickness (μm) and the bubble point (MPa) is 2.0 to 4.

5. Polyolefin microporous membrane.

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

3. The microporous polyolefin membrane of claim 1 , wherein the polyolefin comprises polyethylene.

4. The polyolefin microporous membrane according to claim 1, 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 polyolefin microporous membrane according to claim 1, having a Gurley value of 1 second / 100 mL to 20 seconds / 100 mL.

6. The microporous polyolefin membrane according to claim 1, having a porosity of 60% to 90%.

7. The polyolefin microporous membrane according to any one of claims 1 to 6, which is used as a filter material 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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