Substrate for liquid filter and liquid filter

Abstract

The base material for a liquid filter has a polyolefin microporous membrane having a pore diameter of 1-35 nm and a polyolefin crystallinity of 45% or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a substrate for a liquid filter and a liquid filter. BACKGROUND

[0002] As for a liquid composition used in a photolithography process of a semiconductor, before use, for the purpose of removing minute foreign matters from the liquid composition, filtration is performed using a liquid filter. Inside the liquid filter, a filter material in which a dense porous membrane is subjected to a pleat process, a filter material in which a hollow yarn is bundled, and the like are housed.

[0003] Conventionally, as a porous membrane used in a filter material of a liquid filter, a polyolefin microporous membrane is known. In Patent Documents 1 to 7, a substrate for a liquid filter formed of a polyolefin microporous membrane having a small pore diameter is disclosed.

[0004] Since polyolefin does not contain halogen elements, a liquid filter in which a filter material is a polyolefin microporous membrane is advantageous in terms of less limitation on disposal after use and less environmental load.

[0005] PRIOR ART DOCUMENTS PATENT DOCUMENTS Patent Document 1: International Publication No. 2014 / 181760 Patent Document 2: International Publication No. 2014 / 181761 Patent Document 3: International Publication No. 2014 / 181762 Patent Document 4: Japanese Patent Application Publication No. 2014-217800 Patent Document 5: Japanese Patent Application Publication No. 2014-218563 Patent Document 6: Japanese Patent Application Publication No. 2018-167198 Patent Document 7: International Publication No. 2020 / 022321 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION As for a liquid filter, generally, a filter material is washed after manufacture and before use. By washing the filter material, fine powder generated in the manufacture of the liquid filter and dust in the air or the like are removed from the filter material, and these foreign matters are prevented from being mixed into a treated liquid of the liquid filter.

[0007] In addition, when the filter material is washed, a trace amount of metal contained in the filter material itself is eluted and removed. In a polyolefin microporous membrane, metal (calcium, zinc, or the like. A polymerization catalyst of polyolefin or an additive (for example, a metal soap) after polymerization, which is brought in from polyolefin as a raw material of manufacture, is sometimes contained, and when the filter material is washed, these metals are eluted from the polyolefin microporous membrane and removed.

[0008] Since there is a concern that the metal contained in the filter material is eluted into a liquid composition used in a photolithography process to thereby reduce the production yield of a semiconductor, it is desirable to remove it from the filter material before use.

[0009] However, along with the miniaturization and high densification of the wiring pattern of a semiconductor, the necessity to remove even extremely small foreign matter (for example, a fine particle having a particle diameter of several nm) from a liquid composition used in a photolithography process is increasing. Therefore, there is a demand for a liquid filter having a more dense filter material.

[0010] However, there is a tendency that the more the filter material is densified, the lower the washing efficiency of the filter material becomes. For a polyolefin microporous membrane, the smaller the pore diameter, the lower the efficiency of elution and removal of the metal contained in the polyolefin microporous membrane when the filter material is washed.

[0011] Further, the densification of the polyolefin microporous membrane also makes the elution and removal of the metal difficult due to the following mechanism.

[0012] Generally, the densification of the polyolefin microporous membrane is achieved by using ultrahigh molecular weight polyolefin as a raw material. Therefore, as a countermeasure to further densify the polyolefin microporous membrane, it is conceivable to increase the blending ratio of the ultrahigh molecular weight polyolefin, in other words, to decrease the blending ratio of the high density polyolefin. At this time, however, the proportion of the crystalline of the polyolefin in the polyolefin microporous membrane decreases, and the proportion of the amorphous increases. The metal contained in the raw material polyolefin is easily left in the amorphous of the polyolefin, and the metal inside the amorphous is difficult to elute when the filter material is washed.

[0013] For the above reasons, it is difficult to simultaneously achieve the densification of the polyolefin microporous membrane and the removal of the remaining metal.

[0014] The present disclosure is completed based on the above situation.

[0015] The present disclosure is completed based on the above situation.

[0016] Means for solving the problem Specific means for solving the aforementioned problem include the following modes.

[0017] <1> A substrate for a liquid filter having a polyolefin microporous membrane having a pore diameter of 1 nm to 35 nm and a crystallinity of polyolefin of 45% or more.

[0018] <2> The substrate for a liquid filter according to <1>, wherein the polyolefin microporous membrane has a film thickness of 3 μm to 20 μm.

[0019] <3> The liquid filter substrate according to <1> or <2>, wherein the polyolefin microporous membrane has a porosity of 35 to 70%.

[0020] <4> The liquid filter substrate according to any one of <1> to <3>, wherein the polyolefin microporous membrane has a water flow rate of 0.003 L / min / ft 2 / psi to 0.180 L / min / ft 2 / psi.

[0021] <5> The liquid filter substrate according to any one of <1> to <4>, wherein the polyolefin microporous membrane has a weight average molecular weight of the polyolefin as a whole of 800,000 or more.

[0022] <6> The liquid filter substrate according to any one of <1> to <5>, wherein the polyolefin microporous membrane is a polyethylene microporous membrane.

[0023] <7> A liquid filter comprising the liquid filter substrate according to any one of <1> to <6>.

[0024] Effects of Invention According to the present disclosure, it is possible to provide a liquid filter substrate in which a residual metal is easily removed by washing even if the pore size is relatively small. DETAILED DESCRIPTION

[0025] Hereinafter, embodiments of the present disclosure will be described. These descriptions and examples are examples of the embodiments and do not limit the scope of the embodiments.

[0026] In the present disclosure, a numerical range shown using "~" indicates a range in which the values written before and after "~" are respectively included as the minimum value and the maximum value.

[0027] In the numerical ranges described in stages in the present disclosure, the upper limit value or the lower limit value described in one numerical range can be replaced with the upper limit value or the lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value of the numerical range can be replaced with the value shown in the examples.

[0028] In the present disclosure, the term "step" includes not only a single step, but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0029] In the present disclosure, when the amount of each component in the composition is mentioned, the total amount of the plurality of substances belonging to each component present in the composition is meant, unless otherwise specified.

[0030] In the present disclosure, a plurality of particles belonging to each component can be contained. In the case where a plurality of particles belonging to each component is present in the composition, the particle diameter of each component is the value for the mixture of the plurality of particles present in the composition, unless otherwise specified.

[0031] In the present disclosure, the MD (Machine Direction) means the longitudinal direction in the polyolefin microporous membrane produced in a long strip shape, and the TD (Transverse Direction) means the direction orthogonal to the MD in the planar direction of the polyolefin microporous membrane. In the present disclosure, the TD is also referred to as the "width direction".

[0032] In the present disclosure, in the case where the laminated relationship of each layer constituting the liquid filter or the substrate for liquid filter is presented using "upper" and "lower", the layer closer to the polyolefin microporous membrane is referred to as "lower", and the layer farther from the polyolefin microporous membrane is referred to as "upper".

[0033] In the present disclosure, the volume of the microporous membrane or the porous layer other than the pores is referred to as the "solid component volume".

[0034] <Substrate for liquid filter> The substrate for liquid filter of the present disclosure has a polyolefin microporous membrane.

[0035] The substrate for liquid filter of the present disclosure can be a substrate formed of only a polyolefin microporous membrane, or a substrate formed of a polyolefin microporous membrane laminated with other porous membranes or porous layers. The substrate for liquid filter of the present disclosure can have one polyolefin microporous membrane, or two or more polyolefin microporous membranes (i.e., two or more polyolefin microporous membranes can be overlapped).

[0036] One example of the embodiment of the substrate for liquid filter of the present disclosure is a substrate formed of only a polyolefin microporous membrane.

[0037] One example of the embodiment of the substrate for liquid filter of the present disclosure is a substrate of a single-layer structure formed of only one polyolefin microporous membrane.

[0038] The polyolefin microporous membrane possessed by the substrate for liquid filter of the present disclosure has a pore diameter of 1 nm to 35 nm, and the crystallinity of the polyolefin is 45% or more.

[0039] In the case of the substrate for a liquid filter of the present disclosure, since the polyolefin microporous membrane has a pore diameter of 1 nm to 35 nm, fine particles (for example, particles having a particle diameter of 5 nm) contained in a liquid to be treated can be filtered out.

[0040] In the case of the substrate for a liquid filter of the present disclosure, since the polyolefin microporous membrane has a pore diameter of 1 nm to 35 nm, fine particles (for example, particles having a particle diameter of 5 nm) contained in a liquid to be treated can be filtered out.

[0041] In the case of the substrate for a liquid filter of the present disclosure, since the polyolefin microporous membrane has a pore diameter of 1 nm to 35 nm, fine particles (for example, particles having a particle diameter of 5 nm) contained in a liquid to be treated can be filtered out.

[0042] Hereinafter, the polyolefin microporous membrane possessed by the substrate for a liquid filter of the present disclosure will be described in detail.

[0043] [Polyolefin microporous membrane] The polyolefin microporous membrane refers to a membrane in which fibril-like polyolefin forms a three-dimensional network structure, has a large number of fine pores in the inside, and forms a structure in which the fine pores are connected, and a gas or a liquid can pass through from one side to the other side.

[0044] In the polyolefin microporous membrane, the polyolefin preferably accounts for 90% by mass or more of the polyolefin microporous membrane, more preferably 95% by mass or more of the polyolefin microporous membrane, and further preferably 99% by mass or more of the polyolefin microporous membrane.

[0045] In the polyolefin microporous membrane, an organic filler, an inorganic filler, a surfactant, or the like can be contained within a range that does not affect the effects of the present disclosure.

[0046] The polyolefin microporous membrane can be hydrophobic or hydrophilic. Since the polyolefin is a hydrophobic resin, the polyolefin microporous membrane itself is hydrophobic. The polyolefin microporous membrane can be a hydrophobic polyolefin microporous membrane that is not subjected to a hydrophilization treatment, or a polyolefin microporous membrane that is imparted with hydrophilicity by a hydrophilization treatment.

[0047] As a method of hydrophilizing the polyolefin microporous membrane, for example, physical hydrophilization treatment (plasma treatment, corona discharge treatment, ultraviolet irradiation, electron beam irradiation, or the like), coating treatment of a surfactant or a hydrophilic material (cellulose, polyvinyl alcohol, or the like), graft polymerization of a hydrophilic monomer can be given.

[0048] [Pore diameter] The polyolefin microporous membrane has a pore diameter of 1 nm to 35 nm.

[0049] Since the polyolefin microporous membrane has a pore diameter of 1 nm or more, sufficient liquid permeability can be obtained. From this viewpoint, the polyolefin microporous membrane preferably has a pore diameter of 5 nm or more, more preferably 10 nm or more, further preferably 11 nm or more, and particularly preferably 13 nm or more.

[0050] Since the polyolefin microporous membrane has a pore diameter of 35 nm or less, fine particles (e.g., particles having a particle diameter of 5 nm) contained in a liquid to be treated can be filtered out. From this viewpoint, the polyolefin microporous membrane preferably has a pore diameter of 33 nm or less, more preferably 32 nm or less, further preferably 25 nm or less, and particularly preferably 22 nm or less.

[0051] The pore diameter of the polyolefin microporous membrane is a flow pore diameter measured by the half-dry method described in ASTM E1294-89. The flow pore diameter is measured using a palm porometer (PMI Corporation, Capillary Flow Porometer, Model: CFP-1500A) and a fluorine-based nonreactive liquid (trade name: Fluorinert, surface tension: 16.0 dyn / cm) as an immersion liquid. The measurement temperature is 25°C, and the measurement pressure is varied in the range of 0 psi to 500 psi.

[0052] The flow pore diameters are measured at a total of five points, i.e., the center, two points 50 mm away from the center toward both end portions, and two points 100 mm away from the center toward both end portions along the TD of the polyolefin microporous membrane, and the average value thereof is taken as the pore diameter of the polyolefin microporous membrane.

[0053] [Crystallinity of polyolefin] From the viewpoint of easily removing residual metals by washing, the polyolefin constituting the polyolefin microporous membrane has a crystallinity of 45% or more, preferably 46% or more, more preferably 47% or more, further preferably 48% or more, and particularly preferably 49% or more.

[0054] From the viewpoint of densifying the polyolefin microporous membrane, the polyolefin constituting the polyolefin microporous membrane preferably has a crystallinity of 65% or less, more preferably 60% or less, further preferably 58% or less, and particularly preferably 57% or less.

[0055] In the case where the polyolefin microporous membrane is a polyethylene microporous membrane, the polyethylene also preferably has a crystallinity in the above range.

[0056] The crystallinity of the polyolefin is determined by wide angle X-ray diffraction (WAXD).

[0057] A polyolefin microporous membrane was cut out from the center portion of the TD in a size of 10 mm in MD x 3 mm in TD, and stacked in a thickness of 1 mm with the orientation in MD and TD aligned, which was used as a sample.

[0058] The sample was fixed to a sample holder of a 2D-WAXD device (Model: NANO-Viewer, Rigaku Corporation). At this time, the sample was fixed with the MD in the north-south direction in such a manner that the X-rays could be perpendicularly incident to the face of the polyolefin microporous membrane. Transmission measurement was performed under the following conditions.

[0059] • X-ray generator: ultrax 18 • Tube voltage / tube current: 45 kV / 60 mA • Target: Cu Kα (λ = 0.1542 nm) • Camera length: 95 mm • Measurement time: 10 minutes • Measurement temperature: room temperature • Detector: imaging plate The two-dimensional data obtained using the imaging plate were converted into a 2θ profile, and peak fitting was performed using a Gaussian function / Lorentzian function = 50 / 50 in the range of 2θ = 8° to 33°. The integral intensity of each peak was calculated, and the sum of the peak intensities of the crystalline component, that is, the crystalline integral intensity Ic, and the sum of the peak intensities of the amorphous component, that is, the amorphous integral intensity Ia, were calculated. The crystallinity Xc was calculated by the following equation.

[0060] Xc (%) = Ic ÷ (Ic + Ia) x 100 [Membrane thickness] From the viewpoint of obtaining mechanical strength and durability, the membrane thickness of the polyolefin microporous membrane is preferably 3 μm or more, more preferably 4 μm or more, further preferably 5 μm or more, and particularly preferably 6 μm or more.

[0061] From the viewpoint of obtaining sufficient liquid permeability, the viewpoint of a relatively small filtration pressure, the viewpoint of being able to perform pleat processing and the like to increase the filtration area, the viewpoint of the processing being easy to increase the filtration area, and the like, the membrane thickness of the polyolefin microporous membrane is preferably 20 μm or less, more preferably 18 μm or less, further preferably 16 μm or less, more further preferably 15 μm or less, and particularly preferably 14 μm or less.

[0062] The membrane thickness of the polyolefin microporous membrane was calculated by measuring 10 points using a contact type membrane thickness gauge and averaging the values. The measurement sites were 10 points set at equal intervals along the TD from the vicinity of one end portion to the vicinity of the other end portion.

[0063] [Porosity] The porosity of the polyolefin microporous membrane is preferably 35% or more, more preferably 38% or more, further preferably 40% or more, and particularly preferably 42% or more, from the viewpoint of obtaining sufficient liquid permeability, the viewpoint of a small filtration pressure, and the like.

[0064] The porosity of the polyolefin microporous membrane is preferably 70% or less, more preferably 66% or less, further preferably 60% or less, more further preferably 57% or less, and particularly preferably 55% or less, from the viewpoint of obtaining mechanical strength and durability.

[0065] The porosity ε (%) of the polyolefin microporous membrane is calculated using the following formula.

[0066] ε (%) = {1 - Ws / (ds · t)} x 100 Ws: The weight per unit area of the polyolefin microporous membrane (g / m 2 ): The polyolefin microporous membrane is cut out in a size of MD 10 cm x TD 10 cm from the center portion and the vicinity of both end portions of TD in total of three places, the mass thereof is measured, and the mass is divided by the area. Further, the average value of the center portion and the vicinity of both end portions of TD is calculated, and this is used as Ws.

[0067] ds: The true density of the polyolefin microporous membrane (g / cm 3 ): This is set to 0.96.

[0068] t: The film thickness of the polyolefin microporous membrane (μm): This is calculated as described above.

[0069] [Water flow rate] The water flow rate of the polyolefin microporous membrane is preferably 0.003 L / min / ft 2 / psi or more, more preferably 0.004 L / min / ft 2 / psi or more, further preferably 0.005 L / min / ft 2 / psi or more, and particularly preferably 0.007 L / min / ft 2 / psi or more, from the viewpoint of obtaining sufficient liquid permeability for a long period of time.

[0070] The water flow rate of the polyolefin microporous membrane is preferably 0.180 L / min / ft 2 / psi or less, more preferably 0.170 L / min / ft 2 / psi or less, further preferably 0.150 L / min / ft 2 / psi or less, and particularly preferably 0.140 L / min / ft 2 / psi or less.

[0071] The water flow rate of the polyolefin microporous membrane is determined by the following method.

[0072] The polyolefin microporous membrane was cut out in a square of 40 mm x 40 mm from the center and both end portions of TD, and immersed in ethanol, and dried at room temperature. The polyolefin microporous membrane was set in a liquid permeation cell (liquid permeation area: 10.75 cm 2 ) having a diameter of 37 mm, and 100 ml of pure water was passed at a pressure difference of 90 kPa in a temperature atmosphere of 24°C at room temperature, and the time Tl (min) required for the passage of the entire amount of pure water was measured. From the liquid amount V (100 ml) of pure water, the time Tl (min), and the liquid permeation area S (10.75 cm 2 ), the water flow rate Vs (water flow rate per unit time (min) x unit area (ft 2 ) at a pressure difference of 1 psi) was calculated using the following formula, and the unit was L / min / ft 2 / psi. Further, the average value of the center and both end portions of TD was calculated.

[0073] Vs = (V / 1000) / Tl / (S / 929.03) / (90 / 6.895) [Thermal shrinkage] From the viewpoint of heat resistance, the thermal shrinkage of the polyolefin microporous membrane at a temperature of 105°C is preferably 45% or less, more preferably 40% or less, and further preferably 30% or less in MD.

[0074] From the viewpoint of heat resistance, the thermal shrinkage of the polyolefin microporous membrane at a temperature of 105°C is preferably 20% or less, more preferably 15% or less, and further preferably 10% or less in TD.

[0075] The thermal shrinkage of the polyolefin microporous membrane is determined by the following method.

[0076] The polyolefin microporous membrane was cut out in a size of MD 10 cm x TD 10 cm from the center and both end portions of TD, and used as a test sample. The test sample was left in an oven in which the inside temperature was maintained at 105°C for 30 minutes. The test sample was taken out of the oven, and the MD length and TD length of the test sample were measured. The thermal shrinkage (%) of each of MD and TD was calculated as { (length before heat treatment - length after heat treatment) ÷ length before heat treatment x 100}. Further, the average value of the center and both end portions of TD was calculated.

[0077] [Polyolefin] As the polyolefin constituting the polyolefin microporous membrane, for example, a homopolymer (i.e., polyethylene, polypropylene, polybutene, polymethylpentene, etc.) or a copolymer of ethylene, propylene, butene, methylpentene, etc., or a mixture thereof can be mentioned.

[0078] The polyolefin microporous membrane is preferably a microporous membrane formed using two or more kinds of polyolefins that differ from each other in at least one of the kind of monomer, the degree of polymerization, the degree of branching, the crystallinity, the stretchability, and the molecular orientation. By using two or more kinds of polyolefins, a network structure is easily formed in the polyolefin microporous membrane due to fibrillation at the time of stretching.

[0079] From the viewpoint of densifying the polyolefin microporous membrane, the weight average molecular weight of the polyolefin constituting the polyolefin microporous membrane as a whole is preferably 800,000 or more, more preferably 850,000 or more, further preferably 900,000 or more, more further preferably 950,000 or more, and particularly preferably 970,000 or more.

[0080] From the viewpoint of easily uniformly melting at the time of melt kneading, the weight average molecular weight of the polyolefin constituting the polyolefin microporous membrane as a whole is preferably 3,500,000 or less, more preferably 3,200,000 or less, further preferably 3,100,000 or less, and particularly preferably 3,000,000 or less.

[0081] The weight average molecular weight of the polyolefin constituting the polyolefin microporous membrane as a whole was measured by heating and dissolving the polyolefin microporous membrane in o-dichlorobenzene and using gel permeation chromatography (system: Waters Alliance GPC 2000, column: GMH6-HT and GMH6-HTL) under conditions of a column temperature of 135°C and a flow rate of 1.0 mL / minute, and was calculated. In the calibration of the molecular weight, a molecular weight monodisperse polystyrene (TOSOH) was used.

[0082] The polyolefin constituting the polyolefin microporous membrane is preferably polyethylene. That is, the polyolefin microporous membrane is preferably a polyethylene microporous membrane. In the present disclosure, the polyethylene microporous membrane refers to a microporous membrane in which the resin having the largest mass proportion in the entire resin is polyethylene.

[0083] In the polyethylene microporous membrane, the polyethylene preferably accounts for 90% or more by mass of the polyethylene microporous membrane, more preferably 95% or more by mass of the polyethylene microporous membrane, and further preferably 99% or more by mass of the polyethylene microporous membrane.

[0084] In the polyethylene microporous membrane, an organic filler, an inorganic filler, a surfactant, etc. can be contained within a range that does not affect the effects of the present disclosure.

[0085] The polyethylene microporous membrane is preferably a microporous membrane formed using two or more kinds of polyethylene that differ from each other in at least one of the degree of polymerization, the degree of branching, the crystallinity, the stretchability, and the molecular orientation. By using two or more kinds of polyethylene, a network structure is easily formed in the polyethylene microporous membrane due to fibrillation at the time of stretching.

[0086] As the polyethylene constituting the polyethylene microporous membrane, for example, ultra-high molecular weight polyethylene, high-density polyethylene, and a mixture of ultra-high molecular weight polyethylene and high-density polyethylene can be mentioned.

[0087] From the viewpoint of densifying the polyethylene microporous membrane, the polyethylene microporous membrane preferably contains ultra-high molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million. The polyethylene microporous membrane preferably contains 50 mass% to 95 mass% of ultra-high molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million, more preferably contains 60 mass% to 90 mass% of ultra-high molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million, and further preferably contains 70 mass% to 85 mass% of ultra-high molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million.

[0088] The polyethylene microporous membrane preferably contains ultra-high molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million, and high-density polyethylene having a weight average molecular weight of 2 million to 8 million and a density of 0.92 g / cm 3 to 0.98 g / cm 3 From the viewpoint of improving the crystallinity of the polyethylene and the viewpoint of producing a microporous membrane having a small pore diameter, the mass ratio of the above two kinds of polyethylene contained in the polyethylene microporous membrane (ultra-high molecular weight polyethylene: high-density polyethylene) is preferably 50:50 to 95:5, more preferably 60:40 to 90:10, and further preferably 70:30 to 85:15.

[0089] From the viewpoint of densifying the polyethylene microporous membrane, the weight average molecular weight of the entire polyethylene constituting the polyethylene microporous membrane is preferably 0.8 million or more, more preferably 0.85 million or more, further preferably 0.9 million or more, still further preferably 0.95 million or more, and particularly preferably 0.97 million or more.

[0090] From the viewpoint of easily uniformly melting at the time of melt kneading, the weight average molecular weight of the entire polyethylene constituting the polyethylene microporous membrane is preferably 3.5 million or less, more preferably 3.2 million or less, further preferably 3.1 million or less, and particularly preferably 3 million or less.

[0091] The calcium content contained in the polyolefin constituting the polyolefin microporous membrane depends on the raw material polyolefin, but is preferably, for example, in the following range.

[0092] In one example of the embodiment, the calcium content contained in the polyolefin is preferably 50 ppm or less, more preferably 38 ppm or less, further preferably 35 ppm or less, and particularly preferably 34 ppm or less.

[0093] In one example of the embodiment, the calcium content contained in the polyolefin is preferably 1000 ppb or less, more preferably 500 ppb or less, further preferably 400 ppb or less, and particularly preferably 300 ppb or less.

[0094] The calcium content contained in the polyolefin is, for example, 0 ppb or more, 10 ppb or more, 20 ppb or more, 50 ppb or more, or 100 ppb or more.

[0095] As a method of adjusting the calcium content contained in the polyolefin, there can be mentioned: adjusting the amount of metal soap (calcium stearate or the like) added to the polyolefin after polymerization; subjecting the polyolefin to acid washing; and removing the polymerization catalyst remaining in the polyolefin by a deashing process.

[0096] As for the calcium content of the polyolefin, ultrahigh-purity nitric acid was added to the polyolefin, and subjected to microwave digestion, and the resultant was used as a sample, and subjected to quantification using ICP-MS (inductively coupled plasma mass spectrometry, device name: Agilent 7500cs, Agilent Technologies Co., Ltd.).

[0097] [Method for producing polyolefin microporous membrane] The polyolefin microporous membrane can be produced by, for example, a production method including the following steps (1) to (6).

[0098] Step (1): a step of preparing a polyolefin solution containing a polyolefin and a solvent.

[0099] Step (2): a step of melt-kneading the polyolefin solution, extruding the melt-kneaded product from a die, and cooling the extrudate to thereby solidify, to obtain a gel-like molded product.

[0100] Step (3): a step of extruding the solvent from the gel-like molded product.

[0101] Step (4): a step of stretching the gel-like molded product in at least one direction to thereby obtain a polyolefin microporous membrane.

[0102] Step (5): a step of washing the polyolefin microporous membrane and removing the solvent.

[0103] Step (6): a step of subjecting the polyolefin microporous membrane to an annealing treatment.

[0104] By controlling the conditions of each of the steps (1), (2), and (4), the crystallinity of the polyolefin constituting the polyolefin microporous membrane can be adjusted. In addition, by controlling the conditions of each of the steps (1) to (6), the film thickness, pore diameter, porosity, water flow rate, and the like of the polyolefin microporous membrane can be adjusted.

[0105] - Step (1) - The step (1) is a step of preparing a polyolefin solution containing a polyolefin and a solvent.

[0106] The polyolefin used in the step (1) can be one kind or two or more kinds. The polyolefin preferably contains polyethylene, and more preferably contains ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million and high density polyethylene having a weight average molecular weight of 2 million to 8 million and a density of 0.92 g / cm 3 to 0.98 g / cm 3 .

[0107] The polyolefin is generally traded in the form of a granular pellet or a powder. In the case where the polyolefin powder is used in the step (1), the particle diameter of the powder is preferably 1 μm to 1000 μm, more preferably 5 μm to 500 μm, and further preferably 10 μm to 300 μm from the viewpoint of melt-kneading the polyolefin without omission in the step (2) and increasing the crystallinity of the polyolefin.

[0108] The above particle diameter is a median particle diameter (d50) of a particle size distribution on a volume basis. In terms of the particle size distribution on a volume basis, a laser diffraction type particle size distribution measuring device (device name: Mastersizer 2000, Malvern Corporation) is used, and the median particle diameter is calculated by dry measurement.

[0109] The solvent used in the step (1) is not limited as long as it is a solvent capable of swelling or dissolving the polyolefin. The solvent can be broadly classified into a non-volatile solvent having a boiling point of 210°C or higher at atmospheric pressure and a volatile solvent having a boiling point of less than 210°C at atmospheric pressure.

[0110] As the non-volatile solvent, for example, liquid paraffin, paraffin oil, mineral oil, castor oil, and the like can be given. The non-volatile solvent can be used alone as one kind or in combination with two or more kinds. As the non-volatile solvent, liquid paraffin is preferred.

[0111] As the volatile solvent, for example, tetrahydronaphthalene, ethylene glycol, decalin (alias decaline), toluene, xylene, diethyltriamine, ethylenediamine, dimethyl sulfoxide, hexane, and the like can be given. The volatile solvent can be used alone as one kind or in combination with two or more kinds. As the volatile solvent, decalin or xylene is preferred.

[0112] The solvent used in the process (1) is preferably a mixed solvent of a non-volatile solvent and a volatile solvent, more preferably a mixed solvent of liquid paraffin and decalin or xylene, and further preferably a mixed solvent of liquid paraffin and decalin.

[0113] The mixing ratio (mass ratio, non-volatile solvent:volatile solvent) of the non-volatile solvent and the volatile solvent is preferably 99:1 to 60:40.

[0114] In the case where two or more kinds of polyolefins are used, the order of mixing the materials when mixing the polyolefin and the solvent is not limited. For example, either of the following modes (a) and (b) can be adopted.

[0115] Mode (a): The two or more kinds of polyolefins are mixed one by one into the solvent. The two or more kinds of solvents are mixed in advance to prepare a mixed solvent, and the polyolefins are mixed one by one into the mixed solvent.

[0116] Mode (b): The two or more kinds of polyolefins are mixed to prepare a polyolefin composition, and the polyolefin composition is mixed with the solvent. The two or more kinds of solvents can also be mixed in advance to prepare a mixed solvent, and the mixed solvent can be mixed with the polyolefin composition.

[0117] From the viewpoint of imparting mechanical strength, liquid permeability, and fine particle trapping performance to the polyolefin microporous membrane in a well-balanced manner, 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 further preferably 20% by mass to 30% by mass.

[0118] If the polyolefin concentration of the polyolefin solution is 10% by mass or more, the mechanical strength of the polyolefin microporous membrane can be ensured.

[0119] If the polyolefin concentration of the polyolefin solution is 40% by mass or less, it is easy to form pores in the polyolefin microporous membrane.

[0120] -Process (2)- The process (2) is a process of melt-kneading the polyolefin solution, extruding the melt-kneaded product from a die, and cooling the extrudate to solidify, thereby obtaining a gel-like molded product. The gel-like molded product is preferably formed in a sheet shape.

[0121] The melt-kneading of the polyolefin solution is preferably performed using a kneading extruder. The kneading extruder is a device that continuously conveys a processed material while applying pressure and heat to the processed material. The structure of the kneading extruder is generally roughly divided into a material feeding port, a barrel, and a die in this order from the upstream to the downstream. A screw is provided inside the barrel. A heater that heats the inside of the barrel is provided around the barrel. The screw can be a single-screw type or a twin-screw type, and is preferably a twin-screw type.

[0122] The temperature of the polyolefin solution in the most downstream region inside the cylinder is preferably MP + 30°C to MP + 150°C, more preferably MP + 40°C to MP + 140°C, and further preferably MP + 50°C to MP + 130°C, from the viewpoint of sufficiently melting the polyolefin, when the melting point of the polyolefin is MP°C (in the case where two or more kinds of polyolefins are used, the highest melting point among the melting points of these polyolefins is MP°C).

[0123] The time required for the treated object to pass through the inside of the cylinder is preferably 1 minute to 10 minutes, more preferably 1 minute and 30 seconds to 8 minutes, and further preferably 2 minutes to 7 minutes. The passing time of the treated object can be controlled using the rotation speed of the screw.

[0124] The temperature of the melt-kneaded product in the die is preferably MP + 30°C to MP + 120°C, more preferably MP + 40°C to MP + 110°C, and further preferably MP + 50°C to MP + 100°C, from the viewpoint of improving the film formability of the polyolefin microporous membrane, when the melting point of the polyolefin is MP°C (in the case where two or more kinds of polyolefins are used, the highest melting point among the melting points of these polyolefins is MP°C).

[0125] As a method of cooling the extrudate, for example, the extrudate can be immersed in water or an organic solvent, or the extrudate can be brought into contact with a metal roll that has been cooled. The cooling temperature is preferably 10°C to 40°C. In the case where the extrudate is immersed in water, a water stream is preferably formed on the surface of a water bath, and the solvent released from the extrudate is inhibited from adhering to the extrudate.

[0126] - Step (3) - Step (3) is a step of extruding the solvent from the gel-like molded product.

[0127] Step (3) is preferably achieved by applying pressure to the gel-like molded product. As a method of applying pressure to the gel-like molded product, for example, the gel-like molded product can be conveyed while being pressed on a roll or a belt, or the gel-like molded product can be passed between a pair of rolls. The pressure applied to the gel-like molded product is preferably 0.01 MPa to 0.5 MPa, and more preferably 0.05 MPa to 0.2 MPa. The surface temperature of the roll or the belt is preferably 40°C to 100°C.

[0128] Between Step (2) and Step (3), it is preferable that a heating treatment of the gel-like molded product be performed for the purpose of volatilizing a part of the solvent from the gel-like molded product. The temperature of the heating treatment is preferably 50°C to 100°C. The heating treatment can be one time, or two or more times with a change in temperature. The time of the heating treatment is preferably 5 minutes to 10 minutes per one time.

[0129] By performing the above-described heat treatment in advance, it is possible to shorten the conveyance path of the process (3) or to make the conditions (pressure and / or temperature) of the process (3) mild.

[0130] - Process (4) - The process (4) is a process of stretching the gel-like molded article in at least one direction to obtain a polyolefin microporous membrane.

[0131] The stretching in the process (4) is preferably biaxial stretching. The biaxial stretching can be either of sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed sequentially, and simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously. The biaxial stretching can also be: stretching in the longitudinal direction multiple times followed by stretching in the transverse direction; stretching in the longitudinal direction and stretching in the transverse direction multiple times; further stretching in the longitudinal direction and / or the transverse direction one or more times after performing sequential biaxial stretching; and the like.

[0132] From the viewpoint of imparting liquid permeability and fine particle capturing performance to the polyolefin microporous membrane with good balance, and the viewpoint of increasing the crystallinity of the polyolefin constituting the polyolefin microporous membrane, the draw ratio (the product of the longitudinal draw ratio and the transverse draw ratio) is preferably 40 times to 200 times, more preferably 45 times to 180 times, and further preferably 50 times to 150 times.

[0133] From the viewpoint of increasing the crystallinity of the polyolefin constituting the polyolefin microporous membrane, the stretching temperature is preferably 90°C to 130°C, more preferably 95°C to 125°C, and further preferably 100°C to 120°C.

[0134] It is preferable to perform heat setting treatment after the stretching. From the viewpoint of increasing the crystallinity of the polyolefin constituting the polyolefin microporous membrane, the heat setting temperature is preferably 100°C to 140°C, more preferably 105°C to 135°C, and further preferably 110°C to 133°C.

[0135] - Process (5) - The process (5) is a process of washing the polyolefin microporous membrane and removing the solvent. The process (5) preferably uses a halogenated hydrocarbon such as dichloromethane, a hydrocarbon such as hexane, or the like as a solvent to wash the polyolefin microporous membrane.

[0136] The washing of the polyolefin microporous membrane is preferably performed by immersing the polyolefin microporous membrane in a bath containing the washing solvent. In this case, in order to improve the washing effect, it is preferable that the bath be divided into two or more tanks, and the purity of the washing solvent be increased in the tank on the more downstream side. This can be achieved by injecting the washing solvent into the tank on the most downstream side and allowing the washing solvent to flow upstream. In the case where the bath is divided into two or more tanks, the number of tanks can be two or more than three. From the viewpoint of making the purity gradient of the washing solvent in each tank more gentle, it is preferable that the number of tanks be three or more.

[0137] The time required for the polyolefin microporous membrane to pass through all the baths is preferably 60 seconds to 150 seconds, more preferably 70 seconds to 120 seconds, and further preferably 80 seconds to 100 seconds.

[0138] After the polyolefin microporous membrane is lifted from the bath containing the washing solvent, the washing solvent is preferably removed by drying. The drying temperature can be a temperature slightly higher than the boiling point of the washing solvent.

[0139] - Step (6) - Step (6) is a step of annealing the polyolefin microporous membrane. The annealing is performed, for example, by conveying the polyolefin microporous membrane on a roll having a surface temperature of 100°C to 130°C or in a thermostat tank having a temperature of 100°C to 130°C. During the annealing, it is preferable to perform an operation of fixing the width direction of the polyolefin microporous membrane in order to suppress the shrinkage in the width direction of the polyolefin microporous membrane.

[0140] Further, the polyolefin microporous membrane can also be subjected to processing to impart affinity to the treated liquid of the liquid filter.

[0141] < Liquid filter > The liquid filter of the present disclosure is a device for removing a fine particle from a treated liquid containing or possibly containing the fine particle. The fine particle is contained in the treated liquid in a solid or gel-like form.

[0142] The liquid filter of the present disclosure is provided with the liquid filter substrate of the present disclosure as a filter material.

[0143] In the case of a liquid filter, the filter material is usually washed after its manufacture and before its use, and the liquid filter substrate of the present disclosure is easily removed of the residual metal by washing even in the case where the residual metal is contained. Therefore, if the liquid filter of the present disclosure is washed in advance of the filter material before use, there is little concern that the metal is eluted into the treated liquid when the treated liquid is filtered.

[0144] The liquid filter of the present disclosure has, for example, a pleated liquid filter substrate and a cylindrical housing in which the pleated liquid filter substrate is housed. The liquid filter of the present disclosure is, for example, a filter cartridge that can be attached to and detached from a filtration device.

[0145] The liquid filter of the present disclosure is suitable for the purpose of removing fine particles having a particle size of several nm from a liquid to be treated. The liquid filter of the present disclosure can be used in, for example, a manufacturing process of a semiconductor, a manufacturing process of a display, or the like.

[0146] Embodiments Hereinafter, the liquid filter substrate of the present disclosure will be described more specifically by citing examples. The materials, amounts, proportions, processing steps, and the like shown in the following examples can be appropriately changed without departing from the gist of the present disclosure. Therefore, the scope of the liquid filter substrate of the present disclosure should not be interpreted limitatively based on the specific examples shown below.

[0147] In the following description, unless otherwise specified, the synthesis, processing, manufacturing, and the like are performed at room temperature (25°C ± 3°C).

[0148] [Measurement method, evaluation method] Examples and comparative examples are all manufactured by winding a polyolefin microporous membrane having a width of 270 mm on a winding core having an inner diameter of 3 inches. A test piece cut out from the manufactured polyolefin microporous membrane in an appropriate size is subjected to physical property measurement or performance evaluation. The measurement method of the physical properties and the evaluation method of the performance are as described below.

[0149] [Calcium content of polyolefin] 0.1 g of polyolefin is precisely weighed into a fluororesin container, and ultrahigh-purity nitric acid is added, and microwave digestion is performed. The calcium (Ca) content is quantified in the order of ppb using ICP-MS (inductively coupled plasma mass spectrometry, device name: Aglient 7500cs, Agilent Technologies Co., Ltd.).

[0150] [Particle size of polyolefin powder] The volume-based particle size distribution of the polyolefin powder is found by dry measurement using a laser diffraction type particle size distribution measuring device (device name: Mastersizer 2000, Malvern), and the median particle size (d50) is used as the particle size.

[0151] [Membrane thickness of polyolefin microporous membrane] The film thickness (μm) of the polyolefin microporous membrane was measured at 10 points on the TD at intervals of 26 mm using a contact type film thickness meter (Mitutoyo Corporation) and a cylindrical contact terminal having a bottom surface diameter of 0.5 cm, and the measured values were averaged. The measurement pressure of the contact terminal was set to 0.1 N.

[0152] [Porosity of polyolefin microporous membrane] The porosity (ε) of the polyolefin microporous membrane was calculated from the following formula.

[0153] ε (%) = {1 - Ws / (ds · t)} x 100 Ws: weight per unit area of the polyolefin microporous membrane (g / m 2 ): The polyolefin microporous membrane was cut out in a size of MD 10 cm x TD 10 cm from the center portion and the vicinity of both end portions of the TD, the mass thereof was measured, and the mass was divided by the area. Further, the average value of the center portion and the vicinity of both end portions of the TD was calculated, and this was taken as Ws.

[0154] ds: true density of the polyolefin microporous membrane (g / cm 3 ): 0.96 was assumed.

[0155] t: film thickness of the polyolefin microporous membrane (μm): measured and calculated as described above.

[0156] [Flow pore diameter of polyolefin microporous membrane] The flow pore diameter was measured at a total of 5 points along the TD of the polyolefin microporous membrane at the center, 2 points 50 mm away from the center toward both end portions, and 2 points 100 mm away from the center toward both end portions, and the measured values were averaged. The flow pore diameter was measured as described below.

[0157] The flow pore diameter was measured by the half dry method prescribed in ASTM E1294-89 using a pore size distribution measuring device (PMI Corporation, Capillary Flow Porometer, Model: CFP-1500A) and a fluorine-based nonreactive liquid (trade name: Fluorinert, surface tension 16.0 dyn / cm) as an immersion liquid. The measurement temperature was 25°C, the measurement pressure was varied in the range of 0 psi to 500 psi, and the measurement was performed under the following conditions.

[0158] • Bubble point parameter: BUBLFLOW = 50, F / PT = 100, MINBPPRES = 0, ZEROTIME = 1, PULSEDELAY = 2 • Wet state parameters: V2INCR = 15, PREGINC = 0.9, MINEQTIME = 30, PRESSLEW = 30, FLOWSLEW = 30, EQITER = 50, AVEITER = 10, MAXPDIF = 1, MAXFDIF = 30 • Dry state parameters: V2INCR = 40, PREGINC = 2.4, MINEQTIME = 30, PRESSLEW = 30, FLOWSLEW = 30, EQITER = 40, AVEITER = 10, MAXPDIF = 1, MAXFDIF = 30 [Water flow rate of polyolefin microporous membrane] A polyolefin microporous membrane was cut out in a square of 40 mm x 40 mm from the center and both end portions of the TD in total three places, immersed in ethanol, and dried at room temperature.

[0159] A polyolefin microporous membrane was set in a stainless steel liquid permeation cell (liquid permeation area: 10.75 cm 2 ) having a diameter of 37 mm. After the polyolefin microporous membrane on the liquid permeation cell was moistened with a small amount (0.5 ml) of ethanol, 100 ml of pure water was made to pass at a pressure difference of 90 kPa in a temperature atmosphere of room temperature of 24°C, and the time Tl (min) required for the entire amount of pure water to pass was measured.

[0160] From the liquid amount V (100 ml) of pure water, the time Tl (min), and the liquid permeation area S (10.75 cm 2 ), the water flow rate Vs (water flow rate per unit time (min) per unit area (ft 2 ) at 1 psi pressure difference, unit: L / min / ft 2 / psi) was calculated using the following formula, and the average value of the center and both end portions of the TD was further calculated.

[0161] Vs = (V / 1000) / Tl / (S / 929.03) / (90 / 6.895) [Weight average molecular weight (Mw) of polyolefin] A polyolefin microporous membrane was heated and dissolved in o-dichlorobenzene, and the molecular weight was measured using gel permeation chromatography (system: Waters Corporation Alliance GPC 2000 type, column: GMH6-HT and GMH6-HTL) under conditions of a column temperature of 135°C and a flow rate of 1.0 mL / minute. In the calibration of the molecular weight, a molecular weight monodisperse polystyrene (TOSOH Corporation) was used.

[0162] [Crystallinity of polyolefin] A polyolefin microporous membrane was cut out from the center portion of the TD in a size of 10 mm in MD x 3 mm in TD, and stacked in a thickness of 1 mm with the orientation of MD and TD aligned, which was used as a sample.

[0163] The sample was fixed to a sample holder of a 2D-WAXD device (Model: NANO-Viewer, Rigaku Corporation). At this time, the sample was fixed with the MD in the north-south direction in such a manner that the X-rays could be perpendicularly incident to the face of the polyolefin microporous membrane. Transmission measurement was performed under the following conditions.

[0164] • X-ray generator: ultrax 18 • Tube voltage / tube current: 45 kV / 60 mA • Target material: Cu Kα (λ = 0.1542 nm) • Camera length: 95 mm • Measurement time: 10 minutes • Measurement temperature: room temperature • Detector: imaging plate The two-dimensional data obtained using the imaging plate were converted into a 2θ profile, and peak fitting was performed using a Gaussian function / Lorentzian function = 50 / 50 in the range of 2θ = 8° to 33°. The integral intensity of each peak was calculated, and the sum of the peak intensities of the crystalline component, that is, the crystalline integral intensity Ic, and the sum of the peak intensities of the amorphous component, that is, the amorphous integral intensity Ia, were calculated. The crystallinity Xc was calculated by the following equation.

[0165] Xc (%) = Ic ÷ (Ic + Ia) x 100 [Trapping performance] Colloidal gold having a particle size of 5 nm (Funakoshi Co., Ltd., particle size width: 4.5 to 6.0 nm) was dispersed in water to prepare a dispersion liquid having a colloidal gold concentration of 40 ppb.

[0166] A polyolefin microporous membrane was cut out in a square of 50 mm x 50 mm from the center portion and the vicinity of both end portions of the TD in total three places, immersed in ethanol, and dried at room temperature.

[0167] A polyolefin microporous membrane was set in a stainless steel liquid permeation cell (liquid permeation area: 10.75 cm 2 ) having a diameter of 37 mm. After the polyolefin microporous membrane on the liquid permeation cell was wetted with a small amount (0.5 ml) of ethanol, 200 ml of the colloidal gold dispersion liquid was permeated at a pressure difference of 0.1 MPa.

[0168] The metal concentration of the colloidal gold dispersion liquid after passing through the polyolefin microporous membrane was measured and quantified using the ICP-OES method (high-frequency inductively coupled plasma emission spectrometry, device name: Agilent-ICP-OES-5100, Agilent Technologies, Inc.). The calibration curve for quantification was prepared using a standard dispersion liquid of colloidal gold (5 or more samples were prepared in the concentration range of 0 ppb to 100 ppb).

[0169] The capture rate (%) = {(M1-M2) ÷ M1 x 100} was calculated from the initial metal concentration M1 (i.e., 40 ppb) of the colloidal gold dispersion liquid and the metal concentration M2 of the colloidal gold dispersion liquid after passing through the polyolefin microporous membrane. The average value of the central part and the vicinity of both ends of the TD was further calculated, and the average value was classified as follows.

[0170] A: The average value of the capture rate is 90% or more B: The average value of the capture rate is 80% or more and less than 90%, C: The average value of the capture rate is less than 80% [metal elution] The adherents (fine powder generated in the manufacture of the polyolefin microporous membrane and dust in the air, etc., and substances that are not integrated with the polyolefin microporous membrane) were removed from both sides of the polyolefin microporous membrane. The polyolefin microporous membrane was cut into square pieces (10 cm x 10 cm, 40 pieces, a total area of 4000 cm 2 ), which were used as samples.

[0171] The sample was loaded into a fluorine resin container, and 200 g of hydrochloric acid extract solution (a liquid containing hydrochloric acid at a concentration of 10 mass% in a mixed solution of water: isopropyl alcohol = 40:60 (mass ratio)) was injected to immerse the sample in the hydrochloric acid extract solution. After 24 hours, the sample was removed and dried.

[0172] The dried sample was loaded into another fluorine resin container, and 200 g of propylene glycol monomethyl ether (PGME) was injected to immerse the sample in PGME.

[0173] At 24 hours and at 168 hours, the Ca concentration and the Zn concentration in PGME were quantified to the order of 0.1 ppb using the ICP-OES method (high-frequency inductively coupled plasma emission spectrometry, device name: Agilent-ICP-OES-5100, Agilent Technologies, Inc.).

[0174] The total amount of Ca and Zn dissolved from the polyolefin microporous membrane (μg / m 2 ) was calculated from the quantified Ca concentration and Zn concentration, the mass of PGME, and the area of the sample. The increase rate (%) of the amount of dissolution = {(the total amount of dissolution after 168 hours - the total amount of dissolution after 24 hours) ÷ the total amount of dissolution after 24 hours x 100} was calculated and classified as follows.

[0175] A: The increase rate of the amount of dissolution is less than 5% B: The increase rate of the amount of dissolution is 5% or more and less than 10% C: The increase rate of the amount of dissolution is 10% or more [Thermal shrinkage rate of TD] The polyolefin microporous membrane was cut out in a size of MD 10 cm x TD 10 cm from the center and both end portions of the TD in total three places. The sample was left in an oven in which the inside temperature was kept at 105°C for 30 minutes. The TD length of the sample was measured after the sample was taken out of the oven. The thermal shrinkage rate (%) of TD = {(TD length before heat treatment - TD length after heat treatment) ÷ TD length before heat treatment x 100} was calculated. Further, the average value of the center and both end portions of the TD was calculated and classified as follows.

[0176] A: The thermal shrinkage rate of TD is 10% or less B: The thermal shrinkage rate of TD is more than 10% and 20% or less C: The thermal shrinkage rate of TD is more than 20% [Manufacture of substrate for liquid filter] Hereinafter, the so-called "UHMWPE" means an ultrahigh molecular weight polyethylene having a weight average molecular weight of 3 million to 6 million, and the so-called "HDPE" means a high density polyethylene having a weight average molecular weight of 2 million to 8 million and a density of 0.92 g / cm 3 to 0.98 g / cm 3 .

[0177] [Example 1] - Step (1) • UHMWPE having Mw of 4.6 million and Ca content of 140 ppb: 22 parts by mass (The particle diameter of the powder of UHMWPE is 60 μm) • HDPE having Mw of 0.5 million and Ca content of 230 ppb: 5 parts by mass (The particle diameter of the powder of HDPE is 250 μm) • Liquid paraffin: 72 parts by mass • Decalin: 1 part by mass The above-described materials were prepared. The liquid paraffin was mixed with decaline to prepare a mixed solvent. The entire amount of UHMWPE was added to the mixed solvent and mixed by stirring. Then, the entire amount of HDPE was added and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 27 mass% was prepared. The Ca content in the mixture of UHMWPE and HDPE was 157 ppb.

[0178] - Step (2) - The polyethylene solution was fed to the twin-screw kneading extruder, and the polyethylene solution was subjected to pressure and heat by operating the screw at a screw rotation speed of 200 rpm. The temperature inside the barrel of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region would be 200°C, and the temperature of the polyethylene solution in the die was adjusted so that it would be 190°C.

[0179] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was transported in a water bath having a water temperature of 20°C to cool it, thereby producing a base tape in the form of a gelled sheet.

[0180] - Step (3) - The base tape was taken out of the water bath and transported in a space having a temperature of 60°C for 10 minutes, and then transported in a space having a temperature of 95°C for 10 minutes, thereby removing decaline from the base tape.

[0181] Next, the base tape was transported on a roll having a surface temperature of 90°C while applying a pressure of 0.05 MPa, thereby removing a part of the liquid paraffin from the base tape.

[0182] - Step (4) - The base tape was stretched in the MD at a stretch ratio of 7 (longitudinal stretching) at a temperature of 100°C, and then stretched in the TD at a stretch ratio of 25 (transverse stretching) at a temperature of 115°C, and then immediately heat-set at a temperature of 115°C, thereby obtaining a polyolefin microporous membrane.

[0183] - Step (5) - The polyolefin microporous membrane was continuously immersed in a dichloromethane bath (referred to as the 1st tank, the 2nd tank, and the 3rd tank from the upstream in this order) divided into three tanks for 30 seconds each, thereby extracting the liquid paraffin from the polyolefin microporous membrane. Dichloromethane was injected into the 3rd tank, and the dichloromethane was caused to flow from the 3rd tank to the 1st tank, thereby forming a gradient in the purity of dichloromethane (1st tank < 2nd tank < 3rd tank).

[0184] The polyolefin microporous membrane was taken out of the dichloromethane bath and transported in a space having a temperature of 40°C, thereby removing the dichloromethane from the polyolefin microporous membrane.

[0185] - Step (6) - The polyolefin microporous membrane was annealed by being conveyed in a constant temperature bath at a temperature of 110°C for 1 minute while keeping the TD length constant.

[0186] Next, the polyolefin microporous membrane was conveyed in a space at a temperature of 60°C for 20 seconds, and then conveyed to a space at room temperature to be cooled.

[0187] The obtained polyethylene microporous membrane was a membrane in which fibrillar polyolefin formed a three-dimensional network structure, had a large number of fine pores inside to form a structure connected by the fine pores, and a gas or a liquid could pass through from one side to the other side. The properties of the polyethylene microporous membrane (the liquid filter base material of the present disclosure) are shown in Table 1.

[0188] [Example 2] The polyethylene microporous membrane was produced by operating similarly to Example 1, but changing the process (4) as described below.

[0189] - Process (4) - The base band was stretched in the MD at a temperature of 100°C at a ratio of 6 times (longitudinal stretching), and then stretched in the TD at a temperature of 110°C at a ratio of 16 times (transverse stretching), and then immediately heat-set at a temperature of 120°C to obtain a polyolefin microporous membrane.

[0190] The obtained polyethylene microporous membrane was a membrane in which fibrillar polyolefin formed a three-dimensional network structure, had a large number of fine pores inside to form a structure connected by the fine pores, and a gas or a liquid could pass through from one side to the other side. The properties of the polyethylene microporous membrane (the liquid filter base material of the present disclosure) are shown in Table 1.

[0191] [Example 3] The polyethylene microporous membrane was produced by operating similarly to Example 1, but changing the process (1), process (2), and process (4) as described below.

[0192] - Process (1) - • UHMWPE having Mw of 4.2 million and Ca content of 31 ppm: 15 parts by mass (Powder of UHMWPE having particle diameter of 30 μm) • HDPE having Mw of 400,000 and Ca content of 34 ppm: 15 parts by mass (Powder of HDPE having particle diameter of 20 μm) • Liquid paraffin: 65 parts by mass • Decalin: 5 parts by mass The above-described materials were prepared. Liquid paraffin was mixed with decalin to prepare a mixed solvent. The entire amount of UHMWPE was added to the mixed solvent and mixed by stirring. The entire amount of HDPE was then added and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 30 mass% was prepared. The Ca content in the mixture of UHMWPE and HDPE was 32.5 ppm.

[0193] - Step (2) - The polyethylene solution was introduced into the twin-screw kneading extruder, which was operated at a screw rotation speed of 100 rpm to apply pressure and heat to the polyethylene solution. The temperature inside the barrel of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region became 175°C, and the temperature of the polyethylene solution in the die was adjusted so that it became 170°C.

[0194] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was conveyed in a water bath having a water temperature of 20°C to cool it, thereby producing a base web in the form of a gelled sheet.

[0195] - Step (4) - The base web was stretched in the MD at a temperature of 100°C at a stretch ratio of 5 times (longitudinal stretching), and then stretched in the TD at a temperature of 115°C at a stretch ratio of 12 times (transverse stretching), and then immediately heat-set at a temperature of 124°C, thereby producing a polyolefin microporous membrane.

[0196] The obtained polyethylene microporous membrane was a membrane in which fibril-like polyolefins formed a three-dimensional network structure, and a large number of fine pores were present in the inside to form a structure in which the fine pores were connected, and a gas or a liquid could pass through from one side surface to the other side surface. The properties of the polyethylene microporous membrane (the base material for a liquid filter of the present disclosure) are shown in Table 1.

[0197] [Example 4] The same operations as in Example 1 were performed, but Steps (1), (2), and (4) were changed as described below, thereby producing a polyethylene microporous membrane.

[0198] - Step (1) - • UHMWPE having a Mw of 4.2 million and a Ca content of 31 ppm: 18 parts by mass (The particle diameter of the powder of UHMWPE was 30 μm) • HDPE having a Mw of 400,000 and a Ca content of 34 ppm: 5 parts by mass (The particle diameter of the powder of HDPE was 20 μm) • Liquid paraffin: 75 parts by mass • Decalin: 2 parts by mass The above-described materials were prepared. Liquid paraffin was mixed with decalin to prepare a mixed solvent. The entire amount of UHMWPE was added to the mixed solvent and mixed by stirring. Then, the entire amount of HDPE was added and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 23 mass% was prepared. The Ca content in the mixture of UHMWPE and HDPE was 31.7 ppm.

[0199] - Step (2) - The polyethylene solution was fed into the twin-screw kneading extruder, and the polyethylene solution was subjected to pressure and heat by operating the screw at a screw rotation speed of 220 rpm. The temperature inside the barrel of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region became 200°C, and the temperature of the polyethylene solution in the die was adjusted so that it became 190°C.

[0200] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was transported in a water bath having a water temperature of 20°C to be cooled, thereby producing a base tape as a gel-like sheet.

[0201] - Step (4) - The base tape was stretched in the MD at a stretch ratio of 9 times (longitudinal stretching) at a temperature of 100°C, and then stretched in the TD at a stretch ratio of 18 times (transverse stretching) at a temperature of 105°C, and then immediately heat-set at a temperature of 110°C, thereby obtaining a polyolefin microporous membrane.

[0202] The obtained polyethylene microporous membrane was a membrane in which fibril-like polyolefins formed a three-dimensional network structure, and a large number of fine pores were present in the inside to form a structure in which the fine pores were connected, and a gas or a liquid could pass through from one side surface to the other side surface. The properties of the polyethylene microporous membrane (the base material for a liquid filter of the present disclosure) are shown in Table 1.

[0203] [Example 5] The same operations as in Example 1 were performed, but Steps (1) and (6) were changed as described below, thereby producing a polyethylene microporous membrane.

[0204] - Step (1) - • UHMWPE having an Mw of 4.6 million and a Ca content of 140 ppb: 4 parts by mass (The particle diameter of the powder of UHMWPE was 60 μm) • HDPE having an Mw of 0.5 million and a Ca content of 230 ppb: 16 parts by mass (The particle diameter of the powder of HDPE was 250 μm) • Liquid paraffin: 80 parts by mass The above-described materials were prepared. The entire amount of UHMWPE was added to the liquid paraffin and mixed by stirring. Then, the entire amount of HDPE was added and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 20 mass% was prepared. The Ca content in the mixture of UHMWPE and HDPE was 212 ppb.

[0205] - Step (6) - The polyolefin microporous membrane was annealed by being conveyed in a constant-temperature bath at a temperature of 115°C for 1 minute while keeping the TD length constant.

[0206] Then, the polyolefin microporous membrane was conveyed in a space at a temperature of 60°C for 20 seconds and then conveyed to a space at room temperature to be cooled.

[0207] The resulting polyethylene microporous membrane was a membrane in which fibrillar polyolefin formed a three-dimensional network structure, had a large number of fine pores in the interior, and formed a structure in which the fine pores were connected, and a gas or a liquid could pass through from one side to the other side. The properties of the polyethylene microporous membrane (the substrate for a liquid filter of the present disclosure) are shown in Table 1.

[0208] [Example 6] The same operations as in Example 1 were performed, but Steps (1), (2), and (4) were changed as described below, to produce a polyethylene microporous membrane.

[0209] - Step (1) - • UHMWPE having an Mw of 4.6 million and a Ca content of 140 ppb: 15 parts by mass (Powder of UHMWPE having a particle diameter of 60 μm) • HDPE having an Mw of 0.5 million and a Ca content of 230 ppb: 5 parts by mass (Powder of HDPE having a particle diameter of 250 μm) • Liquid paraffin: 80 parts by mass The above-described materials were prepared. The entire amount of UHMWPE was added to the liquid paraffin and mixed by stirring. Then, the entire amount of HDPE was added and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 20 mass% was prepared. The Ca content in the mixture of UHMWPE and HDPE was 212 ppb.

[0210] - Step (2) - The polyethylene solution was fed into the twin-screw kneading extruder, and the screw was rotated at 400 rpm to apply pressure and heat to the polyethylene solution. The temperature inside the barrel of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region would be 200°C, and the temperature of the polyethylene solution in the die was adjusted so that it would be 210°C.

[0211] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was conveyed in a water bath at a water temperature of 20°C to cool it, thereby producing a base web as a gelled sheet.

[0212] - Step (4) - The base web was stretched in the MD (machine direction) at a stretch ratio of 6 times at a temperature of 110°C (longitudinal stretching), and then stretched in the TD (transverse direction) at a stretch ratio of 13 times at a temperature of 115°C (lateral stretching), and then immediately heat-set at a temperature of 130°C, thereby producing a polyolefin microporous membrane.

[0213] The polyethylene microporous membrane thus obtained was a membrane in which fibrillar polyolefin formed a three-dimensional network structure, and a large number of fine pores were present in the inside to form a structure in which the fine pores were connected, and a gas or a liquid could pass through from one side surface to the other side surface. The properties of the polyethylene microporous membrane (base material for a liquid filter of the present disclosure) are shown in Table 1.

[0214] [Comparative Example 1] The polyethylene microporous membrane was produced by operating in the same manner as in Example 1, but changing Step (2) and Step (4) as described below.

[0215] - Step (2) - The polyethylene solution was fed into the twin-screw kneading extruder, and the screw was rotated at 200 rpm to apply pressure and heat to the polyethylene solution. The temperature inside the barrel of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region would be 160°C, and the temperature of the polyethylene solution in the die was adjusted so that it would be 165°C.

[0216] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was conveyed in a water bath at a water temperature of 20°C to cool it, thereby producing a base web as a gelled sheet.

[0217] - Step (4) - The base web was stretched in the MD (machine direction) at a stretch ratio of 9 times at a temperature of 100°C (longitudinal stretching), and then stretched in the TD (transverse direction) at a stretch ratio of 25 times at a temperature of 105°C (lateral stretching), and then immediately heat-set at a temperature of 105°C, thereby producing a polyolefin microporous membrane.

[0218] The obtained polyethylene microporous membrane is a membrane in which fibrillar polyolefin forms a three-dimensional network structure, has a large number of fine pores in the inside to form a structure connected by the fine pores, and through which a gas or a liquid can pass from one side to the other side. The properties of the polyethylene microporous membrane are shown in Table 1.

[0219] [Comparative Example 2] The same operation as in Example 1 was performed, but the process (4) was changed as described below, to produce a polyethylene microporous membrane.

[0220] - Process (4) - The base band was stretched at a temperature of 100°C in the MD at a rate of 9 times (longitudinal stretching), and then stretched at a temperature of 115°C in the TD at a rate of 25 times (transverse stretching), and then immediately heat-set at a temperature of 100°C, to obtain a polyolefin microporous membrane.

[0221] The obtained polyethylene microporous membrane is a membrane in which fibrillar polyolefin forms a three-dimensional network structure, has a large number of fine pores in the inside to form a structure connected by the fine pores, and through which a gas or a liquid can pass from one side to the other side. The properties of the polyethylene microporous membrane are shown in Table 1.

[0222] [Comparative Example 3] The same operation as in Example 1 was performed, but the process (1) and the process (4) were changed as described below, to produce a polyethylene microporous membrane.

[0223] - Process (1) - • UHMWPE having Mw of 4.6 million and Ca content of 140 ppb: 27 parts by mass (Powder of UHMWPE having particle diameter of 60 μm) • HDPE having Mw of 0.5 million and Ca content of 230 ppb: 2 parts by mass (Powder of HDPE having particle diameter of 250 μm) • Liquid paraffin: 70 parts by mass • Decalin: 1 part by mass The above-described materials were prepared. The liquid paraffin and the decalin were mixed to prepare a mixed solvent. The entire amount of the UHMWPE was added to the mixed solvent, and mixed by stirring. Then, the entire amount of the HDPE was added, and mixed by stirring. In this way, a polyethylene solution having a polyethylene concentration of 29 mass% was prepared. The Ca content in the mixture of the UHMWPE and the HDPE was 146 ppb.

[0224] - Process (4) - The base film was stretched at a temperature of 105°C in the MD at a ratio of 9 times (longitudinal stretching), then stretched at a temperature of 105°C in the TD at a ratio of 25 times (transverse stretching), and then immediately heat-set at a temperature of 108°C, to obtain a polyolefin microporous membrane.

[0225] The obtained polyethylene microporous membrane was a film in which fibrillar polyolefin formed a three-dimensional network structure, had a large number of fine pores in the inside, and formed a structure in which the fine pores were connected, and a gas or a liquid could pass through from one side to the other side. The properties of the polyethylene microporous membrane are shown in Table 1.

[0226] [Comparative Example 4] The same operations as in Example 1 were performed, but the process (1), process (2), process (4), and process (6) were changed as described below, to produce a polyethylene microporous membrane.

[0227] -Process (1)- • UHMWPE having Mw of 4.2 million and Ca content of 31 ppm: 3 parts by mass (Powder of UHMWPE having particle diameter of 30 μm) • HDPE having Mw of 400,000 and Ca content of 34 ppm: 14 parts by mass (Powder of HDPE having particle diameter of 20 μm) • Liquid paraffin: 51 parts by mass • Decaline: 32 parts by mass The above-described materials were prepared. The liquid paraffin and the decaline were mixed to prepare a mixed solvent. The entire amount of the UHMWPE was added to the mixed solvent, and the mixture was stirred. Then, the entire amount of the HDPE was added, and the mixture was stirred. In this way, a polyethylene solution having a polyethylene concentration of 17 mass% was prepared. The Ca content in the mixture of the UHMWPE and the HDPE was 33.5 ppm.

[0228] -Process (2)- The polyethylene solution was fed into the twin-screw kneading extruder, and the polyethylene solution was subjected to pressure and heat by operating the screw at a screw rotation speed of 450 rpm. The temperature inside the cylinder of the twin-screw kneading extruder was adjusted so that the temperature of the polyethylene solution in the most downstream region became 160°C, and the temperature of the polyethylene solution in the die was adjusted so that the temperature of the polyethylene solution in the die became 160°C.

[0229] The polyethylene solution was extruded from the die in the form of a sheet, and the extrudate was transported in a water bath having a water temperature of 20°C to be cooled, to produce a base film as a gelled sheet.

[0230] -Process (4)- The base film was stretched at a temperature of 100°C in the MD at a 4-fold ratio (longitudinal stretching), then stretched at a temperature of 110°C in the TD at a 9-fold ratio (transverse stretching), and then immediately heat-set at a temperature of 135°C, to obtain a polyolefin microporous membrane.

[0231] - Step (6) For the polyolefin microporous membrane, annealing treatment was performed by conveying in a constant-temperature bath at a temperature of 100°C for 1 minute while keeping the TD length constant.

[0232] The polyolefin microporous membrane was then conveyed in a space at a temperature of 60°C for 20 seconds, and then conveyed to a space at room temperature to be cooled.

[0233] The obtained polyethylene microporous membrane was a membrane in which fibril-like polyolefin formed a three-dimensional network structure, had a large number of fine pores in the interior, and formed a structure in which the fine pores were connected, and through which a gas or a liquid could pass from one side to the other side. The properties of the polyethylene microporous membrane are shown in Table 1.

[0234] [Table 1] The comparative example 4 of the polyolefin microporous membrane having a pore diameter of 57 nm was poor in the evaluation item “trapping performance”.

[0235] In contrast, the polyolefin microporous membranes of Examples 1 to 6 (the liquid filter substrates of the present disclosure) having a pore diameter of 35 nm or less were excellent in the evaluation item “trapping performance”. That is, the liquid filter substrates of the present disclosure were excellent in the performance of filtering out fine particles (for example, particles having a particle diameter of 5 nm) included in a liquid to be treated.

[0236] The comparative examples 1 to 3 of the polyolefin having a crystallinity of less than 45% were poor in the evaluation item “metal elution”.

[0237] In contrast, the polyolefin microporous membranes of Examples 1 to 6 (the liquid filter substrates of the present disclosure) having a crystallinity of 45% or more were excellent in the evaluation item “metal elution”. That is, the liquid filter substrates of the present disclosure were easy to remove residual metals by washing, despite the relatively small pore diameter.

[0238] All literature, patent applications, and technical standards cited in this specification are hereby incorporated by reference as if each had been specifically and individually indicated to be incorporated by reference.

[0239] The entire disclosure of Japanese Application No. 2023-052236 filed on March 28, 2023 is hereby incorporated by reference into the present specification.

Claims

1. A substrate for liquid filters, comprising a polyolefin microporous membrane with a pore size of 1 nm to 35 nm and a polyolefin crystallinity of 45% or higher.

2. The substrate for a liquid filter as described in claim 1, wherein, The thickness of the polyolefin microporous membrane is 3μm~20μm.

3. The substrate for a liquid filter as described in claim 1, wherein, The porosity of the polyolefin microporous membrane is 35%~70%.

4. The substrate for a liquid filter as described in claim 1, wherein, The water flow rate of the polyolefin microporous membrane is 0.003 L / min / ft. 2 / psi~0.180L / min / ft 2 / psi.

5. The substrate for a liquid filter as described in claim 1, wherein, The total weight-average molecular weight of the polyolefin constituting the polyolefin microporous membrane is over 800,000.

6. The substrate for a liquid filter as claimed in claim 1, wherein, The polyolefin microporous membrane is a polyethylene microporous membrane.

7. A liquid filter comprising a liquid filter substrate according to any one of claims 1 to 6.

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