Polyolefin microporous membranes and filtration filters

Abstract

To provide polyolefin microporous membranes and filtration filters with excellent foreign matter removal performance by reducing the pore diameter per specific surface area of the microporous membrane while maintaining a high porosity and uniformly refining the fibril structure in contact with the liquid surface.SOLUTION: There is provided a microporous polyolefin membrane having a porosity of 45% or more and 80% or less, wherein a median value of a pore diameter based on a surface area is 1 nm or more and 15 nm or less in a pore diameter distribution of the polyolefin microporous membrane determined by pure water injection porosimeter measurement.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to polyolefin microporous membranes and filtration filters. [Background technology]

[0002] Polyolefin microporous membranes are widely used in various applications such as battery separators, diaphragms for electrolytic capacitors, water treatment membranes, ultrafiltration membranes, microfiltration membranes, reverse osmosis filtration membranes, and breathable waterproof clothing. Among these, in applications requiring solvent resistance, chemical resistance, etc., there is a growing demand to further improve the performance of polyolefin microporous membranes so that they can maintain high-precision separation capabilities while maintaining sufficient resistance.

[0003] For example, in filtration filters for process liquids used in highly integrated semiconductor manufacturing, the demand for finer foreign matter removal performance in process liquids is increasing as the wiring pitch of semiconductors becomes smaller. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 07-228718 [Patent Document 2] Japanese Patent Application Publication No. 11-322988 [Patent Document 3] Japanese Patent Publication No. 2010-053245 [Overview of the project] [Problems that the invention aims to solve]

[0005] The polyolefin microporous membranes disclosed in Patent Documents 1 to 3 describe the formation of a three-dimensional network structure, but since they are not intended for filtering, the pore size is large and the specific surface area is insufficient to obtain the inertial impaction effect and blocking effect, which are the collection mechanisms of filter media. The present invention aims to provide a polyolefin microporous membrane and a filtration filter that maintain high porosity while reducing the pore size per unit surface area of ​​the microporous membrane, uniformly refining the fibril structure in contact with the liquid surface, and exhibiting excellent foreign matter removal performance. [Means for solving the problem]

[0006] As a result of diligent research to solve the aforementioned problems, we have developed the present invention by controlling the fibril structure of polyolefin microporous membranes using advanced film-forming technology.

[0007] In other words, the polyolefin microporous membrane of the present invention is a polyolefin microporous membrane having a porosity of 45% or more and 80% or less, wherein the median pore size based on surface area in the pore size distribution of the polyolefin microporous membrane, as determined by pure water porosimeter measurement, is 1 nm or more and 15 nm or less. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a polyolefin microporous membrane and a filtration filter that have a high porosity while having a fine fibril structure on the surface in contact with the liquid surface, resulting in a pore structure with uniform irregularities and a small pore diameter per unit area, and that have a high specific surface area and long curves, thereby achieving both good water permeability and particle collection performance in the filtration process. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 shows an example of the pore structure on the surface of a polyolefin microporous membrane according to an embodiment of the present invention. [Figure 2] Figure 2 shows an example of the pore structure on the surface of a polyolefin microporous membrane of a comparative example of the present invention. [Modes for carrying out the invention]

[0010] The polyolefin microporous membrane of the present invention is a polyolefin microporous membrane with a porosity of 45% or more and 80% or less. In the pore size distribution of the polyolefin microporous membrane determined by mercury intrusion porosimeter measurement, the median pore size based on the surface area is 1 nm or more and 15 nm or less. The present invention relates to such a polyolefin microporous membrane. Such a polyolefin microporous membrane preferably has at least a first layer made of a polyethylene resin and a second layer made of a polyolefin resin different from the first layer. Hereinafter, the polyolefin microporous membrane of the present invention will be described. In the present invention, the direction parallel to the film-forming direction of the polyolefin microporous membrane is referred to as the film-forming direction, the longitudinal direction or the MD direction, and the direction perpendicular to the film-forming direction in the plane of the polyolefin microporous membrane is referred to as the width direction or the TD direction.

[0011] (Resin of the first layer) It is preferable to use a polyethylene resin for the resin of the first layer constituting the microporous membrane of the present invention.

[0012] The polyethylene resin includes high-density polyethylene (density: 0.942 g / m 3 or more, molecular weight: molecular weight: 1.0×10 4 ten thousand or more and less than 6.0×10 5 ten thousand), ultra-high molecular weight polyethylene (molecular weight: 1.0×10 6 ten thousand or more and 3.0×10 6At least one selected from the group consisting of (less than 10,000) can be used. Among these, it is preferable to contain high-density polyethylene. The high-density polyethylene content of the first layer is preferably 10% by mass or more and 100% by mass or less, more preferably 20% by mass or more and 90% by mass or less, based on 100% by mass of the total polyolefin resin of the first layer. The ultra-high molecular weight polyethylene content of the first layer is preferably 0% by mass or more and 90% by mass or less, more preferably 10% by mass or more and 80% by mass or less, based on 100% by mass of the total polyolefin resin of the first layer. By containing polyethylene within the above range, the fibril structure of the surface layer can be uniformly refined. The weight average molecular weight (Mw) of high-density polyethylene and ultra-high molecular weight polyethylene is determined by a universal calibration curve using a monodisperse polystyrene standard sample by the gel permeation chromatography (GPC) method described later. The polyethylene resin may be used alone or in combination of two or more as a polyethylene mixture. As the polyethylene mixture, in addition to the polyethylene resin as the main component, ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene may be included. In particular, when ultra-high molecular weight polyethylene is included, the strength of the polyolefin microporous membrane of the present embodiment becomes high and high impact resistance can be obtained.

[0013] (Resin of the second layer) The resin of the second layer constituting the microporous membrane of the present invention is preferably composed of polypropylene and polyethylene. As the polypropylene, isotactic polypropylene having a mesopentad fraction of 90% or more can be used. Also, as the polyethylene, at least one selected from the group consisting of high-density polyethylene and medium-density polyethylene can be used. The polypropylene content of the second layer is preferably 40% to 95% by mass, more preferably 45% to 90% by mass, based on 100% by mass of the total polyolefin resin of the second layer. The polyethylene content of the second layer is preferably 5% to 60% by mass, more preferably 10% to 55% by mass, based on 100% by mass of the total polyolefin resin of the second layer. By including high-density polyethylene within the above ranges, good adhesion at the interface with the first layer is achieved.

[0014] (Solvent for film formation) The solvent used for film formation is not particularly limited as long as it is a substance that can be mixed with polyolefin resin or a substance that can dissolve polyolefin resin. Both liquid and solid solvents can be used as film formation solvents. Examples of liquid solvents include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil fractions with corresponding boiling points. To obtain a gel-like sheet with a stable solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. For solid solvents, those with a melting point of 80°C or lower are preferred, and examples of such solid solvents include paraffin wax, ceryl alcohol, stearyl alcohol, and dicyclohexyl phthalate. Liquid and solid solvents may also be used in combination.

[0015] The mixing ratio of polyolefin resin to film-forming solvent is preferably 1 to 60% by mass of polyolefin resin, with the total of polyolefin resin and film-forming solvent being 100% by mass, from the viewpoint of improving the moldability of the extruded product. The ratio of polyolefin resin to the mixture of polyolefin resin and diluent is more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass.

[0016] (characteristic) The polyolefin microporous membrane of this embodiment, by satisfying conditions such as the stretching temperature and stretching ratio described later, has a pore structure with high porosity and small pore diameter per unit surface area, a high specific surface area, and long curves.

[0017] (Specific surface area) The specific surface area is the measured value obtained by measuring the surface area per unit mass of the polyolefin microporous membrane using a pure water intrusion porosimeter. The specific surface area is preferably 200 m 2 / g or more and 1000 m 2 / g or less. More preferably, the specific surface area is 250 m 2 / g or more and 950 m 2 / g or less. When the specific surface area is 200 m 2 / g or more, a sufficient collection area is ensured, and when the polyolefin microporous membrane is used as a filter, good collection performance can be obtained. Also, when the specific surface area is 1000 m 2 / g or less, the strength of the polyolefin microporous membrane is sufficient and film formation becomes easy. The specific surface area can be made to be in the above range by adjusting the ratio of the polyolefin resin and the film-forming solvent, the stretching temperature conditions, the stretching ratio, the heat setting treatment temperature after stretching of the gel sheet, etc. In particular, by setting the heat setting treatment temperature within the range described later, recrystallization can be suppressed and a sufficient specific surface area can be obtained.

[0018] (Median pore diameter based on surface area) The median pore diameter based on surface area is the measured value obtained by measuring using a pure water intrusion porosimeter. It is preferable that the median pore diameter based on surface area is 1 nm or more and 15 nm or less. The median pore diameter based on surface area is more preferably 2 nm or more, even more preferably 3 nm or more, and particularly preferably 4 nm or more. Also, it is more preferably 14 nm or less, even more preferably 13 nm or less, and particularly preferably 12 nm or less.

[0019] By setting the median pore diameter based on surface area to 1 nm or more, good liquid permeability can be obtained. Also, by setting it to 15 nm or less, foreign substances of extremely small sizes required in semiconductor manufacturing are more easily collected.

[0020] (Median pore diameter based on surface area) The median pore size based on surface area is a measurement taken using a pure water porosimeter. The polyolefin microporous membrane of the present invention has a median pore size based on surface area of ​​1 nm or more and 15 nm or less. The median pore size based on surface area is more preferably 2 nm or more, even more preferably 3 nm or more, and particularly preferably 4 nm or more. Furthermore, it is more preferably 14 nm or less, even more preferably 13 nm or less, and particularly preferably 12 nm or less.

[0021] By setting the median pore size based on surface area to 1 nm or more, good liquid permeability can be obtained. By setting it to 15 nm or less, extremely small foreign matter required in semiconductor manufacturing can be easily collected, resulting in good foreign matter collection performance. The pore size based on specific surface area is adjusted to make the fibrils on the surface of the microporous film finer and the pores on the inner layer denser. Specifically, by rapidly cooling two types of polyolefin resins with different crystallinity in the same temperature range, gel-like sheets with different structures are created, and a complex pore structure can be obtained by lowering the stretching temperature. To increase the porosity on the surface layer and make the fibrils finer, the amount of film-forming solvent in the polyolefin resin on the surface layer is increased compared to the inner layer. A resin with a lower degree of crystallinity is placed on the inner layer side and can be densed by stretching it simultaneously with the surface layer. Furthermore, to stabilize the molded pore structure, it is better to heat-set it while it contains the film-forming solvent.

[0022] (curvature rate) The curvature ratio is the ratio of the film thickness of the microporous membrane to the average effective pore length, and is expressed by the formula: curvature ratio = average effective pore length / film thickness of the microporous membrane. A curvature ratio of 1.32 or more and 2.00 or less is preferred. More preferably, it is 1.35 or more and 1.90 or less. By setting the curvature ratio to 1.32 or more, foreign matter comes into contact with the fine fibril structure, and good foreign matter collection performance is obtained. Furthermore, by setting the curvature ratio to 2.00 or less, deterioration of water permeability is suppressed, which is preferable. The curvature ratio can be controlled to the above range by cooling and stretching both the first and second layers, which have different polyolefin resin compositions, and adjusting the effective pore lengths on the surface and inner layers.

[0023] (Surface roughness Rz) Surface roughness is the surface irregularities of the polyolefin microporous membrane measured by the dynamic force mode (DFM) of a scanning probe microscope, as described later. A surface roughness of 120 nm or less is preferred, and more preferably 100 nm or less. When the surface roughness is 120 nm or less, the uniformly refined fibrils provide inertial collision effects and blocking effects, resulting in good foreign matter collection performance in the filtered liquid. Surface roughness can be controlled to the above range by forming an unstretched gel-like sheet, cooling the first layer of polyolefin resin to 35°C at a rate of 100°C / min or more to crystallize it, and then stretching it at a low temperature to form a fine fibril structure, thereby reducing the surface irregularities.

[0024] (film thickness) The thickness of the polyolefin microporous membrane of the present invention is preferably 7 μm to 35 μm. More preferably 9 μm to 30 μm. A film thickness of 7 μm or more ensures a layer for collecting foreign matter, resulting in sufficient collection performance. A film thickness of 35 μm or less provides good water permeability. The film thickness can be adjusted to the above range by appropriately adjusting, for example, the discharge amount from the T-die, the rotation speed of the cooling roll, the line speed, and the stretching ratio.

[0025] (Air permeability resistance) The air permeability resistance of the polyolefin microporous membrane of the present invention is 10 sec / 100 cm. 3 More than 200sec / 100cm 3 The following is preferable. More preferably 20 sec / 100 cm 3 More than 190sec / 100cm 3 More preferably, 30 sec / 100 cm 3 More than 180sec / 100cm 3 The following applies: Air permeability resistance is 10 sec / 100 cm. 3 A value greater than this is preferable because it makes it less likely for the microporous membrane to rupture when foreign matter collides with it. Furthermore, an air permeability resistance of 200 sec / 100 cm is desirable. 3The pressure loss will not become too high if the values ​​are below the specified range, and good water permeability will be obtained. The air permeability resistance can be set within the above range by adjusting the polypropylene content, stretching conditions, and the heat-fixing treatment temperature after stretching the gel-like sheet.

[0026] (porosity) The porosity of the polyolefin microporous membrane of the present invention is 45% to 80%. More preferably, the porosity is 48% to 75%, and even more preferably 50% to 70%. When the porosity is 45% or higher, suitable water permeability is obtained when the polyolefin microporous membrane is used as a filter. When the porosity is 80% or lower, the voids are appropriate, and therefore excellent foreign matter collection is achieved. The porosity can be set to the above range by adjusting the ratio of polyolefin resin to film-forming solvent, stretching temperature conditions, stretching ratio, and heat-fixing treatment temperature after stretching of the gel-like sheet.

[0027] (Bubble point pore size) The bubble point pore diameter (maximum pore diameter) of the present invention is preferably 1 nm to 25 nm. More preferably 3 nm to 23 nm. Setting the bubble point pore diameter to 25 nm or less is preferable because it can suppress the passage of foreign matter. Furthermore, setting the bubble point pore diameter to 1 nm or more can provide excellent water permeability. The bubble point pore diameter can be set to the above range by appropriately adjusting the ratio of polyolefin resin to film-forming solvent, stretching temperature conditions, stretching ratio, and heat-fixing treatment temperature conditions after stretching of the gel-like sheet.

[0028] (Manufacturing method) In this invention, at least the first and second layers of polyolefin resin and a film-forming solvent are heated, melted, and kneaded together. The resulting resin solution is co-extruded through a die and cooled to form an unstretched gel sheet. The resulting unstretched gel sheet is then subjected to simultaneous biaxial stretching in the MD and TD directions. The molding solvent is removed, and the sheet is dried to obtain a polyolefin microporous film. The porous substrate used in the porous film of the present invention is preferably a laminated film having multiple layers. Examples of laminated configurations include, but are not limited to, two layers of A / B, three layers of A / B / A, A / B / A / B, A / B / A / B / A, etc., when using two different olefin compositions A (first layer) and B (second layer). Furthermore, a laminated configuration can be created by adding layers of different compositions in addition to olefin compositions A and B. In particular, to reduce surface roughness, it is preferable to arrange the polyolefin composition A (first layer), described later, so that it is on the surface side of one of the layers. The manufacturing method for the A / B / A configuration will be described below. In sequential stretching, where the material is stretched sequentially in the MD and TD directions, it is difficult to obtain the pore structure necessary for collecting foreign matter due to the progression of recrystallization. Therefore, simultaneous biaxial stretching is preferred to achieve the objectives of the present invention. A method for producing a polyolefin microporous membrane will be specifically explained using simultaneous stretching as an example.

[0029] (mixing, kneading) (a) Polyolefin resin of the first layer A polyolefin solution is prepared by melt-kneading a resin solution containing a polyolefin resin, which is composed of 2% to less than 98% by mass of high-density polyethylene and 2% to less than 98% by mass of ultra-high molecular weight polyethylene, and a film-forming solvent. The ratio of polyolefin resin to film-forming solvent is preferably 5% to 35% of the polyolefin resin, and more preferably 10% to 30%. By increasing the amount of film-forming solvent, the spacing between the microphases separated by the film-forming solvent can be widened, and the porosity can be increased. The method of melt-kneading the polyolefin resin and the film-forming solvent is not particularly limited, but it is preferably carried out in a twin-screw extruder. The preferred temperature range of the polyolefin solution in the twin-screw extruder varies depending on the resin; for example, for polyethylene compositions, it is 140 to 250°C. The temperature of the polyolefin solution in the extruder can be indirectly monitored by installing a thermometer inside the extruder or in the cylinder, and the heater temperature, rotation speed, and discharge rate in the cylinder can be adjusted as appropriate to reach the target temperature. The film-forming solvent may be added before the start of kneading, or it can be added during kneading. During melt-mixing, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin. (b) Polyolefin resin of the second layer A polyolefin solution is prepared by melt-kneading a resin solution containing a polyolefin resin, which is composed of 5% to 60% by mass of high-density polyethylene and 40% to 95% by mass of polypropylene with a mesopentat fraction of 90% or more, and a film-forming solvent. The ratio of polyolefin resin to film-forming solvent is preferably 5% to 50% of the polyolefin resin, and more preferably 10% to 40%. By increasing the amount of film-forming solvent, the spacing between the microphases separated by the film-forming solvent can be widened, and the porosity can be increased. The method of melt-kneading the polyolefin resin and film-forming solvent is not particularly limited, but it is preferably carried out in a twin-screw extruder. The preferred temperature range of the polyolefin solution in the twin-screw extruder varies depending on the resin; for example, for polyolefin compositions, it is 140 to 250°C. The temperature of the polyolefin solution in the extruder can be indirectly monitored by installing a thermometer inside the extruder or in the cylinder, and the heater temperature, rotation speed, and discharge rate in the cylinder can be adjusted as appropriate to reach the target temperature. The film-forming solvent may be added before the start of mixing, or it can be added during the mixing process. During melt mixing, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.

[0030] (Extrusion and casting) An unstretched gel sheet is formed by co-extruding a first layer of polyolefin resin solution and a second layer of polyolefin resin solution, which have been melted and kneaded in an extruder, into a three-layer structure using a multilayer T-die, with the first layer forming the surface layer and the second layer the inner layer, and then cooling the mixture. As a method for forming the unstretched gel sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

[0031] Cooling is preferably carried out at a rate of at least 100°C / min up to the gelation temperature, and down to 35°C or below. It is preferable that the inner layer of the gel sheet be cooled uniformly, and the thickness of the unstretched gel sheet is preferably 2 mm or less. Through the above treatment, the microphase of polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate and temperature are within the above range, the degree of crystallinity is kept within an appropriate range, the surface pore structure has small irregularities, and an unstretched gel sheet suitable for stretching is obtained. As for the cooling method, methods such as contact with a refrigerant such as cold air or cooling water, or contact with a cooling roll can be used, but it is preferable to cool by contact with a roll cooled with a refrigerant.

[0032] (Stretching) Next, the resulting gel-like sheet is stretched in at least two axes: the MD direction and the TD direction. Since the gel-like sheet contains a film-forming solvent, it is stretched uniformly. It is preferable to stretch the gel-like sheet by heating and then performing simultaneous biaxial stretching at a predetermined magnification using the Tenter method.

[0033] The stretching ratio (area stretching ratio) in this process is preferably 9 to 49 times, more preferably 12 to 42 times, and particularly preferably 16 to 36 times. The stretching ratio in this process refers to the area stretching ratio of the microporous membrane immediately before being used in the next process, based on the microporous membrane immediately before this process. The stretching temperature in this process is preferably 80°C or higher and less than 115°C, and more preferably 85°C or higher and less than 114°C. By setting the temperature to 80°C or higher, stretching by the tenter method can be performed effectively, and by setting the temperature to less than 115°C, the pore size can be reduced and the specific surface area can be controlled to be larger.

[0034] The stretching process described above causes cleavage between polyethylene lamellae, resulting in the refinement of the polyethylene phase and the formation of numerous fibrils. These fibrils form a three-dimensionally irregularly interconnected network structure. While stretching improves mechanical strength and expands pores, stretching under appropriate conditions allows for control of through-pore diameter, resulting in a high porosity and a large specific surface area.

[0035] Next, the resulting stretched film is heat-set. For heat-setting before extracting the film-forming solvent, it is preferable to heat the stretched gel-like film to a temperature of 40°C or higher but less than 120°C. If heat-setting is also performed after the extraction of the film-forming solvent, it is preferable to heat it at a higher temperature than the heat-setting temperature after extraction. By setting the heat-setting temperature high before the extraction of the film-forming solvent and low after the extraction, the formed network structure can be maintained without collapse. The heat-setting time is preferably 20 seconds or less. If the heat-setting time is 20 seconds or less, the fiber structure does not thicken due to recrystallization of the polyolefin resin, and a high specific surface area can be achieved. This is thought to be because the presence of the film-forming solvent between the formed network structures maintains the network structure, resulting in high porosity. By setting the heat-setting temperature below 120°C and for 20 seconds or less, good porosity is obtained, resulting in good water permeability.

[0036] (Extraction of film-forming solvent) The biaxially stretched sheet obtained in this manner is then subjected to extraction of the film-forming solvent using a washing solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, the extraction of the film-forming solvent yields a porous film consisting of fibrils that form a fine three-dimensional network structure, with irregularly interconnected pores in three dimensions. Known methods can be used for the washing solvent and the method for extracting the film-forming solvent using it. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Publication No. 2002-256099 can be used.

[0037] (heat fixation) The film from which the film-forming solvent has been extracted is subjected to heat treatment to stabilize the crystals and make the lamellae uniform. The heat treatment can be either heat-fixing or heat-relaxing. Heat-fixing is a heat treatment that heats the film while maintaining its dimensions. Heat-relaxing is a heat treatment that causes the film to shrink in the MD or TD direction during heating. The heat-setting treatment is preferably carried out by the tenter method. For example, the method disclosed in Japanese Patent Publication No. 2002-256099 is an example of a heat relaxation treatment method. The heat-setting treatment is preferably carried out at a temperature of 40°C or higher but less than 111°C. When the film after extraction of the film-forming solvent is heat-set at a temperature of 111°C or lower, recrystallization of the polyolefin resin is less likely to proceed, the thickening of the fiber structure is suppressed, and a good specific surface area can be obtained. Furthermore, at temperatures of 40°C or higher, the film shrinks, and the formed pore structure can be further adjusted to be smaller.

[0038] (Measurement method) The measurement and evaluation methods are described below.

[0039] (molecular weight distribution) The molecular weight distribution of polyolefin raw materials was determined by gel permeation chromatography (GPC) under the following conditions. • Measurement device: Agilent PL-GPC220 high-temperature GPC device • Columns: Agilent PL1110-6200 (20μm MIXED-A) x 2 Column temperature: 160℃ • Solvent (mobile phase): 1,2,4-trichlorobenzene • Solvent flow rate: 1.0 ml / min ·Sample concentration: 0.1% by mass (dissolution conditions: 160℃ / 3.5h) Injection volume: 500 μl • Detector: Agilent differential refractive index detector (RI detector) • Calibration curve: Determined from a universal calibration curve obtained using monodisperse polystyrene standard samples.

[0040] (Specific surface area, curvature ratio, median pore size based on specific surface area) A rectangular specimen was cut from an arbitrary position on the fabricated polyolefin microporous membrane, such that its longest side was parallel to the TD direction and weighed 0.20 ± 0.02 g. The area and weight of the specimen were measured. The volume of the polyolefin microporous membrane was calculated from the area of ​​the specimen and the film thickness measured using the method described later. The trimmed sample was wound onto a sample core in purified water, taking care to prevent air bubbles from entering, and then placed inside the measurement chamber of a pure water pressure porosimeter (model: WIP-3K-A-1) manufactured by POROUS MATERIALS, INC.

[0041] The surface area of ​​the polyolefin microporous membrane was measured by introducing gas into pure water in a chamber, applying various pressures, and measuring the weight of the pure water that permeated at each pressure. From the surface area and the volume of the polyolefin microporous membrane, the specific surface area and curvature ratio were determined. table The median pore size was calculated based on area. The maximum pressure was 13.7 MPa Measurements were taken up to this point.

[0042] (film thickness) The thickness of the polyolefin microporous film was measured at five arbitrary points using a contact thickness gauge, and the film thickness of the test specimen was determined by averaging these five measurement results. The thickness measuring instrument used was a Mitutoyo "Lightmatic" (registered trademark) VL-50B (measuring pressure 0.01N, carbide spherical probe φ10.5mm).

[0043] (porosity) Prepare a 95mm square sample, and the sample volume (cm³) 3 The porosity (%) was calculated using the following formula based on the results obtained by measuring the sample mass (g) and the density value of 0.99 g / cm³. 3 I used it. Porosity = (1 - sample mass / (0.99 × sample volume)) × 100 (Surface roughness Rz) Surface roughness was measured using a scanning probe microscope SPA500 manufactured by Seiko Instruments Inc., and obtained by dynamic force mode (DFM) from surface topography images of a 4 μm x 4 μm area of ​​the polyolefin microporous membrane surface. The sum of the average of the five highest peaks (with a reference length of 4 μm in the width direction) and the average of the five deepest valleys (with a reference length of 4 μm) was used.

[0044] (Bubble point pore size) Test specimens cut from polyolefin microporous membranes were measured using a palm porometer (model: CFP-1500A) manufactured by POROUS MATERIALS, INC., in the order of dry-up and wet-up measurements. In the wet-up measurement, pressure was applied to the polyolefin microporous membrane, which was thoroughly immersed in Galwick (trade name), a material with known surface tension. The pore diameter calculated from the pressure at which air began to penetrate was defined as the bubble point pore diameter (maximum pore diameter). For the average pore diameter, the pore diameter was calculated from the pressure at the point where the curve showing the 1 / 2 slope of the pressure and flow rate curve from the dry-up measurement intersected with the curve from the wet-up measurement. The following formula was used for the conversion between pressure and pore diameter. d = C·γ / P In the formula, "d(μm)" is the pore size of the polyolefin microporous membrane, "γ(mN / m)" is the surface tension of the liquid, "P(Pa)" is the pressure, and "C" is a constant.

[0045] (Air permeability resistance) Using the EGO1 digital air permeability resistance tester manufactured by Asahi Seiko Co., Ltd., the polyolefin microporous membrane of the present invention was fixed to the measurement area without wrinkles, and the air permeability resistance was measured according to JIS P-8117 (2009). The sample was 5 cm square, and the measurement point was set to a single point in the center of the sample. The measured value was used as the air permeability resistance of the sample [sec / 100 cm]. 3 The same measurement was performed on 10 test pieces taken from arbitrary film positions, and the average of the 10 measured values ​​was taken as the air permeability resistance of the polyolefin microporous membrane.

[0046] (Water permeability) A polyolefin microporous membrane was placed in a 39mm diameter stainless steel permeable cell. The polyolefin microporous membrane was moistened with a small amount (0.5ml) of ethanol, then 100ml of pure water was added to the permeable cell. The pure water was filtered using a differential pressure of 90kPa, and the permeability (cm³) after 10 minutes was measured. 3 ) from unit time (min) · unit area (cm 2 The permeability per unit area was calculated. The room temperature during the measurement was 23±2℃, and the pure water temperature was 23±2℃. The permeability was 0.05 ml / min·cm. 2 The above was considered to have water permeability.

[0047] (Collection performance) A gold colloid dispersion with a concentration of 0.2 ppm and a particle size of 2 nm was filtered at a pressure of 1.5 MPa. The concentrations before and after filtration were quantified using ICP-OES, and the removal rate (%) was calculated as (1 - concentration after filtration / concentration before filtration) × 100. A removal rate of 80% or more was designated as A, and a removal rate of less than 80% was designated as B. [Examples]

[0048] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to these examples.

[0049] (Example 1) The first layer of resin has a wattage of 2.2 × 10 6 Ultra-high molecular weight polyethylene 30% by mass and Mw is 4.1 × 10 5 A mixture was prepared by adding 0.2 parts by mass of tetrakis[methylene-3-(3,5-diter-butyl-4-hydroxyphenyl)-propionate]methane as an antioxidant to 100 parts by mass of polyolefin resin consisting of 70% by mass of high-density polyethylene. 28.5 parts by mass of the resulting mixture was fed into a strong-kneading twin-screw extruder, and 71.5 parts by mass of liquid paraffin [35 cSt (40℃)] was supplied from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 200℃ and 300 rpm to prepare a polyolefin resin solution. The second layer of resin has a wattage of 4.1 × 10 5 A mixture was prepared by adding 0.2 parts by mass of tetrakis[methylene-3-(3,5-diter-butyl-4-hydroxyphenyl)-propionate]methane as an antioxidant to 100 parts by mass of a polyolefin resin consisting of 50% by mass of high-density polyethylene and 50% by mass of polypropylene with a pentat fraction of 95%. Thirty parts by mass of the obtained mixture were placed into a twin-screw extruder of the strong kneading type, and seventy parts by mass of liquid paraffin [35 cSt (40°C)] were supplied from the side feeder of the twin-screw extruder. The mixture was melt-kneaded at 200°C and 200 rpm to prepare a polyolefin resin solution. The first and second layers of polyolefin resin solutions were supplied from a twin-screw extruder to a multi-layer T-die in a three-layer configuration, with the first layer of polyolefin resin solution on the surface and the second layer of polyolefin resin solution on the inner layer. The extruded so that the surface layer was 35% of the sheet thickness and the inner layer was 30% of the sheet thickness. The molded body was cooled while being taken up by a cooling roll heated to 30°C at a cooling rate of 210°C / min to form an unstretched gel sheet. The obtained unstretched gel sheet was simultaneously biaxially stretched by 5 times in the MD direction and 5 times in the TD direction using a tenter device set to a temperature of 113°C. Before washing with a film-forming solvent, it was heat-set at 119°C to obtain a biaxially oriented sheet. The obtained biaxially oriented sheet was washed with methylene chloride to extract and remove any remaining liquid paraffin, and then dried. The resulting dried biaxially oriented sheet was heat-treated using a tenter stretcher, heating it to 110°C to obtain a 10 μm thick polyolefin microporous film. The surface pore structure observed by scanning electron microscopy (SEM) is shown in Figure 1.

[0050] (Example 2) The same polyolefin resin solutions for the first and second layers as in Example 1 were supplied from a twin-screw extruder to a multilayer T-die in a three-layer configuration, with the first layer being the polyolefin resin solution and the inner layer being the polyolefin resin solution for the second layer. The material was extruded so that the surface layer was 37.5% of the sheet thickness and the inner layer was 25% of the sheet thickness. The molded body was cooled while being taken up by a cooling roll heated to 30°C at a cooling rate of 210°C / min to form an unstretched gel-like sheet. The obtained unstretched gel-like sheet was stretched in the same manner as in Example 1 to obtain a polyolefin microporous film with a thickness of 10 μm.

[0051] (Example 3) The same polyolefin resin solutions for the first and second layers as in Example 1 were supplied from a twin-screw extruder to a multilayer T-die in a three-layer configuration, with the first layer being the polyolefin resin solution and the inner layer being the polyolefin resin solution for the second layer. The extruded so that the surface layer was 40% of the sheet thickness and the inner layer was 20% of the sheet thickness. The molded body was cooled while being taken up by a cooling roll heated to 30°C at a cooling rate of 210°C / min to form an unstretched gel-like sheet. The obtained unstretched gel-like sheet was stretched in the same manner as in Example 1 to obtain a polyolefin microporous film with a thickness of 10 μm.

[0052] (Comparative Example 1) The unstretched gel sheet from Example 3 was simultaneously biaxially stretched 5 times in the MD direction and 5 times in the TD direction using a tenter apparatus set to a temperature of 115°C. Before washing with a film-forming solvent, it was heat-set at 119°C to obtain a biaxially oriented sheet. The obtained biaxially oriented sheet was washed with methylene chloride to extract and remove any remaining liquid paraffin, and then dried. The dried biaxially oriented sheet was heat-treated in a tenter-type stretcher to 120°C to obtain a polyolefin microporous film with a thickness of 10 μm. The pore structure of the surface observed by SEM is shown in Figure 2.

[0053] (Comparative Example 2) The unstretched gel sheet from Example 1 was simultaneously biaxially stretched by 5 times in the MD direction and 5 times in the TD direction using a tenter apparatus set to a temperature of 114°C. Before washing with a film-forming solvent, it was heat-set at 119°C to obtain a biaxially oriented sheet. The obtained biaxially oriented sheet was washed with methylene chloride to extract and remove any remaining liquid paraffin, and then dried. The dried biaxially oriented sheet was heat-treated in a tenter-type stretcher to 125°C to obtain a polyolefin microporous film with a thickness of 10 μm.

[0054] (Comparative Example 3) The unstretched gel sheet from Example 3 was simultaneously biaxially stretched by 5 times in the MD direction and 5 times in the TD direction using a tenter apparatus set to a temperature of 113°C. Before washing with a film-forming solvent, it was heat-set at 100°C to obtain a biaxially oriented sheet. The obtained biaxially oriented sheet was washed with methylene chloride to extract and remove any remaining liquid paraffin, and then dried. The dried biaxially oriented sheet was heat-treated in a tenter-type stretcher to 125°C, which is higher than the heat-setting temperature before washing with the film-forming solvent, to obtain a polyolefin microporous film with a thickness of 6 μm.

[0055] (Comparative Example 4) The same polyolefin resin solutions for the first and second layers as in Example 1 were supplied from a twin-screw extruder to a multilayer T-die in a three-layer configuration, with the first layer being the polyolefin resin solution and the inner layer being the polyolefin resin solution for the second layer. The extruded so that the surface layer was 35% of the sheet thickness and the inner layer was 30% of the sheet thickness. The molded body was cooled by taking it up with a cooling roll heated to 50°C at a cooling rate of 100°C / min to form an unstretched gel-like sheet. The obtained unstretched gel-like sheet was stretched in the same manner as in Example 1 to obtain a polyolefin microporous film with a thickness of 10 μm.

[0056] (Comparative Example 5) Only the polyolefin resin solution of the first layer of Example 1 was stretched in the same manner as in Example 1 to obtain a 10 μm polyolefin microporous film. Table 1 shows the results of the physical property measurements and evaluations of each obtained polyolefin microporous membrane.

[0057] [Table 1]

Claims

1. A polyolefin microporous membrane having a porosity of 45% or more and 80% or less, wherein the median pore size based on surface area in the pore size distribution of the polyolefin microporous membrane, as determined by pure water porosimeter measurement, is 1 nm or more and 15 nm or less.

2. The specific surface area of ​​the aforementioned polyolefin microporous membrane is 200 m². ２ / g or more 1000m ２ The polyolefin microporous membrane according to claim 1, wherein the amount is less than or equal to / g.

3. The polyolefin microporous membrane according to claim 1 or 2, wherein the surface roughness Rz of at least one surface of the polyolefin microporous membrane is 120 nm or less.

4. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the curvature ratio of the polyolefin microporous membrane, as determined by pure water injection porosimeter measurement, is 1.32 or more and 2.00 or less.

5. A polyolefin microporous membrane according to any one of claims 1 to 4, wherein the bubble point pore diameter determined by a half-dry measurement method using gas-liquid displacement is 1 nm or more and 25 nm or less.

6. The air permeability resistance of the aforementioned polyolefin microporous membrane is 10 sec / 100 cm. ３ Over 200sec / 100cm ３ A polyolefin microporous membrane according to any one of the following claims 1 to 5.

7. The polyolefin microporous membrane according to any one of claims 1 to 6, characterized in that the polyolefin microporous membrane is a polyolefin multilayer microporous membrane having at least a first layer containing polyethylene and a second layer containing a polyolefin different from the first layer.

8. A filtration filter having a polyolefin microporous membrane according to any one of claims 1 to 7.

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