JP7639340B2 - Wound body and method for producing polyolefin microporous membrane - Google Patents

Description

The present invention relates to a polyolefin microporous membrane, a polyolefin microporous membrane roll, and a method for producing a microporous membrane.

Polyolefin microporous membranes (hereinafter sometimes abbreviated as microporous membranes) are used in various fields, such as filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among these, microporous membranes made of polyolefin as the resin material have excellent chemical resistance, insulating properties, mechanical strength, and shutdown properties, and therefore have been widely used in recent years as separators for secondary batteries.

In particular, lithium-ion secondary batteries are widely used in mobile phones, power tools, electric vehicles, etc.
As the trend toward higher power and higher capacity continues, there is a demand for lower resistance and lower cost microporous membranes for use as separators. In particular, to achieve high power output, the resistance of the microporous membrane must be low, and the development of microporous membranes with high porosity is required. Furthermore, in order to reduce material switching losses in the microporous membrane manufacturing process, it is predicted that the roll diameter of the wound body will increase due to the increase in the length of the microporous membrane and the increase in the number of turns of the wound body of the microporous membrane that accompanies the increase in length.

In this specification, large diameter refers to a diameter of 280 mm or more.

International Publication No. 2018/043331 International Publication No. 2015/194504

However, in the manufacturing process of a large-diameter microporous membrane roll with high porosity, the internal pressure due to tightening increases in the parts closer to the core, compressing the membrane and easily resulting in differences in film thickness and porosity between the surface layer and the core.

For this reason, it is expected that in the future, for high-porosity, large-diameter microporous membrane rolls, there will be an increasing demand for rolls of microporous membrane with smaller differences in film thickness and porosity between the surface layer and the core.

On the other hand, increasing the porosity tends to decrease the dielectric breakdown voltage, making it difficult to achieve both.

Patent Document 1 discloses a separator that exhibits little change in performance even when the electrodes expand and contract, and that exhibits a small rate of change in air permeability resistance when subjected to a compression treatment at a temperature of 60°C and a pressure of 4 MPa for 10 minutes. However, a hybrid structure is formed that has a ladder structure in the submicron region and a three-dimensional mesh structure in the micron region, and does not take into account the dielectric breakdown voltage, including areas with large pore diameters.

Patent Document 2 discloses a separator that has a small rate of change in membrane thickness after compression treatment at a temperature of 90°C and a pressure of 5.0 MPa. However, because the porosity is 40% or less, the membrane has high resistance and performance needs to be improved in order to achieve high output.

In other words, the present invention provides a microporous membrane that has a high porosity, a high dielectric breakdown voltage, and a small difference in film thickness between the surface layer and the core layer, even in a large-diameter wound body.

As a result of extensive research to solve the above problems, the present invention was developed by controlling the fibril structure of the microporous membrane using advanced membrane production technology.

That is, the present invention has the following configuration.
(1) A polyolefin microporous membrane having a thickness of 16 μm or more and 30 μm or less, a porosity of 40% or more and 60% or less, a tortuosity of 1.00 or more and 1.42 or less, and a breakdown voltage of 155 V/μm or more and 300 V/μm or less.
(2) The polyolefin microporous membrane according to (1), having an average pore diameter of 20 nm or more and 30 nm or less.
(3) A wound body, characterized by being obtained by winding the polyolefin microporous membrane according to (1) or (2) to an outer diameter of 280 mm or more.
(4) A method for producing the polyolefin microporous membrane according to (1) or (2), comprising the steps of (a) to (f): (a) melt-kneading and extruding a resin solution containing a polyolefin resin containing 20% by mass or more and 80% by mass or less of ultra-high molecular weight polyethylene and a membrane-forming solvent to obtain an unstretched gel-like sheet;
(b) a preheating step of preheating the unstretched gel-like sheet at 95° C. or more and 115° C. or less;
(c) a longitudinal stretching step in which the temperature is gradually increased to a temperature in the range of 1° C. to 15° C. higher than the temperature in the preheating step and stretched in the sheet transport direction to obtain a longitudinally stretched sheet;
(d) transversely stretching the longitudinally stretched sheet in the sheet width direction at a temperature in the range of 1° C. to 20° C. higher than the temperature in the longitudinal stretching step and at a stretch ratio equal to or higher than that in the longitudinal stretching step to obtain a biaxially stretched sheet;
(e) extracting the membrane-forming solvent from the biaxially oriented sheet;
(f) after said extracting step, uniaxially stretching said biaxially oriented sheet;

Despite its high porosity, the polyolefin microporous membrane of the present invention has a small rate of change in membrane thickness even under pressurized conditions such as the core portion of a wound body, and can provide a separator for a lithium ion secondary battery with a high dielectric breakdown voltage.

The polyolefin microporous membrane of the present invention is described below.

(resin)
The polyolefin resin constituting the polyolefin microporous membrane of the present invention is mainly composed of polyethylene resin. The content of polyethylene resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 100% by mass, based on the total mass of the polyolefin resin being 100% by mass. The polyethylene may be a single substance, but is preferably a polyethylene mixture consisting of two or more kinds of polyethylene.

As the polyethylene mixture, at least one selected from the group consisting of ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, branched low density polyethylene, and linear low density polyethylene can be used. The polyethylene may be used alone or in combination of two or more types. These can be selected appropriately depending on the purpose of use.

The polyethylene mixture is preferably a mixture of ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 5×10 ⁵ or more and polyethylene having an Mw of 1×10 ⁴ or more and 5×10 ⁵ or less. The content of ultra-high molecular weight polyethylene in the polyethylene mixture is preferably 25 to 50% by mass from the viewpoint of the change in film thickness during compression. The content of ultra-high molecular weight polyethylene in the polyethylene mixture is more preferably 28 to 45% by mass, and even more preferably 30 to 40% by mass. When the ultra-high molecular weight polyethylene is present in the polyethylene mixture at 25% by mass or more, the mean flow pore size (through hole size) can be reduced, and the rate of change in film thickness during compression can be suppressed. In addition, the molecular weight distribution (Mw/Mn (number average molecular weight)) is preferably 1 to 20, more preferably 3 to 10, from the viewpoint of extrusion moldability and physical property control by stable crystallization control.

(Membrane forming solvent)
The membrane-forming solvent is not particularly limited as long as it is a substance that can be mixed with polyolefin resin or dissolve polyolefin resin. Both liquid and solid solvents can be used as the membrane-forming solvent. 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. In order 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. The solid solvent preferably has a melting point of 80° C. or less, and examples of such solid solvents include paraffin wax, ceryl alcohol, stearyl alcohol, and dicyclohexyl phthalate. The liquid solvent and the solid solvent may be used in combination.

(Production method)
In the present invention, a polyolefin resin and a membrane-forming solvent are heated, melted and kneaded, the resulting resin solution is extruded through a die and cooled to form an unstretched gel-like sheet, the resulting unstretched gel-like sheet is preheated and then stretched in the machine direction, which is the sheet conveying direction, and then stretched in the transverse direction, which is perpendicular to the conveying direction, to obtain a biaxially stretched sheet, the forming solvent is removed, and the sheet is dried to obtain a microporous membrane. In the case of a simultaneous stretching method in which stretching is performed simultaneously in the machine direction and the transverse direction, it is difficult to provide a stepwise temperature difference during stretching in the machine direction and the transverse direction, so a sequential axial stretching method is preferred to achieve the object of the present invention.

The manufacturing method of polyolefin microporous membrane will be specifically explained using the sequential stretching method as an example.

(a) Step of Obtaining an Unstretched Gel-Like Sheet (Mixing and Kneading)
A polyolefin solution is prepared by melt-kneading a resin solution containing a polyolefin resin containing 20% by weight or more and 80% by weight or less of ultra-high molecular weight polyethylene and a membrane-forming solvent. The method of melt-kneading the polyolefin resin and the membrane-forming solvent is not particularly limited, but it is preferable to perform the melt-kneading 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, the polyethylene composition is 140 to 250°C, and the temperature of the polyolefin solution in the extruder is indirectly grasped by installing a thermometer inside the extruder or on the cylinder, and the heater temperature, rotation speed, and discharge amount of the cylinder are appropriately adjusted to reach the target temperature. The membrane-forming solvent may be added before the start of kneading, or it can be added halfway through kneading. In order to prevent oxidation of the polyolefin resin during melt-kneading, it is preferable to add an antioxidant.

(Extrusion and Casting)
The polyolefin resin solution melted and kneaded in the extruder is cooled to form an unstretched gel-like sheet. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used to form the unstretched gel-like sheet.

Cooling is preferably performed at a rate of 50°C/min or more until the gelation temperature. Cooling is preferably performed to 35°C or less. By cooling, the microphase of the polyolefin separated by the membrane-forming solvent can be fixed. If the cooling rate is within the above range, the crystallinity is maintained within an appropriate range, and an unstretched gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, etc. can be used, but it is preferable to cool by contacting with a roll cooled with a refrigerant.

(b) Preheating Step In the preheating step, the unstretched gel-like sheet containing the membrane-forming solvent is gradually heated by passing it through multiple metal rolls until it reaches the longitudinal stretching temperature. Generally, preheating is performed by passing it through multiple metal rolls, but rolls coated with ceramic or Teflon (registered trademark) may also be used. By passing through the preheating step, the unstretched gel-like sheet can be stretched without temperature unevenness in the stretching step, and breakage due to uneven physical properties and sudden temperature changes can be suppressed. However, in the present invention, the temperature immediately before entering the longitudinal stretching step in the preheating step is a temperature of -15°C to -1°C lower than the temperature in the longitudinal stretching step, preferably -12°C to -3°C, and more preferably -10°C to -5°C. The temperature obtained by subtracting the temperature in the longitudinal stretching step from the temperature immediately before entering the longitudinal stretching step may be abbreviated as the preheating temperature difference. Specifically, the preheating temperature is in the range of 95°C to 115°C. It is important to provide a temperature difference between the inside and the surface of the gel-like sheet during longitudinal stretching. By subjecting the unstretched gel-like sheet to a preheating process, the pore size in the central part in the thickness direction can be made relatively smaller than that in the surface layer part, and it is possible to improve the breakdown voltage while maintaining a small tortuosity, which are difficult to achieve at the same time.

(c) Longitudinal Stretching Process In the longitudinal stretching process, the unstretched gel-like sheet is stretched in the longitudinal direction, so that the polymer chains of the polyolefin are oriented in the stretching direction. The longitudinal stretching process is divided into a stretching process and a heat setting process, and the temperature is adjusted for each process. The longitudinal stretching is performed by passing the sheet through multiple metal rolls with different peripheral speeds. When the unstretched gel-like sheet comes into contact with the first stretching roll in the longitudinal stretching process after passing through the preheating section, it is important to minimize the contact area and contact time with the first stretching roll to adjust the orientation of the polymers in the surface layer and inner layer.

The time that the gel-like sheet first comes into contact with the stretching roll after the preheating step is preferably 3 seconds or less. More preferably, it is 2 seconds or less, and even more preferably, it is 1.5 seconds or less. By keeping the time within this range, if a membrane-forming solvent with low thermal conductivity such as liquid paraffin is used, it is possible to longitudinally stretch the sheet while maintaining a constant difference in temperature between the center and the surface layers in the thickness direction.

It is preferable to perform the longitudinal stretching stepwise in two or more stages, from a low ratio to a high ratio. The stretching ratio in the longitudinal stretching process is preferably 1.1 times or more and 3 times or less, more preferably 1.2 times or more and 2 times or less, because if the temperature of the unstretched gel-like sheet is too low, the stretching stress during longitudinal stretching becomes extremely large and the stretching becomes unstable. The longitudinal stretching ratio in the second and subsequent stages is preferably 1.5 times or less than the longitudinal stretching ratio in the previous stage, and it is preferable to increase the number of stages to the desired stretching ratio. By doing so, breakage can be suppressed. The total longitudinal stretching ratio in the longitudinal stretching is preferably 5 times or more and 12 times or less, more preferably 5.5 times or more and 11 times or less. The total longitudinal stretching ratio is the sum of the stretching ratios in each stage. At such a longitudinal stretching ratio, it is easy to achieve a balance between strength and pore size.

By appropriately adjusting the stretch ratio while keeping the difference between the preheating temperature and the longitudinal stretching temperature within the above range, it is possible to control the pore opening, reduce the tortuous path length of the microporous membrane, and make the micropore diameter uniform. In this way, a fibril structure with relatively large pore diameters on the surface and relatively small pore diameters in the center can be obtained, achieving both compression resistance and high dielectric breakdown voltage.

(d) Step for obtaining a biaxially stretched sheet (transverse stretching step)
The longitudinally stretched sheet obtained in the longitudinal stretching step is stretched in the width direction.
In the transverse stretching, the obtained transversely stretched sheet is stretched in a tenter device at a ratio higher than the total longitudinal stretching ratio while both ends of the sheet are held by clips. Specifically, the sheet is preferably stretched at a ratio of 6 to 13, more preferably 6.5 to 12. The transverse stretching temperature is the temperature inside the tenter, and the sheet is stretched at a temperature 1°C to 10°C higher than the longitudinal stretching temperature. The ratios and temperatures of the longitudinal and transverse stretching preferably satisfy the following formulas (a) and (b).
(a) Transverse stretching ratio ≧ longitudinal stretching ratio (b) Transverse stretching temperature ≧ longitudinal stretching temperature

By satisfying formulas (a) and (b), the transverse stretching process, in which the polymer oriented in the longitudinal direction by longitudinal stretching is stretched in the width direction, is performed at a temperature higher than the longitudinal stretching temperature and at a large stretching ratio, thereby aligning the thicknesses of the fibrils in the longitudinal and transverse directions and maintaining a high dielectric breakdown voltage.

The difference between the transverse stretching temperature and the longitudinal stretching temperature in formula (b) is expressed as the transverse stretching temperature - the longitudinal stretching temperature, and is preferably 1°C or more and 10°C or less, and more preferably 3°C or more and 6°C or less.

(e) Membrane-forming solvent extraction process The biaxially stretched sheet thus obtained is subjected to extraction of the membrane-forming solvent using a washing solvent. Since the polyolefin phase is phase-separated from the membrane-forming solvent phase, when the membrane-forming solvent is extracted, a porous membrane is obtained that is composed of fibrils that form a fine three-dimensional network structure and has holes (voids) that are three-dimensionally irregularly connected. As the washing solvent and the method of extracting the membrane-forming solvent using the same, a known method can be used. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Publication No. 2002-256099 can be used.

(Heat Fixation)
The film from which the membrane-forming solvent has been extracted is subjected to heat treatment in order to stabilize the crystals and make the lamellae uniform.

Heat setting or heat relaxation can be used as the heat treatment method. Heat setting is a heat treatment in which the membrane is heated while maintaining its dimensions. Heat relaxation is a heat treatment in which the membrane is thermally shrunk in the longitudinal and transverse directions during heating. Heat setting is preferably performed using a tenter method or a roll method. For example, an example of a heat relaxation method is the method disclosed in JP-A-2002-256099. Heat setting or heat relaxation is performed by stretching at least in one axial direction at a ratio of 1.1 to 2 times to increase the strength of the microporous membrane. Heat treatment of the microporous membrane is preferably performed at a temperature 1 to 20°C higher than the transverse stretching temperature.

(Winding process)
The obtained microporous membrane was wound around an ABS core having an outer diameter of 150 mm to 450 mm at a winding tension of 10 N to 60 N to obtain a jumbo roll. The tension of the jumbo roll is preferably 10 N to 60 N, more preferably 15 N to 55 N. If the winding tension is too low, winding slippage is likely to occur, and if the winding tension is too high, wrinkles and deformation of the film are likely to occur due to tight winding.

(Slitting process)
The microporous membrane wound on the jumbo roll is slit to the desired length and width. In the slitting process, the jumbo roll may be slit to the required width and length in one slit, or may be slit from the jumbo roll in two to four separate slits. The number of slits is preferably five or less, since the probability of foreign matter contamination increases. The slit tension is adjusted according to the film thickness and width, and is preferably 0.1 N or more and 50 N or less. More preferably, it is 0.5 N or more and 45 N or less, and even more preferably, it is 1.0 N or more and 40 N or less. If the winding tension is too low, winding slippage is likely to occur, and if the winding tension is too high, wrinkles and film deformation are likely to occur due to tight winding.

(Other processes)
The obtained microporous membrane can be provided with layers other than the microporous layer as necessary to form a laminated porous membrane. Examples of the layers include a porous layer formed using a filler-containing resin solution containing a filler and a resin binder or a heat-resistant resin solution.

(Characteristics)
The polyolefin microporous membrane of the present embodiment can have high compression resistance and high breakdown voltage while having a high porosity by satisfying the above-mentioned conditions such as the stretching temperature and stretching ratio.

Below, the characteristics of the polyolefin microporous membrane of this embodiment are described.

(Film thickness (μm))
The membrane thickness can be measured with a measuring device with a display resolution of up to 0.01 μm. When using a contact type, it is desirable to perform the measurement without crushing the pores of the microporous membrane, and the measurement can be performed with a measuring force of 0.01 N or more and 0.15 N or less. The polyolefin microporous membrane is 16 μm or more and 30 μm or less, preferably 16 μm or more and 25 μm or less. In recent years, batteries have become higher power, and some use thin microporous membranes of 16 μm or less. If the thickness is less than 16 μm, the difference in membrane thickness between the surface layer part and the core part due to tightening in the manufacturing process is easier to reduce, but coating is often performed to ensure the safety of the battery, and by making the thickness 16 μm or more, a separator that ensures safety can be obtained without performing a coating process. In addition, by making the membrane thickness 30 μm or less, the battery capacity can be ensured and the thickness change between the surface layer part and the core part of the wound body can be reduced.

(Porosity (%))
The porosity is defined as the ratio of the volume of the space to the total volume of the material. Specifically, the thickness and mass of the microporous membrane are measured, and the porosity is calculated using the density of the resin. The density of the resin can be measured according to JIS K 7112:1999. The porosity of the polyolefin microporous membrane is preferably 40% or more and 60% or less. If the porosity is less than 40% when the battery is increased in output, the resistance of the membrane increases, and good output characteristics cannot be obtained when the microporous membrane is used as a battery separator. In addition, if the porosity is 60% or more, the resistance of the membrane is small and good output characteristics can be obtained, but the amount of change in membrane thickness may increase, and if the amount of change in membrane thickness is large, cycle characteristics may deteriorate.

The porosity is more preferably 45% or more and 55% or less, and even more preferably 48% or more and 53% or less.

(Film thickness change (%))
The amount of change in film thickness is the amount of change in the core film thickness relative to the surface layer film thickness, and is expressed by the following formula.
Amount of change in film thickness (%) = (surface layer film thickness - core film thickness) / surface layer film thickness x 100

The amount of change in film thickness is preferably 0% or more and 0.8% or less, more preferably 0% or more and 0.7% or less, and even more preferably 0% or more and 0.6% or less.

When the amount of change in membrane thickness is within the above range, the number of turns in the microporous membrane wound body can be increased during the microporous membrane manufacturing process.

(Curve ratio)
Tortuosity is the ratio of the thickness of the microporous membrane to the average effective pore path length, and is expressed by the formula Tortuosity = average effective pore path length / membrane thickness. Tortuosity can be measured using a porosimeter. Porosimetry can be performed using either non-mercury porosimetry or mercury porosimetry, but it is preferable to use the non-mercury porosimetry, which does not use harmful mercury.

The tortuosity ratio is preferably 1.00 or more and 1.42 or less. A lower tortuosity ratio provides better compression resistance. More preferably, it is 1.05 or more and 1.40 or less, and even more preferably, it is 1.10 or more and 1.38 or less. If the tortuosity ratio exceeds 1.42, there is a high possibility that compression resistance will deteriorate. The closer the tortuosity ratio is to 1.00, the better the output characteristics will be, while a higher tortuosity ratio is advantageous in terms of safety, so the above range is best in terms of the balance with safety. The tortuosity ratio is determined by the pressing method.

(Breakdown voltage (V/μm))
The dielectric breakdown voltage can be measured according to JIS C2110-1:2016. The dielectric breakdown voltage per 1 μm of film thickness is preferably 155 V/μm or more and 300 V/μm or less, more preferably 160 V/μm or more and 290 V/μm or less, and even more preferably 165 V/μm or more and 280 V/μm or less. If the dielectric breakdown voltage is 155 V/μm or less, even if discharge occurs due to static electricity, pinholes are unlikely to open in the microporous membrane. Also, if the dielectric breakdown voltage exceeds 300 V/μm, the average pore size of the microporous membrane becomes too small, making it difficult for the electrolyte to penetrate.

(Average pore diameter (nm))
The average pore size can be measured using a perm porometer. Unlike a porosimeter, a perm porometer can selectively measure the narrowest part of the pores. The average pore size of the polyolefin microporous membrane measured by a perm porometer is preferably 20 nm or more and 30 nm or less, more preferably 22 nm or more and 28 nm or less, and even more preferably 23 nm or more and 27 nm or less. By controlling the pore size of the membrane within the above range, a high dielectric breakdown voltage can be maintained.

[Measurement method]

(Measurement of film thickness (μm))
For the membrane thickness, a test piece was prepared by cutting a microporous membrane at an arbitrary position in a length of 110 cm in the vertical direction and 6 cm in the horizontal direction. The thickness of this test piece was measured at 20 points at 5 cm intervals in the vertical direction using a contact thickness meter, and the average was taken as the thickness of the test piece. The thickness measuring machine used was a Mitsutoyo Litematic VL-50B (feeder, carbide spherical feeder, φ9.5 mm) with a measuring force of 0.01 N. The measurement was performed in an environment of 23±2°C.

(Measurement of film thickness change rate (%))
The polyolefin microporous membrane slit to a width of 60 mm was wound around an ABS core having an outer diameter of 200 mm and an inner diameter of 76.2 mm until the outer diameter reached 280 mm to obtain a wound body. This polyolefin microporous membrane wound body was stored at 23±2° C. for 2 weeks, and the film thickness was measured at 20 points at 50 mm intervals in the inward direction of the winding, starting from a point 2 m from the surface layer, and the average value was taken as the surface layer film thickness.

The polyolefin microporous membrane roll was opened, and the thickness was measured at 20 points at 5 cm intervals from the core toward the outside of the roll, starting from a point 10 m from the core, and the average thickness was taken as the core thickness. The thickness of the polyolefin microporous membrane was measured 5 minutes after being released from the wound state.
Film thickness change rate (%)=(surface layer film thickness−core film thickness)/surface layer film thickness×100

(Measurement of porosity (%))
A 95 mm square sample was prepared, and its volume ( ^cm3 ) and mass (g) were measured, and the porosity (%) was calculated from the results using the following formula: Density was 0.99 g/ ^cm3 measured according to JIS K 7112:1999.
Porosity=(1−sample mass/(resin density×sample volume))×100

(Measurement of air permeability resistance (sec/100 cm ³ ))
Using a digital Oken type air resistance tester EGO1 manufactured by Asahi Seiko Co., Ltd., the polyolefin laminated microporous membrane of the present invention was fixed so as not to wrinkle the measurement part, and was measured according to JIS P-8117 (2009). The sample was 5 cm square, the measurement point was one point in the center of the sample, and the measured value was the air resistance [seconds] of the sample. The same measurement was performed on 10 test pieces taken from any film position, and the average value of the 10 measured values was taken as the air resistance of the polyolefin microporous membrane.

(Measurement of tensile strength (kPa) and tensile elongation (%))
The tensile strength corresponding to each direction was measured under the conditions of a chuck gap of 20 mm and a test speed of 100 mm/min, without marking the microporous membrane, by cutting into a shape conforming to JIS K7127, type 2 test piece with a width of 10 mm. The measurement was performed in an environment of 23±2°C.

(Measurement of Torque Ratio)
The measurement was performed using purified water with a pure water pressure intrusion porosimeter (product name, model: WIP-3k-A-1) manufactured by POROUS MATERIALS, INC.

The pore volume, specific surface area, porosity, and permeability coefficient of the microporous membrane are measured using a pure water intrusion porosimeter, and the tortuosity is calculated from formula (a) using the membrane thickness with the contact thickness gauge. The membrane was cut into a square of 5 cm in the longitudinal direction and 5 cm in the transverse direction from an arbitrary position to prepare a test piece. The thickness of five arbitrary points of this test piece was measured with a contact thickness gauge, and the average was used to determine the thickness of the test piece. The thickness gauge used was a Mitsutoyo Litematic VL-50B (with a carbide spherical probe φ9.5 mm) with a measuring force of 0.01 N.

l _e : average effective pore length l _d : membrane thickness ε: porosity V _p : specific pore volume S _BET : specific surface area k: permeability coefficient

(Measurement of dielectric breakdown voltage (V/μm))
Using a DC withstand voltage tester manufactured by Kasuga Electric Co., Ltd., a voltage of 100 V/sec was applied, and the voltage at which the test piece broke down was measured 20 times. The average value was divided by the film thickness measured with the contact thickness meter described above, and the breakdown voltage was calculated by converting it to a value per 1 μm.

The upper electrode was a 50 g brass cylinder with a diameter of 25 mm and a rounded edge with a radius of 3 mm, and the lower electrode was a brass flat plate. The microporous membrane was cut to 100 mm x 100 mm, and the lower electrode, microporous membrane, and upper electrode were arranged in this order and measurements were performed.

(Measurement of average pore diameter (nm))
Using a Perm Porometer (trade name, model: CFP-1500A) manufactured by POROUS MATERIALS, INC., measurements were taken in the order of dry-up and wet-up. In the wet-up, pressure was applied to a microporous membrane sufficiently soaked in Galwick (trade name) with a known surface tension, and the pore size calculated from the pressure at which air began to penetrate was taken as the bubble point pore size (maximum pore size). The average pore size was calculated from the pressure at the point where the curve showing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The following formula was used to convert pressure and pore size.
d = C γ / P

In the formula, "d (μm)" is the pore size of the microporous membrane, "γ (mN/m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant.

(Measurement of weight average molecular weight (Mw))
The Mw of UHMwPE and HDPE was determined by gel permeation chromatography (GPC) under the following conditions.
Measurement device: GPC-150C manufactured by Waters Corporation
Column: Shodex UT806M manufactured by Showa Denko K.K.
Column temperature: 135° C.
Solvent (mobile phase): o-dichlorobenzene Solvent flow rate: 1.0 ml/min Sample concentration: 0.1 wt% (dissolution conditions: 135°C/1h)
Injection volume: 500 μl
Detector: Waters Corporation differential refractometer (RI detector)
Calibration curve: A calibration curve was prepared using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.

Example 1
A mixture was prepared by blending 100 parts by mass of a polyolefin resin consisting of 30% by mass of ultra-high molecular weight polyethylene (UHPE) having a Mw of 2.0 x ^{10 6} and 70% by mass of high density polyethylene (HDPE: density 0.955 g/cm ³ , melting point 135°C) having a Mw of 5.6 x 10 5 with 0.2 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant. 28.5 parts by mass of the resulting mixture was put into a strong kneading type twin-screw extruder, 71.5 parts by mass of liquid paraffin [35 cSt (40°C)] was fed from the side feeder of the twin-screw extruder, and melt-kneaded under conditions of 230°C and 250 rpm to prepare a polyolefin resin solution.

The polyolefin resin solution was fed from a twin-screw extruder to a T-die, and the extruded molded product was taken up on a cooling roll whose temperature was controlled at 30°C and cooled while being taken up, forming an unstretched gel-like sheet.

The unstretched gel-like sheet was passed through a preheat roll at 113°C, and then stretched using a longitudinal stretch roll with a diameter of 200 mm at a surface temperature of 119°C, 6°C higher than the preheat temperature, in three steps of 1.3 times, 1.8 times, and 2.4 times in the longitudinal direction before the temperature rose to the inside of the unstretched gel-like sheet, with a total longitudinal stretching ratio of 5.6 times. The sheet was then passed through three cooling rolls and cooled to 50°C to form a longitudinally stretched sheet. At this time, the conveying speed was adjusted so that the time the unstretched gel-like sheet was in contact with the longitudinal stretch roll that it first came into contact with after the preheating process was 2.0 seconds so that the temperature did not rise to the inside.

Both ends of the obtained longitudinally stretched sheet were held with clips and stretched 8.9 times in the transverse direction using a tenter device set at 125°C to obtain a biaxially stretched sheet. The obtained biaxially stretched sheet was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried.

The biaxially stretched sheet obtained after drying was heated to 130°C in a tenter-type stretching machine and re-stretched to 1.44 times the width of the stretching machine entrance, and then heat-treated to adjust the lateral magnification to 1.31 times the width of the re-stretching device entrance, to obtain a polyolefin microporous membrane with a thickness of 19.5 μm. The sheet was slit to a width of 60 mm with a winding tension of 4 N and wound around a resin core with an outer diameter of 203 mm to obtain a wound battery separator with an outer diameter of 303 mm.

Example 2
A mixture was prepared by blending 0.2 parts by mass of tetrakis[methylene- ^3- ( ^3,5 -ditertiarybutyl- ⁴ - hydroxyphenyl )-propionate]methane as an antioxidant into 100 parts by mass of a polyolefin resin consisting of 25% by mass of ultra-high molecular weight polyethylene having a Mw of 2.0×10 6 and 75% by mass of high density polyethylene having a Mw of 5.6×10 5 (density 0.955 g/cm 3 , melting point 135° C.). The same procedure as in Example 1 was followed except that 28.5 parts by mass of the resulting mixture was fed into a strong kneading type twin-screw extruder, 71.5 parts by mass of liquid paraffin [35 cSt (40° C.)] was fed from the side feeder of the twin-screw extruder, and melt-kneaded under conditions of 230° C. and 250 rpm to prepare a polyolefin resin solution. A polyolefin microporous membrane having a thickness of 19.3 μm was obtained using this polyolefin microporous membrane in the same manner as in Example 1. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

Example 3
A polyolefin microporous membrane having a thickness of 19.5 μm was obtained in the same manner as in Example ¹ , except that 100 parts by mass of a polyolefin resin consisting of 40% by mass of ultra-high molecular weight polyethylene having a Mw of 2.0 ^× 10 6 and 60% by mass of high density polyethylene (density 0.955 g/cm ³ , melting point 135° C.) having a Mw of 5.6 × 10 5 was blended with 0.2 parts by mass of tetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methane as an antioxidant to prepare a mixture. A wound body was obtained in the same manner as in Example 1 using this polyolefin microporous membrane. A wound body was obtained in the same manner as in Example 1 using this polyolefin microporous membrane.

Example 4
The unstretched gel-like sheet of Example 1 was passed through a preheated roll at 115° C. and stretched in the longitudinal direction using a longitudinal stretching roll having a diameter of 200 mm at a surface temperature of 119° C., which was 4° C. higher than the preheating temperature, in the same manner as in Example 1 to obtain a polyolefin microporous membrane having a thickness of 19.5 μm. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

Example 5
The unstretched gel-like sheet of Example 1 was passed through a preheated roll at 109° C. and stretched in the longitudinal direction using a longitudinal stretching roll having a diameter of 200 mm at 122° C., which was 13° C. higher than the preheating temperature, in the same manner as in Example 1 to obtain a polyolefin microporous membrane having a thickness of 19.6 μm. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

Example 6
A polyolefin microporous membrane having a thickness of 19.4 μm was obtained in the same manner as in Example 1, except that the longitudinally stretched sheet of Example 1 was stretched 8.9 times in the transverse direction using a tenter apparatus set at 120° C. A wound body was obtained in the same manner as in Example 1 using this polyolefin microporous membrane.

(Example 7)
The unstretched gel-like sheet of Example 1 was passed through a preheat roll at 113°C, and a longitudinal stretch roll with a diameter of 150 mm was used at a surface temperature of 119°C, which is 6°C higher than the preheat temperature. The conveying speed was adjusted so that the time that the unstretched gel-like sheet contacts the longitudinal stretch roll that is first contacted after the preheating step is 1.0 seconds, and the sheet was stretched in three steps of 1.3 times, 1.8 times, and 2.4 times in the longitudinal direction at a total longitudinal stretching ratio of 5.62 times. The rest was the same as in Example 1, and a polyolefin microporous membrane with a thickness of 19.4 μm was obtained. Using this polyolefin microporous membrane, a roll was obtained in the same manner as in Example 1.

(Example 8)
The unstretched gel-like sheet of Example 1 was passed through a preheat roll at 113°C, and a longitudinal stretch roll with a diameter of 250 mm was used at a surface temperature of 119°C, which is 6°C higher than the preheat temperature. The conveying speed was adjusted so that the time that the unstretched gel-like sheet contacts the longitudinal stretch roll that is first contacted after the preheating step is 3.0 seconds, and the sheet was stretched in three steps of 1.3 times, 1.8 times, and 2.4 times in the longitudinal direction at a total longitudinal stretching ratio of 5.62 times. The rest was the same as in Example 1, and a polyolefin microporous membrane with a thickness of 19.5 μm was obtained. Using this polyolefin microporous membrane, a roll was obtained in the same manner as in Example 1.

Example 9
The same polyolefin resin solution as in Example 1 was fed from a twin-screw extruder to a T-die, and the extrusion was taken up by a cooling roll adjusted to 30°C, and cooled while being taken up to form an unstretched gel-like sheet. The unstretched gel-like sheet obtained was divided into three stages in the longitudinal direction, 1.8 times, 1.8 times, and 2.4 times, and stretched at a total longitudinal stretching ratio of 7.8 times. A polyolefin microporous membrane having a thickness of 20.0 μm was obtained in the same manner as in Example 1, except that the unstretched gel-like sheet was stretched at a total longitudinal stretching ratio of 7.8 times. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

Example 10
The polyolefin resin solution of Example 1 was fed from a twin-screw extruder to a T-die, and the extrusion was taken up with a cooling roll adjusted to 30° C. and cooled while being taken up to form an unstretched gel-like sheet having a thickness of 80% of that of Example 1. A polyolefin microporous membrane having a thickness of 16.0 μm was obtained in the same manner as in Example 1 except for the thickness of the unstretched gel-like sheet. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 1)
The polyolefin resin solution of Example 1 was fed from the twin-screw extruder to a T-die, and the extrudate was taken up by a cooling roll adjusted to 30°C, and cooled while being taken up to form an unstretched gel-like sheet. The unstretched gel-like sheet obtained was simultaneously stretched 5 times in the longitudinal direction and 5 times in the transverse direction using a tenter device set at 117°C to obtain a biaxially stretched sheet. After obtaining the biaxially stretched sheet, the same procedure as in Example 1 was followed to obtain a polyolefin microporous membrane having a thickness of 19.5 μm. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 2)
A mixture was prepared by blending 0.2 parts by mass of tetrakis[methylene- ^3- ( ^3,5- ditertiarybutyl- ⁴ -hydroxyphenyl)-propionate]methane as an antioxidant with 100 parts by mass of a polyolefin resin consisting of 18% by mass of ultra-high molecular weight polyethylene having a Mw of 2.0 x 10 6 and 82% by mass of high density polyethylene having a Mw of 5.6 x 10 5 (density 0.955 g/cm 3 , melting point 135°C). The procedure was the same as in Example 1 except that a polyolefin resin solution was prepared by feeding 30 parts by mass of the obtained mixture into a strong kneading type twin screw extruder, and 70 parts by mass of liquid paraffin [35 cSt (40°C)] was fed from the side feeder of the twin screw extruder and melt kneading under conditions of 230°C and 250 rpm. A polyolefin microporous membrane having a thickness of 19.2 μm was obtained in the same manner as in Example 1 except that a mixture was prepared. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 3)
The unstretched gel-like sheet of Example 1 was passed through a preheat roll at 119 ° C., and a longitudinal stretching roll with a diameter of 300 mm was used with a surface temperature of 119 ° C., the same as the preheating temperature, and in order to sufficiently heat the inside, the conveying speed was adjusted so that the time that the unstretched gel-like sheet contacted the longitudinal stretching roll that first contacted after the preheating process was 6.0 seconds, and the sheet was stretched in three stages, 1.3 times, 1.8 times, and 2.4 times in the longitudinal direction, at a total stretching ratio of 5.62 times. The rest was the same as in Example 1, and a polyolefin microporous membrane with a thickness of 19.5 μm was obtained. Using this polyolefin microporous membrane, a roll was obtained in the same manner as in Example 1.

(Comparative Example 4)
The unstretched gel-like sheet of Example 1 was passed through a preheat roll at 113°C, and stretched in the longitudinal direction at a total stretching ratio of 5.62 times in three stages of 1.3 times, 1.8 times, and 2.4 times at 125°C, which was 12°C higher than the preheat temperature. The sheet was then passed through three cooling rolls and cooled to 50°C to form a longitudinally stretched sheet.

The obtained longitudinally stretched sheet was held at both ends with clips and stretched 8.9 times in the transverse direction using a tenter device set at 113°C, 12°C lower than the longitudinal stretching temperature, so that the longitudinal stretching temperature was equal to or greater than the transverse stretching temperature, to obtain a biaxially stretched sheet. The rest of the process was the same as in Example 1, and a polyolefin microporous membrane with a thickness of 19.4 μm was obtained. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 5)
A longitudinally stretched sheet was formed in the same manner as in Example 1, except that the unstretched gel-like sheet of Example 1 was preheated and stretched in the longitudinal direction at a stretching ratio of 1.8 times, 1.8 times, and 2.4 times in three stages, and stretched at a total longitudinal stretching ratio of 7.8 times.

The obtained longitudinally stretched sheet was gripped at both ends with clips and stretched 6.8 times in the transverse direction using a tenter device set at 125°C so that the longitudinal stretch ratio was greater than or equal to the transverse stretch ratio, to obtain a biaxially stretched sheet. After obtaining the biaxially stretched sheet, a polyolefin microporous membrane having a thickness of 19.4 μm was obtained in the same manner as in Example 1. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 6)
The polyolefin resin solution of Example 1 was fed from a twin-screw extruder to a T-die, and the extrudate was taken up with a cooling roll whose temperature was controlled at 30° C. and cooled while being taken up, to form an unstretched gel-like sheet having a thickness of 60% of that of Example 1.

The unstretched gel-like sheet was passed through a preheat roll at 113°C, and then stretched using a longitudinal stretching roll with a diameter of 200 mm at a surface temperature of 120°C, 7°C higher than the preheating temperature, in three steps of 1.8 times, 1.8 times, and 2.4 times in the longitudinal direction before the temperature rose to the inside of the unstretched gel-like sheet, with a total longitudinal stretching ratio of 7.8 times. The sheet was then passed through three cooling rolls and cooled to 50°C to form a longitudinally stretched sheet. At this time, the conveying speed was adjusted so that the time that the unstretched gel-like sheet first came into contact with the longitudinal stretching roll after the preheating process was 2.0 seconds.

Both ends of the obtained longitudinally stretched sheet were held with clips and stretched 8.9 times in the transverse direction using a tenter device set at 115°C to obtain a biaxially stretched sheet. The obtained biaxially stretched sheet was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried.

The biaxially stretched sheet obtained after drying was heated to 130°C in a tenter-type stretching machine and re-stretched to 1.44 times the width of the stretching machine entrance, and then heat-treated to adjust the lateral magnification to 1.31 times the width of the re-stretching device entrance, to obtain a polyolefin microporous membrane with a thickness of 11.9 μm. The sheet was slit to a width of 60 mm with a winding tension of 3.5 N and wound around a resin core with an outer diameter of 203 mm to obtain a wound battery separator with an outer diameter of 268 mm.

The blend ratios of each component, manufacturing conditions, evaluation results, etc. of the polyolefin microporous membranes obtained in Examples 1 to 10 and Comparative Examples 1 to 6 are shown in Tables 1 to 3.

Claims (6)

A winding obtained by winding a polyolefin microporous film whose main component is a polyethylene resin having a thickness of 16 μm or more and 30 μm or less, a porosity of 40% or more and 60% or less, a tortuosity ratio of 1.00 or more and 1.42 or less, and a breakdown voltage of 155 V/μm or more and 300 V/μm or less, around an outer diameter of 280 mm or more, wherein the change in the core film thickness relative to the surface layer film thickness of the winding is 0% or more and 0.8% or less. The wound body according to claim 1, wherein the average pore diameter of the polyolefin microporous membrane is 20 nm or more and 30 nm or less. A method for producing a microporous polyolefin membrane, comprising the steps of (a) to (f).
(a) A process of melting and kneading a resin solution containing a polyolefin resin mainly composed of a polyethylene resin containing 20% by mass or more and 80% by mass or less of ultra-high molecular weight polyethylene and a membrane-forming solvent, and extruding the solution to obtain an unstretched gel-like sheet; (b) A preheating process of preheating the unstretched gel-like sheet at 95°C or more and 115°C or less; (c) A longitudinal stretching process of gradually increasing the temperature to a temperature in the preheating process to a temperature in the range of 1°C to 15°C higher than the temperature in the preheating process and stretching the sheet in the sheet transport direction to obtain a longitudinally stretched sheet; (d) A process of transversely stretching the longitudinally stretched sheet in the sheet width direction at a temperature in the range of 1°C to 20°C higher than the temperature in the longitudinal stretching process at a stretch ratio equal to or higher than that in the longitudinal stretching process to obtain a biaxially stretched sheet; (e) An extraction process of extracting the membrane-forming solvent from the biaxially stretched sheet; (f) A process of stretching the biaxially stretched sheet in a uniaxial direction after the extraction process. The method for producing a polyolefin microporous film according to claim 3, wherein the polyolefin microporous film has a thickness of 16 μm or more and 30 μm or less, a porosity of 40% or more and 60% or less, a tortuosity of 1.00 or more and 1.42 or less, and a dielectric breakdown voltage of 155 V/μm or more and 300 V/μm or less. The polyolefin microporous membrane has a weight average molecular weight of 5×10 ⁵ The ultra-high molecular weight polyethylene is contained in an amount of 20% by mass or more and 80% by mass or less, and the weight average molecular weight is 1×10 ⁴ Above 5 x 10 ⁵ 3. The winding of claim 1 or 2, which is made of a polyethylene mixture further comprising the following polyethylene: The polyolefin resin has a weight average molecular weight of 5×10 ⁵ The ultra-high molecular weight polyethylene is contained in an amount of 20% by mass or more and 80% by mass or less, and the weight average molecular weight is 1×10 ⁴ Above 5 x 10 ⁵ The method for producing a microporous polyolefin membrane according to claim 3 or 4, which comprises a polyethylene mixture further comprising the following polyethylene: