WO2021033413A1

WO2021033413A1 - Microporous polyolefin membrane, wound body of same and method for producing same

Info

Publication number: WO2021033413A1
Application number: PCT/JP2020/024297
Authority: WO
Inventors: 三浦由起子; 大友崇裕
Original assignee: 東レ株式会社
Priority date: 2019-08-21
Filing date: 2020-06-22
Publication date: 2021-02-25
Also published as: JPWO2021033413A1

Abstract

A microporous polyolefin membrane which is characterized by having a thickness of from 16 μm to 30 μm, a porosity of from 40% to 60%, a tortuosity of from 1.00 to 1.42 and a dielectric breakdown voltage of from 155 V/μm to 300 V/μm. A wound body which is characterized by being obtained by winding the microporous polyolefin membrane so as to have an outer diameter of 280 mm or more. The present invention provides a microporous membrane which has a high porosity and a high dielectric breakdown voltage, and which is able to be wound into a wounded body that has a small film thickness difference between the surface layer part and the winding core part even if the wound body has a large diameter.

Description

Polyolefin microporous membrane, its wound body and manufacturing method

The present invention relates to a polyolefin microporous membrane, a polyolefin microporous membrane wound body, 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 for filter membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors. Among these, microporous membranes made of polyolefin as a resin material are excellent in chemical resistance, insulating property, mechanical strength, etc., and have shutdown characteristics, and therefore have been widely used as separators for secondary batteries in recent years.

In particular, lithium-ion secondary batteries are widely used in mobile phones, power tools, electric vehicles, etc.
Higher output and higher capacity are progressing, and low resistance and low cost of microporous membranes are required for separator applications. Above all, in order to achieve high output, it is required that the resistance of the microporous membrane is low, and the development of a microporous membrane having a high porosity is required. Further, in the microporous membrane manufacturing process, in order to reduce the material switching loss, the roll diameter of the wound body is increased by increasing the length of the microporous membrane and increasing the number of turns in the wound body of the microporous membrane. It is expected that the diameter will increase.

The large diameter referred to in this specification means a diameter of 280 mm or more.

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

However, in the case of a microporous membrane wound body with a high porosity and a large diameter, the internal pressure due to winding increases as the portion is closer to the winding core in the manufacturing process, the film is compressed, and the film thickness at the surface layer portion and the winding core portion is increased. Differences between the difference and the porosity are likely to occur.

Therefore, in the case of a microporous membrane wound body with a high porosity and a large diameter, a microporous film wound body with a small difference in film thickness and pore ratio between the surface layer portion and the winding core portion will be required in the future. It is expected that.

On the other hand, if the porosity was increased, the breakdown voltage tended to decrease, and it was difficult to achieve both.

Patent Document 1 describes the rate of change in air permeation resistance when pressure compression treatment is performed under the conditions of a temperature of 60 ° C., a pressure of 4 MPa, and 10 minutes as a separator whose performance does not change much even under expansion and contraction of electrodes. Although a separator with a small pore size is disclosed, a hybrid structure having a ladder structure in the submicron region and a three-dimensional network structure in the micron region is formed, and the breakdown voltage including the portion having a large pore diameter is taken into consideration. It has not been.

Patent Document 2 discloses a separator having a small rate of change in film thickness after pressure compression treatment at a temperature of 90 ° C. and a pressure of 5.0 MPa, but since the pore ratio is 40% or less, high output is performed. The resistance of the film was high and it was necessary to improve the performance.

That is, the present invention provides a microporous film having a high porosity and a high dielectric breakdown voltage, and having a small film thickness difference between the surface layer portion and the winding core portion even for a large-diameter wound body. ..

As a result of diligent studies to solve the above problems, the present invention was achieved by controlling the fibril structure of the microporous membrane by advanced film forming technology.

That is, the present invention has the following configuration.
(1) The thickness is 16 μm or more and 30 μm or less, the pore ratio is 40% or more and 60% or less, the bending ratio is 1.00 or more and 1.42 or less, and the dielectric breakdown voltage is 155 V / μm or more and 300 V. A microporous polyolefin membrane characterized by being / μm or less.
(2) The polyolefin microporous membrane according to (1), wherein the average pore diameter is 20 nm or more and 30 nm or less.
(3) A wound body characterized in that the microporous polyolefin membrane according to (1) or (2) is wound to an outer diameter of 280 mm or more.
(4) The method for producing a microporous polyolefin film according to (1) or (2), which comprises the steps (a) to (f). (A) 20% by mass or more of ultra-high molecular weight polyethylene 80 A step of melt-kneading and extruding a resin solution containing a polyolefin resin contained in a proportion of mass% or less and a solvent for film formation to obtain an unstretched gel-like sheet.
(B) A preheating step of preheating the unstretched gel sheet at 95 ° C. or higher and 115 ° C. or lower.
(C) A longitudinal stretching step of obtaining a longitudinally stretched sheet by gradually raising the temperature to a temperature higher than the temperature of the preheating step in the range of 1 ° C. or higher and 15 ° C. or lower and stretching in the sheet transport direction.
(D) The longitudinally stretched sheet is laterally stretched in the sheet width direction at a temperature higher than the temperature of the longitudinally stretching step in the range of 1 ° C. or more and 20 ° C. or less at a stretching ratio of the longitudinally stretching step or more to obtain a biaxially stretched sheet. Process,
(E) An extraction step of extracting a film-forming solvent from the biaxially stretched sheet.
(F) A step of stretching the biaxially stretched sheet in the uniaxial direction after the extraction step.

Despite the high porosity, the polyolefin microporous film of the present invention has a small rate of change in film thickness and a high breakdown voltage even under pressure conditions such as the core of a wound body. A separator for an ion secondary battery can be provided.

Hereinafter, the polyolefin microporous membrane of the present invention will be described.

(resin)
The polyolefin resin constituting the polyolefin microporous membrane of the present invention contains a polyethylene resin as a main component. The content of the polyethylene resin is preferably 70% by mass or more, more preferably 80% by mass or more, still more preferably 100% by mass, assuming that the total mass of the polyolefin resin is 100% by mass. The polyethylene may be a single product, but is preferably a polyethylene mixture composed of two or more types 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. As for polyethylene, one type may be used alone, or two or more types may be used in combination. These can be appropriately selected according to the purpose of use.

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

(Solvent for film formation)
The film-forming solvent is not particularly limited as long as it is a substance that can be mixed with the polyolefin resin or a substance that can dissolve the polyolefin resin. As the film-forming solvent, either a liquid solvent or a solid solvent can be used. Examples of the liquid solvent include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, and mineral oil distillates having corresponding boiling points. In order to obtain a gel-like sheet having 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 lower, and examples of such a solid solvent include paraffin wax, ceryl alcohol, stearyl alcohol, dicyclohexylphthalate and the like. A liquid solvent and a solid solvent may be used in combination.

(Production method)
In the present invention, a polyolefin resin and a solvent for film formation are heated, melted and kneaded, and the obtained resin solution is extruded from a die and cooled to form an unstretched gel-like sheet, and the obtained unstretched gel-like sheet is obtained. After preheating, it is stretched in the vertical direction, which is the sheet transporting direction, and then stretched in the lateral direction, which is the direction perpendicular to the transporting direction, to obtain a biaxially stretched sheet, and the molding solvent is removed and dried to obtain a finely divided sheet. Obtain a porous film. In the case of the simultaneous stretching method in which stretching in the vertical direction and the horizontal direction is performed at the same time, it is difficult to provide a stepwise temperature difference during stretching in the vertical direction and the horizontal direction. Therefore, in order to achieve the object of the present invention, the sequential axial stretching method is used. Is preferable.

The method for producing a microporous polyolefin membrane will be specifically described by taking the sequential stretching method as an example.

(A) Step of obtaining unstretched gel-like sheet (mixing, kneading)
A polyolefin solution containing a polyolefin resin containing ultra-high molecular weight polyethylene in a proportion of 20% by weight or more and 80% by mass or less and a solvent for film formation is melt-kneaded to prepare a polyolefin solution. The method of melt-kneading the polyolefin resin and the film-forming solvent is not particularly limited, but it is preferably performed in a twin-screw extruder. The preferred range of the temperature of the polyolefin solution in the twin-screw extruder varies depending on the resin. For example, the temperature of the polyolefin solution in the polyethylene composition is 140 to 250 ° C., and the temperature of the polyolefin solution in the extruder should be measured by installing a thermometer inside the extruder or in the cylinder. Indirectly grasp with, and adjust the heater temperature, rotation speed, and discharge amount of the cylinder part as appropriate so that the target temperature is reached. The film-forming solvent may be added before the start of kneading, or may be added during the kneading. In melt-kneading, it is preferable to add an antioxidant in order to prevent oxidation of the polyolefin resin.

(Extruded and cast)
An unstretched gel-like sheet is formed by cooling the melted and kneaded polyolefin resin solution in the extruder. As a method for forming the unstretched gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

Cooling is preferably performed at a rate of 50 ° C./min or higher, at least up to the gelation temperature. Cooling is preferably performed up to 35 ° C. or lower. By cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallinity is maintained in 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, or the like can be used, but it is preferable to contact with a roll cooled with the refrigerant for cooling.

(B) Preheating Step In the preheating step, an unstretched gel-like sheet containing a film-forming solvent is gradually heated by passing a plurality of metal rolls until it reaches the longitudinal stretching temperature. Generally, it is preheated by passing it through a plurality of metal rolls, but a roll coated with ceramic or Teflon (registered trademark) may 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 physical characteristic spots 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. or higher and -1 ° C. or lower, preferably −12 ° C. or higher and -3 ° C. or lower, more preferably The temperature is -10 ° C or higher and -5 ° C or lower. The temperature obtained by subtracting the temperature of 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 undergoing the preheating process of the unstretched gel-like sheet, the pore diameter in the central portion in the film thickness direction can be made relatively smaller than that in the surface layer portion, and the dielectric breakdown voltage, which is difficult to achieve at the same time, can be improved while maintaining a small bending ratio. Can be done.

(C) Longitudinal Stretching Step In the longitudinal stretching step, the polymer chains of polyolefin are oriented in the stretching direction by stretching the unstretched gel-like sheet in the longitudinal direction. The longitudinal stretching step is divided into a stretching step and a heat fixing step, and the temperature is adjusted for each. The longitudinal stretching is performed by passing a plurality of metal rolls having different peripheral speeds. When the unstretched gel-like sheet comes into contact with the first stretched roll in the longitudinal stretching step after passing through the preheating part, the contact area and contact time with the first stretched roll are minimized, and the orientation of the polymer in the surface layer and the inner layer is adjusted. It is important to.

The time for the gel-like sheet to come into contact with the stretched roll that comes into contact first from the preheating step is preferably 3 seconds or less. It is more preferably 2 seconds or less, still more preferably 1.5 seconds or less. Within this range, when a film-forming solvent having a low thermal conductivity such as liquid paraffin is used, vertical stretching can be performed while maintaining a constant difference between the temperature of the central portion and the temperature of the surface layer portion in the film thickness direction.

It is preferable that the longitudinal stretching is performed in two or more steps from low magnification to high magnification in stages. Regarding the stretching ratio in the longitudinal stretching step, 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. Therefore, the initial stretching ratio is 1.1 times or more. It is preferably 2 times or less, more preferably 1.2 times or more and 2 times or less. The longitudinal stretching ratio of the second and subsequent stages is preferably 1.5 times or less of the longitudinal stretching ratio of the previous stage, and it is preferable to increase the number of stages to a desired stretching ratio. By doing so, breakage can be suppressed. The total longitudinal stretching ratio of 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 of each stage. With such a longitudinal stretching ratio, it is easy to obtain a balance between strength and pore size.

Pore opening is controlled by appropriately adjusting the stretching ratio while keeping the difference between the preheating temperature and the longitudinal stretching temperature of the longitudinal stretching within the above range, the curved path length of the microporous membrane is reduced, and the fine pore diameter is made uniform. Can be done. By doing so, a fibril structure having a relatively large pore diameter on the surface and a relatively small pore diameter in the central portion can be obtained, and both compressibility and high dielectric breakdown voltage can be achieved.

(D) Step of obtaining biaxially stretched sheet (transverse stretching step)
The longitudinally stretched sheet obtained in the longitudinally stretching step is stretched in the width direction.
In the transverse stretching, the obtained transverse stretching sheet is stretched at a magnification higher than the total longitudinal stretching magnification by a tenter device while grasping both ends of the obtained transverse stretching sheet with clips. Specifically, it is preferably stretched 6 times or more and 13 times or less, and more preferably 6.5 times or more and 12 times or less. The transverse stretching temperature is the temperature inside the tenter, and stretching is performed at a temperature 1 ° C to 10 ° C higher than the longitudinal stretching temperature. The magnification and temperature of longitudinal stretching and transverse stretching preferably satisfy the following formulas (a) and (b).
(A) Transverse stretching ratio ≥ Longitudinal stretching ratio (b) Transverse stretching temperature ≥ Vertical stretching temperature

By satisfying the formulas (a) and (b), the transverse stretching step of stretching the polymer oriented in the longitudinal direction by longitudinal stretching in the width direction is higher than the longitudinal stretching temperature and the stretching ratio is increased. Therefore, a high breakdown voltage can be maintained by making the thicknesses of the fibrils in the vertical direction and the horizontal direction uniform.

The difference between the transverse stretching temperature and the longitudinal stretching temperature of the formula (b) is represented by the transverse stretching temperature-longitudinal stretching temperature, preferably 1 ° C. or higher and 10 ° C. or lower, and more preferably 3 ° C. or higher and 6 ° C. or lower.

(E) Extraction Step of Solvent for Film Formation The biaxially stretched sheet thus obtained is extracted with a solvent for film formation using a washing solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is extracted, it is composed of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the above is obtained. A known method can be used as a cleaning solvent and a method for extracting a film-forming solvent using the same. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(Heat fixation)
The film from which the film-forming solvent is extracted is heat-treated in order to stabilize the crystals and make the lamella uniform.

As the heat treatment method, heat fixing treatment or heat relaxation treatment can be used. The heat fixing treatment is a heat treatment in which the film is heated while being held so that the dimensions of the film do not change. The heat relaxation treatment is a heat treatment in which the membrane is heat-shrinked in the vertical direction and the horizontal direction during heating. The heat fixing treatment is preferably performed by a tenter method or a roll method. For example, as a heat relaxation treatment method, the method disclosed in JP-A-2002-256099 can be mentioned. The heat fixing treatment or the heat relaxation treatment is performed by stretching at least in the uniaxial direction at a magnification of 1.1 times or more and 2 times or less to increase the strength of the microporous film. The heat treatment of the microporous membrane is preferably performed at a temperature 1 ° C. or higher and 20 ° C. or lower higher than the transverse stretching temperature.

(Turning process)
The obtained microporous membrane was wound around a winding core made of ABS having an outer diameter of 150 mm to 450 mm with a winding tension of 10 N to 60 N, and jumbo rolls were collected. The tension of the jumbo roll is preferably 10 N or more and 60 N or less, and more preferably 15 N or more and 55 N or less. If the take-up tension is too low, winding misalignment is likely to occur, and if the take-up tension is too high, wrinkles and film deformation are likely to occur due to winding tightening.

(Slit process)
The microporous membrane wound around the jumbo roll is slit to the desired length and width. In the slitting step, the slit may be performed from the jumbo roll to the required width and length with one slit, or the slit may be performed from the jumbo roll in two to four times. The number of slits is preferably 5 or less because 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 0.5N or more and 45N or less, still more preferably 1.0N or more and 40N or less. If the take-up tension is too low, winding misalignment is likely to occur, and if the take-up tension is too high, wrinkles and film deformation are likely to occur due to winding tightening.

(Other processes)
The obtained microporous membrane can be made into a laminated porous membrane by providing a layer other than the microporous layer, if necessary. Examples of the other layer include a porous layer formed by using a filler-containing resin solution containing a filler and a resin binder or a heat-resistant resin solution.

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

Hereinafter, each characteristic of the polyolefin microporous membrane of the present embodiment will be described.

(Film thickness (μm))
The film thickness can be measured with a measuring device having a display resolution of up to 0.01 μm. When the contact type is used, it is desirable to perform the measurement without crushing the pores of the microporous membrane, and the measuring force can be measured at 0.01 N or more and 0.15 N or less. The microporous polyolefin membrane is 16 μm or more and 30 μm or less, preferably 16 μm or more and 25 μm or less. In recent years, the output of batteries has been increased, and some batteries use a thin microporous film of 16 μm or less. However, if the output is less than 16 μm, the closer to the core in the manufacturing process, the closer the surface layer and the core are to the winding core. Although it is easy to reduce the difference in film thickness, coating is often applied to ensure the safety of the battery, and by setting the thickness to 16 μm or more, a separator that ensures safety can be obtained without performing a coating process. Can be done. Further, by setting the film thickness to 30 μm or less, the battery capacity can be secured, and the amount of change in thickness between the surface layer portion and the winding core portion of the wound body can be reduced.

(Vacancy rate (%))
The porosity is defined as the ratio of the volume of space to the total volume of the substance. Specifically, the film thickness and mass of the microporous film are measured, and the porosity is calculated using the value of the resin density. To do. The resin density 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 output of the battery is increased, the resistance of the membrane increases, and when a microporous membrane is used as the battery separator, good output characteristics cannot be obtained. Further, when the pore ratio is 60% or more, the resistance of the film is small and good output characteristics can be obtained, but the amount of change in film thickness may be large, and when the amount of change in film thickness is large, the cycle characteristics deteriorate. There is a fear.

The vacancy rate is more preferably 45% or more and 55% or less, and further preferably 48% or more and 53% or less.

(Amount of change in film thickness (%))
The amount of change in film thickness is the amount of change in core film thickness with respect to the surface layer film thickness, and is expressed by the following formula.
Film thickness change (%) = (surface film thickness-core film thickness) / surface 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 further preferably 0% or more and 0.6% or less.

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

(Turn rate)
The curvature ratio is the ratio of the 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. The bend ratio can be measured using a porosimeter. The porosimeter can be measured by either the non-mercury injection method or the mercury injection method, but it is desirable to use the non-mercury injection method that does not use harmful mercury.

The curve rate is preferably 1.00 or more and 1.42 or less. Good compressibility can be obtained when the bend ratio is low. More preferably, it is 1.05 or more and 1.40 or less, and further preferably 1.10 or more and 1.38 or less. If the curvature ratio exceeds 1.42, the compressibility is likely to deteriorate. The closer the curve ratio is to 1.00, the better the output characteristics, while the higher the curve ratio is, the more advantageous it is in terms of safety. Therefore, the above range is good from the viewpoint of safety. The curve rate is based on the press-fitting method.

(Dielectric 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 the 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 / μ m or less, and further preferably 165 V / μm or more and 280 V / μ m or less. When the dielectric breakdown voltage is 155 V / μm or less, pinholes are less likely to open in the microporous membrane even if a discharge is generated by static electricity. Further, when the dielectric breakdown voltage exceeds 300 V / μm, the average pore diameter of the microporous membrane becomes too small, and it becomes difficult for the electrolytic solution to permeate.

(Average pore size (nm))
The average pore size can be measured using a palm poromometer. Unlike the porosimeter, the palm poromometer can selectively measure the thinnest part of the pores. The average pore diameter determined from the palm poromometer of the polyolefin microporous membrane is preferably 20 nm or more and 30 nm or less, more preferably 22 nm or more and 28 nm or less, and further 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.

〔Measuring method〕

(Measurement of film thickness (μm))
The film thickness was cut out from an arbitrary position of the microporous membrane in a vertical direction of 110 cm and a horizontal direction of 6 cm to prepare a test piece. Twenty points of this test piece were measured in the vertical direction at intervals of 5 cm with a thickness contact thickness meter, and averaged to obtain the thickness of the test piece. As the thickness measuring machine, a Mitutoyo Lightmatic VL-50B (meter carbide spherical surface stylus φ9.5 mm) having a measuring force of 0.01 N was used. The measurement environment was within the range of 23 ± 2 ° C.

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

The wound body of the polyolefin microporous film was incised, the film thickness was measured at 20 points in the outer winding direction at intervals of 5 cm, starting from a point 10 m from the core, and the average value was taken as the core film thickness. The film thickness of the microporous polyolefin membrane was measured 5 minutes after being released from the wound state.
Film thickness change rate (%) = (surface film thickness-core film thickness) / surface film thickness x 100

(Measurement of pore ratio (%))
A 95 mm square sample was prepared, the sample volume (cm ³ ) and the sample mass (g) were measured, and the porosity (%) was calculated using the following formula from the obtained results. ^{As the density, 0.99 g / cm 3} measured according to JIS K 7112: 1999 was used.
Pore ratio = (1-sample mass / (resin density x sample volume)) x 100

(Measurement of air permeability resistance (sec / 100 cm ³⁾⁾
Using the digital type Oken type air permeability resistance tester EGO1 manufactured by Asahi Seiko Co., Ltd., the polyolefin laminated microporous membrane of the present invention was fixed to the measuring part so as not to cause wrinkles, and JIS P- Measured according to 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 permeability resistance [seconds] of the sample. The same measurement was performed on 10 test pieces collected from arbitrary film positions, and the average value of the 10 measured values was taken as the air permeation resistance of the polyolefin microporous membrane.

(Measurement of tensile strength (kPa) and tensile elongation (%))
The tensile strength corresponding to each direction is cut into a shape conforming to JIS K7127 with a width of 10 mm and test piece type 2, and the microporous membrane is not marked, and the chuck gap is 20 mm and the test speed is 100 mm / min. It was measured. The measurement environment was within the range of 23 ± 2 ° C.

(Measurement of curve rate)
The measurement was carried out with purified water using a pure water press-fit porosimeter (trade name, model: WIP-3k-A-1) manufactured by POROUS MATERIALS, INC.

The pore specific volume, specific surface area, porosity, and permeability coefficient of the microporous membrane are measured using a pure water press-fit porosimeter, and the curvature ratio is obtained from the formula (a) using the film thickness with the above contact thickness meter. Be done. The film thickness was cut out from an arbitrary position of the microporous membrane into a square having a longitudinal direction of 5 cm and a width direction of 5 cm to prepare a test piece. Arbitrary 5 points of this test piece were measured with a thickness contact thickness meter and averaged to obtain the thickness of the test piece. As the thickness measuring machine, a Mitutoyo Lightmatic VL-50B (meter carbide spherical surface stylus φ9.5 mm) having a measuring force of 0.01 N was used.

l _e : Average effective pore length l _d : Film thickness ε _{: Pore ratio V p} : Pore specific volume S _BET : Specific surface area k: Transmission coefficient

(Measurement of dielectric breakdown voltage (V / μm))
Using a DC withstand voltage tester manufactured by Kasuga Electric Co., Ltd., apply at 100 V / sec, measure the voltage at which the test piece undergoes dielectric breakdown 20 times, divide the average value by the film thickness measured with the above contact thickness meter, and per 1 μm. It was converted to the value of and used as the dielectric breakdown voltage.

The upper electrode used a 50 g brass cylinder with a diameter of 25 mm and a radius of 3 mm rounded at the edge, and the lower electrode used a copper flat plate. The microporous membrane was cut into 100 mm × 100 mm, and the lower electrode, the microporous membrane, and the upper electrode were arranged in this order for measurement.

(Measurement of average pore size (nm))
Using a palm polo meter (trade name, model: CFP-1500A) manufactured by POROUS MATERIALS, INC., The measurement was performed in the order of Dry-up and Wet-up. For Wet-up, pressure is applied to a microporous membrane sufficiently immersed in Galwick (trade name), which has a known surface tension, and the pore diameter converted from the pressure at which air begins to penetrate is defined as the bubble point pore diameter (maximum pore diameter). .. For the average pore diameter, the pore diameter was converted from the pressure at the intersection of the pressure and the half slope of the flow rate curve in the Dry-up measurement and the pressure at the intersection of the Wet-up measurement curve. The following formula was used to convert the pressure and pore diameter.
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 the gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by Waters Corporation
-Column: Showa Denko Corporation Shodex UT806M
-Column temperature: 135 ° C
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow rate: 1.0 ml / min-Sample concentration: 0.1 wt% (dissolution condition: 135 ° C./1 h)
-Injection amount: 500 μl
-Detector: Waters Corporation differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(Example 1)
Mw of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene (UHPE) 30% by weight and Mw of 5.6 × 10 ⁵ high density Porichiren (HDPE: density 0.955 g / cm ^3, melting point 135 ° C.) 70 wt% A mixture was prepared by adding 0.2 parts by mass of tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant to 100 parts by mass of a polyolefin resin composed of the above. .. 28.5 parts by mass of the obtained mixture was put into a strong kneading type twin-screw extruder, and 71.5 parts by mass of liquid paraffin [35 cSt (40 ° C.)] was supplied from the side feeder of the twin-screw extruder to 230 ° C. And 250 rpm, melt-kneading was performed to prepare a polyolefin resin solution.

The polyolefin resin solution was supplied from a twin-screw extruder to a T-die, and the extruded product was taken up by a cooling roll whose temperature was adjusted to 30 ° C. and cooled while being taken up to form an unstretched gel-like sheet.

The unstretched gel-like sheet is passed through a preheating roll at 113 ° C., and a vertically stretched roll having a diameter of 200 mm is used at 119 ° C. where the surface temperature is 6 ° C. higher than the preheating temperature. It was divided into three stages of 1.3 times, 1.8 times, and 2.4 times in the direction, and stretched at a total longitudinal stretching ratio of 5.6 times. Then, three cooling rolls were passed and the sheet was cooled to 50 ° C. to form a vertically stretched sheet. At this time, the transport speed was adjusted so that the time during which the unstretched gel-like sheet was in contact with the vertically stretched roll that was first contacted from the preheating step was 2.0 seconds so that the temperature did not rise to the inside.

Both ends of the obtained vertically stretched sheet were gripped with clips and stretched 8.9 times in the lateral direction with 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 residual liquid paraffin, and dried.

The obtained biaxially stretched sheet after drying is heated to 130 ° C. by a tenter type stretching machine, re-stretched so as to be 1.44 times the inlet width of the stretching machine, and then the entrance width of the re-stretching device. The heat treatment was carried out after adjusting the lateral magnification to 1.31 to obtain a polyolefin microporous film having a thickness of 19.5 μm. The winding tension of the slit was 4N to a width of 60 mm, and the slit was wound around a resin core having an outer diameter of 203 mm to obtain a wound body of a battery separator having an outer diameter of 303 mm.

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

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

(Example 4)
Example 1 except that the unstretched gel-like sheet of Example 1 was passed through a preheating roll at 115 ° C. and stretched in the longitudinal direction using a vertically stretched roll having a diameter of 200 mm at 119 ° C., the surface temperature of which is 4 ° C. higher than the preheating temperature. In the same manner as above, a polyolefin microporous film having a thickness of 19.5 μm was obtained. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Example 5)
Example 1 except that the unstretched gel-like sheet of Example 1 was passed through a preheating roll at 109 ° C. and stretched in the longitudinal direction using a vertically stretched roll having a diameter of 200 mm at 122 ° C. where the surface temperature was 13 ° C. higher than the preheating temperature. In the same manner as above, a polyolefin microporous film having a thickness of 19.6 μm was obtained. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Example 6)
A polyolefin microporous film having a thickness of 19.4 μm was obtained in the same manner as in Example 1 except that the vertically stretched sheet of Example 1 was stretched 8.9 times in the lateral direction by a tenter device set at 120 ° C. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Example 7)
The unstretched gel sheet of Example 1 is passed through a preheating roll at 113 ° C., and a longitudinally stretched roll having a diameter of 150 mm at 119 ° C., whose surface temperature is 6 ° C. higher than the preheating temperature, is used to form a longitudinally stretched roll that comes into contact first from the preheating step. The transport speed is adjusted so that the contact time of the unstretched gel-like sheet is 1.0 second, and the total length is divided into three stages of 1.3 times, 1.8 times, and 2.4 times in the vertical direction. It was stretched at a stretching ratio of 5.62 times. A polyolefin microporous film having a thickness of 19.4 μm was obtained in the same manner as in Example 1 except for the above. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Example 8)
The unstretched gel sheet of Example 1 is passed through a preheating roll at 113 ° C., and a longitudinally stretched roll having a diameter of 250 mm at 119 ° C., whose surface temperature is 6 ° C. higher than the preheating temperature, is used to form a longitudinally stretched roll that comes into contact first from the preheating step. The transport speed is adjusted so that the contact time of the unstretched gel-like sheet is 3.0 seconds, and the total length is divided into three stages of 1.3 times, 1.8 times, and 2.4 times in the vertical direction. It was stretched at a stretching ratio of 5.62 times. A polyolefin microporous film having a thickness of 19.5 μm was obtained in the same manner as in Example 1 except for the above. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Example 9)
The same polyolefin resin solution as in Example 1 was supplied from a twin-screw extruder to a T-die, and the extrusion-formed body was taken up by a cooling roll whose temperature was adjusted to 30 ° C. and cooled while being taken up to form an unstretched gel-like sheet. The obtained unstretched gel-like sheet was divided into three stages of 1.8 times, 1.8 times, and 2.4 times in the longitudinal direction, and stretched at a total longitudinal stretching ratio of 7.8 times. A polyolefin microporous film having a thickness of 20.0 μm was obtained in the same manner as in Example 1 except that the unstretched gel-like sheet had 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 is supplied from a twin-screw extruder to a T-die, and the extruded body is taken up by a cooling roll whose temperature is adjusted to 30 ° C. and cooled while being taken up so that the thickness becomes 80% of that of Example 1. An unstretched gel-like sheet was formed in. A polyolefin microporous film 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 supplied from a twin-screw extruder to a T-die, and the extruded product was taken up by a cooling roll whose temperature was adjusted to 30 ° C. and cooled while being taken up to form an unstretched gel-like sheet. The obtained unstretched gel-like sheet was simultaneously stretched 5 times in the vertical direction and 5 times in the horizontal direction with 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 carried out to obtain a polyolefin microporous film 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)
Mw of 2.0 × 10 ⁶ of the ultra high molecular weight polyethylene 18% by weight and Mw of 5.6 × 10 ⁵ high density Porichiren (density 0.955 g / cm ^3, melting point 135 ° C.) a polyolefin resin 100 consisting of 82 wt% A mixture was prepared by adding 0.2 parts by mass of tetrakis [methylene-3- (3,5-ditercious butyl-4-hydroxyphenyl) -propionate] methane as an antioxidant to the parts by mass. 30 parts by mass of the obtained mixture was put into a strong kneading type twin-screw dispenser, 70 parts by mass of liquid paraffin [35 cSt (40 ° C.)] was supplied from the side feeder of the twin-screw extruder, and the conditions were 230 ° C. and 250 rpm. In the same manner as in Example 1 except that a polyolefin resin solution was prepared, and in the same manner as in Example 1 except that a mixture was prepared, a polyolefin microporous film having a thickness of 19.2 μm was obtained. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 3)
In order to pass the unstretched gel-like sheet of Example 1 through a preheating roll at 119 ° C. and use a vertically stretched roll having a diameter of 300 mm at 119 ° C., which is the same as the preheating temperature, so that the inside is sufficiently heated. The transport speed was adjusted so that the time for the unstretched gel-like sheet to contact the vertically stretched roll that was first contacted from the preheating step was 6.0 seconds, and the transport speed was adjusted to be 1.3 times, 1.8 times, and 2. It was divided into 4 times and 3 stages, and stretched at a total magnification of 5.62 times. A polyolefin microporous film having a thickness of 19.5 μm was obtained in the same manner as in Example 1 except for the above. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

(Comparative Example 4)
The unstretched gel sheet of Example 1 was passed through a preheating roll at 113 ° C., and the surface temperature was 125 ° C., which is 12 ° C. higher than the preheating temperature, 1.3 times, 1.8 times, 2.4 times and 3 in the vertical direction. It was divided into stages and stretched at a total magnification of 5.62 times. Then, three cooling rolls were passed and the sheet was cooled to 50 ° C. to form a vertically stretched sheet.

Both ends of the obtained longitudinally stretched sheet are gripped with clips and stretched 8.9 times in the transverse direction with a tenter device set to 113 ° C., which is 12 ° C. lower than the longitudinally stretched temperature so that the longitudinally stretched temperature ≥ the transversely stretched temperature. Then, a biaxially stretched sheet was obtained. A polyolefin microporous film having a thickness of 19.4 μm was obtained in the same manner as in Example 1 except for the above. Using this polyolefin microporous membrane, a wound body was obtained in the same manner as in Example 1.

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

Both ends of the obtained vertically stretched sheet are gripped with clips and stretched 6.8 times in the lateral direction with a tenter device set at 125 ° C. so that the longitudinal stretching ratio ≥ the transverse stretching ratio to obtain a biaxially stretched sheet. It was. After obtaining the biaxially stretched sheet, a polyolefin microporous film 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 is supplied from a twin-screw extruder to a T-die, and the extrusion-formed body is taken up by a cooling roll whose temperature is adjusted to 30 ° C. and cooled while being taken up so that the thickness becomes 60% of that of Example 1. An unstretched gel-like sheet was formed in.

An unstretched gel sheet is passed through a preheating roll at 113 ° C., and a vertically stretched roll having a surface temperature of 120 ° C., which is 7 ° C. higher than the preheating temperature, and a diameter of 200 mm is used. It was divided into three stages of 1.8 times, 1.8 times, and 2.4 times in the direction, and stretched at a total longitudinal stretching ratio of 7.8 times. Then, three cooling rolls were passed and the sheet was cooled to 50 ° C. to form a vertically stretched sheet. At this time, the transport speed was adjusted so that the time for the unstretched gel-like sheet to contact the vertically stretched roll that was first contacted from the preheating step was 2.0 seconds.

Both ends of the obtained vertically stretched sheet were gripped with clips and stretched 8.9 times in the lateral direction with 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 residual liquid paraffin, and dried.

The obtained biaxially stretched sheet after drying is heated to 130 ° C. by a tenter type stretching machine, re-stretched so as to be 1.44 times the inlet width of the stretching machine, and then the entrance width of the re-stretching device. The heat treatment was carried out after adjusting the lateral magnification to 1.31 to obtain a polyolefin microporous film having a thickness of 11.9 μm. The winding tension of the slit was 3.5 N to a width of 60 mm, and the slit was wound around a resin winding core having an outer diameter of 203 mm to obtain a wound body of a battery separator having an outer diameter of 268 mm.

Tables 1 to 3 show the blending ratios, production conditions, evaluation results, etc. of each component of the polyolefin microporous membranes obtained in Examples 1 to 10 and Comparative Examples 1 to 6.

Claims

The thickness is 16 μm or more and 30 μm or less, the pore ratio is 40% or more and 60% or less, the bending ratio is 1.00 or more and 1.42 or less, and the dielectric breakdown voltage is 155 V / μm or more and 300 V / μm or less. A polyolefin microporous membrane characterized by being.
The polyolefin microporous membrane according to claim 1, wherein the average pore diameter is 20 nm or more and 30 nm or less.
A wound body obtained by winding the polyolefin microporous membrane according to claim 1 or 2 to an outer diameter of 280 mm or more.
The method for producing a polyolefin microporous film according to claim 1 or 2, which comprises the steps (a) to (f). (A) A ratio of 20% by mass or more and 80% by mass or less of ultra-high molecular weight polyethylene. A step of melt-kneading and extruding a resin solution containing a polyolefin resin contained in the above and a solvent for forming a film to obtain an unstretched gel-like sheet.
(B) A preheating step of preheating the unstretched gel sheet at 95 ° C. or higher and 115 ° C. or lower.
(C) A longitudinal stretching step of obtaining a longitudinally stretched sheet by gradually raising the temperature to a temperature higher than the temperature of the preheating step in the range of 1 ° C. or higher and 15 ° C. or lower and stretching in the sheet transport direction.
(D) A step of laterally stretching the longitudinally stretched sheet in the sheet width direction at a temperature higher than the temperature of the longitudinally stretching step in the range of 1 ° C. or higher and 20 ° C. or lower at a stretching ratio of the longitudinal stretching step or higher to obtain a biaxially stretched sheet. ,
(E) An extraction step of extracting a film-forming solvent from the biaxially stretched sheet.
(F) A step of stretching the biaxially stretched sheet in the uniaxial direction after the extraction step.