Polyethylene microporous film, wound body, and method for manufacturing wound body

Abstract

This polyethylene microporous film has a film thickness of 50-200 μm, a variation factor of 0.090 or less for the film thickness in the TD direction, and a porosity of 40-70%.

Description

Polyethylene microporous membrane, rolled body, and method for manufacturing the rolled body

[0001] The present disclosure relates to a polyethylene microporous membrane, a roll, and a method for producing the roll.

[0002] Porous membranes such as polyolefins are widely used as reinforcing materials for ion exchange membranes and the like. Using a porous membrane as a reinforcing material is useful for improving the membrane strength and shape stability of ion exchange membranes and the like. The membrane thickness of the reinforcing material varies depending on the application. For example, in redox flow battery or fuel cell applications, low resistance products are preferred, so thin membranes tend to be used. On the other hand, high strength is preferred in water treatment such as desalination, so thick membranes tend to be used. Furthermore, in the thick membrane range, from the perspective of mechanical strength, microporous membranes produced by a phase separation method are preferred rather than high porosity membranes such as nonwoven fabrics. For example, Patent Document 1 discloses an ion exchange membrane in which ion exchange resin is filled into the pores of a porous substrate film produced by a phase separation method.

[0003] Patent Document 1: Patent No. 6517404

[0004] However, in thick polyolefin microporous membranes produced by a phase separation method, unless the variation in thickness in the width direction (TD) of the polyolefin microporous membrane is well controlled, wrinkles and protrusions are generated during the process of winding the polyolefin microporous membrane around a core, resulting in poor appearance of the wound product (wound body). Polyethylene is particularly softer than polypropylene and is more likely to cause poor appearance during the winding process. The present disclosure has been made in view of the above-mentioned conventional circumstances, and an object of one aspect of the present disclosure is to provide a polyethylene microporous membrane that suppresses the occurrence of poor appearance when obtaining a wound body and has high strength. Another object of the present disclosure is to provide a wound body using this polyethylene microporous membrane and a method for producing the wound body.

[0005] Specific means for achieving the above object are as follows. <1> A polyethylene microporous membrane having a membrane thickness of 50 μm to 200 μm, a coefficient of variation of membrane thickness in the TD of 0.090 or less, and a porosity of 40% to 70%. <2> The polyethylene microporous membrane of <1>, having a coefficient of variation of membrane thickness in the TD of 0.050 or less. <3> The polyethylene microporous membrane of <1> or <2>, having a pore size of 20 nm to 100 nm. <4> The polyethylene microporous membrane of any one of <1> to <3>, used as a reinforcing material. <5> The polyethylene microporous membrane of any one of <1> to <4>, used as a substrate for an ion exchange membrane. <6> A wound body comprising a winding core and the polyethylene microporous membrane of any one of <1> to <5> wound around the winding core, wherein the length of the polyethylene microporous membrane is 100 m or more. <7> The wound body according to <6>, wherein the width of the polyethylene microporous membrane is 500 mm or more. <8> The wound body according to <6> or <7>, wherein the length of the polyethylene microporous membrane is 300 m or more. <9> A method for producing a wound body, comprising the steps of producing the polyethylene microporous membrane according to any one of <1> to <5>, and winding the polyethylene microporous membrane around a winding core. <10> The method for producing a wound body according to <9>, wherein in the winding step, the polyethylene microporous membrane is wound around the winding core by gap winding, in which a touch roll is separated from the polyethylene microporous membrane.

[0006] According to one aspect of the present disclosure, it is possible to provide a polyethylene microporous membrane that suppresses the occurrence of appearance defects when a roll is obtained and has high strength. Furthermore, according to another aspect of the present disclosure, it is possible to provide a roll using the polyethylene microporous membrane and a method for producing the roll.

[0007] 1A and 1B are schematic diagrams illustrating a winding device 1 of a first embodiment for winding a polyethylene microporous membrane onto a winding core, and a winding device 2 of a second embodiment for winding a polyethylene microporous membrane onto a winding core.

[0008] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0009] In the present disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another staged numerical range. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit of that numerical range may be replaced with a value shown in the Examples. In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in a composition, the content or amount of each component refers to the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, the terms "layer" and "film" include cases where the layer or film is formed over the entire region when the region in which the layer or film is present is observed, as well as cases where the layer or film is formed only over a portion of the region.

[0010] In this disclosure, with respect to a polyethylene microporous membrane, "longitudinal direction" means the longitudinal direction of a polyethylene microporous membrane produced in a long shape, and "width direction" means the direction perpendicular to the longitudinal direction of the polyethylene microporous membrane. Hereinafter, "width direction" is also referred to as "TD," and "longitudinal direction" is also referred to as "MD." In this disclosure, the "length" of a polyethylene microporous membrane refers to the length of the polyethylene microporous membrane in the longitudinal direction. The "width" of a polyethylene microporous membrane refers to the length of the polyethylene microporous membrane in the width direction.

[0011] <Polyethylene microporous membrane> The polyethylene microporous membrane of the present disclosure has a membrane thickness of 50 μm to 200 μm, a coefficient of variation of membrane thickness in the TD of 0.090 or less, and a porosity of 40% to 70%. The polyethylene microporous membrane of the present disclosure suppresses the occurrence of defective appearance when a wound body is obtained, and has high strength. The reason for this is not clear, but is presumed to be as follows. Setting the membrane thickness of the polyethylene microporous membrane to 50 μm or more tends to ensure the strength of the polyethylene microporous membrane. On the other hand, setting the membrane thickness of the polyethylene microporous membrane to 200 μm or less tends to facilitate winding the polyethylene microporous membrane around a winding core in the step of winding the polyethylene microporous membrane around a winding core, and tends to suppress the occurrence of defective appearance. Also, permeability tends to be ensured, and impregnation of components such as ion exchange resins tends to be ensured. Furthermore, setting the porosity of the polyethylene microporous membrane to 40% or more tends to facilitate impregnation of components such as ion exchange resins into the pores. On the other hand, by setting the porosity of the polyethylene microporous membrane to 70% or less, the strength of the polyethylene microporous membrane tends to be ensured. Furthermore, by setting the coefficient of variation of the membrane thickness in the TD of the polyethylene microporous membrane to 0.090 or less, the uniformity of the membrane thickness is ensured, and the occurrence of defective appearance tends to be suppressed in the step of winding the polyethylene microporous membrane around a core. From the above, it is presumed that the polyethylene microporous membrane of the present disclosure suppresses the occurrence of defective appearance when obtaining a wound body and has high strength.

[0012] A polyethylene microporous membrane is a microporous membrane containing polyethylene. Here, a microporous membrane refers to a membrane having numerous micropores therein that are interconnected, allowing gas or liquid to pass from one surface to the other.

[0013] The resins constituting the polyethylene microporous membrane of the present disclosure, its physical properties, and the like will be described in detail below.

[0014] (Thickness) The thickness of the polyethylene microporous membrane of the present disclosure is 50 μm to 200 μm. From the viewpoint of strength, the thickness of the polyethylene microporous membrane is preferably 55 μm or more, more preferably 60 μm or more, and even more preferably 70 μm or more. From the viewpoint of suppressing the occurrence of defective appearance, the thickness of the polyethylene microporous membrane is preferably 195 μm or less, more preferably 190 μm or less, and even more preferably 180 μm or less. Furthermore, when the thickness of the polyethylene microporous membrane is 200 μm or less, the air permeability is unlikely to increase and the permeability is unlikely to deteriorate, which tends to ensure practical use as a reinforcing material.

[0015] The polyethylene microporous membrane of the present disclosure has a coefficient of variation of membrane thickness in the TD of 0.090 or less, preferably 0.080 or less, more preferably 0.070 or less, and even more preferably 0.050 or less. The coefficient of variation of membrane thickness in the TD may be 0.0015 or more, preferably 0.015 or more, and more preferably 0.020 or more. The coefficient of variation of membrane thickness in the TD is preferably 0.0015 to 0.090.

[0016] In the present disclosure, the thickness and coefficient of variation of thickness in the TD direction of the polyethylene microporous membrane refer to values ​​determined by the method described in the Examples section.

[0017] (Porosity) In the present disclosure, the porosity of the polyethylene microporous membrane is 40% to 70%. From the viewpoint of impregnation of components such as ion exchange resin into the pores, the porosity of the polyethylene microporous membrane is preferably 42% or more, more preferably 45% or more, and even more preferably 48% or more. On the other hand, from the viewpoint of strength, the porosity of the polyethylene microporous membrane is preferably 68% or less, more preferably 65% ​​or less, and even more preferably 60% or less.

[0018] In the present disclosure, the porosity of a polyethylene microporous membrane refers to the value determined by the method described in the Examples section.

[0019] (Pore size) In the present disclosure, the polyethylene microporous membrane preferably has a pore size of 20 nm to 100 nm. When the pore size of the polyethylene microporous membrane is 20 nm or more, the impregnation of components such as ion exchange resins into the pores tends to be improved. The pore size of the polyethylene microporous membrane is more preferably 25 nm or more, and even more preferably 30 nm or more. On the other hand, when the pore size of the polyethylene microporous membrane is 100 nm or less, the strength tends to be improved. The pore size of the polyethylene microporous membrane is more preferably 90 nm or less, and even more preferably 80 nm or less.

[0020] In this disclosure, the pore size of a polyethylene microporous membrane refers to the value determined by the method described in the Examples section.

[0021] (Basis Weight) In the present disclosure, the basis weight of the polyethylene microporous membrane is preferably set so that the porosity and membrane thickness fall within the suitable ranges, and is preferably 20 g / m ２ ~100g / m ２ In the present disclosure, the basis weight of the polyethylene microporous membrane refers to the value determined by the method described in the Examples section.

[0022] (Pin puncture strength) From the viewpoint of strength, the pin puncture strength of the polyethylene microporous membrane of the present disclosure is preferably 500 gf or more, more preferably 550 gf or more, and even more preferably 600 gf or more. The higher the pin puncture strength of the polyethylene microporous membrane of the present disclosure, the better, but from the viewpoint of handleability, it may be 1500 gf or less. The pin puncture strength of the polyethylene microporous membrane of the present disclosure is preferably 500 gf to 1500 gf. In the present disclosure, the pin puncture strength of the polyethylene microporous membrane refers to the value determined by the method described in the Examples section.

[0023] (Water resistance) From the viewpoint of strength, the water resistance of the polyethylene microporous membrane of the present disclosure is preferably 5000 mm or more, more preferably 5500 mm or more, and even more preferably 6000 mm or more. The higher the water resistance of the polyethylene microporous membrane of the present disclosure, the more preferable. In the present disclosure, the water resistance of the polyethylene microporous membrane refers to the value determined by the method described in the Examples section.

[0024] (Air permeability) The air permeability (Gurley value) of the polyethylene microporous membrane of the present disclosure is preferably 100 sec / 100 mL or more, more preferably 150 sec / 100 mL or more, and even more preferably 200 sec / 100 mL or more. The air permeability of the polyethylene microporous membrane of the present disclosure may be 1000 sec / 100 mL or less, from the viewpoint of impregnation of components such as ion exchange resins into the pores. The air permeability of the polyethylene microporous membrane of the present disclosure is preferably 100 sec / 100 mL to 1000 sec / 100 mL. In the present disclosure, the air permeability of the polyethylene microporous membrane refers to a value determined by the method described in the Examples section.

[0025] (Constituent components of polyethylene microporous membrane) The polyethylene microporous membrane of the present disclosure may be composed of only polyethylene, or may be composed of polyethylene and a material other than polyethylene. Of the resin components constituting the polyethylene microporous membrane, the polyethylene content is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, particularly preferably 90% by mass or more, and extremely preferably 99% by mass or more. The polyethylene microporous membrane may contain only one type of polyethylene, or two or more types with different molecular weights, etc.

[0026] When the polyethylene microporous membrane contains a resin component other than polyethylene, examples of the other resin component include polypropylene, polybutylene, polymethylpentene, and copolymers thereof.

[0027] The polyethylene microporous membrane may contain ultra-high molecular weight polyethylene (UHMWPE) as the polyethylene. When the polyethylene microporous membrane contains ultra-high molecular weight polyethylene, the pore size of the polyethylene microporous membrane tends to be not too large and the strength tends to be excellent.

[0028] When the polyethylene microporous membrane contains ultra-high molecular weight polyethylene, the proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is preferably 50% by mass or less. When the proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is 50% by mass or less, the pore size of the polyethylene microporous membrane tends to be not too small and the permeability performance tends to be excellent. The proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is preferably 48% by mass or less, more preferably 45% by mass or less. Furthermore, when the polyethylene microporous membrane contains ultra-high molecular weight polyethylene, the proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is preferably 1% by mass or more. When the proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is 1% by mass or more, the mechanical strength of the polyethylene microporous membrane is easily increased. The proportion of ultra-high molecular weight polyethylene in the resin components constituting the polyethylene microporous membrane is more preferably 3% by mass or more, even more preferably 5% by mass or more. When the polyethylene microporous membrane contains ultra-high molecular weight polyethylene, the proportion of the ultra-high molecular weight polyethylene in the resin component constituting the polyethylene microporous membrane is preferably 1% by mass to 50% by mass.

[0029] When the polyethylene microporous membrane contains ultra-high molecular weight polyethylene and a polyolefin other than ultra-high molecular weight polyethylene (hereinafter also referred to as other polyolefin) as polyethylene, the type of the other polyolefin is not particularly limited. As the other polyolefin, high-density polyethylene (HDPE) is preferred. In the present disclosure, high-density polyethylene refers to polyethylene having a density of 942 kg / m ３ The above polyethylene refers to polyethylene of the above type.

[0030] In the present disclosure, ultra-high molecular weight polyethylene refers to polyethylene having a weight-average molecular weight of 3 million to 6 million. The weight-average molecular weight of the ultra-high molecular weight polyethylene is preferably 3.5 million or more, and more preferably 4 million or more. The weight-average molecular weight of the ultra-high molecular weight polyethylene is preferably 5 million or less, and more preferably 4.8 million or less.

[0031] In the present disclosure, the weight-average molecular weight of polyethylene is measured by gel permeation chromatography. Specifically, the polyethylene to be measured is heated and dissolved in o-dichlorobenzene, and measurement is performed using gel permeation chromatography (system: Alliance GPC 2000 manufactured by Waters Corporation, columns: GMH6-HT and GMH6-HTL) at a column temperature of 135°C and a flow rate of 1.0 mL / min. Monodisperse polystyrene (manufactured by Tosoh Corporation) is used to calibrate the molecular weight.

[0032] The polyethylene microporous membrane may contain additives such as organic fillers, inorganic fillers, and surfactants as materials other than the resin component, as needed, to the extent that the effects of the present disclosure are not affected.

[0033] (Uses of polyethylene microporous membrane) The uses of the polyethylene microporous membrane are not particularly limited. Specific uses include air filters, liquid filters, moisture-permeable waterproof membranes, bags, dust-collection sheet substrates, and ion-exchange membrane substrates.

[0034] <Wound body and method for manufacturing the wound body> The wound body of the present disclosure includes a winding core and the polyethylene microporous membrane of the present disclosure wound around the winding core, wherein the length of the polyethylene microporous membrane is 100 m or more. The polyethylene microporous membrane of the present disclosure has a thickness variation coefficient in the TD of 0.090 or less, which tends to ensure uniformity in the thickness of the polyethylene microporous membrane. Furthermore, the polyethylene microporous membrane of the present disclosure has a thickness of 200 μm or less, which tends to facilitate winding of the polyethylene microporous membrane around the winding core in the step of winding the polyethylene microporous membrane around the winding core. Furthermore, the polyethylene microporous membrane of the present disclosure has a porosity of 70% or less, which tends to ensure the strength of the polyethylene microporous membrane. From the above, it is presumed that the present disclosure can provide a wound body of polyethylene microporous membrane that suppresses the occurrence of appearance defects and has high strength, even when the polyethylene microporous membrane is wound around a winding core to a length of 100 m or more.

[0035] In the roll of the present disclosure, the length of the polyethylene microporous membrane may be 300 m or more, or 500 m or more. In the roll of the present disclosure, the length of the polyethylene microporous membrane may be 1000 m or less.

[0036] The width of the polyethylene microporous membrane in the wound body of the present disclosure is not particularly limited, and is preferably 500 mm or more, more preferably 600 mm or more. The width of the polyethylene microporous membrane may be 1500 mm or less. Generally, as the width of the polyethylene microporous membrane increases, the polyethylene microporous membrane tends to be more prone to have poor appearance when wound around a core. The wound body of the present disclosure obtained by winding the polyethylene microporous membrane of the present disclosure around a core tends to suppress the occurrence of poor appearance, even when the width of the polyethylene microporous membrane is 500 mm or more.

[0037] The method for producing a roll of the present disclosure includes a step of producing the polyethylene microporous membrane of the present disclosure (hereinafter sometimes referred to as the production step) and a step of winding the polyethylene microporous membrane (hereinafter sometimes referred to as the winding step), and may include other steps such as a step of packaging the roll, as necessary. Each step constituting the method for producing a roll of the present disclosure is described below.

[0038] (Production process) The production process is not particularly limited as long as it is a process that can produce the polyethylene microporous membrane of the present disclosure. The production process may include the following steps (I) to (IV):

[0039] Step (I): A step of preparing a solution containing polyethylene and a solvent. Step (II): A step of melt-kneading the solution, extruding the resulting melt-kneaded product through a die, and cooling and solidifying it to obtain a first gel-like molded product. Step (III): A step of stretching the first gel-like molded product in at least one direction (primary stretching) and drying the solvent to obtain a second gel-like molded product. Step (IV): A step of stretching the second gel-like molded product in at least one direction (secondary stretching).

[0040] Step (I) is a step of preparing a solution containing polyethylene and a solvent, and further containing other components as needed. The solvent can be a non-volatile solvent having a boiling point of 210°C or higher at atmospheric pressure, or a volatile solvent having a boiling point of less than 210°C at atmospheric pressure. Examples of solvents used in preparing the solution include non-volatile solvents such as liquid paraffin, paraffin oil, mineral oil, and castor oil, and volatile solvents such as tetralin, ethylene glycol, decalin, toluene, xylene, diethyltriamine, ethylenediamine, dimethyl sulfoxide, and hexane, with liquid paraffin, decalin, and xylene being particularly preferred. The volatile solvent may be used alone or in combination of two or more. Among these, decalin and xylene are preferred.

[0041] The polyethylene used in step (I) may be one type or two or more types, and can be selected depending on the desired physical properties of the polyethylene microporous membrane, etc. Other components include resins other than polyethylene, the above-mentioned additives, etc.

[0042] From the viewpoint of controlling the porous structure of the polyethylene microporous membrane, the solution prepared in step (I) preferably has a polyolefin concentration of 10% to 35% by mass, more preferably 15% to 32% by mass, and even more preferably 25% to 30% by mass. A polyolefin concentration of 10% by mass or more in the solution can suppress the occurrence of breakage in the polyethylene microporous membrane production process, and also increases the mechanical strength of the polyethylene microporous membrane, improving handleability. A polyolefin concentration of 35% by mass or less in the solution makes it easier to obtain the polyethylene microporous membrane of the present disclosure.

[0043] Step (II) is a step of melt-kneading the solution prepared in step (I), extruding the resulting melt-kneaded mixture through a die, and cooling and solidifying it to obtain a first gel-like molded product. In step (II), for example, extrusion through a die is performed at a temperature range from the melting point of polyethylene to the melting point + 65°C to obtain an extrudate, which is then cooled to obtain a first gel-like molded product. The first gel-like molded product is preferably shaped into a sheet. The cooling method is not particularly limited. For example, cooling may be performed by immersion in water or an organic solvent, contact with a cooled metal roll, or the like. Note that fluctuations in film thickness in the TD direction can be suppressed by adjusting the clearance (opening) at each position in the TD direction of the die.

[0044] Step (III) is a step of stretching the first gel-like molded product in at least one direction (primary stretching) and drying the solvent to obtain a second gel-like molded product. The stretching step in step (III) may be either uniaxial stretching or biaxial stretching. Biaxial stretching may be sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed separately, or simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously. From the viewpoint of controlling the porous structure of the polyethylene microporous membrane, the stretching ratio in the primary stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 1.1 to 3 times, more preferably 1.1 to 2.5 times. The temperature during primary stretching is preferably 120°C or lower. Drying of the solvent in step (III) (drying step) is preferably performed at a temperature at which the second gel-like molded product does not deform, more preferably 80°C or lower.

[0045] The stretching step and the drying step in step (III) may be carried out simultaneously or stepwise. For example, the first stretching may be carried out while pre-drying and then main drying, or the first stretching may be carried out between pre-drying and main drying. The first stretching may also be carried out in a state where the drying is controlled and the solvent remains in a suitable state.

[0046] Step (IV) is a step of stretching the second gel-like molded product in at least one direction (secondary stretching). The stretching step in step (IV) may be uniaxial stretching or biaxial stretching. The biaxial stretching may be any of the following: sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed separately; simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously; a step of stretching in the longitudinal direction multiple times and then stretching in the transverse direction; a step of stretching in the longitudinal direction and then stretching in the transverse direction multiple times; or a step of sequential biaxial stretching and then further stretching in the longitudinal and / or transverse directions once or multiple times.

[0047] From the viewpoint of controlling the porous structure of the polyethylene microporous membrane, the stretching ratio in the second stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 4 to 30. From the viewpoint of controlling the porous structure of the polyethylene microporous membrane, the stretching temperature in the second stretching is preferably 70°C to 135°C, more preferably 80°C to 130°C.

[0048] If necessary, heat setting may be performed after step (IV). The heat setting temperature is preferably 110°C to 150°C, more preferably 120°C to 140°C, from the viewpoint of controlling the porous structure of the polyethylene microporous membrane.

[0049] If necessary, the heat setting may be followed by an extraction treatment of the solvent remaining in the polyethylene microporous membrane and an annealing treatment. The extraction treatment of the residual solvent is carried out, for example, by immersing the heat-set sheet in a methylene chloride bath to dissolve the residual solvent in the methylene chloride. After the polyethylene microporous membrane immersed in the methylene chloride bath is removed from the methylene chloride bath, the methylene chloride is preferably removed by drying. The annealing treatment can be carried out after the extraction treatment of the residual solvent by transporting the polyethylene microporous membrane over rollers heated to, for example, 70°C to 140°C, or by transporting the polyethylene microporous membrane in a heated atmosphere at 70°C to 140°C while maintaining a constant width dimension.

[0050] (Winding process) The winding process is a process of winding the polyethylene microporous membrane of the present disclosure produced through the above-described production process onto a winding core. The winding process will be described below with reference to the drawings, but the present disclosure is not limited thereto. Furthermore, the size of each part in each drawing is conceptual, and the relative relationship between the sizes of each part is not limited thereto. In the following description, the same or equivalent parts will be denoted by the same reference numerals, and duplicate descriptions may be omitted.

[0051] Figure 1 is a schematic diagram of a winding device 1 of a first embodiment for winding a polyethylene microporous membrane around a winding core. The winding device 1 is a winding device for winding a polyethylene microporous membrane 10 around a winding core 12 by touch winding to obtain a wound body 14. The winding device 1 includes a winding core 12 that is rotated in the direction of the arrow in Figure 1 by a rotation drive device (not shown), and a touch roll 16. The touch roll 16 is rotatably supported by a position adjustment device (not shown) so that the touch roll 16 contacts the polyethylene microporous membrane 10 wound around the winding core 12 with a predetermined pressure.

[0052] Examples of materials for the winding core include paper, rubber, plastic, metals such as aluminum, and composites thereof. The surface of the winding core may be covered with a sheet made of a soft material such as sponge. The size of the winding core is not particularly limited and can be appropriately set depending on the size of the polyethylene microporous membrane to be wound, etc. The shape of the winding core may be cylindrical or columnar.

[0053] The dimensions of the touch roll, such as its width and outer diameter, are selected appropriately depending on the width of the polyethylene microporous membrane. The outer diameter of the touch roll is preferably uniform in the width direction. The material of the touch roll is not particularly limited as long as it is applicable to winding up the polyethylene microporous membrane, and may be, for example, a rubber roll or a roll in which rubber is wrapped around the outside of a resin, metal, or carbon core.

[0054] The process of winding a polyethylene microporous membrane by touch winding will be described with reference to FIG. 1 . In the winding process, the polyethylene microporous membrane 10 is continuously transported in the longitudinal direction of the polyethylene microporous membrane 10 and supplied to a touch roll 16. The transport speed of the polyethylene microporous membrane 10 may be set so as to achieve a desired winding speed. The polyethylene microporous membrane 10 supplied to the touch roll 16 is wound around the touch roll 16. The touch roll 16 is freely rotatable, and rotates in the circumferential direction by frictional force applied by the wound polyethylene microporous membrane 10. The rotating touch roll 16 guides the polyethylene microporous membrane 10 to a winding core 12. The winding core 12 rotates in the direction of the arrow by a driving force applied by a rotation drive device (not shown). Therefore, the polyethylene microporous membrane 10 guided to the winding core 12 while wound around the touch roll 16 is taken up onto the winding core 12. The winding core 12 and the polyethylene microporous membrane 10 wound around the winding core 12 form a wound body 14. The position of the touch roll 16 is adjusted by a position adjustment device (not shown) so that it contacts the polyethylene microporous membrane 10 wound around the winding core 12. When the polyethylene microporous membrane 10 is wound around the winding core 12, the touch roll 16 presses the wound body 14 in the middle of production in the radial direction of the winding core 12 with a predetermined load due to the biasing force applied by the position adjustment device (not shown). As a result, the polyethylene microporous membrane 10 wound around the winding core 12 is pressed with a predetermined load toward the center of the winding core 12. Therefore, when the polyethylene microporous membrane 10 is wound, entrapment of air between the circumferential surface of the wound body 14 and the polyethylene microporous membrane 10 is suppressed.

[0055] FIG. 2 is a schematic diagram of a winding device 2 of a second embodiment for winding a polyethylene microporous membrane around a winding core. The winding device 2 is a winding device for winding a polyethylene microporous membrane 10 around a winding core 12 by gap winding to obtain a wound body 14. The winding device 2 includes a winding core 12 rotated in the direction of the arrow in FIG. 2 by a rotation drive device (not shown), and a near roll 18. The near roll 18 is positioned so as not to contact the circumferential surface of the wound body 14 when the polyethylene microporous membrane 10 is wound. The distance d between the circumferential surface of the wound body 14 and the surface of the near roll 18 is set appropriately. Note that the distance d is defined as the distance between the center of the winding core 12 and the center of the near roll 18, minus the radius of the near roll 18, the radius of the winding core 12, and the thickness (lamination thickness) of the polyethylene microporous membrane 10 wound around and laminated on the winding core 12. The near roll 18 may be the touch roll 16 constituting the winding device 1 of the first embodiment, whose position has been adjusted by a position adjustment device (not shown) so that the distance between the circumferential surface of the wound body 14 and the surface of the near roll 18 is distance d. Generally, the shorter the distance d, the better, and it is preferably 50 mm or less, more preferably 30 mm or less, and even more preferably 10 mm or less. The distance d may be 0.1 mm or more.

[0056] The operation of winding device 2 in the gap winding process for a polyethylene microporous membrane is the same as that of winding device 1. Generally, gap winding involves entraining air when winding a film, which has the advantage of preventing tight winding but makes the film more susceptible to TD slippage. Touch winding, on the other hand, can prevent air entrainment and therefore prevent TD slippage of the film. However, from the perspective of preventing the occurrence of defective appearance when a polyethylene microporous membrane is wound into a roll, the present inventors conducted extensive research and found that gap winding is more likely to prevent wrinkles than touch winding. Film winding is performed by selecting appropriate conditions, such as a winding speed, taking into account the porosity and film thickness of the film. It was found that gap winding, which makes the film more susceptible to TD slippage compared to touch winding, also helps prevent the occurrence of defective appearance. Although the reason for this is not clear, when the polyethylene microporous membrane has a thick thickness of 50 μm to 200 μm, as the winding length increases and the outer diameter of the wound body increases, it is thought that the difference in outer diameter becomes more pronounced in touch winding, making it more likely to cause poor appearance. On the other hand, in the case of gap winding, it is generally susceptible to the influence of the layers of the wound body, and the difference in outer diameter makes it more likely to cause poor appearance, but because the porosity of the polyethylene microporous membrane is 40% to 70%, the membrane strength is ensured, making it less susceptible to the influence of the layers of the wound body and suppressing the occurrence of poor appearance.

[0057] Examples and comparative examples of the present disclosure will be described below, but the present disclosure is not limited to these examples in any way.

[0058] [Measurement method] (Basis weight) Samples of 10 cm x 10 cm were taken at two points on both ends of the polyethylene microporous membrane in the TD direction and three points equally spaced between the two points, for a total of five points, and the masses were measured using an electronic balance to obtain the arithmetic average. The obtained arithmetic average value was multiplied by 100 to obtain the basis weight in units of m ２ The mass per unit was taken as basis weight.

[0059] (Film Thickness) A contact-type thickness meter (LITEMATIC, manufactured by Mitutoyo Corporation) was used, and a cylindrical measuring terminal with a diameter of 5 mm was used, and the terminal was adjusted so that a load of 7 g was applied during measurement. Using the thickness meter adjusted in this manner, measurements were taken at a total of 10 points in the TD direction of the polyethylene microporous membrane: two points 10 mm from both ends and eight points equally spaced between the two points, and the film thickness was calculated by arithmetic averaging.

[0060] (Coefficient of variation of film thickness in TD direction) The standard deviation and arithmetic mean value of film thickness measured at 10 points when calculating the film thickness were used to determine the coefficient of variation of film thickness in the TD direction of the polyethylene microporous membrane according to the following calculation method: Coefficient of variation = standard deviation at 10 points / arithmetic mean value at 10 points (film thickness)

[0061] (Porosity) The porosity ε (%) of a polyethylene microporous membrane was calculated according to the following method: ε (%) = {1 - (basis weight / true density / membrane thickness} × 100. The density of polyethylene used was 0.96 g / mL.

[0062] [Example 1] 40 parts by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 4.6 million and 100 parts by mass of polyethylene terephthalate having a weight average molecular weight (Mw) of 560,000 and a density of 950 kg / m ３A polyethylene composition was prepared by mixing the polyethylene composition with 60 parts by mass of high-density polyethylene (HDPE). The polyethylene composition was then mixed with liquid paraffin as a solvent to prepare a polyethylene solution with a polyethylene composition concentration of 30% by mass. The polyethylene solution was extruded into a sheet form through a die at 181°C, and the extrudate was then cooled in a water bath at 15°C to obtain a gel-like sheet. The gel-like sheet was pre-dried at 55°C for 22 minutes, then stretched 2.3 times in the MD direction at 100°C, and then stretched 5.1 times in the TD direction at 115°C. Immediately after the secondary stretching, the sheet was heat-treated (heat-set) at 128°C. The heat-set sheet was continuously immersed in three separate methylene chloride baths for a total of 12 minutes to extract the solvent from the sheet. After being removed from the methylene chloride bath, the sheet was dried on a heated roll at 39°C to remove the methylene chloride. The sheet was then annealed in a heated atmosphere at 120°C. In the final winding step, the polyethylene microporous membrane obtained through the above steps was wound by touch winding in which a contact roll was brought into contact with the wound material, to obtain a 1100 mm wide wound body of the polyethylene microporous membrane of Example 1. The contact roll contact pressure during touch winding was 30 N / m, and the winding core used was a paper core with an inner diameter of 6 inches and a thickness of 12 mm. The winding lengths were 100 m and 300 m.

[0063] Example 2 A polyethylene microporous membrane was produced in the same manner as in Example 1, and only in the final winding step, the contact roll was wound using gap winding with a distance of 5 mm so as not to come into contact with the wound product, thereby obtaining a wound body of the polyethylene microporous membrane of Example 2.

[0064] Example 3 A roll of polyethylene microporous membrane of Example 3 was obtained in the same manner as in Example 1, except that the clearance (opening) of the die at each position in the TD direction was adjusted when the polyethylene solution was extruded into a sheet. When adjusting the clearance (opening) of the die at each position in the TD direction, the clearance at each position in the die was adjusted multiple times during production of the polyethylene microporous membrane while monitoring variation in the TD thickness after winding in order to make the membrane thickness more uniform in the TD, thereby obtaining a polyethylene microporous membrane with a small coefficient of variation in the TD thickness.

[0065] Example 4 A roll of the polyethylene microporous membrane of Example 4 was obtained in the same manner as in Example 2, except that the amount of polyethylene solution extruded from the die was reduced to make the thickness of the gel-like sheet thinner, and the clearance (opening) of the die at each position in the TD direction was narrowed accordingly.

[0066] Example 5 A roll of the polyethylene microporous membrane of Example 5 was obtained in the same manner as in Example 4, except that the film thickness of the gel-like sheet was increased by increasing the discharge rate of the polyethylene solution extruded from the die and, accordingly, the clearance (opening) of the die at each position in the TD direction was increased.

[0067] Comparative Example 1 A roll of polyethylene microporous membrane for Comparative Example 1 was obtained in the same manner as in Example 1, except that the amount of polyethylene solution extruded from the die was reduced to make the thickness of the gel-like sheet thinner, and the clearance (opening) of the die at each position in the TD direction was narrowed accordingly.

[0068] [Comparative Example 2] 15 parts by mass of ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 4.6 million and 15 parts by mass of polyethylene terephthalate having a weight average molecular weight (Mw) of 560,000 and a density of 950 kg / m ３A polyethylene composition was prepared by mixing the polyethylene composition with 85 parts by mass of high-density polyethylene (HDPE). The polyethylene composition was then mixed with decahydronaphthalene as a solvent to prepare a polyethylene solution with a polyethylene composition concentration of 30% by mass. The polyethylene solution was extruded into a sheet form through a die at 165°C, and the extrudate was then cooled in a water bath at 15°C to obtain a gel-like sheet. The gel-like sheet was dried at 55°C for 12 minutes, then stretched in the MD direction at 1.3 times the original stretching ratio at 30°C, and then further dried at 55°C for 6 minutes to obtain a sheet. The decahydronaphthalene content of the sheet at this stage was 1% by mass or less. The sheet was then stretched in the MD direction at 80°C at a stretching ratio of 2.5 times, and then stretched in the TD direction at 125°C at a stretching ratio of 6.0 times (secondary stretching). This was followed immediately by heat treatment (heat setting) at 134°C. Thereafter, the sheet was annealed in a heated atmosphere at 80°C, and the final winding step was carried out in the same manner as in Example 2, to obtain a roll of the polyethylene microporous membrane of Comparative Example 2.

[0069] Comparative Example 3 A roll of polyethylene microporous membrane for Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that the film thickness of the gel-like sheet was increased by increasing the discharge rate of the polyethylene solution extruded from the die, the clearance (opening) of the die at each position in the TD direction was increased accordingly, and the stretch ratio in the MD direction in the secondary stretching was changed to 2.0 times and the stretch ratio in the TD direction to 4.5 times.

[0070] Comparative Example 4 A roll of the polyethylene microporous membrane of Comparative Example 4 was obtained in the same manner as in Example 1, except that the clearance (opening) at each position in the TD direction of the die was adjusted when extruding into a sheet.

[0071] [Evaluation] The polyethylene microporous membrane and its roll obtained as described above were evaluated as follows. The obtained results are shown in Table 1.

[0072] (Puncture Strength) A puncture test was performed using a KES-G5 handy compression tester manufactured by Kato Tech Co., Ltd., under conditions of a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec, and the maximum puncture load was taken as the puncture strength. Here, the sample was clamped and fixed in a metal frame (sample holder) with a Φ11.3 mm hole, together with a silicone rubber packing. Measurements were taken at a total of 10 points in the TD direction, including two points 10 mm from both ends and eight points equally spaced between the two points, and the arithmetic average of these values ​​was used to determine the puncture strength. The puncture strength was used as one of the indicators for evaluating the strength of a polyethylene microporous membrane.

[0073] (Water resistance) Water resistance was measured according to JIS L 1092:2009 Method A (low water pressure method). Only the central part of the sample in the TD direction was used. Water resistance was used as one of the indicators for evaluating the strength of the polyethylene microporous membrane.

[0074] (Air permeability (Gurley value)) The Gurley value of the polyethylene microporous membrane was measured in accordance with JIS P8117:2009 using a Gurley densometer manufactured by Toyo Seiki Seisaku-sho. The time required for 200 ml of air to pass through a 28.6 mmφ sample was measured, and this time was halved to convert it into a value per 100 mL. Measurements were taken at five points in the TD direction: two points 10 mm from both ends and three points equally spaced between the two points, and the air permeability was calculated by arithmetically averaging these values.

[0075] (Pore size flow rate pore size (pore size)) The pore size was measured using a PMI Perm Porometer (model: CFP-1200-AEXL) and PMI Galwick (surface tension 15.9 dyn / cm) as the immersion liquid by the half-dry method specified in ASTM E1294-89. The measurement temperature was 25°C, and the measurement pressure was varied in the range of 0 kPa to 600 kPa. In addition, the sample was measured at two points 100 mm from both ends in the TD direction and one point in the middle of the two points, for a total of three points, and the pore size flow rate pore size was calculated by arithmetically averaging these values.

[0076] (Surface appearance of roll) The surface of the roll was visually inspected for the following items: (1) Presence of strong wrinkles on the surface of the roll (2) Strong marks on the polyethylene microporous membrane when the polyethylene microporous membrane was pulled out from the roll (3) Presence of protrusions of 1 mm or more on the surface of the roll The above items (1) to (3) were evaluated, and those that met all the criteria were rated as C, those that met two criteria as B, and those that met none as A.

[0077]

[0078] The evaluation results in Table 1 show that the polyethylene microporous membranes of the Examples have higher pin puncture strength and water resistance and therefore higher strength than the polyethylene microporous membranes of Comparative Examples 1 to 3. The polyethylene microporous membranes of the Examples also show less occurrence of defective appearance than the polyethylene microporous membrane of Comparative Example 4. A comparison of the polyethylene microporous membrane of Example 1 and the polyethylene microporous membrane of Example 2 shows that gap winding suppresses the occurrence of defective appearance more than touch winding.

[0079] The disclosure of Japanese Patent Application No. 2024-055060, filed on March 28, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated by reference into this specification to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A polyethylene microporous membrane having a membrane thickness of 50 μm to 200 μm, a coefficient of variation of membrane thickness in the transverse direction of 0.090 or less, and a porosity of 40% to 70%.

2. The microporous polyethylene membrane according to claim 1, wherein the coefficient of variation of the membrane thickness in the transverse direction is 0.050 or less.

3. The microporous polyethylene membrane according to claim 1, having a pore size of 20 nm to 100 nm.

4. The microporous polyethylene membrane according to claim 1, which is used as a reinforcing material.

5. The microporous polyethylene membrane according to claim 1, which is used as a substrate for an ion exchange membrane.

6. A wound body comprising a winding core and the polyethylene microporous membrane according to any one of claims 1 to 5 wound around the winding core, wherein the length of the polyethylene microporous membrane is 100 m or more.

7. The roll according to claim 6, wherein the width of the polyethylene microporous film is 500 mm or more.

8. The roll according to claim 6, wherein the length of the polyethylene microporous film is 300 m or more.

9. A method for producing a wound body, comprising the steps of producing the polyethylene microporous membrane according to any one of claims 1 to 5, and winding the polyethylene microporous membrane around a winding core.

10. The method for producing a wound body according to claim 9, wherein in the winding step, the polyethylene microporous membrane is wound around the winding core by gap winding, in which the touch roll is separated from the polyethylene microporous membrane.

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