Polyolefin microporous membrane, separator for secondary battery, filter for liquid, secondary battery, and filter unit

Abstract

The present invention addresses the problem of providing a polyolefin microporous membrane having excellent dendrite resistance and output characteristics when used as a battery separator, and excellent filtration accuracy and high permeability when used in a liquid filter. The present invention is a polyolefin microporous membrane having a thickness of 30 [mu] m or less, in which when Ymax is the maximum value of dV / d (LogD) in the pore diameter range of 0.01 [mu] m to 10 [mu] m and X1 and X2 are the small to large pore diameters satisfying Ymax / 2 in the pore diameter range of 0.01 [mu] m to 10 [mu] m in a pore diameter distribution in which the pore diameter is the X axis and the dV / d (LogD) is the Y axis as measured by mercury intrusion method, X2 / X1 is 15 or more.

Description

Technical Field

[0001] The present invention relates to a polyolefin microporous membrane, a separator for a secondary battery, a filter for a liquid, a secondary battery, and a filtering unit. Background Art

[0002] The polyolefin microporous membrane is used as a filter, a separator for a fuel cell, a separator for a capacitor, etc. It is particularly suitable for use as a separator for a lithium ion secondary battery which is widely used in a notebook personal computer, a mobile phone, a digital camera, etc. In addition, since the polyolefin microporous membrane has a uniform and fine pore structure and excellent solvent resistance and chemical resistance, it is widely used in various filter applications such as a water treatment membrane, an ultrafiltration membrane, a microfiltration membrane, a moisture-permeable and waterproof clothing material.

[0003] In the field of lithium ion secondary batteries, in recent years, centered on in-vehicle applications, development has been carried out aiming at the enlargement and high energy density / high capacity / high output of the battery. Along with this, for some batteries, dendrites are liable to be generated due to the precipitation of lithium, and the required characteristics for the dendrite resistance of the separator have become even higher. The so-called dendrites in a lithium ion secondary battery are needle-like crystals generated near the interface between the negative electrode and the separator during charge and discharge. If they grow and penetrate the separator, they sometimes cause a short circuit. Therefore, in a battery where dendrites are liable to be generated, a separator with a small pore diameter through which dendrites are not easily penetrated is required. On the other hand, if the pore diameter of the microporous membrane is made small, the permeability sometimes decreases, and thus the output characteristics decrease.

[0004] In the field of liquid filters, it is required to have both high precision of separation ability and high permeability. For example, in a semiconductor manufacturing process, as the wiring pitch of the semiconductor is miniaturized, the upper limit of the size of foreign matters allowed in the process decreases, and a filter for filtering the liquid used in semiconductor manufacturing is required to be able to capture smaller foreign matters. On the other hand, from the viewpoint of processing capacity, it is not preferable that the permeability deteriorates.

[0005] Patent Document 1 describes a liquid filter substrate having both high liquid permeability and high particle capturing ability under high pressure, and discloses a polyolefin microporous membrane for a liquid filter in which the average flow pore diameter measured by a semi-dry method using gas / liquid phase replacement and the average flow pore diameter dLLP measured by a semi-dry method using liquid / liquid phase replacement are in a specified range.

[0006] Patent Document 2 describes a polyolefin microporous membrane having a small pore diameter and very excellent air permeability, and discloses a laminated polyolefin microporous membrane having an air permeability resistance of 10 to 200 sec / 100 ml and a bubble point pore diameter of 5 to 35 nm.

[0007] In Patent Document 3, a microporous membrane obtained by laminating layers composed of PE and PP with different resin compositions is described, and a polyolefin microporous membrane is described in which the resin compositions and pore structures are different between the layer disposed on the surface layer and the layer disposed on the inner layer, and which has excellent balance of permeability, mechanical strength, melting characteristics, electrolyte absorbency, and electrolyte retention when used as a separator for a battery.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-167198

[0011] Patent Document 2: WO 2018 / 168871

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2008-255307 Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] Patent Document 1 is a technique that can have both particle removal performance and permeability during filtration under high-pressure conditions by suppressing deformation of the pore structure under high pressure, but cannot improve the trade-off state between small pore size and high permeability.

[0015] Patent Documents 2 and 3 are techniques that can improve the trade-off between small pore size and high permeability by functional separation brought about by lamination. However, the layer with a small pore size has a formulation in which PE and PP are blended. For such a design in which different resins are blended, since the pore structure is non-uniform due to phase separation of each resin, there is room for improvement in the balance between small pore size and high permeability.

[0016] Means for Solving the Problems

[0017] In order to solve the above problems and achieve the object, the present invention has the following configuration. In the following description, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0018] [1]

[0019] A polyolefin microporous membrane, in a pore size distribution in which the pore size is set as the X-axis and dV / d(LogD) is set as the Y-axis measured by mercury intrusion porosimetry, the maximum value of dV / d(LogD) in the range of pore size from 0.01 μm to 10 μm is set as Y max , and in the range of pore size from 0.01 μm to 10 μm, satisfying Y maxWhen the pore diameters of / 2 are set as X1 and X2 from small to large, X2 / X1 is 15 or more, and the thickness of the polyolefin microporous membrane is 30 μm or less.

[0020] Among them,

[0021] V: Cumulative pore volume (cm 3 / g)

[0022] D: Pore diameter (μm).

[0023] 〔2〕

[0024] For the polyolefin microporous membrane described in the above 〔1〕, its bubble point pressure is 1900 kPa or more.

[0025] 〔3〕

[0026] For the polyolefin microporous membrane described in the above 〔1〕 or 〔2〕, it contains 90 mass% or more of polyethylene.

[0027] 〔4〕

[0028] For the polyolefin microporous membrane described in any one of the above 〔1〕 to 〔3〕, the air permeability resistance converted to a thickness of 10 μm is 300 seconds or less.

[0029] 〔5〕

[0030] For the polyolefin microporous membrane described in any one of the above 〔1〕 to 〔4〕, it is used for a separator for secondary batteries.

[0031] 〔6〕

[0032] For the polyolefin microporous membrane described in any one of the above 〔1〕 to 〔4〕, it is used for a liquid filter.

[0033] 〔7〕

[0034] A separator for secondary batteries that uses the polyolefin microporous membrane described in any one of the above 〔1〕 to 〔4〕.

[0035] 〔8〕

[0036] A liquid filter that uses the polyolefin microporous membrane described in any one of the above 〔1〕 to 〔4〕.

[0037] 〔9〕

[0038] A secondary battery that uses the separator for secondary batteries described in the above 〔7〕.

[0039] 〔10〕

[0040] A filtration unit that uses the liquid filter described in the above 〔8〕.

[0041] Effects of the Invention

[0042] When the polyolefin microporous membrane of the present invention is used as a separator for a battery, due to its excellent dendrite resistance and output characteristics, it can be suitably used as a separator for secondary batteries such as electric vehicles that require high energy density, high capacity, and high output. In addition, when used for liquid filter applications, due to its excellent filtration accuracy and high permeability, it can be suitably used as a high-precision liquid filter for semiconductor processes and the like that require removal of minute foreign matters. Detailed Embodiments

[0043] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to these.

[0044] The polyolefin microporous membrane of the present invention contains a polyethylene-based resin as a main component. In addition, the so-called main component shown here refers to the substance with the largest content in terms of mass% among the components constituting the polyolefin microporous membrane. The polyethylene-based resin in the above polyolefin microporous membrane is preferably 90% by mass or more, more preferably 95% by mass or more, further preferably 97% by mass or more, and particularly preferably 99% by mass or more. By making the content of the polyethylene-based resin in the above polyolefin microporous membrane within the above range, a polyolefin microporous membrane with excellent pore structure uniformity, excellent filtration accuracy, and high permeability when used as a liquid filter can be obtained. In addition, the content of the polyethylene-based resin in the polyolefin microporous membrane can be measured by the method described later.

[0045] As the above polyethylene-based resin, various polyethylenes can be used, and examples thereof include ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene, etc.

[0046] The above polyethylene-based resin can be a homopolymer of ethylene or a copolymer of ethylene and other α-olefins. Examples of α-olefins include propylene, 1-butene, 1-hexene, 1-pentene, 4-methyl-1-pentene, octene, vinyl acetate, methyl methacrylate, styrene, etc.

[0047] In addition, the above polyethylene-based resin may contain two or more polyethylenes.

[0048] The above polyolefin microporous membrane may contain a small amount of polyolefins other than polyethylene within the range that does not impair the effects of the present invention. For example, if the above polyolefin microporous membrane contains polypropylene, the melting temperature sometimes increases when used as a battery separator. In addition, when used for liquid filter applications, the filtration accuracy is sometimes excellent.

[0049] In addition to homopolymers, block copolymers and random copolymers of the above-mentioned polypropylene can also be used. The block copolymers and random copolymers may contain copolymer components of other α-olefins other than propylene. As the other α-olefin, ethylene is preferably used.

[0050] However, if the above-mentioned polypropylene is contained, the mechanical strength and permeability are likely to decrease compared with the case of using only polyethylene. Therefore, the content of the above-mentioned polypropylene in the above-mentioned polyolefin microporous membrane is preferably 0 to 20% by mass, more preferably 0 to 10% by mass, further preferably 0 to 5% by mass, and most preferably 0% by mass.

[0051] The above-mentioned polyolefin microporous membrane may be a single-layer membrane, but is preferably a laminated membrane having a B layer made of a polyethylene-based resin with different properties with respect to an A layer made of a polyethylene-based resin as a main component. Generally, if the pore diameter of the porous membrane is reduced, the flow path of the fluid becomes thinner, so the pressure drop becomes higher and the permeability decreases. As a preferred embodiment of the polyolefin microporous membrane of the present invention, there can be mentioned a multilayer polyolefin microporous membrane having a small pore diameter and high permeability, which is composed of an A layer having a small pore diameter and a B layer having high permeability, by using a polyethylene-based resin with a high viscosity-average molecular weight (Mv) as the main component of the resin constituting the A layer and a polyethylene-based resin with a low viscosity-average molecular weight (Mv) as the main component of the resin constituting the B layer, and forming a film using the manufacturing conditions described below.

[0052] The resin constituting the A layer contains a polyethylene-based resin as a main component. In addition, the content of the polyethylene-based resin component in the A layer is preferably 90% by mass or more, more preferably 95% by mass or more, further preferably 97% by mass or more, and particularly preferably 99% by mass or more. By making the content of the polyethylene-based resin component in the A layer within the above range, a polyolefin microporous membrane can be obtained, which has excellent uniformity of pore structure, excellent dendrite resistance and output characteristics when used as a battery separator for secondary batteries, and excellent filtration accuracy and high permeability when used as a liquid filter.

[0053] The resin constituting the B layer contains a polyethylene-based resin as a main component. In addition, the content of the polyethylene-based resin component in the B layer is preferably 90% by mass or more, more preferably 95% by mass or more, further preferably 97% by mass or more, and particularly preferably 99% by mass or more. By making the content of the polyethylene-based resin component in the B layer within the above range, a polyolefin microporous membrane can be obtained, which has excellent uniformity of pore structure, excellent dendrite resistance and output characteristics when used as a battery separator for secondary batteries, and excellent filtration accuracy and high permeability when used as a liquid filter.

[0054] The viscosity-average molecular weight (Mva) of the polyethylene-based resin contained in the A layer is preferably 150×104 Above, more preferably 200×10 4 Above, further preferably 250×10 4 Above, further preferably 300×10 4 Above. The larger the Mva, the more preferable, but if it is too large, the film-forming property deteriorates, and thus it is preferably 500×10 4 Below, more preferably 450×10 4 Below. By making the viscosity-average molecular weight of the polyethylene-based resin contained in the A layer within the above range, a polyolefin microporous membrane can be obtained in which the pore structure can be uniformly refined, and which has excellent dendrite resistance and output characteristics when used as a battery separator for secondary batteries, and excellent filtration accuracy and high permeability when used as a liquid filter. In addition, it is easy to control the layer ratio of the A layer and the B layer described later within a preferable range. The value of Mva can be adjusted by the raw material composition and kneading conditions of the A layer.

[0055] The viscosity-average molecular weight (Mvb) of the polyethylene-based resin contained in the B layer is preferably 125×10 4 Below, more preferably 100×10 4 Below, further preferably 50×10 4 Below, further preferably 40×10 4 Below. The smaller the Mvb, the more preferable, but if it is too small, the film-forming property deteriorates, and thus it is preferably 1×10 4 Above, more preferably 5×10 4 Above. By making the viscosity-average molecular weight of the polyethylene-based resin contained in the B layer within the above range, a polyolefin microporous membrane with excellent permeability can be obtained. In addition, it is easy to control the layer ratio of the above A layer and B layer within a preferable range. Mvb can be adjusted by the raw material composition and kneading conditions of the B layer.

[0056] In the polyolefin microporous membrane of the present invention, when the viscosity-average molecular weight of the resin constituting the A layer is set as Mva and the viscosity-average molecular weight of the resin constituting the B layer is set as Mvb, the relationship between Mva and Mvb preferably satisfies the following formula.

[0057] Mva - Mvb ≧ 100×10 4

[0058] (The value of Mva - Mvb) is more preferably 150×10 4 Above, further preferably 200×10 4 Above, further preferably 250×10 4As described above, from the viewpoint of having both a small pore diameter and a high flow rate, the larger the value of (Mva - Mvb), the more preferable. However, if it is too large, the molecular weight and viscosity difference between the resin constituting the A layer and the resin constituting the B layer become excessively large, and the film-forming property during co-extrusion decreases. Therefore, the value of (Mva - Mvb) is preferably 500×10 4 or less, more preferably 400×10 4 or less, and particularly preferably 300×10 4 or less. In order to make the value of (Mva - Mvb) within the above range, it is preferable to make the raw material compositions and the molecular weights of the raw materials of the A layer and the B layer within the above range.

[0059] The melting point of the polyethylene-based resin contained in the A layer is preferably 136°C or lower. More preferably 133°C or lower, further preferably 130°C or lower, and most preferably 129°C or lower. By making the melting point within the above range, a polyolefin microporous membrane can be obtained in which the pore structure can be uniformly refined, and which has excellent dendrite resistance and output characteristics when used as a battery separator for secondary batteries, and excellent filtration accuracy and high permeability when used as a liquid filter.

[0060] The melting point of the polyethylene-based resin contained in the B layer is preferably 129°C or higher. More preferably 132°C or higher, further preferably 134°C or higher, and most preferably 135°C or higher. By making the melting point within the above range, a polyolefin microporous membrane with excellent permeability can be obtained.

[0061] In the polyolefin microporous membrane of the present invention, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, and fillers can be contained within a range that does not impair the effects of the present invention. By appropriately selecting the types and amounts of the antioxidants and heat stabilizers, the characteristics of the microporous membrane can be adjusted or enhanced. In particular, for the purpose of suppressing oxidative degradation caused by the heat history of the polyethylene resin, it is preferable to add an antioxidant. As the antioxidant, it is preferable to use, for example, one or more selected from 2,6-di-tert-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene (for example, "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2), tetra[methylene-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7), etc.

[0062] As the layer structure of the above-mentioned laminated film, examples thereof include an A layer / B layer, an A layer / B layer / A layer, a B layer / A layer / B layer, etc., and among them, the structures of an A layer / B layer / A layer and a B layer / A layer / B layer are preferred. By having the above-mentioned layer structure, it is possible to obtain a polyolefin microporous membrane that is excellent in dendrite resistance and output characteristics when used as a battery separator for secondary batteries, and excellent in filtration accuracy and permeability when used as a liquid filter.

[0063] In the polyolefin microporous membrane of the present invention, the total thickness of the A layer is preferably 0.5 to 15.0 μm. By the thickness of the A layer being 15.0 μm or less, more preferably 5.0 μm or less, further preferably 3.0 μm or less, and further preferably 2.0 μm or less, it is possible to suppress a decrease in permeability. By the thickness of the A layer being 0.5 μm or more, it is possible to suppress the deterioration of filtration accuracy due to an increase in pore size in a portion where the A layer is locally excessively thinned due to lamination unevenness or the like. In the polyolefin microporous membrane of the present invention, by making the raw material composition and the molecular weight of the raw materials of the A layer within the above-mentioned ranges, even if the ratio of the total thickness of the A layer is small, it is possible to effectively make the pore size small.

[0064] In the polyolefin microporous membrane of the present invention, the ratio of the total thickness of the A layer to the total layer thickness is preferably 50% or less. By the thickness ratio of the A layer being 50% or less, more preferably 40% or less, further preferably 30% or less, and further preferably 20% or less, it is possible to suppress the deterioration of permeability, suppress a decrease in output characteristics when used as a battery separator, and suppress a decrease in flow rate when used as a liquid filter. From the viewpoint of permeability, the lower the thickness ratio of the A layer, the more preferable, but if it is too low, the filtration life may decrease or the lamination accuracy may deteriorate, so about 5% is the lower limit. In order to make the thickness ratio of the A layer within the above-mentioned range, it is preferable to make the raw material composition and the molecular weight of the raw materials of the polyolefin microporous membrane within the above-mentioned ranges. In addition, in order to obtain a polyolefin microporous membrane with excellent lamination accuracy and good quality, it is preferable to make the extrusion conditions during the film formation of the polyolefin microporous membrane within the range described later.

[0065] The polyolefin microporous membrane of the present invention preferably has a thickness of 3 μm or more and 30 μm or less. By the above thickness being 30 μm or less, more preferably 20 μm or less, further preferably 15 μm or less, further preferably 10 μm or less, and further preferably 8 μm or less, the permeability is excellent. In addition, by the above thickness being 3 μm or more, more preferably 5 μm or more, the operability is excellent. The thickness of the polyolefin microporous membrane can be adjusted by the film formation conditions.

[0066] In the pore size distribution of the polyolefin microporous membrane of the present invention measured by mercury intrusion porosimetry, with the pore size on the X-axis and dV / d(LogD) on the Y-axis, the maximum value of dV / d(LogD) in the range of pore sizes from 0.01 μm to 10 μm is set as Y max , and when the pore sizes satisfying Y max / 2 in the range of pore sizes from 0.01 μm to 10 μm are arranged in ascending order as X1 and X2, X2 / X1 is 15 or more.

[0067] Wherein,

[0068] V: Cumulative pore volume (cm 3 / g)

[0069] D: Pore size (μm).

[0070] By X2 / X1 being 15 or more, preferably 20 or more, more preferably 30 or more, and further preferably 50 or more, a polyolefin microporous membrane excellent in dendrite resistance and output characteristics when used as a battery separator and excellent in filtration accuracy and permeability when used as a liquid filter can be obtained. In addition, from the above viewpoints, the upper limit of X2 / X1 is not particularly limited, but from the viewpoint of productivity, X2 / X1 is preferably 500 or less, more preferably 100 or less. By making the layer structure, raw material composition, and molecular weight of the raw materials of the polyolefin microporous membrane within the above ranges, and / or making the film-forming conditions within the ranges described later, X2 / X1 can be within the above ranges.

[0071] Regarding the relationship between the layer structure and X2 / X1, if a design is made in which layer A having a fine pore structure and layer B having a coarse pore structure are laminated, fine pores and coarse pores can coexist in the polyolefin microporous membrane, so there is a tendency for X2 / X1 to increase.

[0072] In addition, regarding the relationship between the raw material composition and X2 / X1, if the viscosity-average molecular weight (Mv) of the polyethylene-based resin used in the polyolefin microporous membrane is high, there is a tendency for X1 and X2 to decrease. It is considered that this is because the entanglement points between polyethylene molecular chains increase, resulting in pore refinement. In addition, if the Mv of the polyethylene-based resin is low, there is a tendency for the pores to become larger and X1 and X2 to increase. Therefore, regarding the above layer structure, by designing such that the Mv of the polyethylene-based resin used in layer A is high and the Mv of the polyethylene-based resin used in layer B is low, there is a tendency for X2 / X1 to increase.

[0073] In addition, regarding the relationship between the film-forming conditions and X2 / X1, if the stretching temperature is low, the polyethylene molecular chains are stretched in a state where their entanglement is restricted, so there is a tendency for the pores to be refined and X1 and X2 to decrease. In addition, if the stretching temperature is high, the polyethylene molecular chains are stretched in a state where their entanglement is easily untied, so there is a tendency for the pores to be coarsened and X1 and X2 to increase. Further, when the Mv of the polyolefin resin used for the polyolefin microporous membrane is low, the refinement of the pores is suppressed when the stretching temperature is low. Therefore, in a design in which layer A using a polyethylene resin with a high Mv and layer B using a polyethylene resin with a low Mv are laminated, when the stretching temperature is low, X2 / X1 has a tendency to increase.

[0074] By adjusting the above layer structure, raw material composition, and film-forming conditions, it is easy to control X2 / X1 within the target range. The specific control range is preferably the range described below.

[0075] X2 / X1 represents the breadth of the pore size distribution in the polyolefin microporous membrane. A large X2 / X1 means that in the polyolefin microporous membrane, fine pores and coarse pores are widely distributed. It is considered that by widely distributing fine pores and coarse pores in the polyolefin microporous membrane, the fine pores are responsible for dendrite resistance (as a battery separator) and filtration performance (as a liquid filter), and the coarse pores are responsible for improving output characteristics (as a battery separator) and improving liquid permeability (as a liquid filter), achieving a functional separation effect, and making the polyolefin microporous membrane excellent for use as a battery separator and a liquid filter. In addition, pores with a diameter less than 0.01 μm extremely deteriorate the output characteristics and liquid permeability, and pores with a diameter exceeding 10 μm extremely deteriorate the dendrite resistance and filtration performance. Therefore, it is preferred that both are less.

[0076] The polyolefin microporous membrane of the present invention preferably has a bubble point pressure of 1900 kPa or more determined by the measurement method described below. The bubble point pressure represents the pressure when air first penetrates by applying air pressure to one side of the polyolefin microporous membrane impregnated with a liquid. The part where air first penetrates is the part with the largest pores in the plane direction of the polyolefin microporous membrane. Therefore, a high bubble point pressure indicates that the polyolefin microporous membrane has a small pore diameter. When the bubble point pressure of the above polyolefin microporous membrane is 1900 kPa or more, more preferably 2100 kPa or more, further preferably 2300 kPa or more, and further preferably 2500 kPa or more, the dendrite resistance is excellent when used as a battery separator, and the filtration accuracy is excellent when used as a liquid filter. From the viewpoint of permeability, the above bubble point pressure is preferably 4000 kPa or less. In order to make the bubble point pressure within the above range, it is preferred to make the raw material composition and molecular weight of the raw material of the polyolefin microporous membrane within the above range, and / or make the stretching conditions and heat setting conditions during the formation of the polyolefin microporous membrane within the range described below.

[0077] The polyolefin microporous membrane of the present invention preferably has an air permeability resistance of 300 seconds / 100 cm in terms of a thickness of 10 μm. 3 As follows. The air permeability resistance in terms of a thickness of 10 μm is 300 seconds / 100 cm 3 or less, more preferably 280 seconds / 100 cm 3 or less, further preferably 250 seconds / 100 cm 3 or less, particularly preferably 200 seconds / 100 cm 3 or less, so that the reduction of output characteristics can be suppressed when used as a battery separator, and the reduction of flow rate can be suppressed when used as a liquid filter. From the viewpoint of film strength, the air permeability resistance in terms of a thickness of 10 μm is preferably 10 seconds / 100 cm 3 or more, more preferably 100 seconds / 100 cm 3 or more. In order to make the air permeability resistance within the above range, it is preferable to make the raw material composition and molecular weight of the polyolefin microporous membrane within the above range. In addition, the stretching conditions and heat setting conditions during the film formation of the polyolefin microporous membrane are within the ranges described later.

[0078] The polyolefin microporous membrane of the present invention preferably has a porosity of 35% or more. By having a porosity of 35% or more, more preferably 40% or more, further preferably 45% or more, and further preferably 50% or more, the reduction of output characteristics can be suppressed when used as a battery separator, and the reduction of flow rate can be suppressed when used as a liquid filter. From the viewpoint of operability, the porosity is preferably 70% or less. In order to make the porosity within the above range, it is preferable to make the raw material composition and molecular weight of the polyolefin microporous membrane within the above range. In addition, the stretching conditions and heat setting conditions during the film formation of the polyolefin microporous membrane are within the ranges described later.

[0079] The manufacturing process of the polyolefin microporous membrane preferably consists of the following steps (a) to (e). Hereinafter, an example of the film formation method of the polyolefin microporous membrane using the above raw materials is described, but it is not necessarily limited thereto.

[0080] (a) Knead / dissolve the polyolefin raw material, plasticizer, and additive to adjust the polyolefin solution.

[0081] (b) Extrude the dissolved material and form it into a sheet shape for cooling and solidification.

[0082] (c) Stretch the obtained sheet by a roll method or a tenter method.

[0083] (d) Extract the plasticizer from the obtained stretched film and dry the film.

[0084] (e) Then perform heat treatment / re-stretching.

[0085] The following describes each process.

[0086] (a) Preparation of polyolefin solution

[0087] Prepare a polyolefin solution by heating and dissolving a polyolefin resin in a plasticizer. As the plasticizer, there is no particular limitation as long as it is a solvent that can fully dissolve polyethylene. However, in order to enable high magnification stretching, the plasticizer is preferably a liquid at room temperature. Examples of the plasticizer include aliphatic, cycloaliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, and mineral oil fractions with boiling points corresponding to them, and phthalic acid esters such as dibutyl phthalate and dioctyl phthalate that are liquid at room temperature. In order to obtain a gel-like sheet with a stable plasticizer content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. In the molten kneading state, a solvent that is miscible with polyethylene but is solid at room temperature can be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, hexacosanol, and paraffin. However, if only the solid solvent is used, stretching unevenness may occur.

[0088] In the case where the polyolefin microporous membrane of the present invention is a laminated membrane, in the preparation of the above polyolefin solution, with respect to the total 100% by mass of the polyolefin resin and the plasticizer in layer A, the proportion of the polyolefin resin (hereinafter, referred to as the resin concentration of the polyolefin solution) is preferably 5% by mass or more. By the resin concentration of the polyolefin solution in layer A being 5% by mass or more, more preferably 10% by mass or more, further preferably 15% by mass or more, and further preferably 20% by mass or more, it is easier to make the pore size small. In order to maintain the formability of the sheet and suppress the deterioration of the film-forming property, the resin concentration of the polyolefin solution in layer A is preferably 40% by mass or less, more preferably 35% by mass or less.

[0089] The resin concentration of the polyolefin solution in layer B is preferably 35% by mass or less. By the resin concentration of the polyolefin solution in layer B being 35% by mass or less, more preferably 30% by mass or less, further preferably 25% by mass or less, and further preferably 20% by mass or less, it is easier to improve the permeability. In order to prevent the deterioration of the formability of the sheet and the reduction of the film-forming property, the resin concentration of the polyolefin solution in layer B is preferably 5% by mass or more, more preferably 10% by mass or more.

[0090] The viscosity of the above plasticizer is preferably 20 - 200 cSt (20×10 -6 ~200×10 -6 m 2(s). If the viscosity at 40 °C is 20 cSt or more, the sheet obtained by extruding the polyolefin solution from the die is less likely to become non-uniform. On the other hand, if it is 200 cSt or less, the removal of the plasticizer is easy. In addition, the viscosity of the plasticizer is the viscosity measured at 40 °C using an Ubbelohde viscometer.

[0091] The uniform melt-kneading of the polyolefin solution is not particularly limited, but in the case of wanting to prepare a high-concentration polyolefin solution, it is preferably carried out in a twin-screw extruder. According to needs, various additive materials such as antioxidants can be added within the range that does not impair the effects of the present invention. In particular, in order to prevent the oxidation of polyethylene, it is preferable to add an antioxidant.

[0092] In the extruder, the polyolefin solution can be uniformly mixed at the temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but relative to the melting point Tm (°C) of the polyolefin resin, it is preferably (Tm + 10) °C to (Tm + 120) °C. More preferably, it is (Tm + 20) °C to (Tm + 100) °C. Here, the so-called melting point refers to the value measured by DSC based on JIS K7121 (1987) (the same hereinafter). By making the melt-kneading temperature (Tm + 10) °C or more, the presence of unmelted matter in the extrudate extruded from the die can be prevented, and the occurrence of film breakage and the like in the subsequent stretching process can be prevented. In addition, by making the melt-kneading temperature (Tm + 120) °C or less, the deterioration caused by the thermal decomposition of the polyolefin can be suppressed, and the deterioration of the physical properties of the obtained microporous membrane such as strength and porosity can be suppressed. In addition, it is possible to prevent the decomposition products from precipitating and adhering to the sheet on the cooling roll, the roll in the stretching process, etc., thereby deteriorating the appearance. For example, when the polyolefin resin is polyethylene, since the polyethylene composition has a melting point of about 130 to 140 °C, the melt-kneading temperature is preferably in the range of 140 to 250 °C. More preferably, it is 160 to 230 °C, further preferably 170 to 200 °C, and further preferably 180 to 200 °C.

[0093] In addition, in the case of manufacturing a laminated film as the polyolefin microporous membrane of the present invention, it can be achieved by melt-kneading with a twin-screw extruder according to the resin composition of each layer and supplying it to a multi-manifold type composite T-die for co-extrusion.

[0094] (b) Formation of the extrudate and formation of the gel-like sheet

[0095] Next, the obtained extrudate is cooled to obtain a gel-like sheet. By cooling, the microphase of polyethylene separated by the solvent can be immobilized. It is preferably cooled to 10 to 50 °C in the cooling process. This is because it is preferable that the final cooling temperature is below the crystallization end temperature, so that the higher-order structure is refined, and thus uniform stretching is easy to perform in the subsequent stretching. Therefore, the cooling is preferably performed at a rate of 30 °C / min or more until at least below the gelation temperature. If the cooling rate is less than 30 °C / min, the crystallinity increases, and it is difficult to obtain a gel-like sheet suitable for stretching. Generally, if the cooling rate is slow, larger crystals are formed, so the higher-order structure of the gel-like sheet becomes coarser, and the gel structure formed is also large. On the contrary, if the cooling rate is fast, smaller crystals are formed, so the higher-order structure of the gel-like sheet becomes denser, which not only brings about uniform stretching but also improves the strength and elongation of the film.

[0096] As the cooling method, there are methods of direct contact with cold air, cooling water, other cooling media, methods of contact with a roll cooled by a cooling medium, methods of using a casting drum, etc.

[0097] (c) Stretching process

[0098] The obtained gel-like sheet is stretched. As the stretching method, it can be obtained by the inflation method, the simultaneous biaxial stretching method, or the sequential biaxial stretching method. Among them, in terms of controlling film-forming stability, thickness uniformity, high rigidity and dimensional stability of the film, the simultaneous biaxial stretching method or the sequential biaxial stretching method is preferably used.

[0099] As the combination of the stretching device and the stretching method used, examples include MD (machine direction) uniaxial stretching using a roll stretcher, TD (transverse direction) uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretcher and a tenter or a combination of two tenters, and simultaneous biaxial stretching using a simultaneous biaxial tenter, etc.

[0100] The stretching temperature is preferably at or below the melting point of the gel-like sheet + 10°C, more preferably in the range of (crystalline dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5°C). Specifically, in the case of a polyethylene composition, since it has a crystalline dispersion temperature of about 90 to 100°C, the stretching temperature is preferably 90 to 125°C, more preferably 90 to 120°C, further preferably 90 to 110°C, and particularly preferably 95 to 105°C. The crystalline dispersion temperature Tcd is determined from the temperature characteristics of the dynamic viscoelasticity measured according to ASTM D 4065. Alternatively, it can also be determined by NMR. By setting the stretching temperature to 90°C or higher, sufficient pore formation can occur, the film thickness uniformity is improved, the porosity becomes higher, and the permeability is increased. In addition, by setting the stretching temperature to 125°C or lower, melting of the sheet can be prevented, a decrease in permeability caused by pore blockage can be prevented, and excessive increase in pore diameter can be prevented.

[0101] From the viewpoint of film thickness uniformity, the stretching ratio varies depending on the thickness of the gel-like sheet, but it is preferably stretched 3 times or more in both the MD and TD directions. For the area stretching ratio, it is preferably 9 times or more. By setting the area stretching ratio to 9 times or more, more preferably 16 times or more, and further preferably 25 times or more, orientation proceeds, the crystallinity becomes higher, and it is easy to obtain a uniform film. From the viewpoints of melting point and strength, an excellent microporous membrane can also be obtained. The area stretching ratio is preferably 100 times or less. By setting the area stretching ratio to 100 times or less, breakage that often occurs in the production of the microporous membrane can be prevented, and productivity can be reduced.

[0102] By stretching, the hierarchical structure formed in the gel sheet is broken, the crystalline phase is refined, and a large number of fibrils are formed. The fibrils form a three-dimensionally irregularly connected network structure. Since the mechanical strength is increased by stretching and the fibrils are broken to form pores, the pore diameter becomes smaller, so a polyolefin microporous membrane suitable for a battery separator or a liquid filter is obtained. In addition, by stretching before removing the plasticizer, the polyolefin is in a sufficiently plasticized and softened state, so the breakage of the hierarchical structure proceeds smoothly, and the refinement of the crystalline phase can be carried out uniformly. In addition, since breakage is easy, strain during stretching is not easily retained, and the thermal shrinkage rate can be made lower than in the case of stretching after removing the plasticizer.

[0103] (d) Washing / drying process

[0104] Next, in the washing process, the plasticizer remaining in the gel-like sheet is removed using a washing solvent. Since the polyethylene phase and the solvent phase are separated in the fine structure of the gel-like sheet, a microporous membrane is obtained by removing the plasticizer.

[0105] As the above-mentioned washing solvent, for example, saturated hydrocarbons such as pentane, hexane, and heptane, chlorinated hydrocarbons such as dichloromethane and carbon tetrachloride, ethers such as diethyl ether and di ane, ketones such as methyl ethyl ketone, and chain fluorocarbons such as trifluoroethane can be mentioned. These washing solvents have a low surface tension, specifically 24 mN / m or less at 25°C. By using a washing solvent with a low surface tension, the formation of a microporous network structure is suppressed from shrinking by the surface tension at the gas-liquid interface during drying after washing, and a microporous membrane with porosity and permeability can be obtained. These washing solvents are appropriately selected according to the plasticizer and can be used alone or in combination.

[0106] The washing method can be carried out by a method of immersing the gel-like sheet in the washing solvent for extraction, a method of spraying the washing solvent onto the gel-like sheet, or a method using a combination thereof. The amount of the washing solvent used varies depending on the washing method, but generally, it is preferably 300 parts by mass or more relative to 100 parts by mass of the gel-like sheet. The washing temperature can be 15 to 30°C and can be heated to 80°C or less as needed. In the case of washing by immersion, from the viewpoints of improving the washing effect of the solvent, the physical properties of the microporous membrane in the TD direction and / or MD direction of the obtained microporous membrane not becoming uneven, and improving the mechanical and electrical properties of the microporous membrane, the longer the time for immersing the gel-like sheet in the washing solvent, the better. The above-mentioned washing is preferably carried out until the residual solvent in the washed gel-like sheet, that is, the microporous membrane, becomes less than 1% by mass.

[0107] After the washing step, in the drying step, the solvent in the microporous membrane is dried and removed. As the drying method, there is no particular limitation, and a method using a metal heating roll, a method using hot air, etc. can be selected. The drying temperature is preferably 40 to 100°C, more preferably 40 to 80°C. If the drying is insufficient, the porosity of the microporous membrane decreases and the permeability deteriorates in the subsequent heat treatment.

[0108] (e) Heat treatment / re-stretching step

[0109] The dried microporous membrane can be stretched (re-stretched). The re-stretching can be carried out by a tenter method or the like while heating the microporous membrane in the same manner as the above-mentioned stretching. The re-stretching can be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, it is carried out by combining simultaneous biaxial or / and successive stretching.

[0110] As the combination of the stretching device and the stretching method used in the re-stretching, examples include MD uniaxial stretching using a roller stretching machine, TD uniaxial stretching using a tenter, successive biaxial stretching using a combination of a roller stretching machine and a tenter or a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter.

[0111] The temperature of heat treatment / re-drawing is preferably below the melting point of the polyethylene composition, more preferably in the range of (Tcd - 20°C) to the melting point. Specifically, it is preferably 70 to 135°C, more preferably 80 to 125°C, and further preferably 90 to 120°C. If the temperature of heat treatment / re-drawing is too high, the pore diameter sometimes becomes excessively large.

[0112] The draw ratio during re-drawing is preferably 0.8 times or more and 1.6 times or less based on the film area before re-drawing. When the draw ratio of re-drawing is 0.8 times or more, more preferably 0.9 times or more, and further preferably 1.0 times or more, the permeability becomes better. In addition, when the draw ratio of re-drawing is 1.6 times or less, more preferably 1.5 times or less, and further preferably 1.4 times or less, the excessive increase in the pore diameter can be suppressed. In addition, a ratio less than 1.0 times means that the film is relaxed.

[0113] (f) Other processes

[0114] Furthermore, according to other uses, the microporous membrane can also be subjected to a hydrophilic treatment. The hydrophilic treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, etc.

[0115] In the case of surfactant treatment, nonionic surfactants, cationic surfactants, anionic surfactants, or zwitterionic surfactants can all be used, but nonionic surfactants are preferred. The multilayer microporous membrane is immersed in a solution in which the surfactant is dissolved in water or a lower alcohol such as methanol, ethanol, or isopropanol, or the solution is coated on the multilayer microporous membrane by a doctor blade method.

[0116] The polyolefin microporous membrane can also be crosslinked by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, or electron rays. In the case of irradiating with electron rays, the electron ray dose is preferably 0.1 to 100 Mrad, and the acceleration voltage is preferably 100 to 300 kV. Through the crosslinking treatment, the melting temperature of the polyolefin microporous membrane rises. The above monomer grafting is preferably carried out after the crosslinking treatment.

[0117] For the purpose of imparting functions such as melting characteristics, heat resistance, and adhesiveness, the polyolefin microporous membrane of the present invention can be laminated with other porous layers other than polyolefin by coating, vapor deposition, etc. to form a multilayer polyolefin porous membrane.

[0118] There is no particular limitation for other porous layers. For example, a porous layer such as an inorganic particle layer containing an adhesive and inorganic particles can be laminated.

[0119] As the binder component constituting the inorganic particle layer, there is no particular limitation, and known components can be used. For example, acrylic resins, poly(1,1-difluoroethylene) resins, polyamideimide resins, polyamide resins, aromatic polyamide resins, polyimide resins, etc. can be used.

[0120] As the inorganic particles constituting the inorganic particle layer, there is no particular limitation, and known materials can be used. For example, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon, etc. can be used. In addition, as the multilayer polyolefin porous membrane, it can be a material in which the above-mentioned binder resin made porous is laminated on at least one surface of the polyolefin microporous membrane.

[0121] The polyolefin microporous membrane of the present invention can be used in various applications such as filters, separators for fuel cells, and separators for capacitors. In particular, since it has excellent dendrite resistance and output characteristics when used as a separator for batteries, it can be preferably used as a separator for secondary batteries that require high energy density, high capacity, and high output, such as electric vehicles. In addition, since it has excellent filtration accuracy and high permeability when used for liquid filter applications, it can be preferably used as a liquid filter for semiconductor resist applications that require high-precision filtration. The polyolefin microporous membrane of the present invention can be used as a liquid filter for filter units in the form of sheets, tubes, pleats, etc. Since it can provide a large filtration area, a pleated filter unit is preferred. When incorporated into a pleated filter unit, it is preferable to laminate a net made of a resin raw material and a reinforcing membrane made of a porous body on at least one surface of the polyolefin microporous membrane of the present invention. After bonding the reinforcing membrane to a heating roller or the like, valley creases can be introduced and woven in a pleated manner and incorporated into the filter unit for use.

[0122] Examples

[0123] The present invention will be further described in detail by way of examples. However, the embodiments of the present invention are not limited to these examples. In addition, the evaluations in this application were carried out in an environment of temperature 23°C and humidity 65% unless otherwise specified. The evaluation methods and analysis methods used in the examples are as described below.

[0124] [Film thickness]

[0125] The film thickness of the polyolefin microporous membrane was measured at five randomly selected points within a range of 50 mm × 50 mm using a LITEMATIC (registered trademark) VL-50 manufactured by Mitutoyo Corporation as a contact thickness gauge (super-hard spherical tip, measurement load 0.01 N), and the average value was taken as the film thickness (μm).

[0126] [Porosity]

[0127] Samples were cut from the polyolefin microporous membrane in a 50 mm × 50 mm square, and the mass (g) was measured. The film thickness was measured by the above method, and the volume (cm 3 ) was calculated. In addition to the above mass and volume, the porosity of the polyolefin microporous membrane was calculated from the value of the film density (g / cm 3 ) by the following formula. In addition, the film density was assumed to be a constant value of 0.99 g / cm 3 for the calculation. This measurement cut samples from 3 randomly selected positions on the polyolefin microporous membrane, and the average value of the porosity measured from each was calculated.

[0128] Porosity (%) = [(Volume - Mass / Film density) / Volume] × 100... (Formula).

[0129] [Air Permeability Resistance]

[0130] For the microporous membrane with a film thickness T1 (μm), in accordance with JIS P-8117 (2009), the air permeability resistance P1 (seconds / 100 cm 3 ) was measured using a Wang-type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). In addition, the air permeability resistance P2 (converted to a thickness of 10 μm) (seconds / 100 cm 3 / 10 μm) was calculated by the formula: P2 = (P1 × 10) / T1.

[0131] [Viscosity-Average Molecular Weight (Mv) of Polyolefin]

[0132] The viscosity-average molecular weight Mv was determined in accordance with ISO1628-3 (2010) by the method shown below. First, 20 mg of the polyolefin resin was weighed, purged with nitrogen, and then 20 mL of decalin was added. The mixture was stirred at 150 °C for 2 hours to dissolve the polyolefin resin. The solution was placed in a constant temperature bath at 135 °C, and the falling time (ts) between the calibration marks was measured using a Cannon-Fenske viscometer (manufactured by Shibata Scientific Instruments Co., Ltd.: product number - 100). For samples with the resin amount changed to 10 mg, 5 mg, 2 mg, and 0 mg, the falling time (ts) between the calibration marks was also measured in the same manner. The reduced viscosity (ηsp / C) of the polyethylene resin composition was calculated according to the following formula.

[0133] ηsp / C = (ts / tb - 1) / 0.1 (unit: dL / g)

[0134] The relationship between the concentration (C) (unit: g / dL) and the reduced viscosity (ηsp / C) of the polyethylene resin composition was plotted respectively, and the approximate linear equation was derived by the least squares method. By extrapolating to a concentration of 0, the intrinsic viscosity ([η]) was obtained. Next, the viscosity-average molecular weight (Mv) was calculated from the value of the above intrinsic viscosity [η] using the following formula.

[0135] Mv = (5.34×10 4 )×[η] 1.49 …(Equation).

[0136] [Pore size distribution of the polyolefin microporous membrane measured by mercury intrusion method]

[0137] Using a fine pore size distribution measuring device (AutoPore V9620 manufactured by Micromeritics), the pore distribution was obtained in the range of pore diameters from 0.004 to 200 μm. The fine pore diameter was calculated using the following equation (Washburn equation).

[0138] PD = -4σcosθ…(Equation)

[0139] Where

[0140] P: Pressure (kPa)

[0141] σ: Surface tension of mercury (480 dyn / cm)

[0142] D: Pore diameter (μm)

[0143] θ: Contact angle of mercury with the sample (140°).

[0144] The method for calculating X2 / X1 from the pore size distribution curve (X-axis: pore size, Y-axis: dV / d(LogD)) obtained by mercury intrusion method is shown below in I to III.

[0145] I. Read the maximum value of dV / d(LogD) (Y max ) and the value of the X-axis at this time (X max ) in the range of the X-axis from 0.01 μm to 10 μm.

[0146] II. In the range of the X-axis from 0.01 μm to 10 μm of the above pore size distribution curve, for the pore size that becomes the intersection point with Y = Y max / 2, set the one closest to 0.01 μm as X1 and the one closest to 10 μm as X2.

[0147] III. Calculate X2 / X1 from X1 and X2 (X1 < X2) obtained by the above method.

[0148] [Bubble point]

[0149] Using a pore size distribution analyzer (manufactured by PMI Co., Ltd., CFP-1500A), the bubble point of the polyolefin microporous membrane was determined. As the impregnating liquid for the polyolefin microporous membrane, GALWICK (1,1,2,3,3,3-hexafluoropropylene oxide, surface tension: 15.9 dynes / cm) was used, and the pressure at the moment when the flow rate became 20 cc / min or more was set as the bubble point pressure (kPa).

[0150] [Differential Scanning Calorimetry (DSC)]

[0151] 6.0 mg of the sample was sealed in an aluminum pan, and using a differential scanning calorimeter (PYRIS Diamond DSC manufactured by PerkinElmer), the first heating was carried out from 30°C to 230°C at a rate of 10°C / min, held at 230°C for 5 minutes, cooled to 30°C at a rate of 10°C / min, and the second heating was carried out from 30°C to 230°C at a rate of 10°C / min. The melting point of the raw polyolefin resin was calculated from the temperature distribution curve of the heat absorption measured during the second heating in the above DSC measurement by drawing a baseline between 60°C and 200°C to obtain the crystal melting peak. In addition, the melting point was set as the temperature at the moment when the maximum heat absorption was shown. Furthermore, the content of the polyethylene-based resin in the polyolefin microporous membrane was obtained from the area of the crystal melting peak (ΔH1) obtained by drawing a baseline between 60°C and 155°C and the area of the crystal melting peak (ΔH2) obtained by drawing a baseline between 155°C and 200°C in the temperature distribution curve of the heat absorption measured during the second heating in the above DSC measurement, and calculated by the following formula.

[0152] Content of polyethylene-based resin in polyolefin microporous membrane (%) = 100 × ΔH1 / (ΔH1 + ΔH2) … (formula).

[0153] [Example 1]

[0154] (First polyolefin solution)

[0155] As the polyolefin raw material for layer A, 100 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C) with a viscosity-average molecular weight (Mv) of 3.0×10 6 was used. 0.5 parts by mass of 2,6-di-tert-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane were added thereto as antioxidants to obtain a polyolefin mixture.

[0156] 15 parts by mass of the obtained polyolefin mixture was put into a strong kneading type twin-screw extruder (inner diameter 58 mm, L / D = 42), and the viscosity at 40°C of 35 cSt (35×10-6 m 2 85 parts by mass of liquid paraffin at 210 °C and 200 rpm were melt-kneaded to prepare a first polyolefin solution.

[0157] (Second polyolefin solution)

[0158] As the polyolefin raw material for the B layer, 100 parts by mass of high-density polyethylene (melting point 135 °C) with a viscosity-average molecular weight (Mv) of 3.7×10 5 was used. 0.5 parts by mass of 2,6-di-tert-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane were added thereto as antioxidants to obtain a polyolefin mixture.

[0159] 25 parts by mass of the obtained polyolefin mixture were put into a strongly kneading twin-screw extruder (inner diameter 58 mm, L / D = 42), and 75 parts by mass of liquid paraffin with a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40 °C was supplied from the side feeder of the twin-screw extruder and melt-kneaded at 210 °C and 200 rpm to prepare a second polyolefin solution.

[0160] (Gel-like sheet)

[0161] After removing foreign matters from the first and second polyolefin solutions by passing them through a filter from each twin-screw extruder, they were supplied to a three-layer T-die, and extruded by adjusting the layer composition of the second polyolefin solution / the first polyolefin solution / the second polyolefin solution (B layer / A layer / B layer) so that the discharge amount ratio of each layer became 1 / 2 / 1 (B layer / A layer / B layer). The extruded molded body was cooled while being drawn at a drawing speed of 1.5 m / minute with a cooling roll adjusted to 30 °C to form a gel-like three-layer sheet.

[0162] (Stretching)

[0163] The above gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 100 °C by a stretching machine.

[0164] (Washing / Drying)

[0165] The stretched gel-like three-layer sheet was immersed in a dichloromethane bath adjusted to 25 °C to thoroughly remove the liquid paraffin, and air-dried at room temperature.

[0166] (Heat setting treatment)

[0167] The obtained dried film was heat-set at 100 °C for 3 minutes.

[0168] The thickness of the obtained polyolefin porous membrane is 29 μm. The mixing ratios of the respective components constituting it, manufacturing conditions, evaluation results, etc. are shown in Table 1.

[0169] [Example 2]

[0170] (First polyolefin solution)

[0171] As the polyolefin raw material for layer A, 100 parts by mass of ultra-high molecular weight polyethylene (melting point 135 °C) having a viscosity-average molecular weight (Mv) of 3.5×10 6 was used. The same antioxidant as in Example 1 was added thereto in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0172] 20 parts by mass of the obtained polyolefin mixture was operated in the same manner as in Example 1 and melt-kneaded with 80 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40 °C to prepare a first polyolefin solution.

[0173] (Second polyolefin solution)

[0174] The same substance as the second polyolefin solution used in Example 1 was used as the second polyolefin solution.

[0175] (Gel-like sheet)

[0176] Using the above first and second polyolefin solutions, the pulling speed of the cooling roll was set to 4 m / minute, and adjusted so that the thickness of the polyolefin microporous membrane became 11 μm. Otherwise, the operation was the same as in Example 1 to form a gel-like three-layer sheet.

[0177] (Stretching, washing / drying, heat setting treatment)

[0178] Using the above gel-like three-layer sheet, the following operation was the same as in Example 1 to obtain a polyolefin porous membrane.

[0179] [Example 3]

[0180] In the formation of the gel-like sheet, the discharge amount ratio of each layer was 2 / 1 / 2 (layer B / layer A / layer B). Otherwise, the operation was the same as in Example 1 to obtain a polyolefin porous membrane.

[0181] [Example 4]

[0182] (First polyolefin solution)

[0183] The same substance as the first polyolefin solution used in Example 1 was used as the first polyolefin solution.

[0184] (Second polyolefin solution)

[0185] As the polyolefin raw material for layer B, 30 parts by mass of high-density polyethylene (melting point 133°C) having a viscosity-average molecular weight (Mv) of 2.0×10 6 and 70 parts by mass of high-density polyethylene (melting point 135°C) having an Mv of 3.7×10 5 were used. 0.5 part by mass of 2,6-di-tert-butyl-p-cresol and 0.7 part by mass of tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane were added thereto as antioxidants to obtain a polyolefin mixture.

[0186] 30 parts by mass of the obtained polyolefin mixture was operated in the same manner as in Example 1, and melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40°C to prepare a first polyolefin solution.

[0187] (Gel-like sheet, stretching, washing / drying, heat setting treatment)

[0188] Using the above first and second polyolefin solutions, a polyolefin porous membrane was obtained in the same manner as in Example 1 below.

[0189] [Example 5]

[0190] (First polyolefin solution)

[0191] As the polyolefin raw material for layer A, 70 parts by mass of ultra-high molecular weight polyethylene (melting point 128°C) having a viscosity-average molecular weight (Mv) of 3.0×10 6 and 30 parts by mass of high-density polyethylene (melting point 132°C) having an Mv of 9.0×10 4 were used. The same antioxidants as in Example 1 were added thereto in the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0192] 23 parts by mass of the obtained polyolefin mixture was operated in the same manner as in Example 1, and melt-kneaded with 77 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40°C to prepare a first polyolefin solution.

[0193] (Second polyolefin solution)

[0194] The same substance as the second polyolefin solution used in Example 1 was used as the second polyolefin solution.

[0195] (Gel-like sheet)

[0196] After removing foreign substances from the above-mentioned first and second polyolefin solutions by passing them through a filter in each twin-screw extruder, they are supplied to a T-die for three layers, and are extruded by adjusting the layer composition of the second polyolefin solution / the first polyolefin solution / the second polyolefin solution (B layer / A layer / B layer) so that the discharge amount ratio of each layer becomes 2 / 1 / 2 (B layer / A layer / B layer). The extruded molded body is cooled while being pulled at a pulling speed of 4 m / minute by a cooling roll adjusted to a temperature of 30°C, forming a gel-like three-layer sheet.

[0197] (Stretching)

[0198] The above-mentioned gel-like three-layer sheet is simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 100°C by a stretching machine.

[0199] (Washing / Drying)

[0200] The stretched gel-like three-layer sheet is immersed in a dichloromethane bath adjusted to a temperature of 25°C to thoroughly remove liquid paraffin, and is air-dried at room temperature.

[0201] (Heat Setting Treatment)

[0202] The obtained dried film is subjected to a heat setting treatment at 100°C for 3 minutes.

[0203] [Example 6]

[0204] In the heat setting treatment at 100°C for 3 minutes, re-stretching is performed in the TD direction so that the stretching ratio becomes 1.3 times. Except for this, the operation is the same as in Example 5 to obtain a polyolefin porous membrane.

[0205] [Comparative Example 1]

[0206] The second polyolefin solution is made to have the same composition as the first polyolefin solution, the pulling speed of the cooling roll is made 4 m / minute, and it is adjusted so that the thickness of the polyolefin microporous membrane becomes 11 μm. Except for this, the operation is the same as in Example 1 to obtain a polyolefin porous membrane.

[0207] [Comparative Example 2]

[0208] The first polyolefin solution is made to have the same composition as the second polyolefin solution, the pulling speed of the cooling roll is made 4 m / minute, and it is adjusted so that the thickness of the polyolefin microporous membrane becomes 10 μm. Except for this, the operation is the same as in Example 1 to obtain a polyolefin porous membrane.

[0209] [Comparative Example 3]

[0210] (The First Polyolefin Solution)

[0211] As the polyolefin raw material for the A layer, 30 parts by mass of high-density polyethylene (melting point: 133°C) with a viscosity-average molecular weight (Mv) of 2.0×10 6 and 70 parts by mass of high-density polyethylene (melting point: 135°C) with a viscosity-average molecular weight (Mv) of 3.7×10 5 were used. An antioxidant identical to that in Example 1 was added to them at the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0212] 30 parts by mass of the obtained polyolefin mixture was subjected to the same operation as in Example 1 and melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40°C to prepare a first polyolefin solution.

[0213] (Second polyolefin solution)

[0214] As the polyolefin raw material for the B layer, 50 parts by mass of high-density polyethylene (melting point: 135°C) with a viscosity-average molecular weight (Mv) of 3.7×10 5 and 50 parts by mass of polypropylene (melting point: 163°C) with a viscosity-average molecular weight (Mv) of 2.0×10 6 were used. An antioxidant identical to that in Example 1 was added to them at the same mass ratio as in Example 1 to obtain a polyolefin mixture.

[0215] 30 parts by mass of the obtained polyolefin mixture was subjected to the same operation as in Example 1 and melt-kneaded with 70 parts by mass of liquid paraffin having a viscosity of 35 cSt (35×10 -6 m 2 / s) at 40°C to prepare a second polyolefin solution.

[0216] (Gel-like sheet)

[0217] After removing foreign matters from the above-mentioned first and second polyolefin solutions through a filter in each twin-screw extruder, they were supplied to a three-layer T-die. Using the layer structure of the second polyolefin solution / first polyolefin solution / second polyolefin solution (A layer / B layer / A layer), extrusion was adjusted so that the discharge amount ratio of each layer became 2 / 1 / 2 (A layer / B layer / A layer). The extruded molded body was cooled while being pulled at a pulling speed of 4 m / minute by a cooling roll adjusted to 30°C to form a gel-like three-layer sheet.

[0218] (Stretching)

[0219] The above-mentioned gel-like three-layer sheet was simultaneously biaxially stretched 5 times in both the MD direction and the TD direction at 110°C by a stretching machine.

[0220] (Washing / drying)

[0221] The stretched gel-like three-layer sheet was immersed in a dichloromethane bath adjusted to a temperature of 25°C to thoroughly remove the liquid paraffin, and then air-dried at room temperature.

[0222] (Heat setting treatment)

[0223] The obtained dry film was subjected to a heat setting treatment at 110°C for 3 minutes.

[0224] The thickness of the obtained polyolefin porous membrane was 10 μm.

[0225] [Comparative Example 4]

[0226] The second polyolefin solution was made to have the same composition as the first polyolefin solution, and otherwise, the same operations as in Example 5 were carried out to obtain a polyolefin porous membrane.

[0227] [Comparative Example 5]

[0228] The first polyolefin solution was made to have the same composition as the second polyolefin solution, and otherwise, the same operations as in Example 5 were carried out to obtain a polyolefin porous membrane.

[0229] [Evaluation]

[0230] For the polyolefin microporous membranes of Examples 1 to 6, since X2 / X1 is 15 or more and the thickness is 30 μm or less, they are polyolefin microporous membranes that are excellent in dendrite resistance and output characteristics when used as a battery separator, and excellent in filtration accuracy and permeability when used as a liquid filter. On the other hand, for the polyolefin microporous membranes of Comparative Examples 1 to 3, X2 / X1 is less than 15.

[0231] [Table 1]

[0232]

[0233] Industrial Applicability

[0234] Since the polyolefin microporous membrane of the present invention is excellent in dendrite resistance and output characteristics when used as a battery separator, it can be suitably used as a battery separator for secondary batteries such as electric vehicles that require high energy density, high capacity, and high output. In addition, since it is excellent in filtration accuracy and high permeability when used for liquid filter applications, it can be suitably used as a high-precision liquid filter for removing minute foreign matters such as in semiconductor processes.

Claims

1. A polyolefin microporous membrane, in the pore size distribution measured by mercury intrusion porosimetry with the pore size on the X-axis and dV / d(LogD) on the Y-axis, when the maximum value of dV / d(LogD) in the pore size range of 0.01 μm to 10 μm is set as Y max , and when the pore sizes satisfying Y max / 2 in the pore size range of 0.01 μm to 10 μm are arranged in ascending order as X1 and X2, X2 / X1 is 15 or more, and the thickness of the polyolefin microporous membrane is 30 μm or less. Among them, V: Cumulative pore volume, in cm 3 / g D: aperture diameter, unit is μm.

2. The polyolefin microporous membrane according to claim 1, having a bubble point pressure of 1900 kPa or more.

3. The polyolefin microporous membrane according to claim 1 or 2, comprising 90% by mass or more of polyethylene.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the air permeability resistance converted from a thickness of 10 μm is 300 seconds / 100 cm 3 or less.

5. The polyolefin microporous membrane according to claim 1 or 2, which is used as a separator for secondary batteries.

6. The polyolefin microporous membrane according to claim 1 or 2, which is used as a filter for liquids.

7. A separator for secondary batteries, which uses the polyolefin microporous membrane according to claim 1 or 2.

8. A filter for liquids, which uses the polyolefin microporous membrane according to claim 1 or 2.

9. A secondary battery, which uses the separator for secondary batteries according to claim 7.

10. A filtration unit, which uses the filter for liquids according to claim 8.

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