Polyolefin microporous membrane

Abstract

A polyolefin microporous membrane which contains a polyolefin and which has a Gurley value of from 1 second / 100 mL to 20 second / 100 mL (inclusive), a thickness of from 5 [mu] m to 25 [mu] m (inclusive), a porosity of from 65% to 85% (inclusive), and an average pore diameter of from 0.05 [mu] m to less than 0.30 [mu] m.

Description

Technical Field

[0001] The present disclosure relates to polyolefin microporous membranes. Background Art

[0002] Nonwoven fabrics are widely used as a means for collecting foreign matter in gas or liquid. For example, Patent Document 1 describes a liquid filter made of nonwoven fabric that can be used in an immersion-type filter cartridge.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2019-198833 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] If the size of the target substance decreases to approximately 0.1 μm to 1.0 μm, non-woven filters may not be able to fully capture it. In the case of non-woven filters, substances are entangled on the constituent fibers to capture them. Therefore, to improve the capture performance of small substances, it is necessary to increase the thickness of the non-woven fabric. However, increasing the thickness of the non-woven fabric also leads to problems such as the filter becoming larger. Therefore, filters that can efficiently capture small substances are required.

[0008] Furthermore, when using a filter continuously or utilizing the captured substances for a specific purpose, it is preferable that the proportion of captured substances that can be recovered from the filter is as high as possible. Specifically, a filter with excellent recovery performance for captured substances is required, but a nonwoven fabric filter does not achieve a sufficient recovery rate.

[0009] Furthermore, from the perspective of the filter's processing efficiency, it is more preferable that the amount of undesirable substances that permeate the filter per unit time is greater. In other words, a filter with excellent performance in selectively allowing substances other than the target to be captured to pass through is required.

[0010] There are limits to achieving these required properties using nonwoven fabrics. However, polyolefin microporous membranes are obtained by forming fibrillar polyolefin into a three-dimensional network, and therefore have the potential to simultaneously achieve these required properties.

[0011] In view of the above circumstances, an object of embodiments of the present disclosure is to provide a polyolefin microporous membrane having excellent performance in capturing small-sized substances, recovering the captured substances, and transmitting substances other than the captured substances.

[0012] Means for solving problems

[0013] Specific means for solving the aforementioned problems include the following aspects.

[0014] <1> A polyolefin microporous membrane comprises polyolefin, wherein the polyolefin microporous membrane has a Gurley value of 1 sec / 100 mL to 20 sec / 100 mL, a thickness of 5 μm to 25 μm, a porosity of 65% to 85%, and an average pore size of 0.05 μm to less than 0.30 μm.

[0015] <2> like <1> The polyolefin microporous membrane is characterized in that the polyolefin comprises polyethylene.

[0016] <3> like <2> In the polyolefin microporous membrane, the polyethylene comprises ultra-high molecular weight polyethylene, and the proportion of the ultra-high molecular weight polyethylene in the entire polyolefin is 65% by mass or less.

[0017] <4> like <1> ~ <3> The polyolefin microporous membrane according to any one of the preceding claims has a Gurley value of 5 sec / 100 mL or more and 15 sec / 100 mL or less.

[0018] <5> like <1> ~ <4> The polyolefin microporous membrane according to any one of the preceding claims has a porosity of 70% to 80%.

[0019] <6> like <1> ~ <5> The polyolefin microporous membrane described above has an average pore diameter of 0.10 μm to 0.25 μm.

[0020] <7> The polyolefin microporous membrane according to claim 1, wherein the ratio of the tensile strength at break in the MD direction to the tensile strength at break in the TD direction (MD direction / TD direction) is 0.75 or more and 1.25 or less.

[0021] Effects of the Invention

[0022] According to the embodiments of the present disclosure, a polyolefin microporous membrane having excellent performance in capturing small-sized substances, recovering the captured substances, and allowing substances other than the captured substances to pass therethrough can be provided. DETAILED DESCRIPTION

[0023] The following describes embodiments of the invention. These descriptions and examples are provided for illustrative purposes only and do not limit the scope of the invention. The mechanisms of action described in this disclosure include inferences, and their accuracy does not limit the scope of the invention.

[0024] In the present disclosure, a numerical range expressed using “to” indicates a range including the numerical values ​​described before and after “to” as the lower limit and the upper limit, respectively.

[0025] In the numerical ranges described in stages in this disclosure, the upper limit or lower limit described in one numerical range may be replaced by the upper limit or lower limit of the numerical range described in another stage. In addition, in the numerical ranges described in this disclosure, the upper limit or lower limit of the numerical range may be replaced by the value shown in the Examples.

[0026] In the present disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process can be achieved.

[0027] In this disclosure, each component may include multiple corresponding substances. In this disclosure, when referring to the amount of each component in a composition, if multiple substances belonging to each component are present in the composition, unless otherwise specified, the amount refers to the total amount of the multiple substances present in the composition.

[0028] In this disclosure, the “MD direction (machine direction)” of a polyolefin microporous membrane refers to the longitudinal direction of a polyolefin microporous membrane produced in an elongated shape, and the “TD direction (width direction)” refers to a direction perpendicular to the “MD direction”.

[0029] <Polyolefin Microporous Film>

[0030] The polyolefin microporous membrane of the present disclosure is a polyolefin microporous membrane comprising polyolefin, having a Gurley value of 1 s / 100 mL to 20 s / 100 mL, a thickness of 5 μm to 25 μm, a porosity of 65% to 85%, and an average pore size of 0.05 μm to less than 0.30 μm.

[0031] The polyolefin microporous membrane of the present disclosure exhibits excellent performance in capturing small-sized substances, recovering captured substances, and allowing the permeation of substances other than the target substance. Specifically, the capture performance is particularly excellent when the target substance is 0.1 μm or larger and 1.0 μm or smaller. The target substance can be present in a fluid, which can be either a liquid or a gas.

[0032] Polyolefin microporous membranes with an average pore size of more than 0.05 μm and less than 0.30 μm generally have a tendency to increase in Gurley value. However, in the present disclosure, by making the thickness more than 5 μm and less than 25 μm and making the porosity more than 65% and less than 85%, a physical property such as a Gurley value of more than 1 second / 100mL and less than 20 seconds / 100mL is achieved. Thus, the performance of capturing small-sized substances and the permeability of the fluid are well balanced, and the recovery performance of the captured substance is also achieved at a high level. If the recovery performance of the substance is good, then when, for example, a polyolefin microporous membrane is used as a liquid or gas filter, the reuse of the filter becomes possible, and the environmental load is small. In addition, for example, in the case of using the captured substance for the purpose, recyclability becomes better and productivity is high.

[0033] In this disclosure, the term "polyolefin microporous membrane" refers to a microporous membrane comprising polyolefin. A microporous membrane is a membrane comprising a three-dimensional network structure formed by interconnected fibrils of polyolefin, having a large number of micropores within the membrane, and having a structure formed by interconnected micropores, allowing gas or liquid to pass from one surface to the other.

[0034] Conventional polyolefin microporous membranes have room for improvement in at least one of the performance of capturing small-sized substances (hereinafter also referred to as capture performance), the performance of recovering the captured substances (hereinafter also referred to as recovery performance), and the performance of allowing substances other than the captured objects to pass through (hereinafter also referred to as permeation performance).

[0035] The polyolefin microporous membrane of the present disclosure, which satisfies specific conditions of Gurley value, thickness, porosity, and average pore diameter, satisfies these properties in a well-balanced manner.

[0036] In the present disclosure, the size of the substance to be captured is not particularly limited.

[0037] For example, when the substance to be captured is in a particulate form, the average particle size of the substance to be captured may be in the range of 0.05 μm to 10 μm, or in the range of 0.1 μm to 5 μm.

[0038] In this disclosure, the average particle size of the substance to be captured is determined by image analysis. Specifically, 100 particles are observed using an electron microscope or the like, and the arithmetic mean of the observed particle sizes is taken as the average particle size of the substance to be captured.

[0039] The captured object may be contained in a liquid or a gas.

[0040] [Polyolefin constituting the polyolefin microporous membrane]

[0041] The polyolefin microporous membrane of the present disclosure may be formed solely of polyolefin or may be formed of polyolefin and a material other than polyolefin. When the polyolefin microporous membrane is formed of polyolefin and a material other than polyolefin, the proportion of polyolefin in the overall polyolefin microporous membrane is preferably 50% by mass or greater, more preferably 70% by mass or greater, and even more preferably 80% by mass or greater.

[0042] The polyolefin contained in the polyolefin microporous membrane may be only one kind, or may be two or more kinds different in molecular structure, molecular weight, etc.

[0043] The polyolefin microporous membrane may contain polyethylene as the polyolefin. In this case, the polyolefin microporous membrane may contain only polyethylene as the polyolefin, or may contain polyethylene and a polyolefin other than polyethylene (e.g., polypropylene). When the polyolefin microporous membrane contains polyethylene and a polyolefin other than polyethylene as the polyolefin, the proportion of polyethylene in the total polyolefin is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0044] The polyolefin microporous membrane may contain ultrahigh molecular weight polyethylene (UHMWPE) as the polyethylene contained in the polyolefin. If the polyolefin microporous membrane contains ultrahigh molecular weight polyethylene, the pore size of the polyolefin microporous membrane will not be too large, and there is a tendency for the collection performance to be excellent.

[0045] When the polyolefin microporous membrane comprises ultra-high molecular weight polyethylene, the proportion of the ultra-high molecular weight polyethylene in the total polyolefin is preferably 65% ​​by mass or less. If the proportion of the ultra-high molecular weight polyethylene in the total polyolefin is 65% by mass or less, the pore size of the polyolefin microporous membrane will not be too small, and there is a tendency for excellent permeability. The proportion of the ultra-high molecular weight polyethylene in the total polyolefin is preferably 63% by mass or less, and more preferably 61% by mass or less.

[0046] In addition, when the polyolefin microporous membrane includes ultrahigh molecular weight polyethylene, the proportion of the ultrahigh molecular weight polyethylene in the total polyolefin is preferably 1% by mass or more. If the proportion of the ultrahigh molecular weight polyethylene in the total polyolefin is 1% by mass or more, the mechanical strength of the polyolefin microporous membrane is easily improved. The proportion of the ultrahigh molecular weight polyethylene in the total polyolefin is preferably 3% by mass or more, and more preferably 5% by mass or more.

[0047] When the polyolefin microporous membrane comprises ultra-high molecular weight polyethylene and polyolefins other than ultra-high molecular weight polyethylene (hereinafter referred to as other polyolefins) as polyolefins, the type of other polyolefins is not particularly limited. As other polyolefins, high-density polyethylene (HDPE) is preferred. In this disclosure, high-density polyethylene refers to a polyethylene having a density of 942 kg / m 3 More polyethylene.

[0048] In the present disclosure, ultra-high molecular weight polyethylene refers to polyethylene having a weight average molecular weight of 1,000,000 to 6,000,000.

[0049] The weight average molecular weight of the ultrahigh molecular weight polyethylene is preferably 3 million or more, more preferably 4 million or more. Furthermore, the weight average molecular weight of the ultrahigh molecular weight polyethylene is preferably 5 million or less, more preferably 4.8 million or less.

[0050] In the present disclosure, the weight average molecular weight of polyolefin can be measured by gel permeation chromatography.

[0051] Specifically, the polyolefin to be measured can be obtained by dissolving it in o-dichlorobenzene with heating, and measuring it by gel permeation chromatography (system: Alliance GPC 2000, manufactured by Waters, columns: GMH6-HT and GMH6-HTL) at a column temperature of 135°C and a flow rate of 1.0 mL / min. For molecular weight calibration, monodisperse polystyrene (manufactured by TOSOH) is used.

[0052] [Gurley value]

[0053] The polyolefin microporous membrane of the present disclosure has a Gurley value of 1 sec / 100 mL or more and 20 sec / 100 mL or less.

[0054] The polyolefin microporous membrane of the present disclosure can ensure sufficient collection performance because of a Gurley value of 1 sec / 100 mL or more, and can ensure sufficient permeability because of a Gurley value of 20 sec / 100 mL or less.

[0055] From the viewpoint of collection performance, the Gurley value of the polyolefin microporous membrane is preferably 5 seconds / 100 mL or more, and more preferably 7 seconds / 100 mL or more.

[0056] From the viewpoint of permeability, the Gurley value of the polyolefin microporous membrane is preferably 15 seconds / 100 mL or less, and more preferably 13 seconds / 100 mL or less.

[0057] In this disclosure, the Gurley value of a polyolefin microporous membrane is measured using a Toyo Seiki Gurley Density Meter in accordance with JIS P8117:2009. The measurement is performed by measuring the time it takes for 200 ml of air to pass through a 28.6 mm diameter sample. The value obtained by multiplying this time by half and converting it to a value per 100 ml is used.

[0058] [thickness]

[0059] The polyolefin microporous membrane of the present disclosure has a thickness of 5 μm or more and 25 μm or less.

[0060] The polyolefin microporous membrane of the present disclosure can ensure sufficient collection performance because of its thickness of 5 μm or more, and can ensure sufficient permeability because of its thickness of 25 μm or less.

[0061] From the viewpoint of collection performance, the thickness of the polyolefin microporous membrane is preferably 7 μm or more, more preferably 10 μm or more.

[0062] From the viewpoint of permeability, the thickness of the polyolefin microporous membrane is preferably 22 μm or less, more preferably 20 μm or less.

[0063] In this disclosure, the thickness of the polyolefin microporous membrane is measured using a contact-type film thickness meter (manufactured by Mitutoyo, ABS digital display meter, Model ID: ID-S112X). Specifically, the thickness of the polyolefin microporous membrane is measured at 20 points, and the arithmetic mean thereof is used as the thickness of the polyolefin microporous membrane. As a contact terminal, a cylindrical terminal with a bottom diameter of 6.5 mm is used. In addition, as another method for measuring the thickness of the polyolefin microporous membrane, the following method can be cited: using a contact-type thickness meter (manufactured by Mitutoyo, LITEMATIC), using a cylindrical measuring terminal with a diameter of 5 mm, adjusting it in a manner that applies a 7 g load during the measurement, measuring at 20 points, and performing arithmetic averaging on them, thereby obtaining the thickness.

[0064] [Porosity]

[0065] The polyolefin microporous membrane of the present disclosure has a porosity of 65% or more and 85% or less.

[0066] The polyolefin microporous membrane of the present disclosure can ensure sufficient permeability due to a porosity of 65% or more, and can ensure sufficient recycling performance due to a porosity of 85% or less.

[0067] From the viewpoint of permeability, the porosity of the polyolefin microporous membrane is preferably 67% or more, more preferably 70% or more.

[0068] From the viewpoint of recycling performance, the porosity of the polyolefin microporous membrane is preferably 83% or less, more preferably 80% or less.

[0069] In this disclosure, the porosity of the polyolefin microporous membrane is calculated according to the following calculation method. That is, the constituent materials of the polyolefin microporous membrane are a, b, c, ..., n, and the mass of each constituent material is Wa, Wb, Wc, ..., Wn (g / cm 2 ), the true density of each constituent material is da, db, dc, ..., dn (g / cm 3 ), when the thickness of the polyolefin microporous membrane is set to t (cm), the porosity ε (%) is calculated using the following formula.

[0070] ε={1-(Wa / da+Wb / db+Wc / dc+…+Wn / dn) / t}×100

[0071] [Average pore size]

[0072] The average pore diameter of the polyolefin microporous membrane of the present disclosure is 0.05 μm or more and less than 0.30 μm.

[0073] The polyolefin microporous membrane of the present disclosure can ensure sufficient permeability because the average pore diameter is 0.05 μm or more, and can ensure sufficient recovery performance because the average pore diameter is less than 0.30 μm.

[0074] From the viewpoint of permeability, the average pore diameter of the polyolefin microporous membrane is preferably 0.10 μm or more, and more preferably 0.11 μm or more.

[0075] From the viewpoint of recycling performance, the average pore diameter of the polyolefin microporous membrane is preferably 0.25 μm or less, and more preferably 0.22 μm or less.

[0076] In this disclosure, the average pore diameter of a polyolefin microporous membrane is determined using a PMI pore size distribution analyzer (Model: CFP-1200-AEXL) and a PMI Galwick (surface tension: 15.9 dyn / cm) immersion liquid using the semi-dry method specified in ASTM E1294-89. The measurement temperature was set at 25°C, and the measurement pressure was varied within the range of 0 kPa to 2500 kPa.

[0077] [M / T ratio]

[0078] The polyolefin microporous membrane of the present disclosure preferably has a ratio of the tensile strength at break in the MD direction to the tensile strength at break in the TD direction (also referred to as MD direction / TD direction, M / T ratio) of 0.75 to 1.25.

[0079] When the M / T ratio of the polyolefin microporous membrane is within the above range, the physical strength of the polyolefin microporous membrane can be sufficiently ensured, and the uniformity in the MD / TD directions can be ensured, thereby improving the capture rate.

[0080] The polyolefin microporous membrane of the present disclosure preferably has an M / T ratio of 0.80 or greater, more preferably 0.85 or greater. Furthermore, the polyolefin microporous membrane of the present disclosure preferably has an M / T ratio of 1.10 or less, more preferably 1.00 or less.

[0081] In this disclosure, the tensile strength at break of polyolefin microporous membranes was measured in accordance with JIS K7176:2014 using a Tensilon tester RTG-1225 manufactured by Orientec. The sample width was set to 15 mm, the distance between the chucks (marked line distance) of the tester was set to 500 mm, and the sample was stretched at a rate of 200 mm / min. The load at break was divided by the sample width (kg / cm). The same measurement was repeated three times using three samples, and the average of the values ​​obtained was used as the tensile strength at break.

[0082] <Method for producing polyolefin microporous membrane>

[0083] The polyolefin microporous membrane can be produced, for example, by a production method including the following steps (I) to (IV).

[0084] Step (I): a step of preparing a solution containing a polyolefin and a solvent.

[0085] Step (II): A step of melt-kneading the solution, extruding the obtained melt-kneaded product from a die, and cooling and solidifying the product to obtain a first gel-like molded product.

[0086] 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.

[0087] Step (IV): a step of stretching the second gel-like molded product in at least one direction (secondary stretching).

[0088] Step (I) is a step of preparing a solution containing a polyolefin and a solvent.

[0089] As the solvent, a solution containing 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 lower than 210° C. at atmospheric pressure can be used.

[0090] Examples of the solvent used to prepare 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, diethylenetriamine, ethylenediamine, dimethyl sulfoxide, and hexane. Liquid paraffin, decalin, or xylene are particularly preferred. One volatile solvent may be used alone, or two or more may be used in combination. Among these, decalin or xylene is preferred.

[0091] The polyolefin used in step (I) may be one type or two or more types, and can be selected based on the desired physical properties of the polyolefin microporous membrane.

[0092] The solution prepared in step (I) preferably has a polyolefin concentration of 10% to 35% by mass, more preferably 15% to 30% by mass, and even more preferably 25% to 32% by mass, from the perspective of controlling the porous structure of the polyolefin microporous membrane. When the polyolefin concentration in the solution is 10% by mass or greater, the occurrence of breakage during the polyolefin microporous membrane formation process can be suppressed, and the mechanical strength of the polyolefin microporous membrane is increased, thereby improving operability. When the polyolefin concentration in the solution is 35% by mass or less, the polyolefin microporous membrane of the present disclosure can be easily obtained.

[0093] Step (II) is a step of melt-kneading the solution prepared in step (I), extruding the resulting melt-kneaded product from a die, and cooling and solidifying the product to obtain a first gel-like molded product. In step (II), for example, an extrudate is obtained by extruding the product from a die within a temperature range of the melting point of the polyolefin to the melting point + 65°C, and then cooling the extrudate 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 can be performed by immersion in water or an organic solvent, contact with a cooled metal roll, or the like.

[0094] 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 of step (III) can be either uniaxial stretching or biaxial stretching. Biaxial stretching can be a sequential biaxial stretching in which longitudinal stretching and transverse stretching are respectively performed, or a simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are simultaneously performed. From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the stretching ratio of the primary stretching (the product of the longitudinal stretching ratio and the transverse stretching ratio) is preferably 1.1 to 3 times, and more preferably 1.1 to 2 times. The temperature during the primary stretching is preferably 75°C or less.

[0095] The 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, and more preferably at 60° C. or lower.

[0096] The stretching step and the drying step of step (III) may be performed simultaneously or in stages. For example, a primary stretching may be performed while pre-drying is performed, followed by main drying, or a primary stretching may be performed between pre-drying and main drying. The primary stretching may also be performed while drying is controlled so that the solvent remains in an appropriate state.

[0097] 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) is preferably biaxial stretching. Biaxial stretching can be any of the following methods: 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 the product in the longitudinal direction multiple times followed by stretching the product in the transverse direction; a step of stretching the product in the longitudinal direction and then in the transverse direction multiple times; or a step of sequential biaxial stretching followed by further stretching the product in the longitudinal direction and / or transverse direction one or more times.

[0098] From the perspective of controlling the porous structure of the polyolefin microporous membrane, the stretch ratio (the product of the longitudinal stretch ratio and the transverse stretch ratio) of the secondary stretching is preferably 5 to 90 times, more preferably 10 to 60 times. From the perspective of controlling the porous structure of the polyolefin microporous membrane, the stretching temperature of the secondary stretching is preferably 70°C to 135°C, more preferably 80°C to 130°C.

[0099] If necessary, a heat setting treatment may be performed after step (IV). From the viewpoint of controlling the porous structure of the polyolefin microporous membrane, the temperature of the heat setting treatment is preferably 110°C to 150°C, more preferably 120°C to 140°C.

[0100] If necessary, the solvent remaining in the polyolefin microporous membrane may be extracted and annealed after the heat setting treatment. The extraction of the residual solvent is carried out, for example, by immersing the sheet after the heat setting treatment in a dichloromethane bath to dissolve the residual solvent into the dichloromethane. Preferably, the polyolefin microporous membrane immersed in the dichloromethane bath is lifted out of the dichloromethane bath and then the dichloromethane is removed by drying. The annealing treatment can be carried out after the extraction of the residual solvent by conveying the polyolefin microporous membrane on a roller heated to 70°C to 140°C, for example, or by conveying the polyolefin microporous membrane in a heated atmosphere at 70°C to 140°C while maintaining the width direction constant.

[0101] <Applications of Polyolefin Microporous Membranes>

[0102] The use of the polyolefin microporous membrane is not particularly limited, but specific uses include air filters, liquid filters, moisture-permeable waterproof membranes, bags (bags), and dust-collecting sheet substrates.

[0103] When a polyolefin microporous membrane is used to capture substances, the average particle size of the substances to be captured may be in the range of 0.1 μm to 10 μm, 0.2 μm to 5 μm, or 0.5 μm to 1 μm.

[0104] The substance to be captured may be contained in either liquid or gas.

[0105] Example

[0106] The polyolefin microporous membrane of the present disclosure will be described in more detail below with reference to examples.

[0107] The materials, amounts, ratios, and processing steps shown in the following examples may be modified as appropriate without departing from the spirit of the present disclosure. Therefore, the scope of the polyolefin microporous membrane of the present disclosure should not be interpreted as being limited based on the specific examples shown below.

[0108] [Example 1]

[0109] 60 parts by mass of ultra-high molecular weight polyethylene (UHMWPE) with a weight average molecular weight (Mw) of 4.6 million and 60 parts by mass of polyethylene with a weight average molecular weight (Mw) of 560,000 and a density of 950 kg / m 3 Next, the polyethylene composition was mixed with paraffin as a solvent so that the concentration of the polyethylene composition became 17% by mass to prepare a polyethylene solution.

[0110] The polyethylene solution was extruded from a die in the form of a sheet at a temperature of 178° C., and the extrudate was then cooled in a water bath at a temperature of 20° C. to obtain a first gel-like sheet.

[0111] The first gel-like sheet was pre-dried at 30°C for 6 minutes, then stretched once at 1.2 times in the MD direction, and finally dried at 30°C for 4 minutes to obtain a second gel-like sheet. The residual solvent content in the second gel-like sheet was set to less than 1% by mass.

[0112] Next, as secondary stretching, the second gel-like sheet was stretched in the MD direction at a temperature of 75°C at a magnification of 2.5 times, and then stretched in the TD direction at a magnification of 6.0 times at a temperature of 105°C. Immediately after the secondary stretching, heat treatment (heat setting) was performed at 136°C.

[0113] The heat-set sheet was immersed in a three-well methylene chloride bath for a total of 2.5 minutes to extract the solvent from the sheet. After removing the sheet from the methylene chloride bath, it was dried on heated rollers at 39°C to remove the methylene chloride. The sheet was then annealed in a heated atmosphere at 120°C.

[0114] The above steps yielded a polyolefin microporous membrane of Example 1. Observation of the polyolefin microporous membrane of Example 1 using a scanning electron microscope revealed a three-dimensional network structure composed of interconnected fibrils of polyolefin, with numerous micropores within the membrane formed by interconnected micropores.

[0115] [Examples 2 to 5 and Comparative Examples 1 to 6]

[0116] Polyethylene microporous membranes of Examples 2 to 5 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1, except that the composition of the polyethylene solution and the conditions of the polyolefin microporous membrane production process were changed as described in Table 1. Observation of the polyolefin microporous membranes of Examples 2 to 5 and Comparative Examples 1 to 6 using a scanning electron microscope revealed that the membranes comprised a three-dimensional network structure composed of interconnected fibrillar polyolefins, with numerous micropores internally formed and these interconnected micropores forming a structure.

[0117] [Table 1]

[0118]

[0119] [Measurement of Gurley value]

[0120] The Gurley value of polyolefin microporous membranes was measured in accordance with JIS P8117:2009 using a Toyo Seiki Gurley Density Meter. The time it takes for 200 ml of air to pass through a 28.6 mm diameter sample was measured, and the value was calculated by multiplying this time by half and converting it to a value per 100 ml. The results are shown in Table 2.

[0121] [Measurement of thickness]

[0122] The thickness of the polyolefin microporous membrane was measured using a contact-type membrane thickness meter (manufactured by Mitutoyo, ABS digital meter, Model ID: ID-S112X). Specifically, the thickness of the polyolefin microporous membrane was measured at 20 points, and the arithmetic mean was used as the thickness. A cylindrical contact terminal with a bottom diameter of 6.5 mm was used. The results are shown in Table 2.

[0123] [Porosity Measurement]

[0124] The porosity of the polyolefin microporous membrane is calculated according to the following calculation method. That is, the constituent materials of the polyolefin microporous membrane are a, b, c, ..., n, and the mass of each constituent material is Wa, Wb, Wc, ..., Wn (g / cm 2 ), the true density of each constituent material is da, db, dc, ..., dn (g / cm 3 ), when the thickness of the polyolefin microporous membrane is t (cm), the porosity ε (%) is calculated using the following formula. The results are shown in Table 2.

[0125] ε={1-(Wa / da+Wb / db+Wc / dc+…+Wn / dn) / t}×100

[0126] [Measurement of average pore diameter]

[0127] The average pore diameter of the polyolefin microporous membrane was determined using a PMI pore size distribution analyzer (model: CFP-1200-AEXL) and a PMI Galwick (surface tension 15.9 dyn / cm) immersion liquid using the semi-dry method specified in ASTM E1294-89. The measurement temperature was set at 25°C, and the measurement pressure was varied from 0 kPa to 2500 kPa. The results are shown in Table 2.

[0128] [Determination of tensile breaking strength]

[0129] The tensile strength at break of polyolefin microporous membranes was measured in accordance with JIS K7176:2014 using a Tensilon tester RTG-1225 manufactured by Orientec. The sample width was set to 15 mm, the distance between the chucks of the tester (the distance between the markings) was set to 500 mm, and the sample was stretched at a rate of 200 mm / min. The load at the time of sample breakage was divided by the sample width. The same measurement was performed three times using three samples, and the average value was used to determine the tensile strength at break (kg / cm). The M / T ratio was calculated from the tensile strength values ​​measured in the MD and TD directions of the polyolefin microporous membrane. The results are shown in Table 2.

[0130] [Evaluation of collection performance]

[0131] A 10 cm x 10 cm sample was prepared from a polyolefin microporous membrane. This sample was placed in a testing apparatus equipped with a particle generator. In this state, the following test was conducted: nitrogen gas containing NaCl particles with an average particle size of 0.5 μm was supplied into the apparatus at a filtration rate of 5.3 cm / s, passing through the sample.

[0132] One minute after the start of the test, the concentration D1 of NaCl particles in the space upstream of the sample (the side where nitrogen flows in) and the concentration D2 of NaCl particles in the space downstream of the sample were measured, and the capture rate of NaCl particles was calculated using the following formula. The larger the capture rate, the more NaCl particles were captured by the sample. The results are shown in Table 2.

[0133] Capture rate (%) = (1-D2 / D1) × 100

[0134] [Evaluation of recycling performance]

[0135] Before implementing the test identical with the evaluation of collection performance, the mass Wa of the sample was measured. After 1 minute from the start of the test, the sample was taken out and the mass Wb of the sample was measured. Then, the NaCl particles attached to the sample were patted down by hand and the Wc of the sample was measured. The recovery rate of the NaCl particles was calculated using the following formula. The greater the value of the recovery rate, the greater the proportion of the recovered NaCl particles in the collected NaCl particles. The results are shown in Table 2.

[0136] Recovery rate (%) = {(Wb-Wc) / (Wb-Wa)}×100

[0137] [Evaluation of Transmission Performance]

[0138] The same test as for evaluating collection performance was conducted. One minute after the start of the test, the pressure P1 in the space upstream of the sample and the pressure P2 in the space downstream of the sample were measured. The pressure difference (P1 - P2) was calculated as the pressure loss. A smaller pressure loss indicates that the nitrogen gas supplied to the device more easily permeates the sample. The results are shown in Table 2.

[0139] [Weight average molecular weight of polyolefin]

[0140] The weight average molecular weight of the polyolefin used for producing the polyolefin microporous membrane is measured by gel permeation chromatography.

[0141] Specifically, the polyolefin to be measured was dissolved in o-dichlorobenzene by heating and measured by gel permeation chromatography (system: Alliance GPC 2000, manufactured by Waters, columns: GMH6-HT and GMH6-HTL) at a column temperature of 135°C and a flow rate of 1.0 mL / min. For molecular weight calibration, monodisperse polystyrene (manufactured by TOSOH) was used.

[0142] [determination]

[0143] When the capture rate is 99.99% or more, the capture performance is evaluated as "excellent", when the capture rate is 99.95% or more and less than 99.99%, the capture performance is evaluated as "qualified", and when the capture rate is less than 99.95%, the capture performance is evaluated as "unqualified".

[0144] When the recovery rate was 90% or more, the evaluation of the recovery performance was deemed "acceptable", and when the recovery rate was less than 90%, the evaluation of the recovery performance was deemed "unacceptable".

[0145] When the pressure loss was 5 kPa or less, the permeability evaluation was considered "excellent", when the pressure loss was greater than 5 kPa and less than 10 kPa, the permeability evaluation was considered "acceptable", and when the pressure loss was greater than 10 kPa, the permeability evaluation was considered "unacceptable".

[0146] A was assigned when two of the collection performance, recovery performance, and permeation performance were evaluated as excellent and one was evaluated as acceptable; B was assigned when one was evaluated as excellent and two were evaluated as acceptable; C was assigned when one was evaluated as unacceptable; and D was assigned when two or more were evaluated as unacceptable. The results are shown in Table 2.

[0147] [Table 2]

[0148]

[0149] As shown in Table 2, the polyolefin microporous membranes of Examples whose Gurley value, thickness, porosity, and average pore size respectively satisfied the conditions of the present disclosure were rated A or B based on the evaluation of the collection performance, recovery performance, and permeability.

[0150] The entire disclosure of Japanese Patent Application No. 2023-015406 is incorporated herein by reference. All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as if each document, patent application, or technical standard were specifically and individually described.

Claims

1. A polyolefin microporous membrane comprising polyolefin, wherein the polyolefin microporous membrane has a Gurley value of 1 s / 100 mL to 20 s / 100 mL, a thickness of 5 μm to 25 μm, a porosity of 65% to 85%, and an average pore size of 0.05 μm to less than 0.30 μm.

2. The polyolefin microporous membrane according to claim 1, wherein The polyolefin comprises polyethylene.

3. The polyolefin microporous membrane according to claim 2, wherein The polyethylene includes ultra-high molecular weight polyethylene, and the proportion of the ultra-high molecular weight polyethylene in the entire polyolefin is 65% by mass or less. The polyolefin microporous membrane according to claim 1 , wherein the Gurley value is 5 seconds / 100 mL or more and 15 seconds / 100 mL or less. The polyolefin microporous membrane according to claim 1 , wherein the porosity is 70% to 80%. The polyolefin microporous membrane according to claim 1 , wherein the average pore diameter is 0.10 μm or more and 0.25 μm or less.

7. The polyolefin microporous membrane according to claim 1, wherein The ratio of the tensile strength at break in the MD direction to the tensile strength at break in the TD direction (MD direction / TD direction) is 0.75 or more and 1.25 or less.

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