Separators for nonwoven fabrics and electrochemical elements

Abstract

To provide a nonwoven fabric with a small oil adhesion amount and a separator for an electrochemical element formed by applying hydrophilic treatment on the nonwoven fabric.SOLUTION: The inventors have found that an oil adhesion amount is small on a polypropylene-resin-derived nonwoven fabric having a lowest melting endothermic peak temperature of 160°C or less compared to the other type of nonwoven fabric. Although the reason has not been clarified, it is considered that polypropylene resin including a number of crystalline forms having a melting endothermic peak temperature of 160°C or less has low compatibility with a hydrophobic portion of oil, and consequently, the oil hardly enters the inside of fibers containing polypropylene resin. A separator for an electrochemical element of the present invention is excellent in strength as it is made by applying hydrophilic treatment on the nonwoven fabric with a small oil adhesion amount.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a nonwoven fabric containing polypropylene resin, and a separator for an electrochemical element obtained by hydrophilizing the nonwoven fabric. [Background technology]

[0002] Conventionally, nonwoven fabrics made from fibers containing polyolefin resins such as polypropylene resin and polyethylene resin, which have excellent chemical resistance, have been commonly used as separators for electrochemical elements such as batteries and capacitors. Furthermore, because nonwoven fabrics made from fibers containing polyolefin resins have low hydrophilicity, it is common practice to subject them to hydrophilization treatments such as sulfonation treatment or plasma treatment.

[0003] Oils are added to the surface of the fibers that make up nonwoven fabrics to improve spinnability during spinning, as well as the hydrophilicity and dispersibility of the fibers. However, it is known that when hydrophilization treatments such as sulfonation treatment are applied to nonwoven fabrics made of polyolefin resin fibers to impart hydrophilicity, a large amount of oil present in the nonwoven fabric has an adverse effect. For example, Japanese Patent Application Publication No. 2011-165360 (Patent Document 1) discloses that if oil remains in the nonwoven fabric, the oil will be sulfonated when the nonwoven fabric is subjected to sulfonation treatment, reducing the actual amount of sulfonic acid groups introduced into the nonwoven fabric. Furthermore, the sulfonated oil decomposes inside the battery, reducing the effect of suppressing the self-discharge reaction by sulfonic acid groups. Therefore, it is recommended to control the oil content of the nonwoven fabric before sulfonation treatment to 0.40% by mass or less. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2011-165360 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, in nonwoven fabrics containing fibers made of polypropylene resin, a type of polyolefin resin, for example, wet-laid nonwoven fabrics, a type of nonwoven fabric, are manufactured by forming paper with fibers dispersed in white water. However, during papermaking, the oil contained in the constituent fibers of the wet-laid nonwoven fabric may not easily leach out from the constituent fibers into the white water. Also, the oil added to the white water to ensure sufficient dispersion of the fibers may adhere to the wet-laid nonwoven fabric in large quantities. Furthermore, during the manufacture of dry-laid nonwoven fabrics, a type of nonwoven fabric, the oil contained in the constituent fibers of the dry-laid nonwoven fabric may not easily leach out from the constituent fibers. As a result, nonwoven fabrics with a large amount of oil attached may have a large amount of oil adhering to them. In addition, when a nonwoven fabric with a large amount of oil attached is subjected to hydrophilization treatment, the oil reacts, reducing the amount of hydrophilic groups such as sulfonic acid groups introduced into the nonwoven fabric by the hydrophilization treatment. Moreover, nonwoven fabrics with a large amount of oil attached may experience a decrease in strength after hydrophilization treatment.

[0006] This invention was made under these circumstances, and aims to provide a nonwoven fabric with a low amount of oil adhering to it, and a separator for an electrochemical element obtained by hydrophilizing the nonwoven fabric. [Means for solving the problem]

[0007] The invention according to claim 1 of the present invention is "a nonwoven fabric containing polypropylene resin fibers and an oil, wherein when the melting endothermic peak of the nonwoven fabric is measured using a differential scanning calorimeter at a heating rate of 10°C / min, the lowest melting endothermic peak temperature derived from the polypropylene resin is 160°C or lower."

[0008] The invention according to claim 2 of the present invention is "the nonwoven fabric according to claim 1, wherein the nonwoven fabric is a wet-laid nonwoven fabric."

[0009] The invention according to claim 3 of the present invention is "a nonwoven fabric according to claim 1 or 2, wherein the content of the oil agent contained in the nonwoven fabric is 1.0% by mass or less of the mass of the nonwoven fabric."

[0010] The invention according to claim 4 of the present invention is "a nonwoven fabric according to any one of claims 1 to 3, wherein the oil contained in the nonwoven fabric is a nonionic oil."

[0011] The invention according to claim 5 of the present invention is "a nonwoven fabric according to any one of claims 1 to 4, comprising ultrafine fibers containing polypropylene with a fiber diameter of 1 to 5 μm."

[0012] The invention according to claim 6 of the present invention is "a separator for an electrochemical element obtained by hydrophilizing a nonwoven fabric according to any one of claims 1 to 5." [Effects of the Invention]

[0013] The inventors have found that the nonwoven fabric having the melting endothermic peak temperature described in claim 1 has less oil adhesion compared to nonwoven fabrics without this condition. Although the reason for this is not fully clear, it is thought that polypropylene resins containing many crystalline forms with melting endothermic peak temperatures of 160°C or lower have low compatibility with the hydrophobic parts of the oil, and as a result the oil does not easily penetrate into the fibers containing the polypropylene resin.

[0014] The nonwoven fabric according to claim 2 of the present invention is a wet-laid nonwoven fabric, and therefore is thinner than other nonwoven fabrics, making it suitable for applications requiring a thin nonwoven fabric, such as separators for electrochemical elements.

[0015] The nonwoven fabric according to claim 3 of the present invention has an oil content of 1.0% by mass or less of the mass of the nonwoven fabric, and thus has a low oil content. When the nonwoven fabric is immersed in a liquid, the oil contained in the nonwoven fabric dissolves into the liquid. However, if the oil content of the nonwoven fabric is low, the amount of oil that dissolves into the liquid is small, and the nonwoven fabric has less influence on the liquid. Therefore, it is a nonwoven fabric suitable for applications where the nonwoven fabric is immersed in a liquid, such as separators for electrochemical elements and liquid filters. Furthermore, the nonwoven fabric tends not to decrease in strength due to hydrophilization treatment, and because it contains little oil, there is little risk of the oil adversely affecting the ions contained in the electrolyte of the electrochemical element. Therefore, it is a nonwoven fabric particularly suitable for separators for electrochemical elements. The reason why nonwoven fabrics with a small amount of oil adhered tend not to experience a significant decrease in strength when hydrophilized is not fully understood. However, nonwoven fabrics with a large amount of oil adhered contain polypropylene resin with many crystalline forms having a melting endothermic peak temperature of 160°C or higher. The high compatibility between this resin and the hydrophobic portion of the oil allows the oil to penetrate into the fibers. This is thought to cause the hydrophilization treatment to reach the inside of the fibers via the oil, thus reducing the fiber strength. On the other hand, nonwoven fabrics with a small amount of oil adhered have low compatibility between the polypropylene resin contained in the nonwoven fabric and the hydrophobic portion of the oil. As a result, the oil does not penetrate into the inside of the fibers, and therefore the hydrophilization treatment does not reach the inside of the fibers.

[0016] The nonwoven fabric according to claim 4 of the present invention is a nonwoven fabric with a low oil content, as the oil is a nonionic oil, and nonionic oils are less likely to adhere to fibers compared to other oils. This makes the nonwoven fabric suitable for applications where the nonwoven fabric is immersed in a liquid, such as separators for electrochemical elements and liquid filters. Furthermore, since nonionic oils do not ionize in solution, the nonwoven fabric of the present invention can be suitably used in applications where it is preferable that the oil does not ionize in a liquid. For example, it is particularly suitable as a separator for electrochemical elements because it is less likely to adversely affect the ions contained in the electrolyte of an electrochemical element.

[0017] Since the nonwoven fabric according to claim 5 of the present invention contains ultra-fine fibers including polypropylene with a fiber diameter of 1 to 5 μm, the structure of the nonwoven fabric is dense, and it is a nonwoven fabric suitable for applications that require a nonwoven fabric with a dense structure, such as applications such as separators for electrochemical elements.

[0018] Since the separator for an electrochemical element according to claim 6 of the present invention is a separator for an electrochemical element obtained by hydrophilizing a nonwoven fabric with a small amount of an oil agent attached thereto, it is a separator for an electrochemical element having excellent strength.

Mode for Carrying Out the Invention

[0019] The nonwoven fabric of the present invention is a nonwoven fabric containing fibers containing a polypropylene resin and an oil agent. When the melting endothermic peak temperature of the nonwoven fabric is measured at a heating rate of 10 ° C. / min using a differential scanning calorimeter, the lowest melting endothermic peak temperature derived from the polypropylene resin exists at 160 ° C. or lower. As a result, it is a nonwoven fabric with a small amount of an oil agent attached thereto. The reason for this has not been fully clarified, but it is considered that a polypropylene resin containing many crystal forms having a melting endothermic peak temperature of 160 ° C. or lower has low compatibility with the hydrophobic part of the oil agent, and as a result, it is difficult for the oil agent to enter the fibers containing the polypropylene resin. Since the lower the lowest melting endothermic peak temperature derived from the polypropylene resin of this nonwoven fabric, the smaller the amount of the oil agent attached, 159.9 ° C. or lower is preferable, and 159.7 ° C. or lower is more preferable.

[0020] In addition, the measurement using a differential scanning calorimeter in the present invention is in accordance with the heat flux differential scanning calorimetry (heat flux DSC) described in JIS K 7121 (2012) "Method for Measuring Transition Temperature of Plastics" 4.2 (2), using a Q1000 manufactured by TA Instruments, and is performed under the following (DSC measurement conditions) to draw a DSC curve.

[0021] (DSC measurement conditions) 1. Shape and size of test piece: As a test piece, a circular nonwoven fabric with a diameter of 6 mm is used. 2. Nitrogen gas flow rate: 50 ml / min 3. Heating temperature: 10℃ / min 4.Measurement start temperature: 0℃

[0022] The polypropylene resin-containing fibers in the nonwoven fabric of the present invention may be fibers composed of only one type of polypropylene resin, fibers composed of two or more types of polypropylene resins, or fibers composed of at least one type of polypropylene resin and at least one type of resin other than polypropylene resin. Fibers composed of two or more types of polypropylene resins, or fibers composed of at least one type of polypropylene resin and at least one type of resin other than polypropylene resin, can be in the form of composite fibers, such as core-sheath type, sea-island type, side-by-side type, orange type, bimetal type, etc. Among these composite fibers, it is preferable that the nonwoven fabric contains core-sheath type composite fibers, in which the constituent fibers of the nonwoven fabric can adhere to each other and high mechanical strength can be realized, thereby realizing nonwoven fabrics and separators for electrochemical elements, and that the surface of the constituent fibers of the nonwoven fabric has a resin with a lower melting point than the other resins contained in the constituent fibers. Among the composite fibers in which the surface of the constituent fibers of the nonwoven fabric has a resin with a lower melting point than the other resins contained in the constituent fibers, it is more preferable that the nonwoven fabric contains core-sheath type composite fibers in which the core component is made of polypropylene resin and the sheath component is made of a resin with a lower melting point than polypropylene resin, or core-sheath type composite fibers in which the core component is made of a resin with a higher melting point than polypropylene resin and the sheath component is made of polypropylene resin, in order to realize nonwoven fabrics and separators for electrochemical elements in which the constituent fibers of the nonwoven fabric can adhere to each other and have high mechanical strength. Furthermore, if the nonwoven fabric contains fine fibers obtained by dividing composite fibers such as orange-type fibers that can be separated into fine fibers by external forces such as water flow or needle punching, or fine fibers composed of island components obtained by dissolving the sea component of sea-island type composite fibers, a dense nonwoven fabric and a separator for electrochemical elements can be realized. Therefore, it is preferable that the nonwoven fabric contains fine fibers obtained by dividing composite fibers or fine fibers derived from island components of sea-island type composite fibers.

[0023] Furthermore, the nonwoven fabric may contain one type of polypropylene resin-containing fiber, or two or more types.

[0024] The proportion of polypropylene resin in the constituent resin of the nonwoven fabric of the present invention is preferably 20 mass% or more, more preferably 30 mass% or more, and even more preferably 40 mass% or more, as a higher proportion of polypropylene resin in the constituent resin allows for the realization of a nonwoven fabric with superior chemical resistance and heat resistance.

[0025] The polypropylene resin-containing fibers of the nonwoven fabric of the present invention can be obtained by known methods such as melt spinning, dry spinning, wet spinning, direct spinning (meltblown, spunbond, electrostatic spinning, etc.), a method of extracting fibers with a fine fiber diameter by removing one or more resin components from composite fibers, or a method of obtaining divided fibers by beating the fibers.

[0026] The fiber diameter of the polypropylene resin-containing fibers in the nonwoven fabric of the present invention is preferably 1 to 30 μm, more preferably 2 to 25 μm, and even more preferably 3 to 20 μm, so that the fibers are uniformly dispersed and the mechanical strength of the nonwoven fabric is excellent. Note that "fiber diameter" refers to the diameter of the cross-sectional shape of the fiber if it is circular, and if it is not circular, the diameter of a circle with the same area as the cross-sectional area is considered the fiber diameter.

[0027] Furthermore, when the nonwoven fabric of the present invention is used as a separator for electrochemical elements, the nonwoven fabric becomes denser and can be made suitable for use as a separator for electrochemical elements. Therefore, it is preferable that the nonwoven fabric contains ultrafine fibers containing polypropylene with a fiber diameter of 1 to 5 μm, more preferably 30 mass% or more of the ultrafine fibers, and even more preferably 40 mass% or more.

[0028] Furthermore, while the fiber length of the polypropylene resin-containing fibers is not particularly limited, it is preferably 0.1 to 75 mm, more preferably 1 to 20 mm, and even more preferably 2 to 15 mm, so that the fibers are uniformly dispersed and the mechanical strength of the nonwoven fabric is excellent. In this invention, the fiber length refers to the length measured by the method specified in JIS L 1015 (Staple Test Method for Chemical Fibers):2010, Section 8.4, Method B (Corrected Staple Diagram Method).

[0029] The nonwoven fabric of the present invention may contain fibers other than those containing polypropylene resin. Specifically, it may contain fibers composed of one type of resin other than polypropylene resin, or composite fibers composed of two or more types of resins other than polypropylene resin. The average fiber diameter of the fibers other than those containing polypropylene resin is preferably 2 to 30 μm, more preferably 3 to 25 μm, and even more preferably 4 to 20 μm, similar to the average fiber diameter of the fibers containing polypropylene resin. The fiber length of the fibers other than those containing polypropylene resin is also preferably 0.1 to 75 mm, more preferably 1 to 20 mm, and even more preferably 2 to 15 mm, similar to the fiber length of the fibers containing polypropylene resin.

[0030] The nonwoven fabrics of the present invention include, for example, dry nonwoven fabrics manufactured by the carding method or air-laying method, wet nonwoven fabrics manufactured by papermaking, or directly spun nonwoven fabrics manufactured by accumulating directly spun (meltblown, spunbond) fibers. Among these, wet nonwoven fabrics tend to have less oil adhering to them because the oil contained in the constituent fibers of the nonwoven fabric dissolves in white water during the manufacturing process, and they are thinner than other nonwoven fabrics. Furthermore, when the nonwoven fabric of the present invention is used as a separator for an electrochemical element, for example, its thinness allows for the realization of an electrochemical element with low electrical resistance. For these reasons, the nonwoven fabric of the present invention is preferably a wet nonwoven fabric.

[0031] The oils contained in the nonwoven fabric of the present invention are generally oils added during fiber manufacturing for purposes such as preventing static electricity and reducing fiber friction, or, in the case of a wet-laid nonwoven fabric, oils added to the white water used during papermaking to ensure that the fibers are sufficiently dispersed in the white water when manufacturing the wet-laid nonwoven fabric. The oils can be nonionic surfactants such as nonionic oils, anionic surfactants such as anionic oils, amphoteric surfactants such as amphoteric oils, natural oils and fats, saturated fatty acids, unsaturated fatty acids, etc. If the oil is a nonionic oil, the oil content can be low because nonionic oils do not adhere to fibers as easily as other oils (such as cationic oils), and because nonionic oils do not ionize in solution, nonwoven fabrics containing nonionic oils can be suitably used in applications where it is preferable that the oil does not ionize in liquid. For example, when a nonwoven fabric is used as a separator for an electrochemical element, it is preferable that the oil agent contained in the nonwoven fabric of the present invention be a nonionic oil agent, as this minimizes the risk of adversely affecting the ions contained in the electrolyte of the electrochemical element.

[0032] The oil content in the nonwoven fabric of the present invention is preferably 1.0% by mass or less, because a lower content results in less oil dissolving into the liquid when the nonwoven fabric is immersed in the liquid, and also tends to prevent a decrease in the strength of the nonwoven fabric when it is hydrophilized. A lower oil content in the nonwoven fabric is more preferably 0.80% by mass or less, and even more preferably 0.70% by mass or less, because a lower oil content results in even less oil dissolving into the liquid when the nonwoven fabric is immersed in the liquid.

[0033] The basis weight of the nonwoven fabric of the present invention is not particularly limited, but is 20 to 100 g / m², provided that the mechanical strength of the nonwoven fabric is excellent. 2 Preferably, 40-90 g / m 2 More preferably, 60-80 g / m 2 This is even more preferable. Note that basis weight refers to the main surface 1m, which is the largest surface of the nonwoven fabric. 2 It refers to the mass per unit area.

[0034] Similarly, the thickness of the nonwoven fabric of the present invention is not particularly limited, but is preferably 50 to 500 μm, more preferably 100 to 400 μm, and even more preferably 150 to 300 μm, in order to provide excellent mechanical strength to the nonwoven fabric. The thickness refers to the measurement taken with an outside micrometer under a 4N load.

[0035] Because the nonwoven fabric of the present invention has a small amount of oil adhering to it, the amount of oil that dissolves into the liquid when the nonwoven fabric is immersed in a liquid is small, making it suitable for applications that require immersion in a liquid. For example, it is a nonwoven fabric that can be suitably used as a separator for electrochemical elements such as primary batteries, secondary batteries, and capacitors, and as a liquid filter. Furthermore, since the strength of the nonwoven fabric does not decrease easily even when hydrophilized, the nonwoven fabric of the present invention can be particularly suitable for use as a separator for electrochemical elements after hydrophilization treatment.

[0036] The separator for electrochemical elements of the present invention is obtained by hydrophilizing the nonwoven fabric described above. Examples of hydrophilizing treatments include sulfonation, fluorine gas treatment, vinyl monomer graft polymerization, electrical discharge treatment, or hydrophilic resin imparting treatment. Among these, sulfonation, fluorine gas treatment, vinyl monomer graft polymerization, or electrical discharge treatment are preferred because they result in little reduction of hydrophilicity and provide excellent liquid retention for the separator for electrochemical elements over a long period of time.

[0037] The nonwoven fabric and separator for electrochemical elements of the present invention can be manufactured, for example, as follows.

[0038] First, prepare fibers containing a polypropylene resin whose lowest melting endothermic peak temperature is 160°C or lower, and, if necessary, fibers other than those containing polypropylene resin. It is preferable that at least one of the polypropylene resin-containing fibers is a composite fiber, such as a core-sheath type, having a resin with a lower melting point than the other resins contained in the constituent fibers on its surface, so that the fibers can adhere to each other.

[0039] Next, the fibers are blended to form a fiber web. The method for forming this fiber web is not particularly limited, but it can be formed by a dry method (such as the carding method or air-laying method) or a wet method. Among these, it is preferable to form it by a wet method, which makes it easier to produce a fiber web with uniform fiber dispersion and less fiber unevenness. This wet method can be a conventionally known method, such as a horizontal long mesh method, an inclined wire type short mesh method, a circular mesh method, or a long mesh / circular mesh combination method.

[0040] Next, the constituent fibers of this fiber web can be bonded together to obtain the nonwoven fabric of the present invention. As for methods of bonding the constituent fibers of the fiber web, for example, a method of fusing the resin contained in the constituent fibers of the fiber web, a method of bonding the constituent fibers of the fiber web with a binder, or a method of entangling the constituent fibers of the fiber web with an external force such as a water flow or needles can be employed. When bonding the constituent fibers of the fiber web by fusing the resin contained in the constituent fibers of the fiber web, the fusing method is not particularly limited, but for example, a method of supporting the fiber web with a conveyor and blowing hot air on it, or a method of applying heat with a calender can be employed. Examples of the nonwoven fabric of the present invention include a heat-bonded nonwoven fabric obtained by fusing the resin contained in the constituent fibers of the fiber web, a binder nonwoven fabric obtained by bonding the constituent fibers of the fiber web with a binder, a water-flow entangled nonwoven fabric obtained by entangling the constituent fibers of the fiber web with a water flow, and a needle-punched nonwoven fabric obtained by entangling the constituent fibers of the fiber web with a needle punch.

[0041] Furthermore, if the fiber web contains split fibers that can be divided into fine fibers by external force, such as orange-shaped composite fibers, it is preferable to divide the split fibers contained in the fiber web into fine fibers by external force such as a water stream or needle, so that the nonwoven fabric and the separator for the electrochemical element have a dense structure.

[0042] When manufacturing the separator for electrochemical elements of the present invention, the nonwoven fabric of the present invention is subjected to a hydrophilic treatment. Examples of hydrophilic treatment methods include sulfonation treatment, fluorine gas treatment, vinyl monomer graft polymerization treatment, electrical discharge treatment, surfactant treatment, or hydrophilic resin imparting treatment. [Examples]

[0043] Examples of the present invention are described below, but the present invention is not limited to these examples. The content of nonionic oils contained in the fibers was measured in the (Oil Content Measurement) section described later.

[0044] (Split fiber A) Divided fibers were prepared, which are orange-shaped composite fibers with a fiber diameter of 15.3 μm and a fiber length of 5 mm, made of polypropylene resin A and high-density polyethylene resin spun by a composite spinning method. These fibers have a hollow center in the fiber cross-section and an orange-shaped cross-section. (When divided, they can be divided into eight fine fibers with a fiber diameter of 3.8 μm made of polypropylene resin A with a roughly trapezoidal cross-section and eight fine fibers with a fiber diameter of 3.8 μm made of high-density polyethylene resin with a roughly trapezoidal cross-section. They contain a nonionic oil agent, and the nonionic oil agent content in divided fibers A is 0.72% by mass.)

[0045] (Split fiber B) Divided fibers were prepared, which are orange-shaped composite fibers with a fiber diameter of 15.3 μm and a fiber length of 5 mm, consisting of polypropylene resin B and high-density polyethylene resin spun by a composite spinning method. These fibers have a hollow center in the fiber cross-section and an orange-shaped cross-section. (When divided, they can be divided into eight fine fibers with a fiber diameter of 3.8 μm made of polypropylene resin B with a roughly trapezoidal cross-section and eight fine fibers with a fiber diameter of 3.8 μm made of high-density polyethylene resin with a roughly trapezoidal cross-section. They contain a nonionic oil agent, and the nonionic oil agent content in divided fibers B is 0.91% by mass.)

[0046] (Split fiber C) A split fiber was prepared, which is an orange-shaped composite fiber with a fiber diameter of 15.3 μm and a fiber length of 5 mm, consisting of polypropylene resin C and high-density polyethylene resin, spun by a composite spinning method. The fiber has a hollow center in its cross-section and an orange-shaped cross-section. (It can be split to produce eight fine fibers with a fiber diameter of 3.8 μm made of polypropylene resin C with a roughly trapezoidal cross-section and eight fine fibers with a fiber diameter of 3.8 μm made of high-density polyethylene resin with a roughly trapezoidal cross-section. It contains a nonionic oil agent, and the content of the nonionic oil agent in the split fiber C is 1.27% by mass.)

[0047] (Split fiber D) Divided fibers were prepared, which are orange-shaped composite fibers with a fiber diameter of 15.3 μm and a fiber length of 5 mm, consisting of polypropylene resin D and high-density polyethylene resin spun by a composite spinning method. These fibers have a hollow center in the fiber cross-section and an orange-shaped cross-section. (When divided, they can be divided to produce eight fine fibers with a fiber diameter of 3.8 μm made of polypropylene resin D with a roughly trapezoidal cross-section and eight fine fibers with a fiber diameter of 3.8 μm made of high-density polyethylene resin with a roughly trapezoidal cross-section. They contain a nonionic oil agent, and the nonionic oil agent content in the divided fibers D is 0.42% by mass.)

[0048] (Fused fibers) Polyolefin-based fused fiber with polypropylene resin E as the core component (non-fused component) and low-density polyethylene resin as the sheath component (fused component). (The low-density polyethylene resin covers the fiber surface except at both ends. The volume ratio of core component to sheath component is 5:5. Fiber diameter: 17.4 μm. Fiber length: 10 mm. Total fiber density: 0.92 g / cm³) 3 A nonionic oil was prepared, with a nonionic oil content of 0.26% by mass in the fused fibers.

[0049] (Example 1) (Manufacturing of water-entangled nonwoven fabrics) 80 mass% of segmented fiber A and 20 mass% of polyolefin-based fused fiber were dispersed in white water, and a fiber web containing dispersed segmented fiber A and polyolefin-based fused fiber was formed by a wet method (horizontal long mesh method). Next, the fiber web is supported by a conveyor, sucked from below the conveyor, and while being conveyed in close contact with the conveyor, hot air at a temperature of 135°C is blown onto the fiber web for 10 seconds, and drying is performed by an air-through method in which a sufficient amount of hot air is passed through to perform heat treatment under no pressure. Simultaneously with the drying of the fiber web, only the low-density polyethylene resin contained in the polyolefin-based fusible fiber is fused to form a fused fiber web. By applying a water flow to the fused fiber web, the split fiber A contained in the fused fiber nonwoven fabric is split, and a water flow complex nonwoven fabric (basis weight: 70 g / m 2 , thickness: 0.25 mm, proportion of polypropylene resin in the nonwoven fabric constituent resin: 50 mass%) was formed. (Hydrophilic treatment of the water flow complex nonwoven fabric) The water flow complex nonwoven fabric is subjected to sulfonation treatment with fuming sulfuric acid (15% SO3 solution) at a temperature of 60°C to form a hydrophilic-treated separator for an electrochemical element (basis weight: 75 g / m 2 , thickness: 0.25 mm).

[0050] (Example 2) (Manufacture of the water flow complex nonwoven fabric) A water flow complex nonwoven fabric (basis weight: 70 g / m 2 , thickness: 0.25 mm, proportion of polypropylene resin in the nonwoven fabric constituent resin: 50 mass%) in which the split fiber B is split was formed in the same manner as in Example 1, except that split fiber B was used instead of split fiber A. (Hydrophilic treatment of the water flow complex nonwoven fabric) Next, in the same manner as in Example 1, by subjecting it to sulfonation treatment, a hydrophilic-treated separator for an electrochemical element (basis weight: 75 g / m 2 , thickness: 0.25 mm) was formed.

[0051] (Comparative Example 1) (Manufacture of the water flow complex nonwoven fabric) ​​​ (Hydrophilic treatment of water-entangled nonwoven fabrics) Next, similar to Example 1, the electrochemical element separator (basis weight: 75g / m²) is subjected to a sulfonation treatment to become hydrophilic. 2 A layer with a thickness of 0.25 mm was formed.

[0052] (Comparative Example 2) (Manufacturing of water-entangled nonwoven fabrics) Except for using split fiber D instead of split fiber A, the same procedure as in Example 1 was used to produce a water-entangled nonwoven fabric (basis weight: 70g / m²) in which split fiber D was divided. 2 A nonwoven fabric with a thickness of 0.25 mm and a polypropylene resin content of 50 mass% was formed. (Hydrophilic treatment of water-entangled nonwoven fabrics) Next, similar to Example 1, the electrochemical element separator (basis weight: 75g / m²) is subjected to a sulfonation treatment to become hydrophilic. 2 A layer with a thickness of 0.25 mm was formed.

[0053] (Evaluation of physical properties of water-entangled nonwoven fabrics and separators for electrochemical elements) The oil content of the water-entangled nonwoven fabrics in the examples and comparative examples, DSC measurements of the water-entangled nonwoven fabrics, and tensile strength measurements of the separators for electrochemical elements in the examples and comparative examples were performed using the following methods to evaluate the physical properties of the water-entangled nonwoven fabrics and the separators for electrochemical elements.

[0054] (Measurement of oil content) (1) When measuring the oil content of water-entangled nonwoven fabric, 10g of the water-entangled nonwoven fabric to be used for measurement was cut off and immersed in a beaker containing 175ml of methanol. Similarly, when measuring the oil content of fibers, 10g of the fiber to be used for measurement was taken and immersed in a beaker containing 175ml of methanol. (2) Place the beaker from (1) on a hot plate heated to 150°C, and stir the methanol in the beaker from (1) for 15 minutes to dissolve the oil contained in the water-entangled nonwoven fabric or fibers in the methanol. (3) The methanol in the beaker from (2) was filtered through filter paper, and the filtrate was transferred to an evaporating dish. (4) The water-entangled nonwoven fabric or fibers remaining on the filter paper in (3) were immersed in another beaker containing 175 ml of methanol. (5) Place the beaker from (4) on a hot plate heated to 150°C, and stir the methanol in the beaker from (1) for 15 minutes to dissolve the oil contained in the water-entangled nonwoven fabric or fibers in the methanol. (6) The methanol in the beaker (5) was filtered through filter paper and the filtrate was transferred to the same evaporating dish as in (3). The evaporating dish containing methanol from (7)(3)(6) was placed on a hot plate at 150°C to evaporate the methanol. At this time, the evaporating dish remained oil extracted from the water-entangled nonwoven fabric or fibers. (8) After methanol had completely evaporated, the mass of the extracted oil was measured using the following formula. C=BA A: Mass (g) of the empty evaporating dish before adding the filtrate. B: Mass (g) of the evaporating dish containing the extracted oil. C: Mass of extracted oil (g) (9) The oil content (=R, unit:%) of the water-entangled nonwoven fabric or fiber was determined from the following formula. R = {C / 10} × 100

[0055] (DSC measurement of water-entangled nonwoven fabric) DSC measurements were performed on the water-entangled nonwoven fabrics of the examples and comparative examples using the aforementioned (DSC measurement conditions), and DSC curves were plotted. Furthermore, the lowest melting endothermic peak temperature originating from the polypropylene resin contained in the water-entangled nonwoven fabric was read from the DSC curve obtained by DSC measurement. Since the water-entangled nonwoven fabric of the present invention contains polyethylene resin, which has a significantly lower melting point than polypropylene resin, the lowest melting endothermic peak among the peaks of the DSC curve that appear above 150°C was taken as the lowest melting endothermic peak temperature originating from the polypropylene resin.

[0056] (Measurement of tensile strength of separators for electrochemical elements) (1) Rectangular samples measuring 200 mm in the vertical direction and 50 mm in the horizontal direction were taken from the electrochemical element separators of the examples and comparative examples. The vertical direction refers to the manufacturing direction (machine direction) of the electrochemical element separator, and the horizontal direction refers to the width direction of the electrochemical element separator, which is perpendicular to the vertical direction. (2) The longitudinally oriented sample was subjected to a constant-speed elongation tensile testing machine (Orientec Co., Ltd., Tensilon, initial gripping distance: 100 mm, tensile speed: 300 mm / min), and the maximum strength when the sample was pulled until it broke was measured. The above measurement was performed on three arbitrarily selected samples, and the arithmetic mean of these three points was taken as the tensile strength (N / 50 mm) of the separator for electrochemical elements.

[0057] The results of the evaluation of the physical properties of the water-entangled nonwoven fabric and the separator for electrochemical elements in the examples and comparative examples are shown in Table 1 below.

[0058] [Table 1]

[0059] From a comparison of the examples and comparative examples, it was found that the water-entangled nonwoven fabric of the example, in which the lowest melting endothermic peak derived from the polypropylene resin of the nonwoven fabric was 160°C or lower, had a low oil content, and that when the water-entangled nonwoven fabric was hydrophilized and used as a separator for electrochemical elements, the tensile strength of the separator for electrochemical elements was high. [Industrial applicability]

[0060] The nonwoven fabric of the present invention has a low oil content, making it suitable for applications where the nonwoven fabric is immersed in a liquid, such as separators for electrochemical elements and liquid filters. Furthermore, because it maintains high strength even after hydrophilic treatment, it can be suitably used as a separator for electrochemical elements. Moreover, the separator for electrochemical elements of the present invention can be suitably used as a separator in electrochemical elements such as primary batteries, secondary batteries (nickel-metal hydride batteries, nickel-cadmium batteries, lithium-ion batteries, etc.), and capacitors.

Claims

1. A nonwoven fabric containing an oil agent, which is entangled by a water flow, and is composed of polypropylene resin fine fibers and high-density polyethylene resin fine fibers generated from segmented fibers having an orange-shaped cross-section, made of polypropylene resin and high-density polyethylene resin, and polyolefin-based fused fibers having polypropylene resin as the core component and low-density polyethylene resin as the sheath component, wherein when the melting endothermic peak of the nonwoven fabric is measured using a differential scanning calorimeter at a heating rate of 10°C / min, the lowest melting endothermic peak temperature originating from the polypropylene resin is 159.9°C or lower.

2. Nonwoven fabric basis weight: 40-100 g / m ２ The nonwoven fabric according to claim 1, wherein the thickness is 150 to 300 μm.

3. The nonwoven fabric according to claim 1 or claim 2, wherein the nonwoven fabric is a wet-laid nonwoven fabric.

4. The nonwoven fabric according to any one of claims 1 to 3, wherein the content of the oil agent contained in the nonwoven fabric is 1.0% by mass or less of the mass of the nonwoven fabric.

5. A separator for an electrochemical element, obtained by sulfonating the nonwoven fabric according to any one of claims 1 to 4.

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