Alkaline battery separator

Abstract

To provide an alkaline battery separator containing a polymethylpentene resin, which has excellent oxidation resistance.SOLUTION: The present inventors have found that in a case in which a melting endothermic peak of a nonwoven fabric containing fibers containing a polymethylpentene resin having a sulfur-containing functional group is measured using a differential scanning calorimeter, when the measurement result shows a melting endothermic peak derived from the polymethylpentene resin, the alkaline battery separator equipped with the nonwoven fabric has high oxidation resistance. Although the reason for this has not been completely clarified, the degree of modification of the polymethylpentene resin is low enough that a melting endothermic peak derived from the polymethylpentene resin can be observed.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a separator for an alkaline battery. [Background technology]

[0002] Conventionally, a separator has been provided between the positive and negative electrodes of an alkaline battery to prevent short circuits by separating the positive and negative electrodes, and to hold an electrolyte to facilitate electromotive reactions. The separator for an alkaline battery needs to hold an alkaline electrolyte, such as an electrolyte containing potassium hydroxide. For this reason, a separator with a nonwoven fabric containing fibers containing a polyolefin resin with excellent alkali resistance has been used as the separator for an alkaline battery.

[0003] Among these, polymethylpentene resin has high heat resistance, and because polymethylpentene resin has low compatibility with other polyolefin resins, its use as a constituent resin for splittable fibers makes it easy to prepare fibers with small fiber diameters. For these reasons, alkaline battery separators equipped with nonwoven fabrics containing fibers containing polymethylpentene resin are being considered.

[0004] However, polyolefin resins such as polymethylpentene resins are hydrophobic and have poor electrolyte retention, and therefore nonwoven fabrics containing fibers containing polyolefin resins have the problem of poor electrolyte retention and poor electrochemical reaction in alkaline batteries.

[0005] Therefore, hydrophilization treatment such as sulfonation treatment is performed on nonwoven fabrics containing fibers containing polymethylpentene resin. By performing the sulfonation treatment, sulfur-containing functional groups are introduced into polyolefin resins such as polymethylpentene resin. As a result, the alkaline battery separator has improved electrolyte retention ability and excellent self-discharge suppression effect. As such an alkaline battery separator, the applicant of the present application has proposed an alkaline battery separator in which a nonwoven fabric containing polyolefin ultrafine short fibers containing polymethylpentene resin is hydrophilized by sulfonation treatment, as described in JP 2017-33678 A (Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-33678 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, in an alkaline secondary battery incorporating an alkaline battery separator according to the prior art such as that disclosed in Patent Document 1, the mechanical strength of the alkaline battery separator may decrease as the battery is repeatedly charged and discharged, which may result in a short circuit in the alkaline battery.

[0008] The cause of the decrease in mechanical strength of alkaline battery separators was thought to be that the constituent resins contained in the alkaline battery separators, such as polymethylpentene resin, were denatured by the sulfonation treatment, resulting in a decrease in the oxidation resistance of the alkaline battery separators.

[0009] As a result, the separator for the alkaline battery becomes susceptible to oxidation as the alkaline secondary battery is charged and discharged, which is thought to be a cause of a decrease in mechanical strength.

[0010] The present invention has been made under these circumstances, and an object of the present invention is to provide a separator for an alkaline battery having excellent oxidation resistance. [Means for solving the problem]

[0011] The present invention is "an alkaline battery separator having a nonwoven fabric, the nonwoven fabric including, as constituent fibers, fibers containing a polymethylpentene resin having a sulfur-containing functional group and having a fiber diameter of 5 μm or less, and when the melting endothermic peak of the nonwoven fabric is measured using a differential scanning calorimeter, the measurement result shows a melting endothermic peak derived from the polymethylpentene resin." Effect of the Invention

[0012] The present inventors have found that when a melting endothermic peak of a nonwoven fabric containing fibers containing a polymethylpentene resin having a sulfur-containing functional group is measured using a differential scanning calorimeter, if the measurement results show a melting endothermic peak derived from the polymethylpentene resin, then an alkaline battery separator comprising the nonwoven fabric has high oxidation resistance.

[0013] The reason for this has not been fully elucidated, but it is believed to be due to the following effects:

[0014] The presence of the melting endothermic peak indicates that the degree of modification of the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric is low.

[0015] On the other hand, as will be apparent from the comparative examples described later, the melting endothermic peak was not present in the nonwoven fabric of the alkaline battery separator according to the prior art disclosed in Patent Document 1. This indicates that the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric is highly modified.

[0016] In contrast, in the alkaline battery separator according to the present invention, the degree of modification of the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric of the alkaline battery separator is low enough that a melting endothermic peak derived from the polymethylpentene resin can be observed, and therefore the alkaline battery separator according to the present invention is considered to have excellent oxidation resistance. [Brief description of the drawings]

[0017] [Figure 1] 1 is a schematic cross-section of a splittable fiber that can be used in the alkaline battery separator of the present invention. [Diagram 2] 2 is a schematic cross-section of another splittable fiber that can be used in the alkaline battery separator of the present invention. [Diagram 3] 2 is a schematic cross-section of yet another splittable fiber that can be used in the alkaline battery separator of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The alkaline battery separator of the present invention (hereinafter, sometimes simply referred to as "separator") is a separator with a nonwoven fabric, the nonwoven fabric containing fibers with a fiber diameter of 5 μm or less that contain a polymethylpentene resin having a sulfur-containing functional group, and when the melting endothermic peak of the nonwoven fabric is measured using a differential scanning calorimeter, the measurement result shows the presence of a melting endothermic peak derived from the polymethylpentene resin. It has been found that when the measurement result shows the presence of a melting endothermic peak derived from the polymethylpentene resin, the separator with the nonwoven fabric has high oxidation resistance.

[0019] The reason for this has not been fully elucidated, but it is believed to be due to the following effects:

[0020] The presence of the melting endothermic peak indicates that the degree of modification of the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric is low.

[0021] On the other hand, as will be apparent from the comparative examples described later, the melting endothermic peak was not present in the nonwoven fabric of the separator according to the conventional technology disclosed in Patent Document 1. This indicates that the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric is highly modified.

[0022] In contrast, in the separator according to the present invention, the degree of modification of the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric of the alkaline battery separator is so low that a melting endothermic peak derived from the polymethylpentene resin can be observed, and therefore the separator according to the present invention has excellent oxidation resistance.

[0023] The measurement using a differential scanning calorimeter in the present invention is performed in accordance with 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 under the following (DSC measurement conditions) to plot a DSC curve.

[0024] (DSC measurement conditions) Shape, size and mass of test specimen: A circular nonwoven fabric with a diameter of 6.4 mm is used as the test specimen. Nitrogen gas flow rate: 50ml / min Temperature rise: 10℃ / min Measurement start temperature: 0℃

[0025] The nonwoven fabric provided in the separator of the present invention contains, as constituent fibers, fibers containing a polymethylpentene resin having a sulfur-containing functional group and having a fiber diameter of 5 μm or less.

[0026] The sulfur-containing functional group referred to here is, for example, a sulfonic acid group (-SO 3 H), sulfonyl group (-SO 2 -) etc.

[0027] Furthermore, the fiber containing the polymethylpentene resin having a sulfur-containing functional group may be a single fiber composed only of the polymethylpentene resin having a sulfur-containing functional group, or may be a composite fiber such as a core-sheath type fiber composed of a polymethylpentene resin having a sulfur-containing functional group and a polymethylpentene resin not having a sulfur-containing functional group, or composed of a polymethylpentene resin having a sulfur-containing functional group and a resin other than the polymethylpentene resin having a sulfur-containing functional group, or composed of a polymethylpentene resin having a sulfur-containing functional group and a resin other than the polymethylpentene resin not having a sulfur-containing functional group.

[0028] The fiber diameter of the fiber containing polymethylpentene resin having a sulfur-containing functional group is 5 μm or less so that the specific surface area is large and the separator has excellent electrolyte retention, but the smaller the fiber diameter, the better the separator has electrolyte retention, so it can be 4.5 μm or less, and can be 4.3 μm or less. The lower limit of the fiber diameter is also appropriately selected, but 1 μm or more is realistic. In addition, when the cross-sectional shape is non-circular, the fiber diameter of the ultrafine fiber is the diameter of a circle having the same area as the cross-sectional area.

[0029] The fiber length of the fiber containing the polymethylpentene resin having the sulfur-containing functional group is not particularly limited, but is preferably 20 mm or less, more preferably 15 mm or less, and even more preferably 10 mm or less so that the fibers can be uniformly dispersed and the separator can uniformly retain the electrolyte. The lower limit of the fiber length of the fiber containing the polymethylpentene resin having the sulfur-containing functional group may be that the fibers cannot be uniformly dispersed, so that a practical value is 1 mm or more.

[0030] The fibers containing the polymethylpentene resin having a sulfur-containing functional group preferably have a non-circular cross-sectional shape so that the path they take to reach the other pole is complex and the dendrites are less likely to extend.

[0031] Such ultrafine fibers having a noncircular cross-sectional shape can be generated, for example, by applying an external force to a splittable fiber made of two or more resins having different resin compositions. More specifically, by applying an external force to a splittable fiber 1 having an orange cross-sectional shape as shown in FIG. 1, it is possible to generate ultrafine fibers having a substantially triangular shape made of a resin component 11 and ultrafine fibers having a substantially triangular shape made of a resin component 12. By applying an external force to a splittable fiber 1 having an orange cross-sectional shape as shown in FIG. 2, it is possible to generate ultrafine fibers having a substantially triangular shape made of a resin component 11, ultrafine fibers having a substantially triangular shape made of a resin component 12, and ultrafine fibers having a circular shape made of a resin component 12. By applying an external force to a splittable fiber 1 having an orange cross-sectional shape as shown in FIG. 3 and having a hollow portion 13, it is possible to generate ultrafine fibers having a substantially trapezoidal shape made of a resin component 11 and ultrafine fibers having a substantially trapezoidal shape made of a resin component 12. Examples of the external force include a fluid flow such as a water flow, a calendar, a refiner, a pulper, a mixer, and a beater. Other methods for producing ultrafine fibers having a noncircular cross-sectional shape include a method in which islands-in-sea fibers made of two or more resin components having different resin compositions, in which the island components have a noncircular cross-sectional shape, are prepared, and the islands-in-sea fibers are immersed in a solution that dissolves only the sea component to produce ultrafine fibers having a noncircular cross-sectional shape; and a method in which ultrafine fibers having a noncircular cross-sectional shape are directly spun from a spinning nozzle having a noncircular discharge port.

[0032] The fibers containing polymethylpentene resin having a sulfur-containing functional group contained in the nonwoven fabric provided in the separator of the present invention can be obtained, for example, by carrying out a sulfonation treatment in a conventional manner in the state of fibers or in the state of nonwoven fabric. The polymethylpentene resin having a sulfur-containing functional group may also have a hydrophilic group (e.g., a carboxyl group, a carbonyl group, etc.) other than the sulfur-containing functional group. In addition, in order to have excellent electrolyte retention ability, a surfactant may be further included in addition to the hydrophilic group containing the sulfur-containing functional group.

[0033] The percentage of the mass of the fibers containing polymethylpentene resin having a sulfur-containing functional group relative to the mass of the constituent fibers of the nonwoven fabric provided in the separator of the present invention is preferably 10 mass% or more, more preferably 15 mass% or more, and even more preferably 18 mass% or more so that the separator has excellent alkali resistance. On the other hand, since the polymethylpentene resin has a high melting point, it is difficult to bond the constituent fibers together with the polymethylpentene resin, and since it is preferable that the separator contains fusible fibers in addition to the constituent fibers containing polymethylpentene resin as described below, the percentage is preferably 25 mass% or less.

[0034] The nonwoven fabric provided in the separator of the present invention may contain fibers other than fibers containing polymethylpentene resin having a sulfur-containing functional group. For example, it may contain fusible fibers containing a fusible component having a lower melting point than polymethylpentene resin on the fiber surface that bonds the constituent fibers of the separator and improves the strength of the separator, ultrafine fibers made of resins other than polymethylpentene with a fiber diameter of 5 μm or less that improve the electrolyte retention ability of the separator, and fibers with a fiber diameter of more than 5 μm that improve the strength of the separator. In addition, it is preferable that the constituent fibers of the separator are made of only polyolefin resins such as polymethylpentene resin, polyethylene resin, and polypropylene resin because they have excellent alkali resistance.

[0035] The resin constituting the fusion fiber containing a low-melting fusion component on the fiber surface can be a known resin, but is preferably a polyolefin resin such as a polyethylene resin or a polypropylene resin because of its excellent alkali resistance. In addition, if the low-melting fusion component on the surface of the fusion fiber is a polyethylene resin, it is preferable because the polyethylene resin has a low melting point among polyolefin resins and tends to easily introduce sulfur-containing functional groups by sulfonation treatment. In addition, examples of polyethylene resins include high-density polyethylene resin, medium-density polyethylene resin, low-density polyethylene resin, and linear low-density polyethylene resin. Among these, high-density polyethylene resin is particularly preferable because it has a relatively high melting point and is easy to use the separator at high temperatures, and because high-density polyethylene resin has a certain degree of hardness, it can be made into a separator with tension and stiffness, and is easy to handle.

[0036] The fusion fiber containing a low-melting fusion component on the fiber surface may be a single fiber composed only of the low-melting fusion component, or may be a core-sheath type composite fiber in which the sheath component on the fiber surface is composed of the fusion component and the core component is composed of another resin. Among these, it is preferable to include a core-sheath type composite fiber in which the core component is composed of another resin, because the shape of the separator can be firmly held by the core component and the shape stability is excellent. When the fusion fiber containing a low-melting fusion component on the fiber surface is a core-sheath type composite fiber, the core component may be a known resin, but it is preferable that the core component has a melting point higher than that of the low-melting fusion component of the sheath component, so that the shape of the separator can be firmly held by the core component. In addition, the core component is preferably a polyolefin resin so that the fiber has excellent alkali resistance, and among polyolefin resins, polypropylene resin, which has excellent chemical resistance and is highly versatile, is more preferable.

[0037] When the fusible fiber containing a low melting point fusible component on the fiber surface is made of a polyolefin resin, it is preferable to contain a polyolefin resin having a sulfur-containing functional group so that the separator has excellent electrolyte retention. The fusible fiber containing a polyolefin resin having a sulfur-containing functional group can be obtained by carrying out a sulfonation treatment in a fiber state or a nonwoven fabric state by a conventional method, for example, in the same manner as the fiber containing a polymethylpentene resin having a sulfur-containing functional group.

[0038] The fineness of the fusible fiber containing a low melting point fusible component on the fiber surface is not particularly limited, but is preferably 0.05 to 3.5 dtex, more preferably 0.1 to 2.5 dtex, and even more preferably 0.3 to 2.0 dtex. If the fineness of the fusible fiber is less than 0.05 dtex, the strength of the separator tends to be insufficient, and if the fineness of the fusible fiber is more than 3.5 dtex, the electrolyte retention ability of the separator tends to be insufficient.

[0039] The fiber length of the fusion fibers is not particularly limited, but is preferably 20 mm or less, more preferably 15 mm or less, and even more preferably 10 mm or less so that the fibers can be uniformly dispersed and the separator can uniformly retain the electrolyte. The lower limit of the fiber length of the fusion fibers is realistically 1 mm or more, since there is a risk that the fibers may not be uniformly dispersed.

[0040] Furthermore, the higher the tensile strength of the fused fiber, the stronger the separator tends to be, and the less likely it is to cause a short circuit in an alkaline battery. Therefore, the tensile strength is preferably 3.0 cN / dtex or more, more preferably 4.0 cN / dtex or more, and even more preferably 4.5 cN / dtex or more. The upper limit of the tensile strength of the fused fiber is not particularly limited, but 50 cN / dtex or less is practical. The tensile strength in the present invention refers to a value measured by the method specified in JIS L 1015 (chemical fiber staple test method): 2010, section 8.7.1.

[0041] Furthermore, the higher the Young's modulus of the fusion fiber, the stronger the separator tends to be, and the less likely it is to cause a short circuit in an alkaline battery. Therefore, the Young's modulus is preferably 20 cN / dtex or more, more preferably 30 cN / dtex or more, and even more preferably 40 cN / dtex or more. The upper limit of the Young's modulus of the fusion fiber is not particularly limited, but 110 cN / dtex or less is practical. The Young's modulus in the present invention refers to a value measured by the method specified in JIS L 1015 (chemical fiber staple test method): 2010, section 8.11.

[0042] Furthermore, the lower the heat shrinkage rate of the fusible fiber, the smaller the shrinkage rate when the separator is heated, and the less likely it is to cause a short circuit in the alkaline battery. Therefore, the heat shrinkage rate is preferably 15% or less, more preferably 10% or less, and even more preferably 8% or less. The lower limit of the heat shrinkage rate is not particularly limited, but 0% is ideal. The heat shrinkage rate in the present invention refers to a value measured by heat treatment for 30 minutes using an oven dryer at a temperature of 120°C based on JIS L 1015 (chemical fiber staple test method): 2010, 8.15 (b) dry heat dimensional change rate.

[0043] The percentage of the mass of the fusible fibers containing a low melting point fusible component on the fiber surface relative to the mass of the constituent fibers of the nonwoven fabric provided in the separator of the present invention is preferably 40 mass% or more, more preferably 45 mass% or more, and even more preferably 50 mass% or more so that the constituent fibers of the separator are strongly bonded to each other and the separator has excellent strength. The upper limit of the percentage is preferably 90 mass% or less because it includes fibers containing a polymethylpentene resin having a sulfur-containing functional group and a fiber diameter of 5 μm or less.

[0044] As described above, the nonwoven fabric provided in the separator of the present invention may contain, in addition to the fusible fibers, ultrafine fibers composed of resins other than polymethylpentene having a fiber diameter of 5 μm or less, which improve the electrolyte retention ability of the separator, and fibers having a fiber diameter of more than 5 μm, which improve the strength of the separator. The preferred physical properties of these fibers, such as fineness, fiber length, tensile strength, Young's modulus, and heat shrinkage rate, and the preferred physical properties of fibers containing polymethylpentene resin having a sulfur-containing functional group and having a fiber diameter of 5 μm or less, such as fiber length, tensile strength, Young's modulus, and heat shrinkage rate, are preferably similar to the preferred physical properties of the fusible fibers.

[0045] In the separator of the present invention, the ratio (S / C) of the number of carbon atoms (C) to the number of sulfur atoms (S), which is measured by a flask combustion method, is higher. As a result, the separator's constituent resin contains more sulfur-containing functional groups, and the separator's ability to retain electrolyte is improved. On the other hand, if the (S / C) is too high, the separator's constituent resin is degenerated, and the separator tends to have poor oxidation resistance and mechanical strength. Therefore, the ratio of 1.3×10 -3 ~3.8×10 -3 is preferred, 1.6×10 -3 ~3.5×10 -3 is more preferable, and 1.9 × 10 -3 ~3.2×10 -3 is more preferred.

[0046] The ratio (S / C) of the number of carbon atoms (C) to the number of sulfur atoms (S) in the separator, which is determined by measurement using a flask combustion method, is measured by the following method.

[0047] (1) Preparation of absorbing solution NaHCO 3 Dissolve 84 mg in 100 ml of pure water and add 31% H 2 O 2 Add 1 ml of the solution to prepare an absorption solution. (2) Sample preparation Take 1g of sample from the separator and immerse it in 13 mass% potassium hydroxide aqueous solution for 30 minutes. After immersion, wash with tap water for 30 minutes and then with pure water for 30 minutes. Then, dry at 60℃ for 1 hour to prepare the sample. (3) Oxygen combustion flask method Put 5 ml of the above absorption liquid into the combustion flask and wet the inner wall with pure water. Next, weigh out 20 mg of the above sample, wrap it in ashless filter paper (5C), and set it in a platinum cage. Then fill the flask with oxygen for 30 seconds. Then, apply electricity (ignite) to the platinum cage to burn the sample. After combustion, shake well while occasionally adding pure water to the flask's injection part, and cool for 5 minutes. After cooling, wash the inner wall with the pure water from the injection part and leave it for several minutes. Dilute the absorption liquid in the combustion flask to 50 ml in a measuring flask and collect the measurement solution. (4) Measurement of (S / C) Using an ion chromatograph, the SO 4 2- Measure the concentration of SO 4 2- Calculate (S / C) from the concentration and sample mass.

[0048] As described above, the separator of the present invention includes a nonwoven fabric. Examples of the nonwoven fabric include dry nonwoven fabrics manufactured by a carding method or an air-laying method, wet nonwoven fabrics manufactured by papermaking, and direct spun nonwoven fabrics (melt blown, spunbond) manufactured by accumulating directly spun fibers. Among these, wet nonwoven fabrics are thinner than other nonwoven fabrics and can realize an alkaline battery with low electrical resistance, so that the separator of the present invention is preferably a wet nonwoven fabric.

[0049] The separator of the present invention may be composed of only nonwoven fabric, but may also contain inorganic particles such as alumina and silica in addition to the nonwoven fabric in order to impart functionality such as electrolyte retention and electrical insulation.

[0050] The basis weight of the separator of the present invention is appropriately adjusted, but is preferably 20 to 100 g / m 2 and 25 to 80 g / m 2 and 30 to 70 g / m2 In the present invention, the "weight" is the weight per square meter of the main surface, which is the widest surface. 2 This refers to the mass per unit.

[0051] In addition, the thickness of the separator of the present invention is preferably 10 to 250 μm, more preferably 20 to 230 μm, and even more preferably 30 to 200 μm, since a thinner separator tends to reduce the internal resistance of the battery, but if the thickness is too thin, there is a risk of the internal short circuit resistance and electrolyte retention being deteriorated. Note that the "thickness" in the present invention refers to the average value of the measured values ​​of 10 randomly selected points measured with an outside micrometer (measurement range: 0 to 25 mm) specified in 3.1 of JIS B 7502 (2016) "Micrometer" under a load of 5 N.

[0052] The separator of the present invention can be produced, for example, as follows.

[0053] First, a fiber containing polymethylpentene and, if necessary, other fibers such as a fusible fiber containing a low melting point fusible component on the fiber surface are prepared. At this time, the fiber containing polymethylpentene can be, for example, a splittable fiber containing polymethylpentene, or a fiber containing a polymethylpentene resin and having a fiber diameter of 5 μm or less.

[0054] When the fiber containing polymethylpentene is a splittable fiber, the cross-sectional shape of the splittable fiber containing polymethylpentene is preferably an orange shape as shown in the above-mentioned Figures 1 to 3. The constituent resin other than polymethylpentene contained in the splittable fiber containing polymethylpentene resin is preferably a polypropylene resin that has low compatibility with polymethylpentene resin, has a high melting point, and does not melt with the heat used to bond the fibers together with fusion fibers or the like.

[0055] When the fiber containing a polymethylpentene resin has a fiber diameter of 5 μm or less, it may be, for example, a fiber formed by removing the sea part from an islands-in-sea type composite fiber, or it may be a fiber spun from a spinning nozzle.

[0056] Next, the above-mentioned fibers are blended to form a fiber web. The fiber web can be formed by, for example, a dry method (e.g., carding method, air laying method, etc.) or a wet method. Among these, it is preferable to form the fiber web by a wet method, which is easy to produce a nonwoven fabric (as a separator) in which the fibers are uniformly dispersed and which easily retains the electrolyte uniformly. As the wet method, the fiber web can be formed by a conventionally known method, for example, a horizontal long-net method, an inclined wire-type short-net method, a cylinder method, or a combination of a long-net and a cylinder method. When two or more layers are laminated, it is preferable to laminate fiber webs having the same fiber blend so that a separator with a single layer structure can be produced.

[0057] Next, the fibers constituting the fiber web are bonded together to obtain a nonwoven fabric. Methods for bonding the fibers constituting the fiber web include a method of bonding a fiber web containing fusible fibers containing a fusible component with a low melting point on the fiber surface by fusing the fusible fibers, and a method of applying a binder to the fiber web and bonding the fibers. However, it is preferable to bond the fibers constituting the fiber web only by fusing the fusible fibers of the fiber web containing a fusible component with a low melting point on the fiber surface. When bonding is performed only by fusing the fusible fibers in this way, the voids in the fiber web are not disturbed, so that the electrolyte is uniformly distributed, making it easy to manufacture a separator that can manufacture an alkaline battery with low internal resistance. The fusion of the fibers constituting the fiber web is performed by a heat treatment under no pressure in which hot air is blown onto the fiber web while the fiber web is in close contact with the support by sucking from below the support such as a conveyor, and a sufficient amount of hot air is passed through the support. This tends to result in a thick separator, and the separator has a high electrolyte retention capacity, which is preferable.

[0058] When the fiber web or nonwoven fabric contains splittable fibers containing polymethylpentene, the splittable fibers are split (1) before the formation of the fiber web, (2) after the formation of the fiber web and before bonding the fibers constituting the fiber web, or (3) after bonding the fibers constituting the fiber web to produce a nonwoven fabric, to generate fibers containing polymethylpentene resin and having a fiber diameter of 5 μm or less. When the splittable fibers are split (1) before the formation of the fiber web, for example, the splittable fibers can be fed into a pulper to split the splittable fibers and generate fibers containing polymethylpentene resin and having a fiber diameter of 5 μm or less. When the splittable fibers are split (2) after the formation of the fiber web and before bonding the fibers constituting the fiber web, or (3) after producing a nonwoven fabric, the fiber web or nonwoven fabric can be subjected to a water flow entanglement treatment to split the splittable fibers and generate fibers containing polymethylpentene resin and having a fiber diameter of 5 μm or less. The method of applying the water flow to the fiber web or nonwoven fabric in the hydroentanglement treatment is not particularly limited, but for example, a water flow at a pressure of 1 MPa to 30 MPa may be sprayed onto the fiber web or nonwoven fabric from a nozzle plate having nozzles arranged in one or more rows with a diameter of 0.05 to 0.3 mm and a pitch of 0.2 to 3 mm.

[0059] Next, the nonwoven fabric is subjected to a sulfonation treatment to introduce sulfur-containing functional groups into the polymethylpentene resin contained in the fibers of the nonwoven fabric, thereby obtaining the separator of the present invention. Examples of the sulfonation treatment include a method of immersing the nonwoven fabric as described above in a solution of fuming sulfuric acid, sulfuric acid, chlorosulfuric acid, or sulfuryl chloride, a method of discharging the nonwoven fabric in the presence of sulfur monoxide gas and / or sulfur dioxide gas, and a method of exposing the nonwoven fabric to sulfur trioxide gas. Any of these methods can introduce sulfur-containing functional groups. Among these methods, it is preferable to immerse the nonwoven fabric in a fuming sulfuric acid solution to perform the sulfonation treatment, since this method tends to suppress the modification of the polymethylpentene resin contained in the nonwoven fabric caused by the sulfonation treatment and realize a separator that satisfies the configuration of the present invention.

[0060] By controlling the conditions of this sulfonation treatment (e.g., temperature, reaction time, etc.), it is possible to adjust the degree of modification of the polymethylpentene resin contained in the separator and the ratio (S / C) of the number of sulfur atoms (S) to the number of carbon atoms (C) of the separator. In addition, such a sulfonation treatment does not need to be performed after the nonwoven fabric is formed, and may be performed on the fibers before the fiber web is formed.

[0061] In this manner, sulfur-containing functional groups can be introduced into the nonwoven fabric. However, when the sulfur-containing functional groups are introduced, by-products are also produced. These by-products are removed by washing with sulfuric acid, water, or a dilute alkaline solution. EXAMPLES

[0062] The present invention will be specifically described below with reference to examples, but the scope of the present invention is not limited thereto.

[0063] (Dividable Fiber) As shown in FIG. 3, a stretched splittable fiber having an orange cross section and a hollow portion, a fineness of 1.7 dtex, and a fiber length of 5 mm, was prepared, which was composed of a polypropylene component (symbol 11 in FIG. 3, capable of producing eight polypropylene ultrafine fibers (melting point: 160°C) having a roughly trapezoidal shape and a fineness of 0.11 dtex (fiber diameter: 3.8 μm)) and a polymethylpentene component (symbol 12 in FIG. 3, capable of producing eight polymethylpentene ultrafine fibers (melting point: 235°C) having a roughly trapezoidal shape and a fineness of 0.11 dtex (fiber diameter: 4.1 μm)).

[0064] (Fused fiber) We prepared a core-sheath type composite fused fiber (fiber diameter: 10 μm, fiber length: 5 mm, Young's modulus: 45 cN / dtex, heat shrinkage rate: 7%) with a core component made of polypropylene (melting point: 168°C) and a sheath component made of high-density polyethylene (melting point: 135°C) and a tensile strength of 6.5 cN / dtex.

[0065] (Formation of nonwoven fabric A) A slurry was prepared by mixing and dispersing 40 mass % of splittable fibers and 60 mass % of fusible fibers, and then a fiber web was formed by a wet method (horizontal fourdrinier method). Next, this fiber web was dried at a temperature of 140° C. without applying pressure, and at the same time, the constituent fibers of the fiber web were bonded to each other by the fusible fibers to produce a bonded fiber web. Next, the bonded fiber web was placed on a net with a wire diameter of 0.15 mm, and a water stream at a pressure of 8 MPa was sprayed alternately twice on each side from a nozzle plate with a nozzle diameter of 0.13 mm and a pitch of 0.6 mm to divide the splittable fibers and entangle the fibers three-dimensionally to produce a hydroentangled fiber web. Next, this hydroentangled fiber web was sucked from below the conveyor to bring the fiber web into close contact with the conveyor. In this state, hot air at a temperature of 140°C was blown onto the fiber web, and a sufficient amount of hot air was passed through the fiber web to dry it under no pressure. At the same time, the constituent fibers of the hydroentangled fiber web were fused together again by the fusion fibers, to produce nonwoven fabric A (basis weight: 62.5 g / m 2 , thickness (under 5N load): 250 μm) was produced.

[0066] (Formation of nonwoven fabric B) Nonwoven fabric B (basis weight: 52.5 g / m) was prepared in the same manner as nonwoven fabric A, except that a slurry was formed in which splittable fibers and fusible fibers were mixed and dispersed in a ratio of 50 mass% and 50 mass% and that the basis weight of the formed nonwoven fabric was different. 2 , thickness (under 5N load): 200 μm) was produced.

[0067] Example 1 Nonwoven fabric A was immersed in a fuming sulfuric acid solution (15% SO3 solution) at 55°C for 2 minutes, and then the nonwoven fabric that had been immersed in fuming sulfuric acid was washed in turn with 75% sulfuric acid, 50% sulfuric acid, and water, and a sulfonation treatment was performed to introduce sulfur-containing functional groups into the constituent fibers of the nonwoven fabric. The sulfonated nonwoven fabric was then dried, and the thickness was adjusted using a pressure roll (at room temperature) to produce a separator (basis weight: 65.0 g / m 2 , thickness (under 5N load): 200 μm) was produced.

[0068] Example 2 The sulfonation treatment, drying, and thickness adjustment were carried out in the same manner as in Example 1, except that the temperature of the fuming sulfuric acid was changed to 53°C when sulfonating the nonwoven fabric A, to prepare a separator (basis weight: 65.0 g / m 2 , thickness (under 5N load): 200 μm) was produced.

[0069] Example 3 A separator (basis weight: 55.0 g / m2) was prepared by sulfonation treatment and drying in the same manner as in Example 1, except that nonwoven fabric B was used instead of nonwoven fabric A and the temperature of fuming sulfuric acid was changed to 53°C. 2 , thickness (under 5N load): 200 μm) was produced.

[0070] Example 4 A separator (basis weight: 55.0 g / m2) was prepared by sulfonation treatment and drying in the same manner as in Example 2, except that nonwoven fabric B was used instead of nonwoven fabric A and the temperature of fuming sulfuric acid was changed to 50°C. 2 , thickness (under 5N load): 200 μm) was produced.

[0071] Comparative Example 1 The sulfonation treatment, drying, and thickness adjustment were carried out in the same manner as in Example 1, except that the temperature at which nonwoven fabric A was immersed in fuming sulfuric acid was changed from 55°C to 60°C, to produce a separator (basis weight: 65.0 g / m 2 , thickness (under 5N load): 200 μm) was produced.

[0072] Comparative Example 2 The sulfonation treatment and drying were carried out in the same manner as in Example 3, except that the temperature at which nonwoven fabric B was immersed in fuming sulfuric acid during the sulfonation treatment was changed from 53°C to 58°C, to prepare a separator (basis weight: 55.0 g / m 2 , thickness (under 5N load): 200 μm) was produced.

[0073] Comparative Example 3 The nonwoven fabric A was not subjected to the sulfonation treatment, and the separator of Comparative Example 3 (basis weight: 62.5 g / m 2, thickness (at 5N load): 200 μm).

[0074] Comparative Example 4 Nonwoven fabric B that had not been sulfonated was used as the separator of Comparative Example 4 (basis weight: 52.5 g / m 2 , thickness (at 5N load): 200 μm).

[0075] The physical properties of the separators of the examples and comparative examples are shown in Table 1. The various physical properties were evaluated as follows.

[0076] (Measurement of (S / C)) The (S / C) of the separator was measured by the above-mentioned method.

[0077] (Measurement of melting endothermic peak) Measurements were carried out using a differential scanning calorimeter under the above-mentioned method and conditions, and a DSC curve was drawn. The presence or absence of a melting endothermic peak at 210 to 250° C., which is a melting endothermic peak derived from polymethylpentene contained in the separator, was confirmed according to the following criteria. 〇: A melting endothermic peak can be confirmed from the DSC curve. ×: No melting endothermic peak can be confirmed from the DSC curve.

[0078] (Evaluation of oxidation resistance) (1) 5 mg was cut out from each of the separators of the examples and comparative examples, and weighed to prepare samples. (2) The sample was placed in a thermogravimetric differential thermal analyzer (TG-DTA) apparatus (TA Instruments Japan, SDT Q600) and heated from 25°C to 180°C at a rate of 100°C / min in a nitrogen gas atmosphere. (3) The temperature was increased to 200°C at 10°C / min. (4) Allow to stand for 5 minutes. (5) Oxygen gas was introduced into the TG-DTA apparatus under a nitrogen gas atmosphere. (6) The separator was left standing at 200°C for 30 minutes, and the time from the introduction of oxygen gas in (5) until the exothermic peak of the differential thermal analysis (DTA) due to the oxidation reaction was observed was confirmed. Note that the longer the time until the exothermic peak of the differential thermal analysis was observed under these conditions, the higher the oxidation resistance of the separator. (7) The oxidation resistance of the separator was evaluated according to the following criteria. ○: The heat generation peak of the differential thermal analysis (DTA) due to the oxidation reaction cannot be observed for more than 5 minutes. ×: The heat generation peak of the oxidation reaction by differential thermal analysis (DTA) can be observed in less than 5 minutes.

[0079] (Measurement of pure water retention rate) (1) From the separators of the examples and comparative examples, an area of ​​200 cm 2 Three rectangular test pieces were taken from each specimen. (2) After reaching moisture equilibrium, the mass of the test piece W 0 was measured to the nearest 1 mg. (3) The test piece was spread out and immersed in pure water at 25°C, and the entire surface of the test piece was wetted with an ultrasonic cleaner for 5 seconds, and then the test piece was immersed in the pure water for 30 minutes. (4) The test piece was removed from the pure water, and the test piece was suspended by pinching one end of the test piece with a clip and left for 10 minutes. (5) Mass of the test piece in (4) W 1 was measured to the nearest 1 mg. (6) The pure water retention rate of each of the three test pieces was measured using the following formula, and the arithmetic mean value of the pure water retention rates of the three test pieces was taken as the pure water retention rate of each separator. Note that if the pure water did not penetrate into the separator, it was marked as "X." (Pure water retention rate (%))={(W 1 -W 0 ) / W 0}×100

[0080] (Measurement of specific surface area) Approximately 0.5 g of a sample was taken from the separator and fed to a gas adsorption measuring device [BELSORP28SA, manufactured by BEL Japan Co., Ltd.], and the value measured by the BET method in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption" was taken as the specific surface area. Note that krypton was used as the adsorbed gas.

[0081] (Measurement of penetration strength) A separator was placed on a support base having a cylindrical through hole (inner diameter: 11 mm) so as to cover the cylindrical through hole, and a fixing material having a cylindrical through hole (inner diameter: 11 mm) was further placed on the separator so as to coincide with the center of the cylindrical through hole of the support base, thereby fixing the separator. A needle (radius of curvature at tip: 0.5 mm, diameter: 1 mm, protruding length from jig: 2 cm) attached to a handy compression tester (Kato Tech, KES-G5) was inserted vertically into the separator at a speed of 0.1 cm / s, and the force (gf) required for the needle to penetrate was measured. This measurement was carried out 10 times, and the arithmetic mean value was taken as the penetration strength (gf).

[0082] The physical properties of the examples and comparative examples are shown in Table 1 below.

[0083] [Table 1]

[0084] From a comparison between the Examples and Comparative Examples 1 and 2, when the melting endothermic peak of the nonwoven fabric was measured using a differential scanning calorimeter, the separators of the Examples in which the melting endothermic peak derived from the polymethylpentene resin was observed in the measurement results were excellent in oxidation resistance. As described above, the presence of the melting endothermic peak indicates that the degree of modification of the polymethylpentene resin (polymethylpentene resin having a sulfur-containing functional group) contained in the constituent fibers of the nonwoven fabric is low.

[0085] Moreover, from a comparison between the Example and Comparative Examples 3 and 4, it was confirmed that the liquid retention rate of the separator could be improved by the sulfonation treatment. [Industrial Applicability]

[0086] The separator of the present invention can be suitably used as a separator for primary batteries using an alkaline electrolyte, and alkaline secondary batteries such as nickel-metal hydride batteries, nickel-cadmium batteries, and nickel-zinc batteries. [Explanation of symbols]

[0087] 1. Dividable Fiber 11 Resin components 12 Resin components 13 Hollow part

Claims

[Claim 1] An alkaline battery separator comprising a nonwoven fabric, The nonwoven fabric contains, as its constituent fibers, fibers containing a polymethylpentene resin having a sulfur-containing functional group and having a fiber diameter of 5 μm or less, When the melting endothermic peak of the nonwoven fabric is measured using a differential scanning calorimeter, the measurement result shows that a melting endothermic peak derived from a polymethylpentene resin is present. Separator for alkaline batteries.

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