Separator for lead-acid battery

Abstract

To provide a separator for a lead-acid battery, which is formed of a microporous membrane mainly composed of a polyolefin-based resin, inorganic powder and mineral oil, and in which a maximum voltage during electrode plate formation, particularly of a lead-acid battery containing a metal additive such as antimony that causes a side reaction in a battery reaction, or a voltage after 10 hours from start of the formation is lower than that of a conventional separator for a lead-acid battery, thereby effectively reducing an electric energy cost required for the electrode plate formation during battery manufacturing.SOLUTION: The invention provides a separator for a lead-acid battery that is formed of a microporous membrane mainly composed of a polyolefin-based resin, inorganic powder, and mineral oil, where an organic compound having a phenolic hydroxyl group in a molecular structure thereof is contained in the separator in an amount of 0.5 wt.% or more and 20 wt.% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for a lead-acid battery comprising a microporous membrane. [Background technology]

[0002] Conventionally, separators for lead-acid batteries are made of polyethylene separators, which are usually made of 20 to 60% by weight of polyolefin resin (usually ultra-high molecular weight polyethylene) with a weight-average molecular weight of 500,000 or more and a specific surface area of ​​50m 2 There are separators made of a microporous membrane that consists of 40 to 80 wt% of inorganic powder (usually silica powder) with a molecular weight of 1 / g or more, 0 to 30 wt% of mineral oil as a plasticizer that also acts as a pore-opening agent, 0 to 10 wt% of surfactant (solid content), and 0 to 5 wt% of additives (antioxidants, weathering agents, etc.).

[0003] The microporous membrane separator is typically obtained by extruding a raw material composition, which is a mixture of the polyolefin resin, the inorganic powder, the mineral oil (blended in a larger amount than in the separator composition), the surfactant, and the additives, while heating, melting, and kneading the material composition into a sheet, rolling the sheet to a predetermined thickness, and then extracting and removing all or part of the plasticizer. The microporous membrane separator has a base thickness of about 0.1 to 0.5 mm, an average pore diameter (mercury porosimetry) of about 0.01 to 0.5 μm, and a porosity (mercury porosimetry) of about 50 to 90% by volume.

[0004] In recent years, there has been a demand for more efficient production systems and product designs than before in order to curb carbon dioxide emissions and rising energy costs. In particular, for lead-acid battery manufacturers, rising energy costs are a burden in battery production, as they require electrical energy for the electrode formation of the produced lead-acid batteries in addition to the energy used during production.

[0005] In such an environment, a separator for lead-acid batteries that can reduce the electrical energy required during the electrode formation process of lead-acid batteries is thought to be effective in reducing manufacturing costs and curbing carbon dioxide emissions. However, reducing electrical energy requires suppressing side reactions without inhibiting the charge / discharge reactions of lead-acid batteries, which is not easy.

[0006] For example, antimony-containing plates (grids, active material) are considered essential for industrial batteries such as forklifts and batteries for commercial vehicles such as buses and trucks. This is necessary to increase the hardness of the grid of the cast plates, or to increase the corrosion resistance of the plates during actual battery use, thereby extending their lifespan. However, antimony migrates to the negative electrode plate and reduces hydrogen overvoltage through a side reaction, which causes the electrode plate formation of the lead-acid battery to consume a lot of electrical energy.Patent Document 1 discloses a lead-acid battery in which an adsorption layer containing, as an active ingredient, a carbonaceous adsorbent such as activated carbon, or an adsorbent of a metal oxide such as titanium oxide, tin oxide, or rare earth oxide is provided between the positive and negative electrodes to limit the migration of antimony from the positive to negative electrodes.

[0007] The lead alloys used for the negative electrode current collector are mainly Pb-Sb alloys, Pb-Ca alloys, Pb-Ca-Sn alloys, etc. These lead or lead alloys may further contain metals such as Ba, Ag, Al, Bi, As, Se, and Cu as additive elements to improve workability, but if these metals are added in excess, they may leach out due to the battery reaction, causing side reactions during charging and reducing the efficiency of the electrode plate formation process.

[0008] Furthermore, in conventional battery manufacturing, batteries are assembled using electrode plates that have already undergone a chemical formation process. However, in recent years, a more common method has become container formation, in which batteries are assembled using unformed electrode plates that have not yet undergone the chemical formation process, and after the battery is assembled, an electrolyte is poured into the battery and an electric current is applied to form the unformed electrode plates. The mechanism of battery formation is the same as that of battery charging, but it requires a larger amount of electrical energy (power) than charging a general battery. Patent Document 1 describes limiting the movement of antimony from the positive electrode plate to the negative electrode plate to prevent a decrease in hydrogen overvoltage, but does not consider the voltage consumed (power consumption) during this electrode plate formation. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6519945 Summary of the Invention [Problem to be solved by the invention]

[0010] In view of the above-mentioned conventional problems, the present invention aims to provide a separator for lead-acid batteries that is made of a microporous film mainly composed of a polyolefin resin, inorganic powder, and mineral oil, and that is characterized by being able to complete the electrode plate formation of lead-acid batteries with less electrical energy than current lead-acid battery separators, by focusing in particular on the maximum voltage during electrode plate formation or the voltage 10 hours after the start of formation for lead-acid batteries that contain metal additives such as antimony that cause side reactions in the battery reaction. A lower maximum voltage during plate formation of a lead-acid battery, or a lower voltage 10 hours after the start of formation, means that the power consumption during plate formation is also reduced, which is advantageous in terms of energy costs. [Means for solving the problem]

[0011] As a result of extensive research to solve the above problems, the separator for a lead acid battery of the present invention is a separator for a lead acid battery having the following characteristics. (1) A separator for a lead-acid battery, which is made of a microporous membrane mainly composed of a polyolefin resin, an inorganic powder, and a mineral oil, and which contains an organic compound having a phenolic hydroxyl group in its molecular structure in an amount of 0.5% by weight or more and 20% by weight or less. (2) The separator for a lead-acid battery according to (1) above, characterized in that the organic compound having a phenolic hydroxyl group in its molecular structure is a phenolic resin or a phenolic antioxidant that is soluble in an organic solvent. (3) The separator for a lead-acid battery according to (1) or (2) above, characterized in that the organic compound having a phenolic hydroxyl group in its molecular structure is added by a spraying method, a coating (application) method, or an immersion method. (4) The separator for a lead-acid battery according to any one of (1) to (3) above, characterized in that the maximum voltage during electrode formation or the voltage 10 hours after the start of formation of a lead-acid battery containing antimony as a material is lower than that of a conventional separator. (5) The separator for a lead-acid battery according to any one of (1) to (4) above, characterized in that the separator contains mineral oil in an amount of 0.5% by weight or more and 20% by weight or less. (6) A lead-acid battery characterized by using the separator for lead-acid batteries according to any one of (1) to (5) above. [Effects of the Invention]

[0012] As described above, a separator for a lead-acid battery made of a microporous film mainly composed of a polyolefin resin, inorganic powder, and mineral oil, which contains an organic compound having a phenolic hydroxyl group in its molecular structure at a concentration of 0.5 wt % or more and 20 wt % or less, can provide a separator for a lead-acid battery characterized in that the maximum voltage during plate formation or the voltage 10 hours after the start of formation is lower than that of current lead-acid battery separators, particularly for lead-acid batteries containing metal additives such as antimony that cause side reactions in the battery reaction. Furthermore, a lead-acid battery can be provided by a highly economical manufacturing process in which, in the case of battery container formation, a battery is assembled using unformed electrode plates that have not yet undergone the formation process, and after the battery is assembled, an electrolyte is poured in and a current is applied to form the unformed electrode plates, which consumes less power during electrode plate formation. DETAILED DESCRIPTION OF THE INVENTION

[0013] The microporous membrane preferably has a total content of the polyolefin resin, the inorganic powder, and the mineral oil as a plasticizer of 80% by weight or more, with the polyolefin resin content being 20 to 59% by weight, the inorganic powder content being 40 to 80% by weight, and the mineral oil content being 0.5 to 20% by weight. If the polyolefin resin content is less than 20% by weight or the inorganic powder content is more than 80% by weight, the mechanical strength, oxidation resistance, and sealing properties of the microporous membrane provided by the polyolefin resin will be insufficient. If the polyolefin resin content is more than 59% by weight or the inorganic powder content is less than 40% by weight, it will be difficult to ensure a large porosity or a fine, complex pore structure in the microporous membrane, making it impossible to maintain good electrical resistance characteristics in the separator made of the microporous membrane.

[0014] The polyolefin resin may be a homopolymer or copolymer of polyethylene, polypropylene, polybutene, polymethylpentene, or the like, or a mixture thereof. Among these, polyethylene is preferred as the main component in terms of moldability and economy. Polyethylene has a lower melt molding temperature than polypropylene, resulting in good productivity and reduced manufacturing costs. By adjusting the weight-average molecular weight of the polyolefin resin to 500,000 or more, the mechanical strength of the film can be ensured even in a microporous film containing a large amount of inorganic powder. Therefore, it is preferable that the weight-average molecular weight of the polyolefin resin is 1,000,000 or more, and even 1,500,000 or more. Polyolefin resins also have good mixability with inorganic powders, and in microporous films, they maintain strength while bonding the skeleton of the inorganic powder as an adhesive functional material, while also being chemically stable and highly safe.

[0015] In the present invention, silica fine powder is preferably used as the inorganic powder, and silica having a fine particle size and a pore structure inside or on the surface is more preferably used. Among inorganic powders, silica has a wide range of powder properties such as particle size and specific surface area, is relatively inexpensive and easily available, and contains few impurities. The silica fine powder is preferably used when the specific surface area is 100 m 2 / g or more, the pore structure of the microporous film becomes finer (more dense) and more complex, improving short-circuit resistance, increasing the electrolyte retention capacity of the microporous film, and providing a large number of hydrophilic groups (-OH) on the powder surface, thereby improving the hydrophilicity of the microporous film, which is preferable. For this reason, the specific surface area of ​​the silica fine powder is 150 m 2 It is more preferable that the specific surface area of ​​the silica fine powder is 400 m / g or more. 2 It is more preferable that the specific surface area of ​​the silica fine powder is 400 m / g or less. 2 If it exceeds 1 / g, the surface activity of the particles will be high and the cohesive force will be strong, which is undesirable because it will be difficult to uniformly disperse the silica fine powder in the microporous film.

[0016] The mineral oils used as plasticizers are preferably used because they are compatible with polyolefin resins and can be easily extracted with various solvents, etc. Mineral oils such as industrial lubricants made of saturated hydrocarbons (paraffins) are preferred because they are easily reusable. The mineral oils are preferably blended in an amount of 30 to 70% by weight in a raw material composition mainly composed of polyolefin resins, the inorganic powder, and the mineral oil.

[0017] As described above, the mineral oil used as a plasticizer is melt-kneaded and formed into a film of a predetermined shape using a raw material composition primarily composed of a polyolefin resin, inorganic powder, and mineral oil, and then removed to create a porous structure. The inclusion of an appropriate amount of mineral oil in a separator made of a microporous membrane contributes to improved oxidation resistance and favorably affects short-circuit resistance. From this perspective, the mineral oil content in the separator is preferably 0.5 to 20 wt %, more preferably 1.0 to 20 wt %, and even more preferably 5 to 20 wt %. However, increasing the mineral oil content in the separator reduces the porosity of the microporous membrane and deteriorates the electrical resistance of the separator made of a microporous membrane. From this perspective, the mineral oil content in the separator is preferably 20 wt % or less.

[0018] As the solvent used to extract and remove the mineral oil from the melt-kneaded film-like raw material composition, a saturated hydrocarbon organic solvent such as hexane, heptane, octane, nonane, or decane can be used.

[0019] In the present invention, it is necessary that the microporous membrane contains an organic compound having a phenolic hydroxyl group in its molecular structure, and as the organic compound having a phenolic hydroxyl group in its molecular structure, a phenolic resin or a phenolic antioxidant is preferably used, and a phenolic resin is more preferably used. As the phenolic resin, a phenolic resin that dissolves in an organic solvent, such as Shonol (registered trademark) CRM-0909 or Shonol (registered trademark) CRP-0909E manufactured by Aica Kogyo Co., Ltd., is preferably used. Preferred examples of the phenolic antioxidant include 4,4'-methylenebis(2,6-di-tert-butylphenol), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 4,4'-thiobis(6-tert-butyl-m-cresol), and 2,2'-methylenebis(4-methyl-6-tert-butylphenol). However, the antioxidant is not limited to the above, as long as it is an organic compound having a phenolic hydroxyl group in its molecular structure.

[0020] In the present invention, the content of the organic compound having a phenolic hydroxyl group in its molecular structure in the lead-acid battery separator must be 0.5% by weight or more and 20% by weight or less, preferably 1% by weight or more and 20% by weight or less, and more preferably 1% by weight or more and 15% by weight or less. If the content of the organic compound having a phenolic hydroxyl group in its molecular structure in the lead-acid battery separator is less than 0.5% by weight, the effects of the present invention cannot be obtained. Furthermore, if the content exceeds 20% by weight, the electrical resistance of the separator made of a microporous membrane increases, which is undesirable.

[0021] In the present invention, the organic compound having a phenolic hydroxyl group in its molecular structure can be incorporated into a lead-acid battery separator by adding the compound in a dispersed state to a raw material composition before film formation (internal addition method), or by dissolving the organic compound having a phenolic hydroxyl group in its molecular structure in an organic solvent and then adding the compound to the formed microporous film from which the plasticizer has been removed by an attachment treatment method such as spraying, coating, or immersion (external addition method). Therefore, the organic compound having a phenolic hydroxyl group in its molecular structure must be soluble in an organic solvent at a predetermined concentration, and a phenolic resin that dissolves in an organic solvent is particularly preferred. In the method of producing a microporous membrane by mixing an organic compound having a phenolic hydroxyl group in its molecular structure into the raw material composition and then melt-kneading the mixture, a portion of the mineral oil is extracted and removed together with the mineral oil in the extraction step for extracting and removing the mineral oil, making it difficult to accurately control the amount added and also undesirable from an economic standpoint.

[0022] In the present invention, the organic solvent for dissolving the organic compound having a phenolic hydroxyl group in its molecular structure is preferably benzene, ethyl acetate, or trichloroethylene, and more preferably trichloroethylene. The concentration at which the solution is dissolved may be any concentration that allows for uniform dissolution, but is preferably about 1% by weight to 10% by weight. If the concentration exceeds about 10% by weight, the viscosity of the solution increases, making it difficult to apply a spraying method, while if the concentration is less than about 1% by weight, subsequent drying takes a long time, which is undesirable from an economic standpoint.

[0023] If necessary, additives such as surfactants (hydrophilizing agents), antioxidants, ultraviolet absorbers, weathering agents, lubricants, antibacterial agents, antifungal agents, pigments, dyes, colorants, antifogging agents, and matting agents may be added (blended) or contained in the raw material composition or the microporous membrane, as long as the purpose and effect of the present invention are not impaired.

[0024] The base thickness of the microporous film is preferably 0.1 to 0.70 mm, but if it exceeds 0.70 mm, the electrical resistance will deteriorate, and if it is less than 0.1 mm, it will be difficult to maintain good short-circuit resistance (short-circuit here refers to both a permeation short-circuit called a dendrite short-circuit, and a normal short-circuit caused by holes or cracks due to localized weak points in the substrate, high pressure, impact, or puncture from a convex part of the electrode plate, or oxidative wear due to the oxidative power of the electrode plate, etc.). The base thickness is a term used to distinguish it from the total thickness including the rib-like protrusions, for example, when the microporous membrane has rib-like protrusions, and refers to the membrane thickness excluding the height of the rib-like protrusions (when rib-like protrusions are not provided).

[0025] The average pore diameter (mercury intrusion porosimetry) of the microporous membrane is preferably 0.01 to 0.5 μm, but if it exceeds 0.5 μm, the insulation between electrodes will decrease and promote the occurrence of permeation short circuits, and if it is less than 0.01 μm, the electrical resistance will be too high. In the present invention, the electrical resistance of the microporous membrane is 0.01 Ω·100 cm 2 / sheet or less is preferable, and 0.005Ω·100cm 2 / sheet or less is more preferable, and 0.003Ω·100cm 2 It is more preferable that the number is equal to or less than 1 sheet.

[0026] The method for obtaining the microporous film is preferably a method of melt-kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder such as silica powder, and a mineral oil as a plasticizer to form a film, and then removing part or all of the mineral oil. This results in a film with numerous uniform, fine, and complex interconnected pores throughout the film. An example of a specific manufacturing method is shown below. First, the raw materials, consisting of a predetermined amount of polyolefin resin, silica powder, and mineral oil, plus various additives (surfactants, antioxidants, weathering agents, etc.) as needed, are stirred and mixed in a mixer such as a Henschel mixer or a Lödige mixer to obtain a raw material mixture. This mixture is then fed into a twin-screw extruder equipped with a T-die at the tip, where it is heated, melted, and kneaded while being extruded into a sheet. The extrusion is then passed through a pair of forming rolls, one or both of which have grooves engraved on them, to obtain a film-like product with ribs of a predetermined shape integrally formed on one or both sides of the flat sheet. This film is then immersed in a suitable solvent (e.g., n-hexane) to extract and remove a predetermined amount of the mineral oil, and the resulting product is dried to obtain a microporous membrane. Next, an organic compound containing a phenolic hydroxyl group in its molecular structure is dissolved in a suitable organic solvent (e.g., trichloroethylene), and the resulting mixture is added by spraying, coating, or immersion, followed by drying to obtain the desired microporous membrane. The raw material composition refers to a composition consisting of all raw materials brought into the melt-kneading process, and it simply means "all raw materials (composition)" and does not specifically refer to a raw material mixture or a melt-kneaded product.

[0027] The microporous membrane contains a large amount of inorganic powder such as silica powder, which has a large specific surface area and high hydrophilicity. This powder alone is hydrophilic and has wettability and permeability (soaking ability) for the sulfuric acid electrolyte of a lead-acid battery, which is an aqueous solution. However, when sulfuric acid electrolyte is poured into a laminate in which electrode plates and separators are tightly packed in a battery case, the electrolyte is quickly absorbed into the voids in the separator and the voids in the separator are quickly replaced with the electrolyte. Therefore, it is preferable to add 0.2 to 8 wt % of a surfactant (solid content) to the microporous membrane.

[0028] Methods for incorporating the surfactant into the microporous membrane include adding the surfactant in a dispersed state to the raw material composition before film formation (internal addition method) and post-treating (adhesion treatment) the microporous membrane after film formation and removal of the plasticizer (external addition method). The method of adding the surfactant to the raw material composition beforehand (internal addition method) is preferred because it simplifies the manufacturing process and reduces the likelihood of the surfactant leaching out of the microporous membrane of the present invention. The surfactant (solid content) content (required amount) in the microporous membrane is 0.2 to 8 wt%. Increasing the surfactant (solid content) content beyond this range does not significantly improve the hydrophilicity of the microporous membrane. Conversely, it reduces the porosity of the microporous membrane, resulting in increased internal resistance (electrical resistance) and increased self-discharge when used as a lead-acid battery separator. Therefore, the surfactant (solid content) content in the microporous membrane is more preferably 0.2 to 6.5 wt%, and even more preferably 0.2 to 5 wt%.

[0029] The surfactant may be any material capable of improving the hydrophilicity of the microporous membrane, and any of nonionic surfactants, cationic surfactants, and anionic surfactants may be used. Examples of nonionic surfactants that can be used include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkylaryl ethers, fatty acid monoglycerides, and sorbitan fatty acid esters. Examples of cationic surfactants that can be used include aliphatic amine salts, quaternary ammonium salts, polyoxyethylene alkylamines, and alkylamine oxides. Examples of anionic surfactants that can be used include alkyl sulfonates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, alkyl sulfosuccinates, and dodecyl benzene sulfonates. Among these, alkyl benzene sulfonates, alkyl sulfosuccinates, and dodecyl benzene sulfonates are preferred because they can impart high hydrophilicity to polyolefin resins with a small amount of addition and have relatively high heat resistance, allowing the surfactant to be added to the raw material composition in advance to produce a microporous membrane (production by hot melt molding).

[0030] Next, an embodiment of a lead-acid battery using the lead-acid battery separator of the present invention will be described, but the lead-acid battery of the present invention is not limited to the following embodiment. The lead-acid battery may be either an open type or a sealed type (valve-regulated). In the open type, an organic compound having a phenolic hydroxyl group in its molecular structure may be contained in the pulp separator as well as the polyethylene separator, and in the sealed type, an organic compound having a phenolic hydroxyl group in its molecular structure may be contained in the glass mat separator (AGM).

[0031] [Electrolyte] The electrolyte contains sulfuric acid in an aqueous solution. The electrolyte may be gelled if desired. The electrolyte may optionally contain additives commonly used in lead-acid batteries. The specific gravity of the electrolyte at 20°C in a lead-acid battery in a fully charged state after formation is, for example, 1.10 g / cm 3 or more, and 1.35 g / cm 3 The following is the result.

[0032] [Positive electrode] There are two types of positive electrodes for lead-acid batteries: paste type and clad type. A paste-type positive electrode plate includes a positive electrode current collector and a positive electrode material. The positive electrode material is held by the positive electrode current collector. In a paste-type positive electrode plate, the positive electrode material is the positive electrode plate minus the positive electrode current collector. The positive electrode current collector may be formed in the same manner as the negative electrode current collector, and may be formed by casting lead or a lead alloy or by processing a lead or lead alloy sheet.

[0033] A clad positive electrode plate includes multiple porous tubes, a metal core inserted into each tube, a positive electrode material filled into the tube with the metal core inserted, and a connecting seat connecting the multiple tubes. In a clad positive electrode plate, the positive electrode material is the positive electrode plate excluding the tubes, the metal core, and the connecting seat.

[0034] The lead alloy used for the positive electrode current collector is preferably a Pb-Ca alloy or a Pb-Ca-Sn alloy in terms of corrosion resistance and mechanical strength. The positive electrode current collector may have lead alloy layers with different compositions, or may have multiple alloy layers. The core metal is preferably a Pb-Ca alloy or a Pb-Sb alloy. The positive electrode material contains a positive electrode active material (lead dioxide or lead sulfate) that generates capacity through an oxidation-reduction reaction. The positive electrode material may contain other additives as needed. In the lead-acid battery of the present invention, it is also preferable to include antimony in the positive electrode active material to increase the corrosion resistance of the electrode plates during actual battery use and extend their lifespan. Antimony is included in the form of antimony oxide or the like, and the concentration in the positive electrode active material, calculated as antimony metal, is preferably 0.01 wt% to 1 wt%, more preferably 0.02 wt% to 0.5 wt%, and even more preferably 0.05 wt% to 0.5 wt%. Instead of including antimony in the positive electrode active material, antimony may be introduced by laminating a foil of a Pb-Sb alloy on the positive electrode grid.

[0035] Unformed paste-type positive plates are obtained by filling a positive electrode current collector with positive electrode paste, aging it, and drying it, similar to the case of negative plates. The unformed positive plate is then formed. The positive electrode paste is prepared by kneading lead powder, additives, water, and sulfuric acid. The clad type positive electrode plate is formed by filling a tube with lead powder or lead powder slurry into a core metal inserted into the tube, and then joining a plurality of the tubes together with a connecting member.

[0036] [Negative electrode] The negative electrode plate of a lead-acid battery is composed of a negative electrode current collector and a negative electrode material. The negative electrode material is the negative electrode plate without the negative electrode current collector. The negative electrode current collector may be formed by casting lead (Pb) or a lead alloy, or by processing a lead or lead alloy sheet. Examples of processing methods include expanding and punching. It is preferable to use a negative electrode grid as the negative electrode current collector because it makes it easy to support the negative electrode material.

[0037] The lead alloy used for the negative electrode current collector may be any of a Pb-Sb alloy, a Pb-Ca alloy, and a Pb-Ca-Sn alloy. These lead or lead alloys may further contain at least one additive element selected from the group consisting of Ba, Ag, Al, Bi, As, Se, and Cu.

[0038] The negative electrode material contains a negative electrode active material (lead or lead sulfate) that exhibits capacity through an oxidation-reduction reaction, and may also contain a shrinkage inhibitor, lignin, a carbonaceous material such as carbon black, barium sulfate, etc., and may also contain other additives as needed.

[0039] The negative electrode active material in the charged state is sponge lead, but the unformed negative electrode plate is usually made using lead powder.

[0040] The negative electrode plate can be formed by filling a negative electrode current collector with a negative electrode paste, aging and drying the paste to produce an unformed negative electrode plate, and then chemically forming the unformed negative electrode plate. The negative electrode paste can be produced by adding water and sulfuric acid to lead powder, an organic shrinkage inhibitor, and various additives as needed, and kneading the mixture. In the aging step, the unformed negative electrode plate is preferably aged at a temperature higher than room temperature and at a high humidity.

[0041] The formation can be carried out by immersing an electrode plate assembly including unformed negative plates in an electrolyte containing sulfuric acid in a battery jar of a lead-acid battery and then charging the electrode plate assembly. However, the formation can also be carried out before assembling the lead-acid battery or the electrode plate assembly. The formation produces spongy lead. [Example]

[0042] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[0043] [Example 1] 100 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) resin powder with a weight-average molecular weight of 1.5 million as a polyolefin resin, 180 parts by weight of silica powder as an inorganic powder, 390 parts by weight of paraffinic oil (a type of mineral oil), and 4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet through a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil, and then dried to obtain a microporous membrane. Next, a solution of 15 g of phenolic resin (Shonoru® CRM-0909, manufactured by Aica Kogyo Co., Ltd.) in 750 g of trichloroethylene was applied to the surface of the microporous membrane and dried to obtain a microporous membrane of the present invention containing 3.4 wt% phenolic resin. This was used as the lead-acid battery separator of Example 1.

[0044] [Example 2] The same procedure as in Example 1 was carried out except that the amount of phenolic resin (Shonoru (registered trademark) CRM-0909 manufactured by Aica Kogyo Co., Ltd.) added was changed, to obtain a microporous membrane of the present invention containing 6.7 wt % of phenolic resin. This was used as the lead-acid battery separator of Example 2.

[0045] [Example 3] The same procedure as in Example 1 was carried out except that the amount of phenolic resin (Shonoru (registered trademark) CRM-0909 manufactured by Aica Kogyo Co., Ltd.) added was changed, to obtain a microporous membrane of the present invention containing 10.0 wt % of phenolic resin. This was used as the lead-acid battery separator of Example 3.

[0046] [Example 4] The same procedure as in Example 1 was carried out except that a phenolic resin (Shonoru (registered trademark) CRM-0909 manufactured by Aica Kogyo Co., Ltd.) was added in varying amounts to be dispersed in the raw material composition before film formation, to obtain a microporous membrane of the present invention containing 2.2 wt % of phenolic resin. This was used as the lead-acid battery separator of Example 4.

[0047] [Comparative Example 1] The same procedure as in Example 1 was carried out except that a phenolic resin (Shonoru (registered trademark) CRM-0909 manufactured by Aica Kogyo Co., Ltd.) was added in varying amounts to be dispersed in the raw material composition before film formation, to obtain a conventional microporous membrane containing 0.3 wt% of phenolic resin. This was used as the lead-acid battery separator of Comparative Example 1.

[0048] [Testing and evaluation methods] The lead-acid battery separators of the above Examples and Comparative Examples were evaluated under the following conditions, and the results are summarized in Table 1. Note that MD (machine direction) refers to the manufacturing direction of the manufactured sheet, and CD (cross-section direction) refers to the direction perpendicular to the MD direction.

[0049] [Total thickness (mm)] Using a dial gauge (Peacock G-6 manufactured by Ozaki Seisakusho Co., Ltd.), measurements were taken at several arbitrary points on the microporous membrane (points including rib-like protrusions), and the average value was expressed.

[0050] [Base thickness (mm)] Using a dial gauge (Peacock G-6 manufactured by Ozaki Seisakusho Co., Ltd.), measurements were taken at several arbitrary points on the microporous membrane (points not including rib-like protrusions), and the average value was expressed.

[0051] [Oil amount (%)] The oil amount was measured as follows. If measurements were performed on the separator after the phenolic resin had been added, the contained phenolic resin would also be eluted into trichloroethylene, resulting in errors in the measured values. Therefore, measurements were performed on the microporous membrane before the phenolic resin was added. The separator was cut into strips to prepare test specimens. The test specimens were first dried in a dryer (105±5°C) for 30 minutes, then cooled in a desiccator (30 minutes), and their weight (W1, g) was measured. The test specimens were then placed in an ultrasonic cleaner containing trichloroethylene, and the oil was extracted by applying ultrasonic waves for 10 minutes. After extraction, the test specimens were dried in a dryer (105±5°C) for 30 minutes, then cooled in a desiccator (30 minutes), and their weight (W2, g) was measured. The amount of oil was calculated using the following formula: Oil volume (%) = [(W1-W2) / (W1)] x 100

[0052] [Tensile strength CD (MPa), elongation CD (%)] The tensile strength in the CD direction and the elongation in the CD direction were measured as follows. The separator was cut into a rectangular shape of 10 mm x 70 mm in the CD direction to prepare a test piece. Using a Schopper-type tensile tester, the test piece was attached to the grips of the tester with a distance (a) of approximately 50 mm, and a tensile test was performed at a pulling speed of 200 mm per minute to measure the tensile load (b) and distance (c) at which the test piece broke. The tensile strength was calculated by dividing the tensile load (b) by the cross-sectional area of ​​the test piece. The elongation was calculated by dividing the distance (c) by the distance (a) between the grips of the testing machine.

[0053] [Electrical resistance (Ω·100cm 2 / sheets)] The electrical resistance was measured as follows. The separator was cut into a 70 mm x 70 mm square to prepare a test piece. After impregnating with ethanol, the solution was replaced with an aqueous sulfuric acid solution and measurements were performed using a test device conforming to SBA S 0402.

[0054] [Chemical conversion test] The chemical conversion test was carried out as follows. A single cell was fabricated using two positive plates and one negative plate, primarily made of a Pb-Sb alloy (containing approximately 2% Sb by weight). The negative plate was wrapped in a separator. The fabricated single cells were placed one by one in the cell compartment of a battery container of type 55D23L, as specified in JIS D5301 (lead-acid starting batteries), and wired in series. After wiring, a sulfuric acid solution with a specific gravity of 1.23 (20°C) was poured into the cells, and a chemical formation test was performed by passing a 12A DC current through the cells in a thermostatic water bath at 25°C for more than 20 hours. The maximum voltage (V) and maximum power (W) were measured during the formation test. The voltage (V) and power (W) were also measured 10 hours after the start of the formation test. The maximum voltage (V) during the chemical formation test and the voltage (V) 10 hours after the start of the chemical formation test were expressed as relative values ​​(%), with the value for Comparative Example 1 (conventional product) being set at 100.

[0055] [Table 1]

[0056] The results in Table 1 reveal the following: (1) It was found that by including 0.5% by weight or more of an organic compound having a phenolic hydroxyl group in its molecular structure in the separator, the maximum voltage during electrode formation was reduced (99.4%) compared to the conventional lead-acid battery separator of Comparative Example 1. It was also found that the voltage 10 hours after the start of the formation test was reduced (99.7%) compared to the lead-acid battery separator of Comparative Example 1. In particular, it was found that by adding an organic compound having a phenolic hydroxyl group in its molecular structure by a coating method, as in the lead-acid battery separators of Examples 1 to 3, the maximum voltage during plate formation was reduced to 99.0% or less compared to the conventional lead-acid battery separator of Comparative Example 1, and the voltage 10 hours after the start of the formation test also reduced to 99.0% or less. It was also found that by increasing the content of organic compounds having phenolic hydroxyl groups in their molecular structure, the maximum voltage during electrode formation decreased further (98.5% or less for a coating amount of 6.7%, and 98.0% or less for a coating amount of 10.0%), and the voltage 10 hours after the start of the formation test decreased further (98.5% or less for a coating amount of 3.4%, and 98.0% or less for a coating amount of 6.7% and 10.0%). (2) It was found that the electrical resistance of lead-acid battery separators tends to increase when an organic compound having a phenolic hydroxyl group in its molecular structure is added in a dispersed state to the raw material composition before film formation (internal addition method). This tendency becomes more pronounced when the content of the organic compound having a phenolic hydroxyl group in its molecular structure is increased. The microporous membrane is produced by melt-kneading a raw material composition mainly composed of polyolefin resin, inorganic powder, and mineral oil as a plasticizer, forming it into a film of a predetermined shape, and then immersing it in n-hexane to remove the mineral oil. Since the phenolic resin used in the present invention is soluble in organic solvents, it is thought that some of the phenolic resin dissolves and bleeds onto the surface of the microporous membrane, forming a film and blocking the pores on the surface of the microporous membrane. This is thought to result in an increase in the electrical resistance of the lead-acid battery separator. [Industrial Applicability]

[0057] A separator made of a microporous membrane mainly composed of a polyolefin resin, inorganic powder, and mineral oil, which contains an organic compound having a phenolic hydroxyl group in its molecular structure in an amount of 0.5% by weight or more and 20% by weight or less, can provide a lead-acid battery separator that is characterized in that the maximum voltage during plate formation or the voltage 10 hours after the start of formation of a lead-acid battery containing antimony as a material is lower than that of current lead-acid battery separators.

Claims

1. A separator for a lead-acid battery, which comprises a microporous film mainly composed of a polyolefin resin, an inorganic powder, and a mineral oil, and which contains an organic compound having a phenolic hydroxyl group in its molecular structure in an amount of 0.5% by weight or more and 20% by weight or less.

2. 2. The separator for a lead-acid battery according to claim 1, wherein the organic compound having a phenolic hydroxyl group in its molecular structure is a phenolic resin or a phenolic antioxidant that is soluble in an organic solvent.

3. 2. The separator for a lead-acid battery according to claim 1, wherein the organic compound having a phenolic hydroxyl group in its molecular structure is added by a spraying method, a coating method, or an immersion method.

4. 2. The separator for a lead-acid battery according to claim 1, wherein the maximum voltage during electrode formation or the voltage 10 hours after the start of formation of a lead-acid battery containing antimony as a material is lower than that of a conventional separator.

5. 2. The separator for a lead-acid battery according to claim 1, wherein the separator contains mineral oil in an amount of 0.5% by weight or more and 20% by weight or less.

6. A lead-acid battery characterized by using the separator for lead-acid batteries according to any one of claims 1 to 5.

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