Adhesive composition and adhesive sheet using the same

Abstract

To provide an adhesive composition which has a high adhesive force to a metal separator such as SUS, and can maintain the adhesive force even after immersion in cooling water.SOLUTION: An adhesive composition contains a styrenic thermoplastic elastomer (A), polycarbodiimide (B), and resin-based beads (C). An adhesive sheet uses the adhesive composition.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an adhesive composition, and particularly to an adhesive composition useful as an in-cell seal for fuel cells. The present invention also relates to an adhesive sheet using the adhesive composition.

Background Art

[0002] Generally, fuel cells are used in the form of a fuel cell stack in which a plurality of power generation cells (hereinafter, also simply referred to as "cells") are stacked. A cell has a structure in which separators are laminated on both sides of an electrode. Inside the cell, a gas containing hydrogen is separately supplied to the anode electrode, and a gas containing oxygen is separately supplied to the cathode electrode. If one gas mixes with the other, there is a risk of a decrease in power generation efficiency. Therefore, an in-cell seal for adhering the separators to each other is used so that the gas containing hydrogen and the gas containing oxygen do not leak.

[0003] An excellent adhesive force to the separator is required for the in-cell seal. Examples of separator materials include carbon-based and metal-based materials. In the future, the use of stainless steel (hereinafter also abbreviated as SUS) is expected as a metal-based separator.

[0004] Fuel cells are used, for example, as a power source for automobiles. A fuel cell vehicle, which is an electric vehicle powered by a fuel cell, is expected as a next-generation vehicle because it does not generate greenhouse gas or air pollutants during driving and is useful for countermeasures against global warming and the preservation of the atmospheric environment.

[0005] In a fuel cell mounted on a fuel cell vehicle, the in-cell seal is immersed in cooling water (also referred to as coolant) mainly composed of ethylene glycol at a high temperature for a long time. Fuel cells generally operate at a temperature of about 70 to 90°C.

[0006] Patent Document 1 discloses an adhesive composition containing a carboxyl group-containing styrene-based thermoplastic elastomer, a tackifier, and a polycarbodiimide, relating to a laminate suitable for forming a battery container for a secondary battery. Patent Document 2 discloses an adhesive sealing member containing a thermoplastic elastomer, an olefin-based thermoplastic resin, and a silane coupling agent. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 5589897 [Patent Document 2] Patent No. 5558889 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Adhesive sheets used as cell seals in automotive fuel cells require high adhesion to separators such as stainless steel (SUS). Furthermore, these adhesive sheets must maintain their adhesive strength even after prolonged immersion in high-temperature cooling water.

[0009] Although studies have been conducted on adhesive sheets used in fuel cells, as described in Patent Documents 1 and 2, the effects of prolonged immersion in cooling water have not been investigated.

[0010] Therefore, the present invention aims to provide an adhesive composition that has high adhesive strength to separators such as SUS, and that maintains that adhesive strength even after being immersed in high-temperature cooling water for a long period of time, as well as an adhesive sheet using the adhesive composition. [Means for solving the problem]

[0011] The inventors have discovered that by containing a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C), the adhesive composition has high adhesive strength and maintains that adhesive strength even after being immersed in high-temperature cooling water for a long period of time, thus completing the present invention.

[0012] In other words, the present invention comprises any of the following configurations (1) to (7). (1) An adhesive composition containing a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C). (2) The adhesive composition according to (1) above, wherein the resin beads (C) are at least one selected from the group consisting of acrylic beads, styrene beads, and urethane beads. (3) The adhesive composition according to (1) above, comprising 0.1 to 45 parts by mass of the polycarbodiimide (B) per 100 parts by mass of the styrene-based thermoplastic elastomer (A). (4) The adhesive composition according to (1) above, comprising 1 to 105 parts by mass of the resin beads (C) per 100 parts by mass of the styrene-based thermoplastic elastomer (A). (5) The adhesive composition according to (1) above, wherein the content of the styrene-based thermoplastic elastomer (A) in the total solid content of the adhesive composition is 40 to 98% by mass. (6) The adhesive composition according to (1) above, wherein the styrene-based thermoplastic elastomer (A) is a carboxyl group-containing styrene-ethylene-butylene-styrene block copolymer. (7) An adhesive sheet comprising an adhesive composition layer consisting of any one of the adhesive compositions described in (1) to (6) above. (8) An adhesive sheet having a core material and adhesive composition layers laminated on both sides of the core material, wherein the adhesive composition layer consists of any one of the adhesive compositions of (1) to (6) above. [Effects of the Invention]

[0013] The adhesive composition and adhesive sheet of the present invention have high adhesive strength to separators such as SUS. Furthermore, the adhesive composition and adhesive sheet of the present invention can maintain their high adhesive strength even after being immersed in high-temperature cooling water for a long period of time. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 shows a schematic diagram of a fuel cell stack consisting of multiple power generation cells stacked on top of each other. [Figure 2] Figure 2 shows a schematic diagram of the swelling distance measured in the example. [Modes for carrying out the invention]

[0015] The present invention will be described in detail below, but these are merely examples of preferred embodiments and are not limiting to these. The "~" in a numerical range indicates a range that includes the numbers before and after it. For example, "0 mass%~100 mass%" means a range that is greater than or equal to 0 mass% and less than or equal to 100 mass%.

[0016] An adhesive composition according to one aspect of the present invention contains a styrenic thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C). The styrenic thermoplastic elastomer (A) exhibits good hydrophobic interaction with the surface of a metal substrate such as a metal separator, and the polycarbodiimide (B) does not inhibit the hydrophobic interaction of the styrenic thermoplastic elastomer (A) and forms a crosslinked structure with the styrenic thermoplastic elastomer (A). Thus, it is presumed that the adhesive composition containing the styrenic thermoplastic elastomer (A) and the polycarbodiimide (B) has excellent water resistance even at high temperatures. Further, by containing the resin beads (C) in addition to the styrenic thermoplastic elastomer (A) and the polycarbodiimide (B), penetration (swelling) into the interior from the portion where the cooling water contacts the adhesive composition layer is suppressed, and deterioration of the adhesive composition layer due to penetration of water molecules can be suppressed. Therefore, a high adhesive force to the metal substrate can be maintained even after long-term immersion in high-temperature cooling water. In the present specification, "high temperature" means, for example, a temperature range of 70°C or higher. Also, in the present specification, "long term" means, for example, a time of 1000 hours or longer. In general, inorganic fillers elute metal ions, which may poison the platinum in the catalyst layer in a fuel cell. However, since resin beads do not elute metal ions, poisoning of the platinum in the catalyst layer can be prevented. Thereby, the action of platinum, specifically, the action of promoting the decomposition of hydrogen and the generation of water can be made smooth.

[0017] <Styrenic thermoplastic elastomer (A)> The styrenic thermoplastic elastomer (A) is a thermoplastic elastomer having styrene as a structural unit, and refers to a block copolymer composed of a continuum of a soft component (elastomer) and a hard component (polystyrene). Generally, since styrenic thermoplastic elastomers have hydrophobicity, the adhesive composition which is one aspect of the present invention can have water resistance. Further, since the adhesive sheet provided with the adhesive composition layer composed of the adhesive composition of the present invention has hydrophobicity, even after being immersed in high-temperature cooling water for a long time, the adhesive composition layer is hardly deteriorated, and it is possible to maintain a high adhesive force with respect to a separator such as SUS.

[0018] In the styrenic thermoplastic elastomer (A), it is preferable that 15 to 80% by mass of styrene units are contained, and more preferably 20 to 70% by mass. When the styrene units in the styrenic thermoplastic elastomer (A) are less than 15% by mass, the resistance to high-temperature cooling water tends to decrease, and when they are more than 80% by mass, the adhesive force to a metal substrate tends to decrease.

[0019] Specific examples of the styrenic thermoplastic elastomer (A) include styrene-butadiene block copolymer, styrene-ethylene-propylene block copolymer, styrene-butadiene-styrene block copolymer, styrene-isoprene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, styrene-ethylene-propylene-styrene block copolymer, and the like. Among them, from the viewpoint of the chemical stability of the molecular structure, a styrene-ethylene-butylene-styrene block copolymer is preferable.

[0020] The styrenic thermoplastic elastomer (A) is preferably a modified styrenic thermoplastic elastomer. The styrenic thermoplastic elastomer (A) preferably contains any one or more residues of a carboxyl group, a hydroxyl group or an epoxy group, and particularly preferably contains a carboxyl group from the viewpoint of reactivity with carbodiimide and excellent adhesive force to a metal substrate.

[0021] Therefore, it is particularly preferable that the styrene-based thermoplastic elastomer (A) is a carboxyl group-containing styrene-ethylene-butylene-styrene block copolymer.

[0022] The glass transition temperature (Tg) of the styrene-based thermoplastic elastomer (A) is preferably 100°C or lower, and more preferably 90°C or lower. If the glass transition temperature (Tg) is higher than 100°C, the adhesion to the metal substrate tends to decrease. The glass transition temperature (Tg) can be measured, for example, by dynamic viscoelasticity measurement (DMA).

[0023] The acid value of the styrene-based thermoplastic elastomer (A) is preferably 1 to 30 mg CH3ONa / g, and more preferably 5 to 20 mg CH3ONa / g. If the above acid value is lower than 1 mg CH3ONa / g, resistance to high-temperature cooling water tends to decrease, and if it is higher than 30 mg CH3ONa / g, adhesion to metal substrates tends to decrease. The acid value can be measured, for example, by neutralization titration.

[0024] The weight-average molecular weight of the styrene-based thermoplastic elastomer (A), when measured by gel permeation column chromatography (GPC) and converted to polystyrene equivalent, is preferably in the range of 10,000 to 500,000, and more preferably in the range of 50,000 to 200,000. If the molecular weight is lower than 10,000, cooling water tends to penetrate more easily into the gaps in the molecular structure, resulting in a decrease in adhesive strength after immersion in cooling water. If the molecular weight is higher than 500,000, the adhesive strength to metal substrates tends to decrease.

[0025] Examples of commercially available styrene-based thermoplastic elastomers (A) include Asahi Kasei Corporation's ToughTec® M series, H series, and P series.

[0026] The styrene-based thermoplastic elastomer (A) may be used alone or in any combination of two or more types.

[0027] In the present invention, when an anhydride of an ethylenically unsaturated carboxylic acid is used as the monomer for polymerization of the carboxyl group-containing styrene-based thermoplastic elastomer (A), it may be used in the form of the anhydride of the carboxylic acid, or it may be used in a ring-opened state with water, alcohol, amine, etc.

[0028] Common methods for introducing carboxyl groups into thermoplastic elastomers include copolymerizing an appropriate amount of ethylenically unsaturated carboxylic acid such as maleic acid or maleic anhydride, or an anhydride of the aforementioned unsaturated carboxylic acid, during the polymerization of monomers for producing thermoplastic elastomers, or grafting the thermoplastic elastomer after synthesis using an appropriate amount of ethylenically unsaturated carboxylic acid such as maleic acid or maleic anhydride, or an anhydride of the aforementioned unsaturated carboxylic acid, and a peroxide.

[0029] <Polycarbodiimide (B)> Polycarbodiimide is a compound having multiple carbodiimide groups. One embodiment of the present invention is an adhesive composition containing polycarbodiimide (B) as a curing agent.

[0030] The carbodiimide equivalent (molecular weight per carbodiimide group) of polycarbodiimide (B) is preferably 100 to 700 g / eq, and more preferably 150 to 500 g / eq. If the above carbodiimide equivalent is lower than 100 g / eq, the crosslinking density of the cured product increases, reducing its flexibility and thus decreasing its adhesion to the metal substrate. If the above carbodiimide equivalent is higher than 700 g / eq, the crosslinking density of the cured product is low, making it easier for cooling water to penetrate the gaps in the molecular structure, thus reducing the adhesion after prolonged immersion in high-temperature cooling water.

[0031] Polycarbodiimide (B) may be dissolved in a solvent. Examples of solvents include methyl ethyl ketone, toluene, and N,N-dimethylformamide.

[0032] Examples of commercially available polycarbodiimide (B) include the Carbodilite® series (V-05, V-09, etc.) manufactured by Nisshinbo Chemical Co., Ltd.

[0033] Polycarbodiimide (B) may be used alone or in any combination of two or more types.

[0034] Polycarbodiimides (B) are generally produced by condensation reactions involving decarbonation of aromatic or aliphatic diisocyanates and / or triisocyanates, with or without a terminal encapsulant. Examples of isocyanates used include hexamethylene diisocyanate (HDI), hydrogenated xylylene diisocyanate (H6XDI), xylylene diisocyanate (XDI), 2,2,4-trimethylhexamethylene diisocyanate (TMHDI), 1,12-diisocyanate dodecane (DDI), norbornane diisocyanate (NBDI), and 2,4-bis-(8-isocyanate octyl)-1,3-dioctylcyclobutane (OCDI), and 4,4'-dicyclohexylmethane diisocyanate (HMDI). Examples include tetramethylxylylene diisocyanate (TMXDI), isophorone diisocyanate (IPDI), and compounds derived therefrom, namely, the diisocyanate nurate, trimethylolpropane adduct, biuret type, prepolymers having isocyanate residues (low polymers obtained from diisocyanate and polyol), uretdione, allophanate, or complexes thereof, which may be used individually or in any combination of two or more.

[0035] <Resin-based beads (C)> An adhesive composition according to one aspect of the present invention contains resin beads (C) to ensure high adhesive strength even after prolonged immersion in high-temperature cooling water and to avoid poisoning of platinum in the catalyst. Resin beads refer to particles made of resin having a size ranging from nanometer-order to micrometer-order. Various known resin beads can be used as resin beads (C), and may be a single type of bead or a mixture of beads made of different resins.

[0036] From the viewpoint of heat resistance, for example, the resin beads (C) are preferably at least one selected from the group consisting of acrylic beads, styrene beads, and urethane beads. Furthermore, the resin beads (C) may also contain a predetermined functional group. The functional group is preferably at least one selected from the group consisting of hydroxyl groups, isocyanate groups, and carboxyl groups.

[0037] The maximum particle size of the resin beads (C) is preferably 50% or less of the thickness of the adhesive composition layer. If the maximum particle size is greater than 50% of the thickness of the adhesive composition layer, the adhesion to the metal substrate tends to decrease. The maximum particle size of the resin beads is the cumulative value of 99% (D) in the particle size distribution. 99 This refers to the particle size at a given time, and can be determined by laser diffraction particle size distribution measurement.

[0038] From the viewpoint of preventing cooling water from entering the adhesive composition layer, it is preferable that the resin beads (C) maintain their particle shape during the manufacturing process of the adhesive composition and adhesive sheet, and during use. In other words, it is preferable to select a resin that does not melt under heating during manufacturing or at the temperature of the usage environment.

[0039] The decomposition temperature of the resin beads (C) is preferably 200°C or higher, and more preferably 250°C or higher, from the viewpoint of heat resistance. If the above decomposition temperature is lower than 200°C, there is a risk that the resin beads will decompose due to the amount of heat applied to the product during the manufacturing process or in the usage environment. Here, the decomposition temperature is the temperature at which the tangent line of the TG curve, where there is no initial weight loss or a gradual weight loss, intersects with the tangent line where the weight loss has decreased by 10%. The TG curve can be determined by measuring it using a thermogravimetric differential thermal analyzer at a heating rate of 5°C / min, in an atmospheric environment, and within a measurement temperature range of room temperature to 600°C.

[0040] Examples of commercially available resin-based beads (C) include the "MX series" of cross-linked acrylic particles and the "SX series" of cross-linked styrene particles manufactured by Soken Chemical Co., Ltd., and the "Art Pearl series" (registered trademark) manufactured by Negami Kogyo Co., Ltd.

[0041] Resin-based beads (C) can be prepared by methods such as suspension polymerization, seed polymerization, and emulsion polymerization.

[0042] <Adhesive composition> An adhesive composition according to one aspect of the present invention contains a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C).

[0043] In an adhesive composition according to one aspect of the present invention, it is preferable to contain 0.1 to 45 parts by mass of polycarbodiimide (B) per 100 parts by mass of styrene-based thermoplastic elastomer (A), more preferably 1 to 30 parts by mass, and most preferably 1 to 20 parts by mass. If the amount of polycarbodiimide (B) is less than 0.1 parts by mass per 100 parts by mass of styrene-based thermoplastic elastomer (A), the crosslinking density of the cured product is low, and cooling water easily penetrates the gaps in the molecular structure, so the adhesive strength tends to decrease after prolonged immersion in high-temperature cooling water. If the amount of polycarbodiimide (B) is more than 45 parts by mass, the crosslinking density of the cured product increases, and the flexibility decreases, so the adhesive strength to metal substrates tends to decrease.

[0044] In an adhesive composition according to one aspect of the present invention, it is preferable to contain 1 to 105 parts by mass of resin beads (C) per 100 parts by mass of styrene-based thermoplastic elastomer (A), more preferably 10 to 80 parts by mass, and most preferably 10 to 70 parts by mass. If the amount of resin beads (C) is less than 1 part by mass per 100 parts by mass of styrene-based thermoplastic elastomer (A), cooling water tends to penetrate into the gaps in the molecular structure of the cured product, resulting in a decrease in adhesive strength after prolonged immersion in high-temperature cooling water. If the amount of resin beads (C) is more than 105 parts by mass, the adhesive strength to metal substrates tends to decrease.

[0045] In one embodiment of the present invention, the content of styrene-based thermoplastic elastomer (A) in the total solid content of the adhesive composition is preferably 40 to 98% by mass, and more preferably 75 to 90% by mass. If the content of styrene-based thermoplastic elastomer (A) in the total solid content of the adhesive composition is less than 40% by mass, hydrophobic interaction between the styrene-based thermoplastic elastomer (A) and the metal substrate surface becomes difficult to obtain, and the adhesive strength to the metal substrate after prolonged immersion in high-temperature cooling water tends to decrease. If the content of styrene-based thermoplastic elastomer (A) is more than 98% by mass, the crosslinking density of the cured product decreases, and cooling water can easily penetrate the gaps in the molecular structure, so the adhesive strength to the metal substrate after prolonged immersion in high-temperature cooling water tends to decrease.

[0046] An adhesive composition according to one aspect of the present invention may contain known additives. Examples of additives include antioxidants, tackifiers, softeners, processing aids, waxes, dispersants, leveling agents, and the like.

[0047] In one aspect of the present invention, the adhesive composition preferably contains substantially no epoxy resin. Here, "substantially no epoxy resin" means that in the adhesive composition according to one aspect of the present invention, the amount of epoxy resin is 1 part by mass or less per 100 parts by mass of styrene-based thermoplastic elastomer (A). If more than 1 part by mass of epoxy resin is used per 100 parts by mass of styrene-based thermoplastic elastomer (A), there is a possibility that hydroxyl groups generated by ring-opening of epoxy groups may form a certain amount or more of hydrogen bonds on the surface of the metal separator. Since hydrogen bonds have low energy, there is a concern that if a certain amount or more of hydrogen bonds are formed on the surface of the metal separator, the water resistance of the adhesive composition at high temperatures may decrease.

[0048] An adhesive composition according to one aspect of the present invention can be prepared by mixing a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), a resin-based bead (C), and other components as needed. The method for producing the adhesive composition of the present invention is not limited, but examples include a method in which polycarbodiimide (B), resin beads (C), and a toluene solvent added as appropriate as needed are mixed and stirred to prepare a solution, to which a styrene-based thermoplastic elastomer (A) dissolved in toluene solvent is mixed to prepare the adhesive resin composition, and then the solvent is removed.

[0049] <Adhesive Sheet> An adhesive sheet according to one aspect of the present invention is an adhesive sheet comprising an adhesive composition layer made of an adhesive composition according to one aspect of the present invention (hereinafter also referred to as a "core-less adhesive sheet"). Furthermore, an adhesive sheet according to one aspect of the present invention is an adhesive sheet having a core material and adhesive composition layers laminated on both sides of the core material, wherein the adhesive composition layer is made of an adhesive composition according to one aspect of the present invention (hereinafter also referred to as a "core-attached adhesive sheet"). A core-attached adhesive sheet may be used when there is a wide gap between the members to be bonded.

[0050] The core material used in core-attached adhesive sheets is not particularly limited, but examples include polyethylene naphthalate, polyether ketone, polyethylene terephthalate, perfluoroalkoxyalkane, ethylene tetrafluoroethylene copolymer, and perfluoroethylene propene copolymer. The thickness of the core material is preferably 5 to 300 μm, and more preferably 10 to 100 μm. If the thickness of the core material is less than 5 μm, wrinkles and other appearance defects tend to occur when the core-attached adhesive sheet is transported using a roll-to-roll method. If the thickness of the core material is thicker than 300 μm, it tends to be difficult to continuously process the core-attached adhesive sheet into a roll shape using a roll-to-roll method.

[0051] The combined thickness of the core material and adhesive composition layer can be appropriately set according to the spacing between separators, but the thickness of the adhesive composition layer is preferably 25 μm or more from the viewpoint of adhesive strength to the metal substrate.

[0052] Both core-less and core-adhesive sheets may have their adhesive surface covered with a release film. The release film is used to protect the surface of the adhesive sheet and is removed before use.

[0053] The method for producing an adhesive sheet with a core material according to one aspect of the present invention is not limited, but for example, it can be produced by following the steps 1 to 4 below. 1. A styrene-based thermoplastic elastomer (A), polycarbodiimide (B), resin beads (C), and other components as needed are mixed to prepare an adhesive composition. 2. Apply the adhesive composition obtained in step 1 to the release surface of the release film, and dry it to remove the solvent to prepare an adhesive sheet with a release film. A total of two adhesive sheets with release films are prepared by repeating the same procedure. 3. One of the two adhesive sheets with release film obtained in step 2 is bonded to the core material so that the adhesive composition layer of the adhesive sheet with release film faces the core material film, thereby creating a single-sided adhesive sheet with a core material. 4. The single-sided adhesive sheet with core material obtained in step 3 and the adhesive sheet with release film obtained in step 2 are bonded together such that the adhesive composition layer of the adhesive sheet with release film faces the core material of the single-sided adhesive sheet with core material.

[0054] Alternatively, a method (so-called coating method) can be used to produce a core-attached adhesive sheet without going through steps 2 to 4, by directly applying the adhesive composition prepared in step 1 to both sides of the core material film and drying it to remove the solvent. Furthermore, a core-attached adhesive sheet can also be produced by using the above-described coating method on the side of the core-attached single-sided adhesive sheet prepared in steps 1 to 3 that does not have an adhesive composition layer.

[0055] The method for producing a core-free sheet according to one aspect of the present invention is not limited, but for example, it can be produced by following the steps 1 to 3 below. 1. A styrene-based thermoplastic elastomer (A), polycarbodiimide (B), resin beads (C), and other components as needed are mixed to prepare an adhesive composition. 2. The adhesive composition obtained in 1. is applied to the release surface of the release film, and the solvent is removed by drying to produce an adhesive sheet with a release film. 3. The release side of another release film is bonded to the adhesive composition layer of the release film-attached adhesive sheet obtained in step 2.

[0056] <Application to fuel cells> An adhesive composition and adhesive sheet according to one aspect of the present invention are suitably used for bonding separators stacked inside the power generation cells of a fuel cell. The method for sealing the adhesive composition and adhesive sheet according to one aspect of the present invention to the separators is not limited, but one example is a compression molding method using a mold molding machine with a mold temperature of 160°C, a molding pressure of 3.0 MPa, and a molding time of 30 seconds. In this context, metals can be used as the material for the separator, and examples of such metals include stainless steel (SUS) and titanium.

[0057] An adhesive composition and adhesive sheet according to one aspect of the present invention preferably have an adhesive strength of 8 N / cm or more, and more preferably 15 N / cm or more, after being sealed to a separator. If the adhesive strength of the adhesive composition and adhesive sheet is lower than 8 N / cm, problems may arise such as the separators peeling apart and the sealing performance being impaired. The adhesive strength can be measured by a tensile testing machine.

[0058] An adhesive composition and adhesive sheet according to one aspect of the present invention preferably have an adhesive strength of 8 N / cm or more, and more preferably 15 N / cm or more, even after being sealed to a separator and immersed in cooling water at 70-90°C for 1,000 hours or more. If the adhesive strength of the adhesive composition and adhesive sheet after being immersed in cooling water at 70-90°C for 1,000 hours or more is lower than 8 N / cm, the separators may peel apart, leading to problems such as impaired sealing performance.

[0059] In an adhesive composition and adhesive sheet according to one aspect of the present invention, the adhesive strength retention rate shown in (Formula 1) below is preferably 60% or higher, and more preferably 80% or higher. The higher the adhesive strength retention rate after immersion in cooling water for 1000 hours, the higher the probability that a practically acceptable adhesive strength can be maintained for a long period of time. If the adhesive strength retention rate is lower than 60%, the adhesive strength may fall below a practically acceptable level when immersed in cooling water for a long period of time (1000 hours or more).

[0060] (Formula 1) R=A / B×100 R: Adhesive force retention rate A: Adhesion strength after immersion in 95°C cooling water for 1,000 hours. B: Adhesion before immersion in 95°C cooling water

[0061] In one aspect of the present invention, the adhesive composition and adhesive sheet preferably exhibit minimal swelling of the adhesive due to the cooling water even after being sealed to a separator and immersed in cooling water at 70-90°C for 1,000 hours or more. If significant swelling is observed, the adhesive strength may fall below a practically acceptable level during longer immersion periods. Here, swelling means that the cooling water has permeated the adhesive. When cooling water penetrates the gaps in the molecular structure of the adhesive, it gradually permeates the entire adhesive. It is believed that when the permeated cooling water reaches the interface between the adhesive and the metal substrate of the separator, delamination at the interface is more likely to occur. Therefore, the less the cooling water penetrates the adhesive, the longer it takes for the cooling water to reach the interface, and the higher the probability of maintaining high adhesive strength for a longer period.

[0062] As explained above, the following matters are disclosed in this specification: [1] An adhesive composition containing a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C). [2] The adhesive composition according to [1] above, wherein the resin beads (C) are at least one selected from the group consisting of acrylic beads, styrene beads, and urethane beads. [3] The adhesive composition according to [1] or [2] above, comprising 0.1 to 45 parts by mass of the polycarbodiimide (B) per 100 parts by mass of the styrene-based thermoplastic elastomer (A). [4] The adhesive composition according to any one of [1] to [3] above, comprising 1 to 105 parts by mass of the resin beads (C) per 100 parts by mass of the styrene-based thermoplastic elastomer (A). [5] The adhesive composition according to any one of [1] to [4] above, wherein the content of the styrene-based thermoplastic elastomer (A) in the total solid content of the adhesive composition is 40 to 98% by mass. [6] The adhesive composition according to any one of [1] to [5] above, wherein the styrene-based thermoplastic elastomer (A) is a carboxyl group-containing styrene-ethylene-butylene-styrene block copolymer. [7] An adhesive sheet comprising an adhesive composition layer made of any one of the adhesive compositions described in [1] to [6] above. [8] An adhesive sheet having a core material and adhesive composition layers laminated on both sides of the core material, wherein the adhesive composition layers consist of the adhesive composition described in any one of [1] to [6] above. [Examples]

[0063] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention.

[0064] First, prior to the examples and comparative examples, the following materials were prepared.

[0065] <Styrene-based thermoplastic elastomer (A)> • Carboxyl group-containing styrene-ethylene-butylene-styrene block copolymer (manufactured by Asahi Kasei Corporation, “ToughTec® M1913”)

[0066] <Polycarbodiimide (B)> • Polycarbodiimide (Carbodilite® series manufactured by Nisshinbo Chemical Co., Ltd., “V-05” and “V-09GB”)

[0067] <Resin-based beads (C)> • Acrylic beads (manufactured by Soken Chemical Co., Ltd., "MX-1000" (average particle size: 10 μm)) • Styrene beads (manufactured by Soken Chemical Co., Ltd., "SX-500H" (average particle size: 5.0 μm)) • Urethane beads (manufactured by Negami Kogyo Co., Ltd., "C-600T" (average particle size: 10 μm)).

[0068] <Acrylic polymer (X)> • Carboxyl group-containing acrylic polymer (manufactured by Soken Chemical Co., Ltd., "SK Dyne® 1838").

[0069] <Epoxy resin (Y)> • Glycidylamine-type epoxy resin (manufactured by Mitsubishi Chemical Corporation, “JER® 604”).

[0070] [Preparation of core material-free sheets] • Example 1 2.31 parts by mass of polycarbodiimide (B) (Carbodilite "V-05" manufactured by Nisshinbo Chemical Co., Ltd.) and 12.5 parts by mass of resin beads (C) (crosslinked acrylic particles "MX-1000" manufactured by Soken Chemical Co., Ltd.) were added, and 25 parts by mass of toluene solvent was added and the mixture was stirred until homogenized. Then, 100 parts by mass of styrene-based thermoplastic elastomer (A) (ToughTec M1913) was dissolved in 400 parts by mass of toluene solvent and mixed with this solution to obtain an adhesive composition. The parts by mass of each component are shown in Tables 1 and 2. The obtained adhesive composition was applied to the release surface of a release film (Lintec Corporation's "PET 50GSY40") so that the thickness after drying was 25 μm, and dried at 150°C for 5 minutes to prepare an adhesive sheet in which the adhesive composition layer was laminated onto the release film. The release surface of the same release film was then bonded to the side of this adhesive composition layer to obtain a core-free adhesive sheet.

[0071] Examples 2-11 and Comparative Examples 1-3 The parts by mass of each component in Examples 2-11 and Comparative Examples 1-3 are shown in Tables 1 and 2. Adhesive sheets were prepared in the same manner as in Example 1, except that the adhesive composition was as shown in Tables 1 and 2. In Comparative Example 1, the adhesive sheet was prepared in the same manner as above, except that resin beads (C) were not used. In Comparative Example 2, the adhesive sheet was prepared in the same manner as above, except that acrylic polymer (X) was used instead of styrene-based thermoplastic elastomer (A). In Comparative Example 3, the adhesive sheet was obtained in the same manner as above, except that epoxy resin (Y) was weighed in instead of polycarbodiimide (B).

[0072] [Measurement of initial adhesive strength] For the core material-free sheets of Examples 1-11 and Comparative Examples 1-3, one release film was peeled off, and a 30 μm thick layer of SUS304 was placed on the exposed adhesive composition layer. Then, using a heat-pressure molding machine, the sheets were pressure-molded at a temperature of 70°C and a pressure of 1.0 MPa for 10 seconds to obtain a laminate with a laminate structure of "SUS304 / adhesive composition layer / release film". Next, the release film of the laminate was peeled off, and a 30 μm thick layer of SUS304 was placed on the exposed adhesive. Then, using a heat-pressure molding machine, the sheets were molded at a temperature of 160°C and a pressure of 3.0 MPa for 60 seconds to obtain a test specimen with a laminate structure of "SUS304 / adhesive composition layer / SUS304". The test specimens were then heated in a dryer at 175°C for 1 hour to cure the adhesive composition. This test specimen was cut to a width of 10 mm and a length of 100 mm. The initial adhesive strength was evaluated by pulling the SUS304 in a 180° direction at a speed of 30 mm / min using a tensile testing machine. The results are shown in Tables 1 and 2. The evaluation criteria were as follows. ·Excellent: 15N / cm or more • Good: 8 N / cm or more, less than 15 N / cm Poor: Less than 8N / cm

[0073] [Measurement of adhesive strength after immersion in 95°C cooling water for 1000 hours] Using the same method as described in [Measurement of Initial Adhesion] above, test specimens were obtained using the core material-free sheets of Examples 1-11 and Comparative Examples 1-3, with a laminated structure of "SUS304 / adhesive composition layer / SUS304". The test specimens were then heated in a dryer at 175°C for 1 hour to cure the adhesive composition. These test specimens were cut to a width of 10 mm and a length of 100 mm and immersed in cooling water (CRUZARD® Straight Coolant Red, manufactured by Komeri Co., Ltd.) at 95°C for 1,000 hours. After 1,000 hours of immersion, the test specimens were removed, the cooling water was wiped off, and they were left to stand for at least 12 hours in a constant temperature room at 23±2°C and 50±5% relative humidity. The adhesive strength was then measured within 24 hours of the start of the standing period. The method for measuring the adhesive strength was the same as described in "Measurement of Initial Adhesion" above. The results are shown in Tables 1 and 2. The judgment criteria were as follows. ·Excellent: 15N / cm or more • Good: 8 N / cm or more, less than 15 N / cm Poor: Less than 8N / cm

[0074] [Degree of swelling of the adhesive sheet after immersion in 95°C cooling water for 1000 hours] For the test specimens used in the "Measurement of Adhesion Strength After Immersion in Cooling Water" experiment, the distance from the end of the specimen in contact with the cooling water to the non-swollen portion (swelling distance) was measured. A schematic diagram of the swelling distance is shown in Figure 2. Measurements were taken at three arbitrary points in the region encompassing the swelling from one 100 mm side (Region A in Figure 2) and at three arbitrary points in the region encompassing the swelling from the other 100 mm side (Region B in Figure 2). Furthermore, the swelling distance was calculated using the average value of six points: three arbitrary points in region A and three arbitrary points in region B. The evaluation criteria were as follows: • Poor (swelling present): Swelling distance is greater than 1 mm. • Excellent (no swelling): Swelling distance is 1 mm or less.

[0075] [Calculation of adhesive strength retention rate] Based on the measured initial adhesive strength and the adhesive strength after immersion in cooling water, the adhesive strength retention rate shown in (Equation 2) below was calculated. (Formula 2) r=a / b×100 r: Adhesive force retention rate (%) a: Adhesion after immersion in 95°C cooling water for 1000 hours. b: Initial adhesive strength

[0076] The results of the adhesive strength retention rate are shown in Tables 1 and 2. The evaluation criteria were as follows. ·Excellent:80% or more • Good: 60% or more, less than 80% Poor: Less than 60%

[0077] [Table 1]

[0078] [Table 2]

[0079] Compared to Comparative Example 1, which does not contain resin beads (C), Comparative Example 2, which does not contain styrene-based thermoplastic elastomer (A), and Comparative Example 3, which does not contain polycarbodiimide (B), Examples 1 to 11, which contain styrene-based thermoplastic elastomer (A), polycarbodiimide (B), and resin beads (C), were shown to have excellent initial adhesion, as well as superior adhesion and adhesion retention after immersion in cooling water for 1000 hours. [Industrial applicability]

[0080] An adhesive composition according to one aspect of the present invention contains a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C). Due to this composition, the adhesive composition according to one aspect of the present invention has high adhesive strength. Furthermore, the adhesive composition of the present invention maintains its high adhesive strength even after immersion in cooling water. [Explanation of Symbols]

[0081] 1 Separator 2 electrodes 3 Cell inner seal 4 Cell-to-cell seal

Claims

1. An adhesive composition containing a styrene-based thermoplastic elastomer (A), a polycarbodiimide (B), and resin beads (C), The styrene-based thermoplastic elastomer (A) contains 100 parts by mass of the polycarbodiimide (B), and the polycarbodiimide (B) contains 0.1 to 45 parts by mass of the polycarbodiimide (B) in proportion to 100 parts by mass of the styrene-based thermoplastic elastomer (A). An adhesive composition containing 1 to 105 parts by mass of the resin beads (C) per 100 parts by mass of the styrene-based thermoplastic elastomer (A).

2. The adhesive composition according to claim 1, wherein the resin beads (C) are at least one selected from the group consisting of acrylic beads, styrene beads, and urethane beads.

3. The adhesive composition according to claim 1, wherein the content of the styrene-based thermoplastic elastomer (A) in the total solid content of the adhesive composition is 40 to 98% by mass.

4. The adhesive composition according to claim 1, wherein the styrene-based thermoplastic elastomer (A) is a carboxyl group-containing styrene-ethylene-butylene-styrene block copolymer.

5. An adhesive sheet comprising an adhesive composition layer made of the adhesive composition according to any one of claims 1 to 4.

6. An adhesive sheet having a core material and adhesive composition layers laminated on both sides of the core material, The adhesive sheet comprises the adhesive composition layer according to any one of claims 1 to 4.

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