Epoxy resin composition, epoxy resin cured product, prepreg, fiber-reinforced composite material, repair method of epoxy resin cured product, repair method of fiber-reinforced composite material, re-molding method of epoxy resin cured product, re-molding method of fiber-reinforced composite material, decomposition method of epoxy resin cured product, and reinforcement fiber recovery method of fiber-reinforced composite material

Abstract

To provide an epoxy resin composition which gives a cured product that has high elastic modulus and is excellent in re-moldability or self-recoverability, an epoxy resin cured product obtained by curing the epoxy resin composition, a prepreg, and a fiber-reinforced composite material.SOLUTION: An epoxy resin composition contains a polyfunctional epoxy resin [A] containing at least a structural unit represented by the following general formula (1). In the general formula (1), Y is each independently a dynamic covalent bond portion, at least one of R1a, R1b, R1c and R1d is a bond to Y of other adjacent structural units, others than at least the one are each independently one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group and a halogen atom, and n represents an integer of 2 to 500.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an epoxy resin composition, a cured epoxy resin product, a prepreg, a fiber-reinforced composite material, a method for repairing a cured epoxy resin product, a method for repairing a fiber-reinforced composite material, a method for remolding a cured epoxy resin product, a method for remolding a fiber-reinforced composite material, a method for decomposing a cured epoxy resin product, and a method for recovering reinforcing fibers from a fiber-reinforced composite material. [Background technology]

[0002] Fiber-reinforced composite materials, obtained by combining reinforcing fiber materials such as carbon fiber, glass fiber, or aramid fiber with various matrix resins, are widely used in a variety of fields and applications. Traditionally, in the aerospace and industrial fields, where high mechanical properties and heat resistance are required, thermosetting resins such as unsaturated polyester resin, epoxy resin, or polyimide resin have been mainly used as the matrix resin.

[0003] However, these thermosetting resins, once cured, do not retain fluidity even when heated, and therefore cannot be molded or deformed again, resulting in discarded cured product scraps and defective molded products generated during the manufacturing process. Although various improvements have been made to the manufacturing process in order to reduce the amount of discarded cured product, it has been difficult to reduce the amount of discarded cured product by improving the manufacturing process alone.

[0004] Therefore, in light of the above background, improvements in resin compositions have been investigated. Among these, a method that has attracted attention is to introduce exchangeable chemical bonding sites (also called dynamic covalent bonding sites) into a resin composition by cleavage followed by recombination or by direct recombination of the bond components, thereby reusing offcuts of cured products and defective moldings. In this method, the exchangeable chemical bonding sites are cleaved or continuously exchanged by heating, thereby eliminating the constraints imposed by the network structure inside the cured product, and the resin becomes fluid and can be molded again. For example, Patent Document 1 discloses an epoxy resin composition that uses, as a curing agent, a compound containing a disulfide bond as a chemical bond site that is exchangeable by direct recombination of bond components. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2015 / 181054 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the epoxy resin composition described in Patent Document 1, in order to obtain a cured product with sufficient fluidity, an excessive amount of an expensive compound containing a disulfide bond had to be used. On the other hand, there was also the drawback that if the curing agent is used in an amount greater than the theoretical equivalent relative to the epoxy resin, the heat resistance and mechanical properties, particularly the elastic modulus, of the cured product are significantly impaired.

[0007] The present disclosure has been made in view of the above, and relates to providing an epoxy resin composition that gives a cured product having a high elastic modulus and excellent remoldability or self-repairability, a cured epoxy resin product obtained by curing the epoxy resin composition, a prepreg, a fiber-reinforced composite material, a method for repairing a cured epoxy resin product, a method for repairing a fiber-reinforced composite material, a method for remolding a cured epoxy resin product, a method for remolding a fiber-reinforced composite material, a method for decomposing a cured epoxy resin product, and a method for recovering reinforcing fibers from a fiber-reinforced composite material. [Means for solving the problem]

[0008] Specific means for solving the above problems include the following aspects. <1> An epoxy resin composition comprising a polyfunctional epoxy resin [A] containing at least a structural unit represented by the following general formula (1):

[0009] [ka]

[0010] (In general formula (1), each Y is independently a dynamic covalent bond moiety, and R 1a , R 1b , R 1c , and R 1d at least one of these is a bond to Y in another adjacent structural unit, and the remaining structural units independently represent one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500. <2> the dynamic covalent bonding site is a group containing a disulfide bond; <1> The epoxy resin composition according to claim 1. <3> The polyfunctional epoxy resin [A] includes a polyfunctional epoxy resin containing a structural unit represented by the following general formula (2): <1> or <2> The epoxy resin composition according to claim 1.

[0011] [ka]

[0012] (In general formula (2), R 2a , R 2b , and R 2c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500. <4> The polyfunctional epoxy resin [A] includes a polyfunctional epoxy resin containing a structural unit represented by the following general formula (3): <1> ~ <3> 1. The epoxy resin composition according to claim 1 .

[0013] [ka]

[0014] (In general formula (3), each R3 independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.) <5> <1> ~ <4> 1. A cured epoxy resin product obtained by curing the epoxy resin composition according to any one of 1 to 8. <6> A cured epoxy resin in which 80% or more by mass of the cured product dissolves when it is brought into contact with a decomposing agent containing a reducing agent and heated at 100°C for 48 hours. <7> A cured epoxy resin material that, when brought into contact with a decomposing agent containing a reducing agent and heated, produces a compound containing at least a structure represented by the following general formula (4):

[0015] [ka]

[0016] (In the general formula (4), * indicates a bond. R 4a , R 4b , and R 4c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. <8> <1> ~ <4> A prepreg obtained by impregnating a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition according to any one of the above items, wherein the content of the reinforcing fibers is 25% by volume to 75% by volume relative to the total volume of the prepreg. <9> <5> ~ <7> and a fiber-reinforced substrate having reinforcing fibers, wherein the content of the reinforcing fibers is 25% by volume to 75% by volume relative to the total volume of the fiber-reinforced composite material. <10> Has internal cracks or peeling <5> ~ <7> 1. A method for repairing a cured epoxy resin material, comprising heating the cured epoxy resin material according to any one of claims 1 to 9 at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material, thereby repairing the cracks or peeling. <11> Has internal cracks or peeling <9> 10. A method for repairing a fiber-reinforced composite material, comprising heating the fiber-reinforced composite material according to claim 1 at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, thereby repairing the crack or delamination. <12> <5> ~ <7> 1. A method for remolding a cured epoxy resin material, comprising heating and pressurizing the cured epoxy resin material described in any one of 1 to 3 at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material. <13> <9> 1. A method for remolding a fiber-reinforced composite material, comprising heating and pressurizing the fiber-reinforced composite material described in 1. above at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material. <14> <5> ~ <7> 1. A method for decomposing a cured epoxy resin material, the method comprising contacting the cured epoxy resin material according to any one of 1 to 3 with a decomposition agent containing a reducing agent. <15> <9> A method for recovering reinforcing fibers from a fiber-reinforced composite material, comprising contacting the fiber-reinforced composite material according to claim 1 with a decomposition agent containing a reducing agent. [Effects of the Invention]

[0017] According to the present disclosure, there are provided an epoxy resin composition that gives a cured product having a high elastic modulus and excellent remoldability or self-repairability, an epoxy resin cured product obtained by curing the epoxy resin composition, a prepreg, a fiber-reinforced composite material, a method for repairing an epoxy resin cured product, a method for repairing a fiber-reinforced composite material, a method for remolding an epoxy resin cured product, a method for remolding a fiber-reinforced composite material, a method for decomposing an epoxy resin cured product, and a method for recovering reinforcing fibers from a fiber-reinforced composite material. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a graph showing the results of stress relaxation measurements. [Figure 2] FIG. 2 is a graph showing the results of the stress relaxation mode separation analysis. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of the present disclosure will be described in detail below. However, the present disclosure is not limited to the following embodiment. In the following disclosure, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure.

[0020] In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the lower and upper limits, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples. In the present disclosure, when a composition contains multiple substances corresponding to each component, the content of each component in the composition means the total content of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, when multiple elements are listed using "or" or "or," unless otherwise expressly stated, it does not exclude the selection of a combination of the multiple elements unless a technical contradiction arises. In the present disclosure, even if an element is referred to in the singular, unless expressly stated otherwise, it does not exclude the presence of a plurality insofar as it does not create a technical contradiction. In the present disclosure, multiple exemplary aspects described separately may be combined with each other to form a new aspect, unless they contradict each other.

[0021] <Epoxy resin composition> The epoxy resin composition of the present disclosure contains a polyfunctional epoxy resin [A] containing at least a structural unit represented by the following general formula (1):

[0022] [ka]

[0023] In general formula (1), each Y is independently a dynamic covalent bond moiety, and R 1a , R 1b , R 1c , and R 1d at least one of these is a bond to Y in another adjacent structural unit, and the structural units other than said at least one each independently represent one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.

[0024] The epoxy resin composition of the present disclosure provides an epoxy resin composition that gives a cured product having a high elastic modulus and excellent remoldability or self-repairability. In this disclosure, remoldability refers to the state in which multiple cured products adhere to each other and no grain boundaries are visible on the surface. Self-repairability refers to the state in which cracks in the cured product disappear or shrink. Although the action of the epoxy resin composition of the present disclosure is not clear, it is presumed as follows.

[0025] The epoxy resin composition of the present disclosure contains a multifunctional epoxy resin [A] containing at least a structural unit represented by general formula (1). In general formula (1), each Y is independently a dynamic covalent bond moiety. In the present disclosure, a dynamic covalent bond moiety refers to a covalent bond moiety that is exchangeable by recombination after cleavage or by direct recombination of bond components. More specifically, as shown in the reaction formula below, dynamic covalent bonding sites are classified into dissociative dynamic covalent bonding sites, which undergo complete cleavage under specific conditions and then recombine under different conditions, cooperative dynamic covalent bonding sites, in which dissociation and recombination of bonds proceed in a concerted manner, and associative dynamic covalent bonding sites, in which a dissociation reaction proceeds after bond formation. A dissociative dynamic covalent bond site is a bond site in which the equilibrium constants for the dissociation and recombination reactions are highly temperature-dependent, and the dissociation and recombination reactions occur in different temperature ranges. In other words, the two covalent bonds are dissociated at a high temperature, and then recombined at a low temperature, where the bond partners are exchanged and the bond partners are recombined to form a new pair of covalent bonds. A cooperative dynamic covalent bond site is a binding site in which the equilibrium constants for the dissociation and recombination reactions are temperature-independent and the cleavage and recombination of two pairs of covalent bonds proceeds in a concerted manner within a specific temperature range. In other words, it is a binding site in which the dissociation of two pairs of covalent bonds, the exchange of binding partners, and the recombination proceed simultaneously within a specific temperature range, thereby reversibly forming two pairs of new covalent bonds. Associative dynamic covalent bonds are those that undergo a bond formation reaction followed by a dissociation reaction, i.e., when heated, the cleaved, free covalent bond can reversibly form a new pair of covalent bonds by recombination. That is, the cured epoxy resin material obtained by curing the epoxy resin composition of the present disclosure has excellent fluidity and excellent remoldability or self-repairing properties because the dynamic covalent bond sites can be reversibly exchanged by heating, thereby continuously changing the molecular network inside the cured material.

[0026] [ka]

[0027] Furthermore, in the epoxy resin composition of the present disclosure, not only is the epoxy resin polyfunctional, but a polyfunctional curing agent can also be used, which increases the crosslink density, and therefore the cured product of the epoxy resin composition of the present disclosure has a high elastic modulus.

[0028] As explained above, the epoxy resin composition of the present disclosure provides an epoxy resin composition that gives a cured product having a high elastic modulus and excellent remoldability or self-repairability through the above-mentioned mechanism. However, the present disclosure is not limited in any way to the above-mentioned assumed mechanism.

[0029] <Multifunctional epoxy resin [A]> [General formula (1)] The epoxy resin composition of the present disclosure contains a multifunctional epoxy resin [A] containing at least a structural unit represented by the following general formula (1): The epoxy resin composition of the present disclosure preferably contains a multifunctional epoxy resin [A] consisting of a structural unit represented by the following general formula (1):

[0030] [ka]

[0031] In general formula (1), each Y is independently a dynamic covalent bond moiety, and R 1a , R 1b , R 1c , and R 1d at least one of these is a bond to Y in another adjacent structural unit, and the structural units other than said at least one each independently represent one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.

[0032] (Y) In general formula (1), each Y is independently a dynamic covalent bond moiety. The dynamic covalent bonding site is preferably a group containing at least one bond selected from the group consisting of a Diels-Alder bond, an imine bond, a disulfide bond, an ester bond, a urethane bond, a carbonate bond, a vinylogous urethane bond, a boronic ester bond, and a silyl ether bond.

[0033] Diels-Alder bonds are dissociative dynamic covalent bonds, with the equilibrium constants for the dissociation and recombination reactions being highly temperature-dependent, meaning that two pairs of covalent bonds dissociate at high temperatures and then recombine at low temperatures. In other words, the two covalent bonds cleaved by heating can exchange their bond partners to form two new pairs of covalent bonds, reversibly achieved by a two-step heating process. Imine and disulfide bonds are cooperative dynamic covalent bond sites in which the equilibrium constants for dissociation and recombination reactions are little dependent on temperature, and in which the cleavage and recombination of two pairs of covalent bonds proceeds in a concerted manner within a specific temperature range. In other words, they are bond sites in which the dissociation of two pairs of covalent bonds, exchange of binding partners, and recombination simultaneously occur within a specific temperature range, allowing the reversible formation of two new pairs of covalent bonds. Ester bonds, urethane bonds, carbonate bonds, vinylogous urethane bonds, boronate ester bonds, and silyl ether bonds are bond sites that undergo a bond formation reaction followed by a dissociation reaction. That is, they are bond sites that can be reversibly cleaved and released to form a new set of covalent bonds by heating, resulting in recombination. From the viewpoint that reshaping is possible by one-step heating and strict control of temperature conditions such as cooling is not required, the dynamic covalent bonding moiety is preferably a cooperative dynamic covalent bonding moiety or an associative dynamic covalent bonding moiety. Among these, from the viewpoint that a catalyst for promoting bond exchange is not required, the dynamic covalent bonding moiety is more preferably a group containing at least one bond selected from the group consisting of an imine bond, a vinylogous urethane bond, a silyl ether bond, and a disulfide bond. From the viewpoint that the rate of bond exchange is very fast, the dynamic covalent bonding moiety is preferably a group containing a disulfide bond, and more preferably a disulfide bond.

[0034] The Diels-Alder bond is represented by the following formula (Y-1-a) or (Y-1-b), the disulfide bond is represented by the following formula (Y-2), the imine bond is represented by the following formula (Y-3), the ester bond is represented by the following formula (Y-4), the urethane bond is represented by the following formula (Y-5), the carbonate bond is represented by the following formula (Y-6), the vinylogous urethane bond is represented by the following formula (Y-7), the boronic acid ester bond is represented by the following formula (Y-8) or (Y-9), and the silyl ether bond is represented by the following formula (Y-10).

[0035] [ka]

[0036] In the above formulas (Y-1) to (Y-10), * represents a bond. X in formula (Y-1-a) is preferably oxygen or an amino group, and R in formulas (Y-7) and (Y-10) is preferably 0 or 1. Y1 , R Y2 , and R Y3 are each independently preferably an aliphatic hydrocarbon group, an aromatic group, or an alkoxy group, more preferably an aliphatic hydrocarbon group or an aromatic group having 4 or less carbon atoms, and even more preferably a methyl group, an ethyl group, or a phenyl group. In formula (Y-9), k and l are each independently preferably an integer of 0 to 10, more preferably an integer of 0 to 5, and even more preferably an integer of 0 to 2.

[0037] (R 1a , R 1b , R 1c , and R 1d ) In general formula (1), R 1a , R 1b , R 1c , and R 1dAt least one of these is a bond to Y in another adjacent structural unit, and the other structural units independently represent any one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. From the viewpoint of the heat resistance and mechanical properties of the cured product, it is preferable that the other structural unit than the at least one bond is a hydrogen atom. From the viewpoint of the toughness of the cured product, it is preferable that the other structural unit than the at least one bond is an aliphatic hydrocarbon group. In general formula (1), R 1a , R 1b , R 1c , and R 1d It is more preferable that any one of the above is a bond to Y in another adjacent structural unit, and the three other than the one bond each independently represent any one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. In general formula (1), R 1d is a bond to Y in another adjacent structural unit, and R 1a , R 1b , and R 1c More preferably, each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. In general formula (1), R 1d is a bond to Y in another adjacent structural unit, and R 1a and R 1c is a hydrogen atom, and R 1b It is particularly preferred that represents any one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, and a halogen atom.

[0038] In the general formula (1), R 1a , R 1b , R 1c , and R 1d Among these, one may be a bond to Y in another adjacent structural unit, two may be bonds to Y in another adjacent structural unit, three may be bonds to Y in another adjacent structural unit, or four may be bonds to Y in another adjacent structural unit.

[0039] R in general formula (1) 1a , R 1b , R 1c , or R 1d In the formula (I), the aliphatic hydrocarbon group may be any of a linear aliphatic hydrocarbon group, a branched aliphatic hydrocarbon group, or an alicyclic hydrocarbon group, and is preferably an aliphatic hydrocarbon group having 1 to 18 carbon atoms, more preferably an aliphatic hydrocarbon group having 1 to 15 carbon atoms, and even more preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms. Furthermore, the aliphatic hydrocarbon group may be a saturated aliphatic hydrocarbon group or an unsaturated aliphatic hydrocarbon group, and is more preferably a saturated aliphatic hydrocarbon group.

[0040] (n) In general formula (1), n ​​represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyfunctional epoxy resin [A] increases, making it difficult to mix or handle with a curing agent or other epoxy resins, and reducing impregnation into a fiber-reinforced substrate. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairing ability of the cured product are significantly reduced. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0041] [General formula (1-a)~(1-d)] The polyfunctional epoxy resin [A] preferably comprises a polyfunctional epoxy resin containing a structural unit represented by any one of the following general formulas (1-a) to (1-d). The polyfunctional epoxy resin [A] more preferably comprises a polyfunctional epoxy resin consisting of a structural unit represented by any one of the following general formulas (1-a) to (1-d). The polyfunctional epoxy resin [A] is even more preferably a polyfunctional epoxy resin consisting of a structural unit represented by any one of the following general formulas (1-a) to (1-d). From the viewpoint of the flowability and handleability of the polyfunctional epoxy resin [A], it is extremely preferable that the polyfunctional epoxy resin [A] is a polyfunctional epoxy resin consisting of a structural unit represented by the following general formula (1-a).

[0042] [ka]

[0043] In general formulas (1-a) to (1-d), each Y is independently a dynamic covalent bond moiety; 1e , R 1f , and R 1g each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom; and n represents an integer of 2 to 500.

[0044] (Y) In the general formulae (1-a) to (1-d), each Y is independently a dynamic covalent bond moiety. The explanation of Y in the general formulae (1-a) to (1-d) is the same as the explanation of Y in the general formula (1), including definitions, examples, preferred embodiments, and the like.

[0045] (R 1e , R 1f , and R 1g ) In general formulas (1-a) to (1-d), R 1e , R 1f , and R 1g Each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. From the viewpoint of the heat resistance and mechanical properties of the cured product, R 1e , R 1f , and R 1g is preferably a hydrogen atom. From the viewpoint of toughness of the cured product, R 1e , R 1f , and R 1g is preferably an aliphatic hydrocarbon group.

[0046] R in general formulas (1-a) to (1-d) 1e , R 1f , and R 1gIn the formula (I), the aliphatic hydrocarbon group may be any of a linear aliphatic hydrocarbon group, a branched aliphatic hydrocarbon group, or an alicyclic hydrocarbon group, and is preferably an aliphatic hydrocarbon group having 1 to 18 carbon atoms, more preferably an aliphatic hydrocarbon group having 1 to 15 carbon atoms, and even more preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms. Furthermore, the aliphatic hydrocarbon group may be a saturated aliphatic hydrocarbon group or an unsaturated aliphatic hydrocarbon group, and is more preferably a saturated aliphatic hydrocarbon group.

[0047] (n) The explanation of n in the general formulae (1-a) to (1-d) is the same as the explanation of n in the general formula (1), including definitions, examples, preferred embodiments, and the like.

[0048] [General formula (2)] The epoxy resin composition of the present disclosure preferably contains, as the polyfunctional epoxy resin [A], a polyfunctional epoxy resin containing a structural unit represented by the following general formula (2). It is more preferable that the polyfunctional epoxy resin [A] contains a polyfunctional epoxy resin composed of a structural unit represented by the following general formula (2). It is even more preferable that the polyfunctional epoxy resin [A] is a polyfunctional epoxy resin composed of a structural unit represented by the following general formula (2).

[0049] [ka]

[0050] In general formula (2), R 2a , R 2b , and R 2c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom; and n represents an integer of 2 to 500.

[0051] That is, the polyfunctional epoxy resin [A] of the present disclosure preferably has a phenyl glycidyl ether structure and a disulfide bond as a constituent unit, and the ether group and disulfide bond of each phenyl glycidyl ether structure are at the ortho position.

[0052] (R 2a , R 2b , and R 2c ) In general formula (2), R 2a , R 2b , and R 2c Each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. From the viewpoint of the heat resistance and mechanical properties of the cured product, R 2a , R 2b , and R 2c is preferably a hydrogen atom. From the viewpoint of toughness of the cured product, R 2a , R 2b , and R 2c is preferably an aliphatic hydrocarbon group.

[0053] R in general formula (2) 2a , R 2b , and R 2c In the formula (I), the aliphatic hydrocarbon group may be any of a linear aliphatic hydrocarbon group, a branched aliphatic hydrocarbon group, or an alicyclic hydrocarbon group, and is preferably an aliphatic hydrocarbon group having 1 to 18 carbon atoms, more preferably an aliphatic hydrocarbon group having 1 to 15 carbon atoms, and even more preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms. Furthermore, the aliphatic hydrocarbon group may be a saturated aliphatic hydrocarbon group or an unsaturated aliphatic hydrocarbon group, and is more preferably a saturated aliphatic hydrocarbon group.

[0054] (n) In general formula (2), n represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyfunctional epoxy resin [A] increases, making it difficult to mix or handle with a curing agent or other epoxy resins, and reducing impregnation into a fiber-reinforced substrate. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairing ability of the cured product are significantly reduced. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0055] [General formula (3)] The epoxy resin composition of the present disclosure preferably contains, as the polyfunctional epoxy resin [A], a polyfunctional epoxy resin containing a structural unit represented by the following general formula (3). It is more preferable that the polyfunctional epoxy resin [A] contains a polyfunctional epoxy resin composed of a structural unit represented by the following general formula (3). It is even more preferable that the polyfunctional epoxy resin [A] is a polyfunctional epoxy resin composed of a structural unit represented by the following general formula (3).

[0056] [ka]

[0057] In general formula (3), each R3 independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom; and n represents an integer of 2-500.

[0058] (R3) In general formula (3), R3 each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. From the viewpoint of the heat resistance and mechanical properties of the cured product, R3 is preferably a hydrogen atom. From the viewpoint of the toughness of the cured product, R3 is preferably an aliphatic hydrocarbon group. From the viewpoint of the handleability of the epoxy resin composition (uncured product), R3 is preferably a quaternary carbon.

[0059] In R3 in general formula (3), the aliphatic hydrocarbon group may be any of a straight-chain aliphatic hydrocarbon group, a branched-chain aliphatic hydrocarbon group, or an alicyclic hydrocarbon group, and is preferably an aliphatic hydrocarbon group having 1 to 18 carbon atoms, more preferably an aliphatic hydrocarbon group having 1 to 15 carbon atoms, and even more preferably an aliphatic hydrocarbon group having 1 to 10 carbon atoms. Furthermore, the aliphatic hydrocarbon group may be a saturated aliphatic hydrocarbon group or an unsaturated aliphatic hydrocarbon group, and is more preferably a saturated aliphatic hydrocarbon group.

[0060] (n) In general formula (3), n represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyfunctional epoxy resin [A] increases, making it difficult to mix or handle with a curing agent or other epoxy resins, and reducing impregnation into a fiber-reinforced substrate. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairing ability of the cured product are significantly reduced. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0061] (Examples of compounds represented by general formula (3)) In the general formula (3), R3's each independently represent a hydrogen atom or an aliphatic hydrocarbon group having one carbon atom, which is preferable because the mechanical properties of the cured product are improved. Examples of the compound represented by the general formula (3) include the compound represented by the following chemical formula (3-1).

[0062] [ka]

[0063] In the chemical formula (3-1), n ​​represents an integer of 2 to 500, preferably an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0064] When R3 in general formula (3) is a methoxy group, the mechanical properties, particularly the elastic modulus, of the cured product are improved, which is preferable. Examples of compounds represented by general formula (3) include compounds of the following chemical formula (3-2).

[0065] [ka]

[0066] In the chemical formula (3-2), n represents an integer of 2 to 500, preferably an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0067] Examples of the polyfunctional epoxy resin [A] of the present disclosure include compounds represented by the following chemical formulas (3-3) and (3-4): When R3 in general formula (3) are each independently an aliphatic hydrocarbon group having 4 to 5 carbon atoms and containing a quaternary carbon, this is preferred because the crystallinity of the polyfunctional epoxy resin is reduced, thereby improving the handleability of the epoxy resin composition (uncured product).

[0068] [ka]

[0069] [ka]

[0070] In the chemical formula (3-3) and the chemical formula (3-4), n represents an integer of 2 to 500, preferably an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0071] [Properties of multifunctional epoxy resin [A]] The weight-average molecular weight of the polyfunctional epoxy resin [A] of the present disclosure is preferably 500 to 100,000, more preferably 500 to 10,000, and even more preferably 500 to 5000. In the present disclosure, the weight-average molecular weight is determined by gel permeation chromatography (GPC) measurement.

[0072] The epoxy equivalent of the polyfunctional epoxy resin [A] of the present disclosure is preferably 200 g / Eq to 1000 g / Eq, more preferably 200 g / Eq to 500 g / Eq, and even more preferably 200 g / Eq to 300 g / Eq. Note that, in the present disclosure, the epoxy equivalent of the polyfunctional epoxy resin is determined according to JIS K7236 (ISO 3001:1999).

[0073] [Method for synthesizing multifunctional epoxy resin [A]] The polyfunctional epoxy resin [A] of the present disclosure may be synthesized by any method, for example, by reacting a raw material phenol analog compound with sulfur chloride to obtain an intermediate poly(phenol disulfide) polymer, which is then reacted with an epihalohydrin such as epichlorohydrin in the presence of a base catalyst to obtain a halohydrin, which is then subjected to a cyclization reaction using a basic compound. More specifically, it can be synthesized by the method described in the Examples below.

[0074] The phenol analogue compound used as the raw material is, for example, a compound represented by the general formula (1) R 1a , R 1b , R 1c , and R 1d Phenol in which three of the R 1a , R 1b , R 1c , and R 1d Examples of phenols include cresol, in which two of the groups are hydrogen atoms and one is a methyl group; ethylphenol, in which the group is an ethyl group; cyclohexylphenol, in which the group is a cyclohexyl group; methoxyphenol, in which the group is a methoxy group; aminophenol, in which the group is an amino group; hydroxybenzaldehyde, in which the group is an aldehyde group; hydroxybenzoic acid, in which the group is a carboxy group; and halophenols, in which the group is a halogen group such as a chloro group, a bromo group, or an iodo group. In addition, R in general formula (1) 1a , R 1b , R 1c , and R 1d Among these, those which are not bonds to Y in the adjacent structural unit include: each independently includes an aliphatic hydrocarbon group having 1 to 18 carbon atoms, an unsaturated aliphatic hydrocarbon group having 2 to 18 carbon atoms, an aromatic group having 6 to 12 carbon atoms, or an alicyclic hydrocarbon group having 3 to 10 carbon atoms; Alternatively, they may each independently be an alkoxy group having an aliphatic hydrocarbon group having 1 to 18 carbon atoms, an unsaturated aliphatic hydrocarbon group having 2 to 18 carbon atoms, an aromatic group having 6 to 12 carbon atoms, or an alicyclic hydrocarbon group having 3 to 10 carbon atoms.

[0075] [Type of structural unit of multifunctional epoxy resin [A]] The polyfunctional epoxy resin [A] of the present disclosure may be a polyfunctional epoxy resin containing one type of structural unit represented by general formula (1), or may contain a polyfunctional epoxy resin containing two or more types of structural units represented by general formula (1).

[0076] The proportion of the structural units represented by general formula (1) in all structural units of the polyfunctional epoxy resin [A] is preferably 10 mol % to 100 mol %, more preferably 50 mol % to 100 mol %, and even more preferably 90 mol % to 100 mol %.

[0077] The polyfunctional epoxy resin [A] of the present disclosure may contain a structural unit other than the structural unit represented by general formula (1). Examples of structural units other than the structural unit represented by general formula (1) that may be contained in the multifunctional epoxy resin [A] of the present disclosure include hydrocarbon groups, heterocyclic groups, -NH-, -SO-, -SO2-, -CO-, -O-, -COO-, -OCO-, -S-, -SS-, and groups formed by combining two or more of these. Examples of hydrocarbon groups include alkyl groups and aryl groups. The heterocyclic group may be a non-aromatic heterocyclic group or an aromatic heterocyclic group. The heterocyclic group is preferably a 5- or 6-membered ring. Examples of heteroatoms constituting the heterocyclic group include nitrogen atoms, oxygen atoms, and sulfur atoms. The heterocyclic group may be a monocyclic group or a fused ring.

[0078] The remoldability and self-repairing properties of the cured epoxy resin product obtained by curing the epoxy resin composition of the present disclosure are presumed to be due to the dynamic covalent bond moiety (Y) contained in the above general formula (1), etc. The high heat resistance, elastic modulus, and strength of the cured epoxy resin product obtained by curing the epoxy resin composition of the present disclosure are presumed to be brought about by the rigid aromatic ring contained in the above general formula (1).

[0079] <Epoxy resins other than multifunctional epoxy resin [A]> The epoxy resin composition of the present disclosure may contain, in addition to the polyfunctional epoxy resin [A] represented by general formula (1), a known epoxy resin that is cured with a curing agent. The epoxy resin other than the polyfunctional epoxy resin [A] may be monofunctional, difunctional, or polyfunctional, and may be a glycidyl ether or a glycidyl amine. The epoxy resin other than the polyfunctional epoxy resin [A] may be any epoxy resin that can be cured with a curing agent, and is appropriately selected depending on the intended use, etc.

[0080] Specific examples of epoxy resins other than the multifunctional epoxy resin [A] include: tetrafunctional glycidylamine-type epoxy resins such as tetraglycidyl-4,4'-diaminodiphenylmethane, tetraglycidyl-4,4'-diaminodiphenyl sulfone, tetraglycidyl-3,3'-diaminodiphenyl sulfone, tetraglycidyl-4,4'-diaminodiphenyl ether, tetraglycidyl-3,4'-diaminodiphenyl ether, and tetraglycidyl-4,4'-didithiodianiline, and derivatives thereof; trifunctional epoxy resins such as triglycidyl-m-aminophenol, triglycidyl-p-aminophenol, and triglycidyl isocyanurate, and derivatives thereof; bifunctional glycidylamine-type epoxy resins such as diglycidyl aniline and its derivatives such as diglycidyl-o-toluidine, diglycidyl-m-toluidine, diglycidyl-p-toluidine, diglycidyl-xylidine, diglycidyl-mesidine, diglycidyl-anisidine, diglycidyl-phenoxyaniline, and diglycidyl-naphthylamine and its derivatives; bifunctional glycidyl ether-type epoxy resins such as bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol S diglycidyl ether, resorcinol diglycidyl ether, 1,6-naphthalenediol diglycidyl ether, and 4,4'-dithiodiphenol diglycidyl ether, and derivatives thereof; polyfunctional glycidyl ether epoxy resins such as phenol novolac epoxy resins, cresol novolac epoxy resins, and derivatives thereof; and glycidyl ester type epoxy resins such as phthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, dimer acid diglycidyl ester, and derivatives thereof; Examples include: These epoxy resins may be used alone or in combination.

[0081] Among these epoxy resins, bifunctional glycidyl ether epoxy resins such as bisphenol A-diglycidyl ether, bisphenol F-diglycidyl ether, bisphenol S-diglycidyl ether, resorcinol diglycidyl ether, and 1,6-naphthalenediol diglycidyl ether are particularly preferred because of the ease of handling of the epoxy resin composition and the remoldability of the cured epoxy resin product.

[0082] [Ingredients in epoxy resin composition] The total amount of the polyfunctional epoxy resin [A] contained in the epoxy resin composition of the present disclosure is not particularly limited, and is preferably 1% by mass or more and less than 100% by mass, more preferably 30% by mass or more and less than 100% by mass, and even more preferably 50% by mass or more and less than 100% by mass, based on the epoxy resin composition.

[0083] <Curing agent> The epoxy resin composition of the present disclosure may contain a curing agent [B]. The curing agent [B] is not particularly limited as long as it is a curing agent that can cure epoxy resins, and may be appropriately selected depending on the intended use, etc.

[0084] Specific examples of the curing agent [B] include dicyandiamide, polyhydric phenol compound curing agents, various isomers of aromatic amine curing agents, and aminobenzoic acid esters. Dicyandiamide is preferred because it provides excellent storage stability for the prepreg described below. Furthermore, aromatic diamine compounds such as 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, and 4,4'-diaminodiphenylmethane, as well as their derivatives having non-reactive substituents, are particularly preferred because they can provide cured products with high heat resistance. Furthermore, 3,3'-diaminodiphenyl sulfone is most preferred because it can provide cured products with high heat resistance and elastic modulus. Examples of non-reactive substituents include alkyl groups such as methyl, ethyl, and isopropyl groups, aromatic groups such as phenyl groups, and halogen groups such as alkoxyl groups, aralkyl groups, and chlorine and bromine.

[0085] An epoxy resin composition containing an aromatic amine-based curing agent and a polyfunctional epoxy resin [A] containing at least a structural unit represented by general formula (1) is preferred from the viewpoint that when cured to form a cured product, it has high heat resistance and excellent mechanical properties.

[0086] [Ingredients in epoxy resin composition] The total amount of the curing agent [B] contained in the epoxy resin composition of the present disclosure may be an amount suitable for curing all of the epoxy resins blended in the epoxy resin composition, and may be adjusted as appropriate depending on the types of epoxy resins or curing agents used. The total amount of the curing agent [B] contained in the epoxy resin composition of the present disclosure is not particularly limited, and is preferably 1% by mass or more and less than 80% by mass, more preferably 5% by mass or more and less than 50% by mass, and even more preferably 10% by mass or more and less than 30% by mass, based on the epoxy resin composition.

[0087] In the epoxy resin composition of the present disclosure, the ratio of the molar amount of all epoxy groups to the molar amount of all active hydrogens is preferably 1:0.5 to 1:1.5, more preferably 1:0.75 to 1:1.3, and even more preferably 1:0.9 to 1:1.2. If the molar amount of all active hydrogens is small relative to the molar amount of all epoxy groups, the viscosity of the epoxy resin composition increases, and the impregnation ability into fiber-reinforced substrates tends to decrease. If the molar amount of all active hydrogens is large relative to the molar amount of all epoxy groups, the remoldability of the resulting epoxy resin cured product tends to decrease.

[0088] The molar amount of all epoxy groups is determined from the sum of the epoxy groups derived from the polyfunctional epoxy resin [A] and the epoxy groups derived from resins other than the polyfunctional epoxy resin [A] contained in the epoxy resin composition. That is, the molar amount of all epoxy groups is determined from the sum of the molar amounts of epoxy groups obtained by dividing the mass of each epoxy resin contained in the epoxy resin composition by the epoxy equivalent of each resin. The total molar amount of active hydrogen is determined from the molar amount of active hydrogen derived from the curing agent [B] contained in the epoxy resin composition. That is, the total molar amount of active hydrogen is determined by dividing the molar amount of the curing agent [B] contained in the epoxy resin composition by the number of active hydrogen atoms contained in one molecule.

[0089] In the epoxy resin composition of the present disclosure, the ratio (=(b) / (a)) of the molar amount (b) of all active hydrogens to the molar amount (a) of epoxy groups in the polyfunctional epoxy resin [A] is preferably 0.25 to 1.5, more preferably 0.4 to 1.3, even more preferably 0.7 to 1.2, and particularly preferably 1.0. When the ratio (b) / (a) is 0.25 or more, the performance of the dynamic covalent bond moieties is more easily exhibited, resulting in excellent remoldability and self-repairing properties. When the ratio (b) / (a) is 1.5 or less, the molar balance between epoxy groups and active hydrogens is more favorable, resulting in a sufficient crosslink density of the resulting cured product, and improved mechanical properties such as heat resistance, elastic modulus, and fracture toughness, which is preferred.

[0090] That is, the ratio (b) / (a) is the ratio of the molar amount (a) of epoxy groups in the multifunctional epoxy resin [A] to the molar amount (b) of all active hydrogens contained in a unit mass of the epoxy resin composition. In the present disclosure, the ratio (b) / (a) is the ratio of the molar amount (a) of epoxy groups in the multifunctional epoxy resin [A] to the molar amount (b) of all active hydrogens contained in the epoxy resin composition. 1 It can be determined from the integrated area ratio of the peak derived from the epoxy group to the peak derived from active hydrogen when analyzed by H-NMR.

[0091] <Optional ingredients> The epoxy resin composition of the present disclosure may contain a curing accelerator, a thermoplastic resin, conductive particles, a flame retardant, an inorganic filler, an internal mold release agent, or the like.

[0092] The curing accelerator accelerates the curing reaction of the epoxy resin composition and the prepreg impregnated with it, thereby providing a cured product with high elastic modulus and strength in a short production time. The curing accelerator is not particularly limited as long as it accelerates the curing reaction of the polyfunctional epoxy resin [A], and examples thereof include nitrogen-containing heterocyclic compounds such as imidazoles and tertiary amines, as well as onium salts thereof, phosphines, and phosphonium salts thereof.

[0093] Specific examples of the curing accelerator include imidazoles such as 2-ethyl-4-methylimidazole and 1-(2-cyanoethyl)-2-phenylimidazole, and salts thereof; tertiary amines such as diazabicycloundecene, diazabicyclononene, and tris(dimethylaminomethyl)phenol, and salts thereof; phosphines such as triphenylphosphine and tri(orthotolyl)phosphine; and phosphonium salts such as tetraphenylphosphonium tetraphenylborate.

[0094] The content of the curing accelerator in the epoxy resin composition of the present disclosure is selected as needed and is not particularly limited.

[0095] The thermoplastic resin improves the fracture toughness and impact resistance of the resulting fiber-reinforced composite material. Examples of the thermoplastic resin include epoxy resin-soluble thermoplastic resins and epoxy resin-insoluble thermoplastic resins.

[0096] The epoxy resin-soluble thermoplastic resin adjusts the viscosity of the epoxy resin composition and further improves the impact resistance of the resulting fiber-reinforced composite material. Specific examples of epoxy resin-soluble thermoplastic resins include polyethersulfone, polysulfone, polyetherimide, and polycarbonate. These may be used alone or in combination. The epoxy resin-soluble thermoplastic resin is particularly preferably polyethersulfone or polysulfone having a weight-average molecular weight (Mw) of 8,000 to 100,000 as measured by gel permeation chromatography. If the weight-average molecular weight (Mw) is less than 8,000, the impact resistance of the resulting fiber-reinforced composite material may be insufficient. Furthermore, if the weight-average molecular weight (Mw) is greater than 100,000, the viscosity of the epoxy resin composition may increase significantly, significantly reducing handleability. It is preferable that the molecular weight distribution of the epoxy resin-soluble thermoplastic resin is uniform. In particular, the polydispersity (Mw / Mn), which is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), is preferably in the range of 1 to 10, more preferably 1.1 to 5.

[0097] The epoxy resin-soluble thermoplastic resin preferably has a reactive group reactive with the epoxy resin or a functional group capable of forming a hydrogen bond. Such an epoxy resin-soluble thermoplastic resin can improve the solubility stability of the epoxy resin during the curing process. Furthermore, the epoxy resin-soluble thermoplastic resin can impart toughness, chemical resistance, heat resistance, and moist heat resistance to the fiber-reinforced composite material obtained after curing.

[0098] The reactive group reactive with the epoxy resin is preferably a hydroxyl group, a carboxylic acid group, an imino group, an amino group, etc. Use of a hydroxyl-terminated polyethersulfone is more preferred because the resulting fiber-reinforced composite material is particularly excellent in impact resistance, fracture toughness, and solvent resistance.

[0099] A portion of the epoxy resin-insoluble thermoplastic resin or epoxy resin-soluble thermoplastic resin (epoxy resin-soluble thermoplastic resin that remains undissolved in the matrix resin after curing) becomes dispersed as particles in the matrix resin of the fiber-reinforced composite material (hereinafter, these dispersed particles are also referred to as "interlayer particles"). These interlayer particles suppress the propagation of impacts received by the fiber-reinforced composite material. As a result, the impact resistance of the resulting fiber-reinforced composite material is improved.

[0100] Examples of epoxy resin-insoluble thermoplastic resins include polyamide, polyacetal, polyphenylene oxide, polyphenylene sulfide, polyester, polyamideimide, polyimide, polyether ketone, polyether ether ketone, polyethylene naphthalate, polyether nitrile, and polybenzimidazole. Among these, polyamide, polyamideimide, and polyimide are preferred due to their high toughness and heat resistance. Polyamide and polyimide are particularly effective in improving the toughness of fiber-reinforced composite materials. These may be used alone or in combination of two or more. Copolymers of these may also be used.

[0101] In particular, the heat resistance of the resulting fiber-reinforced composite material can be improved by using polyamides such as amorphous polyimide, Nylon 6 (registered trademark) (a polyamide obtained by a ring-opening polycondensation reaction of caprolactam), Nylon 11 (a polyamide obtained by a ring-opening polycondensation reaction of undecane lactam), Nylon 12 (a polyamide obtained by a ring-opening polycondensation reaction of lauryllactam), Nylon 1010 (a polyamide obtained by a copolymerization reaction of sebacic acid and 1,10-decanediamine), and amorphous nylon (also called transparent nylon, which does not crystallize the polymer or has an extremely slow crystallization rate).

[0102] Examples of conductive particles include conductive polymer particles such as polyacetylene particles, polyaniline particles, polypyrrole particles, polythiophene particles, polyisothianaphthene particles, and polyethylenedioxythiophene particles; carbon particles; carbon fiber particles; metal particles; and particles in which a core material made of an inorganic or organic material is coated with a conductive substance.

[0103] Examples of flame retardants include phosphorus-based flame retardants, which are not particularly limited as long as they contain a phosphorus atom in the molecule, and examples thereof include organic phosphorus compounds such as phosphate esters, condensed phosphate esters, phosphazene compounds, and polyphosphates, as well as red phosphorus.

[0104] Examples of inorganic fillers include aluminum borate, calcium carbonate, silicon carbonate, silicon nitride, potassium titanate, basic magnesium sulfate, zinc oxide, graphite, calcium sulfate, magnesium borate, magnesium oxide, and silicate minerals. In particular, silicate minerals are preferably used. Specific examples of silicate minerals include THIXOTROPIC AGENT DT 5039 (manufactured by Huntsman Japan Co., Ltd.).

[0105] Examples of internal mold release agents include metal soaps, vegetable waxes such as polyethylene wax and carnauba wax, fatty acid ester-based mold release agents, silicone oil, animal wax, and fluorine-based nonionic surfactants. The amount of these internal mold release agents added is preferably 0.1 to 5 parts by mass, more preferably 0.2 to 2 parts by mass, per 100 parts by mass of the epoxy resin. Within this range, the mold release effect from the mold is optimally exhibited.

[0106] Specific examples of the internal mold release agent include "MOLD WIZ (registered trademark)" INT1846 (manufactured by AXEL PLASTICS RESEARCH LABORATORIES INC.), Licowax S, Licowax P, Licowax OP, Licowax PE190, and Licowax PED (manufactured by Clariant Japan), and stearyl stearate (SL-900A; manufactured by Riken Vitamin Co., Ltd.).

[0107] <Method of producing epoxy resin composition> The epoxy resin composition of the present disclosure can be produced by mixing the polyfunctional epoxy resin [A] and, if necessary, other components, in any order.

[0108] The method for producing the epoxy resin composition of the present disclosure is not particularly limited, and any conventionally known method may be used. The mixing temperature can be, for example, in the range of 40°C to 120°C. If the mixing temperature exceeds 120°C, the curing reaction may proceed partially, reducing the impregnation into the fiber-reinforced substrate layer, and the storage stability of the resulting epoxy resin composition and the prepreg produced using it may be reduced. If the mixing temperature is less than 40°C, the viscosity of the epoxy resin composition may be high, making mixing substantially difficult. The mixing temperature is preferably 50°C to 100°C, and more preferably 50°C to 90°C.

[0109] Conventional known mixing machines can be used. Specific examples include a roll mill, a planetary mixer, a kneader, an extruder, a Banbury mixer, a mixing vessel equipped with stirring blades, and a horizontal mixing tank. The components can be mixed in the air or in an inert gas atmosphere. When mixing is carried out in the air, an atmosphere in which the temperature and humidity are controlled is preferred. Although not particularly limited, mixing is preferably carried out in a low-humidity atmosphere, for example, at a temperature controlled to a constant temperature of 30°C or less and a relative humidity of 50% RH or less.

[0110] <Epoxy resin cured product> <Epoxy Resin Cured Product of First Embodiment> The cured epoxy resin product of the first embodiment is obtained by curing the epoxy resin composition of the present disclosure. The method for curing the epoxy resin composition of the present disclosure to obtain the cured epoxy resin product of the present disclosure is not particularly limited, and may be a known method. For example, the curing method may involve degassing in a vacuum, followed by curing at a temperature of 120°C to 200°C for 15 minutes to 24 hours, preferably at a temperature of 120°C to 180°C for 1 to 12 hours, and more preferably at a temperature of 150°C to 180°C for 1 to 6 hours, to obtain a cured epoxy resin product. Specifically, the cured epoxy resin product may be obtained by degassing in a vacuum, followed by curing at 120°C for 2 hours, at 150°C for 2 hours, and at 180°C for 2 hours.

[0111] When the cured epoxy resin material of the first embodiment is brought into contact with a decomposing agent containing a reducing agent and heated at 100° C. for 48 hours, it is preferable that 80 mass % or more of the cured material dissolves. The explanation for the fact that 80 mass % or more of the cured product dissolves when it is brought into contact with a decomposing agent containing a reducing agent and heated at 100°C for 48 hours is the same as that in the <Cured epoxy resin product of the second embodiment> described later.

[0112] When the cured epoxy resin product of the first embodiment is brought into contact with a decomposing agent containing a reducing agent and heated, it is preferable that a compound containing at least the structure shown in the following general formula (4) is generated. When the cured epoxy resin product of the first embodiment is brought into contact with a decomposing agent containing a reducing agent and heated, it is more preferable that the cured epoxy resin product of the first embodiment is decomposed into at least a compound containing the structure shown in the following general formula (4).

[0113] [ka]

[0114] In general formula (4), * represents a bond. In general formula (4), * in "-C(OH)-*" is preferably a bond to a structure derived from a curing agent, and * in "-S-*" is preferably a bond to a structure derived from a reducing agent. R 4a , R 4b , and R 4c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom. The explanation regarding the fact that when a decomposing agent containing a reducing agent is brought into contact with the cured epoxy resin and heated, a compound containing at least the structure represented by the general formula (4) is generated and the fact that the cured epoxy resin is decomposed into a compound containing at least the structure represented by the general formula (4) is the same as the explanation in <Cured epoxy resin material of a third embodiment> described later.

[0115] <Epoxy Resin Cured Product of Second Embodiment> When the cured epoxy resin material of the second embodiment is brought into contact with a decomposing agent containing a reducing agent and heated at 100° C. for 48 hours, 80 mass % or more of the cured epoxy resin material dissolves. The cured epoxy resin material of the second embodiment has excellent solubility, and can be dissolved in a low molecular weight compound by bringing the cured epoxy resin material into contact with a decomposing agent.

[0116] The type of the reducing agent is not particularly limited as long as it dissolves the cured epoxy resin material, and any reducing agent may be used depending on the type of bond contained in the cured epoxy resin material. Examples of the reducing agent include dithiothreitol, 2-mercaptoethanol, 1-thioglycerol, glutathione, 2-mercaptoethylamine hydrochloride, cystine hydrochloride, tris(2-carboxyethyl)phosphine hydrochloride, etc., and reducing agents that reduce disulfide bonds are preferred. These reducing agents may be used alone or in combination.

[0117] These reducing agents may be used without a solvent, or may be used as a decomposition agent solution by diluting with a solvent. The cured epoxy resin material of the second embodiment may be dissolved by immersing in a decomposition agent solution containing a reducing agent. The amount of the decomposer solution used is not particularly limited and is determined depending on the shape of the cured epoxy resin material, dissolution conditions, etc. The amount of the decomposer contained in the decomposer solution used is preferably 0.5 to 1000 times, more preferably 0.5 to 100 times, and particularly preferably 0.5 to 10 times, the amount of the cured epoxy resin material by mass.

[0118] There are no particular restrictions on the concentration of the reducing agent in the decomposing agent solution, and it is preferably 0.1% by mass to 99.9% by mass, more preferably 1% by mass to 80% by mass, and particularly preferably 5% by mass to 50% by mass. The solvent used for dilution is not particularly limited as long as it can dissolve the reducing agent and the cured product, and may be water or an organic solvent. Examples of organic solvents used for dilution include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, 1,4-dioxane, tetrahydrofuran, methanol, ethanol, etc., and polar organic solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide are particularly preferred because they can completely dissolve the cured product.

[0119] In addition to the above, the decomposing agent may contain any additive that promotes dissolution. In particular, nitrogen-containing bases such as triethylamine, DBU (1,8-diazabicycloundecene), or ammonia are preferred because their addition has a high dissolution promoting effect.

[0120] In order to promote dissolution, the dissolution may be carried out by heating at 25°C to 150°C (preferably 25°C to 60°C) for 1 hour to 48 hours (preferably 1 hour to 8 hours), by stirring appropriately, or by ultrasonic treatment if necessary. For example, the dissolution reaction may be carried out at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In order to promote dissolution, the dissolution may be carried out by applying pressure of 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa).

[0121] In the cured epoxy resin material of the second embodiment, upon contact with a decomposing agent containing a reducing agent and heating, preferably 50% by mass or more of the cured epoxy resin material is dissolved, more preferably 80% by mass or more, even more preferably 95% by mass or more, and most preferably 99% by mass or more. In the cured epoxy resin material of the second embodiment, preferably 100% by mass of the cured epoxy resin material is dissolved.

[0122] In the present disclosure, when a cured epoxy resin material is dissolved, the dissolved ratio (mass %) in the cured epoxy resin material is measured by filtering the solution in which the cured epoxy resin material is dissolved and dividing the mass of the residue by the mass of the cured epoxy resin material before dissolution.

[0123] <Epoxy Resin Cured Product of Third Embodiment> When the cured epoxy resin product of the third embodiment is brought into contact with a decomposing agent containing a reducing agent and heated, a compound containing at least the structure shown in the following general formula (4) is generated. It is more preferable that the cured epoxy resin product of the third embodiment is decomposed into at least the compound containing the structure shown in the following general formula (4) when brought into contact with a decomposing agent containing a reducing agent and heated.

[0124] [ka]

[0125] In general formula (4), * represents a bond. In general formula (4), * in "-C(OH)-*" is preferably a bond to a structure derived from a curing agent, and * in "-S-*" is preferably a bond to a structure derived from a reducing agent. R 4a , R 4b , and R 4c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom.

[0126] From the viewpoint of the solubility of the decomposed resin, the molecular weight of the compound represented by general formula (4) is preferably 200 to 5000, more preferably 200 to 4000, and even more preferably 200 to 3000. In the present disclosure, the molecular weight is measured by gel permeation chromatography (GPC) using chloroform or N,N-dimethylformamide as an eluent.

[0127] The description of the decomposer containing a reducing agent in the cured epoxy resin material of the third embodiment is the same as the description of the decomposer containing a reducing agent in the cured epoxy resin material of the second embodiment, including definitions, examples, preferred aspects, etc.

[0128] For example, when a solution of 2-mercaptoethanol in N,N-dimethylformamide is used as the decomposing agent solution and heated, the cured epoxy resin material of the third embodiment produces a compound containing at least the structure represented by the following general formula (4-1).

[0129] [ka]

[0130] In general formula (4-1), * represents a bond. In general formula (4-1), * in "-C(OH)-*" preferably represents a bond to a structure derived from a curing agent. R 4a , R 4b , and R 4c each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom.

[0131] From the viewpoint of the solubility of the decomposed resin, the molecular weight of the compound represented by general formula (4-1) is preferably 200 to 5,000, more preferably 200 to 2,000, and even more preferably 200 to 1,000.

[0132] The conditions for contacting and heating with a decomposing agent containing a reducing agent are not particularly limited, and heating may be performed at 25°C to 150°C (preferably 25°C to 60°C) for 1 hour to 48 hours (preferably 1 hour to 8 hours), with appropriate stirring, or ultrasonic treatment if necessary. For example, the production reaction may be performed at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In the above production, pressure of 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa) may be applied to promote production.

[0133] In the present disclosure, the generation of at least a compound having a structure represented by general formula (4) from the cured epoxy resin material of the third embodiment can be confirmed by LC-MS. 1 This can also be confirmed by H-NMR.

[0134] <Epoxy Resin Cured Products of First to Third Embodiments> In the present disclosure, the embodiment of the "cured epoxy resin material" includes the embodiments of the cured epoxy resin materials of the first to third embodiments. The cured epoxy resin material of the present disclosure preferably has a glass transition temperature (Tg) of 80° C. to 300° C., more preferably 100° C. to 200° C. If the glass transition temperature is less than 80° C., the heat resistance is insufficient, and as a result, voids and the like are likely to form in the resulting fiber-reinforced composite material. The glass transition temperature in this disclosure can be measured using a differential scanning calorimeter. The temperature of a cured product is raised from 30°C to 350°C at a rate of 10°C / min, and the inflection point in the transition process of the obtained curve is taken as the glass transition temperature.

[0135] The cured epoxy resin product of the present disclosure preferably has a flexural modulus (hereinafter sometimes referred to as FM) measured by JIS K7171 method (ISO 178:2019) of 2.70 GPa or more, more preferably 2.80 GPa to 30.00 GPa, even more preferably 3.00 GPa to 20.00 GPa, and may be 3.30 GPa to 5.00 GPa. If the flexural modulus is less than 2.70 GPa, the properties of the resulting fiber-reinforced composite material are likely to deteriorate. If the flexural modulus is 2.70 GPa or more, the cured product can be said to have a high modulus. The flexural modulus in the present disclosure can be measured in accordance with JIS K7171.

[0136] The cured epoxy resin product of the present disclosure preferably has a flexural strength (FS) of 30 MPa or more, more preferably 60 MPa to 200 MPa, and even more preferably 90 MPa to 150 MPa. If the flexural strength is less than 30 MPa, the properties of the resulting fiber-reinforced composite material are likely to deteriorate. The bending strength in the present disclosure is the maximum value of bending stress measured in accordance with JIS K7171.

[0137] The degree of cure of the epoxy resin cured product of the present disclosure is preferably 90% or more, more preferably 95% or more, and even more preferably 99% or more. If the degree of cure is insufficient, poor toughness is likely to occur. The degree of cure can be measured by a conventionally known evaluation method, such as the JIS K7148 method, using a differential scanning calorimeter (DSC).

[0138] <Stress relaxation rate> When an external force is applied to a cured product obtained by curing the epoxy resin composition of the present disclosure to a degree of cure of 90% or more at a temperature 60°C higher than the glass transition temperature of the cured product and the resulting stress is measured, the stress relaxation rate one hour after the start of application of the external force is preferably 90% or higher. The stress relaxation rate one hour after the start of application of the external force is more preferably 99.0% or higher, even more preferably 99.5% or higher, and particularly preferably 99.7% or higher. If the stress relaxation rate one hour after the start of application of the external force is less than 90%, the remoldability, strength, and elastic modulus of the cured epoxy resin product and the fiber-reinforced composite material described below are likely to decrease.

[0139] Here, the stress relaxation rate (%) is a value calculated by the following formula for the change in modulus value G over 3600 seconds that occurs in the measurement of stress relaxation. The modulus value is the value of stress that occurs when an external force is applied to cause distortion. [1-((G(min)) / (G(0)))]×100 Here, G(0) is the modulus value at 0 seconds after the start of application of external force, and G(min) is the minimum modulus value up to 3600 seconds after the start of application of external force. If the modulus value reaches the lower limit of the device's measurement within 3600 seconds, this value is used; otherwise, it is the modulus value at 3600 seconds.

[0140] [Stress relaxation mode separation analysis] The stress relaxation mode separation analysis is described in detail. The stress relaxation behavior of the cured epoxy resin material of the present disclosure when stress is applied is approximated by a generalized Maxwell model, which is a model in which multiple types of two-element models, so-called Maxwell elements, consisting of a dashpot and a spring arranged in series, are arranged in parallel. In this generalized Maxwell model, each constituent Maxwell element corresponds to one relaxation mode. In other words, the stress relaxation behavior when stress is applied to the generalized Maxwell model can be described as a linear sum of the behavior of each constituent Maxwell element (each relaxation mode). Then, by separating the stress relaxation measurement results according to the linear sum equation of the generalized Maxwell model and evaluating the relaxation time for each relaxation mode, it becomes possible to quantitatively evaluate the bond exchange performance within the cured epoxy resin material of the present disclosure.

[0141] The modulus value G(t) for 3600 seconds obtained in the stress relaxation measurement is calculated using the correlation square coefficient R according to the following formula 1. 2 It is approximated to >0.999.

[0142]

number

[0143] In Equation 1, t is time (unit: seconds), and G0 is the modulus value G(0) at t = 0 seconds. i is Σμ i = 1, and is a variable that indicates the contribution (unit: %) of each term (each relaxation mode) in this multi-dimensional linear equation. τ i is the relaxation time (seconds) in each relaxation mode that is arbitrarily set, and is set to include a relaxation mode having a relaxation time that is sufficiently large relative to the measurement time. Furthermore, each relaxation mode is set so that its relaxation time is uniform across the same logarithmic range and uniform within the same logarithmic range. The number and range of relaxation modes to be set are not particularly limited, but it is preferable to have as many as possible in terms of calculation load.

[0144] For example, when performing stress relaxation mode separation analysis for the cured epoxy resin material of the present disclosure in the range of 0 seconds to 3600 seconds, the following 25 types of relaxation modes (τ i It is preferable to analyze it using the value of τ i =1, 2.5, 4, 5.5, 7, 8.5, 10, 25, 40, 55, 70, 85, 100, 250, 400, 550, 700, 850, 10000, 2500, 4000, 5500, 7000, 8500, 10000 In this case, the measurement time of 3600 seconds is included. i The maximum value of τ i = 10000 (seconds). Furthermore, each relaxation mode is set so that its relaxation time is uniform across the same logarithmic range, such as 1, 2.5, 4, 5.5, 7, etc. The cured epoxy resin material of the present disclosure is then approximated by a linear sum of relaxation modes with 25 different relaxation times, and the stress relaxation behavior is measured.

[0145] Next, an approximate equation is obtained using Equation 1 from a graph plotting the logarithm of relaxation time on the horizontal axis and the logarithm of stress on the vertical axis for each measured relaxation mode. i and μ i On the horizontal axis, τ i The logarithm of μ on the vertical axis i τ, which has a peak when plotting the distribution i is the relaxation time of the separated relaxation modes. Furthermore, the τ that constitutes each peak i μ in the group i τ that constitutes any peak for the sum of i μ in i The value of is the contribution (%) of the separated relaxation mode.

[0146] Here, stress relaxation can be measured using conventionally known devices and methods. It is sufficient to be able to track and record the change in stress over time after instantaneous application of an external force under isothermal conditions using a thermostatic device. Specific examples include measurements in shear mode using a rheometer equipped with parallel plates, and measurements in bending or tension mode using dynamic mechanical analysis (DMA). Measurements are preferably carried out in an inert gas atmosphere, with controlled humidity. While not particularly limited, measurements are preferably carried out in a room with a low humidity atmosphere of 50% RH or less, with nitrogen flowing through the measuring device at a flow rate of 10 L / min or more.

[0147] The stress relaxation behavior of the cured epoxy resin obtained by curing the epoxy resin composition of the present disclosure was measured at [glass transition temperature of the cured product + 60] ° C for 1 hour. The results of the stress relaxation mode separation analysis are shown on the horizontal axis. i The logarithm of μ on the vertical axis i In the graph obtained by plotting the above, it is preferable that there are two or more relaxation mode groups with a distribution. More preferably, the graph has at least a relaxation mode group with a relaxation time of less than 200 seconds and a relaxation mode group with a relaxation time of 1000 seconds or more, each with a contribution of 10% or more. If there is only a single relaxation mode group, or if there are multiple relaxation mode groups, only one with a short relaxation time, the creep properties of the epoxy resin cured product or fiber-reinforced composite material under high temperature conditions are likely to be impaired.

[0148] <Prepreg> The prepreg of the present disclosure is obtained by impregnating a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition of the present disclosure, and the content of the reinforcing fibers relative to the total volume of the prepreg of the present disclosure is 25% by volume to 75% by volume.

[0149] The prepreg of the present disclosure is a prepreg in which a fiber-reinforced substrate is partially or entirely impregnated with the epoxy resin composition of the present disclosure. If the reinforcing fiber content in the prepreg of the present disclosure exceeds 75% by volume relative to the total volume of the prepreg, voids and the like may occur in the resulting fiber-reinforced composite material, which may result in reduced mechanical properties. If the reinforcing fiber content in the prepreg of the present disclosure is less than 25% by volume, the reinforcing effect of the reinforcing fibers may be insufficient, resulting in substantially reduced mechanical properties relative to mass. The reinforcing fiber content in the prepreg of the present disclosure is preferably 45% to 70% by volume, more preferably 50% to 65% by volume, relative to the total volume of the prepreg.

[0150] The prepreg of the present disclosure may be in the form of a tow prepreg in which the reinforcing fibers are formed into strands, an SMC (Sheet Molding Compound) in which the reinforcing fibers are formed of short fibers with a fiber length of 5 mm to 100 mm, or a woven fabric prepreg in which the reinforcing fibers are formed of a woven fabric or a multi-layered woven fabric. A tow prepreg is a narrow intermediate substrate obtained by impregnating a reinforcing fiber bundle, in which several thousand to several tens of thousands of reinforcing fiber filaments are aligned in one direction, with a matrix resin composition and then winding the impregnated fiber bundle around a bobbin such as a paper tube. In the present disclosure, the material wound around a bobbin in this manner or the material unwound after being wound is referred to as a "tow prepreg."

[0151] <Fiber reinforced substrate> The fiber-reinforced substrate used in the present disclosure is not particularly limited, and examples thereof include carbon fiber, glass fiber, aramid fiber, silicon carbide fiber, polyester fiber, ceramic fiber, alumina fiber, boron fiber, metal fiber, mineral fiber, rock fiber, and slag fiber.

[0152] Among these fiber-reinforced substrates, carbon fiber, glass fiber, or aramid fiber is preferred. Carbon fiber is more preferred because it has good specific strength and specific modulus, and can provide a lightweight, high-strength fiber-reinforced composite material. Polyacrylonitrile (PAN)-based carbon fiber is particularly preferred because it has excellent tensile strength.

[0153] When PAN-based carbon fibers are used for the fiber-reinforced substrate, the tensile modulus is preferably 100 GPa to 600 GPa, more preferably 200 GPa to 500 GPa, and particularly preferably 230 GPa to 450 GPa, and the tensile strength is preferably 2000 MPa to 10000 MPa, and more preferably 3000 MPa to 8000 MPa. The diameter of the carbon fibers is preferably 4 μm to 20 μm, more preferably 5 μm to 10 μm. By using such carbon fibers, the mechanical properties of the resulting fiber reinforced composite material can be improved.

[0154] The fiber-reinforced substrate is preferably formed into a sheet for use. Examples of reinforcing fiber sheets include sheets in which a large number of reinforcing fibers are aligned in one direction, bidirectional fabrics such as plain weave or twill weave, multiaxial fabrics, nonwoven fabrics, mats, knits, braids, and paper made from fiber-reinforced substrates. Among these, the use of a unidirectionally aligned sheet in which the fiber-reinforced substrate is formed into a sheet as continuous fibers, a bidirectional fabric, or a multiaxial fabric substrate is preferred because it allows for the production of a fiber-reinforced composite material with better mechanical properties. The thickness of the sheet-like fiber-reinforced substrate is preferably 0.01 mm to 3 mm, and more preferably 0.1 mm to 1.5 mm.

[0155] <Prepreg manufacturing method> The method for producing the prepreg of the present disclosure is not particularly limited, and any conventionally known method can be used. Specifically, a hot melt method or a solvent method can be preferably used.

[0156] The hot melt method is a method in which an epoxy resin composition is applied in the form of a thin film onto release paper to form a resin composition film, and the resin composition film is laminated onto a fiber-reinforced substrate and heated under pressure, thereby impregnating the fiber-reinforced substrate layer with the epoxy resin composition.

[0157] The method for forming the epoxy resin composition into a resin composition film is not particularly limited, and any conventionally known method can be used. Specifically, a resin composition film can be obtained by casting the resin composition onto a support such as release paper or film using die extrusion, an applicator, a reverse roll coater, a comma coater, or the like. The resin temperature during film production is determined appropriately depending on the composition or viscosity of the epoxy resin composition. Specifically, the same temperature conditions as the mixing temperature in the above-mentioned method for producing the epoxy resin composition are preferably used. The epoxy resin composition can be impregnated into the fiber-reinforced substrate layer in one step or in multiple steps.

[0158] The solvent method is a method in which an epoxy resin composition is made into a varnish using an appropriate solvent, and the varnish is impregnated into the fiber-reinforced substrate layer.

[0159] Of these conventional methods, the prepreg of the present disclosure can be suitably produced by a hot melt method that does not use a solvent.

[0160] When the epoxy resin composition film is impregnated into the fiber-reinforced substrate layer by the hot melt method, the impregnation temperature is preferably in the range of 50°C to 120°C. If the impregnation temperature is less than 50°C, the viscosity of the epoxy resin composition is high and the fiber-reinforced substrate layer may not be sufficiently impregnated. If the impregnation temperature exceeds 120°C, the curing reaction of the epoxy resin composition may proceed, resulting in a decrease in the storage stability or drapeability of the obtained prepreg. The impregnation temperature is more preferably 60°C to 110°C, and particularly preferably 70°C to 100°C.

[0161] The impregnation pressure when the epoxy resin composition film is impregnated into the fiber-reinforced substrate layer by the hot melt method is appropriately determined taking into consideration the viscosity or resin flow of the resin composition. A specific impregnation pressure is 0.01 N / cm to 250 N / cm, and preferably 0.1 N / cm to 200 N / cm.

[0162] <Fiber-reinforced composite materials> The fiber-reinforced composite material of the present disclosure comprises a cured epoxy resin obtained by curing the epoxy resin composition of the present disclosure and a fiber-reinforced substrate having reinforcing fibers, and the content of the reinforcing fibers relative to the total volume of the fiber-reinforced composite material of the present disclosure is 25% by volume to 75% by volume. The fiber-reinforced composite material of the present disclosure preferably comprises a cured epoxy resin obtained by curing the epoxy resin composition of the present disclosure and a fiber-reinforced substrate having reinforcing fibers. The content of the reinforcing fibers in the fiber-reinforced composite material of the present disclosure is preferably 45% by volume to 70% by volume, more preferably 50% by volume to 65% by volume, relative to the total volume of the fiber-reinforced composite material.

[0163] The epoxy resin composition of the present disclosure has high impregnation properties and handleability for fiber-reinforced substrates, and can be used to produce fiber-reinforced composite materials with excellent properties. Furthermore, even if the cured epoxy resin and fiber-reinforced composite materials of the present disclosure have internal cracks or delamination, the cracks and delamination can be repaired by subjecting them to a process that includes heating.

[0164] <Method of manufacturing fiber-reinforced composite materials> The fiber-reinforced composite material of the present disclosure can be obtained by combining a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition of the present disclosure and curing the resulting composite. Alternatively, the fiber-reinforced composite material can be obtained by curing the prepreg of the present disclosure under specific conditions under heating and pressure. Examples of methods for producing a fiber-reinforced composite material using the prepreg of the present disclosure include known molding methods such as autoclave molding and press molding.

[0165] [Autoclave molding method] An autoclave molding method is preferably used as a method for producing the fiber-reinforced composite material of the present disclosure. Autoclave molding involves sequentially placing a prepreg and a film bag in the lower mold of a metal mold, sealing the prepreg between the lower mold and the film bag, evacuating the space formed by the lower mold and the film bag, and applying heat and pressure in an autoclave molding machine. The molding conditions are preferably a temperature rise rate of 1°C / min to 50°C / min, and heating and pressurizing at 0.2 MPa to 0.7 MPa at 130°C to 180°C for 1 hour to 12 hours.

[0166] [Press molding method] A press molding method is preferably used as a method for producing the fiber-reinforced composite material of the present disclosure. The fiber-reinforced composite material is produced by the press molding method by heating and pressurizing the prepreg of the present disclosure or a preform formed by laminating the prepregs of the present disclosure using a mold. The mold is preferably preheated to a curing temperature.

[0167] The temperature of the mold during press molding is preferably 150°C to 210°C. If the molding temperature is 150°C or higher, the curing reaction can be sufficiently induced, and a fiber-reinforced composite material can be obtained with high productivity. If the molding temperature is 210°C or lower, the viscosity of the resin composition does not become too low, and excessive flow of the resin composition in the mold can be suppressed. As a result, outflow of the resin composition from the mold and meandering of the fibers can be suppressed, and a high-quality fiber-reinforced composite material can be obtained.

[0168] The molding pressure is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa. If the pressure is 0.05 MPa or higher, the epoxy resin can flow appropriately, preventing poor appearance and the occurrence of voids. Furthermore, the prepreg adheres sufficiently to the mold, making it possible to produce a fiber-reinforced composite material with good appearance. If the pressure is 2 MPa or lower, the resin is not forced to flow more than necessary, making it less likely that the resulting fiber-reinforced composite material will have poor appearance. Furthermore, since no more load than necessary is applied to the mold, deformation of the mold is less likely to occur. The molding time is preferably 1 hour to 8 hours.

[0169] <Examples of fiber-reinforced composite materials> Examples of fiber reinforced composite materials include fiber reinforced plastics (FRP) and carbon fiber composite materials (CFRP).

[0170] <Method for repairing cured epoxy resin> The method for repairing a cured epoxy resin material according to the present disclosure includes heating a cured epoxy resin material having cracks or delamination therein at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material, thereby repairing the cracks or delamination. The cured epoxy resin material of the present disclosure has excellent self-repairing properties, and therefore can repair cracks and peeling that occur inside the material during molding or in the environment in which it is used.

[0171] The heating temperature is not particularly limited as long as it is equal to or higher than the glass transition temperature of the cured epoxy resin material, and is preferably 160°C or higher, more preferably 180°C or higher, and particularly preferably 200°C or higher. Conventional heating methods can be used, and the heating time can be set as desired. Treatment at 200°C for 1 hour in an autoclave or a blower dryer is possible. Furthermore, the repair method for the cured epoxy resin material of the present disclosure may include applying pressure, and if necessary, pressure may be applied using a mold or a support frame. The pressure applied is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa, and these pressures may be applied in stages.

[0172] <Repair method for fiber-reinforced composite materials> The method for repairing a fiber-reinforced composite material disclosed herein includes heating a fiber-reinforced composite material having a crack or delamination therein at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, thereby repairing the crack or delamination. The fiber-reinforced composite material of the present disclosure has excellent self-repairing properties, and therefore can repair cracks and peeling that occur internally during molding or in the environment in which it is used.

[0173] The heating temperature is not particularly limited as long as it is equal to or higher than the glass transition temperature of the fiber-reinforced composite material, and is preferably 160°C or higher, more preferably 180°C or higher, and particularly preferably 200°C or higher. Conventional known methods can be used for the heating method, and the heating time can be set as desired. Treatment at 200°C for 1 hour in an autoclave or a blower dryer is possible. Furthermore, the repair method for fiber-reinforced composite materials of the present disclosure may also include applying pressure, and if necessary, pressure may be applied using a mold or a support frame. The pressure during pressing is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa, and these pressures may be applied stepwise.

[0174] <Method for remolding cured epoxy resin> The method for remolding a cured epoxy resin material according to the present disclosure includes heating and pressurizing the cured epoxy resin material according to the present disclosure at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material. The cured epoxy resin material of the present disclosure has excellent remoldability and can be molded again after molding.

[0175] The heating temperature is not particularly limited as long as it is equal to or higher than the glass transition temperature of the cured epoxy resin, and is preferably 160°C or higher, more preferably 180°C or higher, and particularly preferably 200°C or higher. Conventional known methods can be used for the heating method, and the heating time can be set as desired. Treatment at 200°C for 1 hour in an autoclave or a blower dryer is possible. Furthermore, a mold or a support frame may be used for pressurization, if necessary. The pressure during pressurization is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa, and these pressures may be applied stepwise.

[0176] <Method for remolding fiber-reinforced composite material> The method for reshaping a fiber-reinforced composite material of the present disclosure includes heating and pressurizing the fiber-reinforced composite material of the present disclosure at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material. The fiber-reinforced composite material of the present disclosure has excellent remoldability and can be molded again after molding.

[0177] The heating temperature is not particularly limited as long as it is equal to or higher than the glass transition temperature of the fiber-reinforced composite material, and is preferably 160°C or higher, more preferably 180°C or higher, and particularly preferably 200°C or higher. Conventional known methods can be used for the heating method, and the heating time can be set as desired. Treatment at 200°C for 1 hour in an autoclave or a blower dryer is possible. Furthermore, a mold or a support frame may be used for pressurization, if necessary. The pressure during pressurization is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa, and these pressures may be applied stepwise.

[0178] <Method for decomposing cured epoxy resin> The method for decomposing a cured epoxy resin material according to the present disclosure includes contacting the cured epoxy resin material according to the present disclosure with a decomposing agent containing a reducing agent. The cured epoxy resin material according to the present disclosure has excellent resin decomposition properties, and is capable of decomposing the cured epoxy resin material into low molecular weight compounds.

[0179] The type of the reducing agent is not particularly limited as long as it decomposes the cured epoxy resin material, and any reducing agent may be used depending on the type of bond contained in the cured epoxy resin material. Examples of the reducing agent include dithiothreitol, 2-mercaptoethanol, 1-thioglycerol, glutathione, 2-mercaptoethylamine hydrochloride, cystine hydrochloride, tris(2-carboxyethyl)phosphine hydrochloride, etc., and reducing agents that reduce disulfide bonds are preferred. These reducing agents may be used alone or in combination.

[0180] These reducing agents may be used without a solvent, or may be used as a decomposition agent solution by diluting with a solvent. That is, the cured epoxy resin material of the present disclosure may be decomposed by immersing it in a decomposition agent solution containing a reducing agent. The amount of the decomposer solution to be used is not particularly limited and is determined depending on the shape of the cured epoxy resin material, the decomposition conditions, etc. The amount of the decomposer solution to be used is preferably 1 to 1000 times, more preferably 1 to 100 times, and particularly preferably 1 to 10 times the amount of the cured epoxy resin material by mass.

[0181] There are no particular restrictions on the concentration of the reducing agent in the decomposing agent solution, and it is preferably 0.1% by mass to 99.9% by mass, more preferably 1% by mass to 80% by mass, and particularly preferably 5% by mass to 50% by mass. The solvent used for dilution is not particularly limited as long as it can dissolve the reducing agent and decompose the cured product, and may be water or an organic solvent. Examples of organic solvents used for dilution include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, 1,4-dioxane, tetrahydrofuran, methanol, ethanol, etc., and polar organic solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide are particularly preferred because they can completely dissolve the decomposed cured product.

[0182] In addition to the above, the decomposing agent may contain some additives that promote decomposition. In particular, nitrogen-containing bases such as triethylamine, DBU, or ammonia are preferred because their addition has a high decomposition promoting effect.

[0183] In order to accelerate the decomposition, the mixture may be heated at 25°C to 150°C (preferably 25°C to 60°C) for 1 hour to 48 hours (preferably 1 hour to 8 hours), stirred appropriately, or ultrasonically treated if necessary. For example, the decomposition reaction may be carried out at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In order to accelerate the decomposition, the mixture may be pressurized at 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa).

[0184] By the method for decomposing an epoxy resin cured material according to the present disclosure, preferably 50% by mass or more of the cured material is decomposed, more preferably 80% by mass or more, and even more preferably 95% by mass or more. Preferably, 100% by mass of the cured epoxy resin material according to the present disclosure is decomposed.

[0185] <Method for recovering reinforcing fibers from fiber-reinforced composite materials> The method for recovering reinforcing fibers from a fiber-reinforced composite material of the present disclosure includes contacting the fiber-reinforced composite material of the present disclosure with a decomposition agent containing a reducing agent. When the fiber-reinforced composite material of the present disclosure is contacted with the decomposition agent containing a reducing agent, the resin is decomposed and / or dissolved, and the reinforcing fibers can be recovered.

[0186] The description of the decomposition agent containing a reducing agent in the method for recovering reinforcing fibers from a fiber-reinforced composite material, including definitions, examples, preferred embodiments, etc., is the same as the description of the decomposition agent containing a reducing agent in the <<Method for Decomposing an Epoxy Resin Cured Material>>. In order to promote the decomposition and / or dissolution, the decomposition and / or dissolution may be carried out by heating at 25°C to 150°C (preferably 25°C to 60°C) for 1 hour to 48 hours (preferably 1 hour to 8 hours), by stirring appropriately, or by ultrasonic treatment if necessary. For example, the decomposition and / or dissolution reaction may be carried out at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In order to promote the decomposition and / or dissolution, the decomposition and / or dissolution may be carried out by applying pressure of 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa).

[0187] The method for recovering reinforcing fibers from a fiber-reinforced composite material of the present disclosure may include recovering the reinforcing fibers after decomposing and / or dissolving the resin. The method for recovering the reinforcing fibers is not particularly limited, and the long fibers can be recovered by winding or filtration. The recovered reinforcing fibers can be recycled into prepregs or fiber-reinforced composite materials, and in this process, oil such as a sizing agent may be applied again if necessary. [Example]

[0188] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited thereto. The components and evaluation methods used in the examples and comparative examples are as follows.

[0189] <Production of Epoxy Resin Composition> [Synthesis of multifunctional epoxy resin [A]] [Synthesis Example 1] Synthesis of poly(t-amylphenol (glycidyl ether) disulfide) (hereinafter abbreviated as "G-APDS")

[0190] [ka]

[0191] A four-neck flask equipped with a thermometer, dropping funnel, condenser, and stirrer was charged with 600 mL of N,N-dimethylformamide (DMF) and 247 g (1.5 mol) of 4-(1,1-dimethylpropyl)phenol and cooled to 0°C, after which 189 g (1.4 mol) of disulfur dichloride was added dropwise over 2 hours. The temperature was then raised to 25°C over a further 10 hours, and the mixture was stirred at 45-50°C for 2 hours. The resulting tan solution was slowly heated to 170° C. under reduced pressure to distill off the DMF. The recovered material was then dissolved in 700 mL of diethyl ether, and 3500 mL of hexane was added dropwise to separate the gel-like product, which was then dried at 150°C to obtain a brown solid product.

[0192] The resulting brown solid (118 g, 0.5 mol OH) was placed in a four-neck flask equipped with a thermometer, dropping funnel, condenser, and stirrer along with 400 mL of epichlorohydrin. After dissolving at 60°C, 75 mL of 50% aqueous sodium hydroxide was added dropwise over 4 hours while stirring the flask. After stirring for another hour, a Dean-Stark tube was attached and the pressure was reduced to remove water. The flask was then cooled to 45°C, and 1.3 g of powdered sodium hydroxide was added in 10 increments at 5-minute intervals. After the final sodium hydroxide addition, the mixture was stirred for 30 minutes. After the reaction was complete, the flask was stopped from stirring and allowed to stand for 1 hour. The upper layer was separated by decantation, and the lower layer was extracted with dichloromethane. The two layers were combined, washed with water and saturated brine, dried over magnesium sulfate, filtered through Celite, and finally concentrated to yield 160 g of a brown, viscous liquid product. The main product was poly(t-amylphenol (glycidyl ether) disulfide), and the epoxy equivalent weight determined according to JIS K7236 was 294 g / Eq. The weight average molecular weight determined by gel permeation chromatography (GPC) measurement was 3,500.

[0193] [Synthesis Example 2] Synthesis of poly(t-butylphenol (glycidyl ether) disulfide) (hereinafter abbreviated as "G-BPDS")

[0194] [ka]

[0195] The same procedure as in Synthesis Example 1 was carried out, except that 4-(1,1-dimethylpropyl)phenol was replaced with 4-(t-butyl)phenol. The main product of the brown viscous fluid obtained was poly(t-butylphenol (glycidyl ether) disulfide), and the epoxy equivalent determined according to JIS K7236 was 280 g / Eq. The weight-average molecular weight determined by gel permeation chromatography (GPC) measurement was 1,800.

[0196] [Synthesis Example 3] Synthesis of poly(methoxyphenol (glycidyl ether) disulfide) (hereinafter abbreviated as "G-MOPDS")

[0197] [ka]

[0198] The same procedure as in Synthesis Example 1 was carried out, except that 4-(1,1-dimethylpropyl)phenol was replaced with 4-(methoxy)phenol, to obtain a brown viscous liquid. The main product of the resulting brown viscous fluid (85 g) was poly(methoxyphenol (glycidyl ether) disulfide), and the epoxy equivalent determined according to JIS K7236 was 248 g / Eq. The weight-average molecular weight determined by gel permeation chromatography (GPC) was 1700.

[0199] The following polyfunctional epoxy resin was prepared as the polyfunctional epoxy resin used in the comparative example. Phenol novolac epoxy resin (hereafter abbreviated as "PN") (Nippon Kayaku RE-306, epoxy equivalent = 175g / Eq)

[0200] [Hardening agent [B]] As the curing agent [B], the following curing agent was prepared. 4,4'-Diamino-3,3'-diethyl-5,5'-dimethyldiphenylmethane (hereinafter abbreviated as "MED-J", active hydrogen equivalent = 71g / Eq) 4,4'-Diaminodiphenyl sulfone (hereinafter abbreviated as "4,4-DDS", active hydrogen equivalent = 62g / Eq) Diethyltoluenediamine (hereinafter abbreviated as "DETDA", active hydrogen equivalent = 45g / Eq)

[0201] <Examples 1 to 5 and Comparative Example 1> [1] Preparation of epoxy resin composition The polyfunctional epoxy resin and curing agent were weighed in the parts by mass shown in Table 1 below, and mixed at 80°C using a roll mill until homogeneous, to prepare an epoxy resin composition.

[0202] [2] Preparation of cured epoxy resin and measurement of its physical properties (2-1) Preparation of cured epoxy resin The epoxy resin composition prepared in [1] was degassed in a vacuum and then poured into a silicone resin mold set to a thickness of 4 mm using a 4 mm silicone resin spacer. It was cured at 120°C for 2 hours, 150°C for 2 hours, and 180°C for 2 hours to obtain a 4 mm thick cured epoxy resin product.

[0203] (2-2) Measurement of glass transition temperature (Tg) The glass transition temperature was measured in a nitrogen gas flow at a flow rate of 40 ml / min using a TA Instruments DSC Q2000 differential scanning calorimeter. 5 mg±1 mg of the cured epoxy resin prepared in (2-1) was sampled and heated from 30°C to 350°C at a rate of 10°C / min. The inflection point in the transition process on the obtained curve was recorded as the glass transition temperature.

[0204] (2-3) Measurement of stress relaxation rate The epoxy resin composition prepared in [1] was set on 8 mm diameter aluminum parallel plates with a gap distance of 2 mm, and cured at 120°C for 2 hours, 150°C for 2 hours, and 180°C for 2 hours. Next, after holding at [glass transition temperature of the cured product + 60] °C for 2 minutes, -3 The modulus value G generated when a displacement of 10 rad was applied was measured for 3600 seconds, and the resulting plot was recorded. Stress relaxation was measured using a TA Instruments Discovery DHR-2 rheometer in a nitrogen gas flow of 10 L / min. In the obtained time plot, the stress relaxation rate was calculated by the following formula. [1-((G(min)) / (G(0)))]×100 Here, G(0) is the modulus value at 0 seconds after the start of application of external force. G(min) is the minimum modulus value up to 3600 seconds after the start of application of external force. If the modulus value reaches the lower limit of the measurement limit of the device within 3600 seconds, that value is used; otherwise, it is the modulus value at 3600 seconds. In the present disclosure, the modulus value G(0) at 0 seconds after the start of application of external force was considered to be the same as the modulus value G(0.1) at 0.1 seconds after the start of application of external force, and the stress relaxation rate was calculated.

[0205] The results of the stress relaxation measurement are shown in Table 1 and Figure 1. In Figure 1, the dotted line shows the results of Comparative Example 1, and the solid line shows the measurement results of Example 2. In Example 2, stress was relaxed by the application of an external force, and the stress relaxation rate 3600 seconds after the start of external force application was 99.9%. On the other hand, in Comparative Example 1, stress was hardly relaxed even when an external force was applied, and the stress relaxation rate 3600 seconds after the start of external force application was 0.1%.

[0206] (2-4) Stress relaxation mode separation analysis The modulus value G(t) at 200°C recorded in (2-3) was analyzed according to the above-mentioned [Stress relaxation mode separation analysis]. The stress relaxation mode separation analysis was carried out in the range of 0 seconds to 3600 seconds, and the analysis was carried out in the following 25 modes. τ i =1, 2.5, 4, 5.5, 7, 8.5, 10, 25, 40, 55, 70, 85, 100, 250, 400, 550, 700, 850, 10000, 2500, 4000, 5500, 7000, 8500, 10000

[0207] The results of the stress relaxation mode separation analysis of Example 2 are shown in Figure 2. In Figure 2, black circles indicate the contribution of each regressed relaxation mode, and solid lines indicate smooth lines connecting them.

[0208] (2-5) Confirmation of remolding ability The cured epoxy resin prepared in (2-1) was crushed into chips with three sides of 1 mm or less, and 0.7 g of the sample was placed in a mold equipped with an 8 mm diameter cylindrical mold frame and preheated at 200°C for 5 minutes. A pressure of 25 MPa was then applied in a press at 200°C for 30 minutes to prepare a remolded sample. The remolded samples were visually inspected and the remolding properties were evaluated according to the following criteria, with A and B being considered to be within the range acceptable for practical use. A: The surface of the remolded sample was smooth, and no grain boundaries of the chips were visible even when light was transmitted through it. B: The surface of the remolded sample was smooth, but turbidity due to the grain boundaries of the chip was observed when light was transmitted through it. C: The grain boundaries of the chip before molding were observed on the surface of the remolded sample.

[0209] (2-6) Confirmation of self-repairing ability The cured epoxy resin prepared in (2-1) was polished to a thickness of 2 mm, and a crack of approximately 2 mm was inserted using a razor. The sample was then preheated to 120°C and placed on the sample stage of a transmission electron microscope equipped with a heat stage. The sample was then heated to 200°C at a rate of 20°C / min, and finally held at 200°C for 15 minutes to prepare a self-repairing sample. The obtained self-repairing samples were observed using a transmission electron microscope, n = 5. The self-repairing property was judged according to the following criteria, with A and B being in the range acceptable for practical use. A: The cracks in the self-repaired sample have almost completely disappeared. B: The crack in the self-repaired sample was partially reduced. C: No disappearance or shrinkage of cracks was observed in the self-repaired sample.

[0210] (2-7) Confirmation of resin decomposition The cured epoxy resin prepared in (2-1) was cut into a piece of 1 cm x 1 cm x 0.4 mm, and each piece was immersed in 40 mL of N,N-dimethylformamide solution containing 4 mL of 2-mercaptoethanol and stirred at room temperature (25°C) and 80°C. The state of the solution after 8 hours was visually observed. Resin decomposition was judged according to the following criteria, with A and B being considered to be within the range acceptable for practical use. A: The cured epoxy resin completely decomposed at room temperature and became a homogeneous solution. B: The cured epoxy resin did not completely decompose at room temperature, but 80% by mass or more decomposed at 80°C. C: The cured epoxy resin was decomposed by less than 80% by mass at both room temperature and 80°C.

[0211] (2-8) Flexural modulus (FM) and flexural strength (FS) Tests were conducted on the cured epoxy resin prepared in (2-1) according to JIS K7171. Resin test pieces were prepared with dimensions of 80 mm x 10 mm x 4 mm (thickness h). The bending test was conducted with a support distance L of 16 x 4 (thickness) and a test speed of 2 mm / min, and the flexural modulus and flexural strength were measured. When the flexural modulus is 2.70 GPa or more, the cured product can be said to have a high modulus of elasticity.

[0212] The various physical properties of the cured resin products obtained by curing the obtained epoxy resin compositions are shown in Table 1. The epoxy resin compositions obtained in Examples 1 to 5 and Comparative Example 1 had a degree of cure of 99.9% or more as evaluated by DSC.

[0213] [Table 1]

[0214] <Comparative Examples 2 and 3> Various physical properties of the cured resin products obtained by curing the epoxy resin compositions used in Patent Document 1 are shown in Table 2. The epoxy resin compositions obtained in Comparative Examples 2 and 3 had a degree of cure of 99.9% or more as evaluated by DSC. In Comparative Examples 2 and 3, the following epoxy resins and curing agents were used. Epoxy resin: Mitsubishi Chemical BisA type epoxy resin "jER828" (epoxy equivalent weight = 189g / Eq) Hardener: 4,4-dithiodianiline (active hydrogen equivalent = 62g / Eq)

[0215] [Table 2]

[0216] (2-9) Confirmation of decomposition products (2-7) When the decomposition product of the cured epoxy resin of Example 1 obtained in the confirmation of resin decomposition property was analyzed by LC-MS, it was found that the decomposition product was a compound having the following structure and a molecular weight of 2160.

[0217] [ka]

[0218] From the above, Examples 1 to 5 provided epoxy resin compositions that have a high elastic modulus and give cured products with excellent remoldability or self-repairability. Furthermore, a prepreg obtained by impregnating a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition of the present disclosure, and a fiber-reinforced composite material containing a fiber-reinforced substrate having reinforcing fibers, also exhibit excellent remoldability or self-repairability in the cured product. Furthermore, the cured epoxy resin product of the present disclosure was decomposed and dissolved by contact with a decomposing agent containing a reducing agent. The reinforcing fibers of the fiber-reinforced composite material of the present disclosure can be recovered by contacting the fiber-reinforced composite material with a decomposing agent containing a reducing agent.

Claims

1. An epoxy resin composition comprising a multifunctional epoxy resin [A] containing at least a structural unit represented by the following general formula (1): 【Chemical 1】 (In general formula (1), each Y is independently a dynamic covalent bond moiety, and R １ａ , R １ｂ , R １ｃ , and R １ｄ at least one of these is a bond to Y in another adjacent structural unit, and the structural units other than said at least one each independently represent one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.

2. 2. The epoxy resin composition according to claim 1, wherein the dynamic covalent bonding site is a group containing a disulfide bond.

3. 3. The epoxy resin composition according to claim 1, wherein the polyfunctional epoxy resin [A] comprises a polyfunctional epoxy resin containing a structural unit represented by the following general formula (2): 【Chemistry 2】 (In general formula (2), R ２ａ , R ２ｂ , and R ２ｃ each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.

4. 3. The epoxy resin composition according to claim 1, wherein the polyfunctional epoxy resin [A] comprises a polyfunctional epoxy resin containing a structural unit represented by the following general formula (3): 【Chemistry 3】 (In general formula (3), R ３ each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n represents an integer of 2 to 500.

5. A cured epoxy resin product obtained by curing the epoxy resin composition according to claim 1.

6. A cured epoxy resin product in which 80 mass % or more of the cured product dissolves when brought into contact with a decomposing agent containing a reducing agent and heated at 100°C for 48 hours.

7. A cured epoxy resin material that, when brought into contact with a decomposing agent containing a reducing agent and heated, produces a compound having at least a structure represented by the following general formula (4): 【Chemistry 4】 (In general formula (4), * indicates a bond. R ４ａ , R ４ｂ , and R ４ｃ each independently represents one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom.

8. A prepreg obtained by impregnating a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition according to claim 1 or 2, wherein the content of the reinforcing fibers is 25% by volume to 75% by volume relative to the total volume of the prepreg.

9. A fiber-reinforced composite material comprising the cured epoxy resin according to any one of claims 5 to 7 and a fiber-reinforced substrate having reinforcing fibers, wherein the content of the reinforcing fibers is 25% by volume to 75% by volume relative to the total volume of the fiber-reinforced composite material.

10. A method for repairing a cured epoxy resin material, comprising heating the cured epoxy resin material according to any one of claims 5 to 7, which has cracks or peeling therein, at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material, thereby repairing the cracks or peeling.

11. A method for repairing a fiber-reinforced composite material, comprising heating the fiber-reinforced composite material according to claim 9, which has cracks or delaminations therein, at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, thereby repairing the cracks or delaminations.

12. A method for remolding a cured epoxy resin material, comprising heating and pressurizing the cured epoxy resin material according to any one of claims 5 to 7 at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material.

13. A method for remolding a fiber-reinforced composite material, comprising heating and pressurizing the fiber-reinforced composite material according to claim 9 at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material.

14. A method for decomposing a cured epoxy resin material, comprising contacting the cured epoxy resin material according to any one of claims 5 to 7 with a decomposition agent containing a reducing agent.

15. A method for recovering reinforcing fibers from a fiber-reinforced composite material, comprising contacting the fiber-reinforced composite material according to claim 9 with a decomposition agent containing a reducing agent.

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