Epoxy resin composition, epoxy resin cured product, prepreg, fiber-reinforced composite material, method for repairing epoxy resin cured product, method for repairing fiber-reinforced composite material, method for reshaping epoxy resin cured product, method for reshaping fiber-reinforced composite material, method for decomposing epoxy resin cured product, and method for recovering reinforced fiber of fiber-reinforced composite material

Abstract

This epoxy resin composition contains a polyhydric phenol compound [A] containing at least a constituent unit represented by general formula (1) and an epoxy resin [B]. A ratio (a) / (b) of a molar amount (a) of a phenolic hydroxyl group in the polyhydric phenol compound [A] to a molar amount (b) of an epoxy group in the epoxy resin [B] is 0.25-1.5. If stress generated upon application, to a cured product obtained by curing at a curing degree of 90% or more, of an external force at a temperature 60°C higher than the glass transition temperature of the cured product is measured, the stress relaxation rate at a time point of 1 hour from the start of the application of the external force is 90% or more.

Description

Epoxy resin composition, cured epoxy resin product, prepreg, fiber-reinforced composite material, method for repairing cured epoxy resin product, method for repairing fiber-reinforced composite material, method for remolding cured epoxy resin product, method for remolding fiber-reinforced composite material, method for decomposing cured epoxy resin product, and method for recovering reinforcing fibers from fiber-reinforced composite material

[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.

[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 various fields and applications. Conventionally, in the aerospace and industrial fields, which require high mechanical properties and heat resistance, thermosetting resins such as unsaturated polyester resin, epoxy resin, or polyimide resin have mainly been 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 reuse cured offcuts and defective molded products by introducing exchangeable chemical bond sites (also called dynamic covalent bond sites) into resin compositions through cleavage followed by recombination or direct recombination of bond components. In this method, the exchangeable chemical bond sites are cleaved or continuously exchanged by heating, thereby eliminating the constraints imposed by the network structure within the cured product, and the resin becomes fluid and can be remolded. For example, Patent Document 1 discloses an epoxy resin composition that uses a compound containing a disulfide bond as a curing agent, as a chemical bond site that is exchangeable through direct recombination of bond components.

[0005] Patent Document 1: International Publication No. 2015 / 181054

[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 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 which uses a compound having dynamic covalent bonding sites in an amount close to the theoretical equivalent relative to the epoxy resin, and which has excellent remoldability or self-repairing properties in the cured product; 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.

[0008] Specific means for solving the above problems include the following aspects: <1> An epoxy resin composition comprising a polyhydric phenol compound [A] and an epoxy resin [B], each containing at least a structural unit represented by the following general formula (1), wherein the ratio (a) / (b) of the molar amount (a) of phenolic hydroxyl groups in the polyhydric phenol compound [A] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less, and wherein, when a cured product obtained by curing to a degree of cure of 90% or more is subjected to an external force 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 is 90% or more one hour after the start of application of the external force.

[0009]

[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 which is a bond to Y in another adjacent structural unit, and the structural units other than the at least one are each independently any one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom, and n is an integer of 2 to 500.) <2> The epoxy resin composition according to <1>, wherein the dynamic covalent bond moiety is a group containing a disulfide bond. <3> The epoxy resin composition according to <1> or <2>, comprising, as the polyhydric phenol compound [A], a polyhydric phenol compound containing a structural unit represented by the following general formula (2):

[0011]

[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 epoxy resin composition according to any one of <1> to <3>, wherein the polyhydric phenol compound [A] is a polyhydric phenol compound containing a structural unit represented by the following general formula (3):

[0013]

[0014] (In general formula (3), R ３ 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, and n represents an integer of 2 to 500.) <5> The epoxy resin composition according to any one of <1> to <4>, which is a curing agent. <6> A cured epoxy resin product obtained by curing the epoxy resin composition according to any one of <1> to <5>. <7> A cured epoxy resin product in which 80 mass% or more of the cured product dissolves when contacted with a decomposing agent containing a reducing agent and heated at 100°C for 48 hours. <8> A cured epoxy resin product in which, when contacted with a decomposing agent containing a reducing agent and heated, a compound having at least the structure represented by the following general formula (4) is produced:

[0015]

[0016] (In general formula (4), * indicates a bond. R ４ａ , R ４ｂ , and R ４ｃeach independently represents any one of a hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, an alkoxy group, an amino group, or a halogen atom.) <9> A prepreg, obtained by impregnating a fiber-reinforced substrate having reinforcing fibers with the epoxy resin composition according to any one of <1> to <5>, wherein the content of the reinforcing fibers is 25 to 75 volume % relative to the total volume of the prepreg. <10> A fiber-reinforced composite material, comprising the cured epoxy resin according to any one of <6> to <8> and a fiber-reinforced substrate having reinforcing fibers, wherein the content of the reinforcing fibers is 25 to 75 volume % relative to the total volume of the fiber-reinforced composite material. <11> A method for repairing a cured epoxy resin product, comprising heating the cured epoxy resin product according to any one of <6> to <8>, having internal cracks or delaminations, at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin product, thereby repairing the cracks or delaminations. <12> A method for repairing a fiber-reinforced composite material, comprising heating the fiber-reinforced composite material according to <10>, which has internal cracks or delaminations, at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, thereby repairing the cracks or delaminations. <13> A method for remolding a cured epoxy resin material according to any one of <6> to <8>, at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin material, under pressure. <14> A method for remolding a fiber-reinforced composite material according to <10>, at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, under pressure. <15> A method for decomposing a cured epoxy resin material, comprising contacting the cured epoxy resin material according to any one of <6> to <8> with a decomposition agent containing a reducing agent. <16> A method for recovering reinforcing fibers from a fiber-reinforced composite material, comprising contacting the fiber-reinforced composite material according to <10> with a decomposition agent containing a reducing agent.

[0017] According to the present disclosure, there are provided an epoxy resin composition which uses a compound having dynamic covalent bonding sites close to the theoretical equivalent relative to the epoxy resin, yet which has excellent remoldability or self-repairability in the cured product; 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.

[0018] Figure 1 is a graph showing the results of stress relaxation measurement, and Figure 2 is a graph showing the results of stress relaxation mode separation analysis.

[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, when a numerical range is indicated using "to", the numerical values ​​before and after "to" are included as the lower and upper limits, respectively. In the present disclosure, when a numerical range is described in stages, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another staged numerical range. Furthermore, in the present disclosure, the upper or lower limit of that numerical range may be replaced with a value shown in the examples. In the present disclosure, when multiple substances corresponding to each component are present in the composition, the content of each component refers to 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", this does not exclude the combination of multiple elements unless a technical contradiction occurs, unless otherwise specified. In the present disclosure, when an element is described in the singular, this does not exclude the presence of multiple elements unless a technical contradiction occurs, unless otherwise specified. In the present disclosure, multiple exemplary embodiments described separately may be combined with each other to form a new embodiment, unless mutually contradictory.

[0021] <Epoxy Resin Composition> The epoxy resin composition of the present disclosure comprises a polyhydric phenol compound [A] and an epoxy resin [B], each containing at least a structural unit represented by the following general formula (1), wherein the ratio (a) / (b) of the molar amount (a) of phenolic hydroxyl groups in the polyhydric phenol compound [A] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less, and when an external force is applied to a cured product obtained by curing 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 stress generated is measured, the stress relaxation rate is 90% or more one hour after the start of application of the external force.

[0022]

[0023] In general formula (1), each Y is independently a dynamic covalent bond moiety; １ａ , R １ｂ , R １ｃ , and R １ｄat 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 exhibits excellent remoldability or self-repairability in the cured product, despite the use of a compound having dynamic covalent bonding moieties in an amount close to the theoretical equivalent of the epoxy resin. 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. The function of the epoxy resin composition of the present disclosure is unclear, but is presumed to be as follows.

[0025] The epoxy resin composition of the present disclosure contains a polyhydric phenol compound [A] containing at least a structural unit represented by general formula (1) and an epoxy resin [B]. 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 cleavage followed by recombination or by direct recombination of bond components. More specifically, as shown in the reaction formula below, dynamic covalent bond moieties are classified into dissociation-recombination type dynamic covalent bond moieties that undergo complete cleavage under specific conditions and then recombination under different conditions, cooperative type dynamic covalent bond moieties in which bond dissociation and recombination proceed in a concerted manner, and associative type dynamic covalent bond moieties in which bond formation is followed by a dissociation reaction. 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. That is, it is a bond site in which two pairs of covalent bonds are completely dissociated once at a high temperature range, and then the binding partners are exchanged and recombined at a low temperature range, thereby reversibly forming two pairs of covalent bonds in a new combination. A cooperative dynamic covalent bond site is a bond site in which the equilibrium constants for the dissociation and recombination reactions are low in temperature, and the cleavage and recombination of two pairs of covalent bonds occur in a concerted manner in a specific temperature range. That is, it is a bond site in which the dissociation of two pairs of covalent bonds, the exchange of binding partners, and the recombination occur simultaneously in a specific temperature range, thereby reversibly forming two pairs of covalent bonds in a new combination. Associative dynamic covalent bond sites are binding sites that undergo a bond formation reaction followed by a dissociation reaction. That is, when a cleaved and free covalent bond site is heated, it 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]

[0027] Furthermore, in the epoxy resin composition of the present disclosure, the ratio (a) / (b) of the molar amount (a) of phenolic hydroxyl groups in the polyphenol compound [A] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less. If the ratio (a) / (b) is less than 0.25, the performance of the dynamic covalent bonding moieties in the polyphenol compound [A] cannot be fully exhibited, and remoldability or self-repairability is impaired. If the ratio (a) / (b) exceeds 1.5, the molar balance between the epoxy groups in the epoxy resin [B] and the phenolic hydroxyl groups in the polyphenol compound [A] is disrupted, resulting in insufficient crosslink density of the resulting cured epoxy resin product and reduced mechanical properties such as heat resistance, elastic modulus, and fracture toughness.

[0028] Furthermore, when the epoxy resin composition of the present disclosure is cured to a degree of cure of 90% or more and an external force is applied to the cured product at a temperature 60°C higher than the glass transition temperature of the cured product, the resulting stress is measured. The stress relaxation rate one hour after the start of application of the external force is 90% or greater. In the present disclosure, the stress relaxation rate one hour after the start of application of the external force is calculated using the following formula: [1 - (stress one hour after the start of application of the external force) / (stress at the start of application of the external force)] x 100. In other words, a stress relaxation rate of 90% or greater one hour after the start of application of the external force means that the stress, which was 100% at the start of application of the external force, relaxes with the application of the external force and over time, decreasing to less than 10% one hour after the start of application of the external force, and the cured product has become fluid. The fluidity of the cured product allows the cured product to have remoldability or self-repairing properties.

[0029] As explained above, the epoxy resin composition of the present disclosure uses a compound having dynamic covalent bonding moieties in an amount close to the theoretical equivalent of the epoxy resin, and yet provides an epoxy resin composition that exhibits excellent remoldability or self-repairing properties in the cured product. However, the present disclosure is not limited to the above-mentioned assumed mechanism.

[0030] <Polyhydric phenol compound [A]> [General formula (1)] The epoxy resin composition of the present disclosure contains a polyhydric phenol compound [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 polyhydric phenol compound [A] consisting of a structural unit represented by the following general formula (1):

[0031]

[0032] In general formula (1), each Y is independently a dynamic covalent bond moiety; 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.

[0033] (Y) In general formula (1), each Y is independently a dynamic covalent bonding moiety. The dynamic covalent bonding moiety 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.

[0034] Diels-Alder bonds are dissociative dynamic covalent bond sites in which the equilibrium constants for the dissociation and recombination reactions are highly temperature-dependent, and two pairs of covalent bonds are completely dissociated at a high temperature and then recombined at a low temperature. That is, two pairs of covalent bonds are cleaved by heating, and the binding partners are exchanged with each other to form two new pairs of covalent bonds, which is reversible by a two-step heating operation. Imine bonds and disulfide bonds are cooperative dynamic covalent bond sites in which the equilibrium constants for the dissociation and recombination reactions are less temperature-dependent, and are binding sites in which the cleavage and recombination of two pairs of covalent bonds proceeds in a concerted manner in a specific temperature range. That is, they are binding sites in which the dissociation of two pairs of covalent bonds, the exchange of binding partners, and the recombination proceed simultaneously in a specific temperature range, thereby reversibly forming two new pairs of covalent bonds. Ester bonds, urethane bonds, carbonate bonds, vinylogous urethane bonds, boronate ester bonds, and silyl ether bonds are bonding sites where a dissociation reaction occurs after a bond formation reaction has progressed. That is, these are bonding sites where a cleaved and free covalent bond site can reversibly form a new set of covalent bonds by heating and recombining. From the viewpoints of being able to reshape by a single heating step and not requiring strict control of temperature conditions such as cooling, cooperative dynamic covalent bonding sites or associative dynamic covalent bonding sites are preferred as the dynamic covalent bonding site. Among these, from the viewpoint of not requiring a catalyst to promote bond exchange, a group containing at least one selected from the group consisting of an imine bond, a vinylogous urethane bond, a silyl ether bond, and a disulfide bond is more preferred as the dynamic covalent bonding site. From the viewpoint of extremely fast bond exchange, the dynamic covalent bond moiety is preferably a group containing a disulfide bond, more preferably a disulfide bond.

[0035] 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).

[0036]

[0037] In the above formulas (Y-1-a) 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.

[0038] (R １ａ , R １ｂ , R １ｃ , and R １ｄ ) In general formula (1), 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 other bonds 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 bonds than the at least one are hydrogen atoms. From the viewpoint of the toughness of the cured product, it is preferable that the other bonds than the at least one are aliphatic hydrocarbon groups. In general formula (1), R１ａ , R １ｂ , R １ｃ , and R １ｄ It is more preferable that any one of R represents 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. 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. 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.

[0039] In addition, 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.

[0040] R in general formula (1) １ａ , R １ｂ , R １ｃ , or R １ｄIn 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.

[0041] (n) In general formula (1), n ​​represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyhydric phenol compound [A] increases, making it difficult to mix with or handle the epoxy resin, and impregnation into the fiber-reinforced substrate decreases. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairability of the cured product decrease significantly. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0042] [General Formulas (1-a) to (1-d)] The polyhydric phenol compound [A] preferably contains a polyhydric phenol compound containing a structural unit represented by any one of the following general formulas (1-a) to (1-d). The polyhydric phenol compound [A] more preferably contains a polyhydric phenol compound consisting of a structural unit represented by any one of the following general formulas (1-a) to (1-d). The polyhydric phenol compound [A] is even more preferably a polyhydric phenol compound consisting of a structural unit represented by any one of the following general formulas (1-a) to (1-d). From the viewpoint of the solubility and thermal melting properties of the polyhydric phenol compound in an epoxy resin, it is extremely preferable that the polyhydric phenol compound [A] is a polyhydric phenol compound consisting of a structural unit represented by the following general formula (1-a).

[0043]

[0044] In the general formulas (1-a) to (1-d), each Y is independently a dynamic covalent bond moiety; １ｅ , R １ｆ , and R １ｇeach 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.

[0045] (Y) In the general formulae (1-a) to (1-d), Y each independently represents 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, etc.

[0046] (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.

[0047] R in general formulas (1-a) to (1-d) １ｅ , R １ｆ , and R １ｇ 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.

[0048] (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, etc.

[0049] [General Formula (2)] The epoxy resin composition of the present disclosure preferably contains, as the polyhydric phenol compound [A], a polyhydric phenol compound containing a structural unit represented by the following general formula (2). It is more preferable that the polyhydric phenol compound [A] contains a polyhydric phenol compound consisting of a structural unit represented by the following general formula (2). It is even more preferable that the polyhydric phenol compound [A] is a polyhydric phenol compound consisting of a structural unit represented by the following general formula (2).

[0050]

[0051] 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.

[0052] That is, the polyhydric phenol compound [A] of the present disclosure preferably has a phenol structure and a disulfide bond as a constituent unit, and the phenolic hydroxyl group (OH group) of each phenol structure and the disulfide bond are preferably at the ortho position.

[0053] (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.

[0054] R in general formula (2) ２ａ , R ２ｂ , and R ２ｃIn 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.

[0055] (n) In general formula (2), n represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyhydric phenol compound [A] increases, making it difficult to mix with or handle the epoxy resin, and impregnation into the fiber-reinforced substrate decreases. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairability of the cured product decrease significantly. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0056] [General Formula (3)] The epoxy resin composition of the present disclosure preferably contains, as the polyhydric phenol compound [A], a polyhydric phenol compound containing a structural unit represented by the following general formula (3). It is more preferable that the polyhydric phenol compound [A] contains a polyhydric phenol compound consisting of a structural unit represented by the following general formula (3). It is even more preferable that the polyhydric phenol compound [A] is a polyhydric phenol compound consisting of a structural unit represented by the following general formula (3).

[0057]

[0058] 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.

[0059] (R 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. From the viewpoint of the heat resistance and mechanical properties of the cured product, R ３is preferably a hydrogen atom. From the viewpoint of toughness of the cured product, R ３ is preferably an aliphatic hydrocarbon group. From the viewpoint of handling of the epoxy resin composition (uncured product), R ３ is preferably a quaternary carbon.

[0060] R in general formula (3) ３ 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.

[0061] (n) In general formula (3), n represents an integer of 2 to 500. If n exceeds 500, the viscosity of the polyhydric phenol compound [A] increases, making it difficult to mix with or handle the epoxy resin, and impregnation into the fiber-reinforced substrate decreases. If n is less than 2, the heat resistance, fluidity, remoldability, and self-repairability of the cured product decrease significantly. n preferably represents an integer of 3 to 100, and more preferably an integer of 5 to 50.

[0062] (Examples of compounds represented by general formula (3)) R in general formula (3) ３ are each independently 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 compounds represented by general formula (3) include compounds represented by the following chemical formula (3-1).

[0063]

[0064] 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.

[0065] R in general formula (3) ３is a methoxy group, which is preferable because the mechanical properties, particularly the modulus of elasticity, of the cured product are improved. Examples of compounds represented by general formula (3) include compounds represented by the following chemical formula (3-2).

[0066]

[0067] 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.

[0068] Examples of the polyhydric phenol compound [A] of the present disclosure include compounds represented by the following chemical formulas (3-3) and (3-4). ３ are each independently an aliphatic hydrocarbon group having 4 to 5 carbon atoms containing a quaternary carbon, since the crystallinity of the polyhydric phenol compound is reduced, thereby improving the handleability of the epoxy resin composition (i.e., the uncured product), which is therefore preferred.

[0069]

[0070]

[0071] In the chemical formulas (3-3) and (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.

[0072] [Properties of Polyhydric Phenol Compound [A]] The weight-average molecular weight of the polyhydric phenol compound [A] of the present disclosure is preferably 500 to 100,000, more preferably 500 to 10,000, and even more preferably 500 to 5,000. In the present disclosure, the weight-average molecular weight is determined by gel permeation chromatography (GPC) measurement of a sample that has been previously silylated with a silylating agent such as N,O-bis(trimethylsilyl)trifluoroacetamide.

[0073] The softening point of the polyhydric phenol compound [A] of the present disclosure, as determined in accordance with JIS K5601-2-2:1999 (ISO 4625:1980), is preferably 30° C. to 150° C., more preferably 50° C. to 120° C., and even more preferably 70° C. to 110° C. In the present disclosure, the softening point is determined in accordance with JIS K5601-2-2:1999 (ISO 4625:1980).

[0074] The hydroxyl equivalent of the polyhydric phenol compound [A] of the present disclosure is preferably 160 g / Eq to 1000 g / Eq, more preferably 160 g / Eq to 500 g / Eq, and even more preferably 160 g / Eq to 300 g / Eq. In the present disclosure, the hydroxyl equivalent of the polyhydric phenol compound is calculated from the hydroxyl value determined by the acetylation method specified in JIS K0070:1992 (ASTM D2896-15) using the following formula: [Hydroxyl equivalent] = 56.1 × 1000 ÷ [Hydroxyl value]

[0075] [Method for synthesizing polyphenol compound [A]] The polyphenol compound [A] of the present disclosure may be synthesized by any method, for example, by reacting a raw material phenol analog compound with sulfur chloride. More specifically, it can be synthesized by the method described in the Examples below.

[0076] 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 １ｄIn general formula (1), two of the R groups are hydrogen atoms and one is a methyl group, such as cresol, an ethyl group, cyclohexylphenol, a cyclohexyl group, methoxyphenol, a methoxy group, aminophenol, an amino group, hydroxybenzaldehyde, an aldehyde group, hydroxybenzoic acid, a carboxy group, or halophenol, a halogen group such as a chloro group, a bromo group, or an iodo group. １ａ , R １ｂ , R １ｃ , and R １ｄ Among these, those that are not bonds to Y in adjacent structural units may each independently be 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, or 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.

[0077] [Types of Structural Units of Polyhydric Phenol Compound [A]] The polyhydric phenol compound [A] of the present disclosure may be a polyhydric phenol compound containing one type of structural unit represented by general formula (1), or may contain a polyhydric phenol compound containing two or more types of structural units represented by general formula (1).

[0078] The proportion of the structural units represented by general formula (1) in all structural units of the polyhydric phenol compound [A] is preferably 10 mol % to 100 mol %, more preferably 50 mol % to 100 mol %, and even more preferably 90 mol % to 100 mol %.

[0079] The polyhydric phenol compound [A] of the present disclosure may contain a structural unit other than the structural unit represented by general formula (1). Examples of the structural unit other than the structural unit represented by general formula (1) that may be contained in the polyhydric phenol compound [A] of the present disclosure include a hydrocarbon group, a heterocyclic group, -NH-, -SO-, -SO ２Examples of the heterocyclic group include -, -CO-, -O-, -COO-, -OCO-, -S-, -S-S-, and groups formed by combining two or more of these. Examples of the hydrocarbon group include an alkyl group and an aryl group. 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 the heteroatom constituting the heterocyclic group include a nitrogen atom, an oxygen atom, and a sulfur atom. The heterocyclic group may be a monocyclic group or a condensed ring.

[0080] [Polyphenol Compound Curing Agent] The polyphenol compound [A] of the present disclosure is preferably a curing agent. In the present disclosure, a polyphenol compound serving as a curing agent is also referred to as a "polyphenol compound curing agent." The polyphenol compound [A] preferably has a dynamic covalent bond moiety and two or more phenolic hydroxyl groups, and does not have an epoxy group in its chemical structure. As represented by the above general formula (1), the polyphenol compound [A] of the present disclosure has an aromatic ring, and the aromatic ring has a phenolic hydroxyl group (OH group) and a dynamic covalent bond moiety (Y) as a substituent. The aromatic ring may also have other substituents. The remoldability and self-repairability 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). 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 due to the rigid aromatic ring contained in the above general formula (1).

[0081] [Curing Agent Other Than Polyphenol Compound [A]] The epoxy resin composition of the present disclosure may contain a known curing agent for curing epoxy resins, in addition to the polyphenol compound [A] represented by general formula (1). The curing agent other than the polyphenol compound [A] is not particularly limited as long as it is a curing agent that can cure epoxy resins, and is selected appropriately depending on the intended use, etc.

[0082] Specifically, curing agents other than the polyhydric phenol compound [A] include, for example, dicyandiamide, polyhydric phenol compound curing agents other than those represented by the general formula (1) above, various isomers of aromatic amine-based curing agents, and aminobenzoic acid esters. Dicyandiamide is preferred because it provides excellent storage stability to 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 derivatives thereof 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.

[0083] <Epoxy Resin [B]> The epoxy resin [B] contained in the epoxy resin composition of the present disclosure may be a known epoxy resin. The epoxy resin [B] may be monofunctional, bifunctional, or polyfunctional, and may be a glycidyl ether or a glycidyl amine. The epoxy resin [B] may be any resin that reacts with the polyhydric phenol compound [A], and may be appropriately selected depending on the intended use, etc.

[0084] Specific examples of the epoxy resin [B] 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, i.e., diglycidyl-o-toluidine, diglycidyl-m-toluidine, diglycidyl-p-toluidine, diglycidyl-xylidine, diglycidyl-mesidine, diglycidyl-anisidine, diglycidyl-phenoxyaniline, and diglycidyl-naphthylamine, and their 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 their derivatives; polyfunctional glycidyl ether type epoxy resins such as phenol novolac type epoxy resins, cresol novolac type epoxy resins, and their derivatives; and glycidyl ester-type epoxy resins such as phthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, dimer acid diglycidyl ester, and derivatives thereof. These epoxy resins may be used alone or in combination.

[0085] 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.

[0086] [Weight-average molecular weight of epoxy resin [B]] The weight-average molecular weight of the epoxy resin [B] of the present disclosure is preferably 100 to 5,000, more preferably 150 to 2,000, and even more preferably 200 to 1,500.

[0087] [Epoxy equivalent of epoxy resin [B]] The epoxy equivalent of the epoxy resin [B] of the present disclosure is preferably 80 g / Eq to 1000 g / Eq, more preferably 90 g / Eq to 500 g / Eq, and even more preferably 100 g / Eq to 300 g / Eq. In the present disclosure, the epoxy equivalent is determined according to JIS K7236:2009 (ISO 3001:1999).

[0088] [Method for synthesizing epoxy resin [B]] The epoxy resin [B] of the present disclosure may be synthesized by any method. For example, it may be synthesized by reacting raw materials, that is, an aromatic diamine, an aminophenol, or a diphenol, with an epihalohydrin such as epichlorohydrin to obtain a halohydrin compound, and then carrying out a cyclization reaction using an alkaline compound.

[0089] <Incorporation in Epoxy Resin Composition> The total amount of curing agent contained in the epoxy resin composition of the present disclosure may be an amount suitable for curing all of the epoxy resins incorporated in the epoxy resin composition, and is adjusted appropriately depending on the type of epoxy resin or curing agent used. The total amount of the polyhydric phenol compound [A] and the epoxy resin [B] contained in the epoxy resin composition of the present disclosure is not particularly limited, and is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and less than 100% by mass, and even more preferably 90% by mass or more and less than 100% by mass, based on the epoxy resin composition.

[0090] 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 remoldability of the resulting cured epoxy resin product is likely to be reduced. If the molar amount of all active hydrogens is large relative to the molar amount of all epoxy groups, the viscosity of the epoxy resin composition increases, and the impregnation ability into a fiber-reinforced substrate is likely to be reduced.

[0091] 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 each epoxy equivalent. The molar amount of all active hydrogen is determined from the sum of the molar amount of phenolic hydroxyl groups derived from polyphenol compound [A], the molar amount of phenolic hydroxyl groups derived from polyphenol compounds other than polyphenol compound [A], and the molar amount of active hydrogen derived from curing agents (amines, etc.) other than polyphenol compounds, contained in the epoxy resin composition.

[0092] The molar amount of phenolic hydroxyl groups in the polyphenol compound can be determined by dividing the mass of the polyphenol compound by the equivalent weight of each hydroxyl group. The molar amount of active hydrogens in the curing agent (amines, etc.) other than the polyphenol compound can be determined by dividing the molar amount of the curing agent by the number of active hydrogens contained in one molecule.

[0093] In the epoxy resin composition of the present disclosure, the ratio (=(a) / (b)) of the molar amount (b) of epoxy groups in the epoxy resin [B] to the molar amount (a) of phenolic hydroxyl groups in the polyhydric phenol compound [A] is 0.25 to 1.5, preferably 0.4 to 1.3, more preferably 0.7 to 1.2, and particularly preferably 1.0. According to the epoxy resin composition of the present disclosure, even when the ratio (a) / (b) is 0.25 or more and 1.5 or less, an epoxy resin composition having excellent remoldability or self-repairing properties in the cured product can be obtained.

[0094] That is, the ratio (a) / (b) is the ratio of the molar amount (a) of the phenolic hydroxyl groups to the molar amount (b) of the epoxy groups in the polyhydric phenol compound [A] contained in a unit mass of the epoxy resin composition. １ It can be determined from the integrated area ratio of the peak derived from the phenolic hydroxyl group to the peak derived from the epoxy group when analyzed by H-NMR.

[0095] In the present disclosure, using a theoretical equivalent amount of a compound having dynamic covalent bonding moieties relative to an epoxy resin means that the compound is added in an amount such that the molar amount of active hydrogen in the compound having dynamic covalent bonding moieties is 1 relative to the molar amount of epoxy groups in the epoxy resin of 1, i.e., in this case, the ratio (a) / (b) is 1. In the present disclosure, close to the theoretical equivalent means that the ratio (a) / (b) is 0.25 or more and 1.5 or less.

[0096] <Curing Accelerator> The epoxy resin composition of the present disclosure may contain a curing accelerator in addition to the polyhydric phenol compound [A] and epoxy resin [B] described above. The curing accelerator accelerates the curing reaction of the epoxy resin composition and the prepreg impregnated therewith, thereby providing a cured product having a 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 polyhydric phenol compound [A] and the epoxy resin [B]. Examples of the curing accelerator include nitrogen-containing heterocyclic compounds such as imidazoles and tertiary amines, as well as onium salts thereof, phosphines, and phosphonium salts thereof.

[0097] Specific examples of the curing accelerator include imidazoles such as 2-ethyl-4-methylimidazole and 1-(2-cyanoethyl)-2-phenylimidazole, and salts containing these; tertiary amines such as diazabicycloundecene, diazabicyclononene, and tris(dimethylaminomethyl)phenol, and salts containing these; phosphines such as triphenylphosphine and tri(orthotolyl)phosphine; and phosphonium salts such as tetraphenylphosphonium tetraphenylborate.

[0098] The content of the curing accelerator in the epoxy resin composition of the present disclosure is selected as needed and is not particularly limited.

[0099] <Optional Components> The epoxy resin composition of the present disclosure may contain a thermoplastic resin, conductive particles, a flame retardant, an inorganic filler, an internal mold release agent, or the like.

[0100] 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.

[0101] The epoxy resin-soluble thermoplastic resin adjusts the viscosity of the epoxy resin composition and 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. Polyethersulfone or polysulfone having a weight-average molecular weight (Mw) of 8,000 to 100,000 as measured by gel permeation chromatography is particularly preferred as the epoxy resin-soluble thermoplastic resin. If the weight-average molecular weight (Mw) is less than 8,000, the resulting fiber-reinforced composite material will have insufficient impact resistance. Furthermore, if the weight-average molecular weight (Mw) is greater than 100,000, the viscosity of the epoxy resin composition may be significantly increased, significantly reducing handleability. It is preferable that the molecular weight distribution of the epoxy resin-soluble thermoplastic resin be uniform. In particular, the polydispersity (Mw / Mn), which is the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), is preferably in the range of 1 to 10, and more preferably in the range of 1.1 to 5.

[0102] 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.

[0103] 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.

[0104] A portion of the epoxy resin-insoluble thermoplastic resin or epoxy resin-soluble thermoplastic resin (for example, the 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.

[0105] Examples of epoxy resin-insoluble thermoplastic resins include polyamide, polyacetal, polyphenylene oxide, polyphenylene sulfide, polyester, polyamideimide, polyimide, polyetherketone, polyetheretherketone, polyethylene naphthalate, polyethernitrile, 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.

[0106] In particular, the heat resistance of the resulting fiber-reinforced composite material can be particularly 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).

[0107] 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.

[0108] 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.

[0109] 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. Silicate minerals are particularly preferred. Specific examples of silicate minerals include THIXOTROPIC AGENT DT 5039 (manufactured by Huntsman Japan Co., Ltd.).

[0110] 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 blended is preferably 0.1 to 5 parts by mass, and 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.

[0111] 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 K.K.), and stearyl stearate (SL-900A; manufactured by Riken Vitamin Co., Ltd.).

[0112] <Method for producing epoxy resin composition> The epoxy resin composition of the present disclosure can be produced by mixing the polyhydric phenol compound [A], the epoxy resin [B], and, if necessary, other components. The order of mixing these components does not matter.

[0113] 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 reducing the storage stability of the resulting epoxy resin composition and prepreg produced using it. 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.

[0114] As the mixing machine, a conventionally known machine 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 at a temperature controlled to a constant temperature of 30°C or less and a relative humidity of 50% RH or less.

[0115] <Uses> The epoxy resin composition of the present disclosure is preferably a curing agent, i.e., an epoxy resin curing agent. That is, in the present disclosure, the epoxy resin curing agent comprises a polyhydric phenol compound [A] and an epoxy resin [B], each containing at least a structural unit represented by the following general formula (1), wherein the ratio (a) / (b) of the molar amount (a) of phenolic hydroxyl groups in the polyhydric phenol compound [A] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less, and when a cured product obtained by curing to a degree of cure of 90% or more is subjected to application of an external force 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 is 90% or more one hour after the start of application of the external force.

[0116]

[0117] In general formula (1), each Y is independently a dynamic covalent bond moiety; 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.

[0118] The preferred aspects and specific examples of the polyhydric phenol compound [A], the epoxy resin [B], the ratio (a) / (b), and the stress relaxation rate in the epoxy resin curing agent are the same as the preferred aspects and specific examples of the polyhydric phenol compound [A], the epoxy resin [B], the ratio (a) / (b), and the stress relaxation rate in the epoxy resin composition of the present disclosure.

[0119] <<Cured Epoxy Resin Product>> <Cured Epoxy Resin 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 curing may be performed by a known method. For example, the curing method can be such that the epoxy resin is degassed 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 hour to 12 hours, and more preferably at a temperature of 150°C to 180°C for 1 hour to 6 hours, to obtain a cured epoxy resin product. Specifically, the epoxy resin is degassed in a vacuum, followed by curing at a temperature of 120°C for 2 hours, at a temperature of 150°C for 2 hours, and at a temperature of 180°C for 2 hours, to obtain a cured epoxy resin product.

[0120] It is preferable that 80 mass % or more of the cured epoxy resin product of the first embodiment dissolves when brought into contact with a decomposer containing a reducing agent and heated at 100° C. for 48 hours. The explanation for the dissolution of 80 mass % or more of the cured epoxy resin product when brought into contact with a decomposer containing a reducing agent and heated at 100° C. for 48 hours is the same as the explanation in <Cured epoxy resin product of second embodiment> described later.

[0121] 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).

[0122]

[0123] In general formula (4), * represents a bond. In general formula (4), * in "-C(OH)-*" is preferably a bond to a structure derived from an epoxy resin, 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. Note 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 general formula (4) is produced, and the cured epoxy resin is decomposed into a compound containing at least the structure represented by general formula (4) as described below.

[0124] <Cured Epoxy Resin Product of Second Embodiment> When the cured epoxy resin product 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 product dissolves. The cured epoxy resin product of the second embodiment has excellent solubility, and can be dissolved in a low molecular weight compound by bringing the cured epoxy resin product into contact with a decomposing agent.

[0125] The type of 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, and tris(2-carboxyethyl)phosphine hydrochloride, with reducing agents that reduce disulfide bonds being preferred. These reducing agents may be used alone or in combination.

[0126] These reducing agents may be used without solvent, or may be used as a decomposer solution by diluting with a solvent. The cured epoxy resin material of the second embodiment may be dissolved by immersion in a decomposer 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, the 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.

[0127] The concentration of the reducing agent in the decomposer solution is not particularly limited, and 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 dissolves 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.

[0128] In addition to the above, the decomposing agent may contain any additives that promote dissolution. In particular, nitrogen-containing bases such as triethylamine, DBU (1,8-diazabicycloundecene), or ammonia are preferred because their addition has a high effect of promoting dissolution.

[0129] In the dissolution, in order to promote dissolution, 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), stirring may be performed as appropriate, or ultrasonic treatment may be performed if necessary. For example, the dissolution reaction may be performed at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In the dissolution, pressure of 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa) may be applied in order to promote dissolution.

[0130] In the cured epoxy resin product 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 product 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 product of the second embodiment, upon contact with a decomposing agent containing a reducing agent and heating, 100% by mass or less of the cured epoxy resin product may be dissolved.

[0131] 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.

[0132] <Epoxy Resin Cured Product of Third Embodiment> When the epoxy resin cured 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 produced. It is more preferable that the epoxy resin cured 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.

[0133]

[0134] In general formula (4), * represents a bond. In general formula (4), * in "-C(OH)-*" is preferably a bond to a structure derived from an epoxy resin, 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.

[0135] 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 3000, and even more preferably 200 to 2000. In the present disclosure, the molecular weight is measured by gel permeation chromatography (GPC) using chloroform or N,N-dimethylformamide as an eluent.

[0136] 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.

[0137] 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).

[0138]

[0139] In the general formula (4-1), * represents a bond. The * in "-C(OH)-*" in the general formula (4-1) is preferably a bond to a structure derived from an epoxy resin. 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.

[0140] 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 3,000, and even more preferably 200 to 2,000.

[0141] 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), stirring may be performed as appropriate, and ultrasonic treatment may be performed 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.

[0142] In the present disclosure, the generation of at least a compound containing 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.

[0143] <Epoxy Resin Cured Product of First to Third Embodiments> In the present disclosure, the aspect of the "epoxy resin cured product" includes the aspects of the epoxy resin cured product of the first to third embodiments. The epoxy resin cured product 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, heat resistance is insufficient. As a result, voids and the like are likely to form in the resulting fiber-reinforced composite material. The glass transition temperature in the present disclosure can be measured using a differential scanning calorimeter. The temperature of the 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.

[0144] The epoxy resin cured product of the present disclosure preferably has a flexural modulus (hereinafter sometimes referred to as FM) measured according to JIS K7171:2016 (ISO 178:2019) of 2.00 GPa or more, more preferably 2.30 GPa to 30.00 GPa, even more preferably 2.50 GPa to 20.00 GPa, and may even be 2.50 GPa to 5.00 GPa. If the flexural modulus is less than 2.00 GPa, the properties of the resulting fiber-reinforced composite material are likely to deteriorate. The flexural modulus in the present disclosure can be measured in accordance with JIS K7171:2016.

[0145] 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 150 MPa, and even more preferably 80 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. Note that the flexural strength in the present disclosure is the maximum value of flexural stress measured in accordance with JIS K7171:2016.

[0146] The cured epoxy resin product of the present disclosure preferably has a compressive strength (CS) measured according to JIS K7181:2011 (ISO 604:2002) of 200 MPa or more, more preferably 250 MPa to 400 MPa, and even more preferably 280 MPa to 350 MPa. If the compressive strength is less than 200 MPa, the properties of the resulting fiber-reinforced composite material are likely to deteriorate. The compressive strength in the present disclosure can be measured according to JIS K7181:2011.

[0147] 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 K7148-1:2015 (ISO 14322:2012) method, using a differential scanning calorimeter (DSC).

[0148] <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 stress generated is measured, the stress relaxation rate one hour after the start of application of the external force is 90% or more. The stress relaxation rate one hour after the start of application of the external force is preferably 99.0% or more, more preferably 99.5% or more, and even more preferably 99.7% or more. The stress relaxation rate may be 100.0% or less. If the stress relaxation rate one hour after the start of application of the external force is less than 90%, the remoldability of the epoxy resin cured product and the fiber-reinforced composite material described below is impaired, and the strength and elastic modulus are likely to decrease.

[0149] 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 0 seconds after the start of application of the external force. G(min) is the minimum modulus value up to 3600 seconds after the start of application of the external force; if the modulus value reaches the lower limit of the modulus measurement by the device within 3600 seconds, this value is used; otherwise, it is the modulus value at 3600 seconds.

[0150] [Stress Relaxation Mode Separation Analysis] Stress relaxation mode separation analysis will be described in detail. The stress relaxation behavior when stress is applied to the cured epoxy resin material of the present disclosure is approximated by a generalized Maxwell model, which is a model in which multiple types of two-element models, so-called Maxwell elements, 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 behaviors 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.

[0151] The modulus value G(t) for 3600 seconds obtained in the stress relaxation measurement is calculated as the correlation square coefficient R according to the following equation 1: ２ It is approximated to be >0.999.

[0152]

[0153] In Equation 1, t is time (unit: seconds), G ０ is the modulus value G(0) at t = 0 seconds. ｉ is Σμ ｉ τ is an independent variable in the range satisfying τ = 1, and is a variable indicating the contribution (unit: %) of each term (each relaxation mode) in this multi-dimensional linear equation. 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 is 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.

[0154] 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 ｉIt is preferable to analyze 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. In this case, τ is set to include the measurement time of 3600 seconds. ｉ The maximum value of τ ｉ = 10,000 (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, .... The cured epoxy resin material of the present disclosure is then approximated by a linear sum of relaxation modes having 25 different relaxation times, and the stress relaxation behavior is measured.

[0155] Next, an approximate equation is obtained using Equation 1 from a graph in which the logarithm of the relaxation time is plotted on the horizontal axis and the logarithm of the stress is plotted 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, τ 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 isolated relaxation mode.

[0156] Here, the stress relaxation measurement can be performed 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). The measurement is preferably performed in an inert gas atmosphere, in an atmosphere with controlled humidity. While not particularly limited, it is preferable to perform the measurement in a room with a low humidity atmosphere of 50% RH or less, with nitrogen flowing through the measurement device at a flow rate of 10 L / min or more.

[0157] 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 as τ ｉ 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 having a distribution. More preferably, the graph has at least a relaxation mode group having a relaxation time of less than 200 seconds and a relaxation mode group having a relaxation time of 1000 seconds or more, each of which contributes at a rate 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.

[0158] <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.

[0159] 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 content of reinforcing fibers relative to the total volume of the prepreg of the present disclosure exceeds 75% by volume, voids and the like may occur in the resulting fiber-reinforced composite material, resulting in reduced mechanical properties. If the content of reinforcing fibers relative to the total volume of 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 content of reinforcing fibers relative to the total volume of the prepreg of the present disclosure is preferably 45% to 70% by volume, more preferably 50% to 65% by volume.

[0160] 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 bundle around a bobbin such as a paper tube. In the present disclosure, the prepreg wound around a bobbin or the prepreg unwound after being wound is referred to as a "tow prepreg."

[0161] <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.

[0162] 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.

[0163] 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. 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, and more preferably 5 μm to 10 μm. The use of such carbon fibers can improve the mechanical properties of the resulting fiber-reinforced composite material.

[0164] The fiber-reinforced substrate is preferably formed into a sheet and used. 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 a fiber-reinforced substrate. Among these, the use of a unidirectionally aligned sheet, bidirectional fabric, or multiaxial fabric substrate in which the fiber-reinforced substrate is formed into a sheet as continuous fibers is preferred because it results in a fiber-reinforced composite material with superior mechanical properties. The thickness of the sheet-like fiber-reinforced substrate is preferably 0.01 mm to 3 mm, more preferably 0.1 mm to 1.5 mm.

[0165] <Method for Producing Prepreg> There are no particular limitations on the method for producing the prepreg of the present disclosure, and any conventionally known method can be employed. Specifically, a hot melt method or a solvent method can be suitably employed.

[0166] 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.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] 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 of the resin composition, resin flow, etc. A specific impregnation pressure is 0.01 N / cm to 250 N / cm, and preferably 0.1 N / cm to 200 N / cm.

[0172] <Fiber-reinforced composite material> 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 relative to the total volume of 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.

[0173] 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.

[0174] <Method for producing fiber-reinforced composite material> 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 by heating and pressurizing it under specific conditions. 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.

[0175] [Autoclave Molding Method] Autoclave molding is a preferred 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.

[0176] [Press molding method] Press molding 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 press molding 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 heated to a curing temperature in advance.

[0177] 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. Furthermore, 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.

[0178] 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 a 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 and the like are less likely to occur. The molding time is preferably 1 hour to 8 hours.

[0179] <Examples of Fiber Reinforced Composite Materials> Examples of fiber reinforced composite materials include fiber reinforced plastics (FRP) and carbon fiber reinforced plastics (CFRP).

[0180] <<Method for Repairing Cured Epoxy Resin Material>> The method for repairing a cured epoxy resin material according to the present disclosure comprises heating a cured epoxy resin material having internal cracks or delamination 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 according to the present disclosure has excellent self-repairing properties, and therefore can repair cracks and delamination that occur internally during molding or in the environment of use.

[0181] 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 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 cured epoxy resin materials of the present disclosure may also include applying pressure, and if necessary, pressure may be applied using a mold or 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.

[0182] <<Method for Repairing Fiber-Reinforced Composite Material>> The method for repairing a fiber-reinforced composite material of the present disclosure includes heating a fiber-reinforced composite material having internal cracks or delaminations at a temperature equal to or higher than the glass transition temperature of the fiber-reinforced composite material, thereby repairing the cracks or delaminations. Because the fiber-reinforced composite material of the present disclosure has excellent self-repairing properties, it is possible to repair cracks and delaminations that occur internally during molding or in the usage environment.

[0183] 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 techniques can be used as 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 applied during pressing is preferably 0.05 MPa to 2 MPa, more preferably 0.2 MPa to 2 MPa, and these pressures may be applied in stages.

[0184] <<Method for Remolding Cured Epoxy Resin Material>> The method for remolding a cured epoxy resin material of the present disclosure includes heating and pressurizing the cured epoxy resin material of 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 remolded even after molding.

[0185] 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 in stages.

[0186] <<Method for Remolding a Fiber-Reinforced Composite Material>> The method for remolding 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 remolded even after molding.

[0187] 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.

[0188] <<Method for Decomposing Cured Epoxy Resin Material>> 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.

[0189] The type of 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, and tris(2-carboxyethyl)phosphine hydrochloride, with reducing agents that reduce disulfide bonds being preferred. These reducing agents may be used alone or in combination.

[0190] These reducing agents may be used without solvent, or may be used as a decomposer solution by diluting with a solvent. That is, the cured epoxy resin material of the present disclosure may be decomposed by immersion in a decomposer solution containing a reducing agent. The amount of decomposer solution 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 decomposer solution used is preferably 1 to 1,000 times, more preferably 1 to 100 times, and particularly preferably 1 to 10 times the amount of the cured epoxy resin material by mass.

[0191] The concentration of the reducing agent in the decomposer solution is not particularly limited, and 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 dissolves the reducing agent and can 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.

[0192] 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.

[0193] In the decomposition, in order to promote the decomposition, 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), stirring may be performed as appropriate, or ultrasonic treatment may be performed if necessary. For example, the decomposition reaction may be performed at room temperature (25°C) or 80°C for 8 hours while stirring the decomposing agent. In the decomposition, in order to promote the decomposition, pressure may be applied at 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa).

[0194] By the method for decomposing an epoxy resin cured material according to the present disclosure, preferably 50 mass % or more of the cured material is decomposed, more preferably 80 mass % or more, and even more preferably 95 mass % or more. Preferably, 100 mass % of the cured epoxy resin material is decomposed.

[0195] <<Method for recovering reinforcing fibers from a fiber-reinforced composite material>> 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 comes into contact with the decomposition agent containing a reducing agent, the resin is decomposed and / or dissolved, making it possible to recover the reinforcing fibers.

[0196] The description of the decomposition agent containing a reducing agent in the method for recovering reinforcing fibers from a fiber-reinforced composite material is the same as the description of the decomposition agent containing a reducing agent in the <<Method for Decomposing an Epoxy Resin Cured Material>>, including definitions, examples, and preferred embodiments. In the decomposition and / or dissolution, to promote the decomposition and / or dissolution, 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), stirring may be performed as appropriate, or ultrasonic treatment may be performed if necessary. For example, the decomposition and / or dissolution reaction may be performed at room temperature (25°C) or 80°C for 8 hours while stirring the decomposition agent. In the decomposition and / or dissolution, pressure of 0.1 MPa to 10 MPa (preferably 0.1 MPa to 1 MPa) may be applied to promote the decomposition and / or dissolution.

[0197] The method for recovering reinforcing fibers from a fiber-reinforced composite material according to 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 by filtration. The recovered reinforcing fibers can be recycled into prepregs or fiber-reinforced composite materials. In this process, an oil such as a sizing agent may be applied again if necessary.

[0198] Other Embodiments of Epoxy Resin Curing Agent Other embodiments of the epoxy resin curing agent include an epoxy resin curing agent containing a polyhydric phenol compound [A2] containing a structural unit represented by the following general formula (2):

[0199]

[0200] In general formula (2), R ２ａ , R ２ｂ , and R ２ｃeach 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.

[0201] Preferred aspects and specific examples of the polyhydric phenol compound [A2] containing a structural unit represented by general formula (2) are the same as the preferred aspects and specific examples of the polyhydric phenol compound [A] containing a polyhydric phenol compound containing a structural unit represented by general formula (2) in the epoxy resin composition of the present disclosure.

[0202] Another embodiment of the epoxy resin curing agent further comprises an epoxy resin [B], wherein the ratio (a2) / (b) of the molar amount (a2) of phenolic hydroxyl groups in the polyhydric phenol compound [A2] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less, and when an external force is applied to a cured product obtained by curing 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 stress generated is measured, the stress relaxation rate is preferably 90% or more one hour after the start of application of the external force.

[0203] In other embodiments of the epoxy resin curing agent, the preferred aspects and specific examples of the epoxy resin [B], the ratio (a2) / (b), and the stress relaxation rate are the same as the preferred aspects and specific examples of the epoxy resin [B], the ratio (a) / (b), and the stress relaxation rate in the epoxy resin composition of the present disclosure.

[0204] 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.

[0205] <Production of Epoxy Resin Composition> [Synthesis of Polyphenol Compound [A]] [Synthesis Example 1] Synthesis of Poly(t-amylphenol disulfide) (hereinafter abbreviated as "APDS")

[0206]

[0207] 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. After cooling to 0°C, 189 g (1.4 mol) of disulfur dichloride was added dropwise over 2 hours. The temperature was then raised to 25°C over 10 hours, followed by stirring at 45-50°C for 2 hours. The resulting yellow-brown solution was gradually heated to 170°C under reduced pressure, and the DMF was distilled off. 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. Finally, the mixture was dried at 150°C to obtain a brown solid product. The major product was poly(t-amylphenol disulfide), with a yield of 279 g (88% (mass%)) and a purity of 99% (HPLC area%). The hydroxyl value determined in accordance with JIS K0070:1992 was 241, the hydroxyl equivalent was 233 g / Eq, the weight average molecular weight determined by gel permeation chromatography (GPC) measurement was 3,300, and the softening point was 92°C.

[0208] [Synthesis Example 2] Synthesis of poly(t-butylphenol disulfide) (hereinafter abbreviated as "BPDS")

[0209]

[0210] A brown viscous liquid was obtained by carrying out the same procedure as in Synthesis Example 1, except that 4-(1,1-dimethylpropyl)phenol was replaced with 4-(t-butyl)phenol. The main product was poly(t-butylphenol disulfide), and the yield was 252 g (yield 85% (mass%)) and the purity was 99% (HPLC area%). The hydroxyl value determined according to JIS K0070:1992 was 244, and the hydroxyl equivalent was 230 g / Eq. The weight-average molecular weight determined by GPC measurement was 1600, and the softening point was 98°C.

[0211] [Synthesis Example 3] Synthesis of poly(methoxyphenol disulfide) (hereinafter abbreviated as "MOPDS")

[0212]

[0213] 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 was poly(methoxyphenol disulfide), and the yield was 223 g (yield 86% (mass%)) and the purity was 98% (HPLC area%). The hydroxyl value determined according to JIS K0070:1992 was 295, and the hydroxyl equivalent was 190 g / Eq. The weight-average molecular weight determined by GPC measurement was 1500, and the softening point was 115°C.

[0214] The following polyhydric phenol compounds were prepared as polyhydric phenol compounds for use in the comparative examples: Novolac phenolic resin (hereinafter abbreviated as "PN") (TD-2093Y manufactured by DIC Corporation, hydroxyl group equivalent = 104 g / Eq)

[0215] [Epoxy Resin [B]] The following epoxy resins were prepared as epoxy resin [B]: Bisphenol A-diglycidyl ether (jER825 manufactured by Mitsubishi Chemical Corporation, epoxy equivalent = 176 g / Eq, hereinafter abbreviated as "DGEBA") Bisphenol F-diglycidyl ether (jER806 manufactured by Mitsubishi Chemical Corporation, epoxy equivalent = 168 g / Eq, hereinafter abbreviated as "DGEBF") Tetraglycidyldiaminodiphenylmethane (YH-404 manufactured by Nippon Steel Chemical & Material Co., Ltd., epoxy equivalent = 118 g / Eq, hereinafter abbreviated as "TGDDM")

[0216] [Curing accelerator] Triphenylphosphine (manufactured by Tokyo Chemical Industry Co., Ltd., hereinafter referred to as "PPh ３ ") 2-Ethyl-4-methylimidazole (Curesol 2E4MZ manufactured by Shikoku Chemical Industry Co., Ltd., hereinafter abbreviated as "2E4MZ") 1,8-Diazabicycloundecene (Tokyo Chemical Industry Co., Ltd., hereinafter abbreviated as "DBU")

[0217] Examples 1 to 7 and Comparative Example 1 [1] Preparation of Epoxy Resin Compositions A polyhydric phenol compound and an epoxy resin were weighed out in the parts by mass shown in Table 1 below, and mixed at 80°C using a roll mill until uniform. After that, a curing accelerator was added in the parts by mass shown in Table 1 below, and the mixture was kneaded again to prepare epoxy resin compositions.

[0218] [2] Preparation of cured epoxy resin product and measurement of physical properties (2-1) Preparation of cured epoxy resin product 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. The mixture was cured at 120°C for 2 hours, at 150°C for 2 hours, and at 180°C for 2 hours to obtain a cured epoxy resin product with a thickness of 4 mm.

[0219] (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 differential scanning calorimeter DSC Q2000 manufactured by TA Instruments. 5 mg±1 mg of the cured epoxy resin product 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 of the obtained curve was recorded as the glass transition temperature.

[0220] (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. Subsequently, the composition was held at [glass transition temperature of the cured product + 60]°C for 2 minutes, and then subjected to a 6.25 × 10 －３The modulus value G generated when a displacement of 100 rad was applied was measured for 3600 seconds, and the resulting time plot was recorded. Stress relaxation measurements were performed using a TA Instruments Discovery DHR-2 rheometer in a nitrogen gas flow of 10 L / min. From the obtained time plot, the stress relaxation rate was calculated using the following formula: [1-((G(min)) / (G(0))] x 100, where G(0) is the modulus value at 0 seconds after the start of external force application. G(min) is the minimum modulus value up to 3600 seconds after the start of external force application. If the modulus value reached the lower limit of the instrument's measurement within 3600 seconds, this 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 the external force was set to be equal to the modulus value G(0.1) at 0.1 seconds after the start of application of the external force, and the stress relaxation rate was calculated.

[0221] 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 application of an external force, and the stress relaxation rate 3600 seconds after the start of external force application was 99.2%. 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 0.3%.

[0222] (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 performed in the range of 0 to 3600 seconds, and the analysis was performed 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, 1000, 2500, 4000, 5500, 7000, 8500, 10000

[0223] 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.

[0224] (2-5) Confirmation of remoldability The cured epoxy resin product 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 and preheated at 200°C for 5 minutes. A pressure of 25 MPa was applied for 30 minutes in a press at 200°C to prepare a remolded sample. The obtained remolded sample was visually observed. Remoldability was evaluated according to the following criteria, with A and B representing practically acceptable ranges. A: The surface of the remolded sample was smooth, and no grain boundaries of the chips were observed 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 chips was observed when light was transmitted through it. C: The grain boundaries of the chips before molding were observed on the surface of the remolded sample.

[0225] (2-6) Confirmation of self-repairing ability The epoxy resin cured product prepared in (2-1) was polished to a thickness of 2 mm, and a crack of about 2 mm was inserted using a razor. Furthermore, the product was placed on the sample stage of a transmission electron microscope equipped with a heat stage, preheated to 120°C, 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 ability was evaluated according to the following criteria, with A and B being considered to be within a practically acceptable range. A: The cracks in the self-repairing sample almost completely disappeared. B: The cracks in the self-repairing sample partially shrank. C: No disappearance or shrinkage of the cracks in the self-repairing sample was observed.

[0226] (2-7) Confirmation of Resin Decomposition Property The cured epoxy resin product prepared in (2-1) was cut into a piece of 1 cm x 1 cm x 0.4 mm, and one piece was immersed in 40 mL of an N,N-dimethylformamide solution containing 4 mL of 2-mercaptoethanol, followed by stirring at room temperature (25°C) and at 80°C. The state of the solution after 8 hours was visually observed. Resin decomposition property was evaluated according to the following criteria, with A and B representing practically acceptable ranges. A: The cured epoxy resin product was completely decomposed at room temperature to form a homogeneous solution. B: The cured epoxy resin product was not completely decomposed at room temperature, but 80% by mass or more of the product was decomposed at 80°C. C: The cured epoxy resin product was decomposed by less than 80% by mass at both room temperature and 80°C.

[0227] (2-8) Flexural modulus (FM) and flexural strength (FS) Tests were conducted on the cured epoxy resin prepared in (2-1) in accordance with JIS K7171:2016. Resin test specimens 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.

[0228] (2-9) Compressive Strength (CS) The epoxy resin composition prepared in [1] was placed in an 8 mm diameter test tube and cured at 120°C for 2 hours, 150°C for 2 hours, and 180°C for 2 hours. The cured product was removed and cut to a length of 15 mm to obtain a cylindrical cured product measuring 15 mm x 8 mm diameter. A compression test was conducted in accordance with JIS K7181:2011 at a test speed of 0.2 mm / min to measure the compressive strength.

[0229] 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 7 and Comparative Example 1 had a degree of cure of 99.9% or more as evaluated by DSC.

[0230]

[0231] (2-10) Confirmation of Decomposition Products The decomposition products of the cured epoxy resin materials of Examples 1 to 3 obtained in (2-7) Confirmation of Resin Decomposition Properties were analyzed by LC-MS. As a result, it was found that the decomposition products were compounds having the following structure and a molecular weight of 1,072.

[0232]

[0233] From the above, Examples 1 to 7 provided epoxy resin compositions that exhibited excellent remoldability or self-repairability in the cured product. 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 comprising a fiber-reinforced substrate having reinforcing fibers and a cured epoxy resin obtained by curing the epoxy resin composition of the present disclosure, also exhibited excellent remoldability or self-repairability in the cured product. Furthermore, the cured epoxy resin 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 could be recovered by contacting the fiber-reinforced composite material with a decomposing agent containing a reducing agent. Furthermore, the epoxy resin curing agent of the present disclosure was used as a curing agent.

[0234] The disclosure of Japanese Patent Application No. 2024-057150, filed on March 29, 2024, is incorporated herein by reference in its entirety. In addition, all documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually indicated to be incorporated by reference.

Claims

1. An epoxy resin composition comprising a polyhydric phenol compound [A] containing at least a structural unit represented by the following general formula (1) and an epoxy resin [B], wherein the ratio (a) / (b) of the molar amount (a) of phenolic hydroxyl groups in the polyhydric phenol compound [A] to the molar amount (b) of epoxy groups in the epoxy resin [B] is 0.25 or more and 1.5 or less, and wherein, when an external force is applied to a cured product obtained by curing 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 stress generated is measured, the stress relaxation rate is 90% or more one hour after the start of application of the external force. (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. The epoxy resin composition according to claim 1, wherein the dynamic covalent bonding site is a group containing a disulfide bond.

3. An epoxy resin composition according to claim 1 or 2, wherein the polyhydric phenol compound [A] comprises a polyhydric phenol compound containing a structural unit represented by the following general formula (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. An epoxy resin composition according to claim 1 or 2, wherein the polyhydric phenol compound [A] comprises a polyhydric phenol compound containing a structural unit represented by the following general formula (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. The epoxy resin composition according to claim 1 or 2, which is a curing agent.

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

7. A cured epoxy resin in which 80% or more by mass 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.

8. A cured epoxy resin material which, when brought into contact with a decomposing agent containing a reducing agent and heated, produces a compound containing at least the structure shown in the following general formula (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.

9. 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 to 75 volume % relative to the total volume of the prepreg.

10. A fiber-reinforced composite material comprising the cured epoxy resin according to any one of claims 6 to 8 and a fiber-reinforced substrate having reinforcing fibers, wherein the content of the reinforcing fibers is 25% by volume to 75% by volume of the total volume of the fiber-reinforced composite material.

11. A method for repairing a cured epoxy resin material, comprising heating the cured epoxy resin material according to any one of claims 6 to 8, 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.

12. A method for repairing a fiber-reinforced composite material, comprising heating the fiber-reinforced composite material according to claim 10, which has cracks or delaminations therein, at a temperature equal to or higher than the glass transition temperature of said fiber-reinforced composite material, thereby repairing said cracks or delaminations.

13. A method for remolding a cured epoxy resin product according to any one of claims 6 to 8, comprising heating and pressurizing the cured epoxy resin product at a temperature equal to or higher than the glass transition temperature of the cured epoxy resin product.

14. A method for remolding a fiber-reinforced composite material, comprising heating and pressurizing the fiber-reinforced composite material according to claim 10 at a temperature equal to or higher than the glass transition temperature of said fiber-reinforced composite material.

15. A method for decomposing a cured epoxy resin material, comprising contacting the cured epoxy resin material according to any one of claims 6 to 8 with a decomposition agent containing a reducing agent.

16. A method for recovering reinforcing fibers from a fiber-reinforced composite material, comprising contacting the fiber-reinforced composite material according to claim 10 with a decomposition agent containing a reducing agent.

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