Epoxy resins, epoxy resin compositions, prepregs, and fiber-reinforced composite materials

Abstract

The present invention aims to overcome the trade-off between depolymerizability and heat resistance present in the prior art, and to provide an epoxy resin that has excellent recyclability while possessing sufficient heat resistance. Furthermore, it aims to provide epoxy resin compositions, prepregs, and fiber-reinforced composite materials with excellent recyclability by using this epoxy resin. [Solution] An epoxy resin represented by the following formula (I), with an average epoxy equivalent of 90 to 350 g / eq. TIFF2026094706000008.tif46170 (In formula (I), R 1 ~R 4 Each of these independently represents one selected from the group consisting of a hydrogen atom, an aliphatic hydrocarbon group with 1 to 4 carbon atoms, and a halogen atom.

Description

[Technical Field]

[0001] This invention relates to epoxy resins with excellent recyclability, epoxy resin compositions, and prepregs and fiber-reinforced composite materials using these as matrix resins. [Background technology]

[0002] Epoxy resin compositions are suitably used as matrix resins in fiber-reinforced composite materials, taking advantage of their high strength, rigidity, heat resistance, and adhesive properties, and are combined with reinforcing fibers such as carbon fibers, glass fibers, and aramid fibers.

[0003] In the manufacture of fiber-reinforced composite materials, sheet-like intermediate materials (e.g., prepregs) impregnated with epoxy resin are commonly used. After laminating the prepregs, molded products are obtained by heating to cure the epoxy resin. Because a wide range of properties can be expressed depending on the lamination design of the prepregs, they are applied in various fields such as aircraft, automobiles, sports, and medicine.

[0004] In recent years, the applications of fiber-reinforced composite materials have been rapidly expanding. In addition to heat resistance and rigidity, there is a growing demand for epoxy resins and epoxy resin compositions that are carbon-neutral and highly recyclable, as well as fiber-reinforced composite materials obtained using these materials.

[0005] On the other hand, epoxy resin compositions using a tetrafunctional epoxy resin as the matrix resin for fiber-reinforced composite materials are known (Patent Document 1). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2016-84451 [Overview of the project] [Problems that the invention aims to solve]

[0007] In the invention described in Patent Document 1, it is not possible to depolymerize the fiber-reinforced composite material with an organic solvent after curing to obtain an epoxy resin composition. Therefore, it is not possible to recover the reinforcing fibers from the fiber-reinforced composite material, resulting in poor recyclability.

[0008] To improve the heat resistance of epoxy resin cured products, increasing the crosslinking density is effective. However, a high crosslinking density leads to a more complex network, making the product more difficult to decompose and thus more challenging to depolymerize by chemical treatment.

[0009] Therefore, the present invention aims to overcome the trade-off between depolymerization and heat resistance present in the conventional technology and provide an epoxy resin that has excellent recyclability while possessing sufficient heat resistance. Furthermore, it aims to provide epoxy resin compositions, prepregs, and fiber-reinforced composite materials with excellent recyclability by using this epoxy resin. [Means for solving the problem]

[0010] 1. An epoxy resin represented by the following formula (I), with an average epoxy equivalent of 90 to 350 g / eq.

[0011] [ka]

[0012] (In formula (I), R 1 ~R 4 Each of these independently represents one selected from the group consisting of a hydrogen atom, an aliphatic hydrocarbon group with 1 to 4 carbon atoms, and a halogen atom. 2. An epoxy resin composition comprising the epoxy resin and curing agent described in item 1 above. 3. The epoxy resin composition according to item 2 above, further comprising 5 to 50 parts by mass of a biomass-derived bisphenol-type epoxy resin having an average epoxy equivalent of 150 to 220 as component [A], when the total amount of epoxy resin is 100 parts by mass. 4. Further, the epoxy resin composition according to 2 or 3 above, which contains a polyrotaxane as the component [B]. 5. An epoxy resin cured product obtained by curing the epoxy resin composition according to any one of 2 to 4 above. 6. A prepreg containing the epoxy resin composition according to any one of 2 to 4 above and at least one reinforcing fiber selected from the group consisting of carbon fiber, glass fiber, and aramid fiber. 7. The prepreg according to 6 above, wherein the form of the reinforcing fiber is a woven fabric or discontinuous fiber. 8. The prepreg according to 7 above, wherein the form of the reinforcing fiber is discontinuous fiber, which is in a bundle shape and randomly dispersed. 9. The prepreg according to 7 above, wherein the form of the reinforcing fiber is discontinuous fiber, which is in a substantially monofilament shape and randomly dispersed. 10. A fiber-reinforced composite material obtained by curing the prepreg according to any one of 6 to 9 above.

Advantages of the Invention

[0013] According to the present invention, it is possible to obtain an epoxy resin which, while having excellent recyclability for the cured product, also has sufficient heat resistance. Furthermore, by using the epoxy resin, it is possible to provide an epoxy resin composition, a prepreg, and a fiber-reinforced composite material having excellent recyclability.

Modes for Carrying Out the Invention

[0014] The epoxy resin of the present invention is represented by the following formula (I) and has an average epoxy equivalent of 80 to 350 g / eq.

[0015]

Chemical Formula

[0016] (In formula (I), R 1 ~R 4 each independently represents one selected from the group consisting of a hydrogen atom, an aliphatic hydrocarbon group having 1 to 4 carbon atoms, and a halogen atom.) Examples of the epoxy resin represented by the following formula (I) include tetraglycidyl-4,4'-diaminodiphenyl disulfide, tetraglycidyl-4,4'-diamino-3,3'-diethyldiphenyl disulfide, tetraglycidyl-4,4'-diamino-3,3'-dimethyldiphenyl disulfide, tetraglycidyl-4,4'-diamino-3,3'-diisopropyldiphenyl disulfide, tetraglycidyl-4,4'-diamino-3,3',5,5'-tetraethyldiphenyl disulfide, tetraglycidyl-4,4'-diamino-3,3',5,5'-tetramethyldiphenyl disulfide, tetraglycidyl-3,3'-diaminodiphenyl disulfide, tetraglycidyl-2,2'-diaminodiphenyl disulfide, tetraglycidyl-2,2'-diamino-5,5'-diethyldiphenyl disulfide, tetraglycidyl-2,2'-diamino-5,5'-dimethyl diphenyl disulfide, tetraglycidyl-2,2'-diamino-5,5'-dichlorodiphenyl disulfide, tetraglycidyl-2,2'-diamino-5,5'-dibromodiphenyl disulfide or tetraglycidyl-2,2'-diamino-5,5'-difluorodiphenyl disulfide.

[0017] Among them, R 1 ~R 4 is preferably a hydrogen atom, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group or an isobutyl group because of its excellent compatibility with other epoxy resins, and tetraglycidyl-4,4'-diaminodiphenyl disulfide and tetraglycidyl-4,4'-diamino-3,3'-diethyldiphenyl disulfide are preferred because of their excellent heat resistance. Also, tetraglycidyl-2,2'-diaminodiphenyl disulfide and tetraglycidyl-2,2'-diamino-5,5'-diethyldiphenyl disulfide are preferred because of their excellent elastic modulus and depolymerization properties. In addition, since it has excellent flame retardancy, a form in which part or all of R 1 ~R 4 is a halogen atom such as a chlorine atom (Cl) or a bromine atom (Br) is also a preferred form. As exemplified above, R 1 ~R4 Preferably, two of these atoms are halogen atoms.

[0018] In this invention, the average epoxy equivalent can be evaluated by performing potentiometric titration in accordance with JIS-K7236 (2001). Approximately 300 mg of epoxy resin is weighed and placed in a glass beaker, and then 10 mL of chloroform is added. The mixture is stirred using a magnetic stirrer until the weighed components dissolve in the chloroform. 20 mL of acetic acid is added to the solution, followed by 10 mL of tetraethylammonium bromide acetic acid solution (0.4 g / mL acetic acid), and the mixture is stirred. An electrode is immersed in the solution, and potentiometric titration is performed with perchloric acid-acetic acid standard solution (0.1 mol / L) to calculate the average epoxy equivalent. Instead of tetraethylammonium bromide acetic acid solution, tetrabutylammonium iodide or a 10% chloroform solution can also be used.

[0019] By setting the average epoxy equivalent to 80 g / eq or higher, the three-dimensional crosslinking structure of the cured epoxy resin composition containing the epoxy resin represented by formula (I) is prevented from becoming too dense, resulting in a cured product with high toughness.

[0020] On the other hand, by keeping the average epoxy equivalent to 350 g / eq or less, steric hindrance due to bulky substituents is prevented, and a cured product with excellent heat resistance can be obtained. The average epoxy equivalent is preferably 110 g / eq to 130 g / eq, and this range provides an excellent balance between depolymerization properties and heat resistance.

[0021] The melting point of the epoxy resin represented by formula (I) is preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 100°C or lower. A melting point of 200°C or lower reduces the energy required to melt the resin, simplifying the compounding process. The lower limit of the melting point is preferably -20°C. Setting the lower limit of the melting point to -20°C simplifies the resin compounding process.

[0022] Next, a method for producing the epoxy resin represented by formula (I) of the present invention will be described.

[0023] The epoxy resin represented by formula (I) of the present invention can be produced by reacting a diaminodiphenyl derivative represented by the following general formula (II) with epichlorohydrin.

[0024] [ka]

[0025] (In formula (II), R 1 ~R 4 Each of these independently represents one selected from the group consisting of a hydrogen atom, an aliphatic hydrocarbon group with 1 to 4 carbon atoms, and a halogen atom. In other words, similar to the general method for producing epoxy resins, the method for producing the epoxy resin represented by formula (I) consists of an addition step in which four molecules of epichlorohydrin are added to one molecule of diaminodiphenyl derivative to produce a dichlorohydrin compound represented by the general formula (III) below, followed by a cyclization step in which the dichlorohydrin compound is dehydrochlorinated with an alkaline compound to produce an epoxy compound represented by formula (I), which is a tetrafunctional epoxy.

[0026] [ka]

[0027] (In equation (III), R is independent of each other.) 1 ~R 4 (This represents one of the groups selected from those consisting of a hydrogen atom, an aliphatic hydrocarbon group with 1 to 4 carbon atoms, and a halogen atom.) The epoxy resin composition of the present invention comprises an epoxy resin represented by formula (I) and a curing agent as described above. Furthermore, the epoxy resin composition may optionally contain other epoxy resins besides the epoxy resin represented by formula (I), curing accelerators, inorganic fillers, and the like.

[0028] (Hardening agent) Examples of curing agents of the present invention include phenolic curing agents, aliphatic amines, polyetheramines, alicyclic amines, aromatic amines and other amine-based curing agents, acid anhydride-based curing agents, amide-based curing agents, tertiary amines, or imidazoles, but any structure is acceptable as long as it can cure epoxy resin.

[0029] Of these, aromatic amine-based curing agents or acid anhydride-based curing agents are preferred because they can produce cured products with excellent heat resistance, elastic modulus, and hygroscopic properties. The use of imidazoles is also preferred because they can provide excellent heat resistance and rapid curing properties.

[0030] The curing agent may be used alone or in combination of two or more types. When using two or more curing agents in combination, they may be pre-mixed to prepare a mixed curing agent before use, or each component of the curing agent may be added separately when mixing the components of the epoxy resin composition and then mixed simultaneously. <Phenol-based curing agent> Examples of phenolic curing agents include bisphenol A, bisphenol F, bisphenol S, hydroquinone, resorcinol, biphenol, tetramethylbiphenol, dihydroxynaphthalene, dihydroxydiphenyl ether, thiodiphenols, phenol novolac resin, cresol novolac resin, phenol aralkyl resin, biphenyl aralkyl resin, naphthol aralkyl resin, dicyclopentadienephenol resin, and naphthol novolac resin. <Amine-based curing agent> Aliphatic amines include ethylenediamine, 1,3-diaminopropane, 1,4-diaminopropane, hexamethylenediamine, 2,5-dimethylhexamethylenediamine, trimethylhexamethylenediamine, diethylenetriamine, iminobispropylamine, bis(hexamethylene)triamine, triethylenetetramine, and tetraethylenepentamine. Examples include methyl ester, pentaethylenehexamine, N-hydroxyethylethylenediamine, and tetra(hydroxyethyl)ethylenediamine.

[0031] Examples of polyetheramines include triethylene glycol diamine, tetraethylene glycol diamine, diethylene glycol bis(propylamine), polyoxypropylenediamine, and polyoxypropylene triamines.

[0032] Examples of alicyclic amines include isophoronediamine, metacenediamine, N-aminoethylpiperazine, bis(4-amino-3-methyldicyclohexyl)methane, bis(aminomethyl)cyclohexane, and norbornenediamine.

[0033] Aromatic amines include tetrachloro-p-xylenediamine, m-xylenediamine, p-xylenediamine, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, 2,4-diaminoanisole, 2,4-toluenediamine, 2,4-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 4,4'-diamino-1,2-diphenylethane, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, m-aminophenol, m-aminobenzylamine, benzyldimethylamine, 2-(dimethylaminomethyl)phenol, triethanolamine, methylbenzylamine, and diaminodiethyldimethyldiphenylmethane.

[0034] Examples of tertiary amines include diazabicycloundecene, triethylenediamine, benzyldimethylamine, triethanolamine, dimethylaminoethanol, and tris(dimethylaminomethyl)phenol. <Acid anhydride curing agent> Examples of acid anhydride-based curing agents include phthalic anhydride, trimellitic anhydride, pyromellitic anhydride, benzophenonetetracarboxylic anhydride, dodecenyl succinic anhydride, polyadipic anhydride, polyazelaic anhydride, polysebacic anhydride, poly(ethyloctadecanediic acid) anhydride, poly(phenylhexadecanedioic acid) anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, hexahydrophthalic anhydride, methylhymic anhydride, trialkyltetrahydrophthalic anhydride, methylcyclohexenedicarboxylic acid anhydride, methylcyclohexenetetracarboxylic acid anhydride, nadic anhydride, and methylnadic anhydride. <Amid-based hardener> Examples of amide-based curing agents include dicyandiamide or its derivatives, and polyamide resins. <Imidazoles> Examples of imidazoles include 2-phenylimidazole, 2-ethyl-4(5)-methylimidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 1-benzyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyano-2-phenylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, and 2-phenyl-4-methyl-5-hydroxymethylimidazole. <Curing catalyst> The epoxy resin composition of the present invention may contain a curing catalyst. Examples of curing catalysts include urea-based curing catalysts, hydrazide-based curing catalysts, tertiary amines, imidazoles, and phenols. Commercial curing catalysts include DCMU99 (manufactured by Hodogaya Chemical Co., Ltd.), “Omicure®” U-24M, U-52M (manufactured by CVC Thermoset Specialties), UDH-J (manufactured by Ajinomoto Fine Techno Co., Ltd.), CDH, MDH, SUDH, ADH, SDH (manufactured by Nippon Finechem Co., Ltd.), “DDH-S, IDH-S” (manufactured by Otsuka Chemical Co., Ltd.), “Kaolizer®” No. 20 (manufactured by Kao Corporation), and “Curesol®” 1,2DMZ, C11Z, C17Z (manufactured by Shikoku Chemicals Co., Ltd.).

[0035] The curing catalyst is preferably present in an amount of 0.1 to 15 parts by mass relative to the total volume of the epoxy resin composition. This range is preferable because it allows for both accelerated curing and extended pot life.

[0036] (Component [A]) The epoxy resin composition of the present invention preferably contains 5 to 50 parts by mass of a biomass-derived bisphenol-type epoxy resin having an average epoxy equivalent of 150 to 220 g / eq as component [A], when the total amount of epoxy resin is 100 parts by mass.

[0037] Component [A] of the present invention is preferably a biomass-derived bisphenol-type epoxy resin made from epichlorohydrin produced from a bifunctional glycol derived from vegetable oil, and preferably such epoxy resin has an average epoxy equivalent of 150 to 220 g / eq. With such an average epoxy equivalent, it is easy to obtain an epoxy resin composition in which the glass transition temperature of the cured product is 150°C or higher. Furthermore, an average epoxy equivalent of 160 to 180 g / eq is preferable, as this range allows for a resin composition with a high biomass content, and the cured product exhibits even better flexural modulus and flexural strength. Component [A] is preferable because, being biomass-derived, it can reduce greenhouse gas emissions.

[0038] Component [A] is preferably present in an amount of 5 to 50 parts by mass in 100 parts by mass of the total epoxy resin contained in the resin composition according to the present invention. By including such a predetermined amount of epoxy resin as component [A], an epoxy resin composition with an excellent balance of depolymerization properties and heat resistance can be obtained.

[0039] Commercially available biomass-derived bisphenol-type epoxy resins for component [A] include "Briozen®" YD-128G (biomass content: 28%) and "Briozen®" YD-127G (biomass content: 28%), both manufactured by Aditya Birla Chemicals, which are bisphenol A type epoxy resins, and "Briozen®" YDF-170LCG (biomass content: 30%), manufactured by Aditya Birla Chemicals, which is a bisphenol F type epoxy resin. <Other epoxy resins> Other epoxy resins of the present invention include, as bifunctional epoxy resins, bisphenol-type epoxy resins such as bisphenol A-type epoxy resin, bisphenol F-type epoxy resin, or bisphenol S-type epoxy resin, biphenyl-type epoxy resin, naphthalene-type epoxy resin, epoxy resin having a fluorene skeleton, polypropylene glycol-type epoxy resin, polyethylene glycol-type epoxy resin, and long-chain aliphatic epoxy resin.

[0040] Examples of trifunctional or tetrafunctional epoxy resins include naphthalene-type epoxy resins, epoxy resins having a fluorene skeleton, and glycidylamine-type epoxy resins such as tetraglycidyldiaminodiphenylmethane, tetraglycidylxylenediamine, triglycidylaminophenol, and triglycidylaminocresol. In particular, N,N,N′,N′-tetraglycidyldiaminodiphenylmethane, triglycidylaminophenol, and triglycidylaminocresol are preferred because they are liquid at 23°C and yield epoxy resin compositions with excellent heat resistance. These may be used individually or in combination.

[0041] (Component [B]) The epoxy resin composition of the present invention preferably further contains polyrotaxane as component [B]. Component [B] is preferably present in an amount of 1 to 25 parts by mass, and more preferably 5 to 15 parts by mass, per 100 parts by mass of the total epoxy resin. By adjusting the amount of component [B] to 1 to 25 parts by mass per 100 parts by mass of the total epoxy resin, the depolymerizability can be improved. Furthermore, adjusting the amount to 5 to 15 parts by mass is preferable because the cured product of the epoxy resin composition exhibits an excellent balance between depolymerizability and heat resistance.

[0042] Component [B] is a compound having a structure in which a chain-like polymer penetrates the opening of a cyclic molecule, and sealing groups are bonded to both ends of the chain-like polymer to prevent the cyclic molecule from falling off. Therefore, it is preferable as it has an excellent balance of depolymerization properties and heat resistance in epoxy resin compositions.

[0043] Examples of chain polymers constituting polyrotaxanes include molecules that can penetrate the rings of multiple cyclic molecules. Examples of chain polymers include polyethylene glycol, polyethylene oxide, polypropylene glycol, polylactic acid, polycaprolactone, polyethylene, polypropylene, polyvinyl acetal, polyvinyl methyl ether, polyvinylpyrrolidone, polyacrylamide, polymethyl polyacrylate, polymethyl methacrylate, or polystyrene. Chain polymers may have branched chains as long as they are configured to penetrate the rings of the above-mentioned cyclic molecules, and the polymers exemplified above may have branched chains.

[0044] It is preferable that both ends of the chain polymer have reactive groups that can react with a chelating group to prevent the detachment of cyclic molecules from the chain polymer. Examples of reactive groups include amino groups, hydroxyl groups, carboxyl groups, thiol groups, disulfide groups, vinyl groups, acryloyl groups, methacryloyl groups, and sulfo groups. Among these, amino groups or carboxyl groups are preferred, as they facilitate the bonding of the chelating groups, as exemplified later, to both ends of the chain polymer.

[0045] The chelating group bonded to the chain polymer in the polyrotaxane component [B] of the present invention is not particularly limited, as long as it is a group that acts to prevent the detachment of cyclic molecules from the chain polymer. Examples of chelating groups include dinitrophenyl, cyclodextrin, adamantane, trityl, fluorescein, silsesquioxane, pyrene, alkyl-substituted benzene, or steroid. Among these, adamantane or cyclodextrin are preferred, and a cured epoxy resin composition with excellent depolymerization properties can be obtained.

[0046] Examples of cyclic molecules constituting the polyrotaxane of component [B] of the present invention include α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin, and their derivatives, as well as crown ethers, benzocrowns, dibenzocrowns, or dicyclohexanocrowns, and their derivatives. The cyclic molecules may further have reactive groups. Examples of such reactive groups include hydroxyl groups, carboxyl groups, amino groups, epoxy groups, isocyanate groups, thiol groups, and aldehyde groups.

[0047] The inclusion rate of the polyrotaxane in component [B] of the present invention is the mass ratio of the amount of cyclic molecules actually inclusioning the chain polymer to the maximum amount of cyclic molecules inclusioning the chain polymer. The inclusion rate can be determined by nuclear magnetic resonance spectroscopy (NMR). The inclusion rate of the polyrotaxane in component [B] is preferably 0.1 to 35% by mass, and more preferably 5 to 35% by mass. Setting the inclusion rate to 0.1 to 35% by mass imparts regenerative moldability to the epoxy resin composition, and setting it to 5 to 35% by mass improves the depolymerization properties of the epoxy resin composition.

[0048] Examples of commercially available polyrotaxanes for component [B] include “SeRM®” SH3400P (polycaprolactone-modified polyrotaxane, inclusion rate 28% by mass), “SeRM®” PR02 (inclusion rate 2% by mass), “SeRM®” SH2400P (inclusion rate 28% by mass), and “SeRM®” SH1300P (inclusion rate 28% by mass) (all manufactured by ASM).

[0049] By combining the epoxy resin represented by formula (I) of the present invention with the polyrotaxane component [B], an epoxy resin composition with an excellent balance of depolymerization properties and heat resistance in the cured product can be obtained due to the synergistic effect of the disulfide bonds, which are dynamic covalent bonds contained in the epoxy resin, and the molecular pulley effect of the polyrotaxane.

[0050] The epoxy resin composition of the present invention may contain thermoplastic resins, rubber particles, inorganic particles such as silica, nanoparticles such as CNTs and graphene, etc., to adjust viscoelasticity and improve the tack and drape properties of the prepreg, or to enhance the mechanical properties and toughness of the resin composition, as long as the effects of the present invention are not lost. Examples of thermoplastic resins soluble in epoxy resins include polyvinyl acetal resins such as polyvinyl formal and polyvinyl butyral, polyvinyl alcohol, phenoxy resin, polyamide, polyimide, polyvinylpyrrolidone, polysulfone, and polyethersulfone. Examples of rubber particles include crosslinked rubber particles and core-shell rubber particles obtained by graft polymerization of a different polymer onto the surface of crosslinked rubber particles. Furthermore, boric acid esters, etc., may be included for purposes such as improving the storage stability of the resin composition.

[0051] The epoxy resin composition of the present invention may be prepared by kneading using machinery such as a kneader, planetary mixer, three-roll and twin-screw extruder, or by hand using a beaker and spatula, provided that uniform kneading is possible.

[0052] (Cured epoxy resin) The epoxy resin cured product of the present invention is obtained by curing the above epoxy resin composition with heat, light, moisture, etc. In particular, thermal curing at 10°C to 300°C is preferred because it provides excellent mechanical properties.

[0053] (Reinforced fiber) The prepreg of the present invention comprises the epoxy resin composition described above and at least one reinforcing fiber selected from the group consisting of carbon fiber, glass fiber, and aramid fiber. Specifically, the epoxy resin composition is impregnated into the reinforcing fiber. These reinforcing fibers may be used individually or in combination of two or more types. Among these, from the viewpoint of weight reduction, carbon fibers such as polyacrylonitrile (PAN), pitch, and rayon, which have excellent specific strength and specific rigidity, are preferably used.

[0054] Among these, PAN-based carbon fibers, which have excellent mechanical properties such as strength and elastic modulus, can be used more preferably.

[0055] The reinforcing fibers contained in the prepreg of the present invention may be in the form of a woven fabric. If it is a woven fabric, the weave structure is preferably plain weave, twill weave, satin weave, etc. Plain weave, twill weave, satin weave, etc. are preferable because they provide excellent handling as a sheet-like prepreg and excellent shape conformability during lamination, making it easy to form complex-shaped components. Here, excellent shape conformability during lamination means that in the process of laminating the reinforcing fiber woven fabric, etc., onto a three-dimensional mold, the fiber orientation does not collapse and the material can conform to the shape, resulting in a molded product that is satisfactory in terms of quality and performance. In the process of laminating onto a three-dimensional mold, a preform is created using the reinforcing fiber woven fabric, etc., but if the material does not have excellent shape conformability during lamination, a trimming process is required in which parts of the base material that do not conform to the shape of the mold are cut with scissors or a cutter, or parts that will not be used in the final product are fixed with tape. The trimming process consumes a lot of time and effort, causes material loss and increases industrial waste, but this can be reduced by using woven fabric.

[0056] The reinforcing fibers contained in the prepreg of the present invention may be discontinuous fibers. By using discontinuous fibers for the reinforcing fibers, it becomes easier to mold the sheet-like prepreg into complex shapes when external force is applied to form it.

[0057] Furthermore, if the reinforcing fibers are discontinuous, it is preferable that they are randomly dispersed in bundles within the prepreg. This facilitates the molding of complex shapes when external force is applied to the prepreg during the molding process.

[0058] Alternatively, if the reinforcing fibers are discontinuous, it is preferable that they are randomly dispersed in a substantially monofilamental form within the prepreg. By randomly dispersing the discontinuous reinforcing fibers in a substantially monofilamental form, the number of reinforcing fibers existing as fiber bundles in the prepreg is reduced, thereby minimizing the weak points at the fiber bundle ends of the reinforcing fibers and providing excellent reinforcement efficiency and isotropy. Here, substantially monofilamental means that the reinforcing fiber single filaments exist in fine-denier strands of less than 500 fibers. It is more preferable that the reinforcing fibers are dispersed in a monofilamental form, i.e., as single filaments, and it is even more preferable that the monofilamental single fibers are randomly dispersed. If the reinforcing fibers are discontinuous, the reinforcing fibers may also be in a nonwoven fabric form.

[0059] In this invention, the term "a state in which fibers or fiber bundles are randomly dispersed" refers to a state in which fibers or fiber bundles are dispersed without any regularity.

[0060] (Prepreg, fiber-reinforced composite material) One method for producing a prepreg is to impregnate a reinforcing fiber substrate with the epoxy resin composition of the present invention. Examples of impregnation methods include the hot melt method (dry method). In this process, the fiber mass content of the prepreg can be adjusted by changing the amount of resin applied to the release paper.

[0061] Methods for forming the prepreg include, for example, press forming, autoclave forming, bagging forming, wrapping tape forming, and internal pressure forming, which can be used as appropriate.

[0062] Next, the fiber-reinforced composite material of the present invention will be described.

[0063] One aspect of the present invention is a fiber-reinforced composite material that uses a cured epoxy resin composition of the present invention (hereinafter referred to as "the epoxy resin cured product of the present invention") as the matrix resin, and is typically obtained by curing the prepreg of the present invention described above. More specifically, a fiber-reinforced composite material containing the epoxy resin cured product of the present invention as the matrix resin can be obtained by laminating a prepreg having the epoxy resin composition of the present invention as the matrix resin as necessary, and then heating and curing it.

[0064] In this invention, depolymerization refers to the entire process in which a polymerized substance is broken down by external stimuli or other factors, and this process includes the process in which the target substance decomposes and eventually becomes a monomer. In the initial stage of depolymerization of an epoxy resin cured product, only the surface of the cured product is swollen or only the surface is slightly dissolved. In this state, if the epoxy resin cured product is bonded to a different material such as metal, separation and dismantling become easier, and recyclability is granted. As depolymerization progresses further, the target substance becomes plasticized throughout its entire thickness and becomes rubbery. In this state, the target substance can be treated as a plastic resin and can be recycled through remodeling, etc. In the final stage of depolymerization, the target substance is significantly dissolved and no longer retains its original form. Note that even if the target substance appears to retain its original form when depolymerization is performed in a static state, if it collapses when moved and cannot maintain its shape, it is considered to have lost its original form. In such a state, if it is a fiber-reinforced composite material, the reinforcing fibers contained inside can be recovered as recycled fibers (resources).

[0065] Methods for depolymerizing and plasticizing the epoxy resin cured product of the fiber-reinforced composite material of the present invention by heat treatment, treatment with a dissolving solution, or chemical treatment include thermal decomposition, supercritical water treatment, subcritical water treatment, superheated steam treatment, liquid phase decomposition, dissolution, and glycol treatment. From the viewpoint of easy depolymerization by cleaving disulfide bonds, dissolution with a dissolving solution is preferred, and from the viewpoint of eliminating the need for neutralization of the dissolving solution and washing of recovered fibers, superheated steam treatment is preferred. The dissolving solution of the present invention is not particularly limited as long as it can dissolve the epoxy resin cured product of the present invention, but for example, it includes at least one liquid selected from acidic solutions, organic solvents, hydrogen peroxide solution, and ionic liquids. These liquids can dissolve or plasticize the epoxy resin cured product of the present invention, and can efficiently depolymerize the target cured product.

[0066] Examples of acidic solutions used as dissolving solutions for cured epoxy resins include phosphoric acid, sulfuric acid, hydrochloric acid, and nitric acid.

[0067] Examples of organic solvents used as the above-mentioned dissolving solution include aliphatic hydrocarbon solvents, aromatic hydrocarbon solvents, alcoholic solvents, ketone solvents, etheric solvents, amide solvents, or esteric solvents, and N-methyl-2-pyrrolidone (NMP) having a lactam structure. Among these, NMP is preferred because it is highly water-soluble and allows for washing of the recovered fibers with water. Examples of aliphatic hydrocarbon solvents include pentane, hexane, heptane, or octane and glycol. Examples of aromatic hydrocarbon solvents include benzene, toluene, or xylene and tetralin. An example of an alcoholic solvent is benzyl alcohol. Examples of ketone solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, and diacetone alcohol. Among these, acetone and methyl ethyl ketone are preferred because they are highly volatile and allow for easy drying of the recovered fibers. The organic solvent may contain a decomposition catalyst. Examples of decomposition catalysts include iron oxide and alkali metal compounds.

[0068] Examples of ionic liquids used as the above-mentioned dissolving solution include ionic liquids containing at least one cation selected from imidazolium, pyridinium, pyrrolidinium, quaternary ammonium, and quaternary phosphonium systems.

[0069] In any of the above cases, one type of solvent may be used alone, or two or more types of solvents may be used in combination.

[0070] The recycling method according to the present invention involves performing steps (i) to (iii) using the fiber-reinforced composite material of the present invention. (i) The fiber-reinforced composite material is processed into pellets with reinforcing fibers having a length of 2 to 100 mm. (ii) The epoxy resin curing product contained in the pelletized fiber-reinforced composite material obtained in (i) is depolymerized by heat treatment, treatment with a solvent, or chemical treatment. (iii)(ii) The pelletized fiber-reinforced composite material, from which the epoxy resin cured product has been depolymerized, is randomly dispersed and heated and pressed.

[0071] Here, when performing the heating press in (iii), it is preferable to do so under conditions of 100 to 290°C and 0.7 to 15 MPa.

[0072] By performing steps (i) to (iii), it becomes possible to recycle the fiber-reinforced composite material without depolymerizing the matrix resin until it is separated from the reinforcing fibers, which is preferable because it significantly reduces greenhouse gas emissions when recycling the fiber-reinforced composite material. In other words, the method for recycling fiber-reinforced composite materials of the present invention is preferable because it has a low environmental impact during manufacturing.

[0073] The process of (i) processing the fiber-reinforced composite material of the present invention into pellets, with the fiber length of the reinforcing fibers contained therein being 2 to 100 mm, can be carried out using a hammer mill, sand grind mill type wet mill, pelletizer, high-speed rotor type mill, rotary shear type mill, ball mill mill, etc. If pellets with all fibers being 2 to 100 mm in length are obtained from the beginning, the process of (i) can be omitted.

[0074] Conventional recycling methods for fiber-reinforced composite materials often process long fibers into short fibers less than 2 mm in length. However, it is preferable for the reinforcing fibers in recycled composites to have a fiber length of 3 to 20 mm, as this improves fiber dispersion and allows for the creation of a highly rigid composite. Therefore, the application of the present invention is preferable. From another perspective, it is also preferable for such fiber lengths to be 60 to 100 mm, as this simplifies the processing steps and reduces greenhouse gas emissions. By using recycled materials containing such fiber lengths as composite products and then repeatedly recycling them after further processing to a middle range with fiber lengths of 50 mm or less, greenhouse gas emissions can be significantly reduced.

[0075] The fiber-reinforced composite material described above, which is one aspect of the present invention, is preferably used in automotive, sports, unmanned aerial vehicle, commercial aircraft, and general industrial applications. More specifically, automotive components include seats, hoods, roofs, door panels, trunk lids, spoilers, pillars, battery cases, monocoques, and interior materials. Sports components include golf shafts, fishing rods, tennis and badminton rackets, hockey sticks, and ski poles. More specifically, general industrial components include structural materials for bicycles, wind turbines, ships, and railway vehicles, as well as electronic equipment components including housings for information devices such as IC trays and laptop computers. Furthermore, for unmanned aerial vehicle and commercial aircraft applications, more specifically, primary structural components of aircraft including wings, tail wings, and floor beams, and secondary structural components including interior materials are examples.

[0076] The upper and lower limits of the numerical ranges described above can be combined in any way. [Examples]

[0077] The present invention will be further described with reference to the following examples, but the present invention is not limited to the descriptions in these examples.

[0078] 1. Materials used <Epoxy resin> The epoxy resins used in this example and comparative example are listed below. In Tables 1 and 2, epoxy equivalent weight is abbreviated as "EEW".

[0079] [Epoxy resin of formula (I)] • Tetraglycidyl-4,4'-diaminodiphenyl disulfide (tetrafunctional disulfide-type epoxy resin, average epoxy equivalent: 118) • Tetraglycidyl-2,2'-diaminodiphenyl disulfide (tetrafunctional disulfide-type epoxy resin, average epoxy equivalent: 122) [Epoxy resin of component [A]] • “Briozen®” YD-128G (Biomass-derived bisphenol A type epoxy resin, average epoxy equivalent: 190, biomass content: 28%, manufactured by Aditya Birla Chemicals) • “Briozen®” YDF-170LCG (Biomass-derived bisphenol F type epoxy resin, average epoxy equivalent: 170, biomass content: 30%) [Other epoxy resins] • Tetraglycidyl-4,4'-diaminodiphenylthioether (tetrafunctional disulfide epoxy resin, average epoxy equivalent: 10⁸) • “jER(registered trademark)” 825 (Bisphenol A type epoxy resin, average epoxy equivalent: 175, manufactured by Mitsubishi Chemical Corporation) [Component [B]] • “SeRM (registered trademark)” SH2400P (polyrotaxane, inclusion rate 28%) • “SeRM (registered trademark)” SH1300P (polyrotaxane, inclusion rate 28%) [Hardening agent] • 4,4'-Diaminodiphenylsulfone (manufactured by Tokyo Chemical Industry Co., Ltd.) 2. How to create a sample [Method for preparing epoxy resin compositions] In a stainless steel beaker, predetermined amounts of epoxy resin component and component [B] were placed, the temperature was raised to 40-160°C, and the mixture was kneaded as needed until each component became miscible.

[0080] The prepared mixture was cooled to 60°C, a curing agent was added, and the mixture was kneaded at 60°C for 30 minutes to obtain an epoxy resin composition.

[0081] [Method for producing cured resin products] The epoxy resin composition obtained by the above [Method for Preparing Epoxy Resin Composition] was degassed in a vacuum and then injected into a mold set to a thickness of 1 mm or 2 mm using a Teflon® spacer. Next, the mold was placed in a hot air oven and the temperature was raised from 30°C to 180°C at a rate of 1.0°C per minute, and then held at 180°C for 120 minutes to cure the epoxy resin composition. Subsequently, the temperature was lowered to 30°C and the mold was demolded to produce cured resin products with thicknesses of 1 mm and 2 mm.

[0082] [Method for producing fiber-reinforced composite materials] The epoxy resin composition prepared according to the above [Method for preparing epoxy resin composition] is formed into a sheet of carbon fiber "Torayca®" T700S-12K-60E (manufactured by Toray Industries, Inc., basis weight 150g / m²) by aligning the epoxy resin composition in one direction. 2 The material was impregnated to obtain a prepreg. The obtained prepreg was cut into strips of 200 mm lengthwise (in the orientation direction of the carbon fibers) and 200 mm widthwise. Five strips were stacked so that the fiber directions were perpendicular between adjacent prepregs, and then the stacks were placed in a mold and pressed at 1.0 MPa in a press heated to 180°C for 120 minutes to allow for heat curing. After that, the mold was removed from the press and cooled to 30°C on a metal plate to obtain a fiber-reinforced composite material.

[0083] Vf (in %) refers to the volume ratio occupied by reinforcing fibers in a fiber-reinforced composite material, and is specifically calculated using the following formula. Vf = (W × 100) / (ρ × T) W: Reinforced fiber base material 1cm 2 Mass of reinforcing fibers per unit area (g / cm³) 2 ) ρ: Density of reinforcing fibers (g / cm³) 3 ) T: Thickness of fiber-reinforced composite material (cm).

[0084] 3. Evaluation Method [Method for measuring glass transition temperature (Tg)] From the 1 mm thick resin cured product obtained by the above [Method for Preparing Resin Cured Products], 5 mg of the sample was weighed into a sample pan, and measurements were performed using a differential scanning calorimetry device Q-2500 (manufactured by TA Instruments Co., Ltd.) in accordance with JIS-K7121 (1999). Specifically, the resin cured product obtained under the above conditions was heated from 25°C to 300°C at a heating rate of 10°C / min, held at 300°C for 3 minutes, and then rapidly cooled to 25°C at a cooling rate of 30°C / min. In the differential scanning calorimetry chart of the above heating process (with thermal energy on the vertical axis and temperature on the horizontal axis), the point where the straight line equidistant in the vertical direction from the straight line extending from each baseline showing the state before and after the step-like change associated with the glass transition intersects with the curve of the step-like change portion of the glass transition is the glass transition temperature.

[0085] [Method for measuring three-point bending of cured resin products] The 2mm thick resin cured material obtained by the above [Method for Preparing Resin Cured Material] was cut into 10mm wide x 60mm long pieces to create five test specimens. Next, a three-point bending test was performed in accordance with JIS-K7171 (2006) using an Instron 5565 universal testing machine (manufactured by Instron) at a crosshead speed of 2.5mm / min, and the bending strength and bending modulus were measured.

[0086] [Method for evaluating the depolymerization properties of cured epoxy resins] Using the 1 mm thick resin cured material obtained by the above [Method for Preparing Cured Resin], three test pieces were processed into 15 mm wide and 15 mm long squares. These were placed in stainless steel beakers containing 65 g of NMP solution, and the depolymerization properties on NMP / day 7 were determined according to the following criteria and recorded in the Depolymerization Test column of the table. During this time, each stainless steel beaker was covered and stored at 23°C for 7 days. No change in the cured epoxy resin...N The epoxy resin curing material has dissolved significantly and is no longer recognizable...A It has become plasticized and rubbery throughout its entire thickness...B Only the surface is swollen or slightly dissolved...C. [Method for calculating the biomass content (%) in total epoxy resin] The biomass content of each epoxy resin was calculated by comparing the number of organic carbon atoms in the molecule, in accordance with ASTM D6866 (2024). For example, bisphenol A type epoxy resin has 43 organic carbon atoms. When this is substituted with epichlorohydrin derived from vegetable oil, the total number of organic carbon atoms in the glycidyl groups is 12. Therefore, the biomass content is approximately 28% (12 divided by 43). Next, the biomass content of the total epoxy resin was calculated by taking the total epoxy resin as 100 parts by mass and dividing the amount of biomass-derived epoxy resin (parts by mass) by the biomass content (%), and this is listed as the biomass content in the table.

[0087] (Example 1) An epoxy resin composition was prepared according to the above [Method for preparing epoxy resin composition] using 70 parts by mass of tetraglycidyl-4,4'-diaminodiphenyl disulfide, which corresponds to the epoxy resin of formula (I), 30 parts by mass of “jER(registered trademark)” 825 as another epoxy resin, and 47 parts by mass of 4,4'-diaminodiphenyl sulfone as a curing agent.

[0088] Using this epoxy resin composition, an epoxy resin cured product was prepared according to the [Method for Preparing Epoxy Resin Cured Products] and measured according to the [Method for Measuring Glass Transition Temperature Tg]. The glass transition temperature was 195°C, indicating good heat resistance.

[0089] Furthermore, the bending strength measured according to the [three-point bending measurement method for cured resin] was 142 [MPa] and the bending modulus was 3.2 [GPa], indicating good strength and rigidity.

[0090] The depolymerization of NMP / day 7, evaluated according to the [Method for Evaluating the Depolymerization Properties of Cured Epoxy Resin], was rated C, indicating that only the surface of the cured material was slightly dissolved, demonstrating good depolymerization properties.

[0091] Since no biomass-derived epoxy resin was used, the biomass content in the total epoxy resin composition was 0%.

[0092] (Examples 2-7) Except for changing the resin composition as shown in Table 1, epoxy resin compositions and epoxy resin cured products were prepared using the same method as in Example 1. The evaluation results are shown in Table 1.

[0093] [Table 1]

[0094] (Example 8) The epoxy resin composition obtained in Example 1 was impregnated into a sheet of carbon fiber "Torayca®" T700S-12K-60E, which was aligned in one direction, resulting in a carbon fiber basis weight of 189 g / m². 2 A prepreg was obtained. The obtained prepreg had good tack and excellent handling properties. The fiber volume content Vf of the fiber-reinforced composite material obtained according to the [Method for producing fiber-reinforced composite material] was 69%, and the quality was good.

[0095] (Example 9) The prepreg obtained in Example 8 could be used to form a roof-shaped mold for an automobile. However, the shape-following properties were not always good, and complex trimming was required.

[0096] (Example 10) The epoxy resin composition obtained in Example 1 was mixed with carbon fiber "Torayca®" T700S-12K-60E at a basis weight of 189 g / m². 2 A base material woven in a plain weave was impregnated in such a manner to obtain a woven prepreg. When the obtained woven prepreg was used to shape a roof mold for an automobile, it exhibited excellent shape conformability and did not require trimming.

[0097] (Example 11) When carbon fiber "Torayca (registered trademark)" T700S-12K-60E was cut to a length of 12 mm and randomly dispersed in bundles, the basis weight was 152 g / m². 2 A nonwoven fabric was obtained. The obtained nonwoven fabric was impregnated with the epoxy resin composition obtained in Example 1 to obtain a nonwoven fabric prepreg. When the obtained nonwoven fabric prepreg was used to form a roof shape mold for an automobile, it showed excellent shape conformability and did not require trimming.

[0098] (Example 12) When the fiber-reinforced composite material obtained in Example 8 was pulverized using a twin-shaft mill, pellets with reinforcing fibers ranging in length from 5 mm to 100 mm were obtained. When the obtained pellets were treated with superheated steam at 300°C for 30 minutes, the epoxy resin cured product of the fiber-reinforced composite material depolymerized and became plasticized. When the plasticized pellets were randomly dispersed and heated and pressed at 180°C and 10 MPa, a recycled molded plate was obtained.

[0099] (Comparative Example 1) Except for using 70 parts by mass of tetraglycidyl-4,4'-diaminodiphenylthioether (which does not contain disulfide bonds) and 30 parts by mass of "jER(registered trademark)" 825 as the epoxy resin, the epoxy resin composition was as shown in Table 2, and the epoxy resin composition was prepared and the epoxy resin cured product was made in the same manner as in Example 1. The evaluation results are shown in Table 2. The heat resistance, flexural strength, and flexural modulus of the obtained resin cured product were good, but the depolymerization properties were insufficient.

[0100] [Table 2]

Claims

1. An epoxy resin represented by the following formula (I), with an average epoxy equivalent of 90 to 350 g / eq. 【Chemistry 1】 (In formula (I), R １ ~R ４ Each of these independently represents one selected from the group consisting of a hydrogen atom, an aliphatic hydrocarbon group having 1 to 4 carbon atoms, and a halogen atom.

2. An epoxy resin composition comprising the epoxy resin and curing agent described in claim 1.

3. Furthermore, the epoxy resin composition according to claim 2, comprising 5 to 50 parts by mass of a biomass-derived bisphenol-type epoxy resin having an average epoxy equivalent of 150 to 220 as component [A], when the total amount of epoxy resin is 100 parts by mass.

4. Furthermore, the epoxy resin composition according to claim 2, comprising polyrotaxane as component [B].

5. An epoxy resin cured product obtained by curing the epoxy resin composition according to any one of claims 2 to 4.

6. A prepreg comprising an epoxy resin composition according to any one of claims 2 to 4 and at least one reinforcing fiber selected from the group consisting of carbon fiber, glass fiber, and aramid fiber.

7. The prepreg according to claim 6, wherein the form of the reinforcing fibers is a woven fabric or discontinuous fibers.

8. The prepreg according to claim 7, wherein the form of the reinforcing fibers is discontinuous fibers, randomly dispersed in bundles.

9. The prepreg according to claim 7, wherein the form of the reinforcing fibers is discontinuous, substantially monofilamental, and randomly dispersed.

10. A fiber-reinforced composite material obtained by curing the prepreg described in claim 6.

11. Automotive component using the fiber-reinforced composite material according to claim 10.

12. A sports component using the fiber-reinforced composite material described in claim 10.

13. A component for use in unmanned aerial vehicles or civilian aircraft, using the fiber-reinforced composite material according to claim 10.

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