Epoxy resin composition, cured product thereof, prepreg, and fiber-reinforced composite material

Abstract

To provide an epoxy resin composition which has low viscosity, excellent long-term viscosity stability, and excellent short-time curability and also has a high elastic modulus when cured, and to provide a cured product thereof.SOLUTION: The epoxy resin composition is used as a matrix resin of a fiber-reinforced composite material and contains an epoxy resin (A), a cyclic compound (B) having hydroxyl groups, and an acid generator (C) as a photopolymerization initiator for curing the epoxy resin by a cationic polymerization reaction. The epoxy resin (A) is an alicyclic epoxy compound, and the cyclic compound (B) has two or more hydroxyl groups.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a curable epoxy resin composition that has low viscosity, excellent long-term viscosity stability, excellent short-term curing properties, and a high elastic modulus upon curing, and a cured product thereof; a prepreg obtained by impregnating or coating reinforcing fibers with the curable epoxy resin composition; and a fiber-reinforced composite material obtained by curing the prepreg. [Background technology]

[0002] Curable epoxy resin compositions have been widely used in a wide variety of applications, including adhesives, structural materials, composite materials (e.g., CFRP), coating agents, sealants, transparent materials, optical materials, and electronic materials. In recent years, vigorous development has been underway for fiber-reinforced composite materials, which are obtained by further reinforcing a cured product of a curable epoxy resin composition with reinforcing fibers such as carbon fiber. Fiber-reinforced composite materials generally consist of reinforcing fibers such as glass fiber, aramid fiber, or carbon fiber and a thermosetting matrix resin such as unsaturated polyester resin, vinyl ester resin, epoxy resin, phenolic resin, benzoxazine resin, cyanate resin, or bismaleimide resin. Because of their light weight and excellent mechanical properties, including strength, corrosion resistance, and fatigue resistance, they are widely used as structural materials for aircraft, automobiles, civil engineering and construction products, and sporting goods.

[0003] As mentioned above, one of the characteristics of fiber-reinforced composite materials is their excellent strength, and this high strength is largely due to the high elastic modulus of the matrix resin that constitutes the fiber-reinforced composite material. The reason for this is that the resin with a high elastic modulus fills and bonds the spaces between the fiber filaments, transmitting stress to adjacent threads through the resin, allowing many fiber filaments to effectively bear the load and thereby achieving material strength. Therefore, it is believed that by increasing the elastic modulus of the matrix resin, fiber-reinforced composite materials can be made even stronger.

[0004] Furthermore, methods for producing fiber-reinforced composite materials include autoclave molding or press molding using prepregs in which reinforcing fibers have been previously impregnated with a thermosetting matrix resin, resin transfer molding, liquid compression molding, wet layup molding, pultrusion molding, and filament winding molding, which include a step of impregnating reinforcing fibers with a liquid matrix resin and a molding step by thermosetting.

[0005] Among these, the pultrusion molding method is a molding method in which reinforcing fibers are continuously passed through an impregnation tank filled with a liquid thermosetting resin composition, impregnated with the thermosetting resin composition, and molded and cured while being continuously pulled out by a pulling machine through a squeeze die and a heated mold, and has the advantage of being able to continuously mold a fiber-reinforced composite material.

[0006] In pultrusion molding, the resin composition must have a sufficiently low viscosity so that it can be quickly impregnated into the reinforcing fibers in the impregnation tank, and viscosity stability is important from the viewpoint of long-term continuous productivity. In addition, to enable high-speed production, it is also necessary to achieve fast curing properties so that the resin can be quickly cured in the mold.

[0007] A known resin composition for pultrusion is one that consists of a bisphenol A epoxy resin, an acid anhydride as a curing agent, and an imidazole-based curing accelerator (Patent Document 1). However, because the epoxy resin has a high viscosity, the viscosity of the resin composition increases, which reduces impregnation of the reinforcing fibers, making it difficult to achieve mechanical properties and resulting in poor surface quality of the molded product.

[0008] A pultrusion molding material containing, as main components, an epoxy resin made of tetraglycidyl meta-xylylenediamine and / or tetraglycidyl-1,3-bisaminomethylcyclohexane, a curing agent containing a dicarboxylic acid anhydride, and reinforcing fibers is also known (Patent Document 2). However, although the epoxy resin used has low viscosity, it is multifunctional and therefore highly reactive, resulting in poor viscosity stability and reduced continuous productivity.

[0009] An epoxy resin composition for pultrusion containing bisphenol F-type epoxy resin, acid anhydride, and imidazole derivative is also disclosed (Patent Document 3). However, although attention is focused on adhesion to fibers and mechanical properties, no improvement in functionality due to particularly high elasticity is demonstrated.

[0010] An epoxy resin composition containing an alicyclic epoxy compound, a lactone adduct compound or polycarbonate polyol having a hydroxyl group, and a phenolic resin is also disclosed (Patent Document 4). However, with regard to mechanical properties, the focus is on high tensile elongation at break, and high elasticity is not claimed.

[0011] A resin composition containing an alkali-soluble polymer having a phenolic hydroxyl group, an epoxy resin, and a cationic photopolymerization initiator that is a sulfonium salt has also been proposed (Patent Document 5). However, the alkali-soluble polymer is essential, and the purpose of this composition is to improve pattern accuracy as a permanent resist for printed wiring boards, and it does not address the requirements for fiber-reinforced composite material applications. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-343112 [Patent Document 2] Japanese Patent Application Publication No. 5-117412 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-038082 [Patent Document 4] International Publication No. 2014 / 126253 [Patent Document 5] Japanese Patent Application Publication No. 10-97068 Summary of the Invention [Problem to be solved by the invention]

[0013] Accordingly, an object of the present invention is to provide an epoxy resin composition which can be used as a matrix resin for fiber-reinforced composite materials, has low viscosity and excellent long-term viscosity stability, cures quickly, and has a high elastic modulus when cured, and a cured product thereof. [Means for solving the problem]

[0014] As a result of extensive research conducted by the present inventors to achieve the above object, they have found that an epoxy resin composition comprising an alicyclic epoxy resin as a base, a specific compound having a hydroxyl group, and an acid generator can form a cured product that has low viscosity, excellent long-term viscosity stability, excellent short-term curing ability, and a high elastic modulus, and have completed the present invention.

[0015] The present invention, which has achieved the above object, is as follows. (1) An epoxy resin composition used as a matrix resin for a fiber-reinforced composite material, comprising an epoxy resin (A), a cyclic compound (B) having a hydroxyl group, and an acid generator (C) as a polymerization initiator for curing the epoxy resin by a cationic polymerization reaction, wherein the epoxy resin (A) is an alicyclic epoxy compound, and the cyclic compound (B) has two or more hydroxyl groups. (2) The above epoxy resin composition, wherein the content of the hydroxyl group-containing cyclic compound (B) is 5 to 60 parts by mass per 100 parts by mass of the epoxy resin (A). (3) The epoxy resin composition, wherein the content of the acid generator (C) is 0.1 to 20 parts by mass per 100 parts by mass of the epoxy resin (A). (4) The above epoxy resin composition, wherein the acid generator (C) is a thermal acid generator and is at least one selected from the group consisting of sulfonium salt compounds, iodonium salt compounds, and ammonium salt compounds. (5) The above epoxy resin composition, characterized in that the viscosity at 25°C measured with an E-type viscometer is 10,000 mPa·s or less. (6) The epoxy resin composition, wherein the cured product obtained by heat-treating the epoxy resin composition at 130°C for 30 minutes exhibits a glass transition temperature of 100°C or higher. (7) A cured product of the above epoxy resin composition. (8) A prepreg in which the above epoxy resin composition is impregnated or coated on reinforcing fibers. (9) A fiber-reinforced composite material obtained by curing the prepreg. [Effects of the Invention]

[0016] The epoxy resin composition of the present invention has low viscosity, excellent long-term viscosity stability, and can be cured in a short time. Furthermore, by curing the epoxy resin composition of the present invention, a cured product having a high elastic modulus can be formed with high productivity. Furthermore, by impregnating or coating reinforcing fibers with the epoxy resin composition of the present invention to obtain a prepreg, the prepreg can be cured to obtain a fiber-reinforced composite material with excellent productivity and strength. DETAILED DESCRIPTION OF THE INVENTION

[0017] <Epoxy resin composition> The epoxy resin composition of the present invention comprises an epoxy resin (A), a hydroxyl-containing cyclic compound (B), and an acid generator (C) as a polymerization initiator for curing the epoxy resin by cationic polymerization, characterized in that the epoxy resin (A) is an alicyclic epoxy compound, and the hydroxyl-containing cyclic compound (B) is a cyclic compound having two or more hydroxyl groups. Hereinafter, the epoxy resin (A), the hydroxyl-containing cyclic compound (B), and the acid generator (C) are also referred to as component (A), component (B), and component (C), respectively. Each component of the epoxy resin composition of the present invention will be described in more detail below.

[0018] [Epoxy resin (A)] The epoxy resin (A) used in the epoxy resin composition of the present invention is an alicyclic epoxy compound. As long as it is an alicyclic epoxy compound, there are no particular limitations on the type of epoxy resin, and it may be either a monofunctional epoxy compound or a polyfunctional epoxy compound, with polyfunctional epoxy compounds being preferred. The epoxy group-containing compounds may be used alone or in combination of two or more. However, as long as the object of the present invention is not impaired, known epoxy resins other than alicyclic epoxy compounds may be contained. In this case, it is desirable that the alicyclic epoxy compound is contained in an amount of at least 50% by weight, preferably 80% by weight, based on the total amount of epoxy resins.

[0019] Specific examples of the alicyclic epoxy compound used as the epoxy resin (A) include 3,4,3',4'-diepoxybicyclohexyl, 2,2-bis(3,4-epoxycyclohexyl)propane, 2,2-bis(3,4-epoxycyclohexyl)-1,3-hexafluoropropane, bis(3,4-epoxycyclohexyl)methane, 1-[1,1-bis(3,4-epoxycyclohexyl)]ethylbenzene, bis(3,4-epoxycyclohexyl)adipate, 3,4-epoxycyclohexylmethyl(3,4-epoxy)cyclohexanecarboxylate, (3,4-epoxy-6-methylcyclohexyl)methyl-3',4'-epoxy-6-methylcyclohexanecarboxylate, ethylene-1,2-bis(3,4-epoxycyclohexanecarboxylic acid) ester, cyclohexene oxide, 3,4-epoxycyclohexylmethyl alcohol, and 3,4-epoxycyclohexylethyltrimethoxysilane. Commercially available alicyclic epoxy resins include, for example, CELLOXIDE 2000 and CELLOXIDE 2021P manufactured by Daicel Corporation; EPOMIC VG-3101 manufactured by Mitsui Chemicals, Inc.; E-1031S manufactured by Yuka Shell Epoxy Co., Ltd.; TETRAD-X and TETRAD-C manufactured by Mitsubishi Gas Chemical Company, Inc.; and EPB-13 and EPB-27 manufactured by Nippon Soda Co., Ltd.

[0020] Among these, 3,4-epoxycyclohexylmethyl(3,4-epoxy)cyclohexanecarboxylate is particularly preferred from the viewpoints of viscosity and heat resistance of cured products and fiber-reinforced composite materials. Commercially available 3,4-epoxycyclohexylmethyl(3,4-epoxy)cyclohexanecarboxylates include, for example, Celloxide 2021P manufactured by Daicel Corporation.

[0021] [Cyclic compound (B)] The cyclic compound (B) is a cyclic compound having two or more hydroxyl groups. The hydroxyl group of the cyclic compound (B) may be an alcoholic hydroxyl group or a phenolic hydroxyl group. When the hydroxyl group of the cyclic compound (B) is an alcoholic hydroxyl group and the bonded hydroxy group is a carbon atom, the alcoholic hydroxyl group may be a primary alcohol, a secondary alcohol, or a tertiary alcohol.

[0022] The cyclic compound (B) may be an aromatic compound or a non-aromatic compound. In the case of a non-aromatic compound, it may be a carbocyclic ring in which all atoms are carbon, an inorganic cyclic compound in which all atoms are other than carbon, or a heterocyclic compound containing both carbon and atoms other than carbon. The cyclic compound (B) may be a monocyclic compound or a polycyclic compound.

[0023] When a cyclic compound (B) is added to an epoxy resin composition containing an epoxy resin (A) and an acid generator (C), in addition to the cationic polymerization of the epoxy resin (A) that would normally proceed, a ring-opening reaction of the hydroxyl groups of the cyclic compound (B) with the epoxy groups of the epoxy resin (A) occurs, and as these two reactions proceed, a cured product is produced in which the structure of the cyclic compound (B) is incorporated into the crosslinked structure. As a result, the cured product exhibits a high elastic modulus due to the contribution of the rigid skeleton of the cyclic compound (B). Therefore, the content of the cyclic compound (B) is not particularly limited and may be appropriately determined taking into consideration the effects of the cyclic compound (B), but is preferably 5 to 60 parts by mass per 100 parts by mass of the epoxy resin (A). By adding 5 parts by mass or more of the cyclic compound (B) per 100 parts by mass of the epoxy resin (A), it is possible to reliably increase the elastic modulus of the cured product. From the viewpoint of increasing the elasticity of the cured product, the higher the content of the cyclic compound (B), the more preferable it is, and it may be 10 parts by mass or more. On the other hand, the upper limit is preferably 60 parts by mass or less, because even if an excessive amount of the cyclic compound (B) is added, the effect saturates and costs may increase.

[0024] The hydroxyl groups of the cyclic compound (B) can also act as proton donors to the epoxy groups of the epoxy resin (A) during the curing reaction, thereby increasing the electrophilicity of the epoxy groups and promoting the ring-opening reaction. This makes it possible to promote two reactions that proceed during curing: the cationic polymerization of the epoxy resin (A) and the ring-opening reaction of the hydroxyl groups of the cyclic compound (B) with the epoxy groups of the epoxy resin (A), thereby imparting fast curing properties.

[0025] Since a high acidity enhances the rapid curing effect, the hydroxyl group of the cyclic compound (B) is preferably a phenolic hydroxyl group, which has a higher acidity than an alcoholic hydroxyl group. Since having many phenolic hydroxyl groups enhances the rapid curing effect, it is preferable that the cyclic compound (B) has two or more phenolic hydroxyl groups. Examples of the cyclic compound (B) having a phenolic hydroxyl group include dihydric phenols such as bisphenol A, bisphenol F (isomer mixture), 4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenyl ether, 4,4'-dihydroxybenzophenone, 4,4'-dihydroxydiphenyl sulfide, 4,4'-dihydroxydiphenyl sulfone, fluorene bisphenol, 4,4'-dihydroxybiphenyl, 2,2'-dihydroxybiphenyl, hydroquinone, resorcinol, 1,5-naphthalenediol, 1,6-naphthalenediol, 2,6-naphthalenediol, and 2,7-naphthalenediol, as well as trihydric or higher phenols such as tris(hydroxyphenyl)methane mixtures, tris-(4-hydroxyphenyl)methane, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane, phenol novolak, o-cresol novolak, naphthol novolak, and polyvinylphenol. The cyclic compound (B) may be used alone or in combination of two or more.

[0026] The cyclic compound (B) may be a dihydroxy compound having a cyclic ether structure derived from biomass, and examples thereof include isosorbide, isomannide, and isoidet. Alternatively, polycarbonate polyols made from biomass-derived dihydroxy compounds having a cyclic ether structure may be used as raw materials, and examples thereof include those available under the trade name "Benebiol HS0840H" (manufactured by Mitsubishi Chemical Corporation).

[0027] [Acid generator (C)] The present invention is characterized in that an acid generator is used instead of a commonly used curing agent or curing accelerator to cure the epoxy resin (A). Conventionally, in epoxy resin compositions used as matrix resins in fiber-reinforced composite materials, commonly used curing agents such as acid anhydrides have been used, but the present invention does not use such commonly used curing agents. Acid generators are generally polymerization initiators for curable resins, and as disclosed in Patent Document 5, they have been proposed for use in applications such as resists for printed wiring boards, but are not generally used in applications for fiber-reinforced composite materials. In the present invention, by using such an acid generator in the polymerization reaction of an epoxy resin, it is possible to obtain a cured product with low viscosity and a very high elastic modulus, and an epoxy resin composition suitable as a matrix resin for fiber composite materials has been found. In the present invention, the acid generator (C) functions as a cationic polymerization initiator and can initiate the polymerization reaction of the epoxy resin to cause curing. The acid generator (C) may be a thermal acid generator that generates an acid upon heating or a photoacid generator that generates an acid upon irradiation with active energy rays (light rays), and either can be used. A thermal acid generator is preferred.

[0028] The acid generator (C) may be a sulfonium salt, a phosphonium salt, an iodonium salt, an ammonium salt, or a non-ionic compound, and these aromatic compounds are preferred. Examples of the acid generator (C) include sulfonium salts such as triarylsulfonium hexafluorophosphate and triarylsulfonium hexafluoroantimonate; iodonium salts such as diaryliodonium hexafluorophosphate, diphenyliodonium hexafluoroantimonate, bis(dodecylphenyl)iodonium tetrakis(pentafluorophenyl)borate and iodonium [4-(4-methylphenyl-2-methylpropyl)phenyl]hexafluorophosphate; phosphonium salts such as tetrafluorophosphonium hexafluorophosphate; pyridinium salts; diazonium salts; selenium salts; ammonium salts; and boron trifluoride ether complexes such as boron trifluoride-ethyl ether complex.

[0029] Specific examples of thermal acid generators include, for example, products under the trade names "Sanaid SI-45", "Sanaid SI-47", "Sanaid SI-60", "Sanaid SI-60L", "Sanaid SI-80", "Sanaid SI-80L", "Sanaid SI-100", "Sanaid SI-100L", "Sanaid SI-110L", "Sanaid SI-145", "Sanaid SI-150", "Sanaid SI-160", and "Sanaid SI-110L". Commercially available products such as "SAN-AID SI-180L" (all manufactured by Sanshin Chemical Industry Co., Ltd.), "CI-2921", "CI-2920", "CI-2946", "CI-3128", "CI-2624", "CI-2639", and "CI-2064" (all manufactured by Nippon Soda Co., Ltd.), "PP-33", "CP-66", and "CP-77" (all manufactured by ADEKA Corporation), and "FC-509" and "FC-520" (all manufactured by 3M) can also be used. Furthermore, the thermal acid generator may be a compound of a chelate compound of a metal such as aluminum or titanium with acetoacetic acid or a diketone and a silanol such as triphenylsilanol, or a compound of a chelate compound of a metal such as aluminum or titanium with acetoacetic acid or a diketone and a phenol such as bisphenol S.

[0030] Specific examples of photoacid generators include those sold under the trade names "Cyracure UVI-6970", "Cyracure UVI-6974", "Cyracure UVI-6990", and "Cyracure UVI-950" (all manufactured by Union Carbide Corporation, USA), "Irgacure 250", "Irgacure 261", "Irgacure 264", and "CG-24-61" (all manufactured by BASF), and those sold under the trade names "SP- 150", product name "SP-151", product name "SP-170", product name "OPTOMER SP-171" (all manufactured by ADEKA Corporation), product name "DAICAT II" (manufactured by Daicel Corporation), product name "UVAC1590", product name "UVAC1591" (all manufactured by Daicel-Cytec Co., Ltd.), product name "CI-2064", product name "CI-2639", product name "CI-2624", product name "CI-2481", product name "CI-273 4", trade name "CI-2855", trade name "CI-2823", trade name "CI-2758", trade name "CIT-1682" (all manufactured by Nippon Soda Co., Ltd.), trade name "PI-2074" (manufactured by Rhodia, pentafluorophenylborate toluylcumyl iodonium salt), trade name "FFC509" (manufactured by 3M), trade name "BBI-102", trade name "BBI-101", trade name "BBI-103", trade name "MPI-103", trade name Commercially available products such as "TPS-103", "MDS-103", "DTS-103", "NAT-103", and "NDS-103" (all manufactured by Midori Chemical Co., Ltd.), "CD-1010", "CD-1011", and "CD-1012" (all manufactured by Sartomer Corporation, USA), and "CPI-100P" and "CPI-101A" (all manufactured by San-Apro Co., Ltd.) can also be used. In the resin composition of the present invention, the acid generator (C) may be used either alone or in combination of two or more.

[0031] In the resin composition of the present invention, the content of the acid generator (C) is not particularly limited, but is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 15 parts by mass, and even more preferably 0.3 to 5 parts by mass, per 100 parts by mass of the epoxy resin (A). If the content is less than 0.1 part by mass, the curing reaction may not proceed sufficiently. On the other hand, if the content exceeds 20 parts by mass, the heat resistance of the cured product or fiber-reinforced composite material may be insufficient depending on the application. Note that, even when two or more types of acid generators (C) are used in combination, it is preferable to control the total amount of the acid generators (C) within the above range.

[0032] [Solvent (D)] When the acid generator (C) is a powder, the acid generator (C) may be dissolved in the solvent (D) beforehand and then mixed with the resin. Examples of the solvent (D) include diethylene glycol dimethyl ether and γ-butyrolactone. Even when a solvent is blended into the epoxy resin composition of the present invention, the amount may be small, preferably 5.0 parts by mass or less, more preferably 2.5 parts by mass or less, per 100 parts by mass of the epoxy resin (A).

[0033] [Additive (E)] The epoxy resin composition according to the embodiment of the present invention may optionally contain an additive (E) including at least one selected from the group consisting of a thermoplastic resin, a thermoplastic elastomer, an elastomer, an antifoaming agent, a leveling agent, and a mold release agent.

[0034] When a thermoplastic resin, thermoplastic elastomer, or elastomer is contained as additive (E), it not only changes the viscoelasticity of the epoxy resin composition according to the present invention, optimizing the viscosity, storage modulus, and thixotropy, but also improves the fracture toughness of the cured product of the epoxy resin composition. The thermoplastic resin, thermoplastic elastomer, and elastomer may be used alone or in combination of two or more. These additives may be blended in an amount of 1 to 15 parts by mass, preferably 1 to 10 parts by mass, per 100 parts by mass of the total epoxy resin composition. The thermoplastic resin, etc. may be dissolved in the epoxy resin, or may be contained in the epoxy resin composition in the form of fine particles, long fibers, short fibers, woven fabric, nonwoven fabric, mesh, pulp, or the like.

[0035] The thermoplastic resin preferably has at least one bond selected from the group consisting of a carbon-carbon bond, an amide bond, an imide bond, an ester bond, an ether bond, a carbonate bond, a urethane bond, a urea bond, a thioether bond, a sulfone bond, an imidazole bond, and a carbonyl bond in its main chain. More specifically, examples of the thermoplastic resin include engineering plastics such as polyacrylate, polyamide, polyaramid, polyester, polycarbonate, polyphenylene sulfide, polybenzimidazole, polyimide, polyetherimide, polysulfone, and polyethersulfone. Among these, polyimide, polyetherimide, polysulfone, and polyethersulfone are particularly preferred due to their excellent heat resistance. Furthermore, it is preferable for these thermoplastic resins to have a functional group reactive with epoxy resins, from the viewpoint of improving the fracture toughness and maintaining environmental resistance of the cured product of the epoxy resin composition according to the present invention. Preferred functional groups reactive with epoxy resins include a carboxyl group, an amino group, and a hydroxyl group.

[0036] As additive (E), a defoaming agent or leveling agent can be added for the purpose of improving surface smoothness. These additives can be blended in an amount of 0.01 to 3 parts by mass, preferably 0.01 to 1 part by mass, per 100 parts by mass of the total epoxy resin composition. By controlling the blending amount within this range, the effect of smoothing the surface can be fully exerted. Furthermore, pigments and other additives can also be blended as necessary. A release agent can be added as additive (E) to improve releasability from a mold or the like. The amount of the release agent is not particularly limited, but is preferably 1 to 8 parts by mass, more preferably 1.5 to 7 parts by mass, and even more preferably 2 to 6 parts by mass, per 100 parts by mass of the total amount of components (A) to (D). If the amount is less than 1 part by mass, sufficient release properties cannot be obtained. On the other hand, if the amount exceeds 8 parts by mass, the cured product or fiber-reinforced composite material may have insufficient curing properties, resulting in reduced heat resistance and mechanical properties, depending on the application. When two or more types of release agents are used in combination, it is preferable to control the total amount of the release agents within the above range.

[0037] The epoxy resin composition of the present invention preferably contains components (A) to (D) in an amount of 50% by mass or more, and more preferably 80% by mass or more, of the total epoxy resin composition so that the composition remains liquid as a whole. Note that the solvent is not considered an additive.

[0038] The method for producing the epoxy resin composition of the present invention is not particularly limited, and it may be produced by a known method. Specifically, for example, it can be prepared by stirring and mixing the components constituting the curable resin composition in a predetermined ratio. Note that known devices such as a planetary mixer, a planetary mixer, a kneader, a dissolver, etc. can be used to stir and mix the components. The stirring and mixing can be carried out while heating, if necessary.

[0039] <Cured product> The epoxy resin composition of the present invention contains an acid generator (C) instead of a general-purpose curing agent, and the acid generator functions as a cationic polymerization initiator, allowing the epoxy resin to be cured by a cationic polymerization reaction. When the acid generator (C) is a thermal acid generator, the epoxy resin composition of the present invention is cured by heating through the cationic polymerization reaction, and when the acid generator (C) is a photoacid generator, the epoxy resin composition is cured by irradiation with active energy rays (light rays) such as ultraviolet rays, infrared rays, visible light, or electron beams through the cationic polymerization reaction.

[0040] The conditions for curing the epoxy resin composition of the present invention are not particularly limited. For example, when the polymerization reaction is initiated by heating, the heating temperature is preferably 20 to 250°C (more preferably 40 to 200°C) and the heating time is preferably 0.1 to 480 minutes (more preferably 10 to 240 minutes, and even more preferably 30 to 180 minutes). Heating may be performed at a constant temperature, or the temperature may be increased stepwise (for example, in two steps). If the heating temperature is too low or the heating time is too short, curing may be insufficient, resulting in reduced heat resistance and mechanical properties of the cured product. On the other hand, if the heating temperature is too high or the heating time is too long, decomposition or deterioration of the components in the epoxy resin composition may occur. The heating conditions for the polymerization reaction can be controlled by the type of acid generator. The epoxy resin composition of the present invention can also initiate a polymerization reaction by irradiation with active energy rays. Examples of active energy rays include gamma rays, X-rays, electron beams, ultraviolet rays, visible light, and infrared rays, with ultraviolet rays being preferred. Examples of light sources that can be used include high-pressure mercury lamps, metal halide lamps, and LEDs. For example, the epoxy resin composition of the present invention can be prepared by filling a cylindrical container made of polyethylene or the like with the composition and irradiating it from above with an illuminance of 200 mW / cm. 2 A cured product can be obtained by irradiating the composition with ultraviolet light from a high-pressure mercury lamp for 10 seconds.

[0041] The glass transition temperature of the cured product of the epoxy resin composition of the present invention is not particularly limited, but is preferably 100° C. or higher. If the glass transition temperature is lower than 100° C., the heat resistance of the cured product or fiber-reinforced composite material may be insufficient depending on the application.

[0042] <Fiber-reinforced composite materials> The epoxy resin composition of the present invention can be cured to form a cured product having an excellent elastic modulus, and therefore can be preferably used as a resin composition for forming a composite material (fiber-reinforced composite material) of the cured product and reinforcing fibers. Specifically, a fiber-reinforced composite material can be obtained by coating or impregnating reinforcing fibers with the epoxy resin composition of the present invention to obtain a prepreg, and curing the prepreg. The fiber-reinforced composite material of the present invention has excellent elastic modulus and strength.

[0043] The reinforcing fibers may be any known or commonly used reinforcing fiber, and are not particularly limited. Examples include carbon fiber, glass fiber, aramid fiber, boron fiber, graphite fiber, silicon carbide fiber, high-strength polyethylene fiber, tungsten carbide fiber, and polyparaphenylenebenzoxazole fiber (PBO fiber). Examples of the carbon fiber include polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber, and vapor-grown carbon fiber. Among these, carbon fiber, glass fiber, and aramid fiber are preferred from the viewpoint of mechanical properties (toughness, etc.). The reinforcing fibers may be used alone or in combination of two or more. The form of the reinforcing fibers is not particularly limited, and examples thereof include filaments (long fibers), tows, unidirectional materials in which tows are arranged in one direction, woven fabrics, nonwoven fabrics, etc. Examples of reinforcing fiber woven fabrics include plain weave, twill weave, satin weave, and sheets in which fiber bundles, such as non-crimp fabrics, are aligned in one direction, or stitched sheets in which sheets stacked at different angles are stitched to prevent unraveling. The content of reinforcing fibers in the prepreg of the present invention is not particularly limited and can be adjusted appropriately.

[0044] The method for impregnating or coating the reinforcing fibers with the epoxy resin composition of the present invention is not particularly limited, and can be carried out by any known or commonly used impregnation or coating method used in the production of prepregs.

[0045] The prepreg of the present invention may be obtained by impregnating or coating a reinforcing fiber with the epoxy resin composition of the present invention, and then curing (i.e., semi-curing) a part of the epoxy resin composition by polymerization reaction, by heating if a thermal acid generator is used, or by irradiating with active energy rays if a photoacid generator is used. As described above, the fiber-reinforced composite material of the present invention can be obtained by curing the prepreg of the present invention, and the production method thereof is not particularly limited, but it can be produced by known or conventional methods such as the hand lay-up method, prepreg method, RTM method, pultrusion method, filament winding method, spray-up method, pultrusion molding method, etc. In particular, the epoxy resin compound of the present invention is useful as a matrix resin when producing a fiber-reinforced composite material by pultrusion molding.

[0046] The fiber-reinforced composite material of the present invention comprises a cured product of the above-mentioned epoxy resin composition and reinforcing fibers. There are no limitations on the applications of this fiber-reinforced composite material, and it can be used for aircraft structural materials, as well as for automobiles, ships, sports, and general industrial applications such as wind power generation and rolls. The epoxy resin composition of the present invention has an excellent elastic modulus when cured, and therefore can be used for applications other than fiber-reinforced composite materials, taking advantage of this property. [Example]

[0047] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. The raw materials used in this example are as follows: [raw materials] [Epoxy resin (A)] 3,4-Epoxycyclohexylmethyl (3,4-epoxy)cyclohexanecarboxylate (trade name: Celloxide 2021P, manufactured by Daicel Corporation) [ka] Bisphenol A type bifunctional epoxy resin (product name: YD-128, manufactured by Nippon Steel Chemical & Material Co., Ltd.) [Cyclic compound (B)] Bisphenol F [ka] Bisphenol A [ka] Phenol novolac [ka] 4,4-Dihydroxybiphenyl [ka] Isosorbide [ka] Polycarbonate diol (product name: HS0840H, manufactured by Mitsubishi Chemical Corporation) [ka] Here, R is the following group: [ka] [Acid generator (C)] San-Aid SI-150 (manufactured by Sanshin Chemical Industry Co., Ltd.) [ka] San-Aid SI-360 (manufactured by Sanshin Chemical Industry Co., Ltd.) [ka] [Solvent (D)] γ-butyrolactone

[0048] Example 1 100 parts of Celloxide 2021P as component (A) and 10 parts of Bisphenol F as component (B) were placed in a separable flask and mixed with stirring at 90°C for 90 minutes to dissolve component (B) in component (A). Next, 1 part of San-Aid SI-150 as component (C) and 1 part of γ-butyrolactone as component (D) were placed in a 150 mL plastic container and stirred at room temperature for 5 minutes using a vacuum mixer called "Awatori Rentaro" (Thinky Corporation) to dissolve component (C) in component (D). After that, the mixture of components (A) and (B) was added to the mixture of components (C) and (D), and the mixture was again stirred at room temperature for 5 minutes using a vacuum mixer to obtain an epoxy resin composition.

[0049] (Preparation of test specimens for measuring glass transition temperature) This epoxy resin composition was poured into a mold measuring 80 mm in length and 80 mm in width, which had been provided with a 4 mm thick spacer cut into the shape of a flat plate, and cured at 160°C for 1 hour and then at 180°C for 1 hour. After that, it was cut into a size of 50 mm x 10 mm using a bench band saw and used to measure the glass transition temperature. (Preparation of tensile test specimens) This epoxy resin composition was poured into a mold measuring 180 mm in length and 180 mm in width, which had a dumbbell-shaped spacer cut out, and cured at 160°C for 1 hour and then at 180°C for 1 hour to prepare test specimens for tensile testing in accordance with the Type 1B test specimen specified in JIS 7161-2.

[0050] Examples 2 to 11, Comparative Examples 1 to 3 An epoxy resin composition was prepared under the same mixing conditions as in Example 1, except that the raw materials (A) to (D) were used in the compositions shown in Tables 1 and 2. Test pieces for tensile testing and glass transition temperature measurement were prepared in the same manner as in Example 1.

[0051] (Measurement of initial viscosity and viscosity increase rate) The viscosity value at 25°C was measured using an E-type viscometer with a cone and plate. 1.1 mL of the epoxy resin composition was used for the measurement, and the value measured 60 seconds after the start of the measurement was taken as the initial viscosity value. The epoxy resin composition was then allowed to stand in a thermostatic water bath set at 25°C for 8 hours, after which the viscosity was measured in the same manner using an E-type viscometer with a cone and plate. The value measured 60 seconds after the start of the measurement was taken as the viscosity value after 8 hours. The viscosity increase rate was calculated using the formula 100 × (viscosity after 8 hours / initial viscosity). (Gel time measurement) The epoxy resin composition was added to the plate of a gelation tester (manufactured by Nisshin Scientific Co., Ltd.) that had been heated to 160°C, and the mixture was stirred at a speed of two revolutions per second using a fluororesin rod. The time required for the resin composition to harden and lose its plasticity was recorded as the gelation time. If the gelation time is less than 100 seconds (sec), the short-time curing property required for applications such as fiber-reinforced composite materials is satisfied. (Measurement of glass transition temperature) Using a dynamic viscoelasticity tester, the test piece for measuring the glass transition temperature was measured at a heating rate of 5°C / min, bending mode, and a measurement frequency of 10 Hz, and the maximum value of the loss modulus (E'') was taken as the glass transition temperature. (Measurement of tensile modulus) Using an Autograph AGS-X (Shimadzu Corporation), the tensile test specimen was measured in accordance with JIS7161-2 to measure the tensile modulus (GPa). The test results of the examples and comparative examples are shown in Tables 1 and 2.

[0052] [Table 1]

[0053] [Table 2]

Claims

1. An epoxy resin composition for use as a matrix resin in a fiber-reinforced composite material, comprising: an epoxy resin (A); a cyclic compound (B) having a hydroxyl group; and an acid generator (C) serving as a polymerization initiator for curing the epoxy resin by a cationic polymerization reaction, wherein the epoxy resin (A) is an alicyclic epoxy compound, and the cyclic compound (B) has two or more hydroxyl groups.

2. 2. The epoxy resin composition according to claim 1, wherein the content of the hydroxyl group-containing cyclic compound (B) is 5 to 60 parts by mass per 100 parts by mass of the epoxy resin (A).

3. 2. The epoxy resin composition according to claim 1, wherein the content of the acid generator (C) is 0.1 to 20 parts by mass per 100 parts by mass of the epoxy resin (A).

4. 2. The epoxy resin composition according to claim 1, wherein the acid generator (C) is a thermal acid generator and is at least one selected from the group consisting of sulfonium salt compounds, iodonium salt compounds, and ammonium salt compounds.

5. 2. The epoxy resin composition according to claim 1, wherein the viscosity at 25°C measured with an E-type viscometer is 10,000 mPa·s or less.

6. 2. The epoxy resin composition according to claim 1, wherein the cured product obtained by heat-treating the epoxy resin composition at 130°C for 30 minutes has a glass transition temperature of 100°C or higher.

7. A cured product of the epoxy resin composition according to claim 1.

8. A prepreg obtained by impregnating or coating reinforcing fibers with the epoxy resin composition according to claim 1.

9. A fiber-reinforced composite material obtained by curing the prepreg according to claim 8.

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