Prepreg materials, fiber-reinforced composite materials, and injection molding methods for manufacturing objects.

Abstract

The invention proposes a prepreg material exhibiting superior processing properties obtained by using a resin preparation that can cure in a short time and can limit resin flow during molding. This prepreg material consists of an epoxy resin preparation and reinforcing fibers, and differs in that the epoxy resin preparation contains component (A): urea compound, component (B): thermoplastic resin, component (C): oxazolidone-structured epoxy resin and component (D): bisphenol-type epoxy resin that solidifies at 25°C, and satisfies the following conditions (1) and (2). Condition (1): The minimum viscosity of the epoxy resin preparation measured by rheometer under conditions of 2°C / min is between 3.0 and 6.0 Pa-s. Condition (2): The gelation time value of the epoxy resin preparation measured by a Curelastometer (trade name) at 145°C is between 2.0 and 5.0 minutes.

Description

Prepreg, fiber-reinforced composite material, and method for manufacturing molded body

[0001] The present invention relates to a prepreg, a fiber-reinforced composite material, and a method for producing a molded article.

[0002] Fiber-reinforced composite materials (FRPs), which combine resins with reinforcing fibers, are widely used in sports and leisure applications, automobiles, industrial applications, etc., due to their excellent light weight, rigidity, impact resistance, etc. In particular, tubular molded articles made of fiber-reinforced composite materials are widely used in sports and leisure applications, such as fishing rods, golf club shafts, skee-balls, and bicycle frames.

[0003] Fiber-reinforced composite materials can be obtained by curing and molding prepregs in which a matrix resin is impregnated into a reinforcing material made of continuous fibers such as reinforcing fibers. Carbon fibers are widely used as reinforcing fibers because of their strength and elastic modulus. Epoxy resins are widely used as matrix resins because of their adhesiveness to carbon fibers. Specific methods for obtaining fiber-reinforced composite materials from prepregs include, for example, autoclave molding, press molding, internal pressure molding, and oven molding.

[0004] For tubular molded articles of fiber-reinforced composite materials, longitudinal bending strength and diametric crushing strength are important properties. Patent Document 1 proposes a resin composition for fiber-reinforced composite materials that achieves both longitudinal bending strength and diametric crushing strength. Furthermore, for prepregs required for sports and leisure applications, industrial applications, and the like, there is a demand for materials that can be molded at high speeds and contribute to improved productivity and cost reduction. Patent Document 2 proposes a resin composition for fiber-reinforced composite materials that can be cured completely in a short time even at low temperatures and has excellent mechanical properties and heat resistance.

[0005] International Publication No. WO 1998 / 044017 International Publication No. WO 2020 / 080474

[0006] In high-cycle molding, molding is performed under high temperature and pressure, which causes the temperature of the resin composition that makes up the prepreg to rise during molding, significantly reducing the viscosity of the resin composition. Through investigations by the present inventors, it was discovered that with conventional resin compositions, the heating and pressure applied during the molding process in high-cycle molding causes the resin to flow out of the prepreg, i.e., resin flow, resulting in poor appearance such as unimpregnated areas of the resin composition and meandering fibers.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a prepreg that can be cured in a short time and can suppress resin flow during molding.

[0008] The present invention includes the following embodiments.

[0009] [1]: A prepreg comprising an epoxy resin composition and reinforcing fibers, wherein the epoxy resin composition comprises the following components (A), (B), (C), and (D), and satisfies the following conditions (1) and (2): Component (A): a urea compound Component (B): a thermoplastic resin Component (C): an epoxy resin having an oxazolidone structure Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C)) Condition (1): The minimum viscosity of the epoxy resin composition, measured using a rheometer at 2°C / min, is within the range of 3.0 Pa s or more, 3.1 Pa s or more, or 3.2 Pa s or more, and 6.0 Pa s or less, 5.9 Pa s or less, 5.5 Pa s or less, 5.0 Pa s or less, or 4.5 Pa s or less. Condition (2): The gel time value of the epoxy resin composition measured at 145°C using a Curastometer (registered trademark) is within the range of 2.0 min or more, 2.1 min or more, 2.3 min or more, or 2.5 min or more, and 5.0 min or less, 4.9 min or less, 4.5 min or less, or 4.0 min or less. [2]: The prepreg according to [1], wherein the median diameter (D50) of the component (A) measured by laser diffraction is 8.0 μm or less or 7.0 μm or less. [3]: The prepreg according to [1] or [2], wherein the component (A) contains 2,4-bis(3,3-dimethylureido)toluene. [4]: ​​The prepreg according to any of [1] to [3], wherein the component (B) is a thermoplastic resin that is soluble in the mixture of epoxy resins contained in the epoxy resin composition. [5]: The prepreg according to any one of [1] to [4], wherein the weight-average molecular weight of the component (B) is 60,000 or more or 65,000 or more. [6]: The prepreg according to any one of [1] to [5], wherein the weight-average molecular weight of the component (B) is 150,000 or less or 145,000 or less. [7]: The prepreg according to any one of [1] to [6], wherein the component (B) comprises a polyvinyl formal resin. [8]: The prepreg according to any one of [1] to [7], wherein the component (C) is solid at 25°C. [9]: The prepreg according to any one of [1] to [8], wherein the epoxy equivalent of the component (D) is 300 g / eq or more or 350 g / eq or more.

[10] : The prepreg according to any one of [1] to [9], wherein the epoxy equivalent of the (D) component is 1000 g / eq or less or 950 g / eq or less.

[11] : The prepreg according to any one of [1] to

[10] , wherein the (D) component is a bisphenol A-type epoxy resin.

[12] : The prepreg according to any one of [1] to

[11] , wherein the (A) component is contained in an amount of 2 mass% or more, 2.5 mass% or more, or 3 mass% or more, relative to the total mass of the epoxy resin composition.

[13] : The prepreg according to any one of [1] to

[12] , wherein the (A) component is contained in an amount of 10 mass% or less, 9.5 mass% or less, 8 mass% or less, or 5 mass% or less, relative to the total mass of the epoxy resin composition.

[14] : The prepreg according to any one of [1] to

[13] , wherein the (B) component is contained in an amount of 1 mass% or more, 1.5 mass% or more, or 2 mass% or more, relative to the total mass of the epoxy resin composition.

[15] : The prepreg according to any one of [1] to

[14] , wherein the (B) component is contained in an amount of 10% by mass or less, 9.5% by mass or less, 8% by mass or less, or 5% by mass or less, relative to the total mass of the epoxy resin composition.

[16] : The prepreg according to any one of [1] to

[15] , wherein the (C) component and the (D) component are contained in an amount of 30% by mass or more, 32% by mass or more, 35% by mass or more, or 40% by mass or more, relative to the total mass of the epoxy resin composition.

[17] : The prepreg according to any one of [1] to

[16] , wherein the (C) component and the (D) component are contained in an amount of 60% by mass or less, 58% by mass or less, or 55% by mass or less, relative to the total mass of the epoxy resin composition.

[18] : The prepreg according to any one of [1] to

[17] , further satisfying the following condition (3): Condition (3): The viscosity of the epoxy resin composition at 65°C is within the range of 10 Pa·s or more, 20 Pa·s or more, 50 Pa·s or more, 70 Pa·s or more, or 100 Pa·s or more.

[19] : A prepreg according to any of [1] to

[18] , which further satisfies the following condition (3): Condition (3): The viscosity of the epoxy resin composition at 65°C is within the range of 1000 Pa·s or less, 900 Pa·s or less, 500 Pa·s or less, 300 Pa·s or less, or 200 Pa·s or less.

[20] : A prepreg according to any of [1] to

[19] , which further contains the following component (E):Component (E): at least one selected from dicyandiamide and dicyandiamide derivatives.

[21] : The prepreg according to any one of [1] to

[20] , wherein the reinforcing fibers comprise carbon fibers.

[22] : A fiber-reinforced composite material obtainable by curing the prepreg according to any one of [1] to

[21] .

[23] : A prepreg comprising an epoxy resin composition and reinforcing fibers, wherein the epoxy resin composition comprises the following components (A1), (B1), (C), and (D): Component (A1): a urea compound containing 2,4-bis(3,3-dimethylureido)toluene; Component (B1): a polyvinyl formal resin having a weight-average molecular weight of 60,000 or more or 65,000 or more; Component (C): an epoxy resin having an oxazolidone structure; and Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding the component (C)).

[24] : The prepreg according to

[23] , wherein the following condition (1) is satisfied: Condition (1): The epoxy resin composition has a minimum viscosity of 3.0 Pa s or more, 3.1 Pa s or more, or 3.2 Pa s or more, as measured by a rheometer at 2°C / min.

[25] : A prepreg according to

[23] or

[24] , which satisfies the following condition (1). Condition (1): The epoxy resin composition has a minimum viscosity of 6.0 Pa s or less, 5.9 Pa s or less, 5.5 Pa s or less, 5.0 Pa s or less, or 4.5 Pa s or less, as measured by a rheometer at 2°C / min.

[26] : A prepreg according to any of

[23] to

[25] , which satisfies the following condition (2). Condition (2): The epoxy resin composition has a gel time of 2.0 min or more, 2.1 min or more, 2.3 min or more, or 2.5 min or more, as measured by a Curastometer (registered trademark) at 145°C.

[27] : The prepreg according to any one of

[23] to

[26] , which satisfies the following condition (2): the gel time of the epoxy resin composition measured at 145°C using a Curastometer (registered trademark) is within the range of 5.0 min or less, 4.9 min or less, 4.5 min or less, or 4.0 min or less.

[28] : The prepreg according to any one of

[23] to

[27] , wherein the median diameter (D50) of the component (A1) measured by laser diffraction is 8.0 μm or less or 7.0 μm or less.

[29] : The prepreg according to any one of

[23] to

[28] , wherein the component (C) is solid at 25°C.

[30] : The prepreg according to any one of

[23] to

[29] , wherein the epoxy equivalent of the component (D) is 300 g / eq or more or 350 g / eq or more.

[31] : The prepreg according to any one of

[23] to

[30] , wherein the epoxy equivalent of the component (D) is 1000 g / eq or less or 950 g / eq or less.

[32] : The prepreg according to any one of

[23] to

[31] , wherein the component (D) is a bisphenol A type epoxy resin.

[33] : The prepreg according to any one of

[23] to

[32] , which contains the (A1) component in an amount of 2% by mass or more, 2.5% by mass or more, or 3% by mass or more, relative to the total mass of the epoxy resin composition.

[34] : The prepreg according to any one of

[23] to

[33] , which contains the (A) component in an amount of 10% by mass or less, 9.5% by mass or less, 8% by mass or less, or 5% by mass or less, relative to the total mass of the epoxy resin composition.

[35] : The prepreg according to any one of

[23] to

[34] , which contains the (B1) component in an amount of 1% by mass or more, 1.5% by mass or more, or 2% by mass or more, relative to the total mass of the epoxy resin composition.

[36] : The prepreg according to any one of

[23] to

[35] , which contains the (B1) component in an amount of 10% by mass or less, 9.5% by mass or less, 8% by mass or less, or 5% by mass or less, relative to the total mass of the epoxy resin composition.

[37] : The prepreg according to any one of

[23] to

[36] , wherein the sum of the components (C) and (D) is 30% by mass or more, 32% by mass or more, 35% by mass or more, or 40% by mass or more, relative to the total mass of the epoxy resin composition.

[38] : The prepreg according to any one of

[23] to

[37] , wherein the sum of the components (C) and (D) is 60% by mass or less, 58% by mass or less, or 55% by mass or less, relative to the total mass of the epoxy resin composition.

[39] : The prepreg according to any one of

[23] to

[38] , further satisfying the following condition (3):Condition (3): The viscosity of the epoxy resin composition at 65°C is within the range of 10 Pa·s or more, 20 Pa·s or more, 50 Pa·s or more, 70 Pa·s or more, or 100 Pa·s or more.

[40] : The prepreg according to any of

[23] to

[39] , which further satisfies the following condition (3): Condition (3): The viscosity of the epoxy resin composition at 65°C is within the range of 1000 Pa·s or less, 900 Pa·s or less, 500 Pa·s or less, 300 Pa·s or less, or 200 Pa·s or less.

[41] : The prepreg according to any of

[23] to

[40] , which further contains the following component (E): Component (E): at least one selected from dicyandiamide and dicyandiamide derivatives.

[42] : The prepreg according to any of

[23] to

[41] , wherein the reinforcing fibers include carbon fibers.

[43] : A method for producing a molded article, comprising the step of heating a prepreg containing an epoxy resin composition and reinforcing fibers placed in a mold to 130°C or higher or 140°C or higher, wherein the epoxy resin composition comprises the following components (A1) and (B1): Component (A1): 2,4-bis(3,3-dimethylureido)toluene; and Component (B1): a polyvinyl formal resin having a weight-average molecular weight of 60,000 or higher or 65,000 or higher.

[44] : A method for producing a molded article according to

[43] , wherein the prepreg is placed in a tubular shape and heated to produce a tubular molded article.

[45] : A method for producing a molded article according to

[44] , further comprising the step of pressing the tubular precursor, in which the prepreg is placed in a tubular shape, against a mold by expanding a medium from inside the tubular precursor.

[46] : A method for producing a molded article according to any of

[43] to

[45] , wherein the epoxy resin composition further comprises the following component (C): Component (C): an epoxy resin having an oxazolidone structure

[47] : The method for producing a molded article according to any one of items

[43] to

[46] , wherein the epoxy resin composition further contains the following component (D):Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C): an epoxy resin having an oxazolidone structure)

[48] : a method for producing a molded article according to any one of items

[43] to

[47] , wherein, relative to the total mass of the epoxy resin composition, the total of the following components (C) and (D) is 60% by mass or less, 58% by mass or less, or 55% by mass or less. Component (C): an epoxy resin having an oxazolidone structure Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C))

[49] : a method for producing a molded article according to any one of items

[43] to

[48] , wherein, relative to the total mass of the epoxy resin composition, the total of the following components (C) and (D) is 30% by mass or more, 32% by mass or more, 35% by mass or more, or 40% by mass or more. Component (C): an epoxy resin having an oxazolidone structure; Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C));

[50] : The method for producing a molded article according to any one of

[43] to

[49] , wherein the tubular molded article has an annular curved portion, and the method includes a step of bending the tubular precursor into an annular shape.

[0010] According to the present invention, it is possible to provide a prepreg that can be cured in a short time and that can suppress resin flow during molding.

[0011] The present invention will be described in detail below. [Prepreg] One embodiment of the present invention relates to a prepreg. The prepreg of the present invention contains an epoxy resin composition and reinforcing fibers.

[0012] <Composition of Epoxy Resin Composition> The epoxy resin composition contained in the prepreg according to one example of the embodiment contains the following components (A), (B), (C), and (D), and preferably further contains the following component (E): Component (A): urea compound Component (B): thermoplastic resin Component (C): epoxy resin having an oxazolidone structure Component (D): bisphenol-type epoxy resin that is solid at 25°C (excluding component (C)) Component (E): at least one selected from cyandiamide and dicyandiamide derivatives The epoxy resin composition used in this embodiment preferably satisfies the following conditions (1) and (2), and further satisfies the following condition (3): Condition (1): The epoxy resin composition has a minimum viscosity of 3.0 to 6.0 Pa s as measured using a rheometer at 2°C / min. Condition (2): The epoxy resin composition has a gel time of 2.0 to 5.0 min as measured using a Curastometer (registered trademark) at 145°C. Condition (3): The viscosity of the epoxy resin composition at 65° C. is in the range of 10 to 1000 Pa·s.

[0013] (Component (A)) Component (A) is a urea compound. When heated at high temperatures, the urea compound generates isocyanate groups and dimethylamine, which react with epoxy groups, generating heat that further promotes the reaction between the generated —NH groups and the epoxy groups. Therefore, when the epoxy resin composition contains component (A), resin flow during molding of the resulting prepreg can be suppressed.

[0014] The urea compound is not particularly limited, but examples thereof include aromatic dimethylurea in which a dimethylureido group is bonded to an aromatic ring, and aliphatic dimethylurea in which a dimethylureido group is bonded to an aliphatic compound. These may be used alone or in combination of two or more. Among these, aromatic dimethylurea is preferred from the viewpoint of reducing resin flow.

[0015] Examples of aromatic dimethylureas include phenyl dimethyl urea, methylene bis(phenyl dimethyl urea), and tolylene bis(dimethyl urea). More specific examples include 4,4'-methylene bis(phenyl dimethyl urea) (MBPDMU), 3-phenyl-1,1-dimethyl urea (PDMU), 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), 3-(3-chloro-4-methylphenyl)-1,1-dimethyl urea, and 2,4-bis(3,3-dimethyl ureido) toluene (TBDMU). Among these, 2,4-bis(3,3-dimethyl ureido) toluene (TBDMU) is more preferred from the viewpoint of reducing resin flow.

[0016] Examples of the aliphatic dimethylurea include dimethylurea obtained from isophorone diisocyanate and dimethylamine, dimethylurea obtained from m-xylylene diisocyanate and dimethylamine, and dimethylurea obtained from hexamethylene diisocyanate and dimethylamine.

[0017] The median diameter (D50) of component (A) measured by laser diffraction is preferably 8.0 μm or less, and more preferably 7.0 μm or less. By adjusting the median diameter (D50) of component (A) to 8.0 μm or less, the thixotropy of the epoxy resin composition can be enhanced, thereby suppressing resin flow during molding. Furthermore, the surface area relative to the mass of component (A) is increased, thereby improving the reaction rate and enabling the epoxy resin composition to cure in a shorter time. The median diameter (D50) of component (A) can be adjusted by previously pulverizing component (A) using a bead mill or a three-roll mill. For measurement by laser diffraction, an AEROTRAC SPR (Model: 7340) (manufactured by Nikkiso Co., Ltd.) can be used, for example. The lower limit of the median diameter (D50) of component (A) is not particularly limited, but it can be, for example, 0.1 μm or more.

[0018] Furthermore, the median diameter (D50) of component (A) measured by laser diffraction is preferably equal to or less than the average fiber diameter of all the carbon fibers contained in the reinforcing fiber substrate. When the median diameter (D50) is equal to or less than the average fiber diameter of all the carbon fibers, component (A) is not localized on the surface of the prepreg, thereby improving the surface appearance quality. Furthermore, by distributing component (A) throughout the entire fiber-reinforced composite material, poor curing is less likely to occur and resin flow during molding can be suppressed.

[0019] The content of component (A) relative to the total mass of the epoxy resin composition is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. By ensuring that the content of component (A) is equal to or greater than the aforementioned lower limit, resin flow during molding is suppressed. Furthermore, the content of component (A) relative to the total mass of the epoxy resin composition is preferably 10% by mass or less, more preferably 9.5% by mass or less, even more preferably 8% by mass or less, and particularly preferably 5% by mass or less. By ensuring that the content of component (A) is equal to or less than the aforementioned upper limit, an epoxy resin composition with excellent storage stability and mechanical properties can be obtained. The preferred lower and upper limits for the content of component (A) can be arbitrarily combined, and can be, for example, 2 to 10% by mass, 2 to 9.5% by mass, 2.5 to 8% by mass, or 3 to 5% by mass.

[0020] (Component (B)) Component (B) is a thermoplastic resin. The thermoplastic resin is blended to control viscoelasticity. When the epoxy resin composition contains component (B), the viscosity of the epoxy resin composition can be adjusted, and high thixotropy can be exhibited, thereby suppressing resin flow during molding. Component (B) is preferably a component that is soluble in the mixture of epoxy resins contained in the epoxy resin composition. Here, "dissolved" refers to a state in which the state of the particles in the epoxy resin composition cannot be confirmed by optical methods such as an optical microscope or visual inspection.

[0021] The weight-average molecular weight of component (B) is preferably 60,000 or more, more preferably 65,000 or more. By setting the weight-average molecular weight of component (B) to the above-mentioned lower limit or more, the minimum viscosity of the epoxy resin composition is improved, and resin flow during molding can be suppressed. Furthermore, the weight-average molecular weight of component (B) is preferably 150,000 or less, more preferably 145,000 or less. Setting the weight-average molecular weight of component (B) to the above-mentioned upper limit or less can suppress resin flow during molding while improving the handleability of the prepreg at room temperature. The preferred lower and upper limits of the weight-average molecular weight of component (B) can be arbitrarily combined, for example, 60,000 to 150,000 or 65,000 to 145,000. The weight-average molecular weight of component (B) is measured using gel permeation chromatography (GPC) according to the method described below.

[0022] Examples of component (B) include, but are not limited to, polyamide, polyester, polycarbonate, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyetheretherketone, polyetherketone, polyetherimide, polyimide, polytetrafluoroethylene, polyether, polyolefin, polyarylate, polysulfone, polyacrylonitrilestyrene, polystyrene, polyacrylonitrile, polymethyl methacrylate, acrylonitrile-butadiene-styrene copolymer (ABS resin), acrylonitrile-ethylene-propylene-diene-styrene copolymer (AES resin), acrylonitrile-styrene-alkyl (meth)acrylate copolymer (ASA resin), polyvinyl chloride, polyvinyl formal, and phenoxy resin. These thermoplastic resins may be used alone or in combination of two or more. From the viewpoint of suppressing resin flow, polyvinyl formal resin is preferred. By using polyvinyl formal resin, resin flow during molding can be suppressed.

[0023] Examples of commercially available polyvinyl formal resins include, but are not limited to, Vinylec L (catalog value weight average molecular weight: 66,000), Vinylec H (catalog value weight average molecular weight: 73,000), and Vinylec E (all trade names, manufactured by JNC Corporation).

[0024] The content of component (B) relative to the total mass of the epoxy resin composition is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. By ensuring that the content of component (B) is at or above the lower limit, the epoxy resin composition can exhibit high thixotropy, thereby suppressing resin flow during molding. Furthermore, the content of component (B) relative to the total mass of the epoxy resin composition is preferably 10% by mass or less, more preferably 9.5% by mass or less, even more preferably 8% by mass or less, and particularly preferably 5% by mass or less. By ensuring that the content of component (B) is at or below the upper limit, the handleability of the prepreg at room temperature is improved. The preferred lower and upper limits of the content of component (B) can be arbitrarily combined, and can be, for example, 1 to 10% by mass, 1 to 9.5% by mass, 1.5 to 8% by mass, or 2 to 5% by mass.

[0025] (Component (C)) Component (C) is an epoxy resin having an oxazolidone structure. The inclusion of component (C) in an epoxy resin composition increases the viscosity of the epoxy resin composition, thereby suppressing resin flow during molding. The oxazolidone structure is formed by an addition reaction between an isocyanate group and an epoxy group. The method for producing an epoxy resin having an oxazolidone structure is not particularly limited. For example, an epoxy resin having an oxazolidone structure can be obtained in a nearly theoretical amount by reacting an isocyanate compound with an epoxy resin in the presence of a catalyst used for oxazolidone ring formation. The isocyanate compound and the epoxy resin are preferably reacted at an equivalent ratio (isocyanate compound:epoxy resin) in the range of 1:2 to 1:10. When the equivalent ratio of the isocyanate compound to the epoxy resin is within the above range, the heat resistance and water resistance of the cured resin tend to be further improved.

[0026] The isocyanate compound used as a raw material for component (C) is not particularly limited, but in order to incorporate an oxazolidone structure into the skeleton of the epoxy resin, an isocyanate compound having multiple isocyanate groups is preferred. Furthermore, in order to provide a cured resin with high heat resistance, a diisocyanate having a rigid structure is preferred. Specific examples of the isocyanate compound include methane diisocyanate, butane-1,1-diisocyanate, ethane-1,2-diisocyanate, butane-1,2-diisocyanate, transvinylene diisocyanate, propane-1,3-diisocyanate, butane-1,4-diisocyanate, 2-butene-1,4-diisocyanate, 2-methylbutene-1,4-diisocyanate, 2-methylbutane-1,4-diisocyanate, and pentane-1,5-diisocyanate. 2,2-dimethylpentane-1,5-diisocyanate, hexane-1,6-diisocyanate, heptane-1,7-diisocyanate, octane-1,8-diisocyanate, nonane-1,9-diisocyanate, decane-1,10-diisocyanate, dimethylsilane diisocyanate, diphenylsilane diisocyanate, ω,ω'-1,3-dimethylbenzene diisocyanate, ω,ω'-1,4-dimethylbenzene diisocyanate, ω,ω'-1,3-dimethyl cyclohexane diisocyanate, ω,ω'-1,4-dimethylcyclohexane diisocyanate, ω,ω'-1,4-dimethylnaphthalene diisocyanate, ω,ω'-1,5-dimethylnaphthalene diisocyanate, cyclohexane-1,3-diisocyanate, cyclohexane-1,4-diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, dicyclohexylmethane-4,4'-diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 1-methylbenzene-2,4-diisocyanate, 1-methylbenzene-2,5-diisocyanate, 1-methylbenzene-2,6-diisocyanate, 1-methylbenzene-3,5-diisocyanate, diphenyl ether-4,4'-diisocyanate, diphenyl ether-2,4'-diisocyanate, naphthalene-1,4-diisocyanate, naphthalene-1,5-diisocyanate, biphenyl-4,Examples of the isocyanate compound include, but are not limited to, bifunctional isocyanate compounds such as 4'-diisocyanate, 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 2,3'-dimethoxybisphenyl-4,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3,3'-dimethoxydiphenylmethane-4,4'-diisocyanate, 4,4'-dimethoxydiphenylmethane-3,3'-diisocyanate, norbornene diisocyanate, diphenylsulfite-4,4'-diisocyanate, and diphenylsulfone-4,4'-diisocyanate; trifunctional or higher functional isocyanate compounds such as polymethylene polyphenylisocyanate and triphenylmethane triisocyanate; multimers such as dimers and trimers of the above isocyanate compounds; blocked isocyanates masked with alcohol or phenol; and bis-urethane compounds. These isocyanate compounds may be used alone or in combination of two or more.

[0027] Among the above isocyanate compounds, from the viewpoint of improving the heat resistance of the cured resin material, bifunctional or trifunctional isocyanate compounds are preferred, bifunctional isocyanate compounds are more preferred, and bifunctional isocyanate compounds having a skeleton selected from isophorone, benzene, toluene, diphenylmethane, naphthalene, norbornene, polymethylene, polyphenylene polyphenyl, and hexamethylene are even more preferred. When the number of functional groups in the isocyanate compound is appropriately large, a decrease in the storage stability of the epoxy resin composition can be suppressed. Furthermore, when the number of functional groups in the isocyanate compound is appropriately small, a decrease in the heat resistance of the cured resin material can be suppressed.

[0028] Although various epoxy resins can be used as the epoxy resin raw material for component (C), in order to efficiently incorporate an oxazolidone structure into the epoxy resin skeleton, epoxy resins having epoxy groups at both ends of the molecule are preferred. Specific examples of epoxy resins include, but are not limited to, epoxy resins derived from dihydric phenols such as bisphenol A, bisphenol F, bisphenol AD, bisphenol S, tetramethylbisphenol A, tetramethylbisphenol F, tetramethylbisphenol AD, tetramethylbisphenol S, tetrabromobisphenol A, and biphenyl; epoxy resins derived from tris(glycidyloxyphenyl)alkanes such as 1,1,1-tris(4-hydroxyphenyl)methane, 1,1,1-(4-hydroxyphenyl)ethane, and 4,4-[1-[4-[1-(4-hydroxyphenyl)-1-methylethyl]phenyl]ethylidene]bisphenol; and novolac-derived epoxy resins such as phenol novolac, cresol novolac, and bisphenol A novolac. These epoxy resins may be used alone or in combination of two or more. As the epoxy resin, bisphenol A epoxy resin, bisphenol F epoxy resin, or biphenyl epoxy resin is preferred from the viewpoint of preventing an excessive increase in the viscosity of component (C).

[0029] An addition reaction product obtained by mixing and reacting one molecule of a bifunctional isocyanate having a toluene skeleton, such as tolylene diisocyanate, as the isocyanate compound with two molecules of bisphenol A diglycidyl ether as the epoxy resin is particularly preferred, as it improves the workability of the prepreg at room temperature and the heat resistance of the cured resin. Examples of the bifunctional isocyanate having a toluene skeleton include 1-methylbenzene-2,4-diisocyanate, 1-methylbenzene-2,5-diisocyanate, 1-methylbenzene-2,6-diisocyanate, and 1-methylbenzene-3,5-diisocyanate.

[0030] Commercially available epoxy resins having an oxazolidone structure include, but are not limited to, AER4152, AER4151, LSA3301, and LSA2102 (all trade names, manufactured by Asahi Kasei E-materials Corporation); ACR1348 (trade name, manufactured by ADEKA Corporation); DER (registered trademark; the same applies hereinafter) 852 and 858 (trade names, manufactured by Dow Chemical Japan Co., Ltd.); TSR-400 (trade name, manufactured by DIC Corporation); and YD-952 (trade name, manufactured by Nippon Steel Chemical & Material Co., Ltd.). All of these are preferably used in the present invention, with YD-952 being particularly preferred. The component (C) may be used alone or in combination of two or more types.

[0031] Component (C) is preferably a solid at 25° C. When component (C) is a solid at 25° C., the viscosity of the epoxy resin composition increases, suppressing the fluidity of the resin during molding and making it more likely that the resin will not flow out of the prepreg due to heating and pressure.

[0032] (Component (D)) Component (D) is a bisphenol-type epoxy resin that is solid at 25°C, excluding component (C). When the epoxy resin composition contains component (D), the viscosity of the epoxy resin composition increases, thereby suppressing resin fluidity during molding and preventing the resin from leaking out of the prepreg due to heating and pressure. In other words, resin flow in the resulting prepreg can be suppressed. The softening point of component (D) is preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 50°C or higher. There are no particular limitations on the upper limit of the softening point of component (D), but it can be, for example, 150°C or lower.

[0033] The epoxy equivalent of component (D) is preferably 300 g / eq or more, more preferably 350 g / eq or more. By setting the epoxy equivalent of component (D) at or above the lower limit, both the reactivity of the epoxy resin composition and the suppression of resin flow during molding can be achieved. Furthermore, the epoxy equivalent of component (D) is preferably 1000 g / eq or less, more preferably 950 g / eq or less. Setting the epoxy equivalent of component (D) at or below the upper limit improves the impregnation of the epoxy resin composition into the reinforcing fiber substrate. The preferred lower and upper limits of the epoxy equivalent of component (D) can be arbitrarily combined, for example, 300 to 1000 g / eq or 350 to 950 g / eq.

[0034] The component (D) is not particularly limited as long as it is a bisphenol-type epoxy resin that is solid at 25°C. These bisphenol-type epoxy resins may be used alone or in combination of two or more. Examples of bisphenol-type epoxy resins include bisphenol A-type epoxy resins, bisphenol F-type epoxy resins, bisphenol E-type epoxy resins, bisphenol S-type epoxy resins, bisphenol C-type epoxy resins, bisphenol M-type epoxy resins, bisphenol AF-type epoxy resins, and bisphenol BP-type epoxy resins. Among these, bisphenol A-type epoxy resins are preferred from the viewpoint of improving the mechanical properties of the epoxy resin composition.

[0035] Commercially available bisphenol A epoxy resins that are solid at 25°C include, but are not limited to, YD-011, YD-012, YD-013, and YD-014 (all trade names, manufactured by Nippon Steel Chemical & Material Co., Ltd.); jER1001, jER1002, jER1003, and jER1004 (all trade names, manufactured by Mitsubishi Chemical Corporation). One type of component (D) may be used alone, or two or more types may be used in combination.

[0036] The total content of components (C) and (D) relative to the total mass of the epoxy resin composition is preferably 30% by mass or more, more preferably 32% by mass or more, even more preferably 35% by mass or more, and particularly preferably 40% by mass or more. By ensuring that the total content of components (C) and (D) is at or above the lower limit, the minimum viscosity of the epoxy resin composition is improved and resin flow during molding can be suppressed. Furthermore, the total content of components (C) and (D) relative to the total mass of the epoxy resin composition is preferably 60% by mass or less, more preferably 58% by mass or less, and even more preferably 55% by mass or less. By ensuring that the total content of components (C) and (D) is at or below the upper limit, the impregnation of the epoxy resin composition into the reinforcing fiber substrate is improved and the generation of voids in the fiber-reinforced composite material after molding can be suppressed. The preferred lower and upper limits of the total content of the component (C) and the component (D) can be arbitrarily combined, and can be, for example, 30 to 60 mass%, 32 to 60 mass%, 35 to 58 mass%, or 40 to 55 mass%.

[0037] (Component (E)) Component (E) is at least one selected from dicyandiamide and dicyandiamide derivatives. Dicyandiamide and dicyandiamide derivatives have high melting points and low compatibility with epoxy resins in the low temperature range. When the epoxy resin composition contains component (E), an epoxy resin composition having an excellent pot life while suppressing resin flow during molding can be obtained, and a cured resin product having excellent mechanical properties can be obtained. Examples of dicyandiamide derivatives include those obtained by bonding dicyandiamide with various compounds such as epoxy resins, vinyl compounds, acrylic compounds, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. As component (E), dicyandiamide is preferred from the viewpoint of reactivity.

[0038] When the epoxy resin composition contains component (E), the content of component (E) relative to the total mass of the epoxy resin composition is preferably 4% by mass or more, more preferably 4.5% by mass or more, from the viewpoint of suppressing resin flow in the epoxy resin composition. From the viewpoint of storage stability of the epoxy resin composition, the content of component (E) is preferably 14% by mass or less, more preferably 13.5% by mass or less, relative to the total mass of the epoxy resin composition. The preferred lower and upper limits of the content of component (E) can be arbitrarily combined, and can be, for example, 4 to 14% by mass or 4.5 to 13.5% by mass.

[0039] (Other Components) The epoxy resin composition may further contain components other than the above-mentioned components (hereinafter sometimes referred to as "other components"). Examples of other components include curing agents other than component (A) (hereinafter also referred to as "other curing agents"), epoxy resins other than components (C) and (D) (hereinafter also referred to as "other epoxy resins"), rubber particles, additives, etc.

[0040] The other curing agents are not limited in structure as long as they can cure epoxy resins. Examples include amine-based curing agents such as dicyandiamide, aliphatic amines, alicyclic amines, aromatic amines, and imidazole compounds, as well as acid anhydrides, phenols, and boron chloride amine complexes. These curing agents may be used alone or in combination of two or more.

[0041] As other epoxy resins, epoxy resins in a liquid or semi-solid form at 25°C can be used. Examples include, but are not limited to, bisphenol-type epoxy resins, biphenyl-type epoxy resins, naphthalene-type epoxy resins, phenol novolac-type epoxy resins, cresol novolac-type epoxy resins, and glycidylamine-type epoxy resins that are liquid or semi-solid at 25°C. Bisphenol-type epoxy resins, biphenyl-type epoxy resins, naphthalene-type epoxy resins, phenol novolac-type epoxy resins, cresol novolac-type epoxy resins, and glycidylamine-type epoxy resins that are solid at 25°C can also be used. These epoxy resins may be used alone or in combination of two or more.

[0042] The rubber particles can be synthetic rubber or natural rubber, and examples thereof include those having a structure derived from (meth)acrylic acid alkyl ester, butadiene, isoprene, chloroprene, styrene, α-alkylstyrene, acrylonitrile, and methacrylonitrile. Examples of rubber particles include, but are not limited to, silicone rubber particles, fluororubber particles, ethylene vinyl acetate rubber particles, acrylonitrile butadiene rubber particles, styrene butadiene rubber particles, acrylic rubber particles, and core-shell rubber particles. These may be used alone or in combination of two or more. By including rubber particles in the epoxy resin composition, the fluidity of the epoxy resin composition can be suppressed, thereby suppressing resin flow in the resulting prepreg.

[0043] Various known additives may be used as long as they do not impair the effects of the present invention. Examples of additives include inorganic fillers, internal release agents, organic pigments, and inorganic pigments. More specific examples include flame retardants (metal phosphinates, aluminum hydroxide, magnesium hydroxide, etc.), inorganic oxides and other additives (antimony compounds, zinc borate, zinc stannate, Mo compounds, ZrO, zinc sulfide, zeolites, titanium oxide, etc.), silicone oils, wetting and dispersing agents, antifoaming agents, defoaming agents, natural waxes, synthetic waxes, metal salts of straight-chain fatty acids, acid amides, esters, paraffins, and other release agents; inorganic fillers such as powders, glass fibers, and carbon fibers with fiber lengths of approximately 0.01 mm to 10 mm, such as crystalline silica, fused silica, calcium silicate, alumina, calcium carbonate, talc, and barium sulfate; colorants such as carbon black and red iron oxide; and silane coupling agents. These additives may be used alone or in combination of two or more.

[0044] The epoxy resin composition may contain, for example, 2 to 10 mass% of component (A), 1 to 10 mass% of component (B), and 30 to 60 mass% of components (C) and (D) combined relative to the total mass of the epoxy resin composition, and may further contain 4 to 14 mass% of component (E). However, the total of each component must not exceed 100 mass%. The lower and upper limits of the contents of components (A) to (E) can be any combination of the lower and upper limits described above for each component.

[0045] <Physical Properties of Epoxy Resin Composition> As described above, the epoxy resin composition satisfies the above conditions (1) and (2). When this condition is met, the epoxy resin composition exhibits thixotropy and the reaction initiation point is accelerated. As a result, an epoxy resin composition for fiber-reinforced composite materials can be obtained that can be cured in a short time and that can suppress resin flow during molding.

[0046] (Condition (1)) The fluidity of the resin during molding can be suppressed by setting the minimum viscosity of the epoxy resin composition within a specified range. Here, the minimum viscosity refers to the lowest viscosity measured using a rheometer at 2°C / min. As the rheometer, for example, a HAAKE MARS 40 (manufactured by Thermo Fisher Scientific Co., Ltd.) can be used. The minimum viscosity of the epoxy resin composition is 3.0 Pa·s or more, preferably 3.1 Pa·s or more, and more preferably 3.2 Pa·s or more. A minimum viscosity of 3.0 Pa·s or more reduces the fluidity of the resin during molding, allowing resin flow to be significantly suppressed. Furthermore, the minimum viscosity of the epoxy resin composition is 6.0 Pa·s or less, preferably 5.9 Pa·s or less, more preferably 5.5 Pa·s or less, even more preferably 5.0 Pa·s or less, and particularly preferably 4.5 Pa·s or less. A minimum viscosity of 6.0 Pa·s or less improves the impregnation of the epoxy resin composition into a reinforcing fiber substrate. The preferred lower and upper limits of the minimum viscosity of the epoxy resin composition can be arbitrarily combined, and can be, for example, 3.0 to 6.0 Pa·s, 3.0 to 5.9 Pa·s, 3.1 to 5.5 Pa·s, 3.1 to 5.0 Pa·s, or 3.2 to 4.5 Pa·s. The minimum viscosity of the epoxy resin composition can be adjusted to a specified range by the content of component (A), the median diameter, the content of component (B), and the weight-average molecular weight.

[0047] (Condition (2)) The reaction initiation point of the epoxy resin composition can be adjusted by adjusting the gel time value measured at 145°C using a Curastometer (registered trademark) to fall within a specified range. As the Curastometer (registered trademark), for example, Curastometer (registered trademark) 7 Type P (manufactured by JSR Trading Co., Ltd.) can be used. The gel time value of the epoxy resin composition is 2.0 min or more, preferably 2.1 min or more, more preferably 2.3 min or more, and even more preferably 2.5 min or more. A gel time value of 2.0 min or more improves the thermal stability of the epoxy resin composition. Furthermore, the gel time value of the epoxy resin composition is 5.0 min or less, preferably 4.9 min or less, more preferably 4.5 min or less, and even more preferably 4.0 min or less. A gel time value of 5.0 min or less allows molding to be performed in a short time, thereby suppressing resin flow during molding. The preferred lower and upper limits of the gel time value of the epoxy resin composition can be arbitrarily combined, and can be, for example, 2.0 to 5.0 min, 2.1 to 4.9 min, 2.3 to 4.5 min, or 2.5 to 4.0 min. The gel time value of the epoxy resin composition can be adjusted to fall within the specified range by the content and median diameter of component (A), the content of component (E), and the mass ratio of component (A) to component (E).

[0048] (Condition (3)) The viscosity of the epoxy resin composition at 65°C can be measured using a rheometer at a rate of 2°C / min. As the rheometer, for example, a HAAKE MARS 40 (manufactured by Thermo Fisher Scientific Co., Ltd.) can be used. The viscosity of the epoxy resin composition at 65°C is preferably 10 Pa·s or more, more preferably 20 Pa·s or more, even more preferably 50 Pa·s or more, particularly preferably 70 Pa·s or more, and most preferably 100 Pa·s or more. A viscosity of 10 Pa·s or more ensures sufficient tackiness on the prepreg surface. Furthermore, the viscosity of the epoxy resin composition at 65°C is preferably 1000 Pa·s or less, more preferably 900 Pa·s or less, even more preferably 500 Pa·s or less, particularly preferably 300 Pa·s or less, and most preferably 200 Pa·s or less. By keeping the viscosity at 1000 Pa·s or less, the impregnation into the reinforcing fiber substrate and the molding processability of the prepreg are improved. The preferred lower and upper limits of the viscosity of the epoxy resin composition at 65°C can be arbitrarily combined, and can be, for example, 10 to 1000 Pa·s, 20 to 900 Pa·s, 50 to 500 Pa·s, 70 to 300 Pa·s, or 100 to 200 Pa·s. The viscosity of the epoxy resin composition at 65°C can be adjusted to a specified range by adjusting the content and weight-average molecular weight of component (B) and the total content of components (C) and (D).

[0049] <Preferred Example of Epoxy Resin Composition> A preferred example of an epoxy resin composition (hereinafter also referred to as "epoxy resin composition (I)") contained in the prepreg according to this embodiment contains the following components (A1), (B1), (C), and (D), and further contains the following component (E): Component (A1): a urea compound containing 2,4-bis(3,3-dimethylureido)toluene Component (B1): a polyvinyl formal resin having a weight average molecular weight of 60,000 or more Component (C): an epoxy resin having an oxazolidone structure Component (D): an epoxy resin that is solid at 25°C (excluding the component (C)) Component (E): at least one selected from cyandiamide and dicyandiamide derivatives

[0050] The median diameter (D50) of component (A1) measured by laser diffraction is preferably 8.0 μm or less, and more preferably 7.0 μm or less. By adjusting the median diameter (D50) of component (A1) to 8.0 μm or less, the thixotropy of the epoxy resin composition (I) can be enhanced, thereby suppressing resin flow during molding. Furthermore, the surface area relative to the mass of component (A1) is increased, thereby improving the reaction rate and enabling the epoxy resin composition (I) to be cured in a shorter time. The median diameter (D50) of component (A1) can be adjusted by previously pulverizing component (A1) using a bead mill or a three-roll mill. For measurement by laser diffraction, an AEROTRAC SPR (Model: 7340) (manufactured by Nikkiso Co., Ltd.) can be used, for example. The lower limit of the median diameter (D50) of component (A1) is not particularly limited, but it can be, for example, 0.1 μm or more.

[0051] Furthermore, the median diameter (D50) of the (A1) component measured by laser diffraction is preferably equal to or less than the average fiber diameter of all the carbon fibers contained in the reinforcing fiber substrate. When the median diameter (D50) is equal to or less than the average fiber diameter of all the carbon fibers, the (A1) component is not localized on the surface of the prepreg, thereby improving the surface appearance quality. Furthermore, by distributing the (A1) component throughout the entire fiber-reinforced composite material, poor curing is less likely to occur and resin flow during molding can be suppressed.

[0052] The content of component (A1) relative to the total mass of the epoxy resin composition (I) is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. By ensuring that the content of component (A1) is equal to or greater than the aforementioned lower limit, resin flow during molding is suppressed. Furthermore, the content of component (A1) relative to the total mass of the epoxy resin composition (I) is preferably 10% by mass or less, more preferably 9.5% by mass or less, even more preferably 8% by mass or less, and particularly preferably 5% by mass or less. By ensuring that the content of component (A1) is equal to or less than the aforementioned upper limit, an epoxy resin composition (I) with excellent storage stability and mechanical properties can be obtained. The preferred lower and upper limits for the content of component (A1) can be arbitrarily combined, and can be, for example, 2 to 10% by mass, 2 to 9.5% by mass, 2.5 to 8% by mass, or 3 to 5% by mass.

[0053] The weight-average molecular weight of component (B1) is 60,000 or more, more preferably 65,000 or more. By setting the weight-average molecular weight of component (B1) to the above-mentioned lower limit or more, the minimum viscosity of epoxy resin composition (I) is improved, and resin flow during molding can be suppressed. Furthermore, the weight-average molecular weight of component (B1) is preferably 150,000 or less, more preferably 145,000 or less. By setting the weight-average molecular weight of component (B1) to the above-mentioned upper limit or less, resin flow during molding can be suppressed while improving the handleability of the prepreg at room temperature. The preferred lower and upper limits of the weight-average molecular weight of component (B1) can be arbitrarily combined, and can be, for example, 60,000 to 150,000 or 65,000 to 145,000.

[0054] The content of component (B1) relative to the total mass of epoxy resin composition (I) is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. By ensuring that the content of component (B1) is at or above the above-mentioned lower limit, epoxy resin composition (I) can exhibit high thixotropy, thereby suppressing resin flow during molding. Furthermore, the content of component (B1) relative to the total mass of epoxy resin composition (I) is preferably 10% by mass or less, more preferably 9.5% by mass or less, even more preferably 8% by mass or less, and particularly preferably 5% by mass or less. By ensuring that the content of component (B1) is at or below the above-mentioned upper limit, the handleability of the prepreg at room temperature is improved. The preferred lower and upper limits of the content of component (B1) can be arbitrarily combined, and can be, for example, 1 to 10% by mass, 1 to 9.5% by mass, 1.5 to 8% by mass, or 2 to 5% by mass.

[0055] The components (C) to (E) are as described above, and other components may also be included.

[0056] The epoxy resin composition (I) may have a composition containing, for example, 2 to 10 mass% of component (A1), 1 to 10 mass% of component (B1), and 30 to 60 mass% of components (C) and (D) combined, relative to the total mass of the epoxy resin composition (I). It may also contain 4 to 14 mass% of component (E), provided that the total of each component does not exceed 100 mass%. The lower and upper limits of the amounts of components (A1), (B1), and (C) to (E) can be arbitrarily combined from the lower and upper limits described above for each component.

[0057] The epoxy resin composition (I) preferably satisfies the above-mentioned conditions (1) and (2), and more preferably satisfies the above-mentioned condition (3). The preferred aspects of the above-mentioned conditions (1) to (3) can also be appropriately combined with the epoxy resin composition (I).

[0058] <Method for Producing Epoxy Resin Composition> The epoxy resin composition can be obtained, for example, by mixing the components described above. Examples of methods for mixing the components include methods using a mixer such as a three-roll mill, planetary mixer, kneader, homogenizer, or homodisper. The epoxy resin composition can be used to produce a prepreg, for example, by impregnating a reinforcing fiber substrate, as described below. Alternatively, a film of the epoxy resin composition can be obtained by applying the epoxy resin composition to release paper or the like and curing it. When used as a film, an epoxy resin composition having a viscosity of 100 to 1,000,000 Pa·s at 30°C provides excellent control of the tackiness of the prepreg surface and excellent workability.

[0059] <Reinforcing Fibers> The reinforcing fibers are present in the prepreg as a reinforcing fiber substrate, preferably in the form of a sheet. The reinforcing fiber substrate refers to a single fiber aggregate of reinforcing fibers. The reinforcing fibers may be long fibers (continuous fibers) or short fibers of, for example, 0.01 to 30 cm. The orientation of the fibers in the reinforcing fiber substrate may be such that the reinforcing fibers are aligned in one direction or in a random direction. Examples of the form of the reinforcing fiber substrate include woven reinforcing fibers, nonwoven reinforcing fibers, and sheets in which long reinforcing fibers are aligned in one direction. From the viewpoint of being able to mold a fiber-reinforced composite material with high specific strength and specific modulus, it is preferable to use a sheet made of a bundle of reinforcing fibers in which the long fibers are aligned in one direction (hereinafter sometimes referred to as a "UD substrate") for the prepreg. From the viewpoint of ease of handling, it is preferable to use a woven reinforcing fiber for the prepreg. The basis weight of the reinforcing fiber substrate is 10 to 4000 g / m ２ In the case of a UD substrate, the basis weight is 10 to 300 g / m ２ It may also be possible to use the following.

[0060] Examples of reinforcing fibers include glass fiber, carbon fiber, nylon fiber, aramid fiber, and boron fiber. Among these, carbon fiber is preferred from the viewpoint of the mechanical properties and weight reduction of the resulting fiber-reinforced composite material. The number of carbon fibers in the carbon fiber bundle used in the reinforcing fiber substrate is preferably 1,000 to 70,000. A sheet-like reinforcing fiber substrate can be made by using a plurality of carbon fiber bundles and aligning the fibers in one direction.

[0061] From the viewpoint of the rigidity of the resulting fiber-reinforced composite material, the strand tensile strength of the carbon fiber is preferably 1.5 to 9 GPa, and the strand tensile modulus of the carbon fiber is preferably 150 to 260 GPa. The strand tensile strength and strand tensile modulus of the carbon fiber can be measured in accordance with JIS R7601:1986. The fiber diameter of the carbon fiber can be 3 to 15 μm. When the fiber diameter of the carbon fiber is equal to or greater than the above lower limit, the carbon fiber can be prevented from breaking or becoming lint-filled when the carbon fibers move laterally and rub against each other or against the roll surface during processes such as combing or rolling to process the carbon fiber. Furthermore, the curing agent particles can be uniformly dispersed throughout the substrate, thereby suppressing resin flow during molding.

[0062] <Method for producing prepreg> A prepreg can be obtained, for example, by impregnating a reinforcing fiber substrate with the above-mentioned epoxy resin composition. The prepreg obtained in this manner is a reinforcing fiber substrate impregnated with the epoxy resin composition. Methods for impregnating a reinforcing fiber substrate with the epoxy resin composition include, but are not limited to, a wet method in which the epoxy resin composition is dissolved in a solvent such as methyl ethyl ketone or methanol to reduce the viscosity and then impregnated into the reinforcing fiber substrate; and a hot melt method (dry method) in which the epoxy resin composition is heated to reduce the viscosity and then impregnated into the reinforcing fiber substrate.

[0063] The wet method involves immersing a reinforcing fiber substrate in a solution of an epoxy resin composition, then removing it, and evaporating the solvent using an oven or the like. The hot melt method includes a method in which an epoxy resin composition whose viscosity has been reduced by heating is directly impregnated into a reinforcing fiber substrate, and a method in which the epoxy resin composition is first applied to the surface of a substrate such as release paper to form a film, and then the film is placed on both sides or one side of the reinforcing fiber substrate and heated and pressurized to impregnate the reinforcing fiber substrate with the resin. The coating layer obtained by applying the composition to the surface of a substrate such as release paper may be used in the hot melt method in its uncured state, or it may be used in the hot melt method after curing. The hot melt method is preferred because there is essentially no solvent remaining in the prepreg.

[0064] The prepreg can be produced, for example, by the following procedure. First, a matrix resin is applied to one side of a carrier film (first carrier film). Similarly, another carrier film (second carrier film) is prepared, with the matrix resin applied to one side. Next, the matrix resin-coated surfaces of the first carrier film and the second carrier film are oriented toward the reinforcing fiber substrate, forming a laminate in which the reinforcing fiber substrate and matrix resin are sandwiched between the first carrier film and the second carrier film. The obtained laminate is pressurized to impregnate the reinforcing fiber substrate with the matrix resin, thereby obtaining a prepreg.

[0065] The content of the epoxy resin composition in the prepreg (hereinafter also referred to as "resin content") relative to the total mass of the prepreg (100 mass%) is preferably 15 to 50 mass%, more preferably 20 to 45 mass%, and even more preferably 25 to 40 mass%. When the resin content is equal to or greater than the above lower limit, sufficient adhesion between the reinforcing fibers and the epoxy resin composition can be ensured. When the resin content is equal to or less than the above upper limit, the mechanical properties of the fiber-reinforced composite material are further improved.

[0066] [Fiber-reinforced composite material] A fiber-reinforced composite material is obtained by curing a prepreg. That is, the fiber-reinforced composite material contains a cured product of the resin contained in the prepreg and reinforcing fibers. One embodiment of the fiber-reinforced composite material comprises a reinforcing fiber substrate and a cured product of an epoxy resin composition, the epoxy resin composition containing the aforementioned components (A), (B), (C), and (D). Furthermore, if the epoxy resin composition satisfies the aforementioned conditions (1) and (2), it can be effective in a short time and can suppress resin flow during molding.

[0067] One embodiment of the fiber-reinforced composite material comprises a reinforcing fiber substrate and a cured product of the epoxy resin composition (I), and the epoxy resin composition (I) contains the aforementioned components (A1), (B1), (C), and (D).

[0068] <Method for producing fiber-reinforced composite material> A fiber-reinforced composite material can be obtained, for example, by laminating two or more sheets of the above-mentioned prepreg, and then molding the laminate by applying pressure to the resulting laminate while heating and curing an epoxy resin composition. Examples of molding methods include press molding, autoclave molding, bagging molding, wrapping tape molding, internal pressure molding, sheet wrap molding, RTM (resin transfer molding), VaRTM (vacuum-assisted resin transfer molding), filament winding, and RFI (resin film infusion), which impregnate a reinforcing fiber filament or preform with an epoxy resin composition and cure it to produce a molded product, but the molding method is not limited to these.

[0069] Autoclave molding is a method in which prepregs are laminated, covered with a backing film, and then pressurized and heated to harden while degassing the laminate. This allows precise control of fiber orientation and minimizes void generation, resulting in high-quality molded products with excellent mechanical properties.

[0070] The wrapping tape method is a method in which a prepreg is wound around a core metal such as a mandrel to form a tubular fiber-reinforced composite resin molded body (fiber-reinforced composite resin tubular body), and is preferably used when producing rod-shaped bodies such as golf shafts, fishing rods, etc. More specifically, in this method, the prepreg is wound around a mandrel, a wrapping tape made of a thermoplastic film is wound around the outside of the prepreg to fix and apply pressure to the prepreg, the epoxy resin composition in the prepreg is heat-cured in an oven, and the core metal is then removed to obtain the fiber-reinforced composite resin tubular body.

[0071] The internal pressure molding method involves placing a preform, formed by winding a prepreg around an internal pressure-applying body such as a thermoplastic resin tube, in a mold. Then, high-pressure gas is introduced into the internal pressure-applying body to apply pressure, while simultaneously heating the mold and molding. There are no particular limitations on the heating temperature; however, higher temperatures are preferred because they shorten the molding time. Specifically, temperatures of 120°C or higher are preferred, with 140°C or higher being more preferred. However, if the temperature is too high, it takes a very long time to cool the mold. Furthermore, if the prepreg is set without cooling the temperature, curing may begin and the epoxy resin composition may not penetrate every corner of the final molded product. This method is preferably used when molding complex-shaped objects such as golf shafts, bats, and tennis and badminton rackets. While the high molding pressure of the internal pressure molding method results in rapid curing, it also increases resin flow during molding.

[0072] <Applications of Fiber-Reinforced Composite Material> The fiber-reinforced composite material resin of the present invention is suitable for use in sports, general industrial, and aerospace applications. More specifically, in sports applications, it is suitable for use in golf shafts, fishing rods, tennis and badminton rackets, bicycle frames, hockey sticks, and skeeballs. In general industrial applications, it is suitable for use in structural materials for vehicles such as automobiles, ships, and railway vehicles, drive shafts, leaf springs, wind turbine blades, pressure vessels, flywheels, papermaking rollers, roofing materials, cables, and repair and reinforcement materials.

[0073] [Method for producing a molded body] The method for producing a molded body of the present invention includes the following step (1). The method for producing a molded body of the present invention is preferably a method for producing a tubular molded body, and more preferably includes the following step (2). Step (1): A step of heating a prepreg containing an epoxy resin composition and reinforcing fibers, which is placed in a mold, to 130°C or higher. Step (2): A step of pressing the tubular precursor, in which the prepreg is placed in a tubular shape, against a mold by expanding a medium from inside the tubular precursor. Hereinafter, the method will be described using the case of producing a tubular molded body as an example, but the shape of the molded body to be produced is not limited to a tubular shape.

[0074] When producing a tubular molded article, prepregs are arranged in a tubular shape to form a tubular precursor, which is then heated. The tubular precursor can be obtained by winding a prepreg containing a resin composition and reinforcing fibers around a medium (internal pressure-applying body) such as a thermoplastic resin tube. The obtained tubular precursor is set in a mold and heated to 130°C or higher, preferably 140°C or higher, to be molded. Molding can be performed by introducing high-pressure gas into the internal pressure-applying body to expand the medium and press the tubular precursor against the mold from the inside.

[0075] The epoxy resin composition contained in the prepreg used in the method for producing a molded article of the present invention contains the following components (A1) and (B1), and may further contain the following components (C) and (D): Component (A1): 2,4-bis(3,3-dimethylureido)toluene Component (B1): polyvinyl formal resin having a weight-average molecular weight of 60,000 or more Component (C): epoxy resin having an oxazolidone structure Component (D): epoxy resin that is solid at 25°C (excluding the component (C)).

[0076] The contents and preferred embodiments of the (A1) and (B1) components are the same as those of the (A) and (B) components described above. The specific components, contents, and preferred embodiments of the (C) and (D) components are as described above. The total content of the (C) and (D) components relative to the total mass of the epoxy resin composition is preferably 30 to 60 mass%, more preferably 35 to 60 mass%. When the total content of the (C) and (D) components is equal to or greater than the lower limit described above, resin flow during molding can be suppressed. When the total content of the (C) and (D) components is equal to or less than the upper limit described above, sufficient tackiness can be obtained on the prepreg surface. The prepreg used in the method for producing a molded article of the present invention may, for example, be a prepreg containing the above-described epoxy resin composition (I).

[0077] In the method for producing a molded article of the present invention, when the tubular molded article has an annular curved portion, the method may further include a step of bending the tubular precursor into an annular shape. Examples of the tubular molded article having an annular curved portion include molded articles having a tubular frame that forms an annular racket head, such as a tennis or badminton racket.

[0078] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. The values ​​of various production conditions and evaluation results in the following examples represent preferred upper or lower limit values ​​in the embodiments of the present invention, and preferred ranges may be defined by combining the above-mentioned upper or lower limit values ​​with the values ​​in the following examples or values ​​between the examples.

[0079] <Raw Materials> (Component (A)) <Component (A1)> Omicure 24: 2,4-bis(3,3-dimethylureido)toluene, median diameter (D50) 6.0 μm, "Omicure 24" manufactured by PTI Japan Co., Ltd. <Components other than component (A1)> Omicure 94: 3-phenyl-1,1-dimethylurea, median diameter (D50) 9.5 μm, "Omicure 94" manufactured by PTI Japan Co., Ltd. DCMU-99: 3-(3,4-dichlorophenyl)-1,1-dimethylurea, median diameter (D50) 11.4 μm, "DCMU-99" manufactured by Hojiya Chemical Industry Co., Ltd.

[0080] (Component (B)) <Component (B1)> Vinylec E: fine powder of polyvinyl formal resin, "Vinylec E" manufactured by JNC Corporation <Components other than component (B1)> Vinylec K: fine powder of polyvinyl formal resin, "Vinylec K" manufactured by JNC Corporation 5003MP: fine powder of polyethersulfone resin, "Sumikaexcel 5003MP" manufactured by Sumitomo Chemical Co., Ltd.

[0081] (Component (C)) YD-952: Epoxy resin having an oxazolidone structure, epoxy equivalent weight 340 g / eq, "YD-952" manufactured by Nippon Steel Chemical & Material Co., Ltd.

[0082] (Component (D)) YD-012: solid bisphenol A type epoxy resin, epoxy equivalent 655 g / eq, "YD-012" manufactured by Nippon Steel Chemical & Material Co., Ltd. N-680: cresol novolac type epoxy resin, epoxy equivalent 211 g / eq, "N-680" manufactured by DIC Corporation

[0083] (Component (E)) Dicyanex 1400F: dicyandiamide, "Dicyanex 1400F" manufactured by Evonik Japan Co., Ltd. Dicy 15: dicyandiamide, "jER Cure Dicy 15" manufactured by Mitsubishi Chemical Corporation

[0084] (Other ingredients) YD-128: Liquid bisphenol A epoxy resin, epoxy equivalent 189 g / eq, "YD-128" manufactured by Nippon Steel Chemical & Material Co., Ltd. AEROSIL RY-200: Hydrophobic silicon dioxide, "AEROSIL RY-200" manufactured by Nippon Aerosil Co., Ltd.

[0085] (Carbon fiber) Carbon fiber: "Pyrofil TR50S15L" manufactured by Mitsubishi Chemical Corporation (average fiber diameter 7 μm, specific gravity 1.82, tensile modulus 235 GPa)

[0086] <Rheometer Evaluation> The epoxy resin composition was measured at 2°C / min using a rheometer (HAAKE MARS 40, manufactured by Thermo Fisher Scientific Co., Ltd.), and the lowest viscosity was recorded as the minimum viscosity. The viscosity of the epoxy resin composition at 65°C was also measured using the same rheometer.

[0087] <Curelastometer Evaluation> The epoxy resin composition was measured at 145°C using a Curelastometer (registered trademark, manufactured by JSR Trading Co., Ltd., Type 7P) to obtain a gel time value.

[0088] <Resin Flow Evaluation> The resin flow rate during molding was measured according to the following steps 1 to 5. 1. A prepreg was cut into a 100 mm x 100 mm piece, and eight sheets were stacked so that the fiber direction in plan view was [0° / 90° / 0° / 90° / 90° / 0° / 90° / 0°] to produce a laminate. 2. The weight W1 of the resulting laminate was measured. 3. The laminate was heated in an autoclave at a rate of 4°C / min under a pressure of 0.6 MPa, and then held at 145°C for 30 minutes to heat-cure and mold, producing a fiber-reinforced composite material with a thickness of 1.0 mm. 4. Resin flash protruding from the fiber-reinforced composite material was removed, and the weight W2 of the remaining fiber-reinforced composite material was measured. 5. The resin flow rate was calculated from the measured values ​​of W1 and W2 using the following formula: Resin flow rate [%] = [(W1 - W2) / W1] x 100

[0089] <Measurement of Weight Average Molecular Weight> The weight average molecular weight of the components (B), Vinylec E and Vinylec K, was determined by preparing a 0.05% by mass tetrahydrofuran solution and calculating the weight average molecular weight in terms of standard polystyrene under the following analytical conditions. (Analysis conditions) Apparatus: HLC-8320 GPC manufactured by Tosoh Corporation Columns: (1) to (4) below connected in this order (1) TSKguard column Super HZ-L (guard column) 4.6 mm I·D × 2.0 cm L (2) TSKgel Super HZM-M 4.6 mm I·D × 15 cm L (3) TSKgel Super HZM-M 4.6 mm I·D × 15 cm L (4) TSKgel Super HZ2000 4.6 mm I·D × 15 cm L Thermostatic bath temperature: 40°C Mobile phase: tetrahydrofuran (stabilizer: BHT) Flow rate: 0.35 mL / min Injection volume: 10 μL Detector: RI

[0090] The weight-average molecular weight of 5003MP, component (B), was determined as a weight-average molecular weight converted to standard polystyrene by preparing a 0.4% by mass dimethylformamide solution under the following analytical conditions: Apparatus: Tosoh HLC-8420 High-Performance GPC Guard column: Tosoh TSKguard column α (6.0 mm ID x 4.0 cmL) Sample column: Three Tosoh α-M (7.8 mm ID x 30 cmL) columns connected Thermostat temperature: 40°C Mobile phase: Dimethylformamide (containing 20 mM LiBr) Flow rate: 1.0 mL / min Injection volume: 100 μL Detector: RI

[0091] Examples 1 to 6, Comparative Examples 1 to 9 Preparation of Matrix Resin Compositions Epoxy resin compositions were prepared as follows according to the formulations shown in Table 1. First, component (B) and liquid bisphenol A epoxy resin were weighed into a flask, heated to 150°C using an oil bath, and mixed until component (B) was dissolved. The mixture was then cooled to 100°C, and components (C), (D), and silicon dioxide were added and mixed with stirring. The mixture was then cooled to 65°C, and components (A) and (E) were added and mixed with stirring to obtain epoxy resin compositions. The results of rheometer and curastometer evaluations of the epoxy resin compositions are shown in Tables 1 to 3.

[0092] <Production of prepreg> The obtained epoxy resin composition was formed into a film using a hot melt coater (manufactured by Hirano Tecseed Co., Ltd., "R-HC"), and the resin basis weight was 32.3 g / m ２ This resin film was coated with a carbon fiber having a fiber basis weight of 120 g / m2 obtained by aligning the carbon fibers. ２ and impregnated with a heated roll to give a fiber basis weight of 120 g / m. ２ A prepreg having a resin content of 35% by mass was obtained.

[0093] <Production of Fiber-Reinforced Composite Material> The resulting prepreg was cut into 100 mm x 100 mm pieces, and eight sheets were stacked so that the fiber orientation from bottom to top in a plan view was [0° / 90° / 0° / 90° / 90° / 0° / 90° / 0°] to obtain a laminate. This laminate was molded in an autoclave at a pressure of 0.6 MPa, heated at a rate of 4°C / min, and held at 145°C for 30 minutes for heat curing, yielding a fiber-reinforced composite material with a thickness of 1.0 mm. During molding, the resin flow rate was evaluated according to the evaluation method described above. The results are shown in Tables 1 to 3.

[0094]

[0095]

[0096]

[0097] As shown in Tables 1 to 3, in Examples 1 to 6, which used epoxy resin compositions satisfying the requirements of the present invention, the resin flow rate during molding was significantly suppressed in the production of fiber-reinforced composite materials, compared to Comparative Examples 1 to 9, which did not satisfy the requirements of the present invention. It is estimated that the resin flow rate will be similarly suppressed in other molding methods, such as press molding, wrapping tape molding, internal pressure molding, sheet wrap molding, RTM, and VaRTM.

[0098] This application claims priority based on Japanese Patent Application No. 2023-169784, filed on September 29, 2023, the contents of which are incorporated herein by reference.

Claims

1. A prepreg comprising an epoxy resin composition and reinforcing fibers, wherein the epoxy resin composition comprises the following components (A), (B), (C) and (D), and satisfies the following conditions (1) and (2): Component (A): urea compound Component (B): thermoplastic resin Component (C): epoxy resin having an oxazolidone structure Component (D): bisphenol-type epoxy resin that is solid at 25°C (excluding component (C)) Condition (1): the minimum viscosity of the epoxy resin composition measured at 2°C / min using a rheometer is within the range of 3.0 to 6.0 Pa·s. Condition (2): the gel time of the epoxy resin composition measured at 145°C using a Curastometer (registered trademark) is within the range of 2.0 to 5.0 min.

2. The prepreg according to claim 1, wherein the median diameter (D50) of component (A) measured by laser diffraction is 8.0 μm or less.

3. The prepreg according to claim 1 or 2, wherein the component (A) contains 2,4-bis(3,3-dimethylureido)toluene.

4. The prepreg according to claim 1 or 2, wherein the component (B) is a thermoplastic resin that is soluble in the mixture of epoxy resins contained in the epoxy resin composition.

5. The prepreg according to claim 1 or 2, wherein the weight average molecular weight of component (B) is 60,000 or more.

6. The prepreg according to claim 1 or 2, wherein the component (B) comprises a polyvinyl formal resin.

7. The prepreg according to claim 1 or 2, wherein the component (C) is solid at 25°C.

8. The prepreg according to claim 1 or 2, wherein the epoxy equivalent of component (D) is 300 to 1,000 g / eq.

9. The prepreg according to claim 1 or 2, wherein the component (D) is a bisphenol A type epoxy resin.

10. The prepreg according to claim 1 or 2, comprising 2 to 10 mass% of component (A) based on the total mass of the epoxy resin composition.

11. The prepreg according to claim 1 or 2, comprising 1 to 10 mass % of said component (B) based on the total mass of said epoxy resin composition.

12. The prepreg according to claim 1 or 2, wherein the combined amount of said component (C) and said component (D) is 30 to 60 mass% based on the total mass of said epoxy resin composition.

13. The prepreg according to claim 1 or 2, further satisfying the following condition (3): the viscosity of the epoxy resin composition at 65° C. is within the range of 10 to 1,000 Pa·s.

14. The prepreg according to claim 1 or 2, further comprising the following component (E): Component (E): at least one selected from dicyandiamide and dicyandiamide derivatives.

15. The prepreg according to claim 1 or 2, wherein the reinforcing fibers include carbon fibers.

16. A fiber-reinforced composite material obtained by curing the prepreg according to claim 1 or 2.

17. A prepreg comprising an epoxy resin composition and reinforcing fibers, wherein the epoxy resin composition comprises the following components (A1), (B1), (C) and (D): Component (A1): a urea compound containing 2,4-bis(3,3-dimethylureido)toluene Component (B1): a polyvinyl formal resin having a weight average molecular weight of 60,000 or more Component (C): an epoxy resin having an oxazolidone structure Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding the component (C)).

18. The prepreg according to claim 17, which satisfies the following conditions (1) and (2): Condition (1): The epoxy resin composition has a minimum viscosity in the range of 3.0 to 6.0 Pa·s as measured by a rheometer under conditions of 2°C / min. Condition (2): The epoxy resin composition has a gel time in the range of 2.0 to 5.0 min as measured by a Curastometer (registered trademark) under conditions of 145°C.

19. The prepreg according to claim 17 or 18, wherein the median diameter (D50) of said component (A1) measured by laser diffraction method is 8.0 μm or less.

20. The prepreg according to claim 17 or 18, wherein the component (C) is solid at 25°C.

21. The prepreg according to claim 17 or 18, wherein the epoxy equivalent of component (D) is 300 to 1,000 g / eq.

22. The prepreg according to claim 17 or 18, wherein component (D) is a bisphenol A type epoxy resin.

23. The prepreg according to claim 17 or 18, comprising 2 to 10 mass% of the component (A1) relative to the total mass of the epoxy resin composition.

24. The prepreg according to claim 17 or 18, comprising 1 to 10 mass% of the component (B1) relative to the total mass of the epoxy resin composition.

25. The prepreg according to claim 17 or 18, wherein the combined amount of the component (C) and the component (D) is 30 to 60 mass% based on the total mass of the epoxy resin composition.

26. The prepreg according to claim 17 or 18, further satisfying the following condition (3): The viscosity of the epoxy resin composition at 65° C. is within the range of 10 to 1,000 Pa·s.

27. The prepreg according to claim 17 or 18, further comprising the following component (E): Component (E): at least one selected from dicyandiamide and dicyandiamide derivatives.

28. The prepreg of claim 17 or 18, wherein the reinforcing fibers include carbon fibers.

29. A method for producing a molded article, comprising the step of heating a prepreg containing an epoxy resin composition and reinforcing fibers, which is placed in a mold, to 130°C or higher, the epoxy resin composition containing the following components (A1) and (B1): Component (A1): 2,4-bis(3,3-dimethylureido)toluene Component (B1): polyvinyl formal resin having a weight average molecular weight of 60,000 or more 30. The method for producing a molded article according to claim 29, wherein the prepreg is arranged in a tubular shape and heated to produce a tubular molded article.

31. The method for producing a molded article according to claim 30, further comprising a step of pressing the tubular precursor, in which the prepreg is arranged in a tubular shape, against a die by expanding a medium from inside the tubular precursor, in which the prepreg is arranged in a tubular shape, to form the tubular precursor.

32. The method for producing a molded article according to claim 29 or 30, wherein the epoxy resin composition further contains the following component (C): Component (C): Epoxy resin having an oxazolidone structure 33. The method for producing a molded article according to claim 29 or 30, wherein the epoxy resin composition further contains the following component (D): Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C): an epoxy resin that is solid at 25°C).

34. The method for producing a molded article according to claim 29 or 30, wherein the epoxy resin composition contains the following component (C) and component (D) in a total amount of 30 to 60 mass% based on the total mass of the epoxy resin composition: Component (C): an epoxy resin having an oxazolidone structure; Component (D): a bisphenol-type epoxy resin that is solid at 25°C (excluding component (C)).

35. The method for producing a molded body according to claim 31, wherein the tubular molded body has an annular curved portion, the method including a step of bending the tubular precursor into an annular shape.