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

Abstract

The present invention aims to provide epoxy resin compositions, resin films, and prepregs that are suitable for use in fiber-reinforced composite materials due to their excellent handling properties and superior elastic modulus and strength in room temperature, high temperature, and high humidity environments when cured. [Solution] An epoxy resin composition for fiber-reinforced composite materials that includes the following components [A] to [C] and satisfies conditions (1) and (2). [A]: Epoxy resin having reactivity with component [B], [B]: Amine curing agent, [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650. (1): Contains 3 to 30 parts by mass of component [C] per 100 parts by mass of component [A]. (2) When the epoxy resin composition hardens, the component [C] does not form a self-polymerizing polymer.

Description

Technical Field

[0001] The present invention relates to an epoxy resin composition, an epoxy resin film, a prepreg, and a fiber reinforced composite material, which are suitably used for fiber reinforced composite materials for aerospace applications, general industrial applications, sports applications, and the like.

Background Art

[0002] Fiber reinforced composite materials using carbon fibers, aramid fibers, etc. as reinforcing fibers are widely used for structural materials such as aircraft and automobiles, and for sports and general industrial applications such as tennis rackets, golf shafts, fishing rods, bicycles, and housings, by taking advantage of their high specific strength and specific modulus of elasticity. As the resin composition used for this fiber reinforced composite material, thermosetting resins are mainly used from the viewpoints of heat resistance and productivity, and among them, epoxy resins are preferably used from the viewpoint of mechanical properties such as adhesion to reinforcing fibers.

[0003] For the production of fiber reinforced composite materials, various methods are used, such as the prepreg method, the sheet molding compound method, the hand lay-up method, the filament winding method, the pultrusion method, and the RI (Resin Infusion) molding method. In any of these molding methods, in order to apply fiber reinforced composite materials to applications that require further weight reduction, it is necessary to improve various physical properties.

[0004] For the purpose of improving various mechanical properties of fiber reinforced composite materials, an improvement in the elastic modulus and strength of the epoxy resin used as the matrix resin is required. However, a cured product of an epoxy resin having a high elastic modulus is generally brittle and may have low strength and elongation. Also, when applied to structural materials such as aerospace applications and vehicles, physical properties under high temperature and / or high humidity conditions are also important. Therefore, in addition to the elastic modulus and strength of the matrix resin at room temperature, it has been a technical problem to increase the elastic modulus and strength under high temperature and / or high humidity environments.

[0005] Various studies have been conducted to address these issues. For example, it has been described that an epoxy resin composition with excellent resin strength at room temperature after curing can be obtained by incorporating a compound that improves the solubility of dicyandiamide, a curing agent for epoxy resin compositions, and by adjusting the viscosity of the epoxy resin composition (Patent Document 1). It has also been described that an epoxy resin composition with an excellent balance of elastic modulus at room temperature and in a humid heat environment can be obtained by incorporating a phosphate ester compound with a molecular weight of 250 or less (Patent Document 2).

[0006] On the other hand, research is underway to obtain a resin cured product with excellent strength, elastic modulus, and toughness by combining and curing a specific epoxy resin with radically polymerizable bifunctional or more (meth)acrylate compounds (Patent Document 3). Research is also underway to obtain a hot-melt epoxy resin composition with high strength and high elastic modulus by combining and blending two specific types of amide compounds (Patent Document 4). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2019 / 181402 [Patent Document 2] International Publication No. 2022 / 201890 [Patent Document 3] Patent No. 6993549 [Patent Document 4] International Publication No. 2024 / 113391 [Overview of the project] [Problems that the invention aims to solve]

[0008] While the techniques described in Patent Document 1 or Patent Document 2 yielded excellent elastic modulus and strength at room temperature after curing of the epoxy resin composition, even higher levels of mechanical properties were required under high temperature and / or high humidity conditions.

[0009] While the technology described in Patent Document 3 makes it possible to obtain epoxy resin cured products with excellent toughness, further improvements were needed in terms of mechanical properties under high temperature and / or high humidity conditions. In particular, this technology uses radically polymerizable monomers for the purpose of radical polymerization, and for that purpose uses radical polymerization initiators as substantially essential components. Therefore, when using compounds with relatively low molecular weight, it was not possible to obtain cured products in a way that the compounds were not incorporated into the crosslinking structure.

[0010] While it is sometimes possible to obtain high-strength, high-modulus epoxy resin cured products using the technology described in Patent Document 4, it requires a specific amide compound as an essential component, and such compounds tend to precipitate easily, making them difficult to handle as epoxy resin compositions.

[0011] Therefore, the object of the present invention is to provide epoxy resin compositions, resin films, and prepregs that are suitable for use in fiber-reinforced composite materials due to their excellent handling properties and elastic modulus and strength in room temperature, high temperature, and high humidity environments when cured. [Means for solving the problem]

[0012] To solve these problems, the present invention employs the following means. 1. An epoxy resin composition for fiber-reinforced composite materials that contains the following components [A] to [C] and satisfies conditions (1) and (2). [A]: Epoxy resin having reactivity with component [B], [B]: Amine curing agent, [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650. (1): Contains 3 to 30 parts by mass of component [C] per 100 parts by mass of component [A]. (2) When the epoxy resin composition hardens, the component [C] does not form a self-polymerizing polymer. 2. An epoxy resin composition for fiber-reinforced composite materials comprising the following components [A] to [C], wherein component [C] is present in 3 to 30 parts by mass per 100 parts by mass of component [A], and which does not contain radical polymerization initiators or cationic polymerization initiators, and which has a flexural modulus of 3.5 GPa or more when measured at room temperature after curing. [A]: Epoxy resin having reactivity with component [B], [B]: Amine curing agent, [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650. 3. The epoxy resin composition for fiber-reinforced composite materials according to claim 1 or 2, wherein the component [C] has at least one functional group selected from an acryloyl group, a cycloalkene oxide type epoxy group, and an alcoholic hydroxyl group. 4. The epoxy resin composition for fiber-reinforced composite materials according to 3, wherein the component [C] has an acryloyl group. 5. The epoxy resin composition for fiber-reinforced composite materials according to 3, wherein the component [C] has a cycloalken oxide type epoxy group. 6. The epoxy resin composition for fiber-reinforced composite materials according to 3, wherein component [C] has two or more alcoholic hydroxyl groups. 7. An epoxy resin composition for fiber-reinforced composite materials according to any one of 1 to 6 above, wherein the amount of radical polymerization initiator or cationic polymerization initiator blended is less than 0.1 parts by mass per 100 parts by mass of constituent [C]. 8. An epoxy resin composition for fiber-reinforced composite materials according to any one of items 1 to 7 above, wherein the flexural modulus of elasticity when cured is 3.5 GPa or more at room temperature. 9. An epoxy resin composition for fiber-reinforced composite materials according to any one of items 1 to 8, further comprising a thermoplastic resin. 10. An epoxy resin film for fiber-reinforced composite materials comprising the epoxy resin composition for fiber-reinforced composite materials described in any of items 1 to 9 above. 11. A prepreg for fiber-reinforced composite materials comprising at least partially impregnating continuous reinforcing fibers with the epoxy resin film for fiber-reinforced composite materials described in 10 above. 12. A fiber reinforced composite material obtained by curing a fiber reinforced composite material epoxy resin composition according to any one of 1 to 9 above impregnated in continuous reinforcing fibers.

Advantages of the Invention

[0013] According to the present invention, an epoxy resin composition, a resin film, and a prepreg that are excellent in handleability and elastic modulus and strength under room temperature, high temperature, and high humidity environments when cured, and can be suitably used for fiber reinforced composite materials can be obtained.

Modes for Carrying Out the Invention

[0014] Hereinafter, the present invention will be described in detail.

[0015] The epoxy resin composition for a fiber reinforced composite material of the present invention contains components [A] to [C]. In the present invention, "component" means each compound contained in the composition. Further, for any physical property, characteristic, or composition ratio, any upper limit value and any lower limit value within the numerical ranges described below can be arbitrarily combined (for example, for H / E described below, a preferable range can be obtained by combining 0.30 or more and 1.20 or less).

[0016] Component [A] in the present invention is an epoxy resin having reactivity with component [B]. Component [A] preferably contains two or more epoxy groups in one molecule because it can increase the glass transition temperature of the cured product obtained by heat-curing the resin composition and improve the heat resistance. Further, an epoxy resin containing one epoxy group in one molecule may be blended. These epoxy resins may be used alone or in appropriate combinations. Whether component [A] has reactivity with component [B] can be confirmed by differential scanning calorimetry (DSC) of a mixture obtained by mixing component [A] and component [B] in an equimolar amount. As a measuring device, for example, Pyris1 DSC (manufactured by Perkin Elmer) can be used. The mixture is sampled into an aluminum sample pan and heated from 0°C to 180°C at a heating rate of 10°C / min in a nitrogen atmosphere for measurement. In the obtained DSC curve, the presence or absence of reactivity can be determined by checking whether reaction heat generation is observed. However, when the components contain a compound having a boiling point of less than 180°C, it is confirmed by checking whether reaction heat generation is observed from 0°C to its boiling point. When the resin composition has a plurality of compounds corresponding to component [B], if it shows reactivity with any of them, it may be regarded as satisfying the conditions of component [A].

[0017] In addition, cycloalkene oxide type epoxy resins are mentioned as epoxy resins having no reactivity with component [B]. Cycloalkene oxide type epoxy resins have poor reactivity with the amine curing agent of component [B] in the present invention described later and the curing reaction does not substantially proceed, so they are not included in the epoxy resin of component [A]. The amine curing agent related to component [B] that can be combined as an epoxy resin composition for fiber-reinforced composite materials will be described later.

[0018] Examples of epoxy resins for component [A] include diaminodiphenylmethane type, diaminodiphenylsulfone type, aminophenol type, metaxylenediamine type or 1,3-bisaminomethylcyclohexane type glycidylamine type epoxy resins, bisphenol type (e.g., bisphenol A type, bisphenol F type), phenol novolac type, orthocresol novolac type, trishydroxyphenylmethane type, tetraphenyloleethane type or bisnaphthalene type glycidyl ether type epoxy resins, terephthalic acid type or phthalic acid type glycidyl ester type epoxy resins, isocyanurate type epoxy resins, oxazolidone type epoxy resins, and the like.

[0019] In particular, glycidylamine-type epoxy resins or glycidyl ether-type epoxy resins are preferred due to their good balance of physical properties. In the present invention, as component [A], it is preferable that glycidylamine-type epoxy resin or glycidyl ether-type epoxy resin is included in 25 parts by mass or more, more preferably 40 parts by mass or more, and even more preferably 60 parts by mass or more, out of 100 parts by mass of component [A].

[0020] In addition, other epoxy compounds may be appropriately incorporated into the epoxy resin composition of the present invention.

[0021] Component [B] in the present invention is an amine curing agent. The amine curing agent has an amino group that can react with an epoxy group and functions as a curing agent. Examples of amine curing agents include aliphatic amines and aromatic amines. These amine curing agents may be used alone or in appropriate combinations.

[0022] Aliphatic amines include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, diethylaminopropylamine, isophoronediamine, bis(4-aminocyclohexyl)methane, orthoxylendiamine, metaxyllendiamine, paraxyllendiamine, and dicyandiamide.

[0023] Aromatic amines include diethyltoluenediamines, including 2,2'-diethyldiaminodiphenylmethane, 2,4-diethyl-6-methyl-m-phenylenediamine, 4,6-diethyl-2-methyl-m-phenylenediamine and 4,6-diethyl-m-phenylenediamine, 4,4'-methylenebis(N-methylaniline), 4,4'-methylenebis(N-ethylaniline), 4,4'-methylenebis(N-sec-butylaniline), N,N'-di-sec-butyl-p-phenylenediamine, 4,4'-diaminodiphenylmethane, 3,3'-diisopropyl-4,4'-diaminodiphenylmethane, 3,3'-di-t-butyl-4,4'-diaminodiphenylmethane, and 3,3'-diethyl-5,5'-dimethyl-4,4'-di Examples include diaminodiphenylmethanes, including aminodiphenylmethane, 3,3'-di-t-butyl-5,5'-dimethyl-4,4'-diaminodiphenylmethane, 3,3'-5,5'-tetraethyl-4,4'-diaminodiphenylmethane, 3,3'-diisopropyl-5,5'-diethyl-4,4'-diaminodiphenylmethane, 3,3'-di-t-butyl-5,5'-diethyl-4,4'-diaminodiphenylmethane, 3,3'-di-t-butyl-5,5'-diisopropyl-4,4'-diaminodiphenylmethane, and 3,3'-5,5'-tetra-t-butyl-4,4'-diaminodiphenylmethane, as well as diaminodiphenyl sulfones, including 3,3'-diaminodiphenylsulfone and 4,4'-diaminodiphenylsulfone.

[0024] One preferred embodiment is the use of a solid hardening agent, as it can effectively improve the pot life, and one preferred embodiment is that the hardening agent contains at least one of dicyandiamide, 3,3'-diaminodiphenylsulfone, and 4,4'-diaminodiphenylsulfone.

[0025] Furthermore, using an aromatic amine curing agent is also a preferred embodiment because it can improve the heat resistance of the cured product. In particular, it is preferable to use diaminodiphenylmethanes or diaminodiphenylsulfones in terms of the balance between heat resistance and mechanical properties.

[0026] In the present invention, the lower limit of the ratio H / E, the number of moles E of epoxy groups in component [A] to the number of moles H of active hydrogen in the polyamine curing agent, is preferably 0.20 or higher, more preferably 0.30 or higher, and even more preferably 0.50 or higher. The upper limit of H / E is preferably 1.50 or lower, more preferably 1.30 or lower, and even more preferably 1.20 or lower. By keeping H / E within this range, a crosslinked structure can be appropriately formed by the reaction between the epoxy resin and the polyamine curing agent, resulting in a cured resin product with excellent strength and elongation.

[0027] In particular, when aromatic amines such as diaminodiphenylmethanes and diaminodiphenylsulfones are used as component [B] in the present invention, the lower limit of H / E is preferably 0.60 or higher, more preferably 0.70 or higher, and even more preferably 0.80 or higher. The upper limit of H / E is preferably 1.50 or lower, more preferably 1.30 or lower, and even more preferably 1.20 or lower. By keeping H / E within this range, it may be possible to improve the balance between the heat resistance and mechanical properties of the epoxy resin composition of the present invention.

[0028] Furthermore, when dicyandiamide is used as component [B] in the present invention, the lower limit of H / E is preferably 0.20 or higher, more preferably 0.30 or higher, and even more preferably 0.50 or higher. The upper limit of H / E is preferably 1.10 or lower, more preferably 1.00 or lower, and even more preferably 0.80 or lower. By setting H / E within this range, it is sometimes possible to obtain an epoxy resin cured product with an excellent balance of heat resistance and mechanical properties.

[0029] Component [C] is a compound that is substantially reactive with both component [A] (epoxy resin) and component [B] (amine curing agent). If a resin composition contains multiple components [A] and [B], it means that the compound is not reactive with any of them. In other words, in the crosslinked structure formed by the reaction of the epoxy resin and the amine curing agent, component [C] is not incorporated into the crosslinked structure but exists in the voids and maintains this state even after curing. By including component [C], the elastic modulus of the resulting epoxy resin cured product is increased, and the elongation is also maintained, resulting in a high-strength epoxy resin cured product. It is believed that component [C] is not constrained by covalent bonds with the crosslinked structure formed by the reaction of component [A] and component [B], and can effectively fill the voids in the cured product, thereby increasing the elastic modulus of the cured product. Furthermore, whether or not a component [C] is substantially reactive with the epoxy resin and the amine curing agent can be confirmed by differential scanning calorimetry (DSC) of a mixture obtained by mixing component [C] and the epoxy resin, or component [C] and the amine curing agent, in equimolar amounts. The measuring apparatus and method for confirmation are the same as those described above for the reactivity between component [A] and component [B]. Here, compounds such as amines and phenols that can undergo addition reactions with epoxy resins, acid anhydrides that can copolymerize with epoxy resins, imidazoles that can act as initiators for the self-polymerization reaction of epoxy resins, aromatic urea compounds, and tertiary amine compounds are compounds that have the ability to cure epoxy resins and do not fall under component [C].

[0030] Component [C] has a melting point of 150°C or lower, preferably 120°C or lower, and more preferably 80°C or lower. If the melting point is higher than 150°C, component [C] is not easily compatible with other components in the epoxy resin composition, and crystals of component [C] may precipitate during the preparation or storage of the resin composition, making it difficult to reliably obtain the improved mechanical properties of the cured product. Furthermore, it is preferable for component [C] to be liquid at room temperature from the viewpoint of compatibility with other components in the epoxy resin composition. There is no preferred lower limit for the melting point of component [C]; as long as it is liquid at room temperature, there is no problem, so any compound with a melting point of 0°C or higher is preferably used.

[0031] The molecular weight of component [C] is greater than 250 and less than 650. When the molecular weight of component [C] is 250 or less, the resulting cured product has poor mechanical properties in high-temperature environments, and component [C] is prone to volatilization during curing. On the other hand, when the molecular weight of component [C] is 650 or more, the resulting cured product has poor mechanical properties in both room temperature and high-temperature environments. From the viewpoint of obtaining a cured product with excellent flexural modulus and strength in both room temperature and high-temperature environments, the molecular weight of component [C] is preferably greater than 250, more preferably 251 or more, and preferably less than 550.

[0032] Component [C] is preferably a compound having at least one functional group selected from the group consisting of alcoholic hydroxyl groups, amide groups, ketone groups, ether groups, sulfoxide groups, imide groups, ester groups, acryloyl groups, and cycloalkene oxide type epoxy groups within its molecule. More preferably, it is a compound having at least one functional group selected from the group consisting of acryloyl groups and cycloalkene oxide type epoxy groups, or a compound having two or more alcoholic hydroxyl groups. When component [C] has at least one functional group selected from the group consisting of acryloyl groups and cycloalkene oxide type epoxy groups, or when it has two or more alcoholic hydroxyl groups, component [C] is more easily retained in the voids of the crosslinked structure, which can lead to excellent mechanical properties at room temperature and high temperatures when cured, and may also improve the handling of the epoxy resin composition because the melting point tends to be lower.

[0033] The component [C] preferably has at least one cyclic structure within its structure. By using a compound with at least one cyclic structure within its structure, the mechanical properties at room temperature and high temperature when cured tend to be excellent. Specific cyclic structures include cycloalkane structures, cycloalkene structures, cycloalkyne structures, bicyclo structures, benzene structures, naphthalene structures, or anthracene structures.

[0034] The boiling point of component [C] is preferably 210°C or higher, and more preferably 230°C or higher. A boiling point of 210°C or higher for component [C] reduces volatilization during the curing of the epoxy resin composition, suppressing void formation and a decrease in mechanical properties in the cured product. Furthermore, the boiling point of component [C] is preferably 400°C or lower from the viewpoint of compatibility with the epoxy resin and curing agent. In this invention, the boiling point is the value at atmospheric pressure (101 kPa). If the boiling point at atmospheric pressure cannot be measured, the converted boiling point converted to 101 kPa using a boiling point conversion chart can be used.

[0035] The component [C] may be a single compound, or it may be a combination of several compounds as appropriate.

[0036] In the epoxy resin composition of the present invention, component [C] is present in an amount of 3 to 30 parts by mass per 100 parts by mass of component [A]. If the content is less than 3 parts by mass, the mechanical properties of the cured product will not improve. On the other hand, if it is present in an amount exceeding 30 parts by mass, the heat resistance of the cured product will be insufficient. From the viewpoint of the mechanical properties and heat resistance of the cured product, it is preferable that the lower limit of component [C] is 5 parts by mass or more and the upper limit is 20 parts by mass or less per 100 parts by mass of component [A].

[0037] In a first aspect of the present invention, when the epoxy resin composition cures, component [C] does not form a self-polymerizable polymer. The self-polymerizability of component [C] is determined by the following method: component [C] alone, or a mixture containing component [C] and either a radical polymerization initiator or a cationic polymerization initiator, is treated under the same curing conditions as the epoxy resin composition. The molecular weight before and after treatment is measured by gel permeation chromatography, and if the weight-average molecular weight (polystyrene equivalent) after treatment is twice or less than that before treatment, it is determined that "a self-polymerizable polymer is not formed." When a compound having at least one functional group selected from the group consisting of an acryloyl group, a cycloalkene oxide type epoxy group, and an alcoholic hydroxyl group is incorporated as component [C] in the present invention, by using a component that does not form a self-polymerizable polymer as component [C], the reactions between component [C]s do not proceed, and such component [C] is less likely to react with component [A], thus the effects targeted by the present invention described above can be obtained. A second aspect of the present invention can be described as an aspect relating to a means for realizing the characteristics of the first aspect, and specifically, the epoxy resin composition does not contain either a radical polymerization initiator or a cationic polymerization initiator. If a radical polymerization initiator or a cationic polymerization initiator is substantially included, the component [C] is more likely to form a self-polymerizable polymer, in which case the reaction between component [C] and / or between component [A] and component [C] will proceed, and the desired effect cannot be obtained. From this viewpoint, substantially not including radical polymerization initiators and cationic polymerization initiators means including an amount in which the reaction between component [C] and / or between component [A] and component [C] does not proceed, preferably less than 0.1 parts by mass per 100 parts by mass of component [C], more preferably less than 0.05 parts by mass, even more preferably less than 0.01 parts by mass, and particularly preferably not including at all. Examples of radical polymerization initiators include azo compounds, peroxide compounds, and photoradical polymerization initiators. Cationic polymerization initiators include those that generate cationic species or Lewis acids upon exposure to heat or light, such as sulfonium salts, boron-halogenated amine complexes, and their derivatives.

[0038] The epoxy resin composition of the present invention may contain a curing accelerator from the viewpoint of controlling the curing speed. Examples of curing accelerators include urea compounds and imidazole compounds, and urea compounds are particularly preferred from the viewpoint of storage stability of the epoxy resin composition. Examples of urea compounds include 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-(4-chlorophenyl)-1,1-dimethylurea, phenyldimethylurea, and toluenebisdimethylurea.

[0039] Furthermore, the epoxy resin composition of the present invention preferably contains component [D]: thermoplastic resin from the viewpoint of adjusting viscoelasticity and the heat resistance of the cured product. Component [D]: thermoplastic resin is miscible with component [A]: epoxy resin. Miscibility with epoxy resin means that when the thermoplastic resin is mixed with the epoxy resin and heated and stirred at a temperature suitable for preparing a resin composition for impregnation of carbon fibers, a homogeneous phase is formed. A homogeneous phase means that no phase separation is visible. While some undissolved thermoplastic resin is acceptable, it is more preferable that all of it dissolves and forms a homogeneous phase with the epoxy resin. Examples of thermoplastic resins that can be used in the invention include polyamide, polycarbonate, polyphenylene oxide, polyphenylene sulfide, polyamide-imide, polyimide, polyetherimide, polysulfone, polyethersulfone, polyether ketone, polyetherether ketone, polyether nitrile, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, polyvinyl formal, and phenoxy resin.

[0040] The epoxy resin composition of the present invention preferably has a flexural modulus of 3.5 GPa or higher at room temperature, more preferably 3.7 GPa or higher, and even more preferably 3.9 GPa or higher. If the flexural modulus of the cured product is less than 3.5 GPa at room temperature, the mechanical properties of the fiber-reinforced composite material tend to be insufficient. Generally, a flexural modulus of 6.0 GPa or less at room temperature is sufficient. Furthermore, the epoxy resin composition of the present invention preferably has a flexural modulus of 3.0 GPa or higher at high temperature, more preferably 3.5 GPa or higher. Generally, a flexural modulus of 5.5 GPa or less at high temperature is sufficient. In all of the above measurement conditions, the curing conditions are a heating time of 120 minutes and a heating temperature of 180°C. However, if the range of flexural modulus at room temperature is met when cured at a lower heating temperature (e.g., 135°C), it can be considered that the same range of modulus is met under the conditions of 180°C and 120 minutes. If the degree of hardening is less than 90% under heating conditions of 180°C for 120 minutes, a higher temperature may be used. The conditions for room temperature measurement and high-temperature measurement will be described later in the examples.

[0041] Preferred reinforcing fibers used in the prepreg of the present invention include carbon fibers, graphite fibers, aramid fibers, glass fibers, etc., but carbon fibers are particularly preferred. The form and arrangement of the reinforcing fibers are not limited, and fiber structures such as long fibers aligned in one direction, a single tow, a woven fabric, a knit, and a braid can be used. Two or more types of carbon fibers, or glass fibers, aramid fibers, boron fibers, PBO fibers, high-strength polyethylene fibers, alumina fibers, and silicon carbide fibers may be used in combination as reinforcing fibers.

[0042] Examples of carbon fibers include acrylic, pitch, and rayon-based carbon fibers, with acrylic-based carbon fibers, which have particularly high tensile strength, being preferred.

[0043] While twisted, untwisted, and untwisted carbon fibers can be used, twisted fibers are not oriented parallel to each other, which can lead to a decrease in the mechanical properties of the resulting carbon fiber reinforced composite material. Therefore, untwisted or untwisted fibers, which offer a good balance between moldability and strength properties of the carbon fiber reinforced composite material, are preferred.

[0044] The carbon fibers preferably have a tensile modulus of 200 GPa or more and 440 GPa or less. This range is preferable because it allows for a high level of balance between the stiffness and strength of the carbon fiber reinforced composite material. A more preferable lower limit for the tensile modulus is 230 GPa or more, and even more preferably 260 GPa or more. A more preferable upper limit for the tensile modulus is 400 GPa or less, and even more preferably 370 GPa or less. Here, the tensile modulus of the carbon fibers is the value measured in accordance with JIS R7608 (2008).

[0045] The prepreg of the present invention is in a form in which an epoxy resin composition is impregnated into reinforcing fibers in advance, and can be manufactured by various known methods. For example, the prepreg can be manufactured by a hot melt method, in which the resin composition is reduced in viscosity by heating without using an organic solvent and then impregnated into the reinforcing fibers. The hot melt method is preferred over the wet method, which uses organic solvents, because it is less likely to generate voids in the molded product.

[0046] Furthermore, the prepreg of the present invention is not limited to those in which the epoxy resin composition is completely impregnated into the interior of the reinforcing fiber layer. Rather, any prepreg in which the reinforcing fiber layer is partially impregnated, or more specifically, any prepreg in which the surface layer or the reinforcing fiber layer is partially impregnated and the epoxy resin composition and reinforcing fiber can be handled as a single unit, is considered a prepreg. In other words, any prepreg in which the reinforcing fiber layer is at least partially impregnated with the epoxy resin composition is considered a prepreg as defined in the present invention.

[0047] The reinforcing fiber content in the prepreg is preferably 30% by mass or more and 90% by mass or less. By setting the content to 30% by mass or more, more preferably 35% by mass or more, and even more preferably 65% ​​by mass or more, it is easier to obtain the advantages of fiber-reinforced composite materials, such as superior specific strength and specific modulus. Furthermore, by keeping the fiber content within the above range, it is possible to suppress the generation of excessive heat during curing when molding the fiber-reinforced composite material. On the other hand, by setting the content to 90% by mass or less, more preferably 85% by mass or less, it is easier to suppress the generation of voids in the composite material due to poor resin impregnation, and it is easier to maintain the adhesion between prepregs.

[0048] The fiber-reinforced composite material of the present invention can be manufactured, for example, by laminating the prepregs of the present invention described above in a predetermined form and curing the resin by applying pressure and heat. Methods for applying heat and pressure include press molding, autoclave molding, bagging molding, wrapping tape molding, and internal pressure molding.

[0049] Furthermore, the fiber-reinforced composite material of the present invention can also be obtained by using the epoxy resin composition of the present invention and impregnating reinforcing fibers with the epoxy resin composition of the present invention in a molding process such as the filament winding method, pultrusion method, and RI molding method, and then curing the epoxy resin composition by heating and pressurizing.

[0050] The method for producing the fiber-reinforced composite material of the present invention will be described using the RI molding method as an example. The epoxy resin composition of the present invention can be injected into a reinforcing fiber substrate placed in a mold heated to a specific temperature to impregnate it, and then cured in the mold to produce a fiber-reinforced composite material. In this case, the temperature at which the epoxy resin composition is injected and impregnated and the temperature at which it is subsequently cured may be the same or different. The injection pressure of the epoxy resin composition is usually preferably 0.1 to 1.0 MPa, and even when pressurized injection is performed, the generation of voids can be suppressed by reducing the pressure inside the mold before injecting the epoxy resin composition. In addition, a closed mold made of rigid bodies on both sides may be used as the mold, or a rigid open mold and a flexible film (bag) may be used. In the latter case, the reinforcing fiber substrate can be placed between the rigid open mold and the flexible film.

[0051] When obtaining fiber-reinforced composite materials using the RI molding method, in order for the fiber-reinforced composite material to have high specific strength or specific modulus, the volume content of reinforcing fibers in the fiber-reinforced composite material is preferably in the range of 40 to 85%. The fiber volume content referred to here is a value measured in accordance with ASTM D3171 (1999), and refers to the state after the epoxy resin composition has been injected into the reinforcing fiber substrate and cured.

[0052] The aforementioned RI molding method is merely an example and is not intended to be limiting.

[0053] The fiber-reinforced composite material of the present invention can be widely used in aerospace, general industrial, and sports applications. More specifically, in general industrial applications, it is suitably used in structures such as automobiles, ships, and railway vehicles. In sports applications, it is suitably used in golf shafts, fishing rods, and tennis and badminton rackets. [Examples]

[0054] The present invention will be described below with reference to examples. However, the scope of the present invention is not limited to these examples. Unless otherwise noted, the unit "parts" in composition ratios refers to parts by mass. Furthermore, unless otherwise noted, the various properties (physical properties) were measured under conditions of 23°C and 50% relative humidity.

[0055] <Materials used in the examples and comparative examples> (1) Component [A]: Epoxy resin having reactivity with component [B] • "SumiEpoxy (registered trademark)" ELM-434 (Tetraglycidyldiaminodiphenylmethane, epoxy equivalent: 119g / eq, manufactured by Sumitomo Chemical Co., Ltd.) • “jER(registered trademark)” 828 (liquid bisphenol A type epoxy resin, epoxy equivalent: 189 g / eq, manufactured by Mitsubishi Chemical Corporation).

[0056] (2) Components [B]: Amine curing agent • "LonzaCure (registered trademark)" M-MIPA (3,3'-diisopropyl-5,5'-dimethyl-4,4'-diaminodiphenylmethane, active hydrogen equivalent: 78g / eq, number of active hydrogens: 4, manufactured by Lonza Japan Co., Ltd.) DICY7 (dicyandiamide, active hydrogen equivalent: 21g / eq, number of active hydrogens: 4, manufactured by Mitsubishi Chemical Corporation).

[0057] (3) Other components [B']: curing accelerator DCMU99 (3-(3,4-dichlorophenyl)-1,1-dimethylurea, manufactured by Hodogaya Chemical Co., Ltd.).

[0058] (4) Component [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650. • "Celoxide (registered trademark)" 2021P (3,4-epoxycyclohexanecarboxylic acid 3,4-epoxycyclohexylmethyl: containing a cycloalkene oxide type epoxy group, liquid at room temperature, molecular weight: 252 (main component), manufactured by Daicel Corporation, referred to as "CEL2021P" in the table) 4,4'-Isopropylidenebis(2-phenoxyethanol) (having an alcoholic hydroxyl group, melting point: 112°C, molecular weight: 316, manufactured by Fujifilm Wako Chemical Co., Ltd.) • Epoxy ester 3000A (Bisphenol A diglycidyl ether acrylic acid adduct: containing an acryloyl group, liquid at room temperature, molecular weight: 485 (main component), manufactured by Kyoeisha Chemical Co., Ltd.) • Epoxy ester 3000MK (Bisphenol A diglycidyl ether methacrylate adduct: containing an acryloyl group, liquid at room temperature, molecular weight: 513 (main component), manufactured by Kyoeisha Chemical Co., Ltd.). (5) Other components [C']: Compounds not included in component [C] • N-methyl-2-pyrrolidone (liquid at room temperature, molecular weight: 99, manufactured by Tokyo Chemical Industry Co., Ltd., referred to as "NMP" in the table) • Neopentyl glycol diacrylate (liquid at room temperature, molecular weight: 212, manufactured by Tokyo Chemical Industry Co., Ltd.) • PX200 (Aromatic condensed phosphate ester, melting point: 95°C, molecular weight: 687, manufactured by Daihachi Chemical Industry Co., Ltd.) (6) Component [D]: Thermoplastic resin • "Vinylec (registered trademark)" K (polyvinyl formal, manufactured by JNC Corporation).

[0059] <Method for preparing epoxy resin composition> (1) Preparation of epoxy resin composition If component [C] is solid at room temperature, 100 parts by mass of component [A] and component [C], and (if component [D] is included) component [D] were placed in a beaker, heated to 150°C until uniformly miscible, and then allowed to cool to 60°C or below. If component [C] is liquid at room temperature, 100 parts by mass of component [A] and (if component [D] is included) component [D] were placed in a beaker, heated to 150°C until uniformly miscible, and then allowed to cool to 60°C or below, and component [C] or [C'] was added and thoroughly mixed. In either of the above cases, after the above procedure, components [B] and [B'] were added and the mixture was thoroughly stirred at 60°C or below to obtain an epoxy resin composition.

[0060] The epoxy resin compositions of each example were measured using the following measurement method.

[0061] <Various Evaluation Methods> (1) Preparation of epoxy resin cured plates The prepared uncured epoxy resin composition was degassed in a vacuum, then a 2 mm thick Teflon® spacer was inserted, and the mixture was poured into a mold set to a thickness of 2 mm. Subsequently, the mixture was heated from 30°C at a rate of 1.5°C / min to 135°C or 180°C, as indicated in the "Curing Temperature" column for each example in the table, for 120 minutes to obtain a 2 mm thick cured resin plate. (2) Three-point bending measurement of resin cured plate From the resin cured plate obtained in (1), test pieces measuring 10 mm in width and 60 mm in length were cut out. Using an Instron universal testing machine (manufactured by Instron), with a span of 32 mm, a crosshead speed of 2.5 mm / min, and a sample size of n=6, three-point bending was performed according to JIS K7171 (1994). The average values ​​of the elastic modulus and strength obtained were defined as the flexural modulus and strength of the resin cured material, respectively. Measurements were performed under the following three conditions. Room temperature measurement: After vacuum drying the test specimen at 60°C for one week, the bending measurement was performed at room temperature. High-temperature measurement: After vacuum-drying the test specimen at 60°C for one week, bending measurements were taken in an 82°C environment using a constant-temperature bath. Moist heat measurement: After vacuum drying the test specimen at 60°C for one week, it was immersed in 98°C hot water for 48 hours to allow it to absorb water, and then the bending measurement was performed in an 82°C environment using a constant temperature bath. (3) Measurement of glass transition temperature (Tg) of resin cured plate A test specimen measuring 12.7 mm in width and 55 mm in length was cut from the resin-cured plate obtained in (1), and the glass transition temperature was determined by dynamic viscoelasticity measurement (DMA) according to SACMA SRM18R-94. The glass transition temperature was defined as the temperature at the intersection of the tangent line in the region indicating that the cured material is in the glassy state and the tangent line in the region indicating that it is in the transition state, in the curve showing the change in storage modulus G' with respect to temperature. Here, the measurement was performed at a heating rate of 5°C / min and a frequency of 1 Hz. (4) Evaluation of moldability of resin cured boards (1) If the appearance of the resin-cured plate obtained in (1) was uniform and substantially free of voids or irregularities, it was marked with ○, and if voids or irregularities were clearly visible, it was marked with ×.

[0062] <Examples 1-8, Comparative Examples 1-6> A resin composition was prepared by mixing the components in the proportions shown in Table 1 using the method described above. Three-point bending measurements, glass transition temperature measurements, and moldability evaluations of the epoxy resin cured product were then performed using the method described above.

[0063] Examples 1-9 all exhibited excellent bending properties and heat resistance (Tg) in room temperature, high temperature, and humid heat measurements, and there were no problems with moldability. In Comparative Examples 1 and 2, when the constituent [C] was absent or present in only trace amounts, the bending properties were inferior. In Comparative Example 3, when the constituent [C] was present in excess, the bending properties were excellent in room temperature measurements, but the flexural modulus was insufficient in high temperature and humid heat measurements. In Comparative Examples 4 and 5, when a compound with a molecular weight smaller than the constituent [C] was included, it was confirmed that moldability was poor and the flexural modulus was insufficient in high temperature and humid heat measurements after high-temperature curing at 180°C. In Comparative Example 6, when a compound with a molecular weight larger than the constituent [C] was included, the bending properties were not excellent in measurements under any environment.

[0064] <Example 9, Comparative Examples 7 and 8> Examples 9, Comparative Examples 7 and 8 were obtained by changing the types and amounts of components [A] and [B] from Example 2, and further adding component [B']. Compared to Comparative Example 7, which did not contain component [C], Example 9 had superior bending properties in room temperature, high temperature, and humid heat measurements, and also possessed sufficient heat resistance and formability. When component [C] was included in a significantly excessive amount, as in Comparative Example 8, the bending modulus of elasticity in high temperature and humid heat measurements was insufficient. Furthermore, the Tg was also insufficient.

[0065] [Table 1]

Claims

1. An epoxy resin composition for fiber-reinforced composite materials that includes the following components [A] to [C] and satisfies conditions (1) and (2). [A]: Epoxy resin having reactivity with component [B], [B]: Amine curing agent, [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650. (1): Contains 3 to 30 parts by mass of component [C] per 100 parts by mass of component [A]. (2) When the epoxy resin composition hardens, the component [C] does not form a self-polymerizing polymer.

2. An epoxy resin composition for fiber-reinforced composite materials comprising the following components [A] to [C], wherein component [C] is present in 3 to 30 parts by mass per 100 parts by mass of component [A], and which does not contain radical polymerization initiators or cationic polymerization initiators, and whose cured product has a flexural modulus of 3.5 GPa or more at room temperature. [A]: Epoxy resin having reactivity with component [B], [B]: Amine curing agent, [C]: A compound that is substantially unreactive with components [A] and [B], has a melting point of 150°C or less, and a molecular weight greater than 250 and less than 650.

3. The epoxy resin composition for fiber-reinforced composite materials according to claim 1, wherein the component [C] has at least one functional group selected from an acryloyl group, a cycloalkene oxide type epoxy group, and an alcoholic hydroxyl group.

4. The epoxy resin composition for fiber-reinforced composite materials according to claim 3, wherein the component [C] has an acryloyl group.

5. The epoxy resin composition for fiber-reinforced composite materials according to claim 3, wherein the component [C] has a cycloalken oxide type epoxy group.

6. The epoxy resin composition for fiber-reinforced composite materials according to claim 3, wherein the component [C] has two or more alcoholic hydroxyl groups.

7. The epoxy resin composition for fiber-reinforced composite materials according to any one of claims 1 or 3 to 6, wherein the amount of radical polymerization initiator or cationic polymerization initiator blended is less than 0.1 parts by mass per 100 parts by mass of the constituent [C].

8. An epoxy resin composition for fiber-reinforced composite materials according to any one of claims 1 or 3 to 6, wherein the flexural modulus of elasticity when cured is 3.5 GPa or more at room temperature.

9. Furthermore, the epoxy resin composition for fiber-reinforced composite materials according to any one of claims 1 to 6, further comprising a thermoplastic resin.

10. An epoxy resin film for fiber-reinforced composite materials comprising the epoxy resin composition for fiber-reinforced composite materials according to any one of claims 1 to 6.

11. A prepreg for fiber-reinforced composite materials, comprising at least partially impregnating continuous reinforcing fibers with the epoxy resin film for fiber-reinforced composite materials described in claim 10.

12. A fiber-reinforced composite material obtained by impregnating continuous reinforcing fibers with the epoxy resin composition for fiber-reinforced composite materials according to any one of claims 1 to 6 and then curing the impregnation.

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