Sheet molding compounds and fiber-reinforced composite materials

Abstract

This invention provides a sheet molding compound that can thicken a matrix resin without utilizing chemical reactions, and a fiber-reinforced composite material with excellent strength, elastic modulus, and heat resistance obtained from this sheet molding compound. [Solution] A sheet molding compound comprising an epoxy resin composition containing the following components (A) and (B), an epoxy resin curing agent, and reinforcing fibers. A fiber-reinforced composite material which is a press-molded product of this sheet molding compound. Component (A): Epoxy resin Component (B): Aliphatic polyester with a melting point of 20-100°C

Description

Technical Field

[0001] The present invention relates to sheet molding compounds and fiber-reinforced composite materials.

Background Art

[0002] Fiber-reinforced composite materials composed of reinforcing fibers and a matrix resin are widely used in aircraft, automobiles, various industrial applications and the like due to their excellent mechanical properties and the like. In recent years, the scope of application of fiber-reinforced composite materials has been expanding more and more, and at the same time, the required properties thereof have been increasing. In particular, heat resistance is required when applied to structural materials such as aerospace applications and vehicles.

[0003] As a method for producing a fiber-reinforced composite material, there is a method using a prepreg such as a sheet molding compound (hereinafter also referred to as "SMC"). A prepreg is an intermediate material for molding obtained by pre-impregnating a fiber reinforcing material with a resin. As the matrix resin, thermosetting resins, especially epoxy resins, which are excellent in adhesion to reinforcing fibers such as carbon fibers, heat resistance, elastic modulus and chemical resistance, and have small curing shrinkage, are widely used.

[0004] The matrix resin used in SMC is required to have an appropriate viscosity increase. This allows SMC to exhibit appropriate tackiness and drape property (flexibility), thereby improving handling workability during press molding. In order to increase the viscosity of the matrix resin of SMC, for example, Patent Document 1 describes a method using an epoxy resin composition containing an epoxy resin that is liquid at 25°C and an acid anhydride. In Patent Document 1, viscosity is increased by forming ester bonds through the reaction between the epoxy resin and the acid anhydride.

Prior Art Literature

Patent Literature

[0005]

Patent Document 1

Summary of the Invention

[0006] The thickening method using chemical reactions described in Patent Document 1 has several drawbacks: it takes a long time to thicken; it requires precise adjustment of the component ratios and temperature for proper reaction control; and the viscosity after thickening is not stable. Therefore, there is a need for thickening methods that do not utilize chemical reactions.

[0007] The present invention aims to provide a sheet molding compound that can thicken a matrix resin without utilizing chemical reactions, and a fiber-reinforced composite material with excellent strength, elastic modulus, and heat resistance using this sheet molding compound. [Means for solving the problem]

[0008] As a result of repeated studies to solve the above problems, the inventors of the present invention have found that by using an epoxy resin in combination with an aliphatic polyester having a specific melting point, the matrix resin can be thickened without utilizing a chemical reaction.

[0009] The present invention includes, but is not limited to, the following embodiments. [1]: A sheet molding compound comprising an epoxy resin composition containing the following components (A) and (B), an epoxy resin curing agent, and reinforcing fibers. Component (A): Epoxy resin Component (B): Aliphatic polyester with a melting point of 20-100°C [2]: The sheet molding compound according to [1], wherein component (B) comprises a dicarboxylic acid unit and a diol unit. [3]: The sheet molding compound according to [2], wherein component (B) has only one type each of dicarboxylic acid units and diol units. [4]: The sheet molding compound according to [2] or [3], wherein the dicarboxylic acid unit is a linear dicarboxylic acid unit having 2 to 20 carbon atoms. [5]: The sheet molding compound according to any one of [2] to [4], wherein the diol unit is a straight-chain diol unit having 2 to 20 carbon atoms. [6]: The sheet molding compound according to [1] to [5], further comprising the epoxy resin composition below (C). Ingredient (C): Vinyl polymer [7]: The sheet molding compound according to [6], wherein component (C) is in particulate form. [8]: The sheet molding compound according to [6] or [7], wherein the glass transition temperature of component (C) is 30 to 120°C. [9]: A sheet molding compound according to any one of [6] to [8], comprising 0.1 to 10 parts by mass of component (C) per 100 parts by mass of the total of component (A) and component (B).

[10] : A sheet molding compound according to any one of [1] to [9], wherein component (A) is a liquid epoxy resin at 25°C.

[11] : The sheet molding compound according to any one of [1] to

[10] , wherein the epoxy resin curing agent comprises at least one compound selected from aliphatic amines, aromatic amines, secondary amines, tertiary amines, imidazole compounds, epoxy resin amine adducts, and dicyandiamides.

[12] : The sheet molding compound according to

[11] , wherein the epoxy resin curing agent comprises dicyandiamide.

[13] : The sheet molding compound according to any one of [1] to

[12] , wherein the average length of the reinforcing fibers is 0.3 to 10 cm.

[14] : A sheet molding compound according to any of [1] to

[13] , wherein a needle-shaped crystalline structure is confirmed by evaluating the crystalline structure of component (A) and component (B) by the method described below. <Evaluation of crystal structure> The aforementioned components (A) and (B) are mixed in a ratio of mass of component (A) to mass of component (B) of 99:1. The mixture is heated and stirred at 150°C for 6 hours, and then cooled to room temperature (25°C). The resulting sample is examined using a polarizing microscope to confirm its crystal structure.

[15] : The sheet molding compound according to any one of [1] to

[14] , wherein the melting point of the epoxy resin composition is 0 to 100°C.

[16] : A sheet molding compound according to any one of [1] to

[15] , comprising 1 to 20 parts by mass of component (B) per 100 parts by mass of component (A).

[17] : A fiber-reinforced composite material which is a press-molded product of a sheet molding compound as described in any of [1] to

[16] . [Effects of the Invention]

[0010] According to one embodiment of the present invention, a sheet molding compound is provided that can be thickened by a method that does not utilize chemical reactions. Therefore, the problems of conventional methods that utilize chemical reactions, such as the time required for thickening, the need to precisely adjust the mixing ratio and temperature of components for appropriate reaction control, and the difficulty in stabilizing the viscosity after thickening, are solved, and thickening can be achieved stably and efficiently in a short time. Furthermore, according to another embodiment of the present invention, a fiber-reinforced composite material with excellent strength, elastic modulus, and heat resistance is provided using this sheet molding compound. [Modes for carrying out the invention]

[0011] The present invention will be described in detail below. In this specification, a numerical range indicated by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively, and "A~B" means that it is greater than or equal to A and less than or equal to B. Furthermore, in this specification, "weight" and "mass" are synonymous.

[0012] 1. Sheet molding compound One embodiment of the present invention relates to a sheet molding compound. The sheet molding compound according to the present invention comprises an epoxy resin composition containing the following component (A) and component (B) (hereinafter may be referred to as "the epoxy resin composition of the present invention"), an epoxy resin curing agent, and reinforcing fibers. Component (A): an epoxy resin Component (B): an aliphatic polyester having a melting point of 20 to 100°C Furthermore, the epoxy resin composition of the present invention may further contain the following component (C). Component (C): a vinyl polymer Hereinafter, a product obtained by further adding an epoxy resin curing agent to the epoxy resin composition of the present invention containing component (A) and component (B) may be referred to as "the curable epoxy resin composition of the present invention".

[0013] From the viewpoint of handling properties of SMC, the melting point of the epoxy resin composition of the present invention is preferably 0 to 100°C. The lower limit of the melting point of the epoxy resin composition is more preferably 20°C, and still more preferably 30°C. The upper limit thereof is more preferably 80°C, and still more preferably 70°C. Here, the melting point of the epoxy resin composition is measured by the method described in the Examples section below. In addition, the melting point of the aliphatic polyester of component (B) is a value measured by differential scanning calorimetry.

[0014] Furthermore, the epoxy resin composition of the present invention preferably has a complex viscosity of 1,000 to 40,000 Pa·s at 20°C, as measured by the method described in the Examples section below, and a complex viscosity of 10 Pa·s or less at 100°C. When the complex viscosity at 20°C is 1,000 to 40,000 Pa·s, the resulting SMC can exhibit appropriate tackiness and drapeability due to thickening when handled at room temperature. From this viewpoint, a complex viscosity of 1,500 to 35,000 Pa·s at 20°C is more preferable. Also, if the complex viscosity at 100°C is 10 Pa·s or less, sufficient resin fluidity is obtained, and impregnation into reinforcing fibers is good. From the viewpoint of impregnation, the upper limit of the complex viscosity at 100°C is more preferably 8 Pa·s, and even more preferably 5 Pa·s. Also, the lower limit of the complex viscosity at 100°C is preferably 0.1 Pa·s. This suppresses resin leakage and poor fluidity of reinforcing fibers during press molding, resulting in good moldability. The lower limit of the complex viscosity at 100°C is more preferably 0.2 Pa·s or higher, and even more preferably 1 Pa·s or higher.

[0015] The following provides a detailed explanation of each component.

[0016] 1.1. Ingredients (A) Component (A) is epoxy resin. Examples of component (A) include glycidyl ethers of bisphenols (bisphenol A, bisphenol F, bisphenol AD, halogen-substituted derivatives thereof, etc.), glycidyl ethers of polyhydric phenols obtained by condensation reactions of phenols and aromatic carbonyl compounds, and polyglycidyl compounds derived from aromatic amines. In particular, from the viewpoint of easily preparing the epoxy resin composition of the present invention, it is preferable that the composition contains an epoxy resin that is liquid at 25°C.

[0017] As component (A), a bisphenol-type epoxy resin is preferred because it is easy to adjust the viscosity of the epoxy resin composition to a viscosity suitable for impregnation into reinforcing fibers, and it is easy to adjust the mechanical properties of the fiber-reinforced composite material to a desired range. As the bisphenol-type epoxy resin, a bifunctional bisphenol-type epoxy resin is preferred. Bisphenol A type epoxy resin is more preferred in terms of good heat resistance and chemical resistance of the fiber-reinforced composite material. Bisphenol F type epoxy resin is more preferred in terms of having a lower viscosity than bisphenol A type epoxy resin with a similar molecular weight and a higher elastic modulus of the fiber-reinforced composite material. Here, "difunctional bisphenol-type epoxy resin" refers to a bisphenol-type epoxy resin that has two epoxy groups in its molecule.

[0018] Examples of commercially available difunctional bisphenol-type epoxy resins include the following: Mitsubishi Chemical Corporation's jER(registered trademark) 825, 827, 828, 828EL, 828XA, 806, 806H, 807, 4004P, 4005P, 4007P, 4010P; DIC Corporation's Epiclon(registered trademark) 840, 840-S, 850, 850-S, EXA-850CRP, 850-LC, 830, 830-S, 835, EXA-8 30CRP, EXA-830LVP, EXA-835LV, and Nippon Steel & Sumitomo Metal Chemical's Epotote (registered trademark) YD-115, YD-115G, YD-115CA, YD-118T, YD-127, YD-128, YD-128G, YD-128S, YD-128CA, YDF-170, YDF-2001, YDF-2004, YDF-2005RL, etc.

[0019] To improve heat resistance, polyfunctional, novolac, and naphthalene-based epoxy resins are effective as component (A). Examples of commercially available products include jER(registered trademark) 152, 154, 157S70, 1031S, 1032H60, 604, 630, 630LSD, YX4000, YX4000H, YL6121H, YX7399 (all manufactured by Mitsubishi Chemical Corporation), YDPN-638, YDCN-700-3, YDCN-700-5, YDCN-700-7, YDCN-700-10, YDCN-704, YDCN-704A (all) (Manufactured by Nippon Steel & Sumitomo Metal Chemical Co., Ltd.), N-660, N-665, N-670, N-673, N-680, N-690, N-695, N-665-EXP, N-672-EXP, N-655-EXP-S, N-662EXP-2, N-665-EXP-S, N-670-EXP-S, N-685-EXP-S, N-673-80M, N-680-75M, N-690-75M (all manufactured by DIC Corporation) are similar.

[0020] To adjust the viscosity of the epoxy resin composition, a low-viscosity epoxy resin, a so-called reactive diluent, can be mixed as component (A). Examples of commercially available products include jER(registered trademark) 819 (manufactured by Mitsubishi Chemical Corporation), diglycidylaniline (GAN, manufactured by Nippon Kayaku Co., Ltd.), diglycidyltoluidine (GOT, manufactured by Nippon Kayaku Co., Ltd.), ED-502, ED-509E, ED-509S, ED-529, ED-503, ED-503G, ED-506, ED-523T, ED-505 (all manufactured by ADEKA Corporation), Deconaru ( Examples of registered trademarks include EX-512, EX-411, EX-421, EX-313, EX-314, EX-321, EX-201, EX-211, EX-212, EX-810, EX-811, EX-850, EX-851, EX-821, EX-830, EX-911, EX-941, EX-920, EX-141, EX-145, EX-146 (all manufactured by Nagase ChemteX Corporation).

[0021] The epoxy resin of component (A) may be of one type only, or it may be a mixture of two or more different types of epoxy resins.

[0022] 1.2.Component (B) Component (B) is an aliphatic polyester with a melting point of 20-100°C.

[0023] If the melting point of the aliphatic polyester of component (B) is below 20°C or above 100°C, temperature control for thickening by the following mechanism becomes difficult, which is undesirable. The melting point of the aliphatic polyester of component (B) is preferably 25 to 90°C.

[0024] The epoxy resin composition of the present invention contains component (B), which crystallizes within the epoxy resin composition and exhibits a melting point and crystallization temperature derived from component (B). Furthermore, the formation of an anisotropic needle-shaped crystalline structure by component (B) within the epoxy resin composition enables thickening of the epoxy resin composition at a temperature below the melting point derived from component (B). This is thought to be because the formation of the needle-shaped crystalline structure leads to percolation, that is, the formation of a structure in which crystals are linked together, resulting in the emergence of structural viscosity and enabling thickening.

[0025] The feasibility of forming the above-mentioned needle-shaped crystal structure can be confirmed by the following crystal structure evaluation method, and it is preferable that the epoxy resin of component (A) and the aliphatic polyester of component (B) constituting the epoxy resin composition of the present invention show needle-shaped crystal structures when their crystal structures are evaluated by the method described below. <Evaluation of crystal structure> The aforementioned components (A) and (B) are mixed in a ratio of mass of component (A) to mass of component (B) of 99:1. The mixture is heated and stirred at 150°C for 6 hours, and then cooled to room temperature (25°C). The resulting sample is examined using a polarizing microscope to confirm its crystal structure.

[0026] Furthermore, in the above-described needle-shaped crystal structure, it is preferable that the median L / D ratio is 1.5 or greater, particularly 1.55 or greater, when L is the length along the long axis of the crystal and D is the length along the short axis. If needle-shaped crystals with such an L / D ratio are formed, they will thicken appropriately and have good handling properties. There is no particular upper limit to this median L / D ratio, but from the viewpoint of sheet flexibility, it is usually 3.0 or less. The crystal structure and the median L / D ratio can be evaluated by the method described in the Examples section below.

[0027] Component (B) preferably has dicarboxylic acid units and diol units, and more preferably has only one type each of dicarboxylic acid units and diol units. This improves the crystallinity of component (B) and allows for a good thickening effect of the epoxy resin composition. The dicarboxylic acid units of component (B) preferably contain linear aliphatic dicarboxylic acid units having 2 to 20 carbon atoms, and more preferably contain linear aliphatic dicarboxylic acid units having 2 to 12 carbon atoms. Similarly, the diol units preferably contain linear aliphatic diol units having 2 to 20 carbon atoms, and more preferably contain linear aliphatic diol units having 2 to 12 carbon atoms. This allows the melting point of the epoxy resin composition to be easily adjusted to the aforementioned desirable range.

[0028] The content of component (B) in the sheet molding compound of the present invention is preferably 1 to 20 parts by mass per 100 parts by mass of component (A). This range of component (B) content results in good strength and elastic modulus of the resulting fiber-reinforced composite material. The lower limit of the component (B) content is more preferably 3 parts by mass, and even more preferably 5 parts by mass, per 100 parts by mass of component (A). The upper limit is more preferably 15 parts by mass, and even more preferably 10 parts by mass.

[0029] Although two or more aliphatic polyesters with different monomer compositions may be used for component (B), it is preferable to use only one type from the viewpoint of easily adjusting the melting point of the epoxy resin composition according to the present invention to the desired range described above.

[0030] 1.3.Component (C) Component (C) is a vinyl polymer. The epoxy resin composition of the present invention contains component (C), which swells or dissolves near its own glass transition temperature within the epoxy resin composition, causing the epoxy resin composition to thicken. This suppresses leakage of the matrix resin during press molding and improves the fluidity of the reinforcing fibers, resulting in good moldability.

[0031] The vinyl polymer of component (C) is preferably in particulate form from the viewpoint of thickening effect. Examples of particulate vinyl polymers include single-layer particles and core-shell particles.

[0032] Furthermore, component (C) may be in the form of secondary particles formed by the aggregation of primary particles. When component (C) is in the form of secondary particles, from the viewpoint of dispersibility, the upper limit of the secondary particle diameter is preferably 500 μm, more preferably 200 μm, and even more preferably 100 μm. Also, from the viewpoint of ease of controlling the thickening effect, the lower limit of the secondary particle diameter is preferably 1 μm. The secondary particle diameter can be adjusted by operations such as crushing the secondary particles or kneading the epoxy resin composition containing the secondary particles. Here, the secondary particle size of component (C) is the calculated average value of secondary particle sizes measured by electron microscopy observation of several arbitrarily extracted secondary particles.

[0033] The glass transition temperature of component (C) is preferably 30 to 120°C. A glass transition temperature of 30°C or higher prevents the epoxy resin composition from thickening during SMC production, thus suppressing a decrease in impregnation of reinforcing fibers. The lower limit of the glass transition temperature of component (C) is more preferably 40°C, even more preferably 50°C, and particularly preferably 60°C. Furthermore, a glass transition temperature of 120°C or lower allows for rapid thickening during press molding, more effectively suppressing leakage of the matrix resin. The upper limit of the glass transition temperature of component (C) is more preferably 110°C, even more preferably 100°C, and particularly preferably 90°C. The glass transition temperature of component (C) can be measured by the method described in the examples.

[0034] Component (C) is obtained by polymerizing a radically polymerizable vinyl monomer. The composition of component (C) is not particularly limited and may include, for example, a silicone structure or a crosslinked structure.

[0035] Examples of radically polymerizable vinyl monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, i-butyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, phenyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tricyclo(meth)acrylate, and tricyclo(meth)acrylate.6] (meth)acrylates such as decane-8-yl methacrylate and dicyclopentadienyl (meth)acrylate; aromatic vinyl monomers such as styrene, α-methylstyrene, and vinyltoluene; hydroxyl acids such as hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and glycerol mono(meth)acrylate. (Meth)acrylates containing functional groups; glycidyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N-methyl-2,2,6,6-tetramethylpiperidyl (meth)acrylate, and other functional group-containing (meth)acrylates; acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, fumaric acid, isocrotonic acid, salicylic acid, vinyloxyacetic acid, alyloxyacetic acid, 2-(meth)acryloylpropanoic acid, 3-(meth)acryloylbutanoic acid, 4 Examples include carboxyl group-containing vinyl monomers such as vinylbenzoic acid; vinyl cyanide monomers such as (meth)acrylonitrile; (meth)acrylamide; itaconic acid esters such as monomethyl itaconate, monoethyl itaconate, monopropyl itaconate, monobutyl itaconate, dimethyl itaconate, diethyl itaconate, dipropyl itaconate, and dibutyl itaconate; fumarate esters such as monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, monobutyl fumarate, dimethyl fumarate, diethyl fumarate, dipropyl fumarate, and dibutyl fumarate; maleic acid esters such as monomethyl malate, monoethyl malate, monopropyl malate, monobutyl malate, dimethyl malate, diethyl malate, dipropyl malate, and dibutyl malate; and other vinyl monomers such as vinylpyridine, vinyl alcohol, vinylimidazole, vinylpyrrolidone, vinyl acetate, and 1-vinylimidazole. In this specification, (meth)acrylate refers to either acrylate or methacrylate. These monomers can be used individually or in combination of two or more.

[0036] Among these monomers, (meth)acrylate and the aforementioned functional group-containing (meth)acrylates are preferred because they readily undergo radical polymerization and emulsion polymerization.

[0037] The method for producing vinyl polymers is not particularly limited and includes, for example, solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization.

[0038] The content of component (C) is preferably 0.1 to 10 parts by mass, and more preferably 1 to 10 parts by mass, per 100 parts by mass of the total of components (A) and (B), from the viewpoint of the strength and elastic modulus of the resulting fiber-reinforced composite material.

[0039] Component (C) may be used alone, or two or more components with different monomer compositions and physical properties may be mixed and used.

[0040] 1.4. Epoxy resin hardener As the epoxy resin curing agent, at least one compound selected from, for example, aliphatic amines, aromatic amines, secondary amines, tertiary amines, imidazole compounds, epoxy resin amine adducts, and dicyandiamides can be used.

[0041] In particular, dicyandiamide has a high melting point, and its compatibility with the epoxy resin of component (A) is suppressed in the low-temperature range, making it preferable from the viewpoint of pot life. Furthermore, including dicyandiamide as an epoxy resin curing agent tends to improve the strength and elastic modulus of the resulting fiber-reinforced composite material, which is also preferable.

[0042] The amount of epoxy resin curing agent is preferably such that the number of moles of active hydrogen in the epoxy resin curing agent, such as dicyandiamide (hereinafter also referred to as the active hydrogen equivalent ratio), relative to the total number of moles of epoxy groups in the epoxy resin of component (A) contained in the epoxy resin composition, is 0.1 to 1, and more preferably 0.2 to 0.8. By setting the active hydrogen equivalent ratio above the lower limit, the strength, elastic modulus, and heat resistance of the resulting fiber-reinforced composite material are further improved. Furthermore, by setting it below the upper limit, the plastic deformation capacity and impact resistance of the resulting fiber-reinforced composite material are improved.

[0043] 1.5. Reinforced Fibers Examples of reinforcing fibers include carbon fibers, glass fibers, and aramid fibers. Among these, carbon fibers are the most preferred due to their low specific gravity and high strength.

[0044] The average length of the reinforcing fibers is preferably 0.3 to 10 cm. An average length of 0.3 cm or more yields a fiber-reinforced composite material with better strength and elastic modulus. An average length of 10 cm or less improves the fluidity of the matrix resin during press molding. The lower limit of the average length of the reinforcing fibers is more preferably 1 cm, and even more preferably 2.5 cm. The upper limit is more preferably 5 cm or less.

[0045] Here, the average length of the reinforcing fibers is the value measured by the following method. First, approximately 20 mg of reinforcing fiber is weighed out, crushed on white paper, and photographed with a digital camera to acquire an image. Next, the obtained image data is binarized, contour detection is performed on the fiber, the circumscribing rectangle is detected, and noise is removed. The median is calculated using the longer side as the fiber length and is taken as the average length of the reinforcing fiber.

[0046] 1.6. Other ingredients The epoxy resin composition of the present invention may contain other components in addition to the above-mentioned components (A), (B), and (C), within a range that satisfies the aforementioned preferred melting point and complex viscosity, for example, within a range where the total content of components (A) and (B) in the epoxy resin composition of the present invention is 60% by mass or more. Other components include epoxy resin curing accelerators, inorganic fillers, internal mold release agents, surfactants, organic pigments, inorganic pigments, and other resins other than components (A), (B), and (C) (thermoplastic resins, thermoplastic elastomers, and elastomers). From the viewpoint of strength and elastic modulus of fiber-reinforced composite materials, urea compounds are preferred as curing accelerators for epoxy resins. Including an epoxy resin curing accelerator allows for a reduction in the amount of epoxy resin curing agent added. Examples of inorganic fillers include calcium carbonate, aluminum hydroxide, clay, barium sulfate, magnesium oxide, glass powder, hollow glass beads, and aerosil. Examples of internal mold release agents include carnauba wax, zinc stearate, and calcium stearate. Examples of surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and sorbitan fatty acid esters. The inclusion of surfactants improves the release properties of the carrier film from the SMC. Furthermore, it can reduce the voids contained in the SMC.

[0047] Thermoplastic resins, thermoplastic elastomers, and elastomers not only modify the viscoelasticity of epoxy resin compositions to optimize their viscosity, storage modulus, and thixotropic properties, but also play a role in improving the toughness of fiber-reinforced composite materials. Thermoplastic resins, thermoplastic elastomers, and elastomers may be used individually or in combination of two or more types.

[0048] 2. Method for manufacturing sheet molding compound The sheet molding compound according to the present invention can be manufactured, for example, by adding an epoxy resin curing agent to an epoxy resin composition prepared using components (A) and (B), and optionally component (C) and other components, impregnating reinforcing fibers with the resulting curable epoxy resin composition, and thickening the curable epoxy resin composition. The epoxy resin composition of the present invention can be prepared, for example, by melt-kneading a mixture of components (A) and (B) by heating. If the epoxy resin composition contains component (C), it is preferable to mix component (C) after melt-kneading components (A) and (B).

[0049] An example of a method for manufacturing sheet molding compound is described in detail below.

[0050] First, component (B) is added to component (A), and the mixture is heated to 100-200°C and stirred for 1-7 hours to melt it. If the epoxy resin composition contains component (C), component (C) is added and stirred further to prepare the epoxy resin composition. Next, an epoxy resin curing agent is added to the epoxy resin composition and stirred at a temperature of room temperature to 60°C for 2-10 minutes, for example, 5 minutes, to prepare a curable epoxy resin composition. Next, two films uniformly coated with the obtained curable epoxy resin composition are prepared. Reinforcing fibers are randomly scattered onto one of the films coated with the curable epoxy resin composition to form a sheet. The coated surface of the other film coated with the curable epoxy resin composition is then bonded onto this reinforcing fiber-scattered surface, and the curable epoxy resin composition is pressed and impregnated into the sheet at a temperature of room temperature (25°C) to 100°C. After cooling to 0°C and holding for 1 to 2 hours, component (B) in the curable epoxy resin composition crystallizes, forming needle-shaped crystals, which enables thickening.

[0051] Another method for preparing an epoxy resin composition containing component (C) involves pre-melting and mixing a portion of component (A) with component (B) to prepare an epoxy resin solution, separately kneading another portion of component (A) with component (C) to prepare an epoxy resin dispersion, and then mixing the epoxy resin solution, the epoxy resin dispersion, and the remainder of component (A) to obtain the epoxy resin composition. In this way, by dividing and mixing component (A), the particulate component (C) can be uniformly dispersed and mixed.

[0052] 3. Fiber-reinforced composite materials One embodiment of the present invention relates to a fiber-reinforced composite material.

[0053] The fiber-reinforced composite material according to the present invention is a press-molded product of the sheet molding compound according to the present invention, and exhibits excellent strength, elastic modulus, and heat resistance.

[0054] 4. Method for manufacturing fiber-reinforced composite materials The fiber-reinforced composite material according to the present invention can be manufactured by press-molding the sheet molding compound according to the present invention and curing the above-mentioned curable epoxy resin composition.

[0055] An example of a manufacturing method for fiber-reinforced composite materials is described in detail below. A single sheet of SMC, or multiple sheets of SMC stacked together, is set between a pair of molds. The SMC is heated and compressed between the molds at 120-230°C for 2-60 minutes to cure the curable epoxy resin composition and obtain a molded product, which is a fiber-reinforced composite material. A honeycomb structure such as corrugated cardboard may be used as the core material, with SMC arranged on both sides or one side thereof. [Examples]

[0056] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0057] <Ingredients> (Component (A)) • A-1: ​​jER(registered trademark) 828 (Bisphenol A type liquid epoxy resin, epoxy equivalent 184-194 g / eq, manufactured by Mitsubishi Chemical Corporation) A-2: Denacol® EX-421 (Diglycerol polyglycidyl ether, epoxy equivalent 159 g / eq, manufactured by Nagase ChemteX Corporation)

[0058] (Component (B)) B-1: Polybutylenedecanedicarboxylate (aliphatic polyester composed of 1,4-butanediol and dodecanedioic acid, melting point 72°C, Mw 100,000) B-2: Polyhexene adipyrate (aliphatic polyester composed of 1,6-hexanediol and adipic acid, melting point 60°C, Mw 50,000) • B-3: Polyoctenazelate (aliphatic polyester composed of 1,8-octanediol and azelaic acid, melting point 60°C, Mw 89,000) B-4: Polyheptene pimerate (aliphatic polyester composed of 1,7-heptanediol and pimelic acid, melting point 54°C, Mw 29,000)

[0059] (Component (C)) • C-1: Dianaal (registered trademark) LP-3106 (manufactured by Mitsubishi Chemical Corporation, acrylic resin particles, glass transition temperature 90℃)

[0060] (Epoxy resin curing agent) • DICY: Dicyandiamide (manufactured by EVONIC, product name "DICYANEX(registered trademark) 1400F") • PDMU: 3-phenyl-1,1-dimethylurea (manufactured by Chori GLEX Co., Ltd., product name "Omicure(registered trademark) 24")

[0061] <Rating> (Measurement of the glass transition temperature of component (C)) The glass transition temperature of component (C) was measured by the following method. Using a differential scanning calorimeter "ThermoplusEVO2DSC" (manufactured by Rigaku), component (C) was heated from 30°C to 120°C at a rate of 10°C / min and held for 4 minutes, then cooled from 120°C to -20°C at a rate of 10°C / min and held for 4 minutes, and finally heated from -20°C to 120°C at a rate of 10°C / min. The glass transition temperature of component (C) was read from the DSC chart obtained during the final heating from -20°C to 120°C at a rate of 10°C / min.

[0062] (Measuring the melting point of epoxy resin compositions) The melting point (Tm) of the epoxy resin composition was measured by the following method. Using a differential scanning calorimeter "DSC7020" (manufactured by Hitachi High-Tech Corporation), the epoxy resin composition was heated from 20°C to 200°C at a rate of 10°C / min and held for 5 minutes, then cooled from 200°C to 0°C at a rate of 10°C / min and held for 10 minutes, and finally heated from 0°C to 200°C. The Tm was read from the DSC chart obtained during the final heating from 0°C to 200°C at a rate of 10°C / min.

[0063] (Measurement of viscosity of epoxy resin composition at elevated temperatures) The complex viscosity of the epoxy resin composition at temperatures of 20°C and 100°C was measured using a rheometer "DHR-3" (manufactured by TA Instruments Corporation) by the following method. If the epoxy resin composition does not contain component (C), the epoxy resin composition was heated from 20°C to 100°C at a rate of 50°C / min and held for 5 minutes under the following conditions, then cooled from 100°C to 0°C at a rate of 5°C / min and held for 1 hour, and then heated from 0°C to 100°C at a rate of 5°C / min. Measurement mode: Constant stress Stress value: 300 Pa Frequency: 1 rad / sec Plate: 25mm diameter parallel plate Plate gap: 500 μm The complex viscosity at 20°C and 100°C was read from the temperature-complex viscosity curve obtained when the temperature was increased from 0°C to 100°C. When the epoxy resin composition contains component (C), it thickens above the glass transition temperature of component (C). Therefore, the temperature conditions for rheometer measurement were changed as follows, and the measurement was performed in the same manner as when the epoxy resin composition does not contain component (C). The epoxy resin composition was heated from 20°C to 70°C at a rate of 50°C / min and held for 5 minutes, then cooled from 70°C to 0°C at a rate of 5°C / min and held for 30 minutes, and finally heated from 0°C to 150°C at a rate of 5°C / min. As mentioned above, a complex viscosity of 1,000 to 40,000 Pa·s at 20°C indicates that, when handled at room temperature, the SMC can exhibit appropriate tackiness and drape due to increased viscosity. A viscosity of 10 Pa·s or less at 100°C indicates that sufficient resin fluidity is obtained, resulting in good impregnation into reinforcing fibers. Furthermore, a complex viscosity of 0.1 Pa·s or higher, especially 1 Pa·s or higher at 100°C, indicates that resin leakage and poor reinforcing fiber fluidity during press molding are suppressed, resulting in good moldability.

[0064] (Evaluation of crystal structure) The mixture was prepared so that the mass of component (A) : the mass of component (B) was 99:1. The mixture was heated and stirred at 150°C for 6 hours, and then cooled to room temperature. The resulting sample was observed using a polarizing microscope (Nikon) with a 20x lens to obtain image data and confirm the crystal structure. Next, the obtained image data was binarized to distinguish between the crystalline portion and the surrounding area. Contour detection was performed on the crystalline portion to detect its bounding rectangle, and noise was removed. The L / D ratio was calculated using D as the shorter side and L as the longer side of the bounding rectangle, and the median of the obtained L / D ratio was calculated.

[0065] (Evaluation of bending properties of cured resin products) The bending properties of resin cured products obtained by curing a curable epoxy resin composition were evaluated using the universal testing machine "STB1225L" (manufactured by A&D). Tests were conducted under the conditions of a support distance of 40 mm, a load cell of 500 N, and a test speed of 2 mm / min. Strength, elastic modulus, and elongation at break were measured, and the average value of 3 to 5 samples was calculated. If the strength of the cured resin is 80 MPa or higher and the modulus of elasticity is 3 GPa or higher, the resulting fiber-reinforced composite material can be said to exhibit good strength and modulus of elasticity. A break elongation of 3 to 10% is preferable.

[0066] (Evaluation of heat resistance of cured resin products) The heat resistance of the cured resin product obtained by curing a curable epoxy resin composition was evaluated by solid dynamic viscoelasticity measurement. Using a DMA7100 (Hitachi High-Tech Corporation), the cured resin product was heated from 20°C to 200°C at a rate of 3°C / min. The frequency was measured using a 1 Hz plate. The peak of the obtained tanδ curve was defined as the glass transition temperature (Tg). If the Tg of the cured resin is 110°C or higher, the resulting fiber-reinforced composite material can be said to exhibit good heat resistance.

[0067] [Examples 1-3 and Comparative Example 1] <Manufacturing of epoxy resin compositions> Components (A) and (B) were blended according to the composition shown in Table 1, and an epoxy resin composition was obtained by heating and stirring at 150°C for 6 hours. The results of the melting point measurement and temperature-controlled viscosity measurement of the epoxy resin composition are shown in Table 1.

[0068] <Manufacturing of cured resin products> The obtained epoxy resin composition was mixed with the epoxy resin curing agent shown in Table 1 in the active hydrogen equivalent ratio shown in Table 1, and stirred for 300 seconds at a vacuum pressure of 3 kPa and 1500 rpm using a rotational and revolving mixer "Awatori Rentaro (registered trademark)" (manufactured by Shinky Co., Ltd.). The obtained curable epoxy resin composition was poured into a 60 mm × 10 mm × 2.5 mm silicone mold, heated at 100°C for 1 hour using a hot air dryer, then heated at 120°C for 0.5 hours, and further heated at 140°C for 0.5 hours, and then cooled to room temperature to cure the curable epoxy resin composition. After curing, it was polished with a rotary polishing machine to obtain a cured resin product measuring 60 mm in length × 9 mm in width × 2 mm in thickness. The evaluation results of the bending properties and heat resistance of the cured resin product are shown in Table 1.

[0069] [Examples 4-5 and Comparative Example 2] <Manufacturing of epoxy resin compositions> An epoxy resin solution was obtained by mixing a portion of component (A) with component (B), heating and stirring at 150°C for 6 hours, and then slowly cooling. Next, an epoxy resin dispersion was obtained by mixing a portion of component (A) with component (C) and kneading in a three-roll mill (manufactured by Inoue Seisakusho Co., Ltd.). Then, the epoxy resin solution, epoxy resin dispersion, and the remaining component (A) were mixed to obtain the composition shown in Table 1, and stirred for 300 seconds at a vacuum pressure of 3 kPa and 1500 rpm in a rotary-orbit mixer "Awatori Rentaro (registered trademark)" (manufactured by Shinki Co., Ltd.). The results of the temperature-controlled viscosity measurement of the epoxy resin composition are shown in Table 1.

[0070] <Manufacturing of cured resin products> Using the obtained epoxy resin composition, a cured resin product was obtained in the same manner as in Example 1. The evaluation results of the bending properties and heat resistance of the cured resin product are shown in Table 1. In Table 1, "Component (B) / Component (A)" indicates the content of Component (B) per 100 parts by mass of Component (A). Also, "Component (C) / (Component (A) + Component (B))" indicates the content of Component (C) per 100 parts by mass of the total of Component (A) and Component (B).

[0071] [Table 1]

[0072] As shown in Table 1, Examples 1-5, in which the epoxy resin composition contains components (A) and (B), thickened by a method that does not utilize chemical reactions and exhibited excellent handling properties (tackiness and flexibility). In particular, Examples 4-5, in which the epoxy resin composition contains component (C), had a complex viscosity of 1 Pa·s or higher at 100°C and exhibited good moldability. Furthermore, since the cured resin product obtained by curing the curable epoxy resin composition using this epoxy resin composition exhibited excellent strength, elastic modulus, and heat resistance, it can be said that the fiber-reinforced composite material obtained using this epoxy resin composition also exhibited excellent strength, elastic modulus, and heat resistance. On the other hand, Comparative Examples 1 and 2, which did not contain component (B), had a complex viscosity of less than 1,000 Pa·s at 20°C, and no thickening effect was obtained. In particular, Comparative Example 2 had a very high complex viscosity at 100°C, and impregnation into reinforcing fibers was not achieved. [Industrial applicability]

[0073] The sheet molding compound of the present invention exhibits excellent handling properties (tackiness and flexibility). Furthermore, the fiber-reinforced composite material of the present invention, using such a sheet molding compound, exhibits excellent strength, elastic modulus, and heat resistance, making it suitable as a structural material for aerospace applications and vehicles.

Claims

1. A sheet molding compound comprising an epoxy resin composition containing the following components (A) and (B), an epoxy resin curing agent, and reinforcing fibers. Component (A): Epoxy resin Component (B): Aliphatic polyester with a melting point of 20-100°C

2. The sheet molding compound according to claim 1, wherein component (B) has dicarboxylic acid units and diol units.

3. The sheet molding compound according to claim 2, wherein component (B) has only one type each of dicarboxylic acid units and diol units.

4. The sheet molding compound according to claim 2, wherein the dicarboxylic acid unit contains a linear dicarboxylic acid unit having 2 to 20 carbon atoms.

5. The sheet molding compound according to claim 2, wherein the diol unit contains a straight-chain diol unit having 2 to 20 carbon atoms.

6. The sheet molding compound according to claim 1, wherein the epoxy resin composition further comprises the following component (C). Ingredient (C): Vinyl polymer

7. The sheet molding compound according to claim 6, wherein the component (C) is in particulate form.

8. The sheet molding compound according to claim 6, wherein the glass transition temperature of component (C) is 30 to 120°C.

9. The sheet molding compound according to claim 6, comprising 0.1 to 10 parts by mass of component (C) per 100 parts by mass of the total of component (A) and component (B).

10. The sheet molding compound according to claim 1, wherein component (A) contains a liquid epoxy resin at 25°C.

11. The sheet molding compound according to claim 1, wherein the epoxy resin curing agent comprises at least one compound selected from aliphatic amines, aromatic amines, secondary amines, tertiary amines, imidazole compounds, epoxy resin amine adducts, and dicyandiamides.

12. The sheet molding compound according to claim 1, wherein the epoxy resin curing agent comprises dicyandiamide.

13. The sheet molding compound according to claim 1, wherein the average length of the reinforcing fibers is 0.3 to 10 cm.

14. The sheet molding compound according to claim 1, wherein a needle-shaped crystalline structure is confirmed by evaluating the crystalline structure of component (A) and component (B) by the method described below. <Evaluation of crystal structure> The components (A) and (B) are mixed in a ratio of mass of component (A) to mass of component (B) of 99:1, heated and stirred at 150°C for 6 hours, and then cooled to room temperature (25°C). The crystalline structure of the obtained sample is confirmed using a polarizing microscope.

15. The sheet molding compound according to claim 1, wherein the epoxy resin composition has a melting point of 0 to 100°C.

16. The sheet molding compound according to claim 1, comprising 1 to 20 parts by mass of component (B) per 100 parts by mass of component (A).

17. A fiber-reinforced composite material which is a press-molded product of a sheet molding compound according to any one of claims 1 to 16.

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