WO2025004687A1 - Epoxy resin composition, sheet molding compound, and molded article - Google Patents

Abstract

The present invention provides an epoxy resin composition that is used in a sheet molding compound, and realizes, by using the sheet molding compound, manufacturing of a fiber-reinforced composite material that exhibits improved impact resistance. This epoxy resin composition for a sheet molding compound contains an epoxy resin (A), polymer particles (B) having a core-shell structure, dicyandiamide (C), an imidazole-based compound (D) having a melting point of 40°C or higher, and a non-latent epoxy resin curing agent (E). Each of the polymer particles (B) has one or more types of core layers selected from diene-based rubbers, (meth)acrylate-based rubbers, and organosiloxane-based rubbers.

Description

Epoxy resin composition, sheet molding compound, and molded article

The present invention relates to an epoxy resin composition for a sheet molding compound, a sheet molding compound, and a molded body.

Fiber reinforced plastics (FRP) are lightweight yet can be made strong, and are therefore used as a material for a variety of molded products, including fishing boats, sporting goods, bathtubs, and automobile parts.

Fiber-reinforced plastics are manufactured using reinforcing fibers such as carbon fiber or glass fiber and a curable resin composition through methods such as autoclave molding, filament winding molding, resin injection molding, vacuum resin injection molding, and press molding. Demand for press molding has been increasing in recent years, as it is highly productive and can easily produce molded products with excellent design surfaces.

In particular, press molding SMC (sheet molding compound) has the advantage of easily producing molded bodies with complex shapes. SMC refers to a material made by impregnating short fibers with thermosetting resin, thickening the thermosetting resin, and turning it into a B-stage, forming it into a sheet. By cutting the SMC into a specified shape, stacking multiple sheets, and press molding them, the curable resin composition can be cured to obtain the final molded body.

Various resins are known as thermosetting resins that can be used in SMC, one example being epoxy resin. For example, Patent Document 1 discloses that an epoxy resin composition containing an aromatic epoxy resin, an amino compound having a specific structure, dicyandiamide, and an imidazole compound in specific amounts can be used as a curable resin composition for SMC.

Japanese Patent Application Publication No. 06-166742

Molded articles made using SMC have applications that require impact resistance. However, conventional fiber-reinforced composite materials made from SMC containing epoxy resins do not always have sufficient impact resistance, and improvements in this area are needed.

In view of the above-mentioned current situation, the present invention aims to provide an epoxy resin composition for use in a sheet molding compound, which can be used to produce a fiber-reinforced composite material that exhibits improved impact resistance.

The inventors discovered that the above problems could be solved by configuring the epoxy resin composition in a specific blend, leading to the invention.

That is, the present invention relates to an epoxy resin composition for sheet molding compounds, which contains an epoxy resin (A), polymer particles having a core-shell structure (B), dicyandiamide (C), an imidazole compound (D) having a melting point of 40° C. or higher, and a non-latent epoxy resin curing agent (E).
The present invention also relates to a sheet molding compound comprising reinforcing fibers and the epoxy resin composition for sheet molding compounds impregnated into the reinforcing fibers.
The present invention further relates to a molded article obtained by press-molding the sheet molding compound.

According to the present invention, there is provided an epoxy resin composition for use in a sheet molding compound, which can be used to produce a fiber-reinforced composite material exhibiting improved impact resistance.
According to the present invention, it is possible to produce a fiber-reinforced composite material exhibiting improved impact resistance while maintaining the manufacturability (impregnation) and handling properties (tackiness, drapeability) of a sheet molding compound at a good level.

Hereinafter, an embodiment of the present invention will be described in detail.
The epoxy resin composition for a sheet molding compound according to this embodiment contains at least an epoxy resin (A), polymer particles having a core-shell structure (B), dicyandiamide (C), an imidazole-based compound (D) having a melting point of 40° C. or higher, and a non-latent epoxy resin curing agent (E).

<Epoxy resin (A)>
The epoxy resin composition according to the present embodiment contains an epoxy resin (A) as a curable resin. The epoxy resin (A) is preferably liquid at room temperature or under heating. As the epoxy resin (A), various epoxy resins can be used. For example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, bisphenol S type epoxy resin, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, novolac type epoxy resin, glycidyl ether type epoxy resin of bisphenol A propylene oxide adduct, hydrogenated bisphenol A (or F) type epoxy resin, fluorinated epoxy resin, flame retardant epoxy resin such as glycidyl ether of tetrabromobisphenol A, p-oxybenzoic acid glycidyl ether ester type epoxy resin, m-aminophenol type epoxy resin, diaminodiphenylmethane type epoxy resin, various alicyclic epoxy resins, N,N-diglycidylaniline, N,N-diglycidyl-o-toluidinyl Examples of the epoxy resin include epoxy compounds obtained by addition reaction of the above epoxy resins with bisphenol A (or F) or polybasic acids, triglycidyl isocyanurate, divinylbenzene dioxide, resorcinol diglycidyl ether, polyalkylene glycol diglycidyl ether, glycol diglycidyl ether, diglycidyl ester of aliphatic polybasic acid, glycidyl ether of polyhydric aliphatic alcohols having two or more valences such as glycerin, epoxidized products of unsaturated polymers such as chelate-modified epoxy resins, rubber-modified epoxy resins, urethane-modified epoxy resins, hydantoin-type epoxy resins, petroleum resins, etc., glycidylamine-type epoxy resins, and the like, but are not limited thereto, and generally used epoxy resins can be used. These epoxy resins may be used alone or in combination of two or more.

Among these epoxy resins, those having at least two epoxy groups per molecule are preferred because they have high curability and excellent mechanical properties after curing. Among the above epoxy resins, those having two epoxy groups per molecule are particularly preferred because they are highly flexible after curing and have excellent impact resistance improvement effects due to the incorporation of polymer particles (B). The above epoxy resins may also include those having three or more epoxy groups per molecule. Using an epoxy resin having three or more epoxy groups per molecule results in better heat resistance after curing.

Among the above epoxy resins, those having an aromatic ring structure or an alicyclic structure in the molecule are preferred because they have excellent heat resistance after curing. In addition, epoxy resins having an alicyclic structure in the molecule are also preferred because they can reduce the viscosity of the epoxy resin composition.

Among the above epoxy resins, bisphenol A type epoxy resins and bisphenol F type epoxy resins are preferred because the resulting cured product has a high elastic modulus, is excellent in improving heat resistance and impact resistance, and is relatively inexpensive, with bisphenol A type epoxy resins being particularly preferred.

Among the various epoxy resins, epoxy resins with an epoxy equivalent of less than 220 are preferred because the resulting cured product has a high elastic modulus and heat resistance, and epoxy equivalents of 90 or more and less than 210 are more preferred, and 150 or more and less than 200 are even more preferred.

In particular, bisphenol A type epoxy resins and bisphenol F type epoxy resins with an epoxy equivalent of less than 220 are preferred because they are liquid at room temperature and the resulting epoxy resin compositions are easy to handle.

The amount of epoxy resin (A) in the epoxy resin composition is not particularly limited and can be set as appropriate, but in order to obtain a cured product with a high elastic modulus and excellent heat resistance and impact resistance improving effects, it is preferably 20% by weight or more of the total amount of the epoxy resin composition, more preferably 40% by weight or more, even more preferably 50% by weight or more, and particularly preferably 60% by weight or more.

<Core-shell polymer particles (B)>
The epoxy resin composition according to the present embodiment contains polymer particles having a core-shell structure as component (B). Hereinafter, the polymer particles (B) having a core-shell structure are also referred to as core-shell polymer particles (B) or polymer particles (B).

By blending the core-shell polymer particles (B) into the epoxy resin composition, the impact resistance of the fiber-reinforced composite material produced using the sheet molding compound can be improved.

The core-shell polymer particles (B) may not have epoxy groups in the shell layer, but preferably have epoxy groups in the shell layer. In this case, the content of epoxy groups in the shell layer relative to the total amount of the core-shell polymer particles (B) is preferably 0.03 mmol/g or more and 2.0 mmol/g or less, more preferably 0.1 mmol/g or more and 1.5 mmol/g or less, and even more preferably 0.2 mmol/g or more and 1.0 mmol/g or less, from the viewpoint of the effect of improving the impact resistance of the fiber-reinforced composite material. This suppresses aggregation of the polymer particles (B) and allows the polymer particles (B) to be dispersed in the cured product in the state of primary particles, and as a result, the impact resistance of the fiber-reinforced composite material can be improved.

The particle size of the polymer particles (B) is not particularly limited, but considering industrial productivity, the volume average particle size (Mv) is preferably 10 to 2000 nm, more preferably 30 to 600 nm, even more preferably 50 to 400 nm, and particularly preferably 70 to 300 nm. The volume average particle size (Mv) of the polymer particles can be measured for the latex of the polymer particles using a commercially available measuring device, such as Nanotrac Wave EX-150 (manufactured by Nikkiso Co., Ltd.).

The polymer particles (B) are preferably dispersed in the epoxy resin composition in the form of primary particles. In this specification, "core-shell polymer particles dispersed in the form of primary particles" (hereinafter also referred to as primary dispersion) means that the core-shell polymer particles are dispersed substantially independently (without contact) with each other, and the dispersed state can be confirmed, for example, by dissolving a part of the epoxy resin composition in a solvent such as methyl ethyl ketone and measuring the particle size with a particle size measuring device using laser light scattering.

The value of the volume average particle diameter (Mv)/number average particle diameter (Mn) obtained by the particle diameter measurement is not particularly limited, but is preferably 3 or less, more preferably 2.5 or less, even more preferably 2 or less, and particularly preferably 1.5 or less. If the volume average particle diameter (Mv)/number average particle diameter (Mn) is 3 or less, it is considered that the polymer particles (B) are well dispersed, and the obtained fiber-reinforced composite material has a good effect of improving the impact resistance.

The volume average particle size (Mv)/number average particle size (Mn) can be measured using a commercially available measuring device, such as Nanotrac Wave EX-150 (manufactured by Nikkiso Co., Ltd.), and calculated by dividing Mv by Mn.

The "stable dispersion" of the core-shell polymer particles means a state in which the core-shell polymer particles are dispersed steadily for a long period of time under normal conditions without agglomeration, separation, or precipitation in the continuous layer. It is also preferable that the distribution of the core-shell polymer particles in the continuous layer does not change substantially, and that the "stable dispersion" can be maintained even if the composition is heated within a safe range to reduce the viscosity and stirred.
The polymer particles (B) may be used alone or in combination of two or more kinds.

The structure of the polymer particles (B) is not particularly limited, but it is preferable that the polymer particles have two or more layers. It is also possible for the polymer particles to have a structure of three or more layers, which is composed of an intermediate layer that covers a core layer and a shell layer that further covers the intermediate layer.

Each layer of the polymer particles (B) will be specifically described below.
<Core layer>
The core layer is preferably an elastic core layer having rubber properties in order to increase the toughness of the cured product of the epoxy resin composition. In order to have rubber properties, the elastic core layer preferably has a gel content of 60% by weight or more, more preferably 80% by weight or more, even more preferably 90% by weight or more, and particularly preferably 95% by weight or more. In addition, the gel content in this specification means the ratio of the insoluble content to the total amount of the insoluble content and the soluble content when 0.5 g of crumb obtained by solidification and drying is immersed in 100 g of toluene, left at 23° C. for 24 hours, and then the insoluble content and the soluble content are separated.

The core layer preferably contains one or more rubbers selected from the group consisting of diene-based rubbers, (meth)acrylate-based rubbers, and organosiloxane-based rubbers. The core layer preferably contains diene-based rubbers because they have a high effect of improving the impact resistance of the resulting fiber-reinforced composite material, and because they have low affinity with the epoxy resin (A), they are less likely to cause an increase in viscosity over time due to swelling of the core layer caused by the epoxy resin (A).

(Diene rubber)
Examples of the conjugated diene monomer constituting the diene rubber include 1,3-butadiene, isoprene, 2-chloro-1,3-butadiene, 2-methyl-1,3-butadiene, etc. These conjugated diene monomers may be used alone or in combination of two or more kinds.

The content of the conjugated diene monomer is preferably in the range of 50 to 100% by weight of the core layer, more preferably in the range of 70 to 100% by weight, and even more preferably in the range of 90 to 100% by weight. If the content of the conjugated diene monomer is 50% by weight or more, the impact resistance improvement effect of the obtained fiber reinforced composite material can be better.

Vinyl monomers copolymerizable with conjugated diene monomers include, for example, vinyl arenes such as styrene, α-methylstyrene, monochlorostyrene, and dichlorostyrene; vinyl carboxylic acids and their esters such as acrylic acid and methacrylic acid; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinyl bromide, and chloroprene; vinyl acetate; alkenes such as ethylene, propylene, butylene, and isobutylene; and polyfunctional monomers such as diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, and divinylbenzene. These vinyl monomers may be used alone or in combination of two or more. Styrene is particularly preferred.

The content of the vinyl monomer copolymerizable with the conjugated diene monomer is preferably in the range of 0 to 50% by weight of the core layer, more preferably in the range of 0 to 30% by weight, and even more preferably in the range of 0 to 10% by weight. If the content of the vinyl monomer copolymerizable with the conjugated diene monomer is 50% by weight or less, the impact resistance improvement effect of the obtained fiber reinforced composite material can be better.

The diene rubber is preferably butadiene rubber using 1,3-butadiene and/or butadiene-styrene rubber, which is a copolymer of 1,3-butadiene and styrene, because it has a high effect of improving the impact resistance of the resulting fiber-reinforced composite material and is unlikely to increase in viscosity over time due to swelling of the core layer due to its low affinity with the epoxy resin (A), with butadiene rubber being more preferred. Butadiene-styrene rubber is also preferred because it can increase the transparency of the cured product obtained by adjusting the refractive index.

((Meth)acrylate rubber)
The (meth)acrylate rubber is preferably a rubber elastomer obtained by polymerizing a monomer mixture containing 50 to 100% by weight of at least one monomer selected from the group consisting of (meth)acrylate monomers and 0 to 50% by weight of another vinyl monomer copolymerizable with the (meth)acrylate monomer.

The (meth)acrylate monomers include, for example, (i) alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate; (ii) aromatic ring-containing (meth)acrylates such as phenoxyethyl (meth)acrylate and benzyl (meth)acrylate; (iii) 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate; acrylate and other hydroxyalkyl (meth)acrylates; (iv) glycidyl (meth)acrylates such as glycidyl (meth)acrylate and glycidyl alkyl (meth)acrylate; (v) alkoxyalkyl (meth)acrylates; (vi) allyl alkyl (meth)acrylates such as allyl (meth)acrylate and allyl alkyl (meth)acrylate; (vii) polyfunctional (meth)acrylates such as monoethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate. These (meth)acrylate monomers may be used alone or in combination of two or more. As the (meth)acrylate monomers, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred.

Other vinyl monomers copolymerizable with (meth)acrylate monomers include, for example, (i) vinyl arenes such as styrene, α-methylstyrene, monochlorostyrene, and dichlorostyrene; (ii) vinyl carboxylic acids such as acrylic acid and methacrylic acid; (iii) vinyl cyanides such as acrylonitrile and methacrylonitrile; (iv) vinyl halides such as vinyl chloride, vinyl bromide, and chloroprene; (v) vinyl acetate; (vi) alkenes such as ethylene, propylene, butylene, and isobutylene; and (vii) polyfunctional monomers such as diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, and divinylbenzene. These vinyl monomers may be used alone or in combination of two or more. Styrene is particularly preferred because it can easily increase the refractive index.

(Organosiloxane rubber)
Examples of the organosiloxane rubber include (i) polysiloxane polymers composed of alkyl or aryl di-substituted silyloxy units, such as dimethylsilyloxy, diethylsilyloxy, methylphenylsilyloxy, diphenylsilyloxy, and dimethylsilyloxy-diphenylsilyloxy; and (ii) polysiloxane polymers composed of alkyl or aryl mono-substituted silyloxy units, such as organohydrogensilyloxy in which a portion of the alkyl in the side chain is substituted with a hydrogen atom. These polysiloxane polymers may be used alone or in combination of two or more. Among them, dimethylsilyloxy, methylphenylsilyloxy, and dimethylsilyloxy-diphenylsilyloxy are preferred because they can impart heat resistance to the cured product, and dimethylsilyloxy is most preferred because it is easily available. In an embodiment in which the core layer is formed from an organosiloxane-based rubber, the polysiloxane-based polymer moiety preferably accounts for 80% by weight or more (more preferably 90% by weight or more) of the entire organosiloxane-based rubber as 100% by weight, in order not to impair the heat resistance of the cured product.

The glass transition temperature of the core layer (hereinafter sometimes simply referred to as "Tg") is preferably 0°C or lower, more preferably -20°C or lower, even more preferably -40°C or lower, and particularly preferably -60°C or lower, in order to provide an excellent effect of improving the impact resistance of the resulting fiber-reinforced composite material.

The volume average particle diameter of the core layer is preferably 0.03 to 2 μm, and more preferably 0.05 to 1 μm. Within this range, stable production is possible, and the heat resistance and impact resistance of the fiber-reinforced composite material can be improved. The volume average particle diameter can be measured using a commercially available measuring device, such as the Nanotrac Wave EX-150 (manufactured by Nikkiso Co., Ltd.).

The proportion of the core layer is preferably 40 to 97% by weight, more preferably 60 to 96% by weight, even more preferably 70 to 95% by weight, and particularly preferably 80 to 94% by weight, with the entire core-shell polymer particles being 100% by weight. When the proportion of the core layer is 40% by weight or more, the impact resistance improving effect of the fiber-reinforced composite material can be improved. When the proportion of the core layer is 97% by weight or less, the core-shell polymer particles are less likely to aggregate, the epoxy resin composition has a lower viscosity, and workability can be improved. In addition, impregnation into the fibers is further improved, resulting in a better impact resistance improving effect of the fiber-reinforced composite material.

The core layer is often a single layer structure, but may be a multilayer structure consisting of multiple layers having rubber elasticity. In addition, when the core layer is a multilayer structure, the polymer composition of each layer may be different within the range disclosed above.

<Middle Class>
If necessary, an intermediate layer may be formed between the core layer and the shell layer. In particular, the following rubber surface cross-linked layer may be formed as the intermediate layer. From the viewpoint of the effect of improving the impact resistance of the obtained fiber reinforced composite material, it is preferable that the intermediate layer is not contained, and in particular, that the following rubber surface cross-linked layer is not contained.

If an intermediate layer is present, the ratio of the intermediate layer to 100 parts by weight of the core layer is preferably 0.1 to 30 parts by weight, more preferably 0.2 to 20 parts by weight, even more preferably 0.5 to 10 parts by weight, and particularly preferably 1 to 5 parts by weight.

The rubber surface cross-linked layer is made of an intermediate layer polymer obtained by polymerizing a rubber surface cross-linked layer component consisting of 30 to 100% by weight of a polyfunctional monomer having two or more radically polymerizable double bonds in one molecule and 0 to 70% by weight of other vinyl monomers, and has the effect of reducing the viscosity of the epoxy resin composition and improving the dispersibility of the polymer particles (B) in the epoxy resin (A). It also has the effect of increasing the cross-link density of the core layer and the grafting efficiency of the shell layer.

Specific examples of the polyfunctional monomer do not include conjugated diene monomers such as butadiene, and include allyl alkyl (meth)acrylates such as allyl (meth)acrylate and allyl alkyl (meth)acrylate; allyloxyalkyl (meth)acrylates; polyfunctional (meth)acrylates having two or more (meth)acrylic groups such as (poly)ethylene glycol di(meth)acrylate, butanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tetraethylene glycol di(meth)acrylate; diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, and divinylbenzene; with allyl methacrylate and triallyl isocyanurate being preferred. In this specification, (meth)acrylate means acrylate and/or methacrylate.

<Shell layer>
The shell layer present at the outermost part of the core-shell polymer particle is formed by polymerizing a monomer for forming the shell layer, and is made of a shell polymer that plays a role of improving the compatibility between the polymer particle (B) and the epoxy resin (A) and enabling the polymer particle (B) to be dispersed in the form of primary particles in the epoxy resin composition or its cured product.

Such a shell polymer is preferably grafted to the core layer and/or intermediate layer. Hereinafter, when the term "grafted to the core layer" is used, it also includes the case where an intermediate layer is formed on the core layer, in which case the shell polymer is grafted to the intermediate layer. More precisely, it is preferable that the monomer component used to form the shell layer is graft polymerized to the core polymer forming the core layer (when an intermediate layer is formed, the core polymer also includes the intermediate layer polymer forming the intermediate layer. The same applies below) so that the shell polymer and the core polymer are substantially chemically bonded (when an intermediate layer is formed, it is also preferable that the shell polymer and the intermediate layer polymer are chemically bonded). That is, the shell polymer is preferably formed by graft polymerizing the monomer for forming the shell layer in the presence of the core polymer, and in this way, the shell polymer is graft polymerized to the core polymer and covers a part or the whole of the core polymer. This polymerization operation can be carried out by adding the monomer for forming the shell polymer layer to the latex of the core polymer prepared in an aqueous polymer latex state and polymerizing it.

As the monomer for forming the shell layer, from the viewpoint of compatibility and dispersibility of the polymer particles (B) in the epoxy resin composition, for example, an aromatic vinyl monomer, a vinyl cyan monomer, or a (meth)acrylate monomer is preferred, and a (meth)acrylate monomer is more preferred. In particular, from the viewpoint of the effect of improving the impact resistance of the fiber-reinforced composite material, it is preferable that the monomer for forming the shell layer contains an acrylic acid alkyl ester. These monomers for forming the shell layer may be used alone or in appropriate combination.

The total amount of aromatic vinyl monomer, vinyl cyan monomer, and (meth)acrylate monomer is preferably 50 to 100% by weight, more preferably 60 to 100% by weight, even more preferably 70 to 100% by weight, particularly preferably 80 to 100% by weight, and most preferably 90 to 100% by weight, based on 100% by weight of the monomers for forming the shell layer.

The content of the acrylic acid alkyl ester is preferably 5 to 100% by weight, more preferably 20 to 99% by weight, even more preferably 30 to 97% by weight, and particularly preferably 70 to 95% by weight, based on 100% by weight of the monomer for forming the shell layer.

In order to maintain a good dispersion state without agglomeration of the polymer particles (B) in the cured product or epoxy resin composition, and from the viewpoint of chemically bonding with the epoxy resin (A), it is preferable that the shell layer-forming monomer contains a reactive group-containing monomer containing one or more selected from the group consisting of an epoxy group, an oxetane group, a hydroxyl group, an amino group, an imide group, a carboxylic acid group, a carboxylic anhydride group, a cyclic ester, a cyclic amide, a benzoxazine group, and a cyanate ester group, and in particular, a monomer having an epoxy group.

From the viewpoint of improving the impact resistance of the fiber-reinforced composite material, the monomer having an epoxy group is preferably contained in an amount of 0 to 90% by weight, more preferably 0.5 to 50% by weight, even more preferably 1 to 40% by weight, and particularly preferably 5 to 35% by weight, out of 100% by weight of the monomer for forming the shell layer.

Monomers having epoxy groups are preferably used to form the shell layer, and more preferably only in the shell layer.

In addition, using a polyfunctional monomer having two or more radically polymerizable double bonds as the monomer for forming the shell layer is preferable because it prevents swelling of the core-shell polymer particles in the epoxy resin composition and also tends to reduce the viscosity of the epoxy resin composition and improve its handleability. On the other hand, from the viewpoint of the effect of improving the impact resistance of the resulting fiber-reinforced composite material, it is preferable not to use a polyfunctional monomer having two or more radically polymerizable double bonds as the monomer for forming the shell layer.

The polyfunctional monomer may be contained in an amount of, for example, 0 to 20% by weight, preferably 1 to 20% by weight, and more preferably 5 to 15% by weight, based on 100% by weight of the monomer for forming the shell layer.

Specific examples of the aromatic vinyl monomer include vinylbenzenes such as styrene, α-methylstyrene, p-methylstyrene, and divinylbenzene.

Specific examples of the vinyl cyan monomer include acrylonitrile and methacrylonitrile.

Specific examples of the (meth)acrylate monomer include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; and (meth)acrylic acid hydroxyalkyl esters.

Specific examples of the (meth)acrylic acid hydroxyalkyl ester include hydroxy linear alkyl (meth)acrylates (particularly hydroxy linear C1-6 alkyl (meth)acrylates) such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; caprolactone-modified hydroxy (meth)acrylates; hydroxy branched alkyl (meth)acrylates such as α-(hydroxymethyl)methyl acrylate and α-(hydroxymethyl)ethyl acrylate; and hydroxyl group-containing (meth)acrylates such as mono(meth)acrylates of polyester diols (particularly saturated polyester diols) obtained from divalent carboxylic acids (such as phthalic acid) and dihydric alcohols (such as propylene glycol).

Specific examples of the monomer having an epoxy group include glycidyl group-containing vinyl monomers such as glycidyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate glycidyl ether, and allyl glycidyl ether.

Specific examples of the polyfunctional monomer having two or more radically polymerizable double bonds include the same monomers as the polyfunctional monomers described above, with allyl methacrylate and triallyl isocyanurate being preferred.

In this embodiment, it is preferable to use a shell layer that is a polymer of shell layer-forming monomers (total 100% by weight) that combine, for example, 0-50% by weight (preferably 1-50% by weight, more preferably 2-48% by weight) of aromatic vinyl monomer (particularly styrene), 0-50% by weight (preferably 0-30% by weight, more preferably 10-25% by weight) of vinyl cyan monomer (particularly acrylonitrile), 0-100% by weight (preferably 5-100% by weight, more preferably 70-95% by weight) of (meth)acrylate monomer (particularly acrylic acid alkyl ester), and 1-50% by weight (preferably 2-40% by weight, more preferably 3-30% by weight) of monomer having an epoxy group (particularly glycidyl methacrylate). This makes it possible to achieve a good balance between the desired impact resistance improvement effect and mechanical properties.

These monomer components may be used alone or in combination of two or more. The shell layer may be formed by including other monomer components in addition to the above monomer components.

The graft ratio of the shell layer is preferably 70% or more (more preferably 80% or more, and even more preferably 90% or more). If the graft ratio is 70% or more, the epoxy resin composition can have a lower viscosity.

The graft ratio is calculated as follows. First, the aqueous latex containing the core-shell polymer particles is coagulated and dehydrated, and finally dried to obtain a powder of the core-shell polymer particles. Next, 2 g of the powder of the core-shell polymer particles is immersed in 100 g of methyl ethyl ketone (MEK) at 23° C. for 24 hours, after which the MEK soluble matter is separated from the MEK insoluble matter, and the methanol insoluble matter is further separated from the MEK soluble matter. This methanol insoluble matter corresponds to the non-grafted polymer.
The graft ratio is calculated according to the following formula.
Graft ratio=GM−methanol insolubles/methanol insolubles+MEK insolubles/GM
GM: Proportion [%] of the monomer for forming the shell layer when the total of the monomer for forming the shell layer and the core layer polymer used in producing the core-shell polymer is taken as 100

The non-grafted polymer refers to a shell polymer that is not grafted to a core polymer. The number average molecular weight of the non-grafted polymer is not particularly limited, but is preferably 200,000 or less, more preferably 100,000 or less, even more preferably 50,000 or less, and particularly preferably 20,000 or less, so that the epoxy resin composition can have a lower viscosity. The number average molecular weight of the non-grafted polymer can be measured by gel permeation chromatography (GPC) as a polystyrene-equivalent molecular weight, etc.

<Method for producing core-shell polymer particles>
(Method of Manufacturing Core Layer)
The core layer constituting the polymer particles (B) can be produced by, for example, emulsion polymerization, suspension polymerization, microsuspension polymerization, or the like, and for example, the method described in WO 2005/028546 can be used.

(Method of forming shell layer and intermediate layer)
The intermediate layer can be formed by polymerizing the intermediate layer-forming monomer by known radical polymerization. When the rubber elastic body constituting the core layer is obtained as an emulsion, the intermediate layer-forming monomer is preferably polymerized by an emulsion polymerization method.

The shell layer can be formed by polymerizing the monomer for forming the shell layer by known radical polymerization. When the core layer or the polymer particle precursor formed by coating the core layer with the intermediate layer is obtained as an emulsion, it is preferable to polymerize the monomer for forming the shell layer by emulsion polymerization, and it can be produced, for example, according to the method described in WO 2005/028546.

Emulsifiers (dispersants) that can be used in emulsion polymerization include anionic emulsifiers (dispersants) such as alkyl or aryl sulfonic acids, such as dioctyl sulfosuccinic acid and dodecylbenzenesulfonic acid, alkyl or aryl ether sulfonic acids, alkyl or aryl sulfuric acids, such as dodecyl sulfate, alkyl or aryl ether sulfate, alkyl or aryl substituted phosphoric acids, alkyl or aryl ether substituted phosphoric acids, N-alkyl or aryl sarcosinic acids, such as dodecyl sarcosinic acid, alkyl or aryl carboxylic acids, such as oleic acid and stearic acid, alkyl or aryl ether carboxylic acids, and various other acids, as well as alkali metal or ammonium salts of these acids; nonionic emulsifiers (dispersants), such as alkyl or aryl substituted polyethylene glycol; and dispersants, such as polyvinyl alcohol, alkyl substituted cellulose, polyvinylpyrrolidone, and polyacrylic acid derivatives. These emulsifiers (dispersants) may be used alone or in combination of two or more.

As long as it does not impair the dispersion stability of the aqueous latex of the polymer particles, it is preferable to use a small amount of emulsifier (dispersant). In addition, the higher the water solubility of the emulsifier (dispersant), the more preferable it is. High water solubility makes it easier to wash off the emulsifier (dispersant) with water, and makes it easier to prevent adverse effects on the resulting sheet molding compound and fiber-reinforced composite material.

When emulsion polymerization is used, known initiators, such as 2,2'-azobisisobutyronitrile, hydrogen peroxide, potassium persulfate, and ammonium persulfate, can be used as thermal decomposition initiators.

Redox initiators can also be used that use organic peroxides such as t-butyl peroxyisopropyl carbonate, paramenthane hydroperoxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, and t-hexyl peroxide; inorganic peroxides such as hydrogen peroxide, potassium persulfate, and ammonium persulfate, in combination with reducing agents such as sodium formaldehyde sulfoxylate and glucose, and transition metal salts such as iron (II) sulfate, as required, chelating agents such as disodium ethylenediaminetetraacetate, and phosphorus-containing compounds such as sodium pyrophosphate, as required.

When a redox type initiator system is used, polymerization can be carried out even at a low temperature where the peroxide does not substantially decompose thermally, and the polymerization temperature can be set in a wide range, which is preferable. Among them, it is preferable to use an organic peroxide such as cumene hydroperoxide, dicumyl peroxide, or t-butyl hydroperoxide as a redox type initiator. The amount of the initiator used, and the amount of the reducing agent, transition metal salt, chelating agent, etc. used when a redox type initiator is used, can be used within known ranges. Furthermore, when polymerizing a monomer having two or more radically polymerizable double bonds, known chain transfer agents can be used within known ranges. A surfactant can be additionally used, which is also within known ranges.

The polymerization temperature, pressure, deoxygenation, and other conditions during polymerization can be within the publicly known range. Furthermore, polymerization of the intermediate layer monomer can be carried out in one stage or two or more stages. For example, in addition to a method of adding the intermediate layer monomer to the emulsion of the rubber elastomer that constitutes the elastic core layer all at once or a method of adding them continuously, a method of adding an emulsion of the rubber elastomer that constitutes the elastic core layer to a reactor that has already been charged with the intermediate layer monomer and then carrying out polymerization can be employed.

The amount of the core-shell polymer particles (B) in the epoxy resin composition is not particularly limited and can be appropriately set taking into consideration the effect of the incorporation of the (B) component, and may be, for example, about 1 to 80% by weight of the total amount of the epoxy resin composition. However, in order to balance the ease of handling of the epoxy resin composition and the impact resistance improvement of the resulting fiber-reinforced composite material, the amount of the core-shell polymer particles (B) is preferably 5 to 50% by weight of the total amount of the epoxy resin composition. If the amount is 5% by weight or more, the impact resistance improvement effect can be remarkable. On the other hand, if the amount is 50% by weight or less, the viscosity of the epoxy resin composition can be suitable for impregnation between fibers. The resin composition is sufficiently filled between the fibers, and the impact resistance improvement effect tends to be easily obtained. The amount is more preferably 7.5 to 50% by weight. The lower limit may be 10% by weight or more, or 15% by weight or more. The upper limit may be 45% by weight or less, 40% by weight or less, 35% by weight or less, or 30% by weight or less.

<Dicyandiamide (C)>
Dicyandiamide (C) generates cyanamide when heated, which enables the epoxy resin (A) to be crosslinked, and therefore can function as a latent curing agent that becomes active when heated.

The amount of dicyandiamide (C) in the epoxy resin composition can be set appropriately depending on the desired physical properties, but from the viewpoint of the balance between the curability of the epoxy resin composition, the impact resistance improvement effect of the fiber-reinforced composite material, and impregnation properties, it is preferably 2 to 20 parts by weight, more preferably 3 to 18 parts by weight, even more preferably 4 to 16 parts by weight, even more preferably 5 to 14 parts by weight, and particularly preferably 6 to 12 parts by weight, per 100 parts by weight of epoxy resin (A).

<Imidazole Compound (D) Having a Melting Point of 40° C. or More>
The imidazole compound (D) is a component that can function as a potential curing agent for epoxy resins. The imidazole compound (D) has a nitrogen atom having an unshared electron pair in its structure, which activates the epoxy group of the epoxy resin (A) and dicyandiamide (C) to promote the curing reaction. By using dicyandiamide (C) in combination with the imidazole compound (D), it is possible to achieve both high heat resistance of the fiber-reinforced composite material and fast curing of the epoxy resin composition.

The imidazole compound (D) is not particularly limited, but is preferably a compound in which the 1st, 2nd, 4th, and 5th positions of 1H-imidazole are substituted with any substituent. Only one type of imidazole compound (D) may be used, or two or more types may be used in combination.

The imidazole compound (D) is preferably a compound that exhibits the property of being solid at 25°C. By having the imidazole compound (D) be a solid at 25°C, the reaction of the imidazole compound (D) during the production of the SMC and during storage of the produced SMC is suppressed, and the productivity, storage stability, and handleability of the SMC, as well as the fluidity of the epoxy resin composition during press molding, tend to be improved.

The melting point of the imidazole compound (D) is 40°C or higher, preferably 50°C or higher, more preferably 60°C or higher, and even more preferably 65°C or higher. There is no particular upper limit, but it may be, for example, less than 300°C.

Specific examples of imidazole compounds (D) include 1H-imidazole (melting point: 90°C), 2-methylimidazole (melting point: 144°C), 2-undecylimidazole (melting point: 73°C), 2-phenylimidazole (melting point: 142°C), 2-phenyl-4-methylimidazole (melting point: 179°C), 1-cyanoethyl-2-methylimidazole (melting point: 55°C), and 1-cyanoethyl-2-phenylimidazole (melting point: 108°C).

The imidazole-based compound (D) may be a compound obtained by modifying imidazole. Examples of such compounds include compounds of 2-methylimidazole with phenyl glycidyl ether or bisphenol A diglycidyl ether (epoxy resin amine adduct).

Commercially available epoxy resin amine adducts include Ajinomoto Fine-Techno Co., Ltd.'s PN-23 (melting point: 60°C), PN-23J (melting point: 60°C), PN-31 (melting point: 52°C), PN-31J (melting point: 52°C), PN-40 (melting point: 76°C), PN-40J (melting point: 76°C), PN-50 (melting point: 83°C), PN-50J (melting point: 84°C), and P-0505 (melting point: 69°C).

Further examples of commercially available imidazole compounds (D) include:
2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine such as 2MZA-PW (manufactured by Shikoku Chemical Industry Co., Ltd.);
2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid addition salts such as 2MAOK-PW (manufactured by Shikoku Chemical Industry Co., Ltd.);
2-phenyl-4-methyl-5-hydroxymethylimidazole such as 2P4MHZ-PW (manufactured by Shikoku Chemical Industry Co., Ltd.);
2-phenyl-4,5-dihydroxymethylimidazole such as 2PHZ-PW (manufactured by Shikoku Chemical Industry Co., Ltd.);

The imidazole compound (D) begins to dissolve in the epoxy resin (A) as the temperature rises, and the reaction begins accordingly. Therefore, it is preferable for the compound to be in the form of fine particles, since this ensures stability at room temperature.

The particle size of the imidazole compound (D) is preferably 1 μm to 10 μm from the viewpoint of dissolution speed. A particle size of 1 μm or more ensures stability at room temperature, and a particle size of 10 μm or less ensures good curability.

2MZA-PW is particularly preferred as the imidazole compound (D) because it provides both stability at room temperature and rapid curing of the epoxy resin composition.

The content of the imidazole compound (D) in the epoxy resin composition is preferably 1 to 10 parts by weight, more preferably 2 to 8 parts by weight, and even more preferably 2 to 6 parts by weight, per 100 parts by weight of the epoxy resin (A), from the viewpoints of the balance between the curability of the epoxy resin composition, the impact resistance improvement effect of the fiber-reinforced composite material, and impregnation properties.

<Non-latent epoxy resin curing agent (E)>
The non-latent epoxy resin curing agent (E) refers to a curing agent that is active at a relatively low temperature, such as room temperature. Specific examples thereof include, but are not limited to, amine-based compounds, acid anhydrides, mercaptan-based curing agents, etc. Among these, amine-based compounds and/or acid anhydrides are preferred. As the non-latent epoxy resin curing agent (E), only one type may be used, or two or more types may be used in combination.

The amine compound corresponding to the non-latent epoxy resin hardener (E) may be any amine compound other than dicyandiamide (C) or an imidazole compound (D) having a melting point of 40°C or higher. Although not particularly limited, examples include chain aliphatic polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenediamine, diethylaminopropylamine, and hexamethylenediamine; N-aminoethylpiperazine, bis(4-aminocyclohexyl)methane, bis(4-amino-3-methylcyclohexyl)methane, menthenediamine, isophoronediamine, 4,4'-diaminodicyclohexylmethane, and 3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5.5]undecane (spiroacetaldiamine). , norbornane diamine, tricyclodecane diamine, 1,3-bisaminomethylcyclohexane and other cyclic aliphatic polyamines; aliphatic aromatic amines such as metaxylene diamine; polyamine epoxy resin adducts, which are the reaction products of epoxy resin and excess polyamine; ketimines, which are the dehydration reaction products of polyamine and ketones such as methyl ethyl ketone and isobutyl methyl ketone; polyamidoamines produced by condensation of a dimer (dimer acid) of tall oil fatty acid and a polyamine; and amidoamines produced by condensation of tall oil fatty acid and a polyamine.

Amine-terminated polyethers that contain a polyether backbone and an average of 1 to 4 (preferably 1.5 to 3) amino and/or imino groups per molecule can also be used as component (E). Commercially available amine-terminated polyethers include Jeffamine D-230, Jeffamine D-400, Jeffamine D-2000, Jeffamine D-4000, and Jeffamine T-5000 manufactured by Huntsman.

Furthermore, amine-terminated rubbers containing a conjugated diene polymer backbone and having an average of 1 to 4 (more preferably 1.5 to 3) amino and/or imino groups per molecule can also be used as component (E). Here, the backbone of the rubber is preferably a polybutadiene homopolymer or copolymer, more preferably a polybutadiene/acrylonitrile copolymer, and particularly preferably a polybutadiene/acrylonitrile copolymer having an acrylonitrile monomer content of 5 to 40% by weight (more preferably 10 to 35% by weight, and even more preferably 15 to 30% by weight). Commercially available amine-terminated rubbers include Hypro 1300X16 ATBN manufactured by CVC Corporation.

A single type of amine compound may be used, or two or more types may be used in combination. Among these, cyclic aliphatic polyamines are preferred as amine compounds, with isophoronediamine, bis(4-aminocyclohexyl)methane, and 1,3-bisaminomethylcyclohexane being particularly preferred.

Acid anhydrides that can be used as the non-latent epoxy resin hardener (E) are not particularly limited, but examples thereof include polysebacic polyanhydride, polyazelaic polyanhydride, succinic anhydride, citraconic anhydride, itaconic anhydride, alkenyl-substituted succinic anhydride, octenylsuccinic anhydride, dodecenylsuccinic anhydride, maleic anhydride, tricarballylic anhydride, nadic anhydride, methylnadic anhydride, hydrogenated methylnadic anhydride, linoleic acid adduct with maleic anhydride, alkylated terminal adenylate, methylnadic anhydride ... Examples of the acid anhydride include alkylenetetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, trialkyltetrahydrophthalic anhydride, pyromellitic dianhydride, trimellitic anhydride, phthalic anhydride, tetrachlorophthalic anhydride, tetrabromophthalic anhydride, dichloromaleic anhydride, chloronadic anhydride, chlorendic anhydride, and maleic anhydride-grafted polybutadiene. Only one type of acid anhydride may be used, or two or more types may be used in combination.

Among these, alicyclic acid anhydrides are preferred. Of these, substituted or unsubstituted tetrahydrophthalic anhydride, substituted or unsubstituted hexahydrophthalic anhydride, substituted or unsubstituted methylnadic anhydride, and substituted or unsubstituted hydrogenated methylnadic anhydride are more preferred. The substituents that these acid anhydrides may have are not particularly limited, and examples thereof include hydrocarbon groups and alkoxy groups.

The alicyclic acid anhydrides are particularly preferably methyltetrahydrophthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, and hydrogenated methylnadic anhydride.

The amount of non-latent epoxy resin curing agent (E) in the epoxy resin composition is preferably 1 to 100 parts by weight, more preferably 3 to 50 parts by weight, and even more preferably 5 to 20 parts by weight, per 100 parts by weight of epoxy resin (A), from the viewpoints of the balance between the curability of the epoxy resin composition, the impact resistance improvement effect of the fiber-reinforced composite material, and impregnation properties.

When the non-latent epoxy resin curing agent (E) is an amine compound, the amount of the amine compound is preferably such that the active hydrogen equivalent of the amine compound is 0.1 to 0.5 equivalents relative to the epoxy groups of the epoxy resin (A). When the non-latent epoxy resin curing agent (E) is an acid anhydride, the amount of the amine compound is preferably such that the acid anhydride equivalent of the epoxy groups of the epoxy resin (A) is 0.1 to 0.5 equivalents.

<Reactive Diluent (F)>
The epoxy resin composition according to the present embodiment may further contain a reactive diluent (F). The reactive diluent (F) refers to a compound that has a lower viscosity than the epoxy resin (A) and is reactive with the epoxy resin (A). Specific examples thereof include monoepoxides, difunctional epoxides, and polyfunctional epoxides.

Specific examples of monoepoxides include aliphatic glycidyl ethers such as butyl glycidyl ether, aromatic glycidyl ethers such as phenyl glycidyl ether and cresyl glycidyl ether, ethers consisting of an alkyl group having 8 to 10 carbon atoms and a glycidyl group, such as 2-ethylhexyl glycidyl ether, ethers consisting of a phenyl group having 6 to 12 carbon atoms and a glycidyl group that may be substituted with an alkyl group having 2 to 8 carbon atoms, such as p-tert-butylphenyl glycidyl ether, ethers consisting of an alkyl group having 12 to 14 carbon atoms and a glycidyl group, such as dodecyl glycidyl ether; aliphatic glycidyl esters such as glycidyl (meth)acrylate and glycidyl maleate; glycidyl esters of aliphatic carboxylic acids having 8 to 12 carbon atoms, such as versatic acid glycidyl ester, neodecanoic acid glycidyl ester, and lauric acid glycidyl ester; and p-t-butylbenzoic acid glycidyl ester.

Specific examples of bifunctional epoxides include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, etc.

Specific examples of polyfunctional epoxides include sorbitol polyglycidyl ether, glycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, etc.

As the reactive diluent (F), a bifunctional epoxide or a polyfunctional epoxide is preferred because it can maintain high heat resistance after curing. Particularly preferred examples include 1,6-hexanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, neopentyl glycol diglycidyl ether, and trimethylolpropane polyglycidyl ether.

When a reactive diluent (F) is used, the amount of the diluent is preferably 1 to 30 parts by weight, more preferably 2.5 to 25 parts by weight, and particularly preferably 5 to 20 parts by weight, per 100 parts by weight of the epoxy resin (A).

<Other ingredients>
In the epoxy resin composition according to the present embodiment, other compounding components can be used as necessary. Examples of other compounding components include reinforcing agents such as epoxy unmodified rubber polymers, inorganic fillers such as silicic acid and/or silicates, calcium oxide, radical curing resins, photopolymerization initiators, expansion agents such as azo-type chemical foaming agents and thermally expandable microballoons, fiber pulp such as aramid pulp, colorants such as pigments and dyes, extender pigments, ultraviolet absorbers, antioxidants, stabilizers (gelling inhibitors), plasticizers, leveling agents, defoamers, silane coupling agents, antistatic agents, flame retardants, lubricants, viscosity reducers, low-shrinkage agents, organic fillers, thermoplastic resins, desiccants, dispersants, etc.

<Method for producing epoxy resin composition>
The epoxy resin composition according to the present embodiment is preferably a composition in which the core-shell polymer particles (B) are dispersed in the state of primary particles. Such an epoxy resin composition is preferably produced using a polymer particle dispersion composition in which a high concentration of the core-shell polymer particles (B) is dispersed in the state of primary particles. By using the polymer particle dispersion composition, the dispersion of the primary particles in the epoxy resin composition can be easily realized.

The epoxy resin composition according to this embodiment can also be produced by directly mixing and dispersing the powder of the core-shell polymer particles (B) into the epoxy resin (A). However, this method is prone to producing undispersed matter (aggregates), which can inhibit the impregnation of the epoxy resin composition into the fibers, resulting in reduced mechanical properties and reduced impact resistance improvement effects.

Various methods can be used to obtain the polymer particle dispersion composition. For example, a method of contacting the core-shell polymer particles obtained in an aqueous latex state with an epoxy resin (A) and then removing unnecessary components such as water, a method of extracting the core-shell polymer particles once into an organic solvent and mixing them with the epoxy resin (A) and then removing the organic solvent, etc. are included. However, it is preferable to use the method described in International Publication No. 2005/028546. The specific production method is preferably prepared by mixing an aqueous latex containing the core-shell polymer particles (B) (more specifically, a reaction mixture after the core-shell polymer particles are produced by emulsion polymerization) with an organic solvent having a solubility in water of 5% by weight or more and 40% by weight or less at 20°C, and then further mixing with excess water to aggregate the polymer particles, a second step of separating and recovering the aggregated core-shell polymer particles (B) from the liquid phase, and then mixing them again with an organic solvent to obtain an organic solvent solution of the core-shell polymer particles (B), and a third step of further mixing the organic solvent solution with the epoxy resin (A) and then distilling off the organic solvent.

It is preferable that the epoxy resin (A) is liquid at 23°C, since this facilitates the third step. "Liquid at 23°C" means that the softening point is 23°C or lower, and the resin exhibits flowability at 23°C.

The polymer particle dispersion composition obtained through the above process, in which the core-shell polymer particles (B) are dispersed in the epoxy resin (A) in the form of primary particles, can be mixed with additional epoxy resin (A), dicyandiamide (C), imidazole compound (D), non-latent epoxy resin curing agent (E), and other components as necessary to obtain the epoxy resin composition according to this embodiment, in which the core-shell polymer particles (B) are dispersed in the form of primary particles.

On the other hand, the powdered core-shell polymer particles (B) obtained by solidifying the particles by a method such as salting out and then drying them can be redispersed in the epoxy resin (A) using a dispersing machine with high mechanical shear force, such as a triple paint roll, roll mill, or kneader. In this case, applying mechanical shear force at high temperature enables efficient dispersion of the core-shell polymer particles (B). The temperature during dispersion is preferably 50 to 200°C, more preferably 70 to 170°C, even more preferably 80 to 150°C, and particularly preferably 90 to 120°C.

From the viewpoint of fiber impregnation, the epoxy resin composition according to this embodiment preferably has a viscosity at 25°C of 100,000 mPa·s or less, more preferably 50,000 mPa·s or less, even more preferably 20,000 mPa·s or less, even more preferably 10,000 mPa·s or less, and particularly preferably 5,000 mPa·s or less. Similarly, from the viewpoint of fiber impregnation, the epoxy resin composition according to this embodiment preferably has a viscosity at 50°C of 10,000 mPa·s or less, more preferably 5,000 mPa·s or less, even more preferably 2,000 mPa·s or less, even more preferably 1,000 mPa·s or less, and particularly preferably 500 mPa·s or less.

<Sheet molding compound>
One aspect of the present invention relates to a sheet molding compound (hereinafter also referred to as SMC) obtained by impregnating reinforcing fibers with the epoxy resin composition described above.
The sheet molding compound refers to a flexible sheet composed of reinforcing fibers and an epoxy resin composition that has been impregnated into the reinforcing fibers and brought to a B-stage. The sheet molding compound is usually covered on both sides with a release film before use. When producing a fiber-reinforced composite material, the release film is simply peeled off.

Various types of reinforcing fibers can be used depending on the application and intended use of the SMC, but examples include carbon fiber (including graphite fiber; the same applies below), aramid fiber, silicon carbide fiber, alumina fiber, boron fiber, tungsten carbide fiber, glass fiber, etc. Among these, carbon fiber and glass fiber are preferred, with carbon fiber being particularly preferred, from the standpoint of the mechanical properties of the fiber-reinforced composite material.

As the reinforcing fibers, it is preferable to use short fibers. The average length of the short fibers is not particularly limited, but is preferably 0.3 to 10 cm, more preferably 1 to 5 cm, and even more preferably 1.5 to 4 cm. If the average length of the reinforcing fibers is within the above range, it tends to be easier to obtain an SMC that has an excellent balance between moldability and mechanical properties.
The reinforcing fibers are preferably chopped reinforcing fiber bundles made of short fibers. The form of the reinforcing fibers in the SMC is more preferably a sheet-like product in which chopped reinforcing fiber bundles are two-dimensionally randomly stacked.

<Method of manufacturing sheet molding compound>
The SMC can be produced, for example, by thoroughly impregnating a sheet of chopped reinforcing fiber bundles with the above-mentioned epoxy resin composition to thicken the epoxy resin composition.

As a method for impregnating a sheet of chopped reinforcing fiber bundles with an epoxy resin composition, various conventionally known methods can be used depending on the form of the reinforcing fiber. For example, the following methods can be mentioned.

Two films are prepared that are evenly coated with an epoxy resin composition. Chopped reinforcing fiber bundles are randomly scattered on the surface of one of the films that has the epoxy resin composition applied to it, to produce a sheet of chopped reinforcing fiber bundles. The surface of the other film that has the epoxy resin composition applied to it is then attached to the sheet of chopped reinforcing fiber bundles, and the sheet of chopped reinforcing fiber bundles is pressed and impregnated with the epoxy resin composition, to produce a sheet molding compound precursor (SMC precursor).

The SMC precursor, in which the epoxy resin composition is impregnated into chopped reinforcing fiber bundles, is kept at room temperature to about 60°C for several hours to several tens of days, or at about 60 to 80°C for several seconds to several tens of minutes, causing the epoxy groups to react and causing the epoxy resin composition to enter the B stage (thickening). By thickening the epoxy resin composition in this way, the tackiness of the SMC surface is suppressed, making it possible to obtain an SMC suitable for molding operations.

[Molded body]
One aspect of the present invention is a fiber-reinforced composite material obtained by press-molding the above-mentioned SMC. The fiber-reinforced composite material can be produced by cutting the SMC into a predetermined shape as necessary, laminating a plurality of sheets of the SMC, and press-molding the laminate to harden the epoxy resin composition contained in the SMC, thereby producing a molded article having a predetermined shape.

The molded body can be produced, for example, by the following method.
A sheet of SMC or a stack of multiple sheets of SMC cut into a predetermined shape is set between a pair of dies. The SMC is press molded (compression molded) to harden the epoxy resin composition contained in the SMC, and a fiber-reinforced composite material, which is a press molded product of the SMC, is obtained. A honeycomb structure such as cardboard may be used as a core material, and the SMC of the present invention may be arranged on both sides or one side of the core material. In this specification, for example, stacking two sheets of SMC is referred to as "2-ply stacking".
The temperature and time of the press molding can be appropriately set, but may be, for example, 120 to 230° C. and 2 to 60 minutes.

The resulting molded article is not particularly limited, but is preferably used in applications that require light weight, high strength, and impact resistance. Specific examples include automobile tire wheels, automobile bodies or parts, bathtubs, etc.

The following items enumerate preferred aspects of the present disclosure, but the present invention is not limited to the following items.
[Item 1]
Epoxy resin (A),
Polymer particles (B) having a core-shell structure;
Dicyandiamide (C),
An epoxy resin composition for sheet molding compounds, comprising: (D) an imidazole compound having a melting point of 40° C. or higher; and (E) a non-latent epoxy resin curing agent.
[Item 2]
2. The epoxy resin composition for sheet molding compounds according to item 1, wherein the polymer particles (B) having a core-shell structure have one or more core layers selected from the group consisting of diene-based rubbers, (meth)acrylate-based rubbers, and organosiloxane-based rubbers.
[Item 3]
3. The epoxy resin composition for sheet molding compounds according to item 1 or 2, wherein the polymer particles (B) having a core-shell structure have a core layer containing a diene rubber.
[Item 4]
4. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 3, wherein the polymer particles (B) having a core-shell structure include a shell layer obtained by polymerizing one or more monomer components selected from the group consisting of an aromatic vinyl monomer, a vinyl cyan monomer, and a (meth)acrylate monomer.
[Item 5]
5. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 4, wherein the monomer component constituting the shell layer includes an alkyl acrylate.
[Item 6]
6. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 5, wherein the polymer particles (B) having a core-shell structure include a shell layer having an epoxy group.
[Item 7]
7. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 6, wherein the content of the epoxy groups in the polymer particles having a core-shell structure (B) relative to the total amount of the shell layer is 0.03 to 2.0 mmol/g.
[Item 8]
8. An epoxy resin composition for sheet molding compounds according to any one of items 1 to 7, wherein the content of the polymer particles (B) having a core-shell structure is 5 to 50% by weight based on the total amount of the epoxy resin composition.
[Item 9]
9. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 8, wherein the non-latent epoxy resin curing agent (E) is at least one selected from the group consisting of amine-based compounds and acid anhydrides.
[Item 10]
10. The epoxy resin composition for sheet molding compounds according to any one of items 1 to 9, further comprising a reactive diluent (F).
[Item 11]
11. A sheet molding compound comprising reinforcing fibers and the epoxy resin composition for sheet molding compounds according to any one of items 1 to 10 impregnated into the reinforcing fibers.
[Item 12]
12. The sheet molding compound according to claim 11, wherein the reinforcing fibers are carbon fibers.
[Item 13]
Item 13. A molded article obtained by press-molding the sheet molding compound according to item 11 or 12.
[Item 14]
Item 14. The molded body according to item 13, wherein the molded body is a wheel.

The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples and can be modified as appropriate within the scope of the above and below-mentioned aims, and all such modifications are included within the technical scope of the present invention.

<Each ingredient>
(Epoxy resin (A))
A-1: JER828 (manufactured by Mitsubishi Chemical, bisphenol A type epoxy resin that is liquid at room temperature, epoxy equivalent: 184-194)

(Dispersion (M) in which core-shell polymer (B) is dispersed in epoxy resin (A))
1. Measurement of volume average particle size The volume average particle size (Mv) of the core-shell polymer dispersed in the aqueous latex was measured using Microtrac UPA150 (manufactured by Nikkiso Co., Ltd.). The measurement sample was diluted with deionized water. The measurement was performed by inputting the refractive index of water and the refractive index of each polymer particle, adjusting the sample concentration so that the measurement time was 600 seconds and the signal level was within the range of 0.6 to 0.8.

2. Measurement of graft ratio The aqueous latex containing the core-shell polymer was coagulated and dehydrated, and finally dried to obtain a powder of the core-shell polymer. Next, 2 g of the powder of the core-shell polymer was immersed in 100 g of methyl ethyl ketone (MEK) at 23° C. for 24 hours, after which the MEK soluble matter was separated from the MEK insoluble matter, and the methanol insoluble matter was further separated from the MEK soluble matter. This methanol insoluble matter corresponds to the non-graft polymer.
The graft ratio was calculated from the following formula.
Graft ratio=GM−methanol insolubles/methanol insolubles+MEK insolubles/GM
GM: Proportion [%] of the monomer for forming the shell layer when the total of the monomer for forming the shell layer and the core layer polymer used in producing the core-shell polymer is taken as 100

3. Measurement of number average molecular weight of non-grafted polymer The methanol insoluble matter was dissolved in THF, and the number average molecular weight was measured by the following GPC analysis.
Measurement was performed using a system: "HLC-8420GPC" manufactured by Tosoh Corporation, columns: "TSKgel Super H5000", "TSKgel Super H4000", "TSKgel Super H3000", and "TSKgel Super H2000" manufactured by Tosoh Corporation, and a solvent: THF, and the number average molecular weight was calculated in terms of polystyrene.

4. Formation of Core Layer Production Example 1; Preparation of Polybutadiene Rubber Latex (R-2) 200 parts by mass of water, 0.03 parts by mass of tripotassium phosphate, 0.002 parts by mass of disodium ethylenediaminetetraacetate (EDTA), 0.001 parts by mass of ferrous sulfate heptahydrate (FE), and 1.55 parts by mass of sodium dodecylbenzenesulfonate (SDBS) were charged into a pressure-resistant polymerization reactor, and the mixture was thoroughly substituted with nitrogen while stirring to remove oxygen, after which 100 parts by mass of butadiene (Bd) was charged into the system and the temperature was raised to 45°C. 0.03 parts by mass of paramenthane hydroperoxide (PHP) was charged, followed by 0.10 parts by mass of sodium formaldehyde sulfoxylate (SFS) to initiate polymerization. 0.025 parts by mass of paramenthane hydroperoxide (PHP) was charged at 3, 5, and 7 hours after the start of polymerization, respectively. Further, 0.0006 parts by mass of EDTA and 0.003 parts by mass of ferrous sulfate heptahydrate were added at 4, 6, and 8 hours after the start of polymerization, respectively. At 15 hours after the start of polymerization, the remaining monomers were removed by volatilization under reduced pressure to terminate the polymerization, thereby obtaining a polybutadiene rubber latex (R-1) mainly composed of polybutadiene rubber. The volume average particle diameter of the polybutadiene rubber particles contained in the obtained latex was 79 nm.
In a pressure-resistant polymerization reactor, 21 parts by mass of polybutadiene rubber latex (R-1) (including 7 parts by mass of polybutadiene rubber), 185 parts by mass of deionized water, 0.03 parts by mass of tripotassium phosphate, 0.002 parts by mass of EDTA, and 0.001 parts by mass of ferrous sulfate heptahydrate were charged, and the mixture was thoroughly substituted with nitrogen while stirring to remove oxygen, and then 93 parts by mass of Bd were charged into the system and the temperature was raised to 45° C. 0.02 parts by mass of PHP and then 0.10 parts by mass of SFS were charged to initiate polymerization. Every 3 hours from the start of polymerization until the 24th hour, 0.025 parts by mass of PHP, 0.0006 parts by mass of EDTA, and 0.003 parts by mass of ferrous sulfate heptahydrate were charged, respectively. After 30 hours of polymerization, the polymerization was terminated by removing the remaining monomers under reduced pressure through volatilization, to obtain a polybutadiene rubber latex (R-2) mainly composed of polybutadiene rubber. The volume average particle diameter of the polybutadiene rubber particles contained in the obtained latex was 202 nm.

5. Preparation of Core-Shell Polymer (Formation of Shell Layer)
Production Example 2-1: Preparation of Core-Shell Polymer Latex (L-1) 262 parts by mass of the polybutadiene rubber latex (R-2) prepared in Production Example 1 (containing 87 parts by mass of polybutadiene rubber particles) and 57 parts by mass of deionized water were charged into a glass reactor equipped with a thermometer, a stirrer, a reflux condenser, a nitrogen inlet, and a monomer addition device, and stirred at 60° C. while performing nitrogen substitution. After adding 0.004 parts by mass of EDTA, 0.001 parts by mass of FE, and 0.2 parts by mass of SFS, a mixture of shell monomers (6 parts by mass of styrene (ST), 2 parts by mass of acrylonitrile (AN), 5 parts by mass of methyl methacrylate (MMA)) and 0.04 parts by mass of cumene hydroperoxide (CHP) was continuously added over 120 minutes. After the addition was completed, 0.04 parts by mass of CHP was added, and the mixture was further stirred for 2 hours to complete the polymerization, thereby obtaining an aqueous latex (L-1) containing a core-shell polymer (B-1). The polymerization conversion rate of the monomer components was 99% or more. The volume average particle size of the core-shell polymer (B-1) contained in the obtained aqueous latex was 207 nm, and the graft ratio was 98%. The number average molecular weight of the non-graft copolymer was 12,000.

Production Example 2-2: Preparation of Core-Shell Polymer Latex (L-2) Except for changing the shell monomer to 6 parts by mass of n-butyl acrylate (BA), 5.2 parts by mass of methyl acrylate (MA), and 1.8 parts by mass of glycidyl methacrylate (GMA), the same procedure as in Production Example 2-1 was followed to obtain an aqueous latex (L-2) containing a core-shell polymer (B-2). The conversion rate of the monomer components was 99% or more. The volume average particle diameter of the core-shell polymer (B-2) contained in the obtained aqueous latex was 210 nm, and the graft ratio was 98%. In addition, the number average molecular weight of the non-graft copolymer was 11,000, and the content of the epoxy group in the shell group relative to the total amount of the core-shell polymer (B-2) was 0.13 mmol/g.

Production Example 2-3: Preparation of Core-Shell Polymer Latex (L-3) Except for changing the shell monomer to 6 parts by mass of styrene (ST), 2 parts by mass of acrylonitrile (AN), 1 part by mass of methyl methacrylate (MMA), and 4 parts by mass of glycidyl methacrylate (GMA), the same procedure as in Production Example 2-1 was followed to obtain an aqueous latex (L-3) containing a core-shell polymer (B-3). The conversion rate of the monomer components was 99% or more. The volume average particle diameter of the core-shell polymer (B-3) contained in the obtained aqueous latex was 209 nm, and the graft ratio was 95%. In addition, the number average molecular weight of the non-graft copolymer was 8000, and the content of the epoxy group in the shell group relative to the total amount of the core-shell polymer (B-3) was 0.28 mmol/g.

6. Preparation of Dispersion (M) in which Core-Shell Polymer (B) is Dispersed in Epoxy Resin (A) Production Example 3-1; Preparation of Dispersion (M-1) 132 g of methyl ethyl ketone (MEK) was introduced into a 1 L mixing tank at 25 ° C., and 132 g (corresponding to 40 g of polymer fine particles) of the aqueous latex (L-1) of the core-shell polymer (B-1) obtained in Production Example 2-1 was added while stirring. After uniform mixing, 200 g of water was added at a feed rate of 80 g / min. After the end of the supply, the stirring was stopped immediately, and a slurry liquid consisting of floating aggregates and an aqueous phase containing a part of an organic solvent was obtained. Next, 360 g of the aqueous phase was discharged from the discharge port at the bottom of the tank, leaving the aggregates containing a part of the aqueous phase. 90 g of MEK was added to the obtained aggregates and mixed uniformly, and a dispersion in which the core-shell polymer (B-1) was uniformly dispersed was obtained. 60 g of epoxy resin (JER828: liquid bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation) which is component (A) was mixed with this dispersion. MEK was removed from this mixture using a rotary evaporator. In this way, a dispersion (M-1) in which the core-shell polymer (B-1) was dispersed in the epoxy resin (A) was obtained.

Production Example 3-2; Preparation of Dispersion (M-2) A dispersion (M-2) in which the core-shell polymer (B-2) was dispersed in the epoxy resin (A) was obtained in the same manner as in Production Example 3-1, except that (L-2) was used instead of (L-1) as the aqueous latex of the core-shell polymer in Production Example 3-1.

Production Example 3-3; Preparation of Dispersion (M-3) A dispersion (M-3) in which the core-shell polymer (B-3) was dispersed in the epoxy resin (A) was obtained in the same manner as in Production Example 3-1, except that (L-3) was used instead of (L-1) as the aqueous latex of the core-shell polymer in Production Example 3-1.

<Dicyandiamide (C)>
C-1: Dyhard 100S (manufactured by AlzChem)

<Imidazole Compound (D) Having a Melting Point of 40° C. or More>
D-1: 2MZA-PW (manufactured by Shikoku Kasei, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine, melting point 253°C)

<Non-latent epoxy resin curing agent (E)>
E-1: VESTAMIN IPD (manufactured by Evonik, isophorone diamine)
E-2: EPICLON B-570-H (DIC, methyltetrahydrophthalic anhydride)

<Reinforcing fiber (F)>
F-1: ECS 03T-187H (Nippon Electric Glass, glass fiber, fiber length 3 mm)
F-2: T700SC-12000-50C (Toray, carbon fiber, fiber length 25 mm)

<Production of glass fiber reinforced composite material>
(Examples 1 to 5, Comparative Example 1)
According to the formulation shown in Table 1, each component [in each Example, dispersion (M-1) containing (A) and (B), and additional (A), (C), (D), and (E)] was weighed and thoroughly mixed to obtain a curable resin composition. The blending amount of component (A) shown in each Example in Table 1 is the total amount of the added amount of component (A) added as the liquid epoxy resin itself and the content of component (A) contained in the dispersion (M) of the core-shell polymer.
10 g of the curable resin composition was poured into a box-shaped mold made of a polypropylene sheet and spread evenly. 25 g of reinforcing fiber (F) was randomly scattered therein. 15 g of the curable resin composition was further poured in and heated at 60° C. for 10 minutes. A polypropylene sheet was placed on top and pressed through rolls to impregnate the reinforcing fiber with the curable resin composition, thereby obtaining an SMC precursor.
The SMC precursor was allowed to stand at 50° C. for 16 hours to allow the curable resin composition in the SMC precursor to sufficiently thicken, thereby obtaining an SMC.
The SMC was laminated in 2 plies and hot-pressed in a hot press molding machine at a molding temperature of 155° C. and a pressure of 4 MPa for 30 minutes to cure the curable resin composition, thereby obtaining a plate-shaped fiber-reinforced composite material having a thickness of 2.5 mm.

(Impregnation)
The fiber-reinforced composite material was cut, and the cross section was visually observed and evaluated according to the following criteria.
A: There are no voids on the cut surface, and the impregnation is good.
B: There are voids on the cut surface, and impregnation is poor.

(tackiness)
The tackiness of the SMC was evaluated according to the following criteria.
A: When the SMC was touched with the hand, it had a suitable tackiness and was easy to laminate.
B: When the SMC was touched with the hand, it was very sticky or very sticky, making lamination difficult.

(Drapability)
The drapeability of the SMC was evaluated according to the following criteria.
A: When the SMC was touched with the hand, it had a suitable flexibility and was easy to cut and carry.
B: When the SMC was touched with the hand, it was found to have poor flexibility and was difficult to cut and carry.

(Impact resistance)
A fiber-reinforced composite material cut into 60 mm squares was set in a DuPont drop impact tester, and a 700 g weight was dropped from a height of 300 mm. Five pieces were evaluated under the same conditions, and if three or more of the five pieces were penetrated or buckled or broken in the vertical direction by 5 mm or more, it was deemed a failure, and if two or less of the five pieces were penetrated or buckled or broken by 5 mm or more, it was deemed a pass. The height was changed by 100 mm at a time, and the test was repeated until a failure was obtained. The highest drop height that resulted in a pass was taken as the test value.

Table 1 shows that in Examples 1 to 5, in which an SMC was prepared using an epoxy resin composition containing polymer particles (B) with a core-shell structure and a fiber-reinforced composite material was produced using the SMC, the impact resistance was improved compared to Comparative Example 1, which did not contain component (B), while the evaluation items of impregnation, tackiness, and drapeability were the same.

<Production of carbon fiber reinforced composite material>
(Example 6, Comparative Example 2)
A curable resin composition was obtained by weighing each component [in Example 6, dispersion (M-1) containing (A) and (B), and additional components (A), (C), (D), and (E)] according to the formulation shown in Table 2 and thoroughly mixing them. The blending amount of component (A) shown in Example 6 in Table 2 is the total amount of component (A) added as the liquid epoxy resin itself and the content of component (A) contained in dispersion (M) of the core-shell polymer.
10 g of the curable resin composition was poured into a box-shaped mold made of a polypropylene sheet and spread evenly. 15 g of reinforcing fiber (F) was randomly scattered therein. Further, 15 g of the curable resin composition was poured in and heated at 60° C. for 10 minutes. A polypropylene sheet was placed on top and pressed through rolls to impregnate the reinforcing fiber with the curable resin composition, thereby obtaining an SMC precursor.
The SMC precursor was allowed to stand at 60° C. for 16 hours to allow the curable resin composition in the SMC precursor to sufficiently thicken, thereby obtaining an SMC.
The SMC was laminated in 2 plies and hot-pressed in a hot press molding machine at a molding temperature of 140° C. and a pressure of 4 MPa for 30 minutes to cure the curable resin composition, thereby obtaining a plate-shaped fiber-reinforced composite material having a thickness of 2.5 mm.
The impregnation, tackiness and drapeability were evaluated in the same manner as in Examples 1 to 5 and Comparative Example 1.

(Impact resistance)
A fiber-reinforced composite material cut into 60 mm squares was set in a DuPont drop impact tester, and a 2500 g weight was dropped from a height of 300 mm. Five pieces were evaluated under the same conditions, and if three or more of the five pieces were penetrated or buckled or broken in the vertical direction by 5 mm or more, it was deemed a failure, and if two or less of the five pieces were penetrated or buckled or broken by 5 mm or more, it was deemed a pass. The height was changed by 100 mm at a time, and the test was repeated until a failure was obtained. The highest drop height that resulted in a pass was taken as the test value.

From Table 2, it can be seen that in Example 6, in which an SMC was prepared using an epoxy resin composition containing polymer particles (B) having a core-shell structure and a fiber-reinforced composite material was produced using the SMC, the impact resistance was improved compared to Comparative Example 2, which did not contain component (B), while the evaluation items of impregnation, tackiness, and drapeability were the same.

Claims (14)

Epoxy resin (A),
Polymer particles (B) having a core-shell structure;
Dicyandiamide (C),
An epoxy resin composition for sheet molding compounds, comprising: (D) an imidazole compound having a melting point of 40°C or higher; and (E) a non-latent epoxy resin curing agent. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the polymer particles (B) having a core-shell structure have one or more core layers selected from the group consisting of diene-based rubbers, (meth)acrylate-based rubbers, and organosiloxane-based rubbers. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the polymer particles (B) having a core-shell structure have a core layer containing a diene rubber. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the polymer particles (B) having a core-shell structure include a shell layer formed by polymerizing one or more monomer components selected from the group consisting of aromatic vinyl monomers, vinyl cyan monomers, and (meth)acrylate monomers. The epoxy resin composition for sheet molding compounds according to claim 4, wherein the monomer component constituting the shell layer includes an alkyl acrylate ester. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the polymer particles (B) having a core-shell structure include a shell layer having epoxy groups. The epoxy resin composition for sheet molding compounds according to claim 6, wherein the content of the epoxy groups in the shell layer relative to the total amount of the polymer particles (B) having a core-shell structure is 0.03 to 2.0 mmol/g. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the content of the polymer particles (B) having a core-shell structure is 5 to 50% by weight of the total amount of the epoxy resin composition. The epoxy resin composition for sheet molding compounds according to claim 1, wherein the non-latent epoxy resin curing agent (E) is at least one selected from the group consisting of amine compounds and acid anhydrides. The epoxy resin composition for sheet molding compounds according to claim 1, further comprising a reactive diluent (F). A sheet molding compound comprising reinforcing fibers and the epoxy resin composition for sheet molding compounds according to any one of claims 1 to 10 impregnated into the reinforcing fibers. The sheet molding compound of claim 11, wherein the reinforcing fibers are carbon fibers. A molded body obtained by press molding the sheet molding compound according to claim 11. The molded body according to claim 13, wherein the molded body is a wheel.