JP7727398B2 - Curable resin composition and fiber-reinforced composite material using the same - Google Patents

Description

The present invention relates to a curable resin composition with excellent heat resistance, and to a matrix resin for fiber-reinforced composite materials that retains high strength even after long-term thermal exposure.

Fiber-reinforced composite materials are generally composed of reinforcing fibers such as glass fibers, aramid fibers, or carbon fibers, and thermosetting matrix resins such as unsaturated polyester resins, vinyl ester resins, epoxy resins, phenolic resins, benzoxazine resins, cyanate resins, and bismaleimide resins. Because they are lightweight and have excellent mechanical properties such as strength, corrosion resistance, and fatigue resistance, they are widely used as structural materials in aircraft, automobiles, civil engineering and construction, sporting goods, and other applications.

Manufacturing methods for fiber-reinforced composite materials include autoclave molding or press molding, which use prepregs in which reinforcing fibers have already been impregnated with a thermosetting matrix resin; resin transfer molding, which includes a process of impregnating reinforcing fibers with a liquid matrix resin and a molding process by thermosetting; liquid compression molding, wet layup molding, pultrusion molding, or filament winding molding.

In resin transfer molding, liquid compression molding, wet layup molding, pultrusion molding, and filament winding molding, liquid thermosetting resins such as unsaturated polyester resin, vinyl ester resin, urethane resin, and epoxy resin have been used. One example of a thermosetting resin composition made from epoxy resin is a resin composition that uses an epoxy resin as the base resin and an amine compound as the curing agent, and is widely used in applications that require ease of handling at the work site, fast curing, and heat resistance and strength after curing.

Resin compositions using epoxy resins and amine compounds as curing agents are fast-curing and produce cured products with high strength and toughness. However, they have the drawback of undergoing decomposition due to oxidation when exposed to high temperatures for extended periods, resulting in discoloration of the cured product and a decrease in strength and toughness.

Attempts have been made to add antioxidants to epoxy resin compositions in order to suppress oxidative degradation. Patent Document 1 discusses the addition of a phenol novolac compound as an epoxy resin curing agent, and a hindered phenol or phosphorus-based compound as an antioxidant; Patent Documents 2 and 3 discuss the addition of an acid anhydride compound as an epoxy resin curing agent, and a hindered phenol, phosphorus-based compound, sulfur-based compound, or amine-based compound as an antioxidant; and Patent Document 4 discusses the addition of dicyandiamide as an epoxy resin curing agent, and a hindered phenol, sulfur-based compound, sulfur-based compound, or hindered amine-based compound as an antioxidant.

The matrix resin of fiber-reinforced composite materials is sometimes used outdoors or in high-temperature environments, and is required to maintain not only its initial physical properties after curing, but also its strength properties after long-term thermal history.

Japanese Patent Application Publication No. 7-309998 JP 2012-236893 A Patent No. 6501359 Patent No. 6163763

The present invention aims to provide a curable resin composition that can produce molded articles that are resistant to oxidative degradation, exhibit minimal loss in strength even after long-term thermal history, and improve the heat resistance reliability of fiber-reinforced composite materials.

As a result of research conducted by the inventors to solve the above-mentioned problems, they discovered that by using a specific epoxy resin, amine compound, and antioxidant, strength properties can be maintained even after long-term thermal exposure, leading to the completion of the present invention.

That is, the present invention relates to a curable resin composition comprising, as essential components, an epoxy resin (A), an alicyclic amine compound (B), and an antioxidant (C), wherein the antioxidant is a naphthol-based or aromatic amine-based antioxidant, and the amount of antioxidant (C) added is 0.05 to 3 parts by mass per 100 parts by mass of the total of components (A), (B), and (C).

Preferred examples of the antioxidant (C) include compounds represented by the following formula (1) or (2).
(wherein _R1 represents a hydrocarbon group having 1 to 4 carbon atoms).
(In the formula, R ₂ and R ₃ each independently represent a hydrogen atom or a hydrocarbon group having 1 to 4 carbon atoms.)

It is more preferable that the weight ratio of the epoxy resin (A) to the alicyclic amine compound (B) be in the range of 90:10 to 70:30.

The curable resin composition preferably contains core-shell rubber particles (D) in addition to the epoxy resin (A), alicyclic amine compound (B), and antioxidant (C), and the amount of the core-shell rubber particles (D) is preferably 1 to 8 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), and (D).

Furthermore, it is desirable that the tensile strength retention ratio (T2/T1) of the cured product (I) obtained by heat-treating the above-mentioned curable resin composition at 130°C for 1 hour and the cured product (II) obtained by further heat-treating this cured product (I) at 170°C for 20 hours be 90% or more. Here, T1 is the tensile strength of the cured product (I), and T2 is the tensile strength of the cured product (II).

Another aspect of the present invention is a resin composition for a fiber-reinforced composite material, characterized by blending reinforcing fibers with the above-mentioned curable resin composition. Also, a fiber-reinforced composite material obtained from this resin composition for a fiber-reinforced composite material.

The curable resin composition of the present invention has excellent heat resistance, and the fiber-reinforced composite materials or molded articles obtained by curing materials containing the composition exhibit little loss in strength even after long-term thermal history. It is particularly suitable as a curable resin composition used to produce molded articles by resin transfer molding or liquid compression molding.

Hereinafter, embodiments of the present invention will be described in detail.
The curable resin composition of the present invention is a curable resin composition comprising an epoxy resin (A), an alicyclic amine compound (B), and an antioxidant (C). Here, the epoxy resin (A), the alicyclic amine compound (B), and the antioxidant (C) are also referred to as component (A), component (B), and component (C), respectively.

The epoxy resin (A) used in the present invention is preferably a compound having two or more epoxy groups in one molecule, and examples thereof include bisphenol type epoxy resins such as bisphenol F type epoxy resin, bisphenol E type epoxy resin, bisphenol S type epoxy resin, bisphenol Z type epoxy resin, and isophorone bisphenol type epoxy resin; halides, alkyl-substituted products, hydrogenated products, and high molecular weight products having a plurality of repeating units, including but not limited to monomers, of these bisphenol type epoxy resins; glycidyl ethers of alkylene oxide adducts; novolac type epoxy resins such as phenol novolac type epoxy resin, cresol novolac type epoxy resin, and bisphenol A novolac type epoxy resin; and 3,4-epoxy-6-methylcyclohexylmethyl- ... Examples of epoxy resins that can be used include alicyclic epoxy resins such as 1-epoxyethyl-3,4-epoxycyclohexane carboxylate, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, 1-epoxyethyl-3,4-epoxycyclohexane, etc., aliphatic epoxy resins such as trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether, polyoxyalkylene diglycidyl ether, etc., glycidyl esters such as phthalic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, dimer acid glycidyl ester, etc., and glycidyl amines such as tetraglycidyldiaminodiphenylmethane, tetraglycidyldiaminodiphenyl sulfone, triglycidylaminophenol, triglycidylaminocresol, tetraglycidylxylylenediamine, etc. These may be used alone or in combination of two or more.
Among these epoxy resins, from the viewpoints of economy and heat resistance, the use of bisphenol A type epoxy resins, bisphenol F type epoxy resins, phenol novolac type epoxy resins, etc., which have two or more epoxy groups in one molecule, can improve the heat resistance of the molded article.

The alicyclic amine compound (B) used in the present invention is an amine compound having an alicyclic structure. Alicyclic amines have excellent reactivity and can enhance heat resistance during curing.

Specific examples of the alicyclic amine compound (B) include polyamines having an alicyclic structure, such as bis(aminomethyl)cyclohexane, methylenebis(cyclohexylamine), methylenebis(2-methylcyclohexylamine), 1,3,6-trisaminomethylcyclohexane, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, 3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5.5]undecane, bis(aminomethyl)norbornane, and N-(2-aminoethyl)piperazine, as well as derivatives of these alicyclic structures substituted with alkyl groups. The alicyclic amine compound (C) can be used alone or in combination of two or more. Of these, the alicyclic amine compound represented by the following formula (3) is preferred, with bis(aminomethyl)cyclohexane or bis(aminomethyl)norbornane being more preferred in terms of heat resistance and strength after curing.
X-(CH ₂ NH ₂ ) _n (3)
(In the formula, X represents an n-valent organic group having 1 to 16 carbon atoms and an alicyclic structure, and n represents 2 or 3.)

The antioxidant (C) used in the present invention is naphthol-based or aromatic amine-based. Common antioxidants include hindered phenol-based antioxidants, but their antioxidant properties are less effective in cured products made from epoxy resins and amine compounds, and they tend to reduce strength due to oxidative degradation over long-term thermal history. In contrast, the present invention has found that naphthol-based or aromatic amine-based antioxidants function as radical traps to prevent oxidation even in cured products made from epoxy resins and amine compounds, thereby maintaining strength by suppressing oxidative degradation over long-term thermal history.

The amount of antioxidant (C) added is 0.05 to 3 parts by mass per 100 parts by mass of the total of components (A), (B), and (C). If it is less than 0.05 parts by mass, the antioxidant ability will be insufficient, and if it exceeds 3 parts by mass, it will cause a decrease in the initial physical properties of the cured product.

Examples of naphthol-based antioxidants include, but are not limited to, 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, 4-methoxy-1-naphthol, 4-ethoxy-1-naphthol, 4-benzyloxy-1-naphthol, 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 5-hydroxy-1,4-naphthoquinone, 5,8-dihydroxy-1,4-naphthoquinone, and 2,5-dihydroxy-1,4-naphthoquinone.

Examples of aromatic amine antioxidants include, but are not limited to, phenothiazine, 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, N,N'-diphenyl-1,4-phenylenediamine, N,N'-di-sec-butyl-1,4-phenylenediamine, and N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine.

Preferred antioxidants (C) include naphthol-based antioxidants represented by the above formula (1) and aromatic amine-based antioxidants represented by the formula (2).
In formula (1), _R1 represents a hydrocarbon group having 1 to 4 carbon atoms, preferably an alkyl group having 1 to 3 carbon atoms. In formula (2), _R2 and _R3 each independently represent a hydrogen atom or a hydrocarbon residue having 1 to 4 carbon atoms, preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms.

The weight ratio of epoxy resin (A) to alicyclic amine compound (B) is preferably in the range of 90:10 to 70:30, which is preferable from the standpoint of initial heat resistance after curing and maintaining strength after thermal history. If the weight ratio of epoxy resin exceeds 90, heat resistance will decrease, and if it is less than 70, the alicyclic amine structure will be relatively more abundant, making it more susceptible to oxidative degradation due to thermal history and resulting in reduced strength.

As the curing agent, other epoxy resin curing agents besides the alicyclic amine compound (B) can also be used. In this case, the amount is preferably less than 10 parts by mass per 100 parts by mass of the total of components (A), (B), and (C). Examples of these other epoxy resin curing agents include phenol-based and thiol-based epoxy resin curing agents.
In addition, it may contain a curing accelerator, a viscosity modifier, an internal mold release agent, etc.

Phenol-based curing agents used as curing agents for the above-mentioned other epoxy resins are not particularly limited, but examples include bisphenol A, bisphenol F, substituted or unsubstituted biphenol, phenol novolac resin, triazine skeleton-containing phenol novolac resin, naphthol novolac resin, naphthol aralkyl resin, triazine skeleton-containing naphthol resin, biphenyl aralkyl phenol resin, etc. Phenol-based curing agents may be used alone or in combination of two or more types.

Together with the alicyclic amine compound (B) or a curing agent consisting of this and other epoxy resin curing agents, a curing catalyst or curing accelerator can be used to accelerate the curing reaction. Examples of curing catalysts and curing accelerators include tertiary amines, carboxylic acids, Lewis acid complexes, onium salts, imidazole, alcohols, phenol, cresol, allylphenol, nitrophenol, paraaminophenol, metaaminophenol, mono-t-butylphenol, di-t-butylphenol, hydroquinone, methylhydroquinone, dimethylhydroquinone, trimethylhydroquinone, tetramethylhydroquinone, isopropylhydroquinone, methylisopropylhydroquinone, mono-t-butylhydroquinone, di-t-butylhydroquinone, and mono-t-amylhydroquinone. Phenol compounds such as quinone, di-t-amylhydroquinone, nitrohydroquinone, phenylhydroquinone, diphenylhydroquinone, chlorohydroquinone, dichlorohydroquinone, trichlorohydroquinone, tetrachlorohydroquinone, bromohydroquinone, dibromohydroquinone, tribromohydroquinone, tetrabromohydroquinone, catechol, t-butylcatechol, resorcinol, pyrogallol, dinitropyrogallol, and 1,2,4-benzenetriol can be used, and these curing catalysts and curing accelerators may be used alone or in combination of two or more.

The curable resin composition of the present invention can contain core-shell rubber particles (D) in addition to the epoxy resin (A), alicyclic amine compound (B), and antioxidant (C). In this case, the amount of core-shell rubber particles (D) blended is preferably 1 to 8 parts by mass per 100 parts by mass of the total of components (A), (B), (C), and (D). This range can improve the initial toughness after curing and produce a cured product that maintains its strength and toughness even after long-term thermal history.

From the viewpoint of long-term storage, it is desirable that the curable resin composition of the present invention be stored separately as a base agent containing the epoxy resin (A) and a curing agent containing the alicyclic amine compound (B), and then mixed on-site or immediately before use to form the curable resin composition.
The above-mentioned base agent and curing agent may also be appropriately blended with other components such as plasticizers, dyes, organic pigments, inorganic fillers, polymeric compounds, coupling agents, surfactants, and solvents. Other curable resins may also be blended. Examples of such curable resins include, but are not limited to, unsaturated polyester resins, curable acrylic resins, curable amino resins, curable melamine resins, curable urea resins, curable cyanate ester resins, curable urethane resins, curable oxetane resins, and curable epoxy/oxetane composite resins. These may be blended with either the base agent or curing agent, taking into consideration their reactivity with the components contained therein, viscosity, and other factors. Compounds that react with the components contained in the base agent or curing agent should be avoided.

The curable resin composition of the present invention can be blended with coupling agents, conductive particles such as carbon particles and metal-plated organic particles, thermosetting resin particles, inorganic fillers such as silica gel, nanosilica, alumina fiber, and clay, or conductive fillers. The use of conductive particles or conductive fillers can improve the conductivity of the resulting cured resin product or fiber-reinforced composite material.

Conductive fillers include carbon black, carbon nanotubes, fullerenes, and metal nanoparticles, and may be used alone or in combination. Among these, the incorporation of carbon nanotubes in particular is widely known for not only improving conductivity but also increasing the impact strength of fiber-reinforced composite materials, even when incorporated in amounts of less than 1 wt % of the fiber-reinforced composite material, making them particularly suitable for use.

The curable resin composition of the present invention is cured by heat treatment at 130°C for 1 hour to obtain a cured product (I). This product is then further subjected to a thermal history at 170°C for 20 hours to obtain a cured product (II), which preferably retains tensile strength by 90% or more. A tensile strength of 90% or more ensures stable physical properties even after long-term thermal history, making it desirable as a matrix resin for fiber-reinforced composite materials.

The curable resin composition of the present invention can be blended with reinforcing fibers to form a fiber-reinforced composite material composition. Reinforcing fibers can be selected from glass fibers, aramid fibers, carbon fibers, boron fibers, etc., with carbon fibers being preferred for obtaining fiber-reinforced composite materials with excellent strength.

The volume content of the reinforcing fibers in the resin composition for fiber-reinforced composite materials of the present invention is preferably in the range of 45 to 70%, and more preferably 48 to 62%. By setting the volume content within this range, a molded product with few voids and a high volume content of reinforcing fibers can be obtained, resulting in a fiber-reinforced composite material with excellent strength. This resin composition for fiber-reinforced composite materials can be used as a prepreg.

The method for producing a fiber-reinforced composite material from the curable resin composition or resin composition for a fiber-reinforced composite material of the present invention is not particularly limited, but the RTM (Resin Transfer Molding) method or the LCM (Liquid Compression Molding) method is suitable. The RTM method involves placing a fiber substrate or preform made of reinforcing fibers in a molding die, injecting a liquid curable resin composition into the mold to impregnate the reinforcing fibers, forming a resin composition for a fiber-reinforced composite material, and then heating to cure the composition and obtain a molded product. The LCM method involves placing a fiber substrate or preform made of reinforcing fibers that has already been blended with a resin in a molding die with the molding pressure released, clamping the die to simultaneously impregnate and mold the fiber-reinforced composite material precursor, and then heating the die to cure the precursor and obtain a fiber-reinforced composite material (molded product).

Next, the present invention will be described in detail based on examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. Parts indicating blend amounts are parts by weight unless otherwise specified.

The abbreviations for the components used in the examples are as follows:
(A) Component YD-128: Bisphenol A epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
YDF-170: Bisphenol F epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
YDPN-638: Phenol novolac epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd.)
(B) Component CHDA: 1,3-bis(aminomethyl)cyclohexane NBDA: bis(aminomethyl)norbornane (mixture of isomers)
BCDA: methylenebis(cyclohexylamine) (mixture of isomers)
Component (B') TETA: triethylenetetramine (aliphatic amine curing agent)
(C) Component HMN: 4-methoxy-1-naphthol (naphthol-based antioxidant)
PTZ: Phenothiazine (aromatic amine antioxidant)
Component (C') BHT: dibutylhydroxytoluene (hindered phenol-based antioxidant)
(D) Component MX-154: A bisphenol A type epoxy resin (manufactured by Kaneka Corporation) containing 40 wt% core-shell type rubber particles, containing core-shell rubber particles (MX-CSR) and a bisphenol A type epoxy resin component (MX-EP). MX-EP is counted as component (A).

Example 1
83 parts of YD-128 as component (A) and 0.3 parts of HMN as component (C) were used, placed in a 150 mL plastic container, and stirred for 5 minutes using a vacuum mixer "Awatori Rentaro" (manufactured by Thinky Corporation) to uniformly dissolve the HMN. Next, 17 parts of CHDA was added as component (B), and the mixture was stirred for 2 minutes using a vacuum mixer to obtain a curable resin composition.

Examples 2 to 16, Comparative Examples 1 to 8
Curable resin compositions were prepared under the same mixing conditions as in Example 1, except that the raw materials (A) to (C) were used in the compositions shown in Tables 1, 2, and 3 .

The measurement or test methods for each strength property are as follows:

(Preparation of test pieces for measuring tensile strength)
The obtained curable resin composition was poured into a mold hollowed out in the shape specified in JIS K-7161, and cured by heating at 130°C for 1 hour in a hot air circulating oven to obtain a test piece for tensile test.

(Measurement of Tensile Strength Retention Rate)
Using an Autograph AGS-X (Shimadzu Corporation), the tensile test specimens were measured according to a method in accordance with JIS K-7161, and the maximum tensile stress (MPa) was taken as the initial tensile strength (T1). After subjecting the tensile test specimens to a thermal history of 170°C for 20 hours in an oven, the tensile test specimens were removed and subjected to a tensile test according to a method in accordance with JIS K-7161, and the maximum tensile stress (MPa) was taken as the tensile strength after the thermal history (T2). The tensile strength retention rate was then calculated using the following formula:
Tensile strength retention rate = 100 x (T2/T1)

(Preparation of test specimens for fracture toughness measurement)
A mold was prepared by sandwiching a 4 mm thick U-shaped spacer between two steel plates measuring 150 mm long x 150 mm wide x 8 mm thick, and the resin composition was cured by heating at 130°C for 1 hour in a hot air circulating oven to obtain a molded product measuring 140 mm x 130 mm x 4 mm thick. The molded product was then cut into a size of 80 mm x 10 mm using a bench band saw and used for measuring fracture toughness, as described below.

(Measurement of fracture toughness)
Using an Autograph AGS-X (manufactured by Shimadzu Corporation), the fracture toughness test specimen was measured by a method in accordance with ASTM D 5045, and the initial fracture toughness value was recorded. After the fracture toughness test specimen was subjected to a thermal history of 20 hours at 170°C in an oven, the fracture toughness test specimen was removed and subjected to a fracture toughness test by a method in accordance with ASTM D 5045, and the obtained value was recorded as the fracture toughness after the thermal history.

The test results of the Examples and Comparative Examples are shown in Tables 1, 2 and 3 , respectively.
The blend amount is in parts by weight, the unit of tensile strength is MPa, and the unit of fracture toughness is MPa·m ^0.5 .

Claims (6)

A curable resin composition comprising, as essential components, an epoxy resin (A), an alicyclic amine compound (B), and an antioxidant (C), wherein the alicyclic amine compound (B) is an alicyclic amine compound having two or more primary amino groups, the antioxidant is a naphthol-based or aromatic amine- based compound represented by the following formula (1) or (2) , and the amount of the antioxidant (C) added is 0.05 to 3 parts by mass per 100 parts by mass of the total of the components (A), (B), and (C):
(wherein R1 _represents a hydrocarbon group having 1 to 4 carbon atoms).
(In the formula, R ₂ and R ₃ each independently represent a hydrogen atom or a hydrocarbon group having 1 to 4 carbon atoms.) 2. The curable resin composition according to claim 1 , wherein the weight ratio of the epoxy resin (A) to the alicyclic amine compound (B) is in the range of 90:10 to 70:30. 3. The curable resin composition according to claim 1, further comprising: an epoxy resin (A), an alicyclic amine compound (B), an antioxidant (C), and core-shell rubber particles (D), wherein the amount of the core-shell rubber particles (D) is 1 to 8 parts by mass per 100 parts by mass of the total of the components (A), (B), (C), and (D). 4. The curable resin composition according to claim 1, wherein a cured product (I) obtained by heat-treating the curable resin composition at 130°C for 1 hour and curing the curable resin composition, and a cured product (II) obtained by further heat-treating the cured product (I) at 170°C for 20 hours, have a tensile strength retention rate of 90% or more. A resin composition for fiber-reinforced composite materials, characterized in that it is obtained by blending reinforcing fibers with the curable resin composition according to any one of claims 1 to 4 . A fiber-reinforced composite material obtained from the resin composition for a fiber-reinforced composite material according to claim 5 .