JP7587683B2 - Epoxy resin composition, prepreg, and fiber-reinforced plastic using the same - Google Patents

Description

The present invention relates to an epoxy resin composition, a fiber-reinforced epoxy resin composition, a prepreg, and a fiber-reinforced plastic using these.

Fiber-reinforced plastics (FRP) exhibit excellent physical properties such as light weight and high strength, and are used in many fields. Among them, those that use carbon fiber as the reinforcing fiber (CFRP) are known to have particularly excellent mechanical strength.

Epoxy resin is mainly used as the base resin for FRP because it has an excellent balance between price and physical properties. Epoxy resin also forms a good adhesive surface with reinforcing fibers due to its secondary hydroxyl groups, so it is known to exhibit good mechanical properties as a base resin for CFRP.

On the other hand, those that use thermoplastic resin as the base resin are called FRTP, and their development is underway due to their superior mass production, moldability, and recyclability. Nylon, polypropylene, polycarbonate, etc. are mainly used as the base resin for FRTP.

One of the issues with FRTP is that the adhesion between the reinforcing fibers and the resin is low, resulting in low bending strength in the 90-degree direction in unidirectional materials (UD materials). In order to improve the adhesion between the reinforcing fibers and the resin, various methods have been considered for modifying the reinforcing fiber surface, such as cleaning the sizing agent attached to the fibers and oxidizing the fiber surface with ozone or acid, but all of these require additional steps and are not simple (Non-Patent Document 1).

In-situ polymerization thermoplastic epoxy resins have been proposed as thermoplastic resins with improved adhesion to reinforcing fibers. In-situ polymerization thermoplastic epoxy resins have good impregnation properties because they are impregnated into fibers in a low viscosity state before polymerization, making it possible to increase the proportion of reinforcing fibers. In addition, the presence of secondary hydroxyl groups in the epoxy resins is expected to result in good adhesion to reinforcing fibers.

Previous reports on the adhesion between thermoplastic epoxy resins and carbon fibers have shown that increasing the molecular weight of the thermoplastic epoxy improves the interfacial shear strength between the carbon fiber and the matrix (Non-Patent Document 2).

JP 2006-321897 A

Improvement of Bending Strength of Carbon Fiber/Thermoplastic Epoxy Composites (Open Journal of Composite Materials, 2017, 7, 207-217) Journal of the Adhesion Society of Japan VOL.53 No.11 (2017) 375-380

However, according to the inventors' research, in order to sufficiently increase the molecular weight of the thermoplastic epoxy resin within the reinforcing fibers, it is necessary to allow sufficient time for polymerization. Furthermore, if the skeleton of the thermoplastic epoxy resin is made bulky in order to increase heat resistance, the steric hindrance of the reaction becomes large, so the curing time for polymerization becomes even longer and productivity is poor. For this reason, a method has been sought that improves the adhesion of thermoplastic epoxy resin to reinforcing fibers and improves bending strength in the 90-degree direction using a simple method with excellent productivity.

As a result of extensive research conducted by the inventors with the aim of developing adhesive properties for thermoplastic epoxy resins to reinforcing fibers, they have discovered that the use of a specific epoxy resin composition can improve adhesion to reinforcing fibers and increase bending strength in the 90 degree direction.

That is, the present invention provides an epoxy resin composition comprising, as essential components, a bifunctional phenol compound, a bifunctional epoxy resin, and a polymerization catalyst,
The difunctional epoxy resin contains 50% by weight or more of a difunctional epoxy resin (a) represented by the following formula (1):
The amount of the difunctional epoxy resin is 1.01 to 1.05 moles per mole of the difunctional phenol compound,
The epoxy resin composition is characterized in that the polymer obtained from the epoxy resin composition is a thermoplastic epoxy resin, has an epoxy equivalent of 5,000 g/eq. or more and 20,000 g/eq. or less, has a bending strength of 70 MPa or more, and contains 10% by weight or less of a component that becomes insoluble when the polymer is dissolved in tetrahydrofuran.

ここで、式（１）中のＡは式（２）で表され、ｎは繰り返し数でその平均値は０～５である。Ｘは単結合、炭素数１～１３の炭化水素基、－Ｏ－、－ＣＯ－、－ＣＯＯ－、－Ｓ－、－ＳＯ_２－のいずれかであり、Ｙ^１は独立に、炭素数１～４のアルキル基、炭素数６～１０のアリール基のいずれかであり、Ｙ^２及びＹ^３はそれぞれ独立に、水素原子、炭素数１～４のアルキル基、炭素数６～１０のアリール基のいずれかである。

Here, A in formula (1) is represented by formula (2), n is the number of repetitions, the average value of which is 0 to 5. X is any one of a single bond, a hydrocarbon group having 1 to 13 carbon atoms, -O-, -CO-, -COO-, -S-, and _-SO2- , ^Y1 is independently any one of an alkyl group having 1 to 4 carbon atoms, and an aryl group having 6 to 10 carbon atoms, and ^Y2 and ^Y3 are each independently any one of a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an aryl group having 6 to 10 carbon atoms.

The present invention also relates to a reinforcing fiber-containing epoxy resin composition or prepreg obtained by mixing the above-mentioned epoxy resin composition with reinforcing fibers.

The above-mentioned reinforcing fibers are preferably PAN-based carbon fibers, and are preferably contained in the resin composition or prepreg in a proportion of 50% by weight or more and 80% by weight or less.

The present invention also relates to a fiber-reinforced plastic using the above-mentioned reinforcing fiber-containing epoxy resin composition or the above-mentioned prepreg.

The present invention makes it possible to provide a thermoplastic epoxy resin composition having excellent adhesion to reinforcing fibers.

The reason why the adhesion between the reinforcing fibers and the resin was demonstrated is thought to be that by controlling the raw material composition ratio, an epoxy resin with a low epoxy equivalent and a high epoxy group concentration can be obtained, which forms good adhesion with the functional groups on the surface of the reinforcing fibers and the sizing agent.
In addition, when the polymerization is not sufficiently advanced, the bending strength of the polymer is 70 MPa or more, so that the composite material can exhibit sufficient strength. Conventionally, the adhesiveness of epoxy resins is believed to be due to secondary hydroxyl groups, and there have been no reports that mention the effect of the epoxy group concentration of the polymer. According to the present invention, the adhesiveness to reinforcing fibers can be exhibited by a simple method of adjusting the feed ratio of the raw materials.

The present invention will be described in detail below based on preferred embodiments thereof.
The epoxy resin composition of the present invention is a composition that contains a bifunctional phenol compound, a bifunctional epoxy resin, and a polymerization catalyst as essential components and can be polymerized by heating. This composition may also contain additives such as an organic solvent, a filler, and a flame retardant.

The bifunctional epoxy resin contains the epoxy resin (a) represented by the formula (1) as an essential component in an amount of 50% by weight or more, preferably 66% by weight or more, more preferably 75% by weight or more, and even more preferably 80% by weight or more. The epoxy resin (a) constitutes a part of the bifunctional epoxy resin.
The epoxy equivalent of the difunctional epoxy resin is preferably 150 to 350 g/eq.

In formula (1), A is a divalent group represented by the above formula (2). n is the number of repetitions, the average value of which is 0 to 5, preferably 0 to 1.

In formula (2), X is any one of a single bond, a hydrocarbon group having 1 to 13 carbon atoms, -O-, -CO-, -COO-, -S- and -SO ₂ -.
The hydrocarbon group having 1 to 13 carbon atoms is preferably an alkylene group having 1 to 9 carbon atoms or an arylene group having 6 to 13 carbon atoms, for example, -CH ₂ -, -CH(CH ₃ )-, -C(CH ₃ ) ₂ -, -C(CF ₃ ) ₂ -, -CHPh-, -C(CH ₃ )Ph-, 1,1-cyclopropylene group, 1,1-cyclobutylene group, 1,1-cyclopentylene group, 1,1-cyclohexylene group, 4-methyl-1,1-cyclohexylene group, 3,3,5-trimethyl-1,1-cyclohexylene group, 1,1-cyclooctylene group, 1,1-cyclononylene group, 1,2-ethylene group, 1,2-cyclopropylene group, 1,2-cyclobutylene group, 1,2-cyclopentylene group, 1,2-cyclohexyl group, Examples of such alkylene groups include a 1,2-phenylene group, a 1,3-propylene group, a 1,3-cyclobutylene group, a 1,3-cyclopentylene group, a 1,3-cyclohexylene group, a 1,3-phenylene group, a 1,4-butylene group, a 1,4-cyclohexylene group, a 1,4-phenylene group, a 1,1-fluorene group, a 1,2-xylylene group, a 1,4-xylylene group, a tetrahydrodicyclopentadienylene group, and a tetrahydrotricyclopentadienylene group.
Among these, a single bond, -O-, -CO-, -COO-, -S-, -SO ₂ -, -CH ₂ -, -CH(CH ₃ )-, -C(CH ₃ ) ₂ -, -CHPh-, -C(CH ₃ )Ph-, 1,1-cyclohexylene group, 4-methyl-1,1-cyclohexylene group, 3,3,5-trimethyl-1,1-cyclohexylene group, 1,4-cyclohexylene group, 1,4-phenylene group, and 1,1-fluorene group are preferred, and a single bond, -O-, -CO-, -COO-, -S-, -SO ₂ -, -CH ₂ -, -CH(CH ₃ )-, -C(CH ₃ ) ₂ -, and -C(CH ₃ ) Ph-, a 1,1-cyclohexylene group, a 3,3,5-trimethyl-1,1-cyclohexylene group, and a 1,1-fluorene group are more preferred.
In addition, Ph represents a phenyl group, and the alkylene group includes an alkylidene group.

^Y1 is independently any one of an alkyl group having 1 to 4 carbon atoms and an aryl group having 6 to 10 carbon atoms.
Examples of the alkyl group having 1 to 4 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, an isobutyl group, and a t-butyl group.
Examples of the aryl group having 6 to 10 carbon atoms include a phenyl group, a tolyl group, an ethylphenyl group, a xylyl group, an n-propylphenyl group, an isopropylphenyl group, a mesityl group, and a naphthyl group.
Of these, a methyl group, an ethyl group, an n-propyl group, an n-butyl group, a t-butyl group, a phenyl group, a tolyl group, a xylyl group, or a naphthyl group is preferred, and a methyl group, an ethyl group, an n-propyl group, an n-butyl group, a t-butyl group, a phenyl group, or a tolyl group is more preferred.

^Y2 is independently any one of a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an aryl group having 6 to 10 carbon atoms, and is preferably a group other than a hydrogen atom. Examples of the alkyl group and the aryl group are the same as those exemplified for ^Y1 . Preferred ^Y2 is the same as ^Y1 .
^Y3 is independently any one of a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an aryl group having 6 to 10 carbon atoms. Examples of the alkyl group and the aryl group are the same as those exemplified for ^Y1 . Preferred ^Y3 is a hydrogen atom or the same as those exemplified for ^Y1 .

Examples of bifunctional epoxy resins (a) include tetramethylbisphenol F type epoxy resins (e.g., YSLV-80XY (manufactured by Nippon Steel Chemical & Material Co., Ltd.)), tetramethylbiphenol type epoxy resins (e.g., YX-4000 (manufactured by Mitsubishi Chemical Corporation)), and biscresolfluorene type epoxy resins (e.g., OGSOL CG-500 (manufactured by Osaka Gas Chemicals Co., Ltd.)).

In addition, epoxy resins other than the epoxy resin (a) can be used in combination as long as they are bifunctional epoxy resins, and the purity is preferably 95% or more. If the purity as a bifunctional epoxy resin is high, positional isomers and oligomers may be included. Examples of epoxy resins that can be used in combination with the epoxy resin (a) include, but are not limited to, bisphenol type epoxy resins such as bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, bisphenol acetophenone type epoxy resins, diphenyl sulfide type epoxy resins, diphenyl ether type epoxy resins, and bisphenol fluorene type epoxy resins, biphenol type epoxy resins, diphenyl dicyclopentadiene type epoxy resins, alkylene glycol type epoxy resins, dihydroxy naphthalene type epoxy resins, and dihydroxy benzene type epoxy resins.
The bifunctional epoxy resin that can be used in combination with the epoxy resin (a) is preferably less than 50% by weight, and more preferably less than 30% by weight, of the total epoxy resin. If it exceeds 50% by weight, gelation occurs and components that are difficult to dissolve in solvents are generated, which may deteriorate the reshapeability.

When monofunctional impurities are contained in a bifunctional epoxy resin, the molecular weight after polymerization does not increase, and the mechanical properties of the obtained thermoplastic resin product may deteriorate. Therefore, it is preferable that the monofunctional impurities are 2% by weight or less relative to the bifunctional epoxy resin.
When trifunctional or higher impurities are contained, the impurities tend to form crosslinked structures starting from the impurities, which may lead to increased dispersion of the polymer and gelation, resulting in loss of thermoplasticity. Therefore, it is preferable that the trifunctional or higher impurities are 1% by weight or less relative to the bifunctional epoxy resin.
In addition, impurity components that do not have active groups that react with either the epoxy resin or the phenolic hydroxyl group and that do not inhibit the polymerization reaction by themselves may cause the molecular weight after polymerization to decrease if their amount is large. Therefore, it is preferable that the amount of such impurity components is 2% by weight or less relative to the bifunctional epoxy resin.

The bifunctional phenol compound used in the epoxy resin composition of the present invention is a compound having two phenolic hydroxyl groups in one molecule, and its purity is preferably 95% by weight or more. If the purity of the bifunctional phenol compound is high, positional isomers may be contained.
When monofunctional impurities are contained, the molecular weight after polymerization does not increase, and the mechanical properties of the produced thermoplastic resin may deteriorate. Therefore, it is preferable that the monofunctional impurities are 2% by weight or less based on the bifunctional phenol compound.
When trifunctional or higher impurities are contained, the impurities tend to form crosslinked structures starting from the impurities, which may lead to increased dispersion of the polymer and gelation, resulting in loss of thermoplasticity. Therefore, it is preferable that the trifunctional or higher impurities are 1% by weight or less relative to the bifunctional phenol compound.
In addition, impurity components that do not have active groups that react with either the epoxy resin or the phenolic hydroxyl group and that do not inhibit the polymerization reaction by themselves may cause the molecular weight after polymerization to decrease if their amount is large. Therefore, it is preferable that the amount of such impurity components is 2% by weight or less relative to the bifunctional phenol compound.

The bifunctional phenol compound is preferably a bisphenol compound or a biphenol compound. Examples of the bisphenol compound include bisphenol A, bisphenol F (all manufactured by Nippon Steel Chemical & Material Co., Ltd.), bisphenol fluorene (manufactured by Osaka Gas Chemical Co., Ltd.), Bis-E, Bis-Z, BisOC-FL, BisP-AP, BisP-CDE, BisP-HTG, BisP-MIBK, BisP-3MZ, S-BOC (all manufactured by Honshu Chemical Industry Co., Ltd.), and bisphenol S. Examples of the biphenol compound include biphenol, dimethylbiphenol, and tetramethylbiphenol. Other bifunctional phenol compounds include benzenediols such as hydroquinone, methylhydroquinone, dibutylhydroquinone, resorcin, methylresorcin, catechol, and methylcatechol, and naphthalenediols such as naphthalenediol. Of these, bisphenol compounds or biphenol compounds are preferred.

The ratio of the bifunctional epoxy resin contained in the epoxy resin composition of the present invention is 1.01 to 1.05 mol, preferably 1.02 to 1.03 mol, per mol of the bifunctional phenolic compound. In a thermoplastic epoxy resin, the epoxy resin and the phenolic compound react successively to form a linear structure, thereby exhibiting thermoplasticity. If there is an excess of the epoxy resin, the epoxy group ends, and if there is an excess of the phenolic compound, the phenolic group ends, and the reaction ends.
If the ratio of the epoxy resin is less than 1.01 mol, the polymer is likely to have a phenol group terminal, and there is a risk that adhesion to the reinforcing fibers will not be exhibited.
If the ratio of the epoxy resin exceeds 1.05 mol, unreacted epoxy resin components will remain in the resin even after the polymerization reaction is completed, which may adversely affect the strength of the resin.

In an epoxy resin composition, if a phenolic compound is present in a crystalline state in an epoxy resin, the molar ratio will deviate from the design when viewed microscopically. If the reaction is started in this state, the polymerization may not proceed sufficiently. In order to proceed sufficiently with the polymerization, an epoxy resin composition in which the phenolic compound and the epoxy resin are uniformly dissolved (compatible) with each other is preferred.
In addition, it is desirable that the epoxy resin composition before blending with reinforcing fibers, etc., is completely dissolved or in a uniform liquid state, but for example, when the molten mixture is poured into a glass petri dish to a thickness of 2 mm without any air bubbles and the haze value in the thickness direction is measured, if the haze value in the thickness direction is less than 30%, it is judged that the composition has dissolved to a level that does not affect the polymerization reaction or has become a uniform liquid state. The haze value is more preferably less than 20%, and even more preferably less than 10%.

There are no limitations on the polymerization catalyst used in the epoxy resin composition, but any known catalyst can be used. Specific examples include phosphorus-based polymerization catalysts such as triphenylphosphine, tris(paratoluyl)phosphine, tris(orthotoluyl)phosphine, and tris(paramethoxyphenyl)phosphine. Other polymerization catalysts include imidazole compounds such as 1B2MZ, 1B2PZ, and TBZ (manufactured by Shikoku Chemical Industry Co., Ltd.).

The polymerization catalyst is preferably 0.05% by weight or more and 5% by weight or less based on the total amount of the resin composition consisting of a bifunctional epoxy resin and a bifunctional phenolic compound. If it is less than 0.05% by weight, the on-site polymerization takes too long, which may reduce productivity, and it may be deactivated for some reason before the target molecular weight is reached. If it exceeds 5% by weight, the polymerization reaction proceeds quickly, but storage stability may be impaired, which may cause problems with process compatibility, and since it is a component that participates in the reaction but is not incorporated into the skeleton, it may impair the physical properties after polymerization, and it is simply expensive, which is economically disadvantageous.

The epoxy resin composition may contain an organic solvent to adjust the viscosity or as a solvent for the polymerization catalyst. The organic solvent used is not particularly limited as long as it does not inhibit the reaction between the epoxy resin and the phenolic compound, but hydrocarbons, ketones, and ethers are preferred because of their ease of availability. Specific examples include toluene, xylene, acetone, methyl ethyl ketone, isobutyl ketone, cyclopentanone, cyclohexanone, and diethylene glycol dimethyl ether. However, if a large amount of organic solvent is present during the reaction, it inhibits the polymerization reaction. In addition, if organic solvent remains in the polymer, it deteriorates the mechanical properties and heat resistance. For this reason, it is desirable that the proportion of organic solvent is 5% by weight or less based on the total weight of the epoxy resin composition.

The progress of polymerization of an epoxy resin composition can be judged from the change in the epoxy equivalent of the polymer. If the heating is for less than one hour, the epoxy equivalent tends to increase, and it is possible that polymerization has not progressed sufficiently. If the heating is for more than one hour, the epoxy equivalent hardly increases from the value at one hour, and it can be determined that polymerization has progressed sufficiently. Therefore, the polymerization conditions for obtaining a polymer from the epoxy resin composition were set to heating at 160°C for one hour. In this invention, the polymer used to measure the tetrahydrofuran (THF) insoluble content is the one polymerized under these conditions.

The progress of polymerization of the reinforcing fiber-containing epoxy resin composition and prepreg was judged by the change in epoxy equivalent, in the same way as checking the progress of the polymerization product. When heated for less than four hours, the epoxy equivalent tends to increase, and it is possible that polymerization has not progressed sufficiently. When heated for more than four hours, the epoxy equivalent hardly increases from the value at the four-hour mark, and it was judged that polymerization has progressed sufficiently. In the case of an epoxy resin composition containing fibers, the epoxy equivalent is almost the same when the heating time is four times that of the epoxy resin composition alone. This is thought to be because the reaction is suppressed in the fibers. For this reason, the curing conditions for obtaining fiber-reinforced plastic from the reinforcing fiber-containing epoxy resin composition or prepreg were set to heating at 160°C for four hours.

It is important that the flexural strength of the polymer obtained by polymerizing the epoxy resin composition of the present invention without containing fillers or additives such as reinforcing fibers is 70 MPa or more. If the flexural strength of the polymer is below the lower limit of the range, it will not be able to fully exhibit mechanical strength as a fiber-reinforced plastic. The higher this strength, the better the mechanical strength of the fiber-reinforced plastic will be, so there is no need to specify an upper limit.

The epoxy equivalent of the polymer obtained by polymerizing the epoxy resin composition of the present invention is 5000 g/eq. or more and 20000 g/eq. or less. If the epoxy equivalent of the polymer is less than the lower limit of the range, the polymer contains a large amount of epoxy resin that has not been sufficiently polymerized, and there is a risk of the mechanical strength being deteriorated. If the epoxy equivalent of the polymer exceeds the upper limit of the range, the terminal groups become phenol groups, and there is a risk of the adhesion of the reinforcing fibers being deteriorated.

The epoxy resin composition of the present invention may contain additives. Examples of additives include fillers such as fumed silica, flame retardants such as aluminum hydroxide and red phosphorus, and modifiers such as core-shell rubber. From the viewpoint of stabilizing the polymerization reaction, it is desirable to use additives that are different from the resin phase, but plasticizers and compatible flame retardants may be included to the extent that they do not affect the reaction.

The epoxy resin composition of the present invention can be polymerized to form a thermoplastic epoxy resin, which is excellent as a resin component for fiber-reinforced plastics.
The reinforcing fiber-containing epoxy resin composition of the present invention can be obtained by mixing or impregnating the above-mentioned epoxy resin composition with reinforcing fibers. The prepreg can be obtained as follows.

The epoxy resin composition of the present invention can be applied to a release-treated paper or plastic film, and a release-treated cover film can be applied as necessary to obtain an epoxy resin composition film. Any known release paper, release plastic film, or cover film can be used, and is not particularly limited. The thickness of the epoxy resin composition film is determined by the design thickness and resin ratio of the prepreg, but is usually 1 μm or more and 300 μm or less. If the thickness is less than 1 μm, there is a problem that the openings of the fibers become noticeable unless the reinforcing fibers are defibrated cleanly, and if the thickness exceeds 300 μm, it becomes difficult to uniformly impregnate the reinforcing fibers. It is preferably 5 μm or more and 150 μm or less, and more preferably 10 μm or more and 100 μm or less.

The reinforcing fibers used in the present invention are intended to reinforce plastics, such as carbon fibers, aramid fibers, and cellulose fibers, and are not particularly limited. The form of the fibers is also not particularly limited, and examples include UD sheets with aligned fibers, woven fabrics, tows, chopped fibers, nonwoven fabrics, and paper. However, from the viewpoint of impregnation, the thickness of each fiber bundle is 1 mm or less, preferably 0.5 mm or less, and more preferably 0.2 mm or less.

The reinforcing fiber-containing epoxy resin composition or prepreg of the present invention is obtained from the above-mentioned epoxy resin composition and/or epoxy resin composition film and reinforcing fibers.
The ratio by weight of the reinforcing fibers to the epoxy resin composition is preferably 5:5 to 8:2. If the ratio of the reinforcing fibers is too small, the strength required for the fiber-reinforced material may not be fully satisfied, whereas if the ratio of the reinforcing fibers is too large, defects such as voids may occur.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. Unless otherwise specified, "parts" means parts by weight, and "%" means % by weight.

The raw materials, catalysts, solvents, and reinforcing fibers used in the examples are as follows:

[Epoxy resin]
A1: Tetramethylbiphenol type epoxy resin (manufactured by Mitsubishi Chemical Corporation, YX-4000, epoxy equivalent 188)
A2: Bisphenol A type epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd., YD-8125, epoxy equivalent 173)

[Phenol compounds]
B1: Bisphenol A (manufactured by Nippon Steel Chemical & Material Co., Ltd., hydroxyl equivalent: 114)
B2: 4,4'-bis(3,3,5-trimethylcyclohexylidene)bisphenol (Honshu Chemical Industry Co., Ltd., BisP-HTG, hydroxyl equivalent: 155)

[Polymerization catalyst]
C1: Tris(paramethoxyphenyl)phosphine (TPAP, manufactured by Hokko Chemical Industry Co., Ltd.)
C2: 2,3-dihydro-1H-pyrrolo-[1,2-a]benzimidazole (manufactured by Shikoku Chemical Industry Co., Ltd., TBZ)

[solvent]
D1: Cyclohexanone

[Reinforced fiber]
E: PAN-based carbon fiber (Toray Industries, Inc., T700-12K-50C)

The evaluation methods used in the examples are as follows:

Epoxy equivalent:
The measurement was carried out in accordance with Japanese Industrial Standard JIS K7236, and the unit was expressed in "g/eq.".
The polymer was measured as it was, and the resin component of the fiber-reinforced plastic was extracted by the following procedure, and the obtained resin component was used for the measurement.
Approximately 4 g of the sample was weighed into a 110 mL vial, 100 mL of tetrahydrofuran (THF) was added, ultrasonic diffusion was performed at room temperature for 1 hour, and then the mixture was left to stand at room temperature for 23 hours or more to dissolve. The obtained THF solution was filtered under reduced pressure with a 5 μm filter paper, and the filtrate was collected. The collected filtrate was dried in a silicon bat at 20 ° C for 24 hours or more, and then dried in an oven set at 110 ° C for 5 hours or more to obtain a film-formed resin component.

Hydroxyl equivalent:
The measurement was carried out in accordance with JIS K0070 standard, and the unit was "g/eq." Unless otherwise specified, the hydroxyl equivalent of the phenol resin means the phenolic hydroxyl equivalent.

Uniformity;
Whether the phenol compound was uniformly dissolved in the epoxy resin was judged by the haze value. The epoxy resin composition was placed in a colorless and transparent glass petri dish to a thickness of 2 mm, and the haze value was evaluated in five stages, "less than 5% (<5)", "less than 10% (<10)", "less than 20% (<20)", "less than 30% (<30)", and "30% or more (30≦)", with reference to a haze standard plate manufactured by Murakami Color Research Laboratory. If the haze value was less than 30%, it was judged that the phenol compound was uniformly dissolved in the epoxy resin and was marked with ◯, and if it was 30% or more, it was judged that the phenol compound was not uniformly dissolved and was marked with ×.

Gel fraction;
Approximately 1 g of the sample was weighed out in a 100 mL vial, 50 mL of tetrahydrofuran was added, ultrasonic diffusion was performed at room temperature for 1 hour, and the sample was dissolved by leaving it at room temperature for 23 hours or more. A 325 mesh wire net was dried in an oven at 100 ° C for 1 hour, and its weight was measured. The wire net was folded into a funnel shape, and the entire sample solution was poured onto the funnel. The vial was washed with tetrahydrofuran until no insoluble matter of the sample remained, and then poured into the funnel. The insoluble matter on the wire net was further washed with tetrahydrofuran and then dried in an oven at 100 ° C for 4 hours or more. The dry weight of the wire net was subtracted from the weight of the dried sample and the weight of the wire net, and this was divided by the weight of the sample to determine the gel fraction in weight % and evaluated. The gel fraction is equal to the weight % of the THF insoluble matter.

Bend test;
The bending strength of the polymer was measured in accordance with JIS K 7171. The test machine used was an Autograph AGS-X manufactured by Shimadzu Science Co., Ltd. The sample dimensions were 4 mm in thickness, 100 mm in length, and 15 mm in width, the bending span was 70 mm, and the test was performed at a test speed of 1 mm/min.
The 90 degree bending strength of the unidirectional fiber reinforced plastic was measured in accordance with JIS K 7074. The test was performed using a test machine (Shimadzu Science Autograph AGS-X) with sample dimensions of 2 mm thickness, 100 mm length, and 15 mm width, a bending span of 70 mm, and a test speed of 1 mm/min.

Resin adhesion amount:
The cross section of the fiber reinforced plastic after bending test was observed with a SEM (JSM-7900F, manufactured by JEOL Ltd.) to confirm the amount of resin attached to the fiber. If the adhesion between the fiber and the resin is good, it can be confirmed that the resin is well attached to the fiber surface at the fracture surface. Ten fibers were observed with the SEM, and the evaluation was made based on the number of fibers with resin attached to 80% or more of the fiber surface.
9 or more: ○, 8 or less: ×

Example 1
2913 parts of A1, 1000 parts of B1, and 1000 parts of B2 were weighed out and pulverized and mixed using a Henschel mixer. Then, melt mixing was carried out using an S1KRC kneader (manufactured by Kurimoto Iron Works, Ltd.) whose barrel temperature was preheated to 170°C, and the entire amount was collected in a metal can and cooled with stirring to obtain a precursor mixture (F1) of an epoxy resin composition.

Five parts of C1 (polymerization catalyst) were dissolved in five parts of D1 (organic solvent) in advance. The precursor mixture (F1) was placed in a planetary mixer set at 60°C, and the polymerization catalyst solution was added and mixed. After mixing, the mixture was quickly removed and immediately cooled to below 40°C to obtain an epoxy resin composition (G1).

The obtained epoxy resin composition (G1) was heated to about 70°C and stirred, then poured into a chrome-plated iron mold container with a clearance of 4 mm set in advance, and thermally polymerized at 160°C for 60 minutes in a hot air circulating oven to obtain a polymerized product.

The epoxy equivalent of the resulting polymer was measured and found to be 9,900 g/eq.
The bending strength of the resulting polymer was measured and found to be 87 MPa.
The gel fraction of the resulting polymer was measured and found to be 1%.

Examples 2 to 3, Comparative Examples 1 to 4
Epoxy resin compositions and polymers were obtained in the same manner as in Example 1 under the conditions shown in Table 1. The epoxy equivalent, bending strength and gel fraction of the obtained polymers were measured in the same manner as in Example 1, and the evaluation results are shown in Table 1.

However, in Comparative Examples 3 and 4, instead of pulverization and mixing using a Henschel mixer, stirring and mixing were performed in a planetary mixer set at 60°C.
In Comparative Example 3, the barrel temperature during melt mixing was set to 80° C. At this time, the haze value of the obtained epoxy resin composition was 30% or more, and the molten state of the phenol compound was judged to be ×.
In Comparative Example 4, the gel fraction of the product by weight was 95%, and the polymer could not be dissolved in a solvent, so that the epoxy equivalent was not measured.

Example 4
A release-treated release paper was fixed onto a hot plate preheated to 70° C. with the release side facing up, and the epoxy resin composition (G1) obtained in Example 1 was placed on the release paper, and then coated using a bar coater preheated to 70° C. so that the resin area weight became 79 g/ ^m2 . Immediately after coating, the sheet was removed from the hot plate and air-cooled to obtain an epoxy resin composition sheet.
Subsequently, carbon fiber (E) was laminated onto the obtained epoxy resin composition sheet so that the area weight of the fiber was 153 g/ ^m2 , and pressure was applied to the sheet using a hot press preheated to 90°C so that the surface pressure became 0.5 MPa. After 1 minute, the sheet was removed and air-cooled to obtain a prepreg having an Rc of 34%.
Thirteen of the prepregs thus obtained were laminated together with the fibers oriented in the same direction, and then sandwiched between release films and vacuum pressed to obtain a unidirectional fiber-reinforced plastic. The vacuum pressing conditions were 160° C., 0.1 MPa, and 240 minutes.

The 90 degree bending strength of the obtained unidirectional reinforced fiber plastic was measured and found to be 86 MPa. The epoxy equivalent of the resin component of the obtained unidirectional reinforced fiber plastic was measured and found to be 9,800 g/eq.

For Examples 5 to 6 and Comparative Examples 5 to 8, unidirectional fiber-reinforced plastics were obtained using the same procedure as in Example 4. Note that for Comparative Example 8, the epoxy equivalent was not measured because a large amount of insoluble matter was generated during the epoxy equivalent measurement.

From Tables 1 and 2, it can be seen that if the epoxy equivalent of the polymer is 5,000 g/eq. or more and 20,000 g/eq. or less and the resin strength is 70 MPa or more, the 90 degree bending strength of the fiber-reinforced plastic is 80 MPa or more.
From Comparative Example 5 (Comparative Example 1), it can be confirmed that when the epoxy equivalent of the polymer is high, the adhesion between the fiber and the resin is weak, and the 90 degree bending strength is not sufficiently exhibited.
From Comparative Examples 6 and 7 (Comparative Examples 2 and 3), it can be confirmed that if the resin strength of the polymer is not sufficiently high, the 90 degree bending strength is not sufficiently exhibited.
It can be seen from Comparative Example 8 (Comparative Example 4) that if the polymer contains a large amount of insoluble components, the 90 degree bending strength is not sufficiently achieved. Resins containing a large amount of gel components tend to have a high minimum melt viscosity during the polymerization reaction, which is thought to lead to voids remaining in the CFRP molding, adversely affecting the 90 degree bending strength.

Claims (6)

An epoxy resin composition containing a reinforcing fiber and an epoxy resin composition containing a difunctional phenol compound, a difunctional epoxy resin, and a polymerization catalyst as essential components, the epoxy resin composition comprising a reinforcing fiber ,
The difunctional epoxy resin contains 50% by weight or more of a difunctional epoxy resin (a) represented by the following formula (1):
The compounding ratio of the bifunctional phenol compound to the bifunctional epoxy resin is 1.01 to 1.05 moles of the bifunctional epoxy resin per mole of the bifunctional phenol compound,
A polymer obtained by heating the epoxy resin composition at 160°C for 1 hour is a thermoplastic epoxy resin, the polymer having an epoxy equivalent of 5000 g/eq. or more and 20000 g/eq. or less, a bending strength of 70 MPa or more, and a content of a component that is insoluble when dissolved in tetrahydrofuran of 10% by weight or less.
Here, the bending strength of the polymer and the component that becomes insoluble when dissolved in tetrahydrofuran (gel fraction) are values measured by the methods described in paragraphs 0056 and 0055 of the specification, respectively.

Here, A is a divalent group represented by formula (2), n is the number of repetitions and its average value is 0 to 5. X is a single bond, a hydrocarbon group having 1 to 13 carbon atoms, -O-, -CO-, -COO-, -S-, or -SO ₂ -, Y ¹ is independently an alkyl group having 1 to 4 carbon atoms or an aryl group having 6 to 10 carbon atoms, and Y ² and Y ³ are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or an aryl group having 6 to 10 carbon atoms. 2. The reinforcing fiber-containing epoxy resin composition according to claim 1 , comprising PAN-based carbon fibers as reinforcing fibers in an amount of 50% by weight or more and 80% by weight or less. A prepreg made of a mixture containing a reinforcing fiber and an epoxy resin composition containing a difunctional phenol compound, a difunctional epoxy resin, and a polymerization catalyst as essential components ,
The difunctional epoxy resin contains 50% by weight or more of a difunctional epoxy resin (a) represented by the following formula (1):
The compounding ratio of the bifunctional phenol compound to the bifunctional epoxy resin is 1.01 to 1.05 moles of the bifunctional epoxy resin per mole of the bifunctional phenol compound,
A prepreg characterized in that a polymer obtained by heating the epoxy resin composition at 160°C for 1 hour is a thermoplastic epoxy resin, the polymer having an epoxy equivalent of 5000 g/eq. or more and 20000 g/eq. or less, a bending strength of 70 MPa or more, and a content of a component that becomes insoluble when dissolved in tetrahydrofuran of 10% by weight or less.
Here, the bending strength of the polymer and the component that becomes insoluble when dissolved in tetrahydrofuran (gel fraction) are values measured by the methods described in paragraphs 0056 and 0055 of the specification, respectively.

Here, A is a divalent group represented by formula (2), n is the number of repetitions and its average value is 0 to 5. X is a single bond, a hydrocarbon group having 1 to 13 carbon atoms, -O-, -CO-, -COO-, -S-, or -SO 2 -, Y ₁ ^is independently an alkyl group having 1 to 4 carbon atoms or an aryl group having 6 to 10 carbon atoms, and Y ² and Y ³ are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or an aryl group having 6 to 10 carbon atoms. 4. The prepreg according to claim 3 , containing PAN-based carbon fibers as reinforcing fibers in a ratio of 50% by weight or more and 80% by weight or less. A fiber reinforced plastic using the prepreg according to claim 3 . A fiber-reinforced plastic using the reinforcing fiber-containing epoxy resin composition according to claim 1 .