Carbon fiber base material, prepreg, porous structure, production method of the same, preform, fiber-reinforced resin molding, sandwich structure, and aircraft member

Abstract

To provide a carbon fiber base material which achieves heat resistance and moldability at a high level, and a prepreg and a porous structure using the same which have high resistance.SOLUTION: A carbon fiber base material contains carbon fibers (A), and a resin composition (B) coating the surface of the carbon fibers (A), wherein the carbon fibers (A) form a non-woven fabric, and tensile strength after heating at 300°C for 5 minutes is 110% or more of that before heating.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a carbon fiber substrate and a prepreg. [Background technology]

[0002] Fiber-reinforced composite materials, which use a reinforcing fiber substrate such as a carbon fiber substrate as a reinforcing material and impregnate the pores with a matrix resin, are lightweight yet have excellent mechanical properties such as strength and rigidity, and have been applied to many fields such as aerospace, automobiles, railway vehicles, ships, civil engineering and construction, electronic devices, industrial machinery, and sporting goods. In particular, prepregs using carbon fiber nonwoven fabric as a carbon fiber substrate and porous structures made by expanding it have excellent mechanical properties and are easy to process into complex shapes such as thin walls and three-dimensional shapes. In addition, prepregs using nonwoven fabric in which reinforcing fibers are dispersed at the single thread level and are isotropically oriented as a reinforcing fiber substrate have isotropic mechanical properties, so they do not require lamination to compensate for low-strength directions such as in quasi-isotropic lamination, and have a high degree of freedom in designing the lamination structure. Such prepregs can be used as a single thin layer, or can be used for molding as a preform by freely laminating multiple sheets, and can be molded into a void-free fiber-reinforced resin molded body by heating and pressurizing, or molded into a porous body by heating and expanding, so they have the advantage of being excellent in processing into complex shapes such as thin walls and three-dimensional shapes. Patent Document 1 shows a carbon fiber substrate to which a binder has been added and a fiber-reinforced composite material in which the substrate is impregnated with a matrix resin.

[0003] On the other hand, in expanding these applications, achieving both moldability and heat resistance is a continuing challenge. In particular, the market is demanding increasingly high heat resistance for matrix resins, which is accompanied by higher temperatures and higher pressures in the impregnation process, and the development of a carbon fiber substrate with excellent moldability that can withstand such processes has been a challenge. Patent Documents 2 and 3 disclose carbon fibers coated with polyetherimide as a sizing agent as carbon fibers for obtaining fiber-reinforced resins with high heat resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2010 / 013645 [Patent Document 2] Special Publication No. 02-002990 [Patent Document 3] Special Publication No. 2014-500912. Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 does not disclose a carbon fiber substrate that can withstand high-temperature, high-pressure processes that accompany the use of high-melting-point matrix resins, and further improvements in heat resistance and moldability remain issues. Patent Documents 2 and 3 are technologies related to sizing agents, and are insufficient for impregnation or application to reinforcing fibers in the form of nonwoven fabrics.

[0006] An object of the present invention is to provide a carbon fiber base material that combines moldability and heat resistance. [Means for solving the problem]

[0007] The present invention provides a carbon fiber substrate comprising carbon fibers (A) and a resin composition (B) that coats the surfaces of the carbon fibers (A), the carbon fibers (A) forming a nonwoven fabric, and a tensile strength after heating at 300°C for 5 minutes that is 110% or more of that before the heating.

[0008] The present invention also relates to a prepreg (first embodiment) obtained by impregnating a substrate derived from the carbon fiber substrate of the present invention with a matrix resin (C).

[0009] The present invention also provides a prepreg comprising a substrate made of carbon fibers (A) and a binder resin composition (D) impregnated with a matrix resin (C), The carbon fibers (A) are discontinuous fibers forming a nonwoven fabric, The glass transition temperature of the binder resin composition (D) is 120° C. or higher and 450° C. or lower, The melting point of the matrix resin (C) is 250°C or higher and 400°C or lower, The binder resin composition (D) is present at the interface between the carbon fiber (A) and the matrix resin (C), The prepreg satisfies the following conditions (i) and (ii) (second embodiment). (i) The minimum tensile strength is 100 MPa or more. (ii) The thickness variation at 400°C is 10% or less.

[0010] The present invention also relates to a porous structure formed from the prepreg of the present invention.

[0011] The present invention also relates to a method for producing the porous structure of the present invention, comprising a first step of melting and expanding the matrix resin (C), and a second step of solidifying the matrix resin (C).

[0012] The present invention also relates to a preform comprising the prepreg of the present invention as a lamination unit.

[0013] The present invention also relates to a fiber-reinforced resin molding obtained by molding the preform of the present invention by heating and pressurizing it.

[0014] The present invention also relates to a fiber-reinforced resin molding having voids, which is produced by heating and expanding the preform of the present invention.

[0015] The present invention also provides a sandwich structure comprising the fiber-reinforced resin molding of the present invention as a core material, sandwiched between a pair of skin materials to form an integrated structure.

[0016] The present invention also relates to an aircraft member comprising the prepreg of the present invention, the porous structure of the present invention, the fiber-reinforced resin molding of the present invention, or the sandwich structure of the present invention. Effect of the Invention

[0017] The carbon fiber substrate of the present invention has excellent formability that flexibly follows the mold in the early stage of molding of the fiber-reinforced resin. In addition, it is possible to prevent molding defects such as breakage under the high-temperature and high-pressure process of impregnating the matrix resin. Furthermore, the carbon fiber substrate of the present invention can maintain the dispersion state of the carbon fibers, and prepregs, fiber-reinforced resins, and porous structures having excellent heat resistance can be easily obtained.

[0018] Moreover, the prepreg of the present invention can achieve both heat resistance and isotropy of mechanical properties. [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic diagram showing one embodiment of a carbon fiber substrate of the present invention, in which the right-hand diagram is an enlarged view showing carbon fiber (A) and a resin composition (B) adhered to the surface of the carbon fiber. [Diagram 2] FIG. 2 is a schematic diagram showing one embodiment of the prepreg of the present invention together with an enlarged view of the internal cross section thereof. [Diagram 3] FIG. 3 is a schematic diagram showing one embodiment of the porous structure of the present invention together with an enlarged image of the internal cross section thereof. [Figure 4] FIG. 4 is a schematic diagram for explaining the pore diameter of the porous structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] In the present invention, the thickness direction of the prepreg is referred to as the out-of-plane direction, and the direction perpendicular to the out-of-plane direction is referred to as the in-plane direction.

[0021] [Carbon fiber base material] The carbon fiber substrate of the present invention contains carbon fibers (A) and a resin composition (B) that coats the surfaces of the carbon fibers (A).

[0022] Carbon fiber (A) is excellent in mechanical properties and light weight among known reinforcing fibers. Examples of carbon fiber (A) include PAN-based, rayon-based, and pitch-based carbon fibers, and PAN-based carbon fibers are preferred from the viewpoint of mechanical properties and economic efficiency. Among PAN-based carbon fibers, it is preferable to use carbon fibers with an elastic modulus of 200 GPa or more from the viewpoint of the mechanical properties of the resulting fiber-reinforced substrate. It is also preferable to use recycled carbon fibers from the viewpoint of cost.

[0023] The carbon fiber (A) preferably has a thermal mass reduction rate of 1% by mass or less at 400°C. The thermal mass reduction rate is more preferably 0% by mass or more and 1% by mass or less. In the present invention, the carbon fiber (A) is preferably a discontinuous fiber as described below, and is bound with the resin composition (B) to form a carbon fiber substrate. If the carbon fiber (A) has low heat resistance, the decomposition gas may peel off the interface between the carbon fiber (A) and the resin composition (B), resulting in insufficient binding effect. For this reason, it is preferable to set the thermal mass reduction rate of the carbon fiber (A) to the above range. The thermal mass reduction rate can be calculated by the following formula. Thermal mass reduction rate (mass%)=[(M1-M2) / M1]×100 Here, M1 is the mass (mg) of the sample after being isothermally held at 30°C for 10 minutes, and M2 is the mass (mg) of the sample when it is subsequently heated at 10°C / min to 400°C.

[0024] The diameter of the carbon fiber (A) is preferably 1 to 20 μm, more preferably 3 to 10 μm, and further preferably 6 to 8 μm. By setting the diameter within this range, the carbon fiber has excellent dispersibility, and the carbon fiber substrate can effectively achieve both high porosity and mechanical properties. In addition, the mechanical properties of the prepreg obtained from the carbon fiber substrate can be improved.

[0025] In the present invention, the carbon fibers (A) constituting the nonwoven fabric are preferably discontinuous fibers, and the number-average fiber length is more preferably 0.1 to 100 mm, further preferably 1 to 20 mm, and particularly preferably 3 to 10 mm. By setting the length in this range, it becomes easier to widen the intervals between the fibers in the network structure of the nonwoven fabric, and it becomes easier to control the formation of pores. The number-average fiber length of the carbon fibers (A) can be calculated by randomly selecting 400 carbon fibers (A), measuring their fiber lengths with an optical microscope, and dividing the total fiber length by the number of fibers measured.

[0026] The carbon fibers (A) preferably contain 50% by mass or more of fibers having a fiber length of 2 mm or more and 10 mm or less, more preferably 3 mm or more and 8 mm or less, and even more preferably 5 mm or more and 7 mm or less. The content is more preferably 80% by mass or more and 100% by mass or less, and even more preferably 100% by mass. By setting the content within this range, the carbon fibers (A) constituting the network in the nonwoven fabric form are dispersed, and it becomes easier to control the formation of voids and the isotropic fiber orientation.

[0027] Examples of methods for collecting samples in the fiber length measurement method include a dissolution method using an organic solvent, a melting method in which the resin component is heated and melted to prepare a thin-walled sample for observation, and a method in which the resin component is removed by a burn-off method using an electric furnace, and the remaining carbon fibers (A) are filtered out and then measured using an optical microscope.

[0028] The carbon fibers (A) form a nonwoven fabric. This allows the porosity to be designed higher than that of a woven fabric or a fiber substrate oriented in one direction, which has the advantage of allowing a larger amount of matrix resin (C) to be impregnated, as described below. In addition, since the carbon fibers (A) form a nonwoven fabric, even if a by-product gas is generated during the reaction of the resin composition (B), as described below, the by-product gas can be easily exhausted, and a hard carbon fiber substrate can be obtained. In FIG. 1, the carbon fibers (A) 1 are randomly dispersed as discontinuous fibers, forming a nonwoven fabric with a network structure, which forms fine pores 3 between the fibers.

[0029] In general, the strength of a carbon fiber substrate in the form of a nonwoven fabric is lower than that of a woven fabric or a fiber substrate oriented in one direction, and there is an increased risk of the substrate breaking due to the infiltration pressure during melt impregnation with the matrix resin (C). Therefore, in the present invention, carbon fibers with excellent mechanical properties are used. The carbon fiber (A) in the form of a nonwoven fabric can be produced by an airlaid method, a carding method, a papermaking method, or the like.

[0030] The mass ratio of the carbon fiber (A) in the carbon fiber substrate is preferably 90% or more and 97% or less. When the mass ratio is 90% or more, the strength of the carbon fiber substrate is sufficient, and molding defects such as breakage during impregnation with the matrix resin (C) can be effectively prevented. In addition, when the mass ratio is 97% or less, more preferably 96% or less, and even more preferably 92% or less, the carbon fiber (A) is prevented from being bundled into fiber bundles, and the dispersion of the carbon fiber (A) in the prepreg obtained from the carbon fiber substrate can be prevented from becoming insufficient.

[0031] The resin composition (B) has the function of maintaining the nonwoven form by binding the network structure of the carbon fibers (A) at their contact points and protecting the carbon fibers (A) from abrasion. In the diagram on the right side of FIG. 1, the resin composition (B) 2 covers the surface of the carbon fibers (A) to form carbon fibers (A) 4 coated with the resin composition (B). The carbon fiber substrate of the present invention bound with the resin composition (B) can be made into a prepreg while maintaining the network structure such as the fiber orientation and dispersibility of the carbon fibers (A).

[0032] From the viewpoint of maintaining the network structure of the carbon fibers (A), the resin composition (B) preferably covers 50 to 100% of the surface area of ​​all the carbon fibers (A) constituting the carbon fiber substrate, more preferably covers 80 to 100%, and even more preferably covers 100%. In order to achieve such a state, it is preferable to form the carbon fibers (A) into an aggregate such as a nonwoven fabric, and then to carry out a process of immersing this in the resin composition (B).

[0033] The coverage of the resin composition (B) can be determined from the surface oxygen concentration (O / C) by X-ray photoelectron spectroscopy using the following formula, as described in the Examples below. (Coverage rate of resin composition (B))={(O / C of carbon fiber (A))-(O / C of carbon fiber base material)} / {(O / C of carbon fiber (A))-(O / C of resin composition (B))}×100(%).

[0034] The carbon fiber substrate of the present invention preferably has a porosity of 50 to 99%. The porosity of the carbon fiber substrate is more preferably 80 to 99%, and further preferably 90 to 99%. By setting the porosity within this range, the impregnation amount of the matrix resin (C) can be designed to be large. In addition, even if the porosity is high, the carbon fiber substrate of the present invention has excellent mechanical properties at high temperatures, and can prevent molding defects such as breakage of the carbon fiber substrate during the melt impregnation process of the matrix resin (C). The porosity of the carbon fiber substrate is the volume ratio of pores in the carbon fiber substrate, and can be calculated by the following formula. (Porosity of carbon fiber substrate) = {(Thickness of carbon fiber substrate [m]) - (Weight per unit area of ​​carbon fiber substrate [g / m 2 ]) / (density of the carbon fiber substrate components [g / m 3 ])} / (carbon fiber substrate thickness [m]) × 100 [%].

[0035] The carbon fiber substrate of the present invention has a tensile strength of 110% or more after heat treatment at 300°C for 5 minutes. The tensile strength after heat treatment relative to the tensile strength before heating is preferably 200% or more and 600% or less, more preferably 300% or more and 500% or less. As described later, the carbon fiber substrate of the present invention can be processed into a prepreg by melting and impregnating the matrix resin (C). In order to combine with the matrix resin (C), the matrix resin (C) melted by heating is impregnated into the pores of the carbon fiber substrate at high temperature and high pressure, but in this impregnation process, defects such as the breakage of the carbon fiber substrate may occur. On the other hand, if a hard carbon fiber substrate is used before the impregnation process, it may elastically rebound or break when it is shaped to follow the shape of a mold or the like. Therefore, by making it flexible at the start of the impregnation process and having a shapeability that follows the mold or the like, and having a property that hardens so as not to break even under pressure during impregnation, molding defects such as breakage can be prevented even in the high temperature and high pressure impregnation process of the matrix resin (C). The change in tensile strength before and after the heat treatment is calculated by measuring the tensile strength (Ta) at room temperature after heating at 300°C for 5 minutes in one direction of the carbon fiber substrate and the tensile strength (Tao) at room temperature of the carbon fiber substrate not subjected to the heat treatment in the same direction, and calculating it by the following formula. Tensile strength after heat treatment relative to tensile strength before heating (%) = (Ta / Tao) x 100. In this specification, the tensile strength [N / cm] of the carbon fiber substrate is a value obtained by cutting a test piece 50 mm wide and 150 mm long from the carbon fiber substrate, supporting 50 mm lengths on both ends, and subjecting the remaining central 50 mm-long portion to a tensile test at room temperature and at a test speed of 3 m / min, and dividing the obtained maximum point load by the width of the test piece.

[0036] The carbon fiber substrate of the present invention is preferably excellent in isotropy when impregnated with a matrix resin, that is, the strength in one direction of the carbon fiber substrate is close to that in the direction perpendicular to the direction. If the anisotropy is large, the risk of breakage increases in the direction of weak tensile strength during impregnation with the matrix resin (C). As an index of such isotropy, the tensile strength ratio (Ta / Tb) after heat treatment, which is obtained by dividing the Ta by the tensile strength (Tb) measured at room temperature after heat treatment at 300 ° C. for 5 minutes in the direction perpendicular to the test direction, is preferably 0.6 to 1.7. The tensile strength ratio (Ta / Tb) after such heat treatment is more preferably 0.8 to 1.3, and further preferably 0.9 to 1.1. The absolute values ​​of Ta and Tb are preferably 25 N / cm or more and 100 N / cm or less, and more preferably 25 N / cm or more and 45 N / cm or less. By setting them in such ranges, a strong carbon fiber substrate that does not break even during melt impregnation with the matrix resin (C) is obtained.

[0037] In addition, the carbon fiber substrate of the present invention is preferably excellent in isotropy even before impregnation with a matrix resin from the viewpoint of preventing breakage in the shaping process of making the substrate conform to a mold. As such an index, the tensile strength ratio (Tao / Tbo) before the heat treatment, which is obtained by dividing the tensile strength Tao before the heat treatment by the tensile strength (Tbo) measured at room temperature in a direction perpendicular to the test direction, is preferably 0.6 to 1.7. The tensile strength ratio (Tao / Tbo) before the heat treatment is more preferably 0.8 to 1.3, and further preferably 0.8 to 1.1. The absolute values ​​of Tao and Tbo are preferably 8 N / cm or more and 25 N / cm or less, and more preferably 8 N / cm or more and 15 N / cm or less. By setting the range, a carbon fiber substrate that is flexible and has high conformability to a mold is obtained.

[0038] In the present invention, the resin composition (B) is preferably a resin composition that undergoes a dehydration condensation reaction in which the thermal mass reduction rate of the following formula (1) is 5 to 50 mass %. Thermal mass reduction rate (mass%)=[(W1-W2) / W1]×100...(1) Here, W1 is the mass (mg) of the sample after it has been heated from room temperature to 100°C at 10°C / min and then kept isothermal for 30 minutes, and W2 is the mass (mg) of the sample when it has been heated again at 10°C / min and reached 250°C.

[0039] The resin composition (B) has the effect of hardening the carbon fiber substrate by a dehydration condensation reaction accompanying heating. And, since the carbon fiber (A) is in the form of a nonwoven fabric, the by-product gas generated by the dehydration condensation reaction of the resin composition (B) can be easily exhausted. By setting the thermal mass reduction rate of the resin composition (B) within this range, the hardening of the resin composition (B) and the exhaust of the by-product gas can be achieved in a well-balanced manner during the impregnation of the carbon fiber substrate with the matrix resin (C). The mass of the gas generated during heating and its components can be analyzed by a thermogravimetry-mass spectrometry (TG-MS). The thermal mass reduction rate accompanying the dehydration condensation reaction of the resin composition (B) is more preferably 5 to 30 mass%, and even more preferably 5 to 20 mass%.

[0040] When the resin composition (B) undergoes a dehydration condensation reaction, the by-product gas generated contains water, and the by-product gas preferably contains 5% or more of water, more preferably 50% or more, and even more preferably 100%. When the by-product gas contains a large amount of water, it is possible to prevent excessive temperature rise due to pressurization during impregnation with the matrix resin (C) and to improve releasability from the mold.

[0041] More specifically, the dehydration condensation reaction of the resin composition (B) as described above is preferably an imidization reaction, an oxazolization reaction, or an imidazolization reaction from the viewpoint of the heat resistance of the resulting reaction product. In the case of a thermal decomposition reaction such as oxidation or radical cleavage accompanied by mass reduction, the heat resistance and mechanical strength of the resin composition (B) may decrease, and the carbon fiber substrate may break, resulting in molding defects.

[0042] From this viewpoint, it is preferable that the resin composition (B) contains at least one selected from polyamic acid, polyamic acid salt, polyamic acid ester, polyhydroxyamide, and polyaminoamide. Polyamic acid, polyamic acid salt, and polyamic acid ester are precursors that undergo an imidization reaction to become a polyimide resin, polyhydroxyamide is a precursor that undergoes an oxazolization reaction to become a polybenzoxazole resin, and polyaminoamide is a precursor that undergoes an imidazolization reaction to become a polybenzimidazole resin.

[0043] Polyamic acid can be synthesized by reacting tetracarboxylic dianhydride with equimolar diamine compound. Specific synthesis methods include dissolving diamine compound in a solvent such as NMP and adding tetracarboxylic dianhydride while stirring at room temperature. In this method, the carboxylic anhydride, which is the functional group of tetracarboxylic dianhydride, and the amino group, which is the functional group of diamine compound, undergo ring-opening polyaddition reaction in equimolar amounts, i.e., 1:1, to increase the molecular weight, and at this time, carboxyl groups derived from the carboxylic anhydride are by-produced. Polyamic acid salts are salts obtained by neutralizing the by-produced carboxyl groups with a basic compound. Examples of basic compounds used for neutralization here include ammonia, monoalkylamines, dialkylamines, trialkylamines, and tetraalkylammonium hydroxides.

[0044] The polyamic acid ester can be synthesized by reacting the by-produced carboxyl group of the polyamic acid with an alcohol to esterify it. Examples of the alcohol used here include methanol, ethanol, 1-propanol, and 2-propanol. Esterification is preferable because it can reduce hydrolysis of the polyamic acid.

[0045] The polyamic acid, the polyamic acid salt and the polyamic acid ester are cyclized by an intramolecular imidization reaction due to heating, and are converted into polyimide while generating a by-product gas. The resin composition (B) may be a mixture of the polyamic acid, the polyamic acid salt and the polyamic acid ester.

[0046] From the viewpoint of high water solubility and use as an aqueous solution with a relatively low load on equipment, the resin composition (B) is preferably a polyamic acid salt.

[0047] The tetracarboxylic dianhydride may, for example, be 3,3',4,4'-biphenyltetracarboxylic dianhydride. <s-bpda>, 2,3,3',4'-biphenyltetracarboxylic dianhydride <a-bpda>, 2,2',3,3'-biphenyltetracarboxylic dianhydride <i-bpda>, pyromellitic dianhydride <pmda>, 3,3',4,4'-benzophenonetetracarboxylic dianhydride <btda>, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 4,4'-oxydiphthalic anhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride <ntda>, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride <6FDA>, etc. Among these, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) is preferred from the viewpoints of melt processability of the resulting polyamic acid, and compatibility between the heat resistance of the carbon fiber substrate and adhesion to the matrix resin (C).

[0048] The diamine compound is, for example, p-phenylenediamine. <ppd>, m-phenylenediamine <mpd>Phenylenediamines such as 3,5-diaminobenzoic acid, diaminobenzoic acids such as 4,4'-diaminodiphenyl ether <oda>diaminodiphenyl ethers such as 3,4'-diaminodiphenyl ether and 3,3'-diaminodiphenyl ether, diaminodiphenyl methanes such as 4,4'-diaminodiphenyl methane, 3,3'-diaminodiphenyl methane, 3,3'-dichloro-4,4'-diaminodiphenyl methane and 3,3'-dimethyl-4,4'-diaminodiphenyl methane, and diaminobenzophenones such as 4,4'-diaminobenzophenone. Among these, from the viewpoint of the coatability of the obtained polyamic acid and the balance between heat resistance and coatability to carbon fiber (A), phenylenediamines are preferred, and m-phenylenediamine is more preferred.

[0049] Polyhydroxyamide is a precursor of polybenzoxazole resin, and is a prepolymer formed by polycondensation of aromatic diamine diol having amino group and hydroxyl group on adjacent carbon atoms in aromatic ring with dicarboxylic acid derivative such as aromatic dicarboxylic acid, aromatic dicarboxylic acid ester, aromatic dicarboxylic acid dihalogen compound, heterocyclic dicarboxylic acid, etc. to form amide bond. Specific synthesis method includes a method of mixing aromatic diamine diol and dicarboxylic acid derivative in a 1:1 molar ratio in a solvent such as NMP, and stirring at 120 ° C for 12 hours. Polyhydroxyamide is heated at 200 to 400 ° C, and adjacent hydroxyl groups and amide bonds are condensed to form oxazole rings, and reacts with polybenzoxazole resin having oxazole rings and aromatic rings in the molecule. Examples of aromatic diamine diols include 3,3'-dihydroxybenzidine, 3,3'-diamino-4,4'-dihydroxybiphenyl, 4,6-diaminoresorcinol, 2,5-diaminohydroquinone, 4,4'-diamino-3,3'-dihydroxydiphenyl sulfone, 3,3'-diamino-4,4'-dihydroxydiphenyl sulfone, 4,4'-diamino-3,3'-dihydroxydiphenyl ether, 3,3'-diamino-4,4'-dihydroxydiphenyl ether, 4,4'-diamino-3,3'-dihydroxydiphenyl methane, 3,3'-diamino-4,4'-dihydroxydiphenyl methane, 2,2-bis(3-amino-4-hydroxyphenyl)-propane, 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, and their hydrochlorides. Examples of dicarboxylic acid derivatives include isophthalic acid.

[0050] Polyaminoamide is a precursor of polybenzimidazole resin, and is a prepolymer formed by polycondensation of a diamine component of an aromatic tetraamine compound with a dicarboxylic acid derivative such as an aromatic dicarboxylic acid, an aromatic dicarboxylic acid ester, an aromatic dicarboxylic acid dihalide, or a heterocyclic dicarboxylic acid to form an amide bond. A specific synthesis method includes mixing an aromatic tetraamine compound and a dicarboxylic acid derivative in a 1:1 molar ratio in a solvent such as NMP, and stirring at 120°C for 12 hours. When the polyaminoamide is heated at 200 to 400°C, the adjacent amino groups and amide bonds are condensed to form an imidazole ring, and the polyaminoamide reacts with a polybenzimidazole resin having an imidazole ring and an aromatic ring in the molecule. Examples of aromatic tetraamine compounds include 3,3'-diaminobenzidine, 1,2,4,5-tetra-aminobenzene, 2,3,5,6-tetra-aminopyridine, 3,3',4,4'-tetra-aminodiphenylsulfone, 3,3',4,4'-tetra-aminodiphenylether, 3,3',4,4'-tetra-aminodiphenylmethane, 3,3',4,4'-tetra-aminodiphenyldimethylmethane, and their hydrochlorides. Examples of dicarboxylic acid derivatives include isophthalic acid. Polybenzimidazole resins obtained by imidazolization of polyaminoamides are preferred because they have a high glass transition temperature of 400°C or higher and excellent heat resistance.

[0051] The resin composition (B) preferably contains at least one selected from polyamic acid, polyamic acid salt, polyamic acid ester, polyhydroxyamide, and polyaminoamide in a total amount of 50 mass% or more, more preferably 80 mass% or more. More preferably, the resin composition (B) is composed of at least one selected from polyamic acid, polyamic acid salt, polyamic acid ester, polyhydroxyamide, and polyaminoamide. By setting the content in this range, the resin composition (B) adheres to the carbon fiber (A), and a carbon fiber substrate having excellent heat resistance and mechanical strength can be obtained.

[0052] The resin composition (B) may contain other additives as appropriate depending on the application, within the scope of not impairing the object of the present invention, such as flame retardants, conductivity imparting agents, antioxidants, antibacterial agents, insect repellents, release agents, antistatic agents, plasticizers, lubricants, colorants, pigments, dyes, and foam control agents.

[0053] In the carbon fiber substrate of the present invention, the resin composition (B) is preferably 0.5 to 30 parts by mass relative to 100 parts by mass of the carbon fiber (A). By the resin composition (B) being 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 3 parts by mass or more, even more preferably 5 parts by mass or more, and even more preferably 8 parts by mass or more relative to 100 parts by mass of the carbon fiber (A), the carbon fiber substrate has excellent handleability and is less likely to break during transportation. By the resin composition (B) being 30 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less relative to 100 parts by mass of the carbon fiber (A), pores can be sufficiently formed and impregnation of the matrix resin (C) can be facilitated.

[0054] The resin composition (B) preferably covers 50% or more of the surface area of ​​the carbon fiber (A). When the coverage is 50% or more, more preferably 80% or more, and even more preferably 100%, an interface in which the carbon fiber (A) and the matrix resin (C) are firmly adhered to each other can be effectively formed in the prepreg of the present invention described later.

[0055] In order to achieve this coated state, the step of producing the carbon fiber base material preferably includes a step of immersing the nonwoven fabric made of the carbon fibers (A) in the resin composition (B).

[0056] The coverage of the resin composition (B) can be determined from the surface oxygen concentration (O / C) by X-ray photoelectron spectroscopy using the following formula, as described in the Examples below. (Coverage rate of resin composition (B)) (%)={(O / C of carbon fiber (A))-(O / C of carbon fiber base material)} / {(O / C of carbon fiber (A))-(O / C of resin composition (B))}×100.

[0057] By using the carbon fiber base material of the present invention, molding defects such as breakage during impregnation with the matrix resin (C) can be prevented, and a prepreg with excellent expansion coefficient can be obtained. In particular, the carbon fiber base material of the present invention can be impregnated with a highly heat-resistant matrix resin (C) that requires an impregnation process at high temperature and high pressure while preventing molding defects such as breakage. Due to this effect, a porous structure that is excellent in heat resistance and lightweight can be easily obtained by using a highly heat-resistant matrix resin (C).

[0058] [Prepreg] The carbon fiber substrate of the present invention is suitable as a substrate for a prepreg. That is, a first embodiment of the prepreg of the present invention is obtained by impregnating a substrate derived from the carbon fiber substrate of the present invention with a matrix resin (C).

[0059] A second embodiment of the prepreg of the present invention is obtained by impregnating a substrate made of carbon fibers (A) and a binder resin composition (D) with a matrix resin (C).

[0060] Details of the carbon fiber (A) in the second embodiment are common to those of the carbon fiber (A) in the first embodiment, that is, in the carbon fiber substrate of the present invention.

[0061] The details of the matrix resin (C) in the first embodiment and the matrix resin (C) in the second embodiment are also common.

[0062] The binder resin composition (D) in the second embodiment is preferably derived from the resin composition (B) described above. The binder resin composition in the first embodiment, which is the resin composition (B) or is derived from it, and the binder resin composition (D) in the second embodiment are collectively referred to simply as "binder resin composition". Hereinafter, the first embodiment and the second embodiment are common to both unless otherwise specified.

[0063] In Fig. 2, a substrate made of carbon fibers (A) 1 and the binder resin composition 2 has fine pores formed therein, and the pores are impregnated with a matrix resin (C) 5 to form a prepreg. In the prepreg, the carbon fibers (A) 1 are randomly dispersed and exist in the form of a nonwoven fabric forming a network structure, and the fine spaces between the fibers are filled with the matrix resin (C) 5. The binder resin composition 2 exists at the interface between the carbon fibers (A) 1 and the matrix resin (C) 5.

[0064] The mass ratio of the carbon fiber (A) in the prepreg of the present invention is preferably 1% to 50%, more preferably 5% to 40%, and even more preferably 10% to 30%. If it is smaller than this range, the reinforcing effect of the prepreg by the carbon fiber base material may be insufficient. If it is larger than this range, the carbon fiber base material may break during impregnation with the matrix resin (C), resulting in defects.

[0065] The resin contained in the binder resin composition includes phenol resin, urea resin, melamine resin, polyimide resin, polybenzoxazole resin, polybenzimidazole resin, bismaleimide resin, benzoxazine resin, cyanate ester resin, etc., and may be a copolymer, modified product, or a blend of two or more of these. Among these, polyimide resin, polybenzoxazole resin, and polybenzimidazole resin are preferred from the viewpoint of heat resistance. Furthermore, polyimide resin is more preferred from the viewpoint of processability to carbon fiber substrate, and among polyimide resins, polyetherimide resins each having an ether group and an imide group are particularly preferred.

[0066] The binder resin composition is preferably a composition containing a polymer or derivative thereof having at least one structure selected from an etherimide skeleton, a benzoxazole skeleton, a benzimidazole skeleton, a benzoxazine skeleton, and a cyanate ester skeleton. By containing such a compound, the binder resin composition has excellent heat resistance and excellent adhesion to the carbon fiber (A) and the matrix resin (C). The etherimide skeleton means that it has an ether group and an imide group, respectively, and an example of a polymer or derivative thereof having the etherimide skeleton is a polyetherimide resin. The benzoxazole skeleton is a structure in which an aromatic ring and an oxazole ring are bonded to each other while sharing one side, and an example of a polymer or derivative thereof having the benzoxazole skeleton is a polybenzoxazole resin. The benzimidazole skeleton is a structure in which an aromatic ring and an imidazole ring are bonded to each other while sharing one side, and an example of a polymer or derivative thereof having the benzoxazine skeleton is a polybenzimidazole resin. An example of a polymer or derivative thereof having the benzoxazine skeleton is a benzoxazine resin. An example of a polymer or derivative thereof having the cyanate ester skeleton is a cyanate ester resin. Furthermore, from the viewpoint of heat resistance and processability into a carbon fiber substrate, the binder resin composition more preferably contains a polymer having an etherimide skeleton or a derivative thereof.

[0067] The glass transition temperature of the binder resin composition in the first embodiment is preferably 120°C or more and 450°C or less. The glass transition temperature of the binder resin composition (D) in the second embodiment is 120°C or more and 450°C or less. The glass transition temperature of the binder resin composition is more preferably 200°C or more and less than 400°C, and even more preferably 200°C or more and 350°C or less. The binder resin composition has a function of maintaining the nonwoven form by binding the network structure of the carbon fibers (A) at their contact points during impregnation with the matrix resin (C), and exists at the interface between the carbon fibers (A) and the matrix resin (C) inside the obtained prepreg. When the glass transition temperature of the binder resin composition is within the above range, the binder resin composition maintains the form of the carbon fiber base material as a solid at the beginning of impregnation with the matrix resin (C), and then gradually softens to become compatible with the molten matrix resin (C), and an interface in which the carbon fibers (A) and the matrix resin (C) are firmly adhered to each other can be formed.

[0068] The binder resin composition preferably has a thermal mass reduction rate of 5% by mass or less at 400°C. The binder resin composition has a function of maintaining the nonwoven form by binding the network structure of the carbon fibers (A) at their contact points during impregnation with the matrix resin (C), and exists at the interface between the carbon fibers (A) and the matrix resin (C) inside the obtained prepreg. The thermal mass reduction rate of the binder resin composition at 400°C is 5% by mass or less, preferably 0% by mass or more and 3% by mass or less, so that the binder resin composition has high heat resistance and is less susceptible to thermal decomposition, and can prevent the decomposition gas from peeling off the interface between the carbon fibers (A) and the binder resin composition, thereby preventing a decrease in the binding effect. This thermal mass reduction rate can be calculated by the following formula. Thermal mass reduction rate (mass%)=[(M1-M2) / M1]×100 Here, M1 is the mass (mg) of the sample after being isothermally held at 30°C for 10 minutes, and M2 is the mass (mg) of the sample when it is subsequently heated at 10°C / min to 400°C.

[0069] The binder resin composition preferably covers 50% or more of the surface area of ​​the carbon fibers (A). When the coverage is 50% or more, more preferably 80% or more, and even more preferably 100%, an interface in which the carbon fibers (A) and the matrix resin (C) are firmly adhered to each other can be effectively formed in the prepreg of the present invention described later.

[0070] In order to achieve this coated state, the step of producing the carbon fiber base material preferably includes a step of immersing the nonwoven fabric made of the carbon fibers (A) in the resin composition (B).

[0071] The coverage of the binder resin composition can be determined from the surface oxygen concentration (O / C) by X-ray photoelectron spectroscopy using the following formula, as described in the Examples below. (Coverage rate of binder resin composition) (%)={(O / C of carbon fiber (A))−(O / C of carbon fiber base material)} / {(O / C of carbon fiber (A))−(O / C of binder resin composition)}×100.

[0072] Generally, in prepregs, it is difficult to coat the surface of reinforcing fibers with a high coverage rate using a binder resin having a high glass transition temperature because the processing temperature is high. In the present invention, as a means for solving this problem, the above-mentioned resin composition (B) is applied to the surface of carbon fiber (A) as a precursor of binder resin composition (D), and this is reacted by heating to form binder resin composition (D).

[0073] In addition, when a polyimide resin, a polybenzoxazole resin, or a polybenzimidazole resin is used as the binder resin composition (D), a method in which a resin composition (B) is applied as a precursor to carbon fibers (A) and then a dehydration condensation reaction is carried out to obtain the binder resin composition (D) can also be preferably used from the viewpoint of heat resistance and processability.

[0074] The matrix resin (C) is preferably a thermoplastic resin. Examples of the thermoplastic resin include polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyolefins such as polyethylene, polypropylene, polybutylene, and modified polypropylene, polyamides such as polyoxymethylene, polyamide 6, and polyamide 66, polycarbonate, polymethyl methacrylate, polyvinyl chloride, polyarylene sulfides such as polyphenylene sulfide, polyphenylene ether, modified polyphenylene ether, polysulfone, modified polysulfone, polyether sulfone, polyarylene ether ketones such as polyketone, polyether ketone, polyether ether ketone, and polyether ketone ketone, polyarylate, polyether nitrile, and phenoxy resin.

[0075] Furthermore, these thermoplastic resins may be copolymers, modified products, and / or blends of two or more types of resins.

[0076] Among these, from the viewpoint of the balance between moldability and heat resistance, it is more preferable that the matrix resin (C) contains at least one selected from the group consisting of polyetherketone, polyetheretherketone, polyetherketoneketone, and polycarbonate as the main component, even more preferable that it contains at least one selected from the group consisting of polyetherketone, polyetheretherketone, and polyetherketoneketone as the main component, and even more preferable that it contains polyetherketoneketone as the main component. Here, the main component means that it accounts for 50% or more by mass of the constituent components, preferably 80% or more and 100% or less by mass, and more preferably 100% by mass.

[0077] The melting point of the thermoplastic resin used as the matrix resin (C) in the first embodiment is preferably 250°C or more and 400°C or less. The melting point of the thermoplastic resin used as the matrix resin (C) in the second embodiment is 250°C or more and 400°C or less. By making the melting point of the thermoplastic resin 250°C or more, more preferably 280°C or more, and even more preferably 300°C or more, the prepreg and the porous structure to be described later and other products obtained by processing the prepreg can be effectively made excellent in heat resistance. In addition, by making the melting point of the thermoplastic resin 400°C or less, more preferably less than 400°C, even more preferably 360°C or less, and even more preferably 350°C or less, the molding processability of the prepreg can be effectively made excellent.

[0078] The glass transition temperature of the thermoplastic resin used as the matrix resin (C) is preferably 80°C or more and 250°C or less, more preferably 100°C or more and 230°C or less, and even more preferably 140°C or more and 180°C or less. When the glass transition temperature is 80°C or more, preferably 100°C or more, more preferably 120°C or more, and even more preferably 140°C or more, the heat resistance of the prepreg and the porous structure described later can be effectively improved. In addition, when the glass transition temperature is 250°C or less, preferably 230°C or less, more preferably 200°C or less, and even more preferably 180°C or less, the compatibility with the binder resin composition is excellent, and the tensile strength of the obtained prepreg can be effectively improved.

[0079] The matrix resin (C) as a resin composition preferably contains other fillers and additives as appropriate depending on the application, within the scope of not impairing the object of the present invention. For example, inorganic fillers, flame retardants, conductive agents, crystal nucleating agents, ultraviolet absorbers, antioxidants, vibration dampers, antibacterial agents, insect repellents, deodorants, coloring inhibitors, heat stabilizers, release agents, antistatic agents, plasticizers, lubricants, colorants, pigments, dyes, foaming agents, foam control agents, coupling agents, etc. may be mentioned.

[0080] The amount of the matrix resin (C) in the prepreg is preferably 5 to 1000 parts by mass, more preferably 50 to 700 parts by mass, and even more preferably 100 to 500 parts by mass, per 100 parts by mass of the carbon fiber (A). If it is less than this range, the mechanical properties of the prepreg or the porous structure obtained by molding it may be insufficient. If it is more than this range, the amount of the matrix resin (C) is too large relative to the volume of the pores, and the carbon fiber base material may be broken during melt impregnation with the matrix resin (C), resulting in defects.

[0081] The prepreg of the first embodiment preferably has a minimum tensile strength of 100 MPa or more. The prepreg of the second embodiment preferably has a minimum tensile strength of 100 MPa or more. The minimum tensile strength of the prepreg is preferably 200 MPa or more, more preferably 240 MPa or more, and even more preferably 260 MPa or more and 400 MPa or less.

[0082] In general, the tensile strength of a prepreg is strongly influenced by the fiber orientation of the carbon fiber substrate contained therein. Even within the same plane of the prepreg, the fiber orientation is biased, and the more parallel the carbon fibers are in the direction, the higher the tensile strength is, and conversely, the lower the tensile strength is in the direction perpendicular to this. For this reason, even if the tensile strength of a prepreg is high in one direction, the tensile strength may be low in the direction perpendicular to it. From this perspective, in order to obtain a prepreg with high isotropic mechanical properties in the in-plane direction, it is desirable to design the tensile strength based on the minimum value.

[0083] The minimum tensile strength can be measured using test pieces cut out in four directions, with any one direction in the in-plane direction of the prepreg set as the reference 0°, and then including 4 directions: 45°, 90°, and -45°. The number of measurements for each direction is n=5, and the minimum value of all the measured values ​​(n=20) is the minimum tensile strength.

[0084] The prepreg of the present invention preferably has a maximum tensile strength of 260 MPa or more, and more preferably 260 MPa or more and 400 MPa or less.

[0085] In general, the tensile strength of a prepreg increases as the fiber orientation of the carbon fiber substrate contained therein becomes biased and the number of carbon fibers that are close to parallel to the tensile direction increases. By designing the maximum value of the tensile strength within this range and to be approximately the same as the minimum value of the tensile strength described above, a prepreg with more isotropic mechanical properties can be obtained.

[0086] The maximum tensile strength can be measured using test pieces cut out in four directions, with any one direction in the in-plane direction of the prepreg set as the reference 0°, and then including 4 directions: 45°, 90°, and -45°. The number of measurements for each direction is n=5, and the maximum value of all the measured values ​​(n=20) is regarded as the maximum tensile strength.

[0087] In the prepreg of the present invention, the ratio of the maximum tensile strength to the minimum tensile strength is preferably 1 to 3, more preferably 1 to 1.5, even more preferably 1 to 1.2, and particularly preferably 1 to 1.1. By setting the ratio within this range, the prepreg can have equivalent mechanical properties in any in-plane direction of the prepreg, and can have excellent isotropic mechanical properties. This value can be determined by dividing the maximum tensile strength of the prepreg by the minimum tensile strength of the prepreg.

[0088] The prepreg of the present invention preferably has an average tensile strength of 100 MPa or more, more preferably 200 MPa or more, even more preferably 240 MPa or more, and particularly preferably 260 MPa or more and 400 MPa or less. By setting the strength in this range, a prepreg with high isotropic mechanical properties can be obtained. Such average tensile strength can be measured using test pieces cut out in four directions, including 45°, 90°, and -45°, with one direction in the in-plane direction of the prepreg being set as the reference 0°. The number of measurements for each direction is n=5, and the average value of all the measured values ​​(n=20) is taken as the average tensile strength.

[0089] From the viewpoint of obtaining a prepreg with isotropic mechanical properties, it is also preferable to set the in-plane tensile strength variation to 20% or less, more preferably 10% or less, and even more preferably 5% or less. The tensile strength variation is the coefficient of variation (CV) that can be calculated from the average value and standard deviation of all measured values ​​(n=20) by the following formula, and more specifically, can be measured by the method described in the examples below. Tensile strength variation (%) = [(standard deviation of tensile strength) / (average tensile strength)] x 100.

[0090] The prepreg of the present invention preferably has a thickness of 0.1 mm to 5 mm at 400° C., more preferably 0.1 mm to 2 mm, and even more preferably 1 mm to 2 mm. The thickness at 400° C. corresponds to the thickness after expansion caused by springback due to the carbon fiber substrate contained in the prepreg. The prepreg can be used for molding purposes according to the desired thickness by laminating a single layer or multiple sheets, and is preferable because it can be molded into a porous body of various thicknesses by controlling the thickness of the prepreg during expansion within the range.

[0091] The thickness at 400° C. can be calculated as the average value of the thicknesses of the prepreg heated on a hot plate at 400° C. for 5 minutes, which is measured in a non-contact manner using a laser displacement meter.

[0092] The prepreg of the first embodiment preferably has a thickness variation of 10% or less at 400°C. The prepreg of the second embodiment has a thickness variation of 10% or less at 400°C. The thickness variation at 400°C is preferably 8% or less, more preferably 0% or more and 5% or less. When the prepreg of the present invention is heated to 400°C, the matrix resin (C) melts, causing springback and expanding into the void. Such expansion is due to the carbon fiber base material contained in the prepreg, and the variation in thickness after expansion is an index of isotropy in the out-of-plane direction. In other words, when the thickness after expansion of a certain portion is significantly different from the surroundings, it can be said that the fiber orientation and fiber concentration of the contained carbon fiber base material are different. In the case of a prepreg that is isotropic in the out-of-plane direction, the thickness after such expansion is generally equal, and as a result, the variation in thickness is small.

[0093] The fiber-reinforced resin molded body and the void body described later are integrally molded products in which the prepregs constituting the preform are bonded between layers. By using a prepreg that has expandability and small thickness variation when expanded, it is possible to avoid gaps between layers and obtain a fiber-reinforced resin molded body and a void body that are well bonded.

[0094] The thickness variation at 400°C is a CV value (coefficient of variation) that can be calculated from the average thickness value and standard deviation by the following formula, where the thickness of a prepreg heated on a hot plate at 400°C for 5 minutes is measured in a non-contact manner using a laser displacement meter, and more specifically, it can be measured by the method described in the examples below. Thickness variation at 400°C (%) = [(standard deviation of thickness at 400°C) / (average thickness at 400°C)] × 100.

[0095] By heating the prepreg of the present invention to 400 ° C, the matrix resin (C) having a melting point of 250 ° C or more and 400 ° C or less, preferably 280 ° C or more and 400 ° C or less, melts, and the springback force of the carbon fiber base material is released to the maximum extent, and the prepreg is expanded. By making it in such a state, the fiber orientation and fiber density of the carbon fiber base material contained in the prepreg can be grasped. That is, in the inside of the prepreg, the fiber orientation of the carbon fiber (A) is biased and converges into a fiber bundle, and the carbon fiber base material is broken and the concentration of the carbon fiber (A) is thin, and the thickness of the state expanded at 400 ° C is thin. The presence of such sites can be detected as the variation in thickness after expansion. Such defects of the carbon fiber base material are likely to occur during impregnation with the matrix resin (C), and as a method for confirming this after prepreg, the measurement of the variation in thickness at 400 ° C is effective.

[0096] In addition, defects in the carbon fiber substrate contained in the prepreg affect the tensile strength of the prepreg. Therefore, for example, by making the thickness variation at 400°C less than 10%, it is possible to obtain a prepreg with high average tensile strength, or a prepreg that is isotropic, i.e., has a small ratio of the maximum tensile strength to the minimum tensile strength, and has a high maximum tensile strength.

[0097] The prepreg of the present invention is preferably produced by impregnating the matrix resin (C) in the out-of-plane direction from the viewpoint of short impregnation distance and excellent productivity. In such a production method, the carbon fiber substrate first repels compression accompanying impregnation of the matrix resin (C). This repulsive force causes the matrix resin (C) to resin flow in the in-plane direction. During this resin flow, a force pulling the carbon fiber substrate in the in-plane direction acts. When a conventional carbon fiber substrate is used, it breaks against the force in the in-plane direction, resulting in molding defects. By using the carbon fiber substrate of the present invention, such breakage can be prevented.

[0098] The method for producing the prepreg includes a heating impregnation step in which the matrix resin (C) is melted to plasticize it and impregnates the pores of the carbon fiber substrate, and then a cooling solidification step in which the impregnated matrix resin (C) is solidified. In such a production method, the method for compounding the matrix resin (C) with the carbon fiber substrate includes a method in which the molten matrix resin (C) is directly injected into the carbon fiber substrate, and a method in which a film-like, powder-like, or fibrous matrix resin (C) is laminated on the carbon fiber substrate and then impregnated by heating and melting. From the viewpoint of suppressing uneven concentration of the prepreg and bias in fiber orientation and easy production, a method of heating and pressurizing a laminate of a film-like matrix resin (C) and a carbon fiber substrate is preferred.

[0099] The temperature in the heat impregnation step is preferably 250 to 450° C., more preferably 280 to 400° C., and further preferably 300 to 400° C. The pressure in the heat impregnation step is preferably 0.5 to 20 MPa, and more preferably 5 to 15 MPa. Even under such high temperature and high pressure conditions, the prepreg can be efficiently produced by using the carbon fiber base material of the present invention.

[0100] The time in the heat impregnation step is preferably 0.5 to 60 minutes, more preferably 5 to 30 minutes. By setting the heating time in this range, the hardening of the carbon fiber substrate proceeds in parallel with the impregnation of the matrix resin (C), and strong adhesion is possible, which is preferable.

[0101] The temperature in the cooling and solidifying step is preferably 20 to 250° C., and more preferably 100 to 200° C. The pressure in the cooling and solidifying step is preferably 0.5 to 20 MPa, and more preferably 5 to 15 MPa. The time in the cooling and solidifying step is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. By setting the time in this range, the prepreg can be produced efficiently.

[0102] As equipment for realizing such a prepreg manufacturing method, a press molding machine or a double belt press machine can be suitably used. In the case of a batch type, the former is used, and productivity can be improved by using an intermittent press system in which two or more machines, one for heating and one for cooling, are arranged in parallel. In the case of a continuous type, the latter is used, and continuous processing can be easily performed, so that continuous productivity is excellent.

[0103] The prepreg produced by the above-mentioned manufacturing method contains the carbon fiber substrate that is compressed in the out-of-plane direction during impregnation with the matrix resin (C) and stores the repulsive force in the out-of-plane direction. Therefore, when the matrix resin (C) melts, the carbon fiber substrate releases the repulsive force and causes a springback phenomenon in which the substrate tries to return to its original porosity. In other words, the prepreg has the expandability in the out-of-plane direction and can be molded into a porous structure.

[0104] The prepreg of the present invention preferably expands at an average out-of-plane expansion rate of 150 to 1,000%, more preferably 200 to 800%, and even more preferably 450 to 700% when the matrix resin (C) is melted. This value is an expansion rate calculated by (thickness of the porous structure after expansion [mm]) / (thickness of the prepreg before expansion [mm])×100[%], and is the arithmetic average value of five samples.

[0105] The prepreg of the present invention contains a carbon fiber substrate that is compressed in the out-of-plane direction during impregnation with the matrix resin (C) and stores a repulsive force in the out-of-plane direction. Therefore, when the matrix resin (C) melts, the carbon fiber substrate releases the repulsive force and causes a springback phenomenon in which the carbon fiber substrate tries to return to its original thickness. In other words, such a prepreg has an expandability in the out-of-plane direction, and can be molded into a porous body by taking in air during expansion.

[0106] [Porous structure] The prepreg of the present invention can be molded into a porous structure by taking advantage of its expandability when heated. That is, the porous structure of the present invention is formed from the prepreg of the present invention.

[0107] In FIG. 3, the carbon fiber substrate containing carbon fibers (A) and a resin composition (B) covering the surface thereof expands in the out-of-plane direction due to the springback force, and pores generated by this volume expansion are generated inside the matrix resin (C) as micropores. By forming such a structure, the porous structure is made lighter in weight, and the reinforcing effect of the carbon fiber substrate can be exerted. In this way, by having a structure in which the pores in the porous structure are reinforced by the carbon fiber substrate, it is possible to achieve both weight reduction due to an increase in the amount of pores and suppression of deformation of the pores, and it is possible to achieve both weight reduction and mechanical properties at a high level.

[0108] The method for producing the porous structure preferably includes a first step of melting and expanding the matrix resin (C), and a second step of solidifying the matrix resin (C).

[0109] The first step is a step of plasticizing the matrix resin (C) by melting it, and selectively expanding it in the out-of-plane direction by the springback force of the carbon fiber substrate.

[0110] In the first step, the temperature at which the prepreg is heated to melt and expand the matrix resin (C) is preferably 250 to 450°C, and more preferably 300 to 400°C. The pressure in the first step is preferably 0 to 5 MPa, and more preferably 0.1 to 3 MPa. The time spent in the first step is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. By setting the time in this range, the prepreg can be efficiently expanded, and a porous structure having excellent mechanical properties can be obtained.

[0111] The second step is a step of solidifying the matrix resin (C) to fix the carbon fiber base material in a springback state and form a porous structure.

[0112] The temperature in the second step is preferably 20 to 250°C, more preferably 100 to 200°C. The pressure in the second step is preferably 0 to 5 MPa, more preferably 0.1 to 3 MPa. The time in the second step is preferably 0.5 to 60 minutes, more preferably 1 to 30 minutes. By setting the time in this range, the expansion ratio can be controlled, and a porous structure having excellent mechanical properties can be obtained.

[0113] In addition, in the first step and the second step, it is preferable to adjust the thickness of the molded product. As a method for controlling the thickness, a method of constraining the thickness using a metal plate or a method of directly controlling the thickness by adjusting the pressure are preferable from the viewpoint of ease of production. As equipment for realizing such a method, a press molding machine or a double belt press machine can be suitably used. In the case of a batch type, the former is used, and productivity can be improved by using an intermittent press system in which two or more machines for heating and cooling are arranged in parallel. In the case of a continuous type, the latter is used, and continuous processing can be easily performed, resulting in excellent continuous productivity.

[0114] In the present invention, the porous structure has micropores with an average pore diameter of 500 μm or less. The average pore diameter is preferably 200 μm or less, more preferably 10 μm to 150 μm, and even more preferably 30 μm to 100 μm. If it is smaller than this range, the weight reduction effect of the porous structure due to expansion may not be sufficient, and if it is larger than this range, the mechanical properties of the porous structure may be deteriorated. The average pore diameter here is the arithmetic average value of the lengths measured at 50 randomly selected points of the diameter 8 of the largest circle inscribed in the pores in the observation image, as shown in the schematic diagram of the surface of the porous structure in FIG. 4. The surface of the porous structure can be observed, for example, by a scanning electron microscope (SEM).

[0115] The density of the porous structure of the present invention is 0.01 g / cm 3 More than 0.8g / cm 3 It is preferable that the concentration is 0.1 g / cm or less. 3 More than 0.5g / cm 3 More preferably, it is 0.2 g / cm or less. 3 More than 0.3g / cm 3 It is more preferable that the density [g / cm3] is not more than 100%. If the density is smaller than this range, the reinforcing effect of the carbon fiber substrate as a porous structure may be insufficient, and if the density is larger than this range, the weight reduction effect of the porous structure may be insufficient. 3 ] is the mass of the porous structure sample [g] divided by the volume [cm] obtained from the perimeter of the sample 3 ] and can be calculated as the arithmetic mean of the results of measurements on five randomly selected samples.

[0116] The porous structure of the present invention preferably has a bending strength when heated to 140° C. that is 80% or more, and more preferably 90% or more and 100% or less, of the bending strength at 30° C. Such a value can be calculated as the "retention rate of bending strength when heated to 140° C." by the following formula. (Bending strength retention when heated to 140°C) = (Bending strength at 140°C [MPa]) / (Bending strength at 30°C [MPa]) x 100 [%] The bending strength is measured by a three-point bending test in accordance with JIS K7171 (2016) and is the arithmetic average value of five samples for each level. A thermostatic chamber is used to control the temperature of each level, and tests are performed in an atmosphere of 30°C and 140°C.

[0117] If the bending strength retention rate when heated to 140°C is within the above range, it is preferable because it can provide a porous structure with excellent heat resistance that can be used as a structural material even in a high-temperature environment such as 100°C.

[0118] The prepreg of the present invention may be used as a single layer, or may be used as a preform containing the prepreg as a lamination unit. The prepreg of the present invention has isotropic mechanical properties, and therefore does not require lamination that compensates for the direction of low strength, as in quasi-isotropic lamination, and has a feature of having a high degree of freedom in designing the lamination structure. This reduces the labor and time required for the lamination process, and reduces the economic burden.

[0119] [Preform] The preform of the present invention contains the prepreg of the present invention as a lamination unit. The number of laminations in the preform is preferably 2 to 50, more preferably 2 to 10.

[0120] The preform can be molded into a "fiber reinforced resin molding having voids" (hereinafter sometimes referred to as a "void body") or a "fiber reinforced resin molding without voids".

[0121] [Fiber-reinforced resin molding] The porous body, which is one embodiment of the fiber-reinforced resin molding of the present invention, is formed by heating and expanding the preform of the present invention, i.e., by welding the multiple laminated prepregs that constitute the preform of the present invention in an expanded state.

[0122] The method for producing the porous body includes a step (1-A) of melting the matrix resin (C) and a step (2-A) of solidifying the matrix resin (C) while controlling the thickness of the molded article. The prepreg of the present invention and the preform which is a laminate thereof have expandability in the out-of-plane direction, so that it is possible to mold a fiber-reinforced resin molded article having voids therein by molding the molded article while controlling the thickness.

[0123] The step (1-A) is a step in which the matrix resin (C) is plasticized by melting and selectively expanded in the out-of-plane direction by the springback force of the carbon fiber substrate.

[0124] The temperature in step (1-A) is preferably 250 to 450°C, more preferably 280 to 450°C, and even more preferably 300 to 400°C. The pressure in step (1-A) is preferably 0 to 5 MPa, and more preferably 0.1 to 3 MPa. The time in step (1-A) is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. When the conditions in step (1-A) are within the above ranges, the prepregs constituting the preform can be efficiently expanded, and the layers of the prepregs can be firmly bonded to each other.

[0125] The step (2-A) is a step of solidifying the matrix resin (C) to fix the carbon fiber base material in a springback state and form a porous body.

[0126] The temperature in step (2-A) is preferably 20 to 280°C, more preferably 20 to 250°C, even more preferably 20 to 200°C, and even more preferably 20 to 100°C. The pressure in step (2-A) is preferably 0 to 3 MPa, and more preferably 0 to 1 MPa. The time in step (2-A) is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. By setting the conditions in step (2-A) within the above ranges, the expansion coefficient can be controlled, and a porous body excellent in mechanical properties and light weight can be obtained.

[0127] In the step (2-A), the thickness of the molded product is controlled. As a method for controlling the thickness, a method of constraining the thickness using a metal plate or a method of directly controlling the thickness by adjusting the pressure are preferable from the viewpoint of ease of production.

[0128] A press molding machine can be suitably used as a facility for implementing such a method for producing a porous body. By forming an intermittent press system in which two or more machines for heating in step (1-A) and cooling in step (2-A) are arranged in parallel, productivity can be improved.

[0129] The porous body of the present invention is preferably produced by expanding the preform used for molding at an average expansion rate of 150 to 1,000% in the out-of-plane direction, more preferably 200 to 800%, and even more preferably 300 to 400%. This value is an expansion rate that can be calculated by (thickness of the porous body [mm]) / (thickness of the preform [mm])×100[%], and is the arithmetic average value of five samples. By setting the expansion rate in this range, a porous body with an excellent balance between light weight and mechanical properties can be obtained.

[0130] The density of the porous body in the present invention is 0.01 g / cm 3 More than 0.8g / cm 3 It is preferable that the concentration is 0.1 g / cm or less. 3 More than 0.5g / cm 3 More preferably, it is 0.3 g / cm or less. 3 More than 0.5g / cm 3 It is more preferable that the density is not more than 100 g / cm. By setting the density in this range, a porous body having an excellent balance between light weight and mechanical properties can be obtained. 3 ] is the mass of the porous sample [g] divided by the volume [cm] obtained from the circumference of the sample. 3 ] and is calculated as the arithmetic mean of the results of measurements on five randomly selected samples.

[0131] A void-free fiber-reinforced resin molded body, which is one embodiment of the fiber-reinforced resin molded body of the present invention, is molded by heating and pressurizing the preform of the present invention, i.e., by welding the multiple laminated prepregs that constitute the preform of the present invention in an unexpanded state.

[0132] The method for producing the void-free fiber-reinforced resin molding includes a step (1-B) of melting the matrix resin (C) and a step (2-B) of solidifying the matrix resin (C) while suppressing the springback force of the prepreg. The prepreg of the present invention and the preform which is a laminate thereof have expandability in the out-of-plane direction, but by molding at a high pressure which suppresses the springback force, it becomes possible to mold them into a fiber-reinforced resin molding in a non-expanded state.

[0133] The step (1-B) is a step of plasticizing the matrix resin (C) by melting it.

[0134] The temperature in step (1-B) is preferably 250 to 450°C, more preferably 280 to 450°C, and even more preferably 300 to 400°C. The pressure in step (1-B) is preferably 0 to 5 MPa, and more preferably 0.1 to 3 MPa. The time in step (1-B) is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. By setting the conditions in step (1-B) within the above ranges, it is possible to firmly bond the layers of the prepregs constituting the preform together.

[0135] The step (2-B) is a step in which the matrix resin (C) is solidified while suppressing the springback force of the prepregs, thereby forming a fiber-reinforced resin molding in which the laminated prepregs are integrated in a non-expanded state.

[0136] The temperature in step (2-B) is preferably 20 to 280°C, more preferably 20 to 250°C, even more preferably 20 to 200°C, and even more preferably 20 to 100°C. The pressure in step (2-B) is preferably 3 to 10 MPa, and more preferably 5 to 10 MPa. The time in step (2-B) is preferably 0.5 to 60 minutes, and more preferably 5 to 30 minutes. By setting the conditions in step (2-B) within the above ranges, the expansion coefficient can be controlled, and a fiber-reinforced resin molding having excellent mechanical properties can be obtained.

[0137] A press molding machine can be suitably used as a facility for implementing such a method for producing a fiber-reinforced resin molded product. By forming an intermittent press system in which two or more machines for heating in step (1-B) and cooling in step (2-B) are arranged in parallel, productivity can be improved.

[0138] The fiber reinforced resin molding containing no voids has an average expansion rate in the out-of-plane direction of preferably 90 to 150%, more preferably 100 to 120%, and even more preferably 100 to 105%, relative to the preform used for molding. This value is an expansion rate that can be calculated by (thickness of fiber reinforced resin molding [mm]) / (thickness of preform [mm])×100[%], and is the arithmetic average value of five samples. By setting the expansion rate within this range, a fiber reinforced resin molding with excellent mechanical properties can be obtained.

[0139] The density of the fiber reinforced resin molding, which does not include voids, is 0.8 g / cm 3 More than 2.0g / cm 3 It is preferable that the density is 1.2 g / cm or less. 3 More than 1.5g / cm 3 It is more preferable that the density is not more than 100 g / cm. By setting the density in this range, a fiber-reinforced resin molding having excellent mechanical properties can be obtained. 3 ] is the mass [g] of a fiber-reinforced plastic molding sample and the volume [cm] obtained from the outer periphery of the sample. 3 ] and is calculated as the arithmetic mean of the results of measurements on five randomly selected samples.

[0140] The fiber-reinforced resin molded body of the present invention is also preferably molded into a corrugated or honeycomb shape because of its excellent moldability. The corrugated shape is a wave-shaped shape with a plurality of curved surfaces or bends in succession. The honeycomb shape is a structure in which a fiber-reinforced resin molded body with a corrugated shape is used as a cell wall and a plurality of cell walls are joined at the convex parts of the curved surfaces or bends, and is an assembly of a large number of hollow columnar cells partitioned and formed by the cell walls. The shape of the bottom surface of the hollow columnar cells is preferably a triangular shape, a rectangular shape, a hexagonal shape, etc., and a hexagonal shape is more preferable from the viewpoint of mechanical properties.

[0141] [Sandwich structure] The sandwich structure of the present invention is formed by sandwiching the fiber-reinforced resin molded body of the present invention as a core material between a pair of skin materials to form an integrated structure. Since the fiber-reinforced resin molded body of the present invention has excellent heat resistance, when used as a core material, deformation is suppressed even in the heating and pressurizing process for joining the skin materials, and a sandwich structure with excellent dimensional accuracy is obtained. In addition, the sandwich structure of the present invention has an excellent balance between light weight and mechanical properties. There are no particular limitations on the skin material, and examples thereof include prepregs, fiber-reinforced resin molded bodies, metal plates, and resin plates.

[0142] [Aircraft parts] The prepreg of the present invention, and the porous structure, fiber-reinforced resin molded body, and sandwich structure using the prepreg can be suitably used for aircraft parts such as aircraft, artificial satellites, UAM (Urban air mobility), and drones, automobile parts, railroad car parts, ship parts, civil engineering and construction parts, electronic device housings, sporting goods, etc. In particular, the present invention can be suitably used for aircraft parts that require both heat resistance and reliability. EXAMPLES

[0143] The materials used in the examples and comparative examples are as follows.

[0144] [PEKK] The material used was a crystalline polyetherketoneketone ("Kepstan" (registered trademark) 6003 manufactured by Arkema Co., Ltd.) having a melting point of 305°C and a glass transition temperature of 157°C measured in accordance with JIS K7121 (2012).

[0145] [PEEK] The material used was a crystalline polyether ether ketone (Victrex PEEK381G, manufactured by Victrex), which has a melting point of 334°C and a glass transition temperature of 143°C measured in accordance with JIS K7121 (2012).

[0146] [Carbon fiber nonwoven fabric] (CF-1) A carbon fiber bundle with a total of 12,000 single fibers was obtained by spinning, calcining, and surface oxidation of a copolymer mainly composed of polyacrylonitrile. The properties of this carbon fiber bundle were a tensile modulus of 220 GPa measured in accordance with JIS R7608 (2007), and a circular cross section with a single fiber diameter of 7 μm.

[0147] The carbon fiber bundle was cut into 6 mm lengths with a cartridge cutter to obtain chopped carbon fiber. A dispersion liquid with a concentration of 0.1% by mass was prepared from water and a surfactant (Polyoxyethylene lauryl ether (trade name) manufactured by Nacalai Tesques Co., Ltd.), and a carbon fiber nonwoven fabric was produced using this dispersion liquid and chopped carbon fiber. The manufacturing device is equipped with a cylindrical container with a diameter of 1000 mm having an opening and closing cock at the bottom of the container as a dispersion tank, and a linear transport section (inclined angle 30°) connecting the dispersion tank and the papermaking tank. A stirrer is attached to the opening on the top surface of the dispersion tank, and chopped carbon fiber and dispersion liquid (dispersion medium) can be charged from the opening. The papermaking tank is equipped with a mesh conveyor with a papermaking surface with a width of 500 mm at the bottom, and a conveyor capable of transporting carbon fiber nonwoven fabric (papermaking base material) is connected to the mesh conveyor. Papermaking was performed with a carbon fiber concentration in the dispersion liquid of 0.05% by mass. The papermaking conditions were adjusted so that the carbon fibers were dispersed in the form of single threads and were isotropically dispersed in the in-plane direction. The paper-made substrate was dried in a drying oven at 200°C for 30 minutes, and had a thickness of 1.68 mm and a basis weight of 100 g / m 2 The carbon fiber nonwoven fabric was named CF-1. The thermal mass loss rate of CF-1 was 0% by mass.

[0148] (CF-2) Carbon fiber nonwoven fabric CF-2 was obtained in the same manner as CF-1, except that the length cut by the cartridge cutter was changed to 3 mm. CF-2, basis weight 100 g / m 2 , thickness 0.85 mm, and thermal mass reduction rate was 0 mass%.

[0149] [Polyamic acid salts (PAMC)] A polyamic acid was prepared by reacting 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) as a tetracarboxylic dianhydride with m-phenylenediamine as a diamine compound in a molar ratio of 1:1, and further neutralizing the polyamic acid salt with N,N-dimethyl-2-aminoethanol as a trialkylamine. This compound was diluted with purified water and used as an aqueous solution with a concentration of 10% by mass.

[0150] [Polyvinyl alcohol (PVA)] Polyvinyl alcohol (polyvinyl alcohol 500, fully saponified type, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used. This compound was diluted with purified water and used as an aqueous solution with a concentration of 10% by mass.

[0151] [Polyaminoamide (PAA)] Polyaminoamide was synthesized by reacting 3,3'-diaminobenzidine as an aromatic tetraamine compound with isophthalic acid as a dicarboxylic acid derivative in a molar ratio of 1:1. The polyaminoamidation reaction was carried out in NMP solvent. This was used as an NMP solution with a concentration of 10% by mass.

[0152] The methods for evaluating the structure, physical properties, etc. in each of the examples and comparative examples are as follows.

[0153] [Porosity of carbon fiber substrate] The porosity of the carbon fiber substrate is the volume ratio of pores in the carbon fiber substrate, and was calculated from the thickness and basis weight of the carbon fiber substrate and the density of its constituent components using the following formula. The density here can be calculated according to JIS K0061 (2001). (Porosity of carbon fiber substrate) = {(Thickness of carbon fiber substrate [m]) - (Weight per unit area of ​​carbon fiber substrate [g / m 2 ]) / (density of the carbon fiber substrate components [g / m 3 ])} / (carbon fiber substrate thickness [m]) × 100[%].

[0154] [Coverage rate of resin composition (B)] The coverage of the resin composition (B) with respect to the carbon fiber substrate can be determined from the surface oxygen concentration (O / C) by X-ray photoelectron spectroscopy using the following formula. Coverage rate (%) of resin composition (B)={(O / C of carbon fiber (A))−(O / C of carbon fiber base material)} / {(O / C of carbon fiber (A))−(O / C of resin composition (B))}×100. The samples used here included the carbon fiber substrate, as well as a sample of carbon fiber (A) alone before the resin composition (B) was applied, and a sample of the resin composition (B) alone formed into a 0.1 mm thick film.

[0155] The surface oxygen concentration (O / C) of each sample was determined according to the following procedure. First, the sample was cut to a size of 20 mm in width and 20 mm in length, and placed on a sample support. 1,2 The sample chamber was filled with 1×10 -8 Torr and the photoelectron escape angle was 45°. 1s The binding energy of the main peak (peak top) of was set to 284.6 eV. The number of carbon atoms was calculated by drawing a straight line baseline in the range of 282 to 296 eV. 1s The number of oxygen atoms was calculated by drawing a straight line baseline in the range of 528 to 540 eV. 1s The surface oxygen concentration (O / C) was calculated as an atomic ratio obtained by dividing the number of oxygen atoms by the number of carbon atoms. PHI Quantera II manufactured by ULVAC-PHI, Inc. was used as the X-ray photoelectron spectroscopy device.

[0156] [Tensile strength of carbon fiber substrate] A test piece with a width of 50 mm and a length of 150 mm was cut out from the carbon fiber substrate, and a 50 mm length was supported on each end, and the remaining central portion with a length of 50 mm was subjected to a tensile test at a test speed of 3 m / min under a room temperature environment. The obtained maximum point load was divided by the width of the test piece to obtain the tensile strength [N / cm], and the arithmetic average value of five samples for each level was used. The tensile test of the carbon fiber substrate was performed for each level in which heat treatment was performed at 300 ° C. for 5 minutes and for each level in which heat treatment was not performed. Specifically, the tensile strength of the carbon fiber substrate in one direction was determined as the tensile strength of the level in which the heat treatment was performed (Ta) and the tensile strength of the level in which the heat treatment was not performed (Tao). Furthermore, the tensile strength of the level in which the heat treatment was performed (Tb) and the tensile strength of the level in which the heat treatment was not performed (Tbo) were obtained as the tensile strength in the direction perpendicular to the one direction. The change in tensile strength before and after heat treatment was calculated from the Ta and Tao by the following formula. (tensile strength after heat treatment relative to tensile strength before heating) = Ta / Tao x 100 [%] The tensile strength ratio after heat treatment (Ta / Tb) was calculated by dividing the Ta by the Tb, and the tensile strength ratio before heat treatment (Tao / Tbo) was calculated by dividing the Tao by the Tbo.

[0157] [Thermogravimetric mass spectrometry (TG-MS)] The mass and components of the gas generated during heating were analyzed using a thermogravimetric-mass spectrometer (TG-MS). The thermal mass reduction rate of the resin composition (B) was calculated using thermogravimetric analysis (TG) according to the following formula (1). Thermal mass reduction rate (mass%)=[(W1-W2) / W1]×100...(1) Here, W1 is the mass (mg) of the sample after it has been heated from room temperature to 100°C at 10°C / min and then kept isothermal for 30 minutes, and W2 is the mass (mg) of the sample when it has been heated again at 10°C / min and reached 250°C.

[0158] A GC / MSQP2010 (Shimadzu Corporation) was used to analyze the evolved gas by mass spectrometry (MS). The presence or absence of moisture in the evolved gas allowed us to determine whether it was a dehydration condensation reaction or another type of thermal decomposition reaction.

[0159] [Number average fiber length of carbon fiber (A)] 400 carbon fibers were randomly extracted from the carbon fiber nonwoven fabric used as carbon fiber (A), and their lengths were measured using an optical microscope, and the number average fiber length (Ln) was calculated using the following formula. Number average fiber length (Ln) = (ΣLi) / Nf Li: Measured fiber length (i = 1, 2, 3, . . . , n) Nf: The total number of fibers whose fiber length was measured.

[0160] [Average out-of-plane expansion rate of prepreg] The out-of-plane expansion coefficient of the prepreg was calculated from the thickness of the prepreg and the thickness of the porous structure expanded by heating the prepreg for 5 minutes at a temperature obtained by adding 20°C to the melting point of the matrix resin (C), using the following formula. Expansion rate of prepreg in the out-of-plane direction (%)=[thickness of porous structure (mm) / thickness of prepreg (mm)]×100.

[0161] The average expansion coefficient in the out-of-plane direction of the prepreg was calculated as the arithmetic mean value of the expansion coefficients in the out-of-plane direction of the prepreg measured at five randomly selected points.

[0162] [Density of prepreg or porous structure] Density of prepreg or porous structure (g / cm 3 ) is the mass of the sample (g) multiplied by the volume (cm) calculated from the circumference of the sample. 3 ) and was calculated as the arithmetic mean of the results of measurements on five randomly selected samples.

[0163] [Average pore diameter of prepreg or porous structure] The average pore diameter was measured from the diameter of the largest circle inscribed in the pore as shown in the schematic diagram of the surface of the porous structure in Figure 4. The surface of the prepreg or porous structure was observed with a scanning electron microscope (SEM), and the largest circle inscribed in the pore was drawn in the observed image, and the diameter of this circle was calculated as the pore diameter. The average pore diameter was calculated as the arithmetic average of the lengths measured at 50 randomly selected points.

[0164] [Bending strength of porous structures] The bending strength was measured by a three-point bending test in accordance with JIS K7171 (2016) using an "Instron" (registered trademark) 5565 universal material testing machine (manufactured by Instron Japan Co., Ltd.). The environmental temperature during the measurement was adjusted using a thermostatic chamber, and bending strength was measured under temperature environments of 30°C and 140°C. The arithmetic average value of five samples at each level was used for bending strength. Furthermore, the bending strength retention rate when heated to 140°C was calculated using the following formula. (Retention rate of bending strength when heated to 140°C) = (Bending strength at 140°C [MPa]) / (Bending strength at 30°C [MPa]) × 100 [%].

[0165] [Prepreg tensile strength] The tensile strength of the prepreg was measured by a tensile test in accordance with ISO527-3 (1995) using an "Instron" (registered trademark) 5565 universal material testing machine (manufactured by Instron Japan Co., Ltd.). Test pieces were cut out in four directions: 45°, 90°, and -45°, with an arbitrary direction being the 0° direction, and the number of measurements for each direction was n=5. For all the measured values ​​(n=20), the maximum and minimum values, and the average tensile strength, which is the average of these, were calculated. Furthermore, the standard deviation was calculated, and the CV value (coefficient of variation) calculated by the following formula was used as the variation in tensile strength. Variation in tensile strength (%) = [(standard deviation of tensile strength) / (average value of tensile strength)] x 100 Furthermore, the maximum tensile strength was divided by the minimum tensile strength to determine the ratio of the maximum tensile strength to the minimum tensile strength.

[0166] [Thickness and variation of prepreg at 400℃] A hot plate equipped with a laser displacement meter and capable of non-contact thickness measurement was used. The prepreg was placed on the hot plate, heated to 400°C, and held for 5 minutes before the thickness of the prepreg was read. The thickness of the prepreg was read at 20 randomly selected locations, and the average value was taken as the thickness at 400°C. The standard deviation was then calculated, and the CV value (coefficient of variation) calculated using the following formula was taken as the thickness variation at 400°C. Thickness variation at 400°C (%) = [(standard deviation of thickness at 400°C) / (average thickness at 400°C)] × 100.

[0167] [Coverage rate of binder resin composition (D)] The coverage of the binder resin composition (D) with respect to the carbon fiber substrate can be determined from the surface oxygen concentration (O / C) by X-ray photoelectron spectroscopy using the following formula. Coverage rate (%) of binder resin composition (D)={(O / C of carbon fiber (A))−(O / C of carbon fiber base material)} / {(O / C of carbon fiber (A))−(O / C of binder resin composition (D))}×100. The samples used here included the carbon fiber substrate, as well as the carbon fiber (A) alone before the binder resin composition (D) was applied, and a sample of a precursor of the binder resin composition (D) alone that was heat-treated under the same conditions as those used for producing the carbon fiber substrate and formed into a 0.1 mm thick film.

[0168] The surface oxygen concentration (O / C) of each sample was determined in the same manner as in the above-mentioned "coverage rate of resin composition (B)".

[0169] [Thermal mass reduction rate] Using thermogravimetric analysis (TG), the thermal mass reduction rates of the carbon fiber (A) and the binder resin composition (D) were calculated according to the following formula. Thermal mass reduction rate (mass%) = (M1-M2) / M1×100 Here, M1 is the mass (mg) of the sample after being isothermally held at 30°C for 10 minutes, and M2 is the mass (mg) of the sample when it is subsequently heated at 10°C / min to 400°C. For the carbon fiber (A) sample, carbon fiber (A) before the application of binder resin composition (D) was used. For the binder resin composition (D) sample, a precursor of the binder resin composition (D) was used which was heat-treated alone under the same conditions as those for producing the carbon fiber substrate.

[0170] [Glass transition temperature of binder resin composition (D)] The glass transition temperature of the binder resin composition (D) was measured by differential scanning calorimetry (DSC) in accordance with JIS K7121 (2012). For the sample of the binder resin composition (D), the precursor of the binder resin composition (D) was heat-treated alone under the same conditions as those for producing the carbon fiber substrate, and used for evaluation.

[0171] [Fiber length of carbon fiber (A)] 400 carbon fibers (A) were randomly extracted from the carbon fiber substrate, and their lengths were measured using an optical microscope to calculate the number average fiber length (Ln) using the following formula. Number average fiber length (Ln) = (ΣLi) / Nf Li: Measured fiber length (i = 1, 2, 3, . . . , n) Nf: The total number of fibers whose length was measured.

[0172] [Mass ratio of carbon fiber (A) in carbon fiber substrate] The mass proportion of the carbon fiber (A) in the carbon fiber substrate was calculated from the mass of the carbon fiber substrate and the mass of the carbon fiber (A) before application of the precursor of the binder resin composition according to the following formula. Mass ratio (%) of carbon fiber (A) in the carbon fiber base material = (mass (g) of carbon fiber (A)) / (mass (g) of carbon fiber base material) × 100.

[0173] [Mass ratio of carbon fiber (A) in prepreg] The mass proportion of the carbon fiber (A) in the prepreg was calculated from the mass of the prepreg and the mass of the carbon fiber (A) used therein by the following formula. Mass proportion (%) of carbon fiber (A) in prepreg = (mass (g) of carbon fiber (A)) / (mass (g) of prepreg) × 100.

[0174] [Expansion rate of fiber-reinforced plastic body or porous body] The expansion coefficient of the fiber reinforced resin molding or the porous body was calculated from the thickness of the preform used for molding and the thickness of the fiber reinforced resin molding or the porous body obtained by molding this preform, according to the following formula. (Expansion rate) = (thickness of fiber-reinforced resin molding or porous body [mm]) / (thickness of preform [mm]) x 100 [%] The thickness of the fiber-reinforced resin molding or the porous body and the thickness of the preform were each calculated as the average value of thicknesses measured at five randomly selected points.

[0175] [Density of fiber-reinforced plastic body or porous body] Density of fiber-reinforced plastic body or void body [g / cm 3 ] is the mass of the sample [g] multiplied by the volume [cm] obtained from the circumference of the sample 3 ] and was calculated as the average of the results measured on five randomly selected samples.

[0176] Hereinafter, the carbon fiber substrates, prepregs, and porous structures produced in the examples and comparative examples of the present invention will be described.

[0177] [Example 1] (Carbon fiber (A)) As the carbon fiber (A), carbon fiber nonwoven fabric CF-1 cut to a size of 300 mm x 300 mm was used.

[0178] (Resin composition (B)) PAMC was used as the resin composition (B), which was then re-diluted to 1% by mass to give an aqueous solution.

[0179] (Carbon fiber substrate) The carbon fiber nonwoven fabric was immersed in the aqueous solution, and then taken out and dried at 100° C. for 1 hour to obtain a carbon fiber substrate.

[0180] The difference between the mass of the obtained carbon fiber substrate and the mass of the carbon fiber (A) used was the amount of the resin composition (B) attached, and this amount was 5 parts by mass per 100 parts by mass of the carbon fiber (A). The coverage of the resin composition (B) was 100%. The results of each evaluation are shown in Table 1.

[0181] [Example 2] The concentration of the resin composition (B) when it was re-diluted was set to 3 mass%, so that the adhesion amount of the resin composition (B) was set to 10 parts by mass. A carbon fiber base material was obtained in the same manner as in Example 1 except for the above.

[0182] The coverage of the resin composition (B) on the carbon fiber substrate was 100%. The results of each evaluation are shown in Table 1.

[0183] [Comparative Example 1] A carbon fiber substrate was obtained in the same manner as in Example 1, except that PVA was used as the resin composition (B). The coverage of this sample with PVA was 100%. The results of each evaluation are shown in Table 1.

[0184] [Comparative Example 2] The carbon fiber nonwoven fabric CF-1 was directly used for evaluation without using the resin composition (B). The results of each evaluation are shown in Table 1.

[0185] [Example 3] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as that used in Example 1 was used.

[0186] (Resin composition (B)) PAA was used as the resin composition (B), which was re-diluted to 1% by mass to prepare a solution in N-methyl-2-pyrrolidone (NMP).

[0187] (Carbon fiber substrate) The carbon fiber nonwoven fabric was immersed in the NMP solution. The carbon fiber nonwoven fabric was then removed from the NMP solution and immersed in water to cause the resin composition (B) to precipitate on the surface of the carbon fiber (A). The carbon fiber nonwoven fabric was then removed from the water and dried by heating at 100°C for 1 hour to obtain a carbon fiber substrate.

[0188] The amount of the resin composition (B) attached was 5 parts by mass per 100 parts by mass of the carbon fibers (A). The coverage of the resin composition (B) was 100%. The results of each evaluation are shown in Table 1.

[0189] [Table 1]

[0190] [Example 1-2] (Carbon fiber substrate) The carbon fiber substrate obtained in Example 1 was used.

[0191] (Matrix resin (C)) Matrix resin (C) is made of PEKK with a basis weight of 150 g / m 2 Two pieces of 300mm x 300mm resin film were used.

[0192] (Prepreg) The carbon fiber substrate and the resin film were laminated in the order of resin film / carbon fiber substrate / resin film. This laminate was further sandwiched between a 400mm x 400mm SUS flat tooling plate and heated and pressed while being transported. The pressing process was first performed by heating at a temperature of 350°C and a pressure of 10 MPa for 10 minutes. Then, the laminate was cooled and solidified at a temperature of 150°C and a pressure of 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm.

[0193] The appearance of the obtained prepreg showed no tears in the carbon fiber substrate, and no molding defects were confirmed. The results of each evaluation are shown in Table 2.

[0194] [Example 2-2] A prepreg was produced in the same manner as in Example 1-2, except that the carbon fiber substrate obtained in Example 2 was used as the carbon fiber substrate. No breaks were observed in the carbon fiber substrate in the appearance of the obtained prepreg, and no molding defects were confirmed. The results of each evaluation are shown in Table 2.

[0195] [Comparative Example 1-2] A prepreg was produced in the same manner as in Example 1-2, except that the carbon fiber substrate obtained in Comparative Example 1 was used.

[0196] The obtained prepreg had some molding defects due to breaks in the carbon fiber base material. Places with few breaks were selected and the results of each evaluation were shown in Table 2.

[0197] [Comparative Example 2-2] A prepreg was produced in the same manner as in Example 1-2, except that the carbon fiber nonwoven fabric CF-1 evaluated in Comparative Example 2 was used as the carbon fiber substrate. However, the carbon fiber substrate was broken and fragmented during press molding, and no prepreg was obtained. The results are shown in Table 2.

[0198] [Example 3-2] A prepreg was produced in the same manner as in Example 1-2, except that the carbon fiber substrate obtained in Example 3 was used.

[0199] The appearance of the obtained prepreg showed no tears in the carbon fiber substrate, and no molding defects were confirmed. The results of each evaluation are shown in Table 2.

[0200] [Table 2]

[0201] [Examples 1-3] (Prepreg) The prepreg used was that obtained in Example 1-2.

[0202] (Porous structure) Four sheets of the above prepreg, each 300 mm x 300 mm in size and 0.28 mm in thickness, were laminated and used, and the laminate was sandwiched between 400 mm x 400 mm SUS flat tooling plates together with a 4 mm thick SUS spacer to control the expansion ratio, and hot press was performed. In the pressing process, the laminate was first heated at a temperature of 350°C and a pressure of 1 MPa for 10 minutes, and then cooled and solidified at a temperature of 150°C and a pressure of 1 MPa for 10 minutes. The expansion ratio was controlled to 360%, and the thickness of the resulting porous structure was 4 mm. The results of each evaluation are shown in Table 3.

[0203] [Comparative Example 1-3] A porous structure was produced in the same manner as in Example 1-3, except that the prepreg obtained in Comparative Example 1-2 was used. The results of each evaluation are shown in Table 3.

[0204] [Example 3-3] A porous structure was produced in the same manner as in Example 1-3, except that the prepreg obtained in Example 3-2 was used. The results of each evaluation are shown in Table 3.

[0205] [Examples 1-4] (Prepreg) The prepreg used was that obtained in Example 1-2.

[0206] (Porous structure) Three sheets of the above prepreg, each 300 mm x 300 mm in size and 0.28 mm thick, were laminated and sandwiched between 400 mm x 400 mm SUS flat tooling plates together with a 4 mm thick SUS spacer to control the expansion ratio, and hot press was performed. The pressing process was first performed by heating at a temperature of 350°C and a pressure of 1 MPa for 10 minutes, and then cooling and solidifying at a temperature of 150°C and a pressure of 1 MPa for 10 minutes. The expansion ratio was controlled to 480%, and the thickness of the resulting porous structure was 4 mm. The results of each evaluation are shown in Table 3.

[0207] [Comparative Example 1-4] A porous structure was produced in the same manner as in Example 1-4, except that the prepreg obtained in Comparative Example 1-2 was used. However, the expansion ratio in the out-of-plane direction was insufficient, and the desired porous structure having a thickness of 4 mm was not obtained. The results of each evaluation are shown in Table 3.

[0208] [Example 3-4] A porous structure was produced in the same manner as in Example 1-4, except that the prepreg obtained in Example 3-2 was used. The results of each evaluation are shown in Table 3.

[0209] [Table 3]

[0210] [Example 4] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as that used in Example 1 was used.

[0211] (Resin composition (B)) As a solution of the resin composition (B) which is a precursor of the binder resin composition (D), a 1% by mass aqueous solution of PAMC similar to that used in Example 1 was used.

[0212] (Carbon fiber substrate) The carbon fiber nonwoven fabric was immersed in the aqueous solution. The carbon fiber nonwoven fabric was then taken out and dried by heating at 200°C for 1 hour to imidize the PAMC, and a carbon fiber substrate in which the polyetherimide resin was attached to the carbon fiber (A) was obtained as the binder resin composition (D). The fiber length of the reinforcing fiber (A) was 6 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 220°C, and the thermal mass loss rate at 400°C was 1% by mass.

[0213] (Matrix resin (C)) Matrix resin (C) is made of PEKK with a basis weight of 150 g / m 2 Two pieces of 300mm x 300mm resin film were used.

[0214] (Prepreg) The carbon fiber substrate and resin film were laminated in the order of resin film / carbon fiber substrate / resin film. This laminate was further sandwiched between a 400mm x 400mm SUS flat tooling plate and heated and pressed while being transported. In the pressing process, the laminate was first heated at a temperature of 350°C and a pressure of 10 MPa for 10 minutes, and then cooled and solidified at a temperature of 30°C and a pressure of 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm. The results of each evaluation are shown in Table 4.

[0215] [Example 5] A prepreg was obtained in the same manner as in Example 4, except that the concentration of the aqueous solution of PAMC was 3% by mass. The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 220°C, and the thermal mass loss rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0216] [Example 6] A prepreg was obtained in the same manner as in Example 4, except that CF-2 was used as the carbon fiber (A). The fiber length of the carbon fiber (A) was 3 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 220°C, and the thermal mass loss rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0217] [Example 7] A prepreg was obtained in the same manner as in Example 4, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 220°C, and the thermal mass loss rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0218] [Example 8] (Carbon fiber (A)) The same cut carbon fiber nonwoven fabric CF-1 as that used in Example 1 was used.

[0219] (Resin composition (B)) PAA was used as the resin composition (B) which is a precursor of the binder resin composition (D), and was diluted again to 1% by mass to prepare an NMP solution.

[0220] (Carbon fiber substrate) The carbon fiber nonwoven fabric was immersed in the NMP solution. The carbon fiber nonwoven fabric was then taken out and dried by heating at 200°C for 1 hour to convert the PAA into imidazole, and a carbon fiber substrate in which the polybenzimidazole resin was attached to the carbon fiber (A) was obtained as the binder resin composition (D). The coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 425°C, and the thermal mass loss rate at 400°C was 1% by mass.

[0221] (Matrix resin (C)) Matrix resin (C) is made of PEKK with a basis weight of 150 g / m 2 Two pieces of 300mm x 300mm resin film were used.

[0222] (Prepreg) The carbon fiber substrate and resin film were laminated in the order of resin film / carbon fiber substrate / resin film. This laminate was further sandwiched between a 400mm x 400mm SUS flat tooling plate and heated and pressed while being transported. In the pressing process, the laminate was first heated at a temperature of 350°C and a pressure of 10 MPa for 10 minutes, and then cooled and solidified at a temperature of 30°C and a pressure of 10 MPa for 10 minutes to produce a prepreg with a thickness of 0.28 mm. The results of each evaluation are shown in Table 4.

[0223] [Example 9] A prepreg was obtained in the same manner as in Example 8, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 425°C, and the thermal mass reduction rate at 400°C was 1% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0224] [Comparative Example 3] A prepreg was obtained in the same manner as in Example 4, except that PVA was used as the resin composition (B). The coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 60°C, and the thermal mass loss rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0225] [Comparative Example 4] A prepreg was obtained in the same manner as in Comparative Example 3, except that CF-2 was used as the carbon fiber (A). The fiber length of the carbon fiber (A) was 3 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 60°C, and the thermal mass loss rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0226] [Comparative Example 5] A prepreg was obtained in the same manner as in Comparative Example 3, except that PEEK was used as the matrix resin (C). The fiber length of the carbon fiber (A) was 6 mm, and the coverage of the binder resin composition (D) was 100%. The glass transition temperature of the binder resin composition (D) was 60°C, and the thermal mass reduction rate at 400°C was 60% by mass. The thickness of the prepreg was 0.28 mm. The results of each evaluation are shown in Table 4.

[0227] [Table 4]

[0228] [Example 10] (Prepreg) The prepreg obtained in Example 4 was used.

[0229] (Preform) A preform was prepared by laminating four sheets of the above prepreg, each measuring 300 mm x 300 mm and having a thickness of 0.28 mm.

[0230] (Fiber reinforced plastic molding) The preform was sandwiched between 400 mm x 400 mm SUS tooling plates and subjected to hot and pressurized pressing. In the pressing process, the preform was first heated at 350°C and 3 MPa for 10 minutes, and then cooled and solidified at 30°C and 5 MPa for 10 minutes. The obtained fiber-reinforced resin molded body had a thickness of 1.12 mm, an expansion rate of 100%, and a density of 1.4 g / cm. 3 There was a molded product with no voids.

[0231] [Example 11] (Prepreg) The prepreg obtained in Example 4 was used.

[0232] (Preform) A preform was prepared by laminating four sheets of the above prepreg, each measuring 300 mm x 300 mm and having a thickness of 0.28 mm.

[0233] (Fiber reinforced plastic molding) The preform was sandwiched between 400mm x 400mm SUS flat tooling plates together with a 4mm thick SUS spacer to control the expansion rate, and hot press was performed. In the pressing process, the preform was first heated at 350°C and 3MPa for 10 minutes, and then cooled and solidified at 30°C and 1MPa for 10 minutes, yielding a fiber-reinforced resin molding with voids (void body). The resulting porous body had a thickness of 4mm, an expansion rate of 360%, and a density of 0.4g / cm. 3 The bending strength at 30°C was 53 MPa, and the bending strength at 140°C was 48 MPa, and the retention rate of bending strength when heated to 140°C was 91%.

[0234] [Example 12] A porous body was molded in the same manner as in Example 11, except that the prepreg obtained in Example 8 was used. The thickness of the obtained porous body was 4 mm, the expansion rate was 360%, and the density was 0.4 g / cm. 3 The bending strength at 30°C was 53 MPa, the bending strength at 140°C was 50 MPa, and the retention of bending strength when heated to 140°C was 94%.

[0235] [Comparative Example 6] A porous body was molded in the same manner as in Example 11, except that the prepreg obtained in Comparative Example 3 was used. The thickness of the obtained porous body was 4 mm, the expansion rate was 360%, and the density was 0.4 g / cm. 3 The bending strength at 30°C was 47 MPa, and the bending strength at 140°C was 32 MPa, and the retention of bending strength when heated to 140°C was 68%.

[0236] [Example 13] (Prepreg) The prepreg used was that obtained in Example 1-2.

[0237] (Preform) A preform was prepared by laminating two of the above prepregs, each having a length of 300 mm, a width of 300 mm, and a thickness of 0.28 mm.

[0238] (Fiber reinforced plastic molding) The above preform was sandwiched between a pair of molds having a concave-convex shape, and was subjected to a hot press under pressure while controlling the expansion rate. In the pressing process, the preform was first heated at a temperature of 350°C and a pressure of 3 MPa for 10 minutes, and then cooled and solidified at a temperature of 30°C and a pressure of 1 MPa for 10 minutes. The obtained fiber-reinforced resin molded product had a thickness of 2 mm, an expansion rate of 360%, and a density of 0.4 g / cm. 3 Furthermore, the obtained fiber-reinforced resin molding was molded into a corrugated shape by transferring the uneven shape of the mold, and when viewed from one surface, the fiber-reinforced resin molding, each 10 mm long, had a corrugated shape in which it was repeatedly bent at bending angles of 120° / 120° / 240° / 240° in the longitudinal direction.

[0239] [Example 14] The corrugated fiber-reinforced resin molded product obtained in Example 13 was cut into strips with a width of 10 mm. These strip-shaped cut pieces were stacked so that the convex parts of adjacent cut pieces faced each other, thereby bonding the convex parts to each other. An epoxy resin adhesive (120°C curing type) was applied to the bonding surfaces of the convex parts, and the convex parts were bonded by heating and pressurizing at a temperature of 120°C and a pressure of 0.1 MPa for 1 hour, thereby obtaining a fiber-reinforced resin molded product with a honeycomb structure in which regular hexagonal columnar hollow cells with sides of 10 mm were arranged.

[0240] [Example 15] (Core material) The honeycomb-shaped fiber-reinforced resin molding obtained in Example 14 was used as a core material.

[0241] (Skin material) Two sheets of the prepreg obtained in Example 1-2 were used as a pair of skin materials.

[0242] (Sandwich structure) An epoxy resin adhesive (curing type at 120°C) was applied to the opposing surfaces of the two skin materials. The core material was sandwiched between these skin materials and heated and pressurized at a temperature of 120°C and a pressure of 0.1 MPa for one hour to integrate them into a sandwich structure. [Explanation of symbols]

[0243] 1 Carbon fiber (A) 2. Resin composition (B) or binder resin composition 3 Vacancies 4. Carbon fiber (A) coated with resin composition (B) or binder resin composition 5 Matrix resin (C) 6 In-plane directions 7 Out-of-plane direction 8 Pore diameter (diameter of the inscribed circle of the hole)< / oda> < / mpd> < / ppd> < / ntda> < / btda> < / pmda>

Claims

1. A prepreg obtained by impregnating a substrate made of carbon fibers (A) and a binder resin composition (D) with a matrix resin (C), The carbon fibers (A) are discontinuous fibers forming a nonwoven fabric, The glass transition temperature of the binder resin composition (D) is 120°C or higher and 450°C or lower, The melting point of the matrix resin (C) is 250°C or higher and 400°C or lower, the binder resin composition (D) is present at the interface between the carbon fiber (A) and the matrix resin (C); the binder resin composition (D) is a composition containing a polymer or a derivative thereof having at least one structure selected from an etherimide skeleton, a benzoxazole skeleton, a benzimidazole skeleton, a benzoxazine skeleton, and a cyanate ester skeleton, A prepreg that satisfies the following conditions (i) and (ii): (i) The minimum tensile strength is 100 MPa or more. (ii) The thickness variation at 400°C is 10% or less.

2. 2. The prepreg according to claim 1, wherein the binder resin composition (D) contains at least one resin selected from the group consisting of a phenolic resin, a urea resin, a melamine resin, a polyimide resin, a polybenzoxazole resin, a polybenzimidazole resin, a bismaleimide resin, a benzoxazine resin, and a cyanate ester resin.

3. The prepreg according to claim 1, wherein the binder resin composition (D) has a thermal mass loss rate at 400°C of 5% by mass or less.

4. The prepreg according to any one of claims 1 to 3, wherein the matrix resin (C) contains at least one selected from polyether ketone, polyether ether ketone, and polyether ketone ketone as a main component.

5. The prepreg according to any one of claims 1 to 3, wherein the amount of the matrix resin (C) relative to 100 parts by mass of the carbon fiber (A) is 5 to 1,000 parts by mass.

6. The prepreg according to any one of claims 1 to 3, wherein the carbon fibers (A) contain 50 mass% or more of fibers having a fiber length of 2 mm or more and 10 mm or less.

7. The prepreg according to any one of claims 1 to 3, having a thickness at 400 ° C. of 0.1 mm or more and 5 mm or less.

8. The prepreg according to any one of claims 1 to 3, wherein the in-plane tensile strength variation is 20% or less.

9. The prepreg according to any one of claims 1 to 3, wherein the maximum tensile strength is 260 MPa or more.

10. The prepreg according to any one of claims 1 to 3, wherein the ratio of the maximum tensile strength to the minimum tensile strength is 1 to 3.

11. The prepreg according to any one of claims 1 to 3, wherein the mass proportion of the carbon fiber (A) in the prepreg is 1% or more and 50% or less.

12. 4. The prepreg according to claim 1, wherein the matrix resin (C) expands at an average expansion rate of 150 to 1,000% in the out-of-plane direction when melted.

13. A porous structure formed from the prepreg according to any one of claims 1 to 3.

14. 14. The porous structure according to claim 13, wherein the bending strength when heated to 140°C is 80% or more of the bending strength at 30°C.

15. A method for producing the porous structure according to claim 13, comprising a first step of melting and expanding the matrix resin (C), and a second step of solidifying the matrix resin (C).

16. A preform comprising the prepreg according to claim 1 as a lamination unit.

17. A fiber-reinforced resin molded article obtained by molding the preform according to claim 16 by heating and pressurizing it.

18. A fiber-reinforced resin molding having voids, produced by molding the preform according to claim 16 through thermal expansion.

19. The fiber-reinforced resin molding according to claim 17 or 18, which has a corrugated shape or a honeycomb shape.

20. A sandwich structure comprising the fiber-reinforced resin molded article according to claim 19 as a core material, sandwiched between a pair of skin materials and integrated with each other.

21. An aircraft member comprising the prepreg according to any one of claims 1 to 3 or the fiber-reinforced resin molding according to claim 17 or 18.

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