Prepreg, preform, or fiber-reinforced composite material, and method for producing the same

Abstract

To provide a prepreg that uses a thermosetting resin as a matrix resin and that can easily mold a fiber-reinforced composite material having voids.SOLUTION: In a prepreg containing reinforcing fibers and a thermosetting resin, the reinforcing fibers are in a form of a nonwoven fabric, and when heated from 25°C to 120°C, the thermosetting resin softens and expands 1.5 to 100 times in an out-of-plane direction.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a prepreg in which a thermosetting resin is used as a matrix resin and a fiber-reinforced composite material having pores can be easily formed.

Background Art

[0002] A fiber-reinforced composite material using reinforcing fibers as a reinforcing material and impregnating the pores with a matrix resin is lightweight and has excellent mechanical properties such as strength and elastic modulus. It is used in many fields such as aerospace, automotive, railway vehicles, ships, electronic devices, industrial machinery, sports goods, and civil engineering.

[0003] Among them, by using reinforcing fibers in the form of a nonwoven fabric, it is possible to obtain a fiber-reinforced composite material having pores by expanding it by springback. Since such a fiber-reinforced composite material has a structure in which the pores are reinforced by the reinforcing fibers, it is possible to make it very lightweight while maintaining a considerable degree of mechanical properties, and it is expected as a new fiber-reinforced composite material.

[0004] Furthermore, in such a fiber-reinforced composite material, by using a thermosetting resin as the matrix resin, it is possible to obtain a fiber-reinforced composite material having excellent heat resistance and excellent mechanical properties and dimensional stability at high temperatures. From this point of view, for example, Patent Document 1 discloses a method for manufacturing a structure having a thermosetting resin, reinforcing fibers, and voids.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] On the other hand, for a structure such as that in Patent Document 1, it was necessary to adopt a manufacturing method according to a predetermined procedure in order to control the curing reaction of the thermosetting resin. In such a manufacturing method, for example, there was a problem that it took time to raise the temperature of the mold.

[0007] An object of the present invention is to provide a prepreg in which a fiber-reinforced composite material having pores can be easily molded with a thermosetting resin as a matrix.

Means for Solving the Problems

[0008] The present invention for solving such problems is a prepreg containing reinforcing fibers and a thermosetting resin, wherein the reinforcing fibers are in the form of a nonwoven fabric, and when heated from 25°C to 120°C, the thermosetting resin softens and expands 1.5 to 100 times in the out-of-plane direction.

Advantages of the Invention

[0009] By heating the prepreg of the present invention at a predetermined temperature, the thermosetting resin softens, and the reinforcing fibers in the form of a nonwoven fabric release the springback force and expand. Due to such an effect, it is possible to handle it as a thin prepreg and to mold it into a fiber-reinforced composite material having pores by a simple process.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0011] <Prepreg> The prepreg of the present invention is a prepreg containing reinforcing fibers and a thermosetting resin, characterized in that the reinforcing fibers are in the form of a non-woven fabric. FIG. 1 is a schematic diagram showing an embodiment of the prepreg of the present invention together with an enlarged view of its internal cross-section. In this specification, the thickness direction of the prepreg is referred to as the out-of-plane direction, and the direction orthogonal to the out-of-plane direction is referred to as the in-plane direction. In the prepreg, the reinforcing fibers 1 form a randomly dispersed network structure and are surrounded by the thermosetting resin 2.

[0012] Such a prepreg can be formed by impregnating a non-woven fabric-shaped reinforcing fiber (reinforcing fiber base material) with a thermosetting resin. FIG. 2 is a schematic diagram showing an embodiment of the reinforcing fiber base material before impregnation with the thermosetting resin in the present invention. In this embodiment, the reinforcing fibers 1 are randomly dispersed as discontinuous fibers and have a non-woven fabric form in which a network structure is formed, thereby forming fine pores 5 between the fibers. The single filaments of the reinforcing fibers 1 cross each other and are partially in contact, thereby forming a three-dimensional shape in which the fiber orientation is dispersed in both the out-of-plane direction and the in-plane direction.

[0013] As described above, the reinforcing fiber base material containing the reinforcing fiber 1 has fine pores formed therein. In the present embodiment, a prepreg is formed by impregnating the pores with the thermosetting resin 2. Such a prepreg encloses the reinforcing fiber base material in a state where a repulsive force in the out-of-plane direction is stored by receiving compression in the out-of-plane direction during impregnation of the thermosetting resin. At this time, by designing the thermosetting resin to be solid at 25°C, the repulsive force of the reinforcing fiber base material can be kept from being released at room temperature, and the thinness as a prepreg can be maintained. On the other hand, in the heating process, specifically, when heated from 25°C to 120°C to soften the thermosetting resin from solid to liquid, the reinforcing fiber base material releases the repulsive force and causes a springback phenomenon in which it tries to return to its original thickness. Furthermore, the thermosetting resin in the present invention can be cured into a thermosetting resin cured product by continuing heating at a temperature equal to or higher than the curing start temperature. Therefore, the prepreg of the present invention can perform expansion and curing reaction in a single heating process. That is, the prepreg of the present invention can be formed into a fiber-reinforced composite material having pores by a simple heating operation.

[0014] FIG. 3 is a schematic view showing an embodiment of the fiber-reinforced composite material in the present invention together with an enlarged view of its internal cross section. In the present embodiment, the prepreg containing the reinforcing fiber 1 expands in the out-of-plane direction due to its springback force, and the pores 5 generated along with this volume expansion are generated inside the thermosetting resin cured product 6 as pores. By adopting such a structure, the weight can be reduced as a fiber-reinforced composite material having pores, and the reinforcing effect by the reinforcing fiber can be exhibited.

[0015] The prepreg of the present invention can be expanded 1.5 to 100 times in the out-of-plane direction. The expansion using the above-described springback force can be designed by the reinforcing fiber base material to be encapsulated. The expansion ratio of such expansion is preferably 1.5 to 30 times, more preferably 5 to 20 times. By setting it within such a range, it is possible to achieve both light weight and mechanical properties. Such an expansion ratio can be obtained by raising the temperature of the prepreg from 25°C to 120°C at a rate of 20°C / min and dividing the thickness of the prepreg at 120°C by the thickness of the prepreg at 25°C.

[0016] Furthermore, the prepreg of the present application preferably has a deformation in the in-plane direction of 0.8 to 1.2 times. The expansion using the springback force described above is also excellent in direction selectivity. That is, it is possible to greatly expand in the out-of-plane direction while suppressing the deformation in the in-plane direction to a small extent. The deformation rate of such deformation in the in-plane direction is more preferably 0.9 to 1.1 times. The prepreg of the present invention that can be selectively expanded in the out-of-plane direction is excellent in following the mold in pressure molding such as press molding. Such a deformation rate can be obtained by raising the temperature of the prepreg from 25°C to 120°C at a rate of 20°C / min and dividing the in-plane width of the prepreg at 120°C by the in-plane width of the prepreg at 25°C.

[0017] In the prepreg of the present invention, the glass transition temperature of the thermosetting resin is preferably 25°C or higher and less than 120°C. The glass transition temperature of the thermosetting resin is more preferably 25°C or higher and 100°C or lower, and even more preferably 30°C or higher and 50°C or lower. By setting it within such a temperature range, at room temperature, the thickness of the prepreg can be maintained thin by suppressing springback as a solid, and it can be rapidly softened by heat molding and expanded by releasing the springback force.

[0018] In the prepreg of the present invention, the curing start temperature of the thermosetting resin is preferably 50°C or higher and lower than 400°C. The curing start temperature of the thermosetting resin is more preferably 100°C or higher and 300°C or lower, and even more preferably 120°C or higher and 200°C or lower. By setting the temperature range as such, the progress of the curing reaction of the thermosetting resin when stored at room temperature as a prepreg can be suppressed, and the curing reaction of the thermosetting resin to a cured product of the thermosetting resin can be rapidly carried out in the heating process.

[0019] The glass transition temperature and the curing start temperature of the thermosetting resin can be measured using differential scanning calorimetry (DSC).

[0020] In the present invention, examples of the thermosetting resin include epoxy resins, unsaturated polyester resins, vinyl ester resins, phenolic resins, urea resins, melamine resins, thermosetting polyimide resins, bismaleimide resins, benzoxazine resins, cyanate ester resins, etc. Copolymers, modified products, and resins blended with two or more of these may also be used. Among these, an epoxy resin is preferred in view of the balance between the curing temperature and the curing time. Also, it is preferable that the thermosetting resin contains other fillers, additives, elastomers, and rubber components as appropriate within a range that does not impair the object of the present invention. Examples of the fillers and additives include, for example, inorganic fillers, flame retardants, conductivity-imparting agents, crystal nucleating agents, ultraviolet absorbers, antioxidants, vibration damping agents, antibacterial agents, insect repellents, deodorants, anti-coloring agents, heat stabilizers, mold release agents, antistatic agents, plasticizers, lubricants, coloring agents, pigments, dyes, foaming agents, foam control agents, coupling agents, etc.

[0021] In the present invention, the epoxy resin is not particularly limited as long as it is a compound having two or more epoxy groups in the molecule, and examples include glycidyl ether type epoxy resins, glycidyl amine type epoxy resins, glycidyl ester type epoxy resins, and alicyclic epoxy resins. Halogen or alkyl-substituted products thereof may also be used.

[0022] Specific examples of glycidyl ether type epoxy resins include bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, biphenyl type epoxy resins, resorcinol type epoxy resins, novolac type epoxy resins, naphthalene type epoxy resins, epoxy resins having a fluorene skeleton, and epoxy resins made from copolymers of phenol compounds and dicyclopentadiene as raw materials.

[0023] Specific examples of glycidyl amine type epoxy resins include tetraglycidyl diaminodiphenylmethane, glycidyl compounds of aminophenols, and glycidyl anilines.

[0024] Specific examples of glycidyl ester type epoxy resins include diglycidyl phthalate, diglycidyl terephthalate, diglycidyl hexahydrophthalate, and diglycidyl isophthalate.

[0025] These epoxy resins may be used alone or in combination of two or more. From the balance between moldability and heat resistance of the cured product, it is preferable to use a combination of glycidyl ether type epoxy resin and glycidyl amine type epoxy resin.

[0026] The curing reaction of an epoxy resin can be controlled by adding a curing agent. The curing agent is preferably compounded in the range of 25 to 100 parts by mass, more preferably 30 to 50 parts by mass, based on 100 parts by mass of the epoxy resin. Preferred curing agents include amine-based curing agents. An amine-based curing agent refers to a curing agent having a nitrogen atom in the curing agent molecule. Such a curing agent is not particularly limited as long as it has a nitrogen atom in the molecule. For example, aromatic amine compounds having active hydrogen such as 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, m-phenylenediamine, m-xylylenediamine, diethyltoluenediamine, aliphatic amine compounds having active hydrogen such as diethylenetriamine, triethylenetetramine, isophoronediamine, bis(aminomethyl)norbornane, bis(4-aminocyclohexyl)methane, polyethyleneimine, modified amine compounds obtained by reacting these amines having active hydrogen with compounds such as epoxy compounds, acrylonitrile, phenol and formaldehyde, thiourea, etc., tertiary amine compounds having no active hydrogen such as dimethylaniline, dimethylbenzylamine, 2,4,6-tris(dimethylaminomethyl)phenol and 1-substituted imidazole, polycarboxylic acid hydrazides such as dicyandiamide, tetramethylguanidine, adipic acid hydrazide and naphthalenedicarboxylic acid hydrazide, Lewis acid complexes such as boron trifluoride ethylamine complex, etc. can be used.

[0027] Among amine-based curing agents, aromatic amine compounds are preferably used from the viewpoint of the excellent heat resistance of the obtained cured product. When an aromatic amine compound is used, a high temperature of about 180 °C is required to obtain a cured product, but a cured product having a high elastic modulus and heat resistance can be obtained. A fiber-reinforced resin composite material using this as a matrix resin is suitable for structural materials such as aircraft and automobile parts. Among them, 3,3'-diaminodiphenylsulfone and 4,4'-diaminodiphenylsulfone are particularly preferred because they impart heat resistance, especially wet heat resistance, to the fiber-reinforced composite material and have excellent storage stability when mixed and liquefied in the epoxy resin.

[0028] In addition, these curing agents can be combined with a suitable curing accelerator to enhance the curing activity. For example, dicyandiamide can be preferably used in combination with a urea derivative or an imidazole derivative such as 1,1-dimethyl-3-phenyl-urea, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-(3-chloro-4-methylphenyl)-1,1-dimethylurea, or 2,4-bis(3,3-dimethylureido)toluene as a curing accelerator.

[0029] Polymer additives can also be incorporated into the epoxy resin in the present invention for fluidity control and toughness imparting. Examples of such polymer additives include polymethyl methacrylate, polyvinyl formal, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene copolymer, polyamide, polyester, polycarbonate, polyarylene oxide, polysulfone, polyether sulfone, and polyimide. Polysulfone and polyimide may have an ether bond or an amide bond in the main chain.

[0030] Polymethyl methacrylate, polyvinyl formal, polyvinyl butyral, and polyvinyl pyrrolidone have good compatibility with many types of epoxy resins such as bisphenol A type epoxy resin, and are preferable in that they have a great effect on controlling the fluidity of the thermosetting resin. Polyvinyl formal is particularly preferable.

[0031] In addition, polysulfone, polyether sulfone, and polyimide are excellent in heat resistance of the resin itself, and have excellent compatibility with glycidylamine type epoxy resin, which is an epoxy resin excellent in heat resistance of the cured product. Adding these can control the fluidity of the epoxy resin and has the effect of enhancing the impact resistance of the resulting cured product, so it is preferable.

[0032] In the epoxy resin of the present invention, it is preferable to contain 1 to 60 parts by mass, more preferably 5 to 30 parts by mass of a polymer additive with respect to 100 parts by mass of the epoxy resin. If it is less than such a range, the effects of controlling fluidity and imparting toughness may not appear. If it is more than such a range, the drapeability of the prepreg may be impaired, or it may affect aspects such as water absorption.

[0033] In the present invention, examples of the reinforcing fiber include carbon fiber, glass fiber, aramid fiber, alumina fiber, silicon carbide fiber, boron fiber, metal fiber, natural fiber, mineral fiber, etc., and these may be used alone or in combination of two or more. Among these, carbon fiber is preferable from the viewpoint of high mechanical properties and excellent spring-back force. Examples of the carbon fiber include PAN-based, rayon-based, and pitch-based ones, and PAN-based carbon fiber is preferable from the viewpoint of the balance between mechanical properties and economy. Recycled carbon fiber is also preferable from the viewpoint of economy. Further, these reinforcing fibers may be surface-treated. Examples of the surface treatment include, in addition to the deposition treatment of a metal as a conductor, treatment with a coupling agent, treatment with a sizing agent, and binding treatment by adhesion of a binder.

[0034] The diameter of the reinforcing fiber is preferably 1 to 20 μm, more preferably 3 to 10 μm, and even more preferably 6 to 8 μm. Further, the cross-section of the reinforcing fiber is preferably circular. By setting it within such a range, the reinforcing fiber has excellent dispersibility and is likely to form pores in the reinforcing fiber base material.

[0035] In the present invention, the reinforcing fiber constituting the non-woven fabric form is preferably discontinuous fiber. The number average fiber length is preferably 0.1 to 100 mm, more preferably 1 to 20 mm, and even more preferably 3 to 10 mm. By setting it within such a range, it becomes easy to widen the interval between the fibers in the reinforcing fiber base material and easy to control the formation of pores. The number average fiber length of the reinforcing fiber can be obtained by randomly selecting 400 reinforcing fibers, measuring their fiber lengths with an optical microscope, and dividing the total fiber length by the number of measurements.

[0036] Examples of the method for manufacturing a reinforcing fiber base material in the form of a non-woven fabric include the air-laid method, the carding method, and the papermaking method.

[0037] The prepreg of the present invention preferably has a porosity of 0% by volume or more and less than 30% by volume. More preferably, the porosity is 0% by volume or more and 10% by volume or less, and even more preferably 0% by volume or more and 5% by volume or less. By controlling within such a range, it can be handled as a thin prepreg at room temperature and also has excellent out-of-plane expansion magnification during heating.

[0038] The prepreg of the present invention preferably contains 10 to 1,000 parts by mass of a thermosetting resin with respect to 100 parts by mass of the reinforcing fiber. More preferably, the thermosetting resin is 10 to 500 parts by mass, even more preferably 10 to 100 parts by mass, and particularly preferably 10 to 50 parts by mass. If it is less than such a range, the amount of the thermosetting resin is insufficient, and the springback by the reinforcing fiber cannot be sufficiently suppressed, and the thinness as a prepreg may not be maintained. If it is more than such a range, the amount of the reinforcing fiber is insufficient, and the expansion in the out-of-plane direction may be insufficient due to insufficient springback force.

[0039] The prepreg of the present invention preferably has a reinforcing fiber content of 30 to 60% by volume. More preferably, the reinforcing fiber content is 40 to 60% by volume, and even more preferably 50 to 60% by volume. By setting the range in this way, it is possible to increase the ratio of the contained reinforcing fiber and design a high springback force, and a prepreg excellent in out-of-plane expansion magnification can be obtained.

[0040] The prepreg of the present invention preferably has a thickness of 0.01 to 1.5 mm. More preferably, the thickness of the prepreg is 0.1 to 1 mm, and even more preferably 0.1 to 0.5 mm. By setting the range in this way, it is possible to handle it as a thin prepreg, and it is excellent in drapeability and ease of adjusting the thickness by lamination.

[0041] The prepreg of the present invention can be manufactured by a wet method in which a thermosetting resin is dissolved in a solvent such as methyl ethyl ketone or methanol to reduce its viscosity and then impregnated, or by a hot melt method in which the viscosity is reduced by heating and then impregnated.

[0042] The wet method is a method in which a reinforcing fiber is impregnated with a solution of a thermosetting resin, and then the solvent is evaporated by heating to obtain a prepreg.

[0043] The hot melt method is a method in which a thermosetting resin with reduced viscosity by heating is directly impregnated into a reinforcing fiber base material, or a resin film in which a thermosetting resin is coated on a release paper is produced, and then the film is overlapped from both sides or one side of the reinforcing fiber base material and heated and pressed to impregnate the thermosetting resin to obtain a prepreg. From the viewpoint of excellent impregnability, it is preferable to perform impregnation by overlapping the resin film on both sides of the reinforcing fiber base material. Also, in the hot melt method, it is preferable because substantially no solvent remains in the prepreg.

[0044] When obtaining a prepreg by the hot melt method, the temperature in the step of producing the resin film is preferably 30 to 80°C, more preferably 40 to 70°C. If it is lower than this range, the basis weight of the obtained resin film may not be stable because the viscosity is high. If it is higher than this range, the curing reaction may proceed and the viscosity may increase, and film formation may not be possible.

[0045] Furthermore, it is also preferable that the prepreg of the present invention is manufactured by a method for manufacturing a prepreg, which involves impregnating a non-woven fabric containing reinforcing fibers with a thermosetting resin raw material having a glass transition temperature of less than 25°C by heating and pressurizing, and semi-curing until the glass transition temperature rises to 25°C or more and less than 120°C. The thermosetting resin, by keeping the degree of curing lower as a thermosetting resin raw material having a glass transition temperature of less than 25°C, is excellent in impregnability into the pores of the reinforcing fiber base material. The thermosetting resin raw material can be made into the thermosetting resin in the present invention by semi-curing while adjusting the heating temperature and heating time after impregnating the reinforcing fiber base material. The heating temperature in such an impregnation process is preferably 50°C or more and less than 400°C, more preferably 100°C or more and 300°C or less, even more preferably 120°C or more and 200°C or less, and particularly preferably 140°C or more and 160°C or less. If it is lower than such a range, the curing reaction may not proceed, and the glass transition temperature of the thermosetting resin may not reach 25°C or more. If it is higher than such a range, the curing reaction may proceed too much, and the glass transition temperature of the thermosetting resin may exceed 120°C. Also, the heating time in such an impregnation process is preferably 3 to 90 minutes, more preferably 5 to 60 minutes, and even more preferably 10 to 30 minutes. If it is shorter than such a range, the curing reaction may not proceed, and the glass transition temperature of the thermosetting resin may not reach 25°C or more. If it is longer than such a range, the curing reaction may proceed too much, and the glass transition temperature of the thermosetting resin may exceed 120°C.

[0046] By controlling the glass transition temperature of the thermosetting resin to 25°C or more and less than 120°C, it is possible to obtain a prepreg that is thin while storing the spring-back force of the reinforcing fiber base material at room temperature and expands in the out-of-plane direction by heating. Since such a thermosetting resin raw material is more excellent in impregnability than the thermosetting resin, for example, it is also excellent in the productivity of a prepreg having a high fiber content rate of the reinforcing fiber. <Preform> The prepreg of the present invention can also be used as a preform obtained by laminating these prepregs. Since the prepreg of the present invention softens upon heating, good adhesion between layers can be achieved by laminating multiple layers and performing heat treatment. The number of laminated layers is preferably 2 to 100 layers, more preferably 2 to 50 layers, and even more preferably 2 to 10 layers. The prepreg of the present invention in which the reinforcing fibers are in the form of an isotropic nonwoven fabric can be laminated without restrictions such as symmetrical lamination. It is also preferable to form a laminated body with an uneven thickness shape in which the areas of the respective layers are different. <Fiber Reinforced Composite Material> The prepreg of the present invention or the preform can be made into a thermosetting resin cured product having a glass transition temperature of 120°C or higher and 400°C or lower by curing the thermosetting resin, and can be formed into a fiber reinforced composite material. The glass transition temperature of the thermosetting resin cured product is preferably 150°C or higher and 400°C or lower, and more preferably 180°C or higher and 400°C or lower. By setting it within such a range, a fiber reinforced composite material with little deformation during heating and high heat resistance can be obtained.

[0047] Examples of the means for curing the thermosetting resin include heat forming. Examples of heat forming include press forming method, autoclave forming method, bagging forming method, wrapping tape method, internal pressure forming method, and RTM forming method. The press forming method is preferred in terms of the balance between formability and productivity.

[0048] As a method for manufacturing a fiber reinforced composite material, it is preferable to simultaneously perform expansion in the out-of-plane direction and curing of the thermosetting resin by heat forming the prepreg or preform of the present invention in a mold having a surface temperature of 50°C or higher and 400°C or lower. The heating temperature is more preferably 120°C or higher and 350°C or lower, even more preferably 150°C or higher and 300°C or lower, and particularly preferably 180°C or higher and 250°C or lower. The heating time is preferably 1 to 300 minutes, more preferably 10 to 180 minutes, and even more preferably 30 to 120 minutes. By setting such heat forming conditions, the productivity of the fiber reinforced composite material is excellent. Since the matrix resin of such a fiber reinforced composite material is a thermosetting resin cured product, it can be demolded as a molded product without going through a mold cooling process such as that of a thermoplastic resin.

[0049] In the present invention, the porosity of the fiber-reinforced composite material is preferably 30% by volume or more and 98% by volume or less. The porosity of the fiber-reinforced composite material is more preferably 40% by volume or more and 98% by volume or less, still more preferably 80% by volume or more and 98% by volume or less, and particularly preferably 90% by volume or more and 98% by volume or less. Such a porosity can also be adjusted by adjusting the thickness of the mold cavity when spring-back is caused and controlling the expansion ratio. The fiber-reinforced composite material using the prepreg of the present invention is excellent in the expansion ratio in the out-of-plane direction, and thus a high porosity can be achieved. By forming a molded product having a porosity within such a range, a product excellent in light weight can be obtained.

[0050] Since the fiber-reinforced composite material in the present invention is excellent in light weight and mechanical properties, it can be suitably used for aircraft members such as aircraft, artificial satellites, UAM (Urban Air Mobility), and drones, automobile members, railway vehicle members, ship members, electronic equipment casings, sports goods, or building materials.

Examples

[0051] The materials used in the examples and comparative examples are as follows.

[0052] [Reinforcing fiber] (A-1) Carbon fiber non-woven fabric A carbon fiber bundle with a total number of 12,000 monofilaments was obtained by spinning, firing, and surface oxidation treatment of a copolymer mainly composed of polyacrylonitrile. The properties of this carbon fiber bundle were measured in accordance with JIS R7608 (2007), and the tensile elastic modulus was 220 GPa, and the cross-section was a circular cross-section with a single fiber diameter of 7 μm.

[0053] Using the carbon fiber bundle, it was cut into 6 mm lengths with a cartridge cutter to obtain chopped carbon fibers. A dispersion with a concentration of 0.1% by mass consisting of water and a surfactant (manufactured by Nacalai Tesque, Inc., polyoxyethylene lauryl ether (trade name)) was prepared, and using this dispersion and the chopped carbon fibers, a carbon fiber nonwoven fabric was produced. The manufacturing apparatus is a cylindrical container with a diameter of 1000 mm having an opening and closing cock at the lower part of the container as a dispersion tank, and is equipped with a linear transport section (inclination angle 30°) connecting the dispersion tank and the papermaking tank. A stirrer is attached to the opening at the upper surface of the dispersion tank, and chopped carbon fibers and the dispersion (dispersion medium) can be introduced from the opening. The papermaking tank is equipped with a mesh conveyor having a papermaking surface with a width of 500 mm at the bottom, and a conveyor capable of transporting the carbon fiber nonwoven fabric (reinforcing fiber base material) is connected to the mesh conveyor. Papermaking was performed with the carbon fiber concentration in the dispersion being 0.05% by mass. The papermade base material was dried in a drying oven at 200 °C for 30 minutes, and had a basis weight of 100 g / m 2 to obtain a carbon fiber nonwoven fabric. When measuring the number average fiber length, the fiber length was 6 mm.

[0054] [Thermosetting resin] (B-1) Thermosetting resin raw material 50 parts by mass of “jER” 825 (bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Corporation), 50 parts by mass of “Sumiepoxy” ELM-434 (tetraglycidyl diaminodiphenylmethane manufactured by Sumitomo Chemical Co., Ltd.), and 15 parts by mass of “Sumikaexcel” PES5003P (polyethersulfone manufactured by Sumitomo Chemical Co., Ltd.) were mixed with a kneader and stirred at 150 °C for 1 hour to uniformly dissolve them. Next, the temperature was lowered to 60 °C, 40 parts by mass of 4,4'-diaminodiphenyl sulfone was added as an amine-based curing agent, and kneading was performed for 40 minutes while maintaining the temperature at 60 °C. The obtained mixture was taken out and formed into a film with a knife coater to obtain a resin film with a basis weight of 50 g / m 2 made of the thermosetting resin raw material. The glass transition temperature of this resin film measured in accordance with JIS K7121 (2012) was less than 25 °C.

[0055] (B-2) Thermosetting resin raw material 50 parts by mass of “jER” 806 (bisphenol F type epoxy resin manufactured by Mitsubishi Chemical Corporation), 50 parts by mass of “Sumiepoxy” ELM-434 (tetraglycidyl diaminodiphenylmethane manufactured by Sumitomo Chemical Co., Ltd.), and 20 parts by mass of “Sumikaexcel” 5200P (polyethersulfone manufactured by Sumitomo Chemical Co., Ltd.) were mixed in a kneader and stirred at 150 °C for 1 hour to uniformly dissolve them. Next, the temperature was lowered to 60 °C, 40 parts by mass of 3,3'-diaminodiphenyl sulfone was added as an amine-based curing agent, and kneading was carried out for 40 minutes while maintaining the temperature at 60 °C. The obtained mixture was taken out and formed into a film by a knife coater, and a resin film with a mass of 50 g / m 2 was obtained. The glass transition temperature of this resin film measured in accordance with JIS K7121 (2012) was less than 25 °C.

[0056] The evaluation methods for the structure, physical properties, etc. in each example and comparative example are as follows.

[0057] [Glass transition temperature of thermosetting resin] The glass transition temperature of the thermosetting resin was evaluated using the prepreg as a sample. Differential scanning calorimetry was performed in accordance with JIS K7121 (2012), and substances within the range of a glass transition temperature of 25 °C or higher and less than 120 °C were judged as “〇”, and substances outside the range were judged as “×”.

[0058] [Curing start temperature of thermosetting resin] The curing start temperature of the thermosetting resin was evaluated using the prepreg as a sample. Differential scanning calorimetry was performed in accordance with JIS K7148-1 (2022), and for the obtained exothermic peak, the temperature of the intersection of the straight line obtained by extending the baseline on the low-temperature side to the high-temperature side and the tangent line drawn at the point where the gradient of the curve on the low-temperature side of the exothermic peak is maximum was defined as the curing start temperature. Substances with an exothermic peak and a curing start temperature within the range of 50 °C or higher and less than 400 °C were judged as “〇”, and substances outside the range or without an exothermic peak were judged as “×”.

[0059] [Glass transition temperature of thermosetting resin cured product] The glass transition temperature of the thermosetting resin cured product was evaluated using the fiber-reinforced composite material as a sample. Differential scanning calorimetry was performed in accordance with JIS K7121 (2012), and those with a glass transition temperature within the range of 120°C or higher and 400°C or lower were judged as "〇", and those outside the range were judged as "×".

[0060] [Void ratio of prepreg] The void ratio of the prepreg is the ratio of voids in the prepreg, and was determined using the following formula by SEM observation of the cross-section of the prepreg. (Void ratio of prepreg) = (Area of voids in the observed cross-section [μm 2 ) / (Area of the observed cross-section [μm 2 ) × 100 [%] [Content ratio of reinforcing fiber] The content ratio of the reinforcing fiber is the ratio of the reinforcing fiber in the prepreg, and was determined using the following formula by SEM observation of the cross-section of the prepreg. (Content ratio of reinforcing fiber) = (Area of the cross-section of the reinforcing fiber in the observed cross-section [μm 2 ) / (Area of the observed cross-section [μm 2 ) × 100 [%] [Expansion magnification in the out-of-plane direction] The expansion magnification in the out-of-plane direction was obtained by heating the prepreg from 25°C to 120°C at a rate of 20°C / min and using the following formula. (Expansion magnification in the out-of-plane direction) = (Thickness of the prepreg at 120°C [mm]) / (Thickness of the prepreg at 25°C [mm]) [times] [Deformation rate in the in-plane direction] The deformation rate in the in-plane direction was obtained by heating the prepreg from 25°C to 120°C at a rate of 20°C / min and using the following formula. The width of the prepreg was determined as the average value of the four sides. (Deformation rate in the in-plane direction) = (Width of the prepreg at 120°C [mm]) / (Width of the prepreg at 25°C [mm]) [times] [Void ratio of fiber-reinforced composite material] The void ratio of the fiber-reinforced composite material is the ratio of voids in the fiber-reinforced composite material, and was determined using the following formula by SEM observation of the cross-section of the fiber-reinforced composite material. (Void ratio of fiber-reinforced composite material) = (Area of voids in the observed cross-section [μm2 ) / (area of the observation cross-section [μm 2 ) × 100 [%] Hereinafter, examples and comparative examples will be described.

[0061] [Example 1] (Reinforcing fiber) As the reinforcing fiber, one piece of A-1 carbon fiber non-woven fabric cut into a square shape with a side length of 300 mm was used.

[0062] (Thermosetting resin) As the thermosetting resin raw material, six pieces of B-1 resin film cut into a square shape with a side length of 300 mm were used.

[0063] The above-mentioned reinforcing fiber and thermosetting resin were laminated in the order of B-1 × 3 sheets / A-1 × 1 sheet / B-1 × 3 sheets, and press-molded at 150 °C under 1 MPa for 30 minutes to impregnate the thermosetting resin raw material into the reinforcing fiber and obtain a prepreg semi-cured in the thermosetting resin.

[0064] Two pieces of the prepreg were laminated and press-molded in a mold with a cavity thickness of 3 mm. The mold temperature was 180 °C, and it was heated for 120 minutes with the mold closed to cure the thermosetting resin into a thermosetting resin cured product. Then, the mold was opened without going through a cooling process, and the fiber-reinforced composite material was taken out. The results of each evaluation are shown in Table 1.

[0065] [Example 2] B-2 was used instead of B-1. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0066] [Example 3] The number of B-1 sheets was changed from six to two, and the lamination structure was changed to B-1 × 1 sheet / A-1 × 1 sheet / B-1 × 1 sheet. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0067] [Example 4] The number of B-1 sheets was changed from 6 to 1, and the laminated structure was B-1×1 sheet / A-1×1 sheet. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0068] [Example 5] The number of B-1 sheets was changed from 6 to 18, and the laminated structure was B-1×9 sheets / A-1×1 sheet / B-1×9 sheets. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0069] [Comparative Example 1] The press molding time for prepreg formation by impregnating B-1 into A-1 was changed from 30 minutes to 1 minute. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0070] [Comparative Example 2] The temperature of press molding for prepreg formation by impregnating B-1 into A-1 was changed from 150°C to 180°C, and the press molding time was changed from 30 minutes to 120 minutes. Otherwise, in the same manner as in Example 1, a prepreg and a fiber-reinforced composite material were obtained. The results of each evaluation are shown in Table 1.

[0071]

Table 1

Explanation of Symbols

[0072] 1 Reinforcing fiber 2 Thermosetting resin 3 Out-of-plane direction 4 In-plane direction 5 Void 6 Thermosetting resin cured product

Claims

1. A prepreg containing a reinforcing fiber and a thermosetting resin, wherein the reinforcing fiber is in the form of a nonwoven fabric, and when heated from 25°C to 120°C, the thermosetting resin softens and expands 1.5 to 100 times in the out-of-plane direction.

2. The prepreg according to claim 1, wherein when heated from 25°C to 120°C, the in-plane deformation is 0.8 to 1.2 times.

3. The prepreg according to claim 1 or 2, wherein the glass transition temperature of the thermosetting resin is 25°C or higher and lower than 120°C.

4. The prepreg according to claim 1 or 2, wherein the curing start temperature of the thermosetting resin is 50°C or higher and lower than 400°C.

5. The prepreg according to claim 1 or 2, wherein the porosity is 0% by volume or more and less than 30% by volume.

6. The prepreg according to claim 1 or 2, wherein the amount of the thermosetting resin is 10 to 1,000 parts by mass based on 100 parts by mass of the reinforcing fiber.

7. The prepreg according to claim 1 or 2, wherein the content of the reinforcing fiber is 30 to 60% by volume.

8. The prepreg according to claim 1 or 2, wherein the thermosetting resin is an epoxy resin.

9. The prepreg according to claim 1 or 2, wherein the reinforcing fiber is a carbon fiber.

10. A method for producing a prepreg, comprising heating and pressing a nonwoven fabric containing a reinforcing fiber with a thermosetting resin raw material having a glass transition temperature of less than 25°C to impregnate the nonwoven fabric, and semi-curing until the glass transition temperature rises to 25°C or higher and lower than 120°C.

11. A preform obtained by laminating the prepreg according to claim 1 or 2.

12. A fiber-reinforced composite material obtained by curing the thermosetting resin of the preform according to claim 11 to obtain a thermosetting resin cured product having a glass transition temperature of 120°C or higher and 400°C or lower.

13. The fiber-reinforced composite material according to claim 12, wherein the porosity is 30% by volume or more and 98% by volume or less.

14. The fiber-reinforced composite material according to claim 13, which is used as an aircraft member, an automobile member, a railway vehicle member, a ship member, an electronic device housing, a sports good, or a building material.

15. A method for producing a fiber-reinforced composite material, comprising heating and molding the preform according to claim 11 in a mold having a surface temperature of 50°C or higher and 400°C or lower, thereby simultaneously performing expansion in the out-of-plane direction and curing of the thermosetting resin.

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