Matrix resin mixing method and prepreg

Abstract

The object of the present invention is to provide a kneading method for a matrix resin composition with excellent particle dispersibility by controlling the optimal shear stress, resin viscosity, and shear rate for the dispersion of powder particles incorporated into the matrix resin. [Solution] This is a method of kneading powder into the matrix resin used in fiber-reinforced composite materials. When mixing a powder whose D95 particle size, which represents 95% of the cumulative volume particle size distribution measurement of primary particle size, is between 10 μm and 80 μm, in a matrix resin, the shear stress applied to the powder in the matrix resin is between 160,000 Pa and 500,000 Pa. A method for kneading a matrix resin, wherein the maximum diameter of secondary particles of the powder dispersed in the matrix resin is less than 90 μm.

Description

Technical Field

[0001] The present invention relates to a kneading method for a matrix resin composition, which is suitably used for obtaining a lightweight and high-performance fiber reinforced composite material.

Background Art

[0002] Fiber reinforced composite materials composed of reinforcing fibers such as glass fibers, carbon fibers, and aramid fibers and a matrix resin are lightweight and excellent in mechanical properties such as strength, rigidity, impact resistance, and fatigue resistance. They have been applied to many fields such as aircraft, spacecraft, automobiles, railway vehicles, ships, civil engineering, sports goods, and general industrial goods.

[0003] As the matrix resin used in fiber reinforced composite materials, thermosetting resins and thermoplastic resins are used. Among them, epoxy resins are excellent because they have adhesiveness with reinforcing fibers, heat resistance, elastic modulus, and chemical resistance, and have small curing shrinkage.

[0004] In the matrix resin, a thermoplastic resin is preferably blended to improve the processability, handleability, and composite mechanical properties of the prepreg, and a curing agent is preferably blended to improve the curability, mechanical properties, and heat resistance. Furthermore, powder particles or particulate fillers may be preferably blended to provide new functions and added values.

[0005] When particles are blended into the matrix resin, in order to exhibit the functions of the particles, it is necessary for the particles to be uniformly dispersed in the matrix resin. However, depending on the particle size distribution of the particles and the shear stress applied during kneading of the particles, the powder particles may aggregate during kneading of the resin composition. When aggregation occurs, the functions of the particles may not be fully exerted, and in the prepreg manufacturing process, the aggregated particles may also have an adverse effect on the appearance quality.

[0006] Patent Document 1 discloses a technique for stably storing an epoxy resin composition containing cyanamide by incorporating inhibitory particles that prevent or suppress the crystallization of cyanamide.

[0007] Patent Document 2 discloses a technique for defining the particle size distribution of a solid curing agent insoluble in the thermosetting resin in a prepreg consisting of reinforcing fibers and a thermosetting resin composition, thereby suppressing the difference in curing agent concentration between the prepreg surface and the interior of the prepreg. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2019-167462 [Patent Document 2] Japanese Patent Publication No. 2004-51690 [Overview of the project] [Problems that the invention aims to solve]

[0009] The resin compositions described in Patent Documents 1 and 2 both contain powder particles and specify particle size, but they do not take into account the risk of particle aggregation when the particles are kneaded into the matrix resin. Depending on the manufacturing method, it is difficult to control the dispersibility, and the methods described were not sufficient.

[0010] Therefore, the object of the present invention is to provide a kneading method for a matrix resin composition with excellent particle dispersibility by controlling the optimal shear stress, resin viscosity, and shear rate for the dispersion of powder particles incorporated into the matrix resin. [Means for solving the problem]

[0011] 1. This is a method of kneading powder into the matrix resin used in fiber-reinforced composite materials. When mixing a powder whose D95 particle size, which represents 95% of the cumulative volume particle size distribution measurement of primary particle size, is between 10 μm and 80 μm, in a matrix resin, the shear stress applied to the powder in the matrix resin is between 160,000 Pa and 500,000 Pa. A method for kneading a matrix resin, wherein the maximum diameter of secondary particles of the powder dispersed in the matrix resin is less than 90 μm. 2. The method for kneading a matrix resin as described in item 1 above, wherein the complex viscosity of the matrix resin to be kneaded is 400 Pa·s or more and 1,000 Pa·s or less. 3. A method for kneading a matrix resin as described in 1 or 2 above, wherein the shear rate during the kneading process is controlled to be between 400 / s and 1,000 / s. 4. A prepreg formed by impregnating reinforcing fibers with a matrix resin kneaded by any of the kneading methods described in 1 to 3 above. [Effects of the Invention]

[0012] By using the matrix resin kneading method of the present invention, the powdered raw materials are uniformly dispersed in the matrix resin composition, thereby enabling the production of a film with good appearance quality during the prepreg manufacturing process. [Modes for carrying out the invention]

[0013] The present invention will be described in more detail below.

[0014] The matrix resin of the present invention is not particularly limited, but it is preferably mainly composed of a thermosetting resin. Examples of such thermosetting resins include epoxy resins, unsaturated polyester resins, vinyl ester resins, benzoxazine resins, phenolic resins, urea resins, melamine resins, and thermosetting polyimide resins. Modified versions thereof and mixtures of multiple types can also be used.

[0015] Among these thermosetting resins, those containing epoxy resin are preferred because they offer an excellent balance of heat resistance, mechanical properties, and adhesion to carbon fibers. A matrix resin composed of epoxy resin and a curing agent and / or curing accelerator is preferably used.

[0016] The epoxy resin is not particularly limited, and one or more can be selected and used from among bisphenol-type epoxy resins, amine-type epoxy resins, phenol novolac-type epoxy resins, cresol novolac-type epoxy resins, resorcinol-type epoxy resins, phenol aralkyl-type epoxy resins, naphthol-type epoxy resins, dicyclopentadiene-type epoxy resins, epoxy resins having a biphenyl skeleton, isocyanate-modified epoxy resins, tetraphenylethane-type epoxy resins, triphenylmethane-type epoxy resins, etc.

[0017] Here, bisphenol-type epoxy resin refers to a bisphenol compound in which two phenolic hydroxyl groups are glycidylated, and examples include bisphenol A type, bisphenol F type, bisphenol AD ​​type, bisphenol S type, or halogen, alkyl-substituted, and hydrogenated versions of these bisphenols. Furthermore, not only monomers but also high molecular weight compounds having multiple repeating units can be suitably used.

[0018] Commercially available bisphenol A type epoxy resins include "jER(registered trademark)" 825, 828, 834, 1001, 1002, 1003, 1003F, 1004, 1004AF, 1005F, 1006FS, 1007, 1009, and 1010 (all manufactured by Mitsubishi Chemical Corporation). Brominated bisphenol A type epoxy resins include "jER(registered trademark)" 505, 5050, 5051, 5054, and 5057 (all manufactured by Mitsubishi Chemical Corporation). Commercially available hydrogenated bisphenol A type epoxy resins include ST5080, ST4000D, ST4100D, and ST5100 (all manufactured by Nippon Steel Chemical & Material Co., Ltd.).

[0019] Examples of commercially available bisphenol F type epoxy resins include "Epiclon (registered trademark)" 830 (manufactured by DIC Corporation), "jER (registered trademark)" 806, 807, 4002P, 4004P, 4007P, 4009P, 4010P (manufactured by Mitsubishi Chemical Corporation), "Epotoate (registered trademark)" YDF2001, YDF2004 (manufactured by Nippon Steel Chemical & Material Co., Ltd.), and the like. Examples of tetramethyl bisphenol F type epoxy resins include YSLV-80XY (manufactured by Nippon Steel Chemical & Material Co., Ltd.).

[0020] Examples of bisphenol S type epoxy resins include "Epiclon (registered trademark)" EXA-1514 (manufactured by DIC Corporation).

[0021] Among them, bisphenol A type epoxy resin, bisphenol F type epoxy resin, and bisphenol S type epoxy resin are preferred because of their good balance of elastic modulus, toughness, and heat resistance.

[0022] Examples of amine type epoxy resins include, for example, tetraglycidyl diaminodiphenylmethane, triglycidyl aminophenol, triglycidyl aminocresol, tetraglycidyl xylylenediamine, and their halogen, alkynol-substituted products, hydrogenated products, and the like.

[0023] Examples of commercially available tetraglycidyl diaminodiphenylmethane include "Sumiepoxy (registered trademark)" ELM434 (manufactured by Sumitomo Chemical Co., Ltd.), YH434L (manufactured by Nippon Steel Chemical & Material Co., Ltd.), "jER (registered trademark)" 604 (manufactured by Mitsubishi Chemical Corporation), "Araldite (registered trademark)" MY720, MY721 (both manufactured by Huntsman Japan K.K.), and others. Examples of commercially available triglycidyl aminophenol or triglycidyl aminocresol include "Sumiepoxy (registered trademark)" ELM100 (manufactured by Sumitomo Chemical Co., Ltd.), "Araldite (registered trademark)" MY0500, MY0510, MY0600 (manufactured by Huntsman Japan K.K.), "jER (registered trademark)" 630 (manufactured by Mitsubishi Chemical Corporation), and others. Examples of commercially available tetraglycidyl xylylenediamine and its hydrogenated products include TETRAD-X, TETRAD-C (both manufactured by Mitsubishi Gas Chemical Company, Inc.).

[0024] Examples of commercially available phenol novolak type epoxy resins include "jER (registered trademark)" 152, 154 (manufactured by Mitsubishi Chemical Corporation), "Epiclon (registered trademark)" N-740, N-770, N-775 (manufactured by DIC Corporation), and others.

[0025] Regarding the above-mentioned curing agents, although not particularly limited, amine compounds containing aliphatic amines, aromatic amines or alicyclic amines, phenol resins, dicyandiamide or its derivatives, acid anhydrides, polyaminoamides, organic acid hydrazides or isocyanates may be used.

[0026] Examples of aromatic amines include, for example, xylylenediamine, diaminodiphenylmethane, phenylenediamine, diaminodiphenylsulfone.

[0027] Among these, it is preferable to use at least one selected from aromatic amines and phenol resins as the curing agent because of its excellent heat resistance, and more preferably to use diaminodiphenylsulfone because of its excellent balance of heat resistance, storage stability and mechanical properties.

[0028] The curing accelerators mentioned above include urea compounds, tertiary amines and their salts, imidazoles and their salts, triphenylphosphine or its derivatives, metal carboxylate salts, Lewis acids, Brønsted acids and their salts. Among these, urea compounds are preferred due to their balance of storage stability and catalytic activity.

[0029] Examples of urea compounds that can be used include N,N-dimethyl-N'-(3,4-dichlorophenyl)urea, toluenebis(dimethylurea), 4,4'-methylenebis(phenyldimethylurea), and 3-phenyl-1,1-dimethylurea. Commercially available examples of such urea compounds include DCMU99 (manufactured by Hodogaya Chemical Co., Ltd.) and “Omicure®” 24, 52, and 94 (all manufactured by Emerald Performance Materials, LLC).

[0030] Furthermore, in this invention, various powder particles are mixed into the matrix resin. The powder particles may be used as a curing agent or curing accelerator as described above and blended into the thermosetting resin, or rubber particles, thermoplastic resin particles, or inorganic particles, as described later, may be blended into the thermosetting resin and mixed in.

[0031] When a thermosetting resin is used as the matrix resin, a thermoplastic resin may be added to adjust toughness and fluidity, within the limits that do not impair the effects of the present invention. When an epoxy resin is used as the thermosetting resin, the thermoplastic resin may be a thermoplastic resin soluble in epoxy resin, or organic particles such as rubber particles or thermoplastic resin particles that are partially soluble or insoluble in epoxy resin.

[0032] As a thermoplastic resin soluble in epoxy resin, a thermoplastic resin having hydrogen-bonding functional groups that can be expected to improve the adhesion between the matrix resin and carbon fibers is preferably used. Examples of hydrogen-bonding functional groups include alcoholic hydroxyl groups, amide bonds, sulfonyl groups, and carboxyl groups.

[0033] Examples of thermoplastic resins having alcoholic hydroxyl groups include polyvinyl acetal resins containing polyvinyl formal or polyvinyl butyral, polyvinyl alcohol, and phenoxy resins.

[0034] Examples of thermoplastic resins having amide bonds include polyamides, polyimides, polyamideimides, and polyvinylpyrrolidone.

[0035] Examples of thermoplastic resins having sulfonyl groups include polysulfone and polyethersulfone.

[0036] Examples of thermoplastic resins having carboxyl groups include polyester, polyamide, and polyamide-imide. In such thermoplastic resins, carboxyl groups may be present in either the main chain, the terminal, or both.

[0037] Commercially available thermoplastic resins that are soluble in epoxy resins and have hydrogen-bonding functional groups include polyvinyl acetal resins such as "Mowital®" (manufactured by Kuraray Co., Ltd.) and "Vinirec®" (manufactured by JNC Corporation).

[0038] As rubber particles that are partially soluble or insoluble in epoxy resin, crosslinked rubber particles and core-shell rubber particles obtained by graft polymerization of a different polymer onto the surface of crosslinked rubber particles are preferably used from the viewpoint of handling and other factors.

[0039] Polyamide particles and polyimide particles are preferably used as thermoplastic resin particles that are partially soluble or insoluble in epoxy resin.

[0040] In this invention, various inorganic particles, such as silica, carbon, alumina, smectite, and synthetic mica, can be incorporated into the matrix resin to provide it with various functions.

[0041] In this invention, the particles incorporated into the matrix resin have a D95 (hereinafter, D95) particle size, which is the cumulative value of the volume particle size distribution measurement of primary particles before kneading into the matrix resin that accounts for 95%, of 10 μm or more and less than 90 μm, and the maximum diameter of secondary particles dispersed in the resin after kneading into the matrix resin is less than 90 μm. The D95 of the primary particles is preferably less than 50 μm, and more preferably less than 20 μm. The maximum diameter of the secondary particles is preferably 20 μm or more and less than 80 μm, and more preferably 20 μm or more and less than 50 μm. If the maximum diameter of the secondary particles is 90 μm or more, when a resin film is made by coating the matrix resin onto a release sheet in the prepreg manufacturing process, the particles may get caught in the resin discharge part, resulting in resin gaps in the longitudinal direction of the resin film, or the particle ratio in the resin composition may decrease, leading to insufficient expression of function.

[0042] In this context, the D95 of primary particles refers to the particle size value corresponding to 95% of the cumulative percentage distribution in the volume particle size distribution obtained by imaging the supernatant liquid obtained by stirring powder particles in a dispersion liquid with an optical microscope (e.g., ECLIPSE 80i: Nikon, etc.) and then classifying it using image analysis software (e.g., LUZEX_AP: Nireco, etc.). The maximum diameter of secondary particles refers to the largest particle size value obtained in the volume particle size distribution obtained by similarly classifying the particles.

[0043] In the method for producing the matrix resin composition of the present invention, when mixing the matrix resin with particles, the shear stress applied to the powder is controlled to be between 160,000 Pa and 500,000 Pa. By setting the shear stress to 160,000 Pa or higher, the mixture is kneaded with a force stronger than the cohesive force of the particles blended into the matrix resin, thereby preventing particle aggregation. On the other hand, increasing the shear stress increases the load on the kneader, so from the viewpoint of protecting the equipment, the shear stress is set to 500,000 Pa or lower. Preferably, the lower limit of the shear stress is 200,000 Pa or higher, and the upper limit is 300,000 Pa or lower.

[0044] Shear stress is calculated by the product of the complex viscosity of the matrix resin during mixing and the shear rate during mixing of the matrix resin and powder.

[0045] The complex viscosity of a matrix resin refers to the complex viscosity coefficient η* measured using a dynamic viscoelasticity measuring device (e.g., ARES-G2: manufactured by TA Instruments, Inc.) with parallel plates, maintaining the temperature at the mixing temperature, with a strain of 0.1%, a frequency of 0.5 Hz, and a plate spacing of 1 mm.

[0046] Here, the matrix resin preferably has a complex viscosity of 400 Pa·s or more and 1,000 Pa·s or less during kneading. By setting the viscosity to 400 Pa·s or more, the fluidity of the matrix resin is suppressed, the time the powder is in contact with the blades and walls of the kneader is increased, and dispersibility can be improved. On the other hand, if the complex viscosity of the matrix resin is too high, the fluidity of the powder in the matrix resin will also be poor, so 1,000 Pa·s or less is preferable. More preferably, the lower limit is 500 Pa·s or more and the upper limit is 800 Pa·s or less, and within this range, both the fluidity of the matrix resin and the fluidity of the powder can be achieved, so a favorable shear stress can be applied.

[0047] Shear rate is generally expressed by dividing the speed at which one of the plates moves when a sample is sandwiched between two plates by the distance between the plates. When mixing with a kneader equipped with blades, shear rate is calculated from the rotational speed of the kneader's blades and the clearance between the blades and the kneader wall or between the blades themselves.

[0048] Here, the shear rate is preferably 400 / s or more and 1,000 / s or less. By setting the shear rate to 400 / s or more, the number of times the kneader blades come into contact with the powder in the matrix resin can be increased, thereby improving dispersibility. On the other hand, increasing the shear rate requires increasing the rotational speed of the blades, which increases the load on the kneader, so from the viewpoint of protecting the equipment, it is preferable to keep it at 1,000 / s or less. More preferably, the lower limit is 500 / s or more, and the upper limit is 800 Pa·s or less.

[0049] Various known equipment can be used to knead the matrix resin composition of the present invention. Examples include horizontal mixers and planetary mixers. [Examples]

[0050] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following examples. The matrix resins and various measurement methods used in these examples and comparative examples are as follows. <Epoxy resin> • Bisphenol A type liquid epoxy resin ("jER(registered trademark)" 828, manufactured by Mitsubishi Chemical Corporation) • Bisphenol A type liquid epoxy resin ("jER(registered trademark)" 1001, manufactured by Mitsubishi Chemical Corporation) • Bisphenol A type solid epoxy resin ("jER(registered trademark)" 1007, manufactured by Mitsubishi Chemical Corporation) • Phenol novolac type epoxy resin ("Epiclon®" N-740, manufactured by DIC Corporation) <Thermoplastic resin> • Polyvinyl formal ("Vinirec®" K, manufactured by jNC Corporation) <Hardening agent> • Dicyandiamide (DICY7, manufactured by Mitsubishi Chemical Corporation) <Curing accelerator, powder particles> • Urea compounds ("Omicure®" U24M, manufactured by Huntsman Advanced Materials) (1) Method for mixing matrix resin In a horizontal mixer (clearance between blade and wall: 2 mm), epoxy resin and thermoplastic resin in the compositions and proportions listed in Table 1 were added, and the mixture was heated to 160°C while kneading. The mixture was then stirred for 1 hour to dissolve the thermoplastic resin and obtain a matrix resin. After cooling this resin to a temperature range corresponding to a predetermined viscosity while kneading, the curing agent and powder particles listed in Table 1 were added, and the mixture was further kneaded while controlling the blade rotation speed to achieve a predetermined shear rate, thereby obtaining a matrix resin composition. (2) Method for measuring particle size The volume particle size distribution D95 of the primary particles was obtained by placing the powder particles before mixing them into the matrix resin into a dispersion liquid, stirring it, imaging the supernatant liquid with an optical microscope (ECLIPSE 80i: Nikon), and then classifying the volume particle size distribution result using image analysis software (LUZEX_AP: Nireco). The value of the particle size corresponding to 95% of the cumulative percentage distribution was obtained from the value of the largest particle size [mm]. The maximum diameter of the secondary particles was obtained by classifying the volume particle size distribution result of the powder particles dispersed in the resin after mixing them into the matrix resin in the same way as the primary particles, and then obtaining the value of the largest particle size [mm]. (3) Method for measuring the dynamic viscoelasticity of matrix resin The composition used as the matrix resin was measured using a dynamic viscoelasticity analyzer (ARES-G2: TA Instrument Co., Ltd.) with a 25 mm diameter parallel plate, a composition thickness of 1.0 mm between the parallel plates, the measurement temperature maintained at the mixing temperature, and the frequency at 0.5 Hz to determine the complex viscosity [Pa·s]. (4) Method for evaluating the surface quality of film The resulting matrix resin was applied onto a release sheet using a coater, resulting in a film with a width of 1000 mm and a resin basis weight of 33 g / m². 2 A resin film with a length of 1000m was produced. The width of each longitudinal resin gap (hereinafter referred to as "streaks") that occurs during resin film production and the number of streaks were checked. Films with a width of less than 1mm per streak and a maximum number of streaks in the film width direction of less than 50 were classified as "A," and films with one or more streaks with a width of 1mm or more, or a maximum number of streaks of 50 or more were classified as "B."

[0051] Table 1 summarizes the results of kneading the matrix resin and measuring its properties for each example and comparative example using the method described above. <Example 1> A matrix resin was prepared by mixing the components shown in Table 1. During the mixing of the powder particles, the temperature was controlled so that the complex viscosity of the matrix resin was 500 Pa·s, and the blade rotation speed was set to 50 rpm. At this time, the shear rate and shear stress applied to the matrix resin were as shown in the table. The D95 of the primary powder particles was 13 μm, and the maximum diameter of the secondary particles after mixing the matrix resin was 55 μm. The obtained matrix resin composition was applied to a release sheet using a coater to produce a resin film. The obtained resin film had 40 streaks, with a maximum width of 0.5 mm per streak, and the film surface quality was judged as "A". <Example 2> A matrix resin was prepared by mixing the components shown in Table 1. During the mixing of the powder particles, the temperature was controlled so that the complex viscosity of the matrix resin was 400 Pa·s, and the blade rotation speed was the same as in Example 1. The evaluation results are shown in Table 1. The obtained resin film had 30 streaks, with a maximum width of 0.5 mm per streak, and the film surface quality was judged as "A". <Comparative Example 1> Using the same matrix resin as in Example 1, the temperature was controlled during the mixing of powder particles so that the complex viscosity of the matrix resin was 200 Pa·s, and the blade rotation speed was set to 40 rpm. The evaluation results are shown in Table 1. The obtained resin film had 150 streaks, with a maximum width of 3.0 mm per streak, and the film surface quality was judged as "B".

[0052] In Comparative Example 1, the complex viscosity of the matrix resin during powder particle mixing was lower compared to the example, and the blade rotation speed was also lower compared to the example. As a result, the shear rate was lower, and the shear stress applied to the matrix resin was low. This was not sufficient to suppress the aggregation of powder particles, and therefore, the surface quality of the resulting resin film was poor. <Comparative Example 2> Using the same matrix resin as in Example 1, the temperature was controlled during the mixing of powder particles so that the complex viscosity of the matrix resin was 200 Pa·s, and the blade rotation speed was set to 55 rpm. The evaluation results are shown in Table 1. The obtained resin film had 80 streaks, with a maximum width of 2.5 mm per streak, and the film surface quality was judged as "B".

[0053] In the kneading method of Comparative Example 2, the blade rotation speed was higher than that of the example, but similar to Comparative Example 1, the complex viscosity of the matrix resin in the powder particle kneading was lower than that of the example, resulting in lower shear stress being applied. This was not sufficient to suppress the aggregation of powder particles, and therefore, we believe that the surface quality of the resulting resin film was poor. <Comparative Example 3> Using the same matrix resin as in Example 2, the temperature was controlled during the mixing of powder particles so that the complex viscosity of the matrix resin was 500 Pa·s, and the blade rotation speed was set to 20 rpm. The evaluation results are shown in Table 1. The obtained resin film had 120 streaks, with a maximum width of 2.5 mm per streak, and the film surface quality was judged as "B".

[0054] In Comparative Example 3, the compounding method resulted in a complex viscosity of the matrix resin during powder particle compounding that was equivalent to or higher than that of the Examples. However, similar to Comparative Example 1, the blade rotation speed was lower than that of the Examples, resulting in lower shear stress applied to the matrix resin. This was not sufficient to adequately suppress the aggregation of powder particles, which is why the surface quality of the resulting resin film was poor.

[0055] [Table 1] [Industrial applicability]

[0056] The matrix resin obtained by the kneading method of the present invention has powder particles and particulate fillers suitably dispersed in the matrix resin, resulting in a prepreg with excellent functionality and good quality. Therefore, it is preferably used in a wide range of fields as a lightweight, high-strength, and high-rigidity fiber-reinforced composite material, including sports and leisure applications such as fishing rods and golf shafts, as well as industrial applications such as automobiles and aircraft.

Claims

1. This is a method of kneading powder into the matrix resin used in fiber-reinforced composite materials. When kneading a powder in a matrix resin, where the D95 particle size (the particle size at which 95% of the cumulative volume particle size distribution measurement of the primary particle size is obtained) is between 10 μm and 80 μm, the shear stress applied to the powder in the matrix resin is set to between 160,000 Pa and 500,000 Pa. A method for kneading a matrix resin, wherein the maximum diameter of secondary particles of the powder dispersed in the matrix resin is less than 90 μm.

2. The method for kneading a matrix resin according to claim 1, wherein the complex viscosity of the matrix resin to be kneaded is 400 Pa·s or more and 1,000 Pa·s or less during the kneading process.

3. The method for kneading a matrix resin according to claim 1, wherein the shear rate during the kneading process is controlled to be between 400 / s and 1,000 / s.

4. A prepreg comprising a matrix resin kneaded by the kneading method described in any one of claims 1 to 3, impregnated into reinforcing fibers.

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