Prepreg, carbon fiber–reinforced composite material, structure, and production method for prepreg

Abstract

The present invention addresses the problem of providing a prepreg that stabilizes the conductivity between adjacent layers of carbon fibers that have been layered to form the prepreg at or above a prescribed level, even when there can be variation in the thickness between the layers of carbon fibers due to such factors as variation in the amount of resin included in the prepreg in the in-plane direction of the prepreg when the prepreg is produced. To solve this problem, the present invention is a prepreg that includes at least components [A]–[D]. For component [D], X as given by expression (1) is no more than 0.003. Component [A]: Carbon fibers. Component [B]: An epoxy resin. Component [C]: A curing agent. Component [D]: Conductive particles.

Description

Prepreg, carbon fiber reinforced composite material and structure, and method for producing prepreg

[0001] The present invention relates to a prepreg, a carbon fiber reinforced composite material, and a structure using the carbon fiber reinforced composite material.

[0002] Fiber-reinforced composite materials made of reinforcing fibers and a matrix resin have traditionally been used in numerous fields, including aerospace, automobiles, railway vehicles, ships, civil engineering and construction, and sporting goods, due to their lightweight properties and excellent mechanical properties such as strength and rigidity, as well as heat resistance and corrosion resistance. In particular, applications requiring high heat resistance require fiber-reinforced composite materials using continuous reinforcing fibers. Carbon fibers (hereinafter sometimes abbreviated as "CF"), which have excellent specific strength and specific modulus of elasticity, are used as the reinforcing fibers, and thermosetting resins, particularly epoxy resins, which have excellent adhesion to CF, heat resistance, and modulus of elasticity and low cure shrinkage, are often used as the matrix resin. In recent years, as the use of carbon fiber-reinforced composite materials (hereinafter sometimes abbreviated as "CFRP") has increased, the required properties have become even more stringent.

[0003] CFRP can be obtained by impregnating CF sheets, which are made into sheets of CF, with a matrix resin to form prepregs, and then molding the prepregs. Examples of CF sheets include UD sheets, which are made by aligning CF sheets in a unidirectional (UD) plane, and CF fabrics, which are made by arranging CF sheets in multiple directions or randomly. UD sheets are used when the mechanical properties of CFRP are prioritized, and CF fabrics tend to be used when producing CFRP with complex shapes, but they are sometimes mixed. For aircraft structural materials, where mechanical properties are prioritized, CFRPs made by laminating prepregs containing UD sheets in multiple directions and molding them are widely used.

[0004] CF is a conductor, while matrix resin is generally an insulator. Therefore, in CFRP, a matrix resin layer is formed between adjacent CF sheets (between layers), which can act as an insulating layer. Focusing on the CF sheet layers, the conductivity is relatively high in the fiber axis direction of the CFRP (hereinafter sometimes abbreviated as the "fiber direction") because the CF itself serves as a conductive path. On the other hand, in the thickness direction of the CFRP (hereinafter sometimes abbreviated as the "orthogonal direction"), although conductive paths are formed by contact between CFs, the conductivity in the orthogonal direction is approximately 1 / 1000 of the conductivity in the fiber direction. Even the conductivity of CFRP in the fiber direction is generally approximately 1 / 1000 of that of metals such as aluminum. Thus, CFRP is less conductive than metal materials, and its conductivity is anisotropic in the direction perpendicular to the fiber direction. Therefore, when a certain current flows into CFRP, a higher voltage is applied than to metal materials, and furthermore, in CFRP made up of multiple CF sheets with different fiber orientation angles, the current distribution becomes very complex.

[0005] Aircraft made of CFRP, which has such complex electrical properties, are susceptible to lightning damage. Lightning protection systems have been developed to ensure safety, including the addition of metal mesh and the use of sealant to cover areas where sparks may occur. However, these lightning protection systems have the drawback of increasing weight and cost, making it necessary to ensure that the electrical properties of the CFRP itself are consistently maintained.

[0006] One type of spark that occurs around fuel tanks is edge glow, i.e., a light-emitting phenomenon (glow) at the edge of a component, and research is being conducted to clarify the mechanism by which it occurs. Non-Patent Document 1 compares the potential analysis of CFRP with the experimental results of edge glow occurrence, and finds that creating conductivity between layers is effective in suppressing edge glow.

[0007] It is generally believed that improving the conductivity between layers is effective in increasing the conductivity in the thickness direction of CFRP. Many material designs have been proposed for this purpose, including methods for placing conductive particles between layers to achieve electrical conduction. Patent documents 1 to 5 show techniques for improving conductivity by varying the interlayer thickness, distributing conductive particles, and adding conductive additives, which have a significant effect on improving conductivity.

[0008] Patent Documents 1 and 2 disclose techniques for distributing conductive particles with a narrow particle size distribution between adjacent CF sheets (between layers) in CFRP. Patent Document 3 discloses a technique for adjusting the average particle size of conductive particles relative to the interlayer thickness. These inventions enable a greater number of conductive particles to be distributed between layers of a desired thickness for electrical conduction. Patent Document 4 discloses a technique for achieving interlayer electrical conduction even without conductive particles by varying the interlayer thickness of the resin layer or CFRP constituting the outer fiber layer of a prepreg from the average thickness depending on the location. Patent Document 5 discloses a technique for further improving electrical conductivity by using a prepreg with a matrix resin containing conductive particles with a particle size of 1 μm or less in combination with conductive particles with a particle size of 5 μm or more. It should be noted that Patent Document 3 (page 5, lines 28 to 30), Patent Document 4 (page 6, lines 9 to 11), and Patent Document 5 (paragraphs

[0034] to

[0035] ) all suggest that narrowing the particle size distribution of conductive particles is preferable from the viewpoints of conductivity, mechanical properties, and processability.

[0009] Patent Document 1: JP 2011-213991 A, International Publication No. 2008 / 056123, International Publication No. 2011 / 027160, International Publication No. 2012 / 084197, International Publication No. 2012 / 124450

[0010] R. B. Gregor et al. “Finite Element Simulation and Experimental Analysis of Edge Glow for a Generic, 16-Ply Carbon Fiber "Reinforced Plastic Composite Laminate" ICOLSE15 Paper 2015

[0011] A widely used method for producing prepregs involves impregnating a CF sheet with a resin film prepared by coating a resin composition that becomes the matrix resin when the prepreg is made using a coater or the like. When mass-producing prepregs using this method, variations in the coatability of the resin film may not be completely eliminated due to variations in the viscosity of the raw materials used, the thermal history during the production process, and speed deviations of the coating device. This can lead to a loss of uniformity in the amount of matrix resin in the prepreg, resulting in variations in the thickness between adjacent CF layers of the CFRP. For use in aircraft structures, a prepreg must be designed that can consistently exhibit conductivity even in such cases. However, none of the techniques described in Patent Documents 1 to 5 take into account the variation in conductivity of CFRP during mass prepreg production. Even if high conductivity can be achieved under certain conditions, if it cannot be consistently achieved, it increases the risk of structural damage in the event of a lightning strike. Furthermore, if there is a large variation, it will be impossible to accurately estimate the potential conductive performance of the prepreg, and as a result, excessive lightning protection systems will be introduced, which will lead to increased weight and costs for the aircraft.

[0012] In light of the common concept shared by Patent Documents 1 to 5, in which the particle size of conductive particles arranged between adjacent CF layers in CFRP is increased to ensure sufficient contact with the CF layers, allowing them to function as a conductive path, a possible approach to reducing conductivity variations dependent on the prepreg production process is to use conductive particles with a particle size large enough to accommodate the variable maximum interlayer thickness. However, it is known that large-diameter conductive particles penetrate deeply into the CF layers in CFRP, excessively disrupting the linearity of the CF arrangement and thereby degrading mechanical properties. In addition, there are many challenges in terms of processability. Specifically, when using a process for producing a resin composition film containing particles as the prepreg production process, larger particles have a higher difficulty passing through the coater, leading to variations in the amount of large-diameter particles present in the prepreg. Furthermore, clogging of the coater clearance can become the starting point for film streaks, leading to variations in the amount of matrix resin. These challenges present numerous process-related issues related to factors that can affect conductivity. Although a method of highly precisely classifying conductive particles to a particle size greater than the maximum interlayer thickness of the CF layers, which may vary, and within a particle size distribution range that does not impair process or mechanical properties, is conceivable, this method is not practical due to the need for a very high level of classification and the increased cost of raw materials. Furthermore, while it is conceivable to significantly increase the conductivity in light of the variation in conductivity, as described in Patent Document 3, there is a limit to the amount of conductive particles that can be incorporated into the CFRP because they adversely affect the mechanical properties of the CFRP, and the effectiveness of this method is therefore limited. Thus, it has been difficult to reduce the variation in conductivity of molded bodies while maintaining the mechanical properties.

[0013] The problem that the present invention aims to solve is to provide a prepreg that can stably obtain a certain level of interlayer conductivity between adjacent carbon fiber layers in CFRP, even in cases where the amount of resin contained in the prepreg varies during the prepreg production process due to raw materials, production processes, etc., and where this can result in variations in the thickness between adjacent CF layers in the CFRP formed by laminating such prepregs.In this case, the present invention aims to simplify conventional lightning protection systems and contribute to weight and cost reductions for aircraft.

[0014] [1] A prepreg comprising at least the following constituent elements [A] to [D], wherein in the constituent element [D], X represented by the following formula (1) is 0.003 or less.

[0015] Component [A]: carbon fiber Component [B]: epoxy resin Component [C]: hardener Component [D]: conductive particles

[0016]

[0017] where α ｉ a: the number of particles included in the particle size interval of i-0.5 (μm) or more and less than i+0.5 (μm), a: d50, b: d99, i: an integer of d50 or more and d99 or less, N: the total number of particles to be measured, where d50 is the median diameter (50% diameter; D50) of component [D] expressed in micrometers and rounded to the nearest whole number, and d99 is the 99% diameter (D99) of component [D] expressed in micrometers and rounded to the nearest whole number. [2] The prepreg according to [1] above, in which layers containing component [B], component [C], and component [D] and not containing component [A] are laminated on both sides of a layer containing component [A], component [B], and component [C] but not containing component [D]. [3] The prepreg according to [1] or [2] above, wherein D50 of the component [D] is equal to or greater than the average radius of the component [A]. [4] The prepreg according to any one of [1] to [3] above, wherein D99 (μm) of the component [D] is 0.33 × A or less, where A is the basis weight of the component [A] in the prepreg in g / m ２ [5] The prepreg according to any one of [1] to [4] above, wherein D50 (μm) of the component [D] is 0.18 × A or less, where A is the basis weight of the component [A] in the prepreg in g / m ２The values ​​are rounded to one decimal place when expressed. [6] The prepreg according to any one of [1] to [5] above, wherein when 16 plies of the prepreg are quasi-isotropically laminated in a [+45° / 0° / -45° / 90°] 2s configuration and cured in an autoclave under heating and pressure at a temperature of 180°C for 2 hours at a pressure of 0.6 MPa and a temperature increase rate of 1.7°C / min to obtain a carbon fiber reinforced composite material, the coefficient of variation in thickness within one interlaminar resin layer is 26% or more and 45% or less. [7] The prepreg according to any one of [1] to [6] above, wherein component [A] is carbon particles or conductive polyamide particles. [8] The prepreg according to any one of [1] to [7] above, further comprising component [E].

[0018] Component [E]: Non-conductive polyamide particles. [9] The prepreg according to [8] above, wherein the mass ratio of component [E] is 4% by mass or more and 15% by mass or less, when the total mass of the components other than component [A] is taken as 100% by mass.

[10] A carbon fiber reinforced composite material obtained by laminating two or more sheets of the prepreg according to any one of [1] to [9] above and heat-curing the laminate.

[11] The carbon fiber reinforced composite material according to

[10] above, wherein the electrical conductivity of the interlaminar resin layer is 0.1 S / m or more.

[12] The carbon fiber reinforced composite material according to

[10] or

[11] above, wherein the CAI is 230 MPa or more.

[13] A structure obtained by using the carbon fiber reinforced composite material according to any one of

[10] to

[12] above, wherein the shape of the structure is selected from the group consisting of a flat structure, a cylindrical structure, a box-shaped structure, a C-shaped structure, an H-shaped structure, an L-shaped structure, a T-shaped structure, an I-shaped structure, a Z-shaped structure, and a hat-shaped structure.

[14] A method for producing a prepreg, comprising the steps of: impregnating a component [A] with a primary resin composition containing components [B] and [C] but not component [D] to obtain a prepreg intermediate material; and applying a resin film using a secondary resin composition containing components [B], [C], and [D] to both sides of the prepreg intermediate material, wherein X, represented by the following formula (1), for component [D] contained in the secondary resin composition is 0.003 or less, wherein component [A] is carbon fiber, component [B] is epoxy resin, component [C] is a curing agent, and component [D] is conductive particles.

[0019]

[0020] where α ｉ a: the number of particles included in the particle size interval of i-0.5 (μm) or more and less than i+0.5 (μm), a: d50, b: d99, i: an integer equal to or greater than d50 and equal to or less than d99, N: the total number of particles to be measured, where d50 is the median diameter (50% diameter, D50) of the component [D] expressed in micrometers and rounded to the nearest whole number, and d99 is the 99% diameter (D99) of the component [D] expressed in micrometers and rounded to the nearest whole number.

[0021] The present invention makes it possible to obtain a prepreg that can stably maintain a certain level of interlaminar conductivity between adjacent carbon fiber layers in CFRP, even when the amount of resin in the in-plane direction in the prepreg varies due to raw materials, production processes, etc. Furthermore, by applying such prepreg and carbon fiber reinforced composite material to aircraft, the overall efficiency of lightning protection systems can be improved.

[0022] Top view of the CFRP panel produced in the example section

[0023] The present invention will be described in more detail below.

[0024] <Component [A]> In the prepreg of the present invention, component [A], i.e., carbon fiber, is used as a CF bundle in which carbon fibers are bundled together. For example, a tape-like collection of approximately 1,000 to 1,000,000 carbon fiber monofilaments is typically called a "tow," and a sheet-like carbon fiber bundle can be obtained by arranging these tows. The carbon fiber that forms a sheet within the prepreg is called a CF sheet. Depending on the arrangement state, a CF sheet in which the carbon fibers are oriented in one direction (UD) along the longitudinal direction of the carbon fiber is called a UD sheet, and a CF fabric in which the carbon fibers are oriented in multiple directions is called a CF fabric. As the carbon fiber fabric, in addition to woven fabrics and knitted fabrics, fabrics in which carbon fiber is multiaxially arranged in two dimensions, and nonwoven fabrics, mats, paper, and other fabrics in which carbon fiber is randomly oriented can be used.

[0025] <Component [B]> The prepreg of the present invention uses an epoxy resin composition as the matrix resin, which contains at least an epoxy resin (component [B]), a curing agent (component [C]), and conductive particles (component [D]). The epoxy resin is preferably one whose precursor is an amine, a phenol, or a compound having a carbon-carbon double bond. Specific examples of epoxy resins whose precursor is an amine include various isomers of tetraglycidyldiaminodiphenylmethane (N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane), triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, and triglycidylaminocresol; epoxy resins whose precursor is a phenol include bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, phenol novolac epoxy resin, and cresol novolac epoxy resin; and epoxy resins whose precursor is a compound having a carbon-carbon double bond include alicyclic epoxy resins, but are not limited to these. Reducing the crosslink density of the matrix resin is effective in improving the tensile strength of CFRP, but this tends to reduce heat resistance and elastic modulus. Dicyclopentadiene-type epoxy resins and glycidylaniline-type epoxy resins are preferred because they improve heat resistance and elastic modulus. Epoxy resins whose precursors are aromatic amines, such as tetraglycidyldiaminodiphenylmethane, are most suitable for the present invention because of their excellent heat resistance and adhesiveness to reinforcing fibers.

[0026] <Component [C]> As the curing agent, which is component [C], a compound having an active group capable of reacting with an epoxy group can be used. Preferably, a compound having an amino group, an acid anhydride group, or an azide group is suitable. Specifically, dicyandiamide, various isomers of diaminodiphenyl sulfone, and aminobenzoic acid esters are suitable.

[0027] <Component [D]> In the present invention, a prepreg can be obtained in which component [A] is impregnated with a composition (matrix resin) containing at least component [B] through component [D]. In addition to the aforementioned composition, a composition containing component [B] and component [C] but not component [D] can also be used as a matrix resin. This can then be molded to obtain a CFRP. Since the prepreg has at least some matrix resin layers on the upper and lower surfaces of the CF sheet, the prepreg laminate and CFRP have a carbon fiber layer (CF layer) containing the matrix resin and the CF sheet, and a resin layer sandwiched between adjacent CF layers. The interlaminar thickness of adjacent CF layers and the thickness of the resin layer between the CF layers are synonymous. In the present invention, conductive particles, which are component [D], are disposed in the resin layer between adjacent CF layers (such a resin layer is referred to as an "interlaminar resin layer"). These conductive particles form portions that electrically connect adjacent CF layers, thereby providing electrical continuity between the layers, meaning that the conductive particles are substantially in contact with the CF layers present above and below the interlayer resin layer, forming a conductive path.

[0028] The conductive particles, which are the component [D] used in the prepreg of the present invention, may be particles that behave as good electrical conductors, and are not limited to particles consisting solely of conductors. The volume resistivity of the conductive particles is preferably 10 Ωcm or less, more preferably 5 Ωcm or less, and even more preferably 1 Ωcm or less. By setting the volume resistivity within this range, a conductive path can be formed in the interlaminar resin layer, thereby increasing the conductivity in the thickness direction. Here, the volume resistivity can be calculated by placing a sample in a cylindrical cell with four probe electrodes and measuring the thickness and resistance of the sample while applying a pressure of 60 MPa to the sample.

[0029] Specific examples of such conductive particles include metal particles, metal oxide particles, metal-coated inorganic particles, organic polymer particles, conductive polymer particles such as polyacetylene particles, polyaniline particles, polypyrrole particles, polythiophene particles, polyisothianaphthene particles, polyethylenedioxythiophene particles, and polyamide particles, and carbon particles, etc. Among these, carbon particles and polyamide particles that have been given conductivity (conductive polyamide particles) are preferred.

[0030] Carbon particles are preferably used because they do not cause corrosion problems even when used in aircraft. Crystalline carbon and amorphous carbon are preferably used as carbon particles. Specific examples of amorphous carbon include "Bellpearl (registered trademark)" C-600, C-800, and C-2000 (manufactured by Air Water Inc.), "NICABEADS (registered trademark)" ICB, PC, and MC (manufactured by Nippon Carbon Co., Ltd.), glassy carbon (manufactured by Tokai Carbon Co., Ltd.), high-purity artificial graphite SG series, SGB series, and SN series (manufactured by SEC Carbon Co., Ltd.), and spherical carbon (manufactured by Gun-ei Chemical Industry Co., Ltd.).

[0031] Conductive polyamide particles are preferably used because they are conductive and can provide CFRP with the impact resistance of polyamide particles, which will be described later. Conductive polyamide particles can be obtained by imparting conductivity to non-conductive polyamide particles, which will be described later. The means for imparting conductivity to polyamide particles is not particularly limited, but examples include coating with a metal, conductive polymer, or conductive nanomaterial, and dispersing a conductive polymer or conductive nanomaterial within the polyamide particles. Among these, metal coating and dispersing particulate conductive nanomaterials within the particles are preferred from the viewpoint of achieving both conductivity and impact resistance. The conductive nanomaterial is not particularly limited, but examples include carbon nanotubes, carbon black, and graphene. Conductive particles with an aspect ratio of 1.5 or less are preferably used.

[0032] Furthermore, in addition to these conductive particles, conductive substances in a form other than particles, that is, conductive fillers, short fibers, or the conductive nanomaterials, can also be used in combination as needed.

[0033] <Particle Size Distribution of Component [D]> In the component [D] used in the present invention, X represented by the following formula (1) is 0.003 or less. In the following description, this X may be referred to as the particle size change rate X.

[0034]

[0035] where α ｉ a: the number of particles included in the particle size interval of i-0.5 (μm) or more and less than i+0.5 (μm), a: d50, b: d99, i: an integer equal to or greater than d50 and equal to or less than d99, N: the total number of particles to be measured, where d50 is the median diameter (50% diameter, D50) of the component [D] expressed in micrometers and rounded to the nearest whole number, and d99 is the 99% diameter (D99) of the component [D] expressed in micrometers and rounded to the nearest whole number.

[0036] Here, D50 and D99, as well as D10 and D90 described below, D10 is the particle size at which the proportion of particles having a particle size of D10 or less is 10%, D50 is the particle size at which the proportion of particles having a particle size of D50 or less is 50%, D90 is the particle size at which the proportion of particles having a particle size of D90 or less is 90%, and D99 is the particle size at which the proportion of particles having a particle size of D99 or less is 99%. These can be easily determined from a graph with particle size on the horizontal axis and cumulative relative frequency (%) on the vertical axis.

[0037] By adjusting the particle size change rate X of component [D] to 0.003 or less, a CFRP with stable interlayer conductivity can be obtained, even if variations in the interlayer thickness of the CF layers occur due to changes in the amount of matrix resin in the prepreg caused by raw materials, production processes, etc. Methods for obtaining conductive particles with such a predetermined particle size distribution include, for example, classifying a single conductive particle into a desired range, mixing two or more types of conductive particles with different particle size distributions, or, if necessary, mixing and then classifying. When using two or more types of conductive particles with different particle size distributions, the materials may be the same or different. If the desired particle size distribution of the conductive particles is obtained, classification may not necessarily be necessary. As described above, in CFRP, the conductive particles present between the CF layers electrically connect the CF layers, thereby achieving conductivity between adjacent CF layers. Furthermore, as described above, the interlayer thickness may vary depending on the raw materials and production processes of the prepreg, such as the coatability during film production, and this interlayer thickness may be partially thinner. Here, when the mass of conductive particles added is constant, the number of particles is inversely proportional to the cube of the diameter, so the smaller the diameter, the greater the number of conductive particles. In other words, when thin interlayers are possible, by widening the particle size distribution of the conductive particles relative to the small diameter side, the number of conductive particles on the small diameter side can be increased even with the same added mass. Since the conductive particles on the small diameter side can frequently connect with the upper and lower CF layers in the partially thin interlayers, a particle size distribution with a wide distribution on the small diameter side is preferable. On the other hand, by using a particle size distribution with a wide distribution on the large diameter side, the large diameter particles can connect the upper and lower CF layers in the partially thick interlayers.

[0038] When the particle size change rate X exceeds the range specified in the present invention, the particle size distribution of the conductive particles varies significantly. The particle size distribution of such particles tends to be narrow (high kurtosis), as in the conductive particles preferably used in Patent Documents 1 to 5. When the particle size change rate X is large, the number of conductive particles with a particle size capable of electrically connecting adjacent CF layers tends to change significantly with variations in interlayer thickness, resulting in a large variation in the interlayer conductivity of the CF layers. If the number of particles that can be arranged in the interlayer resin layer of a CFRP is sufficiently large, even if the number of particles changes with variations in interlayer thickness, the number of particles sufficient to connect the CF layers and exhibit conductivity can be ensured. However, if the particle amount is too large, problems with mechanical properties and processability may arise, as described above, and therefore there is a limit to the amount of particles that can be suitably arranged. The content of component [D] is preferably 0.05% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 9% by mass or less, and even more preferably 0.5% by mass or more and 8% by mass or less, when the mass of the matrix resin is taken as 100% by mass.

[0039] Therefore, for example, if there is variation in the interlayer thickness of the CF layers and the number of conductive particles capable of electrically connecting adjacent CF layers also changes significantly, the variation in conductivity due to the variation in interlayer thickness may become greater than the allowable range. In the present invention, by setting the particle size change rate X of the component [D] to a range of 0.003 or less, even if the interlayer thickness of the CFRP varies, the change in the number of conductive particles capable of connecting adjacent CF layers is small, and stable interlayer conductivity can be obtained even during mass production.

[0040] The particle size change rate X is more preferably in the range of 0.002 or less, and even more preferably in the range of 0.001 or less. In reality, a component [D] having a lower limit of the particle size change rate X of 0.0001 or more is preferably used because the production process becomes complicated and the cost may increase.

[0041] Next, the procedure for obtaining D10, D50, D90, and D99 in the particle size distribution of conductive particles is described. The surface of the prepreg is observed using a laser microscope (e.g., a digital microscope VHX-5000 manufactured by Keyence Corporation) at a magnification of 200 times or more. The diameter of the circle inscribed on the periphery of the observed image is taken as the particle size, and the diameter is calculated in micrometers to two decimal places. This is then rounded off to the nearest tenth to obtain a value to one decimal place. This is done for each of 1,000 arbitrarily selected particles. However, particles with an inscribed circle diameter of less than 0.5 μm are excluded from the measurement. If a secondary resin composition film containing conductive particles is available, it may also be used as the observation subject. From the particle sizes of 1,000 particles in the prepreg or secondary resin composition film, a graph is obtained with particle size on the horizontal axis and cumulative relative frequency (%) on the vertical axis, and the particle size at which the cumulative relative frequency is 10% is designated D10, the particle size at which the cumulative relative frequency is 50% is designated D50, the particle size at which the cumulative relative frequency is 90% is designated D90, and the particle size at which the cumulative relative frequency is 99% is designated D99.

[0042] The particle size change rate X is calculated using the values ​​d50 and d99 obtained by expressing the D50 and D99 obtained above in micrometers and rounding them to the nearest whole number, and is calculated using formula (1) for particles having a size of (d50-0.5) μm or more and less than (d99+0.5) μm. Here, N is the total number of particles measured, which is 1000 in this procedure.

[0043] Furthermore, in the prepreg of the present invention, the median diameter (D50) of the component [D] is preferably equal to or greater than the average radius of the component [A]. The average radius of the component [A] here refers to the average radius of a single yarn. It is more preferably equal to or greater than twice the average radius of the component [A] (i.e., equal to or greater than the average diameter of the carbon fiber), and even more preferably equal to or greater than four times the average radius of the component [A] (i.e., equal to or greater than twice the average diameter of the component [A]). There is no particular upper limit for the D50 of the component [D] as long as it does not adversely affect the mechanical properties. However, from the viewpoint of reducing interlaminar conductivity variation and achieving a high CAI (compressive strength after impact), it is desirable that the upper limit be 0.18×A (μm) or less, as described below, depending on the basis weight of the carbon fiber.

[0044] This range can reduce the variation in electrical conductivity between adjacent CF layers caused by the penetration of small-diameter particles into the CF layer, and also can prevent cracks in the CFRP upon impact from propagating into the CF layer along the small-diameter conductive particles that have penetrated the CF layer, thereby suppressing a decrease in the CAI (described later).

[0045] Furthermore, in general, when using CFRP as an aircraft component, it is necessary to adjust the basis weight (mass per unit area) of the carbon fiber in the prepreg depending on the part to which the CFRP is applied, and when the basis weight of the CF is changed, the thickness between layers changes if the volume content of the CF in the CFRP is kept constant. In the present invention, the upper limit of the D50 (here, the unit of D50 is μm) of the conductive particles is set to g / m for the basis weight of the carbon fiber in the prepreg. ２ When the value obtained by rounding off to one decimal place when expressed is taken as A, the D50 is preferably 0.18 x A or less, and more preferably 0.15 x A or less. There is no particular lower limit, but it is preferably 0.05 x A or more. Note that although particle size and basis weight have different units, the preferred range of D50 is determined based on the correlation between basis weight and particle size (the same applies to the relationship between D99 and the basis weight of the carbon fiber in the prepreg below).

[0046] By setting the upper limit of D50 of the component [D] within the above range, the number of conductive particles that penetrate too deeply into the CF layer and disrupt the linearity of the CF arrangement can be further reduced, thereby achieving a high CAI, and the number of conductive particles that can function as a conduction path between adjacent upper and lower CF layers increases assuming the same mass, which is preferable.

[0047] Furthermore, D10 of the component [D] is preferably equal to or greater than the average radius of the component [A], more preferably equal to or greater than twice the average radius of the component [A] (i.e., equal to or greater than the average diameter of the CF), and even more preferably equal to or greater than three times the average radius of the component [A] (i.e., equal to or greater than the average diameter of the CF). By setting D10 in the above range, it is possible to reduce the variation in conductivity between adjacent carbon fiber layers caused by the penetration of small-diameter conductive particles into the CF layer, which is preferable. Furthermore, in a CFRP, it is possible to prevent cracks upon impact from propagating into the CF layer along the small-diameter conductive particles that have penetrated the CF layer, and it is preferable to suppress a decrease in the CAI (compressive strength after impact) described below. Furthermore, D99 (here, the unit of D99 is μm) of the component [D] is expressed as g / m 2 in terms of the basis weight of the carbon fiber in the prepreg. ２ When the value rounded to one decimal place when expressed is A, from the viewpoint of processability and the viewpoint of being able to suppress a decrease in CAI due to conductive particles that have embedded too deeply into the CF layer, D99 is preferably 0.33 × A (μm) or less, more preferably 0.30 × A (μm) or less, and even more preferably 0.27 × A (μm) or less. There is no particular restriction as a lower limit, but 0.17 × A (μm) or more is preferable, and 0.20 × A (μm) or more is more preferable. By setting D99 within this range, it is possible to suppress a decrease in CAI, and even if the interlayer thickness varies due to changes in the amount of resin in the prepreg during the production process, the change in the number of conductive particles that can connect adjacent CF layers is small, making it easier to obtain stable interlayer conductivity after molding of the prepreg laminate, which is preferable.

[0048] When multiple types of conductive particles are used, unless otherwise specified, D10, D50, D90 and D99 are values ​​for all the conductive particles.

[0049] <Average Radius of Component [A]> Next, the procedure for obtaining the average radius of component [A] will be described. When the average radius of component [A] is determined from CFRP, it can be measured and calculated by obtaining a cross-sectional observation image of the CFRP. The CFRP is cut in a direction perpendicular to the fiber axis direction of the CF embedded in the CFRP to obtain a cross section. A laser microscope (for example, VHX-5000, manufactured by Keyence Corporation) is used to observe the CF layer (90° layer) perpendicular to the cutting direction of the sample. The diameter of a circle inscribed in the outer periphery of the cross section of the obtained CF is obtained. 300 CFs are selected from the cross-sectional image, and their diameters are measured using the above method. The arithmetic mean is calculated and divided by 2 to obtain the average radius of the CFs.

[0050] When the average radius of CF is determined from a prepreg, it can be measured and calculated from a cross-sectional observation image of the prepreg. Similar to the method for obtaining a cross-section from CFRP described above, the prepreg is cut with a sharp blade in a direction perpendicular to the fiber axis of the CF, and a cross-section perpendicular to the CF is obtained and observed using a scanning electron microscope (SEM). The method for calculating the average radius of CF from the obtained cross-section is the same as the method for calculating from the cross-section of CFRP described above. This method may be used if the resin constituting the prepreg does not flow during the cutting and a cross-section in the perpendicular direction can be obtained stably and accurately.

[0051] <D99 / D50 Ratio in Particle Size Distribution of Component [D]> In the particle size distribution of component [D], the value obtained by dividing D99 by D50, (D99 / D50), is preferably 1.7 or more. D99 / D50 is more preferably 2.0 or more, and even more preferably 2.4 or more. By setting the ratio within this range, even if the thickness of the interlaminar resin layer in CFRP varies, the change in the number of conductive particles capable of connecting adjacent CF layers is small, making it easier to obtain stable interlaminar conductivity even during mass production. Furthermore, from the viewpoints of processability and CAI, D99 / D50 is preferably 8.0 or less. It is more preferably 6.0 or less, and even more preferably 4.0 or less.

[0052] The effect of suppressing the variation in conductivity between layers by controlling the particle size distribution of the conductive particles is 2) and the resin content (Rc (mass%)), A × (Rc / 100) ２ This is particularly noticeable when the value calculated by takes a value between D50 and D99 (where the units of D50 and D99 are μm).

[0053] Here, the resin content refers to the mass proportion of the matrix resin (i.e., components other than CF) when the mass of the prepreg is taken as 100 mass %, and the specific measurement method is as described in the Examples section.

[0054] <Mass Proportion of Component [D] Having a Particle Size of D50 or More> In the prepreg of the present invention, the mass proportion of component [D] having a particle size of D50 or more is preferably 70 mass% or more, and more preferably 80 mass% or more, when the total mass of component [D] is taken as 100 mass%. Within this range, the number of conductive particles that function as conductive paths between adjacent CF layers can be ensured, and the variation in conductivity between the CF layers can be further reduced.

[0055] The mass proportion of the component [D] having a particle size of D50 or more can be determined by the following procedure. Similar to the procedure for measuring the particle size of the component [D] described above, the particle sizes of 1,000 particles are arbitrarily selected on the surface of the prepreg, and the 1,000 particles are arranged in order from smallest to largest. The particle size is divided by 2, and the radius r is calculated as 4 / 3 × π × r ３ The volume of each particle is calculated from the above. The mass ratio of conductive particles having a particle size of D50 or more is calculated by calculating the total volume of the 501st to 1000th particles relative to the total volume of the 1st to 1000th particles. When two or more types of material components [D] are used, their specific gravities are considered to be equal. If a secondary resin composition film is available, it may also be used.

[0056] <Shape of Component [D]> The sphericity of the component [D] used in the present invention is preferably 90% or more, and more preferably 95% or more. By using conductive particles with high sphericity, the influence of variations in the orientation of the major diameter of the particles in the interlaminar resin layer due to the flow of the matrix resin during molding of the prepreg laminate can be suppressed, and as a result, variations in the conductivity between layers after molding of the prepreg laminate are reduced.

[0057] A preferred procedure for obtaining sphericity is as follows. First, the sphericity of each conductive particle is determined. Similar to the procedure for measuring the particle size of the component [D] described above, the surface of the prepreg is observed with a laser microscope, and the diameters of the inscribed and circumscribed circles on the periphery of 1,000 randomly selected particles are measured. As with the particle size measurement, particles with an inscribed circle diameter of less than 0.5 μm are excluded. For each particle, the diameter of the inscribed circle is divided by the diameter of the circumscribed circle, and the result is multiplied by 100 to determine the sphericity (%) of the individual particle. Next, this is performed for 1,000 particles, and the median sphericity of the 1,000 conductive particles, i.e., the average value of the sphericity of the 500th particle and the 501st particle when sorted by smallest sphericity, is determined as the sphericity of the component [D]. If a secondary resin composition film is available, it may also be used.

[0058] <Ratio of Presence of Component [D] from the Prepreg Surface to a Depth of 15%> For the component [D] used in the present invention, S1 / S2, which is an index of the ratio of presence of component [D] from the prepreg surface to a depth of 15%, as determined as follows, is preferably 0.9 or more. Here, S1 is the total area of ​​particles present from the prepreg surface to a depth of 15% when the total thickness of the prepreg is taken as 100% in the cross section of the cured prepreg described below, and S2 is the total area of ​​particles present throughout the entire thickness of the prepreg. S1 / S2 is more preferably 0.95 or more. By adopting such a form, the conductive particles can be stably present between layers after molding into a laminate, and fluctuations in the number of particles forming the portion electrically connecting the upper and lower CF layers are reduced, allowing the prepreg laminate to exhibit stable interlayer conductivity after molding.

[0059] S1 / S2 is determined as follows. A prepreg is sandwiched between two parallel, smooth-surfaced polytetrafluoroethylene resin plates without applying pressure to allow them to adhere tightly to one another. The temperature is gradually increased to 150°C over seven days to gel and cure the prepreg, producing a plate-shaped cured prepreg. After curing, the prepreg is cut in a direction perpendicular to the adhesion surface (thickness direction). The cross section is polished and then photographed using a laser microscope (e.g., a digital microscope VHX-5000 manufactured by Keyence Corporation) at a magnification of 200x or more, ensuring that the top and bottom surfaces of the cured prepreg are within the field of view. The spacing between the polytetrafluoroethylene resin plates is measured at five arbitrarily selected locations in the horizontal direction (width direction of the prepreg) of the cross-sectional photograph, and the average value is taken as the average thickness of the cured prepreg. Two lines parallel to the surface of the cured prepreg are drawn on the photograph of the cured prepreg, at a depth of 15% of the average thickness of the cured prepreg from both surfaces of the cured prepreg. The total area S1 of all conductive particles present between the surface of the cured prepreg and the parallel lines and the total area S2 of all conductive particles present throughout the entire thickness of the cured prepreg are calculated. Note that particles on the parallel lines are not included in the measurement of S1. The abundance ratio of particles present within a depth range of 15% from the surface of the cured prepreg is calculated by dividing S1 by S2. As mentioned above, the cured prepreg was cured under very slow conditions, so the value calculated by this calculation is considered to be the abundance ratio of conductive particles within a depth of 15% from the prepreg surface. Furthermore, when two or more materials [D] are used as constituent elements, their specific gravities are considered to be equal.

[0060] <Thermoplastic Resin> In the present invention, it is preferable to incorporate a thermoplastic resin into the matrix resin, and it is also preferable that such a thermoplastic resin be compatible with the epoxy resin, which is the component [B] used. Epoxy resins generally have the drawback of being brittle but are capable of low-pressure molding in an autoclave. On the other hand, thermoplastic resins generally have the advantage of being tough but are difficult to mold in an autoclave at low pressures. In other words, the two exhibit contradictory properties. However, by mixing these, it is possible to achieve a balance between physical properties and moldability in low-pressure molding in an autoclave. The thermoplastic resin dissolved in the epoxy resin is preferably contained in an amount of 5% by mass or more, more preferably 10% by mass or more, when the total amount of the epoxy resin composition excluding solid mass components such as conductive particles and non-conductive polyamide particles is taken as 100% by mass. From a process standpoint, the upper limit is preferably 20% by mass or less.

[0061] The thermoplastic resin may be a polymer having a bond in its main chain selected from a carbon-carbon bond, an amide bond, an imide bond, an ester bond, an ether bond, a carbonate bond, a urethane bond, a urea bond, a thioether bond, a sulfone bond, an imidazole bond, and a carbonyl bond. Specific examples of the thermoplastic resin include polyacrylate, polyolefin, polyamide (PA), aramid, polyester, polycarbonate (PC), polyphenylene sulfide (PPS), polybenzimidazole (PBI), polyimide (PI), polyetherimide (PEI), polysulfone (PSU), polyethersulfone (PES), polyaryletherketone (PAEK), and polyamideimide (PAI). Examples of PAEK include polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). In fields requiring heat resistance, such as aircraft applications, PPS, PES, PI, PEI, PSU, PEEK, PEKK, PAEK, etc. are preferred. PES is particularly preferred from the viewpoint of mechanical properties and compatibility with epoxy resins. These may be polymers, or oligomers or monomers may be used for the purposes of low viscosity and low-temperature application. Of course, these may be copolymerized depending on the purpose, or various types may be mixed and used as polymer blends or alloys.

[0062] <Non-Conductive Polyamide Particles> In the prepreg of the present invention, it is preferable to dispose non-conductive polyamide particles in the layer that will become the interlaminar resin layer from the viewpoint of toughness. Such polyamide particles are different from the conductive polyamide particles described above and are non-conductive (not treated to impart conductivity). Such non-conductive polyamide particles may be used in combination with the conductive polyamide particles described above. The use of non-conductive polyamide particles can improve the interlaminar toughness of CFRP and further improve the impact resistance of CAI and other properties important in aircraft applications. Suitable polyamides include nylon 12, nylon 11, nylon 6, nylon 66, nylon 6 / 12 copolymers, and nylons (semi-IPN nylons) formed into a semi-IPN (polymer interpenetrating network) structure with an epoxy compound as described in Example 1 of JP-A-01-104624. To achieve a preferred CAI range, the non-conductive polyamide particles are preferably contained in an amount of 4% by mass or more when the mass of the matrix resin is taken as 100% by mass, while from the viewpoint of processability, 60% by mass or less is preferred. Furthermore, from the viewpoint of easily controlling the coefficient of variation of thickness within one interlayer resin layer in the CFRP described below within a preferred range, the non-conductive polyamide particles are preferably contained in an amount of 4% by mass or more and 15% by mass or less, more preferably 5% by mass or more and 12% by mass or less, and even more preferably 6% by mass or more and 10% by mass or less, when the mass of the epoxy resin composition constituting the prepreg is taken as 100% by mass. Since the polyamide particles are not crushed during molding under pressure as described below and maintain their original particle shape, it is more likely that the desired interlayer thickness will be obtained. Therefore, it is more preferred that the polyamide particles be crystalline, having a melting point above the molding temperature.

[0063] When using a two-stage impregnation method as described below, when the mass of the secondary resin composition is taken as 100 mass%, the non-conductive polyamide particles are more preferably 4 mass% or more and 30 mass% or less, and even more preferably 12 mass% or more and 20 mass% or less.

[0064] <Carbon Fiber Reinforced Composite Material (CFRP)> The CFRP of the present invention can be obtained by laminating two or more sheets of the prepreg of the present invention together and heat-curing them, and various known methods can be used for this purpose. For example, a method in which the obtained prepreg is cut to a predetermined size, a predetermined number of prepregs are laminated together, and then heat-cured while applying pressure can be preferably used.

[0065] Methods for heat-curing laminated prepregs while applying pressure include press molding, autoclave molding, bagging molding, wrapping tape method, and internal pressure molding, and are used appropriately depending on the intended use. Of these, autoclave molding is often used for aircraft and spacecraft applications because it can produce CFRP with excellent performance and stable quality.

[0066] The temperature at which CFRP is molded needs to be adjusted depending on the type of curing agent contained in the epoxy resin composition, and when an aromatic amine compound is used as the curing agent, molding is usually carried out at a temperature in the range of 150 to 220° C. If the molding temperature is too low, sufficient rapid curing may not be achieved, and conversely, if it is too high, warping due to thermal distortion may occur.

[0067] The pressure used to mold CFRP using an autoclave molding method varies depending on factors such as the thickness of the prepreg and the volume content of the reinforcing fibers, but is typically in the range of 0.1 MPa to 1 MPa. This allows for the production of CFRP that is free of defects such as voids and has minimal dimensional variation such as warping.

[0068] <Coefficient of Thickness Variation Within One Interlayer Resin Layer in CFRP> In the CFRP of the present invention, focusing on one interlayer resin layer, the coefficient of variation of its thickness is preferably 26% or more and 45% or less. More preferably, it is 28% or more and 43% or less, and even more preferably, it is 30% or more and 40% or less. A coefficient of variation of thickness within one interlayer resin layer of 26% or more ensures a favorable interlayer thickness distribution for the wide particle size distribution of the conductive particles used in the present invention, allowing a greater number of conductive particles, ranging from large to small diameters, present between layers to electrically connect the upper and lower CF layers. As a result, even if the interlayer thickness varies slightly within a CFRP molded product, the variation in the number of conductive particles capable of connecting adjacent CF layers can be reduced, resulting in stable interlayer conductivity. Furthermore, the above range is preferable because it allows for the development of a high CAI. Here, the thickness variation of the interlayer resin layer occurs because the conductive particles present between the layers function as spacers that contact the upper and lower CF layers between the layers. Therefore, the thickness variation coefficient within one interlayer resin layer in CFRP is primarily affected by the particle size and particle size distribution of the conductive particles. However, it may also be affected by solid particles disposed between the layers, including the non-conductive polyamide particles. The coefficient of variation can be within a suitable range depending on the particle size distribution, D50, D99, and mass fraction of the non-conductive polyamide particles in the epoxy resin composition contained in the prepreg.

[0069] In the CFRP of the present invention, it is preferable that interlayer resin layers having a thickness variation coefficient of 26% to 45%, preferably 28% to 43%, and more preferably 30% to 40%, account for 50% or more, preferably 70% or more of the total number of interlayer resin layers, and most preferably all interlayer resin layers satisfy this coefficient.

[0070] The coefficient of variation in thickness within one interlayer resin layer in CFRP can be obtained as follows: The CFRP of the present invention is cut to obtain a cross section perpendicular to the fiber axis of the carbon fiber in the 0° layer. The cross section is then polished and magnified 500 times with an optical microscope. Photographs are taken of the boundary regions of the interlayer resin layer between adjacent CF layers, randomly selecting 20 areas in the in-plane direction for each layer, avoiding overlapping of photographed locations. For each cross-sectional photograph, the thickness of the interlayer resin layer between adjacent CF layers (the distance between the CF layers in the thickness direction of the CFRP at each measurement point) is measured at 100 points at 5 μm intervals along the longitudinal direction between each layer. The average thickness (μm) and standard deviation (μm) of the interlayer resin layer are calculated from the 2,000 measurements. The average thickness (μm) of the interlayer resin layer is the arithmetic mean of the thicknesses of the interlayer resin layer at each measurement point. The standard deviation (μm) is divided by the average thickness (μm) to determine the coefficient of variation (%) of the thickness of the interlayer resin layer being measured.

[0071] Furthermore, when the prepreg of the present invention is prepared by laminating 16 plies quasi-isotropically in a [+45° / 0° / -45° / 90°] 2s configuration and curing them in an autoclave under heating and pressure at 180°C for 2 hours at a pressure of 0.6 MPa and a heating rate of 1.7°C / min to obtain a CFRP, it is preferable that the coefficient of variation in thickness within one interlaminar resin layer be 26% or more and 45% or less. By producing such a prepreg, the interlaminar thickness distribution becomes favorable relative to the particle size distribution of the conductive particles used in the present invention, and the variation in the number of conductive particles capable of connecting adjacent CF layers can be reduced, resulting in stable interlaminar conductivity.

[0072] <Interlayer Electrical Conductivity of CFRP> The electrical conductivity between adjacent CF layers (interlayer resin layers) in the CFRP after laminating and molding the prepreg of the present invention is preferably 0.1 S / m or more, as this may simplify the lightning protection system in aircraft. The electrical conductivity between such CF layers is more preferably 1.0 S / m or more, and even more preferably 1.5 S / m or more. On the other hand, if an attempt is made to make the electrical conductivity between adjacent CF layers very high, the manufacturing process may become complicated and the costs may become very high, so in reality, a conductivity of 1000 S / m or less is preferable.

[0073] The conductivity between adjacent CF layers (interlayer resin layer) can be measured by the following method. A rectangular parallelepiped test piece is cut out from the CFRP. In this case, the thickness direction of the laminated prepreg is defined as the z-axis, and the directions parallel to the other two sides of the rectangular parallelepiped other than the thickness direction are defined as the x-axis and y-axis. The area S3 (m) of the xy plane of the test piece is calculated by multiplying the lengths of the sides in the x-axis and y-axis directions of the test piece. ２ ) is calculated. The surfaces of the test piece parallel to the xy plane are polished until the CF layer is exposed on the surface. Next, the four surfaces other than the polished surface are masked, platinum metal is vapor-deposited on the polished surface to form an electrode, and the masking is removed to obtain a sample for conductivity evaluation. Next, one surface is arbitrarily selected from the masked surfaces (for convenience, this surface is referred to as the "voltage measurement surface") and placed on a moving stage with this surface facing up. A digital multimeter (e.g., an R6451A digital multimeter manufactured by Advantest Corporation) is connected to the platinum electrodes of the placed test piece, and a voltage is applied between the platinum electrodes to achieve a constant current density (current value Ia). Meanwhile, a voltmeter (the digital multimeter may be used) is prepared, and one terminal is connected to one of the platinum electrodes (for convenience, referred to as the "base electrode"), and the other is used as a probe (micromanipulator probe). The probe is scanned from the base electrode side of the voltage measurement surface toward the other platinum electrode, and the relationship between the scanned distance and the voltage between the probe and base electrode is plotted. From the resulting graph, sections corresponding to the CF layers and sections corresponding to the interlayer resin layers are identified, and for each section corresponding to the interlayer resin layer, the thickness of the interlayer resin layer (scanned length) and the absolute value of the difference between the voltage when the probe enters the interlayer resin layer and the voltage when the probe exits the interlayer resin layer (for convenience, referred to as the "voltage change amount") are calculated. The conductivity between CFRP layers can be calculated using the following formula:

[0074] Conductivity between CFRP layers (S / m) = {(total thickness of scanned interlayer resin layers (m)) × Ia (A)} / {(total voltage change amount of scanned interlayer resin layers (V)) × S3 (m ２ This measurement is carried out five times by changing the position of the probe, and the arithmetic mean of the obtained interlayer conductivity of the CFRP is taken as the final interlayer conductivity of the CFRP.

[0075] The probe scans five times so that the sides of the voltage measurement surface parallel to the z-axis are equally spaced. The sections corresponding to the CF layer and the interlayer resin layer are identified by observation if possible, but if observation is difficult, the relationship between the scanned distance and the voltage between the probe and the base electrode is determined by the inflection point of the plot.

[0076] <CAI (Compression Strength After Impact)> The CFRP of the present invention preferably has a compression strength after impact (CAI), an important indicator of impact resistance in aircraft applications, of 230 MPa or more after an impact of 6.7 J / mm, since the strength can be maintained even after a weak impact such as a tool being dropped or a pebble being kicked up. A CAI of 250 MPa or more is more preferable, and 270 MPa or more is even more preferable. On the other hand, since attempting to achieve a very large CAI can complicate the manufacturing process and increase costs, a CAI of 400 MPa or less is actually preferable.

[0077] <Structure> The CFRP obtained by the present invention can be suitably used for aircraft structures. Examples of aircraft structures include shapes selected from flat structures, cylindrical structures, box-shaped structures, C-shaped structures, H-shaped structures, L-shaped structures, T-shaped structures, I-shaped structures, Z-shaped structures, and hat-shaped structures. Combining these structures forms aircraft components. Details of structures of the above shapes are described, for example, in "Structural Design of Aircraft," 5th Edition, by Torikai and Kuze, and the Japan Aeronautical Engineering Association (2003). Such structures can be obtained by shaping prepregs as described, for example, in International Publication No. 2017 / 110991 (paragraph

[0084] ), International Publication No. 2016 / 043156 (paragraph

[0073] ), and International Publication No. 2019 / 0314078 (paragraph

[0088] ). Furthermore, a structure having a desired shape can also be obtained by automatically laminating a prepreg tape onto a mold having the desired shape and then curing it.

[0078] In manufacturing an aircraft, a fuselage, main wings, center wing, tail, etc. are formed from a joined structure in which a plurality of the above-mentioned structures are joined. Fasteners such as bolts and rivets, adhesive films, etc. are used as means for joining the structures. Furthermore, a co-curing method can also be used in which a plurality of uncured or semi-cured prepreg laminates are joined together and then cured.

[0079] Next, an example of a method for obtaining the prepreg of the present invention will be described in detail. Considering that the prepreg of the present invention will be used as a structural material for aircraft, it is preferable to use a UD sheet as the CF sheet form. Of course, CF fabric can also be used when using a cover prepreg to be attached to the surface of a prepreg for structural materials or a CFRP with a complex shape. Below, an example of a UD prepreg using a matrix resin mainly composed of a thermosetting resin will be used for the explanation. It should be noted that the present invention should not be construed as being limited to this example.

[0080] In the present invention, a two-stage impregnation method is preferably used, in which the resin is impregnated twice to form an orderly interlayer structure when the CFRP is produced and to control the number of conductive particles connecting adjacent CF layers. First, a primary resin composition is prepared by kneading a combination of epoxy resin, curing agent, and thermoplastic resin for the prepreg intermediate material (hereinafter simply referred to as the "prepreg intermediate material") obtained by the first impregnation stage. This primary resin composition is then used to produce a primary resin composition film using a roll coater. The CF bundles are then aligned to form a UD sheet, and primary resin composition films are applied from above and below the sheet. After preheating, the sheet is pressed with nip rolls to impregnate the UD sheet with the primary resin composition, thereby obtaining the prepreg intermediate material. At this time, it is preferable to increase the degree of resin impregnation in the prepreg intermediate material.

[0081] Next, to perform the second stage of impregnation, a secondary resin composition is prepared by adding non-conductive polyamide particles and conductive particles to the epoxy resin, curing agent, and thermoplastic resin, and a secondary resin composition film is produced using a roll coater.

[0082] Then, a secondary resin composition film is applied to both the top and bottom surfaces of the prepreg intermediate material, and after preheating, it is pressed and laminated with nip rolls. At this time, it is desirable to perform sufficient preheating to ensure sufficient fluidity of the secondary resin composition. The prepreg is then wound up with a winder to obtain a prepreg. In the prepreg obtained by this manufacturing method, the above particles can be unevenly distributed on the surface if their D50 is equal to or greater than the average radius of the fiber. Note that this does not exclude the formation of a boundary region (a region where the primary resin composition and the secondary resin composition are mixed; also, as a result, a region where component [A], component [B], component [C], and component [D] are mixed) in the two-stage impregnation method.

[0083] The present invention will be described in detail below with reference to examples. However, the present invention should not be construed as being limited to these examples. The unit "parts" used in the composition ratios means parts by mass unless otherwise noted. Furthermore, measurements of various characteristics (physical properties) were carried out in an environment of 23°C and 50% relative humidity unless otherwise noted.

[0084] <Raw Materials> (1) Carbon Fiber (CF) CF with 24,000 filaments per bundle, a tensile strength of 5.8 GPa, and a tensile modulus of 280 GPa was prepared. Measurement was performed according to (14) Average Radius of CF described below, and the average radius of the single filament was found to be 3.5 μm. (2) Epoxy Resin - "Sumiepoxy (registered trademark)" ELM434 (tetraglycidyldiaminodiphenylmethane, manufactured by Sumitomo Chemical Co., Ltd.). - "EPICLON (registered trademark)" 830 (bisphenol F type epoxy resin, manufactured by DIC Corporation). - "GOT (registered trademark)" (glycidylaniline type epoxy resin (N,N-diglycidyl-o-toluidine), manufactured by Nippon Kayaku Co., Ltd.). (3) Curing Agent - Seikacure-S (4,4'-DDS, manufactured by Seika Corporation). (4) Thermoplastic resin (PES) - "Sumikaexcel (registered trademark)" 5003P (PES, manufactured by Sumitomo Chemical Co., Ltd.). (5) Non-conductive polyamide particles. Referring to International Publication No. 2018 / 207728, 200 g of ε-caprolactam (manufactured by Toray Industries, Inc.), 800 g of polyethylene glycol (manufactured by Wako Pure Chemical Industries, Ltd., grade 1 polyethylene glycol 20,000, mass average molecular weight 18,600), and 1,000 g of water were added to a 3 L autoclave equipped with a helical ribbon-type stirring blade to form a homogeneous solution, which was then sealed and purged with nitrogen. Thereafter, the stirring speed of the stirring blade was set to 100 rpm, and the temperature was raised to 240 ° C. At this time, after the pressure of the system reached 1 MPa, the pressure was controlled by slightly releasing the steam so that the pressure remained at 1 MPa. After the temperature reached 240°C, the pressure was released at a rate of 0.02 MPa / min. The polymerization was then completed by maintaining the temperature for 1 hour while flowing nitrogen, and the mixture was discharged into a 2,000 g water bath to obtain a slurry. The unreacted material was dissolved in water in the water bath, followed by filtration. 2,000 g of water was added to the residue, and the mixture was washed at 80°C. The washed slurry was filtered through a 200 μm sieve to remove aggregates. The filtrate from which the aggregates had been removed was filtered again, and the isolated residue was dried at 80°C for 12 hours to produce 140 g of polyamide 6 powder. The melting point of the resulting powder was 218°C, the same as polyamide 6, and the crystallization temperature was 170°C.(6) Conductive Particles: Conductive Particle A: Two types of phenolic resin particles (Marilyn HF type, manufactured by Gun-ei Chemical Industry Co., Ltd.) with different particle size distributions were mixed, fired at 2000°C, and classified (component: carbon, D50: 20.3 μm, D99: 64.4 μm, particle size change rate X: 0.0006). The volume resistivity of conductive particle A was calculated using the method described below and was found to be 0.05 Ωcm. Conductive particles B, C, E, F, G, and H, described below, also exhibited the same volume resistivity as conductive particle A. Conductive particles B, E, F, G, and H: Each was classified under different conditions and obtained in the same manner as conductive particle A, except that they had the physical properties and characteristics listed in Table 2. Conductive particle C: Each was obtained by classifying "Glassy Carbon (registered trademark)" (component: carbon, manufactured by Tokai Carbon Co., Ltd.). The physical properties and characteristics are shown in Table 2. Conductive Particles D: Two types of phenolic resin particles (Marilyn HF type, manufactured by Gun-ei Chemical Industry Co., Ltd.) with different particle size distributions from those used in the example of Conductive Particles A were mixed and fired at 2000°C and classified (component: carbon, D50: 27.6 μm, D99: 66.4 μm, particle size change rate X: 0.0009). The volume resistivity of Conductive Particles D was calculated using the method described below and found to be 0.05 Ωcm. Conductive Particles I: 100 g of the non-conductive polyamide particles obtained in (5) was added to 1000 ml of electroless nickel plating solution Nickel Boomer LP-26LL (manufactured by Nippon Chemical Industry Co., Ltd.), followed by plating at 50°C for 60 minutes and classification to produce Conductive Particles I. The volume resistivity of Conductive Particles I was calculated using the method described below and found to be 0.01 Ωcm. Table 2 shows the properties of the particles before plating (there is no substantial change before and after plating). Conductive particles J, K, and L: These were obtained in the same manner as conductive particles D, except that they were classified under different conditions and had the physical properties and characteristics shown in Table 2. Conductive particles M: "NICABEADS (registered trademark)" ICB-2020 (manufactured by Nippon Carbon Co., Ltd.) described in Patent Document 1 (JP 2011-213991 A). (Component: carbon, D50: 13.0 μm, D99: 32.1 μm, particle size change rate X: 0.0038).

[0085] <Evaluation methods> (1) Preparation of epoxy resin composition An epoxy resin and a thermoplastic resin (PES) were kneaded together, heated to 150°C or higher, and stirred for 1 hour to dissolve the PES in the epoxy resin, yielding a transparent viscous liquid. After lowering the temperature of this liquid while kneading, a curing agent was added and the mixture was further kneaded to obtain a primary resin composition.

[0086] In addition, a transparent viscous liquid similar to that described above was separately prepared, and after lowering the temperature while kneading, a curing agent, non-conductive polyamide particles, and conductive particles were added and kneaded to obtain a secondary resin composition.

[0087] The compositional ratios of the obtained primary resin compositions and secondary resin compositions are shown in Tables 1, 4, 5 and 6 (units in each table are parts by mass).

[0088] (2) Preparation of prepreg with resin content of 33% by mass (hereinafter abbreviated as Rc33%) Using the primary resin composition and secondary resin composition prepared in (1) above, a prepreg was prepared using the two-stage impregnation method as follows. The primary resin composition and secondary resin composition prepared in (1) above were each uniformly applied to a silicone-coated release paper using a roll coater equipped with opposing rolls at a winding speed of 15 m / min to form a primary resin composition film and a secondary resin composition film, respectively. Then, carbon fiber (CF) uniformly aligned in one direction was sandwiched between two primary resin composition films, and heated and pressed using a press roll to obtain a prepreg intermediate material in which the carbon fiber was sufficiently impregnated with the primary resin composition (carbon fiber basis weight was 268 g / m ２ , resin content 19% by mass). Next, both release papers were peeled off from the prepreg intermediate material. Next, the prepreg intermediate material was sandwiched between two sheets of secondary resin composition film, and heated and pressed using a press roll to obtain a prepreg in which the prepreg intermediate material was impregnated with the secondary resin composition (CF basis weight 268 g / m ２ , Rc33% by mass).

[0089] (3) Preparation of prepreg with a resin content of 36% by mass (hereinafter abbreviated as Rc36%). Using the epoxy resin composition prepared in (1) above, a prepreg intermediate material was obtained in the same manner as in (2) (however, the basis weight of the CF was 268 g / m ２, resin content 21% by mass). Next, in the same manner as in (2), a prepreg in which the prepreg intermediate material was impregnated with the secondary resin composition was obtained (CF basis weight 268 g / m ２ , Rc 36% by mass). At this time, the resin content was adjusted by changing the basis weight of the first resin composition film and the second resin composition film.

[0090] (4) Measurement of Resin Content (Rc) of Prepreg Rc was measured according to the test method for "Prepreg Mass per Unit Area, Carbon Fiber Mass per Unit Area, Resin Mass Content, and Fiber Mass Content" described in JIS K7071 (1988). Specifically, a test piece measuring 100 mm long x 100 mm wide was cut out from the sample prepreg, and its mass was measured. Next, at a temperature of 23°C, the test piece was placed in a beaker and approximately 200 ml of methyl ethyl ketone (MEK) was added. After 15 minutes of ultrasonic irradiation and stirring, the CF was collected with tweezers and placed in another beaker. The collected CF was subjected to the same washing procedure as above twice more, and after the third operation, the CF was transferred onto a glass filter whose mass had been measured in advance and subjected to suction filtration. After suction filtration, the CF together with the glass filter was dried in a dryer at a temperature of 105 ° C. for 90 minutes, and then cooled in a desiccator for 45 minutes or more. The mass of the glass filter with the CF still on it was measured, and the value obtained by subtracting the previously measured mass of the glass filter was used as the mass of the CF. Next, the mass of the CF was subtracted from the mass of the prepreg measured initially to obtain the mass of the resin. Rc was calculated by dividing this mass of the resin by the mass of the prepreg. The measurement was performed three times, and the average value was used as the Rc of the prepreg.

[0091] (5) Measurement of conductive particle diameter, particle size change rate X, D50, and D99. The secondary resin composition film for a resin content of 36% by mass prepared in (2) was observed at a magnification of 200 times or more using a laser microscope (digital microscope VHX-5000: manufactured by Keyence Corporation, hereinafter referred to as VHX-5000). For each of 1,000 randomly selected particles, the diameter of the circle inscribed on the periphery of the observed image was measured as the particle diameter, and the value was calculated in micrometers to the first decimal place. However, particles with an inscribed circle diameter of less than 0.5 μm were excluded from the measurement. For particles with a particle diameter of 0.5 μm or more, a graph was created with particle diameter on the horizontal axis and cumulative relative frequency (%) on the vertical axis, and D50, D99, d50, and d99 were obtained from the graph. X was then calculated using formula (1).

[0092] Furthermore, D99 / D50 was calculated by dividing D99 by D50.

[0093] (6) Volume Resistivity of Conductive Particles Using an MCP-PD51 powder resistivity measurement system (manufactured by Nitto Seiko Analytics Co., Ltd.), each conductive particle sample was placed in a cylindrical cell with four probe electrodes. The thickness and resistance of the sample were measured under a pressure of 60 MPa, and the volume resistivity was calculated.

[0094] (7) Sphericity of Conductive Particles The secondary resin composition film for a resin content of 36% by mass prepared in (2) was observed at a magnification of 200 times or more using a VHX-5000. The diameters of the inscribed and circumscribed circles were measured for each of 1,000 randomly selected particles. However, particles with an inscribed circle diameter of less than 0.5 μm were excluded from the measurement. The diameter of the inscribed circle was divided by the diameter of the circumscribed circle, and the result was multiplied by 100 to determine the sphericity (%) of the particle. The median sphericity of the 1,000 conductive particles, i.e., the average value of the sphericity of the 500th particle and the 501st particle when arranged in ascending order of sphericity, was determined to be the sphericity of the conductive particle.

[0095] (8) Presence of conductive particles within a depth of 15% from the prepreg surface. The prepreg with Rc of 36% prepared in (3) above was sandwiched between two parallel, smooth-surfaced polytetrafluoroethylene resin plates without applying pressure, and the temperature was gradually raised to 150°C over 7 days to gel and cure the prepreg, producing a plate-shaped cured product. After curing, the prepreg was cut in a direction perpendicular to the adhesive surface (thickness direction). The cross section was polished and then photographed with a VHX-5000 at a magnification of 200x or more so that the top and bottom surfaces of the prepreg were within the field of view. The spacing between the polytetrafluoroethylene resin plates was measured at five randomly selected locations in the horizontal direction of the cross-sectional photograph (width direction of the prepreg), and the average value was used as the average thickness of the prepreg. In the above photograph, two lines parallel to the surface of the cured prepreg were drawn from both surfaces of the cured prepreg at a depth of 15% of the average thickness of the prepreg, and the total area S1 of all conductive particles present between the surface of the cured prepreg and the parallel lines and the total area S2 of all conductive particles present throughout the entire thickness of the cured prepreg were calculated. Note that particles present on the parallel lines were not included in the measurement of S1. The abundance ratio of particles present within a range of 15% depth from the surface of the prepreg was calculated by dividing S1 by S2, and this was taken as the abundance ratio of conductive particles from the surface of the prepreg to a depth of 15%.

[0096] (9) Measurement of Interlaminar Conductivity of CF Layers The prepreg prepared in (2) was laminated in a [+45° / 0° / -45° / 90°] 2s configuration, with the longitudinal direction of the CF contained in the prepreg prepared in (2) set to 0°, to obtain a quasi-isotropic prelaminate with a total of 16 plies. The obtained prelaminate was placed in an autoclave and heated from room temperature to 180°C at a rate of 1.7°C per minute under a pressure of 0.6 MPa. It was then cured at 180°C for 2 hours to obtain a CFRP with a width of 350 mm and a depth of 350 mm. While maintaining a constant center of gravity, the four sides were cut off to obtain a rectangular CFRP with a width of 300 mm and a depth of 300 mm (Figure 1a). Similarly, a rectangular CFRP with a width of 300 mm and a depth of 300 mm was obtained from the prepreg prepared in (3).

[0097] For each of the obtained CFRP panels, four rectangular parallelepipeds (test piece A, c in Figure 1) measuring 8 mm in length and 8 mm in width similar to the outer shape of the CFRP panel were cut out, with one vertex being the intersection of the lines connecting the midpoints of each side between opposite sides (b, b' in Figure 1). Four rectangular parallelepipeds (test piece B, d in Figure 1) measuring 8 mm in length and 8 mm in width similar to the outer shape of the CFRP panel were also cut out, with one vertex being a corner of the CFRP panel. For each of the cut-out test pieces A and B, the cross section corresponding to the right side of the two sides parallel to b in Figure 1 was used as the interlaminar conductivity measurement surface (voltage measurement surface). Both ends of the test piece in the thickness direction were polished, and the resin on the surface was removed to expose the CF layer. After masking the four cross sections, including the measurement surfaces other than the polished surfaces, platinum was vapor-deposited on both polished surfaces to form electrodes. The masking was then removed to obtain a sample for conductivity evaluation. The test piece was then placed on a movable stage with the voltage measurement surface facing up. A digital multimeter (Advantest Corporation R6451A digital multimeter) was connected to the platinum electrodes of the placed test piece, and a voltage was applied between the platinum electrodes to achieve a constant current density (current value: Ia). A voltmeter (using the digital multimeter) was used, with one terminal connected to one of the platinum electrodes (base electrode), and the other terminal used as a probe (micromanipulator probe). The probe was scanned from the base electrode side of the voltage measurement surface toward the other platinum electrode, and the relationship between the voltage between the probe and the base electrode versus the scanned distance was plotted. From the resulting graph, the sections corresponding to the CF layer and the interlayer resin layer were identified. For each section corresponding to the interlayer resin layer, the thickness of the interlayer resin layer (scanned length) and the absolute value of the difference between the voltage when the probe entered the interlayer resin layer and the voltage when the probe exited the interlayer resin layer (voltage change amount) were calculated. The interlayer conductivity of the CFRP was calculated using the following formula.

[0098] Conductivity between CFRP layers (S / m) = {(total thickness of scanned interlayer resin layers (m)) × Ia (A)} / {(total voltage change amount of scanned interlayer resin layers (V)) × S3 (m ２ )} where S3 is the cross-sectional area of ​​the test piece, 64 × 10 －６ (m ２)

[0099] This measurement was carried out five times by changing the position of the probe, and the arithmetic mean of the obtained interlayer conductivity of the CFRP was taken as the final interlayer conductivity of the CFRP.

[0100] The probe was scanned so that the scanning lines and the sides of the voltage measurement surface parallel to the thickness direction of the sample were spaced equally apart. The sections corresponding to the CF layer and the interlayer resin layer were identified by finding the inflection point in the plot of the voltage between the probe and the base electrode versus the scanning distance.

[0101] The same measurements as above were performed on all four test pieces A, and the arithmetic mean value of the measurement results was used as the interlayer conductivity of the CFRP at the center of the panel. The same measurements as for test piece A were also performed on all four test pieces B, and the arithmetic mean value of the measurement results was used as the interlayer conductivity of the CFRP at the panel edge.

[0102] (10) Determination of Interlayer Conductivity Variation (A) (Variation in Conductivity of Prepregs with Different Rc) For the interlayer conductivity of test piece A obtained in (9), the dependency of the interlayer conductivity of the CF layer on the resin content was calculated using Equation 2 for a CFRP using a prepreg with an Rc of 33% (at Rc 33%) and a CFRP using a prepreg with an Rc of 36% (at Rc 36%). A value of 35% or less was rated as excellent, a value of more than 35% but not more than 50% was rated as good, a value of more than 50% but not more than 70% was rated as acceptable, and a value of excellent, good, or acceptable was rated as pass.

[0103] (Interlayer conductivity at Rc 33%−Interlayer conductivity at Rc 36%) ÷ (Average of interlayer conductivity at Rc 33% and Rc 36%) × 100: (Equation 2).

[0104] (11) Determination of Interlaminar Conductivity Variation (B) (Variation in Conductivity Between the Center and Ends Within a Molded Product) From the interlaminar conductivities of test piece A and test piece B of the CFRP panel using the prepreg with Rc of 36% obtained in (9), the change in interlaminar conductivity within a CFRP molded product was calculated using Equation 3. A value of 25% or less was rated as excellent, a value of more than 25% but not more than 35% was rated as good, a value of more than 35% but not more than 50% was rated as pass, and a value of excellent, good, or pass was rated as pass. |Interlayer conductivity at the center of the panel -Interlayer conductivity at the edge of the panel| ÷ (Average of the interlayer conductivity at the center of the panel and the interlayer conductivity at the edge of the panel) × 100: (Equation 3) (12) CAI (Compression Strength After Impact) The prepreg with Rc of 36% prepared in (3) above was quasi-isotropically laminated in 16 plies in a [+45° / 0° / -45° / 90] 2s configuration, and molded in an autoclave at a temperature of 180°C for 2 hours, a pressure of 0.6 MPa, and a heating rate of 1.7°C / min to produce CFRP. A sample measuring 150 mm long x 100 mm wide was cut from this CFRP, and a drop weight impact of 6.7 J / mm was applied to the center of the sample according to SACMA SRM 2R-94, followed by a compression fracture test to determine the compressive strength after impact (CAI).

[0105] (13) Coefficient of variation in thickness within a single interlayer resin layer: Using the prepreg with Rc of 36% prepared in (3) above, a square CFRP panel measuring 300 mm wide and 300 mm long was prepared in the same manner as in (9). Two test pieces were selected from the four test pieces in the central portion of the CFRP panel (Fig. 1c), which were symmetrical with respect to Fig. 1b'. The cross sections corresponding to Fig. 1b were polished and then magnified 500 times with an optical microscope. Cross-sectional photographs were taken of 20 areas (20 photographs total), 10 areas per test piece, randomly selected so that no overlapping areas were observed, revealing the interlayer between the seventh and eighth plies in the panel thickness direction. For each cross-sectional photograph, the thickness of the interlayer resin layer between adjacent CF layers (the interlayer distance in the CFRP thickness direction at each measurement point) was measured at 100 points at 5 μm intervals along the side corresponding to Fig. 1b. The thickness (μm) and standard deviation (μm) of the interlayer resin layer were determined from a total of 2,000 readings. The average thickness (μm) of the interlayer resin layer was calculated as the arithmetic mean of the thicknesses of the interlayer resin layer at each measurement point. The coefficient of variation (%) was calculated by dividing the standard deviation (μm) by the average thickness (μm).

[0106] (14) Average Radius of Carbon Fiber The prepreg with Rc of 36% obtained in (2) was quasi-isotropically laminated in 16 plies in a [+45° / 0° / -45° / 90°] 2s configuration and molded in an autoclave at 180°C for 2 hours at a pressure of 0.6 MPa and a heating rate of 1.7°C / min to produce CFRP. The obtained CFRP sample was cut parallel to the 0° direction to obtain a cross section. Using a VHX-5000, the CF layer in which the carbon fiber extended perpendicular to the cross section (i.e., the 90° layer) was observed. From the obtained cross section, the diameter of the circle inscribed around the periphery of the carbon fiber single yarn was calculated. 300 carbon fiber single yarns were selected from the cross-sectional image, and the diameters of the inscribed circles were measured. The arithmetic mean was calculated and divided by 2 to obtain the average radius of the carbon fiber.

[0107] (15) Mass ratio of non-conductive polyamide particles in the epoxy resin composition constituting the prepreg (2) The primary resin composition film and the secondary resin composition film used to prepare the prepreg with Rc 36% were cut into 100 mm squares (100 mm ２ ) film mass (g) is measured to find the mass of 1 m ２Resin weight per unit area (g / m ２ The resin basis weight of the secondary resin composition film was multiplied by the blending ratio of the non-conductive polyamide particles in the secondary resin composition in Tables 1, 4, 5, and 6 to calculate the weight per unit area of ​​the secondary resin composition film. ２ The basis weight of the non-conductive polyamide particles contained per unit area was calculated. The mass proportion of the non-conductive polyamide particles in the epoxy resin composition constituting the prepreg was calculated by dividing this basis weight of the non-conductive polyamide particles by the sum of the basis weight of the primary resin composition film and the basis weight of the secondary resin composition film.

[0108] (Example 1) An epoxy resin composition was prepared using the formulation shown in Table 1 and the methods of <Evaluation Methods> (1) to (3), and then a prepreg was produced and evaluated. Conductive particles A were used as the conductive particles.

[0109] When the interlayer conductivity of the CF layers was measured using the method described in (9) of the <Various Evaluation Methods>, the prepreg with an Rc of 36% showed a slight decrease compared to the prepreg with an Rc of 33%, but both were very high. This decrease in interlayer conductivity is thought to be due to the increase in the total amount of resin, which increased the volume occupied by the resin in the CFRP, resulting in a thicker interlayer and a decrease in the number of conductive particles connecting the CF layers. However, the rate of change in conductivity calculated using Equation 2 in (10) of the <Various Evaluation Methods>, which indicates the variation in interlayer conductivity, was only 29%, which is very small. This is thought to be due to the small change in the number of conductive particles connecting the CF layers relative to the change in Rc, due to the small particle size change rate X of conductive particle A. Furthermore, the CAI was also very high for use in primary aircraft structural materials.

[0110] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was a very small 18%.

[0111] Comparative Example 1 An epoxy resin composition was prepared, a prepreg was produced, and evaluations were carried out in the same manner as in Example 1, except that conductive particles B were used as the conductive particles as shown in Table 2. The conductive particles B had a larger particle size change rate X than the conductive particles A.

[0112] When the interlayer conductivity in the thickness direction was measured in the same manner as in Example 1, the CFRP using a prepreg with an Rc of 33% showed a very high value, even higher than that of Example 1, whereas the CFRP using a prepreg with an Rc of 36% showed a significant drop in value, lower than that of Example 1. This suggests that the increase in resin content significantly reduced the number of conductive particles connecting the CF layers. This is likely due to the large particle size change rate X of conductive particle B, which is caused by a large change in the number of conductive particles connecting the CF layers where there is a change in interlayer thickness due to a change in Rc.

[0113] Example 2 An epoxy resin composition was prepared, a prepreg was produced, and an evaluation was carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles C as shown in Table 2. The variation in conductivity between layers was very small, the conductivity between layers was very high, and the CAI was also very high for use as a primary structural material for an aircraft.

[0114] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was a very small 18%.

[0115] Comparative Example 2 An epoxy resin composition was prepared, a prepreg was produced, and evaluations were carried out in the same manner as in Example 1, except that conductive particles E were used as the conductive particles as shown in Table 2. Although conductive particles E had a smaller particle size change rate X than conductive particles B, it was not sufficiently small, and therefore the variation in conductivity between layers was large.

[0116] (Example 3) An epoxy resin composition was prepared, a prepreg was produced, and an evaluation was carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles F as shown in Table 2. As in Example 1, the variation in conductivity between layers was very small. The conductivity between layers was very high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.

[0117] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 38%.

[0118] Example 4 An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that conductive particles G were used as the conductive particles as shown in Table 2. Conductive particles G had a small particle size change rate X. Compared to Example 1, in which conductive particles A with a larger D99 were used, the variation in conductivity between layers was even smaller, demonstrating that in order to reduce the variation in conductivity between layers, it is more important to control the particle size change rate X than to control the number of large-diameter conductive particles such as D99. The conductivity between layers was very high, and the CAI was also very high for use as a primary structural material for aircraft.

[0119] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 22%.

[0120] Example 5 An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that conductive particles H were used as the conductive particles shown in Table 2. Although the interlayer conductivity variation was sufficiently small, a slight increase in variation was observed compared to Example 4, in which conductive particles F with the same particle size change rate X were used. This is thought to be due in part to the fact that conductive particles H have a smaller D50 than conductive particles F, and therefore small-diameter conductive particles penetrate into the CF layers during prepreg production, resulting in greater interlayer thickness variation in the CFRP. Furthermore, the penetration of some particles into the CF layers is thought to have led to variation in the number of conductive particles connecting the CF layers, thereby resulting in variation in interlayer conductivity. In other words, it has been shown that by setting D50 within a certain range relative to the CF fiber diameter, the increase in interlayer conductivity variation can be suppressed with greater precision. The interlayer conductivity was sufficiently high, and the CAI was also high enough for use as a primary structural material for aircraft.

[0121] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was a very small 46%.

[0122] Example 6 An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that the conductive particles used were conductive particles I as shown in Table 2. Conductive particles I were conductive polyamide particles, and by setting the particle size change rate X within a predetermined range, the variation in conductivity between layers was sufficiently small. The conductivity between layers was very high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.

[0123] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 21%.

[0124] (Example 7) An epoxy resin composition was prepared, a prepreg was produced, and an evaluation was carried out in the same manner as in Example 1, except that conductive particles D were used as the conductive particles as shown in Table 2. As in Example 1, the variation in conductivity between layers was very small. The conductivity between layers was very high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.

[0125] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 15%.

[0126] (Example 8) An epoxy resin composition was prepared, a prepreg was produced, and an evaluation was carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles J as shown in Table 2. As in Example 1, the variation in conductivity between layers was very small. The conductivity between layers was very high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.

[0127] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was a very small 24%.

[0128] (Example 9) An epoxy resin composition was prepared, a prepreg was produced, and an evaluation was carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles K as shown in Table 2. As in Example 1, the variation in conductivity between layers was very small. The conductivity between layers was very high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.

[0129] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 3, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 29%.

[0130] Comparative Example 3 An epoxy resin composition was prepared, a prepreg was produced, and evaluations were carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles L as shown in Table 2. Although conductive particles L had a smaller particle size change rate X than conductive particles B, it was not sufficiently small, and therefore the variation in conductivity between layers was large.

[0131] Comparative Example 4 An epoxy resin composition was prepared, a prepreg was produced, and evaluations were carried out in the same manner as in Example 1, except that the conductive particles used were conductive particles M as shown in Table 2. Although conductive particles M had a smaller particle size change rate X than conductive particles B and E, it was not sufficiently small, and therefore the variation in conductivity between layers was large.

[0132] (Example 10) Epoxy resin compositions were prepared and prepregs were produced and evaluated in the same manner as in Example 1, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed as shown in Table 4. The variation in conductivity between layers was very small, the conductivity between layers was very high, and the CAI was also very high for use as a primary structural material for an aircraft.

[0133] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 8, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 18%.

[0134] Example 11 Epoxy resin compositions were prepared and prepregs were produced and evaluated in the same manner as in Example 2, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed as shown in Table 4. The variation in conductivity between layers was very small, the conductivity between layers was very high, and the CAI was also very high for use as a primary structural material for an aircraft.

[0135] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 8, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 18%.

[0136] (Example 12) Epoxy resin compositions were prepared and prepregs were produced and evaluated in the same manner as in Example 1, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed as shown in Table 5. The variation in conductivity between layers was very small, the conductivity between layers was very high, and the CAI was also very high for use as a primary structural material for an aircraft.

[0137] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 8, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 34%.

[0138] (Example 13) An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed as shown in Table 6. The variation in interlayer conductivity was very small, the interlayer conductivity was very high, and the CAI was also acceptable for use as a primary structural material for aircraft; however, it was a lower value than in Examples 1, 10, and 12, which used the same conductive particles.

[0139] Furthermore, with regard to the variation in electrical conductivity between layers within the CFRP panel molding shown in Table 8, the rate of change in electrical conductivity between layers calculated by Equation 3 in <Evaluation Methods> (11) was very small at 16%.

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148] The prepreg of the present invention and CFRP made from the prepreg are widely applicable to industrial fields requiring conductive materials. In particular, when used in aircraft structural components, they can reduce the need for conventional lightning protection systems, static elimination systems, electromagnetic wave shielding systems, etc., such as metal foils and metal meshes, making them suitable for use in these fields.

[0149] a: Square CFRP panel 300 mm wide and 300 mm long b, b': Lines connecting the midpoints of the sides of the CFRP panel between opposite sides c: Part corresponding to the test piece in the center of the CFRP panel d: Part corresponding to the test piece at the end of the CFRP panel

Claims

1. At least the following components [A] or components A prepreg comprising a component [D], wherein in the component [D], X represented by the following formula (1) is 0.003 or less. Component [A]: carbon fiber Component [B]: epoxy resin Component [C]: curing agent Component [D]: conductive particles Here, α ｉ a: the number of particles included in the particle size range of i-0.5 (μm) or more and less than i+0.5 (μm), a: d50, b: d99, i: an integer of d50 or more and d99 or less, N: the total number of particles to be measured, where d50 is the median diameter (50% diameter) of the component [D]. d99 is the 99% diameter (D99) of the component [D] rounded off to the nearest whole number when expressed in micrometers.

2. The prepreg according to claim 1, in which a layer containing the components [A], [B] and [C] but not the component [D] is laminated on both sides of a layer containing the components [A], [B] and [C] but not the component [D], and a layer containing the components [B], [C] and [D] but not the component [A].

3. The prepreg according to claim 1, wherein D50 of the component [D] is equal to or greater than the average radius of the component [A].

4. The prepreg according to claim 1, wherein the D99 (μm) of the component [D] is 0.33×A or less, where A is the basis weight of the component [A] in the prepreg in g / m ２ The value is rounded off to the first decimal place when written.

5. The prepreg according to claim 1, wherein the D50 (μm) of the component [D] is 0.18×A or less, where A is the basis weight of the component [A] in the prepreg in g / m ２ The value is rounded off to the first decimal place when written.

6. The prepreg according to claim 1, wherein when 16 plies of the prepreg are quasi-isotropically laminated in a [+45° / 0° / -45° / 90°] 2s configuration and cured in an autoclave at 180°C for 2 hours under conditions of a pressure of 0.6 MPa and a temperature increase rate of 1.7°C / min to obtain a carbon fiber reinforced composite material, the coefficient of variation in thickness within one interlaminar resin layer is 26% or more and 45% or less.

7. The prepreg according to claim 1, wherein the component [A] is carbon particles or conductive polyamide particles.

8. The prepreg according to claim 1, further comprising a component [E]. Component [E]: non-conductive polyamide particles.

9. The prepreg according to claim 8, in which the mass ratio of component [E] is 4 mass% or more and 15 mass% or less, when the total mass of components other than component [A] is 100 mass%.

10. A carbon fiber reinforced composite material in which two or more sheets of the prepreg according to claim 1 are laminated and heat cured.

11. The carbon fiber reinforced composite material according to claim 10, wherein the electrical conductivity between adjacent carbon fiber layers is 0.1 S / m or more.

12. The carbon fiber reinforced composite material according to claim 10, having a CAI of 230 MPa or more.

13. A structure obtained by using the carbon fiber reinforced composite material according to claim 10, the shape of which is selected from the group consisting of a flat plate structure, a cylindrical structure, a box-shaped structure, a C-shaped structure, an H-shaped structure, an L-shaped structure, a T-shaped structure, an I-shaped structure, a Z-shaped structure and a hat-shaped structure.

14. A method for producing a prepreg, comprising the steps of: impregnating a component [A] with a primary resin composition that contains components [B] and [C] but does not contain component [D] to obtain a prepreg intermediate material; and applying a resin film using a secondary resin composition that contains components [B], [C], and [D] to both sides of the prepreg intermediate material, wherein for component [D] contained in the secondary resin composition, X represented by the following formula (1) is 0.003 or less, where component [A] is carbon fiber, component [B] is epoxy resin, component [C] is a curing agent, and component [D] is conductive particles. Here, α ｉ a: the number of particles included in the particle size interval of equal to or greater than i-0.5 (μm) and less than i+0.5 (μm), a: d50, b: d99, i: an integer equal to or greater than d50 and less than d99, N: the total number of particles to be measured, where d50 is the median diameter (50% diameter, D50) of the component [D] expressed in micrometers, rounded off to the first decimal place, and d99 is the 99% diameter (D99) of the component [D] expressed in micrometers, rounded off to the first decimal place.

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