Prepreg, carbon fiber-reinforced composite material, and structure

Abstract

The present invention addresses the problem of providing a prepreg capable of stably obtaining at least a certain level of electrical conductivity between adjacent carbon fiber layers in a CFRP molded body while maintaining high impact resistance even when the thickness between the layers may vary. Suggested is a prepreg in which carbon fibers are impregnated with a matrix resin, wherein the matrix resin is an epoxy resin composition including at least an epoxy resin, a curing agent, and conductive particles, and the coefficient of variation in the particle size distribution of the conductive particles is 40% or more.

Description

Prepreg, carbon fiber reinforced composite materials and structures

[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 layer, the conductivity is relatively high in the direction of the fiber axis of the CF (hereinafter referred to 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 referred to as the orthogonal direction), although conductive paths are formed by contact between CF fibers, 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 approximately 1 / 1000 of that of metals such as aluminum. Thus, CFRP is less conductive than metal materials and exhibits anisotropy in conductivity in the direction orthogonal 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 disposing 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 disposed 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 side of the fiber layer in 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] The technologies described in Patent Documents 1 to 5, which achieve high conductivity using conductive particles controlled to a narrow particle size distribution through high-precision classification, have issues such as the need for advanced classification technology and a significant increase in raw material costs, as well as the insufficient consideration of conductivity variations that can occur due to the flow of matrix resin within the molded body. Even if high conductivity can be achieved under certain conditions, if it cannot be stably achieved within the molded body, it increases the risk of structural damage in the event of a lightning strike. Furthermore, if the variation is large, the potential of the prepreg cannot be accurately estimated, which results in the introduction of excessive lightning protection systems, leading to increased aircraft weight and costs.

[0012] Patent Documents 1 to 5 all share the idea of ​​placing conductive particles between adjacent CF layers and enlarging them so that they can fully contact the adjacent CF layers to function as a conductive path. Considering this, a possible approach to reducing conductivity variation within a molded body is to use conductive particles with a particle size large enough to accommodate the variable maximum interlayer thickness of adjacent CF layers. However, it has been found that large conductive particles tend to embed deeply into the CF layers in the CFRP, excessively disrupting the linearity of the CF arrangement and thereby degrading mechanical properties. In addition, there are many issues in terms of processability. Specifically, when using a process for producing a film from a resin composition containing particles as a prepreg production step, larger particles have a higher chance of passing through the coater, leading to localization of large particles, or clogging the coater clearance, which can become the starting point for film streaks and lead to variations in resin basis weight. These issues present numerous process-related challenges related to factors that can affect conductivity. Although a method of precisely classifying conductive particles to a particle size greater than the maximum expected thickness between adjacent CF layers and within a particle size distribution range that does not impair mechanical properties or process aspects is conceivable, this method is not practical due to the need for a higher classification level than the aforementioned techniques and the increased cost of raw materials. Furthermore, considering the variability in conductivity, it is conceivable to significantly increase the conductivity. However, as described in Patent Document 3, the inclusion of a large amount of conductive particles adversely affects the mechanical properties of CFRP, so the effectiveness of this method is limited by the limited amount that can be incorporated. Thus, it has been difficult to reduce the variability in conductivity of molded bodies while maintaining their mechanical properties.

[0013] The object of the present invention is to provide a prepreg that can stably obtain a certain level of interlaminar electrical conductivity between adjacent carbon fiber layers of a CFRP molded body while maintaining a high level of the mechanical properties of CFRP, in particular the impact resistance that is important for aircraft structural materials, thereby simplifying conventional lightning protection systems and contributing to weight and cost reductions for aircraft.

[0014] To solve the above problems, the present invention proposes the following: [1] A prepreg in which carbon fibers are impregnated with a matrix resin, the matrix resin being entirely or partially an epoxy resin composition containing at least an epoxy resin, a curing agent, and conductive particles, and the conductive particles having a coefficient of variation of 40% or more in particle size distribution. [2] The prepreg according to [1] above, in which the median diameter (D50) of the conductive particles is equal to or greater than the average radius of the carbon fibers. [3] The prepreg according to [1] or [2] above, in which the D99 (μm) of the conductive particles is 0.33 × A or less, where A is the basis weight of the carbon fibers in the prepreg in g / m ２ [4] The prepreg according to any one of [1] to [3], wherein the D50 (μm) of the conductive particles is 0.18×A or less, where A is the basis weight of the carbon fiber in the prepreg in g / m ２The values ​​are rounded to one decimal place when expressed. [5] The prepreg according to any one of [1] to [4] above, wherein the epoxy resin composition has a minimum viscosity of 0.6 Pa s or more when measured under conditions of an angular frequency of 10 rad / s and a heating rate of 2°C / min. [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 heating rate of 1.7°C / min to obtain a carbon fiber 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 the conductive particles have a particle size distribution with a kurtosis of 5 or less. [8] The prepreg according to any one of [1] to [7] above, wherein the conductive particles are carbon particles or conductive polyamide particles. [9] The prepreg according to any one of [1] to [8] above, further comprising non-conductive polyamide particles.

[10] The prepreg according to [9] above, wherein the mass ratio of the non-conductive polyamide particles is 4% by mass or more and 15% by mass or less, when the mass of the epoxy resin composition is 100% by mass.

[11] A carbon fiber reinforced composite material obtained by heat-curing two or more sheets of the prepreg according to any one of [1] to

[10] above, which are laminated together.

[12] The carbon fiber composite material according to

[11] above, wherein the coefficient of variation in thickness within one interlaminar resin layer is 26% or more and 45% or less.

[13] The carbon fiber composite material according to

[11] or

[12] above, wherein the electrical conductivity between adjacent carbon fiber layers is 0.1 S / m or more.

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

[11] to

[13] 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.

[0015] The present invention makes it possible to obtain a prepreg that exhibits the excellent impact resistance of CFRP while stably maintaining a certain level of interlayer electrical conductivity even when there is variation in the interlayer thickness of adjacent carbon fiber layers after CFRP formation. Furthermore, by applying such prepreg and carbon fiber reinforced composite material to aircraft, the overall efficiency of lightning protection systems can be improved.

[0016] Top view of the CFRP panel produced in the example section

[0017] The present invention will be described in more detail below, but it should be understood that the present invention is not limited to the specific examples shown below.

[0018] <Carbon Fiber (CF)> In the prepreg of the present invention, CF is used as a CF bundle in which CF is bundled. For example, a tape-like collection of approximately 1,000 to 1,000,000 CF single fibers is typically called a "tow," and a sheet-like CF bundle can be obtained by arranging these tows. Depending on the arrangement state, CF sheets are referred to as UD sheets when CF fibers are oriented in one direction (UD) along the longitudinal direction, and as CF fabrics when CF fibers are oriented in multiple directions. CF fabrics include woven fabrics and knitted fabrics, as well as fabrics in which CF fibers are multiaxially arranged in two dimensions, and nonwoven fabrics, mats, paper, and other fabrics in which CF fibers are randomly oriented.

[0019] <Epoxy Resin> The prepreg of the present invention uses an epoxy resin composition containing at least an epoxy resin, a curing agent, and conductive particles as the matrix resin. Note that, as in the case of the two-stage impregnation method described below, an epoxy resin composition containing an epoxy resin and a curing agent but not conductive particles can also be used as the matrix resin. Preferred epoxy resins are those whose precursors are amines, phenols, or compounds having a carbon-carbon double bond. Specifically, epoxy resins whose precursors are amines include various isomers of tetraglycidyldiaminodiphenylmethane, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, and triglycidylaminocresol; epoxy resins whose precursors are phenols 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 precursors are compounds 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 an effective way to improve the tensile strength of CFRP, but this tends to reduce heat resistance and elastic modulus. Therefore, using a dicyclopentadiene-type epoxy resin with a rigid skeleton or a glycidylaniline-type epoxy resin, which is a pendant-type epoxy resin, is preferable because it improves heat resistance and elastic modulus. Epoxy resins whose precursors are aromatic amines, such as tetraglycidyldiaminodiphenylmethane (N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane), are most suitable for the present invention because of their excellent heat resistance and adhesion to reinforcing fibers.

[0020] <Curing Agent> As the curing agent, 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.

[0021] <Conductive Particles> In the present invention, a prepreg is prepared by impregnating a CF sheet with the matrix resin. CFRP can be obtained by molding this prepreg. Since the prepreg has at least one matrix resin layer on the upper and lower surfaces of the CF sheet, the prepreg laminate and CFRP have a carbon fiber layer (hereinafter sometimes referred to as a "CF layer") containing the matrix resin and the CF sheet, and a resin sandwiched between adjacent CF layers. Therefore, the thickness between adjacent carbon fiber layers and the thickness of the interlayer resin layer between the carbon fiber layers are synonymous. In the present invention, conductive particles are disposed in the resin layer between adjacent CF layers (such resin layer is referred to as an "interlayer resin layer"). These conductive particles form a portion that electrically connects adjacent CF layers, thereby providing interlayer conductivity. This means that the conductive particles are substantially in contact with the CF layers above and below the interlayer resin layer, forming a conductive path.

[0022] The conductive particles 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.

[0023] 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.

[0024] Carbon particles are preferably used in aircraft because they do not pose a corrosion problem. Preferred carbon particles include crystalline carbon and amorphous carbon. 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.).

[0025] 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, a conductive polymer, or a conductive nanomaterial, and dispersing a conductive polymer or a conductive nanomaterial within the polyamide particles. Of these, from the viewpoint of achieving both conductivity and impact resistance, metal coating methods and dispersing particulate conductive nanomaterials within the particles are preferred. 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.

[0026] <Particle size distribution of conductive particles> The median diameter (also referred to as D50) of the conductive particles used in the present invention is preferably equal to or greater than the average radius of the CF. Note that the average radius of the CF here refers to the average radius of a single yarn. It is preferably equal to or greater than twice the average radius of the CF (i.e., equal to or greater than the average diameter of the CF), and more preferably equal to or greater than four times the average radius of the CF (i.e., equal to or greater than twice the average diameter of the CF). There is no particular upper limit, but from the viewpoint of reducing the variation in conductivity between layers and achieving a high CAI, it is desirable that the median diameter be equal to or less than 0.18 × A (μm), as described below, depending on the basis weight of the CF.

[0027] Here, D50 and D10, D90, and D99 described below refer to the following: D50 is the particle size at which 50% of the particles have a particle size of D50 or less, D10 is the particle size at which 10% of the particles have a particle size of D10 or less, D90 is the particle size at which 90% of the particles have a particle size of D90 or less, and D99 is the particle size at which 99% of the particles have a particle size of D99 or less. These can be easily determined from a graph with particle size on the horizontal axis and cumulative relative frequency (%) on the vertical axis.

[0028] By setting the thickness within the above range, it is possible to reduce the variation in conductivity between adjacent CF layers caused by the penetration of small-diameter conductive particles into the CF layer, and also to 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 compressive strength after impact (CAI), which will be described later.

[0029] 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, more preferably 0.15 x A or less, and even more preferably 0.13 x A or less. There is no particular lower limit, but it is preferably 0.05 x A or more, and more preferably 0.06 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).

[0030] By setting the upper limit of D50 within the above range, the number of conductive particles that sink 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 conductive path between adjacent CF layers increases assuming the same mass, which is preferable.

[0031] Next, the procedure for obtaining the median diameter of conductive particles will be 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 around the periphery of the conductive particle in the observed image is taken as the particle size, and 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 as the observation target, it may also be used. The median diameter of 1,000 particles in the prepreg or secondary resin composition film (i.e., when the diameters of the 1,000 particles are arranged in ascending order, the average of the diameters of the 500th and 501st particles) is taken as the median diameter (D50) in the particle size distribution of the target conductive particles.

[0032] Furthermore, by arranging the particles in order of particle size from smallest to largest, D10, which will be described later, can also be calculated from the average diameter of the 100th and 101st particles. D10 is preferably equal to or larger than the average radius of CF, more preferably equal to or larger than twice the average radius of CF (i.e., equal to or larger than the average diameter of CF), and even more preferably equal to or larger than three times the average radius of CF (i.e., equal to or larger than 1.5 times the average diameter of CF).

[0033] The above range is preferable because it can reduce the variation in electrical conductivity between adjacent carbon fiber layers caused by the penetration of small conductive particles into the CF layer. Also, in the CFRP, it is preferable because it can prevent cracks during impact from propagating into the CF layer along the small particles that have penetrated the CF layer, thereby suppressing a decrease in the compressive strength after impact (CAI), which will be described later.

[0034] Furthermore, by arranging the particles in order of particle size from smallest to largest, D90 (described later) can be calculated from the average diameter of the 900th and 901st particles. D99 (described later) can be calculated from the average diameter of the 990th and 991st particles in the above order.

[0035] D99 (here, the unit of D99 is μm) is the weight per unit area of ​​the carbon fiber in the prepreg in g / m ２ When the value rounded to one decimal place is designated as A, from the viewpoint of process passability and the ability to suppress a decrease in CAI due to conductive particles being embedded too deeply into the CF layer, D99 is preferably 0.33 × A or less, more preferably 0.30 × A or less, and even more preferably 0.27 × A or less. There is no particular lower limit, but it is preferably 0.20 × A or more, and more preferably 0.23 × A or more. By setting D99 within this range, it is possible to suppress a decrease in CAI, and even if the interlayer thickness within the CFRP molded body varies, the change in the number of conductive particles capable of connecting adjacent CF layers is small, making it easier to obtain stable interlayer conductivity after molding of the prepreg laminate, which is advantageous.

[0036] When multiple types of conductive particles are used, unless otherwise specified, D10, D50, D90 and D99 are values ​​for all the conductive particles.

[0037] Next, the procedure for obtaining the average radius of CFs will be described. When the average radius of CFs 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 CFs 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 the 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.

[0038] 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 direction 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.

[0039] <Coefficient of variation of particle size distribution of conductive particles> The coefficient of variation of the particle size distribution of the conductive particles in the prepreg of the present invention is 40% or more. A coefficient of variation of 40% or more makes it possible to obtain a CFRP having stable interlayer conductivity even when variations occur in the interlayer thickness within the CFRP molded article.

[0040] 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, and, if necessary, mixing and then classifying them. Here, 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.

[0041] When two or more types of conductive particles with different particle size distributions are used, an example of a method for obtaining mixed conductive particles having the above-mentioned particle size distribution coefficient is to use particles that satisfy the following (Formula 1), where two particles are selected from the conductive particles in descending order of their blending ratio, and the particle with the smaller D50 is designated particle α and the particle with the larger D50 is designated particle β.

[0042] D10 of particle β<D90 of particle α (Equation 1).

[0043] The technical significance of setting the coefficient of variation of the particle size distribution to a predetermined value or greater will be explained in detail below. First, by increasing the coefficient of variation, the particle size distribution can become a distribution with a wide range on the small diameter side, or a distribution with a wide range on the large diameter side, or a distribution with a wide range on both the small diameter side and the large diameter side. As described above, in CFRP, conductive particles present between adjacent CF layers electrically connect the adjacent CF layers, thereby achieving conductivity between the adjacent CF layers. However, as described above, when molding a prepreg laminate, the thickness of the interlayer resin layer within the CFRP molded body may change due to the flow of the matrix resin. In addition to interlayer thickness variations within the CFRP molded body, the interlayer thickness may also vary due to factors such as the raw materials and production process of the prepreg. Due to these factors, the interlayer thickness may become partially thinner or thicker.

[0044] On the other hand, the conductivity between adjacent CF layers can be improved by increasing the amount of conductive particles contained in the interlayer resin layer. However, in order to ensure the mechanical properties of the CFRP, there is a natural upper limit, and as described below, it is preferable to set the amount to 10 mass% or less of the total mass of the epoxy resin composition. By using conductive particles with a characteristic particle size distribution, the present invention can provide a prepreg that is less affected by variations in the thickness of the interlayer resin layer, even when a smaller amount of conductive particles is used than when conductive particles without such a particle size distribution are used, and ultimately can provide a CFRP with excellent lightning resistance.

[0045] That is, when the mass of the conductive particles contained in the matrix resin of the prepreg 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, the particle size distribution of the conductive particles can be broadened toward the small diameter side to increase the number of conductive particles on the small diameter side. These conductive particles on the small diameter side can frequently connect adjacent CF layers between partially thin interlayers. On the other hand, by using a particle size distribution with a wide distribution on the large diameter side, large diameter particles can connect adjacent CF layers between partially thick interlayers. Furthermore, the large particle size variation ensures the presence probability of conductive particles corresponding to each interlayer thickness even when the interlayer thickness varies greatly, thereby minimizing the area of ​​portions where electrical connection between layers is not ensured.

[0046] When the coefficient of variation of the particle size distribution of conductive particles is small, as with the conductive particles preferably used in Patent Documents 1 to 5, there is a range in which the frequency of particle presence varies significantly with particle size. A narrow particle size distribution tends to significantly change the number of conductive particles with a particle size sufficient to connect adjacent CF layers at different positions within the CFRP molded body between layers. If the number of particles that can be arranged between layers in CFRP is sufficiently large, even if the number of particles varies at different positions between layers, the number of particles sufficient to connect the CF layers and provide conductivity can be ensured. However, if the particle amount is too large, the mechanical properties and processability problems described above may arise, limiting the amount of particles that can be arranged. In the resin composition according to the present invention, the preferred proportion of conductive particles is preferably 0.05% by mass to 10% by mass, more preferably 0.1% by mass to 9% by mass, and even more preferably 0.5% by mass to 8% by mass, based on 100% by mass of the epoxy resin composition constituting the matrix resin.

[0047] Therefore, for example, if the interlayer thickness of the CF layers varies during molding of the prepreg laminate, and the number of conductive particles capable of connecting adjacent CF layers also varies significantly, this can result in variations in conductivity at each interlayer position. In the present invention, by using a wide particle size distribution with a coefficient of variation of 40% or more, even if the interlayer thickness varies within the CFRP molded body, the change in the number of conductive particles capable of connecting adjacent CF layers is smaller than when conductive particles with a small coefficient of variation in the particle size distribution are used, thereby achieving stable interlayer conductivity within the molded body. The coefficient of variation of the particle size distribution is preferably 50% or more, more preferably 55% or more. Furthermore, from the viewpoint of processability, the coefficient of variation of the particle size distribution is preferably 100% or less, more preferably 70% or less.

[0048] Next, the procedure for obtaining the coefficient of variation of 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. For each of 1,000 randomly selected particles, the diameter of the conductive particle is determined in micrometers to two decimal places, using the diameter of the circle inscribed around the periphery in the observed image as the particle size. This is rounded to the nearest tenth to obtain a value to one decimal place. This procedure is performed for each of 1,000 randomly selected particles. However, particles with a diameter of the inscribed circle less than 0.5 μm are excluded from the measurement. The average diameter (μm) and standard deviation (μm) of the conductive particles are determined for the 1,000 particles. The average diameter (μm) is the arithmetic mean of each particle size. The coefficient of variation (%) is determined by dividing the standard deviation (μm) by the average diameter (μm). When a secondary resin composition film containing conductive particles is available as an observation object, it may be used as the observation object.

[0049] <Kurtosis in particle size distribution of conductive particles> The kurtosis in the particle size distribution of the conductive particles of the present invention is preferably 5.0 or less, more preferably 4.6 or less, and even more preferably 4.0 or less. When the kurtosis is large, the particle size distribution tends to be sharply peaked, and therefore there is a region in which the frequency of particle presence varies greatly depending on the particle size. Therefore, when the interlayer thickness varies within a CFRP molded body, the conductivity between layers tends to vary. From the viewpoint of processability, the kurtosis of the particle size distribution is preferably 1.0 or more.

[0050] Next, a preferred procedure for obtaining the kurtosis 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. For each of 1,000 arbitrarily selected particles, the diameter of the conductive particle is determined in micrometers to two decimal places, using the diameter of the circle inscribed on the periphery of the observed image as the particle size. This is rounded to two decimal places to obtain a value to one decimal place. This is done for each of 1,000 arbitrarily selected particles. However, particles with a diameter of the inscribed circle less than 0.5 μm are excluded from measurement. The kurtosis of the particle size distribution for these 1,000 particles can be calculated using the following equation 3. Here, Dj in equation 3 represents the diameter of the jth particle when the 1,000 conductive particles are arranged in ascending order of particle size determined by the above method. Furthermore, Dave represents the average particle size of 1,000 conductive particles, and is the arithmetic mean diameter of each particle size, the same as the average diameter (μm) obtained in calculating the coefficient of variation of the particle size distribution described above. Note that if a secondary resin composition film containing conductive particles is available as the observation object, it may also be used as the observation object.

[0051]

[0052] <D99 / D50 Ratio in Particle Size Distribution of Conductive Particles> In the particle size distribution of the conductive particles used in the present invention, the value obtained by dividing D99 (μm) by D50 (μm) (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 the CFRP molded body 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 after molding of the prepreg laminate. Since there is a preferred upper limit for D99 from the viewpoints of process passability 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.

[0053] <Mass Proportion of Conductive Particles Having a Particle Size of D50 or More> In the present invention, the mass proportion of the conductive particles having a particle size of D50 or more is preferably 70 mass % or more, and more preferably 80 mass % or more, when the mass of the entire conductive particles is taken as 100 mass %. Within this range, the number of conductive particles that function as a conductive path between adjacent CF layers can be ensured, and the variation in conductivity between the CF layers can be further reduced.

[0054] The mass proportion of conductive particles 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 conductive particles described above, the particle sizes of 1,000 randomly selected particles are calculated, 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 x π x r. ３ The volume of each particle is calculated from the above. The mass proportion of conductive particles having a particle size of D50 or more is calculated by finding the total volume of the 501st to 1000th particles relative to the total volume of the 1st to 1000th particles. Even if two or more types of conductive particles are used and their specific gravities are different, they can be considered to have the same specific gravity and the mass proportion of conductive particles having a particle size of D50 or more can be found.

[0055] <Shape of Conductive Particles> The conductive particles used in the present invention preferably have a sphericity of 90% or more, more preferably 95% or more. By using conductive particles with high sphericity, the influence of variations in particle major axis orientation between CFRP intermediate layers due to flow of the matrix resin during molding of the prepreg laminate can be suppressed, resulting in smaller variations in conductivity between layers after molding of the prepreg laminate.

[0056] 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 conductive particles described above, the surface of the prepreg is observed with a laser microscope, as in the case of measuring particle size. The diameters of the inscribed and circumscribed circles on the periphery of 1,000 randomly selected particles are measured. As with measuring particle size, 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 each 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 conductive particles. Note that if a secondary resin composition film is available, it may also be used.

[0057] <Conductive Particle Abundance from the Prepreg Surface to a Depth of 15%> For the conductive particles used in the present invention, S1 / S2, which is an index of the conductive particle abundance from the prepreg surface to a depth of 15%, as defined below, is preferably 0.9 or more. Here, S1 is the total area of ​​particles present in the cross section of the cured prepreg up to a depth of 15% from the prepreg surface when the total thickness of the prepreg is taken as 100%, 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 there is less variation in the number of particles forming the portion electrically connecting the upper and lower CF layers, allowing the prepreg laminate to exhibit stable interlayer conductivity after molding.

[0058] S1 / S2 can be determined as follows. A prepreg is sandwiched between two parallel, smooth-surfaced polytetrafluoroethylene resin plates without applying pressure to bring them into close contact. The temperature is gradually increased to 150°C over seven days to gel and cure the prepreg, producing a plate-like cured product. After curing, the prepreg is cut in a direction perpendicular to the contact surface (thickness direction). The cross section is polished and then photographed with a laser microscope (e.g., a digital microscope VHX-5000, manufactured by Keyence Corporation) at a magnification of 200x or more so 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 rate 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 above cured prepreg was cured under very slow conditions, so the value calculated by this calculation is considered to be the abundance rate of conductive particles within a depth range of 15% from the prepreg surface.

[0059] <Thermoplastic Resin> In the present invention, it is preferable to incorporate a thermoplastic resin into the epoxy resin composition serving as the matrix resin, and it is also preferable that the thermoplastic resin be compatible with the epoxy resin 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. From the viewpoint of the mechanical properties of the CFRP obtained by curing the prepreg, 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, based on the total amount (100% by mass) of the epoxy resin composition excluding solid mass components such as conductive particles and non-conductive polyamide particles. From the viewpoint of processability, the upper limit is preferably 20% by mass or less.

[0060] Furthermore, when using a prepreg production method in which an epoxy resin composition is separated into a primary resin composition and a secondary resin composition and then impregnated into a CF sheet in two stages as described below, the amount of thermoplastic resin to be blended for both the primary resin composition and the secondary resin composition is preferably 7 parts by mass or more, more preferably 14 parts by mass or more, and even more preferably 19 parts by mass or more, per 100 parts by mass of epoxy resin. From the viewpoint of processability, the upper limit is preferably 30 parts by mass or less.

[0061] When the blending amount of the thermoplastic resin is within the above range, the flow of the matrix resin during the process of molding the prepreg laminate can be suppressed, which makes it easier to obtain a uniform interlayer thickness within the CFRP molded body and suppresses variations in electrical conductivity between layers.

[0062] Suitable thermoplastic resins include polymers having a bond in the main chain selected from carbon-carbon bonds, amide bonds, imide bonds, ester bonds, ether bonds, carbonate bonds, urethane bonds, urea bonds, thioether bonds, sulfone bonds, imidazole bonds, and carbonyl bonds. Specific examples include polyacrylate, polyolefin, polyamide (PA), aramid, polyester, polycarbonate (PC), polyphenylene sulfide (PPS), polybenzimidazole (PBI), polyimide (PI), polyetherimide (PEI), polysulfone (PSU), polyethersulfone (PES), polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), 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.

[0063] <Minimum Viscosity of Epoxy Resin Composition> In the present invention, the epoxy resin composition used as the matrix resin preferably has a minimum viscosity of 0.3 Pa·s or more when measured under conditions of an angular frequency of 10 rad / s and a temperature rise rate of 2°C / min. By adopting such an embodiment, the flow of the matrix resin can be suppressed. Therefore, a uniform interlayer thickness can be easily obtained within the CFRP molded product, and variations in electrical conductivity between layers can be suppressed. The minimum viscosity is more preferably 0.6 Pa·s or more, and even more preferably 0.8 Pa·s or more. The minimum viscosity measured under conditions of an angular frequency of 10 rad / s and a heating rate of 2°C / min refers to the viscosity at the minimum value on a viscosity curve obtained by using a dynamic viscoelasticity measuring device equipped with parallel plates (e.g., ARES manufactured by TA Instruments) to heat a resin composition at a measurement start temperature of 40°C, a parallel plate diameter of 40 mm, a parallel plate spacing of 1 mm, an angular frequency of 10 rad / s, and a heating rate of 2°C / min, and performing heating measurement in autostrain mode until the storage modulus and loss modulus become the same value.

[0064] <Non-Conductive Polyamide Particles> From the perspective of toughness, it is preferable to arrange non-conductive polyamide particles between adjacent CF layers. These polyamide particles are different from the conductive polyamide particles described above and are non-conductive (not treated to impart conductivity). These 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 can also improve the impact resistance of CAI (Common Air Insulation) and other properties important for aircraft applications, as described below. Suitable polyamides include nylon 12, nylon 11, nylon 6, nylon 66, nylon 6 / 12 copolymers, and nylons (semi-IPN nylons) formed with an epoxy compound in a semi-IPN (polymer interpenetrating network) structure, as described in Example 1 of JP-A-01-104624. To achieve a preferred CAI (to be described later), the non-conductive polyamide particles are preferably contained in an amount of 4% by mass or more, based on 100% by mass of the epoxy resin composition constituting the prepreg. From the viewpoint of processability, however, the amount is preferably 60% by mass or less. Furthermore, from the viewpoint of easily controlling the coefficient of variation of the thickness of one interlayer resin layer in the CFRP (to be described later) 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, based on 100% by mass of the epoxy resin composition constituting the prepreg. Since the polyamide particles are not crushed and maintain their original particle shape during pressure-applied molding as described later, thereby making it easier to obtain the desired interlayer thickness, it is more preferable that the polyamide particles be crystalline, having a melting point above the molding temperature.

[0065] <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 and heat-curing them, and various known methods can be used as the lamination and heat-curing method. For example, a method that can be preferably used is to cut the obtained prepreg to a predetermined size, laminate a predetermined number of prepregs, and then heat-cur them while applying pressure.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] <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 diameters to small diameters, present between the CF 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, thickness variation within a single 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 a single interlayer resin layer 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.

[0070] 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.

[0071] The coefficient of variation in thickness within one interlayer resin layer in a 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 20 random regions (20 photographs) in the in-plane direction of each layer, allowing the boundary regions of the interlayer resin layer between adjacent CF layers to be confirmed, with no overlapping of the photographed regions. For each cross-sectional photograph, the thickness of the interlayer resin layer disposed 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 total of 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.

[0072] 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.

[0073] <Interlaminar Electrical Conductivity of Adjacent CF Layers in CFRP> The electrical conductivity between adjacent CF layers (interlaminar 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 so that the five scanning lines and 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] 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 1.6 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 becomes complicated and the costs may become very high, so in reality, a conductivity of 1000 S / m or less is preferred.

[0077] <CAI (Impact Resistance)> The CFRP of the present invention preferably has a compressive strength after impact (CAI), which is an important indicator of impact resistance in aircraft applications, of 230 MPa or more after an impact of 6.7 J / mm is applied. A CAI of 230 MPa or more is preferable because it allows the strength to 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.

[0078] <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.

[0079] 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.

[0080] 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.

[0081] 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 this 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.

[0082] 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.

[0083] 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 on 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.

[0084] Here, when the epoxy resin composition is impregnated into the CF sheet in two stages, separating it into a primary resin composition and a secondary resin composition, the secondary resin composition only needs to contain at least the epoxy resin, curing agent, and conductive particles. For example, the primary resin composition may not contain conductive particles. In an embodiment in which the primary resin composition does not contain conductive particles, the utilization efficiency of the conductive particles can be increased when viewed as the prepreg as a whole, or the amount of conductive particles used can be reduced, which is advantageous in terms of lightning resistance and mechanical properties when made into a CFRP. Furthermore, the fact that the thermoplastic resin can be used arbitrarily depending on the desired properties can be said for both the primary resin composition and the secondary resin composition.

[0085] 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.

[0086] <Raw Materials> (1) Carbon Fiber (CF) CF with a single fiber diameter of 7 μm, 24,000 filaments per bundle, tensile strength of 5.8 GPa, and tensile modulus of elasticity of 280 GPa was prepared. (2) Epoxy Resin "Sumiepoxy" ELM434 (tetraglycidylaminodiphenylmethane, manufactured by Sumitomo Chemical Co., Ltd.).

[0087] "EPICLON (registered trademark)" 830 (bisphenol F type epoxy resin, manufactured by DIC Corporation) "GOT (registered trademark)" (glycidyl aniline 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 uniform solution, which was then sealed and purged with nitrogen. The stirring speed of the stirring blade was then 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 water vapor to maintain the pressure at 1 MPa. After the temperature reached 240 ° C, the pressure was released at a rate of 0.02 MPa / min. The above temperature was then maintained for 1 hour while flowing nitrogen, completing the polymerization, and the mixture was discharged into a 2,000 g water bath to obtain a slurry. The unreacted material was dissolved in water in a 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 passed 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 resulting powder had a melting point of 218°C, the same as polyamide 6, and a crystallization temperature of 170°C. (6) Conductive Particles (Note: The "conductive particles" section is omitted from the particle names in the table): 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, calcined at 2000°C, and classified (component: carbon, D50: 20.3 μm, D99: 64.4 μm, coefficient of variation of particle size distribution: 62%). The volume resistivity of conductive particles A was calculated by the method described below and was found to be 0.05 Ωcm. Conductive particles B, C, E, F and G described below also showed the same volume resistivity as conductive particles A.

[0088] Conductive particles B, E, F and G: These were obtained in the same manner as conductive particles A, except that they were classified under different conditions and had the physical properties and characteristics shown in Table 2.

[0089] Conductive particles C: Obtained by classifying "Glassy Carbon (registered trademark)" (component: carbon, manufactured by Tokai Carbon Co., Ltd.). Table 2 shows the physical properties and characteristics.

[0090] Conductive particle D: Two types of phenolic resin particles (Marilyn HF type, manufactured by Gun-ei Chemical Industry Co., Ltd.) having different particle size distributions from those of the example of conductive particle A were mixed, fired at 2000°C, and classified (component: carbon, D50: 27.6 μm, D99: 66.4 μm, coefficient of variation of particle size distribution: 54%). The volume resistivity of conductive particle D was calculated using the method described below and found to be 0.05 Ω cm.

[0091] Conductive particles H: 100 g of the non-conductive polyamide particles obtained in (5) were added to 1,000 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 H. The volume resistivity of the conductive particles 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).

[0092] Conductive particles I, J, and K: 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.

[0093] <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.

[0094] 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.

[0095] The composition ratios of the epoxy resin compositions used in each of the Examples and Comparative Examples are shown in Tables 1, 3, 4, 6, 7, and 8 (the units in each table are parts by mass).

[0096] (2) Preparation of Prepreg Using the epoxy resin composition prepared in (1) above, a two-stage impregnation method was used to prepare a prepreg as follows. The primary resin composition and secondary resin composition prepared in (1) above were each uniformly applied onto 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 20% 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 , resin content (Rc) 34% by mass).

[0097] (3) Measurement of Resin Content of Prepreg (hereinafter abbreviated as "Rc") 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, test pieces measuring 100 mm long and 100 mm wide were cut out from the prepregs obtained in each Example and Comparative Example, and their masses were measured. Next, at a temperature of 23°C, the test pieces were 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. After the third washing procedure, 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 resin was determined by subtracting the mass of the CF from the mass of the prepreg measured first. 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. The same measurement was also performed on the prepreg intermediate material.

[0098] (4) Minimum Viscosity of Epoxy Resin Composition An epoxy resin composition was prepared by extracting the same area of ​​resin composition from each of the primary and secondary resin composition films used in the prepreg preparation in (2) and mixing them. The epoxy resin composition was subjected to temperature measurement in autostrain mode using a dynamic viscoelasticity measuring device (ARES, manufactured by TA Instruments) at a starting temperature of 40°C, a parallel plate diameter of 40 mm, a parallel plate spacing of 1 mm, an angular frequency of 10 rad / s, and a temperature rise rate of 2°C / min until the storage modulus and loss modulus reached the same value. The viscosity at the minimum value on the obtained viscosity curve was determined to be the minimum viscosity of the epoxy resin composition.

[0099] (5) Measurement of particle diameter of conductive particles, coefficient of variation, kurtosis, D50, D99 The secondary resin composition film was observed using a laser microscope (digital microscope VHX-5000: manufactured by Keyence Corporation, hereinafter referred to as VHX-5000) at a magnification of 200 times or more, and the diameter of the circle inscribed on the periphery of 1,000 randomly selected particles (primary particles) in the observed image was measured as the particle diameter. However, particles with an inscribed circle diameter of less than 0.5 μm were excluded from the measurement. The particle diameter was calculated in micrometers to two decimal places and rounded to the nearest tenth to obtain a value to one decimal place. The average diameter (μm) and standard deviation (μm) of the 1,000 conductive particles were calculated. The coefficient of variation (%) was calculated by dividing the standard deviation (μm) by the average diameter (μm). Kurtosis was calculated using Equation 3 based on the particle diameters of the 1,000 particles. The average diameter (μm) is the arithmetic mean of the diameters of 1,000 particles, and corresponds to Dave in Equation 3. The 1,000 particles were arranged in ascending order of particle diameter, and the average diameter of the 500th and 501st particles was defined as D50 (μm), and the average diameter of the 990th and 991st particles was defined as D99 (μm).

[0100] (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 set as a sample in a cylindrical cell with four probe electrodes. The thickness and resistance of the sample were measured while applying a pressure of 60 MPa to the sample, and the volume resistivity was calculated.

[0101] (7) Sphericity of Conductive Particles The secondary resin composition film was observed at a magnification of 200 times or more using a VHX-5000. For 1,000 randomly selected particles (primary particles), the diameters of the inscribed circle and the circumscribed circle were measured. 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.

[0102] (8) Prevalence of Conductive Particles Within 15% Depth from the Prepreg Surface The prepreg prepared in (2) above was sandwiched between two parallel, smooth-surfaced polytetrafluoroethylene resin plates without applying pressure, and the temperature was gradually raised to 150°C over seven days to gel and cure the prepreg into a plate-like cured product. After curing, the prepreg was cut in a direction perpendicular to the adhesive surface (thickness direction), and the cross section was polished. Photographs were taken with a VHX-5000 at a magnification of 200x or more, ensuring that the top and bottom surfaces of the cured 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 (the width direction of the cured prepreg), and the average value was used as the average thickness of the cured 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. S1 was divided by S2 to determine the presence rate of conductive particles from the surface of the prepreg to a depth of 15%.

[0103] (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).

[0104] 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.

[0105] 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)

[0106] 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.

[0107] The probe was scanned so that the scanning lines and the sides parallel to the thickness direction of the sample on the voltage measurement surface were equally spaced. The sections corresponding to the CF layer and the interlayer resin layer were determined by plotting the relationship between the voltage between the probe and the base electrode versus the scanning distance and using the inflection point of the curve. The same measurements were performed on all four test pieces A, and the arithmetic mean value of the measurement results was used as the interlayer conductivity (Pcc (S / m)) of the CFRP at the center of the panel. The same measurements were performed on all four test pieces B, and the arithmetic mean value of the measurement results was used as the interlayer conductivity (Ptc (S / m)) of the CFRP at the edge of the panel.

[0108] (10) Determination of variation in conductivity between layers For the conductivity between layers obtained in (9), the rate of change in conductivity within the CFRP molded body was calculated from the Pcc and Ptc using Equation 4. 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.

[0109] Rate of change in conductivity (%) = |Ptc - Pcc| ÷ (average of Ptc and Pcc) × 100: (Equation 4) (11) Coefficient of variation of thickness within one interlayer resin layer A square CFRP measuring 300 mm in width and 300 mm in length was prepared in the same manner as in (9), and two test pieces that were line-symmetric with respect to b' in Figure 1 were selected from the four test pieces in the central part of the CFRP panel (c in Figure 1). The cross section with the side corresponding to b in Figure 1 was polished, and then the cross section was magnified 500 times with an optical microscope to obtain cross-sectional photographs in which the interlayer between the 7th and 8th plies in the panel thickness direction could be confirmed, with 10 areas per test piece taken at random so that the photographed areas did not overlap, for a total of 20 areas (20 photographs). For each cross-sectional photograph, the thickness of the interlayer resin layer between adjacent CF layers (the interlayer CF distance in the CFRP thickness direction at each measurement point between the layers) was measured at 100 points at 5 μm intervals along the side corresponding to b in Figure 1. The average thickness (μm) and standard deviation (μm) of the interlayer resin layer were calculated from a total of 2,000 measurements. The average thickness (μm) of the interlayer resin layer was calculated as the arithmetic mean of the thicknesses of each interlayer resin layer. The coefficient of variation (%) was also calculated by dividing the standard deviation (μm) by the average thickness (μm) of the interlayer resin layer.

[0110] (12) CAI (Compression Strength After Impact of CFRP) 16 plies of the prepreg obtained in (2) were quasi-isotropically laminated in a [+45° / 0° / -45° / 90°] 2s configuration, and molded in an autoclave at a temperature of 180°C for 2 hours under 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 out from this CFRP, and a drop weight impact of 6.7 J / mm was applied to the center of the sample in accordance with SACMA SRM 2R-94, followed by a compression fracture test to determine the compressive strength after impact (CAI).

[0111] (13) Average Radius of Carbon Fiber The prepreg 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 under 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 on the outer 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.

[0112] (14) Mass ratio of non-conductive polyamide particles in epoxy resin composition The primary resin composition film and the secondary resin composition film used in the preparation of the prepreg in (2) were cut into 100 mm squares (100 mm ２ ) film mass (g) is measured to find the mass of 1 m ２ Resin 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, 3, 4, 6, 7, and 8 to calculate the weight per unit area of ​​the secondary resin composition film. ２ Weight of non-conductive polyamide particles per unit area (g / m ２ The weight per unit area of ​​the non-conductive polyamide particles was divided by the sum of the resin weight per unit area of ​​the primary resin composition film and the resin weight per unit area of ​​the secondary resin composition film determined above to calculate the mass proportion of the non-conductive polyamide particles in the epoxy resin composition constituting the prepreg.

[0113] Example 1: An epoxy resin composition was prepared using the composition shown in Table 1 and the methods described in (1) and (2) of <Evaluation Methods>. A prepreg was then produced and evaluated. Conductive particles A were used, and the D50 was sufficiently large compared to the average radius of 3.5 μm of the carbon fiber. The coefficient of variation of the particle size distribution of the conductive particles was 62%, which was sufficiently large. The minimum viscosity of the epoxy resin composition was 0.9 Pa s.

[0114] When the interlayer conductivity was measured using the method described in (9) of the <Various Evaluation Methods>, the conductivity Pcc was slightly lower than the conductivity Ptc, but both were very high. This decrease in interlayer conductivity is thought to be due to the flow of the matrix resin during molding of the prepreg laminate, which resulted in a thicker interlayer space at the center of the panel compared to the panel edges, resulting in a decrease in the number of conductive particles connecting the CF layers. However, the rate of change in conductivity calculated using Equation 4 in (10) of the <Various Evaluation Methods>, which indicates the variation in interlayer conductivity, was only 16%, a very small value. This is thought to be due to the small change in the number of conductive particles connecting the CF layers within the CFRP molded body due to the large coefficient of variation in the particle size distribution of the conductive particles. The CAI was also very high for use in primary aircraft structural materials.

[0115] 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. Conductive particles B had an extremely small coefficient of variation in the particle size distribution of the conductive particles.

[0116] When the interlayer conductivity was measured in the same manner as in Example 1, the panel edges showed very high interlayer conductivity, a value higher than that of Example 1, whereas the interlayer conductivity in the panel center dropped significantly and was lower than that of Example 1. This is thought to be because the flow of the matrix resin during molding of the prepreg laminate caused the film to become thicker in the panel center compared to the panel edges, resulting in a decrease in the number of conductive particles connecting the CF layers in the panel center.

[0117] This is thought to be due to the small coefficient of variation of the particle size distribution of conductive particles B, which resulted in a large change in the number of conductive particles connecting the CF layers where there was a change in the interlayer thickness within the CFRP molded body.

[0118] Example 2 A resin film and a prepreg were produced and evaluated 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.

[0119] Comparative Example 2 An epoxy resin composition was prepared, a prepreg was produced, and evaluation was performed in the same manner as in Example 1, except that the conductive particles used were conductive particles E as shown in Table 2. The coefficient of variation of the particle size distribution of conductive particles E was 28%, which was larger than that of conductive particles B but was not sufficiently large, and therefore, similar to Comparative Example 1, there was a large difference in conductivity between the layers at the center and edges of the panel.

[0120] Example 3 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 F 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. In Example 3, the variation in conductivity between layers was reduced compared to Comparative Example 2, which used conductive particles E having a D50 equivalent to that of conductive particles F, and this is thought to be due to the contribution of the coefficient of variation of the particle size distribution of the conductive particles.

[0121] 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 shown in Table 2. The interlayer conductivity variation was sufficiently small. However, compared to Examples 1 to 3, which used conductive particles A, C, or F, a slight increase in the conductivity variation was observed, despite the higher coefficient of variation of the particle size distribution. This is thought to be due in part to the fact that conductive particles G have a smaller D50 than conductive particles A, C, or F, and therefore small-diameter conductive particles penetrate into the CF layer during prepreg production, resulting in greater interlayer thickness variation in the CFRP. Furthermore, the penetration of some particles into the CF layer is thought to lead to variation in the number of conductive particles connecting the CF layer, which in turn leads to variation in interlayer conductivity. In other words, it has been shown that by setting D50 to a value sufficiently large relative to the average radius of the CF, for example, four times the average CF radius, the increase in interlayer conductivity variation can be suppressed with greater precision. The electrical conductivity between the layers was sufficiently high, and the CAI was also high enough for use as a primary structural material for aircraft.

[0122] 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 as shown in Table 2. Conductive particles H are conductive polyamide particles, and by setting the coefficient of variation of the particle size distribution 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] (Example 6) 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 D as shown in Table 2. The interlayer conductivity had very little variation, and the interlayer conductivity was very high. The CAI was also very high for use as a primary structural material for an aircraft.

[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 the conductive particles used were conductive particles I as shown in Table 2. The variation in conductivity between layers was very small, and the conductivity between layers was very high. The CAI was also very high for use as a primary structural material for an aircraft.

[0125] (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. The variation in conductivity between layers was very small, and the conductivity between layers was very high. The CAI was very high for use as a primary structural material for an aircraft.

[0126] Comparative Example 3 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 K as shown in Table 2. The coefficient of variation of the particle size distribution of conductive particles K was 25%, which was larger than the coefficient of variation of the particle size distribution of conductive particles B, but was not sufficiently large. Therefore, similar to Comparative Examples 1 and 2, there was a large difference in conductivity between the layers at the center and edges of the panel.

[0127] (Example 9) As shown in Table 3, an epoxy resin composition was prepared and a prepreg was produced in the same manner as in Example 1, except that the amount of thermoplastic resin in the primary resin composition was changed, and then evaluation was performed. The results are shown in Table 5. The minimum viscosity of the resin composition was 0.6 Pa s. The variation in interlayer conductivity was larger than in Example 1, but was still sufficiently small. The interlayer conductivity was sufficiently high, and the CAI was also sufficiently high for use as a primary structural material for an aircraft.

[0128] (Example 10) As shown in Table 4, an epoxy resin composition was prepared and a prepreg was fabricated and evaluated in the same manner as in Example 1, except that the amount of thermoplastic resin in the primary resin composition was changed. The results are shown in Table 5. The minimum viscosity of the resin composition was 0.3 Pa·s. The variation in interlayer conductivity was greater than in Examples 1 and 9, but was within the acceptable range. As shown in Table 5, the variation in interlayer conductivity increased in the order of Example 1, Example 9, and Example 10. This is thought to be because the lower the minimum viscosity, the greater the flow of the matrix resin during molding, resulting in variation in interlayer thickness between the center and edge of the panel and variation in the number of conductive particles connecting the CF layers. However, the interlayer conductivity was high, and the CAI was sufficiently high for use as a primary structural material for aircraft.

[0129] (Example 11) As shown in Table 6, an epoxy resin composition was prepared and a prepreg was produced in the same manner as in Example 1, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, and then evaluation was performed. The results are shown in Table 9. The variation in conductivity between layers was very small, at 17%. The CAI was also sufficiently high for use as a primary structural material for aircraft.

[0130] (Example 12) As shown in Table 6, an epoxy resin composition was prepared and a prepreg was produced in the same manner as in Example 2, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, and then evaluation was performed. The results are shown in Table 9. The variation in conductivity between layers was very small, at 17%. The CAI was also sufficiently high for use as a primary structural material for aircraft.

[0131] Example 13: Epoxy resin compositions were prepared and prepregs were fabricated 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 7. The results are shown in Table 9. The variation in interlayer conductivity was greater than in Examples 1 and 11, which used the same conductive particles, but was within the acceptable range. Example 13 had a smaller coefficient of variation in interlayer thickness compared to Examples 1 and 11, resulting in a relatively smaller number of conductive particles connecting the CF layers. This is thought to be due to the susceptibility to changes in the number of conductive particles connecting the CF layers due to variations in interlayer thickness between the center and edges of the panel. However, the interlayer conductivity was high, and the CAI was sufficiently high for use as a primary structural material for aircraft.

[0132] (Example 14) As shown in Table 8, an epoxy resin composition was prepared and a prepreg was produced in the same manner as in Example 1, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, and evaluation was performed. The results are shown in Table 9. The variation in conductivity between layers was very small, and the conductivity between layers was very high. The CAI was acceptable for use in primary aircraft structural materials, but was lower than that of Examples 1, 11, and 13, which used the same conductive particles.

[0133]

[0134]

[0135]

[0136]

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143] 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.

[0144] 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. A prepreg in which carbon fibers are impregnated with a matrix resin, the matrix resin being an epoxy resin composition, all or part of which contains at least an epoxy resin, a curing agent, and conductive particles, and the conductive particles have a coefficient of variation of 40% or more in their particle size distribution.

2. The prepreg according to claim 1, wherein the median diameter (D50) of the conductive particles is equal to or greater than the average radius of the carbon fibers.

3. The prepreg according to claim 1, wherein the conductive particles have a D99 (μm) of 0.33×A or less, where A is the basis weight of the carbon fibers in the prepreg in g / m ２ The value is rounded off to the first decimal place when written.

4. The prepreg according to claim 1, wherein the D50 (μm) of the conductive particles is 0.18×A or less, where A is the basis weight of the carbon fibers 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 epoxy resin composition has a minimum viscosity of 0.6 Pa·s or more when measured under conditions of an angular frequency of 10 rad / s and a temperature rise rate of 2°C / min.

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 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 conductive particles have a kurtosis in particle size distribution of 5 or less.

8. The prepreg according to claim 1, wherein the conductive particles are carbon particles or conductive polyamide particles.

9. The prepreg of claim 1, further comprising non-conductive polyamide particles.

10. The prepreg described in claim 9, wherein the mass ratio of the non-conductive polyamide particles is 4 mass% or more and 15 mass% or less when the mass of the epoxy resin composition is 100 mass%.

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

12. The carbon fiber composite material according to claim 11, wherein the coefficient of variation in thickness within one interlaminar resin layer is 26% or more and 45% or less.

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

14. A structure obtained by using the carbon fiber reinforced composite material according to claim 11, the shape of which 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.