Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
  • C08J5/24—Impregnating materials with prepolymers which can be polymerised in situ, e.g. manufacture of prepregs

Definitions

• the present invention relates to a prepreg, a carbon fiber reinforced composite material, and a structure using the carbon fiber reinforced composite material.
• fiber-reinforced composite materials made of reinforcing fibers and matrix resins have been applied to many fields such as aerospace, automobiles, railway vehicles, ships, civil engineering and construction, and sporting goods because they are lightweight yet have excellent mechanical properties such as strength and rigidity, heat resistance, and corrosion resistance.
• fiber-reinforced composite materials using continuous reinforcing fibers are used in applications that require high heat resistance.
• Carbon fibers hereinafter sometimes abbreviated as "CF”
• thermosetting resins especially epoxy resins, which have excellent adhesion to CF, heat resistance, and elastic modulus, and have small cure shrinkage, are often used as the matrix resin.
• CFRP carbon fiber reinforced composite materials
• the required properties have become even stricter.
• CFRP can be obtained by impregnating CF sheets, which are made by forming CF into sheets, with a matrix resin to form prepregs, and then molding them.
• CF sheets include UD sheets, which are made by aligning CF in a unidirectional (UD) plane, and CF fabrics, which are made by arranging CF 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 making CFRP with complex shapes, but they can also be used in combination. Since mechanical properties are prioritized in aircraft structural materials, CFRP made by laminating prepregs containing UD sheets in multiple directions and molding them is widely used.
• CFRP matrix resin
• matrix resin is generally an insulator. Therefore, in CFRP, a layer of matrix resin is formed between adjacent CF sheets (between layers), and this may become an insulating layer. Focusing on the layers of CF sheets, the conductivity is relatively high in the fiber axis direction of the CF (hereinafter abbreviated as "fiber direction") because the CF itself becomes a conductive path.
• fiber direction the fiber axis direction of the CF
• orthogonal direction although a conductive path is formed by contact between CFs, the conductivity in the orthogonal direction is about 1/1000 of the conductivity in the fiber direction.
• CFRP is less conductive than metal materials, and has anisotropy in conductivity in the fiber direction and orthogonal directions. 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.
• 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.
• Patent Document 1 and Patent Document 2 disclose a technique for arranging 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 thickness between layers. By using these inventions, it is possible to arrange more conductive particles for the purpose of conduction within a layer of a desired thickness.
• Patent Document 4 discloses a technique for making the interlayer conductive even without conductive particles by varying the interlayer thickness in the resin layer or CFRP constituting the outer side of the fiber layer in the prepreg from the average thickness depending on the location.
• Patent Document 5 discloses a technique for further improving conductivity by making a prepreg having a matrix resin in which conductive particles with a particle size of 1 ⁇ m or less are used in combination with conductive particles with a particle size of 5 ⁇ m or more.
• Patent Document 3 page 5, lines 28-30
• Patent Document 4 page 6, lines 9-11
• Patent Document 5 paragraphs [0034]-[0035] all suggest that it is preferable to narrow the particle size distribution of the conductive particles from the standpoint of electrical conductivity, mechanical properties, and processability.
• JP 2011-213991 A International Publication No. 2008/056123 International Publication No. 2011/027160 International Publication No. 2012/084197 International Publication No. 2012/124450
• Patent Documents 1 to 5 which use conductive particles controlled to a narrow particle size distribution through highly accurate classification to achieve high conductivity, have issues such as the need for a high level of classification technology and a significant increase in raw material costs, as well as the problem of not fully considering the conductivity variations that can occur due to the flow of the matrix resin within the molded body. Even if it is possible to achieve high conductivity under certain conditions, if it cannot be achieved stably within the molded body, it will increase 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, and as a result, excessive lightning protection systems will be introduced, leading to increased aircraft weight and costs.
• the objective of this 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, particularly the impact resistance that is important for aircraft structural materials, thereby simplifying conventional lightning protection systems and contributing to weight and cost reductions in aircraft.
• the conductive particles having a coefficient of variation of 40% or more in particle size distribution.
• the present invention makes it possible to obtain a prepreg that exhibits the excellent impact resistance of CFRP while maintaining a stable interlayer electrical conductivity above a certain level even when there is variation in the interlayer thickness of adjacent carbon fiber layers after the CFRP is made. Furthermore, by applying such prepregs and carbon fiber reinforced composite materials to aircraft, the overall efficiency of lightning protection systems can be improved.
• the CF is used as a CF bundle in which the CF is bundled.
• a tape-like assembly of about 1,000 to 1,000,000 CF monofilaments is usually called a "tow”
• a sheet-like CF bundle can be obtained by arranging this tow.
• a CF sheet in which the CF is arranged in one direction (UD) along the longitudinal direction is called a UD sheet
• a CF fabric in addition to woven fabrics and knitted fabrics, those in which the CF is arranged multiaxially in two dimensions, and those in which the CF is randomly oriented such as nonwoven fabrics, mats, and paper can be used.
• the prepreg of the present invention uses an epoxy resin composition containing at least an epoxy resin, a curing agent, and conductive particles as a matrix resin.
• an epoxy resin composition containing an epoxy resin and a curing agent but not containing conductive particles can also be used as the matrix resin.
• the epoxy resin those having amines, phenols, and compounds having a carbon-carbon double bond as a precursor are preferable.
• epoxy resins having amines as a precursor include various isomers of tetraglycidyldiaminodiphenylmethane, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, and triglycidylaminocresol
• epoxy resins having phenols as a precursor include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, phenol novolac type epoxy resin, and cresol novolac type epoxy resin
• epoxy resins having a compound having a carbon-carbon double bond as a precursor include alicyclic epoxy resins, but are not limited thereto.
• Epoxy resins using aromatic amines as precursors such as tetraglycidyldiaminodiphenylmethane (N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane), are most suitable for the present invention because of their good heat resistance and good adhesion to reinforcing fibers.
• a compound having an active group capable of reacting with an epoxy group can be used.
• a compound having an amino group, an acid anhydride group, or an azide group is suitable.
• dicyandiamide, various isomers of diaminodiphenylsulfone, and aminobenzoic acid esters are suitable.
• a prepreg is prepared by impregnating a CF sheet with the matrix resin. This can be molded to obtain a CFRP.
• the prepreg has a layer of matrix resin on the upper and lower surfaces of the CF sheet
• the laminate of the prepregs and the CFRP have a layer of carbon fiber (hereinafter, sometimes referred to as a "CF layer") containing the matrix resin and the CF sheet, and a resin sandwiched between the adjacent CF layers. Therefore, the thickness between adjacent carbon fiber layers and the thickness of the interlayer resin layer present between the carbon fiber layers are synonymous.
• conductive particles are disposed in the layer of resin present between adjacent CF layers (such a resin layer is referred to as an "interlayer resin layer").
• the conductive particles form a portion that electrically connects the adjacent CF layers, thereby conducting the interlayers. This means that the conductive particles are substantially in contact with the CF layers present above and below the interlayer resin layer to form a conductive path.
• the conductive particles used in the prepreg of the present invention may be any particles that behave as good electrical conductors, and are not limited to particles consisting of only conductors.
• the volume resistivity of the conductive particles is preferably 10 ⁇ cm or less, more preferably 5 ⁇ cm or less, and even more preferably 1 ⁇ cm or less. By setting the volume resistivity in this range, a conductive path can be formed in the interlayer resin layer, thereby increasing the conductivity in the thickness direction.
• the volume resistivity can be calculated by placing a sample in a cylindrical cell with four probe electrodes, applying a pressure of 60 MPa to the sample, and measuring the thickness and resistance of the sample.
• conductive particles include metal particles, metal oxide particles, inorganic particles with a metal coating, organic polymer particles, conductive polymer particles such as polyacetylene particles, polyaniline particles, polypyrrole particles, polythiophene particles, polyisothianaphthene particles, polyethylenedioxythiophene particles, polyamide particles, and carbon particles. Of these, carbon particles and polyamide particles that have been given electrical conductivity (conductive polyamide particles) are preferred.
• Carbon particles are preferably used in aircraft because they do not cause corrosion problems.
• crystalline carbon and amorphous carbon are preferably used.
• 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.).
• Conductive polyamide particles are preferably used because they are conductive and can provide CFRP with impact resistance, which is a function of polyamide particles described later. Conductive polyamide particles can be obtained by imparting conductivity to non-conductive polyamide particles described later. There are no particular limitations on the means for imparting conductivity to polyamide particles, but coating with metal, conductive polymer, or conductive nanomaterial, or dispersing conductive polymer or conductive nanomaterial within polyamide particles can be used. Of these, from the viewpoint of achieving both conductivity and impact resistance, the method of coating with metal and the method of dispersing particulate conductive nanomaterial within the particles are preferably used. There are no particular limitations on the conductive nanomaterial, but examples include carbon nanotubes, carbon black, and graphene. Conductive particles with an aspect ratio of 1.5 or less are preferably used.
• the median diameter (also referred to as D50) of the conductive particles used in the present invention is preferably equal to or larger than the average radius of the CF.
• the average radius of the CF referred to here is the average radius of the single yarn. It is preferably equal to or larger than twice the average radius of the CF (i.e., equal to or larger than the average diameter of the CF), and more preferably equal to or larger than four times the average radius of the CF (i.e., equal to or larger than the average diameter of the CF).
• 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.
• the upper limit of D50 (note that the unit of D50 here is ⁇ m) of the conductive particles is preferably 0.18 ⁇ A or less, more preferably 0.15 ⁇ A or less, and even more preferably 0.13 ⁇ A or less, when the basis weight of the carbon fiber in the prepreg is expressed in g/m2 and rounded off to the first decimal place, A.
• the lower limit is preferably 0.05 ⁇ A or more, and more preferably 0.06 ⁇ A or more.
• 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 fibers in the prepreg below).
• the upper limit of D50 within the above range, the number of conductive particles that penetrate too deeply into the CF layer and disrupt the linearity of the CF arrangement can be reduced, resulting in 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.
• the procedure for obtaining the median diameter of conductive particles is shown.
• the surface of the prepreg is observed with a laser microscope (for example, a digital microscope VHX-5000: manufactured by Keyence Corporation) at a magnification of 200 times or more, and the diameter of the circle inscribed on the outer periphery of the conductive particle in the observed image is taken as the particle size, which is calculated in micrometers to two decimal places and rounded off to obtain a value to one decimal place. This is done for each of 1,000 particles selected at random. However, particles with a diameter of the inscribed circle of less than 0.5 ⁇ m are excluded from the measurement. Note that if a secondary resin composition film containing conductive particles is available as the observation subject, it may also be used as the observation subject.
• the median diameter of 1,000 particles in the prepreg or secondary resin composition film is the median diameter (D50) in the particle size distribution of the conductive particles in question.
• D10 can be calculated from the average diameter of the 100th and 101st particles.
• D10 is preferably equal to or greater than the average radius of the CF, more 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 even more preferably equal to or greater than three times the average radius of the CF (i.e., equal to or greater than 1.5 times the average diameter of the CF).
• the above range is preferable because it can reduce the variation in electrical conductivity between adjacent carbon fiber layers caused by small conductive particles penetrating into the CF layer. It is also preferable because it can prevent cracks during impact in CFRP from propagating into the CF layer along the small particles that have penetrated into the CF layer, thereby suppressing the decrease in CAI (compressive strength after impact), which will be described later.
• D90 can be calculated from the average diameter of the 900th and 901st particles.
• D99 can be calculated from the average diameter of the 990th and 991st particles in the above order.
• D99 (the unit of D99 here is ⁇ m) is the value obtained by rounding off the first decimal place when the weight of the carbon fiber in the prepreg is expressed in g/ m2. From the viewpoint of process passability and the viewpoint of suppressing the decrease in CAI due to conductive particles that are 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 limit as the lower limit, but it is preferably 0.20 ⁇ A or more, and more preferably 0.23 ⁇ A or more.
• D10, D50, D90 and D99 are values for all conductive particles.
• the procedure for obtaining the average radius of CF is shown.
• the average radius of CF is obtained from CFRP, it can be measured and calculated by obtaining a cross-sectional observation image of the CFRP.
• the CFRP is cut in a direction perpendicular to the fiber axis direction of the CF embedded in the CFRP to obtain a cross section.
• a laser microscope e.g., VHX-5000, manufactured by Keyence Corporation
• 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 the diameters are measured using the above method, the arithmetic average is calculated, and the average radius of the CF is obtained by dividing by 2.
• the average radius of CF When the average radius of CF is obtained 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 in the perpendicular direction of the CF is obtained and observed with 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 that constitutes the prepreg does not flow during the cutting and a cross-section in the perpendicular direction can be obtained stably and accurately.
• SEM scanning electron microscope
• the coefficient of variation in the particle size distribution of the conductive particles in the prepreg of the present invention is 40% or more.
• a CFRP having stable interlayer conductivity can be obtained.
• Methods for obtaining conductive particles with such a predetermined particle size distribution include, for example, classifying a single conductive particle into the desired range, mixing two or more types of conductive particles with different particle size distributions, and, if necessary, mixing and then classifying.
• 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 required.
• an example of a method for obtaining mixed conductive particles with a particle size distribution having the above-mentioned coefficient of variation is to use particles that satisfy the following (Formula 1) when two particles selected in descending order of their blending ratio from among the conductive particles have the particle ⁇ with the smaller D50 and the particle ⁇ with the larger D50.
• the particle size distribution can be a wide distribution on the small diameter side, or a wide distribution on the large diameter side, or a wide distribution on both the small diameter side and the large diameter side.
• the conductive particles present between adjacent CF layers electrically connect the adjacent CF layers, thereby generating conductivity between the adjacent CF layers.
• the thickness of the interlayer resin layer in the CFRP molding may change due to the flow of the matrix resin.
• the interlayer thickness may vary due to the raw materials and production process of the prepreg. Due to such factors, the interlayer thickness may become partially thin or partially thick.
• the conductivity between adjacent CF layers can be increased by increasing the amount of conductive particles contained in the interlayer resin layer.
• the amount in order to ensure the mechanical properties of the CFRP, there is naturally an upper limit, and as described below, it is preferable to set the amount at 10 mass% or less of the total mass of the epoxy resin composition.
• the number of particles contained in the matrix resin of the prepreg is constant, so the smaller the diameter, the greater the number of conductive particles.
• the particle size distribution of the conductive particles can be made wider on 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 in the partially thin interlayer.
• the particle size distribution wider on the large diameter side large diameter particles can connect adjacent CF layers in the partially thick interlayer.
• the probability of the presence of conductive particles corresponding to each interlayer thickness can be ensured even if the interlayer thickness varies greatly, and the area of the part where electrical connection is not ensured between layers can be minimized.
• the coefficient of variation of the particle size distribution of the conductive particles is small, as in the conductive particles preferably used in Patent Documents 1 to 5, there is a region in which the frequency of the particles varies greatly with respect to the particle size.
• the particle size distribution is narrow, the number of conductive particles with a particle size capable of connecting adjacent CF layers tends to vary at each position in the CFRP molded body between the layers. If the number of particles that can be arranged between layers in the CFRP is sufficiently large, the number of particles required to connect the CF layers and exhibit conductivity can be secured even if the number of particles varies at each position between the layers.
• the preferred proportion of the conductive particles is preferably 0.05% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 9% by mass or less, and even more preferably 0.5% by mass or more and 8% by mass or less, relative to 100% by mass of the epoxy resin composition constituting the matrix resin.
• the interlayer thickness of the CF layers varies during the molding of the prepreg laminate, and the number of conductive particles that can connect adjacent CF layers also varies significantly, this can cause variations in conductivity at each position between the layers.
• the coefficient of variation of the particle size distribution is preferably 50% or more, and more preferably 55% or more. From the viewpoint of process passability, the coefficient of variation of the particle size distribution is preferably 100% or less, and more preferably 70% or less.
• the procedure for obtaining the coefficient of variation of the particle size distribution of conductive particles is shown.
• the surface of the prepreg is observed with a laser microscope (for example, a digital microscope VHX-5000: manufactured by Keyence Corporation) at a magnification of 200 times or more, and for each of 1,000 randomly selected particles, the conductive particle diameter is calculated in micrometers to two decimal places, taking the diameter of the circle inscribed in the outer periphery of the observed image as the particle size, and rounded off to the nearest tenth to obtain a value to one decimal place. This is done for each of 1,000 randomly selected particles. However, particles with a diameter of the inscribed circle of less than 0.5 ⁇ m are not measured.
• the average diameter ( ⁇ m) and standard deviation ( ⁇ m) of the conductive particles are calculated for the 1,000 particles.
• the average diameter ( ⁇ m) is the arithmetic mean of each particle size.
• the coefficient of variation (%) is calculated by dividing the standard deviation ( ⁇ m) by the average diameter ( ⁇ m). If a secondary resin composition film containing conductive particles is available as the observation subject, it may be used as the observation subject.
• 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.
• 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 changes significantly depending on the particle size. Therefore, when the interlayer thickness changes in 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.
• the surface of the prepreg is observed with a laser microscope (for example, a digital microscope VHX-5000: manufactured by Keyence Corporation) at a magnification of 200 times or more, and for each of 1,000 particles selected at random, the conductive particle diameter is calculated in micrometers to two decimal places, taking the diameter of the circle inscribed in the outer periphery of the observed image as the particle size, and rounded off to the nearest tenth place to obtain a value to one decimal place. This is done for each of 1,000 particles selected at random. However, particles with a diameter of the inscribed circle of less than 0.5 ⁇ m are not measured.
• the kurtosis of the particle size distribution for the 1,000 particles can be calculated using the following formula 3.
• Dj in formula 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.
• 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 ( ⁇ m) calculated in the calculation of the coefficient of variation for the particle size distribution described above. If a secondary resin composition film containing conductive particles is available as the observation subject, it may be used as the observation subject.
• the value (D99/D50) obtained by dividing D99 ( ⁇ m) by D50 ( ⁇ m) is preferably 1.7 or more.
• D99/D50 is more preferably 2.0 or more, and even more preferably 2.4 or more.
• D99/D50 is preferably 8.0 or less. It is more preferably 6.0 or less, and even more preferably 4.0 or less.
• the mass ratio of the conductive particles having 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%.
• the mass ratio of conductive particles having a particle size of D50 or more can be obtained by the following procedure. As in the procedure for measuring the particle size of conductive particles described above, the particle size of 1000 randomly selected particles is calculated, and 1000 particles are arranged in order from the smallest to the largest. The volume of each particle is calculated from 4/3 x ⁇ x r 3 , with the radius r being the value obtained by dividing the particle size by 2.
• the mass ratio of conductive particles having a particle size of D50 or more is calculated by calculating the total volume of particles 501 to 1000 relative to the total volume of particles 1 to 1000. Note that even if two or more types of conductive particles are used and their specific gravities are different, the mass ratio of conductive particles having a particle size of D50 or more can be obtained by considering them to have the same specific gravities.
• the conductive particles used in the present invention preferably have a sphericity of 90% or more, and more preferably 95% or more.
• the preferred procedure for obtaining sphericity is as follows. First, the sphericity of each conductive particle is determined. As in the procedure for measuring the particle size of the conductive particles described above, the surface of the prepreg is observed with a laser microscope in the same manner as when measuring the particle size, and the diameters of the inscribed circle and the circumscribed circle are measured for 1,000 randomly selected particles. As in the case of measuring the particle size, particles with a diameter of the inscribed circle of less than 0.5 ⁇ m are excluded. For each particle, the diameter of the inscribed circle is divided by the diameter of the circumscribed circle, and multiplied by 100 to obtain the sphericity (%) of each particle.
• the median value of the sphericity of the 1,000 conductive particles that is, the average value of the sphericity of the 500th particle and the 501st particle when arranged in ascending order of sphericity, is the sphericity of the conductive particle.
• a secondary resin composition film it may be used.
• S1/S2 which is an index of the presence rate of conductive particles from the prepreg surface to a depth of 15%, as defined below, is preferably 0.9 or more.
• S1 is the total area of particles present in the cross section of the prepreg cured product up to a depth of 15% from the prepreg surface when the total thickness of the prepreg is 100%
• S2 is the total area of particles present throughout the entire thickness of the prepreg.
• S1/S2 is more preferably 0.95 or more.
• the conductive particles can be stably present between the layers after molding into a laminate, the variation in the number of particles forming the part electrically connecting the upper and lower CF layers is reduced, and a stable interlayer conductivity can be exhibited after molding of the prepreg laminate.
• S1/S2 can be calculated as follows.
• the prepreg is sandwiched between two parallel polytetrafluoroethylene resin plates with smooth surfaces without applying pressure, and the temperature is gradually raised to 150°C over a period of 7 days to gel and harden the prepreg into a plate-shaped hardened product.
• the prepreg is cut in a direction perpendicular to the contact surface (thickness direction), and the cross section is polished and then photographed with a laser microscope (e.g., Digital Microscope VHX-5000: manufactured by Keyence Corporation) at a magnification of 200 times or more so that the top and bottom surfaces of the prepreg hardened product are within the field of view.
• a laser microscope e.g., Digital Microscope VHX-5000: manufactured by Keyence Corporation
• the distance between the polytetrafluoroethylene resin plates is measured at five randomly selected points 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 prepreg hardened product.
• Two lines are drawn on the photograph of the prepreg hardened product, parallel to the surfaces of the prepreg hardened product, at a depth of 15% of the average thickness of the prepreg hardened product from both surfaces of the prepreg hardened product.
• 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 thickness of the cured prepreg are calculated. Note that particles on the parallel lines are not included in the measurement of S1.
• the percentage of particles present within a depth range of 15% from the surface of the cured prepreg is calculated by dividing S1 by S2. As mentioned above, the cured prepreg is cured under very slow conditions, so the value calculated by this calculation is regarded as the percentage of conductive particles present within a depth of 15% from the prepreg surface.
• thermoplastic resin in the epoxy resin composition, which is the matrix resin, and it is also preferable that the thermoplastic resin is compatible with the epoxy resin used.
• Epoxy resins generally have the drawback of being brittle, but can be molded at low pressure in an autoclave.
• thermoplastic resins generally have the advantage of being tough, but are difficult to mold at low pressure in an autoclave. That is, the two exhibit contradictory characteristics.
• by mixing and using these it is possible to balance the physical properties and moldability in low pressure molding in an autoclave.
• the thermoplastic resin dissolved in the epoxy resin is preferably contained in an amount of 5% by mass or more, more preferably 10% by mass or more, based on the total amount 100% by mass of the epoxy resin composition excluding solid mass contents such as conductive particles and non-conductive polyamide particles.
• the upper limit is preferably 20% by mass or less from the viewpoint of processability.
• the amount of thermoplastic resin to be mixed 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 for both the primary resin composition and the secondary resin composition.
• the upper limit is preferably 30 parts by mass or less.
• thermoplastic resin 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. This makes it easier to obtain a uniform interlayer thickness within the CFRP molded body, and suppresses variation in electrical conductivity between layers.
• thermoplastic resin a polymer having a bond 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 in the main chain can be used.
• PA 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).
• PAEK include polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK).
• PPS heat resistance
• PES PES
• PI polystyrene-maleic anhydride
• PEI polystyrene
• PSU polystyrene
• PEEK polystyrene-maleic anhydride
• PAEK PAEK
• 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.
• the flow of the matrix resin can be suppressed. Therefore, it becomes easier to obtain a uniform interlayer thickness in the CFRP molded body, and the variation in electrical conductivity between layers can be suppressed.
• the above 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 heating a resin composition using a dynamic viscoelasticity measuring device equipped with parallel plates (e.g., ARES: manufactured by TA Instruments) 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 auto strain mode until the storage modulus and loss modulus become the same value.
• ARES manufactured by TA Instruments
• Non-conductive polyamide particles In terms of toughness, it is preferable to place non-conductive polyamide particles between adjacent CF layers. Such polyamide particles are different from the conductive polyamide particles described above and are non-conductive (not treated to impart conductivity). Such non-conductive polyamide particles may be used in combination with the conductive polyamide particles described above. By using non-conductive polyamide particles, the interlaminar toughness of CFRP can be improved, and the impact resistance of CAI, which is important for aircraft applications, can be improved.
• nylon 12 nylon 11, nylon 6, nylon 66, nylon 6/12 copolymer, nylon (semi-IPN nylon) that has been semi-IPN (polymer interpenetrating network structure) with an epoxy compound described in Example 1 of JP-A-01-104624, and the like can be suitably used.
• the non-conductive polyamide particles are preferably contained in an amount of 4% by mass or more when the mass of the epoxy resin composition constituting the prepreg is taken as 100% by mass, while from the viewpoint of processability, it is preferably contained in an amount of 60% by mass or less.
• 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, relative to 100% by mass of the epoxy resin composition constituting the prepreg.
• the polyamide particles are not crushed and the original particle shape is maintained, so that the desired interlayer thickness is more easily obtained. Therefore, it is more preferable that the polyamide particles have a crystalline property having a melting point higher than the molding temperature.
• 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 method of laminating and heat curing. For example, a method of cutting the obtained prepreg to a predetermined size, laminating a predetermined number of prepregs, and heat curing them while applying pressure can be preferably used.
• Methods for applying pressure to laminated prepregs while heating and curing include press molding, autoclave molding, bagging molding, wrapping tape method, and internal pressure molding, and are used appropriately depending on the application.
• autoclave molding is often used for aircraft and spacecraft applications, as it produces CFRP with excellent performance and stable quality.
• the temperature at which CFRP is molded must be adjusted depending on the type of curing agent contained in the epoxy resin composition; when an aromatic amine compound is used as the curing agent, molding is usually carried out at a temperature in the range of 150-220°C. If the molding temperature is too low, it may not be possible to obtain sufficiently fast curing, and conversely, if it is too high, warping due to thermal distortion may occur more easily.
• the pressure used to mold CFRP using the autoclave molding method varies depending on factors such as the thickness of the prepreg and the volume content of the reinforcing fibers, but is usually in the range of 0.1 MPa to 1 MPa. This allows the resulting CFRP to have no defects such as voids and to have little dimensional variation such as warping.
• the coefficient of variation of the 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.
• the coefficient of variation of the thickness in the one interlayer resin layer is 26% or more, the distribution of the interlayer thickness becomes suitable for the wide particle size distribution of the conductive particles used in the present invention, and more conductive particles from the large diameter side to the small diameter side present between the CF layers can electrically connect the upper and lower CF layers.
• the thickness variation in one 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, so the thickness variation coefficient in one interlayer resin layer is mainly 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 in a suitable range depending on the particle size distribution, D50, D99, and mass ratio of the non-conductive polyamide particles in the epoxy resin composition contained in the prepreg of the conductive particles used in the present invention.
• 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. Most preferably, all interlayer resin layers satisfy this requirement.
• the variation coefficient of thickness in one interlayer resin layer in CFRP can be obtained as follows.
• the CFRP is cut so as to obtain a cross section perpendicular to the fiber axis of the carbon fiber of the 0° layer, and the cross section is polished.
• the cross section is magnified 500 times with an optical microscope, and 20 areas (20 photos) in which the boundary area of the interlayer resin layer between adjacent CF layers can be confirmed are randomly taken in the in-plane direction for each layer so that the imaged areas do not overlap.
• the thickness of the interlayer resin layer arranged between adjacent CF layers is read at 100 points at 5 ⁇ m intervals in the longitudinal direction between each layer.
• the average thickness ( ⁇ m) and standard deviation ( ⁇ m) of the interlayer resin layer are obtained from the total of 2,000 points read.
• the average thickness ( ⁇ m) of the interlayer resin layer is the arithmetic mean of the thickness of the interlayer resin layer at each measurement point.
• the standard deviation ( ⁇ m) divided by the average thickness ( ⁇ m) gives the coefficient of variation (%) of the thickness of the interlayer resin layer being measured.
• the prepreg of the present invention when the prepreg of the present invention is laminated in 16 plies quasi-isotropically in a [+45°/0°/-45°/90°] 2s configuration and cured in an autoclave at a temperature of 180°C for 2 hours under conditions of 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 interlayer resin layer is 26% to 45%.
• the distribution of interlayer thickness becomes favorable for the particle size distribution of the conductive particles used in the present invention, the variation in the number of conductive particles that can connect adjacent CF layers can be reduced, and a stable interlayer conductivity can be obtained.
• the conductivity between adjacent CF layers can be measured by the following method.
• a rectangular parallelepiped test piece is cut out from the CFRP.
• the thickness direction in which the prepregs are laminated is the z-axis
• the directions parallel to the other two sides other than the thickness direction of the rectangular parallelepiped are the x-axis and y-axis.
• the area S3 (m 2 ) 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.
• the surfaces of the test piece parallel to the xy plane are polished until the CF layer is exposed on the surface.
• the four surfaces other than the polished surfaces are masked, platinum metal is evaporated onto the polished surfaces to form electrodes, and the masking is removed to obtain a sample for evaluating conductivity.
• one surface is arbitrarily selected from the masked surfaces (this surface is referred to as the "voltage measurement surface” for convenience), and the surface is placed on a moving stage so that the upper surface is this surface.
• a digital multimeter (for example, 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 so that a constant current density (current value is Ia) is obtained.
• 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 voltage between the probe and the base electrode versus the scanned distance is plotted.
• the section corresponding to the CF layer and the section corresponding to the interlayer resin layer are determined, 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").
• the electrical conductivity between layers of CFRP is calculated using the following formula:
• 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, they are identified by the inflection point of the plot of the relationship between the voltage between the probe and the base electrode versus the scanned distance.
• the electrical conductivity between adjacent CF layers (interlayer resin layer) in the CFRP is 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.
• the electrical conductivity be 1000 S/m or less.
• the CFRP of the present invention preferably has a compressive strength after impact (CAI), which is an important index of impact resistance in aircraft applications, of 230 MPa or more after an impact of 6.7 J/mm is applied, and a CAI of 230 MPa or more is preferable because the strength can be maintained even after a weak impact such as a tool being dropped or a pebble being kicked up.
• CAI compressive strength after impact
• the CAI is more preferably 250 MPa or more, and more preferably 270 MPa or more.
• a CAI of 400 MPa or less is preferable.
• the CFRP obtained by the present invention can be suitably used for aircraft structures.
• aircraft structures include those having a shape selected from flat structures, cylindrical structures, box structures, C-shaped structures, H-shaped structures, L-shaped structures, T-shaped structures, I-shaped structures, Z-shaped structures, and hat-shaped structures. By combining these structures, aircraft parts are constructed.
• structures of the above shapes see, for example, "Structural Design of Airplanes" 5th Edition, Torikai and Kuze, Japan Aviation Technology Association (2003).
• Such structures can be obtained by shaping prepregs as described in, for example, International Publication No. 2017/110991 (paragraph [0084]), International Publication No.
• a structure having a desired shape can also be obtained by automatically laminating a prepreg tape on a mold having the desired shape and then curing it.
• the fuselage, main wings, center wing, tail, etc. are formed from a joint structure in which multiple of the above-mentioned structures are joined.
• Fasteners such as bolts and rivets, adhesive films, etc. are used as means for joining the structures.
• the co-curing method can be used in which multiple uncured or semi-cured prepreg laminates are used and then cured.
• a method for obtaining the prepreg of the present invention will be described in detail.
• 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.
• CF fabric can also be used when using a cover prepreg that is attached to the surface of a prepreg for structural materials, or when using a CFRP with a complex shape.
• an explanation will be given using an example of a UD prepreg that uses a matrix resin mainly made of a thermosetting resin. Note that the present invention should not be interpreted as being limited to this example.
• a two-stage impregnation method in which the resin is impregnated twice.
• a primary resin composition is prepared by kneading a combination of epoxy resin, hardener, and thermoplastic resin, and a primary resin composition film is produced using the primary resin composition by a roll coater.
• the CF bundles are aligned to form a UD sheet, and the primary resin composition film is applied from above and below this, and after preheating, pressure is applied with a nip roll to impregnate the UD sheet with the primary resin composition to obtain a prepreg intermediate material. At this time, it is preferable to increase the degree of resin impregnation in the prepreg intermediate material.
• a secondary resin composition is prepared by adding non-conductive polyamide particles and conductive particles to the epoxy resin, hardener, and thermoplastic resin, and a secondary resin composition film is produced using a roll coater.
• 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 obtained by winding it up with a winding machine. 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 fibers.
• the secondary resin composition when the epoxy resin composition is impregnated into the CF sheet in two stages by dividing it into a primary resin composition and a secondary resin composition, the secondary resin composition only needs to contain at least the epoxy resin, the curing agent, and the conductive particles, and for example, the primary resin composition does not need to contain the conductive particles.
• the utilization efficiency of the conductive particles can be increased when viewed as a whole prepreg, or the amount of the conductive particles used can be reduced, which is advantageous in terms of lightning resistance and mechanical properties when made into CFRP.
• 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.
• 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 of "parts" in the composition ratio means parts by mass unless otherwise noted.
• measurements of various characteristics (physical properties) were performed in an environment with a temperature of 23°C and a relative humidity of 50% unless otherwise noted.
• Carbon fiber A CF having a single fiber diameter of 7 ⁇ m, 24,000 filaments per bundle, a tensile strength of 5.8 GPa, and a tensile modulus of elasticity of 280 GPa was prepared.
• Epoxy resin "Sumiepoxy" ELM434 tetraglycidyldiaminodiphenylmethane, manufactured by Sumitomo Chemical Co., Ltd.
• EPICLON registered trademark 830 (bisphenol F type epoxy resin, manufactured by DIC Corporation)
• GOT registered trademark
• Glycidylaniline type epoxy resin N,N-diglycidyl-o-toluidine
• Hardener Seikacure-S (4,4'-DDS, manufactured by Seika Co., Ltd.).
• Non-conductive polyamide particles With reference to International Publication No.
• the pressure was released at a rate of 0.02 MPa / min. Thereafter, the above temperature was maintained for 1 hour while flowing nitrogen, polymerization was completed, 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, and then filtration was performed. 2,000 g of water was added to the filtered material, and the mixture was washed at 80° C. The washed slurry was passed through a 200 ⁇ m sieve to remove the coagulated material. The filtrate from which the coagulated material had been removed was filtered again, and the isolated product was dried at 80° C. for 12 hours to produce 140 g of polyamide 6 powder.
• Conductive particles (Note: In the particle names in the table, the term "conductive particles” is omitted): 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, baked at 2000°C, and classified (component: carbon, D50: 20.3 ⁇ m, D99: 64.4 ⁇ m, coefficient of variation of particle size distribution: 62%). The volume resistivity of conductive particle A was calculated using the method described below and was found to be 0.05 ⁇ cm. Conductive particles B, C, E, F, and G described below also showed the same volume resistivity as conductive particle A.
• Conductive particles B, E, F and G Each was classified under different conditions and obtained in the same manner as conductive particle A, except that they had the physical properties and characteristics listed in Table 2.
• Conductive Particle D Two types of phenolic resin particles (Marilyn HF type, Gun-ei Chemical Industry Co., Ltd.) with different particle size distributions from those of the example of Conductive Particle A were mixed and fired at 2000°C and classified (components: carbon, D50: 27.6 ⁇ m, D99: 66.4 ⁇ m, 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 ⁇ cm.
• Conductive particles H 100 g of the non-conductive polyamide particles obtained in (5) were added to 1000 ml of electroless nickel plating solution Nickel Boomer LP-26LL (manufactured by Nippon Kagaku Sangyo Co., Ltd.), then plating was carried out at 50°C for 60 minutes, and the particles were classified 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 ⁇ cm. Table 2 shows the characteristics of the particles before plating (there is no substantial change before and after plating).
• Conductive particles I, J, and K Each was classified under different conditions and obtained in the same manner as conductive particle D, except that they had the physical properties and characteristics listed in Table 2.
• 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.
• composition ratios of the epoxy resin compositions used in each Example and Comparative Example are shown in Tables 1, 3, 4, 6, 7, and 8 (note that the units in each table are parts by mass).
• the epoxy resin composition prepared in (1) above was used to prepare a prepreg using the two-stage impregnation method as follows. On a release paper coated with silicone, the primary resin composition and the secondary resin composition prepared in (1) above were uniformly applied at a winding speed of 15 m/min using a roll coater equipped with an opposing roll to obtain a primary resin composition film and a secondary resin composition film, respectively. Then, a CF uniformly aligned in one direction was sandwiched between two sheets of the primary resin composition film, and heated and pressed using a press roll to obtain a prepreg intermediate material in which the CF was sufficiently impregnated with the primary resin composition (CF basis weight 268 g/m 2 , resin content 20% by mass).
• both release papers were peeled off from the prepreg intermediate material.
• the prepreg intermediate material was sandwiched between two sheets of the 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/ m2 , resin content (Rc) 34 mass%).
• Rc resin content of prepreg
• the test piece was placed in a beaker and about 200 ml of methyl ethyl ketone (MEK) was added, and ultrasonic irradiation was performed for 15 minutes and stirred, and then the CF was collected with tweezers and placed in another beaker.
• MEK methyl ethyl ketone
• the collected CF was washed twice more in the same manner as above, and after the third operation, the CF was transferred onto a glass filter whose mass had been measured in advance and suction filtered. After suction filtration, the CF was dried together with the glass filter 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 mass of the glass filter previously measured was taken as the mass of the CF.
• the mass of the CF was subtracted from the mass of the prepreg measured first to obtain the mass of the resin.
• the mass of this resin was divided by the mass of the prepreg to calculate Rc. The measurement was performed three times, and the average value was taken as the Rc of the prepreg. The same measurement was also performed on the prepreg intermediate material.
• the average diameter ( ⁇ m) and standard deviation ( ⁇ m) were calculated for the 1000 conductive particles.
• the standard deviation ( ⁇ m) was divided by the average diameter ( ⁇ m) to obtain the coefficient of variation (%).
• the kurtosis was calculated using Equation 3 based on the particle size of the 1000 particles.
• the average diameter ( ⁇ m) is the arithmetic mean of the diameters of 1000 particles, and corresponds to Dave in formula 3.
• the 1000 particles were arranged in ascending order of diameter, and the average diameter of the 500th and 501st particles was determined as D50 ( ⁇ m), and the average diameter of the 990th and 991st particles was determined as D99 ( ⁇ m).
• each conductive particle sample was set as a sample in a cylindrical cell having 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.
• the secondary resin composition film was observed at a magnification of 200 times or more using a VHX-5000, and the diameters of the inscribed circle and the circumscribed circle in the outer periphery of 1000 randomly selected particles (primary particles) were measured. However, particles with an inscribed circle diameter of less than 0.5 ⁇ m were excluded from the measurement. The diameter of the inscribed circle was divided by the diameter of the circumscribed circle, and multiplied by 100 to determine the sphericity (%) of the particle.
• the median value of the sphericity of the 1000 conductive particles that is, the average value of the sphericity of the 500th particle and the 501st particle when arranged in ascending order of sphericity, was determined as the sphericity of the conductive particle.
• the distance between the polytetrafluoroethylene resin plates was measured at five randomly selected points in the horizontal direction of the cross-sectional photograph (width direction of the cured prepreg), and the average value was taken as the average thickness of the cured prepreg.
• two lines were drawn parallel to the surface of the cured prepreg 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 obtain the presence rate of conductive particles from the prepreg surface to a depth of 15%.
• the intersection of the lines (b, b' in Fig. 1) connecting the midpoints of each side between the opposite sides was set as one of the vertices, and four rectangular parallelepipeds (c in Fig. 1) of length 8 mm x width 8 mm similar to the outer shape of the CFRP panel were cut out (test piece A), and four rectangular parallelepipeds (d in Fig. 1) of length 8 mm x width 8 mm similar to the outer shape of the CFRP panel were cut out (test piece B), and one of the corners of the CFRP panel was set as one of the vertices.
• the cross section with the right side of the two sides parallel to b in the positional relationship in Fig. 1 was used as the measurement surface for interlayer conductivity (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 evaporated onto both polished surfaces to form electrodes, and the masking was removed to obtain a sample for conductivity evaluation. Then, the test piece was placed on a moving stage so that the voltage measurement surface was the upper surface.
• 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 so as to obtain 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 side to the other platinum electrode, and the relationship between the voltage between the probe and the base electrode versus the scanned distance was plotted, and from the obtained graph, the section corresponding to the CF layer and the section corresponding to the interlayer resin layer were determined, 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 entered the interlayer resin layer and the voltage when the probe exited the interlayer resin layer (voltage change amount) were obtained.
• the electrical conductivity between CFRP layers was calculated using the following formula:
• the probe was scanned so that the scanning line and the side parallel to the thickness direction of the sample on the voltage measurement surface were equally spaced.
• the section corresponding to the CF layer and the section corresponding to 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 taken 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 taken as the interlayer conductivity (Ptc (S/m)) of the CFRP at the end of the panel.
• Rate of change in conductivity (%)
• the cross section was magnified 500 times with an optical microscope to obtain a total of 20 areas (20 photos) of 10 areas per test piece at random so that the imaged areas did not overlap, in which cross-sectional photographs in which the interlayer between the 7th and 8th plies in the panel thickness direction could be confirmed were obtained.
• the thickness of the interlayer resin layer existing between adjacent CF layers was read at 100 points at 5 ⁇ m intervals along the side corresponding to b in FIG. 1.
• the average thickness ( ⁇ m) and standard deviation ( ⁇ m) of the interlayer resin layer were obtained from a total of 2,000 points read.
• the average thickness ( ⁇ m) of the interlayer resin layer was calculated as the arithmetic mean of the thicknesses of the individual interlayer resin layers.
• the coefficient of variation (%) was calculated by dividing the standard deviation ( ⁇ m) by the average thickness ( ⁇ m) of the interlayer resin layer.
• CAI Compressive Strength After Impact of CFRP
• the prepregs obtained in (2) were quasi-isotropically laminated in 16 plies in a [+45°/0°/-45°/90°] 2s configuration, and molded in an autoclave at a temperature of 180°C for 2 hours at a pressure of 0.6 MPa and a heating rate of 1.7°C/min to produce CFRP.
• a sample of 150 mm length x 100 mm width 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, after which a compression fracture test was performed to determine the compressive strength after impact (CAI).
• the mass ratio of the non-conductive polyamide particles in the epoxy resin composition constituting the prepreg was calculated by dividing the resin weight per m 2 of the non-conductive polyamide particles by the sum of the resin weight per m 2 of the primary resin composition film and the resin weight per m 2 of the secondary resin composition film obtained above.
• Example 1 An epoxy resin composition was prepared using the composition shown in Table 1 and the methods of ⁇ Various evaluation methods> (1) and (2), and then a prepreg was produced and evaluated. Conductive particles A were used, and D50 was sufficiently large compared to the average radius of CF of 3.5 ⁇ m, and 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.
• Comparative Example 1 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 B as shown in Table 2.
• the conductive particles B had a very small coefficient of variation in particle size distribution.
• Example 1 When the interlayer conductivity was measured in the same manner as in Example 1, the panel edges showed very high interlayer conductivity, a higher value than in Example 1, whereas the interlayer conductivity in the panel center dropped significantly, being lower than in Example 1. This is thought to be because the flow of the matrix resin during molding of the prepreg laminate caused the center of the panel to become thicker relative to the panel edges, resulting in a decrease in the number of conductive particles connecting the CF layers in the center of the panel.
• 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.
• Comparative Example 2 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 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, like Comparative Example 1, the difference in conductivity between the layers at the center and edges of the panel was large.
• Example 3 Except for using conductive particles F as shown in Table 2, an epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1. 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.
• Example 4 Except for the conductive particles used being conductive particles G as shown in Table 2, the epoxy resin composition was prepared and the prepreg was produced and evaluated in the same manner as in Example 1. The variation in conductivity between layers was sufficiently small. However, compared to Examples 1 to 3 using conductive particles A, C, or F, a slight increase in the variation in the conductivity was observed, despite the higher coefficient of variation in 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 thus small-diameter conductive particles penetrate into the CF layer during prepreg production, resulting in a large variation in the thickness between layers when made into CFRP.
• 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 the conductive particles used were conductive particles H 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.
• Example 6 An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that the conductive particles used were conductive particles D 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 aircraft.
• Example 7 An epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1, except that the conductive particles used were conductive particles I as shown in Table 2. 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 aircraft.
• Example 8 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 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 aircraft.
• 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, and therefore, like Comparative Examples 1 and 2, there was a large difference in conductivity between the layers at the center and edges of the panel.
• Example 9 As shown in Table 3, except that the amount of thermoplastic resin in the primary resin composition was changed, an epoxy resin composition was prepared and a prepreg was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 5.
• the minimum viscosity of the resin composition was 0.6 Pa ⁇ s.
• the variation in electrical conductivity between layers was larger than in Example 1, but was sufficiently small.
• the electrical conductivity between layers was sufficiently high, and the CAI was also sufficiently high for use as a primary structural material for aircraft.
• Example 10 As shown in Table 4, except that the amount of thermoplastic resin in the primary resin composition was changed, the epoxy resin composition was prepared and the prepreg was produced and evaluated in the same manner as in Example 1. 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 larger than that in Examples 1 and 9, but was within the allowable range.
• the variation in interlayer conductivity increases in the order of Example 1, Example 9, and Example 10. This is thought to be because the lower the minimum viscosity, the more the matrix resin flows during molding, causing variation in the interlayer thickness between the center and end parts of the panel, and the number of conductive particles connecting the layers of the CF layer varies.
• the interlayer conductivity was high, and the CAI was sufficiently high for use as a primary structural material for aircraft.
• Example 11 As shown in Table 6, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, an epoxy resin composition was prepared and a prepreg was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 9. The variation in electrical conductivity between layers was very small at 17%. The CAI was also sufficiently high for use as a primary structural material for aircraft.
• Example 12 As shown in Table 6, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, an epoxy resin composition was prepared and a prepreg was fabricated and evaluated in the same manner as in Example 2. The results are shown in Table 9. The variation in electrical conductivity between layers was very small at 17%. The CAI was also sufficiently high for use as a primary structural material for aircraft.
• Example 13 As shown in Table 7, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, an epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1. The results are shown in Table 9. The variation in interlayer conductivity was larger than that in Examples 1 and 11, which used the same conductive particles, but was within the allowable range. Since Example 13 has a smaller coefficient of variation in interlayer thickness than Examples 1 and 11, the number of conductive particles connecting the CF layers was relatively small, which is considered to be due to the fact that it was easily affected by the change in the number of conductive particles connecting the CF layers due to the variation in interlayer thickness at the center and end parts of the panel. However, the interlayer conductivity was high, and the CAI was sufficiently high for use as a primary structural material for aircraft.
• Example 14 As shown in Table 8, except that the amount of non-conductive polyamide particles in the secondary resin composition was changed, an epoxy resin composition was prepared and a prepreg was produced and evaluated in the same manner as in Example 1. 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 as a primary structural material for aircraft, but was lower than that of Examples 1, 11, and 13, which used the same conductive particles.
• the prepreg of the present invention and CFRP made from said prepreg are widely applicable to industrial fields where electrical conductivity is required.
• they when used in structural components of aircraft, they can reduce the need for conventional lightning protection systems, static elimination systems, electromagnetic shielding systems, etc., such as metal foils and metal meshes, making them ideal for use in these fields.

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