Fiber-reinforced resin structure

Abstract

In order to obtain a fiber-reinforced composite material that is lightweight, has excellent mechanical properties, and good impact resistance, a fiber-reinforced resin structure is provided in which a fiber-reinforced resin layer (I) having a thermoplastic resin (I) as a matrix resin and a fiber-reinforced resin layer (II) containing voids having a thermoplastic resin (II) as a matrix resin are joined together, and the fiber-reinforced resin structure satisfies the following conditions (1) and (2). Condition (1): The difference in crystallinity [%] between the thermoplastic resin (I) and the thermoplastic resin (II) is 20 to 60 points. Condition (2): The difference in melting point between the thermoplastic resin (I) and the thermoplastic resin (II) is less than 20°C.

Description

Technical Field

[0001] The present invention relates to a fiber-reinforced resin structure having a thermoplastic resin as a matrix resin and a method for manufacturing the same.

Background Art

[0002] In recent years, market requirements for lightweight properties of industrial products such as automobiles, aircraft, sports products, and electronic devices have been increasing year by year. In order to meet such requirements, fiber-reinforced resin structures such as sandwich structures having excellent lightweight properties and excellent mechanical properties are widely used in various industrial applications.

[0003] Patent Document 1 describes a sandwich structure of a fiber-reinforced resin in which each of a core material and a skin layer is formed from a sheet-like intermediate base material composed of a mat made of reinforcing fibers and a thermoplastic resin. This document describes that a porous structure is formed by molding because the sheet-like intermediate base material used for the core material has heat-expandability. In Patent Document 1, since the applicable temperature ranges of the thermoplastic resins used for the core material and the skin layer overlap, the thermoplastic resins of the core material and the skin material are simultaneously caused to flow to form an anchoring structure at the interface, thereby enhancing the bonding strength between the core material and the skin material.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] }On the other hand, in the sandwich structure disclosed in Patent Document 1, although attention is paid to the bonding strength between the skin and the core, the examination of the impact resistance characteristics of the entire sandwich structure is not sufficient.

[0006] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a fiber-reinforced resin structure that is lightweight and has excellent mechanical properties while also maintaining good impact resistance. [Means for solving the problem]

[0007] The present invention, which aims to solve the above-mentioned problems, is mainly a fiber-reinforced resin structure in which a fiber-reinforced resin layer (I) having a thermoplastic resin (I) as a matrix resin and a fiber-reinforced resin layer (II) including voids having a thermoplastic resin (II) as a matrix resin are joined together, and the fiber-reinforced resin structure satisfies the following conditions (1) and (2): Condition (1): The difference in crystallinity [%] between the thermoplastic resin (I) and the thermoplastic resin (II) is 20 to 60 points. Condition (2): The difference in melting point between the thermoplastic resin (I) and the thermoplastic resin (II) is less than 20°C. The present invention also includes a flying vehicle member selected from the group consisting of an aircraft, an artificial satellite, an urban air mobility (UAM), and a drone, which has the above-described fiber-reinforced resin structure.

[0008] In this specification, the symbol "~" is used to represent a range of values including both ends of the range.

[0009] In addition, the components of the fiber-reinforced resin layer (I) are given the symbol (I), and the components of the fiber-reinforced resin layer (II) are given the symbol (II), and Roman numerals are used in the following in a corresponding relationship. [Effects of the Invention]

[0010] According to the present invention, a fiber-reinforced composite material can be obtained that is lightweight and has excellent mechanical properties as well as good impact resistance. [Brief explanation of the drawings]

[0011]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0012] <Fiber-Reinforced Resin Structure> The fiber-reinforced resin structure of the present invention (hereinafter sometimes simply referred to as "structure") has a structure in which a fiber-reinforced resin layer (I) using a thermoplastic resin (I) as a matrix resin and a fiber-reinforced resin layer (II) containing voids using a thermoplastic resin (II) as a matrix resin are joined.

[0013] The fiber-reinforced resin layer (I) only needs to be present on at least one surface of the structure, but a so-called sandwich structure in which the fiber-reinforced resin layers (I) are present on both surfaces may also be used. By adopting the sandwich structure, the elastic modulus and strength of the fiber-reinforced resin structure can be further increased, and the occurrence of warpage due to thermal shrinkage during molding can be reduced due to the symmetrical structure.

[0014] The reinforcing fiber used in the fiber-reinforced resin layer (I) is not particularly limited, but is preferably a continuous fiber. Here, the continuous fiber is a reinforcing fiber having a number average fiber length exceeding 100 mm. Examples of the form of the continuous fiber in the fiber-reinforced resin layer (I) include a fiber woven fabric composed of a reinforcing fiber bundle composed of a large number of continuous fibers, and a unidirectional continuous fiber base material in which a large number of continuous fibers are arranged in one direction. Among them, from the viewpoint of enhancing the reinforcing effect of the composite material, a unidirectional continuous fiber base material is preferable, and when the composite material has a complex shape, a fiber woven fabric having excellent formability is preferable.

[0015] On the other hand, the reinforcing fibers used in the fiber-reinforced resin layer (II) are preferably discontinuous fibers. Here, discontinuous fibers are fibers with a number-average fiber length of 0.1 to 100 mm. The number-average fiber length of discontinuous fibers is more preferably 1 to 20 mm, and even more preferably 3 to 10 mm. When the reinforcing fibers are discontinuous fibers with the number-average fiber length, the formation of voids by raising as described above can be easily achieved. Methods for measuring the fiber length of reinforcing fibers include, for example, a method in which reinforcing fibers are directly extracted from the reinforcing fiber group and measured under a microscope. When resin is attached to the reinforcing fiber group, the resin is dissolved from the reinforcing fiber group using a solvent that dissolves only the resin contained therein, and the remaining reinforcing fibers are filtered and measured under a microscope (dissolution method). When no solvent that dissolves the resin is available, the resin alone is burned off in a temperature range in which the reinforcing fibers do not oxidize and lose weight, and the reinforcing fibers are separated and measured under a microscope (burn-off method).

[0016] The reinforcing fibers contained in the fiber-reinforced resin layer (II) preferably have a tensile modulus of 200 GPa or more and 1000 GPa or less, more preferably 220 GPa or more and 500 GPa or less, in terms of the rigidity of the fiber-reinforced resin structure. If the tensile modulus of the reinforcing fibers is less than 200 GPa, the rigidity of the fiber-reinforced resin structure may be poor. If it is greater than 1000 GPa, the crystallinity of the reinforcing fibers must be increased, making it difficult to manufacture such reinforcing fibers. If the tensile modulus of the discontinuous reinforcing fibers is within the above range, it is preferable in terms of the rigidity of the fiber-reinforced resin structure and the manufacturability of the reinforcing fibers. The tensile modulus of the reinforcing fibers can be measured by a strand tensile test according to JIS R7601-1986.

[0017] The type of reinforcing fibers contained in the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II) is not particularly limited. Examples include metal fibers such as aluminum fibers, brass fibers, and stainless steel fibers; carbon fibers (including graphite fibers) such as polyacrylonitrile (PAN)-based carbon fibers, rayon-based carbon fibers, lignin-based carbon fibers, and pitch-based carbon fibers; insulating fibers such as glass fibers; organic fibers such as aramid fibers, polyparaphenylenebenzoxazole (PBO) fibers, polyphenylene sulfide fibers, polyester fibers, acrylic fibers, nylon fibers, and polyethylene fibers; and inorganic fibers such as silicon carbide fibers and silicon nitride fibers. These fibers may also be surface-treated. Examples of surface treatments include coating with conductive metals, treatment with a coupling agent, treatment with a sizing agent, treatment with a binder, and treatment with an additive. These reinforcing fibers may be used alone or in combination.

[0018] Among these, from the viewpoint of weight reduction, carbon fibers such as PAN-based carbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers, which have excellent specific strength and specific rigidity, are preferably used, and PAN-based carbon fibers, which have excellent mechanical properties such as strength and elastic modulus, are particularly preferably used. Furthermore, from the viewpoint of improving the economic efficiency of the resulting molded article, glass fibers are preferably used, and in particular, a combination of carbon fibers and glass fibers is preferred from the viewpoint of balancing mechanical properties and economic efficiency. Furthermore, from the viewpoint of improving the impact absorption and shapability of the resulting molded article, aramid fibers are preferably used, and in particular, a combination of carbon fibers and aramid fibers is preferred from the viewpoint of balancing mechanical properties and impact absorption. Furthermore, from the viewpoint of improving the electrical conductivity of the resulting molded article, reinforcing fibers coated with metals such as nickel, copper, and ytterbium can also be used.

[0019] In the fiber-reinforced resin structure of the present invention, the fiber-reinforced resin layer (II) has voids. FIG. 1 is a schematic diagram (a) of the fiber-reinforced resin structure in one embodiment of the present invention and a cross-sectional schematic diagram (b) showing the partial structure of the fiber-reinforced resin layer (II) in the fiber-reinforced resin structure. The voids in the present invention typically refer to the space formed by the reinforcing fibers coated with the thermoplastic resin (II) becoming columnar supports and overlapping or intersecting, where neither the reinforcing fibers nor the thermoplastic resin (II) is present. For example, when obtaining the fiber-reinforced resin layer (II) by heating a fiber-reinforced resin base material in which the reinforcing fibers are pre-impregnated with the thermoplastic resin (II), voids are formed due to the fluffing of the reinforcing fibers caused by the melting or softening of the thermoplastic resin (II) accompanying the heating. This is based on the property that the reinforcing fibers, which were in a compressed state by pressure in the fiber-reinforced resin base material, which is the precursor of the fiber-reinforced resin layer (II), exhibit a restoring force derived from the elasticity of the reinforcing fibers and fluff up when the pressure is released.

[0020] The fiber-reinforced resin layer (II) preferably has a porosity of 10 to 90% by volume. When the porosity is 10% by volume or more, the density of the fiber-reinforced resin layer (II) decreases, so that the lightweight property can be enhanced. On the other hand, when the porosity is 90% by volume or less, the reinforcing effect by the reinforcing fibers and the resin can be made sufficient, and the mechanical properties are improved, which is preferable. Here, when the measurement of the porosity can be clearly discriminated, a method of visually discriminating may be adopted, but in the present invention, as described in the examples below, it is preferable to adopt an image processing method such as binarization.

[0021] Regarding the matrix resins of the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II), in the fiber-reinforced resin structure of the present invention, the difference in crystallinity [%] is 20 to 60 points. By combining fiber-reinforced resin layers with different crystallinities, it is possible to achieve both rigidity and impact resistance without sacrificing the lightweight property. In particular, by combining the fiber-reinforced resin layer (I) with a high crystallinity and the fiber-reinforced resin layer (II) containing voids with a low crystallinity, it is possible to improve the impact resistance while ensuring the rigidity of the entire fiber-reinforced resin structure.

[0022] Furthermore, the crystallinity of the thermoplastic resin (I) constituting the fiber-reinforced resin layer (I) is preferably 20 to 60%. When the crystallinity of the thermoplastic resin (I) is 20% or more, sufficient rigidity is ensured, and when it is 60% or less, productivity is excellent from the viewpoint of crystallization time. From the viewpoint of increasing rigidity, the crystallinity of the thermoplastic resin (I) is more preferably 25 to 60%, and even more preferably 30 to 60%.

[0023] On the other hand, the crystallinity of the thermoplastic resin (II) is preferably 0 to 5%. When the crystallinity of the thermoplastic resin (II) is 5% or less, it is particularly excellent in toughness and impact resistance. From the viewpoint of increasing toughness, the crystallinity of the thermoplastic resin (II) is more preferably 0 to 3%, and even more preferably 0 to 1%.

[0024] And for the thermoplastic resin (I) and the thermoplastic resin (II), either one alone may be within the above-mentioned crystallinity range, but it is particularly preferable that both are within the above-mentioned crystallinity range.

[0025] Incidentally, the crystallinity of the thermoplastic resin can be measured according to the method described in the Examples section below, based on the measured heat of fusion obtained from a differential scanning calorimeter.

[0026] In the present invention, the thermoplastic resin (I) and the thermoplastic resin (II) are selected such that the difference in melting point is less than 20°C. The melting point of the thermoplastic resin in the present invention, in the case of a crystalline thermoplastic resin, uses the melting point measured in accordance with JIS K7120 (1987), and in the case of an amorphous thermoplastic resin, uses the temperature obtained by adding 100°C to the Vicat softening temperature measured in accordance with JIS K7206 (1999). Since the melting point of the resin varies depending on additives and molecular weight distribution and cannot be uniformly defined, suitable combinations of resin types for a design that satisfies the above melting point relationship include polyamide 6 resin (PA6) and polycarbonate resin (PC), low melting point polyaryletherketone resin (LM-PAEK) and polyetherketoneketone resin (PEKK), and the like. By using such a combination of the thermoplastic resin (I) and the thermoplastic resin (II), since the process temperature ranges of the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II) are close, the molding temperature conditions can be freely determined, and the degree of crystallinity can be designed.

[0027] From the viewpoint of the continuous use temperature of the fiber-reinforced resin structure, it is preferable that the glass transition temperatures of both the thermoplastic resin (I) and the thermoplastic resin (II) are 100°C or higher, and more preferably 120°C or higher. As the thermoplastic resin having such a glass transition temperature, a polyaryletherketone or a copolymer of polyaryletherketone is preferable, and further, a compound selected from polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) is preferable.

[0028] These thermoplastic resins (I) and thermoplastic resins (II) may contain an impact resistance improver such as an elastomer or a rubber component, and other fillers and additives, as long as the object of the present invention is not impaired. Examples of the filler and additive include an inorganic filler, a flame retardant, a conductivity-imparting agent, a crystal nucleating agent, an ultraviolet absorber, an antioxidant, a vibration damping agent, an antibacterial agent, an insect repellent, a deodorant, a coloring inhibitor, a heat stabilizer, a mold release agent, an antistatic agent, a plasticizer, a lubricant, a coloring agent, a pigment, a dye, a foaming agent, a foam inhibitor, or a coupling agent.

[0029] Regarding the thicknesses of the fiber reinforced resin layer (I) and the fiber reinforced resin layer (II), when the thickness of the fiber reinforced resin layer (I) is H1 and the thickness of the fiber reinforced resin layer (II) is H2, it is preferable that H2 / H1 is 5 or more. By increasing the proportion of the fiber reinforced resin layer (II) in the structure, a lightweight molded body can be obtained and impact absorption properties are also improved. From the viewpoint of impact absorption properties, H2 / H1 is more preferably 10 or more. Furthermore, the upper limit of H2 / H1 is not particularly limited, but from the viewpoint of the contribution effect of the fiber reinforced resin layer (I) to the rigidity, it is preferably 250 or less, more preferably 200 or less, and even more preferably 100 or less.

[0030] An annealing step is generally known in which a molded thermoplastic resin is left standing for a certain period of time at or above its glass transition temperature to adjust its crystallinity. This step is preferably also applied to the fiber-reinforced resin structure of the present invention. The crystallinity [%] of the thermoplastic resin (I) after being left standing for 2 hours at a temperature 50°C higher than the glass transition temperature of the thermoplastic resin (II) is preferably 20 points or more higher than the crystallinity [%] of the thermoplastic resin (II). Maintaining this crystallinity relationship improves the toughness of the fiber-reinforced resin layer (II), resulting in a structure with excellent impact strength. From the viewpoint of toughness, the difference between the crystallinity of the thermoplastic resin (I) and the crystallinity of the thermoplastic resin (II) under these conditions is more preferably 25 points or more, and even more preferably 30 points or more. Meanwhile, the upper limit of the difference in crystallinity is preferably less than 60 points, in view of the effect of crystallization time on productivity.

[0031] The structure of the present invention can be preferably used as, but not limited to, aircraft components such as aircraft, and flying mobile components such as artificial satellites, UAM (Urban Air Mobility), and drones. In particular, in the above applications, it is necessary to suppress deformation of components due to collisions with sand and dust particles during flight or bird strikes, etc., in order to maintain predetermined aerodynamic characteristics. Therefore, the fiber-reinforced resin structure of the present invention, which has excellent impact resistance, can exhibit higher performance than conventional materials. Furthermore, the fiber-reinforced resin structure of the present invention is excellent in vibration damping due to the use of a thermoplastic resin, and therefore is also excellent in flutter suppression.

[0032] <Method for manufacturing fiber-reinforced resin structure> As an example, the above fiber-reinforced resin structure can be manufactured by a manufacturing method having the following steps in this order. First step: Stack a fiber-reinforced material (I) composed of reinforcing fibers and a thermoplastic resin (I) and a fiber-reinforced material (II) including a non-woven fabric of reinforcing fibers and a thermoplastic resin (II), and place them in a mold. Second step: Pressurize under conditions where both the thermoplastic resin (I) and the thermoplastic resin (II) are melted to join the fiber-reinforced material (I) and the fiber-reinforced material (II). Third step: While the thermoplastic resin (II) is in a molten state, cool and pressurize from the fiber-reinforced material (I) side at a cooling rate such that the crystallinity of the thermoplastic resin (I) becomes 20 to 60%. Fourth step: Release the pressure and expand the fiber-reinforced material (II) in the mold. Fifth step: Cool the fiber-reinforced material (I) and the fiber-reinforced material (II) at a cooling rate such that the crystallinity of the thermoplastic resin (II) becomes 0 to 5% to solidify the thermoplastic resin (II). Sixth step: Take out the fiber-reinforced resin structure that has completely solidified from the mold.

[0033] Note that the fiber-reinforced material (I) and the fiber-reinforced resin layer (I) after molding, and the fiber-reinforced material (II) and the fiber-reinforced resin layer (II) after molding correspond to each other, so they are described with the same Roman numerals. Hereinafter, the Roman numerals are used based on the corresponding relationship according to this.

[0034] The relationship among the pressure, temperature, and time of the above manufacturing method is shown in FIG. 2.

[0035] In the third step and the fifth step, the cooling rate can be determined by measuring the heat of fusion of the thermoplastic resin in advance before molding using a differential scanning calorimeter capable of identifying the crystallinity, and measuring the crystallinity of the thermoplastic resin under various temperature-lowering conditions. Crystallization is a phenomenon in which molecular chains rearrange during the process of cooling the thermoplastic resin from a molten state to form a partially regular crystal structure, and it depends on the cooling rate. Therefore, by setting a plurality of temperature-lowering conditions with a differential scanning calorimeter, the crystallinity for each cooling rate can be obtained, and conversely, the cooling rate required to obtain a specific crystallinity can be acquired. For example, after heating to a temperature 30 °C higher than the melting point of the thermoplastic resin and holding for 5 minutes, cooling to 0 °C at 10 °C / min and holding for 5 minutes, then heating again to a temperature 30 °C higher than the melting point at 1 °C / min, and obtaining the crystallinity at a cooling rate of 10 °C / min from the heat of fusion at this time. This is repeated under different temperature-lowering conditions. The detailed method for measuring the crystallinity from the heat of fusion will be described later in the examples.

[0036] According to such a method, in the state where the thermoplastic resin (I) has already solidified and the fiber-reinforced resin layer (I) has been formed in the third step, since the fiber-reinforced material (II) expands in the fourth step, it is possible to suppress a decrease in surface quality such as streaks on the fiber-reinforced resin layer (I) due to the flow of the expanding thermoplastic resin (II). At this time, from the viewpoint of surface quality, the surface roughness Ra measured according to JIS B 0601 (2001) of the fiber-reinforced resin layer (I) is preferably 10 μm or less, and more preferably 1 μm or less.

[0037] It is preferable to select the thermoplastic resin (I) and the thermoplastic resin (II) such that the difference in melting point is less than 20 °C. With such a combination, since the process temperature ranges of the fiber-reinforced resin material (I) and the fiber-reinforced resin material (II) are close, the molding temperature conditions can be freely determined, and the crystallinity can be designed.

[0038] Note that as the fiber-reinforced material (II), it is preferable to use either a laminate of a non-woven fabric of reinforcing fibers and a sheet made of the thermoplastic resin (II) or a base material obtained by impregnating the non-woven fabric of reinforcing fibers with the thermoplastic resin (II). When using a laminate of a non-woven fabric of reinforcing fibers and a sheet made of the thermoplastic resin (II) as the fiber-reinforced material (II), it is preferable to perform an impregnation operation of impregnating the reinforcing fibers with the thermoplastic resin (II) by applying pressure after any step before the fourth step, preferably after the third step.

[0039] Also, from the viewpoint of maintaining the melting of the thermoplastic resin (II) in the fiber-reinforced material (II), the cooling and pressurizing time in the third step is preferably short, and the ratio T5 / T3 of the cooling time T5 [seconds] in the fifth step to the cooling and pressurizing time T3 [seconds] in the third step is preferably 20 or more.

[0040] The pressure of the mold in the second step and the third step is preferably in the range of 0.5 to 15.0 MPa according to the melt viscosities of the thermoplastic resin (I) and the thermoplastic resin (II). If the pressure is less than 0.5 MPa, it becomes difficult for the thermoplastic resin to be impregnated and flow, and it becomes difficult for the fiber-reinforced material (I) and the fiber-reinforced material (II) to be joined. Also, if the pressure is greater than 15.0 MPa, not only does the thermoplastic resin ooze out from the fiber-reinforced material and damage the appearance, but the reinforcing fibers may be damaged and the expansibility of the fiber-reinforced material (II) may decrease. From the above viewpoints, the pressure of the mold in the second step and the third step is more preferably 0.7 MPa to 12.0 MPa, and even more preferably 1.0 MPa to 10.0 MPa.

Examples

[0041] Hereinafter, the present invention will be described in more detail with reference to examples.

[0042] <Measurement methods for various physical properties> (1) Crystallinity of the thermoplastic resin (I, II) The crystallinity of the thermoplastic resin is calculated as follows using the measured melting heat obtained from a differential scanning calorimeter. Crystallinity (%) = (measured heat of fusion [J / g] / heat of fusion of saturated crystalline body [J / g]) x 100 In this application, the heat of fusion of saturated crystals was experimentally obtained by maintaining the thermoplastic resin at a temperature 50°C higher than the glass transition temperature for three days to allow sufficient crystallization to proceed, and then melting the resin.

[0043] (2) Melting point of thermoplastic resin (I, II) The melting points of the thermoplastic resins (I) and (II) constituting the fiber-reinforced materials (I) and (II) were evaluated as follows. The melting points were measured in accordance with the "Method for Measuring Transition Temperature of Plastics" specified in JIS K7121 (1987). The sheet or nonwoven fabric used to prepare the fiber-reinforced material (I) or fiber-reinforced material (II) was dried for 24 hours or more in a vacuum dryer controlled at an oven temperature of 50°C, and then cut into small pieces to prepare samples. The melting points of these samples were measured according to the above standard using a differential scanning calorimeter (NETZSCH, DSC 200F3 Maia).

[0044] On the other hand, for the amorphous resins of thermoplastic resin (I) and thermoplastic resin (II), the softening points were measured according to the A50 method of "Plastics - Thermoplastics - Vicat Softening Temperature (VST) Test" specified in JIS K7206 (1999). The resin pellets used as the raw material for the sheet or nonwoven fabric used to prepare fiber-reinforced material (I) or fiber-reinforced material (II) were dried for at least 24 hours in a vacuum dryer controlled at a furnace temperature of 50°C and then molded using a twin-screw kneader / injection machine (DSM Xplore, Micro Compounder 15, 12 ml injection molding machine). From the resulting molded pieces, square plates measuring 3.2 mm thick and 12.5 mm long and wide were cut out and used as samples. The softening points of these samples were measured according to the above standard using a heat distortion temperature tester (Toyo Seiki Seisakusho, S3-FH). This operation was repeated three times, and the average of the obtained temperatures was calculated to be the melting point or softening point of the thermoplastic resin (I) and the thermoplastic resin (II).

[0045] (3) Measurement of the porosity of the fiber-reinforced resin layer (II) A test piece measuring 10 mm in length and 10 mm in width was cut from the flat portion of the fiber-reinforced resin structure, and the cross section was observed using a scanning electron microscope (S-4800, manufactured by Hitachi High-Technologies Corporation). Ten equally spaced locations on the fiber-reinforced resin layer (II) were photographed at 1000x magnification. A brightness threshold was set for each image, and binarization processing was performed to determine the voids within the image. Because the brightness threshold differs depending on the image captured, it is necessary to appropriately set the brightness threshold for each image. The brightness threshold was set by taking a histogram of the brightness values of each image and using the intermediate value between the two peaks. Furthermore, the void area was calculated from the binarized image and divided by the area of the entire image to calculate the void area ratio. The void ratio of the fiber-reinforced resin layer (II) was calculated by arithmetic averaging the void area ratios at a total of 50 locations, each of which was photographed at 10 locations on five test pieces. Here, the arithmetic average of the area ratio is calculated, but since it is considered that the fiber reinforced material (II) in the fiber reinforced resin layer (II) expands relatively uniformly in the thickness direction, the value of the area ratio can be treated as volume % as it is.

[0046] (4) Measurement of the thickness of the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II) A test piece measuring 10 mm in length and 10 mm in width was cut out from the fiber-reinforced resin structure, and the cross section was observed using a scanning electron microscope (S-4800 model, manufactured by Hitachi High-Technologies Corporation), and the thickness of the fiber-reinforced resin layer (I) and the thickness of the fiber-reinforced resin layer (II) were measured. The thickness was calculated as the arithmetic average of the thicknesses at five randomly selected points.

[0047] (5) Evaluation of impact resistance of fiber-reinforced resin structures Five test pieces measuring 150mm long x 100mm wide were cut out from the fiber-reinforced resin structure in accordance with the "Method for Compression After Impact of Carbon Fiber Reinforced Plastics" specified in JIS K7089 (1996), and a falling weight impact of 1500 inch-pounds per inch was applied to the center of the fiber-reinforced resin layer (I). Then, using an ultrasonic flaw detector (SDS6500-R manufactured by Nippon Krautkramer Co., Ltd.), flaws were detected at 0.1mm intervals with a 10MHz probe, and the average damage area of the five test pieces was evaluated.

[0048] (6) Measurement of surface roughness of fiber-reinforced resin layer (I) Using a surface roughness meter, the cutoff value and reference length were selected based on JIS-B-0601 (2001), and the surface roughness Ra of the fiber-reinforced resin layer (I) was determined.

[0049] (7) Measurement of bending strength of fiber-reinforced resin structure Five test pieces measuring 10 mm in length and 100 mm in width were cut out from the fiber-reinforced resin structure, and a four-point bending test was carried out in accordance with JIS K7017 (1999) with the fiber-reinforced resin layer (I) side facing up, and the average bending strength of the five test pieces was measured.

[0050] <Material> [Reinforced fiber] A polymer mainly composed of polyacrylonitrile was spun and calcined to obtain continuous carbon fibers with a total filament count of 12,000. The continuous carbon fibers were then electrolytically surface-treated and dried in heated air at 120°C to obtain reinforcing fibers. The properties of the reinforcing fibers were as follows:

[0051] Density: 1.80g / cm 3 Single fiber diameter: 7 μm Tensile strength: 4.9GPa Tensile modulus: 230GPa [6mm Matte] The above-mentioned reinforcing fibers were cut into 6 mm pieces using a cartridge cutter to obtain chopped reinforcing fibers. A dispersion medium with a concentration of 0.1 wt% consisting of water and a surfactant (manufactured by Nacalai Tesque, Inc., polyoxyethylene lauryl ether (trade name)) was prepared in an amount of 40 liters, and such a dispersion medium was charged into a papermaking apparatus. The papermaking apparatus consisted of an upper papermaking tank (capacity 30 liters) equipped with a stirrer with a rotating blade and a lower water storage tank (capacity 10 liters), and a porous support was provided between the papermaking tank and the water storage tank. First, such a dispersion medium was stirred with a stirrer until minute air bubbles were generated. Then, the chopped reinforcing fibers with adjusted weight were put into the dispersion medium in which minute air bubbles were dispersed and stirred so as to obtain a slurry in which the reinforcing fibers were dispersed. Next, the slurry was sucked from the water storage layer and dehydrated through the porous support to obtain a reinforcing fiber sheet. The sheet was dried in a hot air dryer under the conditions of 150 °C for 2 hours to obtain a reinforcing fiber mat (6 mm mat).

[0052] [PA6 sheet] Using nylon 6 resin (manufactured by Toray Industries, Inc., "Amilan" (registered trademark) CM1001), a PA6 sheet with a basis weight of 100 g / m 2 was produced.

[0053] [PC sheet] Using polycarbonate resin (manufactured by Mitsubishi Engineering Plastics Corporation, "Iupilon" (registered trademark) S3000), a PC sheet with a basis weight of 100 g / m 2 was produced.

[0054] [LM-PAEK sheet] Using LM-PAEK resin (manufactured by Victrex plc, AE250), a LM-PAEK sheet with a basis weight of 100 g / m 2 was produced.

[0055] [PEKK sheet] Using polyetherketoneketone resin (manufactured by Arkema S.A., "Kepstan" (registered trademark) 6002), a PEKK sheet with a basis weight of 100 g / m 2 was produced.

[0056] [PA6 prepreg (I)] The reinforcing fibers (carbon fibers) that had been subjected to fiber-opening processing were aligned in parallel and aligned in one direction at a density of 1.4 fibers / cm to form a sheet-like reinforcing fiber group. A PA6 sheet was laminated on the reinforcing fiber group and impregnated with a press heated to 280°C while applying a surface pressure of 10 MPa to obtain PA6 prepreg (I).

[0057] [LM-PAEK prepreg (I)] The reinforcing fibers (carbon fibers) that had been subjected to fiber-opening processing were aligned in parallel and aligned in one direction at a density of 1.4 fibers / cm to form a sheet-like reinforcing fiber group. An LM-PAEK sheet was laminated on the reinforcing fiber group and impregnated with a surface pressure of 10 MPa in a press heated to 330°C to obtain an LM-PAEK prepreg (I).

[0058] [PC fiber reinforced material sheet (II)] One PC sheet and one 6 mm mat were laminated to obtain a PC fiber reinforced material sheet (II).

[0059] [PA6 fiber reinforced material sheet (II)] One PA6 sheet and one 6 mm mat were laminated together to obtain a PA6 fiber-reinforced material sheet (II).

[0060] [PEKK fiber reinforced material sheet (II)] One PEKK sheet and one 6 mm mat were laminated to obtain a PEKK fiber-reinforced material sheet (II).

[0061] Example 1 PA6 prepreg (I) was used as the fiber reinforced material (I), and PC fiber reinforced material sheet (II) was used as the fiber reinforced material (II). A fiber reinforced resin structure was obtained under the following molding conditions. (First step) A preform was fabricated by laminating PA6 prepreg (I), PC fiber reinforced material sheet (II), and PC fiber reinforced material sheet (II) in that order. This preform was placed in a press molding die cavity preheated to 300°C, and the die was closed. (Second step) Next, a pressure of 5 MPa was applied to the mold and maintained for 120 seconds. (Third step) Thereafter, the mold platen was cooled to 280°C while maintaining the pressure for 60 seconds. (Fourth step) The mold cavity was opened to expand the PC fiber-reinforced material sheet (II). At this time, a metal spacer was inserted into the end of the mold cavity, and the thickness of the spacer was adjusted so that the expansion ratio of the PC fiber-reinforced material sheet (II) was 3 times when obtaining the fiber-reinforced resin structure. (Fifth step) Thereafter, the mold cavity was clamped again, and while maintaining the pressure, the cavity temperature was cooled to 50° C. over 20 minutes to solidify the polycarbonate resin. (Sixth step) Then, the mold was opened and the fiber reinforced resin structure was taken out. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0062] Example 2 LM-PAEK prepreg (I) was used as the fiber reinforced material (I), and PEKK fiber reinforced material sheet (II) was used as the fiber reinforced material (II). A fiber reinforced resin structure was obtained by the following press molding conditions. (First step) A preform was fabricated by laminating LM-PAEK prepreg (I), PEKK fiber reinforced material sheet (II), and PEKK fiber reinforced material sheet (II) in that order. This preform was placed in a press molding die cavity preheated to 360°C, and the die was closed. (Second step) Next, a pressure of 10 MPa was applied and maintained for 120 seconds. (Third step) Thereafter, the mold platen was cooled to 300°C while maintaining the pressure for 60 seconds. (Step 4) The mold cavity was released, and the PEKK fiber-reinforced material sheet (II) was expanded. At this time, a metal spacer was inserted at the end of the mold cavity, and the thickness of the spacer was adjusted so that the expansion ratio of the PEKK fiber-reinforced material sheet (II) when obtaining the fiber-reinforced resin structure would be 3 times. (Step 5) Thereafter, the mold cavity was fastened again, and with the pressure maintained, the cavity temperature was cooled to 50°C in 30 minutes to solidify the polyether ketone ketone resin. (Step 6) Thereafter, the mold was opened and the fiber-reinforced resin structure was taken out. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0063] (Example 3) A fiber-reinforced resin structure was produced in the same manner as in Example 2, except that the output of the heater was adjusted so that the cooling rate of the mold in Step 5 would be slower, and the cavity temperature was cooled to 50°C in 60 minutes. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0064] (Example 4) A fiber-reinforced resin structure was produced in the same manner as in Example 2, except that the lamination structure was [LM-PAEK prepreg (I) / PEKK fiber-reinforced material sheet (II) / PEKK fiber-reinforced material sheet (II) / PEKK fiber-reinforced material sheet (II) / PEKK fiber-reinforced material sheet (II)]. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0065] (Example 5) A fiber-reinforced resin structure was produced in the same manner as in Example 2, except that the lamination structure was [LM-PAEK prepreg (I) / PEKK fiber-reinforced material sheet (II) / PEKK fiber-reinforced material sheet (II) / LM-PAEK prepreg (I)]. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0066] (Example 6) A fiber-reinforced resin structure was obtained in the same manner as in Example 2, except that the cooling time in the third step was set to 300 seconds. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0067] Example 7 A fiber-reinforced resin structure was obtained in the same manner as in Example 2, except that the cooling time in the third step was set to 900 seconds and the heater output was adjusted to slow the cooling rate of the mold in the fifth step, thereby cooling the cavity temperature to 50°C over 40 minutes. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0068] Example 8 The fiber-reinforced resin structure obtained in Example 2 was further subjected to an annealing step of being left standing in an oven at 200°C for 2 hours to obtain a fiber-reinforced resin structure. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0069] Comparative Example 1 A fiber-reinforced resin structure was obtained in the same manner as in Example 1, except that PA6 prepreg (I) was used as the fiber-reinforced material (I) and PA6 fiber-reinforced material sheet (II) was used as the fiber-reinforced material (II). The properties of the fiber-reinforced resin structure are shown in Table 1.

[0070] (Comparative Example 2) A fiber-reinforced resin structure was obtained in the same manner as in Example 1, except that the cooling time in the third step was set to 10 seconds. The properties of the obtained fiber-reinforced resin structure are shown in Table 1.

[0071] [Table 1]

[0072] In the fiber-reinforced resin structures obtained in Example 1 and Example 2, the crystallinity of the fiber-reinforcing material (II) is sufficiently low and amorphous, so the impact resistance is high, and the damaged area is smaller than that in Comparative Example (I) where the fiber-reinforcing material (II) is crystalline. In Example 3, the crystallinity of the fiber-reinforcing material (II) is higher compared to Example 2, but the difference is sufficiently large compared to the fiber-reinforcing material (I), so impact resistance can be imparted. In Example 4, by further increasing the thickness of the fiber-reinforcing material (II), the shock absorbency is improved and the damaged area is further reduced compared to Example 2. In Example 5, by adopting a sandwich structure, the rigidity is improved and the flexural strength is improved compared to Example 2. In Example 6, since the cooling time of the fiber-reinforcing material (I) was made longer compared to Example 2, the crystallinity of the fiber-reinforcing material (I) was improved, but since a part of the fiber-reinforcing material (II) solidified before expansion, it could not expand to fill the mold cavity, resulting in a lower porosity. In Example 7, since the cooling time of the fiber-reinforcing material (I) was made even longer, the crystallinity of the fiber-reinforcing material (I) was further improved, and also, the fiber-reinforcing material (II) was further cooled before expansion, resulting in an increase in crystallinity and partial solidification, so the porosity was further reduced. In Example 8, the fiber-reinforced resin structure obtained in Example 2 was annealed to obtain a fiber-reinforced resin structure. As a result of measuring the crystallinity, it was confirmed that crystallization progressed during the annealing process and there was a difference of 20% (i.e., 20 points) or more between the crystallinity of the fiber-reinforcing material (I) and the crystallinity of the fiber-reinforcing material (II). Since the crystallinity of the fiber-reinforcing material (II) is increased up to 20%, the damaged area is larger compared to Example 2, but since the fiber-reinforcing material (II) absorbs the impact against the fiber-reinforcing material (I), the damaged area is smaller compared to Comparative Example (I) and it has excellent impact resistance. Also, in Comparative Example 2, by quenching the fiber-reinforcing material (I), the crystallinity is lowered, but while it has excellent impact resistance, the rigidity is decreased and a decrease in flexural strength is observed.

Explanation of Signs

[0073] 1 Fiber-reinforced resin layer (I) 2 Fiber-reinforced resin layer (II) 3 Reinforcing fiber 4 Void (space) 5 Thermoplastic resin (II)

Claims

1. A fiber-reinforced resin structure in which a fiber-reinforced resin layer (I) having a thermoplastic resin (I) as a matrix resin and a fiber-reinforced resin layer (II) containing voids having a thermoplastic resin (II) as a matrix resin are joined, the fiber-reinforced resin structure satisfying the following conditions (1) and (2). Condition (1): The difference in crystallinity [%] between the thermoplastic resin (I) and the thermoplastic resin (II) is 20 to 60 points. Condition (2): The difference in melting point between the thermoplastic resin (I) and the thermoplastic resin (II) is less than 20°C.

2. The fiber-reinforced resin structure according to claim 1, wherein the crystallinity of the thermoplastic resin (I) is 20 to 60%.

3. The fiber-reinforced resin structure according to claim 1 or 2, wherein the crystallinity of the thermoplastic resin (II) is 0 to 5%.

4. The fiber-reinforced resin structure according to claim 1 or 2, wherein the crystallinity of the thermoplastic resin (I) after standing at a temperature 50°C higher than the glass transition temperature of the thermoplastic resin (II) for 2 hours is 20 points or more higher than the crystallinity of the thermoplastic resin (II).

5. The fiber-reinforced resin structure according to claim 1 or 2, wherein the ratio H2 / H1 of the thickness H1 of the fiber-reinforced resin layer (I) to the thickness H2 of the fiber-reinforced resin layer (II) is 5 or more.

6. The fiber-reinforced resin structure according to claim 1 or 2, having a sandwich structure in which the fiber-reinforced resin layer (I) is present on both side surfaces of the fiber-reinforced resin structure.

7. The fiber-reinforced resin structure according to claim 1 or 2, wherein the average surface roughness Ra of the fiber-reinforced resin layer (I) measured in accordance with JIS B 0601 (2001) is 10 μm or less.

8. The fiber-reinforced resin structure according to claim 1 or 2, wherein the reinforcing fibers contained in the fiber-reinforced resin layer (II) are discontinuous fibers.

9. The fiber-reinforced resin structure according to claim 1 or 2, wherein the porosity of the fiber-reinforced resin layer (II) is 10 to 90% by volume.

10. The fiber-reinforced resin structure according to claim 1 or 2, wherein the glass transition temperatures of both the thermoplastic resin (I) and the thermoplastic resin (II) are 120°C or higher.

11. A flying mobile member selected from the group consisting of an aircraft, a satellite, a UAM (Urban air mobility), and a drone, comprising the fiber-reinforced resin structure according to claim 1 or 2.

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