Fiber-reinforced resin structure

Abstract

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

Description

Fiber-reinforced resin structure

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

[0002] In recent years, market demands for lightweight industrial products such as automobiles, aircraft, sports products, electronic devices, etc. have been increasing year by year. To meet these demands, fiber-reinforced resin structures such as sandwich structures, which have excellent lightweight properties and excellent mechanical properties, have been widely used in various industrial applications.

[0003] Patent Document 1 describes a fiber-reinforced resin sandwich structure in which the core material and skin layers are each formed from a mat of reinforcing fibers and a sheet-like intermediate substrate made of a thermoplastic resin. This document describes that the sheet-like intermediate substrate used for the core material has thermal expansion properties, which allows a porous structure to be formed by molding. Patent Document 1 claims that the usable temperature ranges of the thermoplastic resin used for the core material and the skin layer overlap, allowing the thermoplastic resins of the core material and skin material to flow simultaneously, forming an anchoring structure at the interface and increasing the bonding strength between the core material and skin material.

[0004] International Publication No. 2014 / 162873

[0005] On the other hand, in the sandwich structure disclosed in Patent Document 1, attention is focused on the bonding strength between the skin and the core, but the impact resistance properties of the sandwich structure as a whole are not sufficiently considered.

[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.

[0007] The present invention, which aims to solve the above problems, is 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) having a void and having a thermoplastic resin (II) as a matrix resin are bonded 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 UAM (Urban Air Mobility), and a drone, which has the above-mentioned fiber-reinforced resin structure.

[0008] In this specification, the symbol "to" 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 denoted by the symbol (I), and the components of the fiber reinforced resin layer (II) are denoted by the symbol (II). In the following, Roman numerals are used in a corresponding relationship according to this.

[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.

[0011] 1 is a schematic diagram showing an embodiment of a fiber-reinforced resin structure according to the present invention, and FIG. 2 is a diagram showing changes in pressure and temperature in an embodiment of a method for producing a fiber-reinforced resin structure according to the present invention.

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

[0013] The fiber-reinforced resin layer (I) may be present on at least one surface of the structure, but may also be present on both surfaces in a so-called sandwich structure. The sandwich structure can further increase the elastic modulus and strength of the fiber-reinforced resin structure, and the symmetrical structure can reduce warpage due to thermal shrinkage that occurs during molding.

[0014] The reinforcing fibers used in the fiber-reinforced resin layer (I) are not particularly limited, but are preferably continuous fibers. Here, continuous fibers are reinforcing fibers having a number average fiber length of more than 100 mm. Examples of the form of the continuous fibers in the fiber-reinforced resin layer (I) include a fiber woven fabric composed of reinforcing fiber bundles consisting of a large number of continuous fibers, and a unidirectional continuous fiber substrate in which a large number of continuous fibers are arranged in one direction. Among these, a unidirectional continuous fiber substrate is preferred from the viewpoint of enhancing the reinforcing effect of the composite material, and a fiber woven fabric with excellent shapeability is preferred when the composite material is to have a complex shape.

[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 having 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 performed. As a method for measuring the fiber length of the reinforcing fibers, for example, there is a method in which the reinforcing fibers are directly extracted from the reinforcing fiber group and measured by microscopic observation. 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 by microscopic observation (dissolution method). In addition, when there is no solvent that dissolves the resin, only the resin is burned off in a temperature range where the reinforcing fibers do not oxidize and lose weight, and the reinforcing fibers are separated and measured by microscopic observation (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, and if it is greater than 1000 GPa, it is necessary to increase the crystallinity of the reinforcing fibers, 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 described in JIS R7601-1986.

[0017] The type of reinforcing fiber contained in the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II) is not particularly limited, and examples thereof 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, polyparaphenylene benzoxazole (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, in addition to deposition of conductive metals, treatment with a coupling agent, treatment with a sizing agent, treatment with a bundling agent, and treatment with an additive. These reinforcing fibers may be used alone or in combination of two or more types.

[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 it is particularly preferable to use carbon fibers and glass fibers in combination from the viewpoint of balancing mechanical properties and economic efficiency. Furthermore, from the viewpoint of improving the impact absorption and formability of the resulting molded article, aramid fibers are preferably used, and it is particularly preferable to use carbon fibers and aramid fibers in combination from the viewpoint of balancing mechanical properties and impact absorption. Furthermore, from the viewpoint of improving the 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 a fiber-reinforced resin structure according to one embodiment of the present invention, and a cross-sectional schematic diagram (b) showing a partial structure of the fiber-reinforced resin layer (II) in the fiber-reinforced resin structure. The voids in the present invention typically refer to spaces in which neither the reinforcing fibers nor the thermoplastic resin (II) are present, formed by the reinforcing fibers coated with the thermoplastic resin (II) becoming columnar supports and overlapping or intersecting with each other. For example, when a fiber-reinforced resin substrate in which the reinforcing fibers are pre-impregnated with the thermoplastic resin (II) is heated to obtain the fiber-reinforced resin layer (II), the reinforcing fibers are raised by melting or softening the thermoplastic resin (II) due to heating, forming voids. This is based on the property that the reinforcing fibers that have been compressed by pressure in the fiber-reinforced resin substrate, which is the precursor of the fiber-reinforced resin layer (II), exhibit a restoring force derived from the elasticity of the reinforcing fibers when the pressure is released, causing the fibers to raise.

[0020] The fiber-reinforced resin layer (II) preferably has a porosity of 10 to 90% by volume. A porosity of 10% by volume or more reduces the density of the fiber-reinforced resin layer (II), thereby improving its lightness. On the other hand, a porosity of 90% by volume or less is preferable because it provides sufficient reinforcement by the reinforcing fibers and resin, improving mechanical properties. Here, the porosity may be measured by visual inspection if it can be clearly determined, but in the present invention, it is preferable to use an image processing method such as binarization, as described in the examples below.

[0021] In the fiber-reinforced resin structure of the present invention, the difference in crystallinity [%] between the matrix resins of the fiber-reinforced resin layer (I) and the fiber-reinforced resin layer (II) is 20 to 60 points. By combining fiber-reinforced resin layers with different crystallinity, it is possible to achieve both rigidity and impact resistance without compromising lightness. 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 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 high 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, the thermoplastic resin (II) is particularly excellent in toughness and impact resistance. From the viewpoint of high toughness, the crystallinity of the thermoplastic resin (II) is more preferably 0 to 3%, and even more preferably 0 to 1%.

[0024] Although only one of the thermoplastic resin (I) and the thermoplastic resin (II) may be within the above range of crystallinity, it is particularly preferable that both of them are within the above range of crystallinity.

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

[0026] In the present invention, the thermoplastic resin (I) and the thermoplastic resin (II) are selected so that the difference in melting point is less than 20°C. For a crystalline thermoplastic resin, the melting point measured in accordance with JIS K7120 (1987) is used as the melting point of the thermoplastic resin. For an amorphous thermoplastic resin, the melting point is the Vicat softening temperature measured in accordance with JIS K7206 (1999) plus 100°C. The melting point of a resin varies depending on additives and molecular weight distribution, and therefore cannot be uniformly defined. However, suitable combinations of resins for satisfying the above-mentioned melting point relationship include polyamide 6 resin (PA6) and polycarbonate resin (PC), and low-melting-point polyaryletherketone resin (LM-PAEK) and polyetherketoneketone resin (PEKK). By using such a combination of thermoplastic resin (I) and thermoplastic resin (II), the process temperature ranges of the fiber reinforced resin layer (I) and the fiber reinforced resin layer (II) are close to each other, so that the molding temperature conditions can be freely determined. It becomes possible to design the degree of crystallinity.

[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 a thermoplastic resin having such a glass transition temperature, polyaryletherketone or a copolymer of polyaryletherketone is preferable, and a compound selected from polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) is more preferable.

[0028] These thermoplastic resins (I) and (II) may contain an impact resistance improver such as an elastomer or rubber component, other fillers, and additives, provided that the object of the present invention is not impaired. Examples of fillers and additives include inorganic fillers, flame retardants, conductivity imparting agents, crystal nucleating agents, ultraviolet absorbers, antioxidants, vibration dampers, antibacterial agents, insect repellents, deodorizing agents, color inhibitors, heat stabilizers, release agents, antistatic agents, plasticizers, lubricants, colorants, pigments, dyes, foaming agents, foam control agents, and coupling agents.

[0029] Regarding the thickness 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, H2 / H1 is preferably 5 or more. By increasing the ratio of the fiber reinforced resin layer (II) in the structure, a lightweight molded body can be obtained and impact absorption is also improved. From the viewpoint of impact absorption, H2 / H1 is more preferably 10 or more. In addition, 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 process 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 process is also preferably performed on the fiber-reinforced resin structure of the present invention. The crystallinity [%] of the thermoplastic resin (I) after 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. On the other hand, the upper limit of the crystallinity difference is preferably less than 60 points in terms 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, satellites, urban air mobility (UAM), drones, and other flying mobile body components. In particular, in the above applications, from the viewpoint of maintaining predetermined aerodynamic characteristics, it is necessary to suppress deformation of components due to collisions with sand and dust particles during flight, bird strikes, etc. 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> The fiber-reinforced resin structure can be manufactured, for example, by a manufacturing method having the following steps in this order. First step: A fiber-reinforced material (I) consisting of reinforcing fibers and a thermoplastic resin (I) and a fiber-reinforced material (II) containing a nonwoven fabric of reinforcing fibers and a thermoplastic resin (II) are laminated and placed in a mold. Second step: Pressurization is applied under conditions in which both the thermoplastic resin (I) and the thermoplastic resin (II) are melted, thereby bonding the fiber-reinforced material (I) and the fiber-reinforced material (II). Third step: The fiber-reinforced material (I) and the fiber-reinforced material (II) are cooled and pressurized from the fiber-reinforced material (I) side while the thermoplastic resin (II) is in a molten state, at a cooling rate such that the crystallinity of the thermoplastic resin (I) is 20 to 60%. Fourth step: The pressure is released, and the fiber-reinforced material (II) is expanded within the mold. Fifth step: The fiber-reinforced material (I) and the fiber-reinforced material (II) are cooled 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: The entirely solidified fiber-reinforced resin structure is removed from the mold.

[0033] In addition, the fiber reinforced material (I) corresponds to the fiber reinforced resin layer (I) after molding, and the fiber reinforced material (II) corresponds to the fiber reinforced resin layer (II) after molding, so they are described with the same Roman numerals. Hereinafter, Roman numerals will be used based on this correspondence.

[0034] The relationship between pressure, temperature and time in the above process is shown in FIG.

[0035] The cooling rates in the third and fifth steps can be determined by measuring the crystallinity of the thermoplastic resin under various cooling conditions before molding using a differential scanning calorimeter, which can measure the heat of fusion of the thermoplastic resin and identify the degree of crystallinity. Crystallization is a phenomenon in which molecular chains rearrange as the thermoplastic resin cools from a molten state, forming a partially regular crystalline structure, and is dependent on the cooling rate. Therefore, by setting multiple cooling conditions with the differential scanning calorimeter, the crystallinity can be obtained for each cooling rate, and conversely, the cooling rate required to achieve a specific degree of crystallinity can be determined. For example, the temperature is raised to a temperature 30°C higher than the melting point of the thermoplastic resin and held there for 5 minutes, then cooled to 0°C at 10°C / min and held there for 5 minutes, after which the temperature is raised again to a temperature 30°C higher than the melting point at 1°C / min. The crystallinity at a cooling rate of 10°C / min is obtained from the heat of fusion at this time. This process is repeated under different cooling conditions. A detailed method for measuring the crystallinity from the heat of fusion will be described later in the Examples.

[0036] According to this method, the thermoplastic resin (I) is already solidified in the third step, and the fiber-reinforced resin layer (I) is molded. In this state, the fiber-reinforced material (II) expands in the fourth step. Therefore, deterioration of the surface quality of the fiber-reinforced resin layer (I), such as smearing, due to the flow of the expanding thermoplastic resin (II) can be suppressed. At this time, from the viewpoint of surface quality, the fiber-reinforced resin layer (I) preferably has a surface roughness Ra measured according to JIS B 0601 (2001) of 10 μm or less, more preferably 1 μm or less.

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

[0038] As the fiber reinforced material (II), it is preferable to use either a laminate of a nonwoven fabric of reinforcing fibers and a sheet made of a thermoplastic resin (II), or a substrate in which a nonwoven fabric of reinforcing fibers is impregnated with a thermoplastic resin (II). When a laminate of a nonwoven fabric of reinforcing fibers and a sheet made of a thermoplastic resin (II) is used as the fiber reinforced material (II), it is preferable to carry out an impregnation operation in which the reinforcing fibers are impregnated with the thermoplastic resin (II) by pressure in any step before the fourth step, preferably after the third step.

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

[0040] The pressure of the mold in the second and third steps is preferably in the range of 0.5 to 15.0 MPa depending on the melt viscosity of the thermoplastic resin (I) and the thermoplastic resin (II). If the pressure is less than 0.5 MPa, the thermoplastic resin will be difficult to impregnate and flow, making it difficult to bond the fiber-reinforced material (I) and the fiber-reinforced material (II). Furthermore, if the pressure is greater than 15.0 MPa, the thermoplastic resin may seep out of the fiber-reinforced material, impairing the appearance, and the reinforcing fibers may be damaged, reducing the expansibility of the fiber-reinforced material (II). From the above perspective, the pressure of the mold in the second and third steps is more preferably 0.7 MPa to 12.0 MPa, and even more preferably 1.0 MPa to 10.0 MPa.

[0041] The present invention will be described in more detail below with reference to examples.

[0042] <Methods for measuring various physical properties> (1) Crystallinity of thermoplastic resins (I, II) The crystallinity of a thermoplastic resin is calculated using the measured heat of fusion obtained from a differential scanning calorimeter as follows: Crystallinity (%) = (measured heat of fusion [J / g] / heat of fusion of saturated crystalline form [J / g]) × 100 Here, in the present application, the heat of fusion of saturated crystalline form was experimentally obtained by holding the thermoplastic resin at a temperature 50°C higher than the glass transition temperature for 3 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 resin (I) and thermoplastic resin (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 a sample. The melting points of these samples were measured according to the above standard using a differential scanning calorimeter (DSC 200F3 Maia, manufactured by NETZSCH).

[0044] On the other hand, for the amorphous resins of thermoplastic resin (I) and thermoplastic resin (II), the softening point was measured in accordance with the A50 method of "Plastics - Thermoplastics - Vicat Softening Temperature (VST) Test" specified in JIS K7206 (1999). The resin pellets, which are the raw materials for the sheet or nonwoven fabric used to prepare fiber reinforced material (I) or fiber reinforced material (II), were dried for 24 hours or more 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 obtained molded piece, a square plate with a thickness of 3.2 mm and a length and width of 12.5 mm was cut out, and this was used as a sample. For this sample, the softening point according to the above standard was obtained using a heat distortion temperature measuring instrument (Toyo Seiki Seisakusho, Ltd., 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 planar portion of 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). 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. Since the brightness threshold differs depending on the image captured, it is necessary to appropriately set the brightness threshold for each image. To set the brightness threshold, the intermediate value between the two peaks observed when a histogram of the brightness values ​​of each image was taken was used as the brightness threshold. Furthermore, the area of ​​the voids was determined from the binarized image, and the area ratio of the voids was calculated by dividing it by the area of ​​the entire image. The porosity of the fiber-reinforced resin layer (II) was calculated by arithmetic averaging the area ratio of the voids 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. However, since it is considered that the fiber reinforced material (II) expands relatively evenly in the thickness direction in the fiber reinforced resin layer (II), the area ratio value 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). The thickness of the fiber-reinforced resin layer (I) and the thickness of the fiber-reinforced resin layer (II) were measured. The thickness was determined by the arithmetic average of the thicknesses at five randomly selected points.

[0047] (5) Evaluation of impact resistance of fiber-reinforced resin structure Five test pieces measuring 150 mm long x 100 mm 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 / inch was applied to the center of the fiber-reinforced resin layer (I) side. Then, using an ultrasonic flaw detector (SDS6500-R manufactured by Nippon Krautkramer Co., Ltd.), flaws were detected at 0.1 mm intervals with a 10 MHz 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, a cutoff value and a 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] <Materials> [Reinforcing fiber] A polymer mainly composed of polyacrylonitrile was spun and baked to obtain continuous carbon fiber with a total of 12,000 filaments. The continuous carbon fiber was then subjected to electrolytic surface treatment and dried in heated air at 120°C to obtain reinforcing fiber. The properties of this reinforcing fiber were as follows:

[0051] Density: 1.80g / cm ３ Single fiber diameter: 7 μm Tensile strength: 4.9 GPa Tensile modulus: 230 GPa [6 mm mat] The above-mentioned reinforcing fibers were cut to 6 mm with a cartridge cutter to obtain chopped reinforcing fibers. 40 liters of a dispersion medium with a concentration of 0.1 wt% consisting of water and a surfactant (Nacalai Tesque, Inc., Polyoxyethylene Lauryl Ether (trade name)) was prepared, and this dispersion medium was introduced into a papermaking apparatus. The papermaking apparatus consisted of an upper papermaking tank (capacity 30 liters) equipped with a rotor-equipped agitator and a lower water storage tank (capacity 10 liters), with a porous support provided between the papermaking tank and the water storage tank. First, the dispersion medium was stirred with the agitator until air microbubbles were generated. Then, the chopped reinforcing fibers, the weight of which was adjusted to the desired basis weight, were introduced into the dispersion medium in which air microbubbles had been dispersed, and stirred to obtain a slurry in which the reinforcing fibers were dispersed. The slurry was then sucked from the water reservoir and dehydrated through a porous support to obtain a reinforced fiber sheet. The sheet was then dried in a hot air dryer at 150°C for 2 hours to obtain a reinforced fiber mat (6 mm mat).

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

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

[0054] [LM-PAEK sheet] LM-PAEK resin (AE250, manufactured by Victrex Co., Ltd.) was used to form a sheet having a basis weight of 100 g / m ２ An LM-PAEK sheet was produced.

[0055] [PEKK Sheet] A PEKK sheet having a basis weight of 100 g / m2 was prepared using a polyether ketone ketone resin ("Kepstan" (registered trademark) 6002, manufactured by Arkema Co., Ltd.). ２ A PEKK sheet of the above was prepared.

[0056] [PA6 Prepreg (I)] The reinforcing fibers (carbon fibers) subjected to the fiber-opening process were aligned in parallel and arranged 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, thereby obtaining PA6 prepreg (I).

[0057] [LM-PAEK prepreg (I)] The reinforcing fibers (carbon fibers) described above that had been subjected to fiber-opening processing were aligned in parallel and arranged 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 while applying a surface pressure of 10 MPa using a press machine heated to 330°C, thereby obtaining LM-PAEK prepreg (I).

[0058] [PC fiber reinforced material sheet (II)] One PC sheet and one 6 mm mat were laminated together 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 produced by laminating the PA6 prepreg (I) / PC fiber-reinforced material sheet (II) / PC fiber-reinforced material sheet (II) in this order. This preform was placed in a press-molding mold cavity preheated to 300°C, and the mold was closed. (Second step): A pressure of 5 MPa was applied to the mold and maintained for 120 seconds. (Third step): The mold platen was then cooled to 280°C while maintaining the pressure for 60 seconds. (Fourth step): The mold cavity was opened, and the PC fiber-reinforced material sheet (II) was expanded. 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) when obtaining the fiber-reinforced resin structure was 3 times. (Step 5) The mold cavity was then closed again, and the cavity temperature was cooled to 50°C over 20 minutes while maintaining the pressure, thereby solidifying the polycarbonate resin. (Step 6) The mold was then opened, and the fiber-reinforced resin structure was removed. 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 under the following press molding conditions. (First step) A preform was produced by laminating the materials in the following order: LM-PAEK prepreg (I) / PEKK fiber-reinforced material sheet (II) / PEKK fiber-reinforced material sheet (II). This preform was placed in a press-molding mold cavity preheated to 360°C, and the mold was closed. (Second step) A pressure of 10 MPa was then applied and maintained for 120 seconds. (Third step) The mold platen surface was then cooled to 300°C while maintaining the pressure for 60 seconds. (Fourth step) The mold cavity was opened, and the PEKK fiber-reinforced material sheet (II) was expanded. 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 PEKK fiber-reinforced material sheet (II) when obtaining the fiber-reinforced resin structure would be 3 times. (Step 5) The mold cavity was then closed again, and the cavity temperature was cooled to 50°C over 30 minutes while maintaining the pressure, solidifying the polyether ketone ketone resin. (Step 6) The mold was then opened, and the fiber-reinforced resin structure was removed. 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 heater output was adjusted to slow the cooling rate of the mold in the fifth step, and the cavity temperature was cooled to 50° C. over 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 laminate 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 laminate 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. in 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 in which it was left to stand 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]

[0072] In the fiber-reinforced resin structures obtained in Examples 1 and 2, the crystallinity of the fiber-reinforced material (II) was sufficiently low and amorphous, resulting in high impact resistance and a smaller damage area compared to Comparative Example (I) in which the fiber-reinforced material (II) was crystalline. In Example 3, the crystallinity of the fiber-reinforced material (II) was higher than in Example 2, but the difference was sufficiently large compared to the fiber-reinforced material (I), thereby providing impact resistance. In Example 4, the thickness of the fiber-reinforced material (II) was further increased, thereby improving impact absorption and further reducing the damage area compared to Example 2. In Example 5, a sandwich structure was used, resulting in improved rigidity and improved bending strength compared to Example 2. In Example 6, the cooling time of the fiber-reinforced material (I) was longer than in Example 2, resulting in an improved crystallinity of the fiber-reinforced material (I). However, the fiber-reinforced material (II) partially solidified before expansion, preventing it from fully expanding into the mold cavity, resulting in a low porosity. In Example 7, the cooling time of the fiber-reinforced material (I) was extended, further improving the crystallinity of the fiber-reinforced material (I). Furthermore, the fiber-reinforced material (II) was further cooled before expansion, increasing its crystallinity and partially solidifying, further reducing the porosity. In Example 8, the fiber-reinforced resin structure obtained in Example 2 was subjected to an annealing process to obtain a fiber-reinforced resin structure. Measurement of the crystallinity confirmed that crystallization progressed during the annealing process, resulting in a difference of 20% (i.e., 20 points) or more between the crystallinity of the fiber-reinforced material (I) and the crystallinity of the fiber-reinforced material (II). Since the crystallinity of the fiber-reinforced material (II) was increased to 20%, the damage area was larger than in Example 2. However, because the fiber-reinforced material (II) absorbed the impact on the fiber-reinforced material (I), the damage area was smaller than in Comparative Example (I), resulting in excellent impact resistance. In Comparative Example 2, the crystallinity was reduced by rapidly cooling the fiber reinforced material (I), and although the impact resistance was excellent, the rigidity was reduced and a decrease in bending strength was observed.

[0073] REFERENCE SIGNS LIST 1 Fiber reinforced resin layer (I) 2 Fiber reinforced resin layer (II) 3 Reinforced fiber 4 Void (space) 5 Thermoplastic resin (II)

Claims

1. A fiber-reinforced resin 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) using a thermoplastic resin (II) as a matrix resin and including voids are bonded together, 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. A fiber-reinforced resin structure according to any one of claims 1 to 3, wherein the crystallinity of the thermoplastic resin (I) after standing for 2 hours at a temperature 50°C higher than the glass transition temperature of the thermoplastic resin (II) is 20 points or more higher than the crystallinity of the thermoplastic resin (II).

5. A fiber-reinforced resin structure according to any one of claims 1 to 4, 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. A fiber-reinforced resin structure according to any one of claims 1 to 5, having a sandwich structure in which the fiber-reinforced resin layer (I) is present on both surfaces of the fiber-reinforced resin structure.

7. The fiber-reinforced resin structure according to any one of claims 1 to 6, 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. A fiber-reinforced resin structure according to any one of claims 1 to 7, wherein the reinforcing fibers contained in the fiber-reinforced resin layer (II) are discontinuous fibers.

9. A fiber-reinforced resin structure according to any one of claims 1 to 8, wherein the porosity of the fiber-reinforced resin layer (II) is 10 to 90 volume %.

10. A fiber-reinforced resin structure according to any one of claims 1 to 9, wherein the glass transition temperatures of the thermoplastic resin (I) and the thermoplastic resin (II) are both 120°C or higher.

11. A flying mobile member selected from the group consisting of aircraft, artificial satellites, UAM (urban air mobility), and drones, comprising the fiber-reinforced resin structure according to any one of claims 1 to 10.

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