Method for manufacturing a fiber-reinforced plastic molded article, and fiber-reinforced plastic molded article

Abstract

The present invention provides a novel method for manufacturing a fiber-reinforced plastic molded article, and a novel fiber-reinforced plastic molded article. [Solution] A method for manufacturing a fiber-reinforced plastic molded article, comprising press-molding a nonwoven fabric containing a fiber-reinforced material and a thermoplastic resin to obtain a molded article having a porosity of 1.0 to 29.0%. By this manufacturing method, a fiber-reinforced plastic molded article is produced that contains a thermoplastic resin and a plurality of fiber-reinforced materials oriented in a predetermined direction, and the coefficient of variation (CV) of the bending fracture load measured for each test piece obtained by dividing the molded article into eight equal parts in a width direction perpendicular to the predetermined direction is 6.0% or less.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a fiber reinforced plastic molded body and a fiber reinforced plastic molded body.

Background Art

[0002] In recent years, parts used in automobiles, aircraft, etc. have been promoted for substitution from metal to plastic and weight reduction for the purpose of reducing carbon dioxide emissions, expanding cruising range by electrification, etc. Such metal substitute plastics are usually used as fiber reinforced plastic molded bodies obtained by blending reinforcing materials such as carbon fiber (carbon fiber; CF) and glass fiber (glass fiber; GF) with resin and molding.

[0003] Especially in automobiles, many plastic members are used for both interior and exterior parts, and members molded by injection molding of thermoplastic resin are often used from the viewpoints of productivity and cost. However, at present, the members obtained by injection molding do not have sufficient strength and rigidity for use as metal substitutes. In addition, as a metal substitute plastic for automobile members, SMC (sheet molding compound) using a thermosetting resin is used for the outer panel of automobiles, and molded products obtained by impregnating carbon fiber (CF) or glass fiber (GF) with an epoxy resin or the like and curing with an autoclave or without using an autoclave are used for Formula 1, high-class car bodies, and some sports cars. Furthermore, in recent years, hydrogen tanks used in hydrogen vehicles have been plasticized and contributed to weight reduction. However, thermosetting resins have the problem that once they are cured by molding, the resins cannot be reused and they have poor sustainability.

[0004] Therefore, in recent years, a technology has been used to replace metal in parts such as automobile back doors by using injection molding of pellets (long fiber reinforced pellets; LFTP or LFT) in which long fibers (GF or CF) with extended fiber lengths are dispersed in thermoplastic resin. However, even when using LFTP, the reinforcing material such as GF or CF is subjected to shearing by the screw during injection molding, and the performance of the reinforcing material is not fully realized.

[0005] Conventionally, as a method for manufacturing fiber-reinforced plastic molded articles without using an injection molding machine, a technique has been proposed in which the molded article is produced by press-molding a wet-type nonwoven fabric or a dry-type nonwoven fabric (stampable sheet) that has been compounded with a reinforcing material into a thermoplastic resin (see, for example, Patent Document 1). According to this technique, there is no process in which the reinforcing material is subjected to shearing, as in injection molding, and the fiber length is maintained almost as is. As a result, compared to manufacturing using injection molding, a material with higher strength and rigidity can be obtained, which in turn allows for thinner molded articles and further weight reduction. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2016-074197 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a novel method for manufacturing a fiber-reinforced plastic molded article, and a novel fiber-reinforced plastic molded article. [Means for solving the problem]

[0008] A method for manufacturing a fiber-reinforced plastic molded article according to one embodiment of the present invention is characterized by comprising press-molding a nonwoven fabric containing a fiber-reinforcement material and a thermoplastic resin to obtain a molded article having a porosity of 1.0 to 29.0%.

[0009] Furthermore, another embodiment of the present invention provides a fiber-reinforced plastic molded article containing a thermoplastic resin and a plurality of fiber-reinforcements oriented in a predetermined direction, characterized in that the coefficient of variation (CV) of the bending fracture load measured for each of the test pieces obtained by dividing the molded article into eight equal parts in a width direction perpendicular to the predetermined direction is 6.0% or less. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic diagram illustrating the configuration of the apparatus in the press forming process and the state after press forming. Figure 1(a) is a schematic diagram illustrating the configuration of the apparatus in the press forming process. Figure 1(b) is a schematic diagram illustrating the state in which a molded body 20 is manufactured by press forming a nonwoven fabric 10 in a conventional manufacturing method. Figure 1(c) is a schematic diagram illustrating the state in which a molded body 20 is manufactured by press forming a nonwoven fabric 10 in a manufacturing method according to one embodiment of the present invention. [Figure 2] Figure 2 is a graph showing the stress-strain curves (SS curves) corresponding to eight test specimens when the bending fracture load was measured for the fiber-reinforced plastic molded articles produced in the comparative examples and examples described later. Figures 2(a) to (d) show the results for Comparative Example 1 and Examples 1 to 3, respectively. [Modes for carrying out the invention]

[0011] The embodiments of the present invention described above will be explained below with reference to the drawings as necessary, but the technical scope of the present invention should be determined based on the claims and is not limited to the following forms. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios. In this specification, "X~Y" indicating a range means "X or more and Y or less". Unless otherwise specified, operations and measurements of physical properties, etc., are performed under conditions of room temperature (20~25°C) and relative humidity of 40~50%RH.

[0012] Method for manufacturing fiber-reinforced plastic molded articles In one embodiment, the present invention provides a method for manufacturing a fiber-reinforced plastic molded article, which includes press-molding a nonwoven fabric containing a fiber-reinforced material and a thermoplastic resin to obtain a molded article having a porosity of 1.0 to 29.0%. In this specification, "fiber-reinforced plastic molded article" is also simply referred to as "molded article."

[0013] Conventionally, from the viewpoint of obtaining a molded article with higher strength and rigidity, it was considered preferable to apply a large press pressure to the nonwoven fabric when press-molding the nonwoven fabric to obtain a molded article, so that the resulting molded article would not substantially contain voids. In contrast to this, the present inventors attempted to control the conditions when press-molding the nonwoven fabric to obtain a molded article, contrary to the conventional wisdom in this art, and to allow a predetermined amount of voids to be contained in the resulting molded article. Surprisingly, it was found that the rigidity of the molded article improved depending on the void content, and ultimately a molded article that also contributed to further weight reduction could be provided. The mechanism by which such a surprising effect occurs is not yet fully clear, and it is thought that various factors are involved in combination. However, one possible reason is that by making the press conditions during press molding milder, the cutting and fragmentation of the fiber reinforcing material, especially near the surface of the nonwoven fabric, was suppressed, and this contributed to the improvement in rigidity. It should be noted that this mechanism is based solely on speculation, and its accuracy does not affect the technical scope of the present invention.

[0014] [Press forming] Figure 1 is a schematic diagram illustrating the configuration of the apparatus in the press forming process and the state after press forming. Figure 1(a) is a schematic diagram illustrating the configuration of the apparatus in the press forming process. As shown in Figure 1(a), in the press forming process, the nonwoven fabric 10 is pressure-formed by tightening the upper die of a pair of press dies (upper die 110 and lower die 120) downwards. In the press forming process, both of the pair of press dies 110 and 120 may be tightened in the direction of the arrow, or one of the pair of press dies 110 and 120 (for example, the lower die 120) may be fixed and the other press die (for example, the upper die 110) may be tightened to apply press pressure.

[0015] For example, press molds made of steel, stainless steel, or aluminum can be used in the press forming process. In the press forming process, the press molds are positioned parallel to each surface of the nonwoven fabric.

[0016] In the case of a fiber-reinforced plastic molded article obtained by press-molding a nonwoven fabric containing fiber reinforcement and thermoplastic resin, generally, in order to obtain a molded article of a target thickness (referred to as the "design thickness"), the required amount of nonwoven fabric (fiber-reinforced molded article sheet) is set in a mold and press-molded to obtain the molded article. For example, if the molded article is 30 cm square, 2 mm thick, and has a specific gravity of 1.20, then 30 × 30 × 0.2 × 1.2 = 216 g of nonwoven fabric (fiber-reinforced molded article sheet) is set in a mold and press-molded to obtain a 2 mm thick molded article. Alternatively, if the volume of the molded article to be obtained is fixed, for example, 500 cm³ 3 Assuming that the molded body is made of a material with a specific gravity of 1.20, a 2mm thick molded body can be obtained by setting 500 × 1.2 = 600g of nonwoven fabric (fiber-reinforced molded sheet) into a mold and press molding it.

[0017] Figure 1(b) is a schematic diagram illustrating the state in which a molded body 20 is manufactured by press-molding a nonwoven fabric 10 using a conventional manufacturing method. In the conventional manufacturing method, when press-molding the nonwoven fabric 10 to obtain a molded body 20, the press conditions are set so that the thickness t of the obtained molded body 20 is approximately the same as the design thickness T of the molded body 20. As a result, the obtained molded body 20 contains substantially no voids.

[0018] In contrast, the method for manufacturing a fiber-reinforced plastic molded article according to this embodiment is characterized by press-molding a nonwoven fabric to obtain a molded article having a void ratio of 1.0 to 29.0%. Figure 1(c) is a schematic diagram illustrating the state in which a molded article 20 is manufactured by press-molding a nonwoven fabric 10 in a manufacturing method according to one embodiment of the present invention. In this embodiment, when press-molding the nonwoven fabric 10 to obtain the molded article 20, the press conditions are set so that the thickness t of the obtained molded article 20 is somewhat larger than the design thickness T of the molded article 20. As a result, the obtained molded article 20 contains a predetermined amount of voids 22.

[0019] As described above, it has been found that by including a predetermined amount of voids in the manufactured molded article, the rigidity of the molded article is improved depending on the void content, ultimately resulting in a molded article that contributes to further weight reduction. From the viewpoint of further exhibiting such effects, the void ratio of the obtained molded article is preferably 2.5 to 25.0%, more preferably 3.0 to 23.0%, and even more preferably 3.3 to 22.5%. This void ratio may be, for example, 4.0% or more, 5.0% or more, 6.0% or more, 7.0% or more, 8.0% or more, 9.0% or more, 10.0% or more, 11.0% or more, or 12.0% or more. Alternatively, this void ratio may be, for example, 22.0% or less, 21.0% or less, 20.0% or less, 19.0% or less, 18.0% or less, 17.0% or less, 16.0% or less, 15.0% or less, or 14.0% or less.

[0020] In the press molding process, the press pressure and the temperature of the non-woven fabric used in the press molding process only need to be controlled so that the resulting molded body contains the above-described predetermined amount of voids, and there are no particular restrictions on their specific values. These conditions can be appropriately set in consideration of the viscosity when the resin melts, etc. As an example, the press pressure is preferably 1 MPa or more, more preferably 2 MPa or more, still more preferably 3 MPa or more, and particularly preferably 5 MPa or more. Also, the press pressure in the press molding process is preferably 50 MPa or less, more preferably 40 MPa or less, still more preferably 30 MPa or less, and particularly preferably 28 MPa or less.

[0021] Also, the temperature of the non-woven fabric is preferably at or above the softening temperature of the thermoplastic resin contained in the non-woven fabric, and more preferably at or above the softening temperature of the thermoplastic resin + 20°C. For example, when using polypropylene as the thermoplastic resin, the temperature of the non-woven fabric used in the press molding process is preferably 150°C or above, more preferably 165°C or above, still more preferably 180°C or above, and particularly preferably 190°C or above. Also, the temperature of the non-woven fabric used in the press molding process is preferably 230°C or below, more preferably 220°C or below, and particularly preferably 210°C or below. Note that the softening temperature of the thermoplastic resin can be measured using a microscope, DSC, etc. Usually, the softening temperature of a crystalline resin is the melting point, and the softening temperature of an amorphous resin is the glass transition temperature.

[0022] The pressing process may be a so-called heat and cool process. In the heat and cool process, first, the non-woven fabric is set in the mold at a temperature of about 40 to 100 °C, and then the non-woven fabric is heated to the above-mentioned temperature. Next, after obtaining a molded body by performing a pressing process on the non-woven fabric to a predetermined thickness, the mold is cooled. The molding cycle time in the heat and cool process is preferably within 20 minutes, more preferably within 19 minutes. Also, for the purpose of shortening the molding time in the heat and cool process, a cold press process may be carried out. In the cold press process, a preheating process and a press molding process are carried out. In the preheating process, the non-woven fabric is heated to a temperature above the melting point of the thermoplastic resin using near-infrared rays or a heatable press machine to melt the resin, and then it is rapidly cooled in the press process (mold temperature of about 40 to 130 °C) and press-molded. According to this method, there is an advantage that the preheating (melting process) and the press molding process (cooling) can be separated, and the time required for one cycle can be made 5 minutes or less.

[0023] After the press molding is performed in the press molding process, it is preferable to further provide a process of cooling the fiber reinforced plastic molded body formed from the non-woven fabric. In this case, it is preferable that the fiber reinforced plastic molded body is taken out from the press mold after being cooled to at least -50 °C or lower than the softening temperature of the thermoplastic resin.

[0024] 〈Non-woven fabric〉 The nonwoven fabric used in the press molding process contains fiber reinforcement and thermoplastic resin, and is also referred to as a "stampable sheet" in this art. Preferably, the nonwoven fabric contains carbon fibers and thermoplastic resin fibers, and more preferably, it is a wet-laid nonwoven fabric containing carbon fibers and thermoplastic resin fibers. However, the effects of the present invention can be similarly achieved even if a dry-laid nonwoven fabric is used. Here, the wet-laid method involves dispersing chopped strands of thermoplastic resin fibers and fiber reinforcement in a solvent, and then removing the solvent to form a web. The dry-laid method involves mixing fiber reinforcement and thermoplastic fibers in a gas, and then capturing them on a net to obtain a mat; this method is also called air-laid. Furthermore, the obtained dry-laid nonwoven fabric is often needle-punched to become a nonwoven fabric for press molding.

[0025] (thermoplastic resin) Examples of thermoplastic resins include polyester, polyethylene, polypropylene, polycarbonate (PC), polyamide (PA6, PA66, PA9T), ABS, polyether ether ketone (PEEK), polyamide-imide (PAI), polyphenylene sulfide (PPS), polyether-imide (PEI), polyether-ketone ketone (PEKK), and polystyrene (PS).

[0026] In this specification, thermoplastic resins are also referred to as "matrix resins" because they form bonding points at the intersections of matrix or fibrous components during press molding. Nonwoven fabrics using such thermoplastic resins do not require autoclave treatment and have shorter heating and press molding times compared to sheets using thermosetting resins, thus contributing to improved productivity.

[0027] The thermoplastic resin contained in the nonwoven fabric is preferably in fibrous form. The thermoplastic resin in fibrous form maintains its fibrous structure before press molding. Therefore, in the state before forming the fiber-reinforced plastic molded product, the sheet itself is flexible and drapes well. Consequently, the nonwoven fabric can be stored and transported in a rolled form, offering the advantage of excellent handling.

[0028] When the thermoplastic resin is in fibrous form, the number-average fiber length of the thermoplastic resin fibers is preferably 3 mm or more, more preferably 4 mm or more, and even more preferably 5 mm or more. Furthermore, the number-average fiber length of the thermoplastic resin fibers is preferably 30 mm or less, more preferably 20 mm or less, and even more preferably 15 mm or less. By keeping the number-average fiber length of the thermoplastic resin fibers within the above range, it is possible to suppress the shedding of thermoplastic resin fibers during the manufacture of nonwoven fabrics. In addition, by keeping the number-average fiber length of the thermoplastic resin fibers within the above range, it is possible to manufacture fiber-reinforced plastic molded articles with excellent strength.

[0029] When the thermoplastic resin is in fibrous form, the average fiber diameter of the thermoplastic resin fibers is preferably between 3 μm and 50 μm. By keeping the average fiber diameter of the thermoplastic resin fibers within the above range, the strength of the fiber-reinforced plastic molded article can be increased.

[0030] The thermoplastic resin content in the nonwoven fabric is preferably 46% by volume or more, more preferably 52% by volume or more, and even more preferably 57% by volume or more, relative to the total volume of the nonwoven fabric. Furthermore, the thermoplastic resin content in the nonwoven fabric is preferably 95% by volume or less, more preferably 85% by volume or less, and even more preferably 76% by volume or less, relative to the total mass of the nonwoven fabric. Here, the above content (by volume) is the volume ratio (percentage) of the thermoplastic resin to the total volume of the nonwoven fabric, determined from the true specific gravity of the nonwoven fabric. For example, if the specific gravity of polypropylene (PP) is 0.9 and the specific gravity of carbon fiber (CF) is 1.80, and the mass ratio of these in the nonwoven fabric is 50:50, then the volume of PP is calculated as 50 / 0.9 = 55.6. On the other hand, similarly, the volume of CF is calculated as 50 / 1.8 = 27.8. Therefore, the PP content in the total is calculated to be 55.6 × 100 / (55.6 + 27.8) = 66.7 volume%, and similarly, the CF content is calculated to be 33.3 volume% (the preferred range (volume%) of carbon fiber content in nonwoven fabrics will be discussed later).

[0031] (Fiber-reinforced material) There are no particular restrictions on the specific form of the fiber reinforcement, and conventionally known knowledge in the field of fiber-reinforced plastic molded articles may be referenced as appropriate. In particular, the fiber reinforcement preferably contains one or more types selected from the group consisting of inorganic fibers such as carbon fiber (CF) and glass fiber (GF), and more preferably contains carbon fiber (CF). These fiber reinforcements may be used individually or in combination of two or more types. Of course, other fiber reinforcements may also be used, for example, heat-resistant organic fibers such as aramid fibers or PBO (poly(p-phenylenebenzoxazole)) fibers may be used. In particular, the fiber reinforcement preferably contains carbon fiber, and more preferably consists only of carbon fiber.

[0032] As carbon fibers, polyacrylonitrile (PAN)-based, petroleum / coal pitch-based, rayon-based, and lignin-based carbon fibers can be used. These carbon fibers may be used individually or in combination of two or more types. Among these carbon fibers, polyacrylonitrile (PAN)-based carbon fibers are preferred from the viewpoint of productivity and mechanical properties on an industrial scale. Furthermore, from the viewpoint of particularly long fiber length, it is also preferable that the carbon fibers include recycled carbon fibers (rCF) which are recycled carbon fibers used in fiber-reinforced plastic molded articles applied to aircraft components and the like.

[0033] The number-average fiber length of the carbon fibers is preferably 3 mm or more, more preferably 4 mm or more, and even more preferably 5 mm or more. Furthermore, the number-average fiber length of the carbon fibers is preferably 50 mm or less, more preferably 40 mm or less, and even more preferably 30 mm or less. In other words, the number-average fiber length of the carbon fibers is preferably 3 to 50 mm, and more preferably 5 to 30 mm. By setting the number-average fiber length of the carbon fibers within the above range, it is possible to suppress the shedding of carbon fibers during the manufacturing of the nonwoven fabric. In addition, by setting the number-average fiber length of the carbon fibers within the above range, it is possible to mold a fiber-reinforced plastic molded article with excellent strength.

[0034] The average fiber diameter of the carbon fibers is preferably between 3 μm and 20 μm. By keeping the average fiber diameter of the carbon fibers within the above range, the strength of the fiber-reinforced plastic molded article can be increased.

[0035] The single-fiber strength of carbon fiber is preferably 4150 MPa or higher, and more preferably 4700 MPa or higher. Single-fiber strength refers to the tensile strength of the monofilament. The single-fiber strength can be measured in accordance with JIS R7601:2000 "Carbon fiber - Test method for tensile properties of single fibers".

[0036] The fiber reinforcing material content in the nonwoven fabric is preferably 5% by volume or more, more preferably 15% by volume or more, and even more preferably 24% by volume or more, relative to the total mass of the nonwoven fabric. Furthermore, the fiber reinforcing material content in the nonwoven fabric is preferably 54% by volume or less, more preferably 48% by volume or less, and even more preferably 43% by volume or less, relative to the total mass of the nonwoven fabric. In other words, the fiber reinforcing material content in the nonwoven fabric is preferably 5 to 54% by volume relative to the total mass of the nonwoven fabric. By setting the fiber reinforcing material content within the above range, it becomes easier to obtain fiber-reinforced plastic molded articles with excellent moldability. In addition, by setting the fiber reinforcing material content within the above range, it becomes possible to mold fiber-reinforced plastic molded articles with excellent strength.

[0037] The total content of thermoplastic resin and fiber reinforcing material in the nonwoven fabric is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 98% by mass or more, based on the total mass of the nonwoven fabric.

[0038] The nonwoven fabric used in the press molding process may be a pre-press sheet (prepreg sheet). Here, a pre-press sheet is a sheet obtained by heating and pressing an unpressurized nonwoven fabric containing a fiber reinforcing material and a thermoplastic resin at low pressure. In this specification, a pre-press sheet (prepreg sheet) refers to a sheet for a molded body that has been heated and pressed once or multiple times to the extent that there is room (remaining) for molding.

[0039] (optional ingredient) The nonwoven fabric subjected to the press molding process may contain optional components in addition to fiber reinforcing materials and thermoplastic resins. Examples of optional components include binder components.

[0040] Examples of binder components include polyester resins such as polyethylene terephthalate and modified polyethylene terephthalate, which are commonly used in the manufacture of nonwoven fabrics, as well as binder fibers with a core-sheath structure combining these, acrylic resins, styrene-(meth)acrylic acid ester copolymer resins, urethane resins, epoxy resins, PVA resins, various starches, cellulose derivatives, sodium polyacrylate, polyacrylamide, polyvinylpyrrolidone, acrylamide-acrylic acid ester-methacrylic acid ester copolymers, styrene-maleic anhydride copolymer alkali salts, isobutylene-maleic anhydride copolymer alkali salts, polyvinyl acetate resins, styrene-butadiene copolymers, vinyl chloride-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, and styrene-butadiene-(meth)acrylic acid ester copolymers.

[0041] The binder component content is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, relative to the total mass of the nonwoven fabric. By keeping the binder component content within the above range, the strength of the nonwoven fabric can be increased and its handling properties can be improved.

[0042] In addition, optional components such as coupling agents, antioxidants, light stabilizers, flame retardants, and colorants such as carbon black may be used.

[0043] Fiber-reinforced plastic molded products According to the manufacturing method of one embodiment of the present invention described above, a fiber-reinforced plastic molded article is produced. The inventors' studies have shown that the molded article produced in this manner exhibits less variation in physical properties between the center and the edges compared to molded articles produced by conventional manufacturing methods. That is, according to another embodiment of the present invention, a novel fiber-reinforced plastic molded article is provided, containing a thermoplastic resin and a plurality of fiber reinforcing materials oriented in a predetermined direction, wherein the coefficient of variation (CV) of the bending fracture load measured for each test piece obtained by dividing the molded article into eight equal parts in a width direction perpendicular to the predetermined direction is 6.0% or less. This fiber-reinforced plastic molded article can be produced by the manufacturing method of one embodiment of the present invention described above. However, the fiber-reinforced plastic molded article according to this embodiment is not limited to those produced by the manufacturing method of one embodiment of the present invention.

[0044] Here, if the coefficient of variation (CV) of the bending fracture load in a fiber-reinforced plastic molded article exceeds 6.0%, it can be said that there is a large variation in strength depending on the part of the molded article. It is difficult to apply a molded article that exhibits such strength variation to applications requiring high strength, such as automotive components. On the other hand, a molded article with a small coefficient of variation as described above can be sufficiently suited to applications requiring high strength, such as automotive components. Here, the value of the coefficient of variation (CV) of the bending fracture load described above is preferably 5.0% or less, more preferably 4.0% or less, even more preferably 3.5% or less, particularly preferably 3.0% or less, and most preferably 2.8% or less. There is no particular lower limit for the coefficient of variation, but it is usually 0.5% or more. Furthermore, the value of the coefficient of variation shall be the value calculated using the method described in the Examples section below (the same applies to the various physical properties below).

[0045] Specific gravity of the molded body [g / cm³] 3If the physical properties such as strength and rigidity are the same, a smaller value is preferable from the viewpoint of weight reduction. For example, the specific gravity of the molded article is preferably less than 1.20, more preferably 1.16 or less, even more preferably 1.00 or less, and particularly preferably 0.95 or less. There is no particular limit on the lower limit of the specific gravity, but it is usually 0.85 or higher.

[0046] The bending fracture load [N] of a molded article is an indicator of its strength against bending stress. While it can vary depending on the size of the molded article, the type of constituent components, and the composition at the time of measurement, it is preferably 165.0 or higher, more preferably 168.0 or higher, even more preferably 175.0 or higher, particularly preferably 180.0 or higher, and most preferably 200.0 or higher. There is no particular upper limit on the bending fracture load, but it is usually 300.0 or lower.

[0047] The flexural modulus [GPa] of a molded article is an indicator of its ability to return to its original shape after bending stress, and is preferably 22.0 or higher, more preferably 25.0 or higher, even more preferably 28.0 or higher, and particularly preferably 30.0 or higher. There is no particular upper limit on the flexural modulus, but it is usually 50.0 or lower.

[0048] The rigidity [N / mm] of a molded body is an indicator of its resistance to deformation under external force. While it can vary depending on the size of the molded body, the type of constituent components, and the composition at the time of measurement, it is preferably 25.0 or higher, more preferably 28.0 or higher, even more preferably 30.0 or higher, particularly preferably 35.0 or higher, and most preferably 40.0 or higher. There is no particular upper limit on the rigidity, but it is usually 60.0 or lower.

[0049] The Charpy impact energy [J] (with notch) of a molded body is an indicator of brittleness and toughness. It can vary depending on the size of the molded body, the type of constituent components, and the composition at the time of measurement, but is preferably 0.350 or higher, more preferably 0.390 or higher, and even more preferably 0.440 or higher. There is no particular upper limit on the Charpy impact energy, but it is usually 0.600 or lower.

[0050] The weight reduction ratio of the molded body to iron is an indicator of its usefulness as a substitute material for iron, with smaller values ​​indicating a greater contribution to weight reduction. This weight reduction ratio is preferably 30.0 or less, more preferably 29.0 or less, even more preferably 28.0 or less, even more preferably 27.0 or less, particularly preferably 26.0 or less, and most preferably 25.0 or less. There is no particular lower limit for the weight reduction ratio, but it is usually 20.0 or higher.

[0051] Applications of fiber-reinforced plastic molded articles according to this embodiment include, for example, "casings for portable electronic devices such as office automation equipment, mobile phones, smartphones, personal digital assistants, tablet PCs, and digital video cameras, as well as air conditioners and other home appliances, and reinforcing materials such as ribs attached to the casings, diaphragms for speakers in home and automotive audio equipment and electronic musical instruments, sports and leisure goods such as golf clubs, fishing rods, and running shoes, aircraft materials, civil engineering and building material parts such as pillars, panels, and reinforcing materials, exterior panels or body parts and their reinforcing materials such as various frames, various wheel bearings, various beams, doors, trunk lids, side panels, upper back panels, front bodies, under bodies, various pillars, various frames, various beams, and various supports," and "instrumentation equipment." It is suitably used in components such as interior parts like panels and seat frames, fuel system, exhaust system, or intake system parts like gasoline tanks, various pipes, and various valves, automobile and motorcycle parts such as engine coolant joints, air conditioner thermostat bases, headlamp supports, and pedal housings, aircraft parts such as winglets and spoilers, railway vehicle parts such as seat components for railway vehicles, exterior panels, reinforcing materials attached to exterior panels, ceiling panels, and air conditioner vents, and reinforcing materials for molded bodies made of resin (thermosetting resin, thermoplastic resin), reinforcing materials for molded bodies made of resin and reinforcing fibers, and reinforcing materials for plant-derived sheets (kraft paper, corrugated cardboard, oil-resistant paper, insulating paper, conductive paper, release paper, impregnated paper, glassine paper, cellulose nanofiber sheets, etc.).

[0052] Furthermore, the following items are also included within the scope of the present invention: Item 1: A method for producing a fiber-reinforced plastic molded article, comprising press-molding a nonwoven fabric containing a fiber-reinforced material and a thermoplastic resin to obtain a molded article having a porosity of 1.0 to 29.0%; Item 2: A method for producing a fiber-reinforced plastic molded article according to Item 1, wherein the porosity is 2.5 to 25.0%; Item 3: A method for producing a fiber-reinforced plastic molded article according to Item 2, wherein the porosity is 3.0 to 23.0%; Item 4: A method for producing a fiber-reinforced plastic molded article according to any one of items 1 to 3, wherein the fiber-reinforced material includes carbon fiber (CF); Item 5: A method for producing a fiber-reinforced plastic molded article according to Item 4, wherein the carbon fibers include recycled carbon fibers (rCF); Item 6: A method for producing a fiber-reinforced plastic molded article according to item 4 or 5, wherein the number-average fiber length of the carbon fibers is 3 to 50 mm; Item 7: A method for producing a fiber-reinforced plastic molded article according to any one of items 1 to 6, wherein the nonwoven fabric is a wet-laid nonwoven fabric containing carbon fibers and thermoplastic resin fibers; Item 8: A method for producing a fiber-reinforced plastic molded article according to any one of items 1 to 7, wherein the content of the fiber-reinforcement material in the nonwoven fabric is 5 to 54 volume percent of the total volume of the nonwoven fabric; Item 9: A fiber-reinforced plastic molded article comprising a thermoplastic resin and a plurality of fiber-reinforced materials oriented in a predetermined direction, A fiber-reinforced plastic molded body in which the coefficient of variation (CV) of the bending fracture load measured for each test specimen obtained by dividing the molded body into eight equal parts in a width direction perpendicular to the predetermined direction is 6.0% or less; Item 10: A fiber-reinforced plastic molded article as described in Item 9, having a porosity of 1.0 to 29.0%; Item 11: A fiber-reinforced plastic molded article as described in Item 10, having a porosity of 2.5 to 25.0%; Item 12: The fiber-reinforced plastic molded article according to item 11, wherein the porosity is 3.0 to 23.0%; Item 13: A fiber-reinforced plastic molded article according to any one of items 9 to 12, wherein the fiber-reinforced material includes carbon fiber (CF); Item 14: The fiber-reinforced plastic molded article according to item 13, wherein the carbon fibers include recycled carbon fibers (rCF); Item 15: The fiber-reinforced plastic molded article according to item 13 or 14, wherein the number-average fiber length of the carbon fibers is 3 to 50 mm; Item 16: Specific gravity is 1.20 [g / cm³] 3 A fiber-reinforced plastic molded article as described in any of items 9 to 15, which is less than ]; Item 17: Fiber-reinforced plastic molded articles as described in any of items 9 to 16, having a bending fracture load of 165.0 [N] or more, measured at standard dimensions in accordance with the JIS K7074:1988 three-point bending test (Method A); Item 18: Fiber-reinforced plastic molded articles as described in any of items 9 to 17, having a flexural modulus of 22.0 [GPa] or higher as measured at standard dimensions in accordance with the JIS K7074:1988 three-point bending test (Method A); Item 19: Fiber-reinforced plastic molded articles as described in any of items 9 to 18, having a stiffness of 25.0 [N / mm] or more as measured in standard dimensions in accordance with the JIS K7074:1988 three-point bending test (Method A); Item 20: A fiber-reinforced plastic molded article as described in any of items 9 to 19, wherein the Charpy impact energy (with notch), measured in accordance with JIS K7111-1 / leA:2012 with a test specimen measuring 80 mm in length, 10 mm in width, and 2 mm in thickness, is 0.350 [J] or greater; Item 21: A fiber-reinforced plastic molded article according to any of items 9 to 20, wherein the weight reduction ratio to iron is 30.0 or less. [Examples]

[0053] The present invention will be described in more detail below using examples and comparative examples, but it is not limited in any way to the following examples.

[0054] Fabrication of fiber-reinforced plastic molded products [Comparative Example 1] (Preparation of fiber-reinforced plastic molded sheets (CF nonwoven fabric)) Recycled carbon fibers (rCF) with a number-average fiber length of 12 mm were added to water to a slurry concentration of 0.5%. After initial dispersion by stirring with a pulper for 30 seconds, the slurry was diluted with water to a slurry concentration of 0.15% (carbon fiber slurry).

[0055] Meanwhile, polypropylene fibers containing polypropylene, maleic acid-modified polypropylene, and a resin stabilizer were prepared. These polypropylene fibers were added to the carbon fiber slurry so that the mixing ratio of carbon fibers to polypropylene fibers was 1:1 (by mass) (raw material slurry). This raw material slurry was then continuously supplied to an inclined wire paper machine to produce paper with a width of 1200 mm and a basis weight of 100 g / m². 2 A CF nonwoven fabric was prepared by papermaking, and both ends in the width direction were cut to obtain a CF nonwoven fabric with a width of 1000 mm. The CF nonwoven fabric obtained in this way has fiber orientation. For this reason, the longitudinal direction (fiber flow direction) of the nonwoven fabric roll was defined as the MD direction, and the direction perpendicular to the MD direction (width direction) was defined as the TD direction. This nonwoven fabric was then cut into pieces with dimensions of 300 mm in the MD direction and 210 mm in the TD direction, respectively, and pieces with dimensions of 300 mm in the TD direction and 210 mm in the MD direction.

[0056] (Press forming process) A 210 x 300 mm flat plate mold was prepared as the mold for press molding. A heat-and-cool press molding machine was used as the molding method. The design thickness of the molded body was set to 2 mm, and based on the true specific gravity of the molded body's constituent components of 1.20, 151 g of fiber-reinforced plastic molded body sheet (CF nonwoven fabric) was set in the mold. Specifically, the nonwoven fabric cut in the MD direction and the nonwoven fabric cut in the TD direction were alternately laminated to cancel out the orientation direction, resulting in a total of 24 nonwoven fabric layers being laminated.

[0057] Specifically, first, the CF nonwoven fabric obtained above was placed in a mold heated to 40°C. Next, the upper mold of the press molding machine was lowered to a position where the CF nonwoven fabric would not be completely pressed down, and the mold was heated to 210°C while applying a load of 0.8 tons from the spring attached to the mold, ensuring that the nonwoven fabric would not be damaged. Once 210°C was reached, this temperature was maintained for 3 minutes to melt the CF nonwoven fabric. After that, while maintaining the temperature at 210°C, a press pressure of 28 MPa was applied and pressurized for 2 minutes. Then, while maintaining the same pressure, it was cooled to 40°C, and once 40°C was reached, the mold was opened to obtain the fiber-reinforced plastic molded body of this comparative example. Here, the time from setting the CF nonwoven fabric in the 40°C mold, heating, pressurizing, cooling, and removing the molded body was defined as the molding cycle time. The value of this molding cycle time is shown in Table 1 below.

[0058] [Example 1] The fiber-reinforced plastic molded article of this embodiment was obtained using the same method as in Comparative Example 1 described above, except that the press pressure was changed from 28 MPa to 10 MPa.

[0059] [Example 2] The fiber-reinforced plastic molded article of this embodiment was obtained using the same method as in Comparative Example 1 described above, except that the pressing temperature was changed from 210°C to 190°C.

[0060] [Example 3] The fiber-reinforced plastic molded article of this embodiment was obtained using the same method as in Example 2 described above, except that the press pressure was changed from 28 MPa to 5 MPa.

[0061] Evaluation of the physical properties of fiber-reinforced plastic molded articles To evaluate the physical properties, a contour machine was used to cut out fiber-reinforced plastic molded parts and prepare bending test specimens.

[0062] First, 10 mm was cut from one end of the molded body in the width direction (210 mm) using a contour machine. Next, since the width of the bending test specimen is 100 mm, the molded body was cut at a width of 100 mm from the position where the 10 mm had been removed, resulting in a 100 mm x 300 mm specimen. Furthermore, 15 mm sections were cut along the length direction (300 mm) to obtain 20 samples of 15 mm x 100 mm size. The first and last two samples were removed, and the odd-numbered samples were collected to obtain n=8 bending test specimens.

[0063] Furthermore, since the width of the Charpy impact test specimen is 80 mm, the molded body was cut again at a width of 80 mm from the position where 100 mm had been cut above, resulting in a size of 80 mm x 300 mm. In addition, 15 mm x 2 pieces were cut from both ends in the longitudinal direction (300 mm), and the remaining part was cut into 10 mm pieces, and 24 10 mm samples were sampled in the order of (1) Charpy notch, (2) no Charpy notch, and (3) other, thereby obtaining n=8 test specimens for each. The other specimens were used in a separate test. Unless otherwise noted, the following physical property evaluations were performed on each of the eight test specimens, and the arithmetic mean of the measured values ​​for each specimen was calculated.

[0064] (Measurement of the thickness of the molded body) The thickness of the molded body was measured using a micrometer. The results are shown in Table 1 below.

[0065] (Measurement of specific gravity of molded body) The specific gravity of the molded article was measured in accordance with JIS K7112:1999 (Plastics - Method for determining density and specific gravity of non-foamed plastics). The results are shown in Table 1 below.

[0066] (Calculation of the porosity of the molded body) The porosity [%] was calculated from the measured specific gravity of the molded body obtained above and the true specific gravity of the constituent components of the molded body. The results are shown in Table 1 below.

[0067] (Measurement of bending fracture load, bending modulus, and stiffness) The bending fracture load [N] and bending modulus [GPa] were measured in accordance with the 3-point bending test (Method A) of JIS K7074:1988 (Bending Test Method for Carbon Fiber Reinforced Plastics). These results are shown in Table 1 below. Table 1 also shows the calculated coefficient of variation (CV) of the bending fracture load, which is calculated by dividing the standard deviation of the eight measured values ​​of the bending fracture load by the arithmetic mean of the bending fracture load. Furthermore, the stress-strain curves (SS curves) corresponding to the eight test specimens when the bending fracture load was measured are shown in Figures 2(a) to (d).

[0068] Furthermore, the stiffness [N / mm] was calculated from the slope of the straight line in the elastic region of the stress-strain curve (SS curve) when the bending fracture load was measured for the test specimen closest to the center in the width direction.

[0069] (Charpy impact energy) In accordance with JIS K7111-1 / leA:2012 (Plastics - Determination of Charpy impact properties - Part 1: Uninstrumented impact testing), the Charpy impact energy [J] was measured for test specimens with dimensions of 80 mm in length, 10 mm in width, and 2 mm in thickness. This measurement was performed on both notched and non-notched test specimens.

[0070] (Measurement of the thickness of a molded body equivalent to the rigidity of 1 mm thick iron) The relationship between the flexural modulus of an elastic body, the size of the test specimen, the change in bending load, and the change in deflection is expressed by the following equation 1:

[0071]

number

[0072] In Equation 1, L is the distance between supports, b is the width of the specimen, h is the thickness of the specimen, ΔF is the change in bending load, and Δs is the change in deflection.

[0073] It is known that the flexural modulus of iron is 206 [GPa]. Assuming the thickness of the iron is 1 mm, the stiffness of the iron (ΔF / Δs [N / mm]) can be calculated. Using this, the thickness of the molded body corresponding to the stiffness of 1 mm thick iron was calculated by substituting the value of "stiffness of iron" calculated above, as well as the size of each test piece and the measured value of the flexural modulus, into Equation 1 above. The results are shown in Table 1 below.

[0074] (Calculation of the weight reduction ratio of the molded body relative to iron) First, assuming the thickness of the iron was 1 mm, the mass A per unit area was calculated by multiplying this thickness (1 mm) by the specific gravity of iron (7.8). On the other hand, the mass B per unit area was calculated by multiplying the thickness of the molded body measured above by the specific gravity of the molded body measured above. Then, the weight reduction ratio (relative to iron) was calculated by dividing mass B by mass A and multiplying by 100. The results are shown in Table 1 below. Note that this weight reduction ratio value corresponds to the mass of the molded body required to replace 1 mm of iron, and the smaller this value, the more effectively the molded body contributes to weight reduction.

[0075] [Table 1]

[0076] In general press forming, if the press pressure and temperature during forming are low, the thickness of the molded body will be greater than the design thickness, and voids (air gaps) will be trapped inside the molded body. This has been considered a common practice in this technical field, and it is usually thought to cause problems with the quality of the material properties. Contrary to this common practice, the molded bodies of Examples 1 to 3 showed several improvements in performance compared to the molded body of Comparative Example 1, which did not contain voids, despite containing a certain amount of voids. For example, in Examples 1 to 3, the rigidity and notched Charpy impact energy of the molded bodies were significantly improved compared to Comparative Example 1. Furthermore, in Examples 1 to 3, despite the improved rigidity, the weight reduction ratio compared to iron was even better than that of Comparative Example 1, making them very promising as metal substitute materials for various components such as automotive parts.

[0077] Furthermore, as shown in Figure 2, in Comparative Example 1, the bending fracture load near both ends of the molded body is smaller than that in the center, and the coefficient of variation is also larger, indicating that there is variation in strength depending on the part of the molded body. In Comparative Example 1, the thickness of the molded body is as designed, but if such variation in strength occurs, it will be difficult to apply it to automotive components and the like. In contrast, in Examples 1 to 3, the variation in bending fracture load (coefficient of variation) depending on the part of the molded body is suppressed more than in Comparative Example 1, demonstrating that it is sufficiently practical.

[0078] Furthermore, in Examples 2 and 3, setting the press molding temperature to 190°C shortened the heating and cooling times during the heat-and-cool process, thereby reducing the molding cycle time.

[0079] As described above, according to the present invention, a fiber-reinforced plastic molded article can be obtained that has high strength, high rigidity, improved quality, reduced weight, and lower costs depending on the molding conditions during press molding. In press molding using fiber-reinforced materials with high hardness, such as inorganic fiber reinforcement materials, it is thought that damage occurs due to contact between fibers due to the pressure during press molding. However, by adopting the manufacturing method according to the present invention, this damage is mitigated, and it is presumed that this leads to improved strength and rigidity. In addition, there is the advantage that it does not require the use of foaming agents or equipment for foam molding, as is the case with foam molding. In the above-described example, as one embodiment of the present invention, a fiber-reinforced plastic molded article was manufactured using a wet papermaking method with an equal mass mixture of polypropylene fibers and recycled carbon fibers as raw materials. However, in view of the mechanism by which the effects of the present invention are manifested, it can also be applied to the manufacture of fiber-reinforced plastic molded articles using dry nonwoven fabrics and other thermoplastic resins, and it can be said that it has very high industrial applicability. [Explanation of Symbols]

[0080] 10 Nonwoven fabric, 20 Fiber-reinforced plastic molded bodies, 22 void, 110 Upper mold, 120 Lower mold, T design thickness, t: Thickness of the molded body.

Claims

1. A method for producing a fiber-reinforced plastic molded article, comprising press-molding a nonwoven fabric containing a fiber-reinforced material and a thermoplastic resin to obtain a molded article having a porosity of 1.0 to 29.0%.

2. A method for producing a fiber-reinforced plastic molded article according to claim 1, wherein the porosity is 2.5 to 25.0%.

3. A method for producing a fiber-reinforced plastic molded article according to claim 2, wherein the porosity is 3.0 to 23.0%.

4. A method for producing a fiber-reinforced plastic molded article according to claim 1 or 2, wherein the fiber-reinforced material includes carbon fiber (CF).

5. The method for producing a fiber-reinforced plastic molded article according to claim 4, wherein the carbon fiber includes recycled carbon fiber (rCF).

6. The method for manufacturing a fiber-reinforced plastic molded article according to claim 4, wherein the number-average fiber length of the carbon fibers is 3 to 50 mm.

7. The method for producing a fiber-reinforced plastic molded article according to claim 1 or 2, wherein the nonwoven fabric is a nonwoven fabric containing carbon fibers and thermoplastic resin fibers.

8. The method for producing a fiber-reinforced plastic molded article according to claim 1 or 2, wherein the content of the fiber reinforcing material in the nonwoven fabric is 5 to 54 volume percent as the volume ratio of thermoplastic resin to the total volume of the nonwoven fabric, determined from the true specific gravity of the nonwoven fabric.

9. A fiber-reinforced plastic molded article containing a thermoplastic resin and a plurality of fiber-reinforced materials oriented in a predetermined direction, A fiber-reinforced plastic molded body in which the coefficient of variation (CV) of the bending fracture load measured for each test piece obtained by dividing the molded body into eight equal parts in a width direction perpendicular to the predetermined direction is 6.0% or less.

10. A fiber-reinforced plastic molded article according to claim 9, wherein the porosity is 1.0 to 29.0%.

11. A fiber-reinforced plastic molded article according to claim 10, wherein the porosity is 2.5 to 25.0%.

12. The fiber-reinforced plastic molded article according to claim 11, wherein the porosity is 3.0 to 23.0%.

13. The fiber-reinforced plastic molded article according to claim 9 or 10, wherein the fiber-reinforced material includes carbon fiber (CF).

14. The fiber-reinforced plastic molded article according to claim 13, wherein the carbon fibers include recycled carbon fibers (rCF).

15. The fiber-reinforced plastic molded article according to claim 13, wherein the number-average fiber length of the carbon fibers is 3 to 50 mm.

16. Specific gravity is 1.20 [g / cm³] ３ A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the value is less than ].

17. A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the bending fracture load measured at standard dimensions in accordance with JIS K7074:1988 three-point bending test (Method A) is 165.0 [N] or more.

18. A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the flexural modulus measured at standard dimensions in accordance with JIS K7074:1988 three-point bending test (Method A) is 22.0 [GPa] or more.

19. A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the stiffness measured at standard dimensions in accordance with JIS K7074:1988 three-point bending test (Method A) is 25.0 [N / mm] or more.

20. A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the Charpy impact energy (with notch), measured in accordance with JIS K7111-1 / leA:2012 with a test specimen having dimensions of 80 mm in length, 10 mm in width, and 2 mm in thickness, is 0.350 [J] or more.

21. A fiber-reinforced plastic molded article according to claim 9 or 10, wherein the weight reduction ratio relative to iron is 30.0 or less.

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