Fiber-reinforced composite material, molded article, and production method for fiber-reinforced composite material

Abstract

Provided is a fiber-reinforced composite material using natural fibers. Specifically provided is a fiber-reinforced composite material containing a thermoplastic resin having a melting point of 180-220°C and natural fibers, wherein the natural fibers are impregnated with at least a portion of the thermoplastic resin, and the natural fibers are oriented in at least one direction.

Description

Fiber-reinforced composite material, molded article, and method for manufacturing fiber-reinforced composite material

[0001] The present invention relates to a fiber-reinforced composite material, a molded article, and a method for producing a fiber-reinforced composite material, and more particularly to a fiber-reinforced composite material using natural fibers as reinforcing fibers.

[0002] In recent years, there has been an increasing demand for fiber-reinforced composite materials, such as carbon fiber-reinforced thermoplastic resins (CFRTPs), which are made of thermoplastic resins and reinforcing fibers. Among thermoplastic resins, fiber-reinforced composite materials made of polyamide resins have been studied (Patent Documents 1 and 2).

[0003] JP 2023-147962 A International Publication No. 2020 / 174871

[0004] Meanwhile, in recent years, fiber-reinforced composite materials have been used in molded articles that require aesthetic appeal. Natural fibers are often used as reinforcing fibers for applications requiring aesthetic appeal. However, it has been found that, unlike carbon fibers and glass fibers, natural fibers are sometimes not easy to manufacture fiber-reinforced composite materials. The present invention aims to solve this problem and to obtain a fiber-reinforced composite material using natural fibers that can provide molded articles with excellent aesthetic appeal.

[0005] In light of the above-mentioned problems, the present inventors have conducted research and found that the above-mentioned problems can be solved by adjusting the melting point of the thermoplastic resin impregnated into the natural fibers to fall within a predetermined range. Specifically, the above-mentioned problems have been solved by the following means. <1> A fiber-reinforced composite material comprising a thermoplastic resin having a melting point of 180 to 220°C and natural fibers, wherein at least a portion of the thermoplastic resin is impregnated into the natural fibers, and the natural fibers are oriented in at least one direction. <2> The fiber-reinforced composite material according to <1>, wherein the thermoplastic resin comprises a polyamide resin. <3> The fiber-reinforced composite material according to <1>, wherein the thermoplastic resin comprises a polyamide resin comprising diamine units and dicarboxylic acid units, wherein 70 mol % or more of the diamine units are derived from xylylenediamine. <4> The fiber-reinforced composite material according to <1>, wherein the thermoplastic resin comprises a polyamide resin containing diamine units and dicarboxylic acid units, wherein 70 mol % or more of the diamine units are derived from xylylenediamine, and wherein 70 mol % or more of the dicarboxylic acid units are derived from an α,ω-linear aliphatic dicarboxylic acid having 4 to 20 carbon atoms. <5> The fiber-reinforced composite material according to any one of <1> to <4>, wherein the natural fibers are twisted. <6> The fiber-reinforced composite material according to any one of <1> to <5>, wherein the fiber-reinforced composite material is a prepreg. <7> The fiber-reinforced composite material according to <6>, wherein the natural fibers are continuous fibers oriented in one direction and the prepreg is formed by impregnating the natural fibers with the thermoplastic resin. <8> The fiber-reinforced composite material according to <6>, wherein the natural fibers are a woven fabric and the prepreg is formed by impregnating the natural fibers with the thermoplastic resin. <9> A molded article formed from the fiber-reinforced composite material according to any one of <1> to <8>. <10> The method for producing a fiber-reinforced composite material according to any one of <1> to <8>, wherein the impregnation of the natural fibers with the thermoplastic resin is carried out at a temperature of the melting point of the thermoplastic resin + 5 to 30°C and in an atmosphere having an oxygen concentration of less than 500 ppm. <11> The method for producing a fiber-reinforced composite material according to <10>, wherein the impregnation is carried out while applying a pressure of 0.1 to 2.0 MPa.

[0006] According to the present invention, it is possible to provide molded articles with excellent design properties and obtain fiber-reinforced composite materials using natural fibers.

[0007] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. Note that the following present embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment. Note that in this specification, the word "to" is used to mean that the numerical values ​​before and after it are included as the upper and lower limits. Furthermore, any combination of the upper and lower limit values ​​of numerical values ​​in this specification is cited as an example of the present embodiment. In this specification, various physical property values ​​and characteristic values ​​are those at 23°C unless otherwise specified.

[0008] The fiber-reinforced composite material of this embodiment is characterized by including a thermoplastic resin having a melting point of 180 to 220°C and natural fibers, at least a portion of the thermoplastic resin impregnating the natural fibers, and the natural fibers being oriented in at least one direction. This configuration makes it possible to provide a fiber-reinforced composite material using natural fibers that exhibits excellent design when molded. Fiber-reinforced composite materials using carbon fibers or glass fibers have been easily produced by heating and melting a thermoplastic resin while applying pressure to impregnate the reinforcing fibers. However, when similar attempts were made with natural fibers, it was found that fiber-reinforced composite materials could not be easily obtained. This was presumed to be due to the oxidative decomposition of natural fibers at around 200°C. In this embodiment, it was discovered that by using a thermoplastic resin having a melting point of 180 to 220°C and impregnating the natural fibers with the thermoplastic resin in an atmosphere with a low oxygen concentration, it was possible to impregnate the thermoplastic resin even when natural fibers were used. Furthermore, the molded products exhibited excellent design. The details of this embodiment will be described below.

[0009] <Thermoplastic Resin with a Melting Point of 180 to 220°C> The fiber-reinforced composite material of this embodiment contains a thermoplastic resin with a melting point of 180 to 220°C. The melting point of the thermoplastic resin is preferably 215°C or lower, more preferably 210°C or lower, even more preferably 208°C or lower, and preferably 183°C or higher. By setting the melting point below the upper limit, the molding temperature tends to be kept low, and deterioration of the natural fibers tends to be more effectively suppressed. The fiber-reinforced composite material of this embodiment may contain only one thermoplastic resin with a melting point of 180 to 220°C, or may contain two or more thermoplastic resins. When two or more thermoplastic resins are contained, the melting point of the thermoplastic resin with a melting point of 180 to 220°C is the melting point of the thermoplastic resin with the highest melting point. The melting point of the thermoplastic resin is measured as described in the Examples below.

[0010] The thermoplastic resin having a melting point of 180 to 220 ° C. used in this embodiment is not particularly limited as long as it is a crystalline resin, and examples thereof include polyamide resin, polyacetal resin, and crystalline thermoplastic polyester resin. Polyamide resin is preferred, and xylylenediamine-based polyamide resin, which will be described in detail later, is more preferred. For details of polyacetal resin, please refer to paragraphs 0009 to 0014 of JP 2021-098767 A, the contents of which are incorporated herein by reference. For details of crystalline polyester resin, please refer to paragraphs 0023 to 0029 of JP 2023-037620 A, the contents of which are incorporated herein by reference.

[0011] The type of polyamide resin used in this embodiment is not particularly limited, and may be an aliphatic polyamide resin or a semi-aromatic polyamide resin, but is more preferably a semi-aromatic polyamide resin.

[0012] Examples of aliphatic polyamide resins include polyamide 4, polyamide 46, polyamide 6, polyamide 66, polyamide 666, polyamide 610, polyamide 11, and polyamide 12. As described above, the polyamide resin used in this embodiment preferably contains a semi-aromatic polyamide resin. For example, it is more preferable that 90% by mass or more of the polyamide resin contained in the resin composition of this embodiment is a semi-aromatic polyamide resin. Here, a semi-aromatic polyamide resin refers to a polyamide resin composed of diamine units and dicarboxylic acid units, in which 20 to 80 mol % (preferably 30 to 80 mol %, more preferably 40 to 70 mol %) of the total constituent units of the diamine units and dicarboxylic acid units contain an aromatic ring.

[0013] Examples of semi-aromatic polyamide resins include terephthalic acid-based polyamide resins (polyamide 6T, polyamide 9T, polyamide 10T) and xylylenediamine-based polyamide resins, which will be described later.

[0014] The polyamide resin used in this embodiment may be a recycled polyamide resin product (including recovered products, material recycled products, chemical recycled products, etc.), a rejected product, or scrap material generated when molding a molded product from a resin composition.

[0015] In this embodiment, a polyamide resin containing diamine units and dicarboxylic acid units, in which 70 mol % or more of the diamine units are derived from xylylenediamine (hereinafter, sometimes referred to as a "xylylenediamine-based polyamide resin") is preferred.

[0016] The diamine units of the xylylenediamine-based polyamide resin are more preferably derived from xylylenediamine (preferably paraxylylenediamine and / or metaxylylenediamine) at 75 mol% or more, even more preferably 80 mol% or more, even more preferably 90 mol% or more, still more preferably 95 mol% or more, and particularly preferably 99 mol% or more.

[0017] The xylylenediamine is preferably paraxylylenediamine and / or metaxylylenediamine. The xylylenediamine preferably contains 0 to 100 mol% metaxylylenediamine and 100 to 0 mol% paraxylylenediamine (provided that the total of metaxylylenediamine and paraxylylenediamine does not exceed 100 mol%), more preferably 10 to 100 mol% metaxylylenediamine and 90 to 0 mol% paraxylylenediamine, even more preferably 20 to 100 mol% metaxylylenediamine and 80 to 0 mol% paraxylylenediamine, still more preferably 40 to 100 mol% metaxylylenediamine and 60 to 0 mol% paraxylylenediamine, and even more preferably 60 to 100 mol% metaxylylenediamine and 40 to 0 mol% paraxylylenediamine. In the xylylenediamine-based polyamide resin, the total of the paraxylylenediamine units and metaxylylenediamine units preferably accounts for 80 mol % or more, more preferably 85 mol % or more, even more preferably 90 mol % or more, still more preferably 95 mol % or more, still more preferably 98 mol % or more, and still more preferably 99 mol % or more of the diamine units. The upper limit of the total of the paraxylylenediamine units and metaxylylenediamine units is 100 mol %.

[0018] Diamines other than metaxylylenediamine and paraxylylenediamine that can be used as raw diamine components for xylylenediamine-based polyamide resins include aliphatic diamines such as tetramethylenediamine, pentamethylenediamine, 2-methylpentanediamine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, dodecamethylenediamine, 2,2,4-trimethyl-hexamethylenediamine, and 2,4,4-trimethylhexamethylenediamine; 1,3-bis( Examples of the diamine include alicyclic diamines such as bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, 1,3-diaminocyclohexane, 1,4-diaminocyclohexane, bis(4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, bis(aminomethyl)decalin, and bis(aminomethyl)tricyclodecane; and diamines having an aromatic ring such as bis(4-aminophenyl)ether, paraphenylenediamine, and bis(aminomethyl)naphthalene, and these can be used alone or in combination of two or more.

[0019] On the other hand, the dicarboxylic acid units of the xylylenediamine-based polyamide resin are preferably derived from α,ω-straight-chain aliphatic dicarboxylic acids preferably having 4 to 20 carbon atoms (preferably α,ω-straight-chain aliphatic dicarboxylic acids having 11 or more carbon atoms, and preferably α,ω-straight-chain aliphatic dicarboxylic acids having 14 or less carbon atoms) for 70 mol% or more, more preferably 75 mol% or more, even more preferably 80 mol% or more, still more preferably 90 mol% or more, still more preferably 95 mol% or more, and particularly preferably 99 mol% or more.

[0020] Examples of α,ω-linear aliphatic dicarboxylic acids having 4 to 20 carbon atoms that are suitable for use as the raw dicarboxylic acid component of the xylylenediamine-based polyamide resin include aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, undecanedioic acid, 1,10-dodecanedioic acid, and 1,12-tetradecanedioic acid. These can be used alone or in combination of two or more. Among these, at least one of sebacic acid, 1,10-dodecanedioic acid, and 1,12-tetradecanedioic acid is preferred, as this ensures that the melting point of the polyamide resin falls within a range suitable for molding and processing. 1,10-dodecanedioic acid and / or 1,12-tetradecanedioic acid is more preferred, and 1,10-dodecanedioic acid is even more preferred.

[0021] Examples of dicarboxylic acid components other than those mentioned above include phthalic acid compounds such as isophthalic acid, terephthalic acid, and orthophthalic acid, and isomers of naphthalenedicarboxylic acid such as 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, and these can be used alone or in combination of two or more.

[0022] Although the xylylenediamine-based polyamide resin is primarily composed of diamine units and dicarboxylic acid units, other structural units are not completely excluded. It goes without saying that the resin may contain lactams such as ε-caprolactam and laurolactam, and aliphatic aminocarboxylic acid units such as aminocaproic acid and aminoundecanoic acid. Here, "primary component" refers to the structural units constituting the xylylenediamine-based polyamide resin in which the total number of diamine units and dicarboxylic acid units is the largest among all structural units. In this embodiment, the total of the diamine units and dicarboxylic acid units in the xylylenediamine-based polyamide resin preferably accounts for 90% by mass or more of all structural units, more preferably 95% by mass or more, even more preferably 97% by mass or more, and even more preferably 99% by mass or more.

[0023] It is also preferable to use a polyamide resin produced using biomass raw materials (biomass polyamide resin) as the xylylenediamine-based polyamide resin. Using biomass polyamide resin can reduce the environmental impact. Mass balance certified (ISCC PLUS) raw material monomers can also be used. Mass balance certification means that the amount of renewable raw materials and bio-based raw materials used at each factory or production facility and the amount of products produced and shipped are quantified, and the quality is guaranteed.

[0024] The content of the xylylenediamine-based polyamide resin in 100 parts by mass of the thermoplastic resin having a melting point of 180 to 220°C contained in the fiber-reinforced composite material of the present embodiment is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, and even more preferably 95 parts by mass or more, and may be 99 parts by mass or more, or may be 100 parts by mass or less.

[0025] The thermoplastic resin having a melting point of 180 to 220°C contained in the fiber-reinforced composite material of this embodiment preferably has a number average molecular weight (Mn) of 6,000 or more, more preferably 8,000 or more, and even more preferably 10,000 or more, and preferably 100,000 or less, and more preferably 50,000 or less. The number average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC). More specifically, the number average molecular weight can be measured according to the description in paragraph 0047 of JP 2018-165298 A, the contents of which are incorporated herein by reference.

[0026] The content of the thermoplastic resin having a melting point of 180 to 220°C in the fiber-reinforced composite material of this embodiment is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, even more preferably 60% by mass or less, and even more preferably 50% by mass or less, and is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 28% by mass or more, even more preferably 30% by mass or more, and even more preferably 35% by mass or more, based on 100% by mass of the fiber-reinforced composite material. By setting the content at or below the upper limit, the mechanical properties of the resulting molded article tend to be improved. Furthermore, by setting the content at or above the lower limit, the impregnation time can be shortened, and deterioration of the natural fibers tends to be more effectively suppressed. The fiber-reinforced composite material of this embodiment may contain only one thermoplastic resin having a melting point of 180 to 220°C, or may contain two or more thermoplastic resins. When two or more thermoplastic resins are contained, the total amount is preferably within the above range.

[0027] The fiber-reinforced composite material of the present embodiment may or may not contain an amorphous thermoplastic resin. Preferably, it is substantially free of an amorphous thermoplastic resin. "Substantially free" means that the content of the amorphous thermoplastic resin is less than 10% by mass, preferably less than 5% by mass, more preferably less than 3% by mass, and even more preferably less than 1% by mass of the thermoplastic resin contained in the fiber-reinforced composite material.

[0028] <Natural Fiber> The fiber-reinforced composite material of this embodiment contains natural fibers. Examples of natural fibers include plant fibers and animal fibers, with plant fibers being preferred. Examples of plant fibers include cotton, hemp, kapok, palm fiber, sisal fiber, banana fiber, coconut fiber, and bamboo fiber, while examples of animal fibers include wool, goat hair, cashmere, mohair, angora, alpaca, camel hair, and silk. In this embodiment, the natural fiber preferably contains hemp.

[0029] The natural fibers used in this embodiment are preferably continuous natural fibers. Continuous natural fibers refer to natural fibers having an average fiber length of more than 6 mm, preferably more than 10 mm, more preferably more than 12 mm, more preferably 30 mm or more, and even more preferably 10 cm or more. There are no particular restrictions on the average fiber length of the natural continuous fibers used in this embodiment, but from the viewpoint of improving molding processability, it is preferably 1 m or more, more preferably 100 m or more, even more preferably 1,000 m or more, and preferably 20,000 m or less, more preferably 1,0000 m or less, and even more preferably 7,000 m or less.

[0030] The natural fibers may be treated with a surface treatment agent or a sizing agent. The natural fibers may be twisted or untwisted. By twisting the natural fibers, it becomes possible to produce molded products with good design properties, even if they are not treated with a surface treatment agent or a sizing agent.

[0031] Natural fibers are oriented in at least one direction. An example of natural fibers oriented in one direction is a roving of natural fibers that has been opened. Note that twisted natural roving fibers are also included in the examples of unidirectional orientation in this embodiment. An example of natural fibers oriented in two directions is a woven fabric of natural fibers. Examples of woven fabrics include plain weave, twill weave, and satin weave.

[0032] The content of natural fibers in the fiber-reinforced composite material of this embodiment is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 100 parts by mass or more, even more preferably 110 parts by mass or more, even more preferably 120 parts by mass or more, even more preferably 125 parts by mass or more, and preferably 81 parts by mass or less, more preferably 17 parts by mass or less, even more preferably 160 parts by mass or less, even more preferably 150 parts by mass or less, and even more preferably 140 parts by mass or less, per 100 parts by mass of the thermoplastic resin having a melting point of 180 to 220°C. By setting the content at or above the lower limit, the mechanical properties of the resulting molded article tend to be further improved. Furthermore, by setting the content at or below the upper limit, the impregnation time can be shortened, and deterioration of the natural fibers tends to be more effectively suppressed. The fiber-reinforced composite material of this embodiment may contain only one type of natural fiber, or two or more types. When two or more types of natural fibers are contained, the total amount is preferably within the above range.

[0033] The fiber-reinforced composite material of this embodiment may or may not contain fillers other than natural fibers. An example of a fiber-reinforced composite material of this embodiment is one that is substantially free of fillers other than natural fibers. "Substantially free" means that the content of other fillers in the fiber-reinforced composite material is preferably less than 10% by mass of the content of natural fibers, more preferably less than 5% by mass, even more preferably less than 3% by mass, and even more preferably less than 1% by mass.

[0034] <Other Components> The fiber-reinforced composite material of this embodiment may or may not contain other components in addition to the thermoplastic resin and natural fibers. Examples of the other components include fillers other than natural fibers, nucleating agents, antioxidants, stabilizers such as heat stabilizers, hydrolysis resistance improvers, weathering stabilizers, delustering agents, UV absorbers, nucleating agents, plasticizers, dispersants, flame retardants, antistatic agents, coloring inhibitors, antigelling agents, colorants, and additives such as mold release agents. For details, see paragraphs

[0130] to

[0155] of Japanese Patent No. 4,894,982 and paragraphs

[0047] to

[0103] of International Publication No. 2021 / 241,471, the contents of which are incorporated herein by reference. When the fiber-reinforced composite material of this embodiment contains the other components, it is preferable to melt-knead the other components with the thermoplastic resin to form a resin composition, which is then combined with the natural fibers to form a composite material. For the resin composition, please refer to the description in paragraph 0044 of WO 2023 / 188549, the contents of which are incorporated herein by reference. The content of these other components is preferably less than 10% by mass of the fiber-reinforced composite material, more preferably less than 5% by mass, even more preferably less than 3% by mass, and even more preferably less than 1% by mass. Furthermore, in the fiber-reinforced composite material of this embodiment, the total of the thermoplastic resin (preferably a polyamide resin, more preferably a xylylenediamine-based polyamide resin) and the natural fibers preferably accounts for 90% by mass or more of the fiber-reinforced composite material, more preferably 95% by mass or more, and may account for 99% by mass or more, or 100% by mass or less.

[0035] <Fiber-reinforced composite material> In the fiber-reinforced composite material of this embodiment, natural fibers are impregnated with a thermoplastic resin. That is, one example of the fiber-reinforced composite material of this embodiment is a prepreg. In the prepreg, the natural fibers may be oriented in one direction and the natural fibers may be impregnated with the polyamide resin, or the natural fibers may be woven and the natural fibers may be impregnated with the polyamide resin. Other impregnation forms are also possible. In the continuously reinforced composite material, the impregnation rate of the polyamide resin into the natural fibers is preferably 90% or more, more preferably 95% or more. The upper limit is preferably 100%.

[0036] <<Method for Measuring Impregnation Rate>> For a mixed yarn, cross sections perpendicular to the longitudinal direction of the continuous reinforcing fibers are cut out together, embedded in epoxy resin, and the surfaces corresponding to the cross sections of the mixed yarn are polished. The cross sections are then photographed using an ultra-deep color 3D shape measuring microscope. The cross sections of the mixed yarn embedded in epoxy resin are observed using a digital microscope. From the obtained cross-sectional photograph, areas where the thermoplastic resin fibers have impregnated the continuous reinforcing fibers (areas where the thermoplastic resin fibers have melted and impregnated between the continuous reinforcing fibers) are selected using image analysis software ImageJ, and their areas are measured. The impregnation rate is expressed as the area where the thermoplastic resin fibers have impregnated the continuous reinforcing fibers / cross-sectional area (unit: %). The ultra-deep color 3D shape measuring microscope used was a VK-9500 (controller unit) / VK-9510 (measurement unit) (manufactured by Keyence Corporation).

[0037] The thickness of the fiber-reinforced composite material is not particularly limited, but is preferably 50 μm or more, more preferably 70 μm or more, even more preferably 90 μm or more, and even more preferably 100 μm or more. The upper limit is preferably 2000 μm or less, more preferably 1750 μm or less, and even more preferably 1500 μm or less.

[0038] The fiber-reinforced composite material of this embodiment is preferably produced by impregnating the natural fibers with the thermoplastic resin at a temperature equal to or higher than the melting point of the thermoplastic resin by 5 to 30°C in an atmosphere with an oxygen concentration of less than 500 ppm. Impregnation at a temperature equal to or higher than the lower limit ensures good impregnation. Furthermore, impregnation at a temperature equal to or lower than the upper limit effectively suppresses decomposition of the natural fibers, resulting in a good prepreg. The impregnation temperature is preferably equal to or lower than the melting point by 28°C, more preferably equal to or lower than the melting point by 25°C, and even more preferably equal to or lower than the melting point by 20°C. Furthermore, it is preferably equal to or higher than the melting point by 7°C, more preferably equal to or higher than the melting point by 10°C, and even more preferably equal to or higher than the melting point by 12°C, and may even be equal to or higher than the melting point by 15°C. Setting the temperature equal to or lower than the upper limit tends to effectively suppress deterioration of the natural fibers. Setting the temperature equal to or higher than the lower limit tends to reduce the melt viscosity during impregnation, shorten the molding time, and effectively suppress deterioration of the natural fibers. The impregnation temperature may be constant from the start to the end of impregnation or may vary. Preferably, 90% or more of the time from the start to the end of impregnation, excluding the temperature rise at the start of impregnation and the temperature drop at the end of impregnation, is within the above range.

[0039] The impregnation is preferably carried out in an atmosphere with an oxygen concentration of less than 500 ppm. Since natural fibers undergo oxidative decomposition at about 200° C., impregnation in an atmosphere with a low oxygen concentration makes the natural fibers less susceptible to decomposition, resulting in a good prepreg. Examples of the atmosphere with an oxygen concentration of less than 500 ppm include a nitrogen atmosphere and a vacuum.

[0040] The impregnation may be carried out under pressure, and the pressure applied is preferably 0.1 MPa or more and 2.0 MPa or less.

[0041] <Applications of Fiber-Reinforced Composite Material> As described above, the fiber-reinforced composite material of this embodiment can be preferably used as a prepreg. Furthermore, the fiber-reinforced composite material of this embodiment can be preferably used as a UD (Uni-Directional) tape. The fiber-reinforced composite material of this embodiment can be wound around a core material during storage, shipping, etc. That is, it can be a wound body having a core material and the fiber-reinforced composite material wound around the core material. The fiber-reinforced composite material of this embodiment can be processed and molded as is, or it can be processed and molded by laminating several layers. That is, the form of a molded product formed from the fiber-reinforced composite material is not particularly limited. Examples include a method for manufacturing a molded product, which includes manufacturing a fiber-reinforced composite material, laminating multiple sheets of the fiber-reinforced composite material, and heat-processing the laminate, as well as a molded product obtained by the method. The thickness of the thinnest part of such a molded product can be determined appropriately depending on the application, etc., but can be, for example, 1 mm to 10 mm. The application field of the fiber reinforced composite material of this embodiment is not particularly limited, and it is widely used in transportation equipment parts such as automobiles, general machine parts, precision machine parts, electronic and electrical equipment parts, office automation equipment parts, building materials and housing related parts, medical devices, leisure and sporting goods, play equipment, medical supplies, daily necessities such as food packaging films, defense and aerospace products, etc.

[0042] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments used in the examples are difficult to obtain due to discontinuation or the like, measurements can be made using other instruments with equivalent performance.

[0043] <Raw Materials> Natural fiber: hemp fiber, woven fabric, basis weight 200 gsm Continuous carbon fiber: TR3523, PYROFIL (registered trademark) manufactured by Mitsubishi Chemical Corporation MXD10: polyamide resin synthesized according to Synthesis Example 1 below MP12: polyamide resin synthesized according to Synthesis Example 2 below MXD12: polyamide resin synthesized according to Synthesis Example 3 below MP10(40): polyamide resin synthesized according to Synthesis Example 4 below

[0044] Synthesis Example 1: Synthesis of MXD10 Dodecanedioic acid was placed in a jacketed reactor equipped with a stirrer, partial condenser, cooler, thermometer, dropping tank, and nitrogen gas inlet tube, and the contents were thoroughly purged with nitrogen. After heating and melting at 170°C, metaxylylenediamine (MXDA, manufactured by Mitsubishi Gas Chemical Company, Inc.) was gradually added dropwise to a molar ratio of 1:1 to sebacic acid while stirring the contents, and the temperature was raised to 240°C. After completion of the dropwise addition, the temperature was raised to 260°C and continued for 20 minutes. Thereafter, the pressure inside the reaction system was continuously reduced to 0.08 MPa, and the reaction was continued. After completion of the reaction, a pressure of 0.2 MPa was applied to the reactor with nitrogen gas, and the polymer was taken out as a strand from a nozzle at the bottom of the polymerization vessel. After water cooling, it was pelletized using a pelletizer.

[0045] Synthesis Example 2: Synthesis of MP12 Dodecanedioic acid was placed in a jacketed reactor equipped with a stirrer, partial condenser, condenser, thermometer, dropping tank, and nitrogen gas inlet tube. The contents were thoroughly purged with nitrogen and heated to 180°C to melt. While stirring the contents, a mixed amine (70:30) of metaxylylenediamine and paraxylylenediamine (MPXDA, manufactured by Mitsubishi Gas Chemical Company, Inc.) was gradually added dropwise to a molar ratio of 1:1 to dodecanedioic acid, while the temperature was raised to 290°C. After completion of the dropwise addition, the reaction was continued for 10 minutes while maintaining a liquid temperature of 290°C. The internal pressure of the reaction system was then continuously reduced to 600 Torr over 10 minutes, and the reaction was then continued for 20 minutes. During this time, the reaction temperature was continuously raised to 300°C. After completion of the reaction, a pressure of 0.3 MPa was applied to the reactor with nitrogen gas, and the polymer was removed as a strand from a nozzle at the bottom of the polymerization vessel. The polymer was then cooled with water and pelletized using a pelletizer.

[0046] Synthesis Example 3: Synthesis of MXD12 Dodecanedioic acid was placed in a jacketed reactor equipped with a stirrer, partial condenser, cooler, thermometer, dropping tank, and nitrogen gas inlet tube, and the contents were thoroughly purged with nitrogen. After heating and melting at 180°C, metaxylylenediamine (MXDA, manufactured by Mitsubishi Gas Chemical Company, Inc.) was gradually added dropwise while stirring the contents so that the molar ratio to dodecanedioic acid was 1:1, and the temperature was raised to 290°C. The reaction was continued for 10 minutes while maintaining a liquid temperature of 290°C. Thereafter, the internal pressure of the reaction system was continuously reduced to 600 Torr over 10 minutes, and the reaction was continued for 20 minutes. During this time, the reaction temperature was continuously raised to 300°C. After completion of the reaction, a pressure of 0.3 MPa was applied to the reactor with nitrogen gas, and the polymer was removed as a strand from a nozzle at the bottom of the polymerization vessel. The polymer was then cooled with water and pelletized using a pelletizer.

[0047] Synthesis Example 6: Synthesis of MP10 (40) Sebacic acid was placed in a jacketed reactor equipped with a stirrer, a partial condenser, a thermometer, a dropping tank, and a nitrogen gas inlet tube, and the reactor was thoroughly purged with nitrogen. After heating and melting at 170°C, xylylenediamine (metaxylylenediamine (manufactured by Mitsubishi Gas Chemical Company, Inc., MPXDA) and paraxylylenediamine (manufactured by Showa Denko, PXDA) in a molar ratio of 60:40 was gradually added dropwise to the reactor so that the molar ratio to sebacic acid became 1:1, and the temperature was raised to 240°C. After completion of the dropwise addition, the temperature was raised to 260°C and continued for 20 minutes. Thereafter, the pressure inside the reaction system was continuously reduced to 0.08 MPa, and the reaction was continued. After completion of the reaction, a pressure of 0.2 MPa was applied to the reactor with nitrogen gas, and the polymer was taken out as a strand from a nozzle at the bottom of the polymerization vessel. After water cooling, the polymer was pelletized using a pelletizer.

[0048] <Melting Point of Polyamide Resin> Unless otherwise specified, the melting point (Tm) of the polyamide resin was measured by differential scanning calorimetry (DSC) in accordance with ISO 11357. Specifically, a differential scanning calorimeter was used. The resin was placed in the measurement pan of the differential scanning calorimeter, heated to a temperature above the melting point at a heating rate of 10°C / min under a nitrogen atmosphere, and then rapidly cooled before measurement. The measurement conditions were a heating rate of 10°C / min, held at 280°C for 5 minutes, and then cooled to 100°C at a cooling rate of -5°C / min to determine the melting point (Tm). The differential scanning calorimeter used was a "DSC-60" manufactured by Shimadzu Corporation. Table 1 shows the melting points of the polyamide resins used in each example and comparative example.

[0049] Example 1 <Production of Fiber-Reinforced Composite Material (Prepreg)> A fiber-reinforced composite material was produced using natural fibers and the types of thermoplastic resins shown in Table 1. Specifically, the thermoplastic resin was processed into a 100 μm thick film, and natural fiber woven fabric and a thermoplastic resin film were alternately arranged on top of the film, with the thermoplastic resin film on the top surface. A pressure of 2.0 MPa was applied to obtain a fiber-reinforced composite material with a thickness of 1.0 mm. The proportion of natural fibers in the fiber-reinforced composite material was 57% by mass.

[0050] The manufacturability of the prepreg was evaluated as follows. In the evaluation, Reference Example 1 was used as the standard, with an "S" and the evaluation was made in comparison with this. The evaluation was carried out by five experts and judged by majority vote. S: A prepreg of equal or better quality than Reference Example 1 could be produced. A: A prepreg of practical quality could be produced, although it was inferior to Reference Example 1. B: Production was possible, but it was not practical. C: The natural fibers decomposed and production was not possible.

[0051] <Molding of fiber-reinforced composite material (prepreg)> The prepreg obtained above was vacuum-press molded at a temperature of 15°C above the melting point of the thermoplastic resin under a pressure of 3 MPa to obtain a molded product. The proportion of natural fibers in 100% by mass of the obtained fiber-reinforced composite material was 57% by mass.

[0052] The design of the obtained molded article was evaluated as follows. In the evaluation, Reference Example 1 was used as the standard, with "A" as the rating, and the evaluation was made in comparison with this. The evaluation was carried out by five experts, and the decision was made by majority vote. S: The design was superior to Reference Example 1. A: The design was equivalent to Reference Example 1. B: The design was inferior to Reference Example 1. -: The prepreg could not be produced, so no production was performed.

[0053] Examples 2 to 4, Comparative Examples 1 to 3, Reference Example 1 In Example 1, the changes were made as shown in Table 1, but the rest were carried out in the same manner.

[0054]

[0055] In the above, "Tm" refers to the melting point of the thermoplastic resin. "In the absence of oxygen" means that the experiment was performed in an atmosphere with an oxygen concentration of less than 500 ppm, and "in oxygen" means that the experiment was performed in an air atmosphere. All of the above natural fiber reinforced composite materials had an impregnation rate of 95% or higher. As is clear from the above results, in the present invention, a fiber reinforced composite material (prepreg) can be produced using natural fibers, and the resulting molded articles also had design characteristics equivalent to or superior to that of the natural fiber reinforced composite material (prepreg, Reference Example 1) (Examples 1 to 4). On the other hand, when the impregnation temperature of the thermoplastic resin was high (Comparative Example 1), the natural fibers decomposed, making it impossible to produce a prepreg. Furthermore, when produced under oxygen (Comparative Example 2), the natural fibers decomposed, making it impossible to produce a prepreg. When the melting point of the thermoplastic resin was high, the impregnation temperature inevitably became high (Comparative Example 3), causing the natural fibers to decompose, making it impossible to produce a prepreg.

Claims

1. A fiber-reinforced composite material comprising a thermoplastic resin having a melting point of 180 to 220°C and natural fibers, at least a part of the thermoplastic resin being impregnated into the natural fibers, and the natural fibers being oriented in at least one direction.

2. The fiber-reinforced composite material according to claim 1, wherein the thermoplastic resin contains a polyamide resin.

3. The fiber-reinforced composite material according to claim 1, wherein the thermoplastic resin contains a diamine unit and a dicarboxylic acid unit, and the polyamide resin contains 70 mol% or more of the diamine unit derived from xylylenediamine.

4. The fiber-reinforced composite material according to claim 1, wherein the thermoplastic resin contains a diamine unit and a dicarboxylic acid unit, the polyamide resin contains 70 mol% or more of the diamine unit derived from xylylenediamine, and the polyamide resin contains 70 mol% or more of the dicarboxylic acid unit derived from an α,ω-linear aliphatic dicarboxylic acid having 4 to 20 carbon atoms.

5. The fiber-reinforced composite material according to any one of claims 1 to 4, wherein the natural fibers are twisted.

6. The fiber-reinforced composite material according to any one of claims 1 to 5, which is a prepreg.

7. The fiber-reinforced composite material according to claim 6, wherein the natural fibers are continuous fibers oriented in one direction, and the prepreg is impregnated with the thermoplastic resin.

8. The fiber-reinforced composite material according to claim 6, wherein the natural fibers are a woven fabric, and the prepreg is impregnated with the thermoplastic resin.

9. A molded article formed from the fiber-reinforced composite material according to any one of claims 1 to 8.

10. A method for producing a fiber-reinforced composite material according to any one of claims 1 to 8, wherein the impregnation of the thermoplastic resin into the natural fibers is carried out at a temperature of the melting point of the thermoplastic resin + 5 to 30°C and in an atmosphere having an oxygen concentration of less than 500 ppm.

11. The method for producing a fiber-reinforced composite material according to claim 10, wherein the impregnation is carried out while pressurizing at a pressure of 0.1 to 2.0 MPa.

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