Thermosetting maleimide resin composition for RTM molding, fiber reinforced composite material and radome

Abstract

To provide: a heat-curable maleimide resin composition for RTM molding that is used to produce a fiber-reinforced composite material such as FRP, and is superior in dielectric properties; and a fiber-reinforced composite material using such a composition.SOLUTION: A heat-curable maleimide resin composition for RTM molding contains: (A-1) a maleimide compound having at least one dimer acid skeleton-derived hydrocarbon group in one molecule, with a viscosity of 20 Pa s or less, as measured under the conditions set forth below; (A-2) a maleimide compound having at least one dimer acid skeleton-derived hydrocarbon group in one molecule, with a viscosity of more than 20 Pa s, as measured under the conditions set forth below, the compound exhibiting a fluidity at 25°C; (B) a radical polymerization initiator; and (C) a polymerization inhibitor, wherein the viscosities of the components (A-1) and (A-2) are measured in accordance with a method set forth in JIS Z8803:2011, at a measurement temperature of 25°C, using a Brookfield-type rotary viscometer, with a rotation rate of a spindle set at 5 rpm.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a thermosetting maleimide resin composition for RTM molding, a fiber-reinforced composite material using the same, and a radome having the fiber-reinforced composite material. [Background technology]

[0002] Fiber-reinforced composite materials, including FRP (fiber-reinforced plastics), which are made of reinforcing fibers and matrix resins, can be designed to take advantage of the advantages of reinforcing fibers and matrix resins. As a result, they have many applications in aerospace, sports, general industry, and automotive, and their uses are continuing to expand.

[0003] As the matrix resin, both thermosetting resins and thermoplastic resins are used, but thermosetting resins that are easy to impregnate into reinforcing fibers are often used. As the thermosetting resin, epoxy resins, unsaturated polyester resins, modified (meth)acrylate resins, vinyl ester resins, phenolic resins, cyanate resins, etc. are used (for example, Patent Documents 1 to 4). As the reinforcing fibers, glass fibers, aramid fibers, carbon fibers, boron fibers, etc. are used.

[0004] Methods for manufacturing fiber-reinforced composite materials include the prepreg method, hand lay-up method, spray-up method, filament winding method, and RTM (resin transfer molding) method.

[0005] Meanwhile, in recent years, the next-generation communication system known as 5G has become widespread, and development has also begun for the next-generation communication system known as 6G, which will go beyond the millimeter wave range of 26 GHz to 80 GHz, and will achieve even faster, larger capacity, and lower latency communications than ever before. To achieve this, materials for the high frequency band of 3 to 80 GHz are required, and since reducing transmission loss is essential as a noise countermeasure, there is a demand for the development of insulating materials with excellent dielectric properties (low dielectric constant and low dielectric tangent).

[0006] For FRP, the development of 5G-related materials is also necessary, particularly for radomes, etc. The dielectric constant is particularly important for FRP, and specifically, a low dielectric constant is considered important, but it is becoming very difficult to meet these requirements with currently used materials, and the development of alternative materials is desired.

[0007] In recent years, thermosetting resin compositions using maleimide compounds having a dimer diamine skeleton have been reported as thermosetting resins with excellent dielectric properties (Patent Documents 5 to 8). Although there are descriptions of using these materials for substrate applications, mainly printed wiring boards, there are no descriptions of their application for FRP applications, and even when attempts are made to manufacture radomes using prepregs using these materials, the tackiness when uncured makes them difficult to handle, making their application to FRP applications difficult. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2001-2739 A [Patent Document 2] JP 2012-77124 A [Patent Document 3] International Publication No. 2018 / 105380 [Patent Document 4] JP 2020-94100 A [Patent Document 5] International Publication No. 2016 / 114286 [Patent Document 6] JP 2016-131243 A [Patent Document 7] JP 2016-131244 A [Patent Document 8] JP 2020-45446 A Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, an object of the present invention is to provide a thermosetting maleimide resin composition for RTM molding which has excellent dielectric properties and is for producing fiber-reinforced composite materials such as FRP, and further to provide a fiber-reinforced composite material using the same and a radome having the fiber-reinforced composite material. [Means for solving the problem]

[0010] Means for Solving the Problems The present inventors have conducted intensive research to solve the above problems and have found that the following thermosetting maleimide resin composition can achieve the above object, thereby completing the present invention.

[0011] <1> (A-1) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton per molecule, and having a viscosity of 20 Pa s or less as measured under the following conditions: (A-2) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton in one molecule, having a viscosity of more than 20 Pa s measured under the following conditions, and exhibiting fluidity at 25°C; (B) Radical polymerization initiator and (C) Polymerization inhibitor A thermosetting maleimide resin composition for RTM molding comprising: Measurement conditions: According to the method described in JIS Z8803:2011, the measurement temperature is 25°C, a Brookfield type rotational viscometer is used, and the spindle rotation speed is 5 rpm. <2> The viscosity measured under the following conditions is 3 Pa s or less <1> 2. The thermosetting maleimide resin composition for RTM molding according to claim 1 . Measurement conditions: According to the method described in JIS Z8803:2011, the measurement temperature is 60°C, a Brookfield type rotational viscometer is used, and the spindle rotation speed is 5 rpm. <3> The component (A-1) is a maleimide compound represented by the following formula (1): <1> or <2> 2. The thermosetting maleimide resin composition for RTM molding according to claim 1 . [ka] (In formula (1), A represents a hydrocarbon group derived from a dimer acid skeleton.) <4> The component (A-2) is a maleimide compound represented by the following formula (2): <1> ~ <3> 2. The thermosetting maleimide resin composition for RTM molding according to claim 1 . [ka] (In formula (2), B is independently a tetravalent organic group having a cyclic structure, X is independently a divalent hydrocarbon group having 6 to 200 carbon atoms, at least one of which is a hydrocarbon group derived from a dimer acid skeleton, and n is 1 to 100.) <5> In formula (2), B is any of the tetravalent organic groups represented by the following structural formulas: <4> 2. The thermosetting maleimide resin composition for RTM molding according to claim 1 . [ka] (The bond not bonded to a substituent in the above structural formula is bonded to the carbonyl carbon that forms a cyclic imide structure in formula (2).) <6> The ratio of (A-1) to (A-2) is (A-1):(A-2)=95:5 to 40:60 by mass. <1> ~ <5> 2. The thermosetting maleimide resin composition for RTM molding according to claim 1 . <7> <1> ~ <6> 2. A fiber-reinforced composite material formed from the thermosetting maleimide resin composition for RTM molding according to claim 1 and reinforcing fibers. <8> The reinforcing fiber is quartz glass fiber. <7> Fiber-reinforced composite materials. <9> <8> A radome comprising the fiber-reinforced composite material described in claim 1. Effect of the Invention

[0012] The thermosetting maleimide resin composition for RTM molding of the present invention gives a cured product having a low relative dielectric constant and excellent dielectric properties in the high frequency range, and has good fluidity and excellent moldability. Therefore, the thermosetting maleimide resin composition for RTM molding of the present invention is useful as a fiber-reinforced composite material, and the fiber-reinforced composite material can be suitably used for radomes, etc. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention will now be described in more detail.

[0014] First, RTM (Resin Transfer Molding) will be explained. RTM is a molding method in which a reinforcing fiber substrate is placed in a mold, the mold is closed, resin is injected (injection process), the resin is poured between the fibers of the reinforcing fiber substrate to fill the mold, the resin is cured, and the mold is opened to remove the molded product. At this time, it is preferable that the resin to be injected has a low viscosity during the injection process into the reinforcing fiber substrate, and that the increase in viscosity during the injection process is suppressed and the resin has excellent impregnation properties. At this time, the injection temperature is preferably from room temperature to 100°C or less.

[0015] It is also preferable that the viscosity of the resin to be injected is low. Specifically, it is necessary to inject at a temperature between room temperature and 100°C where the viscosity does not increase easily while maintaining a low viscosity, and it is preferable that the viscosity at the injection temperature is 3 Pa s or less. On the other hand, in the high temperature range of 150 to 180°C, rapid curing is required, so both must be achieved. Therefore, the thermosetting maleimide resin composition for RTM molding of the present invention has the properties of maintaining a low viscosity and not easily increasing in viscosity in the temperature range from room temperature to 100°C, and further, curing rapidly in the temperature range of 150 to 180°C.

[0016] The RTM in the thermosetting maleimide resin composition for RTM molding of the present invention includes VaRTM (vacuum-assisted resin transfer molding), which is said to be vacuum RTM in which the inside of a mold is suctioned by vacuum and resin is injected.

[0017] [(A) Specific maleimide compound] The component (A) is composed of two types of maleimide compounds, component (A-1) and component (A-2), both of which have one or more hydrocarbon groups derived from a dimer acid skeleton in each molecule.

[0018] The dimer acid referred to here is a liquid dibasic acid mainly composed of a dicarboxylic acid with 36 carbon atoms, produced by dimerization of unsaturated fatty acids with 18 carbon atoms, which are derived from natural products such as vegetable oils. Dimer acids do not have a single skeleton, but rather have multiple structures and exist in several types of isomers. Representative dimer acids are classified as linear (a), monocyclic (b), aromatic (c), and polycyclic (d). In this specification, the dimer acid skeleton refers to a group derived from a dimer diamine having a structure in which the carboxy groups of such a dimer acid are substituted with primary aminomethyl groups. That is, the component (A) preferably has, as the dimer acid skeleton, a group in which two carboxy groups in each of the dimer acids shown in the following (a) to (d) are substituted with methylene groups. Furthermore, from the viewpoint of the heat resistance and reliability of the cured product, it is more preferable that the hydrocarbon group derived from the dimer acid skeleton of the component (A) has a structure in which the carbon-carbon double bonds in the hydrocarbon group derived from the dimer acid skeleton are reduced by a hydrogenation reaction.

[0019] [ka]

[0020] [(A-1) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton per molecule and a viscosity of 20 Pa s or less at 25°C] The component (A-1) is a maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton in one molecule. From the viewpoints of availability of raw materials and stability of synthesis, the maleimide compound of the component (A-1) is preferably a bismaleimide compound having two maleimide groups in one molecule.

[0021] In order to improve handleability at room temperature and reduce the viscosity during injection, component (A-1) is a maleimide compound having a viscosity of 20 Pa s or less, preferably 1 to 20 Pa s, measured at 25°C under the following measurement conditions. Measurement conditions: According to the method described in JIS Z8803:2011, a Brookfield type rotational viscometer is used at the specified measurement temperature, with the spindle rotation speed at 5 rpm. In this specification, unless otherwise specified, the viscosity indicates a value measured at a temperature specified as the "designated measurement temperature" under the above conditions.

[0022] The maleimide compound of component (A-1) is preferably a maleimide compound represented by the following formula (1). This maleimide compound represented by the following formula (1) has a low viscosity at room temperature, and when incorporated into a composition, not only does it improve the handleability at room temperature, but also the cured product of the thermosetting maleimide resin composition has a relative dielectric constant of 2.6 or less and a dielectric loss tangent of 0.004 or less at a measurement frequency of 4 to 80 GHz, resulting in excellent dielectric properties. Note that component (A-1) may be used alone or in combination of two or more types. [ka] (In formula (1), A represents a hydrocarbon group derived from a dimer acid skeleton.)

[0023] [(A-2) Maleimide compounds having one or more hydrocarbon groups derived from a dimer acid skeleton per molecule other than (A-1)] Component (A-2) is a maleimide compound that has one or more hydrocarbon groups derived from a dimer acid skeleton per molecule, has a viscosity of more than 20 Pa s at 25°C, and is fluid at 25°C, and is a compound different from component (A-1). In this specification, fluidity can be said to be exhibited if the maleimide compound moves slowly when a bottle containing the compound is tilted sideways, and in terms of viscosity, "having fluidity" means that the viscosity at 25°C under the above measurement conditions is 100 to 1500 Pa·s. From the viewpoint of the handleability of the compound and the resulting composition, the upper limit of the viscosity of component (A-2) is preferably 1200 Pa·s or less, more preferably 1000 Pa·s or less. In addition, the lower limit of the viscosity of component (A-2) is a viscosity exceeding 20 Pa·s because it is other than component (A-1), but is preferably 100 Pa·s or more, and more preferably 200 Pa·s or more.

[0024] The maleimide compound of component (A-2) is preferably a maleimide compound represented by the following formula (2): When the maleimide compound represented by the following formula (2) is used, the adhesive strength to the reinforcing fibers described below is strong after curing, the toughness of the cured product can be increased, and the cured product has a relative dielectric constant of 2.6 or less and a dielectric loss tangent of 0.004 or less at a measurement frequency of 4 to 80 GHz, and thus has excellent dielectric properties. [ka] (In formula (2), B's are independently tetravalent organic groups having a cyclic structure, X's are independently divalent hydrocarbon groups having 6 to 200 carbon atoms, and at least one of them is a dimer acid skeleton. n is 1 to 100.)

[0025] In the formula (2), B independently represents a tetravalent organic group having a cyclic structure, and among these, any of the tetravalent organic groups represented by the following structural formulas is preferable. [ka] (The bond not bonded to a substituent in the above structural formula is bonded to the carbonyl carbon that forms a cyclic imide structure in formula (2).)

[0026] In addition, in the formula (2), X is independently a divalent hydrocarbon group having 6 to 200 carbon atoms, preferably 8 to 100, and more preferably 10 to 50. Among them, it is preferable that the divalent hydrocarbon group is a branched divalent hydrocarbon group in which one or more hydrogen atoms are substituted with an alkyl or alkenyl group having 6 to 200 carbon atoms, preferably 8 to 100, and more preferably 10 to 50 carbon atoms. The branched divalent hydrocarbon group may be either a saturated aliphatic hydrocarbon group or an unsaturated hydrocarbon group, and may have an alicyclic structure or an aromatic ring structure in the middle of the molecular chain. Specific examples of the branched divalent hydrocarbon group include divalent hydrocarbon groups derived from diamines at both ends, called dimer diamines. Note that dimer diamines are compounds derived from dimers (dimer acids) of the above-mentioned unsaturated fatty acids (e.g., oleic acid, etc.), and therefore, X is particularly preferably a branched divalent hydrocarbon group in which the two carboxy groups in each of the dimer acids (a) to (d) above are substituted with methylene groups.

[0027] In the formula (2), n is 1 to 100, preferably 1 to 60, and more preferably 1 to 50. If n is too large, the solubility and flowability may decrease, and moldability may be poor. The (A-2) component may be used alone or in combination of two or more.

[0028] There are no particular restrictions on the number average molecular weight of the maleimide compound of component (A-2), but from the standpoint of ease of handling the composition, it is preferably 1,000 to 30,000, and more preferably 1,200 to 10,000. The number average molecular weight referred to in the present invention refers to the number average molecular weight measured by gel permeation chromatography (GPC) under the following conditions using polystyrene as a standard substance.

[0029] [Measurement conditions] Developing solvent: Tetrahydrofuran (THF) Flow rate: 0.35mL / min Detector: Refractive index detector (RI) Column: TSK Guardcolumn SuperH-L TSKgel SuperHZ4000(4.6mmI.D.×15cm×1) TSKgel SuperHZ3000(4.6mmI.D.×15cm×1) TSKgel SuperHZ2000 (4.6mmI.D.×15cm×2) (All manufactured by Tosoh Corporation) Column temperature: 40℃ Sample injection volume: 5 μL (0.2% by mass in THF solution)

[0030] The ratio of the above-mentioned (A-1) and (A-2) components in the (A) component is, on a mass basis, (A-1):(A-2)=95:5 to 40:60, taking into consideration that the composition is liquid at room temperature before curing, and (A-1):(A-2)=93:7 to 45:55 is preferable, and (A-1):(A-2)=90:10 to 50:50 is more preferable, considering curability. If (A-1) is used alone, the reactivity is too high and the storage stability is lacking. In addition, in the thermosetting maleimide resin composition for RTM molding of the present invention, the blending amount of component (A) is preferably 50 to 99.5 mass %, more preferably 60 to 99 mass %, and even more preferably 65 to 99 mass %.

[0031] [(B) Radical polymerization initiator] The radical polymerization initiator, component (B), is added to promote the crosslinking reaction of the maleimide compound, component (A), and the radical polymerization reaction between the maleimide group in component (A) and a reactive group that can react with the maleimide group. The component (B) is not particularly limited as long as it promotes a radical polymerization reaction, and examples thereof include organic peroxides such as diallyl peroxide, dialkyl peroxide, peroxide carbonate, and hydroperoxide; and azo compounds such as azoisobutyronitrile and 1,1'-azobis(cyclohexanecarbonitrile). Of these, organic peroxides are preferably used. Examples of organic peroxides include dicumyl peroxide, t-butyl peroxybenzoate, t-amyl peroxybenzoate, dibenzoyl peroxide, diuraloyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,1-di(t-butylperoxy)cyclohexane, di-t-butyl peroxide, and dibenzoyl peroxide. The amine compound undergoes an addition reaction between the amino group in the amine compound and the maleimide group in the maleimide compound of component (A), but is not preferred for use in the composition of the present invention because of poor storage stability. Also, although a polymerization reaction using an imidazole compound is possible, it requires a reaction at a very high temperature, and is not preferred for use in the composition of the present invention from the viewpoint of productivity.

[0032] The radical polymerization initiator is preferably blended in an amount of 0.05 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, per 100 parts by mass of the component (A). When other thermosetting resins described later are blended in the composition, it is preferable to blend them in an amount of 0.05 to 10 parts by mass, particularly 0.1 to 5 parts by mass, per 100 parts by mass of the total of the component (A) and the other thermosetting resin components. If the amount is outside the above range, the maleimide resin composition may be cured very slowly or quickly during molding, which is not preferable. In addition, the balance between the heat resistance and moisture resistance of the obtained cured product may be poor. The radical polymerization initiator of the component (B) may use one type alone, or two or more types in combination.

[0033] [(C) Polymerization inhibitor] The component (C) used in the present invention is a polymerization inhibitor. The polymerization inhibitor is added not only to improve the storage stability of the thermosetting maleimide resin composition for RTM molding of the present invention, but also to suppress thickening when the resin is injected at a temperature higher than room temperature, and is not particularly limited as long as it can provide these effects.

[0034] Examples of the polymerization inhibitor include commonly used ones such as catechol, resorcinol, and 1,4-hydroquinone, as well as 2-methylcatechol, 3-methylcatechol, 4-methylcatechol, 2-ethylcatechol, 3-ethylcatechol, 4-ethylcatechol, 2-propylcatechol, 3-propylcatechol, 4-propylcatechol, 2-n-butylcatechol, 3-n-butylcatechol, 4-n-butylcatechol, 2-tert-butylcatechol, and 3-tert-butylcatechol. alkyl catechol compounds such as catechol, 4-tert-butylcatechol, and 3,5-di-tert-butylcatechol; alkyl resorcinol compounds such as 2-methylresorcinol, 4-methylresorcinol, 2-ethylresorcinol, 4-ethylresorcinol, 2-propylresorcinol, 4-propylresorcinol, 2-n-butylresorcinol, 4-n-butylresorcinol, 2-tert-butylresorcinol, and 4-tert-butylresorcinol; methylhydroquinol alkylhydroquinone compounds such as tributylphosphine, trioctylphosphine, tricyclohexylphosphine, triphenylphosphine; phosphine oxide compounds such as trioctylphosphine oxide, triphenylphosphine oxide; phosphite compounds such as triphenylphosphine, tris(nonylphenyl)phosphite; 2,2,6,6-tetramethylpiperidine-1 hindered amine compounds such as 1,4-dihydroxy-2-naphthalenesulfonate ammonium and 4-methoxy-1-naphthol; naphthoquinone compounds such as 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone and anthrone; and phenolic antioxidants such as pyrogallol, phloroglucin and 4,4'-butylidene-bis(6-tert-butyl-m-cresol).

[0035] The content of the polymerization inhibitor in the component (C) is preferably 0.01 to 0.80 parts by mass, more preferably 0.02 to 0.60 parts by mass, and even more preferably 0.03 to 0.50 parts by mass, per 100 parts by mass of the component (A). Furthermore, when the composition of the present invention contains a thermosetting resin having a reactive group capable of reacting with a maleimide group other than the component (A) described below, the content of the component (C) is preferably 0.01 to 0.70 parts by mass, more preferably 0.02 to 0.60 parts by mass, and even more preferably 0.03 to 0.50 parts by mass relative to 100 parts by mass of the sum of the component (A) and the thermosetting resin having a reactive group capable of reacting with a maleimide group other than the component (A) (hereinafter, both components may be collectively referred to simply as the "thermosetting resin component"). When the amount of the (C) component is less than 0.01 part by mass per 100 parts by mass of the (A) component, the polymerization inhibitory effect may be weakened. When the amount is more than 0.80 parts by mass, the effect reaches a plateau, and not only is it difficult to expect significant improvement in storage stability, but curability may also be adversely affected. The component (C) may be used alone or in combination of two or more types.

[0036] [Other additives] The thermosetting maleimide resin composition of the present invention may further contain various additives other than the above components (A) to (C) as necessary, provided that the effects of the present invention are not impaired. Examples of such additives are given below.

[0037] [Thermosetting resin having a reactive group capable of reacting with a maleimide group] In the present invention, a thermosetting resin having a reactive group capable of reacting with a maleimide group may further be added. The thermosetting resin is not limited to a specific type, and examples thereof include various resins other than component (A), such as epoxy resins, phenolic resins, melamine resins, silicone resins, cyclic imide resins including maleimide compounds other than component (A), urea resins, thermosetting polyimide resins, modified polyphenylene ether resins, thermosetting acrylic resins, and epoxy-silicone hybrid resins. Examples of reactive groups that can react with maleimide groups include epoxy groups, maleimide groups, hydroxyl groups, acid anhydride groups, alkenyl groups such as allyl groups and vinyl groups, (meth)acrylic groups, and thiol groups.

[0038] From the viewpoint of reactivity, the reactive group of the thermosetting resin is preferably one capable of undergoing radical polymerization, for example, a maleimide group, an alkenyl group, or a (meth)acrylic group, and from the viewpoint of dielectric properties, an alkenyl group or a (meth)acrylic group is more preferable. However, the amount of the thermosetting resin having a reactive group capable of reacting with a maleimide group is 0 to 60 mass % in the total amount of the thermosetting resin.

[0039] [Additives other than the above thermosetting resins] In addition to the above, non-functional silicone oils, thermoplastic resins, thermoplastic elastomers, organic synthetic rubbers, inorganic fillers, thixotropy-imparting agents, photosensitizers, light stabilizers, flame retardants, pigments, dyes, adhesion aids such as silane coupling agents, release agents, antioxidants, plasticizers, etc. may also be blended.

[0040] [Manufacturing method] The thermosetting maleimide resin composition of the present invention can be produced by the following method. For example, the maleimide compounds (A-1) and (A-2), the radical polymerization initiator (B) and the polymerization inhibitor (C) can be mixed, stirred, dissolved and / or dispersed simultaneously or separately, while optionally carrying out a heat treatment, to obtain a mixture of the components (A) to (C). Depending on the intended use, the above-mentioned other components may be added alone or in combination. In the above-mentioned production method, the device for mixing, stirring and dispersing is not particularly limited. Specifically, a mortar and pestle machine equipped with a stirring and heating device, a two-roll mill, a three-roll mill, a ball mill, a planetary mixer, a mass colloider, or the like can be used, and further, these devices can be used in appropriate combination.

[0041] [Fiber-reinforced composite materials] The composition of the present invention is a thermosetting maleimide resin composition for RTM molding, which becomes a fiber-reinforced composite material (FRP) when combined with reinforcing fibers and cured. Examples of reinforcing fibers include glass fibers such as quartz glass, E glass, T glass, and S glass, organic fibers such as acrylic, PBO, and nylon, carbon fibers, boron fibers, and metal fibers such as copper and iron. One type of reinforcing fiber may be used alone, or two or more types may be used in combination. Among these, quartz glass fibers, which have few impurities and excellent dielectric properties, are preferred.

[0042] The quartz glass fiber is preferably selected from quartz cloth, quartz chopped strands, quartz nonwoven fabric, and quartz wool. It may be in the form of fiber, fabric called glass cloth, quartz chopped strands, nonwoven fabric, or quartz wool, but it is more preferable to use quartz glass cloth because of ease of handling. In addition, the quartz glass cloth may be in the form of plain weave, satin weave, twill weave, etc., but plain weave and satin weave are preferable because of uniformity of thickness. The quartz glass cloth is made, for example, by using a quartz glass strand and / or a quartz glass yarn. The quartz glass strand and / or the quartz glass yarn are made by bundling 50 to 500 of the above-mentioned quartz glass fibers.

[0043] In order to improve the adhesion between the maleimide resin composition and the quartz glass fiber, it is preferable to treat the surface of the quartz glass fiber with a silane coupling agent, such as epoxy group-containing alkoxysilane, amino group-containing alkoxysilane, (meth)acrylic group-containing alkoxysilane, and alkenyl group-containing alkoxysilane. In particular, (meth)acrylic group and / or amino group-containing alkoxysilanes are preferably used, and specific examples include 3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, etc. These may be used alone or in combination of two or more. In addition, the present invention is not limited to these.

[0044] Next, a method for producing a fiber-reinforced composite material by the RTM method using the thermosetting maleimide resin composition of the present invention will be described.

[0045] The fiber-reinforced composite material of the present invention is produced by injecting the heated thermosetting maleimide resin composition into a reinforcing fiber substrate placed in a molding die heated to a specific temperature, impregnating the composition, and curing the composition in the molding die.

[0046] The temperature to which the thermosetting maleimide resin composition is heated is determined from the relationship between the initial viscosity of the resin composition and the viscosity increase in terms of the impregnation into the reinforcing fiber substrate, and is preferably 40 to 100° C., more preferably 50 to 90° C. In this case, as a guideline for the temperature and viscosity, the viscosity at 60° C. is 0.01 to 3 Pa s, more preferably 0.02 to 2 Pa s.

[0047] In addition, in the method for producing a fiber-reinforced composite material, a mold having multiple injection ports is used, and appropriate conditions can be selected according to the fiber-reinforced composite material to be obtained, such as injecting the maleimide resin composition from the multiple injection ports simultaneously or sequentially with a time lag. There are no limitations on the number or shape of the injection ports, but the more injection ports there are, the more preferable they are to enable injection in a short period of time, and the position of the injection ports is preferably such that the flow length of the resin can be shortened according to the shape of the molded product.

[0048] The injection pressure in RTM of the maleimide resin composition is usually 0.1 to 20.0 MPa. However, a VaRTM method in which the resin composition is injected into a mold by vacuum suction can also be used. In the case of VaRTM, the injection pressure is preferably 0.1 to 5.0 MPa from the viewpoints of injection time and equipment economy.

[0049] After the reinforcing fiber substrate is impregnated with the maleimide resin composition, the composition is cured under conditions of a temperature in a molding die of 120 to 200°C, preferably 130 to 190°C, for a time of 20 to 600 minutes, preferably 30 to 300 minutes, to obtain a fiber-reinforced composite material. The fiber-reinforced composite material thus obtained has excellent dielectric properties and moldability, and is therefore useful as fiber-reinforced plastics (FRP) for applications such as radomes, communication antennas, and printed circuit boards, and is particularly suitable for use in radomes.

[0050] Radomes protect antennas from the natural environment, such as rain and wind, and hide the antenna from the outside to prevent contact. In recent years, next-generation communication systems known as 5G have become widespread, and radomes for communication systems in high-frequency ranges, particularly those in the millimeter-wave range of 26 GHz to 80 GHz, are required not only to protect antennas, but also to have dielectric properties such as low dielectric constant and low dielectric dissipation factor in order to reduce transmission and reception losses of radio waves as much as possible. In addition, since radomes come in a wide variety of shapes depending on their installation location and purpose, the radome material must also have excellent formability. Therefore, the thermosetting maleimide resin composition for RTM molding of the present invention gives a cured product with a low dielectric constant and excellent dielectric properties in the high frequency range, and the composition has good fluidity and excellent moldability, and a fiber-reinforced composite material using the same are suitable as materials for radomes.

[0051] Although the resin composition of the present invention is specialized for RTM, it may be used for other molding methods such as a hand lay-up method, if necessary, or for other applications such as an adhesive. EXAMPLES

[0052] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0053] [(A-1) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton per molecule and a viscosity of 20 Pa s or less at 25°C] (A-1-1): A hydrocarbon group-containing bismaleimide compound derived from a dimer acid skeleton represented by the following formula (product name: X-45-6895, manufactured by Shin-Etsu Chemical Co., Ltd., viscosity at 25°C: 3.5 Pa s) [ka] -C 36 H 70 - indicates a structure derived from a dimer acid skeleton.

[0054] [(A-2) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton in one molecule, having a viscosity of more than 20 Pa s at 25°C, and exhibiting fluidity at 25°C] (A-2-1): A hydrocarbon group-containing bismaleimide compound derived from a flowable dimer acid skeleton represented by the following formula (product name: X-45-1400, manufactured by Shin-Etsu Chemical Co., Ltd., viscosity at 25°C: 450 Pa s, number average molecular weight: 1500) [ka] -C 36 H 70- indicates a structure derived from a dimer acid skeleton. n≒2 (average value) (A-2-2): A bismaleimide compound containing a hydrocarbon group derived from a flowable dimer acid skeleton represented by the following formula (product name: X-45-1500, manufactured by Shin-Etsu Chemical Co., Ltd., viscosity at 25°C: 500 Pa s, number average molecular weight: 1500) [ka] n≒2 (average value) -C 36 H 70 - indicates a structure derived from a dimer acid skeleton.

[0055] [(A-3) Comparative Maleimide Compound] (A-3-1): A bismaleimide compound containing a hydrocarbon group derived from a non-flowable dimer acid skeleton represented by the following formula (product name: BMI-5000, manufactured by Designer Molecules Inc., non-flowable (powder form) at 25°C, number average molecular weight 10,000) [ka] -C 36 H 70 - indicates a structure derived from a dimer acid skeleton. n≒10 (average value) (A-3-2): 1,6-bismaleimide-(2,2,4-trimethyl)hexane (trade name: BMI-TMH, manufactured by Daiwa Chemical Industry Co., Ltd., solid at 25°C) (A-3-3): Bisphenol-A-diphenyl ether bismaleimide (trade name: BMI-4000, manufactured by Daiwa Kasei Kogyo Co., Ltd., solid at 25°C)

[0056] [(B) Reaction initiator] (B-1): 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (product name: Trigonox 101, manufactured by Kayaku Nouryon Co., Ltd.) (B-2): 2-ethyl-4-methylimidazole (trade name: 2E4MZ, manufactured by Shikoku Chemical Industry Co., Ltd., for comparative example)

[0057] [(C) Polymerization inhibitor] (C-1) 4,4'-butylidene-bis(6-tert-butyl-m-cresol) (trade name: ANTAGE W-300, manufactured by Kawaguchi Chemical Industry Co., Ltd.)

[0058] [(D) Resin composition for comparative example] (D-1): An epoxy resin composition produced by the following method 100 g of bisphenol A type epoxy resin (trade name: YD-128, manufactured by Nippon Steel Chemical & Material Co., Ltd.), 90 g of a mixture of methylhexahydrophthalic acid and hexahydrophthalic acid (trade name: Rikacid MH-700, manufactured by New Japan Chemical Co., Ltd.), and 10 g of 1,2-dimethylimidazole (trade name: Curesol 1,2-DMZ, manufactured by Shikoku Chemical Industry Co., Ltd.) were placed in a 500 mL flask and stirred at room temperature for 30 minutes to obtain an epoxy resin composition (D-1). (D-2): Epoxy resin composition produced by the following method 100 g of 1,3-bis(aminomethyl)cyclohexane (trade name: 1,3-BAC, Mitsubishi Gas Chemical Co., Ltd.) and 3 g of methanesulfonic acid (manufactured by Toyobo Co., Ltd.) were placed in a 500 mL flask and stirred at room temperature for 1 hour. Furthermore, 141 g of bisphenol A type epoxy resin (trade name: YD-128, manufactured by Nippon Steel Chemical & Material Co., Ltd.) was added to the flask and stirred at room temperature for 30 minutes to obtain an epoxy resin composition (D-2). (D-3): Unsaturated polyester resin composition (product name: U-PICA 4001, manufactured by Nippon U-PICA Co., Ltd.) and curing accelerator (product name: PR-D, manufactured by Nippon U-PICA Co., Ltd.), blending ratio 100:1 (mass ratio)

[0059] <Sample preparation, Examples 1 to 7, Comparative Examples 1 to 10> Resin compositions were prepared by mixing the components in the compounding ratios shown in Tables 1 and 2 using a gate mixer. For Comparative Examples 4 to 7, the components had high melting points and were difficult to mix, so they were mixed at 80°C until the entire mixture was mixed well. For Comparative Example 7, one of the components, (A-3-3), did not dissolve and remained completely separated, so evaluation was not performed. The resin compositions (D-1) to (D-3) used for the comparative examples were prepared by the above-mentioned method.

[0060] <Viscosity, storage stability> According to the method described in JIS Z8803:2011, the viscosity was measured at 25 and 60° C. using a Brookfield type rotational viscometer with a spindle rotation speed of 5 rpm. Next, each resin composition was left at 25° C. for 72 hours and the viscosity was similarly measured at a measurement temperature of 25° C. to confirm storage stability. In addition, each resin composition was left at 60° C. for 10 minutes and the viscosity was similarly measured at a measurement temperature of 60° C.

[0061] <Dielectric properties and curability> A 70mm x 70mm, 200μm thick frame was prepared, and each resin composition was sandwiched between 50μm thick release-treated PET films (E7006, manufactured by Toyobo), and molded at 150°C for 5 minutes using a vacuum press (manufactured by Nikko Materials) to produce a cured product (molded film). The curability was evaluated as "○" for those that cured under these conditions, "×" for those that did not cure, and those that did not cure were not evaluated for dielectric properties. The molded film was post-cured at 180°C for 1 hour to obtain a cured resin film. The cured resin film was then used to connect a network analyzer (manufactured by Keysight, product name: E5063-2D5) and a strip line (manufactured by Keycom Corporation) to measure the relative dielectric constant and dielectric loss tangent of the cured resin film at frequencies of 10 and 28 GHz.

[0062] [Table 1]

[0063] [Table 2] *Since molding at 150°C was not possible due to the rapid curing rate, the material was cured at 50°C for 30 minutes, and the dielectric properties were evaluated using the cured material.

[0064] The thermosetting maleimide resin composition for RTM molding of the present invention has good fluidity, maintains low viscosity particularly in the temperature range from room temperature to 100°C, does not easily thicken, has excellent storage stability and moldability, and the cured product thereof has low relative dielectric constant and dielectric dissipation factor and is excellent in dielectric properties. On the other hand, it was found from Comparative Example 1 that the viscosity of the composition not containing the polymerization inhibitor (C) rose early, and that it had a problem in moldability as a molding material for RTM. Also, from Comparative Examples 2 and 3, it was found that the composition not containing the radical polymerization initiator (B) did not cure under the curing conditions of 150°C for 5 minutes, and that it was unsuitable as a molding material for RTM in terms of curability.

[0065] Next, a fiber-reinforced composite material was produced using each of the resin compositions of Example 1 and Comparative Examples 1 to 4 by the following method, and the appearance and moldability were confirmed. <Production, appearance and moldability of fiber-reinforced composite materials> Four quartz cloths (product name: SQX-2116, manufactured by Shin-Etsu Chemical Co., Ltd.) cut to 495 mm x 495 mm were stacked and set in a mold having a plate-shaped cavity of 500 mm x 500 mm x 0.8 mm, and the mold was clamped. Next, the mold was heated to 60 ° C., and each resin composition preheated at 60 ° C. for 10 minutes was injected into the mold at an injection pressure of 0.2 MPa using a resin injection device, and the quartz cloth was impregnated with the resin composition. After that, the mold was heated at 180 ° C. for 2 hours to harden the resin composition, and then cooled to 25 ° C. to obtain a fiber-reinforced composite material. The fiber-reinforced composite material was then removed from the mold, and its appearance and moldability were confirmed.

[0066] The fiber reinforced composite material of Example 1 had no unfilled portions or voids, and no abnormalities were observed in the appearance. On the other hand, the fiber reinforced composite material of Comparative Example 1 had unfilled portions at the four corners, and the fiber reinforced composite materials of Comparative Examples 2 and 3 had uncured resin compositions to begin with, so fiber reinforced composite materials could not be obtained. The fiber reinforced composite material of Comparative Example 4 had no unfilled portions, but some voids were confirmed, and the appearance was not satisfactory. Furthermore, due to insufficient toughness, only the resin was partially chipped when removed from the mold.

[0067] From the above results, it was confirmed that the thermosetting maleimide resin composition for RTM molding of the present invention can be suitably used for fiber-reinforced composite materials.

Claims

1. (A-1) A maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton in one molecule and having a viscosity of 20 Pa·s or less as measured under the following conditions: (A-2) a maleimide compound having one or more hydrocarbon groups derived from a dimer acid skeleton in one molecule, having a viscosity of more than 20 Pa·s measured under the following conditions, and exhibiting fluidity at 25° C.; (B) Radical Polymerization Initiator and (C) Polymerization inhibitor Measurement conditions: According to the method described in JIS Z8803:2011, the measurement temperature is 25° C., a Brookfield type rotational viscometer is used, and the spindle rotation speed is 5 rpm. A thermosetting maleimide resin composition for RTM molding comprising: The blending amount of component (A) in the composition is 50 to 99.5 mass %, the ratio of (A-1) to (A-2) is (A-1):(A-2)=95:5 to 40:60 on a mass basis, The amount of the (B) component is 0.05 to 10 parts by mass per 100 parts by mass of the (A) component, The amount of the component (C) is 0.01 to 0.80 parts by mass per 100 parts by mass of the component (A), A composition having a viscosity of 0.3 to 2.8 Pa·s at 60° C. as a whole and after standing at 60° C. for 10 minutes, measured under the following conditions: Measurement conditions: According to the method described in JIS Z8803:2011, a Brookfield type rotational viscometer is used at a measurement temperature of 60° C. and a spindle rotation speed of 5 rpm.

2. 2. The thermosetting maleimide resin composition for RTM molding according to claim 1, wherein the component (A-1) is a maleimide compound represented by the following formula (1): 【Chemistry 1】 (In formula (1), A represents a hydrocarbon group derived from a dimer acid skeleton.)

3. 3. The thermosetting maleimide resin composition for RTM molding according to claim 1, wherein the component (A-2) is a maleimide compound represented by the following formula (2): 【Chemistry 2】 (In formula (2), B is independently a tetravalent organic group having a cyclic structure, X is independently a divalent hydrocarbon group having 6 to 200 carbon atoms, at least one of which is a hydrocarbon group derived from a dimer acid skeleton, and n is 1 to 100.)

4. 4. The thermosetting maleimide resin composition for RTM molding according to claim 3, wherein B in formula (2) is any one of tetravalent organic groups represented by the following structural formulas: 【Chemistry 3】 (The bond not bonded to a substituent in the above structural formula is bonded to the carbonyl carbon that forms a cyclic imide structure in formula (2).)

5. A fiber-reinforced composite material formed from the thermosetting maleimide resin composition for RTM molding according to any one of claims 1 to 4 and reinforcing fibers.

6. 6. The fiber-reinforced composite material according to claim 5, wherein the reinforcing fibers are quartz glass fibers.

7. A radome comprising the fiber-reinforced composite material according to claim 6.

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