Sheet molding compound and carbon fiber-reinforced composite material

Abstract

Provided is a sheet molding compound with which it is possible to produce a carbon fiber-reinforced composite material having excellent flame retardancy while maintaining the mechanical characteristics and heat resistance. This sheet molding compound is obtained by impregnating carbon fibers with a resin composition. The resin composition contains a thermosetting resin, a thickener, and a flame retardant. The thermosetting resin contains a vinyl ester resin or an unsaturated polyester resin. The flame retardant contains a phosphonic acid ester that is liquid at 25°C.

Description

Sheet Molding Compound and Carbon Fiber-Reinforced Composite Material

[0001] The present invention relates to a sheet molding compound and a carbon fiber-reinforced composite material. The present application claims priority based on Japanese Patent Application No. 2025-052892 filed in Japan on March 27, 2025, the content of which is incorporated herein by reference.

[0002] Carbon fiber-reinforced composite materials composed of carbon fibers and a matrix resin (hereinafter also referred to as "CFRP") are widely used in aircraft, automobiles, and industrial applications due to their excellent mechanical properties and the like. In recent years, the scope of application of CFRP has been expanding more and more, and at the same time, the required properties thereof have been increasing. In particular, heat resistance is required when the material is applied to structural materials such as those for aerospace applications and vehicles.

[0003] As a method for producing CFRP, a method using a prepreg such as a sheet molding compound (hereinafter also referred to as "SMC") is known (see, for example, Patent Document 1). A prepreg is an intermediate material for molding obtained by impregnating a fiber reinforcing material such as carbon fiber with a matrix resin. For example, CFRP is produced by molding the prepreg into a desired shape through press molding or the like.

[0004] SMC has a structure in which a mat made of, for example, chopped carbon fiber bundles is impregnated with a resin composition containing a thermosetting resin. SMC has excellent fluidity during molding and is suitable for molding members having complex shapes. In addition, SMC has high specific strength, which enables thinning of members. By using a vinyl ester resin as the resin composition of SMC, molding time can be shortened, high-cycle molding becomes possible, and application to mass-produced members for mobility such as automobiles is expected.

[0005] International Publication No. 2021 / 182172

[0006] In the application of SMC to the mobility sector, particularly in battery-related components, high flame retardancy is required for CFRP to prevent fires caused by abnormal heat generation and to slow the spread of fire in the event of a fire. However, CFRP molded from SMC using thermosetting resins such as vinyl ester resin as the resin composition does not always exhibit sufficient flame retardancy, making it difficult to apply to components requiring flame retardancy.

[0007] Flame retardants are sometimes used to improve flame retardancy. Flame retardants come in solid and liquid forms. Solid flame retardants are difficult to uniformly disperse in resin compositions due to aggregation. Also, when impregnating carbon fibers with the resin composition, the flame retardant particles are filtered by the chopped carbon fiber bundles, leading to uneven dispersion in the resin composition. Due to these factors, flame retardancy tends to be uneven or its manifestation suppressed. On the other hand, liquid flame retardants can reduce the mechanical properties and heat resistance of thermosetting resins due to their plasticizing effect. Therefore, it is difficult to incorporate enough flame retardant into the resin composition to achieve sufficient flame retardancy, and adequate flame retardancy cannot be obtained.

[0008] The present invention aims to provide a sheet molding compound and a carbon fiber reinforced composite material that can be manufactured while maintaining mechanical properties and heat resistance, and exhibiting excellent flame retardancy.

[0009] As a result of diligent research, the inventors of this invention discovered that by using a liquid flame retardant with high compatibility with thermosetting resins, it is possible to achieve sufficient flame retardancy while maintaining mechanical properties and heat resistance, thus completing the present invention.

[0010] In other words, the present invention has the following embodiments: [1] A sheet molding compound comprising carbon fibers impregnated with a resin composition, wherein the resin composition contains a thermosetting resin, a thickener, and a flame retardant, the thermosetting resin comprises a vinyl ester resin or an unsaturated polyester resin, and the flame retardant comprises a phosphonic acid ester that is liquid at 25°C. [2] The sheet molding compound according to [1], wherein the 50% weight loss temperature of the flame retardant, measured using a thermogravimetric analyzer under nitrogen, is 250 to 340°C. [3] The sheet molding compound according to [1] or [2], wherein the flame retardant comprises an aliphatic cyclic phosphonic acid ester that is liquid at 25°C. [4] The sheet molding compound according to any one of [1] to [3], wherein the flame retardant comprises a compound represented by the following general formula (1).

[0011]

[0012] In equation (1), x is either 0 or 1.

[0013] [5] The sheet molding compound according to any one of [1] to [4], wherein the resin composition contains the flame retardant in an amount of 1 to 20% by mass relative to the total mass of the resin composition. [6] The sheet molding compound according to any one of [1] to [5], wherein the resin composition contains the flame retardant in an amount of 1.0 to 15.0% by mass relative to the total mass of the resin composition. [7] The sheet molding compound according to any one of [1] to [6], wherein the thermosetting resin comprises a vinyl ester resin. [8] The sheet molding compound according to [7], wherein the resin composition contains 10 to 70 parts by mass of the flame retardant per 100 parts by mass of the vinyl ester resin. [9] The sheet molding compound according to [7] or [8], wherein the thermosetting resin further comprises an unsaturated polyester resin.

[10] The sheet molding compound according to any one of [1] to [9], wherein the thickening agent comprises at least one selected from the group consisting of polyisocyanate compounds, alkaline earth metal oxides, and alkaline earth metal hydroxides.

[11] The sheet molding compound according to any one of [1] to

[10] , wherein the thickening agent comprises a polyisocyanate compound.

[12] The sheet molding compound according to any one of [1] to

[11] , wherein the resin composition further comprises an ethylenically unsaturated monomer.

[13] A carbon fiber reinforced composite material which is a molded product of the sheet molding compound according to any one of [1] to

[12] .

[0014]

[14] A method for producing a sheet molding compound, comprising impregnating carbon fibers with a resin composition to obtain impregnated reinforced fibers, and thickening the resin composition in the impregnated reinforced fibers, wherein the resin composition contains a thermosetting resin, a thickener, and a flame retardant, the thermosetting resin contains a vinyl ester resin or an unsaturated polyester resin, and the flame retardant contains a phosphonic acid ester that is liquid at 25°C.

[15] The method for producing a sheet molding compound according to

[14] , wherein the 50% weight loss temperature of the flame retardant, measured using a thermogravimetric analyzer under nitrogen, is 250 to 340°C.

[16] The method for producing a sheet molding compound according to

[14] or

[15] , wherein the flame retardant contains an aliphatic cyclic phosphonic acid ester that is liquid at 25°C.

[17] The method for producing a sheet molding compound according to any one of

[14] to

[16] , wherein the flame retardant contains a compound represented by the following general formula (1).

[0015]

[0016] In equation (1), x is either 0 or 1.

[0017]

[18] A method for producing a sheet molding compound according to any one of

[14] to

[17] , wherein the resin composition contains the flame retardant in an amount of 1 to 20% by mass relative to the total mass of the resin composition.

[19] A method for producing a sheet molding compound according to any one of

[14] to

[18] , wherein the resin composition contains the flame retardant in an amount of 1.0 to 15.0% by mass relative to the total mass of the resin composition.

[20] A method for producing a sheet molding compound according to any one of

[14] to

[19] , wherein the thermosetting resin contains a vinyl ester resin.

[21] A method for producing a sheet molding compound according to

[20] , wherein the resin composition contains 10 to 70 parts by mass of the flame retardant per 100 parts by mass of the vinyl ester resin.

[22] A method for producing a sheet molding compound according to

[20] or

[21] , wherein the thermosetting resin further contains an unsaturated polyester resin.

[23] A method for producing a sheet molding compound according to any one of

[14] to

[22] , wherein the thickener comprises at least one selected from the group consisting of polyisocyanate compounds, alkaline earth metal oxides, and alkaline earth metal hydroxides.

[24] A method for producing a sheet molding compound according to any one of

[14] to

[23] , wherein the thickener comprises a polyisocyanate compound.

[25] A method for producing a sheet molding compound according to any one of

[14] to

[24] , wherein the resin composition further comprises an ethylenically unsaturated monomer.

[26] A method for producing a carbon fiber reinforced composite material, comprising producing a sheet molding compound by the method for producing a sheet molding compound according to any one of

[14] to

[25] , and molding the obtained sheet molding compound.

[0018] According to the present invention, a sheet molding compound is available that can be used to manufacture carbon fiber reinforced composite materials that are excellent in flame retardancy while maintaining mechanical properties and heat resistance.

[0019] The present invention will now be described in detail. The following embodiments are merely illustrative for illustrating the present invention and are not intended to limit the present invention to these embodiments. The present invention can be implemented in various forms without departing from its spirit. In this specification and in the claims, a numerical range represented by "~" means a numerical range that includes the numbers before and after "~" as the lower and upper limits. For example, "A~B" is synonymous with A or greater and B or less. The numerical ranges of content, various physical properties, and property values ​​disclosed herein can be combined with their lower and upper limits to form new numerical ranges.

[0020] The following definitions of terms apply throughout this specification and the claims. "Sheet Molding Compound (SMC)" is a sheet-like molding material comprising short-fiber reinforcing fibers and a matrix resin. "Liquid at 25°C" means that, under conditions of 1 atmosphere, a sample heated to 60°C as a primary heating step using a differential scanning calorimeter, held for 5 minutes, cooled to -50°C, and then heated to 25°C at a rate of 10°C / min as a secondary heating step, exhibits an endothermic peak below 25°C during the secondary heating step, and the sample removed after measurement is completed at 25°C is liquid. "Thickening agent" is a resin composition prepared by mixing components constituting a resin composition to include a thermosetting resin, and then leaving the prepared resin composition undisturbed in an environment of 25°C for 7 to 14 days to reach the B-stage. "B-stage" means a semi-cured state in which a portion of the resin composition containing the thermosetting resin has begun to harden.

[0021] [Sheet Molding Compound] One embodiment of the present invention relates to a sheet molding compound. The SMC of this embodiment is obtained by impregnating carbon fibers, as shown below, with a resin composition containing a thermosetting resin. That is, the SMC of this embodiment includes carbon fibers and a resin composition. In addition to carbon fibers and a resin composition, the SMC may further contain fibers other than carbon fibers (hereinafter also referred to as "other fibers") as needed, as long as it does not impair the effects of the present invention.

[0022] <Carbon Fibers> Examples of carbon fibers include polyacrylonitrile (PAN) carbon fibers, rayon carbon fibers, and pitch carbon fibers. Carbon fibers may be used individually or in combination of two or more types.

[0023] The carbon fiber form is typically a short fiber bundle. A short fiber bundle is a cut piece obtained by cutting a carbon fiber bundle (tow), in which continuous fibers are aligned in one direction, to a predetermined length. The number of filaments in a carbon fiber bundle is usually around 1,000 to 60,000, preferably 1,000 to 20,000, more preferably 1,000 to 15,000, and even more preferably 1,000 to 5,000. If the number of filaments in a carbon fiber bundle is above the lower limit, the fluidity of the SMC can be improved. If the number of filaments in a carbon fiber bundle is below the upper limit, warping of the CFRP formed from the SMC can be prevented.

[0024] The length of the carbon fibers is preferably the same as the length of carbon fibers commonly used in SMC. The average length of the carbon fibers is preferably 1 to 60 mm, and more preferably 1 to 25 mm. If the average length of the carbon fibers is above the lower limit, the mechanical properties of the CFRP are superior. If the average length of the carbon fibers is below the upper limit, good fluidity is obtained when press molding the SMC. The average length of the carbon fibers is determined in accordance with JIS L 1019:2006, and the average value of the midpoint of three measurements for various samples is taken as the average length of the reinforcing fibers.

[0025] The carbon fiber content in SMC is typically 5 to 90% by mass, preferably 10 to 80% by mass, more preferably 40 to 75% by mass, even more preferably 45 to 70% by mass, and particularly preferably 50 to 65% by mass, based on the total mass of the SMC. If the carbon fiber content is above the lower limit, the reinforcing effect of the carbon fibers is fully exhibited, and the mechanical strength of the resulting CFRP is superior. If the carbon fiber content is below the upper limit, the fluidity of the SMC during molding is superior.

[0026] <Other Fibers> As for other fibers, a variety can be used depending on the application and purpose of SMC, and are not particularly limited, but examples include glass fiber, aramid fiber, silicon carbide fiber, alumina fiber, boron fiber, and tungsten carbide fiber. Other fibers may be used individually or in combination of two or more types.

[0027] The content of other fibers in SMC is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 3% by mass or less, relative to the total mass of SMC. However, the content of other fibers is preferably less than the content of carbon fibers. Furthermore, SMC does not need to contain substantially any other fibers. Here, "substantially free of other fibers" means not actively using other fibers in SMC, excluding those unintentionally included. Specifically, the content of other fibers relative to the total mass of SMC is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, and even more preferably below the detection limit.

[0028] <Resin Composition> The resin composition contains a thermosetting resin, a thickener, and a flame retardant. Preferably, the resin composition further contains an ethylenically unsaturated monomer in addition to the thermosetting resin, thickener, and flame retardant. In addition to the thermosetting resin, thickener, and flame retardant, the resin composition may further contain components other than the thermosetting resin, thickener, flame retardant, and ethylenically unsaturated monomer (hereinafter also referred to as "optional components") as needed, as long as they do not impair the effects of the present invention.

[0029] The resin composition is preferably a thickening agent. If the resin composition contained in SMC is a thickening agent, it is possible to achieve both productivity and handling ease of SMC, and the stability of the B-stage of SMC tends to be better (i.e., the B-stage can be maintained for a long period of time). In addition, the viscosity change of SMC over time is small, and it tends to have excellent storage stability. The individual components contained in the resin composition are described below.

[0030] (Thermosetting Resin) The thermosetting resin includes a vinyl ester resin or an unsaturated polyester resin. From the viewpoint of mechanical properties, it is preferable that the thermosetting resin includes a vinyl ester resin. Furthermore, from the viewpoint of storage stability of SMC, it is more preferable that the thermosetting resin includes both a vinyl ester resin and an unsaturated polyester resin. The thermosetting resin may consist only of a vinyl ester resin and / or an unsaturated polyester resin, or it may contain other thermosetting resins as needed. From the viewpoint of moldability and mechanical properties, the content of the thermosetting resin is preferably 15 to 95% by mass, and more preferably 25 to 80% by mass, relative to the total mass of the resin composition.

[0031] ((Vinyl Ester Resins)) Vinyl ester resins are reaction products (unsaturated monobasic acid epoxy esters) of epoxy resin components and unsaturated monobasic acid components. Epoxy resin components are compounds having two or more epoxy groups in one molecule. Examples of epoxy resin components include diglycidyl ether type epoxy resins with a bisphenol compound as the main skeleton, such as bisphenol A, bisphenol F, and brominated bisphenol A; polyglycidyl ether type epoxy resins with a polynuclear phenol compound as the main skeleton, such as phenol novolac, cresol novolac, and brominated phenol novolac; polyglycidyl ester type epoxy resins with an organic polybasic acid as the main skeleton, such as dimer acid and trimellitic acid; and glycidyl ether type epoxy resins with a diol compound as the main skeleton, such as an ethylene oxide or propylene oxide adduct of bisphenol A, glycol, or hydrogenated bisphenol A. Epoxy resin components may be used individually or in combination of two or more types.

[0032] An unsaturated monobasic acid component is a monobasic acid having an ethylenically unsaturated group. Examples of unsaturated monobasic acid components include acrylic acid, methacrylic acid, crotonic acid, and sorbic acid. An unsaturated monobasic acid component may be used alone or in combination of two or more types.

[0033] As the vinyl ester resin, epoxy (meth)acrylate resin is preferred. Epoxy (meth)acrylate resin is a vinyl ester resin using acrylic acid or methacrylic acid as the unsaturated monobasic acid component, that is, a reaction product of an epoxy resin component and acrylic acid or methacrylic acid. The vinyl ester resin may be used alone or in combination of two or more types.

[0034] Vinyl ester resins have ethylenically unsaturated groups derived from unsaturated monobasic acid components. The number of ethylenically unsaturated groups in one molecule of vinyl ester resin is preferably 1 to 2. If the number of ethylenically unsaturated groups is below the above upper limit, the polymerizability of the resin composition during curing is excellent. Vinyl ester resins typically have hydroxyl groups. The number of hydroxyl groups in one molecule of vinyl ester resin is preferably 1 to 4. If the number of hydroxyl groups is above the above lower limit, the viscosity of the resin composition is excellent. If the number of hydroxyl groups is below the above upper limit, the fluidity of the resin composition is excellent.

[0035] The vinyl ester resin content is preferably 10 to 90% by mass, and more preferably 20 to 70% by mass, relative to the total mass of the resin composition. If the vinyl ester resin content is above the lower limit, the mechanical properties of the resulting CFRP are superior. If the vinyl ester resin content is below the upper limit, the impregnation of the resin composition into carbon fibers during SMC production is superior.

[0036] ((Unsaturated Polyester Resin)) Unsaturated polyester resin is a polyester resin obtained by the condensation reaction of an α,β-olefin-based unsaturated dicarboxylic acid and a divalent glycol, that is, a polycondensate of an α,β-olefin-based unsaturated dicarboxylic acid and a divalent glycol. In the condensation reaction of an α,β-olefin-based unsaturated dicarboxylic acid and a divalent glycol, in addition to the α,β-olefin-based unsaturated dicarboxylic acid and the divalent glycol, other dicarboxylic acids other than α,β-olefin-based unsaturated dicarboxylic acid (hereinafter also referred to as "other dicarboxylic acids"), dicyclopentadiene that reacts with the dicarboxylic acid, and alcohols other than divalent glycols (hereinafter also referred to as "other alcohols") may be used in combination as needed. Unsaturated polyester resin may be used alone or in combination of two or more types.

[0037] Examples of α,β-olefinic unsaturated dicarboxylic acids include maleic acid, fumaric acid, itaconic acid, citraconic acid, and the anhydrides of these dicarboxylic acids. α,β-olefinic unsaturated dicarboxylic acids may be used individually or in combination of two or more types.

[0038] Other dicarboxylic acids include, for example, saturated dicarboxylic acids such as adipic acid, sebacic acid, succinic acid, and gluconic acid; and aromatic dicarboxylic acids such as phthalic anhydride, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, and tetrachlorophthalic acid. These other dicarboxylic acids may be used individually or in combination of two or more.

[0039] Examples of the divalent glycol include alkane diols, oxaalkane diols, and alkylene oxide adducts of bisphenol A. Examples of the alkane diol include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, and cyclohexanediol. Examples of the oxaalkane diol include dioxyethylene glycol, dipropylene glycol, and triethylene glycol. Examples of the alkylene oxide adduct of bisphenol A include an ethylene oxide adduct of bisphenol A and a propylene oxide adduct of bisphenol A. One type of divalent glycol may be used alone, or two or more types may be used in combination.

[0040] Examples of other alcohols include monohydric alcohols (monools) such as octyl alcohol and oleyl alcohol; and trihydric alcohols (triols) such as trimethylolpropane. One type of other alcohol may be used alone, or two or more types may be used in combination.

[0041] The unsaturated polyester resin is derived from an α,β-olefinic unsaturated dicarboxylic acid and has an ethylenically unsaturated group. In addition, the unsaturated polyester resin typically has hydroxyl groups. The number of hydroxyl groups contained in one molecule of the unsaturated polyester resin is preferably 1 to 3. When the number of hydroxyl groups is equal to or more than the above lower limit, the thickening property of the resin composition is excellent. When the number of hydroxyl groups is equal to or less than the above upper limit, the fluidity of the resin composition is excellent.

[0042] In the resin composition, the mass ratio of vinyl ester resin to unsaturated polyester resin (hereinafter also referred to as the "resin ratio"), represented as vinyl ester resin / unsaturated polyester resin, is preferably 0.1 to 9, more preferably 0.15 to 7, even more preferably 0.25 to 4, and particularly preferably 0.5 to 3. If the resin ratio is above the lower limit, the mechanical properties of the CFRP are better. If the resin ratio is below the upper limit, the impregnation of the resin composition into carbon fibers during SMC manufacturing is excellent.

[0043] (Thickening agent) The thickening agent is an additive that thickens to a desired viscosity value in 1 to 400 hours under an ambient temperature of 20 to 80°C, and then stabilizes the viscosity value. There are no particular restrictions on the thickening agent, and known thickening agents can be used. Examples include polyisocyanate compounds such as diisocyanate compounds; oxides or hydroxides of alkaline earth metals (magnesium oxide, magnesium hydroxide, calcium hydroxide, etc.); and acid anhydrides such as carboxylic acid anhydrides. When a resin rich in hydroxyl groups is used as the thermosetting resin, it is preferable to use a polyisocyanate compound as the thickening agent. When a resin rich in carboxyl groups is used as the thermosetting resin, it is preferable to use an oxide or hydroxide of an alkaline earth metal as the thickening agent. When an epoxy resin is used as the thermosetting resin, it is preferable to use an acid anhydride as the thickening agent. The thickening agent may be used alone or in combination of two or more types.

[0044] Among the above, polyisocyanate compounds are preferred as thickeners. When a thickener contains a polyisocyanate compound, the hydroxyl groups of the vinyl ester resin or unsaturated polyester resin react with the isocyanate groups of the polyisocyanate compound to form a linear ethylenically unsaturated group-containing prepolymer. This prepolymer effectively contributes to giving the SMC an appropriate viscosity.

[0045] The polyisocyanate compound is an organic compound having two or more isocyanate groups (-NCO) in one molecule. As an example of the polyisocyanate compound, mention may be made of diisocyanate compounds having two isocyanate groups in one molecule. Examples of the diisocyanate compound include toluene diisocyanate (including 2,4-toluene diisocyanate and 2,6-toluene diisocyanate), diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, and modified products thereof. Examples of polyisocyanate compounds other than diisocyanate compounds include polymeric MDI having three or more isocyanate groups in one molecule. One polyisocyanate compound may be used alone, or two or more polyisocyanate compounds may be used in combination.

[0046] The content of the thickener can be appropriately set according to the type thereof.

[0047] Further, when the thickener is a polyisocyanate compound, the content of the thickener is preferably 1 to 90 parts by mass, more preferably 5 to 70 parts by mass, relative to 100 parts by mass of the thermosetting resin. Further, it is preferably 1 to 99 parts by mass, more preferably 5 to 90 parts by mass, relative to 100 parts by mass of the vinyl ester resin. When the content of the thickener is not less than the above lower limit, the resin composition can be sufficiently thickened. When the content of the thickener is not more than the above upper limit, it is possible to suppress foaming caused by the reaction of excess isocyanate groups with moisture, and the retention of foams inside the CFRP after molding.

[0048] Furthermore, when the thickener is a polyisocyanate compound, the molar ratio (NCO / OH) of isocyanate groups of the thickener to the total amount of hydroxyl groups of the vinyl ester resin and the unsaturated polyester resin is preferably 0.07 to 0.67, and more preferably 0.13 to 0.6. Also, the molar ratio (NCO / OH) of isocyanate groups of the thickener to the total amount of hydroxyl groups of the vinyl ester resin is preferably 0.1 to 1, and more preferably 0.2 to 0.9. If the molar ratio is above the lower limit, the thickening effect is easily obtained, and the handling properties of the SMC are easily improved. If the molar ratio is below the upper limit, it is possible to suppress the SMC from continuously thickening over time, which would lead to a decrease in storage stability and moldability.

[0049] (Flame Retardant) The flame retardant contains a phosphonic acid ester that is liquid at 25°C. Generally, when using a flame retardant that is liquid at 25°C, bleed-out problems occur in SMC sheets and CFRP. In addition, it can reduce the mechanical properties and heat resistance of thermosetting resins. However, by including a phosphonic acid ester that is liquid at 25°C in the flame retardant, compatibility with thermosetting resins is improved, and bleed-out can be suppressed. Furthermore, because phosphonic acid esters have a high phosphorus content in their structure, they can efficiently exhibit flame retardant effects even in small amounts. Moreover, because phosphonic acid esters have a more stable phosphorus-carbon bond in addition to the phosphorus-oxygen bond, they have a high char-forming ability during combustion. Therefore, there is no need to excessively increase the amount of flame retardant, and excellent flame retardancy can be imparted to CFRP while maintaining good mechanical properties and heat resistance. Also, when the thermosetting resin contains vinyl ester resin, it is thought that the compatibility is excellent due to the similar polarity with phosphonic acid ester, and a decrease in various physical properties can be prevented. From the viewpoint of suppressing bleed-out and maintaining mechanical properties and heat resistance, the flame retardant preferably contains an aliphatic cyclic phosphonic acid ester that is liquid at 25°C, and more preferably contains a compound represented by the following general formula (1). The phosphonic acid ester that is liquid at 25°C may be used alone or in combination of two or more types.

[0050]

[0051] In equation (1), x is either 0 or 1.

[0052] The 50% weight loss temperature of the flame retardant, measured using a thermogravimetric analyzer under nitrogen conditions, is preferably 250 to 340°C, more preferably 260 to 320°C, and even more preferably 270 to 300°C. If the 50% weight loss temperature is above the lower limit, the decrease in the heat resistance of the CFRP can be suppressed. If the 50% weight loss temperature is below the upper limit, the flame retardant tends to decompose thermally before the thermosetting resin, making it easier to exhibit a flame retardant effect in the gas phase. The 50% weight loss temperature of the flame retardant is a value measured by thermogravimetric analysis using a thermogravimetric analyzer (TGA). Specifically, using a thermogravimetric analyzer, the temperature is raised from 30°C to 500°C at a heating rate of 10°C / min under a nitrogen atmosphere, and the weight loss at that time is measured. The temperature at which 50% of the initial weight is lost is defined as the 50% weight loss temperature of the flame retardant.

[0053] The flame retardant may consist solely of a phosphonic acid ester that is liquid at 25°C, or it may contain other flame retardants as needed. Examples of other flame retardants include other phosphorus-containing flame retardants other than phosphonic acid esters that are liquid at 25°C, nitrogen-based flame retardants, hydrated metals, and halogen-based flame retardants. The other flame retardants may be used individually or in combination of two or more types.

[0054] Other phosphorus-containing flame retardants include, for example, non-halogenated phosphate esters, halogenated phosphate esters, metal phosphinate salts, red phosphorus, ammonium polyphosphate, melamine phosphate, guanidine phosphate, and guanylurea phosphate. Examples of non-halogenated phosphate esters include trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, and aromatic polyphosphates. Examples of halogenated phosphate esters include tris(chloroethyl) phosphate, tris(dichloropropyl) phosphate, tris(chloropropyl) phosphate, bis(2,3-dibromopropyl)-2,3-dichloropropyl phosphate, tris(2,3-dibromopropyl) phosphate, bis(chloropropyl)octyl phosphate, halogenated alkyl polyphosphate, and halogenated alkyl polyphosphate. As metal phosphinate salts, this includes not only metal salts of phosphinic acid that do not have organic groups, but also metal salts of organic phosphinic acids such as diphenylphosphinic acid, monophenylphosphinic acid, dialkylphosphinic acid, monoalkylphosphinic acid, and alkylphenylphosphinic acid, as well as metal salts of diphosphinic acids such as methane (dimethylphosphinic acid) and benzene-1,4-di(methylphosphinic acid). Examples of dialkylphosphinic acids include dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, and methyl-n-propylphosphinic acid. Examples of monoalkylphosphinic acids include methylphosphinic acid, ethylphosphinic acid, and n-propylphosphinic acid. Examples of alkylphenylphosphinic acid include methylphenylphosphinic acid. Examples of metal phosphinate salts include aluminum phosphinate, zinc phosphinate, calcium phosphinate, and magnesium phosphinate. Other phosphorus-containing flame retardants may be used individually or in combination of two or more types.

[0055] Examples of nitrogen-based flame retardants include melamine compounds such as melamine cyanurate, triazine compounds, guanidine compounds, ammonium phosphate, and ammonium carbonate. Nitrogen-based flame retardants may be used individually or in combination of two or more types.

[0056] Examples of hydrated metals include aluminum hydroxide and magnesium hydroxide. Hydrated metals may be used individually or in combination of two or more types.

[0057] Examples of halogenated flame retardants include hexabromobenzene, hexabromodiphenyl ether, tribromophenol, decabromodiphenyl ether, dibromocresylglycidyl ether, decabromodiphenyl oxide, tetrabromobisphenol A, tetrabromobisphenol A derivatives [tetrabromobisphenol A epoxy oligomer, tetrabromobisphenol A carbonate oligomer, tetrabromobisphenol A bis(dibromopropyl ether), tetrabromobisphenol A bis(aryl ether), etc.], and bis(pentabromopeny). Examples include ethane, 1,2-bis(2,4,6-tribromophenoxy)ethane, 2,4,6-tris(2,4,6-tribromophenoxy)-1,3,5-triazine, brominated polyphenylene ether, brominated polystyrene, polybrominated styrene, brominated polyethylene, ethylene bistetrabromophthalimide, hexabromocyclododecane, hexabromobenzyl acrylate, pentabromobenzyl acrylate, brominated epoxy compounds (monoepoxy compounds obtained by the reaction of brominated phenols with epichlorohydrin), chlorinated paraffin, and chlorinated polyethylene. Halogenated flame retardants may be used individually or in combination of two or more types.

[0058] The content of liquid phosphonic acid ester at 25°C is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and particularly preferably 100% by mass, relative to the total mass of the flame retardant. In other words, it is particularly preferable that the flame retardant consists only of liquid phosphonic acid ester at 25°C.

[0059] The flame retardant content is preferably 1.0 to 20.0% by mass relative to the total mass of the resin composition, with a lower limit of 3.0% by mass, even more preferably 6.0% by mass, and particularly preferably 8.0% by mass. The upper limit is more preferably 16.0% by mass, even more preferably 15.0% by mass, and particularly preferably 11.0% by mass. The lower and upper limits of the flame retardant content can be arbitrarily combined, for example, 1.0 to 15.0% by mass, 3.0 to 16.0% by mass, 6.0 to 15.0% by mass, and 8.0 to 11.0% by mass. If the flame retardant content is within the above range, better flame retardancy can be imparted to the CFRP while maintaining good mechanical properties and heat resistance.

[0060] Furthermore, the flame retardant content is preferably 5 to 60 parts by mass, more preferably 10 to 50 parts by mass, even more preferably 10 to 30 parts by mass, and particularly preferably 15 to 30 parts by mass, per 100 parts by mass of thermosetting resin. Also, per 100 parts by mass of vinyl ester resin, it is preferably 10 to 70 parts by mass, more preferably 15 to 60 parts by mass, even more preferably 15 to 40 parts by mass, and particularly preferably 20 to 40 parts by mass. If the flame retardant content is within the above range, better flame retardancy can be imparted to CFRP while maintaining good mechanical properties and heat resistance.

[0061] (Ethylene-unsaturated monomers) Ethylene-unsaturated monomers primarily serve as reactive diluents. Examples of ethylenically unsaturated monomers include monofunctional (meth)acrylates such as styrene; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-hydroxyethyl methacrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, methylbenzyl (meth)acrylate, phenoxyethyl (meth)acrylate, methylphenoxyethyl (meth)acrylate, morpholino (meth)acrylate, phenylphenoxyethyl acrylate, phenylbenzyl (meth)acrylate, phenyl methacrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl methacrylate. Note that "(meth)acrylate" is a general term for acrylates and methacrylates. Ethylene-unsaturated monomers may be used individually or in combination of two or more.

[0062] The content of ethylenically unsaturated monomers is preferably 1 to 60% by mass, more preferably 5 to 50% by mass, and even more preferably 10 to 40% by mass, based on the total mass of the resin composition. If the content of ethylenically unsaturated monomers is above the lower limit, the impregnation of the resin composition into carbon fibers during SMC production is excellent. If the content of ethylenically unsaturated monomers is below the upper limit, the mechanical properties of CFRP are even better.

[0063] Furthermore, the total content of vinyl ester resin, unsaturated polyester resin, thickener, flame retardant, and ethylenically unsaturated monomer 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 may also be 100% by mass, based on the total mass of the resin composition.

[0064] (Optional components) Examples of optional components include known crosslinking agents, polymerization initiators, polymerization inhibitors, curing agents, inorganic fillers, internal release agents, stabilizers, hygroscopic agents, surfactants, dispersing wetting agents, pigments, and colorants. Optional components may be used individually or in combination of two or more.

[0065] (Manufacturing Method) The resin composition can be prepared by conventionally known methods. For example, it can be prepared by mixing each component that makes up the resin composition. Specifically, the resin composition can be prepared by mixing a thermosetting resin, a thickener, a flame retardant, and optionally an ethylenically unsaturated monomer and other optional components. It is preferable to prepare a mixture in advance by mixing all components except the thickener, and then prepare the resin composition by mixing the previously prepared mixture with the thickener immediately before impregnating the carbon fibers with the resin composition.

[0066] The mixing method only needs to ensure that each component is uniformly dispersed or dissolved, and conventional methods can be used. Mixing equipment such as a three-roll mill, planetary mixer, kneader, universal stirrer, homogenizer, and homodispenser can be used, but are not limited to these. Furthermore, two or more types of mixers may be used in combination.

[0067] (Content) The content of the resin composition in SMC is typically 10 to 95% by mass, preferably 20 to 90% by mass, more preferably 25 to 60% by mass, even more preferably 30 to 55% by mass, and particularly preferably 35 to 50% by mass, based on the total mass of SMC. If the content of the resin composition is above the lower limit, the fluidity of the SMC during molding is excellent. If the content of the resin composition is below the upper limit, the reinforcing effect of the carbon fibers is fully exhibited, and the mechanical strength of the resulting CFRP is even better.

[0068] <Method for Manufacturing SMC> The SMC according to this embodiment is obtained by impregnating carbon fibers with a resin composition to obtain impregnated reinforced fibers, and then increasing the viscosity of the resin composition in the obtained impregnated reinforced fibers. That is, the method for manufacturing SMC includes a step of impregnating carbon fibers with a resin composition to obtain impregnated reinforced fibers (hereinafter also referred to as the "impregnation step"), and a step of increasing the viscosity of the resin composition in the obtained impregnated reinforced fibers (hereinafter also referred to as the "thickening step"). Furthermore, the method for manufacturing SMC may further include a step of preparing the resin composition prior to the impregnation step (hereinafter also referred to as the "preparation step"). The following describes each step.

[0069] (Preparation Step) The preparation step is the step of preparing the resin composition. The resin composition can be obtained, for example, by the resin composition manufacturing method described above.

[0070] (Impregnation Process) The impregnation process is a process of impregnating carbon fibers with a resin composition to obtain impregnated reinforced fibers. The method of impregnating with the resin composition is not particularly limited, and any well-known method suitable for the form of the carbon fibers may be used. An example of an impregnation method is described below. First, the resin composition is coated onto the surface of each of two carrier films (a first carrier film and a second carrier film) to form a resin composition layer on the surface of each carrier film. Next, chopped carbon fiber bundles are randomly scattered on the resin composition layer of the first carrier film to provide a sheet-like fiber base layer consisting of aggregates of carbon fiber bundles. In this fiber base layer, there is usually no regularity in the orientation direction of the carbon fiber bundles, and the orientation direction of the carbon fiber bundles is random. Other fibers may be scattered together when scattering the chopped carbon fiber bundles. Next, the second carrier film is placed on top of the fiber base layer so that the resin composition layers provided on each of the two carrier films face each other via the fiber base layer. Next, the two carrier films are pressed together from above and below, and the resin composition layer is pressed and impregnated between the carbon fiber bundles of the fiber base layer and inside each carbon fiber bundle. This results in a sheet-like impregnated reinforced fiber (hereinafter also referred to as "SMC precursor") in which each resin composition layer and the fiber base layer are integrated.

[0071] (Thickening Process) The thickening process is a process of increasing the viscosity of the resin composition in the impregnated reinforcing fibers. The thickening process can be carried out, for example, by holding the impregnated reinforcing fibers at a predetermined temperature and time. While the impregnated reinforcing fibers are held, the resin composition undergoes the B-stage. The holding temperature and time of the impregnated reinforcing fibers can be appropriately set depending on the type and amount of thickener. Typically, this is several hours to several tens of days at a temperature of about 25 to 60°C, or several seconds to several tens of minutes at a temperature of about 60 to 80°C.

[0072] <Effects> The sheet molding compound according to the present invention contains a resin composition containing the above-mentioned thermosetting resin, thickener, and flame retardant as a matrix resin, and the flame retardant contains a phosphonic acid ester that is liquid at 25°C. Therefore, it is possible to produce CFRP with excellent flame retardancy while maintaining mechanical properties and heat resistance.

[0073] <Applications> The SMC of this embodiment is suitable as an intermediate material for molding CFRP. An example of CFRP using the SMC of the present invention will be described below.

[0074] [Carbon Fiber Reinforced Composite Material] The CFRP of this embodiment is a molded product of SMC according to the present invention, exhibiting excellent flame retardancy while maintaining mechanical properties and heat resistance. CFRP can be manufactured by molding SMC according to the present invention and curing the resin composition contained in SMC. In other words, CFRP is a cured product obtained by curing SMC.

[0075] An example of a CFRP manufacturing method is described in detail below. A single sheet of SMC or a laminate of multiple SMC sheets is set between a pair of molds. The SMC or laminate is heated and compressed at 120 to 230°C for 2 to 60 minutes to cure the resin composition contained in the SMC and obtain a molded CFRP product. A honeycomb structure such as corrugated cardboard may be used as a core material, and SMC may be placed on both sides or one side thereof. Alternatively, the SMC may be cut to a predetermined size in advance.

[0076] Furthermore, the method for forming SMC is not limited to the press forming method described above. For example, SMC may also be formed by autoclave forming, bagging forming, wrapping tape forming, internal pressure forming, or sheet wrap forming.

[0077] While CFRP has no particular limitations on its applications, it is suitably used in sports, general industrial, and aerospace applications. More specifically, in sports applications, it is suitably used for golf shafts, fishing rods, tennis and badminton rackets, bicycle frames, hockey sticks, and ski poles. In general industrial applications, it is suitably used for structural materials in mobility such as automobiles, ships, and railway vehicles, as well as drive shafts, leaf springs, wind turbine blades, pressure vessels, flywheels, papermaking rollers, roofing materials, cables, and repair and reinforcement materials. In particular, it is suitably used in applications where flame retardancy is required, such as in the mobility sector, especially in battery peripheral components such as battery cases.

[0078] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not exceed the spirit of the invention, and various modifications are possible as long as they do not depart from the spirit of the invention.

[0079] [Raw Materials] <Carbon Fiber> The following carbon fiber bundles were used as carbon fiber: ・Carbon fiber bundle A: A carbon fiber bundle with 15,000 filaments (manufactured by Mitsubishi Chemical Corporation, product name "TR50S 15L"), chopped to a length of 25 mm. ・Carbon fiber bundle B: A carbon fiber bundle with 15,000 filaments (manufactured by Mitsubishi Chemical Corporation, product name "TR50S 15L"), partially split and chopped to a length of 25 mm.

[0080] <Thermosetting Resins> As thermosetting resins, reaction products of epoxy resin and acrylic acid (vinyl ester resin) were used. As unsaturated polyester resins, polycondensation products of α,β-olefin-based unsaturated dicarboxylic acid and divalent glycol were used. As epoxy resins, jER® 827 manufactured by Mitsubishi Chemical Corporation and TETRAD-X® manufactured by Mitsubishi Gas Chemical Corporation were used.

[0081] <Thickening Agents> The following compounds were used as thickening agents: ・Cosmonate LL: Modified diphenylmethane diisocyanate (manufactured by Mitsui Chemicals, Inc., product name "Cosmonate LL"). ・HN-2200: 3 or 4-methyl-1,2,3,6-tetrahydrophthalic anhydride (manufactured by Resonaq Corporation, product name "HN-2200").

[0082] <Flame Retardant> The following compounds were used as flame retardants: ・FR-001: A mixture of phosphonic acid esters represented by formula (1) (manufactured by ACCI Specialty Materials, product name "Technirez FR-001", liquid at 25°C, 50% weight loss temperature: 282°C, a mixture of the component where x=0 and the component where x=1 in formula (1)). ・FR-001 LV: A mixture of phosphonic acid esters represented by formula (1) (manufactured by ACCI Specialty Materials, product name "Technirez FR-001 LV", liquid at 25°C, 50% weight loss temperature: 277°C, a mixture of the component where x=0 and the component where x=1 in formula (1)). - CR-733S: Resorcinol bis(diphenyl phosphate) (manufactured by Daihachi Chemical Industry Co., Ltd., product name "CR-733S", liquid at 25°C). - OP935: Organic phosphinate (aluminum diethyl phosphinate type) (manufactured by Clariant, product name "Exolit OP935", solid at 25°C).

[0083] <Ethylene-unsaturated monomers> The following compounds were used as ethylenically unsaturated monomers: ・Styrene (manufactured by Kyoei Chemical Co., Ltd., product name "Styrene Monomer").

[0084] <Optional Components> The following compounds were used as optional components. (Polymerization initiators) ・Perhexa C-75: 75% by mass solution of 1,1-di(t-butylperoxy)cyclohexane (manufactured by Nippon Oil & Fats Co., Ltd., product name "Perhexa C-75 (EB)"). ・Trigonox BPIC-75: 74% by mass solution of t-butylperoxyisopropyl carbonate (manufactured by Kayaku Nurion Co., Ltd., product name "Trigonox BPIC-75"). (Epoxy curing agent) ・2MZA-PW: 2,4-diamino-6-[2'-methylimidazole-(1')]-ethyl-s-triazine (manufactured by Shikoku Chemicals, Ltd., product name "Curesol 2MZA-PW").・PN-23J: Amine adduct-based latent curing agent for epoxy resins (manufactured by Ajinomoto Fine Techno Co., Ltd., product name "Amicure® PN-23J"). (Internal mold release agent) ・INT-EQ-6: Phosphate ester derivative composition (manufactured by Accel Plastic Research Laboratories, Inc., product name "MOLD WIZ INT-EQ-6"). (Stabilizer) ・PBQ (manufactured by Seiko Chemical Co., Ltd., product name "PBQ"). ・Acetone (manufactured by Sankyo Chemical Co., Ltd., product name "Acetone"). (Hygrometer) ・Molecular Sieve A4 (manufactured by Resonac Co., Ltd., product name "Molecular Sieve A4"). (Surfactant) ・SP-O30V (manufactured by Kao Corporation, product name "Leodor SP-O30V"). (Dispersing wetting agent) BYK-W996 (manufactured by Bic Chemie Japan Co., Ltd., product name "BYK-W996").

[0085] [Measurement and Evaluation] <Measurement of 50% Weight Loss Temperature> Using a thermogravimetric analyzer (TA Instruments, product name "TGA550"), the temperature was raised from 30°C to 500°C at a heating rate of 10°C / min under a nitrogen flow of 60 ml / min, and the weight loss during this process was measured. The temperature at which the weight loss from the initial weight reached 50% was defined as the 50% weight loss temperature of the flame retardant.

[0086] <Evaluation of Mechanical Properties> Six test specimens (1) measuring 4 mm thick, 250 mm long, and 25 mm wide were cut from CFRP. Using a universal testing machine (Instron Corporation, product name "Instron 4482"), the tensile strength and tensile modulus of each test specimen (1) were measured in accordance with ISO 527-4 under the following conditions, and the average value of the six specimens was calculated. • Span distance: 150 mm. • Strain gauge: KFGS-20-120-C1-11L1M2R. • Gauge length: 20 mm. • Data recording device: KYOWA DPM-911B. • Crosshead speed: 2 mm / min.

[0087] <Evaluation of Flame Retardancy> Test pieces (2) measuring 3 mm thick, 125 mm long, and 13 mm wide were cut from CFRP, and their flame retardancy was evaluated using the UL94 vertical combustion test method as a reference. Among those conforming to the UL94 standard, the flame retardancy was best in the order of V-0, V-1, and V-2, with V-0 being the most flame retardant, followed by V-1 and then V-2.

[0088] <Heat Resistance Evaluation> Test specimens (3) were cut from CFRP or cured resin. For the CFRP specimens (3), the thickness was 3 mm, the length 57 mm, and the width 12.7 mm. For the cured resin specimens (3), the thickness was 2 mm, the length 57 mm, and the width 12.7 mm. The glass transition temperature (Tg) was measured by performing dynamic viscoelastic analysis of the test specimens (3) in double-support mode using a dynamic viscoelasticity analyzer (TA Instruments Co., Ltd., product name "DMA Q800"). The measurement conditions were: heating rate 5°C / min, frequency 1 Hz, strain 0.01%, temperature 25-250°C for the CFRP, and heating rate 5°C / min, frequency 1 Hz, strain 0.02%, temperature 25-250°C for the cured resin. A higher glass transition temperature indicates that the material can maintain its mechanical strength even in high-temperature environments, i.e., it has superior heat resistance. Note that the "glass transition temperature (Tg)" is the temperature at which the temperature-tanδ curve shows a maximum value.

[0089] [Example 1-1] <Preparation of Resin Composition> Each component was weighed according to the formulation shown in Table 1. The components other than the thickener were mixed in advance using a homodisper to obtain a mixture. Immediately before impregnating the carbon fibers, the thickener was added to the mixture and mixed further to prepare the resin composition.

[0090] <SMC Manufacturing> The previously obtained resin composition is transferred to a first polyethylene carrier film at a rate of 730 g / m² using a doctor blade. ２ The material was coated in such a manner to form a resin composition layer. Carbon fiber bundles A were scattered on the resin composition layer so that the fiber direction of the carbon fibers was random, thereby creating a sheet-like fiber substrate layer. The same resin composition was then applied to a second polyethylene carrier film to a thickness of 730 g / m² using a doctor blade. ２ A resin composition layer was formed by coating the two carrier films in such a manner. The second carrier film was placed on top of the fiber substrate layer so that the resin composition layers on each of the two carrier films faced each other via the fiber substrate layer. The two carrier films were then passed between rolls and pressed together from above and below to impregnate the carbon fiber bundle A with the resin composition, obtaining sheet-like impregnated reinforced fibers with the basis weight and fiber content shown in Table 2. The obtained impregnated reinforced fibers were left to stand at 25°C for 7 days to thicken the resin composition in the impregnated reinforced fibers, thereby obtaining SMC.

[0091] <CFRP Manufacturing> Four sheets of the previously obtained SMC were stacked and placed in a mold heated to a constant temperature of 140°C, then clamped. By compressing at a pressure of 8 MPa for 2 minutes, a CFRP of 300 mm × 300 mm × 4 mm was obtained. The mechanical properties of the obtained CFRP were evaluated. The results are shown in Table 1. Separately, a CFRP of 300 mm × 300 mm × 3 mm was obtained in the same manner except that three sheets of the previously obtained SMC were stacked. The flame retardancy and heat resistance of the obtained CFRP were evaluated. The results are shown in Table 1.

[0092] [Examples 1-2 to 1-3, Comparative Examples 1-1 to 1-3] Resin compositions were prepared in the same manner as in Example 1-1, except that the blending composition of each component was changed as shown in Table 1. Sheet-like impregnated reinforced fibers with the basis weight and fiber content shown in Table 1 were obtained in the same manner as in Example 1-1, except that the obtained resin composition was used. SMC and CFRP were manufactured using the obtained impregnated reinforced fibers in the same manner as in Example 1-1. Mechanical properties, flame retardancy, and heat resistance were evaluated using the obtained CFRP. The results are shown in Table 1.

[0093] [Examples 2-1 to 2-2, Comparative Examples 2-1 to 2-2] <Preparation of Resin Composition> Resin compositions were prepared in the same manner as in Example 1, except that the blending composition of each component was changed as shown in Table 2.

[0094] <SMC Manufacturing> The previously obtained resin composition is transferred to a first polyethylene carrier film at a rate of 470 g / m² using a doctor blade. ２ The material was coated in such a manner to form a resin composition layer. Carbon fiber bundles B were scattered on the resin composition layer so that the fiber direction of the carbon fibers was random, thereby creating a sheet-like fiber substrate layer. The same resin composition was then applied to a second polyethylene carrier film to a thickness of 470 g / m² using a doctor blade. ２ A resin composition layer was formed by coating the two carrier films in such a manner. The second carrier film was placed on top of the fiber substrate layer so that the resin composition layers on each of the two carrier films faced each other via the fiber substrate layer. The two carrier films were then passed between rolls and pressed together from above and below to impregnate the carbon fiber bundle B with the resin composition, obtaining sheet-like impregnated reinforced fibers with the basis weight and fiber content shown in Table 2. The obtained impregnated reinforced fibers were left to stand at 25°C for 7 days to thicken the resin composition in the impregnated reinforced fibers and obtain SMC.

[0095] <Manufacturing of CFRP> CFRP was manufactured in the same manner as in Example 1-1, except that the previously obtained SMC was used. Mechanical properties, flame retardancy, and heat resistance were evaluated using the obtained CFRP. The results are shown in Table 1. [Comparative Example 3-1] <Preparation of Resin Composition> Each component was weighed according to the formulation shown in Table 3. The components other than the thickener were mixed in advance using a stirring and defoaming device to obtain a mixture. The thickener was added to the mixture and mixed further to prepare the resin composition. <Manufacturing of Cured Resin> Two heat-resistant glass plates were prepared. A 2 mm thick silicone spacer was placed between the glass plates to form a uniform space of 2 mm thickness. The previously obtained resin composition was poured into this space and cured by placing it in an oven heated to 130°C and letting it stand for 2 hours. Next, it was removed from the oven and the glass plates were removed to produce a 2 mm thick cured resin. Heat resistance was evaluated using the obtained cured resin. The results are shown in Table 3. [Comparative Example 3-2] A resin composition was prepared in the same manner as in Comparative Example 3-1, except that the blending composition of each component was changed as shown in Table 3. A cured resin product was manufactured in the same manner as in Comparative Example 3-1, except that the obtained resin composition was used. The heat resistance of the obtained cured resin product was evaluated. The results are shown in Table 3.

[0096]

[0097]

[0098]

[0099] As shown in Tables 1 and 2, CFRP with excellent flame retardancy while maintaining mechanical properties and heat resistance was produced from the SMC obtained in Examples 1-1 to 1-2 and Examples 2-1 to 2-2. In contrast, the CFRP molded from the flame retardant-free SMC obtained in Comparative Example 1-1 had poor flame retardancy. Similarly, the CFRP molded from the SMC obtained in Comparative Examples 1-2 to 1-3 and Comparative Examples 2-1 to 2-2, which did not contain phosphonic acid ester as a flame retardant, also had poor flame retardancy. Table 3 shows the case where epoxy resin was used as the thermosetting resin. Compared to Comparative Example 3-1 which did not contain a flame retardant, Comparative Example 3-2 which contained a flame retardant showed a significant decrease in the heat resistance of the resulting cured resin. Because the usable temperature range for molding was limited to low temperatures, it could lead to a decrease in material fluidity and an increase in voids, and was judged unsuitable for CFRP production.

[0100] The sheet molding compound according to the present invention makes it possible to produce carbon fiber reinforced composite materials with excellent flame retardancy while maintaining mechanical properties and heat resistance, and is suitable as a raw material for the mobility sector, particularly for battery peripheral components.

Claims

1. A sheet molding compound comprising carbon fibers impregnated with a resin composition, wherein the resin composition contains a thermosetting resin, a thickener, and a flame retardant, the thermosetting resin comprises a vinyl ester resin or an unsaturated polyester resin, and the flame retardant comprises a phosphonic acid ester that is liquid at 25°C.

2. The sheet molding compound according to claim 1, wherein the 50% weight loss temperature of the flame retardant, measured using a thermogravimetric analyzer under nitrogen, is 250 to 340°C.

3. The sheet molding compound according to claim 1, wherein the flame retardant comprises an aliphatic cyclic phosphonic acid ester that is liquid at 25°C.

4. The sheet molding compound according to claim 1, wherein the flame retardant comprises a compound represented by the following general formula (1). (In equation (1), x is either 0 or 1.) 5. The sheet molding compound according to claim 1, wherein the resin composition contains the flame retardant in an amount of 1 to 20% by mass relative to the total mass of the resin composition.

6. The sheet molding compound according to claim 1, wherein the resin composition contains 1.0 to 15.0% by mass of the flame retardant relative to the total mass of the resin composition.

7. The sheet molding compound according to claim 1, wherein the thermosetting resin comprises a vinyl ester resin.

8. The sheet molding compound according to claim 7, wherein the resin composition contains 10 to 70 parts by mass of the flame retardant per 100 parts by mass of the vinyl ester resin.

9. The sheet molding compound according to claim 7, wherein the thermosetting resin further comprises an unsaturated polyester resin.

10. The sheet molding compound according to claim 1, wherein the thickening agent comprises at least one selected from the group consisting of polyisocyanate compounds, alkaline earth metal oxides, and alkaline earth metal hydroxides.

11. The sheet molding compound according to claim 1, wherein the thickening agent comprises a polyisocyanate compound.

12. The sheet molding compound according to claim 1, wherein the resin composition further contains an ethylenically unsaturated monomer.

13. A carbon fiber reinforced composite material which is a molded product of a sheet molding compound according to any one of claims 1 to 12.

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