Patent Application
20240101775
The present invention relates to a self-assembled carbon fiber bundle (hereinafter sometimes referred to as “SACFB”) and its producing method, and a prepreg using SACFB as a reinforcement and its producing method.
CFRP (Carbon Fiber Reinforced Plastic), which is a fiber-reinforced plastic using carbon fiber as a reinforcement, is a lightweight material with excellent mechanical properties suitable for parts of automobiles, ships, railcars, manned aircrafts, unmanned aircrafts and other transportation equipment. The importance of CFRP has been increasing in recent years.
In a known method of efficiently producing CFRP products, for example, they are compression molded from a prepreg such as a sheet molding compound (SMC). A prepreg is an intermediate material having a structure in which a fiber reinforcement is impregnated with a matrix comprising an uncured thermosetting resin composition.
The fiber reinforcement used in a CF-SMC is a carbon fiber mat formed by cutting a continuous carbon fiber bundle into chopped carbon fiber bundles and spreading them on a carrier film (for example, Patent Literature 1).
The continuous carbon fiber bundle is produced by carbonizing a polyacrylonitrile fiber bundle comprising mechanically bundled fiber filaments. The continuous carbon fiber bundle is sized so that carbon fibers are kept bundled.
A continuous carbon fiber bundle with a large bundle size, such as 48K, called a large tow, has high production efficiency. On the other hand, some applications, including CF-SMC, require carbon fiber bundles with small bundle sizes, eg, 6K or less. Under these circumstances, a technique for dividing a large tow into a plurality of tows has been developed (Patent Literature 2).
A method for producing carbon fiber pellets for use in the production of fiber-reinforced thermoplastics (FRTP) by mixing cut carbon fibers with a solution or a suspension of a sizing agent, pelletizing the resulting agglomerates using a rotating disk pelletizer, and then drying the resulting pellets has been proposed (Patent Literature 3).
A phenomenon has been reported that by adding a small amount of chloroform to water in which short carbon fibers having a length of 3 to 5 mm are dispersed and shaking vigorously, the short carbon fibers agglomerate and form needle-like bundles by self-assembly. A group that reported this phenomenon described that carbon fibers recycled from waste materials can be used again as raw materials for high-performance materials by bundling them using this phenomenon (Non-Patent Literature 1).
The present invention has been made with the aim to accomplish at least one of the following tasks.
This specification may explicitly or implicitly indicate problems that can be solved by each embodiment of the present invention.
According to one aspect of the present invention, there is provided a prepreg comprising a thermosetting resin composition and a self-assembled carbon fiber bundle impregnated with the thermosetting resin composition.
According to another aspect of the present invention, there is provided a prepreg comprising a carbon fiber mat comprising a plurality of carbon fiber bundles including self-assembled carbon fiber bundles and a thermosetting resin composition, wherein the carbon fiber mat is impregnated with the thermosetting resin composition.
According to yet another aspect of the present invention, there is provided a method for producing a prepreg comprising impregnating a self-assembled carbon fiber bundle with a liquid thermosetting resin composition.
According to yet another aspect of the present invention, there is provided a method for producing a prepreg comprising impregnating a carbon fiber mat comprising a plurality of carbon fiber bundles including self-assembled carbon fiber bundles, with a liquid thermosetting resin composition.
According to yet another aspect of the present invention, there is provided a self-assembled carbon fiber bundle comprising a plurality of carbon fibers and an organic binder, wherein the fiber length of the plurality of carbon fibers is 60 mm or less, and wherein the plurality of carbon fibers contain no carbon fiber having a fiber length of less than 5 mm, or even when such a carbon fiber is contained therein, its content is less than 5 wt %.
According to yet another aspect of the invention, there is provided a method for producing a self-assembled carbon fiber bundle, wherein the method comprises mixing carbon fiber fluff and a bundling liquid to obtain a mixture; removing a liquid component of the bundling liquid from the mixture, wherein at least one of the carbon fiber fluff and the bundling liquid contains an organic binder, wherein all carbon fibers in the carbon fiber fluff have a fiber length of 60 mm or less, and wherein the carbon fiber fluff contain no carbon fiber having a fiber length of less than 5 mm, or even when such a carbon fiber is contained therein, its content is less than 5 wt %.
The embodiments of the present invention will be described in detail below.
The SACFB is formed through a process in which a plurality of short carbon fibers gather to form a bundle spontaneously. All of the short carbon fibers may be monofilaments prior to forming the SACFB. That is, the SACFB can be formed by aggregating of a plurality of carbon fiber filaments into a bundle. In another example, the SACFB may be formed by aggregating of a plurality of fine carbon fiber bundles, each comprising a small number of filaments, eg, less than 100 filaments. In yet another example, the SACFB may be formed by aggregating of a plurality of carbon fiber filaments and a plurality of fine carbon fiber bundles.
At an end of the SACFB, the tips of the plurality of carbon fibers comprising the bundle are unevenly positioned, as illustrated in
As shown in
The number of carbon fibers contained in one SACFB (bundle size of the SACFB) is preferably in the range of 1.5K or more and less than 4.5K.
“K” here is a symbol representing 1000. For example, 1K means 1000. 10K means 10000. 100K means 100000.
The shorter the fiber length, the less entanglement occurs when carbon fibers are bundled by self-assembly, and the formed SACFB is easier to impregnate with resin. Therefore, in all the short carbon fibers constituting the SACFB, the fiber length is preferably 60 mm or less, more preferably 40 mm or less, even more preferably 30 mm or less, and may be 20 mm or less.
On the other hand, carbon fiber with too short a fiber length is less effective as a reinforcement when used in FRP. For this reason, the fiber length of carbon fibers constituting the SACFB is preferably 5 mm or more, more preferably 10 mm or more.
The bundle length of the SACFB usually exceeds 5 mm, when most of the carbon fibers contained therein have a fiber length of 5 mm or more. The SACFB typically has a bundle length of more than 10 mm, when most of the carbon fibers contained therein have a fiber length of 10 mm or more.
By limiting the contained amount of a carbon fiber less than a certain length, it is possible to enhance a reinforcing effect of the SACFB when it is used as a reinforcement for FRP. For example, it is preferable that the SACFB does not contain a carbon fiber having a fiber length of less than L1 (mm), or even when such a carbon fiber is contained therein, its content is less than 5 wt % of the carbon fibers constituting the SACFB. Here, L1 can be 5, 6, 7, 8, 9 or 10. The SACFB having a larger L1 exhibits a higher reinforcing effect when used in an FRP.
In one example, by forming the SACFB only from carbon fibers having the same fiber length, it is possible to suppress variations in the quality of the SACFB between production lots. Among the plurality of carbon fibers constituting the SACFB, the difference between the maximum and minimum fiber lengths is preferably 5 mm or less, more preferably 4 mm or less, and even more preferably 3 mm or less.
There is no particular limitation on the filament diameter of the carbon fibers constituting the SACFB. That may be within the range of filament diameters that PAN-based carbon fibers normally have, for example, within the range of 5 μm to 15 mn.
The SACFB contains an organic binder that binds short carbon fibers to each other. A suitable example of the material for the organic binder is a resin used for sizing in a commercially available common carbon fiber bundle. In other words, it can be said to be a component resin of a sizing agent. Examples of such resins include, but are not limited to, epoxy resin, unsaturated polyester resin, vinyl ester resin (also called epoxy acrylate resin), polyurethane resin and polyamide resin. These resins may be used alone or in combination of two or more. The organic binder may be added with a surfactant in addition to the above resins. The organic binder may be the sizing agent itself used in common carbon fiber bundles.
The organic binder content of SACFB may be 0.5 wt % or more and less than 1.5 wt %, 1.5 wt % or more and less than 3 wt %, 3 wt % or more and less than 5 wt %, 5 wt % or more and less than 7 wt %, or 7 wt % or more and less than 10 wt %.
The carbon fibers constituting the SACFB may be all non-thermally degraded carbon fibers, or may be partially non-thermally degraded carbon fibers with the remainder thermally degraded carbon fibers. The carbon fibers constituting the SACFB may be all thermally degraded carbon fibers.
Non-thermally degraded carbon fibers are typically virgin carbon fibers.
Thermally degraded carbon fibers are typically recycled carbon fibers recovered from CFRP waste materials, which have been thermally degraded in the process of thermally decomposing and removing the matrix resin.
The SACFB preferably contains only carbon fibers as a fiber component. However, if there is no particular problem, it is allowed to contain fibers other than carbon fibers. For example, recycled carbon fibers containing glass fibers mixed therein may be preferably used as raw materials for SACFB with the mixed glass fibers still in them to the extent possible from the viewpoint of reducing the burden on the environment.
From the viewpoint of stabilizing the quality of the SACFB, the amount of fibers other than carbon fibers that can be contained in SACFB is preferably less than 10 wt %, more preferably less than 5 wt %, and even more preferably less than 1 wt % of the total fiber component.
Regarding the method of producing the SACFB, an embodiment using virgin carbon fiber as raw materials and an embodiment using recycled carbon fiber as raw materials will be explained.
The SACFB made of virgin carbon fiber can be produced, for example, through the following steps (i) to (iii).
However, as described later, the loosening process (ii) can be omitted.
The details of each process are explained below.
In the chopping process, a continuous carbon fiber bundle formed of virgin carbon fiber, that is, a brand new continuous carbon fiber bundle is cut into a predetermined length using, for example, a rotary cutter to obtain a chopped carbon fiber bundle.
The bundle size of the continuous carbon fiber bundle (the number of carbon fiber filaments constituting the bundle) is, for example, 10K or more, and may be 12K or more, 15K or more, 24K or more, 36K or more, 48K or more, or 50K or more. Although there is no particular upper limit, it is, for example, 100K or less.
The larger the bundle size of the continuous carbon fiber bundle, the greater the number of SACFBs obtained from one piece of chopped carbon fiber bundle, so the production efficiency is high. In addition, the production cost of continuous carbon fiber bundles is also lower for larger bundle sizes. Therefore, the bundle size of the continuous carbon fiber bundle is preferably 24K or more, more preferably 36K or more, and even more preferably 48K or more.
The fiber length of the chopped carbon fiber bundle may be set according to the bundle length of the SACFB to be produced, and is not particularly limited, but is preferably 6 mm or more, 7 mm or more, 8 mm or more, 9 mm or more, and may be 10 mm or more. Also, it is preferably 60 mm or less, more preferably 40 mm or less, even more preferably 30 mm or less, and may be 20 mm or less. Most of the carbon fibers in the chopped carbon fiber bundle are retained in their length during the production of SACFB.
The bundle length of the SACFB, which is formed from a plurality of short carbon fibers mostly having the same fiber length, is approximately the same as or longer than the fiber length of the short carbon fibers. When substantially all short carbon fibers contained in the SACFB have a fiber length of L2 (mm), the bundle length of the SACFB is often within the range of L2+1 (mm) to L2×1.4 (mm). However, there are exceptions.
In one example, chopped carbon fiber bundles having different fiber lengths may be mixed and used. However, in order to suppress variations in the quality of the SACFBs between production lots, it is preferable to use only chopped carbon fiber bundles having the same fiber length.
In the loosening process, carbon fiber fluff is obtained by loosening the chopped carbon bundles obtained in the chopping process.
The carbon fiber fluff may consist of monofilaments, or may contain fine carbon fiber bundles formed of a small number of filaments, for example, less than 100 filaments.
The loosening can be performed preferably using, but not limited to, a common loosening machine.
For example, the loosening can be carried out by putting only the chopped carbon fiber bundles into an agitation mixer such as a Henschel mixer and stirring them in a dry state. In this case, the produced carbon fiber fluff can be used in the next bundling process without being taken out from the agitation mixer.
For example, the chopped carbon fiber bundles can also be loosened by dipping them in an organic solvent, such as acetone, that can dissolve the sizing agent contained in the chopped carbon fiber bundles, and then irradiating them with ultrasonic waves. After washing off the sizing agent, carbon fiber in fluffy form remains. Since at least part of the sizing agent contained in the chopped carbon fiber bundles is lost in this method, it is desirable to supplement the organic binder in the subsequent bundling process.
In the bundling process, the short carbon fibers are spontaneously bundled by mixing the carbon fiber fluff obtained in the loosening process with the bundling liquid. After the bundling, liquid components in the bundling liquid are removed by evaporation.
A preferred liquid component contained in the bundling liquid is water. The strong surface tension of water strongly promotes the bundling of short carbon fibers due to the capillary effect. Moreover, since water is nonflammable, it is preferable in that an explosion-proof stirring device is not required.
The amount of the bundling liquid is, for example, 30 to 100 parts by weight with respect to 100 parts by weight of the short carbon fibers, but is not limited thereto. The amount of the bundling liquid can be appropriately adjusted while observing the state of the mixture.
An organic binder can be added to the bundling liquid as a dispersoid or a solute. When the organic binder is dispersed in the water-based bundling liquid, an organic solvent may be used as a dispersing aid.
When the amount of organic binder is sufficient, the bundle size distribution between SACFBs formed tends to be narrow. This is because formation of SACFBs with relatively small bundle sizes of less than 4.5K is promoted.
When setting the amount of the organic binder to be added to the bundling liquid, it is desirable to also consider the amount of sizing agent contained in the virgin carbon fiber bundles, the raw material. This is because the sizing agent remains in the final product, SACFB, while retaining its ability as an organic binder.
There is no limit to the method of mixing the carbon fiber fluff and the bundling liquid. Stirring is preferred for efficient mixing in a short time.
For stirring, an agitation mixer for powder known as a Henschel mixer can be preferably used. The agitation mixer may be of a type comprising only an agitator blade (stirring blade), or may be of a type with a chopper attached.
In one example, carbon fiber bundles produced by mixing carbon fiber fluff with a bundling liquid by stirring can be reformed by treating them with a tumbling granulator before removing the liquid component in the bundling liquid.
In another example, loosening and bundling by self-assembly can be performed at once by mixing the chopped carbon fiber bundles with the bundling liquid using an agitation mixer, omitting the loosening step. However, in order to produce the SACFB with stable quality, it is preferable to conduct the loosening process.
In the method of producing the SACFB described in 2.1. above, all or part of the virgin carbon fiber as raw material can be replaced with recycled carbon fiber.
A suitable example of recycled carbon fiber is carbon fiber recovered from CFRP scraps derived from products molded from SMC, or carbon fiber recovered from SMC offcuts. For example, the above-mentioned scraps or offcuts are preferably dry distilled at a temperature of 600° C. or higher, and further heated to, for example, 550° C. or higher, preferably 600° C. or higher, under an oxidizing atmosphere to completely thermally decompose the matrix resin. This leaves behind fluffy recycled carbon fiber having a fiber length of 60 mm or less.
Recycled carbon fiber is thermally degraded and has lower strength than virgin carbon fiber, but it has sufficient strength to be used as a reinforcement for FRP.
Another method for recovering carbon fiber from CFRP waste is to decompose the matrix resin using a subcritical or supercritical fluid. The matrix resin is completely removed so that fluffy recycled carbon fibers are obtained. Resin residues that cannot be completely removed by this method may be removed by heat treatment in an oxidizing atmosphere. The recycled carbon fiber obtained by this method using the subcritical or supercritical fluid is also thermally degraded and therefore has lower strength than virgin carbon fiber.
As another method of recovering carbon fiber from CFRP waste materials, it is also possible to use a method including decomposing the matrix resin by microwave heating.
Another method for recovering carbon fiber from SMC offcuts is to wash out the uncured matrix resin using a solvent that may be a subcritical or supercritical fluid. According to this method, it is possible to obtain recycled carbon fiber that is not thermally degraded and has strength equivalent to that of virgin carbon fiber.
A recycled carbon fiber recovered from CFRP molded from SMC or SMC is a short fiber having a fiber length of 60 mm or less and does not require further cutting for use in the production of the SACFB. When this recycled carbon fiber is used as raw material still with the sizing agent removed, (ii) loosening process is also unnecessary, and only (iii) bundling process described above is required. The organic binder must be added to the bundling solution.
A prepreg can be produced by impregnating the SACFB with a liquid thermosetting resin composition.
Impregnation can be performed, for example, by immersing the SACFB in the liquid thermosetting resin composition.
The fiber content of the prepreg using the SACFB can be 20 wt % or more and less than 30 wt %, 30 wt % or more and less than 40 wt %, 40 wt % or more and less than 50 wt %, 50 wt % or more and less than 60 wt %, 60 wt % or more and less than 70 wt %, 70 wt % or more and less than 80 wt %, or 80 wt % or more and less than 90 wt %.
The higher the fiber content, the better the mechanical properties of CFRP obtained by curing the prepreg. The lower the fiber content, the easier the prepreg flows during pressure molding, so the degree of freedom in designing the shape of the CFRP product molded from the prepreg increases.
Examples of base resins for liquid thermosetting resin compositions include, but are not limited to, vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin. One type or two or more types of thermosetting resin may be added to the liquid thermosetting resin composition.
A curing agent is usually added to the liquid thermosetting resin composition in addition to the base resin. In addition, a polymerization inhibitor, a thickener, a reactive diluent, a low-shrinkage agent, an antioxidant, an internal release agent, a coloring agent, a modifier (for example, a rubber, an elastomer or a thermoplastic resin), a flame retardant, an antimicrobial agent, etc. may be added to the liquid thermosetting resin composition if necessary.
The liquid thermosetting resin composition is preferably solvent-free, in other words, not a varnish. If it is a varnish, a process to remove the solvent by evaporation is required after impregnating the SACFB. Reactive diluents are not included in solvents here.
One suitable example of a non-varnish liquid thermosetting resin composition is a vinyl ester resin-based composition containing a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a thickener, a polymerization initiator and a polymerization inhibitor added thereto.
Suitable examples of the vinyl ester resin are a bisphenol A type epoxy vinyl ester resin and a novolac vinyl ester resin. Either one of these may be used, or both may be used. The weight ratio of the vinyl ester resin to the unsaturated polyester resin to be used may be 1:9 to 9:1, 1:7 to 7:1, 1:4 to 4:1, 1:2 to 2:1, etc.
The ethylenically unsaturated monomer is used as a reactive diluent. At least one monofunctional ethylenically unsaturated monomer, at least one polyfunctional ethylenically unsaturated monomer, or both may be used.
A suitable example of a monofunctional ethylenically unsaturated monomer is styrene. Other examples include, but not limited to, monofunctional (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, isobomyl (meth)Acrylates, benzyl (meth)acrylate, methylbenzyl (meth)acrylate, phenoxyethyl (meth)acrylate, methylphenoxyethyl (meth)acrylate, morpholine (meth)acrylate, phenylphenoxyethyl acrylate, phenylbenzyl(meth)acrylate, phenylmethacrylate, dicyclopentenyl(meth)acrylate, dicyclopentenyloxyethyl(meth)acrylate, and dicyclopentanyl methacrylate.
Examples of polyfunctional ethylenically unsaturated monomers include, but not limited to, difunctional (meth)acrylates such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, bisphenol di(meth)acrylate, and 1,4-cyclohexanedimethanol di(meth)acrylate.
The thickener is polyisocyanate. Polyisocyanates are organic compounds with two or more isocyanate groups (—NCO) per molecule. Suitable examples of polyisocyanates are diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate and hexamethylene diisocyanate.
The polymerization initiator includes organic peroxides that are commonly used as curing agents for vinyl ester resins and unsaturated polyester resins. The organic peroxides include, for example, ketone peroxides, hydroperoxides, diacyl peroxides, dialkyl peroxides, peroxyketals, alkyl per-esters, and percarbonates.
The polymerization inhibitor can be appropriately selected from various compounds generally known as polymerization inhibitors. Preferred examples include catechol, hydroquinone and benzoquinone.
Another suitable example of the non-varnish liquid thermosetting resin composition is an epoxy resin-based composition, in which an epoxy resin and an epoxy curing agent are blended, and if necessary, a thickener is further blended.
The types of the epoxy resins are not limited. Various types of epoxy resins including bisphenol-type epoxy resins, naphthalene-type epoxy resins, biphenyl-type epoxy resins, novolac-type epoxy resins, glycidylamine-type epoxy resins, epoxy resins having an oxazolidone ring structure, alicyclic epoxy resins, and aliphatic epoxy resins can be used.
In a preferred example, a bisphenol type epoxy resin such as a bisphenol A type epoxy resin or a bisphenol F type epoxy resin is used. Among commercially available bisphenol type liquid epoxy resins, there are varieties that have a low viscosity of 5 Pa·s or less at 25° C. 50 wt % or more. 60 wt % or more, 65 wt % or more, 70 wt % or 75 wt % or more of the total epoxy resin to be used may be a bisphenol type epoxy resin.
As the epoxy curing agent, it is preferable to use a latent curing agent. The latent curing agent is a solid with low solubility in epoxy resin at room temperature, but when heated, it melts or dissolves in the epoxy resin to express its function as curing agent.
Various imidazoles, dicyandiamide, and boron trifluoride-amine complexes are typical examples of latent curing agents.
Imidazoles are compounds having an imidazole ring. Substituted imidazoles, in which the hydrogen atom of imidazole is substituted with a substituent, as well as imidazolium salts and imidazole complexes, are included in imidazoles.
Suitable examples of substituted imidazoles that act as latent curing agents include substituted imidazoles having an aromatic ring which may be a heteroaromatic ring in the molecule, such as 2,4-diamino-6[2′-methylimidazolyl-(1)]-ethyl-s-triazine, 2-phenyl-4-methylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4-benzyl-5-hydroxymethylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-p-toluyl-4-methyl-5-hydroxymethylimidazole, 2-paratoluyl-4,5-dihydroxymethylimidazole, 2-methatolyl-4-methyl-5-hydroxymethylimidazole, 2-methato|yl-4,5-dihydroxymethylimidazole and 1-cyanoethyl-2-phenylimidazole.
Also imidazolium salts such as 1-cyanoethyl-2-ethyl-4-methylimidazolium trimellitate, 1-cyanoethyl-2-undecylimidazolium trimellitate and 1-cyanoethyl-2-phenylimidazolium trimellitate are suitable examples of imidazole-based latent curing agents.
Isocyanuric acid adducts of various substituted imidazoles such as 2-phenylimidazole, 2-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole and 2-phenyl-4-methyl-5-hydroxymethylimidazole, especially isocyanuric acid adducts of substituted imidazoles having a triazine ring such as 2,4-diamino-6-(2′-methylimidazolyl-(1′))-ethyl-s-triazine, 1-(4,6-diamino-s-triazin-2-yl)ethyl-2-undecylimidazole and 2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolypethyl]-s-triazine are particularly preferred examples of imidazole-based latent curing agents.
Amine adducts are also a preferred example of latent curing agents. Amine adducts are obtained by reacting imidazole and/or tertiary amine with epoxy resin and/or isocyanate to increase the molecular weight, and have relatively low solubility in epoxy resin.
The latent curing agents may be used alone or in combination of two or more. When dicyandiamide is used as the latent curing agent, urea derivatives such as 4,4′-methylenebis(phenyldimethylurea) and 2,4-bis(3,3-dimethylureido)toluene are preferably used as curing accelerators.
In addition to or instead of the latent curing agents, epoxy curing agents other than latent curing agents, such as carboxylic acid anhydrides, aromatic amines and phenolic resins, can also be used.
Among the carboxylic anhydrides, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride and methyl-5-norbornene-2,3-dicarboxylic anhydride (methyl-3,6-endomethylene-1,2,3, 6-tetrahydrophthalic anhydride) have a viscosity of less than 0.5 Pa·s at 25° C., and thus may be used for the purpose of lowering the viscosity of the composition.
The carboxylic anhydride is known to react with epoxy compounds at low temperatures to form bonds with them through the catalytic action of tertiary amines, which may be glycidylamine. Therefore, when 20 weight parts or less of the carboxylic anhydride with respect to 100 weight parts of the epoxy compound is used with tertiary amine, the carboxylic acid anhydride acts as a thickening agent.
Amine compounds also act as thickening agents when used in amounts of 0.1 to 0.5 equivalents of active hydrogen per epoxy group. Examples of amine compounds that may preferably be used as thickener include, but are not limited to, isophoronediamine, bis(4-aminocyclohexyl)methane and 1,3-bis (aminomethyl)cyclohexane.
A polyisocyanate that may be a diisocyanate, particularly a diisocyanate having an aromatic ring in its molecular structure, such as bis(4-isocyanatophenyl)methane and toluenediisocyanate, is a suitable example of a thickener.
The polyisocyanate is preferably used with a polyol to exhibit a higher thickening effect. Examples of polyols include, but are not limited to ethylene glycol, polyethylene glycol, isosorbide, neopentyl glycol, cyclohexanediol, cyclohexanedimethanol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol and 1,6-hexanediol.
The following is an explanation of the flame retardant that can be added to the liquid thermosetting resin composition.
Preferred flame retardants include phosphorus-containing flame retardants.
Examples of phosphorus-containing flame retardants include non-halogen phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate and aromatic polyphosphates.
Other examples of phosphorus-containing flame retardants include halogenated phosphoric acid esters such as tris(chloroethyl)phosphate, tris(dichloropropyl)phosphate, tris(chloropropyl)phosphate, bis(2,3-dibromopropyl)2,3-dichloropropylphosphate, tris(2,3-dibromopropyl)phosphate, bis(chloropropyl)octylphosphate, halogenated alkylpolyphosphate and halogenated alkylpolyphosphonate.
A further example of a phosphorus-containing flame retardant is a phosphinic acid metal salt. The phosphinic acid metal salts here include not only metal salts of phosphinic acids having no 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 acid such as methane-di(methylphosphinic acid) and benzene-1,4-di(methylphosphinic acid).
Examples of the 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.
An example of an alkylphenylphosphinic acid is methylphenylphosphinic acid. Phosphinic acid metal salts can be, but are not limited to, aluminum phosphinate, zinc phosphinate, calcium phosphinate, and magnesium phosphinate.
Further examples of phosphorus-containing flame retardants include red phosphorus, ammonium polyphosphate, melamine phosphate, guanidine phosphate, and guanylurea phosphate.
In addition to the phosphorus-containing flame retardant, a phosphorus-free flame retardant can be added to the liquid thermosetting resin composition. Examples of phosphorus-free flame retardants include melamine compounds such as melamine cyanurate, triazine compounds, guanidine compounds, nitrogen-based flame retardants such as ammonium phosphate and ammonium carbonate, hydrated metals such as aluminum hydroxide and magnesium hydroxide, and organometallic salt flame retardants such as ferrocene and acetylacetone metal complexes.
In a preferred example, halogen-free materials are selected for all materials, including flame retardants, to be added to the liquid thermosetting resin composition. Thereby, a halogen-free flame retardant SACFB prepreg is obtained.
An example of a prepreg that can be produced using the SACFB is a sheet prepreg. In a suitable example, a sheet prepreg can be produced through the following first to fourth steps.
First step: A step of applying a liquid thermosetting resin composition on respective surfaces of a first protective film and a second protective film.
Second step: A step of depositing a plurality of short carbon fiber bundles including the SACFB on the surface of the first protective film on which the liquid thermosetting resin composition has been applied to form a carbon fiber mat.
Third step: A step of laminating the second protective film to the first protective film so that the surfaces coated with the liquid thermosetting resin composition face each other, with the carbon fiber mat in between, to form a laminate.
Fourth step: A step of pressurizing the laminate to impregnate the carbon fiber mat with the liquid thermosetting resin composition to obtain a sheet prepreg.
The first protective film and the second protective film are synthetic resin films, and the material can be appropriately selected from polyolefins such as polyethylene and polypropylene, polyvinylidene chloride, vinyl chloride resin, polyamide, and the like. The first protective film and the second protective film may be multilayer films. The specifications of the first protective film and the second protective film may be the same or different.
In the second step, a carbon fiber mat is formed on the surface of the first protective film coated with the liquid thermosetting resin composition by depositing a plurality of short carbon fiber bundles including the SACFB, for example, by spraying.
The amount of the liquid thermosetting resin composition applied to the first protective film and the second protective film in the first step and the basis weight of the carbon fiber mat formed on the first protective film in the second step are adjusted considering the basis weight and fiber content of the sheet prepreg to be produced.
When the liquid thermosetting resin composition has been added with a thickener, the prepreg is aged after the fourth step until the viscosity of the liquid thermosetting resin composition becomes sufficiently high.
In the above procedure, the first protective film and the second protective film may be carrier films unwound from a roll.
In a suitable example, a long sheet prepreg can be continuously produced using the sheet prepreg producing apparatus shown conceptually in
The sheet prepreg producing apparatus shown in
The basis weight of the sheet prepreg using the SACFB can be appropriately designed depending on the purpose. The basis weight may be, for example, 300 g/m2 or more and less than 500 g/m2, 500 g/m2 or more and less than 1000 g/m2, 1000 g/m2 or more and less than 2000 g/m2, 2000 g/m2 or more and less than 4000 g/m2, 4000 g/m2 or more and less than 6000 g/m2, 6000 g/m2 or more and less than 8000 g/m2, or 8000 g/m2 or more and less than 10000 g/m2.
The thickness of the sheet prepreg can be designed to be, for example, 0.5 mm or more and less than 1.5 mm, 1.5 mm or more and less than 3 mm, or 3 mm or more and 5 mm or less, but is not limited thereto.
As a molding method when producing CFRP products from prepregs using the SACFB, although without limitation, a press molding method can be preferably used. For example, other molding methods, such as autoclave molding, can also be used.
The following are the results of experiments conducted by the present inventors.
A brand new continuous carbon fiber bundle made of carbon fibers with a filament diameter of 7 having a bundle size of 15 K, and containing 1 wt % of a sizing agent was cut with a rotary cutter to obtain chopped carbon fiber bundles having a fiber length of 25 mm (about 1 inch).
1000 g of the chopped carbon fiber bundles were placed in a mixing chamber of an agitation mixer “SP Granulator” (SPG-25T, manufactured by Dalton Co., Ltd.) and agitated without adding liquid to obtain carbon fiber fluff.
Without taking out the carbon fiber fluff, water was added to the mixing chamber of the agitation mixer so that the weight ratio of the carbon fiber fluff and water was 100:40, and the mixture was agitated for 7 minutes. The peripheral speed at the tip of the stirring blade was 4 m/s for the first 1 minute, 8 m/s for the next 3 minutes, and 4 m/s for the next 3 minutes.
After the completion of the agitation, the mixture was observed and it was found that short carbon fibers were spontaneously bundled. The mixture was dried at 110° C. for 30 minutes using a vibrating hot-air dryer to obtain SACFBs.
Since no organic binder was added to the water used as the bundling liquid, the organic binder contained in this SACFB was only the sizing agent contained in the raw material continuous carbon fiber bundles. Since the entire amount of this sizing agent is considered to remain, the organic binder content of the obtained SACFB is 1 wt %.
A photograph of the appearance of the SACFB produced in Experiment 1 is shown in
When three hundred SACFBs out of the obtained SACFBs were sampled and examined, about 80% in terms of number had a bundle size of less than 15K. The bundle size distribution of the SACFBs was wide, and when the frequency distribution of the bundle size was expressed with a class width of 1K, there was no class having the frequency of 10% or more.
When fifty out of the above three hundred SACFBs were examined, the majority of them had a bundle length in the range of 26 mm to 35 mm, that is, the length of the carbon fiber (25 mm) plus 1 mm to 1.4 times the length of the carbon fiber.
When the prepared SACFB was soaked in a liquid epoxy resin (manufactured by Mitsubishi Chemical, JER® 807) and left at room temperature for 3 days, the bundle loosened and more than doubled in width (long diameter of the cross section) as the resin penetrated between the filaments.
SACFBs were prepared in the same manner as in Experiment 1, except that a water dispersion containing a sizing agent for carbon fiber at a concentration of 2.5 wt % was used as the bundling liquid. Epoxy resin was contained in the sizing agent as the main component.
When combined with the sizing agent contained in the raw material continuous carbon fiber bundle, the organic binder content of the resulting SACFB is 2 wt %.
The appearance of the SACFB obtained in Experiment 2 was similar to that obtained in Experiment 1. The bundle size distribution of the SACFBs was narrower than that of Experiment 1. When sampling and examination were performed in the same way as Experiment 1, 90% in terms of number of the SACFBs had a bundle size of less than 8K, and the majority had a bundle size of 1.5K or more and less than 4.5K.
Furthermore, when fifty out of the sampled three hundred SACFBs were examined, 60% or more in terms of number had bundle sizes in the range of 26 mm to 35 mm.
A sheet prepreg was produced by impregnating the SACFBs produced in
Experiment 2 with a liquid thermosetting resin composition containing a vinyl ester resin, an unsaturated polyester resin, styrene, a polyisocyanate, and a radical polymerization initiator.
The procedure was as follows.
Three polyethylene films of 40 cm×40 cm (polyethylene films A to C) were provided, and two of them (polyethylene films A and B) were each coated with the paste made of the above liquid thermosetting resin composition in a 30 cm×30 cm area on one side thereof.
Next, the SACFBs produced in Experiment 2 were sprinkled to deposit a carbon fiber mat on one polyethylene film (polyethylene film C) to which no paste was applied.
Next, one (polyethylene film A) of the polyethylene films coated with the above paste on one side was placed over the carbon fiber mat, with the side coated with the paste facing down.
Next, the stack were turned over upside down, such that the polyethylene film C, which was not coated with the paste, was placed on top, and the polyethylene film A was placed on the bottom. In this state, another polyethylene film B coated with the paste on one side was replaced with the polyethylene film C. By doing so, a laminate was obtained in which the two polyethylene films A and B coated with the paste were laminated together so that the sides coated with the paste faced each other with the carbon fiber mat sandwiched therebetween.
Subsequently, this laminate was pressed from both sides in the thickness direction to impregnate the carbon fiber mat with the paste.
After that, it was left at 25° C. for 7 days to thicken the paste.
The obtained sheet prepreg had a basis weight of 2000 g/m2 and a fiber content of 53 wt %.
Two 26 cm×26 cm prepreg pieces cut out from the produced sheet prepregs were stacked and subjected to press-molding under the conditions of a temperature of 140° C., a pressure of 8 MPa, and a pressing time of 2 minutes to produce a CFRP plate having a length and width of 30 cm and a thickness of 2 mm.
A test piece having a width of 8 mm and a length of 60 mm was cut out from this CFRP plate, and flexural tests were conducted using a universal testing machine (Instron 4465, manufactured by Instron) with a crosshead speed of 2 mm/min and a span of 32 mm (16 times the thickness of the CFRP plate). The average of the measurement results of six test pieces showed a flexural strength of 360 MPa and a flexural modulus of 24 GPa.
A sheet prepreg having a basis weight of 2000 g/m2 and a CF content of 53 wt % was produced in the same manner as in Experiment 3, except that the same chopped carbon fiber bundles as those produced in Experiment 1, that were chopped carbon fiber bundles having a fiber length of 25 mm (about 1 inch) obtained by cutting continuous carbon fiber bundles having a bundle size of 15K with a rotary cutter, were used as they were instead of SACFBs.
A CFRP plate was produced from this sheet prepreg in the same manner as in Experiment 3, and the mechanical properties of the CFRP plate were evaluated. The result were that flexural strength was 251 MPa and flexural modulus was 23 GPa.
Embodiments of the present invention include the following. However, they are not limited to these.
Although the present invention has been described in detail using specific embodiments, it is clear to those skilled in the art that various changes can be made within the scope of achieving the effects of the invention.
The present application is based on Japanese Patent Application No. 2021-174789 filed on Oct. 26, 2021, Japanese Patent Application No. 2021-101844 filed on Jun. 18, 2021, Japanese Patent Application No. 2021-101845 filed on Jun. 18, 2021 and Japanese Patent Application No. 2021-101846 filed on Jun. 18, 2021, the entire contents of which are incorporated herein by reference.
Prepregs using the SACFB can be preferably used when producing various CFRP products for automobiles, motorcycles, bicycles, ships, railroad cars, manned aircraft, unmanned aircraft and other transportation equipment, sporting goods, leisure goods, home appliances, agricultural equipment, construction materials, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-101844 | Jun 2021 | JP | national |
| 2021-101845 | Jun 2021 | JP | national |
| 2021-101846 | Jun 2021 | JP | national |
| 2021-174789 | Oct 2021 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/024336 | Jun 2022 | US |
| Child | 18528651 | US |
