JP3106261B2 - Sizing agent for carbon fiber and carbon fiber treated with the sizing agent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sizing agent for an epoxy resin-containing carbon fiber which can be formed into a stable and uniform aqueous dispersion, and a carbon fiber which is treated with the sizing agent and which is excellent in formability and physical properties. .

[0002]

2. Description of the Related Art In general, epoxy resins are widely used as carbon fiber sizing agents in carbon fiber reinforced composite materials (hereinafter referred to as CFRP) because epoxy resins are widely used as matrix resins.
Since this epoxy resin is generally hydrophobic, it has been difficult to stably and uniformly disperse it in water. For this reason, at the beginning of the industrialization of the carbon fiber production technology, an organic solvent solution such as methyl ethyl ketone was used as it was as a sizing agent. However, since organic solvents have a high risk of fire and are toxic, there has been a strong demand for providing an aqueous epoxy resin dispersion from the viewpoint of safety and health.

As a method of dispersing an epoxy resin in water, Japanese Patent Application Laid-Open No. 57-171767 discloses a method in which a composition comprising a diglycidyl ether / bisphenol A type epoxy resin which is solid at room temperature and another epoxy resin is added to water. A way to do that has been proposed. However, since the epoxy resin composition is prepared to have a viscosity at 50 ° C. of a low viscosity in the range of 10 to 1000 poise, the carbon fiber treated with the epoxy resin composition is subjected to a winding and unwinding step and a forming step by filament winding. When subjected to strong abrasion, the fibers are liable to be opened and fluff and yarn breakage are likely to occur, and the physical properties are impaired. Further, when the viscosity of the epoxy resin composition is increased, the stability of the aqueous dispersion is deteriorated as the viscosity increases, and there is a disadvantage that it is difficult to uniformly attach the dispersion to carbon fibers.

Japanese Patent Application Laid-Open No. 58-137981 proposes an aqueous dispersion containing a quaternary ammonium base-containing polyurethane resin and an epoxy resin as a sizing agent for carbon fibers. However, since the carbon fibers treated with the aqueous dispersion become harder, a more flexible sizing agent has been demanded for fabric forming. US Patent No. 4,47
No. 4,906 discloses a high molecular weight completely thermoplastic polyurethane resin as a sizing agent for carbon fibers. However, since this sizing agent is used after being dissolved in an organic solvent, there is a problem in that the working environment is deteriorated. Further, the sizing agent has poor compatibility with a thermosetting matrix resin such as an epoxy resin, and further has no group reactive with the matrix resin, so that the mechanical properties of CFRP are sufficiently improved. No carbon fiber could be obtained.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a sizing agent for carbon fiber which has no safety and health problems.
An object of the present invention is to provide an epoxy resin sizing agent that does not harden carbon fibers and can be used as a stable and uniform aqueous dispersion. Another object of the present invention is to prevent the fibers from being opened to cause fluff and yarn breakage even when subjected to strong abrasion in the winding or unwinding process, and to have excellent formability and properties that can be formed by filament winding. It is to provide an excellent carbon fiber.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a composition having a viscosity at 50 ° C. of more than 1,000 poise,
Achieved by a sizing agent for carbon fibers comprising an epoxy resin having a molecular weight of 000 poise or less, a urethane compound having a hydroxyl group obtained from a polyol having an oxyalkylene unit and a polyisocyanate, and carbon fibers treated with the sizing agent. be able to.

In the present invention, the viscosity of the epoxy resin is
Brookfield viscometer (BH type, rotor number N)
o. 4 to 7, the number of rotations is 2 rpm). Examples of the type of epoxy resin that can be used in the present invention include, for example, "Handbook of Epoxy Resins: Henry Lee et al., Published by McGraw-Hill Book Company, 196
7th edition, pp. 1-1 to 3-20 ". Specifically, an aromatic ring-containing epoxy resin,
Alicyclic group-containing epoxy resin, aliphatic ether epoxy resin, ester epoxy resin, epoxidized soybean oil, etc.
The viscosity at 50 ° C. exceeds 1,000 poise,
Epoxy resins of 000 poise or less can be mentioned. These may be used in combination of two or more.

The aromatic ring-containing epoxy resin includes glycidyl etherified phenols and aromatic glycidylamine epoxy resins. Examples of glycidyl etherified phenols include bisphenol A type epoxy resin, bisphenol AD type epoxy resin, halogenated bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenol novolak type epoxy resin, cresol novolak type epoxy resin, halogenated There is a phenol novolak type epoxy resin and the like. Examples of the aromatic glycidylamine epoxy resin include aniline, diaminodiphenylmethane, aminophenols (o-, m-, p-aminophenol, 2-amino-p-cresol, 6-amino-p-cresol, etc.). , O-, m-, p-xylylenediamine, halogenated aniline (o-, m-, p-chloroaniline, o-,
polyglycidyl ethers such as m-, p-bromoaniline, o-, m-, p-iodoaniline) and condensates of aromatic amines such as bisaminomethylcyclohexane with epichlorohydrin.

The alicyclic group-containing epoxy resin includes bis (3,4-epoxy-6-methylcyclohexyl) adipate, 3,4-epoxycyclohexylmethyl-
3,4-epoxycyclohexanecarboxylate, 2
-(3,4-epoxy) cyclohexyl-5,2-spiro (3,4-epoxy) cyclohexyl-m-dioxane, and the hydrogenated nuclei of glycidyl etherified phenols.

Examples of the aliphatic ether epoxy resin include a condensate of an aliphatic polyol such as an aliphatic polyhydric alcohol and a polyether polyol with epichlorohydrin. As the ester-based epoxy resin, there is a copolymer of glycidyl (meth) acrylate and an ethylenically unsaturated monomer such as acrylonitrile, hydroxy (meth) acrylate, N, N-dimethylaminoethyl (meth) acrylate. .

Of these, epoxy resins containing an aromatic ring such as glycidyl etherified phenols and aromatic glycidylamine epoxy resins are preferred, and glycidyl etherified phenols are more preferred. Is good. The urethane compound used in the present invention is derived from a urethane-forming reaction between a polyol having an oxyalkylene unit and a polyisocyanate.

The polyol includes, for example, an alkylene oxide adduct of a compound having at least two active hydrogen groups. This polyol may be either a polyether polyol or a polyester polyol, and has an average molecular weight of 500 to 50,000.
00, preferably 500 to 30,000, and more preferably 500 to 10,000.

The compounds having at least two active hydrogen groups include aliphatic polyhydric alcohols, polyhydric phenols, polyamines, and polycarboxylic acids. Among them, aliphatic polyhydric alcohols and polyamines, particularly, aliphatic polyhydric alcohols are preferably used.
Examples of the aliphatic polyhydric alcohols include dihydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, and 1,3-butanediol, and trihydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, and souce. There are more alcohols.

Examples of the polyhydric phenols include bisphenol A, pyrogallol, hydroquinone, and condensates of phenol and formaldehyde described in Japanese Patent No. 3,265,641. Examples of the polyamines include alkanolamines such as triethanolamine, N-methyldiethanolamine, and monoethanolamine; aliphatic polyamines such as ethylenediamine, diethylenetriamine, and triethylenetetramine; tolylenediamine, methylenedianiline, and polymethylenepolyphenylamine. Aromatic polyamines.

Examples of the above polycarboxylic acids include aliphatic polycarboxylic acids such as succinic acid, adipic acid, sebacic acid, maleic acid and dimer acid, and aromatic polycarboxylic acids such as phthalic acid, terephthalic acid and trimellitic acid. is there.
The oxyalkylene unit has 2 to 2 carbon atoms.
4, oxyethylene units (EO), oxypropylene units (PO), and oxybutylene units (BO). Two or more of these oxyalkylene units can be used in combination. In this case, the oxyalkylene unit may be random or block. Preferably, oxyethylene units (EO) and oxypropylene units (PO), more preferably oxyethylene units (EO), are added to the total amount (by weight) of oxyethylene units (EO) and oxypropylene units (PO). Is preferred.

As the polyisocyanate forming the urethane compound of the present invention, there are polyisocyanates having 4 to 100 carbon atoms, such as aromatic polyisocyanates, aliphatic polyisocyanates and alicyclic polyisocyanates. These can be used in combination of two or more. Of these, aromatic polyisocyanates are preferably used.

The aromatic polyisocyanates include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylylene diisocyanate,
Tetramethylxylylene diisocyanate, diphenylmethane-4,4'-diisocyanate (MDI), phosgenated crude diaminophenylmethane, that is, a condensation reaction product of formaldehyde with an aromatic amine such as aniline (diaminophenylmethane) and a small amount ( 5-20
Wt%) of a phosgenate of a mixture of triamine or higher polyamines, 1,3-bis (phenylmethyl) benzene-4,
4 ', 4''-triisocyanate, naphthylene diisocyanate and the like.

The aliphatic polyisocyanates include ethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 1,6,11-undecane triisocyanate, 2,2,4-trimethylhexane diisocyanate, lysine diisocyanate, and the like. 2,6-
Examples include diisocyanatomethylcaproate, bis (2-isocyanatoethyl) fumarate, bis (2-isocyanatoethyl) carbonate, and 2-isocyanatoethyl-2,6-diisocyanatohexanoate.

Examples of the alicyclic polyisocyanates include isophorone diisocyanate, cyclohexane-1,4-diisocyanate, and 1,4-methylenebis (cyclohexyl isocyanate). The urethane compound used in the present invention has an equivalent ratio of polyol to polyisocyanate (OH / NCO ratio) of 3: 1.
The reaction is carried out in a range of from 1: 1, preferably 3: 1.06 to 1: 1, and more preferably from 2.2: 1 to 1.8: 1. By setting the equivalent ratio to 1 or more, a urethane compound containing substantially no free isocyanate group and containing a hydroxyl group can be obtained. By using such a urethane compound, the ethoxy resin can be stably dispersed in water.

The above urethane compound can be synthesized according to a known method. Usually, a polyol having an oxyalkylene unit and a polyisocyanate are used in an amount of 40 to 1
The reaction is carried out at a temperature of 50 ° C, preferably 60 to 100 ° C. When a polyol containing a nitrogen atom is used, the reaction temperature is preferably 80 ° C. or lower, more preferably 0 to 70 ° C.

The above reaction may be carried out in the presence or absence of an organic solvent. As an organic solvent,
Ketones such as acetone, methyl ethyl ketone and isobutyl ketone; esters such as ethyl acetate and butyl acetate; ethers such as dioxane and tetrahydrofuran; aromatic hydrocarbons such as toluene and xylene; amides such as dimethylformamide and dimethylacetamide; Examples thereof include sulfoxides such as dimethyl sulfoxide and a mixed solvent of two or more of these. Of these, acetone, methyl ethyl ketone, ethyl acetate,
It is preferable to use butyl acetate, tetrahydrofuran, toluene, xylene and a mixed solvent of two or more thereof.

The urethane compound can be synthesized in the presence or absence of a catalyst. As a catalyst,
For example, there are amines such as triethylamine, N-ethylmorpholine and triethylenediamine, and tin compounds such as dibutyltin dilaurate and dioctyltin dilaurate. The obtained urethane compound has a high water dispersing ability, and thus has a function of easily and stably dispersing a high-viscosity epoxy resin in water.

The aqueous dispersion of the sizing agent for carbon fibers of the present invention can be obtained by dispersing the above-mentioned epoxy resin and urethane compound in water. Also, before or after dispersing in water, add a water-soluble organic solvent,
If desired, a stable dispersion can be obtained by heat emulsification. Examples of water-soluble organic solvents include alcohols such as methanol, ethanol, isopropanol, ethylene glycol, diethylene glycol, and glycerin; ethers such as tetrahydrofuran, dioxane, ethylene glycol dimethyl ether and ethylene glycol diethyl ether; ketones such as acetone and methyl ethyl ketone. There are esters such as methyl acetate and ethyl acetate.

The ratio of the urethane compound to the epoxy resin in the sizing agent of the present invention is 1 to 100% by weight, preferably 10 to 40% by weight. If it is less than 1% by weight, a sufficiently stable aqueous dispersion cannot be obtained. 100% by weight
If the ratio exceeds the above range, the proportion of the epoxy resin decreases and the performance as a sizing agent decreases. The sizing agent of the present invention has a solid content of 30 to 70% by weight from the viewpoint of economy and ease of dispersion.
It is desirable that the aqueous dispersion be in the range of This can be used by appropriately diluting it with water. The amount of the sizing agent applied to the carbon fiber is usually 0.1 to 10% by weight, preferably 0.3 to 5% by weight in terms of solid content.
0% by weight. If the content is less than 0.1% by weight, the convergence effect is reduced and fluff is likely to occur. On the other hand, when the content is 10% by weight or more, the yarn becomes too hard, and the composite characteristics are deteriorated.

As a method for applying the sizing agent to the carbon fiber, an impregnation method using a dip roller, a kiss roll method by contact with a rotating roller, a spray method, and the like can be applied. The carbon fiber provided with the sizing agent of the present invention is dried and heat-treated at a temperature of 100 to 250C. 10
If the temperature is lower than 0 ° C., the evaporation rate of water decreases, the drying time becomes longer, and a long dryer is required, which is economically disadvantageous.
Conversely, if the temperature exceeds 250 ° C., the sizing agent is undesirably deteriorated by heat. Suitable drying methods include a hot air system, an infrared system, and a hot roller contact system.

According to the present invention, a high-viscosity epoxy resin can be stably dispersed in water by the urethane compound.
A sizing agent suitable for a carbon fiber manufacturing process can be obtained. In addition, the resin component contained in the sizing agent has excellent adhesiveness to carbon fiber, so that the treated carbon fiber opens even if it is subjected to strong rubbing in the winding or unwinding process, resulting in fluff or yarn breakage. Is unlikely to occur. That is, it is excellent in unwinding property and scratch resistance. Further, the carbon fiber treated with the sizing agent of the present invention is flexible and has excellent moldability and physical properties, so that it can be molded by filament winding.

[0027]

【Example】

Example 1 A mixture of Epikote 828 and Epikote 1001 manufactured by Yuka Shell Epoxy Co., Ltd.
40 parts of epoxy resin [A-1] to [A-3] and a PO-EO-block adduct of propylene glycol (average molecular weight = 8000, PO: EO (Molar ratio = 1: 5) 10 parts of a urethane compound [B-1] obtained from 2 moles and 1 mole of tolylene diisocyanate (TDI) are charged into a high-viscosity emulsifying apparatus, and heated to 60 to 90 ° C. to be uniform. After mixing
10 parts of water was added, mixed well, and emulsified for phase inversion. After the phase inversion, 40 parts of water was gradually added to obtain uniform white emulsion sizing agents [1], [2] and [3].

The aqueous dispersion stability of these sizing agents was evaluated by the following method, and the results are shown in Table 1. Water dispersion stability : sizing agent 25 having a resin concentration of 5% by weight
The cc is placed in a 50 cc glass centrifuge tube, and centrifuged at 4,000 rpm for 10 minutes using a centrifuge. After the separation, the supernatant is removed by a gradient method, and the precipitate at the bottom of the centrifuge tube is dissolved with methyl ethyl ketone. This is transferred to a petri dish, and after evaporation to dryness, the weight is measured and indicated by weight. There is no problem if the aqueous dispersion stability of the sizing agent is 50 mg or less. Example 2 Urethane in which the molar ratio of the epoxy resin [A-2] of Example 1 to the PO-EO-block adduct of propylene glycol and TDI was changed in the range of 3: 1 to 1: 1 as shown in Table 1. Sizing agents [4] and [5] were obtained in the same manner as in Example 1 except that compounds [B-2] to [B-3] were used. Table 1 shows the aqueous dispersion stability of these sizing agents.

Example 3 The procedure of Example 1 was repeated except that the weight ratio of the epoxy resin [A-2] to the urethane compound [B-1] was changed from 100: 1 to 50:50. Sizing agents [6], [7] and [8] were obtained. Table 1 shows the aqueous dispersion stability of these sizing agents. Examples 4, 5, 6 TDI was converted to MDI and hexamethylene diisocyanate (H
DI) and isophorone diisocyanate (IPDI), respectively, except that urethane compounds [C-1], [C-2] and [C-
3], and using these three types of urethane compounds, a sizing agent was prepared in the same manner as in Example 1.

[9], [1
0] and [11]. The water dispersion stability of these three sizing agents was evaluated, and the results are shown in Table 1.

[0030]

Examples 7, 8, 9 The urethane compounds [D-1] and [D] were prepared in the same manner as in Example 1 except that polyols having the average molecular weight and the EO / PO molar ratio shown in Table 2 were used. -2] and [D
-3] and sizing agents [12] and [1] using these three types of urethane compounds in the same manner as in Example 1.
3] and [14] were produced. The water dispersion stability of these three sizing agents was evaluated, and the results are shown in Table 1.

[0032]

Examples 10 and 11 The viscosity at 50 ° C. was changed to 20,000 poise and 100 by changing the mixing ratio of “Epicoat” 828 and “Epicoat” 1001 manufactured by Yuka Shell Epoxy.
Epoxy resin [A-4] and [A-
5] was produced. Using these two types of epoxy resins and the urethane compound [B-1] of Example 1, Example 1
Sizing agents [15] and [16] were prepared in the same manner as described above. The water dispersion stability of these two sizing agents was evaluated, and the results are shown in Table 1.

Example 12 The sizing agents [1] to [8] obtained in Examples 1 to 3 were diluted with water to prepare aqueous dispersions <1> to <8> having a concentration of 3% (sizing agents). And the number of the aqueous dispersion correspond to each other). In these aqueous dispersions, carbon fiber "Treca" (trademark) T300-12K (trade number: 12) manufactured by Toray Industries, Inc.
2,000 yarns) was impregnated with the yarn bundle, dried in hot air at 180 ° C. for 2 minutes, and wound around a bobbin. For the carbon fiber wound on the bobbin, the amount of the sizing agent adhered, and the unwinding property, abrasion resistance, yarn hardness,
The composite physical properties were evaluated, and the results are shown in Table 3.

Unwinding property: 50 carbon fibers wound on a bobbin
Rolling test unwinding length at m / min speed 10
^It is displayed as the number of yarn breaks per ⁵ m. Number of thread breaks is 1
It is preferably 0 times or less.

Scratch resistance : Five stainless steel rods each having a smooth surface and a diameter of 10 mm were arranged in a zigzag manner so that the carbon fiber yarns were passed in parallel at 50 mm intervals while being in contact with each other at an angle of 120 °. This device has a
00D-A carbon fiber yarn having 12,000 filaments is passed at a speed of 3 m / min while applying an initial tension of 300 g, and a laser beam is irradiated from a direction perpendicular to the fiber yarn. The number of fluffs generated is counted from the number of times the laser beam is shielded, and the number is expressed in pieces / m. 50 fluff
Pcs / m or less.

Hardness of yarn : Torque developed when carbon fiber having a test length of 10 cm and 12,000 filaments having a number of filaments of 10 is twisted 10 times is 20 cm in length and 0.3 mm in thickness.
To a stainless steel wire. It is displayed with the twist angle of the wire. Preferably, the twist angle is 30 ° or less.

Composite physical properties : Carbon fibers were aligned in one direction and placed in a mold, which was then coated with Epicoat-828 (10
0) / BF ₃ MEA (3 parts), add the blended resin, and impregnate in vacuum. At this time, the amount of the carbon fiber is adjusted so that the volume content of the fiber becomes 60%. After impregnation, 150 ℃
Cure for 1 hour under pressure, take out from mold, and post cure at 140 ° C for 4 hours.

For a test piece having a thickness of 2.5 mm and a width of 6.0 mm, ASTM D-3039 was used.
The tensile strength was measured according to -72-T, and ASTM D-
The interlaminar shear strength (ILSS) was measured according to 2344. Comparative Example “Epicoat” manufactured by Yuka Shell Epoxy Co., Ltd. so that the viscosity at 50 ° C. becomes 100 poise and 500 poise.
Sizing agents [BG-1], [BG-1] in the same manner as in Example 1 except that epoxy resins [A-6] and [A-7] were used in which the ratio of 828 to “Epicoat” 1001 was changed.
-2]. The aqueous dispersion stability is shown in Table 1.

The sizing agent [BG-1],
Except that the aqueous dispersions [17] and [18] each having a concentration of [BG-2] diluted to 3% were used (however, the drying temperature was 120 ° C.), the same procedure as in Example 12 was carried out. A company-made carbon fiber "Treca" (trademark) T300-12K is wound around a bobbin, the sizing agent adhesion amount, unwinding property, abrasion resistance, yarn hardness, and composite physical properties are evaluated. It was shown to.

[0041]

Examples 13 to 20 Sizing agents obtained in Examples 4 to 11

[9] to [16] were diluted with water to prepare aqueous dispersions <9> to <16> having a concentration of 3% (the number of the sizing agent and the number of the aqueous dispersion correspond to each other). In these aqueous dispersions, carbon fiber manufactured by Toray Industries, Inc.
The “Treca” yarn bundle was immersed in the yarn bundle, dried in hot air at 180 ° C. for 2 minutes, and wound around a bobbin. The carbon fiber wound around the bobbin was evaluated for the amount of the sizing agent attached, unwinding property, abrasion resistance, yarn hardness, and composite physical properties. The results are shown in Table 3.

Example 21 The aqueous dispersion <2- using the sizing agent [2] of Example 1 and changing its concentration to 0.1 to 20% as shown in Table 4.
1>, <2>, <2-2>, and <2-3> were obtained. Except for using these aqueous dispersions, in the same manner as in Example 12, carbon fiber "Treca" (trade name) T300-12K manufactured by Toray Industries, Inc.
After being treated, it was wound up on a bobbin, and the sizing agent adhesion amount, unwinding property, abrasion resistance, yarn hardness, and composite physical properties were evaluated. The results are shown in Table 4.

[0044]

[0045]

According to the present invention, an epoxy resin can be stably and uniformly dispersed in water by using a hydroxyl group-containing urethane compound obtained from a polyol having an oxyalkylene unit and a polyisocyanate. A high viscosity sizing agent can be obtained.
In addition, this sizing agent forms a low-hardness film on the carbon fiber, thereby preventing the generation of fluff and yarn breakage, and obtaining a carbon fiber that is flexible, has excellent moldability and physical properties, and is moldable by filament winding. Can be.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI // D06M 101: 40 (72) Inventor Moto Ito 1515 Tsutsui, Matsumae-cho, Iyo-gun, Ehime Prefecture Toray Ehime Plant (72) Invention Person Isotani Taniguchi 1 Sanyo Chemical Industry Co., Ltd., 11-11, Hitotsubashi-Honcho, Higashiyama-ku, Kyoto-shi, Kyoto (72) Inventor Hiroshi Hasegawa 1-11-11 Hitotsubashi-Honcho, Higashiyama-ku, Kyoto, Kyoto, Japan (72) ) Inventor: Manabu Saito 11 Sanyo Chemical Industry Co., Ltd., 11-11, Hitotsubashi-Honcho, Higashiyama-ku, Kyoto-shi, Kyoto (56) References JP-A-1-314786 (JP, A) JP-A-62-110984 (JP, A) JP-A-4-41779 (JP, A) JP-A-60-90215 (JP, A) JP-A-4-82969 (JP, A) JP-A-57-171767 (JP, A) (58) (Int.Cl. ^7, D Name) D06M 15/00 - 15/715

Claims (7)

(57) [Claims] 1. A sizing agent for carbon fibers comprising an epoxy resin having a viscosity at 50 ° C. of more than 1,000 poise and not more than 20,000 poise, and a hydroxyl group-containing urethane compound obtained from a polyol having oxyalkylene units and a polyisocyanate. . 2. The carbon fiber according to claim 1, wherein the epoxy resin is a glycidyl etherified product of a phenol, and the urethane compound is a reaction product of an alkylene oxide adduct of an aliphatic polyhydric alcohol and an aromatic polyisocyanate. Sizing agent. 3. The sizing agent for carbon fibers according to claim 1, wherein an equivalent ratio of the polyol to the polyisocyanate is in a range of 1.8 to 2.2. 4. The carbon fiber according to claim 1, wherein the content of the urethane compound is in the range of 1 to 100% by weight based on the weight of the epoxy resin. Sizing agent. 5. The sizing agent for carbon fibers according to claim 1, wherein the sizing agent is dispersed in water. 6. A carbon fiber treated with the sizing agent for carbon fiber according to any one of claims 1 to 5. 7. The carbon fiber according to claim 6, wherein the amount of the sizing agent adhered is in the range of 0.1 to 10% by weight in terms of solid content.