阅读说明：本技术 碳纤维前体用处理剂以及碳纤维前体 (Treatment agent for carbon fiber precursor and carbon fiber precursor ) 是由 前田基树 铃木洋平 大岛启一郎 于 2020-06-29 设计创作，主要内容包括：提供一种提高耐燃化处理工序中的耐燃化纤维的集束性的碳纤维前体用处理剂。碳纤维前体用处理剂含有：(A)包含氨基改性硅酮的平滑剂；(B)选自有机硫酸鏻盐、有机磺酸鏻盐、在分子中具有碳原子数为3以上的烷基的有机硫酸的季铵盐、在分子中具有碳原子数为3以上的烷基的有机磺酸的季铵盐中的至少一种鎓盐；以及(C)非离子表面活性剂。(Provided is a treatment agent for a carbon fiber precursor, which improves the bundling properties of flame-resistant fibers in a flame-resistant treatment step. The treating agent for a carbon fiber precursor contains: (A) a smoothing agent comprising an amino-modified silicone; (B) at least one onium salt selected from the group consisting of organic phosphonium sulfates, organic phosphonium sulfonates, quaternary ammonium salts of organic sulfates having an alkyl group having 3 or more carbon atoms in the molecule, and quaternary ammonium salts of organic sulfonic acids having an alkyl group having 3 or more carbon atoms in the molecule; and (C) a nonionic surfactant.)

1. A carbon fiber precursor treating agent comprising:

a smoothing agent comprising an amino-modified silicone;

at least one onium salt selected from the group consisting of organic phosphonium sulfates, organic phosphonium sulfonates, quaternary ammonium salts of organic sulfates having an alkyl group having 3 or more carbon atoms in the molecule, and quaternary ammonium salts of organic sulfonic acids having an alkyl group having 3 or more carbon atoms in the molecule; and

a nonionic surfactant.

2. The treatment agent for a carbon fiber precursor according to claim 1, wherein the onium salt comprises at least one selected from the group consisting of an organic phosphonium sulfate salt and an organic phosphonium sulfonate salt.

3. The agent for treating a carbon fiber precursor according to claim 2, wherein the onium salt comprises at least one selected from the group consisting of an organic phosphonium sulfate salt in which all substituents bonded to phosphorus atoms in a molecule are alkyl groups having 3 or more carbon atoms and an organic phosphonium sulfonate salt in which all substituents bonded to phosphorus atoms in a molecule are alkyl groups having 3 or more carbon atoms.

4. The agent for treating a carbon fiber precursor according to any one of claims 1 to 3, wherein the smoothing agent comprises an amino-modified silicone and a polyether-modified silicone.

5. The treatment agent for a carbon fiber precursor according to claim 3 or claim 4, wherein the onium salt is contained in a proportion of 0.01 to 20 parts by mass, assuming that the total content of the smoothing agent and the onium salt is 100 parts by mass.

6. The agent for treating a carbon fiber precursor according to any one of claims 1 to 5, wherein the nonionic surfactant contains a substance obtained by adding ethylene oxide to 1 to 20 moles of an aliphatic saturated alcohol having 4 to 20 carbon atoms.

7. A carbon fiber precursor to which the treatment agent for a carbon fiber precursor according to any one of claims 1 to 6 is attached.

Technical Field

The present invention relates to a carbon fiber precursor treating agent and a carbon fiber precursor having the carbon fiber precursor treating agent attached thereto.

Background

Carbon fibers are generally used in various fields such as building materials and transportation equipment as carbon fiber composite materials combined with a matrix resin such as an epoxy resin. Carbon fibers can be produced, for example, as follows: the carbon fiber precursor is produced by performing a spinning step of spinning an acrylic fiber and a drawing step of drawing the fiber, and the carbon fiber precursor is subjected to a flame resistance treatment step and a carbonization treatment step, thereby producing a carbon fiber. In carbon fiber precursors, a carbon fiber precursor treatment agent is sometimes used in order to suppress adhesion or fusion between fibers generated in a carbon fiber production process.

International publication No. 2013-129115 discloses a technique in which a carbon fiber precursor treating agent contains a modified silicone having a modifying group containing a nitrogen atom and an acidic phosphate ester, whereby the solution stability of the treating agent can be improved, the blocking during spinning of the carbon fiber precursor can be suppressed, and the fusion of the carbon fiber during the firing treatment can be suppressed.

Disclosure of Invention

Problems to be solved by the invention

In the conventional treating agent for a carbon fiber precursor, the effect of imparting bundling property in the flame resistance treatment step to the carbon fiber precursor is insufficient.

Therefore, an object of the present invention is to improve the bundling property of flame-resistant fibers in a flame-resistant treatment step.

Means for solving the problems

The treatment agent for a carbon fiber precursor for solving the above problems contains: a smoothing agent; at least one onium salt selected from the group consisting of organic phosphonium sulfates, organic phosphonium sulfonates, quaternary ammonium salts of organic sulfates having an alkyl group having 3 or more carbon atoms in the molecule, and quaternary ammonium salts of organic sulfonic acids having an alkyl group having 3 or more carbon atoms in the molecule; and a nonionic surfactant.

The onium salt preferably contains at least one selected from the group consisting of organic phosphonium sulfates and organic phosphonium sulfonates.

The onium salt preferably contains at least one selected from the group consisting of an organic phosphonium sulfate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms and an organic phosphonium sulfonate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms.

The smoothing agent preferably contains an amino-modified silicone.

The above-mentioned smoothing agent preferably contains an amino-modified silicone and a polyether-modified silicone.

When the total content of the smoothing agent and the onium salt is 100 parts by mass, the onium salt is preferably contained in the carbon fiber precursor treatment agent in a proportion of 0.01 to 20 parts by mass.

The nonionic surfactant preferably contains ethylene oxide added in a proportion of 1 to 20 moles to 1 mole of an aliphatic saturated alcohol having 4 to 20 carbon atoms.

The carbon fiber precursor treating agent for solving the above problems is attached to a carbon fiber precursor.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, the bundling property of the flame-resistant fibers in the flame-resistant treatment step can be improved.

Detailed Description

(embodiment 1)

Next, embodiment 1 of a carbon fiber precursor treating agent (hereinafter simply referred to as a treating agent) embodying the present invention will be described.

The treating agent of the present embodiment contains (a) a smoothing agent, (B) a specific onium salt, and (C) a nonionic surfactant. The components contained in the treating agent of the present embodiment will be described in detail below.

(A) Smoothing agent

Examples of the smoothing agent contained in the treatment agent of the present embodiment include silicone and ester.

The silicone used as the smoothing agent is not particularly limited, and examples thereof include dimethyl silicone, phenyl-modified silicone, amino-modified silicone, amide-modified silicone, polyether-modified silicone, amino polyether-modified silicone, alkyl aralkyl-modified silicone, alkyl polyether-modified silicone, ester-modified silicone, epoxy-modified silicone, carbinol-modified silicone, mercapto-modified silicone, and the like.

The ester used as a smoothing agent is not particularly limited, and examples thereof include: (1) ester compounds of aliphatic monoalcohols and aliphatic monocarboxylic acids such as octyl palmitate, oleyl laurate, oleyl oleate, and isotetradecyl oleate; (2) ester compounds of aliphatic polyhydric alcohols and aliphatic monocarboxylic acids such as 1, 6-hexanediol dicaprate, glycerol trioleate, trimethylolpropane trilaurate and pentaerythritol tetracaprylate; (3) ester compounds of aliphatic monoalcohols and aliphatic polycarboxylic acids such as dioleyl azelate, dioleyl thiodipropionate, diisohexadecyl thiodipropionate and diisostearyl thiodipropionate; (4) ester compounds of aliphatic monocarboxylic acids and aromatic monoalcohols such as benzyl oleate and benzyl laurate; (5) a full ester compound of an aliphatic monocarboxylic acid and an aromatic polyol such as bisphenol a dilaurate or a dilaurate of an alkylene oxide adduct of bisphenol a; (6) full ester compounds of aliphatic monoalcohols such as bis (2-ethylhexyl) phthalate, diisostearyl isophthalate and trioctyl trimellitate with aromatic polycarboxylic acids; (7) natural oils such as coconut oil, rapeseed oil, sunflower seed oil, soybean oil, castor oil, sesame oil, fish oil, and beef tallow; and so on. Further, a known smoothing agent used for a synthetic fiber treatment agent may be used.

The number of the smoothing agents may be 1 or more.

The smoothing agent preferably comprises an amino-modified silicone. The amino equivalent of the amino-modified silicone is preferably 500 to 10000g/mol, for example. When the smoothing agent contains the amino-modified silicone, the strength of the carbon fiber obtained from the carbon fiber precursor to which the treatment agent is added can be improved. In addition, the smoothing agent more preferably contains an amino-modified silicone and a polyether-modified silicone. In this case, the bundling property in the spinning step described later can be improved, and the bundling property in the flame resistance treatment step described later can be further improved.

Kinematic viscosity of smoothing agentThe degree is preferably 10mm, for example at 25 DEG C²/s～100000mm²/s。

(B) Specific onium salts

The specific onium salt contained in the treating agent of the present embodiment is at least one selected from the group consisting of organic phosphonium sulfate salts, organic phosphonium sulfonate salts, quaternary ammonium salts of organic sulfuric acids having an alkyl group having 3 or more carbon atoms in the molecule, and quaternary ammonium salts of organic sulfonic acids having an alkyl group having 3 or more carbon atoms in the molecule.

The number of carbon atoms of the alkyl group is preferably 4 or more, more preferably 5 or more. The number of carbon atoms of the alkyl group is preferably 20 or less. The alkyl group may be either linear or branched, and is preferably linear.

Examples of the organic sulfuric acid constituting the organic phosphonium sulfate salt include: (1) alkylsulfates such as ethylsulfuric acid, octylsulfuric acid, lauryl sulfuric acid, tetradecylsulfuric acid, hexadecylsulfuric acid, and octadecylsulfuric acid; (2) polyoxyalkylene alkyl ether sulfuric acid such as polyoxyethylene lauryl ether sulfuric acid (average number of moles of added polyoxyalkylene is, for example, 1 to 25), polyoxyalkylene alkylphenyl ether sulfuric acid such as polyoxyethylene nonylphenyl ether sulfuric acid (average number of moles of added polyoxyalkylene is, for example, 1 to 25).

Examples of the organic sulfuric acid constituting the quaternary ammonium salt of an organic sulfuric acid having an alkyl group having 3 or more carbon atoms in the molecule include: (1) alkyl sulfates such as octyl sulfate, lauryl sulfate, tetradecyl sulfate, hexadecyl sulfate, octadecyl sulfate, and isooctadecyl sulfate; (2) polyoxyalkylene alkyl ether sulfuric acid (average number of moles of added polyoxyalkylene is, for example, 1 to 25), such as polyoxyethylene lauryl ether sulfuric acid, polyoxyethylene tetradecyl ether sulfuric acid, polyoxyethylene hexadecyl ether sulfuric acid, polyoxyethylene octadecyl ether sulfuric acid, and polyoxyethylene isostearyl ether sulfuric acid, and polyoxyalkylene alkylphenyl ether sulfuric acid (average number of moles of added polyoxyalkylene is, for example, 1 to 25), such as polyoxyethylene nonylphenyl ether sulfuric acid.

Examples of the organic sulfonic acid constituting the organic sulfonic acid phosphonium salt include: (1) alkylsulfonic acids such as methanesulfonic acid, ethanesulfonic acid, hexylsulfonic acid, heptylsulfonic acid, 2-ethylhexylsulfonic acid, octylsulfonic acid, nonylsulfonic acid, decylsulfonic acid, undecylsulfonic acid, dodecylsulfonic acid, and tridecylsulfonic acid; (2) alkylarylsulfonic acids such as p-toluenesulfonic acid, ethylbenzenesulfonic acid, decylbenzenesulfonic acid, undecylbenzenesulfonic acid, dodecylbenzenesulfonic acid, tridecylbenzenesulfonic acid, tetradecylbenzenesulfonic acid, pentadecylbenzenesulfonic acid, hexadecylbenzenesulfonic acid and dibutylnaphthalenesulfonic acid; (3) diphenyl ether sulfonic acids such as hexadecyl diphenyl ether disulfonic acid; (4) ester sulfonic acids such as dioctyl sulfosuccinate, dibutyl sulfosuccinate, dodecyl sulfoacetate, and nonylphenoxy polyethylene glycol sulfoacetate; and so on.

Examples of the organic sulfonic acid constituting the quaternary ammonium salt of an organic sulfonic acid having an alkyl group having 3 or more carbon atoms in the molecule include: (1) alkylsulfonic acids such as hexylsulfonic acid, heptylsulfonic acid, 2-ethylhexylsulfonic acid, octylsulfonic acid, nonylsulfonic acid, decylsulfonic acid, undecylsulfonic acid, dodecylsulfonic acid, and tridecylsulfonic acid; (2) alkyl aryl sulfonic acids such as decyl benzene sulfonic acid, undecyl benzene sulfonic acid, dodecyl benzene sulfonic acid, tridecyl benzene sulfonic acid, tetradecyl benzene sulfonic acid, pentadecyl benzene sulfonic acid, hexadecyl benzene sulfonic acid, and dibutyl naphthalene sulfonic acid; (3) diphenyl ether sulfonic acids such as hexadecyl diphenyl ether disulfonic acid; (4) ester sulfonic acids such as dioctyl sulfosuccinate, dibutyl sulfosuccinate, dodecyl sulfoacetate, and nonylphenoxy polyethylene glycol sulfoacetate; and so on.

Examples of the phosphonium constituting the organic phosphonium sulfate and the organic phosphonium sulfonate include quaternary phosphonium such as tetramethylphosphonium, tetraethylphosphonium, tetrabutylphosphonium, tetraoctylphosphonium, dibutyldihexylphosphonium trihexyltetradecylphosphonium, triethyloctylphosphonium, and triphenylmethylphosphonium.

Examples of quaternary ammonium constituting the quaternary ammonium salt of an organic sulfuric acid having an alkyl group having 3 or more carbon atoms in the molecule and the quaternary ammonium salt of an organic sulfonic acid having an alkyl group having 3 or more carbon atoms in the molecule include tetrabutylammonium and trihexyltetradecylammonium.

The specific onium salt may be used alone in 1 kind, or may be used in combination in 2 or more kinds.

The specific onium salt preferably contains at least one selected from the group consisting of organic phosphonium sulfates and organic phosphonium sulfonates. Further, the specific onium salt more preferably contains at least one selected from the group consisting of an organic phosphonium sulfate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms and an organic phosphonium sulfonate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms. In this case, antistatic properties can be imparted to the carbon fiber precursor to which the treatment agent has adhered.

(C) Nonionic surfactant

The nonionic surfactant contained in the treating agent of the present embodiment is not particularly limited, and examples thereof include those obtained by adding an alkylene oxide to an alcohol or a carboxylic acid.

Specific examples of the alcohols used as the raw material of the nonionic surfactant include: (1) linear alkyl alcohols such as methanol, ethanol, propanol, butanol, pentanol, hexanol, octanol, nonanol, decanol, undecanol, dodecanol, tridecanol, tetradecanol, pentadecanol, hexadecanol, heptadecanol, octadecanol, nonadecanol, eicosanol, heneicosanol, docosanol, tricosanol, tetracosanol, pentacosanol, hexacosanol, heptacosanol, octacosanol, nonacosanol, and triacontanol; (2) branched alkyl alcohols such as isopropyl alcohol, isobutyl alcohol, isohexyl alcohol, 2-ethylhexanol, isononyl alcohol, isodecyl alcohol, isododecyl alcohol, isotridecyl alcohol, isotetradecyl alcohol, isotridecyl alcohol, isohexadecyl alcohol, isoheptadecyl alcohol, isostearyl alcohol, isononadecyl alcohol, isoheneicosyl alcohol, isodocosanol, isotridecyl alcohol, isotetracosanol, isopentacosanol, hexacosanol, heptacosanol, isooctacosyl alcohol, nonacosanol, and pentadecanol; (3) linear alkenyl alcohols such as tetradecenol, hexadecenol, heptadecenol, octadecenol, and nonadecenol; (4) branched alkenyl alcohols such as isocetylenol and isostearyl alcohol; (5) cyclic alkyl alcohols such as cyclopentanol and cyclohexanol; (6) aromatic alcohols such as phenol, nonylphenol, benzyl alcohol, monostyrenated phenol, distyrenated phenol, and tristyrenated phenol; and so on.

Specific examples of carboxylic acids used as a raw material of the nonionic surfactant include: (1) linear alkyl carboxylic acids such as octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, nonadecanoic acid, eicosanoic acid, heneicosanoic acid, and behenic acid; (2) branched alkyl carboxylic acids such as 2-ethylhexanoic acid, isododecanoic acid, isotridecanoic acid, isotetradecanoic acid, isohexadecanoic acid, and isooctadecanoic acid; (3) linear alkenyl carboxylic acids such as octadecenoic acid, octadecadienoic acid, and octadecatrienoic acid; (4) aromatic carboxylic acids such as benzoic acid; and so on.

Specific examples of the alkylene oxide used as a raw material of the nonionic surfactant include ethylene oxide, propylene oxide, and the like.

The nonionic surfactant may be used alone in 1 kind, or may be used in combination of 2 or more kinds.

The nonionic surfactant preferably contains ethylene oxide added at a ratio of 1 to 20 moles to 1 mole of an aliphatic saturated alcohol having 4 to 20 carbon atoms such as a linear alkyl alcohol. In this case, the stability of the treating agent is improved.

(D) Other ingredients

The treatment agent of the present embodiment may further contain other components generally used in treatment agents within a range not to impair the effects of the present invention. Examples of the other components include a thickener, a stabilizer for maintaining the quality of the treatment agent such as an antioxidant and an ultraviolet absorber, and a charge control agent. The other components may be used alone in 1 kind, or 2 or more kinds may be used in combination.

Next, the content ratio of each component in the treating agent of the present embodiment will be described. The content ratio of each component in the treatment agent of the present embodiment is not particularly limited, and is preferably the following content ratio.

The content of the smoothing agent (A) is preferably 29.9 to 95 parts by mass, more preferably 60 to 90 parts by mass, when the total content of the smoothing agent (A), the specific onium salt (B) and the nonionic surfactant (C) (hereinafter referred to as the total of the main components) is 100 parts by mass.

The content of the specific onium salt (B) is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 25 parts by mass, based on 100 parts by mass of the total of the main components. The content of the specific onium salt (B) is preferably 0.01 to 20 parts by mass, assuming that the total content of the leveler and the specific onium salt is 100 parts by mass. In this case, the effect of imparting antistatic properties and the effect of improving the strength of the carbon fibers are easily obtained.

The nonionic surfactant (C) is preferably 1 to 70 parts by mass, more preferably 5 to 30 parts by mass, per 100 parts by mass of the total of the main components.

The total content ratio of the smoothing agent (a), the specific onium salt (B), and the nonionic surfactant (C) is preferably 50 parts by mass or more, and more preferably 75 parts by mass or more, assuming that the content ratio of the nonvolatile component in the treatment agent of the present embodiment is 100 parts by mass.

(embodiment 2)

Next, embodiment 2 of the carbon fiber precursor embodying the present invention will be described.

The carbon fiber precursor of the present embodiment includes a fiber portion obtained by spinning a raw material fiber, and the treating agent of embodiment 1 attached to the fiber portion. The carbon fiber precursor of the present embodiment is subjected to a flame-resistant treatment step of converting the carbon fiber precursor into a flame-resistant fiber in an oxidizing atmosphere at 200 to 300 ℃, preferably 230 to 270 ℃, and a carbonization step of carbonizing the flame-resistant fiber in an inert atmosphere at 300 to 2000 ℃, preferably 300 to 1300 ℃.

Examples of the raw material fiber include acrylic fiber and the like. The acrylic fiber is preferably composed of a fiber mainly composed of polyacrylonitrile obtained by copolymerizing at least 90 mol% or more of acrylonitrile with 10 mol% or less of a flame-retardant acceleration component. As the component for promoting the flame retardancy, for example, a vinyl-containing compound copolymerizable with acrylonitrile can be suitably used.

The amount of the treatment agent of embodiment 1 deposited on the carbon fiber precursor is not particularly limited, but is preferably 0.1 to 2 mass%, more preferably 0.3 to 1.2 mass%, based on the carbon fiber precursor. The concentration is a solid content concentration without containing a solvent.

The single fiber fineness of the carbon fiber precursor is not particularly limited, and is preferably 0.1 to 2.0dTex in terms of balance between performance and production cost. The number of single fibers constituting the fiber bundle of the carbon fiber precursor is not particularly limited, and is preferably 1,000 to 96,000 in terms of balance between performance and production cost.

The carbon fiber precursor can be produced by a reeling process of spinning and drawing a raw material fiber. In the reeling step, for example, a spinning step of spinning the raw material fiber, an adhesion treatment step of adhering the treatment agent of embodiment 1 to the spun fiber, and a drawing step of drawing the spun fiber are sequentially performed. The raw fiber is drawn immediately after spinning, but the high-magnification drawing after the adhesion treatment step is particularly referred to as a "drawing step".

The adhesion treatment step is a step of adhering the treatment agent of embodiment 1 after spinning the raw fiber. That is, in the adhesion treatment step, the treatment agent of embodiment 1 is adhered to the raw fiber.

A known method can be applied to the method for adhering the treating agent according to embodiment 1 in the adhesion treatment step. Examples of the known adhesion method include a spray oil feed method, a dip oil feed method, a roll oil feed method, and a guide oil feed method using a metering pump. The form of the treating agent in the case of adhering the treating agent of embodiment 1 to the fibers may be, for example, an organic solvent solution, an aqueous solution, or the like.

The stretching method in the stretching step may be a known method. Examples of the known stretching method include a wet heat stretching method using high-temperature water vapor, a dry heat stretching method using hot rolls, and the like.

In the reeling step, the timing of the adhesion of the treating agent of embodiment 1 and the number of times of the adhesion of the treating agent of embodiment 1 are not particularly limited. For example, the fibers may be attached to the raw material fibers before the spinning step or may be attached after the drawing step. The timing of adhesion after the stretching step may be, for example, immediately after the stretching step, at the winding stage after the stretching step, or immediately before the flame-retardant treatment step. The treating agent according to embodiment 1 is preferably once attached before the stretching step, and more preferably once attached before the stretching step and again attached immediately after the stretching step.

Effects of embodiment 1 and embodiment 2 will be described.

(1) The treating agent of embodiment 1 contains (a) a smoothing agent, (B) a specific onium salt, and (C) a nonionic surfactant. The carbon fiber precursor of embodiment 2 has the treating agent of embodiment 1 attached thereto. According to the above configuration, the bundling property of the flame-resistant fibers in the flame-resistant treatment step in producing carbon fibers from the carbon fiber precursor to which the treatment agent has adhered is improved. Further, according to the above configuration, it is possible to impart antistatic properties to the carbon fiber precursor to which the treatment agent has adhered, to improve the strength of the carbon fiber obtained from the carbon fiber precursor to which the treatment agent has adhered, and to improve the bundling property in the spinning step in the production of the carbon fiber precursor.

(2) The specific onium salt (B) may include at least one selected from the group consisting of organic phosphonium sulfates and organic phosphonium sulfonates. With the above configuration, the effect (1) can be more remarkably obtained.

(3) The specific onium salt (B) may contain at least one selected from the group consisting of an organic phosphonium sulfate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms and an organic phosphonium sulfonate salt in which all substituents bonded to phosphorus atoms in the molecule are alkyl groups having 3 or more carbon atoms. According to the above configuration, the effect of imparting antistatic property to the carbon fiber precursor to which the treatment agent is attached can be improved. This can reduce the electricity generated when the carbon fiber precursor is moved or wound up, and facilitate handling of the carbon fiber precursor.

(4) The (a) smoothing agent may comprise an amino-modified silicone. According to the above configuration, the strength of the carbon fiber obtained from the carbon fiber precursor to which the treatment agent is attached can be improved.

(5) The (a) smoothing agent may comprise an amino-modified silicone and a polyether-modified silicone. According to the above configuration, in addition to the effect of the above (4), the bundling property in the spinning step in the production of the carbon fiber precursor can be improved, and the bundling property of the flame-resistant fibers in the flame-resistant treatment step can be further improved.

(6) The treating agent for a carbon fiber precursor may contain the specific onium salt (B) in an amount of 0.01 to 20 parts by mass, based on 100 parts by mass of the total content of the smoothing agent (a) and the specific onium salt (B). According to the above configuration, the effect (3) and the effect (4) can be easily obtained.

(7) The nonionic surfactant (C) may contain 1 to 20 moles of ethylene oxide added to 1 mole of an aliphatic saturated alcohol having 4 to 20 carbon atoms. According to the above constitution, the stability of the treating agent can be improved.