Treatment agent for synthetic fibers, and synthetic fibers

Abstract

The objectives are to more effectively suppress the scattering of synthetic fiber treatment agents attached to synthetic fibers during the spinning and drawing process, and to improve the adhesion of synthetic fibers to rubber materials. The synthetic fiber treatment agent contains a smoothing agent, a nonionic surfactant, and an ionic surfactant. The synthetic fiber treatment agent has a maximum hydrolytic viscosity of 10,000 mPa·s or more in the range of nonvolatile content concentration of 40% by mass or more and 80% by mass or less, and the kinematic viscosity at 30°C of the synthetic fiber treatment agent adjusted to a nonvolatile content concentration of 25% by mass is 2 mm 2 / s or more 12mm 2 The viscosity is less than or equal to / s. The synthetic fiber treatment agent is applied to synthetic fibers in the form of an aqueous solution with a non-volatile content concentration of 5% by mass or more and 25% by mass or less. Hydrolyzed viscosity is the viscosity at each concentration when the non-volatile content concentration of the synthetic fiber treatment agent is changed in 5% by mass increments from 30% by mass to 100% by mass.

Description

[Technical Field]

[0001] This invention relates to a treatment agent for synthetic fibers and synthetic fibers. [Background technology]

[0002] In processes such as spinning and drawing synthetic fibers, treatments are sometimes performed to apply a treatment agent to the surface of the synthetic fibers, for example, from the viewpoint of improving smoothness, antistatic properties, etc. Synthetic fibers are also widely used as industrial materials. For example, they are used in the automotive sector (tire cords, airbags, etc.), the construction sector (carpets, tents, etc.), the commercial sector (advertising cloth, etc.), the agricultural and fisheries sector (ropes, fishing nets, etc.), and the civil engineering sector (conveyor belts, safety lines, etc.). Among these, they are widely used as reinforcing materials for rubber products. When used as reinforcing materials for rubber products, an adhesive is applied to the synthetic fiber and then bonded to the rubber material.

[0003] Conventionally, synthetic fiber treatment agents disclosed in Patent Documents 1 to 4 are known. Patent Document 1 discloses a synthetic fiber treatment agent containing a smoothing agent component and an emulsifier component, with a maximum hydration viscosity of less than 10,000 mPa·s at 25°C. Patent Document 2 discloses a spinning oil for synthetic fibers with a stock viscosity of 100 cst or less at 25°C, a viscosity of 50% aqueous emulsion of 50 cst or less, and an emulsion viscosity at 20°C to 50°C that does not have a maximum value at a concentration of 50% or less. Patent Document 3 discloses an oil containing 50 to 80% by weight of a smoothing agent component having a sulfur-containing ester compound and 15 to 30% by weight of a nonionic surfactant, with a maximum hydration viscosity of 30,000 centipoise or less. Patent Document 4 discloses a fiber treatment agent with a smoothing agent, an ester compound, a phenolic antioxidant, and a phosphite antioxidant as essential components. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2012-92482 [Patent Document 2] Patent No. 2540438 [Patent Document 3] Japanese Patent Application Publication No. 8-269870 [Patent Document 4] Japanese Patent Application Publication No. 9-188968 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, the conventional synthetic fiber treatment agents described above do not take into account the scattering of the treatment agent attached to the synthetic fiber during the spinning and drawing process. With the recent increase in spinning and drawing speeds, there is a need to more effectively suppress the scattering of the treatment agent attached to the synthetic fiber. Furthermore, the conventional synthetic fiber treatment agents described above do not take into account the adhesion of the synthetic fiber to the rubber material when the treated synthetic fiber is used as a reinforcing material for rubber products. There is a need to improve the adhesion of the synthetic fiber to the rubber material. [Means for solving the problem]

[0006] As a result of research conducted to solve the aforementioned problems, the inventors have found that a treatment agent for synthetic fibers containing a smoothing agent, a nonionic surfactant, and an ionic surfactant, and having a specific hydrated viscosity and a specific kinematic viscosity, is indeed suitable.

[0007] The following describes various methods for solving the above problems. The synthetic fiber treatment agent of embodiment 1 is a synthetic fiber treatment agent containing a smoothing agent (A), a nonionic surfactant (B), and an ionic surfactant (C), The aforementioned synthetic fiber is for reinforcing rubber products, The maximum viscosity after hydration is 10,000 mPa·s or more in the range of non-volatile content concentration between 40% by mass and 80% by mass. Furthermore, when the non-volatile content concentration is adjusted to 25% by mass, the kinematic viscosity at 30°C is 2 mm 2 / s or more 12mm 2The gist of this method is that it applies to processes in which synthetic fibers are applied in an aqueous solution with a non-volatile content of 5% by mass or more and a non-volatile content of 25% by mass or less, and with a non-volatile content of 5% by mass or more.

[0008] Hydrated viscosity is the viscosity at each concentration when the non-volatile content concentration of a synthetic fiber treatment agent is changed in 5% by mass increments from 30% by mass to 100% by mass. Embodiment 2 is the synthetic fiber treatment agent described in Embodiment 1, wherein the hydrolyzed viscosity of the synthetic fiber treatment agent is 10,000 mPa·s or less when the non-volatile content concentration is in the range of 85% by mass or more and 100% by mass or less.

[0009] Embodiment 3 is a synthetic fiber treatment agent according to Embodiment 1 or 2, wherein the smoothing agent (A) contains an ester compound (A1) having a sulfur atom in its molecule. Embodiment 4 is a synthetic fiber treatment agent according to any one embodiment of Embodiments 1 to 3, wherein the maximum value of the hydrated viscosity is 15,000 mPa·s or more.

[0010] Embodiment 5 is a synthetic fiber treatment agent according to any one embodiment of Embodiments 1 to 4, wherein the maximum value of the hydrated viscosity is 20,000 mPa·s or more. Embodiment 6 is a synthetic fiber treatment agent according to any one embodiment of Embodiments 1 to 5, wherein the maximum value of the hydrated viscosity is less than 100,000 mPa·s.

[0011] Embodiment 7 is a synthetic fiber treatment agent according to any one embodiment of Embodiments 1 to 6, wherein, when the total content of the smoothing agent (A), the nonionic surfactant (B), and the ionic surfactant (C) is 100% by mass, the smoothing agent (A) is contained in a proportion of 30% by mass or more and 70% by mass or less, the nonionic surfactant (B) in a proportion of 20% by mass or more and 60% by mass or less, and the ionic surfactant (C) in a proportion of 0.1% by mass or more and 10% by mass or less.

[0012] Appearance 8 For reinforcing rubber products The essence of the synthetic fiber is that it has been treated with a synthetic fiber treatment agent described in any one of the embodiments 1 to 7. 。 [Effects of the Invention]

[0013] According to the present invention, it is possible to more suitably suppress the scattering of the treating agent for synthetic fibers attached to the synthetic fibers in the spinning and drawing process. Further, it is possible to improve the adhesion of the synthetic fibers to which the treating agent for synthetic fibers is attached to the rubber material.

Mode for Carrying Out the Invention

[0014] <First Embodiment> Hereinafter, a first embodiment in which the treating agent for synthetic fibers of the present invention (hereinafter, also simply referred to as a treating agent) is embodied will be described. The treating agent of this embodiment contains a smoothing agent (A), a nonionic surfactant (B), and an ionic surfactant (C).

[0015] (Smoothing Agent (A)) The smoothing agent (A) is not particularly limited, and a known smoothing agent used in treating agents can be used. Examples of the smoothing agent (A) include an ester compound (A1) having a sulfur atom in the molecule, an ester compound (A2) having an ester bond in the molecule and no ether bond, and the like. Among these, the smoothing agent (A) preferably contains the ester compound (A1). By containing the ester compound (A1), the adhesion of the synthetic fibers to which the treating agent is attached to the rubber material can be further improved.

[0016] (Ester Compound (A1)) The ester compound (A1) preferably has a structure of an ester compound formed from a polyvalent carboxylic acid and a monohydric alcohol and having a sulfur atom in the molecule.

[0017] The ester compound (A1) is preferably an ester compound represented by the following formula (1). <好ましくは、式(1)中、R​​​​​3 and R 4 Each of the following independently represents a hydrocarbon with 12 to 24 carbon atoms, and m and n independently represent integers between 1 and 4. In the above equation (1), R 3 , R 4 These are hydrocarbon groups having 12 to 24 carbon atoms, such as lauryl group, tridecyl group, isotridecyl group, myristyl group, isomiristyl group, cetyl group, isocetyl group, stearyl group, isostearyl group, arachidyl group, isoarachidyl group, behenyl group, isobehenyl group, lignoceryl group, isolignoceryl group, palmitrail group, oleyl group, eicosenyl group, docosenyl group, and tetracosenyl group, with branched hydrocarbon groups being preferred.

[0020] Specific examples of ester compounds (A1) include, for example, di(2-hexyl-1-decyl)thiodipropionate, di(2-decyl-1-tetradecyl)thiodipropionate, dioleylthiodipropionate, diisostearylthiodipropionate, isostearyl(laurylthiopropionate), and diesters of compounds obtained by adding 3 moles of ethylene oxide (hereinafter also referred to as EO) to 1 mole of dodecanol with thiodipropionic acid.

[0021] (Ester compound (A2)) Ester compounds (A2) are ester compounds that have an ester bond in their molecule but no ether bond. Examples of ester compounds (A2) include polyhydric alcohol fatty acid ester compounds (a1), polyhydric carboxylic acid aliphatic alcohol ester compounds (a2), and ester compounds of aliphatic monohydric alcohols and fatty acids (a3).

[0022] Polyhydric alcohol fatty acid ester compounds (a1) include, for example, esters of an aliphatic dihydric alcohol having 2 to 6 carbon atoms, an aliphatic trihydric alcohol having 3 to 6 carbon atoms, or an aliphatic tetrahydric alcohol having 5 carbon atoms, with a fatty acid having 4 to 32 carbon atoms, and which do not have an ether bond in the molecule. Among these, those having a branched hydrocarbon group are preferred.

[0023] Examples of aliphatic dihydric alcohols having 2 to 6 carbon atoms include ethylene glycol, propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, 1,5-pentanediol, and 1,6-hexanediol.

[0024] Examples of aliphatic trihydric alcohols with 3 to 6 carbon atoms include glycerin and trimethylolpropane. Examples of aliphatic tetrahydric alcohols with 5 carbon atoms include pentaerythritol.

[0025] The fatty acids (aliphatic monocarboxylic acids) constituting the polyhydric alcohol fatty acid ester (a1) may be saturated or unsaturated. The number of carbon atoms in the fatty acids is preferably 8 to 30, more preferably 10 to 28, and even more preferably 12 to 24. One or more types of fatty acids may be used, and saturated and unsaturated fatty acids may be used in combination.

[0026] Specific examples of ester compounds (A2) include, for example, trimethylolpropane trioleate, trimethylolpropane trilaurate, pentaerythritol tetraoctanate, sorbitan monooleate, 2-decyl-1-tetradecyl elcinate, and 2-decyl-1-tetradecyl oleate.

[0027] The above-mentioned smoothing agent (A) may be used as a single type of smoothing agent (A), or two or more types of smoothing agents (A) may be used in appropriate combinations. (Nonionic surfactant (B)) Examples of nonionic surfactants (B) include compounds having a (poly)oxyalkylene structure obtained by adding alkylene oxide to alcohols or carboxylic acids, ether ester compounds having a (poly)oxyalkylene structure obtained by adding alkylene oxide to ester compounds of carboxylic acids and polyhydric alcohols, compounds obtained by adding alkylene oxide to natural oils and fats or compounds obtained by esterifying such compounds with carboxylic acids, amine compounds such as compounds having a (poly)oxyalkylene structure obtained by adding alkylene oxide to primary organic amines, compounds having a (poly)oxyalkylene structure obtained by adding alkylene oxide to fatty acid amides, amide compounds obtained by condensing amine compounds and carboxylic acids, and partial ester compounds of carboxylic acids and polyhydric alcohols, etc.

[0028] Specific examples of alcohols used as raw materials for nonionic surfactants (B) include, for example, (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, heptadecanol, octacosanol, nonacosanol, and triacontanol; (2) isopropanol, isobutanol, isohexanol, 2-ethylhexanol, isononanol, isodecanol, isododecanol, isotridecanol, isotetradecanol, isotriacontanol, isohe Examples include: (3) branched alkyl alcohols such as xadecanol, isoheptadecanol, isooctadecanol, isononadecanol, isoeicosanol, isoheneicosanol, isodocosanol, isotricosanol, isotetracosanol, isopentacosanol, isohexacosanol, isoheptacosanol, isooctactasanol, isononacosanol, and isopentadecanol; (4) linear alkenyl alcohols such as tetradecenol, hexadecenol, heptadecenol, octadecenol, and nonadecenol; (5) branched alkenyl alcohols such as isohexadecenol and isooctadecenol; (6) cyclic alkyl alcohols such as cyclopentanol and cyclohexanol; and aromatic alcohols such as phenol, nonylphenol, benzyl alcohol, monostyrene-modified phenol, distylenide, and tristyrene-modified phenol.

[0029] Specific examples of carboxylic acids used as raw materials for nonionic surfactants (B) include, for example, (1) linear alkyl carboxylic acids such as octic 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 docosanoic 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 (5) hydroxycarboxylic acids such as ricinoleic acid.

[0030] As the alkylene oxide used as a raw material to form the (poly)oxyalkylene structure of the nonionic surfactant (B), an alkylene oxide having 2 to 4 carbon atoms is preferred. Specific examples of alkylene oxides include EO, propylene oxide (hereinafter also referred to as PO), butylene oxide (hereinafter also referred to as BO), etc. The number of moles of alkylene oxide to be added is set as appropriate, but is preferably 0.1 moles to 250 moles, more preferably 1 mole to 200 moles, and even more preferably 2 moles to 150 moles. Ranges arbitrarily combining the above upper and lower limits are also conceivable. The number of moles of alkylene oxide to be added indicates the number of moles of alkylene oxide per mole of the compound to be added in the raw materials. One type of alkylene oxide may be used alone, or two or more types of alkylene oxide may be used in appropriate combination. When two or more alkylene oxides are applied, their addition methods may be block addition, random addition, or a combination of block addition and random addition, and there are no particular restrictions.

[0031] Specific examples of polyhydric alcohols used as raw materials for nonionic surfactants (B) include, for example, ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,2-propanediol, 1,5-pentanediol, 1,6-hexanediol, 2,5-hexanediol, 2-methyl-2,4-pentanediol, 2,3-dimethyl-2,3-butanediol, glycerin, diglycerin, 2-methyl-2-hydroxymethyl-1,3-propanediol, trimethylolpropane, sorbitan, pentaerythritol, sorbitol, and the like.

[0032] Specific examples of aliphatic amines or primary organic amines used as raw materials for nonionic surfactants (B) include, for example, methylamine, ethylamine, butylamine, octylamine, laurylamine, octadecylamine (stearylamine), octadecenylamine, and coconut amine.

[0033] Specific examples of fatty acid amides used as raw materials for nonionic surfactants (B) include, for example, octylic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, oleic acid amide, behenic acid amide, and lignoceric acid amide.

[0034] Specific examples of nonionic surfactants (B) include, for example, a compound obtained by adding 10 moles of EO to 1 mole of hydrogenated castor oil, a compound obtained by adding 12 moles of EO to 1 mole of hydrogenated castor oil, a compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil, a compound obtained by adding 25 moles of EO to 1 mole of hydrogenated castor oil, a compound obtained by esterifying 1 mole of the compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil with 2 moles of oleic acid, a compound obtained by esterifying 1 mole of the compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil with 3 moles of oleic acid, and a compound obtained by adding 25 moles of EO to 1 mole of hydrogenated castor oil. Examples include compounds obtained by esterifying 1 mole of a compound with added moles with 0.5 moles of adipic acid and 1 mole of stearic acid, compounds obtained by adding 7 moles of EO to 1 mole of sorbitan monooleate, compounds obtained by adding 20 moles of EO to 1 mole of sorbitan monostearate, diesters of polyethylene glycol (weight-average molecular weight 600) and oleic acid, compounds obtained by randomly adding 10 moles of EO and 10 moles of PO to 1 mole of dodecanol, compounds obtained by adding 10 moles of EO to 1 mole of stearylamine, and compounds obtained by adding 15 moles of EO to 1 mole of stearylamine.

[0035] These nonionic surfactants (B) may be used individually or in combination of two or more nonionic surfactants (B) as appropriate.

[0036] (Ionic surfactant (C)) Examples of ionic surfactants (C) include anionic surfactants, cationic surfactants, and amphoteric surfactants.

[0037] As an anionic surfactant, any known one can be used as appropriate. Specific examples of anionic surfactants include, for example, (1) phosphate ester salts of aliphatic alcohols such as lauryl phosphate salt, cetyl phosphate salt, octyl phosphate salt, oleyl phosphate salt, and stearyl phosphate salt; (2) phosphate ester salts of aliphatic alcohols to which at least one alkylene oxide selected from ethylene oxide and propylene oxide has been added, such as polyoxyethylene lauryl ether phosphate salt, polyoxyethylene oleyl ether phosphate salt, and polyoxyethylene stearyl ether phosphate salt; (3) aliphatic sulfonates or aromatic sulfonates such as lauryl sulfonate, myristyl sulfonate, cetyl sulfonate, oleyl sulfonate, stearyl sulfonate, tetradecane sulfonate, dodecylbenzene sulfonate, secondary alkyl sulfonic acid (C13 to C15) salt, secondary alkyl sulfonic acid (C11 to C14) salt, and α-olefin sulfonate; (4) lauryl sulfate salt, oleyl sulfate salt, stearyl (5) Sulfate salts of aliphatic alcohols such as polyoxyethylene lauryl ether sulfate, polyoxyalkylene (polyoxyethylene, polyoxypropylene) lauryl ether sulfate, polyoxyethylene oleyl ether sulfate, etc., to which at least one alkylene oxide selected from ethylene oxide and propylene oxide has been added, (6) Castor oil fatty acid sulfate, sesame oil fatty acid sulfate, (7) Sulfate salts of fatty acids such as sulfate salts of soybean oil, sulfate salts of rapeseed oil, and sulfate salts of palm oil, (8) Sulfate salts of fats and oils such as sulfate salts of castor oil, sulfate salts of sesame oil, sulfate salts of tall oil, sulfate salts of soybean oil, sulfate salts of rapeseed oil, and sulfate salts of palm oil, (9) Sulfosuccinate salts of aliphatic alcohols such as dioctyl sulfosuccinate,(10) Examples include carboxylic acid ester salts obtained by adding at least one alkylene oxide selected from ethylene oxide and propylene oxide to an aliphatic alcohol. Examples of counterions for anionic surfactants include alkali metal salts such as potassium salts and sodium salts, ammonium salts, and alkanolamine salts such as triethanolamine.

[0038] As cationic surfactants, known ones can be used as appropriate. Specific examples of cationic surfactants include, for example, lauryltrimethylammonium chloride, cetyltrimethylammonium chloride, stearyltrimethylammonium chloride, behenyltrimethylammonium chloride, and didecyldimethylammonium chloride.

[0039] Any known amphoteric surfactant can be used as appropriate. Specific examples of amphoteric surfactants include, for example, betaine-type amphoteric surfactants. Specific examples of ionic surfactants (C) include, for example, sodium α-olefin (14 to 18 carbon atoms) sulfonate, sodium secondary alkyl (12 to 18 carbon atoms) sulfonate, salts of oleyl phosphate ester and compounds obtained by adding 4 moles of EO to 1 mole of laurylamine, and salts of isocetyl phosphate ester and compounds obtained by adding 10 moles of EO to 1 mole of laurylamine.

[0040] These ionic surfactants (C) may be used individually or in combination of two or more ionic surfactants (C) as appropriate. (Content ratio of lubricant (A), nonionic surfactant (B), and ionic surfactant (C)) The proportions of the smoothing agent (A), nonionic surfactant (B), and ionic surfactant (C) in the treatment agent are not particularly limited. When the total proportion of the smoothing agent (A), nonionic surfactant (B), and ionic surfactant (C) is taken as 100% by mass, it is preferable that the smoothing agent (A) is contained in proportions of 30% to 70% by mass, the nonionic surfactant (B) in proportions of 20% to 60% by mass, and the ionic surfactant (C) in proportions of 0.1% to 10% by mass.

[0041] When the proportions of the smoothing agent (A), nonionic surfactant (B), and ionic surfactant (C) in the treatment agent are within the above numerical range, it becomes easier to suppress the scattering of the treatment agent attached to the synthetic fibers during the spinning and drawing process. In addition, it becomes easier to improve the adhesion of the synthetic fibers to the rubber material to which the treatment agent has been attached.

[0042] In one embodiment of this product, the content of the smoothing agent (A) in the treatment agent is, for example, 38% by mass or more, 40% by mass or more, 50% by mass or more, 54.5% by mass or more, or 55% by mass or more. Similarly, the content of the smoothing agent (A) in the treatment agent is, for example, 55% by mass or less, 54.5% by mass or less, 50% by mass or less, 40% by mass or less, or 38% by mass or less.

[0043] In one embodiment of this product, the content of nonionic surfactant (B) in the treatment agent is, for example, 30% by mass or more, 38% by mass or more, 39% by mass or more, 40% by mass or more, 41% by mass or more, 43% by mass or more, 44% by mass or more, 44.5% by mass or more, 45% by mass or more, 47% by mass or more, 53% by mass or more, 55% by mass or more, or 57% by mass or more. Similarly, the content of nonionic surfactant (B) in the treatment agent is, for example, 57% by mass or less, 55% by mass or less, 53% by mass or less, 47% by mass or less, 45% by mass or less, 44.5% by mass or less, 44% by mass or less, 43% by mass or less, 41% by mass or less, 40% by mass or less, 39% by mass or less, 38% by mass or less, or 30% by mass or less.

[0044] In one embodiment of this product, the content of ionic surfactant (C) in the treatment agent is, for example, 2% by mass or more, 2.2% by mass or more, 2.5% by mass or more, 3% by mass or more, 3.3% by mass or more, 4% by mass or more, 4.5% by mass or more, 6% by mass or more, or 6.5% by mass or more. Similarly, the content of ionic surfactant (C) in the treatment agent is, for example, 6.5% by mass or less, 6% by mass or less, 4.5% by mass or less, 4% by mass or less, 3.3% by mass or less, 3% by mass or less, 2.5% by mass or less, 2.2% by mass or less, or 2% by mass or less.

[0045] (Hydrated viscosity) The treatment agent has a maximum hydrous viscosity of 10,000 mPa·s or more when the non-volatile content concentration is between 40% and 80% by mass. Hydrous viscosity is the viscosity at each concentration when the non-volatile content concentration of the treatment agent is changed in 5% by mass increments from 30% to 100% by mass.

[0046] Within a non-volatile content range of 40% by mass or more and 80% by mass or less, the maximum value of the hydrated viscosity is preferably 15,000 mPa·s or more, and more preferably 20,000 mPa·s or more. Furthermore, the maximum value of the hydrated viscosity is preferably less than 100,000 mPa·s.

[0047] When the non-volatile content concentration is in the range of 40% to 80% by mass, and the maximum hydrous viscosity is 15,000 mPa·s or higher, it becomes easier to suppress the scattering of the treatment agent attached to the synthetic fibers during the spinning and drawing process. In addition, it becomes easier to apply adhesive to the surface of the synthetic fibers to which the treatment agent has been attached, thereby improving the adhesion of the synthetic fibers to the rubber material. When the maximum hydrous viscosity is 20,000 mPa·s or higher, it becomes even easier to suppress the scattering of the treatment agent attached to the synthetic fibers during the spinning and drawing process.

[0048] The hydrated viscosity of the treatment agent is preferably 10,000 mPa·s or less, with a non-volatile content concentration in the range of 85% to 100% by mass. When the hydrated viscosity is 10,000 mPa·s or less, with a non-volatile content concentration in the range of 85% to 100% by mass, the adhesion of the synthetic fibers to which the treatment agent is attached to the rubber material can be further improved.

[0049] In this process, synthetic fibers treated with a treatment agent during the spinning and drawing process undergo further processes, such as twisting and weaving, before being coated with adhesive to bond them to the rubber material. As the twisting and weaving processes progress, the non-volatile content of the treatment agent adhering to the synthetic fibers tends to increase. In other words, the non-volatile content of the treatment agent adhering to synthetic fibers after the weaving process tends to be relatively higher than that of the treatment agent applied during the spinning and drawing process. When the non-volatile content of the treatment agent is relatively high, the viscosity of the surface of the synthetic fiber increases, making it difficult to apply the adhesive uniformly. Therefore, by ensuring that the non-volatile content of the treatment agent is in the range of 85% by mass to 100% by mass and the hydrated viscosity is 10,000 mPa·s or less, the adhesive can be applied more uniformly to the synthetic fibers. This improves the adhesion of the treated synthetic fibers to the rubber material.

[0050] The non-volatile content of the treatment agent refers to the oven-dried material obtained by heat-treating the treatment agent at 105°C to remove solvents and other substances, and reaching a constant weight. The state of the non-volatile content of the treatment agent is defined as a non-volatile content concentration of 100% by mass. When the non-volatile content is diluted by adding deionized water and stirring, the ratio of the non-volatile content to the total mass including the deionized water is defined as the non-volatile content concentration (mass%).

[0051] The viscosity with added water was measured using an E-type viscometer at 30°C, starting from a state where the non-volatile content of the treatment agent was 100% by mass. This measurement was performed three times at each non-volatile content concentration, and the average value was calculated.

[0052] (Kinematic viscosity) When the non-volatile content concentration of the treatment agent is adjusted to 25% by mass, the kinematic viscosity at 30 °C is 2 mm 2 / s or more and 12 mm 2 / s or less. When the kinematic viscosity at 30 °C of the treatment agent with the non-volatile content concentration adjusted to 25% by mass is 2 mm 2 / s or more and 12 mm 2 / s or less, the scattering of the treatment agent adhered to the synthetic fiber can be more preferably suppressed. In addition, the adhesiveness of the synthetic fiber to which the treatment agent is adhered to the rubber material can be improved.

[0053] The kinematic viscosity of the treatment agent was measured using the treatment agent diluted with ion-exchanged water so that its non-volatile content concentration was 25% by mass. It was measured under the condition of 30 °C by the Cannon-Fenske method. The treatment agent was diluted with ion-exchanged water by adding the treatment agent little by little to the ion-exchanged water stirred at 550 rpm using a stirring blade and stirring until it was completely dissolved.

[0054] In the present invention, the method for adjusting the values of the addition viscosity and the kinematic viscosity to the above numerical ranges is not particularly limited. For example, it can be adjusted by selecting the types of the smoothing agent (A), the nonionic surfactant (B), and the ionic surfactant (C), or by adjusting the content ratios. It can also be adjusted by containing a viscosity modifier or the like in the other component (D) described later.

[0055] (Other component (D)) The treatment agent may contain the other component (D). Examples of the other component (D) include components usually used in treatment agents such as stabilizers, antistatic agents, linking agents, antioxidants, ultraviolet absorbers, pH adjusters, viscosity modifiers, and appearance adjusters.

[0056] Specific examples of other components (D) include, for example, malic acid, tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanurate, bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionic acid](ethylenebisoxy)bisethylene, dibutylethanolamine, ethylene glycol, glycerin, etc.

[0057] In the non-volatile components of the treatment agent, the content of other component (D) is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and even more preferably 2.5% by mass or less. Other component (D) may be 0% by mass.

[0058] (Preservation form) The treatment agent may be configured as a single dosage form containing the above-mentioned components (A) to (D), or, from the viewpoint of improving formulation stability, it may be configured as a two-dosage or three-dosage treatment agent.

[0059] (solvent) The treatment agent of this embodiment may be mixed with a solvent as needed to prepare a treatment agent-containing composition for synthetic fibers (hereinafter also referred to as the "treatment agent-containing composition"), which may then be stored or distributed in the form of the treatment agent-containing composition.

[0060] A solvent is a solvent whose boiling point at 1 atmosphere is 105°C or lower. Examples of solvents include water and organic solvents. Specific examples of water include, for instance, deionized water, distilled water, hard water, and soft water. Among these, deionized water or distilled water is preferred.

[0061] Specific examples of organic solvents include lower alcohols such as ethanol and propanol, and low-polarity solvents such as hexane. These solvents may be used individually or in combination of two or more as appropriate. Among these, polar solvents such as water and lower alcohols are preferred from the viewpoint of excellent dispersibility or solubility of each component, and water is more preferred from the viewpoint of excellent handling.

[0062] (Application form) The treatment agent of this embodiment is applied to synthetic fibers in the form of an aqueous solution with a non-volatile content concentration of 5% to 25% by mass. For example, in the spinning and drawing process, the treatment agent of this embodiment is applied to synthetic fibers in the form of an aqueous solution with a non-volatile content concentration of 5% to 25% by mass. By applying the treatment agent to the synthetic fibers in the form of an aqueous solution with a non-volatile content concentration of 5% to 25% by mass, the scattering of the treatment agent can be more effectively suppressed. In addition, the adhesion of the synthetic fibers to which the treatment agent is attached can be improved to the rubber material.

[0063] The above aqueous solution refers to a treatment agent diluted with a solvent containing water. The proportion of water in the solvent is preferably 90% by mass or more. The above aqueous solution can be prepared by adding a solvent containing water to the treatment agent. Alternatively, the above aqueous solution may be prepared by adding a solvent containing water to the non-volatile components.

[0064] <Operation and Effects of the First Embodiment> (1-1) The treatment agent of the first embodiment contains a smoothing agent (A), a nonionic surfactant (B), and an ionic surfactant (C). Furthermore, the maximum viscosity with water is 10,000 mPa·s or more in the range of nonvolatile content concentration of 40% by mass or more and 80% by mass or less, and the kinematic viscosity at 30°C when the nonvolatile content concentration is adjusted to 25% by mass is 2 mm 2 / s or more 12mm 2 The time interval is less than or equal to / s. Furthermore, the treatment agent is applied to the synthetic fiber in the form of an aqueous solution with a non-volatile content concentration of 5% by mass or more and 25% by mass or less.

[0065] Therefore, the scattering of the treatment agent attached to the synthetic fibers during the spinning and drawing process can be more effectively suppressed. In addition, the adhesion of the synthetic fibers to the rubber material can be improved.

[0066] (1-2) The viscosity of the treatment agent after hydration is 10,000 mPa·s or less when the non-volatile content is in the range of 85% by mass or more and 100% by mass or less. Therefore, the adhesion of the synthetic fibers to which the treatment agent is attached to the rubber material can be further improved.

[0067] (1-3) The lubricant (A) contains an ester compound (A1) having a sulfur atom in its molecule. Therefore, the adhesion of the synthetic fiber to which the treatment agent is attached to the rubber material can be further improved.

[0068] <Second Embodiment> Next, a second embodiment of the synthetic fiber according to the present invention will be described. The synthetic fiber of this embodiment is a synthetic fiber to which the treatment agent of the first embodiment is attached. That is, according to this embodiment, a treated synthetic fiber is provided, comprising a synthetic fiber and a treatment agent attached thereto. The treatment agent may be applied to the synthetic fiber in the form of a diluted solution obtained by diluting it with the above-mentioned solvent, for example, an organic solvent solution, an aqueous solution, etc. The synthetic fiber is obtained by a process in which a diluted solution such as an aqueous solution is attached to the synthetic fiber, for example in a spinning and drawing process. The diluted solution attached to the synthetic fiber may have its solvent evaporated in a drawing or drying process after the spinning process.

[0069] There are no particular limitations on the specific synthetic fibers to which the treatment agent of this embodiment is applied. Examples include (1) polyester fibers such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, and composite fibers containing these polyester resins; (2) polyamide fibers such as nylon 6 and nylon 66; (3) polyacrylic fibers such as polyacrylic and modacrylic; and (4) polyolefin fibers such as polyethylene and polypropylene. Among these, it is preferable to apply it to polyester fibers and polyamide fibers.

[0070] There are no particular restrictions on the proportion of the treatment agent to be attached to the synthetic fibers, but it is preferable to attach the treatment agent in a proportion of 0.1% by mass or more and 3% by mass or less relative to the synthetic fibers (a proportion that does not include solvents such as water). This configuration further improves the effects of the present invention. Furthermore, there are no particular restrictions on the method of attaching the treatment agent, and known methods such as roller lubrication, guided lubrication using a metering pump, immersion lubrication, and spray lubrication can be employed.

[0071] In the present invention, the use of synthetic fibers is not particularly limited, but it is preferable that they be used in industrial materials. For example, they can be used as fibers for airbags, seat belts, tire cords, carpets, tents, advertising fabrics, fishing nets, conveyor belts, ropes, etc. They can also be used in fields such as automobiles, construction, commerce, agriculture / fisheries, and civil engineering.

[0072] When the above-mentioned industrial materials are used to reinforce rubber products such as tire cords, the adhesive used for bonding to the rubber material is not particularly limited. Any known adhesive used for bonding to rubber materials can be used. Examples of known adhesives used for bonding to rubber materials include epoxy compounds, isocyanate compounds, and resorcinol-formaldehyde-latex (RFL) solutions. Among these, epoxy compounds are preferred. Mixtures of these adhesives or mixtures of adhesives and surfactants can also be used. When applying the adhesive, it may be applied as a diluted solution with water or an organic solvent, or it may be applied in its undiluted state.

[0073] The procedure for applying the treatment agent and adhesive of the present invention to synthetic fibers is not particularly limited. For example, during the spinning and drawing process, a treatment agent may be applied to the synthetic fibers, then the twisted synthetic fibers may be woven to produce a fabric, and an adhesive may be applied to this fabric to bond the fabric to the rubber material. Alternatively, the treatment agent and adhesive may be mixed in advance to prepare a mixed solution, the mixed solution may be applied to the synthetic fibers during the spinning process, then the twisted synthetic fibers may be woven to produce a fabric, and this fabric may be bonded to the rubber material. Alternatively, the treatment agent and adhesive may be applied to the synthetic fibers individually during the spinning process, then the twisted synthetic fibers may be woven to produce a fabric, and this fabric may be bonded to the rubber material.

[0074] <Operation and Effects of the Second Embodiment> (2-1) The synthetic fibers are treated with the treatment agent of the first embodiment. Therefore, the adhesion of the synthetic fibers to the rubber material can be improved. It can be suitably used for reinforcing rubber products. [Examples]

[0075] The following examples illustrate the structure and effects of the present invention in more detail, but the present invention is not limited to these examples. In the following examples and comparative examples, % refers to mass%.

[0076] Test category 1 (Preparation of treatment agent) (Treatment agent 1) As shown in Table 1, the lubricant (A) consists of 45% trimethylolpropanetrioleate (A-1) and 5% di(2-hexyl-1-decyl)thiodipropionate (A1-7), and the nonionic surfactants (B) consist of 10% of compound (B-2) obtained by adding 12 moles of EO to 1 mole of hydrogenated castor oil, 5% of compound (B-3) obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil, 20% of compound (B-5) obtained by esterifying 1 mole of the compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil with 2 moles of oleic acid, and compound (B-7) obtained by esterifying 1 mole of the compound obtained by adding 25 moles of EO to 1 mole of hydrogenated castor oil with 0.5 moles of adipic acid and 1 mole of stearic acid. Treatment agent 1 was prepared by mixing the following components to a total of 100% by mass: 5% of ( ), 4% of a compound (B-12) obtained by adding 10 moles of EO to 1 mole of stearylamine, 1.8% of a secondary alkyl (12 to 18 carbon atoms) sodium sulfonate (C-2) as an ionic surfactant (C), 1.5% of a salt (C-4) of isocetyl phosphate ester and a compound obtained by adding 10 moles of EO to 1 mole of laurylamine, and 0.7% of bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionic acid](ethylenebisoxy)bisethylene (D-3), 1% of dibutylethanolamine (D-4), and 1% of ethylene glycol (D-5) as other components (D).

[0077] (Treatment agent 2-30) Treatment agents 2 to 30 were prepared in the same manner as treatment agent 1, containing a smoothing agent (A), a nonionic surfactant (B), an ionic surfactant (C), and other components (D) in the proportions shown in Tables 1 and 2.

[0078] The types and amounts of lubricants (A), nonionic surfactants (B), ionic surfactants (C), and other components (D) are shown in the "Lubricants (A)", "Nonionic Surfactants (B)", "Ionic Surfactants (C)", and "Other Components (D)" columns of Tables 1 and 2, respectively.

[0079] [Table 1]

[0080] [Table 2]

[0081] The details of the lubricants (A), nonionic surfactants (B), ionic surfactants (C), and other components (D) shown in Tables 1 and 2 are as follows. <Smoothing agent (A)> A-1: Trimethylol Propane Trioleart A-2: Trimethylolpropane trilaurate A-3: Pentaerythritol tetraoctanate A-4: Sorbitan Monooleart A-5:2-decyl-1-tetradecyl elcinato A-6:2-decyl-1-tetradecyloleate A1-7: Di(2-hexyl-1-decyl)thiodipropionate A1-8: Di(2-decyl-1-tetradecyl)thiodipropionate A1-9: Georail thiodipropionate A1-10: Diisostearylthiodipropionate A1-11: Isostearyl (laurylthiopropionate) A1-12: Diester of a compound obtained by adding 3 moles of EO to 1 mole of dodecanol and thiodipropionic acid. Furthermore, the smoothing agents A-1 to A-6 are ester compounds (A2) that have ester bonds in their molecules but no ether bonds. The smoothing agents A1-7 to A1-12 are ester compounds (A1) that have sulfur atoms in their molecules and are formed from a polycarboxylic acid and a monohydric alcohol.

[0082] <Nonionic surfactant (B)> B-1: A compound obtained by adding 10 moles of EO to 1 mole of hydrogenated castor oil. B-2: A compound obtained by adding 12 moles of EO to 1 mole of hydrogenated castor oil. B-3: A compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil. B-4: A compound obtained by adding 25 moles of EO to 1 mole of hydrogenated castor oil. B-5: A compound obtained by esterifying 1 mole of a compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil with 2 moles of oleic acid. B-6: A compound obtained by esterifying 1 mole of a compound obtained by adding 20 moles of EO to 1 mole of hydrogenated castor oil with 3 moles of oleic acid. B-7: A compound obtained by esterifying 1 mole of a compound obtained by adding 25 moles of EO to 1 mole of hydrogenated castor oil, 0.5 moles of adipic acid, and 1 mole of stearic acid. B-8: A compound obtained by adding 7 moles of EO to 1 mole of sorbitan monooleate. B-9: A compound obtained by adding 20 moles of EO to 1 mole of sorbitan monostearate. B-10: Diester of polyethylene glycol (mass-average molecular weight 600) and oleic acid B-11: A compound obtained by randomly adding 10 moles of EO and 10 moles of PO to 1 mole of dodecanol. B-12: A compound obtained by adding 10 moles of EO to 1 mole of stearylamine. B-13: A compound obtained by adding 15 moles of EO to 1 mole of stearylamine. <Ionic surfactant (C)> C-1: Sodium α-olefin sulfonate (14 to 18 carbon atoms) C-2: Secondary alkyl (12 to 18 carbon atoms) sodium sulfonate C-3: Salt of oleyl phosphate ester and a compound obtained by adding 4 moles of EO to 1 mole of laurylamine. C-4: Salt of isocetyl phosphate ester and a compound obtained by adding 10 moles of EO to 1 mole of laurylamine. <Other ingredients (D)> D-1: Malic acid D-2: Tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanurate D-3: Bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionic acid](ethylenebisoxy)bisethylene D-4: Dibutylethanolamine D-5: Ethylene glycol D-6: Glycerin Test Category 2 (Evaluation of kinematic viscosity and hydrous viscosity of the treatment agent) The kinematic viscosity of each treatment agent was measured using an aqueous solution prepared by diluting each treatment agent with deionized water to a non-volatile content concentration of 25% by mass. The measurement was performed using the Cannon-Fenske method under conditions of 30°C. Dilution of the treatment agent with deionized water was carried out by adding the treatment agent in small amounts to deionized water stirred at 550 rpm using a stirring blade, and stirring until completely dissolved.

[0083] The viscosity of the treatment agents with added water was measured using an E-type viscometer at 30°C, starting from a state where the non-volatile content of each treatment agent was 100% by mass. This was done for every 5% decrease in the non-volatile content by adding deionized water. The viscosity with added water was measured three times at each non-volatile content concentration, and the average value was calculated. The measurement results for kinematic viscosity and viscosity with added water are shown in Tables 3 and 4.

[0084] [Table 3]

[0085] [Table 4]

[0086] In Tables 3 and 4, values ​​marked with an asterisk (*) in the upper left corner represent the maximum hydration viscosity. Values ​​marked with ** indicate that the viscosity exceeded 100,000 mPa·s and therefore could not be measured. In the above-mentioned viscosity with added water, in regions with high concentrations of the treatment agent, the treatment agent is dispersed as an oily component, and the viscosity is kept relatively low. As the concentration of the treatment agent decreases, the treatment agent undergoes a phase transition to a state where it is dispersed in water, and the viscosity becomes relatively higher. Furthermore, in regions with even lower concentrations of the treatment agent, the treatment agent is dispersed in water, and the viscosity becomes relatively lower.

[0087] Test category 3 (Evaluation of the amount of treatment agent applied) For each treatment agent, aqueous solutions with non-volatile content concentrations of 5% by mass, 10% by mass, 15% by mass, 20% by mass, 25% by mass, and 70% by mass were prepared and designated as Examples 1 to 100 and Comparative Examples 1 to 80.

[0088] Polyethylene terephthalate chips were dried by conventional methods, then melt-spun using an extruder. After extrusion from the die and cooling to solidify, aqueous solutions of the treatment agents for each example and comparative example were applied to the running yarn using a guided lubrication method with a metering pump. The amount of treatment agent applied was 0.6% by mass (amount excluding diluent and water). The yarn was then gathered with a guide and stretched through a stretching roll and a slackening roll at 250°C to a total stretching ratio of 2.0 times, and wound at a speed of 5,000 m / min. The amount of treatment agent applied to the stretched yarn was measured, and evaluated according to the following evaluation criteria, with 100% representing the amount applied as targeted. The closer the amount of treatment agent applied is to the target amount, the more effectively the scattering of the treatment agent is suppressed. The results are shown in the "OPU" column of Tables 5-7. OPU stands for Oil Pickup.

[0089] • Criteria for evaluating adhesion amount 4 (Excellent): When the percentage is 95% or higher 3 (Good): 85% or more but less than 95% 2 (OK): If the percentage is between 75% and 85% 1 (Not acceptable): If less than 75% Test category 4 (Evaluation of the adhesive properties of rubber materials) The aqueous solutions of the treatment agents for each example and comparative example were applied to oil-free polyethylene terephthalate fibers with a density of 1670 decitex, 288 filaments, and an intrinsic viscosity of 0.93 during the spinning process using an oiling roller lubrication method, so that the amount of non-volatile content was 5.0% by mass. After that, the moisture was dried to obtain test yarns. Two test yarns were twisted together with a twist count of 40 turns / 10cm for the undertwist and 40 turns / 10cm for the overtwist to produce twisted cords.

[0090] This twisted cord was immersed in a first adhesive (epoxy compound (product name Denacol EX-512, manufactured by Nagase ChemteX Corporation) / blocked isocyanate (product name Elastron BN-27, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) = 5 / 5 (solids ratio)) and then heat-treated.

[0091] Next, the reinforcing cord was immersed in a second adhesive (RFL solution of resorcinol (product name Resorcinol, manufactured by Kishida Chemical Co., Ltd.), formalin (product name Formaldehyde Solution (37%), manufactured by Kishida Chemical Co., Ltd.), and latex (product name Nipol 2518FS, manufactured by Nippon Zeon Co., Ltd.) = 1.5 / 0.5 / 8 (solids content ratio)), then heat-treated to obtain a reinforcing cord treated with the adhesive. This reinforcing cord was arranged without gaps on an unvulcanized rubber sheet measuring 2.5 cm vertically and 12.5 cm horizontally, and another layer of unvulcanized rubber was placed on top. This was then press-vulcanized at 150°C and 4 MPa for 30 minutes to create a test specimen for evaluating rubber adhesion. After the test specimen cooled, the reinforcing cord and rubber were peeled apart at a peeling speed of 50 mm / min, and the degree of rubber adhesion to the reinforcing cord was visually observed. The adhesion was evaluated according to the following evaluation criteria. The results are shown in the "Rubber Adhesion" column of Tables 5-7.

[0092] • Evaluation criteria for the adhesion of rubber materials 3 (Good): When the rubber is so attached that the reinforcing cord is not visible. 2 (OK): When the rubber is attached to the reinforcing cord so that it is only slightly visible. 1 (Not allowed): When the reinforcement cord is clearly visible.

[0093] [Table 5]

[0094] [Table 6]

[0095] [Table 7]

[0096] As shown in Tables 1-7, the kinematic viscosity values ​​of treatment agents 21-24 (comparative examples 21-44) were outside the range of the present invention, indicating that they failed to suppress the scattering of the treatment agent and that the synthetic fibers to which the treatment agent adhered exhibited poor adhesion to the rubber material.

[0097] For treatment agents 25-30 (comparative examples 45-80), the maximum hydration viscosity in the range of 40% to 80% by mass of non-volatile content was outside the numerical range of the present invention, confirming that the scattering of the treatment agent was not suppressed and that the adhesion to rubber material was poor.

[0098] Furthermore, among the treatment agents 1 to 30, the treatment agent with a concentration of 70% by mass when applied to synthetic fibers (Comparative Examples 1 to 20, 26, 32, 38, 44, 50, 56, 62, 68, 74, 80) exhibited poor adhesion due to the excessively high concentration of the treatment agent when applied to the synthetic fibers. Therefore, it was confirmed that the evaluation of the amount of adhesion was poor, and the adhesion of the treated synthetic fibers to the rubber material was also poor.

[0099] On the other hand, according to the treatment agents 1 to 20 (Examples 1 to 100) of the present invention, the scattering of the treatment agent attached to the synthetic fiber during the spinning and drawing process can be more effectively suppressed. Furthermore, the adhesion of the synthetic fiber to which the treatment agent is attached to the rubber material can be improved.

Claims

1. A treatment agent for synthetic fibers containing a smoothing agent (A), a nonionic surfactant (B), and an ionic surfactant (C), The aforementioned synthetic fiber is for reinforcing rubber products, The maximum viscosity after hydration is 10,000 mPa·s or more in the range of non-volatile content concentration of 40% by mass or more and 80% by mass or less. Furthermore, when the non-volatile content concentration is adjusted to 25% by mass, the kinematic viscosity at 30°C is 2 mm ２ / s or more 12mm ２ / s or less, It is characterized by being applied to synthetic fibers in the form of an aqueous solution with a non-volatile content concentration of 5% by mass or more and 25% by mass or less. The aforementioned hydrolytic viscosity is the viscosity at each concentration when the non-volatile content concentration of the synthetic fiber treatment agent is changed in 5% by mass increments from 30% by mass to 100% by mass, wherein the synthetic fiber treatment agent is a synthetic fiber treatment agent.

2. The synthetic fiber treatment agent according to claim 1, wherein the hydrated viscosity of the synthetic fiber treatment agent is 10,000 mPa·s or less in the range of 85% by mass or more and 100% by mass or less for the non-volatile content concentration.

3. The synthetic fiber treatment agent according to claim 1, which contains an ester compound (A1) having a sulfur atom in its molecule as the smoothing agent (A).

4. The synthetic fiber treatment agent according to claim 1, wherein the maximum value of the hydrated viscosity is 15,000 mPa·s or more.

5. The synthetic fiber treatment agent according to claim 4, wherein the maximum value of the hydrated viscosity is 20,000 mPa·s or more.

6. The synthetic fiber treatment agent according to claim 1, wherein the maximum value of the hydrated viscosity is less than 100,000 mPa·s.

7. The synthetic fiber treatment agent according to claim 1, wherein, when the total content of the smoothing agent (A), the nonionic surfactant (B), and the ionic surfactant (C) is 100% by mass, the smoothing agent (A) is contained in a proportion of 30% by mass or more and 70% by mass or less, the nonionic surfactant (B) in a proportion of 20% by mass or more and 60% by mass or less, and the ionic surfactant (C) in a proportion of 0.1% by mass or more and 10% by mass or less.

8. A synthetic fiber for reinforcing rubber products, characterized in that it has a synthetic fiber treatment agent according to any one of claims 1 to 7 attached to it.