TWI775021B - Synthetic fiber treatment agent and method of producing synthetic fiber - Google Patents

Abstract

An object of the present invention is to provide a treatment agent for synthetic fibers and a method for producing a synthetic fiber capable of reducing yarn slack in the spinning step of imparting the treatment agent for synthetic fibers by direct oil supply. The treatment agent for synthetic fibers of the present invention is a treatment agent for synthetic fibers used for direct oil feeding, characterized by containing a compound selected from the group consisting of ester compounds having three or more ester bonds in the molecule and ester compounds having a sulfur element in the molecule. At least one ester compound is used as a smoothing agent, and the water content is 0.1 to 5 mass %. In addition, the method for producing a synthetic fiber of the present invention is characterized in that, in the spinning step, the synthetic fiber is directly oiled with a treatment agent for synthetic fiber.

Description

Treatment agent for synthetic fibers and method for producing synthetic fibers

The present invention relates to a treatment agent for synthetic fibers that can effectively reduce yarn flutter in the spinning step of synthetic fibers using a device for direct oil feeding, and a method for producing synthetic fibers to which the treatment agent adheres.

In general, in the spinning step of synthetic fibers, in order to reduce friction and prevent fiber damage such as thread breakage, a treatment for adhering a synthetic fiber treatment agent to the surface of the synthetic fiber filaments may be performed. The adhesion treatment can be performed by diluting the treatment agent for synthetic fibers in water (emulsion oiling), or in the case of diluting the treatment agent for synthetic fibers with a diluent such as low-viscosity mineral oil or undiluted as it is. The given situation (direct oiling).

Conventionally, there have been known treatment agents for synthetic fibers disclosed in JP 11-158770 A, JP 6-57541 A, JP 61-19871 A, and JP 2013-7141 A. Japanese Patent Application Laid-Open No. 11-158770 discloses a treatment agent for synthetic fibers that is degassed by decompression treatment. Japanese Patent Application Laid-Open No. 6-57541 discloses a treatment agent for synthetic fibers containing a smoothing agent, an emulsifier, and a charge control agent, and having a limited viscosity. Japanese Patent Laid-Open No. 61-19871 discloses a treatment agent for synthetic fibers containing mineral oil, fatty acid ester, alkylene oxide adduct of higher alcohol, and higher aliphatic unsaturated monocarboxylic acid. Japanese Patent Application Laid-Open No. 2013-7141 discloses a treatment agent for synthetic fibers containing an organozinc compound and containing at least one of an ester compound, a polyether compound, and a mineral oil.

The problem to be solved by the invention However, when these conventional treatment agents for synthetic fibers are used for direct oil feeding, the yarn slack in the spinning step cannot be sufficiently reduced.

The problem to be solved by the present invention is to provide a treatment agent for synthetic fibers and a method for producing synthetic fibers capable of reducing yarn slack in the spinning step of applying the treatment agent for synthetic fibers by direct oiling.

means of solving problems The present inventors have studied to solve the above-mentioned problems, and as a result, have found that a specific ester compound is contained as a smoothing agent in a treatment agent for synthetic fibers that is directly oil-fed in the spinning step and has a predetermined ratio of moisture.

That is, one aspect of the present invention provides a treatment agent for synthetic fibers, which is a treatment agent for synthetic fibers used for direct oil feeding, characterized in that the treatment agent contains an ester compound selected from the group consisting of ester compounds having three or more ester bonds in the molecule. And at least one ester compound among the ester compounds which have a sulfur element in a molecule|numerator as a smoothing agent, and the water content in the said processing agent for synthetic fibers is 0.1-5 mass %.

Preferably, the above-mentioned treatment agent for synthetic fibers further contains a nonionic surfactant and an ionic surfactant.

Preferably, the ionic surfactant contains a secondary alkane sulfonic acid compound, and the secondary alkane is preferably 100% by mass when the total content of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100% by mass. The content ratio of the sulfonic acid compound is 0.01 to 10 mass %.

Preferably, the ionic surfactant contains an organic phosphate compound, and when the total content of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100% by mass, the amount of the organic phosphate compound is preferably 100% by mass. The content ratio is 0.01 to 10 mass %.

Preferably, the ionic surfactant contains a diluent in a ratio of 0.1 to 400 parts by mass when the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100 parts by mass.

It is preferable that the water content in the said processing agent for synthetic fibers is 0.5-2.0 mass %.

Another aspect of the present invention provides a method for producing a synthetic fiber, characterized in that, in the spinning step, the synthetic fiber is directly oiled with the above-mentioned treatment agent for synthetic fiber.

Invention effect According to the present invention, it is possible to reduce the sloshing of yarn generated in the spinning step.

(first embodiment) First, the first embodiment of the treatment agent for synthetic fibers (hereinafter also referred to as treatment agent) that realizes the present invention concretely will be described. The treatment agent of the present embodiment contains a predetermined ester compound as a smoothing agent, is a treatment agent for synthetic fibers for direct oil feeding, and has a water content of 0.1 to 5 mass % in the treatment agent. Preferably, a nonionic surfactant and an ionic surfactant are further contained, and the water content is 0.1 to 5 mass %. More preferably, the water content in the treatment agent is 0.5 to 2.0 mass %. It is particularly preferable that the water content in the treatment agent is 0.7 to 1.5 mass %. With this configuration, the effect of the present invention, that is, it is possible to further reduce the waving of the yarn that occurs in the spinning step can be obtained.

The water content contained in the synthetic fiber treatment agent can be adjusted by adding water to the synthetic fiber treatment agent at the time of production or immediately before spinning, and the water content can be adjusted by a generally known dehydration method such as decompression and heating. Make adjustments. The water added as water in the treatment agent for synthetic fibers may be industrial water, tap water, etc., water purified by ion exchange resin, membrane, distillation, etc., or other materials in the raw material of the treatment agent composition. The water is brought into the form of ingredients etc. Among these, water with a conductivity of 300 μS/cm or less is preferable, water with a conductivity of 100 μS/cm or less is more preferable, and pure water with a conductivity of 10 μS/cm or less is most preferable. Alternatively, the hardness of water (as calcium carbonate) is preferably 100 mg/L or less, more preferably 10 mg/L or less, and more preferably 5 mg/L or less. The water to be mixed may be degassed by a known method such as depressurization, and may contain oxygen, nitrogen, carbon dioxide, and the like in the atmosphere. In addition, pH is preferably in the vicinity of neutrality, for example, in the range of 5.5 to 8.0. The water content in the treatment agent for synthetic fibers can be calculated and obtained from the amount of water added and the water content in the raw materials of the constituent substances of the synthetic fiber treatment agent. When the water content in the raw material is unclear, it can be determined by a known analytical method such as Karl Fischer titration or gas chromatography.

As the smoothing agent to be supplied to the treating agent of the present embodiment, a reagent containing at least one ester compound selected from the group consisting of ester compounds having three or more ester bonds in the molecule and ester compounds having a sulfur element in the molecule is used. With this configuration, the effect of the present invention, that is, it is possible to reduce yarn backlash that occurs in the spinning step. In addition, roll contamination can be further reduced, thereby enabling more uniform dyeing in post-processing steps. Examples of ester compounds having three or more ester bonds in the molecule include (1) trimethylolpropane trilaurate, trimethylolpropane trioleate, glyceryl trioleate, neopentyl Ester compounds of polyhydric alcohols and monocarboxylic acids such as tetraol tetracaprylate, trimethylolpropane-coconut oil fatty acid ester; (2) polycarboxylic acids such as trioctyl trimellitate, triethyl citrate, etc. Ester compounds of monohydric alcohols; (3) natural oils and fats such as castor oil, palm oil, rapeseed refined oil; etc. Among these ester compounds, trimethylolpropane-coconut oil fatty acid ester and palm oil are preferable. Examples of ester compounds having a sulfur element in the molecule include (4) di(isohexadecyl)thiodipropionate, di(isostearyl)thiodipropionate, and di(oleyl)dipropionate. ) ester compounds of dicarboxylic acids such as thiodipropionate and monohydric alcohols; (5) monocarboxylic acids and monohydric alcohols such as lauryl mercaptopropionate, octyl mercaptopropionate, and oleyl lauryl mercaptopropionate ester compounds; etc. Among these ester compounds, di(isohexadecyl)thiodipropionate and di(isostearyl)thiodipropionate are preferred. These components may be used alone or in combination of two or more.

Specific examples of the nonionic surfactant to be supplied to the treatment agent of the present embodiment are not particularly limited, and examples thereof include (1) addition of an alkylene oxide having 2 to 4 carbon atoms to an organic Compounds obtained from at least one of acids, organic alcohols, organic amines and organic amides, more specifically such as polyoxyethylene laurate, polyoxyethylene oleate, polyoxyethylene laurate methyl ether, polyoxyethylene laurate Ethylene octyl ether, polyoxypropylene lauryl ether methyl ether, polyoxybutylene oleyl ether, polyoxyethylene polyoxypropylene lauryl ether, polyoxyethylene polyoxypropylene nonyl phenyl ether, polyoxyethylene lauryl amino Ether, polyoxyethylene lauryl amide ether and other ether-type nonionic surfactants; (2) sorbitan monooleate, sorbitan trioleate, glycerol monolaurate and other polyol partial ester type Nonionic surfactants; (3) polyethylene glycol dioleate, polyoxyethylene sorbitan monooleate, polyoxybutylene sorbitan trioleate, polyoxyethylene glyceryl trioleate Esters, polyoxypropylene castor oil, polyoxyethylene hydrogenated castor oil, polyoxyethylene propylene hydrogenated castor oil trioleate, polyoxyethylene hydrogenated castor oil trilaurate, ethylene oxide selected from castor oil (hereinafter referred to as Polyoxyalkylene polyol fatty acid ester type nonionic surfactants such as ether ester compounds formed by condensing at least one compound of EO adducts and EO adducts of hydrogenated castor oil with monocarboxylic and dicarboxylic acids ; (4) Alkyl amide type non-ionic surfactants such as diethanolamine monolauryl amide; and so on. These components may be used alone or in combination of two or more.

As the ionic surfactant to be supplied to the treatment agent of the present embodiment, an anionic surfactant is preferable. Specific examples of the anionic surfactant to be used in the treatment agent of the present embodiment are not particularly limited, and examples thereof include (1) carboxylic acid soaps such as potassium octoate, potassium oleate, and potassium alkenyl succinate. (2) Sulfonate type ionic surfactants such as secondary alkane sulfonic acid sodium salt, dodecylbenzene sulfonic acid sodium salt, dioctyl sulfosuccinic acid sodium salt; (3) polymer Sulfate-type ionic surfactants such as oxyethylene lauryl sulfate sodium salt, hexadecyl sulfate potassium salt, tallow sulfide oil, castor oil sulfide oil, etc. Among the anionic surfactants, it is preferable to contain a secondary alkanesulfonic acid compound and an organic phosphoric acid compound. As the secondary alkanesulfonic acid compound, the secondary alkanesulfonic acid salt represented by the following chemical formula 1 is more preferable, and the sodium salt thereof is more preferable. As the organic phosphoric acid compound, oleyl phosphate EO5 (represents the number of added molars of EO. The same applies hereinafter) stearyl amino ether salt and isohexadecyl phosphate EO10 lauryl amino ether salt are more preferable. These components may be used alone or in combination of two or more.

（化學式1中， a、b分別為0以上的整數且a+b=5～17， M為鹼金屬、銨基或有機胺基。）Chemical formula 1

(In Chemical Formula 1, a and b are each an integer of 0 or more, a+b=5 to 17, and M is an alkali metal, an ammonium group, or an organic amine group.)

The processing agent of this embodiment may further contain a diluent. Specific examples of the diluent to be supplied to the treatment agent of the present embodiment include (1) aromatic hydrocarbons such as xylene, toluene, benzene, and phenol; (2) ketones such as acetone and methyl ethyl ketone; (3) alcohols such as methanol, ethanol, propanol, 2-ethylhexanol, dodecanol; (4) hexane, heptane, octane, decane, undecane, dodecane, tridecane , tetradecane, octadecane and other linear saturated hydrocarbons; (5) isohexane, isoheptane, isooctane, isodecane, isoundecane, isododecane, isotridecane, isotetradecane Branched chain hydrocarbons such as alkane, isooctadecane and isoeicosane; (6) cyclic saturated hydrocarbons such as cyclohexane, cyclooctane, cyclododecane, cyclooctadecane, and cycloeicosane; (7) Low molecular weight ester compounds such as methyl octanoate, ethyl laurate, propyl palmitate, isobutyl stearate, n-butyl stearate; (8) selected from paraffinic hydrocarbons, naphthenic hydrocarbons, and olefinic hydrocarbons Solvents other than water such as mineral oil in hydrocarbons and aromatic hydrocarbons. These components may be used alone or in combination of two or more. The diluent is preferably volatilized in the spinning step, more preferably about 70 mass % or more volatilized before and after spinning. In addition, the diluent may be added not only at the time of production of the synthetic fiber treatment agent, but also immediately before spinning. Among these, mineral oil, straight-chain saturated hydrocarbon (C10-C15), isobutyl stearate, and n-butyl stearate having a Raymond viscosity at 40°C of 100 seconds or less are preferred.

When the processing agent of this embodiment further contains a nonionic surfactant and an ionic surfactant in addition to the above-mentioned smoothing agent, the content ratio of each component is not particularly limited. When the treating agent of the present embodiment contains a secondary alkanesulfonic acid compound as an ionic surfactant, when the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100% by mass, the secondary The content of the alkanesulfonic acid compound is preferably 0.01 to 10% by mass. With this configuration, the effect of the present invention, that is, it is possible to further reduce the waving of the yarn that occurs in the spinning step can be obtained. In addition, dyeing can be performed more uniformly in post-processing steps.

When the treating agent of the present embodiment contains an organic phosphate compound as an ionic surfactant, the organic phosphate compound will be The content ratio of 0.01 to 10% by mass is preferred. With this configuration, the effect of the present invention, that is, it is possible to further reduce the waving of the yarn that occurs in the spinning step can be obtained. In addition, roll contamination can be further reduced, thereby enabling more uniform dyeing in post-processing steps.

When the processing agent of this embodiment contains a diluent, when the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100 parts by mass, the ratio is preferably 0.1 to 400 parts by mass Contains diluent. By limiting to this blending amount range, the effect of the present invention, that is, the yarn backlash occurring in the spinning step can be further reduced. In addition, roll contamination can be further reduced, thereby enabling more uniform dyeing in post-processing steps.

(Second Embodiment) Next, a description will be given of a second embodiment of the method for producing a synthetic fiber that specifically realizes the present invention. The manufacturing method of the synthetic fiber of this embodiment is a manufacturing method including the step of adhering the treating agent of the first embodiment to the synthetic fiber by direct oil supply in the spinning step. Specific examples of the synthetic fibers to be produced are not particularly limited, and examples thereof include (1) polyester fibers such as polyethylene terephthalate, polytrimethylene terephthalate, and polylactate; ( 2) Polyamide fibers such as nylon 6 and nylon 66; (3) Polyacrylic fibers such as polyacrylic acid and modified acrylic acid; (4) Polyolefin fibers such as polyethylene and polypropylene; and so on. The fineness of the synthetic fiber to be produced is not particularly limited, but is preferably 150 dtex or more, more preferably 500 dtex or more, and particularly preferably 1000 dtex or more. In addition, the strength of the synthetic fiber to be produced is not particularly limited, but is preferably 5.0 cN/dtex or more, more preferably 6.0 cN/dtex or more, and particularly preferably 7.0 cN/dtex or more.

The ratio of adhering the treatment agent of the first embodiment to the synthetic fibers is not particularly limited, but the treatment agent of the first embodiment is preferably a ratio of 0.1 to 3 mass % (excluding the diluent and water) with respect to the synthetic fibers. to attach. With this configuration, the effects of the present invention can be further enhanced. In addition, the method for adhering the treatment agent of the first embodiment is not particularly limited, and for example, known methods such as the roller oiling method, the guiding oiling method using a metering pump, the immersion oiling method, the spraying oiling method, etc. The spinning step is not particularly limited. For example, in the stretching or heat treatment step, the effect of the invention can be further expected by using a manufacturing facility and a step having a step of passing through a roll of 150° C. or higher.

According to the processing agent of the said embodiment, and its manufacturing method, the following effects can be acquired.

In the treatment agent of the above-described embodiment, the treatment agent used by direct oiling contains a specific ester compound as a smoothing agent, and the water content contained in the treatment agent is limited. Therefore, it is possible to effectively reduce the sway of the yarn generated by the godet in the spinning step of the synthetic fiber. In addition, the coking contamination generated around the godet can be effectively reduced, thereby enabling uniform dyeing in post-processing steps. The synthetic fiber of the present embodiment thus obtained can exhibit excellent step passability. In addition, when a nonionic surfactant and an ionic surfactant are further blended in the treatment agent and constituted, the above-mentioned effects are further enhanced.

However, the above-mentioned embodiment may be modified as follows.

・In the treatment agent of the present embodiment, a stabilizer, a charge control agent, a linking agent, an antioxidant, an ultraviolet absorber, etc. for maintaining the quality of the treatment agent may be further blended within a range that does not impair the effects of the present invention. A component used in a treatment agent for synthetic fibers.

Example

Hereinafter, in order to demonstrate the structure and effect of this invention more concretely, an Example etc. are given, but this invention is not limited to these Examples. However, in the following Examples and Comparative Examples, part means parts by mass, and % means mass %.

Test Category 1 (Manufacture of Treatment Agents for Synthetic Fibers)

・Manufacture of treatment agent for synthetic fibers (Example 1) The following components are uniformly mixed to obtain a mixture, and the above components are: 50 parts of trimethylolpropane-coconut oil fatty acid ester (L-1) as a smoothing agent, di(isostearyl) thiodipropionate (LS-1) 5 parts; 10 parts of adduct (N-1) obtained by adding 10 mol of EO to 1 mol of hydrogenated castor oil as a nonionic surfactant, and 10 parts of adduct (N-1) to 1 mol of hydrogenated castor oil 10 parts of compound (N-3) obtained by adding 20 mol of EO to the adduct obtained by esterification with 3 mol of oleic acid, 10 parts of compound (N-3) obtained by adding 25 mol of EO to 1 mol of hydrogenated castor oil. 5 parts of compound (MW5000) (N-6) obtained by crosslinking with adipic acid and end-esterification with stearic acid, and 10 parts of polyethylene glycol (molecular weight 600) dioleate (N-7); 5 parts of sodium alkane sulfonate (a+b=8-11) (S-1), oleyl phosphate (EO5) stearyl amino ether salt (P -1) 5 servings.

Further, 100 parts by mass of ion-exchanged water was added, and 10 parts by mass of linear saturated hydrocarbon (C12-15) (M-1) was added as a diluent, and the mixture was uniformly mixed. The moisture content in the compound was 0.9% to prepare the treatment agent for synthetic fibers of Example 1. As for the ion-exchanged water used, ion-exchanged water having a conductivity of 0.2 μS/cm and a hardness of 0 mg/L was used.

・Preparation of treatment agent for synthetic fibers (Example 2) It was prepared by the same method as the synthetic fiber treatment agent of Example 1. Among them, in addition to the raw materials in Table 1, 1,1,3-tris(2-methyl-4-hydroxy-5) was added as an antioxidant at a ratio of 0.8 part to 100 parts of the treatment agent before the addition of the diluent. - tert-butylphenyl)butane.

・Preparation of treatment agents for synthetic fibers (Examples 3 to 8 and Comparative Examples 1 to 3) The treatment agents for synthetic fibers of Examples 3 to 8 and Comparative Examples 1 to 3 were prepared in the same manner as the preparation of the synthetic fiber treatment agent of Example 1, and Table 1 shows the results. In addition, Table 1 shows the kind of each component in the processing agent for synthetic fibers, and shows the compounding ratio (%) when components other than the diluent and water are 100%. The addition ratio (parts) of the diluent and water in the case where the components other than the diluent and water in the processing agent for synthetic fibers are set to 100 parts is also shown. The content (%) of water in the treatment agent is also shown.

Table 1

Each symbol in Table 1 represents the following substances. L-1: Trimethylolpropane-coconut oil fatty acid ester L-2: Palm Oil LS-1: Di(isostearyl)thiodipropionate LS-2: Di(isohexadecyl)thiodipropionate rL-1: Isostearate Oleate N-1: an adduct obtained by adding 10 mol of EO to 1 mol of hydrogenated castor oil N-2: An adduct obtained by adding 20 mol of EO to 1 mol of hydrogenated castor oil N-3: A compound obtained by esterifying an adduct obtained by adding 20 mol of EO to 1 mol of hydrogenated castor oil with 3 mol of oleic acid N-4: A compound obtained by esterifying an adduct obtained by adding 40 mol of EO to 1 mol of hydrogenated castor oil with 2 mol of oleic acid N-5: Compound obtained by adding 25 mol of EO to 1 mol of castor oil and esterified with 3 mol of lauric acid N-6: Compound obtained by adding 25 mol of EO to 1 mol of hydrogenated castor oil and cross-linked with adipic acid and end-esterified with stearic acid (MW5000) N-7: polyethylene glycol (molecular weight 600) dioleate N-8: Sorbitan monooleate S-1: Sodium secondary alkane sulfonate (a+b=8～11 in the above chemical formula 1) S-2: Sodium secondary alkane sulfonate (a+b=11～14 in the above chemical formula 1) rS-1: Sodium Dioctyl Sulfosuccinate P-1: Oleyl Phosphate (EO5) Stearyl Amino Ether Salt P-2: Isohexadecyl Phosphate (EO10) Lauryl Amino Ether Salt M-1: Straight-chain saturated hydrocarbon (C12-15) M-2: Mineral oil (Ray type viscosity at 40°C for 80 seconds) M-3: isobutyl stearate M-4: n-butyl stearate

Test Category 2 (Evaluation of Synthetic Fiber Treatment Agents)

・Evaluation of heat resistance coking After drying the polyethylene terephthalate chips by a conventional method, melt spinning was performed using an extruder, and the above-mentioned treatment agent was adhered to the above-mentioned processing agent discharged from the die by the pilot oil feeding method using a metering pump, and cooled and solidified. of moving yarn. Oil feeding was performed so that the adhesion amount of the synthetic fiber treatment agent was 0.6 mass % (excluding diluent and water). After that, it was bundled by guiding, and stretched so that the full stretch ratio reached 5.5 times through a stretching roll and a relaxation roll at 245°C, and a 1100dtex192 filament stretched yarn was obtained by winding it into a 10kg flat yarn. . The heat resistance (coking) was evaluated as follows in terms of contamination (coking) of the godet roll (GR) after 48 hours of spinning. The results are shown in Table 1.

・Evaluation criteria for GR pollution ⊚ (excellent): almost no contamination (coking) was observed. ○ (good): Contamination (coking) was slightly observed. × (poor): Contamination (coking) was observed.

・Evaluation of yarn wobble In the above-mentioned spinning step, the sway of the yarn on the godet was observed and evaluated according to the following criteria. The results are shown in Table 1.

・Evaluation criteria for yarn flutter ◎ (Excellent): Yarn shaking on the roller was not observed ○ (good): Shaking of yarn on the roller is slightly observed × (defective): Yarn shaking on the roll was observed

・Evaluation of dyeability Using 360 fibers obtained in the above-mentioned spinning step as warp yarns, using polyester filaments of 560dtex-96 filaments as weft yarns, using grey fabrics for healds with a weft yarn density of 21 / inch, 51mm wide, dipping without refining In the following dyeing liquids (solutions obtained by adding 3.4 g of Dianix Red S-4G, 3.3 g of Dianix Yellow S-6G, and 3.3 g of Dianix S-2G to 1 L of water), continuously in a color developing tank at 220°C The dyeing was performed by performing the treatment for 2 minutes. The dyeability was evaluated by the following criteria based on the number of dyeing defects per 2000 m of healds at this time. The results are shown in Table 1.

・Evaluation criteria for dyeability ◎ (Excellent): The number of dyeing defects is 0 to 3 ○ (good): The number of dyeing defects is 4 to 10 × (defective): The number of dyeing defects is 11 or more

As can be seen from the results in Table 1, the synthetic fiber treatment agents of the respective Examples were all good in the evaluation of yarn waving, roll contamination, and dyeability. According to the present invention, the following effects are obtained: yarn sway on the godet roll and contamination of the godet roll can be reduced in the spinning step of synthetic fibers, and synthetic fibers with good dyeability can be obtained.

Claims (9)

A treatment agent for synthetic fibers, which is a treatment agent for synthetic fibers for directly supplying oil to synthetic fibers having a fineness of 500 dtex or more, characterized in that it contains an ester compound selected from the group consisting of ester compounds having 3 or more ester bonds in the molecule and an ester compound having three or more ester bonds in the molecule. At least one of the ester compounds having sulfur element is used as a smoothing agent, and the water content in the treatment agent for synthetic fibers is 0.1 to 5 mass %. The treatment agent for synthetic fibers according to claim 1, further comprising a nonionic surfactant and an ionic surfactant. The treatment agent for synthetic fibers according to claim 2, wherein the ionic surfactant contains a secondary alkane sulfonic acid compound, and the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is set as follows: When it is 100 mass %, the content rate of the said secondary alkanesulfonic acid compound is 0.01-10 mass %. The treatment agent for synthetic fibers according to claim 2, wherein the ionic surfactant contains an organic phosphate compound, and the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100. In mass %, the content rate of the said organic phosphate compound is 0.01-10 mass %. The treatment agent for synthetic fibers according to claim 2, wherein, when the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100 parts by mass, the amount is 0.1 to 400 parts by mass. Scale contains diluent. The treatment agent for synthetic fibers according to any one of claims 1 to 5, wherein the water content in the treatment agent for synthetic fibers is 0.5 to 2.0% by mass. The treatment agent for synthetic fibers according to any one of claims 1 to 5, wherein the water content in the treatment agent for synthetic fibers is 0.8 to 5.0% by mass. A method for producing synthetic fibers, characterized in that, in the spinning step, the synthetic fibers having a fineness of 500 dtex or more are directly oiled with the treatment agent for synthetic fibers according to any one of claims 1 to 7. A method for treating synthetic fibers having a fineness of 500 dtex or more, comprising directly oiling a treatment agent for synthetic fibers, the treatment agent for synthetic fibers comprising an ester compound having three or more ester bonds in a molecule and an ester compound having three or more ester bonds in the molecule. At least one kind of ester compound among the ester compounds of sulfur element is used as a smoothing agent, and the water content in the said processing agent for synthetic fibers is 0.1-5 mass %.