TW202022191A - Synthetic fiber treatment agent and method of producing synthetic fiber - Google Patents

Abstract

The present invention addresses the problem of providing a synthetic fiber treatment agent and a synthetic fiber production method with which it is possible to reduce yarn wobbling that occurs in a spinning step in which the synthetic fiber treatment agent is applied by directly supplying oil. The synthetic fiber treatment agent according to the present invention is a synthetic fiber treatment agent for directly supplying oil, and is characterized by containing at least one ester compound, as a smoothing agent, selected from the group consisting of ester compounds having three or more ester bonds in the molecule and ester compounds having elemental sulfur in the molecule, wherein moisture content is 0.1-5% by mass. Furthermore, the production method is characterized in that in the spinning step, the synthetic fiber treatment agent is used for carrying out direct oil supplying on the synthetic fibers.

Description

Synthetic fiber treatment agent and synthetic fiber manufacturing method

The present invention relates to a synthetic fiber processing agent capable of effectively reducing yarn sloshing in a synthetic fiber spinning step using a direct lubricating device, and a method for producing synthetic fibers to which the processing agent adheres.

Generally, in the spinning step of synthetic fibers, from the viewpoint of reducing friction and preventing fiber damage such as thread breakage, treatment of attaching a synthetic fiber treatment agent to the surface of synthetic fiber filaments is sometimes performed. The adhesion treatment methods include the case of diluting the synthetic fiber treatment agent in water (emulsion oiling), and the synthetic fiber treatment agent being diluted with a diluent such as low-viscosity mineral oil or undiluted and kept as it is. The condition of granting (oil directly).

Conventionally, treatment agents for synthetic fibers disclosed in Japanese Patent Application Publication No. 11-158770, Japanese Patent Application Publication No. 6-57541, Japanese Patent Application Publication No. 61-19871, and Japanese Patent Application Publication No. 2013-7141 are known. Japanese Patent Application Laid-Open No. 11-158770 discloses a synthetic fiber treatment agent that is degassed by a reduced pressure treatment. Japanese Patent Application Laid-Open No. 6-57541 discloses a treatment agent for synthetic fibers that contains a smoothing agent, an emulsifier, and a charge control agent and has a defined viscosity. JP 61-19871 A discloses a synthetic fiber treatment agent 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 synthetic fiber treatment agent containing an organozinc compound and at least one of an ester compound, a polyether compound, and a mineral oil.

The problem to be solved by the invention However, these conventional synthetic fiber treatment agents cannot sufficiently reduce yarn sloshing in the spinning step when used for direct lubrication.

The problem to be solved by the present invention is to provide a synthetic fiber processing agent and a synthetic fiber manufacturing method that can reduce yarn sloshing generated in the spinning step of applying the synthetic fiber processing agent by directly applying oil.

Means to solve the problem The inventors of the present invention have conducted studies to solve the above-mentioned problems, and as a result, found that the treatment agent for synthetic fibers, which is directly oiled in the spinning step, contains a specific ester compound as a smoothing agent and has a predetermined ratio of moisture.

That is, one aspect of the present invention provides a synthetic fiber treatment agent, which is a synthetic fiber treatment agent for direct oil application, and the treatment agent is characterized in that it contains an ester compound selected from ester compounds having three or more ester bonds in the molecule. And at least one of the ester compounds having a sulfur element in the molecule is used as a smoothing agent, and the water content in the synthetic fiber treatment agent is 0.1 to 5% by mass.

It is preferable that the said processing agent for synthetic fibers further contains a nonionic surfactant and an ionic surfactant.

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

Preferably, the ionic surfactant includes 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 organic phosphate compound The content ratio is 0.01 to 10% by mass.

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

Preferably, the water content in the treatment agent for synthetic fibers is 0.5 to 2.0% by mass.

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

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

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

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 manufacture or immediately before spinning. The water content can be adjusted by commonly known dehydration methods such as decompression and heating. Make adjustments. The water added as water to the treatment agent for synthetic fibers may be industrial water, tap water, etc., may be water purified by ion exchange resin, membrane, distillation, etc., or may be other materials used in the treatment agent composition. The ingredients and other forms are brought into the water. 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. Or the hardness of water (calculated 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 mixed water may be degassed by a known method such as reduced pressure, and may contain oxygen, nitrogen, carbon dioxide, etc. in the atmosphere. In addition, the pH is preferably near neutral, for example, in the range of 5.5 to 8.0. The water content in the synthetic fiber processing agent can be calculated and determined from the added amount of water and the water content in the raw material of the synthetic fiber processing agent. When the water content in the raw material is not clear, it can be determined by a known analysis method such as Karl Fischer titration or gas chromatography.

As the smoothing agent supplied to the treatment agent of the present embodiment, an agent containing at least one ester compound selected from an ester compound having three or more ester bonds in the molecule and an ester compound having a sulfur element in the molecule is used. With this configuration, it is possible to obtain the effect of the present invention, that is, to reduce yarn sloshing that occurs in the spinning step. In addition, roller contamination can be further reduced, and furthermore, dyeing can be performed more uniformly in the post-processing step. Examples of ester compounds having three or more ester bonds in the molecule include: (1) trimethylolpropane trilaurate, trimethylolpropane trioleate, glycerol trioleate, and pivalic acid Tetraol tetracaprylate, trimethylolpropane-coconut oil fatty acid ester and other polyhydric alcohols and monocarboxylic acid ester compounds; (2) Trioctyl trimellitate, triethyl citrate and other polycarboxylic acids Monohydric alcohol ester compounds; (3) natural 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 preferred. Examples of ester compounds having sulfur in the molecule include: (4) di(isohexadecyl)thiodipropionate, di(isostearyl)thiodipropionate, di(oleyl) ) Ester compounds of dicarboxylic acids and monohydric alcohols such as thiodipropionate; (5) Monocarboxylic acids and monohydric alcohols such as lauryl mercaptopropionate, octyl mercaptopropionate, oleyl lauryl mercaptopropionate, etc. The ester compound; etc. Among these ester compounds, di(isohexadecyl)thiodipropionate and di(isostearyl)thiodipropionate are preferred. These components may be used individually by 1 type, and may be used in combination of 2 or more types.

There are no particular limitations on the specific example of the nonionic surfactant supplied to the treatment agent of this embodiment, and examples include: (1) Addition of an alkylene oxide having 2 to 4 carbon atoms to an organic Acid, organic alcohol, organic amine, and organic amide at least one compound, more specifically such as polyoxyethylene laurate, polyoxyethylene oleate, polyoxyethylene laurate methyl ether, polyoxyethylene laurate Ethylene octyl ether, polyoxypropylene lauryl ether methyl ether, polyoxybutene oleyl ether, polyoxyethylene polyoxypropylene lauryl ether, polyoxyethylene polyoxypropylene nonyl phenyl ether, polyoxyethylene lauryl amino Ether, polyoxyethylene lauramide ether and other ether type nonionic surfactants; (2) sorbitan monooleate, sorbitan trioleate, glycerol monolaurate and other polyol partial ester type Non-ionic surfactant; (3) Polyethylene glycol dioleate, polyoxyethylene sorbitan monooleate, polyoxybutylene sorbitan trioleate, polyoxyethylene glycerol 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 Is EO) adduct and hydrogenated castor oil EO adduct at least one compound and monocarboxylic acid and dicarboxylic acid condensed ether ester compound and other polyoxyalkylene polyol fatty acid ester type nonionic surfactant (4) Alkyl amine type nonionic surfactants such as diethanolamine monolaurin; etc. These components may be used individually by 1 type, and may be used in combination of 2 or more types.

As an ionic surfactant supplied to the processing agent of this embodiment, an anionic surfactant is preferable. The specific example of the anionic surfactant to be supplied to the treatment agent of the present embodiment is not particularly limited, and examples include: (1) Carboxylic acid soaps such as potassium caprylate, potassium oleate, and potassium alkenyl succinate. Type ionic surfactant; (2) secondary alkane sulfonate sodium salt, dodecylbenzene sulfonate sodium salt, dioctyl sulfosuccinate sodium salt and other sulfonate type ionic surfactants; (3) poly Sulfate type ionic surfactants such as oxyethylene lauryl sulfate sodium salt, cetyl potassium sulfate, tallow vulcanized oil, castor oil vulcanized oil, etc. The anionic surfactant preferably contains a secondary alkane sulfonic acid compound and an organic phosphoric acid compound. As the secondary alkane sulfonic acid compound, in particular, the secondary alkane sulfonate represented by the following chemical formula 1 is more preferred, and the sodium salt is most preferred. As the organic phosphoric acid compound, oleyl phosphate EO5 (representing the number of added moles of EO. The following is the same.) Stearyl amino ether salt and isocetyl phosphate EO10 lauryl amino ether salt are more preferred. These components may be used individually by 1 type, and may be used in combination of 2 or more types.

（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 greater than or equal to 0 and a+b=5-17, and M is an alkali metal, ammonium group or organic amine group.)

The processing agent of this embodiment may further contain a diluent. Specific examples of the diluent supplied to the treatment agent of this embodiment include: (1) aromatic hydrocarbons such as xylene, toluene, benzene, and phenol; (2) ketones such as acetone and methyl ethyl ketone; (3) Methanol, ethanol, propanol, 2-ethylhexanol, dodecanol and other alcohols; (4) Hexane, heptane, octane, decane, undecane, dodecane, tridecane , Tetradecane, octadecane and other linear saturated hydrocarbons; (5) isohexane, isoheptane, isooctane, isodecane, isundecane, isododecane, isotridecane, istetradecane Branched chain hydrocarbons such as alkanes, isoctadecane, and isoeicosane; (6) Cyclic saturated hydrocarbons such as cyclohexane, cyclooctane, cyclododecane, cyclooctadecane, and cycloeicosane; (7) Low molecular weight ester compounds such as methyl octoate, ethyl laurate, propyl palmitate, isobutyl stearate, n-butyl stearate, etc.; (8) selected from paraffinic hydrocarbons, cycloalkane hydrocarbons, and olefins Solvents other than water such as mineral oil in hydrocarbons and aromatic hydrocarbons. These components may be used individually by 1 type, and may be used in combination of 2 or more types. The thinner is preferably volatilized in the spinning step, and more preferably about 70% by mass or more before and after spinning. In addition, the diluent can be added not only during the manufacture of the synthetic fiber treatment agent, but also just before spinning. Among these, mineral oil, linear saturated hydrocarbon (C10-C15), isobutyl stearate, and n-butyl stearate having a Ray's viscosity at 40°C of 100 seconds or less are preferred.

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

When the treatment agent of this embodiment contains an organophosphate compound as an ionic surfactant, when the total content of the smoothing agent, nonionic surfactant, and ionic surfactant is set to 100% by mass, the organophosphate compound The content ratio of is preferably 0.01-10% by mass. With this configuration, it is possible to obtain the effect of the present invention, that is, it is possible to further reduce yarn sloshing that occurs in the spinning step. In addition, roller contamination can be further reduced, and furthermore, dyeing can be performed more uniformly in the post-processing step.

When the treatment agent of the present embodiment contains a diluent, when the total content of the smoothing agent, nonionic surfactant, and ionic surfactant is 100 parts by mass, it is preferably in the ratio of 0.1 to 400 parts by mass Contains thinner. By limiting the blending amount to this range, it is possible to obtain the effect of the present invention, that is, it is possible to further reduce yarn sloshing that occurs in the spinning step. In addition, roller contamination can be further reduced, and furthermore, dyeing can be performed more uniformly in the post-processing step.

(Second embodiment) Next, a second embodiment of the synthetic fiber manufacturing method that specifically realizes the present invention will be described. The manufacturing method of the synthetic fiber of this embodiment is a manufacturing method including the process of attaching the processing agent of 1st Embodiment to synthetic fiber by directly supplying oil in a spinning process. There are no particular limitations on the specific examples of the synthetic fibers produced, and examples include (1) polyester fibers such as polyethylene terephthalate, polytrimethylene terephthalate, and polylactate; 2) Polyamide fibers such as nylon 6, nylon 66; (3) Polyacrylic fibers such as polyacrylic acid and modified acrylic; (4) Polyolefin fibers such as polyethylene and polypropylene; etc. The fineness of the synthetic fiber 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 produced is not particularly limited, but it 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 the treatment agent of the first embodiment to the synthetic fiber is not particularly limited, but it is preferable to make the treatment agent of the first embodiment at a ratio of 0.1 to 3% by mass (excluding diluent and water) relative to the synthetic fiber Make attachment. With this configuration, the effect of the present invention can be further enhanced. In addition, the adhesion method of the treatment agent of the first embodiment is not particularly limited. For example, well-known methods such as roller lubrication method, guided lubrication method using a metering pump, immersion lubrication method, spray lubrication method and the like can be used. 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 equipment or step having a step of passing through a roll of 150°C or higher.

According to the processing agent of the above-mentioned embodiment and the manufacturing method thereof, the following effects can be obtained.

In the treatment agent of the above-mentioned embodiment, the treatment agent used by directly 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 yarn sloshing caused by the godet roller in the spinning step of synthetic fibers. In addition, the coking pollution generated around the godet roller can be effectively reduced, and the dyeing can be evenly performed in the post-processing step. The synthetic fiber of this embodiment obtained in this way can exhibit excellent process passability. In addition, when a nonionic surfactant and an ionic surfactant are further blended to constitute the treatment agent, the above-mentioned effects are further improved.

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

・In the treatment agent of this embodiment, within the range that does not hinder the effects of the present invention, a stabilizer, charge control agent, linking agent, antioxidant, ultraviolet absorber, etc. can be further blended to maintain the quality of the treatment agent. It is a component used in synthetic fiber processing agent.

Example

Hereinafter, in order to explain the structure and effects of the present invention more specifically, examples and the like are given, but the present invention is not limited to these examples. Among them, in the following examples and comparative examples, parts refer to parts by mass, and% means mass%.

Test category 1 (manufacturing of treatment agent for synthetic fibers)

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

Further add 1.00 parts by mass of ion-exchange water when the above mixture is 100 parts by mass, and then add 10 parts by mass of linear saturated hydrocarbon (C12-15) (M-1) as a diluent and mix them uniformly, according to the treatment agent The water content in it was 0.9%, and the synthetic fiber processing agent of Example 1 was prepared. Regarding the ion exchange water used, ion exchange water having a conductivity of 0.2 μS/cm and a hardness of 0 mg/L was used.

・Preparation of synthetic fiber treatment agent (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) as an antioxidant was added at a ratio of 0.8 parts to 100 parts of the treatment agent before adding the diluent. -Tertiary butylphenyl) butane.

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

Table 1

The symbols in Table 1 indicate the following substances. L-1: Trimethylolpropane-coconut oil fatty acid ester L-2: Palm oil LS-1: Two (isostearyl) thiodipropionate LS-2: Di(isohexadecyl)thiodipropionate rL-1: Isostearyl 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 moles of EO to 1 mole of hydrogenated castor oil N-3: A compound obtained by esterifying an adduct obtained by adding EO20 mol 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: A compound obtained by esterifying an adduct obtained by adding EO25 mol to 1 mol of castor oil with 3 mol of lauric acid N-6: Compound obtained by adding 25 mol of EO to 1 mol of hydrogenated castor oil, crosslinked with adipic acid and end-esterified with stearic acid (MW5000) N-7: Polyethylene glycol (molecular weight 600) dioleate N-8: Sorbitan monooleate S-1: secondary sodium alkane sulfonate (a+b=8-11 in the above chemical formula 1) S-2: secondary sodium 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 (the mine viscosity at 40°C is 80 seconds) M-3: Isobutyl stearate M-4: n-butyl stearate

Test category 2 (evaluation of synthetic fiber treatment agent)

・Evaluation of heat resistance coking After the polyethylene terephthalate chips are dried by a conventional method, they are melt-spinned using an extruder, and the treatment agent is adhered to the die and cooled and solidified by the guide oil feeding method using a metering pump. Moving yarn silk. Lubrication was performed so that the adhesion amount of synthetic fiber treatment agent was 0.6% by mass (excluding diluent and water). After that, it was bundled by guiding, and stretched through a stretching roll at 245°C and a relaxation roll so that the full stretching magnification reached 5.5 times. The stretched yarn of 1100dtex192 filament was obtained by winding it into a 10kg flat cone. . Regarding the heat resistance (scorching), the following evaluation was performed in the form of contamination (scorching) of the godet roller (GR) after 48 hours of spinning. The results are shown in Table 1.

・Evaluation criteria for GR pollution ◎ (Excellent): Almost no contamination (coking) is observed. ○ (good): Contamination (scorching) is slightly observed. × (bad): Contamination (scorching) is observed.

・Evaluation of yarn shaking In the above-mentioned spinning step, the yarn sloshing on the godet roller was observed, and the evaluation was made according to the following criteria. The results are shown in Table 1.

・Evaluation criteria for yarn sloshing ◎(Excellent): No yarn shaking on the roller is observed ○ (good): The yarn sway on the roller is slightly observed × (bad): the yarn swaying on the roller is observed

・Evaluation of dyeability 360 fibers obtained in the above spinning step were used as warp yarns, 560 dtex-96 filament polyester yarn was used as weft yarns, and a grey fabric for heddles with a weft density of 21 yarns/inch and a width of 51 mm was used, impregnated without refining In the following dye solution (a solution 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 1L of water), continuously in the color developing tank at 220°C The treatment is performed for 2 minutes, thereby performing dyeing. The dyeability was evaluated by the following criteria based on the number of dyeing defects per 2000 m of healds. The results are shown in Table 1.

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

From the results of Table 1, it can be seen that the synthetic fiber treatment agents of each example have good evaluations of yarn sloshing, roll contamination, and dyeability. According to the present invention, the following effects can be produced: in the spinning step of synthetic fibers, yarn sloshing on the godet roller and contamination of the godet roller can be reduced, and synthetic fibers with good dyeability can be obtained.

Claims (7)

A treatment agent for synthetic fibers, which is a treatment agent for synthetic fibers used for direct oiling, and is characterized in that: Containing at least one ester compound selected from ester compounds having three or more ester bonds in the molecule and ester compounds having sulfur in the molecule as a smoothing agent, The water content in the treatment agent for synthetic fibers is 0.1 to 5% by mass. The synthetic fiber treatment agent according to claim 1, wherein It further contains nonionic surfactants and ionic surfactants. The synthetic fiber treatment agent according to claim 2, wherein The above-mentioned ionic surfactant includes a secondary alkane sulfonic acid compound, When the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100% by mass, the content of the secondary alkane sulfonic acid compound is 0.01 to 10% by mass. The synthetic fiber treatment agent according to claim 2 or 3, wherein The above-mentioned ionic surfactant includes an organophosphate compound, When the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100% by mass, the content of the organophosphate compound is 0.01 to 10% by mass. The synthetic fiber treatment agent according to any one of claims 2 to 4, wherein When the total content ratio of the smoothing agent, the nonionic surfactant, and the ionic surfactant is 100 parts by mass, the diluent is contained in a ratio of 0.1 to 400 parts by mass. The synthetic fiber treatment agent according to any one of claims 1 to 5, wherein The water content in the synthetic fiber treatment agent is 0.5 to 2.0% by mass. A method for manufacturing synthetic fibers, characterized in that: In the spinning step, the synthetic fiber treatment agent described in any one of claims 1 to 6 is directly lubricated to the synthetic fiber.