TWI894553B - Thermoplastic polyurethane elastic fiber - Google Patents

Abstract

The present invention provides a thermoplastic polyurethane elastic fiber with excellent _NOx gas yellowing resistance and heat resistance. The present invention relates to a thermoplastic polyurethane elastic fiber characterized by containing at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% to 5.00 wt%, wherein the metal compound comprises an alkali metal or an alkaline earth metal.

Description

Thermoplastic polyurethane elastic fiber

The present invention relates to a thermoplastic polyurethane elastic fiber.

Polyurethane elastic fibers are commonly used in clothing and sanitary materials. Yellowing resistance and heat resistance are required for polyurethane elastic fibers used in clothing and sanitary materials. Patent Document 1 below discloses that dry-spinning polyurethane elastic fibers containing hindered amine compounds can improve _NOx gas yellowing resistance. Patent Document 2 below discloses that the _NOx gas yellowing resistance of polyurethane resins can be improved by combining a phenolic antioxidant with a hindered amine light stabilizer, a polyester compound, and a benzotriazole light stabilizer. Patent Document 3 below discloses that the heat resistance of polyurethane elastic fibers can be improved by melt-spinning a thermoplastic polyurethane resin obtained by reacting a polyol with a diisocyanate, and a polyol, a diisocyanate, and a low-molecular-weight diol, with a prepolymer having hydroxyl groups at both ends. Patent Document 4 below discloses that the heat resistance of polyurethane elastic fibers can be improved by using an oil containing polydimethylsiloxane and a phenolic antioxidant in combination. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2006-342448 [Patent Document 2] Japanese Patent Publication No. 2009-19062 [Patent Document 3] Japanese Patent Publication No. 2006-307409 [Patent Document 4] Japanese Patent Publication No. 2003-20521

[Identify the problem you want to solve]

However, references 1 to 4 do not disclose thermoplastic polyurethane elastic fibers having both _NOx gas yellowing resistance and heat resistance.

In view of the above-mentioned problems of the prior art, the problem to be solved by the present invention is to provide a thermoplastic polyurethane elastic fiber with excellent resistance to yellowing caused by _NOx gas and heat resistance. [Technical Means for Solving the Problem]

To address the aforementioned issues, the inventors conducted diligent research and repeated experiments, resulting in the unexpected discovery of a thermoplastic polyurethane elastic fiber that can address the aforementioned issues. The thermoplastic polyurethane elastic fiber is characterized by containing from 0.05 wt% to 5.00 wt% of at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides, wherein the metal compound comprises an alkali metal or alkaline earth metal. This led to the completion of the present invention.

That is, the present invention is as follows. [1] A thermoplastic polyurethane elastic fiber characterized by containing at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% to 5.00 wt%, and the metal compound contains an alkali metal or an alkali earth metal. [2] The thermoplastic polyurethane elastic fiber as described in [1] above, wherein the metal compound contains an alkali earth metal. [3] The thermoplastic polyurethane elastic fiber as described in [2] above, wherein the alkali earth metal is magnesium. [4] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [3] above, wherein the metal compound is magnesium hydroxide. [5] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [4] above, wherein the polyurethane constituting the thermoplastic polyurethane elastic fiber is a polyurethane obtained by polymerization of a polymer polyol, a diisocyanate, and a chain extender containing an active hydrogen compound. [6] The thermoplastic polyurethane elastic fiber as described in [5] above, wherein the chain extender is a diol having a molecular weight of 60 or more and 120 or less. [7] The thermoplastic polyurethane elastic fiber as described in [5] or [6] above, wherein the diisocyanate is 4,4'-diphenylmethane diisocyanate (MDI). [8] The thermoplastic polyurethane elastic fiber as described in any one of the above [5] to [7], wherein the ratio of the hard chain segment of the chain extender to the diisocyanate (Mh fraction) is 20% or more and 40% or less. [9] The thermoplastic polyurethane elastic fiber as described in any one of the above [5] to [8], wherein the total molar number of the chain extender and the polymer polyol relative to the molar number of the diisocyanate is 1.001 times or more and 1.100 times or less. [10] The thermoplastic polyurethane elastic fiber as described in any one of the above [1] to [9], wherein the total fiber fineness is 160 dtex or more and 2000 dtex or less. [11] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [10] above, which is a multifilament. [12] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [11] above, wherein the fiber unevenness coefficient in the filament length direction is 3.0% or more and 10.0% or less. [13] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [12] above, wherein the difference between the maximum fiber length and the minimum fiber length in the filament length direction is 10 dtex or more and 150 dtex or less. [14] The thermoplastic polyurethane elastic fiber as described in any one of [1] to [13] above, wherein the outflow starting temperature of the thermoplastic polyurethane elastic fiber measured by a flow tester is 150°C or higher and 220°C or lower. [Effects of the Invention]

The thermoplastic polyurethane elastic fiber of one embodiment of the present invention has the above-mentioned structure and thus has excellent _NOx gas yellowing resistance and heat resistance.

The following describes an embodiment of the present invention (hereinafter referred to as "this embodiment") in detail. The present invention is not limited to the following embodiment and can be implemented with modifications within the scope of its gist.

[Metal Compound] The thermoplastic polyurethane elastic fiber of this embodiment is characterized by containing at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% to 5.00 wt%, preferably 0.10 wt% to 1.00 wt%, and more preferably 0.30 wt% to 0.50 wt%. By containing at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% to 5.00 wt%, the fiber exhibits excellent resistance to yellowing due to _NOx gas and heat resistance. The reason why the inclusion of at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% to 5.00 wt% improves both _NOx gas yellowing resistance and heat resistance is unclear, but the inventors speculate as follows. The inclusion of at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides in an amount of 0.05 wt% or more effectively enhances _NOx gas yellowing resistance. Furthermore, the inclusion of at least one metal compound in an amount of 5.00 wt% or less prevents excessive reduction in the polymer ratio in the polyurethane elastic fiber, maintaining heat resistance.

The metal compound preferably contains an alkali metal or an alkali earth metal as the metal element. Furthermore, it is more preferred that it contains an alkali earth metal. Furthermore, as the alkali earth metal, calcium or magnesium is preferred, and magnesium is more preferred. If the metal element is an alkali metal or an alkali earth metal, the effect of improving the resistance to _NOx gas yellowing is further enhanced. The reason why the resistance to _NOx gas yellowing can be improved by setting the metal element of the metal compound to an alkali earth metal is not yet clear, but the inventors presume as follows. It is presumed that since alkali earth metals have a larger charge, they are more likely to adsorb _NOx gas, thereby suppressing the attack of _NOx gas on the thermoplastic polyurethane elastic fiber, and thus the resistance to _NOx gas yellowing of the thermoplastic polyurethane elastic fiber is improved.

If the metal compound is magnesium hydroxide, the effect of improving resistance to _NOx gas yellowing is particularly advantageous. The reason why using magnesium hydroxide as the metal compound improves resistance to _NOx gas yellowing is not yet clear, but the inventors speculate as follows. It is speculated that magnesium hydroxide, a solid base with a relatively strong alkaline strength, readily reacts with acidic _NOx gas, resulting in improved resistance to _NOx gas yellowing.

[Thermoplastic Polyurethane] In this embodiment, the thermoplastic polyurethane constituting the thermoplastic polyurethane elastic fiber is not particularly limited as long as it has a structure formed by polymerizing, for example, a diisocyanate, a polymer polyol, a diol, and a diamine, and exhibits thermoplastic properties. Furthermore, the polymerization method is not particularly limited. Examples of thermoplastic polyurethanes include those formed by polymerizing a diisocyanate, a polymer polyol, and a low-molecular-weight diamine as a chain extender containing an active hydrogen compound. Furthermore, examples of thermoplastic polyurethanes include those formed by polymerizing a diisocyanate, a polymer polyol, and a low-molecular-weight diol as a chain extender containing an active hydrogen compound (hereinafter also referred to as "polyurethane urethane"). Trifunctional or higher-functional diols or isocyanates may also be used as long as they do not hinder the desired effects of the present invention. Furthermore, in this specification, "thermoplastic" refers to a material having the reversible property of being able to melt upon heating below its decomposition temperature, exhibiting plastic flow while molten, and solidifying upon cooling. Polyurethane resins typically begin to decompose at temperatures above 230°C.

[Polymer Polyol] Examples of polymer polyols include, but are not limited to, polyether diols, polyester diols, and polycarbonate diols. From the perspective of hydrolysis resistance, polyether polyols are preferred.

Examples of polyether polyols include polyethylene glycol, polypropylene glycol, polytetramethylene ether glycol, copolymers of tetrahydrofuran (THF) and neopentyl glycol, and copolymer glycols of THF and 3-methyltetrahydrofuran. These polyether polyols may be used alone or in combination. Furthermore, from the perspective of easily obtaining elastic fibers with excellent elongation, stretch recovery, and heat resistance, the number average molecular weight of the polymer glycol is preferably from 1000 to 8000. From the perspective of light embrittlement, polytetramethylene ether glycol, copolymers of THF and neopentyl glycol, and polyols blended with these are preferred as polyether polyols.

[Diisocyanates] Examples of diisocyanates include aromatic diisocyanates, alicyclic diisocyanates, and aliphatic diisocyanates. Examples of aromatic diisocyanates include, but are not limited to, diphenylmethane diisocyanate (hereinafter also referred to as "MDI"), toluene diisocyanate, 1,4-phenylenediisocyanate, xylylenediisocyanate, and 2,6-naphthalene diisocyanate. Examples of cycloaliphatic and aliphatic diisocyanates include methylenebis(cyclohexyl isocyanate) (hereinafter also referred to as "H12MDI"), isophorone diisocyanate, methylcyclohexane 2,4-diisocyanate, methylcyclohexane 2,6-diisocyanate, cyclohexane 1,4-diisocyanate, hexahydroxylenediisocyanate, hexahydrotoluene diisocyanate, and octahydro-1,5-naphthalene diisocyanate. These diisocyanates may be used alone or in combination. In particular, from the perspective of elastic fiber stretch recovery, aromatic diisocyanates are preferred, and MDI is particularly preferred. Furthermore, by using MDI, a ring structure is introduced into the polymer skeleton, thereby increasing rigidity and improving heat resistance.

[Chain Extender] The chain extender comprising an active hydrogen compound is preferably at least one selected from the group consisting of low molecular weight diamines and low molecular weight diols. Furthermore, the chain extender may contain both hydroxyl and amino groups in the molecule, such as ethanolamine. From the perspective of obtaining a thermoplastic polyurethane suitable for melt spinning, a low molecular weight diol is preferred as the active hydrogen compound.

Examples of low-molecular-weight diamines used as chain extenders for active hydrogen compounds include hydrazine, ethylenediamine, 1,2-propylenediamine, 1,3-propylenediamine, 2-methyl-1,5-pentanediamine, 1,2-butylenediamine, 1,3-butylenediamine, 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane, 2,2-dimethyl-1,3-diaminopropane, 1,3-diamino-2,2-dimethylbutane, 2,4-diamino-1-methylcyclohexane, 1,3-pentanediamine, 1,3-cyclohexanediamine, bis(4-aminophenyl)phosphine oxide, hexamethylenediamine, 1,3-cyclohexyldiamine, hexahydro-m-phenylenediamine, 2-methylpentanediamine, and bis(4-aminophenyl)phosphine oxide.

Examples of low-molecular-weight diols used as chain extenders for active hydrogen compounds include ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, dihydroxyethoxybenzene, dihydroxyethylene terephthalate, 1-methyl-1,2-ethanediol, 1,6-hexanediol, and 1,8-octanediol. These low-molecular-weight diols may be used alone or in combination.

Regarding the chain extender, from the perspective of improving the stretch recovery of the elastic fiber and improving heat resistance and _NOx gas yellowing resistance, diols with a molecular weight of 60 to 120 are preferred. Preferred active hydrogen compounds of the diols with a molecular weight of 60 to 120 are ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, and 1,8-octanediol. More preferred are 1,3-propylene glycol, 1,4-butanediol, and 1,6-hexanediol, with 1,4-butanediol being the most preferred.

[Thermoplastic Polyurethane Synthesis Method] Thermoplastic polyurethane can be obtained using known polyurethane esterification techniques, or it can be produced via either the single-tank process or the prepolymer process. In the prepolymer process, a polymer polyol and a diisocyanate are added to a reaction vessel equipped with nitrogen flushing, a hot water jacket, and a stirrer, and reacted at a molar ratio of preferably 1.0:1.8-3.0, more preferably 1.0:2.0-2.5, to obtain a prepolymer containing isocyanate groups at both ends. A chain extender is then added to the prepolymer containing isocyanate groups at both ends to effect chain extension. This reaction is followed by solid-phase polymerization to obtain a polyurethane of a desired molecular weight. Alternatively, after uniformly mixing the prepolymer and chain extender, the polymer can be obtained continuously or semi-continuously using a cylindrical tube or a twin-screw extruder, followed by solid-phase polymerization.

If the combined molar number of the chain extender and the polymer polyol relative to the molar number of the diisocyanate is 1.001 to 1.100 times, both heat resistance and resistance to _NOx gas yellowing are achieved, which is preferable. The reason why the combined molar number of the chain extender and the polymer polyol relative to the molar number of the diisocyanate is 1.001 to 1.100 times the molar number of the diisocyanate improves both resistance to _NOx gas yellowing and heat resistance is not yet clear, but the inventors speculate as follows: If the combined molar number of the chain extender and the polymer polyol relative to the molar number of the diisocyanate is 1.001 to 1.100 times the molar number of the diisocyanate, the amount of diisocyanate-derived structures within the molecules, which are susceptible to _NOx gas adsorption, is reduced, thereby improving resistance to _NOx gas yellowing. On the other hand, if the combined molar ratio of the chain extender and polymer polyol relative to the molar ratio of the diisocyanate is less than 1.100, ligand exchange between the hydroxyl groups of the thermoplastic polyurethane and the metal salt is less likely to occur, allowing the metal salt's NOx _- resistant yellowing effect to be more effectively exerted, thereby improving NOx _- resistant yellowing properties. Furthermore, if the combined molar ratio of the chain extender and polymer polyol relative to the molar ratio of the diisocyanate is greater than 1.001, the molecular weight of the thermoplastic polyurethane is more readily increased, thereby improving heat resistance.

[Method for Producing Thermoplastic Polyurethane Elastic Fiber] The spinning method is not particularly limited as long as the desired physical properties are achieved. For example, in addition to feeding thermoplastic polyurethane chips into an extruder, heating, and melt-spinning, other methods include: melting the chips and then mixing them with a polyisocyanate compound for spinning; and adding a reaction product of a double-terminal isocyanate prepolymer and an active hydrogen compound to a double-terminal isocyanate prepolymer for continuous spinning without fragmentation.

The polyurethane is metered into the extruder by a metering pump and fed into the spinning head. Filtered as needed through a metal mesh or glass beads to remove foreign matter, the yarn is then ejected from the head, cooled in a cold air chamber, treated with a treatment agent, and then taken up by a guide roller.

During the spinning process, the die head temperature, cold air velocity, cold air temperature, bundle position, and spinning speed are adjusted to closely control the fiber temperature distribution and spinning tension. The die head temperature is preferably between 180°C and 220°C, more preferably between 200°C and 210°C. The cold air is blown perpendicularly to the direction of yarn travel from directly below the spinning nozzle, using conventional melt-spinning cooling methods. The cold air velocity is preferably between 0.2 m/s and 2.0 m/s, more preferably between 0.5 m/s and 1.2 m/s, and the cold air temperature is preferably between 5°C and 20°C, more preferably between 7°C and 15°C. As a method of bundling multifilaments, the following method can be exemplified: a false twisting machine is installed between the head and the godet roller, and the twist is propagated from the bottom according to the strength of the twist, so that the filaments are bundled with each other, and the height of the bundling point is controlled. The method of false twisting can be selected from the usual methods, such as air false twisting using an air nozzle, or a ring false twisting machine that is in contact with a rotating ring.

The method for incorporating at least one metal compound selected from metal hydroxides, metal carbonates, and metal oxides into the thermoplastic polyurethane elastic fiber of the present embodiment at a concentration of 0.05 wt % to 5.00 wt % is not particularly limited. Examples include adding the metal compound to the prepolymer prior to the reaction of the polymer polyol with the diisocyanate, adding the metal compound to the prepolymer during the chain extension reaction between the prepolymer and the active hydrogen compound, and adding a masterbatch containing the metal compound to the spinning process.

The thermoplastic polyurethane elastic fiber of this embodiment may contain polymers other than polyurethane, or additives such as antioxidants, light stabilizers, ultraviolet absorbers, gas discoloration inhibitors, dyes, surfactants, matting agents, pigments, lubricants, etc., as long as the desired effects of the present invention are not impaired.

From the perspectives of release and ease of application, the thermoplastic polyurethane elastic fiber of this embodiment may also contain a treatment agent such as an oil. Examples of treatment agents include, but are not limited to, silicone oils such as dimethyl silicone, mineral oils, and combinations thereof. The treatment agent application method is not particularly limited; for example, application using an oil roller is possible.

In the thermoplastic polyurethane elastic fiber of this embodiment, the ratio of the hard chain segment containing the chain extender and the diisocyanate (hereinafter referred to as the Mh fraction) is preferably 20% to 40%, more preferably 20% to 35%, and even more preferably 22% to 30%. When the Mh fraction is 20% to 40%, both heat resistance and resistance to _NOx gas yellowing are improved. The reason why a Mh fraction of 20% to 40% improves both _NOx gas yellowing resistance and heat resistance is not yet clear, but the inventors speculate as follows. If the Mh fraction is 20% or higher, the number of hydrogen bonds between urethane bonds increases, improving heat resistance. Furthermore, the presence of metal salts around the hard chain segments increases, improving resistance to _NOx gas yellowing. On the other hand, if the Mh fraction is 40% or lower, when the diisocyanate contains aromatic rings, the amount of aromatic rings that yellow due to _NOx gas adsorption is reduced, thereby improving resistance to _NOx gas yellowing. The detailed calculation method for the Mh fraction is described below.

The total fiber density of the polyurethane elastic fiber of this embodiment is preferably 160 dtex to 2000 dtex, more preferably 300 dtex to 1500 dtex, and even more preferably 600 dtex to 1000 dtex.

The polyurethane elastic fiber of this embodiment can be either monofilament or multifilament, preferably multifilament. When the polyurethane elastic fiber is multifilament, the number of monofilaments is preferably 14 or more and 140 or less.

The thermoplastic polyurethane elastic fiber of this embodiment preferably has a fiber-length-direction nonuniform variation coefficient of 3.0% to 10.0%, more preferably 3.0% to 9.5%, and even more preferably 3.5% to 9.0%. A fiber-length nonuniform variation coefficient of 3% to 10% improves both _NOx gas yellowing resistance and heat resistance. While the reason for the improved _NOx gas yellowing resistance and heat resistance achieved by a fiber-length nonuniform variation coefficient of 3.0% to 10.0% is not yet clear, the inventors speculate as follows. If the fiber variation coefficient is 3.0% or higher, light is easily diffused on the fiber surface, making the fiber appear opaque. This makes yellowing inside the fiber less visible and the yellowing appears less noticeable. If the fiber variation coefficient is 10.0% or lower, breakage caused by heat in fine fiber areas is suppressed, improving heat resistance. There are no particular limitations on methods for controlling the coefficient of variation in fiber density, as long as the desired properties can be achieved. Examples include: increasing the diameter of the spinning nozzle used in melt spinning to produce tensile resonance; increasing the jet flow to produce sharkskin or melt fracture; and varying the cooling intensity during the spinning step to induce yarn wobble.

The difference between the maximum and minimum fiber lengths of the thermoplastic polyurethane elastic fiber of this embodiment is preferably 10 dtex to 150 dtex, more preferably 15 dtex to 100 dtex, and even more preferably 20 dtex to 80 dtex. A maximum fiber length difference of 10 dtex to 150 dtex improves both _NOx gas yellowing resistance and heat resistance. While the reason for the improvement in _NOx gas yellowing resistance and heat resistance by limiting the maximum fiber length difference to 10 dtex to 150 dtex is unclear, the inventors speculate as follows. If the difference between the maximum and minimum fiber sizes is 10 dtex or greater, light is diffusely reflected from the fiber surface, making the fiber appear opaque. This makes yellowing within the fiber less noticeable and the yellowing appears less noticeable. If the difference between the maximum and minimum fiber sizes is 150 dtex or less, breakage caused by heat exposure in finer fiber areas is suppressed, improving heat resistance. There are no particular limitations on methods for controlling fiber density differences, as long as the desired properties are achieved. Examples include: increasing the diameter of the spinning nozzle used in melt spinning to produce tensile resonance; increasing the jet flow to produce sharkskin or melt fracture; and varying the cooling intensity during the spinning step to induce yarn wobble.

From the perspective of improving heat resistance and resistance to yellowing due to _NOx gas, the thermoplastic polyurethane elastic fiber of this embodiment preferably has a flow start temperature of 150°C to 220°C, more preferably 150°C to 200°C, as measured using a flow tester. The reason why a flow start temperature of 150°C to 220°C improves heat resistance and resistance to yellowing due to _NOx gas is not yet clear, but the inventors speculate as follows. By setting the outflow start temperature above 150°C, the structural changes of the thermoplastic polyurethane due to heat are reduced, thereby improving heat resistance. On the other hand, by setting the outflow start temperature below 220°C, the viscosity during melting is reduced, and the wettability is improved. As a result, the metal salt is evenly dispersed, thereby improving the resistance to yellowing caused by _NOx gas. [Example]

The present invention is described in detail with reference to the following examples and comparative examples. However, the scope of the present invention is not limited by these examples. First, the evaluation methods for physical properties, etc., used in the examples and comparative examples will be described.

<Quantification of Thermoplastic Polyurethane Components> The structures of the chain extender containing an active hydrogen compound and the diisocyanate contained in the thermoplastic polyurethane elastic fiber were determined using NMR. Specifically, NMR measurements were performed under the following conditions to determine the structures of the diisocyanate and chain extender. The structures of the diisocyanate and chain extender can be determined based on the peak positions obtained by NMR measurements. Measurement equipment: Bruker Biospin Avance600 Detected nucleus: ¹ H Resonance frequency: 600 MHz Accumulated times: 256 Measurement temperature: Room temperature Solvent: Deuterated dimethylformamide Measurement concentration: 1.5 wt% Chemical shift reference: Dimethylformamide (8.0233 ppm)

<Calculation method of the ratio of the total molar number of chain extender and polymer polyol to the molar number of diisocyanate (hereinafter referred to as OH/NCO)> The OH/NCO ratio of thermoplastic polyurethane elastic fiber is calculated by the integral value of the corresponding peak obtained by NMR measurement according to the following formula (1): OH/NCO＝{(Hh＋Hs)/4}/(Hi/x)…Formula (1) Wherein, Hh: integral value of methylene groups derived from active hydrogen compounds adjacent to the urethane bond Hs: integral value of methylene groups derived from polymer polyol adjacent to the urethane bond Hi: integral value of hydrogen compounds in diisocyanate x: total number of hydrogen atoms in diisocyanate.

<Quantitative method of the ratio of hard chain segments (Mh fraction)> The Mh fraction of thermoplastic polyurethane elastic fiber is calculated by solving the following simultaneous equations of formulas (2) to (5): Ms＝{Mdo＋Mdi(N1－N0)}/(N1－N0－1)－2Mdi…Formula (2) Mh＝{Mda(N1－1)＋Mdi×N0}/(N1－N0－1)＋2Mdi…Formula (3) N0＝0.03806N1 ⁴ －0.3997N1 ³ ＋1.617N1 ² －2.144N1 ¹ ＋0.8795…Formula (4) Mh fraction (%)＝{Mh/(Ms＋Mh)}×100…Formula (5). Wherein, Ms: number-average molecular weight of the soft segment; Mdo: number-average molecular weight of the polymer polyol; Mdi: molecular weight of the isocyanate; N1: molar ratio of isocyanate to polymer polyol; N0: molar ratio of unreacted isocyanate to polymer polyol; Mh: number-average molecular weight of the hard segment; Mda: molecular weight of the chain extender (the number-average molecular weight when two or more are mixed); and Mdi: molecular weight of the isocyanate.

<Method for Identification and Quantification of Metal Compounds> Thermoplastic polyurethane elastomer fibers are wrapped around a glass plate and analyzed using XRD (RIGAKU Ultima-IV). The chemical composition of the metal compounds present is determined by comparing the analyzed spectrum with data in the database. After XRD identification of the metal compounds, a sample is prepared by seamlessly wrapping the thermoplastic polyurethane elastomer fiber around a porous PP film. This sample is then analyzed using XRF (RIGAKU ZSX-100e). The metal compound content is quantified based on the detection intensity of the elements that make up the metal compound. A calibration curve using the same metal compound as the metal compound can be used for quantification.

<Measurement of the Flow Initiation Temperature of Thermoplastic Polyurethane Elastomer Fiber> The flow initiation temperature of thermoplastic polyurethane elastomer fiber was measured using a flow tester, model CFT-500D (manufactured by Shimadzu Corporation). The thermoplastic polyurethane elastomer fiber was not pretreated with degreasing agents or other treatment agents. A 1.5 g sample was taken per measurement to determine the flow initiation temperature. A die (nozzle) with a diameter of 0.5 mm and a thickness of 1.0 mm was used. An extrusion load of 49 N was applied. At an initial set temperature of 120°C, after a preheating period of 240 seconds, the stroke length (mm) versus temperature curve was calculated when the temperature was raised at a constant rate of 3°C/minute to 250°C. As the temperature rises, the polymer in the color enhancer is heated and begins to flow out of the die. The temperature at this point is considered the flow start temperature.

<Fiber Gage Variation Coefficient Measurement Method> The fiber gage variation coefficient is measured by adjusting the rotational speed of two guide rolls to hold a thermoplastic polyurethane elastic fiber at a two-fold elongation. The following device is installed between the guide rolls. The elastic fiber's outer diameter is measured in two perpendicular directions using a laser. The fiber gage variation coefficient is calculated as the ratio of the average deviation of the diagonal lengths to the average value, calculated using the Pisces law. The measurement data is the average of 50,000 points measured at 160 points/second. Measuring device: LS9006D (Keyence Corporation) Measurement type: Outside diameter Minimum display unit: 0.0001 mm Number of measuring points: 50,000 Accumulation cycle: ×100

<Fiber Determination Method> The fiber was determined by cutting the thermoplastic polyurethane elastic fiber vertically, observing the cross section of the yarn using the following apparatus and conditions, and calculating the total cross-sectional area of the yarn by automatic area measurement. The fiber density per unit length was calculated using the following formula (6): d = D × 1.1 (g/cm ³ ) × 10 ⁶ ... Formula (6) {wherein, d is the fiber density (dtex), and D is the total cross-sectional area of the yarn (cm ² )}. Measuring device: VHX-7000 (manufactured by KEYENCE Corporation) Used lens: VH-Z100R Magnification: 500 times Measurement: Automatic area measurement Extraction method: Brightness <Method for determining the difference between the maximum and minimum fiber lengths of polyurethane elastic fibers> The difference between the maximum and minimum fiber lengths is determined by cutting the thermoplastic polyurethane elastic fiber vertically and observing the yarn cross section using the following device and conditions. The total cross-sectional area of the yarn is calculated by automatic area measurement. The fiber length is calculated at 10 points with an interval of 5 mm in the yarn length direction using the above formula (6). The difference between the maximum and minimum fiber lengths at this time is used to calculate the fiber length. Measuring device: VHX-7000 (manufactured by KEYENCE Corporation) Lens: VH-Z100R Magnification: 500x Measurement: Automatic area measurement Extraction method: Brightness

Heat Resistance Evaluation Method: Thermoplastic polyurethane elastic fiber was held stretched to 200% and pressed against a 110°C heat source. The time (heat-cut seconds) required for the fiber to break was used as an indicator of heat resistance.

<Evaluation Method for Yellowing Resistance to _NOx Gas> 1. ΔYI Value Yellowing resistance was evaluated using thermoplastic polyurethane elastic fibers according to the JIS-L-0855 test method for color fastness to nitrogen oxides and the weak test method. The yellowness YI value obtained using a MacBest colorimeter (manufactured by MacBest) was compared with the untreated sample's YI0, and the ΔYI value calculated using the following formula (7) was used for evaluation: ΔYI = YI - YI0... Formula (7). The smaller the ΔYI value, the less likely it is to yellow, while the larger the ΔYI value, the more likely it is to yellow.

2. Yellowing Visibility The yellowed thermoplastic polyurethane elastomer fiber from method 1 above was compared to a color code and assigned a 10-point rating. Specifically, 18 participants (10 in their 20s and 2 in their 30s to 60s, respectively) were asked to select the color code that most closely resembled the yellowed thermoplastic polyurethane elastomer fiber. The average score was used as the yellowing visibility rating. The color code and rating used are as follows: higher scores indicate less yellowing. #FFD500: 1 point #FFD91A: 2 points #FFDD33: 3 points #FFE14D: 4 points #FFE666: 5 points #FFEA80: 6 points #FFEE99: 7 points #FFF2B3: 8 points #FFF7CC: 9 points #FFFBE6: 10 points

Both the ΔYI value and the yellowing perceptibility evaluation measure resistance to _NOx gas yellowing. Since the ΔYI value is not affected by human visual perception, whereas the yellowing perceptibility is, comparing the yellowing perceptibility of samples with the same ΔYI value allows for evaluation of _NOx gas yellowing resistance based on human visual perception.

[Example 1] <Synthesis of Thermoplastic Polyurethane Resin> 2400 g of polytetramethylene ether glycol (number average molecular weight: 1800) and 750.78 g of 4,4'-diphenylmethane diisocyanate were stirred at 60°C for 3 hours under a dry nitrogen atmosphere to produce a polyurethane prepolymer terminated with isocyanate groups. 151.20 g of 1,4-butanediol was added to the polyurethane prepolymer and stirred for 15 minutes to produce a polyurethane with a viscosity of 2000 poise (at 30°C). The polyurethane is then placed on a Teflon (registered trademark) tray and annealed in a hot air oven at 110°C for 16 hours to obtain a thermoplastic polyurethane resin.

<Masterbatch Preparation> The thermoplastic polyurethane resin obtained in this manner was pulverized into a powder approximately 3 mm in diameter using a Horai UG-280 pulverizer. The pulverized fragments were dried in a dehumidifier at 110°C to a moisture content of 100 ppm. The polyurethane resin powder and magnesium hydroxide were then added to a hopper at a specified ratio and melted in an extruder to form strands. The strands were then cooled in a 20°C water bath and pelletized using an Isuzu Kakoki SCF-100 plastics processing machine to produce a masterbatch containing 10 wt% magnesium hydroxide as an active ingredient.

Production of Thermoplastic Polyurethane Elastic Fibers A magnesium hydroxide-containing polyurethane resin powder, prepared by mixing thermoplastic polyurethane resin powder and a magnesium hydroxide masterbatch at a weight ratio of 95:5, is metered and pressurized by a gear pump mounted on the die head. After filtering through filter paper, the powder is ejected from a 0.23 mm diameter, 60-hole nozzle at a die temperature of 210°C to a density of 620 dtex. Cold air is then blown perpendicularly to the fibers from a cold air chamber at a temperature of 15-17°C and a velocity of 0.8-1.0 m/s, thereby melt-spinning the fibers. After that, a ring-type false twisting machine is used to spread the twist to the multifilament. On one side, a treatment agent containing polydimethylsiloxane and mineral oil as the main components is applied, and on the other side, it is rolled into a paper tube to obtain a coiled body of 620 dtex/60 F thermoplastic polyurethane elastic fiber. The thermoplastic polyurethane elastic fiber contained 0.50 wt% magnesium hydroxide, had an Mh fraction of 24%, a fiber inhomogeneity coefficient of 4.0%, an OH/NCO ratio of 1.010, a thermal cutting time of 600 seconds or more, a ΔYI value of 8, a difference between the maximum and minimum fiber sizes of 30 dtex, an outflow starting temperature of 160°C, and a yellowing visual evaluation score of 10. The results are also shown in Table 1 below.

[Examples 2-6] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the ratio of polyurethane resin to masterbatch was adjusted and the amount of magnesium hydroxide contained in the polyurethane elastic fiber was increased or decreased. The results are shown in Table 1 below.

[Examples 7-12] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the metal compound was replaced with magnesium carbonate (Example 7), magnesium oxide (Example 8), calcium hydroxide (Example 9), calcium carbonate (Example 10), sodium carbonate (Example 11), and potassium carbonate (Example 12). The results are shown in Table 1 below.

[Examples 13-17] Thermoplastic polyurethane elastic fibers were obtained by the same method as Example 1, except that the chain extender containing an active hydrogen compound was replaced with ethylene glycol (Example 13), 1,3-propylene glycol (Example 14), 1,6-hexanediol (Example 15), 1,8-octanediol (Example 16), and 1,10-decanediol (Example 17). The results are shown in Table 1 below.

[Examples 18 and 19] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that methylene bis(cyclohexyl isocyanate) (H12MDI) (Example 18) and 1,6-hexamethylene diisocyanate (HDI) (Example 19) were used instead. The results are shown in Table 1 below.

[Examples 20-26] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the molar ratio of the polymer polyol to the diisocyanate was adjusted to increase or decrease the Mh fraction of the thermoplastic polyurethane elastic fibers. The results are shown in Table 1 below.

[Table 1] [Table 1] metal compounds chain extenders Diisocyanate Mh fraction OH/NCO Fiber inequality variation coefficient Poor fineness Total fiber density Outflow starting temperature Thermal cutting seconds ΔYI value Yellow color visual recognition Kind Addition rate (wt%) Kind Molecular weight Kind (%) (%) (dtex) (dtex) (℃) (Second) Example 1 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 8 10 Example 2 Magnesium hydroxide 0.05 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 31 5.4 Example 3 Magnesium hydroxide 0.10 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 12 9.2 Example 4 Magnesium hydroxide 0.30 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 9 10 Example 5 Magnesium hydroxide 1.00 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 553 8 10 Example 6 Magnesium hydroxide 5.00 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 144 8 10 Example 7 Magnesium carbonate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 12 9.2 Example 8 Magnesium oxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 10 9.1 Example 9 Calcium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 41 3.9 Example 10 calcium carbonate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 36 4.4 Example 11 Sodium carbonate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 45 2.7 Example 12 Potassium carbonate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 45 2.8 Example 13 Magnesium hydroxide 0.50 Ethylene glycol 62.07 MDI twenty four 1.010 4.0 30 620 160 245 9 10 Example 14 Magnesium hydroxide 0.50 1,3-Propanediol 76.09 MDI twenty four 1.010 4.0 30 620 160 478 9 10 Example 15 Magnesium hydroxide 0.50 1,6-Hexanediol 118.2 MDI twenty four 1.010 4.0 30 620 160 462 9 10 Example 16 Magnesium hydroxide 0.50 1,8-Octanediol 146.2 MDI twenty four 1.010 4.0 30 620 160 273 9 10 Example 17 Magnesium hydroxide 0.50 1,10-Decanediol 174.3 MDI twenty four 1.010 4.0 30 620 160 89 9 10 Example 18 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 H12MDI twenty four 1.010 4.0 30 620 160 176 9 10 Example 19 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 HDI twenty four 1.010 4.0 30 620 160 162 9 10 Example 20 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 18 1.010 4.0 30 620 150 213 16 8.8 Example 21 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 20 1.010 4.0 30 620 160 576 9 10 Example 22 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty two 1.010 4.0 30 620 160 More than 600 seconds 9 10 Example 23 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 30 1.010 4.0 30 620 160 More than 600 seconds 9 10 Example 24 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 35 1.010 4.0 30 620 160 More than 600 seconds 14 9.2 Example 25 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 40 1.010 4.0 30 620 160 More than 600 seconds 19 8.4 Example 26 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI 45 1.010 4.0 30 620 160 More than 600 seconds 29 6.2

[Examples 27-33] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the molar ratio of the polymer polyol, diisocyanate, and diol was adjusted to alter the OH/NCO ratio of the thermoplastic polyurethane elastic fibers. The results are shown in Table 2 below.

[Examples 34-41] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the spinning temperature, nozzle diameter, spray rate, cooling conditions, and take-up conditions were adjusted to alter the fiber density variation coefficient. The results are shown in Table 2 below.

[Examples 42-49] Thermoplastic polyurethane elastic fibers were obtained by the same method as in Example 1, except that the spinning temperature, nozzle diameter, spray rate, cooling conditions, and take-up conditions were adjusted to vary the fiber density difference (maximum fiber density - minimum fiber density). The results are shown in Table 2 below.

[Table 2] [Table 2] metal compounds chain extenders Diisocyanate Mh fraction OH/NCO Fiber inequality variation coefficient Poor fineness Total fiber density Outflow starting temperature Thermal cutting seconds ΔYI value Yellow color visual recognition Kind Addition rate (wt%) Kind Molecular weight Kind (%) (%) (dtex) (dtex) (℃) (Second) Example 27 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.000 4.0 30 620 160 More than 600 seconds 15 8.8 Example 28 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.001 4.0 30 620 160 More than 600 seconds 13 9.4 Example 29 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.005 4.0 30 620 160 More than 600 seconds 9 10 Example 30 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.030 4.0 30 620 160 More than 600 seconds 9 10 Example 31 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.050 4.0 30 620 160 588 9 10 Example 32 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.100 4.0 30 620 160 571 13 9.1 Example 33 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.500 4.0 30 620 160 469 19 8.1 Example 34 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 1.0 10 620 160 More than 600 seconds twenty three 4.3 Example 35 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 3.0 20 620 160 More than 600 seconds 12 8 Example 36 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 3.5 twenty four 620 160 More than 600 seconds 9 10 Example 37 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 5.0 35 620 160 More than 600 seconds 9 10 Example 38 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 9.0 60 620 160 More than 600 seconds 9 10 Example 39 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 9.5 64 620 160 563 9 10 Example 40 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 10.0 66 620 160 477 8 10 Example 41 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 15.0 98 620 160 391 8 10 Example 42 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 3.5 5 620 160 More than 600 seconds 8 4.9 Example 43 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 3.5 10 620 160 More than 600 seconds 8 8.2 Example 44 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 20 620 160 More than 600 seconds 8 10 Example 45 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.5 30 620 160 More than 600 seconds 8 10 Example 46 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 5.0 80 620 160 More than 600 seconds 8 10 Example 47 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 7.0 100 620 160 More than 600 seconds 8 10 Example 48 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 8.0 148 620 160 560 8 10 Example 49 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 9.0 199 620 160 478 8 10

[Examples 50-54] Thermoplastic polyurethane elastic fibers were obtained by the same method as Example 1, except that the molecular weight of the thermoplastic polyurethane was adjusted by adjusting the molecular weight of the polymer polyol, thereby altering the outflow starting temperature of the thermoplastic polyurethane elastic fiber. The results are shown in Table 3 below.

[Comparative Example 1] A thermoplastic polyurethane elastic fiber was obtained by the same method as in Example 1, except that no metal compound was added. The results are shown in Table 3 below.

[Comparative Example 2] Thermoplastic polyurethane elastic fiber was obtained by the same method as Example 1, except that the amount of masterbatch added was adjusted to change the amount of magnesium hydroxide contained in the polyurethane elastic fiber to 10.0 wt%. The results are shown in Table 3 below.

[Comparative Examples 3-6] Thermoplastic polyurethane elastic fibers were obtained by the same method as Example 1, except that the metal compounds were replaced with magnesium stearate (Comparative Example 3), calcium stearate (Comparative Example 4), zinc oxide (Comparative Example 5), and aluminum hydroxide (Comparative Example 6). The results are shown in Table 3 below.

[Table 3] [Table 3] metal compounds chain extenders Diisocyanate Mh fraction OH/NCO Fiber inequality variation coefficient Poor fineness Total fiber density Outflow starting temperature Thermal cutting seconds ΔYI value Yellow color visual recognition Kind Addition rate (wt%) Kind Molecular weight Kind (%) (%) (dtex) (dtex) (℃) (Second) Example 50 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 140 512 9 10 Example 51 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 150 More than 600 seconds 9 10 Example 52 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 200 More than 600 seconds 9 10 Example 53 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 220 More than 600 seconds 12 9.4 Example 54 Magnesium hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 225 570 13 9.1 Comparative example 1 without 0 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 55 1.4 Comparative example 2 Magnesium hydroxide 10.00 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 Behead immediately 9 10 Comparative example 3 Magnesium stearate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 54 1.3 Comparative example 4 Calcium stearate 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 55 1.4 Comparative example 5 Zinc oxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 56 1.2 Comparative example 6 Aluminum hydroxide 0.50 1,4-Butanediol 90.12 MDI twenty four 1.010 4.0 30 620 160 More than 600 seconds 54 1.3 [Industrial Availability]

The thermoplastic polyurethane elastic fiber of the present invention can be suitably used in underwear, socks, compression garments and other clothing materials, as well as pleated components or sanitary materials such as diapers.

Claims (12)

A thermoplastic polyurethane elastic fiber contains from 0.05 wt% to 5.00 wt% of at least one metal compound selected from the group consisting of metal hydroxides, metal carbonates, and metal oxides, wherein the metal compound comprises an alkali metal or an alkali earth metal. The thermoplastic polyurethane elastic fiber has a fiber density variation coefficient in the filament length direction of from 3.0% to 10.0%, and a difference between a maximum fiber density and a minimum fiber density in the filament length direction of from 10 dtex to 150 dtex. The thermoplastic polyurethane elastic fiber of claim 1, wherein the metal compound comprises an alkaline earth metal. The thermoplastic polyurethane elastic fiber of claim 2, wherein the alkaline earth metal is magnesium. The thermoplastic polyurethane elastic fiber according to any one of claims 1 to 3, wherein the metal compound is magnesium hydroxide. The thermoplastic polyurethane elastic fiber of any one of claims 1 to 3, wherein the polyurethane constituting the thermoplastic polyurethane elastic fiber is a polyurethane polymerized by polymerization of a polymer polyol, a diisocyanate, and a chain extender containing an active hydrogen compound. The thermoplastic polyurethane elastic fiber of claim 5, wherein the chain extender is a diol having a molecular weight of 60 to 120. The thermoplastic polyurethane elastic fiber of claim 5, wherein the diisocyanate is 4,4'-diphenylmethane diisocyanate (MDI). The thermoplastic polyurethane elastic fiber of claim 5, wherein the ratio of the hard chain segment containing the chain extender and the diisocyanate (Mh fraction) is not less than 20% and not more than 40%. The thermoplastic polyurethane elastic fiber of claim 5, wherein the total molar number of the chain extender and the polymer polyol relative to the molar number of the diisocyanate is not less than 1.001 times and not more than 1.100 times. The thermoplastic polyurethane elastic fiber according to any one of claims 1 to 3, wherein the total fiber fineness of the thermoplastic polyurethane elastic fiber is not less than 160 dtex and not more than 2000 dtex. The thermoplastic polyurethane elastic fiber according to any one of claims 1 to 3, which is a multifilament. The thermoplastic polyurethane elastic fiber of any one of claims 1 to 3, wherein the outflow starting temperature of the thermoplastic polyurethane elastic fiber measured by a flow tester is 150° C. to 220° C.