JP4343351B2 - Polyurethane fiber - Google Patents

Description

【００４０】
【表２】

【００４１】
【発明の効果】
本発明のポリウレタン弾性繊維は、高伸度を示すので、加工ドラフト（カバーリングドラフト）を大きくできるので、カバリング糸、コアヤーンの製造が経済的に行うことができる。本発明のポリウレタン弾性繊維は、整経工程及び編立工程においても糸切れ回数が極めて少ない加工が可能である。
【００４２】
更に、耐摩耗性に優れた糸が得られるため、編み機での糸切れ、編成後の生地の着用時の糸切れが少ない優れた弾性繊維が得られる。
本発明によるポリウレタン弾性繊維は、３００％モジュラスが低いこと及び伸長応力回復率が小さいという優れた弾性機能を有する。さらに、常温以下（０℃）の条件で、従来のポリウレタン弾性繊維では見られない弾性機能に優れた残留歪みの小さな特性を示し、寒冷地における冬季での加工トラブルが減少する。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a high elongation at break, a low modulus, and a high stress recovery when stretched using a copolymerized prepolymer of a low degree of unsaturation polypropylene ether glycol (PPG) and polytetramethylene ether glycol (PTMG). The present invention relates to a polyurethane fiber which has excellent properties, low residual strain after stretching under low temperature, excellent stretch frictional wear resistance, and has low thread breakage during spinning and excellent productivity.
[0002]
[Prior art]
Polyurethane elastic fibers are called spandex and are widely used in applications that require stretch properties such as swimwear, foundations, stockings and the like. However, in applications where stretchability is required, the present situation is that further improvement is desired for various elastic functions of polyurethane elastic fibers. For example, regarding the elastic function at room temperature, high fracture elongation, small stress against deformation strain, and low hysteresis loss of stress during elongation are low. Improvement is desired.
[0003]
For the purpose of improving the aforementioned elastic function of polyurethane elastic fibers, elastic fibers from polyurethanes produced using various low melting point diols are known. For example, a polyurethane polymer using a copolymerized ether ester diol as a low melting point diol component (JP 55-160016, JP 58-59213, JP 63-178131), The use of ether carbonate diols (Japanese Patent Laid-Open No. 1-252630) is known, but elastic fibers obtained using diols containing these ether components have resistance to hydrolysis because ester bonds exist in the diol components. It is inferior in durability such as decomposability and mold resistance, and the field of use of the polyurethane polymer is limited.
[0004]
Also, attempts to improve the elastic function of polyurethane elastic fibers using a diol component that does not contain an ester bond and contains only an ether bond are disclosed in, for example, JP-A-60-26021 and JP-A-63-235320. JP-A-58-125718. However, the polyurethane elastic fiber using a diol component containing only these ether bonds has a problem that its friction and wear resistance is low even if its elastic function is improved.
[0005]
Japanese Patent Laid-Open No. 1-284518 and Japanese Patent Laid-Open No. 2-49022 disclose polyurethanes composed of alternating block copolymers of urethane portions and urea portions using a copolymerized polyalkylene ether diol as a raw material. However, the copolymerized polyalkylene diol is expensive and the obtained spandex has a problem in basic performance as an elastic fiber material having a low initial modulus and a lot of thread breakage during processing. .
[0006]
Conventionally, polyurethanes using PPG have terminal unsaturated molecules (monools) contained in PPG that hinder chain extension, making it difficult to increase the molecular weight of the polymer and obtaining effective fiber properties. Is not on the market. Polyurethane fibers using PPG having a low degree of unsaturation are known from, for example, Japanese Patent Publication No. 8-511297 and US Pat. No. 5,723,563. However, these elastic fibers have low heat resistance, and a significant decrease in strength is observed in normal fiber processing. US Pat. No. 5,691,441 discloses a polyurethane fiber composed of PPG and PTMG with low unsaturation, but the modulus is the same as that of spandex with a known modulus, and it is used for secondary products such as tights. When you do this, you will feel the tension of the elbows and knees.
[0007]
[Problems to be solved by the invention]
The object of the present invention is a polyurethane fiber with improved physical properties of a conventional low modulus elastic yarn, that is, high breaking strength, low modulus, high recovery at elongation, low residual strain after stretching at low temperature, excellent It is intended to provide a polyurethane fiber that also has anti-stretch friction and wear resistance.
[0008]
[Means for Solving the Problems]
As a result of various investigations on the above problems, the inventors of the present invention consisted of polyalkylene ethers of A and B. A is an unsaturation degree of less than 0.030 meq / g, a hydroxyl value of 56 mgKOH or less, and a number average molecular weight of 1, PPG of 000 to 20,000, B is a PTMG having a number average molecular weight of 1,000 to 3,000, and a polyalkylene ether diol mixture having a molar ratio (A: B) of 1:19 to 1: 1.86 In a dry-spun elastic yarn of a polymer obtained by chain-extending a prepolymer consisting of a diisocyanate and a difunctional diamine-based chain extender, the physical properties of conventional low modulus elastic yarns are particularly high at breaking strength and elongation. Has been found to be improved to a polyurethane fiber having both high recoverability, low residual strain after stretching at low temperature, and excellent stretch frictional wear resistance. It was. It has also been found that polyurethane fibers having a lower modulus can be prepared by setting the polydispersity of PTMG in the range of 1.3 to 2.0.
[0009]
Generally, the elastic function of the obtained polyurethane fiber is improved by including two or more types of alkylene as compared with the case of using an alkylene ether diol containing only one type of alkylene. Among them, copolymer polytetramethylene glycol composed of PTMG and other alkylene ether glycol is preferable from the viewpoint of water resistance, light resistance and abrasion resistance of the obtained polyurethane fiber, and moreover, PTMG and PPG are used from the viewpoint of elastic function. A combination is preferred.
[0010]
The PPG used in the present invention has a degree of unsaturation of less than 0.030 meq / g and a hydroxyl value of 56 mgKOH / g or less, preferably 0.010 meq / g or less, more preferably 0.008 meq / g or less. The hydroxyl value is preferably 40 mgKOH / g, particularly preferably 35 mgKOH / g or less. If the degree of unsaturation is 0.030 meq / g or more, the polyurethane molecular weight is difficult to increase, and the spinning is not stable because the viscosity is low. The number average molecular weight is 1,000 to 20,000, preferably 2,000 to 12,000, and more preferably 4,000 to 10,000. When the molecular weight is less than 1,000, the breaking strength is remarkably lowered. When the molecular weight is 20,000 or more, it is difficult to obtain a PPG having an unsaturation level of less than 0.030 meq / g. PPG may be a mixture of two or more types, and even in that case, the average degree of unsaturation and the hydroxyl value are as described above.
[0011]
PPG having a low degree of unsaturation and a low hydroxyl value can be produced using an alkali catalyst such as potassium or sodium, but preferably a cesium catalyst such as cesium, which is an alkali metal having a larger atomic weight, or a complex metal cyanide complex. It is preferable to produce using a catalyst.
Examples of the cesium catalyst include cesium metal, cesium hydroxide, cesium methoxide, cesium ethoxide, cesium propoxide, and other cesium alkoxides, cesium carbonate, and the like. As the double metal cyanide complex catalyst, a known compound such as a zinc hexacyanocobaltate complex catalyst can be used.
[0012]
PTMG used in the present invention has a number average molecular weight of 1,000 to 3,000, preferably a number average molecular weight of 1,500 to 2,500, and more preferably 1,600 to 2,200. If the number average molecular weight is less than 1,000, the elongation required for the elastic fiber cannot be obtained. PTMG having a number average molecular weight of 3,000 or more has a wide molecular weight distribution, and the breaking strength and elongation of the obtained elastic yarn are remarkably lowered. It is important that the polydispersity d, which is the ratio of weight average molecular weight / number average molecular weight, is 1.3 to 2.0, and the cyclic oligomer content of 9 or less repeating units does not exceed 3% by weight. Is 2% by weight or less, more preferably 0.5% by weight or less. When the polydispersity is 2.0 or more, the high molecular weight PTMG is increased, and the elongation recovery at low temperatures is lowered. On the other hand, when the degree of multi-branching is 1.3 or less, the separation yield of PTMG becomes low and the economic efficiency is impaired.
[0013]
PTMG used in the present invention may be produced by any method. A method of ring-opening polymerization of tetrahydrofuran (hereinafter referred to as THF) using a heteropolyacid disclosed in JP-A-58-95036 as a catalyst, and fluorosulfone disclosed in JP-A-07-228855 A method of hydrolyzing the terminal group after ring-opening polymerization of THF using an acid as a catalyst can be mentioned as an example. PTMG used as a raw material is used as it is without adding a purification treatment when the degree of polymerization and the polydispersity indicating the degree of molecular weight distribution and the content of impurities are within the range of the present invention described above. be able to. When the polydispersity of the raw material PTMG is not within the predetermined range of the present invention, the polydispersity and the molecular weight are determined by applying the method disclosed in JP-A-50-75697 exemplified below. Prepared to range.
[0014]
In the present invention, the molar ratio of PPG and PTMG (PPG: PTMG) is preferably in the range of 1:19 to 1: 1.86. More preferably, it is 1: 11.5-1: 2.33. When the mole fraction of PPG ({PPG / (PPG + PTMG)} × 100) is 35 mol% or more, the polymerization time is remarkably slow, which is not economical. Moreover, if it is 5 mol% or less, extensional stress recovery property will fall.
[0015]
In the prepolymer synthesis of the present invention, a method of mixing a reaction prepolymer of PPG and diisocyanate and a reaction prepolymer of PTMG and diisocyanate, and a method of obtaining a block prepolymer by reacting a premixed mixture of PPG and PTMG with diisocyanate Is possible. In the former method, the PPG prepolymer and the PTMG prepolymer are phase-separated, and the resulting urethane fiber has a low elongation at break and many yarn breaks in covering. Polyurethane fibers made from a prepolymer in which PPG and PTMG are combined in a block form have better recoverability at low temperatures than conventional PPG / PTMG copolymer urethane fibers, and even when used in cold regions, It is possible to obtain an elastic fiber secondary product that is less likely to come off and fall out of the knee.
[0016]
Examples of the diisocyanate used in the present invention include aliphatic, alicyclic or aromatic organic diisocyanates having two isocyanate groups in the molecule. Specifically, methylene-bis (4-phenylisocyanate), Illustrative organic diisocyanates such as 2,4- or 2,6-tolylene diisocyanate, p-phenylene diisocyanate, 1,5-naphthalene diisocyanate, xylylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate Is done. Further, as the organic diisocyanate, a compound having a blocked isocyanate group that is converted into a free isocyanate group may be used. As a diisocyanate that gives good physical properties, methylene-bis (4-phenylisocyanate) is preferred because of its high breaking strength and elongation.
[0017]
As the active hydrogen-containing compound that reacts with the isocyanate group used in the present invention, a conventional chain growth agent in polyurethane, that is, a low-molecular compound having a molecular weight of 500 or less containing at least two hydrogen atoms that can react with isocyanate is used. it can. Specific examples of chain extenders include diamines such as ethylenediamine, propylenediamine, tolylenediamine, m-xylylenediamine, 1,3-diaminocyclohexane, isophoronediamine, hydrazine, 4,4'-diaminodiphenylmethane, dihydrazide, piperazine and the like. And diols such as ethylene glycol, propylene glycol and 1,4-butanediol. These compounds may be used alone or in admixture of two or more. Moreover, you may use together with the compound containing one active hydrogen which can react with isocyanate depending on the case. Preferred diamines that give good physical properties are hydrazine, polyhydrazide, polyol, ethylene diamine, propylene diamine, bis- (4-aminohexyl) methane, m-xylylenediamine, and p-xylylenediamine, especially the use of ethylenediamine. Is preferable for obtaining a polyurethane having good heat resistance and light resistance.
[0018]
As a method for producing a polyurethane using the diisocyanate, copolymerized polyalkylene ether diol and active hydrogen-containing compound of the present invention, a known urethanization technique can be employed. A preferred method is to heat a predetermined amount of the polyalkylene ether glycol mixture used in the present invention and methylene-bis (4-phenyl isocyanate) in the presence or absence of a solvent at 40-80 ° C. with stirring. . The molar ratio n value of glycol and diisocyanate needs to be within the range of 1: 1.5 to 1: 2.3, more preferably 1: 1.60 to 1: 1.85. Below, the heat resistance of polyurethane is inferior, and when it is 2.3 or more, the polyurethane is easily gelled.
[0019]
In this way, a prepolymer having isocyanate groups at both ends is obtained, dissolved in a solvent, and then stirred and mixed with a solution of organic diamine and organic monoamine, whereby a viscous high polymerization degree solution can be prepared.
Compared with PTMG, the polyalkylene ether glycol mixture used in the present invention contains PPG having a methyl group in the side chain, so that the crystallization temperature (freezing point) is remarkably low. Therefore, another feature of the present invention is that the viscosity of the prepolymer and the high polymer solution is remarkably lower than that of the known method of the conventional method, and handling in film forming and spinning becomes very easy.
[0020]
If necessary, the polyurethane solution of the present invention may further contain a stabilizer such as an antioxidant, a light stabilizer, a gas yellowing inhibitor, an ultraviolet absorber, inorganic fine particles such as titanium oxide and zinc oxide, calcium stearate. Anti-adhesive agents such as magnesium stearate, polytetrafluoroethylene, and organopolysiloxy acid, spinners, and other compounding agents can be appropriately blended.
[0021]
The polyurethane polymer thus obtained can be formed into a fiber by any of the conventionally known dry spinning method, wet spinning method, and melt spinning method, but the dry spinning method is preferred economically.
As for the physical properties of the polyurethane elastic yarn of the present invention, the 300% modulus is 0.20 g / d or less, preferably 0.18 g / d or less, more preferably 0.15 g / d or less. When the modulus is larger than 0.20 g / d, softness is not exhibited in pantyhose, woven fabric, and the like. The residual strain at low temperature (0 ° C.) is 30% or less, preferably 20% or less, more preferably 15% or less. If the residual strain at a low temperature is greater than 30%, problems such as missing elbows and knees are likely to occur when used in cold regions. The elongation stress recovery rate is 70.0% or more, preferably 80.0% or more. When the elongation stress recovery rate at a low temperature is 70.0% or more, thread breakage frequently occurs during covering processing in a cold region.
[0022]
The polyurethane fiber obtained in this way may be used as bare yarn as it is in practice, and other fibers such as polyamide fiber, wool, cotton, recycled fiber, polyester fiber and the like are conventionally known fibers. And can be used as a coated elastic fiber, and is useful for foundations, socks, rubber bands, corsets, surgical bandages, woven and knitted swimsuits, stockings, pantyhose and the like.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to this. In addition, the measuring methods of various characteristic values in the following examples and reference examples are as shown below.
(1) Measurement of hydroxyl value It was determined by the pyridine-phthalic anhydride method, and from this, the number average molecular weight was determined.
[0024]
(2) Measurement of viscosity of polymer solution: Measured at 30 ° C. using an E-type viscometer RE110U system manufactured by Toki Sangyo Co., Ltd.
(3) Measurement of breaking strength, elongation and 300% modulus Tensilon UTM-100 type tensile tester manufactured by Orientec Co., Ltd. was used, and measurement was performed under the conditions of a temperature of 20 ° C and a humidity of 65%. A sample having a length of 5 cm is measured for strength and elongation at break and 300% modulus when stretched at a speed of 50 cm / min.
[0025]
(4) Measurement of recovery rate of elongation stress Tensilon UTM-100 type tensile tester manufactured by Orientec Co., Ltd. was used, and measurement was performed under conditions of a temperature of 20 ° C. and a humidity of 65%. When elongation up to 300% is repeated three times at a rate of 1,000% / min, this is a value (%) obtained by dividing the return elongation stress at 200% for the third time by the past elongation stress.
[0026]
(5) Residual strain measurement temperature: 0 ° C., at a rate of 1,000% / minute, when elongation recovery up to 300% is repeated three times, the residual when the stress at the third recovery is zero It is distortion. In addition, the measurement under low temperature attaches and measures the temperature-controlled bath TFL-1-40B-S by Orientec Co. to the said tensile tester.
[0027]
(6) Measurement of friction and abrasion resistance Nylon yarn (manufactured by Asahi Kasei Kogyo Co., Ltd., Leona 10/5 BOP) and a sample (spandex yarn) are aligned to produce a tubular knitted fabric having a length of about 15 cm at a yarn feed of 35 m / min. To do. The knitted fabric is KW dyed at a free length, washed with water for 10 minutes, and dehydrated for 5 minutes. For KW dyeing, BLANKOPHOR CL (manufactured by Bayer) 1.2% owf (1.2 g with respect to 100 g of knitted fabric), Ionette Wrap 50 (manufactured by Sanyo Chemical Co., Ltd.) 0.4 g / l, ammonium acetate A solution in which 0.5 g / l and acetic acid 2 ml / l are dissolved is used as a dyeing solution, and the dyeing solution is immersed in the knitted fabric for 45 minutes under a boil (100 ° C.) so that the bath ratio is 1:30.
[0028]
The above-described cylindrical knitted fabric subjected to the above treatment is air-dried for 12 hours, stretched 220% (0-220% repeated stretching), and repeatedly stretched using a demache tester at 200 rpm. After a predetermined number of times, the cutting state of the spandex yarn cut by frictional wear with nylon yarn is observed, and the time until the spandex yarn is cut is taken as a measure of the friction wear resistance.
[0029]
[Reference Example 1] Production Example of PPG A pressure-resistant stirring vessel was charged with 1,235 g of PPG having a molecular weight of 1,000 obtained by reacting propylene oxide (PO) with glycerin and 126 g of a zinc hexacyanocobaltate complex catalyst in a nitrogen atmosphere. This was heated to 120 ° C. and dehydrated. Next, the temperature was lowered to 100 ° C., PO was continuously introduced while maintaining this temperature, and 6,040 g of PO was introduced over 7 hours. After completion of the reaction, unreacted PO was distilled under reduced pressure, and then an acid and an adsorbent were added and stirred to neutralize and adsorb the catalyst, followed by filtration and drying to obtain PPG. The obtained PPG had a molecular weight of 10,000, a hydroxyl value of 28.0 KOH, and an unsaturation degree of 0.020 meq.
[0030]
Reference Example 2 Production Example of PTMG 547 g of tetrahydrofuran is charged into a container equipped with a stirrer and a reflux condenser. Add 500 g of phosphotungstic acid heated at 340 ° C. for 3 hours to an anhydrous state, store in a dry nitrogen gas atmosphere at 60 ° C., react for 10 hours with stirring, and leave at room temperature to separate into two phases To do. The upper layer was separated, and unreacted tetrahydrofuran was removed by distillation to obtain 145 g of transparent and viscous PTMG. The number average molecular weight of the obtained PTMG was 2,000, and the molecular weight distribution was 1.4.
[0031]
〔Example〕
789 g of PPG having a number average molecular weight of 10,000 prepared in Reference Example 1, 184 g of PTMG having a number average molecular weight of 2,000 prepared in Reference Example 2, and 400 g of methylene-bis (4-phenyl isocyanate) were dried. The reaction was carried out under stirring at 80 ° C. for 3 hours under nitrogen to obtain a prepolymer. After this was cooled to room temperature, 4457 g of dimethylacetamide was added and dissolved while stirring at room temperature to obtain a uniform prepolymer solution. On the other hand, a solution prepared by dissolving 34.2 g of ethylenediamine and 4.38 g of diethylamine in 1,233 g of dimethylacetamide was added to the above prepolymer solution at a high speed with rapid stirring, and further reacted at room temperature for 1 hour to obtain a polyurethane polymer solution. . From this solution, 40d polyurethane fiber was obtained by ordinary dry spinning at a spinning speed of 400 m / min. The physical properties are shown in Table 2.
[0032]
[Examples 2 to 6]
According to Reference Example 1 and Reference Example 2, a polyalkylene diol mixture was produced with a composition comprising the molecular weight of PPG and PTMG described in Tables 2 to 6 and the molar ratio of PPG and PTMG, and then each of them. The prepolymer reacted at the ratio (n value) of the polyalkylenediol and diisocyanate described in Table 1 was chain extended in the same manner as in Example 1 to obtain a polyurethane polymer liquid. From this solution, 40d polyurethane fiber was obtained by ordinary dry spinning at a spinning speed of 400 m / min. The physical properties are shown in Table 2.
[0033]
Example 7
A tubular knitted fabric for measuring friction and abrasion resistance was produced from the polyurethane fibers of Example 1, processed, and the knitted fabric was cut open. The 4 cm square knitted fabric was stretched and fixed 200% in length and width, and rubbed against a cotton friction cloth at 300 rpm under a contact pressure of 300 g. At this time, it took 450 minutes for the polyurethane elastic yarn to break.
[0034]
[Comparative Examples 1-5]
According to Reference Example 1 and Reference Example 2, a polyalkylene diol mixture was produced with a composition comprising the molecular weights of PPG and PTMG described in Comparative Examples 1 to 5 of Table 1 and the molar ratio of PPG and PTMG, and then, respectively. A prepolymer reacted at a ratio (n value) of the polyalkylenediol and diisocyanate described in Table 1 was chain extended in the same manner as in Example 1 to obtain a polyurethane polymer liquid. From this solution, 40d polyurethane fiber was obtained by ordinary dry spinning at a spinning speed of 400 m / min. The physical properties are shown in Table 2.
[0035]
When the polyurethane polymer solution of Comparative Example 2 was stored at room temperature, it gelled on the next day of production and could not be spun.
[Comparative Example 6]
Only PPG having a number average polymerization degree of 10,000 produced according to Reference Example 1 was reacted with diisocyanate, but it took 24 hours to obtain a prepolymer. When the molar fraction of the high molecular weight PPG becomes high, it takes time for the polymerization and is not economical.
[0036]
[Comparative Example 7]
According to Reference Example 1 and Reference Example 2, a polyalkylene diol mixture was produced with a composition comprising the molecular weight of PPG and PTMG described in Comparative Example 5 of Table 1 and the molar ratio of PPG and PTMG, and then each of Table 1 The polypolymer diol and diisocyanate ratio (n value) described in 1 was extended in the same manner as in Example 1 to obtain a polyurethane polymer solution. From this solution, 40d polyurethane fiber was obtained by ordinary dry spinning at a spinning speed of 400 m / min. The physical properties are shown in Table 2.
[0037]
A tubular knitted fabric for measuring friction and abrasion resistance was prepared from polyurethane fibers, processed, and the knitted fabric was cut open. The 4 cm square knitted fabric was stretched and fixed 200% in length and width, and rubbed against a cotton friction cloth at 300 rpm under a contact pressure of 300 g. At this time, the polyurethane elastic yarn was broken in 20 minutes.
[Comparative Example 8]
PPG having a molecular weight of 4,000 produced according to Reference Example 1 and methylene-bis (4-phenylisocyanate) were reacted at an n value of 1.6 to obtain a PPG prepolymer. Further, PTMG having a molecular weight of 2,000 and methylene-bis (4-phenylisocyanate) were reacted at an n value of 1.6 according to Reference Example 2 to obtain a PTMG prepolymer.
[0038]
PPG prepolymer and PTMG prepolymer were mixed at a weight ratio of 30/70. This prepolymer mixture was chain extended in the same manner as in Example 1 to obtain a polyurethane polymer solution. From this solution, 40d polyurethane fiber was obtained by ordinary dry spinning at a spinning speed of 400 m / min. When the physical properties of the polyurethane fiber were measured, the elongation at break was 400%.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
【The invention's effect】
Since the polyurethane elastic fiber of the present invention exhibits high elongation, the processing draft (covering draft) can be increased, so that the covering yarn and the core yarn can be produced economically. The polyurethane elastic fiber of the present invention can be processed with very few yarn breaks in the warping process and the knitting process.
[0042]
Furthermore, since a yarn excellent in abrasion resistance can be obtained, an excellent elastic fiber with little yarn breakage at the time of knitting machine and wearing of the knitted fabric can be obtained.
The polyurethane elastic fiber according to the present invention has an excellent elastic function of low 300% modulus and low elongation stress recovery rate. Furthermore, under normal temperature (0 ° C.) or less, it exhibits a small residual strain characteristic excellent in the elastic function that is not found in conventional polyurethane elastic fibers, and processing troubles in winter in cold regions are reduced.

Claims (5)

A and B are polyalkylene ethers. A is a polypropylene ether glycol (PPG) having an unsaturation level of less than 0.030 meq / g, a hydroxyl value of 56 mgKOH or less and a number average molecular weight of 1,000 to 20,000, and B is The number average molecular weight is 1,000 to 3,000, the polydispersity d, which is the ratio of weight average molecular weight / number average molecular weight, is 1.3 to 2.0 , and the content of cyclic oligomers having 9 or less repeating units is 3% by weight. A polytetramethylene ether glycol (PTMG) not exceeding the molar ratio (A: B) of a polyalkylene ether diol mixture having a molar ratio (A: B) of 1:19 to 1: 1.86 and a diisocyanate, wherein the ratio of diisocyanate to polyalkylene diol n Chain extension of prepolymers with values ranging from 1.5 to 2.3, mainly with bifunctional diamine chain extenders Polyurethane polymer dry-spun elastic fiber having 300% modulus of 0.20 g / d or less, residual strain at low temperature (0 ° C.) of 30% or less, and elongation stress recovery rate of 70.0% or more fiber.   2. The polyurethane fiber according to claim 1, wherein the prepolymer comprises A and B connected on the block. B is polyurethane fibers according to claim 1, wherein the number-average molecular weight of 1,600～2,200.   2. The polyurethane fiber according to claim 1, wherein the diisocyanate is mainly methylene-bis (4-phenyl isocyanate).   The diamine is mainly a diamine such as hydrazine, polyhydrazide, polyol, ethylenediamine, propylenediamine, bis- (4-aminocyclohexyl) methane, m-xylylenediamine, and p-xylylenediamine. Item 1. A polyurethane fiber according to Item 1.