JP2003306831A - Hygroscopic polyester fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyester fiber exhibiting a soft touch feeling and having excellent hygroscopicity by regulating the alkali weight reduction rate of fibers within the range without actual problems and carrying out a uniform weight reduction processing. <P>SOLUTION: The hygroscopic polyester fiber comprises a polyalkylene terephthalate containing a polyethylene glycol (X) having 4,000-10,000 average molecular weight and a polyether compound or a polyether compound derivative (Y) having ≤30 mass% solubility in water at 20°C so as to simultaneously satisfy formulae (1) to (3): 5 mass%≤X≤15 mass% (1), 5 mass%≤Y≤20 mass% (2) and 10 mass%≤X+Y≤30 mass% (3). An aromatic dicarboxylic acid having a sulfonate group or an aromatic dicarboxylic acid having a phenolic hydroxy group or both are copolymerized in an amount of 0.5-5 mol% based on the whole acid components. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyester fiber having excellent hygroscopicity and capable of exhibiting a soft texture.

[0002]

2. Description of the Related Art Polyester represented by polyethylene terephthalate has excellent mechanical properties and chemical properties, and is used in a wide range of fields, and particularly has a very wide range of uses as a synthetic fiber.

However, polyester fibers have a very low hygroscopicity because they are hydrophobic, and they are worn in direct contact with the skin or in a state close to the skin side, such as innerwear, inner garments and sports clothing. When it is used for, it causes stuffiness and stickiness due to sweating from the skin, and is significantly inferior to natural fibers in terms of comfort, and its use in the field of clothing is greatly restricted.

Conventionally, various attempts have been made to impart hydrophilicity or hygroscopicity to polyester fibers. For example, JP-A-62-267352 discloses a polyester composition containing 50 to 70% by mass of polyalkylene glycol. Fibers composed of such a composition are inferior in chemical resistance, and have the disadvantage that the weight loss rate due to alkali becomes extremely fast.

Generally, polyester fibers are subjected to alkali weight reduction processing to develop a soft texture. However, in the case of such a fiber whose weight reduction rate by alkali is high, weight reduction processing is performed by controlling the weight reduction rate. However, there was a problem in that the fibers having a soft texture could not be obtained.

As a method of solving such a problem,
For example, Japanese Patent Application Laid-Open No. 2-99612 proposes a core-sheath type composite fiber in which a hygroscopic polymer having a moisture absorption rate of 10% by mass or more at room temperature is used as a core, and the sheath is covered with polyester. However, even with such a method, the hygroscopic polymer in the core part swells greatly with hot water during alkali weight reduction processing, so cracks occur on the surface of the composite fiber, and the core polymer with a high affinity for water flows out. There was a problem of doing.

Further, in JP-A-9-228157, an intermediate layer of a hygroscopic polymer containing a polyether compound is compounded between an outer layer made of polyester and an inner layer,
Moreover, a hollow three-layer composite fiber having a hollow portion has been proposed. Such a fiber has a high affinity for water, because the surface of the composite fiber does not crack due to deformation of the hollow part even if the hygroscopic polymer of the intermediate layer swells due to hot water during alkali weight reduction processing. The polymer of the layer is prevented from flowing out. By taking such a fiber form, it is possible to carry out alkali weight loss processing to some extent, but there is a problem in that the stability of the yarn making is poor and the operability is extremely poor due to the complicated cross-sectional shape of the fiber.

Therefore, the present applicant has proposed in JP-A-2001-115335 a hygroscopic polyester fiber which has an excellent hygroscopicity and can set the alkali weight loss rate to a practically unproblematic range. This fiber contains a polyether compound and the like, and is made of a polyester in which a sulfonate group-containing compound and dimer acid are copolymerized. By adding a polyether compound, hygroscopicity is imparted, and an alkali weight loss rate is obtained. To dimerize the dimer acid. The sulfonate group-containing compound serves to improve the hydrophilicity of the polyester, and also serves to mitigate the deterioration of the hygroscopic property due to the hydrophobicity of the dimer acid.

However, in this hygroscopic fiber, since the dimer acid is hydrophobic, the hygroscopicity is diminished even though the hydrophobicity of the dimer acid is alleviated by copolymerizing the sulfonate group-containing compound. It could not be prevented from decreasing, and the hygroscopicity of the fiber was not sufficiently satisfactory.

[0010]

DISCLOSURE OF THE INVENTION According to the present invention, the alkali weight loss rate of the fiber is set to a range that practically does not cause a problem, and a soft texture can be developed by performing uniform weight reduction processing, and the fiber has excellent hygroscopicity. It is intended to provide polyester fibers.

[0011]

The present invention is to solve the above-mentioned problems, and the gist thereof is that the average molecular weight is 400.
0 to 10000 polyethylene glycol (X) and 20
A perfume having a sulfonate group, which contains a polyether compound or a polyether compound derivative (Y) having a solubility in water at 30 ° C of 30% by mass or less so as to simultaneously satisfy the following formulas (1) to (3). A hygroscopic polyester fiber characterized in that an aromatic dicarboxylic acid having a group dicarboxylic acid and / or a phenolic hydroxyl group is composed of polyalkylene terephthalate copolymerized in an amount of 0.5 to 5 mol% based on all acid components. is there. 5% by mass ≤ X ≤ 15% by mass (1) 5% by mass ≤ Y ≤ 20% by mass (2) 10% by mass ≤ X + Y ≤ 30% by mass (3)

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The polyalkylene terephthalate forming the fiber of the present invention contains polyethylene glycol having an average molecular weight of 4,000 or more and 10,000 or less (hereinafter referred to as PEG) in an amount of 5 to 5.
It contains 5 to 20% by mass of a polyether compound or a polyether compound derivative having a solubility in water of 15% by mass and 20 ° C. of 30% by mass or less, and the total content of both is 10 to 30% by mass. It is necessary.

When the average molecular weight of PEG is less than 4,000, it does not exhibit sufficient hygroscopicity, and even if the amount added is increased, fibers having the desired hygroscopicity cannot be obtained. When the average molecular weight is more than 10,000, the compatibility with polyalkylene terephthalate is poor and the PEG component is easily eluted during alkali weight reduction processing, and as a result, the polyester fiber containing the PEG component has a high alkali weight loss rate. Therefore, it is not preferable.

When the content of PEG is less than 5% by mass, a sufficient moisture absorption rate is not exhibited. On the other hand, when it exceeds 15% by mass, the compatibility with polyalkylene terephthalate is deteriorated and the rate of alkali weight reduction is high. Become a fiber.

Next, for a polyether compound or a polyether compound derivative having a solubility in water at 20 ° C. of 30% by mass or less, if the solubility exceeds 30% by mass, this component is likely to be eluted during alkali reduction processing. Not preferable. The dissolution rate in the present invention is as follows. First, 1 g of a polyether compound or a polyether compound derivative is added to 100 g of water adjusted to 20 ° C.
g and stir for 5 minutes. When completely dissolved, the operation of adding 1 g of a polyether compound or a polyether compound derivative and stirring for 5 minutes is repeated, and the amount is calculated by the following formula from the amount of addition at the time of complete dissolution. Dissolution rate (mass%) = [total addition amount / (total addition amount + 10
0)] × 100

Examples of the polyether compound or polyether compound derivative used in the present invention include polypropylene glycol and polytetramethylene glycol. Further, a copolymer of ethylene oxide and propylene oxide having an average molecular weight of 9,000 or more is also preferable because it has low solubility in water. Particularly, in this case, the copolymerization ratio of propylene oxide is preferably 20 to 80 mol%.

It is also preferable that a part or all of the terminals of such a polyalkylene oxide reacts with an isocyanate compound or the like to be crosslinked, and as a commercial product, "Aqua Coke" manufactured by Sumitomo Seika Co., Ltd., etc. Is mentioned. Also,
A polyether ester amide composed of a polyether component and a polyamide component is also preferable, and examples of commercially available products include "Pelestat" manufactured by Sanyo Kasei Co., Ltd.

When the content of the polyether compound or the polyether compound derivative is less than 5% by mass, sufficient hygroscopicity is not exhibited, while when it exceeds 20% by mass, the operability is remarkably deteriorated.

In the present invention, it is important to use PEG having an average molecular weight of 4000 or more and 10000 or less and a polyether compound or a polyether compound derivative having a solubility in water at 20 ° C. of 30% by mass or less. is there.

That is, in the former case, the moisture absorption performance of the fiber is improved by increasing the addition amount thereof, but the addition amount must be limited in order to suppress the alkali weight loss rate because of high hydrophilicity. On the other hand, in the latter, the elution during alkali reduction processing is small, but the hygroscopicity of the compound itself is lower than that of the former, and if the latter compound is used alone to obtain the desired hygroscopicity, the amount added will increase and the operability will increase. There is a problem of getting worse. Therefore, by using both of them together, it becomes possible to suppress the addition amount of each and compensate for the drawbacks.

The average molecular weight is 4000 to 10000.
If the total amount of PEG and the polyether compound or polyether compound derivative having a solubility in water at 20 ° C. of 30% by mass or less is less than 10% by mass, sufficient moisture absorption performance cannot be obtained. If it exceeds 30% by mass, the physical properties of the polymer are deteriorated, and the operability is remarkably deteriorated.

Next, as the polyalkylene terephthalate forming the fiber of the present invention, specifically, polyethylene terephthalate (hereinafter abbreviated as PET), polybutylene terephthalate, polypropylene terephthalate and the like are preferable.

In this polyalkylene terephthalate, the aromatic dicarboxylic acid having a sulfonate group or the aromatic dicarboxylic acid having a phenolic hydroxyl group is used alone or in combination, and the amount thereof is 0.5% based on all acid components.
It is necessary that the copolymerization is ˜5 mol%. Specifically, the former is preferably isophthalic acid having a sulfonate group such as 5-sodium sulfoisophthalic acid, and the latter is preferably 5-hydroxyisophthalic acid.

First, the aromatic dicarboxylic acid having a sulfonate group and the aromatic dicarboxylic acid having a phenolic hydroxyl group have an effect of hydrophilizing polyalkylene terephthalate, and thereby a fiber having excellent hygroscopicity can be obtained. It is a thing.

The aromatic dicarboxylic acid having a phenolic hydroxyl group can prevent an increase in alkali weight loss rate. That is, as described above, in order to adjust the alkali weight loss rate, PEG having a specific molecular weight and characteristics
However, these components that are not copolymerized by hot water elute and increase the surface area of the fiber during the alkali weight reduction processing. Hydrolysis proceeds. Therefore, by copolymerizing an aromatic dicarboxylic acid component having a phenolic hydroxyl group, these components in a state of being dispersed in the polyester without being copolymerized can be fixed in the polyester, and during alkali reduction processing The elution of this component is suppressed. That is, since elution of the polyether compound and the like can be prevented, it is possible to prevent an increase in the alkali weight loss rate, and it is possible to retain the hygroscopic component, so that a fiber having excellent hygroscopicity can be obtained. Furthermore, the aromatic carboxylic acid component having at least one phenolic hydroxyl group has a hydrophilic functional group, and by copolymerization, the hydrophobicity of the polyester itself is relaxed.

The copolymerization ratio of the aromatic dicarboxylic acid having a sulfonate group and the aromatic dicarboxylic acid component having a phenolic hydroxyl group is 0.5 to 5 mol% with respect to the total acid component, either alone or in combination. It is necessary to be in the range of, and more preferably 0.5 to 2 mol%. When the copolymerization ratio is less than 0.5 mol%, the effect of improving the hydrophilicity of the fiber and the effect of preventing an increase in the alkali weight loss rate are insufficient as described above. On the other hand, if it exceeds 5 mol%, the melt viscosity of the polymer becomes high at the time of polymerization and the operability is lowered. Further, when the copolymerization amount of the aromatic dicarboxylic acid component having a sulfonate group alone exceeds 5 mol%, the hydrophilicity of the polyester becomes too high and the alkali weight loss rate becomes fast.

In the present invention, the polyalkylene terephthalate may contain a small amount of other copolymerization components as long as the essential characteristics are not impaired, and these copolymerization components include isophthalic acid and phthalic anhydride. Acid, 5-sulfoisophthalic acid, naphthalenedicarboxylic acid and other aromatic dicarboxylic acid components, adipic acid, sebacic acid and other aliphatic dicarboxylic acid components, diethylene glycol, 1,4-cyclohexanone dimethanol, dimer diol, bisphenol A ethylene oxide addition Body, glycol component such as ethylene oxide adduct of bisphenol S, 4-
Examples thereof include hydroxycarboxylic acid components such as hydroxybenzoic acid and ε-caprolactone.

Since the fiber of the present invention comprises the above polyester, the alkali weight loss rate is 1.5% by mass / min or less, the temperature is 34 ° C., and the humidity is 90%.
The moisture absorption rate in the RH atmosphere is 4.0% by mass or more. When the alkali weight loss rate exceeds 1.5% by mass / minute,
Since the progress of weight reduction is too fast, it becomes difficult to control the weight reduction rate, uniform weight reduction processing cannot be performed, and fibers having a soft texture cannot be obtained.

The value of the alkali weight loss rate is a value measured by the following method. The fiber is tubularly knitted, and it is determined by the following formula from the weight loss mass when treated with a 5% by mass aqueous sodium hydroxide solution at 70 ° C. for 40 minutes at a bath ratio of 1:50. Weight reduction rate (mass% / min) = [(weight before weight-weight after weight) / weight before weight] x 100/40

When the moisture absorption rate is less than 4.0% by mass in an atmosphere of a temperature of 34 ° C. and a humidity of 90% RH, sufficient hygroscopicity cannot be obtained as compared with natural fibers such as cotton, and when used as clothing. It is not preferable because it is likely to cause discomfort when worn.

The value of the moisture absorption rate is a value measured by the following method. Fibers were cylindrically knitted, which 25 ° C., measuring the mass W ₀ and humidified at 60% RH. Then, this tubular knitted fabric
It was dried at 80 ° C. for 6 hours under a reduced pressure of 2 hPa and then at 34 ° C. for 9 hours.
After being left for 6 hours in a thermo-hygrostat set to 0% RH, the mass W ₁ is measured. Then, the moisture absorption rate is calculated by the following equation. Moisture absorption rate (mass%) = [(W ₁ −W ₀ ) / W ₀ ] × 100

Next, the method for producing the fiber of the present invention will be described with reference to an example. By directly esterifying terephthalic acid and a diol and transesterifying a lower alkyl ester of terephthalic acid with a diol, a polyester oligomer is synthesized, on which ethylene glycol, a polyether compound or a polyether compound derivative, and a sulfonate group And an aromatic dicarboxylic acid having a phenolic hydroxyl group are added to carry out a polycondensation reaction. The polycondensation reaction is usually carried out in the presence of a metal compound such as antimony, germanium, tin, titanium, zinc, aluminum and cobalt under a reduced pressure of about 0.12 to 12 hPa at a temperature of 220 to 290 ° C. and an intrinsic viscosity of 0. ．
It is preferable to carry out until 5 or more. In addition, antioxidants such as hindered phenol compounds, cobalt compounds, fluorescent agents, color improvers such as dyes, pigments such as titanium dioxide, light resistance such as cerium oxide, etc. There is no problem even if it contains an additive such as a property improving material. Next, the obtained polyester is dried by an ordinary method and is supplied to an ordinary melt spinning machine to be melt spun at a temperature higher than the melting point of the polyester by 20 ° C. or more. After the yarn is cooled, the undrawn yarn or the semi-undrawn yarn is obtained. As the above, the target fiber is obtained by temporarily winding it up, or by performing drawing and heat treatment without winding up.

As long as the effects of the present invention are not impaired,
It may be a composite fiber with other components. Further, the form of the polyester fiber may be a long fiber or a short fiber,
If necessary, post-processing such as crimping, false twisting, and treatment with a chemical solution may be performed before use.

[0034]

The inventor believes that the polyester fiber of the present invention can be uniformly subjected to alkali reduction processing and has excellent hygroscopicity as follows. In order to impart hygroscopicity to polyester, it is necessary to contain a polyether compound or a polyether compound derivative having a certain segment length. However, such high molecular weight polyether compounds generally have poor compatibility with polyesters,
Not all were copolymerized with the polyester, and a certain amount was dispersed in the polyester. When a fiber made of such polyester is subjected to alkali weight reduction processing, first, the polyether compound or polyether compound derivative not copolymerized by hot water is eluted to increase the surface area of the fiber, and as a result, the polyester produced by the alkali is It is considered that the hydrolysis of sucrose rapidly progresses. Therefore, when it is attempted to suppress the alkali weight loss rate to such an extent that there is no practical problem, the amount of addition is limited, so that the moisture absorption performance of the obtained fiber is also limited. The polyester fiber of the present invention contains a PEG having a molecular weight of 4000 to 10000 capable of improving hygroscopicity, and further has a solubility in water at 20 ° C. of not more than 30% by mass, which is a sparingly soluble polyether compound or poly. By including the ether compound derivative, both alkali resistance and hygroscopicity can be imparted to the polyester. That is, the polyether compound or the polyether compound derivative used in the present invention can impart hygroscopicity to the polyester, although it is inferior to PEG, and this component is substantially sparingly soluble in water. Even in the dispersed state, the degree of elution during alkali reduction processing can be kept low. Therefore, by adding the sparingly soluble polyether compound or polyether compound derivative in water within a range not deteriorating the operability,
It is possible to improve the moisture absorption performance of the fiber and reduce the alkali weight loss rate while reducing the PEG content. Furthermore, since the aromatic compound having a sulfonate group and the aromatic compound having a phenolic hydroxyl group are copolymerized in the polyester, hydrophilicity is improved, and an increase in alkali weight loss rate can be prevented. Moisture absorption performance improves. That is, the aromatic compound having a sulfonate group can improve the hydrophilicity of the polyester itself by copolymerization, and the phenolic hydroxyl group can easily generate a radical at a high temperature. By interacting with PEG and polyether compounds or polyether compound derivatives that are not polymerized and dispersed in polyester and immobilizing them, it is possible to prevent this component from eluting during alkali reduction processing. This is because the polyester has a hydrophilic functional group, and therefore the hydrophobicity of the polyester itself can be relaxed. As a result of these, the fiber is provided with excellent hygroscopicity, and the alkali weight loss rate is 1.5% by mass.
It is presumed that it is possible to set the range to be less than or equal to /, which is practically no problem.

[0035]

EXAMPLES Next, the present invention will be described in detail with reference to examples. In addition, the measurement and evaluation of various values in the examples were performed as follows. (A) Intrinsic viscosity [η] It was measured at a temperature of 20 ° C using an equal mass mixture of phenol and tetrachloroethane as a solvent. (B) Strength and Elongation Tensileon RTC-1210 manufactured by Oriental Co., Ltd. was used to perform a tensile test on a material of 50 cm at a speed of 50 cm / min, and the stress-strain curve was obtained. (C) Alkali weight loss rate It was measured by the above method. (D) Moisture absorption rate The moisture absorption rate was evaluated according to the above-described measurement method of the moisture absorption rate. (E) Cylindrical knitting of the textured fiber, and 7 with a 5% by mass aqueous sodium hydroxide solution
The treatment was performed at 0 ° C. and a bath ratio of 1:50 so that the weight loss was 20% by weight, and the texture after the treatment was evaluated by a sensory test by 10 panelists in the following three stages. It should be noted that, in the table, those for which uniform weight reduction processing could not be performed and texture evaluation was not performed are shown as XX. ◯: More than 7 out of 10 felt soft. Δ: 4 to 6 out of 10 felt soft. X: The number of people who felt soft was 3 or less out of 10.

Example 1 A slurry of terephthalic acid and ethylene glycol in a molar ratio of 1 / 1.6 was continuously fed to an esterification reactor having a PET oligomer, and the temperature was 250 ° C. and the pressure was 0.1M.
The esterification reaction was carried out under the conditions of Pa and a residence time of 8 hours to continuously obtain a PET oligomer having a reaction rate of 95%.
46.7 kg of this PET oligomer was transferred to a polycondensation reaction can, and 6.0 kg of PEG having an average molecular weight of 6000 (10% by mass based on the polymer produced) and a copolymer of ethylene oxide and propylene oxide (copolymerization). Ratio 5
0/50, average molecular weight 9,000, random diol) 9.0k
g (15% by mass with respect to the polymer produced),
As an antioxidant, 180 g of "Irganox 245" (manufactured by Ciba Specialty Chemicals, hindered phenolic antioxidant) was added in an amount of 0.8 g for the polymer produced.
3% by mass) was added and stirred and mixed for 40 minutes. Then, 20 g of antimony trioxide (amount of 3 × 10 −4 mol based on 1 mol of all acid components) and 416 g of ethylene glycol ester of 5-sodium sulfonic acid (hereinafter abbreviated as SIPG) (based on all acid components) 0.5 mol%) and the temperature in the polycondensation reaction vessel is adjusted to 27 in 30 minutes.
The temperature is raised to 0 ° C, the pressure is gradually reduced, and after 60 minutes, 1.2 hours.
It was set to Pa or less. Under these conditions, the polycondensation reaction was carried out for 4 hours with stirring, and the product was discharged by a conventional method to form pellets.
Next, the pellets are dried by a conventional method and then melt-spun at a spinning temperature of 290 ° C. using a usual melt-spinning apparatus to
The undrawn yarn was wound up at a speed of 400 m / min. This undrawn yarn was fed to a drawing machine and preheated at 80 ° C.
It was drawn 3.5 times while being in contact with a heat plate at 0 ° C., heat-treated and wound up to obtain a hygroscopic polyester fiber of 83 decitex / 36 filaments.

Examples 2 and 3, Comparative Examples 3, 4, 7 to 9 Polyether compounds or polyether compound derivatives (hereinafter, hardly soluble in water) having a solubility of 30% by mass or less in PEG and water at 20 ° C. Example 1 was repeated, except that the type and content of abbreviated as a polyether compound) and the copolymerization ratio of SIPG were variously changed as shown in Table 1.

Example 4, Comparative Examples 1, 2, 5, 6 The type and content of PEG and a sparingly water-soluble polyether compound were variously changed as shown in Table 1, and 5-hydroxy was used instead of SIPG. Example 1 was repeated except that isophthalic acid (hereinafter abbreviated as HIPA) was added and the copolymerization ratio thereof was changed as shown in Table 1.

Table 1 shows the [η] of the polyesters obtained in Examples 1 to 4 and Comparative Examples 1 to 9 and the physical properties and evaluations of the fibers.

[0040]

[Table 1]

As is clear from Table 1, the polyester fibers of Examples 1 to 4 have an alkali weight loss rate of 1.5% by mass.
/ Min or less, which is a practical range for weight reduction processing, it is possible to develop a soft texture by weight reduction processing, and the moisture absorption rate is 4% by mass or more, which is excellent in hygroscopicity. It was a thing. In addition, yarn production and operability were also good. On the other hand, in Comparative Example 1, the molecular weight of PEG to be added is small and the hygroscopicity of itself is low. In Comparative Example 3, the amount of PEG added was too small. Therefore, in Comparative Example 5, a sparingly water-soluble polyether compound was used. In Comparative Example 8, since the amount of addition of was too small, the copolymerization ratio of SIPG was too small, and thus the fibers obtained together had insufficient moisture absorption. Further, in Comparative Example 2, the molecular weight of PEG was large, and P
Since the compatibility with ET is poor, the amount of PEG added in Comparative Example 4 was too large, and the copolymerization ratio of SIPG in Comparative Example 9 was too large. However, uniform weight reduction processing could not be performed. In Comparative Example 6, the amount of the sparingly water-soluble polyether compound was too large, and in Comparative Example 7, the total amount of the PEG and the sparingly water-soluble polyether compound was too large, resulting in yarn breakage during spinning. It occurred frequently and the fiber could not be obtained.

[0042]

EFFECT OF THE INVENTION The polyester fiber of the present invention has excellent hygroscopicity and can exhibit a soft texture by being subjected to weight reduction processing. Therefore, the fabric obtained from this fiber can be used for various purposes, especially for clothing. When used as, a comfortable wearing feeling can be obtained.

Claims (2)

[Claims] 1. A polyethylene glycol (X) having an average molecular weight of 4000 to 10000 and a polyether compound or a polyether compound derivative (Y) having a solubility in water at 20 ° C. of 30% by mass or less are represented by the following formula (1): ) To (3) at the same time, the aromatic dicarboxylic acid having a sulfonate group and / or the aromatic dicarboxylic acid having a phenolic hydroxyl group is contained in an amount of 0.5 to 5 mol% based on all acid components. A hygroscopic polyester fiber comprising a polyalkylene terephthalate copolymerized. 5% by mass ≤ X ≤ 15% by mass (1) 5% by mass ≤ Y ≤ 20% by mass (2) 10% by mass ≤ X + Y ≤ 30% by mass (3) 2. The alkali weight loss rate is 1.5% by mass / minute or less, and the moisture absorption rate in an atmosphere of a temperature of 34 ° C. and a humidity of 90% RH is 4.0% by mass or more. Hygroscopic polyester fiber.