JPH0978357A - Modified polyester fiber, intermediate thereof, and method for producing the same - Google Patents

Abstract

(57) [Summary] [Object] To provide a fibrillar polyester fiber having a high water absorbency, a rayon-like texture, a dry feeling, and a high fiber strength. [Structure] The main repeating unit is an ethylene terephthalate unit, and a polyester polymer containing 0.5 to 5% by weight of the organic sulfonic acid metal salt represented by the general formula (1) is contained in the sheath portion, and 95 mol% or more is ethylene. A core-sheath type composite fiber in which a polyester polymer composed of a repeating unit of terephthalate is arranged in a core portion is melt-spun to obtain a drawn yarn, which is then subjected to a dry heat treatment at a temperature of 120 ° C. or more for 10 seconds or more, and thereafter. By reducing the weight of the fiber by 1% or more with an aqueous solution of an alkaline compound, the width of the fiber surface is reduced to 0.
1 to 3 μm, adjacent to the length of 5 μm or more, the interval between arbitrary stripe-shaped grooves is 5 μm or less, has a large number of continuous stripe-shaped grooves highly oriented in the fiber axis direction, strength 1.5 g / d or more, Elongation 1
A fibrillated polyester fiber showing 0% or more is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a modified polyester fiber in which fibrillar ultrafine fibers are generated in a divided manner, an intermediate thereof, and a method for producing the same. For more details,
A modified polyester fiber in which fibrillar ultrafine fibers are split and generated, which has a rayon-like unique texture and dry feeling, exhibits high water absorption, and can retain fiber strength, and an intermediate thereof It also relates to a method for industrially stably producing the fiber.

[0002]

2. Description of the Related Art Polyester fibers are widely used for various clothing applications because they have many excellent properties. However, since polyester fibers are produced by the melt spinning method, they generally have a smooth surface, a hard touch, and a unique slimy feel, which are disadvantages in terms of feeling.

On the other hand, conventionally, on the basis of the recognition that the fibrillation phenomenon of polyester fibers is generally a defect in the quality of polyester fibers, technological development has been made to suppress fibrillation. It was On the other hand, on the other hand, it has been proposed to positively produce fibrillated fibers for the purpose of improving the above-mentioned drawbacks of polyester fibers. For example, JP-A-55-1
In 6906 and the like, a method of blending polyester and a polymer incompatible with polyester and melt spinning is proposed, and in JP-B-61-9427 and the like, a method of composite spinning polyamide and polyester is proposed. Proposed.

Polyester fibers also have the drawback of being inferior in water absorbency and sweat absorbability as compared with natural fibers such as cotton, silk and wool, and semi-synthetic fibers such as rayon and acetate. . In order to improve these, by treating a hollow fiber made of a polyester to which a metal salt of an organic sulfonic acid is added with an alkaline aqueous solution, a water-absorbent polyester fiber having fine pores partially communicating with the hollow portion on the fiber surface is obtained. It has been proposed (Japanese Patent Publication No. 61-60188, Japanese Patent Publication No. 62-44065, etc.).

However, in the method of simply blending different kinds of polymers, a large amount of a polymer other than polyester is used to produce fibers, so that it is not possible to expect a sufficient feeling improving effect of a woven or knitted fabric made of the fibers, and dyeing is also possible. However, it has a drawback in that the water absorption is poor. Further, the manufacturing process of the woven or knitted fabric made of these fibers also has a drawback in the manufacturing process in that it requires a complicated process such as treatment with a swelling agent of polyamide polymer. In addition, the hollow polyester fibers with a microporous surface have improved water absorption, but the texture has not been improved to a satisfactory degree, and a special spinneret is required to make the hollow polyester. However, there is a drawback that the manufacturing cost increases. That is, in the above technique, it is difficult to obtain a modified polyester fiber having both a texture such as a dry feeling and a function of water absorption by highly controlling the fine structure of the fiber such as the fibril structure in the polyester fiber. Yes,
Moreover, fibrillation is generated by an industrially simple and stable method, and a new feeling due to the fibrillation phenomenon,
It was difficult to give the texture or the like to the woven or knitted fabric.

As a technical means for improving the above-mentioned drawbacks, the inventors of the present invention have disclosed in Japanese Unexamined Patent Publication No. 6-248512, etc. that a fiber surface having continuous highly oriented streak grooves is provided, and at least one of them is provided inside the fiber. A fibrillar polyester fiber having a unique rayon-like texture and dry feeling and high water absorption due to the irregular voids communicating with the fiber surface was proposed.

[0007]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the problems in the texture surface, which polyester fibers conventionally have, that is, the surface of the fiber has a unique slimy feel and the touch is hard. In particular, while further maintaining the characteristics such as feeling of the fibrillar polyester fiber obtained in JP-A-6-248512, the fiber strength thereof is further increased,
It is intended to further enhance the practical performance of fibers when used for general clothing applications.

[0008]

The gist of the present invention is to provide a fiber having a width of 0.1 to 3 μm and a length of 5 μm or more, and a distance between adjacent arbitrary linear grooves of 5 μm or less on the fiber surface. It has a large number of continuous grooves that are highly oriented in the axial direction, and the strength is 1.5g.
/ D or more and an elongation of 10% or more, which is a modified polyester fiber.

The stripe-shaped grooves highly oriented in the fiber axis direction on the surface of the polyester fiber of the present invention have a width of 0.1 to 3 μm and a length of 5 μm or more, preferably 10 μm or more, and any adjacent stripes. It is necessary that the interval between the groove portions is 5 μm or less. If the width of the streak groove is less than 0.1 μm or more than 3 μm, or if the length thereof is 5 μm or less, the slimy feel of the woven or knitted fabric made of the fibers is not sufficiently improved and the texture is not sufficiently improved.

More preferably, at least a part of the polyester fiber of the present invention has fibrillar ultrafine fibers split and generated, and a woven or knitted fabric made of the fiber is provided with a fluffy surface feel and a soft texture. However, the fibrils have a diameter of 0.05 to 5 μm and a length of 10 μm or more. In order to generate this fibrillar ultrafine fiber, it is preferable to have irregular voids inside the fibers of the sheath, and in the fiber cross section of any sheath perpendicular to the fiber axis, the total fiber cross-sectional area of the sheath On the other hand, the void cross-sectional area is preferably 2% or more, more preferably 5% or more, and at least a part of the irregular voids is preferably in communication with the fiber surface.

Further, the fiber of the present invention has a strength of 1.5 g /
d or more, preferably 2.0 g / d or more, more preferably 2.5 g / d or more, elongation is 10% or more, preferably 20.
It has mechanical properties of not less than%, and has excellent physical properties such as tear strength of the fabric in the final product when used in the field of general clothing.

In the modified polyester fiber of the present invention, the main repeating unit is an ethylene terephthalate unit, and a polyester containing 0.5 to 5% by weight of the organic sulfonic acid metal salt represented by the following general formula (1). It can be obtained from an intermediate of a core-sheath type composite fiber in which a polymer is contained in a sheath portion and a polyester polymer having 85 mol% or more of ethylene terephthalate repeating units is disposed in the core portion. R-SO ₃ M (1) (wherein R represents an alkyl group having 3 to 30 carbon atoms, or an aryl group or alkylaryl group having 7 to 40 carbon atoms, and M represents an alkali metal) Show.)

That is, in the modified polyester fiber of the present invention, the main repeating unit is an ethylene terephthalate unit, and a polyester polymer containing 0.5 to 5% by weight of the organic sulfonic acid metal salt represented by the general formula (1). In a sheath portion and a polyester polymer composed of a repeating unit of 85 mol% or more of ethylene terephthalate in the core portion is melt-spun into a stretched yarn as a core-sheath type composite fiber, and then a temperature of 120 ° C. or higher. It can be produced by performing a dry heat treatment for 10 seconds or more and then reducing 1% or more of the fiber weight with an aqueous solution of an alkaline compound.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The polyester, which is the main constituent of the core and sheath of the fiber of the present invention, has ethylene terephthalate as the main repeating unit. A typical polyester is a polyester having at least one glycol selected from ethylene glycol or 1,4-butanediol or an ester-forming derivative thereof as a glycol component. May be replaced by the dicarboxylic acid component and / or a part of the glycol component may be replaced by another glycol component.

Other dicarboxylic acid components include diphthalic acids such as isophthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, adipic acid, sebacic acid and 1,4-cyclohexanedicarboxylic acid, or ester formation thereof. Sex derivative,
A metal sulfonate group-containing acid component such as an alkali metal salt of 5-sulfoisophthalic acid or 2-sulfoisophthalic acid, 1,8-dicarboxynaphthalene-3-sulfonic acid, p-oxybenzoic acid, p-β- Examples thereof include oxycarboxylic acids such as oxyethoxybenzoic acid, and ester-forming derivatives thereof.

The other glycol component has 3 to 10 carbon atoms.
10 lower alkylene glycols, 1,4-cyclohexanedimethanol, neopentyl glycol, 1,4-
Examples thereof include bis (β-oxyethoxy) benzene, bisglycol ether of bisphenol-A, polyethylene glycol or polypropylene glycol having various molecular weights, ethylene oxide-propylene oxide copolymer, and various derivatives of polyalkylene glycol.

Further, within the range in which the polyester is substantially linear, polycarboxylic acids such as trimellitic acid and pyromellitic acid, trimethylolpropane or its ethylene oxide addition derivative, pentaerythritol, glycerin or its ethylene oxide addition. A polyol such as a derivative, a monohydric polyalkylene oxide having various molecular weights or a derivative thereof, and a polymerization terminator such as phenylacetic acid may be used.

The polyester may be synthesized by any known method. In the synthesis of the polyester of the present invention, known catalysts, antioxidants, anti-coloring agents, ether bond by-product inhibitors, flame retardants, and other additives may be appropriately used.

The inclusion of the metal salt of an organic sulfonic acid represented by the general formula (1) in the polyester component constituting the sheath portion of the fiber of the present invention causes streak-like grooves on the surface of the fiber or fibrils. It is an indispensable condition for expressing the chemical phenomenon. Specific examples of the organic sulfonic acid metal salt used in the present invention include C3 to C30 alkyl sulfonic acid sodium salt or potassium salt, lithium salt, or a mixture thereof, or toluene sulfonic acid or dodecylbenzene sulfonic acid. Examples thereof include sodium or potassium salts of alkylbenzene sulfonic acid, lithium salts, and mixtures thereof. Among them,
Preferred specific examples of the organic sulfonic acid metal salt used in the present invention include alkyl sulfonic acid sodium salt. Such an organic sulfonic acid metal salt does not have to be a single compound and may be a mixture of various organic sulfonic acid metal salts represented by the general formula (1).

The organic sulfonic acid metal salt may be used alone, but it is preferable to add a small amount of a known stabilizer, antioxidant or the like for the purpose of improving thermal stability. is there. Examples of the stabilizer to be added to the organic sulfonic acid metal salt include hindered phenol compounds and thioether compounds. These compounds may be added alone, but in order to improve heat resistance,
Co-addition is also preferable.

Organic sulfonic acid metal salts which are usually commercially available contain inorganic impurities such as sodium sulfate and sodium chloride. Therefore, when melt-spinning a polyester containing a metal salt of an organic sulfonic acid, these impurities are easily trapped by the filter in the spinneret pack and are deposited on the filter over time. As a result, the back pressure of the filter is increased, which further causes thread breakage. These phenomena appear as remarkable phenomena as the blending amount of the organic sulfonic acid metal salt in the polyester increases. Therefore, the organic sulfonic acid metal salt used in the present invention is
The inorganic content is preferably 1,500 ppm or less. In particular, when sodium alkyl sulfonate is used, the sodium oxide content is 500 ppm or less and the sodium sulfate content is 1,000 ppm.
The following is preferred.

The organic sulfonic acid metal salt contained in the polyester component constituting the sheath of the fiber of the present invention is 0.5 to
It is necessary to be 5% by weight, preferably 1 to 4% by weight. If the blending amount is less than 0.5% by weight, the sheath portion of the finally obtained core-sheath polyester fiber does not become a fibrillar fiber, and if the blending amount is more than 5% by weight, the process passage stability deteriorates. Is not preferred.

As a method of blending the organic sulfonic acid metal salt with the polyester, it is not necessary to use a particular method, and it may be carried out by any method at any stage until the melt spinning of the polyester fiber is completed. it can.
For example, a method of adding and blending in a raw material of polyester, a method of adding and mixing in a reaction vessel during synthesis of polyester,
A method of compounding and kneading in an extruder, a method of adding and mixing during melt spinning, and the like can be adopted. However, it is preferable to use such an addition means that the organic sulfonic acid metal salt and the polyester are sufficiently kneaded in a molten state after the addition.

Preferably, the melt stream of the main component polyester polymer forming the sheath is supplied from the main extruder, and the melt modifier component flow containing the organic sulfonic acid metal salt as the main component can be depressurized to 50 Torr or less. From a twin-screw extruder having various degassing holes, and the polyester polymer stream and the melt modifier component stream supplied through the main extruder are combined to form a single combined stream. Immediately thereafter, the mixture is passed through a static mixing and stirring element, the composite flow is metered by a first metering pump, and the polymer mixed with the modifier is composite-spun through a composite spin pack equipped with a spinneret. It is preferable that the sheath component is melt-spun. At this time, the control of the polymer flow forming the core is
The control methods used for spinning ordinary polyester fibers are sufficient.

The core-sheath component ratio (volume ratio) of the core-sheath polyester fiber of the present invention is core / sheath = 3/1 to 1/8, preferably 2/1 to 1/4. If the ratio of the core component is too large, a satisfactory texture cannot be obtained, and if the ratio of the sheath component is too large, a satisfactory texture can be obtained, but on the other hand, the fiber strength decreases and the final product is obtained. Therefore, the mechanical properties of can not be satisfied.

In the production of the polyester fiber of the present invention, the fiber obtained after melt spinning or, if necessary, subjected to drawing heat treatment, false twisting, etc., has a temperature of 120 ° C. or higher, preferably 140 ° C. or higher. It is necessary to carry out dry heat treatment at a temperature for 10 seconds or longer, preferably 20 seconds or longer. This dry heat treatment is necessary to generate streak-like grooves on the fiber surface or to generate fibrillar ultrafine fibers separately from the polyester fiber after alkali reduction. In dry heat treatment of less than 120 ° C or less than 10 seconds,
The generation of streak-like grooves or the formation of fibrillar ultrafine fibers on the fiber surface is not sufficiently or not at all performed, so that the object of the present invention, which is the improvement of the feeling, cannot be achieved. It is advantageous in terms of heat treatment efficiency to perform this dry heat treatment in the form of a woven or knitted fabric after weaving or knitting.

Then, after the dry heat treatment, a weight reduction treatment of 1% or more, preferably 5% or more, more preferably 10% or more, still more preferably 15% or more of the weight of the fiber is performed with an aqueous solution of an alkaline compound. is necessary. This alkali weight reduction treatment is an essential treatment step for forming streak-like grooves on the fiber surface and for forming fibrillar ultrafine fibers. If the weight loss rate is less than 1% by weight, the formation of streak grooves on the fiber surface becomes insufficient, and the weight loss rate is 1 to 5% by weight.
In the case of 1, the formation of the streak-like grooves on the fiber surface is sufficient, but the formation of the fibrillar ultrafine fibers is insufficient or not formed at all. Therefore, a reduction treatment of 5% or more is required to form the fibrillar ultrafine fibers.
Generally, when the weight loss rate becomes large, the strength decrease becomes large, but in the present invention, the composite fiber causes little strength reduction even if the weight loss rate becomes large. The rate of decrease in strength is greatly affected by the core-sheath component ratio (volume ratio), but the fiber of the present invention has a weight loss ratio of 1 to 18% and a ratio of 2.5.
Strength of g / d or more, 1.5 g / d even if the weight loss rate is 19% or more
The above strength is shown. Examples of the alkaline compound used in the alkali weight reduction treatment include sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, sodium carbonate, potassium carbonate and the like, and sodium hydroxide and potassium hydroxide are particularly preferable. As the alkali weight loss treatment method, any known method generally applied as weight loss processing to polyester woven and knitted fabrics can be applied, but during treatment, the woven and knitted fabrics are contacted with each other or between the woven and knitted fabrics and the treatment device, thereby It is preferable to apply a method in which a rubbing action is added such that division and fibrillation of the polyester fiber occur during the alkali weight loss treatment step.

In the production method of the present invention, a mechanical external force may be applied to the polyester fiber at the same time as the dry heat treatment and the alkali weight loss treatment. By applying a mechanical external force, it is possible to more effectively fibrillate the polyester fibers and to generate the ultrafine fibers in a divided manner. As a method of applying a mechanical external force, for example, a means using frictional force such as rubbing or beating, an impact force, or a raising process can be used.

The modified polyester fiber of the present invention obtained as described above has, as shown in one example in FIGS. 1 and 2, streak grooves oriented in the fiber axis direction on the fiber surface, and Fibril-shaped ultrafine fibers are generated separately. FIG. 3 shows an electron micrograph of the side surface of a fiber showing a situation in which fibrillar ultrafine fibers are split and generated.

When the polyester fiber of the present invention is used as a woven or knitted fabric, the polyester fiber of the present invention may be used alone, but the purpose is to maintain the strength of the woven or knitted fabric or to improve the texture. Then, other fibers may be mixed and used.

[0031]

EXAMPLES The present invention will be described more specifically with reference to the following examples. The characteristic values and judgment values in the examples are measured or judged by the following methods. In the examples, “parts” means “parts by weight” and “%” means “% by weight”.

(Intrinsic viscosity) It is a value measured by dissolving a sample in a phenol / tetrachloroethane (50/50) mixed solvent and using an Ubbelohde viscometer at 25 ° C.

(Formation of Fiber Surface Streaky Grooves) A surface photograph of the fiber finally obtained is magnified by a scanning electron microscope at a magnification of 2.
It was photographed at 000 times and visually judged. ◯: Striated grooves highly oriented in the fiber axis direction are well formed. Δ: Although formation of streak grooves oriented in the fiber axis direction is recognized, the degree of orientation is low and the grooves are shallow. X: Almost no streak formation is observed.

(Generation state of ultrafine fibers) A surface photograph of the fibers finally obtained is magnified by a scanning electron microscope at a magnification of 1,000.
It was photographed at 0 times and visually judged. ◯: The fibrillar ultrafine fibers are satisfactorily generated, and the occurrence frequency is high. B: Some fibrillar ultrafine fibers are derived. X: Generation of fibrillar ultrafine fibers is hardly observed.

(Hand) O: A soft and rayon-like unique texture, which is good. Δ: Fairly good. X: Like a conventional polyester fiber, defective.

(Synthesis of Polyester of Shell Component: A) 100 parts of terephthalic acid and 52 parts of ethylene glycol were charged into an esterification tank, and an esterification reaction was carried out at 260 ° C. under a pressure of 4 kg / cm ² . Subsequently, 0.01% of trimethyl phosphate, 0.04% of antimony trioxide and 0.5% of titanium dioxide were added to the obtained reaction product as ethylene glycol solution or dispersion, respectively.
Further, sodium alkylsulfonate having an average carbon number of 15 (RSO ₃ Na) was also added as an ethylene glycol solution so as to be 2.0% by weight with respect to the produced polymer, and then transferred to a polymerization tank. Liquor Then, the polycondensation reaction was allowed to proceed at 285 ° C. under high vacuum to obtain a polyester polymer having an intrinsic viscosity of 0.68. This polyester is referred to as "Polymer A".

(Synthesis of Sheath Component Polyester: B) 100 parts of terephthalic acid and 52 parts of ethylene glycol were charged into an esterification tank, and the esterification reaction was carried out at 260 ° C. under a pressure of 4 kg / cm ² . Subsequently, 0.01% of trimethyl phosphate, 0.04% of antimony trioxide and 0.5% of titanium dioxide were added to the obtained reaction product as an ethylene glycol solution or dispersion, respectively, and then the polymerization tank was added. Was transferred to. Subsequently, the pressure in the polymerization tank was gradually reduced, and 30 minutes after the degree of vacuum in the polymerization tank became 1 Torr or less, 70 minutes after the polycondensation reaction was started,
Average carbon number is 15.5, sodium chloride as impurities 30ppm, sodium sulfate 100ppm
Containing sodium alkyl sulfonate (RSO ₃ N
a) was added in a molten state while the polymerization tank was in a high vacuum state so as to be 2.0% by weight based on the produced polymer.
Then, the polycondensation reaction was allowed to proceed further at 285 ° C. under high vacuum to obtain a polyester polymer having an intrinsic viscosity of 0.70.
This polyester is referred to as "Polymer B".

Examples 1 to 5 and Comparative Examples 1 to 2 Polyethylene terephthalate (Polymer C) containing 0.5% of titanium dioxide and having an intrinsic viscosity of 0.71 was used as the core component polymer and Polymer A as the sheath component polymer. A spinning temperature of 28 was passed through a spinneret having 24 circular spinning holes with a hole diameter of 0.3 mm.
At 8 ° C., fibers having different core-sheath ratios as shown in Table 1 were melt-spun. After the discharged yarn was cooled and solidified with a cooling air flow, an oil agent was applied, and an undrawn yarn of 195 denier / 24 filament was wound at a spinning speed of 1,800 m / min. Next, this unstretched yarn was stretched to about 2.3 times by a conventional method to obtain a core / sheath fiber stretched yarn of 75 denier / 72 filaments. In Example 5, the discharge yarn was immediately cooled below the glass transition temperature of the polymer, stretched by running in a heating zone installed between the spinneret and the take-up roller, and 4,000 m / min. The core-sheath fiber drawn yarn was directly wound at a winding speed. All of these fibers were subjected to dry heat treatment at 180 ° C. for 60 seconds, and then subjected to alkali weight loss treatment at a boiling water temperature using an aqueous sodium hydroxide solution. The results are shown in Table 1.

(Examples 6 to 8) Intrinsic viscosity of 0.56 obtained by copolymerizing 2.3 mol% of sodium 5-sulfoisophthalate
The copolymerized polyester (Polymer D) is used as the core component polymer and the polymer B is the sheath component polymer, and the spinning temperature is 28
The same procedure as in Examples 1 to 4 was repeated except that the spinning speed was 5 ° C., the spinning speed was 1,600 m / min, and the draw ratio was 70% of the maximum undrawn yarn breaking ratio. A changed fiber was obtained. The results are shown in Table 1.

(Examples 9 to 11) Isophthalic acid was added to 8.3.
The same procedure as in Examples 6 to 8 was repeated except that the core component polymer was the copolymerized polyester (polymer E) having an intrinsic viscosity of 0.69 and the polymer B was the sheath component polymer. Fibers having different core-sheath ratios were thus obtained. The results are shown in Table 1.

[0041]

[Table 1]

[0042]

EFFECTS OF THE INVENTION The core-sheath fibrillated polyester fiber of the present invention has a continuous streak groove highly oriented in the fiber axis direction on the fiber surface, and the fibrillar ultrafine fibers are generated in a divided manner. The woven or knitted fabric made of the fibers can have a unique rayon-like feel and dry feel, as well as a fluffy surface feel and a soft feel. Further, due to the core-sheath composite fiber, the strength retention rate after alkali reduction is also high.

[Brief description of drawings]

FIG. 1 is a scanning electron micrograph showing an example of a cross section of a fibrillated polyester core-sheath fiber of the present invention. (Magnification 2,000 times)

FIG. 2 is a scanning electron micrograph showing another example of the cross section of the fibrillated polyester core-sheath fiber of the present invention.
(Magnification 2,000 times)

FIG. 3 is a scanning electron micrograph showing an example of a side surface of the fibrillated polyester core-sheath fiber of the present invention. (Magnification 1,000 times)

Claims (4)

[Claims] 1. A fiber surface having a width of 0.1 to 3 μm and a length of 5
The spacing between any adjacent streak grooves of 5 μm or more is 5 μm
The following is a modified polyester fiber having a large number of continuous streaky grooves highly oriented in the fiber axis direction and having a strength of 1.5 g / d or more and an elongation of 10% or more. 2. The modified polyester fiber according to claim 1, which has fibrils having a diameter of 0.05 to 5 μm and a length of 10 μm or more on the fiber surface. 3. The main repeating unit is an ethylene terephthalate unit, and the polyester polymer containing 0.5 to 5% by weight of the organic sulfonic acid metal salt represented by the following general formula (1) is contained in the sheath portion in an amount of 85 mol% or more. Is a core-sheath type conjugate fiber in which a polyester polymer composed of a repeating unit of ethylene terephthalate is arranged in a core portion.
An intermediate of the described modified polyester fiber. R-SO ₃ M (1) (wherein R represents an alkyl group having 3 to 30 carbon atoms, or an aryl group or alkylaryl group having 7 to 40 carbon atoms, and M represents an alkali metal) Show.) 4. The main repeating unit is an ethylene terephthalate unit, and the polyester polymer containing 0.5 to 5% by weight of the organic sulfonic acid metal salt represented by the general formula (1) is contained in the sheath portion at 85 mol% or more. Is melt-spun as a core-sheath type composite fiber in which a polyester polymer composed of a repeating unit of ethylene terephthalate is arranged in the core to obtain a stretched yarn, and then a dry heat treatment is performed at a temperature of 120 ° C. or more for 10 seconds or more, After that, 1% or more of the fiber weight is reduced with an aqueous solution of an alkaline compound, and the method for producing a modified polyester fiber according to claim 1 or 2, characterized in that.