JPS5915512A - Production of polyester fiber - Google Patents

Abstract

PURPOSE:An aromatic polyester is combined with a low molecular weight compound which is unreactive and compatible with the polyester and has a specific melting point and molecular weight and they are subjected to melt spinning, then a part of the filaments is leached out to give the titled porous fiber which gives dyed fabrics with deep color and bright tone. CONSTITUTION:100pts.wt. of an aromatic polyester such as polyethylene terephthalate are combined with 0.1-30pts.wt. of a low molecular weight compound which is stable, unreactive and compatible with the polyester under conditions where the polyester is melted causes no phase separation, when the composition is solidified, melts over 100 deg.C and has a molecular weight of less than 1,000 such as 1-phthalimide-3-phthalimidemethyl-3,5-5-trimethylcyclohexane, then, the composition is subjected to melt spinning. The resultant fiber is treated with an aqueous alkali to effect elution by more than 2wt% to give the objective fiber.

Description

[Detailed description of the invention] The present invention relates to a method for producing polyester fibers.

More specifically, it has special fine pores and is colored 1. Regarding the production of porous F1 polyester fibers exhibiting significantly improved color depth and clarity, polyesters have many excellent properties and are therefore suitable for use with W synthetic fibers. is widely used. However, compared to natural fibers such as wool and silk, cellulose fibers such as rayon and acetate, and acrylic fibers, polyester fibers have a lower depth of color when dyed. There are some disadvantages in terms of quality and sharpness.

(From now on, in order to solve this drawback, 1.
Attempts have been made to chemically modify llr q polyester, but none of them have been sufficiently effective. Also proposed are a method of forming a transparent thin film on the surface of polyester fibers and a method of applying plasma irradiation of 80 to 500 ynA-sec/7 to the surface of a woven or knitted fabric to form fine irregularities on the surface of the fiber. However, even with these methods, the effect of improving the depth of color is insufficient, and in addition, the transparent thin film formed on the fiber surface easily falls off when washed, etc., and its durability is insufficient. The irradiation method does not create unevenness on the surface of the fiber in the fiber part that will be in the shadow of the irradiated surface, so the smooth fiber surface will come to the surface due to the rolling of threads within the fiber structure that occurs during wear, resulting in color spots. There is a drawback.

On the other hand, as a method of imparting irregularities to the surface of polyester m fibers, polyoxyethylene glycol or polyoxyethylene glycol and fabric 1. A method for producing hygroscopic fibers in which wrinkle-like micropores arranged in the axial direction of the fibers are formed on the fiber surface by treating fibers made of polyester containing a fonic acid compound with an alkaline aqueous solution, or methods using materials such as zinc oxide, calcium phosphate, etc. Fine particles 7 of an inert inorganic substance are added and blended into the polyester reaction system (-filtered).The polyester fibers are treated with an alkaline aqueous solution (2) and fine pores are formed by eluting the non-+rj P particles. Hygroscopic properties #j#, fiber manufacturing methods, etc. have been proposed.
However, the fibers obtained by these methods are not effective in improving the depth of color, and on the contrary, a decrease in visual density is observed. That is, in these methods, if the treatment with the alkaline aqueous solution is not sufficient, the effect of improving the color depth is not seen at all, and when the treatment with the alkaline aqueous solution is sufficient, the color depth, 2/ Far from improving, the visual density decreased, perhaps due to the diffuse reflection of light by the micropores, and the dark color KM color 1. Even if it's white, it will look even brighter, and the strength of the # fibers will be noticeably stronger. <Lower L, it becomes easily fibrillated and cannot be put to practical use.

In addition, non-fJ f such as silica with a particle size of 80 mμ or less
Fibers made of polyester are subjected to alkali reduction treatment to give irregular irregularities of 0.2 to 0.7 μm on the fiber surface, and to form irregularities of 50 to 200 μm within these irregularities.
A method has been proposed to improve the depth of color by creating fine irregularities. However, even with this method, the effect of improving the depth of color is insufficient, and on top of that, it may be due to the extremely complicated uneven shape.
The concavities and convexities are destroyed by external physical actions such as friction, and the destroyed areas become significantly discolored or have a difference in gloss compared to other undestructed areas, and furthermore, they easily form into pressure fibrils. There is.

The inventor of the present invention has conducted extensive research in an effort to provide a polyester fiber that does not have the above-mentioned drawbacks and has excellent color depth and clarity by adding micropores to the polyester fiber, and as a result, the inventors have discovered that a specific phosphorus compound and this phosphorus Synthesis I was carried out by adding a metal compound to the polyester reaction system in a specific ratio without reacting the compound in advance. By forming irregularities smaller than the size of the fibers on the entire surface of the fiber, the depth and vividness of the color when colored 1. is excellent, and the discoloration due to friction is sufficiently small, making it fibril resistant. It was discovered that 13-polynystle fibers with excellent properties could be obtained (1. Previously proposed 1.).

In the course of such studies, the present inventors have come to the conclusion that as the size of the irregularities formed on the fiber surface becomes finer, both the color improvement effect and the fibril resistance improve. Based on this knowledge of the present invention, we have developed a method for imparting even finer irregularities to the surface of polyester fibers.As a result of one sharp and difficult experiment, we have found that the molecule By applying an alkali treatment to the Hbori ester fiber obtained by melt-spinning a polyester composition blended with a compound, improvements in the uneven morphology of the fiber 14F surface and an improvement in the color improvement effect were achieved. The present invention is based on this knowledge.
, is completed.

Vll, 4, the invention is 1. 1 part of 100 boxes of aromatic polyester is blended with 0.1 to 30 parts of a low molecular weight compound that satisfies F1l14moxibustion1'No~(4). A method for producing porous polyester fibers characterized by melt spinning a polyester composition obtained by melt-spinning, and treating the obtained polyester fibers with an aqueous solution of an alkaline compound to elute (spread) at least 2 Jit of the fibers. It is.

(1) Melting Q of aromatic polyester! It is substantially stable under conditions, non-reactive with the polyester, and compatible with the polyester.

(2) Phase separation from the polyester does not occur when the polyester composition is cooled and solidified.

(3) The melting point is 100°C or higher.

(4) The number of molecular moths is 1000 or less.

The aromatic polyester used in the present invention has an aromatic dicarboxylic acid as a main acid component, and the aromatic dicarboxylic acid and (7) terephthalic acid, isophthalic acid,
Naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid. diphenyl ether dicarboxylic acid, :) phenoxyethane dicarboxylic acid,
Examples include methyl terephthalic acid and methyl isophthalic acid. In addition, glycol components include ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, octamethylene glycol, decamethylene glycol, etc. (1) 6[]p aliphatic glycol, 1,4- Alicyclic glycols such as cyclohexane dimetatool, etc.
and heid p-quinone, methylhydro-p-quinone, fupur hydroquinone, 7-mil hydroquinone.

resorcin, 2,2-his(4-hydroxyphenyl)
Propane [bisphenol A], 1,1-his(4-human 1-xyphenyl)cyclohexane [bisphenol Z
) l'' (4-bitroxyphenyl) ether and the like can be used.An oxycarboxylic acid component may be used in addition to the above-mentioned dicarboxylic acid component and glycol component, and a nuclear oxycarboxylic acid and 1.
Examples include oxybenzoic acid, oxynaphthoic acid, and β-hydroxyethoxybenzoic acid.

In addition to the above-mentioned components, the aromatic polyester used in the present invention also contains aliphatic dicarboxylic acids such as dipic acid, sebacic acid, hexahydroterephthalic acid, etc. in small proportions (usually 10 mole or less based on the total acid components). It can contain alicyclic dicarboxylic acids and 5-sodium sulfoisophthalic acids such as, etc., and a small amount (usually 10% by weight or less) of polyoxyfulkylene glycol may be copolymerized.Furthermore, aromatic polyesters Even if polycarboxylic acids such as trimellitic acid, bilumellitic acid, etc., polyols such as glycerin, trimethylolbutane, pentaerythritol, etc. are copolymerized within a range in which is substantially linear (usually 1 molar or less). No. [, not supported.

Among the above-mentioned aromatic polyesters, polyesters mainly containing repeating units represented by the following general formula (%) are preferable, with polyethylene terephthalate and polybutylene terephthalate being particularly preferred. Such an aromatic polyester may be synthesized by any method.For example, in the case of polyethylene/terephthalate 1, it is usually a direct esterification reaction of terephthalate with an acid and ethylene glycol, or a synthesis reaction of dimethyl terephthalate. A glycol ester of terephthalic acid and/or a low polymer thereof is obtained by transesterifying a lower alkyl ester of terephthalic acid and ethylene glycol, or by reacting de!/phthalic acid with ethylene oxide. The reaction of the first stage to be generated and the first
C is produced by a second reaction step in which the reaction product of the step is heated under reduced pressure and subjected to polycondensation reaction until the desired degree of polymerization is achieved.

The low-molecular compound compounded in the above-mentioned aromatic polyester in Kishinmei is based on (1) melting conditions of the aromatic polyester-F
For example, at a temperature of 120°C, the melting point of polyester,
It needs to be substantially stable, non-reactive with the aromatic polynisful, and homogeneous and pressure compatible which will cause phase separation when blended with the aromatic polyester. "Substantially stable and non-reactive with the aromatic polyester" means that it does not degrade itself or degrade the aromatic polyester or react with the aromatic polyester. do.

The low-molecular compound can be obtained by (2) melt-mixing the aromatic polyester and the low-molecular compound, and then cooling and solidifying the resulting homogeneous melt, without causing phase separation from the aromatic polyester. It is necessary that they be compatible (, maintain their original state (, obtain).

This can be done, for example, by cooling the homogeneous transparent melt described above at a cooling rate sufficient to give an indecent solid. When a non-quality solid is obtained, it is necessary to obtain a homogeneous transparent solid or to cause phase separation.
．． This can be easily determined by observing whether it gives an opaque solid.

Further, the low molecular weight compound needs to have (3) an M point of 100° C. or higher, and (4) a molecular weight of 1000 or lower. When the melting point of the low-molecular compound is 100°C or less, melt mixing with the aromatic polyester 1. However, the secondary transition point of the aromatic polyester is significantly lowered by 7 degrees, making it difficult to handle the compost during molding.

The low molecular weight compound used in the present invention meets the above conditions (1).
It is sufficient if P is added to (4), but in addition, from the viewpoint of preventing volatilization during melt mixing, the boiling point at normal pressure is 250 ° C. or higher,
Particularly preferred is 300°C or higher.1. <Used.

The low-molecular compound used in the method of the present invention may be one that satisfies the following conditions, for example, the following formula (1)! Examples include an imide compound represented by 1, an imide compound represented by the following formula (1), and the like.

The compound of the above formula (1) and 1. Then, in the above formula (1), (
Compounds in which at least one of - or R1 is an optionally substituted aromatic residue, particularly compounds in which R1 is an optionally substituted divalent aromatic residue, are preferred. use

Furthermore, the expression for the compound of the above formula value) preferably uses a compound in which R1 is a monovalent, optionally substituted aliphatic residue in the above formula (1).

In the above-mentioned formula (2), the divalent aromatic residue representing 1. is, for example, 1.2-phenylene group, 1.2-
．． 2.3- or 1,8-naphthylene groups; examples of divalent aliphatic residues include linear alkaline radicals such as ethylene or trimethylene, or 1,2-cyclohexylene groups. Cycloalkylene groups such as

These groups may be substituted with a substituent that is non-reactive with respect to the aromatic polyester (3). Such substituents include, for example, lower argyl groups such as methyl and ethyl, lower alkoxy groups such as methoxy and ethoxy, chlorine,
Gen atoms such as bromine, nido ag, phenyl group, phenoxy group.

Examples include cyclohexyl which may be substituted with a methyl group.

A monovalent group representing H+ (as an aromatic residue with n=1 or 20, for example, a phenyl group, a naphthyl group, or the formula <:)
-2(y (where 2 is →-1-8Os- or -CH,
- is a monovalent aromatic residue such as a group of 1,2
-phenylene group, 1,2-. 2.3- or 1,8-
Divalent aromatic residues such as naphthylene groups or groups of the formula C nido z-/:j (where 2 is +, -8ow * or -CH,-) can be mentioned; (n
= 1 'j or 2) as an aliphatic residue, for example, methyl, ethyl.

1 carbon number such as butyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, myridityl, stearyl
~18 linear furkyl group or cyclohexyl is also L <
is a 5- or 6-membered cyclic alkyl group such as cyclopentyl, yatake ethylene, trimethylene, tetramethylene, hexamethylene, octamethylene, decamethylene.

A linear alkylene group having 2 to 12 carbon atoms such as dodecamethylene, a cyclic alkylene group such as 1,3- or 1,4-cyclohexylene, or Rt1 These groups are the same as described for AI. may be substituted with a substituent.

In (II) above, in 6, a tetravalent aromatic residue representing and 1, or, for example, (2 is the same as the above definition), It is possible to list a tetravalent aromatic group as b7 + + -.

A tetravalent template-like or cyclic aliphatic residue representing Rt and 1
- may also be the same chain alkyl group having 1 to 18 carbon atoms as exemplified for R1 in the above formula +Il, or 1 to 5. In addition, a 6-membered cyclic alkyl group can be mentioned.

Example 1 regarding the above AI and Rf K. The groups may be substituted with the same substituents as described for AI.

As the imide compound represented by the above formula (1), for example, in the formula (1), as a compound when 1==i, N
-Methylphthalimide, N-ethylphthalimide, N-butylphthalimide, N-ethyl-1,8-
Phthalimide, N-butyl-1,8-naphthalimide, etc.; Compounds where n=2 and 1. N,N'-ethylene bisphthalimide, N,N'-tetramethylene bisphthalimide.

N, N'-hexamethylene bisphthalimide.

N, N'-octamethylene bisphthalimide.

N, N'-tecamethylene bisphthalimide,
N,N'-dodecamethylene bisphthalimide, N,N
'-Neopentylenebisphthalimide, N,N'-tetramethylenebis(1,S-naphthalimide).

N,N'-hexamethylenebis(1,8-naphthalimide), N,N'-octamethine/bis(1,8-naphthalimide), N,N'-decamethylenebis(1,
8-naphthalimide), N,N'-dodecamethylenebis(l,8-naphthalimide), N,N'-dodecamethylenebissuccinimide, N,N'-dodecamethylenebishexahydrophthalimide, I'J ,N'
-1,4-cyclohexylene bisphthalimide, 1-
phthalimido 3-phthalimidomethyl-3,s,
s-) Limethylcyclohexane, 4.4'-bisphthal imidodiphenyl ether 1°1.4'-bisphthalimidodiphenyl ether, 3.3'-bisphthalimidodiphenyl sulfone, 4. Examples include c-hisphthalimidodiphenylsulpone, 4,4'-bisphthalimidodiphenylsulpone, and nilmetaso.

”: An imide compound represented by the formula +11,
For example, N,N'-diethyl bilmeritoimide.

N, N'-dibutylpyromellitimide, N, N'-
didecylpymeritimide, N,N'-dioctylbimeritimide, N,N'-didecylpymeritimide, N,N'-dicyclohexylpyromellitimide, N,N'-his(3,3,5 -trimethylcyclohexyl)pyromellitimide, N,N'-
Diethyl-1,4,5,8-naphthalenetetracarboxylic acid 1.11-. 4.5-diimide, etc. can be mentioned.

The imide compounds represented by the above formulas (1) and +1) can be easily produced from the corresponding acid anhydrides and organic amines by known methods.

In the present invention, first, 0.1 to 30 parts by weight of a low-molecular compound is blended into a portion of aromatic polyester 100, and then mixed to obtain a polyester composition. If the amount of the low molecular weight compound is less than 0.1 part by weight, the color exploration and clarity of the final polyester fiber obtained will be insufficient, and as this amount is increased, the depth and clarity of the color will increase. If the amount exceeds 30 parts by weight, the abrasion resistance and durability will no longer be significantly improved, and the abrasion resistance and durability will deteriorate]1, and the melt viscosity of the resulting composition will decrease significantly, making melt spinning difficult. The amount of the low-molecular compound to be blended is more preferably 1° or 3 to 20 parts by weight.

A composition consisting of an aromatic polyester and a low-molecular-weight compound can be prepared by adding the low-molecular-weight compound to the polyester after the synthesis of the polyester is completed.
The polyester may be synthesized at any stage of the process, for example, by adding it to the raw polyester or the reaction mixture, and then completing the synthesis of the polyester. It may be melt-mixed in advance in a melt extruder.Also, a low-molecular compound heated and melted above the lock point is added to chips of aromatic bo-11 ester. After mixing with
1. The low molecular weight compound obtained by cooling is deposited on the surface of the aromatic polyester chip.1. Also preferably used.

” Class 2! Optional additives may be added to the material as needed.

example, touch, oxidation stabilizer, ultraviolet stabilizer, retardant,
It may contain a brightening agent, a coloring agent, etc., and a high degree of polymerization during melt-spinning or a nail density f14-1.
- From the viewpoint of inhibition, 2,2'-bis(2-oxazoline), 2,2'-bis(3,1-benzoxazine-4
Chain extenders such as (-one) are also preferred, and (1'-2) can also be used.

In order to melt-spun the polyester composition obtained in step 1 into fibers, it is not necessary to employ any special method, and any ordinary melt-spinning method for polyester fibers may be employed. The fibers to be spun here may be either solid fibers or hollow fibers having hollow portions.

Further, the outer shape and the shape of the hollow portion in the cross section of the fiber to be spun may be circular or irregular.

A part of the polyester fiber thus obtained is removed [7] To obtain a porous polyester fiber, a stretching heat treatment or a false twisting process is performed as necessary, or the fabric is further processed in 1. This can be easily carried out by treating with an aqueous solution of an alkaline compound after treatment.

Examples of the alkali compounds used here include sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, botrium carbonate, potassium carbonate, and the like.

Among these, sodium hydroxide and potassium hydroxide are particularly preferred.

The concentration of such an aqueous solution of an alkali compound varies depending on the type of alkali compound, the amount of treatment, etc., but is usually 0.01.
The range of 0.1 to 40 weight ratio is preferable [5, and the range of 0.1 to 30 weight ratio is particularly preferable.

The processing temperature is preferably in the range of room temperature to 100°C! L, <, The treatment time is usually carried out in the range of 1 minute to 4 hours. In addition, the pupil to be eluted and removed by treatment with the aqueous solution of the alkaline compound should be in the range of 2% or more based on the fiber type. By treating with an aqueous solution of an alkaline compound in this way, special micropores can be formed on the fiber surface and its vicinity, resulting in excellent color depth when dyed. The polyester fibers obtained by the method of the present invention can be treated with an organic solvent as necessary to extract and remove low molecular weight compounds remaining in the fibers. Such an organic solvent and 1. Examples include toluene, xylene, punidocumene, dioquiline, phosphorform, methylene chloride, dichloroethane, ethanol, ethyl acetate, acetone, methyl ethyl ketone, methyl isobutyl ketone, and coolbenzene. These organic solvents are preferably selected appropriately depending on the type of aromatic compound, polyester compound, and low-molecular compound, especially imide compound.

Although this extraction treatment using an organic solvent can be carried out at room temperature, it is preferable to carry out heating within a range that does not impair the morphology and physical properties of the polyester fiber, for example, under reflux conditions of the organic solvent.

Further explanation will be given below with reference to Examples. In the examples, the parts and divisions indicate the mold weight, and the depth of color and resistance to abrasion discoloration when the resulting polyester fibers are dyed are measured by the following method (-).

(1) Deepness of color A scale indicating the depth of color (- or bathochromicity (K/S). This value is the spectral reflectance of the sample cloth @) Shimadzu RC
It was measured with a -330 type self-recording spectrophotometer and calculated from the Kubelka-Munk equation shown below. The larger this value is, the greater the deep color effect is.

Note that K represents an absorption coefficient and S represents a scattering coefficient.

(11) Using a single-oscillation flat abrasion machine for friction discoloration and friction fastness tests.
Using a georgette made of polyethylene terephthalate ioθ% as a friction cloth, the test cloth was subjected to surface abrasion for a predetermined number of times under a load of 50°. The degree of discoloration was determined using a gray scale for discoloration and fading. When the wear resistance was extremely low, it was ranked as 1st grade, and when it was extremely high, it was rated as 6th grade. For practical purposes, a grade 4 or above is required.

Examples 1-6 Intrinsic viscosity 0,645. 1f to 100 parts of polyethylene terephthalate chips with a softening point of 260°C! l! A predetermined amount of the low-molecular compound shown in 1-1 was dry-blended, and the resulting mixture was melt-spun 1-1 at 275°C using a spinneret with 36 circular spinning holes with a hole diameter of 0.3 fins. Then, it was drawn according to a conventional method at a draw ratio of 4.5 times to obtain a raw yarn of 75 denier/24 filaments.

This raw yarn KS twist 2500' f/m and 2 twist 250
Strongly twisted yarn at 0 T/m [1, then the strongly twisted yarn was heated at 80°C
The twist was fixed by steaming treatment 12 for 30 minutes.

A satin georgette fabric was woven by alternately arranging two S and Z twists with a warp density of 47 yarns/width and a weft density of 32 yarns/fold.

The obtained gray fabric is heated to boiling temperature in a rotary washer for 2 hours.
The cloth was subjected to a relaxing treatment for 0 minutes, then grained, preset by a conventional method, and treated with a 3.5-aqueous sodium hydroxide solution to boiling temperature to obtain a fabric with a weight loss rate of 10 inches.

The fabric after this alkali treatment is Dianix Black.
HG-F8 (Mitsubishi Chemical Industries ■ product) 15% owf
After dyeing at 30° C. for 60 minutes, the fabric was photowashed at 70° C. for 20 minutes in an aqueous solution containing 1 y/L of sodium hydroxide and 1 f/l of hydrosulfite to obtain a black dyed fabric. Table 1 shows the color depth and friction discoloration resistance of this black cloth after 200 wears.

In addition, the surface of the constituent fibers of the black cloth obtained in Example 1 was
Figure 1 is an electron micrograph magnified 500 times.

Example 7 Used in Example 1 1. Instead of polyethylene terephthalate chips, 2.5 mol of 5-Na sulfoisophthalic acid was copolymerized [-] with an intrinsic viscosity of 0.490. The same procedure as in Example 1 was conducted except that 100 parts of copolymerized polyethylene terephthalate chips having a softening point of 258° C. were used.

The knots are shown in Table 1.

Example 8 Instead of the polyethylene terephthalate chips used in Example + IC, 4% by weight of polyoxyethylene glycol with an average molecular weight of 600 was copolymerized [, and the intrinsic viscosity was 0].
,640. The same procedure as in Example 1 was carried out except that 100 parts of copolymerized polyethylene terephthalate 0 chips having a softening point of 259°C were used.

The results are shown in Table 1.

Example 9 Polytetramethylene terephthalate chips with an intrinsic viscosity of 0.88 were used in place of the polyethylene terephthalate chips used in Example I, and the melt spinning temperature was set to 23.
The same procedure as in Example 1 was conducted except that the temperature was 5°C. The results are shown in Table 1.

Comparative Examples 1 to 4 Comparative Examples 1 to 4 were carried out in the same manner as Examples 1, 7.8 and 9, respectively, except that the low molecular weight compounds used in Examples 1, 7.8 and 9 were not used. The results are shown in Table 1.

4 Inside the drawing, t! F, light Figure 1 shows the surface of polyester fiber used in the method of the present invention>
Magnified 3500 times with a Neel electron microscope 1. 4).

Procedural amendment dated August 11, 1980, Commissioner of the Japan Patent Office 1, Indication of the case, Patent Application No. 1987-120571, 2 Name of the invention, Process for manufacturing polyester fiber 3 Person making the amendment Relationship with the case Patent, i'F application 1-IJ, Minamihonmachi, Higashi-ku, Osaka (300) Teijin Ltd. Representative Tomohito Tokusue (1) On page 14, line 13 of the specification, [1 for polyester is corrected to read ``for polyester.''

(2) 15jj, line 11K, "butyl" is corrected to "butyl".

(3) In the same page, page 17, line 6, the text "1-N-butylphthalimide" is corrected to "1N-butylphthalimide."

(4) On page 17, line 7, the phrase "N-butyl" is corrected to "N-butyl."

(5) In the same page, page 18, line 15, "dipylpyromellitimide" is corrected to "dibutylpyromellitimide."

(6) Page 24, line 17, K “36 pieces” is U
Correct it to 24 blue.

Above - Above

Claims (1)

[Scope of Claims] 1. 100 parts by weight of aromatic polyester and the following conditions (1
) to (4) A polyester composition blended with 0.1 to 30 parts by weight of a low molecular weight compound is melt-spun, and the resulting polyester fibers are treated with an aqueous solution of an alkali compound to reduce the weight of the fibers by 0.1 to 30 parts by weight. 17. A method for producing porous polyester fibers, characterized by dissolving the above. (11) Substantially stable under the melting conditions of the aromatic polyester, non-reactive with the polyester, and compatible with the polyester. (2) The polyester composition is cooled and solidified. Does not cause phase separation from polyester. (3) Melting point is 100°C or higher. (4) Molecular weight is 1000 or less. 2. Aromatic polyester mainly consists of repeating units represented by the following general formula 6 (%) 2. A method for producing a polyester fiber according to claim 1, which is made of polyester.