JP7619899B2 - Method for producing polyester fiber and method for producing same - Google Patents

Description

The present invention relates to spun-dyed polyester fibers and a manufacturing method thereof. More specifically, the present invention relates to spun-dyed polyester fibers made of polyester containing a colorant with a specific particle size and content, in which the single fibers are thin and have high toughness, and to a manufacturing method thereof.

In recent years, ultrafine fibers have been widely used for a variety of purposes, including eyeglass cleaning wipers, household cleaning wipers, filters, and artificial leather with a suede or nubuck texture similar to that of natural leather. In particular, artificial leather is used as a material for clothing, shoes, and furniture, so it is dyed in a post-process. The dyeing process uses a large amount of chemicals and energy, and requires treatment of the dye wastewater, which increases manufacturing costs. In addition, dyeing has the problem that the dye molecules only penetrate the amorphous parts of the fibers and cause coloring, making them prone to fading. Therefore, a method of coloring fibers by pre-dyeing, in which a dye or pigment is added to the fibers, has been studied. (For example, Patent Documents 1 to 3)

In both Patent Documents 1 and 2, two incompatible resins are mixed and spun into yarn using an islands-in-the-sea type co-spinning method, and then the sea component is removed from the resulting fiber using an organic solvent and discarded, yielding doped ultrafine fibers that are the island component. However, there are concerns about the burden on the natural and working environments.

Patent Document 3 describes a composite fiber in which polyester containing carbon black and a polyolefin polymer or a polyamide polymer are combined to form a specific cross-sectional shape. However, the use of different resin types poses an issue with the recyclability of the product.

Japanese Patent Application Publication No. 9-59881 JP 2002-146624 A JP 2011-208336 A

The present invention was made against the background of the above-mentioned conventional technology, and its purpose is to obtain dyed polyester fibers that have good dispersibility in water and high toughness, and to provide an environmentally friendly manufacturing method.

As a result of extensive research into solving the above problems, the inventors have completed the present invention by using a colorant with a specific primary particle size.

That is,
1. A doped polyester fiber made of polyester containing 0.5 to 3.0% by weight of a colorant having an average primary particle size of 5 to 30 nm, the fiber having a single fiber fineness of 0.01 to 0.5 dtex, a toughness of 10 or more, and a fiber length of 2 to 25 mm;
2. A method for producing spun-dyed polyester fibers as described in 1 above, comprising: mixing polyester and a colorant having an average primary particle size of 5 to 30 nm at a blending ratio of 0.5 to 3.0% by weight based on polyester fiber; melting the mixture; passing the melt through a spinneret having an outlet hole at a spinning speed of 300 to 1000 m/min to produce undrawn polyester fibers; flow drawing the resulting fibers at a ratio of 5.0 to 15.0 times; neck drawing the resulting fibers at a ratio of 1.5 to 4.0 times; and cutting the resulting fibers;
3. The method for producing spun-dyed polyester fibers according to 2 above, wherein the polyester is polyethylene terephthalate and the colorant is carbon black.

According to the present invention, polyester fibers having good toughness and excellent dispersibility in water can be obtained, even though the fiber diameter of the single fiber is extremely fine, and therefore suitable for wet-laid nonwoven fabrics. Furthermore, since the extremely fine dyed fibers of the present invention are obtained by the melt spinning method, a manufacturing method that is excellent in terms of both the environment and cost can be provided.

The present invention will be described in detail below.
Examples of the polyester used in the present invention include polyesters of aromatic dicarboxylic acids and aliphatic diols, such as polyalkylene terephthalates such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, and polyalkylene naphthalates such as polyethylene naphthalate, polyesters of alicyclic dicarboxylic acids and aliphatic diols, such as polyalkylene cyclohexane dicarboxylate, polyesters of aromatic dicarboxylic acids and alicyclic diols, such as polycyclohexane dimethanol terephthalate, polyesters of aliphatic dicarboxylic acids and aliphatic diols, such as polyethylene succinate, polybutylene succinate, and polyethylene adipate, and polyhydroxycarboxylic acids, such as polylactic acid and polyhydroxybenzoic acid.

The diol components constituting the polyester may be diethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, 2,2-bis(p-β-hydroxyethylphenyl)propane, polyethylene glycol, poly(1,2-propylene) glycol, poly(trimethylene) glycol, or poly(tetramethylene) glycol, which may be copolymerized as one or more components.

Depending on the purpose, one or more of the following may be copolymerized as acid components: isophthalic acid, phthalic acid, adipic acid, sebacic acid, α,β-(4-carboxyphenoxy)ethane, 4,4-dicarboxyphenyl, 5-sodium sulfoisophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, or esters thereof; and one or more of the following diol components: diethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, polyalkylene glycol, etc.; and three or more carboxylic acid components or components having hydroxyl groups, such as pentaerythritol, trimethylolpropane, trimellitic acid, trimesic acid, etc., may be copolymerized to provide branching. Also included are mixtures of polyesters with different compositions, such as those exemplified above. These polyesters may contain known additives, such as matting agents, antifouling agents, antibacterial agents, deodorants, fluorescent whitening agents, flame retardants, stabilizers, UV absorbers, and lubricants.

The dyed polyester fiber of the present invention must contain a colorant in a ratio of 0.5 to 3.0% by weight relative to the fiber. If the colorant is less than 0.5% by weight relative to the fiber, it is difficult to obtain sufficient color development and clarity. If the ratio exceeds 3.0% by weight, stretchability will deteriorate in the stretching step described below, which is not preferable. A more preferable ratio of the colorant is 0.7% by weight or more and 1.5% by weight or less, and even more preferable 0.8% by weight or more and 1.2% by weight or less.

The coloring agent may be a pigment or a dye, but a pigment is preferably used in terms of weather resistance and clarity. The pigment may be either a pigment containing carbon black as a main component for the purpose of coloring black, or a pigment containing a chromatic color for the purpose of coloring chromatic colors. Among them, a blackening agent containing carbon black as a main component is preferred.
The carbon black blended is not particularly limited, but carbon black having a particle size in the fiber of 3 μm or less is preferred, and carbon black having a particle size of 1 μm or less is particularly preferred.

The average primary particle size of the colorant used in the present invention must be 5 to 30 nm, and is preferably 5 to 20 nm. If the average primary particle size of the colorant is less than 5 nm, sufficient deep color cannot be obtained, and if it exceeds 30 nm, the colorant is difficult to disperse uniformly, which inhibits stretching and makes it easier for color spots to appear.

In the present invention, when carbon black is used as a colorant, coloring components other than carbon black may be included, but it is preferable that the content of carbon black in the entire colorant is 30% by weight or more. In addition, examples of coloring components that can be used in combination include nigrosine (an azine condensation mixture described in the COLORINDEX as C.I. SOLVENT BLACK 5 and C.I. SOLVENT BLACK 7).

The dyed polyester fiber in the present invention may be a solid fiber, a hollow fiber, or a composite fiber, so long as the cross-sectional circumference is round.

(Fiber diameter)
The fineness of the spun-dyed polyester fiber of the present invention described above must be 0.01 to 0.5 dtex, preferably 0.04 to 0.3 dtex, and more preferably 0.1 to 0.25 dtex. If the fineness is less than 0.01 dtex, the fibers tend to be entangled with each other, resulting in poor texture, while if the fineness exceeds 0.5 dtex, the bending rigidity of the fiber increases, resulting in insufficient softness in the feel, which is undesirable.

(Fiber length)
The fiber length of the spun-dyed polyester fiber of the present invention must be 2 to 25 mm, preferably 2 to 15 mm, and more preferably 2 to 10 mm. If the fiber length is shorter than 2 mm, the cutting resistance increases, the fibers are likely to become entangled, and the fiber quality becomes uneven. On the other hand, if the fiber length is longer than 25 mm, the dispersibility of the fiber in water during papermaking is deteriorated, which is not preferable.

(toughness)
The toughness of the spun-dyed polyester fiber of the present invention must be 10 or more, preferably 11 or more, and more preferably 14 or more. In the present invention, the toughness is a value calculated from the strength (cN/dtex) and elongation (%) of the fiber by using the following formula.
Toughness = Strength x √(Elongation)
If the toughness is less than 10, the quality of the fiber during processing and use is not sufficient.

(Production method)
Next, the production method of the present invention will be described.
The polyester pellets and the colorant are dried separately in a conventional manner, and the polyester pellets and the colorant are each weighed in a predetermined amount, melted in a melt spinning apparatus equipped with a screw extruder capable of adjusting the colorant concentration in the resin, and spun and taken out in a conventional manner to obtain an undrawn polyester fiber. In this case, the spinning speed for taking up the undrawn yarn must be 300 to 1000 m/min. If the take-up speed is less than 300 m/min, it is difficult to obtain the mechanical strength required for taking up and moving to the next step, which is not preferable. On the other hand, if it exceeds 1000 m/min, the orientation of the fiber proceeds too much and it is not suitable for flow drawing, which is not preferable. The spinning speed for taking up the undrawn yarn is more preferably 400 to 900 m/min.

In addition, when at least one of the polyester pellets and the colorant is used in a wet state, a screw extruder equipped with a vacuum suction device may be used. From the viewpoint of the kneading property of the polyester pellets and the colorant, a twin-screw type screw extruder is preferable to a single-screw type.

Next, the undrawn polyester fiber is drawn at a temperature higher than its glass transition point (hereinafter referred to as Tg). In the present invention, for more stable drawing, it is desirable to perform flow drawing in a medium at least 10°C higher than the Tg of the polyester used. For example, it is performed in wet heat at 80 to 100°C, that is, in warm water at 80 to 99°C or in wet heat at 100 to 150°C. The draw ratio of the flow drawing at this time must be 5.0 to 15.0 times, and is preferably 7.0 to 10.0 times. By making it 5.0 times or more, it is possible to obtain extremely fine fibers by the non-island-in-sea melt spinning method, such as those that can be obtained by sea-island mixed type or sea-island composite type spinning (for example, a method in which the sea component is dissolved and removed in a solution and the remaining island component is obtained as fiber).

If the draw ratio for flow drawing is less than 5.0 times, it will be impossible to obtain extremely fine fibers using the non-island-in-sea melt spinning method, such as those that can be obtained using sea-island mixed or sea-island composite spinning. If the draw ratio exceeds 15.0 times, the number of yarn breakages during flow drawing will increase, and the quality and processing processability will be significantly deteriorated, which is not preferable.

The resulting fibers are then neck-drawn to produce polyester fibers with a single fiber fineness of 0.5 dtex or less. The neck-drawing ratio is usually 1.5 to 4.0 times. Neck-drawing is preferably performed in warm water at 60 to 80°C. Temperatures below 60°C are undesirable because the drawing point is not stable, increasing the incidence of yarn breakage during drawing. On the other hand, temperatures above 80°C are undesirable because the fibers stick together immediately after immersion in warm water, causing fuzz.

In the present invention, heat treatment may be performed after neck drawing to adjust the shrinkage properties of the fiber. There are no particular limitations on the heat treatment method, and any heat shrinkage treatment such as tension heat treatment, restricted shrinkage treatment, and relaxation heat treatment can be performed. In addition, if necessary, a surfactant is added to the polyester fiber after drawing, and the fiber is cut to a predetermined cut length. Known surfactants can be used, such as polyester-polyether copolymers.

The method of applying the surfactant is not particularly limited, but an example is a method in which the fiber bundle is immersed in an emulsion containing a surfactant and then gripped between rollers. In the above-mentioned production method, since there is no need to dissolve and remove the sea component as in the sea-island blend spinning method or the sea-island composite spinning method, the surfactant can be applied uniformly to the surface of each fiber, and high dispersibility in water can be obtained. In the sea-island blend spinning method or the sea-island composite spinning method, it is possible to apply a surfactant to the surface of the sea-island composite yarn before dissolving and removing the sea component, but the surfactant on the surface of the sea-island composite yarn is simultaneously eluted in the process of dissolving and removing the sea component. As a result, no surfactant that improves dispersibility in water is applied to the surface of each island component fiber obtained, and the dispersibility in water is inferior to that of the method according to the present invention.
The spun-dyed polyester fiber of the present invention described above can be applied to various processing methods, but is particularly suitable for use as a substrate for artificial leather.

The present invention will be explained in more detail below with reference to the following examples. Each item in the examples was measured by the following methods.

(1) Intrinsic viscosity [η]
0.12 g of the polymer sample was dissolved in 10 mL of a mixed solvent of tetrachloroethane/phenol (volume ratio 1/1), and the intrinsic viscosity (dL/g) at 35° C. was measured.

(2) Glass transition point, melting point A Thermal Analyst-2200 differential scanning calorimeter (DSC) manufactured by TA Instruments Japan was used. The measurement was performed by heating 10 mg of a sample in a nitrogen atmosphere from room temperature to 300° C. at a heating rate of 20° C./min, according to the method described in JIS K7121-1987.

(3) Average primary particle diameter of colorant After etching a polymer containing a pigment or its molded product (fiber), the particle size was observed using a Hitachi SEM (S3500-N). For each particle observed, the maximum length (Dmax) and minimum length (Dmin) were measured, and the average value (Dave) was measured. The same procedure was then repeated to determine the average value (Dave) for 100 particles, and the average value per 100 particles was defined as the average primary particle diameter (D).

(4) Single Yarn Fineness After drawing, 1,800 mm of the fiber bundle was sampled and dried for 40 minutes in a hot air dryer at 120° C., and the measured bone dry mass was multiplied by 5,000 to measure the total fineness (unit: denier) of the fiber bundle. The total fineness obtained was divided by the number of single filaments constituting the fiber bundle to obtain the single filament fineness (unit: denier), which was then multiplied by 1.111 to calculate the single filament fineness (unit: dtex).

(5) Fiber Length The side of the short fiber was magnified under a microscope, and its length was measured for N=10 samples, and the average value was calculated.

(6) Strength and Elongation From the fiber bundle after drawing, a fiber bundle of about 2,000 de was collected, and a length of 1,800 mm was taken out and dried in a hot air dryer at 120°C for 40 minutes. The measured bone dry mass was then multiplied by 5,000 to measure the total fineness (unit: denier) of the fiber bundle.
The same fiber bundle was used to measure the strength (g) and elongation (%) at a gripping distance of 20 cm and a pulling speed of 20 cm/min according to JIS L 1013 8.5.1. The strength obtained was divided by the total fineness of the fiber bundle and then multiplied by 0.8826 to obtain the strength (cN/dtex).

(7) Toughness: Calculated from the fiber strength (cN/dtex) and elongation (%) obtained in (6) using the following formula.
Toughness = Strength x √(Elongation)

(8) Dispersibility in Water Put 500 mL of tap water into a 1000 mL measuring cylinder, and add 0.1 g net of short fibers to it. When the fibers reach the bottom of the measuring cylinder, cover the opening of the measuring cylinder, hold the top and bottom with both hands, and invert the measuring cylinder once to disperse the fibers. The dispersibility in water is evaluated according to the following criteria.
Good: There are no undispersed fiber bundles, and each single fiber is neatly spread out in the water. Poor: There are several or more undispersed fiber bundles, and the single fibers are often entangled with each other.

[Example 1]
A master batch containing 20% by weight of polyethylene terephthalate (Tg70°C) dried at 170°C and carbon black having an average primary particle size of 20 nm was mixed in a ratio of 95:5, melted at 290°C in a melt spinning apparatus equipped with a screw extruder, discharged at 378 g/min through a spinneret having 2504 discharge holes, and taken up at a speed of 500 m/min. After adding a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. The undrawn polyester fiber bundle was flow-drawn 7.9 times in 95°C hot water, neck-drawn 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to add 0.3% by weight to the fiber, and further cut to 3 mm to obtain a spun-dyed polyester fiber.
The fineness was 0.16 dtex, the strength was 2.0 cN/dtex, the elongation was 35%, the toughness was 11.8, and the dispersibility in water was good.

[Example 2]
A master batch containing 20% by weight of polyethylene terephthalate (Tg70°C) with an intrinsic viscosity of 0.64 dL/g and carbon black with an average primary particle size of 5 nm, dried at 170°C, was mixed in a ratio of 95:5, melted at 290°C in a melt spinning device equipped with a screw extruder, discharged at 378 g/min through a spinneret with 2504 discharge holes, and taken up at a speed of 500 m/min. After adding a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. The undrawn polyester fiber bundle was flow-drawn 7.9 times in 95°C hot water, then neck-drawn 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to add 0.3% by weight to the fiber, and further cut to 3 mm to obtain a spun-dyed polyester fiber.
The fineness was 0.16 dtex, the strength was 3.1 cN/dtex, the elongation was 22%, the toughness was 14.5, and the dispersibility in water was good.

[Example 3]
A master batch containing 20% by weight of polyethylene terephthalate (Tg70°C) with an intrinsic viscosity of 0.64 dL/g and carbon black with an average primary particle size of 30 nm, dried at 170°C, was mixed in a ratio of 95:5, melted at 290°C in a melt spinning apparatus equipped with a screw extruder, discharged at 378 g/min through a spinneret with 2504 discharge holes, and taken up at a speed of 500 m/min. After adding a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. The undrawn polyester fiber bundle was flow-drawn 7.9 times in 95°C hot water, neck-drawn 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to add 0.3% by weight to the fiber, and further cut to 3 mm to obtain a spun-dyed polyester fiber.
The fineness was 0.16 dtex, the strength was 2.5 cN/dtex, the elongation was 34%, the toughness was 14.6, and the dispersibility in water was good.

[Example 4]
Polyethylene terephthalate (Tg70°C) with an intrinsic viscosity of 0.64 dL/g, dried at 170°C, and a master batch containing 20% by weight of carbon black with an average primary particle size of 20 nm were mixed in a ratio of 97.5:2.5, melted at 290°C in a melt spinning device equipped with a screw extruder, discharged at 378 g/min through a spinneret with 2504 discharge holes, and taken up at a speed of 500 m/min, and a polyester-polyether copolymer was added thereto, to obtain an unstretched polyester fiber bundle. The unstretched polyester fiber bundle was flow-stretched 7.9 times in 95°C hot water, then neck-stretched 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to impart 0.3% by weight of the emulsion to the fibers. The fibers were then cut to 3 mm to obtain spun-dyed polyester fibers.
The fineness was 0.16 dtex, the strength was 2.5 cN/dtex, the elongation was 40%, the toughness was 15.8, and the dispersibility in water was good.

[Example 5]
A master batch containing 20% by weight of polyethylene terephthalate (Tg70°C) with an intrinsic viscosity of 0.64 dL/g and carbon black with an average primary particle size of 20 nm, dried at 170°C, was mixed in a ratio of 85:15, melted at 290°C in a melt spinning apparatus equipped with a screw extruder, discharged at 378 g/min through a spinneret with 2504 discharge holes, and taken up at a speed of 500 m/min. After adding a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. The undrawn polyester fiber bundle was flow-drawn 7.9 times in 95°C hot water, neck-drawn 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to add 0.3% by weight to the fiber, and further cut to 3 mm to obtain a spun-dyed polyester fiber.
The fineness was 0.16 dtex, the strength was 1.8 cN/dtex, the elongation was 33%, the toughness was 10.3, and the dispersibility in water was good.

[Example 6]
A master batch containing 20% by weight of polyethylene terephthalate (Tg70°C) with an intrinsic viscosity of 0.64 dL/g and carbon black with an average primary particle size of 20 nm, dried at 170°C, was mixed in a ratio of 95:5, melted at 290°C in a melt spinning apparatus equipped with a screw extruder, discharged at 378 g/min through a spinneret with 2504 discharge holes, and taken up at a speed of 500 m/min. After adding a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. The undrawn polyester fiber bundle was flow-drawn 5.3 times in 95°C hot water, neck-drawn 2.5 times in 70°C hot water, and further subjected to a 5% limited shrinkage treatment in 95°C hot water. The obtained fiber bundle was immersed in an emulsion of polyether-polyether copolymer, and then held with a pair of rollers to add 0.3% by weight to the fiber, and further cut to 3 mm to obtain a spun-dyed polyester fiber.
The fineness was 0.24 dtex, the strength was 2.8 cN/dtex, the elongation was 35%, the toughness was 16.6, and the dispersibility in water was good.

[Comparative Example 1]
Polyethylene terephthalate (Tg 70°C) with an intrinsic viscosity of 0.64 dL/g, dried at 170°C, and a master batch containing 20% by weight of carbon black with an average primary particle size of 40 nm were mixed in a ratio of 95:5, melted at 290°C in a melt spinning apparatus equipped with a screw extruder, extruded at 378 g/min through a spinneret with 2504 nozzles, and taken up at a speed of 500 m/min. After imparting a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. As in Example 1, the undrawn polyester fiber bundle was subjected to flow drawing in 95°C warm water, but many single yarn breaks occurred and drawing was not possible.

[Comparative Example 2]
Polyethylene terephthalate (Tg 70°C) with an intrinsic viscosity of 0.64 dL/g, dried at 170°C, and a master batch containing 20% by weight of carbon black with an average primary particle size of 20 nm were mixed in a ratio of 80:20, melted at 290°C in a melt spinning device equipped with a screw extruder, extruded at 378 g/min through a spinneret with 2504 nozzles, and taken up at a speed of 500 m/min. After imparting a polyester-polyether copolymer, an undrawn polyester fiber bundle was obtained. As in Example 1, the undrawn polyester fiber bundle was subjected to flow drawing in 95°C warm water, but many single yarn breaks occurred and drawing was not possible.

[Comparative Example 3]
A master batch containing polyethylene terephthalate (Tg 70°C) with an intrinsic viscosity of 0.64 dL/g and 20% by weight of carbon black with an average primary particle size of 20 nm was mixed in a ratio of 95:5 for the island component and melted at 290°C in a screw extruder, and a modified polyethylene terephthalate copolymerized with 4 mol% of 5-sodium sulfoisophthalic acid for the sea component was melted at 280°C in another screw extruder and spun in a weight ratio of sea component:island component=35:65 using a melt spinning apparatus equipped with a known sea-island composite spinning die with 19 islands, and taken up at a spinning speed of 1250 m/min to obtain an undrawn yarn. This undrawn yarn was flow drawn 2.52 times in warm water at 70°C and neck drawn 1.15 times in warm water at 90°C. After reducing the weight by 35% with a 4% NaOH aqueous solution at 95°C, 0.3% by weight of a polyether-polyether copolymer was added to the fiber and cut to 3 mm to obtain a dyed polyester fiber. The fineness was 0.16 dtex, the strength was 1.8 cN/dtex, the elongation was 30%, and the toughness was 9.9. The dispersibility in water was poor, with many defects of non-dispersion.

The dyed polyester fibers obtained by the present invention are polyester fibers that, despite being extremely fine fibers, have good toughness and excellent dispersibility in water, and therefore can be suitably used for wet-laid nonwoven fabrics, particularly artificial leather substrates. Furthermore, the present invention can provide a manufacturing method for dyed polyester fibers that is also excellent in terms of the environment and cost.

Claims (3)

A dyed polyester fiber made of polyester containing 0.5 to 3.0% by weight of a colorant with an average primary particle size of 5 to 30 nm, with a single fiber fineness of 0.01 to 0.5 dtex, toughness of 10 or more, and fiber length of 2 to 25 mm. A method for producing the dyed polyester fiber according to claim 1, in which a molten mixture of polyester and a colorant having an average primary particle size of 5 to 30 nm is mixed at a blending ratio of 0.5 to 3.0% by weight based on the polyester fiber, and the molten mixture is passed through a spinneret having an outlet hole to form an undrawn polyester fiber at a spinning speed of 300 to 1000 m/min, which is then flow-drawn at 5.0 to 15.0 times, neck-drawn at 1.5 to 4.0 times, and then cut to produce the dyed polyester fiber. The method for producing dyed polyester fibers according to claim 2, wherein the polyester is polyethylene terephthalate and the colorant is carbon black.