Synthetic fiber

Abstract

Provided is a synthetic fiber which has excellent dye-absorbing ability in a dyeing step and thus can be densified, has a high dyeing speed, has an excellent texture, and can be suitably used as a fiber structure. The synthetic fiber of the present invention has a loss tangent peak temperature of 100 DEG C or more and 150 DEG C or less, a loss tangent peak value of 0.15 or more, and a dry heat shrinkage rate of 5% or more and less than 15%.

Description

Technical Field

[0001] The present invention relates to a synthetic fiber. Background Art

[0002] Synthetic fibers, particularly polyester fibers, have excellent mechanical properties and dimensional stability and are therefore widely used in a variety of applications, from clothing to materials and medical uses, and other non-clothing applications.

[0003] As people's lives become more enriched, they desire higher functionality and promote the development of fiber products with various functions. In addition, in recent years, due to environmental concerns, resource conservation and energy conservation are required in various industries, and the fiber industry is no exception.

[0004] As a way to save resources and energy for synthetic fibers, the recycling of non-product parts in the manufacturing process, the reuse of final products, the reduction of the amount of supplies used in various parts of the manufacturing process, and the improvement of energy efficiency are being promoted. In the manufacturing process, there is a particular demand for saving resources and energy in the dyeing process that consumes a lot of water and energy. As such methods, it is considered that by improving the dye exhaustion capacity, even if the amount of dye used is low, it can show high color rendering, or improve the energy efficiency in the dyeing process.

[0005] For example, a technology has been proposed in which, in a polyester fiber dyeing process, the fiber structure before dyeing is changed to develop excellent dye exhaustion ability, thereby saving resources and energy to produce a textile with excellent color development properties.

[0006] That is, Patent Document 1 discloses that before dyeing, a high-temperature heat treatment using steam or a water-soluble non-swelling medium is applied, thereby imparting changes to the fiber structure and controlling the loss tangent (tanδ) based on dynamic viscoelasticity measurement, thereby increasing the dyeing seat, improving the dye absorption capacity, and thus improving the dye utilization efficiency.

[0007] In addition, Patent Document 2 discloses that: focusing on the control of the tanδ peak temperature based on dynamic viscoelasticity measurement, a modified polyester fiber copolymerized with an aliphatic dicarboxylic acid is produced, thereby improving the dyeability of the polyester fiber under normal pressure. In addition, after high-temperature treatment, it also has high shrinkage, overcoming the high temperature and high pressure conditions required for the dyeing of conventional polyester fibers.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Laid-Open No. 5-279917

[0011] Patent Document 2: Japanese Patent Laid-Open No. 10-204723 Summary of the invention

[0012] Problems to be solved by the invention

[0013] In the technology of patent document 1, although the dye utilization efficiency is improved, a loose fiber structure is formed, so the shrinkage of the fiber in the high temperature heat treatment becomes too large, and the density between fibers of the textiles etc. to which the technology is applied is excessively improved, and a fiber structure with a hardened feel is sometimes formed. In addition, it is necessary to add a process that is not usually present, and when all fiber manufacturing processes are considered, energy is sometimes excessively consumed.

[0014] In the technology of Patent Document 2, it can be confirmed that the normal pressure dyeability is brought about by the control of the tanδ peak temperature based on the dynamic viscoelasticity measurement, but it is also a technology aimed at high shrinkage. By improving the dry heat shrinkage rate, similar to the synthetic fibers described in Patent Document 1, textiles and the like sometimes have a hardened feel due to their dense structure.

[0015] In view of the above problems of the prior art, the purpose of the present invention is to provide a synthetic fiber which has excellent dye absorption ability and fast dyeing speed in the dyeing process, and is suitable for manufacturing a fiber structure that combines dark dyeing processing and soft hand feel that is difficult to achieve with the prior art.

[0016] Technical means of solving problems

[0017] The above problems can be solved by the following (1) to (3).

[0018] (1) A synthetic fiber having a loss tangent peak temperature of 100°C to 150°C, a loss tangent peak value of 0.15 or more, and a dry heat shrinkage of 5% to less than 15%.

[0019] (2) The synthetic fiber according to (1), wherein the area of ​​the loss tangent from 30°C to 130°C is 4.0°C or more and 7.5°C or less.

[0020] (3) A synthetic fiber according to (1) or (2), wherein, when the cross-section of the fiber after dyeing to an L* value of less than 20 is displayed as a grayscale image with 256 gray levels, the absolute value of the difference between the average pixel value of a circle which is concentric with the center of the inscribed circle and has an area of ​​10% of the area of ​​the cross-section of the fiber and the average pixel value of the peripheral portion which has an area of ​​10% of the area of ​​the cross-section of the fiber is less than 15.

[0021] Effects of the Invention

[0022] Since the synthetic fiber of the present invention has excellent dye exhaustion ability, it can be dyed in a deep color at a fast speed in the dyeing process, and can produce a fiber structure that maintains a soft feel even after dyeing. DETAILED DESCRIPTION

[0023] The synthetic fiber of the present invention has a loss tangent peak temperature of 100° C. to 150° C., a loss tangent peak value of 0.15 or more, and a dry heat shrinkage of 5% to less than 15%.

[0024] The synthetic fiber of the present invention refers to a fiber produced by chemical synthesis such as polyester fiber, polyamide fiber, acrylic fiber, etc. The synthetic fiber of the present invention is preferably a polyester fiber, which is easy to introduce a copolymer component in the production process, and disperse dyes can be dyed to amorphous parts. Furthermore, in the present invention, the main components of the preferred polyester fiber are terephthalic acid and ethylene glycol, and it may also have a copolymer component. Only one copolymer component may be used, or two or more copolymer components may be used in combination.

[0025] The synthetic fiber of the present invention needs to have a peak temperature of the loss tangent of 100° C. or higher and 150° C. or lower.

[0026] The loss tangent mentioned here is also called loss coefficient or tan δ and is the ratio of the loss elastic coefficient to the storage elastic coefficient obtained by dynamic viscoelasticity measurement, and represents the magnitude of the micro-Brownian motion of the amorphous part in the fiber structure.

[0027] The peak temperature of the loss tangent in the present invention refers to the value obtained as the peak temperature of the loss tangent when the loss tangent (dimensionless quantity) is measured under the conditions of a temperature increase rate of 3°C / min, a frequency of 110 Hz, and a temperature increase from 30°C to 200°C using a dynamic viscoelasticity measuring device, with the synthetic fiber clamped at a distance of 30 mm between the chucks, and a tension of 0.15 g / dtex. Examples of the dynamic viscoelasticity measuring device include "Rheovibron DOV-II-EP" manufactured by ORIENTEC.

[0028] If the peak temperature of the loss tangent is 100°C or higher, the micro-Brownian motion of the amorphous part is sufficiently large during the dyeing process, and the dyeing property of the disperse dye to the amorphous part is improved, so the dye exhaustion ability is excellent, so that the color can be darkened and the dyeing speed is fast. In addition, the shrinkage during heat treatment is suppressed, and the feel is excellent.

[0029] On the other hand, when the peak temperature of the loss tangent exceeds 150°C, the micro-Brownian motion of the amorphous part is small in the dyeing process, the dyeing property of the disperse dye to the amorphous part is low, and therefore the dye exhaustion ability is low, so it is difficult to achieve a deep color, and sometimes it is not possible to dye sufficiently. If the peak temperature of the loss tangent is 150°C or less, the dye exhaustion ability is excellent in the dyeing process, so a deep color can be achieved, and the color development is excellent.

[0030] The peak temperature of the loss tangent is preferably 110° C. or higher, more preferably 120° C. or higher, and preferably 140° C. or lower.

[0031] The synthetic fiber of the present invention needs to have a peak value of loss tangent of 0.15 or more.

[0032] When the peak value of the loss tangent is 0.15 or more, the micro-Brownian motion is large in the dyeing process, the dyeing property of the disperse dye to the amorphous part is improved, and the dye exhaustion ability is excellent, so that the color can be deep, the dyeing speed is fast, and the color development property is excellent.

[0033] From the viewpoint of suppressing an increase in dry heat shrinkage, the peak value of the loss tangent is preferably 0.20 or less, and more preferably in the range of 0.15 to 0.20.

[0034] The peak value of the loss tangent can be measured by the above-mentioned method using a dynamic viscoelasticity measuring device.

[0035] The synthetic fiber of the present invention preferably has an area of ​​loss tangent from 30°C to 130°C of 4.0°C or more and 7.5°C or less when the relationship between temperature and loss tangent is graphed.

[0036] The so-called area of ​​loss tangent from 30°C to 130°C refers to the following area, that is, using a dynamic viscoelasticity measuring device, clamping the synthetic fiber with a distance between the chucks of 30 mm, applying a tension of 0.15 g / dtex, and measuring the loss tangent under the conditions of heating rate of 3°C / min, frequency of 110 Hz and heating from 30°C to 130°C. When plotting with the horizontal axis as temperature (°C) and the vertical axis as loss tangent (dimensionless quantity), the horizontal axis is divided into partitions of 1°C, and the graph is integrated to obtain the area.

[0037] The area of ​​the loss tangent from 30°C to 130°C represents the accumulation of the micro-Brownian motion of the amorphous part of the fiber structure in the dyeing process of the synthetic fiber. The higher the value, the more active the micro-Brownian motion in the dyeing process, the higher the dye exhaustion ability, and the deeper the color.

[0038] If the area of ​​the loss tangent from 30°C to 130°C is 4.0°C or more, the micro-Brownian motion in the dyeing process is sufficiently large, the dyeing property of the disperse dye to the amorphous part is improved, and the dye exhaustion ability is improved, the color can be concentrated, the color development is excellent, the dyeing speed is fast, and the color development is excellent, so it is preferred. On the other hand, when the area of ​​the loss tangent is a value greater than 7.5°C, the micro-Brownian motion is large, the dyeing property of the disperse dye to the amorphous part is further improved, and the dye exhaustion ability is improved, and the color development is further improved, but the heat shrinkage of the amorphous part is also improved, the dimensional change becomes large, and the feel is sometimes solidified, so the area of ​​the loss tangent is preferably 7.5°C or less.

[0039] The area of ​​the loss tangent from 30°C to 130°C is more preferably 4.2°C or more, further preferably 4.5°C or more, and more preferably 7.0°C or less.

[0040] The synthetic fiber of the present invention needs to have a dry heat shrinkage of 5% or more and less than 15%.

[0041] If the thermal shrinkage is 5% or more, there will be sufficient amorphous parts for dye introduction when dyeing with disperse dyes, and the color development will be excellent. On the other hand, if the thermal shrinkage is less than 15%, there will be no excessive shrinkage when heat treatment is applied, and the hand feel will be hardened and deteriorated.

[0042] The dry heat shrinkage ratio is preferably 14% or less.

[0043] Regarding the dry heat shrinkage rate, a skein containing synthetic fibers (10 winds) was produced using a 1 m / week length measuring machine in an environment of 20°C and 65% RH, and the dry heat shrinkage rate was calculated using the following formula: length L0, which is the length of the skein when a load of 0.03 cN / dtex is applied after the skein has been allowed to stand for 24 hours at 20°C and 65% RH; and length L1, which is the length of the skein when a load of 0.03 cN / dtex is applied after the skein has been heat treated at 160°C for 5 minutes without a load and then allowed to stand for 24 hours at 20°C and 65% RH.

[0044] Dry heat shrinkage (%) = {(L0-L1) / L0}×100

[0045] Furthermore, in order to sufficiently judge whether the dye has penetrated into the fiber cross section, the synthetic fiber of the present invention is preferably such that when the fiber cross section after being dyed to an L* value of 20 or less is displayed as a grayscale image of 256 gray levels, the absolute value of the difference between the average pixel value of a circle concentric with the center of the inscribed circle and having an area of ​​10% of the area of ​​the fiber cross section and the average pixel value of the peripheral portion having an area of ​​10% of the area of ​​the fiber cross section is 15 or less. In addition, the L* value represents the brightness in the L*a*b* color space, and is measured using a spectrophotometer with a D65 light source, a viewing angle of 10°, and an optical condition of excluding specular component exclusion (SCE) (specular component exclusion method).

[0046] If the absolute value of the average difference in pixel values ​​is less than 15, the dye penetrates deep into the fiber. Unlike the case where only the fiber surface is dyed, the dye is evenly absorbed by the amorphous part of the entire fiber. Therefore, the dye absorption capacity is improved, the color can be intensified, and the color rendering is excellent, so it is preferred.

[0047] The absolute value of the difference is more preferably 10 or less. When the dye uniformly permeates the inside, the absolute value of the difference is 0, so the lower limit is not particularly limited.

[0048] Next, the form of the synthetic fiber of the present invention will be described.

[0049] The fiber form of the synthetic fiber of the present invention is not particularly limited and may be any form such as monofilament, multifilament, or staple fiber. In order to exert the excellent dye exhaustion ability and other characteristics, the form of multifilament or staple fiber is also preferred.

[0050] In the synthetic fiber of the present invention, the single fiber fineness and the number of filaments are appropriately selected according to the application or required properties. When considering the practical range, the fineness of the multifilament is preferably 10 dtex to 3000 dtex.

[0051] The fineness (dtex) in the present invention is calculated using the following formula by measuring the weight of 100 m of fibers obtained by hanking with an electric length measuring machine under an environment of a temperature of 20° C. and a humidity of 65% RH.

[0052] Density (dtex) = weight of 100m of fiber (g) × 100

[0053] If the fineness of the synthetic fiber is 10 dtex or more, there is less yarn breakage, the process passability is good, and in addition, when used, there is less generation of fluff and excellent durability, so it is preferred. On the other hand, if the fineness of the synthetic fiber is 3000 dtex or less, the softness of the fiber and the fiber structure will not be damaged, so it is preferred.

[0054] The elongation of the synthetic fiber of the present invention can be adjusted according to the application or required characteristics by the manufacturing method described below. Here, the elongation in the present invention is calculated in accordance with Japanese Industrial Standards (JIS) L1013: 2010 (Testing methods for chemical fiber yarns) 8.5.1. Specifically, the elongation (%) is calculated by dividing the stress (cN) at the point showing the maximum load by the fineness (dtex) by the fineness (dtex) in a tensile test at a temperature of 20°C and a humidity of 65%RH to calculate the strength (cN / dtex). The elongation (L1) at the point showing the maximum load and the initial sample length (L0) are used to calculate the following formula.

[0055] Elongation (%) = {(L1-L0) / L0} × 100

[0056] The higher the elongation, the less likely the fiber will be stretched and deformed and break even if a sharp deformation is applied. However, the fiber will be stretched and deformed during the molding process, and the properties of the fiber product may become unstable. Therefore, in view of the handleability of the fiber, the elongation of the synthetic fiber of the present invention is more preferably 30% to 60%. In particular, if the elongation is 60% or less, the dimensional stability of the fiber and the fiber structure becomes good, which is suitable.

[0057] In addition, the synthetic fiber of the present invention can be adjusted in combination with the elongation required for its use. When used for clothing purposes, the elongation is preferably adjusted to 30% to 50%, and when used for non-clothing purposes, it is particularly preferably adjusted to 20% to 40%.

[0058] In the synthetic fiber of the present invention, the cross-sectional shape of the fiber is not particularly limited and can be appropriately selected according to the application or required properties. The cross-sectional shape may be a true circular cross-section or a non-circular cross-section.

[0059] As the specific example of the non-circular cross section described herein, polylobate, polygon, flat, elliptical, C-shaped, H-shaped, S-shaped, T-shaped, W-shaped, X-shaped, Y-shaped, field-shaped, well-shaped, hollow etc. can be listed, but are not limited to these. When it is envisaged that the thick dyeing as the characteristic of the synthetic fiber of the present invention is effectively utilized to manufacture textiles with higher quality, it is suitable to suppress the so-called glare of light from a certain angle with high intensity reflection as the characteristic of the synthetic fiber, and from the said viewpoint, it is preferably set to the complicated cross-sectional shapes such as polylobate, H-shaped, S-shaped, T-shaped, W-shaped, X-shaped, Y-shaped, field-shaped, well-shaped. When it is set to the said cross-sectional shape, when textiles are made, there is no flat surface on the surface, and light reflection is dispersed, which can make the effect of the thick dyeing of the present invention more significant.

[0060] The synthetic fiber of the present invention can be processed by false twisting or twisting in the same manner as ordinary fibers, and can also be woven or braided in the same manner as ordinary fibers.

[0061] The fiber structure containing the synthetic fiber of the present invention can be in various forms by effectively utilizing known methods, and specifically, woven fabrics, knitted fabrics, fleece cloth, nonwoven fabrics or spun yarns, cotton wool, etc. can be cited. In addition, in these fiber structures, various weaves or braids can be used according to the application, and plain weave, twill weave, satin weave or variations of these weaves, or warp weave, weft weave, circular weave, flower weave or variations of these weaves can be appropriately used. In addition, it goes without saying that the synthetic fiber of the present invention can be combined with other fibers by interweaving or cross-weaving when it is made into a fiber structure, and it can also be made into a fiber structure after being made into a mixed yarn with other fibers.

[0062] Next, a method for producing the synthetic fiber of the present invention is described below.

[0063] As a method for producing the synthetic fiber of the present invention, a known melt spinning method, drawing method, or false twisting method can be used.

[0064] In the present invention, it is preferred to set the moisture content of the raw material to 0.3 wt% or less before melt spinning, so it is preferred to dry the raw material as needed. If the moisture content of the raw material is 0.3 wt% or less, foaming due to water will not occur during melt spinning, and spinning can be performed stably, which is preferred. In addition, it is preferred because the reduction in mechanical properties or deterioration in color tone caused by hydrolysis can be suppressed depending on the type of raw material. The moisture content of the raw material is more preferably 0.1 wt% or less.

[0065] In the case of melt spinning of synthetic fibers, as a method for ejecting from a spinning die to form a fiber sliver, the following examples can be cited, but are not limited to these. As a first example, it can be cited that after the chips with the same composition as the final fiber are dried as needed, the chips are supplied to a melt spinning machine for melting, and metering is performed using a metering pump. Thereafter, the chips are introduced into a spinning assembly heated in a spinning head, and after the molten polymer is filtered in the spinning assembly, the fiber sliver is ejected from the spinning die. As a second example, it can be cited that after the chips with different compositions are dried as needed, the chips are mixed into the composition of the final fiber in the state of the chips, and the mixed chips are supplied to a melt spinning machine for melting, and metering is performed using a metering pump. Thereafter, the chips are introduced into a spinning assembly heated in a spinning head, and after the island portion of the molten polymer is mixed and filtered in the spinning assembly, the fiber sliver is ejected from the spinning die. As a third example, it can be cited that after the chips with different compositions are dried as needed, the chips are supplied separately and melted, and metering is performed using a metering pump. Thereafter, the molten polymer is introduced into a spinning pack heated in a spinning head, kneaded in the spinning pack to have the same composition as the final fiber, filtered, and then ejected from a spinning die to form a fiber sliver.

[0066] The fiber yarn ejected from the spinning die is cooled and solidified by a cooling device, extracted by a first guide roller (godet roller), and wound by a winder via a second guide roller to form a wound yarn. In addition, an oil supply device may be used to supply oil to the fiber yarn, and an intertwining device may be used to give intertwining to the fiber yarn.

[0067] The spinning temperature in melt spinning can be appropriately selected according to the melting point or heat resistance of each component in the fiber, and is preferably 240°C to 300°C. If the spinning temperature is 240°C or higher, the elongation viscosity of the fiber sliver ejected from the spinning die is sufficiently reduced, so the ejection is stable, and the spinning tension does not become too high, and yarn breakage can be suppressed, which is preferred. On the other hand, if the spinning temperature is 300°C or lower, thermal decomposition during spinning can be suppressed, and the reduction in mechanical properties or coloring of the obtained synthetic fiber can be suppressed, which is preferred.

[0068] The spinning speed in melt spinning can be appropriately selected according to the composite ratio of each component in the fiber, the spinning temperature, etc., and in the case of the two-step method, it is preferably 1000m / min to 3000m / min. If the spinning speed in the two-step method is above 1000m / min, the advancing yarn is stable and yarn breakage can be suppressed, so it is preferred. On the other hand, if the spinning speed in the two-step method is below 3000m / min, the yarn will not break due to the suppression of the spinning tension, and stable spinning can be performed, so it is preferred. In addition, in the one-step method in which spinning and stretching are performed simultaneously without winding temporarily, the spinning speed is preferably set to 1000m / min to 3000m / min for the low-speed roller and 2500m / min to 6000m / min for the high-speed roller. If the low-speed roller and the high-speed roller are within the above range, the advancing yarn is stable, and yarn breakage can be suppressed, and stable spinning can be performed, so it is preferred.

[0069] In order to set the peak temperature of the loss tangent of the synthetic fiber to 100°C or more and 150°C or less, set the peak value of the loss tangent to 0.15 or more, and set the dry heat shrinkage to 5% or more and less than 15%, it is preferred to perform stretching or false twisting by a one-step method or a two-step method, and the stretching in the process may be performed by a one-step stretching method or a multi-step stretching method of two or more stages. There is no particular limitation on the heating method in the stretching or false twisting process as long as it is a device that can directly or indirectly heat the traveling yarn.

[0070] The stretching temperature during stretching can be appropriately selected according to the glass transition temperature or melting point of each component in the fiber, the strength and elongation of the stretched fiber, etc., and is preferably 60°C to 120°C. If the stretching temperature is above 60°C, the preheating of the yarn supplied to the stretching is fully carried out, the thermal deformation during stretching becomes uniform, the occurrence of uneven fineness can be suppressed, and high-quality fibers with excellent uniformity in the long side direction of the fiber can be obtained, so it is preferred. On the other hand, if the stretching temperature is below 120°C, the fusion or thermal decomposition of the fibers with each other accompanying the contact with the heating roller can be suppressed, and the process passability or quality is good, so it is preferred. In addition, the sliding property of the fiber relative to the stretching roller becomes good, so the yarn breakage can be suppressed, and stable stretching can be performed, so it is preferred.

[0071] The stretching ratio during stretching can be appropriately selected according to the elongation of the fiber before stretching or the strength or elongation of the fiber after stretching, and is preferably 1.02 to 5.0 times. If the stretching ratio is 1.02 times or more, the mechanical properties such as the strength or elongation of the fiber can be improved by stretching, so it is preferred. On the other hand, if the stretching ratio is 5.0 times or less, yarn breakage during stretching can be suppressed, and stable stretching can be performed, so it is preferred. Specifically, in order to be practical and set the peak value of the loss tangent to be 0.15 or more, it is preferred that the stretching ratio after stretching is around 40%.

[0072] The stretching speed during stretching can be appropriately selected according to whether the stretching method is a one-step method or a two-step method. In the case of the one-step method, the speed of the high-speed roller of the spinning speed is equivalent to the stretching speed. The stretching speed during stretching by the two-step method is preferably 100m / min to 1000m / min. In the case of the two-step method, if the stretching speed is above 100m / min, the traveling yarn is stable and yarn breakage can be suppressed, so it is preferred. If the stretching speed is below 1000m / min, yarn breakage during stretching can be suppressed, and stable stretching can be performed, so it is preferred.

[0073] In addition, it is preferred to perform heat setting at 100°C to 150°C. Heat setting refers to a treatment that performs heat treatment after stretching of synthetic fibers to stabilize their dimensions. If heat setting is performed at a temperature above 100°C, the fibers are fully crystallized during heat setting, the peak temperature of the loss tangent is above 100°C, and the dry heat shrinkage is less than 15%. Therefore, changes in physical properties or changes in feel caused by shrinkage due to heat treatment can be suppressed, so it is preferred. On the other hand, if the heat setting temperature is below 150°C, the peak temperature of the loss tangent is below 150°C, the dry heat shrinkage is above 5%, and the process passability or quality is good, so it is preferred.

[0074] When false twisting is performed, the elongation of the unstretched yarn or stretched yarn used can be appropriately selected according to the application or required characteristics, and is preferably in the range of 30% to 300%. If the elongation is 30% or more, the generation of fuzz of the false twisted yarn containing synthetic fibers or yarn breakage during false twisting can be suppressed, and if the elongation is 300% or less, false twisting can be performed stably.

[0075] As the device used in the false twist processing, the following examples can be cited, but are not limited to these. As a first example, a false twist processing device including 1DR (1 stretching roller), 1HT (1 heater), a cooling plate, a false twist device, 2DR (2 stretching rollers), 3DR (3 stretching rollers), 4DR (4 stretching rollers), and a winding machine is used. The processing ratio between 1DR and 2DR can be selected according to the elongation of the fiber used in the processing or the elongation of the false twist processing yarn containing synthetic fibers, preferably in the range of 1.1 to 3.0 times. The heater can be either contact or non-contact. The temperature of 1HT can be appropriately selected according to the glass transition temperature or melting point of each component of the synthetic fiber, the strength, elongation, dry heat shrinkage, and expansion recovery rate of the fiber after false twist processing. The upper limit of the temperature of 1HT is as long as the temperature at which the unstretched yarn or stretched yarn used is not melted in the heater. The false twist device is preferably a friction false twist type, and examples include a friction disk type and a band clamp type. Preferably, it is a friction disk type. The ratios between 2DR and 3DR and between 3DR and 4DR can be appropriately set according to the false twist yarn containing synthetic fibers, and are usually preferably set to 0.9 to 1.0 times. In any of between 2DR and 3DR, between 3DR and 4DR, and between 4DR and the winder, in order to improve the high-order passing property of the false twist yarn, intertwining can be imparted by an intertwining nozzle or oiling can be performed by an oil supply guide.

[0076] As a second example, a false twist processing device including 1DR, 1HT, a cooling plate, a false twist device, 2DR, 3DR, 2HT (2 heaters), 4DR, and a winding machine is used. The processing ratio between 1DR and 2DR can be selected according to the elongation of the fiber used in the processing or the elongation of the false twist processed yarn containing synthetic fibers, and is preferably in the range of 1.1 to 3.0 times. The temperature of 1HT can be appropriately selected according to the glass transition temperature or melting point of each component of the synthetic fiber, the strength, elongation, dry heat shrinkage, and expansion recovery rate of the fiber after false twist processing. The upper limit of the temperature of 1HT is only required to be the temperature at which the unstretched yarn or stretched yarn used does not melt in the heater. The false twist device is preferably a friction false twist type, and examples include a friction disk type and a band clamp type. Preferably, it is a friction disk type. Between 2DR and 3DR, in order to improve the high-order passability of the false twist processed yarn, intertwining can also be performed using an intertwining nozzle. The processing ratio between 3DR and 4DR can be selected according to the elongation of the fiber used in the processing or the elongation of the false twisted yarn containing synthetic fibers, and is preferably in the range of 0.8 to 1.1 times. The temperature of 2HT can be appropriately selected according to the glass transition temperature or melting point of each component of the synthetic fiber, the strength, elongation, dry heat shrinkage, and stretch recovery of the fiber after false twisting. The upper limit of the temperature of 2HT can be a temperature at which the undrawn yarn or drawn yarn used does not melt in the heater. In the 4DR-winder, oiling can also be performed using an oil supply guide.

[0077] Example

[0078] Hereinafter, the present invention will be described in more detail by way of examples. In addition, each characteristic value in the examples was obtained by the following method.

[0079] A. Melt Viscosity

[0080] The polymer in the form of chips, which was set to a moisture content of less than 200 ppm by a vacuum dryer, was used as a sample and measured using a capillary rheometer (Capilograph) 1B manufactured by Toyo Seiki Seisaku-sho Co., Ltd. The period from the introduction of the sample into the heating furnace to the start of the measurement was set to 5 minutes, and the strain rate was changed stepwise under a nitrogen environment. The measurement temperature was set to 290°C to measure the melt viscosity. In addition, in the examples or comparative examples, 1216s -1 of melt viscosity.

[0081] B. Fineness

[0082] 100 m of the fiber obtained in the Examples or Comparative Examples was skeined using an electric length measuring machine manufactured by INTEC in an environment of 20°C and 65%RH. The weight of the skein obtained was measured and the fineness (dtex) was calculated using the following formula.

[0083] Fineness (dtex) = weight of 100 m of fiber (g) × 100. The measurement was performed 5 times for one sample, and the average value was defined as the fineness.

[0084] C. Strength, elongation

[0085] The strength and elongation are calculated by using the fibers obtained by the examples or comparative examples as samples according to JIS L1013:2010 (Testing methods for chemical fiber yarns) 8.5.1. The tensile test was conducted under the conditions of an initial sample length of 20 cm and a tensile speed of 20 cm / min using Tensilon UTM-III-100 manufactured by ORIENTEC at a temperature of 20°C and a humidity of 65% RH. The strength (cN / dtex) is calculated by dividing the stress (cN) at the point showing the maximum load by the fineness (dtex), and the elongation (%) is calculated by the following formula using the elongation (L1) at the point showing the maximum load and the initial sample length (L0).

[0086] Elongation (%) = {(L1-L0) / L0} × 100

[0087] The measurement was performed 10 times for one sample, and the average values ​​were used as the strength and elongation.

[0088] D. Dry heat shrinkage

[0089] In an environment of temperature 20°C and humidity 65% ​​RH, a 1m / week length measuring machine is used to produce a skein (10 wraps) of fibers obtained by the embodiments or comparative examples, and the skein is left to stand for 24 hours. Thereafter, under the above environment, a load of 0.03 cN / dtex is applied to the skein to measure the sample length L0. Subsequently, the skein is heat-treated in an oven at 160°C for 5 minutes without load, and then left to stand for 24 hours in an environment of temperature 20°C and humidity 65% ​​RH. Thereafter, a load of 0.03 cN / dtex is applied to the skein to measure the sample length L1. The dry heat shrinkage rate (%) is calculated using the sample length L0 and sample length L1 before and after the treatment in the oven using the following formula.

[0090] Dry heat shrinkage (%) = {(L0-L1) / L0}×100

[0091] The measurement was performed three times for one sample, and the average value was defined as the dry heat shrinkage rate.

[0092] E. Melting point (Tm) and heat of fusion (ΔHm)

[0093] The fiber obtained by the embodiment or comparative example was used as a sample, and a differential scanning calorimeter (DSC) Q2000 manufactured by TA Instruments of Japan was used to increase the temperature from 30°C to 280°C at a heating rate of 16°C / min, and DSC measurement was performed. The melting point (Tm) and the heat of fusion (ΔHm) were calculated based on the melting peak observed during the heating process. The measurement was performed 3 times for one sample, and the average value was taken as the heat of fusion. In addition, when multiple melting peaks were observed, Tm and ΔHm were read in sequence from the top of the melting peak on the lowest temperature side and recorded separately.

[0094] F. Peak temperature, peak value, and area of ​​loss tangent

[0095] The fibers obtained by the examples or comparative examples were used as samples, and the dynamic viscoelasticity was measured using Rheovibron DOV-II-EP manufactured by ORIENTEC. The sample was clamped at a distance of 30 mm between the chucks, a tension of 0.15 g / dtex was applied, and the temperature was measured from 30°C to 200°C under the conditions of a heating rate of 3°C / min and a frequency of 110 Hz. The horizontal axis was set to temperature (°C), and the vertical axis was set to loss tangent for plotting, and the peak temperature, peak value, and area from 30°C to 130°C of the loss tangent were evaluated.

[0096] GL* value

[0097] The fibers obtained in the examples or comparative examples were used as samples, and about 5 g of cylindrical knitted fabrics were made using a circular knitting machine NCR-BL (kettle diameter 3.5 inches (8.9 cm), 27 gauge) manufactured by Eiko Sangyo Co., Ltd., and then scoured at 80° C. for 20 minutes in an aqueous solution containing 1 g / L of sodium carbonate and 1 g / L of Granup US-20, a surfactant manufactured by Meisei Chemical Industry Co., Ltd., and then washed with running water for 5 minutes and dried in a hot air dryer at 60° C. for 30 minutes. 3.0% by weight of Dianix Navy S-2G 200% manufactured by DyStar was added to the scoured cylindrical knitted fabric as a disperse dye, and dyed at 130° C. for a predetermined time (5 minutes, 15 minutes, 30 minutes) at a bath ratio of 1:50 in a dyeing solution adjusted to pH 5.0, and then washed with running water for 5 minutes. The dyed cylindrical braid was reduced and cleaned in an aqueous solution containing 0.7 g / L of sodium hydroxide and 2 g / L of sodium dithionite at a bath ratio of 1:50 at 80°C for 20 minutes, then washed with running water for 5 minutes and dried in a hot air dryer at 60°C for 30 minutes. Thereafter, it was trimmed and shaped at 160°C for 1 minute. The trimmed cylindrical braid was used as a sample, and the spectrophotometer CM-3700d manufactured by Konica Minolta Japan was used to measure the L* value with a D65 light source, a viewing angle of 10°, and an optical condition of SCE (regular reflection light elimination method). In addition, the measurement was performed 3 times for 1 sample, and the average value was taken as the L* value.

[0098] H. Pixel value after dyeing

[0099] The cylindrical braided fabric prepared in G with a dyeing time of 30 minutes was used as a sample after finishing. The fibers in the cylindrical braided fabric were cut perpendicularly to the fiber axis direction at any position in the fiber axis direction, and a microscope photograph was taken of the cut surface at a magnification of 2000 times using an optical microscope manufactured by Olympus Corporation as a background, and the pixel value of the white background after conversion to the grayscale image below was 200 to 230. The photographed photograph was converted into a grayscale image of 256 gray levels using image processing software (WINROOF manufactured by Mitani Shoji Co., Ltd.), and the average pixel value of the circle concentric with the center of the inscribed circle of the fiber cross section and with an area of ​​10% of the area of ​​the fiber cross section was calculated for 10 fiber cross sections randomly selected from the same photograph, and the average pixel value was used as the central pixel value, and the average pixel value of the peripheral portion with an area of ​​10% of the area of ​​the fiber cross section was calculated, and the average pixel value was used as the peripheral pixel value.

[0100] I.Feel

[0101] The cylindrical knitted fabric after finishing prepared in G was evaluated by five inspectors with more than five years of experience in quality evaluation according to the following four levels, and the average of the five inspectors was used as the evaluation result of the feel of the fabric. S evaluation, A evaluation and B evaluation were acceptable, and C evaluation was unacceptable.

[0102] (Evaluation Criteria)

[0103] S: Excellent hand feel was felt.

[0104] A: Feel the good feel.

[0105] B: Feel the feel.

[0106] C: The hand feel becomes worse.

[0107] J.Rubbing fastness (contamination)

[0108] The evaluation of the friction fastness was carried out according to the drying test of the friction tester II type (Gakushin type) method in accordance with JIS L0849:2013 (Test method for color fastness to friction) 9.2. The cylindrical knitted fabric prepared in G was used as a sample, and the sample was subjected to friction treatment with white cotton cloth (cotton No. 3-1) specified in JIS L0803:2011 using the Gakushin type friction tester RT-200 manufactured by Daiei Kagaku Seiki Seisakusho Co., Ltd., and the degree of contamination of the white cotton cloth was graded and determined using the contamination gray scale specified in JIS L0805:2005, thereby evaluating the friction fastness (contamination).

[0109] K.Recovery rate (CR)

[0110] Evaluation of the stretch recovery rate (CR) is carried out in accordance with JIS L1013 (2010) 6 (Sample collection and preparation) and 8.12 (Stretch recovery rate). For the false twisted yarn, a load of 0.176 mN × fineness (dtex) × 10 is applied while making it into a skein with a skein length of 40 cm and wound 10 times. Then, an initial load of 0.176 mN × 20 × fineness (dtex) × 10 is applied to the skein, and after being treated with hot water at 90°C for 20 minutes, it is dehydrated with filter paper and naturally dried for more than 12 hours. Thereafter, in the state where the initial load is applied, it is immersed in water at 20°C (range of 18°C ​​to 22°C), and a standard load of 8.82 mN × 20 × fineness (dtex) × 10 is additionally applied. After being left for 2 minutes, the length of the skein after being left is measured and is set as the skein length a. After that, the standard load was removed from the water, and the sample was left standing for 2 minutes with only the initial load applied. The length of the skein after standing was measured and set as the skein length b. The skein length a and skein length b were measured 5 times with different samples, and the stretch recovery rate (CR) was calculated according to the following formula, and the average value was taken.

[0111] CR (%) = {(hank length a - hank length b} / hank length a) × 100

[0112] (Example 1)

[0113] Polyethylene terephthalate (melt viscosity 112 Pa·s) obtained by copolymerizing 7.0 mol% of isophthalic acid and 4.0 mol% of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane was used as chip A, and polyethylene terephthalate (melt viscosity 120 Pa·s) was used as chip B. The chips were mixed in advance in the form of chips at a mixing ratio of 25 wt% of chip A and 75 wt% of chip B so that the copolymerization amount of isophthalic acid was 1.8 mol% and the 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane was 1.0 mol%. The mixed chips were supplied to an extruder type melt spinning machine for melt mixing, and discharged from a spinning die (discharge hole diameter 0.23 mm, discharge hole length 0.30 mm, number of holes 36, round hole) at a spinning temperature of 290° C. and a discharge amount of 42.0 g / min to obtain a spun yarn. The spun yarn was cooled by cooling air at a wind temperature of 20°C and a wind speed of 25 m / min, and sized by applying oil by an oil supply device. The yarn was extracted by a first guide roller rotating at 2500 m / min, and wound by a winder via a second guide roller rotating at the same speed as the first guide roller to obtain an undrawn yarn of 168 dtex-36f. The undrawn yarn obtained was drawn at a draw ratio of 2.0 times between a first hot roller set at 90°C and a second hot roller set at 130°C, and heat-set to obtain a drawn yarn of 84 dtex-36f.

[0114] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 1. The dry heat shrinkage of the obtained drawn yarn was 10.4%, and the hand feeling was excellent even after finishing and setting after dyeing. In addition, it was confirmed that the higher the peak value of the loss tangent, the larger the area, the more fully dyed at the dyeing time of 5 minutes, the faster the dyeing speed, and the inner and outer layer difference of the pixel value after dyeing was 3.1 and dyed to the center, so the dye was fully absorbed and the color development was excellent.

[0115] (Example 2 to Example 4, Comparative Example 1, Comparative Example 2)

[0116] A drawn yarn was produced in the same manner as in Example 1 except that the ratio of the copolymer component in the polyethylene terephthalate composition was changed as shown in Table 1 (adjusted by adjusting the blending ratio of the chips A and the chips B).

[0117] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 1. According to the results, it was confirmed that the higher the peak value of the loss tangent, the larger the area of ​​the loss tangent, and the darker the color of the dyeing. On the other hand, in Comparative Example 1, the peak value of the loss tangent is low, and if the dyeing is not performed for 30 minutes, it cannot be fully dyed. In addition, the difference between the inner and outer layers of the pixel value after dyeing is large, and the color rendering is poor. In Comparative Example 2, the peak value and area of ​​the loss tangent are excellent and the color rendering is good, but the dry heat shrinkage rate is large. If the dyeing is performed after finishing, the feel is deteriorated. In addition, the friction fastness (staining) is also deteriorated.

[0118] (Example 5, Example 6)

[0119] A drawn yarn was produced in the same manner as in Example 1 except that the temperature of the second hot roller during drawing was changed as shown in Table 1.

[0120] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 1. It was confirmed that the peak value of the loss tangent became higher and the area became larger, thereby improving the color development property by setting the second heated roll temperature at a low temperature during drawing.

[0121] (Example 7)

[0122] After the chips A and B were supplied to different extruders for melting, they were ejected from a core-sheath type composite spinning die (ejection hole diameter 0.30 mm, ejection hole length 0.50 mm, number of holes 36, round holes) through a composite spinning machine, and the composite ratio of the core component to the sheath component was set as shown in Table 2. The stretched yarn was produced in the same manner as in Example 3. In addition, the chips A were used as the sheath component, and the chips B were used as the core component.

[0123] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 2. Since it is a core-sheath structure, it cannot be fully dyed unless dyeing is performed for 30 minutes. The difference between the inner and outer layers of the pixel value after dyeing is large. Since the peak of the loss tangent is high and the area is large, the color development is good after 30 minutes of dyeing. Compared with Example 1 in which the fragments are mixed in advance to form a single structure, polyethylene terephthalate obtained by copolymerizing 7.0 mol% of isophthalic acid and 4.0 mol% of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane as a sheath component is concentrated on the surface, so it is confirmed that the friction fastness (staining) is worse than that of the single structure.

[0124] (Example 8, Comparative Example 3, Comparative Example 4)

[0125] A drawn yarn was produced in the same manner as in Example 7 except that the conjugation ratio was changed as shown in Table 2.

[0126] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 2. From the above results, it was confirmed that in the core-sheath structure, the larger the area of ​​the loss tangent, the darker the color of the dyeing. In addition, it was confirmed that as the ratio of polyethylene terephthalate copolymerized with 7.0 mol% of isophthalic acid and 4.0 mol% of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane as the sheath component increased, the friction fastness (staining) decreased.

[0127] (Example 9, Example 10)

[0128] A drawn yarn was produced in the same manner as in Example 7 except that the temperature of the second hot roller during drawing was changed as shown in Table 2.

[0129] The evaluation results of the fiber properties and fabric properties of the obtained drawn yarn are shown in Table 2. It was confirmed that the area of ​​the loss tangent was increased and the color development property was improved by setting the second heat roll temperature at a low temperature during drawing.

[0130] (Example 11)

[0131] The unstretched yarn obtained in Example 1 was false twisted by a false twisting device including 1DR, 1HT, cooling plate, false twisting device, 2DR, 3DR, 4DR, and winder to obtain a false twisted yarn containing synthetic fibers. The false twisting conditions are as follows.

[0132] 1DR speed: 150m / min, processing ratio between 1DR-2DR: 2.0 times, 1HT (hot plate type contact heater, length 2500mm): 180℃, cooling plate length: 1050mm, friction disc type friction false twisting device, ratio between 2DR-3DR: 1.0 times, ratio between 3DR-4DR: 1.00 times, 4DR-winding machine ratio: 0.98 times.

[0133] The evaluation results of the fiber properties and fabric properties of the obtained false twist textured yarn are shown in Table 3. The dry heat shrinkage of the obtained false twist textured yarn was 5.2%, and the hand feeling was excellent even after finishing and setting after dyeing. In addition, it was confirmed that the higher the peak value of the loss tangent, the larger the area, the more fully dyed at the dyeing time of 5 minutes, the faster the dyeing speed, and further, the inner and outer layer difference of the pixel value after dyeing was 2.2 and dyed to the center, so the dye was fully absorbed and the color development was excellent.

[0134] (Example 12 to Example 14, Comparative Example 5)

[0135] Except having changed the 1HT temperature in the false twisting process as shown in Table 3, a false twisted yarn was produced in the same manner as in Example 11.

[0136] The evaluation results of the fiber properties and fabric properties of the obtained false twisted yarn are shown in Table 3. It was confirmed that the peak value of the loss tangent increased by setting the 1HT temperature during false twisting to a low temperature, but the peak temperature of the loss tangent also increased, so the area of ​​the loss tangent decreased and the color development decreased. In addition, it was confirmed that the color development decreased as the dry heat shrinkage rate decreased by setting the 1HT temperature during false twisting to 200°C, and further, fusion between single fibers occurred, and the hand feeling deteriorated.

[0137] (Comparative Example 6)

[0138] A false twisted yarn was produced in the same manner as in Example 11 except that the ratio of the copolymer component in the polyethylene terephthalate composition was changed as shown in Table 3 (only the chip B was used).

[0139] Evaluation results of the fiber properties and fabric properties of the obtained false twisted yarn are shown in Table 3. According to the results, the peak value of the loss tangent was low, and sufficient dyeing was not possible without dyeing for 30 minutes. In addition, the inner and outer layer difference of the pixel value after dyeing was large, and the color development was poor.

[0140] (Example 15)

[0141] The undrawn yarn obtained in Example 7 was subjected to the same procedure as in Example 11 to produce a false twisted yarn.

[0142] The evaluation results of the fiber properties and fabric properties of the obtained false twisted yarn are shown in Table 4. Since it is a core-sheath structure, it cannot be fully dyed unless dyeing is performed for 30 minutes. The difference between the inner and outer layers of the pixel value after dyeing is large. Since the peak of the loss tangent is high and the area is large, the color development is good after 30 minutes of dyeing. Compared with Example 11 in which the fragments are mixed in advance to form a single structure, polyethylene terephthalate obtained by copolymerizing 7.0 mol% of isophthalic acid and 4.0 mol% of 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane as a sheath component is concentrated on the surface, so it is confirmed that the friction fastness (staining) is worse than that of the single structure.

[0143] (Example 16, Example 17, Comparative Example 7, Comparative Example 8)

[0144] A false twist yarn was produced in the same manner as in Example 15 except that the 1HT temperature during false twisting was changed as shown in Table 4.

[0145] The evaluation results of the fiber properties and fabric properties of the obtained false twist yarn are shown in Table 4. In Comparative Example 8, since the 1HT temperature during false twisting was set to 200°C, fusion between single fibers occurred frequently, and false twist yarn could not be obtained stably.

[0146] (Example 18)

[0147] The unstretched yarn obtained in Example 1 was false twisted by a false twisting device including 1DR, 1HT, a cooling plate, a false twisting device, 2DR, 3DR, 2HT, 4DR, and a winder to obtain a false twisted yarn containing synthetic fibers. The conditions for the false twisting are as follows.

[0148] 1DR speed: 150m / min, processing ratio between 1DR-2DR: 2.0 times, 1HT (hot plate type contact heater, length 2500mm): 140℃, cooling plate length: 1050mm, friction disc type friction false twisting device, 2DR-3DR ratio: 1.0 times, 2HT (hot plate type contact heater, length 2000mm): 120℃, 3DR-4DR ratio: 0.92 times, 4DR-winding machine ratio: 0.98 times.

[0149] The evaluation results of the fiber properties and fabric properties of the obtained false twist textured yarn are shown in Table 5. The dry heat shrinkage of the obtained false twist textured yarn was 13.5%, and the hand feeling was excellent even after finishing and setting after dyeing. In addition, it was confirmed that the higher the peak value of the loss tangent, the larger the area, the more fully dyed at the dyeing time of 5 minutes, the faster the dyeing speed, and further, the inner and outer layer difference of the pixel value after dyeing was 2.4 and dyed to the center, so the dye was fully absorbed and the color development was excellent.

[0150] (Example 19 to Example 22)

[0151] A false twist yarn was produced in the same manner as in Example 18 except that the 1HT temperature and the 2HT temperature during false twisting were changed as shown in Table 5.

[0152] The evaluation results of the fiber properties and fabric properties of the obtained false twist textured yarn are shown in Table 5. In Example 20, since the 2HT temperature was higher than the 1HT temperature, heat setting was performed, and both the dry heat shrinkage rate and the stretch recovery rate were reduced.

[0153] [Table 1]

[0154] Table 1

[0155]

[0156] PET: polyethylene terephthalate, IPA: isophthalic acid, BHPP: 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane [Table 2]

[0157] Table 2

[0158]

[0159] PET: polyethylene terephthalate, IPA: isophthalic acid, BHPP: 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane [Table 3]

[0160] Table 3

[0161]

[0162] PET: polyethylene terephthalate, IPA: isophthalic acid, BHPP: 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane [Table 4]

[0163] Table 4

[0164]

[0165] PET: polyethylene terephthalate, IPA: isophthalic acid, BHPP: 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane

[0166] [Table 5]

[0167] Table 5

[0168]

[0169] PET: Polyethylene terephthalate, IPA: Isophthalic acid, BHPP: 2,2-bis[4-(2-hydroxyethoxy)phenyl]propane Industrial Applicability

[0170] The synthetic fiber of the present invention provides a synthetic fiber which is excellent in dye exhaustion ability in the dyeing step, can achieve deep color, has a fast dyeing speed, and has excellent texture, and can be suitably used as a fiber structure.

[0171] The present invention is described in detail using a specific form, but it is clear to those skilled in the art that various changes and modifications can be made without departing from the intent and scope of the present invention. In addition, this application is based on Japanese patent applications (Japanese Patent Application No. 2022-134679) filed on August 26, 2022 and Japanese patent applications (Japanese Patent Application No. 2023-31721) filed on March 2, 2023, and is cited in its entirety by reference.

Claims

1. A synthetic fiber having a loss tangent peak temperature of 100° C. or higher and 150° C. or lower, a loss tangent peak value of 0.15 or higher, and a dry heat shrinkage of 5% or higher and less than 15%.

2. The synthetic fiber according to claim 1, in, The area of ​​the loss tangent from 30°C to 130°C is 4.0°C or more and 7.5°C or less.

3. The synthetic fiber according to claim 1 or 2, in, When the fiber cross-section dyed to an L* value of less than 20 is displayed as a grayscale image with 256 gray levels, the absolute value of the difference between the average pixel value of a circle concentric with the center of the inscribed circle and with an area of ​​10% of the area of ​​the fiber cross-section and the average pixel value of the peripheral part with an area of ​​10% of the area of ​​the fiber cross-section is less than 15.

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