Polyester fiber, method for producing same, and fiber structure

Abstract

Provided is an ultrafine polyester fiber capable of exhibiting excellent dispersibility regardless of an aspect ratio. The polyester fiber may have an average cross-sectional area x of 1 to 40 µm2, a standard deviation σ of a cross-sectional area of 10 µm2 or less, a standard deviation σ of a cross-sectional area of 10 µm2 or less, and a ratio Ao / Ai of a sum Ao of cross-sectional areas of data outside a range of x ± 2.5σ to a sum Ai of cross-sectional areas of data within the range of x ± 2.5σ of 10% or less, wherein these values are calculated from data of cross-sectional areas of single fibers. The polyester fiber may have an average fiber diameter of single fibers of 1 to 10 µm, and a roundness of a fiber cross-section of 0.90 to 1.00.

Description

CROSS REFERENCE TO THE RELATED APPLICATION

[0001] This application is based on and claims Convention priority to Japanese patent application No. 2023-084700, filed May 23, 2023 and Japanese patent application No. 2023-084701, filed May 23, 2023, the entire disclosures of which are herein incorporated by reference as a part of this application.FIELD OF THE INVENTION

[0002] The present invention relates to an ultrafine polyester fiber which can be used as a papermaking fiber or the like and a method for producing the same, and a fiber structure including such a polyester fiber.BACKGROUND OF THE INVENTION

[0003] Conventionally, cellulose-based fibers such as pulp and rayon, polypropylene fibers, polyacrylonitrile fibers, polyvinyl alcohol fiber, and the like have been used as papermaking fibers. Recently, a paper-like material made of polyester fibers in part or all as raw materials by a papermaking method have been more commonly used because the polyester fibers have excellent physical properties such as mechanical properties, electrical properties, heat resistance, dimensional stability, and hydrophobicity.

[0004] Papermaking fibers have been widely utilized in industrial material applications for filters, etc., and ultrafine fibers have been recently used in response to demand for improved flexibility, increased density, and thinner sheets. For example, Patent Document 1 (JP Patent No. 6022054) discloses an organic resin non-crimped staple fiber for a wet-laid nonwoven fabric having a fiber fineness of 0.0001 to 0.6 dtex, a fiber length of 1.0 to 5.0 millimeters, a moisture content of 10 to 100 wt%, a cut-end coefficient of 1.00 to 1.07, and a coefficient of variation (CV%) of a fiber length of 0.0 to 15.0%.

[0005] Patent Document 2 (JP Laid-Open Patent Publication No. 2012-92458) discloses an ultrafine fiber for binder including a thermoplastic resin and having an average fiber diameter of 10 to 2000 nm, a coefficient of variation of a fiber diameter of 0 to 25%, and a degree of crystallinity of 20% or less.

[0006] Patent Document 3 (JP Laid-Open Patent Publication No. 2012-193476) discloses an ultrafine polyester fiber having a single fiber diameter of 10 to 1000 nm, a variation (CV%) of the single fiber diameter of 0 to 25%, a tensile strength of an ultrafine fiber bundle of 4.0 cN / dtex or more, an elongation at break of 10 to 80%, and a dry heat shrinkage ratio at 150°C of 5% or less.CONVENTIONAL ART DOCUMENTSPATENT DOCUMENTS

[0007] [Patent Document 1] JP Patent No. 6022054 [Patent Document 2] JP Laid-Open Patent Publication No. 2012-92458 [Patent Document 3] JP Laid-Open Patent Publication No. 2012-193476 SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] However, although in Patent Document 1, dispersibility is measured, it is described that the fibers become more likely to be intermingled with each other as an aspect ratio of fibers (ratio of fiber length to fiber diameter) increases, and in consideration of this, the dispersibility is evaluated with the aspect ratio adjusted depending on the fineness of the fiber. Thus, Patent Document 1 does not describe improvement in fiber dispersibility regardless of the aspect ratio. In addition, Patent Document 1 provides no description regarding the evaluation of the wet-laid nonwoven fabric, since the wet-laid nonwoven fabric was not actually produced using the organic resin non-crimped staple fiber.

[0009] Patent Document 2 describes that uniformity in fiber diameter allows uniform dispersion of the ultrafine fibers among subject fibers during papermaking, and Patent Documents 2 and 3 describe variations in fiber diameter of the ultrafine fibers. However, in Patent Documents 2 and 3, the ultrafine fibers are produced by a method in which a sea component is removed from a sea-island composite fiber having a large number of island domains. With respect to such a production method, although the flow of an island-component polymer divided into countless ultrafine domains is highly unstable, two kinds of polymers having different flow properties (viscosities) are combined. Thus, for example, due to slight fluctuations which occur when the island-component polymer is inserted into sea components through a plurality of pipe tips of a spinneret for an island component, island components do not enter the intended position to cause adjacent ones of the island components to merge and coarsen, or experience similar development, so that a desired cross section cannot be obtained in some cases. That is, production conditions such as the combination of polymers and the ratio between sea and island components are extremely limited, and thus improvement in fiber diameter uniformity is limited.

[0010] Therefore, an object of the present invention is to provide an ultrafine polyester fiber capable of exhibiting excellent dispersibility regardless of an aspect ratio of the fiber.

[0011] Another object of the present invention is to provide an ultrafine polyester fiber from which a wet-laid nonwoven fabric having improved surface smoothness can be produced.MEANS FOR SOLVING THE PROBLEMS

[0012] The inventors of the present invention have conducted extensive studies in order to achieve the aforementioned object, and have found that, since cross sections of single fibers in an ultrafine polyester fiber are extremely small, slight variations in size and shape significantly affect fiber performance, and probably due to this fact, adjustment of the fiber cross-sections in the ultrafine polyester fiber can exhibit excellent dispersibility even when the aspect ratio of the fiber is high, to obtain a wet-laid nonwoven fabric having improved surface smoothness by using such fibers, leading to the completion of the present invention.

[0013] That is, the present invention may include the following aspects.[Aspect 1]

[0014] A polyester fiber having an average cross-sectional area (x) of 1 to 40 µm 2< (preferably 2 to 35 µm 2< , and more preferably 5 to 30 µm 2< ), a standard deviation (σ) of a cross-sectional area of 10 µm 2< or less (preferably 8 µm 2< or less, and more preferably 7 µm 2< or less), and a ratio (Ao / Ai) of a sum Ao of cross-sectional areas of data outside a range of x ± 2.5σ to a sum Ai of cross-sectional areas of data within the range of x ± 2.5σ of 10% or less (preferably 8% or less, and more preferably 6% or less), wherein the average cross-sectional area (x), the standard deviation (σ) of the cross-sectional area, and the ratio (Ao / Ai) are calculated from data of cross-sectional areas of single fibers.[Aspect 2]

[0015] The polyester fiber according to aspect 1, having a coefficient of variation of the cross-sectional area of 50% or less (preferably 45% or less, more preferably 40% or less, and further preferably 36% or less), as calculated from the data of the cross-sectional areas of the single fibers.[Aspect 3]

[0016] The polyester fiber according to aspect 1 or 2, having an average fiber diameter of the single fibers of 1 to 10 µm (preferably 1.5 to 9 µm, and more preferably 2 to 8 µm), and a roundness of a fiber cross-section of 0.90 to 1.00 (preferably 0.91 to 1.00, more preferably 0.92 to 1.00, and further preferably 0.93 to 1.00).[Aspect 4]

[0017] The polyester fiber according to aspect 3, having a coefficient of variation of the roundness of the fiber cross-section of 7% or less (preferably 6% or less, more preferably 5% or less, and further preferably 4% or less).[Aspect 5]

[0018] A polyester fiber having an average fiber diameter of single fibers of 1 to 10 µm (preferably 1.5 to 9 µm, and more preferably 2 to 8 µm), and a roundness of a fiber cross-section of 0.90 to 1.00 (preferably 0.91 to 1.00, more preferably 0.92 to 1.00, and further preferably 0.93 to 1.00).[Aspect 6]

[0019] The polyester fiber according to aspect 5, having a coefficient of variation of the roundness of the fiber cross-section of 7% or less (preferably 6% or less, more preferably 5% or less, and further preferably 4% or less).[Aspect 7]

[0020] The polyester fiber according to any one of aspects 1 to 6, having an average fiber length of 0.1 to 110 mm (preferably 0.5 to 50 mm, and more preferably 1 to 20 mm).[Aspect 8]

[0021] The polyester fiber according to any one of aspects 1 to 7, having an aspect ratio of 100 or higher (preferably 300 to 10000, and more preferably 500 to 3000).[Aspect 9]

[0022] A fiber structure including the polyester fiber as recited in any one of aspects 1 to 8.[Aspect 10]

[0023] The fiber structure according to aspect 9, wherein the fiber structure is a wet-laid nonwoven fabric.[Aspect 11]

[0024] A method for producing the polyester fiber as recited in any one of aspects 1 to 8, the method including: a spinning step of melt-spinning a resin material containing a polyester-based resin to obtain a fiber; and a drawing step of drawing the fiber obtained in the spinning step, wherein in the drawing step, flow drawing and subsequent neck drawing are performed, and a drawing tension is 0.01 cN / dtex or less (preferably 0.009 cN / dtex or less, and more preferably 0.008 cN / dtex or less) at least in the flow drawing.

[0025] As used herein, the singular forms, "a," "an", and "the" are intended to include plural forms including "at least one", unless the content clearly indicates otherwise. As used herein, the terms "and / or", "at least one", and "one or more" include any and all combinations of the relevant listed items.

[0026] Any combination of at least two constructions, disclosed in the appended claims and / or the specification should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention.EFFECT OF THE INVENTION

[0027] The polyester fiber according to the present invention can achieve excellent dispersibility regardless of an aspect ratio of the fiber, and such a fiber is useful for producing a wet-laid nonwoven fabric having improved surface smoothness.DESCRIPTION OF EMBODIMENT[Polyester fiber]

[0028] A polyester fiber according to a first embodiment of the present invention has an average cross-sectional area (x) of 1 to 40 µm 2< and a standard deviation (σ) of a cross-sectional area of 10 µm 2< or less, and a ratio (Ao / Ai) of a sum Ao of cross-sectional areas of data outside a range of x ± 2.5σ to a sum Ai of cross-sectional areas of data within the range of x ± 2.5σ of 10% or less, wherein the average cross-sectional area (x), the standard deviation (σ) of the cross-sectional area, and the ratio (Ao / Ai) are calculated from data of cross-sectional areas of single fibers.

[0029] According to the present invention, since the fiber cross-section of an ultrafine polyester fiber is extremely small, variations in size thereof are expected to significantly affect fiber properties, and thus, instead of focusing on the fiber diameter, focus is placed on the cross-sectional area which can more clearly reflect the variations. Then, it has been found that the distribution of the cross-sectional areas of single fibers significantly affects fiber dispersibility, and surface smoothness of a wet-laid nonwoven fabric produced using the fibers.

[0030] In the present specification, the fiber cross-section means a cross section orthogonal to a fiber axis direction, and the data of the cross-sectional areas of single fibers is data of values obtained by measuring the cross-sectional areas of 250 single fibers using the method described in Examples below. The average cross-sectional area (x) and the standard deviation (σ) represent an average value and a standard deviation based on the data of the cross-sectional areas of single fibers, respectively. Ai represents the sum of the cross-sectional areas of all data within the range of x ± 2.5σ (i.e., a range of 5 times the standard deviation (σ) centered around the average cross-sectional area (x); x - 2.5σ or more and x + 2.5σ or less), and Ao represents the sum of the cross-sectional areas of all data outside the range of x ± 2.5σ. The ratio (Ao / Ai) of Ao to Ai represents a numerical value obtained by dividing Ao by Ai and expressed as a percentage (i.e., a numerical value calculated by dividing Ao by Ai and multiplying by 100), and serves as an index indicating the extent to which a numerical value, as the cross-sectional area data (outlier) which are outside the range of x ± 2.5σ, deviates from the average cross-sectional area and how many such numerical values exist. According to the present invention, since the range of x ± 2.5σ is used as a reference, the data of relatively small cross-sectional areas can be included in the range, thus the outliers can form a range composed mainly of the data of the cross-sectional areas exceeding the upper limit value, and the numerical value represented by Ao allows single fibers having abnormal large cross-sectional areas to be primarily recognized. In addition, as in Patent Documents 2 and 3, the coefficient of variation (CV value) of the single fiber diameter does not allow recognition of the presence of the above-described outliers in the data of the cross-sectional areas of single fibers. Therefore, according to the present invention, the ratio (Ao / Ai) of Ao to Ai is used as an index to recognize whether single fibers having abnormal large cross-sectional areas exist.

[0031] In the ultrafine polyester fiber, in a case where a large number of single fibers having cross-sectional areas outside the range of x ± 2.5σ, based on the average cross-sectional area (x) and the standard deviation (σ) of the cross-sectional area, exist, entanglement in which single fibers having cross-sectional areas larger than x + 2.5σ serve as nuclei occurs when the single fibers are dispersed in water, so that the surface smoothness of the resulting wet-laid nonwoven fabric is deteriorated. Thus, according to the present invention, it has been found that adjustment is performed such that the ultrafine polyester fiber includes a smaller number of such thick single fibers, to have the ratio (Ao / Ai) of the sum Ao of the cross-sectional areas of the data outside the range of x ± 2.5σ to the sum Ai of the cross-sectional areas of the data within the range of x ± 2.5σ of 10% or less, whereby dispersibility of such a fiber can be improved, and the surface smoothness of the wet-laid nonwoven fabric obtained using such a fiber can be improved.

[0032] From the viewpoint of improving fiber dispersibility, the polyester fiber according to the first embodiment of the present invention has the average cross-sectional area (x) of 1 to 40 µm 2< , and may have the average cross-sectional area (x) of preferably 2 to 35 µm 2< and more preferably 5 to 30 µm 2< .

[0033] From the viewpoint of improving the surface smoothness of the resulting wet-laid nonwoven fabric, the polyester fiber according to the first embodiment of the present invention has the standard deviation (σ) of the cross-sectional area of 10 µm 2< or less, and may have the standard deviation (σ) of the cross-sectional area of preferably 8 µm 2< or less and more preferably 7 µm 2< or less. The lower limit of the standard deviation of the cross-sectional area is not particularly limited to a specific one, and may be, for example, about 1 µm 2< .

[0034] From the viewpoint of improving fiber dispersibility and the surface smoothness of the resulting wet-laid nonwoven fabric, the polyester fiber according to the first embodiment of the present invention has the ratio (Ao / Ai) of Ao to Ai of 10% or less, and may have the ratio (Ao / Ai) of preferably 8% or less and more preferably 6% or less. The lower limit of Ao / Ai is not particularly limited to a specific one, and may be, for example, about 0.1%.

[0035] The polyester fiber according to the first embodiment of the present invention may has a coefficient of variation of the cross-sectional area of 50% or less, preferably 45% or less, more preferably 40% or less, and further preferably 36% or less, as calculated from the data of the cross-sectional areas of the single fibers, from the viewpoint of improving the surface smoothness and the pore size uniformity of the resulting wet-laid nonwoven fabric. The lower limit of the coefficient of variation of the cross-sectional area is not particularly limited to a specific one, and may be, for example, about 0.1%. In the present specification, the coefficient of variation of the cross-sectional area can be calculated from a formula of the standard deviation (σ) of the cross-sectional area / the average cross-sectional area (x) × 100.

[0036] A polyester fiber according to a second embodiment of the present invention has an average fiber diameter of single fibers of 1 to 10 µm, and a roundness of a fiber cross-section of 0.90 to 1.00.

[0037] The fiber cross-section of the ultrafine fiber is extremely small, and slight variations in the roundness of the fiber cross-section significantly depend on the shape of the fiber and, probably due to this fact, affect fiber dispersibility. The roundness of the fiber cross-section also affects the surface smoothness of the wet-laid nonwoven fabric produced using the fiber. Thus, according to the present invention, it has been found that adjustment is performed such that the ultrafine polyester fiber has the fiber cross-section closer to a perfect circle, whereby the fiber exhibits good dispersibility even when the aspect ratio of the fiber is high, and the surface smoothness of the wet-laid nonwoven fabric obtained using such a fiber can be improved.

[0038] The polyester fiber according to the second embodiment of the present invention may have the average fiber diameter of the single fibers of preferably 1.5 to 9 µm and more preferably 2 to 8 µm. In the present specification, the average fiber diameter of single fibers means the average value of circumscribed circle diameters (i.e., maximum diameters) of the fiber cross-sections, and is a value measured by the method described in Examples below.

[0039] In the polyester fiber according to the second embodiment of the present invention, the lower limit of the roundness of the cross section may be preferably 0.91 or more, more preferably 0.92 or more, and further preferably 0.93 or more. Further, it is preferable as the fiber cross-section is closer to a perfect circle, but the upper limit of the roundness is not particularly limited to a specific one, and may be, for example, 0.99 or less or 0.98 or less. In the present specification, the roundness of the fiber cross-section is the ratio of the minimum diameter to the maximum diameter (minimum diameter / maximum diameter) of a cross section orthogonal to the fiber axis direction, and is a value measured by the method described in Examples below.

[0040] The polyester fiber according to the second embodiment of the present invention may have a coefficient of variation of the roundness of the fiber cross-section of 7% or less, preferably 6% or less, more preferably 5% or less, and further preferably 4% or less, from the viewpoint of improving the surface smoothness and the pore size uniformity of the resulting wet-laid nonwoven fabric. The lower limit of the coefficient of variation of the roundness of the fiber cross-section is not particularly limited to a specific one, and may be, for example, about 0.1%. In the present specification, the coefficient of variation of the roundness of the fiber cross-section can be calculated from a formula of the standard deviation of the roundness / the average value of the roundness, and is a value measured by the method described in Examples below.

[0041] The polyester fiber according to the first embodiment of the present invention may have the same structure as that of the polyester fiber according to the second embodiment. For example, the polyester fiber according to the present invention may have the average cross-sectional area (x) of 1 to 40 µm 2< and the standard deviation (σ) of the cross-sectional area of 10 µm 2< or less, and the ratio (Ao / Ai) of the sum Ao of the cross-sectional areas of the data outside the range of x ± 2.5σ to the sum Ai of the cross-sectional areas of the data within the range of x ± 2.5σ of 10% or less, wherein the average cross-sectional area (x), the standard deviation (σ) of the cross-sectional area, and the ratio (Ao / Ai) are calculated from data of cross-sectional areas of single fibers, and may have the average fiber diameter of the single fibers of 1 to 10 µm, and the roundness of the fiber cross-section of 0.90 to 1.00.

[0042] The polyester fiber according to the present invention may be a non-composite fiber. Further, the polyester fiber according to the present invention may be a solid fiber.

[0043] The polyester fiber according to the present invention may be a staple fiber depending on the type of the fiber structure or the like, and may have an average fiber length of, for example, from 0.1 to 110 mm, preferably from 0.5 to 50 mm, and more preferably from 1 to 20 mm. The polyester fibers may have the average fiber length of preferably 0.1 to 50 mm, more preferably 0.5 to 25 mm, and further preferably 1 to 20 mm, when intended for use in a production of a paper-like material which is one example of the wet-laid nonwoven fabric, from the viewpoint of improving processability during production of the paper-like material and the physical properties such as strength of the resulting paper-like material. On the other hand, in a production of a dry-laid nonwoven fabric using a carding machine or the like, a web including fibers needs to pass through a line continuously without causing a breakage in a traveling direction, and thus the polyester fiber may have the average fiber length of preferably 10 to 110 mm, more preferably 15 to 100 mm, and further preferably 20 to 90 mm, when intended for use in the production of the dry-laid nonwoven fabric. In the present specification, the average fiber length is a value measured by the method described in Examples below.

[0044] The polyester fiber according to the present invention may have an aspect ratio of 100 or higher, preferably 300 to 10000, and more preferably 500 to 3000. In the present specification, the aspect ratio means a ratio of the average fiber length to the average fiber diameter (average fiber length / average fiber diameter).

[0045] The polyester fiber according to the present invention includes a polyester-based resin. According to the present invention, the polyester-based resin is a polycondensate of a dicarboxylic acid and a diol. Examples of the dicarboxylic acid may include: aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 4,4'-biphenyldicarboxylic acid; and aliphatic dicarboxylic acids such as azelaic acid and sebacic acid. Examples of the diol may include: aliphatic diols such as ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, polyethylene glycol, and polytetramethylene glycol; aromatic diols such as ethylene oxide adducts of bisphenol A or bisphenol S; and alicyclic diols such as cyclohexanedimethanol.

[0046] The polyester-based resin constituting the polyester fiber according to the present invention may be preferably a polyester-based resin having an aromatic dicarboxylic acid as a main dicarboxylic acid component. Examples of such a polyester-based resin may include a polyethylene terephthalate, a polytrimethylene terephthalate, a polytetramethylene terephthalate, a polycyclohexanedimethylene terephthalate, a polyethylene naphthalate, and a polytetramethylene terephthalate. These polyester-based resins may be copolymers obtained by copolymerizing an additional diol and / or an additional dicarboxylic acid such as isophthalic acid as a third component. Among these, the polyethylene terephthalate is more preferable.

[0047] The polyester fiber according to the present invention may include thermoplastic resin as long as the effect of the present invention is not impaired. The polyester fiber according to the present invention may also include a delustering agent, a heat stabilizer, an ultraviolet absorbent, an antistatic agent, a terminating agent, a fluorescent brightening agent, etc., as necessary.

[0048] The polyester fiber according to the present invention may contain the polyester-based resin at a proportion of 50 wt% or more, preferably 80 wt% or more, more preferably 90 wt% or more, and further preferably 95 wt% or more.

[0049] The polyester fiber according to the present invention is preferably a drawn fiber from the viewpoint of improving the mechanical properties of the fiber structure. For example, the polyester fiber according to the present invention may be a polyester fiber in which amorphous components crystallize through drawing, and no crystallization peak is observed in a DSC curve obtained during a heating run by differential scanning calorimetry (DSC). In the present specification, if a crystallization peak (exothermic peak) is extremely broad and cannot be identified clearly as a peak, it may be determined that no crystallization peak is observed.[Method for producing polyester fiber]

[0050] A method for producing the polyester fiber according to the present invention may include a spinning step of melt-spinning a resin material containing a polyester-based resin to obtain a fiber, and a drawing step of drawing the fiber obtained in the spinning step.(Spinning step)

[0051] The spinning step can be performed by melt-spinning in a known method, and for example, a known spinning machine can be used. Specifically, a resin material in the form of pellets, powders, etc. to be used as a raw material for a polyester fiber is fed into a hopper to be supplied to an extruder through a feeder as a feeding device. As the polyester-based resin contained in the resin material, the above-described polyester-based resin can be used. From the viewpoint of spinnability and the physical properties of the resulting fiber, the polyester-based resin may have an intrinsic viscosity of from 0.4 to 1.0 dL / g, preferably from 0.45 to 0.9 dL / g, and more preferably from 0.5 to 0.8 dL / g.

[0052] As an extruder, a known extruder such as a single-screw extruder or a multi-screw extruder (twin- or more-screw extruder) can be used. For example, in a twin-screw extruder, the resin material is conveyed from the upstream side to the downstream side while being melt-kneaded by the rotation of parallel twin screws rotatably supported in a barrel, and the resin material is heated and melted by a known heating means such as a heater installed in the barrel, thereby forming a molten resin. A temperature for melt-kneading depends on the type of the polyester-based resin, etc., and may be 260 to 320°C and preferably 270 to 310°C, in order to ensure uniform and stable melt-kneading and stable spinning of the resin material.

[0053] Then, the molten resin is metered using a gear pump, is introduced into a spinning head, and is discharged through spinneret holes in a nozzle to form filaments, and the filaments can be taken up to produce undrawn fibers (as-spun fibers). Although it depends on the melt viscosity of the resin material, the average fiber diameter of the resulting undrawn fibers can be adjusted as desired by adjusting a size of the spinneret holes in the nozzle, a discharge rate, a take-up speed (spinning speed), etc. For example, a hole diameter of the spinneret holes in the nozzle may be from 0.05 to 0.30 mm, and the discharge rate per single hole may be from 0.01 to 2.0 g / min. Further, the take-up speed may be from 500 to 3000 m / min, preferably 600 to 2000 m / min, and more preferably 800 to 1500 m / min.

[0054] According to the present invention, it is possible that the undrawn fiber obtained in the spinning step is once wound up, and then is unwound and is drawn, or the undrawn fiber is drawn such that the spinning step is directly connected to the drawing step without winding. From the viewpoint of adjusting a drawing tension in the drawing step as described below, it is preferable that the undrawn fiber is once wound up and then is subjected to the drawing step. For example, the average fiber diameter of the undrawn fiber that has been wound up once may be from 8 to 17 µm, preferably from 9 to 16 µm, and more preferably from 10 to 15 µm.(Drawing step)

[0055] According to the present invention, in order to obtain the ultrafine polyester fiber, it is preferable to draw the undrawn fiber to make it finer. In the drawing step, flow drawing capable of making it finer while maintaining low molecular orientation is performed, and the drawing tension is adjusted to fall into a specific range, whereby the size and shape of fiber cross-sections can be made uniform. Specifically, it is preferable that, in the drawing step, the obtained undrawn fiber is subjected to flow drawing and subsequent neck drawing, and the drawing tension is 0.01 cN / dtex or less.

[0056] In general, a polyester fiber can be made finer by drawing an undrawn fiber at a high ratio. However, according to the present invention, it has been found that probably since a plurality of fibers cannot be uniformly drawn in a case where high drawing tension is applied in the drawing step, in the resulting polyester fibers, single fibers having abnormal large cross-sectional areas coexist, or the shape of the fiber cross-sections is changed. Further, as a result of studying conditions for the drawing step, it has been found that in a case where the flow drawing capable of making the undrawn fibers finer while maintaining low molecular orientation is performed, and the drawing tension is adjusted to fall into the specific range so as to avoid applying much tension to the polyester fibers, the polyester fibers can be made finer as well as the size and shape of the fiber cross-sections can be controlled.

[0057] In the drawing step, a multifilament obtained in the spinning step may be directly drawn, or from the viewpoint of improving productivity, a plurality of multifilaments may be bundled together to form a fiber bundle (tow) and then may be drawn.

[0058] The flow drawing is a method in which the obtained undrawn fiber is drawn at a temperature higher than a glass transition temperature (Tg) of the undrawn fiber, and can make it finer while maintaining the molecular orientation of the undrawn fiber. According to the present invention, it is considered that, in such flow drawing, very small and more uniform drawing tension can be applied in a fiber axis direction by performing drawing in a state of reduced drawing tension, and probably due to this fact, the size and shape of the fiber cross-sections can be controlled. In the flow drawing, drawing can be performed under heating according to various embodiments described below, and the drawing tension can be checked upstream of a heating zone and can be adjusted by controlling drawing speed, etc. The drawing tension may be preferably 0.01 cN / dtex or less, more preferably 0.009 cN / dtex or less, and further preferably 0.008 cN / dtex or less. On the other hand, from the viewpoint of productivity, the drawing tension may be 0.001 cN / dtex or more.

[0059] According to the present invention, the flow drawing is not particularly limited to a specific method as long as drawing can be performed under a specific low drawing tension at a temperature higher than the glass transition temperature of the undrawn polyester fiber, and may be performed by hot bath drawing, dry heat drawing, or moist heat drawing, and the hot bath drawing is preferable from the viewpoint of uniform heating. As a heating medium for the hot bath, a liquid such as water and an organic solvent (various oils, etc.) can be used, and the hot water bath is preferable from the viewpoint of thermal conductivity, cleanability, etc.

[0060] A temperature in the flow drawing is not particularly limited to a specific one as long as the temperature is higher than the glass transition temperature (Tg) of the undrawn polyester fiber, and may be from Tg + 5°C to Tg + 45°C and preferably from Tg + 10°C to Tg + 40°C. Although it depends on the type of polyester-based resin, etc., the temperature may be, for example, from 80 to 120°C and preferably from 85 to 115°C. Further, the temperature in the flow drawing is more preferably from 90 to 110°C from the viewpoint of performing more stable drawing while reducing the drawing tension.

[0061] From the viewpoint of making the undrawn fibers finer and controlling the size and shape of the fiber cross-sections, a draw ratio in the flow drawing may be from 1.5 to 10 times, preferably from 2.0 to 8.0 times, and more preferably from 2.5 to 6.0 times.

[0062] After the flow drawing, neck drawing is performed on the fiber subjected to the flow drawing, whereby further finer fineness can be achieved, as well as crystallinity can be changed to improve mechanical strength. The neck drawing can be performed by drawing at a temperature that is equal to or lower than the glass transition temperature of the polyester fiber, and may be performed by, for example, hot bath drawing, dry heat drawing, or moist heat drawing, and the hot bath drawing is preferable from the viewpoint of uniform heating. As a heating medium for the hot bath, a liquid such as water and an organic solvent (various oils, etc.) can be used, and the hot water bath is preferable from the viewpoint of thermal conductivity, cleanability, etc., similar to the flow drawing.

[0063] A temperature in the neck drawing may be a temperature that is equal to or lower than the glass transition temperature (Tg) of the polyester fiber, and may be from Tg - 50°C to Tg, preferably from Tg - 30°C to Tg - 1°C, and more preferably from Tg - 20°C to Tg - 5°C. From the viewpoint of improving the molecular orientation and performing more stable drawing, the temperature may be, for example, from 30 to 80°C, preferably from 50 to 75°C, and more preferably from 60 to 73°C.

[0064] From the viewpoint of making the fibers finer, improving mechanical strength, and controlling the size and shape of the fiber cross-sections, a draw ratio in the neck drawing may be from 1.1 to 5.0 times, preferably from 1.5 to 4.0 times, and more preferably from 2.0 to 3.5 times.

[0065] The flow drawing and the neck drawing may be performed separately in the respective drawing processes. For example, fibers subjected to the flow drawing may be wound up once, and then may be subjected to the neck drawing using another drawing device. In a case where the fibers subjected to the flow drawing are wound up once and are subjected to the neck drawing using the other drawing device, it is preferable to also adjust the drawing tension in the neck drawing, as in the above-described flow drawing. Alternatively, both the flow drawing and the neck drawing may be performed using a continuous drawing device. From the viewpoint of improving production efficiency and adjusting drawing tension, both the flow drawing and the neck drawing are preferably performed using the continuous drawing device.

[0066] A total draw ratio in the drawing step (the product of the draw ratio in the flow drawing and the draw ratio in the neck drawing) depends on the average fiber diameter of as-spun fibers (undrawn fibers) and / or the desired average fiber diameter of drawn fibers, and may be, for example, from 2.0 to 20 times, preferably from 5.0 to 18 times, and more preferably from 7.0 to 15 times.

[0067] A ratio (flow drawing / neck drawing) of the draw ratio in the flow drawing to the draw ratio in the neck drawing in the drawing step may be, for example, from 0.3 to 5.0, preferably from 0.4 to 4.0, and more preferably from 0.5 to 3.0.

[0068] In the method for producing the polyester fiber according to the present invention, for example, in order to improve bundling properties of fibers, to prevent fluffing, fiber breakage, and the like, to prevent fibers from fusion, etc., known various oiling agents may be applied by known methods, during the spinning step, before the drawing step, and / or after the drawing step.

[0069] In the method for producing the polyester fiber according to the present invention, the drawn fiber may be cut to a predetermined fiber length by a known method depending on the application.[Fiber structure]

[0070] The polyester fiber according to the present invention may be used as a subject fiber of a fiber structure. The fiber structure is not limited to a specific form as long as the fiber structure includes at least the polyester fiber. Examples of the fiber structure include various fabrics such as a nonwoven fabric, a woven fabric, and a knitted fabric, and the nonwoven fabric is preferable. The fiber structure may include fibers other than the polyester fiber according to the present invention. For example, the fiber structure may include: cellulose-based fibers such as pulp and rayon; polyolefin-based fibers formed from polyolefin-based resins such as polyethylene and polypropylene; polyamide-based fibers formed from polyamide-based resins such as polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 610, and polyamide 612; polyacrylonitrile-based fibers; and polyvinyl alcohol-based fibers. In addition, as necessary, binder fibers, for example, water-soluble polymer fibers such as polyvinyl alcohol-based fibers, and heat-fusible fibers such as polyester-based fibers, may be included.

[0071] In the fiber structure according to the present invention, a basis weight and / or a thickness can be adjusted depending on the type, the application, and the like. For example, the fiber structure may have the basis weight of from 1 to 120 g / m 2< , preferably from 3 to 100 g / m 2< , and more preferably from 5 to 90 g / m 2< .

[0072] The fiber structure according to the present invention may be preferably a wet-laid nonwoven fabric and more preferably a paper-like material. A paper-like material produced using the polyester fiber according to the present invention has excellent surface smoothness. The surface roughness of the paper-like material may be, for example, 9.0 µm or less, preferably 8.5 µm or less, and more preferably 8.3 µm or less, and the lower limit thereof is not particularly limited to a specific one and may be 1.0 µm or more. In the present specification, the surface roughness refers to arithmetic mean roughness Ra measured in accordance with ISO1997, and is a value measured by the method described in Examples below.

[0073] In a case where the wet-laid nonwoven fabric is produced as the fiber structure, the wet-laid nonwoven fabric may include the binder fiber, and a mass ratio (polyester fiber / binder fiber) of the polyester fiber according to the present invention to the binder fiber (e.g., polyester binder fiber) which compose the wet-laid nonwoven fabric may be from 95 / 5 to 5 / 95, preferably from 80 / 20 to 20 / 80, more preferably from 75 / 25 to 25 / 75, further preferably from 70 / 30 to 30 / 70, and even more preferably from 70 / 30 to 50 / 50.

[0074] The polyester fiber according to the present invention has excellent dispersibility, and thus can be used to produce the wet-laid nonwoven fabric in particular. The wet-laid nonwoven fabric can be produced by forming a web using water in a production process and subsequently drying the web as necessary, followed by heating the web to bond the subject fibers with the binder fiber. Examples of the specific method for forming the web using water in the production process include a papermaking method in which fibers are dispersed in water to produce a paper-like web, and a hydroentanglement method in which a web is formed using no water, and then fibers in the web are entangled using water.

[0075] Any known method can be employed as the papermaking method. For example, a cylinder machine papermaking process, a fourdrinier machine papermaking process, a Tanmo machine papermaking process, or other process can be employed.

[0076] After the paper-like web is formed, the web may be dried. The web can be dried by using a known device. After the web is dried, the web may then be subjected to pressing process at a high temperature.

[0077] The wet-laid nonwoven fabric including the polyester fiber according to the present invention is excellent in surface smoothness, air permeability, and liquid permeability, and thus can be used as various industrial materials, for example, a filter, a separator, a support for a reverse osmosis membrane, a wiping material, an artificial leather, and other various types of functional paper.EXAMPLES

[0078] Hereinafter, the present invention will be described in more detail on the basis of Examples, but the present invention is not limited to these Examples whatsoever. In Examples and Comparative Example presented below, various physical properties were measured according to the following methods.(Intrinsic viscosity)

[0079] An intrinsic viscosity (dL / g) of a resin before spinning was measured using an automatic viscometer ("SS-600-L1" available from Shibayama Scientific Co., Ltd.). A solvent used for the measurement was a mixed solvent of phenol and tetrachloroethane (volume ratio: 1 / 1) at a temperature of 30°C.(Glass transition temperature)

[0080] As a measurement sample, 3 to 8 mg of undrawn polyester fibers (as-spun fibers) were weighed, and measurement was performed on the measurement sample using a differential scanning calorimeter ("DSC-60" available from Shimadzu Corporation) under a nitrogen atmosphere at a heating rate of 10°C / min from a room temperature to 300°C. A glass transition temperature was determined from the obtained DSC curve, as a temperature at the intersection of a straight line obtained by extending a baseline on the low-temperature side toward the high-temperature side and a tangent line drawn at the point where the gradient of the curve is maximum in a stepwise change region corresponding to the glass transition.(Drawing tension)

[0081] Using a handheld model tension meter (available from Nidec Shimpo Corporation), a tension (cN / dtex) of a fiber tow immediately before the fiber tow was introduced into a drawing bath was measured.(Single fiber cross-sectional area)

[0082] Fibers obtained in each of Examples and Comparative Example were embedded in paraffin, and were cut by using a microtome in a direction perpendicular to the fiber axis direction, to prepare a sample for measurement. A cut surface of the sample for measurement was observed using a digital microscope ("VHX-5000" available from KEYENCE CORPORATION), and an automatic area measurement function was used. In order to exclude inclined fibers, were selected 250 single fibers that have cross sections exhibiting high uniformity in shadow (contour visibility) and that independently exist, and an area of each single fiber cross-section was measured. An average value and a standard deviation were calculated from the obtained data of the cross-sectional areas of 250 points, to determine an average cross-sectional area x and a standard deviation σ of a cross-sectional area, respectively. In addition, a coefficient of variation of the cross-sectional area was calculated from the following formula.

[0083] Among the obtained data of the cross-sectional areas of 250 points, a sum Ai of cross-sectional areas of all data within a range of x ± 2.5σ based on the average cross-sectional area x and the standard deviation σ, and a sum Ao of cross-sectional areas of all data outside the range were calculated. Then, the ratio of Ao to Ai was calculated from the following formula. Ratio of Ao to Ai = Ao / Ai × 100(Roundness)

[0084] Fibers obtained in each of Examples and Comparative Example were embedded in paraffin, and were cut by using a microtome in a direction perpendicular to the fiber axis direction, to prepare a sample for measurement. A cut surface of the sample for measurement was observed using a digital microscope ("VHX-5000" available from KEYENCE CORPORATION), a maximum diameter and a minimum diameter of a fiber cross-section were measured using an automatic area measurement function, and a roundness was calculated from the following formula. In order to exclude inclined fibers, were selected 50 fibers that have fiber cross-sections exhibiting high uniformity in shadow (contour visibility) and that independently exist, and an average value of the roundness calculated from such fiber cross-sections was used as the measured value. Roundness = minimum diameter / maximum diameter

[0085] In addition, a standard deviation of roundness was calculated from the data of the roundness of the 50 fibers, and a coefficient of variation of the roundness was calculated from the following formula. (Average fiber diameter)

[0086] With respect to the maximum diameters of the fiber cross-sections measured during measurement of roundness in the above, an average fiber diameter (µm) was defined as an average value of the maximum diameters of the 50 fibers selected in the above.(Average fiber length)

[0087] Fiber lengths of 50 fibers randomly selected were measured, and an average fiber length was defined as an average value of the measured values.(Aspect ratio)

[0088] An aspect ratio was calculated from the average fiber diameter and the average fiber length obtained through the above measurement, using the following formula. Aspect ratio = average fiber length / average fiber diameter(Crystallization peak)

[0089] As a measurement sample, 3 to 8 mg of fibers obtained in each of Examples and Comparative Example were weighed, and measurement was performed on the measurement sample using the differential scanning calorimeter ("DSC-60" available from Shimadzu Corporation) under a nitrogen atmosphere at a heating rate of 10°C / min from a room temperature to 300°C. The obtained DSC curve was read for the presence or absence of a crystallization peak attributed to the polyester resin and caused by crystallization during heating.(Dispersibility)

[0090] Fibers obtained in each of Examples and Comparative Example were dispersed in water to prepare a slurry having a concentration of 0.1%, and the slurry was stirred and subjected to papermaking. The obtained paper was visually checked and evaluated in accordance with the following criteria. A: Single fibers are uniformly dispersed B: Converged fibers (fibers bundled together) or entangled fibers (fibers entangled and not dispersed) exist as undispersed fibers (Arithmetic mean roughness Ra)

[0091] Using a portable surface roughness tester ("Surf Test SJ-410" available from Mitutoyo Corporation), an arithmetic mean roughness Ra of each of both surfaces of a wet-laid nonwoven fabric obtained in each of Examples and Comparative Example were measured with reference to ISO1997. For the measurement, a stylus having a tip shape with a conical taper angle of 60° and a radius of curvature at the tip of 2 µm was used. Five measurements were performed at different points on each surface with a sampling length of 0.8 mm, and an average value of the measured values was calculated.[Example 1]

[0092] Pellets of polyethylene terephthalate having an intrinsic viscosity of 0.65 dL / g were fed to a twin-screw extruder and were melt-knead at 300°C, and the molten resin was discharged at a discharge rate of 20 g / min through a spinneret having 150 holes (hole diameter: 0.12 mm) and was taken up at a speed (spinning speed) of 1000 m / min to produce undrawn polyester fibers having an average fiber diameter of 11 µm. The glass transition temperature of the undrawn polyester fibers was 76.3°C. The undrawn polyester fibers were aligned to form a fiber tow, a drawing tension of the fiber tow was adjusted to 0.006 cN / dtex, flow drawing was performed at a draw ratio of 3.0 times in a hot water bath at 99°C, and then neck drawing was sequentially performed at a draw ratio of 3.0 times in a hot water bath at 70°C. An oiling agent was applied to the fiber tow, and then the fiber tow was squeezed such that the fiber tow had a moisture content of about 20 wt%, and was cut into 3 mm in length using a cutter, to obtain a polyester fiber. Table 1 shows the evaluation results of the physical properties of the obtained polyester fibers.

[0093] The obtained polyester fibers at 60% by mass and binder fibers ("N720H" produced by Kuraray Co., Ltd., single fiber fineness: 2.2 dtex, fiber length: 5 mm) at 40% by mass were put into a water bath of a pulp disintegrator (available from Tester Sangyo Co., Ltd.). The fibers were dispersed in water at 3000 rpm for 6000 cycles, and then were subjected to papermaking using a TAPPI papermaking machine (available from Kumagai Riki Kogyo Co., Ltd.) so as to obtain a web having a basis weight of 10 g / m 2< . Then, the obtained web was pressed under a pressure of 3.0kg / cm 2< for 30 seconds using a press machine (available from Kumagai Riki Kogyo Co., Ltd.) to adjust the moisture content thereof, and then was dried at 120°C for 1 minute using a rotary dryer (available from Kumagai Riki Kogyo Co., Ltd.), to obtain a paper-like wet-laid nonwoven fabric. Table 1 shows the evaluation results of the physical properties of the obtained wet-laid nonwoven fabric.[Example 2]

[0094] Polyester fibers were obtained in the same manner as in Example 1, except that as drawing conditions, the drawing tension of the fiber tow was adjusted to 0.005 cN / dtex, flow drawing was performed at the draw ratio of 2.5 times in the hot water bath at 99°C, and then neck drawing was sequentially performed at the draw ratio of 3.0 times in the hot water bath at 70°C. Table 1 shows the evaluation results of the physical properties of the obtained polyester fibers. In addition, a paper-like wet-laid nonwoven fabric was obtained using the obtained polyester fibers in the same manner as in Example 1. Table 1 shows the evaluation results of the physical properties of the obtained wet-laid nonwoven fabric.[Example 3]

[0095] Polyester fibers were obtained in the same manner as in Example 1, except that as drawing conditions, the drawing tension of the fiber tow was adjusted to 0.006 cN / dtex, flow drawing was performed at the draw ratio of 4.0 times in the hot water bath at 99°C, and then neck drawing was sequentially performed at the draw ratio of 3.0 times in the hot water bath at 70°C. Table 1 shows the evaluation results of the physical properties of the obtained polyester fibers. In addition, a paper-like wet-laid nonwoven fabric was obtained using the obtained polyester fibers in the same manner as in Example 1. Table 1 shows the evaluation results of the physical properties of the obtained wet-laid nonwoven fabric.[Comparative Example 1]

[0096] Polyester fibers were obtained in the same manner as in Example 1, except that as drawing conditions, the drawing tension of the fiber tow was adjusted to 0.15 cN / dtex, flow drawing was performed at the draw ratio of 3.0 times in the hot water bath at 97°C, and then neck drawing was sequentially performed at the draw ratio of 3.0 times in the hot water bath at 70°C. Table 1 shows the evaluation results of the physical properties of the obtained polyester fibers. In addition, a paper-like wet-laid nonwoven fabric was obtained using the obtained polyester fibers in the same manner as in Example 1. Table 1 shows the evaluation results of the physical properties of the obtained wet-laid nonwoven fabric. [Table 1]Polyester fiber production conditions (drawing conditions)Drawing tension (cN / dtex)Flow drawingNeck drawingTemperature (°C)Draw ratio (times)Temperature (°C)Draw ratio (times)Ex. 10.006993.0703.0Ex. 20.005992.5703.0Ex. 30.006994.0703.0Comp. Ex. 10.15973.0703.0 [Table 1-continued] Polyester fiberCross-sectional areaAverage fiber diameter (µm)RoundnessCoefficient of variation of roundness (%)Average cross-sectional area (µm 2< )Standard deviation (µm 2< )Coefficient of variation (%)Ao / Ai (%)Ex. 117.27.69944.85.025.820.913.25Ex. 225.88.44332.75.185.460.903.17Ex. 317.46.11335.25.914.480.905.57Comp. Ex. 130.711.34937.010.296.020.876.32 [Table 1-continued] Polyester fiberWet-laid nonwoven fabricAverage fiber length (mm)Aspect ratioCrystallization peakDispersibilityArithmetic mean roughness Ra (µm)Ex. 13.0510.3NoneA8.00Ex. 23.0544.0NoneA8.13Ex. 33.0662.9NoneA7.63Comp. Ex. 13.0502.5NoneB9.68

[0097] As shown in Table 1, in Examples 1 to 3, the drawing tension was adjusted to be low, and flow drawing and neck drawing were performed, so that, while the fibers can be made significantly finer, as well as formation of fewer abnormal thick single fibers, which are outside the range of x ± 2.5σ, can be achieved, and the fiber cross-section can be made closer to a perfect circle. Such polyester fibers have excellent dispersibility, and the wet-laid nonwoven fabric (paper) produced using these polyester fibers has excellent surface smoothness.

[0098] On the other hand, in Comparative Example 1, flow drawing and neck drawing were performed at the same draw ratios as those of Example 1, but the drawing was performed in a state where the drawing tension was higher than those of Examples 1 to 3. Thus, single fibers having abnormal large cross-sectional areas, with respect to the average cross-sectional area, coexist, and the roundness is low. Although the aspect ratio of the obtained polyester fibers was lower compared to those of Examples 1 to 3, the dispersibility was poor, and the wet-laid nonwoven fabric (paper) produced using such polyester fibers exhibited inferior surface smoothness compared to those of Examples 1 to 3.INDUSTRIAL APPLICABILITY

[0099] The polyester fiber according to the present invention can be used for various fabrics such as a nonwoven fabric, a woven fabric, and a knitted fabric (particularly, a wet-laid nonwoven fabric). For example, the wet-laid nonwoven fabric including the polyester fiber according to the present invention can be used as various industrial materials such as a filter, a separator, a support for a reverse osmosis membrane, a wiping material, and other various functional papers.

[0100] Although the present invention has been described above in connection with the preferred embodiment thereof, numerous additions, changes, and deletions can be made without departing from the gist of the present invention. Therefore, such additions, changes, and deletions are also construed as included within the scope of the present invention.

Claims

1. A polyester fiber having an average cross-sectional area x of 1 to 40 µm2, a standard deviation σ of a cross-sectional area of 10 µm2 or less, and a ratio Ao / Ai of a sum Ao of cross-sectional areas of data outside a range of x ± 2.5σ to a sum Ai of cross-sectional areas of data within the range of x ± 2.5σ of 10% or less, wherein the average cross-sectional area x, the standard deviation σ of the cross-sectional area, and the ratio Ao / Ai are calculated from data of cross-sectional areas of single fibers.

2. The polyester fiber according to claim 1, having a coefficient of variation of the cross-sectional area of 50% or less, as calculated from the data of the cross-sectional areas of the single fibers.

3. The polyester fiber according to claim 1, having an average fiber diameter of the single fibers of 1 to 10 µm, and a roundness of a fiber cross-section of 0.90 to 1.00.

4. The polyester fiber according to claim 3, having a coefficient of variation of the roundness of the fiber cross-section of 7% or less.

5. A polyester fiber having an average fiber diameter of single fibers of 1 to 10 µm, and a roundness of a fiber cross-section of 0.90 to 1.00.

6. The polyester fiber according to claim 5, having a coefficient of variation of the roundness of the fiber cross-section of 7% or less.

7. The polyester fiber according to any one of claims 1 to 6, having an average fiber length of 0.1 to 110 mm.

8. The polyester fiber according to any one of claims 1 to 6, having an aspect ratio of 100 or higher.

9. A fiber structure comprising the polyester fiber as recited in any one of claims 1 to 6.

10. The fiber structure according to claim 9, wherein the fiber structure is a wet-laid nonwoven fabric.

11. A method for producing the polyester fiber as recited in any one of claims 1 to 6, the method comprising: a spinning step of melt-spinning a resin material containing a polyester-based resin to obtain a fiber; and a drawing step of drawing the fiber obtained in the spinning step, wherein in the drawing step, flow drawing and subsequent neck drawing are performed, and a drawing tension is 0.01 cN / dtex or less at least in the flow drawing.

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