Liquid crystal polyester fiber

Abstract

Provided is a liquid crystal polyester fiber having excellent fatigue resistance against tensile deformation and compressive deformation. The liquid crystal polyester fiber has a ketone bond amount of 0.050 mol% or less, a tensile strength of 18 cN / dtex or more, and a single fiber compressive strength of 0.55 cN / dtex or more. For example, the liquid crystal polyester fiber may have a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg and 85.0 mEq / kg or less. Further, the liquid crystal polyester fiber may include a liquid crystal polyester having a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more based on a total content of all structural units.

Description

CROSS REFERENCE TO THE RELATED APPLICATION

[0001] This application is based on and claims Convention priority to Japanese patent application No. 2023-061332, filed April 5, 2023, the entire disclosure of which is herein incorporated by reference as a part of this application.FIELD OF THE INVENTION

[0002] The present invention relates to a liquid crystal polyester fiber.BACKGROUND OF THE INVENTION

[0003] A liquid crystal polyester fiber is a fiber made from a liquid crystal polyester having a rigid molecular structure, and it has been known that the liquid crystal polyester fiber which is produced by subjecting an as-spun fiber obtained by melt-spinning to heat treatment can exhibit the highest strength among synthetic fibers obtained by melt-spinning. The liquid crystal polyester fiber is used for applications such as industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing, utilizing such a characteristic of having high strength and a characteristic of being lightweight, which is a characteristic of synthetic fibers. Liquid crystal polyester fibers with improved durability have been developed for these various applications.

[0004] For example, Patent Document 1 (JP Laid-open Patent Publication No. 2004-107826) discloses a polyarylate fiber including 0.03 to 5.0% by mass of inorganic fine particles having a Mohs hardness of 4 or less, mainly including silicic acid and magnesium and having an average particle diameter of 0.01 to 15 µm attached to a fiber surface, and describes that the polyarylate fiber has excellent flexural fatigue resistance and abrasion resistance.

[0005] Patent Document 2 (WO2020 / 166316) discloses a liquid crystal polyester multifilament having a fiber compressive yield stress of 15 to 40 mN / dtex, and describes that high flexural fatigue resistance can be exhibited when the liquid crystal polyester multifilament is used in high-order processed products such as ropes, slings, and tension members.

[0006] Patent Document 3 (WO2022 / 113802) discloses a liquid crystal polyester fiber including at least one metallic element selected from the group consisting of metallic elements belonging to from Groups 8 to 11 in Periodic Table, and describes that the liquid crystal polyester fiber is excellent in heat aging resistance.CONVENTIONAL ART DOCUMENTPATENT DOCUMENT

[0007] [Patent Document 1] JP Laid-open Patent Publication No. 2004-107826 [Patent Document 2] WO2020 / 166316 [Patent Document 3] WO2022 / 113802 SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] For example, in applications as reinforcing materials used for rubber materials such as automotive oil brake hoses and tires, electronic components such as board materials for circuit boards, durability against stretching and vibration is required in addition to mechanical properties. However, while a liquid crystal polyester fiber is strong against a stress in a tensile direction, the liquid crystal polyester fiber is relatively weak against a stress in a compressive direction, and when the liquid crystal polyester fiber is used as a reinforcing fiber in such applications, forces are applied not only in the tensile direction but also in the compressive direction of the fiber axis, so that fatigue resistance is required for both stresses.

[0009] In Patent Document 1, abrasion resistance and flexural fatigue resistance are improved by attaching the inorganic fine particles to the fiber surface, but fatigue resistance against both stresses in the tensile direction and the compressive direction is not described. Even if the fiber surface is covered with the inorganic fine particles in order to reduce damage to the fiber, when the fiber is used as a reinforcing fiber, stresses in the tensile direction and the compressive direction are not distributed but are transmitted to the reinforcing fiber in a composite material, so that this does not lead to improved fatigue resistance against both stresses.

[0010] In Patent Document 2, fatigue resistance against repeated bending is improved by enhancing the compressive strength in the diameter direction of the fiber and the flexibility of the fiber, but fatigue resistance against both stresses in the tensile direction and the compressive direction is not described. Even if the flexibility of the fiber is enhanced, when the fiber is used as a reinforcing fiber, the reinforcing fiber fixed to a matrix by adhesion in a composite material cannot sufficiently distribute the stress during repeated bending in a yarn, so that this does not lead to improved fatigue resistance against both stresses in the tensile direction and the compressive direction.

[0011] In Patent Document 3, by containing a metal catalyst in the liquid crystal polyester fiber, tensile strength is improved through heat treatment at a lower temperature for a shorter time, and the heat aging resistance of a heat-treated fiber is improved, but compressive strength is not described. Even if tensile strength can be improved by the metal catalyst, it is not necessarily possible to sufficiently improve compressive strength at the same time. In addition, in Patent Document 3, fatigue resistance against both stresses in the tensile direction and the compressive direction is not described.

[0012] An object of the present invention is to solve the above problems and to provide a liquid crystal polyester fiber having excellent fatigue resistance against tensile deformation and compressive deformation.MEANS FOR SOLVING THE PROBLEMS

[0013] The inventors of the present invention have conducted extensive studies in order to achieve the aforementioned object, and found that a liquid crystal polyester fiber having high tensile strength and compressive strength in specific ranges and having a low ketone bond amount in a specific range has excellent fatigue resistance in a disc fatigue test in which tensile deformation and compressive deformation are alternately applied (hereinafter sometimes referred to as "disc fatigue resistance"), leading to the completion of the present invention.

[0014] That is, the present invention may include the following aspects.[Aspect 1]

[0015] A liquid crystal polyester fiber having a ketone bond amount of 0.050 mol% or less (preferably 0.010 mol% or less, more preferably 0.005 mol% or less, further preferably 0.003 mol% or less, and even more preferably less than 0.0020 mol%), a tensile strength of 18 cN / dtex or more (preferably 20 cN / dtex or more, more preferably 22 cN / dtex or more, further preferably 25 cN / dtex or more, even more preferably 26 cN / dtex or more, and particularly preferably 28 cN / dtex or more), and a single fiber compressive strength of 0.55 cN / dtex or more (preferably 0.60 cN / dtex or more, more preferably 0.65 cN / dtex or more, further preferably 0.66 cN / dtex or more, even more preferably 0.70 cN / dtex or more, and particularly preferably 0.80 cN / dtex or more).[Aspect 2]

[0016] The liquid crystal polyester fiber according to aspect 1, having a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg (preferably 6.0 mEq / kg or more, more preferably 8.0 mEq / kg or more, further preferably 10.0 mEq / kg or more, and even more preferably 15.0 mEq / kg or more) and 85.0 mEq / kg or less (preferably 80.0 mEq / kg or less, more preferably 75.0 mEq / kg or less, further preferably 65 mEq / kg or less, and even more preferably 60.0 mEq / kg or less).[Aspect 3]

[0017] The liquid crystal polyester fiber according to aspect 1 or 2, including a liquid crystal polyester including a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more (preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more) based on a total content of all structural units.[Aspect 4]

[0018] A fiber structure at least partially including the liquid crystal polyester fiber as recited in any one of aspects 1 to 3.[Aspect 5]

[0019] A composite material including the liquid crystal polyester fiber as recited in any one of aspects 1 to 3 as a reinforcing fiber.

[0020] 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.

[0021] 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

[0022] The liquid crystal polyester fiber according to the present invention has excellent disc fatigue resistance.DESCRIPTION OF EMBODIMENT[Liquid Crystal Polyester Fiber]

[0023] The liquid crystal polyester fiber includes a liquid crystal polyester. The liquid crystal polyester includes structural units derived from, for example, aromatic diols, aromatic dicarboxylic acids, aromatic hydroxycarboxylic acids, etc. As long as the effect of the present invention is not impaired, the structural units derived from aromatic diols, aromatic dicarboxylic acids, and aromatic hydroxycarboxylic acids are not limited to a specific chemical composition. The liquid crystal polyester may include the structural units derived from aromatic diamines, aromatic hydroxy amines, or aromatic aminocarboxylic acids in the range which does not impair the effect of the present invention. For example, preferable structural units may include units shown in Table 1. [Table 1] In the formula, X is selected from the following m is an integer from 0 to 2, Y is a substituent selected from hydrogen atom, halogen atoms, alkyl groups, aryl groups, aralkyl groups, alkoxy groups, aryloxy groups, aralkyloxy groups.

[0024] In the structural units in Table 1, m is an integer from 0 to 2, and Y in the formula independently represents, as from one substituent to the number of substituents in the range of the replaceable maximum number of aromatic ring, a hydrogen atom, a halogen atom (for example, fluorine atom, chlorine atom, bromine atom and iodine atom), an alkyl group (for example, an alkyl group having 1 to 4 carbon atoms such as methyl group, ethyl group, isopropyl group and t-butyl group), an alkoxy group (for example, methoxy group, ethoxy group, isopropoxy group, n-butoxy group, etc.), an aryl group (for example, phenyl group, naphthyl group, etc.), an aralkyl group [for example, benzyl group (phenylmethyl group), phenethyl group (phenylethyl group), etc.], an aryloxy group (for example, phenoxy group, etc.), an aralkyloxy group (for example, benzyloxy group, etc.), and others.

[0025] As more preferable structural units, there may be mentioned structural units as described in Examples (1) to (20) shown in the following Tables 2, 3, and 4. It should be noted that where the structural unit in the formula is a structural unit which can show a plurality of structures, combination of two or more types may be used as structural units for a polymer.

[0026] In the structural units shown in Tables 2, 3, and 4, n is an integer of 1 or 2, among each of the structural units, n = 1 and n = 2 may independently exist, or may exist in combination; each of the Y 1 and Y 2 independently represents, a hydrogen atom, a halogen atom (for example, fluorine atom, chlorine atom, bromine atom, iodine atom, etc.), an alkyl group (for example, an alkyl group having 1 to 4 carbon atoms such as methyl group, ethyl group, isopropyl group, and t-butyl group, etc.), an alkoxy group (for example, methoxy group, ethoxy group, isopropoxy group, n-butoxy group, etc.), an aryl group (for example, phenyl group, naphthyl group, etc.), an aralkyl group [for example, benzyl group (phenylmethyl group), phenethyl group (phenylethyl group), etc.], an aryloxy group (for example, phenoxy group, etc.), an aralkyloxy group (for example, benzyloxy group, etc.), and others. Among these, the preferable one may include a hydrogen atom, a chlorine atom, a bromine atom, and a methyl group.

[0027] Z may include substitutional groups denoted by following formulae.

[0028] In one embodiment, the liquid crystal polyester may include a structural unit derived from a hydroxycarboxylic acid as a main component. The liquid crystal polyester may preferably include a structural unit (A) derived from hydroxybenzoic acid and a structural unit (B) derived from hydroxy naphthoic acid. For example, the structural unit (A) may have a structural unit derived from 4-hydroxybenzoic acid (the following formula (A)), and the structural unit (B) may have a structural unit derived from 6-hydroxy-2-naphthoic acid (the following formula (B)). In order to improve melt-formability, the ratio of the structural unit (A) and the structural unit (B) may preferably be in a range of former / latter of 9 / 1 to 1 / 1, more preferably from 7 / 1 to 1 / 1, and still more preferably from 5 / 1 to 1 / 1.

[0029] The liquid crystal polyester may include the structural unit derived from 4-hydroxybenzoic acid. In the case where the liquid crystal polyester includes both the structural unit (A) and the structural unit (B), a content of the structural unit derived from 4-hydroxybenzoic acid based on a total content of all the structural units may be 50 mol% or more, preferably 53 mol% or more, more preferably 60 mol% or more, further preferably 65 mol% or more, and even more preferably 70 mol% or more. The upper limit of the content of the structural unit derived from 4-hydroxybenzoic acid in the liquid crystal polyester is not particularly limited to a specific value, and may be, for example, 90 mol% or less, preferably 88 mol% or less, and more preferably 85 mol% or less.

[0030] The liquid crystal polyester may include the structural unit derived from 6-hydroxy-2-naphthoic acid. In the case where the liquid crystal polyester includes both the structural unit (A) and the structural unit (B), a content of the structural unit derived from 6-hydroxy-2-naphthoic acid based on the total content of all the structural units may be 4 to 45 mol%.

[0031] Furthermore, a total content of the structural unit (A) and the structural unit (B) based on the total content of all the structural units may be, for example, 65 mol% or more, more preferably 70 mol% or more, and further preferably 80 mol% or more.

[0032] In another embodiment, the liquid crystal polyester may include a structural unit represented by the following formula (I) (structural unit (I)), a structural unit represented by the following formula (II) (structural unit (II)), and at least one structural unit selected from the group consisting of a structural unit represented by the following formula (III) (structural unit (III)) and a structural unit represented by the following formula (IV) (structural unit (IV)).         -O-Ar 1< -CO-     (I)         -CO-Ar 2< -CO-     (II)         -O-Ar 3< -O-     (III)         -O-Ar 4< -NH-     (IV) (wherein Ar 1< represents at least one group selected from the group consisting of a phenylene group, a naphthylene group, and a biphenylylene group, each of Ar 2< , Ar 3< , and Ar 4< independently represents at least one group selected from the group consisting of a phenylene group, a naphthylene group, a biphenylylene group, and a diphenyl ether diyl group, and hydrogen atoms in each aromatic ring in Ar 1< , Ar 2< , Ar 3< , and Ar 4< may be independently substituted with a halogen atom, an alkyl group, an alkoxy group, an aryl group, an aralkyl group, an aryloxy group, or an aralkyloxy group)

[0033] The structural unit (I) is a structural unit derived from an aromatic hydroxycarboxylic acid, and may be preferably a structural unit in which Ar 1< is a 1,4-phenylene group (structural unit derived from 4-hydroxybenzoic acid) and a structural unit in which Ar 1< is a 2,6-naphthylene group (structural unit derived from 6-hydroxy-2-naphthoic acid).

[0034] The structural unit (II) is a structural unit derived from an aromatic dicarboxylic acid, and may be preferably a structural unit in which Ar 2< is a 1,4-phenylene group (structural unit derived from terephthalic acid), a structural unit in which Ar 2< is a 1,3-phenylene group (structural unit derived from isophthalic acid), a structural unit in which Ar 2< is a 2,6-naphthylene group (structural unit derived from 2,6-naphthalenedicarboxylic acid), and a structural unit in which Ar 2< is a diphenyl ether-4,4'-diyl group (structural unit derived from diphenyl ether-4,4'-dicarboxylic acid).

[0035] The structural unit (III) is a structural unit derived from an aromatic diol, and may be preferably a structural unit in which Ar 3< is a 1,4-phenylene group (structural unit derived from hydroquinone), a structural unit in which Ar 3< is a 4,4'-biphenylylene group (structural unit derived from 4,4'-dihydroxybiphenyl), a structural unit in which Ar 3< is a phenyl-1,4-phenylene group (structural unit derived from phenylhydroquinone), and a structural unit in which Ar 3< is a diphenyl ether-4,4'-diyl group (structural unit derived from 4,4'-dihydroxydiphenyl ether).

[0036] The structural unit (IV) is a structural unit derived from an aromatic hydroxy amine, and may be preferably a structural unit in which Ar 4< is a 1,4-phenylene group (structural unit derived from 4-aminophenol) and a structural unit in which Ar 4< is a 4,4'-biphenylylene group (structural unit derived from 4-amino-4'-hydroxybiphenyl).

[0037] A content of the structural unit (I) in the liquid crystal polyester based on a total content of all the structural units may be 20 to 80 mol%, preferably 30 to 75 mol%, and more preferably 40 to 70 mol%.

[0038] A content of the structural unit (II) in the liquid crystal polyester based on a total content of all the structural units may be 10 to 40 mol%, preferably 12.5 to 35 mol%, and more preferably 15 to 30 mol%.

[0039] A total content of the structural units (III) and (IV) in the liquid crystal polyester based on a total content of all the structural units may be 10 to 40 mol%, preferably 12.5 to 35 mol%, and more preferably 15 to 30 mol%.

[0040] A molar ratio of the content of the structural unit (II) to the total content of the structural units (III) and (IV), as (II) / [(III) + (IV)], may be 90 / 100 to 100 / 90, preferably 95 / 100 to 100 / 95, more preferably 98 / 100 to 100 / 98, and further preferably 100 / 100.

[0041] The liquid crystal polyester may include two or more types of each of the structural units (I) to (IV). The content of each structural unit represents the total content of all structural units corresponding to the structural unit, and, for example, in the case where the liquid crystal polyester includes two or more types of the structural unit (I), the content of the structural unit (I) represents the total content of these types.

[0042] In the liquid crystal polyester, the total content of the structural units (I) to (IV) based on the total content of all the structural units may be, for example, 90 mol% or more, preferably 95 mol% or more, more preferably 99 mol% or more, and further preferably 100 mol%.

[0043] From the viewpoint of enhancing disc fatigue resistance of the liquid crystal polyester fiber, the liquid crystal polyester preferably has a combination including a structural unit having a naphthalene skeleton. In the case of including the structural unit having the naphthalene skeleton, it is considered that disc fatigue resistance can be improved, perhaps since molecular chains are densely packed and staggering between the molecular chains is less likely to occur. In addition, from the viewpoint of decreasing the ketone bond amount, the liquid crystal polyester preferably includes the structural unit having the naphthalene skeleton. The inventors of the present invention have found that when a liquid crystal polyester is exposed to a high temperature, side reactions in which ketone bonds are formed from ester bonds in the molecular chains occur, and that the tendency for such side reactions to occur varies depending on the composition of the structural units of the liquid crystal polyester. Further, the inventors of the present invention have found that in the case of including the structural unit having the naphthalene skeleton, there is a tendency to be able to suppress the formation of ketone bonds in the liquid crystal polyester. For example, a total content of a structural unit including a 2,6-naphthylene group in the liquid crystal polyester based on the total content of all the structural units may be 28 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more. Furthermore, from the viewpoint of improving melt-formability, the total content of the structural unit including the 2,6-naphthylene group in the liquid crystal polyester based on the total content of all the structural units may be 95 mol% or less, preferably 90 mol% or less, and more preferably 85 mol% or less. As the structural unit including the 2,6-naphthylene group, a structural unit derived from 6-hydroxy-2-naphthoic acid (the structural unit (B) represented by the above formula (B), a structural unit (I) in which Ar 1< is a 2,6-naphthylene group) and a structural unit derived from 2,6-naphthalenedicarboxylic acid (a structural unit (II) in which Ar 2< is a 2,6-naphthylene group) are preferable.

[0044] The higher the melting point of the liquid crystal polyester is, the higher the heat resistance of the obtained liquid crystal polyester fiber is. Therefore, for example, when the liquid crystal polyester fiber is used as a rubber reinforcing fiber at a high temperature or is used for applications in which heat is generated due to vibration or the like during use, the thermal durability of the liquid crystal polyester fiber can be improved by using the liquid crystal polyester having the higher melting point. For example, the liquid crystal polyester may have a melting point (hereinafter sometimes referred to as Mp 0 ) of 250°C or higher, preferably 260°C or higher, more preferably 280°C or higher, further preferably 290°C or higher, and even more preferably 300°C or higher. On the other hand, from the viewpoint of improving melt spinnability, the liquid crystal polyester may have the melting point of 380°C or lower, preferably 370°C or lower, more preferably 360°C or lower, further preferably 350°C or lower, and even more preferably 340°C or lower. In the present specification, the melting point refers to a main endothermic peak temperature determined and observed using a differential scanning calorimeter (DSC) in accordance with the JIS K 7121 test method. Specifically, 4 to 6 mg of a sample is encapsulated in an aluminum pan and taken into the DSC device. Then, the temperature is elevated at a rate of 10°C / min from room temperature (e.g., 25°C) while supplying nitrogen as a carrier gas at a flow rate of 200 mL / min to measure an endothermic peak. Depending on the type of polymer, some polymers may not show a clear peak in the 1st run of DSC measurement. If no clear peak appears in the 1st run of DSC measurement, the sample is heated up to a temperature 50°C higher than the expected flow temperature in a temperature elevation rate of 50°C / min. After keeping the temperature for 3 minutes so as to make the sample completely molten, the sample is cooled at a cooling rate of 80°C / min to 50°C, and then is elevated at 10°C / min to measure the endothermic peak thereof.

[0045] It should be noted that the liquid crystal polyester fiber may contain thermoplastic polymers, such as a polyethylene terephthalate, a modified-polyethylene terephthalate, a polyolefin, a polycarbonate, a polyamide, a polyphenylene sulfide, a polyether ether ketone, a fluoro-resin, and others, as long as the effects of the present invention are not impaired. In addition, the liquid crystal polyester fiber may contain various additives including: inorganic substances such as titanium oxide, kaolin, silica, and barium oxide; carbon black; a colorant such as dyes and paints; an antioxidant; an ultraviolet-ray absorbent; a light stabilizer; etc.

[0046] As long as the effects of the present invention are not impaired, the liquid crystal polyester fiber may be a mixed spun fiber obtained by mixing and spinning the liquid crystal polyester with the above-described thermoplastic polymer and various additives, or may be a composite spun fiber obtained by simultaneously spinning the liquid crystal polyester and different components of the above-described thermoplastic polymer from separate spinnerets. The liquid crystal polyester fiber may be a non-composite spun fiber or may be a composite spun fiber. In particular, it is preferable that the liquid crystal polyester is present on the fiber surface of the liquid crystal polyester fiber.

[0047] The fiber cross-sectional shape of the liquid crystal polyester fiber is not particularly limited to a specific one as long as the effects of the present invention are not impaired, and may, for example, be a circular cross-sectional shape or a modified cross-sectional shape such as a hollow cross-sectional shape, an elliptical shape, a hexagonal shape, a pentagonal shape, a quadrangular shape, a triangular shape, a star shape, and a flat shape.

[0048] The liquid crystal polyester fiber may contain a liquid crystal polyester at a proportion of 50 wt% or more, preferably 80 wt% or more, more preferably 90 wt% or more, still more preferably 95 wt% or more, and even more preferably 99.9 wt% or more.

[0049] The liquid crystal polyester fiber has a ketone bond amount of 0.050 mol% or less. In the present specification, the ketone bond amount refers to a ratio of a molar amount of ketone bonds based on a total molar amount of ester bonds and ketone bonds (molar amount of ketone bonds / (molar amount of ester bonds + molar amount of ketone bonds)), and is a value measured by the method described in Examples below. Ketone bonds are heterogeneous bonds formed by side reactions from ester bonds during production of the liquid crystal polyester fiber, and disc fatigue resistance can be improved by decreasing the ketone bond amount. Although the mechanism for this is not fully understood, it is considered that where ketone bonds are present instead of ester bonds, perhaps since the molecular chains are distorted, staggering between the molecular chains is likely to occur when tensile stress and compressive stress are applied. Therefore, it is considered that a liquid crystal polyester fiber having a decreased ketone bond amount can suppress distortion in the molecular chains and improve disc fatigue resistance. From the viewpoint of enhancing disc fatigue resistance, the liquid crystal polyester fiber may have the ketone bond amount of preferably 0.010 mol% or less, more preferably 0.005 mol% or less (e.g., 0.0054 mol% or less), further preferably 0.003 mol% or less (e.g., 0.0034 mol% or less), and even more preferably less than 0.0020 mol%. The lower limit value of the ketone bond amount is not particularly limited to a specific value, and may be, for example, 0.0001 mol% or more.

[0050] The liquid crystal polyester fiber has a tensile strength of 18 cN / dtex or more. The tensile strength may be preferably 20 cN / dtex or more, more preferably 22 cN / dtex or more, further preferably 25 cN / dtex or more, even more preferably 26 cN / dtex or more, and particularly preferably 28 cN / dtex or more. The upper limit value of the tensile strength is not particularly limited to a specific value, and may be, for example, about 40 cN / dtex. The tensile strength of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0051] The liquid crystal polyester fiber has a single fiber compressive strength of 0.55 cN / dtex or more. The single fiber compressive strength may be preferably 0.60 cN / dtex or more, more preferably 0.65 cN / dtex or more, further preferably 0.66 cN / dtex or more, even more preferably 0.70 cN / dtex or more, and particularly preferably 0.80 cN / dtex or more. In addition, the upper limit value of the single fiber compressive strength is not particularly limited to a specific value, and may be, for example, about 1.5 cN / dtex. The single fiber compressive strength of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0052] The liquid crystal polyester fiber having the tensile strength and the compressive strength in the above ranges can exhibit sufficient reinforcing performance when used as a reinforcing fiber in a composite material, and high disc fatigue resistance.

[0053] The liquid crystal polyester fiber may have a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg and 85.0 mEq / kg or less. In the present specification, the total CEG amount means the amount of carboxy groups existing at the ends of the liquid crystal polyester molecules in 1 kg of the liquid crystal polyester fiber, and is a value measured by the method described in Examples below. For example, as the carboxy end groups in liquid crystal polyester, carboxy groups that do not participate in reaction and remain in the terminal structural units derived from monomers having carboxy groups, such as aromatic hydroxycarboxylic acids and aromatic dicarboxylic acids, may be exemplified. When carboxy groups are present at the ends of the liquid crystal polyester molecules, perhaps since the carboxy ends interact with each other between the molecules, it is considered that disc fatigue resistance can be further improved. From the viewpoint of further improving disc fatigue resistance, the liquid crystal polyester fiber may have the total CEG amount of preferably 6.0 mEq / kg or more, more preferably 8.0 mEq / kg or more, further preferably 10.0 mEq / kg or more, and even more preferably 15.0 mEq / kg or more. On the other hand, since the molecular weight tends to decrease as the end group amount increases, from the viewpoint of appropriately increasing the molecular weight to enhance strength and from the viewpoint of suppressing gas generation during thermoforming processing for a composite material using the liquid crystal polyester fiber as a reinforcing fiber, the liquid crystal polyester fiber may have the total CEG amount of preferably 80.0 mEq / kg or less, more preferably 75.0 mEq / kg or less, further preferably 65 mEq / kg or less (e.g., 65.0 mEq / kg or less), and even more preferably 60.0 mEq / kg or less.

[0054] The liquid crystal polyester fiber may include a metal catalyst such as a melt polymerization catalyst for synthesizing a raw material liquid crystal polyester and a solid phase polymerization catalyst for an as-spun fiber. From the viewpoint of suppressing a decrease in the single fiber compressive strength, a content of metal elements belonging to Groups 8 to 11 in Periodic Table may be less than 1 ppm by weight. The metal elements belonging to Groups 8 to 11 in Periodic Table act as solid phase polymerization catalysts and can enhance strength of the liquid crystal polyester fiber through a heat treatment at a lower temperature for a shorter time. However, when a raw material including a large amount of these metal elements is melt-spun, perhaps since compounds derived from the metal elements act as obstacles during fiber formation to prevent polymer chains from being highly oriented and to reduce interactions between the polymer chains, the single fiber compressive strength decreases. In addition, perhaps since, due to the presence of compounds derived from the metal elements in the fiber, buckling is likely to occur starting from the portions where the metal elements are present when repeated compressive and elongation stresses are applied, such a fiber tends to be poor in disc fatigue resistance. The content of the metal elements indicates a ratio of the total weight of the above-described metal elements based on the total weight of the liquid crystal polyester fiber, and in the case where the metal elements are contained as metal compounds, indicates a content in terms of metal atoms. Here, the content of the above metal elements is a content of the metal elements in the components constituting the fiber itself, excluding components adhering to the fiber surface such as an oil agent.

[0055] The liquid crystal polyester fiber may have a melting point of 260 to 380°C, preferably 280 to 375°C, more preferably 300 to 370°C, further preferably 320 to 365°C, and even more preferably 335 to 360°C. The melting point of the liquid crystal polyester fiber increases from a melting point (Mp) of an as-spun fiber due to solid phase polymerization. The melting point of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0056] The liquid crystal polyester fiber may have a strength retention rate of 30% or more, preferably 40% or more, more preferably 50% or more, and further preferably 60% or more in the disc fatigue test by the method described in Examples below.

[0057] The liquid crystal polyester fiber may have an adjusted single fiber fineness depending on the application, etc. The single fiber fineness, for example, may be 50 dtex or less, preferably 15 dtex or less, and more preferably 10 dtex or less, and from the viewpoint of adapting to downsizing in electronic component application, etc., the single fiber fineness is preferably a low fineness and may be, for example, 7 dtex or less. On the other hand, the lower limit of the single fiber fineness is not particularly limited to a specific one, and may be, for example, about 0.01 dtex. The single fiber fineness is a value measured by the method described in Examples below.

[0058] The liquid crystal polyester fiber may be a monofilament or a multifilament. In the case of multifilament, the number of filaments may be adjusted depending on the application, etc. For example, the number of filaments may be 2 to 5000 filaments, preferably 3 to 4000 filaments, and more preferably 5 to 3000 filaments.

[0059] The total fineness of the liquid crystal polyester fiber may be adjusted depending on the application, etc. For example, the total fineness may be 50000 dtex or less, preferably 10000 dtex or less, more preferably 2000 dtex or less, and further preferably 600 dtex or less. From the viewpoint of adapting to downsizing in electronic component application, etc., the total fineness is preferably a low fineness and may be, for example, 300 dtex or less. On the other hand, the lower limit of the total fineness is not particularly limited to a specific one, and may be, for example, about 1 dtex.[Method for Producing Liquid Crystal Polyester Fiber]

[0060] A method for producing the liquid crystal polyester fiber is not particularly limited to a specific one as long as each of the ketone bond amount, the tensile strength, and the compressive strength of the liquid crystal polyester fiber can be adjusted to the above specific range, and may include at least melt-spinning a liquid crystal polyester to obtain an as-spun fiber and heat-treating the obtained as-spun fiber.

[0061] The inventors of the present invention have found that when a liquid crystal polyester is exposed to a high temperature, side reactions in which ketone bonds are formed from ester bonds in the molecular chains occur, and that the tendency for such side reactions to occur varies depending on the composition of the structural units of the liquid crystal polyester. As the liquid crystal polyester to be subjected to melt-spinning, a liquid crystal polyester having the above-described structural units may be used. For example, the liquid crystal polyester preferably has a combination including the structural unit having the naphthalene skeleton, and such a liquid crystal polyester is less likely to undergo side reactions in which ester bonds are converted into ketone bonds, possibly due to steric hindrance, etc., caused by a bulky naphthalene ring being bonded to the ester bonds. Further, the liquid crystal polyester that is less likely to undergo such side reactions may be more preferably a liquid crystal polyester including a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more based on a total content of all structural units. Furthermore, the total content of the structural unit including the 2,6-naphthylene group based on the total content of all the structural units may be preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more.

[0062] Furthermore, the liquid crystal polyester to be subjected to melt-spinning may be a liquid crystal polyester including: the above structural unit (I); the above structural unit (II); and at least one structural unit selected from the group consisting of the above structural unit (III) and the above structural unit (IV).

[0063] The liquid crystal polyester can be synthesized by a known polycondensation method. Examples of monomers for polycondensation may include various aromatic hydroxycarboxylic acids, aromatic dicarboxylic acids, aromatic diols, and aromatic hydroxy amines, and hydroxy group acylates and carboxylic acid derivatives such as carboxyl group esters, acid halides, and acid anhydrides by activating the end thereof.

[0064] Polycondensation may be performed in the presence of various polymerization catalysts, such as organotin-based catalysts (dialkyl tin oxide, etc.), antimony-based catalysts (antimony trioxide, etc.), titanium-based catalysts (titanium dioxide, etc.), alkali metal salts or alkaline earth metal salts of carboxylic acids (potassium acetate, etc.), and Lewis acids (BF 3 , etc.).

[0065] The above-described thermoplastic polymers and various additives may be added to the liquid crystal polyester, as long as the effects of the present invention are not impaired.

[0066] When melting-spinning the liquid crystal polyester and when heat-treating the obtained as-spun fiber, side reactions occur by heating at a high temperature. Therefore, to suppress the formation of ketone bonds, it is effective to decrease the spinning head temperature and the heat-treatment temperature. On the other hand, to improve tensile strength and compressive strength, it is preferable to take into consideration orienting the molecules through melt-spinning and allowing solid phase polymerization to sufficiently proceed through heat treatment. In the present invention, by setting the spinning conditions and the heat-treating conditions depending on the melting point of the liquid crystal polyester that is the raw material resin and the composition of the structural units, it is possible to obtain a liquid crystal polyester fiber in which each of the ketone bond amount, the tensile strength, and the compressive strength are adjusted to the above specific range.

[0067] In the spinning step, the liquid crystal polyester is fed into an extruder and is melt-kneaded in the extruder. Then, the melt-kneaded material of the liquid crystal polyester is conveyed to a spinning head and discharged through a nozzle. The obtained yarn is wound, whereby an as-spun fiber can be obtained. During winding, an oil agent may be applied from the viewpoint of preventing static electricity.

[0068] As the extruder, known extruders such as single-screw extruders and multi-screw extruders (twin- or more-screw extruders) can be used, and twin-screw extruders are preferable from the viewpoint of improving kneadability and degassability.

[0069] In the spinning step, from the viewpoint of suppressing an increase in the ketone bond amount, where the melting point of the liquid crystal polyester that is the raw material resin is denoted by Mp 0 , a spinning head temperature may be from Mp 0 to Mp 0 + 60°C, preferably from Mp 0 to Mp 0 + 50°C, more preferably from Mp 0 + 5°C to Mp 0 + 45°C, and further preferably from Mp 0 + 10°C to Mp 0 + 40°C. In the present specification, the spinning head temperature means the maximum temperature in the spinning head.

[0070] Melt-spinning can be performed by a known or common method, and the melt-kneaded material can be discharged from the nozzle of the spinning head, and wound by a godet roller or the like to obtain an as-spun fiber.

[0071] By performing heat treatment on the as-spun fiber, solid phase polymerization of the liquid crystal polyester can be advanced so as to improve tensile strength and compressive strength. In the heat-treating step, the method of the heat treatment is not particularly limited to a specific one, and may be, for example, a batch-type heat treatment or a continuous heat treatment by conveyance.

[0072] For example, the batch-type heat treatment may be carried out in a state where the as-spun fiber is wound onto a bobbin in the form of a package, or in a state of hank as well as tow. The heat treatment may be preferably carried out in a package because it can be carried out in simpler equipment and improved productivity. The bobbin needs to be endurable to a temperature of solid phase polymerization, and may be preferably from a metal, such as aluminum, brass, iron, and stainless steel.

[0073] In the case of the continuous heat treatment by conveyance, the conveyance method may be carried out by either contact conveyance (for example, a conveyor type, a support roll type, a heated roller type), or non-contact conveyance (a roll-to-roll type). The processing course may be linear or nonlinear, and may be arranged using a folding roller and / or a guide to suitably change a length, an angle, a curvature of processing course, etc.

[0074] For the heat-treating step, a publicly known method can be used, and examples thereof include atmosphere heating, contact heating, and other heating procedure. Preferable atmosphere may include an atmosphere such as air, inactive gas (for example, nitrogen, argon), or a combined air thereof. In addition, there is no problem even if the heat treatment is carried out under vacuum.

[0075] In the heat-treating step, it is preferable to perform heat treatment at as high a temperature as possible in order to accelerate solid phase polymerization reaction and improve tensile strength and compressive strength. On the other hand, from the viewpoint of suppressing an increase in the ketone bond amount, it is preferable to perform heat treatment at a lower temperature to inhibit occurrence of side reactions. For example, where the melting point of the liquid crystal polyester that is the raw material resin is denoted by Mp 0 , a heat treatment temperature may be preferably from Mp 0 - 30°C to Mp 0 + 30°C, more preferably from Mp 0 - 25°C to Mp 0 + 25°C, and further preferably from Mp 0 - 20°C to Mp 0 + 20°C. In the heat-treating step, since the melting point of the liquid crystal polyester fiber increases with progress of solid phase polymerization, from the viewpoint of efficient strength improvement, the heat treatment temperature may be step-wisely raised in accordance with progress of solid phase polymerization, so that the heat treatment may be carried out at a temperature beyond the melting point (melting point of as-spun fiber) at the time of starting the heat-treating step.

[0076] Depending on the heat treatment procedure and / or the heat treatment temperature, a heat treatment period of the heat-treating step may be set as appropriate. From the viewpoint of enhancing tensile strength and compressive strength and suppressing an increase in the ketone bond amount, the heat treatment period may be preferably from 15 minutes to 30 hours, more preferably from 4 to 25 hours, and further preferably from 8 to 20 hours. Here, the heat treatment period refers to a retention time at a predetermined heat treatment temperature.

[0077] In the method for producing the liquid crystal polyester fiber, for example, in order to improve bundling properties of fibers and to prevent fibers from fusing during the heat treatment, an oil agent may be applied before the heat-treating step. In addition, after the heat treatment, a finishing oil agent may be applied as appropriate depending on the application of the liquid crystal polyester fiber.[Fiber Structure]

[0078] The liquid crystal polyester fiber according to the present invention can be used as a reinforcing fiber for producing a composite material. In the case of using the liquid crystal polyester fiber as a reinforcing fiber, a fiber structure at least partially including the liquid crystal polyester fiber can be used as an intermediate material in the production of the composite material.

[0079] The fiber structure including the liquid crystal polyester fiber according to the present invention can be used as various fiber configurations such as staple fibers, short-cut fibers, filament yarns, spun yarns, cordage, ropes, etc., and also used as various fabrics such as nonwoven fabrics, woven fabrics, and knitted fabrics, using the liquid crystal polyester fibers. Such fibers and fabrics can be produced by known methods using the liquid crystal polyester fibers.

[0080] The fiber structure according to the present invention may be made by combining the liquid crystal polyester fibers with other fibers as long as the effects of the present invention are not impaired. The fiber structure may be, for example, a combined yarn using the liquid crystal polyester fibers and other fibers (e.g., a commingled yarn made from the liquid crystal polyester fibers and other fibers, or others). The fiber structure may also be a blend fabric using the liquid crystal polyester fibers and other fibers (e.g., a combined fabric in which the liquid crystal polyester fibers and other fibers are used in combination, a layered material in which a fabric of the liquid crystal polyester fibers and a fabric of other fibers are used in combination, or others). In the case of using the fiber structure to produce a composite material, the fiber structure may be a combined yarn or blend fabric, including heat-fusible fibers which form the matrix of the composite material as other fibers.

[0081] The liquid crystal polyester fiber according to the present invention can be used in various forms of fiber structures for various applications such as general industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing. For example, the liquid crystal polyester fiber according to the present invention can be used as various fiber products including tension members (e.g., electric cables, optical fibers, etc.), heater wire core yarns, cords for various electrical products such as earphone cords, ropes, sling belts, climbing ropes, life lines, fishing lines, fishing nets, longlines, land nets (safety nets, nets for golf practice ranges, etc.), catheters, reinforcing materials for plastics, concrete, and rubber, base cloth for printed circuit boards, sailcloth, protective clothing, and protective gloves. In particular, the liquid crystal polyester fiber having a low fineness (e.g., a single fiber fineness of 7 dtex or less) can be used for electronic component application such as base cloth for printed circuit boards.[Composite Material]

[0082] According to the present invention, the composite material may be any material which can be obtained by using the liquid crystal polyester fiber as a reinforcing fiber and molding a matrix. The liquid crystal polyester fiber can be molded using the above fiber structure.

[0083] As the matrix, a resin generally used for composite materials can be used, and a thermosetting resin or a thermoplastic resin can be used. Examples of the thermosetting resin include epoxy-based resins, unsaturated polyester-based resins, vinyl ester-based resins, bismaleimide-based resins, phenolic-based resins, urea-based resins, melamine-based resins, thermosetting polyimide-based resins, thermosetting polyurethane-based resins, and benzoxazine-based resins. The type of the thermoplastic resin is not particularly limited to a specific one as long as a softening temperature of the thermoplastic resin is lower than the melting point of the liquid crystal polyester fiber according to the present invention, and examples of the thermoplastic resin include: vinyl-based resins (polymers or derivatives obtained from monomers having vinyl groups CH 2 =CH- or vinylidene groups CH 2 =C<); polyamide-based resins such as aliphatic polyamide-based resins (polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 610, polyamide 612, etc.), and semi-aromatic polyamide-based resins; polyester-based resins such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; fluorine-containing resins such as polytetrafluoroethylene-based resins; polysulfone-based resins such as polysulfonic-based resins and polyethersulfic-based resins; polyether ketone-based resins such as polyether ketone-based resins, polyether ether ketone-based resins, and polyether ketone ketone-based resins; polycarbonate-based resins; polyphenylene ether-based resins; amorphous polyarylate-based resins; and liquid crystal polyester-based resins such as wholly aromatic polyester-based resins. These matrix resins may be used singly, or in combination of two or more.

[0084] Rubber may also be used as the matrix. Examples of the rubber include natural rubber (NR), synthetic natural rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPM, EP, EPDM), chloroprene rubber (CR), acrylic rubber (ACM, ANM), chlorosulfonated polyethylene rubber (CSM), urethane rubber (PUR, U), silicone rubber (Si, Q, VMQ, SR), fluororubber (FKM, FPM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO, ECO), polysulfide rubber (T), and urethane-based, styrene-based, olefin-based, vinyl chloride-based, ester-based, and amide-based thermoplastic elastomers. These matrix rubbers may be used singly, or in combination of two or more.EXAMPLES

[0085] Hereinafter, the present invention will be demonstrated by way of some examples that are presented only for the sake of illustration, which are not to be construed as limiting the scope of the present invention. It should be noted that in the following Examples and Comparative Examples, various properties were evaluated in the following manners.(Melting Point of Resin Chips (Granular Molded Bodies) and Fibers)

[0086] In accordance with JIS K 7121, a melting point was determined as a main endothermic peak temperature observed in measurement using a differential scanning calorimeter (DSC; "DSC60A Plus", produced by SHIMADZU CORPORATION). Specifically, 4 to 6 mg of a sample was taken and sealed in an aluminum pan in the DSC device, and then, the temperature was elevated at a rate of 10°C / min from 25°C while supplying nitrogen as a carrier gas at a flow rate of 200 mL / min to measure an endothermic peak derived from the liquid crystal polyester.(Total Fineness and Single Fiber Fineness)

[0087] In accordance with 8.3.1 Method A in JIS L 1013: 2010, a liquid crystal polyester fiber was reeled into a hank (100 m in total) with 100 rounds each of which had 1 meter using a sizing reel "Wrap Reel by Motor Driven" produced by DAIEI KAGAKU SEIKI MFG. Co., Ltd., to measure a weight of the liquid crystal polyester fiber. The measurement was conducted twice for each sample. Each of the weights (g) was multiplied by 100, and the average value was regarded as a total fineness (dtex) of the liquid crystal polyester fiber. Thus-obtained total fineness was divided by the number of filaments in the liquid crystal polyester fibers so as to give a single-fiber fineness (dtex).(Ketone Bond Amount)

[0088] Ketone bond amount was calculated by the pyrolysis-gas chromatography method described in Polymer Degradation and Stability, 76, 85-94 (2002). Specifically, each of the liquid crystal polyester fiber samples was heated using a thermal decomposition device ("PY2020iD" produced by Frontier Laboratories Ltd.) in the presence of tetramethylammonium hydroxide (TMAH) so as to generate a gas by pyrolysis / methylation. The generated gas was analyzed using a gas chromatography ("GC-6890N" produced by Agilent Technologies, Inc.), and the ketone bond amount (mol%) was calculated from a peak area derived from ketone bonds and a peak area derived from ester bonds.(Total Amount of CEG(s))

[0089] Each of liquid crystal polyester fiber samples was subjected to freeze-grinding until having a particle size of d90 = 100 µm or less (d90: the particle size at which the cumulative volume reaches 90% in the particle size distribution), then to the ground sample was added an excess amount of n-propylamine, followed by heating under agitation at 40°C for 90 minutes to decompose the sample. In this process, the ester bonds present inside the polymer chain are decomposed into carboxylic acid n-propyl amides and hydroxy groups, while the carboxy end groups (CEG) and hydroxy end groups in the polymer chain are unchanged from the carboxy groups and hydroxy groups. The decomposition products were separated by the HPLC method, and peak areas of the separated decomposition products with carboxy groups were compared with calibration curves prepared by HPLC analysis of the respective standard samples so as to quantify the amount (mEq / kg) of the carboxy end groups derived from each of the monomers. For example, the amount of CEG derived from monovalent carboxylic acids such as 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid can be determined by directly quantifying 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. The amount of CEG derived from divalent carboxylic acids such as terephthalic acid, isophthalic acid, and 2,6-naphthalene dicarboxylic acid can be determined by quantifying the amount of amidation products in which one of two carboxy groups is amidated, such as terephthalic acid mono-n-propyl amide, isophthalic acid mono-n-propyl amide, or 2,6-naphthalene dicarboxylic acid mono-n-propyl amide. The sum of the amounts of all carboxy end groups contained in each sample was regarded as the total carboxy end group amount (total CEG amount) (mEq / kg) of the sample.(Tensile Strength)

[0090] With reference to JIS L 1013: 2010 8.5.1, using an autograph "AGS-100B" produced by SHIMADZU CORPORATION, a tensile test was carried out under the conditions of a test sample length of 20 cm and a tensile speed of 10 cm / min 8 times for each sample yarn to obtain an average tensile force (cN), and a tensile strength (cN / dtex) was calculated by dividing the average tensile force (cN) by the total fineness (dtex) measured by the above-described method.(Compressive Strength)

[0091] One single fiber was taken from liquid crystal polyester fibers (multifilament), and this single fiber was placed on a glass plate so as to protrude from an end portion thereof and was fixed thereto using an adhesive. The protruding single fiber was cut using a focused ion beam (FIB) device at a position away from the end portion of the glass plate by a length that was 1.5 times the fiber diameter. Accordingly, a sample, for measurement of the compressive strength of the single fiber, having a cross-section exactly orthogonal to the fiber axis was produced. Using a single fiber compression testing machine ("Model B20-049", produced by THK PRECISION CO., LTD.), an indenter was pressed against the cut surface of the sample for measurement at a speed of 200 nm / sec to obtain a pressing force-strain curve. This measurement was performed for each of five samples, and the average value of the pressing forces at the yield points of the pressing force-strain curves obtained from the five tests was calculated. The average pressing force was divided by the single fiber fineness measured by the above-described method to determine the single fiber compressive strength (cN / dtex).(Metal Element Content)

[0092] Analysis liquid was prepared in accordance with a procedure described in "Microwave Digestion" as indicated below, and a content of metal elements belonging to Groups 8 to 11 in Periodic Table (ppm by weight) was determined by performing ICP-MS measurement.- Microwave Digestion

[0093] Microwave digestion was performed using a microwave digestion device "ETHOS-1" produced by Milestone General K. K. Each of the liquid crystal polyester fiber samples (0.1 g) was weighed and inserted to a quartz insert, and then 6 mL of nitric acid (1.42 mol / L) was added. The quartz insert was put into a digestion vessel containing 5 mL of water and 2 mL of hydrogen peroxide (concentration: 30 to 36 wt%) and sealed, and then microwave digestion was performed. After leaving it to be cooled, the resultant was volumed up to 50 mL and filtered through a filter (pore size: 0.45 µm), and the filtrate was subjected to ICP-MS measurement.- ICP-MS Measurement

[0094] The metal element content of each of the sample solution prepared by the above microwave digestion was analyzed using an ICP-MS analyzer "Agilent 7900" produced by Agilent Technologies, Inc. Under the conditions of a carrier gas flow rate of 0.7 L / min and an RF output of 1500 W, measurement was performed three times on the same sample solution in comparison with XSTC-622 (standard solution produced by SPEX CertiPrep), and the content of each metal element was determined from the average value of these measurements.

[0095] For samples such as fibers to which an oil agent adheres, if the oil agent might contain some metal elements to affect the measurement, microwave digestion may be performed after the oil agent is removed by the following method.- Oil Agent Removal

[0096] Into an aqueous solution in which 2 g of a nonionic surfactant ("Actinol F-9" available from Matsumoto Yushi-Seiyaku Co., Ltd.) was dissolved in 1 L of ion exchange water, was added a liquid crystal polyester fiber sample in an amount of 100 g or less, followed by temperature control in a range of 60 to 90°C, and then the resultant mixture was shaken for 40 minutes. Thereafter the liquid crystal polyester fiber sample was taken out from the aqueous solution, and rinsed two times at each time for 40 minutes with 1 L of ion exchange water with a controlled temperature in a range of 60 to 90°C. The liquid crystal polyester fiber sample was taken out and dried at 80°C under air atmosphere for 3 hours or longer using a hot air dryer "DN63HI" produced by Yamato Scientific Co., Ltd. so as to obtain a liquid crystal polyester fiber sample from which the oil agent was removed.(Disc Fatigue Test)

[0097] Six liquid crystal polyester fibers each having a total fineness of 280 dtex were combined and were first-twisted (Z-twisted) with a twist number of 328 T / m, and three first-twisted strands were second-twisted (S-twisted) with a twist number of 268 T / m to produce a cord.

[0098] This cord was subjected to the following dipping process to obtain a dip cord.First Time:

[0099] A dipping process was performed using a dipping liquid prepared by mixing 1 wt% of MARPOMERCE (produced by Matsumoto Yushi-Seiyaku Co., Ltd.), 4 wt% of DENACOL EX313 (produced by Nagase ChemteX Corporation), 0.3 wt% of a sodium hydroxide aqueous solution (10 wt%), and 94.7 wt% of water. Then, the dipped cord was dried at 150°C for 30 seconds, and then was heat-treated at 240°C for 30 seconds.Second Time:

[0100] A dipping process was performed using an RFL liquid as a dipping liquid. Then, the dipped cord was dried at 150°C for 30 seconds, and then was heat-treated at 240°C for 30 seconds. The RFL liquid was prepared as follows. A liquid A was prepared by mixing 3.3 wt% of resorcinol, 2.46 wt% of formaldehyde (37 wt%), 1.4 wt% of a sodium hydroxide aqueous solution (10 wt%), and 32.2 wt% of water and was left to mature at a temperature of 25°C for 6 hours. A liquid B was prepared by mixing 52.86 wt% of a VP latex (JSR-0650; produced by JSR Corporation) and 7.78 wt% of water, was mixed with the matured liquid A, and then was left to mature at a temperature of 25°C for 16 hours to prepare the RFL liquid.

[0101] The obtained dipped cord was embedded in a rubber (SBR : NR = 1:1) and was vulcanized at 150°C for 30 minutes to produce a rubber composite specimen. With reference to JIS L 1017: 2002, a disc fatigue test was conducted by the Goodrich method under the following conditions. Device: disc fatigue tester (produced by Mys-Shikenki Co., Ltd.) Disc interval: 24.5 mm Strain amount: 2% (elongation rate, compression rate) Rotation speed: 2500 rpm Temperature: 100°C Number of times: 300,000

[0102] For each dipped cord taken out from the rubber composite specimen before and after the disc fatigue test, with reference to JIS L 1013: 2010 8.5.1, using an autograph "AGS-100B" produced by SHIMADZU CORPORATION, a tensile test was carried out under the conditions of a specimen length of 20 cm and a tensile speed of 10 cm / min 6 times for one sample, so as to determine an average tensile force thereof as a tensile force (N) of each dipped cord before and after the disc fatigue test. A strength retention rate (%) was calculated from the following formula. [Example 1]

[0103] Chips (granular molded bodies) of a liquid crystal polyester (α) (Mp 0 : 309°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 60 / 20 / 15 / 5 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder (Φ 15 mm) (produced by TECHNOVEL CORPORATION) to be melt-kneaded, and the melt-kneaded material was fed to a spinning head. The spinning head was equipped with a spinneret having 50 holes each having a hole diameter of 0.10 mm φ, a spinning head temperature was set to 330°C, and the melt-kneaded material was discharged at a discharge rate of 28 g / min and wound onto a bobbin at a winding rate of 1000 m / min to obtain as-spun fibers of liquid crystal polyester fibers.

[0104] Next, 500 m of the as-spun fibers obtained in the above process was rewound onto an aluminum bobbin at a winding density of 0.6 g / cm 3< , and subjected to a heat treatment process under a nitrogen atmosphere using a closed oven at 300°C for 16 hours to obtain heat-treated fibers of liquid crystal polyester filaments. The evaluation results of the obtained liquid crystal polyester fibers are shown in Table 5.[Example 2]

[0105] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (β) (Mp 0 : 275°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 45 / 27.5 / 13.75 / 13.75 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.[Example 3]

[0106] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 300°C for 24 hours.[Example 4]

[0107] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the spinning head temperature was set to 360°C.[Example 5]

[0108] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 360°C for 32 hours.[Example 6]

[0109] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (γ) (Mp 0 : 278°C) having a structural unit derived from 4-hydroxybenzoic acid and a structural unit derived from 6-hydroxy-2-naphthoic acid at a ratio of 73 / 27 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 320°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.[Example 7]

[0110] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that heat treatment was performed on the as-spun fibers obtained in Example 6 under a nitrogen atmosphere at 270°C for 24 hours.[Example 8]

[0111] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 290°C for 16 hours.[Example 9]

[0112] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 270°C for 16 hours.[Comparative Example 1]

[0113] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that the spinning head temperature was set to 360°C.[Comparative Example 2]

[0114] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that heat treatment was performed on the as-spun fibers obtained in Example 6 under a nitrogen atmosphere at 320°C for 32 hours.[Comparative Example 3]

[0115] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 240°C for 16 hours.[Comparative Example 4]

[0116] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the spinning head temperature was set to 380°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 360°C for 40 hours.[Comparative Example 5]

[0117] To 5 L of acetonitrile (available from FUJIFILM Wako Pure Chemical Corporation, special grade reagent), were added two kinds of reagents, copper(I) iodide (available from FUJIFILM Wako Pure Chemical Corporation, special grade reagent) at an amount of 1 mol and 1,10-phenanthroline (available from FUJIFILM Wako Chemical Corporation) in equal molar amount with copper(I) iodide, stirred in a condition of suspension for 1 hour, filtered, and dried at 100°C for 3 hours to obtain an orange solid (melting point: 300°C).

[0118] This solid was added as a polymerization catalyst to resin chips of the liquid crystal polyester (γ) at a proportion of 10 ppm by weight in terms of copper atoms (a content of copper element based on a total content of the resin chips and the polymerization catalyst), and the mixture was sufficiently mixed using a shaking device. Thus-obtained blend of the resin chips and the polymerization catalyst was dried by hot air at 120°C for at least 4 hours. As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the blend was used as a raw material and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 250°C for 3 hours. [Table 5]Ex. 1Ex. 2Ex. 3Ex. 4Ex. 5Ex. 6Ex. 7Ex. 8Ex. 9Raw material resinLiquid crystal polyester typeαβαααγγααMelting point Mp 0 [°C]3092753093093092782783093092,6-Naphthylene group-containing structural unit [mol%]8072.580808027278080Metal element type---------Metal element content added [ppm by weight]---------Production conditionsSpinning head temperature [°C]330310330360330320320330330Heat treatment temperature [°C]300270300300360270270290270Heat treatment period [h]161624163216241616Total fineness [dtex]280280280280280280280280280Single fiber fineness [dtex]5.65.65.65.65.65.65.65.65.6Number of filaments505050505050505050Melting point [°C]345330350352360330333340332Liquid crystal polyester fibersKetone bond amount [mol%]0.00110.00050.00200.00350.00540.02560.03340.00100.0008Total CEG amount [mEq / kg]44.430.326.214.59.43.12.762.677.5Tensile strength [cN / dtex]312829272624232824Single fiber compressive strength [cN / dtex]0.980.850.760.680.670.650.610.800.66Metal element content [ppm by weight]---------Disc fatigue test625957464538355839Strength retention rate [%] [Table 5-continued] Comp. Ex. 1Comp. Ex. 2Comp. Ex. 3Comp. Ex. 4Comp. Ex. 5Raw material resinLiquid crystal polyester typeγγααγMelting point Mp 0 [°C]2782783093092782,6-Naphthylene group-containing structural unit [mol%]2727808027Metal element type----CopperMetal element content added [ppm by weight]----10Production conditionsSpinning head temperature [°C]360320330380310Heat treatment temperature [°C]270320240360250Heat treatment period [h]163216403Total fineness [dtex]280280280280280Single fiber fineness [dtex]5.65.65.65.65.6Number of filaments5050505050Melting point [°C]335340315373330Liquid crystal polyester fibersKetone bond amount [mol%]0.05540.07220.00030.05420.0180Total CEG amount [mEq / kg]2.82.386.69.82.3Tensile strength [cN / dtex]1917132225Single fiber compressive strength [cN / dtex]0.620.600.500.470.54Metal element content [ppm by weight]----9Disc fatigue test2220232429Strength retention rate [%]

[0119] As shown in Table 5, the liquid crystal polyester fibers of Examples 1 to 9 have a ketone bond amount, a tensile strength, and a single fiber compressive strength in specific ranges, and thus have a strength retention rate of 30% or more in the disc fatigue test and excellent disc fatigue resistance.

[0120] In particular, the liquid crystal polyester fibers of Examples I to 5 and 8 include a liquid crystal polyester having a large number of structural units including a 2,6-naphthylene group, have a lower ketone bond amount (0.005 mol% or less), a higher tensile strength (26 cN / dtex or more), and a higher single fiber compressive strength (0.67 cN / dtex or more), and thus have a strength retention rate of 40% or more in the disc fatigue test and more excellent disc fatigue resistance.

[0121] Furthermore, among these, the liquid crystal polyester fibers of Examples 1 to 3 and 8 include a liquid crystal polyester having a large number of structural units including a 2,6-naphthylene group, are obtained by adjusting the spinning head temperature, the heat treatment temperature, and the heat treatment period, have a very low ketone bond amount (0.002 mol% or less), a very high tensile strength (28 cN / dtex or more), and a very high single fiber compressive strength (0.76 cN / dtex or more), and thus have a strength retention rate of 50% or more in the disc fatigue test and very excellent disc fatigue resistance.

[0122] On the other hand, in Comparative Example 1, although the raw material resin of the same liquid crystal polyester (γ) as in Examples 6 and 7 was used, since the spinning head temperature was high, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 1 have inferior disc fatigue resistance compared to those of Examples 6 and 7.

[0123] In Comparative Example 2, although the raw material resin of the same liquid crystal polyester (γ) as in Examples 6 and 7 was used, since the heat treatment temperature was high and the heat treatment period was long, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions and a low tensile strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 2 have inferior disc fatigue resistance compared to those of Examples 6 and 7.

[0124] In Comparative Example 3, although the raw material resin of the same liquid crystal polyester (α) as in Examples 1, 3 to 5, 8, and 9 was used, since the heat treatment temperature was low, solid phase polymerization did not proceed, and liquid crystal polyester fibers having low tensile strength and compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 3 have inferior disc fatigue resistance compared to those of Examples 1, 3 to 5, 8, and 9.

[0125] In Comparative Example 4, although the raw material resin of the same liquid crystal polyester (α) as in Examples 1, 3 to 5, 8, and 9 was used, since the spinning head temperature was high, the heat treatment temperature was high, and the heat treatment period was long, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions and a low compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 4 have inferior disc fatigue resistance compared to those of Examples 1, 3 to 5, 8, and 9.

[0126] In Comparative Example 5, since the raw material resin contains a copper element-containing compound, perhaps since polymer chains could not be highly oriented during spinning, and the interactions between the polymer chains were reduced, liquid crystal polyester fibers having a low compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 5 have inferior disc fatigue resistance compared to those of Examples 1 to 9 and cannot be considered sufficient when used as reinforcing fibers in a predetermined composite material application.INDUSTRIAL APPLICABILITY

[0127] The liquid crystal polyester fiber according to the present invention can be used for various applications such as general industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing, and can be used, for example, as a reinforcing fiber for an automotive rubber material, an electronic circuit board, or the like that require durability against vibration.

[0128] Although the preferred embodiments of the present invention have been described, various additions, modifications, or deletions may be made without departing from the scope of the invention. Accordingly, such variants are included within the scope of the present invention.

Examples

example 1

[Example 1]

[0103]Chips (granular molded bodies) of a liquid crystal polyester (α) (Mp 0 : 309°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 60 / 20 / 15 / 5 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder (Φ 15 mm) (produced by TECHNOVEL CORPORATION) to be melt-kneaded, and the melt-kneaded material was fed to a spinning head. The spinning head was equipped with a spinneret having 50 holes each having a hole diameter of 0.10 mm φ, a spinning head temperature was set to 330°C, and the melt-kneaded material was discharged at a discharge rate of 28 g / min and wound onto a bobbin at a winding rate of 1000 m / min to obtain as-spun fibers of liquid crystal polyester fibers.

[0104]Next, 500 m of the as-spun fibers obtained in the abov...

example 2

[Example 2]

[0105]As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (β) (Mp 0 : 275°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 45 / 27.5 / 13.75 / 13.75 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.

example 3

[Example 3]

[0106]Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 300°C for 24 hours.

CROSS REFERENCE TO THE RELATED APPLICATION

[0001] This application is based on and claims Convention priority to Japanese patent application No. 2023-061332, filed April 5, 2023, the entire disclosure of which is herein incorporated by reference as a part of this application.FIELD OF THE INVENTION

[0002] The present invention relates to a liquid crystal polyester fiber.BACKGROUND OF THE INVENTION

[0003] A liquid crystal polyester fiber is a fiber made from a liquid crystal polyester having a rigid molecular structure, and it has been known that the liquid crystal polyester fiber which is produced by subjecting an as-spun fiber obtained by melt-spinning to heat treatment can exhibit the highest strength among synthetic fibers obtained by melt-spinning. The liquid crystal polyester fiber is used for applications such as industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing, utilizing such a characteristic of having high strength and a characteristic of being lightweight, which is a characteristic of synthetic fibers. Liquid crystal polyester fibers with improved durability have been developed for these various applications.

[0004] For example, Patent Document 1 (JP Laid-open Patent Publication No. 2004-107826) discloses a polyarylate fiber including 0.03 to 5.0% by mass of inorganic fine particles having a Mohs hardness of 4 or less, mainly including silicic acid and magnesium and having an average particle diameter of 0.01 to 15 µm attached to a fiber surface, and describes that the polyarylate fiber has excellent flexural fatigue resistance and abrasion resistance.

[0005] Patent Document 2 (WO2020 / 166316) discloses a liquid crystal polyester multifilament having a fiber compressive yield stress of 15 to 40 mN / dtex, and describes that high flexural fatigue resistance can be exhibited when the liquid crystal polyester multifilament is used in high-order processed products such as ropes, slings, and tension members.

[0006] Patent Document 3 (WO2022 / 113802) discloses a liquid crystal polyester fiber including at least one metallic element selected from the group consisting of metallic elements belonging to from Groups 8 to 11 in Periodic Table, and describes that the liquid crystal polyester fiber is excellent in heat aging resistance.CONVENTIONAL ART DOCUMENTPATENT DOCUMENT

[0007] [Patent Document 1] JP Laid-open Patent Publication No. 2004-107826 [Patent Document 2] WO2020 / 166316 [Patent Document 3] WO2022 / 113802 SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] For example, in applications as reinforcing materials used for rubber materials such as automotive oil brake hoses and tires, electronic components such as board materials for circuit boards, durability against stretching and vibration is required in addition to mechanical properties. However, while a liquid crystal polyester fiber is strong against a stress in a tensile direction, the liquid crystal polyester fiber is relatively weak against a stress in a compressive direction, and when the liquid crystal polyester fiber is used as a reinforcing fiber in such applications, forces are applied not only in the tensile direction but also in the compressive direction of the fiber axis, so that fatigue resistance is required for both stresses.

[0009] In Patent Document 1, abrasion resistance and flexural fatigue resistance are improved by attaching the inorganic fine particles to the fiber surface, but fatigue resistance against both stresses in the tensile direction and the compressive direction is not described. Even if the fiber surface is covered with the inorganic fine particles in order to reduce damage to the fiber, when the fiber is used as a reinforcing fiber, stresses in the tensile direction and the compressive direction are not distributed but are transmitted to the reinforcing fiber in a composite material, so that this does not lead to improved fatigue resistance against both stresses.

[0010] In Patent Document 2, fatigue resistance against repeated bending is improved by enhancing the compressive strength in the diameter direction of the fiber and the flexibility of the fiber, but fatigue resistance against both stresses in the tensile direction and the compressive direction is not described. Even if the flexibility of the fiber is enhanced, when the fiber is used as a reinforcing fiber, the reinforcing fiber fixed to a matrix by adhesion in a composite material cannot sufficiently distribute the stress during repeated bending in a yarn, so that this does not lead to improved fatigue resistance against both stresses in the tensile direction and the compressive direction.

[0011] In Patent Document 3, by containing a metal catalyst in the liquid crystal polyester fiber, tensile strength is improved through heat treatment at a lower temperature for a shorter time, and the heat aging resistance of a heat-treated fiber is improved, but compressive strength is not described. Even if tensile strength can be improved by the metal catalyst, it is not necessarily possible to sufficiently improve compressive strength at the same time. In addition, in Patent Document 3, fatigue resistance against both stresses in the tensile direction and the compressive direction is not described.

[0012] An object of the present invention is to solve the above problems and to provide a liquid crystal polyester fiber having excellent fatigue resistance against tensile deformation and compressive deformation.MEANS FOR SOLVING THE PROBLEMS

[0013] The inventors of the present invention have conducted extensive studies in order to achieve the aforementioned object, and found that a liquid crystal polyester fiber having high tensile strength and compressive strength in specific ranges and having a low ketone bond amount in a specific range has excellent fatigue resistance in a disc fatigue test in which tensile deformation and compressive deformation are alternately applied (hereinafter sometimes referred to as "disc fatigue resistance"), leading to the completion of the present invention.

[0014] That is, the present invention may include the following aspects.[Aspect 1]

[0015] A liquid crystal polyester fiber having a ketone bond amount of 0.050 mol% or less (preferably 0.010 mol% or less, more preferably 0.005 mol% or less, further preferably 0.003 mol% or less, and even more preferably less than 0.0020 mol%), a tensile strength of 18 cN / dtex or more (preferably 20 cN / dtex or more, more preferably 22 cN / dtex or more, further preferably 25 cN / dtex or more, even more preferably 26 cN / dtex or more, and particularly preferably 28 cN / dtex or more), and a single fiber compressive strength of 0.55 cN / dtex or more (preferably 0.60 cN / dtex or more, more preferably 0.65 cN / dtex or more, further preferably 0.66 cN / dtex or more, even more preferably 0.70 cN / dtex or more, and particularly preferably 0.80 cN / dtex or more).[Aspect 2]

[0016] The liquid crystal polyester fiber according to aspect 1, having a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg (preferably 6.0 mEq / kg or more, more preferably 8.0 mEq / kg or more, further preferably 10.0 mEq / kg or more, and even more preferably 15.0 mEq / kg or more) and 85.0 mEq / kg or less (preferably 80.0 mEq / kg or less, more preferably 75.0 mEq / kg or less, further preferably 65 mEq / kg or less, and even more preferably 60.0 mEq / kg or less).[Aspect 3]

[0017] The liquid crystal polyester fiber according to aspect 1 or 2, including a liquid crystal polyester including a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more (preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more) based on a total content of all structural units.[Aspect 4]

[0018] A fiber structure at least partially including the liquid crystal polyester fiber as recited in any one of aspects 1 to 3.[Aspect 5]

[0019] A composite material including the liquid crystal polyester fiber as recited in any one of aspects 1 to 3 as a reinforcing fiber.

[0020] 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.

[0021] 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

[0022] The liquid crystal polyester fiber according to the present invention has excellent disc fatigue resistance.DESCRIPTION OF EMBODIMENT[Liquid Crystal Polyester Fiber]

[0023] The liquid crystal polyester fiber includes a liquid crystal polyester. The liquid crystal polyester includes structural units derived from, for example, aromatic diols, aromatic dicarboxylic acids, aromatic hydroxycarboxylic acids, etc. As long as the effect of the present invention is not impaired, the structural units derived from aromatic diols, aromatic dicarboxylic acids, and aromatic hydroxycarboxylic acids are not limited to a specific chemical composition. The liquid crystal polyester may include the structural units derived from aromatic diamines, aromatic hydroxy amines, or aromatic aminocarboxylic acids in the range which does not impair the effect of the present invention. For example, preferable structural units may include units shown in Table 1. [Table 1] In the formula, X is selected from the following m is an integer from 0 to 2, Y is a substituent selected from hydrogen atom, halogen atoms, alkyl groups, aryl groups, aralkyl groups, alkoxy groups, aryloxy groups, aralkyloxy groups.

[0024] In the structural units in Table 1, m is an integer from 0 to 2, and Y in the formula independently represents, as from one substituent to the number of substituents in the range of the replaceable maximum number of aromatic ring, a hydrogen atom, a halogen atom (for example, fluorine atom, chlorine atom, bromine atom and iodine atom), an alkyl group (for example, an alkyl group having 1 to 4 carbon atoms such as methyl group, ethyl group, isopropyl group and t-butyl group), an alkoxy group (for example, methoxy group, ethoxy group, isopropoxy group, n-butoxy group, etc.), an aryl group (for example, phenyl group, naphthyl group, etc.), an aralkyl group [for example, benzyl group (phenylmethyl group), phenethyl group (phenylethyl group), etc.], an aryloxy group (for example, phenoxy group, etc.), an aralkyloxy group (for example, benzyloxy group, etc.), and others.

[0025] As more preferable structural units, there may be mentioned structural units as described in Examples (1) to (20) shown in the following Tables 2, 3, and 4. It should be noted that where the structural unit in the formula is a structural unit which can show a plurality of structures, combination of two or more types may be used as structural units for a polymer.

[0026] In the structural units shown in Tables 2, 3, and 4, n is an integer of 1 or 2, among each of the structural units, n = 1 and n = 2 may independently exist, or may exist in combination; each of the Y 1 and Y 2 independently represents, a hydrogen atom, a halogen atom (for example, fluorine atom, chlorine atom, bromine atom, iodine atom, etc.), an alkyl group (for example, an alkyl group having 1 to 4 carbon atoms such as methyl group, ethyl group, isopropyl group, and t-butyl group, etc.), an alkoxy group (for example, methoxy group, ethoxy group, isopropoxy group, n-butoxy group, etc.), an aryl group (for example, phenyl group, naphthyl group, etc.), an aralkyl group [for example, benzyl group (phenylmethyl group), phenethyl group (phenylethyl group), etc.], an aryloxy group (for example, phenoxy group, etc.), an aralkyloxy group (for example, benzyloxy group, etc.), and others. Among these, the preferable one may include a hydrogen atom, a chlorine atom, a bromine atom, and a methyl group.

[0027] Z may include substitutional groups denoted by following formulae.

[0028] In one embodiment, the liquid crystal polyester may include a structural unit derived from a hydroxycarboxylic acid as a main component. The liquid crystal polyester may preferably include a structural unit (A) derived from hydroxybenzoic acid and a structural unit (B) derived from hydroxy naphthoic acid. For example, the structural unit (A) may have a structural unit derived from 4-hydroxybenzoic acid (the following formula (A)), and the structural unit (B) may have a structural unit derived from 6-hydroxy-2-naphthoic acid (the following formula (B)). In order to improve melt-formability, the ratio of the structural unit (A) and the structural unit (B) may preferably be in a range of former / latter of 9 / 1 to 1 / 1, more preferably from 7 / 1 to 1 / 1, and still more preferably from 5 / 1 to 1 / 1.

[0029] The liquid crystal polyester may include the structural unit derived from 4-hydroxybenzoic acid. In the case where the liquid crystal polyester includes both the structural unit (A) and the structural unit (B), a content of the structural unit derived from 4-hydroxybenzoic acid based on a total content of all the structural units may be 50 mol% or more, preferably 53 mol% or more, more preferably 60 mol% or more, further preferably 65 mol% or more, and even more preferably 70 mol% or more. The upper limit of the content of the structural unit derived from 4-hydroxybenzoic acid in the liquid crystal polyester is not particularly limited to a specific value, and may be, for example, 90 mol% or less, preferably 88 mol% or less, and more preferably 85 mol% or less.

[0030] The liquid crystal polyester may include the structural unit derived from 6-hydroxy-2-naphthoic acid. In the case where the liquid crystal polyester includes both the structural unit (A) and the structural unit (B), a content of the structural unit derived from 6-hydroxy-2-naphthoic acid based on the total content of all the structural units may be 4 to 45 mol%.

[0031] Furthermore, a total content of the structural unit (A) and the structural unit (B) based on the total content of all the structural units may be, for example, 65 mol% or more, more preferably 70 mol% or more, and further preferably 80 mol% or more.

[0032] In another embodiment, the liquid crystal polyester may include a structural unit represented by the following formula (I) (structural unit (I)), a structural unit represented by the following formula (II) (structural unit (II)), and at least one structural unit selected from the group consisting of a structural unit represented by the following formula (III) (structural unit (III)) and a structural unit represented by the following formula (IV) (structural unit (IV)).         -O-Ar 1< -CO-     (I)         -CO-Ar 2< -CO-     (II)         -O-Ar 3< -O-     (III)         -O-Ar 4< -NH-     (IV) (wherein Ar 1< represents at least one group selected from the group consisting of a phenylene group, a naphthylene group, and a biphenylylene group, each of Ar 2< , Ar 3< , and Ar 4< independently represents at least one group selected from the group consisting of a phenylene group, a naphthylene group, a biphenylylene group, and a diphenyl ether diyl group, and hydrogen atoms in each aromatic ring in Ar 1< , Ar 2< , Ar 3< , and Ar 4< may be independently substituted with a halogen atom, an alkyl group, an alkoxy group, an aryl group, an aralkyl group, an aryloxy group, or an aralkyloxy group)

[0033] The structural unit (I) is a structural unit derived from an aromatic hydroxycarboxylic acid, and may be preferably a structural unit in which Ar 1< is a 1,4-phenylene group (structural unit derived from 4-hydroxybenzoic acid) and a structural unit in which Ar 1< is a 2,6-naphthylene group (structural unit derived from 6-hydroxy-2-naphthoic acid).

[0034] The structural unit (II) is a structural unit derived from an aromatic dicarboxylic acid, and may be preferably a structural unit in which Ar 2< is a 1,4-phenylene group (structural unit derived from terephthalic acid), a structural unit in which Ar 2< is a 1,3-phenylene group (structural unit derived from isophthalic acid), a structural unit in which Ar 2< is a 2,6-naphthylene group (structural unit derived from 2,6-naphthalenedicarboxylic acid), and a structural unit in which Ar 2< is a diphenyl ether-4,4'-diyl group (structural unit derived from diphenyl ether-4,4'-dicarboxylic acid).

[0035] The structural unit (III) is a structural unit derived from an aromatic diol, and may be preferably a structural unit in which Ar 3< is a 1,4-phenylene group (structural unit derived from hydroquinone), a structural unit in which Ar 3< is a 4,4'-biphenylylene group (structural unit derived from 4,4'-dihydroxybiphenyl), a structural unit in which Ar 3< is a phenyl-1,4-phenylene group (structural unit derived from phenylhydroquinone), and a structural unit in which Ar 3< is a diphenyl ether-4,4'-diyl group (structural unit derived from 4,4'-dihydroxydiphenyl ether).

[0036] The structural unit (IV) is a structural unit derived from an aromatic hydroxy amine, and may be preferably a structural unit in which Ar 4< is a 1,4-phenylene group (structural unit derived from 4-aminophenol) and a structural unit in which Ar 4< is a 4,4'-biphenylylene group (structural unit derived from 4-amino-4'-hydroxybiphenyl).

[0037] A content of the structural unit (I) in the liquid crystal polyester based on a total content of all the structural units may be 20 to 80 mol%, preferably 30 to 75 mol%, and more preferably 40 to 70 mol%.

[0038] A content of the structural unit (II) in the liquid crystal polyester based on a total content of all the structural units may be 10 to 40 mol%, preferably 12.5 to 35 mol%, and more preferably 15 to 30 mol%.

[0039] A total content of the structural units (III) and (IV) in the liquid crystal polyester based on a total content of all the structural units may be 10 to 40 mol%, preferably 12.5 to 35 mol%, and more preferably 15 to 30 mol%.

[0040] A molar ratio of the content of the structural unit (II) to the total content of the structural units (III) and (IV), as (II) / [(III) + (IV)], may be 90 / 100 to 100 / 90, preferably 95 / 100 to 100 / 95, more preferably 98 / 100 to 100 / 98, and further preferably 100 / 100.

[0041] The liquid crystal polyester may include two or more types of each of the structural units (I) to (IV). The content of each structural unit represents the total content of all structural units corresponding to the structural unit, and, for example, in the case where the liquid crystal polyester includes two or more types of the structural unit (I), the content of the structural unit (I) represents the total content of these types.

[0042] In the liquid crystal polyester, the total content of the structural units (I) to (IV) based on the total content of all the structural units may be, for example, 90 mol% or more, preferably 95 mol% or more, more preferably 99 mol% or more, and further preferably 100 mol%.

[0043] From the viewpoint of enhancing disc fatigue resistance of the liquid crystal polyester fiber, the liquid crystal polyester preferably has a combination including a structural unit having a naphthalene skeleton. In the case of including the structural unit having the naphthalene skeleton, it is considered that disc fatigue resistance can be improved, perhaps since molecular chains are densely packed and staggering between the molecular chains is less likely to occur. In addition, from the viewpoint of decreasing the ketone bond amount, the liquid crystal polyester preferably includes the structural unit having the naphthalene skeleton. The inventors of the present invention have found that when a liquid crystal polyester is exposed to a high temperature, side reactions in which ketone bonds are formed from ester bonds in the molecular chains occur, and that the tendency for such side reactions to occur varies depending on the composition of the structural units of the liquid crystal polyester. Further, the inventors of the present invention have found that in the case of including the structural unit having the naphthalene skeleton, there is a tendency to be able to suppress the formation of ketone bonds in the liquid crystal polyester. For example, a total content of a structural unit including a 2,6-naphthylene group in the liquid crystal polyester based on the total content of all the structural units may be 28 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more. Furthermore, from the viewpoint of improving melt-formability, the total content of the structural unit including the 2,6-naphthylene group in the liquid crystal polyester based on the total content of all the structural units may be 95 mol% or less, preferably 90 mol% or less, and more preferably 85 mol% or less. As the structural unit including the 2,6-naphthylene group, a structural unit derived from 6-hydroxy-2-naphthoic acid (the structural unit (B) represented by the above formula (B), a structural unit (I) in which Ar 1< is a 2,6-naphthylene group) and a structural unit derived from 2,6-naphthalenedicarboxylic acid (a structural unit (II) in which Ar 2< is a 2,6-naphthylene group) are preferable.

[0044] The higher the melting point of the liquid crystal polyester is, the higher the heat resistance of the obtained liquid crystal polyester fiber is. Therefore, for example, when the liquid crystal polyester fiber is used as a rubber reinforcing fiber at a high temperature or is used for applications in which heat is generated due to vibration or the like during use, the thermal durability of the liquid crystal polyester fiber can be improved by using the liquid crystal polyester having the higher melting point. For example, the liquid crystal polyester may have a melting point (hereinafter sometimes referred to as Mp 0 ) of 250°C or higher, preferably 260°C or higher, more preferably 280°C or higher, further preferably 290°C or higher, and even more preferably 300°C or higher. On the other hand, from the viewpoint of improving melt spinnability, the liquid crystal polyester may have the melting point of 380°C or lower, preferably 370°C or lower, more preferably 360°C or lower, further preferably 350°C or lower, and even more preferably 340°C or lower. In the present specification, the melting point refers to a main endothermic peak temperature determined and observed using a differential scanning calorimeter (DSC) in accordance with the JIS K 7121 test method. Specifically, 4 to 6 mg of a sample is encapsulated in an aluminum pan and taken into the DSC device. Then, the temperature is elevated at a rate of 10°C / min from room temperature (e.g., 25°C) while supplying nitrogen as a carrier gas at a flow rate of 200 mL / min to measure an endothermic peak. Depending on the type of polymer, some polymers may not show a clear peak in the 1st run of DSC measurement. If no clear peak appears in the 1st run of DSC measurement, the sample is heated up to a temperature 50°C higher than the expected flow temperature in a temperature elevation rate of 50°C / min. After keeping the temperature for 3 minutes so as to make the sample completely molten, the sample is cooled at a cooling rate of 80°C / min to 50°C, and then is elevated at 10°C / min to measure the endothermic peak thereof.

[0045] It should be noted that the liquid crystal polyester fiber may contain thermoplastic polymers, such as a polyethylene terephthalate, a modified-polyethylene terephthalate, a polyolefin, a polycarbonate, a polyamide, a polyphenylene sulfide, a polyether ether ketone, a fluoro-resin, and others, as long as the effects of the present invention are not impaired. In addition, the liquid crystal polyester fiber may contain various additives including: inorganic substances such as titanium oxide, kaolin, silica, and barium oxide; carbon black; a colorant such as dyes and paints; an antioxidant; an ultraviolet-ray absorbent; a light stabilizer; etc.

[0046] As long as the effects of the present invention are not impaired, the liquid crystal polyester fiber may be a mixed spun fiber obtained by mixing and spinning the liquid crystal polyester with the above-described thermoplastic polymer and various additives, or may be a composite spun fiber obtained by simultaneously spinning the liquid crystal polyester and different components of the above-described thermoplastic polymer from separate spinnerets. The liquid crystal polyester fiber may be a non-composite spun fiber or may be a composite spun fiber. In particular, it is preferable that the liquid crystal polyester is present on the fiber surface of the liquid crystal polyester fiber.

[0047] The fiber cross-sectional shape of the liquid crystal polyester fiber is not particularly limited to a specific one as long as the effects of the present invention are not impaired, and may, for example, be a circular cross-sectional shape or a modified cross-sectional shape such as a hollow cross-sectional shape, an elliptical shape, a hexagonal shape, a pentagonal shape, a quadrangular shape, a triangular shape, a star shape, and a flat shape.

[0048] The liquid crystal polyester fiber may contain a liquid crystal polyester at a proportion of 50 wt% or more, preferably 80 wt% or more, more preferably 90 wt% or more, still more preferably 95 wt% or more, and even more preferably 99.9 wt% or more.

[0049] The liquid crystal polyester fiber has a ketone bond amount of 0.050 mol% or less. In the present specification, the ketone bond amount refers to a ratio of a molar amount of ketone bonds based on a total molar amount of ester bonds and ketone bonds (molar amount of ketone bonds / (molar amount of ester bonds + molar amount of ketone bonds)), and is a value measured by the method described in Examples below. Ketone bonds are heterogeneous bonds formed by side reactions from ester bonds during production of the liquid crystal polyester fiber, and disc fatigue resistance can be improved by decreasing the ketone bond amount. Although the mechanism for this is not fully understood, it is considered that where ketone bonds are present instead of ester bonds, perhaps since the molecular chains are distorted, staggering between the molecular chains is likely to occur when tensile stress and compressive stress are applied. Therefore, it is considered that a liquid crystal polyester fiber having a decreased ketone bond amount can suppress distortion in the molecular chains and improve disc fatigue resistance. From the viewpoint of enhancing disc fatigue resistance, the liquid crystal polyester fiber may have the ketone bond amount of preferably 0.010 mol% or less, more preferably 0.005 mol% or less (e.g., 0.0054 mol% or less), further preferably 0.003 mol% or less (e.g., 0.0034 mol% or less), and even more preferably less than 0.0020 mol%. The lower limit value of the ketone bond amount is not particularly limited to a specific value, and may be, for example, 0.0001 mol% or more.

[0050] The liquid crystal polyester fiber has a tensile strength of 18 cN / dtex or more. The tensile strength may be preferably 20 cN / dtex or more, more preferably 22 cN / dtex or more, further preferably 25 cN / dtex or more, even more preferably 26 cN / dtex or more, and particularly preferably 28 cN / dtex or more. The upper limit value of the tensile strength is not particularly limited to a specific value, and may be, for example, about 40 cN / dtex. The tensile strength of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0051] The liquid crystal polyester fiber has a single fiber compressive strength of 0.55 cN / dtex or more. The single fiber compressive strength may be preferably 0.60 cN / dtex or more, more preferably 0.65 cN / dtex or more, further preferably 0.66 cN / dtex or more, even more preferably 0.70 cN / dtex or more, and particularly preferably 0.80 cN / dtex or more. In addition, the upper limit value of the single fiber compressive strength is not particularly limited to a specific value, and may be, for example, about 1.5 cN / dtex. The single fiber compressive strength of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0052] The liquid crystal polyester fiber having the tensile strength and the compressive strength in the above ranges can exhibit sufficient reinforcing performance when used as a reinforcing fiber in a composite material, and high disc fatigue resistance.

[0053] The liquid crystal polyester fiber may have a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg and 85.0 mEq / kg or less. In the present specification, the total CEG amount means the amount of carboxy groups existing at the ends of the liquid crystal polyester molecules in 1 kg of the liquid crystal polyester fiber, and is a value measured by the method described in Examples below. For example, as the carboxy end groups in liquid crystal polyester, carboxy groups that do not participate in reaction and remain in the terminal structural units derived from monomers having carboxy groups, such as aromatic hydroxycarboxylic acids and aromatic dicarboxylic acids, may be exemplified. When carboxy groups are present at the ends of the liquid crystal polyester molecules, perhaps since the carboxy ends interact with each other between the molecules, it is considered that disc fatigue resistance can be further improved. From the viewpoint of further improving disc fatigue resistance, the liquid crystal polyester fiber may have the total CEG amount of preferably 6.0 mEq / kg or more, more preferably 8.0 mEq / kg or more, further preferably 10.0 mEq / kg or more, and even more preferably 15.0 mEq / kg or more. On the other hand, since the molecular weight tends to decrease as the end group amount increases, from the viewpoint of appropriately increasing the molecular weight to enhance strength and from the viewpoint of suppressing gas generation during thermoforming processing for a composite material using the liquid crystal polyester fiber as a reinforcing fiber, the liquid crystal polyester fiber may have the total CEG amount of preferably 80.0 mEq / kg or less, more preferably 75.0 mEq / kg or less, further preferably 65 mEq / kg or less (e.g., 65.0 mEq / kg or less), and even more preferably 60.0 mEq / kg or less.

[0054] The liquid crystal polyester fiber may include a metal catalyst such as a melt polymerization catalyst for synthesizing a raw material liquid crystal polyester and a solid phase polymerization catalyst for an as-spun fiber. From the viewpoint of suppressing a decrease in the single fiber compressive strength, a content of metal elements belonging to Groups 8 to 11 in Periodic Table may be less than 1 ppm by weight. The metal elements belonging to Groups 8 to 11 in Periodic Table act as solid phase polymerization catalysts and can enhance strength of the liquid crystal polyester fiber through a heat treatment at a lower temperature for a shorter time. However, when a raw material including a large amount of these metal elements is melt-spun, perhaps since compounds derived from the metal elements act as obstacles during fiber formation to prevent polymer chains from being highly oriented and to reduce interactions between the polymer chains, the single fiber compressive strength decreases. In addition, perhaps since, due to the presence of compounds derived from the metal elements in the fiber, buckling is likely to occur starting from the portions where the metal elements are present when repeated compressive and elongation stresses are applied, such a fiber tends to be poor in disc fatigue resistance. The content of the metal elements indicates a ratio of the total weight of the above-described metal elements based on the total weight of the liquid crystal polyester fiber, and in the case where the metal elements are contained as metal compounds, indicates a content in terms of metal atoms. Here, the content of the above metal elements is a content of the metal elements in the components constituting the fiber itself, excluding components adhering to the fiber surface such as an oil agent.

[0055] The liquid crystal polyester fiber may have a melting point of 260 to 380°C, preferably 280 to 375°C, more preferably 300 to 370°C, further preferably 320 to 365°C, and even more preferably 335 to 360°C. The melting point of the liquid crystal polyester fiber increases from a melting point (Mp) of an as-spun fiber due to solid phase polymerization. The melting point of the liquid crystal polyester fiber is a value measured by the method described in Examples below.

[0056] The liquid crystal polyester fiber may have a strength retention rate of 30% or more, preferably 40% or more, more preferably 50% or more, and further preferably 60% or more in the disc fatigue test by the method described in Examples below.

[0057] The liquid crystal polyester fiber may have an adjusted single fiber fineness depending on the application, etc. The single fiber fineness, for example, may be 50 dtex or less, preferably 15 dtex or less, and more preferably 10 dtex or less, and from the viewpoint of adapting to downsizing in electronic component application, etc., the single fiber fineness is preferably a low fineness and may be, for example, 7 dtex or less. On the other hand, the lower limit of the single fiber fineness is not particularly limited to a specific one, and may be, for example, about 0.01 dtex. The single fiber fineness is a value measured by the method described in Examples below.

[0058] The liquid crystal polyester fiber may be a monofilament or a multifilament. In the case of multifilament, the number of filaments may be adjusted depending on the application, etc. For example, the number of filaments may be 2 to 5000 filaments, preferably 3 to 4000 filaments, and more preferably 5 to 3000 filaments.

[0059] The total fineness of the liquid crystal polyester fiber may be adjusted depending on the application, etc. For example, the total fineness may be 50000 dtex or less, preferably 10000 dtex or less, more preferably 2000 dtex or less, and further preferably 600 dtex or less. From the viewpoint of adapting to downsizing in electronic component application, etc., the total fineness is preferably a low fineness and may be, for example, 300 dtex or less. On the other hand, the lower limit of the total fineness is not particularly limited to a specific one, and may be, for example, about 1 dtex.[Method for Producing Liquid Crystal Polyester Fiber]

[0060] A method for producing the liquid crystal polyester fiber is not particularly limited to a specific one as long as each of the ketone bond amount, the tensile strength, and the compressive strength of the liquid crystal polyester fiber can be adjusted to the above specific range, and may include at least melt-spinning a liquid crystal polyester to obtain an as-spun fiber and heat-treating the obtained as-spun fiber.

[0061] The inventors of the present invention have found that when a liquid crystal polyester is exposed to a high temperature, side reactions in which ketone bonds are formed from ester bonds in the molecular chains occur, and that the tendency for such side reactions to occur varies depending on the composition of the structural units of the liquid crystal polyester. As the liquid crystal polyester to be subjected to melt-spinning, a liquid crystal polyester having the above-described structural units may be used. For example, the liquid crystal polyester preferably has a combination including the structural unit having the naphthalene skeleton, and such a liquid crystal polyester is less likely to undergo side reactions in which ester bonds are converted into ketone bonds, possibly due to steric hindrance, etc., caused by a bulky naphthalene ring being bonded to the ester bonds. Further, the liquid crystal polyester that is less likely to undergo such side reactions may be more preferably a liquid crystal polyester including a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more based on a total content of all structural units. Furthermore, the total content of the structural unit including the 2,6-naphthylene group based on the total content of all the structural units may be preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, even more preferably 55 mol% or more, particularly preferably 60 mol% or more, more particularly preferably 65 mol% or more, and further particularly preferably 70 mol% or more.

[0062] Furthermore, the liquid crystal polyester to be subjected to melt-spinning may be a liquid crystal polyester including: the above structural unit (I); the above structural unit (II); and at least one structural unit selected from the group consisting of the above structural unit (III) and the above structural unit (IV).

[0063] The liquid crystal polyester can be synthesized by a known polycondensation method. Examples of monomers for polycondensation may include various aromatic hydroxycarboxylic acids, aromatic dicarboxylic acids, aromatic diols, and aromatic hydroxy amines, and hydroxy group acylates and carboxylic acid derivatives such as carboxyl group esters, acid halides, and acid anhydrides by activating the end thereof.

[0064] Polycondensation may be performed in the presence of various polymerization catalysts, such as organotin-based catalysts (dialkyl tin oxide, etc.), antimony-based catalysts (antimony trioxide, etc.), titanium-based catalysts (titanium dioxide, etc.), alkali metal salts or alkaline earth metal salts of carboxylic acids (potassium acetate, etc.), and Lewis acids (BF 3 , etc.).

[0065] The above-described thermoplastic polymers and various additives may be added to the liquid crystal polyester, as long as the effects of the present invention are not impaired.

[0066] When melting-spinning the liquid crystal polyester and when heat-treating the obtained as-spun fiber, side reactions occur by heating at a high temperature. Therefore, to suppress the formation of ketone bonds, it is effective to decrease the spinning head temperature and the heat-treatment temperature. On the other hand, to improve tensile strength and compressive strength, it is preferable to take into consideration orienting the molecules through melt-spinning and allowing solid phase polymerization to sufficiently proceed through heat treatment. In the present invention, by setting the spinning conditions and the heat-treating conditions depending on the melting point of the liquid crystal polyester that is the raw material resin and the composition of the structural units, it is possible to obtain a liquid crystal polyester fiber in which each of the ketone bond amount, the tensile strength, and the compressive strength are adjusted to the above specific range.

[0067] In the spinning step, the liquid crystal polyester is fed into an extruder and is melt-kneaded in the extruder. Then, the melt-kneaded material of the liquid crystal polyester is conveyed to a spinning head and discharged through a nozzle. The obtained yarn is wound, whereby an as-spun fiber can be obtained. During winding, an oil agent may be applied from the viewpoint of preventing static electricity.

[0068] As the extruder, known extruders such as single-screw extruders and multi-screw extruders (twin- or more-screw extruders) can be used, and twin-screw extruders are preferable from the viewpoint of improving kneadability and degassability.

[0069] In the spinning step, from the viewpoint of suppressing an increase in the ketone bond amount, where the melting point of the liquid crystal polyester that is the raw material resin is denoted by Mp 0 , a spinning head temperature may be from Mp 0 to Mp 0 + 60°C, preferably from Mp 0 to Mp 0 + 50°C, more preferably from Mp 0 + 5°C to Mp 0 + 45°C, and further preferably from Mp 0 + 10°C to Mp 0 + 40°C. In the present specification, the spinning head temperature means the maximum temperature in the spinning head.

[0070] Melt-spinning can be performed by a known or common method, and the melt-kneaded material can be discharged from the nozzle of the spinning head, and wound by a godet roller or the like to obtain an as-spun fiber.

[0071] By performing heat treatment on the as-spun fiber, solid phase polymerization of the liquid crystal polyester can be advanced so as to improve tensile strength and compressive strength. In the heat-treating step, the method of the heat treatment is not particularly limited to a specific one, and may be, for example, a batch-type heat treatment or a continuous heat treatment by conveyance.

[0072] For example, the batch-type heat treatment may be carried out in a state where the as-spun fiber is wound onto a bobbin in the form of a package, or in a state of hank as well as tow. The heat treatment may be preferably carried out in a package because it can be carried out in simpler equipment and improved productivity. The bobbin needs to be endurable to a temperature of solid phase polymerization, and may be preferably from a metal, such as aluminum, brass, iron, and stainless steel.

[0073] In the case of the continuous heat treatment by conveyance, the conveyance method may be carried out by either contact conveyance (for example, a conveyor type, a support roll type, a heated roller type), or non-contact conveyance (a roll-to-roll type). The processing course may be linear or nonlinear, and may be arranged using a folding roller and / or a guide to suitably change a length, an angle, a curvature of processing course, etc.

[0074] For the heat-treating step, a publicly known method can be used, and examples thereof include atmosphere heating, contact heating, and other heating procedure. Preferable atmosphere may include an atmosphere such as air, inactive gas (for example, nitrogen, argon), or a combined air thereof. In addition, there is no problem even if the heat treatment is carried out under vacuum.

[0075] In the heat-treating step, it is preferable to perform heat treatment at as high a temperature as possible in order to accelerate solid phase polymerization reaction and improve tensile strength and compressive strength. On the other hand, from the viewpoint of suppressing an increase in the ketone bond amount, it is preferable to perform heat treatment at a lower temperature to inhibit occurrence of side reactions. For example, where the melting point of the liquid crystal polyester that is the raw material resin is denoted by Mp 0 , a heat treatment temperature may be preferably from Mp 0 - 30°C to Mp 0 + 30°C, more preferably from Mp 0 - 25°C to Mp 0 + 25°C, and further preferably from Mp 0 - 20°C to Mp 0 + 20°C. In the heat-treating step, since the melting point of the liquid crystal polyester fiber increases with progress of solid phase polymerization, from the viewpoint of efficient strength improvement, the heat treatment temperature may be step-wisely raised in accordance with progress of solid phase polymerization, so that the heat treatment may be carried out at a temperature beyond the melting point (melting point of as-spun fiber) at the time of starting the heat-treating step.

[0076] Depending on the heat treatment procedure and / or the heat treatment temperature, a heat treatment period of the heat-treating step may be set as appropriate. From the viewpoint of enhancing tensile strength and compressive strength and suppressing an increase in the ketone bond amount, the heat treatment period may be preferably from 15 minutes to 30 hours, more preferably from 4 to 25 hours, and further preferably from 8 to 20 hours. Here, the heat treatment period refers to a retention time at a predetermined heat treatment temperature.

[0077] In the method for producing the liquid crystal polyester fiber, for example, in order to improve bundling properties of fibers and to prevent fibers from fusing during the heat treatment, an oil agent may be applied before the heat-treating step. In addition, after the heat treatment, a finishing oil agent may be applied as appropriate depending on the application of the liquid crystal polyester fiber.[Fiber Structure]

[0078] The liquid crystal polyester fiber according to the present invention can be used as a reinforcing fiber for producing a composite material. In the case of using the liquid crystal polyester fiber as a reinforcing fiber, a fiber structure at least partially including the liquid crystal polyester fiber can be used as an intermediate material in the production of the composite material.

[0079] The fiber structure including the liquid crystal polyester fiber according to the present invention can be used as various fiber configurations such as staple fibers, short-cut fibers, filament yarns, spun yarns, cordage, ropes, etc., and also used as various fabrics such as nonwoven fabrics, woven fabrics, and knitted fabrics, using the liquid crystal polyester fibers. Such fibers and fabrics can be produced by known methods using the liquid crystal polyester fibers.

[0080] The fiber structure according to the present invention may be made by combining the liquid crystal polyester fibers with other fibers as long as the effects of the present invention are not impaired. The fiber structure may be, for example, a combined yarn using the liquid crystal polyester fibers and other fibers (e.g., a commingled yarn made from the liquid crystal polyester fibers and other fibers, or others). The fiber structure may also be a blend fabric using the liquid crystal polyester fibers and other fibers (e.g., a combined fabric in which the liquid crystal polyester fibers and other fibers are used in combination, a layered material in which a fabric of the liquid crystal polyester fibers and a fabric of other fibers are used in combination, or others). In the case of using the fiber structure to produce a composite material, the fiber structure may be a combined yarn or blend fabric, including heat-fusible fibers which form the matrix of the composite material as other fibers.

[0081] The liquid crystal polyester fiber according to the present invention can be used in various forms of fiber structures for various applications such as general industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing. For example, the liquid crystal polyester fiber according to the present invention can be used as various fiber products including tension members (e.g., electric cables, optical fibers, etc.), heater wire core yarns, cords for various electrical products such as earphone cords, ropes, sling belts, climbing ropes, life lines, fishing lines, fishing nets, longlines, land nets (safety nets, nets for golf practice ranges, etc.), catheters, reinforcing materials for plastics, concrete, and rubber, base cloth for printed circuit boards, sailcloth, protective clothing, and protective gloves. In particular, the liquid crystal polyester fiber having a low fineness (e.g., a single fiber fineness of 7 dtex or less) can be used for electronic component application such as base cloth for printed circuit boards.[Composite Material]

[0082] According to the present invention, the composite material may be any material which can be obtained by using the liquid crystal polyester fiber as a reinforcing fiber and molding a matrix. The liquid crystal polyester fiber can be molded using the above fiber structure.

[0083] As the matrix, a resin generally used for composite materials can be used, and a thermosetting resin or a thermoplastic resin can be used. Examples of the thermosetting resin include epoxy-based resins, unsaturated polyester-based resins, vinyl ester-based resins, bismaleimide-based resins, phenolic-based resins, urea-based resins, melamine-based resins, thermosetting polyimide-based resins, thermosetting polyurethane-based resins, and benzoxazine-based resins. The type of the thermoplastic resin is not particularly limited to a specific one as long as a softening temperature of the thermoplastic resin is lower than the melting point of the liquid crystal polyester fiber according to the present invention, and examples of the thermoplastic resin include: vinyl-based resins (polymers or derivatives obtained from monomers having vinyl groups CH 2 =CH- or vinylidene groups CH 2 =C<); polyamide-based resins such as aliphatic polyamide-based resins (polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 610, polyamide 612, etc.), and semi-aromatic polyamide-based resins; polyester-based resins such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; fluorine-containing resins such as polytetrafluoroethylene-based resins; polysulfone-based resins such as polysulfonic-based resins and polyethersulfic-based resins; polyether ketone-based resins such as polyether ketone-based resins, polyether ether ketone-based resins, and polyether ketone ketone-based resins; polycarbonate-based resins; polyphenylene ether-based resins; amorphous polyarylate-based resins; and liquid crystal polyester-based resins such as wholly aromatic polyester-based resins. These matrix resins may be used singly, or in combination of two or more.

[0084] Rubber may also be used as the matrix. Examples of the rubber include natural rubber (NR), synthetic natural rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPM, EP, EPDM), chloroprene rubber (CR), acrylic rubber (ACM, ANM), chlorosulfonated polyethylene rubber (CSM), urethane rubber (PUR, U), silicone rubber (Si, Q, VMQ, SR), fluororubber (FKM, FPM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO, ECO), polysulfide rubber (T), and urethane-based, styrene-based, olefin-based, vinyl chloride-based, ester-based, and amide-based thermoplastic elastomers. These matrix rubbers may be used singly, or in combination of two or more.EXAMPLES

[0085] Hereinafter, the present invention will be demonstrated by way of some examples that are presented only for the sake of illustration, which are not to be construed as limiting the scope of the present invention. It should be noted that in the following Examples and Comparative Examples, various properties were evaluated in the following manners.(Melting Point of Resin Chips (Granular Molded Bodies) and Fibers)

[0086] In accordance with JIS K 7121, a melting point was determined as a main endothermic peak temperature observed in measurement using a differential scanning calorimeter (DSC; "DSC60A Plus", produced by SHIMADZU CORPORATION). Specifically, 4 to 6 mg of a sample was taken and sealed in an aluminum pan in the DSC device, and then, the temperature was elevated at a rate of 10°C / min from 25°C while supplying nitrogen as a carrier gas at a flow rate of 200 mL / min to measure an endothermic peak derived from the liquid crystal polyester.(Total Fineness and Single Fiber Fineness)

[0087] In accordance with 8.3.1 Method A in JIS L 1013: 2010, a liquid crystal polyester fiber was reeled into a hank (100 m in total) with 100 rounds each of which had 1 meter using a sizing reel "Wrap Reel by Motor Driven" produced by DAIEI KAGAKU SEIKI MFG. Co., Ltd., to measure a weight of the liquid crystal polyester fiber. The measurement was conducted twice for each sample. Each of the weights (g) was multiplied by 100, and the average value was regarded as a total fineness (dtex) of the liquid crystal polyester fiber. Thus-obtained total fineness was divided by the number of filaments in the liquid crystal polyester fibers so as to give a single-fiber fineness (dtex).(Ketone Bond Amount)

[0088] Ketone bond amount was calculated by the pyrolysis-gas chromatography method described in Polymer Degradation and Stability, 76, 85-94 (2002). Specifically, each of the liquid crystal polyester fiber samples was heated using a thermal decomposition device ("PY2020iD" produced by Frontier Laboratories Ltd.) in the presence of tetramethylammonium hydroxide (TMAH) so as to generate a gas by pyrolysis / methylation. The generated gas was analyzed using a gas chromatography ("GC-6890N" produced by Agilent Technologies, Inc.), and the ketone bond amount (mol%) was calculated from a peak area derived from ketone bonds and a peak area derived from ester bonds.(Total Amount of CEG(s))

[0089] Each of liquid crystal polyester fiber samples was subjected to freeze-grinding until having a particle size of d90 = 100 µm or less (d90: the particle size at which the cumulative volume reaches 90% in the particle size distribution), then to the ground sample was added an excess amount of n-propylamine, followed by heating under agitation at 40°C for 90 minutes to decompose the sample. In this process, the ester bonds present inside the polymer chain are decomposed into carboxylic acid n-propyl amides and hydroxy groups, while the carboxy end groups (CEG) and hydroxy end groups in the polymer chain are unchanged from the carboxy groups and hydroxy groups. The decomposition products were separated by the HPLC method, and peak areas of the separated decomposition products with carboxy groups were compared with calibration curves prepared by HPLC analysis of the respective standard samples so as to quantify the amount (mEq / kg) of the carboxy end groups derived from each of the monomers. For example, the amount of CEG derived from monovalent carboxylic acids such as 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid can be determined by directly quantifying 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. The amount of CEG derived from divalent carboxylic acids such as terephthalic acid, isophthalic acid, and 2,6-naphthalene dicarboxylic acid can be determined by quantifying the amount of amidation products in which one of two carboxy groups is amidated, such as terephthalic acid mono-n-propyl amide, isophthalic acid mono-n-propyl amide, or 2,6-naphthalene dicarboxylic acid mono-n-propyl amide. The sum of the amounts of all carboxy end groups contained in each sample was regarded as the total carboxy end group amount (total CEG amount) (mEq / kg) of the sample.(Tensile Strength)

[0090] With reference to JIS L 1013: 2010 8.5.1, using an autograph "AGS-100B" produced by SHIMADZU CORPORATION, a tensile test was carried out under the conditions of a test sample length of 20 cm and a tensile speed of 10 cm / min 8 times for each sample yarn to obtain an average tensile force (cN), and a tensile strength (cN / dtex) was calculated by dividing the average tensile force (cN) by the total fineness (dtex) measured by the above-described method.(Compressive Strength)

[0091] One single fiber was taken from liquid crystal polyester fibers (multifilament), and this single fiber was placed on a glass plate so as to protrude from an end portion thereof and was fixed thereto using an adhesive. The protruding single fiber was cut using a focused ion beam (FIB) device at a position away from the end portion of the glass plate by a length that was 1.5 times the fiber diameter. Accordingly, a sample, for measurement of the compressive strength of the single fiber, having a cross-section exactly orthogonal to the fiber axis was produced. Using a single fiber compression testing machine ("Model B20-049", produced by THK PRECISION CO., LTD.), an indenter was pressed against the cut surface of the sample for measurement at a speed of 200 nm / sec to obtain a pressing force-strain curve. This measurement was performed for each of five samples, and the average value of the pressing forces at the yield points of the pressing force-strain curves obtained from the five tests was calculated. The average pressing force was divided by the single fiber fineness measured by the above-described method to determine the single fiber compressive strength (cN / dtex).(Metal Element Content)

[0092] Analysis liquid was prepared in accordance with a procedure described in "Microwave Digestion" as indicated below, and a content of metal elements belonging to Groups 8 to 11 in Periodic Table (ppm by weight) was determined by performing ICP-MS measurement.- Microwave Digestion

[0093] Microwave digestion was performed using a microwave digestion device "ETHOS-1" produced by Milestone General K. K. Each of the liquid crystal polyester fiber samples (0.1 g) was weighed and inserted to a quartz insert, and then 6 mL of nitric acid (1.42 mol / L) was added. The quartz insert was put into a digestion vessel containing 5 mL of water and 2 mL of hydrogen peroxide (concentration: 30 to 36 wt%) and sealed, and then microwave digestion was performed. After leaving it to be cooled, the resultant was volumed up to 50 mL and filtered through a filter (pore size: 0.45 µm), and the filtrate was subjected to ICP-MS measurement.- ICP-MS Measurement

[0094] The metal element content of each of the sample solution prepared by the above microwave digestion was analyzed using an ICP-MS analyzer "Agilent 7900" produced by Agilent Technologies, Inc. Under the conditions of a carrier gas flow rate of 0.7 L / min and an RF output of 1500 W, measurement was performed three times on the same sample solution in comparison with XSTC-622 (standard solution produced by SPEX CertiPrep), and the content of each metal element was determined from the average value of these measurements.

[0095] For samples such as fibers to which an oil agent adheres, if the oil agent might contain some metal elements to affect the measurement, microwave digestion may be performed after the oil agent is removed by the following method.- Oil Agent Removal

[0096] Into an aqueous solution in which 2 g of a nonionic surfactant ("Actinol F-9" available from Matsumoto Yushi-Seiyaku Co., Ltd.) was dissolved in 1 L of ion exchange water, was added a liquid crystal polyester fiber sample in an amount of 100 g or less, followed by temperature control in a range of 60 to 90°C, and then the resultant mixture was shaken for 40 minutes. Thereafter the liquid crystal polyester fiber sample was taken out from the aqueous solution, and rinsed two times at each time for 40 minutes with 1 L of ion exchange water with a controlled temperature in a range of 60 to 90°C. The liquid crystal polyester fiber sample was taken out and dried at 80°C under air atmosphere for 3 hours or longer using a hot air dryer "DN63HI" produced by Yamato Scientific Co., Ltd. so as to obtain a liquid crystal polyester fiber sample from which the oil agent was removed.(Disc Fatigue Test)

[0097] Six liquid crystal polyester fibers each having a total fineness of 280 dtex were combined and were first-twisted (Z-twisted) with a twist number of 328 T / m, and three first-twisted strands were second-twisted (S-twisted) with a twist number of 268 T / m to produce a cord.

[0098] This cord was subjected to the following dipping process to obtain a dip cord.First Time:

[0099] A dipping process was performed using a dipping liquid prepared by mixing 1 wt% of MARPOMERCE (produced by Matsumoto Yushi-Seiyaku Co., Ltd.), 4 wt% of DENACOL EX313 (produced by Nagase ChemteX Corporation), 0.3 wt% of a sodium hydroxide aqueous solution (10 wt%), and 94.7 wt% of water. Then, the dipped cord was dried at 150°C for 30 seconds, and then was heat-treated at 240°C for 30 seconds.Second Time:

[0100] A dipping process was performed using an RFL liquid as a dipping liquid. Then, the dipped cord was dried at 150°C for 30 seconds, and then was heat-treated at 240°C for 30 seconds. The RFL liquid was prepared as follows. A liquid A was prepared by mixing 3.3 wt% of resorcinol, 2.46 wt% of formaldehyde (37 wt%), 1.4 wt% of a sodium hydroxide aqueous solution (10 wt%), and 32.2 wt% of water and was left to mature at a temperature of 25°C for 6 hours. A liquid B was prepared by mixing 52.86 wt% of a VP latex (JSR-0650; produced by JSR Corporation) and 7.78 wt% of water, was mixed with the matured liquid A, and then was left to mature at a temperature of 25°C for 16 hours to prepare the RFL liquid.

[0101] The obtained dipped cord was embedded in a rubber (SBR : NR = 1:1) and was vulcanized at 150°C for 30 minutes to produce a rubber composite specimen. With reference to JIS L 1017: 2002, a disc fatigue test was conducted by the Goodrich method under the following conditions. Device: disc fatigue tester (produced by Mys-Shikenki Co., Ltd.) Disc interval: 24.5 mm Strain amount: 2% (elongation rate, compression rate) Rotation speed: 2500 rpm Temperature: 100°C Number of times: 300,000

[0102] For each dipped cord taken out from the rubber composite specimen before and after the disc fatigue test, with reference to JIS L 1013: 2010 8.5.1, using an autograph "AGS-100B" produced by SHIMADZU CORPORATION, a tensile test was carried out under the conditions of a specimen length of 20 cm and a tensile speed of 10 cm / min 6 times for one sample, so as to determine an average tensile force thereof as a tensile force (N) of each dipped cord before and after the disc fatigue test. A strength retention rate (%) was calculated from the following formula. [Example 1]

[0103] Chips (granular molded bodies) of a liquid crystal polyester (α) (Mp 0 : 309°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 60 / 20 / 15 / 5 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder (Φ 15 mm) (produced by TECHNOVEL CORPORATION) to be melt-kneaded, and the melt-kneaded material was fed to a spinning head. The spinning head was equipped with a spinneret having 50 holes each having a hole diameter of 0.10 mm φ, a spinning head temperature was set to 330°C, and the melt-kneaded material was discharged at a discharge rate of 28 g / min and wound onto a bobbin at a winding rate of 1000 m / min to obtain as-spun fibers of liquid crystal polyester fibers.

[0104] Next, 500 m of the as-spun fibers obtained in the above process was rewound onto an aluminum bobbin at a winding density of 0.6 g / cm 3< , and subjected to a heat treatment process under a nitrogen atmosphere using a closed oven at 300°C for 16 hours to obtain heat-treated fibers of liquid crystal polyester filaments. The evaluation results of the obtained liquid crystal polyester fibers are shown in Table 5.[Example 2]

[0105] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (β) (Mp 0 : 275°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 45 / 27.5 / 13.75 / 13.75 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.[Example 3]

[0106] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 300°C for 24 hours.[Example 4]

[0107] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the spinning head temperature was set to 360°C.[Example 5]

[0108] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 360°C for 32 hours.[Example 6]

[0109] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (γ) (Mp 0 : 278°C) having a structural unit derived from 4-hydroxybenzoic acid and a structural unit derived from 6-hydroxy-2-naphthoic acid at a ratio of 73 / 27 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 320°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.[Example 7]

[0110] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that heat treatment was performed on the as-spun fibers obtained in Example 6 under a nitrogen atmosphere at 270°C for 24 hours.[Example 8]

[0111] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 290°C for 16 hours.[Example 9]

[0112] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 270°C for 16 hours.[Comparative Example 1]

[0113] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that the spinning head temperature was set to 360°C.[Comparative Example 2]

[0114] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 6, except that heat treatment was performed on the as-spun fibers obtained in Example 6 under a nitrogen atmosphere at 320°C for 32 hours.[Comparative Example 3]

[0115] Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 240°C for 16 hours.[Comparative Example 4]

[0116] As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the spinning head temperature was set to 380°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 360°C for 40 hours.[Comparative Example 5]

[0117] To 5 L of acetonitrile (available from FUJIFILM Wako Pure Chemical Corporation, special grade reagent), were added two kinds of reagents, copper(I) iodide (available from FUJIFILM Wako Pure Chemical Corporation, special grade reagent) at an amount of 1 mol and 1,10-phenanthroline (available from FUJIFILM Wako Chemical Corporation) in equal molar amount with copper(I) iodide, stirred in a condition of suspension for 1 hour, filtered, and dried at 100°C for 3 hours to obtain an orange solid (melting point: 300°C).

[0118] This solid was added as a polymerization catalyst to resin chips of the liquid crystal polyester (γ) at a proportion of 10 ppm by weight in terms of copper atoms (a content of copper element based on a total content of the resin chips and the polymerization catalyst), and the mixture was sufficiently mixed using a shaking device. Thus-obtained blend of the resin chips and the polymerization catalyst was dried by hot air at 120°C for at least 4 hours. As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that the blend was used as a raw material and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 250°C for 3 hours. [Table 5]Ex. 1Ex. 2Ex. 3Ex. 4Ex. 5Ex. 6Ex. 7Ex. 8Ex. 9Raw material resinLiquid crystal polyester typeαβαααγγααMelting point Mp 0 [°C]3092753093093092782783093092,6-Naphthylene group-containing structural unit [mol%]8072.580808027278080Metal element type---------Metal element content added [ppm by weight]---------Production conditionsSpinning head temperature [°C]330310330360330320320330330Heat treatment temperature [°C]300270300300360270270290270Heat treatment period [h]161624163216241616Total fineness [dtex]280280280280280280280280280Single fiber fineness [dtex]5.65.65.65.65.65.65.65.65.6Number of filaments505050505050505050Melting point [°C]345330350352360330333340332Liquid crystal polyester fibersKetone bond amount [mol%]0.00110.00050.00200.00350.00540.02560.03340.00100.0008Total CEG amount [mEq / kg]44.430.326.214.59.43.12.762.677.5Tensile strength [cN / dtex]312829272624232824Single fiber compressive strength [cN / dtex]0.980.850.760.680.670.650.610.800.66Metal element content [ppm by weight]---------Disc fatigue test625957464538355839Strength retention rate [%] [Table 5-continued] Comp. Ex. 1Comp. Ex. 2Comp. Ex. 3Comp. Ex. 4Comp. Ex. 5Raw material resinLiquid crystal polyester typeγγααγMelting point Mp 0 [°C]2782783093092782,6-Naphthylene group-containing structural unit [mol%]2727808027Metal element type----CopperMetal element content added [ppm by weight]----10Production conditionsSpinning head temperature [°C]360320330380310Heat treatment temperature [°C]270320240360250Heat treatment period [h]163216403Total fineness [dtex]280280280280280Single fiber fineness [dtex]5.65.65.65.65.6Number of filaments5050505050Melting point [°C]335340315373330Liquid crystal polyester fibersKetone bond amount [mol%]0.05540.07220.00030.05420.0180Total CEG amount [mEq / kg]2.82.386.69.82.3Tensile strength [cN / dtex]1917132225Single fiber compressive strength [cN / dtex]0.620.600.500.470.54Metal element content [ppm by weight]----9Disc fatigue test2220232429Strength retention rate [%]

[0119] As shown in Table 5, the liquid crystal polyester fibers of Examples 1 to 9 have a ketone bond amount, a tensile strength, and a single fiber compressive strength in specific ranges, and thus have a strength retention rate of 30% or more in the disc fatigue test and excellent disc fatigue resistance.

[0120] In particular, the liquid crystal polyester fibers of Examples I to 5 and 8 include a liquid crystal polyester having a large number of structural units including a 2,6-naphthylene group, have a lower ketone bond amount (0.005 mol% or less), a higher tensile strength (26 cN / dtex or more), and a higher single fiber compressive strength (0.67 cN / dtex or more), and thus have a strength retention rate of 40% or more in the disc fatigue test and more excellent disc fatigue resistance.

[0121] Furthermore, among these, the liquid crystal polyester fibers of Examples 1 to 3 and 8 include a liquid crystal polyester having a large number of structural units including a 2,6-naphthylene group, are obtained by adjusting the spinning head temperature, the heat treatment temperature, and the heat treatment period, have a very low ketone bond amount (0.002 mol% or less), a very high tensile strength (28 cN / dtex or more), and a very high single fiber compressive strength (0.76 cN / dtex or more), and thus have a strength retention rate of 50% or more in the disc fatigue test and very excellent disc fatigue resistance.

[0122] On the other hand, in Comparative Example 1, although the raw material resin of the same liquid crystal polyester (γ) as in Examples 6 and 7 was used, since the spinning head temperature was high, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 1 have inferior disc fatigue resistance compared to those of Examples 6 and 7.

[0123] In Comparative Example 2, although the raw material resin of the same liquid crystal polyester (γ) as in Examples 6 and 7 was used, since the heat treatment temperature was high and the heat treatment period was long, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions and a low tensile strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 2 have inferior disc fatigue resistance compared to those of Examples 6 and 7.

[0124] In Comparative Example 3, although the raw material resin of the same liquid crystal polyester (α) as in Examples 1, 3 to 5, 8, and 9 was used, since the heat treatment temperature was low, solid phase polymerization did not proceed, and liquid crystal polyester fibers having low tensile strength and compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 3 have inferior disc fatigue resistance compared to those of Examples 1, 3 to 5, 8, and 9.

[0125] In Comparative Example 4, although the raw material resin of the same liquid crystal polyester (α) as in Examples 1, 3 to 5, 8, and 9 was used, since the spinning head temperature was high, the heat treatment temperature was high, and the heat treatment period was long, liquid crystal polyester fibers having a ketone bond amount increased due to side reactions and a low compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 4 have inferior disc fatigue resistance compared to those of Examples 1, 3 to 5, 8, and 9.

[0126] In Comparative Example 5, since the raw material resin contains a copper element-containing compound, perhaps since polymer chains could not be highly oriented during spinning, and the interactions between the polymer chains were reduced, liquid crystal polyester fibers having a low compressive strength were obtained. Therefore, the liquid crystal polyester fibers of Comparative Example 5 have inferior disc fatigue resistance compared to those of Examples 1 to 9 and cannot be considered sufficient when used as reinforcing fibers in a predetermined composite material application.INDUSTRIAL APPLICABILITY

[0127] The liquid crystal polyester fiber according to the present invention can be used for various applications such as general industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing, and can be used, for example, as a reinforcing fiber for an automotive rubber material, an electronic circuit board, or the like that require durability against vibration.

[0128] Although the preferred embodiments of the present invention have been described, various additions, modifications, or deletions may be made without departing from the scope of the invention. Accordingly, such variants are included within the scope of the present invention.

Examples

example 1

[Example 1]

[0103]Chips (granular molded bodies) of a liquid crystal polyester (α) (Mp 0 : 309°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 60 / 20 / 15 / 5 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder (Φ 15 mm) (produced by TECHNOVEL CORPORATION) to be melt-kneaded, and the melt-kneaded material was fed to a spinning head. The spinning head was equipped with a spinneret having 50 holes each having a hole diameter of 0.10 mm φ, a spinning head temperature was set to 330°C, and the melt-kneaded material was discharged at a discharge rate of 28 g / min and wound onto a bobbin at a winding rate of 1000 m / min to obtain as-spun fibers of liquid crystal polyester fibers.

[0104]Next, 500 m of the as-spun fibers obtained in the abov...

example 2

[Example 2]

[0105]As-spun fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that a liquid crystal polyester (β) (Mp 0 : 275°C) having a structural unit derived from 6-hydroxy-2-naphthoic acid, a structural unit derived from 2,6-naphthalenedicarboxylic acid, a structural unit derived from hydroquinone, and a structural unit derived from 4,4'-dihydroxybiphenyl at a ratio of 45 / 27.5 / 13.75 / 13.75 (mol%) was used instead of the liquid crystal polyester (α) and the spinning head temperature was set to 310°C. Next, heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the obtained as-spun fibers under a nitrogen atmosphere at 270°C for 16 hours.

example 3

[Example 3]

[0106]Heat-treated fibers of liquid crystal polyester filaments were obtained in the same manner as in Example 1, except that heat treatment was performed on the as-spun fibers obtained in Example 1 under a nitrogen atmosphere at 300°C for 24 hours.

Claims

1. A liquid crystal polyester fiber having a ketone bond amount of 0.050 mol% or less, a tensile strength of 18 cN / dtex or more, and a single fiber compressive strength of 0.55 cN / dtex or more.

2. The liquid crystal polyester fiber according to claim 1, having a total amount of carboxy end groups (total CEG amount) of more than 5.0 mEq / kg and 85.0 mEq / kg or less.

3. The liquid crystal polyester fiber according to claim 1 or 2, comprising a liquid crystal polyester comprising a structural unit including a 2,6-naphthylene group at a proportion of 28 mol% or more based on a total content of all structural units.

4. A fiber structure at least partially comprising the liquid crystal polyester fiber as recited in claim 1 or 2.

5. A composite material comprising the liquid crystal polyester fiber as recited in claim 1 or 2 as a reinforcing fiber.

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