Liquid crystal polyester fiber and its manufacturing method

Abstract

Provided is a liquid crystal polyester fiber that has excellent tensile strength and compressive strength and has excellent disk fatigue performance. The liquid crystal polyester fiber has a melting point of 335 °C to 360 °C (exclusive of 360 °C), as measured by a differential scanning calorimeter under a nitrogen atmosphere at a heating rate of 10 °C / min, and a crystallinity of 45-60%. The total number of voids and unmelted substances, with a major diameter of at least 5 μm, contained per 5 cm of single yarn, is at most 3. For example, the liquid crystal polyester fiber may have an orthorhombic crystallinity of at least 20% in a crystalline component.

Description

Related Applications

[0001] This application claims priority from Japanese Patent Application No. 2022-199789, filed on December 14, 2022, the entire contents of which are incorporated herein by reference. [Technical Field]

[0002] The present invention relates to a liquid crystal polyester fiber and a method for producing the same. [Background technology]

[0003] Liquid crystal polyester fibers are made from liquid crystal polyesters with a rigid molecular structure. By highly aligning the molecular chains along the fiber axis during melt spinning and then subjecting the fibers to prolonged heat treatment at high temperatures, they are known to exhibit the highest strength of any synthetic fiber obtained by melt spinning. Furthermore, heat treatment increases the molecular weight and melting point of liquid crystal polyester fibers, improving their heat resistance and dimensional stability. These fibers can be used in a variety of applications, including general industrial materials, civil engineering and construction materials, various reinforcing materials, electrical and electronic component materials, and protective clothing. For example, reinforcing material applications require good mechanical properties, while electronic component applications, such as circuit board materials, require not only the mechanical properties required for reinforcing materials but also heat resistance sufficient to withstand the thermal processing required during component assembly.

[0004] Patent Document 1 (WO 2019 / 142692) describes that by heat treating a liquid crystal polyester fiber having a longitudinal orientation degree of 89% or more and 95% or less at a temperature of 250°C or more, a liquid crystal polyester fiber having a tensile strength of 25 cN / dtex or more and 30 cN / dtex or less and having high strength can be obtained.

[0005] Patent Document 2 (JP 2010-43380 A) discloses a material for fiber production, which comprises a liquid crystal polyester having, as repeating units, structural units derived from an aromatic hydroxycarboxylic acid, structural units derived from an aromatic dicarboxylic acid, and structural units derived from an aromatic diol, in which 40 mol % or more of these structural units are 2,6-naphthalenediyl groups, and which has a flow initiation temperature of 280 to 320°C, and describes that a liquid crystal polyester fiber having high heat resistance can be obtained.

[0006] Patent Document 3 (JP 2022-6590 A) describes a manufacturing method in which liquid crystal polyester is processed into fibers by melt spinning and heated at a temperature of 330°C or higher, and the maximum crystallite size is 120 × 10 -10 The patent describes a liquid crystal polyester fiber having a viscosity of 1000 MPa or more and having improved heat resistance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2019 / 142692 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-43380 [Patent Document 3] Japanese Patent Publication No. 2022-6590 Summary of the Invention [Problem to be solved by the invention]

[0008] However, while liquid crystal polyester fibers are strong against stress in the tensile direction, they are relatively weak against stress in the compressive direction, and further improvement in their mechanical properties has been necessary depending on the application. For example, reinforcing fibers used in rubber materials such as tires, timing belts, and rubber hoses are subjected to stress in both the tensile and compressive directions relative to the fiber axis, so high strength and fatigue resistance against stress in both directions are required.

[0009] Patent Document 1 describes that the tensile strength of the liquid crystal polyester fiber is improved with respect to its mechanical properties. However, there is room for further improvement in the tensile strength, and it does not describe the compressive strength or fatigue resistance.

[0010] Patent Document 2 describes that the viscosity of the liquid crystal polyester can be reduced during melt spinning, thereby making it possible to easily form fibers and obtain fibers having high heat resistance. Although the spinnability of the liquid crystal polyester is evaluated, the physical properties of the obtained fibers are not evaluated, and no mechanical properties are described.

[0011] Patent Document 3 describes increasing the melting point of the liquid crystal polyester fiber, but does not describe the mechanical properties of the liquid crystal polyester fiber that is actually obtained.

[0012] The present invention has been made to solve the above problems, and an object of the present invention is to provide a liquid crystal polyester fiber that is excellent not only in tensile strength and compressive strength but also in fatigue resistance against tensile deformation and compressive deformation. [Means for solving the problem]

[0013] The inventors of the present invention conducted extensive research to achieve the above-mentioned object and found that, depending on the raw material liquid crystal polyester used and the conditions during melt spinning of the liquid crystal polyester fiber, unmelted materials and voids may occur in the fiber, resulting in a decrease in the mechanical properties of the fiber. Further research led to the discovery that by using a liquid crystal polyester having a specific weight-average molecular weight and melting point and adjusting the conditions in the extruder and the spinning temperature to apply shear to the molten resin during melt mixing and spinning, the occurrence of unmelted materials and voids can be suppressed and high orientation can be achieved. Furthermore, by heat-treating this highly oriented spun raw yarn with few unmelted materials and voids, the melting point and crystallinity can be increased, resulting in a liquid crystal polyester fiber that not only has sufficient tensile strength and compressive strength but also has excellent fatigue resistance (hereinafter sometimes referred to as disk fatigue resistance) in a disk fatigue test in which tensile deformation and compressive deformation are alternately applied. This discovery led to the completion of the present invention.

[0014] That is, the present invention can be configured in the following manner. [Aspect 1] A liquid crystal polyester fiber having a melting point of 335°C or higher but lower than 360°C (preferably 337°C or higher but lower than 358°C, more preferably 340°C or higher but lower than 355°C), as measured with a differential scanning calorimeter in a nitrogen atmosphere at a heating rate of 10°C / min, a crystallinity of 45 to 60% (preferably 46 to 58%, more preferably 48 to 55%), and a total number of unmelted particles and voids with a major diameter of 5 μm or higher per 5 cm of a single yarn of 3 or less (preferably 2 or less, more preferably 1 or less). [Aspect 2] The liquid crystal polyester fiber according to aspect 1, wherein the degree of orthorhombic crystallinity in the crystalline component is 20% or more (preferably 26% or more, more preferably 27% or more). Aspect 3 The liquid crystal polyester fiber according to aspect 1 or 2, wherein the degree of orientation in the fiber axis direction of the crystalline component is 97% or more and less than 100%. Aspect 4 A liquid crystal polyester fiber according to any one of aspects 1 to 3, having a tensile strength of 27 cN / dtex or more (preferably 28 cN / dtex or more, more preferably 30 cN / dtex or more). Aspect 5 A liquid crystal polyester fiber according to any one of aspects 1 to 4, wherein the compressive strength of a single fiber is 0.70 cN / dtex or more (preferably 0.75 cN / dtex or more, more preferably 0.80 cN / dtex or more). Aspect 6 A liquid crystal polyester fiber according to any one of aspects 1 to 5, wherein the single fiber fineness is 7 dtex or less, and the coefficient of variation of the single fiber fineness is 7% or less (preferably 6% or less, more preferably 5% or less). Aspect 7 A fiber structure at least partially comprising the liquid crystal polyester fiber according to any one of the first to sixth aspects. Aspect 8 A composite material comprising the liquid crystal polyester fiber according to any one of the first to sixth aspects as a reinforcing fiber. Aspect 9 A method for producing the liquid crystal polyester fiber according to any one of aspects 1 to 6, A liquid crystalline polyester having a weight average molecular weight in terms of polystyrene measured by GPC of 50,000 or more and 160,000 or less (preferably 60,000 or more and 150,000 or less, more preferably 80,000 or more and 130,000 or less) and a melting point Mp0 measured by a differential scanning calorimeter under a nitrogen atmosphere at a heating rate of 10°C / min of 300°C or more (preferably 300 to 340°C, more preferably 305 to 330°C), is provided, which is selected from the following (1) to (3): (1) Extruder temperature: Mp0+15°C to Mp0+35°C (preferably Mp0+16°C to Mp0+30°C, more preferably Mp0+17°C to Mp0+25°C) (2) Extruder screw peripheral speed: 10 to 40 m / min (preferably 15 to 35 m / min, more preferably 17 to 25 m / min) (3) Spinneret temperature: Extruder temperature -15°C to Extruder temperature -5°C a step of obtaining a raw spinning yarn by spinning under conditions satisfying the above; a step of heat treating the obtained raw spinning yarn; A method for producing a liquid crystal polyester fiber, comprising at least Aspect 10 The method for producing a liquid crystalline polyester fiber according to embodiment 9, wherein the extruder is a twin-screw extruder. Aspect 11 The method for producing a liquid crystal polyester fiber according to aspect 9 or 10, wherein the heat treatment temperature is 250 to 350°C (preferably 255 to 320°C, more preferably 260 to 315°C, and even more preferably 280 to 310°C).

[0015] As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms, including "at least one," unless the content clearly dictates otherwise. As used herein, the terms "and / or," "at least one," and "one or more" include any and all combinations of the associated listed items.

[0016] It should be noted that any combination of at least two elements disclosed in the claims and / or the specification is included in the present invention, and in particular any combination of two or more of the claims set forth in the claims is included in the present invention. [Effects of the Invention]

[0017] The liquid crystal polyester fiber of the present invention is excellent in tensile strength and compressive strength, and is also excellent in disk fatigue resistance. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Liquid crystal polyester fiber] The liquid crystal polyester fiber of the present invention includes a liquid crystal polyester. The liquid crystal polyester is composed of structural units derived from, for example, aromatic diols, aromatic dicarboxylic acids, aromatic hydroxycarboxylic acids, etc., and the structural units derived from aromatic diols, aromatic dicarboxylic acids, and aromatic hydroxycarboxylic acids are not particularly limited in chemical structure as long as the effects of the present invention are not impaired. Furthermore, the liquid crystal polyester may also contain structural units derived from aromatic diamines, aromatic hydroxyamines, or aromatic aminocarboxylic acids, as long as the effects of the present invention are not impaired. For example, preferred structural units are shown in Table 1.

[0019] [Table 1]

[0020] In the structural units in Table 1, m is an integer of 0 to 2, and Y in the formula, in the range of 1 to the maximum number of possible substitution, each independently represents a hydrogen atom, a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc.), an alkyl group (for example, an alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, an isopropyl group, a t-butyl group, etc.), an alkoxy group (for example, a methoxy group, an ethoxy group, an isopropoxy group, a n-butoxy group, etc.), an aryl group (for example, a phenyl group, a naphthyl group, etc.), an aralkyl group (for example, a benzyl group (phenylmethyl group), a phenethyl group (phenylethyl group), etc.), an aryloxy group (for example, a phenoxy group), an aralkyloxy group (for example, a benzyloxy group, etc.).

[0021] More preferred structural units include the structural units described in Examples (1) to (20) shown in the following Tables 2, 3, and 4. When the structural unit in the formula is a structural unit that can exhibit multiple structures, two or more of such structural units may be combined and used as structural units that constitute the polymer.

[0022] [Table 2]

[0023] [Table 3]

[0024] [Table 4]

[0025] In the structural units of Tables 2, 3, and 4, n is an integer of 1 or 2, and each of the structural units n=1 and n=2 may exist alone or in combination, and Y1 and Y2 may each independently be a hydrogen atom, a halogen atom (e.g., a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc.), an alkyl group (e.g., an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, an isopropyl group, or a t-butyl group), an alkoxy group (e.g., a methoxy group, an ethoxy group, an isopropoxy group, an n-butoxy group, etc.), an aryl group (e.g., a phenyl group, a naphthyl group, etc.), an aralkyl group (e.g., a benzyl group (phenylmethyl group), a phenethyl group (phenylethyl group), etc.), an aryloxy group (e.g., a phenoxy group), an aralkyloxy group (e.g., a benzyloxy group), etc. Among these, a hydrogen atom, a chlorine atom, a bromine atom, or a methyl group is preferred.

[0026] Furthermore, examples of Z include substituents represented by the following formulas.

[0027] [ka]

[0028] The liquid crystal polyester may preferably contain at least one structural unit selected from the group consisting of a structural unit represented by the following formula (I) (structural unit (I)), a structural unit represented by the following formula (II) (structural unit (II)), 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) (In the formula, Ar 1 is at least one selected from the group consisting of a phenylene group, a naphthylene group, and a biphenylylene group, and Ar 2 , Ar 3 and Ar 4 are each independently at least one selected from the group consisting of a phenylene group, a naphthylene group, a biphenylylene group, and a diphenyletherdiyl group, and Ar 1 , Ar 2 , Ar 3 and Ar 4 The hydrogen atoms of the aromatic ring may each independently be substituted with a halogen atom, an alkyl group, an alkoxy group, an aryl group, an aralkyl group, an aryloxy group, or an aralkyloxy group.

[0029] The structural unit (I) is a structural unit derived from an aromatic hydroxycarboxylic acid, and Ar 1 However, a structural unit which is a 1,4-phenylene group (a structural unit derived from 4-hydroxybenzoic acid) and a structural unit which is a 2,6-naphthylene group (a structural unit derived from 6-hydroxy-2-naphthoic acid) are preferred.

[0030] The structural unit (II) is a structural unit derived from an aromatic dicarboxylic acid, and Ar 2 However, preferred are structural units which are 1,4-phenylene groups (structural units derived from terephthalic acid), structural units which are 1,3-phenylene groups (structural units derived from isophthalic acid), structural units which are 2,6-naphthylene groups (structural units derived from 2,6-naphthalenedicarboxylic acid), and structural units which are diphenylether-4,4'-diyl groups (structural units derived from diphenylether-4,4'-dicarboxylic acid).

[0031] The structural unit (III) is a structural unit derived from an aromatic diol, and Ar 3 However, preferred are structural units which are 1,4-phenylene groups (structural units derived from hydroquinone), structural units which are 4,4'-biphenylylene groups (structural units derived from 4,4'-dihydroxybiphenyl), structural units which are phenyl-1,4-phenylene groups (structural units derived from phenylhydroquinone), and structural units which are diphenylether-4,4'-diyl groups (structural units derived from 4,4'-dihydroxydiphenyl ether).

[0032] The structural unit (IV) is a structural unit derived from an aromatic hydroxyamine, and is represented by Ar 4 However, a structural unit which is a 1,4-phenylene group (a structural unit derived from 4-aminophenol) and a structural unit which is a 4,4'-biphenylylene group (a structural unit derived from 4-amino-4'-hydroxybiphenyl) are preferred.

[0033] The content of the structural unit (I) in the liquid crystal polyester may be 20 to 80 mol %, preferably 30 to 75 mol %, and more preferably 40 to 70 mol %, based on the total amount of all structural units.

[0034] The content of the structural unit (II) in the liquid crystal polyester may be 10 to 40 mol %, preferably 12.5 to 35 mol %, and more preferably 15 to 30 mol %, based on the total amount of all structural units.

[0035] The total content of the structural units (III) and (IV) in the liquid crystal polyester may be 10 to 40 mol %, preferably 12.5 to 35 mol %, and more preferably 15 to 30 mol %, based on the total amount of all structural units.

[0036] From the viewpoint of easily increasing the molecular weight of the liquid crystal polyester and improving its mechanical properties, the molar ratio of the content of the structural unit (II) to the total content of the structural units (III) and (IV), expressed 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 even more preferably 100 / 100.

[0037] The liquid crystal polyester may contain two or more of each of the structural units (I) to (IV). The content of each structural unit represents the content of all structural units corresponding to that structural unit. For example, when the liquid crystal polyester contains two or more types of structural unit (I), the content of the structural unit (I) represents the total content thereof.

[0038] The liquid crystal polyester may have a total content of the structural units (I) to (IV) of, for example, 90 mol% or more, preferably 95 mol% or more, more preferably 99 mol% or more, and even more preferably 100 mol%, based on the total amount of all structural units.

[0039] In order to provide the liquid crystal polyester with a highly crystalline structure and increase the melting point and crystallinity, it is preferable that the liquid crystal polyester has a naphthalene skeleton as a structural unit.For example, the total content of structural units containing 2,6-naphthylene groups in the liquid crystal polyester may be 40 mol% or more, preferably 50 mol% or more, more preferably 65 mol% or more, even more preferably 68 mol% or more, even more preferably 70 mol% or more, and may be 90 mol% or less, preferably 85 mol% or less, more preferably 83 mol% or less, even more preferably 80 mol% or less, based on the total amount of all structural units.As the structural unit containing 2,6-naphthylene groups, Ar 1 is a 2,6-naphthylene group (structural unit (I) derived from 6-hydroxy-2-naphthoic acid) and Ar 2 is a 2,6-naphthylene group (structural unit (II) derived from 2,6-naphthalenedicarboxylic acid) is preferred.

[0040] The liquid crystal polyester fiber may contain thermoplastic polymers such as polyethylene terephthalate, modified polyethylene terephthalate, polyolefin, polycarbonate, polyamide, polyphenylene sulfide, polyether ether ketone, fluororesin, etc., within the range that does not impair the effects of the present invention. Furthermore, the liquid crystal polyester fiber may contain various additives such as inorganic substances such as titanium oxide, kaolin, silica, barium oxide, etc., colorants such as carbon black, dyes, and pigments, antioxidants, ultraviolet absorbers, and light stabilizers.

[0041] The liquid crystal polyester fiber of the present invention may contain 50% by weight or more of liquid crystal polyester, preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 95% by weight or more, and still more preferably 99.9% by weight or more.

[0042] The liquid crystal polyester fiber of the present invention has a melting point of 335°C or higher and lower than 360°C, as measured using a differential scanning calorimeter in a nitrogen atmosphere at a temperature increase rate of 10°C / min. The liquid crystal polyester fiber of the present invention has a high melting point and is excellent in heat resistance. The melting point of the liquid crystal polyester fiber may be preferably 337°C or higher and 358°C or lower, more preferably 340°C or higher and 355°C or lower. The melting point of the liquid crystal polyester fiber is a value measured by the method described in the examples below.

[0043] The liquid crystal polyester fiber of the present invention has a crystallinity of 45 to 60%. Since the crystallinity of the liquid crystal polyester fiber of the present invention is within a specific range, it has excellent mechanical properties. If the crystallinity is too low, the orientation in the fiber axis direction tends to be low, resulting in low tensile strength and compressive strength. If the crystallinity is too high, toughness tends to be impaired, resulting in poor compressive strength and disc fatigue resistance. The crystallinity of the liquid crystal polyester fiber may be preferably 46 to 58%, more preferably 48 to 55%. In this specification, the crystallinity of the liquid crystal polyester fiber is calculated from the diffraction peaks (e.g., diffraction peaks appearing at diffraction angles 2θ=19 to 21° and 2θ=27°, etc.) derived from each crystal in the X-ray diffraction profile obtained by wide-angle X-ray diffraction measurement, and is a value measured by the method described in the Examples below.

[0044] The liquid crystal polyester fiber of the present invention has a total number of unmelted particles and voids with a major axis of 5 μm or more per 5 cm of a single yarn of 3 or less. In this specification, unmelted particles are derived from the liquid crystal polyester that did not melt during spinning and generally refer to microcrystals with an orientation different from the crystals oriented in the fiber axis direction, and can be confirmed by observation with a polarizing microscope. Furthermore, voids are residual bubbles generated by decomposition gases generated from the resin during spinning or bubbles generated due to poor melt-kneading, and refer to bubbles present inside or on the side of the fiber, and can be confirmed by observation with a microscope using transmitted light.

[0045] The liquid crystal polyester fiber of the present invention not only has a crystallinity within a specific range, but also has few unmelted particles and voids, resulting in excellent mechanical properties. When unmelted particles or voids are present in the fiber, these areas are locally weak defects, and when a tensile force is applied, these areas become the starting point for breakage, resulting in low tensile strength. Furthermore, when a compressive force is applied to liquid crystal polyester fibers, buckling deformation known as kink bands occurs, resulting in a decrease in strength. Since kink bands originating from unmelted particles or voids are likely to occur, the compressive strength is consequently low. Furthermore, since unmelted particles and voids are defects that occur during spinning, the presence of such defects (particularly voids) can easily change the single fiber fineness, impairing the uniformity of the resulting fibers.

[0046] When these unmelted particles and voids are present at a certain size, deterioration in mechanical properties and fiber uniformity is likely to occur. Therefore, in the present invention, unmelted particles and voids with a major diameter (maximum diameter) of 5 μm or more are measured. The total number of unmelted particles and voids with a major diameter of 5 μm or more contained per 5 cm of a single fiber is preferably 2 or less, more preferably 1 or less. The total number of unmelted particles and voids is a value measured by the method described in the Examples below. When targeting short fibers of liquid crystal polyester fibers with a fiber length of less than 5 cm, multiple short fibers with a total length of 5 cm are observed, and the number of unmelted particles and voids with a major diameter of 5 μm or more contained in the multiple short fibers is counted, thereby measuring the total number of unmelted particles and voids with a major diameter of 5 μm or more contained per 5 cm.

[0047] The liquid crystal polyester fiber of the present invention may have a degree of orthorhombic crystallinity in the crystalline component of 20% or more. Liquid crystal polyesters have crystalline components such as orthorhombic and hexagonal crystallinity, and a high degree of orthorhombic crystallinity in these crystalline components tends to further improve heat resistance and mechanical properties. The degree of orthorhombic crystallinity can be adjusted by the composition of the liquid crystal polyester used and production conditions such as spinning conditions and heat treatment conditions. The degree of orthorhombic crystallinity may be preferably 26% or more, more preferably 27% or more. The upper limit of the degree of orthorhombic crystallinity is not particularly limited, but may be, for example, 35% or less. In this specification, the degree of orthorhombic crystallinity of the liquid crystal polyester fiber is calculated from diffraction peaks derived from orthorhombic and hexagonal crystallinity in the X-ray diffraction profile obtained by wide-angle X-ray diffraction measurement (for example, diffraction peaks appearing around a diffraction angle 2θ of 19 to 21°, etc.), and is a value measured by the method described in the Examples below.

[0048] The liquid crystal polyester fiber of the present invention may have a degree of orientation of the crystalline component in the fiber axis direction of 97% or more but less than 100%. The degree of orientation is an index showing the degree to which the crystalline component is oriented in the fiber axis direction, and the higher the degree of orientation, the more improved the mechanical properties tend to be. In this specification, the degree of orientation of the liquid crystal polyester fiber is calculated from the azimuth angle profile obtained by wide-angle X-ray diffraction measurement and is a value measured by the method described in the Examples below.

[0049] The liquid crystal polyester fiber of the present invention may have a tensile strength of 27 cN / dtex or more, preferably 28 cN / dtex or more, and more preferably 30 cN / dtex or more. The upper limit of the tensile strength is not particularly limited, but 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 the Examples below.

[0050] The liquid crystal polyester fiber of the present invention may have a compressive strength of 0.70 cN / dtex or more, preferably 0.75 cN / dtex or more, and more preferably 0.80 cN / dtex or more, per single fiber. The upper limit of the compressive strength is not particularly limited, but may be, for example, about 1.5 cN / dtex. The compressive strength of the liquid crystal polyester fiber is a value measured by the method described in the Examples below.

[0051] The liquid crystal polyester fiber of the present invention may have a strength retention rate of 50% or more, preferably 55% or more, and more preferably 60% or more, in a disk fatigue test performed by the method described in the Examples below.

[0052] The single fiber fineness of the liquid crystal polyester fiber of the present invention can be appropriately selected depending on the application, etc. For example, the single fiber fineness may be 50 dtex or less, preferably 15 dtex or less, more preferably 10 dtex or less. However, from the viewpoint of responding to miniaturization in electronic component applications, a small fineness is preferred, for example, 7 dtex or less. In addition, the lower limit of the single fiber fineness is not particularly limited, but may be, for example, about 0.01 dtex. The single fiber fineness is a value measured by the method described in the Examples below.

[0053] The liquid crystal polyester fiber of the present invention may have a coefficient of variation of single fiber fineness of 7% or less, preferably 6% or less, and more preferably 5% or less. In applications such as electronic components, thinner fibers and woven fabrics are required as products become smaller in size. However, when the single fiber diameter is smaller, uniformity of the fiber is particularly necessary, so even with a finer fiber, it is preferable that the variation in single fiber fineness is small. The coefficient of variation of single fiber fineness can be calculated by the formula: standard deviation of single fiber fineness (σ) / single fiber fineness (average value) (x)×100, and is a value measured by the method described in the Examples below.

[0054] The liquid crystal polyester fiber of the present invention may be a monofilament or a multifilament. In the case of a multifilament, the number of filaments can be appropriately selected depending on the application, etc. For example, the number of filaments may be 2 to 5,000, preferably 3 to 4,000, and more preferably 5 to 3,000.

[0055] The total fineness of the liquid crystal polyester fiber of the present invention can be appropriately selected depending on the application, etc. For example, the total fineness may be 50,000 dtex or less, preferably 10,000 dtex or less, more preferably 2,000 dtex or less, and even more preferably 600 dtex or less. From the viewpoint of responding to miniaturization in electronic component applications, a fine fineness is preferred, and for example, it may be 300 dtex or less. In addition, the lower limit of the total fineness is not particularly limited, but may be, for example, about 1 dtex.

[0056] [Method of manufacturing liquid crystal polyester fiber] The method for producing the liquid crystal polyester fiber of the present invention comprises: A liquid crystalline polyester having a weight average molecular weight of 50,000 or more and 160,000 or less in terms of polystyrene measured by GPC and a melting point Mp0 of 300°C or more measured by a differential scanning calorimeter at a heating rate of 10°C / min under a nitrogen atmosphere is prepared by the following (1) to (3): (1) Extruder temperature: Mp0+15℃ to Mp0+35℃ (2) Extruder screw peripheral speed: 10 to 40 m / min (3) Spinneret temperature: Extruder temperature -15°C to Extruder temperature -5°C a step of obtaining a raw spinning yarn by spinning under conditions satisfying the above; The method may also include at least a step of heat-treating the obtained raw spinning yarn.

[0057] In the present invention, by using a liquid crystal polyester having a specific weight average molecular weight and melting point as the raw material resin, it is easy to increase the crystallinity in the spinning process and the heat treatment process, and therefore it is possible to obtain a liquid crystal polyester fiber having a high crystallinity and melting point as a fiber after heat treatment. On the other hand, the inventors of the present invention have found that such liquid crystal polyesters are prone to the generation of unmelted material and voids. Specifically, liquid crystal polyesters that have a high melting point relative to a specific weight-average molecular weight range tend to have low melt viscosities, which makes it difficult to apply shear during melt-kneading, resulting in incomplete melting and the remaining unmelted material remaining as foreign matter. Furthermore, if the melt-kneading temperature is set high to reduce the unmelted material, decomposition gas is generated, resulting in voids. The generation of unmelted material and / or voids frequently causes breakage during spinning, making it difficult to consistently obtain uniform fibers and may also hinder the improvement of the fiber's crystallinity. Furthermore, the inventors have found that the low melt viscosity of this liquid crystal polyester makes it difficult to discharge stably, resulting in uneven fineness, and that the difficulty of applying shear during discharge makes it difficult to highly orient the material and increase its crystallinity. Therefore, in the present invention, the temperature in the extruder and the peripheral speed of the screw are adjusted to apply shear at a temperature at which decomposition gas is not generated and the liquid crystal polyester is not deteriorated, thereby promoting melting and suppressing the occurrence of unmelted materials and voids. Furthermore, by adjusting the spinneret temperature to be lower than the temperature of the extruder upstream thereof in a specific relationship, the viscosity of the liquid crystal polyester is increased, the discharge is stabilized, and unevenness in fineness is suppressed. Furthermore, shear is applied during discharge to orient the liquid crystal polyester and promote crystallization. Thus, in the present invention, even when a liquid crystal polyester having a specific weight average molecular weight and melting point is used as a raw material, the above-mentioned liquid crystal polyester fiber can be obtained by adjusting the spinning conditions to reduce unmelted materials and voids, and by heat-treating the spun raw yarn with adjusted crystallinity.

[0058] The weight-average molecular weight of the liquid crystal polyester used in the present invention is preferably 50,000 or more and 160,000 or less, more preferably 60,000 or more and 150,000 or less, and even more preferably 80,000 or more and 130,000 or less. Liquid crystal polyesters having such weight-average molecular weights have high molecular chain mobility, and the molecular chains tend to pack closely together during heat treatment, resulting in an increased degree of crystallinity. In this specification, the weight-average molecular weight is calculated as a polystyrene-equivalent weight-average molecular weight by gel permeation chromatography (GPC), and is a value measured by the method described in the Examples below.

[0059] The melting point (hereinafter, sometimes referred to as Mp0) of the liquid crystal polyester used in the present invention is preferably 300°C or higher, more preferably 300 to 340°C, and even more preferably 305 to 330°C. By using a liquid crystal polyester having such a melting point as a raw material, the melting point of the liquid crystal polyester fiber after heat treatment can be increased, thereby improving heat resistance. In this specification, the melting point is measured using a differential scanning calorimeter (DSC) in accordance with the JIS K 7121 test method and refers to the main absorption peak temperature observed. Specifically, 4 to 6 mg of a sample is placed in an aluminum pan and sealed in a DSC apparatus. Nitrogen is then passed as a carrier gas at a flow rate of 200 mL / min, and the endothermic peak is measured when the temperature is increased from room temperature (e.g., 25°C) at a rate of 10°C / min. If a clear peak does not appear in the first run of DSC measurement due to the type of polymer, it is recommended to increase the temperature at 50°C / min to a temperature 50°C higher than the expected flow temperature, allow it to completely melt at that temperature for 3 minutes, then decrease the temperature to 50°C at a rate of 80°C / min, and then measure the endothermic peak at a heating rate of 10°C / min.

[0060] The liquid crystal polyester used in the present invention has a shear rate of 1216 sec at Mp0+15°C, for example. -1 The melt viscosity at 2000 kJ / min may be 3 to 28 Pa·s, preferably 4 to 25 Pa·s, and more preferably 5 to 20 Pa·s. The melt viscosity is a value measured by the method described in the examples below.

[0061] The liquid crystal polyester used in the present invention may be a liquid crystal polyester having the above-mentioned structural units. From the viewpoint of obtaining a liquid crystal polyester fiber having further improved heat resistance and mechanical properties, a combination having a naphthalene skeleton as a structural unit is preferred, and more preferably, the liquid crystal polyester may be one in which the total content of structural units containing 2,6-naphthylene groups is 40 mol% or more relative to the total amount of all structural units.

[0062] The liquid crystal polyester can be synthesized by a known polycondensation method. As the monomer to be subjected to polycondensation, various aromatic hydroxycarboxylic acids, aromatic dicarboxylic acids, aromatic diols, aromatic hydroxyamines, or carboxylic acid derivatives such as acylated products of the hydroxyl groups, esterified products of the carboxyl groups, acid halides, and acid anhydrides, which are obtained by activating the terminals of these monomers, may be used.

[0063] The polycondensation may be carried out in the presence of various polymerization catalysts, such as organotin catalysts (dialkyltin oxides, etc.), antimony catalysts (antimony trioxide, etc.), titanium catalysts (titanium dioxide, etc.), alkali metal salts or alkaline earth metal salts of carboxylic acids (potassium acetate, etc.), and Lewis acid salts (BF, etc.).

[0064] The liquid crystal polyester may contain the above-mentioned thermoplastic polymers and various additives within the range that does not impair the effects of the present invention.

[0065] In the spinning process, the liquid crystal polyester is introduced into an extruder, heated in the extruder, and melt-kneaded by the rotation of the screw. The molten mixture is then metered by a gear pump, transported to a spinning head, and discharged through a spinneret. The resulting yarn is wound up to obtain a spun yarn.

[0066] Heating can be performed inside the extruder using a known heating means such as a heater, and the extruder temperature may be Mp0+15°C to Mp0+35°C, preferably Mp0+16°C to Mp0+30°C, and more preferably Mp0+17°C to Mp0+25°C, relative to the melting point Mp0 of the liquid crystal polyester. By adjusting the temperature within such a range, melting can be promoted within a range in which decomposition gas is not generated and the liquid crystal polyester is not deteriorated. In this specification, the extruder temperature means the maximum temperature inside the extruder.

[0067] In addition to heat transfer from the heating means of the extruder, the rotation of the screw applies shear between the screw and the inner wall of the cylinder (barrel) and between the screws themselves. The peripheral speed of the screw may be 10 to 40 m / min, preferably 15 to 35 m / min, and more preferably 17 to 25 m / min. By adjusting the peripheral speed to this range, mechanical energy can be applied within a range that does not deteriorate the liquid crystalline polyester, thereby promoting melting of the liquid crystalline polyester. That is, when the peripheral speed of the screw is equal to or greater than the lower limit, sufficient kneading is possible and melting can be promoted, so that unmelted material is less likely to remain. When the peripheral speed of the screw is equal to or less than the upper limit, air bubbles are less likely to be trapped in the kneaded resin, so that voids are less likely to remain, and heat generation due to shear is not too great, so that decomposition gas generation and resin degradation are less likely to occur.

[0068] The total residence time in the device from when the liquid crystal polyester is fed into the extruder until when it is discharged from the spinneret may be 60 minutes or less, preferably 40 minutes or less, and more preferably 30 minutes or less, from the viewpoint of suppressing resin deterioration and decomposition.

[0069] Since bubbles may be trapped in the extruder during melt-kneading due to gas generation by decomposition or air entrapment, in order to prevent the formation of voids, it is preferable to degas the extruder by, for example, providing a vent and connecting a vacuum pump or the like to the extruder to reduce the pressure inside the extruder. For example, the degree of vacuum may be 100 kPa or less, preferably 80 kPa or less, and more preferably 60 kPa or less, in absolute pressure.

[0070] As the extruder, known extruders such as a single-screw extruder and a multi-screw extruder (two or more screws) can be used, and a twin-screw extruder is preferred from the viewpoint of improving kneading properties and degassing properties.

[0071] The molten mixture obtained by melt-kneading in the extruder is fed to a spinning head and extruded from a spinneret at a specific temperature. The spinneret temperature may be extruder temperature −15°C to extruder temperature −5°C. By adjusting the spinneret temperature to be lower than the temperature in the extruder upstream of the spinning head and to have a specific relationship, the viscosity of the liquid crystal polyester during extrusion can be increased to stabilize the extrusion, and shear can be applied to promote oriented crystallization. Furthermore, the spinneret temperature may be Mp0+5°C to Mp0+15°C, where Mp0 is the melting point of the liquid crystal polyester.

[0072] The melt spinning can be carried out by a known or conventional method, and the molten mixture is discharged from a spinneret having the above-mentioned specific temperature and wound up by a godet roller or the like to obtain a spun raw yarn.

[0073] By subjecting the raw spun yarn to heat treatment, solid-state polymerization of the liquid crystalline polyester proceeds, the melting point rises from the melting point (Mp) of the raw spun yarn, and the crystallinity improves. In the heat treatment step, the heat treatment method is not particularly limited, and may be, for example, a batch-type heat treatment or a continuous heat treatment by conveyance. The melting point (Mp) of the raw spun yarn can be measured by the same method as the melting point of the liquid crystalline polyester fiber.

[0074] For example, in a batchwise heat treatment, the material may be wound around a bobbin in a packaged state, or in a skein or tow state, and the heat treatment is preferably performed in a packaged state in terms of simplifying the equipment and improving productivity. The bobbin must be able to withstand the temperature of solid-state polymerization, and is preferably made of a metal such as aluminum, brass, iron, or stainless steel.

[0075] In the case of continuous heat treatment by conveyance, the conveyance method may be either contact conveyance (for example, conveyor system, support roll system, or heat treatment system using heated rollers) or non-contact conveyance (roll-to-roll system). The treatment path does not have to be straight, and heat treatment may be performed by appropriately changing the length, angle, curvature, etc. of the treatment path by arranging return rollers or guides within the device.

[0076] The heat treatment step can be carried out by a known method, such as atmospheric heating or contact heating. The atmosphere is preferably air, an inert gas (e.g., nitrogen, argon), or a combination thereof. The heat treatment may also be carried out under reduced pressure.

[0077] In the heat treatment step, the heat treatment temperature may be 250 to 350°C, preferably 255 to 320°C, more preferably 260 to 315°C, and even more preferably 280 to 310°C. By setting the temperature within this range, the crystallinity can be appropriately increased. Furthermore, the heat treatment temperature may be lower than the melting point (Mp) of the raw spun yarn subjected to the heat treatment step to prevent melting. For example, within the range of 250 to 350°C, the heat treatment temperature may be Mp-50°C or higher and lower than Mp°C, preferably Mp-40°C or higher and lower than Mp°C, and more preferably Mp-30°C or higher and lower than Mp°C. In the heat treatment step, the melting point of the liquid crystalline polyester fiber increases as the solid-state polymerization proceeds, so the initial heat treatment temperature in the heat treatment step may be set to be lower than the melting point (Mp) of the raw spun yarn. From the viewpoint of efficient strength improvement, the heat treatment temperature may be increased stepwise according to the progress of the solid-state polymerization, and the heat treatment may be performed at a temperature higher than the melting point at the time of the heat treatment step (the melting point of the raw spun yarn).

[0078] The heat treatment time in the heat treatment step can be set appropriately depending on the heat treatment method and heat treatment temperature, for example, in the range of 15 minutes to 30 hours, preferably 2 to 24 hours, and more preferably 3 to 20 hours, and the heat treatment time here refers to the holding time at a predetermined heat treatment temperature (for example, the maximum temperature).

[0079] In the method for producing a liquid crystal polyester fiber of the present invention, the strength ratio of the liquid crystal polyester fiber before and after the heat treatment step may be 1.5 times or more, preferably 1.8 times or more, and more preferably 2.0 times or more. The upper limit of the strength ratio of the liquid crystal polyester fiber before and after the heat treatment step is not particularly limited, but may be, for example, 10 times or less. Here, the strength ratio before and after the heat treatment step refers to the value obtained by dividing the tensile strength of the liquid crystal polyester fiber after the heat treatment step by the tensile strength of the liquid crystal polyester fiber (raw spinning yarn) before the heat treatment step.

[0080] In the method for producing the liquid crystal polyester fiber of the present invention, for example, a known oil agent may be applied before the heat treatment step in order to improve the bundling property of the fiber and to prevent fusion during the heat treatment.

[0081] [Fiber structure] The liquid crystal polyester fiber of the present invention can be used as a reinforcing fiber for producing a composite material. When used as a reinforcing fiber, a fiber structure at least partially containing the liquid crystal polyester fiber can be used as an intermediate material for producing the composite material.

[0082] The fiber structure containing the liquid crystal polyester fiber of the present invention can be used in any fiber form such as staple fiber, short-cut fiber, filament yarn, spun yarn, string-like material, rope, etc., and can also be used as various fabrics using the liquid crystal polyester fiber such as nonwoven fabric, woven fabric, knitted fabric, etc. Such fibers and fabrics can be produced using the liquid crystal polyester fiber by known methods.

[0083] The fiber structure of the present invention may be a combination of liquid crystal polyester fibers and other fibers, as long as the effects of the present invention are not impaired. For example, a composite fiber using liquid crystal polyester fibers and other fibers (e.g., a mixed yarn in which liquid crystal polyester fibers are mixed with other fibers) can be used. Also, composite fabrics using liquid crystal polyester fibers and other fibers (e.g., mixed fabrics in which liquid crystal polyester fibers are mixed with other fibers, and laminates of fabrics made of liquid crystal polyester fibers and fabrics made of other fibers) can be used. When the fiber structure is used to produce a composite material, the fiber structure may be a composite fiber or composite fabric containing fusion fibers that form the matrix of the composite material as the other fibers.

[0084] The liquid crystal polyester fiber of the present invention can be used in the form of various 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, it can be used as various fiber products such as tension members (electric wires, optical fibers, etc.), heater wire cores, cords for various electrical products such as earphone cords, ropes, sling belts, ropes, lifelines, fishing lines, fishing nets, longlines, land nets (safety nets, golf driving range nets, etc.), catheters, reinforcing materials for plastics, concrete, and rubber, base fabrics for printed circuit boards, sail cloth, protective clothing, and protective gloves. In particular, liquid crystal polyester fibers having a finer fiber count (e.g., a single fiber fineness of 7 dtex or less) can be used for electronic component applications such as base fabrics for printed circuit boards.

[0085] [Composite material] In the present invention, the composite material may be any material that can be obtained by molding a matrix resin using liquid crystal polyester fibers as reinforcing fibers. The liquid crystal polyester fibers can be molded using the above-mentioned fiber structure.

[0086] The matrix resin may be a resin generally used in composite materials, such as a thermosetting resin or a thermoplastic resin. Examples of thermosetting resins include epoxy resins, unsaturated polyester resins, vinyl ester resins, bismaleimide resins, phenolic resins, urea resins, melamine resins, thermosetting polyimide resins, thermosetting polyurethane resins, and benzoxazine resins. The thermoplastic resin is not particularly limited as long as it has a softening temperature lower than the melting point of the liquid crystal polyester fiber of the present invention. Examples of the thermoplastic resin include vinyl resins (polymers or derivatives made of a monomer having a vinyl group CH═CH— or a vinylidene group CH═C<); polyamide resins such as aliphatic polyamide resins (polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 610, polyamide 612, etc.) and semi-aromatic polyamide resins; polyester resins such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; fluorine-containing resins such as polytetrafluoroethylene resins; polysulfone resins and polyethersulfone resins; polyetherketone resins, polyetheretherketone resins, and polyetherketoneketone resins; polycarbonate resins; polyphenylene ether resins; amorphous polyarylate resins; and liquid crystal polyester resins such as wholly aromatic polyester resins. [Example]

[0087] The present invention will be described in more detail below with reference to examples, but the present invention is not limited by these examples. In the following examples and comparative examples, various physical properties were measured by the following methods.

[0088] (Weight average molecular weight) The liquid crystal polyester was dissolved in a mixed solvent of pentafluorophenol / chloroform = 1 / 2 (weight ratio) to a concentration of 0.03 wt% to prepare a sample for GPC measurement. This was measured under the following conditions using a high-speed GPC measurement device (manufactured by Tosoh Corporation, "HLC-8420GPC"), and the weight-average molecular weight was calculated in terms of standard polystyrene. Column: TSKgel guard column SuperH-L 4.6mm x 3.5cm 1 piece TSKgel SuperH2000 6.0mmID×15cm 1 tube, TSKgel SuperHM-H 6.0mmID×15cm 2 pieces Detector: RI detector Temperature: 40℃ Flow rate: 0.6mL / min Injection volume: 100μL

[0089] (Melting point of resin chips (granular moldings) and fibers) Measurement was performed using a differential scanning calorimeter (DSC; "DSC60A Plus" manufactured by Shimadzu Corporation) in accordance with JIS K 7121, and the observed main absorption peak temperature was taken as the melting point. Specifically, 4 to 6 mg of the sample was placed in an aluminum pan and sealed in the DSC apparatus. Nitrogen was then passed through the DSC apparatus at a flow rate of 200 mL / min as a carrier gas, and the endothermic peak derived from the liquid crystal polyester was measured when the temperature was raised from 25°C at a rate of 10°C / min.

[0090] (Melt viscosity of resin chips (granular molded body)) The melt viscosity of the raw resin was measured using a melt viscosity measuring device (Capillograph 1C manufactured by Toyo Seiki Co., Ltd.). The resin chips were filled into the device cylinder, and a 1.00 mm diameter × 10 mm capillary was used to measure the melt viscosity of the liquid crystal polyester (Mp0) at a shear rate of 1216 s under various temperature conditions of Mp0 + 15 °C relative to the melting point of the liquid crystal polyester (Mp0) (the melting point of the liquid crystal polyester in the resin chips measured above). -1 The melt viscosity was measured at 100°C.

[0091] (crystallinity) The liquid crystal polyester fiber was attached to a fiber holder, and wide-angle X-ray diffraction (WAXD) measurements were performed using the transmission method under the following measurement conditions, with X-rays incident perpendicular to the fiber axis. Measurement equipment: Bruker "D8 Discover IμS" Detector: 2D PSPC・VANTEC-500 X-ray source:Cu Current: 1mA Voltage: 50kV Exposure time: 10 minutes Collimator diameter: 0.5 mm Camera length: 17cm Detector position (2θ): 20° Sample position (ω): 10° Inclination angle (Ψ): 90° Measurement temperature: Room temperature (approx. 25℃)

[0092] An X-ray diffraction profile was obtained under the following conditions, with the horizontal axis representing the diffraction angle (2θ) and the vertical axis representing the intensity. Integration range: 2θ=5~35°, γ (azimuth angle)=250~290° Step width: 0.05° (diffraction angle)

[0093] In the obtained X-ray diffraction profile, a baseline was set by connecting the values ​​at 2θ = 5° and 35° with a straight line. Based on this newly set baseline, the difference in intensity on the vertical axis from the actual measurement data to the baseline was converted into profile data with the new intensity.

[0094] After baseline correction, the amorphous peak was fitted to the profile data using a pseudo-Voigt function (Lorentzian function ratio: α = 0) with the peak height, peak top position, σ, and asymmetry parameter as variables. The initial value of the peak top position of the fitting function was set to around 20.6°. The peak area of ​​this fitting function was calculated as the amorphous amount (D).

[0095] For Peak A (peak top position near 19°), Peak B (peak top position near 20.5°), and Peak C (peak top position near 27°) of the baseline-corrected profile data, the crystal peaks were fitted using the following function, with the peak height, peak top position, and σ as variables. All crystal peaks were assumed to be symmetric. Peak A: Pseudo-Voigt function (α=1) Peak B: Pseudo-Voigt function (α=0) Peak C: Pseudo-Voigt function (α=0.5)

[0096] The sum of all fitting functions, including the fitting function for the amorphous peak obtained above, was then fitted using the least squares method to minimize the difference between the baseline-corrected profile data. The peak areas of these fitting functions were calculated as the amounts of crystallinity (A), (B), and (C), respectively.

[0097] Using the above amorphous amount and crystalline amount, the crystallinity was calculated according to the following formula. Crystallinity (%)={(A)+(B)+(C)} / {(A)+(B)+(C)+(D)}×100

[0098] (orthorhombicity) The degree of orthorhombic crystallinity was calculated from the following formula using the amount of crystallinity (A) of peak A derived from hexagonal crystals and the amount of crystallinity (B) of peak B derived from orthorhombic crystals, which were calculated from the X-ray diffraction profile (profile data after baseline correction) in the above crystallinity measurement. Orthorhombicity (%)=(B) / {(A)+(B)}×100

[0099] (Orientation degree) In the wide-angle X-ray diffraction measurement under the above crystallinity measurement conditions, an azimuth angle profile was obtained under the following conditions, with the horizontal axis representing the azimuth angle (γ) and the vertical axis representing the intensity. Integration range: γ = 225 to 315°, 2θ = 19.0 to 21.0° Step width: 0.5°

[0100] In the obtained azimuth angle profile, a baseline was set by connecting the values ​​of γ = 225° and 315° with a straight line. Based on this newly set baseline, the profile data was converted into a new intensity, which was the difference in intensity on the vertical axis from the actual measurement data to the baseline.

[0101] The half-width of the peak observed in the baseline-corrected profile data was calculated, and the degree of orientation (%) was calculated using the following formula: Orientation degree (%) = (180 - half width) / 180 x 100

[0102] (Number of voids) A single fiber was taken from the liquid crystal polyester fiber (multifilament), and microscopic observation was performed using transmitted light with a microscope (Nikon Corporation's "ECLIPSE Ci-E") over a 5 cm length of the fiber, and the number of voids with a major diameter (maximum diameter) of 5 μm or more was counted. The number of voids was counted for five single fibers, and the average number was taken as the number of voids with a major diameter of 5 μm or more per 5 cm of single fiber.

[0103] (Number of unmelted pieces) The same samples as those used in the void measurements were observed under a polarizing microscope (crossed Nicols). Specifically, using the same microscope, a polarizer (Sensitive Color Polarizer Unit C-TP) and an analyzer (Sensitive Color Polarizing Analyzer Unit C-IA) were set above and below the microscope stage. The polarizer and analyzer transmission axes were aligned perpendicular to each other (crossed Nicols). Because liquid crystal polyester fibers are highly oriented and exhibit optical anisotropy (birefringence), the entire fiber appears luminous when observed at a certain angle (bright field). Rotating the stage 45° from this position, the entire fiber becomes dark (dark field), revealing the presence of unmelted particles. Using this method, the number of unmelted particles with a major diameter (maximum diameter) of 5 μm or larger was counted within a 5 cm length of the fiber. The number of unmelted particles was counted for five single fibers, and the average count was taken as the number of unmelted particles with a major diameter of 5 μm or larger per 5 cm of single fiber.

[0104] (total fineness, single fiber fineness) Based on JIS L 1013:2010 8.3.1 A method, the liquid crystal polyester fiber was wound onto a skein of 1m x 100 skeins (total 100m) using a measuring device "Wrap Reel by Motor Driven" manufactured by Daiei Scientific Instruments Manufacturing Co., Ltd., and the weight (g) was multiplied by 100 and measured twice per level, and the average value was taken as the total fineness (dtex) of the obtained liquid crystal polyester fiber. The quotient obtained by dividing this value by the number of filaments was taken as the single fiber fineness (dtex).

[0105] (tensile strength) Referring to JIS L 1013:2010 8.5.1, tensile tests were conducted eight times for each yarn sample using an autograph "AGS-100B" manufactured by Shimadzu Corporation under conditions of a test length of 20 cm and a tensile speed of 10 cm / min, and the average tensile strength (cN) was divided by the total fineness (dtex) measured by the above-mentioned method to calculate the tensile strength (cN / dtex).

[0106] (Compressive strength) A single fiber was removed from a liquid crystal polyester fiber (multifilament) and placed on a glass plate with its end protruding. The protruding fiber was then cut using a focused ion beam (FIB) scanner at a distance 1.5 times the fiber diameter from the end of the glass plate. This resulted in a sample for measuring single fiber compressive strength, with a cross section precisely perpendicular to the fiber axis. Using a single fiber compression tester (THK Precision Co., Ltd., Model B20-049), an indenter was pressed against the cut surface of the measurement sample at a speed of 200 nm / sec to obtain a pressure-strain curve. This measurement was performed on five samples, and the average pressure at the yield point of the pressure-strain curve obtained from the five tests was calculated. The average pressure was divided by the single fiber fineness measured using the method described above to calculate the single fiber compressive strength (cN / dtex).

[0107] (Coefficient of variation of single fiber fineness) Fifteen single fibers were taken from a liquid crystal polyester fiber (multifilament). Using a Denier Computer DC-11 manufactured by Search Control Electric Co., Ltd., the single fiber fineness was measured under the condition that an initial load of 0.1 g per 0.9 dtex was applied to a single fiber with a measurement length of 50 mm. The standard deviation of the measured single fiber fineness values ​​of the 15 fibers was divided by the average value, and the result was multiplied by 100 to calculate the coefficient of variation of the single fiber fineness (%).

[0108] (Disc fatigue test) Six liquid crystal polyester fibers with a total fineness of 280 dtex were doubling up (10 fibers were doubling up for a total fineness of 170 dtex; doubling was performed to obtain a total fineness of approximately 1700 dtex) and a first twist (Z twist) of 328 T / m was applied, and three of these first twisted yarns were doubling up and a final twist (S twist) of 268 T / m was applied to produce a cord.

[0109] This was subjected to the following dipping treatment to obtain a dipped cord. First time: The specimen was dipped in a dip solution containing 1% by weight of Marpomace (70%; Matsumoto Yushi Pharmaceutical Co., Ltd.), 4% by weight of Denacol EX313 (Nagase ChemteX Corporation), 0.3% by weight of a 10% aqueous solution of sodium hydroxide, and 94.7% by weight of water. The specimen was then dried at 150°C for 30 seconds and then heat-treated at 240°C for 30 seconds. Second try: The RFL liquid was used as the dipping liquid for the dipping treatment. The sample was then dried at 150°C for 30 seconds and then heat-treated at 240°C for 30 seconds. The RFL liquid was prepared as follows: Liquid A, which was a mixture of 3.3 wt% resorcinol, 2.46 wt% formaldehyde (37%), 1.4 wt% sodium hydroxide aqueous solution (10%), and 32.2 wt% water, was aged at 25°C for 6 hours. Liquid B, which was a mixture of 52.86 wt% VP latex (JSR-0650; 40%; manufactured by JSR Corporation) and 7.78 wt% water, was mixed with the aged Liquid A and then aged at 25°C for 16 hours to obtain the RFL liquid.

[0110] The prepared dipped cord was embedded in rubber (SBR:NR = 1:1) and vulcanized at 150°C for 30 minutes to prepare a rubber composite specimen. A disk fatigue test was carried out using the Goodrich method under the following conditions, with reference to JIS L 1017:2002. Equipment: Disc fatigue testing machine (manufactured by Mize Testing Machine Co., Ltd.) Disk spacing: 24.5mm Distortion: 2% (elongation rate, compression rate) Rotation speed: 2500 rpm Temperature: 100℃ Number of times: 300,000

[0111] Tensile tests were performed six times per sample on the dipped cords removed from the rubber composite specimens before and after the disc fatigue test using an autograph "AGS-100B" manufactured by Shimadzu Corporation, with a test length of 20 cm and a tensile speed of 10 cm / min, in accordance with JIS L 1013:2010 8.5.1. The average tensile strength was measured as the tensile strength (N) of the dipped cord before and after the disc fatigue test. The strength retention (%) was calculated using the following formula. Strength retention rate (%) = (tensile strength of dipped cord after disc fatigue test) / (tensile strength of dipped cord before disc fatigue test) × 100

[0112] [Reference example 1] A liquid crystalline polyester (LCP) was obtained by adding 20.66 g (60 mol%) of 6-hydroxy-2-naphthoic acid, 7.91 g (20 mol%) of 2,6-naphthalenedicarboxylic acid, 3.02 g (15 mol%) of hydroquinone, 1.70 g (5 mol%) of 4,4'-dihydroxybiphenyl, 20.55 g of acetic anhydride, and 3.77 mg of potassium acetate as a polymerization catalyst to a reaction vessel. The mixture was acetylated under a nitrogen atmosphere at 160°C under reflux for approximately 2 hours. The mixture was then heated to 280°C for 0.5 hours, 320°C for 1 hour, and 360°C for 1 hour. The mixture was then subjected to a vacuum treatment (100 Pa) for 30-120 minutes until foaming ceased. The resulting liquid crystalline polyester had a weight-average molecular weight of 109,000 and a melting point of 309°C.

[0113] [Reference example 2] A liquid crystalline polyester (LCP) was obtained by adding 19.74 g (60 mol%) of 6-hydroxy-2-naphthoic acid, 7.56 g (20 mol%) of 2,6-naphthalenedicarboxylic acid, 0.96 g (5 mol%) of hydroquinone, 4.88 g (15 mol%) of 4,4'-dihydroxybiphenyl, 19.64 g of acetic anhydride, and 3.77 mg of potassium acetate as a polymerization catalyst to a reaction vessel. The mixture was acetylated under a nitrogen atmosphere at 160°C under reflux for approximately 2 hours. The mixture was then heated to 280°C for 0.5 hours, 320°C for 1 hour, and 360°C for 2 hours. The mixture was then subjected to a vacuum treatment (100 Pa) for 30-120 minutes until foaming ceased. The resulting liquid crystalline polyester had a weight-average molecular weight of 91,000 and a melting point of 302°C.

[0114] [Reference example 3] A liquid crystalline polyester was obtained in the same manner as in Reference Example 1, except that the maximum temperature during the polymerization reaction was changed to 365°C. The obtained liquid crystalline polyester had a weight average molecular weight of 167,000 and a melting point of 320°C.

[0115] [Reference example 4] 22.64g (73 mol%) of 4-hydroxybenzoic acid, 11.41g (27 mol%) of 6-hydroxy-2-naphthoic acid, 23.38g of acetic anhydride, and 3.77mg of potassium acetate were added to a reaction vessel and acetylated under a nitrogen atmosphere (160°C, reflux for approximately 2 hours). After acetylation, the mixture was heated to 250°C for 0.5 hours, 280°C for 1 hour, and 320°C for 1 hour. The mixture was then subjected to a 30-minute vacuum treatment (100Pa). After confirming that foaming had ceased, the atmosphere was purged with nitrogen to obtain a liquid crystalline polyester. The resulting liquid crystalline polyester had a weight average molecular weight of 159,000 and a melting point of 278°C.

[0116] [Example 1] The liquid crystal polyester chips (granular molded bodies) obtained in Reference Example 1 were dried with hot air at 120 ° C for more than 4 hours. Then, they were placed in a Φ15 mm twin-screw extruder (manufactured by Technovel Co., Ltd., "KZW15TW-45MG-NH (-700)"), melt-kneaded at a heater temperature of 330 ° C and a screw peripheral speed of 18 m / min, and the molten mixture was fed to the spinning head while being metered with a gear pump. At this time, a vacuum pump (manufactured by Orion Machinery Co., Ltd., dry pump "KRF40A-V-01B") was connected to the vent part midway through the twin-screw extruder via a metal tube, and the resin-unfilled space in the twin-screw extruder was decompressed to 50 kPa. The temperature setting from the extruder outlet to the spinning head was 330 ° C, and the spinneret temperature setting was 315 ° C. The spinning head was equipped with a spinneret with a hole diameter of 0.10 mm, a land length of 0.14 mm, and 50 holes. The molten mixture was discharged at a discharge rate of 28 g / min and wound onto a bobbin at a winding speed of 1000 m / min to obtain a liquid crystal polyester fiber spun yarn. During this process, a 2 wt% aqueous solution of sodium dodecyl phosphate (Wako First Grade, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was applied to the spun yarn from an oiling guide located directly below the spinneret. The application rate of this aqueous solution was 1.4 g / min, and the calculated deposition ratio of sodium dodecyl phosphate to the spun yarn was 0.1 wt%.

[0117] Next, 500 m of the spinning yarn obtained here was wound at a winding density of 0.6 g / cm 3 The fiber was rewound onto an aluminum bobbin so that the fiber became 100% pure, and the temperature was gradually increased from room temperature to 300°C in a closed oven under a nitrogen atmosphere. After the temperature reached 300°C, the fiber was heat-treated at 300°C for 8 hours to obtain a heat-treated liquid crystalline polyester filament yarn. The analytical results of the obtained liquid crystalline polyester fiber are shown in Table 5.

[0118] [Example 2] A liquid crystalline polyester fiber was obtained in the same manner as in Example 1, except that the liquid crystalline polyester obtained in Reference Example 2 was used, and the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head were set to 320°C during spinning, and the spinneret temperature was set to 310°C.

[0119] [Example 3] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that a spinneret with a hole diameter of 0.08 mmφ, a land length of 0.11 mm, and 100 holes was used and the output rate during spinning was 17 g / min.

[0120] [Example 4] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the screw peripheral speed of the extruder during spinning was set to 15 m / min, the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head were set to 325°C, and the spinneret temperature was set to 315°C.

[0121] [Example 5] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the spinneret temperature during spinning was set to 320°C.

[0122] [Example 6] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the raw spinning yarn was heat-treated at 330° C. for 8 hours.

[0123] [Comparative Example 1] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the peripheral screw speed of the extruder during spinning was set to 7 m / min.

[0124] Comparative Example 2 A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the peripheral screw speed of the extruder during spinning was set to 45 m / min.

[0125] Comparative Example 3 Liquid crystal polyester fibers were obtained in the same manner as in Example 1, except that the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head during spinning were set to 320°C, and the spinneret temperature was set to 310°C.

[0126] Comparative Example 4 A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the spinneret temperature during spinning was set to 340°C.

[0127] Comparative Example 5 Liquid crystal polyester fibers were obtained in the same manner as in Example 1, except that the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head during spinning were set to 360°C, and the spinneret temperature was set to 350°C.

[0128] Comparative Example 6 A liquid crystalline polyester fiber was obtained in the same manner as in Example 1, except that the liquid crystalline polyester obtained in Reference Example 3 was used and the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head were set to 340°C during spinning, and the spinneret temperature was set to 325°C.

[0129] Comparative Example 7 A spun raw yarn was obtained in the same manner as in Example 1, except that the liquid crystal polyester obtained in Reference Example 4 was used and the heater temperature of the extruder and the temperature from the extruder outlet to the spinning head were set to 310° C. during spinning, and the spinneret temperature was set to 310° C. Next, a liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the obtained spun raw yarn was heat-treated at 270° C. for 5 hours.

[0130] [Comparative Example 8] A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the raw spinning yarn was heat-treated at 245°C for 8 hours.

[0131] Comparative Example 9 A liquid crystal polyester fiber was obtained in the same manner as in Example 1, except that the raw spinning yarn was heat-treated at 355°C for 8 hours.

[0132] [Table 5]

[0133] As shown in Table 5, in Examples 1 to 6, a specific liquid crystal polyester was used as the raw resin, and melt-kneading and melt-spinning were performed under specific conditions, thereby reducing unmelted materials and voids and adjusting the melting point and crystallinity to specific ranges. Therefore, the liquid crystal polyester fibers of Examples 1 to 6 have excellent tensile strength and compressive strength, excellent disk fatigue resistance, and excellent uniformity of single fiber fineness.

[0134] On the other hand, in Comparative Examples 1 to 5, although the same liquid crystal polyester raw resin as in Examples 1 and 3 to 6 was used, the temperature in the extruder, the peripheral speed of the screw, or the spinneret temperature was not within a specific range, resulting in the formation of many unmelted materials or voids. As a result, the liquid crystal polyester fibers of Comparative Examples 1 to 5 had lower tensile strength and compressive strength, poorer disk fatigue resistance, and a larger coefficient of variation in single fiber fineness, resulting in variation, compared to Examples 1 to 6.

[0135] In Comparative Example 6, the liquid crystal polyester raw material resin had a large weight-average molecular weight, which made it difficult to melt in the extruder, resulting in unmelted material remaining. Furthermore, perhaps due to the low mobility of the molecular chains, the molecular chains were unable to pack closely together during heat treatment, and the crystallinity could not be increased. Therefore, the liquid crystal polyester fiber of Comparative Example 6 had lower tensile strength and compressive strength, poorer disk fatigue resistance, and a large coefficient of variation in single fiber fineness, resulting in variation.

[0136] In addition, in Comparative Example 7, since the melting point of the liquid crystal polyester raw material resin is low, the melting point cannot be sufficiently increased even when the raw spun yarn is heat-treated. Therefore, the liquid crystal polyester fiber of Comparative Example 7 has lower heat resistance than those of Examples 1 to 6.

[0137] In Comparative Examples 8 and 9, although there were few unmelted materials or voids, the crystallinity was outside the specific range, and therefore the liquid crystal polyester fibers of Comparative Examples 8 and 9 had lower tensile strength and compressive strength than those of Examples 1 to 6, and were inferior in disk fatigue resistance. [Industrial Applicability]

[0138] The liquid crystal polyester fiber of the present invention can be used for various purposes 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 composite materials.

[0139] As described above, the preferred embodiment of the present invention has been described, but various additions, modifications, or deletions can be made without departing from the spirit of the present invention, and such additions, modifications, or deletions are also included within the scope of the present invention.

Claims

1. A liquid crystal polyester fiber having a melting point of 335°C or higher and lower than 360°C as measured by a differential scanning calorimeter in a nitrogen atmosphere at a temperature rise rate of 10°C / min, a crystallinity of 45 to 60%, and a total number of unmelted particles and voids having a major axis of 5 μm or higher per 5 cm of a single fiber being 3 or less.

2. 2. The liquid crystal polyester fiber according to claim 1, wherein the degree of orthorhombic crystallinity in the crystalline component is 20% or more.

3. 3. The liquid crystal polyester fiber according to claim 1, wherein the degree of orientation in the fiber axis direction of the crystalline component is 97% or more but less than 100%.

4. 3. The liquid crystal polyester fiber according to claim 1, having a tensile strength of 27 cN / dtex or more.

5. 3. The liquid crystal polyester fiber according to claim 1, wherein the compressive strength of a single fiber is 0.70 cN / dtex or more.

6. 3. The liquid crystal polyester fiber according to claim 1, wherein the single fiber fineness is 7 dtex or less and the coefficient of variation of the single fiber fineness is 7% or less.

7. A fiber structure comprising at least a part of the liquid crystal polyester fiber according to claim 1 or 2.

8. A composite material comprising the liquid crystal polyester fiber according to claim 1 or 2 as a reinforcing fiber.

9. A method for producing the liquid crystal polyester fiber according to claim 1 or 2, The weight average molecular weight in terms of polystyrene measured by GPC is 50,000 or more and 160,000 or less, and the melting point Mp measured by a differential scanning calorimeter in a nitrogen atmosphere at a heating rate of 10°C / min ０ The liquid crystal polyester having a melting point of 300° C. or more is selected from the following (1) to (3): (1) Extruder temperature: Mp ０ +15℃~Mp ０ +35℃ (2) Extruder screw peripheral speed: 10 to 40 m / min (3) Spinneret temperature: extruder temperature -15°C to extruder temperature -5°C a step of obtaining a raw spinning yarn by spinning under conditions satisfying the above; a step of heat treating the obtained raw spinning yarn; A method for producing a liquid crystal polyester fiber, comprising at least

10. The method for producing a liquid crystal polyester fiber according to claim 9, wherein the extruder is a twin-screw extruder.

11. The method for producing a liquid crystal polyester fiber according to claim 9, wherein the heat treatment temperature is 250 to 350°C.

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