EP4636143A1

EP4636143A1 - Melt-anisotropic aromatic polyester fiber and method for manufacturing same

Info

Publication number: EP4636143A1
Application number: EP23903397.0A
Authority: EP
Inventors: Kota TOGII; Shunichi Hasegawa; Yuji Ogino; Toshiaki Kobayashi
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2022-12-14
Filing date: 2023-12-07
Publication date: 2025-10-22
Also published as: WO2024128110A1; JPWO2024128110A1; KR20250114511A; CN120303453A; WO2024128110A8; JP7745782B2; TW202432922A

Abstract

Provided is a melt-anisotropic aromatic polyester fiber having excellent creep properties. The melt-anisotropic aromatic polyester fiber has a degree of orthorhombic crystallinity of 15.0% or more in a crystal component. For example, the melt-anisotropic aromatic polyester fiber may have a density of 1.4080 g/cm³ or more as determined using a density gradient tube. The melt-anisotropic aromatic polyester fiber may include a melt-anisotropic aromatic polyester having a structural unit derived from 4-hydroxybenzoic acid at a proportion of 50 mol% or more.

Description

CROSS REFERENCE TO THE RELATED APPLICATION

This application is based on and claims Convention priority to Japanese patent application No. 2022-199790, filed December 14, 2022 , the entire disclosure of which is herein incorporated by reference as a part of this application.

FIELD OF THE INVENTION

The present invention relates to a melt-anisotropic aromatic polyester fiber and a method for manufacturing the same.

BACKGROUND OF THE INVENTION

General-purpose fibers such as general-purpose polyester fibers are commonly used for tension members, but there is a demand for reducing the diameters of cables and cords in order to downsize electrical products, and thus super fibers which have sufficient strength even with a small diameter have attracted attention. Further, in applications such as optical fiber cables, the communication speed decreases acceleratingly with even slight elongation of optical fibers, so that liquid crystal polymer fibers (melt-anisotropic aromatic polyester fibers, aramid fibers, etc.) having high dimensional stability are used. It is known that a melt-anisotropic aromatic polyester fiber can have an increased degree of crystallinity by spinning to obtain an as-spun fiber highly oriented in a fiber axis direction and then heat-treating the as-spun fiber to cause solid-phase polymerization, and thus can have excellent dimensional stability.
For example, regarding the crystallinity of a melt-anisotropic aromatic polyester fiber, Patent Document 1 ( JP Laid-open Patent Publication No. 2010-150694 ) discloses a liquid crystalline polyester fiber for which a half width of a peak having a local maximum at 18 to 22° in an equatorial direction in wide-angle X-ray diffraction measurement using CuKα radiation as a radiation source is 3.5° or more.
Regarding a method for producing a melt-anisotropic aromatic polyester fiber, Patent Document 2 ( JP Laid-open Patent Publication No. H03-227407 ) discloses a method for spinning a melt-anisotropic aromatic polyester, the method including extruding and spinning the melt-anisotropic aromatic polyester using an extruder with a vent, wherein a pressure in the vent portion is reduced to 100 to 760 mmHg to adjust a head pressure of the extruder to 5 to 30 kg/cm², and then the head pressure is increased to 40 to 200 kg/cm² by a gear pump having a volumetric efficiency of 50 to 90% to pass the melt-anisotropic aromatic polyester through a filter and spinning it.

CONVENTIONAL ART DOCUMENT

PATENT DOCUMENT

[Patent Document 1] JP Laid-open Patent Publication No. 2010-150694
[Patent Document 2] JP Laid-open Patent Publication No. H03-227407

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

However, in order to achieve longer lifetimes for various applications such as electrical products, there is a demand for further improvement of dimensional stability over conventional melt-anisotropic aromatic polyester fibers, and it is necessary to improve creep property of melt-anisotropic aromatic polyester fibers, but Patent Documents 1 and 2 do not describe improvement of creep property.
Therefore, an object of the present invention is to solve the above problems and to provide a melt-anisotropic aromatic polyester fiber having excellent creep property.

MEANS FOR SOLVING THE PROBLEMS

The inventors of the present invention have conducted extensive studies in order to achieve the aforementioned object, and found that where kneading conditions in melt-spinning are changed, there are differences in the creep property of melt-anisotropic aromatic polyester fibers obtained by heat-treating as-spun fibers depending on the kneading conditions.
Further, the inventors of the present invention have focused on a crystal structure of a melt-anisotropic aromatic polyester fiber in consideration of the relationship with creep property and found that a melt-anisotropic aromatic polyester fiber having more excellent creep property has a higher degree of orthorhombic crystallinity.
As a result of further studies, the inventors of the present invention have found that probably because in melt-spinning, kneading a melt-anisotropic aromatic polyester at a low temperature in a twin-screw extruder enables to apply a shear force to the melt-anisotropic aromatic polyester in a state where the viscosity thereof is high, and to obtain an as-spun fiber having a more uniform microcrystal structure, the molecular chains are densely packed during solid-phase polymerization by heat-treating such an as-spun fiber, and it allows to increase the proportion of orthorhombic crystals, which have a more dense crystal structure, leading to the completion of the present invention.
That is, the present invention may include the following aspects.

[Aspect 1]

A melt-anisotropic aromatic polyester fiber having a degree of orthorhombic crystallinity of 15.0% or more (preferably 16.0% or more, more preferably 17.0% or more, and further preferably 18.0% or more, and 25.0% or less, preferably 24.0% or less, and more preferably 23.0% or less) in a crystal component.

[Aspect 2]

The melt-anisotropic aromatic polyester fiber according to aspect 1, having a density of 1.4080 g/cm³ or more as determined using a density gradient tube.

[Aspect 3]

The melt-anisotropic aromatic polyester fiber according to aspect 1 or 2, comprising a melt-anisotropic aromatic polyester having a structural unit derived from 4-hydroxybenzoic acid at a proportion of 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).

[Aspect 4]

The melt-anisotropic aromatic polyester fiber according to any one of aspects 1 to 3, having a melting point of 260 to 380°C (preferably 270 to 360°C, more preferably 275 to 340°C, and further preferably 275 to 330°C) as measured by a differential scanning calorimeter under a nitrogen atmosphere at a temperature elevation rate of 10°C/min.

[Aspect 5]

A fiber structure at least partially including the melt-anisotropic aromatic polyester fiber as recited in any one of aspects 1 to 4.

[Aspect 6]

A method for producing a melt-anisotropic aromatic polyester fiber, the method at least including:

melt-kneading a melt-anisotropic aromatic polyester using a twin-screw extruder at a barrel temperature, from a resin feed portion to a kneading portion outlet, of lower than a melting point Mp₀ (preferably Mp₀ - 5°C or lower, more preferably Mp₀ - 10°C or lower, and further preferably Mp₀ - 15°C or lower) of the melt-anisotropic aromatic polyester to obtain a melt-kneaded material, the melting point Mp₀ being measured by a differential scanning calorimeter under a nitrogen atmosphere at a temperature elevation rate of 10°C/min;
spinning the melt-kneaded material to obtain an as-spun fiber; and
heat-treating the obtained as-spun fiber.

[Aspect 7]

The method according to aspect 6, wherein the melt-kneading is performed under a condition where a residence time in the kneading portion of the twin-screw extruder is 10 seconds or longer (preferably 15 seconds or longer and more preferably 20 seconds or longer).
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.
Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawing 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

The melt-anisotropic aromatic polyester fiber according to the present invention has excellent creep property. In addition, with the production method according to the present invention, a melt-anisotropic aromatic polyester fiber having a high degree of orthorhombic crystallinity can be produced.

BRIEF DESCRIPTION OF THE DRAWING

In any event, the present invention will become more clearly understood from the following description of a preferred embodiment thereof, when taken in conjunction with the accompanying drawing. However, the embodiment and the drawing are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. The drawings are not necessarily shown at a consistent scale and are exaggerated in order to illustrate the principle of the present invention. In the figures,
Fig. 1 is a schematic diagram for describing a method for producing a melt-anisotropic aromatic polyester fiber according to one embodiment of the present invention.

DESCRIPTION OF EMBODIMENT

[Melt-Anisotropic Aromatic Polyester Fiber]

The melt-anisotropic aromatic polyester fiber according to the present invention includes a melt-anisotropic aromatic polyester. The melt-anisotropic aromatic 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 melt-anisotropic aromatic 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. 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.
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.
As more preferable structural units, there may be mentioned structural units as described in Examples (1) to (18) 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.
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 Y₁ and Y₂ 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.
Z may include substitutional groups denoted by following formulae.
A preferable melt-anisotropic aromatic polyester may have a combination including a structural unit having a naphthalene skeleton. An especially preferable one may include both 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 the following formula (A), and the structural unit (B) may have the following formula (B). From the viewpoint of improving melt-formability, the ratio of the structural unit (A) and the structural unit (B) may be in a range of former/latter of preferably 9/1 to 1/1, more preferably 7/1 to 1/1, and further preferably 5/1 to 1/1.
The total proportion of the structural units (A) and (B) may be, based on all the structural units, for example, 65 mol% or more, more preferably 70 mol% or more, and further preferably 80 mol% or more. A melt-anisotropic aromatic polyester having the structural unit (B) at a proportion of 4 to 45 mol % is especially preferred among polymers.
The melt-anisotropic aromatic polyester may include a structural unit derived from 4-hydroxybenzoic acid, at a proportion of preferably 50 mol% or more, more preferably 53 mol% or more, further preferably 60 mol% or more, even more preferably 65 mol% or more, and particularly preferably 70 mol% or more. The upper limit of the content of the structural unit derived from 4-hydroxybenzoic acid in the melt-anisotropic aromatic polyester is not particularly limited to a specific one, and may be, for example, 90 mol% or less, preferably 88 mol% or less, and more preferably 85 mol% or less.
The melt-anisotropic aromatic polyester used in the present invention may preferably have a melting point (hereinafter sometimes referred to as Mp₀) in the range from 250 to 380°C, more preferably from 255 to 370°C, further preferably from 260 to 360°C, and even more preferably 260 to 330°C. 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.
It should be noted that the melt-anisotropic aromatic 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 melt-anisotropic aromatic 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.
The melt-anisotropic aromatic polyester fiber according to the present invention may contain a melt-anisotropic aromatic 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.
The melt-anisotropic aromatic polyester fiber according to the present invention has a degree of orthorhombic crystallinity of 15.0% or more in a crystal component. The melt-anisotropic aromatic polyester has a crystal component such as an orthorhombic or hexagonal crystal system, and an orthorhombic crystal have a crystal structure in which molecular chains are more densely packed, so that a melt-anisotropic aromatic polyester fiber having a high proportion of the orthorhombic crystal in the crystal component rather than only a high degree of crystallinity and degree of orientation has excellent creep property due to its dense crystal structure. The degree of orthorhombic crystallinity may be preferably 16.0% or more, more preferably 17.0% or more, and further preferably 18.0% or more. On the other hand, the degree of orthorhombic crystallinity may be, for example, 25.0% or less, preferably 24.0% or less, and more preferably 23.0% or less. In the present specification, the degree of orthorhombic crystallinity of the melt-anisotropic aromatic polyester fiber is calculated from diffraction peaks derived from orthorhombic and hexagonal crystals in an X-ray diffraction profile obtained by wide-angle X-ray diffraction measurement (e.g., diffraction peaks appearing around a diffraction angle 2θ = 19 to 21°, etc.), and is a value measured by the method described in Examples below.
The melt-anisotropic aromatic polyester fiber according to the present invention has a high density due to the high proportion of the orthorhombic crystal in which molecular chains are more densely packed, and, for example, may have a density of 1.4080 g/cm³ or more as determined using a density gradient tube. The upper limit of the density is not particularly limited to a specific one, and depends on the composition of the melt-anisotropic aromatic polyester, etc., but may be, for example, 1.4200 g/cm³ or less. In the present specification, the density of the melt-anisotropic aromatic polyester fiber is a value measured by the method described in Examples below.
The melt-anisotropic aromatic polyester fiber according to the present invention may have a melting point of 260 to 380°C, preferably 270 to 360°C, more preferably 275 to 340°C, and further preferably 275 to 330°C. The melting point of the melt-anisotropic aromatic polyester fiber can increase from the melting point (Mp) of an as-spun fiber due to solid-phase polymerization. The melting point of the melt-anisotropic aromatic polyester fiber is a value measured by the method described in Examples below.
The melt-anisotropic aromatic polyester fiber according to the present invention may have a tensile strength of 20 cN/dtex or more, preferably 24 cN/dtex or more, and more preferably 25 cN/dtex or more. On the other hand, the upper limit of the tensile strength is not particularly limited to a specific one, and may be, for example, about 40 cN/dtex. The tensile strength of the melt-anisotropic aromatic polyester fiber is a value measured by the method described in Examples below.
For the melt-anisotropic aromatic polyester fiber according to the present invention, a time until the fiber breakage in a creep test by the method described in Examples below may be 30 hours or longer, preferably 60 hours or longer, and more preferably 68 hours or longer.
The melt-anisotropic aromatic polyester fiber according to the present invention 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 electric 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.
The melt-anisotropic aromatic polyester fiber according to the present invention 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.
The total fineness of the melt-anisotropic aromatic polyester fiber according to the present invention 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 1000 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 Melt-Anisotropic Aromatic Polyester Fiber]

A method for producing the melt-anisotropic aromatic polyester fiber according to the present invention at least includes:

melt-kneading a melt-anisotropic aromatic polyester using a twin-screw extruder at a barrel temperature, from a resin feed portion to a kneading portion outlet, of lower than a melting point Mp₀ of the melt-anisotropic aromatic polyester to obtain a melt-kneaded material, the melting point Mp₀ being measured by a differential scanning calorimeter under a nitrogen atmosphere at a temperature elevation rate of 10°C/min;
spinning the melt-kneaded material to obtain an as-spun fiber; and
heat-treating the obtained as-spun fiber.

According to the present invention, by using the twin-screw extruder to improve the kneadability of the melt-anisotropic aromatic polyester, and in addition, by kneading the melt-anisotropic aromatic polyester in a kneading portion in the twin-screw extruder in a state where the viscosity thereof is high by setting the temperature to a low temperature lower than the melting point thereof, it is made possible to efficiently apply a shear force to the melt-anisotropic aromatic polyester. It should be noted that whereas it is common to heat the resin at a temperature equal to or higher than the melting point of the resin to promote melting, the temperature condition based on the melting point of the resin in the present invention is opposite to the usual temperature condition.
Probably because an as-spun fiber having a more uniform microcrystal structure can be obtained by spinning the melt-kneaded material to which a shear force has been applied during kneading at the low temperature as described above, the molecular chains are densely packed during solid-phase polymerization by heat-treating the as-spun fiber, and it allows to increase the proportion of orthorhombic crystals, which have a more dense crystal structure.
Furthermore, probably because compared to heating the resin at a temperature equal to or higher than the melting point thereof to add thermal energy to the resin as in the conventional conditions, applying the shear force during kneading at the low temperature makes it possible to add more mechanical energy than the thermal energy, the melt-kneading under the conditions can promote melting of the resin more than that under conventional conditions. As a result, in addition to the formation of the crystal structure, it is also possible to reduce the amount of unmelted materials remaining, so that a filter provided to remove foreign materials is less likely to become clogged, and fiber breakages during spinning can be suppressed.
It is also generally known that a single-screw extruder can be used in the melt spinning method, but in the case where an extrusion is performed in the single-screw extruder at the extruder temperature of lower than the melting point as described above, an improperly feeding of a resin, pressure fluctuations, and an increase in torque occur, so that it is difficult to stably perform spinning. This is thought to be due to the fact that, whereas the idea behind the twin-screw extruder is to melt the resin using the heat generated by high shear, the contribution of heat transfer from an extruder barrel during melting is significant in the single-screw extruder. Therefore, it is not possible to obtain the same effects using the single-screw extruder in the present invention.
The method for producing the melt-anisotropic aromatic polyester fiber will be described below with reference to Fig. 1. Fig. 1 is a schematic diagram showing an internal structure from a side surface of a twin-screw extruder 100 used for producing the melt-anisotropic aromatic polyester fiber according to one embodiment of the present invention. As shown in Fig. 1, the twin-screw extruder 100 includes a hopper 11 for feeding the melt-anisotropic aromatic polyester, a barrel 12, a screw 13 which rotates in the barrel 12, and a vent 14, and has a resin feed portion 21, a kneading portion 22, and a conveying portion 23 from the upstream side to the downstream side in the barrel 12. The resin feed portion 21, the kneading portion 22, and the conveying portion 23 have screw elements 13a, 13b, and 13c of the screw 13, respectively. The screw 13 (screw elements 13a, 13b, and 13c) represents one of the screws in the twin-screw extruder 100. Although Fig. 1 is shown by a simple structure in order to describe the method for producing the melt-anisotropic aromatic polyester fiber according to the present invention, a plurality of various apparatuses may be provided as necessary, and an apparatus commonly used in a twin-screw extruder may be provided in addition to the shown apparatuses.
In Fig. 1, a solid-state melt-anisotropic aromatic polyester fed from the hopper 11 is conveyed in the barrel 12 in an X direction, which is a travelling direction, by rotation of the screw 13, and is heated by a known heating means such as a heater installed at the barrel 12. In addition to the heat transfer from the heating means, a shear force is applied between an inner wall of the barrel 12 and the screw 13 and between the screws 13, thereby melting the solid-state melt-anisotropic aromatic polyester as the solid-state melt-anisotropic aromatic polyester travels in the X direction. The melt-anisotropic aromatic polyester may be fed into the twin-screw extruder 100 as a resin composition containing the above-described thermoplastic polymer, various additives, etc.
In the resin feed portion 21, the solid-state melt-anisotropic aromatic polyester fed from the hopper 11 moves in the X direction while being pressed and compacted in the solid state by the rotation of the screw 13, and can be gradually melted by the heat transfer from the heating means of the barrel 12 and the application of the shear force by the rotation of the screw 13. Examples of the screw element 13a used in the resin feed portion 21 include a full flight screw.
In the kneading portion 22, the melt-anisotropic aromatic polyester conveyed from the resin feed portion 21 and containing solids is kneaded by using a shear-applying-type screw element such as a kneading disc as the screw element 13b, thereby allowing promotion of melting of the melt-anisotropic aromatic polyester. In Fig. 1, one kneading portion 22 is provided, but a plurality of kneading portions may be provided.
In the conveying portion 23, the viscosity of the melt-kneaded material obtained in the kneading portion 22 can be adjusted when conveying the melt-kneaded material to a spinning head. Examples of the screw element 13c used in the conveying portion 23 include a full flight screw.
In the present invention, by setting the temperatures of the resin feed portion 21 and the kneading portion 22 in the barrel 12 to a low temperature lower than the melting point Mp₀ of the melt-anisotropic aromatic polyester fed into the twin-screw extruder 100, it is made possible to efficiently apply the shear force to the highly viscous melt-anisotropic aromatic polyester. The temperature of the barrel 12 from the resin feed portion 21 to the outlet of the kneading portion 22 may be preferably Mp₀ - 5°C or lower, more preferably Mp₀ - 10°C or lower, and further preferably Mp₀ - 15°C or lower. On the other hand, from the viewpoint of promoting melting by heating, the temperature of the barrel 12 may be Mp₀ - 100°C or higher, preferably Mp₀ - 90°C or higher, and more preferably Mp₀ - 80°C or higher. In the production method according to the present invention, in the case where there are a plurality of kneading portions, the barrel temperature from the resin feed portion to the kneading portion outlet refers to the barrel temperature to the outlet of the most downstream kneading portion adjacent to the conveying portion.
The heating means installed at the barrel 12 may be controlled to have a different temperature in each region in the travelling direction from the upstream side to the downstream side, and it is preferable to adjust the temperature of the barrel 12 from the resin feed portion 21 to the outlet of the kneading portion 22 such that the temperature of the barrel 12 gradually increases within the above temperature range.
In the present invention, it is sufficient to adjust the temperatures of the resin feed portion 21 and the kneading portion 22 in the barrel 12 to be in the above temperature range, and, from the viewpoint of adjusting the viscosity of the melt-kneaded material for spinning, the temperature of the barrel 12 in the conveying portion 23 after that may be Mp₀ or higher, preferably Mp₀ + 10°C or higher, and more preferably Mp₀ + 20°C or higher. On the other hand, from the viewpoint of suppressing the decomposition of the melt-anisotropic aromatic polyester, the temperature of the barrel 12 in the conveying portion 23 may be 400°C or lower, preferably 370°C or lower, and more preferably 350°C or lower.
From the viewpoint of efficiently applying the shear force to the melt-anisotropic aromatic polyester to improve spinnability, a residence time in the kneading portion 22 may be 10 seconds or longer, preferably 15 seconds or longer, and more preferably 20 seconds or longer. On the other hand, from the viewpoint of suppressing the degradation of the melt-anisotropic aromatic polyester, the residence time in the kneading portion 22 may be 300 seconds or shorter, preferably 180 seconds or shorter, and more preferably 130 seconds or shorter. The residence time in the kneading portion can be calculated by the following formula based on a volume of the kneading portion and a discharge rate. (Space in barrel [cm³]) = (Volume in barrel in kneading portion [cm³]) - (Volume occupied by screw in kneading portion [cm³]) (Residence time in kneading portion [s]) = (Space in barrel [cm³])/(Volume discharge rate [cm³/s])
Depending on the type of the melt-anisotropic aromatic polyester, spinning conditions, etc., an embodiment such as a capacity, a distribution, and a configuration of the resin feed portion, the kneading portion, and the conveying portion; shapes of the screws; intervals between the screws; or the like may be designed as appropriate, and in addition to the screw elements such as full flight screws and kneading discs, screw elements for returning in the direction opposite to the travelling direction from the upstream side to the downstream side, such as back kneading discs, may be placed. Further, a seal ring may be used as an element at the kneading portion outlet for the purpose of efficiently retaining the resin in the kneading portion and for the purpose of enhancing the sealability at a vent upstream portion.
In the twin-screw extruder 100, bubbles may be included during melt-kneading due to air entrainment, etc. Thus, it is preferable to carry out degassing by, for example, installing the vent 14 in the twin-screw extruder 100, connecting a vacuum pump or the like to the vent 14, and reducing the pressure in the twin-screw extruder 100. For example, the degree of vacuum in absolute pressure may be 100 kPa or lower, preferably 80 kPa or lower, and more preferably 60 kPa or lower.
The melt-kneaded material obtained by melt-kneading in the twin-screw extruder 100 is then metered by a gear pump (not shown) from the conveying portion 23 and conveyed to the spinning head. In order to avoid contamination with foreign materials such as unmelted materials included in the melt-kneaded material, a filter may be provided in front or back of the gear pump after conveyance from the twin-screw extruder 100. In the present invention, the amount of unmelted materials remaining can be reduced by melt-kneading in the twin-screw extruder, so that the filter is less likely to become clogged, and the spinning can be stabilized.
After being conveyed to the spinning head, the melt-kneaded material can be discharged through a nozzle at a predetermined spinning temperature, and the obtained yarn can be wound by a godet roller or the like to produce an as-spun fiber of the melt-anisotropic aromatic polyester fiber. The spinning temperature (spinneret temperature) relative to the melting point Mp₀ of the melt-anisotropic aromatic polyester may be from Mp₀ - 30°C to Mp₀ + 60°C, preferably from Mp₀ - 25°C to Mp₀ + 50°C, and more preferably from Mp₀ - 20°C to Mp₀ + 45°C. Further, the spinning temperature may be from barrel temperature in kneading portion + 10°C to barrel temperature in kneading portion + 120°C, preferably from barrel temperature in kneading portion + 15°C to barrel temperature in kneading portion + 100°C, more preferably from barrel temperature in kneading portion + 20°C to barrel temperature in kneading portion + 90°C, and further preferably from barrel temperature in kneading portion + 30°C to barrel temperature in kneading portion + 80°C. In the case where the spinning temperature (spinneret temperature) is the above upper limit or lower, the degradation of the resin in the vicinity of the spinneret can be suppressed, so that process defects due to fiber breakages and yellowing of the product are less likely to occur.
Probably because a more uniform microcrystal structure is formed in the as-spun fiber, by performing heat treatment on the as-spun fiber, solid-phase polymerization of the melt-anisotropic aromatic polyester can be advanced, so as to increase the melting point from the melting point (Mp) of the as-spun fiber, as well as improve the degree of orthorhombic crystallinity in the heat-treated fiber. 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. The melting point (Mp) of the as-spun fiber can be measured by the same method as for the melting point of the melt-anisotropic aromatic polyester fiber.
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.
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.
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.
In the heat-treating step, the heat treatment temperature may be from 250 to 350°C, preferably from 255 to 320°C, more preferably from 260 to 315°C, and further preferably from 280 to 310°C. In addition, in order to prevent a heat-treated fiber from being fused during heat treatment, the heat treatment temperature may be lower than the melting point (Mp) of the as-spun fiber to be subjected to the heat-treating step. For example, the heat treatment temperature may be, in the range of 250 to 350°C, 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-treating step, since the melt-anisotropic aromatic polyester fiber enables to enhance the melting point with progress of solid phase polymerization, it is sufficient to carry out the heat-treating step at a first heat treatment temperature of less than the melting point (Mp) of the as-spun fiber. 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.
Depending on heat treatment procedure and/or the heat treatment temperature, a heat treatment period of the heat-treating step may be set as appropriate. For example, the heat treatment period may be set in the range of from 15 minutes to 30 hours, preferably from 2 to 24 hours, and more preferably from 3 to 20 hours. Here, the heat treatment period indicates a retention time at a predetermined heat treatment temperature (e.g., the maximum temperature).
In the method for producing the melt-anisotropic aromatic polyester fiber according to the present invention, for example, in order to improve bundling properties of fibers and to prevent fibers from fusion during the heat treatment, a known oil agent or anti-fusion agent may be applied before the heat-treating step.

[Fiber Structure]

The melt-anisotropic aromatic polyester fiber according to the present invention can be used for various applications as a fiber structure at least partially including the melt-anisotropic aromatic polyester fiber. The fiber structure including the melt-anisotropic aromatic polyester fiber according to the present invention can be used as various fiber configurations such as staple fibers, shortcut fibers, filament yarns, spun yarns, cordage, ropes, etc. Further, the fiber structure can also be used as various fabrics such as nonwoven fabrics, woven fabrics, and knitted fabrics, using the melt-anisotropic aromatic polyester fibers. Such fibers and fabrics can be produced by known methods using the melt-anisotropic aromatic polyester fibers.
The fiber structure according to the present invention may be made by combining the melt-anisotropic aromatic 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 melt-anisotropic aromatic polyester fibers and other fibers (e.g., a commingled yarn made from the melt-anisotropic aromatic polyester fibers and other fibers, or others). The fiber structure may also be a blend fabric using the melt-anisotropic aromatic polyester fibers and other fibers (e.g., a combined fabric in which the melt-anisotropic aromatic polyester fibers and other fibers are used in combination, a layered material in which a fabric of the melt-anisotropic aromatic polyester fibers and a fabric of other fibers are used in combination, or others).
The melt-anisotropic aromatic 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 various fiber products. For example, the melt-anisotropic aromatic polyester fiber according to the present invention can be used for highly processed products such as tension members (e.g., electric cables, optical fibers, umbilical cables, heater wire core yarns, cords for various electrical products such as earphone cords, etc.), sailcloth, ropes (marine ropes, mountaineering ropes, crane ropes, yacht ropes, tug ropes, etc.), climbing ropes, land nets, slings, life lines, fishing lines, sewing threads, cords for screen doors and screen windows, fishing nets, longlines, geogrids, protective gloves, ripstop for protective clothing and outdoor clothing, rider suits, sports rackets, guts, medical catheter reinforcing materials, sutures, screen gauzes, filters, base cloth for printed circuit boards, mesh-like conveyor belts, paper-making belts, dryer canvas, airships, balloons, airbags, speaker cones, reinforcing materials for various hoses and pipes, and reinforcing materials for rubbers and plastics such as tires and conveyor belts. In particular, the melt-anisotropic aromatic polyester fiber according to the present invention has excellent creep property and thus can be suitably used as tension members.

EXAMPLES

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 Points of Resin Chips (Granular Molded Bodies) and Fibers)

In accordance with JIS K 7121, measurement was performed using a differential scanning calorimeter (DSC; "DSC60A Plus", produced by SHIMADZU CORPORATION), and the observed main endothermic peak temperature was determined as a melting point. 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 melt-anisotropic aromatic polyester.

(Amount of Pressure Increase)

50 kg of a melt-anisotropic aromatic polyester obtained by melt-kneading under each condition in a twin-screw extruder was conveyed to and passed through a container to which a φ76-size metal non-woven fabric filter with an opening size of 30 µm was set. The pressure was monitored using a resin pressure gauge GC75, produced by NAGANO KEIKI CO., LTD., which was installed in the container, and the amount of pressure increase before and after the passing was recorded.

(Spinnability)

A melt-anisotropic aromatic polyester was discharged and wound, and a spinning test was conducted for three days after the start of winding. The number of times the yarn broke during the spinning test was counted, and the results were evaluated based on the following criteria.

A: Number of yarn breakages of 0 times/day
B: Number of yarn breakages of more than 0 times/day and less than 0.7 times/day
C: Number of yarn breakage of 0.7 times/day or more and less than 2.0 times/day
D: Number of yarn breakages of 2.0 times/day or more

(Degree of Orthorhombic Crystallinity)

A wide-angle X-ray diffraction (WAXD) measurement was performed by setting a melt-anisotropic aromatic polyester fiber in a holder dedicated for fibers, and irradiating the melt-anisotropic aromatic polyester fiber with X-rays in a direction perpendicular to a fiber axis under the following measurement conditions by a transmission method.

Measurement device: "D8 Discover IµS", produced by Bruker
Detector: two-dimensional PSPC·VANTEC-500
X-ray source: Cu
Current: 1 mA
Voltage: 50 kV
Time of exposure to light: 10 minutes
Collimator diameter: 0.5 mm
Camera length: 17 cm
Detector position (2θ): 20°
Sample position (ω): 10°
Tilt angle (Ψ): 90°
Measurement temperature: room temperature (about 25°C)

An X-ray diffraction profile with a horizontal axis as a diffraction angle (2θ) and a vertical axis as intensity was obtained under the following conditions.

Integration range: 2θ = 5 to 35°, γ (azimuth angle) = 250 to 290°
Step width: 0.05° (diffraction angle)

In the obtained X-ray diffraction profile, two values at 20 = 5° and 35° were connected by a straight line to set a baseline. Based on this newly set baseline, the profile was converted into profile data with the difference in intensity along the vertical axis from an actual measured data to the baseline as new intensity.
An amorphous peak was fitted to the profile data after baseline correction using a pseudo-Voigt function (Lorentz function ratio: α = 0) with a peak height, a peak top position, σ, and an asymmetry parameter as variables. At this time, an initial value of the peak top position of the fitting function was set to around 20.6°. A peak area of this fitting function was determined as an amorphous amount (D).
Crystal peaks were fitted to a peak A (peak top position: around 19°), a peak B (peak top position: around 20.5°), and a peak C (peak top position: around 27°) of the profile data after baseline correction using the following functions with a peak height, a peak top position, and σ as variables. All of the crystal peaks were assumed to be symmetrical.

Peak A: pseudo-Voigt function (α = 1)
Peak B: pseudo-Voigt function (α = 0)
Peak C: pseudo-Voigt function (α = 0.5)

Then, fitting was performed by the least-squares method such that the difference between the sum of all fitting functions, including the fitting function for the amorphous peaks obtained above, and the profile data after baseline correction was minimized. The peak areas of these fitting functions were determined as crystal amounts (A), (B), and (C), respectively.
A degree of orthorhombic crystallinity was calculated from the following formula, using the crystal amount (A) of the peak A derived from the hexagonal crystal and the crystal amount (B) of the peak B derived from the orthorhombic crystal. $Degree of orthorhombic crystallinity (%) = (B) / \{(A) + (B)\} \times 100$

(Fiber Density)

A density was measured with reference to the density gradient tube method (JIS L 1013: 2010 8.17.2). In order to compare the slight density differences caused by the packing differences in fibers due to orthorhombic crystallization, the average of five measurements for each sample was calculated and rounded to four decimal places.

(Total Fineness and Single Fiber Fineness)

In accordance with 8.3.1 Method A in JIS L 1013: 2010, 10 m of melt-anisotropic aromatic polyester fibers were reeled into a hank using a sizing reel "Wrap Reel by Motor Driven" produced by DAIEI KAGAKU SEIKI MFG. Co., Ltd., to measure the weight of the liquid crystal polyester fibers. The measurement was conducted three times. Each of the weights (g) was multiplied by 1000, and the average of the three measurements was regarded as a total fineness (dtex) of the melt-anisotropic aromatic polyester fibers. Thus-obtained total fineness was divided by the number of filaments in the melt-anisotropic aromatic polyester fibers so as to give a single fiber fineness (dtex).

(Tensile Strength)

With reference to JIS L 1013: 2010 8.5.1, using a strength and elongation measuring machine "TENSORAPID5" produced by USTER Technologies AG, a tensile test was carried out under the conditions of a test sample length of 30 cm, a tensile speed of 15 cm/min, and an initial load of 0.33 g/dtex 5 times for one sample to obtain an average tensile strength (cN), and a tensile strength (cN/dtex) was calculated by dividing the average tensile strength (cN) of the five tests by the total fineness (dtex) measured by the above-described method.

(Creep Properties)

Using the creep testing machine "525-R CREEP TESTER" produced by Mys-Shikenki Co., Ltd., measurement was started from the time at which a load of 0.18 N/dtex was applied to fibers having a test length of 300 mm at a measurement temperature of 20°C, and the time until the fiber broke was measured. The measurement was performed three times for one sample to determine the average value.

[Example 1]

Chips (granular molded bodies) of a melt-anisotropic aromatic polyester (Mp₀: 278°C) having a structural unit (A) derived from 4-hydroxybenzoic acid and a structural unit (B) derived from 6-hydroxy-2-naphthoic acid in a ratio of (A)/(B) = 73/27 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder having a space volume in a barrel in a kneading portion of 14.7 cm³ to be melt-kneaded with a barrel temperature from a resin feed portion to a kneading portion outlet being set to 260°C and a barrel temperature in a conveying portion downstream of the kneading portion outlet being set to 325°C. At this time, a residence time in the kneading portion was adjusted to 21 seconds. In this process, a vacuum pump was connected via a metal pipe to a vent portion, which was provided in the middle of the twin-screw extruder, to reduce the pressure in the space not filled with the polymer in the twin-screw extruder to 30 kPa. Then, the melt-kneaded material was fed from the twin-screw extruder to a spinning head while being metered by a gear pump. The spinning head was equipped with a spinneret having 100 holes each having a hole diameter of 0.10 mm φ, a spinneret temperature was set to 320°C, and the melt-kneaded material was discharged at a volume discharge rate of 42.0 cm³/min to obtain as-spun fibers with 560 dtex/100 f. Then, the obtained as-spun fibers were heat-treated under a nitrogen atmosphere at 275°C for 16 hours to obtain heat-treated fibers of melt-anisotropic aromatic polyester fibers. The analysis results of the obtained melt-anisotropic aromatic polyester fibers are shown in Table 5.

[Example 2]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the space volume in the barrel in the kneading portion was set to 44.1 cm³, the barrel temperature from the resin feed portion to the kneading portion outlet was set to 240°C, the volume discharge rate was set to 126.1 cm³/min, and a spinneret having 300 holes was used.

[Example 3]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 220°C.

[Example 4]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 200°C.

[Example 5]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the space volume in the barrel in the kneading portion was set to 29.4 cm³, the barrel temperature from the resin feed portion to the kneading portion outlet was set to 230°C, the volume discharge rate was set to 30.2 cm³/min, and the residence time in the kneading portion was set to 58.5 seconds.

[Example 6]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 250°C, the volume discharge rate was set to 7.0 cm³/min, the residence time in the kneading portion was set to 126 seconds, and a spinneret having 10 holes was used.

[Example 7]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 230°C, the volume discharge rate was set to 58.8 cm³/min, and the residence time in the kneading portion was set to 15 seconds.

[Example 8]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the volume discharge rate was set to 96.9 cm³/min and the residence time in the kneading portion was set to 9.1 seconds.

[Example 9]

Chips (granular molded bodies) of a melt-anisotropic aromatic polyester (Mp₀: 311°C) having a structural unit (A) derived from 4-hydroxybenzoic acid and a structural unit (B) derived from 6-hydroxy-2-naphthoic acid in a ratio of (A)/(B) = 78/22 (mol%) were dried by hot air at 120°C for at least 4 hours. Then, the chips were fed into a twin-screw extruder having a space volume in a barrel in a kneading portion of 14.7 cm³ to be melt-kneaded with a barrel temperature from a resin feed portion to a kneading portion outlet being set to 270°C and a barrel temperature in a conveying portion downstream of the kneading portion outlet being set to 340°C. At this time, a residence time in the kneading portion was adjusted to 21 seconds. In this process, a vacuum pump was connected via a metal pipe to a vent portion, which was provided in the middle of the twin-screw extruder, to reduce the pressure in the space not filled with the polymer in the twin-screw extruder to 30 kPa. Then, the melt-kneaded material was fed from the twin-screw extruder to a spinning head while being metered by a gear pump. The spinning head was equipped with a spinneret having 100 holes each having a hole diameter of 0.10 mm φ, a spinneret temperature was set to 340°C, and the melt-kneaded material was discharged at a volume discharge rate of 42.0 cm³/min to obtain as-spun fibers with 560 dtex/100 f. Then, the obtained as-spun fibers were heat-treated under a nitrogen atmosphere at 280°C for 16 hours to obtain heat-treated fibers of melt-anisotropic aromatic polyester fibers.

[Comparative Example 1]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that a single-screw extruder (Φ 50 mm) having no vent was used as the extruder and the barrel temperature except for the resin feed portion was set to 320°C.

[Comparative Example 2]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 300°C.

[Comparative Example 3]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 1, except that the barrel temperature from the resin feed portion to the kneading portion outlet was set to 285°C.

[Comparative Example 4]

Melt-anisotropic aromatic polyester fibers were obtained in the same manner as in Example 9, except that a single-screw extruder (Φ 50 mm) having no vent was used as the extruder and the barrel temperature except for the resin feed portion was set to 350°C.

[Table 5]

		Ex. 1	Ex. 2	Ex. 3	Ex. 4	Ex. 5	Ex. 6	Ex. 7	Ex. 8	Ex. 9
Production	Melt-anisotropic aromatic polyester melting point Mp₀ [°C]	278	278	278	278	278	278	278	278	311
	Extruder	Twin-screw	Twin-screw	Twin-screw	Twin-screw	Twin-screw	Twin-screw	Twin-screw	Twin-screw	Twin-screw
	Barrel temperature from resin feed portion to kneading portion outlet [°C]	260	240	220	200	230	250	230	260	270
	Residence time in kneading portion [sec]	21	21	21	21	58.5	126	15	9.1	21
	Amount of pressure increase [MPa]	0.5	0.7	0.4	0.7	0.2	0.3	0.6	0.7	0.7
	Spinnability	A	A	A	A	A	A	B	B	B
Fiber	Total fineness [dtex]	560	1670	560	560	560	56	560	560	560
	Single fiber fineness [dtex]	5.6	5.6	5.6	5.6	5.6	5.6	5.6	5.6	5.6
	Number of filaments	100	300	100	100	100	10	100	100	100
	Melting point [°C]	319.1	320.3	319.4	321.5	320.8	320.2	317.6	318.5	335.0
	Degree of orthorhombic crystallinity [%]	17.0	16.5	18.4	18.5	21.3	22.7	15.9	17.8	16.2
	Density [g/cm³]	1.4083	1.4083	1.4083	1.4082	1.4084	1.4085	1.4083	1.4082	1.4089
	Tensile strength [cN/dtex]	25.8	25.2	25.1	26.1	26.9	27.8	25.8	25.6	23.4
	Creep properties Breakage time [hour]	72.3	50.8	80.3	115.4	69.6	77.2	65.9	74.4	62.3

[Table 5-continued]

		Comp. Ex. 1	Comp. Ex. 2	Comp. Ex. 3	Comp. Ex. 4
Production	Melt-anisotropic aromatic polyester melting point Mp₀ [°C]	278	278	278	311
	Extruder	Single-screw	Twin-screw	Twin-screw	Single-screw
	Barrel temperature from resin feed portion to kneading portion outlet [°C]	320	300	285	350
	Residence time in kneading portion [sec]	-	21	21	-
	Amount of pressure increase [MPa]	2.5	2.1	1.8	4.1
	Spinnability	D	C	B	D
Fiber	Total fineness [dtex]	560	560	560	560
	Single fiber fineness [dtex]	5.6	5.6	5.6	5.6
	Number of filaments	100	100	100	100
	Melting point [°C]	310.8	312.9	314.0	335.0
	Degree of orthorhombic crystallinity [%]	8.6	11.4	12.3	13.3
	Density [g/cm³]	1.4075	1.4077	1.4077	1.4080
	Tensile strength [cN/dtex]	23.2	23.5	23.7	20.4
	Creep properties Breakage time [hour]	2.5	6.4	11.9	7.2

As shown in Table 5, in Examples 1 to 9, since melt-kneading was performed with the barrel temperature from the resin feed portion to the kneading portion outlet of the twin-screw extruder being set to a specific low temperature condition, melt-anisotropic aromatic polyester fibers having a high degree of orthorhombic crystallinity can be obtained. Therefore, the melt-anisotropic aromatic polyester fibers of Examples 1 to 9 have excellent creep property.
On the other hand, in Comparative Examples 1 and 4, since a single-screw extruder was used and the barrel temperature from the resin feed portion to the kneading portion outlet was high, the degree of orthorhombic crystallinity cannot be increased. Also, in Comparative Examples 2 and 3, although a twin-screw extruder was used, since the barrel temperature from the resin feed portion to the kneading portion outlet was high, the degree of orthorhombic crystallinity cannot be increased. Therefore, the melt-anisotropic aromatic polyester fibers of Comparative Examples 1 to 4 have inferior creep properties compared to Examples 1 to 9.

INDUSTRIAL APPLICABILITY

The melt-anisotropic aromatic 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 various fiber products, and can be used as a tension member, for example.
Although the present invention has been described above in connection with the preferred embodiment thereof with reference to the drawing, numerous additions, changes, and deletions can be made without departing from the gist of the present invention. Therefore, such additions, changes, and deletions are also construed as included within the scope of the present invention.

REFERENCE NUMERALS

100: twin-screw extruder
11: hopper
12: barrel
13: screw
13a, 13b, 13c: screw element
14: vent
21: resin feed portion
22: kneading portion
23: conveying portion
X: flow direction

Claims

A melt-anisotropic aromatic polyester fiber having a degree of orthorhombic crystallinity of 15.0% or more in a crystal component.
The melt-anisotropic aromatic polyester fiber according to claim 1, having a density of 1.4080 g/cm³ or more as determined using a density gradient tube.
The melt-anisotropic aromatic polyester fiber according to claim 1 or 2, comprising a melt-anisotropic aromatic polyester having a structural unit derived from 4-hydroxybenzoic acid at a proportion of 50 mol% or more.
The melt-anisotropic aromatic polyester fiber according to claim 1 or 2, having a melting point of 260 to 380°C as measured by a differential scanning calorimeter under a nitrogen atmosphere at a temperature elevation rate of 10°C/min.
A fiber structure at least partially comprising the melt-anisotropic aromatic polyester fiber as recited in claim 1 or 2.
A method for producing a melt-anisotropic aromatic polyester fiber, the method at least comprising:
melt-kneading a melt-anisotropic aromatic polyester using a twin-screw extruder at a barrel temperature, from a resin feed portion to a kneading portion outlet, of lower than a melting point Mp₀ of the melt-anisotropic aromatic polyester to obtain a melt-kneaded material, the melting point Mp₀ being measured by a differential scanning calorimeter under a nitrogen atmosphere at a temperature elevation rate of 10°C/min;

spinning the melt-kneaded material to obtain an as-spun fiber; and

heat-treating the obtained as-spun fiber.
The method according to claim 6, wherein the melt-kneading is performed under a condition where a residence time in the kneading portion of the twin-screw extruder is 10 seconds or longer.