Sheath-core fiber and method for producing the same

Abstract

To provide a sheath-core fiber that can retain a high concentration of a functional agent in a core portion of the fiber and has high breaking strength, and a method for producing the same.SOLUTION: A sheath-core fiber is obtained that comprises a sheath of a first polymer comprising wholly aromatic polyamide and a core of a second polymer comprising wholly aromatic polyamide and a functional agent, and has a breaking strength of 1.5 cN / dtex or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a sheath-core fiber and a method for producing the same. More specifically, the present invention relates to a sheath-core fiber having a sheath of a first polymer containing a wholly aromatic polyamide and a core of a second polymer containing a wholly aromatic polyamide and a functional agent, and having a breaking strength of 1.5 cN / dtex or more, and a method for producing the same. [Background technology]

[0002] It has long been known that wholly aromatic polyamides produced from aromatic diamines and aromatic dicarboxylic acid dihalides have excellent heat resistance and flame retardancy. Among such wholly aromatic polyamides, fibers of meta-type wholly aromatic polyamides (hereinafter referred to as meta-aramids), typified by polymetaphenylene isophthalamide, are particularly useful as heat-resistant and flame-retardant fibers. These properties have led to their use in, for example, disaster prevention and safety clothing, such as protective clothing, and industrial applications, such as filters and electronic components.

[0003] In recent years, with the advancement of society, there has been a demand for even higher functionality in each of the above applications, and there is an increasing need for functions that have not been achieved with conventional meta-aramid polymers, such as high electrical conductivity, high heat resistance, electromagnetic wave shielding, etc. In order to achieve these functions, possible methods include copolymerizing a functional comonomer with a meta-aramid polymer to impart functionality to the polymer itself, kneading a functional agent into the polymer during spinning, or applying a functional agent to the surface of the fiber after spinning.

[0004] However, copolymerization is subject to restrictions on the monomer structure, and functional monomers are generally expected to be very expensive, making it unrealistic from a manufacturing standpoint. Furthermore, when a functional agent is kneaded into the fiber and spun, there is a limit to the concentration that can be contained in the fiber from the standpoint of spinnability, and there is a high possibility that the functional agent will not be fully expressed. Furthermore, applying a functional agent to the fiber surface has the problem of significantly poor durability against friction and washing.

[0005] Another method for producing novel meta-aramid fibers with the above-mentioned functionality is to introduce a sheath-core structure. The sheath-core structure is a structure in which a core structure made of a polymer containing a functional agent or having a different polymer structure is covered with a polymer having a different functional agent or polymer structure from the core polymer. Many examples of this structure exist in general-purpose polymer fibers such as polyester and nylon, and have been industrially commercialized.

[0006] By using this sheath-core structure, the sheath structure prevents the core structure, which contains a large amount of functional agents, from losing significant strength, making it possible to produce high-performance fibers with sufficient breaking strength to withstand use.

[0007] Here, the following methods are available for spinning meta-aramid fibers. (a) Dry spinning, in which a fibrous polymer solution is spun from a spinneret, and the solvent is evaporated and dried from near the surface of the formed fibrous material to form fibers (for example, Japanese Patent Publication No. 1960-14399). (b) Wet spinning in which a meta-aramid polymer solution substantially free of salts is discharged into a coagulation bath containing an amide solvent and water, coagulated to form a fibrous material (thread), and then stretched in a plastic stretching bath containing an amide solvent and water, followed by water washing and heat treatment (Japanese Patent Laid-Open Nos. 2001-303365, 2003-301326, 2003-342832, etc.). (c) Wet spinning in which meta-aramid polymer powder is redissolved in an amide-based solvent, and then a 15 to 25 mass% meta-type wholly aromatic polyamide solution is spun into an aqueous coagulation bath containing 35 to 45 mass% of a highly concentrated inorganic salt to coagulate (JP 48-17551 B).

[0008] Furthermore, the following methods for producing aramid sheath-core fibers have been reported. U.S. Patent No. 10,982,353 discloses a method for obtaining, by dry spinning, sheath-core fibers consisting of an aramid polymer core containing 5 to 10 wt% carbon particles and an aramid polymer sheath containing a masking pigment. U.S. Patent No. 10,590,567 discloses a method for obtaining, by dry spinning, heterogeneous polymer sheath-core fibers consisting of an aramid polymer core containing 5 to 10 wt% carbon particles and a modacrylic polymer sheath containing a masking pigment.

[0009] Although methods for producing aramid fibers with a sheath-core structure have been reported, all of them are dry spinning methods, and no methods for producing sheath-core fibers by wet spinning have been reported. This is because the production of sheath-core fibers by wet spinning is difficult due to the nature of the coagulation process.

[0010] That is, when a polymer solution is spun into a coagulation liquid and coagulated, in most processes, the coagulation and solidification of the fiber proceeds through mutual diffusion, in which the solvent in the polymer solution diffuses into the coagulation liquid and the coagulation liquid penetrates into the polymer solution. Considering the mechanism of the above process, even if the polymer solution is spun into a sheath-core structure, the sheath-core structure will likely collapse or its boundaries will become unclear during the coagulation process, preventing the expected functionality of the sheath-core structure fiber from being fully realized. Therefore, it is difficult to produce a perfect sheath-core structure fiber by wet spinning.

[0011] On the other hand, as mentioned above, the production of sheath-core fibers by dry spinning has been achieved, and this method makes it possible to produce high-performance meta-aramid fibers with functional agents added to the core. However, because dry spinning involves spinning a polymer solution into a high-temperature gas atmosphere and solidifying the polymer by evaporating the solvent, there is a limit to the number of filaments that can be spun from one spinneret, making it difficult to achieve the high production efficiency of wet spinning, and there are also significant equipment constraints, which are fatal problems. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Special Publication No. 35-14399 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-303365 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-301326 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-342832 [Patent Document 5] Special Publication No. 48-17551 [Patent Document 6] U.S. Patent No. 10,982,353 [Patent Document 7] U.S. Patent No. 10,590,567 Summary of the Invention [Problem to be solved by the invention]

[0013] As described above, an object of the present invention is to provide a method for producing a sheath-core fiber by wet spinning, which has been considered difficult to achieve so far, as an alternative to the production of a sheath-core fiber by dry spinning, which is difficult to achieve high production efficiency and has significant equipment limitations. [Means for solving the problem]

[0014] As a result of extensive research into solving the above problems, the present inventors have found a method for obtaining sheath-core fibers even in a wet spinning method using an aqueous coagulation liquid containing an amide solvent, and have completed the present invention.

[0015] That is, according to the present invention, 1. A sheath-core fiber comprising a sheath portion made of a first polymer containing a wholly aromatic polyamide and a core portion made of a second polymer containing a wholly aromatic polyamide and a functional agent, the sheath-core fiber having a breaking strength of 1.5 cN / dtex or more. 2. The sheath-core fiber according to claim 1, wherein the first and second polymers both contain polymetaphenylene isophthalamide. 3. A method for producing a sheath-core fiber according to item 1 above, comprising the following steps in sequence: a) preparing a first polymer solution, which is a solution of a first polymer containing a wholly aromatic polyamide in an amide-based solvent; b) preparing a second polymer solution, which is a solution of a second polymer including a wholly aromatic polyamide in an amide-based solvent, and which includes a functional agent in the solution; c) introducing the first polymer solution and the second polymer solution into a spinneret having separate inlets and multiple outlets for spinning filaments, and d) discharging the first and second polymer solutions so that they meet at the outlet of the spinneret to form a plurality of dope filaments each having a core made of the second polymer solution and a sheath made of the first polymer solution; e) spinning the dope filament into an aqueous coagulation bath containing 20 to 50% by mass of an inorganic salt and 1 to 20% by mass of an amide-based solvent to form a coagulated thread; f) The coagulated yarn is drawn in a boiling water drawing bath, followed by dry heat treatment and then hot drawing. and, 4. The method for producing a sheath-core fiber according to 3 above, wherein the first and second polymers both contain polymetaphenylene isophthalamide; is provided. [Effects of the Invention]

[0016] The sheath-core fiber obtained by the present invention can retain a high concentration of functional agents in the fiber core, thereby achieving functionality not previously possible. Furthermore, the protection provided by the sheath structure prevents the functional agents from being removed by washing or wear. Even with a high concentration of functional agents inside due to the sheath-core structure, the fiber has a breaking strength of 1.5 cN / dtex or more, making it less susceptible to problems during processing into fabrics and practical use, making it suitable for use in protective clothing and other applications. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an electron microscope photograph illustrating an example of a cross-sectional shape of a sheath-core fiber of the present invention. [Figure 2] 1 is an electron microscope photograph showing an example of the cross-sectional shape of a sheath-core fiber obtained by a method other than the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below. The sheath-core fiber of the present invention is characterized by having a breaking strength of 1.5 cN / dtex or more.

[0019] Examples of polymers constituting such sheath-core fibers include wholly aromatic polyamides (hereinafter sometimes referred to as aramids), and particularly preferred are meta-type wholly aromatic polyamides composed of meta-type aromatic diamine components and meta-type aromatic dicarboxylic acid components, which are synthesized by polymerization.

[0020] Particularly preferred for use in the present invention is a wholly aromatic polyamide made of an aramid polymer having a structure containing metaphenylene isophthalamide units, from the viewpoints of mechanical properties, heat resistance, and flame retardancy.

[0021] Examples of aromatic diamine components that serve as raw materials for wholly aromatic polyamides include metaphenylenediamine or paraphenylenediamine, 3,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl sulfone, and derivatives thereof having substituents such as halogens and alkyl groups having 1 to 3 carbon atoms on the aromatic ring.

[0022] Examples of raw materials for the aromatic dicarboxylic acid component constituting the wholly aromatic polyamide of the present invention include aromatic dicarboxylic acid halides. Examples of meta-type aromatic dicarboxylic acid halides include isophthalic acid halides such as isophthalic acid chloride and isophthalic acid bromide, and derivatives thereof having a substituent such as a halogen or an alkoxy group having 1 to 3 carbon atoms on the aromatic ring.

[0023] The polymerization method for the wholly aromatic polyamide of the present invention includes, but is not limited to, a method in which an organic solvent system (e.g., tetrahydrofuran) that is not a good solvent for the resulting polyamide containing metaphenylenediamine and isophthalic acid chloride is contacted with an aqueous solution system containing an inorganic acid acceptor and a soluble neutral salt, thereby isolating a powder of polymetaphenylene isophthalamide polymer (interfacial polymerization, JP-B-47-10863), or a method in which the above diamine and acid chloride are solution polymerized in an amide solvent, followed by neutralization with calcium hydroxide, calcium oxide, or the like (solution polymerization, JP-A-8-074121, JP-A-10-88421).

[0024] The weight-average molecular weight of the wholly aromatic polyamide polymer (also referred to as aramid polymer) used in the present invention must be 400,000 to 1,000,000, as determined by the analytical method described below, in order to form fibers with practically sufficient breaking strength. If the weight-average molecular weight is less than 400,000, not only will the breaking strength be significantly reduced, but stable spinning will also be impossible. Furthermore, if the molecular weight exceeds 1,000,000, the viscosity will be too high to handle when preparing and spinning a wholly aromatic polyamide solution described below, requiring specialized equipment.

[0025] The polymer within the molecular weight range specified in the present invention may be a mixture of a low molecular weight polymer and a high molecular weight polymer, and the overall molecular weight may be within the range specified by adjusting the mixing ratio. For example, if a polymer with a weight-average molecular weight of 200,000 is mixed with a polymer with a weight-average molecular weight of 800,000, and the weight-average molecular weight of the resulting polymer is 600,000, it can be used without any problems because it is within the molecular weight range specified in the present invention.

[0026] The wholly aromatic polyamide fiber of the present invention is produced using the wholly aromatic polyamide obtained by the above-mentioned production method through the following steps: spinning solution preparation step, spinning / coagulation step, washing step, boiling water drawing step, dry heat treatment step, and hot drawing step.

[0027] [Spinning solution preparation process] In the spinning solution preparation step, the wholly aromatic polyamide of the present invention is dissolved in a solvent to prepare a spinning solution (dope). An amide solvent is typically used to prepare the spinning solution, and examples of such solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethylacetamide (DMAc). Among these, NMP or DMAc is preferred from the viewpoints of solubility and handling safety.

[0028] The solution concentration can be appropriately selected from the viewpoints of the coagulation rate in the subsequent spinning and coagulation step and the solubility of the polymer, and is usually in the range of 10 to 30% by mass, more preferably 15 to 25% by mass to achieve stable spinning.

[0029] In the present invention, an inorganic salt may be incorporated into the dope, and the content of the inorganic salt is preferably 0 to 20% by mass relative to the dope, and more preferably 0 to 10% by mass in order to obtain stable spinnability.

[0030] If the inorganic salt content exceeds 20% by mass, the coagulation rate becomes too fast, resulting in the formation of numerous voids in the fiber, making it impossible to obtain a fiber with the desired physical properties. It is preferable to use chloride salts such as calcium chloride, magnesium chloride, and lithium chloride as the inorganic salt.

[0031] In the present invention, functional agents introduced into the dope are diverse, including pigments, flame retardants, weathering agents, metal powders, and conductive materials, and are selected to impart desired functionality. From the standpoint of spinnability, the functional agent is in the form of a solid soluble in a liquid or amide-based solvent, or in the form of a powder if insoluble in the solvent. When using a powdered functional agent, its particle size is acceptable to be 5 μm or less, preferably 2 μm or less, and more preferably 1 μm or less.

[0032] [Spinning and coagulation process] In the spinning and coagulation step, the dope obtained above is spun into a coagulation liquid and coagulated. The spinning device is not particularly limited, and a conventionally known wet spinning device can be used. As long as it can perform stable wet spinning, there is no need to particularly limit the number of spinning holes, the diameter of the spinning holes, the arrangement state, etc. of the spinneret. For example, a spinneret with 10 to 30,000 spinning holes and a spinning hole diameter of 0.03 to 0.2 mm can be used.

[0033] On the other hand, with regard to the structure of the spinneret, it is necessary to prepare a spinneret having separate inlets for the first polymer solution and the second polymer solution and multiple outlets for spinning filaments, and a structure is required that can discharge the first and second polymer solutions so that they join at the spinneret outlet and form multiple dope filaments each having a sheath of the first polymer solution and a core of the second polymer solution. The temperature of the dope when it is spun out from the spinneret is preferably in the range of 20 to 90°C, more preferably 70 to 90°C.

[0034] In the present invention, by forming a sheath-core fiber composed of a sheath portion made of a first polymer containing only the above-mentioned wholly aromatic polyamide and a core portion made of a second polymer containing the above-mentioned wholly aromatic polyamide and also containing a functional agent, a high concentration of the functional agent can be retained in the core portion, allowing sufficient functionality to be exhibited, and the protection of the sheath structure prevents the functional agent from falling off due to washing or wear. Depending on the application, the above-mentioned functional agent can also be added to the first polymer to exhibit some function.

[0035] The coagulation bath used to obtain the fiber of the present invention must be an aqueous solution containing 1 to 20 mass % of an amide solvent, preferably 1 to 15 mass %. The temperature of this aqueous solution is preferably in the range of 50 to 90°C.

[0036] It is also essential that the coagulation bath contains 20 to 50 mass % of an inorganic salt such as calcium chloride or magnesium chloride, and preferably 35 to 45 mass %. As described above, the dope is spun from the spinneret into the coagulation liquid and passed through the coagulation bath to obtain a coagulated thread.

[0037] [Washing process, boiling water stretching process] The coagulated yarn thus obtained is thoroughly washed in an aqueous washing bath and then sent to a boiling water drawing step. The draw ratio in the boiling water draw bath is preferably in the range of 1.1 to 5.0, more preferably 1.1 to 3.0. By performing drawing within this range and increasing the molecular chain orientation, the strength of the final fiber can be ensured.

[0038] [Dry heat treatment process] The fibers that have been subjected to the above washing and drawing steps are preferably subjected to a dry heat treatment step. In the dry heat treatment step, the fibers that have been washed in the above washing step are dry heat treated at a temperature in the range of 100 to 250°C, preferably 100 to 200°C. The dry heat treatment is preferably carried out at a fixed length. The temperature of the dry heat treatment refers to the set temperature of a fiber heating means such as a hot plate or a heated roller.

[0039] [Hot stretching process] In the present invention, it is preferable to subject the fiber that has undergone the dry heat treatment step to a hot drawing step. In the hot drawing step, drawing is carried out while applying heat treatment in the range of 290 to 380°C. The treatment temperature is preferably in the range of 290 to 350°C. Temperatures below 290°C are inappropriate because high-ratio drawing is not possible, while temperatures above 380°C may cause discoloration of the fiber or breakage of the yarn. In the hot drawing step, the draw ratio is preferably in the range of 1.1 to 5.0 times, more preferably in the range of 1.2 to 3.0 times. The temperature of the hot drawing treatment refers to the set temperature of the fiber heating means, such as a hot plate or a heated roller.

[0040] It is essential that the sheath-core fiber obtained by the above method has a breaking strength of 1.5 cN / dtex or more. If the breaking strength is less than 1.5 cN / dtex, the strength is insufficient as a sheath-core fiber targeted by the present invention, and problems of insufficient strength arise in actual processing and use. On the other hand, since the fiber is made of meta-aramid and contains a functional agent inside the fiber, it is difficult to increase the breaking strength, and in many cases it is difficult to achieve a breaking strength of 4.0 cN / dtex or more. [Example]

[0041] The present invention will be described in detail below with reference to examples and comparative examples, but the scope of the present invention is not limited to the following examples and comparative examples. The physical properties in the examples and comparative examples were measured by the following methods.

[0042] [Weight-average molecular weight Mw and molecular weight dispersity] Analysis was performed using a high-performance liquid chromatography system equipped with a size-exclusion chromatography column in accordance with JIS-K-7252, using dimethylformamide (containing 0.01 mol% lithium chloride) as the developing solvent. Sigma-Aldrich polystyrene sets (peak top molecular weights Mp = 400-2,000,000) were used as standard molecular weight samples. The molecular weight dispersity was calculated as the weight-average molecular weight Mw / number-average molecular weight Mn.

[0043] [Single fiber fineness] Measurements were carried out in accordance with Method A of correct fineness in accordance with JIS-L-1015, and the results were expressed as apparent fineness.

[0044] [Breaking strength, breaking elongation] Measurement was carried out using a tensile tester (Instron, model: 5565) in accordance with JIS-L-1015 under the following conditions. (Measurement conditions) Grip spacing: 20mm Initial load: 0.044cN (1 / 20g / dtex) Pulling speed: 20 mm / min

[0045] [Example 1] An aramid polymer powder consisting of metaphenylenediamine and isophthaloyl monomer was synthesized by interfacial polymerization according to Japanese Patent Publication No. 47-10863. The weight-average molecular weight was 720,000. This polymer powder was dissolved in N-methyl-2-pyrrolidone (NMP) to obtain a transparent polymer solution. The mass concentration of the aramid polymer in the polymer solution was adjusted to 19%. This polymer solution was designated the first solution.

[0046] Next, the aramid polymer powder synthesized by the above method and conductive carbon black particles were dissolved and dispersed in NMP to obtain a polymer solution. At this time, the mass concentration of the aramid polymer in the polymer solution was adjusted to 16% and the mass concentration of the conductive carbon black in the aramid polymer was adjusted to 30%. This polymer solution was designated as the second solution.

[0047] The first polymer solution and the second polymer solution were heated to 85°C to form a spinning dope, and the first and second polymer solutions were joined at the nozzle of the spinneret to form multiple dope filaments each having a core of the second polymer solution and a sheath of the first polymer solution. The dope filaments were extruded from the spinneret and spun into a coagulation bath at 85°C. A spinneret with 100 circular nozzles and a hole diameter of 0.1 mm was used. The ratio of the extrusion rates of the first polymer solution and the second polymer solution was 2:1.

[0048] The composition of the coagulation bath was 43% by mass of calcium chloride, 3% by mass of NMP, and the remainder 54% by mass of water. The fiber was passed through an immersion length (effective coagulation bath length) of 100 cm at a yarn speed of 5.0 m / min, and then drawn out into the air. This coagulated yarn was washed in first and second water washing baths for a total immersion time of 180 seconds. The water temperatures in the first and second aqueous washing baths were 20°C and 30°C, respectively. This washed yarn was stretched 1.5 times in boiling water at 90°C, and then immersed in warm water at 90°C for 40 seconds for washing.

[0049] Next, the fiber was wound around a roller with a surface temperature of 170°C for dry heat treatment, and then stretched 1.5 times on a hot plate with a surface temperature of 320°C to obtain sheath-core fiber. The resulting fiber had a fineness of 2.5 dtex, a breaking strength of 1.7 cN / dtex, and a breaking elongation of 18%. The electrical conductivity of the fiber was measured and found to be 850 Ωcm. A cross-sectional photograph of the resulting fiber is shown in Figure 1.

[0050] [Example 2] An aramid polymer powder composed of metaphenylenediamine and isophthaloyl monomer was synthesized by interfacial polymerization according to Example 1. The weight-average molecular weight was 650,000. The aramid polymer powder and black pigment were dissolved and dispersed in NMP to obtain a polymer solution. The mass concentration of the aramid polymer relative to the polymer solution was adjusted to 20%, and the black pigment relative to the aramid polymer was adjusted to 2.0% by mass. This polymer solution was designated the first solution.

[0051] Next, the aramid polymer powder and the flame retardant were dissolved in NMP to obtain a polymer solution. The mass concentration of the aramid polymer in the polymer solution was adjusted to 18% and the mass concentration of the flame retardant in the aramid polymer was adjusted to 10%. This polymer solution was designated as the second solution. The first polymer solution and the second polymer solution were discharged into a coagulation bath and spun in the same manner as in Example 1. At this time, the ratio of the discharge amounts of the first polymer solution and the second polymer solution was 1:1.

[0052] The composition of the coagulation bath was 41% by mass of calcium chloride, 4% by mass of NMP, and the remainder 55% by mass of water. The fiber was passed through an immersion length (effective coagulation bath length) of 100 cm at a yarn speed of 5.0 m / min, and then drawn out into the air. This coagulated yarn was washed in first and second water washing baths for a total immersion time of 180 seconds. The water temperatures in the first and second aqueous washing baths were 20°C and 30°C, respectively. This washed yarn was stretched 1.8 times in boiling water at 90°C, and then immersed in warm water at 90°C for 40 seconds for washing.

[0053] Next, the fiber was wound around a roller with a surface temperature of 170°C for dry heat treatment, and then stretched 1.5 times on a hot plate with a surface temperature of 320°C to obtain sheath-core fiber. The resulting fiber had a fineness of 2.7 dtex, a breaking strength of 1.9 cN / dtex, and an elongation at break of 23%. The LOI of this fiber was measured and showed a very high flame retardancy of 55.

[0054] [Comparative Example 1] Sheath-core fibers were obtained by dry spinning in accordance with US 10982353. The concentrations were adjusted so that the sheath contained 2.0% by mass of black pigment relative to the aramid polymer, and the core contained 30% by mass of conductive carbon black relative to the aramid polymer. The obtained fiber had a fineness of 2.5 dtex, a breaking strength of 1.2 cN / dtex and a breaking elongation of 20%, and was therefore poor in breaking strength.

[0055] Comparative Example 2 First and second polymer solutions were prepared by a method similar to Example 1. The first and second polymer solutions were heated to 30°C to form a spinning solution. The first and second polymer solutions were joined at a spinneret outlet and extruded from a spinneret capable of forming multiple dope filaments having a core of the second polymer solution and a sheath of the first polymer solution into a coagulation bath at 35°C for spinning. A spinneret with 100 circular extrusion holes and a hole diameter of 0.1 mm was used. The extrusion rate ratio of the first polymer solution to the second polymer solution was 2:1.

[0056] The composition of the coagulation bath was 10% by mass of calcium chloride, 45% by mass of NMP, and the remaining 45% by mass of water. The fiber was passed through an immersion length (effective coagulation bath length) of 100 cm at a yarn speed of 5.0 m / min, and then drawn out into the air. This coagulated yarn was washed in first and second water washing baths for a total immersion time of 180 seconds. The water temperatures in the first and second aqueous washing baths were 20°C and 30°C, respectively. This washed yarn was stretched 1.1 times in boiling water at 90°C, and then immersed in warm water at 90°C for 40 seconds for washing.

[0057] Next, the fiber was wound around a roller with a surface temperature of 170°C for dry heat treatment, and then stretched 1.2 times on a hot plate with a surface temperature of 320°C to obtain sheath-core fiber. The obtained fiber had a fineness of 2.6 dtex, a breaking strength of 0.8 cN / dtex and a breaking elongation of 10%, and was therefore poor in breaking strength. A cross-sectional photograph of the obtained fiber is shown in Figure 2. The obtained fiber did not have a clear sheath and core, and many void structures were observed. [Industrial Applicability]

[0058] The sheath-core fiber obtained by the present invention can retain a high concentration of functional agents in the core portion of the fiber, thereby achieving functionality not previously possible. Furthermore, the protection provided by the sheath structure prevents the functional agents from being removed by washing or wear. Even with a high concentration of functional agents inside due to the sheath-core structure, the fiber has a breaking strength of 1.5 cN / dtex or more, making it less susceptible to problems during processing into fabrics and practical use, making it suitable for use in protective clothing and other applications.

Claims

1. A sheath-core fiber comprising a sheath portion made of a first polymer containing a wholly aromatic polyamide and a core portion made of a second polymer containing a wholly aromatic polyamide and a functional agent, the sheath-core fiber having a breaking strength of 1.5 cN / dtex or more.

2. 2. The sheath-core fiber of claim 1, wherein the first and second polymers both comprise polymetaphenylene isophthalamide.

3. 2. The method of claim 1, comprising the steps of: a) preparing a first polymer solution which is a solution of a first polymer containing a wholly aromatic polyamide in an amide-based solvent; b) preparing a second polymer solution, which is a solution of a second polymer including a wholly aromatic polyamide in an amide-based solvent, and which includes a functional agent in the solution; c) introducing the first polymer solution and the second polymer solution into a spinneret having separate inlets and multiple outlets for spinning filaments, and then d) discharging the first and second polymer solutions so that they meet at the outlet of the spinneret to form a plurality of dope filaments each having a core made of the second polymer solution and a sheath made of the first polymer solution; e) spinning the dope filament into an aqueous coagulation bath containing 20 to 50% by mass of an inorganic salt and 1 to 20% by mass of an amide-based solvent to form a coagulated thread; f) The coagulated yarn is stretched in a boiling water stretching bath, then dry-heat treated, and then hot stretched.

4. 4. The method for producing a sheath-core fiber according to claim 3, wherein the first and second polymers both comprise polymetaphenylene isophthalamide.

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