Method for producing meta-type wholly aromatic polyamide fiber

Abstract

To provide a method for easily producing a recycled meta-type wholly aromatic polyamide fiber by reducing microfibrils present in a recycled meta-type wholly aromatic polyamide polymer solution as much as possible.SOLUTION: A method for producing a meta-type wholly aromatic polyamide fiber comprises: a dissolution step of subjecting a meta-type wholly aromatic polyamide fiber to a dissolution treatment in a solvent containing 50 to 90 mass% of an amide-based solvent and 0 to 5 mass% of an inorganic salt at 60 to 90°C for 2 to 30 hours, thereby obtaining a polymer solution in which the weight of microfibrils relative to the total amount of the meta-type wholly aromatic polyamide polymer in the solution is 0.015 mass% or less; a spinning step of discharging the meta-type wholly aromatic polyamide polymer solution into a coagulation bath to obtain an undrawn yarn; and a drawing step of drawing the undrawn yarn obtained in the spinning step to obtain a drawn yarn.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing recycled meta-type wholly aromatic polyamide fibers by redissolving, spinning, and drawing meta-type wholly aromatic polyamide fibers recovered from used fiber structures composed of meta-type wholly aromatic polyamide fibers, in particular, meta-type wholly aromatic polyamide fibers.Furthermore, the present invention relates to a method for producing recycled meta-type wholly aromatic polyamide fibers, in which, when redissolving the meta-type wholly aromatic polyamide fibers, a meta-type wholly aromatic polyamide polymer other than the meta-type wholly aromatic polyamide fibers constituting the fiber structure is dissolved in the solution, and the solution is spun and drawn to obtain recycled meta-type wholly aromatic polyamide fibers having practically sufficient breaking strength. [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, and among such wholly aromatic polyamides, fibers of meta-type wholly aromatic polyamides (hereinafter sometimes referred to as meta-aramids), represented by polymetaphenylene isophthalamide, are particularly useful as heat-resistant and flame-retardant fibers.

[0003] Meta-type wholly aromatic polyamide fibers exhibit these properties and are used, for example, in disaster prevention and safety clothing applications such as protective clothing, and in industrial applications such as filters, electronic components, etc. Such meta-aramid fibers are mostly produced as staple fibers, and are processed into fabrics and spun yarns for various applications.

[0004] On the other hand, when disposing of these fabrics and spun yarns, incineration or the like is not possible from the viewpoint of chemical stability and flame retardancy, so that they are generally disposed of by landfilling. However, since these fabrics and spun yarns are used as industrial fibers, a large amount of aramid fiber waste poses a large environmental load, which has become a problem.

[0005] In response to the above problems, several methods for recycling meta-aramid fibers have been reported. For example, a method has been reported in which a used heat-resistant, high-performance spun yarn product is washed in advance if desired, then subjected to a crushing treatment, the crushed product is opened to form a cotton-like material, and the cotton-like material is spun to form a yarn to obtain a recycled heat-resistant, high-performance spun yarn (JP-A-2004-100066, JP-A-2006-37240, JP-A-2006-233409, JP-A-2005-105491, etc.).

[0006] On the other hand, instead of reusing fibrous products as they are as described above, methods have also been reported in which the fibrous products are chemically treated to produce a fiber soluble material and then used.For example, JP 2014-70133 A and JP 2007-321069 A report a method of producing an aramid soluble material (dope) by mixing aramid fiber waste generated in the process of producing and processing aramid fibers with an inorganic salt-containing solvent and applying shear or the like.

[0007] In addition, Japanese Patent Application Laid-Open No. 7-286061 and Japanese Patent Application Laid-Open No. 2007-321310 report a method in which the aramid dope prepared as described above is fibridized by stirring it in water while applying shear to produce aramid pulp that can be used as a structural material. Recycling flame-retardant aramid fibers in this way is significant because it reduces the environmental load and is a high-value-added product.

[0008] However, each of the above-mentioned methods is based on the premise of using fabrics, spun yarns, and fiber waste that are not soiled or only slightly soiled, and it is difficult to reuse fabrics that are severely soiled due to harsh usage environments or fabrics whose chemical structure has partially decomposed, etc. For example, aramid fibers are often used in heat-resistant bag filters used to filter hot exhaust gases from production factories, and a large amount of them are discarded, but due to the harsh usage environment, in addition to being soiled, the molecular weight is greatly reduced due to chemical decomposition, and the original strength of the fiber cannot be maintained.

[0009] Therefore, as a method for recycling wholly aromatic polyamide fibers, a method of producing recycled wholly aromatic polyamide fibers by newly spinning a recycled wholly aromatic polyamide polymer solution produced by redissolving used fabrics, spun yarns, and fiber waste is considered, but even among fibers having the same meta-type wholly aromatic polyamide structure, there are those that are easy to produce recycled wholly aromatic polyamide fibers from and those that are difficult to produce. And, when those that are difficult to produce are used as recycled raw materials, there are cases where it is difficult to produce fibers, such as frequent yarn breakage occurring when newly spinning the wholly aromatic polyamide polymer solution. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2004-100066 A [Patent Document 2] JP 2006-37240 A [Patent Document 3] JP 2006-233409 A [Patent Document 4] JP 2005-105491 A [Patent Document 5] JP 2014-70133 A [Patent Document 6] JP 2007-321069 A [Patent Document 7] Japanese Patent Application Publication No. 7-286061 [Patent Document 8] JP 2007-321310 A Summary of the Invention [Problem to be solved by the invention]

[0011] The object of the present invention is to provide a method for easily producing recycled meta-type wholly aromatic polyamide fibers by reducing as much as possible the microfibrils present in the recycled meta-type wholly aromatic polyamide polymer solution, focusing on the fact that the presence or absence of crystalline portions (microfibrils) contained in the meta-type wholly aromatic polyamide polymer solution obtained from meta-type wholly aromatic polyamide fibers affects the spinning process in the above-mentioned method for recycling wholly aromatic polyamide fibers. [Means for solving the problem]

[0012] As a result of intensive research to solve the above problems, the inventors have discovered that the weight of microfibrils in a polymer solution can be reduced as much as possible by treating used meta-type wholly aromatic polyamide fibers in a solvent containing a specific amount of an amide solvent and an inorganic salt at a specific temperature for a specific period of time, thereby arriving at the present invention.

[0013] That is, according to the present invention, 1. A method for producing a meta-type wholly aromatic polyamide fiber, comprising: a dissolving step of dissolving a meta-type wholly aromatic polyamide fiber in a solvent containing 50 to 90 mass% of an amide solvent and 0 to 5 mass% of an inorganic salt at 60 to 90°C for 2 to 30 hours to obtain a polymer solution having a microfibril weight of 0.015 mass% or less relative to the total amount of the meta-type wholly aromatic polyamide polymer in the solution; a spinning step of discharging the meta-type wholly aromatic polyamide polymer solution into a coagulation liquid to obtain an undrawn yarn; and a drawing step of drawing the undrawn yarn obtained in the spinning step to obtain a drawn yarn. 2. The method for producing a meta-type wholly aromatic polyamide fiber according to the above item 1, wherein the meta-type wholly aromatic polyamide fiber is recovered from a used textile product containing meta-type wholly aromatic polyamide fiber and / or fiber waste generated in the process of producing and processing the meta-type wholly aromatic polyamide fiber; 3. A method for producing a meta-type wholly aromatic polyamide fiber according to the above 1 or 2, in which a meta-type wholly aromatic polyamide polymer other than fibers is further mixed into the meta-type wholly aromatic polyamide polymer solution. 4. The method for producing a meta-type wholly aromatic polyamide fiber according to the above item 1, wherein the turbidity of the meta-type wholly aromatic polyamide polymer solution is 3 ppm or less. And, 5. A method for producing a meta-type wholly aromatic polyamide fiber according to the above 1, wherein the coagulation liquid is an amide-based solvent containing an inorganic salt and kept at 70 to 100°C, and the drawing process includes water washing at 20 to 30°C, hot water drawing at a magnification of 1.5 to 3.0 times at 85 to 95°C, hot water washing at 90 to 100°C, dry heat treatment with a roller having a surface temperature of 150 to 250°C, and drawing at a magnification of 1.0 to 3.0 times with a hot plate having a surface temperature of 300 to 350°C. is provided. Effect of the Invention

[0014] According to the present invention, a recycled meta-type wholly aromatic polyamide polymer solution suitable for recycling can be obtained, and by using the same, a method for producing recycled meta-type wholly aromatic polyamide fiber which is easy to spin and has the inherent strength of the fiber can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The meta-type wholly aromatic polyamide constituting the recycled meta-type wholly aromatic polyamide fiber of the present invention is composed of a meta-type aromatic diamine component and a meta-type aromatic dicarboxylic acid component, and may be copolymerized with other copolymerization components such as para-type within a range that does not impair the object of the present invention.

[0016] In the present invention, from the viewpoints of mechanical properties, heat resistance, and flame retardancy, a fiber made of a meta-type wholly aromatic polyamide mainly composed of metaphenylene isophthalamide units is particularly preferably used. In the meta-type wholly aromatic polyamide composed of metaphenylene isophthalamide units, the metaphenylene isophthalamide units preferably account for 90 mol % or more of all repeating units, more preferably 95 mol % or more, and particularly preferably 100 mol.

[0017] Examples of meta-type aromatic diamine components that are raw materials for the meta-type wholly aromatic polyamide used in the present invention include metaphenylenediamine, 3,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl sulfone, etc., and derivatives having a substituent such as halogen or an alkyl group having 1 to 3 carbon atoms on the aromatic ring, such as 2,4-toluylenediamine, 2,6-toluylenediamine, 2,4-diaminochlorobenzene, 2,6-diaminochlorobenzene, etc. Among these, it is preferable to use only metaphenylenediamine, or a mixed diamine containing 85 mol % or more, preferably 90 mol % or more, and particularly preferably 95 mol % or more of metaphenylenediamine.

[0018] Examples of the raw material of the meta-type aromatic dicarboxylic acid component constituting the meta-type wholly aromatic polyamide include meta-type aromatic dicarboxylic acid halides. Examples of the 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 halogen or an alkoxy group having 1 to 3 carbon atoms on the aromatic ring, such as 3-chloroisophthalic acid chloride. Among them, isophthalic acid chloride itself or a mixed carboxylic acid halide containing 85 mol% or more, preferably 90 mol% or more, particularly preferably 95 mol% or more of isophthalic acid chloride is preferable.

[0019] In the present invention, a fiber made of the meta-type wholly aromatic polyamide or a fiber structure made of the meta-type wholly aromatic polyamide is used. Here, the fiber structure refers to fabrics such as monofilaments, yarns, spun yarns, cotton, braids, woven and knitted fabrics, and nonwoven fabrics made of meta-type wholly aromatic polyamide fibers or containing meta-type wholly aromatic polyamide fibers, and includes new products, defective products, and products used in the market.

[0020] In the present invention, a fiber structure containing a mixture of fibers other than meta-type wholly aromatic polyamide fibers can also be used. However, after the dissolving operation in an amide-based solvent described below, it is necessary to remove insoluble other fiber components by filtration.

[0021] The meta-type wholly aromatic polyamide fiber constituting the fiber structure used in the present invention is assumed to have undergone chemical decomposition due to the usage environment, and the weight-average molecular weight of the meta-type wholly aromatic polyamide fiber is sufficient to be at least 200,000, and preferably at least 300,000. On the other hand, if the molecular weight is less than 200,000, in order to obtain a fiber with sufficient strength, it is necessary to mix a large amount of unused meta-type wholly aromatic polyamide polymer in the process described below, which reduces the effect of recycling.

[0022] The meta-type wholly aromatic polyamide fiber of the present invention is produced by using a meta-type wholly aromatic polyamide fiber or a fiber structure composed of a meta-type wholly aromatic polyamide fiber through the following steps: spinning solution preparation step, spinning / coagulation step, washing step, wet (hot water) drawing step, dry heat treatment step, and hot drawing step.

[0023] In the spinning solution preparation step, meta-type wholly aromatic polyamide fibers or a fiber structure composed of meta-type wholly aromatic polyamide fibers is dissolved in a solvent to prepare a meta-type wholly aromatic polyamide polymer solution (dope) for spinning. In preparing the meta-type wholly aromatic polyamide polymer solution, an amide-based solvent is usually used, and examples of the solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethylacetamide (DMAc). Another example is dimethylsulfoxide (DMSO), which is an aprotic polar solvent. Among these, it is preferable to use NMP or DMAc from the viewpoints of solubility and handling safety.

[0024] In the spinning solution preparation step, the amount of meta-type wholly aromatic polyamide fiber or fiber structure composed of meta-type wholly aromatic polyamide fiber dissolved in the solvent is preferably 10 to 30% by mass. If the amount of the fiber or fiber structure is less than 10% by mass, the fiber or fiber structure will not form a thread, will become devitrified, and will break during solidification. On the other hand, if the amount of the fiber or fiber structure exceeds 30% by mass, the weight of microfibrils described below may become too large.

[0025] In the above spinning solution preparation step, it is essential that the content of the amide solvent in the meta-type wholly aromatic polyamide polymer solution is 50 to 90% by mass, and preferably 60 to 80% by mass.

[0026] In the present invention, it is necessary to introduce an inorganic salt to dissolve the fibers, and it is essential to use an amide solvent containing 0 to 5 mass % of an inorganic salt in the meta-type wholly aromatic polyamide polymer solution. The preferred content of the inorganic salt is 0 to 3 mass %.

[0027] Here, for example, if the inorganic salt is contained in an amount exceeding 10 mass %, the dissolution rate of the fiber increases, but it becomes difficult to adjust the concentration of the spinning solution, which will be described later. As the inorganic salt, it is preferable to use a chloride salt such as calcium chloride, magnesium chloride, or lithium chloride.

[0028] In the present invention, in order to prepare a spinning solution that is easy to recycle, it is important to completely dissolve the meta-type wholly aromatic polyamide fiber. Since the insoluble meta-type wholly aromatic polyamide is transparent, it is difficult to confirm visually, but it can be confirmed using a polarizing microscope. Furthermore, the undissolved microfibrils can be separated from the spinning solution by centrifugation, and the smaller the amount of microfibrils, the easier it is to prepare recycled meta-type wholly aromatic polyamide fiber.

[0029] In the present invention, it is essential to obtain a polymer solution in which the microfibril weight relative to the total amount of meta-type wholly aromatic polyamide polymer is 0.015 mass% or less. In order to obtain such a meta-type wholly aromatic polyamide polymer solution in which the microfibril weight is reduced as much as possible, it is necessary to dissolve meta-type wholly aromatic polyamide fibers in an amide solvent containing an inorganic salt at 60 to 90°C for 2 to 30 hours.

[0030] Here, if the dissolution temperature is less than 60°C, it becomes difficult to completely dissolve the meta-type wholly aromatic polyamide fiber, while if the dissolution temperature exceeds 90°C, the risk of fire increases due to the problem of the flash point. Furthermore, if the dissolution time is less than 2 hours, it becomes difficult to completely dissolve the meta-type wholly aromatic polyamide fiber, while if the dissolution time exceeds 30 hours, the hue of the raw cotton is affected, and a problem occurs in which the target hue cannot be obtained.

[0031] In addition, the preparation of the meta-type wholly aromatic polyamide polymer solution is also affected by the form of the meta-type wholly aromatic polyamide fiber product when it is dissolved. For example, in the case of a cloth, it is difficult to dissolve the product if it is more than 5 cm square, so it is preferable to cut it to a size smaller than that.

[0032] As described above, in the present invention, a fiber structure containing a mixture of fibers other than meta-type wholly aromatic polyamide fibers can also be used. However, when dissolving the fiber structure in an amide-based solvent containing the inorganic salt, it is necessary to remove insoluble other fiber components by filtration.

[0033] Furthermore, in the present invention, the turbidity of the meta-type wholly aromatic polyamide polymer solution is preferably 3 ppm or less. Here, the turbidity is an index representing the amount of undissolved fibers, and if the turbidity exceeds 3 ppm, a large amount of poorly dissolved fibers will be present in the spin dope, which may cause fiber breakage during spinning.

[0034] In the present invention, for example, as described above, when using fibers that are used in a harsh environment and are very dirty or fibers whose chemical structure has been partially decomposed, a meta-type wholly aromatic polyamide polymer other than fibers, which is different from the meta-type wholly aromatic polyamide fibers that constitute the fiber structure, may be mixed in the spinning solution to adjust the concentration and weight average molecular weight.

[0035] When a meta-type wholly aromatic polyamide polymer other than the fiber is used in combination, the meta-type wholly aromatic polyamide polymer preferably has the same composition as the meta-type wholly aromatic polyamide fiber and a weight average molecular weight of 600,000 or more. Any polymerization method may be used to obtain the meta-type wholly aromatic polyamide polymer. The meta-type wholly aromatic polyamide polymer is preferably used in an amount of 0 to 330% by mass relative to the meta-type wholly aromatic polyamide fiber constituting the fiber structure.

[0036] The mass ratio of the meta-type wholly aromatic polyamide polymer in the above meta-type wholly aromatic polyamide polymer solution must be determined based on the weight average molecular weight of the meta-type wholly aromatic polyamide fiber constituting the fiber structure to be used, and it is preferable to adopt a mass ratio that will give sufficient fibers in the spinning process described below.

[0037] In order to obtain sufficient spinnability and fiber strength in the spinning step described below, it is preferable that the weight average molecular weight of the meta-type wholly aromatic polyamide fiber constituting the fiber structure and the meta-type wholly aromatic polyamide polymer to be mixed therewith are both 500,000 or more. If no meta-type wholly aromatic polyamide polymer is mixed, problems may occur, depending on the weight average molecular weight of the meta-type wholly aromatic polyamide fiber constituting the fiber structure, such as insufficient strength being not achieved due to poor drawing, or unstable spinning, making it impossible to carry out production.

[0038] The concentration of the meta-type wholly aromatic polyamide in the spinning solution may be appropriately selected from the viewpoints of the coagulation rate in the subsequent spinning / coagulation step and the solubility of the meta-type wholly aromatic polyamide, and is usually preferably in the range of 10 to 30% by mass, more preferably in the range of 15 to 25% by mass in order to achieve stable spinning.

[0039] The thus obtained meta-type wholly aromatic polyamide spinning solution is spun into the coagulation solution in the spinning and coagulation step to coagulate. 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, the number and arrangement of the spinneret spinning holes do not need to be particularly limited, and for example, a multi-hole spinneret for staple fibers having 10 to 30,000 holes and a spinning hole diameter of 0.03 to 0.2 mm may be used. The temperature of the dope when spun from the spinneret is suitably in the range of 20 to 90°C, and particularly preferably 70 to 90°C.

[0040] The coagulation bath used to obtain the meta-type wholly aromatic polyamide fiber of the present invention is an aqueous solution containing 30 mass %, preferably 35 to 45 mass %, of an inorganic salt such as calcium chloride, lithium chloride, magnesium chloride, etc., and 1 to 20 mass %, preferably 3 to 15 mass %, of an amide solvent, at a temperature in the range of 70 to 100° C. Also known is a method using an aqueous solution of an amide solvent that is substantially free of inorganic salts, but such a coagulation solution makes it impossible to perform stable spinning or to obtain yarn with sufficient strength.

[0041] The coagulated yarn thus obtained is thoroughly washed in an aqueous washing bath at 20 to 30°C and sent to a wet drawing step. Usually, wet drawing is performed in a hot water drawing bath at 85 to 95°C, and the appropriate draw ratio is 1.5 to 3.0 times, and more preferably 2.0 to 3.5 times. In the present invention, wet drawing is performed within this range of draw ratio to increase the molecular chain orientation, thereby ensuring the breaking strength of the finally obtained fiber.

[0042] The fibers that have been subjected to the above washing and wet stretching steps are preferably washed with hot water at 90 to 100°C, and then subjected to a dry heat treatment step. In the dry heat treatment step, the fibers that have been washed and stretched in the above washing and wet stretching steps are dry heat treated with a roller at a surface temperature of preferably 150 to 250°C, more preferably 100 to 200°C. Here, the dry heat treatment is not particularly limited, but is preferably performed at a fixed length. The temperature of the above dry heat treatment refers to the set temperature of a fiber heating means such as a hot plate or a heated roller.

[0043] In the present invention, the fiber that has been subjected to the above-mentioned dry heat treatment step is subjected to a hot drawing step. In the hot drawing step, drawing is performed while applying heat treatment with a hot plate having a surface temperature of 300 to 350° C. The appropriate draw ratio is 1.0 to 3.0 times, and more preferably in the range of 1.5 to 2.5 times.

[0044] The breaking strength of the meta-type wholly aromatic polyamide fiber obtained by the above method is preferably 3.0 cN / dtex or more, more preferably 3.0 to 6.0 cN / dtex. Furthermore, the breaking elongation is preferably 20% or more. Furthermore, the heat shrinkage rate of the undrawn yarn is preferably 10% or less. Furthermore, the single fiber fineness is preferably 0.5 to 15.0 dtex, more preferably 1.4 to 2.5 dtex. If the single fiber fineness is less than 0.5 dtex or more than 15 dtex, the processability is poor and stable spinning cannot be achieved. Furthermore, the limiting oxygen index (LOI) is preferably 26 or more. EXAMPLES

[0045] 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.

[0046] [Single yarn size] Measurements were carried out in accordance with JIS L 1015, Method A for corrected fineness, and the results were expressed as apparent fineness.

[0047] [Breaking strength, elongation] Measurements were performed 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: 20mm / min

[0048] [Microfibril weight measurement] Meta-type fully aromatic polyamide fibers were dissolved in N-methyl-2-pyrrolidone (NMP) and calcium chloride, and the resulting polymer was centrifuged (rotation speed: 500-2000 rpm, centrifugation time: 30-60 minutes) to obtain microfibrils. The weight of the obtained microfibrils was measured using a digital precision balance manufactured by LACHOI.

[0049] [Turbidity measurement] Meta-type wholly aromatic polyamide fibers were dissolved in N-methyl-2-pyrrolidone (NMP) and calcium chloride to obtain a polymer. The polymer was placed in a quartz cell and set in a CENTRAL KAGAKU SS / TURBIDITY METER ST-100 turbidity meter to measure the turbidity.

[0050] [Measurement of heat shrinkage of undrawn yarn] Uncrimped yarn was heat-treated in an oven at 300°C for 15 minutes, the length before and after treatment was measured, and the heat shrinkage rate was calculated. Heat shrinkage rate of undrawn yarn = (before treatment - after treatment) / before treatment x 100

[0051] [Limiting Oxygen Index (LOI) measurement] Flame retardancy was measured by measuring the LOI value according to the LOI measurement method of JIS K-7201. An LOI value of 26 or more is considered to be excellent in flame retardancy.

[0052] [Example 1] 21.5% by mass of used polymetaphenylene isophthalamide fiber fabric cut into 5 cm squares was dissolved in 76.0% by mass of N-methyl-2-pyrrolidone (NMP) containing 2.5% by mass of calcium chloride powder at 80°C for 10 hours to obtain a transparent polymer solution (meta-type wholly aromatic polyamide polymer solution). The microfibril weight relative to the total amount of meta-type wholly aromatic polyamide polymer contained in this polymer solution was 0.012% by mass, and the turbidity was 2.8 ppm.

[0053] This polymer solution was heated to 89°C and used as a spinning dope, which was discharged from a circular spinneret with 100 discharge holes each having a hole diameter of 0.12 mm into a coagulation bath at 70°C for spinning. The composition of this coagulation bath was 35% by mass of calcium chloride, 3% by mass of NMP, and the remaining 62% by mass of water. The immersion length (effective coagulation bath length) was 120 cm, and the yarn was passed through at a yarn speed of 5.0 m / min, and then drawn out into the air.

[0054] The coagulated yarn was washed in the first and second water washing baths, with the total immersion time being 230 seconds. The temperatures of the first and second aqueous washing baths were 20 and 30° C., respectively. The washed yarn was then stretched 2.5 times in hot water at 90° C., and subsequently immersed in hot water at 95° C. for 40 seconds and washed.

[0055] The fiber was then wound around a roller with a surface temperature of 170°C for dry heat treatment, and then stretched 1.5 times using a hot plate with a surface temperature of 340°C to obtain a meta-type wholly aromatic polyamide fiber. The fiber had a single fiber fineness of 2.2 dtex, breaking strength of 4.1 cN / dtex, breaking elongation of 51%, LOI value of 29, and heat shrinkage of 7.0%.

[0056] [Example 2] 15.0% by mass of used polymetaphenylene isophthalamide fiber fabric cut into 5 cm squares was dissolved in 82.5% by mass of N-methyl-2-pyrrolidone (NMP) containing 2.5% by mass of calcium chloride powder at 80°C for 10 hours to obtain a transparent polymer solution (meta-type wholly aromatic polyamide polymer solution). The microfibril weight contained in this polymer was 0.009% by mass, and the turbidity was 1.5 ppm.

[0057] This polymer solution was spun, washed with water, drawn in hot water, dry-heated, and drawn on a hot plate to obtain a meta-type wholly aromatic polyamide fiber in the same manner as in Example 1. The obtained fiber had a single fiber fineness of 2.2 dtex, a breaking strength of 4.3 cN / dtex, a breaking elongation of 52%, an LOI value of 29, and a heat shrinkage rate of 6.0%.

[0058] [Example 3] 6.5% by mass of used polymetaphenylene isophthalamide fiber fabric cut into 5 cm squares was dissolved in 76.0% by mass of N-methyl-2-pyrrolidone (NMP) containing 2.5% by mass of calcium chloride powder at 80°C for 10 hours. 15.0% by mass of polymetaphenylene isophthalamide polymer powder other than fibers was further dissolved in this solution to obtain a transparent polymer solution (meta-type wholly aromatic polyamide polymer solution). The weight of microfibrils contained in this polymer was 0% by mass, and the turbidity was 1.0 ppm.

[0059] This polymer solution was spun, washed with water, drawn in hot water, dry-heated, and drawn on a hot plate to obtain a meta-type wholly aromatic polyamide fiber in the same manner as in Example 1. The obtained fiber had a single fiber fineness of 2.1 dtex, a breaking strength of 4.4 cN / dtex, a breaking elongation of 52%, an LOI value of 29, and a heat shrinkage rate of 5.5%.

[0060] [Example 4] A transparent polymer solution (meta-type wholly aromatic polyamide polymer solution) was obtained in the same manner as in Example 2, except that 7.0% by mass of used polymetaphenylene isophthalamide fiber fabric and 14.5% by mass of polymetaphenylene isophthalamide polymer powder other than fibers were used. The weight of microfibrils contained in this polymer was 0.001% by mass, and the turbidity was 1.5 ppm.

[0061] This polymer solution was spun, washed with water, drawn in hot water, dry-heated, and drawn on a hot plate to obtain a meta-type wholly aromatic polyamide fiber in the same manner as in Example 1. The obtained fiber had a single fiber fineness of 1.7 dtex, a breaking strength of 5.6 cN / dtex, a breaking elongation of 55%, an LOI value of 29, and a heat shrinkage rate of 6.0%.

[0062] [Example 5] A transparent polymer solution (meta-type wholly aromatic polyamide polymer solution) was obtained in the same manner as in Example 2, except that the dissolution conditions of the fabric were changed to 75° C. and 20 hours in Example 2. The weight of microfibrils contained in this polymer was 0.01% by mass, and the turbidity was 2.5 ppm.

[0063] This polymer solution was spun, washed with water, drawn in hot water, dry-heated, and drawn on a hot plate to obtain a meta-type wholly aromatic polyamide fiber in the same manner as in Example 1. The obtained fiber had a single fiber fineness of 2.3 dtex, a breaking strength of 4.0 cN / dtex, a breaking elongation of 50%, an LOI value of 29, and a heat shrinkage rate of 7.0%.

[0064] [Comparative Example 1] A polymer solution (meta-type wholly aromatic polyamide polymer solution) was obtained in the same manner as in Example 2, except that 9.0 mass% of used polymetaphenylene isophthalamide fiber fabric and 12.5 mass% of polymetaphenylene isophthalamide polymer powder other than fibers were used and the dissolution conditions of the fabric were changed to 80° C. and 1.5 hours. The weight of microfibrils contained in this polymer solution was 0.2 mass%, and the turbidity was 8.5 ppm, and a transparent polymer solution was not obtained.

[0065] An attempt was made to spin this polymer solution, wash it with water, draw it in hot water, dry heat treat it, and draw it on a hot plate in the same manner as in Example 1, but a large amount of yarn breakage occurred in the drawing process, and it was not possible to stably obtain meta-type wholly aromatic polyamide fibers.

[0066] [Comparative Example 2] A polymer solution (meta-type wholly aromatic polyamide polymer solution) was obtained in the same manner as in Example 2, except that the dissolution conditions for the fabric were changed to 50° C. and 10 hours. The weight of microfibrils contained in this polymer solution was 1.0 mass %, and the turbidity was 10.0 ppm, and a transparent polymer solution was not obtained.

[0067] An attempt was made to spin this polymer solution, wash it with water, draw it in hot water, dry heat treat it, and draw it on a hot plate in the same manner as in Example 1, but a large amount of yarn breakage occurred in the drawing process, and it was not possible to stably obtain meta-type wholly aromatic polyamide fibers.

[0068] [Comparative Example 3] A polymer solution (meta-type wholly aromatic polyamide polymer solution) was obtained in the same manner as in Example 2, except that 32.0% by mass of used polymetaphenylene isophthalamide fiber fabric was used, the amount of N-methyl-2-pyrrolidone (NMP) was 65.5% by mass, and the fabric dissolution conditions were changed to 90° C. and 30 hours. The weight of microfibrils contained in this polymer solution was 0.05% by mass, and the turbidity was 7.0 ppm, and a transparent polymer solution was not obtained.

[0069] An attempt was made to spin this polymer solution, wash it with water, draw it in hot water, dry heat treat it, and draw it on a hot plate in the same manner as in Example 1, but a large amount of yarn breakage occurred in the drawing process, and it was not possible to stably obtain meta-type wholly aromatic polyamide fibers. The evaluation results of the above examples and comparative examples are shown in Table 1.

[0070] [Table 1] [Industrial Applicability]

[0071] According to the present invention, a method for easily producing recycled meta-type wholly aromatic polyamide fibers can be provided by reducing as much as possible the microfibrils present in a recycled meta-type wholly aromatic polyamide polymer solution, which can greatly contribute to reducing the environmental load and has extremely great industrial value.

Claims

1. The method for producing a meta-type wholly aromatic polyamide fiber includes the steps of: dissolving a meta-type wholly aromatic polyamide fiber in a solvent containing 50 to 90 mass % of an amide solvent and 0 to 5 mass % of an inorganic salt at 60 to 90° C. for 2 to 30 hours to obtain a polymer solution having a microfibril weight of 0.015 mass % or less relative to the total amount of the meta-type wholly aromatic polyamide polymer in the solution; a spinning step of discharging the meta-type wholly aromatic polyamide polymer solution into a coagulation liquid to obtain an undrawn yarn; and a drawing step of drawing the undrawn yarn obtained in the spinning step to obtain a drawn yarn.

2. The method for producing meta-type wholly aromatic polyamide fibers according to claim 1, wherein the meta-type wholly aromatic polyamide fibers are recovered from used textile products containing meta-type wholly aromatic polyamide fibers and / or from fiber waste generated in the process of producing and processing meta-type wholly aromatic polyamide fibers.

3. 3. The method for producing meta-type wholly aromatic polyamide fibers according to claim 1, wherein a meta-type wholly aromatic polyamide polymer other than fibers is further mixed with the meta-type wholly aromatic polyamide polymer solution.

4. 2. The method for producing meta-type wholly aromatic polyamide fibers according to claim 1, wherein the turbidity of the meta-type wholly aromatic polyamide polymer solution is 3 ppm or less.

5. 2. The method for producing a meta-type wholly aromatic polyamide fiber according to claim 1, wherein the coagulation liquid is an amide-based solvent containing an inorganic salt and kept at 70 to 100°C, and the drawing process includes water washing at 20 to 30°C, hot water drawing at a magnification of 1.5 to 3.0 times at 85 to 95°C, hot water washing at 90 to 100°C, dry heat treatment with rollers having a surface temperature of 150 to 250°C, and drawing at a magnification of 1.0 to 3.0 times with a hot plate having a surface temperature of 300 to 350°C.

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